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Teledentistry: A literature review of evolution and ethicolegal aspects

How to cite this article: Bhargava A, Sabbarwal B, Jaggi A, Chand S, Tandon S. Teledentistry: A literature review of evolution and ethicolegal aspects. J Global Oral Health 2019;2(2):128-33.
Teledentistry is about delivering data from one point (spoke site) to another point (hub site) using telecommunications technology. Teledentistry is a relatively new field that combines telecommunication technology and dental care. It provides new opportunities for education and delivery of care that offers much potential and challenges. Teledentistry is also useful in long-distance clinical training and continuing education, screening, and dentist laboratory communication. In rural areas, where there is a shortage of specialists, lack of comprehensive and sophisticated health-care teledentistry can extend care to remote patient populations at a reasonable cost as well as ease the problem of a shortage of specialized dental consultants.
Telecommunication
Teledentistry, electronic health records.

- INTRODUCTION
Teledentistry is a developing area of dentistry that links dental providers to their patients. Cook defined teledentistry as “the practice of using videoconferencing technologies to diagnose and provide advice about treatment over a distance.” [ 1 ] Interactive access to specialist opinion is provided with the use of telecommunication and computer technologies which is not limited by time and space. [ 2 ] It uses telecommunication technology to send data, graphics, audio, and video images between participants who are physically separated for the purpose of clinical care [ 3 ] (Association of American Medical Colleges). [ 4 ] Telemedicine program was launched in 1994 by the United States military. [ 5 ] Teledentistry harnesses the capability of modern telecommunications to allow offsite dentists of any specialty to assist their colleagues in providing care. [ 6 ] This consultation could be direct (between the patient and the expert) or indirect (between the patient’s doctor and the expert). [ 7 ]
- EVOLUTION OF TELEDENTISTRY
The changes within the past decade in the speed and method of data transfer have prompted clinicians and information technology experts to reevaluate teledentistry as a highly valuable tool. For example, cases submitted to the dental laboratories have subtle complications or esthetic nuances that require direct contact between the dentist and the laboratory technician. In these instances, the ability to send color images of the patient’s teeth and then to talk about the images can help to prevent making improperly constructed appliances, thereby saving time and money. [ 8 ] The following steps show the use of technological advancements for better execution of teledentistry over the decade.
- Step 1: Image file transfer through modem
In a study conducted at Fort Gordon, Georgia, a dental image management system was used in conjunction with an intraoral camera to capture color images of a patient’s mouth. These images were then transmitted over a 9600-band modem from the Dental Clinic in Fort McPherson, Georgia, to Fort Gordon, a distance of 120 miles. Fifteen patients were referred for periodontal surgery to Fort Gordon. After healing had been determined to be complete, the patients then reported back to Fort McPherson for suture removal and intraoral imaging. Color still images of the surgical sites were then transmitted to the periodontist who performed the surgery. In conclusion, 14 patients were saved from visiting the hospital again. The patients uniformly felt that they had received better care and time was saved. The dentists were also comfortable in their ability to make proper decisions and diagnosis using the equipment. [ 9 ]
- Step 2: File image transfer through satellite
The second study was performed in Haiti in 1995. In this study, a video teleconferencing system was used over international maritime satellite (Inmarsat) allowing the deployed dentists to talk face to face with specialists at Walter Reed Army Medical Center in Washington. The images were obtained with a high-resolution still camera and transmitted to Walter Reed, where the specialists reviewed them. This study showed that the video quality was insufficient for dental diagnosis of most pathologic conditions but that the Kodak gave diagnostic images. Due to the encouraging results of these two studies, it was decided to expand the scope of the Fort Gordon study to an actual research project. [ 10 ]
- Step 3: Integrated services digital network (ISDN)-based teledentistry system
For this project, the Army posts of the Southeast Dental Service Support Area were networked using desktop video teleconferencing equipment and ISDN lines at 128 kbps data rates, an intraoral camera, and a document camera. This equipment allows live videoconsulting as well as capability to send still images. Whiteboarding is a feature of this system, which allows users to do annotation on an image.
In 1996, the US Department of Defense established a medical network in Bosnia that connected army field dentists at five regional military medical centers in the United States (Washington, Texas, California, District of Columbia, and Hawaii). Using commercially available technology, dentists transmitted radiographs, color images, and full-motion videos to remote field hospitals for diagnostic support and obtained results and prescription support by utilizing digitalized medical logistics and online clinical information. [ 5 ]
For primetime III, an integrated frame relay ISDN architecture was used. Landstuhl Regional Medical Center in Germany was the center, integrated into the internet and the commercial ISDN gateway link to the world. The Department of Defense concluded that the ISDN- based systems provide invaluable support for clinical decision making in time-critical contexts, though its major drawback is expense, both in initial purchase and equipment maintenance. [ 11 ] However, the savings in travel and work hours lost were recouped in <2 years.
- Step 4: Plain old telephone system (POTS)-based teledentistry system
A POTS-based teledentistry network was tested in 1997 and implemented in Germany, Italy, Belgium, England, Spain, and Portugal. This system was mainly deployed to smaller dental clinics in Europe, which did not have internet access at the time. The POTS-based network has been established for the US Army, Navy, and Air Force dental clinics at over 52 sites in Europe and 16 sites in the United States. [ 10 ]
The POTS-based systems consist of a desktop computer, a 28.8 kbps modem, software and hardware (Share vision PCS 3000), intraoral camera, and a document camera. An online database was developed to gather information on user satisfaction, number of consults, frequency of use, and video or audio problems occurred and avoided travel. Dental providers complete this form each time they use the system. The data collected from the use of the POTS-based systems had been analyzed. The advantage of the POTS-based systems was that it was user friendly, low in cost, and easy to maintain.
- Step 5: Web-based teledentistry systems
The web-based teledentistry system (October, 1997) consists of a laptop, a digital camera, a web browser, and requires internet access. Since most of the dental clinics in Europe have a local area network and access to internet through the medical hospitals, this system is being used in over 50 triservice dental clinics in Europe. A web-based clinical database has been developed for storing the consults. This system uses MS SQL server 7.0 for storing the consults as the database server and MS Internet information server 4.0 as web server. [ 12 ]
The referring dentist after logging into a secure server chooses a specialty (orthodontics, oral medicine, oral and maxillofacial surgery, endodontics, oral pathology, periodontics, prosthodontics, and pediatric dentistry). He then sends the patient’s details including complaint, images, and radiographs to the specialist. The data gets sent to the database and an electronic mail notifies the specialist of the pending consult, which he will access through the internet. A plug-in developed in visual C++ enables him to do image manipulations such as contrast and brightness changes of the radiographs within the web browser. The specialist after reviewing the details writes diagnosis and treatment plan which is sent through email to the referring dentist. [ 12 ]
In this practice, the data collected on the web-based teledentistry referrals show an average of forty consults per month. Advantages of a web-based teledentistry consultation system include low cost, expandable to a wide range of locations, and more complete information for data analysis.
Advancement in the technology has broadened the scope of utilization of teledentistry.
- Patient care
In some of the remote clinics, a patient must travel hundreds of miles to receive specialty care. Often pre- and post-operative visits take only a few minutes of actual appointment time but require hours of travel by the patient. Cost and travel time required by the patient are reduced. Referral to specialists, consultations, and laboratory communications are some of the clinical areas where improvement can be done.
Teledentistry in consulting can present in two forms: Real-time consulting and store-and-forward consulting. In real time consulting, a videoconferencing format is used. In store and forward consulting, the referring dentist collects applicable data and sends them to the consulting doctor through an electronic medium. [ 13 ] The consultant reviews the material and returns as opinion through the same route. Either way, the potential for access to specialized care is increased. On the other hand, the potential for error could also be increased, as well as the potential for practitioners to incur additional liability. In either case, two or more practitioners are involved in the consultation. It is not advised to make a diagnosis based only on a telephone conversation with a patient. It is equally inadvisable to make a diagnosis, treatment recommendation, or both without an examination by a licensed practitioner. [ 14 ]
- Continuing dental education
Through the use of video teleconferencing equipment, the lectures could be broadcasted to any clinic where continuing dental education is difficult to obtain.
- LEGAL AND ETHICAL ISSUES IN PRACTISING TELEDENTISTRY
The universal use of teledentistry, accessibility to information has raised a number of legal concerns. Dentistry can be viewed as falling under telemedicine laws by inclusion. Alabama has recommended amendment of its Dental Practice Act to include the Federation of State Medical Award’s interstate licensure model. [ 15 ]
The importance of the World Wide Web and its effect on the dental profession will be profound. There are no “rules” on the internet – there is no license and verification. [ 16 ]
Even simply sending as E-mail message to a colleague could be considered a teledentistry referral and may come under legal scrutiny. It is each practitioner’s responsibility to understand the implications of the use of information technologies and their associated legal ramifications for the dental practice. Each practitioner should seek the advice of a qualified attorney who is familiar with teledentistry and its implications. [ 17 ]
Earlier practitioners were free to communicate and exchange information with colleagues in other states. Inherently, a level of confidentiality was assumed when using these methods – the information was exchanged with only a single individual or office. Many states have decided that such referrals constitute the practice of medicine or dentistry in those states. Therefore, practitioners engaged in telemedicine/teledentistry must be licensed in each state in which they practice. In the United States currently, 20 states have restrictive licensure laws that required the health-care practitioner to obtain a full license (with some exceptions) to participate in teledentistry across state lines. This allows a practitioner who is licensed and good standing in one state to avail himself or herself of reciprocity with any of the states. If the matter is adjudicated, the practitioner could be found guilty of practicing without a license in those states in which he or she is practicing teledentistry. The risk may extend to software and hardware manufacturers as well.
- Jurisdiction
To establish jurisdiction, a state court must establish that an act was committed and that either the act resulted in injury within one jurisdiction or the party has sufficient “minimum contacts” with the jurisdiction so that it would reasonably be expected to defend itself there.
- Malpractice
Any practitioner offering an opinion over the internet, either to a colleague or a layperson, through e-mail or formal consultation has indeed established a doctor-patient relationship. [ 18 ] With respect to real-time videoconferencing consultations, technology now makes it possible for the patient, dentist, and consultant to all be “present” at the same time, with the consultation being rendered both with the patient’s consent and on behalf of that patient. Under these circumstances, it is increasingly likely that the courts will determine that a doctor-patient relationship has been established through the electronic medium. Once this relationship is established, the consultant has a duty to act within the parameters of the standard of care.
Establishment and acceptance of an altered standard of care, be it higher or lower than that for a traditional consultation, carries with them certain risks. Might providers be guilty of negligence for not availing themselves of the specialized care available through teledentistry? Failure to refer may constitute negligence. Telemedicine may affect the standard of care by elevating the standard to the point that not having telemedical capacity is in fact substandard.
The next concern is establishing the injuries a patient might suffer (or claim to suffer) stemming from a teledentistry referral. A consultant may be liable for the ultimate treatment undertaken by the referring dentist, especially if the consultant failed to independently inform the patient of his or her findings (termed “negligent supervision”).
- Technological issues
With teledentistry, it is no longer solely not only the failure of the practitioner (to treat or diagnose) but also the failure of the technology itself that can have a negative impact on outcomes of care. Understanding the technology and its potential problems by the patients are areas of concern. [ 19 ] Health- care professionals may find themselves sued for equipment failure or malfunction, if that failure results either directly or indirectly in an injury to a patient. In addition, the equipment manufacturer, hardware and software distributors, and utility and equipment service companies could be joined in a claim for equipment failure or malfunction and could be held liable for technological problems associated with their products under one of the five theories of product liability, including strict liability. There is currently little case law available discussing product liability as it relates to telemedicine.
Patients should be informed that the potential exists for their medical or dental information to be accessed by unauthorized people, despite the best efforts of the physician or dentist treating the patient. Clearly, there are steps the practitioner can take to make it more difficult for a transmission to be intercepted. For example, data encryption, password protection, and user access logs can help deter most people and protect patient confidentiality. [ 20 ] Physicians and dentists engaged in telemedicine and teledentistry must make every effort to ensure the security of their systems, as well as any data they may transmit.
Patients should be made aware that their medical information might be subject to the differing laws and differing jurisdictions depending on where and how the information is transmitted.
While it is certainly true that unauthorized users could access paper records (and that disclosure of that fact is not the norm), electronic storage increases the number of potential unauthorized viewers; hence, disclosure of this possibility is advisable.
It should be stressed that no security protocol will be 100% effective, but a good faith effort to maintain system and data security will be important should a practitioner be challenged in a court of law.
Patients should be informed of the security measures that a practitioner uses to ensure the safety of patient data. An explanation that there is the potential for a breach of that security should be included with this statement. Computer viruses, hacking, hardware and software failure, and disasters such as fire or theft may result in loss of patient information. Again, the consent form should mention that while efforts are made to ensure data protection, some loss of information might be inevitable should an extreme event occur.
No discussion of data security would be complete without a discussion of the importance of regular, secure data backup. This becomes a critical issue when patient records are involved and exposes a practice owner to a negligence suit in the event of lost data when a backup is unavailable. Should patient data be unrecoverable, it would be difficult for the practitioner to avoid being held liable for negligence.
- Informed consent
Patients should be informed that there may be the risk of an inaccurate diagnosis, treatment or both as a result of a failure in the technologies. For those practitioners who choose to venture toward this new frontier and its considerable additional legal exposure, a teledentistry consultation form is recommended and is required in some states. While not a guarantee of protection, this form could offer some substantiation of a good faith effort at patient informed consent. It should become a standard part of our patient documentation. Such documentation involves a description of the teledentistry arrangement and of the credentials of the consulting doctors. [ 21 ] The patient should understand that he or she would have access to all information transmitted during a teledentistry interaction and that he or she may withhold or withdraw at any time without affecting the right to future treatment.
- Ethical issues
Maintenance of security during internet commerce and internet fraud are some of the ethical issues. These ethical concerns extend to teledentistry. Of particular concern in the area of telemedicine are public protection and fraud.
- Public protection
Health-care providers have an obligation to ensure that they as individuals as well as the profession as a whole provide the best possible health care to patients. While the internet offers the promise of an unprecedented improvement in patient care, it also can be easily abused. There are standards available helping internet users decipher the immense quantity of information available, but no simple way to verify the credentials of the provider of that information.
When an inquiry is posted (perhaps by a patient seeking treatment advice), anyone is free to respond – dentists, educators, other patients, and manufacturers. Unfortunately, there is no way to verify the credentials of the person responding. Given the anonymous nature of the internet, a person can answer a question even when he or she has a formal dental degree or not and even if the person wishes to falsify credentials by impersonating a dentist. [ 22 ] There is no simple way for a reader to substantiate any information about the person answering questions on a newsgroup.
While one can argue whether these replies were appropriate, it becomes apparent that the responses do create an impression of our profession. Hence it is important to remain cognizant of the impact, ethical implications and the impressions the words of a professional carry whether written or digitally encoded.
Fraudulent alteration of medical records has been a long-standing problem. Unfortunately with the advent of electronic data storage, the potential exists for an undetectable alteration of an electronic record. As most states do not consider an electronic record an acceptable form of medical information storage; electronic records should be supported with a written copy of patient information. In addition, the original record should be identified and maintained as such. [ 23 ] While a printed copy of the radiograph can be made, this negates some of the advantages of digital imaging (for example, less storage space required).
The ethical implications of fraud for insurance carrier are even graver. Alterations could be used to falsify completion of procedures without ever having treated the patient. Indeed, a person familiar with this technology could alter digital dental radiographs at will. We as a professional must work cautiously and with the best interests of our patients in mind. [ 24 ]
- Recommendations
Attention to detail, good communication, and excellent documentation can help reduce a teledentistry practitioner’s medium-associated liability. E-mail sent to the practitioner by the patient in reference to his or her condition or treatment must be kept by the practitioner as part of the patient’s medical record and that the content of such e-mails can be made a part of the discovery process in a potential legal proceeding. Before practicing teledentistry, the potential provider should clarify and document the parties responsible for installation, maintenance, access, security, and privacy efforts associated with the equipment used. Transmission verification procedures should be developed and documented at both the local and remote sites. Documented contingency plans should be developed, including a description of the backup protocols used. Furthermore, clinical guidelines should be established and documented that are at least equal to the accepted standards of care in the dental community. [ 25 ]
Teledentistry is a relatively new and exciting field that has endless potential. It is useful in long-distance clinical training and continuing education, screening and dentist laboratory communication. In rural areas where there is a shortage of specialists, lack of comprehensive and sophisticated health care teledentistry can extend care to remote patient populations at a reasonable cost as well as ease the problem of a shortage of specialized dental consultants.
Declaration of patient consent
Patients consent not required as patients identity is not disclosed or compromised.
Financial support and sponsorship
Conflicts of interest.
There are no conflicts of interest.
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Wer wüsste besser, was Jugendliche gern lesen, als diese selbst? Diesem Gedanken folgten Bibliothekslehrerin Dagmar Hofmann und Marianne Merle, hauptamtliche Kraft und „Seele“ der Schulbibliothek, als sie die „Otto“ beim Programm „Lesescouts“ der Stiftung Lesen anmeldeten. Die Idee dahinter ist, die Expertise der Jugendlichen zu nutzen, damit diese Gleichaltrige für Bücher begeistern. Die Wege hierbei können vielfältig sein: „Lesescouts“ können Leseevents organisieren, Lieblingsbücher bewerben oder selbst vorlesen. Tamanna Kumar und Niruvika Mahendran aus der Klasse 9Ga haben den Anfang... [mehr]
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Socio-economic, marketing and gender aspects of village chicken production in the tropics: A review of literature
This paper ( PDF ) by ILRI reviews the literature that focuses on the socio-economic, gender and marketing aspects of chicken production in the tropics. Chicken production is an essential agricultural activity practiced by almost all rural communities throughout the developing world. Most importantly, chicken is a valuable asset to the local population, especially for the disadvantaged groups and less favoured areas of rural Africa and elsewhere in the developing world. This is because chicken production contributes to food security, poverty alleviation and gender equality. However, the level of production and productivity is still low and constrained by many social, economic, and market related factors. In most areas of the world (e.g. Africa, Latin America and Asia), the routine management of poultry are undertaken by women, Nevertheless, there are still big gaps to get gender-based disaggregated data to figure out women’s roles and responsibilities in the family poultry production. Furthermore, the review indicated that in most areas, chicken research was focused on production and productivity, with little attention for marketing. A few studies mentioned that biological aspects of chicken production, such as feeding and breeding, are efficient if it is accompanied by efficient marketing systems since, efficient marketing system is rewarding to all agents involved in the production, marketing and consumption of chicken. Therefore, an efficient marketing system has paramount importance in the chicken production. The role of poultry in escaping extreme poverty has frequently been indicted there are many constraints to the development of the smallholder poultry production. The main challenges for the development of smallholder chicken production include: disease control; protection against predators; better feeding; genetic improvement; better marketing; training and management; access to production inputs; poor infrastructure and access to capital; the lack of farmer organizations and the creation of conducive institutions and governmental policies. In this respect policies and actions need to focus on reducing the constraints related to chicken production.
Curated from cgspace.cgiar.org

This report highlights options for sustainably promoting growth in the livestock sector, drawing from what four African countries have done successuflly in terms of institutional and policy innovation as well as programmatic interventions. By adapting the lessons, African governments can meet their national and international commitments to agricultural growth and transformation. »
This archive reflects on 50 years of research on pastoralism at IDS. Much has changed, but there are also important continuities. The capacity to respond to today’s turbulent world, to make productive use of marginal environments, to make use of mobility to respond to heightened uncertainty, and to adapt and innovate are all features of pastoralism that can be important in meeting wider, global challenges. »
Dissertation Genius
Avoiding Common Errors in your Literature Review
October 29, 2016 by Dissertation Genius
As so many doctoral students working on their dissertations have discovered, writing the literature review chapter is a tedious process involving many steps and pitfalls. To help save you time and heartache, this article will show you the most common mistakes encountered when putting together your literature review. These common literature review errors are broken down into three general categories:
- Writing & Stylistic Issues
- Issues with Literature Review Structure & Elements
- Credibility/Validity Issues
Common Errors Category #1: Writing & Stylistic Issues
Using emotional phrases – Avoid using emotional phrases in your literature review section. Remember, you are writing a literature review for the purpose of presenting the existing currents of thought on your research topic. This means you are only relaying what already exists in the research world of your topic. Thus, there should be no subjectivity and, hence, no words of emotion.
Giving personal opinions in a literature review – Although this issue should not be an issue, I see it all too often. Again, for the same reasons you do not use emotional phrases in a literature review, you also don’t insert your own personal opinions. The literature review is supposed to be an unbiased display of already-existing thought and research around your topic. It is supposed to be objective, never subjective.
Unwittingly plagiarizing – I will not focus on all the aspects of plagiarism that exist in a dissertation or thesis but, for literature reviews in general, there are a few issues you should take important note of:
- Unjustified claims
- No referenced page numbers for direct quotations
First, you must be very wary of any unjustified claims you may make. To illustrate this common mistake of doctoral students, refer to the following example of an unjustified claim:
In general, rice is known to be a staple food in Taiwan. The Taiwan Council of Agriculture states that there were originally no indigenous species of rice from Taiwan. Rice cultivation and technology was brought into Taiwan before the Qing Era. Shortly after, the Indica rice became the most productive and efficiently-grown rice grain, especially because of the ample construction of water reservoirs near rice fields. During the Japanese colonial period.
This example is full of unjustified claims and bring up several questions like how did the author know all this? And where was the information taken from? Remember, no matter what the claim and information, you must reference it. No reference for information like the example above means it may be considered as plagiarism , so be extra wary.
Using an author’s first and last name inside the text – When discussing authors and/or researchers inside your study, do not use the first and last name (whether inside or outside parentheses). Use the last name only, followed by the year of the study’s publication is you are required to use APA style.
Inserting long URLs in the body of a study – Often times, doctoral students insert long URLs to provide links to mentioned research studies or articles inside their paper. Do not do this. Instead, cite the source with the last name(s) of the author(s) followed by the year of the work. Then, in the references section of your paper, under the particular reference to that citation, you would insert the URL.
Common Errors Category #2: Issues with Literature Review Structure
Giving no background/definition section – This is an all-too-often occurrence and, unfortunately, many students forget to insert a pre-literature review section that gives relevant background information and key definition of terms. If you don’t define key concepts and present the necessary background information about your topic, you will alienate a huge potential audience to your work, especially if you aim to publish your dissertation in the future. When defining certain concepts, do not provide too many definitions lest you confuse your audience. Keep it simple and cite only the most common definitions for any relevant terms and concepts.
Bad organization and structure – This is perhaps the most common error since badly organizing a literature review requires doctoral students to rewrite and restructure many parts. To prevent a bad structure from penetrating your literature review, use sub-headings and then organize your literature reviews under each of these sub-headings. Doing this will help you maintain focus on the details while keeping focus on the big picture and the overall chain of logic inherent in your review. This will help you avoid illogical structure and bad organization.
Irrelevant content – It is common that doctoral students get caught up in the details of their literature review especially while reading other studies. As a result, many students tend to mention studies or points that are unrelated to the topic and the research question(s) at hand. Therefore, make sure your literature review mentions studies fully relevant and, at the same time, make sure that the relevant points you mention about a study are also relevant to your sub-heading and research question(s). Just because a study is important to your topic does not mean that all the details within it are relevant to your task.
Not going over the methodology section of a reviewed article – Usually, doctoral students working on their dissertations like to focus only on certain aspects of articles they review such as the abstract, results, and discussion sections. However, these students fail to realize that reading through the methodology section of any reviewed articles, even a cursory reading, provides immensely valuable information to help them produce a top-notch dissertation. Do not lose the opportunity to gain valuable insight into honing your research study by learning from the methodology of others. Remember, the research methodology, the way you conduct your research, is perhaps the most difficult aspect of any PhD dissertation. Learn from others. If you ignore this advice you will waste time because your will surely need to add these details later (after your chair or mentor tells you it is required)!
Common Errors Category #3: Credibility/Validity Issues
Writing a narrowly-focused literature review – Too many doctoral students write their literature reviews in terms of general categories instead of writing on focused topics (and subtopics) sufficiently narrowed down. This results in a literature review that is too general and not directly related to their research questions. Thus, you should avoid topics or categories that would require an entire book to sufficiently cover. In addition, as previously discussed, you must make sure that the studies you include in your review are framed in terms of their relation to your research question(s). Finally, make good use of subheadings in your literature review and make sure these subheadings and their respective content are relevant to your research question(s) as well.
Relying on direct quotations – Another bad habit commonly found in doctoral students working on their dissertations involves the insertion of too many direct quotations. Although it is ok to insert direct quotations in your study, you should not rely on them too much. Doing so will prevent you from using your critical thinking skills and applying them to appropriately analyze, synthesize, and evaluate the studies you include.
Using non-scholarly sources – Something I see too often is doctoral students relying too much on professional opinion articles rather than searching for more authoritative or scholarly sources. They tend to avoid authoritative sources because these sources are usually the most arduous to read. Do not take shortcuts (or what seem like shortcuts). Spend quality time reading authoritative sources, no matter how much effort this requires. Doing so will make your literature review sparkle and compelling to read. It will also give you extremely valuable insight into appropriately conducting the rest of your study, making the long road ahead so much easier.
Citing only ‘supportive’ source – It is human nature to be attracted only to those arguments that support our own point of view. You must remember that your dissertation committee fully anticipates opposing opinions to exist in your literature review and they expect you to discuss them. Understand that citing dissenting opinions will only strengthen your argument, not weaken it. Although it is not necessary to focus too much on them, it is vital that you mention some dissenting studies and explain why they depart from your own thinking.

A Final Word – Get Help When You Need It!
Following the aforementioned guidelines to avoid common errors in the literature review will go a long way to help you create an awesome chapter. However, no matter how much you try to do so, you may still find yourself frustrated.
Your dissertation advisor or mentor may be unwilling, or unable, to give you the help you need. If you refer to your colleagues, you may find them unable to explain the process appropriately or, if they can explain it to you, their process may not work for you, since there is no one single strategy to write a literature review.
In these cases, you don’t have to worry. You can find the dissertation help you need. Just contact a dissertation consultant who can give you the expert help you need, or at least guide you far enough to do it on your own. Remember, your doctoral dissertation is not only a precursor for your PhD, but it is also a key that can open many doors for you. Don’t waste your time and energy because you were hesitant to seek assistance!
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INTRODUCTION -...
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INTRODUCTION

This chapter deals with a review of the literature on the various aspects of biofibres
and biocomposites. Biocomposites are finding applications in many fields ranging from
construction industry to automotive indusby. The pros and cons of using biofibres are
enumerated. The classification of biocomposites into green composites, hybrid
biocomposites and texhle biocomposites has been discussed. New d evdopments
dealing with cellulose based nanocomposites and electrospinning of nanofibres have
been presented. The importance of the interface in determining the properks of the
composite and various methods of characterization have been reviewed. The
mechanics and properties of short fibre and biofibre reinforced natural rubber
composites have been discussed. Finally, the applications of fibre reinforced rubber
composites have been highlighted.

1 .I .I COMPOSITES
A composite can be defiried as a material having two or more
chemically distinct phases, which at the microscopic scale are separated by a
distinct interface. We can also say thal a composite is a combination of
properties of two or more components held together by the same type of matrix.
In essence, a composite is a commodity having superior properties than the
individual constituents. Composites, the wonder materials with light weight, high
strength to weight ratio and stiffness properties have come a long way in
replacing the conventional materials such as metals and wood!
The classification of composites is represented in Figure 1.1.1
Composites can be mainly categorized on the basis of matrix and type of
reinforcement. The classifications according to matrix type are ceramic matrix
composites (CMC), polymer matrix composites (PMC) and metal matrix
composites (MMC). The classifications according to type of reinforcement are
particulate composites (composed of particles), fibrous composites (composed of
fibres) and laminate composites (composed of laminates). Fibrous composites
can be further sub-divided on the basis of qatural I biofibre or synthetic fibre.
EioEbre encompassing composites are referred to as biocomposites.
Biocomposites can be again divided Qn the basis of matrix i.e. non-biodegradable
matrix and biodegradable matrix. Biocomposites made from natural I biofibre and
biodegradable polymers are referred to as green composites. These can be
further sub-divided as hybrid composites and textile composites. Hybrid
composites comprise of a combination of two or more types of fibres?

In fibrous composites, fibres are the load carrying members, while the
surrounding matrix keeps them in desired location and orientation. The matrix
acls as a load transfer medium, provides shape to the composite structure and
protects the fibres from environmental damage. The matrix also gives toughness
and compressional strength to the composite. The selection of suitable fibres is
determined by the required value of stiffness and tensile strength of the
composite. Other deciding factors are thermal stability, adhesion of fibres and
matrix, dynamic and long term behaviour, price and processing costs.
Natural or8iofibre I Composite BIOCOMPOSITE
Synthetic fibre 1 composite
Bio fiber-petroleum Biofiber-bioplastic (PLA) based plastic (PE, PP) GREEN COMPOSITE i s
HYBRID/TEXTILE BIOCOMPOSITE [Fibre blendinglmatrix blending]
Figure 1 .I .I Classification of composites

1 .I .2 BlOFlBRESl LIGNOCELLULOSIC FIBRESINATURAL FIBRES
Natural fibres are subdivided based on their origins, coming from plants,
animals or minerals. All piant fibres are composed of cellulose while animal
fibres consist of proteins (hair, silk and wool). Plant fibres include bast (or stem
or soft sclerenchyma) fibres, leaf or hard fibres, seed, fruit, wood, cereal straw
and other grass fibres. Over the last few years, a number of researchers have
been involved in investigating the exploitation of natural fibres as load bearing
constituents in composite materials. The use of such materials in composites
has increased due to their relative cheapness, their ability to recycle and for the
fact that they can compete well in terms of strength per weight of material.
Natural fibres can be considered as naturally occurring composites consisting
mainly of cellulose fibrils embedded in lignin matrix. The cellulose fibrils are
aligned along the length of the fibre, which render maximum tensile and flexural
strengths, in addition to providing rigidity. The reinforcing efficiency of natural
fibre is related to the nature of cellulose and its crystallinity. The main
components of natural fibres are cellulose (a-cellulose), hemicellulose, lignin,
pectins and waxes.
Cellulose is a natural polymer .consisting of D-anhydroglucose
(C6tt1105) repeating units joined by e-1,4-glycosidic linkages at CI & C4
position3. The degree of polymerization (DP) is around 10,000. Each repeating
unit contains three hydroxyl groups. These hydroxyl groups and their ability to
hydrogen bond play a major role in directing the crystalline packing and also
govern the physical properties of cellulose. Solid cellulose forms a
microcrystalline structure with regions of high order i.e. crystalline regions and
regions of low order i.e. amorphous regions. Cellulose is also formed of slender

rod like crystalline microfibrils. The crystal nature (monoclinic sphenodic) of
naturally occurring cellulose is known as cellulose I. Cellulose is resistant to
strong alkali (1 7.5 wt%) but is easily hydrolyzed by acid to water-soluble sugars.
Cellulose is relatively resistant to oxidizing agents.
Hemicellulose is not a form of cellulose and the name is a misnomer.
They comprise a group of polysaccharides composed of a combination of 5- and
6- carbon ring sugars. Hemicellulose differs from cellulose in three aspects.
Firstly, they contain several different sugar units whereas cellulose contains
only 1,4--P-D-glucopyranose units. Secondly, they exhibit a considerable
degree of chain branching containing pendant side groups giving rise to its non
crystalline nature, whereas cellulose is a linear polymer. Thirdly, the degree of
polymerization of native cellulose is 10-100 times higher than that of
hemicellulose. DP of hemicellulose is around 50 to 300. Hemicelluloses form
the supportive matrix for cellulose microfibrils. Hemicellulose is very hydrophitic,
soluble in alkali and easily hydrolyzed in acids.
Lignin is a complex hydrocarbon polymer with both aliphatic and
aromatic constituents. They are totally insoluble in most solvents and cannot be
broken down to monomeric units. Lignin is totany amorphous and hydrophobic
in nature. It is the compound that gives rigidity to the plants. It is thought to be a
complex, three-dimensional copolymer of aliphatic and aromatic constituents
with very high molecular weight. Hydroxyl, methoxyl and carbonyl groups have
been identified. Lignin has been found to contain five hydroxyl and five methoxyl
groups per building unit. It is believed that the structural units of lignin molecule
are derivatives of 4-hydroxy-3-methoxy phenylpropane3. The main difficulty in
lignin chemistry is that no method has been established by which it is possible

to isolate lignin in its native state from the fibre. Lignin is considered to be a
thermoplastic polymer exhibiting a glass transition temperature of around 90°C
and melting temperature of around 170°C4. It is not hydrolyzed by acids, but
soluble in hot alkali, readily oxidized and easily condensable with phenols.
Pectins are a collective name for heteropolysaccarides. They give
plants flexibility. Waxes make up the last part of fibres and they consist of
different types of alcohols.
Biofibres can be considered to be composites of hollow cellulose fibrils
held together by a lignin and hemicellulose matrix? The cell wall in a fibre is not
a homogenous membrane (Figure 1 . I .2). Each fibril has a complex, layered
structure consisting of a thin primary wall that is the first layer deposited during
cell growth encircling a secondary wall. The secondary wall is made up of three
layers and the thick middle layer determines the mechanical properties of the
fibre. The middle layer consists of a series of helically wound cellular microfibrils
formed from long chain cellulose molecules: the angle belween the fibre axis
and the microfibrils is called the microfibrillar angle. The characteristic value for
this parameter varies from one fibre to another. +

Secondary wall S3 Lumen
'. Helically
crystatline microfibrils \\-
i ,. Secor~da r y wall 52
Spiral anqle
Secondary ,#all S1 of cellulose
Amorphous rogion mainly consrsting of lignin a n d hemicellulose
O~sarderly arrr nge --, crysta i l~ne celltllos
~ ~ l i c r o f ; b r ~ l s netwot
Figure 1.1.2 Structure of biofibre
[Reference: Rong M.Z. et al., Comp. Sci. Tech., 61, 1437, 20011
Such microfibrils have typically a diameter of about 10-30 nrn and are
made up of 30 to 100 cellulose molecules in extended chain conformation and
provide mechanical strength lo the fibre. The amorphous matrix phase in a
cell wall is very complex and consists of hemicellulose, lignin and in some
cases pectin. The hemicellulose molecules are hydrogen bonded to cellulose
and act as cementing matrix between the cellulose microfibrils, forming the
cellulose-hemicellulose network, which is thought to be the main structural
component of the fibre cell. The hydrophobic lignin network affects the
properties of other network in a way that it acts as a coupling agent and
increases the stiffness of the celluloselhemicellulose composite.
The structure, microfibrillar angle, cell dimensions,defects and the
chemical composition of fibres are the most important variables that determine
the overall properties of the fibres7. Generally, tensile strength and Young's
modulus of fibres increases with increasing cellulose con tent. The microfibrillar
angle determines the stiffness of the fibres. Plant fibres are more ductile if the

microfibrils have a spiral orientation to the fibre axis. If the microfibrils are
oriented parallel to the fibre axis, the fibres will be rigid, inflexible and have high
tensile strength. Some of the importanl biofibres are listed in Table 1 . I . I
Table 1.1.18 List of important biofibres
[Reference: Eichorn et al. J. Mat. Sci., 36, 2107, 20011
Fibre source Abaca Bagasse Bamboo Banana
Cantala Caroa China jute Coir
I Dale palm 1 Phoenix Dactylifera Leaf 1
Musa texfis
(> 1 250 species) ~ u s a indica
Muhlenbergia macroura
Cotton Curaua
1 Flax 1 Linum usitatissimurn I Stem
Leaf Grass Grass Leaf Root
Agave canfala Neoglaziovia variegata Abutilontheophrasti Cocos nucifera
I Hemp I Cannabis safiva I Stem
Leaf Leaf Stem Fruit
Gossypium sp. Ananas erectifoiius
r enequen 1 Agave fourcroydes 1 Leaf _) lsora Helicteres isora Stern
I lstle / Samue~a carnerosana r ~ e a f 1 r -- I I
1 Corchorus caosularis 1 Stem I Kapok I Ceiba penfranda I Fruit 1
pz - 1 H i b i s ~ n a ~ u s 1 F5 I
Pueraria thunbergiana Mauritius hemp Furcraea gigantea Nettle Urtica dioica Stem

1 0 j 1 7 e ; s gujn;e*Tt -1 Piassava A ttalea funifera Leaf Pineapple Ananus comosus Leaf Phormium Phormium tenas Leaf
1 Rosetle 1 Hibiscus sabdariffa I Stem I 1 Ramie -1 - - Boehmeria nivea Istern I 1 Sansevieria Sansevieria 1 Leaf I
(Bowstring hemp) I sisal I Agave sis~lana I ~ e a f /
Sponge gourd Straw (Cereal)
I Wood 1 (>10,000 species) 1 Stem
1.1.2.1 Biofibres: Advantages 8 Disadvantages
Sun hemp Cadillol Urena
The growing interest in lignocellulosic fibres is mainly due b their economical
Luffa cylinderica
production with few requirements for equipment and low specific weight, which results
Stalk Crorolaria juncea Urena lobata
in a higher specific strength and stiffness when compared to glass reinforced
composites. They also present safer handling and working conditions compared to
synthetic reinforcements. Biofibres are nonabrasive to mixing and molding
equipment, which can contribute to significant cost reductions. The most interesting
aspect about natural fibres is their positive environmental impact. Biofibres are a
renewable resource with production requiring little energy. They are carbon dioxide
neutral i.e. they do not return excess carbon dioxide into the atmosphere when they
are cornposted or combusted. The processing atmosphere is friendly with better
working conditions and therefore there will be reduced dermal and respiratory
irritation. Biofibres possess high electrical resistance. Thermal recycling is also

possible. The hollow cellular structure provides good acoustic insulating properties.
The worldwide availability is an additional factor.
The inherent polar and hydrophilic nature of lignocellulosic fibres and the
non-polar characleristics of most thermoplastics results in compounding difficulties
leading to non-uniform dispersion of fibres within the matrix which impairs the
efticiency of the composite. This is a major disadvantage of natural fibre reinforced
composites. Another problem is that the processing temperature of composites is
restricted to 200°C as vegetable fibres undergo degradation at higher temperatures;
this restricts the choice of matrix material. Another setback is the high moisture
absorption of natural fibres leading to swelling and presence of voids at the interface,
which results in poor mechanical properties and reduces dimensional stability of
composites. Another restriction to ihe successful exploitation of bioftbres for durable
composite application is low microbial resistance and susceptibility to rotting. These
properties pose serious problems during shipping, storage and composite processing.
The non-uniformity and variation of dimensions and of their mechanical properties
(even between individual plants in the same cultivation) poses another serious
It is quite clear that the advantages outweigh the disadvantages and most of
the shortcomings have remedial measures in the form of chemical treatments. The
lignocellulosic fibres have an advantage over synthetic ones since they buckle rather
than break during processing and fabrication. In addition, cellulose possesses a
flattened oval cross section that enhances stress transfer by presenting an effectively
higher aspect ratio.
Researchers9 at the Institute of Natural Fibres, Poland have developed fire
resistant upholstery using fire-relarded flax nonwoven (LinFR) FT. The non-woven

used in the composites plays the role of fire barrier that reduces the vulnerability of
filling material to the development and spread of fire. The softness of the upholstery
system is also enhanced. In another interesting study, researchers10 investigated ihe
influence of natural and synthetic fibres which covered the forearm muscles on the
activity of motor units in these muscles. The electrophysiological parameters of motor
units were measured with electromyographical methods. The results indicated that
temporaly covering of examined muscles in the forearm with the synthetic clothing
changed the pattern of motor units activity of muscles. Clothing made of natural fibre
did not evoke this kind of effect.
Recently, natural cellulose fibres having properties between cotton and linen
and suitable for various industrial applications were extracted from comhusksil. High
quality cellulosic fibres from comhusks mean food, clothing and other major industrial
products from the same source without the need for any additional natural resources.
Using cornhusks for fibrous applications would save the land and other natural
resources required to grow fibre crops and will conserve the non-renewable
petroleum sources required to produce synthetic fibres. More than 9 million tons of
natural cellulose fibres with a potential sale value of $19 billion and with a value
addition of at least $12 billion could be produced from the cornhusks available every
year. Utilizing comhusks as a source for natural cellulose fibres will significantly
benefit the agriculture, fibre, food and energy needs of the future and will also benefit
the environment. The structure and composition of the natural cellulose fibres
obtained from cornstalks were found to be different from that of the common bast
fibres such as flax and kenaf. Cornstalk fibres were found to have relatively lower
percentage of crystallinity but similar microfibrillar angle as that of the common bast

1.1.3. BIOCOMPOSITES
The history of mankind has witnessed several surges in the field of research
an3 development. The rampant use of pelroleurn products has created a twin
dilemma; depletion of petroleum resources and entrapment of plastics in the food
chain and environment. The increasing pollution caused by the use of plastics and
emissions during incineration is affecting the food we eat, water we drink, air we
breathe and threatening the greatest right of human beings, the right to live. The
exhaustive use of petroleum based resources has initiated the efforts to develop
biodegradable plastics. This is based on renewable biobased plant and agricultural
products that can compete in the markets currently dominated by petroleum based
products. The production of 100 % biobased materials as substitute for petroleum
based products is not an economical solution. A more viable solution would be to
combine petroleum and biobased resources to develop a cost-effective product
having immense applications. Biopolymers or synthetic polymers reinforced with
natural or bio fibres (termed as biocomposites) are a viable alternative to glass fibre
composites. Scientists are looking at the various possibilities of combining biofibres
such as sisal, flax, hemp, jute, banana, wood and various grasses with polymer
matrices from non-renewable and renewable resources to form composite materials
to make the biocomposite revolution a reality'?
Broadly defined, biocomposites are composite materials made from
naturallbio fibre and petroleum derived non-biodegradable polymers (PP,PE) or
biodegradable polymers (PLA, PHA) . The latter categoly i.e. biocomposites derived
from plant derived fibre (naturallbiofibre) and croplbioderived plastic
(biopolymerlbioplastic) are likely to be more eco-friendly and such composites are
termed as green composites.

The best known renewable resources capable of making biodegradable
plastics are starch and cellulose14. Starch is one of the least expensive biodegradable
materials available in the world market today. It is a versatile polymer with immense
potential for use in non-food industries. Cellulose from trees and cotton plants is a
substitute for petroleum feedstocks to make cellulose plastics. Another aspect that
has gained global attention is the development of biodegradable plastics from
vegetable oils like soybean oil, peanut oil, walnut oil, sesame oil and sunflower oil.
Green composites from soy protein based bioplaslics and natural fibres show
potential for rigid packing and housing and transportation applicationsl5. Fish oil
based polymers have also attracted the attention of researchers due to their high
degree of unsaturation. Fish oil based polymers also possess unique good damping
and shape memory properties16.
The history of fibre reinforced plastics began in 1908 with cellulose fibre in
phenolics, later extending to urea and melamine and reaching commodity status with
glass fibre reinforced plastics. The fibre-reinforced composites market is now a
multibilliondollar business" (Figure 1 . I .3). Though hailed as a miraculous discovery
long back, plastic products now enjoy an ambiguous reputation. Scientists at the
BioComposites Centre at the University of Wales, Bangor are developing high-quality
packaging of goods, using starch from corn and potatoes to tackle the problem of
high cost waste disposal'*. Researchers19 are also exploring the aspects of producing
plastics from limonene which is extracted from cilrus. After decades of obscurity,
biofibre reinforced composites are being touted as the material of the millennium.

Electronic components
products 31 %
Automotives
Figure 1.1.3 Fibre reinforced plastic composites used in 2002-2.28 X l o 9 lb. [Reference: Adapted from Plast. News. August 26,20021
1 .I .3.1 Classification of Biocomposites 1 .I .3.1 .I Green Composites
Research efforts are currently being harnessed in developing a new class of
fully biodegradable green' composites by combining (natural I bio) fibres with
biodegradable resins2Q. The major attractions about green composites are that they
are environmentally-friendly , fully degradable and sustainable, i.e. they are truly
'green' in every way. At the end of their life they can be easily disposed of or
cornposted without harming the environment. The design and life cycle assessment
of green composites have been exclusively dealt with by Baillie.2t Green composites
may be used effectively in many applications such as mass produced consumer
products with short life cycles or products intended for one time or short time use
before disposal, Green composites may also be used for indoor applications with a

useful life of several years. A number of natural and biodegradable matrices hat are
available for use in such green cornposi tes22 are listed in Table 1.1.2
Table 1.1.2 20 Natural and biodegradable matrices
[Reference: Stevens E.S., Green Plastics, Princeton University Press, Princeton 20021
Biodegradable polymer matrices -
1. Polysaccharides
1 4. Poly(viny1 alcohol)
1. Poly (amides)
3. Poly (amide-enamines)
1 2. Proteins 1 5. PoIy (vinyl acetate) I 1 Collagen/ Gelatin 1 6. Polyesters
I Casein, Albumin,Fibrogen, Silks / 6.1 Poly (glycolic acid)
1 4. Other Polymers
3. Polyesters
Polyh yd roxyalkanoates
1 6.4 Poly (orthoesters)
6.2 Poly (lactic acid)
6.3 Poly (caprolactone)
I Lipids 1 8. Poly (phosphazines) I Lig n in
Natural rubber
7..Poly (ethylene oxides)
The reinforcement of biofibres in green composites has been
highlighted by Bismarck et al.5 Researchers recently investigated the effect of

stearic acid on tensile and thermal properties of ramie fibre reinforced soy
protein isolate (SPI) resin green composites23. It was observed that part of the
stearic acid cryslallized in SPI resin and that the crystallizability was affected by
the addition of glycerol as a plasticizer. The fabricated green composite was
found to have enormous potential for certain indoor applications.
The interfacial adhesion in the composites was apparent from SEM
photomicrographs of the fracture tensile surfaces of the ramielSPl and ramie1
modified soy protein isolate (MSPI) composiles tested in the axial direction is
shown in Figure 1.1.4 a & b respectively. Some of the fibres at the fracture
surface do not show any resin, where as a few other fibres clearly show resin
sticking on the surface. The presence of resin on some fibre surfaces for the
SPI and MSPI resins clearly indicates good interfacial interaction between the
fibre and the resins.
Figure 1.1.4 (a & b) SEM photomicrographs of the fracture tensile surfaces of untreated and treated composites
(Reference: Lodha and Netravali, Cornp. Sci. Tech. 65, 121 1, 20051
In an interesting study, researchers24 modified soy flour (SF) by cross-
linking it with glutaraldehyde (GA). The cross-link'ed soy flour (CSF) polymer

was characterized for its tensile and thermal properties. The effect of glycerol
on the mechanical properties of the soy flour was also characterized and
optimized. CSF polymer showed improved tensile properties and thermal
stability, compared to unmodified SF resin, for use as a resin to fabricate
composites. Unidirectional green composites using flax yarn and CSF resin
were fabricated and characterized for their tensile and flexural properties. The
composite specimens exhibited fracture stress and Young's modulus of 259.5
MPa and 3.71 GPa, respectively, and flexural strength of 174 MPa, in the
longitudinal direction.
Recently green composites were fabricated using pineapple leaf fibre and
soy based plastic25. The addilion of compatabilizer (polyester amide grafted
g lycid yl methacry late (PEA-g-G MA) was seen to increase the mechanical
properties of composites. In another interesting study involving biocomposites,
the effect of alkali treatment on the thermal properties of Indian grass fibre
reinforced soy protein green composites was studied by the same
The thermal stability of biodegradable composite films prepared from
blends of poly(viny1 alcohol), cornstarch, and lignocellulosic fibre was investigated
by Imam e l al.27. Thermogravimetric analysis (TGA) indicated the suitability of
formulations for melt processing, and for application as mulch films in fields at
much higher temperatures. The study also revealed that both starch and
lignocellulosic fibre degraded much more rapidly than PVA. The addition of fibre
to the formulations was found to enhance the PVA degradation. In another
interesting study, the thermal behavior of green composites fabricated from
bagasse fibre and polyvinyl alcohol was investigated by Fernandes et a128. They
observed an increase of thermal stability upon incorporation of bagasse fibre.

In another study biocomposites*9 were fabricated using a non-woven
fibre mat (90% hemp fibre with 10% thermoplastic polyester binder) as
reinforcement, and unsaturated polyester (UPE) resin as well as blends of UPE
and functionalized vegetable oils as the polymer matrix. All composites were
made with 30% volume fraction of fibre, which was optimized earlier. The
structure-property relationships of this system as well as the thermo-
mechanical properties of these composites were measured. The notched lzod
impact strength of biocomposites from biobased resin blends of UPE and
functionalized vegetable oil and industrial hemp fibre mat were enhanced by
90% as compared to that of the pure UP€-industrial hemp fibre mat composites.
The tests also showed an improvemenl in the tensile properties of the
composite as a result of the incorporation of the derivatized vegetable oil.
Researchers have also developed green composites from jute fabric
and Biopol composites30. Chemical modification of the fabric was carried out lo
improve interfacial properties. A significant amount of research has been done
at the German Aerospace Centre (DLR) in Braunschweg on biodegradable
plastics and composites31. Starch and modified resins have also been used as
matrix to form green composites32.
A major hurdle to the commercialization of green composites is the high
cost of biodegradable matrices. Most of the biodegradable resins cost
significantly more than the commonly used resins. Inexpensive production of
oils for resins through biotechnology would certainly be of help in expediting
their commercialization. The manufacturing costs can also be reduced by faster.
better and more efficient processing.

Efforts are on the anvil for the development of "advanced green
composites" made out of high strength protein fibres (spider silk) and
biodegradable matrices Biotechnology is being used to increase the yield of
specific triglycerides and oils in beans for producing resins. These resins will be
inexpensive compared to those available today and if suitably modified, could
be biodegradable. Research is also being conducted to develop new pathways
to synthesize inexpensive biodegradable resins33 with better mechanical
properties and thermal stability using nanote~hnology3~.
1 .I .3.1.2 Hybrid Biocomposites
The incorporation of several different types of fibres into a single matrix
has led to the development of hybrid biocomposites. The behavior of hybrid
composites is a weighed sum of the individual components in which there is a
more favorable balance between the inherent advantages and disadvantages.
Also, using a hybrid composite that contains two or more types of fibre, the
advantages of one type of fibre could complement with what are lacking in the
other. As a consequence, a balance in cost and performance could be achieved
through proper material design? The properties of a hybrid composite mainly
depend upon the fibre content, length of ind iv id~a l fibres, orientation, extent of
intermingling of fibres, fibre to matrix bonding and arrangement of both the
fibres. The strength of the hybrid composite is also dependent on the failure
strain of individual fibres. Maximum hybrid results are obtained when the fibres
are highly strain compatible36.
The properties of the hybrid system consisting of two components can
be predicted by the rule of mixtures.

where PI{ is the property to be investigated, PI the corresponding property of the
first system and P2 the corresponding property of the second system. VI and V?
are the relative hybrid volume fractions of the first and second system and
A positive or negative hybrid effect is defined as a positive or negative
deviation of a certain mechanical property from the rule of hybrid mixture.
The term hybrid effect has been used to describe the phenomenon of an
apparent synergistic improvement in the properties of a composite containing
two or more types of fibre? The selection of the components that make up the
hybrid composite is determined by the purpose of hybridization, requirements
imposed on the material or the construction being designed. The problem of
selecting the type of compatible fibres and the level of their properties is of
prime importance when designing and producing hybrid composites. The
successful use of hybrid composites is determined by the chemical, mechanical
and physical stability of the fibre I matrix system.
Hybrid biocomposites can be designed by the combination of a
synthetic fibre and natural fibre (biofibre) in a matrix and a combination of two
natural fibre I biofibre in a matrix. Hybridization with glass fibre provides a
method to improve the mechanical properties of natural fibre composites and its
effect in different modes of stress depends on the design and construction of
the composites38. The effect of hybridization of glass fibre in thermoset
biocom posi tes has been discussed in detai139.

The tensile and impact behavior of oil palm fibre-glass fibre-reinforced
epoxy resin was investigated by Bakar et al.40. The hybridization of oil palm
fibres with glass fibres increased the tensile strength, Young's modulus, and
elongation at break of the hybrid composites. A negative hybrid effect was
observed for the tensile strength and Young's modulus while a positive hybrid
effect was observed for the elongation at break of the hybrid composites. The
impact strength of the hybrid composites increased with the addition of glass
Cellular biocomposile cores fabricated from industrial hemp or flax
fibres with unsaturated polyester were hybridized with woven jute, chopped
glass, and unidirectional carbon fabrics4'. Material characterizalion showed
improved stifiness, strength, and moisture-absorption stability, while flexural
tests on laboratory-scale plates demonstrated enhanced structural behavior.
These hybrid cellular biofibre-based composites were found to provide an
economic and environmentally friendlier alternative to entry-level synthetic
composites.
Hybridization also has a profound effect on the water absorption
property of composites.
An attempt to study the moisture uptake characteristics of hybrid
systems was performed by Mishra et aI.42. The composite systems chosen were
sisal I glass and pineapple lglass fibre reinforced polyester composites.
Composites were prepared by varying the concentration of glass fibre and by
subjecting the bio-fibres to different chemical treatments. The authors observed

that water uptake of hybrid composites were less than that of unhybridized
A comparative study of the water absorption of the glass fibre (7 wt. %)
1 natural biofibre (13 wt. %) with that of non-hybrid composites is given in Table
1 .I .3 where a lowering in water absorption is evident4?
Table 1.1.3 Comparative study of the water absorption of the glass fibre I natural biofibre with that of non-hybrid composites
[Reference: Rout J. et at., Composites Science & Technology 61, 1303,20011
T- - Water Absorption %
Sample Non hybrid [Coir-Polyester composite]
Hybrid [CoirlGlass -
Polyester composite]
/ Untreated
I PMMA grafted (5 %) I 3.98 1 2.663 1 Alkali- treated (5 %)
4.994 I 3.147 1 5.186
PAN grafted (10 Oh)
/ Bleached 1 5 -8 I 3.718 1 1 Cyanoethylated 1 3.6
The novel development of a photofabrication process of biofibre
composites, based on oil palm empty fruit bunch fibres was recently reported44.
The process consisled of the following steps: (1) Lhe preparation of nonwoven
mat of biofibre, either alone or in combination with glass and nylon; (2) drying
the mat; (3) preparation of photocurable resin matrix, consisting of vinyl ester
and photoinitiator; (4) impregnation of the mat by photocurable resin; and (5)

irradiation of the impregnated mat by UV radiation to effect the cure of the
composite. Oil palm fibre, glass, and nylon fibres were mixed in different
proportions. A "mixture experimental design" was used to generate
experimental compositions of the reinforcing fibres and to model dependency of
the response variables on the components through mathematical relationships.
Scientists45 at the Affordable Composites from Renewable Resources
(ACRES) program at the University of Delaware investigated the mechanical
properties of glasslflax hybrid composites based on a novel modified soybean
oil matrix material. Composites with different glasslflax ratios and different fibre
arrangements were made using a modified soybean oil matrix material. The
fibre arrangement was varied to make symmetric and unsymmetric composites.
The latter were tested in different modes in flexural tests and drop weight
impact tests. The mechanical properties of the composites were found to
depend upon the glasslflax ratio and the arrangement of fibres in the composite.
On proper selection of the arrangement of fibres in the composite, the glass
fibres and flax fibres were found to act synergistically resulting in an improved
flexural and impact performance.
Another innovative approach to hybrid Eornposites is the incorporation
of two natural fibres I biofibre in a matrix system. Recently, the mechanical
performance of short randomly oriented banana and sisal hybrid fibre reinforced
polyester composites46 was investigated with reference to the relative volume
fraction of the two fibres at a constant total fibre loading of 0.40 volume fraction
(Vr), keeping banana as the skin material and sisal as the core material. A
positive hybrid effect was observed in the flexural strength (Figure 1.1.5) and
flexural modulus of the hybrid composites. The tensile strength of the

composites showed a positive hybrid effect when the relative volume fraction of
the two fibres was varied, and maximum tensile strenglh was found to be in the
hybrid composite having a ratio of banana and sisal 4 : 1.
Volume fraction of the fibre (banana+sisal)
Figure 1.1.5 Variation of flexural strength with volume fraction of fibre
[Reference: ldicula M., et al. J. Appl. Polym. Sci. 96, 5,1699,20051
As a continuation to ihe above study the dynamic and static mechanical
properties of randomly oriented intimately mixed banana and sisal hybrid fibre
reinforced polyester composites were reported47. Maximum stress transfer
between the fibre and matrix was obtained in composites having volume ratio of
banana and sisal as 33. The storage modulus was found to increase with fibre
volume fraction above the glass transition temperature of the composites.

Researchers have also explored the incorporation of a fruit and leaf
fibre in natural rubber48. The addition of sisal and coir fibres offered good
rcinforccmen t and resulted in improvement of properties.
1 .I .3.1.3 Textile Biocomposites
The development of textile technologies such as weaving, knitting and
braiding has resulted in the formation of composites that have superior
mechanical properties, as continuous orientation of fibres is not restricted at
In applications where more than one fibre orientation is required, a
fabric combining 0" and 90" fibre orientations is useful. Woven fabrics are
produced by the interlacing of warp (0") fibres and weft (90") fibres in a regular
pattern or weave style. The fabric's integrity is maintained by the mechanical
interlocking of the fibres. Drape (the ability of a fabric to conform lo a complex
surface), surface smoothness and stability of a fabric are controlled primarily by
the weave style.
1 .I ,3,1.3.1 Common Weave Architectures
The following is a description of some of the more commonly found
weave styles:
Each warp fibre passes alternately under and over each weft fibre. The
fabric is symmetrical, with good stability and reasonable porosity. However, it is
the most difficult of the weaves to drape, and the high level of fibre crimp
imparts relatively low mechanical properties compared with the other weave

styles. With large fibres (high (ex) this weave style gives excessive crimp and
therefore it is not generally used for very heavy fabrics.
One or more warp fibres alternately weave over and under two or more
weft fibres in a regular repeated manner. 'This produces the visual effect of a
straight or broken diagonal 'rib' to the fabric. Superior wet out and drape is seen
in the twill weave over the plain weave with only a small reduction in stability.
With reduced crimp, the fabric also has a smoother surface and slightly higher
mechanical properties.
Satin weaves are fundamentally twill weaves modified to produce fewer
intersections of warp and weft. The 'harness' number used in the designation
(typically 4 , 5 and 8) is the total number of fibres crossed and passed under,

before the fibre repeats the pattern. A 'crowsfoot' weave is a form of satin
weave with a different stagger in the repeat pattern. Satin weaves are very flat,
have good wet out and a high degree of drape. The low crimp gives good
mechanical properties. Satin weaves allow fibres to be woven in the closest
proximity and can produce fabrics with a close 'tight' weave. However, the
style's low stability and asymmetry needs to be considered. The asymmetry
causes one face of the fabric to have fibre running predominantly in the warp
direction while the other face has fibres running predominantly in the weft
Basket weave is fundamentally the same as plain weave except that two
or more warp fibres alternately interlace with. two or more weft fibres. An
arrangement of two warps crossing two wefts is designated 2x2 basket, but the
arrangement of fibre need not be symmetrical, Therefore it is possible to have
8x2, 5x4, etc. Basket weave is flatter, and, though less crimp, stronger than a
plain weave, but less stable. It must be used on heavy weight fabrics made with
thick (high tex) fibres to avoid excessive crimping.

28 Cltrrp frr 1
Leno weave improves the stability in 'open' fabrics which have a low
fibre count. A form of plain weave in which adjacent warp fibres are twisted
around consecutive weft fibres to form a spiral pair, effectively 'locking' each
weft in place. Fabrics in leno weave are normally used in con-junction with other
weave styles because if used alone their openness could not produce an
effective composite component.
A version of plain weave in which occasional warp fibres, at regular
intervals but usually several fibres apart, deviate from the alternate under-over
interlacing and instead interlace every two or more fibres. This happens with
similar frequency in the weft direction, and the overall effect is a fabric with
increased thickness, rougher surface, and additional porosity.

Researchers have looked into tensile strength of ramie-cotton hybrid
fibre reinforced polyester cornposites49. They observed that tensile behaviour
was dominated by volume fraction of ramie fibres aligned in the tesl direclion.
The fabric and diameter of the thread did not play any role in tensile
characteristics. Cotton fabric was found to have minor reinforcement effect due
to weak cottonlpolyester interface. Similar studies were performed by
Mwaikambo and Bisandaso on kapok-cotton fibre reinforced polyester
Pothen et al.51 conducted tensile and impact studies of woven sisal
fabric reinforced polyester composites prepared by RTM technique. It was found
that the weave architecture was a crucial factor in determining the response of
the composites. Researchers have studied the micromechanics of moisture
diffusion in woven cornposites52. The weave pattern of the fabric was found to
have a profound effect on the water uptake of the composites. They observed
that woven composites exhibited quicker diffusion than that of a unidirectional
laminate with the same overall fibre volume fraction. The plain weave with a
lenticular tow and large waviness was seen to exhibit the quickest diffusion

Novolac type phenolic composites reinforced with jutelcotton hybrid
woven fabrics were fabricated and its properties were investigated as a function
of fibre orientation and rovinglfabric characteristics53. Results showed that the
composite properties were strongly influenced by test direction and
rovingslfa bric characteristics. The an isotropy degree was shown to increase
with test angle and to strongly depend on the type of architecture of fabric used,
i.e., jute rovings diameter, relative fibre content, etc. The best overall
mechanical properties were obtained for the composites tested along the jute
rovings direction. Composites tested at 45" and 90' with respect to the jute
roving direction exhibited a controlled brittle failure combined with a successive
fibre pullout, while those tested in the longitudinal direction (0") exhibited a
catastrophic failure mode. The researchers are of the opinion that jute fibre
promotes a higher reinforcing effect and cotton fibre avoids catastrophic failure.
Therefore, this combination of natural fibres is suitable to produce composites
for lightweight structural applications.
The thermal diffusivity, thermal conductivity and specific heat of
jutelcotton, sisallcotton and ramielcotton hybrid fabric-reinforced unsaturated
polyester composites were inveslgated by Alsina et a15? These properties were 4
measured both parallel and perpendicular to the plane of the fabrics. The
results obtained show that higher values were obtained parallel to the plane of
the fibres. Sisallcotton composites showed a particular behavior, with thermal
properties very close to those of the resin matrix. The thermal properties of the
fabrics, i.e. without any resin, were also evaluated and were used to predict the
properties of the composites from the theoretical series and parallel model
equations. The effect of fabric pre-drying on the thermal properties of the

composites was also evaluated. The results showed that the drying procedure
used did not bring any relevant change in the properties evaluated.
1.1.3.2 Applications of Biocomposites
The use of biofibre reinforced composites has extended to almost all
fields. Recently three-layer particleboards were produced from a mixture of
sunflower stalks and poplar wood at certain ratios utilizing urea-formaldehyde
(UF) adhesives. Panels with a density of 0.7glcm3 were manufactured with the
ratios of 25, 50, and 75 percent particles from sunflower stalks or poplar. Panels
were subjected to various tests for physical properties. Results show that all the
panels provide properties required by the normal standards for general purpose-
use particleboardsss.
Bio-based composite roof slructures were successfully fabricated from
soy oil-based resin and cellulose fibres in the form of paper sheets made from
recycled card board boxes. This recycled paper was previously tested in
composite sheets and structural unit beams and was found to give the required
stiffness and strength required for roof construction56.
In a study encompassing many applications, the flame retardancy of
biodegradable polymers and biocomposites was inves tigateda. For the
comparison, flame retarded lignocellulosic filler reinforced biocomposites were
prepared using polypropylene (PP), polyurethane (PUR) and fully biodegradable
starch matrices. The compatibility of wood flake with PP was improved by
application of an alkoxy silane based reactive surfactant. The silylation
improved not only the compatibility but also the thermal stability of the wood
flake according to TG measurements. Raman spectroscopic analysis of the

silylated product showed that the improved thermal stability is the result of
reduced ratio of the amorphous phase of cellulose. The phosphorus additives in
flame retarded PUK biocomposites, comprising waste bio fillers and recycled
polyol, proved to be very effective because both the matrix and the filler
components participate in mechanism of flame retardancy.
Researchers58 developed a new low dielectric constant material suited to
electronic materials applications using hollow keratin fibres and chemically
modified soybean oil. The unusual low value of dielectric constant was due to the
air present in the hollow microcrystalline keratin fibres and the triglyceride
molecules. The authors are of the opinion that the low cost composite made from
avian sources and plant oil has the potential to replace the dielectrics in
microchips and circuit boards in the ever growing electronics materials field. In an
extension of the above study the authors have also observed that the coefficient
of thermal expansion (CTE) of the composite was low enough for electronic
applications and similar to the value of silicon materials or polyimides used in
printed circuit boards?
Plasticlwood fibre composites are being used in a large number of
applications in decks, docks, window frames and molded panel components60. It
has been reported that 460 million pounds of plasticlwood fibre composites
were produced in 199961. Statistics show that the production of these
composites in 2001 has increased to 700 million pounds62. Over the last three
decades considerable research has been committed to finding an alternative
fibre to replace asbestos in fibre cement products. Australian research was
centred on natural fibres and ultimately it was a natural fibre, wood pulp fibre,
which was responsible for the greatest replacement of asbestos in the

beleaguered global fibre cement industry63. As these fibres are cheap and
readily available the energy required for the processing of these composites is
low; also the incorporation of random vegetable fibres in cement matrices
requires only a small number of trained personnel in the construction industry.
Recently, researchers64 have explored the use of bamboo fibre as reinforcement
in structural concrete elements. Pulp from eucalyptus waste and residual sisal
and coir fibres have also been studied as a replacement for asbestos in roofing
components65. The use of cactus pulp as a stabilising agent to improve the
behaviour of the sisal fibre reinforced soil has been studied66. Chopped barley
straw has also been used as a suitable reinforcement for composite soil67.
Biocomposites also offer immense opportunities for an increasing role
as alternate material, especially as wood substitutes in the furniture market68.
Yet biofibre as construction material for buildings were known long before. For
centuries, mixtures of straw and loam, dried in the sun, were employed as
construction composite in Egypt69. Pipes, pultruded profiles and panels with
polyester matrices are also quite popular. Large projects have been promoted
where jute-reinforced polyester resins are used for buildings, e.g. the Madras
House and grain elevators. Today, a renaissance in the use of natural fibres as
reinforcement in technical applications is taking place, mainly in the automobile
and packaging industries (e.g., egg boxes).
In the automotive industry70, cotton fibres embedded in polyester matrix
was used in the body of the East German "Trabant" car. The use of flax fibres in
car disk brakes to replace asbestos fibres is also another example. Daimler-
Benz has been exploring the idea of replacing glass fibres with natural fibres in
automotive components since 1991. A subsidiary of the company, Mercedes

8enz pioneered this concept with the "Beleem project" based in Sao Paolo,
Brazil. In this case, coconut fibres were used in the commercial vehicles over a
9 year period. Mercedes also used jute-based door panels in its E-class
vehicles in 1996. In September 2000, Daimler Chrysler began using natural
fibres for their vehicle production. The bast fibres are primarily used in
automotive applications because they exhibit greatest strength. The other
advantages of using bast fibres in the automotive industry include weight
savings of between 10 and 30 X and corresponding cost savings. Recent
studies have also indicated that hemp-based natural fibre mat thermoplastics
are promising candidates in automotive applications where high specific
stiffness is required?
Virtually all the major car manufacturers in Germany (Daimler Chrysler,
Mercedes, Volkswagen, Audi Group, BMW, Ford and Opel) now use
biocomposites in various applications. Interior trim components such as
dashboards and door panels using polypropylene and natural fibres are
produced by Johnson Controls, lnc for Daimler Chrysler72. In 2000 Audi
launched the A2 midrange car in which door trim panels were made of
polyurethane reinforced with mixed flaxlsisal mat. DaimlerChrysler has now
increased its research and development in flax reinforced pol yes ter composites
for exterior applications73.
The end of life vehicle (ELV) directive in Europe states that by 2015,
vehicles must be constructed of 95 % recyclable materials, with 85 % recoverable
through reuse or mechanical recycling and 10 % through energy recovery or
thermal recycling74. This will definitely lead to an increased use of biofibres. The
diverse range of products utilizing biofibres and biobased resins derived from soy

beans is giving life to a new generation of composites for a number of
applications like hurricane-resistant housing and structures75.
1 .I .3.3 Designing Biocomposites
Although biofibre reinforced polymer composites are gaining interest,
the challenge is to replace conventional glass reinforced plastics with
biocomposites that exhibit structural and functional stability during storage and
use and yet are susceptible to environmental degradation upon disposal. An
interesting approach in fabricating biocomposites of superior and desired
properties include efficient and cost effective chemical modification of fibre,
matrix mod ifica lion by functional izing and blending and efficient processing
techniques. (Figure 1 . I .6)
Another interesting concept is that of "engineered natural fibres" to
obtain superior strength biocomposites76.This concept explores the suitable
blending of bast (stem) and leaf fibres. The judicious selection of blends of
biofibres is based on the fact that the correct blend achieves optimum balance
in mechanical properlies for e.g., the combination of bast and leaf fibre is
expected to provide a stiffness-toughness balance in the resulting
biocom posites. •

/ EFFICIENT BIOCOMPOSITE PROCESSING
Figure 1.1.6 'Tricorner approach in designing of high performance biocornposites.
[Reference: Adapted from Natural fibres, Biopolyrners and Biocomposites, 37, Edited by Mohanty A.K., Misra M., Drzal L.T., CRC Press, 20051
1.1.4. ELECTROSPJNNING (ELSP)
Electrospinning has been recognised as one of the most efficient
techniques for the fabrication of nanopolymer fibres. When the diameter of
polymer fibre is shrunk from micrometer to nanometre range, amazing new

properties are exhibited in the nanopolymer fibres. The large surface area of the
fibres contributes to various applications ranging from medical prosthesis to
composite preparation. In addition, the electrospun fibres find applications in
various other fields such as tissue template, protective clothing, cosmetics, drug
delivery, conducting nanofibres, sensors, and optical shutters etc.
Electrospinning has recently drawn strong attention in biomedical
engineering77, providing a basis for the fabrication of unique matrices and
scaffolds for tissue engineering. ELSP is a spinning method that can produce
polymer fibres with diameters ranging from several micros to 100 nm or less
under a high-voltage electrostatic field operated between a metallic nozzle of a
syringe and a metallic collector in air. The fibres are typically deposited in the
form of a non-woven fabric onto a target metallic collector through a random
projected jet of polymer solution, the so-called cone or instable jet. The
accumulated charges on the polymer solution ejected from the nozzle induce
the radial charge repulsion in the electric field, which induces the instable jet. If
the random deposition process of fibres in the instability jet is appropriately
con trolled on the mesoscopic scale, the higher-ordered spatial placement of the
nano- and microfibres in the ELSP fabrics will be realized. In addition, the high-
speed movement of the collector or nozzle may produce oriented meshes and
more hig her-ordered macroscopic devices.
Researchers recently developed two novel ELSP techniques:
multilayering ELSP and (multicomponent) mixing ELSP, both of which are
composed of different nano- and rnicrofibres78. In the rnultilayering ELSP, after
electrospinning the first polymer, the second polymer is sequentially electrospun
on the same target collector (See Figure 1.1.7). Such a sequential spinning

process can produce a multilayered fibre mesh, in which a hierarchically
ordered structure composed of different kinds of polymer mesh could be
obtained. For the mixing ELSP, two different polymers are simultaneously
electrospun from different syringes under special conditions. The produced
polymer fibres are mixed on the same target collector, resulting in the formation
of a mixed fibre mesh.
Figure 1.1.7
[Reference: Kidoaki et at. Biornaterials 26, 37, 20051
Tremendous amount of studies are being undertaken in the field of
efectrospinning. The effect of molecular weight on fibrous PVA prepared by
electrospinning has been reported by Koski et a179. Zarkoob e l a1.80 has reporled
on the structure and property of electrospun silk fibres. The preparation of
PVAlZirconiumoxychloride composite fibres by using sol-gel process and

electrospinning was reported by Shao et ata1. In the above work, fibres with
diameter 50-200 nm could be obtained.
Recently scientists82 have looked into the governing parameters in the
electrospinning of polymer fibres. Researchers have studied the mechanical
characterisation of electrospun gelatin nanofibres83. Another innovative
approach has been the preparation of blend biodegradable nanofibrous non-
woven mats via multi jet electro spinninga4. Recently a leading group85 at the
Chonbuk National University, Republic of Korea has reported on the preparation
of nano to submicron fibres of ruthenium doped titanium dioxide lpoly (vinyl
acetate) hybrid fibres by electrospinning.
Another interesting study was the electrospinning of chitosan nanofibres
from aqueous chitosan solution using concentrated acetic acid solution as a
solvent. A uniform nanofibrous mat of average fibre diameter of 130 nm was
obtained from 7% chitosan solution in aqueous 90% acetic acid solution in an
electric field of 4 kVlcm. An aqueous acetic acid concentration higher than 30%
was prerequisite for chitosan nanofibre formation, because more concentrated
acetic acid in water progressively decreased surface tension of the chitosan
solution and concomitantly increased charge density of jet without significant
effect on solution viscositya6.
In an innovative study, ultrafine gelatin (Gt) fibres were successfully
produced with the use of the electrospinning technique by using a fluorinated
alcohol of 2,2,2-trifluoroethanol (TFE) as the dissolving solvent87.

1.1.5. CELLULOSE BASED NANOCOMPOSITES
The concept of nanostructured materials design is gaining widespread
importance among the scientific cornmunityaa. The strong reinforcement effects
at low volume fraction resulted in a lremendous interest from the industry and
research circles. With this as an inspiration, the potential of nanoscale cellulose
structures as reinforcement in novel composite materials was extremely
interesting. The concept of cellulose nanocomposites for load bearing
applications is fairly new. Property enhancements are expected due to higher
Young's modulus of pure cellulose reinforcement and finely distributed
reinforcing microfibrils. A major problem in the commercial use of cellulose
microfibrils in structural materials is the disintegration of cellulose from plant cell
wall at reasonable cost and without severe degradation. Another major problem
is dispersion of cellulose microfibrils in a polymer matrix.
A simple model of the cellulose microfibril structure is presented in
Figure 1 . I .8. Cellulose nanocomposites are usually fabricated by utilizing these
microfibrils of 10-50 nm on width as reinforcement in a polymer matrix. Although
many studies provide detailed knowledge regarding the morphology and
crystallography of different types of cellulose, the Young's modulus of
microfibrils from different sources and subjected to different types of hydrolysis
is seldom discussed.

Ceilulose elementary fibril
Figure 1.1.8 Model of the cellulose microfibril structure [Reference: Berglund L., Natural fibres, Biopolymers and Biocornposites, 808, Edited by
Mohanty A.K., Misra M., Drzal L.T., CRC Press, 20051
The research group at CERMAV-CN~S has presented numerous
studies based on cellulose whiskers reinforced nanocomposites89. The whiskers
were disintegrated from an edible tunicate, a sea animal. Another area of
interest is that of microfibrillaled cellulose (MFC) nanocomposites. MFC based
on parenchyma cells (i.e. sugar beets*, potato tuber) tend to show small
microfibril diameter and the fibrillar structures resemble swirled mats of
connected, microfibrils rather than discrete rods. A review of the recent

research into cellulosic whiskers, their properties and their applicalion in
nanocomposite field has been presented by the same groupql.
Recently scientists92 achieved uniform polypyrrole nanocoating on
natural cellulose fibres without disrupting the hierarchical network structures of
individual cellulose fibres by means of polymerization-induced adsorption
(Figure 1.1.9).
Cellul~$e fiber coated with Cellulose C b e r coated wilh polypyrrde layer titania and polypyrrole layers
Figure 1.1.9
[Reference: Huang et al,, Chemical Communications, 13,1717, 20051
High performance composites from low-cost plant primary cell walls
were developed by Bruce et al93. Purified cell wall fragments and cellulose
microfibrils were developed from Swede root to form novel composite materials.
The composites fabricated from purified cell wall fragments and acrylic matrix
gave the best tensile properties. Researchers94 have also reinforced natural
rubber with nanofibres of sepiolite. The composites also contained silica
particles generated in situ by the sol-gel process. The level of reinforcement
was assessed from mechanical and orientation behaviour.

Algae are also a potential source of cellulose for use in composite
materials. Researchers have concentrated on the morphology of cellulose
powder from Caldophora spatza algae from Ihe Baltic sea95. The effective
stiffness of the algae MFC (which is the combined effect of the microfibril
modulus and its aspect ratio) was found to be higher than for the wheat straw
cellulose. Bacterial or microbial cellulose (BC) have also found their way as
reinforcement in composites. Cellulose can be produced from some bacteria
such as A.xylinum, vinegar or acetic acid. Bacterial cellulose is an extracellular
product which is excreted into the culture medium. Acetic acid bacteria are not
photosynthetic but can convert glucose, sugar, glycerol to pure cellulose.
Bacterial cellulose has been frequently studied to clarify the mechanism for
biosynthesis96. Researchers have looked into the development of cellutose
nanocomposites based on bacterial cellulose and cellulose acetate butyrateg7.
Though BC composites have evoked widespread interest, the industrial use of
BC composites requires the development of efficient large-scale fermentation
technology.
Nanoscale-modifed plant structures such as paper or bast fibres also
present a different approach lo preparation of nanocomposite structures, An
interesting study concentrated on the preserving the microfibril organization of
wood veneer in composites98. Pine veneer was used as the starting material.
The basic idea was to increase the volume fraction of cellulose microfibrils by
partially removing the lignin and hemicellulose wood polymers and by
compressing the veneer. Phenol formaldehyde resin (PF) was used to preserve
the compressed microstructure of the material and to bond microfibrils.

1.1.6. INTERFACE
A strong interface is critical for superior mechanical properties of the
composites. Interface has been dubbed as the "heart" of the composite. The
term interface is defined as a two dimensional region between the fibre and
matrix having properties intermediate between those of fibre and matrix. Matrix
molecules can be anchored to the fibre surface by chemical reaction or
adsorption, which determine the extent of interfacial adhesion. In certain cases,
the interface may be composed of additional constituent as a bonding agent or
as an interlayer between the two components of the composite.
The region separating the bulk polymer from the fibrous reinforcement
is of utmost importance to load transfer. This region was originally dubbed as
interface but is now viewed as interphase because of its three dimensional
heterogeneous nature. It is not a distinct phase, as the interphase does not
have a clear boundary. It is more accurately viewed as a transition region that
possesses neither the properties of fibre nor those of the matrix. An
interphase that is softer than the surrounding polymer would result in lower
overall stiffness and strength, but greater resistance to fracture. On the other
hand an interphase that is stiffer than the surrpunding polymer would give the
composite less fracture resistance but make it very strong and stiff. The
nature of the interphase varies with the specific composite system. The
interphase is generally thought to Lhin (less than 5 pm) with the differences in
property between bulk polymer and the interphase very subtle. The
developments of atomic force microscope and nano-indentation devices have
facilitated the investigation of the interphasegg.

The word interphase is used as a general term to categorize the
polymeric region surrounding a fibre. It consists of polymeric material made
from the chemical interaction of sizing or coating on the fibre and the bulk
matrix during the curing process. The interphase is also known as the
mesophase (Figure 1 .1.10).
- BULK U4tRD:
p O L v u U l d ,-. o l r r c w x
P R r n R T I M
r m cauarpv ~ E R U ~ L . '---FI~wT-
Q i m J ' a L . "-#aC.Q - M Y
U E C W l [ j U . BULK F l E R
FIBER-MATRIX INTERPHASE ..,-,,
Figure 1.1.10 Schematic model of interface
[Reference: Laly A Pothen, Ph.D Thesis]
The strength and toughness of fibre-reinforced materials are
determined by the interface between the reinforcing fibres and matrix.
Extensive research has been carried out in order to understand the nature of
the interfacial bond and ils characterization. A strong interface creates a
material that displays exemplary strength and stiffness but which is very
brittle in nature with easy crack propagation through the matrix and fibre. A
weaker interface reduces the efficiency of stress transfer from the matrix to
the fibre and as a result the strength and stiffness are not high.

The fibre1 matrix interface in a continuous filament composite transfers
an externally applied load to fibres themselves. Load applied directly to the
matrix at the surface of the composite is transferred to the fibres nearest the
surface and continues from fibre to fibre via matrix and interface. If the
interface is weak, effective load distribution is not achieved and the
mechanical properties of the composite are impaired. On the other hand, a
strong interface can assure that the composite is able to bear load even after
several fibres are broken because the load can be transferred to the intact
portions of broken as well as unbroken fibres. A poor interface is also a
drawback in situations other than external mechanical loading e.g. because of
differential thermal expansions of fibre and matrix, premature failure can occur
at a weak interface when the composite is subjected to thermal stress. Thus,
adhesion between fibre and matrix is a major factor in determining the
response of the interface and its integrity under stress. Additionally the
interface maybe more vulnerable to moisture and solvents than are fibre and
matrix. The fracture of an interface between two materials depends on the
geometry, the constitutive properties of the adherends and the details of
bonding across the interface.
1.1.6.1 Characterization Techniques
The characterization of the interface gives the chemical composition
as well as information on interactions between fibre and matrix. Various
methods are available for characterization of the interface.
1. Micro-mechanical, Techniques
The extent of fibre-matrix interface bonding in terms of interfacial
shear strength can be determined by different micro-mechanical testssuch as

single fibre pull out, micro debond test, micro-indentation /micro-compression 1
fibre push out test and single fibre fragmentation test.
2. Spectroscopic Techniques.
Electron spectroscopy for chemical analysis 1 X-ray photoelectron
spectroscopy (ESCAI XPS), fourier transform spectroscopy (FTIR), Laser
Raman Spectroscopy (LRS), nuclear magnetic resonance (NMR) and
photoacoustic spectroscopy have been shown to be successful in polymer
surface and interfacial characterization.
3. Microscopic Techniques
Microscopic studies such as optical microscopy, scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and atomic
force microscopy (AFM) can be used to study the morphological changes on
the surface and can predict the extent of mechanical bonding at the
interface. The adhesive strength of fibre to various matrices can be
determined by AFM studies.
4. Thermodynamic Methods
The frequently used thermodynamic methods for characterization in
reinforced polymers are wettability study, idverse gas chromatography
measurement, zeta potential measurements etc. Contact angle measurements
have been used to characterize the thermodynamic work of adhesion be tween
solids and liquids as well as surface of solids.
5. Other Techniques
The interface can also be characterized qualitatively by other methods
like dynamic mechanical analysis and stress relaxation technique. Swelling

techniques have been used to assess the level of interfacial adhesion in the
case of fibre reinforced elastomer composites.
1 .I .6.1 .I Micromechanical Techniques
(a) Single fibre pull out test
One of the most common methods is to measure the force required to
pull out a fibre embedded in a matrix. This method, in addition to its relative
simplicity of sample preparation and measurement is expected to give realistic
information when one considers the pull out of fibres from the fracture
surfaces of composites. The two main aspects are the debonding which
destroys the bond between fibre and matrix and the other is the pull out in
which a fibre is extracted from a broken matrix against friction. The pull out
test is considered to be the best method of evaluating the interfacial shear
load as it can directly measure the interfacial shear strength between the fibre
and matrix independent of their properties. The interfacial shear strength is a
critical factor that controls the toughness, mechanical properties and
interlaminar shear strength of composite materials. The fibre pull out problem
has been investigated extensively for purposes of studying the interfacial
adhesion quality and elastic slress transfer between fibres and matrix. From
the loadldisplacernent curves, the average interfacial shear strength (IFSS) r
is given by
where F is the load needed to debond the fibre from the microbead and d and
I are the diameter and embedded length of the fibre respectively. It is assumed
that the shear stress along the interface is uniform. Though the single fibre

pull-out has an apparently unrealistic character when compared to the
complexity of the composite material, much information can be derived that is
related to the most fundamental aspects of the fibretmatrix mechanical
interaction. The drawback of single fibre pull out test is that it involves only a
single fibre. As the role of neighboring fibres is not taken into account, the
thermal stresses and the polymer morphology around the fibre are not the
same as in a real composite. Real composites contain multiple fibres and the
pull out fibre is surrounded by a composite medium. In single fibre model
composites, the effect of the composite medium surrounding the pull out fibre
has been ignored. Therefore, owing to the influence of the composite medium,
surrounding the pull out fibre, the interfacial debonding process in multi-fibre
composites and the interfacial properties obtained would very likely deviate
from those of the single-fibre composite pull out test.
(b) Micro-debond test
The microbond technique, which is a modification of the single fibre pull
out, involves depositing a micro-bead of the matrix onto a fibre. The fibre with
the micro-bead is then mounted in a micro-vise by placing the microbead under
the micro-vise blades and the fibre is pulled out. The interfacial shear strength
is calculated from equation 1 .I .3.
The specimen preparation for the micro droplet test whereby a single
fibre is pulled out of a small droplet of resin suffers from several difficulties. For
instance, the reliability of the data is affected by the shape of the droplet*
Symmetric, round droplets are easier to test and analyze than droplets with flat
surfaces, produced when the specimens solidify on a flat substrate. Also the
size of the droplet is critical. If the length of the droplet exceeds a critical value,

the fibre will fracture prior to debonding and pull out. An additional complication
with some thermoset materials is that the anticipated curing characteristics may
not manifest themselves in a droplet of small size, and hence comparison on a
microstructural level between micro and macro specimens may not be possible.
Another defect is that this test is not applicable to matrices that are soft.
(c) Microindentation test
The micro indentation test was initially developed for fibre -reinforced
ceramics but has been extended to the other fibre-matrix systems. It is also
known as the fibre-push out test. This is the only single fibre test, which is able
to analyze actual composite specimens. The use of a real composite allows a
more realistic simulation of thermal stresses, polymer morphology and the
influence of neighboring fibres. The presence of other fibres is an advantage,
but it complicates the calculation of stress state around the fibre and the choice
of an appropriate failure criterion. Nevertheless, the more realis tic tes ling
conditions of the micro-indentation method make it an attractive new technique
for many researchers. In this method, a compression force is applied on a single
fibre in a well-prepared specimen of a real composite.
(d) Single fibre fragmentation test
The SFF technique involves embedding a single fibre along the
centerline of a dog-bone shaped specimen of matrix material. The specimen is
then strained along the fibre axis. The shear in the resin exerts a tensile stress
on the fibre. At some stress the fibre fractures at its weakest point. With
increasing specimen strain the fibre fractures repeatedly at different locations.
No additional breaks can occur when the fragments become so short that the
shear transfer along the length of the broken fibre can no longer make the

tensile strength higher to cause additional fractures. This fragment length at the
end of the test is known as critical length l c . The average IFSS is then
calculated using the force balance equation:
The ratio I, Id is called the critical aspect ratio. This is the most realistic
test from the point of view of the interfacial pressure. The fibre is neither pushed
nor pulled directly, and so fibre Poisson effects are similar to that occurring in a
fibre composite.
A disadvantage of this technique is that the failure strain of the matrix
must be much larger than the failure strain of the fibre to promote multi-
fragmentation of the fibre. This requires the use of matrices, which can undergo
large deformations. Consequently commercial resins utilized in actual
composite systems, which typically have low strains to failure, cannot be used
for this test. Therefore the interfacial shear strength determined is not directly
applicable to the actual composite system.
Another aspect is that friction plays an important role in the debonding
process and this is governed by two additional u~knowns, i.e, the coefficient of
friction 11 and the pressure across the interface P. Although some progress has
been made with this, value for debonding rely rather heavily on the correctness
of the assumptions about 11 and P.
1.1.6.2 Characterization of Interfaces in Biocomposites
Tremendous amount of work has been carried out for the past twenty
years on the characterization of the fibre I matrix interface. The key issue has
been the development of mathematical models that by taking into account the

stress transfer at the interface would provide the calculalion of a parameter that
could assess the interfacial bond. Biofibres unlike synthetic ones are very
heterogeneous ~naterials and are still not completely understood. A major
problem for the natural fibres is the fibre diameter, which is not constant even for
the same single fibre. Therefore the characterization of interfaces in natural fibre
reinforced composites is a difficult task.
The role and the importance of interfaces and interphases in
rnulticomponenl materials have been enumerated by Pukanszky1004 Recently
researchers have looked into the applicability of different micromechanical tests
to interface strength characterization and the advantages and disadvantages of
stress based and energy based rnodels'o! In an interesting studyjO2, the
correlation between post-mortem fracture surface morphology of short fibre
reinforced thermoplastics and interfacial adhesion was presented. The authors
are of the opinion that fracture surface morphology is dependent on both fibre-
matrix interfacial adhesion strength and matrix shear yield strength. Jacob et
al.Io3 have reviewed the recent advances in characterization of interfaces in
biofibre reinforced composites.
The interfacial morphology in wood-fibre and maleated HDPE
composites was studied by Lu et allo4. FTlR and ESCA analyses presented the
evidence of a chemical bridge between the wood fibre and polymeric matrix
through esterification. The interface effects on the mechanical properties and
fracture toughness of sisal fibre reinforced vinyl-es ter composites were
invesligated by Li et allo5. The interfacial shear strength was calculated by
single fibre pull out technique. It can be clearly seen from Figure 1 . I .ll that after
fibre surface treatments by using different chemical agents (except silane-I ), the

IFSS was greatly improved. KMn04 treatment showed the best effect and the
IFSS between permanganate-treated sisal fibre and vinyl-ester resin was almost
three limes that of the untreated fibre, then followed by siiane-2 and DCP
treatments. For silane-I treated sisal fibres, IFSS was similar to that of
untreated sisal fibre reinforced vinyl-ester.
Untreated Silane 1 DCP Silane 2 KMnO,
Figure 1.1.11 Variation of interfacial shear strength with chemical modification
[Reference: Li et al., Comp. Interf., 12,l-2,141, 20051
Gassan'" has attempted an interesting study on fibre and interface
parameters affecting the fatigue behaviour of natural fibre reinforced composites.
Composites comprising of flax and jute yams and wovens as reinforcements for epoxy
resin, polyester and polypropylene were analyzed. The studies revealed that when the
interface bonding is weak, debonding and frictional sliding occur readily upon crack
extension. Upon cyclic loading of composites, the frictional sliding at the interface

changes with interface properties such as roughness. Furthermore, unidirectional
composites were found to be less sensitive lo fatigue induced damage than woven
reinforced ones.
A study utilizing single fibre pull-out was atiempted by VandeVelde et al.lo7.
The authors used dew retted hackled long flax treated with propyltrimethoxy silane,
phenyl isocyanate and maleic anhydride polypropylene. The studies revealed that
composite prepared with flax fibre treated with MAA-PP exhibited the highest interfacial
shear strength. Joseph et al.108 have conducted an interesting study on the comparison
of interfacial properties of banana fibre and glass fibre reinforced phenol-formaldehyde
composites. They observed that the interfacial shear strength is higher for banana I PF
system than for glass IPF system. This was attributed to the hydrophilic nature of
cellulose and PF resin. Hydrophilicity of fibre arises from the hydroxyl groups of lignin
and cellulose which can easily form bonds with methyl01 and phenolic hydroxyl groups
of the resole resulting in a strong interlocking between the two. In the case of glass IPF
composites this kind of bonding is not possible and hence the interfacial shear strength
The interfacial adhesion of flax fibre reinforced polypropylene and
polypropyleneelhylene propylene diene terpolymer blends were investigated by
Manchado e l al.l@ In this study both the matrices were modified with maleic anhydride.
Single fibre pullout tests were conducted to investigate the extent of interfacial
adhesion. They observed that the addition of small proportions of maleic anhydride to
the matrices significantly increased the shear strength. The authors were of the opinion
that introduction of functional groups in the matrix reduced the interfacial stress
mncen tra tions preventing fi bre-fibre interactions which are responsible for premature
composite failure. Also, in presence of rnaleic anhydride functional groups the

esterification of flax fibres takes place increasing the surface energy of the fibres to a
level closer to that of the matrix. Hence, a better wettability and interfacial adhesion was
Zafeiropoulos et al.1'0 characterized the interface in flax fibre reinforced
polypropylene composites by means of single fibre fragmentation test. Flax fibre was
modified by means of two surface treatments: acetylation and stearation. The authors
observed that acetylation improved the stress transfer efficiency at the interface. Stearic
acid treatment was also found to improve the stress transfer efficiency but only for lower
times. They also found that stearation for longer durations deteriorated the interface.
This was attributed to decrease of fibre strength upon treatment of longer duration and
the fact that excess of stearic acid acted more as a lubricant than as a compatabiliser.
In an interesting study, dynamic mechanical analysis has been used to analyse
the interfacial adhesion in banana fibre reinforced polyester composites by Pothan and
Thomas"'. The authors observed an additional peak in their tan delta curve and have
reported it to be due to an additional polymer layer which probably is the interphase.
The authors also observed that there was increase of storage modulus with treatment.
Slorage modulus is found to be directly proportional to the interfacial strength of
composite. Also the T, was found to shift to higher temperatures for treated composiles
indicative of good interfacial. adhesion.
The effect of different chemical modifications on the interfacial characteristics
of aspen fibre reinforced HDPE composite was conducted by Colom et al.112 The
interaction between aspen fibres and HDPE was improved by the addition of two
coupling agents; maleated polyethylene (epolene C-18) and y-methacryloxy-propyl
trimethoxy silane (silane A-174). Interfacial morphology was studied by means of FTIR.

1.1.7. NATURAL RUBBER [NR]
Natural rubber (NU) belongs to a class of compounds known as elastomers.
The Mayans in the Western Hemisphere used NR for centuries before it was introduced
into Europe by Columbus. The term rubber was however coined by Joseph Priestly.
Natural rubber is indispensable in our daily lives. The main uses of NR are
concentrated in four key areas, namely: medical devices, industrial products, domestic
and recreational goods and foremost automobile products. The current elastomer
consumption in the world is 18 million tonnes per year. NR supplies about one-third of
the world demand for elastomers. It is also used as an industrial raw material. NR is a
naturally occurring elastomeric polymer of isoprene ( 2-methyl-1,Sbutadiene). It can be
extracted from latex of only one kind of tree, the Hevea braziliensis. Hevea rubber is
produced in many tropical regions of Southeast Asia, Africa and Central and South
America. There is practically only one other potential source of NR, that is the guayule
shrub (Parthenium argentaturn). Polyisoprene exists naturally in the form of two stereo-
isomers, namely cis-l,4-plyisoprene and trans-1.4-poly isoprene ( Figure 1 .I . I 2)
Trans-l,4-polyi soprene
Figure 1.1.12 Structure of natural rubber

Chemically NR is cis 1,4-polyisoprene. A linear, long chain polymer with
repeating isoprenic units (CsHa), it has a density of 0.93 at 20" C . Natural trans-
1, 4-polyisoprene is a crystalline thermopiastic polymer, which is mainly used in
golf ball covers, root canal fillings in dentistry, special adhesives and to a lesser
extent in wire and cable coverings. It is obtained as gutta percha from
Palaquium oblongofolium in South East Asia. I I is much harder and less soluble
than Hevea rubber. Certain trees like Chicle (Achras sapota) produce latex,
which is a physical mixture of cis and trans polyisoprene. Cis-I , 4-polyisoprene
can be produced synthetically by stereospecific polymerization reaction using
Zieglar-Natta-hetergenous catalyst. The important components of NR are given
in Table 1.1.4113
Table 1.1.4 Typical Analysis of Natural Rubber [Reference: Barfow F.W., Rubber Compounding, Principles, Materials and Techniques,
M. Dekker, 19981
1 components 1 YO
1 Rubber hydrocarbon
1 Acetone extract 1 2.9
1 Moisture --

The most important factor governing the properties of polyisoprene is
the stereoregularity of the polymer chain. The very unique characteristic of NU
is the ability to crystaltize under strain, the phenomenon known as strain-
induced crystallization~~4. Stretching of vulcanizates of polyisoprene having at
least 90 % cis content leads to crystallization, which in turn leads to
strengthening of rubber.
Another interesting aspect of isoprene rubbers is their low hystereses
giving low heat build up during flexing. The combination of high tensile
properties and low hysteresis explains the requirement of NR as the primary
rubber in heavy vehicle tyres. In addition to this, NR has excellent tensile and
tear properties, good green strength and building tack. However, NR is not very
resistant to oxidation, ozone weathering and a wide range of chemicals and
solvents, mainly due to its unsaturated chain structure and non-polarity.
1.1.7.1 Grades and Grading115
All types of NR that are not modified (such as oil extended NR) or
technically specified rubbers (TSR1s) are considered to be international grades.
Grade designations usually use color or how the rubber was made; typical
grade description are pale crepe, rubber smoked sheet and thin brown crepe.
The main disadvantage of this system is that grading is done on visual aspects.
Almost exclusively, the darker the rubber, the lower the grade. Other grading
criteria such as the presence or absence of rust, bubbles, mould and cut spots
are subjective in nature. Perhaps the most valid assumption is that the darker
the rubber the more dirt it contains. In the 19601s, Malaysia developed a grading
scheme that was more sophisticated and useful to customers. A major criticism
of NR is the large and varying dirt content. The cornerstone of the new system

was grading according to dirt content measured in hundredths of 1%. For eg:
Standard Malaysian rubber (SMR) is a rubber whose dirt conlent does not
exceed 0.5%. Dirt is considered to be the residue on a 45pm sieve after a
rubber sample has been dissolved in an appropriate solvent, washed through
the sieve and dried. The specification has other parameters including source
material for the grade, ash and nitrogen content, volatile matter, plasticity
retention index (PRI) and initial plasticity. Acceptance of these standards is
vigorous and other rubber producing countries followed suit. Letter
abbreviations identify the rubber source, SMR denotes rubber from Malaysia,
SIR indicates Indonesian product, SSR indicates Singapore product and ISNR
denotes Indian Standard Natural Rubber.
1.1.7.2 Modifications of Natural Rubber
The main modifications of NR are:
Deprotenized rubber (DPNR) - This is very useful when low water
absorption is wanted, vulcanizates with low creep are needed or more than
ordinary reproducibility is required. Normally NR has between 0.25 and 0.5 %
nitrogen as protein; deprotenized rubber has only about 0.07%. A drawback is
that since protein matter in the rubber accelerates cure, deprotenized rubber
requires more acceleration. DPNR is made by treating NU latex with bioenzyme
which hydrolyzes the proteins to water-soluble forms. A protease like bacillus
subtilis is used at about 0.3 phr. When the enzymolysis is completed the latex is
diluted to 3 % total solids and coagulated by adding a mixture of phosphoric and
sulphuric acid. The coagulated rubber is then pressed free of most of the water,
crumbed, dried and baled.

Another modification is oil extended NR (OENR). There are three ways
lo make this kind of rubber a) co-coagulation of latex with oil emulsion
b) Banbury mixing of oil and rubber c) allowing the rubber to absorb the oil in
pans until almost all is absorbed, then milling to incorporate the remaining oil.
Recently rubber and oil have been mixed using an extruder. A comparatively
new modification of NR is epoxidized NR. The rubber molecule is partially
epoxidised with the epoxy groups randomly distributed along the molecular
chain. The main advantage of ENR over NR is improved oil resistance and low
gas permeability. Another new modification is thermoplastic natural rubbers
(TPNR). These are physical blends of natural rubber and polypropylene, mixed
in different proportions to give rubbers with different stiffness properties. They
are suitable for injection moulding into products for automotive applications
such as flexible sight shields and bumper components. Many other chemically
modified forms of natural rubber were available in the past. These included
cyclized rubber, chlorinated rubber, hydrochlorinated rubber, isomerized or an
anticrystallizing rubber and depolymerised rubber.
1.1.8 SHORT FIBRE REINFORCED RUBBER COMPOSI'TES
Short fibre rubber composite can be defined as a compounded rubber d
matrix containing discontinuous fibres that are distributed within the rubber to
form a reinforcement phase. A tremendous amount of research has been
carried out on natural and synthetic fibre reinforced rubber composites. The
reinforcement of an elastomer with fibres combines the elastic behavior of
rubber with the strength and sliffness of the reinforcing fibre. Short fibres are
also used to improve or modify certain thermodynamic properties of the rubber
for specific applications or to reduce the cost of the fabricated articles.

According lo O'Connorll6, the limitations of using short fibres in rubber
compounding applications have been due to difficulties in achieving uniform
dispersion of fibres, fibre breakage during processing and difficulties in
handling, incorporating and bonding the short fibres into the rubber matrix. In
order to use a fibre-reinforced elastomeric composite most effectively, a basic
understanding must be obtained of how the various properties depend on the
fibre properties, the matrix properties and the processing methods13. The early
use of fibres (bast fibres) in rubber was merely as a cheapening aid. The
presence of fibres in a rubber increases the modulus but other improvements
are also sought with fibre addition. The possibility of much improved properties
brought about by the alignment of fibres, to give a composite with some
measure of anisotropy is also of great interest.
Gulh et al.ll7 derived an equation for calculating the modulus of a fibre-
reinforced matrix, applicable to fibre-reinforced rubbers, which has been much
quoted. This equation is commonly referred to as the 'Guth-Gold' equation and
is expressed as:
where G = modulus of composite material
Go = modulus of matrix material
f = length to diameter ratio (aspect ratio) of fibre
c = volume concentration of fibre
When the fibre aspect ratio is in the range 10-50, moduli ratio of
102- 103 can be achieved if there is good adhesion between the fibre and

matrix. A later modification taking into account the fibre orientation is the
Boustany -Corm equation:
where €cornp = Modulus of the composite
El = Modulus of rubber
K = constant
f = a function of fibre orientation
Ild = fibre aspect ratio
Vr = fibre volume fraction
Anthoine et all18 dealt mainly with the use of natural short length
cellulose fibre as rubber reinforcement. They claimed thal this fibre being
naturally available in short lengths (from wood pulp) it would circumvent the
complicated route in making short fibre from synthetic polymer and thus would
be more cost effective. The authors reported that the oriented cellulose fibres
gave an increase in tensile strength but the most important changes were an
increased modulus and reduced elongation at break. The modulus of the fibre-
reinforced materials as defined by Young's modulus, showed a steady increase
with increasing fibre concentration. Tensile strength however first fell with
increasing fibre level and then increased beyond that of the matrix material at
higher fibre loadings. This was explained that at low concentrations the matrix is
not restrained by enough fibres and high strains occur in the matrix at low
stresses, causing debonding. The matrix strength is thus reduced by the
debonded fibres.

The advantages of using short fibres are that they can be easily
incorporated into the rubber compound. Short fibres also provide high green
strength and dimensional stability during fabrication. Almos l all standard rubber
processing operations such as extrusion, calendering, compression moulding,
injection moulding & transfer moulding can be used for fabrication of
composites. Short fibre composites also possess design flexibility resulting in
the fabricalion of complex shaped articles. These composites possess high
specific strength, reduced shrinkage, controlled damping properties, improved
swell resistance, increased abrasion resistance and creep resistance. They are
also more economically viable since dipping, wrapping, laying and placing of
fibres associated with long fibre composites are avoided.
'The disadvantages of short fibre reinforced rubber composites are that
it is quite difficult to achieve uniform dispersion, the problem of fibre breakage
during processing (though biofibres undergo less breakage than synthetic
ones). Also certain processing techniques like filament winding, autoclave and
vacuum bag processing techniques cannot be used for short fibre composites.
1.1.8.1 Theory of Reinforcement
The most common assumption in the case of short fibre reinforced
elastomers is that either the stress state or the strain is uniform everywhere in
the deformed composite material, Stiffness is usually considered in terms of the
linear elastic Young's modulus. Even though elastomers do not exhibit
substantial linear elastic regions of deformation in comparison to their useful
extensions, the analysis of elastomer-based composites can proceed on this
basis, because in the reinforced state, linear elasticity is more closely followed.
In the case of reinforcements with fibres of sufficiently high aspect ratio, it is

found that as long as a substantial orientation is obtained in the direction of the
applied stress, the assumption of constant strain is valid.
Thus, linear elasticity is generally applicable, but it must be realized that
in actuality the matrix exists in a rather complex state of non-uniform tension
and shear. The high elastic moduli of the fibres (> 14GPa) differ widely from
those of the elaslomeric matrix (generally about 3 MPa), which might yield a l o 3 greater Young's modulus ratio Ef I Em than for a hard plastic with the same fibre
and have an effect on reducing the efficiency of reinforcement.
The tensile stress generated in a fibre-reinforced elastomer will in
general be distributed belween the fibre (f) and matrix (m) phases,
where c refers to the fibre-rubber composite. If each phase is assumed to be
linearly elastic, this relationship can be further written as
Gc = Et Et $f + E m Ern $rn
where E is the Young's modulus, E the tensile elongation, and 4 the volume-
fraction of each phase.
Although the micromechanical stress state is highly complex in a non-
uniform composite, the role of discontinuous fibres in providing reinforcement
can be gained by considering them to be well aligned in the direction of applied
stress. It is through the shear stresses applied through the matrix that a tensile
stress is generated in the fibre. As these shearing stresses are larger at the
fibre ends and diminish to zero in the midsection of the fibre, the tensile stress
builds up from a small value given by the matrix-supported tensile stress at the

fibre ends toward a constant elevated value in its midsection. The fibre tensile
stress does not continue to increase in unbounded form with increasing
distance from the fibre end but will rather reach a peak at the point where the
fibre strength is reached or will plateau at some high level due to the eventual
decay of the shear stress as the matrix shear deformation goes to zero in
regions of fully developed stress state between fibres. In general, it has been
found that low aspect ratio, non-uniform dispersion and partial or improper fibre
orientation limit the stress development in short-fibre composites.
1 .I 3 . 2 Factors Affecting Reinforcement
'There are many parameters which affect the performance of a fibre-
reinforced rubber composite"? The degree and type of adhesion cannot be
estimated quantitatively even though its importance is well recognized. Aspect
ratio has a considerable effect on composite properties, hence it is important to
conserve fibre length as much as possible during rubber composite processing
operations. Fibre aspect ratio must be in the range of 100-200 for optimum
effectiveness. Fibre orientation however has the largest effect on composite
properties. During processing, the fibres tend to orient along the flow direction
causing mechanical properties to vary in different directions. 4
Poor fibre dispersion results in a loose bundle, embracing an effectively
lower aspect ratio with less reinforcing potential than a single fibre. In addition,
the bundle itself may be low in strength due to poor adhesion. Both the above
factors reduce the overall strength of the composite. The development of
strength in a composite depends on the existence of a strong interface, which
can be enhanced through the selection of an optimum bonding system. A
suitable reaction or a high degree of physical compatibility with both the fibre

CIt (~pter 1
and matrix is required. Though the presence of a covalent bond would increase
the levels of adhesion tremendously, many of the bonding systems rely only
upon physical forces. The extent of physical forces is quite large in the case of
polar polymers and hydrophilic surfaces. The typical bonding systems tried out
in the case of most cellulosic fibres is resorcinol-based adhesives such as
resorcinol-formaldehyde-latex (RFL) cord-dip formulation or resorcinol-
hexamethylene tetramine-hydrated silica (HRH) bonding system. Another
version substitutes hexamethoxyrnethyl-melamine as the methylene bridge
donor. Isocyanate based resins and organofunctional silanes or tilanales act as
good bonding systems for the more polar classes of fibre-rubber composites.
1.1.8.3. Biofibre Reinforced Natural Rubber Composites
The primary effects of short fibre reinforcement on the mechanical
properties of natural rubber composites include increased modulus, increased
strength with good bonding at high fibre concentrations, decreased elongation
at failure, greatly improved creep resistance over particulate-filled rubber,
increased hardness and a substantial improvement in cut, tear and puncture
resistance. Hysteresis and fatigue strength are also improved. When fibres are
aligned parallel to the stress direction, tensile strength develops a characteristic +
drop with increasing fibre volume contenl until a critical fibre level is reached.
Higher reinforcement concentrations generally cause the strength to increase.
The critical concentration level at which the unreinforced matrix strength is
recovered varies directly with the critical fibre aspect ratio. The stiffness and
modulus of short fibre composite increase with fibre concentration though it may
not be necessarily linear. Tear strength is highly dependent on fibre loading and
is seen to increase with concentration.

The mechanical properties of lignocellulosic fibre reinforced natural
rubber composites have been extensively studied. It has been reported by
Dzyura119 that the minimum amount of fibres to restrain the matrix is smaller if
the matrix strength is higher. Natural rubber is a very strong matrix because of
its strain-induced crystallization. Generally it has been seen that the tensile
strength initially drops down to a cerlain amount of fibre and then increases.
The minimum volume of fibre is known as the critical volume above which the
fibre reinforces the matrix, The critical volume varies with the nature of fibre and
matrix, fibre aspect ratio and fibre I matrix interfacial adhesion.
At low fibre concentrations, the fibre acts as a flaw in the rubber matrix
and the matrix is not restrained by enough fibres causing highly localized strains
to occur in the matrix at low stress. This makes the bond between fibre and
rubber to break leaving the matrix diluted by non-reinforcing debonded fibres.
As the fibre concentration increases, the stress is more evenly distributed and
the strength of composite increases. 'The incorporation of fibre in to rubber
matrix increases the hardness of the composite, which is related to strength and
toughness. The close packing of fibres in the compounds increases the density
while resilience decreases.
The reinforcement of coir fibre in natural rubber has been extensively
studiedl20. Upon incorporation of coir fibre, it was seen that the tensile strength
decreased sharply with increase in fibre loading up to 30 phr and then showed a
slight increase for composites containing 40 and 60 phr fibre loading. This trend
was observed in both longitudinal and transverse directions. The extent of coir
fibre orientation from green strength measurements was also determined. It was

6 8 - .--."
Clt npter I
observed that orientation was lowest when fibre loading was small and
increased with loading. Maximum orientation was observed at 30 phr loading.
Researchers have also investigated the reinforcement effects of a leaf
fibre - sisal fibre -in natural rubber'*'. They observed that the tensile strength
decreased up to 17.5 % volume loading and then increased. The tear strength
and modulus values showed a consistent increase with loading. Thomas and
co-workers have also investigated the effect of chemical modificalion of banana
fibre in natural rubber. It was found that modification of banana fibre resulted in
superior mechanical properties122.
Attempts to incorporate oil palm fibre in rubber matrix have also been
successful. 'The effect of fibre concentration on the mechanical properties of oil
palm reinforced natural rubber composites was investigated by lsmail e t a1.123 They
observed the general trend of reduction in tensile and tear strength with increasing
fibre concentration. The modulus and hardness of composites registered an
increase with fibre loading. Another interesting study by the same group'24 reported
on the fatigue and hysteresis behavior of oil palm wood flour filled natural rubber
composites. It was observed that the composite with the highest loading was the
most sensitive towards changes in strain energy and exhibited the highest
hysteresis.
An interesting report on the reinforcement effect of grass fibre - bagasse
- in natural rubber was presented by Nassar et aI.'25 Aging experiments
revealed tensile strength retention of 97 %. Scientists have also developed
composites comprising of kenaf fibre and natural rubber126. An increase in

rheometric and mechanical properties was observed. The kenaf fibre loaded
composites also possessed good thermal stability and swelling resistance.
Pineapple127 and jute fibre128 have also found their way as a potential
reinforcement in natural rubber. In an interesting study, researchers have used
a novel fibre isora fibre-in natural rubber129. lsora fibres are present in the bark of the
Helicteres isora plant and are separated by retting process, lsora fibre resembles jute in
appearance but surpasses it in strength, durability and lustre. The effects of different
chemical treatments, including mercerisation, acetylation , benzo yla tion and treatment with
toluene diisocyanate and silane coupling agents, on isora fibre properties and mechanical
properties were analyzed, lsora fibre was seen to have immense potential as
reinforcement in natural rubber. The variation of tensile strength with chemical
modification is given in Figure 1.1 -13.
Figure 1.1.13 Variation of tensile strength with chemical modification
[Reference: Mathew et al., Prog. Rubb. Plast. Recycl. Tech., 20, 4, 337, 20041

Researchers have also designed novel rubber biocomposites by using a
combination of leaf and fruit fibre in natural rubber-130. The incorporation of sisal and
coif fibre in NR was seen to increase the dielectric constant of the composites.
These hybrid biocomposites were found to have enormous applications as antistatic
agents. In another interesting study, the preparation of composites comprising of
waste paper in natural rubber along with boron carbide and paraffin wax, for
radiation shielding applications, was investigated13'.
1.8.4 Biofibre ! Natural Rubber Adhesion
The compatibility of hydrophobic rubber matrix and h ydrophilic cellulose
fibre can be enhanced through the modification of polymer or fibre surface. The
extent of adhesion is usually increased by the use of bonding agents and chemical
modification of fibres. The common bonding systems employed are: silica-phenol
formaldehyde and resorcinol-hexamethylenetetramine-silica. Silica is believed to
act as controller for resin formation and helps in developing adhesion between
rubber and fibre. The importance of silica is still a matter of controversy.
Studies by Murty et al.132 have indicated that silica is an essential
component for enhancing the development of adhesion between fibre and rubber in
the case of jute-NR composiles. But studies by Varghese el al.1" indicated that
bonding between sisal and NR can be enhanced by use of resorcinol and
hexamethylenetetramine in the ratio 53.2. It was found that adhesion imparted by
this system was found to be better than that of normal HRH system.
Another interesting report134 claimed that silica was not essential in producing
good adhesion between coir fibre and NU. It was observed that the tensile strength of
mixes that did not contain silica was significantly higher than that of the mixes containing

silica. Thus, it is clear that nature of both rubber matrix and reinforcing fibre determines
whether silica is needed or not as one of the components.
The adhesion of short oil palm fibre with natural rubber was
investigated135 using various bonding agents like phenol-formaldehyde;
resorcinolformaldehyde: silica; hexarnethylenetetrarnine: resorcinolformaldehyde:
silica. It was observed that different bonding agents gave different cure
characteristics and mechanical properties. Better mechanical properties were
obtained for the bonding system RF: Sil : Hex ( 5:2:5).
Equilibrium swelling technique was used by Mathew et aIa136 to study the
interfacial adhesion in isora fibre reinforced natural rubber composites. The authors
observed that Ihe presence of bonding agent increased the interfacial adhesion
between the fibre and matrix. The composites containing bonding agents were less
prone lo solvent sorption (Figure 1.1.14)
Without bonding agent
With bonding agent
Figure 1.1.14 Variation of Qt with chemical modification
[Reference: Mathew L. et a!. In Proceedings from ICBC 2005, March 21-23, 61, 20051

72 Clt np frr 1
Another interesting study, using the swelling technique to estimate
interfacial adhesion was reported in the case of sisal I coir fibre reinforced
natural rubber compositesl3? The bonding agent added mixes showed
enhanced restriction to swelling and it was seen that the ratio of change in
volume fraction of rubber before and after swelling to the volume fraction of
rubber before swelling (V, - VrNo) was lower for bonding agent added
composites, when compared to an unbonded one.
1 .I .9. PROCESSING138
Some of the common and important processing techniques for rubber
composites are given below.
The first step in mil ling is to oven-dry the whole fibre lo reduce moisture
to below 0.1 %. The fibres can also be modified by chemical treatments to make
it more compatible with the rubber matrix. The second step is the mixing of the
treated fibre into the rubber formulation during the rubber compounding
operation in an intensive (banbury) or two roll mill. The product from this step is
a homogenous rubber compound reinforced with fibre. The compound is heated
on a mill roll into manageable sheets for handling. The final process step is the
compression molding at elevated temperature and pressure to cure the rubber.
During mixing in a two roll mill, high shear forces get developed leading to fibre
breakage and the breakage pattern be studied by means of fibre length
distribution curve. It has been reported that the breakage is more common for
synthetic fibres than natural fibres.

b. Calendering
Calendering usually converts the rubber mix into a uniform continuous
sheet. The product of this step is a roll of thin flexible rubber that can be
trimmed with scissors or knives to fit into any desired mould. The addition of
fibres renders a rubber compound less elastomeric and extensible and therefore
the technique of calendering is quite difficult.
c. Injection moulding
The moulding of short fibre-reinforced rubber composites follows closely
the technology of plastic composites. The natural fibre reinforced rubbers are
less abrasive to machine and loo1 surfaces causing less wear than is common
with synthetic fibre-plastic resins. The highly automated injection moulding
requires fibres that are shorter and less concentrated than in compression
moulding. An advantage of injection moulding is its less labor intensive
operation under well-controlled sequencing for good reproducibility at low cost.
The typical parts that are made from short fibre reinforced rubber composites
include diaphragms, gaskets and certain flexible automotive parts.
d. Extrusion
Rubber extrusion is assisted by the lower rigidity of the organic fibre
reinforcement commonly used. In this area, a number of processability criteria
take on added importance with short fibre reinforced rubber. These are (he
direction of fibre orientation, surface appearance, and flow balancing in the die
to minimize tearing and ihe control of downstream post-die sizing operations.
Elastomeric matrices possess the advantage of green strength that is essential
to allow a free-surface forming operation such as extrusion. A unique
application of this technology for extruding short fibre reinforced hose allows the

hose to be curved into a shape as i t emerges from the expanding die by
mechanically adjusting the die geometry to effect local variations in the flow
uniformity within the die itself. Another attractive feature is that the fibre
reinforced green hose retains its shape during handling and the bend areas are
nearly as strong as the straight sections. At present, hose reinforced with
oriented natural short length cellulose fibre is available commercially in Europe.
I .I .I 0. APPLICATIONS OF FIBRE REINFORCED RUBBER COMPOSITES
Short fibres have the potential for reinforcing low-performance tires. In
automotive and truck tires they find application in better abrasion resistance for the
chafer strip and in improved cut resistance to treads especially for trucks and OTR
vehicles. As short fibres have higher green strength and cut, tear and puncture
resistance they can be used for sheeting. Some applications for such reinforced
sheeting are in roofing membranes. Short fibres can be utilized as the sole
reinforcement for a moderate -performance hose or as an auxiliary reinforcement
with cord constructions. They can provide stiffening to soft inner tubes for the
application of metal braids and can extend hose life by bridging the stresses across
weakened filaments. Other uses are as belts diaphragms and gaskets. Some of the
other applications are roofing; hose; dock and ship fenders and general uses such
as belts, tires and other industrial articles, Short fibres can reinforce and stiffen
rubber in fenders and other impact applications in accordance with simple design
1.1.11 IMPORTANCE OF THE WORK
1.1.1 1 .I Sisal and Oil Palm Fibres
The use of agricultural by-products solves the problem of disposal of
agricultural waste. Sisal and oil palm fibres are extremely promising materials

because of the high tensile strength of sisal fibre139 and hardness of oil palm fibre.
Therefore, the composite comprising of these two fibres is anticipated to exhibit the
above desirable properties of the individual constituents. Oil palm fibre is the waste
by-product that is amassed after palm oil production140. The utilization of oil palm
fibre is therefore an ecological and economical answer to the problem of waste
Another major attraction is that both the matrix and reinforcing fibres are
naturally available. Hence, products based on this work will be both cheaper
and sustainable.
1 .I .11.2. Scope of the present work
The present work deals with the fabrication of hybrid and textile
biocomposites. Designing biocomposites comprising of different blends of
b iofi bres will mutt in high performance biocompssites. Research work is going on with other
natural fibres like jute, flax, hemp, kenaf etc. A unique combination of s'ial and oil palm fibres
in natural rubber matrix has been expbred. Sisal fibre has relatively high cellulosic mtent and
low rnimfibrillar angle Mile oil palm fibre passess high hardness value. Presently they are
underutilized and can be considered lo be a g o d candidate as reinforcement in pdymeric
and cement mabices. Careful analysis of the literature Indicates that no systematic studies
have been reported on hybnd composites of sisal and oil palm.
1 .I -12 Major Objectives
The objectives of the present work include
1.1.12.1. Chemical modification of sisal and oil palm fibres and woven sisal fabric
The inherent polar and hydrophilic nature of lignocellulosic fibres and
the non-polar characteristics of most polymeric matrices results in weak bonding

between fibre and matrix which impairs the efficiency of the composite. Another
major setback of natural fibres is their high moisture absorption leading to
swelling and presence of voids at the interface, which results in poor
mechanical properties. This can be remedied by the addition of bonding agents
and chemical modification of natural fibres which decreases the hydrophilic
character of the fibres thereby reducing their affinity for moisture and
consequently increasing fibrel matrix interaction.
1.1.12.2. Optimisation of sisal/ oil palm length, ratio and loading in the preparation of hybrid biocomposites
Composites were prepared by varying the lengths of sisal and oil palm.
The fibre ratio and loading were also changed. 'The mechanical properties were
evaluated to choose the optimum fibre length, ratio and loading.
1.1.12.3. Processability characteristics and green strength measurements
The elastic properties of polymer melts are of prime importance in
processing and thereby in the fabrication of polymer products. Hence, studies
were conducted using Monsanto R-100 rheometer. Green strength
measurements also provide information about processa bility .
1.1.12.4. Macroscale examination of the* composites to evaluate fiber- matrix interactions and swelling measurements
The mechanical properties of composite are clear indications of the
strength of the interface and the fibrel matrix interaction. The fracture
mechanism is an indication of the interfacial adhesion in a composite. Therefore
a comprehensive analysis of the macroscopic properties of the composite is
very important. Equilibrium and anisotropic swelling measurements reveal a
great deal about the crosslink density and fibre orientation of composites.

1 . I . 12.5. Analysis of dynamic mechanical properties of hybrid biocomposites
The nature of fibrel matrix interaction can be evaluated on the basis of
dynamic mechanic analysis. The nature of modulus curves and damping peaks
are good indications of fibrel matrix interactions and nature of interface.
1 .I .I 2.6. Thermogravimetric analysis of hybrid biocomposites
Thermal analysis gives an indication about the thermal stability of the
composites. Generally it has been seen that the incorporation of plant fibres into
different ma trices increases the thermal stability of the system. The kinetics of
thermal degradation of the composites was also evaluated. The kinetics of
thermal degradation of the composites provides a clear idea regarding their
performance under different thermal environments.
1 .I .12.7. Water sorption behaviour of hybrid biocomposites
'The water sorption behaviour of cornposiles is an extremely important
characteristic for applications where contact with moisture becomes essential.
Generally it has been seen that chemical modification of fibres reduces the
water uptake of the composites.
1.12.8. Solvent sorption behaviour of hybrid biocomposites
Diffusion in organic solvents is an important study in elastorneric
compounds because it can be used as a measure of their long term behaviour in
a liquid environment. It can also be an indirect estimation of the interfacial
adhesion in rubber composites. The factors that affect swelling are type of solvent
used, temperature, structure of rubber compound and presence of fillers or fibres.

1 .I .12.9. Ageing and Bio-degradation studies
The information of the mechanical behaviour of natural fibre reinforced
composites during long term environmental exposure is crucial to be utilized in
outdoor applications. Also, the accumulation of plastic wastes causing
environmental pollution has led to an increasing interest and demand for
biodegradable polymers"? Biodegradable polymers can be divided into two
classes: completely biodegradable polymers and bio-disintegrable polymers.
Completely biodegradable polymers are known to be the best materials for
reduction of environmental pollution caused by plastic waste but they often have
inferior properties. A bio-disintegrable polymer, on the other hand, is a blend
material composed of a completely biodegradable polymer and a non-
biodegradable polymer
1 .I .12.10 Dielectric measurements of hybrid biocomposites
For a given polymer, the electrical properties are determined by the
amount and type of conductive additives. Parameters like dielectric constant,
volume resistivity and dielectric loss factor of the composites were evaluated as
a function of fibre loading, frequency and chemical modification of fibres. These
parameters are an indication of the conducting capacity of the composites.
1 .I .12.11. Stress relaxation behaviour of hybrid biocomposites
Long term behaviour of the composites can be evaluated by studying
stress relaxation behaviour of the composites. The stress relaxation properties
of rubber composites are important as they can influence the useful life of
components made from such a polymer system. Of particular interest is the
understanding of the changes in properties with time and whether the

mechanisms responsible for stress relaxation are able to induce other material
property changes.
1 .I $1 2.1 2. Fabricat ion of textile biocomposi tes f rom woven s isa l fabric and natural rubber
Woven fabric composites are widely used in laminated composites for
engineering and aerospace applications. Sisal fabric - natural rubber textile
composites were prepared by sandwiching a single layer of woven sisal fabric
between two layers of pre-weighed rubber sheets which was then compression
moulded. The mechanical and viscoelastic properties of the textile composites
were evaluated as a function of chemical modification of the fabric.
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Experimental
This chapter deals with the details of materials used, chemical modification
used on fibres, biocornposite preparation and the various experiments done for
evaluating the performance of hybrid sisal and oil palm fibre reinforced natural
1 rubber biocomposites and woven sisal fabric reinforced natural rubber textile
biocomposites.

EXPERIMENTAL 1.2.1. Materials
Natural rubber used for the study was ISNR 3L grade obtained from
Rubber Research Institute of India, KO ttayam, Kerala. The molecular weight,
molecular weight distribution and non-rubber constituents of natural rubber are
affected by clonal variation, season, use of yield stimulants, tapping system and
method of preparation! Hence rubber obtained from the same lot has been used in
this study. The properties are given in Table 1.2.1. All the other ingredients were of
commercial grade and were obtained from local sources. The various silanes such
as silane A174, silane A1 100, silane A151 were obtained from Sigma-Aldrich, India.
Silane F8261 was obtained from ABCR MBH and Co, Mumbai.
Table 1.2.1 Properties of rubber
I Dirt content, % by mass 0.03 1
1 Nitrogen, % by mass 1 0.30 1 Volatile matter, % by mass 0.50
/ Initial plasticity number, Po 1 38 1 Ash, % by mass
- - - - - - 1 Plasticity reten tion index 1 78
Sisal fibre was obtained from Sheeba Fibres, Poovancode, Tamil Nadu. Oil
palm fibre was obtained from Oil Palm lndia Limited, Punalur, Kerala. Sisal rabric
was hand-woven in collaboration with Khadi Village Industries Corporation,
Sreekariyam, Thiruvananthapuram, India.

1.2.2. Fibre Preparation
Sisal and oil palm fibres were first separated from undesirable foreign
matter and pith material. The fibres were then chopped lo different lengths viz.,
2,6,10 and 15 mm. The chemical constituents and physical properties of fibres2 are
given in Table 1.2.2. (a) & 1.2.2.(b)
A unidirectional type of sisal fabric weave having a count of 20 was used.
The properties of sisal fabric are given in Table 1.2.3.
Table 1.2.2. (a) Properties of Sisal Fibre
Chemical constituents (%)
Hemicellulose
Physical properties d sisal fibre -
1 Diameter (pm)
I Tensile strength ( MPa)
1 Young's modulus (GPa)
I Microfibrillar angle (")

Table 1.2.2. (b) Properties of Oil Palm Fibre
I Chemical constituents (%)
1 Cellulose
I Hemicellulose
1 - Physical properties of oil palm fibre 1
I Tensile strength ( MPa) I 248 I ( Diameter (prn) 150-500
Young's modulus (GPa)
1 Microfibrillar angle (") 1 46 1
Elongation at break (%)
Table 1.2.3 Properties of sisal fabric
, properties of sisal fabric ,, , , b .
- . I .ir, , ,, ' ' ' , , , "' . : - ; ' , , . , " _ , , :.. <
Yarn distance (warp)
Yarn distance
Twist (turns per mm)
Area density (slm2)

1.2.3. Chemical modification of sisal, oil palm fibres and sisal fabric
1.2.3.1 Alkali treatment
Sisal and oil palm fibres of lengths 10 mm and 6 mm were treated for 1
hour with NaOH solutions of concentrations 0.5,1,2,4 and 8 % respectively. The
fibres were further washed with water containing acetic acid. Finally the fibres
were washed again with fresh water and dried in an oven.
Sisal fabric was treated for 1 hour with NaOH solution of concentration
4 %. The fabric was further washed with water containing acetic acid. Finally,
the fabric was washed again with fresh water and dried in an oven at 70°C until
the fabric was completely dry.
1.2.3.2 Silane treatment
The silanes used were Silane F8261 [1,1,2,2-Perfluorooctyl triethoxy
silane], Silane A1 100 [y-Aminopropyltriethoxy silane ] and Silane A151 [vinyl
triethoxy silane]. 0.4 % of the respective silanes was prepared by mixing with an
ethanol I water mixture in the ratio 6 1 4 and was allowed to stand for 1 hour.
The pH of the solution was maintained at 4 with the addition of acetic acid. Sisal
and oil palm fibres were immersed in 1% NaOH 'solution and then dipped in the
silane solution and were allowed to stand for 1.5 hours. The ethanol I water
mixture was drained out and the fibres were dried in air and then in an oven at
70°C until the fibres were completely dry.

The silanes used for chemically modifying sisal fabric were Silane A1 100 [y-
Arninopropyltriethoxy silane] and Silane A174 [y-Metharyloxypropyltrimethoxysilane].
Sisal fabric was immersed in 1 % NaOH solution and then soaked in 0.4 %
silane solution and were allowed to stand for 1.5 hours. The ethanol - water
mixture was drained out and the fabric was dried in air and then in an oven at
70°C until the fabric was completely dry.
The structures of the silane coupling agents are given in Figure 1.2.1
Figure 1.2.1 Structures of silane coupling agents
1.2.3.3. Thermal treatment
Thermal treatment was carried out by keeping the woven sisal fabric in
the oven for 8 hours at 150°C. The fabric, directly from the oven was used for
composite preparation.

1.2.4. Spectroscopic Techniques
The surfaces of the treated sisal and oil palm fibres were characterized
by Perkin Elmer spectrometer model 1720-X FTIR. Fibres were ground well and
embedded in KBr crystals before taking the spectra.
1.2.5. Preparation of hybrid biocomposites and textile biocomposites
Formulation of mixes [hybrid biocomposites and textile biocomposites]
used in the present investigation are shown in Tables 1.2.4 (a, b & c) and
Table 1.2.5. Natural rubber was masticated on a two-roll mixing mill (1 50 x 300
mm) for 2 minutes followed by the addition of the ingredients. The nip-gap, mill
roll, speed ratio, and the number of passes were kept the same in all the mixes.
The samples were milled for sufficient time to disperse the fibres in the matrix at
a mill opening of 1.25 mm.
The bonding system consisting of resorcinol and hexamethylene
tetramine was incorporated for mixes containing treated fibres. The mixed sisal
and oil palm fibres (untreated and chemically treated) were added at the end of
the mixing process, taking care to maintain the direction of compound flow, so
that the majority of fibres followed the direction of the flow.
The sisal fabric - natural rubber textile composites were prepared by
sandwiching a single layer of sisal fabric between two layers of pre-weighed
rubber sheets which was then compression moulded at 150" C for 8 minutes. A
schematic sketch is given in Part Ill, Chapter I.

Table 1.2.4. (a) Formulation of hybrid biocomposites [mixes A to €1
I Mixes (phr) by weight 1
a- 2,2,4 -trimethyl-1,2-dihydroxy quinoline
Ingredients
Natural Rubber
Stearic acid
Sisal fibre Fibre length, ( 1 Omm)
Oil palm fibre Fibre length
b- N-cyclohexyl-2-benzothiazyl sulphenamide

Table 1.2.4. (b) Formulation of hybrid biocomposites [mixes ( I to R)]
a - 2, 2, 4-trimethyl-l , 2-di hydroxy quinoline
Ingredients I Q R J ---
Hexamethylene tetramine
Sisal fibre Fibre length (10 mm)
Oil palm fibre Fibre lenglh (6 mm)
0.4% Silane

Table 1.2.4 (c) Formulation of hybrid biocomposites [mixes (A1 to A5)]
lngredien ts
Oil palm fibre Fibre length (6 mm)
I Mixes [phr] by weight
a - 2, 2,4-trimethyl-1, 2-dihydroxy quinoline
b- N-cyclohexyl-2-benzothiatyl sulphenamide

Table I .2.5. Formulation of textile biocomposites
a - 2, 2, 4-trimethyl-1, Zdihydroxy quinoline
b - N-cyclohexyl-2-benzothiazyl sulphenamide
ZnO - , . - - . --
Sisal fabric
Mixes [phr] by weight
100.0 - . --

1.2.6. Measurement of Properties
1.2.6.1. Scanning electron microscopic studies (SEM)
The fracture morphology of the composites was observed by means of
SEM. Scanning electron microscopic studies were conducted using JEOL, JSM
5800 to analyse the fracture behaviour of the composites. The fracture ends of
the tensile and tear specimens of the composites were mounted on aluminium
stubs and gold coated to avoid electrical charging during examination.
1.2.6.2. Fibre Breakage Analysis
'The fibre breakage analysis of hybrid biocornposites was carried out by
dissolving l g of the uncured composite in toluene, followed by separation of
fibres from Re solution. The distribution in fibre length was determined using a
traveling microscope.
1.2.6.3. Processing Characteristics
The rheological properties of the hybrid biocomposites under very low
shear rate were evaluated using an Oscillating Disc Rheorneter (model
Monsanto R-100). This consists of a bi-conical shaped disc. The upper and
lower dies constituting the cavities were maintained, at 150°C. The cavity was
opened and closed pneumatically. The torque required to oscillate the rotor and
thus shearing stress experienced by rubber was measured by a stress
transducer consisting of strain gauges.
Two circular pieces of the vulcanizable rubber compound having
approximately 30 rnrn in diameter, 12.5 mm in thickness or equivalent to a volume
of 8 cm3, was cut and introduced into the test cavity. The disc is oscillated through

small rotational amplitude, 3", at a frequency of 100 oscillations per minute and
this action exerts a low shear strain on the test piece.
The force required to oscillate the disc was continuously recorded as a
function of time. This force is proportional to the shear modulus (stiffness) of the
test piece. This stiffness first decreased as it warms up and then increased due
to the increased cross-link density as a result of vulcanization. The test was
completed when the recorded torque increased to an equilibrium value. The
time required to obtain a cure curve is a function of the characteristics of the
rubber compound and the test temperature3.
1.2.6.4. Green Strength Measurements
Green strength measurements of hybrid biocomposites were carried
using dumb-bell shaped samples obtained from unvulcanized composites on an
lnstron Universal Testing Machine (UTM) at a stretching rate of 500 %min-1.
1.2.6.5. Mechanical Property Measurements
Stress-strain measurements were carried out at a crosshead speed of
500 mmlmin. Tensile strength and tear strength was measured according to
ASTM methods D412-98 and D624-00 respectively using lnstron UTM. The
tensile tests were done using dumbbell samples cut at different angles with
respect to the orientation of fibres. Tear testing was done using crescent
shaped specimens. Minimum of four samples were tested in each case and the *
average value is reported.

1.2.6.6. Swelling Studies
1.2.6.6.1. Anisotropic Swelling
Anisotropic swelling studies of sisal - oil palm hybrid fibre reinforced
natural rubber biocomposites were carried out using rectangular samples cut at
different angles with respect to orientation of the fibre from the specimen sheets
and swollen in toluene at room temperature for 3 days. Length, breadth and
thickness of the samples were measured before and after swelling.
1.2.6.6.2. Equilibrium Swelling
Equilibrium swelling measurements of the composites was performed in
toluene at room temperature. Circular shaped samples having a thickness of 2
mm, punched out of specimen sheets were used. The samples were allowed lo
swell for 72 h in toluene. The weight of the samples before and after equilibrium
swelling was measured.
1.2.6.7. Hardness and Abrasion Resistance Studies
Wear properties such as hardness and abrasion resistance of hybrid
and textile biocomposites were determined. Hardness measurements were 4
measured by Shore A type Durometer according to ASTM D2240-03. Abrasion
resistance was measured on an abrader according to DIN 53516.
1.2.6.8. Dynamic Mechanical Analysis
DMA measurements were carried out on Universal V2.6D TA
Instruments. The test specimen was clamped between the ends of two parallel
arms, which are mounted on low force flexure pivots allowing motion only in

horizontal plane. The samples were measured at an operating frequency of
0.1,l and 10 Hz and a heating rate of 2°C I rnin. The samples were evaluated in
the temperature range from -80°C to +40°C.
1.2.6.9. Thermogravimetric Studies
Thermal analysis of the composites was carried out by using a
Universal V2.3C TA Instrument with temperature programmed at 20°C I min
heating rate, from room temperature to 700 " C in presence of nitrogen.
1.2.6.1 0. Water Sorption Experiments
Water sorption is evaluated in terms of weight increase for composite
specimens immersed in distilled water at temperatures 30, 50 & 70 "C. Circular
specimens were dried in vacuum at room temperature for two days and the
weight of dried specimen was measured using an electronic balance. The
thickness of the samples was also measured. The weighed specimens were
then immersed in distilled water at different temperatures. The specimens were
periodically removed from water bath and the surface moisture was wiped off.
The weight gain of the specimen has been measured as a function of lime until
equilibrium or saturated state of water uptake' has been reached. Moisture
absorption was determined by weighing the specimen on an electronic balance.
The molar percentage uptake Qt for water by 1009 of the polymer was plotted
against square root of time. The Qt value is expressed as:

where Me (w) is the mass of water sorbed by the sample, M,(w) is the relative
molecular mass of water (18) and M~(s) is the initial mass of the sample. When
equilibrium was reached, QI was taken as the molar percentage uptake at
infinite time i.e. Q,.
1.2.6.1 1. Solvent Sorption Measurements
Solvent sorption is evaluated in terms of weigh1 increase for composite
specimens immersed in solvents benzene, toluene and xylene at room
temperature. Circular specimens were dried in vacuum at room temperature for
two days and the weight of dried specimen was measured using an electronic
balance. The thickness of the samples was also measured. The weighed
specimens were then immersed in the solvents. The specimens were
periodically removed from water bath and the surface solvent was wiped off.
The weight gain of ihe specimen has been measured as a function of time until
equilibrium or saturated state of solvent uptake has been reached. The molar
percentage uptake Qt for the composite samples was determined using the
following equation:
where W2 is the weight of the sample after swelling , W1 is the weight of the
sample before swelling and M, is the molecular mass of the solvent. The
sorption data were evaluated by plotting the mole percentage uptake of the
composite versus square root of time for different solvents.

E-vperir~r ert t r r l --- - 10s
1.2.6.1 2. Ageing and Bio-degradation Studies
Biodegradation experiments were carried out by soil burial
experiments. This test was performed for 6 and 12 months of soil exposure.
Dumbbell tensile specimens were completely buried in natural soil at 90%
water holding capacity (WHC) and a 50% soil moisture content. The samples
were in permanent contact with the soil. The soil test was set up using plastic
bins. All the specimens were vertically buried in the bins, using spatula and
forceps. Tensile strength of the exposed samples was measured according to
ASTM methods D412-98.
The resistance to ageing was determined by keeping tensile specimens
in an air circulated oven, maintained at a temperature of 90°C for seven days.
The tensile properties of the samples were determined after the ageing period
and compared with those of the original samples.
1.2.6.1 3. Dielectric Measurements
Rectangular specimens of 1.9 mm thickness were used. Samples
were prepared by cutting from the composite specimens using a die. The
test samples were coated with conductive graphite paint on either side and
copper wires are fixed on both sides of the samples as electrodes. The
capacitance, resistance and dielectric loss factor were measured directly at
room temperature using an Impedance Analyzer by varying the frequencies
(500Hz - 6 MHz).

1.2.6.14. Stress relaxation Experiments
This experiment was carried out on a Zwick universal testing machine
(model 1465) (Zwick EmbH & Company,Ulm-Ein, Germany) in uniaxial tension
mode at 25°C. The dumbbell specimens were extended to different strain
levels, namely, 5 and 50%. When the appropriate strain was reached, it was
maintained, and the stress was recorded for a time span of 10,800 seconds.
Graphs were plotted with stress at a specific time (or) divided by the maximum
stress when the required strain was attained (00) versus time. The slope was
obtained by regression analysis of the data for the best fit to a straight line.
1 . Subramanyam A,. Rubber Chem. Technol. 45 346 1972
2, Bismarck A., Mishra S., Lampke T., Natural Fibres, Biopolymers, and
Biocomposites, Edited by Mohanty A.K., Misra M., Drzal L.T., 39, 2005
3. ASTM D 2084-87

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Remember, a literature review can be termed as discursive prose, not a list summarizing several pieces of literature. Economic Data Literature Review of Aspects of the Primary Curriculum' title='A Literature
This paper reviews the literature that focuses on the socio-economic, gender and marketing aspects of chicken production in the tropics
As so many doctoral students working on their dissertations have discovered, writing the literature review chapter is a tedious process involving many steps and pitfalls. To help save you time and heartache
Modernism (in literature) focused on all of the following aspects except: question 2 options: a) appreciation of nature b) alienation of the individual c) - 268…
Hemicellulose differs from cellulose in three aspects. INTRODUCTION Chapter I Abstract This chapter deals with a review of the literature on the various aspects of biofibres and biocomposites