Polymer Composites, Biocomposites

Editors

Sabu Thomas

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills P.O.

School of Chemical Sciences

Kottayam 686 560

Kerala

India

Kuruvilla Joseph

Indian Institute of Space Science and

Technology

ISRO P. O.

Veli, Thiruvananthapuram 695 022

Kerala

India

Dr. S. K. Malhotra

Flat-YA, Kings Mead

Srinagar Colony

South Mada Street 14/3

Srinagar Colony

Saidapet, Chennai 600 015

India

Prof. Koichi Goda

Faculty of Engineering

Yamaguchi University

Tokiwadai 2-16-1

Yamaguchi University

755-8611 Ube, Yamaguchi

Japan

Dr. M. S. Sreekala

Department of Chemistry

Sree Sankara College

Kalady 683 574

Kerala

India

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The Editors

Sabu Thomas is a Professor of Polymer Science and Engineering at Mahatma Gandhi University (India). He is a Fellow of the Royal Society of Chemistry and a Fellow of the New York Academy of Sciences. Thomas has published over 430 papers in peer reviewed journals on polymer composites, membrane separation, polymer blend and alloy, and polymer recycling research and has edited 17 books. He has supervised 60 doctoral students.

Kuruvilla Joseph is a Professor of Chemistry at Indian Institute of Space Science and Technology (India). He has held a number of visiting research fellowships and has published over 50 papers on polymer composites and blends.

S. K. Malhotra is Chief Design Engineer and Head of the Composites Technology Centre at the Indian Institute of Technology, Madras. He has published over 100 journal and proceedings papers on polymer and alumina–zirconia composites.

Koichi Goda is a Professor of Mechanical Engineering at Yamaguchi University. His major scientific fields of interest are reliability and engineering analysis of composite materials and development and evaluation of environmentally friendly and other advanced composite materials.

M. S. Sreekala is an Assistant Professor of Chemistry at Post Graduate Department of Chemistry, SreeSankara College, Kalady (India). She has published over 40 papers on polymer composites (including biodegradable and green composites) in peer reviewed journals and has held a number of Scientific Positions and Research Fellowships including those from the Humboldt Foundation, Germany, and Japan Society for Promotion of Science, Japan.

List of Contributors

Kurungattu Arjunan Nair Ajith Kumar

Mahatma Gandhi University

Department of Chemistry

Sree Sankara College

Mattoor, Kalady 683 574

Kerala

India

Preetha Balakrishnan

Mahatma Gandhi University

Department of Chemistry

Sree Sankara College

Mattoor, Kalady 683 574

Kerala

India

Dongwan Cho

Kumoh National Institute of Technology

Department of Polymer Science and Engineering

Polymer/Bio-Composites Research Lab

61 Daehak-ro

Gumi, Gyeongbuk 730-701

Republic of Korea

Taek-Jun Chung

Seoul National University

Laboratory of Adhesion and Bio-Composites

1 Daehak-ro, Gwanak-gu

Seoul 151-921

Republic of Korea

Lawrence T. Drzal

Michigan State University

Composite Materials and Structures Center

428 S. Shaw Lane, 2100 Engineering Building

East Lansing, MI 48824

USA

Alain Dufresne

Grenoble INP-Pagora

Laboratoire Génie des Procédés Papetiers (LGP2)

461 rue de la Papeterie

CS 10065

38402 Saint-Martin d'Hères cedex

France

Raju Francis

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills

Kottayam 686 560

Kerala

India

Toru Fujii

Doshisha University

Department of Mechanical Engineering and Systems

Kyoutanabe-city

Kyoto 610-0394

Japan

Koichi Goda

Yamaguchi University

Department of Mechanical Engineering

2-16-1 Tokiwadai

Ube 755-8611, Yamaguchi

Japan

Preetha Gopalakrishnan

LGP2, CNRS, UMR 5518

Grenoble INP

Rue de la Papeterie

BP-65

38402 St. Martin d'Hères Cedex

France

Geethy P. Gopalan

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills

Kottayam 686 560

Kerala

India

Hirokazu Ito

Yamaha Livingtec Corporation

Business Planning Division

1370, Nishiyama-cho, Nishi-ku

Hamamatu 432-8001

Japan

Kuruvilla Joseph

Indian Institute of Space Science and Technology

ISRO P.O.

Veli, Thiruvananthapuram 695 022

Kerala

India

Balbir Singh Kaith

Dr. B. R. Ambedkar National Institute of Technology (Deemed University)

Department of Chemistry

Jalandhar 144 011

Punjab

India

Susheel Kalia

Bahra University

Department of Chemistry

Waknaghat (Shimla Hills)

173 234, District Solan (H.P.)

India

Inderjeet Kaur

H.P. University

Department of Chemistry

Shimla 171 005, (H.P.)

India

Hyun-Joong Kim

Seoul National University

Laboratory of Adhesion and Bio-Composites

1 Daehak-ro, Gwanak-gu

Seoul 151-921

Republic of Korea

and

Seoul National University

Research Institute for Agriculture and Life Sciences

1 Daehak-ro, Gwanak-gu

Seoul 151-921

Republic of Korea

Masatoshi Kubouchi

Tokyo Institute of Technology

Graduate School of Science and Engineering

Department of Chemical Engineering

2-12-1, S4-5, O-okayama, Meguro-ku

Tokyo 152-8552

Japan

Hyeok-Jin Kwon

Seoul National University

Laboratory of Adhesion and Bio-Composites

1 Daehak-ro, Gwanak-gu

Seoul 151-921

Republic of Korea

Louis Laberge Lebel

Bombardier Aerospace

1800 Marcel Laurin

Saint-Laurent, Québec

H4R 1K2

Canada

Hyun-Ji Lee

Seoul National University

Laboratory of Adhesion and Bio-Composites

1 Daehak-ro, Gwanak-gu

Seoul 151-921

Republic of Korea

Sant Kumar Malhotra

Flat-YA, Kings Mead Srinagar Colony

14/3, South Mada Street

Saidapet

Chennai 600 015

India

Luiz H.C. Mattoso

Laboratório Nacional de Nanotecnologia para o Agronegócio (LNNA)

Embrapa Instrumentação Agropecuária (CNPDIA)

Rua XV de Novembro

1452 Centro

13.560-970 São Carlos, SP

Brazil

Eliton S. Medeiros

Universidade Federal da Paraíba (UFPB)

Departamento de Engenharia de Materiais (DEMAT)

Cidade Universitária

58.051-900 João Pessoa, PB

Brazil

Dionysis E. Mouzakis

Higher Technological Educational Institute of Thessaly

Department of Mechanical Engineering

Larissas-Trikalon Highway

41110 Thessaly

Greece

Ajalesh Balachandran Nair

Cochin University of Science and Technology

Department of Polymer Science and Rubber Technology

Kochi 682 022

Kerala

India

Asami Nakai

Gifu University

Dept. of Mechanical and Systems Engineering

Gifu City, 501-1193

Japan

Rie Nakamura

Nihon University

Department of Mechanical Engineering

1 Nakakawahara Tamura

Koriyama, 963-8642, Fukushima

Japan

Anil N. Netravali

Cornell University

Department of Fiber Science and Apparel Designs

Fiber Science Program

201 MVR Hall

Ithaca, NY 14853

USA

Takashi Nishino

Kobe University

Department of Chemical Science and Engineering

Graduate School of Engineering

Rokko, Nada-ku, Kobe 657-8501

Japan

James Njuguna

Cranfield University

Department of Sustainable Systems

Bedfordshire MK43 0AL

UK

Kazuya Okubo

Doshisha University

Department of Mechanical Engineering and Systems

Kyoutanabe-city

Kyoto 610-0394

Japan

William J. Orts

United States Department of Agriculture (USDA)

Western Regional Research Center (WRRC)

Bioproduct Chemistry and Engineering (BCE)

800 Buchanan Street

Albany, CA 94710

USA

Jean Marc Saiter

Jubail Industrial College

Department of Chemical and Process Engineering Technology

Al-Jubail, 31961

Kingdom of Saudi Arabia

and

Université de Rouen

Laboratoire Polymères

Biopolymères et Membranes

Institut des Matériaux Rouen

FRE 3101, équipe LECAP

BP 12

76801 Saint Etienne du Rouvray

France

Amélia S.F. Santos

Universidade Federal do Rio Grande do Norte (UFRN)

Departamento de Engenharia de Materiais (DEMAT)

Avenida Salgado Filho

3000 – Lagoa Nova

59078-970 Natal, RN

Brazil

Soumya Sasikumar

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills

Kottayam 686 560

Kerala

India

Yoshinobu Shimamura

Shizuoka University

Department of Mechanical Engineering

3-5-1 Johoku

Naka-ku, Hamamatsu

Shizuoka 432-8561

Japan

Palanisamy Sivasubramanian

Mahatma Gandhi University

Department of Mechanical Engineering

Saint GITS College of Engineering

Pathamuttom, Kottayam 686 532

Kerala

India

Meyyarappallil Sadasivan Sreekala

Mahatma Gandhi University

Department of Chemistry

Sree Sankara College

Mattoor

Kalady 683 574

Kerala

India

Parambath Madhom Sreekumar

Jubail Industrial College

Department of Chemical and Process Engineering Technology

Al-Jubail, 31961

Kingdom of Saudi Arabia

Tatsuya Tanaka

Doshisha University

Department of Mechanical and Systems Engineering

1-3 Tatara Miyakodani

Kyotanabe City 610-0394

Japan

Sabu Thomas

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills

Kottayam 686 560

Kerala

India

Terence P. Tumolva

University of the Philippines-Diliman

Department of Chemical Engineering

Quezon City 1101

Philippines

William Tai Yin Tze

University of Minnesota

Department of Bioproducts and Biosystems Engineering

2004 Folwell Avenue

Saint Paul, MN 55108

USA

1

Advances in Polymer Composites: Biocomposites – State of the Art, New Challenges, and Opportunities

Koichi Goda, Meyyarappallil Sadasivan Sreekala, Sant Kumar Malhotra, Kuruvilla Joseph, and Sabu Thomas

1.1 Introduction

Environmental compatibility of polymer composites has become an important characteristic as the need to reduce environmental hazards is increasing worldwide. Many incidents taking place around the world are enough to bring us around to this point of view. A catastrophic earthquake and tsunami devastated the Pacific coast of north-eastern Japan on 11 March 2011. The earthquake, which was the most powerful earthquake ever measured in Japan, was of magnitude 9.0 on the Richter scale. About 19 000 were dead and missing. Three prefectures in the Tohoku (north-eastern) region of Japan, Miyagi, Iwate, and Fukushima, were most severely damaged. Reconstruction is yet to take place in many of the affected cities and towns. The area around the Fukushima Daiichi Nuclear Power Plant was evacuated owing to radioactive contamination. It is said that complete restoration will take more than 30 years, because the influence of the Chernobyl nuclear power plant disaster, which happened more than 25 years ago, continues to be felt. In Fukushima prefecture, many residents are still forced to lead lives as long-term refugees, and the residents in certain areas outside the refuge zone continue to live under threat of radiation that is much higher than is normal. The damage caused by radioactivity has also been considerable: it has already affected the soil of schoolyards, tapwater, grass, agricultural products, marine products, and so on, in large areas within the Fukushima prefecture. It is not clear how much of this damage is due to sea pollution and how long its effects will last in the future.

Against such a background, a planned conversion to renewable natural power sources as recommended by the energy policy, depending on nuclear power generation, attracts attention. For instance, it has been decided to abolish nuclear power generation systems in Germany; they propose to convert from 16% of total energy generation from the natural power sources at present to 35% by 2020 and to 80% by 2050 [1]. In the report The Green New Deal published in 2009 [2], promotion of use and development of alternative and renewable energy, improvement in energy efficiency, greenhouse gas reduction, and so on, have also been proposed. Today, technologies for various natural power sources, such as solar power, hydraulic power, woody biomass, and wind force power generation, are already in practical use. The authors believe that many people in the world desire realization of a sustainable society that uses such renewable energy power generation technologies.

To realize a sustainable society, various supplies around our life also need to be made from renewable materials. Biomass-derived materials are one of the most sustainable materials, which can also be used as industrial materials. On the other hand, most engineering plastic products are petroleum-derived products. As is well-known, the use of fossil resources causes difficulties in recycling and induces the problem of waste plastic and petroleum products, of which the incineration also causes an increase in carbon dioxide linking to global warming. In addition, fossil resources are an exhaustible resource. To maintain a sustainable society, we are of the opinion that biomass resources may be suitably exploited socially/ecologically as much as possible, by their replacing fossil resources. Since the arrival of such a society will result in a carbon-neutral system, this would also greatly contribute to global environmental protection. It is said that biodegradable plastics, for example, polylactic acid (PLA) and polyhydroxyalkanoic acid (PHA), are among the leading biomass-derived materials, which are finally decomposed by microorganisms into water and carbon dioxide. Therefore, there are only a few impacts on natural environment compared with those of conventional petroleum-derived plastics. Such biomass-derived materials are expected to be more widely applicable for the commodities used by us on a daily basis, for industrial products, and so on.

The main drawbacks of biodegradable resins are low strength and stiffness, and therefore, it is not appropriate to apply resins directly for structural components. Plastics are often reinforced with inorganic fibers such as glass or carbon, as described in Volume I of this series. Carbon fiber-reinforced plastic matrix composites (CFRP), in particular, have been recently used for primary structural components in airplanes and automobiles as well as sport goods and construction materials, because of their excellent mechanical properties. Biodegradable resin may also be reinforced with such fibers, similarly to the conventional petroleum-derived plastics. However, let us recall here how we should construct a sustainable society. If the final products do not really require high strength and durability, do we need to use strong artificial fiber-reinforced composites? Cellulosic materials, namely, plant-based natural fibers such as flax, hemp, bamboo, and wood, have low densities, are biodegradable, and inexpensive, and they have relatively high stiffness and less wear/abrasion to material partners. If such cellulosic materials are used as reinforcements of biomass-derived plastics, this material would be a quite suitable for building a sustainable society. We call such a biomass-based composite material a biocomposite. This idea of using natural fibers had already been adopted in the experimentally developed automotive body in 1940s by Henry Ford [3]. Fifty years later, Mercedes-Benz applied composites produced from natural fibers and polypropylene to their car interior parts in the 1990s. Although the matrix used in the cars was petroleum-derived thermoplastic resin, this business should be evaluated as an advanced measure in terms of practical and large-scale production. The use of natural fiber-reinforced composites using biomass-based biodegradable resin began in the 2000s. Toyota Motors first applied these composites to their spare tire covers in 2003, the constituents of which were kenaf fibers and PLA resin [4]. NEC and UNITIKA collaborated to develop a biocomposite of the same system, applicable for the body of mobile phones in 2005 [5]. Today, novel biocomposites are further being developed in research institutes and industries.

Technological innovation, which replaces all petroleum-based materials by biomass-based ones would be the task imposed on scientists and engineers of the twenty-first century, because it is anticipated that fossil resources will disappear in the near future. We consider that this innovation also includes the improvement and development of biomass-based fibers and resins, of which the mechanical properties are comparable to those of artificial fibers and petroleum-based resins, respectively. In this sense, the study of biocomposites must not end with just their being environmental-friendly, but must be advanced in the future in the quest toward establishing a sustainable society.

1.2 Development of Biocomposite Engineering

Biocomposites (the title of Volume III), are often interpreted as either biomass-based or biomedical materials. The former have a wider meaning than the latter, because they are available for various industrial purposes. A biomass-based composite consists of biomass and/or biomass-derived substance. On the other hand, a biomedical composite is a specified material because it is limited merely to biomedical use. In this use, the constituents are not necessarily biomass-based or biodegradable, but should be biocompatible. In the present volume, as stated earlier, by biocomposites, we mean biomass-based composites.

In this volume, the application of biocomposites is premised on structural use rather than functional one. From this point of view, we need to know exactly the mechanical properties, such as tensile strength and Young's modulus, of natural fibers and wood flours, similarly to the case of artificial reinforcing materials such as carbon and glass fibers. Tensile properties of natural fibers such as cotton, flax, wool, and silk have been examined in detail in the field of textile engineering. According to the book titled Physical Properties of Textile Fibers [6] published in 1962, the strength of fibers is mainly evaluated as maximum load divided by fiber specimen weight, denoted by the tenacity (g tex−1) or the specific strength (g denier−1). In textile engineering, a continuous filament called a spun yarn is a basic configuration [7]. Spun yarns are produced by spinning short fibers using a spinning machine or wheel, because most natural fibers are finite in length. Some of the spun yarns are further processed into twisted or blended yarns. To evaluate the various and complicated configurations, the concept of normalization by load per weight may be convenient to understand. The relation between the basic structure of spun yarns and their mechanical properties had already been clarified in the 1970s. The field of such study, called yarn mechanics, was extended to regenerated and chemical fibers, as well as natural fibers [8]. In the 1980s, natural fibers began to attract attention as a sustainable material, in addition to textile use, which is deeply related to the solution of environmental and energy problems. India, especially, played a pioneering role regarding production and application of several materials containing jute fibers, as shown in many papers [9–13] and review articles [14, 15]. During this period, jute fiber composites using thermosetting resins had been the main targets of research; thus the idea of hybrid composites with glass fibers was proposed [9, 16, 17]. Application of plant-based natural fibers into cement concrete had also been reported by several Indian institutes [18, 19].

Meanwhile, the project known as Poverty and Environment Amazonia (POEMA) in Brazil, established by Daimler-Chrysler, also started in 1981 [20]. This organization contracted with the residents of the Amazon valley, and encouraged them to apply natural resources such as coconut fibers to car interior parts. In the 1980s, however, the natural fiber composites were not biodegradable, because the resins applied were petroleum based. In the 1990s, a new type of fibrous composites was reported, in which the reinforcement and matrix of the composites were both biodegradable; the constituents were respectively natural fibers and polyvinyl alcohol (PVA) [21]. Netravali et al. [22] also developed in 1998 the composite system of natural fibers and biomass-derived resin, and these were termed as fully green composites. Since then, green composites have been recognized as one of the representative biodegradable materials reinforced with natural fibers. Various production methods and properties of green composites have been studied, and they are applied for several industries, as mentioned above. In the studies of green composites, most of researchers treat the tensile strength of natural fibers as load per cross-sectional area. To estimate the exact strength, even the morphology of the fiber cross-section has often been investigated, because it is quite complicated and different from the circular cross-section seen in many artificial fibers [23–28]. In relation to such mechanics or strength estimation, several researchers have further extended it to the numerical [29–31] or stochastic [24, 32] viewpoint. Not only such academic points of view of natural fibers but also the mechanics of composites reinforced with textile yarns such as spun or twisted yarn is on the rise [33, 34] (see, also Chapter 10.1).

The study on the above-mentioned natural fiber strength is one example concerning the progress of biocomposite engineering, in which natural fibers are evaluated as a structural material. Meanwhile, studies on the improvement of interface between wood fibers/flour (WF/F) and polymeric resin have also been progressing, of which the material is known by the name of wood–plastic composites (WPCs) [35] (see, also Chapter 5.2). This material is an in-between field that needs knowledge of both polymer chemistry and wood science. We also consider that WPC is in a category of biocomposites. The compatibility between WF/F and polymeric resin is quite poor, which leads to nonuniform dispersion of WF/F and low mechanical properties. The relation between wood and plastic is similar to that between oil and water – they do not mix so easily. Thermoplastic resins often used as a matrix material are hydrophobic, while WF/F is hydrophilic. These two contrary properties result in poor interfacial strength. As in the development of the silane-coupling agent linking glass fibers to polymeric resin, studies on the effect of various chemical treatments on the interfacial strength, such as cross-liking and acetylation of cellulose, grafting, use of coupling agent, have been conducted since the 1970s. It has been reported in many papers [36–38] (see Chapter 5.2) that, for example, WPC is improved in strength and impact properties by addition of a compatibilizer such as maleic anhydride-grafted-polypropylene (MAPP). WPCs were first introduced into the decking market in the early 1990s, in which 50% wood flours and 50% low density polyethylene (LDPE) were combined. Today, the WPC industry has grown into one of the greatest in the various fields of biocomposites. Although surface treatments on inorganic filler or reinforcement have been developed in conventional composite engineering, the above polar and nonpolar interface improving technology, a common subject to natural fibers, has also been creating biocomposite engineering (see, Chapter 4). In the 2000s, such chemical treatment in the WPC production process has been extended to achieve compatibility with biodegradable resins such as PLA [39]. This research progress quite matches the idea of as-mentioned fully green composites. WPC is further progressing through a technology fibrillating WF/F into the nanoscale [40].

As in the above, hitherto unknown issues inherent in biocomposites are being solved and meanwhile the appropriate evaluation methods are also being built up. We believe that development of various researches and technologies, such as the following would lead to an unwavering future for biocomposite engineering:

Structure–property relationships in biopolymers (Chapter 2)

Basic and applied researches of cellulose (Chapter 3)

Interface improvement technology (Chapter 4)

Production technology for thermoplastic–resin matrix biocomposites (Chapter 5)

Production technology for thermoset–resin matrix biocomposites (Chapter 6)

Biofiber reinforcement in thermoplastics (Chapter 7)

Biofiber reinforcement in natural rubber (Chapter 8)

Cellulose containing effect for improvement of interfacial strength (Chapter 9)

Yarn optimization for natural fiber composites (Chapter 10)

Bionanocomposites (Chapter 11)

Fully biodegradable green composites (Chapter 12)

Applications and future scope of natural fiber composites (Chapter 13)

Biomedical applications of polymer composites (Chapter 14)

Environmental effects, biodegradability, and life cycle analysis of biocomposites (Chapter 15).

1.3 Classification of Biocomposites

In this section, we try to clarify where biocomposites are positioned among the whole composite materials. In the previous section, the importance of green composite studies was described, and related to the biocomposite-engineering field. The combination of natural fibers and biomass-derived biodegradable resin is common to both biocomposites and green composites. What is the difference between biocomposites and green composites? PLA containing hydroxyapatite, a representative bioabsorbable biomedical composite, is expected to be applied widely as a bone-connecting material. Hydroxyapatite is a mineral-derived natural resource, but it is neither biodegradable nor biomass-based. Therefore, this composite cannot be denoted as a fully green composite, though it is partially biodegradable and biomass-based. Meanwhile, carbon fiber had originally been made from a biomass, as represented in a carbonized bamboo fiber filament developed by Thomas Edison. Although most of them are made from petroleum-derived acrylic fibers in the present technology, even now, some carbon fibers are made from pulp-originated rayon fibers. When we apply such a carbonized bamboo fiber to reinforcement, this may be called a fully biomass-based composite by combining it with biomass-derived resin even if the resin used is nonbiodegradable. Wood ceramics [41] is also a carbonized composite material composed of wood flour and phenolic resin, which is produced by sintering its precursor at high temperatures under inert atmosphere. (The precursor of ceramics is often called green body.) This material is expected to be applied to electromagnetism shields, tribological components, and heat-resistant and corrosion-resistant materials, because of their excellent properties. Wood ceramics is not green, but the great part of this material is biomass-based. Such carbonized biomass materials also attract attention from the viewpoint of carbon fixation technology, called biochar. It seems from the aforementioned that biocomposites can be defined as a biomass-based composite occupying a larger category than green composites.

On the other hand, we must not forget that green often means environment-friendly as well as biodegradable. Many unnecessary textiles and discarded composite products are often treated as industrial waste, but we understand that these are recyclable. For example, when waste uniform clothes composed of polyester and wool fibers are combined with PVA, they can be used as an agricultural material [42]. Growth of plants is promoted more effectively through this application. In this case, this could be termed green composite material. Discarded glass fiber-reinforced plastic (GFRP) or CFRP products are decomposed thermally and/or chemically, and can be used as a recycle glass or carbon fiber [43, 44]. If such fibers are used again as reinforcement of composites, then we could also call these green composites. Green chemistry means chemical technology aiming at lower environmental impact, in which one of the purposes is to improve life cycle efficiency for petroleum-based plastics. Another purpose is furthermore directed to refining of bioethanol from biomass resources and even polyolefin materials production using this ethanol. Composites made from such improved petroleum-based plastics or biopolyolefins may also be called green composites, despite the fact that they are not biodegradable. Thus, we should know that green composites are not necessarily a subset of biocomposites, but consist of the intersection of biocomposites and a disjoint part.

From such a point of view, we have classified biocomposites and green composites, as shown in Figure 1.1. This classification is based on the various matrix and reinforcement (or filler) properties as shown in Table 1.1a,b, and the meaning of green is defined as biodegradability.

Figure 1.1 Classification of biocomposites and green composites.

Table 1.1 Various matrix materials and reinforcements (filler included).

PCL, poly( -caprolactone); PAN, polyacrylonitrile.

A ∩ B is the intersection of biocomposites and green composites. In this category, the materials of matrix and/or reinforcement consist of a biomass-based and biodegradable substances. The group combinations are given as BM1/BF1, BM1/PF1, PM1/BF1, BM1/BF2, BM1/PF2, PM1/BF2, BM2/BF1, BM2/PF1, and PM2/BF1. The first combination, i.e., BM1/BF1, leads to a fully biomass-based and biodegradable composite material.

c01-math-0001 is the intersection of biocomposites and nongreen composites. In this category, the materials of matrix and reinforcement consist of a biomass-based and nonbiodegradable substance. In addition, in case the material of either matrix or reinforcement satisfies this substance, its counterpart must not be biomass-based or biodegradable. The group combinations are given as BM2/BF2, BM2/PF2, and PM2/BF2.

c01-math-0002 is the intersection of nonbiocomposites and green composites. In this category, the materials of matrix and/or reinforcement consist of a petroleum-derived (or inorganic) and biodegradable substance. However, the materials of matrix and reinforcement must not be petroleum derived (or inorganic) and nonbiodegradable. The group combinations are given as PM1/PF1, PM1/PF2, and PM2/PF1.

c01-math-0003 is the compliment of biocomposites or green composites. In this category, the materials of matrix and reinforcement are both petroleum derived (or inorganic) and nonbiodegradable. The group combination is given as PM2/PF2. However, in case the material of reinforcement consists of recycled textiles or fibers or that of the matrix consists of recycled resin, it can be called a green composite. If the matrix material is an excellent life cycle resin in conformity with the concept of green chemistry, this material is also accepted as green composite even if it is petroleum derived and nondegradable. This category is presented as c01-math-0004 ∩ B.

Meanwhile, biocomposite research needs many tasks to identify. Identification and effective utilization of renewable resource materials are needed for biocomposite preparation. Finding out effective and economic processing methods is necessary for the separation of starting biomaterials into their pure forms for the production of biocomposites. Performance of the biocomposites is dependent on the inherent properties of the matrix and reinforcement and their interface characteristics. We can tailor the properties of the biocomposites by optimizing processing parameters and by employing suitable physical or chemical modifications to improve the interface. Identifying the thrust areas for the application of biocomposites and manufacture of prototypes and fabrication of useful products has an important role in biocomposite research. The biocomposites will play a major role in replacing nonbiodegradable synthetic materials in the near future.

References

1. Law for the Priority of Renewable Energies (Renewable-Energy-Law) – EEG, http://www.gesetze-im-internet.de/bundesrecht/eeg_2009/gesamt.pdf (accessed 1 April 2013).

2. UNEP Global Green New Deal, http://www.unep.org/pdf/G20_policy_brief_Final.pdf (accessed 1 April 2013).

3. Popular Mechanics Magazine (Dec. 1941), Vol. 76, No. 6.

4. Inoh, T., Industrial products of plant origin material – effective use of plant origin plastics for recycling society, J. Jpn. Soc. Mech. Eng., 109, 51–52 (2006) (in Japanese).

5. Iji M., Serizawa, S. and Inoue, K., Development of polylactic acid with kenaf and its application to electronic products, Seikei-Kakou (J. JSPP), 15, 602–604 (2003) (in Japanese).

6. Morton, W.E. and Hearle, J.W.S., Physical Properties of Textile Fibers, (1962) The Textile Institute & Butterworths, Manchester and London.

7. Hearle, J.W.S., Grosberg, P. and Backer, S., Structural Mechanics of Fibers, Yarns and Fabrics, Vol. I (1969) John Wiley & Sons, Inc., New York.

8. Hearle, J.W.S. and Konopasek, M., On united approaches to twisted yarn mechanics, Appl. Polym. Symp., 27 253–273 (1975).

9. Shah, A. N. and Lakkad, S. C., Mechanical properties of jute-reinforced plastics, Fibre Sci. Technol., 15, 41–46 (1981).

10. Mukherjea, R.N., Pal, S.K., Sanyal, S.K., Studies on jute fiber composites with polyesteramide polyols as interfacial agent, J. Appl. Polym. Sci., 28, 3029–3040 (1983).

11. Pal, P.K., Jute reinforced plastics: a low cost composite material, Plast. Rubber Process. Appl., 4, 215–219 (1984).

12. Sanadi, A.R., Prasad, S.V., Rohatgi, P.K., Natural fibers and agro-wastes as fillers and reinforcements in polymer composites, J. Sci. Ind. Res., 44, 437–442 (1985).

13. Prashant, K., Mechanical behavior of jute fibers and their composites, Indian J. Technol., 24, 29–32 (1986).

14. Chand, N., Tiwary, R.K., Rohatgi, P.K., Bibliography resource structure properties of natural cellulosic fibres – an annotated bibliography, Mater. Sci., 23, 381–387 (1988).

15. Mohanty, A.K. and Misra, M., Studies on jute composites – a literature review, Polym. Plast. Technol. Eng., 4, 729–792 (1995).

16. Mohan, R., Kishore, A., Shridhar, M.K., and Rao, R.M.V.G.K., Compressive strength of jute-glass hybrid fibre composites, J. Mater. Sci. Lett., 2, 99–102 (1983).

17. Varma, I.K., Anantha Krishnan, S.R. and Krishnamoorthy, S., Composites of glass/modified jute fabric and unsaturated polyester resin, Composites, 20, 383–388 (1989).

18. Parameswaran, V.S., Krishnamoorthy, T.S. and Balasubramanian, K., Current research and applications of fiber reinforced concrete composites in India, Transp. Res. Rec., 1226, 1–6 (1989).

19. Sethunarayanan, R., Chockalingam, S., Ramanathan, R, Natural fiber reinforced concrete, Transp. Res. Rec., 1226, 57–60 (1989).

20. Tomari, M. and Harago, Y., Amazon no Hatake de Toreru Mercedes-Benz (1997) Tsukiji-Shokan, Tokyo (in Japanese).

21. Herrmann, A.S., Nickel, J. and Riedel, U., Construction materials based upon biologically renewable resources, Polym. Degrad. Stab., 59, 251–261 (1998).

22. Luo, S. and Netravali, A.N., Interfacial and mechanical properties of environment-friendly ‘green’ composites made from pineapple fibers and poly (hydroxybutyrate-co-valerate) resin, J. Mater. Sci., 34, 3709–3719 (1999).

23. Suzuki, K., Kimpara, I., Saito, H. and Funami, K., Cross-sectional area measurement and monofilament strength test of kenaf bast fibers, J. Soc. Mater. Sci. Jpn., 54, 887–894 (2005) (in Japanese).

24. Tanabe, K., Matsuo, T., Gomes, A., Goda, K. and Ohgi, J., Strength evaluation of curaua fibers with variation in cross-sectional area, J. Soc. Mater. Sci. Jpn., 57, 454–460 (2008) (in Japanese).

25. Silva, F. A., Chawla, N., Filho, R. D. T., Tensile behavior of high performance natural (sisal) fibers, Compos. Sci. Technol., 68, 3438–3443 (2008).

26. (a) Virk, A.S., Hall, W. and Summerscales, J., Multiple data set (MDS) weak-link scaling analysis of jute fibres, Composites Part A, 40, 1764–1771 (2009);(b) Virk, A. S., Hall, W. and Summerscales, J., Tensile properties of jute fibres, Mater. Sci. Technol., 25, 1289–1295 (2009).

27. Xu, X.W. and Jayaraman, K., An image-processing system for the measurement of the dimensions of natural fiber cross-section, J. Comput. Appl. Technol., 34 (2), 115–121 (2009).

28. Goda, K., Current status and future prospects of biocomposites II: strength evaluation of plant-based natural fibers for green composites, J. Soc. Mater. Sci. Jpn., 59 (12), 977–983 (2010) (in Japanese).

29. Gassan, J., Chate, A. and Bledzki, A.K., Calculation of elastic properties of natural fibers, J. Mater. Sci., 36, 3715–3720 (2001).

30. Baley, C., Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Composites Part A 33 939–948 (2002).

31. Xu, P. and Liu, H., Models of microfibril elastic modulus parallel to the cell axis, Wood Sci. Technol., 38, 363–374 (2004).

32. Andersons, J., Porike, E. and Sparnins, E., The effect of mechanical defects on the strength distribution of elementary flax fibres, Compos. Sci. Technol., 69, 2152–2157 (2009).

33. Shioya, M., Itoh, T., Kunugi, T. and Takaku, A., Variation of longitudinal modulus with twist for yarns composed of high modulus fibers, Text. Res. J., 71 (10), 928–936 (2001).

34. Yoshida, K., Kurose, T., Nakamura, R., Noda, J., and Goda, K., Effect of yarn structure on mechanical properties of natural fiber twisted yarns and green composites reinforced with the twisted yarn, J. Soc. Mater. Sci. Jpn., 61 (2), 111–118 (2012) (in Japanese).

35. Ashori, A., Wood–plastic composites as promising green-composites for automotive industries! (Review Paper) Bioresour. Technol., 99, 4661–4667 (2008).

36. Felix, J.M. and Gatenholm, P., Nature of adhesion in composites of modified cellulose fibers and polypropylene, J. Appl. Polym. Sci., 42, 609–620 (1991).

37. Kazayawoko, M., Balatinecz, J. and Matuana, L.M., Surface modification and adhesion mechanisms in woodfiber-polypropylene composites, J. Mater. Sci., 34, 6189–6199 (1999).

38. La Mantia, F.P. and Morreale, M., Green composites: a brief review, Composites Part A, 42, 579–588 (2011).

39. Takatani, M. and Okamoto, T., Wood/plastic composite of high filler content, Mol. Cryst. Liq. Cryst., 483, 326–338 (2008).

40. Abe, K., Iwamoto, S. and Yano, H., Obtaining cellulose nanofibers with a uniform width of 15 nm from wood, Biomacromolecules, 8, 3276–3278 (2007).

41. Okabe, T., et al., Wood Ceramics, (1996) Uchida Rokakuho, Tokyo (in Japanese).

42. Sekkuden, M., Yamamura, T., Okazawa, T., Sano, T., Tanaka, K., Goda, K., Ogawa, K., and Okabe, T. (2012) Eco-friendly utilization of uniform cloths waste – composites with PVA and anti-fungal biomass oil. Proceedings of the 10th International Conference on Ecomaterials (ICEM-10), pp. 203–206.

43. Shima, H., Takahashi, H., and Mizuguchi, J., Recovery of glass fibers from fiber reinforced plastics, Mater. Trans., 52, 1327–1329 (2011).

44. Liu, Y., Liu, J., Jiang, Z. and Tang, T., Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water: synergistic effect of phenol and KOH on the decomposition efficiency, Polym. Degrad. Stab., 97 (3), 214–220 (2012).

2

Synthesis, Structure, and Properties of Biopolymers (Natural and Synthetic)

Raju Francis, Soumya Sasikumar, and Geethy P. Gopalan

2.1 Introduction

Natural biodegradable polymers are, in general, called biopolymers and they have broad applications in various fields of the economy. They are large macromolecules composed of single or many repeating monomer units. These polymers are of very high molecular weight and monomer composition influences their material characteristics. Polysaccharides such as starch and cellulose signify the most characteristic family of these natural polymers. Other natural polymers such as proteins can also be used to produce biodegradable materials [1–8]. However, the synthetic applications of polynucleotides are limited owing to the fact that the field is still in the growing stage. Starch, fibers, and so on are the most common biopolymers and they can be incorporated into a variety of biological materials. Each biopolymer has its own material-specific properties, for example, toughness, crystallinity, barrier properties such as oxygen permeability. The packaging industry has special significance because more than 60% of the synthetic polymer products are used in this industry, according to a US statistics (Figure 2.1). Figure 2.1 also shows that the maximum postconsumer waste is produced by the packaging industry, according to the data prepared in the year 2000. Here, more than 90% of the material corresponds to nonbiodegradable thermoplastics. But recently, there have been a large number of reports on the use of biodegradable polymers in the packaging industry [9]. The barrier properties are relevant to the choice of biopolymers for the packaging of particular products. Bioplastics have very promising prospects for use in packaging in-flight catering products, for packaging dairy products, and in pesticide soil pins. The predominant mechanism in the case of biodegradable materials, which makes them capable of undergoing decomposition into carbon dioxide, water, inorganic compounds, methane, or biomass, is the enzymatic action of microorganisms. This biodegradation can be measured by reported procedures and thereby establish the natural processes of disposal [9].

Figure 2.1 A projection of postconsumer plastic waste is shown for different sectors in the year 2000.

Over the last decade, depletion in the petroleum reserves has resulted in the emergence of biodegradable polymers as a possible alternative to traditional plastics [9]. In addition, increased production of these plastics has proven to be a severe threat to the environment. Plastic disposal is a major concern because plastics are nonbiodegradable in nature. Hence, biodegradable polymers are considered to be a potential solution because of their biocompatibility, making them suitable for various applications [10]. Natural polymers or agro-polymers are renewable sources that are formed in nature during the growth cycles of all organisms. The use of biorenewable resources for the production of biopolymers has gained a huge amount of interest over the past decade because of their low cost and ready availability [11].

The interest toward biopolymers is mainly due to their biodegradability. Nonbiodegradable polymers have raised questions on sustainability of the environment from all walks of life. Biodegradability is sometimes initiated by nonbiological degradation such as photodegradation, oxidation by atmospheric oxygen, and hydrolysis. Biological degradation takes place through the actions of enzymes or by-products (such as peroxides and acids) secreted by microorganisms (bacteria, fungi, and yeast) [12–14]. Hydrolyzable or oxidizable groups are present along the main chain in most of the natural biodegradable polymers and synthetic biodegradable polymers [15].

Biopolymers are most commonly used for the following purposes:

1. Packaging (wraps, food containers, nets, foams)

2. Plastic bags for collection and composting of food waste and as carrier bags

3. Catering products (plates, cutlery, cups straws, etc.)

4. Agriculture (plant pots, mulch films, nursery films, etc.)

5. Hygiene products (wound healing pads, diapers, etc.)

6. Medical and dental implants (artificial teeth, sutures, etc.).

2.2 Classification

Biopolymers are polymers that are environmentally benign. They contain monomeric units that are covalently bonded to form larger structures. There are two main classes of biopolymers:

1. Natural biopolymers

2. Synthetic biopolymers.

Figure 2.2 presents the general classification of biopolymers. Natural biopolymers are further divided into various types of proteins, polysaccharides, and nucleic acids. On the other hand, a general classification is not possible for synthetic biopolymers. However, they can be classified in a broad sense according to the method of preparation. For instance, biopolymers synthesized by condensation and addition polymerization reaction are listed separately.

Figure 2.2 General classification of biopolymers.

2.3 Natural Biopolymers

Natural polymer materials serve as fundamental templates for the existence of life on earth. All of them have specific functions and structural identity. They are broadly divided into proteins, polysaccharides, and nucleic acids. Among the three basic natural biopolymers, the type, units, and function can be given broadly as follows (Table 2.1) [16].

Table 2.1 The type, units, and function of the three basic natural biopolymers.

Here, it is interesting to note that one amino acid on combining with itself or with another amino acid gives only one type of compound called a dipeptide. Similarly, the combination of two nucleotides will give a dinucleotide. In contrast to this, the combination of two sugar molecules will give large number of variants of oligosaccharides – approximately 20. This clearly indicates that there are structurally different molecules of polysaccharides, whereas such variations are limited in the case of proteins and polynucleotides. Hence it is important to see that the bulk of the biopolymers for commercial exploitation come from polysaccharides.

2.3.1 Proteins

The basic structural units in proteins are amino acids connected via peptide bonds that are nothing but amide linkages. All the 20 natural amino acids are called oligopeptides and polypeptides according to the size of the peptide. Among them, glycine is the only amino acid that is not optically active. The basic amino acid sequence represents the primary structure and the secondary structure is represented by the helical or β-pleated structures formed by hydrogen bonds. The tertiary structure of a protein is represented by the formation of disulfidic linkages or other intramolecular bonds that represent the three-dimensional structure of a protein. Finally, the quaternary structure is formed by the specific joining of more than one protein molecule. The primary functions of living systems in terms of skeleton, growth, and regulation are controlled by the protein molecules. Biologically, proteins are synthesized by the command obtained from the deoxyribonucleic acid (DNA). Depending on the codon present, each protein molecule is synthesized for a particular function. The synthesis is carried out on ribonucleic acid (RNA) by attaching specific amino acids through peptide linkages one by one, when the required polypeptide is formed. The various steps involved in protein synthesis, namely, initiation, elongation, and termination are depicted in Figure 2.3. These steps are very similar to the steps in a polymerization reaction except for the fact that protein synthesis occurs in a physiological environment. The structure of a protein has a great influence on its function. Hence, understanding the structure of proteins is of supreme importance. The most modern techniques such as X-ray crystallography and other microscopic techniques are employed to develop the structure of proteins. The α-helix and β-pleated sheets are represented along with the random arrangement. The given picture shows the crystal structure of a protein considerably simplified to represent only the overall shape and groves present in it (Figure 2.4). Here, the straight ribbonlike portion represents a β-pleated sheet and the helical ribbon represents the α-helix. In a protein, these are generally crystalline regions and the loose bonds are amorphous.

Figure 2.3 Cartoon showing protein synthesis in a cell.

Figure 2.4 Simplified skeletal structure of a protein.

Proteins and other amino acid-derived polymers have been a favored biomaterial for sutures, scaffolds for tissue engineering, drug delivery vehicles, and hemostatic agents, as they serve as a major component of natural tissues. For instance, silk was studied for various biomedical applications decades ago [17, 18]. Furthermore, protein-based biomaterials are known to undergo naturally controlled degradation processes.

2.3.1.1 Collagen

Collagen is a major structural protein that provides structure to all animal bodies, shielding and supporting the softer tissues and connecting them with the skeleton. The tendons and vast, resilient sheets that support the skin and internal organs are strengthened by the molecular strands formed by collagen. Bones and teeth are composed of collagen combined with mineral crystals, mainly hydroxyapatites (HAPs). Collagen is a rod-type polymer nearly 300 nm long with a molecular weight of 300 000 Da and each chain contains 1000 amino acids. Collagen may be processed into a variety of formats, including sheet, sponge, and gel and can be cross-linked with chemicals or put through physical treatments to make it stronger or alter its degradation rate [19–22]. There are more than 22 different types of collagen in the human body. The most common types of collagen are Types I–IV and Type I collagen is the single most abundant protein present in mammals. Type I collagen is composed of three polypeptide subunits with similar amino acid compositions. Each polypeptide is made up of about 1050 amino acids, containing approximately 33% glycine, 25% proline, and 25% hydroxyproline with a relative abundance of lysine [23]. These collagen types can be found as part of fibrillar structures that form an essential part of tissue architecture and integrity [24–29]. The ability to form fibers with extra strength and stability through its self-aggregation and cross-linking is the major reason for the usefulness of collagen in biomedical applications. It has been comprehensively investigated for the localized delivery of low molecular weight drugs including antibiotics [30]. Figure 2.5 shows the cartoon of collagen fibers joined in the nanoscale to form a macrostructure. Collagen is built up of single strands that form triple helices, which then further assemble into bundles and fibers [31]. A more representative structure of collagen with the amino acid sequences is presented in Figure 2.6.

Figure 2.5 Cartoon structure of collagen showing the packing.

Figure 2.6 Collagen Type II fragment showing the amino acid sequences.

Collagen has been extensively used for the regeneration of tissues, predominantly for the repair of soft tissues. It favors cell adhesion and provides cellular recognition for regulating cell attachment and function. Collagen undergoes enzymatic degradation in the body by means of enzymes, such as collagenases and metalloproteinases, to yield the corresponding amino acids [30]. This enzymatically degradable biopolymer shows unique physicochemical, mechanical, and biological properties in various applications. It can be processed in sheets, fleeces, injectable viscous solutions, dispersions, tubes, sponges, foams, and nanofibrous powders. The degradation rate can be modified by various treatments. Novel spongy collagen matrix containing oxidized cellulose has been recently introduced in US and European markets for treating exuding diabetic and ulcer wounds [32, 33]. Degradable collagen sponges have been broadly studied as scaffold material for accelerated tissue reproduction, owing to their excellent biocompatibility and porous structure. The composite of collagen, HAP, and TCP (tricalcium phosphate) is used as a biodegradable synthetic in bone graft replacement. It has wide applications as scaffold for musculoskeletal and nervous tissue engineering. Bovine or porcine skin or bovine or equine Achilles tendons are the major sources of collagen currently used for biomedical applications. One major disadvantage of these collagen-based biomaterials, which is a limiting factor for the extensive clinical application, is their mild immunogenicity imparted by the composition of the terminal region and the antigenic sites in the central helix [26]. Collagen shows variable physical, chemical, and degradation properties and is difficult to handle owing to the risk of infection [27]. Oral drug molecules, for instance, proteins coated with collagen, have already been marketed.

2.3.1.2 Elastin

Elastin contains glycine, proline, and alanine as the main amino acids and it is an extracellular matrix (ECM) protein, which is major component of skin, blood vessels, such as the aorta, and lung tissues in mammals. Elastin is a highly cross-linked insoluble polymer composed of a number of covalently bonded tropoelastin molecules. Elastin has specific cross-linkages – desmosine and isodesmosine, which account for the formation of a three-dimensional network with 60–70 amino acids between two cross-linking points [34, 35]. Multifunctional cross-linking agents that are also amino acids are presented in it. The cross-linking agents commonly found in elastin are shown in Figure 2.7. The tropoelastin molecules are produced intracellularly by smooth muscle cells and fibroblasts and they are cross-linked extracellularly to form a secondary structure with β-turns [36]. The tropoelastin is composed of numerous repeating sequences of the pentapeptide VPGVG, hexapeptide APGVGV, nonapeptide VPGFGVGAG, and the tetrapeptide VPGG. Among these, the pentapeptide VPGVG recurs up to 50 times in a single molecule [36]. Elastin has been evaluated as biological coating for synthetic vascular grafts owing to its minimal interaction with platelets [37].

Figure 2.7 Cross-linking agents in elastin: the amino acids desmosine (a) and isodesmosine (b).

Although it is extremely difficult to blend elastin with synthetic polymers, it forms an excellent basis for biomaterials, such as dermal substitute, arterial prosthesis, hydrogels, and for scaffolds [38–42].

2.3.1.3 Albumin

Albumin has a high content of cystine and charged amino acids, such as aspartic and glutamic acids, lysine and arginine, and a low content of tryptophan and methionine [43], and it is the most abundant water-soluble protein in human blood plasma, accounting for almost 50% of total plasma mass. It has a molecular weight of 66 kDa. A portion of human serum albumin is shown in Figure 2.8. Albumin can be easily processed into various shapes and forms such as membranes, microspheres, nanofibers, and nanospheres owing to its solubility and the presence of functional groups along the polymer chain. The basic function of albumin is to carry hydrophobic fatty acid molecules through the bloodstream and maintain blood pH. Preproalbumins are synthesized in the liver and undergo further processing before getting released into the circulatory system. Owing to its excellent blood compatibility, albumin has been extensively investigated as a carrier vehicle for intravenous drug/gene delivery [44]. Albumin-based surgical adhesives have also been approved by the US Federal Drug Administration (FDA) for reapproximating the layers of large vessels, such as femoral and carotid arteries and aorta, and are composed of bovine serum albumin, glutaraldehyde, and gelatin. Albumin is also used as coating in cardiovascular devices [45].

Figure 2.8 A portion of human serum albumin.

2.3.1.4 Fibrin

Fibrin, a biopolymer similar to collagen, is involved in the natural blood-clotting process. Fibrinogen, which is a 360 kDa protein, composed of three pairs of polypeptide chains is the source of fibrin. The structure of fibrin can be divided into three major sections consisting of a central domain composed of fibrinopeptide E with two pairs of fibrinopeptide A and B molecules and two terminal domains of fibrinopeptide D. Spontaneous fibrillogenesis is the formation of a linear fibril by the cleavage of fibrinopeptide A and B in the presence of enzyme thrombin, which undergoes lateral association to form fibers of varying diameters ranging from 10 to 200 nm, according to the changes in environmental conditions. These fibrin clots, once formed, can undergo degradation called fibrinolysis in the body, initiated by a complex cascade of enzymes present in the human body [46].

Owing to the excellent biocompatibility, biodegradability, injectability, and the presence of several ECM proteins, such as fibronectin (FN), that favorably affect cell adhesion and proliferation, fibrin has broad applications as a biomaterial. One of the first products developed from fibrin was a fibrin sealant. Worldwide, various fibrin sealant products are being used clinically for hemostasis and tissue-sealing applications in various surgical procedures. Owing to its injectability and biodegradability, fibrin has also been investigated as a carrier vehicle for bioactive molecules. It has been found that proteins interact differently with fibrin clots, with certain growth factors demonstrating a strong interaction with fibrin matrices [47].

2.3.1.5 Fibronectin

FN is a very large glycosylated protein of about 450–550 kDa. It forms a main constituent of ECM around and beneath many cells containing a complex structural organization made of two chains connected at their carboxyl termini by a pair of disulfide bonds. Each subunit is composed of three types of repeating modules, called FNI, FNII, and FNIII (12, 2, and 15 copies, respectively). The modules are linearly connected by short chains of variable flexibility. Characterization methods such as X-ray crystallography and NMR spectroscopy were used to elucidate the structure of some individual modules or short fragments. The structure of these modules is found as being mainly β-sheet, and the three types of modules are considered globular [48].

FN is largely responsible for the functional properties of ECM, including cell adhesion, migration, and proliferation [49, 50]. It has broad applications in cellular research into proliferation, development, differentiation, and morphology. Precoating of plastic, glass, and microcarrier beads aids cell attachment, particularly of that cell which is deficient in cell surface FN. The proteolytic digests of FN are mitogenic for fibroblasts that will bind to commonly used cell-culture surfaces. The various cell-connecting domains of FN are given in Figure 2.9. It can increase plating efficiency of fastidious cells and cloning efficiency. Variation in plating efficiency found in different serum batches can be minimized by using FN [19].

Figure 2.9 Functional domains of fibronectin.

2.3.1.6 Zein

Zein is an alcohol-soluble protein contained in the endosperm tissue of Zeamais and occurs as a by-product of corn processing. It is a natural film-forming polymer and is inherently resistant to water and grease penetration. Although zein has been empirically used as an edible coating for foods and pharmaceuticals for many decades, it has not attracted significant attention as a potential alternative for film-forming agents in drug formulations such as derivatives of cellulose or polyacrylates [51] (Figure 2.10). It might serve as an inexpensive and most efficient substitute for the fast-disintegrating synthetic and semisynthetic film coatings currently used for the formulation of substrates that allow extrusion coating [52]. Biodegradable plastics or thermoplastic can be made from zein. It is nonallergenic, which allows its use in food products.

Figure 2.10 Zein powder.

2.3.1.7 Gluten

Gluten, a fully biodegradable, nontoxic, protein composite found in foods processed from wheat and related grain species, including barley and rye (a type of wheat), is a storage endosperm protein, which is used today as a wheat protein marker. Gluten is a direct gene product, having a complex genetic determination and is a big natural biopolymer. The two basic structural characteristics of gluten are its evolutionary conservatism and, at the same time, its relatively high degree of polymorphism [53]. It is readily obtainable in high quantity and at low cost.

Wheat gluten is a combination of a gliadin and a glutelin (Figure 2.11). Gliadins, the glycoproteins containing only intramolecular disulfide links are mainly present in wheat and several other cereals within the grass genus. These glycoproteins are slightly soluble in ethanol. They have low molecular weight and a low level of aminoacids with charged side groups. Glutelins have a three-dimensional structure and are soluble in dilute acids and bases. Because of their complicated structure, their molecular weight is at least 10 times higher than that of gliadins. Wheat gluten materials have the fastest degradation rates.

Figure 2.11 Structure of gluten, a hybrid of gliadin and glutenin.

Excellent films can be formed with wheat gluten. While preparing wheat gluten films, a plasticizer must be used to decrease the brittleness of the films [54]; there are reports on the effects of glycerol, water, and sorbitol on the glass-transition temperature of wheat gluten [55]. Glycerol and sorbitol have high molecular weights and low evaporation rates and this reduces their plasticizing effect to less than that of water. When gluten is plasticized with glycerol, a malleable phase [56, 57] resembling a structured viscoelastic solid with pseudoplastic behavior is obtained. As cross-linking reactions occur at temperatures higher than 60 °C, the temperature range of the use of wheat gluten is limited [54].

2.3.1.8 Gelatin

Gelatin, which is obtained by the partial hydrolysis of collagen inside animals' skin and bones, is a transparent, colorless, brittle (when dry), flavorless, solid, water-soluble protein compound containing 19 amino acids and a mixture of peptides and proteins. Since collagen is the main fibrous protein constituent in bones, cartilages, and skins, the source, age of the animal, and type of collagen, are all important factors influencing the properties of gelatins [58]. A general repeat unit found in gelatin is presented in Figure 2.12. In water, gelatin forms a highly viscous solution that sets to a gel on cooling. The chemical composition of gelatin resembles that of collagen. Gelatin has the ability to form a homogeneous gel for concentrations in the range of approximately 1–50% and below this range there are insufficient molecules to support an infinite three-dimensional gel network [59–61].

Figure 2.12 A general repeat unit found in gelatin.

The mechanical and barrier properties of the gelatin films are influenced by the physical and chemical characteristics of gelatin, especially the amino acid composition and the molecular weight distribution. The physical properties of gelatin films can be improved by combining gelatin with other biopolymers such as oils and fatty acids, soy protein, or certain polysaccharides [62]. Mechanical and water vapor barrier properties of gelatin films are also influenced by the plasticizers used [63]. Gelatin can also be modified by means of grafting [64].

Gelatin is mainly used in cosmetic industry, photography, for decorating food items, and it has broad applications in the biomedical field as coatings and for microencapsulating various drugs [65]; these applications are based mainly on its gel-forming and viscoelastic properties. Gelatin has excellent applications in the food industry as emulsifiers, foaming agents, fining agents, colloid stabilizers, biodegradable packaging materials, and microencapsulating agents [66].

2.3.1.9 Soy Protein

Soy protein, a protein isolated from soybean (Figure 2.13), is a widely used food ingredient owing to its emulsification and texturizing properties. It is generally considered as the storage protein held in discrete particles called protein bodies. Soy protein can aid in preventing heart problems and therefore, of late, its popularity has increased at a fast rate. Depending on the method of production, the different categories of soy proteins are soy protein isolate, soy protein concentrate, and textured soy protein (TSP). Soy protein isolate contains about 90% protein and is the most refined form of soy protein. Soy protein concentrate is basically soybean without the water-soluble carbohydrates, containing about 70% protein [67]. TSP is made from soy protein concentrate by giving it some texture. The major proteins stored in soybean are globulins [68]. Because of their hydrophilic nature, soy protein films do not have good mechanical and barrier properties as do most protein films, and they are used to produce flexible and edible films.

Figure 2.13 Soy powder.

Soy protein has been regarded as a practical substitute to the petroleum polymers in the manufacture of plastics, adhesives, and packaging materials [69] Water sensitivity and poor mechanical performance, which limit the applications of soy protein materials [70], could be resolved, for example, by blending with polyurethane (PU) [71], gelatin [72], natural rubber [73], cellulose derivates [74], chitosan [75, 76], lipid [77], and beeswax [78], filling with nanoparticles [79], or by chemical modification [80].

2.3.1.10 Whey Protein

Caseins and whey are the two major sources of protein in milk (Figure 2.14). After processing occurs, the whey remains in an aqueous environment, while caseins are the proteins responsible for making curds. Whey protein is a mixture of globular proteins isolated from whey. Whey was considered a cure-all used to heal ailments ranging from gastrointestinal complaints to joint and ligament problems. Whey is a popular dietary protein supplement supposed to provide immune modulation, antimicrobial activity, improved body composition and muscle strength, and to prevent cardiovascular disease and osteoporosis. The components of whey include β-lactoglobulin, immunoglobulins, lactoperoxidase enzymes, α-lactalbumin, bovine serum albumin, lactoferrin, glycomacropeptides, lactose, and minerals [81]. Whey proteins have all the essential amino acids and in higher concentrations when compared to various vegetable protein sources such as soy, corn, and wheat gluten. It has strong antioxidant activity, by contributing cysteine-rich proteins that help in the synthesis of glutathione (GSH), a powerful intracellular antioxidant [81] containing glycine, cysteine, and glutamate.

Figure 2.14 Whey protein powder.

2.3.1.11 Casein

Caseins are a family of phosphoproteins (αs1, αs2,