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Volume 1: Composites
By Maya J. John, Sabu Thomas
The Royal Society of ChemistryCopyright © 2012 The Royal Society of Chemistry
All rights reserved.
Natural Polymers: An Overview
MAYA JACOB JOHN AND SABU THOMAS
The scarcity of natural polymers during the world war years led to the development of synthetic polymers like nylon, acrylic, neoprene, styrene–butadiene rubber (SBR) and polyethylene. The increasing popularity of synthetic polymers is partly due to the fact that there are unlimited and economic avenues for modification of chemical structures to obtain a product with specific properties. However, this rampant use of petroleum products has created a twin dilemma: depletion of petroleum resources (Figure 1.1) and entrapment of plastics in the food chain and environment. The exhaustive use of petroleum-based resources has initiated efforts to develop biodegradable plastics. This is based on renewable bio-based plant and agricultural products that can compete in the markets currently dominated by petroleum-based products. Table 1.1 presents a selected list of the common synthetic polymers.
Another issue is that the disposal of plastics in landfills creates a serious aesthetic problem in large urbanized areas of the world. The chemical stability of plastic prevents plastic waste from decomposing into the environment at a rate comparable to the rate of waste generation. In the long run, the incentive to preserve the local environment is reduced and the costs of cleaning and recovery of contaminated sites rise. Large streams can also transport excess plastic waste to other areas, creating a mobile contamination problem. Plastic waste comprises 60–80% of the marine debris litter accumulated in ocean shores. The problem of marine waste is aggravated by the low reliability of removal mechanisms aimed at reducing marine plastic residual concentration in the oceans. The effects of plastic waste on marine life include the entanglement and ingestion of harmful plastics by marine vertebrates and the bioaccumulation of toxicants along the food chain.
Natural polymers are those which are present in, or created by, living organisms. These include polymers from renewable resources that can be polymerized to create bio-plastics. There are two main types of natural polymers: those that come from living organisms (these include carbohydrates and proteins) and those which need to be polymerized but come from renewable resources (e.g. lactic acid and triglycerides). Both types are used in the production of bio-plastics.
Among the different types of natural polymers, the best known resources capable of making biodegradable plastics are starch and cellulose. Cellulose is the most abundant carbohydrate in the world (40% of all organic matter is cellulose). It is the main constituent of plants, serving to maintain their structure, and is also present in bacteria, fungi, algae and even in animals. Cellulose from trees and cotton plants is a substitute for petroleum feedstocks to make cellulose plastics.
Starch is a condensation polymer made up of hundreds of glucose monomers, which release water molecules as they chemically combine. Starch is a member of the basic food group of carbohydrates and is found in cereal grains and potatoes. It is also referred to as a polysaccharide, because it is a polymer of the monosaccharide glucose. Starch molecules include two types of glucose polymers, i.e. amylose and amylopectin, the latter being the major starch component in most plants, making up about three-quarters of the total starch in wheat flour. Amylose is a straight-chain polymer with an average of about 200 glucose units per molecule. 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. The annual world production of starch is well over 70 billion pounds weight, with much of it being used for non-food purposes, like making paper, cardboard, textile sizing and adhesives.
Chitin, a polysaccharide similar to cellulose, is Earth's second most abundant polysaccharide. It is present in the cell walls of fungi and is the fundamental substance in the exoskeletons of crustaceans, insects and spiders. The structure of chitin is identical to that of cellulose, except for the replacement of the OH group on the C-2 carbon of each of the glucose units with an –NHCOCH3 group. The principal source of chitin is shellfish waste. Commercial uses of chitin waste include the making of edible plastic food wraps and the cleaning up of industrial wastewater.
Chitin is the main source of production of chitosan, which is used in a number of applications, such as a flocculating agent, a wound healing agent, a sizing and strengthening agent for paper, and a delivery vehicle for pharmaceuticals and genes. Chitin deacetylation leads to the formation of chitosan. The process involves the use of strong alkali solutions for the removal of N-acetyl groups, both at room and elevated temperatures. The amount of chitin obtained annually from harvested shellfish is estimated to be over 39 000 tonnes. At least 10 billion tonnes of chitin are produced in the biosphere each year, chiefly in marine environments.
Collagen is one of the most plentiful proteins present in the bodies of mammals, including humans. In fact, it makes up about 25% of the total amount of proteins in the body. It has found increasing applications in tissue engineering and repair. The ability of collagen to polymerize into a three-dimensional fibrous matrix makes it an appealing material for extensive therapeutic applications, including medical implants.
Some of the other important natural polymers that are under scrutiny by the research community, but beyond the scope of this book, include lignin, shellac and natural rubber. In the category of natural polymers which need to be polymerized is the interesting development of biodegradable plastics from edible and non-edible vegetable oils like soybean oil, peanut oil, walnut oil, sesame oil, sunflower oil, tung oil and castor oil.
Table 1.2 presents a selected list of the common natural polymers.
The production of 100% bio-based materials as substitutes for petroleum-based products is not an economical solution. Some of the possible solutions are blending biopolymers with synthetic polymers and reinforcing natural fibres with synthetic polymers (termed bio-composites), which are a viable alternative to glass fibre composites.
1.2 Natural Polymer Research
The aim of this book is to examine the research conducted worldwide on the use of different types of natural polymers. The book looks at the different processing techniques of natural polymers as well as applications in advanced industrial sectors. The structure, mechanical and thermal characteristics of selected natural polymers are highlighted.
1.2.1 Natural Fibres
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. Natural fibres are subdivided based on their origins, coming from plants, animals or minerals. All plant 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. Knowledge of the structure of natural fibres is crucial in understanding the structural parameters (number, size and shape of cells, chemical constituents) and fracture mechanisms in fibres.
Some of the important natural fibres used as reinforcement in composites are listed in Table 1.3.
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 the fact that they can compete well in terms of strength per weight of material.
Volume 1 focuses on different sources and applications of natural fibres. One chapter deals with novel renewable sources from which natural fibres can be extracted. Another chapter looks at relating the structural anisotropy of natural fibres to mechanical properties. One of the challenges of using natural fibres in aerospace applications is the airworthiness requirements. Currently, natural fibres are being explored for use in secondary structures in aircraft for which flame, smoke and toxicity (FST) requirements are very stringent. This has led to a lot of developmental research being undertaken in this field. A further chapter therefore explores the flammability properties of natural fibre reinforced composites. A crucial problem associated with the use of natural fibres in composites is their hydrophilic properties. This aspect is dealt with in a chapter on probing the water sorption characteristics of natural fibres. The chemical modification of natural fibres has been well documented in the literature, but ideally it would also be desirable that the chemicals used for modification should also be from renewable resources as it would preserve the biodegradable nature of natural fibres. A chapter therefore focuses on environmentally friendly coupling agents for natural fibre-reinforced composites. Other chapters include examining the characterization techniques of the interfacial properties of natural fibre-reinforced composites and the increasing applications of natural fibre composites in the automotive sector.
1.2.2 Protein Fibres
The book also deals with the properties of selected protein fibres. Protein fibres are formed by natural animal sources through condensation of α-amino acids to form repeating polyamide units with various substituents on the α-carbon atom. The sequence and type of amino acids making up the individual protein chains contribute to the overall properties of the resultant fibre. In general, protein fibres possess moderate strength, resiliency and elasticity. They have excellent moisture absorbency and transport characteristics and do not build up static charge. Some of the common protein fibres include wool, spider silk, cashmere, etc. Among natural fibres, silk exhibits exceptional properties, especially in toughness and biocompatibility properties. A chapter therefore focuses on the studies and properties of silk fibre-reinforced composites. Other chapters include studies on collagenous waste-based composites and exploring the properties and applications of mussel byssus fibres. Important advancements in the field of zein fibres are also discussed in another chapter.
Volume 2 deals with the properties and characterization of selected natural polymer nanocomposites. Cellulose nanowhiskers (CNWs) have emerged as one of the most interesting bio-based nano-reinforcements in the last decade. Cellulose nanowhiskers can be generated from various plant sources with transverse dimensions as small as 3–30 nm, giving a high surface-to-volume ratio. It has also been shown that since the nanowhiskers are rod-like, they can be self-assembled into chiral nematic liquid crystalline structures, not only in solution but also in the dry state. The volume begins by exploring nanocellulose as a potential reinforcement in composites. Chitosan (a natural polymer) is a good candidate for the development of conventional and novel drug delivery systems. Chitosan has been found to be used as a support material for gene delivery, cell culture, and tissue engineering. However, practical use of chitosan has been mainly confined to the unmodified forms. For a breakthrough in utilization, especially in the field of controlled drug delivery, graft copolymerization onto chitosan will be a key point, which will introduce desired properties and enlarge the field of the potential applications of chitosan by choosing various types of side chains. The properties and applications of chitosan and soy protein-based nanocomposites are discussed in subsequent chapters. Other chapters include studies on furanic-based nanocomposites, the characterization of molecular interactions in amylose/starch nanocomposites, and unique properties of nacre from mollusc shells. The last two chapters touch upon the industrial and biomedical applications of natural polymer nanocomposites.CHAPTER 2
Biomimetics: Inspiration from the Structural Organization of Biological Systems
KALPANA S. KATTI, CHUNJU GU AND DINESH R. KATTI
Department of Civil Engineering, North Dakota State University, Fargo, ND 58105, USA
2.1 Introduction: Hierarchy and Structural Order
Structural materials found in nature exhibit hierarchical structures that span a structural order over length scales from molecular to macroscopic. Optimization of properties and structural redundancy as well as growth are all ramifications of the hierarchical structures that result from eons of evolution. The following sections elucidate the hierarchical details of structure in bone, teeth, seashells and spider silk and fabrication attempts at duplication of these structures.
2.2 Biological Materials Systems
Bone is a remarkable biological material, providing skeletal stability, support and protection of vital organs. The structure of bone has been extensively studied historically as well as recently, owing to its perfect adaption of mechanical properties to metabolic functions. In addition, as a protein/mineral nanocomposite, bone combines the optimal properties of both components: stiffness and toughness. This rather unusual combination of material properties is a combination of rigidity and resistance against fracture. The unique mechanical properties of bone and the structure–property relationship have attracted significant attention.
Excerpted from Natural Polymers by Maya J. John, Sabu Thomas. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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