Chapter 1
Keratin as Renewable Material to Develop Polymer Composites: Natural and Synthetic Matrices

Flores-Hernandez C.G., Murillo-Segovia B., Martinez-Hernandez A.L.* and Velasco-Santos C

Division of Graduate Studies and Research, Technological Institute of Querétaro, Querétaro, Mexico

*Corresponding author: almh72@gmail.com

Abstract

Keratin is a structural fibrous protein, considered as the main constituent of wool, hair, horns, feathers and other outer coverings of mammals, reptiles and birds. This protein represents an inexhaustible source of non-contaminant materials for possible diverse applications. In the last decade the use of keratin in different forms to elaborate polymer composites has opened a novel and outstanding research field. Ongoing research have been developed keratin materials from diverse sources as reinforcements. These have been in the form of fibers, particles, nanoparticles or powder, among others. Thus, this chapter reviews different studies related to the use of keratin materials obtained from feathers, wool, hair and other renewable sources in order to reinforce polymer matrices. The properties obtained in these polymer composites are discussed separately depending on the nature of the matrix, natural or synthetic. The possible applications and the future of these kinds of composites are also discussed.

Keywords: Keratin, natural fiber, polymer composites, biodegradable polymer

1.1 Introduction

Biocomposites can be obtained from plant or living beings (natural/biofiber) and crop-derived plastics (bio-plastic). Actually, these are considered novel materials, still in development during the beginning of the twenty-first century (Singha & Thakur, 2009a-c; 2010a-c). The study of these materials started as an answer to a growing environmental threat and as attempt to supply solutions for the coming problem about petroleum supply (Mohanty et al., 2002; Thakur et al., 2016). It was reported that since the 1960s the demand for non-continuous components of composites has been growing incessantly. For example, in 1967, in the United States, necessities for fillers by the plastic production were around 525,000 tons, whereas in 1998, 1,925,000 tons were required by the same industry (Eckert, 1999). By this century, in 2000 the US market for natural composites exceeded $150 million (Mohanty et al., 2002), but for 2010, the projected requirement for fillers for the United States plastic production was to 3.85 billion kilograms, from which 0.31 billion kilograms (8%) were expected to be bio-based fibers (Farsi, 2012).

Natural fibers are the support to develop high performing fully biodegradable eco or green composites (Thakur et al., 2013a-e). Natural fibers are considered as biodegradable and environmentally friendly, mainly due to their plant-based cellulosic or lignocellulosic fibers. Much research is being undertaken of these as natural prospects for reinforcing (or filling) polymers to make them less aggressive towards the environment (Netravali & Chabba, 2003). In agreement with Thakur et al., (2014), one of the most successful emerging areas of interest in polymer engineering and materials science is precisely related to the proper application of raw natural fibers as an essential element towards achievement of new low-cost green composites.

In reality, many scientists have found an interesting research field by using plant-based fibers due to their ready availability. However, different prospects exist if high-strength protein fibers are taken into account. For example, keratin can be obtained from chicken feathers, wool, hair and horns. Keratin, a non-food protein, is an abundant biopolymer, and because of its animal origin, it is a renewable and low-priced feedstock. It is also assessed that worldwide there are some million tons per year of material-based keratin disposed in landfills that comes from non-used residues of wools, hairs, feathers, horns and nails (Bertini et al., 2013).

This chapter reviews the latest advancements in the field of composites with synthetic and natural matrices using keratin as reinforcement. The first section begins with a brief description of the structural characteristics of keratin. Subsequently, different natural materials that contain keratin are compared. In the second section, composites with synthetic matrices and different sources of keratin as reinforcement are detailed. The methods, techniques and properties are described for these composites. The last part discusses composites with natural matrices reinforced with keratin from different natural sources.

It is worthy of mention that there are many matrix systems that have been reinforced with keratin materials; therefore these novel composites are versatile to different applications depending on the desired properties. However, important criteria in the synthesis procedures must be carefully observed, since natural characteristics of keratin represent certain processing restrictions. Examples of these criteria could be: processing methods, morphological structures of keratin reinforcements, quantity of keratin used to reinforce matrices, among others. Thus, this review aims to describe the development of different polymeric composites using natural and synthetic matrices and applying renewable keratin reinforcements obtained from different natural sources.

1.2 Keratin

Keratin is present in almost all animals that have a backbone; this protein is the product of the keratinization process, which occurs because the skin cells die and accumulate in the surface layer. This protein can be considered as soft or hard, according to the diverse mechanism of biosynthesis (Meyers et al., 2008). Mammals have diverse tissues formed by hard keratin (skin, hair, wool, nail, claw, quill, horn, hoof and whale baleen), all of which are sophisticated epidermal appendages, differentiated not only by their external morphology and physical properties but also in their amino acid compositions, especially the content of amino acids like cysteine and tyrosine (Meyers et al., 2008; Gillespie & Frenkel, 1974). Generally, the keratin class of proteins is mechanically strong, designed to be unreactive and resistant to most forms of stress encountered by animals (Whitford, 2005). There are 30 different variants of keratin in mammals; these have been identified according to cells in a tissue-specific manner. In spite of the basic unit of keratin being an a helix, this structure is slightly distorted as a result of interactions with a second helix that leads to the formation of a left handed coiled-coil. Commonly, the arrangement for keratin is a coiled-coil of two a helices, although three helical stranded arrangements are known for extracellular protein domains, whereas those in bugs have been found as four-stranded coiled coils (Whitford, 2005).

There are two major groups of keratin that can be identified: a- and ß-keratin, depending on keratin's molecular structure (Meyers et al., 2008). Hard a-keratin is a hierarchically ordered material, with a fibrillar organization from the micrometer to the nanometer scale. In addition, a-keratin is rich in cysteine residues that form disulfide bonds linking adjacent polypeptide chains (Kreplak et al., 2004; Whitford, 2005). a-keratin is found in skin, hoof, baleen and wool, whereas ß-keratin is found in feather, beaks, claw and silk fibroin structures (Meyers et al., 2008; Whitford, 2005). The term "soft" or "hard" refers to the sulfur content of keratins, but also originates from the keratins' biosynthesis process, which is related to their mechanical properties. In fact sulfur presence is due to cysteine amino acid, hard keratin has high content of this amino acid and it is resistant to deformation. Hard keratin is found in nails and hair, whereas a low content of cysteine residues induces keratin with less mechanical resistance to stress (Whitford, 2005).

Keratin assembles in its primary structure around 18 different amino acids; these form polypeptide chains by condensation reactions. These biopolymer chains have molecular weights in the range from 59,000 to 65,000 (Mercer, 1961). Amino acids perform as monomers to construct the biopolymer, in this sense the polypeptide chain is assembled by 16% of serine, 12% of proline, 11% of glycine, 9% of valine, 7% of cysteine, and other amino acids comprise smaller percentages (Huda & Yang, 2008). Figure 1.1 shows a schematic representation of the main chain of keratin with the most abundant amino acids. The amino acid content in keratins depends on diverse factors directly related to the animals, the primary source of this protein, among these, breed, diet and environment. Despite of the diversity in composition, a common arrangement can be observed, since keratin contains a two-phase structure involving nanometric filaments embedded in diverse quantities of filamentous matrix. One of the most important amino acids in keratin is cysteine, due to stabilize the structure through disulphide cross-linkages. If these bonds are disrupted around 90% of the keratinous tissues can be extracted and easily separated into three types of proteins with different composition: a low sulphur protein, which originates in the filaments and is partly a-helical, a high sulphur protein, which is rich in cysteine, and finally the high tyrosine protein; the last two kinds are identifiable...