Handbook of Composites from Renewable Materials, Polymeric Composites

By Vijay Kumar Thakur, Manju Kumari Thakur and Michael R. Kessler

The Handbook of Composites From Renewable Materials comprises a set of 8 individual volumes that brings an interdisciplinary perspective to accomplish a more detailed understanding of the interplay between the synthesis, structure, characterization, processing, applications and performance of these advanced materials. The handbook covers a multitude of natural polymers/ reinforcement/ fillers and biodegradable materials. Together, the 8 volumes total at least 5000 pages and offers a unique publication.

This 6th volume Handbook is solely focused on Polymeric Composites. Some of the important topics include but not limited to: Keratin as renewable material for developing polymer composites; natural and synthetic matrices; hydrogels in tissue engineering; smart hydrogels: application in bioethanol production; principle renewable biopolymers; application of hydrogel biocomposites for multiple drug delivery; nontoxic holographic materials; bioplasticizer - epoxidized vegetable oils-based poly (lactic acid) blends and nanocomposites; preparation, characterization and adsorption properties of poly (DMAEA) – cross-linked starch gel copolymer in waste water treatments; study of chitosan crosslinking hydrogels for absorption of antifungal drugs using molecular modelling; pharmaceutical delivery systems composed of chitosan; eco-friendly polymers for food packaging; influence of surface modification on the thermal stability and percentage of crystallinity of natural abaca fiber; influence of the use of natural fibers in composite materials assessed on a life cycle perspective; plant polysaccharides-blended  ionotropically-gelled alginate multiple-unit systems for sustained drug release; vegetable oil based polymer composites; applications of chitosan derivatives in wastewater treatment; novel lignin-based materials as a products for various applications; biopolymers from renewable resources and thermoplastic starch matrix as polymer units of multi-component polymer systems for advanced applications; chitosan composites: preparation and applications in removing water pollutants and  recent advancements in biopolymer composites for addressing environmental issues.

• Language: English
• Publisher: Wiley
• Release date: Mar 27, 2017
• ISBN: 9781119224433

Book preview

Preface

The concept of green chemistry and sustainable development policy impose on industry and technology to switch raw material base from the petroleum to renewable resources. Remarkable attention has been paid to the environmental friendly, green and sustainable materials for a number of applications during the last few years. Indeed the rapidly diminishing global petroleum resources, along with awareness of global environmental problems, have promoted the way to switch towards renewable resources based materials. In this regards, bio-based renewable materials can form the basis for variety of eco-efficient, sustainable products that can capture and compete markets presently dominated by products based solely on petroleum based raw materials. The nature provides a wide range of the raw materials that can be converted into a polymeric matrix/adhesive/reinforcement applicable in composites formulation. Different kinds of polymers (renewable/nonrenewable) and polymer composite materials have been emerging rapidly as the prospective substitute to the ceramic or metal materials, due to their advantages over conventional materials. In brief, polymers are macromolecular groups collectively recognized as polymers due to the presence of repeating blocks of covalently linked atomic arrangement in the formation of these molecules. The repetitive atomic arrangements forming the macromolecules by forming covalent links are the building block or constituent monomers. As the covalent bond formation between monomer units is the essence of polymer formation, polymers are organic or carbon compounds of either biological or synthetic origin. The phenomenon or process of polymerization enables to create diverse forms of macromolecules with varied structural and functional properties and applications. On the other hand, composite materials, or composites, are one of the main improvements in material technology in recent years. In the materials science field, a composite is a multi-phase material consisting of two or more physically distinct components, a matrix (or a continuous phase) and at least one dispersed (filler or reinforcement) phase. The dispersed phase, responsible for enhancing one or more properties of matrix, can be categorized according to particle dimensions that comprise platelet, ellipsoids, spheres and fibers. These particles can be inorganic or organic origin and possess rigid or flexible properties.

The most important resources for renewable raw materials originate from nature such as wood, starch, proteins and oils from plants. Therefore, renewable raw materials lead to the benefit of processing in industries owing to the short period of replenishment cycle resulting in the continuous flow production. Moreover, the production cost can be reduced by using natural raw materials instead of chemical raw materials. The waste and residues from agriculture and industry have been also used as an alternative renewable resources for producing energy and raw materials such as chemicals, cellulose, carbon and silica. For polymer composites applications, an intensifying focus has been directed toward the use of renewable materials. Bio-based polymers are one of the most attractive candidates in renewable raw materials for use as organic reinforcing fillers such as flex, hemp, pine needles, coir, jute, kenaf, sisal, rice husk, ramie, palm and banana fibres which exhibited excellence enhancement in mechanical and thermal properties. For green polymer composites composed of inorganic reinforcing fillers, renewable resources based polymers have been used as matrix materials.

Significant research efforts all around the globe are continuing to explore and improve the properties of renewable polymers based materials. Researchers are collectively focusing their efforts to use the inherent advantages of renewable polymers for miscellaneous applications. To ensure a sustainable future, the use of bio-based materials containing a high content of derivatives from renewable biomass is the best solution.

This volume of the book series "Handbook of Composites from Renewable Materials is solely focused on the Polymeric Composites". Some of the important topics include but not limited to: Keratin as renewable material for developing polymer composites; natural and synthetic matrices; hydrogels in tissue engineering; smart hydrogels: application in bioethanol production; principle renewable biopolymers; application of hydrogel biocomposites for multiple drug delivery; nontoxic holographic materials; bioplasticizer - epoxidized vegetable oils-based poly (lactic acid) blends and nanocomposites; preparation, characterization and adsorption properties of poly (DMAEA) – cross-linked starch gel copolymer in waste water treatments; study of chitosan cross-linking hydrogels for absorption of antifungal drugs using molecular modelling; pharmaceutical delivery systems composed of chitosan; eco-friendly polymers for food packaging; influence of surface modification on the thermal stability and percentage of crystallinity of natural abaca fiber; influence of the use of natural fibers in composite materials assessed on a life cycle perspective; plant polysaccharides-blended ionotropically-gelled alginate multiple-unit systems for sustained drug release; vegetable oil based polymer composites; applications of chitosan derivatives in wastewater treatment; novel lignin-based materials as a products for various applications; biopolymers from renewable resources and thermoplastic starch matrix as polymer units of multi-component polymer systems for advanced applications; chitosan composites: preparation and applications in removing water pollutants and recent advancements in biopolymer composites for addressing environmental issues.

Several critical issues and suggestions for future work are comprehensively discussed in this volume with the hope that the book will provide a deep insight into the state-of-art of "Polymeric Composites" of the renewable materials. We would like to thank the Publisher and Martin Scrivener for the invaluable help in the organisation of the editing process. Finally, we would like to thank our parents for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

Washington State University - U.S.A

Manju Kumari Thakur, M.Sc., M.Phil., Ph.D.

Himachal Pradesh University, Shimla, India

Michael R. Kessler, Ph.D., P.E.

Washington State University - U.S.A

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: [e-mail]

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 α 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 α 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: α- and β-keratin, depending on keratin’s molecular structure (Meyers et al., 2008). Hard α-keratin is a hierarchically ordered material, with a fibrillar organization from the micrometer to the nanometer scale. In addition, α-keratin is rich in cysteine residues that form disulfide bonds linking adjacent polypeptide chains (Kreplak et al., 2004; Whitford, 2005). α-keratin is found in skin, hoof, baleen and wool, whereas β-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 from matrix (Gillespie & Frenkel, 1974). The high content of cysteine causes stability in the protein, because the α-amino group and α-carbonyl are useful functional links capable to weave a network between the nearby structures of polypeptides (Schmidt, 1998; Martinez-Hernandez et al., 2005a).

Figure 1.1 Schematic representation of most important aminoacids linked to keratin backbone chain.

Keratin is a fibrillar protein, a product of the keratinization process, which generates a highly structured protein with arrangement in different levels. The primary structure is constituted by the assembly of amino acids forming the polypeptide chain; this is folded upon itself, acquiring three dimensions and forming the called secondary structure, which represents its molecular structure (α-keratin and β-keratin). The first one could be arranged as a spiral, known as protein α-helix. The shape of this structure is maintained by hydrogen bonding and hydrophobic forces that hold together the amino acids of protein, which gives that special characteristic hardness. The α-keratin is also called mammalian keratin, and the β-keratin is found in avian and reptilian keratin. However, it is possible to find the two types of keratin in the same tissues, for example, the hair has α-keratin based on fiber cortex and β-keratin based cuticle (Hill et al., 2010). The main difference between both kinds of keratins is basically the intermediate filament (IF). The α-keratin has an IF arranged as an α-helix folding pattern with diameter of 8 nm, while β-keratin is based on a folding pattern β-sheet and its diameter is 4 nm. Although there are fundamental differences, both molecular structures have similar mechanical behavior and are linear elastic. In fact these two structures could be rearranged according to the environment needs, for instance during mechanical stretching the α-keratin structure changes into β-keratin (Meyers et al., 2008). Figure 1.2 represents both kinds of molecular structures of keratin. Other general features of keratin are important in materials field, since because of hierarchical and stable structure, this protein is characterized by durability, non-soluble in organic solvents, chemically non-reactive and flexible. In addition keratin is able to recover its original mechanical properties, after repeated deformations and only with minimal loss (Martinez-Hernandez et al., 2005a).

Figure 1.2 Molecular representation of α-keratin and β-keratin arrangements.

Keratin is a main protein component in many external appendages in different animal species. Thus, natural different keratinous materials can be divided according to their function (McKittrick et al., 2012), for example:

Protection and/or covering: skin, hair and wool, quills, spines and pangolin armor

Defense and/or aggression: Horns, claws, nails, beaks, teeth, hagfish slime

Motion: hooves and feathers

In the next sections four of these types of keratin, feathers, hair and wool, and horns will be described briefly in order to understand their structure and most important characteristics.

1.2.1 Feathers

Feathers are about 90% protein, mostly β-keratin (Stettenheim, 2000). Feathers are non-homogeneous arrangements distributed in almost all living birds’ bodies. Feathers are used in a diversity of functions and are enormously variable in structure and color (Norell & Xu, 2005). Feathers are probably one of the most complex appendages that have evolved since the dinosaur era (Martinez-Hernandez & Velasco-Santos, 2011; Bartels, 2003).

The evolution of feathers has not been resolved completely because there is no morphological history of its origin. The first recorded history structurally corresponds to modern feathers of Archaeopteryx. Xu et al., have studied in depth some important discoveries of fossilized feathers, and they argued that primitive feathers had their beginning in certain filamentous integumental appendages observed on some theropod dinosaurs. However, there is not an exact homology between these primary structures and feathers, resulting in the theory that two taxa with true feathers (Caudipteryx and Protarchaeopteryx) have been proposed to be flightless birds (Xu et al., 2001). They observed the filamentous primitive feather of the basal dromaeosaurid dinosaur Sinornithosaurus millenii, and indicated those appendages were constituted by complex structures formed by multiple filaments, which exhibit two types of branching arrangement. These are distinctive in avian feathers. The first structure is based on filaments joined in a basal tuft, and the other configuration involves filaments joined at their bases in series along a central filament. These observations are useful to conclude that integumental appendages of Sinornithosaurus and avian feathers can be considered structurally homologous (Xu et al., 2001). In contrast, the question about the evolutionary cause of avian flight is tangled due to first birds (i.e., feathered vertebrates) may not necessarily have had the capability for flight, because primitive feathers were improbable to have been specialized flight appendages (Maderson & Homberger, 2000).

Research has reported a classification of feathers on living birds: contour, down, semiplumes, filoplumes, powder-down and bristles (see Figure 1.3). Contour feathers are the most familiar kind. They cover most of the bird’s body as well as the bird’s integumentary flight surfaces. Contour feathers comprise hollow tubes, called the calamus proximally and the rachis distally. Barbs are branches growing from the rachis or calamus (Norell & Xu, 2005).

Figure 1.3 Different types of feathers and most significant parts of their hierarchical structure.

Feathers from chicken (Gallus gallus) are complex hierarchical arrangements of a three level branched structure. A feather is structured by a central shaft, which is inserted in a follicle by an initial segment called calamus or quill. This is a short, tubular basal segment. The next part is called rachis, and this is a much longer, pith-filled section. Quill or calamus is cylindrical, transparent, and hollow. Barbs grow up from shaft or rachis, this last one is compacted on the sides in order to support the growing of barbs and is roughly rectangular in cross-section, which is the main difference with respect to quill. Internally it is packed with a succinct material containing air cells. The branches (barbs) grow up sideways the rachis; these constitute structures in the shape of sheets or vanes, although some feathers also have series of lateral barbs growing from the upper end of the quill. The barbs (so called rami) originate from flattened sides of rachis and grow in parallel ordered rows opposing one another and directing outward and toward the tip of the feather. Barbs morphology is characterized by a slight ovoid cross-section, broader near their origin in the rachis, but flattening and narrowing close the tip. In similar way to rachis, barbs are also filled with a kind of light material containing air cells. A feather may have only a couple of dozen barbs or several hundred. This natural material follows a natural, hierarchical order, since like a complete feather, also each barb replies most of this ordered structure. Thus, barbs have a central axes (ramus) from which barbules are originate as closely spaced branches on either side. The barbs and their barbules determine the nature of the vanes. Each barbule is basically a stalk of single cells, serially differentiated along its length. Barbules, also named radii, come from either side of the barbs. According to several morphologic studies, the shape of barbules changes depending on the part where they are growing. For example in the area from the base to about half way to their tip, barbules are similar to ribbons and can be called basal lamella. The next variation depends on the distal half and here barbules are identified as the pennulum. In addition rising direction also depends on distance over surface of barbs: the barbules on the upper edge extend outward perpendicularly to the barb, but barbules on the lower edge occur more parallel with the barb (Luallen, 1996; Stettenheim, 2000; Meyers et al., 2008).

In domestic chickens, the so-called hard-feathered breeds have short contour feathers that are located nearly alongside the bird’s body surface (Bartels, 2003). The primary structure is established by the amino acid arrangement; in chicken feathers this is very similar to that of other birds’ feathers and also to that found in reptilian keratins from claws. This primary structure is mostly composed of the amino acids cystine, glycine, proline, and serine, with minimal quantities of histidine, lysine, or methionine (Schmidt, 1998; Kock, 2006).

Chicken feathers possess distinctive structure and unique properties including excellent compressibility and resiliency and good thermal and acoustic insulating properties. In addition, density of chicken feathers has been reported around 0.8 g/cm³, which represents a lower value compared to that shown by cellulose fibers (1.5 g/cm³) and wool (1.3 g/cm³). This feature makes them suitable for use in various applications, including the proposal of new material alternatives and not just as problem to dispose of them or to be included in animal feed (Kock, 2006; Reddy & Yang, 2007). So, technologies for processing chicken feathers into fibrous (feather fiber) and particulate (quill) fractions have been developed due to interesting characteristics that distinguish them from synthetic counterparts (Martinez-Hernandez & Velasco-Santos, 2011; Meyers et al., 2011), some of these features include:

Natural structures are assembled from the bottom-up process using surrounded components. This means self-assembly is dictated by nature to have the necessary conformation to survive and evolved.

The ability to serve more than one purpose, for instance feathers involves diverse functions: mechanosensory, ornament, flight, thermal regulation, acoustic insulation, etc. This kind of structure is called multifunctional, which is worthy to be imitated and could be basis for new and high developed materials or devices. In addition could provide multifunctionality to diverse simple materials if it is used as reinforcement.

Physicochemical properties are strongly conditioned by the level of water in the structure. Specifically, keratin fiber has three different types: free water, loosely bounded water and chemically bounded water. The last is one of the most important elements to achieve the conformational stability in keratin, since interactions between negative and positive groups of polypeptide chains and therefore ability to create hydrogen bonds. Thus water is prevailing to create the three-dimensional complex of interactions inside the keratin structure. Secondary structure is stabilized with the structural support of both water interactions and disulfide bonds from cysteine.

The morphology and properties of natural structures are determined by the evolution, environmental restrictions and availability of materials and elements.

The synthesis and development of almost all biological materials are realized with a surrounding aqueous environment, and at normal temperature and pressure conditions.

The structures are hierarchical, which means they are ordered by different scale levels. Hierarchy confers distinct and unique properties.

1.2.2 Hair and Wool

Hair and wool consist of circumferential covers of dead cells produced by follicles in certain living beings’ skin. These keratin appendages act as skin protection. Their hierarchical structure comprises three distinctive parts: an outer layer called the cuticle, which maintains the root adhered into the follicle; the next layer, called the cortex, is in the middle and consists of keratinized cells; and finally the last part includes the medulla or hollow core, as shown in Figure 1.4. The hollow hair structure can be considered as a material with thermal insulation properties (McKittrick et al., 2012).

Figure 1.4 Basic hierarchical structure from hair or wool.

As it was mentioned before, the cuticle is the surface layer in the human hairthat is formed by smooth covering cells, which are nearly 0.5 µm thick, 45–60 µm long and are located at 6–7 µm intervals. Beneath this, it is found the cortex, which comprises an ordered group of cells that growth parallel to the fiber axis and are named cortical cells. These are 1–6 µm in diameter and 50–100 µm in length (Yang et al., 2014).

Wool fibers have a slightly elliptical hierarchical structure. One of the most important components in this structure is the scaly outer coat, called the cuticle; it is covered with a waxy protection film composed mainly of lanolin, and this confers wool’s resistance to mist and light rain. The diameters of wool fibers are between 15 µm and 50 µm, their density is around 1.3 g/cm³ and its decompose temperature is around 130 ºC.

Figure 1.5(a) shows the tensile stress-strain graphic for hydrated wool. In this curve, it is appreciated a first uncramping section, from which a linear elastic region is initiated and continues until it reaches up to the yield point, at approximately 2% strain. Almost 3% strain, slope increases abruptly and when 50% and 60% strain were reached the fracture was observed. Wool and hair offer a notable characteristic: an ample recovery is achieved if the fibers are successively soaked in warm water after testing. This recovery behavior can be compared to that shown by shape-memory alloys. Figure 1.5(b) shows tensile stress–strain curves obtained for wool tested in water and varying temperatures from 0 ºC to 100 ºC (Figure 1.5(b) (i)) or tested changing relative humidity from 0% to 100% (Figure 1.5(b) (ii)) (McKittrick et al., 2012).

Figure 1.5 (a) Schematic representation of stress-strain curve for wool fibers (b) Stress–extension curves for wool: (i) in water at various temperatures and (ii) at different relative humidity values (Adapted from McKittrick et al., 2012. Copyright Springer).

1.2.3 Horn

As it was mentioned before, keratin develops from dead cells produced by biological systems. Horns involve a structural core formed by cancellous bone covered with skin that grows up from the posterior side of the skull. They are structured in a complex arrangement, which comprises two main elements: tubules and intertubular material having a laminated assembly, where the tubules are organized as dense pack, forming a laminated composite (Figure 1.6). This laminated composite comprises fibrous keratin, with 2–5 µm of thickness in the lamellae zones. Also a porosity gradient was appreciated through the thickness of the horn, pore sizes were measured from 60–200 µm along the long axis of the pores (Tombolato et al., 2010). The laminated structure has a strong influence in the mechanical properties. For instance an ample anisotropy was appreciated through strength, which was around 10 times higher in longitudinal direction (parallel to the tubules), than in the transverse direction (perpendicular to the tubules). Additionally the elastic modulus and yield strength were associated to the orientation of tubules, showing an anisotropic behavior. In fact, horns absorb high quantities of energy before breaking, showing a critical crack length around 60% of the transverse element of the horn for crack propagation. Meanwhile, work fracture was found to be greater than many other biological or even synthetic materials. The fracture resistance and tensile failure can be attributed to crack arrest, matrix separation and keratin fiber pullout (McKittrick et al., 2012).

Figure 1.6 Schematic representation of structural elements in horns (Tombolato et al., 2010. Copyright Elsevier).

On the other hand, it is worthy of mention that elastic and shear modulus diminished significantly as the moisture content increased, like other keratin-based materials (Figure 1.7) (Tombolato et al., 2010).

Figure 1.7 Elastic modulus and maximum bending strength determined by three-point tests of ambient dried and rehydrated horn specimens in longitudinal (L) and transverse (T) directions. (Tombolato et al., 2010. Copyright Elsevier).

1.3 Natural Fibers to Reinforce Composite Materials

The current lifestyle, in almost all the industrialized world, is strongly related to the development of materials that can provide an ample range of characteristics related to high mechanical resistance, light weight, or fire retardant behavior. This can be accomplished with polymer composites, which have an infinite potential applications including in the automotive, aerospace, construction, civil, medical, and the sports and leisure industries.

The study of novel composite materials developed using natural fibers as reinforcements has grown into an area of pronounced interest, mostly because environmentally friendly materials have a real opportunity when applied to commodities to daily life, with properties comparable to those offered by completely synthetic composites.

Composites reinforced with fibers have been used for diverse applications due to specific properties, as shown in Table 1.1, which describes the most important properties observed by common natural fibers used in the manufacture of polymeric composite materials. Natural fiber-reinforced composites have become an outstanding alternative in the development of inexpensive and biodegradable materials with improved mechanical and thermal properties. Natural fibers have attracted the attention of scientists and industries because they have remarkable advantages over conventional synthetic reinforcements. Some of these advantages include low cost, biodegradability, high toughness, low density, high structural performance and good specific strength properties.

Table 1.1 Properties of some natural fibers commonly used in polymeric composite materials. (Adapted from Netravali & Chaba, 2003)

*Proposed by Pickering et al., 2015.

**According with Martinez-Hernandez & Velasco-Santos, 2011.

1.4 Keratin, an Environmental Friendly Reinforcement for Composite Materials

Different research around the world describes the use of natural fibers in plasticized polymers, and is focused mainly on using cellulose-based fibers, which can be obtained through an adequate exploitation from renewable plant resources such as wood, cotton, sisal, kenaf, hemp, flax, ramie, coir and bamboo, among many others. However, in addition to cellulose, there exists one supplementary resource for natural fibers, which is extremely abundant: animal filaments. These have attracted the interest of materials scientists due to their exceptional structural properties, meaning a remarkable alternative as reinforcement materials. Keratin represents one of the clearest examples of this trend (Martinez-Hernandez & Velasco-Santos, 2011). The following sections show the efforts realized by different research groups to incorporate this novel fiber in both synthetic or natural matrices. Specifically this chapter deals with four novel materials as fiber: wool, feathers, horn and human hair, as shown in Figure 1.8.

Figure 1.8 Scanning Electron Micrographs of four keratin materials applied as reinforcements in polymer composites: (a) Wool fiber (Rivera-Gomez et al., 2014. Creative Commons Attribution License); (b) Feather fiber; (c) Horn (Tombolato et al., 2010. Copyright Elsevier); (d) Human hair (Vazquez Villa et al., 2013. Creative Commons Attribution License).

1.4.1 Synthetic Matrices

In the manufacturing process for polymer composites a great diversity of synthetic polymers has been amply studied to achieve the needs established by consumers. However, when natural fibers are used, the complexity of processing is intensified by the inherent characteristics of such materials; one of the most important is the thermal behavior. This restricts strongly the options of suitable matrices that can be reinforced with natural fibers, and keratin is not an exception. Therefore, thermal behavior of matrices and keratin fibers, adequate dispersion features, critical longitude of fibers, among others, are important challenges to overcome in order to take advantage of the most important advantages of keratin fibers in polymer composites.

1.4.1.1 Petroleum-Based Polymers Reinforced with Chicken Feathers

Considering the characteristics of chicken feathers, such as ready availability, low cost, low density, thermal insulation behavior, and hydrophobicity, among others, fibers obtained from this material are being used successfully by several authors to reinforce polymer composites. The following descriptions include only research from the last decade, but this field has attracted each day more interest.

Barone and Schmidt, (2005), used fibers obtained from chicken feathers as reinforcements and varying their aspect ratio. The reinforcements and low-density polyethylene (LDPE) as matrix were incorporated by a Brabender mixing head. The results showed that composites increased elastic modulus almost 3 times with respect to LDPE, also composites have higher values in yield stress. In addition, these authors observed well dispersed fibers and achieved proper interactions between reinforcements and matrix. In particular, this fact is important to achieve good physical properties. In this way, Barone and Schmidt found interactions between fiber (as reinforcement) and polymer (matrix), without using coupling agents or chemical treatments. These interactions have a clear influence in the physical properties and morphological characteristics. Therefore, they conclude that the fibers obtained from feathers can be suitable for incorporation into a polymeric matrix using thermomechanical mixing techniques (Barone & Schmidt, 2005). In addition, Barone et al. (2005), also reported the study of keratin fibers as reinforcements, but using 20 wt% of fibers with high-density polyethylene (HDPE). In this case the composite were prepared using the same equipment: a Brabender mixing head. Diverse parameters such as fiber dispersion, compounding time, processing temperature and speed were analyzed. The influence of compounding and molding variables were analyzed by means of tensile testing and morphology through scanning electron microscopy. Barone et al. 2005, reported keratin fibers showed thermal stability for long periods above 200 ºC. The authors demonstrated that composites achieved the best properties using processing temperature around 205 ºC and 75 rpm in a Brabender mixing. Finally, both studies showed that feather fiber can be an excellent material to increase the tensile properties of polyolefins (Barone et al., 2005).

Bullions et al., (2006), also decided to use a polyolefin, in this case polypropylene (PP) and reinforce it with keratin obtained from chicken feather. In this research various natural reinforcing materials were compared, according to those authors, the reinforcements were called feather fiber (Ff), recycled kraft pulp fiber (Pf) recycled new pulp fiber (Nf) and retted kenaf bast fiber (Kf). The composites were synthesized with different quantities of these reinforcements, made by compression molding by using several plies of nonwoven, fabric-like prepreg. In this research eleven PP-fiber blends were made, and divided into 4 groups. PP was included constantly in all of them, and the reinforcement fibers were combined varying their quantities and type, but Ff was included in 10 of these composites. In spite of cellulose fibers being found as important contributors to the mechanical features of composites, Ff does not function in the same way in order to have better mechanical performance. The authors conclude that combining cellulose fibers and Ff was not convenient, since tensile modulus for Ff is much lower than the value presented by cellulose fibers. The results showed that mixtures of cellulose fibers reached similar modulus whereas keratin fiber induced a reduction in the modulus observed for the composites. Therefore, the effect observed with the three types of cellulose fibers and Ff was ordered as: Pf > Nf > Ff > Kf. The highest influence was observed by Pf, but the lowest was found for Kf. The main parameters that play definitive roles in the strength performance of composites were fiber aspect ratio and fiber strength (Bullions et al., 2006).

Huda and Yang (2008) focused on studying the mechanical and acoustic properties of composites obtained with polypropylene (matrix) and ground chicken quill (reinforcement), and compared their properties with composites made of polypropylene (PP) and jute. This research was the first published about studying the quill as reinforcing material and not only the fiber as it was reported by previous authors. The authors studied the effect of powdered quill concentration, holding temperature and density upon two outstanding parameters: mechanical behavior and sound absorption. The results obtained in this research showed that PP-quill composites produced similar values in flexural strength and offset yield load to those obtained from PP-jute composites; however, composites made of keratin have lower tensile properties and flexural modulus than those with PP-jute composites. Additionally, density has an important role in defining mechanical and acoustics properties, as shown in Figure 1.9. Noise reduction coefficient of ground quill composites is 71% higher compared with jute composites. This allows proposing quill reinforced composites as good materials for acoustic panels and headliner substrates (Huda & Yang, 2008).

Figure 1.9 Effect of density and void content on the flexural properties of quill–PP composites at 35% quill, holding temperature of 185 ºC, and holding time of 80 s (Huda & Yang, 2008. Copyright Elsevier).

On the other hand, Jimenez-Cervantes Amieva et al., (2015), used also PP, but in this case recycled polyolefin with the same type of reinforcement (quill) that was used by Huda and Yang (2008). The main difference was that the recycled-PP-quill composite was obtained by extrusion, using a laboratory extruder of single screw (Beutelspacher E 1930), with a 19 mm 3-hole circular die and control of temperature by Matlow controllers. Different percentages of quill as reinforcements were employed: 5, 10 and 15 wt. In this research, the authors studied the behavior of thermal, thermo-mechanical and morphological properties of composites. The results revealed good dispersion achieved in the composites, which was confirmed by the scanning electron microscope (SEM) images (Figure 1.10). It was also found that the presence of quill as reinforcing material increases the storage modulus obtained by dynamic mechanical analysis (DMA); for example, the composite reinforced with 5% of quill presented the maximum storage modulus (E′ = 1451 MPa) compared to pure PP (E′ = 965 MPa). Additionally this composite has a lower density (0.75 g/cm³). In spite of the other synthesized composites also have good results, they were not as good as with 5%, since the addition of 10% and 15% of quill decreases E′ when compared to that obtained with 5% of reinforcement. The storage modulus was 1369 MPa for 10% and 1167 MPa for 15%, but remained above the storage modulus for pure PP. These results make recycled-PP-quill composites appropriate materials in automotive and aerospace applications (Jimenez-Cervantes Amieva et al., 2015).

Figure 1.10 SEM images for fractured surfaces of (a) Recycled-PP and Recycled-PP/Quill composites with (b) 5, (c) 10 and (d) 15 wt% of quill (Jimenez-Cervantes Amieva et al., 2014. Copyright SAGE).

In order to take advantage of the outstanding features of keratin as reinforcements, such as good mechanical properties, low cost, low density, high toughness, among others, Martinez-Hernandez et al., (2005) studied poly(methyl methacrylate) (PMMA) reinforced with different percentages of keratin biofibers obtained from chicken feathers. The keratin biofiber-PMMA composites were formed by bulk polymerization. The composites were analyzed by a series of tensiles tests, optical and scanning electron microscopy. Keratin reinforcement and PMMA matrix showed good compatibility, which can be attributed to hydrophobic nature of biofiber (Martinez-Hernandez et al., 2005a). The mechanical tests revealed that PMMA’s elastic properties were modified through the use of keratin biofibers as reinforcements. This fact is even more meaningful due to PMMA is a rigid polymer. Therefore, tensile tests results demonstrated keratin biofibers can be exploited as suitable elements to modify mechanical characteristics for less rigid polymers. This also can be observed in the improved Young’s modulus and other results showed in Table 1.2 (Martinez-Hernandez et al., 2005b). In addition, Martinez-Hernandez et al., (2007), reported the thermal characteristics of the keratin biofiber-PMMA composites and for the first time the thermo-mechanical properties of these composites were evaluated. This research validated that keratin fibers can be used as reinforcements for polymeric matrix. It was observed that keratin biofiber-PMMA composites have good thermal stability and glass transition temperature (Tg), found to be higher than standard PMMA. The composites reinforced with 5 wt% of keratin biofiber showed a Tg of 109 ºC, composites with 1 wt% of reinforcement presented a Tg of 95 ºC, whereas for pure PMMA it was observed a value of 72 ºC in Tg. In addition, dynamic mechanical analysis showed that composites reinforced using 1 and 2 wt% of keratin biofiber achieved higher storage modulus. For example composite with 1 wt% showed 2076 MPa in E′ and composite with 2 wt% presented 2029 MPa; both composites were above pure PMMA, which demonstrated only 1926 MPa in E′. Besides keratin biofibers embedded by this rigid polymer produced that tan delta signal was decreased, since for pure PMMA the corresponding value was 1.08, whereas composite with 1 wt% and 2 wt% showed 0.80 and 0.76 respectively. This fact suggests that a strong interface could be achieved. Optical images showed in Figure 1.11 supported these results (Martinez-Hernandez, et al., 2007).

Table 1.2 Mechanical properties in PMMA-keratin biofiber composites.

Figure 1.11 Optical image of feather biofiber-PMMA composite (Martinez-Hernandez et al., 2007. Copyright Elsevier).

On the other hand, chicken feathers can also be used as reinforcements in epoxy matrices, one of the most extensive applied polymers. In 2007, Ananda et al., observed the erosive wear behavior of poultry feather reinforced epoxy, which underwent multiple impact conditions. The matrix used was epoxy resin (Araldite LY 556) with its corresponding hardener (HY951), and 20 wt% of short fiber obtained from poultry feathers was used as reinforcement. The authors reported that although the fabrication of composites was achieved using a simple technique, the composites have only a minor improvement in tensile strength from 70 MPa (pure epoxy resin) to 70.45 MPa (feather fiber-epoxy resin composite). However, they showed a significant improvement in the erosion wear performance of the composite reinforced with feathers, since 25 units were detected as the difference between unreinforced epoxy resin and the composite. In addition, reinforcement induces a clear reduction in the density which achieved 13%, since neat epoxy has 1.12 g/cm³ and composite was measured to be 0.97 g/cm³.

The same polymer matrix was employed by Mishra and Nayak in 2010, in order to develop epoxy composites reinforced with chicken feathers. They studied dielectric properties of the short fiber-epoxy composites. In this research, the polymer matrix used was epoxy LY556. The authors found that the amount of reinforcement (10, 20 and 30 wt%), temperature and frequency ranges influence the dielectric properties. For example, dielectric constant (k) decreases significantly if frequency and feather fibers are augmented (Mishra & Nayak, 2010). It is possible to observe that for pure epoxy at a frequency log (f) value of 6, the dielectric constant is 3.7, whereas if fiber is included to 10, 20 and 30 wt% the values of k are 2.7, 2.5 and 2.0 respectively. However in the case of temperature the opposite effect was observed: a slight increment of k was appreciated for each concentration of keratin; for example, for pure epoxy at 25 ºC the k value is 4.8, but at 125 ºC the value increases as to 6.5. Similar behavior was observed in composites with 10, 20 and 30% of keratin reinforcement, which from 3.4, 2.8 and 2.3 respectively at 25 ºC, were incremented achieving values of 5, 4 and 3 respectively. These results have a positive impact for the future potential application of these composites reinforced with keratin fibers.

1.4.1.2 Synthetic Matrices Reinforced with Hair or Wool

Research aimed to the use of natural fibers in materials field has become of great interest for diverse scientists, mainly due to these materials offering innumerable advantages over synthetic reinforcements that can impulse ecological, renewable, low cost alternatives for commonplace materials. In recent years, studies involving hair or wool as reinforcements in matrices have had important advances in specialized areas, such as biotechnological and biomedical disciplines. For instance, advanced researches for tissue engineering development and affinity membranes production are suitable subject matter. This specific interest is related to keratin’s biocompatibility and its ability to support fibroblast growth, but in addition its capacity to absorb heavy metal ions and volatile organic compounds is worthy to take into account (Aluigi et al., 2008).

Different techniques used by diverse authors have transformed keratin from human hair or wool into a useful component for composite materials. For instance, Tonin et al., produced nanofibers using wool as source of natural keratin and electrospinning. They generated a blended composite with polyethylene oxide (PEO), a synthetic polymer. In their research, these authors found that PEO affects wool keratin self-assembling and induces changes in thermal behavior of this fibrillar protein, but also crystalline structure of PEO is influenced by keratin content. The processing technique also plays an important role; for example nanofibers obtained by electrospinning of the PEO/Keratin blend (30 wt.% and 70 wt.%) have an increase of 26% of α-helix structure with respect to that obtained from the film synthesized by casting with the same blend. In addition it is shown that the use of electrospinning destabilizes the β-sheet conformation, in this case, a decrease of 27% was observed (Aluigi et al., 2008; Tonin et al., 2007).

The research published by Reichl (2009) reported the use of two different approaches to create films taking advantage of human hair keratin as support for cell culture and tissue engineering. In that study, the author reported two approaches used to create substrates for in vitro cell culture procedures. The first one uses precipitation of trichloroacetic acid (TCA), while the second was obtained with keratin dialysate (keratin CLEAR) through wetting and drying processes. The study proved that the keratin CLEAR coated substrates showed good behavior as support for the attachment and proliferation of different cell types, as well as offer advantages over the common polystyrene method (see Figure 1.12). This new material would result an economical and interesting alternative to conventional and commonly used coating materials. The results of this research suggested that keratin produces a very good seeding efficacy and high saturation densities when demanding and valuable cell cultures are required (Reichl, 2009).

Figure 1.12 Effect of fiber content on tensile properties of Hair Fiber/PP composites: (a) stress-strain graphs of tensile test, (b) ultimate tensile strength vs. fiber content, (c) tensile modulus vs. fiber content and (d) breaking strain vs.fiber content (Reichl, 2009. Copyright Elsevier).

In order to produce composite materials with novel properties, Bertini et al., in 2013, investigated keratin obtained from wool wastes. Keratin micro-sized particles were produced by using microwave-assisted green hydrolysis with superheated water. The composite materials were made with isotatic polypropylene (iPP), maleic anhydride grafted polypropylene (PPg) and keratin micro-particles (Ker). Different combinations of these three components are described in Table 1.3. Their results demonstrated that keratin powder acts as reinforcing material and stabilizes and protects matrix to possible damage generated during thermo-mechanical processing. Indeed, reinforced composites exhibited excellent compatibility, which was improved by use of maleic anhydride. This was added to the matrix, generating an increase in the mechanical properties of reinforced composites compared to the iPP matrix. For example, the composites reinforced with 20% of keratin showed 10% increase in elastic modulus compared to the reference iPP, as shown in Table 1.3. However, composites reinforced with 5 and 10% had similar values to the matrix. Furthermore, it was observed that the composites reinforced with keratin showed an increase also in thermal resistance due to thermal degradation was achieved at higher temperatures; the composites reinforced with 20% of keratin reported an increment by 20 ºC with respect to the matrix (Bertini et al., 2013).

Table 1.3 Composition by weight of the investigated materials and tensile data for polypropylene composites. (Adapted from Bertini et al., 2013)

1.4.1.3 Synthetic Matrices Reinforced with Horn

Bio-waste horn fiber, likewise other keratin materials, can be studied as useful material to manufacture composite materials. As in other keratin reinforcements, their uses reduce environmental pollution, replace synthetic fibers and are cost effective. Some authors have investigated the mechanical properties, influence of humidity or dielectric behavior of this singular material (Marzec & Kubisz, 1997; Trim et al., 2011; Zhang et al., 2015). In spite of its promising features, only a few works have been aimed towards application of keratin from horns. One example of this fact is the research published by Kumar and Boopathy (2014). They studied mechanical and thermal properties of reinforced composites synthesized with oxen horn as fiber (HF) and polypropylene (PP) as matrix. In addition, in those composites, maleic anhydride grafted polypropylene (MAPP) was used to improve the compatibility between the components. Four different weight proportions were synthesized: 5, 10, 15 and 20 wt.%. The results showed that fiber reinforcement (HF) has higher mechanical properties than pure PP and HF/PP composites. Because of that HF/PP composites showed a slight increase in tensile yield, but flexural strength and flexural modulus increased by 5.85 MPa (16.95%) and 274.18 MPa (59.69%) respectively. Besides, composites showed a decreasing in ultimate tensile strength by 4.9 MPa (15.03%). On the other hand the tensile modulus increased up to 73.09 MPa representing 15.74%. In addition, it was observed that if fiber content is increased then melt flow index for HF/PP composites decreases. At the same time, fiber content was found as related directly to thermal behavior of HF/PP composites, which was observed through thermogravimetric analysis; thermal stability was augmented as function of horn fiber content (Kumar & Boopathy, 2014).

1.4.2 Natural Matrices

One of the most outstanding challenges nowadays is create sustainable bio-based products, in this sense eco-friendly bio-composites from renewable sources have attracted the attention of many scientists. These sustainable novel materials can be an important alternative to solve many problems related to petroleum derived polymers and the commodities obtained from them. Thus, sustainable eco-composite development is a frontier topic for the twenty-first century and it is an emergent field with great importance to the materials area, where specific knowledge about processing, microstructure, interface, mechanical performance among other specific demands would be understandable. In order to take advantage of the many useful properties of biopolymers, not only natural fibers should be considered but also matrices obtained from renewable sources. Hence the so called green bio-composites are being developed by using biofibers as reinforcements for biopolymers such as cellulosic plastics, polylactides, starch plastics, polyhydroxyalkanoates, and soy-based plastics (Mohanty et al., 2002). It is worthy of mention that both natural reinforcements and matrices could be combined in an almost infinite range of compositions with many diverse purposes.

1.4.2.1 Natural Matrices Reinforced with Chicken Feathers

The development of natural matrix composites reinforced with fibers obtained also from natural sources is an option to develop eco-friendly materials to assist in the environmental impact and reduce the use of petroleum-based plastics. Fibers from chicken feathers have been used in this context with diverse eco-matrices. For example, Cheng et al., in 2009, used two different types of fibers: flight and down feathers, which were obtained from corresponding regions of chicken feathers. Figure 1.13 shows scanning electron microscope (SEM) images obtained from the cross-section of flight and down fibers. In that research, poly(lactic acid) (PLA) was employed as matrix; this can be obtained from renewable agricultural materials. The PLA-chicken feather fiber composites were formed by extrusion and injection molding methods, wherein fiber was varied from 2 to 10 wt%. The authors reported that composites showed elastic moduli considerably higher than pure PLA. On the other hand, the stiffness increased from 3.6 to 4.2 GPa when the chicken feather fibers were present. Furthermore, the presence of the keratin fiber increases the storage modulus (stiffness) and decrease tan δ values. Finally, the thermogravimetric analysis showed PLA-chicken feather fibers composites have better thermal stability than pure PLA and the thermomechanical analysis reveal that composite with 5 wt% of chicken feather fiber has the best thermal stability over all composites and even above PLA (Cheng, et al., 2009).

Figure 1.13 Scanning Electron Micrographs of: a) flight feather fiber and b) down feather fiber (Cheng et al., 2009. Copyright Elsevier).

Ahn et al., in 2011, studied the biodegradable behavior in bioplastic pots made of PLA and poultry feather fiber (PFB). Those composites were synthesized using extrusion process and injection molding. The biodegradable behavior of composites was evaluated through quantifying the production of carbon dioxide (CO2) by a laboratory controlled composting with incubation process at 58 ºC for 60 days. The composites were obtained as follows: A) 100% PLA; B) PFF: 5 wt%; PLA: 80 wt%; starch: 15 wt%; and C) PFF: 50 wt%, urea: 25 wt% and glycerol: 25 wt%. Cumulative CO2–C quantification from containers including the compost inoculum alone and combined with each of three different composites can be observed in Figure 1.14. The results showed that the CO2 production of composite A and the inoculum-only treatment was very close. On the other hand, composites B and C showed comparable amount of CO2–C during the first 25 days, then the composite B produced more CO2–C than composite C. Elsewhere 85% of the CO2–C quantification from mixture B and C was detected during the first 38 days. Finally, although PLA-based composites are confirmed as biodegradable materials due to composting, the feather fibers presented a dissimilar behavior, since they did not easily decompose with the conditions used by these authors (Ahn et al., 2011).

Figure 1.14 Cumulative CO2–C production from compost inoculum and bioplastic pots (bioplastic A: 100% PLA, bioplastic B: 5% PFF, 80% PLA, 15% starch, bioplastic C: 50% PFF, 25% urea, 25% glycerol). Values are means ± SE of three replicates (Ahn et al., 2011. Copyright Elsevier).

Pardo-Ibañez et al., in 2014, developed via melt-compounding a biodegradable composite by using a blend of polyhydroxyalkanoates (PHAs) with keratin obtained from poultry feathers. The characterization by SEM demonstrated that a good interaction between both components in the biocomposite material was achieved. The results obtained by these authors showed elastic modulus increases over the matrix as a result of keratin additive loading. On the other hand, the elongation at break decreased as keratin was augmented, showing less ductility for PHAs-keratin composites. Reductions in water, limonene and oxygen permeability were reached when the composites contained 1 wt% of keratin additive (Pardo-Ibañez et al., 2014).

Recently, Flores-Hernandez et al., (2014) developed a new kind of eco-composite synthesized with a polymer mix of chitosan and starch and reinforced with keratin fibers obtained from chicken feathers (short and long biofibers and quill particles). The results obtained by Flores-Hernandez et al., (2014) proved the compatibility between the three natural materials. These authors studied morphology and mechanical and thermal properties of this polymer blend, and the reinforced composites. The dynamical mechanical analysis reported that the storage moduli (E′) for all composites are considerably higher than E′ for chitosan-starch matrix as result of keratin content. In fact, influence of keratin reinforcement towards matrix properties originated in this order: short fiber > long fiber > ground quill. As can be observed in Figure 1.15, the composite with 5 wt% of short fiber showed E′ around 1142 MPa at 35 ºC, it means 763% higher than chitosan-starch matrix whose E′ is 133 MPa. Thermal analysis exposes better thermal stability of composites with respect to the matrix. For example, in thermogravimetric analysis (TGA) the composites reinforced with 10, 15 and 20 wt% show better stability than matrix in the range of temperature from 325 ºC to 475 ºC. When the analysis reached 600 ºC all the composites exhibited lower weight loss (between 85 and 93%) than the matrix (95%). In addition, differential scanning calorimetry showed changes in the endothermic peak, which corresponds to water evaporation. In the matrix this endothermic peak appears around 98 ºC, whereas the composite reinforced with 5 wt% of keratin fiber presents this peak nearby 105 ºC (Flores-Hernandez et al., 2014). The morphological characteristics of composites are shown in Figure 1.16, which presents the SEM micrographs. These images show the good distribution reached by keratin reinforcements and the good compatibility that exist between keratin fiber and natural matrix.

Figure 1.15 Storage modulus (E′) for composites (ChS-SB), with 5%–20% of feather biofiber included.

Figure 1.16 Scanning Electron Micrographs of chitosan-starch composites reinforced with feather fibers: (a-d) 5 wt% of short keratin fiber, (e-h) 5 wt% of long keratin fibers, (i-l) 5 wt% of ground quill.

In the quest to find an application for feather keratin, Dou et al., in 2015, studied films based on feather keratin and polyvinyl alcohol (PVA) cross-linked by dialdehyde starch (DAS). This novel composite was proposed for potential drug release procedure. The variables studied were the ratio of feather keratin, PVA and DAS. The results showed that compatibility between feather keratin and PVA was enhanced when it was applied cross-linked by DAS, since covalent bonds were achieved between keratin and PVA. On the other hand, the influence of DAS cross-linking on the dye release rate from feather keratin-PVA films was observed through Rhodamine B as model element. The DAS cross-linking reaction produced not only a decrease of the total soluble mass in the blended films, but also it is responsible for the development of interpenetrating linkages and reducing the RB release rate, which indicate that this is strictly related to cross-linking networks achieved (Dou et al., 2015).

1.4.2.2 Natural Matrices