Microcrystalline Keratin: From Feathers To Composite Products
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Microcrystalline Keratin: From Feathers To Composite Products Walter F. Schmidt, Environmental Quality Laboratory, Agricultural Research Service, USDA, Beltsville, MD.
SUMMARY Molecular order and morphological order are two characteristics of feather fiber that make it unique. The fiber is highly microcrystalline and is very durable, i.e., resistant to both mechanical and thermal stress. Feather structure at the microscopic level is a complicated mixture of straight and branched fibers held together by a central fiber to form specific macroscopic structures. The proper design of processes to incorporate feathers into composite products 1) preserves the fiber’s molecular properties and 2) alters its micro-/macroscopic morphology to conform to existing composite process requirements.
MICROCRYSTALLINE STRUCTURE Feathers, like wool, are composed of the protein fiber keratin. DNA sequence unambiguously determines the chemical composition of this keratin. Published amino acid sequences for keratin in a variety of avian species contain the same 95 amino acids. About 85% of the sequences are homologous among avian species. The molecular weight of one keratin monomer is 10,168 (1). The most abundant amino acid is the Ser, followed by Pro, Gly, Val and Cys. Thus, the fiber at the molecular level has both hydrophilic and hydrophobic properties. The monomers of keratin in fibers align as α-helices. Disulfide (-S-S-) cross-linkage bonding and hydrogen bonding between Ser and other hydrophilic sites define the precise molecular level order from which macroscopic morphology originates. The monomer structure of most plant-based biopolymers is the carbohydrate unit (mol. wt. 180). In cellulose, the polymer is composed of glucose, with 10,000 units arranged in an ordered structure with a molecular weight of 1.8 million (2). In contrast, only about 180 cross-linked keratin molecules are required to achieve the same molecular weight product. For cellulose to have uniform properties at the molecular level, all 10,000 units need to have a similar orientation. For keratin to have uniform properties at the molecular level, the difficulty is in first arranging any one keratin into an α-helix; thereafter, only 180 need to be similarly ordered. Knowing the DNA sequence, one can, in principle, grow keratin with the correct sequence. However, keratin fiber cannot occur unless the monomer is in the correct conformation. Biochemically, avian species form microscopically ordered keratin easily, routinely, and abundantly. The U.S. poultry industry generates more than one billion kilograms of feathers annually (3). No additional mechanical or chemical processing is required to form or reform the chemical structure of this superabundance of individual feather fibers. There are, however, two forms of microcrystalline keratin in feathers: the fiber and the quill. The quill structure is consistent with beta-sheet molecular order. The phase transition temperature of keratin (Figure 1), as determined by differential scanning U1.5.1
calorimetry, is 220
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