According to the survey conducted recently, a chicken processing plant produces about 4000 pounds of chicken feathers every hour. In most western countries, these feathers are used as (1) feather fiber feed; (2) air filter elements that replace traditional wood pulps (retarding the tree cut-down rate); and (3) lightweight feather composites. Chicken feathers are approximately 91% protein (keratin), 1% lipids, and 8% water. The amino acid sequence of a chicken feather is very similar to that of other feathers and also has a great deal in common with reptilian keratin from claws . The sequence is largely composed of cystine, glycine, proline, and serine, and contains almost no histidine, lysine, or methionine. In fact, a CFF is made up of two parts, the fibers and the quills (see Fig. 1.5). The fibers are thin filamentous materials that merge from the middle core material called quills. In simple terms, the quill is a hard, central axis off which soft, interlocking fibers branch. Smaller feathers have a greater proportion of fiber, which has a higher aspect ratio than the quill. The presence of quill among fibers results in a more granular, lightweight, and bulky material. A typical quill has dimensions on the order of centimeters (length) by millimeters (diameter). Fiber diameters were found to be in the range of 5—50 цш. The density of CFF is lighter than the other synthetic and natural reinforcements, thus, CFF inclusion in a composite could potentially lower composite density, whereas the density of a typical composite with synthetic reinforcing increases as fiber content increases. Hence, lightweight composite materials can be produced by the inclusion of CFF to plastics which even reduces the transportation cost. The barbs at the upper portion of the feather are firm, compact, and closely knit, while those at the lower portion are downy, i.e., soft, loose, and fluffy. The down feather provides insulation, and the flight feather provides an airfoil, protects
Figure 1.5 (A) SEM image of the tertiary structure of a chicken feather. (B) A typical chicken feather fiber.
Source: Park SJ, Lee KY, Ha WS, Park SY. Structural changes and their effect on mechanical properties of silk fibroin/chitosan blends. J Appl Polym Sci 1999;74:2571-5.
Figure 1.6 SEM images of a chicken down feather fiber.
the body from moisture, the skin from injury, and provides colors and shapes for displays. Fig. 1.6 shows the cross-sectional views of the flight and down feather fibers. It is obvious that flight feather fiber exists in a hollow form while down fiber is solid. In terms of fiber reinforcement, the use of down fiber appears much better than the use of flight fiber.
The moisture content of CFF is an important factor that can highly influence their weight and mechanical properties. The moisture content of processed CFFs can vary depending upon processing and environmental conditions. The glass transition temperature (Tg) of the feather fibers and inner quills is approximately 235°C while for the outer quills it is 225°C. High Tg represents that a tighter keratin structure is formed to which water is more strongly bonded. Fibers and inner quills do not begin to lose water below 100° C. The moisture evolution temperature of the CFF and quill occurs in the range of 100—110°C. This suggests that it may be possible to have fully-dry fibers and inner quills at 110°C.
The length and diameter (sometimes in the form of bundles) of CFF would highly affect their properties and the impregnatability of resin into a resultant composite. Therefore, the control of resin temperature (thus, its viscosity), while at the same time managing the sonication (ultrasonic vibration) time to facilitate the resin penetration rate into the fibers are essential. Short or longer fibers would highly affect the stress transferability as well as the shear strength of the composites. The fibers, themselves, also would be a barrier to the movement of polymer chains inside the composites and may result in increasing the strength and thermal properties, but reduce the fracture toughness. These properties will be studied in detail in this section.