Fibers, synthetic degradation

Natural fibers based on cellulose or protein are more prone to degradation due to humidity and temperature than synthetic organic fibers. Photo-degradation occurs when exposed to light (both visible as well as ultraviolet) which shows up as a discoloration and loss in mechanical properties. Most polymeric fibers (natural or synthetic) swell due to moisture absorption. The swelling is generally more in natural fibers. We discuss this topic again in Chapter 4. 

Biodegradable composite materials are materials obtained from nature or by synthetic methods, whose chemical bonds can be cleaved by bacteria or other microorganisms present in the biosphere. Composites of biodegradable polyester can be classified as composites made of polyester reinforced by carbon and glass fibers, natural fibers, and degradable polymers. 

As discussed in Chapter 10, a wide variety of additives is used in the polymer industry. Stabilizers, waxes, and processing aids reduce degradation of the polymer during processing and use. Dyes and pigments provide the many hues that we observe in synthetic fabrics and molded articles, such as household containers and toys. Functional additives, such as glass fibers, carbon black, and metakaolins can improve dimensional stability, modulus, conductivity, or electrical resistivity of the polymer. Fillers can reduce the cost of the final part by replacing expensive resins with inexpensive materials such as wood flour and calcium carbonate. The additives chosen will depend on the properties desired. 

Geotextiles are available as mats, textiles, webs, nets, grids, and sheets. When retention of the contained material is desired synthetic polymers such as polypropylenes, polyesters, nylons, PEs, and PVCs are used because they resist rapid degradation. When only shortterm retention is needed natural materials such as cotton are used. Geotextiles are not always made from fibers, but include film materials such as PE and polypropylene sheets used to retain moisture but retard weed growth in gardens. 

The first really successful artificial material used in the manufacture of synthetic blood vessels was Dacron , a polyester fiber made from polyethylene terephthalate (PET). The material is woven or knitted into thin tubes with dimensions similar to those of a natural blood vessel. The tubes are then treated with coagulated blood or with albumin, an important blood protein, to block the tiny holes in the fabric of which they are made. Over time, cells migrate into the blood or albumin trapped within the Dacron matrix and deposit collagen. As the blood or albumin degrades, it is replaced by the collagen, producing a vessel with some properties similar to those of natural blood vessels. 

Polyglycolide was one of the first synthetic polymers used as a degradable surgical suture [122]. Fig. 8 shows the glycolide monomer and polymer structures. This aliphatic polyester is biodegradable and exhibits negligible toxicity when implanted in tissue. It is also possible to fabricate a strong fiber of this polyester with satisfactory mechanical properties. 

Synthetic fibers have been characterized by a resistance to degradation over forensically relevant timescales (Table 7.4). Nylon (polyamide), polyester, and acrylic fibers show considerable resistance to soil burial. Regenerated cellulose fibers (rayon viscose), however, share the vulnerability of natural cellulose to decomposition (Rowe 1997). However, they do show a higher degree of resistance to biodegradation compared with natural fibers or regenerated cellulose, with the exception of triacetate. 

Other fibers. The other major class of synthetic fibers, the polyacrylonitriles (orlon, acrilon, etc.) like the cellulosics (rayon, cotton) show no thermal activity up to 300 °C. Above these temperatures degradation of sample accompanies any characteristic transitions or curing exotherms. To minimize this effect, the samples are run in an inert environment such as N2, as seen in figure 16. Under these conditions reproducible characteristic endotherms were obtained for identifying wool, cotton and rayon. In roughly the same temperature region,... 

One of the more recent studies on cotton, polyester, and nylon (24) demonstrated that cotton was superior to the synthetics in outdoor performance in areas of low air pollution but that its performance was reduced considerably in areas of high air pollution. Specific effects of air pollutants are discussed later under chemical agents causing fiber degradation. 

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