Natural fibers, degradation

Whichever application of natural fiber or natural fiber-reinforced plastics will be used depends on the different environmental conditions, which are likely to add to the aging and degrading effects. On the other hand, such effects are often desirable, as is the case with com-... 

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. 

Natural fibers may be adversely affected by sunlight, air pollutants, acids and alkali, bleaches and other oxidizing agents, and chemical finishing processes. However, the effects of photochemical degradation are by far the most problematic in the conservation of textiles and therefore have received the most attention. 

Photochemical degradation of natural fibers depends on the atmospheric conditions in which the textiles are exposed, stored, or displayed, on... 

The last type of chemical degradation to be considered is that encountered by natural fibers in their purification by agents such as acids, alkali, and bleaches. Several excellent reviews and books are available on this subject (96,97,98,99), all of which stress the proper choice of temperature, pH, and other reaction conditions to optimize removal of impurities from fibers while minimizing damage. 

Up to the present time conservators were interested in preserving textiles made from natural fibers. With time, however, as present day materials incorporating man-made fibers are collected, concerns will arise as to their preservation and display. In this chapter, I survey the different types of man-made fibers, discuss environmental factors that can degrade them, and indicate methods for the identification or estimation of such damage. Also, I indicate where precautions need to be taken in their cleaning and display. 

Changes in the fiber and fiber/cement interfacial region due to environmental interactions can affect the long-term performance of cement-based composites reinforced with natural fibers. A significant mechanism of changes in composite properties is pulp fiber 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. 

The natural fibers of cotton and wood are the most important commercial sources of cellulose. Thus, knowledge of their structure is essential to any discussion of its enzymatic degradation. Several excellent reviews of cellulose structure have been published recently those on cotton by Hamby (22) and Warwicker et al. (76), and on wood in the volumes edited by Zimmerman (79) and C6t6 (7), as well as the discussions of the ultrastructure of plant Cell walls by Miihlethaler (47) and Rollins (59) make more than a general summary here unnecessary. 

To appreciate fully the influence of the structural features of natural fibers on their susceptibility and resistance to enzymatic degradation, it is necessary to understand the relationship between cellulolytic microorganisms, their extracellular enzymes, and the fiber substrate itself. 

The organisms that degrade natural fibers live either on the exterior surface of the fibers, as most often is the case with cotton, or in the fiber lumina as is necessary in the case of wood. Here they secrete extracellular enzymes that catalyze the dissolution of the high-polymeric constituents of the fiber to soluble products that can be assimilated and metabolized by the organisms. 

Size and Diffusibility of Cellulolytic Enzymes in Relation to the Capillary Structure of Cellulose. As discussed earlier, enzymatic degradation of cellulose requires that the cellulolytic and other extracellular enzymes of the organisms diffuse from the organism producing them to accessible surfaces on or in the walls of the fiber. This accessible surface is defined by the size, shape, and surface properties of the microscopic and submicroscopic capillaries within the fiber in relation to the size, shape, and diffusibility of the enzyme molecules themselves. The influence of these relationships on the susceptibility and resistance of cellulose to enzymatic hydrolysis has not been verified experimentally in natural fibers but the validity of the concepts that follow is demonstrated by the work of Stone, Scallan, Donefer, and Ahlgren (69). 

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