Cotton fibers degradation

Figure 13. Fracture tip surface of a typical cotton fiber degraded by exposure to a 100-Mraa dose of high-voltage electrons...

The effect of microorganisms results in noticeable changes in chemical composition and physical stracture of cotton fibers. As found by electron microscopy, cotton fiber degradation by enzymes is most intensive in the zones of lower fibril stracmre density [24],... 

Braun et al. (20) demonstrated a drop in FEV. This correlated with fine dust levels and with concentration of chymotryp-sln-llke enzymes. The main source of these enzymes is said to be fungi, probably Aspergillus species. Many fungi are found on cotton plants and are related to degradation of the cotton fiber(22). The fungi found include Aspergillus. Peniclllium. Mucor and others. 

Cotton fibers are quite resistant to thermal degradation. After about 5 hours at 250°F (121°C). material yellows. Decomposes above 300°F (150°C). 

A11 assays contained the same number of Cx (CM-cellulase) units of activity (26). 200 fig of Ci protein was added where applicable. Neither of the Ci components could degrade cotton fiber significantly when acting alone. 

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. 

Figure 15. Degraded surface of typical Peruvian cotton fibers, ca. a.d. 1200...

Figure 16. Degraded surface of typical cotton fiber from the Initial Period, ca. 1000 b.c., from test pit 8...

Studies over a number of years have now shown that papers of adequate permanence can be made from wood pulps if the wood is deligni-fied without undue degradation and is carefully bleached and purified. Wood pulp papers sometimes do not have the initial high physical test values, particularly folding endurance, that can be achieved with sheets of cotton fibers, but the relative rates of loss on aging often approach those exhibited by cotton papers. 

Despite of their lack in hydrolytic activity, the CBDs of the cellulases CenA and Cex from C. fimi have been found to be capable of disrupting cotton fibers and releasing small particles from the substrate (Kilburn et al., 1993 Din et al., 1994a). This disruptive effect does not seem to be a general characteristic of CBM, since it was only found with CBD from cellulose from a Penicillium sp. (Gao et al., 2001). Anyhow, a synergism with the catalytic module and enhanced degradation capacity were reported (Din et al., 1994a). 

The formation of starch complexes with other polysaccharides is perhaps best evidenced by the sorption of starch on cellulose and its derivatives. This sorption is selective with respect to amylose and amylopectin.1065 1066 Cotton fibers selectively adsorb amylose and not amylopectin.1667-1071 However, this method cannot be used for starch fractionation because cotton was considered to induce degradation (hydrolysis).1067 On the other hand, a method of purifying amylopectin that involves sorption of amylose on defatted cellulose has been published.1072 Reversible adsorption of amylopectin on cellulose occurs when starch is equilibrated with urea in 32-35% ethanol. There are also published attempts to separate starch components on filter paper.1073-1076 The reverse idea is applied in the stabilization of nitrocellulose by its sorption on starch.1077... 

The response of the cotton fiber to heat is a function of temperature, time of heating, moisture content of the fiber and the relative humidity of the ambient atmosphere, presence or absence of oxygen in the ambient atmosphere, and presence or absence of any finish or other material that may catalyze or retard the degradative processes. Crystalline state and DP of the cotton cellulose also affect the course of thermal degradation, as does the physical condition of the fibers and method of heating (radiant heating, convection, or heated surface). Time, temperature, and content of additive catalytic materials are the major factors that affect the rate of degradation or pyrolysis. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

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