Crystallinity of cellulosic fibers

Chemical reaction methods are effective in destroying the crystallinity of cellulose. For example, xanthation can decrystallize cellulose (34), but such elaborate processes appear much too costly unless cellophane or rayon fibers are the objectives. One route that might be effective and economical would be treatment of crystalline cellulose with alcohol in the presence of acid catalysts. Low-molecular-weight acetals may be formed, which could plasticize the cellulose at the same time that the degree of polymerization is reduced. 

The degree of crystallinity of the fibers and the structure of the approximately 80% crystallinity, kraft with 60%, and regenerated cellulose fiber with around 50% show differing degrees of accessibility. A cotton-based paper does have longer life, under adverse conditions, than one made from rayon. However, given acid conditions, all the cellulose fibers finally do degrade and become brittle. 

Depolymerization of cellulose fibers during irradiation is accompanied by a reduction in crystallinity, and, at high doses, extensive decomposition occurs. A dose of 5 X 10 equivalent roentgens brings about marked degradation and is sufficient to convert cotton linters into water-soluble materials. After irradiation, cellulose is more susceptible to acid hydrolysis and exhibits an after-effect. When irradiation is terminated, the intrinsic viscosity of cupriethylenediamine solutions of the irradiated cellulose continues to decrease. This behavior is initiated by oxygen and terminated by water. A similar effect is encountered with pectins after irradiation. 

Much of the chemical behavior of cellulose fiber can be attributed to cellulose structure. Since cellulose is a highly crystalline polymer, it can absorb mechanical energy efficiently for mechanical stress reaction ( 5, 19). The mechanically activated thermal energy, in addition to rupture of main chains, may alter morphology or microstructure of cotton cellulose. Accordingly, the crystallinity and accessibility of cotton fiber may be influenced. 

Definition Isolated, colloidal crystalline portion of cellulose fibers partially depolymerized acid hydrolysis prod, of purified wood cellulose... 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

Synonyms Cellulose gel MCC Dehnition Isolated, colloidal crystalline portion of cellulose fibers partially depolymerized acid hydrolysis prod, of purified wood cellulose Properties Wh. fine cryst. powd., odorless partly sol. with swelling in dil. alkali insol. in water, dil. acids, and most org. soivs. bulk dens. 18-19 Ib/ft ref. index 1.55 pH 5-7 Toxicology LD50 (oral, rat) > 5 g/kg, no significant hazard irritant by inhalation (dust) may be damaging to lungs TSCA listed... 

After treatment with NaOH 2% (w/v), a weight loss of 17.93% was observed with respect to the initial weight of the fiber. Lignin and hemicellulose were partially removed from the fiber, promoting better packing of the cellulose chains. This leads to increased crystallinity of the fiber [1]. 

The use of cellulosic fibers in polymer composites has increased in the last decade. The reinforcing ability of natural fibers is governed by the nature of cellulose and its crystallinity. Natural fibers are generally lignocellulosic in nature [1], The main constituents of cellulosic (natural fibers) fibers are shown in Figure 12.1. 

---