Acid hydrolysis of cotton

Recently, Elazzouzi-Hafraoui et al. (2008) studied the size distribution of nanocellulose resulting from sutfuric acid hydrolysis of cotton treated with 65% sulfuric acid over 30 min at different temperatures, ranging from 45 to 72°C. By increasing the temperature, they demonstrated that shorter crystals were obtained however, no clear influence on the width of the crystal was revealed. 

The effect of ball milling on the dilute acid hydrolysis of cellulose is similar in many ways to the effect electron irradiation has on enhancing saccharification (18). In the case of cotton linters, for example, maximum irradiation lO d) resulted in a half life of hydrolysis of about 7 min... 

The relative influence of vibratory milling on the course of enzymatic and dilute acid hydrolysis of four cellulosic substrates was investigated. The four substrates—cotton linters, newsprint, Douglas fir, and red oak— were vacuum-dried and then milled for various time periods ranging up to 240 min. Assays were then made of rate and extent of hydrolysis, maximum yield of reducing sugar, and cellulose crystallinity. 

Vibratory milling also yields substantial increases in the rates of dilute acid hydrolysis of all four substrates nearly nine-fold for cotton linters and roughly five-fold for the three lignocelluloses. Increases in maximum sugar yields under simple batch conditions ranged from 60% to 140% over the yields for the unmilled materials. 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

Figure 6. Rate of homogeneous hydrolysis of cotton cellulose in 81.25% phosphoric acid (D), chromate-oxidized cellulose (O), and SBH-reduced cellulose (R). (Reproduced with permission from Ref. 24. Copyright 1961...

Figure 1.7 TEM micrographs of nanocrystals obtained by sulfuric acid hydrolysis of (a) cotton (b) avicel and (c-e) tunicate cellulose. The insets of (a) and (b) provide higher resolution images of some characteristic particles. Reprinted with permission from [47, 53]. Copyright 2010 John Wiley and 2008 American Chemical Society.

Hypochlorite solutions are generally less damaging to cotton than chlorine water, as a result of the avoidance of acid hydrolysis of the cellulose, but operation too close to neutrality can weaken the fabric due to excesive oxidation by the elevated HCIO concentrations that result as the pH falls. The effects of pH on the acid-base equilibria, stability of the bleach solutions, and cellulose degradation are discussed separately below. 

The ease of hydrolysis of a DMEU-treated fabric has been used to produce bicolored cotton fabrics. This was accompHshed by applying a thickened DMEU solution in a print configuration to the pile of fabric, curing the resin, and dyeing the fabric. The DMEU-treated areas resisted dyeing because of the cross-links. Subsequendy, the DMEU-crosslinks were removed via an acid hydrolysis and the entire fabric was overdyed to achieve the desired bicolored effect (69). 

Cellulose acetate [9004-35-7] is the most important organic ester because of its broad appHcation in fibers and plastics it is prepared in multi-ton quantities with degrees of substitution (DS) ranging from that of hydrolyzed, water-soluble monoacetates to those of fully substituted triacetate (Table 1). Soluble cellulose acetate was first prepared in 1865 by heating cotton and acetic anhydride at 180°C (1). Using sulfuric acid as a catalyst permitted preparation at lower temperatures (2), and later, partial hydrolysis of the triacetate gave an acetone-soluble cellulose acetate (3). The solubiUty of partially hydrolyzed (secondary) cellulose acetate in less expensive and less toxic solvents such as acetone aided substantially in its subsequent commercial development. 

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