Hydrocellulose degradation

Figure 3 Effect of alkali concentration on the extent of hydrocellulose degradation at 120°C for 1 h. (From Ref. 108.)...

Cellulose may be degraded by a number of environments. For example, acid-catalysed hydrolytic degradation will eventually lead to glucose by rupture of the l,4-(3-glucosidic linkages. Intermediate products may also be obtained for which the general term hydrocellulose has been given. 

Use of low viscosity cellulose. Cellulose which has been degraded by acids or by other means may be used as a starting material. Hydrocellulose and cellulose regenerated from viscose have been used. 

Partially degraded cellulose is called hydrocellulose or oxycellulose, depending on the agent used for degradation. The term holocellulose is used to describe the residue after lignin has been removed from wood pulp. 

Degradation of cellulose. In contact with hydrolysing or oxydizing agents, cellulose undergoes hydrolysis, or oxidation accompanied by hydrolysis, to form hydrocellulose or oxycellulose. 

Nitrocellulose with a decreased molecular weight may be obtained as the result of depolymerization (degradation) of the cellulose before nitration, e.g. by keeping it at a temperature of 150-170°C or by treating it with acids. The resultant hydrocellulose, which usually possesses a lower molecular weight than cellulose, is then subjected to nitration to produce a more soluble substance as compared with the nitration product of a non-depolymerized raw material. 

In his introduction to the Lehrbuch, Heuser states objectively Even the methods of treatment adopted by Schwalbe and by Heuser seem to be fundamentally unprofitable, because they do not lead to definite results. These authors have divided the subject along the lines of the various derivatives of cellulose, such as cellulose hydrate, hydrocellulose, oxycellulose, etc. Such discussions are of little value because the substances to which the above names have been applied are not homogeneous chemical individuals, but are mixtures of the most varied degradation products of cellulose and may react very differently under apparently similar conditions. Cross and Bevan have also made the same unfruitful and dangerous mistake of trying to build a system of cellulose chemistry on the basis of such mixtures.. . ... 

When cotton is treated with very dilute solutions of hydrogen chloride in an aprotic solvent such as benzene it suffers severe degradation. This is because the small amount of hydrogen chloride in the solvent is redistributed in the water adsorbed on the cotton, forming a very concentrated aqueous solution of hydrochloric acid [454]. At low moisture contents, the sites of the consequent hydrolysis are near the ends of the cellulose chains. The relation between DP and copper number therefore differs from that for normal aqueous hydrolysis. However, as the moisture content of the cotton increases, the type of hydrocellulose produced approaches that found with aqueous systems. 

Accessibility. Many reports [98-103] indicated that when hydrocellulose was treated with an alkali the stable residues still contained noticeable amounts of reducing endgroups [94,101-103]. This phenomenon was ascribed to a physical stopping process [100] when a degrading end reached a crystalline region inaccessible to the alkali. 

Such recombination studies in our laboratory and others 17, 48, 54) have shown clearly the multi-enzymic character of the complete cellulase system. When recombined with fraction I enzymes, fraction II or fraction III enzymes from chromatography of the water fraction proteins on DEAE-Sephadex caused the degradation of hydrocellulose. 

In 1972 Ogawa and Toyama (56) purified three components— A-I-a, A-I-b, and A-II-1—which were adsorbed on a gauze column during purification from Cellulase Onozuka P1500, a commercial preparation of T. viride cellulase. These three components had molecular weights of 32,000, 48,000, and 48,000 as determined by gel filtration and contained 7-16% carbohydrate. Each is reported to carry out the random hydrolysis of CM-cellulose and to degrade hydrocellulose (Avicel) and cellooligosaccharides except for cellobiose. The order of reactivity toward either cotton or Avicel was A-II-1 > A-I-b > A-I-a. The proteins adsorbed on cellulose comprised 38% of the total cellulase protein. 

Methylene Blue test can also be used to differentiate between oxycellulose and hydrocellulose. Methylene Blue is a cationic dye. Standard cellulose generally has no affinity for Methylene Blue, but oxycellulose with the formation of carboxyl groups confer an affinity and can be sorbed onto cotton. For this purpose. Methylene Blue absorption tests are carried out both at pH of 7.0 and at a pH of 2.7 (acidic). Two pieces of fabrics to be tested are taken and treated one with Methylene Blue at a pH of 2.7 and the other at pH 7.0. If oxycellulose is present, the material will absorb less dye in the acid than in the neutral solution, whilst reverse is the case if hydrocellulose is present. The degree of staining will indicate the extent of degradation. 

This qualitative test is also referred to as Harrison s test since he was the first to describe it in 1912. The test specimen is either boiled or padded with a reagent containing a mixture of silver nitrate (1%), sodium thiosulphate (4%) and sodium hydroxide (4%) and then steamed. Those parts where degradation takes place due to oxycellulose or hydrocellulose in a fabric will be stained black or dark grey due to the formation of silver by reduction. 

Fehling s solution can also be used as a qualitative test for the presence of aldehyde groups due to either hydrocellulose or osycellulose formation. AA hen cotton is boiled gently with the I ehling s solution for about 10 minutes a red deposit of cuprous oxide can be observed either in local patches or as a uniform stain, according to the distribution of the degradation. 

Hydrocellulose, as expected, is degraded more rapidly in alkaline solutions, and, likewise, prehydrolysis of wood pulps accelerates and increases their alkali absorption during pulping. Loss of weight from the boiling of hydrocelluloses with dilute alkaline solutions can be greatly diminished by prereduction with sodium borohydride. ... 

Figure 7. Relationship of the rate of degradation of crystalline hydrocellulose particles to their surface area (5)...

In the present study, the role of cellulose physical structure in alkaline reactions was investigated by comparing the alkaline degradation of highly crystalline (cellulose I) fibrous hydrocellulose with that of amorphous (noncrystalline) hydrocellulose. The amorphous substrate was taken as a cellulose model the reactivity of which would most closely approximate that of alkali-soluble cellulose. The availablity of such an approximation to the inherent reactivity of cellulose allowed evaluation of the effects of the more highly ordered structure of the fibrous hydrocellulose. 

Experimental Approach. The experimental study was a comparison of the alkaline degradations of fibrous and amorphous hydrocelluloses in oxygen-free 1.0 NaOH, at 60 and 80 C. The fibrous hydrocellulose was predominantly crystalline (cellulose I) and therefore served as a substrate which would undergo alkaline reactions with significant physical structure effects. In contrast, the amorphous hydrocellulose was noncrystalline (9,10). Thus, it was a substrate which would experience substantially less structural constraint during its alkaline reactions. 

During the course of the alkaline degradations, both physical and chemical structures of the hydrocelluloses were monitored. Hydroxyl accessibility (13) was determined as a practical measure of the fraction of molecules accessible to the alkaline medium. The crystalline structure was characterized by x-ray diffraction (14). 

Alkaline Degradations - Change in Physical Structure. The hydroxyl accessibility of the fibrous hydrocellulose was initially 51.4 0.8%. In contrast, the amorphous substrate had an accessibility of 99.2 1.0%. Exposure of the fibrous hydrocellulose to the alkaline media caused the accessibility to decrease slightly to 50.7 1.0% and 49.1 1.2% at 60 and 80°C, respectively, but accessibility did not change significantly during the reaction periods (0-168 hr). 

---