PROPERTIES AND USES OF CELL WALLS 



93 



Native celluloses have a degree of polymerization (DP) as high 

 as 100,000 (MW ca. 20,000,000), whereas regenerated cellulose 

 gives DP 600 or more, MW 100,000 or more. When cellulose is 

 treated with acetic acid-acetic anhydride and a catalyst (H 2 S0 4 ,/ 

 ZnCl 2 ), the acetylated product may be formed into bulk or sheet 

 plastics (MW 80,000) or fibers {MW > 100,000). Acetate deri- 

 vatives are of comparatively low crystallinity. As plastics, their 

 tensile strength is low, 2-6 kg/mm 2 but the fibers ("acetate rayon") 

 may range up to 75 kg/mm 2 . Cellulose acetate is noted for its 

 resistance to sunlight, humidity, weathering and biological attack. 



The alkylation of celluloses to their ethers yields a series of 

 compounds of considerable interest. Alkylation is carried out by 

 autoclaving alkali cellulose (or cellulose + NaOH) with alkyl 

 sulfates or chlorides. Solubility and other properties vary with 

 the akyl group; the ratio of ether to free OH; chain length (number 

 average molecular weight); and uniformity. In the ethyl celluloses, 

 for example, alkali-solubility occurs when the ethoxy content 

 lies between 5 and 15 per cent; water solubility between 20 and 

 30 per cent; and organic solvent solubility between 40 and 50 per 

 cent. Various viscosity grades of water-soluble methyl celluloses 

 have been introduced commercially for use as water soluble fibers 

 and films, as adhesives and sizing agents and as stabilizing agents 

 in place of natural substances such as aga^. The methyl celluloses 

 exhibit the interesting physical property of thermogel formation. 

 When dilute (for example, 2 per cent) solutions are heated, a gel 

 is reversibly formed at temperatures of 50-60°C. Salts and organic 

 additives lower the gel-point. 



Under dehydrating conditions, cellulose reacts with nitric acid 

 to yield several nitroesters. The principal types and their uses 

 include plastics, fibers (low tensile strength), explosives, etc: 



Representative 

 ester 



% Nitrogen 



Molecular wt. 



Uses 



Mononitrate 

 Dinitrate 



Trinitrate 



10.5-11.5 



11.5-12 



12-14 



50,000 

 150,000 



250,000 



Plastics, lacquers 

 Lacquers, films, 



fibers 

 Explosives 



