Purification of Wood Cellulose FRED OLSEN Cellulose Research Corporation, Western Cartridge Company, East Alton, Ill.
The use of wood pulp instead of cotton for conversion to cellulose derivatives, especially the acetate, apparently introduces severe problems of purification. If the course of the conventional processes of bleaching and caustic treatment is extrapolated t o give higher purity, so much depolymerization may occur that the strength and other properties of the finished article may be seriously reduced. Wood cellulose has been prepared from conifers such as spruce, western hemlock, etc. s and from such deciduous woods as maple, birch, aspen, cottonwood, etc. , which compares favorably with cotton. An alpha-cellulose content of 99+ per cent, pentosans of less than 1.5 per cent,
A
LTHOUGH wood cellulose is being used to a very large extent in various esterification processes, cotton has long remained a standard against which the purity and reactivity of wood celluloses can be compared. I n the case of the xanthation processes resulting in viscose rayon and the cellophane type of sheeting, wood pulp constitutes the major proportion of the cellulose employed. Although wood cellulose has been utilized in experimental and even to a limited extent in commercial applications in nitration, most of the nitrocellulose manufactured in this country for use in smokeless powder or in lacquers is still made from cotton. Cotton linters today constitutes the sole commercial cellulosic raw material for acetylation. This is true in spite of the fact that numerous experiments have been conducted and a large number of patents have been published disclosing methods of preparing cellulose acetate from wood cellulose. Wood cellulose has failed to meet the commercial requirements in acetylation mainly because of lack of reactivity. It is not intended to convey the idea that wood cellulose will not acetylate, although it has been a frequent experience that this is almost literally the case with many samples of commercially available wood pulps. It has often been found that, if wood cellulose is subjected to the acetylation technic that results in acetic acid dopes of good color and haze characteristics when cotton linters are used as the base, a mass of unacetylated or only partially acetylated fibers is formed which is useless for conversion into sheeting, yarn, or plastics.
and a loss in 7.14 per cent sodium hydroxide of less than 2 per cent have been obtained. A t the same time the viscosity can be held, if desired, at very high levels, even higher than that of any of the available samples of purified cotton linters. The reactivity of the purified wood cellulose is very high, as indicated by the rapidity and smoothness of the acetylating reaction and the clarity and good color of the resulting acetate dopes. The topochemical character of the reactions with cellulose make it imperative that thoroughly uniform contact of the solid phase with the liquid reactants be provided. This involves particular emphasis on permeability of the cellulose.
equal that of cotton linters. Increased amounts of catalyst are frequently employed with various types of pretreatment, sometimes a t elevated temperatures. These devices usually give rise to an acetylation reaction which proceeds much more smoothly than is encountered with the wood pulps of lesser purity or with less drastic pretreatments of catalysts. However, in spite of these features, the quality of the resulting cellulose acetate has not been such as to fulfill the commercial requirements for sheeting, yarn, or the like. The price that has been paid in effecting additional purification has almost invariably been that of greatly increased degradation of the cellulose-more specifically, the depolymerization which accompanies the drastic purification processes. Similarly, the action of the increased amounts, of catalyst or the more extended pretreatments perhaps a t higher temperatures result in degradation of the cellulose acetate, which is usually evidenced by yarn of poor strength or nonuniform dyeing character, and by brittle sheeting. A comparison of the ordinary chemical constants of cotton linters and of various wood celluloses may serve to d e h e more clearly the relationships which are under consideration :
Cellulose
or-Cellulose
%
1. Cottonlinters 98.5 2. Bleached sulfite Pulp 85-88 3. High or-cellulose 93-96 PdPS 4. Acetylatable wood pulp from Cellulose Researahcorp. >98.5
Purification Problems The patent art has recognized this condition, and there are frequent references to devices for activating the wood pulp. Sometimes these methods call for extended purification treatment in which the alpha-cellulose has been carried up to values higher than 96 per cent, but rarely, if ever, do they
Soln. in Boiling 7% PentoNaOH san
Ash
Permanganate No.
Viscosity i n 2.5% Concn. Csntipoises
0.25
500-10000
0.5 0.5
50-100 50-300
%
%
