US1941154A - Process of reducing the solution viscosity of cellulose fiber - Google Patents

Description

Fatenteci Dec, 26, 1933 IPIRQCESS @F REDUCHNG THE SEPlLUTItON VKSCGSK'ILYI F GELLUILQSE F$1ER Maine No Drawing. Application September 0, 1930 Serial No. 480,199

0 Claims,

This invention relates to a process of reducing the solution viscosity of cellulose fiber, its object being to eiiect such a reduction in solution viscosity without degrading the alpha cellulose con- 0 tent of the fiber into lower forms of cellulose,

which when converted along with the alpha cellulose content of the fiber into cellulose derivatives contaminate the derivatives and impair the quality of products, such as silk, lacquers, filaments, or the like, prepared therefrom.

In some branches of the cellulose derivative industry, it is desirable that the cellulose fiber used as a raw material be of low solution viscosity, by which it is meant that cellulose derivatives prepared from the fiber, such as the nitrate and xanthate, be soluble in the usual solvent media to form solutions of low viscosity. It is knownthat acids, even when dilute, are effective as agencies for reducing the solution viscosity of cellulose fiber, but the treatment of cellulose fiber therewith is accompanied by a marked reduction in the alpha cellulose content of the fiber. This is particularly true in the case of cellulose fibers of high alpha cellulose content, such as cotton or refined wood pulps, as such fibers are sensitive to degradation by the hydrolyzing action of acid reagents,-far more so than the usual commercial wood pulps, such as kraft or sulphite, whose nonalpha cellulose components "are more readily reacted upon and hydrolyzed than its alpha cellulose components. Even when comparatively mild acids in dilute aqueous solution are used to lower the solution viscosity of cellulose fiber, the desired efiect is realized only at a sacrifice of alpha cellulose. For instance, when a refined wood pulp having an alpha cellulose content of 94.8% and a solution viscosity of 5.3 is digested for three hours in a 0.5% sulphurous acid solution at 100 0., the resulting product has a solution viscosity of 0.43,

91%. Hence, while a marked decrease in solution viscosity has been brought about, it is coupled with a drastic cut in alpha cellulose content of the fiber. If the temperature of digestion is raised to 125 0., reduction in solution viscosity is enhanced, but so too the alpha cellulose content of the fiber is brought down to below 90%. It might be reasoned that the alpha cellulose content of the fiber would be preserved while at the same time causing the desired lowering of the solution viscosity of the fiber by using much weaker sulphurous acid solution, but this is not the case. In fact, the degradation of fiber ensuing from the use of a 0.1% sulphurous acid solution is practically as bad as when much stronger sulbut its alpha cellulose content is brought down to (G l. se n) phurous acid solutions, for instance of 0.5% strength, are employed. It the temperature of digestion in sulphurous acid solutions be raised to above 100 C., in order to attain viscosities as low as 0.5 or lower, the degradation of the fiber by hydrolysismay be so marked that the resulting product may be more or less crumbly or pulverulent.

In an attempt to do away with the hydrolyzing effect of the sulphurous acid solution, I have gone so far as to use water alone as the agent for lowering the solution viscosity of cellulose fiber.

- But a refined wood pulp having the characteristics hereinbefore given, when digested in water alone at 100 0., undergoes little change in its characteristics; and if the water is raised to a temperature of 150 C. and the pulp is kept in suspension therein for four hours, the desirable low solution viscosity value of 0.3 is reached in the product only to find that its alpha cellulose content is 90%. The tumbling of the alpha cellulose content of the fiber to the extent of almost 5% is doubtless traceable to the hydrolyzing effect of the organic acids generated by the action or" the Water at 150 C. on the less-resistant components of the fiber.

. I have discovered that the hydrolyzing effect of a sulphurous acid solution. on cellulose fiber may be checked suificiently to avoid a significant reduction in the alpha cellulose content of the fiber by the presence in the solution of a suitable amount of a sulphite, for instance oi calcium, magnesium, potassium, sodium, or the like. I have further discovered that in order to attain the desired lowering of the solution viscosity by the use of such sulphurous acid solutions containing a suitable amount of sulphite or mixture or sulphites in a reasonable period of time, it is necessary that digestion of the fiber insuch solution be carried out at above about 150 C. By the presence of a suitable amount of sulphite in the solution, I mean that the solution should con tain no more free than combined SO22 in other words, the amount of S02 which responds to neutralization by an alkali, such as caustic soda, should not exceed the amount of S02 which is combined in the form of the neutral sulphite, for when there is free S02 in excess of combined S02 in the solution, there is a marked tendency toward degradation of the fiber by hydrolysis, which is evidently caused by such excess free S02. The limiting or highest value of free S02 which I employ in the solution is that corresponding to a bisulphite solution, in which the free S02 is present in amount equal to the combined S02.

When solutions containing both free and combined S02 and falling within this limiting definition are employed, the free S02 content of the solution may be much higher than 0.1% without materially injuring the fiber either physically or chemically. From the foregoing discoveries, it appears that the hydrogen ion is what causes the hydrolysis of the fiber, and further that the hydrogen ion. furnished by the bisulphite radical is far less effective as a hydrolyzing agent than is the first hydrogen ion furnished by sulphurous acid, perhaps by reason of the fact that ionization of the bisulphite radical to generate hydrogen ion takes place to a much less extent than ionization of sulphurous acid to generate hydrogen ion and the bisulphite radical or ion.

I shall now give examples of procedure falling within the purview of the present invention, using a bleached, refined wood pulp especially sensitive to hydrolysis as a raw material. The refined wood pulp which was of the kind hereinbefore described, had an alpha cellulose content of 94.8% and a solution viscosity of 5.3. It was digested for three hours at 175 C. in a solution containing 0.5% free and 0.5% combined S02 in the form of sodium bisulphite. The resulting product, when washed, had an alpha cellulose content of 94.3% and a solution viscosity of 0.7, and bore no visible signs of having been severely attacked, such as loss of fiber structure. When a step of digestion was carried out under similar conditions' but using a solution containing 0.5% free S02 and 1% combined S02 in the form of a mixture of sodium bisulphite and sodium sulphite, the digested, washed product had an alpha cellulose content of 95% and a solution viscosity of 0.7. It is thus seen that the presence of more combined than free S02 in the solution serves to preserve or even to increase somewhat the alpha cellulose content of the fiber, evidently because of the action of the sulphite in removing residual lignin or other non-alpha cellulose components from the fiber.

A solution containing combined S02 alone in the form of sulphite is capable of preserving, or even increasing, the alpha cellulose content of the fiber, but it is not especially effective as an agent for reducing the solution viscosity of the fiber at temperatures as high as 175 C. Thus, when the exemplary procedure given was repeated but using an 0.5% sodium sulphite solution, it was found that the washed product, while it had an alpha cellulose content of 96, also had the still high solution viscosity of 2.85. It is thus seen that the conjoint use of both free and combined S02 in the solution is far more effective than the use of combined S02 alone at a given elevated temperature in reducing the solution viscosity of cellulose fiber, although the refining action of a straight sulphite solution (i. e., one lacking a free S02 content), coupled with its solution-viscositylowering effect on cellulose fiber more particularly at temperatures above 175 0., is especially advantageous in connection with unrefined pulps which ofier considerable opportunity for increase in alpha cellulose content. I do not, however, herein claim the treatment of cellulose pulps with straight sulphite solutions at temperatures above 175 C., as this is disclosed and claimed in my application Serial No. 480,198, filed September 6, 1930.

I claim:

1. A process which comprises digesting cellulosefiber of the character of cotton and preliberated wood pulp at temperatures above 150 C. in a sulphurous acid solution of a sulphite containing no more free than combined S02.

2. A process which comprises digesting cellulose fiberof the character of cotton and preliberated wood pulp at temperatures above 150 C. in a sulphurous acid solution of a sulphite containing less free than combined S02.

3. A process which comprises digesting a refined wood pulp of high alpha cellulose content at temperatures above 150 C. in a sulphurous acid solution of a sulphite containingno more free than combined S02 for at least about three hours, whereby the solution viscosity of said pulp is reduced to only a small fraction of its original solution viscosity, whereas the alpha cellulose content of said pulp is practically unaltered.

4. A process which comprises subjecting preliberated chemical Wood pulp at a temperature of about 175 C. to the action of an acid sulphite solution whose acid content available for neutralization is not in excess of that forming the bisulphite, thereby markedly reducing the solution viscosity of the pulp while substantially preserving its'alpha cellulose content.

5. A process which comprises digesting a white, chemical wood pulp of high alpha cellulose content at a temperature of about 175 C. in a sulphurous acid solution of sodium sulphite whose acid content available for neutralization is not in excess of that forming the sodium bisulphite, thereby markedly reducing the solution viscosity of the pulp while substantially preserving its high alpha cellulose content.

6.-A process which comprises subjecting preliberated chemical wood pulp at a temperature of about 175 C. to the action of an acid sulphite solution whose acid content available for neutralization ranges from 50% to 100% of that forming the bisulphite, thereby markedly reducing the solution viscosity of the pulp while substantially preserving its alpha cellulose content.

GEORGE A. RICHTER.