US2420499A - Process of esterifying cellulose fibers - Google Patents

Description

Patented May 13, 1947 UNITED STATES PATENT OFFICE PROCESS OF ESTERIFYING CELLULOSE FIBERS No Drawing. Application February 5, 1943, Serial No. 474,894. In Switzerland January 14, 1942 7 Claims. 1

The esterification of regenerated cellulose with anhydrides of organic acids with retention of structure is known; in this connection, compare, among others, the U. S. Patents Nos. 1,922,287, 1,930,895 and 2,103,018 and the French Patents Nos. 755,267, 758,185, 770,806 and 783,303 as well as the Swiss Patent 157,681.

In the technical execution of this esterification on a large scale difliculties have previously been encountered on account of the lack of a suitable continuously working process. Naturally, a process of this nature must be characterized by a reaction time which is as short as possible and must work as rationally as possible in regard to the quantity of esterifying agent consumed. However, a short reaction time necessitates working at high temperatures; but if esterifying baths are exposed for a long period to high temperatures in the presence of cellulose and an esterifying catalyst, they will become unusable in a comparatively short space of time, due to discolouration, accumulation of impurities, and alteration of the composition. A continual replacement of the esterifying bath is therefore necessary. From this cause, a large volume of used baths accumulates which must be regenerated; this implies a heavy economic burden for the process.

If, in order "to avoid the difliculties mentioned, work is carried out using esterifying agents in the gaseous state, other and no less serious difiiculties are met with. The esterifying reaction is an exothermic process and, when working in the gaseous phase-in contrast to esterifying in a liquid medium-local overheatings easily take place, causing irregularities -in the material. Furthermore, the boiling points of the organic acid anhydrides coming in question are very high, so that the work must be carried out under reduced pressure in order to avoid having to carry out the esterification at exaggeratedly high temperatures. It is unnecessary to explain that extraordinary difficulties would be met with if a continuous process had to be carried out under reduced pressure.

It has now been found that it is possible to carry out the esterification in a short time, using small quantities of esterifying agent, if regenerated cellulose (in continuous filament form or in form of staple fibres) which is loaded with a suitable catalyst be impregnated or otherwise loaded with a liquid esterifying agent and exposed to the action of hot gases which are in a state of motion. The hot gases are preferably circulated in a closed apparatus, so that the esterification is carried out in a gas circulation which is saturated with the esterifying agent. It has been ascertained--and this was not to be foreseenthat, by this method of working, an extensive and surprisingly regular esterification of the regenerated cellulose takes place in a short time, and this without any of the disadvantages being met with which would otherwise occur when working with gaseous esterifying agents.

In order to be able to carry out the esterification within the required short period of reaction it is preferable that a comparatively high temperature, for example, somewhat in the neighbourhood of C. or higher, should be chosen. By a short time is to be understood a period which is shorter than 30 minutes, preferably shorter than ten minutes, since for longer periods of time the size of the continuous apparatus would become too great.

The quantity of esterifying agent used doesv not need to be great; in general, the quantity of acid anhydride which remains in the regenerated cellulose after impregnation with the liquid anhydride and squeezing is sufiicient. Loading with liquid esterifying agent can take place by impregnation, irrigation or spraying, or in any other manner.

The incorporation of the catalyst in the regenerated cellulose takes place before the esterification. As catalysts, those mentioned in the patents listed above may be employed; especially suitable for an esterification at high temperature are alkali metal salts of weak acids, such as lithium, sodium and, particularly, potassium acetate. The regenerated cellulose can be impregnated with an aqueous solution of these salts and the water removed by drying. Other catalysts and other methods for their incorporation into the regenerated cellulose can also be chosen. When using a catalyst which is dissolved in water or in a solvent containing hydroxyl groups, the solvent must be removed before the esterification, either by drying or by displacement by a suitable liquid, so that no large quantities of anhydride will be used up by reaction with the solvent.

A particularly advantageous form of the process is that in which the excess acid anhydride, i. e., the anhydride which is not'used in the esterifying reaction, is separated by cooling and thus recovered at a suitable point in the circulatory system. The continuous esterification can be carried out according to the present invention on all types of structures consisting of dissolved and reprecipitated cellulose materials, no matter whether these be continuous structures of filaments, threads, woven materials, knitted materials, films or discontinuous structures, for example, staple fibre in the form of flocks. In the last case, the loose material must be conveyed through the continuous apparatus by means of a conveyor belt or other suitable arrangement. As diSSOlVed and reprecipitated cel lulose materials there come into consideration first of all regenerated cellulose, such as viscose, cuprammonium and Chardonnet structures; but there can also be used as parent material dissolved and reprecipitated cellulose derivatives which contain reactive hy'droxyl groups, 'e. g., incompletely esterified or etherified cellulose, 'for example dissolved and reprecipitated partially acetylated or low methylated cellulose.

As esterifying agents, use may be made of-ac'id anhydrides of organic acids, such as acetic anhydride, propionic anhydride, butyric anhydride, or of mixed anhydrides, such as acetic-butyric anhydride, or of mixtures of anhydrides. They can be used dissolved in a suitable solvent, preferably however, undiluted.

As a gas which is caused to act in a hot state on the material impregnated with a catalyst and an organic acid anhydride, air is preferred above all; however, there may also be used other gases, especially iridifierent gases, such as, for example, nitrogen.

The following examples illustrate the new process but are not to be regarded as limiting it in any way:

Example 1 Viscose solution is spun in known manner on a staple fibre apparatus to form a rope of parallel viscose filaments, and this latter, before drying, is passed through a solution of potassium acetate of 40 per cent. strength. The rope, impieg'n'ated with potassium acetate and dried, is led through a vessel containing undiluted acetic animate; on leaving the vessel it passes between two adjustable squeezing rollers, the pressure of the rollers being adjusted to the desired degree of esterification and to other conditions of the esterifying process. The rope, loaded with acetic anhydride, passes from the squeezing rollers into the interior of a closed apparatus, in which it is led through a series of driven pairs of rollers. During this passage it is exposed to the-action of hot air which is circulated through the apparatus by means of a fan. In this section of the apparatus the actual esterification takes place. The time taken by the esterification reaction and thus the speed of passage of the filaments is dependent on the temperature of reaction and can amount, for example, to 3 minutes at 110 C. or to 6 minutes at 100 C. In 'a further section of the apparatus the filamerits meet with a new air circulation, in the path of which are to be found a cooler and an air heater. In this section of the apparatus the excess acetic anhydride, which is still being carried by the rope of filaments is removed by the hot air and is separated in the cooler. cooled air is again heated in the air heater and is again blown on to the filaments by means of the fan, preferably in the 'form of a countercurrent. Apart from the recovery of the excess acetic anhydride, a certain portion of the esterification can also take place in this section of the apparatus.

The esterified rope of filaments, which still contains potassium acetate and a'certain amount or acetic acid (formed during the esterifying The reaction) combined as acid potassium acetate, is washed, after leaving the esterifying apparatus, in a series of successive baths with water, the counter-current principle being adopted, after which it is finished and dried. In this manneifl-a continuous rope of filaments, consisting no longer of regenerated cellulose but of acetyl cellulose, is obtained. The filaments can be cut to staple fibre if desired at any stage of the process, for example, directly before the drying or just as they leave the esterification apparatu s.

In place of acetic anhydride, use may be made of butyric anhydride or of the mixed aceticbutyric anhydride in an analogous manner, allowance being made for the reduced reactivity of the anhydrides mentioned by increasin the temperature of reaction, extending the reaction time or addition of an accelerator, such as pyridine.

Example 2 Work is carried out as described in Example 1, the only difference being that a woven ribbon of rayon is esterified in place of the rope of filaments. The ribbon mustbe Woven lighter than usual, in order that it will not become too dense on account of the increase in volume of the individual fibres, caused by the esterification.

Example 3 Work is carried out as described in Example 1, with the difference that, in place of the rope of filaments, staple fibre consisting of regenerated cellulose in the already cut state, that is, in the form of flocks, is Subjected to the treatment. The staple fibre, impregnated with potassium acetate and dried, is carried in a loose layer on a conveyor belt; it is sprayed with acetic armydride, the excess being removed by suetioii or by squeezing, and is treated in the esterification apparatus with hot air in a state of motion exactly as described in Example 1. In place of several vertically arranged pairs of rollers, horizontal conveyor bands, arranged one above the other and running in alternating opposite directiohs, are used to convey the staple fibre. The flocks, on reaching the end of one conveyor band, fall on to the band below and are thereby turned over, which ensures a regular course of the es'terifylng reaction. After removal and recovery of the excess acetic anhydride, the esterified fibres are scrooped and dried in a manner analogous to that described in Example 1.

What I claim is:

1. In the process for the continuous esterification of dissolved and repr'ecipitated cellulose ma terial with retention er its structure to produce at least monoacylat'e'd cellulose esters which are insoluble in organic solvents by impregnating the material with a solution of a catalyst, removing the solvent, impregnatin with a liquid anhydrld'e of an organic acid containing at the most 4 carbon atoms, and heating to effect the 'acylation, the special method of working which comprises of said liquid anhydride followin the impregna tion with the latter and prior to the heating step, and subjecting the separated and still unac lated impregnated material to the action of a hot circulating gas, which is inert to the desired re action, at a temperature of at least about C., unti1the desired acylation is efiected.

2. in the process for thecontinuous esterification of dissolvedand reprecipitated cellulose material with retention of its structure to produceat least monoacylated cellulose esters which are insoluble in organic solvents by impregnating the material with a solution of a catalyst, removing the solvent, impregnating with a liquid anhydride of an organic acid containing at the most 4 carbon atoms, and heating to effect the acylation, the special method of working which comprises separating the impregnated material from excess of said liquid anhydride following the impregnation with the latter and prior to the heating step, and subjecting the separated and still unacylated impregnated material to the action of a hot circulating gas, which is inert to the desired reaction, at a temperature of about 100-110 C. for a period of less than 30 minutes, until the desired acylation is effected.

3. In the process for the continuous esterification of regenerated cellulose in continuous filament form with retention of its structure to produce at least monoacylated cellulose esters which are insoluble in organic solvents by impregnating the material with a solution of a catalyst, removing the solvent, impregnating with a liquid anhydride of an organic acid containing at the most 4 carbon atoms, and heating to effect the acylation, the special meth'od of working which comprises separating the impregnated material from excess of said liquid anhydride following the impregnation with the latter and prior to the heating step, and subjecting the separated and still unacylated impregnated material to the action of a hot circulating gas, which is inert to the desired reaction, at a temperature of about 1001l0 C. for a period of less than 30 minutes, until the desired acylation is effected.

4. In the process for the continuous esterification of viscose in the form of staple fibers with retention of its structure to produce at least monoacylated cellulose esters which are insoluble in organic solvents by impregnating the material with a solution of a catalyst, removing the solvent, impregnating with a liquid anhydride of an organic acid containing at the most 4 carbon atoms, and heating to effect the acylation, the special method of working which comprises separating the impregnated material from excess of said liquid anhydride following the impregnation with the latter and prior to the heating step, and subjecting the separated and still unacylated impregnated material to the action of a hot circulating gas, which is inert to the desired reaction, at a temperature of about IOU- C. for a period of less than 30 minutes, until the desired acylation is efiected.

5. The process according to claim 1, wherein the catalyst is an alkali metal salt of a weak acid.

6. The process according to claim 1, wherein the catalyst is an alkali metal salt of a weak aliphatic acid and the anhydride is acetic anhydride.

7. The process according to claim 1, wherein the catalyst is potassium acetate and the anhydride is acetic anhydride.

ANDREAS RUPERTI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,930,895 Haller et al Oct. 17, 1933 1,958,315 Rheiner May 8, 1934 2,103,018 Ruperti Dec, 21, 1937 2,172,475 Kaase et a1 Sept. 12, 1939 1,861,320 Rheiner May 31, 1932 2,270,658 Linnhoff Jan. 20, 1942 2,343,920 Maxwell Mar. 14, 1944 224,330 Cross et al Sept. 28, 1907 FOREIGN PATENTS Number Country Date 5,016 Great Britain 1907 419,918 Great Britain Nov. 21, 1934 540,790 Great Britain Oct. 30, 1941 622,304 France May 28, 1927