US2391569A - Process of treating cellulosic material - Google Patents

Description

Patented Dec. 25, 1945 PROCESS OF TREATING CELLULOSIC MATERIAL Clifford I. Haney and Mervin E. Martin, Cumberland, Md., assignors to Celanese Corporation of America, a corporation of Delaware No Drawing. Application January 20, 1942. Serial No. 427,440

19 Claims.

This invention relates to the treatment of cellulosic materials in fibrous form for the purpose of rendering them more suitable for esterification, especially acetylatlon, and also to the production of cellulose acetates or other cellulose esters from such treated materials.

An object of our invention is to treat cellulosic materials so as to render them more reactive to acetylation or other esteriflcation.

Another object of our invention is to obtain cellulose esters yielding solutions of improved clarity.

Still another object of our invention is the preparation of cellulose esters from said treated materials whereby the course of the reaction may be more readily controlled and the heat generated during the reaction dissipated or removed with ease and economy.

Other objects of our invention will appear from the following detailed description.

While our invention will be more particularly described in connection with the preparation of been proposed to pretreat the cellulosic materials employed so as to render said materials more reactive toesterlfying agents so that cellulose esters of improved properties might be obtained. In pretreating said cellulosic materials, various expedients have been employed. In some instances, the cellulosic materials. have been given a pretreatment with alkaline swelling agents to cause them to become more easily esterifiable and in other instances it has been proposed to pretreat said materials with organic acids, either in liquid or vapor form. These pretreatments have been of some value in increasing the reactivity of the cellulosic materials so treated, but the properties, such as clarity, of the resulting cellulose esters have not always beenwholly satis factory.

We now have discovered that cellulose esters yielding solutions of increased clarity, and which may be formed into yarns of improved properties. may be prepared from cellulosic materials by pretreating said cellulosic materials, prior to esteriflcation, with a mixture of lower aliphatic acids, one of which is formic acid, and having incorporated therein a small amount 01' a mineral I in accordance with our invention, cellulose aceta e 01' improved clarity may be prepared from cellulosic materials by pretreating said materials with an acid mixture containing formic acid, acetic acid and sulphuric acid, and then, after pretreatment, acetylating said cellulosic material with acetic anhydride in accordance with the usual procedure. Due to the reactivity of the materials treated in this way, the acetylation takes place rapidly and completely and the resulting cellulose acetate possesses unusually desirable characteristics.

In pretreating said cellulosic materials prior to esterification, various lower aliphatic acids may be employed in combination with formic acid and sulphuric acid to produce the desired reactivity in the cellulosic materials. While we preferably employ acetic acid in said pretreatment mixtures, particularly in pretreating prior to acetylation, other acids such as proplonic acid and butyric acid may likewise be employed.

The quantity as well as the composition of the acid mixture used in the pretreatment may be varied within rather wide limits. It has been found that if glacial acetic acid is employed in the pretreatment, excellent results may be obtained by employing 20 to 200 parts by weight of the glacial acetic acid to 100 parts by weight of cellulose. With the glacial acetic acid there may be employed from 1 up to 20 parts by weightof formic acid and from V: to 2 parts by weight of sulphuric acid. Preferably, we employ formic acid in amounts of from 1 to 10 parts by weight per 100 pads of cellulose, but the best results are obtained with from 1 to 5 parts by weightof formic acid per 100 parts of cellulose. Sulphuric acid is most preferably present in the pretreatment mixture in amounts of about 0.5 to 1 part per 100 parts of cellulose. While the acids employed may be anhydrous, we have found that the presence of small amounts of water is not harmful. Thus, we preferably employ formic acid of strength and acetic and sulphuric acids of a strength of from 98 to 100%, but more dilute acids may also, if desired, be emp yed.

The pretreatment is preferably carried out at room temperature, for example, at from 16 to 30 C. but higher or lower temperatures may also be employed. Thus, the pretreatment may be carried out at temperatures as low as 15 C. or-at temperatures as high as 50 C. The time of treatment maybe varied from A hour to 3 hours. depending on the concentration, amount and nature of the acid mixture employed as well as the temperature of treatment.

In applying the pretreatment of our invention, the acid mixture is distributed evenly over the cellulosic material as, for instance, by spraying, and the cellulosic material may be turned or tumbled from time to time if necessary to insure uniform impregnation. The material to which the acid mixture has been applied is then allowed to stand, for the time indicated, until a product having the desired degree of reactivity is obtained;

As cellulosic materials, we may employ, cotton cellulose or wood pulp, or similar fibrous cellulosic materials derived from various other sources such as, for example, bamboo, reeds, jute, esparto, straws, such as linseed and hempseed straw, grasses, peanut hulls, barks, etc.

The esteriflcation of the pretreated cellulosic materials may, if desired, be carried out in the usual way, that is, by adding a mixture of acid anhydride, an esterification catalyst and a solvent to the pretreated cellulosic materials, or entering the latter into a bath comprising such mixture, and allowing the esterification to proceed to any desired stage. The cellulose ester thus formed is then ripened, precipitated from solution, washed, and stabilized, if desired. This method of esterification, however, necessitates the use of intensive refrigeration since the esterification reaction is an exothermic one and substantial quantities of heat ar generated. For reasons of temperature control not only must the mixture containing the esterification agent be cooled substantially before the reaction but the reaction mass as well must be positively cooled during this reaction so that the resultant rise in temperature will not be sufficiently great to cause a partial degradation of the cellulose. This use of intensiverefrigeration is not only costly but frequently it is difllcult, in large scale esterification, to obtain satisfactory temperature control even with the use of refrigeration.

This difllculty may be overcome, however, and the esterlfication of the cellulosic materials carried out under precise temperature control if the fibrous cellulosic materials, activated as heretofore described, are esterified in a plurality of stages by the progressive addition of the acid anhydride to the pretreated cellulosic materials. Thus, partial esterification may be caused to take place in the pretreatment chamber by the addition of a portion of the acid anhydride needed for esterification to the activated cellulosic material in the presence of the sulphuric acid employed in the acid pretreatment mixture. The small amount of sulphuric acid present enables a partial esterification of the cellulosic material to take place without generation of a large amount of heat. The partially esterifled cellulosic material, which still retains its original fibrous form, may then be treated with the major portion of the esterification catalyst and the acid anhy-,

dride, and the esterification reaction then allowed to proceed to the desired degree. Since a substantial amount of the total heat of reaction is dissipated inithe first stage of the acetylation or other esterification in the pretreater,

the amount of heat of reaction generated in the;

main esterification stage is not so great and can be dissipated with relatively little difllculty. Since only a limited amount of catalyst is present when the initial addition of acid anhydride takes place in the pretreater, the resulting esterification reaction does not-proceed with undue rapidity and this, of course, serves to limit the amount of heat evolved per. unit time. The refrigeration system does not have to cope with the sudden generation of large quantities of heat, as is the case where all the anhydride is added in one step in the presence of the full amount of catalyst. Since no sudden large rise in temperature occurs, the temperature and course of the main esteriflcation reaction are always under precise control and, as a result, cellulose esters of improved characteristics can be obtained.

In the preparation of cellulose acetate, for example, by progressive esterification in accordance with our process, the cellulose materials to be acetylated preferably are first subjected to a pretreatment, as described, with a mixture of acetic acid, formic acid and sulphuric acid. After pretreatment, and without the removal of the pretreating acids, the activated cellulosic materials are then progressively acetylated by the addition of acetic anhydride thereto. In carrying out our novel acetylation process, as stated, only a limited portion of the total quantity of anhydride which is necessary to attain the desired final degree of acetylation is added in the pretreater or pretreatment chamber. The proportion of the anhydride which may be added for the initial esterification may be varied. Thus, for example, desirable results may be obtained when from 10 to 50% of the total quantity of acetic anhydride necessary for acetylation is added in the pretreater, but, preferably, we add from 30 to 50%. In general, this corresponds to from 'l'lto 130% based on the weight of the cellulosic materials being acetylated. The acetic anhydride, either alone or mixed with a diluent such as acetic acid, may be applied to the pretreated fibrous cellulosic material by spraying, sprinkling Or in any other manner. The cellulosic material may be tumbled or mixed during such application in order to insure a uniform distribution of the anhydrid liquid. The anhydride may be sprayed or sprinkled onto the pretreated cellulosic material continuously until the desired amount has been added or it may be sprinkled or sprayed in several increments at short intervals. When a diluent such as acetic acid is used, it may be present in amounts up to based on the anhydride,

Both th pretreated cellulosic materials and the acetic anhydride for the initial acetylation are preferably cooled prior to .the addition of the anhydride to the pretreater containing the cellulosic materials. Usually, satisfactory control of the temperature may be obtained when the pretreated cellulosic material is cooled in the pretreater to from 20 to 35 C. before the addition of the anhydride, and the anhydride solution cooled to from (minus) 10 to +15 C. before being added. The cooling of the pretreated cellulosic materials in the pretreatment chamber may be accomplished by circulating water, cooled to about 10 C. through a cooling jacket surrounding the chamber. Cooling the cellulosic materials to below about 20' C, is usually unnecessary since adequate temperature control may usually be achieved even with such moderate cooling. The temperatur of the reacting cellulose and anhydride is preferably maintained at from 20 u to 40 c. during the first stage of the acetylation in the pretreater, which reactionmay usually be completed in from 30 to 60 minutes. The 'degree of acetylation in the pretreater may vary but is usually carried to the point where the cellulose material has an scetyl value of from 17 up to 35% (calculated as acetic acid). Preferably, the acetyl value is 26 to 30%.

The final acetylation of the partially acetylated fibrous cellulosic material may then be carried out by charging the latter into a suitable acetylizer containing an acetic acid solution of the remaining acetic anhydride to be added, together with the. desired amount of esterification catalyst, usually sulphuric acid. Other esterification catalysts such as perchloric acid or sulfoacetic acid may also be employed in the final acetylation. The mixture in the acetylizer is preferably cooled before the fibrous, partially acetylated cellulosic material is entered and the temperature to which it is cooled prior to charging may be from to +5 C. By cooling the esterification mixture prior to charging, the rise in temperature during the course of esterification reaction is suiiiciently counteracted so that the temperature rise is held to a minimum and further cooling of the acetylizer may be dispensed with. I

The esteriflcation may be so carried out that the resulting cellulose ester is obtained in solution, or, if desired, the cellulose ester may be obtained in suspension by carrying out the esterification in the presence of a relatively large proportion of a suitable diluent in which the cellulose ester is insoluble. In the case of cellulose acetate, diluents such as toluene, benzene or other aromatic hydrocarbons are suitable.

In order further to illustrate our invention, but without being limited thereto, the following examples are given:

Example I 700 parts by weight of cotton linters contained in a suitable chamber are pretreated by spraying said linters with an acid mixture containing 3.5 parts by weight of 98% sulphuric acid, 175 parts by weight of 99.8% acetic acid and 70 parts by weight of 85% formic acid. The linters are tumbled during the spraying operation to ensure uniform distribution and action of the liquid and the linters are then allowed to stand for 3 hours at a temperature of 22 C. The pretreatment causes the resulting linters to be highly reactive and they may then be esterifled, if desired, in the yzsyual manner to yield a product of unusual clar- More advantageously, however, the pretreated cellulosic materials are acetylated by the progressive addition of acetic anhydride as follows. The 700 parts of pretreated cotton linters containing the pretreating liquid are cooled by circulating water at a temperature C. through the cooling jacket of the pretreater. After the treated linters are cooled, a mixture of 700 parts by weight of 97% acetic anhydride and 455 parts by weight of 99.8% acetic acid is cooled to 10 C. and sprayed onto the linters in three approximately equal parts over a period of about 8 hours. During the addition of the cooled anhydride solution, the temperature rises to a maximum of 22 C. and after the anhydride has been completely added the temperature drops to 18 C. While the addition of each portion of the anhydride solution causes a further reaction with the cellulosic materials and hence some evolution of heat due to the exothermic character of the reaction, the fact that the anhydride solution is cooled prior to being added and that the degree of esterification is limited to a partial acetylation serves to conated cotton linters have an acetyl value of 28%, calculated as acetic acid.

After the addition of this portion of the acetic anhydride solution is completed, and the temperature remains constant, the partially acetylated cotton linters are ready for the final acetylation in an acetylizer. Before charging, there is added to the acetylizer 1,600 parts by weight of 99.8% acetic acid, 1,295 parts by weight of 97% acetic anhydride and 66.5 parts of 98% sulphuric acid, which mixture is cooled to 0.5 C. The partially acetylated cotton linters are then charged into the acetylizer and the acetylation reaction then proceeds with the reaction temperature reaching a peak of 37 C. after 1% hours with no additional cooling being necessary.

The resulting solution contains the primary cellulose acetate which may then be ripened as desired to any degree of solubility, e. g. acetone solubility, after which the cellulose acetate may then be precipitated, washed and dried. When spun from an acetone solution, this cellulose acetate gives yarns of superior tenacity.

Example II 700 parts by weight of cotton linters in a suitable chamber are pretreated by spraying said linters with an acid mixture containing 3.5 parts by weight of 98% sulphuric acid, 7 parts by weight of 85% formic acid and 245 parts by weight of 99.8% acetic acid. The acid mixture is uniformly distributed by tumbling the linters during the spraying and after all the acid mixture is added, the linters are allowed to stand for 8 hours at 25 C. The pretreated linters may then be acetylated, if desired, in the usual manner,.but preferably the acetylation is carried out progressively as follows:

The cotton linters, pretreated as above, are cooled to 19 C. and are then sprayed continuously with 735 parts by weight of acetic anhydride cooled to 3 C. No acetic acid is added with the anhydride. The temperature drops upon addition of the anhydride and then rises gradually to a maximum of 33.5 C. which is reached 35 minutes after all the acetic anhydride has been added, at whichpoint the temperature begins to drop. The resulting acetylated cellulose has an acetyl value of 17.7% (calculated as acetic acid). An acetylizer is then charged with a mixture of 1,085 parts by weight of 97% acetic anhydride, 2,185 parts by weight of 99.8% acetic acid and 94.5 parts by weight of 98% sulphuric acid, the mixture being cooled to 0 C. The cotton linters,

pretreated as above, are then charged into the acetylizer. After the acetylation has proceeded for 1% hours, an additional 100 parts-by weight of acetic anhydride are added to the reaction mixture to ensure complete acetylation of all the linters present. The temperature rises to a peak of 41 C. in about 1 hour and the acetylation is completed. about 2 hours after charging. The acetic acid solution of cellulose acetate is then ripened to acetone solubility, precipitated,

trol the temperature of the reaction, thus prewashed and dried. The resulting cellulose acetate is of very high clarity.

It is to be understood that'the foregoing detailed description is merely given by way of illus tration and that many variations may be made therein without departing from the spirit of our invention.

Haying described our invention, what we desire to secure by-Letters Patent is:

1. In a process for the manufacture of a lower fatty acid ester of cellulose ,with the aid of a lower fatty acid anhydride, the step which comprises pretreating a cellulosic material-in fibrous form with a medium comprising a mixture of lower aliphatic acids, one of which is formic acid, and having incorporated therein a small amount of a mineral acid esterification catalyst, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose.

2. In a process for the manufacture of a lower fatty acid ester of cellulose with the aid of alower fatty acid anhydride, the step which comprises pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, one of which is formic acid, and having incorporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose.

3. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the step which comprises pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of a mineral acid esterification catalyst, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose.

4. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the step which comprises pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose.

5. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the step which comprises pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, said mixture containing, on the weight of the cellulosic material, from 1 to 20% of formic acid, from 20 to 200% of acetic acid and from /2 to 2% of sulfuric acid.

6. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps which comprise pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, said mixture containing, on the weight of the cellulosic material, from 1 to 20% offorr'nic acid, from 20 to 200% of acetic acid and from /3 to 2% of sulfuric acid, and maintaining said pretreated cellulosic material at a temperature of from 16 to 50 C. for 30 to 180 minutes before effecting the acetylation of said cellulosic material.

7. In a process for the manufacture of a lower fatty acid ester of cellulose with the aid of a lower fatty acid anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, one of which is formic acid, and having incorporated therein a small amount of a mineral acid esterification catalyst, the formic acid 1 being present in an amount of from 1 to 20% by weight of the cellulose, mixing the pretreated cellulosic material with an organic acid anhydride under such conditions that the cellulosic material is partially esterified while retaining its fibrous form, and then continuing the esterification by mixing said partially esterified fibrous cellulosic material with an esterification mixture comprising said organic acid anhydride and esteriflcation catalyst.

8. In a process for the manufacture of a lower fatty acid ester of cellulose with the aid of a lower fatty acid anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, one of which is formic acid, and having inc'orporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing the pretreated cellulosic material with an organic acid anhydride under such conditions that said cellulosic material is partially esterified while retaining its fibrous form, and then continuing the esterification by mixing said partially esterified fibrous cellulosic material with an esterification mixture comprising said organic acid an hydride and sulfuric acid.

9. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating .a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of a mineral acid esterification catalyst, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing said pretreated cellulosic material with acetic anhydride under such conditions that said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and an esterification catalyst.

10. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing said pretreated cellulosic material with acetic anhydride under such conditions that said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

11. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing said pretreated cellulosic material with from '77 to of its weight of acetic anhydride whereby said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid,

12. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps which comprise pretreating a cellulosic ma-' terial in fibrous form with a medium comprising a mixture of formic acid and aceticacid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid in said pretreatment mixture being present in an amount equal to from 1 to 20% on the weight of the cellulosic material, mixing said pretreated cellulosic material with from 77 to 130% of its weight of acetic anhydride, whereby said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

13. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid in said pretreatment mixture being present in an amount equal to from 1 to 20% on the weight of the cellulosic material, mixing said pretreated cellulosic material with from 77 to 130% of its weight of acetic anhydride diluted with not more than an equal amount of acetic acid, whereby said cellulosic material is partially acetylated while still retaining its fibrous form,-and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

I said cellulosic material is partially acetylated while still retaining its fibrous form. and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

15. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of lower aliphatic acids, said mixture containing, on the weight of the cellulosic material, from 1 to 20% of formic acid, from 20 to 200% of acetic acid andfrom A to 2% of sulfuric acid, maintaining said cellulosic material at a temperature of from 16 to 50 C. for 30 to 180 minutes, mixing said pretreated cellulosic material with from 77 to 130% of its weight of acetic anhydride whereby said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

16. In-a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the

steps of pretreating a cellulosic material in fibrous form with a medium comprising amixture of.

lower aliphatic acids, said mixture containing. on

the weight of the cellulosic material, from 1 to and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising ace tic anhydride and sulfuric acid.

17. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising a mixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of a mineral acid esterification catalyst, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing said pretreated cellulosic material with acetic anhydride diluted with not more than an equal amount of acetic acid under such conditions that said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and an esterification catalyst.

18. In a process for the manufacture of cellulose acetate with the aid of acetic anhydride, the steps of pretreating a cellulosic material in fibrous form with a medium comprising amixture of formic acid and acetic acid, said mixture having incorporated therein a small amount of sulfuric acid, the formic acid being present in an amount of from 1 to 20% by weight of the cellulose, mixing said pretreated cellulosic material with acetic anhydride diluted with not more than an equal amount of acetic acid under such conditions that said cellulosic material is partially acetylated while still retaining its fibrous form, and then continuing the acetylation by mixing said partially acetylated fibrous cellulosic material with an acetylation mixture comprising acetic anhydride and sulfuric acid.

19. The method for activating cellulose to prepare it for esterification, which comprises treating the cellulose with a mixture of a fatty acid of 3-4 carbon atoms. #20 part of formic acid based on the weight of the-cellulose, and a small proportion of sulfuric acid until the desired activation has occurred.

CLIFFORD I. HANEY. IIERVIN E. MARTIN.