US2164416A - Process for the production of cellulosic products - Google Patents

Description

'20 terial in making cellulose esters.

Patented July 4, 1939 UNITED STATES PROCESS FOR THE PRODUCTION OF CELLULOSIC PRODUCTS Arnold Marschall, Baden, Germany, assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 4, 1937, Serial No. 162,504

12 Claims.

The present invention relates to the disintegration and processing of dense commercial wood pulp sheets to produce a finelydivided cellulosic mass which may be readily esterified to produce a cellulose ester solution of excellent quality,

free from lumps or graininess.

A great many procedures have been outlined heretofore, for preparing wood pulp suitable for esterification purposes, since wood pulp represents a substantially cheap source of cellulosic raw material. These previously suggested preparation procedures, however, have generally suffered the drawbacks either of giving a cellulosic material unsatisfactory for the production of quality cellulose esters or of being so expensive as to render the wood pulp as costly or more so than cotton linters. As a result of these drawbacks, insofar as known, wood pulp has never become a commercial source of cellulosic raw ma- In addition, at the present time, due to economy and convenience in production, shipping, handling and other reasons, wood pulp is sold to the trade in the form of rolls or bundles of very dense, thoroughly dried sheets. Such dense wood pulp sheets are widely used in making viscose, but are not used in making cellulose acetate, since wood pulp sheets having a density over .4, and particularly over .6, are relatively inactive to an esterifying mixture such as acetic acid, acetic anhydride, and sulfuric acid. By density, reference is made to the ratio of themass of the wood pulp sheet to the mass of an equal volume of water at 4 C.

In order to overcome the inactivity of the dense wood pulp sheets, it has been found necessary to disintegrate the sheet into a finely divided state so that it may react uniformly and smoothly with the esterifying liquids. The wood pulp sheets may not, however, be disintegrated in the dried state with such well-known mechanical devices as attrition mills, hammer mills, or shredders, since the resulting finely divided wood pulp is found to be inactive to an esterifying mixture for another reason. Likewise, attempts to secure the requisite degree of subdivision by wetting the dense wood pulp sheets with water and disintegrating the same in a wet con- 50 dition has resulted in undesirable conditions. In

this latter case, the wood pulp containing a considerable quantity of water is unsatisfactory and very expensive in that the water must either be removed by special dehydration equipment or by 55 the use of relatively expensive anhydride in considerable excess of that required for the esterification.

It is therefore an object of the present invention to prepare a chemically reactive, finely divided wood pulp from dense wood pulp sheets "5 which can be satisfactorily esterified to produce a cellulose ester of excellent quality.

It is another object of this invention to prepare, in an economical manner, a disintegrated wood pulp which readily and uniformly esterifies -10 to a. cellulose ester gum free from lumps and graininess.

More specifically, it is an object of the present invention to provide a process for the disintegration of dense wood pulp sheets in such a manner 15 as to activate the same toward esterification with materials used to esterify the wood pulp and without degrading the cellulose to such an extent as to produce an inferior product.

Other objects of the invention will appear 20 hereinafter.

The objects of this invention may be accomplished in general by agitating the dense wood pulp sheets for less than two hours in a liquid organic acid at such a temperature that the 5 dense wood pulp sheets are disintegrated to a slurry consisting of individual fibers and loose aggregates of individual fibers.

In the preferred method of operating according to the present invention, one part by weight 30 of thoroughly dried high alpha wood pulp sheets, having a density over .4, is placed in an acetator containing a mixture comprising at least four parts by weight of acetic acid, at least threh quarters of one per cent by weight of sulfuric acid 35 based on the weight of wood pulp sheets, and suflicient acetic anhydride to remove substantially all the water present. The resulting mixture is tumbled for a period slightly less than two hours while the temperature is maintained be- 40 tween 25 and C., and preferably between 30 and 45 C. By such treatment, the dense wood pulp sheets are disintegrated into a slurry comprising individual cellulose fibers and loosely held aggregates thereof. The substantially unacetylated fibers are highly reactive and may be esterified by any of the known esterification processes. The following examples illustrating certain preferred methods of operating in accordance with the present invention are given to more clearly outline the present invention; however, these examples are not to be considered in any way limitative.

Example I One hundred parts by weight oLcommercial rayon wood pulp sheets, previously dried at a temperature above 40 C. to a moisture content of less than 5% and having a density greater than about .6, are placed with 600 parts by weight of glacial acetic acid in a container so arranged that, by tumbling, the contents can be continuously agitated. The mixture is heated to a tem.- perature between 35 and 45 C. and kept there while being continuously tumbled. After approximately one hour of such agitation, the wood pulp sheets are found to be quite uniformly softened and disintegrated into substantially unacetylated individual fibers and loosely held aggregates thereof.

If desired, the disintegrated wood pulp thus obtained may be readily acetylated in thefollowing manner: 2 of sulfuric acid based on the weight of cellulose and an amount of acetic anhydride just sufiicient to react with the amount of water in the system are added to the disintegrated slurry in an acetator and the heat of reaction between acetic anhydride and water dissipated by external cooling. The substantially anhydrous mixture is brought to a temperature of 15-18 C., 250 parts by weight of acetic anhydride are added, and the temperature: permitted to rise gradually to 40-45 C. until complete solution of the mixture to a clear viscous liquid is obtained. The reaction is usually completed in two to four hours. Acetylation is then halted by the addition of aqueous acetic acid and the cellulose acetate subsequently hydrolyzed to acetone soluble products by known methods.

Ezcample II One hundred parts of thoroughly dried cellulose wood pulp. sheets having a density greater than about .6, 600 parts by weight of glacial acetic acid, 4% of sulfuric acid based on the weight of cellulose, and suflicient acetic anhydride to barely react with the amount'of water in the system, are placed in a container so arranged that the contents can be continuously agitated by stirring or tumbling. The mass is heated to a temperature of' 35-45 C., and is maintained at such temperature while being continuously agitated. After one hours agitation, the cellulose wood pulp sheets are quite uniformly softened and disintegrated into substantially unacetylated individual fibers and loosely held aggregates thereof.

The substantially anhydrous slurry of wood pulp fibers is cooled to l5-18 C., and 2% of sulfuric acid based on the weight of cellulose is added. Two hundred and fifty parts of acetic anhydride are then added and the temperature is gradually raised to 40-45 C. until a completely clear and viscous acetylation solution is obtained after approximately two to four hours. Acetylation is then halted by the addition of aqueous acetic acid and the cellulose triacetate is subsequently hydrolyzed to acetone-soluble products by known methods.

Example III The procedure of Example II is followed, except any water present is not removed with acetic anhydride until after the dense wood pulp sheets have been quite uniformly softened and disintegrated into substantially unacetylated individual fibers and loosely held aggregates thereof. The disintegration results after one hour of tumbling are quite comparable to those of Example II, and

the same process of acetylation during approximately two to four hours also yields a completely clear and viscous acetylation liquid.

Example IV The same procedure is followed as in Example II, except that disintegration is performed in the absence of any sulfuric acid and the sulfuric acid based on the weight of cellulose is increased to'2 /2% for the esterification. Again the desired type of disintegration is obtained, and acetylation for approximately two to four hours of the disintegrated wood pulp yields a clear viscous solution.

It will be evident to the skilled technician that the process of the invention is susceptible to considerable variation. Generally speaking, the character of the disintegrated wood pulp will vary with factors such as the time and temperature of disintegration, the amounts of acetic acid and sulfuric acid present, and thetype and time of agitation. All these factors are related to each other, and a change in one necessitates a compensating change in the others to secure the same type of disintegration. Bearing such in mind, the following variations may be made from the procedures of the above examples:

As to the amount of acetic acid'for the disintegration, one preferably employs a quantity by weight six times that of the cellulose, but depending upon the proposed use of the disintegrated wood pulp and the adjustment of the above mentioned factors, the acetic acid may be intro,- duced in quantities either as low as four or considerably more, such as 10-20 times the weight of cellulose. Although it is greatly to be preferred that no more acetic anhydride be used in the disintegration treatment than is necessary to react with the water present, slightly larger quantities may be used if care be taken that the wood pulp is not materially degraded or esterified'during this treatment.

The disintegration may be performed in the absence of sulfuric acid, but it is preferable to use at least based on the weight of cellulose in order to secure both activation and disintegration of the dense wood pulp sheets. If no sulfuric acid is added during the disintegration step, it is necessary to introduce a correspondingly larger quantity of sulfuric acid in the later esterification step. Depending upon the proposed use of the disintegrated wood pulp and the suitable adjustment of the amount of acetic acid and other factors, quantities of sulfuric acid up to 540% may be added during the disintegration of the dense wood pulp sheets,

The temperature is preferably held at 3545 C. during the disintegration for the conditions cited in the above examples, but a temperature as low as about 25 C. or as high as C. may be successfully employed provided the other factors are suitably adjusted in accordance therewith. That is to say, for an agitation period of less than two hours, with a given type of agitation and a given amount of acetic acid with or without a given amount of sulfuric acid, the mixture must be kept over a certain minimum temperature in order to disintegrate thedense wood pulp sheets into a slurry of individual fibers and loosely held aggregates thereof, and under 1 separate mixing container. In either case, provision must be made for continuous and thorough agitation by tumbling, stirring, or like means. Preferably, the disintegration is carried out in a rotating drum such as described in Grillet U. S. 1,566,398, but it may also be performed in a Werner-Pfieiderer shredder or in a kneading and mixing machine such as described in U. S. 1,989,126, wherein the sheet is disintegrated into individual fibers or loosely held aggregates thereof Without a grinding action.

The time necessary for satisfactory disintegration of the dense wood pulp sheets may vary considerably in accordance with the treating liquids used and the source and quality of wood pulp. The disintegration treatment should be carried out for a period of at least 20 minutes and should be completed within two hours, and preferably within one hour, so as to obtain in the subsequent esterification a cellulose ester as slightly degraded as possible.

For the purpose of facilitating the later esterification of the wood pulp, it is preferred that the disintegration be performed in a medium that is substantially anhydrous. However, such is not absolutely essential and the advantages of the invention are not completely lost as long as not more than 10% water is present during the dis integration of the Wood pulp sheets. Likewise, the- Wood pulp sheets may be subjected to a preliminary soaking with a liquid medium prior to disintegration by agitation, but such procedure is seldom required and does not represent the preferred method of operation. Similarly, the liquid medium may be removed from the disintegrated wood pulp prior to the esterification step, but such represents an unnecessarily costly and distinctly less preferred method of operation.

In the esterification step, sulfuric acid is preferred as the esterification catalyst. However, it will be recognized that the benefits of the invention may be secured with suitable concentrations of other familiar esterification catalysts such as phosphoric acid, zinc chloride, hydrochloric acid, or magnesium perchlorate. Such catalysts may be used alone or in combination with sulfuric acid.

For convenience in the above specification, the esterification of the disintegrated wood pulp has been described in terms of acetylation with acetic acid and acetic anhydride for the production of cellulose acetate. The process of the invention is, of course, equally applicable for the production of organic esters of cellulose in general, such as cellulose propionate, cellulose butyrate, cellulose acetopropionate, cellulose acetobutyrate, cellulose acetate-propionate-butyrate, cellulose crotonate, cellulose acetostearate, etc. by the use of the corresponding organic acid and acid anphysicochemical inactivity of the individual Wood pulp fibers.

It will be recognized, of course, by persons skilled in the art, that the fibrous product prepared in accordance with the invention may be esterified to give a clear gum solution or esterified in the presence of a liquid nonsolvent to give directly a fibrous cellulose ester. Likewise, the fibrous disintegrated wood pulp may be esterified in the presence of sufficient liquid medium and properly adjusted quantity of esterification catalyst that a high viscosity cellulose ester results.

By the process of the invention, the most common .and cheapest commercial form of rayon wood pulp, namely, sheets, may be economically disintegrated without degradation and directly esterified by methods commonly employed for the esterification of cotton linters. Besides using re agents which later form animportant component of the esterification medium, the disintegration procedure quite definitely activates the wood pulp fibers. Such activation makes possible the use of dense wood pulp sheets exhibiting inactivity from too drastic drying or similar causes. The disintegration process is not only relatively rapid, but also separates the individual fibers of the dense wood pulp sheets sufficiently that the esterifying liquid rapidly penetrates to each fiber. In contrast to dry disintegration methods, the present invention yields no lumps or fiber dust which interferes with the quality of the esterified product. In contrast to aqueous disintegration methods, the invention obviates any need of intermediate pressing or extracting treatments to remove water. The present invention eliminates the necessity of disintegration and pretreatment as separate steps in the process of making dense Wood pulp sheets esterifiable. Since there is no waste of reagents, since no special apparatus is required, and since the disintegration maybe performed rapidly and as a part of the esterification procedure, the process of the present invention represents a practical and economical method for the preparation of cellulose esters of excellent quality from dense sheets of wood pulp cellulose.

Since it is obvious that many changes and modifications can be made in the process as above described without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited except as set forth in the appended claims.

I claim:

1. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the step which comprises agitating said sheets in a liquid, cellulose ester-forming, lower aliphatic acid to disintegrate the same and form a slurry of substantially unesterified individual fibers and loosely held aggregates of fibers which are highly lose ester from diflicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the step which comprises agitating said sheets for a period of between twenty minutes and two hours in a liquid, cellulose ester-forming, loweraliphatic acid at a temperature of at least 25 C. to

disintegrate the same and form a slurry of substantially unesterified individual fibers and loose- 1y held aggregates of fibers which are highly reactive and readily esterifiable.

4'. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the step which comprises agitating said sheets for a period of. between twenty minutes and two hours in at least four times their weight of a liquid, cellulose ester-forming, lower aliphatic acid to disintegrate the same and form a slurry of substantially unesterified individual fibers and loosely heldaggregates of fibers which are highly reactive and readily esterifiable.

5'. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the step which comprises agitating said sheets for a period of between twenty minutes and two hours in at least four times their weight of a liquid, cellulose ester-forming, lower aliphatic acid containing at least 4% (based on the weight of wood pulp sheets) of an esterification catalyst to disintegrate the same and form a slurry of substantially unesterified individual fibers and loosely held aggregates of fibers which are highly reactive and readily esterifiable.

6'. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulpsheets having a density or at least 0.4, the step which comprises agitating said sheets for a period of between twenty minutes and two hours in at least four times their weight of a liquid, cellulose ester-forming, lower aliphatic acid containing at least (based on the weight of the wood pulp sheets) of an esterification catalyst and approximately a stoichiometric amount of an organic acid anhyd'ride to react with the water present in the wood pulp to disintegrate the same and form a slurry of substantially unesterified individual fibers and loosely held aggregates of fibers which are highly reactive and readily esterifiable.

7. In a process for the production of a cellulose ester from difl-lcultly esterifiable, dry Wood pulp sheets having a density of at least 0.4, the steps comprising disintegrating said wood pulp sheets by continuously agitating the same in at least four times their weight of a glacial acetic acid'at a temperature of at least 25 C. to form a slurry of substantially unacetylated fibers which are highly reactive and readily esterifiable, and. esterifying the distintegrated wood pulp mass.

8'. In-a process for the production oi a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the steps comprising disintegrating said wood pulp sheets by continuously agitating the same for a period ofbetween twenty minutes and two hours in at least four times their weight of a glacial acetic acid at a temperature of at least 25 C. to term a slurry of substantially unacetylated fibers which are highly reactive and readily esterifiable, and esterifying the disintegrated wood pulp mass.

9. In a process for the productionof a cellulose ester from difiicultly esterifiable, dry woodzpul'p sheets having a density of at least 0.4, the steps comprising disintegrating said wood pulp sheets by continuously agitating the same for a period of between twenty minutes and two hours in at least four times their weight of a glacial acetic acid containing at least (based on the weight of the wood pulp) of sulfuric acid at a temperature of at least 25 C. to form a slurry of substantially unacetylated fibers which are highly reactive and readily esterifiable, and esterifying the disintegrated wood pulpmass.

10. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the steps comprising disintegrating said wood pulp sheets by continuously agitating the same for a period of between twenty minutes and two hours in at least four times their weight of a glacial acetic acid containing at least (based on the weight of the wood pulp) of sulfurc acid and approximately a stoichiometric amount of an acetic anhydride to react with the water present in the wood pulp at a temperature of at least 25 C; to form a slurry of substantially unacetylated fibers which are highly reactive and readily esterifiable, and esterifying the disintegrated wood pulp mass.

11. In a process for the production of a cellulose ester from difiicultly esterifiable, dry wood pulp sheets having a density of at least 0.4, the steps comprising disintegrating said wood pulp sheets by continuously agitating the same for a period of between twenty minutes and two hours in at least four times their weight of a glacial acetic acid containing at least 4% (based on the weight of the Wood pulp) of sulfuric acid and approximately a stoichiometric amount of an acetic anhydride to react with the water present in the wood pulp at a temperature of between 35 and C'. to form a slurry of substantially unacetylated fibers which are highly reactive and readily esterifiable, and esterifying the disintegrated wood pulp mass.

12. In a process for the production of a cell'ulose ester from difiicultly esterflable, dry wood pulp sheets having a density of at least 0.4, the step comprising disintegrating said wood pulp sheets by continuously agitating the same for a period of between twenty minutes and two hours in at least four times their weight of a glacial acetic acid containing between and 10% sulfuric acid to form a slurry of substantially unacetylated fibers which are highlyreactive and readily esterifiable- ARNOLD MARSCHALL.