US2735846A

US2735846A - Production of alkali cellulose

Info

Publication number: US2735846A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

United States Patent PRODUCTION OF ALKALI CELLULOSE No Drawing. Application December 8, 1951,

Serial No. 260,777

5 Claims. (Cl. 260-233) This invention relates to the production of soda-cellulose fOIjl-ISG in the preparation of cellulose xanthate solutions or \viscose syrups, cellulose ethers, or other cellulose derivatives.

In the customary manufacture of soda-cellulose, the cellulose pulp is subjected generally to the following steps, the purpose of which is as indicated:

Sheeting.The pulp is formed into an integral compressed sheet in which the fibers are interfelted, such sheeting being required in order to make it possible to conveniently handle the pulp in the subsequent operations.

Steeping.The pulp sheets are immersed in a large excess of sodium hydroxide solution at mercerizing temperature, usually 18 C., the weight ratio of caustic solution to cellulose being at least :1. The purpose of steeping is mainly to provide the pulp fibers with a certain amount of caustic soda. The sodium hydroxide concentration depends on the purpose for which the soda-cellulose is intended. If it is to be converted to viscose for the manufacture of regenerated cellulose threads, 18% sodium hydroxide is generally used, whereas if the soda-cellulose is to be used in the manufacture of cellulose ethers, higher concentrations of caustic are employed.

Pressing.After the pulp sheets are thoroughly permeated by the sodium hydroxide solution, they are removed in the sheet form which still remains after completion of the steeping step, and are pressed. The object of the pressing is to remove excess caustic solution. Pressing is continued until the desired weight ratio of the pressed sheet to original cellulose is obtained and usually until it is about 3 :1 (press factor 3).

Dialyzing.-The residual liquor, including that from the pressing step, is dialyzed. The sodium hydroxide is recovered for reuse in subsequent steeping operations.

- Shredding.-The purpose of this step is to comminute the sheet structure comprising the alkali cellulose resulting from .the; steeping. It is usually performed in a shredder or pfleiderer until the sheet is disintegrated and a mass of alkali cellulose crumbs is obtained.

-When the soda-cellulose is to be used in the production of viscose and spun into regenerated cellulose threads or cast to films, the alkali cellulose crumbs are aged by storing them for about 48 hours at about 18 C., whereby depolymerization or degradation of the cellulose molecules takes place with resulting lower viscosity for the viscose.

The. production of viscose also involves the additional steps of xanthating the soda-cellulose, dissolution of the cellulose xanthate, ripening of the viscose, and filtering.

The above outlined procedure for producing sodacellulose has important disadvantages. In the first place, the manufacture of strong coherent pulp sheets which will retain the sheet structure during steeping and pressing is" an expensive operation and moreover requires the exclusive use of long-fibered pulp. This automatically limits'the pulp sources which can be utilized by excluding the extremely .short-fibered pulps such as those derived from hard woods. If it is attempted to use the short- 2,735,846 Patented Feb. 21, 1956 F ice fibered pulps, great diiiiculty is experienced in forming the sheets on conventional sheeting machines. Also, because the cellulose is converted to soda-cellulose while it is in the sheet form, large excesses of the caustic solution are required in order to insure access of the caustic to all parts of the cellulose. The use of excess caustic solution necessitates the pressing operation, which is both expensive and time consuming. The need for excess caustic and pressing in turn requires recovery of the excess sodium hydroxide so that the overall cost of producing the soda-cellulose will be reduced as far as possible. This recovery is generally accomplished by dialysis and, in a commercial plant, requires a large number of dialyzers and considerable time.

These disadvantages are all avoided by the present invention, in accordance with which the cellulose pulp, in bulk, and a heated sodium hydroxide solution in amount and concentration theoretically calculated to produce a final soda cellulose of the desired caustic and cellulose contents, are mixed in a shredder at a relatively high temperature at which there is little, if any, swelling of the cellulose fibers or conversion of the cellulose to sodacellulose until an intimate physical mixture of the two is obtained, the temperature of the mixture is reduced to the temperature at which soda-cellulose is produced, and the mixture is held at the reduced temperature until a mass of substantially uniform soda-cellulose crumbs is obtained, usually about two hours.

In this method of producing the soda-cellulose the amount of sodium hydroxide solution required to thoroughly wet and permeate the fibers is small compared to the amount the cellulose is capable of absorbing, or the amount required when the solution is added to the cellulose at swelling and mercerizing temperature.

The temperature inside the mixer may be maintained at the selected mixing and shredding temperature by providing it with a jacket or coils through which a hot liquid, such as hot water is circulated continuously. Reduction of the temperature to mercerizing temperature, after the mixing has been accomplished, can be effected by circulating ice water through the jacket or coils. Any other suitable means for controlling the temperature of the mass at either stage of the process may be employed.

In the preferred embodiment of the invention, both reduction of the temperature of the physical mixture of cellulose pulp and sodium hydroxide solution, to mercerizing temperature and conversion of the cellulose to soda-cellulose at the reduced temperature, are effected with continuous shredding or grinding of the mass.

The sodium hydroxide solution mixed with the pulp at the higher temperature wets all of the fibers and all portions of the individual fibers because it is not tightly held by swollen fibers nor prevented from penetrating the fibers by soda-cellulose formed prematurely at the fiber surfaces. All of the solutions added is consumed in the chemical reaction when the temperature is reduced. There is no further addition of caustic, and no removal of caustic from the mass or system.

The concentration of sodium hydroxide solution required to obtain soda-cellulose crumbs containing predetermined amounts of sodium hydroxide, cellulose and water can be calculated in advance using the following formula where X is the concentration of the sodium hydroxide solution to be used expressed in percent by weight of sodium hydroxide, S is the percent by weight of sodium hydroxide required in the crumbs, and Y represents the parts by weight of sodium hydroxide solution of X concentration required per unit weight of air dry cellulose as calculated from the formula where M represents the percent by weight of moisture in the pulp and C represents the percent by weight of cellulose desired in the crumbs.

Applying the formula, and assuming that it is desired to produce soda-cellulose crumbs containing 30% cellulose, 15% sodium hydroxide and 55% water from a pulp containing moisture, the concentration of the sodium hydroxide solution required is 21.9% determined as follows:

These formulae apply only when the present method of producing the alkali cellulose is used, and do not apply to any process which involves a steeping and pressing operation.

Generally, the sodium hydroxide solution will be used in concentrations of from 18 to 40% by weight.

The temperature in the shredder during intial mixing or grinding of the cellulose pulp and sodium hydroxide solution must be above the temperature at which the fibers are swollen. Such temperatures are above the normal mercerizing temperatures and thus there is little, if any, mercerization of the cellulose during the physical mixing. The actual temperature employed during the intial grinding will depend upon the rate of degradation desired (degree of polymerization of the soda-cellulose), and in the case when viscose is to be produced as a final product, the viscosity of the viscose and the concentration of the sodium hydroxide solution used. In general, the lower the concentration of sodium hydroxide, the higher must be the temperature during the initial mixing and shredding in order to produce uniform soda-cellulose crumbs when the temperature is lowered to mercerizing temperature, and the longer the mixing period required, at the elevated temperature, in order to insure a given D. P. (degree of polymerization). In other words, the temperature in the mixing and shredding stage is inversely proportional to the concentration. In any case, the temperature during the initial mixing resulting in the intimate physical mixture must be at least 30 C., and is usually between 30 and 50 C.

The following table gives the preferred minimum ternperatures at which initial mixing and shredding of the cellulose pulp and sodium hydroxide solution is performed, using sodium hydroxide solutions of the stated concentrations:

%NaOH Preferred minimum temperature C. 18 45 21 40 23.5 38 28 35 35 and over 30 The minimum temperature to which the physical mixture is reduced to effect conversion of the cellulose to soda-cellulose, is just above the freezing point of the liquid in the mass (about 5C). The maximum temperature of mercerization depends upon the concentration of the sodium hydroxide solution used. in general, the lower the concentration of sodium hydroxide, the lower is the temperature required for mercerization, that is, during the reaction period the temperature should be substantially directly proportional to the concentration. The following table gives the preferred maximum temperatures to which the physical mixture is reduced, using sodium hydroxide solutions of the stated concentration:

%NaOH Preferred maximum temperature C. 18 5 21 12 23.5 18 28 21 35 and over 25 It will be apparent that when a sodium hydroxide solution of comparatively low concentration is mixed with the cellulose pulp in the initial step, the mass must be maintained at a higher temperature during the initial mixing, and must be cooled to a lower temperature for mercerization, than is necessary when a sodium hydroxide solution of relatively high concentration is used.

The advantages and improvements which the present invention makes possible in the production of soda-cellulose crumbs are as follows:

Sheeting.This step is eliminated. The pulp is used in the bulk condition, by which is meant loose bulk pulp or small masses of fiber aggregates, such as chips, strips, or the like, in which the individual fibers are more or less loosely associated. The pulp may be supplied in the loose bulk form or in bales, or it may be formed into loose webs and rolled for shipment, the webs being torn into fragments for use in the present process. Such webs of loosely associated fibers can be formed without difliculty and at comparatively small expense. If desired, of course, the pulp may be sheeted in the usual way, and then subdivided to chips, strips, or the like for use in the present process. Elimination of the sheeting step, however, is an important feature of the invention because it makes possible the use of very short-fibered pulps, as well as the conventional long-fibered pulps, and thus increases the sources of cellulose pulp available to rayon or other producers.

Steeping.-This step is eliminated. The pulp in bulk condition and the sodium hydroxide solution are intimately physically admixed simultaneously with shredding or grinding of the mass, such shredding being initiated as soon as the sodium hydroxide solution has been added to the pulp or vice versa. When cellulose pulp is steeped in or mixed with sodium hydroxide at temperatures around 18 C. as in the conventional processes, the caustic is absorbed and held tenaciously at the surface of the cellulose fibers, with swelling of the fibers and localized production of soda-cellulose at the fiber surfaces. hampers penetration of the caustic solution to the interior of the fibers. Unless a large excess and long steeping periods are resorted to, the cellulose is not uniformly wetted by the caustic and the final product is non-uniform. When operating according to the present method, there is little if any, chemical reaction between the cellulose and caustic, or swelling of the fibers, due to the elevated temperature at which the mixing is performed. The

caustic solution is thus free to contact all of the cellulose" fibers and all portions of the individual fibers, and it is only necessary therefore, to use the amount ofsodium hydroxide solution of given concentration required, theoretically, to produce the soda-cellulose, as explained in detail hereinabove.

Pressing.-This step is eliminated by the present method. Excess sodium hydroxide solution is not used; there is no necessity for a pressing operation.

Dialyzing.There is no residual liquor, so that dialyzing for recovery of excess sodium hydroxide is' not required.

The soda-cellulose crumbs obtained by the present process can be xanthated and dissolved in sodium hydroxi e solution to produce'viscoses of standard compositions, fiber count, and filterability.

In the conventional practice, and especially when vis cose is to be produced, the soda-cellulose crumbs are" stored for ageing, during which ageing period the sodacellulose is depolymerized to the extent necessary to give a viscose of the desired viscosity. Since, in the present amused process, degradation of the cellulose "is favored by the high temperatures prevailing during initial fo'fthe cellulose "pulp and "sodium *hydroxide "solution; by *increasing the time of initial mixing; -and by/increasing "the concentration of the sodium hydroxide solution, these :conditions can be selected, "within the "limits stated,

so that the desired degradation of the :cellulose is ob- :tained, in :whole or 'in vpart, during the initial mixing and grindingfstep. 'This eliminates the need for ageing the soda-cellulose crumbs or permits a "reduction in the time required for such ageing.

Another important .advantage of the method is that special equipment "is not required. The process can be conducted in an ordinary shredder or .p fieiderer. The

mixing and-shredding are-performed in a single vessel.

600 parts of air-dried wood pulp (Novocell) containing about moisture by weight, in the form of chips obtained by subdivision of a pulp sheet, were introduced into a shredder provided with a jacket through which hot water was circulated continuously to maintain the temperature inside the shredder at about 40 C. 1300 parts of 23.3% sodium hydroxide solution heated to 70 C. were rapidly added, and the shredder was started. Grinding of the mass comprising the pulp and sodium hydroxide solution was continued at about 40 C. for about two hours. The temperature in the shredder was then reduced to about C. over a period of one hour, by circulating ice water through the jacket, the shredding being continued. The mass was held at about 15 C. for two hours with continuous shredding, after which the product comprised a mass of fluffy soda-cellulose crumbs of the uniform composition: 16.9% caustic, 30% cellulose, by weight.

The crumbs were stored in a churn for about twelve hours at 18 C., and then mixed with 185 parts of carbon disulfide (45% based on cellulose) for two hours at C. The cellulose xanthate crumbs so obtained were mixed with 1303 parts of 17.8% sodium hydroxide solution and 3072 parts of water, to produce a viscose containing about 7.0% cellulose and about 7.5% sodium hydroxide, and having, immediately after the mixing, the following properties: ball fall seconds; fiber count 7; salt test 12.2; which properties are substantially the same as those for a viscose of the same cellulose and sodium hydroxide content but produced by the conventional method involving steeping of cellulose pulp sheets. Immediately after its preparation, the viscose was filtered using a filter pad comprising one No. 3 sheeting, three layers of swansdown, and two of Gamagee. The filtration rate was at least as great as that for a freshly prepared viscose made by the conventional process (about 1500 gms./hour). The viscose was spun into filaments, using an acid coagulating and regenerating bath of normal composition. No difficulty wasexperienced in spinning the viscose and the threads obtained had properties, such as tenacity, etc., substantially the same as the properties of threads obtained from a viscose made from a soda-cellulose prepared by the conventional method.

Example 11 Exactly 600 parts of air-dried wood pulp (Novocell) containing about 5% by weight of moisture, in the form of strips Vs" wide were introduced into a shredder, the inside temperature of which was maintained at about 6 40 CIT-3'00 partsof 21% sodium hydroxide solution at"70 :C. were rapidly added, and grinding was'initiat'ed "immediatelyand continued at the elevated temperature for about three hours. The temperature of the mass was then reduced to aboutl'S" at which .temperatureygr indingwas continued for about itwohours. Amass of flutfy, uniform crumbs having the composition "14.9% caustic; 30% cellulose, "byiweight was obtained. the crumbs were stored .in a closed vessel for about 112 hours at 18 C. The crumbs were then xanthated, 1'85 parts: of carbon disulfide (45% 'basedon cellulose.) )for 2 hours at 25 "C. "The xanthated crumbs were dissolved in 2933 parts of water .and .1122 parts of 117.8% sodium hydroxide,fto.o'btain a viscose containing approximately 7.0% sodium hydroxideand 7.4% cellulose. Z'Ehe viscose had the following properties immediately .after mixing: Iball fall 55 seconds (25 C.,) fiber count salt test 12.2. It was filtered and .spunjntoihreads, as described in Example 1.

Example III I The method was th'e same as in Example I, except -that a 28% solution of sodium hydroxide was added *to A mass of flufiy crumbs having the composition 19.1%

caustic, 30% cellulose, by weight'was obtained. The viscose contained about 7.0% sodium hydroxide and 7.4% cellulose, and, immediately after its preparation, had the following properties; ball fall 10 seconds (25 C.); fiber count 13; salt tests about 12.5.

Since modifications and variations may be made in practicing the invention without departing from the spirit and scope of the invention, it is to be understood that the invention is not to be limited except as defined in the appended claims.

I claim:

1. The method for the production of soda-cellulose which comprises mixing and shredding cellulose pulp, in bulk condition, with a sodium hydroxide solution the amount and concentration of which is calculated to produce a soda-cellulose of predetermined caustic and cellulose content, to produce an intimate physical mixture of the pulp and solution, the temperature throughout the mixing and shredding being maintained at a value of at least 30 C., reducing the temperature of the physical mixture to a temperature of about 5 C. to 25 C., and maintaining the mass at the reduced temperature until a mass of substantially homogeneous soda-cellulose is obtained.

2. The method for the production of soda-cellulose which comprises mixing and shredding cellulose pulp, in bulk condition, with a sodium hydroxide solution the amount and concentration of which is calculated to produce a soda-cellulose of predetermined caustic and cellulose content, to produce an intimate physical mixture of the pulp and solution, the temperature throughout the mixing and shredding being maintained at a value of at least 30 C., reducing the temperature of the physical mixture to a temperature of about 5 C. to 25 C., and maintaining the mass at the reduced temperature while continuously shredding it until a mass of substantially homogeneous soda cellulose is obtained.

3. The method for the production of soda-cellulose which comprises mixing and shredding cellulose pulp, in bulk condition, with a sodium hydroxide solution of 18 to 40% concentration the amount of which is calculated to produce a soda-cellulose of predetermined caustic and cellulose content, to produce an intimate physical mixture of the pulp and solution, the temperature throughout the mixing and shredding being maintained at a value between 30 and 45 C. which is inversely propor 1300 parts of spam-see 7 tional to the concentration of the sodium hydroxide, reducing the temperature of the physical mixture to a temperature of about to C., and maintaining the mass at the reduced temperature until a mass of substantially homogeneous soda-cellulose is obtained.

4. The method for the production of soda-cellulose which comprises mixing and shredding cellulose pulp, in bulk condition, with a sodium hydroxide solution the amount and concentration of which is calculated to produce a soda-cellulose of predetermined caustic and cellulose content, to produce an intimate physical mixture of the pulp and solution, the temperature throughout the mixing and shredding being maintained at a value of at least C., then reducing the temperature of the physical mixture to a value between about 5 C. and 25 C. which is directly proportional to the concentration of the sodium hydroxide, and maintaining the mass at the reduced temperature until a mass of substantially homogeneous soda-cellulose is obtained.

5. The method for the production of soda-cellulose which comprises mixing and shredding cellulose pulp, in bulk condition, with a sodium hydroxide solution of from 1 8 to concentration at a temperature between 30 and C. which is inversely proportional to the concentration of the sodium hydroxide, the amount of the sodium hydroxide being that calculated to produce a soda-cellulose of predetermined caustic and cellulose content, to produce an intimate physical mixture of the pulp and solution, reducing the temperature of the physical mixture to a temperature between about 5 and 25 C. which is directly proportional to the concentration of the sodium hydroxide, and maintaining the mass at the reduced temperature while continuously shredding it until a mass of substantially homogeneous soda-cellulose is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 2,079,313 Clary et al May 4, 1937 2,602,083 Savage et al July 1, 1952 FOREIGN PATENTS 286,620 Great Britain Oct. 11, 1928 OTHER REFERENCES Ott: Cellulose and Cellulose Derivatives, pp. 812-813, Interscience Publisher Inc., 1943.

Claims

1. THE METHOD FOR THE PRODUCTION OF SODA-CELLULOSE WHICH COMPRISES MIXING AND SHREDDING CELLULOSE PULP, IN BULK CONDITION, WITH A SODIUM HYDROXIDE SOLUTION THE AMOUNT AND CONCENTRATION OF WHICH IS CALCULATED TO PRODUCE A SODA-CELLULOSE OF PREDETERMINED CAUSTIC AND CELLULOSE CONTENT, TO PRODUCE AN INTIMATE PHYSICAL MIXTURE OF THE PULP AND SOLUTION, THE TEMPERATURE THROUGHOUT THE MIXING AND SHREDDING BEING MAINTAINED AT A VALUE OF AT LESS 30* C., REDUCING THE TEMPERATURE OF THE PHYSICAL MIXTURE TO A TEMPERATURE OF ABOUT 5* C. TO 25* C., AND MAINTAINING THE MASS AT THE REDUCED TEMPERATURE UNTIL A MASS OF SUBSTANTIALLY HOMOGENEOUS SODA-CELLULOSE IS OBTAINED.