US3029232A - Production of amide containing cellulose - Google Patents

Description

This invention is concerned with the chemical modification of cellulose. More particularly, it relates to an improved method of modifying cellulose with such reactants as acrylamide and N-substituted acrylamides.

In the past, various proposals have been made for modifying cellulose with acrylamide. Therein, cellulose fibers were heated with aqueous alkali solutions and acrylamide. However, these treatments were usually in the presence of high alkali concentrations of some ten to forty percent. Their purpose was to produce solutions of the modified cellulose, i.e., carboxyethylated cellulose, which could be subsequentlly precipitated by acid.

Subsequently, attempts were made to avoid hydrolysis of the substituted group and produce carbamylethylated cellulose, rather than carboxyethylated cellulose. These proposals required the use of lower alkali concentrations of some four to ten percent. Some useful products were obtained. However, the operation was never successful in introducing nitrogen contents as high as were desired. Nor were the resultant unavoidable carboxyethyl contents as low as desired.

There remained, then, a need for a process which was capable of producing carbamylethylated cellulose having a high carbamylethyl content without at the same time attaining an undesirably high carboxyethyl content. It is an advantage of the present invention that this has been simply and easily accomplished without introducing unusual apparatus requirements or the necessity for abnormally stringent conditions.

Moreover, previous attempts to accomplish analogous results with N-substituted acrylamides were highly unsatisfactory. Using acrylamides it was possible to obtain maximum nitrogen contents of about 2.2 percent in the product. However, the eificiency was very poor. When an N-substituent was introduced, or as the substituent increased in size, the reaction became less effective. Using N,N'-methylenebisacrylamide, for example, the maximum nitrogen content obtainable was less than about one percent. When attempts were made to use such materials as N-terL-butylacrylamide, no reaction occurred. It is an advantage of the present invention that it is not so-lirnited.

in general, the process of the present invention may be simply described. Cellulose fibers are treated with aqueous alkali solutions of low concentration and with the desired acrylamide in the presence of a suitable Watersoluble salt present in sufficient amount. Heating at moderately elevated temperatures of from about C. to about 130 C., preferably about 55 to about 95 C., produces the desired result. Nitrogen contents as high as four to five or more percent are easily obtained with acrylamide. When the required degree of substitution is attained, the treatment is stopped and the product washed and dried.

It is an advantage of the present invention that it is applicable to the production of the desired product Without being restricted to a particular type of cellulose. It may be in particulate form, such as from Wood pulp, cotton linters, rayon flock and the like; as fibers; as yarn or thread; or as woven fabrics.

It is a further advantage of the present invention that no special processing or apparatus is required. Where 35,929,232 Fatenteel Apr. 19, 1962 2 the form of cellulose permits, simply stirring and heating the cellulose with the treating liquor containing the alkali, the salt and the acrylamide in any available vessel is adequate. Where the material is a yarn or fabric the treatment may be easily carried out in conventional equipment such as is used for dyeing operations.

It is still anotheradvantage oi the invention that the cellulose may be treated in a single operation with a single treating bath or it may be prewetted with a solution of the alkali and the salt before adding the acrylamide. It is desirable but not essential in such cases to have the salt present during the prewetting. It need not be all added at that stage. A portion, or if necessary all, the salt may be added as an intermediate treatment or with the acrylamide.

As noted above, the process is quite flexible as to temperature requirements. Reaction is of course faster at the higher temperatures within the range noted above. However, the caustic, or other alkali, is present in sufiiciently dilute solution that excessive hydrolysis does not occur in most cases, even over long periods.

Reaction is slower with N-substituted acrylamides than when using acrylamide per so. However, the process is also improved and more flexible in this respect. For example, where in the prior art no reaction was obtained in attempting to utilize such an acrylamide as N-tert-butylacrylamide, in the present process even this reactant, which is only partially soluble in the treating liquor, may be made to react with the cellulose. However, in general practice the N-substituent will seldom contain more than two carbon atoms. Typical N-substituted acrylamides which may be reacted with cellulose include N-methyL, N-ethyl, N-propyL, N-isopropyl-, N-n-butyl-, N-isobutyl-, N-t-butyl-, N-methylol, N-N methylenebis-, N-hydroxyethyl-, and N-hydroxypropylacrylamide and the like.

Accordingly, in this discussion, the term an acrylamide is used generically to designate these materials and the expression carbamylethylated is used to designate not only amide-containing cellulose: derived from acrylamide per se but also the analogs obtained using these N-substituted acrylamides.

In general, the alkali used will be sodium hydroxide. However, this is not a limitation. Any strongly reactive alkali may be used, for example, potassium hydroxide; or such strongly basic quaternary bases as benzyltrialkylammonium hydroxide, dibenzyldimethylammonium hydroxide and the like. It is to be understood that the term alkali solution will in most cases be an aqueous caustic alkali solution and concentrations herein are discussed on that basis. However, the other alkalies may be substituted therefor in equivalent amounts.

Concentrations of aqueous alkali in the treating liquor of this invention will be less than four percent and usually much lower than those previously used. In general practice, the concentration will range from about onehalf to about three parts per hundred parts of Water. Within this range, higher concentrations do produce higher reaction rates.

The weight of acrylamide used will depend to a major extent on the amount of carbamylethylation which it is desired to produce. While less sometimes may be used, in general practice the amount will range from about five parts per hundred parts of water up to the solubility limit. As noted above, nitrogen contents above four percent are readily obtainable. However, it is not necessary to produce this degree of substitution to obtain the advantages of the invention. The degrees of substitution previously attained are achieved more readily and the product is much more effectively produced. This can be measured by the ratio of the carboxyethyl content to the nitrogen content. The latter is conveniently exsnaaass pressed as the weight. The former is readily assayed as milli-equivalents per gram of product. in the products previously obtainable, this could not be reduced below about 1:10. In the process of the present invention, ratios of 1:15 to 1:20, or better, are readily obtainable.

The ratio of amount of solution to amount of cellulose is not critical except in one respect. It is necessary that the treating liquor be in intimate and effective contact with the cellulose at all times. Therefore, the total 1 amount of treating liquor must be adequate to maintain this contact. For example, to maintan an equally effective contact while treating yarn in a package dyeing machine, a much greater total volume of treating liquor is required than when stirring a particulate form in an open kettle. That portion of the liquor in immediate contact with the fiber should contain effective reactant amounts in terms of concentration and the minimum volume should furnish at least the required weight of reactants. However, the total volume and hence the total weight may and often will be greatly in excess of this.

Probably more important as a factor in effecting the desired result than any other is the use of the correct amount of a suitable salt. The present invention depends on the discovery that certain water-soluble salts, which are substantially unreactive with acrylarnide under reaction conditions, are capable of accelerating the reaction and increasing the efficiency. These are salts which in aqueous solution have a swelling effect on cellulose. In general, the useful salts are substantially neutral alkali-metal salts of strong acids. However, it is not essential that the salt be neutral in the sense that when dissolved in water it produces exactly pH 7. A slight departure from complete neutrality does not interfere. Of course, strongly acidic salts are not useful.

The useful salts apparently are not acting as alkaline catalysts. Those which display some slight alkalinity in water, such as sodium benzoate, do so to a degree of alkalinity insufficient for useful catalysis. In fact, the effectiveness of such salts is not as great as those that are more nearly completely neutral, such as the alkali metal iodides and thiocyanates. The useful group includes such neutral to slightly alkaline salts as lithium, sodium and potassium iodide; lithium and sodium perchlorate; the alkali metal salts of such aryl sulfonic acids as benzene-, tolueneand xylene-sulfonic acids; and the alkali metal thiocyanates and benzoates.

While the salts used in the present invention are hydrotropes, hydropicity is not a critical factor. The relative effectiveness of different salts is in no sense directly proportional to their hydrotropic powers. Other factors play an important role. The swelling effect of the salt on the cellulose is important and well may be the major factor. Since the exact mechanism is not known the invention is not intended to be limited to any particular theory.

As noted above, this invention produces an increase in reactivity. it may be measured, for example, by the increased carbamylethylation obtainable under fixed conditions. Identical reaction conditions which, without salts, yield no more than two percent nitrogen, yield products of four or more percent nitrogen when the salts are presout during reaction. Moreover, these new products are substantially water-soluble. In other words, reaction conditions which would normally produce an insoluble product or a product soluble only in aqueous alkali solution, yield products with a nitrogen content of from three to about six percent nitrogen with the salt present.

The present invention also presents marked advantages in increasing the efiiciency of reaction and in shortening the reaction time required to obtain products of less than the maximum nitrogen content. It is thus an important advantage of the present invention that it is extremely flexible and better results can be obtained without introducing undesirable characteristics.

The amount of salt which is added can be varied over wide ranges. Thus, for example, sodium iodide may be used in amounts from as little as 20 grams, or less per 100 grams of sodium hydroxide solution, up to amounts as high as about 230 grams, which is a saturated solution at C. In general, the more active salts, of which sodium thiocyanate and sodium iodide are typical, reach a ver favorable concentration well below saturation. Additional amounts increase the reaction efiiciency ratio only a little. For example, in the case of sodium iodide at 55 C., the maximum efiect is reached at about 100 grams per 100 grams of Na-OH'solution and adding more does not produce a commensurate increase in effect.

in general then, it is desirable to add as little salt as practicable and still obtain the desired degree of carbamylethylation, in a reasonable time with a good carboxyethylation ratio. The minimum useful amount will be that which will produce swelling of the cellulose in addition to the degree of swelling produced by the alkali. There is no maximum since, if so desired, an amount in excess of that which will produce saturation may be present. Very precise control is not essential. This large range of operative concentrations is an additional advantage of the invention.

As noted above, one of the most important features of the present invention is the production of the desired degree of carbamylethylation with a greatly reduced degree of carboxyethylation. This has been referred to above as the carboxyethylation ratio. Since it is a relative term, the units are not important as long as they are consistent. One readily expressed numerical ratio is that of the carboxyethyl content, in milli-equivalent per gram of product, to weight percent nitrogen content. By the use of the present invention, as noted above, this ratio may be reduced by 100% or more below those previously possible, making a much better and efficient usage of the acrylamide or its derivative. This will be illustrated in the following examples wherein all parts and percentages are by weight, and all temperatures in C., unless otherwise noted.

Example 1 Five parts of cotton linters are heated with stirring for one hour at C., in a solution consisting of 88 parts of Water, 2 parts of sodium hydroxide and 10 parts of acrylamide. A trace of N-phenyl-beta-naphthylamine is also present to prevent polymerization of the acrylamide. Neutralization is effected with dilute acetic acid and the product is thoroughly washed with water and alcohol. Kjeldahl analysis showns a nitrogen content of only 0.5%. The carboxyethyl content is 0.05 milliequivalent per gram of product and the carboxyethyl ratio is about 1:10.

Example 2 To demonstrate the effect of the added salt, the procedure of Example 1 is repeated exactly with the exception that the parts of sodium thiocyanate is omitted. The nitrogen content is 1.3% and the carboxyethyl content is only 0.05 rnilliequivalents per gram, a ratio of about 1:10.

The use of the added salt has produced an increase in nitrogen content of about with a greatly improved carboxyethyl ratio.

Example 3 Twenty-three parts of alpha-cellulose from Wood pulp is allowed to swell for 45 minutes at room temperature in a solution made up of 295 parts of 2% sodium hydroxide solution, 295 parts of sodium thiocyanate and a trace of polymerization inhibitor. Acrylamide, 147 parts, is then added and the mixture stirred at 7278 C. for 3 hours. The solution is then poured into a large excess of methanol, filtered and washed with more alcohol. The product is almost completely water-soluble both in basic and acidic solutions, which distinguishes it from carboxyethyl cellulose which is soluble only in basic solution. The nitrogen content is 4.5%; the carboxyethyl content only 0.22 milliequivalents per gram, a ratio of about 1:21.

Example 4 Four parts of alpha-cellulose is stirred for about 10 minutes in a solution comprising 1 part of sodium hy droxide, 49 parts of Water and 50 parts of sodium lithiocyanate. Thirty parts of acrylamide containing hydroquinone as polymerization inhibitor is then added and reaction allowed to proceed at 80 C. for 2 hours. The mixture is then neutralized with an alcoholic solution of acetic acid. The product is thoroughly washed with alcohol, then reslurried in a small quantity of water and washed again with alcohol. Analysis shows a nitrogen content of 3.8%. The carbamylethylated cellulose is partially soluble in Water.

Example 5 Ten parts of cotton linters is allowed to react for 2 hours at 80 C., in a solution consisting of 2 parts of sodium hydroxide, 200 parts of water, 200 parts of potas sium iodide, 50 parts of acrylamide and a trace of hydroquinone. After workup as described in Example 5, the product contains 1.2% nitrogen.

Example 6 Five parts of alpha-cellulose is treated at 80 C., for 2 hours with a solution consisting of 2 parts of sodium hydroxide, 98 parts of water, 35 parts of sodium thiocyanate, 25 parts of N,N-methylenebisacrylamide and a trace of hydroquinone. After workup as described in Example 5, the product contains 2.9% nitrogen.

Example 7 Five parts of cotton linters are allowed to react in a solution comprising 2 parts of sodium hydroxide, 70 parts of water, 35 parts of sodium thiocyanate, 30 parts of N-methylolacrylamide and a trace of hydroquinone. After workup as described in Example 5, the product contains 0.6% nitrogen.

Example 8 Example 6 is repeated substituting for the N,N-methylenebisacrylamide an equal weight of N-n-propylacrylamide. Paper made from treated cellulose is stronger than from the untreated pulp.

As may be noted in the above examples, it is desirable to insure against polymerization of the acrylamide, or substituted acrylamide. This may be done by addition to the treating liquor of a known polymerization inhibitor such for example as the naphthylamine, hydroquinone used in the examples and their known equivalents.

The products of this invention have utility in a wide number of fields, for example in producing paper or fabrics of increased strength. The water-solub1e products have particular utility as thickners for printing pastes and the like, in coating and sizing compositions, as flocculating agents and the like.

We claim:

1. In a process of carbamylethylating cellulose by heating it With an aqueous treating liquor comprising water, an acrylamide and a strong alkali, the improvement which comprises; carrying out the reaction at from about 45 to about 130 C.; maintaining in said treating liquor, per hundred parts of water, an amount of alkali stoichiometrically equivalent to from about one-half to about four parts of sodium hydroxide, an amount of an acryl amide of from about five parts to about the solubility limit at the operating temperature, and an amount eifective to produce swellin of the cellulose of a watersoluble salt of a strong alkali and a strong acid, said salt having in Water alone a substantially neutral to slightly alkaline pH and being in said treating liquor substantially unreactive with the acrylamide; maintaining said cellulose in intimate reactive contact with a suflicient amount of said treating liquor to thoroughly wet the cellulose; and maintaining said conditions until substitution is completed, whereby the amide-containing cellulose is obtained with a minimized carboxyethyl content.

2. A process according to claim 1, wherein said acrylamide is selected from the group consisting of acrylamide and the N-alkyl substituted acrylamides, N,N-alkylene bisacrylamides and N-alkylolacrylamides wherein said alkyl, alltylene and alkylol substituents contain from one to four carbon atoms.

3. A process according to claim 1 in which the cellulose is cotton.

4. A process according to claim 1 in which the cellulose is derived from Wood pulp.

5. A process according to claim 1 in which the cellulose is a regenerated cellulose.

6. A process according to claim 1 in whi h the salt is an alkali metal iodide.

7. A process according to claim 6 in which the reaction is carried out in the temperature range of between about 55 and about C.

8. A process according to claim 1 in which the salt is an alkali metal thiocyanate.

9. A process according to claim 8 in which the reaction is carried out at a temperature in the range between about 55 and about 95 C.

l0. A process according to claim 1 in which the reaction is continued until the carbamylethylation has proceeded beyond a nitrogen content of about three percent.

11. The process according to claim 10 in which the salt is an alkali metal iodide.

12. A process according to claim 10 in which the salt is an alkali metal thiocyanate.

13. A process according to claim 10 in which the salt is an alkali metal salt of a mononuclear aryl sulfonic acid.

14. A process according to claim 10 in which the salt is an alkali metal xylene sulfonate.

References Cited in the tile of this patent UNITED STATES PATENTS Rock at al. Jan. 4, 1944 Gardner Feb. 8, 1949 OTHER REFERENCES

Claims (1)

1. IN A PROCESS OF CARBAMYLETHYLATING CELLULOSE BY HEATING IT WITH AN AQUEOUS TREATING LIQUOR COMPRISING WATER, AN ARCYLAMIDE AND A STRONG ALKALI, THE IMPROVEMENT WHICH COMPRISES; CARRYING OUT THE REACTION AT FROM ABOUT 45* TO ABOUT 130* C; MAINTAINING IN SAID TREATING LIQUOR, PER HUNDRED PARTS OF WATER, AN AMOUNT OF ALKALI STOICHIOMETRICALLY EQUIVALENT TO FROM ABOUT ONE-HALF TO ABOUT FOUR PARTS OF SODIUM HYDROXIDE, AN AMOUNT OF AN ACRYLAMIDE OF FROM ABOUT FIVE PARTS TO ABOUT THE SOLUBILITY LIMIT AT THE OPERATING TEMPERATURE, AND AN AMOUNT EFFECTIVE TO PRODUCE SWELLING OF THE CELLULOSE OF A WATERSOLUBLE SALT OF A STRONG ALKALI AND A STRONG ACID, SAID SALT HAVING IN WATER ALONE A SUBSTANTIALLY NEUTRAL TO SLIGHTLY ALKALINE PH AND BEING IN SAID TREATING LIQUOR SUBSTANTIALLY UNREACTIVE WITH THE ARCYLAMIDE; MAINTAINING SAID CELLULOSE IN INTIMATE REACTIVE CONTACT WITH A SUFFICIENT AMOUNT OF SAID TREATING LIQUOR TO THROUGHLY WET THE CELLULOSE; AND MAINTAINING SAID CONDITIONS UNTIL SUBSTITUTION IS COMPLETED, WHEREBY THE AMIDE-CONTAINING CELLULOSE IS OBTAINED WITH A MINIMIZED CARBOXYETHYL CONTENT.