US1668944A - Cellulose esters of fatty acids and process of making the same - Google Patents

Description

Patented May 8, 1928.

- UNITED STATES PATENT oFFicE.

HANS .T. CLARKE AND CARL J. MALM, ROCHESTER, NEW YORK, ASSIGNORS TO EASTMAN KODAK COMPANY, OF ROCHE YORK.

STER, NEW YORK, A CORPORATION OF NEW CELLULOSE ESTERS OF FATTY ACIDS AND PROCESS MAKING THE SAME.

No Drawing. Application filed July 12,

This invention relates to cellulose esters of fatty acids and processes of making the same.

One object of the invention is to provide fatty acid esters of cellulose in which approximately one molecular proportion of acid (especially one having more than one carbon atom) is combined with an amount of cellulose corresponding to twenty-four carbon atoms. Another object is to provide an inexpensive, stable, fatty acid ester of cellulose which is very susceptible to further esterification. Still another object is to pro vide such an ester in a purified condition. A

further object is to provide an inexpensive and simple process for producing such esters. Another object is to provide a process for producing such esters in which no anhydrid is used, the fatty acid being the sole acylating agent. A still further object is to provide a process of makin fatty acid cellulose esters in one stage of W ich esterification is carried on without the aid of afatty acid .anhydrid, and in a subsequent stage of which further esterification is effected with the aid of such an anhydrid but in only economical amounts. Other objects will hereinafter appear. We have found that'hitherto undescribed fatty acid esters of cellulose can be prepared in which approximately one molecular proportion of-acid, containing less than eight carbon atoms, 1s comblned with anamount J of cellulose corresponding to twenty-four cellulose from which they have been carbon atoms. Aside from the scientific bearing of this fact upon the theory of the structure of the cellulose molecule, "these esters have properties of technical importance. They are relatively inexpensive, can' be obtained in a sufficiently pure state and are very susceptible to further esterification.

Moreover, those derived from fatty acids having more than one and less than eight carbon atoms are very stable, as indicated, for example, by their resistance to hydrolysis under prolonged treatment with boiling water. As a rule, the new compounds are superficially indistinguishable from the repared; for example, esters prepared rom cotton cellulose still have the appearance of cotton fibers. w

We have likewise found that the process 1926. Serial No. 122,028.

of producing these esters is very simple and inexpensive, and can be carried out wlthout the aid of anhydrids of the fatty acids,

which are relatively expensive. It is merely necessary to heat the cellulose with the tion, with the acid as the sole acylating agent, to the point where the above mentioned ester is obtained, usefulpractical re 7 sults are effected by esterifying to the point where at lea'st4% of acyl group is present in the compound:

We have also found that when th ese one in twenty-four esters are further esterified,

say by using anhydrids or acyl chlorides in the ways previously described for the aoylation of cellulose itself, technically useful economies in anhydrid or acyl chloride can be obtained. Savings of 10% to 25% of the amount of anhydrid used have been noted in, this work. i

Broadly speaking, our invention is concerned with estersof the fatty acids having more than one and less than eight carbon atoms, such as acetic, propionic, butyric, valeric, caproic, heptylic, etc. But since the rate of esterification falls off rapidly upon ascending the above cited homologous series, we have found that economy of time is obtained best when using'fatty acids having more than one and less than eight carbon atoms. Of course, acetic acid, propionic acid and butyric acid are preferred, because of their relatively greater commercial availability and their greater speed of reaction.

Our process can utilize cellulose from any of the sources customarily used in the manufacture of esters, such, for instance, as sulfite wood pulp (preferably bleached), cotton,

surgical cotton wool, tissue paper especially prepared from cotton, etc. It is an advantage that our process can utilize relatively impurer cellulose than is often employed in essential that it be employed, because of the whitening tendency of our process when applied to the unbleached or partially bleached pulp. It will be seen that the above list relates to cellulose and does not include hydrocellulose or hydracellulose.

The process may be conducted over a considerable temperature range. We have found that the production of one in twenty-four esters takes place rapidly with the lower members of the fatty acid series at 117 C, but the working temperatures can range up to the point where the cellulose or the prodnets suffer from degradation. Ve find it inadvisable, as a general proposition, to use temperatures above 17 0 C. Within the working range the temperature will vary with the particular acid or mixture ofacids which is employed and with the pressure under which the process is carried out. For instance, Working at atmospheric pressure and employing acids which boil below 170 C. at such pressure, we find it useful to conduct the process by boiling the cellulose in the acid, preferably in a considerable excess of acid over the amount which will enter into the ester. But the time can be very considerably shortened by using a superatmospheric pressure, provided the corresponding temperature does not reach the point where the product is degraded.

While we have found it practical to carry out the process by heating the cellulose in the acid or mixtures of acids, as the sole treating bath, nevertheless, useful amounts of the acyl group may be introduced, say Well above 4%, for example, when the acids are mixed with non-acylating liquids, such as chloroform, carbon tetrachloride or benzene.

The amount of water present during the reaction is important because it has a depressing effect upon the total percentage of acyl group which can be introduced. Taking into account both the initial water in the reagents and the water formed during the esterification, it should never be allowed to reach the point where it prevents the introduction of more than 4% of the acyl group. We prefer, in fact, to maintain the conditions during the reaction such that water. tends to be remoyed or diminished in quantity rather than increased in amount. The use-of a reflux condenser, which will condense and. return to the bath the vapors of the fattv acids without condensing the water vapor, is one way of effecting this result. Such a reflux condenseraction is not always essential, especially when working with the lower members of the above recited series, provided the acid and cellulose are sufficiently low in moisture. With the higher members of the series it is best to eliminate the water during the reaction.

We shall now describe various embodiments of our process by way of illustration, but it will be understood that the nvent on is not limited to such details except as indieated in the appended claims.

As the boiling continues, in this example,. the esterification gradually takes place and eventually the percentage ofacetyl group in the ester reaches the point where it is not materially increased by further boiling, say about 6%. This time varies somewhat with the type and condition of cellulose em ployed, representative times, for example, being 160 to 200 hours. But a distinctly useful product is obtained after a much shorter time, say about 48 hours, for instance. At that time the amount of acetyl group is very considerably above 1% and the ester is eminently suited for rapid and ecomical acetylation in which acetic anhydrid is used. As indicated above, the maximum amount of acetyl group, which can be introduced by prolonged boiling, is substantially one molecular proportion of the fatty acid combined with an amount of cellulose corresponding to twenty-four carbon atoms. It is convenient to note that the percentage of acyl group in a given ester corresponds to a still higher percentage of combined acid, the relation being in the proportion of the molecular weight of the acyl group to the molecular weight of the acid. For example, 4% of acetyl group means 5.58% of combined acetic acid.

,After the esterification has reached the desired point, the excess of acid is removed if the ester is to be further esterified by treatment with an anhydrid and a catalyst.

Enough of the acid can be left with the one in twenty-four ester to properly function as a diluent or solvent along with the anhydrid and catalyst, as is customary in the acetylation of plain cellulose. But where the lower ester is desired by itself. the excess of acid is completely wrung and washed out of it and the latter then dried. Acetyl cellulose, thus obtained, has substantially the superficial appearance of the cellulose from which it was prepared,'in other words, 1t looks like cotton when 1t is prepared from cotton fibers, as in the above example. Moreover, such an ester is remarkably stable and withstands rather prolonged boilingin w'at'er wi thoutloss of the acetyl group. Of course, in the above and in the acid, the latter may contain up to about of water. But in this case it requires a very much longer time before the proportion of acetyl group rises above 4%. It' is, therefore, highly preferable to keep the water content lower than 10% and preferably below 1%. v

In a further example, one part of cellulose, say cotton fibers of the usual purified form, is boiled under atmospheric pressure with. 30 parts by weight of propionic acid, provision being made for the removal of the water during the boiling, say by a suitable reflux condenser. In general the percent- 'e of propionyl group rises to about 6% fter 48 hours and reaches a maximum of about 8% after between 100 and 200 hours boiling In another example one part by weight of cellulose, say cotton fibers somewhat puri fied, is boiled at atmospheric pressure with 30 parts by weight of butyric acid. After 48 hours boiling the product contains in the neighborhood of 5% of butyryl group and isuitable kind 'of cellulose is heated with 30;

after 200 hours has well above 10% of this ac 1 group present.

n another example one part of cellulose,

such as purified cotton fibers, say surgical cotton wool, is heated with 30 parts by weight of heptylic acid, the materials during the reaction being kept between the temperatures of 145 and 165 C. This is below the boilin point of the acid, but'a notable esteri cation, nevertheless, takes place. By observing these temperature recautions the tendency to degrade the pro uct is lessened.

In all of the above examples the opera tions are conducted at atmospheric pressure. The reaction may be considerably hastened by carrying out the action under pressure, say in a suitable acid-resisting autoclave. For example, one part of any part of acetic acid of 99 to 100%- strength at a temperature of 140 to 160 (1, and under a corresponding superatmospheric pressure. Under such circumstances up.-

wards of 5% of 'acetyl'group will be found.

combined in about 2 hours,thus enormously shortening the time of the'operation.

The-esterification may beconducted with the acid in'the vapor phase. For example, cotton fibers, may be placed in a glass tube, heated by any suitable means, such as an oil b'ath, and vapor of acetic acid conducted through the tube. It is convenient to operate at atmospheric pressure with the It will be noted that no catalysts are used 7 in any of the hereinabove described examples; and consequently 'there can beno degradation from that source, even upon prolonged heating.

It is a remarkable fact that all of the esters of low esterification'produoed in the above examples are especially susceptible to further esterification, say by the methods customarily employed in esterifying cellulose itself. Moreover, the acyl group, which is further introduced by the conventional methods, employing anhydrids and cata lysts, need not be the same acyl grou as that originally present in the lower or one in twenty-four ester. Thus, for example, the cellulose propionate, described above, may be acetylated and the final product will contain both propionyl and acetyl groups. Obviously a wide combination of mixed esters may be obtained in this manner.

It is a characteristic of our new esters that further esterification can be carried out with in acetic anhydri'd ranging between 10 and 25%. Because of the susceptibility of our low esters to further esterification, the latter! operation can beconducted with milder catalysts, such as zinc chloride, thus avoiding any danger of degradation of the product through careless or incorrect use of sulfuric acid as the condensing agent. As these methodsof esterification with anhyrid and a weak catalyst are well known to those skilled in the art, it is unnecessary to describe them in detail here as they can be used in their original form but with a useful lowering of the amount of aceticanhydrid.

Having thus described our invention,

what we claim as new and desire to secure 2. Substantially unhydrolyzed cellulose acetate having the appearance of cellulose and being stable to boiling water at atmospheric pressure, the proportion of the acetyl group therein being between 4.% and 6.5%.

3, In the process of [making a cellulose ester of a fatty acid, heating together cellulose and a fatty acid having more than one and less than eight carbon atoms at a temperature-between 117 and 17 0 C. inclusive until substantially one molecular proportion of said fatty acid combines with an amount of cellulose corresponding to twenty-four carbon atoms, said acid being the sole acylating agent.

4. In the process of making a cellulose ester of a fatty acid, heating together substantially unhydrolyzed cellulose and a fatty acid having more than one and less than eight carbon atoms, said acid being the sole acylating agent, the temperature during the reaction being kept below the decomposition points of the ingredients and product, and the proportion of water in the reaction mass permitting the acyl group in the ester to reach at least 4.72,.

5. In the process of making a cellulose ester of a fatty acid, heating together cellulose and an excess of a fatty acid having more than one and less than eight carbon atoms, said acid being the sole acylating agent, the reaction being conduct-ed at a pressure above atmospheric but the temperature being maintained below the decomposition points of the ingredients and product.

6. In the process of making cellulose acetate, heating cellulose in the absence of a catalyst in an excessof acetic acid, the latter being the sole acetylating agent, until the acetyl group in the ester reaches at least 4.%.

7. In the process of makin a fatty acid cellulose ester, partially esterif ying cellulose with a fatty acid having more than one and less than eight carbon atoms in theabsence both of an anhydrid and a catalyst, and thereafter esterifying it. further [with the aid of an anhydrid of a fatty acid.

8. In the process of making a fatty acid cellulose ester, heating cellulose, together With a fatty acid having more than one and less than eight carbon atoms until substantially one molecular'proportion of said fatty acid combines with an amount ofcellulose corresponding to twenty-four carbon atoms, said acid being the sole acylatin agent, and thereafter further esterifying 1t with the aid of an anhydrid of a fatty acid.

Signed at Rochester, New York, this 21st day of June 1926.

HANS r. CLARKE. CARL .i. MALM.