US2490754A - Activation of cellulose for acylation - Google Patents

Description

Patented Dec. 6, 1949 ACTIVATION OF CELLULOSE FOR ACYLATION William B. Hincke, Kingsport, Tenn, and George A. Richter, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application August 4, 1945,

Serial No. 609,044

This invention relates to the activation of cellulose sheets in which the sheet is dampened or wetted, stored for a time under non-drying conditions, and then dewatered with a lower fatty acid.

It is well known that cellulose is more readily susceptible to acylation if it is first pretreated or activated. When the cellulose is in a loosely assembled form such as picked cotton linters, treatment for a time with acetic acid has been found to impart sufficient activation thereto, par- 3 Claims. (Cl. 260-212 ticularly when acetic anhydride is employed for the esterifying agent.

That water has a swelling action on cellulose has been recognized in the prior art. Its use for activating cellulose necessitates a long soaking period thus tieing up equipment and in addition anhydride is consumed if the moist cellulose is employed directly in the esterification process.

It has been recognized that a convenient form in which cellulose may be handled is in sheet form. The formation of sheets is accomplished by laying down the pulp slurry on a sheet forming surface and then drying. This drying shrinks the fiber resulting in loss of chemical reactivity due probably to a hardening of the fiber wall structure. As has been recognized in the prior art, cellulose which has been sheeted is generally less susceptible to esterification than cellulose which has not been so treated. The dry sheet cellulose even after mechanical distintegration is very difilcult to react by the ordinary commercial processes with lower fatty acid anhydrides so as to result in complete acylation. The residual unreacted material can be removed only with great difiiculty from the cellulose ester during the subsequent processing steps.

We have found that cellulose in sheet form can be activated while in that form to a degree that it is readily susceptible to esterification. Our activating method is limited to the use of cellulose sheet which has been dried to a point that unreactive hornified material is present therein, such as sheets having a moisture content of less than We have found that moistening the dry sheet, storing it under humidity conditions such that drying does not occur and then dewatering the sheet with concentrated lower fatty acid, a cellulose is obtained which readily enters in its entirety into an acylation reaction so that no residual unreacted material remains. In the manufacture of high butyryl cellulose esters (at least butyryl) butyric anhydride is ordinarily employed as the esterifying agent. As this anhydride is sluggish in its cellulose esterifying action compared with acetic anhydride, it is particularly important that the cellulose to be esterified with butyric anhydride be completely reactive.

One object of our invention is to provide a process for activating cellulose in sheet form in which the convenient sheet form is retained throughout the activation. Another object of our invention is to provide a process for activating cellulose in which the cellulose when ready for esterification is substantially free of moisture. A further object of our invention is to provide a method of activating cellulose in sheet form whereby the cellulose is rendered completely susceptible to the usual acylation procedure. Other objects of our invention will appear herein.

Our invention in its broadest aspects comprises applying moisture to cellulose sheet so that a moisture content of about 35-65% (based on the wet sheet) is imparted thereto, storing the sheet under conditions of suflicient humidity to prevent drying thereof (such as a relative humidity of at least for from 1 day to approximately 2 weeks; and then dewatering the sheet with a lower fatty acid of 2-4 carbon atoms, followed by acylation of the so-treated cellulose, such as in an esterification mixture in which butyric anhydride is the esterifying agent.

An outstanding feature of our invention is the reactivation of cellulose in sheet form. It is ordinarily very convenient to form cellulose into a dry sheet for either storing or shipping. For instance as wood pulp is marketed it is usually in the form of rolls of the dried pulp. In sheet form cellulose may be compacted into a small space. Activation of cellulose in sheet form avoids the necessity of providing mixers or containing vessels in which to carry out the pretreatment. By our method the moisture may be directly imparted to the cellulose sheet, which may then be rolled up or otherwise compacted, in which form it may be stored for the required time.

The cellulose pretreated by our process may be any acetylation grade cellulose in sheet form. For instance it may be cotton cellulose such as linters, wood pulp cellulose such as sulfite pulp, kraft pulp or other cellulose material of sufficient viscosity for making cellulose esters of the desired properties. Our invention is also applicable to the activation of mercerized cellulose, which is even less reactive in dry form than unmercerized cellulose. Our invention is directed to the activation of dry cellulose sheet, that is having a moisture content of less than 20%.

The cellulose sheet may be either thin. medium or thick as desired. For instance the thickness of the pulp sheet may be from paper thickness (.005") up to /8 inch or even more. The sheet may be composed of substantially unbeaten fiber or in some instances may be composed of pulp which has undergone a preliminary beating or jordanning. The sheet activated may be tightly compacted or it may be one in which the fibers are loosely assembled (or bulky).

The water employed is preferably one which has been purified such as by one of the usual water purification treatments, .to. remove any organic material in suspension or inorganic material in solution which may be present therein. Distilled water is ideally suited for use in this connection. If desired the water may contain a small proportion of preservative such as sodium benzoate or acetic acid (approximately 5%) as the presence of these materials will allow longer storage of the cellulose sheet without spoiling. In lieu of the use of preservative, the dry sheet may be sterilized by exposing to ultraviolet light such as on its way to the water applicator roll for this purpose. The water used should have no chemicals (except as specified) added thereto,

it: being preferable to employ pure water for activating the cellulose (-20 P. P. M. mineral content).

The water may be applied to the cellulose sheet by any desired method. such as spraying, steamcases warm or hot water may be applied with or without a subsequent chilling roll. In the use of spraying it is desirable to us spore-free air, if air is to be used, to assure the absence of organisms. After wetting the sheet it is preferred to roll it into a compactroll and allow it to stand for the time necessary .to impart .good activation to the cellulose. Thev pH of wet cellulose sheet may range from -8 depending on the wood pulp used. As the addition. of chemicals is to be avoided, the pH should be kept atthat naturally obtained with the pulp used.

' The time neededito obtain maximum. activation depends on several factors such. as the temperature, the amount of water present, the history of the cellulose fiber and theuse to whichv it is. to be put. We have found that under the most favorable conditions activation of the cellulose. may be obtained in as little as 24 hours. These conditions include the use as the starting material of a pulp which has notbeen severely dried (such as having a. moisture content-of -20%),

conducting the pretreatment at a relatively high.

temperature (such as 180 F.) after imparting a moisture content of 50-65% to the cellulose pulp sheet. The range of time within which activation may be obtained is from 1 day to 2 weeks. We have found howeverthat ordinarily a time within the rangeof 5-10 days is satisfactory for pretreating a pulp' sheet in accordance with our invention.

After the pulp sheet has been moistened so that the water content is -65% (based on the wet sheet) it is preferably compacted to save space, facilitate handling, and facilitate the pretreatment operation. If the sheet is continuous it is preferably rolled up into units of suitable.

size such as weighing from 100 to 500 lbs. or more. If the sheet is discontinuous the cut sheets may be piled one on the other to the desired height. The pulp sheet is then stored dehydrated by displacing the water therein with a lower fatty acid such as acetic, propionic, buiwric or a mixture of any two or all of these acids. This dehydration is conveniently carried out in the case of the continuous sheet by passing it, preferably while supported on a continuous perforated stainless steel belt, over a series of suction boxes and spraying or flooding the sheet with the lower fatty acid between suction boxes. If desired, acid of increasing concentrations may be applied to the sheet as it progresses through the series of suction boxes.

The acid applied to the pulp. to dewater it, serves the additional function of removing coloring matter contained in the pulp. In some cases it may be desirable to apply the acid at a somewhat elevated temperature (l-l80 F. for ex ample) to further enhance the removal of coloring matter contained in the pulp.

If the wet pulp is in the form of cut sheets, these may be dewatered by passing lower fatty acid through a column in which these sheets are stacked until substantially complete removal of the water is obtained. In any procedure for the removal of the Water, acetic acid, butyric acid or a mixture of acetic and butyric acids may be employed.

After the cellulose has been activated and a cellulose pulp sheet containing lower fatty acid results, it may be sent directly to the esterification. vessel either as a continuous sheet or after. it has passed through a mechanical shredder. If it is to be butyrylated so as to impart butyryl content thereto, it is usually desirable that the amount of acetic acid present be kept to a minimum. Our invention is particularly directed to the preparation of butyric acid esters of cellulose having a high butyryl content such as cellulose acetate butyrates or cellulose butyrates having a butyryl content greater than 25%. However our invention relates broadly to the activation of cellulose and cellulose activated in accordance with our invention also may be acetylated or propionated to form any of the following esters using the corresponding anhydride or anhydrides: cellulose acetate, cellulose propionate, cellulose acetate propionate.

The following examples illustrate our invention:

Example I One part of rolled sheet wood pulp of acetylation grade having the following composition:

or cellulose per cent 96.

p cellulose do 2.2

Gamma cellulose do 1.8 Solubility in 10% KOH do 6.1

Cuprammonium viscosity poises 7-20 uous countercurrent water displacement appara- The water was removed over a series of 8 tus. suction boxes by butyric acid of increasing concentrations. A total of i l -5 parts of 100% butyric acid (based on the dry weight of the sheet) was sprayed or flowed onto the sheet on the last stage of the suction box. About 1-2 parts of acid and from 0.01 to 0.04 part of water remain in the dewatered cellulose sheet. The other part of the acid is caused to-pass countercurrentwise from suction box to box and alternately through Parts Cellulose (dry weight) 1 Butyric acid 2 Acetic anhydride 0.76 Butyric anhydride 2.8 Sulfuric acid catalyst 0.03

This mass reacts rapidly in 4 to 6 hours, the temperature being so controlled that it does not exceed 100 F., the reaction being carried out in a sigma bladed jacketed mixer, in the jacket of which cooling water circulates to control the temperature. There was obtained by this process a very clear, haze-free, smooth, acid dope of cellulose acetate b-utyrate having a butyryl content of 38%. The resulting cellulose acetate butyrate was of fairly high viscosity and was particularly suitable for use in the manufacture of plastics.

Example II One part of rolled sheeted cotton linters of acetylation grade having the following composition was used.

Alpha cellulose per cent 99.0-99.5

Solubility in KOH solution do 1.5-2.5 Cuprammonium viscosity (A. C. S.

method) -seconds- 200-500 This rolled sheet was dampened with 40% water (based on the weight of the wet cellulose) by passing it over a water applicator and rerolling. The rolled sheet was then let stand for seven days in a high humidity room at 100 to 130 F. The damp pulp sheet was then dewatered by spraying it countercurrentwise with acetic acid in the same manner as described in Example I.

The loosely picked apart pulp, wet with two parts acetic acid, was charged to an acetylation reactor to which cold butyric anhydride had previously been charged. The reaction mass, having an initial temperature of about F., was of the following composition:

Parts Cellulose (dry weight) 1 Acetic acid 2 Butyric anhydride 4 Sulfuric acid catalyst .03

This mass reacted rapidly with increasing temperature but the temperature was controlled so as not to exceed 190 F. A sigma bladed jacketed mixer was used through the jacket of which cooling water was circulated to control the temperature. After from 4 to 6 hours, a smooth clear acid dope was obtained of a 26% butyryl acetate butyrate mixed ester particularly suitable for the manufacture of plastics.

Example III One hundred parts of rolled sheeted cotton linters of the same acetylaton grade as shown in Example II was dampened with 60 parts of water, re-rolled, and let stand five days in a room at 120-130 F. and -90% relative humidity. The so activated cellulose was then dewatered as described in Example I but using propionic acid instead of butyric.

The dewatered sheet, wet with two parts of propionic acid, was then picked apart and charged to a jacketed acylation mixer to which cold propionic anhydride had previously been added. The reaction mass, having an initial temperature of about 50 F., was of the following composition Parts Cellulose (dry weight) 1 Propionic acid 2 Propionic anhydride 3.4 Sulfuric acid catalyst 0.03

' After initiating the reaction, this mass acylated rapidly and smoothly. The temperature, controlled by jacket cooling, was not permitted to raise over F. In 4 to 6 hours a smooth completely reacted acid dope of a tripropionate cellulose ester resulted. The ester prepared was of high vscosity and of good clarity.

Example IV One part of a wood pulp rolled sheet of the same analysis as shown in Example I was activated by wetting and letting stand. After 5-8 days it was then dewatered as described in Example I. This pulp sheet, wet with two parts of butyric acid, was then picked apart and charged to a sigma bladed acylation mixer to which four parts of cold butyric anhydride had been added. The reaction mass at an initial temperature of 50-55 F. was of the following composition:

Parts Cellulose (dry weight basis) 1 Butyric acid 2 Butyric anhydride 4 Sulfuric acid catalyst 0.03

After initiating the reaction, the mass reacted rapidly to a smooth clear acid dope. At a temperature increasing as reaction ensued but not exceeding 80-90 F., the reaction became complete in from 4-6 hours to a high viscosity acid 1 dope of cellulose tributyrate.

Example V Cellulose (dry Weight basis) 1 Propionic acid 2 Butyric acid 4 Sulfuric acid 0.03

The reaction, initiated at about 50 F.,

75 gradually accelerated as the temperature was al- 7- lowed; to raise; Apeaktemperature of over- 90 F. was avoided. After 4 to 8 hours the reaction was complete to a clear thick acid dope of a propionate-butyrate mixed cellulose ester of a high viscosity.

The cellulose esters of high butyryl and propionyl content prepared from cellulose activated in accordance with our invention are, eminently suitable for the manufacture of molding compositions, melt-coating compositions or the like. by mixing with suitable plasticizers. If thought desirable, these esters. are readily susceptible to; treatment by the, method described and claimed in Malm and Crane Patent No. 2,346,498, granted April 11-, 1944.

We claim:

1'. A process for pretreating dry fibrous cellu-- lose sheet which has av moisture content of less than and is resistant to the action of esterifying, reagents, which comprises moistening the sheet to a point where the, moisture content of the whole is 65%, placing the sheet in an atmosphere having a relative humidity-of at least 60% and a temperature of 100-180 F. for one day to two weeks whereby the cellulose is activated, and then dewatering the sheet with fatty acid of 2-4 carbon atoms so as to give a cellulose containing fatty acid but substantially free of Water and readily susceptible to the action of acylating reagents.

2. A process for, pretreating dry fibrous cellulose sheet which has a moisture content of less than 20% and is resistant to the action of esterifying, reagents, which comprises moistening the sheet to a point where the, moisture content of the whole is- 30-65%, placing" the sheet in, an atmosphere having a. relative, humidity of; at least and a; temperature of 100-180-F. forone day; to two; weeks whereby the cellulose is activated, and then dewatering, the sheet with acetic acid so as to give a, cellulose containing aceticacid' but substantially free of water and, readily suseptible to the action of acylating re,- agents.

3. A process for pretreating; dry fibrous cellulose; sheet. having a moisture content: of 0-20%, which comprises moistening the sheet-to a, point where the moisture content of the whole is 50-65%, placing thesheet in an atmosphere-having a: relative humidity of -100% and a, tem -l perature of -180 F. for one day to two weeks: whereby: thezcelluloseis activated and then dewatering the sheetwith acetic acid so as to give a; cellulose containing acetic acidbut substantially free of water and readily susceptible to theaction of acylating reagents.

WILLIAM B. HINCKE. GEORGE A. RICHTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,752,596 Hubert Apr; 1, 1930 2,112,115 Richter Mar. 22, 1938 2,315,973. Malm Apr. 6, 1943 2,379,310 Malm etal June 26, 1945 2,380,706 Richter July 31, 19.45