preparation of cellulose acetate

Preparation of Cellulose Acetate ACTION OF SULFURIC ACID

CARL J. MALM, LEO J. TANGHE, AND BARBARA C. LAIRD Eastman Kodak Company, Rochester, N . Y .

The amount of combined sulfur has been determined at all stages in the manufacture of cellulose acetate, includ- ing pretreatment, acetylation, and hydrolysis. Little combined sulfur was found during the pretreatment, but after the addition of acetic anhydride, the sulfuric acid combined quantitatively with the cellulose in the intermediate stages of acetylation. A t the point where substantially all the hydroxyl groups had been esterified, the combined sulfur content began to drop gradually, and as the reaction time was extended beyond the point where it is normally interrupted to initiate the hydrolysis stage, it continued to drop and was replaced by acetyl. The combined sulfur dropped rapidly during the addition of water for hydrolysis. The lowering of the sulfur content was sharply influenced by the temperature glnd the rate of addition of water. The amount of sulfuric acid catalyst was varied over a thirty-two fold range, and the amount of combined sulfur at the completion of esterification and during hydrolysis was roughly proportional to the concentration of sulfuric acid used. When part of the sulfuric acid was neutralized at the start of hydrolysis, the amount of combined sulfur in the product was determined by the amount of soluble sulfate (SO4--) present during hydrolysis.

OMMERCIAL acetylations of cellulose are carried out al- c most exclusively with sulfuric acid catalyst. Although numerous other catalysts have been recorded in the patent literature, sulfuric acid is one of the few which are really effective when used in moderate amounts a t comparatively low temperatures for.short reaction times. Thus 5-10y0 of sulfuric acid based on the cellulose is sufficient to complete the acetylation in 1-3 hours at 60-100' F. It is low in cost and does not present corro- sion problems in the recovery system.

A disadvantage of this catalyst, however, is that it enters into combination with the cellulose so that the cellulose ester requires a stabilizing treatment such as removal of the sulfate groups or salt formation with certain metallic ions.

Commercial acetylation schedules include usually a pretreatment, the acetylation proper, and hydrolysis. In the pretreatment, the cellulose, acetic acid, and all or part of the sulfuric acid catalyst are intimately mixed before the addition of the anhydride. After cooling, the acetylation proper takes place upon the addition of acetic anhydride and the remaining catalyst, if this was not all added a t the start of the pretreatment. During this interval the temperature is controlled to permit a gradual rise, and the acetylation phase is completed when a reaction dope, free from fiber and grain, and of the desired viscosity is obtained. At this point 50-75% aqueous acetic acid is added. The acetic acid serves merely as a diluent to prevent' local precipitation of the cellulose acetate during the addition of water. The signif& cant component of the mixture is the water which destroys the excess of anhydride and partially hydrolyzes the cellulose acetate. The product is then precipitated, washed, and, if neceasary, stabilized.

The behavior of the sulfuric acid catalyst throughout these stages of cellulose ester preparation has never been adequately

reported in the literature, but such fragmentary data as are avail- able indicate that it combines, in part at least, with the cellulose.

Caille (2) reported the introduction of combined sulfur when cellulose was treated with high concentrations of sulfuric acid in acetic acid under conditions comparable to the pretreatment stage. Clement and Rivihre (3) observed a decrease in the concentration of sulfuric acid in acetic acid solution upon the introduction of cellulose but were unable to find any sulfur in the cellulose recovered from such treatment by washing in distilled water. Marschall and Stauch (Q), using large amounts of sulfuric acid in acetic acid, found sulfur remaining on the fibers after washing in distilled water and attempted to differentiate between absorbed and combined sulfur, based on the easy removal of the former with dilute ammonium hydroxide.

Cross and Bevan (4) examined products a t the completion of the acetylation stage and found large amounts of combined sulfur, Ost ( I O ) extended the reaction time after complete dissolu- tion of fibers and found a considerable drop in sulfur content and a slight increase in acetyl content. Caille ( I ) observed a gradual increase in combined sulfur content throughout the course of the acetylation, which is a t variance with the findings of this paper.

The sulfur content throughout a limited range of hydrolysis has been reported. The effect of the amount of water added at the end of the esterification has been studied (3, 5, 6), but the effects of temperature and the rate of addition of the water have been neglected. The addition of a slight excess of water over that required to react with the remaining anhydride was found to provide the most favorable conditions for the cleavage of sulfate groups. Larger amounts of water caused the retention of considerable amounts of combined sulfate groups which were then slowly split off during the subsequent hydrolysis of the cellulose ester.

In the present work the sulfur content was studied throughout the various phases of cellulose acetate manufacture described above. Most of the work was carried out using an amount of sulfuric acid equal to 7% of the weight of the cellulose. This quantity of catalyst sufficed t o give a rapid but easily controllable esterification. Esterifications were carried out, however, using multiples and fractions of this quantity of catalyst, over the range from 0.875 to 28y0 based on the weight of the cellulose. Celluloses from various sources were compared, and minor variations were made in the acetylation schedule a t a given catalyst concentration. The temperature during the addition and the rate of addition of the water were found to be important factors in- fluencing the sulfur content of the product during hydrolysis. Finally the effect of partial neutralization of the catalyst a t the beginning of hydrolysis was studied.

ACETYLATION PROCEDURE

The acetylations were carried out in a water-jacketed stainless steel mixer of the Werner and Pfleiderer type, having a capacity of 5 gallons. Acetylations were carried out according to the following schedule, using 2.5 pounds of cellulose for each acetylation. One part of cellulose, moisture content about 5%, was added to 2.4 parts of acetic acid and the mixer run for a n hour a t 100' F.

.Then four parts of acetic acid and 0.88yo sulfuric acid, based on the weight of the cellulose, were added, and the mixing was continued at the same temperature for 45 minutes before cooling t o 65' F. The cooling was limited at this stage by the crystalliaa- tion of acetic acid but was continued to 60' F. after the addition

17

78 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 1

TABLE I. SULFUR CONTENT DURING ESTERIFICATION

A B C D E P 0 H I J K L M N 0

' R S 5

A B C D E F G H I J

A , Normal Reaction 10 min. pretreatment a t 100' F. . . 26 min. pretreatment a t 100' F. . . 45 min. pretreatment a t 100' F. . . Following C, cooled to 75O F. Following C cooled to 65' F. 0 : 9

8 . 1 30 min. after adding AczO 10 .0

5-min. reaction; full catalyst 14 .8 15-min. reaction 27.0 30-min. reaction 34 .7 60-min. reaction 42.8

1.5-hr. reactjon 2.0-hr. reaction 4318 3.0-hr. reaction 4.5-hr. reaction , 4 k : 1 6.0-hr. reaction

11.0-hr. reaction 24.0-hr. reaction 45: 1

10 min. afte; adding AcnO

7.6-hr. reaction 44:2

B . Slow Reaction 10-min. reaction: full catalyst 11.0 30-min. reaction 31 .1 60-min. reaction 39.2 2-hr. reaction 42.8 3-hr. reaction 43 .4 4.25-hr. reaction . . 6-hr. reaction . . 8-hr. reaction . .

11-hr. reaction . . 21-hr. reaction . .

Sulfur, c/o Found in Based on product cellulose

0.0109 0.0236 0.0307 0.0360 0.0353 0.227 0 ,254 0.987 1 .28 1 .50 1 .28 1 .08 0.935 0.748 0.607 0.401 0,336 0.190 0,062

1.31 1 .55 1 .37 1.29 1.20 1 .09 0.805 0.676 0.393 0.134

0,0109 0.0236 0.0307 0,0360 0.0356 0.247 0.282 1 .16 1 .75 2 .30 2.24 1 .91 1.66 1 .34 0.91 0.721 0.607 0.344 0.112

1 .47 2.26 2 .26 2.26 2 .13 1.81 1.44 1.03 0 .71 0.24

of 2.7 parts of 98y0 acetic anhydride. The remaining6.12% of sulfuric acid was then added. The jacket temperature was controlled t o permit a gradual rise to 90-95' F. during an interval of 1.5 to 2 hours. At this stage the reaction dope was very viscous and free from fibers. A mixture of one part of water and two parts of acetic acid was added during an interval of one hour. The reaction of the excess anhydride with the water gave a sharp temperature rise of about 10" F. After thorough mixing and adjusting the temperature to 100" F., the dope was transferred to a hydrolysis bath held a t 100' F.

ISOLATION OF SAMPLES. Samples for sulfur analysis required different treatment, depending on the interval in which they were taken. As long as the fibers remained intact, they were isolated by immersion in distilled water with vigorous stirring and then washing to neutrality in distilled water with moderate stirring. At intervals of 10 to 15 minutes the water was changed, and the sample was squeezed out by hand in a cloth bag. Samples were considered neutral when one drop of 0.5 N alkali imparted a pink color to phenolphthalein indicator in 100 cc. of wash water. Four to six washes were sufficient.

Samples taken later than 10 minutes !allowing the last addition of catalyst to the mixer did not retain sufficient fiber struc- ture to be isolated in this way but required the addition of a diluent. During the interval in which acetic anhydride was present in the mixer, i t was found very important to maintain anhydrous conditions during the dilution. This was achieved by using a diluent consisting of acetic acid (melting point 16.1-16.3' C.) to which was added 2% of acetic anhydride and 0.05y0 of perchloric acid. The perchloric acid is added to catalyze the reaction of the acetic anhydride with the slight amount of water in the acetic acid. The diluent was added in small portions with good mixing to yield a slurry or a dope thin enough to give a good precipitate in distilled water with high speed stirring. Two to five parts of diluent for one part of dope were sufficient, depending on the point where the sample was taken. When anhydrous conditions were not maintained, as in the use of ordinary acetic acid or acetone for dilution, results were very erratic, depending on whether sufficient water was present to destroy the excess anhydride.

Samples taken in this interval were high in combined sulfur, and the precipitates were very gelatinous in texture and almost transparent in water suspension. After centrifuging or squeezing as dry as possible by hand in cloth bags, they could be readily washed to neutrality but required special care in drying. Before drying they contained only about 3% solids. The bulk of the moisture was removed in a current of air a t 160" F., and the drying was completed at 120" F. Even a t this temperature the prod-. ucts discolored readily if not removed as soon as dry. Acetic acid was split off on storage in closed bottles for a few days.

After addition of the aqueous acetic acid, samples were isolated by dilution of the hydrolysis dope with two parts of aqueous ace-

tic acid and precipitation in distilled water. Samples were washed to neutrality and dried a t 160" F.

I n the range of high sulfur content, separate samples were prepared for acetyl analyses. These were prepared by diluting the reaction mixtures 'with 99% acetone and stirring the mixture for a few minutes at room temperature. The acetates were then precipitated and washed in the usual manner. This reduced the sulfur content to such a low level (about 0.05% sulfur) that it did not interfere with the determination of acetyl by the modified Eberstadt method (7'). Also, the samples vere much more stable toward drying.

SULFUR CONTENT DURING ESTERIFICATION

Sulfur analyses were carried out as previously reported (8) , but smaller samples were used when the sulfur content was high. Table IA presents the results of sampling from a cellulose acetylation carried out according to the schedule outlined above. After 2-hour reaction with full catalyst the reaction dope was free of fibers, and a portion was removed from the mixer and held in a closed bottle a t 100' F. for further sampling.

A similar acetylation was carried out with additional cooling t o prolong the reaction. The mixture was cooled to 50' F. before the addition of the last portion of catalyst, and the temperature was permitted to rise gradually to 95 O F. during an interval of 6 hours. Samples taken from the reaction mixture are described in Table IB. Figure 1 presents composite data from these two sam- plings and also the sulfur content during hydrolysis.

Since the maximum possible sulfur content, based on cellulose, is 2.27% when using 7% catalyst, the above values indicate that the sulfuric acid combines quantitatively with the cellulose in the intermediate stages of the acetylation process, In support of this conclusion, it was found that the wash water from the precipitation of samples C and D, Table IB, gave no qualitative test for sulfate upon the addition of barium chloride solution.

TREATMENT OF SAMPLES HIGH IN COMBINED SULFUR. The gelatinous nature of samples very high in sulfur has already been mentioned. The following evidence indicates that the sulfur ia chemically combined rather than physically retained in a "poor" precipitate. First, the sulfur is not readily removed on cxtended washing. Second, considerable latitude is permissible in the amount of anhydrous diluent taken without affecting the sulfur content. Third, the samples can be dissolved in acetic acid and reprecipitated without intermediate drying and with only slight loss in sulfur content. Fourth, deacetylation with 14% ammonium hydroxide for 2 days a t room temperature removes only about 20% of the total sulfur. Fifth, titration and ion displacement reactions show one equivalent of acidity for each atom of sulfur. The following table shows the latitude permissible in the precipitation of samples high in combined sulfur, with respect t o the amount of anhydrous diluent used and the time interval b e tween dilutfon and precipitation.

Sample A B C D E F

Parts Diluent (Anhydrous AcOH)

2 .5 4 6 4 4 4

Interval Minuted

5 5 5

10 20 30

Sulfur, %

1.07 1 .08 1 .01 1.02 0.94 1.07

Samples high in combined sulfur can be reprecipitated and deacetylated, without intermittent drying, with only slight loss of sulfur :

Sulfur Content, 70 ----Deacetylated- Found in Based on cellu-

Sample Original Reprecipitated product lose acetate A B c

1.03 0.956 1 .57 0.881 1.09 0.921 1 .41 0.789 1 .08 . . . 1.52 0.850

This table and some experiments previously published (8) clarify some-of the confusion in the literature due to failure to take ac- count of cleavage of combined sulfur during drying. The decrease in sulfur content on reprecipitation, reported by Taniguchi (11) is due, in part a t least, to cleavage during drying.

January, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 19

- 2

ACETYLATION SCHEDULE

A.MIXER CHARGED WITH LINTERS AND AcOh A 8 C F G

IF - .

2 2 . 4 - 3

1.0 7

E

z 0.2

6.MINOR PORTION OF CATALYST ADDED C. BEGAN COOLING TO 6 5 F.

.MAJOR PORTION O F CATALYST ADDED ,RETICAL MAXIMUM 2.27e,e D. AcLO ADDED; CONTINUED COOLING TO 60F

F-G.WATER ADDED DURING ONE HOUR

W 2 2 0 -

I 2 3 5 20 HOURS HOURS FROM BEGINNING

I 2 3 4 5 DAYS HYDROLYSIS AT 100 F.

OF MIXER CYCLE

ties used in the acetylation, were allowed to react for the time and at the temperatures used in the a c e t ylation. A sample of the reaction mixture was diluted with water and the remaining sulfate precipitated with barium chloride. It was found that only 3,6y0 of the sulfuric acid had been converted to sulfoacetic acid.

A sample of reaction dope at the completion of esterification was precipitated ih 1 N hydrochloric acid, and the suspension was digested on the steam bath overnight to degrade the cellulose ester t o water solubility. A small amount of residual fiber . was filtered off, and the sulfuric acid originally added was accounted for quantitatively by precipitation with barium chloride. A control experiment in which sulfoacetic acid was heated under the same conditions showed that i t did not revert to sulfuric acid.

Finally, an acetylation was carried out using sulfoacetic acid as the catalvst. Even with the use of

.

-

Figure 1. Combined Sulfur during Preparation of Cellulose Acetate an amount of SulfoaCetiC acid equal to m% of the weight of the cellulose, higher temperature and

A repetition of the experiments of Marschall and Stauch (9) showed that dilute ammonium hydroxide was effective in lowering the sulfur content only after drying of the sample. They were apparently observing a partial cleavage of combined sulfur during drying, rather than differentiating between absorbed and combined sulfate.

NATURE OF COMBINED SULFATE. Samples high in combined sulfur and washed free from acidity could be titrated in aqueous sodium chloride suspension with dilute alkali. Furthermore, an ion displacement reaction could be forced to completion by repeated treatments with sodium chloride:

cellulose-OS020H + NaCl + cellulose-OSOzONa + HCl For the titration experiments a 25-gram, wet sample (2.94%

solids) was shaken with 100 cc. of 1% sodium chloride solution and titrated with 0.075 N sodium hydroxide, using bromothymol blue indicator. The end point was established rapidly and did not give troublesome fading. On holding overnight, the blue color was discharged but could be re-established with the addition of 1 or 2 drops of alkali.

In the ion displacement experiments the sample, as above, was given repeated treatments with sodium chloride solution until no further acidity was found in the filtrates. After each treatment the cellulose ester was filtered off on a fritted glass funnel, and the hydrochloric acid in the filtrate was titrated. For 25 grams of wet sample, five treatments with 100-cc. portions of 1% sodium chloride were required to drive the reaction to completion.,

The calculations of sulfur content from the titration data are based on one equivalent of acidity for each combined sulfate. The following results were obtained on a sample taken at the completion of a normal acetylation:

Method Sulfur Content, % Direat snalyds 1 06 1 08

Ion diiplaoement 1.01 Titration 1:01: 1:02

The term cellulose sulfoacetate has been used loosely in the literature in reference t o products poor in stability and high in sulfur content. I n strict usage this term should be applied only to an ester of cellulose with sulfoacetic acid which conceivably could be formed by the reaction between sulfuric acid and acetic anhydride. However, this reaction is retarded by the presence of acetic acid. Experiments showed that under the conditions pre- vailing in the cellulose acetylation described, the conversion of sulfuric to sulfoacetic acid was small, and that during an acetylation the conversion was negligible.

Acetic acid, sulfuric acid, and acetic anhydride, in the quanti-

longer time were required than with 7% of sulfuria acid. Products isolated throughout the reaction and early stages of the hydrolysis contained only 0.02 t o 0.03% of sulfur.

SULFUR CONTENT DURING HYDROLYSIS

The hydrolysis stage in a cellulose acetylation is initiated by the addition of a n acetic acid-water mixture to decompose the remaining anhydride and to provide water for the hydrolysis of the cellulose acetate. In these experiments three parts of 67% acetic acid for one part of cellulose were added over a period of one hour to provide 7 to 8% of water in the hydrolysis dope.

The sulfur content of samples over a wide range of hydrolysis at 100 F. is illustrated by the data of Table 11. The sulfur content always passed through a minimum during hydrolysis; an experiment relating t o this behavior is described toward the end of thq paper.

TABLE 11. SULFUR CONTENT DURING HYDROLYSIS . Acetyl, % Sulfur, % Sample Treatment

A B C D E F G H

Just before adding water Just after adding water

6-hr. hydrolysis 17-hr. hydrolyeis 41-hr. hydrolyeis 65-hr. hydrolyeis 89-hr. hydrolysis

113-hr. hydrolysis

.. 1.02 0.0214

4i :g 0.0130 40.9 0.0125 38.5 0.0139 35.4 0.0138 34.0 0.0141 31.8 0.0160

No difference could be detected in cellulose from different sources. I n each case samples were taken throughout hydrolysis, but the results were so similar that only representative data are given :

Sulfur Content, % -. Just before Just after Minimum addition additioii during

Source Cellulose of water of water hydrolysis A Linters 1.02 0.021 0.013 B Linters 1 .08 0.037 0.012 C Pulp 1 .06 0.020 0.013 D Pulp 1 .09 0.028 0.012

The variations in the sulfur content just after addition of water are not characteristic of the sample of cellulose since, at this stage, variations of the same magnitude were encountered in duplicate runs with the same cellulose. At all other stages of sampling, good duplication of results could be achieved. The variation in sulfur content just after the addition of water is more understandable in the light of further experiments in which the temperature and the rate of addition of the water were vaned. Minor variations in the acetylation schedule were without affect on the sulfur content throughout the hydrolysis.

Vol. 38, No, 1 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY 80

0.15

IT 3

A 5 0.IC

g

a z w (D

0

d 0.0 e

0.02

0.01

\ A D D I T I O N AT END O F TIME ESTER IF1 C A T I O N FOR PARTS %AcOH ADDITION

3 67 2HOURS 3 67 I HOUR

-- - 3 6 7 30MlNUTES 3 67 IOMINUTES \ n 4 5 0 I O M I N U T E S

k C

k -: - v - I 1 I I I I 5 IO 20 40 6 0 80 I O 0 12

HOURS HYDROLYSIS AT 100' F.

further aqueous acetic acid was introduced, after the addition of three parts of 67% acetic acid during one hour, high values for sulfur content did not ensue. The higher values for sulfur content throughout Table IV are duch to the rapid addition of the water.

An experiment in which alternate additions of water, anhydride, and water were made at the completion of acetylation demonstrated the extreme case with which sulfuric acid combines with reactive hydroxyl groups in celluloqe. At first just enough water was added to destroy the excess anhydride. This resulted in cleavage of over 95% of the combined sulfur. But with the addition of acetic anhydride at this point, the sulfuric acid re-entered the cellulose almost quantitatively, despite the competition of the anhydride for the same hydroxyl groups (Table V). There was a slight temperature rise with each of these additions, but cooling was applied to maintain

Figure 2. Sulfur Content during Hydrolysis of Cellulose Acetate after the temperature at loo F* Addition of Water at Different Rates TEMPERATURE DURING ADDITION OF WATER.

Considerable variation was permissible in thc amount and na- furl: of the diluent used for dope samples taken during hydrolysis. Usually about two parts of 50-807, acetic acid for one part of dope were used for dilution, the amount of water in the diluent being increased as hydrolysis progressed. In one experiment ali- quot portions of dope were diluted with one, two, and four parts of 75% acetic acid, tn-o parts of 757, acetone, and two parts of 75% pyridine. Identical values were found for combined sulfur content. One and four parts of diluent rcpresented, respectively, minimum and maximum amounts suitable to give good precipitates in distilled mater a t room temperature. All samples of diluted hydrolysis dopes were filtered through felt to assure freedom from gelatinous lumps. Samples could be reprecipitated and dc- acetylated without intermittent drying, and no combined sulfatc was lost.

I n the following experiments variations were made in the time interval for the addition of water. First, portions of 67% acetic acid were added s loi~ly to permit sampling near the point where just enough watcr had been added to destroy the excess of acetic anhydride. Anhydrous or aqueous acetic acid dilucnt was used, depending on whether a test portion of the dope showed the presence of anhydride or 17 ater, as indicated by a temperature rise upon the addition of a small amount of water or acetic anhydride, respectively. Results in Table I11 show the abrupt drop in sulfur content with the disap- pearance of excess acetic anhydride.

Table IV shows the effect of adding water a t accelerated rates and, in one instance, the effect of an increased amount of water. Figure 2 shows graphically the tremendous effect of variations in the rate of addition of water. The inset, covering a wider range of sulfur content, shows the effect of rapid additions of water. T h e n

RATE OF A 4 ~ ~ ~ ~ ~ WATER. #

TABLE 111. SULFUR CONTENT AFTER SLOW ADDITION OF TT'ATER Parts 67%

Acetic AceticAcid Acetyl, Sulfur, Gamde Time of Sampling Acida Diluent % 70 .~

A' Just before adding water . . . Anhydrous

B After 20 min. 0 , 2 Anhydrous C After 40 min. 0 . 4 Aqueous D After 60 min. 0 . 6 Aqueous E After 120 min. 3 . 0 Aqueous F 7-hr. hydrolysis 3 . 0 Aqueous G 24-hr. hydrolysis 3 . 0 Aqueous H 48-hr. hydrolysis 3 . 0 Aqueous I 80-hr. hydrolysis 3 . 0 Aqueous J 96-hr. hydrolysis 3 , 0 Aqueous K 120-hr. hydrolysis 3 . 0 Aqueous

0 Added for one part of cellulose.

, . 1.17 , . 1.06 , . 0.0315 , . 0.0201

0.0195

4 0 . 5 0.01oL; 3 7 . 8 0.0127 34 .7 0.0152 3 3 . 1 0.0162 XI 1 0.0172

4i:a o.oioo

I n the runs described so far, the temperature was about 95' F. before the addition of aqueous acetic

acid, with a rapid temperature rise of 10 O F. upon reaction of the water with the excess anhydride. I n the following experiments variations were made in this temperature range by heating or cooling the mixture at the completion of normal esterification but before the addition of water. I n the first of these the temperature was maintained in the range 75-82' F. and in the second, 100-124" F. In both cases the water was added during one hour; before the mixer was emptied, the temperature was brought to 100" F., where it was maintained during hydrolysis. The data in Table VI and Figure 3 show that an increase in temperature during the addition of water gives products of con- siderably lower sulfur content, especially in the early stages of hydrolysis.

TABLE I l r . SULFUR CONTENT .%YPER FAST ADDITIOK OF \T-ATER Interval Aqueous for 1 2 0 AcOH Acetyl, Sulfur,

Sample Addition Added Time of Sampling 70 % C-1 30 min. 3 parts Just before adding

c -2 Just after adding 67Yc AcOH water . . 1.06

wa TPI' 44 x n 0357 c-3 c-4 C-5

G-h;..hydrolysis 43. E 0,0202 47-hr. hydrolysis 40 .7 0.0144 95-hr. hydrolysis 37 .5 0.0167

D-1 10 min. 3 parts Just after adding 67Y0 AcOIl water .. 0.788

D-2 2.5-hr. hydrolysis 0.615 D-3 7.5-hr. hvdrolvsis 4 i : 1 0,348 D-4 D-5 1 D-6 n-7 0-8 E-1

E-2 E-3 E-4 E-5 E-6

10 min 4 parts 50% AcOH

12-hr. hydrolyiis 41 .9 0 ,231 24-hr. hydrolysis 0 ,081 36-hr. hydrolysis 39' 6 0.030 72-hr. hydrolysis 37,O 0.0146

120-hr. hydrolysis 32 .4 0,0155

12-hr. hvdrolviis 41 .9 0 231 24-hr. h)drol$sis 0 ,081 36-hr. hydrolysis 39' 6 0.030 72-hr. hydrolysis 37,O 0.0146

120-hr. hydrolysis 32 .4 0,0155 Just before adding

water 4-hr. hJ-drolysis

22-hr. hydrolysis 45-hr. hydrolysis 94-hr. hydrolysis

118-hr. hydrolysis

. . . . 3 L 4 32 .0 2 9 . 1

1.13 1 .06 0.756 0.418 0.181 0.115

COSCGNTRATION OF CATALYST. Acetylations were carried out with wide variations in the amount of catalyst and appropriate modifications in the acetylation schedule. Reactions using lower concentrations of catalyst required higher temperatures and longer reaction times. I n all cases the other ingredients of tho reaction were held constant. At the end of the esterification thrcse parts of 67% acetic acid were added a t average temperatures of 95-105 F. during one hour. Samples were taken just before addition of water and throughout hydrolysis. Results were comparable to those of Table I1 with regard to the rapid drop in sulfur

January, 1946 * I N D u s T R I AL" X N' D E N G I N E E R ~t N G' c H E M I s T R Y 81

n \ content during addition of water and the ' - minimum during hydrolysis. A summary of the data follows:

Total Catalyst in Sulfur in Product, % Esterification (Based Just before Min. during

on Cellulose), % adding water hydrolysis 0 .88 1.75 3 . 5 7 . 0

14.0 28 .0

0 .18 0.32 0.50 1.06 1.97 3 .79

0.0030 0.0045 0.0070 0.0125 0 029 0:059 ',

The sulfur content of the product is roughly proportional to the amount of sulfuric acid used in the esterification, with slightly more than a proportionate amount at low levels of catalyst concentration.

PARTIAL NEUTRALIZATION OF CATALYST. The neutralization of part of the catalyst a t the beginning of hydrolysis retarded the rate of hydrolysis and lowered the amount, of combined sulfur in the product, in the event an insoluble sulfate was formed. I n the solvent mixture used for hydrolysis, magnesium and calcium sul-

0.15

A. WATER ADDED AT 75- 82' F E. WATER ADDED AT 97-106'F C.WATER ADDED AT 11O-l&?4*F e

LL

In P Y r I 0 0

6 0.10

." 0.05

0.0 2 0.0 I

5 IO 20 40 60 80 100 I HOURS HYDROLYSIS AT 100'F.

Figure 3. Sulfur Content during Hydrolysis of Cellulose Acetate after Addition of Water at Different Temperatures

fates were substantially insoluble whereas pyridine sulfate and sodium sulfate were soluble. The addition of chlorides did not have a neutralizing action but permitted an exchange of sulfuric acid for hydrochloric acid. With the addition of sodium chloride, no insoluble sulfate was formed, and no appreciable change was observed in the rate of hydrolysis or the sulfur content of the product. The amount of sulfuric acid initially present in these acetylations was 7% based on the cellulose. Results are summar- ized as follows:

Molar Ratio Minimum S of Salt to Content of Rate of

Salt Sulfuric Acid Product, % Hydrolysis None MgCOs MgCOa Ca(0Ac)z CaClz Pyridine acetate NaCl

. . . 1:a 3:4 .1:2 1:l 1:2 1:1

0.0125 0.0065 0.0045 0.0065 0.0024 0.0123 0.0139

Rk'ddck'd ' Reduced Reduced Accelerated Reduced Unchanged

With the neutralization of 50 or 75% of the catalyst with magnesium carbonate, the combined sulfur content of the product sought the same level as in reactions which used amounts of sulfuric acid corresponding to the residual amounts of catalyst. With the complete neutralization of catalyst either before or after the addition of water, the combined sulfur content was fixed at its existing level.

HYDROLYSIS O F SULFUR-FREE CELLULOSE ACETATE

In reactions carried out with sulfuric acid catalyst, a minimum in combined sulfur content during hydrolysis was always observed, and the subsequent rise was not entirely explained on the basis of the increasing cellulose content of the samples of hydrolyzed acetate. An experiment was carried out in which a sulfur- free cellulose acetate was dissolved in acetic acid and hydrolyzed by the addition of sulfuric acid and water.

The starting material was prepared by acetylating cellulose with zinc chloride as catalyst, followed by slight hydrolysis to give a product of good solubility in acetic acid. It was precipitated in distilled water, washed, dried, and redissolved in acetic acid alone. A mixture of water, acetic acid, and sulfuric acid was then added to this dope to duplicate the conditions of hydrolysis in a normal reaction carried out with 7% of sulfuric acid based on the cellulose. The temperature during hydrolysis was 100" F., and the sulfur content ultimately reached the same value as a

Q

reaction carried out with 7% sulfuric acid throughout. Introduc- tion of sulfur into the product is shown by the following table:

Sample Hr. of Hydrolyeink Sulfar, % Acetyl, % A Starting material 0.0006 41.4 B 6 0.0048 40 .3 C 12 c ,,-,..., D 24 ( E 48 C F 96 c

I . UWOB ).0091 ).0113 ).0141

39 : s

34;3

I n esterifications catalyzed by sulfuric acid, the catalyst combines rapidly with the cellulose, and since the latter is present in the fibrous state, the combination may not be uniform. If some portions of the cellulose are relatively sulfur-free, the final rise of combined sulfur in the latter stages of hydrolysis may repre- sent an equilibration of sulfuric acid with all portions of the cellulose.

'

TABLE V. EFFECT OF ALTERNATE ADDITIONS OF WATER, ACETIC ANHYDRIDE, AND WATER

Acetic' Acetyl, Sulfur, Sample Time of Sampling Diluent % %

At end of esterification 0.4 part 67% AcOH added

during 20 min. 0 .6 part Am0 added:

sampled after 15 min. 3 parts 67% AcOH added

during 1 hr. 42-hr. hydrolysis

113-hr. hydrolysis

Anhydrous

Aqueous

Anhydrous

Aqueous Aqueous Aqueous

. . 43.7

.. 44.0 39 .8 30 .6

1.21

0.0440

1.11

0.0282 0.0182 0.0188

TABLE VI. EFFECT OF ADDING WATER AT DIFFERENT TEMPER- ATURES ON SULFUR CONTENT DURING HYDROLYSIS

Temp. during Water Addition Acetyl, Sulfur,

Sample Initial Max. Time of Sampling % % A- 1 A-2

A-4 A-3

A-5 A-6 A-7 A-8

75' F. 82O F. Just before adding water

Just after adding water 4-hr. hydrolysis

12-hr. hydrolysis 20-hr. hydrolysis 28-hr. hydrolysis 49-hr. hydrolysis

116-hr. hydrolysis

. . 42:4 41 . O . . . . . . . .

1.05

0.398 0.160 0.0796 0.0368 0.0235 0,0160 0.0166

C-1 110: F. 124" F. Just befoye adding water . . 0.737 ,

c-2 Just after adding water 43 9 0.0091

c -3 7-hr. hydrolysis . . 0 0081 c -4 24-hr hydrolysis 0 0117 c - 5 48-hr: hydrolysis 38 '7 0 0146 C-6 96-hr. hydrolysis 34.7 0 0158

82 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 1

CONCLUSIONS

1. The combination of sulfuric acid with cellulose during cellulose acetate manufacture has long been known but has now been found to be quantitative during the intermediate stages of acetylation. Toward the completion of acetylation and with extended time of reaction, the combined sulfur is gradually replaced by acetyl.

2. The loss in sulfur content during isolation of samples from acetylation dopes is avoided by maintaining anhydrous conditione during the dilution prior to precipitation. , 3. Sulfuric acid combines with cellulose during acetylation to form a ceIlulose acetate acid sulfate. This acid sulfate is remark- ably resistant toward hydrolysis with 14% ammonium hydroxide. 4. Samples high in combined sulfur can be reprecipitated

without intermittent drying with only slight loss of sulfur. 5 . Extreme variations in combined sulfur content may be

found during the early stages of hydrolysis. Rapid addition of water and low temperature favor the retention of combined sulfur.

6. During hydrolysis the combined sulfur drops rapidly to a minimum and then increases slightly on prolonged hydrolysis. The same !eve1 of sulfur content was introduced into a sulfur-free cellulose acetate when hydrolyzed with acetic acid, water, and sulfuric acid in amounts equal to those resulting from a cellulose acetylation with sulfuric acid catalyst.

The amount of combined sulfur a t the completion of acetylation and during hydrolysis is roughly proportional over a wide range to the amount of sulfuric acid catalyst used. I n the

7.

event of partial neutralization of catalyst a t the start of hydrolysis, the amount of combined sulfur in the product is determined by the amount of soluble sulfate present during hydrolysis.

The conversion of sulfuric to sulfoacetic acid during these cellulose acetylations was negligible. Sulfoacetic acid is a very weak acetylation catalyst which does not combine appreciably with cellulose during acetylation.

8.

LITERATURE CITED

(1) Caille, A., Chimie & industrie, 12, 441-8 (1924).

(3) Clement, L., and RiviBre, C., Bull. SOC. chirn., 4, 889-80 (1937). (4) Cross, C. F., and Bevan, E. J., Researches on Cellulose, 1900-

1905, pp. 83-93, London, Longmans, Green and Co., 1906. (5 ) Deripaako, A., Cellulosechem., 12, 254-63 (1931). (6) Fabriek van chemische Producten, French Patent 858,324,

(Jan. 25, 1929), reproduced in Fausts Celluloseverbindun- gen, pp. 848-50, Berlin, Julius Springer, 1935.

(7) Genung, L. B., and Mallatt, R. C., IND. ENG. CHEM., ANAL. ED., 13, 369-74 (1941).

(8) Malm, C. J., and Tanghe, L. J., Ib id . , 14, 940-2 (1942). (9) Marschall, A,, and Stauch, H., J . mabromol. Chem., 1, 56-73

(10) Ost, H., 2. angew. Chem., 32,66-70.78-9,82-9 (1919). (11) Taniguchi, M., J . SOC. Chem. Ind. Japan, 44, Suppl. binding,

PEEsENTan on the program of the Division of Cellulose Chemistry of the 1845 Meeting-in-Print, AMERICAN CHmxrrcar, SOCIETY.

( 2 ) Ibid. , 13, 11-13 (1925).

(1943).

83-5 (1941).

CONTROLLING ORANGE DECAY Thiourea, Thioacetamide, 2-Aminothiazole, and Quinosol

in Aqueous Solution J. F. L. CHILDS AND E. A. SIEGLER

U . S . Department of Agriculture, Orlando, Fla.

RELIMINARY reports have been made recently on the P effectiveness of thiourea (2) and several other organic com- pounds (3) in controlling decays of Florida orange fruits. Tests with thiourea over two seasons and additional tests with thioacetamide, 2-aminot%iazole, and quinosol @-hydroxyquinoline sulfate) confirm the results previously reported. Although the investigations are still in progress, the problem of decay control in citrus fruits is so important economically that it is desirable t o report at this time on the status of the work.

The first comprehensive publication on control of orange decay appeared in 1908 (17) . Since then citrus fruits have taken first place in economic value, and improved methods of handling and increased use of refrigeration in transit have decreased the loss from decay on a per box basis; nevertheless the total loss has increased with the tremendous increase in production. Re- frigeration in transit, however essential for the delivery of fruit in a fresh condition to the wholesale markets, merely delays the incidence of decay and transfers the main loss to the retailer, the consumer, and ultimately back to the producer. For this reason there have been many attempts to develop a treatment which would decrease the spoilage of fruit after its arrival at the wholesale market.

I n this problem most of the critical etiological factors have been known for many years. The stem-end rots are caused by the fungi Dipbdia natalensis .and Phomopsis citri; the blue and green molds, respectively, are caused by the fungi Penici l l ium itaEicum and P. digitatum. A11 of these organisms are dissemi- inated by spores. The stem-end rot organisms infect the LLbutton (receptacle, calyx, and stem parts) of the fruits some time before picking but remain inactive until after harvest; the two Penicil-

l i u m species, whose spores are ubiquitous, generally infect abrasions on the fruit after harvest.

During the past thirty years a number of control measures have been advocated. Some of these have been of practicral value; others have not justified the expense involved. Decay of Florida grapefruit caused by Diplodia and Phomopsis has been greatly reduced by the general adoption of Winstons recom- mendation (22) that the fruit be harvested by pulling, which separates the buttons from the fruits. Orange buttons are not easily removed, but i t has been demonstrated (14) that pulling oranges is feasible in Florida a t certain seasons when the fruit is tree ripe. However, experience has shown that, when fruit is pulled, there is often increased loss from green mold.

Despite the degree of control secured by remedial measures applied in the grove, the need for a method which will practically ensure 100% control is apparent when the problem is viewed from the standpoint of the various groups that comprise the citrua industry. Each group recognizes that intangible liabilities are in- herent in fruit which arrives at the consumer market with latent, invisible infection; but since the loss by each group is only a fraction of the total, there has been little appreciation of the enormity of the aggregate loss. I n experiments at this laboratory over two seasons i t was found that oranges stored three weeks at 70 F. with about 70 to 80% relative humidity showed 20 to 60% decay. The major portion of such losses is borne by the consumer.

One of the earliest control measures used commercially was the borax dip, developed about 1923 (8) This has remained the standard for comparative tests with hundreds of other antiseptics and fungicides. To be most effective under Florida conditions, i t is essential that borax be applied to the fruit shortly after harvest

preparation of cellulose acetate

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