Preparation of Cellulose Acetate ACTION OF SULFURIC ACID CARL J. MALM, LEO J. TANGHE, AND BARBARA C. LAIRD Eastman Kodak Company, Rochester, N . Y . T h e amount of combined sulfur has been determined at all stages in the manufacture of cellulose acetate, including 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.
reported in the literature, but such fragmentary data as are available 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 t o 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 i n acetic acid, found sulfur remaining on the fibers after washing in distilled water and attempted t o 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 dissolution 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 t o 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. I n 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 b u t 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 t o be important factors influencing 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.
c
OMMERCIAL acetylations of cellulose are carried out almost 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 5-10y0of sulfuric acid based on the cellulose is sufficient to complete the acetylation i n 1-3 hours at 60-100' F. It is low in cost and does not present corrosion 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. I n 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 t o 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
ACETYLATION PROCEDURE
The acetylations were carried out i n 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 t o 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 crystalliaation of acetic acid but was continued t o 60' F. after the addition 17
78
INDUSTRIAL AND ENGINEERING CHEMISTRY DURING ESTERIFICATION TABLE I. SULFURCONTENT
Sulfur, c/o Found in Based on product cellulose
A
B
C
D E
P 0 H
I J K L M N 0
5
'R
S
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. 0 :9 Following C cooled t o 65' F. 8.1 10 min. afte; adding AcnO 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 ,4k:1 6.0-hr. reaction 7.6-hr. reaction 44:2 11.0-hr.reaction 24.0-hr. reaction 45: 1
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
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
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 ..
1.31 1.55 1.37 1.29 1.20 1.09 0.805 0.676 0.393 0.134
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 t o 90-95' F. during a n interval of 1.5 t o 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 t o six washes were sufficient. Samples taken later than 10 minutes !allowing the last addition of catalyst t o the mixer did not retain sufficient fiber structure 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.) t o 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 t o 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 t o 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 t o 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-
Vol. 38, No. 1
tic acid and precipitation in distilled water. Samples were washed t o 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 t o 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 F. during an interval of 6 hours. Samples taken from the reaction mixture are described in Table IB. Figure 1presents composite data from these two samplings 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, I n 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 HIGHIN COMBINEDSULFUR. 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. O
Sample A
B
C
D
E
Parts Diluent (Anhydrous AcOH) 2.5 4 6 4 4
F
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 Sample A
B
c
Original 1.03 1.09 1.08
Reprecipitated 0.956 0.921
...
----DeacetylatedFound in Based on celluproduct lose acetate 1.57 0.881 1.41 0.789 1.52 0.850
This table and some experiments previously published ( 8 ) clarify some-of the confusion in the literature due to failure t o take account 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.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1946
ties used i n the acetylation, were allowed t o react for the time and at the temperatures used in the a c e t A 8 C F G - . A.MIXER CHARGED WITH L I N T E R S AND AcOh ylation. A sample of the reaction mixture was 2 6.MINOR PORTION OF CATALYST ADDED diluted with water and the remaining sulfate pre2 2.4C. BEGAN COOLING TO 6 5 ’ F. cipitated with barium chloride. It was found t h a t 3 ,RETICAL MAXIMUM 2.27e,e D. AcLO ADDED; CONTINUED COOLING TO 60°F €.MAJOR PORTION O F CATALYST ADDED only 3,6y0of the sulfuric acid had been converted F-G.WATER ADDED DURING ONE HOUR t o sulfoacetic acid. . A sample of reaction dope at the completion of esterification was precipitated ih 1N hydrochloric acid, and the suspension was digested on the steam bath overnight t o degrade the cellulose ester t o water solubility. A small amount of residual fiber . 7 1.0 was filtered off, and the sulfuric acid originally E added was accounted for quantitatively by precipitation with barium chloride. A control experiW 2 20z ment in which sulfoacetic acid was heated under 0.2 t h e same conditions showed t h a t i t did not revert t o sulfuric acid. I 2 3 5 2 0 HOURS HOURS FROM BEGINNING Finally, an acetylation was carried out using sulI 2 3 4 5 OF MIXER CYCLE foacetic acid as the catalvst. Even with the use of DAYS HYDROLYSIS AT 100’ F. an amount of SulfoaCetiC acid equal t o m% of the Figure 1. Combined Sulfur during Preparation of Cellulose Acetate weight of the cellulose, higher temperature and longer time were required than with 7% of sulfuria acid. Products isolated throughout the reaction and early stages A repetition of the experiments of Marschall and Stauch (9) showed that dilute ammonium hydroxide was effective in lowerof the hydrolysis contained only 0.02 t o 0.03% of sulfur. ing the sulfur content only after drying of the sample. They SULFUR CONTENT DURING HYDROLYSIS were apparently observing a partial cleavage of combined sulfur during drying, rather than differentiating between absorbed The hydrolysis stage in a cellulose acetylation is initiated by and combined sulfate. the addition of a n acetic acid-water mixture t o decompose the NATUREOF COMBINED SULFATE. Samples high in combined remaining anhydride and t o provide water for the hydrolysis of sulfur and washed free from acidity could be titrated in aqueous the cellulose acetate. In these experiments three parts of 67% sodium chloride suspension with dilute alkali. Furthermore, an acetic acid for one part of cellulose were added over a period of ion displacement reaction could be forced t o completion by reone hour t o provide 7 t o 8% of water i n the hydrolysis dope. peated treatments with sodium chloride: The sulfur content of samples over a wide range of hydrolysis at 100’ F. is illustrated by the data of Table 11. T h e sulfur concellulose-OS020H NaCl + cellulose-OSOzONa HCl tent always passed through a minimum during hydrolysis; an For the titration experiments a 25-gram, wet sample (2.94% experiment relating t o this behavior is described toward the end solids) was shaken with 100 cc. of 1%sodium chloride solution of thq paper. and titrated with 0.075 N sodium hydroxide, using bromothymol blue indicator. The end point was established rapidly and did TABLE 11. SULFUR CONTENT DURING HYDROLYSIS not give troublesome fading. On holding overnight, the blue Sample Treatment Acetyl, % Sulfur, ’% color was discharged but could be re-established with the addiA Just before adding water 1.02 tion of 1 or 2 drops of alkali. B Just after adding water 0.0214 I n the ion displacement experiments the sample, as above, was C 6-hr. hydrolysis 4i:g 0.0130 D 17-hr. hydrolyeis 40.9 0.0125 given repeated treatments with sodium chloride solution until no E 41-hr. hydrolyeis 38.5 0.0139 65-hr. hydrolyeis F 35.4 0.0138 further acidity was found in the filtrates. After each treatment G 89-hr. hydrolysis 34.0 0.0141 the cellulose ester was filtered off on a fritted glass funnel, and the H 113-hr. hydrolysis 31.8 0.0160 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 t o drive the reaction t o completion., No difference could be detected in cellulose from different The calculations of sulfur content from the titration data are sources. I n each case samples were taken throughout hydrolysis, based on one equivalent of acidity for each combined sulfate. but the results were so similar t h a t only representative d a t a are The following results were obtained on a sample taken at the comgiven : Sulfur Content, % pletion of a normal acetylation:
-
‘
19
ACETYLATION SCHEDULE
IF
+
+
..
Method Direat snalyds Titration Ion diiplaoement
Sulfur Content, % 1 06 1 08 1:01: 1:02 1.01
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 t o a n 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 t h e presence of acetic acid. Experiments showed t h a t under the conditions prevailing in the cellulose acetylation described, the conversion of sulfuric t o sulfoacetic acid was small, and t h a t during an acetylation the conversion was negligible. Acetic acid, sulfuric acid, and acetic anhydride, in the quanti-
Source
A B C
D
Cellulose Linters Linters Pulp Pulp
Just before addition of water 1.02 1.08 1.06 1.09
Just after additioii of water 0.021 0.037 0.020 0.028
-.
Minimum during hydrolysis 0.013‘ 0.012 0.013 0.012
The variations in t h e 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 t h e temperature and the rate of addition of the water were vaned. Minor variations in t h e acetylation schedule were without affect on the sulfur content throughout the hydrolysis.
.
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
80
\
0.15
A D D I T I O N AT END O F E S T E R IF1 C A T I O N PARTS %AcOH 3 67
\ n
3 IT
3 3 3 4
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 t o the rapid addition of the water. An experiment in which alternate additions of water, anhydride, and water were made a t 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 t o maintain
TIME FOR ADDITION 2HOURS I HOUR 30MlNUTES IOMINUTES IOMINUTES
--
-
67 67 67 50
A
5
0.IC
a
w
z
(D
g 0 d 0.0e
k 0.02 0.01
C
k
-
:
I
1
I
I
I
5
IO
20
40
60
v
I
80
12
IO0
HOURS HYDROLYSIS AT 100' F.
Figure 2.
#
Vol. 38, No, 1
the temperature at loo F* TEMPERATURE DURING ADDITIONOF WATER. 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 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. I n both cases the water was added during one hour; before the mixer was emptied, the temperature was brought t o 100" F., where it was maintained during hydrolysis. The data in Table VI and Figure 3 show that a n increase in temperature during the addition of water gives products of considerably lower sulfur content, especially in the early stages of hydrolysis.
Sulfur Content during Hydrolysis of Cellulose Acetate after Addition of Water at Different Rates
Considerable variation was permissible in thc amount and nafurl: 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. I n one experiment aliquot 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 t o 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 dcacetylated without intermittent drying, and no combined sulfatc was lost. experiments RATEOF A 4WATER. ~ I n the ~ following ~ ~ ~ variations were made in the time interval for the addition of water. First, portions of 67% acetic acid were added s l o i ~ l yto 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 disappearance of excess acetic anhydride. Table IV shows the effect of adding water a t accelerated rates and, in one instance, the effect of a n 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
O
TABLE I l r . SULFUR CONTENT .%YPER FAST ADDITIOKO F Sample C-1
Interval for € 1 2 0 Addition 30 min.
c-2
Aqueous AcOH Added 3 parts 67Yc AcOH
Acetyl, Time of Sampling Just before adding water Just after adding wa
c-3 c-4
C-5 D-1
10 min.
D-2 D-3
3 parts 67Y0 A c O I l
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
TPI'
G-h;..hydrolysis 47-hr. hydrolysis 95-hr. hydrolysis Just after adding water 2.5-hr. hydrolysis 7.5-hr. hvdrolvsis 12-hr. hvdrolviis hydrolyiis 24-hr. hydrolysis h)drol$sis 36-hr. hydrolysis 72-hr. hydrolysis 120-hr. hydrolysis Just before adding water 4-hr. hJ-drolysis 22-hr. hydrolysis 45-hr. hydrolysis 94-hr. hydrolysis 118-hr. hydrolysis
70 ..
44
\T-ATER
Sulfur,
%
1.06
x
43. E 40.7 37.5
..
n
0357 0,0202 0.0144 0.0167
4i:1 41.9
0.788 0.615 0,348 231 0,2 31
39' 6 37,O 32.4
0.030 0.0146 0,0155
.. ..
0,081
3L4 32.0
1.13 1.06 0.756 0.418 0.181
29.1
0.115
TABLE 111. SULFUR CONTENT AFTER SLOW ADDITION OF TT'ATER Parts 67% Acetic Time of Sampling Acida .~ Just before adding water . B After 20 min. 0,2 C After 40 min. 0.4 D After 60 min. 0.6 E After 120 min. 3.0 F 7-hr. hydrolysis 3.0 G 24-hr. hydrolysis 3.0 H 48-hr. hydrolysis 3.0 I 80-hr. hydrolysis 3.0 J 96-hr. hydrolysis 3 ,0 K 120-hr. hydrolysis 3.0 Added for one part of cellulose.
Gamde A'
0
..
AceticAcid Diluent Anhydrous Anhydrous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous
Acetyl,
%
,.
Sulfur,
70
, . , . , .
1.17 1.06 0.0315 0.0201
4i:a
o.oioo
0.0195
40.5 37.8 34.7 33.1 XI 1
0.01oL; 0.0127 0.0152 0.0162 0.0172
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 ~tN G'
content during- addition of water and the minimum during hydrolysis. A summary of the d a t a follows: Total Catalyst in Esterification (Based on Cellulose), % 0.88 1.75 3.5 7.0 14.0 28.0
Sulfur in Product, % Just before Min. during adding water hydrolysis 0.18 0.0030 0.32 0.0045 0.0070 0.50 0.0125 1.06 0 029 1.97 3.79 0:059 ',
cHE M I sTR Y
81
n\
'
0.15 A. WATER ADDED AT 75- 82' F E. WATER ADDED AT 97-106'F C.WATER ADDED A T 11O-l&?4*F
e LL
6
In
0.10
Y P
r
I 0
." The sulfur content of the product is roughly 0.05 proportional t o the amount of sulfuric acid used in the esterification, with slightly more than a proportionate amount at low levels of 0.0 2 catalyst concentration. 0.0 I PARTIAL NEUTRALIZATION OF CATALYST.The neutralization of part of the catalyst a t the 5 IO 20 40 60 80 100 I Q beginning of hydrolysis retarded the rate of HOURS HYDROLYSIS AT 100'F. hydrolysis and lowered the amount, of comFigure 3. Sulfur Content during Hydrolysis of Cellulose Acetate bined sulfur in the product, in the event an insoluafter Addition of Water at Different Temperatures ble sulfate was formed. I n the solvent mixture used for hydrolysis, magnesium and calcium sulfates were substantially insoluble whereas pyridine sulfate and reaction carried out with 7% sulfuric acid throughout. Introducsodium sulfate were soluble. The addition of chlorides did not tion of sulfur into the product is shown by the following table: have a neutralizing action but permitted a n exchange of sulfuric Sample Hr. of Hydrolyeink Sulfar, % Acetyl, % acid for hydrochloric acid. With the addition of sodium chloride, A Starting material 0.0006 41.4 no insoluble sulfate was formed, and no appreciable change was B 6 0.0048 40.3 C 12 c , , , . . . , I . UWOB observed in the rate of hydrolysis or the sulfur content of the D 24 ().0091 39:s E 48 C).0113 product. The amount of sulfuric acid initially present in these F 96 c).0141 34;3 acetylations was 7% based on the cellulose. Results are summarized as follows: I n esterifications catalyzed by sulfuric acid, the catalyst combines rapidly with the cellulose, and since the latter is present in Molar Ratio Minimum S the fibrous state, the combination may not be uniform. If some of Salt to Content of R a t e of Salt Sulfuric Acid Product, % Hydrolysis portions of the cellulose are relatively sulfur-free, the final rise None 0.0125 ... of combined sulfur in the latter stages of hydrolysis may repre1:a 0.0065 Rk'ddck'd ' MgCOs sent an equilibration of sulfuric acid with all portions of the cellu3:4 0.0045 Reduced MgCOa .1:2 0.0065 Ca(0Ac)z Reduced lose. 1:l 0.0024 CaClz Accelerated Pyridine acetate NaCl
1:2 1:1
0.0123 0.0139
'
Reduced Unchanged
TABLE V. EFFECT OF ALTERNATE ADDITIONS OF WATER, ACETIC 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 t o 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.
ANHYDRIDE, AND WATER
Sample
Time of Sampling At end of esterification 0.4 part 67% AcOH added during 20 min. 0 . 6 part A m 0 added: sampled after 15 min. 3 parts 67% AcOH added during 1 hr. 42-hr. hydrolysis 113-hr. hydrolysis
Acetic' Diluent Anhydrous Aqueous Anhydrous Aqueous Aqueous Aqueous
Acetyl,
%
..
43.7
.. 44.0 39.8 30.6
Sulfur,
% 1.21 0.0440 1.11 0.0282 0.0182 0.0188
HYDROLYSIS O F SULFUR-FREE CELLULOSE ACETATE
I n 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 sulfurfree 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 t o 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 t o 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
TABLE VI. EFFECT OF ADDING WATERAT DIFFERENTTEMPERATURES O N SULFUR CONTENT DURING HYDROLYSIS Sample A- 1
Temp. during Water Addition Initial Max. 82O F. 75' F.
A-2 A-3 A-4 A-5 A-6 A-7
A-8
C-1 c-2 c-3 c-4 c-5 C-6
110: F.
124" F.
Acetyl, Time of Sampling 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 J u s t befoye adding water Just after adding water 7-hr. hydrolysis 24-hr hydrolysis 48-hr: hydrolysis 96-hr. hydrolysis
%
.. 42:4 41 . O
.. ..
.. .. .. ..
43 9 38'7 34.7
Sulfur,
%
1.05 0.398 0.160 0.0796 0.0368 0.0235 0,0160 0.0166 0.737
0.0091 0 0081 0 0117 0 0146 0 0158
,
82
INDUSTRIAL A N D ENGINEERING CHEMISTRY 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 t o form a ceIlulose acetate acid sulfate. This acid sulfate is remarkably 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 t o those resulting from a cellulose acetylation with sulfuric acid catalyst. 7. 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
Vol. 38, No. 1
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. 8. The conversion of sulfuric t o 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. LITERATURE CITED (1) Caille, A., Chimie & industrie, 12, 441-8 (1924). ( 2 ) Ibid., 13, 11-13 (1925). (3) Clement, L., and RiviBre, C., Bull. SOC. chirn., 4,889-80 (1937). (4) Cross, C. F., and Bevan, E. J., “Researches on Cellulose, 19001905”, 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 Faust’s “Celluloseverbindungen”, 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., Ibid., 14,940-2 (1942). (9) Marschall, A,, and Stauch, H., J . mabromol. Chem., 1, 56-73
(1943).
(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, 83-5 (1941). on the program of the Division of Cellulose Chemistry of the 1845 Meeting-in-Print, A M E R I C A N CHmxrrcar, SOCIETY.
PEEsENTan
CONTROLLING ORANGE DECAY Thiourea, Thioacetamide, 2-Aminothiazole, and Quinosol in Aqueous Solution J. F. L. CHILDS AND E. A. SIEGLER U . S . D e p a r t m e n t of Agriculture, Orlando, Fla.
P
RELIMINARY reports have been made recently on the effectiveness of thiourea (2)and several other organic compounds (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 i n 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. Refrigeration in transit, however essential for the delivery of fruit i n a fresh condition t o 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 D i p b d i a natalensis .and Phomopsis citri; the blue and green molds, respectively, are caused by the fungi Penicillium itaEicum and P. digitatum. A11 of these organisms are dissemiinated 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 Winston’s recommendation (22) that the fruit be harvested by pulling, which separates the buttons from the fruits. Orange buttons are not easily removed, but it has been demonstrated (14) t h a t 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 t h a t comprise the citrua industry. Each group recognizes that intangible liabilities are inherent 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 t h a t 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. T o be most effective under Florida conditions, i t is essential that borax be applied t o the fruit shortly after harvest
