Chemical Reactions in the Making of Cellulose Acetate U
CARL J. MALM AND LEO J. TANGHE Cellulose Acetate Development Division, Eastman Kodak Co., Rochester, A'. Y .
T
reaction was first described by 0 s t ( 7 ) and has been used (8) to remove the combined sulfate from heparin and related products. This reaction has now been studied in more detail a t the end of a cellulose esterification to compare the behavior of sulfate groups on primary and secondary hydroxyls. This was achieved by removing the sulfate groups with a small amount of water under conditions such that acetyl groups were not affected. The amounts of total and primary hydroxyl, thereby liberated, were measured. The location of the sulfate groups could thus be determined.
HE manufacture of cellulose acetate is almost exclusively based on the acetylation of cellulose with acetic anhydride, using sulfuric acid catalyst. A preliminary investigation of some of the chemical reactions involved was reported several years ago ( 3 ) . This paper gives a more detailed study of the process. The process can be divided into two parts, acetylation and hydrolysis. The acetylation is carried out under anhydrous conditions and the hydrolysis in the presence of water. REACTIONS DURING ACETYLATION
Sulfuric acid combines rapidly and quantitatively with the cellulose when acetic anhydride is added to start the esterification, whereas the actylation reaction is comparatively slow ( 3 ) . Preferential Sulfation of Primary Hydroxyls. The introduction of sulfate during acetylation was more conveniently studied starting with hydrolyzed cellulose acetate rather than with cellulose itself. The methods OC tritylation and carbanilation (4)cannot be used to determine the amounts of primary and total hydroxyl remaining after introduction of sulfate, because the combined sulfate is slowly removed during tritylation and carbanilation in hot pyridine solutions. An indirect method was, therefore, used to show that under certain conditions the primary hydroxyls sulfate faster than secondary. Sulfation reactions were carried out on two samples of cellulose acetate. Both had approximately 40% acetyl; in one 5001, of the hydroxyl was primary, and in the other only 8% was primary (6). These cellulose esters were dissolved in acetic acid. Methylene chloride was added in the experiments a t 0" C. or below to prevent crystallization of the acetic acid. Sulfuric acid and acetic anhydride were added to the solutions a t the temperatures indicated in Table I. After 5 minutes the cellulose acetate sulfates were precipitated from solution by pouring into a large exceas of methanol.
a
Table I.
Temp.,
O
25 0 -113 ~.
- zn
$15
\
A - P r i m a r y hydroxyl after removing sulfate
Sulfation of Cellulose Acetate by Sulfuric -4cid in Presence of Acetic Anhydride C.
Solvent
AcOH
AcOH AcOH . .
+ MeCIz + MeClz +
4 r O_ H . _ , _ MeClz __
fitiiirnum for reaction of all sulfuric acid Maximum for reaction of all primary hydroxyls
Primary OH in Starting Material High Low % Sulfur 1.10 0.99 0.73 0.68
0
0.85 0.46 0.40
1.14
1.14
0.43
I 2
I
I
I
1
4
6
8
IO
Hours a f t e r completion of acetylation Figure 1. Loss of combined sulfate after completion of cellulose acetylation 7 96 sulfuric acid catalyst, based on cellulose
1.13
2.7
I 1
Cellulose was acetylated as previously described (S), but the reaction was continued far beyond the point where trhe solution was free from fiber and grain, in order to follow the replacement of sulfate by acetyl. Samples were taken in pairs, one to retain and one to remove the combined sulfate. To retain the combined sulfate, an excess of pyridine was added over that required to neutralize the sulfuric acid. To remove the combined sulfate, the water content was adjusted to about 0.5y0of the solvent, and the solution was allowed to stand for 2 hours a t room temperature. All samples were precipitated and washed in distilled water and stabilized by adding sodium carbonate to tKe last wash. Figure 1 shows the amounts of sulfate and the amounts of total and primary hydroxyl after removal of sulfate when 7%
At room temperature, all the sulfuric acid combined with the cellulose acetate in both cases. I n the starting material with low primary hydroxyl, more sulfuric acid combined than corresponded to the amount of primary hydroxyl present. At lower temperatures not all the sulfuric acid reacted. Less sulfate was found in the sample having the lower amount of primary hydroxyl. This indicates that the sulfation reaction is preferential to the primary hydroxyl groups. Replacement of Sulfate by Acetyl. At the end of the esterification the combined sulfate is slowly replaced by acetyl. This
995
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INDUSTRIAL AND ENGINEERING CHEMISTRY ,200
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Vol. 41, No. 5
sulfuric acid, based on cellulose, was used as catalyst. The first samples were taken as soon as the reaction solution was free from fiber and grain. Throughout the interval studied, the amount of combined sulfate was very nearly the same as the amount of primary hydroxyl after removing sulfate. The total hydroxyl was slightly greater than the combined sulfate, probably indicating that a small amount of secondary hydroxyl never had been esterified. Similar results were obtained when the acetylation was carried out a t a much higher level of viscosity (by omitting the catalyst treatment prior to the acetylation proper), also with cellulose propionate and cellulose acetate butyrate, Figure 2 shows the results of a similar experiment carried out with 14y0 sulfuric acid based on cellulose. I n this case the amount of sulfate agreed with the total hydroxyl after removal of sulfate, thereby indicating that all the hydroxyls had been esterified. However, a small amount of the sulfate was apparently on secondary hydroxyl. An interesting experiment was run in which a larger proportion of the combined sulfate was on secondary hydroxyl. Hydrolyzed cellulose acetate (32% acetyl) was dissolved in acetic acid and partially re-esterified with acetic anhydride in the absence of catalyst. As shown in Figure 5, these conditions promote a highly selective acetylation of the primary hydroxyls. When the acetyl content had reached 38.6% (4.4% OH), only 10% of the hydroxyls were primary. At that Doint an amount of sulfuric acid was added, corre-
hydroxyl a f t e r removing sulfate
v)
c
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3
m
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Primary hydroxyl a f t e r removing sulfate
Hours r e a c f i o n Figure 3. Loss of combined sulfate after reacetylation 'of cellulose acetate having low primary hydroxyl 7 0 sulfuric acid catalyst,'hased on cellulose
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INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955
ondary (Figure 4). This is undoubtedly due to the topochemical nature of the reaction rather than to almost equal reactivity of the different hydroxyls, The reacetylation of hydrolyzed cellulose acetate with acetic anhydride in solution was, therefore, studied (6). I n the absence of catalyst a t high temperature, the primary hydroxyls reacted about five times as fast as the secondary. At room temperature the reaction was still more preferential for the primary hydroxyls. I n Figure 5, cellulose acetate, 33.4y0 acetyl (0.46 primary and 0.67 secondary hydroxyls per glucose unit), was dissolved in 10 parts of acetic acid, and 1part of acetic anhydride was added. Samples were taken, after the number of days indicated, a t room temperature. This shows that the acetylation is preferential to the primary hydroxyl groups.
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Table 11. Hydroxyl Content after Dilution to 0.3% Water in the Solvent Time of Holding Hours at 25O C.' 1 2 4
7 24
OH, Weight % Total 0.47 0.46 0.47 0.49 0.47
Primary 0.40 0.38 0.35 0.35 0.31
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Primory hydroxyls available at start
3 -
Primarv hvdroxvls
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Secondary hydroxyls available at start
.70
REACTIONS DURING HYDROLYSIS
The addition of water a t the end of the esterification destroys the excess anhydride and promotes rapid desulfation and slow deacetylation. Optimum conditions for removal of sulfate and acetyl are different. Since products of lowest possible sulfur content are desirable for optimum stability, careful attention must be given to this stage. Desulfation. It was known from earlier work ( 3 ) that the combined sulfate was readily removed in the presence of small amounts of water. The optimum conditions for this removal were established by the following experiments: Cellulose was acetylated as previously described (3)using 7y0 sulfuric acid, based on the cellulose. At the end of the scetylation, sufficient water (as 67% acetic acid) was added slowly and with good mixing to destroy most but not all of the acetic anhydride. Small amounts of anhydride or water in such solutions may be determined empirically by measuring the temperature rise on addition of water or anhydride, respectively, to samples of the solution. A portion of the solution was diluted quickly with 2 volumes of acetic acid containing sufficient water to provide 0.3% water in the solvent after mixing. Another portion was treated in the same way t o contain 1.0% water. Samples of cellulose acetate were isolated a t intervals up to 24 hours and analyzed for sulfur (Figure 6). With o.3y0 water there was a sharp drop in sulfur content during the first few minutes. With 1.0% water the drop was not so rapid a t first, but lower sulfur content was obtained later. The rate of removal of combined sulfate is sharply influenced by the amount of water in the acetic acid solution. Figure 7 gives the sulfur content 10 minutes after adjustment to small concentrations of water or acetic anhydride in solution. A trace of anhydride was sufficient t o retain the combingd sulfate. Minimum sulfur content was found in the presence of 0.1 to 0.2y0 water. When the solutions were held for 2 hours, the amount of combined sulfate was much lower, and the minimum was found in the presence of about 0.5% water in solution (Figure 8). Araki ( 1 ) found the minimum sulfur content with somewhat more water in solution. He isolated cellulose acetate sulfate and redissolved it in acetic acid containing various amounts of water. Upon holding the solutions for 20 hours a t 20" C., he found the minimum sulfur content with 2% water in solution. With less than 1% water in acetic acid solution, no acetyl is
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30
50
40
60
70
100
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Days reaction Figure 5. Re-esterification of primary and secondary hydroxyl in cellulose acetate
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esterification (bosed on cellulose)
14% H2S04
P
7% H2S04
.o
0.5
Percent Ac,O Figure 7.
in solvent
0
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0.2
0.5
Percent HO ,
1.0
in solvent
-
Combined sulfate in cellulose acetate
After adjustment of acetylation solution to various percentages of water and acetic anhydride and holding at 2 5 O C. for 10 minutes
lost during the interval required to remove the combined sulfate. On holding for as long as 24 hours, the total hydroxyl remained unchanged, although the secondary increased slightly a t the expense of the primary ( 5 ) . The amounts of total and primary hydroxyl found after dilution to O.3y0 water in the solvent are given in Table 11. From the data it follows that the hydroxyl8 found after dilution to approximately 0.3% water are those which previously bore sulfate groups or which never had been esterified a t all.
INDUSTRIAL AND ENGINEERING CHEMISTRY
998 .le
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acid with respect t o the water became high toward the end of the drying, causing some of it to recombine with the cellulose acetate. Deacetylation. The small amounts of water which are so effective for desulfation are totally inadequate for deacetylation. In industrial practice amounts of water from 5 to 207@of the solvent are commonly used. Lesser amounts of water cause considerable breakdown of the cellulose chain a t the temperatures and catalyst concentrations used. The upper limit of water is determined by the water tolerance of the particular ester a t the operating temperature. The amount of primary hydroxyl in the hydrolyzed product increases with the percentage of water in the hydrolysis bath ( 5 ) . I n aqueous acetic acid there is a n equilibrium between esterification and hydrolysis, and in the presence of a small amount of water there is a preferential partial reacetylation of the primary hydroxyls. A logical consequence of this equilibrium is that different amounts of water, depending on the ester, will maintain a balance
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Table IV.
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0.2
0.5
Percent HO ,
Drying No. of before ReprecipitaReprecipitation tions None 1 .... None 2 None 1 day a t 70' C. 1 1 2 days a t 70' C. 1 3 days a t 70' C. 3 days a t 70' C.. followed by 1 day a t 100' C. 1 3 days at 70' C., followed 1 by 2 days a t 100' C. MgCOa in last wash 1 day a t 70' C. 1 2 days a t 70' C. 1 2 days a t 70' C followed by 1 day a t iOOo C. 1 2 days a t 70° C., followed 1 by 2 days a t 100' C. a 50-gram sample used for analysis when sulfur content was 0.01 7 0 .
....
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in s o l v e n t
Figure 8. Combined sulfate in cellulose acetate After adjustment of acetylation solution to various percentages of water and holding at 25' C. for 2 hours
Equilibrium of Combined Sulfate during Hydrolysis. The equilibrium amount of combined sulfate is determined by the soluble sulfate content of the hydrolysis bath (3). A second factor has been found to be the hydroxyl content of the cellulose acetate. Two samples of sulfur-free cellulose acetate having differect amounts of hydroxyl were hydrolyzed with sulfuric acid catalyst under identical conditions (Table 111). The sample starting with the greater amount of hydroxyl (lower acetyl content) had the greater amount of combined sulfate. This explains the slight rise in combined sulfur content previously observed on prolonged hydrolysis of cellulose acetate ( 3 ) .
+I2
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L" .. Table 111. Effect of Hydroxyl Content on Introduction of Sulfate Hours Hydrolysis Starting material 7 24 48
Low Hydroxyl in Starting Material Acetyl, Sulfur,
%
%
42.2 41.5 40.4 39.6
None 0.0015 0.0030 0.0049
Recombination of Sulfate during Drying
Stabilization before Drying
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Vol. 47, No. 5
High Hydroxyl in Starting Material Acetyl, Sulfur,
%
%
37.5 36.9 36.0 34.7
None 0,0030 0,0059 0.0075
%
0.0082
0,0091 0.0127 0.0118 0.0085 0.0075 less than
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Equilibrium of Combined Sulfate during Drying. Another example of t,he equilibrium nature of this reaction is the recombination of sulfate during extended drying. Cellulose acetate containing about 0.0125% combined sulfur was dried without stabilizing salt. After drying for different lengths of time, samples were reprecipitated t o remove the sulfate which had split off. A low value for combined sulfate was found after mild drying far one day but, on continued drying, larger amounts of combined sulfate were present (Table IV). When the ester was stabilized by adding magnesium carbonate to the last wash, there was only a slight, gradual loss in combined sulfate during extended drying. A possible explanation for this behavior is that in t,he absence of stabilizing salt the concentration of sulfuric
Sulfur, 0.0124 0.0128 0 . 0043a 0.0052 0.0061
-16 0.4
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0.6
0.8
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1
1.2
Hydroxyls per glucose unit Figure 9. Optical rotation during hydrolysis of cellulose acetate in 98 and 50% acetic acid
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INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955 50 I
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-
Start, high primary OH I
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50% HO , 1
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2.
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6
35
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~i~~ Hour; Cellulose acetate, 4 3 . 5 % acetyl Start
24 Cellulose acetate, 3 2 . 3 % acetyl Start 6 24 6 24 6 24
W
-
Table V. Behavior of Cellulose Acetate in Aqueous Acetic Acid at 100’ C.
6 24 6 24 6 24
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30
Water,
% 0:5 0.5 1.0 1.0 2.0 2.0 4.0 4.0
..
1.0 1.0 2.0 2.0 4.0 4.0
Hydroxyl, W e k x Total Primary 0.91 0.63 0.91
1.18 0.98 1.31 1.07 1.30 1.27 2.03 8.73 7.95 7.70 8.16 7.80 8.80 8.79
0.60 0.60 0.66
0.57 0.61 0.60
0.66 0.79 2.82 2.01 1.57 2.30 1.71 2.35 2.00
z1 X
2
D
2
25
CONCLUSIONS
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.;L c
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2 a
15
A s t a r t , tow primary OH
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Hydroxyls per glucose unit Figure 10. Primary hydroxyl during hydrolysis of cellulose acetate in 98 and 50% acetic acid
between esterification and hydrolysis. Two samples of cellulose acetate containing 43.5 and 32.3% acetyl, respectively, were heated on the steam bath in acetic acid containing small amounts of water (Table V). The sample containing 43.5% acetyl lost a slight amount of acetyl even in the presence of 0.5% water, but had no loss of acetyl from primary hydroxyl with u p to 2% water. The sample containing 32.3y0 acetyl esterified slightly in the presence of 1 or 2% water and maintained the same amount of acetyl in the presence of 4% water. Though the total hydroxyl was constant in the presence of 4% water, a true equilibrium was not established, since the secondary hydroxyl increased a t the expense of the primary. It is interesting to observe the behavior of different samples of cellulose acetate having the same amount of acetyl but low, medium, and high percentages of primary hydroxyl. Such esters were dissolved in 98% acetic acid containing 0.025M hydrochloric acid, and samples were taken over a period of 6 weeks a t room temperature. The samples with medium and low primary hydroxyl hydrolyzed slowlv, whereas the sample with high primary hydroxyl esterified slightly before beginning to hydrolyze (Figures 9 and 10). The same esters were dissolved in 60% acetic acid at steam bath temperature. After 1 hour, water was added to give Soy0 acid. Samples taken during an interval of 7 hours showed that all three esters hydrolyzed readily. I n each solvent the three esters approached the same values for optical rotation (Figure 9) and percentage primary hydroxyl (Figure 10). The limiting optical rotation in chloroform-ethyl alcohol (85 to 15) was about +8” for hydrolysis in 98% acetic acid and -3” for hydrolysis in 50% acetic acid. The limiting amount of primary hydroxyl was about 24% of the total amount of hydroxyl present for hydrolysis in 98Yo acetic acid, and 45% for hydrolysis in 50% acetic acid.
Acetylation Step. At the start of the esterification the acetylation and sulfation reactions compete for the hydroxyl groups of the cellulose. The sulfation reaction is very rapid compared t o the acetylation. Both reactions have a preference for the primary hydroxyl. A slow desulfation takes place a t the end of the esterification, with sulfate being replaced by acetyl. Hydrolysis Step. The optimum amount of water for removal of sulfate a t the end of the esterification is less than 1 % of the aoetic acid, whereas deacetylation is best accomplished by much larger amounts of water. Both of these reactions are reversible. The equilibrium amount of sulfate in the hydrolyzed product is determined by the soluble sulfate content of the hydrolysis bath ( 3 )and by the hydroxyl content of the product. The hydrolysis stage in the manufacture of cellulose acetate is the net result of simultaneous deacetylation and reacetylation. With increasing amounts of water in the hydrolysis bath, the reacetylation is retarded and greater amounts of primary hydroxyl are found in the product. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of B. C. Laird, H. M. Herzog, M. H. Stewart, and G. D. Smith in carrying out much of the experimental work reported herein. LITERATURE CITED
(1) Araki, T., Tertile Research J.,22, 630 (1952). (2) Hearon, W. M., Hiatt, G. D., and Fordyce, C. R., J . A m . Chem. SOC.,65, 2449 (1943). (3) Malm, C. J., Tanghe, L. J., and Laird, B. C., IND. ENG.CHEY.,
38, 77 (1946). (4) Malm, C. J., Tanghe, L. J., Laird, B. C., and Smith, G. D., A d . Chem., 26, 188 (1954). (5) Malm, C. J., Tanghe, L. J., Laird, B. C., and Smith, G. D., J . Am. Chem. SOC.,74, 4105 (1952). (6) Ibid., 75, 80 (1953). (7) Ost, H., 2. angew. Chem., 32, 66 (1919). (8) Wolfrom, M. L., and Montgomery, R., J . Am. Chem. SOC.,72, 2859 (1950). ACCEPTED December 28, 1954. R~CEIYE forD review December 9, 1954, Presented before the X I I I t h International Congress of Pure and Applied Chemistry, Stockholm, Sweden, August 1, 1963.
Correction In the article, “Textiles and the Chemical Industry,’’ by Robert W. Philip [Resources for the Chemical Industry-South Atlantic States, IND.ENG.CHEX.,47, Part I, 422-47 (1955)], the heading for the last column of Table 7 (p. 439) should read, “Estimated Capacity (1954 or 1955), Thous. Lh.”
