Acetylation of Homogeneously Sulfated Cellulose

9 Acetylation of Homogeneously Sulfated Cellulose W. B. RUSSO and G. A. SERAD Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 14, 2018 | https://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0058.ch009

Celanese Fibers Co., Charlotte, N.C. 28232

The most common method for the preparation of cellulose acetate involves the heterogeneous reaction of cellulose with an acetic anhydride-acetic acid solution i n the presence of a sulfuric acid catalyst. The heterogeniety of the reaction i s reflected i n that the interior regions of the cellulose become available to the reagents only as accessible regions i n the cellulose react and become soluble. A consequence i s that to prepare a useful cellulose acetate the cellulose must be completely esterified to a soluble triacetate and then subsequently hydrolyzed to the desired acetyl value. A desirable alternative to prepare cellulose acetate would be to dissolve the cellulose i n a suitable solvent, react it homogeneously to the desired acetyl value, and quench the reaction to give the product. Unfortunately the traditional solvents for cellulose do not lend themselves to direct acetylation. Recently a dimethyl formamide/dinitrogen tetroxide solvent system was reported for cellulose (1). I t was also reported that cellulose could be sulfated to give a water soluble, homogeneous cellulose sulfate i n this solvent. With this former work as a basis, a program was i n i t i a t e d to prepare secondary cellulose acetate using a cellulose solvent system. Results and Discussion Direct Acetylation of Dissolved Cellulose. Based on the published reports (1). a variety of wood pulps were dissolved by bubbling N O gas into a dimethyl formamide - pulp slurry. A clear viscous solution and color from light yellow to blue-green indicated complete solutioning. Attempts were made to acetylate the dissolved cellulose directly using a variety of reagents (see Table I ) . The reagents were used with and without added catalysts, at room temperature and at elevated temperatures. In all cases the regenerated material showed no evidence of acetylation. As a f i n a l attempt, 2

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Turbak; Solvent Spun Rayon, Modified Cellulose Fibers and Derivatives ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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116

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 14, 2018 | https://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0058.ch009

cellulose solutions were treated with ketene. Although color changes suggested reaction, no evidence of acetylation was observed after regeneration. We believe that none of these reagents was capable of replacing the n i t r i t e groups attached to the cellulose to form an acetate* Instead, i t was decided to concentrate efforts on the regenerated cellulose to circumvent this problem. Table I: Attempted Acetylation of DMF-N^ Reagent

Result

Acetic Anhydride

No Reaction

Acetic Anhydride/Sulfuric Acid

No Reaction

Acetic Anhydride/Pyridine

No Reaction

Sodium Acetate

No Reaction

Potassium Acetate

No Reaction

Acetyl Chloride

No Reaction

Ketene

No Reaction

Acetylation of Regenerated Cellulose, The accessibility of the cellulose to reagents i s a major consideration i n the preparation of cellulose acetate. A study was made to determine the combined effects of dissolution of cellulose i n a DMF-N 0i solution and regeneration as a pretreatment for acetylation. An acetate grade wood pulp was dissolved i n a DMF-N 0it solution and then regenerated with acetic acid. I t was further solvent exchanged with acetic acid to remove residual DMF-N 0it by products. Acetylation of the regenerated pulp at a low consistency ( 2 ) ( 2 % solids) was much faster than for the same pulp pretreated under standard conditions. However, the turbidity of the f i n a l solution was much higher (see Figure 1 ) . This Indicated that although the rate of acetylation substantially improved, the triacetate product quality was poorer. Thus, merely increasing the accessibility of the cellulose to reagents i s not sufficient to produce a good acetylation product. Since this was f e l t not to be a viable route to preparing useful cellulose esters, i t was decided to explore more uniform sulfation of the cellulose. 2

f

2

2

Acetylation of Regenerated Cellulose with Homogeneous Sulfation, The mechanism for the dissolution of cellulose i n DMF-N 0it i s reported to be due to the formation of a n i t r i t e ester. I t has also been reported that sulfur trioxide i s capable of displacing the n i t r i t e groups to form cellulose n i t r i t e sulfate, with cellulose sulfate obtained after regeneration ( 1 ) . The high s o l u b i l i t y of this regenerated material i n water was 2



RUSSO AND SERAD

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117

Acetylation of Cellulose

0

Standard Acetylation

•

Homogeneous Sulfation

5

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Figure 2. Comparative acetlyations of an acetylation grade wood pulp processed with standard and homogeneous sulfation procedures



S O L V E N T SPUN R A Y O N , MODIFIED C E L L U L O S E FD3ERS

15 20 TIME - MINUTES

Figure 3. Comparative acetulations of a homogeneously sulfated, acetylation grade wood pulp having different equivalent sulfuric acid concentrations

O Standard Acetylation •


15 20 TIME - MINUTES

Figure 4. Comparative acetylations of a softwood sulfite pulp processed with standard and homogeneous sulfation procedures



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cited as evidence of the homogeneous nature of the sulfate distribution. The acetylation behavior of cellulose sulfate prepared i n this manner became the main focus of this study. To provide samples for acetylation, an acetate grade wood pulp was dissolved i n D M F - ^ O i * and treated with sufficient sulfur trioxide (3) to provide an equivalence of 6% or 12% sulfuric acid. These samples were regenerated with acetic acid to form a gelatinous solid, then solvent exchanged with additional acetic acid. The gelatinous solid obtained was acetylated at a 2% consistency using a standard acetylation test (except for the omission of sulfuric acid.) The reaction was very rapid and gave a f i n a l solution turbidity normally associated with good f i l t e r a b i l i t y . Figure 2 shows a comparison of the same pulp acetylated by the homogeneous sulfation procedure (12% equiva lent sulfuric acid) and acetylated with standard procedures. The rate of esterification of the pulp homogeneously sulfated (containing a 6% sulfuric acid equivalency) was also very rapid (see Figure 3). It was observed that the viscosity of the f i n a l cellulose triacetate was unusually high, when either a 6 or 12% equivalent sulfuric acid level was used. Thus i t appeared that acetylation grade wood pulps could be acetylated to an equiva lent product quality. However, the acetylation occured more rapid than by the traditional process and i t occurred with less degradation of the pulp i n t r i n s i c viscosity. Acetylation v i a the homogeneous sulfation process was examined with several pulps having purities below that of a dissolving grade. Properties of the pulps and cellulose acetate flake prepared from them by conventional procedures have been described previously (4). A softwood s u l f i t e pulp (α-cellulose, ^85%) showed improved acetylation performance (see Figure 4). The acetylation time was substantially reduced (from eighteen minutes to four minutes). The solution turbidity was lower and increased only slightly with time, i n contrast to the same pulp acetylated by standard procedures. In fact, i t performed only s l i g h t l y poorer than the acetylation pulp prepared by the homo geneous acetylation conditions. A softwood kraft pulp (α-cellu lose, ^80%) also showed a markedly improved acetylation perfor mance (see Figure 5). There was a large reduction i n solution turbidity as compared to conventional acetylation. The reactivity was also significantly improved. The solution turbidity was better than that obtained using the softwood s u l f i t e pulp. A hardwood kraft pulp showed an unusual result (see F i g ure 6). The reaction of this type of pulp with conventional acetylation conditions i s slow and the reaction mixture remains quite turbid. The homogeneous sulfation procedure increased the rate of acetylation as with the other pulps and led to a much lower i n i t i a l turbidity. However, as the reaction continued the turbidity suddenly increased rapidly. The high xylan fraction of the hardwood pulp, linearized by the pulping process crystallized from solution leading to an observable turbidity.


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Figure 5. Comparative acetylations of a softwood Kraft pulp processed with standard and homogeneous sulfation procedures

Figure 6. Comparative acetylations of a hardwood Kraft pulp processed with standard and homogeneous sulfation procedures

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T h i s i s s i m i l a r to the behavior o f a c e t y l a t e d hardwood x y l a n s described by Gardner and Chang (5) and c o n s i s t e n t w i t h the s i g n i f i c a n t q u a l t i t y o f x y l a n s found i n i n s o l u b l e a c e t a t e r e s i d u e s w i t h acetone and methylene c h l o r i d e ( 4 ) . The low c o n s i s t e n c y a c e t y l a t i o n t e s t i s e x c e l l e n t f o r p r e d i c t i n g a c e t y l a t i o n performance at h i g h e r c o n s i s t e n c y f o r a c e t y l a t i o n grade wood p u l p s . However, because o f the h i g h t u r b i d i t y l e v e l s w i t h lower p u r i t y p u l p s , i t s v a l u e f o r these pulps i s sometimes q u e s t i o n a b l e . I n a d d i t i o n , i t i s d i f f i c u l t to i s o l a t e e i t h e r a secondary o r t r i a c e t a t e product from the r e a c t i o n f o r f u r t h e r c h a r a c t e r i z a t i o n . For t h i s reason f u r t h e r e v a l u a t i o n s were conducted w i t h a s m a l l - s c a l e , h i g h c o n s i s t e n c y (6% s o l i d s ) a c e t y l a t i o n procedure t o produce a secondary a c e t a t e f l a k e product. A . h y d r o l y s i s step was included t o reach a t a r g e t 55.0% combined a c e t i c a c i d . An a c e t y l a t i o n grade wood pulp was used i n the i n i t i a l attempts t o prepare a secondary a c e t a t e f l a k e u s i n g the homogeneous s u l f a t i o n process. F l a k e was prepared u s i n g standard procedures f o r h y d r o l y z i n g the t r i a c e t a t e t o a secondary a c e t a t e but the r e s u l t s were not s u c c e s s f u l . The a c e t y l v a l u e s (combined a c e t i c a c i d ) were c o n s i s t e n t l y lower than expected. Longer a c e t y l a t i o n times d i d not improve the s i t u a t i o n . A r e d u c t i o n i n the h y d r o l y s i s time was s u c c e s s f u l i n i n c r e a s i n g a c e t y l v a l u e . However, the h y d r o l y s i s time r e q u i r e d t o reach t a r g e t a c e t y l v a l u e was so short that l i t t l e more than d e s u l f a t i o n occurred. Hence, based on these d a t a , i t was concluded that a secondary a c e t a t e was prepared d i r e c t l y . The s o l u t i o n p r o p e r t i e s of t h i s f l a k e i n acetone were q u i t e good (see Table I I ) compared to that obtained by standard a c e t y l a t i o n procedures i n the same equipment. An experiment was conducted u s i n g DMF as a p r e t r e a t i n g agent to a c t i v a t e the pulp p r i o r t o c o n v e n t i o n a l a c e t y l a t i o n . T h i s was done t o assure that i t was the homogeneous s u l f a t i o n process that was a c t u a l l y r e s p o n s i b l e f o r the improved a c e t y l a t i o n and not the DMF pretreatment. A sample o f the softwood s u l f i t e pulp was p r e t r e a t e d by s l u r r y i n g i t i n DMF, solvent exchanging w i t h a c e t i c a c i d , and a c e t y l a t i n g by the normal process u s i n g s u l f u r i c a c i d as c a t a l y s t . A second sample was d i s s o l v e d i n DMF-^Oi*, regenerated as c e l l u l o s e , and a c e t y l a t e d u s i n g s u l f u r i c a c i d as the c a t a l y s t . In both cases, the products obtained were i n s o l u b l e i n acetone and could not be evaluated. Thus, as i n the low c o n s i s t e n c y a c e t y l a t i o n s , making the c e l l u l o s e a c c e s s i b l e was not the major f a c t o r i n improving the acetylation. F u r t h e r e v a l u a t i o n s w i t h the homogeneous s u l f a t i o n technique were made w i t h the softwood s u l f i t e and the softwood k r a f t pulps which performed w e l l i n low c o n s i s t e n c y t e s t s . A c e t y l a t i o n of e i t h e r pulp under standard c o n d i t i o n s has always provided a f l a k e which gave cloudy dope s o l u t i o n s and which blocked f i l t r a t i o n media v e r y r a p i d l y ( 4 ) . A c e t y l a t i o n o f the regener a t e d c e l l u l o s e s u l f a t e prepared from these pulps provided



Wood

Softwood

Softwood

Grade

Acetylation

Acetylation

Sulfite

Sulfite

Process


Standard

Acetylation Process 101 143

54.1

6 Weight % Solution Viscosity - c p 8

55.5

Combined Acetic Acid X

2

103

8

Filterability in 95/5 Acetone/Water gm/cm

Acetate Flake Properties

Small Scale, High Consistency Acetylation

Cellulose Acetate Flake Properties

Table II


RUSSO A N D SERAD


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c e l l u l o s e a c e t a t e f l a k e which was almost comparable t o t h a t obtained from the a c e t y l a t l o n pulp (see Table 111)·


Conclusions I t has been shown that I t I s p o s s i b l e to prepare an acetone s o l u b l e secondary c e l l u l o s e a c e t a t e d i r e c t l y without the need t o f i r s t prepare c e l l u l o s e t r i a c e t a t e . To accomplish t h i s , i t i s necessary to d i s t r i b u t e the c a t a l y s t u n i f o r m l y on the c e l l u l o s e . T h i s i s r e a d i l y accomplished by d i s s o l v i n g the c e l l u l o s e i n DMF-^Oit and adding s u f f i c i e n t s u l f u r t r i o x i d e . T h i s technique i s a p p l i c a b l e to a wide range of α-cellulose content wood p u l p s . For an a c e t y l a t l o n grade p u l p , the r a t e of r e a c t i o n i s increased probably as a r e s u l t of making the c e l l u l o s e h i g h l y a c c e s s i b l e to the reagents. However, the f i l t r a t i o n p r o p e r t i e s of the c e l l u l o s e a c e t a t e were s i m i l a r t o t h a t obtained by c o n v e n t i o n a l a c e t y l a t l o n techniques. For low p u r i t y pulps (α-cellulose, 80-90%), there i s a d u a l advantage. F i r s t , a c e t y l a t l o n r a t e i s more r a p i d due t o the a c c e s s i b i l i t y of the c e l l u l o s e . Secondly, improved s o l u t i o n p r o p e r t i e s r e s u l t from the uniform d i s t r i b u t i o n of the s u l f a t e c a t a l y s t . I t i s b e l i e v e d t h a t i n the standard a c e t y l a t l o n procedure a d i s p r o p o r t i o n a t e amount of the c a t a l y s t i s absorbed by the h e m i c e l l u l o s e s , l e a v i n g the c e l l u l o s e w i t h i n s u f f i c i e n t c a t a l y s t f o r a good r e a c t i o n . T h i s i s overcome by d i s t r i b u t i n g the c a t a l y s t u n i f o r m l y on the d i s s o l v e d c e l l u l o s e . Literature Cited 1. 2. 3. 4.

5.

Schweiger, R. G . , Tappi Dissolving Pulp Conference (1973) Preprints, 167-174. Rosenthal, A. J., International Symposium on Dissolving Pulps Helson Κ (1966), Symposium Lectures 535-549. Schweiger, R. G . , Carbohydrate Research (1972) 21 225. Russo, W. Β., and Serad, G. Α . , Characterization of Insoluble Cellulose Acetate Residues, presented at the 173rd ACS National Meeting, C e l l u l o s e , Paper, and T e x t i l e D i v i s i o n , New Orleans, L a . , March 1977. Gardner, P. Ε . , and Chang, Μ. Υ . , Tappi Dissolving Pulp Conference (1973) Preprints, 83-96.


Acetylation of Homogeneously Sulfated Cellulose

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