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Effects of solvent addition to acetylation medium on cellulose triacetate prepared from low-grade dissolving pulp
29 Effects of solvent addition to acetylation medium on cellulose triacetate prepared from
low-grade dissolving pulp
S Saka and T Takahashi - Department of Wood & Paper Science, Faculty of Agriculture, Kyoto University, Kyoto, Japan 606-01
Shiro Saka and Tadashi Takahashi- Department of Hood & Paper Science, Faculty of Agriculture. Kyoto University, Kyoto, Japan 606-01
ABSTRACT As cellulose triacetate is prepared from low-grade dissolving pulp, a considerable amount of the insoluble residue is present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. As one of the remedies for reducing the insoluble residue, a solvent was added to the acetylation medium and the effects of the solvent addition on the amount of insoluble residue formed were studied. To do so, 17 different solvents were selected so as to cover a wide range of solubility parameters. The obtained results clearly indicated that the addition of the solvent affects the amount of insoluble residue and that nitromethane, nitroethane, dichloroacetic acid and methylene chloride were very effective for its reduction. A new acetylation system with such an appropriate solvent would provide a clue as to an industrial usage of the low-grade dissolving pulps for cellulose acetate production.
INTRODUCNON
Cellulose triacetate being important in the fiber and textile industries requires high quality cellulose as a starting material. ') This is because the low-grade dissolving pulps contain more hemicelluloses and the formation of hemicellulose acetates such as xylan acetate and glucomannan acetate, results in industrial problems such as filterability, turbidity (haze) and false viscosity. Although a considerable effort has been made since the mid-1950s to explain the relationships between the solution properties of the cellulose acetate and contaminated hemicel lulose acetate,2-8) viscose grade wood pulps with an a-cellulose content of less than 90% still cannot be used for manufacturing cellulose acetate. 219
220 Derivatives of cellulose and their properties In our previous work,'* l o ) we have studied cellulose triacetate prepared from softwood sulfite dissolving pulps with an a-cellulose content of 87.5%. The first problem faced was a substantial amount of insoluble residue present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. This insoluble residue retained a fiber structure of swollen form, resulting in one of the industrial problems, filterability. Characterization of the insoluble residue9) showed that it was composed of cellulose triacetate (CTA) and glucomannan triacetate (GTA) in aggregate with each other in the acetylation medium due to their compatible nature. In addition to such chemical effects of the molecular interactions, ultrastructural effects of glucomannan distribution in pulp fibers were found to be involved in the formation of insoluble residues. Therefore, the amount of insoluble residue can be expected to be reduced if chemical effects and/or ultrastructural effects are decreased during acetylation. As one of remedies, the pretreatment of low-grade dissolving pulps with mixtures of acetic acid and sulfuric acid was Found to reduce both the chemical and ultrastructural effects of pulp fibers in the following acetylation, resulting in reducing the amount of the insoluble residue. l o ) The use of higher amounts of sulfuric acid as a catalyst was also found t o reduce the insoluble residue formed. ") In this study, a solvent was added to the acetylation medium of acetic acid / acetic anhydride / sulfuric acid system and the effects of' the solvent addition on the amount of the insoluble residue formed were studied. To do so, seventeen different solvents were selected to cover a wide range of solubility parameters of the solvents.
EXPERIMENTAL Sulfite softwood dissolving pulp with an a-cellulose content of 87.5% was used for preparing cellulose triacetate as described in the previous paper. ') As cellulose triacetate was prepared from this pulp, a large amount of insoluble residue was present in the acetylation medium of an acetic acid / acetic Therefore, in this work, a solvent was anhydride / sulfuric acid system.') added to this acetylation medium to study the effects of the solvent on the amount of insoluble residue formed. The solvents used in this study are given in Table 1. The low-grade dissolving pulps (1 part) were thus acetylated with a solution of acetic acid (160 parts). acetic anhydride (7parts) and sulfuric acid (0.1 part) with a solvent (51 parts) for 3 h at 40 C. followed by stirring overnight at 20 C. The solutions were then spun in a centrifuge at 7000 rpm for 30 min. After the tubes were removed carefully, the supernatants pipetted away and precipitated substances were washed repeatedly with mixtures of fresh acetic acid and the solvent used in the system (160:51 in a weight ratio) by centrifugation to obtain the insoluble portions. The supernatants collected were, on the other hand, concentrated and poured into deionized water to precipitate the soluble portions. The insoluble and w!uble portions were washed as doscribed previously. The degree of substitution (DS) of these samples was determined by a titration method, I I ) while neutral sugar compositions were determined by an alditol-acetate procedure'2' using conditions described in a previous paper. ') For structural observations, some micrographs of the insoluble residues were taken in an acetylation system with a different solvent.
Effects of solvent additiofi 221 RESULTS AND DISCUSSION
It has been known in our previous work"' that, when cellulose triacetate is prepared from low-grade sulfite dissolving pulp, a substantial amount of insoluble residue is present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. Therefore, in this study, a solvent was added to the above acetylation medium with 1:3 weight ratio of solvent to acetic acid to study the effects of the solvent addition on the reduction of the insoluble residue content in the acetylation medium. Table 1 shows selected solvents excluding the alkaline, unstable and reactive solvents in the reaction medium. The acetylation system with acetic acid (No.10 in Table 1) was also included just for comparison as a control reaction system without any addition of the solvent. Compared with acetic acid, 20.7 (MPa)i'2 in its solubility parameter (SP), the selected solvents could cover a Also included in Table wide range of the SP values from 16.6 to 26.0 (MPa)'/'. 1 is the insoluble residue content in the acetylation medium with a designated so 1vent. Figure 1 shows a relationship between the amount of insoluble residue and the SP of the solvent added to the acetylation medium. Numbers correspond to solvents as in Table 1. It is very apparent that compared with 15.1% in a control reaction system (acetic acid, No.10). the results are varied from 0.6% to 36.2% in insoluble residue contents, indicating that large effects of the solvent addition exist on the amount of the insoluble residue in the acetylation medium. Furthermore, it is noted that with an increase in the SP
Table 1 The insoluble residue content in the acetylation medium with a solvent added and its chemical compositions. Solvent 1. 2. 3. 4.
5. 6. 7. 8. 9.
10. 11.
12. 13. 14. 15. 16. 17.
R-Butyl chloride 4-Chlorotol uene 1.2-Dichloroethane Methylcellosolve acetate Ethylbromide Methylene chloride 2-Ni tropropane Bromobenzene Ni trobenzene Acetic acid 1-N i t ropropane Methylbenzoate 1-Bromonaphtalene Dimethyl phthalate Dichloroacetic acid Nitroethane Nitromethane
Solubility parameter ((MPa)"')
Chemical Compositions ( mol% )
Insoluble residue ( wtX )
Glucose
Mannose
Xylose
16.6 18.0 18.2 18.8
36. 2 28.1 9. 2 29. 3
80.0 79.4 55.2 78. 7
18. 9 20. 6 44.8 21. 3
1.1
19.6 19.8 20.3 20.3 20. 5 20.7 21. 1 21. 5 21. 7 21. 9 22. 5 22. 7 26.0
19.9 4.8 9. 1 17.2 7.4
66. 5 58. 2 71.8 73. 6 57. 9 71. 1 70. 3 80.1 68.3 70. 5
33. 5 29. 9 28.2 25. 3 42.1 20. 2 29. 7 19. 9 26.8 29. 5
69. 2 42. 6
30. 8 57.4
15. 1
7.1 24.1 20. 2 15. 6 0.8 3.0 0. 6
-
-
-
-
-
11.9
-
1. 1
-
0.7
-
4.9
-
222 Derivatives of cellulose and their properties
0'
16
I
18
1
20
22
0
I
24
2G
27
Solubility parameter (( MPa)'l2) Fig.
1
The soluble residue content vs. solubility parameter of the solvent added to the acetylation medium. Numbers correspond to the solvents as in Table 1.
I
,-.I
U
I
I
I
10
20
30
Insoluble residue content (%) Fig. 2
40
The sugar content vs. insoluble residue content for acetylation medium with a solvent added. Numbers correspond to solvents as in Table 1.
Effects of solvent addition 223 values, the amount of insoluble residue has a tendency to decrease. The effective solvents for reducing the insoluble residue are, therefore, nitromethane (0.6%). dichloroacetic acid (0.8%). nitroethane (3.0%), methylene chloride (4.8%), 1-nitropropane (7.1%), nitrobenzene (7.4%), 2-nitropropane (9.1%) and 1.2-dichloroethane (9.2%). The chemical compositions of the insoluble residue in each reaction system are also given in Table 1. It is apparent that, in addition to glucose, the insoluble residue contains a high proportion of mannose, with a limited amount of xylose except for the reaction system with methylene chloride. This result is in g o d agreement with the findings of our previous worke' that the insoluble residue is composed of cellulose triacetate (CTA) and glucomannan triacetate (GTA) in aggregate with each other at the molecular level by their mutual interactions. Therefore, GTA is involved in the formation of the insoluble residue for all acetylation systems studied. However, a closer inspection of the results in Table 1 shows a large variation in mannose content from a maximum of 57.4% to a minimum of 18.9%. Therefore, a relationship between the insoluble residue content and its sugar content for glucose and mannose was studied (Fig.2). From Fig. 2, it is apparent that the glucose content decreases with a decrease in the insoluble residue content, whereas the mannose content increases. Based on a study on the chemical compositions of the isolated glucomannan from the low-grade pulp used in this study, the molar ratio of mannose to glucose is known to be 3.2:1.@' With the value of 3.2, therefore, the contents of the GTA and CTA can be computed. Figure 3 shows the obtained ratio of CTA to GTA against the insoluble residue content. It is apparent that with a decrease in the insoluble residue content, the ratio of CTA to GTA decreases. Comparative studies of acetylation in the model experiments"' indicated that the insoluble residue from the low-grade dissolving pulp is formed by
5 4 -
3 -
2l
0
2 -
@@
1-
0 Fig. 3
@@
0
@@ @@ I
I
1
10
20
30
Insoluble residue content (%)
40
The ratio of the CTA to GTA content in the insoluble residue vs. insoluble residue content. Numbers correspond to solvents as in Table 1.
224 Derivatives of cellulose and their properties
ultrastructural effects of the pulp fibers and the chemical effects of the molecular interactions of CTA and GTA. The former effects originate from the ul trastructural distribution of residual glucornannan in the dissolving pulp, whereas the latter effects are due to molecular aggregation of CTA and GTA by their compatible nature. It is , therefore, possible to get a reduction in the amount of insoluble residue if chemical effects and/or ultrastructural effects are decreased during acetylation. Furthermore, our previous study'O ) indicated that as the ultrastructural effects are decreased, the ratio of CTA to GTA content in the insoluble residue is decreased. Therefore, the results in Fig. 3 suggest the reduction of the ultrastructural effects as the insoluble residue content decreases. Figure 4 shows the light micrographs of the insoluble residue after acetylation. The structural changes are evident as the insoluble residue content decreases; the residues in the acetylation system with 4-chlorotoluene with the residual content of 28.1% are more likely to be original pulp fibers with a limited swelling. However, fiber residues in the control acetylation system with acetic acid (15.1%) are greatly swollen with retaining the fiber structure. In the acetylation system with 2-nitropropane with the lower residual content of 9.1%. the acetylated fibers are reduced to fiber fractions and finally such fiber fractions disappear in the reaction system with nitromethane (0.6%). Such changes of the fiber structure would make the pulp fibers swell w r e readily during acetylation and allow them t o solubilize greater to the acetylation medium. These lines of evidence seem to be parallel to a process to reduce the ultrastructural effects of the pulp fibers, thus supporting the suggestion above from Fig.3.
4 - Cliloro t ol uene
A cct,jc n ci tl
28.1 96
15.1 o/o
Fig. 4
2- N i 1ropropane
9.1
% I
Nit roinet liane
0.G %
Light micrographs of the insoluble residue after acetylation with a solvent designated. The value shows the insoluble residue content.
Effectsof solvent addition 225
0
6 - 0 0
e
5-
0
4-
0
,-.
U
g
t.
I
Q
10
20
0
Insoluble
0 0 I
30
Insoluble residue content (%) Fig. 5
40
Insoluble residue content vs. mannose content of the original pulp (7.0%) divided into soluble and insoluble portions.
Based on these results, it may be concluded that, in the reaction system with a small insoluble residue content, the great reduction of the insoluble residue is not only due to the ultrastructural effects reduced as proved above but also due to the chemical effects reduced in which the solubilities of CTA and GTA are improved with mixtures of the acetylation medium with the solvent added. The determination of the chemical compositions of the original pulps used in this.study indicated that it contains 7.0% mannose. Thus, the proportions of the total mannose in the soluble portion and insoluble residue were computed as shown in Fig. 5. It is apparent from Fig. 5 that as the insoluble residue decreases in content, the proportion of the mannose in the insoluble portion decreases, whereas that in the soluble portion, in turn, increases. This implies that some of the GTA which has formed the insoluble residue in the acetylation medium is no longer involved in its formation and moved to the soluble portion. Therefore, the addition of the appropriate solvent to the acetylation medium could solve a problem on the formation of the insoluble residue as the low-grade dissolving pulps are used for cellulose acetate preparation. In conclusion, a new acetylation system with an appropriate solvent can provide a clue as to an industrial usage of the low-grade dissolving pulps for cellulose acetate production.
REFERENCES 1) 2) 3) 4)
Ichino, M. : ffikkakyo Geppo, 39.25-32 (1986). Steinmann, H. W. ; White, B. B. : TAPPI, 37,225-232 (1954). Bradway, K. E. : ibid ,37,440-446 (1954). Wells, F. L. ; Schattner, W. C. ; Walker, A. Jr. : i b i d , 46,581-586 (1963).
226 Derivatives of cellulose and their properties
5) Nea1.J.L. : J. Appl. Polym. Sci, 9, 947-961 (1965). Wilson, J. D. ; Tabke, R. S. : ibid , 57, 77-80 (1974). 7) Ueda, K. ; Saka, S. ; Funaki, Y. ; Soejima, S . : Mokuzai Cakkaishi, 34,346-353
6)
8) 9) 10) 11) 12)
(1988).
Ueda, K.; Saka, S . ; Soejima. S.: TAPPI, 71. 183-187 (1988). Matsumura, H. ; Saka, S. : Mokuzai Gakkaishi, 38, 270-276 (1992). Matsumura, H. ; Saka, S. : ;bid., 38,862-868 (1992). Atsuki, K. : "Sen' isokagaku oyobi Kogyo" Maruzen 1956, p. 418-419. Borchardt, L. G. ; Piper, C. V. : TAPPI, 53,257-260 (1970).
low-grade dissolving pulp
S Saka and T Takahashi - Department of Wood & Paper Science, Faculty of Agriculture, Kyoto University, Kyoto, Japan 606-01
Shiro Saka and Tadashi Takahashi- Department of Hood & Paper Science, Faculty of Agriculture. Kyoto University, Kyoto, Japan 606-01
ABSTRACT As cellulose triacetate is prepared from low-grade dissolving pulp, a considerable amount of the insoluble residue is present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. As one of the remedies for reducing the insoluble residue, a solvent was added to the acetylation medium and the effects of the solvent addition on the amount of insoluble residue formed were studied. To do so, 17 different solvents were selected so as to cover a wide range of solubility parameters. The obtained results clearly indicated that the addition of the solvent affects the amount of insoluble residue and that nitromethane, nitroethane, dichloroacetic acid and methylene chloride were very effective for its reduction. A new acetylation system with such an appropriate solvent would provide a clue as to an industrial usage of the low-grade dissolving pulps for cellulose acetate production.
INTRODUCNON
Cellulose triacetate being important in the fiber and textile industries requires high quality cellulose as a starting material. ') This is because the low-grade dissolving pulps contain more hemicelluloses and the formation of hemicellulose acetates such as xylan acetate and glucomannan acetate, results in industrial problems such as filterability, turbidity (haze) and false viscosity. Although a considerable effort has been made since the mid-1950s to explain the relationships between the solution properties of the cellulose acetate and contaminated hemicel lulose acetate,2-8) viscose grade wood pulps with an a-cellulose content of less than 90% still cannot be used for manufacturing cellulose acetate. 219
220 Derivatives of cellulose and their properties In our previous work,'* l o ) we have studied cellulose triacetate prepared from softwood sulfite dissolving pulps with an a-cellulose content of 87.5%. The first problem faced was a substantial amount of insoluble residue present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. This insoluble residue retained a fiber structure of swollen form, resulting in one of the industrial problems, filterability. Characterization of the insoluble residue9) showed that it was composed of cellulose triacetate (CTA) and glucomannan triacetate (GTA) in aggregate with each other in the acetylation medium due to their compatible nature. In addition to such chemical effects of the molecular interactions, ultrastructural effects of glucomannan distribution in pulp fibers were found to be involved in the formation of insoluble residues. Therefore, the amount of insoluble residue can be expected to be reduced if chemical effects and/or ultrastructural effects are decreased during acetylation. As one of remedies, the pretreatment of low-grade dissolving pulps with mixtures of acetic acid and sulfuric acid was Found to reduce both the chemical and ultrastructural effects of pulp fibers in the following acetylation, resulting in reducing the amount of the insoluble residue. l o ) The use of higher amounts of sulfuric acid as a catalyst was also found t o reduce the insoluble residue formed. ") In this study, a solvent was added to the acetylation medium of acetic acid / acetic anhydride / sulfuric acid system and the effects of' the solvent addition on the amount of the insoluble residue formed were studied. To do so, seventeen different solvents were selected to cover a wide range of solubility parameters of the solvents.
EXPERIMENTAL Sulfite softwood dissolving pulp with an a-cellulose content of 87.5% was used for preparing cellulose triacetate as described in the previous paper. ') As cellulose triacetate was prepared from this pulp, a large amount of insoluble residue was present in the acetylation medium of an acetic acid / acetic Therefore, in this work, a solvent was anhydride / sulfuric acid system.') added to this acetylation medium to study the effects of the solvent on the amount of insoluble residue formed. The solvents used in this study are given in Table 1. The low-grade dissolving pulps (1 part) were thus acetylated with a solution of acetic acid (160 parts). acetic anhydride (7parts) and sulfuric acid (0.1 part) with a solvent (51 parts) for 3 h at 40 C. followed by stirring overnight at 20 C. The solutions were then spun in a centrifuge at 7000 rpm for 30 min. After the tubes were removed carefully, the supernatants pipetted away and precipitated substances were washed repeatedly with mixtures of fresh acetic acid and the solvent used in the system (160:51 in a weight ratio) by centrifugation to obtain the insoluble portions. The supernatants collected were, on the other hand, concentrated and poured into deionized water to precipitate the soluble portions. The insoluble and w!uble portions were washed as doscribed previously. The degree of substitution (DS) of these samples was determined by a titration method, I I ) while neutral sugar compositions were determined by an alditol-acetate procedure'2' using conditions described in a previous paper. ') For structural observations, some micrographs of the insoluble residues were taken in an acetylation system with a different solvent.
Effects of solvent additiofi 221 RESULTS AND DISCUSSION
It has been known in our previous work"' that, when cellulose triacetate is prepared from low-grade sulfite dissolving pulp, a substantial amount of insoluble residue is present in the acetylation medium of the acetic acid / acetic anhydride / sulfuric acid system. Therefore, in this study, a solvent was added to the above acetylation medium with 1:3 weight ratio of solvent to acetic acid to study the effects of the solvent addition on the reduction of the insoluble residue content in the acetylation medium. Table 1 shows selected solvents excluding the alkaline, unstable and reactive solvents in the reaction medium. The acetylation system with acetic acid (No.10 in Table 1) was also included just for comparison as a control reaction system without any addition of the solvent. Compared with acetic acid, 20.7 (MPa)i'2 in its solubility parameter (SP), the selected solvents could cover a Also included in Table wide range of the SP values from 16.6 to 26.0 (MPa)'/'. 1 is the insoluble residue content in the acetylation medium with a designated so 1vent. Figure 1 shows a relationship between the amount of insoluble residue and the SP of the solvent added to the acetylation medium. Numbers correspond to solvents as in Table 1. It is very apparent that compared with 15.1% in a control reaction system (acetic acid, No.10). the results are varied from 0.6% to 36.2% in insoluble residue contents, indicating that large effects of the solvent addition exist on the amount of the insoluble residue in the acetylation medium. Furthermore, it is noted that with an increase in the SP
Table 1 The insoluble residue content in the acetylation medium with a solvent added and its chemical compositions. Solvent 1. 2. 3. 4.
5. 6. 7. 8. 9.
10. 11.
12. 13. 14. 15. 16. 17.
R-Butyl chloride 4-Chlorotol uene 1.2-Dichloroethane Methylcellosolve acetate Ethylbromide Methylene chloride 2-Ni tropropane Bromobenzene Ni trobenzene Acetic acid 1-N i t ropropane Methylbenzoate 1-Bromonaphtalene Dimethyl phthalate Dichloroacetic acid Nitroethane Nitromethane
Solubility parameter ((MPa)"')
Chemical Compositions ( mol% )
Insoluble residue ( wtX )
Glucose
Mannose
Xylose
16.6 18.0 18.2 18.8
36. 2 28.1 9. 2 29. 3
80.0 79.4 55.2 78. 7
18. 9 20. 6 44.8 21. 3
1.1
19.6 19.8 20.3 20.3 20. 5 20.7 21. 1 21. 5 21. 7 21. 9 22. 5 22. 7 26.0
19.9 4.8 9. 1 17.2 7.4
66. 5 58. 2 71.8 73. 6 57. 9 71. 1 70. 3 80.1 68.3 70. 5
33. 5 29. 9 28.2 25. 3 42.1 20. 2 29. 7 19. 9 26.8 29. 5
69. 2 42. 6
30. 8 57.4
15. 1
7.1 24.1 20. 2 15. 6 0.8 3.0 0. 6
-
-
-
-
-
11.9
-
1. 1
-
0.7
-
4.9
-
222 Derivatives of cellulose and their properties
0'
16
I
18
1
20
22
0
I
24
2G
27
Solubility parameter (( MPa)'l2) Fig.
1
The soluble residue content vs. solubility parameter of the solvent added to the acetylation medium. Numbers correspond to the solvents as in Table 1.
I
,-.I
U
I
I
I
10
20
30
Insoluble residue content (%) Fig. 2
40
The sugar content vs. insoluble residue content for acetylation medium with a solvent added. Numbers correspond to solvents as in Table 1.
Effects of solvent addition 223 values, the amount of insoluble residue has a tendency to decrease. The effective solvents for reducing the insoluble residue are, therefore, nitromethane (0.6%). dichloroacetic acid (0.8%). nitroethane (3.0%), methylene chloride (4.8%), 1-nitropropane (7.1%), nitrobenzene (7.4%), 2-nitropropane (9.1%) and 1.2-dichloroethane (9.2%). The chemical compositions of the insoluble residue in each reaction system are also given in Table 1. It is apparent that, in addition to glucose, the insoluble residue contains a high proportion of mannose, with a limited amount of xylose except for the reaction system with methylene chloride. This result is in g o d agreement with the findings of our previous worke' that the insoluble residue is composed of cellulose triacetate (CTA) and glucomannan triacetate (GTA) in aggregate with each other at the molecular level by their mutual interactions. Therefore, GTA is involved in the formation of the insoluble residue for all acetylation systems studied. However, a closer inspection of the results in Table 1 shows a large variation in mannose content from a maximum of 57.4% to a minimum of 18.9%. Therefore, a relationship between the insoluble residue content and its sugar content for glucose and mannose was studied (Fig.2). From Fig. 2, it is apparent that the glucose content decreases with a decrease in the insoluble residue content, whereas the mannose content increases. Based on a study on the chemical compositions of the isolated glucomannan from the low-grade pulp used in this study, the molar ratio of mannose to glucose is known to be 3.2:1.@' With the value of 3.2, therefore, the contents of the GTA and CTA can be computed. Figure 3 shows the obtained ratio of CTA to GTA against the insoluble residue content. It is apparent that with a decrease in the insoluble residue content, the ratio of CTA to GTA decreases. Comparative studies of acetylation in the model experiments"' indicated that the insoluble residue from the low-grade dissolving pulp is formed by
5 4 -
3 -
2l
0
2 -
@@
1-
0 Fig. 3
@@
0
@@ @@ I
I
1
10
20
30
Insoluble residue content (%)
40
The ratio of the CTA to GTA content in the insoluble residue vs. insoluble residue content. Numbers correspond to solvents as in Table 1.
224 Derivatives of cellulose and their properties
ultrastructural effects of the pulp fibers and the chemical effects of the molecular interactions of CTA and GTA. The former effects originate from the ul trastructural distribution of residual glucornannan in the dissolving pulp, whereas the latter effects are due to molecular aggregation of CTA and GTA by their compatible nature. It is , therefore, possible to get a reduction in the amount of insoluble residue if chemical effects and/or ultrastructural effects are decreased during acetylation. Furthermore, our previous study'O ) indicated that as the ultrastructural effects are decreased, the ratio of CTA to GTA content in the insoluble residue is decreased. Therefore, the results in Fig. 3 suggest the reduction of the ultrastructural effects as the insoluble residue content decreases. Figure 4 shows the light micrographs of the insoluble residue after acetylation. The structural changes are evident as the insoluble residue content decreases; the residues in the acetylation system with 4-chlorotoluene with the residual content of 28.1% are more likely to be original pulp fibers with a limited swelling. However, fiber residues in the control acetylation system with acetic acid (15.1%) are greatly swollen with retaining the fiber structure. In the acetylation system with 2-nitropropane with the lower residual content of 9.1%. the acetylated fibers are reduced to fiber fractions and finally such fiber fractions disappear in the reaction system with nitromethane (0.6%). Such changes of the fiber structure would make the pulp fibers swell w r e readily during acetylation and allow them t o solubilize greater to the acetylation medium. These lines of evidence seem to be parallel to a process to reduce the ultrastructural effects of the pulp fibers, thus supporting the suggestion above from Fig.3.
4 - Cliloro t ol uene
A cct,jc n ci tl
28.1 96
15.1 o/o
Fig. 4
2- N i 1ropropane
9.1
% I
Nit roinet liane
0.G %
Light micrographs of the insoluble residue after acetylation with a solvent designated. The value shows the insoluble residue content.
Effectsof solvent addition 225
0
6 - 0 0
e
5-
0
4-
0
,-.
U
g
t.
I
Q
10
20
0
Insoluble
0 0 I
30
Insoluble residue content (%) Fig. 5
40
Insoluble residue content vs. mannose content of the original pulp (7.0%) divided into soluble and insoluble portions.
Based on these results, it may be concluded that, in the reaction system with a small insoluble residue content, the great reduction of the insoluble residue is not only due to the ultrastructural effects reduced as proved above but also due to the chemical effects reduced in which the solubilities of CTA and GTA are improved with mixtures of the acetylation medium with the solvent added. The determination of the chemical compositions of the original pulps used in this.study indicated that it contains 7.0% mannose. Thus, the proportions of the total mannose in the soluble portion and insoluble residue were computed as shown in Fig. 5. It is apparent from Fig. 5 that as the insoluble residue decreases in content, the proportion of the mannose in the insoluble portion decreases, whereas that in the soluble portion, in turn, increases. This implies that some of the GTA which has formed the insoluble residue in the acetylation medium is no longer involved in its formation and moved to the soluble portion. Therefore, the addition of the appropriate solvent to the acetylation medium could solve a problem on the formation of the insoluble residue as the low-grade dissolving pulps are used for cellulose acetate preparation. In conclusion, a new acetylation system with an appropriate solvent can provide a clue as to an industrial usage of the low-grade dissolving pulps for cellulose acetate production.
REFERENCES 1) 2) 3) 4)
Ichino, M. : ffikkakyo Geppo, 39.25-32 (1986). Steinmann, H. W. ; White, B. B. : TAPPI, 37,225-232 (1954). Bradway, K. E. : ibid ,37,440-446 (1954). Wells, F. L. ; Schattner, W. C. ; Walker, A. Jr. : i b i d , 46,581-586 (1963).
226 Derivatives of cellulose and their properties
5) Nea1.J.L. : J. Appl. Polym. Sci, 9, 947-961 (1965). Wilson, J. D. ; Tabke, R. S. : ibid , 57, 77-80 (1974). 7) Ueda, K. ; Saka, S. ; Funaki, Y. ; Soejima, S . : Mokuzai Cakkaishi, 34,346-353
6)
8) 9) 10) 11) 12)
(1988).
Ueda, K.; Saka, S . ; Soejima. S.: TAPPI, 71. 183-187 (1988). Matsumura, H. ; Saka, S. : Mokuzai Gakkaishi, 38, 270-276 (1992). Matsumura, H. ; Saka, S. : ;bid., 38,862-868 (1992). Atsuki, K. : "Sen' isokagaku oyobi Kogyo" Maruzen 1956, p. 418-419. Borchardt, L. G. ; Piper, C. V. : TAPPI, 53,257-260 (1970).