I
R. L. MELLIES, C. L. MEHLTRETTER, and I. A. WOLFF Northern Utilization Research and Development Division, U. S. Department of Agriculture, Peoria, 111.
Behavior..
.
Periodate- and Hypochlorite-Oxidized Starches V I s c m s m Y DATA are important for finding uses of periodate oxystarches as adhesives, textile- and paper-sizing assistants. Hypochlorite-oxidized starches rather than native starches are now used in paper-sizing because of their excellent adhesive powers, high fluidities, lower gelatinization temperatures, and lesser tendency to retrograde.
Materials and Methods Several hypochlori'te-oxidized starches and periodate oxystarches, prepared in the laboratory, provided materials with a known history, and are representative of the useful industrial range. The same commercial undefatted corn starch was used for both series. Oxidatioh with sodium hypochlorite is described for the most highly modified member. To a stirred suspension of 648 grams (dry basis) of corn starch in 2250 ml. of distilled water, adjusted to p H 9.5 with sodium hydroxide, was added in 1.5 hours 540 ml. of sodium hypochlorite solution containing 7.2 grams sodium hypochlorite per 100 ml. The latter was prepared by passing chlorine gas into an ice-cold solution of sodium hydroxide of such strength that the amount of free sodium hydroxide still present would be 1 to 1.5 grams per 100 ml. Sodium hypochlorite was estimated by previous methods (7,2). During the 8-hour oxidation period more sodium hydroxide solution was added to keep the pH at 8.5 or above. Unreacted oxidant was destroyed with sodium thiosulfate. The slurry was adjusted to pH 6.5 with hydrochloric acid and filtered. Inorganic salts were removed by repeatedly suspending the product in distilled water and refiltering, finally slurrying with 95% ethyl alcohol. The filtered product was dried over calcium chloride in vacuo; traces of alcohol were removed by exposure to 60%. relative humidity for several days, followed by equilibration for 2 days. Yield was 91%) based on the dry weight of starch used. For the other oxystarches, yields ranged from 94 to 97%. The method for oxidizing with sodium metaperiodate was similar to that already reported (75). Approximately 4% sodium metaperiodate solution was
added dropwise to the 10% cold (0' C.) starch slurry with vigorous stirring. For the two highly oxidized starches, the starch was added to the ice-cold oxidant. The calculated amount of oxidant to give the extent of oxidation desired was used, except for 100% oxidation, where the molar ratio of oxidant to starch was 1.5 to 1. Completion of the reaction was shown by a qualitative test for periodate in the supernatant or by analysis for residual periodate (3). Reaction times varied from 2 to 76 hours. Washing and drying procedures were similar to those described. All samples were ground to pass an 80mesh screen. The final moisture contents ranged from 9 to 13%. Carbonyl content in periodate and hypochlorite oxystarches was determined by the hydroxylamine method (5), with a preliminary treatment with calcium acetate to remove free carboxyl in the hydrochlorite oxystarches (Tables I and 11). Analysis for carboxyl in the hypochlorite oxystarches was by paste titration (7 7). Since the path of oxidation of starch with hypochlorite is uncertain, the method used earlier (75) to indicate extent of modification of periodate oxystarches was revised. Instead of being expressed as a percentage of dicarbonyl units (moles dicarbonyl per 100 AGU) modification is given as moles of carbonyl per 100 AGU (moles carbonyl plus carboxyl per 100 AGU for the hypochlorite oxystarches), or as moles functional groups (MFG) per 100 AGU. For the periodate oxystarches, division of this value by two gives the previously used value (75). Effect on viscosity of carboxyl groups and lesser amounts of carbonyl groups in the hypochlorite oxystarches can then be compared more directly with carbonyl groups in the periodate oxystarches. For viscosity measurements, the Corn Industries Viscometer (8) was used, and except where noted, distilled water was used. A starch concentration of 5% (dry basis) was chosen, so that results could be directly compared on a concentration basis. Bath temperature was 92' C., and the slurry was added a t room temperature. Total time allowed was at least 25 minutes. The term
viscosity is used; however, the term apparent viscosity or consistency is probably preferable.
Viscosity Behavior Periodatedized. For
us.
Hypochlorite- Oxi-
periodate oxystarches (Table 111; Figure 1) the maximum viscosity was reached rapidly in samples V I to I X and as rapidly declined, indicating extensive degradation. These pastes on cooling were thin, as contrasted with heavy to medium pastes in I to IV. Opacity of pastes decreased with increase in modification, the lowest members resembling ordinary starch paste. The very thin paste from X was almost clear but was yellow. Hypochlorite oxystarches C and D gave almost clear, thin, colorless pastes with much less opacity than pastes from the more highly modified oxystarches V I and VII. Oxystarches X and X I required longer times and higher temperatures to gelatinize. Degradation in X I led to a maximum viscosity of only 7 gram-cm. Decrease in maximum viscosity of oxystarches with extent of oxidation
Table 1.
Carbonyl Contents of Periodate-Oxidized Starches Moles Carbonyl/lOOAGU by HydroxylPeriodate Sample amine consumed
... ...
I 0.1 I1 0.2 111 0.4 IV 1.4 1.0 V 2 1.6 VI 6 6 VI1 8 8 VI11 11 12 IX 26 20 'X 122 120 XI 1805 206 186 By Na borohydride reduction (18).
...
Table II. Functional Groups in Hypochlorite-Oxidized Starches MFG/100 AGU Sample Carbonyl Carboxyl Total A
B C
D
VOL. 50, NO. 9
0.14 0.12 0.98 0.87
0.25 0.72 2.05 3.46
SEPTEMBER 1958
0.4 0.8
3.0 4.3
131 1
240
200
v
r n
0
160
-STARC
0: >-
k 120 In
0
z'
0 TIME, MINUTES
Figure 1. As modification of periodate oxystarches increased, gelatinization began at slightly lower temperatures and in a shorter time than for unmodified starch. Rate of gelatinization increased along with extent of paste breakdown probably stems from two main factors. One is related to the degree of swelling of the granule and depends partly upon carbonyl content. The other factor may relate to the instability and fragmentation of the swollen granule, which is increased by the presence of carbonyl groups. Periodate-oxidized starches, when heated in water, undergo primary bond scission as shown by striking decreases in molecular size (9). Cooled pastes from X and XI deposited a curdy material which did not redisperse on heating. Cooled pastes from oxystarches I to IV formed gels, as did pastes from A and B of the hypochlorite series. Greater acidities at the higher levels of oxidation of the periodate oxystarches may have caused greater degradation. A series of periodate oxystarches prepared from a different corn starch of about the same viscosity also behaved as shown in Figure 2. The actual maximum viscosity values in the two series did not agree, except for two of the less oxidized members. The type of curve obtained may be a function of oxidation level and not of the particular lot of corn starch used. The hypochlorite oxystarches showed marked lowering of beginning time and Table 111. Sample Starch
I I1 I11
IV V VI VI1 VI11
IX X XI A B C D a
MFGjlOO AGU
... 0.1
0.2 0.4 1.0 1.6 6 8 12 20 120 206 0.4 0.8 3.0 4.3
40
0 0
8
A
temperature of gelatinization as compared with unmodified starch or periodate oxystarches. For the higher members a rapid development of maximum viscosity was followed by a slower decline than with the periodate oxystarches. The higher maximum viscosities obtained than with unmodified starch or some of the periodate oxystarches may be related to the polyelectrolytic character of the hypochlorite oxystarches. Members of the two series with about the same MFG per 100 AGU values can be compared in Figure 3. The hypochlorite starch began gelatinizing in a shorter time and at a lower temperature than did the corresponding periodate oxystarch. The higher maximum viscosity was shown by the hypochlorite oxystarch. De-ashed Hypochlorite-Oxidized Starches. Hypochlorite oxystarches were de-ashed by the method previously used for analysis ( 7 7). Analysis for carboxyl gave about the same values as previously found for the sodium salts (Table 11). Trace amounts of sodium showed that de-ashing had been complete (Table IV and Figure 3). Maximum viscosities of de-ashed
Viscometric Data for Oxidized Starches Gelatinization Maximum Time range, Temp. (" C.) Viscosity, pHb min." at start G.-Cm. Slurry Past; 8.5-25 8-24 8-17
86 85.5 86
8-16
86
9-14 6.75-12.5
86 84 82 83 82.5 83.5 90.7 90
5-5.5 5-5.5
5-5.5 6-6.5 20.75-32.75 48-48.5 4.5-15 3.25-6.5 2.5-3.75 2-2.75
80
.
73 69 63
146 125 100 83 8 62 158 150 113 15 128 7 142 186 223 124
From time gelatinization began until maximum velocity was reached.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
5.3 5.7 5.6 5.7 5.5 5.6 5.6 5.6 5.2 5.8 5.1 4.4 8.2 7.5 7.8 8.1 25' C.
5.4 5.8 5.8 5.8 5.6 5.6 5.8 5.3 5.2 5.1 3.9 3.6 6.6 5.1 5.1 5.3
12
16 20 TIME, MINUTES
24
28
starches were much lower than for the sodium salts. Pastes were also more opaque, but addition of alkali decreased the opacity. Addition of hydrochloric acid to pastes of the sodium salts produced greater opacity. Time and temperature of beginning gelatinization were increased by de-ashing. Low p H values of slurries of the de-ashed starches may cause considerable degradation on heating before any appreciable viscosity can develop. Degradation during the de-ashing is not an important factor in reduction of viscosity. A 5% slurry of the last member of the de-ashed series was restored by addition of sodium hydroxide to the former p H level of the sodium salt. The maximum viscosity was then within 92% of the former value for the sodium salt. Good agreement between maximum viscosity and the shape and location of the pasting curve is shown (Figure 3) for two members of the de-ashed series [A (D) and B (D)] as compared with two periodate oxystarches (I11 and IV) a t nearly the same levels of modification. At these low levels, a carboxyl group is apparently almost equivalent to a carbonyl group in its effect on maximum viscosity. The assumption must be made that the effect of degradation is roughly parallel in the two series. Effect of Storage. After 2 months' storage, maximum viscosity of the periodate oxystarches was lower in some samples (Table V). Only slight alterations were found in the time or temperature range of gelatinization, in the shape of the pasting curves, or in p H values of slurries and pastes. Analysis of periodate oxystarches for carbonyl content showed only slight changes. Highly oxidized periodate oxystarches, after storage under various conditions, dissolve less readily in alkali and show anomalous behavior to reduction by the sodium borohydride method (72). No definite relation was found between extent of oxidation and percentage change in maximum viscosity on aging.
HY PO C HLOR ITE-OX IDIZED 240
200
4
HYPOCHLORITE
STARCHES
Figure 2. When maximum viscosities of periodate and oxystarches are plotted against moles of functional groups, the curve dips, rises, and then dips again
I w 160
L7.
>
e
s 120 Y
5 80
40
0
Periodate oxystarches are stable a t low oxidation levels; unmodified starch and hypochlorite oxystarches at all levels are unchanged, with the exception of the last member. Since viscosity is dependent upon the degree of granule swelling, the latter is apparently reduced by some unknown factor in several of the oxystarches upon storage. Effect of Electrolytes. Various theories have been proposed to explain why viscosity of dilute solutions of polyelectrolytes is lowered when electrolytes are added. Viscosity of potato starch but not corn starch is affected by electrolytes (73, 74). However, the theory generally accepted ( 4 ) is that introduction of counter ions from the added salt reduces the degree of ionization of the polyelectrolyte. Repulsive forces existing between charged centers along the polymer chains are reduced, and the polymer chain tends to coil, which decreases the viscosity. An increase in concentration of the polyelectrolyte has a similar effect. These effects are probably operative even in the more concentrated solutions used in the present work. When salts are added, hypochlorite oxystarches should behave as typical polyelectrolytes. Periodate oxystarches should, however, resemble neutral polymers, should not be much affected by salt addition, and might prove useful industrially. Instead of distilled water, solutions of
0
Sample
sodium chloride (Table VI) were used. O n a dry weight basis of starch plus sodium chloride, percentages of salt were about 20, IO, and 2 . Re-determined maximum viscosities for oxidized starches without added salt do not coincide with those found earlier, partly because of aging. Salt increased the gelatinization time and temperature of unmodified starch, especially a t the higher salt concentrations. The maximum viscosity was virtually unchanged. The maximum viscosities of periodate oxystarches decreased in 0.2 N salt solution, and in the 0.02 N salt solution a t the higher oxidation level (Table VI). Gelatinization was delayed, and the time range was broadened for 11. At the higher oxidation level (VII) time and temperature of beginning gelatinization were unchanged, but the maximum viscosity was lowered. Greater decreases in maximum viscosity were noted, as expected, on using salt solutions with hypochlorite oxystarches (A and D). Increases in beginning gelatinization time and tem-
Viscometric Data for De-ashed Oxystarches Gelatinization Temp. MaxiMFG/ (" C.) mum 100 Range, at Viscosity, PH (26'c.) AGU min." start G.-Cm. Slurry Paste
A 0.4 A (D) 0 . 4
IIIb B B (D) IVb
4.5-15 6.5-15 8-16 3.25-6.5 8-14 9-14 2.5-3.75
0.4 0.8 0.8 1.0 C 3.0 0 C(D) 3.0 D 4.3 2-2.75 c D (D) 4.3 D = de-ashed. From
viscosity was reached. viscosity.
80 a3 86 73 83 86 69
142 84 83 186 23 8 223
63
124
0
E
c
0
16 20 TIME, MINUTES
28
24
Figure 3. Pasting curves for periodate, and hypochlorite(including de-ashed) oxidized starches HC = hypochlorite; HC(D) = hypochlorite, de-ashed; PO = periodate
Table IV.
Sample
12
8
4
8.2 4.5 5.7 7.5 4.2 5.5 7.8 4.0 8.1 3.8
6.6 3.7 5.8 5.1 3.2 5.6 5.1 2.8 5.3 2.7
time gelatinization began until maximum Periodate oxystarch. No measurable hot
A AP) 111 B B(D)
Oxidation Type HC HC(D)
MFG/lOO AGU 0.4 0.4 0.4 0.8 0.8
PO HC HCP) PO
IV
1 .o
perature are possible factors in reducing the viscosity through increased tendency toward degradation. Increases in acidity occurred in slurries made using sodium chloride solutions, for example, from p H 8.4 to 6.6, but were much smaller with periodate oxystarches. The p H decrease in hypochlorite oxystarches slurried in salt solution is not alone responsible for loss in maximum viscosity. A 5% suspension of A in 0.02 N sodium chloride was brought with sodium hydroxide to p H 8.4, which corresponds to that found for A in distilled water. Maximum viscosity was 34 gram-cm., compared to 150 in distilled water. The presence of sodium chloride and its action on the polyelectrolyte, and not the p H decrease noted earlier must be mainly responsible. A viscosity curve with unmodified starch and tap water was also run.
Table V.
Changes in Maximum Viscosity on Storage
Sample
Max. Visc., G.-Cm. Original 2 mos. 3 mos.
I I1 I11 IV V VI VI1 VI11 A
B
C
D
125 100 a3 8 62 158 150 113
128 95 16 98 103 57
142 186 223 124
140 179 211 122
to
Based on original viscosity.
69 G.-cm. after 4 months' storage.
VOL. 50, NO. 9
... 99 ... ... ...
...
89O
... 150
... ... 111
% Change in Max.' Visc. 2 mos. 3 mos. +2 -5 - 28
...
- 74 -38 -31 - 50 -1 -4 -5 -2
... -1 ... ... ... 9 . .
-41
... +6 ... ...
- 11
No measurable hot viscosity.
SEPTEMBER 1958
1313
4 240
Somple
D r y i n g Conditions
200
z U 160 a >.
-
;;120
Figure 4. Effect of drying conditionson viscosity of periodate oxystarches. High drying temperatures for extended periods of time should be avoided
Y 80
40
0 0
4
8
12
16 20 TIME,. MINUTES
(2 moles of carbonyl per 100 AGU) from another corn starch was prepared a t room temperature. The final alcohol wash was omitted. The filtered starch, subdivided and dried, contained 11.9 to 12.6% moisture (Figure 4). Starting gelatinization time and temperature, as well as p H levels of the slurries and of the pastes, were of the same order for all four samples. The curve and other characteristics of the unmodified starch varied little from that (Figure 1) of the first unmodified starch, although on prolonged heating a slight drop in maximum viscosity occurred. Analytical data (ash, nitrogen, and phosphorus) for the two unmodified starches are nearly identical. Drying of the second unmodified starch for 15 to 20 hours in vacuo at 100' C. decreased the maximum viscosity by 16%. When this result is compared with that for IV the presence of carbonyl groups is seen to have considerable influence on the viscosity. High drying temperatures for extended periods should be avoided.
The p H of the tap water was 7.3; of the slurry, 6.95. Gelatinization occurred in 5 to 8.5 minutes, and in a temperature range of 80' to 82' C. Maximum viscosity was 137 gram-cm., about 8% below that with distilled water, and declined on continued heating. Data obtained with the oxystarches would probably have been considerably altered if tap water had been used. No general relation was found between concentration of salt used with the oxidized starches and the loss in maximum viscosity. Higher salt concentration in two instances (Table VI) had less effect than did the lower concentration. Partial dependence rests upon the modification of the starch; the more highly oxidized the starch, the greater the effect a t the same salt concentration. As expected, salts had a greater effect upon hypochlorite than upon periodate oxystarches. Effect of Drying Conditions. The method of drying already cited would be impractical commercially. Therefore, a fresh lot of periodate oxystarch
Table VI.
Sample Starch
I1 VI1
A D
28
24
Effect of Sodium Chloride upon the Viscosity of Oxidized Starches Gelatinization Normality Temp., Maximum Maximum MFG/100 XaC1 Time range, (" C.) Viscosity, Viscosity, AGU Soh. min.a at start G.-cm. Change, 70
... ... ... ...
...
0.02 0.1 0.2
0.2
...
0.2
0.02 0.2
0.2 8 8 8 0.4 0.4 0.4 4.3 4.3 4.3
...
0.02 0.2
...
0.02 0.2
...
0.02 0.2
8.5-25 8.5-24 10.75-36 11-46 8-19 8.5-24 11-31 5-5.7 5.5-6 5.5-6
86 86 87 88 86 86 88 83 83.5 83.5
148 153 151 151 99
4-14 10-41 13-65 2-3 3-3.5
78.5 86 88.5 63 71
150 51 57c 111 18
d
d
101
78 89 39b 59
d
... ... ...
...
...f 2
-22 ... -44
- 34
... - 66
- 62 - 84 - 100
...
From time gelatinization began until maximum viscosity was reached. b Maximum viscosity without salt was 69 g -cm., redetermined when the viscosity in 0.02N NaCl was also No measurable hot viscosity. determined. Viscosity still rising very slowly.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Another lot of this particular oxystarch, prepared by the same method, was dried 44 hours at 40' C., and had a maximum viscosity of 165 gram-cm. compared with 262 gram-cm. after 20 hours. Conclusions Differences in gelatinization rate and temperature, and in paste appearance may be related to the kind and degree of modification. More highly oxidized members of the two series gelatinize rapidly and break down as rapidly. At Iow oxidation levels a carbonyl group is about equal to a carboxyl group in the effect on viscosity. De-ashing of hypochlorite starches gives materials with much less potential viscosity than the sodium salts, either by reduction in the polyelectrolytic character, or by degradation because of high acidity. Addition of sodium chloride lowers markedly the potential viscosity of hypochlorite starches, and of periodate oxystarches a t high oxidation levels or a t high salt concentrations. Extended storage, and temperature and time of drying during preparation are factors which may also be detrimental to periodate oxystarches. Acknowledgmenf The assistance of C. S. Wise is gratefully acknowledged. References (1) Am. Pharm. ASSOC., National Formulary, 9th ed., p. 482, Washington, D. C., 1950. (2) Assoc. Offic. Agr. Chemists, Official Methods of Analvsis. 7th ed.. D. 79. Washington, D. C.: 1450. (3) Fleury, P., Lange, J., J. Pharm. Chem. 17, 107-113 (1933). (4) FUOSS,R. M., Strauss, U. P., J. Polymer Sci. 3. 246-63. 602-3 (1948): Ann. N . Y.Acari. Sci. 51.'836-51 11949)': (5) Gladding, E. K:, Purves; C. B., Paper Trade J . 116, No. 14, 26-31 (1943). (6) . . Hullinger, C. H., Whistler, R. L., Cereal Ch;m..28, 153-.7 (1951). . (7) Jackson, R. L., Hudson, C. S., J . Am. Chem. Sac. 60. 989-91 119381. (8) Kesler, C. 'C., Bechtel, W. C., Anal. Chem. 19,16-21 (1947). (9) Levine, S., Griffin, H. L., Senti, F. R., Abstracts of Paoers. P. 5D. Div. of Carbohydrate ChkmistG, 131A Meeting, ACS, Miami, Fla., April 1957. (IO) McKillican, Mary E., Purves, C. B., Can. J. Chem. 32, 316-32 (1954). (11) Mattisson, M. F., Legendre, K. A,, Anal. Chem. 24, 1942-4 (1952). (12) Rankin, J. C., Mehltretter, C. L., Anal. Chem. 28, 1012-14 (1956). (13) Riiggeberg, H., Die Starke 5 , 109-113 (1953). (14) Samec, M., Kolloid-Beihefte 3, 123-60 (1911). (15) Sloan, J. W., Hofreiter, B. T., Mellies, R. L., Wolff, I. A., IND.ENC. CHEM.48,1165-72 (1956). (16) Whistler, R. L., Linke, G., Kazeniac, S., J . Am. Chem. SOG.78,4704-9 (1956). (17) Yelland, W. E. C . (to Lauhoff Grain Co.) U. S. Patent 2,606,188 (Aug. 5, 1952). RECEIVED for review December 4, 1957 ACCEPTED April 30, 1958
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