False Viscosity in Cellulose Acetate Solutions
F
CELLULOSE ACETATE prepared from good quality acetylation grade cotton linters, viscosities of concentrated solutions in acetone can be predicted from intrinsic viscosities of dilute solutions. This relationship, however, cannot be routinely applied to acetates from wood pulps; for most of these acetates viscosities of concentrated solutions are not only higher than those predicted, but vary according to processing history of the wood cellulose. This difference between viscosities predicted from experience with linters and that found experimentally is called in the industry “false viscosity.” False viscosity is important in manufacturing acetate yarn-for normal handling, it requires lower solution concentration and increases cost; or if normal solution concentrations are used, it sacrifices yarn strength by requiring a lower ester degree of polymerization. Better methods for measuring this viscosity and means for its reduction in wood cellulose acetates are important to both pulp and acetate producers. Although false viscosity has been attributed to materials in cellulose, such as pentosans and mannans (3), it is probably related also to orientation and association of cellulose and/or other carbohydrate polymers in the fiber before acetylation and in acetate products. This article, however, is concerned primarily with detection and measurement. That solution viscosity of cotton acetate relates directly to intrinsic viscosity is shown by plotting the logarithm of solution viscosity under standard temperature and concentration against intrinsic viscosity (Figure 1). The resulting linear relationship is characteristic and applies to intrinsic viscosities well below those found in commercial acetates. Figure 1 includes acetates from wood pulps with false viscosities both acceptable and unacceptable for commercial use. These lines, together with that for cotton linter acetate, form a family of lines having on axis y a common intercept which for Figure 1 is -0.8. Because the lines are determined empirically, no special significance need be
attached to the intercept; however, it is approximately the logarithm of the viscosity of acetone measured under conditions used for the solutions. Because these lines in Figure 1 are characteristic of celluloses from which the acetates were made, their slopes may be used to measure false viscosity effects. If linters acetate is used for comparison, its slope of 1.19 is the base for comparing all other celluloses. Under similar conditions, wood pulp from western hemlock gives for the acetylation grade a slope of 1.27 and for the viscose grade a slope of 1.57. The relationship of this family of lines may be used to measure false viscosity which for a given cellulose is the ratio of its line to that for cotton linters as the base cellulose. Thus, the false viscosity number, obtained by subtracting 1.OO from the ratio of slopes and then multiplying by 100 to eliminate decimals, is defined by the equation, False viscosity No. = slope of cellulose line - 1.00) 100 slope of linters line
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Applying this equation to wood pulp, the viscose type shows a false viscosity number of 32, and the acetylation grade a number of 7. As a working limit, numbers below 15 indicate the cellulose can be used in preparing commercial acetates while higher numbers are intolerable. After the slope for the cotton linters line has been established, the false viscosity number for any cellulose may be determined by a single acetylation and hydrolysis; its position in Figure 1 may be located by any point and the common intercept a t zero intrinsic viscosity. The term, false viscosity number, is preferred to Steinmann and White’s “viscosity ratio” ( 3 ) . The advantage of this number is principally that it can be evaluated a t any degree of degradation that accompanies acetylation. For many celluloses, it is difficult to predict conditions necessary to produce the desired intrinsic viscosity, and preparing samples may entail a number of acetylations and tests. Also, yarn properties of the acetate correlate somewhat with intrinsic viscosity; thus, intrinsic vis-
cosity is frequently a routine determination and measuring viscosity of the concentrated acetone dope is the only added determination needed to calculate the false viscosity number. If solution viscosities are plotted on the ‘/*th power scale suggested by Mitchell and Umberger (2), linear relationships are similar to those using the logarithmic scale. Slopes of the lines are different because of different treatments, but false viscosity numbers from both plots are practically identical. Because of simplicity, the logarithmic scale is preferred. False viscosity numbers are not affected by changing intrinsic viscosity through using a different solvent. Thus, these numbers, calculated from Figure 2, using intrinsic viscosities measured in cupriethylenediamine, are the same as those from Figure 1 using acetone. Of course, intrinsic viscosiJies used in determining the linters reference line must be measured in the same solvent used for that of the test sample. High false viscosities can be considerably reduced by subjecting cellulose to alkaline refinement. This can be done by either mercerization or treatment with dilute alkali a t temperatures above 100’ C. For example, when a viscosetype cellulose was mercerized and its line redetermined (Figure 2), the false viscosity number dropped from 32 to 15. The conditions of acetylation were, of course, much milder for the mercerized than for the unmercerized sample; consequently, the two acetates are different. The alkaline treatment, however, greatly improved false viscosity of the cellulose. This represents a broad field for study; false viscosities are probably sensitive to conditions of alkaline treatment and minimum values may be found in narrow ranges of time, temperature, or alkali concentration. In producing commercial acetates having suitable intrinsic viscosities, conditions of acetylation are also changing along with conditions of alkali treatment; therefore, a fine balancing of these conditions against prior history of the cellulose is necessary. Only minor variations in false viscosity have thus far been produced by improving or altering conditions of acetylation. Slopes for the lines of cotton linters have varied from 1.14 to 1.19. Using similar variations for wood pulp from western hemlock that was refined with hot dilute alkali, slopes for the acetylation grade were 1.24 to 1.34 and for viscose-type cellulose from the same wood, 1.54 to 1.63. In measuring false viscosity of unmercerized celluloses, a linters line determined by an acetylation procedure similar to that used on the test sample is preferred laboratory practice. This failure in responding to variations in acetylation conditions partially verifies the theory that false viscosity is a characteristic of cellulose from which the VOL. 49,
NO. 9
SEPTEMBER 1957
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Figure 1. Relationship of solution viscosity to intrinsic viscosity, both measured in acetone One part of cellulose acetate in five parts of acetone
acetate is made, and is carried through acetylation, partial hydrolysis, precipitation into water, and dissolution in acetone. Therefore, it follows that false viscosity must appear in the acetic acid solution where the acetylation has been performed. Figure 3 shows a logarithmic plot where viscosities of these acetic acid solutions correlate with those obtained later in acetone. This correlation permits viscosities of this acetic acid solution to be used in calculating the false viscositl-number.
Experimental
Air-dry cellulose shreds adjusted to 4.0% moisture were treated for 1 hour
Intrinsic viscosity in acetone
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Figure 2. Relationship of solution viscosity in acetone to intrinsic viscosity in cupriethylenediamine
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Intrinsic viscosity in cupriethylenediomine
Figure 3. Acetone solution viscosity plotted against viscosity of original acetate sobtion in acetic acid
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1 = acetylation cotton linters
H A = hot alkaline-refined acetylation wood pulp from western hemlock V = viscose-type sulfite wood pulp from western hemlock MV = V that has been mercerized b y treating with 1870 sodium hydroxide solution at
K
20° c. = bleached prehydrolyzed
at 38' C. with 2.4 parts of glacial acetic acid followed by tumbling for 1 hour in a solution of 0.03 part of sulfuric acid in a total of 5.3 parts of acetic acid. The cooled system was then esterified by adding 0.04 part of sulfuric acid dissolved in 0.5 part of acetic acid and 3.70 parts of acetic anhydride, and tumbling for I hour at 38" C. After hydrolysis of the excess acetic anhydride, the acetates were hydrolyzed to 40 j= 0.5% acetyl. Distilled water was used throughout to avoid salt effects (7). Mercerized celluloses required an overnight adjustment with 0.5 part of acetic acid before treatment with sulfuric acid solution to avoid residues of unacetylated fiber at the end of esterification. Different viscosities for acetates from a single cellulose were produced either by changing time periods in the sulfuric acid solution before adding anhydride or by changing the time for esterification after adding anhydride. Concentrated acetone solutions were prepared by tumbling 5 parts of acetone (0.4'% water) by weight with 1 part of acetate. For both acetic acid solutions before hydrolysis and concentrated acetone solutions, viscosities were measured by the falling ball method, using Hercules glass beads and expressing viscosity in seconds. Intrinsic viscosities in both cupriethylenediamine and acetone were measured in Fenske-Ostwald viscometers using cellulose instead of acetate concentration in the calculations. Literature Cited (1) Malm, C. J., Tanghe, L. J., Smith. G. D., IND. ENG. CHEM.42, "30
(1950). ( 2 ) Mitchell, J. A . . Umberger, J. C., 130th S .J . , Meeting, ACS, Atlantic City, i September 1956. ( 3 ) Steinmann, H. W., White, B. B., TAPPI 37, 225 (1954).
kraft pulp from southern pine
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
C A = cold alkaline-refined acet1,000
G. A. RICHTER and 1. E. HERDLE
ylation cellulose from spruce
Eastman Kodak Co., Rochester, N. Y.
