November 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
culent precipitate was filtered and the light transmission of the two solutions compared. ACKNOWLEDGMENT
The sample of styrene-isobutylene copolymer was supplied through the courtesy of R. G. Newberg, Chemical ~ i ~ i Esso New Products Laboratory, Linden, N. J. LITERATURE CITED
(1) Dow Chemical Go., Brit. Patent applic. 28,871 and 28,876-8 (1949).
2693
(2) FUOSS, R.M., and Straws, U.P., Ann. N . Y . Acid Sci., 51, 836 (1949). (3) Hawdon, A. R.H., Marmio, N. D., Powell, G. R., and Thomas, S., paper presented at Ion Exchange Symposium, A.1.Ch.E. Meeting, Pittsburgh, Pa., April 1951. (4) McBurney, C. H., U. S. Patent 2,591,573 (April 1, 1952). ~ (5)i Skner, ~ ~ R., , and Demagistri, A., J. c h k . PhW., 47, 704 (1950). (6) Wheaton, R.,and Bauman, W., IND.ENG.Crrmf, 43, 1088 (1951). RECEIVED for review September 13, 1951. ACCEPTED August 19, 1952. Presented before the Division of Paint, Varnish, and Plastics Chemistry at the 121st meeting of the AMERICAN CHEMICAL SOCIETY, Milwaukee, Wis.. Maroh 30-April 3. 1952.
Fractionation of Cellulose Acetate A. J. ROSENTHAL AND B. B. WHITE Central Research and Development Department, Celanese Corp. of America, Summit, N . J .
c
ELLULOSE acetate is a polymer containing molecules of cal portions of a cellulose acetate in acetone solution by succesvarying chain length and chemical composition distribusive additions of n-heptane in one case and water in the other. tions. The distributions depend on the source of the cellulosic I n both cases the fractions varied in both acetyl value and visraw material as well as on the esterification and hydrolysis techcosity. When n-heptane was used as precipitant, the fraction niques employed. Because acetate yarn properties may be of highest viscosity had lowest acetyl value, whereas when water dependent on the distributions of both molecular weights and was used as precipitant, the fraction of highest viscosity had acetyl values, it is important that methods be available for their highest acetyl value. determination. DUAL DEPENDENCE OF SOLUBILITY ON The methods which have been used for fractionation of polyACETYL VALUE AND VISCOSITY meric substances have been reviewed by Cragg and Hammerschlag (2). For the most part, fractionations have been based on Schematically, a heterogeneous cellulose acetate with its spread the differential solubilities of the components of a polymer in of acetyl value and intrinsic viscosity can be represented by a solvent-precipitant mixtures. circle in Figure la. The actual distribution within the limits of The choice of solvent-precipitant system is worthy of emphasis the circle can be represented by intensity of shading. The shading is implied, but not included, in the subsequent diagrams for from several standpoints, Morey and Tamblyn (7)obtained variations in sensitivity to a molecular weight difference in two the sake of clarity. cellulose acetate fractions when using various combinations of If the solubility of the cellulose acetate in a solvent-precipitant mixture depends solely on its intrinsic viscosity, one would expect solvents and precipitants. Howlett and Urquhart (6) examined a number of solvent-precipifractions 1, 2, 3, and 4 to tant systems with a view to precipitate successively as extending the range of per shown in Figure lb. These cent precipitant over which fractions would be of concellulose acetate fractions stant average acetyl value could be obtained. Clement 3&; and would decrease remand Rivibre (1) characterlarly in intrinsic viscosity. ised cellulose acetate by Similarly, if the solventINTRINSIC precipitant combination fractional extraction with VISCOSITY could be chosen such that aqueous ethyl alcohol and found that the fractions the solubility of the cellulose varied in both viscosity and acetate depends solely on acetyl value. On the other its acetyl value, then frachand, Sookne, Rutherford, A.V. 4 3 2 I tions related as shown A.V.I{@, A.t[@' Mark, and Harris (8) fracschematically in Figure IC tionally precipitated cellu(b) (C) (d I would be expected. These lose acetate from an acetone fractions would have a consolution with ethyl alcohol I.v. I .v. I .v. stant average intrinsic vist o yield fractions of varying cosity and would differ prcviscosity with only slight gressively in acetyl value. acetyl value variations. Actually, the solubility of The importance of the A.V.1 a cellulose acetate molecule A .V. dual dependence of celluin a solvent-precipitant mixlose acetate solubility on ture is found in most cases (e) acetyl value and chain t o depend on both its inlength became apparent I .v. I.V. I.V. trinsic viscosity and its when McGoury and White Figure 1. Dual Dependence of Solubility of Cellulose acetyl value. Thus, in an (6) fractionated two identiAcetate on Acetyl Value and Intrinsic Viscosity acetone-water mixture low
/o?
[@,
p&I
-
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2694
intrinsic viscosity and low acetyl value both contribute toward increased solubility. With increasing water addition t o an acetone solution, the successive fractions will decrease in both average acetyl value and average intrinsic viscosity. This is shown schematically as fractions 1, 2, 3, and 4 in Figure I d . The acetyl value-intrinsic viscosity relationships of the cellulose acetate fractions are not always the same, but depend upon the choice of solvent-precipitant combination. In an acetonen-pentane system, the successive fractions precipitated will decrease in average intrinsic viscosity, but will increase in average acetyl value as shown schematically by fractions 1, 2, 3, and 4 in Figure I e. If a cellulose acetate solution in acetone were fractionated by successive additions of water, assuming that several refractionations were employed to minimize coprecipitation and that 16 fractions were obtained, the results would be schematically represented by Figure If. It is apparent that in spite of the most painstaking technique, each of the fractions would have relatively wide spreads of both intrinsic viscosity and acetyl value, as shonn by the limits of a shaded fraction in Figure If. If the fractionation were performed using n-pentane as precipitant, similar wide fractions would be obtained.
Vol. 44, No. 11
and acetyl value spreads, as represented schematically by fractions 1,2,3, and 4 of Figure lg. Refractionation of these cross-fractionated fractions with acetone-water showed relatively narrow limits of both acetyl value and intrinsic viscosity, as evidenced by the data obtained on refractionation of fraction B2 (Table 111).
TABLE111, ACETONE-WATER FRACTIONATION Fraction Original c1 C2 c3 c4
Acetone’Water Ratio
...
63/37 61/39 54/46 Evaporated
Weight, G. 33
..
20 7 4
Acetyl Value,
%
38.6
...
38.8 38.5 38.4
OF
FRACTION BZ Intrinsic Viscosity, Dl./G. 2.38
. ..
2.48 1.77 1.62
From a group of narrow fractions of varying acetyl value and intrinsic viscosity prepared by this cross-fractionation procedure two series of samples were selected, as shown in Table IV, for further study. Series A samples had identical intrinsic viscosity but varied in acetyl value. Series B samples had identical acetyl value but varied in intrinsic viscosity.
EXPERIMENTAL
MATERIALS.The solvents used in these experiments were c.P., redistilled prior to use. The cellulose acetate samples were Celanese Corp. Pilot Laboratory batches made from cotton linters. CROSSFRACTIONATION. I n order to obtain cellulose acetate fractions with narrow acetyl value and intrinsic viscosity distributions, a “cross-fractionation” scheme n-as developed. A cellulose acetate sample x-as fractionated by making four successive water additions to a 5y0 solution in 99.5% acetone. The fifth fraction was obtained by evaporation of the total filtrate after removal of the fourth fraction. The procedure was carried out with the results shown in Table I.
TABLE I. ACETONE-WATER FRBCTIONATION Fraction Original A1 A2 A3 A4 A5
AcetoneWater Ratio
... 63/37 59/41 52/48 43/57 Evaporated
Weight,
Acetyl Value,
100 9.6 50.8 19.4 3.0 2.5
38.6 38.9 38.9 37.9 36.9 37.0
G.
76
Intrinsic Viscosity, Dl./G. 1.75 2.31 2.10 1.09 0.72 0.46
The successive fractions described in Table I fit the schematic representation of Figure Id. Successive n-pentane additions were made to a 7% solution of fraction A2, combined with an equivalent fraction from a duplicate determination, in acetone. New fractions were isolated as shown in Table 11.
TABLE 11. ACETONE-PENTANE FRACTIONATIOX OF FRACTIOK A2 Fraction Original B1 B2 B3 B4
AcetonePentane Ratio
...
87/13 84/ 16 82/18 Evaporated
Weight, G. 85 15 43 16
5
Acetyl Value,
%
38.9 37.9 38.6 39.2 39.1
Intrinsic Viscosity, DI./G. 2.09 2.59 2.38
1.80 0.87
The fractionation data of Table I1 indicate the broad heterogeneity of the fractions obtained from the single acetone-water fractionation. The fractions obtained from this second fractionation were expected to have relatively narrow intrinsic viscosity
TABLE IV. CELLULOSE ACETATE FRACTIONS Sample Series A 1
2 3 Series B 4 5 6
Acetyl Value,
Viscosity, Intrinsic
%
Dl./G.
37.9 33.4 39.9
1.66 1.67 1.63
39.4 39.5 39.5
0.75 1.77 2.86
INTRINSIC VISCOSITYDETERMINATION. Intrinsic viscosities were obtained from relative viscosities of 0.1% solutions in 957, aqueous acetone, measured in 100-second Ostwald viscometers, by means of a nomograph based on the Baker-Philippoff equation: q:.,135 -1 ‘’I = 0.135 X C where
intrinsic viscosity, 100 ml. per gram relative viscosity concentration, grams per 100 ml. ACETYLVALUEDETERMINATIONS. Acetyl values were determined by an alcoholic alkali procedure ( 3 ) . TURBIDIMETRIC TITRATIOSS. One per cent stock solutions in the desired solvents were prepared from each of the cellulose acetates listed in series A and series B (Table IV); 0.02% solutions for turbidimetric titrations were prepared by diluting 2 ml. of stock solution to 100 ml. in a volumetric flask with the desired starting mixture of solvent and precipitant. The initial solvent-precipitant mixture was chosen so that the saturation point would be conveniently achieved in the subsequent titration with precipitant. The titration was performed in a glass cell in a Lumetron colorimeter, using a blue (4400 A.) filter; 100 ml. of the 0.02% solution of the cellulose acetate in the desired solvent was added t o the cell. h stirrer operated in the solution just above the path of the light beam. The colorimeter was adjusted to read 100% transmittance with the cellulose acetate solution in place with the stirrer operating. A precipitant was added in small increments from a buret until turbidity appeared. The point a t which a small increment of precipitant caused the first abrupt reduction in transmittance of the solution was referred to as the saturation point. In one series of experiments, in which the precipitant-solvent ratio was very great, it was found more convenient to start with a turbid suspension of the cellulose acetate in a mixture containing an excess of precipitant. The solvent was then added in small increments from a buret and the decrease in turbidity was followed by measuring the increase in transmittance. rlt the saturation point the transmittance readings abruptly became constant; this corresponded to complete solution of the cellulose acetate. [q] =
qrel = C =
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1952
All solutions were adjusted to 25” C. before each titration, and the titrations were conducted in a temperature-controlled room a t 24’ =k 1’ C. An attempt was made to maintain the manipulative technique constant, since the temperature oi the photoelectric apparatus was not controlled during the titration. All titrations were carried out in duplicate or triplicate. Individual saturation point determinations usually agreed within 1%. Averaged data of the multiplicate determinations are presented in this report. The following solvents and precipitants were investigated. Solvents Acetone Acetic acid Ethyl lactate
The ratio “A intrinsic viscosity/A acetyl value” in Table VI is a measure of the relative dependence of the saturation point on acetyl value and intrinsic viscosity of the dissolved cellulose acetate for each solvent-precipitant system. A high ratio indi100
90
Precipitants Water Ethyl alcohol-water (2 to 1) Ethyl alcohol 1-Propanol-water (1 to 1 )
80
P
1-Propanol
$
Methanol Ethyl acetate Butyl acetate Isopropyl ether
f
Carbon tetrachloride
c
8
RESULTS AND DISCUSSION
I n Figure 2 transmittances of 0.02% solutions in acetone of the cellulose acetates of Table IV are plotted as a function of volume of n-pentane added to the solutions. These curves are typical of the data obtained using various combinations of solvent and precipitant. The per cent acetone present a t the saturation points when acetone solutions of each of the samples in series A and series B were titrated with precipitants are tabulated in Table V. It is readily seen that the saturation point varies with the acetyl value and intrinsic viscosity of the cellulose acetate, and with the choice of solvent-precipitant, To resolve a heterogeneous cellulose acetate into narrow fractions by means of solubility differences, a solvent-precipitant system must be chosen such that the saturation point varies progressively with change in cellulose acetate intrinsic viscosity, but does not vary with change in acetyl value. Similarly, to resolve a cellulose acetate into narrow acetyl value fractions a system must be chosen such that the saturation point varies progressively with acetyl value, but does not vary with change in intrinsic viscosity. From the data for the series A samples in Table V the influence
(Cellulose acetate samples of Table IV) Constant I.V. Constant A.V. Variable A.V. Variable I.V. Series A Series B Precipitants 1 2 3 4 5 6 Water 54.2 54.6 57.1 52.6 69.0 59.7 23.5 28.5 34.5 31.4 36.0 34.2 Ethyl alcohol-water (2 to 24.7 23.8 34.1 27.2 32.9 34.5 Propanol-water (1 to 1) 30.2 28.5 28.9 25.0 29.7 34.3 Ethyl alcohol 42.2 4 0 . 4 40.2 37.2 40.4 4 3 . 8 Propanol 28.0 Ethyl acetate 0 0 0 0 5 8 . 6 42.4 3 4 . 4 2 8 . 7 3 3 . 3 4 1 . 2 Butyl acetate 84.3 81.0 76.7 73.7 76.5 80.3 Pentane 83.4 76.6 71.7 69.3 71.2 75.3 Isopropyl ether a Initial cellulose acetate concentration was 0.06% instead of 0 . 0 2 ~ o . Turbid suspension titrated with acetone.
of acetyl value variation at constant intrinsic viscosity on the saturation point for a large variety of solvent-precipitant systems can be assessed. The change in saturation point acetone concentration for each system over the range from 37.9 to 39.9 acetyl value is tabulated as “A acetyl value” in Table VI. From the data for the series B samples in Table V the influence of intrinsic viscosity variation at constant acetyl value on the saturation point for the same solvent-precipitant systems can be assessed. The change in acetone concentrations at the saturation point over the range from 0.75 to 2.86 intrinsic viscosity is tabulated as ‘‘A intrinsic viscosity” in Table VI.
70
3
%-Pentane
CONCENTRATION AT SATURATION POINT (%) TABLE V. ACETONE
2695
60
50
P
40
30
t I
\ ’ PO
30
40
Pentane,
50
60
MI.
Figure 2. Per Cent Transmittance of 100 M1. of 0.020% Solutions of Cellulose Acetate Fractions in Acetone on Addition of Pentane of constant intrinsic viscosity _ _ _ - Fractions Fractions of constant acetyl value
Curve A B C D
E F
Acetyl Value 52.9 53.6 55.7 55.1 55.1 55.0
Intrinsic Viscosity 1.66 1.67 1.63 2.86 1.77 0.75
cates a solvent-precipitant combination that is more sensitive to intrinsic viscosity distribution than to acetyl value distribution of the dissolved cellulose acetate; it, therefore, indicates a solvent-precipitant combination which may be suitable for use in an intrinsic viscosity fractionation scheme. A low ratio “A intrinsic viscosity/A acetyl value” in Table VI indicates a solvent-precipitant combination that is more sensitive to acetyl value differences than to intrinsic viscosity differences of the dissolved cellulose acetate. It, therefore, indicates a solvent-precipitant combination which may be suitable for use in an acetyl value fractionation scheme. Of the low ratio systems, acetone-ethyl alcohol-water (2 to I ) has the lowest ratio and should prove to be the most satisfactory
TABLEVI. RELATIVE SENSITIVITYOF SOLVENT-PRECIPITANT SYSTEMSTOWARD INTRINSIC VISCOSITYAND ACETYLVALUE VARIATIONS (Solvent acetone) Precipitant
A I.V.
A I.V. A A.V.
Water 7.1 2 . 9 (L) Ethyl alcohol-water (2 to 1) 4.6 1 1 . 0 (L) Propanol-water (1 to 1) 7.3 1 0 . 3 L) Ethyl alcohol 9.3 1 . 7 ((H) 1-Propanol 6.6 2 . 0 (HI Butyl acetate 12.5 2 4 . 2 (H) Pentane 6.6 7 . 6 (H) Isopropyl ether 6.0 1 1 . 7 (H) (L Signifies lower acetyl value samples are more soluble. Signifies higher acetyl value samples are more soluble.
(2)
2.5 0.41 0.71 5.5 3.3 0.52 0.87 0.51
INDUSTRIAL AND ENGINEERING CHEMISTRY
2696
Vol. 44, No. 11
56
55 54
53 52 2 0
I O
30
10
INTRINSIC VISCOSITY
A
INTRINSIC
B
Figure 3.
20 VISCOSITY
D
Saturation Point us. Acetyl Value and Intrinsic Viscosity
Solvent acetone Nonsolvent ethyl alcohol B . Nonsolvent ethyl alcohol-water (2 to 1) A.
for an acetyl value fractionation procedure. I n the “A acetyl value” column of Table VI a notation (H) or (L) has been included to indicate whether higher or lower acetyl cellulose acetate is the more soluble in the solvent-precipitant system being considered. I n the acetone-ethyl alcohol-water (2 to 1) system, the lower acetyl cellulose acetate is the more soluble. For some reason it may be desirable to choose a fractionation system in which the higher acetyl cellulose acetate is the more soluble. I n that case the acetone-butyl acetate or acetone-isopropyl ether combination should prove most satisfactory of those investigated. N o advantageous differences in ratio “A intrinsic viscosity/ A acetyl value” were obtained when solvents other than acetone were studied. I n Figure 3 the location of a circle is determined by the acetyl value and the intrinsic viscosity of the fraction it represents. The number within a circle is the per cent acetone at the saturation point, obtained from Table V. The lines, based on the points shown, are the loci of acetyl value-intrinsic viscosity combinations having constant saturation points in the particular solventprecipitant system under consideration. The resemblances between these figures and Figure l b-e are readily apparent. Thus, the vertical lines in Figure 3A show that fractionation of an acetone solution with ethyl alcohol is dependent only on intrinsic viscosity, which is in line with the findings of Sookne et al. (8). The almost horizontal lines in B indicate that the acetoneethyl alcohol-water system is dependent on acetyl value differences and almost independent of intrinsic viscosity. The oppositely slanted lines of constant saturation point for the acetonewater system (Figure 3C), and the acetone-n-pentane system (Figure 3 0 ) can readily account for the results of McGoury and White (6). SOLUBILITY PARAMETERS. The parameter, 6, has been found useful in predicting solubilities of nonelectrolytes (4). Maximum solubility of polymer in a solvent can be expected when the solubility parameter of the polymer, 6,, and the solubility parameter of the solvent, So, coincide. Values for 6,, the square root of internal pressure, 6, =
30
c
-
where AE = molal energy of vaporization for a liquid to a gas a t zero pressure; V = molal volume of liquid calculated from heat of vaporization data for the group of solvents and precipitants dealt with in these experiments, are tabulated in Table VII. On comparing these 6, values with the data of Table VI1 it is apparent that more precjpitant is required to reach the polymer saturation point when the precipitant 6, parameter is within the range 9 to 15, than is required above and below this range. Comparison of values of Table VI1 with polymer saturation point values for series A in Table V shows that for precipitants with 6o < 10, lower acetyl cellulose acetate is more readily pre-
C. D.
Nonsolvent pentane Nonsolvent water
cipitated from solution; in the range 6, = 12 to 13 there is little variation in solubility with acetyl value and for a0 > 14 the higher acetyl cellulose acetate is more readily precipitated from solution. This indicates that the solubility parameter for the polymer, 6,) lies between 10 and 14, and that 6, decreases with increasing acetyl value.
TABLE VII. SOLUBILITY PARAMETERS’ Liquid
80
’
Water 23 E t h y l alcohol-mater (2 t o 1) 15 Propanol-water (1 to 1) 14.5’ Ethanol 12.8 1-Propanol 11.9 a Solubility parameter values kindly provided these laboratories. Approximate value from proportionate parts
Liquid 60 Acetone 9.8 E t h y l acetate 9.1 Butyl acetate 8.6 Pentane 7.1 Isopropyl ether 6.9 b y Herman D . Noether of by volume.
CONCLUSIONS
The solubility of cellulose acetate in solvent-precipitant systems depends to various extents on the interaction of acetyl value and intrinsic viscosity. Additional factors such as the ratio of free primary to free secondary hydroxyl groups, and the combined sulfur content also influence cellulose acetate solubility; these could be assessed by a study similar to that described in this paper. The use of a cross-fractionation scheme to obtain narrower cellulose acetate fractions has been demonstrated. The applaiction of cross fractionation to other polymers which possess both chemical and chain length distribution-e.g., copolymers-should prove fruitful. ACKNOWLEDGMENT
The authors are indebted to colleagues and former colleagues in the Celanese Corp., specifically T. E. McGoury, H. J. Philipp and J. L. Riley, for their data and ideas which have been incorporated in the work reported here, and to June illley for technical assistance. LITERATURE CITED
(1) CiBment, L., and Rivibre, C., Bull. SOC. chim., (5) 1, 1075 (1934). (2) Cragg, L. H., and Hammerschlag, H., Chern. Revs., 39, 79
(1946).
(3) Genung, L. B., and Mallatt, R. C., IND.ENC.CHEM.,AXAL.ED., 13, 372 (1941). (4) Hildebrand, J. H., and Scott, R. L., “Solubility of Non-Electrolytes,” 3rd ed., p. 424, New York, Reinhold Publishing Corp., 1950. (5) Howlett, F., and Urquhart, A. R., J. Tertile Inst., 37, 189
(1940).
(6) hlcGoury, T. E., and White, B. B., unpublished data, Celanese Corp. of America. (7) Morey, D. R., and Tamblyn, J. W., J . Phys. Chem., 50, 12
(1946).
(8) Sookne, A. M., Rutherford, H. A., Mark, H., and Harris, M., J . Research Natl. Bur. Standards, 29, 123 (1942). RECEIVED for review September 17, 1951. ACCEPTED June 3, 1952. Presented before the Division of Cellulose Chemistry at the 11Qth Meeting of the AMERICAN CHEMICAL SOCIETY, Boston, Mass.
