Rubber-
-ELASTOMERS-Synthetic
CONCLUSION
Table 111. Rate of Emulsion Polymerization of Styrene (Two parts of soap) Initiator system Initiation temperature of latex,
c.
Propagation system Propagation temperature of latex C. convkrsion of latex. % Polymerization rate, Yoper hour Psrticle diameter, -4. (D,) Particle diameter, A . (Dn ralc.) No. of partioles/ml. €110 ( X Polymerization rate, grains/second/ml. HzO X 105 Rate/particle, grams/second X
*
CHPTETA'
KaSrOsCHPTETAb
40 40 KzSs08 CHP-TETAa
5 CHP-TETAa
Same 5 74 17.0 1000
830
5 67 9.2 1150 850
0.89
1.18
1 89
1.42
2.6
1.2
1.4
40 72 13.3 1500 1100
0 54
10-15)
1090
Potassium Persulfate
5 50
7.0
1130
3.5
1.1
6.5
1.2
5
740
Curncne hydroperoxide-triethyienetetrarnine. persulfate-cumene hydroperoxide-triethylenetetramine.
b .Potassium
doubled by the addition of a ferrous salt to such a system. It can be seen from Figure 3 that the addition of free fm-ous ion results in a definite though small increase in rate of polymerization, the rate changing from 24 to 30% per hour. Similarly, for the sulfite addition, the rate increases from 21.5 to 26.5% per hour. I t is entirely likely that the addition of an active reducing agent into a styrene system a t 40" C. may lead to the formation of some new particles from styrene in the aqueous phase or in the free monomer phase, which would account for the increased rate. Although it would be of great interest to confirm such a formation of new particles, the soap titration method used for particle size determination is not considered sensitive enough to detect such relatively small changes in number of particles. Smith (8) also found that the rate of polymerization of a seeded latex showed a small increase when excessive amounts of persulfate were added.
The emulsion polymerization of styrene, with the various initiator systems which have been tried, appears t o follow the theoretical prediction that the rate of polymerization depends only on the number of particles and on the temperature. It is therefore possible to correlate particle size and rate of polymerization of polystyrene latex. In the case of butadiene, however, the different initiator systems tried exhibit varying degrees of efficiency in the propagation process, the peroxamine system being the only one which appears to conform to theory. Hence, for butadiene, the particle size and rate of polymerization can only be correlated separately for each initiator type. It is obvious that the only reliable method of evaluating the efficiency of any initiator system is to determine the rate per particle of latex. LITERATURE CITED
(1) H a r k i n s , R. D., J . Am. Chem. Soc., 69, 1428 (1947). (2) H a r k i n s , W.D., J . Polvmer Sci., 5 , 217 (1950). (3) Kolthoff, I. &I., a n d Meehan, E. J., private communication to Office of R u b b e r Reserve. (4) h f o r t o n , A I . , Salatiello, P. P., a n d Landfield, H., J . Polymer Sci., 7, 111 (1952). (5) Ibid., p. 215. (6) I b i d . , in press. (7) S m i t h , W. V., J . Am. Chem. Soc., 70,3695 (1948). (8) Smith, W. V., a n d Ewart, R. H.. J . Chem. Phus., 16, 592 (1948). (9) Whitby, G. S., Wellman, N., Flouta, V. TT., a n d Stephens, H. L., I X D . ENG. CHEM., 42,445 (1950).
RECEIVED for review November 5 , 1951.
ACCEPTED December 11, 1951. This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corp., in connection with the government synthetic rubber program.
Sonie Effects of Polymerization Temperature andMolecular Weight J. R. BEATTY, Research Center, B. F. Goodrich Co., Brecksville, Ohio B. M. G.ZWICKER, B. F. Goodrich Chemical Co., Cleveland, Ohio The advantages of 5" C. cold rubber over hot GR-S in high-black vulcanizates are well established. This paper describes some changes in micro and macro structure observed in butadiene-styrene copolymers made at five polymerization temperatures (--18', -IO", 5", 20°, and 50" C.) and their relation to changes in vulcanizate properties as a function of polymerization temperature and molecular weight. The major differences in polymer structure of cold rubber as compared with GR-S appear to be: a narrower molecular weight distribution with less soupy polymer, a slight increase in crystallization tendency, and an increase in homogeneity of the polymer composition. Improvement in stress-strain and resilience with reduction of polymerization temperature from 50,. t o 5" C. was proportionately much greater than that resulting from further reduction to -18" C. Reduction in the amount of low molecular weight polymer softener in cold rubber as compared with GR-S increased its sensitivity to hot aging and changed the cure rate. Vulcanizates of fractionated polymers showed a marked decrease in hysteresis as molecular weight increased. There was a simultaneous increase in dynamic rate or modulus, but 742
the product, as measured by temperature rise, decreased with increasing molecular weight in nearly all cases. Dynamic flexibility and crystallization tendency at -25" C . of OR-S vulcanizates are advemely influenced by the presence of low molecular weight material.
S
EVERAL authors (13-16) have reported that as poly-
merization temperature is reduced, butadiene-styrene copolymers display improved vulcanizate properties in treadtype recipes. The quality advantages of the cold rubber vulcaniaates are offset somewhat by inferior factory processability as compared to hot GR-S polymers. A fundamental study was undertaken to characterize the changes in polymer structure which might explain the observed differences. POLYMER PREPARATIONS
Polymerization. The polymers evaluated in this program were prepared in pilot plant equipment with recipes designed to make polymerization temperature the primary variable over the range of -18' to +50" C. Several charges mere prepared in a 175-gallon reactor a t each temperature and the latices were
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
R L A S T O M E R S - S y n t h e t i c RubberTable I. Experimental Polymers Prepared in 175-Gallon Reactor Reference No., 2005-X144 142 143 Polymerization Temp., e C. - 10 5 20
146 Polymerization recipea Waterb Methanol (synt.hetic) Butadiene (98.5%. alkali washed) Styrene (99+ %. as received) Tertiary-Cln mercautan Cumene hydroperoxide (78%) Dresinate 731 C Dreninate 214C Dresinate 9-134 (hydrogenated resin soap) ORR soap, sodium salt O R R soap, potassium salt K laurate
- 18 150 *50 72 28 0.21 0.20
.. ..
167 33 72 28 0.20 0.164 3176
180 72 ' 28 0.21 0.10 3.76
..
,
1so. . 72 28 0.21 0.12 3 .76
..
145 50-
180 72. 28 0.20 0.10 3.76
..
T o separate this variable, a large scale fractional precipitation was performed on five of the polymers to provide a quantity of each with approximately similar molecular weight ranges. The high and low solubility fractions were separated, leaving a middle cut of approximately 2.0 intrinsic viscosity.
The large scale fractionation technique employed for the study 0: 94 .. .. .. was conducted in 210-gallon 0:94 aluminum-coated jacketed FeSOa.7H20 0.36 0:31 0: 10 0: 10 0:lOd vessels. A p p r o x i m a t e l y 1 5 Na4P901 0.24 0.42 0.35 .. K4PzO7 .. .. .. 0:14 .. pounds of polymer were dissolved 0.85 Dextrose 0..: s o .. .. in nitration grade toluene to give KCI 0:soe 0.40 KzSOa 0:20 .. .. .. .. a 1% solution. Approximately KzSO4 0.283 .. .. 24 hours were required t o comKOH 0.287 0 :126 0: 10 plete solution a t 25' C. Two NaOH .. .. 0:06 0:06 per cent of diphenyl paraphenylNaaPOi .. .. 0: 10 .. .. Daxad 11 (polyalkylaryl sulfonate) 0.10 enediamine was added on the Santovar A shortstopf 0:40 61 :. 45 00 0.30 0:30 0:SO weight polymer. The addition Stalite antioxidant (e)# 1.50 1.50 1.50 1.50 Av. reaction time (4-5 chgs.), hours , 3 2 . 9 =t 2 . 1 24.2 f 7 . 8 14.9 -I: 2 . 1 1 4 . 7 =t4 . 3 1 5 . 6 =t 2.0 of 0.5% di-tert-butyl hydroAv. conversion, % 60 f 2 60 2z 2 60 2z 2 60 f 2 60 st 2 quinone on the polymer weight Av.micro.), latex A particle diam. (electron 400 provides additional protection 345 420 410 635 Av. Mooney viscosity (blend), MLfrom air oxidation during the 2120 4' 44 45 42 48 54 f r a c t i o n a t i o n . A laboratory a All figures i n parts of pure ingredients. fractionation test was conducted b Zeolite softened except for polymers prepared below f 5 ' C.; steam condensate used for latter to control on a ~ O O - ~ I portion . of the electrolyte more closely. cement to determine the amount 0 Sodium and potassium soap, respectively, of disproportionated rosin. d Added t o latex prior t o coagulation. of synthetic methanol precipitant * Added stepmise during reaction. required to separat,e the high I Di-tert-amyl hydroquinone. 0 Alkarylated diphenylamine. m o 1e cu 1a r w e i g h t material. Graduated tubes were used t o assist in determining volume and viscosity of the precipitat,e to be blended after short,-stopping and stripping. Polymerization removed from the large tank. Measurements of total solids recipes and average reaction data are summarized in Table 1, and intrinsic viscosity on the supernatant cement as compared withthe original V?hole polymer alloWed estimation Of the exThe di-t&-amy] hydroquinone shortstop was added immediately tent of removal of high molecular weight cemponent. after diluting a 2% solution in methanol with half its volume of ,accomplished by sioMr addiThe large scale precipitation water; stripping Of the latex was accomplished a t 26-inch vacuum. tion of methano] a t the vortex of the stirring cement. The preThe Stalite antioxidant was added as a n emulsion t o the cipitate was dissolved b y warming the mixture t o 35" C. The fraction was reprecipitated by decreasing the temperature to 25' blended polymer latices, which were then coagulated with salt f 1' c. with stirring. The mass was then allowed t o settle for and sulfuric acid, and the decanted crumbs weye ,wash-milled 48 to 72 hours. The separated gel was slowly drawn off the and dried a t 26-inch vacuum* Approximately 900 pounds of bottom as a thick cement, The was removed in a each polymer were prepared. similar manner. Recovery of the lowest molecular weight fracStandard Rubber Reserve chemical tests -;ere made on the tions required concentration of the cement. The cements five polymer blends with the results shown in Table I1 as compared with commercial X-478 cold rubber and GR-S 10. Table 11. Chemical Tests on Experimental Polymers I n the course of the molecular weight evaluaQR-S GR-S10 143 145 Reference No.. 2005-X- 146 144 142 tion it developed that the 50' C. experimental X-478 Polymerization polymer 2005-X-145 was abnormal in its tendency temp., C. 5 48 - 18 - 10 5 20 50 t o form gel rapidly. This was attributed to the Heat loss, % 0.21 0.08 0.16 0.18 0.35 0.44 0 51 0.56 1.47 l.2i 1.04 1.02 0.50 Ash, % addition of uncomplexed ferrous sulfate to the SoaD (as rosin) Yo 0.14 0.10 0.16 0.06 0.04 0.12 0.11 Acid (as rosin),' % ' 5.96 5.51 6 . 4 5 6 . 4 5 6 . 3 0 6 . 1 6 6.30 latex prior t o coagulation. Consequently another E T A extracta 6.28 6.36 6.62 5 26 7.17 6.63 6.27 control polymer for laboratory evaluation was 0.045 0.03 0.04 0.025 0.05 0.11 Iron, % Nil iii Nil Nil Gel, % Nil Nil 5 prepared a t 50' C. in a 15-gallon reactor. This Intrinsic viscosity in benzene (uncorpolymer, 2005-X-155, differed from 2005-X-145 reoted) 1.64 1.54 1.47 1.57 1.66 1.76 1.90 in the following respects: a Ethanol-toluene azeotrope. 3.76
..
..
0:94
0:94
0:94
'
O
Polymerization rerine: 0.19 tertiary Clz mercaptan, no ferrous sulfate (no iron added prior t o coagulation). 57y0 conversion in 15.75 hours. Fifty-four Moonev (ML-212-4): 1.57 intrinsic viscosity. This polvmer was used for studies of molecular -~ weight distribution and polymer structure. Large Scale Fractionation. Substantial differences in molecular weight distribution would probably affect physical properties of the polymers made a t the various polymerization temperatures. April 1952
Table 111. Separation of Butadiene-Styrene Copolymers into Middle Cuts of Similar Molecular Weight Original Properties Mooney Original Polvmer No.
v1sc..
Polym. ML Temp., 4 min., 212O F. C. 50 70 2005-X-145 2005-X-155 50 52 2005-X-163" 50 57 2005-X-142 5 46 2005-X-146 -18 55 a X-412 GR-8 10 control.
%
gel 70 5 7 5 5
I.V. (uncorrected) 0 80 1.57 1.86 1.37 1.37
Properties of Fraction Wt.-Sfqoney Polymer % visc., I.V. Fraction of 4 min. % (uncorNo. Orig. 212' F. gel rected) 2005-X-156 54 105 5 1 60 35 150 5 2 32 2005-X-165 2005-X-164 30 97 5 1 82 2005-X-157 50 127 5 2 23 2005-X-158 45 97 9 2.02
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
743
E L A S T O M E R S 4 y n t h e t i c RubberTable IV.
2005 X 185 2005 X 186 2005 X 187
2005 X 188 2005 X 189 2005 X 190
zation temperature. Styrenc content was divergent only in tlic case of the 50' C. polymer. The data on cis-trans-1,4- and 1,2-butadiene polymerization are in qualit'ative agreement with those published by other laboratories (6). The specific technique used for the infrared evaluation of polymerized butadiene unit configurations was as follows : The nieasurements on the polymer were obtained with solid samples. A large number of thicknesses were used and the region was selected which gave best conformance to Beer's law. Model compounds used for calibration of the Perkin-Elmer Model 12-8 spectrophotonieter byere:
Fractionation
Slooney I.V. Visc. (Uncorrected) Gel ML 4 ' , 2 i 2 0 F. Fractions of Hot GR-S 10 X-412 (1.94 I.V., 55 M L 4 Min., bl2' F.) 18 1.19 5 2.64 5 109 Too tough 3.97 90% Fractions of Cold GR-S X-558 (1.61 I.V., 55 ML 4 Min.,' 212' F.) 12 1.os 5 79 5 1.73 96 MS 5 2.51
Configuration trcns-1,4 trcns-1,4 cis-1,4
were dried in large stainless steel trays at room temperature. Last traces of solvent were removed in a vacuum oven. Results of large scale fractionation of the five polymers are summarized in Table 111. Four additional large scale fractionations were made to provide more material for evaluation of commercial cold rubber and GR-S 10 molecular weight fractions, and these are summarized in Table IT'. In the case of the latter materials, the low, middle, and high molecular weight fractions were evaluated. POLYMER EVALUATIONS
Moiecule trans-boctene trans-3-hexene cis-2-pentene 3.3-Dimethyi-1-butexie
1,2
Wave Length, Microns 10.30 10.30 14.40 10.95
Lleasurements were conducted in the same sample cell arid n-ith identical instrument adjustments. Corrections were applied to reduce the errors due t o reflection and general absorption. Because of its large concentration in butadiene rubbers, the trans-1,4 structure is the most critical measurement. Therefore the K value selected u-as the result of averaging ratios obtainctl for both of the trans-1,4 model compounds.
Raw Polymer Composition.
The changes in chemical composition of these butadiene-styrene copolymers with polymeriza; tion temperature were evaluated by molecular weight fractionation and optical butadiene-styrene analyses. Llolecular x-eight
em5 X I S S 8O'C. 2005 X 142 ...', , ' ' 5OC. eoos x 14s IE"C.
-- - - -
60
0
B
0-
\
5
30
0 0
10
- 20
0
20
40
2.0
1.0
60
F i g u r e 1. Configuration of B u t a d i e n e U n i t s
5.0
In GR-S polymerized at various t e m p e r a t u r e s
F i g u r e 2. Effect of Polymerization T e m p e r a t u r e o n Differential Molecular Weight D i s t r i b u t i o n
(infrared)
Tertiary m e r c a p t a n modified
was estimated by intrinsic viscosity. Bound styrene mas determined on solid samples by the National B~~~~~of Standards refractive index procedure for GR-9 rubbers. Orientation of the polymerized butadiene unit was estimated from infrared absorpxion m e a s u r e ments as coinpared with model molecules. POLYMER ComosiAVERAGE TIOK. Average polymer compositions, summarized in Table V, indicate a regular but relatively small change in structure of combined butadiene units with reaction temperature. The data are plotted in Figure 1 t o show the linearity of the changes with polymeri144
4.0
tr\I 0
POLYM. TEMP "C.
Table V.
Average Polymer Compositiona
R+ference S o . , 2005-Xi Polmeriaation temp., C. -CH%-CH-
146 -18
*c
(styrene) -CHz-CH(1>2) -CHz
Wt.
'CH=CH 'CHz(tians-1,4) CHz'CH=CH /
142 5
143
14s
13.3
23.2 13.5
23.1 13.5
23.1 13.5
21.0
17.0 19.1 22.0
16.7 18.8 21.7
18.0 20.2 23.4
18.4 20.7 23.9
21.0 23.6 26.9
56.6 63.6 73.4
71.8
55.2 62.0
52.8 69.5 68.6
50 6 36.9 65.8
45.3 $50.8 58.2
7.9 8.9 10.3
11.6 13.0
22.8
W t . yc Mole % CaHs polymerized, %
kHz
144 -10
yc
Mole % C4HBpolymeI'ized, Wt. yo Mole 70 CcH6 polymerized, yo
3 5
4.0 4.6
5.0
5.65 6.5
6.15 6.8 8.0
20
50
12.7
14.9
(cis-1.4) These data neglect the relatively small percentawe of branched units in the polymer chain, for which there is still no direct method of measurement. Data b y JEhnson and M'olfanqel (8) indicate parameter c in equation [nl = K M a to be increased by about 40% as polymerization temperature is reduced from 50' to -20' C. a
1;NfD U S T R I A
L
A ND E N G I NE E R ING C HE M I STR Y
Vol. 44, No. 4
-ELASTOMERS-Synthetic I M WEIGHT ~ DISTR~BUTION ~ BY INTRINSIC ~ VISCOSITY ~ [710. Three of the experimental polymers (-Bo, 5', and 50" C. polymerization temperature) were subjected to a nine-step fractional precipitation by the technique described by Yanko (18) for GR-S. k 4 PO05 X 166-
TERT. 0-IP R S H
x . @ l BR.S""""'PRIM.
"
Rubber-
~ ~ ~ ~ T a b l e VI. Precipitation F r a c t i o n a t i o n of C r u d e Polymers Wt. % x [TI 100 rn 1 55 ML X-55 GR-S. 74 % CONVERSION, 50' C., 0.6 PART DDM 83 261 6.57 0.93 2.2 69 137 3.85 0.55 3.7 0 .. 2.86 0.34 4.2 3 .. 1.86 0.17 5.4 0 .. 1.07 0.12 10.5 0 .. 0.90 0.09 11.7 0 .. 0.54 0.04 14.6 0 .. 0.30 0.03 38.: 0 0.08 0.01 (44) 0 .. 2.36 2.28 65 ML 2005-X-155. 60% CONVERSION, 50° C., 0.19 PART~ - C H MERCAPTAN 1 8.1 5 4.67 0.38 1.7 2 9.8 0 3.77 0.37 2.6 3 10.6 0 3.12 0.33 3.4 4 11.2 0 2.43 0.27 4.6 5 11.7 0 1.84 0.22 6.4 6 10.6 0 1.43 0.15 7.4 7 8.9 0 1.04 0.09 8.6 8 10.6 0 0.73 0.08 14.5 9 9.6 0 0.04 20.: 0.46 10 8.9 0 0.03 (53) Whole 100.0 2 1 :93 1.95 43 M L 2005-X-142. 60% CONVERSION, 5' C., 0.18 PART t-C~zMERCAPTAN 1 9.2 5 85 4.07 0.38 2.3 2 12.5 0.5 .. 0.38 3.07 4.1 3 12.0 0 0.31 2.58 4.7 4 10.4 0 0.22 2.10 5.0 5 0 8.4 0.14 1.65 5.1 6 10.6 0 .. 7.9 0.14 1.34 7 8.2 0 .. 0.08 8.0 1.02 8 8.1 0 .. 0.06 10.7 0.76 9 12.2 0 .. 0.05 29.2 0.42 10 8.4 0 .. 0.03 (40) 100.0 Whole 0 1:76 1.81 42 ML 2005-X-146. 60% CONVERSION,1 8 O C., 0.21 PART t-Ciz MERCAPTAN ~. 1 12.1 45 300 0.43 3.4 3.55 2 10.3 8 1000 3.41 0.35 3.0 3 10.0 0.5 .. 2.78 0.28 3.6 4 1 10.1 ., 2.15 0.22 4.7 10.8 5 0 .. 0.18 1.66 6.5 6 10.4 0 .. 0.14 1.32 7.9 7 9.1 0 .. 0.08 10.6 0.86 8 8.3 0 .. 0.05 14.1 0.59 9 10.0 0 , . 0.04 27.: 0.36 8 . 9 1 10 . . 0.03 (45) Whole 100.0 1.5 1 :77 1.99 Corrected for soap, fatty acid, and antioxidant content. Inaccuracy of 1710 determination makes analytical use of these figures undesirable. Wt.. %
Fraction
% Gel
Swelling Index
[si0
..'
..
I
I
3.0
2.0
4.0
6
.o
[r\ 10
F i g u r e 3. Effect of Modifier o n Differential Molecular Weight D i s t r i b u t i o n 80' c. POlyMerS
.. .. I
The results of the fractionation are summarized in Table VI together with the values found by Yanko for 50' C. GR-S X-55. Figure 2 graphically illustrates the differential molecular weight distribution curves obtained for the three experimental polymers between [7100.03 and the uncertain microgelled portion. Figure 3 compares the distribution curve for X-55 (primary dodecyl mercaptan modified) with t h a t for the tertiary Cl9 mercaptan modified experimental 50' C. polymer. These curves were obtained by the graphical technique and corrected by planimeter measurements, each of the points for the nine fractions being given full weight. As fractions were not redissolved and reprecipitated, detailed curves may not have real meaning. Table VI1 summarizes the distributian curve data for the four polymers between selected [q lo limits. The larger percentage of very low molecular weight material obtained with the primary mercaptan modifier is the obvious difference in molecular weight distribution between GR-S and cold rubber. Two facts should be stressed in comparing these data: The X-55 polymer was carried to a substantially higher conversion (74%) as compared with the pilot plant rubbers (60%), and the Mooney viscosities of X-55 and 2005-X-155 were more than 10 points higher than t h a t of the low temperature rubbers, although the average intrinsic viscosities of the three experimental rubbers were comparable. These facts could account for the increase in both sol and gel high molecular weight fractions in the hot rubbers. However, it is evident t h a t the pattern of at least three [710 peaks is sufficiently consistent t o form a basis for further investigation. The low [rlo peak appears t o be associated directly with type and amount of modifier, while the high [710 peak may be a t least partially a function of polymerization temperature. The possibility of modified precipitation characteristics of t h e low temperature polymers cannot be neglected and further work with more precise fractionation techniques is indicated. The distribution curve for the 5' C. rubber appears t o be less complicated than for either the 50' or -18' C. rubbers. It would be interesting to have fractionation data on a subzero rubber made with a n antifreeze other than methanol as well as a 5' C. rubber polymerized in the presence of methanol, t o see if the increased complexity is related to a modifier activity by the methanol. MOLECULAR COMPOSITION.Several of the precipitated fractions from the -Bo, 5", and 50' C. polymers were analyzed optically for bound styrene and polyfierised butadiene units in April 1952
.
..
-
~
I
.
the manner described for the whole polymers above. The results summarized in Table VI11 show no significant differences in pplymer unit composition as a function of molecular weight. Values for bound styrene content were practically within experimental error for all fractions of each rubber (both low and high molecular weight polymers are difficult to purify and measure on the refractometer). Yanko's data ( 2 8 ) on X-55 with its larger percentage of "soup" showed a mild trend toward higher styrene content with increasing molecular weight, which agreed qualitatively with polymerization data. Evidently the tertiary mercaptan modified experimental rubbers, which have less soup, do not have a molecular weight-conversion relation that is sufficiently distinct to be resolved by the single-stage precipitation fractionation technique. Table VII. Molecular Weight D i s t r i b u t i o n
. (From d W / d 15710 us. [slo curves, Figures 2 and 3) Polymer X-55a 2005-X-155 2005-X-142 2005-X-146 - 18 Polym. temp., O C . 50 50 5 t-c12 Modifier p-Cia t-Ciz t-C12 [?lo range Weight Yo between Indicated [ V ] O Limitsb 0.03-0.50 20.2 13.3 15.0 16.5 25.2 0.50-1.25 28.8 25.2 26.5 1.25-2.0 9.9 21.2 21.1 19.8 2.0-3.0 12.8 18.2 24.5 15.7 8.2 3 .O-4 .O 14.1 14.4 19.90 4 . 0 (sol) 4.2 6.1 0.7 4 . 0 (gel) 10.0 1.6 4.0 2:5 a Yrtnko (18). b Excluding fraction with
