L L A S T O M E R S - S y nt hetic Rubber Crystallization tendency as measured by the increase in modulus with time a t low temperature under stress is shown in Figure 17 for the fractions of X-412 and X-558 GR-S. This tendency is more pronounced for t h e low molecular weight materials. This may be due to a greater number of unattached chain ends in the vulcanizate, which can be more easily forced into some degree of orientation under the imposed stress. The higher molecular weight fractions of X-412, GR-S, and X-558 cold rubber were essentially equivalent in spite of the difference in the whole polymers. Thus it can be concluded t h a t decreasing the temperature of polymerization increases the tendency t o formation of crystals for GR-S. This increase in crystallization tendency may be due to increased linearity of the chains However, in view of the data shown in Figure 17, i t appears that molecular weight distribution has an even greater influencr on this property. ACKNOWLEDGMENT
The assistance and counsel of G. R. Mitchell, J. J. Shipman, D. M. Kurtz, W. L. Wittig, J. hi. Kime, C. B. Howard, A. E. Juve, L. 0. Schroyer, and D. L. Loughborough in the euperimental work and preparation of this manuscript are appreciated. LITERATURE CITED
(1) Beatty, J. R., I n d i a Rubber World, 125, KO 4, 438 (1952). (2) Beatty, J. R., and Davies, J. RI., J . A?jpZzed Phys., 20, 533 (1949).
BekkedahI, N-, and Wood, L. A,, IND.ENG.CREM.,33, 382 (1941). Beu, K. E., Reynolds, W. B., Fryling, C. F., snd McRIurry, E. L., J. Polumer Sci., 3, 465 (1948). Cornell, D. H., and Beatty, J. R., Trans. Am. Soc. N e e h . Engrs., 69,799 (1947); Rubber Age, 60,679 (1947). Hart, E. J., and Meyer, A . W.,J . Am. Chem. Sot., 71, 1980 (1949). Johnson, E. L.. ID. ENG.CHEX.,40,351 (1948). Johnson, E. L., and Wolfangel, R. D., Ibid., 41,1580 (1949). Juve, A. E., I b i d . , 39, I494 (1947). Juve, A. E., and Hay, D. C., Indaa Rubber World, 117, 62 (194'7) Lessig, E. T., IND.EXG.CHEM.,ANAL. ED., 9, 582 (1937); A.S.T.M. Standards for Rubber, 1950. ENC.C R E v . , Parks, C. R., Cole, 0. D., and D'Ianni, J. D., IND. 42, 2553 (1950). Schulae, W. A , , Reynolds, W.B., Fryling, C. F., Sperberg, L. R., and Troyan, J. E., I n d i a Rubber W o r l d , 117,739 (1948). Sjothun, I. J., and Cole, 0. D., IND. ENG.CHEW., 41, 1564 (1949). Tuley, W. F., Rubber A g e , 64, I93 (1948). White, L. M., IND.ENG.CHEM.,41, 1554 (1949). Wood, L. A., '"Advances in Colloid Science," 1'01. 11, pp. 57-93, New York, Inteiscience Publishers, 1946. Yanko, J. A., J. Polgnzer Sci., 3, 576 (1948). Yerzley. F. L., Proc. Am. Soc. Testing Materials, 39, 1180' (1939); Rubbev Chem. and Teehnol., 13, 149 (1940); A.S.T.M. Standards, 1950. RECEIVED fur review September 17. 1951. ACCEPTEDFebluary 8. 1962. Investigation carried out under sponsorship oP the ORce OF Rubher Reserve. Reconstruction Finance Carp.
lecular ight Polybutadiene EFFECT OF POLYMERIZATION CONDITIONS, O N BRANCHING AND C R O S S LINKING €3.
L. JOHNSON AND R. D. WOLFANGEL The Pirestone Tire and Rubber Co., Akron, Ohio
Variation in polymerization temperature has been shown previously to influence the relationship between intrinsic viscosity and molecular weight. Research on this relationship has been continued in order to determine i f the change'in molecular weight exponent can be correlated with a particular change in polymer structure. The exponent a in the relationship [ T ] = KIM" was found to decrease as the hydrocarbon conversion of emulsion polybutadiene was increased from 20.5 t o 81q' at a constant polymerization temperature of 50" C. Values of 0.45, somewhat below the theoretical lower limit of 0.50, were obtained for the molecular weight exponent a t conversions which would normally be considered to be at or beyond the gel point as judged by intrinsic viscosity US. conversion curves. The molecular weight exponent of a series of polybutadienes made with progressively less modifier was also found to decrease as the modifier concentration was reduced. A more direct measure of the effect of cross linking upon the molecular weight exponent was obtained by copolymerization of increasing amounts of divinylbenzene with styrene in the GR-S system a t 50" C. As the number of divinylbenzene units or potential crosslinking sites per molecule was increased there was a rapid decrease in the molecular weight exponent from the value of 0.92 for polystyrene t o a value of 0.5 as the divinylbenzene concentration approached one cross-linking unit per
752
molecuIe. On the basis of these results and t h e fact that microstructural changes in poIybutadiene were not significant, it is beIieved that cross linking is the predominant change in structure responsible for the drop in moIecular weight exponent as conversion progresses 0.p modifier is reduced. While this method does not provide an absolute means for measurement of cross linking, it is of significance as a measure of the degree of cross linking in soluble polymers and is helpful in the proctuction of polymers of maximum chain length without gelation.
H E exponent a in t h e widely used relationship, [ T ] = KMa, between intrinsic viscosity and molecular weight (5, 9, 11), has been shown previously t o be dependent on the temperature a t which polymers were polymerized ( I , I O , I S ) . I n the case of emulsion polybutadiene the exponent was found t o increase as t h e temperature of polymerization was lowered ( I O ) . This increase in exponent was considered t o be in agreement with the structural changes known t o accompany t h e reduction in polymerization temperature. However, lowered polymerization temperature results in several simultaneous structural changes which have been listed by Meyer ( I d ) . These changes consist of a shift in the cis-trans ratio in t h e direction of more trans 1,4-polymer and a smaller decrease in t h e number of side vinyl groups as polymerization temperature is lowered. I n addition,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic Table I.
Fractions of Polybutadiene Stopped at Various Conversions
Polymer Fraction
Intrinsic viscosities a n d molecular weights F action Purified &eight, Polymer, Viscosity Intrinsic G. % A.
Whole 1 2 3 4 5 6 7 8 9 Unprecipitated
1
9 Unprecipitated
1
2 3 4 5 6 7 8 9
10
Unprecipitated
40%
1.12
63.5
2.65 2.10 1.42 0.93 0.65
169 138 85.2
60.5y0
81.0%
86 8
1.6 9.9
3.45
361
3.8 8.6 10.0 14.4 14.7 10.1 10.5 16.4
2.39 2.03 1.67 1.37 0.89 0.68
221 130
56
CONVERSION 5.8 10.8 1.7 14.9 11.2 12.5 11.4 9.7 8.5 13.5
1.35 2.53 0.39 3.47 2.61 2.91 2.66 2.27 1.98 3.17
2.05
94
4.67
1110
3.47 2.46 1.67 1.47 1.12 0.86
315 130 50
CONVERSION 4.4 5.8 21.1 8.5 6.4 10.5 9.1 10.3 6.1 6.7 11.1
2.40
121
4.05 3.37 2.92 2.05 1.21 1.00
670 298 153 93.7 66
the degree of branching and cross linking is thought t o decrease b y an undetermined amount as polymerization temperature is reduced. T h e study of t h e relationship between intrinsic viscosity and molecular weight was continued in order t o determine if the change in molecular weight exponent could be correlated with a n y one of these structural changes whicb occur upon lowering the polymerization temperature. It would be expected that increased hydrocarbon conversion would involve branching and cross linking (4) as the chief variable, t o t h e exclusion of other changes such as were encountered in t h e study of the effect of polymerization temperature. Therefore, t h e first step in continuation of this study was t o evaluate a series of polybutadienes made at a constant polymerization temperature but stopped a t various points in conversion. EXPERIMENTAL
Polymerization Details. Several polybutadienes modified with 0.5 part of dodecyl mercaptan (DDM) were prepared in the GR-S system a t 50" C. t o hydrocarbon conversions of 20.5, 40.0, 60.5, 81, and 97%. The stabilized latex was coagulated with salt-acid and the polymer was dried overnight at 70' C. All except the 97% conversion polymer were gel free by the static solubility test, using toluene as the solvent, The polybutadienes in which modification was varied were also prepared in the GR-S system at 50' C., but with 0.3 and with 0.7 part of dodecyl mercaptan. These polymers were stopped at 70 and 72% conversion, respectively.
April 1952
Polystyrene and the co olymers of polystyrene and divinylbenzene used for study of &e effect of cross linking were also produced to about 70% conversion i n the GR-S system a t 50" C. and modified with 0.5 p a r t dodecyl mercaptan. Measurement of Solution Properties. About 30 grams of the whole polymer were first purified by precipitation from 6 liters of C.P. toluene by the dropwise addition of 3 liters of C.P. methanol. The recovered polymer was redissolved in 5 liters of C.P. toluene which contained 0.0075 gram per liter of phenyl-2naphthylamine. The intrinsic viscosity and osmotic molecular weight was determined on a portion of this solution. Ten t o eleven fractions were also separated from this solution b y the fractionation technique described in a n earlier paper (IO). The same reference dpcribes the method for determination of intrinsic viscosity of the fractions by use of Ostwald viscometers. The method for determination of the number-average molecular weight using osmometers of t h e static type has also been described ( 2 4 ) . RESULTS AND DISCUSSION
1.45
1.07 1.46 5.57 2.15 1.62 2.55 2.29 2.61 1.55 1.67 2.80
37 3
CONTERSION
0.36 ; ::}2.27 0.87 1.97 2.28 3.29 3.35 2.30 2.42 3.74
D. W!ole
3.3 2.5 10.2 14.4 16.9 18.7 14.7 9.3 10.0
0.55 0.43 0.64 1 . 1 4 j1, 7 8 2.50 2.94 3.26 2.56 1.61 , 1.74
c. Whole
CONVERSION
26.5%
B. Whole 1 2 3 4 5 6 7 8 9 10 Unprecipitated
Molecular
wt. x.10-3
Rubber
Influence of Degree of Conversion. The fractionation data, intrinsic viscosities, and number-average molecular weights of polybutadiene made a t 50" C . and stopped over a conversion range of from 20.5 t o 81% are recorded in POLYBUTAOIENE AT I O ' C Table I. T h e rela1 I1/1//1/ I I lll1Ili I 1 : I l l tionshin between the intrinsic viscosities and molecular weights of the fractions obtained from polymer a t each eoa CONVERSION h l . , 5 3 . , 0 4 Melo c o n v e r s i o n was ob0 4 0 % CONVERSION e 8 1 . 4 5 5 . IO-^ MO'O tained from t h e log-log m 6 o a CONVERSION MoS4 plots of Figures 1and 2 . hl.252 Pol ybutadiene stopped MOLECULAR WEIGHT RnxIO+ a t 20.5% . - conversion Figure I. Viscosity-Molecular has a relationship of [?I Weight Relation as Function of = 1.53 10-4x L u ~ . ~ . Conversion of 50" C. PolybutaAs t h e c onversion diene is increased the intercept increases and t h e exponent of molecular weight decreases. T h e plot of Figure 1 for polymer made t o 40.0% conversion indicates a relationship of [ v ] = 4.55 X X M0.70for this polymer. At 60.5% conversion, t h e relationship is [?] = 25.2 X lov4 X M o 54, as is also indicated in Figure 1. The final soluble polymer, obtained a t 81.0% ; . ~ ; U , ~ ~ N I A : conversion, has a relationship which is prob:,IO0 E
~
~
~~,1
ably best represented by [ q ] = 95.3 X ['il.SS 3 X !O-' "'M X &P45. However, 5 IO the two lower fractions MOLECULAR WEIGHT 8. X 10" of this polymer, plotted in Figure 2, fall on a Figure 2. Viscosity-MoleculAr line which is essentially Weight Relation for 81Yo Conparallel but has a lower version, 50" C. Polybutadiene intercept. Their estimated relationship, [?I = 53.8 X X is consistent with t h e thought t h a t t h e lower molecular weight fractions of polymer would be less * highly branched or cross linked and, consequently, would have a highgr molecular weight exponent. I n t h e consideration of molecular weight exponents being made i n this paper, a n average value of 0.46 is satisfactory for t h e 81% conversion polybutadiene and well within t h e limits of accuracy of t h e method. T h e individual values for t h e molecular weight exponent of polymer made a t increasing conversion levels become of more interest as they are plotted versus conversion, as is shown in Figure 3, and a s they are considered in relation t o t h e structural changes which take place a s polymerization progresses. T h e data show t h a t a linear relation exists between conversion of
INDUSTRIAL AND ENGINEERING CHEMISTRY
153
ELASTOMERS-Synthetic
io,?: \ , 0.n-
-
r
+
polybutadiene, The gel point, indicated b y the maximum in the plot of i n t r i n s i c viscosity v e r s u s (series
GEL POINT
0.6
i1
A
o.b--
0.4
-
I
I
I
I
1::
A
I
I
I
l
l
SERIES I
0
- _
SERIES 2
IC
z l-
!
l
I
I
I
I
Rubber-
I
l
!
I
,
at 2, Figure 75% conversion 4), falls for t h e present series. This is a t
v i s e o s i t jr-conversion plot (series 1, Figure 4) of the 50" C. p o l y ' b u t a diene reported earlier in connection with the investigation of poly-
1
2
3 4 5
3.12
3.37 1.27
13.7 14.8 5.4 6.0 8.1
;i
9 10 Unprecipitated
1.34 1.83 1.24 1.89 1.59 2.23 2.49 2.40
Whole 2 1
01.03 .72
3.1 4.8
3 4
0.92 1.60
4.3
$
B.
5.4 8.4
6.9
9.8 11.0 10.3 0.7 PARTDDRI
7.4
4.70
4.12 3.50 2.60
1.87 1.40
1150 480
260 157 87
'
3.20
drops off as the polymer approaches t h e gel point. Microstructure. A determination of the microstructure of the polymer a t each conversion was made by infrared adsorption t o confirm t h e opinion that increased branching and cross p,,e linking was t h e major struct,ural change t o occur as t,he := degree of p 01 y m e r i z a t i o n o,ss
-ELASTOMERS-Synthetic Microstructure of Polybutadiene 50" c.
Table I J I . Conversion,
1,2Addition,
%
%
20.5 40.0 60.5 81.0
Cis, %
1,4Addition Trans, yo
As Function of Conversion (0.5Part DDM) 15.8 19.2 15.1 19.2 15.9 19.2 18.2 18.8
65 65.7 64.9 63 .O
Rubber-
of fractions of t h e copolymer which contained 0.05 part divinylbenzene was [ q ] = 52.0 x 10-6 x M0.68. This copolymer had a molecular weight of 207,000 when stopped a t 76.0% conversion. Complete data on fractions of these polymers are included in Table V and Figure 7.
. '--I
STIRLNEIDVB COPOLVMLR8
I
As Function of Modifier (60% Conversion)
Part DDM 0.3 0.7
16.6 18.4
26.1 21.4
57.3 60.2
obtained were redissolved in methyl ethyl ketone, reprecipitated with excess methanol, and recovered by filtration through a Buchner funnel. The fractions were vacuum dried a t room temperature and dissolved in toluene for the intrinsic viscosity and molecular weight determinations. The fractionation, viscosity, and molecular weight data for this polystyrene are reI , I corded in Table I V and the K)O intrinsic viscosity-molecular weight data are plotted in Figure 6. A relationship bePOLYSTYRLNL tween intrinsic viscosity and ~].ea?.,o-'u*" f molecular weight of [7] = 6 8.57 X lo-* X was MOLECULAR WEION? m l o ' found for the polystyrene in Figure 6. Viscositytoluene solution. This high Molecular Weight Relavalue of 0.92 for the molection * O f Polystyrensular weight exponent of GR-S System at 50" c. polystyrene, which is above that of any of the polybutadienes examined, is characteristic of a more linear polymer. A similar value for the intrinsic viscosity-molecular weight relationship of polystyrene was calculated from data obtained b y Frank and Mark ( 7 ) on polystyrene fractions circulated b y t h e International Union of Chemistry. Their data on toluene solutions result in plots for which t h e following relationship exists:
.,,;
/ I / I l ,
1
8""
MOLE&MULIR
WEIGHT
I IO-*
7. ViscosityFigure Molecular Weight Relation of Styrene-Divinylbenzene Copolymers
"'I11
[ q ] = 22.5 X
10-6 X
Cross-linking sites were next introduced by preparation of copolymers of styrene with known amounts of divinylbenzene (DVB). Polymerization bottles were loaded with styrene and commercial divinylbenzene adjusted t o actual ratios of 99.975 t o 0,025 and 99.95 t o 0.05 styrene-divinylbenzene. The polymer polymerized with 0.025 part divinylbenzene was stopped a t 67% conversion and the molecular weight of the whole polymer was found t o be 205,000. Upon fractionation of this polymer t h e relationship between t h e intrinsic viscosity and molecular weight of t h e fractions was [?I = 5.35 X 10-6 X MO.18. Similarly, the relationship between intrinsic viscosity and molecular weight
If one assumes t h a t all of the divinylbenzene enters the copolymer because of the large excess of styrene and the greater reactivity of divinylbeneene (16),the number of divinylbenzene units per molecule can be estimated from t h e molar proportion of divinylbenzene t o styrene and t h e number average molecular weight of the resulting polymer. The results of such calculations, recorded in Table VI, illustrate the influence of this cross linking upon t h e molecular weight exponent. A rapid drop in molecular weight exponent as t h e divinylbenzene concentration approaches one cross-link unit per molecule is also evidqnt in Figure 8. Although these values for the number of divinylbenzene units per molecule cannot be considered to be quantitative because the exact number of divinylbenzene units in the chain is not known, it is of interest t h a t t h e molecular weight exponent rapidly approaches 0.5 as the divinylbenzene
Table V.
Fractions of Styrene-Divinylbenzene Copolymers
Polymer Fraction
Intrinsic viscosities and molecular weights Fraction Purified Intrinsic Polymer, Weight, Viscosity % G.
Table IV.
Fractions of Polystyrene
Intrinsic viscosities and molecular weights Fraction Polymer in Weight, Fraction, Intrinsic Molecular Fraction G. % Viscosity Weight X 10-8 Whole 0.54 175 1 8.9 1.32 470 2 3 4.28 7.0 4 7.46 12.2 5 10.77 17.6 0.73 240 6 1.48 2.4 7 9.80 16.0 0.56 185 8 8.00 13.1 9 4.38 14.4 0.30 75.4 10 4.38}8.76 3.2 11 1.96 Unprecipitated 3.18 5.2
Molecular Wt. X 10-3
0.025 PARTDVB Whole 1 2 3 4 5 6 7 8 Unprecipitated
7.0 3.6 2.7 5.8 8.4 12.6 7.4 3.8 5.7
12.3 6.3 4.8 10.2 14.7 22.0 13.0 6.7 10.0
1.11 3.50
205 1570
2.11 1.37
735 442
0.53
-
132
0.05 PARTDVB Whole 1 2 3
April 1952
Figure 8. Effect o n Molecular Weight Exponent of Copolymerization of DivinylbenBene with Styrene
r*.
11.7
0.91 2.50
207 555
4 5 6 7 8 9 Unprecipitrtted
Table VI. Effect of Divinylbenzene Cross-Linking Agent o n Molecular Weight Exponent of Polystyrene Divinylbenzene Molar Ratio Molecular Part DVB/Styrene Weight None Polystyrene 175,000 0.025 1/4999 205,000 0.05 1/2499 207,000
INDUSTRIAL AND ENGINEERING CHEMISTRY
DVB Units/Molecule, Basis of100% Actu?l conversion conversion
0.39 0.78
0.53 1,05
~ ~ 1 weight Exponent, a 0.92 0.78 0.58
155
.
-ELASTOMERS-Latexunits per molecule approach unity. These values seem t o be consistent with each other in t h a t they both have been indicated t o be characteristics of t h e gel point. Flory ( 4 ) has indicated that very little cross linkage is required t o cause gelation and the present work (Figures 3 and 4) on polybutadiene indicates that polymers which have a molecular weight exponent in the range of 0.5 are near the gel point. The numerical value of t h e molecular weight exponent is then of significance as a measure of the degree of cross linking in soluble polymers and in the establishment of polymerization conditions which lead to maximum chain length without gelation. The degree of cross linking which is measured by this method is of lesser magnitude than that required for gelation and would not, therefore, be directly reflected in physical properties t o as great an extent as is the cross linking represented in highly gelled polymer. However, a low molecular weight exponent of 0.5, characteristic of polybutadiene made t o high conversion in the GR-S system a t 50” C., should no doubt be avoided as indicative of incipient gelation and its accompanying difficulties in processing and stability.
LITERATURE CITED
ACKNOWLEDGMENT
Alfrey, T., Bartovics, A., and Mark, H., J . Am. Chem. SOC, 65, 2319 (1943). Binder, J. L., private communication, The Firestone Tire and Rubber Co. to Office of Rubber Reserve. Doty, P., Brownstein, hl., and Schlener, W., J . PhzJs.& Colloid Chem., 53, 213 (1949). Flory, P. J., J . Am. Chem. Soc., 63,3096 (1941);69,2893 (1947). Flory, P. J., Ibid., 65, 372 (1943). Flory, P. J., J . Chem. Phys., 17, 303 (1949). Frank, H. P., and Mark, H., J . Polymer Sci., 6 , 243 (1951). Holdsworth, R. S., private communication, U. S.Rubbel Co. to Office of Rubber Reserve. Houwink, R., J . pralct. Chem., 157, 15 (1940). Johnson, B. L., and Wolfangel, R. D., IND.ENG.CHEM.,41, 1580 (1949). Mark, H., “Derfeste Korper,” p. 103, Leipzig, S. Hersel, 1938. IND.ENG.CHEM.,41, 1570 (1949). Meyer, A. W., Mochel, W.E., and Sichols, J. B., J . Am. Chem. Soc., 71, 3435 (1949). Sands, G. D., and Johnson, B. L., Bnal. Chem., 19, 261 (1947). Staudinger, H., and Heuer, W., Be?., 67, 1164 (1934); 68, 1618 (1935). (16) Ueberreiter, K., and Kanig, G., J . Chem. Phyu., 18, 399 (1950).
T h e authors gratefully acknowledge the interest of F. W. Stavely in this work and the assistance of Carol Liebler in determination of the intrinsic viscosities.
RECEIVED for review September 17, 1951. ACCEPTEDJanuary 29, 1952. This work was sponsored b y the Office of Rubber Reserve, Reconstruction Finance Corp., i n connection with the government synthetic rubber program.
Latex
E. A. MURPHY Dunlop Rubber Co., Ltd., Port Dunlop, Erdington, Birmingham, England
T h e paper is concerned mainly with progress made in t h e past few years in the technology of natural latex. The relative values of recent methods of testing latex are critically discussed and t h e conclusion is reached that n o available tests of chemical stability are capable of predicting t h e suitability of latex for all applications. The part played by metallic ions in latex is briefly surveyed with t h e plea that a more fundamental investigation be carried out on t h e part played by zinc in latex technology. Latest methods of preparing latex concentrates are reviewed and
a description is given of the concentrate prepared by the new electrodecantation process. Experimental work is also recorded on t h e influence of sodium pentachlorophenates on the mechanical stability and gelling properties of latex. Examples are given of controlled modification of t h e stability of latex for particular applications. The development of one uniform type of high stability concentrate is favored, together with methods of imparting t o it a t will the particular degree and nature of stability required for each industrial application.
N T H I S paper it is not proposed to consider progress that has
The increase in natural latex consumption semis to hr due mainly t o the rapidly expanding demand for latex foam rubber, which, had it not been for the present world unrest, was cstimated to reach an output of 185,000,000 pounds by 1960 (52). Although this was perhaps at first partly due to the fact that after the war the cost of latex as a raw material had not risen so steeply as the costs of raw materials employed in the older types of upholstery, latex foam rubber is now firmly established in its own right because of its superior intrinsic value as a cushioning medium. Whether it will be able to hold its position in spite of the increase in the price of natural latex remains to be seen, but this draaTback may be only of a temporary character in view of the possible near development of synthetic latices as suitable alter-
been made in latex technology historically, as this has already been adequately done in a number of postwar publications ( 4 7 ) . The rapidly growing importance of latex as a raw material is illustrated by the fact t h a t world consumption in 1947 m s 25,000 long tons, which had increased t o 87,500 tons by 1950. The original estimate for 1951 was nearIy 109,000 tons, but this was before the present unsettled conditions which necessitated U. S. Government purchase and price control. Provision is, however, being made for considerably increased outputs in the East Indies and Ceylon and the maximum annual capacity for latex concentrates by the end of 1951 has been estimated a t 175,000 tons. 756
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
