ELASTOMERS-Lateu becomes slightly weaker in the swollen state but crumbles on the application of large shearing stresses only to a slightly greater extent than the dry vulcanized film. When a vulcanized latex dries, sulfur and the other compounding ingredients are deposited a t the interfaces between particles. The extent to which sulfur bonds form from one particle to the next would seem to depend on the amount of interfering adsorption material present and on the subsequent treatment of the film. If little or none of such interfering material is present, it seems difficult to suppose that interparticle sulfur bonds would not form on subsequent heating of the film or even on standing a t room temperature for a sufficient length of time. If such sulfur bridges do exist between particles, then the problem of explaining the high tensile strength of prevulcanized films is much less difficult, since both prevulcanized and dry vulcanized latex films then possess an essentially similar primary valence network, despite the presence of larger capillary openings in the prevulcanized film. It would not be necessary for bonds t o form a t every interface between particles for a complete three-dimensional network to exist.
ACKNOWLEDGMENT
The author wishes to express his appreciation to L. A. Wohler and Ernst Schmidt, both of the Firestone Tire and Rubber Co., for their advice during the course of this work. LITERATURE CITED
(1) A.S.T.M. Standards, Part 111, Nonmetallic Materials, p. 1697, Figure 3 (c), 1944. (2) Dalfsen, J. W.van, Rubber Chem. and Technol., 16, 318 (1943). (3)Ibid., p. 388. (4) Doty, P. M.,Aiken, W. H., andaMark, H., IND.ENG.CHEM., ANAL.ED.. 16. 686 (1944). ( 5 ) Evans, W. D., and Critchfield, C. L., J . Research NutZ. Bur. Standards, 11, 151 (1933)(RP683). (6) Hauser, E.A.,India-Rubber J., 68, 725 (1924). (7) Hauser, E. A., le Beau, D. S., and Kao, J. Y . L., J . Phys. C h m . , 46, 1099 (1942). (8) Humphreys, N. C. H., and Wake, W. C., T ~ a n s Inst. . Rubber Ind., 25, 334 (1950). (9) Schultz, T.H., Miers, J. C., Owens, H. S., and Maclay, W. D., J . Phys. Colloid Chem., 53, 1320 (1949). RWEXVED for review Saptember 17, 1951.
ACCEPTED January 26, 1962.
GR-S LATICES IN F O A M RUBBER LEON TALALAY AND ANSELM TALALAY The Sponge Rubber Products Co., Shelton, Conn.
T
This procedure is basedon H E advent of low temT h e superiority of cold OR-Shigh solids latex over latex U. S. Patent 2,432,353 (IS). perature polymerized polymerized at 120' F. has been demonstrated in foam It was found to produce conGR-S latex has greatly benerubber in terms of greater tensile strength, better elongasistently foam rubber of fited the foam rubber techtion at break, higher modulus, improved rebound elasgood structure from a great nology. Prior to the deticity, and enhanced low temperature properties. The variety of polymers. velopment of high solids cold The standard curing formagnitude of the effect observed is far greater than the mula contained uniformly 5 latex, foam rubber has been corresponding improvement reported in tire tread stocks partsof zinc oxide, l l / z parts made, of necessity, predom(5, 15). A number of polymer modifications in low temof antioxidant (Agerite inantly from Hevea. Such perature polymerized latex were investigated. I t was White), and l l / a parts each GR-S latex, as had been shown that by the proper choice of styrene content (apof zinc diethyldithiocarbac used, was blended in largely mate and zinc mercaptobenproximately 10% bound), OR-S foam rubber superior to zothiazole per 100 parts of for economic reasons, perHevea foam in subzero properties can be made. The elastomer. The sulfur was forming a function not modulus of foam rubber improve'd significantly with vraied from 1 to 3 parts, as much different from a dilrising Mooney viscosity of the contained polymer, while indicated on the individual uent or extender. The supegraphs. the elongation at break was affected adversely. The deMinor adjustments in riority of cold GR-S latex gree of conversion was found to have little bearing on the soap stabilization, alkali over hot in all physical propproperties of the resultant foam rubber. content, and viscosity were erties, its better wet gel made, to compensate for strength, and the much revariations in the colloid duced odor level (9). . have serviceable foam rubber made entirely of GR-S. It is, therefore, COMPARISON OF HEVEA, 41' F. G R - 5 , AND 120' F. O R - S unfortunate that as yet the production of cold high solids GR-S Type V GR-S was chosen as a typical 120' F. latex. It was latex is so limited. compared to a 41 'F. latex of the same charge ratio (70-30 butadiA study was undertaken to compare the physical properties of ene-styrene) and Mooney viscosity (MS-4') of 53. The latter foam rubber made from Hevea latex with foam produced from was a pilot plant latex, made by the Copolymer Corp. as PF 500high solids GR-S latices, polymerized a t 41 ' and 120' F., respec489. It is referred to as polymer 19 in Table I. Foam latex, made tively. In addition, a number of polymer modifications were infrom 62% centrifuged Hevea latex, was used as a standard of refvestigated in low temperature latex. erence. The physical properties of the foam rubber evaluated included Stresestrain measurements were made by the method detensile strength and elongation a t break, modulus of compression, scribed by Conant and Wohler ( I ) . The plotted results are averflexibility a t subzero temperatures, and rebound elasticity. ages of the best six out of ten data a t optimum cure. The tensile values obtained, in pounds per square inch, were PREPARATION OF SAMPLES divided by the density in pounds per cubic inch, and are so plotted Slab foam samples of 1 inch thickness were molded at a density in Figure 1. This method compensates for minor density difof approximately 0.003 pound per cubic inch. ferences between samples, and is permissible since the tensileThe compounded latex was foamed by the catalytic decomposition of hydrogen peroxide and then rapidly frozen. The frozen density relationship is linear within the range encountered (0.003 structure was elled by permeation with a coagulating gas, carpound per.cubic inch f 10%). bon dioxide. %he gel waa vulcanized at 215" F., removed from In the left half of the graph the foam tensile density is plotted the mold, washed, and finally dried a t 180' F. in circulating hot against parts of sulfur per 100 parts of elastomer. There is a difair. , I
April 1952
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
791
-ELASTOMERS-Latex Table I. High Solids GR-S Peroxamine Latices Investigated Ref. 1 2 3 5 6 7 8 9 10 11 12 13 14 15
Origin Copolymer Copolymer Copolymer Copolymer Naugatuok Naugatuok Naugatuck Naugatuck Naugatuck Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer
16 17 .I8 19 20 21 22 23 24 25 26 27 28
Copolymer Copolymer Naugatuck Copolymer Naugatuck Naugatuok Naugatuok Saugatuok Kaugatuok Saugatuok Naugatuck Naugatuck Naugatuck
4
,
Exptl. Polymer Number 500-494 102-515 500-493 101-503 54437-A 54437-B 54437-C 54437-2 54668-A
Charge Ratio,
ws
100/0 95/5 90/10 88/12 87/13 87/13 87/13 87/13 87/13
101-502 101-510 54248 500-489 5416&.4 544734 54862-A 54845-A 54828 54807 54798 54473-B 54791
80/20 SO/ZO
ference in the trend. The tensile strength of I-Ievea foam decreases with rising sulfur content while both GR-S foams improve slightly. The marked superiority of 41 ' F. latex over 120 F. latex is very apparent. At 2 parts sulfur the hot latex has barely 20% of the Hevea foam tensile. The cold GR-S foam has close t,o 60% of the strength of Hevea. In blends with Hevoa-on the right of Figure 1-cold latex again proves far superior to hot GR-S. In fact, substituting one t,hird of the natural rubber with Type V GR-S results in a tensile no better than that of foam made entirely of cold GR-S. Film tensiles, as distinct from foam t,ensiles, were also measured on the same elastomer blends. These are shown in broken lines. 111 view of the known difficulty of obtaining good cast, films of GR-S latex, the rolled-ring technique described by Pirot (8) was used. A very thin film is dipped onto a glass tube, permitted to dry, and then rolled into a ring. Because of the thinness of the fi h i deposited, i t dries without cracking. While this method appears to give more consistent results in GR-S than the making of cast films, it is unfortunately not universally useful, in view of the fact that some polymers adhere tenaciously to the glass, while others are too dry to roll up successSully. The film tensile scale on the right ordinate of Figure 1, and t,he O
Bound Styrene,
%
0 2.8
7 0
8.7 11.5 11.5 11.8 11.8 12.0 12.6 11.9 11.3 11.8 12.2 16 3 14 6
Polymerization Temp., ' F. 41 41 41 41 50 50 50 50 ,
50 41
41 41 41 41 41
Conversion,
%
61 62 55 60 50 60 70 80 60
50 53 50 55 49
41
55
41 50 41 50 50 50 50 50 50 50 50 50
58 80 59 80 80 80 70 60 60 60 80 60
MS-4' Mooney 47 78 71 67 15 21 37 52 83 19 30 47 62 83 93 38 43
Solids,
%
59.5 63.9 63.5 62.3 53.0 56.6 49.5 50.5 61.4 63.; 60.u
62.0 59.9 62.2 62.1 61 4 63 2 48 8 60.0 63.5 63.5 60.5 61.5 60.9 61.0 6.3.5 62.2 62.4
foam tensile scale on the left, n-ere macle to coincide for the allHevea compound. KO ready explanation is available for the fact that for both 41 and 120' F. lat,ex the film tensiles fall off more rapidly than do t,he corresponding foam tensiles, a8 the GR-S content is increased. It might be argued that because of the stereo-reticulate structure of foam the tensile property measured in a cellular material may not be true tensile strength. Elongation a t break (Figure 2 ) again demonstrates the marked superiority of cold over hot GR-S latex in foam. No correction for density variation between samples need be made in this instance, since the foam elongation is independent of density in the operating mnge. -4n expected t'rend of lower elongations wit'h increasing sulfur ratio is observed. On the right of Figure 2, in blends v-ith natural rubber, replacing one third of the Hevea wit,h hot GR-S reduces the elongation of the result,ant foam to the value of foam made entirely of rold GR-S. This elongation ia about two t,hirds of that of Hevea foam. Film (rolled-ring) elongations of the same elastomer blends are shown in broken lines. Here, as distinct from tensiles, the film elongat,ions fall off less severely than do the corresponding foam elongations, f t the ~ GR-S content is increased. O
2% I SULFUR
,o
Lo PARTS
'392
eo SULFUR
100
0
1
1
80
60 40
20
40 60
H E V E A : OR-S
20 80
0
too
.Figure 1. Comparison of Foam and Film0 Tensile Strength of Hevea with 41 O and 120 F. OR-S
Figure 2. Comparison of Foam and Film Elongation at Break of Hevea with 41" and 120"F. OR-S
A t varying sulfur ratios and i n b l e n d s
At varying s u l f u r ratios a n d in blends
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Latex
I
I
I
001
002
003
DENSITY G,
I
I
I
004
005
006
LBS/IN?
Figure 3. Effect of Density on Compression Resistance of Hevea Foam
Figure 5. Comparison of Rebound Elasticity of Hevea F o a m with 41 ' and 120' F. GR-S Foam At varying sulfur ratios and in blends
I
Figure 4. Comparison of Specific Compression Resistance of Hevea Foam with 41 ' and 120"F. GR-S F o a m At varying sulfur ratios and in blends
An important property in foam rubber is resistance to compressive deformation. This is usually expressed as the pounds per square inch necessary to compress the sample 25% of its height. It is normally measured in accordance with the indentation test described in ASTM procedure D552-46aT. The increase in compression resistance with rising density (Figure 3) has been shown by Talalay (1.2) to be satisfied by P parabolic function. The mathematical relationship between compression resistance and density is givep in the formulas (insert of Figure 3), where h is the force in pounds per square inch required to produce the 25% deflection, G, is the density of foam in pounds per cubic inch, and p is a parameter, which is primarily a function of the modulus of the latex compound, and to a minor extent a function of cell structure. In lieu of the density Gf, a cube root function of density, 8,is substituted for convenience. The reference parabola shown (Figure 3) is for Hevea foam a t 2% sulfur, and has a p value of 10. For a given density of foam rubber, the compression values represented by the parabola ( p = 10) are considered to be standard. Specific compression resistance of any foam sample is expressed as a percentage of standard compression. In this manner, the effect df density variations is eliminated. As might be expected, the specific compression resistance of foam goes up with sulfur,content (Figure 4). At equal Mooney viscosity cold GR-S produces a firmer foam than the hot polymer. It is a coincidence that the specific compression resistance of 41 F. GR-S foam is equal t o that of Hevea foam a t 2% sulfur. This is due, as will be seen later, to the particular Mooney viscosity of the GR-S polymer. I p Hevea-GR-S blends small additions of Type V do not appreciably affect the specific compression resistance of foam. The improved molecular architecture (7) of low temperature O
April 1952
,
I
I
I
I
I
I
1
Figure 6. Comparison of Flexibility at Sub; zero Temperatures of Hevea Foam with 41 and 120" F. OR-S Foam At varying sulfur ratios and in blends
polymerized latex has a beneficial effect upon the elastic properties of the foam produced. This was measured (Figure 5) by the rebound of a falling ball a t room temperature. The rising trend with increasing sulfur ratio is shown on the left of this chart; the effect in blends with natural latex is presented on the right. It was found that in samples of very fine cell structure, more consistent rebound data were obtained by carrying out the test in vacuum, In this manner, the pneumatic effect of air leaving and re-entering the sample was eliminated. A S/8-inch diameter steel ball was used and was released magnetically inside an evacuated graduated glass cylinder. The flexibility at subzero temperatures of 41' F. GR-S is also superior t o that of 120' F. GR-S a t essentially the same styrene content (Figure 6). The per cent retention of flexibility is expressed as the percentage of deflection retained when applying at the temperature of the test, the load capable of compressing the sample 25% of its height at room temperature. The test was conducted in the following manner. A specimen 4 square inches in area and approximately 1 inch thick was compressed at room temperature to 75% of its original height. The load required was noted. The sample was then conditioned for a period of 3 hours a t the subzero test temperature. A new height determination was made and the same load as above applied again while the sample remained in the test cabinet. Thirty seconds after load application a new compressed height (deflection) measurement was taken. Per cent retention of flexibility
-
deflection a t subzero temperature deflection a t room temperature
INDUSTRIAL AND ENGINEERING CHEMISTRY
x
100
793
L L A S T O M E R S - L a t e L I
I
I
1
I
I
I
i I
T0i I
300
m z
W
n
4
2 % SULFUR
8
4
Figure 7.
o!
12 % BOUND STYRENE
% B O U N D STYRENE
Effect of Styrene Content on Foam Tensile
Ib
j0
,b
j0
I
5b ,b
I
70 MOONEY VISCOSITY ( M S - 4 ' )
,b
,b
lLo
Figure 8. Effect of Styrene Content on Flexibility of Foam Rubber a t Subzero Temperatures
U 00
20
I
1
I
40
MOONEY VISCOSITY
I
I
60 (MS-4'1
'
a0
I
I 00
Figure 9. Effect of Mooney Viscosity on Specific Compression Resistance of Foam Rubber
Figure 10. Effect of Mooney Viscosity of Cold OR-S on Elongation at Break of Foam Rubber
A t the less severe test temperature of -40" F., increased sulfur has a beneficial effect upon the flexibility of Hevea and cold GR-S. This effect is lost a t -70" F. In blends with natural rubber it is evident that the elastomer of poor flexibility has a dominant effect. For example, the substitution of a little over 10 parts of Hevea with 120' F. GR-S reduced the flexibility of the blend to that of a foam made entirely of 41" F. GR-S latex. This holds true a t boch test temperatures of -40" and -70" F.
centrated a t lower solids. These were creamed to approximately 60% before using. I n succeeding charts, these polymers are referred to by the order number given in the first column of Table I. It was found that generally one particular polymer variable dominates a specific physical property. As an example, the styrene content of a low temperature latex dominates two properties of a foam-the tensile strength and the flexibility a t subzero temperatures. The bound styrene content of a low temperature polymer has essentially linear effect upon the tensile of the foam (Figure 7). However, the effect is not as great as might be expected. Reducing the styrene content from 26% to 11 or 12% lowers the tensile by only 207& The styrene content us. foam tensile correlation is generally satisfactory except for polymer 10 which exhibits abnormally high tensile a t 12% bound styrene. In a preliminary attempt to shed light on this discrepancy, it was found that despite its low Mooney viscosity, 11s-4'of 19, the unmilled polymer is not free of gel. In addition, the soluble portion appears to have an unusually, narrow molecular weight distribution, as indicated by fractional precipitation from benzene solution. Work on this phase is continuing. Styrene content has a major bearing on the flexibility of foam a t subzero temperature. It has previously been shown (5, 6) that reducing the styrene ratio in a polymer improves its flexibility at low temperature. Eight styrene levels from 0 to 26%, as represented by polymers 1-4, 13, 16, 17, and 19, were investigated at 2 parts of sulfur (Figure 8). A definite optimum was found to exist at around 10% bound styrene. The trend persisted down to the lowest test temperature investigated, - 9 5 " F. A t this optimum styrene level foam
POLYMER MODIFICATIONS
The comparison of foam properties has been based thus far on a single low temperature latex, which is polymer 19 in Table I, and has 26% bound styrene, a small rotor Mooney viscosity of 53, and a conversion of 59%. I n order to ascertain the effect of different styrene contents, Mooney viscosities, and percentages of conversion, the 28 experimental polymers shown in Table I were selected and studied in foam rubber. ' They are primarily arranged in order of ascending styrene charge ratios, and within each group, in order of increasing Mooney viscosities (MS-4'). The polymers were produced in pilot plant quantities by the Naugatuck Synthetic Division, and the Copolymer Corp., respectively, and are identified with their pilot plant number. All polymers were made in high solids peroxamine recipes ( I O , IC), at either 41" or 50" F., as indicated, and emulsified in a fatty acid soap-Daxad system. As indicated in the last column of Table I, most latices were received as heat concentrates a t around 60% total solids. A few, such as polymers 5, 6, 7, 8, and 18, were received uncon794
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
ELASTOMERS-Latex
,
aoo
'
The effect of conversion upon the stress-strain properties is a controversial issue, While it can be shown that in high solids cold GR-S latex, film properties decline with higher conversion (Q),i t has not been possible to show this effect in foam rubber. Two conversion series, one at a 70-30 butadiene-styrene charge ratio and constant Mooney viscosity, the other at an 87-13 charge ratio and varying Mooney viscosities were investigated in low temperature GR-S foam (Figure 11). The 70-30 series ranged from 60 to SOYo conversion, the other from 50 to 80%. Both lines lie at appropriate tensile levels for their respective styrene contents, but fail to show any significant trend with conversion. The broken line, on the other hand, which shows film tensiles of the 70-30 polymers, falls off significantly with increasing conversion. Here, again, is an instance of poor correlation between film and foam properties. Considerab1e.interest has focused recently on oil-modification of high Mooney cold GR-S in solid rubber (S,4, 11). Increasing amounts of Circosol 2xH were incorporated into a foam rubber recipe. A low temperature.GR-S latex with a small rotor Mooney of 91 (polymer 28, Table I ) was used in this series. This polymer had a gel content of 36y0. The elongation at break increases substantially, from about 200 to 4ooy0 upon the addition of 50 pslrts of Circosol 2xH per 100 parts of polymer. At the same time, the specific compression resistance drops substantially. It crosses the 100 (Hevea) reference line at about 13 parts of Circosol. T h e foam tensile is affected surprisingly little. I n fact, it appears to increase very slightly.
:
040
w
10
60
80
so
% COHVERSION
Figure 11. Effect of Per Cent Conversion o n Tensile Strength of Cold GR-S Foam Rubber rubber can be made which is completely flexible at -40" F., is more flexible than Hevea foam a t -70" F., and still retains an appreciable degree of flexibility a t -95' F. The flexibility was measured in the test cabinet after conditioning the sample for 3 hours. This exposure is long enough to ensure uniform temperature throughout the samples and probably also to crystallize the more readily crystallizable polymers. It may be too short, however, for all long-term crystallization effects to take place, A clear-cut linear relationship appears to exist between the Mooney viscosity of a low temperature polymer and the specific compression resistance of the resultant foam (Figure 9). Mooney viscosities from 15, small rotor, to 90, small rotor, were investigated and are plotted at 2% sulfur. The specific compression resistance more than doubles in this range. The 1 and 3% sulfur lines would lie parallel and equidistantly below and above the 2% sulfur line shown. The fact that the relationship is dominated by Mooney and is essentially independent of styrene content can be inferred by comparing polymers 1, 8, and 19, which at practically identical specific compression resistance contain 0, 11, and 26y0 bound styrene, respectively. Two of the polymers, 10 and 20, do not fit the trend. Polymer 20 was highly cross linked with divinylbeneene during polymerization and contains SOTo of very tight gel. Polymer 10 is the same polymer which gave abnormally high tensiles in Figure 7. Mooney viscosity also dominates the elongation at .break of low temperature GR-S foam. The trend is shown in Figure 10 for foam rubber vulcanized with 2% sulfur. As the Mooney viscosity is increased, the elongation at break falls off. This reduction appears to level off a t about 70 small rotor Mooney. Again, the styrene content is not a major influence. Polymers 3 and 21, of practically identical Mooney viscosity and elongation, contain 7 m d 26% styrene, respectively.
ACKNOWLEDGMENT
The authors wish to express their appreciation to L. H. Howland and V. C. Neklutin of the Naugatuck synthetic plant, and to J. P. McKenzie of the Copolymer Corp., for making this study possible by providing the experimental latice's. They are also indebted to C. J. Bradway, J. L. Gretz, G. F. Waters, W. D. Coffey, and T. F. Bush of the technical staff of the latex division of The Sponge Rubber Products Co., for their valuable assistance in carrying out the experimental work. LITERATURE CITED
Conant, F. S., and Wohler, L. A., India Rubber World,121, 179 (1949). D'Ianni, J. D., Hoesly, J. J., and Greer, P. S., Rubber Age, N . Y.,69, 317 (1951). Gehman, 6. D.. Jones. P. J.. Wilkinson. C. S.. and Woodford. D. E., IND.ENO.CHEM.,42,475 (1950). Harrington, H. D.,Weinstock, K. V., Legge, N. R., and Storey, E. B., India Rubber World, 124,435 (1951). Howland, L. H., Messer, W. E., Neklutin, V. C., and Chambers. V. S., Rubber Age, N . Y., 64,459 (1949). Liska, J. W., IND.ENQ.CHEM.,36,40 (1944). Meyer, A. W., Ibid., 41,1570 (1949). Pirot, M., Rubber Chem. and Technol., 21,168 (1948). Smith, H. S.,Werner, H. G., Madigan, J. C., and Howland, L. H.. IND.ENO.CHEM..41.1584 (1949). , Smith, H. S., Werner, H. b.,Westerhoff, C. B., and Howland, L. H.,Ibid., 43,212(1951). Swart, G. H., Pfau, E. S., and Weinstock, K. V., India Rubber World, 124,309 (1951). Talalay, J. A.,private communication (1949). Talalay, J. A.,U. S. Patent 2,432,353(1947). Whitby, G. S.,Wellman, N., Flouta, V. W., and Stephens, H. L., IND.Earo. CHEM.,42.445 (1950). White, L. M., Ibid., 41, 1554 (1949). RECP~IYP~D for review September 17, 1951. ACCEPTED February 7, 1952. \----I-
.-
$
P F
~
oo
'
,o
'
I 90 SO CIRC080L 2XH
I 40
Figure 12. Effect of Oil-Extension of a High Mooney-Cold GR-S Latex on Foam Properties
April 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
795
