February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
from mold growth after 32 weeks' exposure in a cycled tropical humidity chamber. A formulation containing 1%p-nitrophenol plus 2Oj, salicylanilide showed no fungus growth after 26 weeks. Five per cent salicylanilide and a multiple fungicide containing 5% salicylanilide and 5% 2,2'-dihydroxy-5,5'-dichlorodiphenylmethane showed slight growth after 32 weeks. The 3,5-dinitroo-cresol treatment which gave excellent fungus resistance in tests A and B broke down in the tropical test chamber. This lack of protection of the dinitro-o-cresol is probably due to the loss of fungicide by aqueous leaching during periods of condensations which occurred during the air temperature change from 24" to 29" C. in the cycling of the tropical chamber. The dew point temperature of the introduced air (which was maintained for 8 hours) was higher than the surface temperature of the cork. The treated cork gaskets had been cooled for 16 hours at 24" C. during the previous portion of the cycle. Formulations containing 1.8% dinitro-o-cresol and 1.5% dinitro-o-cresol plus 5% paraffin wax showed only slight growth after 12 weeks. The paraffin wax increased the tenacity or resistance to weathering of the chemical protectant. All the organic mercurials and malachite green oxalate were very susceptible to fungus growth in the tropical room. EVALUATION BASEDON TnRnE TYPES OF EXPOSURE. Three treatments incorporating a single fungicide, p-nitrophenol in a
267
2% concentration, in the treating bath offered complete protection to the protein-bonded cork. The remaining 13 fungicides, singly and in various combinations, were eliminated by one or more organisms in the three types of exposure. The following formulations showed only slight growth with one test organism or in the tropical room exposure and may be considered satisfactory for fungus-proofing cork: 2% p-nitrophenol plus 5% paraffin wax or 5% aluminum stearate, 1% p-nitrophenol plus 2% salicylanilide, and 1.8%8,5-dinitro-o-cresol. ACKNOWLEDGMENT
. Appreciation is expressed to C. C. Fawcett, E. R. Rechel, M. Frager, L. Teitell, and A. Kaplan of the Frankford Arsenal Laboratory, and to the Ordnance Department for permission t o publish thii; paper. LITERATURE CITED
Berk, S., A.S.T.M. Bull., 145, 73-6 (Mar. 1947). (2) Cooke, T. F., and Vicklund, R. E., 1x0. ENC.CHEM.,ANAL.ED., (1)
18,59-60 (1946). (3) Flett, L. H., Oil and Soap, 22,245-9 (Oct. 1945). (4) Hoisfall, J. G.,"Fungicides and Their Action," Waltham, Mass.,
Chronica Botanic? Co., 1945.
RECEIVED December 14, 1946.
Influence of Cloud Points of Coumarone-Indene Resins on Their Use 'in Rubber Compounding PALMER B. STICKNEY, LAVERNE E. CHEYNEY, AND PAUL 0. POWERS Battelle Memorial Institute, Columbus, Ohio Coumarone-indene resins of varying cloud points and melting points have been investigated in several rubbers. It is found that the cloud point is more reliable than melting point as an index of the plasticizing action of the resin. There is a critical cloud point, varying with the type of rubber, at which no change of plasticity occurs over a limited concentration range.
R
ESINOUS products from coal tar fractions have been widely used as softeners and softener-extenders for rubbers (6, 9). These materials, usually grouped as coumarone-indene resins, vary widely in composition, and hence in properties and usefulness in rubber compounding. This variation arises from the fact that different resin-forming monomers enter into their structure, depending on the coal tar fraction used. Many of these resins are composed predominately of indene units. They also contain varying proportions of styrene, indene, coumarone, cycolpentadiene, and similar materials. A further difference in resins commercially available results from variation in molecular weights of chemically similar polymers. Whitby and Kats (7) showed that melting points of polyindenes increase in a regular way with molecular weight. It is probable that a similar relation holds for other resins of this general class of similar chemical type. With commercial resins the variations in chemical nature make characteriaation by melting point of dubious value. The effects of such resins in rubber compounding have usually been related t o their melting points ( 1 ) . It was the purpose of this investigation to determine the variation in properties of several
rubbers, using different grades of commercial coumarone-indene resins. It was particularly intended to determine whether the cloud point-the temperature a t which a heated mixture of resin and a selected mineral oil clouds on cooling (@-of a resin is a better criterion of its effect on rubber compounds than is its melting point. This was accomplished by incorporating varying amounts of several resins in typical recipes of a nitrile-type rubber (Chemigum N-3), a styrene-butadiene-type rubber ( G R S 38), and natural rubber. The Williams plasticity of the compounded batches was measured, and tensile properties and Shore hardness of the vulcanixates were determined. EXPERIMENTAL PROCEDURES
The resins used are given in Table I and the recipes in Table 11. American Society for Testing Materials methods of preparing and testing samples were employed throughout this study, except for modifications in order and temperature of milling to avoid scorch. The basic recipe, without resins, was milled as a master batch. This base master batch contained all ingredients except accelerator and 6 parts per 100 (6 P.H.R.) of the rubber. The remaining rubber was master-batched with the accelerator. The finely powdered resins were milled into the base master batch a t roll temperatures of 130" t o 180" F., as required. The accelerator master batch was added after cooling, and the completed batch thoroughly blended again. Williams parallel-plate plasticity values (8) were used as a measure of softening action. The plasticity numbers were determined on conventional samples after a 3-minute compression under 5-kg. load a t 70' C., on samples which had been milledfrom 18 to 24 hours previously. The recovery numbers were measured after a 15-minute recovery period a t the same temperature. A t
INDUSTRIAL AND ENGINEERING CHEMISTRY
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30
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Figure 3. Cloud Point of Resins and Williams Plasticity of Chemigum Samples
(MIN.)
Figure 1. Typical Plot of Physical Properties at Various Times of Cure for Chemigum Sample 1-15
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Figure 4. Cloud Point of Resins and Williams Plasticity of GR-S Samples
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: r 20
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40
~
€0
OF CURE AT 305 'E (MIN.)
Figure 2. Tensile Strength of GR-S Samples
least 8 hours were allon-ed for the plastometer t o come to temperature, and all samples were conditioned a t 70" C. for 15 minutes prior t o compression. MELTING POINT
EXPERIBIENT4L RESCLTS
Physical test data for the Chemiguni samples are given in Table 111. Tensile and hardness values are averages of 40- and 60minute cures. The optimum cure in all cases Tyas betveen 20 and 10 minutes, but data froin the later, flatter portion of the curing curve Tyere used in order to minimize minor variations in curing rate. Although there mag be a slight retardation of cure caused by the resins, there Tyas no evidence of a systematic effect of resin type or concentration on curing rate. A typical example of change of physical propeities with vulcanization time is shown in Figure 1. Table I V gives physical test data for the GR-Ssamples. Interpretation of these data \vas complicated by variation in curing rates. This is illustrated by Figure 2, which s h o w the change of tensile strength with curing time for (a)the control; ( b ) sample 11-3 (25 parts resin B ) ; (c) sample 11-12 (25 parts resin P ) . In general, time to optimum tensile cure increased with resin concentration and was greater for lower cloud-point resins. Tensile strength is only one index of state of cure, and it is realized that optimum cures as indicated by some other property might be somewhat different. I t is believed that the general
('CJ
Figure 5. Melting Point of Resins and Williams Plasticity of Chemigum Samples with 23 Parts Resin
effects would be the same Although coumarone-indene resins have been listed by Buchan (8) with acid-containing softeners, such as stearic acid and pine tar oil, the acid value of such resins is actually low (8) 811 of the six resins used in this study had acid numbers less than 1, which coiresponds to neutral equivalents of 50,000 or higher. I t is probable that the cure retardation results from unsaturation of the resins. I n practice, it would be preferable to compensate for slow cure by adjustment of the recipe, but it was preferred in this study to extend curing times for the sakc of simplicity. The data given in Table IT are for the tinies giving the maximum tensile products. When two times of cure gave approximately equal tensile products, the average values of the properties for the two times mere givec. The data in such cases can be considered comparable only to a first approximation, as there are obvious variations in the state of cures, even a t the selected times.
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
Physical test data for the natural rubber samples are given in Table V. Again interpretation of the data is complicated by variation in rate of cure. On account of the strong tendency of natural rubber to revert in long curing cycles, anomalous results are obtained. The plasticity data are less reliable in this case, and reflect the greater sensitivity of natural rubber to time and temperature of milling. All of the samples containing resin F stuck to the mold badly, even in tight cures. Resin particles were visible in the vulcanized samples. These stocks all had a fibrous appearance and tended to split into layers. All of the stocks showed a tendency toward resin separation when put on the mill cold, although the resin appeared to redisperse as the samples warmed up. These difficulties were not unexpected, as usual practice with these resins in natural rubber has been to use smaller concentrations than reported here and to use only resins with low melting points (4). DISCUSSION
Plasticity numbers of the Chemigum and GR-S samples are plotted against resin cloud point in Figures 3 and 4. (The cloud point of resin A has been plotted as 0" C. in all graphs, although it is reported by the supplier only as "lower than 0" C.") The cloud point is a good criterion of softening action of the resins in these two synthetic rubbers. The stiffening action of resin F is perhaps best explained on the basis of aggregation of the resin molecules in the composition. The steady variation of plasticity numbers with cloud point in both G R S and Chemigum seem to indicate that a progressively greater degree of aggregation occurs as higher cloud-point resins are used. This is not surprising, since cloud point is a measure of oil solubility and should, therefore, be an approximate measure of solubility in rubber. That the cloud point is actually a better criterion of softening action than the melting point is established by Figure 5. For resins A through D, which appear to be fractions of increasing molecular weight from the same basic resin, the melting point, which parallels the cloud point, is also a good criterion of softening effect. However, with
269
TABLE I. PROPERTIES OF RESINSUSED Resin Designation A B C
I
5
Cloud Point", OC. 0 46 49 62 D 93 E 116 F Data furnished by manufacturer.
Melting Point", OC. 84.5 120.8 134.5 150.1 102.0 133.3
Specific Gravity" 1.09 1.13 1.13 1.14 1.08
TABLE 11. RECIPESOF STOCKS STUDIED I Chemigum N-3 GR-S 38 Smoked sheet SRF black EPC black MPC black Stearic acid Zinc oxide Sulfur Phenyl-8-naphthylamine Benzothiazyl disulfide Mercaptobenzothiazole Diphenylguanidine Resin
I1
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resins E and F , which appear to be chemically different from the others, the melting point is no indication of softening action. The various concentration curves in Figures 3 and 4 cross over a t a critical cloud point for both the GR-S and Chemigum stocks. The effects wiih natural rubber are qualitatively similar. This critical cloud point is about 120 for Chemigum samples, about 100 for GR-S, and probably lower for natural rubber. It is significant that these critical cloud points increase as the polarity of the polymers increases-that is, natural rubber>GR-S>Chemigum. Sample 1-19, containing 35 parts of resin F in Chemigum, had an abnormally high plasticity number which lies well above the smooth curve found for the other resins a t this concentration. In this case there appears to be a critical concentration between 25
TABLE 111. PHYSICAL TESTDATAFOR CHEMIQUM N-3 SAMPLES Sample No. 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 1-12 1-13 1-14 1-15 1-16 1-17 1-18 1-19
Resin
..
A A A
B B B C
C C
D D
D
E E E F
F F
(Average of 40- and 60-minute cures a t 305O F.) Resin Williams PlasticityTensile Concn., Plasticity Recovery Shore Strength, P.H.R. No. Difference Hardness Lb./Sq. In. .. 169 65 2320 27 139 55 2840 15 16 127 50 2470 25 10 35 2300 118 8 15 13 139 60 49 2660 25 133 15 58 2560 130 35 16 54 2110 148 15 18 58 2660 137 25 56 2530 19 35 56 2190 19 138 151 15 16 60 2540 140 25 12 58 2280 142 35 12 60 2120 160 15 60 2490 28. 157 25 63 2100 l,2 152 35 64 1880 15 li 166 68 2380 25 13 166 72 2220 35 20 199 72 1740
300% Ultimate Stress, Elongation, Lb./Sq. In. % 2120 1600 1210 1020 1690 1420 1040 1580 1340 1140 1720 1460 1380 1580 1310 1080 1980 1550 1400 440 ~
TlBLE
Sample No. 11-1 11-2 11-3 11-4 11-5 11-6 11-7 11-8 11-9 11-10 11-11 11-12 11-13 11-14 11-15
Resin
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A Dioctyl phthalate
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IV. PHYSICAL TESTDATAFOR GR-S 38 SAMPLES
WiIliams Plasticity Plasticity Recovery No. difference 171 27 146 13 125 5 110 9 145 11 136 8 121 5 165 10 159 9 156 7 175 11 180 8 182 0 136 16 123 11
~i~~ of cure at 305O F., Min. 10 20 40 ' 60 20 20 (20 & 40) 20 20 (20 & 40) 20 20 20 10 20
Shore Hardness 54 55 54 53 55 49 48 65 65 68 68 70 71 42 54
Tensile Strength Lb./Sq. Ih. 2940 3060 2900 2580 2880 2500 2290 2690 2620 2600 2400 2290 1920 2250 2790
300% Stress Lb./Sq. i n . 1200 980 980 770 1050 610 580 1330 1140 1080
1310 1260 1130 ROO ...
1300
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Permanent Set,
660
..
510
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..
15 14 13
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21 21
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 8. Hardness
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and 35 parts of resin, for which a rapid increase of plasticity number occurs. It is possible that this is generally true, but a t concentrations outside the range st'udied for the other resins. Aside from the different critical cloud-point values of the polymers, another variation in effect occurs. With GR-Sthe rate of change of plasticity with resin concentration of a given resin is roughly constant over the range studied. However, with Chemigum the softening in the low conceiitmt,ion range is proportionally much greater than at high concentrations. The high plast'icity number of sample 1-19 is probably a pronounced example of this same tendency. A11 of the resins in natural rubber begin to have a st'iffening effect within the range studied. If the variation of softening action with resin cloud point is indicat'ive of a varying degree of aggregation, it is interedng to compare the resins with an est,er-type plasticizer. The data for a GR-S sample (11-15) containing 15 parts of dioctyl phthalate have been included in Table IV for this purpose. This ester is more efficient than resin A , the lowest cloud-point resin used in this study. Since dioctyl phthalate has a molecular weight in the low range of the resins used, it might seem that even resin A is aggregated to some extent. The properties of the vulcanizates are not strictly comparable, particularly with GR-S and natural rubber, but, some trends are readily observed. I n all cases the hardness parallels the behavior of t,he plasticity data. Hardness is directly proportional to the cloud point of the resin and to t,he resin concentration. Low cloud-point resins soften all three polymers while high cloudpoint, resins cause an increase in hardness. The critical cloud point for hardness occurs at a lower value than that for plasticity i n GR-S and Chemigum. This is probably due to a decrease in &hesolubility of the resins in vulcanized rubber as coinpared to the
" 7
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- I5 PARTS RESIN - 25 PARTS RESIN
A - 35 PARTS RESIN
I A.
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uncured stock. The irregularit'y oi the plasticity values for the natural rubber samples makes it difficult to determine a critical cloud point. However, the hardness values definitely cross over at a cloud point of about 60. A4gain,critical cloud points appear to increase with polarity of polymer. Tensile properties are also strongly affected by the resin used. This is shown in Figure 6 , where tensile strengths of the Chemigum samples are plot.ted against resin cloud point. The tensile strengths fall off with increasing resin cloud point or with increasing resin concentration beyond 1.3 parts. That all resins give an increase in tensile strength a t low concentrations is not surprising, since it is known that coumarone-indene resins have a reinforcing action in GR-S type rubbers, even in channel black stocks ( 1 ) . A similar effect is apparent in the GR-S stocks, but, the regularity is somewhat obscured by variations in curing rate. It is evident also in all three polymers that stress increases and elongation falls off with increasing cloud point. Resin E in Chemiguin is unusual in this respect, as it gives stress values similar to those for resin A and elongations almost as high. Further indication of the changing character of the vulcaniaates is given by the stress-strain diagrams in Figure 7. Resin B of
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
271
which indicates that more flow occurs at high elongations. Tensile properties of the natResin Williams Plasticity Tensile 300% Ultimate Sample Concn., Plasticity Recovery 287O F., Shore Strength stress Elongation, Urd rubber samples show conNo. Resin P.H.R. No. differenoe Min. Hardness Lb./Sq. 16. Lb./Sq. in. % siderable irregularity due to 111-1 121 5 30 55 3640 1110 620 the variations in curing rate. 15 97 5 120 54 a800 840 590 111-2 B’ 111-3 B 25 88 11 100 53 2370 560 660 A notable difference appears in 1690 350 660 B 35 95 11 120 111-4 the trend of tensile strengths, 53 770 650 47 3070 111-5 D 15 108 11 100 119 10 120 54 2450 111-6 D 25 which rise to a maximum with 590 640 660 54 1820 1060 111-7 D 35 130 11 150 670 resin E . The failure of thelower 15 111 8 40 59 3530 910 E 111-8 630 111-9 E 25 118 9 60 62 3390 960 cloudpoint resins to attain 630 35 114 8 60 62 3180 980 111-10 E 111-11 F 60 61 3090 1180 540 higher tensile strengths is prob15 127 10 520 25 141 7 100 65 2970 1310 111-12 F 111-13 F a5 138 9 120 66 1640 1040 410 ably a result of the long cures necessary. In this case none of the resins gave a higher tensile strength than did the control. Hardness of vulcanizates is determined more by cloud point low cloud point softens at all concentrations over the entire range than by resin concentration, as indicated by Figures 8 to 10. of strains. Resin F of high cloud point causes a transition in the Although there are individual variations among these curves, the nature of the stress-strain curve, giving progressively higher general trend is similar. The Chemigum curves indicate a sharp stresses for low strains but lower stresses a t elongations beyond rise in hardness as cloud points above about 90” C. are reached; about 350%. It is believed that the low stress values a t high this discontinuity is not evident with the other rubbers. elongations are indications of an incipient yield point. AIThe same hardness data are plotted in a different manner in though permanent set determinations were not made for the Figures 11 to 13. Here again it can be seen that the effect of entire series, they were measured for the GR-S samples conresin concentration upon hardness is dependent upon the resin taining these two resins and are included in Table IV. The Samas well as the rubber stock. For example, increasing concenples containing the resins of high cloud point have a higher set, tration of resin D had virtually no effectupon the hardness of the natural rubber vulcanizates but had a pronounced effect in the GR-S series: whereas the Chemigum series was intermediate. Cure retardation with resins of low cloud point seems logically to depend on the unsaturation of the resins. The variation of effect with cloud point may well depend on aggregation of the resin molecules, since this would presumably reduce the concentration of double bonds available for reaction. However, there is no apparent reason for the lack of rePARTS RESIN tardation in Chemigum samples. The similarity of the effects in natural rubber and GR-S leads Figure 11. Effect of Resin on Hardness of Chemigum N-3 Sample to speculation as to whether nitrile rubbers of the type studied may vulcanize by a different reaction than those of the other two polymers.
TABLE v.
PHYSIC.4L
TESTDATAFOR N.4TURAL RUBBERSAMPLES
rzt :
ACKNOWLEDGMENT
PARTS RESIN
Figure 1 2 .
5
The resin samples employed in this study and their physical properties were supplied by the Barrett Division, Allied Chemical & Dye Corporation, to whom thanks are due. Helpful suggestions and criticisms of this manuscript were received from the Pennsylvania Industrial Chemical Corporation and the Armstrong Cork Company.
Effect of Resin on Hardness of GR-S 38 Sample
PARTS RESIN
Figure 13.
Effect of Resin on Hardness of Natural Rubber Sample
LITERATURE CITED
(1) Barrett Div., Allied Chemical L% Dye Corp., “CUMAR in Bum-S” (March 22, 1943). (2) Buohan, S., Trans. Inst. Rubber Ind., 20, 93 (1944). (3) Langton, H. M., in R. S. Morrell’s “Synthetic Resins and Allied Plastics,’’ p. 238, London, Oxford University Press, 1943. (4) Miller, S. P. (to Barrett Co.), U. S. Patent 1,782,693 (Nov. 26, 1930). (5) Powers, P. O., IND.ENG.CHEM.,ANAL.ED., 14, 387 (1942). (6) Powers, P. O., “Synthetic Resins and Rubbers,” p. 171, New York, John Wiley & Sons, Ino., 1943. (7) Whitby, G. S., and Kate, J., J . Am. Chem. SOC.,50, 116 (1928). (8) Williams, I., IND.EA-G.CHEM.,16, 362 (1924). (9) Zeek, C. J., Plastics (Chicago), 1, No. 7, 76 (1944). RSCEIVBD May 15, 1947. Presented before the Division of Rubber Chemistry at the 112th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y.
