Swelling of Synthetic Rubbers

treated in the same manner as described for the oil solvent- extracted ... P. 0. POWERS AND H. A. ROBINSON. Armstrong Cork Company, Lancaster, Penna. ...
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25 to 30 minutes were required to bring the oil to 200” C.; then it was maintained a t this temperature for 30 minutes. The resulting oil had a viscosity of U (or 6.5 poises) a t room temperature by the Gardner standards, a Wijs iodine value of 145.7, a Browne heat test value of 13 minutes, and a refractive index at 25” C. of 1.5097. A sample of oil solvent-extracted from tung kernels was treated in the same manner as described for the oil solventextracted from press cake. Before treatment the oil was solid at room temperature, had a Wijs iodine value of 165.0, and gave a Browne heat test of 11.75 minutes. After heat treatment the oil had a Wijs iodine value of 152.5, had a Browne

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heat test value of 11.25 minutes, and was liquid at room temperature (Table 11).

Literature Cited (1) Drake, N. L., a n d Spies, J. R., IND.ENC.CHEW,h u 1 , . ED.,5, 284 (1933). ( 2 ) Gardner, H. A , , Am. Paint Varnish Mfrs. Assoc., Circ. 256 (1925). (3) Ibid.,309 (1931). (4) Gardner, H. A., a n d Sward, G. G., “Physical a n d Chemical Examination of Paints, Varnishes, Lacquers a n d Colors”, 9th ed., pp. 299, 300, Washington, I n s t i t u t e of P a i n t a n d Varnish Research, 1939. CONTRIBUTION 42 from the Agricultural Chemical Research Division, Bureau of Agricultural Chemistry and Engineering.

Swelling of Synthetic Rubbers in Mineral Oils’ Swelling in Mineral Oils Containing Polyolefins and in Mixtures of Nujol and Diphenyl P. 0. POWERS AND H. A. ROBINSON Armstrong Cork Company, Lancaster, Penna. Synthetic rubber compositions have been subjected to immersion tests in mineral oils containing polyolefins, and the degree of swelling has been measured. The addition of polyolefins to a mineral oil reduces its tendency to swell synthetic rubber. This tendency is measured by the aniline point of the mineral oil-polyolefin mixture. The same relationship found to hold for mineral oils applies; namely, the logarithm of the percentage swelling varies inversely with the 50 per cent aniline point. The aniline point must be carefully distinguished from the simple haze point caused by the incompatibility of the polyolefin and the aniline. Mixtures of Nujol and diphenyl have been studied as reference liquids, and the relations between aniline point and swelling determined. The swelling in the mixtures is greater than in mineral oils of the same aniline point. It is therefore believed that white mineral oil-diphenyl mixtures offer no advantages over mineral oils of lrnown aniline point i n determining the swelling of synthetic rubber compositions for specific industrial purposes. Immersion tests in mineral oils having the same aniline point as oils which will be encountered in service are believed to be more reliable and significant for evaluating behavior under service conditions.

N EARLIER paper (3) in this series showed that the aniline point of a mineral oil is a useful index of its tend-


ency to swell synthetic rubber compositions. That article brought out that one oil (No. 9) of the series investigated gave a differentslope for the cloud point curve a t various concentrations of aniline from the other oils investigated. It

was suggested that the unusual behavior of this oil might be due to the presence of added polyolefins. This view has been substantiated by the study of mixtures of mineral oil and polyolefins and of commercial lubricating oils known to contain such added materials. Polyolefins are added to mineral oils to improve their viscosity index (6).

Effect of Mineral Oil-Polyolefin Mixtures o n Synthetic Rubber A proposed method for the determination of the aniline point of mineral oils (1) called attention to the possible interference of a turbidity caused by ‘[petroleum products which are not strictly petroleum oil”. This method is essentially that used by the authors except that the A. S. T. M. determination was made on a 50 per cent mixture by volume rather than by weight and that agitation was mechanical rather than manual. Fraser (4) noted the “pseudo aniline point” which occurs when polyolefins are present in mineral oils. He recommends taking the temperature a t which “permanent cloud” appears on cooling and also the temperature a t which it disappears on warming. The two cloud points should not differ by more than 0.5” F. DETERMINATION OF ANILINEPOINT.The method of the earlier paper (3) was used in determining the aniline point. This is the temperature a t which the aniline and mineral oil separate, and must not be confused with a haze point which occurs a t a higher temperature due to separation of the polyolefin from the mineral oil-aniline mixture. In most cases this is only a slight haze and does not obscure the aniline point, 1 Strictly speaking, the term “synthetic rubbers” is incorrect, since these rubberlike substances are not the molecular duplioates of natural rubber Other names have been proposed, such as “elastomers” (by H. L. Fisher) but in the absence of any accepted and uniform nomenclature, industry seems t o have adopted the term “Synthetic rubber”, and that designation has become oommon parlance


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ppint - - - Cloud Haze point



- - Haze Dointa 0 Low-aniline-point mineral oil


Low-aniline-point mineral oil with 2.5 per cent polybutene Low-aniline-point mineral oil with 10 per cent polybutene

40' % SWELLlNG

Composition E was a Thiokol DX composition containing a total of 125 volumes of carbon black and cork dust per 100 volumes of Thiokol. All these compounds were prepared in sheet form by vulcanizing in a tensile sheet mold to the same extent as used in commercial practice. MINERALOILS. Two oils were obtained on the open market. One was a h y d r a u l i c oil, t h e other, an SAE 30 motor oil; both were known to contain p o l y b u t e n es . M i x t u r e s of a l o w aniline-point mineral oil with Vistanex 7000 (a polybutene of approximately 7000 molecular weight) were made; 2.5 and 10 parts of Vistanex were used per 100 parts of mineral oil.

which is marked by a sudden and pronounced turbidity. It has been found that a definite two-layer separation of the liquids takes place just below the true aniline point. Since this does not happen a t the haze point, the two-phase separation is a valuable check on whether or not the true aniline point has been reached. This separation takes place from 1O to 2" C. below the aniline point and occurs as soon as agitation is stopped. Figures 1 and 2 show cloud and haze points of the oils used. The haze temperatures approach the cloud temperatures a t low concentrations of aniline. Where the haze is pronounced, the shape of the cloud point curve a t a low concentration of aniline helps indicate the cloud temperatures a t higher concentrations of aniline. At 60 per cent WEEKS aniline and above, the haze may make the aniline 2 6 10 point difficult of observation, but since the cloud FIGURE 3. SWELLING OF BLACKpoint a t 50 per cent aniline by weight has been PROCEDURE AND RELOADEDNEOPRENE (above) AND OF found to be a satisfactory index of the swelling SULTS. Duplicate samCOMPOSITIONS c AND E IN O I L S CONcharacteristics of mineral oils (S), the critical ples of each stock, 1 x TAINING POLYOLEFINS solution temperature of these oil-aniline mixtures 1. Motor oil 2 X 0.1 inch (2.5 x 5 x 2. Hydraulic oil was not determined. 0.25 em.) were placed in 3. Low-aniline-point mineral oil with 10 per cent added polybutene R U B B E RCOMPOSITIONS. Three synthetic the different oils, and 4. Low-aniline-point mineral oil rubber stocks were used. They had been 5. Low-aniline-point mineral oil with 2.5 per per cent volume increase cent added polybutene evaluated previously in swelling tests and are was determined after believed to be representative of commercial 1, 2, 3, 4, 6, and 12 stocks. weeks of immersion in Composition A was a neoprene Type G compound containsmall individual stoppered bottles held in a constant-temperaing 70 volumes of soft carbon black (Thermax) per 100 volture cabinet a t 70" 1" C. Volume change was determined umes of Neoprene Type G. by the displacement method according to the procedure of Composition C was a Perbunan Extra compound containthe A. S. T. M. (3). ing 53 volumes of carbon black and 9 volumes of extractable The swelling-time curves are shown in Figure 3. For Figsoftener per 100 volumes of Perbunan. ure 4 three curves were taken from the previous article @).









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Diphenyl is readily available in a high state of purity and, as a result of its aromatic structure, should show a high tendency to swell synthetic rubbers. PREPARATION OF hfIXTURES. Diphenyl was obtained from the Dow Chemical Company (melting point 70" C.) and was used without further purification. Nujol was purchased on the open market. The mixtures of Nujol and diphenyl were made on a weight basis, and WBI'M homogeneous a t the temperature of the test. Crystallization of the diphenyl occurs when over 20 per cent is present. Since mixtures rich in diphenyl are practically solid a t room temperature, they are not so easily handled as mineral oils.

'C , - 5 o % AMINE POINT


They show the aniline point-swelling relations for the three synthetic rubbers discussed in this paper. The points plotted are from the data obtained with the mineral oils containing polyolefins. It is clear that the per cent swelling follows the true aniline point as in the previous work. Fraser (4) studied the swelling of a Neoprene Type G composition in lubricating and hydraulic oils containing polyolefins. He found that swelling in these oils could be measured by the aniline point. The tests were made a t 100" C. with five different types of oils. The results with the synthetic mixture (Figure 4, above) show that the addition of a polyolefin to a mineral oil results in slightly lower swelling. The results indicate that haze point is not a measure of swelling tendency, but that aniline point is a useful index and has the same significance that it does with other mineral oils.


The aniline points of the Nujol-diphenyl mixtures were determined by the method described ( 3 ) . I n all cases the percentages are by weight. Figure 5 shows the 50 per cent (by weight) aniline point for various Xujol-diphenyl mixtures. Figure 6 shows the cloud temperatures in the ternary system Nujol-diphenyl-aniline.


Swelling of Neoprene in Nujol-Diphenyl Mixtures A series of standard liquids for measuring the swelling of various synthetic rubbers in the laboratory would be very useful. Such liquids should be obtainable in pure form and be highly reproducible. Pl'ujol-diphenyl mixtures seem to fulfill these requirements. Nujol, a white refined mineral oil, is readily available. Examination of several samples showed a 3" C. maximum variation in the aniline point. Its swelling effect on various samples on synthetic rubber compositions may therefore vary by the amount expected from this range. A highly refined material of higher aniline point than Nujol is not readily available. Paraffin wax probably comes closest to meeting this requirement, but commercial samples have been found to vary by as much as 10' C. in aniline point. Many automotive lubricating oils have a higher aniline point than Nujol. It is not definitely known how much variation can be expected in the aniline point of various samples of any one commercial oil of this type. If such a high-anilinepoint oil is available with no greater variation than is encountered with Nujol, mixtures of this oil with diphenyl mould have a wider range of aniline points than the mixtures employed in this study, but would probably have many of the same deficiencies.



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diphenyl mixtures, but it is apparent that synthetic rubber compositions generally do not show the same correlation between aniline point and swelling as do mineral oils.



PROCEDURE AND RESULTS. Composition A, containing 70 volumes of soft carbon black (Thermax) per 100 volume of Neoprene Type G, was used. It was prepared in sheet form by vulcanizing in a standard tensile sheet mold t o the same extent a s used in commercial practice. Samples of the neoprene compound were immersed in Nujol-diphenyl mixtures containing 20, 45, 60, 70, and 80 per cent Nujol. The per cent volume increase was measured after 1, 2, 3,4, 6, and 12 weeks of immersion in small individual stoppered bottles in a constant-temperature cabinet a t 70" * 1' C. Volume change was determined by the A. S. T. M. method ( 2 ) . ' The time-swelling curves are shown in Figure 7 for the various mixtures. Figure 8 correlates the swelling of the neoprene in the diphenyl-Nujol mixtures and in the mineral oil with the same aniline point. The swelling a t high concentrations of Nujol obviously approaches that of mineral oil, since the swelling in pure Nujol must fall on the mineral oil curve. Since diphenyl is of lower molecular weight than the mineral oils, it may diffuse into the synthetic rubber composition more rapidly and thus give the effect of a lower aniline point oil. Such selective absorption of a solvent was noted p r e viously ( 5 ) . When high swelling was encountered, as when mixtures rich in diphenyl were used, equilibrium was not reached in 12 weeks (Figure 7). This effect was also noted with mineral oils of low aniline point. Perbunan and Thiokol compounds containing carbon black were tested in the mixture containing 30 per cent diphenyl. Both compounds gave appreciably greater swelling than would be expected from mineral oils of the same aniline point. These compounds were not tested with other Nujol-

The true cloud temperature of a 50-50 weight mixture of aniline and mineral oil is a reliable index of the amount of swelling a synthetic rubber will undergo when immersed in mineral oil. This result is also true for those mineral oils which contain polyolefins. Since the presence of both haze and cloud points is characteristic of such oils when mixed with aniline, a method has been worked out for distinguishing between them. Conversely, the presence of both haze and cloud points is believed to be indicative of the fact that polyolefins have been added to the mineral oils. The addition of polyolefins t o mineral oils will reduce the tendency of that oil to swell synthetic rubbers. The logarithm of the perc e n t a g e swelling varies inversely with the aniline point. While mixtures of lvujol and diphenyl can be duplicated fairly well, they offer no advantage over mineral oils for test50% / * y p P O "I ing the tendency of s y n t h e t i c rubber '$0 80 100 compositions t o FIGURE8. ANILINEPOINT VS. P E R swell. W h i l e t h e CENT SWELLINGOF NEOPRENE logarithm of COMPOSITION a. Nujol-diphenyl mixtures ing decreases as the b. Mineral oil8 50 per cent aniline of the mixture inc r e a s e s , t h e relationship is not the same as that found for mineral oils. For evaluating behavior under service conditions, immersion tests in oils having the same aniline point as oils which may be encountered in service are believed to be more reliable and more significant.


Literature Cited (1) Am. Soo. Testing Materials, Proceedings, 40, Appendix 11, 3B (1940). (2) Am. SOC. Testing Materials, Standards, P t . 3. p. 837. designation D471-37T (1939). (3) Carman, F. H., Powers, P . O., and Robinson, H. A., IND. ENQ. CHEM.,32, 1069 (1940). (4) Fraser, D . F., Rubber Chem. Tech., 14, 204 (1941). (5) Smith, C. N., U. S. Patent 2,209,940 (1940). (6) Thomas, R . M., Zimmer, J. C.,Turner, L. B., Rosen R., and Frolich, P. K., IND.ENO.CHEX.,32, 301 (1940).