Swelling of Synthetic Rubbers in Mineral Oils

>^EVERAL methods have been suggested to measure the tendency of mineral oils to swell synthetic rubber compositions. The aniline point (8) ofthe oils ...
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The swelling of compositions from typical oil-resistant rubbers has been measured i n hydrocarbons having a wide range of solvent power. Compositions loaded with the same volume of SRF carbon black and no extractable softeners have been studied at P5*, 70", and 100" C. The aniline point of the hydrocarbon measures the tendency to swell, and up t o 100% swelling the logarithm of the volume increase varies inversely with the aniline point of the oil. Swelling was found to b e greatest with the composition from Neoprene GN and was progressively lower with those from Stanco Perbunan, Hycar OR-15, and Thiokol FA. The swelling was found to increase as temperature was raised, but the rate of increase was different with the various synthetic rubbers. Changes in composition during test may b e responsible for these differences. consideration of these results and published data indicates that the slope of the log swelling-aniline point plots of compositions from a particular synthetic rubber does not change with temperature, loading, or degree of cure. There i s apparently a small difference in the slope as the type of synthetic rubber i s changed. Tensile strength, elongation, and durometer readings of the swollen comporitlonr show a decrease with increased swelling, a slight decrease at l070, and a very pronounced drop at 100% swelling. Thiokol compositions have little strength when immersed at 100".









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Armstrong Cork Company, Lancarter, Pa,


EVERAL methods have been suggested to measure the tendency of mineral oils to swelI synthetic rubber compositions. The aniline point (3) of the oils has proved to be a simple and reliable index. It has already been adopted by two engineering societies (2, 2 1 ) and by several private companies as a specification for mineral oils which may come in contact with synthetic rubber compositions. This study wns made to include oils with low aniline points, and to measure the swelling a t different temperatures. Volatile hydrocarbons, an aromatic fuel blend, and kerosene were included in the list of swelling liquids. Since the earlier studies, Americanmade acrylonitrile-butadiene copolymers have become available, and two such rubbers were used in this study. Four oil-resistant synthetic rubbers, Neoprene GN, Stanco Perbunan, Hycar OR-15, and Thiokol F.4 were compounded with about 28 volumes of SRF carbon black, using no extractable softeners. Thus the swelling of these compounds is comparable. The compounds are shown in Table I. The compounds were cured as 0.075-inch slabs for 40 minutes a t 300" F. (149" (3.). The Thiokol compound was allowed to cool in the press.

January, 1945




A T 25'C.



















Four oils were chosen as swelling liquids to cover a wide range of solvent power, from a highly paraffinic oil to the mineral oil of lowest aniline point yet found. Kerosene and 40% aromatic fuel blend, similar to PPF 813 fuel, were selected as representing the range of swelling likely to be encountered in volatile hydrocarbons. The properties of these oils are shown in Table 11. The aniline point was determined ab 50% by weight. The usual methods {S) specify determination a t 50% by volume, but the mixture can be more accurately made by weighing (3)the aniline and oil. The difference in aniline point by the two methods is not great in most cases. Recently the monomethylaniline point (8) has been suggested as a measure of the swelling tendTable


Synthetic Rubber Compositions

Neoprene G N (GR-M) Stanoo Perbunan H oar OR-15 Ttiokol FA Stearic acid Zinc oxide Sulfur Light MgO Altax DPG SRF (Gastex) Sp r. of polymers 6 r. of compound &I. black/100 vol. polymer Vol. polymer/100 vol. compound



.... .... 0.5 5.0


.... 40 . . I .

1.26 1.41 27.8 77.2


100 .... .... 0.5 5.0 2.0

. . e .

1.5 .... 50

0.97 1.19 27.0 76.6


. 100 ... .... 0.6 5.0 1.6

.... 1.6 .... 50

1.00 1.22 27.8 76.3

ency. Since this value for petroleum lubricating oils is approximately 77" C. below the aniline point, it is apparent that it can be used in the same manner to predict the swelling of such petroleum products. Duplicate \samples of each compound, 1 X 2 X 0.075 inch (25 X 50 X 1.9 mm.), were placed in separate jars a t 25", 70°, and 100" C. The volume change was measured by the displacement method ( 1 ) and hardness by the Shore durometer after 1,2,3,and 4 weeks of immersion.

.... .... .... 100


0.6 10.0

.... .... 0.3

0.1 37.5 1.34 1.61 27.9 76.3

Typeof Oil SAE 30 motor White Plasticiaing Kerosene Aromatic fuel blend Plasticizing

II. Swelling Liquids

Saybolt Specific univereal Gravit6. Viscositx at 20' at37.8 C. 0.8806 455 0.8810 210 0.9156 180 0.7545 ... 0.7685 1.040 iii

Distillation Range 5% 96%

.. .. .. .. .. a60i 2857 . . . 140 ..

50% Aniline Point, 0


108.8 104.0 68.0 61.4 1.5 -11.4



Dumbbells, cut to 0.25 X 1 inch (6.3 X 25 mm.) in the restricted portion, were also placed in each liquid and were tested after 4 weeks of immersion. Tests were made in the swollen condition ( 1 ) before the volatile materials could evaporate. A Scott tester was used at 20 inches pull per minute. The immersed samples were conditioned a t 21" C . (70"F.) and tested at the same temperature. SWELLING RESULTS

The time-swelling curves of the Neoprene GN compositions at 25' and 70' C. are shown in Figures 1 and 2. The numbers on the curves refer to the aniline points of the oils. These curves are typical of compositions of other synthetics and show that with low-aniline-point oils, equilibrium may not be reached in 4 weeks at 25' C. With the less viscous volatile oils, equilibrium is reached in 1 to 2 weeks. The swelling at 25' in the -11.5' C. aniline point plasticizing oil may be slightly below the equilibrium value. At 70" and 100° C. equilibrium is reached in less than 4 weeks, and a slight decrease following the maximum swelling is noted in some instances. This effect was previously observed (10) and may be due to further curing of the sample. It is planned to study the effect of the degree of cure on the amount of swelling encountered. %Theresults of the swelling tests a t 25' are shown in Figure 3. With each compound the logarithm of swelling varies inversely with the aniline point of the swelling liquid up to 1 0 0 ~ oswelling. Hanson (9) had previously suggested from the results with one oil that the swelling of Neoprene GN was not linear above 100%; our results confirm this opinion. I n most cases compounds which swell to this degree in service are of questionable utility. As noted above, the aompounds showing a high degree of swelling at 25' may not have reached equilibrium, but i t does not appear that equilibrium swelling (Figure 1) would be enough greater to make the relation between aniline point and swelling linear. The swelling tests at 70' C. are shown in Figure 4. The compositions all had a greater degree of swelling than at 25'; the synthetic rubbers are arranged in the same relative order, Neoprene GN showing the greatest swelling, and Stanco Perbunan, Hycar OR-15, and Thiokol FA progressively less swelling. The aniline point varies inversely with the logarithm of swelling up to 100% volume increase. At 100" the volatile hydrocarbons were not used. The results a t this temperature are shown in Figure 5 ; Thiokol is not included since the compounds were badly softened under conditions of test and Thiokol is not usually recommended for service at this temperature. The equations of the flat portion of the curves (Figures 3, 4, and 5 ) were derived to be as follows: Neoprene: l o g s = 2.75

t6 Pt. +c - A. 50

t A. Pt. + lx2 - 58 t A. Pt. log S = 1.40 + 126 - 58

Perbunan: l o g s = 2.10 Hycar.: Thiokol: where


- A. 6Pt. 0

log S = 1.18 4- K~

S = % volume increase t = temperature, O C. A. Pt. = 50% aniline point of oil

These equations must be regarded as a first approximation, but i t is believed significant that the effect of a change of aniline point has the same effect on the logarithm of swelling, regardless of temperature. A decrease of 50" C. in the aniline point of the oil gives ten times the swelling in the case of Neoprene GN, within the linear portions of the curves; with Perbunan and

Vol. 37, No. 1

January, 1945



Hycar compounds the value is 58', and with the Thiokol compound it is 60'. The temperature coefficient seems to be somewhat larger with the compounds which exhibit less swelling. The results with the Neoprene GN composition were compared with the reported values of Fraser ( 4 4 7 , and of Hanson (9) and an earlier study (3)from this laboratory (A-70 and B-70). These results are shown in Figure 6. Hanson used the maximum aniline point in his figures, but his results have been redrawn using the 50% aniline point. Since these compounds have various amounts of filler and other eompounding ingredients and since the conditions of cure vary somewhat, it is not surprising that the curves do not always coincide. It is surprieing that the slope of the curves is so nearly the same in almost every instance. This is in agreement with the finding above that the slope of the curve was independent of temperature, but also indicates that it is apparently independent of loading and normal variations of the degree of cure.

The hardness decreases in most instances with incrcased swelling, but there is a noticeable increase in some of the Perbunan and Hycar samples a t 100' C. This suggests further vulcaniration under the conditions of test.


The authors wish to thank S. W. Eby for obtaining the swelling data and for determining the physical properties.

Tensile test pieces were immersed in the various liquids for 4 weeks, and tensile strength and hardness were determined in the swollen condition (Table 111). The tensile strength is computed on the dimensions of the unswollen sample to make the results more nearly comparable. It will be noted that the volatile swelling liquids generally reduce the tensile strength for a given amount of swelling much more than the nonvolatile oils. Low-molecular-weight plasticizers usually have a greater softening effect than those of high molecular weight. Fraser (6) showed k h a t volatile materials soon evaporate from Neoprene GN, the tensile strength reaching that of the unswollen composition. The tensile strength and elongation are not adversely affected by moderate swelling, but as swelling approaches 100% the values show an appreciable decrease.


The aniline point of hydrocarbon oils and solvents is a satisfactory index of the swelling of oil resistant synthetic rubber compositions. The logarithm of the per cent volume increase varies inversely with the aniline point up to 100% swelling. The slope of the swelling curve is apparently characteristic of each synthetic rubber and is not affected by loading, temperature or degree of cure. Slight swelling does not greatly decrease the tensile strength, but above 100% swelling the strength is greatly reduced. ACKNOWLEDGMENT


(1) Am. SOC.for Testing Materials, Standards on Rubber Products, p. 135,Designation D471-43T (Feb., 1944). (2)Ibid., Supplement, Part 111,p. 267,Designation 611-41 (1941). (3) Carman, F. H., Powers, P. O., and Robinson, H. A., IND.ENQ. CZ~EM., 32,1069 (1940). (4) Fraser, D.F., Zbid., 32, 320 (1940). (5) Ibid., 34, 1298 (1942). (6) Ibid., 35,948 (1943). (7) Fraser, D.F., Rubber C h m . Tech., 14,204(1941). ( 8 ) Geddes, B. W., Wilcox, L. Z., and McArdle, E. H., IND.ENG. CHEM,,ANAL.ED.,15,487(1943). (9) Hanson, A. C.,IND.ENQ.CHEM.,34, 1326 (1942). (10) Juve, A. E.,and Garvey, Jr., B. S., Ibid.,34,1316 (1942). (11) Son. of Automotive Engrs., Aeronautical Materials, Gpec. AM 322311 (1943).

HUGH WlNN AND J. REID SHELTON Case School of A p p l i e d Science, Cleveland, O h i o

HE fundamental nature of fatigue failure in rubber during flexing has been widely studied and is known to be complex. Among the more important factors involved (aside from variations in compounding) are state of cure, temperature of test, and oxidation. The object of this paper is to review and extend these studies as they apply to GR-S. Several investigators ( I , 6,6, 10, 11) have studied the effect of time of cure upon the flex cracking resistance of rubber. Busse (6) states that, as the degree of cure was advanced, the flex resistance reached a maximum and then decreased. Similar studies on GR-S (3,4,8,10) show that undercures flex better than either normal cures or overcures. Prettyman (IO), using a different type of machine, showed a maximum near the optimum cure. Cadwell and co-workers (8)and Rainier and Gerke (11) report that the rate of cracking of rubber increased with the temperature of flexing. Similar results have been demonstrated with GR-Sby Breckley (3)and by Carlton and Reinbold (4). Neal and Northam (9)flexed rubber in air, oxygen, and nitrogen on a Du Pont type flexing machine. They report no difference between the flexing behavior in air and oxygen. I n nitrogen, however, an uninhibited stock flexed five times longer


than in air before any failure was apparent, The presence of an, antioxidant did not change the behavior in nitrogen but doubled the normal flex life in air. These workers have concluded that the failure of rubber on flexing is due to oxidation rather than mechanical fatigue. No comparable work on GR-S has yet been reported. The present authors (1.9)previously showed that oxygen plays an important part in the hardening of GR-S vulcanizates during accelerated aging tests in the 100' C. air oven. Since high temperatures are developed during flexing which might well result in a similar hardening by oxidation, it seemed desirable to determine whether oxygen plays a part in the flex cracking of GR-S. For this purpose a typical tread stock was selected and flexed in air and in nitrogen which contained various small amounts of oxygen. The effect of state of cure and testing temperatures upon the resistance of GR-S to flex cracking was also included in the study. EXPERIMENTAL PROCEDURE

The tread type stock employed in this study was compounded according to the following formula from GR-S made according to