Improved oil resistance of natural rubber.

Improved oil resistance of natural rubber

Natural rubber (NR) is known to have poor oil resistance. However, recent efforts to chemically modify NR, through controlled epoxidation of the double bond, have produced a new rubber called the epoxidized natural rubber (ENR) as shown by the reaction scheme below: [Figure Omitted]

As the fraction of the epoxide group increases, the swelling resistance in oil increases[1]. The oil resistance of ENR is comparable to some of the specialty synthetic elastomers[2]. In addition, other changes in physical properties were reported, such as decreases in air permeability, rebound resilience and increase in hysteresis and wet traction[3-6].

This article discusses the results of a study on oil resistance of three types of rubber viz. ENR-25, ENR-50 and nitrile butadiene rubber (NBR) in three types of oil, namely, ASTM No. 1, ASTM No. 3 and a commercial grade engine oil GTX. The investigation was carried out at 70 [degrees] C, and the time of immersion was varied from 50 to 250 hours.

Experimental

Materials The following are the rubbers and oils used.

* ENR-25 (25 mole % epoxidation)

* ENR-50 (50 mole % epoxidation)

* NBR (Krynac 802, low ACN nitrile rubber 27% bound acrylonitrile)

* ASTM No. 1 (aniline point [degrees] C - 124 [plus or minus] 1.0)

* ASTM No. 3 (aniline point [degrees] C - 70 [plus or minus] 1.0)

* Engine oil GTX (aniline point [degrees] C - 110 [plus or minus] 1.0)

The ENRs were prepared in the Rubber Research Institute of Malaysia (RRIM) pilot plant and NBR was a commercial sample. The aniline points of ASTM Oil No. 1 and No. 3 were obtained from the Annual Book of ASTM Standards[7]. The aniline point of the engine oil GTX was determined in accordance to ASTM method D-611. The aniline point provides an estimate of the aromatic hydrocarbon content in the mixtures (oil). It also characterizes the swelling action of oils. In general, the lower the aniline point the more severe the swelling action.

Compounding The formulations of the mixes are given in table 1. The mixes were based on a semi-EV sulphur vulcanization system, filled with carbon black N220 as a reinforcement filler. The base (sodium carbonate) was added to generate consistent scorch safety and to prevent poor aging characteristics due to the presence of sulphur acids produced by the oxidation process[8]. The acids attack the epoxide groups ultimately causing crosslink formation, which results in a substantial increase in modulus, reductions in tensile strength and elongation at break.

The mixing was carried out on a two-roll mill with an initial temperature of approximately 50 [degrees] C. The rubber was first masticated for approximately three minutes before sodium carbonate was added. This was followed by other ingredients and filler. The curatives were added at the final stage of mixing, before sheeting. The final temperature was approximately between 60 to 65 [degrees] C. The cure characteristics of the compound were determined with a Monsanto Rheometer at 160 [degrees] C. The rubber was cured at 160 [degrees] C to [t.sub.90].

Swelling and testing Swelling was carried out in accordance with BS903: Part A16, 1971. The samples were immersed in oil, in a 200 ml test tube carrying a sample holder which was then placed in an oil bath heated to 70 [degrees] C. The immersion periods were varied from 50 to 250 hours. At the end of the immersion period the percentage of volume swell and the physical properties were determined in accordance to ISO standards - tensile strength (ISO 37), hardness (ISO 48), trouser tear strength (ISO 34) and compression set (ISO 815).

Results and discussion

Swelling The oil resistance of NR is shown to improve through the epoxidation process. The oil resistance improved as the level of epoxidation increased. The determination of swelling was carried out over every 50 hour interval for 250 hours. Figures 1, 2 and 3 show the percentage of volume increase of the three rubbers in the three types of oil. It was clearly observed that there was no significant difference between ENR-50 and NBR. Both the rubbers showed less than 2 percent in volume increase after 250 hours immersion period. For ENR-25, the percentage of volume swell increased with the increase in immersion period, and appeared to stabilize after 200 hours of immersion.

A significant difference was observed between ENR-50 and NBR in ASTM No. 3 as shown in figure 2. ENR-50 showed a higher volume swell, approximately double that of NBR. The NBR stabilized after 50 hours, and ENR showed a sharp increase for the first 50 hours, thereafter the increase was gradual. Minimum percentages of volume swell of the rubber were 123.3, 38.8 and 18.8 for ENR-25, ENR-50 and NBR, respectively. In the commercial grade engine oil GTX, the percentage of volume swell of ENR-50 was just slightly higher than NBR. The volume was unchanged after 150 hours of immersion and the maximum percentage of volume increases were 6.2 and 2.5, for ENR-50 and NBR respectively. ENR-25 was significantly inferior compared to that of the two rubbers with respect to swelling.

From this study it was shown that oils of low aniline point have greater swelling action, regardless of the rubber. It was observed that the swelling resistance was of the following order, NBR, ENR-50 and ENR-25, with ENR-25 being least resistant. However for oils of higher aniline point, the differences between NBR and ENR-50 were marginal.

Tensile properties The physical properties of NBR, ENR-25 and ENR-50 vulcanizates are given in table 2. The initial tensile strength and elongation at break for both ENRs were much higher as compared to NBR however NBR gave higher modulus as compared to ENR-50 and ENR-25.

Figures 4, 5 and 6 show the percentage retention of tensile strength of the three rubbers in different types of oil. The tensile strength of NBR and ENR-50 were found to increase over the immersion period, which was illustrated by the retention exceeding 100%, in ASTM No. 1 shown in figure 4. The increase in tensile strength of ENR-50 appeared to be higher when compared to NBR, however at 250 hours of immersion the tensile strength of NBR was slightly higher then ENR-50.

Figure 5 shows the change in tensile strength in ASTM No. 3. Although the increase in % of volume swell of ENR-50 was higher than NBR as shown in figure 2, the retention in tensile strength remained superior. The retention of tensile strength for both rubbers decreased with immersion time stabilizing at approximately 82% and 68% for ENR-50 and NBR, respectively. For ENR-25, the retention of tensile strength decreased to approximately 30% as the immersion started, followed by a gradual decrease with the increase in immersion time.

A slightly different result was observed for engine oil GTX; the retention of tensile strength for NBR was slightly higher compared to ENR-50, as shown in figure 6. The decrease in tensile strength was gradual for both rubbers and similar in characteristic to that of ASTM No. 3. The difference in retention between NBR and ENR-50 was within 10%. For ENR-25, a sharp decrease in tensile strength was observed for the first 100 hours of immersion and stabilized at about 20% retention thereafter.

Figure 7 illustrated the retention in elongation at break of the rubbers after immersion in ASTM No. 1. The ENR-50 exhibited higher retention in elongation at break compared to NBR and ENR-25. The decrease was approximately 7% as compared to approximately 22% for ENR-25 and NBR; even up to 250 hours of immersion. Figure 8 shows the retention of elongation at break after immersion in ASTM No. 3. The ENR-50 performed better than NBR, with ENR-50 and NBR showing the lowest retention of 87% and 69%, respectively. The elongation at break of ENR-25 decreased to below 50% at 50 hours of immersion, and was virtually unchanged above 50 hours of immersion.

The result of % retention in elongation at break in the engine oil GTX is shown in figure 9. The plot is approximately similar to that of ASTM No. 3, except for ENR-25 which showed much poorer retention of about 23%. The modulus of all types of rubber increased in ASTM No. 1 and GTX. The increase in modulus for ENR-50 was the least compared to ENR-25 and NBR; the highest increase was for ENR-25 in all types of oil. The retention of modulus of ENR-50 and NBR were below 100% in ASTM No. 3.

Tear strength The tear strength of all the rubbers decreased significantly after immersion in all types of oil. In ASTM No. 1 the tear strength retention of ENR-50 was as low as 55% at 50 hours of immersion. Above 50 hours, the tear strength increased again; this could be due to experimental errors. Above 150 hours the tear strength of ENR-50 was comparable to NBR. The tear retention of ENR-25 was better than ENR-50 and NBR up to 150 hours of immersion; above 200 hours ENR-50 and NBR were better compared to ENR-25. In ASTM No. 3 the results were systematic; initially all the rubbers have a large decrease in retention, followed by insignificant changes after 50 hours of immersion. The retention of tear strength was of the following order: NBR [is greater than] ENR-50 [is greater than] ENR-25. Tear strength retention in GTX results were scattered, and therefore inconclusive.

Hardness It was observed that the change in hardness of ENR-50 after immersion in ASTM No. 1 oil was insignificant. The increase in hardness of NBR was, however, significant, amounting to approximately 10%. For ENR-25 there was a decrease in hardness by about 10%. The retention in hardness for ENR-25 was lower in ASTM No. 3 oil. The changes in hardness of all the three rubbers in engine oil GTX were approximately similar to that in ASTM No. 1 oil. There was a significant change for NBR but not for ENR-50. The overall results show that the hardness of ENR rubbers were significantly reduced after immersion in ASTM No. 3.

Compression set Generally, the percentage retention of compression sets were low for all the rubbers, after immersion in oils, with ENR-25 giving the poorest set. The retention in compression set of NBR and ENR-50 were approximately similar in all types of oil.

Conclusion

The modification of NR by epoxidation has improved the oil resistance of NR, and increasing the level of epoxidation also improves the oil resistance of the rubber. This significant change in the characteristics of NR was brought about by the presence of epoxy group in the main chain of the elastomer. The percentage of volume swell after immersion of ENR-50 and NBR in ASTM No. 1 and engine oil GTX were small and within the acceptable limits of most specifications. The difference between NBR and ENR-50 is significant in ASTM No. 3, but this is not a drawback for the ENR-50 which showed better and comparable physical properties to NBR, despite higher swelling.

ENR-25 appeared to be less oil resistant compared to NBR, however it may be suitable for applications which do not involve continuous immersion in oil.

The most distinct feature of ENR-50 was good retention in physical properties, particularly in ASTM No. 1, GTX and ASTM No. 3, such as in the tensile strength and elongation at break which were better than NBR. Other advantages of ENR-50 include relatively lower changes of modulus and hardness compared to NBR. This indicates that the crosslinking system of the ENR-50 vulcanizate is relatively more stable. As for the compression set, NBR and ENR-50 were comparable. [Table 1 and 2 Omitted] [Figure 1 to 9 Omitted]

References [1]Davies, C.K.L., Wolfe, S.W., Gelling, I.R. and Thomas, A.G. (1983), Polymer 24,107. [2]Gelling I.R., NR Technol., Vol. 16, part I, 1, 1985. [3]Baker, C.S.L., Gelling, I.R. and Newell, R., Rubb. Chem. Technol., 58, (1), 67, 1985. [4]Gelling, I.R., NR Technol., vol. 18, part 2, 1987. [5]Baker, C.S.L., Gelling, I.R. and Azemi Bin Samsuri, J. Nat. Rubb. Res., Vol. 1, No. 2, 1986. [6]Abu Amu, Sidek Dulngali and Gelling, I.R., Rubb. Plas., 1152, 1986. [7]Annual Book of ASTM Standards, Section 9, Vol. 09.01, Designation D471-79, pg. 118, 1984. [8]Gelling, I.R. and Morrison, N.J., Rubb. Chem. Technol., 85, (2), 1985.