Metallic coagents for curing elastomers.

The use of coagents in conjunction with peroxides to cure elastomers has been common practice in the rubber industry for many years. Coagents are typically multifunctional monomers that are highly reactive in the presence of free radicals and readily graft to elastomer chains to form a complex crosslink network. With peroxide-cured elastomers they not only increase the crosslinking efficiency of the vulcanization process, but increase the crosslink density as well. The increase in crosslink density is directly related to the coagent concentration and has a major impact on the mechanical and physical properties of the cured elastomer. Some of the most common coagents in use today are esters of methacrylic acid (refs. 1 and 2). Trimethylolpropane trimethacrylate (TMPT-MA) and 1,3 butyleneglycol dimethacrylate (BGDMA) are typical examples of the methacrylate ester class of coagents.

Recently, there has been interest in using zinc dimethacrylate as a coagent to enhance the properties of hydrogenated nitrile rubber (HNBR). In research conducted by the Army to develop improved rubbers for tank treads, it was found that zinc dimethacrylate greatly improves die tear strength, abrasion resistance and high temperature performance of HNBR (ref. 3). Also, Zeon Chemicals has reported that curing HNBR with a mixture of zinc oxide and methacrylic acid, which are precursors to zinc dimethacrylate, results in a rubber with very high tensile strength and excellent abrasion resistance (refs. 4-6).

Metallic salts of acrylic acid and methacrylic acid have also been used for many years to crosslink polybutadiene in forming rubber cores for two piece golf balls (refs. 7-9). Crosslinking polybutadiene with metals salts of either acrylic acid or methacrylic acid results in very hard rubber with high compression and high coefficient of restitution. Zinc salts have proved to be the most effective of the metal salts and are used extensively in the manufacture of golf ball cores today.

We have found that the zinc salts of acrylic acid and methacrylic acid behave as coagents in much the same way as their ester counterparts do as described above. They also are highly reactive in the presence of free radicals and readily react to form crosslinks with both saturated and unsaturated elastomers. However, they differ from conventional ester coagents in that they impart unusually high hardness and tensile properties to the elastomer, while retaining the rubber-like characteristics of the elastomer. The work described in this article shows the utility of zinc diacrylate and zinc dimethacrylate as coagents for curing EPDM and nitrile rubbers.

Experimental

Materials

A masterbatch containing 100 phr Hycar 1042, 65 phr N365 carbon black, 15 phr doctyl phthalate, 5 phr zinc oxide and 1 phr stearic acid was used in all experiments pertaining to nitrile rubbers. Hycar 1042 was supplied by Zeon Chemicals and is a general purpose nitrile rubber with good processing characteristics and high physical properties.

A masterbatch containing 100 phr Nordel 1040, 100 phr N762 carbon black, 50 phr Sunpar 2280, 5 phr zinc oxide and 1 phr stearic acid was used in all experiments related to EPDM rubbers. Nordel 1040 was obtained from Du Pont.

The zinc salts of acrylic acid and methacrylic acid were supplied by Sartomer. The scorch retarded versions of zinc diacrylate and zinc dimethacrylate are marketed by Sartomer under the tradenames of Saret 633 and Saret 634, respectively.

Formulations

The rubbers were compounded according to the formulations given in tables 1 and 2 using a laboratory six-inch two roll mill. The masterbatches described above were masticated on a two roll mill until a flux was created at the nip of the rollers. At this point, the Agerite Resin D, dicumyl peroxide and coagent were slowly added to the flux roll. The band was then sheeted and folded and then rebanded for mixing. This process was repeated six times to ensure thorough mixing. The coagent concentration was varied from 0 to 20 phr, unless otherwise noted. The compounded rubbers were then cured in a chelsea mold for twenty minutes at 160 [degrees] C.

Measurement

Physical tests were conducted for all molded compounds after molding and again after heat aging at 1000 [degrees] C for 70 hours. Tensile strength, modulus and elongation were determined according to ASTM method D-412 using a Thwing Albert model 1451-42 tensile tester at a crosshead speed of 20 in./min. Shore A hardness tests were determined for samples after molding using a hand-held Shore A durometer.

Cure characteristics which include scorch time, cure rate and torque values were measured over a twenty minute period at 160 [degrees] C using a Monsanto oscillating-disk rheometer according to ASTM method D-1084.

Tear strength was determined according to ASTM method D-624 using a die C specimen.

Results and discussion

Physical properties

Zinc diacrylate (ZDA) and zinc dimethacrylate (ZDMA) are the difunctional salts of their respective acids. They are free-flowing solids that can be readily compounded with a variety of elastomers. They exhibit excellent high temperature stability as well as outstanding fluid resistance. They derive many of their unique properties from ionic interactions between the zinc cation and the carboxylate anions. Metal cations, particularly zinc, are known to increase the tensile properties of metal-neutralized ionomers by forming ionic crosslinks (ref. 10). The same ionic crosslink mechanism is believed to occur with elastomers that have been cured with ZDA and ZDMA.

Scorch safety

As noted previously, the value of using ZDA and ZDMA with peroxide curing is to enhance the mechanical and physical properties of the cured elastomer. However, while ultimately curing to high durometer and high tensile elastomers, these coagents also accelerate the curing process to such an extent that the scorch time is reduced. For example, the rheometer curves in figure 1 show the cure characteristics of ZDA with peroxide relative to peroxide alone for nitrile rubber. It can be seen that the scorch time is significantly less for ZDA with peroxide than for peroxide alone. This reduction in scorch time could lead to scorchy behavior in applications where injection molding is required. Also, the cure rate is significantly faster with the ZDA/peroxide combination which allows for more rapid processing where scorch is not a concern. It should also be noted that the maximum torque is much higher with ZDA present, which indicates a greater crosslink density and therefore higher mechanical properties.

To overcome the scorchy behavior described above, scorch retarded versions of both ZDA and ZDMA were developed and are marketed under the tradenames of Saret 633 and Saret 634, respectively (ref. 11). Both products contain non-nitroso scorch retarders, which are safe and effective for rubber compounding. The effect of scorch retarded ZDA relative to ZDA for the same peroxide cured nitrile rubber is shown in figure 2. The scorch time of ZDA with the retarder has increased significantly and approaches the scorch time obtained with the peroxide alone. Even though the scorch time has increased with the ZDA/retarder coagent, the cure rate and the maximum torque remain essentially unchanged.

The same behavior occurs with the scorch retarded version of ZDMA and is illustrated in table 3 with EPDM rubber. In this example, a 40% increase in [TS.sub.1] is obtained with the scorch retarded version of ZDMA without any significant loss in maximum torque (MHF). Thus, with the Saret coagents it is possible to significantly increase the crosslink density and the cure rate while maintaining good scorch safety.

Tensile properties

The increase in tensile properties obtained with coagents in peroxide curing is always accompanied by a corresponding decrease in elongation. This behavior is noted for all coagents in commercial use today. One important property of metallic coagents is that they increase the tensile properties of elastomers as other coagents do, but with much greater retention of elongation. This characteristic of ZDA and ZDMA is demonstrated in the following examples with EPDM and nitrile rubbers.

The scorch retarded versions of ZDA and ZDMA were used in all examples. Trimethylolpropane trimethacrylate (TMPTMA) is representative of methacrylate coagents and was included for comparison purposes. The graph in figure 3 shows the effect of coagent concentration on the hardness of peroxide cured EPDM. The data show that the hardness increased and elongation decreased in all cases as the coagent concentration was increased from 0 to 20 phr. However, the data also show that the elongation decreased at a slower rate for ZDA and ZDMA than for TMPTMA. Also, both ZDMA and ZDA gave significantly higher hardness values than TMPTMA, particularly at high coagent concentrations. ZDMA gave the highest hardness of any of the coagents at 700% elongation, while ZDA gave the highest hardness value of 80 Shore A at 400% elongation.

Figure 4 shows the relative effect of ZDA and ZDMA coagents versus TMPTMA on the modulus and elongation of peroxide cure EPDM. In all cases, as the coagent concentration was increased from 0 to 20 phr the modulus increased and the elongation decreased. However, both ZDA and ZDMA gave higher modulus values than TMPTMA for a specific elongation value. Although ZDMA gave a slightly better balance of modulus and elongation at low modulus values, ZDA gave the highest modulus for a given coagent concentration and the highest modulus overall.

Coagents normally have very little effect on the tensile strength of peroxide cured elastomers. However, we have found that with ZDA and ZDMA as coagents, the tensile strength of both EPDM and nitrile rubbers can be increased dramatically. This effect is shown for nitrile rubber in figure 5. In this graph, the tensile strength is plotted as a function of coagent concentration for ZDA and ZDMA relative to TMPTMA. ZDA gave the highest tensile strength of any of the coagents at each concentration. It is interesting to note that, with ZDA, the increase in tensile strength is linear, suggesting that higher tensile values could be obtained at higher coagent concentrations. The tensile strength also increased with ZDMA, but not as dramatically. And, as expected, with TMPTMA the tensile strength remained essentially unchanged.

Figure 6 shows that ZDA and ZDMA have a similar effect on the tensile strength of EPDM rubber. Again, ZDA resulted in the highest tensile strength at each concentration of coagent. ZDMA was more effective with EPDM than with nitrile rubber, however, and closely followed ZDA in performance. With TMPTMA, only a minimal change in tensile strength was observed. In contrast to the nitrile data in figure 5, the curves for EPDM are not linear and actually show maximum tensile values at approximately 10 phr.

Tear strength

The effect of ZDA and ZDMA on the tear strength of EPDM rubber is graphically displayed in figure 7. With both ZDA and ZDMA, the tear strength increased as the coagent concentration was increased, reaching maximum strength at concentrations near 10 to 15 phr. This is in contrast to the behavior of conventional coagents, where tear strength normally declines as the crosslink density is increased. An example of this behavior is shown with TMPTMA in figure 7. In this case, the tear strength decreased sharply with 5 phr TMPTMA and then gradually levelled-off at about 10 phr coagent. Similar behavior was noted for ZDMA and TMPTMA with nitrile rubber. ZDA, however, had no effect on the tear strength of nitrile rubber.

Metal adhesion

Both the ZDA and ZDMA dramatically improve the shear adhesion of cured EPDM to metal surfaces. The improved adhesive strength occurred with a variety of untreated metal substrates such as steel, stainless steel, brass, zinc and aluminum as well as nylon fiber. ZDA exhibits a higher lap shear strength than ZDMA which in turn is superior to peroxide alone. The adhesive strength is dependent on coagent concentration. However, very strong lap shear strengths are obtainable even at low concentrations with 1 phr coagent. Both ZDA and ZDMA are superior to conventional coagents such as TMPTMA.

Conclusions

Based on the data reported in this article, it is evident that the zinc salts of acrylic acid and methacrylic acid are effective coagents for the peroxide curing of EPDM and nitrile rubbers. They appear to be unique among coagents in their ability to maintain high elongation while increasing the hardness and tensile properties of these elastomers. Zinc diacrylate was found to be the most effective in providing high hardness, high tensile properties and adhesion to metal substrates. Zinc dimethacrylate gave a slightly better balance of properties by providing the highest elongation for a given hardness or modulus value.

References

[1.] F.R. Eirich "Science and technology of rubber, Academic Press, Inc. 1978). [2.] J.T. Howath, Rubber World, 69, August 1963). [3.] P. Touchet, et. el, U.S. patent 4,843,114, June 27 (1989). [4.] R.C. Klingender, M. Oyama and Y. Saito, Rubber World, 26, June 1990). [5.] R.C. Klingender, M. Oyama and Y. Saito, 135th Rubber Division Meeting, American Chemical Society, Mexico City, Mexico, May 9-12 1989). [6.] Y. Saito, A. Fujino and A. Ikeda, SAE SP787, "Elastomer developments: Materials, applications, processing and performance," 89D359 (1989). [7.] F.S. Martin, et.el., U.S. patent 4,266,772, May 12 (1988). [8.] C.M. Roland, U.S. patent 4,720,526, Jan. 19 (1988). [9.] R. P. Melter, U. S. patent 4,726,590 Feb. 23 (1988). [10.] H. Xie and Y. Feng, Polymer, 29, 1216, July (1988).