The peroxide curable Vamac^® AEM copolymers made by DuPont offer improved productivity since there is no need to post-cure when crosslinking with organic peroxides. In a recent RubberWorld[i] technical article, Vamac DP and Vamac DHC copolymers were evaluated using the standard grades of dicumyl peroxide and di(t-butylperoxy)diisopropylbenzene. In this paper, we provide data showing how the new DC40P-SP2 and F40P-SP2 (Enhanced Scorch Protected) peroxide grades can provide improved productivity and cost-savings when compounding and crosslinking the DP and DHC copolymers.

AEM is used to produce automotive transmission seals, coolant and power steering hose, wire jacketing (e.g., ignition harnesses), dust boots and in various dynamic applications, e.g., motor mounts and vibration dampers[ii]. Producing these precision components requires complete mold filling prior to any crosslinking, which can be difficult when curing at higher temperatures needed for optimum productivity. Our data in Table 1, shows that DC40P-SP2 and F40P-SP2 provide a significant ~30% to ~50% increase in scorch time when curing at temperatures of 180°C and 190°C. These longer scorch times allow complete mold filling prior to any significant increase in viscosity. Thus, use of the SP2 peroxide technology will help eliminate hot-tear, poor weld lines, scrap, and uneven/non-uniform curing, that hurt physical properties, e.g., % compression set.

The SP2 peroxides provide significant increases in scorch time without increasing cure time to provide optimum productivity. The data in Table 1 shows how the cure temperatures of the standard peroxides were reduced by 8°-10°C with the goal of obtaining the same ts2 scorch time produced using the SP2 peroxides at the original higher temperature. Lowering the cure temperature for the standard peroxides did provide the same increased scorch time as the SP2 peroxides, but significantly slowed the crosslinking reaction, i.e., tc90 cure time was nearly doubled, thus adversely affecting productivity. Clearly, the SP2 peroxides provide a wider processing window to reduce scrap and improve molded part quality while maintaining good productivity.  Thus SP2 peroxides lower the overall production cost of crosslinking AEM copolymers.

The M_L (minimum torque) is equivalent to the rubber viscosity at the reported RPA test temperature. As shown in Table 1, the SP2 peroxides provide a 20+% lower M_L on average compared to the standard peroxide when curing DP and DHC copolymers. Thus the SP2 peroxides not only provide a longer scorch time, but also a significantly lower elastomer viscosity during molding to faithfully fill the mold thereby producing highly engineered automotive components with more consistent physical dimensions, and reducing the amount of “out-of-spec” scrap. The lower elastomer viscosity is due to the enhanced scorch protection provided by the SP2 technology, which prevents chain extension and/or gelling of the elastomer during mold filling.

Compounding the DP and DHC copolymers with DC40P-SP2 or F40P-SP2 provided nearly a three-fold increase in ts2 scorch time compared to the standard DC40 and F40 peroxides as shown in Table 2 and Figure 1. The SP2 peroxide technology permits a 10°C increase in AEM compounding drop-temperature with no threat of scorch versus the standard peroxide. Thus the SP2 peroxide formulation is very useful when going from two-pass to one-pass mixing operations or for increasing mixing speed to increase mixing capacity, thereby reducing the cost of making the compounded AEM.

Figure 1
ts2 Scorch Time @130°-145°C: Vamac DHC using DC40P-SP2 or F40P-SP2