The Impact of Plastic pH on Silicone Elastomer Compression Set 2021-01-0355

Sealing applications in electrified vehicle powertrains present a unique set of boundary conditions when contrasted with typical transmission or internal combustion engine applications, including changes in fluidic exposure, operating pressure, temperature profiles, etc. This novel powertrain environment opens the gasket material spectrum to elastomers uncommon in standard powertrain joints, which allows for more optimized, higher-value sealing solutions. However, this also introduces new risks, including the risk of excessive compression set in silicone elastomers due to acidity in adjacent plastics (which can result from shifting to non-halogenated flame retardants from halogenated flame retardants). To understand this phenomenon, compression set testing was conducted with plastic resins ranging from pH = 3.4 to pH = 7.3 and three high-consistency rubber (HCR) silicone elastomers. For each silicone, compression set was plotted linearly against concentration of hydrogen ions ([H+]) in the plastic and plotted exponentially against plastic pH. This produced a regression model relating compression set in the elastomer to acidity of the adjacent plastic. Then, a threshold pH value - after which the plastic begins to significantly increase compression set in the silicone - was determined by overlaying silicone-only compression set results with the silicone + plastic regression model. The resulting threshold value given the associated conditioning was a pH of 4.5 for all three silicone samples tested, indicating that a plastic resin accelerates compression set in the neighboring silicone if the plastic has a pH ≤ 4.5. Studies into the impact of (1) plastic sample form factor (raw material pellets, colorized material pellets, or molded plastic sheets) and (2) time on plastic pH were evaluated; both showed no statistical impact on pH measurements. Overall, this effect is believed to be due to acid-induced cleavage of the siloxane bond, which causes depolymerization in the silicone by breaking the backbone chain, thereby resulting in gasket compression set.