Factors influencing curing, physical, electrical, and processing properties of automotive EPDM compounds – Effect of different carbon black types
The replacement of steel car doors by Mg/Al doors for light-weighting results in electrochemical corrosion of the EPDM sealing profiles. This can be remedied, if the volume resistivity of EPDM profiles is increased by a reduction of carbon black content to below its percolation threshold. In this paper, the influence of different carbon black types onto the overall electrical and mechanical properties of the final compounds is reported.
V. Thakur, S. Wu
The stricter EU regulations limit the carbon emission (CO2) by a passenger car to 78 g CO2/km by 2025. Lightweighting and partial or full electrification are the two major trends in the automotive industry to reduce emissions. In terms of car light-weighting, it can be done by either reducing the structural weight of the car body or by reducing the weight of the car body parts (e.g. weather-strip).
One of the key light-weighting trends driving the automotive industry is the use of light-weight metals (Mg/Al) to replace steel doors in the car body. The change from steel to Mg/Al doors results in significant weight savings in the car body, therefore, resulting in lower carbon emissions and higher mileage. However, the change from steel to Al/Mg has a significant impact on the car body sealing.
The traditional EPDM WS profiles have a CB filler loading of up to 130 phr. This relatively high CB loading enables a galvanic contact between the car body (steel) and the car door (Mg/Al) through the conductive CB particles and thereby, allowing electrochemical corrosion of the EPDM profile and partly of the metal surface. This phenomenon necessitates the increase of volume resistivity of the EPDM profile in order to avoid the electrochemical corrosion to take place. This increase in the volume resistivity can be readily achieved with reduction of the CB content to below its percolation threshold. Since CB makes about one third of the traditional EPDM compounds, the lowering of CB content has a significant impact on the curing, mechanical and processing properties of the compound. The CB reduction results in lowering of physical properties due to the loss of filler reinforcement as well as due to increase in non-reinforcing insulating white fillers.
Compounds with high hardness (85 – 95 Shore A) are difficult to formulate in the absence of enough CB content. The lowering of filler-filler breakdown occurring during the profile extrusion process also results in poor shear thinning properties of the EPDM compounds, and hence, it is difficult to produce complex shapes. The insulating compounds with lower CB and higher white filler content also show porosity and curing gradient across the profile. Therefore, it is crucial to understand the role of CB (type and content) on the curing, physical, electrical, and processing properties of the EPDM compounds.
This report is focused to understand the influence of CB type and content onto compound properties and curing properties besides looking at only electrical properties. Different CBs were studied above and below their percolation threshold along with same white filler type. The reduction in carbon black loading and consequent increase in white filler loading results in lower Mooney viscosity, lower elasticity, lower tensile strength, lower modulus and hardness. Furthermore the shear thinning is less and compression set at 100 °C is high. Overall, the CBs with lower surface area and lower structure performed the best. Some are able to pass the VR requirement of Al doors even at 90 phr CB loading.
V. Thakur, S. Wu, RFP Rubber Fibres Plastics, 03 2020, 124-131.