Development and optimisation of adhesion mechanisms between rubber and thermoplast materials for 2K injection moulding

Successful two-component (2K) injection moulding of multi-material products requires an efficient production process and an optimal material combination. Specifically for thermoset rubbers with thermoplastics, this is challenging as during injection moulding these materials have opposite temperature requirements. Therefore, a novel process with a mould containing thermally separated heat cavities was developed by Bex et al. [1]. Available research mainly concerned process optimisation aspects, but little research has focused on material optimisation in such two-component injection moulding. Furthermore, knowledge concerning the adhesion mechanism is scarce. Therefore, a study of the influence of material composition and the accompanying adhesion mechanisms was deemed necessary to optimise the process and to improve the economic feasibility.

Ethylene-propylene-diene rubber (EPDM) adhered to thermoplastics is ideal for two-component sealing applications thanks to its resistance to compression, chemical environment and high temperatures. Therefore, the EPDM composition was studied to enhance the adhesion with thermoplastics. The rubber curing system appears to enable adhesion mechanism like interdiffusion and/or co-vulcanisation. The most efficient curing system composition is found to depend on the rubber-thermoplastic material combination. Furthermore, adhesion benefits from a rubber molecular structure with high accessibility to reaction sites and a high number of rubber monomer units which are also present in the molecular structure of the thermoplastic. The adhesion can be improved as well with higher contents of paraffinic oil enhancing mobility of molecular chains at the interface. The choice of filler mainly depends on the desired rubber product properties.

Adhesion between a thermoset rubber and a thermoplastic is a complex phenomenon consisting of multiple adhesion mechanisms induced during the injection moulding process. For adhesion to occur at a polymer interface, intimate contact is a prerequisite. Therefore, a more sophisticated contact angle methodology, closer-to-processing-conditions, is presented to evaluate wettability. The developed technique indicated that wetting does not directly control the amount of adhesion, possibly due to the high pressures used during 2K injection moulding forcing contact between the rubber and thermoplastic. Furthermore, the occurrence of co-vulcanisation can be predicted with a newly developed reactive wetting methodology. During co-vulcanisation several reactions may occur between the thermoplastic and EPDM. Therefore, possible chemical reaction mechanisms are clarified and discussed. Finally, interfacial characterisation suggests that the interdiffusion width is limited to maximum a few micrometres. Presumably, entanglements contribute highly to a strong adhesion.

Eventually, when formulating the rubber composition, adhesion needs to be optimised within product property requirements and minimal cycle time. Therefore, material selection guidelines in terms of radar charts are provided. Additionally, a detailed materials selection grade map is provided for EPDM with polypropylene as this material combination has the highest industrial relevance, e.g. sealing applications in electrical vehicles. This selection map was validated with an industrial case study emphasising the required synergy between product properties and optimal adhesion. Furthermore, even though good adhesion complicates recyclability, end-of-life potential was found by recycling low concentrations of 2K granulates in thermoplastic vulcanisates.