The role of surface coatings in recyclability of plastic products

One key strategy for achieving industrial sustainable development is the transition towards a circular economy, which includes the establishment of circular material flows. Recycling of products at their end-of-life is one option in this context. However, mechanical recycling requires the separation of materials, while technical products commonly consist of a mix of different materials and compounds. Additionally, surface coatings are often applied for the purpose of protection from external influences, as a design feature or for functionalization. Currently, it is often necessary to remove this coating in order to recycle coated products and to obtain unmixed materials. However, this is often not carried out due to cost considerations, or the coating materials cause contamination and accumulate in recycled materials over time. Consequently, these products are treated as waste at end-of-life and can only be used for energy recovery. In order to facilitate circular material flows, it is necessary to consider the role of surface coatings in conjunction with the base material. To demonstrate and investigate the potential impact of surface coatings on the mechanical properties of plastic materials, an experimental procedure is developed and executed for the base material polypropylene (PP) in combination with a PP-foil coated with a hydrophobic layer using hexamethyldisiloxane (HMDSO) as precursor in an atmospheric pressure plasma. The results show a change in mechanical properties and a higher variability induced by the surface coating in the simulation of up to ten closed-loop mechanical recycling flows. Based on these findings, this study presents an overview on the role of surface coatings in recyclability of plastic products and their impact on the transition towards circular economy systems. Subsequently, current and future challenges are identified and mitigation strategies formulated.