Enhanced Interfacial Adhesion of Polystyrene Bead Foams by Microwave Sintering for Microplastics Reduction

Due to weak adhesion among polystyrene (PS) beads, PS foam products easily break during the transport, usage, and recycle processes and thus generate massive PS microplastics in the environment. For the traditional steam-chest molding technique, the elevated temperature and lengthy steaming time are always required to ensure high-quality adhesion, while the weak physical links between beads inevitably appear. Herein, a novel, clean, and energy-saving bead foaming technology assisted by microwave heating was proposed to simultaneously realize the favorable expansion and sufficient welding of expandable PS (EPS) beads in one single step. Water, one of the most promising dielectric materials for microwave absorption, was coated on EPS bead surfaces and acted as the interfacial “heat source” under microwave irradiation. When irradiated, such localized heating induced strong welding among the neighboring beads by promoting molecular diffusion and entanglement across the bead interfaces. The increase in temperature also caused the rapid escape of residual gas and further expansion of EPS beads, which facilitated the intrabead bonding and molding into a three-dimensional foam part. The results indicated that the microwave-sintered bead foam showed a significantly enhanced tensile strength of 2.3 MPa, more than 2 times that of the steam-chest molded one. Therefore, this study provided an insight into the manufacturing of PS bead foams with high resistance to break up at the interfaces and thus effectively reduce the generation of PS microplastics.