An ultrafast and clean method to manufacture poly(vinyl alcohol) bead foam products
Poly (vinyl alcohol) (PVA) foam is a promising environment?friendly packaging material due to the good biodegradability and excellent mechanical properties. Besides, PVA can be produced on a large scale viathe non?petroleum routes. However, the preparation of complex?shaped PVA foam products has not been realized, because PVA is a water?soluble and semi?crystalline polymer with a high melting temperature (226°C), which cannot be welded through the conventional bead foaming technology. In this article, a clean and efficient strategy based on microwave foaming and sintering was innovatively developed to manufacture the PVA bead foam products. First, the expandable PVA beads were prepared through polar solvent?plasticization, followed by supercritical carbon dioxide (scCO2)?impregnation in solid?state. The impregnated beads were then surface plasticized with polar solvent by simple coating. Thus, the incorporated polar solvent in the internal and superficial regions of PVA beads was rapidly heated upon exposure to the microwave irradiation, which simultaneously induced the CO2 foaming and interfacial melting, respectively. In this way, the expansion and welding of PVA beads were completed in a one?step procedure. Meanwhile, the complex?shaped PVA bead foam products with excellent elasticity and intra?bead adhesive strength were prepared within a short period of 30 seconds. Therefore, the microwave heating can be considered as an efficient strategy for preparing the high?performance polymer bead foam products, especially for these high?melting temperature or glass?transition temperature polymers.