Interfacial flame retardance of Poly(vinyl alcohol) bead foams through surface plasticizing and microwave selective sintering

Highlights: • KSi was selectively heated and located at the interface of PVA through microwave irradiation. • The KSi coating layer induced the surface melting and cohesion of the neighboring PVA beads. • The PVA/KSi composite foam possessed excellent flame retardant and mechanical properties. Due to the presence of massive air and porous structure, the high flammability of polymeric foams severely limits their wider application in household, automobile, and aerospace fields. Herein, an interfacial flame-retardant mode based on surface plasticizing and microwave selective sintering was proposed to simultaneously realize the significantly reduced flammability and the enhanced mechanical properties of poly (vinyl alcohol) (PVA)/potassium silicate (KSi) bead foams. First, the foamed PVA beads with unique cell structure were prepared by supercritical carbon dioxide extrusion foaming and pelletizing. Then the KSi aqueous solution was coated on the surfaces of as-prepared PVA beads through surface plasticizing and acted as the excellent microwave absorber. Upon exposure to microwave irradiation, the interfacial heating caused by KSi coating layer induced the surface melting and cohesion of the neighboring PVA beads. The intense molecular diffusion occurred between PVA and KSi in the interfacial regions, improving the mechanical properties of foam parts. Meanwhile, KSi enriched at the bead interface and quickly decomposed into silicon-containing compounds once ignition, which constituted a dense physical Si and C barrier preventing the propagation of combustible substance and oxygen. Thus, the PVA/KSi bead foam reached the UL-94 level of V-0 and LOI value as high as 33.7%.