Lightweight and tough polymer foam with defect-free surface and bimodal cell prepared by thermally expandable microspheres injection molding

Thermally expandable microspheres (TEMs) with different foaming temperatures were utilized to develop bimodal polypropylene (PP)/TEM composite foams through injection molding. The introduction of TEMs significantly enhanced the crystallization process of PP and optimized its viscoelastic behavior. As the TEM content increased, the foam density decreased. An optimal state where the cell structure remained intact was obtained at a 6.0 wt% TEM content and the composite foam exhibited the best comprehensive mechanical properties. Furthermore, with the introduction of a second type of high-temperature TEM as the co-blow agent, bimodal cells with size centralized at 12 μm and 57 μm were generated in the PP matrix. When the ratios of two TEMs were controlled at 3.0 wt% each, the PP/TEM 3 + 3 % foam with an equal amount of co-blowing agent achieved 7 %, 94 %, and 101 % improvement in tensile strength, toughness, and strain-at-break compared with the PP/TEM 6 % foam with sole DU300x TEM blowing agent, owing to the synergetic effects of stress dissipation and redirection within the bimodal cell structures and the transverse toughening of the rigid TEM microspheres. In addition, the composite foam exhibits a smooth surface appearance with a low surface roughness since the TEM shell could effectively prevent the burst of the microspheres during foaming. This work provides a simple and effective approach for manufacturing bimodal foams with high toughness and high-quality surfaces.