Achieving high impact toughness in injection-molded SMMA foams via the synergistic effects of cell size and SBS

Optimizing the cellular structure of polymeric foams has long been considered one of the most economical and effective methods for enhancing their impact properties. However, the relationship between cell size and impact strength has rarely been systematically studied. In this study, styrene-butadiene-styrene (SBS) was used as a toughening agent to improve the toughness of Poly(styrene-co-methyl methacrylate) (SMMA). The expansion ratio of pure SMMA foams and their blended counterparts were controlled by fixing the mold-opening distance, while the cell size was adjusted by changing the blowing agent content and packing time. Notably, the impact strength of the optimized SMMA/SBS blend foam reached 15.5 kJ/m², representing a significant increase of 1400 % compared to that of the pure SMMA foam. In addition, the impact strength of pure SMMA foams did not change significantly as the cell size increased from 29.7 μm to 278.9 μm. However, for SMMA/SBS blend foams, an optimal cell range (100−150 μm) was identified, where the impact strength was approximately twice that of foams with smaller cell sizes (1−25 μm). Then, examination of the impact-fractured surfaces at different cell sizes revealed that the synergistic effects of an appropriate cell size (100−150 μm) and the presence of SBS particles promoted craze initiation and expanded the plastic deformation area during crack initiation, leading toenhanced impact toughness in the blended foam. This work not only systematically elucidates the relationship between cell size and impact strength but also provides new insights into the synergistic effects of rubber particle and cell size on the mechanical properties of polymeric foams.