本发明提供一种发泡成形体之抗裂性优异、发泡剂之保持性优异且可长期维持高发泡性之发泡性聚苯乙烯系树脂粒子、其制造方法及发泡成形体。上述发泡性聚苯乙烯系树脂粒子之特征在于:其系于一聚苯乙烯系树脂粒子中含有挥发性发泡剂而得之发泡性聚苯乙烯系树脂粒子,该聚苯乙烯系树脂粒子系借由将苯乙烯系单体于聚烯烟系树脂粒子中含浸聚合而生成聚苯乙烯系树脂所获得者,且相对于100质量份聚烯烃系树脂粒子,系使用140~600质量份上述苯乙烯系单体,并且将自树脂粒子表面通过中心对半平分所得之切片浸渍于四氢喃中萃取出聚苯乙烯系树脂成分后,于对该切片之切断面进行拍摄所得之扫描式电子显微镜图像中观察到平均厚度为15~150 ���m之表皮层,且含有5.5~13.0重量%之挥发性发泡剂。

Foamable polystyrene resin particles having superior resistance to cracking of a foam molded product, having superior retention of foaming agent and maintaining foamability over a long period of time, a production process thereof, and a foam molded product are provided. These foamable polystyrene resin particles contain a volatile foaming agent in polystyrene resin particles obtained by impregnating and polymerizing a styrene monomer in polyolefin resin particles to form a polystyrene resin, wherein together with the styrene monomer being used at 140 to 600 parts by weight to 100 parts by weight of the polyolefin resin particles, the average thickness of the surface layer observed in scanning electron micrographs obtained by immersing sections cut into halves through the center from the surface of the resin particles in tetrahydrofuran followed by extracting the polystyrene resin component and capturing cross-sections of said sections is 15 to 150 &mgr;m, and the volatile foaming agent is contained at 5.5 to 13.0% by weight.