Foamability of Sensitized Radiation Cross-Linked Polypropylene Sheets with Supercritical Carbon Dioxide as the Foaming Agent

Polypropylene (PP) was mixed with crosslinking sensitizer trimethylolpropane trimethacrylate (TMPTMA) and then pressed into sheet. The PP sheet was irradiated by γ-rays, and then foamed with supercritical carbon dioxide (scCO₂). The effects of γ-rays radiation on the PP structure and properties were investigated by means of gel fraction test, Differential Scanning Calorimeter (DSC), and Advanced Rheology Expand System (ARES). The morphology of PP foam was investigated by Scanning Eelectron Microscope (SEM). Results showed that PP was successfully cross-linked by γ-irradiation with TMPTMA as the crosslinking sensitizer. However, the viscosity of cross-linked PP decreased. The radiation cross-linked PP could be foamed with ideal cell structures. The suitable absorbed dose for crosslinking PP sample was in the range of 10~20 kGy, and correspondingly, the gel mass fraction of PP was in the range of 25%~35%. The foaming temperature window of the radiation modified PP with an absorbed dose of 10 kGy was 12 ℃.