Numerical simulation on bubble wall shape evolution and uniformity in poly(ethylene terephthalate) foaming process

Abstract

The rheological behaviors of commercial foaming-grade poly(ethylene terephthalate) (PET) were measured and modeled, on the basis of which quantitative simulations on the evolution of the bubble wall shape during PET foaming were conducted. A bubble wall stretching uniformity (BSU) curve was proposed to describe the wall uniformity. A fast dropping BSU curve indicated a nonuniform bubble wall that would lead to the wall rupture. It was shown that the strain hardening and fast relaxation were beneficial for maintaining a uniform bubble wall. Simulations also revealed the initial bubble wall geometry had a strong impact on the bubble wall uniformity. Open-cell foam was produced by aggravating the initial nonuniformity through addition of polytetrafluoroethylene (PTFE) particles. With the analysis on the synergistic effects of bubble rupture and coalescence, the optimized foaming temperature (Topt) for the PET could be determined, which was consistent with the experimental value.