The Process Investigation Of Polyproylene Foaming Pleet By Technology Of Supercritical Fluid

Due to its virtue of environmental protection, polyproylene foaming pleet has been the fatest-growing foaming plastic product in recent years. As a crystalline polymer, its processing course is closely related to rheological behavior, and the change of melt pressure and viscosity during die runner is of great importance to foaming effect of polyproylene. The rheological behavior of PP/CO₂ solution is studied in this paper by self-designed in-line rheological experimental system. The rheological behavior of PP/CO₂ solution is measuremnted by Labview data acquisition system. Results show that viscosity curves of PP/CO₂ solution linearly change with melt temperature and gas concentration increase. According to Time-temperature Correspondence Principle, conclusion of the relationship between viscosity of PP/CO₂ solution and CO₂ concentration can be drawn, and changing principle of viscosity at unknown CO₂ concentration under this experimental system can be gained by this equation. What's more, critical foaming pressure (known as phase separation) of PP/CO2 solution under different melt temperature has been detected, which can provide guidance for melt pressure in real production. On the basis of experimental results above, using finite element software POLYFLOW, flow field inside the special foaming die is studied systematically, such as pressure field, viscosity field and temperature field. The influence of feed,gas concentration,out-wall temperature of die as well as die runner's sturcture on foaming performance of PP is discussed. Results of simulation demonstrate that the melt pressure inside die runner is not high enough to maintain homogeneous state if die exit is too big (>4mm), which will result in phase seperation. Finally, using twin-screw extruder combined with melt pump, relationship between front & back pump pressure and perfomance density is studied. Experimental results show that compared with front-pump pressure, it is the back-pump pressure that influences PP foaming pleet's performance greatly.