Research On The Batch-foaming Behavior Of Polypropylene Using Supercritical Carbon Dioxide

The foamed PP was prepared by using supercritical CO2 with self-made high melt-strength polypropylene as starting material. Effect of foaming temperature and foaming pressure on cell structure was studied. The results indicate that a suitable foaming temperature zone, from 155℃to 170℃, was gained. Cell density increases gradually with the decreasing of foaming temperature, while average cell diameter changes along the opposite trend. When the foaming temperature is 160℃, cell density, average cell diameter and apparent density are 1.03×107 cell/cm3,73.86μm and 0.016 g/cm3 respectively. Furthermore, foam expansion ratio reached the largest, about 55 times. Foaming temperature controlled the cell structure and foam expansion ratio by influencing the crystallization rate, viscoelasticity as well as the solubility and diffusivity of the blowing agents in the polypropylene melt.Two different experimental program reached the same conclusion that there was an optimum pressure range in which the best foam was gained. For program one, the optimum foaming pressure range was between 19 and 23MPa. For program two, there was a different foaming pressure range, when the foaming temperature varied. When the foaming temperature was at 175℃, the best foaming pressure range was from 12.5 to 15.5MPa. There was a synergistic effect between the temperature and pressure, and the foam under high temperature and low pressure was better than that under high pressure and low temperature.Blends of branched/Linear polypropylene (Dapoly/T36F) were batch-foamed using CO2 as a blowing agent as a function of blend composition. Evaluation of the resulting cellular morphology revealed an enhanced foamability of the blends with Dapoly contents up to 50 wt% as indicated by a similar average cell diameter but an increased cell density. This behavior was related to an improved coalensence phenomenon and a reduced initial nucleation. The maximum cell density for the blends was achieved at about 50% of Dapoly, about 4.48×107(cell/cm3). The results also showed that the average cell diameter increases obviously, while the cell density changed along the opposite trend with a further increasing of Dapoly. The differences in the foaming behavior can be correlated with the melt rheological properties. There was a transition between 50 and 60 wt% Dapoly content in the shear rheological results, G'plots and Cole-Cole plots performed significantly obvious. The uniaxial elongational investigations revealed an similar transition. The strain hardening behavior was highlighted particularly at the Dapoly content above 60 wt%.Thirdly, the compatibility and foamability of polypropylene-based WPC was explored. A conclusion was reached that when the content of wood flour was very low, about 10%, the compatibility was good before and after foaming, and a large expansion ratio was gained. When the wood flour content reached 30%, the macro-compatibility was good before foaming. But after foaming, the wood flour was excluded to migrate towards the surface obviously. Despite the expansion ratio was high, it relied entirely on the foaming properties of the matrix; The compatibility between the matrix and wood flour had been greatly improved, after the wood flour was treated by NaOH solution. When the wood flour content was at 30%, there was also a very good compatibility after foaming. Meanwhile the foamability was reduced and the expansion ratio decreased.