Multiple SCF In BF / PP Microcellular Foaming Process Of Adsorption And Diffusion

In this study, a method of quick heating was applied to research the process of free foaming and constrained foaming of polymers using supercritical carbon dioxide/cosolvent mixtures as blowing agents. The bamboo fiber/polypropylene composite (BF/PP) was chosen as matrix. The effects of cosolvent, nucleating agent, saturation pressure and foaming temperature on the microcellular structure were analyzed by SEM. On the basis of interfacial compatibility between bamboo fiber and polypropylene, the effects of foaming process conditions and heat treatment temperature on the impact strength of constrained foamed BF/PP were researched.The results showed that the addition of nucleating agent could promote cell nucleation and increase cell density. The influence laws of various factors on the microcellular structure of constrained foaming were similar to that of free foaming. The cell size increased with increasing the foaming temperature, saturation pressure or cosolvent content, and the phenomenon of cell coalescence gradually appeared. In addition, new small cells were generated near the previously nucleated and growing cells. The cell density remained nearly the same in various conditions. Since the polymer chains were confined by bamboo fiber, their mobility was restricted, which increased the melt viscosity of the composite and suppressed the cell growth and/or cell coalescence in the foaming process. In contrast, barrier constraints introduced by constrained foaming were almost ignored. Alkali treatment of bamboo and the addition of coupling agent could improve the interfacial compatibility of BF/PP composite. The impact strength of constrained foamed BF/PP first increased with increasing foaming temperature, saturation pressure or cosolvent content, and then decreased with a further increase in these conditions. Compared with unfoamed samples, the appearance of small cells could improve the impact strength, but bigger cells decreased the impact strength. The impact strength had a constant declining trend with increasing of foaming temperature. This paper provides experimental basis for the further research of microcellular BF/PP with excellent performance.