Preparation And Characterization Of Foamed Polypropylene With High Melt Strength

The foamed PP has been promising for commercial applications such as packing, automobile and architecture owing to its unique properties including high thermal resistance, excellent mechanical property and environment friendly. However, the further development of PP foams is restricted by the rigorous preparing technology due to the high crystallization temperature and weak melt strength of PP. Therefore, the application of PP foams with excellent properties will be expanded by reinforcing the melt strength of PP matrix in a cost-effective way. Here, we have made the effort to improve the melt strength and foamed property two aspects as followings.Firstly, the foamed PP with excellent properties was prepared by the physical blending method. The modified PP was yielded by mixing the linear PP with the flexibilizers including the nitrile butadiene rubber (NBR) and chlorinated polyethylene (CPE) in the Haake torque rheometer. The nano-SiO2 worked as nucleating agent for foaming. The effects of blending weight ratio on the melt strength and foamability of PP matrix have been studied systematically. The foamed PP with excellent mechanical and thermal properties was obtained by adding moderate content of the NBR or CPE. The results showed that the melt strength increased continuously with an increase of CPE or NBR contents, while the foamability increased firstly, and then decreased. Furthermore, the blending modification of the CPE was better than that of NBR. The foamed PP/CPE blending with excellent properties was obtained by addition of 20 wt.% CPE, the optimal melt strength of CPE/PP blending system was 0.183MPa, the density was 0.37g/cm3, the thermal conductivity was 0.035W/(m.K), the Vicat temperature was 112℃, the tensile strength was 17.67MPa, and the compact strength was 16.51kJ/m2.Secondly, the melt strength and foamability of PP matrix were reinforced by chemical crosslinking-modified PP, which was yielded by peroxide-initiated crosslinking of linear PP with the double bonds of the butyl acrylate (BA) or diethyl meleate (DM). The nano-SiO2 worked as a nucleating agent for foaming, and the BA or DM worked as the auxiliary crosslinking agent. The effects of BA or DM contents on the melt strength and the melt flowing ratio of the modified PP matrix were studied. The foamed PP with excellent properties was prepared by adding appropriate content of the auxiliary crosslinking agents (BA or DM). The results showed that the melt strength of PP matrix increased with an increase of BA or DM contents, and the foamability increased firstly, and then decreased. Besides, the crosslinking modified effect of the BA was better than that of DM. The foamed PP with excellent properties was obtained by incorporation 10 wt.% DM, and the density of the foamed PP was 0.234g/cm3, the thermal conductivity was 0.037W/(m.K), and the Vicat temperature was 125℃.In a word, the melt strength of PP matrix has be improved effectively by both physical blending technology and chemical crosslinking modification, however, the latter in better than the former with a lower density, more uniform closed-cells and lower thermal conductivity. Besides, the foamed PP prepared by the physical blending technology has more excellent mechanical property than that of the chemical crosslinking modification.