Study On Crystallization Behavior And Foaming Property Of Modified Polypropylene

Based on many outstanding properties, foamed polypropylene (PP) can be widely used in many fields, which has a bright prospect. However, PP belongs to semicrystalline polymer, that results in poor foaming effect and makes cellular structure hard to control. In order to improve the situation, this paper started with material modification, polypropylene homopolymer (PP-H)/polypropylene block copolymer (PP-B) blend and PP/hydroxylation multi-walled carbon nanotubes (HO-MWCNTs) nanocomposite were obtained respectively, which were foamed with supercritical carbon dioxide as physical foaming agent by high-temperature impregnation and rapid pressure reduction process, crystallization behaviors and foaming properties of PP before and after modification were comparatively studied, and the impacts of crystallization on foaming process of PP were analyzed.Results show that the crystallization type of pure PP belonged to homogeneous nucleation. That led to low crystallization temperature, slow nucleation rate and structured spherocrystal with large size. However for PP-H/PP-B blend and PP/HO-MWCNTs nanocomposite, crystal nuclei formed by PP-B at higher temperature and homodisperse carbon nanotubes both can be as nucleator to induce heterogeneous nucleation, that resulted in higher crystallization temperature and faster nucleation rate, made crystalline grain density higher and crystal size smaller. In PP-H/PP-B blend, except as crystallization nucleation sites of PP-H, PP-B crystal nuclei can form a network structure in polymer melt, that can limit the movement of molecular chain to improve entanglement degree of polymer melt, thus led to the improvement of melt strength. Similarly, in PP/HO-MWCNTs nanocomposite, carbon nanotubes can play a significant potentiation role to enhance the mechanical properties of system. Blending modification and nanometer compound modification made the spherocrystal of PP more fine and uniform, also increased the strength of system, the improvements of two aspects can effectively enhance the cell nucleation, increase the nucleation rate and limit the growth of cell in the process of foaming, thus made the cell of foams more dense and small, the foaming property became better. When PP-H/PP-B blend ratio was 70/30 and carbon nano-tubes content was 1%, the foaming property was improved most significantly. Except to improve the foaming effect by inducing crystallization and cell nucleation, and enhancing mechanical property, the addition of carbon nanotubes can make the foaming property more stable, weaken the influences of technological conditions.