Fabrication And Properties Of PP Foam Composites Reinforced And Toughened By Short Fibers And POE

The green and environmental-friendly foamed polypropylene (PP) has received extensive interests at home and abroad because of its outstanding physico-chemical and mechanical properties as well as the environmental-adaptability, such as easy reused and decomposed and so on. Firstly, the present status and progress of polymer matrix foam composites were reviewed. Then, compounded chemical-foaming agents were developed and the formulae and processing parameters for preparing PP foams via post-foaming process in designed dies were optimized. Furthermore, the infuluences of short fibers, elastomer and interfacial compatilizers on the reinforcement and toughening of PP foam composites were investigated systematically. The evolutions of the cellular structures, interfacial bonding and mechanical properties of PP foam composites were studied by means of SEM, DSC, FT-IR, XRD and Image-Pro Plus software in detail in this thesis.The developed compounded chemical-foaming agents are of lower decomposition temperature and less exothermal enthalpy as well as more rapid decomposition. The crystallinity and melt strength of co-blend PP are declined and increased respectively when PP is co-blended with low density polyethylene (LDPE), which results in the widening of foaming temperature range effectively and improving of the foamability of LDPE/PP co-blends by comparison with pure PP. Subsequently, the processing parameters of PP foams with optimal foamed effect are experimentally determined via optimizing process of premixing of raw materials, preparing of foamable granules and foaming moulding technology. The optimum parameters are confirmed as: mass ratio of LDPE/PP 20/80, mass fraction of the foaming agent in the co-blends 8%, foaming temperature 170~185℃, foaming duration 135~150 min and apparent density of foams 0.46~0.48 g/cm~3.The investigations of foamed effect indicate that the introduction of short glass fiber (SGF) or ethylene-1-octene copolymers (POE) is able to efficiently improve the melt strength and the foamed effects of SGF/PP and POE/PP co-blends. The modified PP foam composites exhibite a closed and spherical-like cell structure, and average cellular diameters are reduced from 0.946 mm for pure PP to 0.646 mm and 0.560 mm for the composites with SGF and POE, and cell density increases 2 - 4 times. Furthermore, the SGF/POE/PP ternary foam composites with average cell size of 0.38 mm and cell density of 16447 cells/cm~3 (fifteen-fold more increased than unmodified PP foams) can be obtained by adjusting the coexistence proportion of SGF and POE. The individual introduction of optimized amounts of PP-g-MAH or POE-g-MAH interfacial compaitilizer both facilitates the formation of excellent cellular structures and uniform distribution of the cell size in SGF/PP binary composites, whereas the influence of PP-g-MAH or POE-g-MAH on the foamed effect of SGF/POE/PP ternary system is entirely different. The addition of PP-g-MAH could indeed improve the foamed effect of ternary system, but the presence of POE-g-MAH results in the negative response of partial irregular cellular structure and cell coalescence or rupture.The analysis of interfacial properties shows that the SGF pretreated by silane coupling agent significantly decreases interfacial tension with PP, facilitates the wettability of PP to SGF and improves the distribution of reinforcing fibers in the matrix. Furthermore, chemical bonding between SGF and PP matrix takes place, which is resulted from interfacial reaction between MAH and amido groups by the incorporation of compatilizers. The novel morphology of POE-g-MAH particles adhered to the surface of SGF in SGF/PP composites is observed, indicating their interfacial properties are greatly enhanced. Co-blend processing facilitates POE particles dispersing uniformly in the matrix and better interfacial compatibility is represented between POE and PP in POE/PP. The addition of PP-g-MAH into the SGF/POE/PP composites seems no visible influence on the distribution of POE particles; nevertheless, the interfacial compatibility between POE and PP is further improved in the existence of POE-g-MAH, resulting in much closer interfacial bonding than that in POE/PP system.The mechanical testings indicate that modified SGF can remarkably increase the flexural strength and impact toughness of SGF/PP foam composites. Moreover, a positive hybrid effect for improving the mechanical properties could be acquired while two different reinforcing fibers, short carbon fiber (SCF) and SGF, are applied. With the increased mass fraction of POE, the impact toughness of POE/PP foam composites is increased dramatically and the peak value of six-fold higher than PP sample is obtained, but their flexural and compressive strength gradually decline. However, a suitable addition of SGF in POE/PP can simultaneously reinforce and toughen PP foam composites. The studies also show that the effects of PP-g-MAH and POE-g-MAH on the mechanical properties of PP foam composites are absolutely different, i.e. the former is more favorable for improving the flexural and compressive strength with the biggest increase of 42% and 25%, respectively, while the latter seems good at increasing the impact toughness of PP foam composites with a maximum amplification of 77%, which is far more than that of PP-g-MAH (35%). Under the 20% mass fraction of POE, the compressive-energy absorption properties of SGF/POE/PP ternary foam composites are improved considerably with suitably increasing SGF content. The capability of energy absorption is more double when SGF added up to 30%, while the peak value of energy absorption efficiency (80.27%) is obtained with 40% SGF, which is higher than that of POE/PP foam composites (77.17%).