Microcellular foam of polymer blends of HDPE/PP and their composites with wood fiber

Microcellular foams of HDPE/PP blends and their composites with wood fiber were investigated to determine the effects of batch processing conditions (foaming time and temperature), blend composition, and wood fiber content on the crystallinity, sorption behavior of CO₂, void fraction, and cellular morphology (cell size and cell density) of the blends and composites. Differential scanning calorimetry (DSC) was used to investigate the heat of fusion, the melting temperature and crystallinity of the samples. Optical microscopy was employed to investigate the miscibility and crystalline morphology of the HDPE/PP blends. Environmental scanning electron microscopy (ESEM) was used to investigate the phase separation, interphase adhesion and cellular morphology of the samples. The microcellular structure of foamed HDPE/PP blends and their composites is strongly dependent on the foaming temperature, foaming time and blend composition as well as wood fiber content. Blending decreased the crystallinity of HDPE and PP and facilitated the formation of microcellular structures in polyolefins due to the poorly bonded interfaces of immiscible HDPE/PP blends, which favored cell nucleation. Well-developed microcellular structures were produced in HDPE/PP blends with ratios such as 50:50 and 30:70 at a foaming temperature of 175°C for 30 sec, but when wood fiber was introduced, a uniform and well-developed microcellular structure was not produced. The cell morphology had a strong relationship with the impact strength of foamed samples. Improvement in impact strength was associated with well-developed microcellular morphology. The effects of HDPE/PP blending on crystallinity as a function of HDPE melt index were also studied. The melting temperature and total amount of crystallinity in HDPE/PP blends were lower than those of pure polymers regardless of blend composition and melt index. The void fraction of the foamed 30:70 HDPE/PP blend was always higher than that of the foamed 50:50 HDPE/PP blend, regardless of HDPE melt flow index. The microcellular structure can be greatly enhanced by using a suitable ratio of HDPE/PP in the blend and foaming above the melting temperature for sufficient time; however, using high melt index HDPE in the blend has a deleterious effect on both the void fraction and cell morphology of the blend, probably related to cell coalescence in the highly softened matrix.