Abstract
The polypropylene (PP)/wood flour (WF) composites were prepared using a co-rotating twin-screw extruder followed by a single-screw extruder foaming system in this paper. Polymers, such as polyolefin elastomer (POE), high-density polyethylene (HDPE) or microcrystalline wax, were blended with PP in the preparation of composites to improve the melt strength. And a cavity transfer mixer was introduced to increase the distribution uniformity of components in composites. Meanwhile, the effect of the polymer blends on the microstructure and mechanical properties of samples was investigated. The experimental results show that the addition of POE and HDPE resulted in the second melting peak in the differential scanning calorimeter (DSC) curves and a great decrease in the cell size was caused by the added POE. However, due to the velocity difference of composites in the die, the shape of bubbles gradually became irregular. Moreover, the impact strength of samples significantly increased by 85% for the added POE and the apparent density decreased by 6.7%. And the minimum Vicat softening temperature of 133.7 °C was obtained when the mass ratio of HDPE to PP was 4/6.
Highlights
Compared with neat polymers, the wood-polymer composites have advantages in material costs and stiffness, which helps wood-polymer composites to be gradually accepted by the plastics industry [1]
These advantages are obtained at the expense of a loss in ductility and impact strength of the composites, and the addition of wood flour will lead to a higher density of composites compared to neat polymers and wood, which have limited the use of wood-polymer composites in some areas in recent years
Most of the wood-plastic foamed products on the market use amorphous polymer as the resin matrix, such as polyvinyl chloride (PVC) [2,3], polystyrene (PS) [4]. This is because the melt strength of amorphous polymers decreases slowly with the increase of processing temperature during the transition from high elastic state to viscous flow state, which makes PVC and PS can effectively restrain the excessive expansion of bubbles in the foaming process
Summary
The wood-polymer composites have advantages in material costs and stiffness, which helps wood-polymer composites to be gradually accepted by the plastics industry [1]. A uniform microcellular structure was introduced into the wood-polymer composites to improve ductility and decrease density. This can be attributed to the fact that small bubbles can effectively suppress crack growth by passivating the crack tip and increasing the energy required for crack growth. Most of the wood-plastic foamed products on the market use amorphous polymer as the resin matrix, such as polyvinyl chloride (PVC) [2,3], polystyrene (PS) [4] This is because the melt strength of amorphous polymers decreases slowly with the increase of processing temperature during the transition from high elastic state to viscous flow state, which makes PVC and PS can effectively restrain the excessive expansion of bubbles in the foaming process. The effect of polymer blends on the cell morphology and mechanical properties of samples was investigated
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