Mechanical Properties Research On Chemical Foaming Wheat Straw Powder/HDPE Composites Of Injection Molding

In recent years, as a research hotspot of environmentally friendly composite, the microcellular foaming wood/plastic composites not only has all the advantages of the general wood/plastic composites, but also has low density, high toughness, sound insulation, heat insulation and other advantages. So it is widely used in construction, car interior and even military fields. Combined with the characteristics of regional economy, the possibility of conventional injection molding for the wheat straw powder/HDPE composites which was made and processed by blending HDPE and wheat straw powder was studied. Through this research, the cost of raw materials can be reduced, the forest resources would be protected, the utilization and value-added of the agricultural waste (wheat straw) can be increased, and the molding efficiency of the wood/plastic composites also can be improved. Because the processing temperature of HDPE is higher than 200℃, while the decomposition temperature of wheat straw powder is around 170℃, the chemical blowing agent is used to improve the melt flow rate (MFR) of the wheat straw powder/HDPE composites and reduce the processing temperature of the wheat straw powder/HDPE composites.In this study, foaming polyolefin and wheat straw powder/HDPE foaming composites sample were prepared using the injection molding. Firstly, the influence of the content and form (powder vs. masterbatch) of chemical foaming agent azodicarbonamide (AC) on the density, mechanical properties, dynamic mechanical properties and cell morphology of foaming polyolefin (HDPE、LDPE、PP) were studied. Secondly, the influence of the content and mesh of wheat straw powder and the content of silane coupling agent on the melt flow rate and the thermal property of wheat straw powder/HDPE composites were studied. Finally, the influence of the content of AC, the content and mesh of wheat straw powder and the content of silane coupling agent on density, mechanical performance, dynamic mechanical properties and cell morphology of the wheat straw powder/HDPE foaming composites which made by injection molding were studied.The main results are as follows: 1、For foaming polyolefin, the AC form has little effect on the density, mechanical properties and cell morphology of foaming polyolefin.Compared with the mechanical properties of pure polyolefin, the properties of foaming polyolefin show no significantly improvement. On the contrary, the tensile properties and impact properties of the foaming polyolefin are declined, but its flexural properties are increased. When AC content is 2wt%, the comprehensive performances of foaming polyolefin is better than others. Compared with the loss modulus of pure polyolefin, the storage modulus and the loss modulus of foaming LDPE and foaming HDPE is declined, while the storage modulus and the loss modulus of foaming PP is increased.The uniformity of the foaming polyolefin is bad, the foaming area is narrow and mainly concentrated in the center of the sample. With the matrix resin different, cell morphology and surface morphology of non-foaming part are quite different.2、For rheological property and thermal property of the wheat straw powder/HDPE composites, the melt flow rate (MFR) of the wheat straw powder/HDPE composites declined with the increasing of wheat straw powder content and silane coupling agent content. However, MFR of the wheat straw powder/HDPE composites increased with the increasing of mesh of wheat straw powder. The effect of silane coupling agent content and mesh of wheat straw powder on the melting temperature (Tm) and the decomposition temperature (Td) of wheat straw powder/HDPE composites are very limited.3、For density of the wheat straw powder/HDPE foaming composites, the density of the wheat straw powder/HDPE foaming composites increased with the increasing of wheat straw powder content. When wheat straw powder content is 50wt%, the density of the wheat straw powder/HDPE foaming composites is increased by 18%. The effect of silane coupling agent content, mesh of wheat straw powder and AC content on the density of the wheat straw powder/HDPE foaming composites are very limited.4、For mechanical properties of the wheat straw powder/HDPE foaming composites, compared with the mechanical properties of the foaming HDPE, the tensile strength of the wheat straw powder/HDPE foaming composites declined, small changes in the flexural properties, the impact strength increased. At the same time, the storage modulus and the loss modulus of it increased. With the increasing of wheat straw powder content, the tensile and impact properties of wheat straw powder/HDPE foaming composites decreased, the flexural properties increased. At the same time, the storage modulus and the loss modulus of it increased. Silane coupling agent treated to make the mechanical properties of the wheat straw powder/HDPE foaming composites increased. When content of silane coupling agent is 6wt%, its comprehensive mechanics performance is the best. Compared with the wheat straw powder/HDPE foaming composites without treated by silane coupling agent, the storage modulus and the loss modulus of the wheat straw powder/HDPE foaming composites with treated by silane coupling agent are increased. With the increasing of mesh of wheat straw powder, the mechanical properties of the wheat straw powder/HDPE foaming composites first increased and then decreased. When particle size of wheat straw powder is 40-60 mesh, the mechanical properties of wheat straw powder/HDPE foaming composites is the best. When particle size of wheat straw powder is 60-80 mesh, the storage modulus and the loss modulus of wheat straw powder/HDPE foaming composites are higher than others.5、For microstructure, SEM observation shows that the morphology of wheat straw powder does not change which treated by silane coupling agent. With the increasing of wheat straw powder content, the dispersion of wheat straw powder gets worse, the size of cell increases and the uniformity of the cell gets worse. The use of silane coupling agent can improve the phenomenon of wheat straw powder pull out from the HDPE and can improve the uniformity of the cell.With the increasing of particle size of wheat straw powder, the compatibility between wheat straw powder and HDPE is improved.When particle size of wheat straw powder is 40-60 mesh, the uniformity of cell is better than others.