The Morphology Evolution And Control On Microcellular Molding Of Polyolefin Composites

Microcellular foam is a lightweight technology of introducing microcellular into polymer matrix. It can reduce the density of the materials and keep the mechanical properties of the polymer materials with a larger limit. Currently, this technology has been applied widely into packaging and automobile industry. Among that, the typical example was the polyolefin microcellular foam materials. But, the existing conventional technology of microcellular foam needs to be used special equipments and complex operation, so there were some restricted conditions in production. It is very significant for fabrication and production of polyolefin microcellular foam materials that try to overcome these shortcomings and find simpler processing method.The purpose of this paper is to produce microcellular foam product with conventional injection molding machine. It aims at adjusting the formula and the process parameters, without any modification on process equipments and devices, to manufacture samples with low density and excellent properties in conventional injection molding machine. The main research work includes the following things:Firstly, a chemical foaming agent was used to manufacture super ductile PP/LDPE microcellular foam samples. The main process was that AC was used as foaming agent and fabricated foaming master-batch with LDPE. At the same time, PP and LDPE were mixed and fabricated the uniform PP/LDPE composites. Then, blending the foaming master-batch with uniform PP/LDPE composites became mixed materials. The mixed materials were put into a conventional injection molding machine to produce PP/LDPE microcellular foam samples. Through trial and error, finally, the optimal process was that 0.6% AC was put into 75/25 PP/LDPE composites, the melt temperature of 200℃, holding pressure of 10 MPa, holding time of 1s. The density of foaming samples compared with common samples can be reduced more than 6%. Through the observation of SEM picture, the cells inside samples were very small and uniform. Particularly, under the similar tensile strength and modulus with the common samples, its elongation at break can reached 715% with excellent ductility. This result can be at or above four multiples than common samples. In addition, it was further studied about the mechanism of the super ductility of PP/LDPE microcellular foam samples, through the analysis of the cell structure and the tensile process of the stretched sample.Secondly, H₂O was used as physical foaming agent. And the activated carbon was fabricated into carrier master-batch, then, became carrier water master-batch with water. The blend of carrier water master-batch and PP/LDPE composites was put into conventional injection molding machine. The water foaming samples had excellent ductility. The typical parameters were that H₂O of the total mass of 0.6%, 75/25 PP/LDPE, the melt temperature of 200℃, holding pressure of 10 MPa, holding time of 1s. The density of water foaming samples than common samples reduced more than 6%, and its elongation at break can reached 610%.Finally, to further study the polyolefin water foaming injection molding process while the carrier was super absorbent resin. The implement method was that water was added into super absorbent resin and became carrier water master-batch, then blended with PP, LDPE and HDPE. The composites of polymer/water can be directly put into conventional injection molding machine and fabricated water foam samples. The results indicated that this way was simple and feasible. And, compared with corresponding chemical foam samples, the density and mechanical properties was similar. So, it was a feasible molding process method of polymer foam samples.These works overcame many difficulties and inconveniences of foaming samples with conventional injection molding machine, and explored several new ways to fabricate polyolefin and its composites foaming samples. Through analyzed the foam structure and tested mechanical properties of samples, the feasibility and simplicity of these ways were proved. These new technology methods can be directly applied to actual production. At the same time, the characteristic of the super ductile PP/LDPE microcellular foam samples can provide a new visual angle to improve mechanical properties of polymer composites. And it can also promote the study of related theoretical of polymer alloy and broaden the way of polymer modifications. Therefore, the study of this paper is very theoretically and practically significant for improving mechanical properties and promoting lightweight technology of polymer products.