Study On Preparation Of Microcellular Plastics Of High Density Polyethylene Through Reaction Extrusion

Microcellular plastics ?MCP? is aimed at light-weighting design for material, reducing raw material costs, meanwhile it can keep or even improve mechanic properties of materials. MCP of High Density Polyethylene was prepared through in situ foaming and thermal foaming in extrusion processing.Single screw extruder was improved according to the requirements of the subject. The extruder was equipped with shield. New die base used in testing melt pressure and die element with different length-diameter ratio was designed. And low velocity was performed for the drafter.MCP was prepared through reaction of calcium carbonate and acid to generate CO2 in situ during extrusion of HDPE. The components of calcium carbonate masterbatch were analyzed. The preparation of adipic acid masterbatch was investigated. It suggests that the optimum formula is LDPE/LLDPE powder/adipic acid in a ratio of 65/25/10, the optimum process conditions of producing adipic acid masterbatch is setting barrel temperature as 120, 132,148,120? and screw speed as 40 rpm. Effect of die temperature, screw speed and content of adipic acid masterbatch on foaming of HDPE was studied. Fine cell structure in HDPE microcellular plastics with average cell diameter of 150 um was extruded under barrel temperature as 175,190,200,160?, screw speed at 60 rpm, and the content of adipic acid as 0.6%. The density of the product is 0.62 g/cm3. And certain proportion of calcium carbonate and HDPE was blended at first, the results show that the cell structure of microcellular sample is improved dramatically with extrusion of adipic acid masterbatch and the blend.MCP was prepared through thermal decomposition reaction of blowing agent H to generate N2 during extrusion of HDPE. Blowing agent H was modified by Stearic acid, and blowing agent H masterbatch was extruded with the formula of LDPE/LLDPE powder/blowing agent H/Stearic acid in a ratio of 70/30/10/2,under the barrel temperature as 120,135,140,120? and screw speed as 40 rpm. Thermal foaming of HDPE 5000S was studied with the masterbatch. The results show that average cell diameter of the product decreases and cell density increases with processing temperature, screw speed or the content of the masterbatch increasing. Microcellular sample was achieved with lower crystallinity and faster crystallization rate, under the condition of barrel temperature as 175,190,200,160? and the content of blowing agent H masterbatch as 14 wt%. The density of the sample is 0.740 g/cm3, and average cell diameter of 65 um, cell density of 7.0×105 cell/cm3.Thermal extrusion foaming of MCP of HDPE DMDC with different length-diameter die was investigated. The experiment results suggest that microcellular samples extruded from die with length-diameter of 9.6 and 6.8 can obtain better cell structure, with the average cell diameter of 69 um, cell density of 8.6X 105 cell/cm3. Meanwhile the relations between melt pressure at die and cell structure were studied. The results show that the smaller pressure differential between die pressure of extruding pure resin and that of extruding foaming formula, the better cell structure of microcellular samples.Effect of Nano SiO₂ on thermal foaming of HDPE was investigated. The cell structure of microcellular sample can be improved with the content of Nano SiO₂ in a rational range.