The Research On Diffusion Mechanism Of Supercritical Fluid And The Development Of Polypropylene Imitation Rattan Foams

The dissolution of SCF in polymer melt is the key of the formation of polymer-gas homogeneous system. Polypropylene imitation rattan foaming has strict requirements for surface quality and foaming quality. To solve above two problems, this graduation thesis will do systematic research on the time of dissolution of SCF in polymer melt and the development of polypropylene imitation rattan foaming products based on the former theory.In the process of polymer foaming, there are two stages for the dissolution of SCF in polymer melt, dispersive mixing stage and physical diffusion stage. At the stage of dispersive mixing, we established the physical and mathematical model of bubble breakup time in simple shear flow, which was proportional to interfacial energy, and inversely proportional to interfacial viscosity and the square of shearing rate, and it was also connected with division number. At the stage of physical diffusion, we established the quantitative relation between the diffusion time and the bubble balance radius, diffusion coefficient, shearing rate.According to the dissolution time of SCF in polymer melt and the axial velocity, we calculated the minimum length of homogenization section and the appropiiate length of slotted section in theory, through which we could guarantee the formation of homogeneous system and the conveying efficiency of polymer melt. We took PP/CO2 and PS/CO2 as examples in this graduation thesis to calculate the dissolution time of each gas in corresponding polymer melt and the length of homogenization section and slotted section in different conditions. The results showed that the influence of the screw speed on the time required in the first stage was not significant, and the length of slotted section was usually short. The physical diffusion time fell rapidly as screw speed increased. The physical diffusion time of PS/CO2 was longer than PP/CO2, and the homogenization section length of PS/CO2 was longer too.We studied the influence factors of surface roughness (Ra) for PP extrusion products in this thesis. The results showed that Ra increased as temperature decreased and shearing rate increased, and it increased as L/D ratio of die inceased. In the aspect of formula, Ra increased with adding PE and grinding agent and decreased with adding silicone master-batches.A quantitative characterization——surface crack coefficient was proposed in this thesis to study the extent of surface crack generated by folding extrusion foaming products. Surface crack would accur when deformation exceeded yield point elongation. The results showed that yield point elongation had nothing to do with extension rate, and it would increase with adding PE, and the cell size had the optimal value, when yield point elongation was maximum value and surface crack coefficient was minimum.A CO2 metering pump system with low flow rate, high pressure and high precision and polypropylene imitation rattan foaming experiment system were established. The optimal temperature window and CO2 content was dicussed in this thesis, and the results showed that when the temperature of homogenization section and die was 165～170℃, and the CO2 content was wt1%, we could get the better quality of the products.