Study On Morphology And Permeability Of Water-assisted Injection Molded PP/EVOH Parts

The performance of polymer blends is closely related to the dispersed phase morphology.Based on the equipment of water-assisted injection moldings(WAIM)developed by our study group,the dissertation aimed to study the influence of viscosity ratio and water assisted injection molding parameters on the dispersed phase morphology and crystal morphology and structure of WAIM polypropylene /poly(vinyl alcohol)(PP/EVOH)blend parts,measure and analyze the permeability of the WAIM blend parts.Flow fields in WAIM process were simulated,the results showed that melt shear rate in water-assisted molding process was greater than that in short shot process,the shear rate reached its maximum near the mold wall,and gradually reduced along the radial direction of cavity hollow in mold,and got its minimum near the water channel;strong elongational rate existed in the melt of the front area propelled by high pressure water,reached maximum near water channel and minimum near mold wall.Three kinds of PPs with different viscosities were blended with EVOH to prepare PP/EVOH blends by single screw extruder,SEM micrographs of PP/EVOH blends indicated that the diameters of dispersed phase decreased with the reduced viscosity ratio.Three kinds of WAIM PP/EVOH blends parts with different viscosities were made.SEM micrographs of WAIM PP/EVOH blends parts showed that dispersed phase morphology or average diameters of dispersed phase fibers changed with the decreasing of viscosity ratios.Short dispersed phase fibers appeared in outer layer of WAIM part of the blend with high viscosity ratio,droplets in the core layer and long fibers in the inner layer.But slim and short fibers existed in the outer layer and long fibers in the core and inner layers for WAIM part of the blend with low viscosity ratio.Following the decreasing of viscosity ratios,the average diameters of EVOH fibers of WAIM blend parts gradually reduced.WAIM blend parts had more deformed dispersed phase at position P2 than that at position P1.With the improvement of melt temperature,melt shear and elongational rates increased in water-assisted molding phase,and the deformation of the dispersed phase increased in the outer and inner layers in both P1 and P2 positions of WAIM blend parts,but decreased in the core layer for the slowed cooling rate.With the increasing of water pressure,the change of dispersed phase morphology of each layer in P1 position was not obvious,but the deformation of dispersed phase in P2 position improved.When the water pressure rose from 6MPa to 18 MPa,the decrease of average diameter of dispersed phase exceeded 50%.The crystallization behavior of three PPs and crystal morphology and structure of WAIM PP and PP/EVOH blend parts were researched.POM results showed that the molecular weight of PP matrix,and the content of EVOH and melt temperature had no significant influence on crystal morphology of WAIM PP/EVOH blend parts,but DSC results showed that EVOH content of WAIM PP/EVOH blend parts had certain influence on crystalline structure.When EVOH content increased to 15 wt %,β crystal formed in outer layer of parts.In addition,the data of crystallinity showed that EVOH had heterogeneous nucleation effect on PP.In general,the addition of EVOH had no large effection on morphology and structure of the crystallization of PP matrix.The test of toluene and oxygen permeabilities of WAIM PP and PP/EVOH blend parts showed that slim and long EVOH fibers existed in WAIM PP/EVOH blend part with low viscosity ratio,which could greatly improve the barrier property of part.WAIM PP/EVOH blend parts had lower permeability at position P2 than that at position P1.WAIM PP3/EVOH blend part had the best barrier property to toluene compared with the WAIM corresponding PP part at position P2(up to 9.8 times).The improvement of barrier property could also be achieved by appropriate increasing of melt temperature and water pressure.