The Effect Of Twin-screw Configurations On Mixing And Flame Retardation Performances For PP Blending

The effect of the screw configurations of the twin-screw extruder on themorphology and performance of polymer blends and particle filled polymercomposites were systematically studied, aiming at screw configurations by whichbetter distributive and dispersive mixing can be obtained..To elucidate the relationship between screw configurations and flame retardingperformances, the influences of screw configuration on the particle morphologymust be investigated, so ten typical screw configurations were devised and produced.The effect of number and angle of kneading blocks on the particle size anddistribution in the melting zone of polymer blends and polymer composites wereconsidered.For PA6/PP blends: when the content of PA6 is 5wt%, the dispersed particle sizecould be effectively reduced by enhancing the shear effect and residence time in theformer zone; increasing of numbers of shear mixing zone shows no significant effecton the dispersed particle size in the surface layer, but can greatly reduce the particlesize in the inner layer; for two section screw configurations, increasing the sheareffect and prolonging the residence time in the latter zone, better than in the formerzone, can remarkably reduce the dispersed particle sizes and the distributioncharacteristics in the surface and inner layer of extruded blends, the angle ofkneading blocks used in the latter zone of on the screw also shows no significanteffect on the dispersed particle size, but K30°kneading blocks can improve thedistribution of the particle size; for the two sections design of the screw, highershear stress and longer residence time in the latter sections can effectively reduce thedispersed particle size and improve its distribution, and the K30°kneading blocks setin the former zone were helpful for particle size reduction and uniformity. In a word,the configurations of more mixing sections or higher shear stress and longerresidence time in the back zone section are all good for particle size reduction and uniform distribution. At the same time, K45°kneading blocks are better than otherangles in particle size reduction and distribution mixing.When the content of PA6 is 20wt%, left-handed elements, larger angle kneadingblocks in the former section of melting zone and several kneading zone in the lattersection is good for the mixing state and decreasing the dispersed particle size; largerdifference of kneading blocks shows greater effect of particle size reduction.For Mg(OH)₂ filled polypropylene composties, when the loading content is low,the effect of screw configuration on distribution is neglectable. Setting left-handedscrew elements and intensive shear kneading elements can reduce the agglomeration,and at the same time, the number of spherical particle increases and the distributionis more even. Long residence time and higher shear force in the back zone showsome effect on the uniformity of distribution although the shape of particles are strip.Additionally, the effect of kneading block angles on the morphology of the surfacelayer shows that the more shear mixing section in the back zone is better. But for thetwo stage design, there is no significant change. When the content of dispersivephrase is higher, the capability of keeping round particles is lower. Increasing theshear mixing zone, the distributing and dispersive mixing are better but two stageconfigurations design does not work well. The more difference of angles ofkneading blocks, the more uniform for the whole system.The mechanical properties of the composites prepared using these screwconfigurations also show some difference. For the tensile strength, the compositewith low content (20%) prepared using two section screw design with low shearstress and short residence time shows brittle fracture behavior. For the compositesprepared using configuration with high shear force and long residence time in theback zone of the screw, the tensile strength increased. With the addtion of shearmixing sections in back zone, the composite shows ductile fracture and the tensilestrength becomes larger. With the change of kneading block angles, more shearmixing sections also perform better than two sections design. When the loadingcontent of the filler is higher (＞60%), all the materials prepared fracture brittlely.The fracture strength of the composites prepared using configurations with moreshear mixing sections is higher than those prepared by two sections configurations.For the dependence of flame retarding properties, when the content of Mg(OH)₂is lower than 70%, the composites can reach the standard of FV-1 and when thecontent of Mg(OH)₂ is higher than 70%, FV-0, using all these screw configurationsexcept for two sections configurations. The best design is also that with the mostsbear mixing sections. The effects of kneading block angle are neglectable, but thefire blaze is smaller when using large kneading block angle.