Toughening Effect Of Co-grafted Polypropylene With Maleic Anhydride And Divinyl Benzene On Wood-Polypropylene Composites

Melt grafting modification of polypropylene matrix with maleic anhydride(MAH)is a rapid and effective method for preparing high-performance wood plastic composites(WPCs),because it can improve the interface compatibility between wood flour and polypropylene.However,under the action of free radical initiator,the severe degradation of polypropylene(PP)will greatly reduce the toughness of PP matrix and PP-based WPCs.To solve this problem,this paper took 60 wt.%of wood filled PP-based WPCs as the research object,MAH grafting modification was used to improve the polarity and fluidity of the PP matrix.Based on the above process,multifunctional monomer divinyl benzene(DVB)was further added to introduce branch chain into the PP molecular chain,and further blending block copolymer elastomers(OBC)to improve the toughness of MAH grafted PP.Then,the modified PP was compounded with wood flour to prepare high toughness WPCs.The influences of wood flour dispersion,matrix properties and interface layer properties on the mechanical properties,creep behavior and dimensional stability of the resulting WPCs were systematically studied.Finally,the modification method of OBC blending and MAH/DVB co-grafting was used for waste PP-based WPCs to evaluate its feasibility.The main contents and results are as follows:(1)The effects of adding DVB on the structure and performance of PP matrix were studied during MAH grafting modification process.Firstly,a small amount of graft product was prepared by torque rheometer to explore the reactivity of monomer attacking PP molecular chain.The torque curve showed that adding DVB could inhibit the degradation of PP and caused a cross-linking reaction.The gel generating in the product proved that cross-linking structure existed in DVB grafting system.However,no gel was detected in MAH/DVB co-grafting system.It indicates that the addition of DVB could introduce branch chain onto PP molecular chain and improve MAH grafting degree.Secondly,PP matrix with different MAH grafting degree and fluidity was prepared by twin screw extruder.In MAH/DVB co-grafting system,no gel was detected in all products,MAH grafting degree improved with the increasing of DVB content,and the fluidity of modified PP was decreased.Compared with MAH grafted PP,the elastic response of co-grafted PP was significantly enhanced in the low-frequency region,and the radius of Cole-Cole curve was decreased in the high-viscosity region.These differences proved the existence of branched chain structure on PP chain.Crystallization analysis showed that the changes of MAH grafting degree had little effect on the crystallization nucleation rate of PP,but the branched chain structure could significantly improve it and refine the crystal.Therefore,grafting of PP with MAH and DVB resulted in a marked improvement on the impact strength of 118%than that of MAH grafted PP.(2)The effects of polarity,fluidity and chain structure of PP matrix on the properties of WPCs were studied.Compared with traditional compatibilizer maleated PP(MAPP)compatibilized PP-based WPCs,grafting of PP matrix with MAH improved the tensile and flexural properties of WPCs,while the impact strength remained unchanged.Grafting of PP matrix with 0.4wt.%DVB and MAH,the impact strength of co-grafted PP-based WPCs increased by 32%than that of MAH grafted PP-based WPCs,and the tensile and flexural properties were also improved.Further increasing the content of DVB,the mechanical properties of co-grafted PP-based WPCs decreased.With the increasing of the MAH content,the tensile and flexural strength of the resulting composites improved,and the impact strength and modulus remained unchanged.Dynamic mechanical analysis showed that grafting of PP with MAH could improve the dispersion of wood in the matrix.The energy loss caused by the motion of molecular chains was reduced due to the effective interfacial layers and an appropriate number of branched chain structures.Compared with MAH grafted PP-based WPCs,co-grafting modification also improved the creep resistance and dimensional stability of WPCs.Scanning electron microscopy analysis showed that the dispersion of wood in the PP matrix and theses interfacial compatibility were improved after the suitable graft modification.(3)The effect of MAH/DVB co-graft modification on the structure and performance of PP/OBC blends was studied.Crystallization analysis and morphology analysis showed that grafting of the blends with MAH improved the interfacial compatibility between PP and OBC.The crystallization rate of PP in the blends decreased.In MAH grafting system,when OBC was added over 10wt.%,the end of Cole-Cole curves of PP/OBC blends began to rise.The above phenomenon indicated that there were two relaxation mechanisms in the melt molecular chain of the blends.In MAH/DVB co-grafting system,with the increasing of DVB content,MAH grafting degree and the amount of branched chains were increased.After co-grafting modification,the melt elasticity of PP/OBC blends improved,indicating that the molecular weight distribution widened.The results of mechanical properties showed that the impact strength of MAH grafted PP/OBC blends was 41%higher than that of MAH grafted PP/POE blends,and less decrease in the tensile properties.Adding 10 wt.%OBC could significantly improve the impact strength of MAH grafted PP,which was 65%higher than that of unmodified PP.Adding and increasing DVB content could further improve the impact strength of MAH grafted PP/OBC blends.(4)The effects of PP/OBC blends matrix with MAH and DVB on the toughness and strength of the PP/OBC/wood composites were studied.When the interfacial compatiblity between wood flour and PP was poor,the addition of OBC would reduce the toughness and strength of wood flour/PP composites.Compared with MAH grafted PP-based WPCs,grafting of PP/OBC blends matrix with MAH improved the impact strength of resulting composites by 38%,and the tensile and flexural properties were remained.In addition,under the same amount addition of MAH,the impact strength of WPCs toughened with OBC further increased of 12%than that of POE toughened composites.Increasing the OBC content from 10 wt.%to 20 wt.%,the impact strength and tensile modulus of the composites increased by 21%and 11%,respectively.In MAH/DVB co-grafted PP/OBC blends system,the mechanical properties of WPCs further improved by adding appropriate contents of MAH and DVB.Morphology analysis showed that the interfacial compatibility between wood,PP and OBC was enhanced by grafting modification.Dynamic mechanical analysis showed that the existence of effective interface layer could promote the shear yield process of the blends matrix,the loss modulus of WPCs decreased and the loss factor increased.The creep resistance and dimensional stability of PP/OBC blends-based WPCs were improved by grafting modification.(5)The feasibility of OBC blending and MAH/DVB grafting modification for waste polypropylene(rPP)-based WPCs was investigated.After MAH/DVB co-grafting modification,MAH group and branched chain structure were successfully introduced onto the molecular chain of rPP/OBC blends.Compared with MAH grafted rPP-based WPCs,MAH/DVB co-grafted rPP WPCs had obviously higher tensile,flexural and impact strength;the modulus of the composites was increased to the same level as that of unmodified rPP-based WPCs.Further blending with OBC,the tensile modulus and impact strength of the resulting co-grafted rPP/OBC composites were increased by 60%and 11%,respectively.Dynamic mechanical analysis showed that the synergy between the effective interface layer and the branched chain structure enhanced the shear yield process of rPP/OBC matrix.The loss modulus of the composites decreased,and the loss factor increased.In summary,the addition of DVB can obviously inhibit the degradation of PP,and improve the impact strength and MAH grafting degree of MAH grafted PP.The toughness and strength of MAH/DVB co-grafted PP-based composites are obviously higher than that of traditional MAPP/PP/wood flour composites.After blending PP matrix with OBC and further co-grafting with MAH/DVB,the impact strength of the resulting WPCs is further improved;meanwhile,the tensile and flexural properties of the composites were remained.Therefore,the modification method of OBC blending and MAH/DVB co-grafting is helpful to restrain the adverse effect of low-modulus elastomer on the mechanical strength of composites.This matrix modification methods of OBC blending and MAH/DVB co-grafting can be also used to prepare high toughness and high tensile modulus rPP-based WPCs.