Preparation Of Long-chain Branched Polypropylene Based On Blending And The Strengthening Modification By Lignin

Polypropylene(PP)is widely used because of its good temperature resistance,chemical resistance,processing properties and recyclability.However,the shortcomings of conventional PP(such as low melt strength,low surface polarity and poor impact resistance)limit its application in high-performance fields.Lignin,as a natural renewable and easily degradable polymer material,contains a large number of reactive groups in its molecular chain.Lignin can blend with PP to prepare composite materials,which not only reduces the use of petroleum-based plastics,but also lowers the R&D cost of high-performance biomass-polymer composites.At the same time,lignin gives functional properties to PP,which is in line with the sustainable development strategy and becomes one of the hot spots for research.In this paper,high performance PP/LS blended composites were prepared by a combination of y-irradiation and melt blending using PP as the matrix,polybutadiene latex(PB)and carboxylated nitrile butadiene rubber emulsion(xNBR)as the toughening unit,acrylamide(AM)as the graft modifier,and lignosulfonate(LS)as the reinforcing unit with the addition of antioxidants,respectively.The physical and mechanical properties,thermal properties,surface structure,crystallization behavior,rheological properties and foaming properties of PP blends were investigated,and the interaction mechanism of lignin-reinforced PP was revealed through interfacial compatibility,free radical capture and melt structure and relaxation.(1)Adding latex particles can improve the mechanical properties and heat resistance of PP to some extent.Hydrogen bonding was formed by polyacrylamide(PAM)functional groups and strengthened the interfacial bonding,by which increased the impact strength of PP by 6.6 times,reduced damping loss factor,improved elasticity and heat resistance,and induced ?-crystal.Addition of LS promotes the dispersion of polar xNBR latex particles in the PP matrix,resulting in the interaction between LS polar groups and PAM polar graft chains and the formation of network structure.The enhanced interfacial bonding leads to the movement restricts of PP molecular chains and the reduction of modulus E' and E" but the increase of loss factor,heat resistance,maximum thermal decomposition rate(by nearly 60?),OIT and grain size,the fine crystallite size,and the enhance of heterogeneous nucleation.Polarity increase of modified PP reduces the contact angle but increases the surface energy.(2)Because the interaction between PB latex particles and PP matrix is better than that of xNBR latex particles,PP/PB and PP/PB/AM have more reductions of complex viscosity,energy storage modulus,loss modulus,loss angle tangent decrease,and frequency dependence in the low frequency region than PP/xNBR and PP/xNBR/AM,respectively.Besides,PP/PB/AM blends show solid-like non-terminal rheological behavior.Synergistical interaction of LS and AM promotes the dispersion of xNBR latex particles in PP matrix and forms a heterogeneous network structure,which also endows the PP blends with long-chain branching behavior.(3)AM active monomer can produce self-compatible action by hydrogen bonding in the PP matrix.Addition of AM can stabilize the dispersal of latex particles in PP matrix,enhance the interfacial bonding and physical crosslinking,stabilize the pores by increased the melt strength and strain hardening,and form improved pore morphology with uniformed size and thin wall thickness.These improved characteristics show good potential to be used as a modified masterbatch for foaming general-purpose polypropylene.Introducing LS enhances the interfacial bonding of the system and the significant strain hardening phenomenon,which results in more stable pore system with increased pore density and bubble ratio,decreased average diameter of the pores,and a foaming multiplicity up to 44.44 times.(4)Both interface compatibility and aging performance are improved by the effect of lignin surfactants,hydrogen bonds and capture of free radicals.The active surfaction of LS can improve the film-forming property of xNBR latex,the radiation grafting reaction occurs between radiated xNBR and LS while the imidation in PP blends happens when heated at 300?,and the reconstruction of hydrogen bonding improves interfacial compatibility of PP blends.Peroxyl radicals are generated after PP radiation,and still remains good activity in 2 weeks.The more LS content results in the stronger the radical capturing ability and the longer the reaction time,which in turn improves the aging property.Due to the increased hydrogen bonding interactions of polar groups in LS molecules,the distance between G' and G" of PP blends decreases and the relaxation rate of melt structure becomes faster under combinations of time scan,pre-shear and cyclic stress.In summary,LS helps to constructs the long-chain branching behavior of PP,promotes the dispersion of both PB and xNBR nanogel particles in PP matrix,and therefore improves the mechanical properties,heat resistance,surface polarity,melt strength and foaming properties of PP.The LS-reinforced PP provides a theoretical basis for the preparation of high-performance biomass-plastic composites.