Multi-Phase Polypropylene Structure Design And Performance Optimization Based On Dispersed Phase Regulation

Multiphase polypropylene is used in a wide variety of applications because of its excellent impact,heat and corrosion resistance properties.With the increase in thinwall and lightweight requirements,there is a strong demand for polypropylene products with both high stiffness and high toughness,and both academia and industry are interested in producing thermoplastic polymers with broad viscoelastic properties,i.e.,the highest possible rubber content in multiphase polypropylene particles.The improvement of the stiffness and toughness of multiphase polypropylene and the stable production of polypropylene with high rubber content are closely related to the molecular chain structure,phase structure,phase morphology of the dispersed phase and its distribution in the particles,but there is a lack of efficient means to regulate them.Therefore,the study of the control techniques of the dispersed phase and thus the design and optimization of the multiphase structure of polypropylene has been an important research topic in the field of polyolefins.The paper investigates the dispersed phase regulation techniques of multiphase polypropylene from multiple scales,including chain segment,condensed state,particle and reactor.Firstly,the microphase separation pattern of ethylene propylene block copolymer(EbP)is revealed,and then a method to induce the enrichment of chain segments of EbP to the interface is proposed to enhance the stiffness performance of multiphase polypropylene.Secondly,we discovered the Induced Condensing Agent(ICA)-induced capillary condensation phenomenon in propylene polymerization,whereby the mechanism of regulation of the microchemical environment/dispersed phase properties of particle was established,and developed a surface stickiness reduction technique(i.e.,Damp mode)to regulate the distribution of dispersed phase in particles by constructing particle-scale microzones.Finally,the effects of multiple temperature zones and temperature switching processes on the microstructure of polyolefins in a liquid-containing gas-solid fluidized bed reactor were investigated to guide the liquid-containing operation and product development of Damp mode in industry.The main research work and results of this thesis are as follows:(1)With the assistance of analytical techniques like preparative temperature rising elution fractionation and nano-infrared,a comparative study of the molecular structure,copolymer distribution,and microphase separation behavior of EbP in multiphase polypropylene with similar modulus but different toughness was conducted.The results show that the microphase separation of EbP determines its distribution within the dispersed phase,which in turn affects the toughness of multiphase polypropylene.EbP with a crystallization temperature higher than that of isotactic polypropylene(iPP)was dispersed in the polypropylene matrix and dispersed phase before the spherical crystals of iPP were formed,thus forming crystal defects containing ethylene-rich chain segments and reducing the toughening effect of EbP.EbP with lower molecular weight,lower ethylene content,and longer number-average propylene and ethylene chain segments are more prone to microphase separation due to the presence of more crystalline nucleation sites.(2)The effects of polyethylene and its introduction sequence on the phase interface and mechanical properties of multiphase polypropylene were investigated by introducing polyethylene(PE)in situ during propylene co-polymerization through sequential polymerization.The results show that EbP,as an interfacial component or capacitant,can form co-crystals with PP and PE.The in situ introduction of PE can drive EbP to enrich the multiphase interfaces of PP/EPR and EPR/PE,enhancing the interfacial interactions and improving the compatibility.It was also found that the sequence of in situ introduction of PE has an important influence,and the components of PP/PE/EPR alloys polymerized in situ in the PP-PE-EPR polymerization sequence have excellent initial dispersion,well-defined phase structure and minimal dispersed phase size.The technique of inducing EbP enrichment to the multiphase interface provides new opportunities for the rational design of next-generation stiffness and toughness balanced impact polypropylene copolymer.(3)The effects of introducing ICA in the gas phase copolymerization stage of multiphase polypropylene on the polymerization microchemical environment,the characteristics of the dispersed phase,and the particle growth were investigated.The results showed that the rubber content gradually increased with the increase of ICA molar concentration,the ethylene content of rubber decreased dramatically,and the molecular weight of rubber increased significantly.On this basis,the mechanism of ICA-induced capillary condensation within the porous polypropylene particles was proposed.It was also found that the particle size of multiphase polypropylene particles would show a bimodal distribution bounded by the critical characteristic length required to maintain capillary condensation.Increasing the concentration of ICA can reduce the fine particle content and the heterogeneity between particles.(4)To address the problems of sticky surface and low copolymerization activity of gas-phase process particles,the Damp mode was used to form micro-zones with concentration gradients at the particle scale based on the induced capillary condensation mechanism to achieve the regulation of rubber phase distribution of multi-phase polypropylene particles.The multiphase polypropylene particles prepared by the Damp mode have a multi-zone structure with the core-zone rich in rubber phase and a shell zone containing a very small amount of rubber phase,which suppresses the surface stickiness of the multiphase polypropylene particles.The rubber phase content limit of the Dry mode product is about 30 wt%before the particles become sticky,while the Damp mode can raise it to more than 45 wt%.The results of the study create a new process route for the production of high rubber content polypropylene resins in general purpose polypropylene industrial plants.(5)A liquid-containing gas-solid fluidized bed was used as the study object,and the temperature of the reactor was changed temporally to simulate the temperature alternation of particles in the gas-liquid-solid and gas-solid zones spatially,and then the effects of the multi-temperature zone and temperature switching process on the microstructure of polyolefins were investigated.The results show that the effect of ICA on the polymer microstructure in the gas-solid and gas-liquid-solid zones is the same pattern,indicating that the induced capillary condensation effect of ICA in porous polymers is independent of the phase state(vapor or liquid phase)in which ICA exists.The effect of temperature dynamic switching in the condensed mode is significantly greater than that in the Dry mode.In the condensed mode,the temperature dynamic switching causes an increase in the content of lamellar crystals of various thicknesses of the polymer,a broadening of the molecular weight distribution,a significant decrease in the co-monomer insertion,and a significant increase in the content of thick lamellar crystals in particular.The results of the study provide guidance for the industrialization and product development of the Damp mode.