| Polypropylene alloy is one of the important methods to optimize the properties and expand the application of polypropylene(PP)materials.The existing synthesis process of polypropylene alloy is mainly focused on the preparation of propylene/ethylene alloy.On the other hand,there is a large amount of cheap 1-butene resources to be utilized in the domestic petrochemical industry.Therefore,the exploration of the production method and performance optimization of propylene/1butene alloy has a high application prospect and research value.In this paper,the polypropylene/1-butene alloy was prepared by the monomer composition switching method in the bulk polymerization environment.The methods to manipulate its structure and properties were systematically explored.Referred to the technology of multi-zone circulating reactor,the Ziegler-Natta catalyst was used in a 10L autoclave to produce rigid PP particles;then 1-butene was added to start the copolymerization process.During the copolymerization stage,propylene was pulsed into the reactor multiple times to maintain the relative stability of the monomer composition and produce propylene-butene alloy.The influences of process parameters,such as switching frequency,hydrogen addition,propylene addition,and prepolymerization,on the polymerization activity,molecular weight,composition and primary particle morphology of the alloy were comprehensively analyzed.It is found that by adjusting the process conditions,the butene content of the copolymer can be adjusted between 5%and 18%,and the molecular weight can be adjusted between 300,000 and 700,000.The inner core of the primary alloy particles was mainly polypropylene while the outer part was polypropylene primary particles and an amorphous rubber phase.The product particles obtained in this study have a more obvious rigid phase compared with the simple copolymerization product of propylene and 1-butylene,and have more rubber phases compared with the products of the sequential polymerization process.The technological means to increase the rubber phase content include the prepolymerization,and increasing switching frequency or hydrogen concnetration.The crystallization behaviors of polypropylene and various propylene alloys have been characterized.Compared with PP,the crystallinity of all kinds of propylene/1butene alloys has decreased.Among them,the propylene-butylene simple copolymer and the product of the periodic switching method had more rubber phases,so the crystallinity decreased more.Polarized microscope observation showed that the number of intact spherulites was decreased and the crystals were finely divided.The increase of the content of rubber phase makes the crystallization more broken.Sequential polymerization products had relatively high isotacticity poly-1-butene segments,so the degree of crystallinity decline was small.A non-isothermal crystallization kinetics study was carried out,in which several kinetic models were used to calculate the kinetic parameters of the corresponding samples.It has been found that secondary crystallization occured in the polymer,so the Jeziorny method cannot fit the process.The Ozawa method has a good fit for the crystallization kinetics of the alloy produced by the monomer composition switching method.The Mo method can describe the nonisothermal crystallization behavior of polypropylene,propylene-butylene copolymer and sequential polymerization products more accurately.The relationship between the structure of the propylene/1-butene alloy and the mechanical properties and thermal properties was revealed.It was found that the glass transition temperature of the product decreased with the increase of the rubber phase content.Moreover,the rubber phase content and the "sea-island" structure are important factors that determine the mechanical properties of materials.Compared with PP,the impact resistance of all kinds of propylene/1-butene alloys has been significantly improved.Compared within three types of alloys:sequential polymerization products have fewer rubber phases and no "sea-island" structure,which leads to the highest Young’s modulus and tensile strength but the relatively inferior impact resistance.Simple copolymerization products have high rubber-phase content,but no "sea-island"structure,which results in the reduced rigidity and the impact resistance equivalent to that of the sequential polymerization product.The product of the monomer composition switching method has a rubber phase content similar to that of propylene/1-butene copolymer,and at the same time forms a "sea-island" structure.Therefore,it has the best impact resistance which is about 200%higher than the other two types of alloys.On the other hand,too high rubber content causes the rubber particles to be connected to each other and no longer to be distribute d in the form of spherical particles in the rigid matrix,which decreases the toughening effect.In the monomer composition switching polymerization process,pre-polymerization,switching frequency,and hydrogen content are all important means to control the rubber phase content and mechanical properties of the product. |
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