Tailoring Polyethylene Microstructure By The Phase State Of The Inert Condensing Agent And Liquid-Containing Particle Polymerization Mode

Gas phase fluidized bed technology is currently the main process for producing ethylene/α-olefin copolymers.Since the polymerization of ethylene is a strongly exothermic reaction,the inability to remove the polymerization heat in time will greatly limit the space-time yield of the gas phase fluidized bed reactor.In order to improve the heat removal capacity and space-time yield of the reactor,the condensed mode operation came into being.The condensed mode operation uses the balance between the fluidizing gas to absorb the sensible heat of temperature rise and the latent heat of vaporization by the inert condensing agent by spraying the inert condensing agent into the reactor,thereby increasing the space-time yield of the reactor.Not only that,scholars have discovered that the injected inert alkane condensing agent has an important impact on the properties of the polymerized product,and proposed that the inert condensing agent is not inert for ethylene polymerization.It is generally believed that the inert condensing agent will quickly evaporate in the fluidized bed,then take away the reaction heat and form a gas-solid phase reaction zone.The inert condensing agent in the gas phase will change the reaction environment near the active center during the growth of the polymer particles,thereby changing the polymerization kinetics and the structure of the polymerization product.However,it is generally believed that there can be no liquid phase zone in the fluidized bed to prevent unstable operation of the fluidized bed because of the limited cognition that the inert condensing agent will evaporate quickly when entering the fluidized bed.There is only a gas-solid zone in the gas phase fluidized bed under condensed mode operation,and its single reaction environment limits the high performance of polymer products.Scholars have studied the difference between the ethylene polymerization reaction in the gas phase environment and the liquid phase environment,and found that there are significant differences in the ethylene polymerization reaction kinetics and product properties in the two environments.Inspired by this,it is possible to construct two different polymerization environments of the gas phase and the liquid phase by adjusting the phase state of the inert condensing agent in a single reactor,then study the law of the influence of the phase state of the inert condensing agent on the polymerization reaction and the properties of the product.At the same time,a liquid-containing particle polymerization mode under the inert condensing agent gas-liquid equilibrium state is established in a single reactor with the change of the content of the inert condensing agent to realize the switching polymerization of catalyst particles and polyethylene particles under different reaction environments.Use this method to design polyethylene product structure and produce high-performance polyolefin materials.In this paper,Ziegler-Natta catalyst is used to study the influence of the phase state of the inert condensing agent in the laboratory autoclave on the polymerization reaction and its products.Then,high-performance polyethylene products are produced in industry based on the pre-theory.Finally,industrial polyethylene products are analyzed by different characterizations to set up the relationship between structure,performance and process.It is mainly divided into the following four parts:(1)The gas phase polymerization reaction of ethylene with the participation of the gas phase inert condensing agent was studied.With the increase of the inert condensing agent content,the polymerization reaction rate decreases in the early stage and increases in the later stage.The overall polymerization activity increases,and the molecular weight of the polyethylene product increases,and the crystallinity increases.After eliminating the thermal history,the secondary crystallinity is still high because it is affected by the inert condensing agent,called solvent annealing effect.The"cosolubility" effect of the inert condensing agent on the polymerization is weakened under higher ethylene pressure or higher reaction temperature,and the polymerization system tends to a dry state polymerization mode.The polymerization system tends to the dry mode,the polymerization rate and the molecular weight of the polyethylene product increase with the increase of the inert condensing agent content.The larger molecular size of the inert condensing agent(heavier component),the more significant of the "cosolubility" effect.The polymerization rate increases more.The chemical dormancy effect of hydrogen on the polymerization reaction is more obvious with the increase of the inert condensing agent content,the polymerization rate is reduced,the molecular weight of polyethylene does not change much,the crystallinity is slightly increased,and the secondary crystallinity is greater than the primary crystallinity,and the secondary crystallinity is no longer affected by the annealing effect of the inert condensing agent.(2)The ethylene polymerization reaction with gas phase α-olefin used as comonomer and the effect of gas phase inert condensing agent are studied in the autoclave.With the increase of the ’comonomer concentration,the comonomer effect is significant,the polymerization reaction rate first increases and then decreases,especially in the early stage of the polymerization reaction.Owing to the comonomer is easy to condense inside the catalyst and particle pores and produce oligomers to cause diffusion resistance’ the rate of polymerization is greatly suppressed.The larger the molecular size of the comonomer,the weaker the promoting effect on the polymerization reaction.As the concentration of comonomers increases,the molecular weight of polyethylene decreases,the crystallinity decreases,and the degree of branching increases.Under the condition of the coexistence of inert condensing agent and comonomer,with the content of inert condensing agent increases,the content of oligomer decreases,the polymerization rate increases,the molecular weight of the polyethylene product increases,the crystallinity increases,and the degree of branching decreases slightly.Adding 1-butene in the polymerization can increase reaction-rate,the oligomer content decreases,and the branching degree of the polyethylene increases,especially the insertion rate of 1-hexene increases.(3)The best 1-hexene/ethylene initial molar ratio of the polymerization rate in the gas phase and slurry environment is investigated,which are about 2.43%and 138.33%.Furthermore,three reaction atmospheres are established:a pure gas phase environment,a poor-hexane environment,and a rich-hexane environment.The polymerization reaction rate in the pure gas phase environment gradually increases with the content of n-hexane increases,the molecular weight of polyethylene increases,the crystallinity increases,and the degree of branching decreases slightly.The particles in the poor-hexane environment can switch between dry state and wet state to realize the liquid-containing particle polymerization.The reaction rate in the poor-hexane environment gradually increases,and reaches the maximum when the 1-hexene/ethylene initial molar ratio of the gas and liquid phases are both around the optimal initial value.The molecular weight of polyethylene decreases,the crystallinity decreases,and the degree of branching increases.The polymerization reaction rate in the rich-hexane environment decreases slightly,the molecular weight of polyethylene decreases,the crystallinity decreases,and the degree of branching decreases.The liquid-containing particle polymerization can broaden the distribution of the branches between the molecular chains,so that the branch content of the high molecular weight part increases.(4)Comparing the liquid-containing particle polymerization with the traditional condensed mode polymerization,it is found that the ethylene/1-butene/1-hexene terpolymer contains the same oligomer content,and the inert condensing agent has no inhibitory effect on the oligomer.For the ethylene/1-hexene copolymer,the inert condensing agent of the liquid-containing particle polymerization has a significant inhibitory effect on the oligomer.At the same time,it is found that the polyethylene of the liquid-containing particle polymerization has high molecular weight,wide branching degree distribution,narrow lamella size distribution,and excellent mechanical and optical properties.It can be successfully realized the design and control of the polyethylene product with constructing different gas-liquid phase environments in a single reactor.