| Ultra-high molecular weight polyethylene(UHMWPE)has excellent mechanical properties,such as impact resistance,abrasion resistance,high modulus,corrosion resistance,and so on.However,there are a large number of chain entanglements in the nascent UHMWPE particles,which limit the orientation,diffusion,and relaxation behavior of molecular chains,not only increasing the melt viscosity dramatically and limiting the melt processing;but also leading to mechanical properties to merely 1/3 of the theoretical value,far from the expected performance.Therefore,reducing chain entanglements of nascent UHMWPE is an important development direction for the high performance of UHMWPE.Chain entanglements in the nascent UHMWPE particles originate from its preparation process:in industrial production,UHMWPE is produced by a supported catalyst in a slurry polymerization process above 60℃.On one hand,the active sites are randomly distributed on the surface of the carrier,and the distance is close,which makes the adjacent active chains entangle.On the other hand,the high polymerization temperature(>60℃)results in the chain growth rate much faster than the chain crystallization rate,and the nascent chain segments do not have time to crystallize,leading to intramolecular entanglements.At industrial reaction temperature,existing research can only inhibit the formation of intermolecular chain entanglement by increasing the distance between active sites.Currently,it is still a challenge to reduce the entanglements based on adjusting the competition between chain propagation and crystallization,especially when the temperature is at the industrially preferred range(>60℃).Thus,the methods for reducing chain entanglement at the industrial polymerization temperature are still relatively lacking,which restricts the improvement of UHMWPE processing performance and mechanical performance.In this work,from the perspective of reaction engineering,three inert media(nitrogen,fluorosilicone and liquid propane)were introduced into the ethylene slurry polymerization respectively.In the above process,the mass transfer path of the solvent to the surface of the polyethylene particle intermittently cut off the supply of the reaction raw materials at the active center through the contact between the inert media and growing polyethylene particles,inhibiting the chain growth process.Therefore,the chemical environment where the chain crystallinity rate is greater than the chain growth rate at the industrial reaction temperature is established,which is beneficial to regulate the chain entanglement structure of the nascent UHMWPE.The main work and research results are as follows:(1)The effect of nitrogen and fluorosilicone oil on ethylene gas-liquid mass transfer process was studied,and the contact behavior between these two inert media and polyethylene particles was analyzed,which provided basis for the subsequent study of ethylene polymerization regulated by microbubbles and microdroplets.The gasliquid equilibrium state of the ethylene-heptane system in the presence of nitrogen was simulated.The study found that with the change in nitrogen/ethylene ratio,the distribution coefficient of nitrogen or ethylene component in the gas and liquid phase remained unchanged,indicating that the binary interaction between nitrogen and ethylene was small,and the nitrogen component did not affect the gaseous distribution of ethylene in heptane.The kinetics of ethylene dissolution and absorption with the assistance of nitrogen bubbles was measured.In the nitrogen bubble-ethylene-heptane system,the kinetic curves of ethylene dissolution and absorption in the nitrogen macrobubble(7551±998 μm)system and the nitrogen microbubble(543±198 μm)system almost completely overlap,indicating that the size of nitrogen bubbles does not affect the gas-liquid mass transfer characteristics of ethylene.The contact behavior between bubbles and particles was investigated by high-speed camera.The residence time of bubble-particle attachment in nitrogen microbubble case(～10-3 s)is comparable to that of nitrogen microbubble case,but the interval time of bubble-particle attachment in nitrogen microbubble case(～10-2 s)is an order of magnitude higher,indicating that the contact frequency in nitrogen microbubble case is higher.The study found that the ethylene dissolution and absorption kinetic curves of the pure heptane system and the fluorosilicone-heptane system almost overlapped,indicating that the fluorosilicone did not affect the solubility of ethylene in heptane and the gas-liquid mass transfer process.Under the action of mechanical shearing(300-1000 rad/min),the fluorosilicone is dispersed in heptane in the form of microdroplets(300-80 μm).The residence time of microdroplet-particle attachment(～10-2 s)is an order of magnitude higher than that in the nitrogen microbubble case,but the interval time between the two systems is similar,indicating that the microdroplets can also collide with polyethylene particles at a higher frequency.(2)The regulation effect of nitrogen microbubble on ethylene polymerization under different polymerization reaction conditions(nitrogen flow rate,polymerization time,reaction temperature)was investigated,and the influence of chain entanglement density on the mechanical properties of UHMWPE was studied.Meanwhile,the pilot test of nitrogen microbubble assisted ethylene polymerization was completed.The results of infrared thermal imaging temperature distribution showed that during the polymerization process,the average temperature of the nitrogen-rich microbubble region(57.4 ℃)was 0.5 ℃ lower than that of the non-bubble region,indicating that nitrogen microbubbles could reduce the polymerization reaction rate.With the increase of nitrogen flow rate(200-600 mL/min),the chain entanglement density of the synthesized polymer gradually decreased;with the prolongation of polymerization time(0.5-2 h),the chain entanglement density of the synthesized polymer could be maintained at a low level;with the increase of the reaction temperature(60-85℃),the chain entanglement density of the synthesized polymer shows a slow linear upward trend,indicating that nitrogen microbubbles can reduce the chain entanglements.Compared with the nitrogen macrobubble system,the synthesized polymer with the assistance of nitrogen microbubble showed a lower chain entanglement density,indicating that reducing the size of the bubbles can strengthen the regulation of the nitrogen bubbles on the chain entanglement formation.The aggregated structure of the polymerized product was further analyzed by solid-state NMR technology.It was found that compared with the nitrogen macrobubble case,the synthesized polymer with the assistance of the nitrogen microbubble had a higher content of monoclinic phase and a lower content of the amorphous phase.Based on the above results,a mechanism for regulating the entanglement of polymer chains by nitrogen bubble-ethylene slurry polymerization technology is proposed:nitrogen bubbles contact polyethylene particles,block the liquid-solid mass transfer path of the reactants,and intermittently cut off the reactants supply of the active center.Thus,the chain growth rate is decreased,creating a metastable crystallization environment for the primary chain segment,and the molecular chain is arranged to form a monoclinic crystal phase at this stage,thereby reducing chain entanglement.Compared with commercial UHMWPE with similar molecular weight,the tensile strength,tensile modulus,elongation at break,and impact strength of the synthesized polymer can be increased to 36.9 MPa(+16%),366.5 MPa(+19%),638.9 MPa,%(+35%),and 111.1 KJ/m2(+48%),respectively.In the pilot test of the 300 L reactor,nitrogen microbubbles can still exert their "dormancy" effect.Thus,UHMWPE with a low entangled state and high performance can be prepared.(3)The regulation effect of fluorosilicone microdroplets on ethylene polymerization under different stirring speed and reaction temperature was investigated,and the influence of chain entanglement density and molecular weight on the mechanical properties of the UHMWPE was studied.The study found that the ethylene polymerization activity regulated by microdroplets was reduced and the entanglement density of the synthesized polymer was lower,indicating that the microdroplets can block the liquid-solid mass transfer path of the reactants,causing a "dormancy" state for living chains,consequently,reducing the chain growth rate and inhibiting the formation of chain entanglement.Increasing the stirring speed,the polymerization activity and the chain entanglement density of the synthesized polymer were further reduced,indicating that strengthening the mechanical shearing effect and reducing the size of microdroplets can increase the contact frequency between microdroplets and polyethylene particles,strengthening the "dormancy" effect of microdroplets.Under the regulation of microdroplets,the chain entanglement density of the synthesized polymer showed a slow linear upward trend with the increase of polymerization activity at elevated temperature(65-85℃).The ethylene polymerization activity regulated by fluorosilicone is higher(>3 times),but the entanglement density of the synthesized polymer increases linearly with the increase of activity by an order of magnitude smaller,indicating that the fluorosilicone microdroplets have a stronger effect on reducing the chain entanglement.The reason is that the fluorosilicone microdroplets have better dispersibility,longer action time in the system,and their residence time(～10-2 s)in contact with polyethylene particles is an order of magnitude higher than that of the nitrogen microbubble-particle system,which makes the fluorosilicone microdroplets have a stronger "dormancy" effect on living polyethylene particles.For UHMWPE with molecular weights of 250×104,350×104,and 650×104 g/mol,the mechanical properties of UHMWPE were significantly improved with the decrease of the chain entanglement density,indicating that reducing chain entanglements is an effective means to realize the high performance of UHMWPE.As the molecular weight increases,the intermolecular force increases,and the mechanical strength of UHMWPE increases.When the molecular weight increases to 650×104 g/mol,the movement of molecular chains is severely restricted,the sintering efficiency decreases,and the mechanical strength decreases significantly,and further reducing chain entanglements can improve its mechanical properties.Therefore,for the UHMWPE with a molecular weight above 650×104 g/mol,reducing chain entanglements is one of the important means to exert the advantages of mechanical strength.(4)Using propane as the inert medium,the liquid-phase propane of low temperature and high pressure(greater than the reactor pressure)was introduced into the high-temperature reactor,and inert bubbles were generated by flash evaporation.An exploratory study was carried out on the regulation of propane bubbles in ethylene polymerization.It was found that at the industrial reaction temperature(70℃),the ethylene polymerization regulated by propane bubbles can also prepare UHMWPE with a low entangled state and high performance.Under the same reaction conditions,compared with nitrogen microbubbles,propane bubble-assisted ethylene polymerization can synthesize the weakly entangled polymer with higher polymerization activity,indicating that liquid-phase propane flashing has a better effect on regulating the formation of chain entanglement.The reason is that on the one hand,the propane bubbles can also inhibit the chain growth process through the "dormancy"effect on the polyethylene particles and gain time for the crystallization of the nascent segments;on the other hand,the propane flash absorbs the heat,reducing the temperature of the surrounding environment,further increasing the chain crystallization rate,consequently strengthening the regulation effect of propane bubbles on the chain entanglement formation. |
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