Study On The Low-Temperature Molding Method And Properties Of Oriented Uhmwpe Products Prepared By Synergistic Effect Of Pulse Vibration Molding And Melt Flow

Ultra-high molecular weight polyethylene(UHMWPE)products have excellent mechanical properties and wear resistance.Due to its high viscosity,however,it usually adopts the static compression molding(CM)method relying on heat conduction molten to manufacture UHMWPE products,which generally has problems of long molding time,high energy consumption,weak mechanical properties,and poor wear resistance.For the dynamic compression molding,UHMWPE products produced by high velocity compaction(HVC)also have some problems,such as poor fracture toughness,weak fracture strength and ductility,while ultrasonic energy in the process of ultrasonic vibration molding is prone to cause chain fracture and thermal degradation,which reduces the performance of products sharply.Therefore,in this paper,the pulse vibration molding(PVM)method is introduced into the process of manufacturing UHMWPE,and the low-temperature molding process method of UHMWPE products prepared by synergistic effect of pulse vibration molding and orientation induced by melt flow(OPVM)is proposed.The low-temperature efficient molding mechanism of UHMWPE products under action of pulse vibration pressure and the regulation method of melt flow are systematically explored and clarified.The internal relationship between"molding process parameters-microstructure-macroscopic properties"during the process of PVM and OPVM is studied.This provides a new method and technology for the molding process of UHMWPE.The influence of mold temperature T and pulse vibration frequency f_m at the molten hot-pressing stage is investigated by comparison of the micromorphology and macroscopic properties of UHMWPE samples manufactured by CM and PVM(CM-UHMWPE and PVM-UHMWPE)molding,additionally,the mechanism of pulse vibration pressure on the consolidation of particle interfaces is also revealed.The results show that with pulse vibration pressure,the compaction and exhaust can be quickly performed,thus the particle interfaces can be fully contacted.Furthermore,the friction heat between particles and dissipated heat generated by plastic deformation of particles are beneficial to accelerate melting and promote fusion.Meanwhile,the structure and property of the core layer and surface layer of products are homogenized.This successfully achieves efficient molding of UHMWPE products at low temperature,which effectively preserves the structural characteristics of high crystallinity and regularity of UHMWPE nascent phase to enhance the mechanical strength of products.With the increase of molding temperature T and pulse vibration frequency f_m,the movement ability and diffusion rate of molecular chains increase.In particular,the formation of crystal structure perpendicular to the interfaces at particle interfaces results in the effective interface fusion and good consolidation quality.However,when the molding temperature exceeds 170°C or the frequency f_m exceeds 3Hz,the structure characteristics of UHMWPE nascent phase are seriously damaged by reducing the crystallinity and lamellae thickness,which impaires the strength of UHMWPE products.In order to improve the toughness of PVM-UHMWPE products,the innovational method of oriented UHMWPE products prepared by OPVM(OPVM-UHMWPE)is proposed,and melt flow is initiated at a different time of molten hot-pressing stage to prepare OPVM-UHMWPE samples.The influence of melt flow at a different time on the microstructure and macroscopic properties of OPVM-UHMWPE is compared and studied,and the relationship between microstructure,such as consolidation of particle interfaces,orientation degree of molecular chain segments,crystal structure,and melt flow at different time is revealed.The results show that the closer the melt flow initiated time is to the end of the hot-pressing stage,the higher the degree of diffusion of molecular chain segments formed at the interfaces is,which is more conducive to maintaining the induced orientation of molecular chains.Therefore,amounts of highly regular lamellae structures perpendicular to the flow direction at particle interfaces are produced to enhance the interfacial strength and improve the macroscopic performance.The influence of pulse vibration frequency f_m at the hot-pressing stage on the melt flow at the end of hot-pressing stage is further explored,and the effect of OPVM with different frequency f_m on the microstructure of OPVM-UHMWPE samples,especially on the consolidation at particle interfaces,is also investigated.The response law of microstructure to melt flow at different frequency f_m is revealed,and the influence mechanism of microstructure on macroscopic properties is expounded.The results show that the impact energy of pulse vibration pressure increases with the augment of f_m,which intensifies the melt flow behavior,increases the orientation degree of molecular chain segments,and significantly improves the overall crystallinity.With the increase of f_m to high frequency(3 Hz,5 Hz),many highly regular lamellae structures derived from more intense melt flow perpendicular to the flow direction are produced at particle interfaces,which substantially improves the consolidation strength of particle interfaces and remarkably reinforces the tensile properties and wear resistance.Compared with CM-210°C-60min sample,the crystallinity and melting temperature of OPVM-170°C-3Hz sample are about 18%and 1.4°C higher,respectively.The yield strength,tensile modulus,tensile stress at break,and work-to-failure(WF)of OPVM-170°C-680s sample are increased by approximately 11%,52%,71%,and 45%,respectively;and the wear rate and index are decreased by about 61%and 39%,respectively.The low-temperature molding process method of oriented UHMWPE products promotes the consolidation of UHMWPE particles through heat generation under the action of pulse vibration pressure and synergistic effect of pulse vibration molding and melt flow,the forming cycle and temperature are notably shortened and reduced,the strengthen and toughen UHMWPE products are successfully manufactured.This provides a new methodology and technology for the molding of polymer materials with extreme rheological properties such as UHMWPE,which is in favor of promoting the development,popularization and application of dynamic molding technology and its equipment for polymer materials,and enriching the basic theory of polymer materials dynamic processing.