Study On The Self-reinforcement Mechanism Of The Polyolefins Pipe Prepared By Pulsatile Extrusion

Plastic pipes are widely and importantly used in many fields including building, municipal construction, hydraulic engineering and gas transportation etc.. Compared with traditional pipes (cast iron pipe, concrete pipe etc.), plastic pipes have multiple advantages such as low weight, small flow resistance, excellent corrosion resistance, good sealing performance, convenient installation, low energy consumption and long service life. So plastic pipes play a very important role among industrial materials and the consumptions every year are very tremendous. However, the strength of plastic pipe is much lower than traditional pipe. And in engineering applications, plastic pipes have to be reinforced to meet the high demands on strength. Besides the additive-based reinforcement plastic pipes, the self-reinforcement plastic pipes have become one of the hot topics at home and abroad based on continuous development.The electromagnetic dynamic plasticating processing technology and equipment for polymer provide a new research direction for polymer processing theory and technology, which introduces a vibration force field into the entire polymer process by the axial vibration of the screw. The particular processing technology has a great impact on the polymer plasticating and extrusion processing, which makes the polymer processing technology, application and theory change greatly. The previous research showed that the vibration force field can improve the mechanical properties and apparent quality of the products. But at present, no investigation has been carried out on the effect of the vibration force field on the properties of plastic pipes prepared by the electromagnetic dynamic plasticating processing technology. The detailed research on the relationship between microstructures and mechanical properties, and the self-reinforcement mechanism of plastic pipes obtained by pulsatile extrusion, and study on the impact of pulsatile extrusion on polymer by experiment and theory ways can provide the theory guidance for polymer dynamic extrusion processing, which has an important scientific and realistic significance.An electromagnetic dynamic plasticating extruder equipped with a specially designed spiral mandrel die for pipe was used in this work. The melt flow characteristic of the spiral mandrel die for pipe was studied by numerical simulation, and the reliability of numerical simulation was validated by experiment. Based on the orthogonal experiment, the optimum design of spiral mandrel die for pipe was achieved by aiming at the distribution uniformity of the exit velocity of the spiral distribution system. The whole experiment device for pipe extrusion was builded according to industrial production.The vibration force field was introduced into the whole extrusion process for polyolefins pipe under industrial production condition, and the effect of the vibration parameters on the mechanical properties, thermal properties and microstructure of plastic pipes was studied. The results showed that the circumferential strength of pipes increased significantly, and biaxial self-reinforcement pipes could be obtained by pulsatile extrusion, which has an important significance for the pipe application. When compared with the static extruded pipes, for HDPE pipes, the mechanical properties testing showed that the maximum increase of bursting pressure, tensile yield strength, and impact strength were 34.2%, 5.3%, and 20.2%, respectively. For PP pipes, they were 27.03%, 7.3%, and 16.2%, respectively. The heat resistance of HDPE pipe was also improved. When the vibration frequency is 6Hz and vibration amplitude is 200μm, the Vicat softening temperature of HDPE pipe increased 3.1℃compared with the static extrusion pipe. The DSC, WAXD and SEM analysis showed that the polyolefins pipe prepared by pulsatile extrusion had higher melting temperature, higher crystallinity, more perfect crystals and higher orientation degree of crystalline planes, but no change in the crystalline form.The slow crack growth resistance is the key factor to improve the plastic pipe application. The plastic pipe should have excellent slow crack growth resistance in order to ensure the service life. The slow crack growth resistance of polyolefins pipe obtained by pulsatile extrusion was investigated in this paper, and the effect and mechanism between pulsatile extrusion and slow crack growth resistance of HDPE pipe was studied. The results showed that the HDPE pipes prepared by pulsatile extrusion had lower crack growth rate, namely the slow crack growth resistance was increased by pulsatile extrusion, which has an important realistic significance for the pipe application. The vibration force field introduced by axial vibration of the screw can make the HDPE melt form crystal nuclei more easily to facilitate the growth of HDPE crystals. The HDPE pipes prepared by pulsatile extrusion had higher crystallinity, higher melting temperature, larger lamellar thickness, larger crystal sizes and more perfect crystals, and the molecular chains formed a network structure. All these were favorable for the improvement of slow crack growth resistance of HDPE pipes.The mathematical model was established based on reasonable assumption and time-dependent flow considered. The polymer melt flow of spiral mandrel die for pipe was firstly studied by three-dimensional non-Newtonian and isothermal numerical simulation under pulsatile extrusion, and the numerical simulation result was well consistent with the experiment result, which provided a new method and idea for the numerical simulation of the polymer melt flow characteristic in the dies under vibration force field. The research showed that the melt shear rate, viscosity, shear stress, pressure and velocity had the cycle change when the vibration force field was introduced. On the other hand, The average value of shear rate increased with the increase of the vibration frequency or amplitude, but the average value of the viscosity, shear stress and pressure decreased.Finally, the microstructure results and the melt rheological properties obtained by numerical simulation were discussed and summarized, and the self-reinforcement mechanism of polyolefins pipe was analyzed based on the relationships among the pulsatile extrusion process and microstructure and property. The improvement of mechanical properties of polyolefins pipe using the pulsatile extrusion process was attributed to the higher crystallinity and the improvement of the molecular orientation and of the crystalline morphology under the influence of a vibration force field. Also, the significant increase of the circumferential strength of pipe was mainly assigned to the axial vibration of the screw which improved the circumferential degree of orientation of molecular chains.The above research achievements enrich the theory and content of polymer dynamic processing technology, and the knowledge about the effect of the vibration on the melt rheological behavior, the product structure and properties during pulsatile extrusion. Also, the experimental data and theoretical basis are provided for the optimization of the vibration parameters and mold design. The research has an important scientific and realistic significance for the further research and popularization of the polymer dynamic processing technology and the production of high performance polymer products.