The polypropylene (PP) pipes were prepared using a self-made electromagnetic dynamic plasticating extruder, which introduced a vibration force field into the whole plasticating and extrusion process by the axial vibration of the screw. The effects of the vibration frequency and the vibration amplitude on the mechanical properties and microstructure of PP pipes were investigated using bursting pressure testing, tensile testing, impact testing, differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The mechanical properties testing showed that the circumferential strength of PP pipes increased significantly, and the biaxial selfreinforcement pipes could be obtained. Also, the impact strength was improved. When compared with the conventional static extruded specimens, the maximum increase of bursting pressure, tensile yield strength, and impact strength were 27.03%, 7.3%, and 16.2%, respectively. DSC and WAXD analysis showed that the PP pipes obtained by vibration plasticating extrusion (VPE) had higher crystallinity, higher melting temperature, more perfect crystals, and smaller crystal sizes, but no new polymeric crystalline peak appeared. The improvement of mechanical properties of the PP pipes prepared by VPE was attributed to the higher crystallinity and the improvement of the molecular orientation and of the crystalline morphology under the action of the vibration force field.

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom-made electromagnetic dynamic plasticating extruder was adopted to extrude high-density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high-circumferential-strength pipes.RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial selfreinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide-angle X-ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology.