Structure And Properties Of Self-reinforced PE-HD Pipes Extruded From Specially Applied Fields

In this dissertation,the fundamental theories,processing methods,physical properties and morphologies of high-performance self-reinforced high-density polyethylene (PE-HD) pipes were described and discussed in detail. By controlling of the applied fields,including temperature,pressure,shear and tensile stress,both uniaxially and biaxially self-reinforced PE-HD pipes were achieved.According to the principles of polymer processing and morphology controlling technology,three types of extrusion dies were designed and manufactured to study the effect of uniaxial and biaxial tensile flow and couette flow on the structure and properties of self-reinforced PE-HD pipes. The distribution of stress-strain of polymers in the dies was analysed using plastic mechanics. Two types of PE-HD materials (5000S,6100M) were employed to produce self-reinforced pipes. Physical properties and microstructure were measured comprehensively. The experimental results showed that:a. Uniaxially self-reinforced PE-HD pipes were extruded through uniaxial tensile flow die,showing a 223-760% improvement of the tensile strength in axial direction and a 91% increase of the tensile strength in hoop direction. The pipes were transparent and hard,with a 76-80% improvement of surface hardness. The softening point of the self-reinforced pipes was 30 higher than that of the conventional ones,exhibiting very good temperature tolerance. The morphology of the self-reinforced pipes was studied by means of scanning electron microscope (SEM) atomic force microscope (AFM), and wide angle X-ray diffraction (WAXD). There existed a shish-kebab morphology in the uniaxially self-reinforced PE-HD pipes.b. Biaxially self-reinforced PE-HD pipes were produced through an extrusion die with an expanding channel geometry. They showed a 69% increase of tensile strength in both axial and hoop directions,together with significant improvement of elongation at break. The softening point of the self-reinforced pipes was 16'C higher than that of the conventional ones. And there was 120% improvement of surface hardness. SEM and AFM observation and WAXD patterns depicted a morphology of ellipsoidal crystals. The long axial of the ellipsoid was at an angle of 45 to the axial direction of the pipe.c. Hoop enhancement of PE-HD pipes was achieved with the mandrel-rotating die under low melt pressure (P<8MPa). The pipes exhibited 4-fold improvement of hoop tensile strength and 70% increase of burst pressure .The formation of fibril structure along hoop direction and elimination of weld-lines resulted in the high hoop strength of the pipes.The uniaxially self-reinforced pipes are suitable for structural application. The biaxially self-reinforced pipe,with excellent stiffness and toughness,are prospective for the industrial application as pressure pipelines.