High Performance High Density Polyethylene Based Materials Prepared Through Stress-Induced Reactions

Functionalization of HDPE through activated initiation by ultrasonic irradiation during melt extrusion was studied. The effects of ultrasonic intensity and rotation speed of twin screw extruder on the functionalization reaction of HDPE with maleic anhydride(MAH), with MAH and γ-methacryloxy-propyltrimethoxysilane(A174) as well as the morphology and mechanical properties of functionalized HDPE based materials (HDPE/CaCO3, glass fiber(GF) reinforced HDPE) were considered. The experimental results show that the method proposed is clean, efficient and easy in operation. Conclusions are as follows:1. The macroradicals produced due to stress-induced scission of HDPE chain caused by ultrasonic vibration during melt-extruding process will lead to the functionalization reaction of HDPE with MAH or with MAH and A174 mixture, some graft (block) polymers will be formed therefrom. The functionalized products with higher percentage of grafting(blocking), a higher melt flow rate and a lower gel content can be obtained.2. Experimental results of FTIR and 1H-NMR indicated that stress-induced macroradicals from scission of HDPE chains reacted with MAH to produce macroradicals with anhydride end group, and some of which contain a doublebond on the end chain. In contrast, the functionalized products prepared through peroxide initiation mainly contain an anhydride ring grafted on the side chain.3. Compared with the peroxide initiated product, the melt flow rate of the ultrasonic initiated product is increased from 0.05 g/lOmin to 1.8 g/lOmin at 270 "Cand 275W ultrasonic intensity. The cross-linking reaction can be effectively retarded through adjusting ultrasonic intensity. The percentage of grafting(blocking) of the product increased from 0.11% to 0.5% while 275W ultrasonic intensity was imposed at 270 °C.4. The percentage of grafting(blocking) of the functionalized products prepared through high shear stress-induced process increased from 0.08% to 0.68% with increasing of screw rotation speed of the twin screw extruder from 50 rpm to 600 rpm during melt-extruding process, The cross-linking reaction can be prevented by optimizing screw rotation speed.5. The functionalized products obtained by stress-induction show much higher reactivity with the coupling agents coated on the surface of glass fiber than that obtained by peroxide initiation. The mechanical properties of HDPE based materials can be greatly enhanced by the functionalized products both from ultrasonic activation and high shear stress-induced reaction. The tensile strength, flexural strength, flexural modular and Izod impact strength of HDPE/GF material modified by ultrasonic-induced product increased from 38.7MPa, 44.1MPa, 2950 MPa and 5.3 kJ/m2 to 55.8 MPa, 64.0 MPa, 4320MPa and 12.7 kJ/m2, respectively. The tensile strength, flexural strength, and Charpy notched impact strength of the HDPE/GF material modified by high shear stress-induced product increased from 48.6MPa, 78.3MPa, 16.3kJ/m2 to 62.2MPa, 95.0MPa and 20.5 kJ/m2, respectively.6. Both stronger ultrasonic intensity and higher screw rotation speed are favorable to the distribution of CaCO3 particles in HDPE matrix and enhance the toughness of HDPE/CaCO3 blend. The Izod impact strength of the HDPE/CaCO3 (70/30) blend increased from 25.4kJ/m2 to 33.6kJ/m2 when ultrasonic intensity increased from 0W to 160W, and increased from 36.5kJ/m2 to 50.5 kJ/m2 when screw rotation speed increased from 100 rpm to 200 rpm. Meanwhile, the tensilestrain at break, the flexural strength and the flexural modulus of the blends increased slightly.7. SEM observation of the fractured surface of HDPE/GF composites displays that there is an oriented crystal layer existing between the interface of GF and the matrix, which is favorable to the improvement of the mechanical properties. The interfacial bonding strength between GF and HDPE matrix is the key factor determining the formation and thickness of the oriented crystal layer. A suitable cooling rate of the specimens of HDPE/CaCO3 blends during molding process can promote interfacial stress-induced crystallization of the matrix, which is advantageous to both toughening effect and crystallization of the matrix.