Study On The Correlation Between Structure And Properties And Mechanism Of Good Creep Resistance For PPR Used Pipes

The use of Polypropylene (PP) pipes for both warm and cold water transport in housing construction was about 2 decade ago. Isotactic PP homopolymer (PPH), ethylene-propylene block copolymer (PPB) and ethylene-propylene random copolymer (PPR) have been successively developed for pipes. The amount of PPR consumption capacity for pipes has been increased by 15 percent each year since 1990 in the Europe. PPR pipes were recommended by China construction ministry and substituted for steel pipes used for discharging and supplying water in the house.PPR resins for pipes in which co-monomer is usually ethylene, have a larger molecular weight, broad molecular weight distribution (MWD) and melting point of (140-150)°C. The long-term strength in hot water for PPR pipes is larger than that for PPH and PPB. Now PPR pipe grades used in China are mainly RA130E, BA202E, Vestolem 9421 and R200P and imported from abroad due to disadvantages of PPR pipe resins produced by home company in the long-term mechanical properties. Therefore, it is urgent to study and establish correlation between structure and properties so that PPR with good pipe properties can be produced in commercial PP plants used Ziegler-Natta catalyst in China.In the present study, PPR, PPB for pipes and PPH and PPB with ultra-high molecular weight (UHPPH, UHPPR) produced with Ziegler-Natta catalyst in the industrial PP plant are studied and compared with the imported PPR in the molecular structure, crystallization and melting behavior, rheological properties. On the other hand, the mechanism of high long-term strength at high temperature for PPR pipes is put forward. The main goal of these works is to establish if crystallization kinetics may be areliable means to foresee PPR durability in relation to molecular architecture.1. The related literatures on structure, crystallization and melting behavior and properties of PPR and the outline of this dissertation have been summarized in detail.2. PPR pipe resins and PPH, PPB and PPR with ultra-low melt flow rate (MFR) were produced with Ziegler-Natta catalyst under condition of no molecular weight modifier hydrogen in the PP plant of year capacity 200kt. Effect of the process variables such as hydrogen concentration and ethylene feed volume in the reactors on MFR and ethylene content of PPR was also discussed.3. The degrading behavior during extrusion in a twin-screw extruder of model ZSK-300 and its influence on mechanical properties for ethyl ene-propylene random and block copolymer PPR and PPB for pipe were studied. The effects of stabilized agent, MFR and melt temperature on degrading behavior were also discussed. The results show that degrading rate markedly decreases only if melt temperature falls for PPR and PPB. Degrading rate of PPR and PPB with lower MFR is larger than that with higher MFR. That degradation is favor for the longer molecule chains results in decrease of molecular weight, narrow MWD and worse of mechanical properties.4. The isothermal and non-isothermal crystallization and subsequent melting behaviors of PPR 1, PPR2, PPB1 pipe resins, UHPPRl and UHPPH1 have been studied by the differential scanning calorimetry (DSC), of which UHPPRl is for the first time. For isothermal crystallization and their melting properties, the experimental results show that crystallization rate of UHPPH1 is larger that of UHPPRl and the ultra-high molecular weight chains don't behave as nucleate agent. Avrami mode can successfully describe PPR1, PPR2, PPB1, UHPPRl and UHPPH1 crystallization kinetics. Their Avrami exponents for PPR1 and UHPPH1 are as high as 3.5 and larger than these of isotactic polypropylene with the conventional molecular weight.For the non-isothermal crystallization process, the fraction of ultra-high molecular weight chains of UHPPH1 acts as nucleate agent under lower cooling rates, but UHPPRl does not, which indicates that co-monomer ethylene in the UHPPRl increases the chain flexibility and does not contribute to nucleus in the process of non-isothermal crystallization. Avrami exponents n of five samples studied for non-isothermalcrystallization are larger than those for isothermal crystallization respectively, of which the largest n value is close to 6. In addition to PPB1, PPR1, PPR2, UHPPRl and UHPPH1 totally show secondary kinetics process, which results in the failure of the Ozawa model describing the non-isothermal crystallization kinetics.It was found the facts that the increase of ethylene content of PPR doesn't result in the appearance of more prominent multi-melting peaks in the DSC melting thermographs, which is contrary to the results reported in the literatures. The multi-melting mechanism of PPR and UHPPR is melting-recrystallization-remelting in the heating process after isothermal and non-isothermal crystallization.5. The crystalline morphologies, rheological and dynamic mechanical properties were analyzed and compared with one another by means of wide-angle X-ray scattering (XRD), polarized light microscopy (PLM), dynamic mechanical analysis (DMA) and capillary rheometer. The results demonstrate that a phase, no y phase are observed for five samples after treatment of fast cooling or annealing at 126°C for 30min from melt. It was found that the melt all show pseudo-plastic flow. Because of the existence of ultra-high molecular weight chains, the critical shear rates of melt fracture for UHPPRl and UHPPH1 are much lower than those for PPR1, PPR2 and PPB1.6. The viscoelastic behaviors of UHPPR2, UHPPH2 and PPR1, produced with Ziegler-Natta catalyst, have been investigated by means of oscillatory rheometry at 180 °C, 200 °C and 220 °C. Loss modulus (G") curves of 180°C and 200°C present a pronounced maximum peak at 38.10rad/s and 84.70rad/s for UHPPR2, 34.35rad/s and 69.21rad/s for UHPPH2, respectively. This makes it possible to directly determine theplateau modulus (C?°) of UHPPR2 and UHPPH2 with broad MWD from a certain experimental temperature G"(co) curve for the first time. The G°N at 180°C and 200°Cdetermined to be 4.51 X 105Pa and 3.67X 105Pa for UHPPR2, 4.28 X 105Pa and 3.62 X 105Pa for UHPPH2, respectively, decrease with increase in temperature and are independent of molecular weight, which directly confirms repation theoretical prediction relating G°N to molecular weight. These results also suggest that the random...