Effect of adding high molecular weight PE-HD on sag resistance of bimodal polyethylene
According to the problems that the bimodal polyethylene for pipes exhibits insufficient sag resistance,leading to uneven wall thickness in large-diameter thick-walled pipes,firstly,the high-temperature gel permeation chromatography was employed to characterize average molecular weight,molecular weight distribution,and short-chain branch distribution of the bimodal polyethylene.By analyzing the characterization results,it was found that the significant proportion of low molecular weight fractions is a crucial factor affecting sag resistance of the bimodal polyethylene.Subsequently,a melt flow rate(MFR)tester,a micro compounder rheometer,and a rotational rheometer were used to investigate the effects of adding high molecular weight high density polyethylene(PE-HD)on sag resistance of the bimodal polyethylene,then the effects of the high molecular weight PE-HD on the crystallization and melting behaviors of the bimodal polyethylene were also examined.The results reveal that incorporating a small amount of PE-HD with an average molecular weight of 800 000 substantially reduces the MFR of the bimodal polyethylene.This addition increases the torque and melt viscosity during extrusion processing and enhances the zero shear viscosity(η0)and relaxation time(λ)of the system.A lower MFR,coupled with higher η0 and λ,indicates sag resistance of the bimodal polyethylene is improved.Consequently,the experimental results demonstrate that the addition of high molecular weight PE-HD can effectively enhance the sag resistance of the bimodal polyethylene.The incorporation of a small amount of PE-HD with an average molecular weight of 800 000 can lower the crystallization peak temperature of the bimodal polyethylene while increase its melting peak temper-ature.The results of above study provide valuable insights for sag resistance modification and pipe application of the bimodal poly-ethylene.
bimodal polyethylenehigh-temperature gel permeation chromatographysag resistancerheological behav-iorcrystallization behavior
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