高密度聚乙烯的熔融支化及其对性能的影响木

在引发剂过氧化二异丙苯、二官能度单体新戊二醇二丙烯酸酯和自由基活性调控剂二甲基二硫代氨基甲酸锌的存在下,使高密度聚乙烯进行熔融支化反应.研究表明,转矩曲线上的反应峰顶对应最佳反应时间,由此获得了凝胶量低的长链支化高密度聚乙烯.熔融支化反应使聚乙烯的分子量分布变宽,其支化程度随单体含量的增加而增大,呈现出更加明显的剪切变稀行为;长链支化结构的引入使改性聚乙烯的结晶度降低,长支链的成核作用使起始结晶温度增加,球晶尺寸明显减小.改性聚乙烯的支化程度和大分子拓扑结构的变化对耐环境应力开裂性能的影响显著,当单体含量超过0.6 phr时,长链支化分子形态从类似不对称星形转变为梳形,使得高密度聚乙烯的耐环境应力开裂时间产生突变,达1000 h以上,同时强度、模量和冲击韧性均得到明显提高.

Melt Branching of High Density Polyethylene and Influences on Its Performances

Melt-branching of high density polyethylene (HDPE),through free radical reaction,was carried out in the presence of dicumyl peroxide (DCP) as the initiator,neopentyl glycol diacrylate (NPGDA) as difunctional monomer,and zinc dimethyldithiocarbamate (ZDMC) as the free radical concentration regulator.Results showed that HDPE with long branching chains and low gel content was obtained under optimized reaction time,which was corresponding to the reaction peak-the second peak in the torque curve.After this melt branching reaction,the molecular weight of the modified HDPE increased slightly and its molecular weight distribution became wider compared with the virgin linear HDPE.The branching degree of modified HDPE increased with increased monomer content,with a more apparent shear thinning in the complex viscosity versus frequency (rη*-ω) plots.The regularity of the polyethylene molecule was broken up after the long branching chains being introduced,which decreased the degree of crystallinity in the modified polyethylene.While the crystallization onset temperature improved and the size of spherocrystal obviously decreased with the nucleation of the long branching chains in the modified HDPE,a threshold value for the branching was shown to exist by the environmental stress cracking resistance test on the modified polyethylene.When monomer content exceeded 0.6 phr,the molecule shape of the long chain branched polyethylene transformed from asymmetric star-like to comb-like,which made the environmental stress cracking resistance time be larger than 1000 h,the strength,the modulus and the impacttoughness were all enhanced at the same time.However,the branching degree was reduced with an overlong time mixing and melt-branching reaction,the comb-like branching chains were not formed by consequence,which made an inapparent enhancement for the environmental stress cracking resistance performance of the modified polyethylene.