Comparison Of Crystallization Behaviors Between Unimodal And Bimodal Polyethylene For High Grade Plastic Pipe Materials

As a result of the great commercial importance of high grade plastic pipe materials such as PE80 and PE100, a lot of research work have been done regarding the molecular structures of these PE pipe materials with either unimodal or bimodal molecular weight distribution (MWD) in order to enhance the long-term property which is controlled by the so called slow crack growth (SCG). Normally, a better SCG resistance performance could be expected in pipe resins having bimodal MWD and this improved property is usually attributed to the prefered incorporation of co-monomer units in the long chains of the bimodal resins. This kind of co-monomer units distribution is suggested to favour the occurrence of intercrystalline tie-molecules during crystallization. In another word, the long-term property is determined by both the molecular structure and the crystallization process. However, limited research work has been found about the crystallization behavior of these pipe resins.In this work, the crystallization behaviors of one commercial PE100 high density polyethylene (HDPE) pipe resin and one PE80 HDPE pipe resin, both having unimodal MWD, and that of three different commercial PE100 pipe resins with bimodal MWD were investigated for a comparison by means of differential scanning calorimetry (DSC) in order to understand the difference of the crystallization kinetics between the two kinds of pipe resins. A correlation between the crystallization kinetics and the slow crack growth resistence performance of the uni-and bi-modal HDPE pipe resins have been observed. Furthermore, the same correlation have also been found between those unimodal pipe resins with similar molecular structures but different SCG resistance performance. Finally, the crystallization behaviors of the solution blending composites composed of unimodal and bimodal pipe resins have been investigated.It was found that the addition of bimodal pipe resins into unimodal ones could imporve the crystallization rate.