The Crystallization Behaviors Of Bimodal HDPE/LDPE Under Different Thermal And Pressure Histories

Since bimodal polyethylene is a new type of material after the appearance of metallocene polyethylene and was not successfully developed and commercialized until 1995,the process and use of it are still in the experimental stage.Study on bimodal polyethylene at home and abroad is mainly on the development of catalyst used in the manufacturing of bimodal polyethylene and research reports on the performance of bimodal polyethylene are scarce.To expedite ordinary polyethylene cost-effective and function of the pace,it is necessary to study the performance of bimodal polyethylene,and then provide the basis for the popularization and application of the bimodal polyethylene.In this article the melting and crystallizing behaviors of bimodal HDPE/LDPE blends of different composition ratios are discussed by DSC analysis under different test conditions.Also a direct observation is made on the crystalline morphologies of bimodal HDPE with different thermal and pressure histories.Via analyzing DSC curves under different thermal and pressure histories, When the bimodal HDPE content was less than 40%,the liquid-liquid phase separation was occurred firstly,drove,bimodal HDPE and LDPE chain segments to form their individual rich domains,bimodal HDPE and LDPE could crystallize separately and two peaks were observed on the DSC trace; when the bimodal HDPE content exceeded 40%,there was only single crystallizing peak appeared on the DSC trace which indicated that co-crystallization occurred.When applying different cooling rates, cocrystallization of bimodal HDPE/LDPE was greatly stimulated by quenching;on the contrary,the liquid-liquid phase separation was occurred distinctly,the crystallinity increased,and then prone to form more perfect crystal when applying slow cooling rates.Samples were etched using KMnO₄ etching procedures,the SEM micrographs showed that the crystalline morphologies of bimodal polyethylene and its blends under different thermal and pressure histories existed obvious diversities.When the blends rapidly quenched,the SEM shows that Neighbouring spheruliters impinged each other,made banded spheruliters fill all the spaces.On the contrary,when the blends slowly quenched,it can be clearly seen that a polycrystalline array of equivalent radiating,branching units tend to produce a spherical envelope,it begin with a fibre and evolve through sheaf-like embryos before attain a spherical envelope,immature spherulites and mature spherulites can be found in blends with different composition.The morphology of injection molded pure bimodal HDPE shows that the structure of fiber after the injection orientation can be clearly observed as being composed of numerous juxtaposed fibers with lengths ranging from a few microns to several dozens of microns,and the thickness of these crystalline lamellae is only 0.05～0.1μm.The tightly packed lamellae are formed through regular arraying of crystal chains,and formed a single population.On the contrary, the morphology of injection molded bimodal HDPE/LDPE(20/80 by weight) is different.Although the structure of fiber after injection molding can be clearly seen,it is obviously different from the morphology of pure bimodal HDPE formed after the injection molding.It is the result of numerous fibers entangled with each other and is in the form of irregularly distributed network structure.