Optimization Of Rotaional Molding Crosslinked Polyethylene Products

Polyethylene (PE) was crosslinked significantly improving impact strength, high temperature resistance and other properties of its products manufactured by rotational molding. However, the polyethylene is crosslinked so that the forming process is complex. And it has not been systematic to understand the influence of crosslinking on crystallization which hindering the engineering applications. In this thesis, on the bases of thermal analysis, rheological testing and microscopic observation, systematically studied was conducted on the crosslinkable polyethylene (XLPE) crosslinking, crystallization and their interactions. The molding process was optimized by varying the heating and cooling procedures for rotational molding. The main work and conclusions are as follows:1. The thermal parameters of XLPE used in rotational molding was tested by DSC, after which its melting temperature, crosslinking temperature, crystallization temperature and other basic thermal parameters was further obtained through numerical analyses. The modified Avrami equation was used for quantitatively analysis of non-isothermal crystallization kinetics, crystal growth at nearly three-dimensional growth mode in XLPE non-isothermal crystallization. It was found that the non-isothermal crystallization rate increased with the cooling rate. The complex viscosity of XLPE was tested by rotary rheometer. Comparison of XLPE complex viscosity with that of HDPE added with DCP showed cross-linking temperature of XLPE rised with 10℃ implying wider the rotational molding processing window of XLPE. With the increase of heating rate and heating temperature, the cross-linking reaction time was shortened while the degree of cross-linking was increased.2. The crystallization behavior of XLPE and gel was studied by polarizied microscopy and DSC. The surface folding free energy of the molecular chain and crystal particle size was improved while the crystal nucleus and crystallinity was decreased after being cross-linked according to microscopic observation and quantitative analysis of isothermal crystallization. The molecular chain of polyethylene could not be inserted into the gel network in the gel crystallization process.The crystallization temperature and crystallinity of gel were lower than that of XLPE under the same crosslink degree. Crystallite parameters and crosslink network density of XLPE were quantitative calculated which showed the density of the crosslinked network increased and the crystallite surface spacing decreased. The lamellar structure of XLPE was observed by atomic force microscopy. With the increase of crosslinking degree, the state of lamellae changed from order to disorder and the internal defects of crystal increased. The crystallization temperature of XLPE was improved by increasing pressure.3. The process of rotational molding of XLPE includes material preheating and fusing, excluding air and compacting pore, crosslinking, cooling crystallization and solidification, which was classified by the temperature of mold inner air. Orthogonal experiments were conducted to optimize processing parameters. The results indicated that the heating temperature and time should be controlled within 270℃-290℃ and 28-32min respectively. Moreover, there are equivalent relationship between the heating temperature and time within the certain range. Innovatively analyzed were the crosslinking degree, crystallinity and crystal size along the thickness direction. The crosslinking degree of inner layer in product was minimum. The crystal size increased gradually from outer side to inner side of product. This shows that there is the negative correlation between the distribution of degree of crosslinking and that of crystallinity4. The influence of the crosslinking degree, crystallinity and crystal size on products’ mechanical properties was investigated showing that products’ bending modulus and tensile modulus depended on its crystallinity to highr degree than on other factors. The crosslinking degree determined impact strength of the products while the crystal size has crucial effect on tensile strength and strain of the products.This research contributes to further understanding of the polymer interaction mechanism of crosslinking and crystallization. Optimization of processing provides the reference to the engineering application.