Research On Optimization Of Cross-linked Polyethylene(XLPE) Rotational Molding Process
Cross-linking modification of polyethylene can significantly improve the impact resistance, high temperature resistance and other properties of its rotational molded products. However, the cross-linking of polyethylene complicates the molding process; the current understanding of issues such as the impact of cross-linking on crystallization is not systematic, which hinders the widespread application of engineering. This paper systematically studies the cross-linking behavior, crystallization behavior and influence of cross-linking on crystallization of cross-linked polyethylene (XLPE) based on thermal, rheology and microscopic observation; by changing the heating and cooling procedures, rotational molding tests are carried out to optimize the Molding process.The main work and conclusions are as follows:
1. Apply DSC to measure the thermal parameters of XLPE for rotational molding, and quantitatively analyze the non-isothermal crystallization kinetics of increases and decreases; a rotational rheometer was used to test the complex viscosity and other parameters of XLPE for rotational molding. By comparing with the complex viscosity of HDPE with only DCP added, it was shown that the starting cross-linking temperature of XLPE increased by about 10°C, broadening the processing Window; increasing the heating rate and cross-linking temperature can shorten the cross-linking reaction time and increase the cross-linking degree of the product. When the heating rate is greater than 7℃/min and the cross-linking temperature is above 190℃, products with a higher cross-linking degree can be obtained .
2. Use polarizing microscope and DSC to study the crystallization behavior of XLPE and gel. Quantitative analysis shows that cross-linking of XLPE increases the surface folding free energy of molecular chains, reduces crystal nuclei, increases crystal particle size, and decreases the crystallization rate and crystallinity; the crystallization DSC curve of a mixture of gel and HDPE will have double crystallization peaks, indicating crystallization The molecular chains of HDPE cannot be inserted into the gel network; at the same cross-linking degree, the crystallization temperature and crystallinity of gel are lower than that of XLPE; the crystallite size and cross-linking network density of XLPE were quantitatively measured, and the cross-linking degree was increased , the crystal plane spacing decreases; the XLPE lamellar structure is observed through atomic force microscopy, and cross-linking changes the lamellae from ordered arrangement to disorder, and defects increase; the application of ultrasonic test analysis shows that increasing pressure can promote crystallization and improve the crystallization of XLPE starting temperature and crystallization end temperature.
3. Using the air temperature inside the mold as an indicator, XLPE rotational molding is divided into stages such as material preheating, material melting, bubble removal and compaction, cross-linking, cooling crystallization and solidification; the heating temperature is 270℃~290℃, and the heating time is 28min~ 32min is the optimal XLPE rotational molding process. Within a certain range of heating temperature and heating time, there is a time-temperature equivalent relationship between a 10°C increase in heating temperature and a 1-min extension in heating time; the distribution of cross-linking degree, crystallinity and crystal particle size of XLPE rotational molded products along the thickness direction was quantified. It was found that the innermost layer of the product has the lowest cross-linking degree; the crystal particle size of the product gradually increases from the outer layer to the inner layer; the crystallinity and cross-linking degree are distributed in a negative correlation.
4. The relationship between the mechanical properties of XLPE products and the degree of cross-linking, crystallinity and crystal particle size was analyzed. The crystallinity has the greatest impact on the flexural modulus and tensile modulus of the product; adjusting the cross-linking degree can significantly change the impact strength of the product; the tensile strength and tensile strain of the product are determined by the cross-linking degree and the crystal particle size. Function: After polyethylene is cross-linked, its high-temperature resistance is significantly improved. The greater the degree of cross-linking, the stronger its ability to resist deformation at high temperatures. This paper deepens the understanding of the interaction mechanism of polymer cross-linking and crystallization, and the optimization process has reference value for its engineering applications.