Principle of polyethylene crosslinking foaming
DATE：2022/5/26 8:34:40 / READ： / SOURCE：This station
Principle of polyethylene crosslinking foaming
1、 Characteristics of polyethylene processingPolyethylene is a crystalline polymer, which hardly flows below the crystallization melting temperature, while its melt viscosity drops sharply above the melting temperature, so the gas produced in the foaming process is difficult to be wrapped. In addition, a large amount of crystallization heat is released when polyethylene changes from molten state to bonded state, and the specific heat capacity of molten polyethylene is large, so the time from molten state to solid state is relatively long; Due to the high permeability of polyethylene, the foaming gas is easy to escape, and the temperature range suitable for foaming is narrow. Therefore, the industrial production of polyethylene foam is late. In order to produce high-quality polyethylene foams, it is necessary to crosslink between polyethylene molecules, adjust the viscosity and elasticity of molten materials within a certain range, so as to significantly improve the foaming performance and improve the physical and mechanical properties of polyethylene foams.
Another factor affecting the stability of polyethylene plastic melt foaming process -- the permeability coefficient of gas is not easy to be too large. Permeability coefficient is the product of diffusion coefficient and solubility. Polyethylene plastic melt is easy to penetrate gas, and the permeability coefficients of several gases in polyethylene melt are shown in table 6-2.
It can be seen from table 6-2 that low-density polyethylene is easier to penetrate gas than high-density polyethylene, and the most easily penetrated gas is carbon dioxide gas. Polyethylene crosslinking foaming method
The commonly used crosslinking methods include chemical crosslinking and radiation crosslinking. Chemical crosslinking generally uses organic peroxide as crosslinking agent, which is widely used in industry because of its low price and convenient operation; Radiation crosslinking refers to the crosslinking reaction of polyethylene under high-speed electron ray or radiation to form network macromolecules. Radiation crosslinking equipment has a large investment, but the quality of the sheet is good, which is generally only used for
Produce 5~10mm thick cross-linked polyethylene foam sheet.
1. Chemical crosslinking foaming
When the polyethylene resin is mixed with the organic peroxide and heated, the organic peroxide decomposes into free radicals with high chemical activity, seizes the hydrogen atom in the polyethylene molecule, and converts some carbon atoms in the main chain into active free radicals. The active free radicals on the two macromolecular chains combine with each other to form a cross-linking bond.
The first step of chemical crosslinking foaming is crosslinking, and the second step is expansion foaming. The foaming methods of cross-linked polyethylene foams can be divided into one-step and two-step methods.
① One step is to heat the sheet to a temperature higher than the decomposition temperature of the foaming agent, and then cross-linked foaming molding.
② In the two-step process, a certain proportion of polyethylene resin, foaming agent and chemical crosslinking agent are plasticized and refined into sheet materials, and then the blank is heated and pressurized, and then the blank is foamed and formed.
2. Radiation crosslinking
Polyethylene and Azodicarbonamide equivalent to 10% by mass of polyethylene are mixed evenly and extruded into a 3mm thick and 10cm long expandable sheet, which is then irradiated by electron ray. The radiation process is carried out in the air at room temperature and crosslinked with radiation measured as 10 ~ 100kgy. Radiation crosslinking can be carried out at high or low temperature. Unlike chemical crosslinking, it needs to be carried out above the melting point of the material.
There are two kinds of energy sources used in radiation crosslinking, gamma ray and electron radiation. The main difference between the two is the speed and depth of penetration into the material. Produced by radioisotopes.
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