Polyethylene (PE) resin is a crystalline polymer with a linear structure. When it is heated and melted, the force between macromolecules is very small, and the temperature range of the high elastic state is very narrow. When the resin is melted, the melt viscosity is low, so it foams. When the decomposition gas of the blowing agent is not easily maintained in the resin, it is difficult to control the foaming process; PE has high crystallinity and fast crystallization speed, and a large amount of heat of crystallization is released from the molten state to the crystalline state, plus the heat capacity of the molten PE Larger, it takes a long time to cool to a solid state, which is not conducive to maintaining the gas during the foaming process. In addition, PE resin has a high gas permeability, among which low-density polyethylene (LDPE) is more easily permeable to gas than high-density polyethylene (HDPE). These factors will promote the escape of foaming gas.
In order to overcome the above shortcomings, it is usually necessary to cross-link the molecules to form a partial network structure to increase the viscosity of the resin and slow down the tendency of the viscosity to decrease with the increase of temperature, that is, to adjust the viscoelasticity of the resin to meet the foaming requirements.
Polyethylene foam can be divided into two types: cross-linked and non-cross-linked. Cross-linked PE foam is divided into two processes: radiation cross-linking and chemical cross-linking. If it is classified according to the external pressure when the blowing agent is decomposed, it can be divided into normal pressure foaming and pressure foaming.
The development of foamed polyolefin materials has always been a hot spot of research. Compared with the traditional foaming materials of PS and PU foams, the development history of polyolefin foaming materials such as PE and polypropylene (PP) is shorter.
In 1941, a patent from DuPont of the United States proposed the preparation of PE foam, which opened the prelude to PE foam. Later, the Callent Cable Construction Company and other companies also obtained patents for the preparation of PE foam. In 1955, DuPont of the United States produced low-foaming PE. In 1958, the Dow Chemical Company of the United States first adopted the non-crosslinking extrusion foaming method to realize the industrial production of high-foaming PE foam. At present, most of the high-foaming PE in the United States is still produced by this method. In the early 1960s, Japanese companies such as Sanwa Chemical Industry, Furukawa Electric and Sekisui Chemical successively developed and developed cross-linked high-foaming PE, and started producing high-foaming PE products in 1965. At present, most manufacturers are based on the technology developed in Japan. Europe began to produce cross-linked PE foam materials from about the 1970s.