A kind of polymer material formed by dispersing a large amount of gas micropores in solid plastic has the characteristics of light weight, heat insulation, sound absorption, shock absorption, etc., and has better dielectric properties than matrix resin and has wide application. Almost all kinds of plastics can be made into foam, and foam molding has become an important field in plastic processing. The structural foam developed in the 1960s is characterized by core foaming, non-foaming of the skin layer, external hard and toughness, high specific strength (strength per unit mass), low consumption, and increasingly extensive replacement of wood. Used in the construction and furniture industries. The chemical or radiation cross-linked foaming technology of polyolefins has been successful, resulting in a significant increase in the production of foamed plastics. The foamed plastic obtained by blending, filling, reinforcing and other modified plastics has better comprehensive performance and can meet the needs of various special purposes. For example, a glass fiber reinforced polyurethane foam obtained by reaction injection molding has been used as a structural component of an airplane, a car, a computer, etc., and a foam made of polybenzimidazole filled with hollow glass beads is light and resistant to high temperatures. , has been used in spacecraft.
The open pores of the classified micropores are called open-cell foams, and the closed ones are called closed-cell foams. The foam is both hard and soft. According to the American Society for Testing and Materials, at a temperature of 18 to 29 ° C, within a period of 5 s, around a 2.5 cm diameter round rod, if it does not break, the test sample belongs to a flexible foam; otherwise, it is a rigid foam. . Foam plastics can also be classified into low foaming and high foaming. Generally, the expansion ratio (the multiple after the expansion is increased by a multiple of the volume before foaming) of less than 5 is referred to as low foaming, and the ratio of more than 5 is referred to as high foaming.
Foaming method No matter what method is used for foaming, the basic process is: 1 introducing gas into liquid or molten plastic to produce micropores; 2 increasing micropores to a certain volume; 3 fixing microporous structure by physical or chemical means . According to the method of introducing gas, the foaming method includes mechanical method, physical method and chemical method.
The mechanical method introduces a large amount of air or other gas into the liquid plastic by means of strong agitation. The industry mainly uses this method to produce urea-formaldehyde foam, which can be used as a thermal insulation material or a set material in a movie (such as artificial snow).
The physical method often dissolves low-boiling hydrocarbons or halogenated hydrocarbons into plastics, and the plastic softens when heated, while the dissolved liquid volatilizes and expands and foams. For example, in the case of polystyrene foam, in the suspension polymerization of styrene, pentane is first dissolved in the monomer, or the polystyrene resin which has been polymerized into a bead is treated with pentane under heat and pressure.
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