The physical principle of high temperature resistant foam foaming: often dissolve low boiling point hydrocarbons or halogenated hydrocarbons into plastics. When heated, the plastics will soften, and the dissolved liquid will evaporate and expand and foam. For example, polystyrene foam can be prepared by dissolving pentane into the monomer during suspension polymerization of styrene, or treating the polystyrene resin polymerized into beads with pentane under heating and pressure to produce a so-called Expandable polystyrene beads.
The beads are pre-expanded in hot water or steam, and then placed in a mold to pass steam to make the pre-expanded particles expand and fuse with each other. After cooling, a product with the same shape as the mold cavity is obtained. Core material foamed plastics They are widely used as insulation and shockproof materials in packaging. The extrusion molding method can also be used. In this case, the expandable beads can be used to foam and extrude them into a sheet at one time.
Ordinary polystyrene pellets can also be used, and halogenated hydrocarbons are added to the appropriate parts of the extruder to mix them evenly with the plastic melt. When the materials leave the machine head, they will expand and foam. The extrusion method is often used to make sheets or plates, and the sheets can be made into food packaging boxes and trays after vacuum forming. Polyethylene can also be used in a similar way to make extruded foamed products. The physical methods for introducing gas include the dissolution method and the hollow microsphere method.
The dissolution method is to mix soluble substances such as salt, starch, etc. with resin, and form a product, and then place the product in water for repeated treatments to dissolve the soluble substance to obtain an open-cell foam product, which is mostly used as a filter material. The hollow microsphere method is to mix hollow glass microspheres with a high melting temperature with a plastic melt. Under the molding conditions that the glass microspheres will not break, a special closed-cell foam can be prepared.