From synthesis to application: introduction to foam
1. How are foam plastics made? Foam plastic is a type of polymer material formed by a large number of gas micropores dispersed in solid plastic. It has the characteristics of light weight, heat insulation, sound absorption, corrosion resistance, and shock absorption, and its dielectric performance is better than that of matrix resin. Compared with pure plastic, foam plastic has low density, light weight, high specific strength (its strength increases with increasing density) and the ability to absorb impact loads. It also has excellent cushioning, shock absorption, sound insulation, and thermal conductivity. Low, good thermal insulation performance, also has excellent electrical insulation, corrosion resistance, and mildew resistance. Flexible foam has excellent elasticity and other properties. Almost all kinds of plastics can be made into foam plastics, and foam molding has become an important field of plastic processing. Cells are divided into closed cell type and open cell type. The pores in the closed cell type are isolated from each other and have floating properties; the pores in the open cell type are connected to each other and have no floating properties. It can be made of polystyrene, polyvinyl chloride, polyurethane and other resins. No matter what method is used, the basic process is foaming: ① Introduce gas into liquid or molten plastic to produce micropores; ② Make the micropores grow to a certain volume; ③Fix the microporous structure by physical or chemical methods. 2. What are the foaming methods? According to the way of introducing gas, foaming methods include mechanical, physical and chemical methods. 1. Mechanical law: With vigorous stirring, a large amount of air or other gases are introduced into the liquid plastic. In industry, this method is mainly used to produce urea-formaldehyde foam plastic, which can be used as heat insulation material or scenery material in movie theaters (such as artificial snowflakes). 2. Physical Law: 1) Dissolve low boiling point hydrocarbons or halogenated hydrocarbons into plastics: When heated, the plastic softens, and the dissolved liquid volatilizes, expands and foams. 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. The 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. They are widely used as anti-shock materials in insulation and packaging. 2) Extrusion molding method can also be used: At this time, either expandable beads can be used to foam and extrude them into a sheet at one time; or ordinary polystyrene pellets can be used, and halogenated hydrocarbons are added to the appropriate part of the extruder to make them melt with the plastic. The mixing is uniform, and when the material leaves the machine head, it expands and foams. The extrusion method is often used to make sheets or plates. 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. 3) The physical methods of introducing gas include dissolution method, hollow microsphere method, etc.: 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.