No matter what method is adopted for foaming, the basic process is as follows:
① Gas is introduced into liquid or molten plastics to produce micropores;
② Make the micropore grow to a certain volume;
③ The microporous structure is fixed by physical or chemical methods.
According to the way of introducing gas, the foaming methods include mechanical method, physical method and chemical method.
Mechanical method: introduce a large amount of air or other gases into the liquid plastic by means of strong stirring. In industry, urea formaldehyde foam is mainly produced by this method, which can be used as thermal insulation materials or scenery materials in movie theaters (such as artificial snowflakes).
Physical method: the low boiling point hydrocarbon or halogenated hydrocarbon is often dissolved in the plastic, when heated, the plastic softens, and the dissolved liquid volatilizes, expands and foams. For example, polystyrene foam, when styrene suspension polymerization, first dissolve pentane into monomer, or under heating and pressure, polymerize the bead shaped polystyrene resin with pentane to prepare the so-called foaming polystyrene beads. The bead is pre foamed in hot water or steam, and then placed in the mold to pass steam, so that the pre foamed particles expand twice and fuse with each other. After cooling, the product with the same shape as the mold cavity is obtained (see Figure). They are widely used as shock proof materials in heat preservation and packaging. The extrusion molding method can also be used. At this time, the foamable beads can be used to extrude the foamable beads into sheets; the ordinary polystyrene granules can also be used to add halogenated hydrocarbons to the appropriate parts of the extruder, so as to mix them evenly with the plastic melt. When the materials leave the extruder head, they will expand and foam. Extrusion method is often used to make sheet or plate. Sheet can be made into food packaging box and tray by vacuum molding. Polyethylene can also be made into extruded foam products by similar methods. The physical methods of introducing gas include dissolution, hollow microspheres and so on. The dissolution method is to mix soluble substances, such as salt, starch and resin, and then form them into products. After that, the products are repeatedly treated in water to dissolve the soluble substances, that is, open foam products are used as filter materials. The hollow microsphere method is a mixture of hollow glass beads with high melting temperature and plastic melt, and special closed cell foam plastics can be made under the condition that the glass beads are not broken.
Chemical method: it can be divided into two categories: ① using chemical foaming agent, they decompose and release gas when heated. Common chemical foaming agents, such as Azodicarbonamide, azodiisobutyronitrile, N, n ′ - dinitro pentamethylene tetramine, sodium bicarbonate, etc. Many thermoplastic plastics can be made into foam plastics by this method. For example, PVC foam shoes, that is, resins, plasticizers, foaming agents and other additives made into the injection molding machine, the foaming agent in the cylinder decomposition, materials in the mold foaming. Foam artificial leather is mixed with PVC agent in PVC paste, coated or scraped or rolled on the fabric, through continuous tunnel heating furnace, the material is plasticized and melted, foaming agent decomposes foaming, cooling and surface finishing, then the foam artificial leather is obtained. Rigid polyvinyl chloride low foamed sheet, tube or profile is extruded to form, foaming agent is decomposed in the machine barrel. When the material is off the head, the pressure drops to atmospheric pressure. When the material is dissolved into the gas, it is expanded and foamed. If the foaming process is properly cooperated with the cooling and shaping process, the structure foam product can be obtained. (2) using the by-product gas in polymerization process, a typical example is polyurethane foam plastic. When isocyanate and polyester or polyether are polycondensation, some isocyanates will react with water, hydroxyl or carboxyl to form carbon dioxide. As long as the gas release rate and polycondensation reaction rate are properly adjusted, it can be made into a highly foamed product with very uniform bubbles. There are two types of polyurethane foam. The soft open hole is like sponge. It is widely used in various seats, sofa cushions, sound absorbing and filtering materials, etc. the hard closed hole type is ideal insulation, insulation, shock absorption and floating materials.
Kaiping city Feng Xin Foam Plastic Technology Co., Ltd. http://www.fengxin-china.com/