PE can also be made into pre foamed particles first, and then molded and foamed to obtain high foaming molded products, similar to PS. In addition, it also undergoes two-step molding and application through other technological processes, with a foaming ratio of up to 30 times and a formed foam board thickness of up to 100mm. Below is an introduction to the two-step molding foaming of PE.
(The phase of process 1 and S is to first make expandable polyethylene beads. After pre foaming, they are placed in a locking mold with small holes and foamed into foamed plastics of the desired shape in water vapor. The volume weight of the product obtained is about 0.05 g/cm. Therefore, the secondary processing of general polyethylene molding sheet materials is omitted in application, and the generation of corner waste is eliminated.
Generally speaking, the processing and molding of expandable polyethylene beads is not as easy as that of expandable polystyrene beads, as it requires higher temperatures.
(i) The manufacturing of expandable polyethylene beads involves using organic peroxides as crosslinking agents to dissolve or disperse into particles in a pressure vessel containing an aqueous solution of surfactants. Add insoluble inorganic salts as dispersants, then add polyethylene particles to suspend them in the aqueous phase. Heat the pressure vessel to 100 ℃, and the crosslinking agent can be immersed in the polyethylene resin. Further heating at high temperatures will cause cross-linking of polyethylene. Add foaming agent to the water suspension of cross-linked polyethylene glycol (with a crosslinking degree of about 10% -70%), and then heat it to 130 ℃ to immerse the foaming agent into the polyethylene particles. After cooling, remove the resin puller from the label: filter, wash, and dry to obtain the expandable particles. These particles can be put into use by using foaming agents. The main raw materials used are as follows: high-pressure steam or other methods are used to heat and foam into pre foamed particles, which are then impregnated again in a pressure vessel
① Surfactants. The organic peroxides can be dissolved in the surfactant aqueous solution in a uniform fine particle state, which is a necessary condition for the preparation of foamed plastics with uniform and small pores. Otherwise, the degree of crosslinking of each polyethylene particle may be inconsistent, and the pores of the expandable particles will be uneven, resulting in a weakened foaming ability. The selection of surfactants should not significantly hinder the crosslinking reaction and the suspension stability of polyethylene particles. Surfactants can be used as one or both. Non ionic surfactants such as polyoxyethylene octylphenol ether, anionic surfactants sodium dodecylbenzenesulfonate, other zwitterionic surfactants, and cationic surfactants can all be used. Its dosage is generally 5~50 ℃ of organic peroxides. In addition, organic peroxides can also be dissolved in lower alcohols such as methanol, and then used together with the above-mentioned surfactants.
② Dispersant. Dispersants are not only used to prevent resin particles from melting, sticking, and clumping when the temperature exceeds the melting point of polyethylene, keeping the particles in a stable suspended state, but also to make the crosslinking reaction more rapid through high temperature. Dispersants are inorganic metal salts, such as calcium phosphate, zinc phosphate, etc. in phosphate compounds; Aluminum hydroxide in hydroxide compounds; Oxides and phosphorus hydrates. The amount of dispersant used varies with the type of inorganic metal salt, processing temperature, type of resin, type of surfactant, and dosage, generally less than 1% of the weight of the resin.
③ Foaming agent. The foaming agents used for expandable polyethylene beads are physical foaming agents, such as butane, propane, isopentane, isobutene, cyclopentane, dichlorodifluoromethane, dichlorotetrafluoroethane, chloromethane, chloroethane, etc. These foaming agents are fluorine based foaming agents that are in a gaseous state under normal conditions.
④ Crosslinking agent. Crosslinking agents can be used with diisopropylbenzene peroxide and 2,5-dimethyl-2.5-di tert amounts ranging from 1% to 20%.
⑤ Polyethylene resin. Polyethylene resin can be made from LDPE, MDPE, and HDPE. The pre foaming of cross-linked polyethylene with particle size as a pre containing agent for producing polymer beads is best carried out using hot air, water vapor, infrared radiation, and high-temperature gases that do not melt bubble substances at 100-300 ℃. The initial pre foaming rate (multi foaming rate) obtained from this is 5-30 times. Load the expandable particles into a pressure tag, and then maintain a certain pressure, temperature, and time. The temperature change should be maintained within the range of expandable particles and the pressure should be maintained to completely crush the expandable particles. The particles can be carried out once or several times in the mixture of foaming agent gas, complex gas, and oxygen rich gas, depending on the requirements for the final foam washing plastic and the type of ethylene oxide.
