Polyethylene wax (PE wax) is a kind of polyolefin synthetic wax. Its appearance is white or light yellow block, flake or powder solid, and its relative molecular mass is 1000-4000. It has low toxicity, high softening point, low melt viscosity, wear resistance, The advantages of heat resistance, good lubricity, dispersion and fluidity, can effectively improve the processing efficiency of pipes, profiles, films, cables and other plastics and rubbers, and have a good demolding effect.
At present, there are mainly three types of polyethylene wax production methods: one is polyethylene wax synthesized from ethylene monomer through oligomerization, such as through free radical oligomerization; the other is polyethylene wax prepared by degradation of high polymer; The third is the by-product in the synthesis of polyethylene, such as polyethylene wax obtained by separating the by-product in the synthesis of polyethylene by the high-pressure method.
1. Ethylene polymerization method
There are three main methods for producing polyethylene wax by ethylene polymerization. One is polymerization under high temperature and high pressure with free radical catalyst; the other is polymerization under low pressure with Ziegler catalyst; and the third is polymerization with metallocene catalyst.
2. Polyethylene cracking method
The polyethylene wax produced by the polymerization method has a narrow molecular weight distribution, and the relative molecular weight can be controlled artificially, but it must be carried out on a large device and the capital investment is large. Domestic manufacturers generally use the thermal cracking method of high molecular weight polyethylene. The raw materials used in this method can be polyethylene resin or polyethylene waste plastics. The former produces high-grade products, while the latter produces low-grade products. High-molecular-weight polyethylene can be thermally cracked into low-molecular-weight polyethylene wax under the isolation of air. The structure-related properties (such as crystallinity, density, hardness and melting point) of the polyethylene wax prepared in this way are all affected by the cracking raw materials. Cracking processing methods are divided into cracking kettle method and extrusion method. The pyrolysis kettle method is a batch processing method, which is suitable for low-volume and small-capacity manufacturers; the extrusion method is a continuous production, which is suitable for large-volume and high-capacity enterprises.
Polyethylene wax can be prepared by using recycled polyethylene lysate. The source of this technology is very rich and cheap, the process is relatively simple, and the operation cost is low.
3. Purification of polyethylene by-product
From the reaction of ethylene polymerization to produce polyethylene, the polyethylene wax product can be recovered from the mixture of low-molecular-weight components and solvents obtained as a by-product. After the solvent and initiator are removed from the by-products of the polyethylene plant, the molecular weight distribution of the product is still very wide, which limits its application fields and requires further purification by solvent separation. This by-product polyethylene wax product usually contains molecules with a relative molecular weight of about 1,000, so it is lower than the polyethylene wax product produced by ethylene polymerization in terms of mechanical strength, heat resistance and other physical properties.
4. Modification of polyethylene wax
Polyethylene wax is a non-polar molecule, if polar groups can be added to the molecule, it will greatly expand the application field. These functionalized polyethylene waxes can be produced by copolymerization of ethylene and oxygen-containing monomers, or the polyethylene wax can be oxidized, grafted and other chemical methods to introduce carboxyl groups, and then further processed by chemical reactions such as esterification, amidation, and saponification. Modified, these functionalized polyethylene waxes can meet the requirements of different applications.
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