There are three main production methods of polyethylene wax: one is polyethylene wax synthesized by oligomerization of ethylene monomer, such as by free radical oligomerization; the second is polyethylene wax prepared by the degradation of high polymers; By-products in the polyethylene synthesis process, such as polyethylene wax obtained by separation of the by-products in the high-pressure polyethylene synthesis.
1. Ethylene polymerization method
There are three main methods to produce polyethylene wax by ethylene polymerization. One is polymerization under high temperature and high pressure with free radical catalyst; the second is polymerization under low pressure with Ziegler catalyst; the third is polymerization with metallocene catalyst.
2. Polyethylene cracking method
The molecular weight distribution of polyethylene wax produced by the polymerization method is narrow, 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 to produce. The raw material used in this method can be polyethylene resin or polyethylene waste plastic. The former has high product grade, while the latter has low product grade . High molecular weight polyethylene can be thermally cracked into low molecular weight polyethylene wax in the absence of air, and the structure-related properties (such as crystallinity, density, hardness and melting point) of the prepared polyethylene wax are all affected by the cracking raw materials. The cracking processing method is divided into cracking kettle method and extrusion method. The cracking kettle method is a batch processing method, which is suitable for manufacturers with low production volume and small production capacity; the extrusion method is a continuous production method, which is suitable for enterprises with large production volume and high production capacity.
Polyethylene wax can be prepared by recycling polyethylene lysate. The raw material 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-products
Polyethylene wax product can be recovered from the mixture of low molecular weight components and solvent obtained by the polymerization of ethylene to produce polyethylene. After the solvent and initiator are removed from the by-product of the polyethylene plant, the molecular weight distribution of the product is still very wide, which limits its application field and needs to be further purified by solvent separation. This by-product polyethylene wax product usually contains molecules with a relative molecular mass of about 1000, so its physical properties such as mechanical strength and heat resistance are lower than those of polyethylene wax products produced by ethylene polymerization.
Polyethylene wax is a non-polar molecule, and if polar groups can be added to the molecule, the application field will be greatly expanded. These functionalized polyethylene waxes can be produced by copolymerization of ethylene and oxygen-containing monomers, or by chemical methods such as oxidation and grafting of polyethylene wax to introduce carboxyl groups, and then further through chemical reactions such as esterification, amidation, and saponification. Modified, these functionalized polyethylene waxes can meet the requirements of different applications.
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