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Polyethylene wax, as alow molecular weight product, is an important processing aid in chemical production. Because of its good compatibility with polyolefin resin and other waxes, it is widely used in themodification of plastics and special waxes.
Polyethylene Wax (pe wax) is a kind of polyolefin synthetic wax, generally refers to the homogeneous polyethylene wax with the relative molecular weight less than 10000, the polyethylene wax has high softening point, low melting viscosity, good chemical stability, and good lubricity and fluidity, as a processing aid, it is widely used in PVC pipe, film, cable and other plastic rubber products to improve the processing performance and improve the appearance of the products.
Preparation methods of pe wax
There are three main synthetic methods of polyethylene wax. The first is the polyethylene cracking method, the polyethylene resin was cracked into low molecular weight polyethylene wax at high temperature. The second is the by-product refining method, which collects the by-products of low molecular weight components produced in the production process of ethylene polymerization and obtains polyethylene wax. The third method is ethylene synthesis method, using ethylene as raw material to directly synthesize polyethylene wax, according to its polymerization mechanism and the use of different types of catalysts, and can be divided into free radical polymerization, Ziegler-Natta (Z-N) catalytic polymerization, metallocene catalytic polymerization, etc.
1. Cracking method
Cracking method is the main method to produce polyethylene wax in China. High molecular weight pure polyethylene or waste polyethylene plastic is cracked into polyethylene wax at high temperature. The product quality and properties (such as hardness, melting point and apparent color, etc.) are greatly affected by the source of cracking raw materials. The cracking process is simple, the source of raw materials is rich, the operation cost is low, the waste polyethylene can be reused, and the economic benefit is good. However, the cracking process is difficult to control, the product molecular weight distribution is wide, the quality of polyethylene wax is difficult to control, there are many black spots, in color masterbatch and other low-end applications are more popular.
The existing pyrolysis processes include thermal cracking, solvo-assisted cracking and catalytic cracking, among which the thermal cracking is the simplest and only needs to control the reaction temperature and time to produce polyethylene wax products, but it requires considerable energy consumption. Ping Xue et from Beijing University of Chemical Technology studied the preparation of PE wax by pyrolysis of PE resin in a single screw extruder at high temperature. A heater was added on the connecting pipe between the extruder and the cooling tank to make the material pyrolysis in it. The optimal cracking temperature was 420℃, and the continuous production of PE wax prepared by pyrolysis of PE resin was realized. Zhang Jianyu Zhang et . studied the preparation of polyethylene wax by catalytic cracking of waste polyethylene in autoclave with Al-MCM-48 as catalyst. The reaction temperature was 360~380℃ and the reaction time was 4h. The use of the catalyst reduces the activation energy of the reaction, reduces the temperature needed for cracking, and reduces the energy consumption. Lulu Wang et al. improved the yield of polyethylene wax by solvent-assisted pyrolysis of polyethylene waste plastics, and investigated the effects of different solvents and reaction conditions on the yield and properties of the wax. The results show that the yield of polyethylene wax can be increased by using aromatic solvents, and the yield can reach 87.88% when using mixed xylene as solvent.
When recycling waste polyolefin plastics to crack polyethylene wax, various additives in the raw material, such as fillers and colorants, will produce pungent odor in the cracking process, affecting the quality and color of the wax. Giseleazimi and others have explored the use of Supercritical fluid extraction to remove organic and inorganic contaminants from polyolefin waster in order to achieve the goal of lightening the color, reducing the pungent smell and improving the quality of polyethylene wax. The experimental results show that under the appropriate parameters, the organic extraction rate of polyethylene wax reaches 80%, the electrodeless extraction rate is 91% of calcium salt, 69% of sodium salt and 65% of aluminum salt, respectively. SEM image shows that the content of metal particles in the polyethylene wax after extraction is significantly reduced. As an environment-friendly and efficient purification and decolorization process of polyethylene wax, SCFE can be widely used in polymer industry to produce high purity water-white polyethylene wax from recycled waste polyolefin plastics.
2. By-product recovery polyethylene wax
The by-products of low molecular weight components in the process of ethylene polymerization are collected, and the solvents and initiators are removed to obtain polyethylene wax. This kind of polyethylene wax product has complex components, wide molecular weight distribution, poor quality stability, and low molecular weight components are easy to precipitate in terminal application, and affect the heat resistance and mechanical strength of wax products. To some extent, its application is limited and it needs to be further purified and cut into products with different melting point range for re-use.
Based on the difference of solubility of polyethylene wax with different molecular weight components in organic solvents, a suitable solvent was used to selectively dissolve the low molecular weight components in polyethylene wax, so as to realize the separation of high and low melting point polyethylene wax.
After solvent separation, the melting point range of high melting point polyethylene wax is 82.6~96.3℃, which can be used in the fields of color masterbatch and paint and ink. The melting point range of low melting point polyethylene wax is 45.3~69.5℃, which can be used in cosmetics industry and capsule materials. Separating polyethylene wax with different melting point can improve its quality, broaden its application field and increase its added value. Long Zhang et al. also studied the method to improve the melting point of by-product polyethylene wax. By adding nucleating agent and polyolefin with high melting point and recrystallization of by-product polyethylene wax, the melting point of the product can be increased to 100.7~129.2℃, which can be used in the production of special wax.
3. Synthetic method
The polyethylene wax synthesized directly from ethylene has the advantages of high purity, small molecular weight distribution, narrow melting path, adjustable performance and stable quality, which can be used to produce high quality and diversified polyethylene wax. According to the polymerization mechanism and the different types of catalysts used, ethylene synthesis methods can be divided into free radical polymerization, Ziegler-Natta (Z-N) catalytic polymerization, metallocene catalytic polymerization and so on.
(1) Free radical polymerization
Free radical polymerization method is under high temperature and high pressure (the general temperature is more than 100℃, the pressure is more than 20MPa), by free radical induced ethylene polymerization to get polyethylene wax. The molecular weight of polyethylene wax prepared by free radical polymerization is generally between 1000~6000, and the relative molecular weight distribution is between 2~5, usually containing branched chains and double bonds. The product has low crystallinity, low melting point, low melting viscosity, and good wettability to pigments and fillers. However, the relative molecular weight distribution is wide, and the melting process is wide, so it is difficult to control the product quality.
(2) Ziegler-Natta (Z-N) catalytic polymerization
Ziegler - karnataka (Z - N) catalytic polymerization preparation of polyethylene wax, is refers to under the condition of low pressure, using the traditional ziegler - tower ethylene polymerization catalyst, the polyethylene wax is a linear molecule with short side chain, number average molecular weight is controlled in thousands, relative molecular mass distribution for 3 ~ 4, high density polyethylene wax. Ziegler-Natta catalytic polymerization can adjust the molecular weight and crystallinity of the product, and the product performance is good.
(3) Metallocene catalytic polymerization
Compared with Ziegler-Natta catalyst, using metallocene to prepare polyethylene wax has higher reactivity, less catalyst dosage, no need for removal process, high product purity, lower polymerization temperature, more economical production process, narrower product molecular weight distribution, generally 1~3, the disadvantage is that the price of catalyst is expensive. The metallocene catalyst can be used to regulate the molecular structure of the polymer and produce polyethylene wax with better properties.
(4) Other catalyst polymerization methods
Ethylene polymers with different molecular weight and branching structure can be obtained by adjusting the structure and polymerization conditions of the late transition metal catalysts represented by nickel and palladium. Sun Wenhua et al designed and synthesized a series of iminopyridine nickel catalysts. Alkyl aluminum was used as catalyst to obtain the narrow dispersion and high branched polyethylene wax under the polymerization temperature of 20~50℃ and the polymerization pressure of 10atm. The maximum polymerization activity was 5.33×106g(Pe)mol(Ni)-1h-1. The average molecular weight of the products ranged from 1000 g/mol to 4000g/mol, the relative molecular weight distribution ranged from 1.42 to 1.92, and the melting point of polyethylene wax ranged from 10 to 80℃. This polymerization method can synthesize branched polyethylene wax with high purity and high quality using only ethylene as monomer. The product quality is stable and the melting point of polyethylene wax can be adjusted by controlling the polymerization conditions. Therefore, it has a broad application prospect in the fields of pigment dispersant and resin lubricant.
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