Manufacturing Methods of Polyethylene
I. INTRODUCTION
Polyethylene is a widely used plastic material with excellent chemical resistance, electrical insulation and processing properties, and therefore has an important position in the field of raw materials for the production of chemicals. In this paper, we will introduce the manufacturing methods of polyethylene, including high-pressure method, low-pressure method and vapor-phase method.
II. High Pressure Method
The high pressure method is the earliest method of producing polyethylene and is one of the methods still in use. This method polymerizes ethylene monomer to produce polyethylene under the conditions of high pressure (150-300MPa) and high temperature (180-200°C), using initiators such as oxygen or peroxide. The specific steps are as follows:
1. Add ethylene monomer, initiator and conditioner to the high pressure polymerization kettle.
2. Heat the polymerization kettle to the desired temperature and simultaneously pressurize to the desired pressure.
3. Maintain the time and temperature for the polymerization of the ethylene monomer to produce polyethylene.
4. Cool down, depressurize and remove the polymer from the polymerization kettle.
The polyethylene produced by the high pressure method has a lower molecular weight, a wider molecular weight distribution, more branched chains, and a lower degree of crystallinity, resulting in better flexibility and processing properties. However, the method has higher energy consumption and lower production efficiency, and is gradually replaced by the low-pressure method.
Three, low-pressure method
Low-pressure method is the main method of producing polyethylene at present, including solution method, suspension method and vapor phase method. These methods are carried out under the conditions of low pressure (0.1-1.0MPa) and lower temperature (70-90°C), using catalysts such as Ziegler-Natta catalyst or chromium catalyst to initiate the polymerization of ethylene monomer to produce polyethylene. The steps are as follows:
1. ethylene monomer, catalyst and other additives are added to the polymerization reactor.
2. polymerize the ethylene monomer to form polyethylene at a certain temperature and pressure.
3. Separate the polymer from the unreacted monomer and catalyst by flash evaporation, extrusion pelletizing and other processes to obtain the finished polyethylene product.
The polyethylene produced by the low-pressure method has a higher molecular weight, a narrower molecular weight distribution, fewer branched chains, and a higher degree of crystallinity, resulting in better mechanical properties and heat resistance. The method has lower energy consumption and higher production efficiency, and has become the main method of producing polyethylene.
Four, gas-phase method
Gas-phase method is a new method developed in recent years to produce polyethylene. The method is to add ethylene monomer and catalyst together into the fluidized bed reactor, under the condition of lower pressure (0.1-0.5MPa) and higher temperature (80-100℃), to make the polymerization of ethylene monomer to produce polyethylene. The steps are as follows:
1. Add catalyst to the fluidized bed reactor and heat to the desired temperature.
2. Ethylene monomer is passed in and polymerized to form polyethylene under the action of the catalyst.
3. By controlling the reaction conditions and the type and amount of catalyst, polyethylene of different molecular weights and molecular structures can be obtained.
4. The polymer is separated from the unreacted monomer and catalyst by processes such as cooling and separation to obtain the finished polyethylene product.
The polyethylene produced by gas phase method has the advantages of high molecular weight, narrow molecular weight distribution, less branched chain, high crystallinity, etc. At the same time, this method has the advantages of low energy consumption, high production efficiency and good product quality, so it is widely used in polyethylene production.
V. CONCLUSION
Polyethylene is an important raw material for the production of chemicals, and its manufacturing methods include high-pressure method, low-pressure method and gas-phase method. Polyethylene obtained by different methods differs in molecular structure, properties and applications. With the continuous progress of technology and the increasing demand of applications, the manufacturing methods of polyethylene are also being developed and improved.
