high pressure-low density polyethylene (HP-LDPE, abbreviated as LDPE) has been industrialized for more than 70 years, although with the discovery and development of olefin polymerization catalysts, the varieties and yields of polyethylene have made great progress. However, high-pressure polyethylene still plays an important role. Ethylene is polymerized into high molecular weight polyethylene by free radical mechanism under high temperature and high pressure, which has the following characteristics:
(1) The process of ethylene polymerization is a strong exothermic process. The polymerization heat of ethylene is about 93.5kJ/mol (or 3.3kJ/g), and the specific heat capacity of ethylene at 235MPa and 150-300 ℃ is 2.51~2.85J/(g·℃). If the reaction heat cannot be removed in time, the temperature will rise by 12~13℃ for every 1% of ethylene polymerization. If the temperature is too high, it will also cause ethylene to decompose.
(2) Under high pressure, ethylene has been compressed to a density of 0.5g/mL, similar to a liquid that can no longer be compressed, at this time the distance between ethylene molecules is significantly shortened, thereby increasing the probability of collision between free groups and ethylene molecules, so it is easy to polymerization.
(3) At high temperature, the free radical activity of the growth chain is large, and chain transfer reaction is easy to occur, and the obtained polymer has a linear structure with more branch chains. Usually every 1000 carbon chain atoms contain 20 to 30 branch chains, crystallinity is 45% to 65%, density is 0.910 to 0.925g/mL, light weight, flexible, low temperature resistance, impact resistance is good.
Low density polyethylene has the following characteristics:
(1) The film is slightly opal transparent and soft. The strength is smaller than high-density polyethylene, and the impact strength is larger than high-density polyethylene.
(2) Cold resistance, low temperature resistance and high temperature resistance. The thicker film can withstand the sterilization process of 90℃ hot water immersion.
(3) moisture resistance is better, chemical performance is stable, insoluble in general solvents.
(4) It has large breathability, so when it is used as a food packaging that is easily oxidized, the storage period of its contents should not be too long.
(5) The oil resistance is poor, and the product can be slowly dissolved. When packaging food containing fat, it will make the food appear rancid after long storage.
(6) Long-term exposure to ultraviolet light and heat will age, affecting its physical properties and dielectric properties.
(7) The melting point is 110~115 ° C, the processing temperature is 150~210 ° C, if in the inert gas, the temperature can reach 300 ° C is still stable. However, contact between melt and oxygen is easy to degrade.
Chinese name low density polyethylene
Foreign name Low Density Polyethylene
Alias LDPE
Density 0.91 to 0.93 g/cm³
Classification of plastic materials
Polymerization method High pressure method and low pressure method
Reactor types Kettle and tube
