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 variety and output of polyethylene have been huge. However, high-pressure polyethylene still occupies an important position. 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 ethylene polymerization reaction process is a strongly exothermic process. The heat of polymerization 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 heat of reaction cannot be removed in time, every time Polymerization of 1% ethylene will increase the temperature by 12 to 13°C. If the temperature is too high, it will also cause ethylene to decompose.
(2) Under high pressure, ethylene has been compressed to an airtight phase with a density of 0.5g/mL, which is similar to a liquid that cannot be compressed. At this time, the distance between ethylene molecules is significantly shortened, thereby increasing free radicals and ethylene molecules. The collision probability is high, so polymerization reaction is prone to occur.
(3) The free radical activity of the growing chain at high temperature is large, and the chain transfer reaction is prone to occur, and the obtained polymer has a linear structure with more branched chains. Usually every 1000 carbon chain atoms contains 20-30 branches, crystallinity is 45%-65%, density is 0.910-0.925g/mL, light weight, flexible, low temperature resistance and impact resistance.