Low-Density Polyethylene

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. Polymerizing ethylene can occur either in a tubular or in a stirred autoclave reactor. In the stirred autoclave, the heat of the reaction is absorbed by the cold ethylene feed. Stirring keeps a uniform temperature throughout the reaction vessel and prevents agglomeration of the polymer. In the tubular reactor, a large amount of reaction heat is removed through the tube walls.

Reaction conditions for the free radical polymerization of ethylene are 100–200°C and 100–135 atmospheres. Ethylene conversion is kept to a low level (10–25%) to control the heat and the viscosity. However, overall conversion with recycle is over 95%.

The polymerization rate can be accelerated by increasing the temperature, the initiator concentration, and the pressure. Degree of branching and molecular weight distribution depend on temperature and pressure. A higher density polymer with a narrower molecular weight distribution could be obtained by increasing the pressure and lowering the temperature.

The crystallinity of the polymer could be varied to some extent by changing the reaction conditions and by adding comonomers such as vinyl acetate or ethyl acrylate. The copolymers have lower crystallinity but better flexibility, and the resulting polymer has higher impact strength.