**3. Properties**

Polyethylene as a polymeric material possesses excellent properties, which makes it useful as an engineering material. These properties depend on the density, molecular weight, molecular weight distribution, degree of long-chain branching, and short branching. It possesses low water absorption, moderate to low gas permeability, good toughness, flexibility at low temperatures, relatively low heat resistance, and many chemicals. The use of PE as an engineering material is due to these excellent properties greatly affected by density, molecular weight, and molecular weight distribution, as enumerated in **Table 2** [15].

Polyethylene has the following engineering properties in near-absolute terms. The crystallinity of LDPE is between 40 and 60%, depending on the degree of branching and thermal history, while that of HDPE is between 60 and 90%, depending on the cooling rate and thermal history. It melts in the absence of air at 300°C to form transparent liquid except for its cross-linked polymer, which does not melt. Its specific heat at 20°C is 1330–2400 JKg−1 K−1 for HDPE, and 1900–2300 JKg−1 K−1 for LDPE, coefficient of linear expansion is 0.00017–0.00022 K−1 for LDPE and 0.00013–0.00020 K−1 for HDPE, Thermal conductivity at 23°C is
