High Density Polyethylene (HDPE) Polyethylene materials are divided into categories based on their specific gravity: (a) low density polyethylene having specific gravity of 0.910 — 0.925, (b) medium density polyethylene, specific gravity of 0.926 — 0.940, and. (c) high density (linear) polyethylene, specific gravity 0.941 — 0.965.