The three basic popular available type of PE are HDPE, MDPE and LDPE.   LDPE grades have a very minimal role in pipe production and HDPE remains the dominant player in pipe manufacture.  The Properties of polyethylene are primarily determined by density, molecular weight distribution.  With increasing density (higher crystallinity), the following properties also increase:  Yield Stress (tensile Strength), Modulus of Elasticity (Rigidity), Hardness, Solvent Resistance, Impermeability to gases and vapors.    On the other hand, impact strength, transparency and stress cracking resistance decrease with increasing density.