HDPE is a high crystallinity, non-polar thermoplastic resin, and also the main raw material of PE pipes. Its density directly affects the performance of pipes. Density is the main variable that determines the properties of HDPE, although the four variables mentioned do interact.
Ethylene is the main raw material of polyethylene. A few other comonomers, such as 1-butene, l-hexene or 1-octene, are also often used to improve polymer properties. For HDPE, the content of the above few monomers generally does not exceed 1% - 2%. The addition of comonomer slightly reduced the crystallinity of the polymer. This change is generally measured by density, which is linear with the crystallinity. According to ASTM D1248, the density of HDPE is above 0.940g/C;
The density range of medium density polyethylene (MDPE) is 0.926~0.940g/CC. Other classifications sometimes classify MDPE as HDPE or LLDPE. Homopolymers have high density, large stiffness, good impermeability and high melting point, but generally have poor resistance to environmental stress cracking (ESCR).
ESCR is the ability of PE to resist cracking caused by mechanical or chemical stress. Higher density generally improves mechanical strength, such as tensile strength, stiffness and hardness; Thermal properties such as softening point temperature and thermal deformation temperature; Impermeability, such as air permeability or water vapor permeability.
The lower density improves the impact strength and E-SCR. The density of polymer is mainly affected by the addition of comonomer, but to a lesser extent, it is also affected by molecular weight. The percentage of high molecular weight slightly reduces the density. For example, homopolymers have different densities in a wide range of molecular weights.
