This is the main variable determining HDPE characteristics, although referred 4 variables indeed play interactions. Ethylene is a polyethylene raw material, a small number of other monomers, such as 11 butene, hexene or 11 l octene, is often used to improve polymer properties of HDPE, above a few monomer content is generally not more than 1%-2%. Monomers join slightly decreases the polymer crystallinity. This change is generally measured by the density, density and crystallization rate showed a linear relationship. United States General classification according to ASTM D1248, HDPE density is above 0.940g/CC; medium density polyethylene (MDPE) density range 0.926~0.940g/CC. Other classifications are sometimes categorized in HDPE or MDPE LLDPE. Homopolymer has the highest density, maximum stiffness, good resistance to permeability and high melting point, but typically have very poor resistance to environmental stress cracking (ESCR). ESCR is PE resistance by mechanical or chemical stress cracking caused by capacity. General improved the mechanical strength of higher density, such as tensile strength, rigidity and hardness thermal properties softening temperature and hot deformation temperature non permeable, such as air or water vapor permeability.