Polyethylene is the largest volume thermoplastic used today.  Polyethylene is available in a variety of grades that have a wide range of properties and are classified according to density:

LDPE   = 0.910 – 0.930 g/cm³

LLDPE = 0.915 – 0.940 g/cm³

HDPE  = 0.940 – 0.950 g/cm³

For polyethylene the density and crystallinity are directly related.  The higher the degree of crystallinity, the higher the resin density which influences numerous properties.  As density increases, heat softening point, resistance to gas and moisture vapor permeation and stiffness increase.   Increased density generally results in a reduction of stress cracking resistance and low temperature toughness. 

The exception to this rule of thumb is a special category of HDPE known as high-molecular-weight HDPE (HMW HDPE) that offers outstanding environmental stress crack resistance, toughness and durability, particularly at low temperatures. These characteristics result from a unique combination of high average molecular weight and a bimodal molecular-weight distribution.  As a result, this material is often used in demanding applications such as automotive fuel tanks, drums, pallets, and industrial containers.

Reference the table below or click on Polyethylene Effects As Density Increases to understand the effect of density on appearance, properties and processing of polyethylene.

Polyethylene Appearance, Properties & ProcessingAs Density Increases
GlossImproves
StiffnessIncreases
ToughnessDecreases
Heat ResistanceImproves
Stress Crack ResistanceDecreases
Durometer Surface HardnessIncreases
Permeability ResistanceImproves
Mold ShrinkageIncreases
Molding SpeedIncreases
WarpageIncreases