What Is Density and Why Does It Matter in Plastic Selection?

Density is the amount of mass per unit volume; for polymers it is generally expressed in g/cm³ (or kg/m³). Common measurement standards are ASTM D792 and ISO 1183.

In polyolefins, density is an indicator of how tightly the material is 'packed' — which is directly related to the degree of crystallinity: the more regular/linear the chains, the more tightly they pack, and crystallinity and density increase together.

As branching increases in polymer chains, it becomes harder for the chains to form a regular crystalline structure — molecules sit together more loosely in amorphous (disordered) regions. This leads to lower density.

This relationship is exactly the basis for the classification of the polyethylene family: heavily branched AYPE has the lowest density, while nearly unbranched YYPE has the highest (see the 'What Is PE' article).

Rigidity/stiffness: as density increases, the material generally becomes stiffer and less flexible.

Barrier properties: a higher-density/more crystalline structure generally reduces moisture and gas permeability — this matters for barrier performance in packaging films.

Optical properties: high crystallinity generally scatters more light, which can mean lower transparency/more haze — lower-density, more amorphous structures typically appear clearer.

Yield calculation: the volume (e.g. film area) that can be produced from the same weight of raw material is inversely proportional to density — a lower-density material yields more volume/area per kilogram.