A polymer is a unique molecule that is made up of many identical units. Each individual unit is called a monomer (“mono” means one and “mer” means unit). The prefix “poly” means "many" -- a polymer is many units. Often, however, different polymers are blended together to impart unique or desirable chemical or physical properties. Each type of polymer has a certain density (mass per unit volume). The density of a polymer blend is the sum of the mass fractional density of each type of polymer.

Determine the chemical composition of a polymer blend. For instance, polypropylene could be blended with polyethylene. If the blend has 70% polypropylene and 30% polyethylene, then the mass fractions are 0.70 for polypropylene and 0.30 for polyethylene

Determine the density of each polymer type using the specific gravity and comparing that to water (specific gravity = 1.0). The "Polymer Technology Dictionary" contains the specific gravities of most common polymers. Polypropylene has a specific gravity of 0.89 and polyethylene has a specific gravity of 0.92. Since the density of water is 62.37 pounds per cubic foot, the specific gravity of other materials is multiplied by this density to determine a density relative to water. For polypropylene, this works out to be 0.89 x 62.37, or 55.51 pounds per cubic foot. For polyethylene, it works out to be 0.92 x 62.37, or 57.38 pounds per cubic foot.

Determine the density for the polymer blend by adding the mass fraction densities together. This is done with the formula 0.70 x 55.51 (polypropylene) + 0.30 x 57.38 (polyethylene) for an answer of 56.07 pounds per cubic foot for the blend.