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.
Things You'll Need
- Polymer Technology Dictionary: Compound Blending, Page 83. Tony Whelan. Springer. 1994.
About the Author
Brian Baer has been writing since 1982. His work has appeared on Web sites such as eHow, where he specializes in technology, management and business topics. Baer has a Bachelor of Science in chemical engineering from the University of Arkansas and a Master of Business Administration from the University of Alabama, Huntsville.