Polypropylene is a workhorse in the plastics industry, The general physical properties of polypropylene are similar to those of high-density polyethylene but they differ in four important areas: 1. Polypropylene has a lower specific gravity. 2. The service temperature of polypropylene is higher. 3. Polypropylene has a harder durometer and is more rigid. 4. Polypropylene is more resistant to environmental stress cracking. Polypropylene comes in two forms, a homo-polymer and a co-polymer. Typically, the co-polymer offers higher impact strength than co-polymer. Typical applications include tanks for fluid handling, lockers or cases and braces for medical use.