Polypropylene filaments. Coloured scanning electron micrograph (SEM) of filaments of the plastic polypropylene, formed by the polymerization of propylene. This material is similar to high-density polythene, but is stronger, lighter and more rigid. It is also a thermoplastic, meaning that it can be repeatedly softened by heating and hardened by cooling. Polypropylene is used to make solid products and fibrous materials. Magnification: x3 at 6x7cm size.