Chemical and physical properties: Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low density polyethylene ( LDPE ) and high density polyethylene ( HDPE ); its Young's modulus is also intermediate. PP is normally tough and flexible, especially when copolymerised with ethylene. This allows polypropylene to be used as an engineering plastic, competing with materials such as ABS. Polypropylene is reasonably economical, and can be made translucent when uncolored but is not as readily made transparent as polystyrene, acrylic or certain other plastics. It is often opaque and/or coloured using pigments. Polypropylene has good resistance to fatigue.