Polypropylene (PP)

Properties and Applications

Polypropylene (PP) is the largest volume commodity thermoplastic in the world, greater than low-density, linear low-density, and high-density polyethylene individually but less than the three polyethylenes combined. Most commercial polypropylene is isotactic with an intermediate level of crystallinity. Isotactic polypropylene (i-PP) has high impact resistance, flexibility, and optical clarity and is relatively easy to mold. It competes in many areas with HDPE. It has a higher melting point, better or comparable tensile strength, superior stress crack and temperature resistance but lower impact resistance and is more rigid. It is often the better choice for more demanding applications, unless a higher resistance against thermo-oxidative degradation is required.

The packaging industry is by far the largest consumer of i-PP followed by the textile and automotive industry. Because of its low price, ease of processing, chemically inertness, and many other attractive properties, it has replaced metals, glass, wood, cellophane, and paper in countless applications. Products made of i-PP include disposable cups and cutlery, drinking straws, bottle caps, flexible food packaging, garbage bags, houseware articles (buckets and bowls), crates, tapes, labels, toys, drainage pipes, pump parts, automotive bumpers, chemical tanks, gas cans, medical components, carpet fibers, and nonwovens for diapers, sanitary products, and filters among countless other products.

Two other forms of polypropylene that have found some commercial applications are syndiotactic and atactic polypropylene. Atactic polypropylene has a irregular structure which greatly reduces the amount of crystallinity. It is much softer and much more flexible than i-PP. Its high tack makes it attractive for sealant and adhesive applications. Syndiotactic polypropylene (s-PP) is mostly crystalline due to its regular structure. However, syndiotactic polypropylene has a noticeably lower melting point and glass transition temperature than i-PP. Since its mechanical properties are inferior, it has found no or only very few commercial applications.¹

Manufacturers