Polypropylene

Lore & Background

Polypropylene is a white, mechanically rugged thermoplastic polymer that is partially crystalline and non-polar. It belongs to the polyolefin group and is the second-most widely produced commodity plastic after polyethylene. While similar to polyethylene, polypropylene is slightly harder and more heat-resistant, and it possesses high chemical resistance. Its density ranges from 0.895 to 0.93 g/cm³, making it the commodity plastic with the lowest density, which allows for lighter molded parts. The material is normally tough and flexible, especially when copolymerized with ethylene, and it has good resistance to fatigue. Its Young’s modulus falls between 1300 and 1800 N/mm². The melting point of commercial isotactic polypropylene varies depending on its crystallinity and atactic content, while syndiotactic polypropylene with 30% crystallinity melts at a lower temperature. Below 0 °C, polypropylene becomes brittle. Chemically, it resists fats and most organic solvents at room temperature, though strong oxidants attack it, and it is less resistant than polyethylene due to its tertiary carbon atoms. Most commercial polypropylene is isotactic, with an intermediate crystallinity between low-density and high-density polyethylene. The three general types are homopolymer, random copolymer, and block copolymer, with ethylene commonly used as a comonomer. The material’s melt flow rate indicates its molecular weight and influences processing ease, though higher flow rates reduce impact strength.

Reader's Guide

Polypropylene is a partially crystalline, non-polar thermoplastic that is the second most produced commodity plastic globally, after polyethylene. Its mechanical and thermal properties are governed by molecular weight, molecular weight distribution, crystallinity, the presence and proportion of comonomer, and tacticity. The polymer is white, mechanically rugged, and offers high chemical resistance, being resistant at room temperature to fats and nearly all organic solvents except strong oxidants; non-oxidizing acids and bases can be stored in polypropylene containers. It has good fatigue resistance and is normally tough and flexible, especially when copolymerized with ethylene, allowing it to compete as an engineering plastic with materials like ABS. Density ranges from 0.895 to 0.93 g/cm³, the lowest among commodity plastics, and Young’s modulus falls between 1300 and 1800 N/mm². The melting point of commercial isotactic polypropylene varies with atactic content and crystallinity, while syndiotactic polypropylene with 30% crystallinity melts at a lower temperature; below 0 °C the material becomes brittle. The discovery of stereoselective polymerization by Giulio Natta and Karl Rehn in 1954 led to large-scale commercial production starting in 1957. Most commercial polypropylene is isotactic, with methyl groups aligned on one side of the carbon backbone, yielding higher crystallinity and stiffness than atactic or syndiotactic forms. Copolymerization with ethylene can reduce crystallinity, lower melting point, and increase transparency, while adding ethylene-propylene rubber improves low-temperature impact strength. Ongoing research in the 21st century explores making polypropylene from bio-based resources.

Did You Know?

• Atactic polypropylene is amorphous and lacks crystal structure, making it soluble even at moderate temperatures.
• The melt flow rate (MFR) of polypropylene helps determine how easily the molten material flows during processing; higher MFR reduces impact strength.