PP homopolymer polypropylene is a semi-crystalline polymer that is hazy in its natural state due to the presence of crystals. PP is generally characterized as having a higher stiffness at a lower density, resistance to higher temperatures, and an excellent strength to weight ratio. The three main grades of PP resin allow for the improvement of specific properties as determined by the composition of the polymer chain:

Homopolymer Polypropylene

Homopolymer PP can be considered the most ‘basic’ grade of PP resin. This grade of PP resin is more rigid and has good resistance to high temperatures, making it particularly suitable for high-heat applications such as hot fill and microwave applications or steam sterilization applications. However, the tradeoff to this is lower impact resistance, particularly at low temperatures. For this reason, homopolymer resins are not recommended for freezer applications or other applications where the material will undergo extremely low temperatures for an extended period of time.

Additional characteristics of this grade of PP resin include a high melting point leading to excellent stiffness, superior scratch resistance, and good chemical resistance against most inorganic acids, alkalis, and salts. In addition, this grade of PP resin provides strong resistance to environmental stress cracking when in contact with alcohols, esters, detergents, or polar hydrocarbons. Homopolymer resins can be clarified through the use of additives as exemplified by Impact’s UltraClear polypropylene sheet.

Impact Copolymer Polypropylene

Similar to random copolymers, impact copolymer PP resins are derived through copolymerization with ethylene, but with this grade, the ethylene content is much higher. This results in a much tougher material with excellent impact strength even at low temperatures. For this reason, impact copolymer PP resins are particularly suitable for the freezer and freezer-to-microwave applications. In the past, using an impact copolymer came with a tradeoff on transparency as traditional impact copolymers could not be clarified, even though the use of additives.  However, Impact Plastics now carries a specialty clarified impact copolymer PP that offers the best of both worlds in terms of performance and aesthetics.  

As with every application, the end goal for the packaging application will guide your decision-making when it comes to what grade of PP to use. Homopolymer PP resins can withstand high heat and exhibit excellent stiffness, but the tradeoff is impact strength; random copolymers offer improvements to impact strength but have a lower melting point, and block copolymer PP resins offer increased impact strength at very low temperatures as well as high heat resistance.

With continued advancements in technology, both on the resin production side and the extrusion side, traditional differences in the primary grades of pp homopolymer polypropylene have blurred. Impact Plastics recommends testing of the material for every new application and custom tailors PP formulations to meet the customer's ultimate goals for their project.