Polypropylene

The three main types of polypropylene generally available are: 

• Homopolymers are made in a single reactor with propylene and a catalyst. It is the stiffest of the three propylene types and has the highest tensile strength at yield. In the natural state (no colorant added), it is translucent and has excellent see-through or contact clarity with liquids. In comparison to the other two types it has less impact resistance, especially below 0 C. 

• Random copolymer (homophasic copolymer) is made in a single reactor with a small amount of ethylene (, 5%) added, which disrupts the crystallinity of the polymer allowing this type to be the clearest. It is also the most flexible with the lowest tensile strength of the three. It has better room temperature impact than homopolymer but shares the same relatively poor impact resistance at low temperatures. 

• Impact copolymers (heterophasic copolymer), also known as block copolymers, are made in a two reactor system, in which the homopolymer matrix is made in the first reactor and then transferred to the second reactor, where ethylene and propylene are polymerized to create ethylene propylene rubber in the form of microscopic nodules dispersed in the homopolymer matrix phase. These nodules impart impact resistance at both ambient and low temperatures to the compound. This type has intermediate stiffness and tensile strength and is quite cloudy. In general, the more ethylene monomer is added, the greater the impact resistance, with correspondingly lower stiffness and tensile strength.

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