Plastic Knowledge Base

Other names:
Polypropene
Polipropene 25 [USAN]
Propene polymers
Propylene polymers
1-Propene

Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging(e.g. plastic bottle caps, textiles (e.g. ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. An addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases and acids.

Polypropylene was first polymerized on March 11th 1954 by Giulio Natta. At first it was thought that it would be cheaper than polyethylene.

The Chemical and physical properties of polypropylene
Molecular formula:  (C₃H₆)_x

Density:
0.855 g/cm3, amorphous
0.946 g/cm3, crystalline

Melting point: ~ 160 °C

Polypropylene has a melting point of ~160°C (320°F), as determined by Differential scanning calorimetry (DSC).

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.

The MFR (Melt Flow Rate) or MFI (Melt Flow Index) is a measure of PP’s molecular weight. This helps to determine how easily the melted raw material will flow during processing. Higher MFR PPs fill the plastic mold more easily during the injection or blow molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.

There are three general types of PP: homopolymer, random copolymer and block copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber or EPDM added to PP homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to PP homopolymer decreases the polymer crystallinity and makes the polymer more transparent.