Polypropylene (PP): Chemical and physical properties

Most commercial polypropylene (PP) is isostatic and has an intermediate level crystallinity between that of low density polyethylene (LDPE) and high density polyethylene (HDPE); its Young's modulus is also intermediate. Through the incorporation of rubber particles, Polypropylene (PP) can be made both tough and flexible, even at low temperatures. This allows Polypropylene (PP) to be used as a replacement for engineering plastics, such as ABS. Polypropylene (PP) is rugged, often somewhat stiffer than some some other plastics, reasonably economical, and can be made transparent as polystyrene, acrylic or certain other plastics. It can be also be made opaque and have many kinds of colors through the use of pigments. Polypropylene (PP) has very good resistance to fatique, so that most plastic living hinges, such as those on flip top bottles, are made from this material. Very thin sheets of Polypropylene (PP) are used as a dielectric within certain high performance pulse and low RF capasitors.

Polypropylene(PP) has melting point of about 160 degree Celcius, as determined by Differential Scanning Calorimetry (DSC). Many plastic items for medical or laboratory use can be made from Polypropylene (PP) because it can withstand the heat in an autoclave. Food containers provide a good hands on example of difference in modulus, since the rubbery (softer, more flexible) feeling of Linear density polyethylene (LDPE) with respect to Polypropylene (PP) of the same thickness is readily apparent. Rugged, translucent, reuseable plastic containers made in a wide variety of shapes and sizes for consumers from various companies such as Rubbermaid and Sterilite are commonly made of polypropylene, although the lids are often made of somewhat more flexible Linear density polyethylene (LDPE) so they can snap on to the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered or similar consumer products, although High density polyethylene (HDPE) and Polyethylene terephthlate (PET) are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, cooler containers, dishes and pitchers are often made of Polypropylene (PP) or High density polyethylene (HDPE), both of which commonly have rather similar appearance, feel, and properties at ambient temperature.

The Melt Flow Rate (MFR) or Melt Flow Index (MFI) is an indication of polypropylene's (PP's) molecular weight. This helps to determine how easily the melted raw material will flow during processing. Higher Melting Flow Rate (MFR) Polpropylenes (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 Polypropylene (PP): homopolymer, random copolymer and impact or block copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber added to Polypropylene (PP) homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to Polypropylene (PP) homopolymer decreases the polymer crystallinity and makes the polymer more transparent.