Chemical & physical properties
Most commercial polypropylene 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. Although it is less tough than HDPE and less flexible than LDPE, it is much less brittle than HDPE. This allows polypropylene to be used as a replacement for engineering plastics, such as ABS. Polypropylene is rugged, often somewhat stiffer than some other plastics, reasonably economical, and can be made translucent when uncolored but not completely transparent as polystyrene, acrylic or certain other plastics can be made. It can also be made opaque and/or have many kinds of colors. Polypropylene has very good resistance to fatigue, so that most plastic living hinges, such as those on flip-top bottles, are made from this material. Very thin sheets of polypropylene are used as a dielectric within certain high performance pulse and low loss RF capacitors.
Polypropylene has a melting point of 320 degrees Fahrenheit (160 degrees Celsius). Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminium foil (both heat-resistant materials). After the product has cooled, the tubs are often given lids of a cheaper (and less heat-resistant) material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to PP of the same thickness is readily apparent. Rugged, translucent, reusable 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 LDPE so they can snap on to the container to close it. When liquid, powdered, or similar consumer products come in disposable plastic bottles which do not need the improved properties of polypropylene, the containers are often made of slightly more economical polyethylene, although transparent plastics such as polyethylene terephthalate are also used for appearance. Plastic pails, car batteries, wastebaskets, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature.
MFI (Melt Flow Index) identifies the flow speed of the raw material in the process. It helps to fill the plastic mold during the production process. The higher MFI increases, the weaker the raw material gets.
It also has Copolymer and Random Copolymer. Copolymer helps stiffness of the PP (Polypropylene). Random Copolymer helps transparent look.
Copolymer is more expensive than Homopolypropylene. Random Copolymer is even higher than copolymer PP.
A rubbery PP can also be made by a specialized synthesis process, as discussed below. Unlike traditional rubber, it can be melted and recycled, making it a thermoplastic elastomer.