Polypropylene is a crystalline material from the same family of polymers as polyethylene, the polyolefins. Polyethylene and polypropylene have some very similar properties such as resistance to water, good chemical resistance and good dielectric properties, but there are some important differences that set it aside. Polypropylene's relative density runs in the 0.90 range versus polyethylene which is in the 0.940 to 0.965 range. Polypropylene has a higher service temperature. It is more rigid and is more resistant to environmental stress cracking. Typically, you will hear of two different types of polypropylene. There is the homopolymer P/P and the random copolymer P/P. One of the homopolymer's biggest uses is in fibers and filaments. It is also used in packaging as a film, due to its water barrier characteristics. When you need a tougher more impact resistant part, you need to look at the copolymer. Here, a flexible ethylene component is added to the molecular structure, giving the material better impact, flexibility, and clarity properties. The copolymers are also used in films because of their toughness, but when you want interior or exterior trim pieces or parts for furniture, the copolymer is the material of choice. Polypropylene can be difficult to bond, normally flammable, and has poor weatherability, but Polypropylene's properties can be enhanced by adding fillers such as glass, talc or a flame retardant.
Polypropylene (PP) has become a major plastic, especially for injection molding, since its introduction in the late 1950s. PP can be synthesized in isotactic, syndiotactic, or atactic structures, the first of these being the most important. It is the lightest of the plastics, and its strength-to-weight ratio is high. PP is frequently compared with HDPE because its cost and many of its properties are similar. However, the high melting point of polypropylene allows certain applications that preclude use of polyethylene—for example, components that must be sterilized. Other applications are injection molded parts for automotive and houseware, and fiber products for carpeting. A special application suited to polypropylene is one-piece hinges that can be subjected to a high number of flexing cycles without failure.
Polypropylenes have better resistance to heat (heat distortion temperature at 66 psi: 200-250°F) and resist more chemicals than do other thermoplastic materials of the same cost. Also, polypropylenes have negligible water absorption and excellent electrical properties, and they are easy to process.
In much the same way that density is important in determining the mechanical properties of polyethylenes, the stereoregularity (related to the repeated units in the stereoregular molecular chain) of a polypropylene very often determines the characteristics of the material. An increase in the stereoregularity of a polypropylene will sharply increase the yield strength of the material. The hardness, stiffness, and tensile strength also increase. On the other hand, as stereoregularity decreases, elongation and impact strength increase.
The ability to carry light loads for long periods and over wide temperature ranges is one of the properties that make polypropylenes valuable in engineering applications. Polypropylenes do not have outstanding long-term creep resistance, but their fatigue endurance limit is excellent. In fact, polypropylene often is referred to as the "living hinge" thermoplastic.
One of the limitations most often mentioned for polypropylenes is their low-temperature brittleness (−4°F). However, polypropylene copolymers have been developed with brittleness points of about −20°F.
Like all other polyolefins, polypropylenes have excellent resistance to water and to water solutions, such as salt and acid solutions that are destructive to metals. They also are resistant to organic solvents and alkalis. Above 175°F, polypropylene is soluble in aromatic substances such as toluene and xylene, and in chlorinated hydrocarbons such as trichlorethylene.
Polypropylenes have excellent electrical resistivity (both volume and surface), and their dielectric strength is high.
The greatest commercial uses for homopolymer PP are in fibers and filaments. PP fibers are woven into fabrics and carpets, and the also are used to produce nonwoven fabrics for disposables. Slit tape-filaments are used as jute replacements in carpet backings and sacks. PP also is made into unoriented and oriented films for packaging, which have largely replaced cellophane and glassine. Homopolymer PP is injection-molded into caps and closures, appliance components, and auto parts.
Random copolymer PP (with up to 7% ethylene) has higher impact strength and better clarity than the homopolymer. Its heat distortion temperature is lower than that of the homopolymer, 150 to 205°F under 66 psi load. These materials are used in blow-molded containers (including oriented and multilayer bottles), injection-molded packaging, and flexible monolayer and coextruded films.
Impact copolymers are produced in a secondary reactor. The ethylene comonomer provides a flexible component in the otherwise rigid, crystalline structure. Impact copolymers have replaced impact PPs made by blending the homopolymer with ethylene-propylene rubber.
PP impact copolymers are tough, even at low temperatures, and yet retain a high percentage of the stiffness of the homopolymer. Injection molding applications include automobile battery cases, interior and exterior trim parts, housewares, and furniture. Coextruded with barrier polymers such as EVOH and PVDC, impact PP is made into multilayer sheet that is thermoformed into food packages that can withstand freezer storage and microwave cooking.
The rigidiy of polypropylenes can be improved significantly by the addition of inexpensive mineral fillers. Filled PPs and even glass-reinforced PPs are made into chemically bonded compounds that are truly engineering thermoplastics.
It is a semi-crystalline thermoplastic, with a continuous use temperature range of up to 180°F (82°C). It has a low difficulty to machine. It has high impact resistance, high chemical resistance, and good corrosion resistance. It has very low water absorption. It has superior resistance to stress cracking and has a high surface hardness. It has good electrical insulation properties.
History
This polyolefin has turned out to be the most versatile of the family, accounting for the continuing rapid increase in its use. First produced in the 1950s, early polypropylenes (PPs) suffered from low yields in polymerization, high percentage (about 10%) of atactic polymer, and poor control of molecular weight.
The development of high-activity catalysts in the 1970s improved the yields dramatically and almost eliminated production of the atactic form. The resulting isotactic polymers were highly stereoregular.