Polypropylene

Polypropylene is a thermoplastic polymer, used in a wide variety of applications, including food packaging and textiles for high performance outdoor clothing.  Propylene is extracted from petroleum and then transformed into polypropylene which is melted and then spun into fibers, which are solidified by cooling and stretched. Plastic parts and reusable containers of various types can be formed, as wel as laboratory equipment, loudspeakers, automotive components, and polymer banknotes. The material is rugged and unusually resistant to many chemical solvents, bases and acids. Propylene is also used as a fuel gas for various industrial processes.

Most commercial polypropylene has a level of crystallinity intermediate between that of low density polyethylene (LDPE) and high density polyethylene (HDPE). Mechanically speaking, its Young's modulus is also intermediate. Although it is less tough and 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 (acrylonitrile butadiene styrene).

Polypropylene is rugged and  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 different colors. Polypropylene has very good resistance to fatigue, so that most plastic living hinges, such as those on flip-top bottles (e.g. Tic Tacs) are made from this material.

Very thin sheets of polypropylene can also used as for their dielectric properties in 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 which is autoclavable so that 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.

A rubbery PP can also be made by a specialized synthesis process. Unlike traditional rubber, it can be melted and recycled, making it a thermoplastic elastomer.

An important concept in understanding the link between the structure of polypropylene and its properties is tacticity. The relative orientation of each methyl CH₃ group relative to the methyl groups on neighboring monomers has a strong effect on the finished polymer's ability to form crystals, because each methyl group takes up space and constrains backbone bending.

                                                          Polypropylene n-mers:                                                              

                                         Isotactic (above) and syndiotactic (below) tacticity.

Like most other vinyl polymers, useful polypropylene cannot be made by radical polymerization. The material that results from such a process has methyl groups arranged randomly, and so is called atactic. The lack of long-range order prevents any crystallinity in such a material, giving an amorphous material with very little strength and few redeeming qualities.

A Ziegler-Natta catalyst seems to be able to limit incoming monomers to a specific orientation, only adding them to the polymer chain if they face the right direction. Most commercially available polypropylene is made with titanium chloride catalysts, which produce mostly isotactic polypropylene (the upper chain in the figure above). With the methyl group consistently on one side, such molecules tend to coil into a helical shape; these helices then line up next to one another to form the crystals that give commercial polypropylene its strength.

More precisely-engineered Kaminsky catalysts have been made, which offer a much greater level of control. Based on metallocene molecules, these catalysts use organic groups to control the monomers being added, yielding the desired tacticity: isotactic, syndiotactic, or atactic polypropylene, or even a combination of these. Aside from this qualitative control, they allow better quantitative control, with a much greater ratio of the desired tacticity than previous Ziegler-Natta techniques. They also produce higher molecular weights than traditional catalysts, which can further improve chemical and physical properties.

To produce a rubbery polypropylene, a catalyst can be made which yields isotactic polypropylene, but with the organic groups that influence tacticity held in place by a relatively weak bond. After the catalyst has produced a short length of polymer which is capable of crystallization, light of the proper frequency is used to break this weak bond, and remove the selectivity of the catalyst so that the remaining length of the chain is atactic. The result is a mostly amorphous material with small crystals embedded in it. Since each chain has one end in a crystal but most of its length in the soft, amorphous bulk, the crystalline regions serve the same purpose as vulcanization.