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Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including food packaging, textiles, plastic parts and reusable containers of various types, thermal pants and shirts made for the military, 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.

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.

 

Synthesis

Short segments of polypropylene, showing examples of isotactic (above) and syndiotactic (below) tacticity.
Short segments of polypropylene, showing examples of isotactic (above) and syndiotactic (below) tacticity.

An important concept in understanding the link between the structure of polypropylene and its properties is tacticity. The relative orientation of each methyl group (CH3 in the figure at left) 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.

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.

A ball-and-stick model of syndiotactic polypropylene.
A ball-and-stick model of syndiotactic polypropylene.

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, so that a proper choice of catalyst can produce 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 provious Ziegler-Natta techniques. They also produce higher molecular weights than traditional catalysts, which can further improve 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.

Practical applications

In the US military, polypropylene, or 'polypro', is the material used for the fabrication of cold-weather gear, such as a long-sleeve shirt or long underwear, in addition to warm-weather gear such as Under Armor clothing, which can easily wick away sweat. Of course these polypro clothes are not flammable, but they will burn and melt, which can result in severe burns if the servicemember is involved in an explosion or fire of any kind.[1]

References

  1. ^ USAF Flying Magazine. Safety. Nov. 2002.

External links

The above article is licensed under the GNU Free Documentation License. From Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Polypropylene).  Modified by Apparel Search 4/10/07

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