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.
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.
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.
|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|