Another “workhorse” of the plastics industry, polypropylene is one of the high-volume “commodity” thermoplastics. Polypropylene was developed out of the Nobel award-winning work of Karl Ziegler and Professor Natta in Europe, and came to the United States in 1957. It belongs to the “olefins” family, which also includes the polyethylenes, but it is quite different in its properties. It has a low density, is fairly rigid, has a heat distortion temperature of 150 to 200 degres F (making it suitable for “hot-fill” packaging applications), and excellent chemical resistance and electrical properties. Polypropylenes are also very easy to process in all conventional systems. (For information on processing methods, see: processing methods.) Major applications of commercial PP are packaging, automotive, appliances and carpeting. Polypropylene is made by polymerizing propylene [CH3CHCH2] and in the case of copolymers with monomers, with suitable catalysts, generally aluminum alkyl and titanium tetrachloride mixed with solvents. The monomer unit in polypropylene is asymmetric and can assume two regular geometric arrangements: isotactic, with all methyl groups aligned on the same side of the chain, or syndiotactic, with the methyl groups alternating. All other forms, where this positioning is random, are called atactic. Commercial polypropylene contains 90-97% crystalline or isotactic PP with the remainder being atactic. Most processes remove excess atactic PP. This by-product is used in adhesives, caulks, and cablefilling compounds.