Polypropylene

Since the mid-1950s when Ziegler-Natta catalysts that yielded high-molecular-weight resin were discovered, the demand for polypropylene has increased dramatically. The conventional catalyst systems used in its manufacture are quite versatile and can be adapted to produce the homopolymer, block copolymers, and random copolymers with other monomers. With increased production, polypropylene has become cost effective to a point where it could be used in a variety of low-value packaging applications such as containers, closures, biaxially oriented film, and tape. The United States presently accounts for about 25% of the global production capacity of the resin. The recently developed metallocene catalysts have been used for the polymerization of propylene as well. This novel route allows close control of the degree of stereoregularity of the resin and the copolymerization of the propylene with a much wider selection of co-monomers (compared to conventional catalyst systems). Metallocene grades of polypropylene (mPP), including syndiotactic resins, are already available commercially but still constitute only a small fraction of the total production.

Polypropylene is typically manufactured by the direct polymerization of propy-lene in a low-pressure process employing Ziegler-Natta catalyst systems (typically aluminum alkyls and titanium halides with optional ether, ester, or silane activators). The process can be carried out in liquid or slurry in conventional manufacturing or in the newer gas-phase stirred-bed or fluidized-bed reactors. The polymerization generally yields an isotactic index (generally measured as the percent insolubles in heptane) of 85-99. The isotactic form of the polymer with a high degree of crystallinity (40-60%) is preferred for most practical applications. Isotactic polypropylene (iPP), the principal type used by the polymer processing industry, has a density of about 0.92-0.94 g/cm3. The weight-average molecular weight of polypropylene from these processes is in the range of 300,000-600,000 with a polydispersity index of about 2-6 [13]. Some atactic polypropylene results as a by-product9 of the process and has found limited practical use [14]. The atactic form is mostly amorphous and has a density of only about 0.85-0.90 g/cm3. Small amounts of the syndiotactic form of polypropylene (where the methyl groups on repeat units are located on alternate sides of the chain on adjacent

8 ACGIH TLV: The threshold limit value, averaged over an 8-h workday, determined by a private professional group, the American Conference of Governmental Industrial Hygienists. The Occupational Safety and Health Administration (OSHA) adopted many of ACGIH's recommended limits in the 1970s, but since then, ACGIH has revised some of the standards, and some of these may be different, often less stringent, compared to OSHA requirements.

9 Atactic (amorphous) polypropylene can be directly synthesized as well. Resins in low- to moderate-molecular weight resins are commercially available (Eastman Chemical Company) for hot-melt adhesive and other applications.

units) are made commercially using the single-site metallocene catalyst and are being evaluated in various applications. The syndiotactic resin has lower crys-tallinity (30-40%) and are softer, tougher, stronger (higher impact strength and elastic modulus), and relatively more transparent than the isotactic resin.

As with the polyethylenes, about a quarter of the polypropylene is manufactured as copolymers, especially with ethylene. The presence of ethylene units at a level as low as 0.5-3% by weight serves to break up the crystallinity in polypropylene matrix and leads to improved flexibility and clarity. A random copolymer that contains about 1-7% ethylene yields a lower melting, optically clearer material with improved flexibility. Incorporating larger amounts of the ethylene co-monomer gives a heterophasic impact-grade copolymer. Mechanical blending of polyethylene or ethylene-propylene rubber with polypropylene can also yield a comparable high-impact polymer. These can have a higher overall ethylene content of up to 15 wt % as the random copolymer in this case is made with much higher levels of ethylene comonomer. As the name implies, the resin has excellent impact characteristics but a somewhat lower flexural modulus.

As the demand for polypropylene continues to create a large supply of low-cost resin, there is a potential for the resin to be used in applications presently using other types of commodity thermoplastics. Its replacement of polyethylene, however, is unlikely except in applications that demand high-temperature resistance where the use of iPP and its copolymers might be advantageous. With rigid PVC products the chance of such substitution appears to be reasonable except in applications that require routine exposure of the material outdoors. The relatively poor weathering resistance of iPP, despite its comparable mechanical properties (in filled or fiber-reinforced compounds) to that of PVC, limits its use outdoors. However, with rapidly improving polypropylene technology and better compounding techniques, such substitutions and even the replacement of some expensive engineering thermoplastics with polypropylenes might become feasible [15].

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