Polypropylene (PP)

Oleh Designer3dmax 26 Mar, 2011 Posting Komentar

Polypropylene is a versatile semicrystalline thermoplastic offering a useful balance of properties

which are typically higher than for HDPE. It is prepared by the catalyzed polymerization of propylene.Crystallinity is key to the properties of PP. The degree of crystallinity is a function of the degree of geometric orientation (stereoregularity) of the methyl groups on the polymer chain (backbone). There are three possible geometric forms of PP.

Isotactic PP has predominantly all the methyl groups aligned on the same side of the chain (above or below the chain). In syndiotactic PP the methyl groups alternate above and below the polymer chain. Finally, atactic PP has the methyl groups randomly positioned along the chain. Both isotactic and syndiotactic PP will crystallize when cooled from the molten states; however, commercial PP resins are generally isotactic.

Isotactic PP is highly crystalline thermoplastic that exhibits low density; rigidity; good chemical resistance to hydrocarbons, alcohols, and oxidizing agents; negligible water absorption; excellent electrical properties; and excellent impact/ stiffness balance. PP has the highest flexural modulus of the commercially available polyolefins. In general, PP has poor impact resistance. However, PP–elastomer blends have improved impact strength. Unfilled PP has poor flame resistance and is degraded by sunlight. Flame-retardant and UV-stabilized grades are available. Typical properties appear in Table 3.