ABSTRACT
By variations on the form of the catalysts used, Natta was able to produce a number of different types of high-molecular-weight PPs, which differed extensively in their properties. One form, now known as isotactic PP, had a higher softening point, rigidity, and hardness, while another form, the atactic polymer, was amorphous and had little strength [9-12]. Montecatini marketed the isotactic PP grade in 1957, which followed rapid commercial exploitation of the polymer. PP has been used in tremendous amounts in a number of applications since then, namely, in bers, lms, and injection moldings. PP is a linear hydrocarbon polymer containing little or no unsaturation. The presence of a methyl group attached to alternate carbon atoms on the chain backbone can alter the properties of PP as compared to PE in a number of ways. A slight stiffening of the chain occurs in the structure owing to the pendant alternate methyl group and it also interferes with the molecular symmetry. The rst effect leads to an increase in the crystalline melting point whereas the interference with molecular symmetry tends to depress it. In the case of the most regular PPs, the net effect is a melting point around 50°C higher than that of the most regular PE grades. Some aspects of chemical behavior are also affected by the methyl group. For example, the tertiary carbon atom provides a site for oxidation causing PP to be less stable than PE to the in©uence of oxygen. In addition, thermal and high-energy treatment leads to chain scission rather than cross-linking. The most signicant in©uence of the methyl group is its generation of different tacticity in the products, ranging from completely isotactic and syndiotactic structures to atactic molecules. The morphological structure of PP is rather complex and at least four different types of spherulite structures have been observed. The properties of the polymer depend on the size and type of crystal structure formed, which in turn is dependent on the relative rates of nucleation to crystal growth. The ratio of these two rates can be controlled by varying the rate of cooling and by the incorporation of nucleating agents. In general, the smaller the crystal structures, the greater the transparency and ©ex resistance and lesser rigidity and heat resistance.
