Several different polymerisation methods can be used to produce PP, but all rely on exposing the propylene monomer to high temperatures and pressures in the presence of an active metal catalyst. By selecting particular catalysts and polymerisation methods, the molecular configuration of the polymer can be directed to one of three forms: atactic, isotactic or syndiotactic. The images here show the monomer and polymer structure of PP and three types of PP.
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Atactic polymers are characterised by their tacky texture, amorphous behaviour and low molecular weights. Where present they act as a plasticiser by reducing the overall crystallinity of the PP. From a commercial viewpoint, isotactic PP is the most important form of the polymer. In comparison to the atactic and syndiotactic forms, isotactic PP has the greatest degree of stereo-regularity; as a result, a higher degree of crystallinity is achieved. Many of the mechanical properties and processing characteristics of pp are determined by the level of isotacticity and thus crystallinity.
The useful properties of the polymer, combined with its comparatively low price, make it suitable for a wide range of applications. PP is the lightest of all man-made fibres (0.91 g/cm3) and has the lowest moisture absorption. Dirt will not penetrate the fibre and can easily be removed from the surface. Polypropylene fibres have the same tensile strength, wet or dry, and are also highly resistant to both acids and alkalis and to most organic solvents. The fibre does not rot and is not readily attacked by micro-organisms or moulds.