Polypropylene impurity levels

In contrast to polyethylene production, solution polymerisation at high temperature is rarely applied for isotactic polypropylene, but some special-purpose polypropylene grades are manufactured (Figure 3.57) [51]. However, the solution process, which yields isotactic polypropylene with a very low level of impurities, is characterised by high overall costs. The solution process is being used to make atactic polypropylene, to which it seems more suited [43],... 

In separate work aimed at elucidating filler effects on surface, oxidation Vesely and coworkers [39] have examined the oxidation of polyethylene and polypropylene containing high levels of calcium carbonate. The grade of filler used was found to decrease polymer thermal stability, an effect they associated with transition metal impurities. They also believe that, in the presence of fillers, heteregeneous oxidation reactions become more important. They observed black polyenes to be formed above about 320 °C and postulated that these could be acting as photo-sensitisers. 

Colour is also changed by the mineral filler pure talc gives translucent, almost colourless compounds while high quality calcium carbonate gives a white colour with some opacity. Increasing levels of impurities in both minerals causes colour degradation. Colour will also be caused by any instability introduced into the polypropylene by the mineral. 

Low pressure polyethylene might contain minute traces of oxygen or nitrogen or sulfur chain transfer catalyst residues. These are usually labile and can be ignored from the foodstuff contamination point of view. Polymers such as polyethylene, polypropylene and polystyrene manufactured by a catalysed low pressure route will however, usually contain appreciable catalyst residues usually appearing in the form of aluminium, chromium, magnesium and titanium, also possibly lithium and sodium. As the example quoted in Table 3.1 indicates these impurities can occur in the polymer at levels approaching 100 ppm. 

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