Thermal or Heat Stability

The mechanism of thermal oxidation in PP is through the formation of free radicals which react with environmental oxygen to produce peroxides. It could also occur due to radiation, light or the presence of metal residues. Primary antioxidants inhibit the oxidation reaction by combining with free radicals. Hindered phenolics (e g., butylated hydroxytoluene (BHT)) are commonly used as primary antioxidants. BHT is FDA approved for many applications but suffers from high-temperature volatility. Furthermore, phenolics suffer from oxidation in the presence of metal residues left from catalysts and produce a yellow colour. Different residues or different amounts of 

Secondary antioxidants, also called peroxide decomposers, inhibit oxidation of PP by decomposing hydroperoxides. Phosphites and thioesters are commonly used as secondary antioxidants. Secondary antioxidants are usually combined with primary antioxidants to produce a synergistic effect on oxidation. With proper selection of two antioxidants, it is possible to achieve protection against oxidation which is greater than the sum of protection given by the two antioxidants when working separately. 

Stabilisation with antioxidants may render PP unsuitable for food contact application as they may directly migrate into the food products, hydrolysing and imparting odour or taste to the food. Careful selection of a suitable stabiliser system is necessary for food contact applications. In addition, heat stabilisers could adversely affect the working of light stabilisers. 

In applications involving no undue mechanical stresses, PP articles will withstand 100 °C for a long period of time, depending on the stabiliser systems. Consequently, the heat and thermal stability of PP is closely related to its maximum continuous use temperature (Section 4.2.2). Short-term exposure to 140 °C is also possible. It has been observed that a properly heat stabilised and properly processed material ean undergo up to five processing cycles without noticeable reduction in molecular weight or the level of antioxidant content. 

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