Antioxidants function during processing

Pokomy, J. and Schmidt, S. (2001) Natural antioxidant functionality during food processing, in Antioxidants in Food, Pokomy, J., Yanishlieva, N., and Gordon, M., Eds., CRC Press, Boca Raton, EL, 331. 

Phosphites. Tertiary phosphites are also commonly used and are particularly effective in most mixed metal stabilizers at a use level of 0.25—1.0 phr. They can take part in a number of different reactions during PVC processing they can react with HC1, displace activated chlorine atoms on the polymer, provide antioxidant functionality, and coordinate with the metals to alter the Lewis acidity of the chloride salts. Typical examples of phosphites are triphenyl phosphite [101 -02-0], diphenyl decyl phosphite [3287-06-7], tridecyl phosphite [2929-86-4], and polyphosphites made by reaction of PC13 with polyols and capping alcohols. The phosphites are often included in commercial stabilizer packages. 

It may be anticipated that the antioxidant consumption during use leads to a shorter remaining life-time and that it is necessary to add more. Recycled plastics suffer from the consequences of degradation during processing and first-life application. This leads to the introduction of new functional groups, which in particular for oxidisable polymers such as PE, enhance the sensitivity of the recyclate to thermal- and photo-degradation. With the formation of new pro-oxidative moieties, a substantial part of the stabilisers are simultaneously consumed [14]. 

Heat stabilizers are used to prevent degradation of polymers when they are exposed to heat, especially during processing. For most polymers, the primary mode of reaction is oxidation, so antioxidants function effectively as heat stabilizers. For a few sensitive polymers, however, other forms of reaction are more important. In particular, a major disadvantage of PVC is its poor thermal stability. Degradation takes the form primarily of dehydrochlorination, yielding HCl and resulting in formation of a double bond in the main polymer chain. 

The term primary antioxidant is used to denote those additives that suppress oxidation over the lifetime of the product, whereas the main function of secondary antioxidants is to protect the polymer for the much shorter period when they are being processed. Nevertheless secondary antioxidants do have a lesser function during service life, as mentioned below. Primary antioxidants are also called chain-breaking antioxidants because they break the ehain of events that lead to oxidation. 

Reaction 6, where R contains a hindered phenol or a UV absorber function, has also been used to introduce antioxidant and UV stabiliser groups into PVC during processing (see Section 7). 

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