Plastic additive 18 (P-EPQ) is a phenolic antioxidant that may be used as a stabiliser in the processing of plastic materials. It consists of seven components, all of which contribute to the efficiency of the additive. However, it is a product-by-process (reaction product) rather than a physical mixture. It comprises one main component and the isomers of that component.

 The members of European Pharmacopoeia (Ph. Eur.) Group 16 (the group responsible for texts on plastic materials and containers) are not certain if this additive – which is only mentioned in Ph. Eur. general chapters Polyolefins (3.1.3) and Plastic additives (3.1.13) – is still in use. In addition, from an analytical point of view, detection and quantification of the components of the additive is complex. This is especially true in the presence of other plastic additives, with some of the components being prone to degradation (oxidation). This makes it difficult to establish reasonable quality criteria that can be measured with a standardised compendial method for products-by-process.

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