Polymer-bound Antioxidants and Light Stabilisers

It was observed in the last section that most antioxidants and stabilisers are relatively low molar mass organic compounds which can be easily extracted by solvents or volatilised from the surface of the polymer at elevated temperature. This is a disadvantage in two ways. Firstly, the additives may migrate into the human environment, causing toxicity in foodstuffs or, as in the case of prostheses, directly in the body. Secondly, because they are lost from the polymer, their effectiveness is reduced. 

A more versatile method of achieving antioxidant substantivity in rubbers is to chemically attach the antioxidant to the polymer. A commercial nitrile-butadiene rubber contains a copolymerised antioxidant, N-methacrylamidodiphenylamine (MADPA) which is much more resistant to oil extraction. A similar result has been achieved by forming an adduct of the analogous mercaptoacryloyl-amidodiphenylamine, MADA [reaction (3.18)] by processing of MADA with nitrile-butadiene rubber. 

For polyolefins used under aggressive conditions (e.g. polypropylene fibres subject to dry-cleaning or detergent action) solvent-resistant TMP systems have been developed by covalent chemical attachment of UV stabilisers to the polymer. Two synthetic routes have been developed to do this. The first involves the modification of the polymer e.g. polypropylene) by copolymerisation with maleic anhydride to give an anhydride group in the polymer chain. This can be subsequently reacted with an alcohol or amine group in the light absorber  

However, the same effect can be achieved by grafting TMP esters of unsaturated carboxylic acids e.g. BPF) to normal polyolefins by reactive processing in the presence of dialkyl peroxides  

These modified polymers may be added to the main polymer matrix as a concentrate and the antioxidant cannot be removed from the resulting polymer products by solvent leaching. This makes it possible to wash and dry-clean fibres containing TMP light stabilisers without sacrificing durability. 

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