Antioxidants substantivity

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. 

High molecular weight antioxidants were subsequently developed, some in industrial laboratories, and some of these commercial products (5-9) are given in Table 1. With the exception of (9), all these products contain the same antioxidant function but differ in the chemically inert part of the molecule which gives credence to the importance of non-volatility as a criterion for antioxidant substantivity and performance. 

Three factors determine the effectiveness of antioxidants in polymers, namely (i) intrinsic molar activity (ii) substantivity in the polymer and (iii) solubility in the polymer. Multifunctional AOs combine multiple functions in one molecule. Sterically hindered amine stabilisers (HAS), such as Chimassorb 944, Tinuvin 622 and Tinuvin 783 are prime examples. 

Furthermore, concentrates may also be used as conventional additives in the same polymer or in different polymers. In this way a substantive antioxidant (or modifier) system can be produced with very high effectiveness especially under aggressive environments. In comparison, under such demanding conditions, conventional antioxidants will not only lead to premature failure of the polymer but also to dangerous situations such as in the case in food contact application (packaging), medical uses of polymers (artificial joints), and failure of aircraft tyres where human lifes are at risk. 

Another example where antioxidant performance can be improved dramatically lies in the mechanochemlcally initiated addition of reactive antioxidants on rubbers (5.10) or unsaturated thermoplastics such as ABS (12). For example, using thiol antioxidants 2 and 3 as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see Equation 1) the polymer becomes much more substantive under aggressive environments. 

Low molar mass antioxidants are known to suffer from physical loss and research in recent years has, therefore, focused on finding substantive alternatives. For example, reactive antioxidants, which can be chemically anchored on the polymer backbones to prevent physical loss during processing or in-service, have been explored as nonmigratory antioxidants. Clearly, there are many factors that need to be considered before choosing antioxidants in end-use applications. The success of an antioxidant package is critically dependent on the underlying chemical and physical factors that influence antioxidant performance in different substrates and environments. 

Scott, G. Substantive antioxidants. In Developments in Polymer Stabilisation, Scott, G., Ed. Applied Science London, 1981 Vol. 4, 181-221. 

Other areas where physical loss of additives from polymers is increasing in importance is in food contact applications and in medical uses of polymers. In Europe, legislation is steadily moving toward the situation where loss of any additives, irrespective of their toxicity, may be prohibitive above a certain level. It is, therefore, no longer an academic exercise to retain antioxidants and stabilizers in polymers merely to satisfy unrealistic accelerated tests. It is a practical necessity to improve the substantivity of antioxidants and stabilizers for a variety of quite different practical reasons. 

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