Antioxidant structure

One of the present authors (31) has developed a series of additives which combine the features of both free radical inhibitors and flame retardants of the tetrabromophthalimide or chlorendic imide type with hindered phenol antioxidant structures such as the following compounds ... 

We have recently evaluated the chlorendic imide/hindered phenol for its effect on the oxygen index of polyethylene, and we found only a miniscule increase, not considred statistically significant, in comparison to the same loading of chlorine as chlorendic anhydride. We believe that if the antioxidant approach to flame retardancy is to be successful, special high temperature antioxidant structures must be designed for this purpose. 

Besides the reactions between phenols and peroxidic bodies, other factors can influence the activity of antioxidants—e.g., compatibility with substrate and volatility. The results show that under the conditions used the influence of the antioxidant structure is dramatic. In this connection we note agreement of the general conclusions dealing with the influences of pyrocatechol antioxidant structure on the activity in polypropylene at 180°C. and those influences found in Tetralin (28) at 80 °C. Despite great differences in experimental conditions, the sequences of the activities of pyrocatechol antioxidants I-VI were in agreement. Great similarities were also found within each particular group of antioxidants. 

Figure 4.32 Various antioxidant structures currently used in polyether polyol...

Natella, R, Nardini, M., Felice, M.D., and Scaccini, C., Benzoic and cinnamic acid derivatives as antioxidants Structure-activity relation. J. Agric. Food Chem., 47, 1453-1459, 1999. 

Al-Malaika, S., Scott, G. and Wirjosentono, B. (1993) Mechanisms of antioxidant action Polymer-bound hindered amines by reactive processing, part III - Effect of reactive antioxidant structure. Polymer Degrad. Stab, 40, 233-238 Al-Malaika, S., Ibrahim, A.Q. and Al-Malaika, S. (1988) Mechanisms of antioxidant action Photoantioxidant activity of polymer-bound hindered amines I Bis maleate esters. Polymer Degrad. Stab., 22, 233-239 Al-Malaika, S. and Scott, G. (1995) US Patent 5382633. 

Natella F, Nardini M, Di Felice M, Scaccini C (1999) Benzoic and cinnamic acid derivatives as antioxidants structure-activity relation. Free radical research group, national institute of nutrition, Roma, Italy. J Agric Food Chem 47 1453-2323... 

Vitamin E is a collective term for tocopherols and tocotrienols, a series of potent antioxidants structurally derived from 6-chromanol (Fig. 6). Tocopherols and tocotrienols differ in the degree of saturation of their respective isoprenoid side chains. Among the tocopherols, tocol can be regarded as the unsubstituted parent molecule, whereas 8-, P + 7-, and a-tocopherol form a homologous series of mono-, di-, and trimethyltocols, respectively, P- and y-tocopherols being positional isomers (46). 

Most thermal analysis methods for studying polymeric stabilizer systems are based on the antioxidant s ability to delay the oxidation process. Usually a sample is heated to a specified temperature and the induction time, or period of time before the onset of rapid thermal oxidation, is determined [see discussion of oxidative induction time (OIT) in Section 3.4.2 of this chapter]. The end of the induction period is marked by an abrupt increase in the sample s temperature, evolved heat, or mass and can be detected by DTA, DSC or TGA, respectively (Bair 1997). The effect of antioxidant structure and its concentration on prolonging a sample s induction period can be used to determine the most effective antioxidant system for a polymer such as polyethylene. Extensive data have shown that thermal information such as this can be used successfully to estimate the lifetime of polyethylene at processing temperatures (Bair 1997). 

Twenty-two numbered antioxidant structures are given in this article and some of the more important examples are mentioned in the next few paragraphs. 

TGA analysis of two secondary antioxidants evaluated in this study also indicated that loss of stabilizer would not be expected under typical curing conditions. The phosphite, PHOS-1, has a 5% weight loss at 189 C and a 90% weight loss at 266"C. The thioester, DTDTDP, has a 5% weight loss at 227°C and a 90% weight loss at 328°C. Some antioxidant structures are given in Appendix 1. 

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