Elastomers secondary antioxidants

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

Benefos 1680 is a secondary antioxidant. It is particularly useful in polyolefins and olefin-copolymers such as polyethylene, polypropylene, polybutene and ethylene vinyl acetate copolymers as wall as polycarbonate and polyamids. Other applications Include use in linear polyesters, high impact polystyrene, ABS, SAN, adhesives, natural and synthetic tackifier resins, elastomers such as BR, IR, euid other organic siibstrates. 

A combined use of primary and secondary antioxidants is state-of-the-art in research and practical applications for stabilizing elastomers against thermal-oxidative degradation - just as in thermoplastics. Mainly phenolic antioxidants or phenols with sulfur compounds are used. 

Secondary aromatic amines, such as phenyl beta-naphthylamine. have been used as antioxidants in elastomers, but the preferred antioxidants for plastics have been 2,6-disubstituted and 2,4,6-trisubstituted phenols. These hindered phenols serve as chain transfer agents with the macroradicals which are produced by the degradation of polymers. 

Natural rubber or synthetic elastomers can be blended vith a range of additional materials that can be defined as secondary polymer systems. This vould include such materials as resin systems, high molecular veight green strength promoters, and polymeric antioxidant systems. 

Antioxidant Types. Commercially available antioxidants may be divided into three general classes secondary amines, phenolics, and phosphites. In general, the amines are more active than the phenolics which in turn are more active than the phosphites. Amine antioxidants, however, often cause staining problems and are therefore used mainly in black-colored compounds. The phenolics and phosphites are relatively nonstaining and are normally used in light-colored elastomer formulas. Many antioxidants in these classes are volatile to some extent at elevated temperatures and almost all antioxidants are readily extracted from their vulcanizates by the proper solvent. These disadvantages have become more... 

---