Alternatively, it was suggested that any methylene group that is B-positioned to the amide can be initially oxidized [365. There are few mechanisms in the literature that explain discolor ation (yellowing) of nylons. UV/visible active chromophores are attributed either to pyrrole structures, or Some reactions occurring during the thermal or thermo-oxidative decomposition lead to the cross- linking of polyamides. Nylon-6.6 cross-links relatively easily, especially in the presence of air whereas nylon-ll and nylon-12 cross-link very little, Strong mineral acids, strong bases, and some oxides or salts of transition metals catalyze the thermal decomposition of nylons, but minimize cross-linking. In contrast, many fire-retardant additives promote secondary reactions, cross-linking, and charring of aliphatic polyamides. 2.5.5.4 Polymer Stabilization The question regarding degradation and stabilization of polyamides has been dealt with elsewhere. Lanska et al. [16gl reviewed aging and durability, degradation, stabilization, and resistance to biological effects. The degradation reactions comprise reactions initiated by radiation, light, oxygen, heat, and both mechanical and thermal energies. The extent of these