Secondary phosphites steric effects

By combining primary and secondary antioxidants, synergetic effects are frequently observed. Both the phosphites and thioesters are synergetic with hindered phenols. A significant technical example of synergism is observed on combining sterically hindered phenols with phosphites or phosphonites for the stabilization... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Primary antioxidants act by trapping radicals and preventing the initiation of oxidative chain reactions. Sterically hindered phenols are widely valued for their lower nonstaining properties. Secondary stabilizers (e.g., phosphite esters and special thioesters) decompose peroxides, which are responsible for autocatalytic effects. 

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