Antioxidants synergetic effect

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... 

Recent developments in the field of AOs include improvements in effectiveness at higher temperatures, lower levels of volatility and extractability as well as improved handling and processing. The newer AOs are higher molecular and contain additional multifunctional groups that enhance the synergetic effect (antioxidants see also under stabilizers). 

The synergetic effect of stabilizing couples (Fig. 12) is defined by the relative values (Ati 2) of oxidation induction times obtained for sample stabilized with antioxidant couple (ti 2), and separate couple components ... 

For outdoor applications of stabilized PVC, phosphites, antioxidants, and UV absorbers are also added to produce synergetic effects. 

In most cases, carbon blacks show synergism when combined with other antioxidants, mainly with sulfur-containing compounds [53,54,57,58]. The mechanism of this synergetic effect is not very well understood. 

Ion deactivating agents usually have negligible antioxidizing properties but sometimes they show synergetic effects with antioxidants. They are used mainly together with phenolic antioxidants. 

Phenol/phosphite combinations typically exhibit synergetic effects during polyolefin processing and are preferred. The selection of antioxidants, their total concentration, and the ratio of individual concentrations have to be adapted to each specific application [186]. 

Because of the variety of degradation reactions, POM is stabilized using multi-component systems consisting of antioxidants and compounds that bind the secondary products (e.g., formaldehyde, protons) of autoxidation. The stabilizers allow numerous combinations that often have synergetic effectiveness. Sterically hindered phenols are mostly used as antioxidants here, polynuclear phenols are preferred because of their low migration tendency. In addition, sterically hindered amines are used as radical scavengers. 

Phosphites used as co-stabilizers with phenolic antioxidants exhibit synergetic effects. 

Some antioxidants also give protection to polymers against the effect of light both by their inherent structure or by a synergetic effect with light stabilizers [47a] while the sterically hindered amines are quite efficient as antioxidants [47b]. 

The synergetic effects have been explained by the following coupled mechanisms diffusion of antioxidant from the volume protected by UV into the oxidizing surface layer quenching of the excited state of antioxidants by light stabilizers increase of solubility and/or redistribution of stabilizer in the presence of other additives protection of the light stabilizer by antioxidant and stabilization by regeneration of another additive. 

The above-mentioned antioxidants are also suitable for the stabilization of PP thin films. However, the synergetic effects of phenolic antioxidants with phosphorus-containing antioxidants [e.g., pentaerytritol-tetra-kis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propio-nate and tris(2,3-di-ter/-butylphenyl)phosphate] was not observed. On the contrary, typical high molecular stabilizers sueh as polymeric HALS have a pronounced contribution to the stabilization of PP thin films. 

It can be combined with UV absorbers. By means of such combinations, further improvement of TPO light stability can frequently be achieved as a result of synergetic effects. Sulfur-containing stabilizers such as thiodipropionic acid esters, or sulfur-containing phenolic antioxidants have sometimes been found to have a negative influence on its effectiveness. 

The more efficient some antioxidant (component 1) and photostabilizer (component 2) combinations present synergetic effects (Table 2) [11]. 

Transformation products of phenolic antioxidants act differently and in many cases contrary to under photo- and thermo-oxidative conditions, also influencing the efficiency of stabilizer mixtures in different ways. Phenol-HAS mixtures also behave synergetically during thermo-oxidation, but show antagonistic effects during photo-oxidation. 

---