Interactions of stabilizers during oxidation processes

Abstract

The antioxidative action of mixtures of phenols, phosphites, HALS, and some of their transformation products in various compositions has been studied in the thermo-and photooxidation of hydrocarbons and polypropylene under different conditions. In the AIBN-initiated oxidation of hydrocarbons at low temperatures (< 8OoC), hindered phenols, hindered aryl phosphites and the nitroxyl derivatives of HALS act antioxidatively when used individually in appropriate concentrations. Secondary HALS do not show any induction period, but a certain retardation of the oxidation process after some reaction time. The inhibiting efficiency of nitroxyls observed cannot be explained completely by the currently accepted action mechanisms of HALS, but is also related to the reaction of the nitroxyls with alkylperoxyl radicals.In mixtures with hindered phenols, HALS have almost no influence on the rate of thermooxidation at low temperatures. Their nitroxyl derivatives, however, always exhibit synergism, most pronounced when both stabilizers are used in equimolar ratios. During the photooxidation phenols lower the efficiency of HALS.The influence of mixtures of stabilizers on the oxidative stability of polypropylene is rather different and depends on the oxidation conditions, the structure, the concentration and the ratio of the stabilizers. Synergistic as well as antagonistic effects are observed. Both aliphatic and aromatic phosphites studied act synergistically when used together and with phenols. This demonstrates that for acting as synergist for phenols, the hydrogen peroxide decomposing capability of the phosphites, but not their chain breaking activity is important. HALS-phosphites and phosphonites, containing amine and phosphorus units in one molecule, are highly effective inhibitors of photo-and thermwxidation and exhibit lower critical antioxidant concentrations and longer induction periods than phosphites alone. They even exceed the efficiency of phenols in many cases.Transformation products of phenolic antioxidants investigated act differently and in many cases contrarily under photo-and thermooxidative conditions. Therefore, they influence the efficiency of stabilizer mixtures also in a different way. a) HALS: Hindered Arnine Light Stabilizers

Preparation of a novel stabilizer and its thermal‐oxidative stabilization effect on polyamide 6

“…Ohkatsu et al have proposed a synergistic action mechanism of these additives for preventing the photo‐degradation process of plastics, which including regeneration of phenols from quinones under UV irradiation in the presence of HALS, which is promoted by UV light. It was also suggested that the mechanism for synergism between hindered phenol and HALS in bifunctional stabilizer AG‐HDM differed from that in a mixture of hindered phenol and HALS . In this article, one of advantages of AG‐HDM in SPA6‐2 over the combination of AG and HDM in SPA6‐1 is that less condensation water is formed during processing and thermal‐oxidative degradation.…”

“…Hindered phenol is an important class of primary antioxidants for PA6, although the efficiency of which is not as good as that in polyolefins . Binary systems composed of a hindered phenol as the primary AO and a phosphite‐based compound as the secondary AO, which have significant synergistic effect, are often used as traditional thermal‐oxidative stabilizing systems for PA6 . Hindered amine light stabilizers (HALS) have been used widely as antioxidants .…”

“…They were originally developed as light stabilizers, but now they are also used as multifunctional additives, since they have been found to work as a radical‐trapping agent, a heavy metal deactivator, a hydroperoxide decomposer, and so on . The HALS and phenolic antioxidants are frequently used together practically , but previous studies have shown either antagonism or synergism . Habicher et al proposed that, mostly, a synergistic interaction has been observed in thermal‐oxidation but an antagonistic one in photo‐oxidation.…”

“…Habicher et al proposed that, mostly, a synergistic interaction has been observed in thermal‐oxidation but an antagonistic one in photo‐oxidation. However, there are some exceptions . The interaction between HALS and hindered phenol remains unclear.…”

Comparative evaluation of the efficiency of a series of commercial antioxidants studied by kinetic modeling in a liquid phase and during the melt processing of different polyethylenes

“…in the case of the sterically hindered phenols and amines) and their inhibiting action in the hypoxic polymer environment are undoubtedly more pronounced than that in the liquid phase. But on the other hand they cause discoloration of the polymer during processing [17,18].…”