Influence of Ultraviolet Absorbers on Decomposition of Hydroperoxide by Hindered Amine Light Stabilizers

Abstract

The interaction between hindered amine light stabilizers (HALS), and ultraviolet absorbers (UVA) was investigated, in order to clarify the reason why they generally show synergism, although UVA belong to kinds of phenols. A homolytic decomposition of cumene hydroperoxide (CHP) by amine-type HALS (HALS NH) in the presence of UVA is the antagonism bringing about autoxidation. This antagonism is found considerably weaker for UVA than that of 3,5-di-t-butyl-4-hydroxytoluene. This is based on a weak proton-donating ability of UVA to HALS due to their intramolecular hydrogen bond. Furthermore, a good relationship was found between the inhibition of photo-oxidation by UVA and the initial rate of homolytic decomposition of CHP by HALS in the presence of UVA, and a strong synergism of UVA with HALS was observed. As a result, UVA are able to exhibit an apparent synergism in a combination with HALS, because of no or little acceleration (antagonism) of homolytic hydroperoxide decomposition caused by the interaction with HALS, as well as the estimated regeneration (synergism) of new UVA from the corresponding quinoide-type compounds, even if formed, by the action of HALS or the derivatives.

Search for Unified Action Mechanism of Hindered Amine Light Stabilizers

“…Interestingly, an equimolar mixture of UVA and HALS NH shows similar or greater photo-antioxidant activity compared with that of a double amount of only UVA 41) . This synergism is ascribable to the regeneration of UVA, because HALS has no particular photoantioxidant functions.…”

Mechanisms of Uv Stabilization

“…73 It is also considered, based on experimental data, that UV absorbers of benzophenone and benzotriazole types occasionally lose phenolic hydrogen during repeated UV absorption and they become quinoid compounds which do not participate in energy dissipation cycle. 74 HAS is thought to be able to reduce the quinoid structure to the regular phenolic form which can exhibit its normal UV absorption activity. 74 The combination Tinuvin 662 (polymeric HAS)/Chimassorb 81 (benzophenone) yields a quite pronounced synergism.…”

“…74 HAS is thought to be able to reduce the quinoid structure to the regular phenolic form which can exhibit its normal UV absorption activity. 74 The combination Tinuvin 662 (polymeric HAS)/Chimassorb 81 (benzophenone) yields a quite pronounced synergism. In the presence of titanium dioxide, UVA no longer contributes to UV stability.…”

“…82 Both stabilizer types were consumed faster when used in combination, and once the concentration of the effective form of the stabilizers dropped below a critical value there is a rapid chemical change of the polymer. 82 The reasons for this antagonistic behavior is not completely understood but there are several hypotheses brought forward: 74,75,[80][81][82] • HALS nitrosonium salt, a derivative of the parent amine, is the key intermediate for the observed antagonism as it leads to the fast consumption of the phenolic antioxidant and homolytical decomposition of hydroperoxides, thereby accelerating the degradation of polymeric materials • oxidation of phenolic antioxidants by nitroxyls from HAS …”

Interaction of Hindered Amine Light Stabilizers and Ultraviolet Absorbers

“…However, these mechanisms do not seem to explain the synergism perfectly. On the other hand, the authors reported a good relationship between the inhibition of photooxidation by UVA and the initial rate of homolytic decomposition of a hydroperoxide by HALS in the presence of UVA, and concluded no or little antagonism of UVA with HALS 6) . In this study, we confi rm it experimentally and propose a new synergism of HALS and UVA by comparing the action of UVA (2-hydroxybenzophenone) with that of phenol (BHT) with respect to both antagonism and synergism with HALS.…”