The reason is discussed, why ultraviolet absorbers (UVA) having phenolic moiety show the synergism with HALS apparently, on the contrarily to phenolic antioxidants. A HALS nitrosonium is a substrate causing the useless oxidation of a phenol, and is formed from HALS more easily in the presence of a phenol. However, 2-hydroxybenzophenone (2-HBP) did not accelerate the formation of the nitrosonium so fast as 3,5-di-t-butyl-4-hydroxytoluene (BHT). A HALS nitrosonium, even if formed, did not oxidize 2-HBP so much as BHT. That is, 2-HBP can continue to work longer and more effi ciently in the presence of HALS. This fact suggests a weak antagonism of 2-HBP and a strong antagonism of BHT with HALS. On the other hand, quinones, derived from 2-HBP and BHT as result of peroxy radical-catching, were reduced to the corresponding hydroquinones by the action of HALS derivatives, such as HALS hydroxylamine. This reaction occurs strikingly faster and more easily for 2-HBP than for BHT. This fact shows a strong synergism of 2-HBP and a weak synergism of BHT with HALS. The above-mentioned results well explain the apparent synergism of UVA (2-HBP) with HALS, contrarily to the apparent antagonism of BHT with HALS.

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.