Hindered amines light stabilizing activity

Figure I. Illustration of the non-hiocidal activity of the hindered amine light stabilizer LS I and the benzotriazole UFabsorber LS II on a modified styrenic terpolymer. Key left, unaged and right, Florida exposure for 500,000 langleys (approx. 4 years).

The most recent class of light stabilizer is the Hindered Amine Light Stabilizer (HALS). These materials have been shown to function as radical traps, thus interrupting the radical chain degradation mechanism. The cyclic stabilization mechanism proposed for HALS involves multiple regeneration of the active nitroxyl stabilizer. The surprising performance of HALS at relatively low concentrations supports this non-sacrificial mechanism. 

As shown in Table I, these stable radicals showed strikingly higher light stabilization activity in polypropylene than that of the UV absorber tested. We felt that their activity was related to their radical scavenging ability. This hypothesis is supported by the observation that the coupled products (32) and (33) were obtained by the reaction of the nitroxyl radicals (2) and (27), respectively, with a C-radical derived from AIBN (10). The radical scavenging ability of the stable nitroxyl radicals is now well known to play a major role in the mechanism of light stabilization by hindered amine compounds (13). 

Elimination of the yellowing introduced by the stable nitroxyl radicals was essential for commercial development of these excellent stabilizers. Since phenolic antioxidants are necessary for the thermal stabilization of polymers during processing, we turned our attention to ways in which unfavorable interactions between the hindered phenols and the stabilizing nitroxyl radicals could be avoided. In this section we describe the discovery of the light-stabilizing activity of hindered amine compounds, an improved synthetic method for these compounds, the synthesis of a number of derivatives, and the evaluation of their stabilizing activity. 

These results suggest that hindered amine compounds could be converted to stable nitroxyl radicals through the corresponding amino radicals in polymers that is, they are the precursors of the stable radicals. In fact a key compound, 2,2,6,6-tetramethyl-4-oxopiperi-dine (42), showed high light-stabilizing activity in polypropylene, comparable to that of the nitroxyl radicals, lending support to this interpretation. 

Synthesis and Stabilizing Activity of Hindered Amines. As mentioned previously, the hindered piperidine compounds showed excellent light-stabilizing activity in polypropylene. In order to find more efficient compounds, various derivatives of 2,2,6,6-tetramethyl-4-oxopi-peridine (42) were synthesized and tested. In this section we describe some typical examples from the great number of derivatives prepared in our laboratory. 

Table II. Light-stabilizing Activity of Hindered Amines...

In addition, we attempted to prepare piperidine compounds bonded with a hindered phenol moiety, since phenolic antioxidants are essential for processing and long-term stabilization. The proposed compounds were expected to have light-stabilizing activity and at the same time thermal stabilizing activity in the polymers. Bifunctional compounds were also synthesized. The corresponding diols were obtained by the reaction of the amines with ethylene oxide (Chart 11). The stabilizing activities of the above compounds are summarized in Table V. 

As a partially hindered amine, 3 showed light-stabilizing activity lower than that of piperazine or piperidine HALS, but comparable to commercial benzophenones. Like HALS, they are strongly synergistic with benzophenones and benzotriazoles in polyolefins. 

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