August 5, 2022 Longchang Chemical

Structure, principle of action and common types of hindered amine light stabilizers (HALS)

The function of free radical trapping agents is mainly to capture the free radicals generated during the photoaging of coatings, including alkyl radicals, alkoxy radicals, peroxy radicals, etc., and block the further oxidative damage of these reactive groups on organic polymers. Free radical trapping agents include hindered phenol light stabilizers and hindered amine light stabilizers ( HALS) two categories. The former is a phenolic structure, on the free radical polymerization may have a blocking effect, light stabilization effect is usually not as good as hindered amine light stabilizer, and as an antioxidant use is more common. Hindered amine light stabilizer is a class of high-efficiency light stabilizer, is currently one of the most widely used light stabilizers in the anti-light aging process of polymer materials, often has a very significant promotion of polymer light stabilization, through the capture of polymer photo-oxidation, degradation process of radicals, and decomposition of alkyl hydrogen peroxide, burst excited state energy and other ways to achieve the purpose of light stabilization of polymers, but also It is also a common and efficient light stabilizer in current light-curing coatings.

 

Structurally, hindered amines include piperidine series, imidazolidinone series, azetidinone series and other derivatives, among which 2,2,6,6-tetramethylpiperidine and its substituted derivatives series are dominant, and the basic structures are shown in the left figure (a) and (b).

 

Among the various structures of HALS, the N-H structure and N-methyl substituted derivatives are common, and the N acetyl derivatives have poor performance. The tetramethylpiperidine structure does not have conjugated structure and chromogenic group, does not absorb any light with wavelength higher than 250 nm, and does not have the properties of ultraviolet absorber and excited state bursting agent. Its photostabilization mechanism is quite complex, as shown in the figure on the left. It is generally believed that it is the hindered amine nitrogen-oxygen radicals that really play a direct role in the photostabilization of polymers, as shown in (c) above left. HALS is only a precursor of the active photostable structure, and under photooxidation conditions, oxidizing species such as ozone, excited singlet state oxygen molecules hydrogen peroxide, peroxyl radicals and alkyl hydrogen peroxide are necessarily present or generated in the polymer. tetramethylpiperidine structure, as shown in (a) and (b) above left are susceptible to oxidation by these reactive species to the nitrogen-oxygen radical structure, as demonstrated in (c) above left. The nitroxide radical structure is a relatively stable radical that can be isolated and purified under conventional conditions.

HALS nitrogen-oxygen radicals can capture the free radicals generated during photo-aging and block their further harmful reactions. The schematic diagram of the effect is shown above.

The chain radicals generated by photoaging may also interact with oxygen molecules within the cured film to form peroxyl radicals, i.e., the nitrogen-oxygen radicals and oxygen molecules compete for binding to the chain radicals, and the binding reaction between oxygen molecules and carbon-centered radicals is slightly dominated by the rate constant. Fortunately, within solid polymer films, especially within cross-linked polymer coatings, the diffusion of oxygen molecules is constrained and the concentration of oxygen molecules within the film is much lower than the concentration of nitrogen oxygen radicals. Therefore, the nitrogen-oxygen radicals are relatively dominant in the closure reaction to the chain radicals. The nitroxyl radical initially only captures polymer chain radicals, forming a hindered piperidine oxygen-sealed polymer chain segment, denoted as NOR, which may also continue to interact with the peroxide radicals generated by photoaging in the polymer system, consuming peroxide radicals and regenerating active nitroxyl radicals and hindered amine structures at the same time, forming a virtuous cycle of HALS. This is probably the main mechanism by which HALS exerts its photostabilizing effect. As shown in the diagram above left.

HALS has been developed many grades of hindered amine light stabilizers, the variety of commonly used light stabilizers occupy a dominant position, HALS due to the characteristics of the amine, showing a certain alkaline, acid protonation, the conversion to nitrogen oxygen radical activity will be reduced. Therefore, HALS with high alkalinity should not be used in acid or acid-catalyzed coating formulations, and similar problems exist for halogen-containing flame retardant coatings. The structure of HALS has a significant effect on its acidity and alkalinity, and the relationship between structure and acidity is shown in the table below.

 

The alkalinity of N-alkylated hindered amine (N-CH3) is slightly weaker than that of hindered amine with secondary amine structure (N-H), and hydroxylamine structure, O-alkylated hydroxylamine structure and acetylated hindered amine line organisms even exhibit some weak acidity. While reducing the basicity of hindered amine light stabilizers, factors such as their reactivity should also be considered. From the application point of view, the weaker alkaline hindered amine light stabilizer is generally more O-alkylated hydroxylamine structure and acetylated hindered amine derivatives, these HALS suitable for acidic environment coating formulations.

 

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