1, Mildew leads to discoloration, stickiness, perforation, damage and peeling of the coating film and loss of adhesion
2, Decrease in viscosity and blackening, unpleasant smell, etc.
3, Shorten the service life and cause harm to environmental hygiene.
The nitroxide radicals only capture the polymer chain free radicals at first to form a hindered piperidinoxy-terminated polymer chain segment, which is expressed as NOP. NOP may continue to interact with the peroxide radicals generated by photoaging in the polymer system and consume the Oxidizing free radicals simultaneously regenerate active nitroxide free radicals and hindered amine structures, forming the virtuous cycle of HALS, which may be the main mechanism for HALS to exert its light stabilization effect. The ptfe white powder specific process can be expressed by the following formula.
Hindered amines can react with alkyl hydrogen peroxide produced by photo-aging, consume hydrogen peroxide and produce hydroxylamine [52.53), and hydroxylamine can react with hydrogen peroxide to regenerate nitroxide free radicals.
Hindered amine light stabilizers have been developed in many brands, and the variety is dominant in commonly used light stabilizers. The main change is to use the hydroxyl group at the 4-position of piperidinol or the amino group at the 4-position of piperidinamine for derivatization. Reaction, improve the compatibility with the coating system, increase the molecular weight, reduce the volatilization migration, and improve the light stabilization effect. This is the case for the two-center hindered amines Tin uv in 770 and Tin uv in 292. Due to its amine characteristics, HALS shows a certain alkalinity. When it is protonated with acid, the activity of transforming into nitroxide radicals decreases. Therefore, HALS with high alkalinity is not suitable for use in acidic or acid-catalyzed coating formulations. Flame-retardant coatings also have similar problems. The overly alkaline HALS may also promote the slow hydrolysis of the ester bond in the polyester coating system and aggravate the aging of the coating. When added to the emulsified water-based coating, it may also cause the instability of the emulsion system due to the change of pH. The structure of HALS has a significant influence on its acidity and alkalinity, and the relationship between structure and acidity and alkalinity.
The basicity of N-alkylated hindered amines (NCH;) is slightly weaker than that of hindered amines with secondary amine structure. The hydroxylamine structure, O-alkylated hydroxylamine structure and acetylated hindered amine derivatives even show a certain degree of weak acidity. . While trying to reduce the alkalinity of hindered amine light stabilizers, factors such as its reactivity should also be considered. From an application point of view, hindered amine light stabilizers with weaker alkalinity generally use O-alkylated hydroxylamine structures and acetylated ones. Hindered amine derivatives , these HALS are suitable for coating formulations in acid environments.
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