1, To withstand the damage of environmental factors such as oxygen, heat, light, chemical aggressive substances
2, The temperature is too high, high humidity
3, The additives added in the paint lead to
It is generally believed that hindered amine nitroxide radicals really play a direct role in the light stabilization of polymers. HALS is only the precursor of active light stable species. Under photo-oxidation conditions, ozone must be present or generated in the polymer to excite singlet oxygen. Molecules, hydrogen peroxide, free radicals and alkyl hydrogen peroxide and other oxidizing species, tetramethylpiperidine [Figure 14-13(a)] and N-methyltetramethylpiperidine [Figure 14-13 (b)] These active species are easily oxidized to nitroxide radical structure, which has been proved by paramagnetic resonance studies. This is a relatively stable free radical that can be separated and purified under conventional conditions.
Peroxide and free radicals will inevitably be generated during the photo-aging of polymers. If they are allowed to develop and unfold, the polymer chains will rapidly degrade and cause yellowing. ptfe powder wax HALS nitroxide radicals can capture the free radicals generated during the photoaging process and block its further harmful reactions. The reaction process is as follows:
The chain free radicals generated by photoaging may also interact with the oxygen molecules in the cured film to form peroxide free radicals, that is, nitroxide free radicals compete with oxygen molecules and combine with chain free radicals, and the combination reaction of oxygen molecules and carbon center free radicals It has a slight advantage in the rate constant. Fortunately, in solid polymer membranes, especially in cross-linked polymer coatings, the diffusion of oxygen molecules is restricted, and the concentration of oxygen molecules in the membrane is much lower than the concentration of nitroxide radicals. Therefore, the blocking reaction of nitroxide radicals on chain radicals is relatively dominant.
Studies have also pointed out that if we simply rely on HALS nitroxide radicals to capture polymer chain radicals to achieve the purpose of polymer light stabilization, the results are likely to be disappointing. This also implies the complexity of the light stabilization mechanism of HALS nitroxide radicals.
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