Four different chemical methods can be used against plastics deterioration
Theoretically, there are four different chemical methods that can be used against plastics deterioration.
The first technique is to introduce additives that absorb light before they can contact the degradation-initiating center and the energy is dissipated as heat.
The second way consists in deactivating the excited state of the initiating center.Again, such reactions produce energy, which is generated either in the form of heat or as fluorescent radiation.
In the third method, the hydroxyperoxides that have been formed are transformed into other stable forms that do not generate free radicals. Metals complexes of sulfur-containing compounds are very efficient in this regard and can be effectively used at low concentrations. Dialkyldithiocarbamates, dialkyldithiophosphates and thiobisphenolates are used for this specific purpose.
The fourth method is achieved by introducing in the system new additives (free radicals scavengers) whose purpose is to stop further reaction of the formed free radicals, by simply reacting with them and forming a stable compound. The hindered amine light stabilizers (HALS) most likely operate by this chemical mechanism. Beside HALS, nickel complexes, which are considered excited state quenchers, are also being used.
However, the addition of UV absorbers alone may not always give adequate protection to all type of plastics; generally they are used in combination with HALS compounds.
The degradation process mostly affects the surface of polyester coating, at least in the initial stages, and results in chalking, discoloration and sometimes even loss of physical and electrical properties. The product bulk properties of impact strength, tensile strength and elongation may all deteriorate because of degradation process.
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