Hindered amine light stabilizers exposure

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

As with thermal stabilizers, photostabilizers must satisfy basic chemical and physical requirements (see the section titled ""Antioxidant permanence effects of chemical and physical factors ). In addition, they must be photo-stable, i.e., stable to UV-light, to withstand continuous periods of UV-exposure, without being prematurely destroyed or effectively transformed into sensitizing products. There are essentially three classes of compounds that are categorized as photostabilizers/ photoantioxidants UV-absorbers and pigments, peroxide decomposers including nickel complexes, and sterically hindered amine light stabilizers. 

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 number of studies in this area is limited, but a few have shown exposure of coatings to atmospheric contaminants to be detrimental. Sampers [35] reports that, in a study of polyolefin samples exposed both in Florida and on the Mediterranean coast of France, a dramatic difference was seen in polymer lifetime. Samples exposed on the Mediterranean had only half the life of those in Florida. The two stations had broadly similar weathering parameters the differences should have led to longer lifetimes in France. Sampers concluded that constituents in the rain or wind had chemically interacted with the hindered amine light stabilizers in the polymers exposed in France, causing these samples to be especially vulnerable to UV degradation. 

A combination of additives, namely hindered amine light stabilizers (HALS), an UV filter and antioxidant, was used for improving the durability of WPCs [53]. The mixture displayed a higher efficiency than HALS alone, as all components contributed to the protective effect by specific mechanisms. WPCs exhibited no cracks or fractures during exposure and, although some discoloration was noticed, the colour modification in stabilized formulations was significantly lower than in neat composites. 

The solventbome clearcoat is spray applied over the basecoat at a dry film thickness of 1.8 to 2.0 mils. The clearcoat, responsible for UV protection, chemical resistance, chip resistance, and so on, is most often composed of a hydroxy functional acrylic binder together with a melamine or isocyanate crossUnker. Ultraviolet light absorbers and hindered amine light stabilizers are added to achieve better free-radical oxidative degradation resistance that results from exposure to sunlight. 

For example, a high MW hindered amine (eg, CYASORB UV-3346 Light Stabilizer) combined with CYASORB UV-2908 provides synergistic performance in PP slit tape applications during Xenon ARC WOM exposure (Fig. 11) (64). The use of this combination also allows improved processability in production of the slit tapes because of reduction in water carryover compared with HALS alone. It has also been demonstrated that, in thick section PP, long-term stability can be dramatically improved with CYASORB UV-3346 combined with CYASORB UV-2908. 

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