Shedding light on the intricacies of light stabilizersShedding light on the intricacies of light stabilizers

Coatings and paints have become indispensable for the protection and embellishment of a wide array of surfaces, including metal, plastic, wood, and mineral surfaces.

In addition to stricter environmental requirements, it is especially the quality requirements for the coated end product that have increased, both in terms of mechanical as well as chemical long-term stability. It is crucial to use light stabilizers, or combinations of them, that match individual applications, to effectively protect the surface and the carrier material below from weathering and UV degradation.

There is a distinction between lightfastness and weatherfastness. Lightfastness is solely defined as fastness against photolytic decomposition as a result of exposure to sunlight. It occurs wherever sunlight reaches. It is in particular the UV-B radiation in the range between 315 and 280 nm wavelength of the solar energy spectrum that causes decomposition of binders. UV-A radiation, on the other hand, only causes marginal polymer decomposition. The effect of water and oxygen, both of which are naturally present everywhere, adds to the deterioration of the coating film through weathering. The ensuing chemical processes, such as hydrolysis, as well as the property lightfastness, occur under the collective term "weatherfastness."

Over time, weathering makes the surface of a clearcoat lose its brilliance, and it can even cause brittleness, which in turn leads to cracks and the yellowing of the binder. Further deterioration also destroys the pigments. The formation of bubbles and chalking (e.g., the decomposition of the binder in the basecoat) finally results in the complete delaminating of the clearcoat. To avoid or at least delay the effect of weathering, light stabilizers have to be an integral part of any coating formulation for permanent outdoor use.

Weatherfastness is a necessary requirement for many different applications, such as auto body coatings and refinishes, wood coatings, furniture coatings and veneers, window and door frames, coil coatings, and other industrially manufactured cladding panels, as well as industrial coatings for outdoor use and protective coatings for buildings.

For use in coating systems, light stabilizers need to have a number of properties. These include:

Good solubility in the binder that is to be stabilized.No additional secondary properties that could affect the coating.No or only slight coloring.Easy integration into the formulation.Thermal stability and no condensation.Migration resistance.Modern light stabilizers are a combination of UV absorbers, radical scavengers such as HALS (hindered amine light stabilizers), and sometimes even an additional antioxidant.Different stabilizers work differentlyUV absorbers work by absorbing high-energy UV radiation and releasing it as heat. According to the Lambert-Beer law, the thickness of the film, its concentration, and the specific absorbance of the UV absorber significantly determine the degree of protection. It is therefore crucial that the coating film be applied not less than at the minimal thickness. The more the UV radiation penetrates, the more it is absorbed. As a result, the protective effect is the lowest right at the surface, whereas the lower layers of the coating are effectively protected from UV radiation. The most common UV absorbers are: oxalanilides, 2-hydroxy-benzophenones, benzotriazoles, and 2-hydroxy-phenyltriazines. Each of these UV absorbers has a specific absorbance capacity.Radical scavengers follow a completely different pattern. Together with oxygen, sterically hindered amine light stabilizers (HALS) form stable nitroxyl radicals when exposed to light. These nitroxyl radicals can then serve as a radical trap for the free radicals that have been formed by the binder under the effect of UV radiation. The ability of the nitroxyl radicals to regenerate prevents a chain reaction, which would otherwise destroy the coating film. Unlike UV absorbers, light protection with HALS works on a chemical basis, not on a physical basis. The effectiveness of HALS depends, among other factors, on their optimal distribution in the binder. The necessary concentration of HALS is largely independent of film thickness.While all sterically hindered amine light stabilizers used today go back to the basic structure of the 2,2,4,4 tetramethyl piperidine, they are different in their substituents of endocyclic nitrogen. HALS are divided into nonsubstituted, methylated, etherified, and acylated HALS. They differ especially in their basicity and activation speed.In addition to conventional UV stabilization with UV absorbers and HALS, lately a third type of light stabilizers is increasingly being used as a costabilizer: antioxidants. Like the sterically hindered amines, this group of organic compounds also prevents the early oxidation of binders. But unlike HALS, antioxidants'' function is not based on a cycle process, which is why they are used up faster. Antioxidants are also very suitable as thermal stabilizers for processes with high baking temperatures, and where additional thermal stabilization is required to protect the polymer, as it is the case with coil coatings and powder coatings. Coated surfaces that can get very hot when in use also require this kind of protection.The most widely used materials are substituted phenolic antioxidants, phosphites, or phosphonites.Combining for optimal protection To achieve optimal light stabilization, your strategy has to be matched with the individual coating system and application. Preliminary tests serve to determine the right light stabilizer package for each system.When looking at the differences in the functioning of light stabilizers, it becomes obvious that clearcoats can only be stabilized with UV absorbers and additional radical scavengers. These two stabilizing effects are very often not only combined, but they also create synergies.In case of a pigmented coating, however, the pigments to a great extent assume the role of the UV absorber in the clearcoat described above. Inorganic pigments such as TiO2, FeO(OH), Fe2O3, or carbon black dissipate the radiation of the visible as well as the UV spectrum. Sometimes the inorganic pigments themselves are good UV absorbers, in which case it is unnecessary to add an extra UV absorber.In this context it is important to bear in mind the opacity of the coating in the UV spectrum. Mainly organic pigmented coatings, such as lead-free yellow or red shades, have a very low opacity, which allows the deteriorating UV radiation to penetrate deeply into the coating film. In these cases at least a small amount of UV absorber is required.A similar situation occurs when the pigment volume concentration in thick coatings is too low and an expensive pigment has been substituted with a low-price filler. While the thick film visually appears as an opaque coating of the surface, UV radiation can penetrate deeply into it. As with coatings with lower opacity, additional UV absorber is required for optimal protection. It has to be pointed out, though, that increasing the pigment concentration in the coating is often more economical than adding an additional UV absorber.In all applications mentioned above, the use of a radical scavenger is required.The situation, however, is different when a photosensitive substrate needs to be protected from photo degradation, as is the case when coating wood with scumble or clearcoat. Here, the UV absorber performs an important function, as the radical scavenger can protect the underlying material only indirectly. Its only task is to stabilize the coating itself and thus maintain its ability to protect the underlying material.To summarize, the basics for optimizing light stabilizers in coatings are:The right choice and combination of light stabilizers in order to ensure good light stabilization of organic coatings. Preliminary tests are strongly recommended.The first rule of using light stabilizers: none of them should generate undesired side effects.The requirement for UV absorbers is determined by the opacity of the coating.Radical scavengers are mandatory in all outdoor applications. UV protection of photosensitive surfaces is an exception.Gerd Faoro, senior technical marketing manager light stabilizers, Clariant Huningue, Pigments & Additives Div. [email protected]