Why Are UV Absorbers Used for Coatings
The sun's ultraviolet light takes a severe toll on any surface exposed to it for an extended time. UV absorbers, stabilizers, and quenchers reduce this damage through a variety of actions. Using these additives offers multiple benefits for products that must stand in the sun and need to maintain their integrity. Knowing the nuanced differences between these three types of UV protection products will make choosing the right one for a specific application easier.
In this article you will learn:
What Is a UV Absorber
UV absorbers for coatings mitigate the damaging effects of the sun. Adding these products to a surface preserves the color brilliance and lifespan of ultraviolet-sensitive products. Do not confuse UV absorbers with blockers. These do not block ultraviolet light. Instead, UV absorbers sacrifice themselves to preserve the polymers they protect. These products compete with the surface material for the UV radiation from the sun. Because these additives have a proclivity for drawing in ultraviolet rays, they prevent the rays from reaching the substrate. By absorbing sunlight instead of letting it reach the polymer, UV absorbers protect the coating.
Because nothing can destroy energy, the radiation the UV absorber takes in must go somewhere. These products typically change the energy from UV radiation into heat. UV absorbers transmit the absorbed energy through radiating the heat. In this manner, these absorbers safely release the UV radiation in a way that does not cause damage to the material they protect.
The amount of UV absorber needed depends on its concentration and the surface's thickness. The relationship between absorber concentration and surface thickness is not linear. Higher levels reduce effectiveness. Therefore, thicker materials will need higher-than-predicted concentrations. To find the right absorber, formulators need to conduct extensive tests to find the concentration required to prevent photodegradation. Not all products absorb across the full UV wavelength spectrum. Since these products typically absorb UV rays and not visible light, they appear transparent. Above 400 nm in wavelength, UV absorbers should not block as many rays to avoid blocking blue light waves, making the material beneath appear yellow.
While other forms of UV protection regenerate themselves, UV absorbers will wear out over time. Though the wearing takes place very slowly, it still happens. To avoid problems with UV damage to the material, reapplication of the coating with absorbers must occur periodically or another means of protection sought. Alternatively, use UV absorbers for short-term solutions, such as with disposable products or those only subjected to periodic sun exposure.
Uses of UV Absorbers
Any material that requires protection from ultraviolet radiation needs some treatment. UV absorbers have a wide range of materials they can protect. However, not all products in this category have the same efficiency.
UV absorbers work best when used with a thick material because these products rely on depth as part of their effectiveness. Generally, very thin films that need heavy ultraviolet protection would not benefit from UV absorbers as much as they would from another type of light stabilizer or additive. Because UV absorbers vary in their effectiveness depending on thickness, concentration, and formulation, engineers must carefully choose the best absorber for a particular application.
The most effective of these absorbers are organically based, though certain types of plastics will require different absorbers. For example, propylene typically receives the most effective protection from aryl esters, benzophenone, benzotriazoles, and formamidines. UV exposure testing will ensure the correct choice of absorber for a particular material. Regular testing of the material's coating will also provide the continued performance of the absorbers.
When working with polyphenylene sulfide, any other form of UV stabilizer will fail. The only type effective in protecting this polymer is the UV absorber class.
UV absorbers are an ideal first line of defense against ultraviolet radiation damage. Often, manufacturers will use these products in combination with quenchers or light stabilizers for the longest-lasting, most significant benefit. UV absorbers will eventually experience a reduction in their ability to preferentially absorb ultraviolet light. When they do, if the material does not also include another form of damage prevention, it will begin to show photodegradation. To avoid this problem, engineers can incorporate light quenchers or stabilizers into the material or coating, depending on the type of polymer protected.
While UV absorbers have their purposes in protecting various materials, stabilizers and additives also preserve the coloring of plastics. Depending on the application, a stabilizer or additive may be a better option or adjunct to UV absorbers.
What Is a UV Stabilizer
UV stabilizers protect a surface through arresting the photodegradation process. Hindered amine light stabilizers, or HALS, are a type of light stabilizer. Unlike UV absorbers that take in the ultraviolet light and keep it from reaching the plastic, HALS have a distinct operation that involves free radicals and chemical reactions.
When light hits a surface, it sends energy to the atoms in the plastic. These atoms absorb the energy, which excites their components. The UV light also activates the HALS to create nitroxide. This substance takes in alkyl radicals the excited atomic particles produce. The alkyl reacts with the nitroxide to form amino ethers. These ethers then find peroxyl radicals in the material. The amino ethers destroy the peroxyl radicals, and the products of the reaction regenerate the HALS nitroxides. Without neutralizing these alkyl and peroxyl radicals, the material would experience damage through oxidation.
In layman's terms, HALS work by producing products that look for and "eat" the free radicals released from the material. Without the free radicals, the material does not wear down from photooxidation. After consuming the free radicals, the resulting products produce the components needed by the HALS to work again. By regenerating, these light stabilizers can stop UV damage innumerable times, ensuring their long-term effectiveness.
Because these light stabilizers regenerate themselves, they have extremely long lives in addition to extended efficiency. With the addition of HALS, artificial turf can remain bright and green for a decade or longer. Even low concentrations of these light stabilizers prove effective at keeping the material protected from photodegradation and thermal wear.
HALS do have limitations, however. These light stabilizers do not work well with any polymers containing halogen, such as PVC. Combining them with other additives such as UV absorbers can help improve the performance of HALS in polymers they do not have ideal use with.
Why Do Manufacturers Use UV Stabilizers
Though limited by the types of polymers they work with, UV stabilizers have greater efficiency in protecting materials from ultraviolet damage. Stabilizers also have the benefit of protecting against thermal damage, something absorbers and quenchers don't do. When a project also needs protection from the heat, HALS offer dual heat and light damage resistance. Interestingly, predictions put HALS as the type of UV additive most likely to be the most popular and grow the most from 2016 through 2021. The multiple applications for HALS, in addition to a growing market for plastics, make this prediction a high likelihood.
When thin products require ultraviolet protection, HALS work better than UV absorbers. This product also has a wide-ranging versatility. Its myriad of molecular weights allows it to have an application in coatings and plastics. Polypropylene and many other polymers can use HALS during their production, including at the stages of sheet or profile extrusion and blow or injection molding.
Because thickness does not affect the performance of HALS, the percentages used and depth of the product do not need to have large measures. Whereas UV absorbers function best when they can have a thicker layer or higher concentration to improve protection, HALS work well even with lower concentrations. For applications on thin polymer layers or when the longevity of UV protection is imperative, stabilizers such as HALS are a better choice when working with compatible polymers. For questions about the compatibility of polymers with HALS or other UV additives, reach out to us online.
How UV Additives Work
The reaction of sunlight with the coloring of a material causes a fading of the shades. This color reduction occurs over time through a reaction between the UV rays and the atoms of the coloring. When ultraviolet light hits a colorant, the electrons get excited. These excited electrons are more volatile and prone to oxidizing, eventually causing the coloring to fade.
Additives include anything added to the polymer to prevent the speed and degree of this damage. UV stabilizers, quenchers, and absorbers are among those in this category of additives. Some of these prevent UV light from reaching the coloring, while others jump into the middle of the process to stop it before it leads to long-term wear and fading.
Quenchers are a type of additive that take in the energy from the excited electrons and release it through quenching, deactivating the process and preventing wear. Depending on the type, the released energy may come in the form of fluorescent or phosphorescent radiation or heat. These products are among the most effective of the UV additives.
Because some products require resistance to other chemical additives to the polymer or that the material has exposure to, not all absorbers or HALS will suffice. Some quenchers work well in these extreme conditions. For example, nickel-based quenchers appear frequently as greenhouse films. Nickel quenchers work best in agricultural applications because they protect against UV damage and resist harm to themselves from pesticides. To improve the performance of these quenchers, they often combine with UV absorbers.
Like many other UV products, additives may combine with other methods of preventing damage to improve performance and effectiveness.
UV stabilizers include both absorbers and HALS. The former absorb some of the UV light, reducing the amount that reaches the surface. The latter scavenges free radicals created partway through the photodegradation process. Depending on the type and chemical makeup, both forms of stabilizers have different degrees of effectiveness.
While UV absorbers have proven effectiveness for a variety of materials, they may not last as long as needed. For longevity, HALS serves as a preferred option for a UV stabilizer.
HALS can boost the performance of UV absorbers when used in conjunction with them. HALS and UV absorbers functions differ. HALS create free radical scavengers, nitroxyl radicals. These take up the free radicals, preventing their reaction that causes the photodegradation of the material. These high molecular weight HALS regenerate themselves, ensuring their longevity. In addition to protecting the color, HALS also provide thermal stability to the plastic or coating.
Unfortunately, not all polymers adapt well to having HALS stop the photodegradation process. For those, UV absorbers or other additives may function better.
Usefulness of UV Absorbers
UV absorbers have two means of protecting the surface, through a surface coating or by mixing into the material before formation. Both methods have advantages for certain situations and instances where a different use works better.
Coating the surface only with a stabilizing agent mixed with lacquer offers ease of application. However, this method protects only the plastic surface to the depth of the coating. Mechanical damage, such as scratches, can affect the performance of the UV absorber, leaving the uncovered base vulnerable to discoloration and wear.
To ensure the best performance of absorbers in coatings, thoroughly mixing the additive with the coating is essential. For instance, when choosing HALS for a powder coating, those with a higher molecular weight and lower volatility work better when mixed using ribo-static processes. Higher molecular weight UV absorbers have better performance when used with polar polymers and the rigors of high-heat curing processes.
When forming the plastic base, a UV absorber may combine with the plastic while still in a fluid form. While this method overcomes the problem of only treating the surface, it still poses the chance of not completely protecting the plastic. Carefully combining the stabilizer with the plastic can prevent this hazard, which explains why this method is the most popular means of treating plastic with a UV absorber.
To ensure proper UV stabilization of the plastic, the treatment product must disperse evenly into the base material and have compatibility with the plastic. Improperly mixed or selected stabilizers have the risk of separating from the base. The different molecular weights of the absorber and plastic could lead to the former evaporating or wearing away during formation of the plastic.
When mixed correctly, absorbers applied using this stabilization "in mass" method can treat the entirety of the plastic, not just the surface.
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