High solids, powder, waterborne and UV curable coatings—the “green” coatings—have been developed to replace conventional high VOC and solvent-based coatings. Although the latest technology has overcome the disadvantages of traditional solvent-based coatings, light stabilizers in green coatings still play an important role helping to avoid degradation from sun light exposure and improve weathering stability. Everlight has developed several products specifically for green coatings.
Green coatings and light stabilizers
Green coatings can be as diversified as high-solids, powder, waterborne and UV curable coatings and so on. They are designed to eliminate the release of VOCs from conventional solvent-based coatings.
The cause of coating degradation such as discoloration, delamination, loss of gloss, cracking and chalking is due to the chromophores, or impurity content, found in the material triggered by UV light, which is damaging. Therefore, UV absorbers and light stabilizers are both recommended in order to prevent coating decomposition.
Light stabilizers can be divided into two groups: UV absorbers (UVA) and the hinder amine light stabilizers (HALS). UVA’s function is to absorb UV light and transform it into heat; HALS is best known to capture free radicals and prevent material degradation.
Light stabilizers recommended for green coatings
The objective with high-solids coatings is to reduce the requirement of organic solvents and increase solids content. Most coating systems need to use acidic catalysts to improve film hardness during thermo-curing processes.
In choosing light stabilizers for high solids coatings, it is necessary to consider the pKb of the coatings; therefore, NOR HALS become very important in some cases as to work with a low basicity system. Also, solubility is another key element for coatings. A liquid type light stabilizer is a better choice, which has no dispersion problem and is easy to use. Before and after exposure of delta Y measurements (as shown in Figure 1) of adding UVA and NOR HALS, coatings would receive different degrees of protection according to its use level. Evidently, UVA mixed NOR HALS was proven to provide a better protection as seen in this test.
Figure 1. UVA and NOR HALS with acrylate clear resin for high solids coating after 2,000 hours QUV-313nm exposure. Results show UVA mixed NOR HALS was able to minimize discoloration to the coatings. (See Figure 1 above)
Powder coatings are applied by high pressure and static electrostatic spray. The resin and additives are formed and attached to the surface of the object and then cured by high temperature heat to form a durable coating. This process results in a solvent-free coating.
Particle size is crucial in powder coatings. It is very difficult for light stabilizers to reach good homogenous dispersion in a short time due to the small dosage. UVA-H and HALS-H, both are novel micron grades, and developed for effectively overcoming the obstacle of dispersion. Before and after exposure of delta Y measurements (as shown in Figure 2) and adding UVA-H and HALS-H, coatings would receive different degrees of the protection according to each consumption level. Evidently, UVA-H mixed with HALS-H was proven to provide a better protection.
Figure 2. UVA-H and HALS-H with polyester resin for powder coating after 2,000 hours QUV-313nm exposure. Results show UVA-H mixed with HALS-H was able to minimize discoloration to the coatings. (See Figure 2 above)
Waterborne coatings are an eco-friendly coating system based on water-soluble or water dispersed resins. They are non-toxic, odorless and nonflammable.
Liquid light stabilizers in many cases are known to not easily disperse in water-based coatings. Adding UV stabilizers directly to water-based paint systems will cause condensation in coatings and make it difficult to spray without filtration. Everlight offers a series of new high-performance liquid light stabilizers that works with waterborne coatings. They can be used directly and homogenously disperse in waterborne coatings. Before and after exposure of delta E measurements (as shown in Figure 3) and adding UVA-W, coatings would receive different degrees of the protection according to each consumption level. Evidently, a higher use level provides a better protection.
Figure 3. UVA-W is with polyurethane clear resin for waterborne coating after 1,000 hours QUV-313nm exposure. Results show UVA-W was able to minimize discoloration to the coatings. (See Figure 3 above)
UV Curable Coatings
UV curable coatings require energy directly from UV light to initiate monomers and oligomers. Through this process, it will then convert the liquid coating into the solid film. One thing that makes UV curable coatings become more and more popular is they reduce energy costs and drying speeds during production are faster.
Photoinitiators and light stabilizers usually are in conflict with one another in the UV curable system. UR series are high-performance liquid-based light stabilizers and they do not affect curing speed. Before and after exposure of delta Y measurements (as shown in Figure 4) and adding UR series, coatings would receive different degrees of protection according to its use level. Evidently, a higher use level provides a better protection.
Figure 4. UR1 is with epoxy acrylate UV curable clear resin for metal coating after 100 hours QUV-313nm exposure. Results show UR1 was able to minimize discoloration to the coatings. (See Figure 4 above)
Environmentally friendly coatings are the trend for future development in the coatings industry. In this article we highlighted the benefits of using light stabilizers in various coating systems. Coating manufactures may still need to run the experiments to confirm their coating systems are compatible. The company who can offer tailor-made UV absorber and light stabilizer products will have the key to providing UV protection in the manufacturer’s coating products.
Light Stabilizers Make the UV Protection of Environmentally Friendly Coatings Easier
Today's class of "green" coatings depend on light stabilizers to protect against UV degradation.
By Yung-Chi Yang, Steven Lee and Dr. Yao-Hsing Huang, Everlight Chemical Industrial Corporation