Changes in legislation and the reduction of VOCs have enabled the emergence of waterborne coatings for numerous indoor wood coating applications like furniture and parquetry, as well as outdoor applications like joinery, exterior deco or decking. In order to emphasize the natural beauty and the color of the wooden substrates, most coatings are transparent. These coatings have to protect the wood, however, the coatings themselves need to be protected against aggressive environmental factors like UV radiation, rain and change in temperature.1 Light stabilizers such as UV absorbers (UVAs) and hindered amine light stabilizers (HALS) play an important role in the inherent protection of these coatings. Their respective roles have been described previously.2-4 For outdoor applications, there is a clear advantage to combining UVA and HALS. UV absorbers perform as deep coating layers and wood protectors, due to the increasing UV absorbance with increasing coating thickness (Beer-Lambert law). HALS, which are effective in the whole film thickness, act as free radical scavengers that inhibit the photo-oxidative reactions that occur within the binder matrix and help maintain flexibility, gloss and water repellency. HALS are effective at coating surfaces, providing higher chalking resistance (in pigmented systems), better gloss retention, less loss of adhesion to the substrate and less crack formation.
As long as coatings were still solvent-based, there were no issues incorporating the hydrophobic light stabilizers into formulations, as both were compatible with each other. The challenge came as more and more waterborne coatings were introduced to the market and long-term stability requirements became even higher. Incorporating the hydrophobic light stabilizers into waterborne coatings could only be achieved through the use of co-solvents and high-energy dispersing methods. Addition of co-solvents leads to possible labeling issues, as well as a lowering of the overall coating properties due to plasticization and exudation.