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2020

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How to Use UV Stabilizers in Water-Based Coatings

　　In addition to high efficiency, UV stabilizers must meet several basic yet critically important requirements. To achieve this high efficiency and complete compatibility, UV stabilizers must exhibit excellent solubility in the film-forming base material. This necessitates that they possess both high performance and low volatility—particularly when used in baking coatings. While exhibiting these properties, they must also avoid any adverse side effects, such as discoloration, reactions with other components in the coating, or interference with crosslinking.
　　For obvious reasons, UV stabilizers must be water-repellent. This is because, in situations involving settling, condensation, humidity, or other conditions that generate moisture, the stabilizers can easily be leached out of the coating film. However, in water-based systems, hydrophobic UV stabilizers can indeed be quite tricky to work with.
　　Method of adding water-repellent UV stabilizers
　　In the conventional preparation of water-based coatings, these additives can be incorporated at three stages: (1) added to the grinding paste; (2) added during the paint formulation process; (3) prepared as water-compatible components and then added.
　　Although adding grinding agents to the formulation is often considered a simple and safe method, it is actually not recommended. The problem lies in the fact that UV stabilizers tend to adsorb onto the pigment surface, causing the additives to leach out of the base material and resulting in at least partial loss of effectiveness.
　　Add during paint mixing.
　　This method of incorporating UV stabilizers requires an extended mixing time. However, prolonging the mixing time can introduce air and carbon dioxide; in extreme cases, it may even lead to flocculation of the base material. Moreover, paint chemists will face additional challenges: the range of UV-stabilizer products suitable for this application is quite limited.
　　Prepare by adding to the aqueous-compatible component.
　　In practice, this addition method is frequently used because it is simple and safe. Typically, before adding the additive to the coating, it is pre-mixed or diluted with other components in the formulation. However, this approach requires additional effort and time. Moreover, a particular pre-mixed ingredient is, in most cases, suitable only for one specific coating formulation system.
　　Diluting hydrophobic UV stabilizers in water-miscible solvents poses another risk—the “gold-washing effect.” When the solution is poured into an aqueous phase, the hydrophilic solvent is extracted, releasing the hydrophobic additive and causing the UV stabilizer to precipitate. In many cases, this phenomenon can lead to even more serious problems.
　　Dispersion of UV stabilizers
　　In addition to high efficiency, UV stabilizer dispersions must meet other relevant requirements: broad compatibility, ease of handling, safe storage, and environmental, safety, and health considerations.
　　The requirement for a wide range of applications means that it is essential to strike an optimal balance. For UV stabilizers, the highest demand is for them to be suitable for high-end coatings such as OEM and automotive refinish paints. However, achieving universal compatibility to meet these requirements seems impossible.
　　A dispersion with appropriate viscosity can be easily incorporated. At the same time, it is essential to add a retention aid and a microbial contamination stabilizer, which will enable the storage life to exceed two years.
　　Whether or not to affix a hazardous materials label depends on the dispersed UV stabilizer. All other components in the dispersion should pose no hazards requiring labeling. This includes VOCs as well. The typical composition of UV-stabilizer dispersions is shown in Table 1.
　　Resin-based or surfactant-based?
　　In most cases, surfactant-based products offer a broader compatibility range. Generally speaking, resin-based formulations typically allow for a smaller amount of additives. The primary reason for choosing surfactant-based products is their wide compatibility and environmental friendliness. In most situations, surfactant-based products also biodegrade more rapidly.
　　UV stabilizer
　　The optimal conditions are a UV stabilizer concentration of 52%, a D50 of 0.5–2.0 μm, and a D90 less than 3.5 μm. When the particle size is smaller, addition becomes easier; however, this also leads to an increase in viscosity.

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