The exterior durability of most organic coatings is highly dependent upon the use of light stabilizers. The two main stabilizer categories are ultraviolet light absorbers (UVAs) and hindered amine light stabilizers (HALS). UVA chemical classes include benzophenones, cyanoacrylates, oxanilides, benzotriazoles and triazines. Today, the latter two of these classes are by far the most commercially significant to the coatings industry due to their excellent spectral coverage, high extinction coefficients and excellent photo-permanence. HALS fit into a broader chemical family known as antioxidants but are distinguished from other materials of that class such as phosphites, lactones and hindered phenols by their cyclical mechanism, which allows for long-term efficacy in a similar way that keto-enol tautomerism permit the same for UVAs.
Recent UVA developments have focused on increasing molecular weight and/or adding functionality, which minimizes migration out of coatings, furthering improvements on increasing extinction coefficients, increasing photo-permanence, and designing encapsulation techniques that facilitate use in waterborne systems without the need for solvents or pre-emulsification. Likewise new HALS developments have mirrored these and include functionalizing to prevent exudation out of films, development of non-interacting materials that are suitable for use with acidic pigments and acid-catalyzed coatings, and encapsulation to accommodate formulating zero-VOC coatings.
Coating UV Protection
UV radiation is known to contribute to the chemical modification of exposed paint surfaces. The deleterious effects of radiation include loss of gloss, color change, chalking, flaking and film delamination. Choice of light stabilizers becomes even more critical for applications over sensitive substrates such as wood and plastics. Coating wood can be especially problematic because of the rapid degradation of the substrate when exposed to light due to photo-oxidation of lignin and sensitivity to humidity.Light-induced damage can be reduced significantly by using UV screeners. Certain pigments reflect and/or absorb UV and visible light. They contribute to reduce the irradiation of deep coating layers and substrates and their consequent degradation effects.
In clear systems, organic UV absorbers have the same role by absorbing UV light and are typically used at 1-5% active substance on resin solids depending on the coating thickness and the desired degree of protection. Organic UV absorbers allow the coating film to be totally colorless without any compromise on film clarity. Pigments like titanium dioxide and iron oxides are good UV screeners but impart color and opacity. Micronized versions of these pigments can be used as more or less transparent UV screeners to overcome these limitations in clearcoat applications. However, a noticeable drawback of titanium dioxide is the tendency to reagglomerate, still leading to reduction in film transparency above a certain use level and to a hazy, whitish-bluish aspect.
For waterborne systems another drawback is the high amount of dispersant required for the stabilization of these fine pigment dispersions and the use of cosolvents to reduce dry-out effects. This may reduce gloss and affect dry film properties such as blocking resistance or water sensitivity. In the case of iron oxides, a significant color shift towards yellow to brown shades occurs when they are used at levels required for effective UV protection.
For exterior applications, synergistic combinations of UV absorbers and HALS are optimal for the stabilization of coatings. UV absorbers are governed by the Beer-Lambert Law, thus absorbance is linearly related to the concentration of UVA, its molar absorptivity (extinction coefficient) and path length (i.e., coating thickness). Thus, for clearcoats, they provide the predominant mechanism for coating stabilization. HALS, by contrast, are free radical scavengers, which are not subject to Beer’s law and work anywhere in the coating system. These inhibit the coating binder’s photo-oxidative reactions and help to maintain their initial film properties such as flexibility and water repellency. HALS are especially effective at coating surfaces, providing better gloss retention, higher chalking resistance in pigmented systems while avoiding crack formation in clearcoats. Thus for pigmented systems, HALS provide the primary mechanism of stabilization because most UV radiation is blocked by pigment from penetrating beyond the first few microns of coating. The selection of the appropriate UV absorber/HALS combinations and concentration is dependent on the chemistry of the coating system, the presence of pigments and fillers, film thickness, and exposure conditions.
Novel Dispersion Technology
The challenge for the stabilizer producer is to develop product forms that enable easy incorporation and unproblematic dispersion into the broadest range of aqueous binder systems of relatively hydrophobic materials. So far the selection of light stabilizers, which can be incorporated by simple post-addition, has been limited to a few hydrophilically modified products. The suitability of such products for the growing waterborne coating resin ranges may be restriced by new labeling requests and higher technical performance requirements.t One of the most frequently used UV absorbers is a hydrophilically modified hydroxyphenylbenzotriazole (BTZ), which requires cosolvents for proper incorporation and to obtain acceptable storage stability in coatings. For outdoor applications the combination with a HALS is needed and the incorporation of both additives requires cosolvents or additional emulsifiers. In the literature there are further ways mentioned to get light stabilizers into water-based formulations, i.e., through polymer emulsions.
As most of the available products are hydrophobic, one approach is to mix them to the monomer feed before polymerization.(1) Conditions can be chosen so that after complete reaction the hydrophobic UV absorber is homogeneously distributed in the polymer particles. Depending on their chemistry, additives and polymers can be selected to allow the controlled release of the active substance from the particles. By contrast, reactable UV absorbers can be copolymerized into the polymer matrix, allowing the particle to completely retain the UV absorbers and provide highly UV absorbing emulsion polymers.(2,3) UV absorbers containing hydroxyl groups are suitable for reactions with polyisocyanates to design polyurethane dispersions.(4) This approach assures that the UV absorber remains compatible even at high dosage and prevents migration during service life of the coating, which is important particularly for thin film applications. Both methods are convenient to produce modified dispersion resins for improved polymer and substrate protection.(5)
Thus a new technology was developed to make water-insoluble UV absorbers compatible with water-based coating systems by using a mini-encapsulation technique. The preparation of the new product form needs two steps. First, stable emulsions with submicron droplet size consisting of monomers and UV absorbers are produced using high-shear emulsification techniques. Secondly, the polymerization of these emulsions yields fine particle size, low viscosity and stable aqueous dispersions.
The dispersion characterization was done by analytical ultra-centrifugation, providing information about particle size, distribution and density as well as density distribution. Measurements indicated that the UV absorber is homogeneously distributed in the polymer particles. Particle size measurements were also performed with dynamic light scattering, which showed that the particle size of the new products was in the range of 0.03 - 0.20 µm, typical for polymer dispersions.
Light Stabilization of Waterborne Wood Coatings
Recently a new stabilization concept has been presented, which provides improved wood color stability in interior applications and long-term durability to clear and transparent pigmented wood coatings in exterior applications.6-9 The concept consists of using first a specific water-soluble HALS compound in the wood primer. Being a lignin stabilizer this compound has to be used in dilute aqueous solution as a pretreatment in order to impregnate the wood surface (See Figure 3).Optimal wood color protection results are obtained when the lignin stabilizer is used in association with UV absorbers. The best method is when the UV absorber is added to a subsequently applied topcoat and acts as an external UV filter towards the wood surface. In case no topcoat is applied, the lignin stabilizer and the UV absorber have to be used in the same wood penetrating treatment, and the UV absorber has to act as an internal filter in the wood surface layer. As UV and also visible light up to 450 nm cause lignin degradation, best results are obtained with UV absorbers having the broadest absorption spectrum. Therefore the new red-shifted TRT chromophore-based UV absorber preparation with high UV-A ray absorption provides significantly better results than the currently used benzotriazoles (BTZ) in terms of color protection and durability.
To illustrate this effect, some test results are presented. Accelerated weathering tests with a spray-nozzle-modified QUV testing device were performed according to a cycle comprised of 5 hours of UVA-340 light at 50 °C followed by 1 hour of water spray at room temperature.
In a first example, a clear coating based on a self-crosslinking acrylic dispersion resin is applied on pine panels in two layers over a primer coat containing or not the lignin-stabilizing HALS described previously. In this experiment the color change is used to record the protection effect provided by the new UV absorber compared to the standard hydrophilic BTZ absorber.
After a total exposure of 1200 h, the unstabilized coating starts to show severe crack formation, while the substrate has strongly darkened as a result of lignin photo-oxidation. Cracking has not yet occurred on the panel that has been treated with the lignin stabilizer, and its color variation is much lower. Both stabilized coatings are in excellent condition and do not exhibit cracking.
It is recognized that in some cases it may be desirable to only apply a monocoat system to wood to accommodate a given cost structure or production constraints. Thus some work was performed on pine wood panels with selected light stabilizer additive options in a clear, film-forming varnish for interior use. These panels provided side-by-side comparisons of the effects of accelerated weathering (1000 hours CAM 0 in a xenon arc weatherometer) for each of the following cases:
• uncoated pine;
• coated pine (no light stabilizer);
• coated pine, 3% w/w standard light stabilizer package (UVA/HALS); and
• coated pine, 2% w/w CGL-362 (water-based dispersion of TRT) + 1% w/w CGL-355 (water-based dispersion of NOR type HALS).
In each case, the light stabilizers were added on an equivalent active ingredient concentration basis. Visual inspection and color measurements of the exposed panels showed that the water-based dispersions of UVA/HALS outperformed the hydrophobic analogues (See Figure 3).
Light Stabilization of Industrial Waterborne Coatings
A final example is given with a waterborne coating based on a self-crosslinking PU dispersion applied and dried at room temperature over white polycarbonate panels. The panels are exposed to a Weatherometer device equipped with a filtered Xenon light source and running according to a cycle of 102 minutes light irradiation followed by 18 minutes light and water spray. Black panel temperatures are 65 °C during the dry phase and room temperature during the spray period. It appears that the HPT dispersion type UV absorber provides significantly better cracking resistance to the coating and color protection to the polycarbonate substrate at the same concentration and, even at lower concentration in active substance, compared to the BTZ UV absorber. While the unstabilized system is totally cracked after 6400 h, the 3% BTZ-stabilized clearcoats resists up to 8000 h while the HPT preparation goes to 8800 h with 2% and far over 9000 h for the 3% active UV absorber level with a significantly better yellowing resistance. Figure 4 summarizes the results of this testing. Note that the UVA-stabilized coatings also contain 2% HALS. The lowest substrate color change is obtained with the 3% HPT UVA preparation.Conclusion
A new mini-emulsion polymerization technique was developed to produce aqueous preparations of hydrophobic UV absorbers enabling their easy incorporation and effective use in waterborne coatings. The products can be added as supplied by simple stir-in to the formulations without any cosolvent at any stage of the manufacturing process and are ideal for post addition. The new product form allows dispersion of hydrophobic substances into aqueous systems without separation problems upon storage. The dispersed additive does not influence the optical properties of clear coatings such as gloss and transparency. Thus it overcomes the disadvantages of solid organic or mineral UV absorber pastes, which cause haze and gloss reduction and can lead to sedimentation in the liquid formulation during longer storage periods.A further advantage is that the dispersed hydrophobic compound which, in comparison to products which were made water-compatible through chemical modification with polar or hydrophilic groups, shows very high water leaching resistance and excellent long-term protection effects. Preliminary results show that the performance obtained with such preparations is comparable to the protection resulting from hydrophobic additives in solvent-based coating systems. This indicates that the degree of dispersion is sufficient to deliver the expected level of performance and durability.
It should be noted that this technology is being extended to other light stabilizers beyond the triazine and NOR HALS products discussed in this report. As this technology is applied to other hydrophobic light stabilizer additives the formulator will have flexibility in developing environmentally compliant solvent-free formulas tailored to the cost structure and performance level required.
This paper was presented at The Waterborne Symposium sponsored by The University of Southern Mississippi School of Polymers and High Performance Materials and The Southern Society for Coatings Technology, 2008, New Orleans, LA.
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