Advanced light stabilizers for the next century's needs.

Today's coatings industry is driven by three main factors: technological advancement, environmental compliance, and economical performance. Light stabilizer chemistry has advanced over the years and has entered a new era with the introduction of products relating to these factors. Advanced triazines take advantage of the known high epsilon values as well as photopermanence and have extended now to longer wavelengths with even higher absorption characteristics. The latest hindered amine light stabilizers (HALS) are able to react into resin technologies so that their migration is limited to the layer they are intended to protect. Finally, a new technology has been developed to render these intrinsically hydrophobic chemistries compatible with water for enhanced performance and ease of use in today's low-VOC systems.

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INTRODUCTION

Coatings manufacturers are facing many challenges in the industry today and are expected to fill these requirements to be successful. In order to meet these needs, producers are looking to new technology from material suppliers to help differentiate their systems. With the ever changing economic climate and increased globalization, the need to implement new performance properties or effects at a faster pace also places significant pressure on manufacturers as well as their suppliers.

Today, the development of paints and coatings focuses mostly on three main areas, including the increase in overall performance, meeting environmental regulations, and maintaining low costs for end users. Increased functionality, longer durability, and use of thinner films are some main areas where performance improvements are occurring. It is anticipated that governments will continue to place pressure on the coatings industry to create products with less impact on the surrounding environment. Technology will help to support this change and eliminate older, less environmentally sustainable systems. Reducing costs with technology usually relates to the elimination of specific manufacturing steps in the preparation or application of paints. However, some approaches allow for the overall reduction in the amount of dry films required in a system, therefore also lowering costs.

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Light stability and durability to weathering during service is a key characteristic of coatings. Maintaining exterior appearance translates to more valuable automobiles, buildings, equipment, decorative articles, etc. Substrates, resins, pigments, and other coatings components are sensitive to UV light due to the presence of chemical functionalities and/or impurities that absorb specific electromagnetic radiation. Light stabilizers help to mitigate degradation through the prevention of light absorption as well as minimize any subsequent radical oxidation reactions.

Light stabilizers based on 2-hydroxybenzophenone were some of the earlier chemistries used for UV absorption. (1) These products also found use in coatings (2) and are still being used today, based on the need for performance. 2-Hydroxy-2-phenylbenzotriazole chemistry was introduced (3) and increased the performance of organic materials. The introduction of ortho-tert-alkyl substituents (4) and, later, ortho-cumyl groups, (5) promoted UV absorption to some of the greatest stabilization potential today. Triazine chemistry also added another facet to stabilization from UV light by providing significant improvements in durability. (6)

Hindered Amine Light Stabilizers (HALS) were first introduced in the 1970s into polymers, (7) and n-alkyl based versions were used in coatings in the early 1980s. (8) In the early 1990s, commercial versions of n-alkoxy HALS became available for use in coatings for better performance and less interaction with other coatings components. (9) All of these improvements have led to very durable coatings and plastics due to changes in the fundamental and auxiliary chemistry related to the core light stabilizing molecules.

PROTECTION FROM UV LIGHT

Pushing the envelope for the protection of substrates, coatings resins and components requires maximizing three critical factors related to the chemistry of a UV absorber. First, the absorbing chromophore must be significantly stable or photopermanent, simply to continue performing as a light stabilizer. Beer's Law dictates that the amount of light absorbed is directly proportional to the concentration of absorber, the path length (thickness of the coating, in this case), and the extinction coefficient of the absorbing molecule. Maximization of this extinction through chemical design is the second priority for product design and can be used to help reduce the concentration required in the formulation.

Finally, the absorption profile remains a key factor for protecting materials from UV light. Here it is important to take into consideration the UV (or even visible) light that reaches the substrate or coating as well as the relative sensitivity of the substrate and coating. For example, a coating resin could be very sensitive to 250 nm light, but the amount of 250 nm radiation in sunlight may not be sufficient by itself to create any photochemistry. The exact amount of radiation in sunlight, coupled with the absorption sensitivity of resins, pigments, the substrate or basecoat, etc., will determine where a UV absorber should function to alleviate any degradation. It could be anticipated that longer wavelength light absorption could be used to reduce or eliminate any photochemistry from occurring. This theory generally holds true, but as absorption approaches the low end of the visible spectrum at 400 nm, visible absorption occurs. The result would be a yellow tint from the additive's absorption spectrum.

As mentioned in the introduction, triazine chemistry exhibits one of the best photopermanence behaviors of any known UV absorber chemistry. This stability is reported to be related to the robustness of the intramolecular hydrogen bridge present in triazine UV absorbers. (10) The proton in benzotriazoles is reportedly more susceptible to base-induced deplanarization, eliminating the excited state-induced proton transfer (ESIPT) pathway to the ground state and energy release. The ranking of stability of UV absorber chemistry can however deviate significantly from studies in inert solvents or model systems when used in a specific application. The overall durability of a system can be more dependant upon other factors, such as absorption profile, yielding a more robust system using a less stable UV absorber.

The synthesis of extensions to existing versions of triazines is also fairly general, starting from cyanuric chloride. (11) UVA1 and UVA2 are commercial materials that were developed based on the variation of aromatic constituents around the triazine core. Electron donating groups and extended pi systems were used to vary absorption maxima and increase molar extinction, respectively.

Figure 1 shows the absorption spectra of UVA1 and UVA2 in non-donor solvents. The absorption of a standard hydroxyphenyl benzotriazole and triazine UV absorbers are included for comparison. Both products exhibit very high extinction coefficients for reduced loadings or thinner film thicknesses. The nominal absorption maxima allow for the use of these products in different applications where substrate or resin sensitivities are of varying wavelengths. For example, UVA1 can be used in clearcoats for wood where visible light can degrade highly absorbing lignins. UVA2 shows very high extinction (67,500 L/mol/cm) and has an absorption maximum at 322 nm. This material imparts very little color to a formulation and can be used at lower concentrations than existing chemistries or in thinner films where high absorption would be required. The absorption power of these new products versus standard chemistry used in many applications today is significantly higher.

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Figure 2 illustrates the performance of UVA2 versus known chemistry in a thin film clearcoat application over a black basecoat. The coatings containing different UV absorbers were exposed for 54 months at 5[degrees] South Florida with 1% standard HALS in all formulations. Standard triazine UVA shows superb performance at a 40 [micro]m layer thickness at 1% concentration, as expected (Figure 2d). However, if thinner films are required, both standard benzotriazoles and triazines show degradation and delamination at 20 [micro]m dry film thicknesses and 1.4% concentration (Figure 2a-b). Using UVA2 at only 1% concentration and 20 [micro]m layer thickness, Figure 2c shows that the film holds up well to weathering. Significant savings can be realized by using only half the coating previously required--and thin film applications previously not possible can now be achieved.

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The photopermanence of UVA2 relative to known benzotriazole chemistry can be observed in Figure 3. Transmission curves of 2K clearcoats over silver metallic basecoats containing these materials were plotted versus wavelength. Again, 54-month 5[degrees] South Florida exposure shows a significant impact on a coating containing a commercial benzotriazole UV absorber. In stark contrast is the amazing photopermanence of UVA2, with virtually no change in transmission after 54 months of exposure to the elements. What is even more significant is that the coatings containing UVA2 are only half the thickness of the samples containing the commercial benzotriazole material (20 [micro]m versus 40 [micro]m). These data suggest that not only can these advanced UV absorbers be used for thin film applications, but will perform under very harsh conditions.

RADICAL SCAVENGING 

Hindered amine light stabilizers are used in coatings and plastics formulations to eliminate radical chain degradation processes initiated by light. Together with UV absorbers, stabilization of organic materials with synergistic results can be realized. The overall effect of HALS is generally related to improved surface gloss and reduced chalking for more durable final articles.

As amines, HALS can be basic, depending on the nitrogen substituent in the molecule. Basicity from unsubstituted and n-alkyl HALS can interact with acidic components of a formulation or metal catalysts present to initiate crosslinking reactions. Acidic components can vary from urethane crosslinking catalysts to carboxylic resins, dispersants, and rheology control agents. The introduction of n-alkoxy substituted additives, known as NOR HALS, opened the doors to stabilization potential for new resin chemistry. In addition, introducing a NOR species delivered the active form of a HALS stabilizer, eliminating the need for the "activation" by oxidation required with unsubstituted, n-alkyl, or n-acyl HALS.

More recently, it became apparent that migration of light stabilizers can occur, relocating valuable chemistry to an undesired location. For example, some NOR HALS can be observed migrating from clearcoat layers into plastic substrates. This mobility results in reduced protection at the surface where the additive is needed most.

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HALS1 is an example of a material where reduced mobility can be obtained using a primary hydroxyl for reaction with isocyanates or other hydroxy crosslinking moieties. HALS1 is also based on NOR chemistry, allowing use in formulations containing acid components or metal driers. Figure 4 illustrates the comparison of standard NOR HALS with HALS1 mobility in a 2K urethane clearcoat over a TPO substrate (with basecoat) after 3000 WOM exposure. Here, HALS1 remains in the bulk of the coating to perform its function, while the standard NOR HALS finds its way through a significant portion of the substrate.

The result of the migration of a HALS can be seen in the gloss retention of the coating. The differences in performance of formulations containing a triazine UV absorber with HALS1, with standard NOR HALS, and NOR HALS together with a benzotriazole UV absorber over plastic substrates can be observed in Figure 5. The gloss retention of the formulation containing HALS1 is significantly better than the standard NOR HALS over plastic, due to the retention of the chemistry to the surface. The overall gloss retention in this case is similar to standard NOR HALS over metal substrates, where migration is not possible. This effect is important in consideration of part design where plastic and metal substrates are placed together and should therefore age at similar rates.

WATER COMPATIBILITY

Waterborne coatings formulations have traditionally contained sufficient co-solvent for incorporation of organic components for various effects. With the increase in regulations and desire to minimize the amount of VOCs released into the air and workplaces, the need has come to develop new chemistry compatible with pure water. Chromophore-containing UV absorbers and functionalized HALS consist of organic functionalities inherently incompatible with water. This paradox created the necessity for out-of-the-box thinking and a new technology to yield compatibility without any compromise of light stability.

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Recent developments in polymer chemistry combined with emulsion polymerization techniques yielded a technology where traditional organic-based materials can be used in waterborne systems. (12) Unlike historical approaches using surfactant technology and water dispersion, polymers can be used to encapsulate additives. The resulting composition contains sub-micron-size capsules consisting of at least 50% active material, available for delivery to waterborne formulations. The overall solids content of such water emulsions is typically in the range of 40%. Figure 6 shows a micrograph of a typical product containing a hydrophobic triazine UV absorber. These materials do not contain any VOC components and the technology can be used to synthesize compositions containing UV absorbers and/or HALS light stabilizers. Formulation using this delivery form of light stabilizer can occur easily by stir-in use in waterborne formulations of varying chemistries (including waterborne UV).

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Some approaches to the compatibalization of hydrophobic chemistries with water can yield products which later separate upon application and film formation. The encapsulation technology developed here uses specific polymers that allow for efficient incorporation of the hydrophobic additives into the resin without exudation or separation. This effect is illustrated in Figure 7 with the product incorporated into a waterborne coating over glass. The clarity of the resulting film is no different than systems without additives.

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CONCLUSION

Technology will continue to give solutions to coatings needs, raising the bar for the industry and creating better products for consumers. New UV absorbers discussed in this article are examples of new performance alternatives that allow for better stability and new applications to exist. HALS can be confined to the areas where needed with the use of crosslinking functional groups in the molecule. Finally, an encapsulation technology can be used to combine low-VOC waterborne coatings with hydrophobic additive performance. Developments will continue and these, as well as other technologies, will be used to create new standards for additive effects.

References

(1) Armitage, J.B., Dessauer, R., and Hyson, A.M., U.S. Patent 3,098,842; and references cited thererin, 1963.

(2) Rothstein, E., J. PAINT TECH., 39, No. 513, 621 (1967).

(3) Heller, H., Rody, J., and Keller, E., U.S. Patent 3,629,191, 1971.

(4) Rody, J. and Slongo, M., U.S. Patent 4,853,471, 1989.

(5) Dexter, M. and Winter, R.A.E., U.S. Patent 4,226,763; 1980.

(6) Hardy, W.B., Milionis, J.P., and Pinto, F. G.; U.S. Patent 3,118,887, 1964.

(7) (a) Heller, H.J. and Blattmann, H.R., Pure Applied Chem., 36 (1-2) 141 (1973). (b) Gilg, B., Mueller, H., and Rody, J., DE 2,417,535 (1974).

(8) Berner, G., U.S. Patent 4,314,933, 1982.

(9) Behrens, R.A., Winter, R.A.E., Mar, A., Schirmann, P.J., Seltzer, R., and Malherbe, R.E, U.S. Patent 5,112,890, 1992.

(10) Elbe, F, Keck, J., Fluegge, A.P., Kramer, H.E.A., Fischer, P., Hayoz, P., Leppard, D., Rytz, G., Kaim, W., and Ketterle, M., J. Phys Chem. A, 104, 8296 (2000).

(11) See Migdal, C.A., Hines, I.B., and Kluger, E.W., U.S. Patent 4,826,978, 1989, for examples of this reaction.

(12) Schellenberg, C., Auschra, C., Peter, W., Renoux, D., and Hayoz, P., WPT Application WO 2005023878, 2005.

by Jonathan Rogers,** Brett Moyer, and Greg Kalscheur Ciba Specialty Chemicals* and Norbert Emig Ciba Specialty Chemicals ([dagger])

Presented at the Federation of Societies for Coatings Technology's 2006 FutureCoat! Conference, November 1-3, 2006, in New Orleans, LA.

* 205 South James St., Newport, DE 19804.

([dagger]) Klybeckstrasse 141, K-147.2.33, 4002 Basel, Switzerland.

** Author to whom correspondence should be addressed. Voice: 302.992.5636; email: [e-mail].