Synergy in the sunlight

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1 Seite/Page: 1 Synergy in the sunlight Hindered amine light stabilisers (HALS) are often used to enhance exterior durability of coatings and plastics. Tests with a gloss paint containing different grades of titanium dioxide showed that HALS had by far the greatest effect with the most durable grades, while the type of HALS used was less significant. HALS are most effective when used with highest durability grades of TiO2 Christian Schaller Light stabilisation of coatings and plastics has been a challenge for industry for a long time [1]. Throughout their service life, coatings are exposed to light, heat and temperature changes (i.e., weathering). To prolong the life of coatings and protect their decorative and protective character it has been common practice to combine UV absorbers (UVA) with hindered amine light stabilisers (HALS) [2]. The role of the UVA is to filter out the harmful wavelengths of the light spectrum and to prevent photochemical reactions from causing degradation of the coating and consequently of the substrate. Today, UVAs based on 2-(2-hydroxyphenyl)-benzotriazole (BTZ) chemistry are considered the most important for coatings, plastics etc.. However, for some applications the paint industry has found it advantageous to adopt the latest UVA class based on 2-hydroxyphenyl-s-triazine (HPT) chemistry [3]. According to the Lambert-Beer law, UVAs are most effective within the depth of the coating and cannot efficiently protect its surface against defects such gloss loss, chalking and crack formation. UVAs are thus combined with HALS, which are mainly derivatives of 2,2,6,6-tetramethylpiperidine [4]. HALS compounds effectively scavenge free radicals at the coating surface, retard the photo-oxidative degradation of polymers (both coatings and plastics) and thus help to prevent surface defects. The stabilisation mechanism of HALS has been extensively studied and is reported to be a cyclic chain-breaking antioxidant process, termed the "Denisov cycle" as shown in Figure 1 [5]. In the presence of oxygen and radiation, the HALS compound (1) is first converted into the corresponding nitroxyl radical (2), which then traps a free radical with formation of an aminoether function (3). This interacts with a peroxide radical with formation of intermediate structures which then decompose into harmless alcohols and ketones while the nitroxyl radical (2) is re-formed. HALS types and applications Today a variety of monomeric, dimeric and oligomeric HALS compounds with different properties and application profiles are available. Monomeric HALS are mostly used in wood treatment for direct lignin stabilisation. This concept combines a HALS derivative and UVA to provide effective colour stabilisation for pale wood species in indoor applications and allow clear coatings to be used for highperformance exterior applications [6]. For coating applications, the most important HALS types are difunctional piperidine derivatives linked by diesters or triazine rings as shown in Figure 2 (HALS 1-3). The most important multi-purpose HALS for coating applications, with R = CH3 (HALS-1) exhibits excellent longterm stability and solubility in most paint applications. More recently, the aminoether (NOR) functionalised HALS-2 has gained wider acceptance. The NOR HALS-3 has a reactable primary hydroxyl function enabling it to co-condense, e.g., with melamine and isocyanate crosslinkers, to improve compatibility and resistance to migration in many systems, such as coatings over plastics [7]. Other HALS compounds are functionalised to introduce new performance properties. For example, a combination of HALS with an antioxidant moiety of the sterically hindered phenol type shows triboelectric charging activity and is therefore recommended mainly for powder coating applications [8]. Oligomeric HALS are the choice for all applications calling for low volatility, high resistance to extraction and minimal migration because of their high molecular weight. These HALS also exhibit antioxidant properties and contribute significantly to the long-term heat stability of polyolefins and other plastic substrates [9]. Basicity of HALS may affect performance HALS properties such as basicity, solubility, migration resistance and thermal stability are controlled by the molecular structure, i.e. the molecular weight, the linkage between the piperidine rings and the piperidine N substituents. The HALS basicity given by the N substituent is one key property and partly determines the suitability of the HALS for particular applications. Here H or alkyl substituted HALS are basic (pkb ~ 4 to 6) whereas aminoether functionalised HALS are considered as non-basic (pkb ~ 8 to 10). Basic types can undergo acid/ base interactions with paint components such as biocides, surfactants and pigments or pigment surface treatments. These interactions can alter wet paint properties such as formulation stability, they can interfere with acid-catalysed cross-linking reactions such as those involving melamine or epoxy resins, or they can retard the curing of some airdrying alkyds or oil-based paints. Finally, these interactions reduce the long term performance of the coating due to loss of additives such as biocides or HALS. Non-basic NOR HALS (e.g. HALS-2/3) do not show such interactions and so can be used in applications where traditional basic HALS have failed. Pigment content affects optimum HALS and UVA levels The key to extending the service lifetime of coatings is the optimisation of UV light protection by the right combination of UVA and HALS. Today, many industrial and decorative coating systems are semi-transparent or opaque finishes

2 Seite/Page: 2 where the pigments and fillers act partly as UV and visible light protectors. Pigmentation is a key property in terms of light protection. Performance depends strongly on the pigment chemistry; organic pigments tend to absorb light and thus photodegrade over time whereas inorganic fillers or pigments simply reflect or scatter UV light and thus limit degradation [10]. The optimal ratio of UVA and HALS thus depends strongly on the concentration of pigments (acting as UVAs) in the coating; clear coatings require higher amounts of UVA (and lower HALS) whereas opaque pigmented coatings require higher amounts of HALS (and lower UVA levels) [11]. However, knowledge about pigment-hals interactions is still limited. It is reported that some HALS compounds are chemisorbed on the pigment surface and so lose their free radical scavenging ability [12]. The degree of chemisorption is a function of HALS basicity as well as the surface charge of the pigment [13]. Today most pigments used in coatings are finished with a surface treatment to improve properties such as weather resistance, dispersibility, rheology and to reduce photocatalytic effects. Surface modifications therefore affect the tendency to interact with additives such as dispersants, wetting agents and HALS. Titanium dioxide in particular, one of the most widely used coating pigments, is available in different qualities and with different surface treatments [14]. Knowledge about possible interactions is the key to efficient and economic use of HALS in pigmented coatings. A systematic study was carried out to determine the interactions between HALS types and TiO2 grades in a solvent-borne white coating for industrial application. The TiO2 grades used, their different surface treatments and qualities are summarised in Table 1 while the structures of the HALS are shown in Figure 2 and itemised in Table 2. The aim was to see how the differing weathering stability of the TiO2 series as well as the difference in HALS basicity influenced the long term performance of the coating. Experimental methods summarised The test coating was a solventborne white pigmented 2K- PUR based on hydroxy functional acrylic resin and an aliphatic polyisocyanate (HDI-trimer) with a pigment volume concentration (PVC) of 16 %. It was applied by spray on white coil coated panels to obtain full opacity with a dry film thickness of 50 to 60 µm. Application parameters were 4:1:1 paint:hardener:solvent, viscosity adjusted with solvent to 22 s DIN cup 4. Drying procedure was 24 h flash off at room temperature (RT), then 30 min at 80 C. Artificial weathering evaluations according to DIN EN ISO A (Xe-WOM CAM 7 cycle) with the "Atlas Weather- Ometer Ci-65 A" (outer and inner filter borosilicate) (0.35 W/ 340 nm: black panel temperature (60 ± 2) C, RH (35 ± 5) %; 102 min light, 18 min light and spray cycle. Colour was measured with a "Minolta CM-3600d" (gloss included) and L*, a*, b*, C*, h and?e* were calculated with "CGREC" software according to DIN The gloss evaluation was performed at 20 with a "Byk/ Gardner Micro-Tri-Gloss" according to DIN Cracking evaluation was performed visually using crack formation scale 353 from TNO (Netherlands Organisation for Applied Scientific Research) Colour changes during exposure are minor The colour deviation DE* of the white pigmented coatings after 4000 h Xe-WOM CAM 7 exposure is shown in Figure 3; DE* is below 1.5 for all coatings. The use of HALS improves the colour retention by around 30 % to 40 % for C, D and E only, with no significant improvement for A and B. This can be explained by the improved weather stability due to surface treatment with combinations of SiO2, Al2O3 and ZrO2 or SiO2 and Al2O3 with a dense skin which increases HALS efficiency and therefore improved colour retention of the coating. This seems to be independent of HALS basicity, indicating that no acid/base interactions occurring in the wet paint or dry state affected the protective properties of the HALS. Pigment has more effect than HALS type on gloss loss The gloss retention of the coatings during exposure is shown in Figure 4. Here it can be seen that the initial gloss is not a function of the TiO2 surface modification. All coatings exhibit an initial gloss of to 88±2. The difference in gloss retention profiles reflect the quality ranking of the TiO2 given by the supplier. The time to 50 % gloss loss of the unstabilised coatings (1.1 A-D) increases from the lowest performing TiO2 at 1800 h (A) to the best performance with 3800 h (E) (see Table 3). A and B show complete loss of gloss and chalking even after 3000 h. C and D show cracking between 3500 and 4000 h. Only E shows no surface defects after 4000 h exposure. This indicates that the TiO2 quality strongly affects coating properties. The next step was to see how the coating quality can be improved by the addition of different HALS types as shown in Figure 4 ( ). The use of HALS significantly improved the gloss retention of the coatings independent of HALS type and basicity for coatings B to E. For pigment A, with the lowest durability, even the use of HALS does not significantly improve the overall performance and coatings based on TiO2 type A exhibit severe gloss loss and chalking after 3000 h. B and C show a clear response to HALS addition, with far better gloss retention but matting after 5500 h and 7000 h respectively. The best gloss retention is obtained by D and E without any surface defects after 7000 h and 9000 h. The improvement in long-term performance due to HALS during Xe-WOM CAM 7 exposure is shown in Table 3 as the time to reach 50 % gloss loss. The improvements achieved range from only around 25 % for A, to around 90 %, for B, 100 % for C and D and more than 130 % for E. Differences between the HALS types are less pronounced and again indicate that there are no interactions between the coating system itself or HALSconsuming paint components in the coating. HALS-3 does show slight advantages compared to HALS-1 and 2, but this difference in performance is not significantly related to basicity. High pigment quality maximises HALS performance The results of the artificial weathering studies show that coating performance depends strongly on the quality of the TiO2 used. The durability of the coatings can be significantly improved (> 100 %) by the use of HALS so long as the TiO2 quality is adequate. This is more or less independent of the HALS basicity, though HALS-3 shows slight advantages.

3 Seite/Page: 3 This leads to the simple rule: a coating is only as good as its weakest component. Do not use low quality pigments and rely on HALS for high quality paints, but rather select high grade pigments and additives to achieve optimum durability. í REFERENCES [1] Valet, A., (1997) Light Stabilizers for Paints, ISBN , C.R. Vincentz Verlag, Hannover, Germany, [2] Decker, C., Biry, S., Zahouily, K., (1995) Photostabilisation of organic coatings, Polym. Degrad. & Stab. 49, [3] Schaller, C., Rogez, D., Braig, A., (2008) Hydroxyphenyl-s-triazines: Advanced multi-purpose UV-absorbers for coatings, JCTR 5(1), [4] Kurumada, T., Ohsawa, H., Yamazaki, T., (1987) Synergism of hindered amine light stabilizers and UVabsorbers, Polym. Degrad. & Stab., 19(3), [5] Pospisil, J., Klemchuk, P. P., (1990) Oxidative deterioration processes in organic materials in: Pospisil, J., Klemchuk, P. P., editors, Oxidation Inhibition in Organic Materials, Vol. 1. Boca Raton: CRC Press, [6] Schaller, C., Rogez, D., (2006) Defended from the sun, Europ. Coat. Jnl., 12, [7] Cliff, N., Adamson, K., Kanouni, M., Yaneff, P., (2004) Migration of reactable UVA and HALS in automotive plastic coatings, JCTR, 1(3), [8] Zeren, S., (2002) UV stabilisation of powder clear coats, Proceedings from the XXVI FATIPEC Congress, Dresden, Germany. [9] Sampers, J., (2002) Importance of weathering factors other than UV radiation and temperature in outdoor exposure, Polym. Degrad. & Stab., 76, [10] Turton, T. J., White, J. R., (2001) Effect of stabilizer and pigment on photo-degradation depth profiles in polypropylene, Polym. Degrad. & Stab., 74, [11] Schaller, C., Rogez, D., (2007) New approaches in wood coating stabilization, JCTR, 4(4), [12] Liauwa, C. M., et al, (1999) Effect of interactions between stabilisers and silica used for anti-blocking applications on UV and thermal stability of polyolefin film, Polym. Degrad. & Stab., 65, [13] Haacke, G., Longordo, E., Andrawes, F. F., Campbell, B. H., (1998) Interactions between light stabilizers and pigment particles in polymeric coatings, Prog. in Org. Coat., 34, [14] Kittel, H., Spille, J., (2003) Lehrbuch der Lacke und Beschichtungen, Band 5: Pigmente, Füllstoffe und Farbmetrik, "Nachbehandlung", 2.Auflage, ISBN: , Hirzel S. Verlag, Stuttgart, Germany, grade of TiO2 used. HALS addition was most effective with the highest grades of pigment and almost ineffective with the lowest one. Although HALS which are basic can lose their efficiency by reacting with the surface treatment of pigments, no such effect was found in these tests. Corresponding Author: Dr. Christian M. Schaller BASF SE christian.schaller@basf.com ACKNOWLEDGEMENTS Thanks are offered to Dr. Josef Schmelzer from Kronos International, Inc., Technical Service Department, for providing the wet paint samples and for fruitful discussions during this study. Results at a glance Hindered amine light stabilisers (HALS) are often combined with UV absorbers (UVA) for optimum exterior protection of coatings and plastics. The type and concentration of pigments modifies additive levels. Coatings containing pigments may only require HALS, as the pigments act as UVAs.Coating durability also depends on the binder and pigments. Tests were carried out with a white gloss paint containing different grades of titanium dioxide and different types of HALS.The greatest influence on durability (in terms of gloss reduction) after accelerated weathering was the

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5 Seite/Page: 5 Figure 1: Mode of action of hindered amine light stabilisers (HALS) according to the "Denisov cycle"

6 Seite/Page: 6 Figure 2: Chemical structures of HALS commonly used for coating applications

7 Seite/Page: 7 Figure 3: Colour deviation?e* of white pigmented coatings with HALS after 4000 h Xe-WOM CAM 7 exposure

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9 Seite/Page: 9 Figure 4: 20 gloss retention of white pigmented coatings without (1.1) and with HALS stabilisation ( ) during Xe-WOM CAM 7 exposure

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