Hindered amine light stabilizers in pigmented coatings.
Keywords UV stabilization, Coating stabilization, UV absorbers (UVA), Hindered amine light stabilizers (HALS), Pigmented coatings, Titanium dioxide
History of light stabilizers
Light stabilization of coatings and plastics has been a challenge for the industry for long time. (1) Automotive clearcoats are, for example, expected to ensure gloss retention and retain a colorless appearance for many years, while protecting the basecoat or coated plastic parts. (1), (2) Throughout their service life, these coatings are exposed to light, heat, and temperature changes (i.e., weathering) and mechanical stress such as scratch and mar impact. Apart from color deviation and loss of gloss, this may lead to surface defects such as cracking and even delamination. (1-3) To ensure the long life of coatings and to prevent its decorative and protective character, it has been common practice to combine UV absorbers (UVA) and hindered amine light stabilizers (HALS). (4) The role of the UVA is to filter out the harmful wavelengths of the light spectrum and to prevent photochemical reactions leading to the degradation of the coating and consequently of the substrate. Today, UVA based on 2-(2-hydroxyphenyl)-benzo-triazoles (BTZ) chemistry are considered the most important light stabilizers for coatings, plastics, and other end uses. (5) Still some limitations of benzotriazole-based UVA have, for certain applications, pushed the paint industry to adopt the latest UVA class based on the 2-hydroxyphenyl-s-triazine (HPT) chemistry with a number of advantages. (6), (7)
However, according to the Lambert-Beer law, UVA are, on their own, inefficient to protect the surface of a coating with the consequence that they cannot prevent effectively surface defects like gloss loss, chalking, and crack formation. Therefore, UVA are combined with HALS, which are mainly derivatives of 2, 2, 6, 6-tetra-methylpiperidine. (8) HALS compounds do effectively scavenge free radicals at the coating surface where minor protection of the UVA is given, retard the photo oxidative degradation of polymers (i.e., coatings and plastics), and thus help to prevent surface defects. The stabilization mechanism of HALS is extensively studied and reported to be a cyclic chain-breaking antioxidant process; the so-called "Denisov cycle." (9-11) The mode of action of HALS according to the "Denisov cycle" is shown in Fig. 1. In a first step, in the presence of oxygen and radiation, the HALS compound (1) is converted into the corresponding nitroxyl radical (2) as reactive species, which then traps a free radical under formation of an aminoether function (3). (3) interacts with a peroxide radical under formation of intermediate structures (4) which then decompose into harmless alcohols and ketones while the nitroxyl radical (2) is re-formed.
[FIGURE 1 OMITTED]
HALS types and typical application
Today, a variety of HALS compounds with different structures and application profiles are commercially available and shown in Fig. 2. The monomeric HALS-1 is mostly used in the wood treatments area for direct lignin stabilization. The lignin stabilizing concept consists of a combination of the HALS-1 derivative used in a wood impregnating solution, associated to UVA either used in a subsequently applied topcoat or in the same impregnating layer. HALS-1 is designed to trap the radicals formed by the degradation of lignin at the wood surface caused by visible light which is not screened by the UVA (essentially in the 400-500 nm range). This concept provides effective color stabilization for pale wood species in indoor applications and allows the use of clearcoatings for high-performance exterior applications where long lasting coatings are required, for example, wooden window frames, which often failed in the past due to insufficient light protection from traditional light stabilizers. (12-14)
[FIGURE 2 OMITTED]
For coating applications, the most important HALS types are di-functional piperidine derivatives linked by diesters or triazine rings as shown in Fig. 2 (HALS 2-6). One of the first commercial HALS, still used to a degree, had R=H (HALS-2). Later versions where with R=[CH.sub.3] (HALS-3) exhibit better long-term stability and solubility in most paint applications. More recently, aminoether functionalized (HALS-4/5) have gained wide acceptance in the paint industry where other HALS species failed. Other HALS compounds are functionalized to introduce new properties for an improved performance profile. HALS-6 contains an antioxidant moiety of the sterically hindered phenol type and also shows triboelectric charging activity and is therefore mainly recommended for powder coating applications. (15) HALS-5 exhibits a readable primary hydroxyl function enabling to co-condense, for example, with melamine and isocyanate crosslinkers, to exhibit improved compatibility and resistance to migration in many systems, such as coatings over plastics. (16) Oligomeric HALS (HALS-7/8) are the choice for all applications calling for low volatility, high resistance to extraction and minimal migration. In addition to light stabilizing properties, these HALS exhibit antioxidant properties and contribute significantly to the long-term heat stability of polyolefines and other plastics substrates. (17-19)
Most of the light stabilizers used today were initially developed for solventborne (SB) systems, whereas the number of waterborne (WB) systems is constantly increasing in most coating applications. To fulfill this increasing demand for water compatible additives, the state-of-the-art products are supplied as solid or liquid UVA or HALS dispersed or emulsified in water. These product forms exhibit some disadvantages such as settling of the additive in the product form or the wet paint, the necessary use of co-solvents (VOC) for incorporation, an inhomogeneous additive distribution in paint films and therefore haziness and reduced coaling transparency. Therefore a Novel Encapsulated Additive Technology (NEAT) has been developed to render organic water-insoluble additives compatible with WB coating systems. (20), (21) The NEAT products are based on a stable 20-30% active aqueous dispersion of UVA and/or HALS with excellent storage stability in the paint and full coating transparency after drying. All products exhibit a clean tox/eco-tox profile and are especially suited for all WB applications where low to zero VOC is required.
HALS properties like basicity, solubility, migration resistance, and thermal stability are controlled by the molecular structure, that is, 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 determines, to a certain extent, the suitability of the HALS in particular applications. The influence of N substituent on HALS basicity is shown in Fig. 3. Here, H or alkyl substituted HALS are basic (p[K.sub.b] ~ 4-6), whereas aminoether functionalized HALS are considered as nonbasic (p[K.sub.b] ~ 8-10). Basic HALS (e.g., HALS-2) can undergo acid/base interactions with paint components like biocides, tensides, as well as pigments or pigment surface treatments. These acid/base interactions can alter wet paint properties, for example, formulation stability, they can interfere with acid-catalyzed cross-linking reactions, for example, such as those involving melamine or epoxy resins, or they can retard the curing of some air-drying systems, like alkyds or oil-based paints. Finally, these interactions reduce the long-term performance of the coating due to consumed additives like biocides or HALS and therefore reduced protection. Nonbasic aminoether functionalized N-OR HALS do not show such behavior and can be considered as noninteracting (e.g., HALS-4/5), which allows the use in applications where traditional basic HALS failed.
[FIGURE 3 OMITTED]
How to get best UVA/HALS synergism
The key for getting best service lifetime of coatings is the optimization of UV light protection by the right combinations of UVA and HALS. Today, various coating systems with different layer setups (i.e., one coat, base coat/clearcoat, etc...), different binder qualities, and different pigmentations are available for a great variety of application. The degree of pigmentation is a key property and has to be strongly considered when thinking about light protection. Besides the clearcoatings, many paints used in industrial and decorative applications are semitransparent or opaque finishes where the used pigments and fillers act partly as UV and VIS light protectors. The light protection properties of pigments in coatings depend strongly on the pigment chemistry; that is, the organic or inorganic nature. Organic pigments tend more to absorb light and consequently photodegrade by time, whereas inorganic fillers or pigments simply reflect or scatter UV light and thus limit degradation. (22) The protection effect of pigments can be increased by combination with organic UVA and HALS. The optimal ratio of UVA and HALS strongly depends on the concentration of pigments (acting as UVA) used in the coating; that is, clearcoatings require higher amounts of UVA (and lower HALS), whereas opaque pigmented coatings require higher amounts of HALS (and lower UVA). A systematic study for an efficient and economic use of light stabilizers for wood coatings regarding the opacity of the coating, that is, the degree of pigmentation, is described elsewhere. (23) However, the knowledge about pigment HALS interactions is still limited today. It is reported that some HALS compounds are chemisorbed on the pigment surface under losing its free radical scavenging ability. (24) The degree of chemisorption is a function of the HALS basicity as well as the nature of the pigment, that is, the surface charge. (25) Today, most of the pigments used in coatings applications are finished with a surface treatment to improve properties like weather resistance, dispersibility, rheology, and to reduce photocatalytic effects. Surface modifications alter the pigment surface nature and therefore the tendency to interact with additives like dispersants, wetting agents, and HALS. Especially titanium dioxide [TiO.sub.2], reflecting one of the mostly used pigments in numerous colors for all coating applications, is available with different surface treatments. (26)
For an efficient and economic use of HALS in pigmented coatings, the knowledge about possible interactions is key. To learn more about HALS and pigment interactions, a systematic study was made together with the company Kronos International. Kronos supplied different SB and WB wet paint samples pigmented with different [TiO.sub.2] grades. The used [TiO.sub.2] grades with different surface treatments are summarized in Table 1 (see Experimental part). The target was to see how these paints, that is, the used [TiO.sub.2] in the paint, interact with different HALS types. This article discusses the influence of HALS basicity on long-term performance in the opaque white pigmented SB and WB coatings for industrial applications. Furthermore, a correlation between pigment surface modification and HALS interaction should be investigated. HALS-2, 3, and 4 are used in a SB, and HALS-3 and the encapsulated version of HALS-4 (i.e., HALS-4DW) are used in a WB paint.
Table 1: [TiO.sub.2] grades in SB and WB paints Ti BET IEP PSD Surface Recommended application [O.sub.2] (a) (b) (c) treatment A 15.6 7.8 0.31 [Al.sub.2] SB/WB applications: inks, [O.sub.3] coatings for interior use, packaging coatings B 16.8 8.2 0.36 [Al.sub.2] SB/WB applications: medium [O.sub.3], stability, suitable for Zr[O.sub.2] most outdoor purposes C 12.0 8.2 0.35 [Al.sub.2] SB/WB applications: [O.sub.3], high stability, Si[O.sub.2], automotive, powder Zr[O.sub.2] coatings, coil coatings D 15.5 7.2 0.35 [Al.sub.2] SB/WB applications: [O.sub.3], highest stability, Si[O.sub.2] automotive, powder dense skin coatings, coil coatings (a) BET surface in [m.sup.2]/g (b) Iso electric point (c) Particle size distribution in nm
The supplied pigmented WB industrial varnish is based on a hydroxyl functional polyester and hexamethoxym-ethylmelamine as crosslinker. The white pigmented SB varnish is a 2K-PUR based on hydroxyl functional acrylic resin and an aliphatic polyisocyanate (HDI-trimer). The used [TiO.sub.2] grades for pigmentation of the paints are summarized in Table 1. As HALS compounds bis/mono (1,2,2,6,6-pentamethyl-4-piperidyl) sebacate (HALS-3), decanedioic acid, bis(2, 2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl) ester (HALS-4) for SB and in encapsulated version for WB (HALS-4DW) and 2,4-bis [N-Butyl-N-1(1-cyclohexyloxy-2,2,6,6,-tetramethylpiperidin-4-yl)amino]-6-(2-hydroxyethylamine)-1,3,5-triazine (HALS-5) are tested and summarized in Table 2. (See structures in Fig. 2.) For comparison reasons the amount of HALS is adjusted according to the active nitrogen content under respect of the molecular weight.
Table 2: Tested HALS in SB and WB paints HALS CAS name Basicity Area Mn Percentage (a) (b) HALS-3 bis/mono (1,2,2,6,6- Basic SB/WB 470 1.0 pentamethyl-4- piperidyl) sebacate HALS-4 bis(2,2,6,6- Nonbasic SB 737 1.5 tetramethyl-1- (octyloxy) -4-piperidinyl)ester HALS-5 2,4-bis [N-Butyl-N-(1- Nonbasic SB 757 1.5 cyclohexyloxy-2,2,6,6- tetramethylpiperidin- 4-yl)amino]-6-(2 -hydroxyethylamine) -1,3,5-triazine HALS- NEAT version of Nonbasic WB 737 4.5 (c) 4DW HALS-4 (a) Molecular weight in g/mol (b) Delivery form on total paint (c) HALS-4DW is 30% active
The paint is applied by spray on white coil coated panels to achieve full opacity. For the SB paint application, the parameters are: 4:1:1 paint:hardner:solvent, viscosity adjustment according to DIN Cup 4 with solvent to 22 s, drying procedure: 24 h flash at RT, 30 min at 80[degrees] C, DFT: 50-60 [micro]m. For the WB paint application, the parameters are: viscosity adjustment according to DIN Cup 4 with water to 25 s, drying procedure: 120 min flash at RT, 10 min at 80[degrees]C and 20 min at 160[degrees]C. DFT: 50-60 [micro]m.
Artificial weathering studies are done according to DIN EN ISO 11341 A (Xe-WOM CAM 7 cycle) with the Atlas Weather-Ometer Ci-65 A (outer filter borosilicate/inner filter borosilicate) (0.35 W/[m.sup.2] at 340 nm: 102 min light. BPT (60 [+ or -]2)[degrees]C, RH (35 [+ or -] 5)%; 18 min light and spray, BPT (60 [+ or -] 2)[degrees]C, RH (35 [+ or -] 5)%). The color was measured with a Minolta CM-3600d (gloss included) and the calculation of L*, a*, b*, C*, h, and [DELTA]E* with CGREC software according to DIN 6174. The gloss evaluation was performed at 20[degrees] with Byk/Gardner Micro-Tri-Gloss according to DIN 67530. Cracking evaluation is performed visually according to the crack formation scale 353 from TNO (Netherlands Organization for Applied Scientific Research).
Result and discussion
Coloristics during exposure
The color deviation [DELTA]E of the white pigmented SB and WB coatings based on different [TiO.sub.2] grades on white coil panels with and without stabilization after 4000 h Xe-WOM CAM 7 exposure is shown in Fig. 4. [DELTA]E is below 1.5 for the SB as well as the WB coatings. For the SB coatings, the use of HALS improves the color retention only for (C) and (D), whereas no or no significant improvement can be seen for (A) and (B). Therefore it can be stated that [SiO.sub.2] treatment increases HALS efficiency (independent of HALS basicity) and therefore the color retention in the SB coating. In contrast, no or only small improvement in color retention due to the use of HALS can be seen for the WB systems. This indicates that surface treatment and HALS basicity do not significantly influence the coloristic properties during exposure in the WB system.
[FIGURE 4 OMITTED]
Gloss retention during exposure
The gloss retention of the white pigmented SB and WB coatings based on different [TiO.sub.2] grades on white coil panels without stabilization during Xe-WOM CAM 7 exposure is shown in Fig. 5. Here, it can be seen that for the SB system the initial gloss is not a function of the [TiO.sub.2] grade. All coatings exhibit an initial gloss of to 88 [+ or -] 2. In contrast, the initial gloss of the WB systems is related to [TiO.sub.2]; that is, the surface modification. Pigments have an influence on the gloss of coatings, for example, by flocculation or flooding or floating phenomena. These undesired appearances reflect the interaction of the binder-system with the pigment surface (wetting behavior). In the case of consideration, the pigments (A) and (D) exhibit an initial gloss of 69 [+ or -] 1, whereas (B) and (C) lead to a reduction of the initial gloss to 61 [+ or -] 1. The gloss retention profiles during exposure reflect the quality ranking of the used [TiO.sub.2] grades given by the supplier. For the SB coating, the 50% gloss loss is increasing from lowest performance with 1900 h (A), to 2400 h (B), to 3500 h (C), and finally best performance with 4000 h (D). This indicates that surface modification with [SiO.sub.2] obviously improves, as it is intended, the long-term performance of the pigment and this determines the performance of the SB coatings. This distinction is not as pronounced for the WB coatings where the 50% loss is increased from 2800 h (A) to 3500-3600 h for (B-D). Here, no significant difference between the medium and high quality [TiO.sub.2] grades can be seen. Furthermore, all WB coatings show an almost linear loss in gloss about 30% within the first 1000 h exposure. This is most probably related to intrinsic coating properties.
[FIGURE 5 OMITTED]
The gloss retention of the coatings based on different [TiO.sub.2] grades with HALS during Xe-WOM CAM 7 exposure is shown in Figs. 6 (SB) and 7 (WB). The use of HALS obviously improves the color retention of the SB coatings independent of the HALS type and basicity except for coating (A) with lowest durability. For (A) even the use of HALS does not significantly improve the overall performance (shift of 50% gloss loss from 2000 to 2500 h); all samples exhibit severe gloss loss and chalking after 3000 h. The coatings (B-D) show clear response to HALS with far better gloss retention. After 4000 h exposure, the HALS stabilized coatings show gloss retention of (62 [+ or -] 6)% for coating (B), (90 [+ or -]4)% for coating (C), and 92-93% for coating (D) with highest durability. Coatings (B-D) exhibit no cracking or chalking without significant differences in HALS performance related to basicity. This result indicates that there are no interactions between the coating system itself or HALS consuming paint components in the SB coating. In that case the difference in HALS performance is not significantly related to basicity.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
For the WB coatings, the use of HALS can increase the initial gloss around 15%. It is quite obvious that the basic HALS-3 worsen the gloss retention compared to the nonstabilized sample for all tested coatings. This reverse effect of HALS-3 can be explained by acid/base interactions with the melamine binder and/or other acidic paint components. The use of the nonbasic NEAT HALS-4DW show improved gloss retention profiles with reduced initial gloss drop, more flat profiles, and an extension of the 50% gloss loss from 2800 to 3500 h (A) and from 3500/3600 h to more than 4000 h (B-D). These results nicely show the possible problems related to HALS basicity and indicate the noninteracting character of nonbasic HALS. The encapsulated version of the nonbasic HALS-4 (HALS-4DW) is an ideal solution for WB paints where acid/base interactions can take place, low VOC and/or a clean tox/eco-tox profile is required.
Based on the results of the artificial weathering studies, the following statements can be made. There seems to be a correlation between [TiO.sub.2] grade and color deviation of the coatings. For the SB coatings, HALS stabilization improves [DELTA]E as long as [SiO.sub.2] treatment is present. No or only small improved color retention due to the use of HALS can be seen for the WB systems. For the WB and SB system it can be stated that a properly selected surface treatment obviously improves the durability of the coatings during exposure. The durability of the SB coatings can be significantly improved by the use of HALS independent of the HALS basicity. For the WB paint, only the use of the nonbasic NEAT HALS (HALS-4DW) shows improved durability, whereas the basic HALS-3 worsen the overall performance. As final summary it can be stated that the use of HALS obviously improves the long-term performance of coatings in terms of gloss retention and surface defects. However, the strong differences of the results between SB and WB indicate that it is key to know about possible HALS-paint interactions to avoid any HALS basicity-related problems. Especially in the WB coating area where electrostatic processes are widely distributed, the use of nonbasic noninteracting HALS is strongly recommended.
This derives to the simple rule: a coatings is only as good as its weakest component. Do not use low stabilized pigments and rely on performance of HALS for high-quality paints but rather select high-grade pigments and additives for optimal durability performance.
Acknowledgment The authors thank Dr. Josef Schmelzer from Kronos International, Inc., Technical Service Department, for providing the wet paint samples and for fruitful discussions during this study.
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C. Schaller [*], D. Rogez, A. Braig
Ciba Inc., Klybeckstr 141, 4002 Basel, Switzerland
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|Author:||Schaller, Christian; Rogez, Daniel; Braig, Adalbert|
|Date:||Mar 1, 2009|
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