Resisting the heat New bismuth vanadate pigment minimises degradation from over-stoving. Bismuth vanadate pigments meet the requirements for powder coatings in respect of colour properties, hiding power, weathering and temperature stability. However, over-stoving of these pigments leads to discoloration and serious surface degradation. The development and properties of an over-stoving stable bismuth vanadate are reviewed. Kirill G. Bramnik, Eckhard Korona* * Corresponding Author. Contact: Eckhard Korona, BASF Aktiengesellschaft, D-67056 Ludwigshafen, Germany, Tel. +49 621 60-72907, eckhard.korona@basf.com Lead chromate pigments have been progressively replaced by other inorganic and/or organic alternatives during recent decades. More and more application fields have become "lead free". European powder coating producers responded in the very early stages of this process by developing lead-free colour formulations. Inorganic pigments of nickel titanate (Colour Index P.Y. 53) and chrome titanate (P.Br. 24) types in combination with organic yellow, orange and red pigments allowed even very brilliant colour shades with high hiding power to be achieved. The development of bismuth vanadate pigments (P.Y. 184) significantly extended the range of applications, especially with regard to colour and weathering properties. This new form of inorganic pigment is well suited to both wet and powder coatings. Thick substrates increase risk of coating degradation Commercially available bismuth vanadate pigments meet the requirements of producers of powder coatings in respect of their colour properties, hiding power, weathering resistance and temperature stability. However, the stoving conditions need to be strictly controlled at those recommended by the resin and accelerator producers, that is, 15-20 min at 180 C. In the last decade, new coating substrates have been put forward for powder coating. Massive metal bodies and thick substrates in particular require significantly higher temperatures and notably longer curing times in the stoving furnaces. Such conditions mean in practice that object temperatures may reach up to 220 C over a stoving time of up to 60 minutes. These conditions pose too great a challenge for current bismuth vanadate pigments. In particular, the use of bismuth vanadates in polyester/hydroxyalkylamide and polyester/glycidyl ester systems leads to discolouration and strong surface degradation under the conditions quoted above. (see examples in Figure 1). The direct influence of the BiVO 4 pigments on the effects described is certain, but the exact relationships between cause and effects are still unknown. Therefore, a yellow BiVO 4 pigment has been developed, that has improved resistance to over-stoving, specifically for applications in powder coatings. Admittedly, even over-stoving of a white powder coating (TiO 2 pigment) results in a significant colour shift compared with the same powder coating stoved under milder conditions. For example, a system based on "Crylcoat 840/XL 552" crosslinker with 20% pigment experiences a colour shift E* of 3.3 units when the stoving cycle is changed from 15 min at 180 C to 60 min at 220 C, probably caused by degradation of the resin/accelerator system. However, as will be shown below, this change is less severe than that encountered when using normal bismuth vanadates. Experimental Powder coatings on a polyester/hydroxyalkylamide base were chosen to test the over-stoving stability of BiVO 4 samples. The components shown in Table 1 were mixed intimately at 1000 rpm for 3 minutes in a laboratory mixer ("Mixaco" Dr. Herfeld GmbH & Co. KG). The mixture was then treated in an extruder running at 100 rpm. The output temperature reached approx. 110 C. The product was milled after cooling. The application of powder to the aluminium plates was carried out by a corona powder spray-gun at 80 kv voltage. The coated plates were stoved for 60 min at 210 C. Colour properties were evaluated by "Spectrolino" spectrophotometer (Gretag-Macbeth) in comparison to the plates coated with the same powder stoved at 180 C for 15 min. Al 3+ dopant level affects bismuth vanadate stability Commercial BiVO 4 pigments comprise in general: - A BiVO 4 pigment core, which can be doped with other ions, e.g. Al 3+, Fe 3+ ; - An additional stabilising shell of different components. Proper and careful choice of the dopants and components of the stabilisation shell as well as the synthesis conditions are very important to "tune" the colourant and weathering properties of these pigments. For example, it is well known that a coating with dense amorphous silica improves the temperature stability of BiVO 4 pigments in plastics considerably [1]. The over-stoving properties of the new pigment have been significantly improved, based on some recent development, which had already been applied to another pigment ("Sicopal Yellow L 1120") [2]. Temperature stability in powder coatings was enhanced by adjusting just one of the pigment components. A strong correlation was observed between the Al 3+ content in the stabilisation shell or as a dopant in the pigment core and temperature stability in several powder coatings systems [3]. To confirm the stabilising effect of the aluminium component, pigments were synthesised and tested with differing Al content in the pigment core and in the stabilisation shell. These results are displayed in Figure 2. Obviously, the Al content in the stabilisation shell plays an important role in achieving over-stoving stability in powder coatings. In addition, the increased Al content in the BiVO 4 pigment core acts as a stabiliser for the powder coating, as shown in Figure 2d. The surface structure and flow of the prepared powder coatings can be significantly improved by increasing the aluminium content in the pigment. The stabilising effect of aluminium ions was confirmed by over-stoving tests carried out with undoped and doped BiVO 4 pigment samples coated solely with aluminium oxide. The higher Al content in the pigment is thus necessary to achieve higher temperature stability of bismuth vanadate pigments. Colour stability and appearance are improved Powder coatings using the new product "Sicopal Yellow EH 0117" are shown in Figure 3. No coating degradation was observed on the plates coated using the new product, even after the over-stoving under the harder conditions (220 C, 60 min). The coating finishes obtained were characterised
by an outstandingly homogeneous appearance. They are not brittle, exhibit a non-structured surface (no craters) and are bonded hard-and-fast to the substrates. A comparison of the colour changes between plates stoved with different commercially available BiVO 4 pigments at 180 C for 15 minutes and at 210 C for 60 minutes, is given in Table 2. The colour values for the two competitors" products are shown for only one resin system, because those prepared using the PT 910 system showed significant surface degradation of the cured coatings. It can be seen from Table 2 that the magnitude of the change in E (last column) which can be taken as a reasonable measure of the total colour change, is less than 3 units for coatings based on the new pigment, but approximately 19 units for the competitors" samples. A colour shift of 3 units under these conditions is comparable to that noted earlier for a white powder coating. Referring to the other columns in the table, it can also be seen that this colour shift is comprised mainly of a decrease in chroma (loss of colour intensity) and in lightness (darker shade) while the change in overall tone or hue angle is relatively small. REFERENCES [1] US 4 063 956 [2] M. Johansson, O. Seeger, Farbe und Lack, 2/2004, S. 30-33. [3] WO 2006/079616 ACKNOWLEDGEMENTS The authors wish to thank P. John, J. Mohrhardt, Ph. Raff, Th. Siegmund and M. Zech for their contributions to this project. Results at a glance - Bismuth vanadate pigments have excellent hiding power and weathering resistance, and allow many effective 'lead-replacement' colour formulations to be produced. - If the typical stoving schedule for powder coatings (15-20 minutes at 180 C) is exceeded, bismuth vanadates may change colour and cause degradation of the coating, yet many large objects and thick substrates require higher temperatures and longer stoving to achieve full cure. - It is shown that the level of aluminium in a bismuth vanadate pigment is critical to maximise its resistance to over-stoving. - A new pigment has been developed based on this work, which shows very little colour change or coating degradation when stoved for 1 hour at 210 C. The authors: -> Eckhard Korona was born in 1956 and studied chemical engineering for coatings at the Fachhochschule Niederrhein, Krefeld, Germany. He joined BASF AG in 1980 as technical marketing manager for coatings pigments. Today he is involved mainly with pigments for powder coatings. ->Dr.rer.nat. Kirill G. Bramnik was born in 1974 and studied chemistry at Moscow State University, then at the University of Technology, Darmstadt. He joined the R&D department of BASF AG in 2004. He works mainly on inorganic coloured and effect pigments.
Figure 1: Powder coatings in Crylcoat 840/XL 552 in a green RAL shade stoved for 60 min at 210 C, (left) "Sicopal Yellow L1120", (right) competitive sample.
Figure 2: Powder coatings in "Primid XL 552" in a green RAL shade stoved for 60 min at 210 C. Optimised sample with the new pigment, with different Al contents: (upper left) 0.2 %; (upper right) 1.3%; (lower left) 2.02%; (lower right) 2.12%.
Figure 3: Powder coatings in a green RAL shade prepared with "Sicopal Yellow EH 0117" stoved for 60 min at 220 C (left) Crylcoat 840/PT 910 (right) Crylcoat 840/XL 552.
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