2K water-borne PU for furniture coatings Water-borne 2K polyurethane systems are a new technology which is already successfully used in many applications also for wood coatings and more specifically furniture finishes, without having to sacrifice quality. Some properties like the ease of introduction by hand of the hardener and the determination of the pot-life through the viscosity increase can be optimised by choosing the right solvent to dilute the polyisocyanate. C. Varron, V. Granier, C. Kwee The application of a lacquer or paint is the ultimate protection of wood. Sustrate preparation, like in any paint application, is essential to ensure good adhesion and durability of the final coating. Polyurethane-based coatings have an established place in the coating industry because of their very high level of quality. Polyurethanes come from the reaction between a hydroxylated polymer and an aliphatic or aromatic isocyanates, the latter being used when yellowing due to UV exposure is not a drawback. 2K water-borne PU As furniture coatings are applied indoors, the main requirements include low emission of solvents during application and drying for health and safety reasons. Good resistance to chemicals and durability are also important. In addition to these film performances, the user requires an easy homogenisation of the hardener and binder (in case of a two-component system), and a good balance between pot-life and drying time. In the case of light-coloured wood, low yellowing of the film is also required to preserve the natural beauty of the substrate. Water-borne coatings offer new challenges to formulate properly, as: - they are complex fluids, due to the fact that entities are often dispersed into water in order to optimise water dispersibility and water sensitivity, and not any longer simple solutions - system compatibility is more difficult to optimise, when compared to solvent-borne coatings - side reactions may occur, due to the fact that the solvent (water) may react with polyisocyanates - they raise new issues and require new working habits, like every technological change does. But as always, everything is just a matter of time and training and in recent years, the market share of waterborne systems has grown very fast due to their favourable properties such as mild odour, ease of application and fast drying for example. Chemical reactions of polyisocyanates The reaction between the hydroxyl groups of the polyol and the isocyanate (reaction 1 in Figure 1) is predominant to form the polyurethane network. But secondary reactions also occur in the system, especially the reaction between isocyanate and water (reaction 2a in Figure 1). The reaction with water plays an important role, as it affects film properties because of carbon dioxide release and urea bond formation (reaction 2b). NCO/OH ratio between 1.2 and 1.8 gives good properties The reaction of organic acid groups with isocyanates (reaction 1 in Figure 2) needs to be mentioned because it destroys the hydrophilic nature of the polyol (if it contains acid stabilising groups) and contributes to carbon dioxide formation. Other side reactions (Figure 2) are less important as they play a far less significant role in the crosslinking and properties of the coating. The relative reaction rates of functional groups with aliphatic isocyanates are: Amine (primary/secondary)>hydroxyl>water>urea>urethane>>carbo xylic [1] This reactivity scale explains why it is possible to use aliphatic polyisocyanates to formulate water-borne coatings with an acceptable pot-life. In order to compensate for these side reactions, the NCO to OH ratio of water-borne formulations has to be higher than 1, as it is normally not the case for solvent-borne coatings. If the NCO/OH ratio is too low, then the polyol may not be fully crosslinked, thus resulting in poor properties. Overall ratio varies between 1.2 and 3, depending on the polyol and polyisocyanate used. Normal ratios stay between 1.2 and 1.8. View of hydrophobic polyisocyanates Aromatic polyisocyanates must be avoided because of their high reactivity with water, therefore it will be focus on aliphatic polyisocyanates, and more specifically HDI-trimer derivatives. There are two types of polyisocyanates which can be used in water-borne formulations. The first approach is to use conventional poly- isocyanates which were developed for solvent-borne systems, and so in theory are not suitable for use in aqueous systems. These products are hydrophobic in nature, but in some cases, depending on the polyol, good performances can be achieved. Isocyanurate trimers of hexamethylene di-isocyanate such as "Tolonate HDT-LV" or "HDT-LV2" are normally used, due to their lower intrinsic viscosity compared to biurets. In such a case, water-soluble polyols and some dispersions are hydrophilic enough to act as dispersing agent for the hydrophobic polyisocyanate. In order to achieve a good introduction of the polyisocyanate into such a system, a low viscosity of the polyisocyanate is highly recommended. Water-compatible solvent can be used to reduce the polyisocyanate viscosity [2]. It is also possible to use high shear equipment to introduce these hydrophobic polyisocyanates into the aqueous phase. View of hydrophilic polyisocyanates The second approach is to modify the polyisocyanate in a way that it becomes hydrophilic and thus water-dispersible under low shear mixing conditions or self-emulsifying. The most common way to increase the hydrophilicity of a polyisocyanate is to graft hydrophilic polyether groups [3, 4], and thus to lower the functionality of the polyisocyanate. Such hydrophilic groups can either be non-ionic or ionic if terminated by a specific group which can be dissociated above a specific ph [5]. Another way to produce hydrophilic polyisocyanates is to formulate them with NCO-non reactive component able to render them water-dispersible. "Rhodocoat WT2104" is a water-dispersible isocyanate based on HDI-trimer with 19% NCO, at 100% solids. Hardener easy to use
The hardener has to be easily dispersible in the binder. This can be obtained by using self water-dispersible polyisocyanate crosslinkers. The major advantage of these new products is clearly their ease to use, as they will not request high shear mixing. Figure 3 illustrates the difference in dispersibility between hydrophobic and hydrophilic polyisocyanates. The left-hand picture shows a hydrophilic isocyanate which has been incorporated into an aqueous medium containing a polyacrylic polyol. The dispersion of the polyisocyanate has been done by simple shaking. The droplets have a diameter lower than half a micron and the particle size is evenly distributed. The right hand side shows the distribution of droplets of a low viscosity hydrophobic polyisocyanate, mechanically introduced into the same polyol. In this case the particle diameter is about 2-3 microns. Solvents assit gloss, chemical resistance and hardness In order to satisfy wood coatings requirements, polyols can be polyacrylic dispersions, emulsions,polyacrylate-grafted polyurethane dispersions or polyurethane dispersions or combinations of these. The choice of one polyol or of a mixture of polyols is in fact imposed by the balance between the level of performance of the film and the cost of the formulation. Why using a solvent in the water-borne formulation? The use of a solvent to pre-dilute a hydrophilic isocyanate is not necessary, however is recommended to reduce the hardener's viscosity so that it is closer to the polyol's, and is therefore dispersed easily into the polyol. Other advantages have been noticed, such as higher gloss, better chemical resistance and hardness. The choice of dilution solvent is important as the solvent much be free of hydroxylic or carboxylic groups, suceptible of reacting with isocyanate, allowing good storage stability. The selection of the dilution solvent is made by measuring the particle size of the pre-diluted isocyanate in water; one particle size is necessary and the smaller the particle, the better the dispersion in a water-borne formulation. Table 1 shows a large choice of solvents and that a wide range of dilution ratios can be used, provided the solvents are urethane grades. NMP is not suitable as it causes swelling of the particles in water. The dilution solvent can also influence the coating's properties, as our experiments showed. Choice of suitable solvent Table 2 shows a typical matt furniture clear-coat formulation based on "Worleecryl 7816" (from Worlée Chemie); this resin is a hydroxylated acrylic dispersion, which was obtained from a solvent-borne resin, dispersed into water using emulsifiers. In this case, several solvents are suitable to pre-dilute the isocyanate; the choice was made by stirring the prediluted isocyanate into the water-based clear-coat with a spatula (hand mixing). The clear-coat is then applied onto glass panels and gloss is measured once the coating is dry. Results are displayed in Table 3. In this clear-ocat formulation, Persoz hardness is influenced by the type of solvent used to dilute the hardener; here, EEP and MPA give suitable results for furniture coatings requirements. Figure 4 shows that once the clear-coat is touch dry (a few hours), it takes several days for the polyurethane coating to be fully cured at room temperature (23 C, 55%RH). Chemical resistance a key point of wood finishes performance The nature of the solvent used to dilute the hydrophilic isocyanate has no effect on the level of chemical resistance of the clearcoat, which gives more flexibility to adjust the formulation to some specific needs. Various chemicals which can be found for example in a kitchen, are left on the clear-coat following DIN 68 861. All the samples show a excellent chemical resistance, except with coffee. More tests were conducted on a 2K water-borne polyurethane based on "Neocryl XK101" (from Avecia NeoResins); this time, the polyol is a hydroxylated acrylic emulsion, which can be cross-linked with "Rhodocoat WT2104" prediluted in MPA. Refer to Table 4 for starting formulation. The hardener is easily dispersed into the polyol phase, giving excellent levelling and high gloss. Then the formulation can be used for several hours and its viscosity may vary or not. In this case, the water-borne system behaves like a solvent-borne one, as the pot-life can be measured as doubling of the initial viscosity. Table 5 shows the various charateristics of the clear-coat applied on glass. The clear-coat was then applied onto maple wood panels. Two coats were applied and dried at room temperature, chemical resistance tests were perfomed as described previously. Figure 5 shows the results of the chemical resistance test performed according to the DIN 68 861 standard. Rating is : 0 (excellent) to 5 (very poor). The resistance to most chemicals is excellent, except for some solvents. References [1] C. R. Hegedus, D. C. Lawson and D. L. Lindenmuth, Two-component waterborne polyurethane coatings: chemistry and application, Proc. Of the International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans, Feb. 1998 [2] E. Brinkman, M. Van Wingerde, Two-component PU paints: a comparison between solvent and waterborne, Proc. 3rd Nürnberg Congress, 1995, Paper 37 [3] L. Kahl, M. Bock, Waterborne two-component PU clear coats for automotive coatings: development of raw materials and mixing technology, Proc. 3rd Nürnberg Congress, 1995, Paper 7 [4] US patent 5252696, Oct. 1993 [5] US Patent 5583176, Dec. 1996 Result at a glance Two examples of formulations show that aqueous 2K PU coatings can be used for wood coatings, without having to sacrifice quality. By choosing the right polyol, a requested balance between cost and performance can be reached. Some properties like the ease of introduction by hand of the hardener and the determination of the pot-life through the viscosity increase can be optimised by choosing the right solvent to dilute the polyisocyanate. The authors: -> Corinne Varron, Rhodia, graduated from the E.N.S.S.S.P.I.C.A.M. in organic chemistry in 1994. In 1995 she joined Rhodia in Manchester. She moved to Rhodia's research center in Lyon/France dealing with application & development on aliphatic isocyanates, position she is currently occupying. -> Vincent Granier, Rhodia, graduated from the Ecole Normale Supérieure de St Cloud (France) in chemistry in 1986. He passed his PhD at Bordeaux University in physico-chemistry in 1989. He joined Rhodia in Aubervilliers/France in1990. Since 2001 he is also World Wide Technical Manager for aliphatic isocyanates.
Figure 1: Reaction of isocyanate groups with hydroxy-functions.
Figure 2: Further possible reaction of isocyanate groups in water-borne systems.
Quelle/Publication: European Coatings Journal 06/2003 Ausgabe/Issue: 36 Seite/Page: Figure 3: Comparison under an optical microscope (x 400).
Figure 4: Persoz hardness developement.
Figure 5: Chemical resistance of the clearcoats.
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