Coatings on furniture, flooring and interior wood substrates are the main applications for radiation-curing lacquers. Increasing adhesion is an important goal in developing UV coatings on wood. Very good adhesion results in better whitening performance (coin test) and abrasion resistance.
Guidelines for improving wood adhesion are:
Swells wood fibers with an aqueous UV primer to increase the surface area of the substrate and its interaction with the paint.
Better wetting to oily surfaces such as teak, mahogany or mahogany by using a low surface tension coating.
Chemical crosslinking of coatings and substrates.
"From a theoretical point of view, adhesion is the result of surface forces, wetting, and the action of the coating on the substrate-coating interface during curing."1 The porosity of wood is the main difference compared to other substrates. As metal and plastic. The liquid paint is partially absorbed into the wood by capillary forces. This presents an additional challenge for UV curable coatings, as absorbed material will only be cured if sufficient UV energy is available. Uncured material - especially low molecular weight reactive diluents - can cause sticking and sweat loss of the cured coating.
Wood surfaces are rarely even. Different surface tensions (caused, for example, by oily components from wood) and the inhibitory effect of phenolic components on UV curing have a negative impact on the adhesion of the coating. A further problem may arise from the very fast UV curing reaction, during which a large amount of internal stress builds up in the coating.
Waterborne UV Primer
Proper selection of waterborne UV curing primers solves many of the aforementioned adhesion problems. Water is rapidly absorbed by wood pores and capillaries together with dispersed or dissolved polymers. The fibers swell and are lifted, which increases the substrate surface area. Waterborne UV-curable coatings are available as water-reducible/aqueous resins, emulsions or dispersions of different chemistries2. Figures 1-2 compare the relative properties of emulsions and dispersions. Polyurethane systems achieve very good overall properties.
Polyurethane dispersions are generally prepared by polyaddition of diisocyanates with divalent compounds which react with the isocyanates. Emulsifying groups such as polyethers or ammonium carboxylates (internal emulsifiers) are also incorporated into the polyurethane structure. A key factor in UV curing PUR dispersions is how the acrylic double bonds are incorporated into the polymer. Previously known dispersions were those in which a hydroxy-functional acrylate monomer such as 2-hydroxyethyl acrylate was used as the UV-curing component. Since these acrylate monomers are monofunctional components that prevent the formation of polyurethane polymers, it is difficult to produce high molecular weights with sufficient density of double bonds.
More recent developments move away from the use of monomeric acrylates as double bond components, and instead use oligomeric or polymeric compounds with their own hydroxyl functionality, such as polyesters and epoxy acrylates. Figure 3 shows the structure of a high molecular weight PUR dispersion.
The combination of different components allows the specific design of UV-curable polyurethane dispersions. By changing saturated and unsaturated polymer building blocks, by choosing different diisocyanates, by using monomeric diols or diamines - also monofunctional or polyfunctional building blocks - and by changing the process technology, the polymer's nature. Examples of these properties are physical drying, UV reactivity, hardness and elasticity, glass transition temperature (Tg) and other solid-related properties of paint films.
Urethane acrylates can also be formulated with about 10% water for roller application and are ideal for use on wood (see Figure 4). Here, the amount of water that needs to be squeezed out of the coating during drying is minimized. Figure 5 summarizes the advantages of PUR dispersions, PUR emulsions and water-dilutable urethane acrylates.
Optimized UV Resin
If it is not possible to use aqueous materials for a given coating process, resins with reduced shrinkage and improved wettability during polymerization are required. Figure 6 compares standard polyester acrylates and unsaturated polyesters in DPGDA. With the latter, good adhesion can be achieved - especially with oily woods such as mahogany, teak or mahogany. However, lower double bond density means reduced UV reactivity.
Wood contains many phenolic compounds, which have a similar chemical structure to those commonly used to stabilize acrylate resins. In model experiments, commercial grade hexanediol diacrylate (HDDA) was used to extract shredded teak (3pbw. HDDA, 1pbw. teak). Analysis of the extract by GC-MS revealed a large amount of the expected phenolic species. Based on the above-mentioned unsaturated polyester in DPGDA, different amounts of teak HDDA extracts were used to prepare paints. The UV reactivity of the test formulations was related to the amount of teak HDDA extract added. Only a reduction in reactivity was found with a high amount of teak HDDA extract compared to formulations with standard HDDA.
This simple experiment demonstrates the inhibitory effect of wood extractables. But under conditions closer to the actual coating process, this suppression effect can be estimated to be low.
chemical crosslinking
The chemical cross-linking between the coating and the wood substrate can be achieved with a "dual-cure" formulation. Dual-cure formulations as used here are those that crosslink not only by free-radical photopolymerization but also by an additional mechanism, namely the reaction of isocyanates with highly polar groups. The polar group may be a component of one of the components in the formulation, or may be formed by the effect of atmospheric moisture on the isocyanate. Alternatively, they can be incorporated in the matrix (see Figure 7).
The use of urethane acrylates with free isocyanate groups significantly improves the adhesion of coatings to wood. We performed model experiments to find out exactly how chemical crosslinking takes into account three possible mechanisms.
Moisture from the wood leads to the formation of urea groups at the coating-substrate interface,
2. The aliphatic hydroxyl group of cellulose fiber reacts with isocyanate to obtain aliphatic carbamate group or
3. Phenolic hydroxyl groups, eg aromatic carbamates from lignin.
We applied an aliphatic hexamethylene diisocyanate isocyanurate polymer on wood and, as a control, on glass. Both coated substrates were stored under a dry nitrogen atmosphere to exclude any influence from atmospheric moisture. Infrared spectra were recorded using a probe head that was moved across the coating along the z-axis. After one week, the control on glass showed no change in the IR spectrum. However, after a few hours, the coating on the wood showed a new peak at 1687 cm-1. This occurs for those spectra recorded near (<2 Km) the coated wood interface. The peaks were evaluated as typical for aromatic carbamates by comparison with reference spectra in the literature. Aliphatic urethanes and ureas were not detected in this experiment. therefore,
For dual-cure applications, the range of isocyanate-functional urethane acrylates for outdoor use (Figure 8) is complemented by a new range of low-viscosity products for indoor use (see Figure 9).
Figure 10 gives an example of the application of the parquet coating method. The use of dual-cure adhesion promoters results in a dramatic improvement in adhesion. The disadvantage of primer formulations is the reduced pot life. However, there are two advantages to using isocyanate-modified urethane acrylates instead of just adding polyisocyanates to acrylate resin formulations. Since unmodified polyisocyanates are not reactive under UV irradiation, it reduces the UV reactivity of the formulation. Second, some polyisocyanates have not yet reacted when the topcoat is applied. Thus, polyisocyanates can be extracted and pot life induced on topcoat roller coaters that apply one-component systems.
in conclusion
Adhesion of UV curable coatings to wood can be improved by different methods. Adhesion can be significantly improved by using a water-based UV primer. Dual-cure adhesion promoters can be similarly modified if water is not required as a solvent. Here, isocyanate functional urethane acrylates are the resin of choice. If the pot life of the dual cure system is not acceptable, a 100% resin can be obtained with optimized adhesion properties but lower UV reactivity.
For more information on the adhesion of UV coatings to wood contact Mike Johnson of Bayer Polymers at 412/777.5573.
refer to
1 Cited in H. Pecina, O. Paprzycki, "Lack auf Holz", Vincentz Verlag, Hannover, 1995
.2 In physics, the term emulsion generally describes insoluble liquid oligomers and polymers dispersed in water. The (umbrella) term dispersed cannot draw conclusions about the state of aggregation of the dispersed phase. In the coatings industry, these two terms are often used to describe the state of a film after water has evaporated. Emulsions produce viscous and dispersible non-tacky films.
