The low viscosity and low volatile organic compound (VOC) emission of UV curable polyurethane dispersions make them ideal for spray, curtain and vacuum coater applications on wood. By Dr Marc Heylen; Dr Matteo Vasconi; Jean-Pierre Bleus; Dr Xavier Deruyttere, Cytec IndustriesUV technology had been used in wood coatings since the late 1970s. These applications had been based primarily on 100 percent UV curable products, formulated mainly for roller coater application, suited only for coating flat panels.
With innovative developments, UV technology now offers a broad range of 100 percent UV curable resins and reactive diluents and has become the preferred technology for industrial parquet coatings, providing excellent adhesion and abrasion performances.
The furniture industry needs to coat flat panels, slightly profiled pieces (for example: doors) and 3D pieces such as chairs or assembled furniture. Spray, curtain or vacuum coaters are better suited for these applications than roller coater.
Unfortunately, the low viscosity 100 percent UV curable binders do not perform well in the spray formulations. Many attempts had been made to cut the viscosity with organic solvents with some success. However, the very low VOC emission, a major advantage of UV technology, was lost.
During the 1990s, co-solvent free UV curable polyurethane dispersions in water (UV-PUD) were developed. The flexibility, adhesion and stain resistance of these coatings created new interest. The low viscosity and very low VOC of these dispersions also made them very attractive for spray, curtain and vacuum coater applications on wood.
Indeed, a broad range of UV-PUDs are available for use as primer (adhesion and wood wetting) and for topcoats (good appearance and excellent stain resistance). Binders with good outdoor performance and binders for pigmented and high scratch resistance coatings had been developed.
Properties Of UV-PUD
UV-PUDs are typically low viscosity colloidal dispersions (< 200 mPa.s) with a solid content around 35-40 percent. They can be easily formulated for spray applications. They provide good appearance to wood open pore finishes.
Before UV curing, water needs to be evaporated completely to avoid blisters, whitening of the film and other defects. After water evaporation, the film may be tacky or tack-free depending on the polymer structure. This property is not so essential for the primer coat. For the topcoat, it is preferable to have a tack-free surface in order to avoid dust pick-up and a uniform gloss level.
UV-PUDs designed for primers have excellent wood wetting and provide excellent adhesion, even on tropical wood. In topcoats, it is possible to obtain the required chemical and stain resistance as well as very high scratch resistance or outdoor durability.
Formulation Of UV-PUD
As with all UV formulations, the choice of the photoinitiator is important. A liquid photoinitiator can be more easily dispersed than a solid one. A solid photoinitiator must be dissolved in a suitable solvent before addition to the formulation.
A rheology modifier is necessary, even for low viscosity spray applications, to give ‘cohesion’ to the liquid film. This will help flow and film uniformity. The rheology modifier needs to be pre-dispersed at 50 percent in water, before adding to the formulation step by step.
Wetting and defoaming agents may also be needed. A typical starting point formulation is given below:
| Parts in weight |
UV-PUD (35-40%) | 100.0 |
Matting agent (100%) | 1.5 – 3.0 |
Wax dispersion (35 %) | 2.0 – 3.0 |
Photoinitiator(s) | 1.5 – 2.0 |
Rheology modifier (50%) | 1.0 – 2.0 |
Wetting agent | 0.4 – 0.5 |
Defoamer | 0.1 – 0.2 |
Physical Drying Of UV-PUD
Physical drying is the longest step in the wood finishing process with UV-PUD. A classical drying method is to use air at 35-40°C with a low velocity (0.5-1.0 m/s) for a few minutes in the first stage and then air at 45-50°C with high velocity (jet-dryer).
This drying process is far more efficient when using dry air. One of the most efficient drying processes is to combine microwave water evaporation with jet-dryers.
Infrared technology is suitable only on closed pore wood surfaces but will induce bubbles in open pore finishing systems.
Wood Coating Performances
In terms of how the various UV-PUD formulations perform, the very low VOC emission during physical drying is an essential advantage.
Colloidal stability (tested at least 10 days at 60°C) of the dispersion is another important parameter. It allows dispersion of matting agents and other additives in the formulation without the formation of grit, allowing longer storage of the dispersion and of the formulated product.
Low viscosity applications (spray or curtain coater) are the first perceived advantage of the UV-PUD technology compared to 100 percent UV curable formulations. The application equipment is easier to clean when using products that are re-emulsifiable compared to tack-free products (after water evaporation).
Strong adhesion to wood is achieved due to the fibre swelling and the right balance of molecular design (see Chart 1). For high performances finishes such as parquet (mainly primer) or kitchen cabinets (complete UV-PUD system), high scratch resistance (according to the Erichsen pen or ‘Hamberger’ test) is obtained. Typical values are 12 - 15 N (Erichsen pen) or 25 -30 N (Hamberger).
Wood wetting is important for the surface appearance, especially for furniture finishing. Wood is a living material that may not be transformed to a ‘plastic-like’ material. This is particularly true for dark wood species. It is also possible to combine UV-PUD 1 with other water compatible materials in order to get a better wood wetting.
High levels of stain and micro-scratch resistance can be obtained with the latest developed UV-PUD 6. And in comparison of their properties, while UV-PUD 1 is easier to formulate than UV-PUD 2 and 3, the formulation actually has less chemical and scratch resistance.
On the other hand, UV-PUD 6 combines ease of formulation and exceptional scratch and chemical resistance. It has a high reactivity (35 m/min with 1 Hg lamp of 80 W/cm, compared to 5 m/min for UV-PUDs of the first generation) and is suitable for white pigmented systems. It passes 25 steel wool double rubs (white pigmented film of 110 g/m² wet) while a UV-PUD of the first generation only passes 5 rubs.
Also, a clear coating (120 g/m² wet based on UV-PUD 6) has a Persoz hardness of 320 s compared to 230 s for the first generation of UV-PUDs.
And as Chart 2 shows, the level of chemical resistance (DIN 68 861 – EN 12720) is also very high (no visual stain with NH3, red wine, mustard, alcohol 50 percent, black marker, tar, shoe polish.)
UV-PUDs compare favourably for outdoor wood applications as well. In ageing testing with UV-PUD 5, the European norm EN 927 was used as a guideline. For outdoor exposure, wood requires special protection, especially the wood-coating interface, which is very sensitive to the combined action of UV light and humidity.
The first coats need to impregnate the wood and traditional acrylic emulsions are well suited to this purpose. UV-PUD brings advantages for the topcoat: excellent weathering, chemical and physical properties and good blocking with 150 g/m² (wet) instead of 250 g/m² (wet) for a conventional topcoat (see Chart 3).
By comparison, UV-PUD 5 has a better performance than UV-PUD 1 of the first generation. Chemical, scratch and blocking resistances of both UV-PUDs are superior to conventional water based acrylic.
UV-PUDs cover a very broad range of applications for wood coatings: clear and pigmented, indoor and outdoor. Due to their low viscosity, they can be easily applied by spray or curtain coater on non-flat surfaces.
The existing finishing lines for solvent (if rust resistant) or conventional water based can be easily adapted by adding on UV curing equipment. The very low VOC emission is in accordance with new VOC regulations.
UV-PUDs therefore show exceptional properties for wood coatings such as stain resistance, chemical resistance, nice appearance and very good physical properties.
