UV-Curable Powder Coatings on Wood Benefits and Performance
Both technological and economic benefits explain the growing interest in the technology. Ease of handling of powder coating formulations, spray application of a powder to easily coat complex shapes, fast heating and curing process (limited space requirements) are some of the typical technological advantages.3 Material usage of 95%, high productivity, low energy use, reduced labor and maintenance costs contribute to the economic attractiveness of UV powder coating technology. Finally, UV powders can be considered as an ideal technology from the viewpoint of environmental friendliness. No volatiles are present in a powder formulation, or are generated during the coating process, meaning that it is an excellent answer to increasingly stringent environmental legislation.
UCB Chemicals has developed special resins and formulations giving excellent performance when used on wood-based substrates, including MDF and natural hardwood. The properties of some new systems developed recently will be described.
Finishing of MDFThe two most widely used finishing techniques for MDF boards are:
A range of surface appearances and qualities can be obtained with these technologies. However, there can be some drawbacks with both liquid coating systems and overlays, which can be overcome by use of UV powders.
When using liquid coatings, normally a multi-coat system with interlayer sanding is required to achieve attractive finishes. This means that labor, maintenance, energy and space requirements are important cost factors. Also, depending on the type of liquid coating system used, waste disposal costs can be quite high. Environmental pressures will oblige companies to replace conventional solventborne paints by alternatives in the near future.
Paper-based laminates are characterized by their excellent surface properties such as wear, heat and chemical resistance. The development of new presses allows the quick and efficient lamination of panels. However, for the finishing of profiled or shaped MDF boards, the use of this type of laminate is excluded because of lack of flexibility.
The use of thermoplastic foil, however, can overcome this problem. The resistance of these types of overlay is considered to be lower than that of paper-based laminates and the cost of the better qualities is known to be quite high. The costs of the glue and of the waste produced by non-matching foils should also be taken into account.
UV Powder on Wood: BenefitsUV powder coating technology offers a fast, clean and economic attractive method to achieve high-quality finishes on wood-based substrates.
The coating process consists of the following steps: First the article is hung or placed onto a conveyor belt and the powder is sprayed electrostatically onto the object. Then the coated object enters the oven (temperatures of 90-140 degC are sufficient) where the powder melts and flows together to form a film. This step takes 30-150 seconds, depending on the desired finish. The molten film is finally cured in a few seconds by irradiating with UV light.
Using this new concept results in a combination of interesting benefits.
Substrate PreparationTo achieve good results with powder coatings on MDF, some properties of the board are of high importance.
For applications on natural hardwood, the surface is smoothly sanded prior to coating. The types of wood tested were: beech, ash, oak, walnut and sapele.
Resins / FormulationsUCB Chemicals has developed two new resins for applications on wood-based substrates: UvecoatTM 3000 is an amorphous resin; Uvecoat 9010 is a semi-crystalline resin. Some typical formulations based on these products are shown in Table 1.
Extrusion and GrindingSystems based on Uvecoat 3000 only (Formulation A, B):
Uvecoat-Based UV Powders: Coating Process SprayingThe powders can be sprayed on most MDF types using conventional corona or tribo charging guns. In the latter case, the addition of a tribo additive to the formulation is recommended. Applied layer thickness is between 35 and 100 Km, depending on the type of application. Pre-heating of the board can aid powder application in some cases.
HeatingTo melt the applied powder efficiently, medium wave infrared radiation (IRM) is used. Further flow out is achieved by using a combination of IRM and convection heat. When coating shaped MDF substrates, the use of convection is specifically recommended to achieve uniform warming-up of the entire panel surface. A typical heating cycle used to melt/flow out formulations A-C is represented in Figure 1. The curve represents the temperature measured at the surface of the non-coated MDF board.
CuringThe above formulations, both clear and pigmented, can be cured with normal mercury vapor lamps. For our tests, two electrodeless lamps of 160 W/cm each irradiate the molten film, passing under the lamps at a line speed of 4-9 m/min. For clear formulations, complete cure can be achieved with a UV dose of 800-1,000 mJ/cm2 (UVA); for pigmented systems, a dose of 2,000 mJ/cm2 (UVA) is recommended.
Possibilities and Coating Performance Textured Finish on MDFFor both furniture and building applications, texture finishing foils are commonly used on wood-composite substrates. Comparable finishes can be obtained when using UV-curable powder coatings such as formulation A or B. One of the advantages of using the latter approach is the high flexibility in terms of coating aspects that can be achieved.
A nice aspect can be obtained with only one powder layer on most types of flat MDF substrates. When the rounded edges of the board have to be coated in one and the same step, the use of MDF with a sufficiently high density in the bulk of the board and sanding of the rougher edges is recommended.
For certain applications such as for kitchen furniture, the surface quality of the coating is of major importance. An overview of the tests done on a textured coating based on formulation A, applied at a thickness of 80-90 Km, is given below.
Coating PerformanceTo assess the quality of the achieved finish, a standardized test was used: Deutsche Industry Norm (DIN) 68861. According to their performance, the different systems can be divided in different groups; coatings/ laminates classified in Group A show the best performance; coatings/laminates classified in Group D-F show worse performance.
For each test, the minimum requirements for high pressure laminate (HPL) finished panels for kitchen applications (both working surface and other surface) are given. This can be considered as a reference of the UV powder coated surfaces tested below.
Chemical resistance was tested according to DIN 68861, Part 1.To assess chemical resistance, the chemical agent is placed upon the surface of the test specimen and covered with a glass. The prepared test specimen is then allowed to stand undisturbed for 16 hours (Group A) or the time indicated (Group B). After exposure, the surface is cleaned and evaluated. The most interesting chemicals that were tested are shown in Table 2 (in total, 26 chemicals were tested).
From these results, it is clear that formulations based on Uvecoat 3000 exhibit excellent chemical resistance; the only solvent damaging the surface, after an exposure of 16 hours, was acetone.
Abrasion Resistance was tested according to DIN 68861, Part 2.This standard specifies a method for determining the resistance to surface abrasion by using the Taber abrader (Model 5151). Abrasion is achieved by rotating a test piece in contact with a pair of loaded cylindrical wheels covered with abrasive paper (S33). The number of revolutions required by the test piece to cause a defined degree of abrasion (in our test, this is when 50% of the MDF becomes visible) is used as a measure of resistance to surface abrasion. Every 25 rotations, the test piece is examined for wear and the abrasive paper for clogging with abraded particles.
A textured finish UV powder coating fulfils the requirements of the kitchen industry standards, including working surface (Table 3).
Scratch Resistance was tested according to DIN 68861, Part 4.For this test, a Universal Scratch Tester Model 413 (Erichsen) was used. The minimum weight power that causes the formation of a scratch on the surface is measured.
A textured finish UV powder fulfils the requirements of the kitchen industry standards, including working surface (Table 4).
Heat Resistance was tested according to DIN 68861, Part 7 (Dry Heat) and Part 8 (Wet Heat).A heavy cone of AlMgSi is placed in an oven at a certain set temperature. Once the object has reached its temperature, it is placed on the coating surface for 20 minutes. For the wet heat resistance test, a filter paper soaked in deionized water is situated between the heated object and the coating surface. With the naked eye, the formation of blisters, the matting and discoloration of the surface, and the deformation of the surface are evaluated (Table 5).
A textured finish UV powder fulfils the requirements of the kitchen industry standards for surfaces other than the working surface.
Apart from the white and black formulations A and B detailed earlier in this paper, Uvecoat 3000 can be formulated in other attractive shades; some basic bright colored formulations are available on request. Typical application areas for these textured finishes are furniture (bookcases, TV/Hi-fi cabinets, kitchen & bathroom doors, etc.) and decorative panels for the building industry.
Clear Finishes on Paper-Laminated MDFAnother interesting application is the use of clear UV powders on wood-composite panels laminated with wood-grain d?r paper. The resulting coatings form an excellent protective layer at low coating thickness and the new approach as a whole can be considered as a replacement for thermoplastic foils showing the same type of wood grain effect. The powder layer thickness applied normally ranges between 30 and 50Km.
While a formulation of type C, containing 0-2% matting additive, results in a high to semi-gloss finish with good flow, the clearcoats obtained by melting and curing formulation C, containing 3-6% matting/texture additive, give matt, fine textured coatings.
Coating PerformanceThe performance of these clearcoats, in terms of chemical, scratch, abrasion and heat resistance, is very similar to those for the white textured finish described previously.
Clear Finishes on Natural Hardwood/ Veneered Composite BoardThe possibility of using UV-curable powder coatings for the coating of natural wood or veneered composite board has been a point of discussion during the last couple of years. The development of the resin combination (Uvecoat 3000 and 9010) described in this paper allows us to lower the melting temperature considerably. When using the heating cycle shown in Figure 2 (40 seconds, temperature stabilizing at 90-100 degC), formulations of type C can melt and flow-out properly and result in attractive matt and semi-gloss finishes. The preferred layer thickness is 30-40 Km; depending on the final properties required, a second layer of 30-60 Km can be applied on the smoothly sanded first layer giving smoother finishes with improved abrasion resistance.
This approach was successfully tested on different types of hardwood veneers such as beech, ash, oak and walnut.
ConclusionBoth technological and economic benefits explain the growing interest in UV powder coating technology on wood composite substrates.
The development of new resins and formulations can further increase the attractiveness due to improved coating surface properties and increased application possibilities. The use of new Uvecoat 3000 and 9010 means it is possible to obtain the following.
The author would like to thank the Institut f?r Holztechnology Dresden (IHD) and L. Boogaerts for their contribution to this paper.
This article was originally presented at the 6th Nurnberg Congress, April 2001 in Germany.
For more information on powder coatings on wood, contact UCB Chemicals, Anderlechtstraat 33, B-1620 Drogenbos, Belgium; phone +32 (0)2 3345637; fax +32 (0)2 3783944; e-mail firstname.lastname@example.org; or Circle Number 83.
References1 Mihalic, S.D.; Mills, P. Radtech Report, July/August 1999, p 26-30.
2 J. Bailey. Industrial Paint & Powder, June 2000, p 30-34.
3 Zune, C.; Buysens, K. European Coatings J., May 2000, p 18-30.
4 J. Kiene, Woodcoatings Congress The Hague, October 2000, paper 15.