Improving Manufacture Efficiency with Water-Based UV-Curable Polyurethanes in Wood Coatings

By Bob Wade and Mike Jeffries, Covestro

High-performance UV-curable coatings have been used in the manufacture of flooring, furniture and cabinets for many years. For most of this time, 100% solids and solvent-based UV-curable coatings have been the dominant technology in this market, but in recent years, water-based UV-curable coatings have become a growing technology. Water-based UV-curable resins have proven to be a useful tool for the manufacturer for a variety of reasons, including passing KCMA stain- and chemical-resistance testing and reducing VOCs. For this technology to continue growing, several drivers have been identified as key areas where improvements need to be made. These will take water-based UV-curable resins beyond the “must haves” that most resins possess and instead will see them begin adding valuable properties to the coating, bringing value to each position along the value chain – from coating formulator to factory applicator, through to the installer and, finally, the owner.

This paper will discuss new developments in water-based UV-curable polyurethanes, which offer much improved 50° C paint stability in clear and pigmented coatings. It also discusses how these resins address the desired attributes of the coating applicator in increasing line speed through fast water release, improved block resistance and solvent resistance off the line, which improves speed for stacking and packing operations. This paper also discusses improvements demonstrated in stain and chemical resistance, which are important to installers and owners.

Background

The expectations for the coatings industry are ever evolving, and the “must have” of just passing the specifications at a reasonable price per applied mil is simply not enough. The landscape for companies that are factory-applying coatings to cabinetry, joinery, flooring and furniture also is changing. Formulators that supply coatings to the factories are being asked to make coatings safer for employees to apply, remove substances of high concern, replace VOCs with water and even provide coatings with less fossil carbon and more bio carbon. The reality is that each customer along the value chain is asking the coating to do more than simply meet specifications.

Seeing an opportunity to create more value for the factory, the authors’ team began to investigate the challenges these applicators were facing at the factory level. After many interviews, the team began to hear some common themes, including:

• Permitting obstacles are preventing expansion goals;
• Costs are increasing and capital budgets are decreasing;
• Costs are increasing for both energy and personnel;
• Experienced employees are being lost;
• It’s challenging to meet corporate SG&A goals, as well as those of the customer; and
• Overseas competition increasingly is a concern.

These themes lead to value proposition statements that resonate with applicators of water-based UV-curable polyurethanes, especially in the joinery and cabinetry market space:

• Manufacturers of joinery and cabinetry are seeking improvements in factory efficiency.
• Manufacturers want the ability to expand production on shorter production lines with less rework damage due to the coatings with slow water-releasing properties.

Table 1 illustrates how, for the manufacturer of coating raw materials, improvements in certain coating attributes and physical properties lead to efficiencies, which can be realized by the end user.

By designing UV-curable polyurethane dispersions (PUDs) with these attributes, end use manufacturers will be able to address the needs they have in improving plant efficiencies. This will allow them to be more competitive and potentially allow them to expand current production.

Experimental Results and Discussion

UV-curable polyurethane dispersions history

In the 1990s, the commercial uses of anionic polyurethane dispersions containing acrylate groups attached to the polymer began to be expanded into industrial applications. ¹ Many of these applications were in packaging, inks and wood coatings. Figure 1 illustrates a generic structure of a UV-curable PUD, showing how these coating raw materials are designed.

UV-curable polyurethane dispersions (UV-curable PUDs, as they often are called) are made up of the typical components used to make polyurethane dispersions: aliphatic diisocyanates are reacted with the typical esters, diols, hydrophilization groups and chain extenders. ² The difference is the addition of an acrylate functional ester, epoxy or ether that is incorporated into the prepolymer step in making the dispersion. Choice of materials used as building blocks, as well as polymer architecture and processing, dictate a PUDs’ performance and drying characteristics. These choices in raw materials and processing will lead to UV-curable PUDs that can be non-film-forming, as well as those which are film-forming. ³ The film-forming, or drying, types are the subject of this paper.

Film forming will yield coalesced films, which are dry to the touch before UV curing. Because applicators wish to limit airborne contamination of the coating due to particulates, as well as needing speed in the production process, these often are dried in ovens as part of a continuous process prior to UV curing.

Spray application methods most typically are used; however, knife-over-roll and flood coat have been used. Once applied, the coating usually will go through a four-step process before it is handled again.

1. Flash: This can be done at room or elevated temperatures for several seconds to a couple of minutes.
2. Oven Dry: This is where the water and co-solvents are driven out of the coating. This step is critical and usually consumes the most time in a process. This step is typically performed at >140° F and lasts for up to eight minutes. Multi-zoned drying ovens also may be utilized.
   • IR lamp and air movement: Installation of IR lamps and air movement fans will accelerate the water flash process.
3. UV Cure
4. Cool: Once cured, the coating will need to cure for some amount of time to achieve blocking resistance. This step may take as long as 10 minutes before blocking resistance is achieved

Experimental

This study compared two UV-curable PUDs currently used in the cabinet and joinery market to the authoring company’s new development, PUD # 65215A. In this study, Standard #1 and Standard #2 were compared to PUD #65215A (hereafter referred to as Development #1) in drying, blocking and chemical resistance. Viscosity stability and pH stability also were evaluated, which can be critical when considering reuse of overspray and shelf life. The physical properties of each resin are shown in Table 2. All three systems were formulated to similar photoinitiator levels, VOCs and solids levels. All three resins were formulated with 3% co-solvent.

In the interviews conducted by the team, responses indicated that most water-based UV coatings in the joinery and cabinetry markets dry on a production line, which takes between 5-8 minutes before UV cure. By contrast, a solvent-based UV line dries between 3-5 minutes. In addition, for this market, coatings typically are applied 4-5 mils wet. A major drawback for waterborne UV-curable coatings, when compared to UV-curable solvent-based alternatives, has been the time it takes to flash water on a production line. ⁴ Film defects, such as white spotting, will occur if water has not been properly flashed from the coating before UV cure. This also can occur if the wet film thickness is too high. These white spots are created when water becomes trapped inside the film during UV cure. ⁵

For this study, a curing schedule similar to one that would be utilized on a UV-curable solvent-based line was chosen. Figure 2 shows the application, drying, curing and packaging schedule used for the study. This drying schedule represents a 25% to 31% improvement in drying time over the current market standard in joinery and cabinetry applications.

The application and curing conditions used for the study are as follows:

• Spray application over maple veneer with a black basecoat.
• 30-second room temperature flash
• 140° F drying oven for 2.5 minutes – convection oven
• UV cure – Intensity about 800 millijoules/cm²
  • Clear coatings were cured using a Hg lamp
  • Pigmented coatings were cured using a combination Hg/Ga lamp
• 1-minute cool down before stacking

For the study, three different wet film thicknesses were sprayed to see if other advantages such as fewer coats, also would be realized. Wet coating applications of 6 mils and 8 mils were included, as well as the typical 4 mils wet for water-based UV.

Curing Results

Results for Standard #1, a high-gloss clear coating, are shown in Figure 3. The water-based UV clear coating was applied to medium-dense fiberboard (mdf) previously coated with a black basecoat and cured according to the schedule shown in Figure 2. At 4 mils wet, the coating passes. However, at 6 and 8 mils wet application, the coating cracked, and 8 mils easily was removed due to poor water release before UV curing. A similar result also is seen in Standard #2, shown in Figure 4.

Shown in Figure 5, using the same curing schedule, Development #1 demonstrated significant improvement in water release/drying. At 8 mils wet film thickness, slight cracking was observed on the lower edge of the sample.

Additional testing of Development #1 in a low-gloss clear coating and pigmented coating over the same mdf with a black basecoat was done to evaluate water-release characteristics in other typical coating formulations. As shown in Figure 6, the low-gloss formulation at 5 and 7 mils wet application released the water and formed a good film. However, at 10 mils wet, it was too thick to release the water under the drying and curing schedule shown in Figure 2.

In a white pigmented formula, Development #1 performed well in the same drying and curing schedule described in Figure 2, except when applied at 8 wet mils. As shown in Figure 7, the film cracks at 8 mils due to poor water release. Overall, in clear, low-gloss and pigmented formulations, Development #1 performed well in film formations and drying when applied up to 7 mils wet and cured at the accelerated drying and curing schedule described in Figure 2.

Blocking Results

Blocking resistance is a coating’s ability not to stick to another coated article when stacked. In manufacturing, this often is a bottleneck if it takes time for a cured coating to achieve block resistance. For this study, pigmented formulations of Standard #1 and Development #1 were applied to glass at 5 wet mils using a drawdown bar. These each were cured according to the curing schedule in Figure 2. Two coated glass panels were cured at the same time, and the panels were clamped together four minutes after cure. They remained clamped together at room temperature for 24 hours. If the panels easily were separated without imprint or damage to the coated panels, then the test was considered a pass. Figure 8 illustrates the improved blocking resistance of Development #1.

Acrylic Blending Results

Coating manufacturers often blend water-based UV-curable resins with acrylics to lower cost. For this study, Development #1 was blended with NeoCryl® XK-12 (NeoCryl is a registered trademark of the Covestro group), a water-based acrylic often used as a blending partner for UV-curable water-based PUDs in the joinery and cabinetry market. For this market, KCMA stain testing is considered the standard. Depending on the end-use application, some chemicals will become more important than others for the manufacturer of the coated article. A rating of 5 is the best and a rating of 1 is considered the worst.

As shown in Table 3, Development #1 performs very well in KCMA stain testing as a high-gloss clear, low-gloss clear and a pigmented coating. Even when blended 1:1 with an acrylic, the KCMA stain testing is not drastically affected; and the coating recovered to an acceptable level after 24 hours even in staining with agents such as mustard.

In addition to KCMA stain testing, manufacturers also will test for cure immediately after UV curing off the line. Often, the effects of acrylic blending will be noticed immediately off the curing line in this test. The expectation is to not to have coating breakthrough after 20 isopropyl alcohol double rubs (20 IPA dr). Samples are tested one minute after UV cure. In the team’s testing, a 1:1 blend of Development #1 with an acrylic did not pass this test. However, Development #1 could be blended with 25% NeoCryl® XK-12 acrylic and pass the 20 IPA dr test.

Resin Stability

The stability of Development #1 also was tested. A formulation is considered shelf stable if, after four weeks at 40° C, the pH does not drop below 7.0 and the viscosity remains stable when compared to the initial. For the testing,  the samples were subjected to the harsher conditions of up to six weeks at 50° C. Under these conditions, Standard #1 and #2 were not stable.

High-gloss clear, low-gloss clear and low-gloss pigmented formulations were used in this study. The pH stability of all three formulations remained stable and above the 7.0 pH threshold, and a minimal viscosity change was observed after six weeks at 50° C.

Another test demonstrating stability performance of Development #1 was to test again the KCMA stain resistance of a coating formulation that had been aged for six weeks at 50° C and compare that to its initial KCMA stain resistance. Coatings that do not exhibit good stability will see drops in staining performance. As shown in Figure 9, Development #1 maintained the same level of performance as it did in the initial chemical/stain resistance testing of the pigmented coating shown in Table 3.

Continuing Evaluations

Because of the improvements seen in factory-applied wood applications, the team also began investigating to see if Development #1 could be useful in other applications, such as factory-applied plastic coating. Here, formulations using Development #1 were a drawdown using a 48-wire bar. The coating was allowed to dry at room temperature overnight before UV curing the next day. As shown in Figure 10, Development #1 had good adhesion to polycarbonate (PC), as well as to T-85. Blending with a nonfunctional acrylic hybrid allowed it to achieve adhesion to ABS as well.

Automotive “Sun and Bug” testing also was completed. Here, slight wrinkling and down-glossing occurred across all samples; no softening was observed.

Another area of interest is for UV-curable site-applied flooring applications. In this application, the ability to release water at room temperature is a key factor in determining how quickly a job can be completed. Current products available today for water-based UV-curable site-applied flooring dry in 2-4 hours before UV curing. In the testing, Development #1 will dry in 1-2 hours in ambient conditions. This would represent a 100% improvement in drying compared to what is available today.

Conclusion

For applicators of UV-curable water-based coatings, Development #1 will enable them to meet the current performance standards in the joinery, wood and cabinet markets. In addition, it will enable line-speed improvements in the coating process greater than 50-60% over current standard UV-curable water-based coatings. For the applicator, this may mean faster production; increased film thickness, which reduces the need for additional coats; shorter drying lines; energy savings due to reduced drying needs; less scrap because of fast blocking resistance; and reduced coating waste due to resin stability.

With VOCs less than 100 g/l, manufacturers also are more able to meet their VOC targets. For manufacturers that may be having expansion worries because of permit issues, the fast water release of Development #1 will enable them to more easily meet regulatory obligations without performance sacrifices.

In the beginning of this article, interviews with applicators of solvent-based UV-curable materials indicated they typically would dry and cure coatings in a process that took 3-5 minutes. This study demonstrated that, according to the process shown in Figure 2, Development #1 will cure up to 7 mils wet film thicknesses in four minutes with an oven temperature of 140° C. This is well within the window of most solvent-based UV-curable coatings. This potentially could enable current applicators of solvent-based UV-curable materials to switch to a water-based UV-curable material with little change to their coating line.

For manufacturers considering production expansion, coatings based on Development #1 will enable them to save money through the use of a shorter water-based coating line, resulting in a smaller coating line footprint in the facility; have a reduced impact on current VOC permits; and see energy savings due to reduced drying needs. In conclusion, Development #1 will help to improve the manufacturing efficiency of UV-curable coating lines through high physical-property performance and fast water-releasing characteristics of the resin when dried at 140° C.

References

1. Meier-Westhues, Danielmeier, Kruppa, Squiller; “Polyurethane Coatings Adhesives and Sealants,” 2^nd Revised Edition, Hanover: Vincentz Network 2019; p 151.
2. EP-B 753 351, Bayer MaterialScience AG
3. Meier-Westhues, Danielmeier, Kruppa, Squiller; “Polyurethane Coatings Adhesives and Sealants,” 2^nd Revised Edition, Hanover: Vincentz Network 2019; p 131.
4. Robert Wade, Bayer Material Science, “Wood Furniture Coatings Market and Technology Options,” BIFMA Conference, 2010.
5. Lawrence C. Van Iseghem, “Wood Finishing with UV-Curable Coatings,” RadTech Report, May/June 2006 Issue; p 35.