By WC Peter Tsang and Christopher A. Baumgart, Red Spot Paint and Varnish Company, Inc.
Dual-cure coatings once were perceived to deliver the best combined performance from both thermal and UV-cured paints. However, such technology comes with a multitude of application challenges typically addressed with unsustainable reworking. Some dual-cure paints also are extremely line-specific. The next-generation, single-component, UV monocoat provides a high-gloss jet-black automotive interior finish that rivals the performance of traditionally thermally cured basecoat/clearcoat systems.
Introduction
Coatings are used in almost every part of a transportation vehicle. From personal sedans and SUVs to commercial trucks, coatings applied by Original Equipment Manufacturers (OEMs) need to be durable and resistant to expected elements they would be exposed to, whether they are part of the interior or the exterior of a vehicle. In many cases, they also serve decorative and functional purposes that enhance the overall sales value. In order to ensure the paints applied on new vehicles meet all the stringent requirements, automotive paints need to pass OEM specification requirements.
Thermally cured technologies have dominated automotive finishes for over 70 years. At least part of this dominance can be attributed to their lower cost of ownership, lower processing complexity and ease of incorporation of multiple classes of pigments while meeting all OEM specifications. As scratch- and chemical-resistance requirements become more demanding, it was envisioned that some variations of UV coatings would be able to fill market needs. UV coatings with higher crosslink density contribute to better performance in both scratch and chemical resistance.
Dual-cure coatings once were recognized to yield a good compromise from both thermally cured and UV-cured paints. The former cures with no line-of-sight challenges, while the latter provides improved gloss, abrasion and chemical resistance. However, current state-of-the-art dual-cure products in the market appear to suffer from process complexities that could result in an incompletely cured part.
If the line process is not fully optimized, some encountered issues could be attributed to the very nature of dual cure, where one mode of curing takes place before the other, which could transiently generate a cured material matrix and “lock up” components to be cured subsequently. The film-forming complexity and challenges that come along with it could manifest in the form of a sticky surface, poor appearance or adhesion failure. Such challenges typically are addressed by passing the parts through the curing process a second time, which rarely is sustainable in manufacturing. With a dual-cure system, two different, competing cure mechanisms need to be precisely managed. Any deviation from optimal process conditions could result in less than desirable coating cure and performance. This narrows the process window considerably when applying these systems. As dual-cure coatings gradually phased into the market, it also was realized that the long-standing assumption that either one of the two modes of curing is sufficient in delivering quality coated surfaces is not quite valid. The search for a less complex technology that meets OEM specifications became an important product development objective.
Pigmented UV Coatings
While it is reasonable to assume that heavily pigmented coatings with good hide properties would be challenging to fully cure by UV, especially in cases where dry film thickness is expected to be 20 µm or higher, there were many examples of properly cured pigmented UV coatings. For example, pigmented UV topcoats were applied over a metal finish from physical vapor deposition to generate a luxurious satin or copper-alloy appearance on plastic substrates in the packaging industry. The advent of thermal tri-coats also demonstrated that color positions could be attained where individual component coating does not need to be self-sufficient in delivering the full-color position or hide.
As proof of concept, the authors developed a one-component (1K) black UV coating and applied it on a conventional black plastic substrate. The result is a high-gloss jet-black appearance comparable to thermally cured high-gloss black coatings at comparable film thickness. Coated panels were evaluated in selected, more challenging automotive interior specification testing, benchmarking against unfortified thermal basecoat/clearcoat and dual-cure systems.
As summarized in Table 1, the high-gloss jet-black UV coating passed scratch-, moisture- and selected chemical-resistance testing. The occasional fails observed from the dual-cure system probably were due to the different levels of cure attained. Both systems with UV cure performed better than the fully thermal basecoat/clearcoat system.
Merits of Black UV Coating
Less complicated curing mechanism and more robust performance aside, the next-generation universal black UV coating could be applied and processed with greater ease, smaller equipment footprint, shorter cycle time and lower overall energy consumption compared to current dual-cure products in the market.
Regardless of the sequence in curing, one part of the cure cycle for typical two-component (2K) dual-cure coatings requires a 30-minute oven bake at a minimum of 180° F (see Figure 2), which is not needed for the UV monocoat (see Figure 3). As a one-component monocoat, no pre-application mixing of moisture-sensitive hardener is required.
Further optimization delivered a black UV monocoat with excellent distinctness of image, no orange-peel concerns and a wide process window in film thickness. Painted panels received the same level of color approval for Universal Black typically achieved by thermally cured basecoat/clearcoat systems.
With the ever-changing designs for the automotive interior, coupled with part shapes that may have light-inaccessible areas, thermally cured paint systems with improved performance will continue to play a major role. However, for high-touch surfaces where individual parts easily can be cured under UV prior to assembly, the high-gloss 1K black UV monocoat could provide a competitive high-performance finish with luxurious appearance at a lower energy cost and reduced carbon footprint.
Conclusion
Pigmented UV coatings could be optimized to deliver a high-gloss jet-black interior automotive finish comparable to the Universal Black color position typically achieved in thermally cured systems. The optimized 1K UV monocoat has a wide process window and delivers excellent scratch-, moisture- and chemical-resistance with a smaller equipment footprint, shorter cycle time and lower overall energy consumption compared to thermally cured coating systems.
Red Spot Paint and Varnish Company, Inc., with corporate headquarters in Evansville, Indiana, has a mission to develop specialty chemicals that are both functional and harmless to the environment. For more information, visit www.redspot.com or email ptsang@redspot.com.
