Renewed Interest in UV Powder Coating

By Cal EzeAgu

The development of UV resins technology in the 1990s was designed to reduce high energy consumption and expand the area of powder coating applications. The high material and capital equipment costs coupled with complications in curing complex- shaped substrates made it challenging to convert potential end users.

Why Low Temperature Cure Powder Coatings?
Lately there is renewed interest in UV curable powder, which is being driven by both the global demand to meet corporate social responsibilities as well as the significant need for low temperature cure technology to broaden the applications of powder coatings. Minimal heat, as low as 120 degrees Celsius (248 degrees Fahrenheit), is required to melt and flow-out the powder, making UV powder a desirable technology.

According to an analysis by Irfab in 2017, the total market size of the global domestic, kitchen, and office wood furniture coating resins is estimated at 580 kilo metric tons (KMT) (1.3 million pounds), with domestic furniture representing about 71% of the total market size.

What Are the Drivers for the Renewed Interest?
The ability to coat non-metallic substrates such as MDF, HDF, glass, gypsum fiber board, ceramics, and plastic is one of the reasons for the new interest in UV powder coatings.

Shifts in regulatory policies to switch away from solvent-borne to more environmentally friendly coatings, as well as the ability to coat thinner heat-sensitive metal sheets, are also viewed as contributing factors in the renewed interest. Furthermore, there is a need to cure slow-to-heat components. By nature, massive metal parts are slow to heat.

The use of UV technology means massive components can be coated faster, thereby increasing both throughput and productivity. Lower cure temperatures also increase applicator production efficiency and energy savings. Finally, UV technology allows end users to coat pre-assembled parts without damaging sensitive components.


How the Technology Works
The film building and curing process takes place in two steps. First, the powder is melted with medium wavelength IR lamps or a combination system with IR/convection capability. The process of melt and flow typically takes one to two minutes depending on the application needs. There is no cure that takes place during the melting step.

The second step is the UV cure, which typically takes less than 10 seconds. The source of the curing light energy is the high energy UV lamps, and there are different types of UV lamps available in the industry. Arc (or medium pressure) lamps have dominated the market over the years. Recently there has been development in the production of UV light emitting diode (LED) with higher UV energy output.

In UV cured powder, the melt and cure phases are independent processes. Please refer to the viscosity curve of the powder cure (Figure 1). This allows for smoother films at lower temperatures due to the longer relaxing time after melting. This process is different from a standard thermoset powder, which begins to cure as the powder is melting.

During the heating cycle of the UV technology, the UV powder is melted and cured in approximately two minutes. The temperature measured just underneath the surface is limited to 43 degrees Celsius (110 degrees Fahrenheit) during the entire melt operation. The final film properties are as good as that of the low bake thermoset system.

UV Resin Range and Properties
UV powder resins are made up of a combination of different chemistries such as (meth)acrylated polyester, (meth)acrylated polyurethane resin, and (meth)acrylated epoxy/polyester resin blends. The typical glass transition varies from -15 to 56 degrees Celsius (5 to 138 degrees Fahrenheit) and viscosity range of 350 mPa s at 100 degrees Celsius (212 degrees Fahrenheit) to 5500 mPa s at 200 degrees Celsius (392 degrees Fahrenheit). Refer to Table 1 for the typical resin properties of (meth)acrylated epoxy and (meth) acrylated polyester.

The range of UV resins used in a powder system are solid polyesters containing (meth)acrylic double bonds. The (meth) acrylic bonds are cured by free radical polymerization (FRP) from the light energy source of a high energy UV lamp. UV curable powder technology does not have a separate hardener. Rather, it uses a photo-initiator to activate the polymerization reaction. The two grades of photo-initiators used in certain UVECOAT® based systems are alpha-hydroxy-ketone type (AHK) for surface cure and bis-acylphosphine oxide type (BAPO) for through-cure. These grades were preferred based on low volatility, no discoloration, no volatile by-product release during cure, through-cure in opaque film, photo- bleaching, and absorption at longer wavelengths. Depending on the thickness of the coating, the use of the combined photo-initiators is recommended to ensure complete cure of the coatings layer.

Despite all the benefits of UV curable powder coatings on heat-resistant and heat sensitive substrates such as steel, wood, plastics, alloys, ceramics, and glass, there are a few notes to take into consideration when formulating and applying UV powder to ensure optimal performance. For example, the choice of pigments can limit the system’s performance. Titanium dioxide requirements should be reduced as it hinders the photo-initiators. To cure grey coatings well, the ratio between the titanium dioxide and carbon black must be optimized. Coatings containing red and yellow pigments are difficult to cure as the pigments absorb at the same wavelength as the photo-initiators. A degassing agent such as benzoin is not welcome in UV powder formulations as it can cause pre-reactions during extrusion. Many fillers used in standard thermoset coatings may be used in UV powder technology.

Another area of consideration is curing conditions. Insufficient cure will affect the adhesion, chemical resistance, and weathering performance of the finished coating. Coatings designed for high film applications need more UV light energy to penetrate the coating to achieve optimum performance. Complex geometric shaped objects require special handling as hidden parts are not readily visible to the curing UV light energy.

Advancement in UV Curable Resins
As UV curable technology continues to mature, resin manufacturers have developed innovative technologies to address some of the challenges of UV curable systems (Figure 2). These latest advances have prompted formulators and end- users to take a second look at UV powder technology.

  Significant advancement has been made in the areas of UV curable powder additives. Semi-crystalline resin, when used as an additive, will help to achieve excellent smoothness. There is advancement to increase the reactivity of formulation and to achieve higher cross-link density. Lately, resin manufacturers have made a demonstrable improvement in the additive for metal adhesion.

Typical Powder Formulation and Preparation
The processing and application conditions of UV curable coatings are similar to those of thermosetting powders (Table 2). This is a significant benefit as the production of UV powder can be done through the conventional powder coating manufacturing process. The cross-linking of formulated UV powder coating is not initiated by heat and therefore there are no concerns of chemical advancement.

Like in standard thermosetting powder preparation, photo-initiators, additives, and pigments (where applicable) are added to the solid resin as a premix and homogenized by extrusion. The finished UV powder is applied to the substrate by corona electrostatic spray gun. UV curable powder can also be applied using a tribo-charging spray gun, especially when spraying more complex parts. Including a tribo additive in the formulation is recommended where a tribo application is desired.

Chemical Resistance
UV curable powder is suited for coating kitchen and bathroom cabinet door fronts. A study comparing the resistance of UV powder to a variety of chemicals against DIN 66861-1, the German inspection standard for furniture surfaces, (Table 3) shows these coatings satisfy the norm for furniture coating as designated by the standard. They fully meet Part B of the standard and nearly meet the requirements of Part A. The UV resins are the backbone of the coating and improvement in the surface properties of the coating could be achieved with further optimization of the powder formula used in this study.

Weathering Resistance
The weathering performance of different UV curable powders versus the outdoor standard durable TGIC systems were evaluated under QUV-A bulb per ASTM G53-88. The coating based on UV2100 did not perform as well as the TGIC standard system. The formulation based on UV2200 performed much better than the TGIC control.

Figure 4 illustrates the gloss retention of different UV2200 pigmented systems sent to South Florida natural exposure testing at 45 degrees. The results demonstrate that UV resins based on UV2200 are suited for outdoor powder coatings applications. Figure 5 shows the comparative South Florida exposure data of a typical pigmented super durable versus standard durable UV system.

Conclusion
Interest in UV curable powder coatings will continue to be elevated not only with the ongoing advancements in UV powder technology and comprehensive range of UV powder coating resins available to cover several applications, but also with the heightened interest to expand the area of powder coating applications such as MDF, HDF, plastics, natural wood, pre-assembled components, large mass steel, glass, ceramic, PVC, and gypsum fiber board. The manufacturing of UV resins and cost of materials will continue to decrease as usage increases. Recently, there has been advancement in the production of UV light emitting diode (LED) with higher UV energy output. Over time, this efficient system will gradually phase out traditional arc and medium pressure lamps.

Cal EzeAgu is technology manager - Americas PCR with allnex USA, Inc.