There are currently two main limitations to powder coatings technology. First, the high bake temperatures necessary to bring about crosslinking of the resins can run up big energy bills in comparison with water- and solventborne coatings. Second, the high curing temperatures also prevent curing on heat-sensitive substrates such as plastic. As promising as powder coatings appear, the net result of these drawbacks severely limits their more extensive use. This situation is changing with the advent of UV-curable powders.
Today, powder coatings are enjoying an increasing popularity. The double bonus of a system that almost eliminates volatile organic compound (VOC) emissions while at the same time reduces overall process costs is proving to be irresistible. What happened to prompt such a turnaround?
Powder coatings were first developed in the 1960s for use on metallic substrates, using the fluidized bed application process. The introduction of the electrostatic spray process allowed substrates to be coated cold. When the Clean Air Act and OSHA began to regulate the use of liquid coatings using solvents, this pushed powder coatings to the fore with a clear advantage. Their popularity increased with the recognition that powder coatings offer durable finishes with high resistance to corrosion, heat, impact and abrasion, as well as an attractive look — all with an array of textures and colors (including metallics, fluorescents) and clearcoats.
Powder coating laid claim to general metal finishing, its biggest applications area, followed by the appliance and automotive markets. Detroit’s Big Three auto makers established the Low Emissions Paint Consortium, in part to carry out research on the use of powder in clear topcoat applications. In the meantime, the automotive industry has used powder for such applications as engines, roof racks and radiators. Here, polyester urethanes are primarily used because of their good UV stability (especially for roof racks). Furniture became another growth area for powder coatings, while other applications were found in machinery, pipe and electrical equipment.
The underlying reason for this high growth is the fact that powders offer a low VOC-generating process that also delivers high-performance coatings. High performance is fortified by the use of additives in the powder coating formulation. These additives are designed to improve properties such as making it easier to spray the powder on the substrate as well as enhancing the durability of the coating once it is cured. Additives can increase the service life of the coating by reducing the effects of UV damage, by providing better high temperature stability, and by improving corrosion resistance.
Absorbers and Scavengers Battle Free Radicals
While a refrigerator may not necessarily need photo-oxidation protection, for painted items that are exposed to the exterior environment, all bets are off. The next challenge for conventional powder may be parked in your driveway or on your deck. Exterior applications, such as automotive and outdoor furniture surfaces, involve battling the elements. The sun’s UV rays can initiate photo-oxidation, the process that breaks down the chemical bonds in a coating polymer’s structure and degrades the binder, which can lead to color fade, cracking, chalking, yellowing, and loss of physical properties.In addition to their aesthetic value, the use of pigments in coatings had, for many years, been the method used to protect against UV light damage. Paint films were stabilized using titanium dioxide (TiO2), carbon black and trans-iron oxides. Unfortunately, even TiO2 can actually promote photo-oxidative degradation of polymers, depending on how it has been treated and modified. When carbon black performance was compared to hindered amine light stabilizers (see the figure), the latter soundly trounced the former with regard to longevity and cost savings, not to mention the additional value of offering a color other than black (our apologies to Mr. Henry Ford). Clearly, pigments could only act as UV absorbers in a very limited way. This set the stage for exploring other methods that would add protection without the undesirable side effects.
Ciba Specialty Chemicals has developed effective coating protection and pioneered benzotriazole ultraviolet absorbers. These UV absorbers (UVAs) show a synergistic effect when used in combination with hindered amine light stabilizers (HALS). Each one operates by interfering at a different step of photo-oxidation; together they can enhance the performance of most coating systems.
The UVAs’ modus operandi is to preferentially absorb the harmful UV radiation and, through a process known as “keto-enol” tautomerism, harmlessly dissipate it as heat. This means it is regenerative. A UVA is especially important in clear and translucent coatings to protect the substrate or basecoat. TINUVIN® 928 is a UVA that offers low volatility, high permanence, and just the right melting temperature to be highly suited for powder coatings. In clearcoats, particularly, it is suggested that a UVA concentration of 1–3% be used to protect the substrate or basecoat. In general, all UVAs should be used in combination with a HALS. Users should make their own tests to determine the suitability of these products for their own particular purposes.
While the number of free radicals formed due to photo-oxidation can be minimized by the use of UVAs, they cannot absorb 100% of the incident UV. The radicals that do form can be controlled by highly effective free radical scavengers, or HALS. Also, some radicals may already be present in the coating raw materials, or formed through heat. HALS “defuse” free radicals by way of several mechanisms, an important one being the termination of the high energy peroxy radicals. Like UVAs, HALS are also regenerative. This allows them to furnish superior long-term protection, even at low concentrations (1–3%). HALS such as TINUVIN 144 or TINUVIN 111 FD should be used in both clear and pigmented coatings.
Like a well-rehearsed team, UVAs and HALS work in tandem for a mutual goal. The UVAs keep the bad guys out while the HALS mop up any that manage to get in.
Combating Corrosion
The majority of conventional powder coatings end up on metal substrates. Just add water, oxygen and electrolytes, and you have a formula for rust and corrosion, especially if the surface of the coating is not sufficiently stabilized. For most exterior uses, corrosion inhibitors are an absolute necessity. Most metals are primed first for corrosion protection before being topcoated. Until now, the most effective corrosion inhibitors used in coatings for metal protection have been based on heavy metals such as chromium, lead and zinc. The best performing anticorrosive, hexavalent chromate, is regarded as carcinogenic and will soon be phased out. The use of such heavy-metal compounds in coating formulations is, or will be, severely regulated by legislation. Furthermore, where the powder coating is both primer and topcoat (direct to metal), a high-gloss finish is frequently desired, which can be negatively impacted by some anticorrosive pigments. Ciba has developed corrosion inhibitors that can deliver high performing alternatives to conventional anti-corrosive pigments. IRGACOR® 252 LD is an organic additive suitable for powder coatings. Used at additive levels of just 2–4% based on total coating solids, it displays excellent results in both direct-to-metal topcoats and primers. In some cases it can be used synergistically with anticorrosive pigments based on nontoxic metals.
Spray Applications of Powder Coatings
Powder coatings can be applied onto a substrate two ways. The predominant one in the United States is the corona process, where the sprayed powder particles are forced through an electrical field to get negatively charged. The charged particles are attracted and deposited on the grounded object to be painted, which is generally a metallic substrate. The second application process is the tribostatic process. In this case, the particles are forced through a Teflon tube and are positively charged by friction onto the tube walls. The tribocharging spray process has some benefits over the corona one, such as more uniform and thinner layers, and less overspray. Depending on their composition, tribostatic applicable powders require additives to improve their chargeability. Certain HALS such as TINUVIN 144 and TINUVIN 111 FD can improve tribocharging.Antioxidants: Breaking the Chain of Degradation
Many readers may have experienced this scenario. After applying a conventional powder to an appliance part — for example, the door for a white refrigerator — the item is put in the oven, and after staying in there longer than it should, it develops a yellow color. This may well be how decorator colors such as “Harvest Gold” were created! At any rate, the result is an over-baked yellow shade. This happens because heat can lead to the destruction of chemical bonds, which leads to loss of aesthetic and performance properties such as color, adhesion, and flexibility. In this process, called autoxidation, free radicals are generated and these react rapidly with oxygen to form peroxy radicals. These may further react with the polymer chains to form hydroperoxides and even more free radicals. This cycle can propagate almost indefinitely.For indirect gas fired, or electric ovens, this chain of degradation can be stopped by primary antioxidants (AOs), typically hindered phenols, that react rapidly with peroxy radicals to break the cycle. In addition to using a primary AO, improved protection at a lower cost can be achieved by using a phosphite- or phosphonite-based secondary AO. New products that provide extra performance against overbake yellowing at relatively low concentrations include IRGANOX® HP 2921 and IRGANOX XP 621. These products are synergistic blends of a new class of stabilizer called lactones with a hindered phenolic AO and a phosphite secondary AO. Typical use levels would be around 0.1–0.5%. If one uses Primid as a crosslinker, IRGANOX XP 490 is the primary choice.
When using direct gas fired ovens, another “bad actor” needs to be controlled — oxides of nitrogen referred to as NOx. These also lead to yellowing. When NOx is present, hindered phenolic AOs can cause a phenomena in powder coatings referred to as “pinking.” In this situation, the synergistic, phenol-free blend of lactone and phosphite or phosphonite such as Ciba® IRGAFOS® XP 30, XP 40, or XP 60 are the best choices for prevention of yellowing while avoiding the pinking.
The bottom line on antioxidants is this: their use produces an improved thermal stability for resins and coatings during manufacture, processing and service lifetime — and they’re especially useful for protecting powder and coil coatings, since they are cured at high bake temperatures and usually used in severe end-use applications. The figure illustrates a basic additive selection guide.
The Limits of Conventional Powders
Industrial powder coatings have grown steadily during the last decades. The success of conventional powder coatings is the result of excellent properties and many economic and other advantages over traditional solvent-base paints. However, powder coating problems, including performance on temperature-sensitive substrates such as wood, medium-density fiberboard (MDF) and plastics led companies to research ways to remove these limitations. UV-curable powder coating is the technology that brings powder coatings one step further.UV-Curable Powders
The idea of curing a powder using UV radiation dates back from 1971 when BASF filed a patent on this topic. A Japanese and an American patent followed in ‘73 and ‘76. Subsequently, nothing happened commercially for more than 10 years. In the early 90s, Hoechst resurrected the development of UV-curable powder coatings. Since then, resin manufacturers DSM and UCB have emerged as technology innovators. Meanwhile, Ciba was developing the additives that would make UV curing of powders possible and their performance outstanding. These concerted efforts yielded a breakthrough product that reduced oven bake time from 15–20 minutes to 5 minutes, and lowered overall process costs.Comparison Shopping
Manufacturers know that a coating not only makes a product more attractive, it should also enable a product to do the job it was designed to perform. Likewise, a poor coating can ruin a good product. Choosing the appropriate coating technology is, therefore, of vital importance, and today’s manufacturers have a range of choices. Solventborne coatings are chosen less often these days because of their VOC emissions during the coating manufacturing and application processes. UV-curable liquid coatings are largely free of solvents, and well suited to heat-sensitive materials, but handling the sticky lacquers, overspray waste, and greater volatility are drawbacks.Conventional powder coating can be a suitable choice for metal objects that can withstand a bake cycle of 20 minutes in a temperature range of 160–200ºC (320–392ºF). On the plus side, these powder coatings flow together in an oven to form a smooth, pinhole-free film, then crosslink to create a tough, durable, scratch-resistant surface. The overspray can be recycled by mixing it with fresh powder and using it for the next job.
On the minus side, coated items must cool for long times before they can be removed from the hangers. A John Deere tractor, for example, takes 24 hours to cool sufficiently. Both the thermal energy required and the floor space needed to cool items are considerable, and only articles made of metal can withstand the oven heat.
Combining UV curing with powder technology delivers the benefits of conventional powder coating, as well as the advantages of UV-curable coatings. The thermal exposure and prolonged heating at higher temperatures needed to cure conventional powder coatings are now absent. Manufacturers only need to apply enough heat to melt the powder, allowing the coating of fully assembled items. The much lower temperatures opens up all kinds of opportunities, allowing UV-powder to be used for coating non-metal substrates, ranging from medium-density fiberboard, paper and cardboard to plastics, leather, wood, and items containing heat-sensitive materials. UV-powder coating can also produce a medley of decorative colors, textures and finishes.
Very little floor space is needed for coating and cooling, and manufacturers can take control of the entire coating process rather than using outside paint shops. Because the five-minute coating application can be integrated into the production process, products can be painted and shipped the same day! Products don’t have to be limited to two dimensions either. If sufficient care and consideration is given to the design of the coating installation, even 3-D objects can benefit from the UV-curable powder technology. UV powder has been successfully used with the two most common types of powder spray processes.
Photoinitiators: The Heart of the Process
Without a photoinitiator, there would be no UV-curable powders. This additive is the crucial component that sets the polymerization process in motion. Here’s how a photoinitiator works:A bank of UV lamps is stationed above a conveyor belt, directing ultraviolet light onto the coating to be cured. The UV energy from the light source is absorbed by the photoinitiators. This kicks off a chemical reaction that swiftly converts the powder coating into a solid, cured film in less than a second.
While the cure mechanism can be either free radical or cationic, most systems are based on generating free radicals that react with the unsaturated compounds in the melt and, in an almost instantaneous reaction, cause them to polymerize. The area of resin development is very active as the technology blossoms into newer application areas.
The bulk of the resin formulation is made up of oligomers and monomers, which contain polymerizable double bonds. For example, unsaturated polyesters and vinyl ethers can be used to produce UV-curable powders. Alternatively, acrylate or methacrylated prepolymers and monomers also have been successfully used to make UV-curable powders. In all cases, the UV-curable powder coating formulation requires a photoinitiator to activate the polymerization process with light.
Ciba has worked closely with resin producers and lamp suppliers to optimize the use of photoinitiators in UV-curable powders, and develop new markets for their use. Consequently, the broadest line of photoinitiators available today is offered by this pioneer of UV-curable powder coatings.
Two powerful photoinitiators for commercial applications were developed in response to customer demand. Both photoinitiators are stable solids that are super-reactive when photoexcited. IRGACURE® 819, a bisacylphosphine oxide (BAPO) photoinitiator delivers excellent through-cure. The recent development of the BAPO class of photoinitiators permits, for the first time, the curing of thick films of pigmented powders. BAPO has the ability to absorb long wavelength light (in the 400–430 nm range), after which it generates initiating free radicals. Pigments can be selected that do not greatly interfere with the light absorption of IRGACURE 819, which means highly colored thick powder coatings can be produced with the very cost-competitive UV technology.
IRGACURE 2959, an alpha-hydroxy ketone (AHK), works in tandem with IRGACURE 819 to supply excellent properties with minimal deleterious effect on the storage stability of the powder. Fortunately, AHK photoinitiators have a synergistic effect with BAPOs, which leads to good surface cure and excellent through-cure in these powder-coating resins. IRGACURE 2959 also has a terminal hydroxyl group that can be reacted into the polymer backbone. It offers low volatility and very low odor. The photoinitiator combination of IRGACURE 819 and IRGACURE 2959 is so successful that good cure can even be obtained with a variety of colors. Together, these products represent a watershed development in solid form photoinitiators for UV-curable powder coatings.
Choosing Pigments for Performance
The lure of an economical alternative cure method made it almost inevitable that applications long dominated by the slower-curing traditional two-part polyurethanes would move toward powders. It’s not surprising that today’s demand for an increasingly larger chromatic palette is narrowing the choice to UV-curable powders coatings. With conventional thermally cured powder coatings, pigments need to be selected based on their thermal stability. They’ll need to stand up to temperatures in the range of 180–200ºC (356–392ºF) for crosslinking to occur. This sets in before the surface flow has totally stopped. The ability of UV powders to swiftly coat heat-sensitive substrates in temperatures of about 100–140ºC (212–284ºF) just long enough to achieve the necessary flow allows for greater freedom in pigment selection giving rise to an expanded palette of color options. Good cure performance relies on the absorption spectrum of the photoinitiator coinciding as closely as possible with the emission spectrum of the UV lamps. When light strikes a pigmented coating, some light is reflected off the coating’s surface, and some is either scattered by the pigment, absorbed by the pigment, or passed through the coating without being absorbed. It follows then that the pigments selected for the coating color should offer as little interference as possible with the photoinitiator’s absorption of UV light.
All Pigments Are Not Created Equal…
…at least not where light curing is concerned. As the use of UV powder coating grows, so does pigment usage. Organic pigments now allow for the replacement of lead chromates, leading to greater ecological harmony and a desire for more saturated colors and different textures. Initial work has already begun to fulfill these style demands. Different characteristics make pigments highly application-dependent, so it’s best to choose them based on the kind of performance the powder coating will need to provide. Pigments offer varying degrees of opacity and therefore UV protection. For example, opaque TiO2 white reflects virtually all UV light, offering more UV protection; black, which tends to have good hiding power, absorbs light and offers no UV protection. It might surprise some people to find that black is not an impossible color to cure with UV light. Gray is actually far more difficult — the TiO2 absorbs the UV light and the black absorbs the remaining visible light, leaving little light available for the photoinitiator. However, as troublesome as gray can be, yellow gets the prize for the most uncooperative color in the spectrum. In reductions with TiO2, one can cure yellow at a low color strength, but anyone looking for a high color strength will be sorely disappointed. At this point, it is very difficult to cure a strong yellow. This color’s intransigence even extends to its neighbors in the color spectrum: orange, brown, certain greens and the aforementioned gray do not take to curing easily.
Demand is up for a wider range of colors, very durable finishes and more transparent colors. Transparency can be accomplished with transparent pigments or dyes, along with the use of pearlescents, to formulate decorative powder coatings that are showing up on motorcycles, bicycles and cars. Besides their applications specific requirements, pigments are also formulation dependent. In this regard, while a pigmented epoxy system often deteriorates quickly, an acrylic will deliver durability. Of course, acrylic is more expensive. There’s always a choice to be made.
A Bright, Colorful Future
UV-curable powder coating lines have been opened in the United States and Europe, signaling the acceptance of this new paint technology. Also, lower-temperature UV-curable powders have taken a foothold in the wood-furniture and paper-coatings industries. The technology is now being used in Europe on television and audio equipment tables and stands, which benefit from UV-powder coating’s ability to provide exactly the same finish on both MDF and metal. Further installations in Europe are planned in which UV powder is applied to MDF panels and moldings. Plastics is yet another area that beckons with tremendous potential.Resin manufacturers are investigating possibilities for markets such as office furniture, parquet and PVC flooring, electric motors, automobile radiators, and shaped MDF products. Meanwhile, Ciba continues R&D work on additives that will endow conventional and UV-powder coatings with the power to perform as needed, for a variety of applications. With equipment available at competitive prices, more developments can be expected to emerge from those hardy manufacturers pioneering the use of UV-powder coatings.
While the initial cost of installing a UV-curable powder line will depend on the size, shape and diversity of the items to be coated, the long-term benefits can hardly be overstated. This cost-effective technology uses less energy, less space, greater transfer efficiency and almost instantaneous UV curing. It also combines low VOC generation in the manufacturing process with high end performance. For these reasons we see an exciting future for the UV-curable powder technology.
For more information on powder coatings, contact Imaging & Coating Additives, Ciba Specialty Chemicals Corp., 540 White Plains Road, Tarrytown, NY 10591; phone 800/200.8224 or 914/ 785.2000; fax 800/259.7065 or 914/785.4224; visit www.cibasc.com; e-mail addcustsvc@cibasc.com.
TINUVIN, IRGACOR, IRGANOX and IRGAFOS are registered trademarks of Ciba Specialty Chemicals Corp.
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