OEM finishers are in good company when it comes to the challenge of continuous improvement. It seems that to stay competitive, everyone is looking for an edge. To finishers, continuous improvement calls for periodically reviewing coating systems used to determine if new entries to the market might offer better performance or application efficiency.
Understanding the nuts and bolts of what makes coatings perform the way they do can make this process easier, whether the goal is improving productivity, enhancing the quality of a finish, or meeting new government regulations.
Selecting the right coating for the job is about matching a coating’s advantages to the needs of your customer and the capabilities of your facility. Of course, this kind of matchmaking requires knowing both sides of the story – the needs of your customer, the capabilities of your facility and a knowledge of what each coating brings to the table in terms of application and performance characteristics.
The Big Picture
All coatings contain pigments and binders. Pigments are finely ground, non-volatile particles that provide hiding, sheen, color and brightness. Pigments also contribute physical integrity and volume solids protection for the substrate. Binders are non-volatile resins that provide the adhesive, protective and durability qualities of the paint, including its film-forming mechanism. Commonly used binders may include emulsions (acrylic or vinyl), dispersions (acrylic, alkyd or polyurethane), or solutions (alkyds, polyesters, epoxies, acrylics, and urethanes).
The resin backbone of many product finishes typically consists of polyesters, acrylics, polyurethanes or epoxies. Hybrids such as acrylic urethanes and polyester urethanes are common in all coating categories. Alkyd resins are also common in solvent-borne and waterborne coatings.
The primary factor for using one resin over another is performance and cost. For most decorative finishes, low-cost acrylics are well suited. However, finishes that require high performance and durability such as corrosion resistance might call for epoxy or urethane resins which tend to be more expensive. To meet customer needs, coating manufacturers can modify and combine resins to accommodate performance qualities. When looking at resin content in a coating, the key is to understand the resin’s contribution to the formulation’s performance.
Liquid coatings also contain solvents, which are the medium in which the solid ingredients are carried. Solvents lower the viscosity of the binder and pigment mixture to make application easier. Solvents dissolve or disperse the resin, control the consistency and thickness of the paint, and evaporate during drying to leave a dry film. While the film is forming, chemical reactions are occurring during the curing process. After the solvents evaporate, a hard, durable surface is left behind. Like other solvents, water is a medium that evaporates as the paint dries. Co-solvents help to drive water out of the paint film. Without co-solvents, evaporation would take a long time.
Volume solids is the term used to describe the percentage of solid ingredients in a coating that remains on the surface once the paint has dried. It used to be true that the higher the percentage of solids, the more material remains on the surfaces providing more durability, better hiding and longer protection. But high film build is not always desirable, so today there are high solids coatings that offer lower film build. High solids coatings are frequently specified given today’s stringent environmental regulations, as they provide both extended durability at low film builds and low VOCs and HAPs. Additionally, they are very attractive from a cost
per square foot of coverage perspective.
Unlike liquid coatings, powder coatings contain only trace amounts of solvents. Powder coatings comprise pigment and resin particles that are electrostatically
charged and sprayed onto parts that have been electrically grounded so the charged particles are attracted to and adhere to the surface. To make a quality powder, each particle must be of uniform size, shape, density and chargeability. Specific
gravity is a key indicator of a powder coating’s application characteristics. The lower the specific gravity of a powder coating, the better its transfer efficiency.
Powder coatings are applied using pneumatic pumps and either manual or automatic electrostatic powder spray guns. The coated piece is then heated in a gas, electric, or infrared oven until the powder particles melt and fuse into a durable, solid paint film.
The Next Level
The three basic coating types, solvent-borne and waterborne liquid coatings, and powder
coatings, each have a long list of advantages. But what are they?
The Solvent-borne Profile. Often thought of as the workhorse of the industry, solvent-borne coatings can be trusted to provide tough, durable finishes that dry quickly, even without baking ovens, to meet production demands. Over the years, sophisticated solvent-borne systems have set the standard for premium finish quality for both metal and plastic substrates.
Solvent-borne systems encompass the whole range of coating products including fillers, primers, sealers and topcoats. These coatings are often the first choice of metal finishers for military, industrial, agricultural, and construction equipment and for consumer products. Plastics finishers also find that solvent coatings are excellent choices for applications where exceptional film durability, long-term performance and superior texturability are required. As a result, solvent-borne coatings are often selected by plastics finishers requiring a very durable coating that has the ability to cure below 300°F, the deformation temperature for many plastics.
Available in a wide range of formulations that meet specific demands in application and performance, solvent-bornes offer a full gloss range with little or no orange peel, excellent color and gloss retention and chemical resistance. These qualities make solvent-borne coatings popular for a variety of uses including high performance exterior applications on agricultural, industrial and construction equipment. The manufacturers of metal office furniture and metal building products also favor these systems for their superior durability and attractive finishes.
Although one might think that strict governmental regulations on the use of solvents in coatings have put a damper on the industry, solvent-borne coatings that meet Federal standards still dominate the $6.5 billion dollar finishing market. High solids, solvent-reduced coatings are often specified because of their low HAPS, low VOCs and improved cost per square foot benefits.
Recent advances in solvent-borne coatings have included the development of ultra-low
VOC polyester baking enamels that emit less than one lb of VOCs and still provide excellent application and performance characteristics. Today’s high-solids solvent coatings may be applied with a wide range of equipment—including air-assisted airless spray equipment as well as conventional, airless, HVLP and electrostatic spray.
Waterborne Performance. There was a time when waterborne coatings were considered inferior to their solvent-borne counterparts. But after more than 50 years of research and advancements in technology, some waterborne coatings have properties that equal or even surpass solvent-bornes.
Today’s waterborne coatings, especially those with very low VOCs and zero HAPS, have proven themselves a good choice for manufacturers facing strict regulatory requirements and those wanting to reduce fire hazards and the high costs associated with solvent cleanup and disposal. The lower solvent levels in waterborne coatings make them desirable in the workplace because of low odors which improves working conditions, and because they offer easy clean up with soap and water.
As their name suggests, these coatings primarily use water instead of organic solvents to deliver resins, pigments, and other additives. To give waterborne coatings their performance characteristics, special resins (usually synthetic polymers) are used in the formulation. Coupled with other technological engineered capabilities, these new waterbornes have fast dry times, and provide excellent adhesion to plastic substrates. Waterbornes are also ideal for application on metal, wood, glass, and masonry surfaces using most types of application equipment including spray, dip or flow coating.
As with any coating, waterborne performance depends upon formulation. For example, waterborne
acrylic latex enamels are quick setting, but they can be limited in the range of glosses offered and on which substrates they are used. These coatings are very suitable for high speed production with the use of force dry ovens in the temperature range of 120–180°F for curing. Fortunately, this range is below the heat deformation temperature of most engineered plastics.
With the addition of modified resins and additives that improve their resistance to corrosion and foaming during application, new
waterborne epoxies offer performance as good as solvent-based epoxies. In addition, waterborne epoxies containing emulsion
polymers offer a high level of hardness and chemical resistance, overcoming the problem of unreacted co-reactant in the paint film. Emulsion polymers also help the coating perform well in tests involving salt spray corrosion and synthetic cutting oils, which are common requirements for the machine tool industry. Faster cure times, longer pot life, improved resistance to yellowing and better gloss stability are other advantages.
Water-reducible polyurethane dispersions and polyurethane-acrylic
hybrids offer performance qualities similar to two-component urethanes with the application ease of a one-component coating. These finishes are tough and durable and require little or no heat to cure, although one drawback can be slightly lower chemical resistance. Among the substrates that are ideal for these coatings are structural foam or injection molded plastic (such as polycarbonate), ABS and high-impact polystyrene for end products such as communications equipment and electronic enclosures.
While waterborne coatings have many advantages, there are some trade-offs. For example, waterborne
acrylic air-dry coatings are affected by relative humidity, so a temperature and humidity-controlled environment is most efficient. Optimal conditions call for 30–70% relative humidity, along with good air circulation for proper drying. In general, waterborne coatings do not have the application versatility of solvent-borne coatings and waterborne coatings are not as forgiving to marginally clean substrates.
When Powder Makes Sense. The technology for powder coatings was developed in Europe and came to the United States in the 1950s. During the 1990s, powder coating sales were driven mostly by strict air quality regulations, which served as a catalyst for finishers to consider alternatives to solvent-based coatings. The growth in popularity of powder coatings in today’s market is driven by technological advances and new applications.
Powder finishes are widely used in the appliance industry as well as on products that range from vacuum cleaners to automotive parts. New innovations in powder technology that allow metal or pearlescent
mica flakes to be bonded to the base coat powder instead of simply being mixed together with it, produce brighter, more attractive metallic finishes without the combustibility of dry, blended metallic powders. With some exceptions, bonded metallic powders also are a more efficient option for reclamation and re-use than blended metallic powders.
Another innovation is lower-temperature cure powder coatings that require less energy. Most powders cure at temperatures between 300–400ºF. But, newer, low-cure products only require temperatures of 250ºF. Continued research on this front should soon yield powder coatings that cure at even lower temperatures for glass, wood and plastic products.
Two other innovations are thin-film powders and new
automotive body clearcoats. Powder coatings are currently applied at a film thickness of 1.5– 4 mils DFT. But the industry is currently working toward achieving thin-film powders applied at closer to 1 mil DFT. Less film to cure means less paint and energy used, shorter cure times, and a more attractive appearance.
There are many advantages associated with using powder coatings including little to no VOCs, solvents, or water. The result is little or no air pollution and little waste when finishers retrieve and reuse overspray that does not adhere during application. And since these coatings do not run, drip, or sag, their use can actually reduce reject rates for manufacturers. Some finishers cite durability—especially the immediate impact-resistance of powder—as the key reason for using it, as it eliminates the need for touch-up due to rough handling in the plant and during the delivery process.
A barrier to entry for powder finishes is the major capital investment required to set up a powder coating line, often in the $100,000-plus range. Also, introducing powder into a facility requires technical assistance and training to ensure that a powder coating line is operating at maximum efficiency. Finally, while new powders have improved aesthetics, they are not as smooth, and are not available in the wide range of colors and textures so abundant in liquid coatings.
With so many choices available to finishers, the task of selecting just the right coating might seem overwhelming. However, by keeping abreast of the latest trends and developments in the coatings industry, and working closely with coatings representatives to learn more about new products, finishers can make good choices that enhance their productivity and customer satisfaction.
