Powder coatings, whether dry or slurry, are being used with excellent results for the painting of car bodies, either as primers, single-layer coatings or as a clear topcoat. In the United States, powder coatings are in full commercial use as primer surfacers at 13 GM and DaimlerChrysler plants. In Germany, BMW has proven powder clearcoats to be a commercial reality. In fact, there are millions of vehicles worldwide with powder coating as a layer of their body paint.
Development Work ContinuesSo what are the issues, and what has to be done to further advance powder's use by the automobile manufacturers? Powder coatings are undeniably attractive from an environmental standpoint, but as always, they also have to be economically justifiable and practical in their utilization.
In the United States, there are four types of powder chemistries that meet the OEM's powder primer specifications. Each powder manufacturer will argue advantages in terms of performance, but obviously, based on the length of time these products have been used at the OEM customer, they all meet the basic requirements. The application process, although never perfect, is also well established. Thus one can consider powder-primer technology to be fairly well accepted.
As with most established technologies, the focus for most OEMs is economics. Most plant managers agree that powder has yet to match the economics of liquid primers on a film-thickness basis. Total savings are not always considered in this cost equation. It is believed that there is still room for improvement in providing better performance at lower film thickness.
In respect to powder clearcoat, the focus is still on making the system adaptable to the OEM's in-plant requirements. That is not to say that system economics are unimportant, as they surely will be in the future. Research continues on the refinement and advancement of both powder-primer and powder clearcoat technology. More specifically, research is taking a look at the use of these products in combination with E-coat and waterborne basecoats as part of a total environmentally acceptable finishing system (Figure 2).
Some of the greatest
advances have been made in the development of clear powder topcoats. The
challange now is to bring them together as a system to commercial reality.
ApplicationPowder primers are already being applied successfully using conventional application technology. The future direction is toward thinner films for powder primers and similarly with powder clearcoats. This will be accomplished by the combination of improvements in resin system rheology and through particle-size engineering. The manufacturers of ap-plication equipment will also make con-tinuing improvements in their equipment to provide more consistent powder application.
Improving the melt-flow rheology of the powder coating is one approach to providing smoother films at lower film thickness. The powder chemist is always being challenged to do this, but very often this cannot be achieved without some compromise in the powder coatings' physical stability. The consequence of lowering powder stability is the potential for application problems such as poor powder fluidization, inconsistent feed to the spray guns and the potential for impact fusion--all of which make film-thickness control by the OEM more difficult.
The future use of powder clearcoats is inextricably linked to the further development of engineered, fine particle-sized powders. Some powder manufacturers are already applying this technology in the production of both powder primers and powder clearcoat materials. In fact, these fine particles provide film smoothness and image clarity similar to liquid clearcoats.
While this would appear to be a very simple solution to appearance issues with OEM powders, fine particle-sized powders do have some associated problems, especially in the area of application. It is typically found that as powder particle size is reduced, powder fluidization and transfer efficiency decrease.
Despite these concerns,
research continues and is now bearing fruit. Through an understanding of these
physical interrelationships and the chemistry of the powder clearcoat, fine
powders that overcome these typical problems have been engineered, and pioneered
AppearanceWith application being a key challenge, the OEM must be able to produce a powder clearcoat finish that is equivalent and/or superior to liquid paint systems used on car bodies.
The vertical and horizontal surfaces on the whole vehicle are emphasized. It is relatively easy to produce a very smooth powder clearcoat film in the laboratory when the powder coating is baked in the horizontal position over a basecoat and primer that have also been baked in the same mode. The surface smoothness can be very different when all of the system components are baked in the vertical position. These comments are made based on the assumption that sanding of either the primer or basecoat to enhance its smoothness is not even a consideration due to economics.
The variations of final
appearances due to film-thickness inconsistencies and orientation during the
bake cycle are as numerous as those with liquid clearcoat systems. Other
factors, such as powder electrostatics and differences due to part grounding and
basecoat color, add to the complexity of this issue. The point is that a powder
clearcoat cannot be designed in a vacuum because its ultimate perfor-mance will
depend very much on the underlying layers. It is very important that the OEM
carefully define these layers before evaluating potential powder clearcoat
candidates for appearance. If this is done, the powder coating chemist is in a
much better position to deliver a robust topcoat that will satisfy the OEM's
Light Colors/YellowingYellowing of powder coating clearcoats over water-reducible basecoats has been widely reported. Most powder clearcoats are based on GMA (glycidyl methacrylate acrylic) resin chemistry that has a strong tendency to yellow in the presence of nitrogen-containing compounds. The presence of these materials in any of the underlying paint layers, or in the powder composition itself, can cause the clearcoat to yellow. This effect is more obvious in white and very light colors.
The yellowing effect depends on film thickness and is very much a function of bake temperature. However, film properties are also bake-dependent. For a product to be feasible for use in an OEM's plant, it must meet all requirements, including color over a range of bake temperatures likely to be experienced in a commercial application.
Again, a thorough understanding of the chemistries of the underlying layers is important. Powder manufacturers have extensively studied these interactions and worked with their raw-material suppliers in the advancement of new materials for powder clearcoats.
At this point,
high-reflectance whites can be color-matched in a complete powder system, but
they are less robust than liquids in color variation due to bake temperature
variations. Continuous improvement in this area of the technology can be
Basecoat/Topcoat InteractionsA powder clearcoat for OEM automotive bodies has to work in conjunction with a basecoat. The basecoat provides not only the color effect, but also contributes to the total system's appearance and performance.
At present, most basecoats under consideration for automotive applications are typically waterborne paints that have to be flash dried before the application of the powder clearcoat. Because of space limitations within an automotive plant, the OEM has to achieve this flashing process in a very short period of time. However favorable these conditions are, there is the potential for the basecoat to retain some water and/or solvent, which the powder must accommodate in its film-forming process. If this does not happen, a defect known as popping will occur.
Popping is not a new problem and occurs within liquid systems as well. The extent of this problem directly relates to the OEM's line conditions (total basecoat process time, application equipment, air velocities, humidity, oven type and temperature of bake/flash). It is also a function of basecoat chemistry, its atomization, its film thickness and the ability of the powder clearcoat to accommodate any evolving materials during its film-forming process.
These are just some of the
issues facing the marriage of the basecoat and the powder clearcoat, which
emphasizes the need for a good understanding of the two technologies and the
specifics of the targeted OEM finishing line. Despite all of these issues, a
good marriage of these two technologies has been demonstrated in both
laboratories and on large-scale pilot lines.
PerformancePowder clearcoats under consideration for pilot-line evaluation are typically based on GMA acrylics cured with either a diacid or an anhydride. The basic powder chemistry dates back to the early 1970s and is well proven in respect to exterior durability. However, the performance of the total finishing system is also a function of the basecoat and the primer surfacer. Any weakness in either of these two underlying layers will be detected in both physical properties, such as chip and impact resistance, as well as water and humidity resistance.
Properties such as acid-rain and scratch-and-mar resistance have gradually improved over the past few years as the polymer chemists have developed better GMA acrylic systems. Acid-rain resistance typical of a two-component liquid paint is now obtainable with GMA acrylic systems.
The polymer chemists
continue to look at alternatives to GMA acrylic resin chemistry for OEM
automotive topcoat applications, but it is believed that it will be a number of
years before any of these
materials reach pilot capability. It is quite possible that any new chemistry
will be tied to future improvements in powder-processing techniques.
A Clear FutureAutomotive powder coating clearcoats have been widely discussed and evaluated for many years. The chemistry is well established, and there is plenty of application experience coming from the present users of powder primer surfacers.
The big "unknown" for powder clearcoats is their ability to work in conjunction with liquid basecoats as part of complete systems. Will they be able to provide the appearance and performance desired by the automobile company? Research and BMW's present commercial experience say the answer is "yes," but only if the underlying basecoat and primer have been engineered as integral parts of that paint system.