by Patrick Smith and Nick Strauss, Graco Inc.
Plastics can be challenging materials to decorate. Different surface properties, lower surface energies and lower polarities are just some of the attributes that provide challenges not seen with metal. Not only is the paintability of plastics different from that of other substrates, it even varies among different plastics. Various materials are added to plastics to give desired physical and chemical properties. The problem is that the nature of these additives changes the paintability of the part. Paintability problems are not limited to the makeup of the plastic alone. Molding release agents and other products used in the forming process also can play a part in making it difficult to paint plastics.
The chemical makeup of the plastic largely determines its surface energy. In general, an exterior with a higher surface energy is more readily wetted by paint and is more paintable. This also means the coating adhesion will be better. The low polarity of the molecules in plastics, such as polyethylene and polypropylene, is the cause of the low surface energy and poor paint adhesion. Increasing the surface energy – and, therefore, paint adhesion is one of the major purposes for pretreating plastics.
Surface energy is measured by analyzing the “wettability” of the substrate. Surface wettability testing involves measuring the contact angle of the paint droplets on the part. When a liquid doesn’t completely wet a part, a contact angle is formed, causing what is commonly known as surface tension. Surface tension prevents a paint droplet from spreading across the surface; it instead continues to hold its shape on top of the part. Think of rain droplets on a newly stained deck or on the hood of a freshly waxed car. The droplets sit high and rounded on top of the surface. This is exactly what plastics decorators dont want when trying to provide a clean and consistent finish. So, what can be done?
The first step to better adhesion on plastic surfaces is removing all soils, dirt and other foreign materials. Common soils include fingerprints, dust, lint and mold release residues. In most instances, cleaning can be taken care of with a power wash followed by a solvent wipe. However, this process should be audited regularly because the detergent residues can sometimes inhibit adhesion, resulting in water spotting or other appearance problems.
An innovative alternative to traditional plastics cleaning is the use of CO2 integrated spray, comprising a jetted stream of CO2 particles and a propellant gas focused at the surface.
Cool Clean Technologies, the global leader in CO2-based cleaning technology, has installed and implemented more than 50 systems for automotive plastics cleaning. CO2 spray cleaning has been mandated for use in cleaning interior plastic parts by BMW and other German OEMs. Also, customers have concluded that CO2 integrated spray cleaning yields equivalent or better cleaning results to traditional aqueous-based systems with economic benefits of significant energy and other operating cost reductions up to 80 percent, with a factory footprint that is less than 20 percent of a powerwash system.
CO2-integrated spray cleaning is a dry, non-condensing, waterless technology that leaves behind no residue post cleaning. Surface condensation is prevented by utilizing a heated air stream as a propellant for the CO2 particles. The effect is that CO2 integrated spray delivers a clean, dry and no-touch cleaning method that can be “dropped-in” to any production line. Automating the CO2 spray offers better and more consistent cleaning, while reducing scrap costs.
“Rework and scrap reductions can be significant,” according to Nick Heisler of Cool Clean Technologies. For example, one leading Tier 1 customer was experiencing 40 percent rework of a large plastic exterior part and, after implementing an automated four-robot/CO2 cleaning system, this rework rate dropped by more than 60 percent. Heisler added, “By implementing CO2 cleaning, they eliminated four of their six rework stations.”
Plastics also have the tendency to build up static charges that attract lint and dust particles. Static can be a problem despite the best detergent cleaning because much of the debris is attracted to the part after the wash cycle. For the best results, keep the paint booth at around 50 percent relative humidity, and supply it with filtered air. Make sure anyone entering the booth is wearing lint-free coveralls, hairnets and shoe covers. If static attraction is still a problem, consider using static neutralizing equipment or an ionized air gun. These products filter intake air and blow ion-laden air across the part so positive and negative ions can neutralize all static charges.
Christian Trimborn of Hildebrand Technologies added, “Applying ionized air to a plastic part prior to painting not only removes debris from the part, but it also neutralizes the part and debris to ensure it doesn’t reattach to the part before reaching the booth air filter.” Cleaning with ionized air should be performed directly before painting the part to ensure maximum cleanliness and to prevent any recontamination.
Another common culprit of poor plastic paintability is mold release agents used to facilitate the separation of plastic parts from the molds. A number of mold release agents exist, and some can cause major adhesion problems for paint. Water-soluble mold releases are preferred, because removal is easily accomplished with detergent washing. Wax-type mold releases are sometimes removed by solvent cleaning, but it is costly and has negative environmental impacts.
Mold releases also are sometimes blended into the plastic formulations. These are called internal mold releases. Internal mold releases have a tendency to migrate to the surface and cause paint adhesion failure months after the part is painted. For finishing purposes, it is often recommended that this process be avoided altogether.
Plasticizers also have an adhesion-limiting effect similar to that of mold releases. These chemical additives are used to increase the strength of the plastic, but they migrate slowly to the surface and cause various types of separation from the paint surface long after it has been painted. Make sure to understand every aspect of the chemical makeup of the plastic, as it alone may cause adhesion problems.
Overcoming problems to achieve the best paint adhesion
Now that the common causes of poor paint adhesion and how to prevent them have been discussed, there are a few other methods for increasing the surface wettability of plastics.
Plastic surfaces are naturally smooth, making paint adhesion difficult. This can be overcome by slightly roughing the surface by chemical or mechanical means. The most common way of overcoming surface smoothness on plastics is to etch the surface with a chemical agent. In best-case scenarios, the solvent is already present in the paint to avoid extra processing steps; however, different plastics require different solvents and the etching solvent might not be the same as the paint solvent. Etching needs to be controlled, as over-etching and under-etching can be detrimental to the process and the plastic.
Sometimes when part shapes are molded, certain areas produce significant frictional heat due to rapid plastic injection. This can cause a glazed skin that is resistant to solvent etching. To combat this overly hard plastic skin, blast the surface with a mildly aggressive grit material or use a solvent vapor immersion treatment.
When deglazing or etching is not an effective or viable option, it may be necessary to induce a chemical reaction by running it through an open flame. This is common for extremely non-polar plastic surfaces like polypropylene and polyethylene. The open flame initiates an oxidative chemical reaction that forms enough polarity to the surface to provide excellent paint adhesion.
Passing plastic parts through an electrical corona discharge also can be an effective oxidation technique. It activates the surface using high voltage electricity. Not only is this better for adhesion, it also creates an ion field that neutralizes static to help remove dust and other particle contaminants.
Plastics with low polarity also can be surface oxidized using light-sensitive chemicals called photosensitizers. This process is followed by exposure to ultraviolet light.
When a gas is forced to absorb energy, it becomes ionized and is considered plasma. Gas plasma treatment, typically performed in a tightly sealed vacuum vessel, creates micro etches and activates the surface. This method creates such excellent adhesion that it makes plastics compatible with the same paints used on metal surfaces. This can be an added bonus for manufacturers decorating both plastic and metal parts.
Once preparation is completed, there are numerous options for actually applying the paint. All types of spray guns used for metals and various other materials are acceptable on plastic. Conventional air spray guns have always been an industry standard, but companies that choose this type of technology may be missing out on the cost savings, waste reduction and lower VOC emissions obtained by using electrostatic spraying technology.
Electrostatic spray guns electrically charge atomized paint material, which is then attracted to the grounded part. This phenomenon greatly reduces overspray and allows the particles to “wrap” around the back of the part to provide paint coverage on both sides. The resulting benefit is increased transfer efficiency, meaning more of the coating sprayed actually lands on the part instead of ending up in the booth filters. In fact, field studies have shown electrostatic applications provide 55 to 75 percent transfer efficiency, while non-electrostatic guns only yield about 30 to 45 percent.
In order for electrostatic spraying to work, the substrate typically needs to be conductive to provide a proper ground for attracting particle. However, the conductivity requirement for electrostatic spraying doesn’t entirely exclude non-conductive plastics from utilizing this technology. Many customers utilize both electrostatic spray guns and rotary atomizers for painting plastic parts.
Painting non-conductive plastics can be accomplished in a few different ways. With the many options available, it is important to find what works best for each plant. Additives like conductive sensitizers, adhesion promoters or conductive primers can be applied prior to finishing, creating a conductive surface on the part. Typically, these additives have a two-fold benefit of improving surface adhesion, but also making the part conductive enough for electrostatic spraying.
Another technique, commonly used in the tier one automotive industry, is called ground imaging. With imaging, the plastic part is placed over a conductive metal, which acts as a grounded image. Atomized particles are then attracted to the image behind the part and, in turn, cause greater transfer efficiency on the non-conductive plastic part. The benefits of imaging are twofold. It not only improves transfer efficiency, but also serves as a fixture, ensuring the part does not warp during the oven process and places the part in the correct position for robotic painting.
For most plants, painting is a key process that can have challenges with plastic materials. The key is to make sure that the plastic has a clean and adhesion-friendly surface and that the application of paint is done in the most efficient manner. All of these techniques for both surface preparation and painting can be used individually or combined, depending on the makeup of the substrate and measures needed to create a strong attraction and efficient application.
Nick Strauss is a global markets manager responsible for growing and developing the automation market for the Industrial Products Division at Graco. He has been with Graco since 2005, performing in a variety of roles including engineering, operations management, product marketing and business development. Since 1926, Graco Inc. has been a leading provider of premium pumps and spray equipment for fluid handling in the construction, manufacturing, processing and maintenance industries. Headquartered in Minneapolis, Minnesota, Graco works closely with distributors around the world to offer innovative products that set the quality standard for spray finishing, paint circulation, lubrication, sealant and adhesives dispensing, process application and contractor power equipment. For more information, email [email protected] or visit www.graco.com/automation.