Exterior UV-Curable Topcoat For Physical Vapor Deposition Applications
by Kristy Wagner and Jennifer R. Smith, Red Spot Paint and Varnish Company, Inc.
Growing environmental concerns with chrome plating have caused finishers to request a greener alternative. Coatings for multi-purpose decorative and automotive lighting PVD applications have been used in the UV-curable coating industry for over twenty years. However, these coatings do not have the required durability to replace chrome plating. Recent developments with a UV-curable topcoat for PVD that provides the performance characteristics needed to pass the OEMs toughest requirements.
Chrome plating can be classified as either hard chrome plating or decorative chrome plating. Hard chrome plating is typically used on steel and is very durable; it is applied as a thick layer on items such as hydraulic cylinder rods, piston rings, thread guides and gun bores. Decorative chrome plating is generally used on plastic materials and can vary in its durability, depending on the process. An electrodeless nickel layer must be applied first. The number of subsequent layers can vary, which can determine the finished quality. The fewer steps utilized, the lower the durability of the end product which can lead to quality defects of the chromed part in the field.
The process of chrome plating requires multiple steps. For optimum appearance, cleaning, polishing, buffing and rinsing should be done for each step in the process. In addition to the length of the process, chrome plating is not environmentally friendly. All of the waste products (including rinse water) are regulated and must be disposed of legally as hazardous waste. Furthermore, chrome plating is done with hexavalent chrome, which is an extreme health hazard. Without proper safeguards, very small amounts can leach into the ground water and contaminate large areas very quickly. Closely regulated by the EPA, it is becoming increasingly difficult to operate a chrome plating facility in the United States.
Despite all of the hazards of chrome plating, the end product is aesthetically pleasing. When decorative chrome plating is processed properly, it is a durable product. When short cuts are taken, the end product can fail in the field. Finishers have been asking for a safer, greener, quicker alternative without sacrificing appearance and performance.
Chrome Alternatives Using UV/PVD Coatings
Physical Vapor Deposition (PVD) on thermosets and thermoplastics has been around for many years. The automotive lighting market has been vacuum metallizing plastic reflectors and bezels for more than 20 years. PVD for non-automotive finishing has been in use even longer. Cosmetic packaging, interior decorative finishing, cell phones and lighting louvers are being finished with PVD; this technique provides a value-added aesthetic to the end user by making plastic components appear to be metal. A variety of PVD metals can be applied. Aluminum is the most common metal used in automotive lighting; tin is used for cell phones because it is non-conductive. Stainless steel, chrome, titanium, silver and nickel chrome also are used in the PVD industry.
PVD with a UV topcoat also can replace chrome plating for interior automotive applications. Since interior parts are not subjected to as severe exterior requirements, aluminum or other metals can be used instead of chrome or chrome alloy. However, the coating must pass a series of chemical resistance and abrasion resistance tests. Since UV topcoats can become tightly crosslinked when cured, they are a natural choice for interior automotive coatings.
Although automotive lighting is an exterior market, the reflectors are protected by a polycarbonate lens that has a weatherable and scratch-resistant coating. The basecoat/PVD/topcoat need only withstand the rigors of a high-heat environment. For a true exterior durable product, the requirements are more stringent. In order to match the appearance of chrome plating, PVD chrome is naturally the first choice. PVD aluminum could be used to match the appearance of chrome plating, but its lack of exterior durability can be a potential problem. Although the metal will be protected by a clear coat, if the topcoat becomes chipped, the chance of moisture reaching the metal layer increases. When PVD aluminum is exposed to water, it can start to oxidize at best and lose adhesion to the basecoat at worst. PVD chrome does not have this tendency; therefore, chrome or a chrome alloy is both the aesthetic and the durable choice to replace chrome plating for exterior applications.
The main target of the UV-curable topcoat development for PVD was to replace traditional inorganic chrome with a layer system of organic and inorganic materials. As illustrated below, this involves applying a UV basecoat on the substrate followed by a PVD metal layer, and lastly the UV protective topcoat. Challenges associated with the development of each of these layers and the processing of each are explained in the following paragraphs.
Substrate: A wide variety of thermoset and thermoplastic substrates can be coated with the UV basecoat. PC, ABS, PC/ABS, PA/PPE and PC/PBT are commonly used plastics for exterior rigid and semi-rigid automotive parts.
UV Basecoat: The surface that the metal is deposited on must be smooth and continuous. If it is not, the metal will not be reflective leading to a dull appearance. Some parts are direct metallized; however, this requires a higher grade of thermoplastic. It also demands that molds must be kept in optimum condition and polished regularly to ensure the surface of the parts are free from defects. To compensate for less than stellar conditions, a basecoat is applied to ensure optimum smoothness.
For the most robust system and for adequate performance of an exterior durable coating UV/PVD system, a UV-curable basecoat is necessary. A successful coating must have excellent adhesion to a variety of substrates as well as be able to accept PVD metals. Chrome is more durable than aluminum, so the basecoat was formulated to target the PVD chrome metal. Due to chrome being a very rigid metal, many commercial basecoats that work well with aluminum may not work with chrome. Stress cracking is a very common failure mode if it is not formulated specific for PVD chrome. Many thermal cure coatings lack the proper crosslink density to be used with the more rigid metals. The UV basecoat serves as a leveling coat to provide a smooth surface, which helps to contribute to the brightness of the metal.
PVD: PVD (Physical Vapor Deposition) is the deposition of a metal onto a substrate through changes in the physical state of the metal (solid to gas to solid). A very thin layer of metal, approximately 600 1000 angstroms, is deposited onto the basecoat layer. A wide variety of metals can be deposited including aluminum, chrome, titanium, stainless steel, nickel chrome and tin, etc. The PVD layer can be deposited by a variety of methods, the two most common being thermal evaporation and sputtering. Both are done in a vacuum, but the metals are deposited differently.
Thermal Evaporation is the deposition of a metal via thermal vaporization in a vacuum environment. The metal is in the form of a cane. It is placed inside a tungsten coil; the number of coils can vary depending on size of the chamber. Once the chamber is pumped down to a vacuum, the tungsten filaments are heated to 1200°F (for aluminum) enough to melt the metal. The power to the filaments is then increased to roughly double the temperature and the metal is evaporated. The metal then re-condenses on the parts in the chamber.
Sputtering is the deposition process where atoms on a solid metal target are ejected into the gas phase due to bombardment of the material by high energy ions. The bombardment releases atoms from the metal target, which are deposited directly onto the part within the vacuum chamber. Metal thickness will vary depending on the cycle time and power applied to the target. Alloys can be used with either method, but they will be deposited differently. With thermal evaporation, the metal with the lowest melt temperature will evaporate first and deposit onto the part. Rather than having a deposition of an alloy, there will be two distinct metal layers.
UV Topcoat: To protect the metal, a topcoat needs to be applied. This can vary from a thin layer of in-chamber siloxane to a thicker thermal or UV-curable topcoat. The choice will vary depending on application and needed performance requirements. For exterior purposes, there are currently OEM-approved thermal, two-component coatings and thermal powder coatings on the market. However, these coatings are not a panacea. The 2K coatings lack both environmental and processing friendliness. Powder coatings are more environmentally friendly. However, not only do the long bake times hinder productivity, the high temperatures required to cure the powder will not work with most thermoplastic substrates. A UV-cured coating would meet both the environmental and process requests.
Until recently, there have not been acceptable UV topcoats to protect the PVD chrome. PVD aluminum, being less rigid than chrome, is much easier to adhere to. Although initial adhesion to chrome is relatively easy to achieve, maintaining that adhesion after humidity, water immersion and weathering can be a bigger challenge. In order to obtain proper adhesion to chrome, the coating needs to have low shrinkage and lower crosslink density. However, these characteristics can lead to a soft, easily marred coating that cannot withstand the testing rigors of an OEM specification. It is imperative to find the balance between too rigid to get adhesion and too soft to pass resistance testing.
The properly formulated UV-curable topcoat can meet these demands. It can pass up to five pints of gravel chip resistance, at room temperature and -30°C; resistance to various solvents and cleaners; 49°C humidity for 240 hours; 80°C water soak for 3 hours; 3000 kJs Xenon accelerated weathering and 2 years natural weathering. The topcoat serving as a protective layer is necessary due to the metal being deposited so thinly.
Advantages of UV/PVD Coatings
In a direct comparison, PVD chrome samples with a UV-curable topcoat have shown to be the same performance as decorative chrome plating, and superior in hydroflouric acid tests. Chrome plating has shown to have superior scratch resistance; however, if the UV topcoat is compared to approved systems in the market today 2K clear coat for automotive bumpers and fascias; thermal cure powder for automotive clear coats there is no difference in scratch resistance. Additionally, there are alternative processing methods for the topcoat that increase the surface hardness to be comparable to that of chrome plating. Other advantages include environmentally friendly process, increased throughput, wide range of plastic substrates can be utilized, large range of appearances is obtainable. UV/PVD also allows for design flexibility and lower capital investment.
Testing has been completed for several automotive and heavy truck OEM specifications. Targeted parts are components that are currently classified as decorative chrome plating on plastic substrates. This includes interior, exterior and under the hood components, such as badges, various trim pieces, door handles, console parts, mirrors housings, wheel hubs, grilles and air filter housings, etc.
The PVD/UV-cured technology has been approved for some vehicle applications, such as the new Ford Taurus tail light surrounds and the interior door parts for the Jeep Compass and Patriot. Several end-component applications at various OEMs are in the process of obtaining part approvals.
The automotive market continues to value bright finishes for exterior and interior components. Decorative chrome plating can provide this look aesthetically. However, the associated environmental issues and the limited design flexibility have created a need for a new technology alternative. With the proper formulation, UV coatings along with the PVD process can offer an environmental, economic and performance alternative. n
Special thanks to Chris Mack, UV chemist, and Dave Ingle, UV applications engineer with Red Spot for their assistance with the UV/PVD technology. Red Spot Paint and Varnish Company, Inc. is a global leader in the development and production of high-performance coatings for a variety of industries including automotive, packaging and appliances. With over 20 years of UV experience, Red Spot is an industry leader in the design and development of high-performance UV formulations, application support and line design. For more information on UV-curable topcoats for PVD, call 800.457.3544 or visit www.redspot.com.