Technology of the automotive refinish future: a look at the potential for UV coatings in each of the application steps for an automotive refinish repair.

The basic challenges with automotive refinish coatings are to prepare invisible repairs and these repairs are carried out in several application steps. These steps usually consist of primer, basecoat and clearcoat applications. Generally each of these applications, done at room temperature, takes a combined amount of time ranging from four hours to several days. In some instances, there are other steps such as body filler, wash primers, adhesion promoters and sealers that can add additional time. In all cases, the user needs to wait for the coating to cure and time is wasted.

Most of the conventional refinish coatings consist of two-component (2K) material and have limited pot lifes for applications. This limits the amount of time the coating can be used and applied. When using two-component chemistry there is also more waste generated from not knowing how much material is needed for each repair. Waste is also generated from the clean-up of the application equipment when used. There is also time wasted mixing material and having to clean application equipment after each of the application steps.

There is another concern with conventional refinish coatings dealing with consistency. With an air-dry coating, temperature and humidity have an affect on the cure of the coating. If the coating is under-cured in any of the steps, there could be factors such as adhesion loss, dieback, swelling and appearance imperfections that are not seen for several days. When this happens, the repairs usually have to be repeated, taking up more valuable time.

With UV technology being applied to automotive refinish applications, the issue of time, waste and consistency can be overcome. UV technology will assure the user that a repair has been performed correctly, in a relatively short amount of time with minimal waste.

UV POTENTIAL IN REFINISH APPLICATION STEPS

In this paper four application steps will be discussed:

* Body fillers

* Primers

* Basecoats

* Clearcoats

Each of these steps are dependent on proper preparation and application of the previous steps. If any of the steps are done improperly, the repair area is visible and would need to be redone.

In this paper, some of the common advantages of using UV coatings in automotive refinish are discussed in each of these steps. Assuming all the UV coating in each of the steps are one-component (1K) materials, several common advantages can be listed. These advantages are mixing of materials, unlimited application window, limited waste and less application equipment cleaning.

However, with UV coatings in automotive refinish, there are several challenges that need to be mentioned. The biggest challenge is UV light sources. Due to the hazards of UV light exposure, it is recommended that lights that are only UVA be used for this industry. With this stipulation, the cure time, repair area and distance of the light to the substrate are all major concerns for each step. With the low irradiance and wavelength of UVA lamps, several challenges are presented in each of the application steps. A challenge for light manufacturers for refinish markets, is to meet the need for larger UVA lights for repairing larger areas. Ideally, the ultimate goal would be to be able to cure a whole vehicle, no matter what the shape of the car body was.

With each of these steps, the advantages and challenges of UV technology will be evaluated. The challenges for the UV industry will also be mentioned to make UV the technology of the future for automotive refinish coatings.

BODY FILLERS

Body fillers are used in automotive refinish applications to fill in imperfections on the automobiles body. Currently, these body fillers consist of two-component, peroxide-cured materials that have a five-minute application window. This limits the application time and limits the areas that can be filled in a single application. With one-component UV-cured body filler, application time is not a concern. Some of the advantages of UV-cured body fillers include:

* Shorter cure times. With conventional styrene/peroxide-cured body fillers, the reaction was relatively fast, however the cure times were 15 minutes before sanding. With UV body filler the dry times are two-minutes or less before sanding, thus saving ten plus minutes per application.

* Shape to body. With 1K material, the material can be shaped to the contour of the body until it is perfect. Once the user is satisfied with the appearance of the body filler, the UV light would be used to cure the UV body filler to the perfect shape.

* Less post-preparation. Due to the nature of being able to shape the body filler to the contour, less sanding and preparation of the repair needs to be done after cure. This allows the repair to be processed faster.

* Air release. Since the cure of the body filler does not take place until the UV light is exposed to the area, air trapped in the body filler, due to the application process, has time to be released out of the filler.

* Removable. At any point of the application at the repair area, the filler has the capability of being completely removed before being cured. This allows for areas not meant for the filler to be removed before filler is cured.

Some of the challenges of UV-cured body filler include:

* Curing thick films. Due to the nature of body fillers and the thick application, there is a need to cure thick films, sometimes up to three centimeters thick. The challenge with UV cure is that light will only penetrate a certain thickness of the film before being absorbed. Therefore the challenge is to be able to cure these thick films with UVA-only lights.

* Shrinking of filler. With UV coatings, these films seem to shrink due to the stress of the reaction. With a body filler, shrinking of the film would affect adhesion and finish of the body filler. UV fillers with little or no shrinking would need to be developed for refinish applications.

* Adhesion to substrates. With body fillers being used on several different substrates, adhesion directly to these would be a major challenge. UV body fillers would need to be capable of going over all substrates.

* Pigment loading. Most pigments absorb UV light, therefore the pigment loading of the body filler would have to be relatively low. The challenge is to develop a body filler with high pigment load that would still cure at thick levels.

* Hard to sand. UV body fillers would be completely cured after UV exposure. Due to this high crosslink density, UV curable body fillers may be hard to sand. Also, because of the pigment loading in the body filler being low, the sanding properties would rely heavily on the resin characteristics.

PRIMERS

Primers are used in automotive refinish applications to fill in imperfections of the body filler or automobiles body. Primers are also used for adhesion to the substrate and for normalization of the repair area surface for acceptance of the basecoat/clearcoat system. Currently, most durable primers are two-components. These two-component materials are either urethanes, epoxies or non-isocyanate. The materials have a one-hour to eight-hour application window, depending on the chemistry. Usually the cure times, before the next step can be applied, are one-hour to overnight cure at room temperature, depending on the chemistry. These materials can take up to several weeks to reach complete cure and film properties can change in this time. With a one-component UV-cured primer, application time is not a concern and complete cure can be obtained in several minutes. Some of the advantages of UV-cured primers include:

* Shorter cure time. In conventional 2K urethane or epoxy systems, the cure times are at least 45 minutes or longer before sanding or applying topcoat. With UV primer the dry times are in two minutes or less before sanding, thus saving 35-plus minutes per application.

* Lower VOC. With UV primers, the VOC of the coating could be reduced at least 50% and could eliminate VOC completely.

* Solvent resistant. Immediately after cure of the UV coating, the film is completely solvent resistant and resists lifting or swelling.

* Shrinking or dieback. Conventional 2K basecoats continue to cure over time and can show imperfections, sanding marks or edge lines after several days. Because of the complete cure with UV, films will not shrink or dieback, eliminating these factors.

Some of the challenges of UV-cured primers include:

* Pigment hiding. Due to UV light having to penetrate the films, pigments that absorb UV cannot be used in order to cure films.

* Curing thick films. UV light will only penetrate so far into the coating causing uncured bottom layers when thick films are applied.

* Flexibility over plastic. The high crosslink density of cured UV coatings must be balanced with flexibility for use over plastic bumpers. This is necessary in order to prevent adhesion loss or cracking.

* Cured surface. With oxygen inhibition, surface cure is a challenge in order to be able to sand the primer or for topcoats to go directly over the primed surface.

* Adhesion to all substrates. With several different substrates used in the automobile industry, it is necessary to have adhesion to all substrates.

BASECOATS

Basecoats are used in automotive refinish applications to apply the color portion of the repair to match the color of the automobile body. Basecoats need to have adhesion to the primer. Basecoats can contain aluminum metals, micas or pearls as well as colored pigments. Currently, most basecoats are one-component systems based on acrylics or polyesters and may be solvent- or water-based. These basecoats are not resistant to solvent and can swell if clearcoat is applied too quickly. These materials have an unlimited application window and require several coats for application. Usually the cure/dry times, before the next step can be applied, are at least 15 minutes and up to overnight cure at room temperature, depending on the chemistry. These materials are usually solvent sensitive even after cure and are usually the weak part in a refinish repair. With a one-component UV-cured basecoat, the main advantage would be complete cure in several minutes and chemical resistance. Some of the advantages for UV-cured basecoat include:

* Shorter cure times. Conventional 1K basecoats cure times are at least 15 minutes or longer before applying clearcoat. With UV basecoats the dry times could be in two minutes or less before clearcoating, saving ten plus minutes per application.

* Lower VOC. With UV basecoats, the VOC of the coating could be reduced 50-80%.

* Solvent resistant. Conventional basecoats are not solvent resistant and when clearcoat is applied, the solvents from the clearcoat can affect the basecoat. With UV basecoats, the film is completely solvent resistant immediately after cure and there is less potential of lifting. UV basecoat films will also allow for multiple color applications to be done due to the solvent resistance.

* Shrinking or dieback. Conventional 1K materials continue to shrink as solvent leaves the film, which over time can cause the film to dieback or lose gloss. Because of the complete cure with UV, films will not shrink or dieback, eliminating this factor.

Some of the challenges for UV-cured basecoats include:

* Curing different pigmented films. Most refinish basecoats consist of tinting bases blended together to makeup the automotive color. These different color bases will need to have photoinitiators that can cure all colors at the same rate.

* Adhesion to all primers. With several different primers used in the automobile industry, it is necessary to have adhesion to all different types of primers.

* Adhesion to all clearcoats. With several different clearcoats used in the automobile industry, it is necessary for all different clearcoats to have adhesion to a UV basecoat.

* Metallic orientation. Most refinish colors have metallics in the coatings and orientation of the metallics is important to get proper color match. With UV basecoats, the fast cure can alter the orientation of these metallic flakes.

CLEARCOATS

Clearcoats are used in automotive refinish applications to protect the basecoat and for the high-gloss finish. Clearcoats need to have adhesion to the basecoat and need to protect the whole coating system from the exterior. Currently, most clearcoats are two-component systems based on acrylics or polyesters and crosslink with isocyanates. These clearcoats usually contain additives to protect the coating from UV light from the sun. These materials have a limited application window and require several coats for application. Usually the cure times for clearcoats are at least one-hour and up to overnight cure at room temperature, depending on the chemistry. These materials are usually solvent sensitive and can shrink up or dieback even after several days. With a one-component UV-cured clearcoat, the main advantage would be complete cure in several minutes and chemical resistance after cure. Some of the advantages of UV-cured clearcoats include:

* Flow and leveling. Conventional 2K clearcoats start to cure immediately after mixing and if material has gained viscosity this increase can affect the flow and leveling. With UV clearcoats, the material will stay at a constant viscosity and will allow the coating to flow and level before being exposed to UV light.

* Shorter cure times. Conventional 2K clearcoats cure times are at least 45 minutes or longer before the film is dry to the touch. With UV clearcoats the dry times could be in two minutes or less, saving over 40 minutes per application.

* Lower VOC. With UV clearcoats, the VOC of the coating could be reduced 50-80%.

* Solvent resistant. Conventional 2K clearcoats are not solvent resistant for several days and can even be affected by water within one day. Immediately after cure, UV clearcoat films are completely solvent and water resistant.

* Shrinking or dieback. Conventional 2K clearcoats continue to cure and shrink over time causing the film to dieback or lose gloss. Because of the complete cure with UV, films will not shrink or dieback, eliminating this factor.

* Scratch resistance. UV clearcoats can be scratch resistant within a relatively short period of time. This will allow UV clearcoats to be more abrasion resistant from dirt and scratches than conventional 2K clearcoats.

Some of the challenges of UV-cured clearcoats include:

* Exterior durability. Due to UV clearcoats being cured by UV light, they also have to be resistant to UV light from the sun. Most of the problems that can occur over time are cracking, loss of adhesion or yellowing of the film.

* Oxygen inhibition. The use of UVA lamps can produce uncured layers that are caused by oxygen inhibition. Since the clearcoat is the last layer of the final film, the surface needs to be cured and completely tack-free. Overcoming this challenge is necessary in clearcoats.

* Photo-bleaching. The photo-bleaching of the UV needs to happen immediately after cure, since most repairs are evaluated for color match after the film is cured.

* Adhesion to all basecoats. With several different basecoats used in the automobile industry, it is necessary to have adhesion to all different types of basecoats.

* Flexibility. With a high crosslink density of UV coatings, flexibility of clearcoat is necessary to prevent adhesion loss or cracking. The clear will have to be flexible in different temperature conditions and be flexible to stone chips.

In conclusion, there is a tremendous potential for UV coatings to be used in the automotive refinish market. UV technology can help to eliminate mixing of materials and pot lifes, allowing the user to have an unlimited application window. From an ecological perspective, UV coatings in refinish will lower VOC emissions and reduce material and cleaning waste. The major advantages of using UV coatings in refinish are time-saving and the reduction of poor quality repairs. Hopefully a better understanding of the automotive refinish market is now known and better resins, photoinitiators, additives and pigments suited for this market can enhance the UV technology. The challenge for the UV industry will be to make UV technology the safe future of automotive refinish coatings.

BY THOMAS J. LAGINESS

BASF CORPORATION