Developments in U.V. Curable Coatings for Automotive Plastics 940858
In the past five to ten years, there have been many new and helpful coating and processing technologies made available to the automotive engineer.
The most common of the current technologies include vacuum metallizing systems for headlamp and taillamp reflectors. Accompanying the reflectors are plastic lenses coated with scratch resistant coatings to protect and sustain the assemblies. Included also are turn signal housings, claddings and moldings, all of which require coating protection.
Sound engineering and processing are most important for producing quality products. U.V. curable coatings require certain radiation dosages to afford proper cure, and thus dictate input from engineering, processors, equipment and coatings suppliers in order to do the job adequately. Contemporary automotive manufacturing demands the marriage of all constituents to insure that “Quality is Job 1.”
New and exciting technology is continually being discovered to enable U.V. curable coatings to be not only one of the fastest growing current technologies, but is expected to increase in specification and consumption in the future as well. Higher temperature resistant reflector coatings will allow for more streamlined designs. Coatings are being developed to maintain low gloss control on instrument panel cladding to provide anti-glare surfaces for the customer.
Curing coatings by high intensity ultraviolet light makes the future of high performance automotive plastic components exciting and innovative to our industry.
Prior to discussing new developments in U.V. curable coatings, one must have an understanding of certain foundation definitions and principles. When referring to coatings systems the three basic categories are:
Lacquer - has its' ultimate properties when the carrier (solvent or water) is evacuated from the film.
Varnish - cures by oxidation/polymerization of the oil length portion of the coating.
Enamel - undergoes an actual crosslinking, or conversion, of multiple constituents to create a new and different molecule.
Ultraviolet curing offers a new and exciting technology for producing resistant, cross-linked coatings.
Baking enamels have long been utilized as high-quality finishes for products that require stability, such as headlamp reflectors.
Typically, enamels are applied by spray or flowcoating and require a high temperature for crosslinking or curing. Typically 30-60 minutes at 250° - 400° F is common for curing, depending upon the enamel and the substrate.
As one can see by the curing schedule, incredible energy costs, tooling and labor penalties would be realized to effect a high volume production line with enamels.
The passage of the Clean Air Act in the mid-1970's restricted solvent usage with conventional coatings (approximately in the 18-28% solids range at application) therefore forcing the utilization of coatings with less volatiles (more in the 30-40% solids range).
Even newer air pollution legislation in California is so severe that the small amounts of coupling solvents found in latex paint is said to be too great a concentration.
Very soon governmental regulation will force decorating industries to use 60-70% solids-coatings, thus heavily restricting solvent emission.
Ultraviolet cured coatings possess superior physical properties such as:
Chemical resistance to solvents, oils stains, perfumes and perspiration.
Abrasion resistance using test media such as Taber Abrasion and 0000 steel wool.
Hardness and scratch resistance (4-6H pencil hardness).
Improved exterior durability for plastics that yellow when tested to QUV and Weather-O-Meter environments.
Flexibility for impact resisance on automotive engineering plastics as well as recreational equipment.
Reduced iridescence, drop-thru and stress cracking with vacuum metallizing due to undercure of the basecoat.
Ultra-violet cured coatings greatly enhance speed of curing in seconds versus minutes or hours, increased thru put and reduced floor space for equipment. U.V. coatings also do not distort low-temperature plastics because there is no heat build-up. They have very high reflectivity for vacuum metallized and sputtered parts due to high film build. Higher solids coatings are used, which affords less solvent handling and emissions problems. In short, U.V. curable coatings are a giant technological step forward for many industries.
This paper will be concerned about two specific types of U.V. coatings:
Vacuum Metallizing Systems for headlamp/taillamp reflectors and interior automotive trim.