As published in the November 2005 issue
Waterborne Coatings for Use on Glass
By Ken Burton, President of Finishes Unlimited
A couple of centuries ago, writer and philosopher Charles Caleb Colton was quoted as observing, “Where we cannot invent, we may at least improve.”
The same can be said about glass: Nature invented it, man discovered it around 5,000 B.C. and has been working to improve upon it ever since.
Glass-related research still continues, and on of the most recent enhancements has been the application of specially formulated waterborne coatings to sheets of clear glass, which provides them with a variety of colors and hues. The furniture industry has been the first to adapt this concept but when the creativity of product development professionals, designers, architects and contractors becomes stimulated, it could have a range of other applications.
These of course are still just technology-driven enhancements as glass historians say that colored glass was accidentally discovered sometime around 3,500 B.C., architectural glass was introduced about 100 A.D. by the Romans and the 11th century saw German glass craftsmen create glass sheets.
When a manufacturer of small, painted glass tabletop items for restaurants was looking for a safer, more environmentally friendly way to coat their tables, they went to Finishes Unlimited about a solution. At the time, the tables were being coated with solvent-based paint, which provided adequate coverage and appearance, but the manufacturer wanted to avoid the environmental, health and storage issues these coatings presented.
A waterborne coating was developed that incorporated the dyes initially developed for solvent-based coatings. It had a high level of solids, making it suitable for spraying or dipping and achieved coverage levels of between 500 and 625 ft2 per gal.
Minimum drying times were important, and the waterborne formula allowed painted items to be handled in 6 min after spraying in ambient temperatures, depending on humidity, with no forced air circulation. When placed in an infrared oven at 125°F, coated items dried in 2 minutes. Field use determined that after a 1 minute flash-off, coated items would dry in less than 40 seconds when passed through air being circulated at 160°F.
It was important that the waterborne coating withstand cold, heat and high humidity without discoloring, cracking or peeling. For example, the table top times were frequently stored for extended periods in warehouses without climate controls and were eventually used in indoor and outdoor locations with similar environments. During one adhesion test, items painted with the waterborne coating were placed in a 125°F oven for 72 hours with no change in performance properties. In another test, coated products were placed in a humidity cabinet under ASTM D-4585 criteria to determine adhesion failure points. The water-based coating did not fail until 24 hours had passed.
Painted objects were also tested in accordance with ASTM D-2803-82 specifications by placing them in a 10-gallon humidity chamber at 85 percent relative humidity. After 30 days, the water-based coating exhibited superior tape pull performance.
At the same time, “real life” conditions were replicated. No color deterioration or appearance change occurred in items exposed to direct sunlight or rain during a one-month outdoor test. They also resisted damage from variety of liquids such as coffee and alcohol.
The waterborne coating offered important production advantages as it present no fire hazard and may safely be stored without special combustion precautions. All cleanup of the painting area can be accomplished with tap water, eliminating the need for any solvent cleaners. From an environmental standpoint, the VOC levels of the coating ranges from 1.1 to 1.4 lb/gal sans water compared to 4 to 6 lb/gal sans water for solvent coatings.
Not Just for Appearance Sake
Waterborne glass paint has a range of product design and architectural applications. It allows glass to become a design element for many types of electronic, office and home entertainment products for which appearance is a selling point.
While one customer wanted color coatings, another was only interested in a clear coating for use in protecting etched glass windows, mirrors and furniture. These products are often magnets for soil and grime. Most people put their hands on glass entry doors when leaving or entering a building, leaving dirt deposits from their hands or gloves. When frosted glass banquet dividers are used in restaurants, airborne pollutants like smoke, grease and food particles settle onto the glass. Also, some people like to experience the tactile sensation of running their hands over the sandblasted portions. Even workers installing glass can leave dirt, oil and other marks so they can start out dirty.
Cleaning dirty, frosted glass is very difficult, if not impossible, in some cases. The only viable option is to remove the glass and re-sand blast it, taking away yet another layer of glass.
For this company, a totally transparent waterborne coating was formulated that could be sprayed over the glass immediately after it had been etched to prevent soil and grease from penetrating into the porous designs. It had to be totally transparent so as not to counter the appearance of the etching but still allow the glass to be cleaned with a soft cloth and common glass detergent. As the client says, “Most people put glass over art to protect it. We put coating over glass for the same reason.”