"With coal tar contractors sometimes push the limits of the material because they've established a comfort level over time with its expected performance," he says. "But if they push the limits with asphalt-base sealers, which are more fragile than coal tar, they very well could encounter consequences not otherwise experienced in the past using coal tar. It's very important that they stay strictly within the limitations of the material as far as mix design and weather conditions are concerned."
Dubey says that because of asphalt's thermoplasticity, it needs to be applied when the temperature is 50°F or higher and contractors can't risk applying it in lower temperatures.
"Thermoplasticity is the temperature at which a material becomes fluid," Dubey says. "Asphalt-based sealer requires higher temperatures to become fluid than does coal tar sealers. It requires higher temperatures for asphalt particles to come together to form a continuous film, which is why asphalt sealers can't be applied unless the temperature is at least 50°F. If it's not at least 50°F the film formation will be jeopardized, the film won't form properly, and it will wear out prematurely."
Dubey adds that asphalt-based sealers can't be diluted more than 25% with water and they also dry slower than coal tar sealer. He recommends that, unless weather conditions are ideal, and particularly in spring and fall, contractors should allow as much time between coats as possible, 8 hours at least but 24 hours is preferable.
"That's something contractors need to be aware of because they should factor it into their bid," he says. "A delay that long will require a second trip to the jobsite, which is something contractors using coal tar sealers are not used to."
He says contractors need to make their customers aware of the additional time, too, as customers used to one-stop contractor visits or night time sealcoating might now be forced to consider other schedules to get the maximum performance out of the asphalt-based sealers.
In fact, Mariani says that he thinks many of the difficulties contractors may have experienced when they tried asphalt sealers for the first time were because they handled them like coal tar sealers.
"The challenge will be to re-educate the market," he says. "Asphalt-based sealers can be useful but they will require a whole different way of handling."
Manufacturers say handling and use of blended products which contain both asphalt and coal tar is determined by which product dominates the blend.
For the last five years Vance Brothers has produced its asphalt/coal tar blend to adjust to the coal tar shortage.
"The percentage of asphalt to coal tar changes because we want to sell coal tar to our customers whenever possible because it's a better product," Vance says. "We think blends are better than straight asphalt base because we think that coal tar works better than asphalt. If a contractor asks me whether he should put down a blend or an asphalt emulsion sealer I'll tell him to use the blend because he will be getting some of the coal tar benefits."
Vance says his company tells contractors exactly what they're getting, and they let contractors know if the percentage formula changes from one delivery to the next.
"If it changes from 80/20 (80% coal tar to 20% asphalt) to 75/25 the contractor won't notice much of a difference and the material won't act differently. But if we were to go from 80/20 to 20/80 they would notice a huge difference and would need to alter their operation accordingly," Vance says. "A blend heavy in coal tar will act more like coal tar sealer and a blend heavy in asphalt will act more like an asphalt sealer. With an 80/20 blend they won't notice much difference from pure coal tar."
Bill Maclean, at The Brewer Co., which produces coal tar sealer for the contractor market and an asphalt-based sealer for the retail market, produced a blended product last year simply out of necessity.
"We produced a blend last year to make certain we had enough material to help our customers keep working," Maclean says. "It is our view that, even with technology that has become available in the last few years, any blend of asphalt and coal tar is a compromise with material that is 100% coal tar. For us, the issue last year was that if we didn't produce a blended product the people that rely on us for their livelihood would be short the material they needed."