Understanding Sealer Basics

Asphalt parking lots and driveways are capital investments, increasing the value and functionality of a property. Like any infrastructure investment, the asphalt surface must be maintained to keep both value and functionality over time. Maintenance options include resurfacing or replacing the asphalt periodically or extending the asphalt’s service life by sealcoating, which reduces resurfacing/replacement expenses.

Sealcoating extends the useful life of the capital asset – an asphalt parking lot – by protecting the pavement from natural aging caused by sunlight, water and debris. An added benefit is that sealcoating adds to the “curb appeal” of a paved surface, giving it a clean, uniform look.

There are two main options for sealcoating: refined coal tar-based sealers and asphalt-based sealers. Refined coal tar-based sealers (RTS) are based on a selectively refined fraction of crude coke oven tar, which is a byproduct of the steel making process. Asphalt-based sealers (ABS) are based on a selectively refined fraction of crude petroleum oil.

There are several important differences between refined tar-based sealers and asphalt-based sealers. The most important difference is RTS protects the underlying asphalt pavement from leaking oil, gas spills and other petroleum products, which are chemically and physically similar to the petroleum from which the asphalt itself is derived. Obviously, ABS cannot, by definition, be fuel resistant. Other advantages of RTS use are greater resistance to natural aging processes caused by exposure to sun, rain, freeze-thaw etc.; stronger cohesion (“sticks” to itself better), and stronger adhesion (“sticks” to the underlying pavement better). Finally, RTS are manufactured to a performance-based specification defined by federal specifications and ASTM specifications using ASTM approved testing protocols. There is no industry-accepted ABS formulary, performance-testing protocol or specification.

Most sealer manufacturers make both RTS products and ABS products, and most recommend RTS for most applications because the superior performance of tar-based sealcoat allows the manufacturers to stand behind the performance of their products, enhancing the reputations of their businesses. Research and development projects continue to improve the performance of ABS, but there remains a long way to go.

Some say that RTS is a health threat. But across the two-, three- and four-generation memories of the many family-owned companies in the business of making or applying sealcoat, there are no reports of adverse chronic health effects that can be attributed to exposure to sealcoat.

In addition, every day millions of people world-wide use coal tar soaps, shampoos and creams approved for use as over-the-counter medicines to treat skin disorders such as eczema, psoriasis and dandruff. Coal tar and coal tar derivatives are listed by the U.S. Food and Drug Administration (FDA) as “generally recognized as safe and effective” active ingredients for use to treat these skin ailments with coal tar concentrations up to 5% in over-the-counter products. Because of its use in medicines, many studies have been performed over nearly a century to see if the patients who intentionally expose themselves to high level doses of coal tar for long periods of time have increased risk of cancer. All the studies have reached the same conclusion: there is no evidence of cancer.

Controversies about the safety of RTS began because one of the components of coal tar-derived materials is a class of chemical compounds called polycyclic aromatic hydrocarbons (PAHs), which occur naturally and are made whenever something organic is heated up or burned: smoke from forest fires, wood burning fireplaces and plants decaying in a swamp or a compost pile all make PAHs. Emissions from planes, trains, cars, cooking food, and petroleum products, contain PAHs, as well as materials derived from coal tar.


More Study Summaries

  • There is no evidence that low-level or intermittent exposure to coal tar or coal tar pitch has caused cancer in humans.
  • There is little evidence that high-level, repeated exposures has caused cancer in humans, and what evidence there is is largely reports from the past, such as chimney sweeps in London in the 18th Century. But, chimney sweeps in other European countries at about the same time did not display high cancer rates. Also, studies from these earlier times are when industrial hygiene practices were virtually non-existent.
  • Some studies conducted in modern factories with high temperature (1000s of degrees Fahrenheit) industrial processes show some adverse effects. But, these are not relevant to pavement sealer.
  • Studies performed in 1991 by refined tar producers, emulsion producers and contractors using both spray and squeegee methods proved that PAH exposures were well below and sometimes even unmeasurable using OSHA evaluation methods and established threshold limit values.


Some other FAQs

Why doesn’t the West Coast use RTS?

Crude coal tar is a byproduct of making steel. The steel industry is largely located east of the Rocky Mountains. To be close to the source of their raw materials, coal tar refineries that make the base material for RTS are located near where steel has historically been made. Transportation costs make locally produced asphalt-based sealers the cost-effective choice on the West Coast.

Are asphalt-based sealers cheaper?

All else being equal, asphalt-based sealers are generally cheaper on the West Coast because of transportation issues. The pricing of the asphalt-based product is more volatile as it fluctuates with the price of crude oil. Other cost factors can be that manufacture of refined tar-based emulsion is a one-stage process and that ABS typically require additives to improve performance.

“Drying” isn’t “curing.”

Like latex paints, RTS and ABS are applied as water-based emulsions. Evaporation of the water starts the process of adhering the sealcoat particles to each other and to the pavement. Sealer that is dry to the touch means that the surface can be open to foot traffic, but not vehicle traffic. Sealcoat can be driven on once the process of curing is well underway, meaning that the sealer particles are sticking to each other and the pavement. Curing takes more time than drying because it takes longer to drive out moisture that remains within the coating after the initial surface drying.

What is that coal tar sealer smell?

The odor of RTS is easily identifiable because it is distinctive and the human nose is able to detect it at extremely low concentrations. But, just because it may smell bad doesn’t mean it is bad! The smell is primarily the presence of one substance – naphthalene (Yes! Your grandmother’s closet moth balls). The odor threshold for naphthalene is below three parts per billion (ppb). To put this into perspective, the odor threshold for nail polish remover is 7,000 ppb. That is 2,333 times greater than naphthalene.

According to the American Conference of Governmental Industrial Hygienists, the level of naphthalene that is considered safe for workers is 10,000 ppb. So the difference between being able to smell it and worrying about it is huge – 3,333 times greater, to be exact.

Why not sealcoat in cold or rain?

For the same reason that exterior painting is not recommended in cold or wet weather, sealcoat is not applied in those conditions because the water in the emulsion won’t evaporate. If water doesn’t evaporate, sealcoat particles can’t begin the drying/curing process.

Is it poisonous?

Dried sealcoat doesn’t wash off and is no longer sticky. If any sealcoat – ABS or RTS – washes off before it dries it can suffocate fish and other aquatic life, but the fish are not poisoned. Autopsies show the fish suffocate because sealcoat particles cover the fish’s gill plates. Dried sealcoat particles are not bio-available to people and most other species.


Anne LeHuray is executive director of Pavement Coatings Technology Council, 2308 Mount Vernon Avenue, Suite 134 Alexandria, Virginia 22301. She can be reached at www.pavementcouncil.org.