Asphalt-based sealers are asphalt emulsions that primarily consist of asphalt, pure water, and an emulsifying agent.
- Asphalt: Asphalt is a selectively refined form of crude petroleum oil and makes up about 50 to 75 percent of a typical emulsion. Asphalt tends to be slightly more variable than coal tar in its production, meaning that different refineries will produce asphalt with different properties, which can affect the viscosity of the sealant. Additionally, since asphalt is petroleum-based, it is chemically similar to petroleum-based automotive fuels and fluids — this means that asphalt is, by definition, not resistant to fuel.
- Emulsifying agent: While these emulsifying agents achieve the same result as those used in coal tar emulsions, emulsifying agents used in asphalt tend to differ based on the compositional differences and chemical properties of the asphalt being used. While coal tar is standardized, asphalt is more variable. Emulsifying agents are employed for chemical compatibility to create a stable emulsion.
- Additives: The most common additives for asphalt emulsions are polymer and non-polymer sealants. Typically, the additive used is either a waterborne acrylic resin or waterborne vinyl — the local environment and temperature gradients will determine which type is used in the final mixture. Mixing additives should be done very carefully, however, since asphalt-based sealers are not standardized and may contain chemicals that are incompatible with certain types of additives. For example, an asphalt emulsion made with an anionic emulsifying agent cannot be mixed with a cationic polymer emulsion, or else the emulsifying agents will gel (ionic bonding) and negate the beneficial properties each produce in ionically balanced mixtures.
After mixing in additives and diluting with water, the sealant mixture is applied to a surface in a slurry, allowing the asphalt and any added particles to spread evenly across the surface. At this point, the emulsifying agent destabilizes, allowing the asphalt and additives to bond as the water evaporates. Asphalt emulsions typically take eight hours or more to dry, depending on the water content and weather.
The important performance assets to remember with asphalt are that it is more thermoplastic, yet more susceptible to humidity, compared to coal tar. Due to its chemical composition, asphalt requires higher temperatures to become fluid than coal tar sealers, requiring heat application at temperatures above 50°F. If the temperature drops below this threshold, the asphalt will remain solid and will not form a proper film, resulting in an ineffective seal. Additionally, it should only be applied in dry weather — rain and humidity can prevent the asphalt from forming a consistent film.
Even after application, asphalt-based emulsions tend to be more susceptible to changes in the temperature and weather. This, along with asphalt's natural susceptibility to automotive fluids, means that asphalt should be reapplied every two years.