Asphalt sealer products can vary from manufacturer to manufacturer. Sure, most meet or exceed Federal Specifications and American Society for Testing and Materials Specifications, but to understand the performance property differences of asphalt sealers, certain tests that go beyond just the composition specification requirement tests are necessary.
Remember, product specifications are written in many ways and they do not necessarily nail down all the relevant aspects for a sealer’s performance. Federal Specifications require sealers to be measured in a consistent manner in regards to the resistance of water, kerosene and heat; but none of these relates to the longevity of the coating itself after it has been through rain, salt and day-to-day traffic.
Some manufacturers perform their own testing procedures to verify performance property. For example, Professional manufacturers will use their laboratory to compare up to 12 different testing points in quality assurance including but not limited to: solids content, ash content, wet color, viscosity, specific gravity, water absorption/desorption, stability in a Weather-O-Meter, durability scrub cycles, comparative dry time, pH level, microscopic particle size measurements and other physical/visual pavement tests. The knowledge gained from this battery of tests can give a much better impression of how a sealcoating will likely perform and provides a baseline against the live testing of products in service.
Let’s take a look at some of the crucial tests and concepts that apply to both refined tar and asphalt sealcoatings, which will prove the performance of the sealer as a protective coating.
Particle Size Analysis
Asphalt sealers are dispersions of tars, clays, fillers and specialty chemicals in water. The performance such coatings depends directly upon how finely the clay and tar particles are broken down during manufacturing. If a batch of sealer is rushed though at any stage of the manufacturing process, then it will likely have a poor “grind” of the raw materials, meaning that particles are not broken down to their smallest possible size, which will greatly change the performance of the cured sealer.
The finer the tar particles are, the tighter the cured film will be. This translates directly to better performance as a protection on the pavement surface. The tar particles are cigar shaped and the particle size is measured as length in microns (one millionth of a meter). Tar particles that are three to four microns in length are considered the optimum size. Clays should be ground down to much smaller sizes and should measure is less than a single micron. The particle size is easily measured under a laboratory microscope, using a water-thinned sample of the sealer.
So what happens on the asphalt? During the curing process, water evaporates out of the curing sealer creating turbulence in the wet film. Brownian motion causes the tar and clays to bump into each other and eventually they are forced into a tighter and tighter space. The clay particles, which should be much smaller than the tar particles, become enveloped by the larger tar particles and form a complete film. If the clay particles are too large, they are not enveloped in the tar properly and you have water sensitivity issues. Tar particles that are too large do not fuse well and leave micro-voids through which water can gain access to clay/filler particles in the cured film and may render it water sensitive, cause softening and potentially making it prone to tracking.
Water Absorption/Desorption Profile
Testing water sensitivity of a cured sealer can tell a great deal about how the cured film will preform in service. The testing measures the amount of water absorbed when the sealer is submerged for a set amount of time, and the rapidity with which the absorbed water is shed once the water is removed is measured.
An ideal sealer will absorb about two percent (maximum) of its own dry weight in six hours of continuous submersion and will shed almost all the absorbed water in the same amount of time after removing the water source. Poor quality sealers absorb as much as eight to ten percent water and retain about four percent after removal of the water source in the same six hours.
This behavior depends largely on the type of clays used in the sealer formulation, but manufacturing processes can also be a factor in this equation. Some clays are more absorbent and hang on to water for a longer period of time. Conversely more desirable clays absorb relatively smaller amounts of water over a longer period of time and give up the water they hold quite quickly when the water source is removed. Water absorption that occurs too quickly and is retained is bad for the sealer because the clay particles in the sealer swell after soaking in the water, thus rendering the sealer soft and quite easy to remove under traffic (foot or vehicle). This generally results in unsightly tracking.
Scrub Resistance – Cycles To Coatings Failure
A scrub test reveals valuable insight in the toughness of the cured sealer film and its susceptibility to softening and re-emulsifying under water. This type of test is useful in comparing the potential longevity of a sealer against another one or two similar sealcoatings with a variation in formulation.
Simply stated, the dry film of sealer is applied to a test substrate, cured in controlled lab conditions for a specified length of time, and is then scrubbed with a stiff brass-bristled brush under a water/sand slurry solution. The number of cycles (back and forth strokes) are noted when a visible portion of the film is lost as a result of the abrasion. The test furnishes an objective comparison between samples in a controlled environment.
Some Other Relevant Criteria
COLOR - The wet sealer must be a chocolate brown color. The brown color is indicative of the finer tar particles and is the sign of a good grind in the batch making process. It is one way to visually tell that your sealer is properly manufactured. This is true for Asphalt, Refined Tar and the new Petroleum Resin based sealers.
ALKALINITY or ACIDITY – pH must be between 7.0 and 8.0. Sealers with a pH figure higher than 8.0 have problems drying and have potential water sensitivity issues. If pH is below 7.0 (acidic), the sealer will be unstable as shown by some gassing or excessive thickening.