cumulative water loss happens rapidly in the early stages of film drying—in other words the first 80-90% of the film dries relatively fast, but the release of the last 15-20% of water takes much longer, requiring the right set of ambient and pavement conditions. For optimum performance, all water must leave the film: 80-90% is not good enough.
As most sealcoaters know, water is the major component of sealcoatings. The sealer supplied as a concentrate contains typically 60% water by volume. After adding 30% water to 100 gallons of the concentrated sealer the water proportion increases to nearly 70%, by volume. Sealcoatings, like other water-based coatings, dry and cure by releasing water to the atmosphere. The optimum cure and the full strength of the sealcoatings are reached after the release of all the water.
By understanding how water is released from the sealer film and how the pavement itself and ambient conditions influence that release, contractors will be better able to obtain optimum cure and improve the overall performance of their sealcoat. If not allowed to cure properly even the very best of sealer could experience poor performance and premature failure.
How Sealer Film is Formed
Sealcoatings, like most water-borne coatings, start releasing water into the ambient atmosphere as soon as applied. The surrounding air acts as a blotting paper to soak up the released water, its capacity depending upon the relative humidity of the atmosphere (more on this later). Sealcoatings attain full cure through the loss of all the water from the wet film. As the water leaves, the volume of the wet film shrinks, in direct proportion to its water content (by volume) in the mix. For example, if the mix design has 70% water by volume, the wet film will shrink by 70%, i.e. 30% of its original volume.
The evaporation of water from the wet film produces a steady turbulence in the film and forces the suspended particles to move closer to one another (Figure 1). As this happens the film becomes progressively denser (1B), eventually forcing the binder particles to touch each other and fuse into a continuous film, encapsulating the filler particles in the process (1C). At the same time the excess binder in the matrix allows the film to effectively bond to the pavement surface.
Descriptions such as the full cure, final set or optimum strength mean that the sealcoating has reached its full strength and is capable of performing its task as a protective coating. A properly cured sealcoating forms a continuous film free of voids or imperfections that stops water, chemicals, salts, etc. from penetrating and damaging the asphalt pavement underneath. Any deficiency in the curing process will prevent the binder from fusing properly and leave voids in the film, thus resulting in inferior performance or failure.
During the curing process, sealcoating films transition through various stages (Figure 2) of water evaporation from the applied film. First it attains initial drying when the film becomes “tack free” to the light touch, then it becomes firmer (about 80-90% cured) to take light pedestrian traffic, and finally, when all the water is lost through evaporation, full firmness to withstand light vehicular traffic.
To better understand the drying and cure process, visualize the wet film not as one solid entity but as a composite of numerous layers of molecularly thin films (imagine a sheet of plywood). Like most water-based coatings, sealcoating dries in successive layers, from top to bottom. As each layer dries, it shrinks in volume, becomes tight and relatively impervious, thereby impeding the evaporation of water from the bottom layers. Cumulative water loss happens rapidly in the early stages of film drying—in other words the first 80-90% of the film dries relatively fast, but the release of the last 15-20% of water takes much longer, requiring the right set of ambient and pavement conditions. For optimum performance, all water must leave the film: 80-90% is not good enough. The uncured bottom layers of the sealcoat will be torn or dislodged if traffic is allowed on it too soon. The percentages noted above are strictly to explain the phenomenon of the cure stages. The final cure will depend on many factors: mix design, coverage rate, and ambient conditions of temperature, humidity and the wind velocity.
Ambient Cure Conditions
Ambient conditions play the decisive role in determining the thoroughness of the overall cure process, and sealcoating performance. The following conditions are the recommendations of the industry and its research association, Pavement Coatings Technology Council.
Temperature (ambient & pavement). Sealer should not be applied unless the pavement temperature is at least 50°F and the air temperature is 50°F and rising. The fusion of the binder particles (in sealcoatings) to form a uniform and continuous film depends on their ability to soften under the ambient and pavement temperatures. The process of fusion is greatly enhanced at higher temperatures, say 75°F to 85°F. Conversely, the fusion process is significantly reduced at temperatures below 50°F.
Cold Temperature Application. When sealcoating is applied below 50°F, refined tar (or asphalt) particles do not soften and form a continuous film – so they leave clay and filler particles uncoated. The color of the sealcoating cured under such conditions usually turns out grey and blotchy and never returns to its characteristic dark slate/black color, even when pavement temperatures rise later; the pavement temperature does not reach high enough to re-melt the binder particles and force them to flow and form a continuous film. Even if the pavement temperatures reached high enough it still will not be sufficient to re-mobilize the binder particles to flow and envelope the clay and filler particles to form a continuous film. Needless to say sealcoating cured under cold weather conditions lacks the integrity and is liable to fail prematurely.
Hot Temperature Application. Sealcoatings should not be applied under the summer sun (over 90°F) without first cooling the surface with a fine mist of water, also called “fogging.” Water should be used to dampen the pavement — without leaving puddles.
If applied to a hot pavement without “fogging” it, the sealcoating film almost gets “baked” as soon as it hits the pavement. With the sudden loss of the film fluidity, the binder particles are immobilized and do not fuse properly. Devoid of the proper fusion process, binder particles do not effectively envelop the clay and filler particles in the sealer film to attain the proper hardness. They stay as thermoplastic entities in the film, which become sticky and soft under hot ambient and pavement temperature conditions, thus causing potential “tracking” problems.
Relative Humidity or Humidity
Relative Humidity (R.H.) also plays a significant role in the cure mechanism because it directly influences the rate of water loss from the sealcoating film. Relative humidity is the ratio of the actual moisture content of the air, at a specified temperature, to its total capacity. For example, 50% R.H. means that only half of the air’s total capacity to hold water has been used and it is capable of absorbing another 50% of moisture or vapor from the surroundings.
Conversely, 90% R.H. means the ambient air is loaded with moisture and has very little (only 10%) room left to hold additional water. Sealcoating applied under highly humid conditions takes a long time to cure because there is very little room for the air to absorb water released from the film – the film will release only the amount of water that can be accommodated by the atmosphere. (The atmosphere and the surrounding environment can be thought of as a sheet of paper towel: When dry it will soak up the spill but will not mop up if the towel is too wet.) So sealcoatings will cure faster at lower humidity than at higher humidity. Under highly humid conditions, sealcoatings must be allowed longer drying time before the application of subsequent coats and finally opening to traffic.
The interdependence of temperature and relative humidity on the water evaporation rate is depicted in Figure 3. You will notice that the water evaporates more than three times faster at 40% R.H. than at 80%, at a given temperature. Along the same lines, the capacity of the air to hold water to the point of saturation increases with the increase in temperature.
The drying and cure times specifications do not take wind velocity or air movement under consideration. But air movement, especially under highly humid conditions, helps sealer dry faster than without any air movement. A light breeze assists in the dissipation of the water and volatiles from the immediate area. Conversely, under low humidity conditions (below 20-25%) the air movement might cause the sealcoat to dry a bit too fast.
Girish C. Dubey is president of STAR Inc., Columbus, OH, which has affiliate sealer producing operations throughout the U. S.