Sealcoatings, being water-based, 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 proportion to the amount of water (by volume) in the mix. For example, if the mix design has 70% water by volume the wet film will shrink by 70%, or down to 30% of the original volume.
As the water evaporates, it creates a steady turbulence in the sealer film, which forces the suspended particles (binder, filler) into a closer proximity. The film becomes progressively denser, thus forcing the binder particles to touch each other and fuse into a continuous film, encapsulating the filler particles in the process. Simultaneously, the binder bonds to the pavement.
Properly cured sealcoatings form a continuous film, free of voids or imperfections, which stop water, chemicals, salts, etc. from penetrating and damaging the asphalt pavement underneath. Understandably, any deficiency in the curing process will not allow the binder to fuse properly and leave voids in the film, thus resulting in inferior performance or failure.
A closer look at the sealer film
The wet sealcoating film is not one solid film but a composite of several layers of very 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 and becomes tight and relatively impervious to water vapor, therefore, impeding the evaporation of water from the bottom layers.
The first 80% to 85% of the film dries relatively fast, but the release of the last 10% to 15% of water becomes difficult and requires the right set of ambient and pavement conditions. It's a fact that the release of water from the bottom layers of most water-based coatings becomes progressively difficult as the film cures. For optimum performance, all water must leave the film; 80% to 90% is not good enough. The uncured 10% to 15% of the coating will be torn or dislodged if traffic is allowed on it too soon. The percentages are mentioned here only to explain the phenomenon. They will, of course, vary with material and mix designs, e.g. mix designs with fillers and/or aggregates will dry faster than sealcoatings without them.
The cure conditions
Pavement and ambient temperature, relative humidity, and wind velocity have a very profound effect on the rate of water release from wet sealer film and the final cure.
Temperature, both ambient and pavement, influences both aspects in the proper film formation and final cure - release of water and fusion of binder particles.
Cold Temperatures: The minimum recommended application temperature for a sealer is 50°F and rising. The fusion of the binder particles to form a continuous film is greatly enhanced at higher temperatures. Conversely, it is significantly reduced at temperatures below 50°F.
When sealcoating is applied below 50°F, tar (or asphalt) particles do not soften and form a continuous film, thus leaving clay and filler particles uncoated. The color of the sealer cured under such conditions usually turns out grey and blotchy in appearance and never returns to its normal slate black appearance, even at higher pavement temperatures, because the temperature of the pavement normally 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 temperature reaches high enough it still will not assure proper fusion. The binder particles will melt but will not have the capability to flow and envelope the clay and filler particles already set in a rigid matrix.
Needless to say sealcoating cured under cold weather conditions lacks the film integrity and the expected performance properties.
Mix design alteration suggestion:
Consider using an additive (1% to 2% on sealer volume) that will help the sealer dry faster. Such additives contain fast-evaporating solvents which combine with water in the sealer, accelerating the evaporation rate, at low temperatures. Most rubberizing (performance-boosting) type additives also speed up the water evaporation. Such additives form a polymer network within the sealer film, which forces the water out of the film at a uniform rate.
Another possibility is to reduce the water of dilution as much as possible. Use the lower limit recommended by the sealer producer.