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Cold Weather Hazards
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When applying coal tar emulsion sealer in cold weather you should be aware of the risks involved. There are several reasons that the cold effects coal tar emulsion sealer and they are outlined below. Understanding these factors will help you to understand the risk you may or may not take!

The three major components of all coal tar emulsion settlers are: coal tar, clay and water The coal tar is not in solution. It is present in the form of discrete individual microscopic particles. These particles must not touch each other until the coating is applied and begins to dry and cure, since, if they do, the emulsion will agglomerate, break and become unuseable.
The coal tar particles are normally from about 2 to 10 or 15 microns in size (0.000079" to 0.00059"). Because in most cases the manufacturing process involves high shear emulsification, the particle shape will range from spherical to elongated ovoids (fat cigar shapes).

In order for these particles to coalesce and form a film during the drying slags and in order to obtain full contact (rather than single point contact) with the pavement, the particles must deform, flow together and flatten out on the surface.
During drying, as the particles gel very close together, little capillary tubes are formed between them and the surface tension of the evaporating water (on a microscopic scale, but amounting to many pounds per square inch in total) helps to distort and pull the particles from their rounded shapes into a continuous film, if they are soft and plastic enough al the time the drying is going on. Coal tar as with many materials, is thinner and more fluid when warm, and stiller and more resistant to flow when cold. ii the particles are too still and hard from low temperatures to flow together while the film is drying, · the coal tar particles will never again have the same opportunity to be forced together into a continuous film while in a flowable state, even though it warms up later. The capillary forces are gone.
Two other factors also work against a subsequent film formation if it does not happen during initial drying. The clay that is used to form and stabilize the emulsion from manufacture to application, is plastic and flowable while still wet and during it is plastic enough to allow the tar particles to move together. Once dry without coalescence, the claly sets up a relatively rigid matrix that also prevents the tar particles from combining at a later date when the temperature is higher unless the temperature would be raised so high (in the order of 200 degrees F) that the tar would become a thin flowing liquid.
The third factor that helps coalescence and adhesion during initial drying, but does not help once the coating has dried is in the composition of coal tar. Coal tar is composed of several thousand individual types of molecules. A small number of these (usually 2 to 7%) are lower molecular weight compounds that act as solvents to plasticize and soften the rest of the mass. These lower molecular weight fractions are, however, by their very nature, considerably more volatile.
After the emulsion coaling is applied, these lighter fractions gradually begin to evaporate from the coating so that the remainder becomes harder and less plastic. The rate of loss of his 2 to 7% of the coal tar is a time-temperature-film thickness-controlled phenomenon and to be complete takes a rather long time, but at no time alter initial drying will the tar particles be quite as soft and plastic at any given temperature as they were when first applied.
For all the above reasons, application of unmodified coal tar emulsion sealers at temperatures below 50 degrees F for application and during the time required for complete drying, is risking premature failure  due to inadequate adhesion, poor film strength or both. Depending on conditions (sun vs. no sun - wind chill vs. still - surface type evaporative cooling etc.) the very real risk of partial or complete failure of the coating to perform as desired must be recognized and weighed against any hoped for benefit of application under adverse weather conditions.
weather hazard
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