What is Sealcoat - Two Types
Currently there are two primary types of sealcoating materials on the market: Those made from refined coal tar and those made from asphalt.
Refined coal tar —a byproduct of the coking process— is a very complex mixture of thousands of chemicals and quite different in its molecular structure than asphalt. The coal tar molecules have a predominantly closed ring (aromatic) structure with a minor degree of un-saturation. Being stable in molecular structure, these chemicals do not allow the destructive elements of weather and chemicals to affect the property of coal tar. So for a variety of reasons, the most commonly used sealcoatings have, until recently, been based on refined coal tar. These sealcoatings act as "barrier coats" to protect asphalt surfaces.
Sealcoatings based on refined coal tar were introduced in the 1950s and have been used extensively to protect off-street pavements. These often are referred to as C.T.P.E. (Coal Tar Pitch Emulsions), denoting that these coatings are water based, obtained by dispersing refined coal tar in a matrix of clay and water. The finished product is a stable, homogeneous composition that is applied with ease and safety in handling. The mineral fillers serve to extend the softening range of the refined coal tar so that the coating will be functional at normal pavement temperatures. The CTPE incorporates the protective features of refined coal tar and the reinforcing effect of mineral fillers, to offer a "barrier coating" that will flex with the pavement’s movements while protecting the asphalt from the destructive elements of weather, gas, oil, fat, chemicals etc.
In recent years asphalt emulsion based coatings have been introduced with varying degrees of success. In fact, many sealer manufacturers that previously produced only refined coal tar sealers now also produce asphalt-based sealers or even asphalt/refined coal tar blends.
The asphalt emulsions deliver most of the same properties as refined coal tar-based coatings —except for the resistance to color fading due to ultraviolet degradation, salts, and petrochemicals like oils, fats, grease and solvents. These deficiencies are inherent in the asphalt binder itself. Being a petroleum derivative, asphalt has a natural affinity for petrochemicals, so it is easily dissolved by them.
Asphalt emulsion-based coatings are made using either a soap emulsion (SS-1-H, for example) or clay stabilized emulsions. Both types are suitable but the properties of clay stabilized emulsions can be controlled better through selection of the right ingredients.
In recent years sealcoat manufacturers have been quite successful refining the performance of asphalt emulsion sealers through the use of specialty chemicals and pigments. But asphalt emulsion’s resistance to petrochemicals and solvents — while improved — has yet to be overcome. In spite of this deficiency, sealcoaters have recognized some definite advantages of asphalt emulsion over refined coal tar sealers: Asphalt emulsion sealcoats are more "user friendly," practically odorless, and do not irritate and burn the skin (features especially important for people with respiratory conditions or sensitive skin). It is noteworthy, however, that the Federal Aviation Administration Specifications (P-625, 627 and 628) accept only refined coal tar-based sealcoatings for use on airport projects, because the refined coal tar based sealcoatings offer greater resistance to jet fuel.
Asphalt paving technology gradually evolved and today the vast majority of all roads are constructed using asphalt as the binding material for the aggregates. Due to its waterproofing properties, asphalt protects the aggregates from absorbing water, thus preserving their strength and load-carrying capacities. Today’s asphalt pavement is a mixture of stone aggregate and mineral filler combined with 4.5% to 12% (average of 6%) asphaltic binder (asphalt cement). The strength of an asphalt pavement is directly related to the pavement design from the ground up.
The asphalt pavement people see is only the "roof," so to speak, of the entire pavement. This "roof" covers a bed of graded stone aggregates of varying depths according to ground conditions as well as traffic requirements. This base of aggregate is what really carries the load of the traffic. The same theory applies to off-street parking lots or drive-ways. A firm resilient surface that provides a roof over the stone base will keep the pavement bed dry. It is important to have an elastic characteristic in this pavement so that it can expand and contract and still remain intact.