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As manufacturers of both types of sealcoatings, coal tar and asphalt-based, we would like to present certain facts that we consider relevant
for our customers to help them choose the type of sealcoating that they want to recommend and use for their projects. We will start with the very
basics in differentiating between coal tar and asphalt and lead to the composition, manufacturing and properties. Let us start with the
explanation of the terms; |
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a. Asphalt is a dark brown-black cementitious material, solid or semi-solid in consistency, in which the main constituents are bitumens which either occur in
nature as such or are obtained as residue in petroleum refining. Asphalt is a very complex mixture of compounds (thousands) which are predominantly aliphatic
or open chain type. Asphalt has;
- Excellent water-proofing and adhesive properties. It is used extensively for cold (slurry seal) or hot (HMA-Hot Mixed Asphalt) paving.
- Poor resistance to petrochemicals; oil, fat, grease, solvents, etc.
- Poor resistance to ultraviolet radiations and weathering, indicated by the change in the color of the asphalt pavement from black to gray as it ages, and the
development of cracks.
b. Coal tar is a black viscous liquid or solid and is obtained by destructive distillation of bituminous coal in coke oven (steel industry). One ton of coal yields 8.8
gallons of crude coal tar which is further distilled, under controlled conditions, to separate the lighter fractions like benzene, phenols, cresols, tar oils etc. and
what remains is coal tar pitch. The pitch undergoes further refining and various grade of the pitch are separated, subsequently. The coal tar used in the
formulation of sealer is, therefore, a very select fraction of the pitch from which the majority of the lighter fractions have already been removed.
Coal tar has altogether different origin and chemistry. The major difference comes from the very origin of coal versus asphalt. Coal, the source of coal tar, finds its
origin in plant and vegetable matters which were successively buried under the layer of earth and degraded over millions of years. Petroleum, the source of asphalt,
originates from the decay of marine life over millions of years. The chemistries and structures of plants and animals are significantly different and possibly that is the
reason for the major difference in properties of coal tar and asphalt.
Coal tar is also a very complex mixture of thousands of chemical compounds which are entirely different than the compounds in asphalt. The chemical compounds
in coal tar are predominantly closed ring (aromatic) in structure and quite stable. By virtue of their molecular structure coal tar has;
- Excellent water-proofing and adhesive properties.
- Excellent resistance to petrochemicals; oil, fat, grease, solvents, etc.
- Excellent resistance to ultraviolet radiations and weathering, evidenced by no change in color or loss of properties.
c. Sealcoatings - Sealcoatings are specialty coatings that are used for the protection and preservation of asphalt surfaces. The name itself defines the role;
a coating that acts as a shield to protect asphalt binder in the pavement, thus “locking-in” or “sealing-in” the redeeming properties of asphalt. |
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Sealcoatings are semi-fluid (like house paint), water-based compositions which are essentially fine dispersions of a binder component (coal tar or asphalt),
into a clay/filler water slurry. Such compositions are stabilized by specialty chemicals and additives. Then four major components are described below; |
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1. |
Binder - Binder is the backbone, responsible for the desired protection and preservation properties to the asphalt pavement. The binder, therefore,
in sealcoating is either refined coal tar or asphalt. |
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2. |
Water is the dispersion medium in which all the ingredients stay suspended. Water based systems are easy to handle and safe to store. Majority of
sealcoatings are water-based. |
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3. |
Clay and Fillers impart dimensional stability to the coating, i.e. the formulated coatings shall be suitable for application and dry/cure to form a
tough film that will effectively bond to the pavement and provide protection. |
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4. |
Specialty Chemicals perform specific functions in the coating and are essential for the stability of the sealcoating. |
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- Emulsifier and surfactants are necessary to keep the dispersed particles in suspension, and not allowing them to coalesce, in the wet coating.
- Thickeners impart proper body and fluidity to sealcoatings.
- Preservatives are essential to protect asphalt emulsion based coatings from bacterial attack and spoilage. Not used in coal tar based sealer.
- Carbon Black Dispersion is commonly used to improve the fade resistance of asphalt emulsion based coating. Not used in coal tar based sealer.
- Other ingredients-proprietary to improve the sealcoating properties.
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