Knowing The Facts

Manufacturers of Pavement Maintenance Products and Equipment

Knowing The Facts

As a sealcoating professional, you must know the facts, to choose the right sealer. Do not just go by the appearance, ask tough questions about asphalt sealer’ performance from your supplier. Ask your supplier how long will it last, how will it resist chemical and weathering attack, what special precautions will have to be taken, and lastly and we believe least importantly, how much will it cost.

Sealcoatings Based on
Refined Coal Tar & Asphalt Emulsions

Their Descriptions, Compositions, Manufacturing Methods, and Properties

Girish C. Dubey, President, S.T.A.R, INC.

October 2006

	General Discussions Terms, Differences, Pro's & Con's, Sealcoatings
	How Are The Seal Coatings Made?
	Background of Asphalt Emulsions Based Sealers & Comparison with Coal Tar Based Sealers
	Table-I - major characteristics of both asphalt & refined coal tar based sealcoatings

General Discussions

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;

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.

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;

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.

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.

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.

4. Specialty Chemicals perform specific functions in the coating and are essential for the stability of the sealcoating.

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.

How are the seal coatings made?

a. Refined Coal Tar Based Sealcoatings are made using two different processes;

Batch Process where ingredients are added individually and ground in a high speed dispersion process, for prescribed length of time. This method gives excellent control and batch to batch consistency.

Continuous Process where the stream of water/clay filler, tar or asphalt, emulsifiers, etc., are continuously fed into a colloid mill, at a precisely controlled rate.

b. Asphalt Based Sealcoatings - The most commonly used method for making asphalt emulsion based sealcoatings is to blend a pre-emulsified asphalt (e.g. (tack coat, SS-1-H, Soap emulsion, etc) , with fillers, sand, clay, paper fiber, preservative, black color, etc in a low speed mixer. Such coatings are generally inferior in properties due to the fact that their performance is dictated by the deficiencies in the properties of the pre-emulsified asphalt itself. The most commonly encountered problems are re-emulsification under water, softness and tracking.

Background of Asphalt Emulsions Based Sealers & Comparison with Coal Tar Based Sealers

During the early years, there were some high-end asphalt coatings that were fortified with specialty chemicals, polymers, etc. and designed to perform like coal tar, but they were also significantly more expensive than the traditional asphalt emulsion coating made from pre-emulsified asphalt. They are no longer available for the cost competitiveness reasons.

The void was filled by STAR’s Micro-Pave line of performance-driven asphalt-based sealcoatings where none of the traditional ways in formulating and processing asphalt sealers was used. STAR has taken the advantage of the state-of-the-art technologies of raw materials and processing methods to formulate asphalt based sealcoatings under its Micro-Pave line which rival coal tar sealers in most of the critical aspects of performance; color retention, flexibility, longevity, fuel resistance, etc. Performance has its price and from the very onset, we realized that the cost of Micro-pave will be at about the same price level as coal tar- based sealers. We did not get lured into creating an inferior coating simply due to the cheaper price of the asphalt binder (AC).
Micro-Pave has a solid track record, spanning over several years. Star Micro-Pave Supreme and Micro-Pave Aviator are designed to last as long as the best coal tar sealers and remain color-fast as well. They are also resistant to chemical attack. They are premium asphalt pavement coatings and therefore have a higher price, but one well worth paying when one calculates the long term cost of not maintaining, or ineffectively maintaining asphalt pavement. In the absence of high quality coal tar sealants Micro-Pave Supreme and Aviator fill the bill. We have jobs down over 5 years showing no fade and very little wear with minimal damage from fuel, the references of which are available, upon request.

The following tabulation (Table-I) lists major characteristics of both asphalt and refined coal tar based sealcoatings, in very general terms. A typical asphalt based sealer usually does not have the same level of resistance to chemicals and weathering, as compared to refined coal tar based sealers. This statement is generally true and has been the experience of many sealcoaters as well.

TABLE 1

Test Refined Coal Tar Emulsion Sealer Typical Asphalt Emulsion Sealer

Binder Component Refined Coal Tar, mostly aromatic,
closed ring compounds (stable) Asphalt Cement (AC), mostly aliphatic,
open chain compounds (unstable)

Origin of the binder Coal-Decay of vegetation Petroleum-Marine life decay

Ultraviolet Resistance Excellent Fair-Color Fades

Gas Resistance Excellent-No Effect Poor-Dissolves

Motor Oil Resistance Excellent-No Effect Poor-Dissolves

Kerosene Resistance Excellent-No Effect Poor-Dissolves

Oil, Fat, Grease Excellent-No Effect Poor-Dissolves

Scrub Resistance - (Durability test) 4000 cycles 2400 cycles

Water re-absorption 1.00% 3.2%

(The Water re-absorption test is an indicator for durability since water attacks seal coatings causing re-emulsification, which can lead to tracking sealer into your place of business. Low numbers are better.)

Odor Some - Characteristic coal tar Milder - Asphalt odor

Skin Irritant yes yes

Photosensitive yes no

(This is what causes burning to some contractors but in no way affects the consumer)

Flexibility pass pass

Drying time pass pass

OSHA Emission testing pass pass

(Both showed no detectable amounts or less than 2% of the permissible limits by OSHA of carcinogenic compounds including Benzene, CTPV’s, PNAH’s, or any other compounds of concern.)

Environmental Non-Hazardous, industrial waste in both Dry and Wet state Non-Hazardous, industrial waste in both Dry and Wet state

Handling Precautions Similar per MSDS Similar per MSDS