Coal Tar Paints and Coatings for Concrete and Steel Surfaces

Turkchem 05 Apr 2018 87 9 dk okuma

TURKCHEM

Bituminous coal, a highly complex chemical mixture, decomposes into simpler components when heated without air above 700°C in distillation columns. It is drawn off as gas, water vapor, and coal tar, leaving coke as the residue. Coal tar is dewatered by heating in retorts to yield oil and coal tar pitch. Depending on the source of the coal tar and the amount of heat applied, different coal tar pitches are obtained. When used as a base material for high-performance paints and coatings, coal tar is reprocessed and corrosion-accelerating substances are removed from the structure. Various types of coal tars are then blended together. The extremely low permeability of coal tar paints, high dielectric resistance, and unique durability against water's damaging effects are the result of this product's exceptional quality. There is scarcely any material, whether water-resistant, old or new, like properly prepared coal tar paint and coating. They are unaffected by mineral oils, but they can dissolve in vegetable and animal oils, greases, and detergents when in direct contact. Their resistance to weak mineral acids, alkalis, salts, salt water solutions, and other active chemicals is good. Furthermore, coal tar paints are more economical than other protective paints and coatings. This fact should not be overlooked when selecting paint for a specific application.

There are five basic types of coal tar paint:

1. Thin coal tar pitch solution prepared without any filler, 2. Heavy coal tar pitch solution prepared with inert fillers, 3. Very heavy coal tar pitch paint and coatings in thixotropic gel form, but containing inert fillers with only moderate natural viscosity, 4. Heavy coal tar emulsions with inert fillers and low natural viscosity, 5. Hot-applied coal tar coatings. The first three types are solutions and have approximately the same chemical and water resistance. These vary in film thickness primarily based on their ability to be applied as a single coat. The fourth type, coal tar emulsions, are dispersions of coal tar pitch in water and have lower corrosion resistance compared to solution types. This is not a fault of the coal tar itself, but results from higher permeability of the applied film. Pitch particles dispersed in water are relatively large and do not fully coalesce after drying like much smaller dissolved pitch particles. However, coal tar emulsions have other excellent properties to be discussed later. The fifth type, coal tar pitch, reinforced with inert fillers and applied in molten state over a primer, is referred to as commercial coal tar enamel. These enamels possess all the good qualities of coal tar paints, but are of a much higher quality grade due to the very thick coating and its independence from solvent evaporation. Type 1, with 60 to 70 percent solids content, is a low-viscosity thin coal tar solution with a spreading rate of 7 to 10 square meters per liter. It provides a thickness of approximately 25 to 50 microns per coat. This thickness cannot be increased because the thin solution cannot be applied at a lower rate without sagging. Type 2 is designed to obtain a heavier coat. In addition to higher solids content, a filled coal tar solution must be used that can be applied at 3-4 square meters per liter without sagging. This yields a dry film thickness of approximately 150 microns. For brush application of heavier coatings, a gel should be selected that can be applied at rates as low as 2 square meters per liter without sagging. This will provide approximately 400 microns dry film thickness as a single coat. Coal tar emulsions, with a coverage of 2 square meters per liter, will provide approximately 300 microns dry film thickness without sagging and in a single coat.

Coal tar paints provide protection through the mechanical exclusion of moisture and air. If applied as a continuous uninterrupted film, they provide nearly perfect protection. In single-coat applications, multiple coats will be necessary since it is not possible to avoid pinholes and surface defects.

These paints dry by solvent evaporation only. As a rule, concrete can be protected with thin coal tar solutions, but steel requires heavier coatings that will form an almost impermeable barrier against serious corrosive effects. All coal tar paints may exhibit alligatoring in the sun, resembling crocodile skin more or less. This name derives from the paint's resemblance to alligator skin. This is a surface defect. It occurs when the upper layer of the film hardens due to heat from the sun's rays. This causes the upper layer to shrink, crack, and slip on the still-soft lower layer beneath it. If insufficient coats are applied, this alligatoring can extend downward from the metal surface in a manner that can lead to atmospheric corrosion. Alligatoring formation does not occur (or only to a limited degree) underwater, where the paint is protected from the sun's rays. In coal tar emulsions, the pitch particles do not bond as tightly as in solution types, so this alligatoring does not occur. For this reason, coal tar emulsions can be used as final coat applications over heavily alligatored coal tar paints. It should not be forgotten that these emulsions have less protection capacity in immersion applications and are not recommended for this use. There are many popular methods to prevent alligatoring of heavy coal tar paints that will be temporarily exposed to sun and air before immersion. One of them is the use of lime whitewash, an old method. As a more current solution, an acrylic latex paint or similar emulsion paint can be applied to the surface of the coal tar paint. However, the color change of this paint film with the oils in the coal tar paint should not be accepted as a reason for failure. The solvents used in coal tar pitches have a strong odor, so adequate ventilation is required during application and drying.

Coal tar emulsions, which use water as a thinner, are superior in this regard and should be used in situations where adequate ventilation is not possible. Coal tar paints provide excellent protection at low cost in dams and flood control facilities, dam gates, piers, marine work, and similar applications.

Type 5 paints, coal tar enamels, are widely used for external protection of steel pipelines, whether supported by wrapping or not. They are applied by machine or hand brush. For the internal lining of steel or concrete pipes, coal tar enamels are applied by centrifugal spinning. This process provides an extremely smooth surface and reduces friction between the pipe surface and flowing liquid. Coal tar enamel paints are produced in different types depending on service conditions such as extreme high and low temperatures or changes in soil stresses. These stresses result from specific soil types encountered in underground pipeline installation. Modifications primarily involve different degrees of plasticizer and the amount of filler given to the coal tar enamel. It is possible to protect many other surfaces with coal tar enamel, but generally heavy coal tar coatings applied cold are preferred in practice because they are easier to apply, can better tolerate temperature changes compared to coal tar enamels, do not require specially trained work crews, and result in lower total cost.

Coal Tar Epoxies

Coal tar-based paints and coatings have traditionally demonstrated extraordinary performance as a protective barrier against corrosive effects. These materials are very low in cost and resistant to water and aqueous corrosives. The only drawback associated with unmodified coal tars in terms of protective paints is their lack of solvent resistance and the paint's tendency to soften and flow under high temperature conditions. Both of these drawbacks are overcome by mixing liquid epoxy resins with coal tar and curing the mixture with typical amine hardening agents. Coal tar epoxies, containing approximately 60 percent coal tar, are low-cost and resistant to a wide variety of aqueous corrosives, including hydrochloric acid, sodium hydroxide, salt water, and crude petroleum. Additionally, these compositions are resistant to sagging up to 196°C and are less brittle and more impact-resistant than unmodified coal tar. A typical formulation is given below:

Production Method

Coal tar pitch is heated to 49°C. Epoxy resin is thinned with xylene and secondary butyl alcohol and added to the coal tar pitch. The mixture is stirred until homogeneous. The solids content of the epoxy/pitch solution is adjusted and a dry mixture of magnesium silicate and colloidal silica is dispersed in the solution through vigorous mixing. To prepare the hardener component, diethylene triamine is mixed with secondary butyl alcohol and packaged separately. For spray application, the mixture formulation can be thinned with xylene or toluene at rates as low as approximately 10 percent. Coal tar epoxies are designed for use in ship underwater hull paints and in tanks and other equipment in highly corrosive environments. They are also widely used in corrosion protection of metallic structures such as underground storage tanks, pipelines, and piles. Other application areas include steel walls above ground, acidic concrete tanks, and sewage tanks.

Coal Tar Epoxy Paint Standard

Scope: This specification covers coal tar-containing epoxy polyamide paint suitable for use on structural steel or concrete surfaces. This specification meets the requirements of Paint Specification No. 16, Type 1, Class II, prepared by the Steel Structures Painting Council.

Composition:

The paint shall be a two-component system with one component containing tar, filler, and catalyst, and another component containing resin. Each component of this paint prepared with the specified materials shall be homogeneous, free of grit and coarse particles, and stable in storage. The components shall contain the following types of materials and their proportions:

Production Process

Talc and thickener powder additive are dispersed in component A through grinding equipment with significant ability to improve shear values. The thickener is mixed with an equal amount of talc and then wetted by mixing in alcohol. The resulting mixture is added to component A and thoroughly mixed. The viscosity of component A is significantly affected by the degree of dispersion of talc and thickener. Inputs shall have the properties specified below.

Coal Tar Pitch

Coal tar pitch is a product obtained by high-temperature distillation of raw coke oven tar, a product obtained during coal destructive distillation in slot furnaces operating at temperatures above 700°C. Coal tar pitch shall have the following properties:

Thickener:

The thickener or thixotropy-providing additive shall be magnesium montmorillonite or an organic derivative of hydrogenated castor oil. It shall be a cream-colored white powder containing a maximum of 3 percent water and a bulk density of 1.75 ± 0.2 g/cm³. Activator: If used, it shall be methanol, ethanol, or propylene carbonate. Catalyst: The accelerating catalyst shall be 2, 4, 6-tri(dimethylamino methyl) phenol. Epoxy Resin: The epoxy resin shall be a di-epoxide with terminal epoxide groups, a condensation product of bisphenol A and epichlorohydrin. It shall be light-colored, non-cloudy, free of crystalline and particulate matter, and have the following properties: Polyamide Resin: The polyamide resin shall be a condensation product of dimerized fatty acid in polyamides. It shall be transparent and non-cloudy. It shall have the following properties: Magnesium Silicate (Talc): Magnesium silicate shall conform to ASTM Standard D 605 "Magnesium Silicate Pigment (Talc)." When dark red paint is specified, the magnesium silicate shall be replaced by 50 percent or more (by volume) by synthetic red iron oxide conforming to ASTM Standard D 3721 for dark red paint. Red paint shall have all test conditions specified for black paint, except for volatile content of Component A, which shall be a certain amount reflecting the higher specific gravity of iron oxide pigment. M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya Ltd. Şti.