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> Coating Guide

Design Guide: Epoxy Coatings

High performance epoxy coating materials are used to protect concrete and steel from chemical and mechanical attack in interior, exterior and marine environments. Epoxy coating systems consist of two or more components which, when blended and applied as a film, react chemically to form a protective film of high integrity, excellent adhesion, toughness and impact resistance.

These coatings are applied in films ranging in thickness from several to over 150 mils. Coatings provide a barrier to elements which otherwise may attack the substrate. The chemical formulation of the coating material will determine its resistance to specific chemicals while resistance to mechanical loads, such as abrasion and impact, is a function of the physical properties and the thickness of the coating film.

Solvent-born coatings generally have a long pot life and are "thinner" and therefore, easier to apply. The "dry film" thickness or the film remaining after solvent evaporation is proportional to the solids content of the coating material and the applied or "wet film" thickness.

In order to avoid sag or runs and to allow proper solvent evaporation, solvent-borne coatings are applied as thin films. Application at low temperature or in thick films may cause solvent entrapment, giving rise to blisters or peeling.

Solvent-born coatings are particularly useful as primers, sealers, surface penetrants or where high film thickness is not required. In recent years, epoxy coatings have been developed which use water as the vehicle. Such products contain the coating materials as emulsions or dispersions, which upon application and evaporation of the water coalesce to form the protective film.

Solventless ("100% solids") coatings contain no volatile ingredients, therefore, the final thickness of the cured coating is the same as the applied thickness. Advantages of solventless over solvent-borne coating materials are:

> Fewer coats are required to achieve a desired film thickness.
> Low temperature cure can be achieved.
> Air ventilation requirements are reduced during application and curing.
> Air pollution and fire risks are minimized.
> VOC compliance

When used on floors and decks, epoxy coatings can be skid-proofed by imbedding grit into the surface layer or reinforced by incorporating glass cloth. Trowelable coating materials of nonsag or mastic consistency are used for applications of thick layers on vertical and overhead surfaces.

Some high performance epoxy coatings for steel and concrete surfaces may chalk and discolor upon exposure to direct sunlight that causes their finish to turn dull. These features should be considered in exterior architectural coating applications.

Protecting Against Highly Aggressive Chemicals

Most applications requiring a high degree of chemical resistance are found in the manufacturing and chemical processing industries. Properly formulated and applied coating materials can stop the action of chemicals that rapidly deteriorate concrete and steel. Applications requiring high chemical resistance include:

> Refineries, electroplating plants, chemical processing plants, battery handling rooms.
> Food processing plants, meat packing areas, tanneries and dairies.
> High nuclear radiation exposure areas.
> Manufacturing plants, particularly where cutting oils are used.

Protecting Against Moderately Aggressive Chemicals

Epoxy coatings protect concrete and steel which are exposed to mild chemical attack from sources such as acid mist, organic acids and water containing corrosive chemicals. Concrete surfaces are often porous, subject to bacterial growth and difficult to clean. The glossy, tile-like, impervious natures of these coatings provide ideal protection in the following areas:

> Institutional and commercial kitchens and laundries.
> Sewage and water treatment tanks and digesters.
> Breweries and winery facilities.
> Protection of concrete columns, piers and abutments from intrusion of de-icing salts.
> Protection of concrete and steel in marine structures above the splash zone.
> Protection of concrete against erosion from high velocity water.

Providing Skid Resistance on Traffic Surfaces

The skid resistance of concrete, steel and asphalt surfaces can be improved by the application of a skid resistant coating. This consists of selected aggregate imbedded into the coating. The aggregate dramatically increases the coefficient of friction of the otherwise smooth coating.

Areas that receive heavy wear due to high traffic speed and volume require a surface texture that will not be worn away or polished smooth. Skid resistant coatings are used to surface:

> Industrial floors
> Parking garage decks and ramps
> Loading docks
> Concrete bridge decks
> Toll plazas

Coating Marine Structures

Epoxy coatings are used to protect concrete and steel marine structures as formulations are available to adhere to wet substrates. Marine coating use can be divided into two areas, splash zone application and underwater application. Both applications require the ability of the coating material to be workable, adhere and cure underwater; in the splash zone the material must resist wave action during application and after cure. Typical applications for the protection of marine structures include:

> Coating piles, piers, sea walls and abutments in the splash zone.
> Coating water tanks, fish ladders, dams, and outfall structures.
> Coating piles, piers, seawalls and abutments underwater.

Design Guide

1.0 Design Considerations
1.1 Effect of Temperature
1.1.1 Application Temperature: The temperature at the time of application will affect the handling characteristics of the material, the evaporation rate of the solvent from the film and the curing characteristics of the coating.

In an exterior application, the optimum timing for coating placement is during a period of declining surface temperature to minimize the potential for blistering caused by water vapor transmission (see 1.2.2).
1.1.2 Service Temperature: Most physical properties of epoxies are affected by temperature. It is important to identify the service temperature range for a proposed application in order that the performance characteristics of a candidate product can be evaluated.
1.2 Moisture
1.2.1 Surface Moisture: Although certain materials will adhere to a moist surface, superior adhesion is achieved in dry conditions.
1.2.2 Rising Moisture: Water vapor will travel through a porous substance such as concrete from an area of high humidity to one of low humidity. When the flow of this moisture is interrupted by an impervious layer such as a epoxy, it may cause blistering of uncured material. Moisture that collects beneath a cured overlay can also cause spalling should freezing occur. This condition is particularly prevalent with slabs on grade with no moisture barrier between the soil and slab.

Rising moisture can be detected by taping a 3 foot square sheet of polyethylene to the floor. After 24 hours, if the concrete under the sheet appears dark or condensation forms on the underside of the sheet, a rising moisture condition exists and corrective measures should be evaluated prior to coating. The flooring industry has established a minimum standard of 3 pounds or less of water transmission per thousand square feet per 24 hours. If moisture is suspected, several test kits should be placed at random intervals around the floor slab.
1.2.3 Condensing Moisture: When the temperature of the slab is at or below the dew point of the atmosphere, moisture will condense on it. To alleviate this condition, the temperature of the slab must be raised by a sufficient amount so the surface temperature remains above the atmospheric temperature during the application of the coating material.
1.3 Downtime: The area to receive a coating must be taken out of service during installation and cure. It is recommended that a realistic appraisal be made of the time interval that a given area can be closed for repairs.
1.4 Food and Potable Water
1.4.1 Contact: Where food or potable water may come into contact with the coating, generally ANSI standard 61 applies with testing performed by NSF or UL.
1.4.2 Odor: Chemicals associated with some coating products may taint the taste of exposed food and beverages; and appropriate precautions, including removal of such foods, are recommended during application.
1.5 Mechanical Loading: Abrasion, wear and impact are resisted by the toughness of the coating. Coating thickness tends to aid resistance to impact.
1.6 Chemical Exposure: The nature and severity of chemical exposure depends on the chemical compounds, their concentration, the duration of the exposure, the frequency of cleaning or flushing, the service temperature of the environment and the temperature of the solution.
1.7 Skid Resistance
1.7.1 The tractive forces to be resisted by surfaces exposed to foot traffic are generated by the foot gear worn. The relative slipperiness of the surface, wet and dry, is measured for both leather and rubber (see section 6.5. 1 ).
1.7.2 Vehicular Traffic: Besides surface texture, the skid resistance for vehicular traffic surfaces depends on the velocity of the vehicle and the slope of the surface. Test methods that take these variables into account have been developed and are in use by several highway agencies (see section 6.5.2). When exposed to vehicular traffic, the surface texture can be worn and polished causing a reduction in skid resistance; therefore, skid resistance is monitored periodically to assure that it is maintained within safe limits.
1.8 Architectural: The ultraviolet component of sunlight may cause chalking and discoloration of certain epoxy materials in exterior applications. These changes are a surface phenomenon and do not influence performance of the coating material in hostile environments. It is important that the designer is aware of these potential changes.
2.0 Material Considerations
2.1 Application Characteristics
2.1.1 Solvent-borne Coatings: Solvent borne coatings are best applied with either air-atomized or conventional airless spray, although application by brush, roller or squeegee may also be employed. Wet film thickness should not exceed the maximum recommended for the individual product in order to avoid solvent entrapment or sag.
2.1.2 Solventless Coatings: Solventless coatings are most often applied by brush, roller, squeegee or trowel but may also be applied with plural component, heated, meter-mix, air-atomized or airless spray equipment.
2.2 Curing Characteristics: Epoxy cure is accelerated by an increase and retarded by a decrease in temperature. The rate of evaporation of solvent is also retarded by a decrease in temperature.
2.2.1 Pot Life: Pot life is the useful life of the mixed epoxy system in the bulk mixing container and is dependent on material temperature.
2.2.2 Recoat Time: Subsequent coats must be applied within minimum and maximum limits. The minimum recoat time is the time required for the coat to become tack free. The maximum recoat time is the time after which a subsequent coat will not adhere to a previous coat without mechanically roughing the cured surface.
2.2.3 Hard Dry Time: The time required for a coating to become hard enough to support light duty traffic (e.g. foot traffic).
2.2.4 Full Cure Time: The time required for a coating to achieve its full potential mechanically and chemically.
2.3 Cured Characteristics
2.3.1 Adhesion: The bond strength of a coating is dependent on the surface preparations, the bonding ability of the coating material to the substrate in question and coating thickness. The substrate could be wet or dry concrete, steel, prime coated concrete, prime coated steel and various other substrates. The bond strength can be determined with a suitable adhesion tester (see section 6.3).
2.3.2 Chemical Resistance: The resistance to chemicals is dependent on the inherent resistance of the resin/curing agent system in the coating formulation and integrity of the coating. Acceptable coatings are free of pinholes, holidays, crazing cracks and other surface defects.
2.3.3 Toughness: To maintain integrity in use, coatings must be tough and impact resistant to resist gouging and chipping.
2.3.4 Skid Resistance: Both aggregate and coating material contribute to skid resistance properties: The amount of aggregate and its gradation, wear, polish and hardness characteristics all affect the degree and longevity of skid resistance. The thickness, cohesive and bond strength of the coating material all contribute to the capacity of the coating to bind the aggregate in place.
3.0 Estimating Quantities
3.1 When the desired dry film thickness is known:
a. Determine the number of gallons per square foot to achieve the required dry film thickness:
Gallons per square foot = mils of dry film
% solids by volume x 16.04
Multiply this value by the number of square feet to be coated to obtain the total number of gallons of material required.
3.2 When using the manufacturer's recommended spreading rate:
a. Determine the dry film thickness per coat by:
mils of dry film = %solids by volume x 16.04
spreading rate (sq. ft. per gallon)
b. Determine the number of coats required by dividing dry film thickness desired by mils of dry film per coat.
c. Divide the recommended spreading rate (in square feet per gallon) into the total area to be coated and multiply by the number of coats to get gallons required.
4.0 Surface Preparation
4.1 Surface Preparation
4.1 General. Surfaces to be bonded must be clean and sound, which in all cases requires some form of preparation.
4.2 Surface Evaluation: The following tests can be used to evaluate the condition of the substrate and the effectiveness of the surface preparation procedures.
4.2.1 Strength: The direct tensile strength of the surface may be determined by a pipe cap pull-off test or a commercially available adhesion tester. The strength of concrete bond should be a minimum of 200 psi.
4.2.2 Contaminants: The presence of grease, wax or oil may be detected by dropping a small amount of muriatic acid onto the surface. No reaction indicates that contaminants are present. If oil has penetrated into a concrete surface, it may be detected by raising the temperature of a small area to about 150F with a heat lamp. Presence of the contaminant is indicated if oil appears or the area becomes "greasy" to the touch.
4.3 Cleaning Procedures
4.3.1 Concrete Surfaces:
a. Grease, wax and oil contaminants may be removed by scrubbing with an industrial grade detergent or degreasing compounds followed with mechanical cleaning.
b. Weak or deteriorated concrete must be removed to sound concrete. This may be accomplished by bush hammering, gritblasting, scarifying, waterblasting or other suitable mechanical means.
c. Dirt, dust, laitance and curing compounds are removed by gritblasting, sanding or etching with 15% hydrochloric acid. Acid etching should only be used if there is no practical alternative. It must be followed by scrubbing and flushing with copious amounts of clean water. Check for removal of acid with moist pH paper. Reading should be more than 10.
d. Mechanical cleaning is followed with vacuum cleaning or oil-free, dry air blast.
4.3.2 Steel Surfaces:
a. Dirt, grease and oil are removed with suitable industrial grade cleaning and degreasing compounds.
b. Rust and mill scale are removed by gritblasting to white metal. Gritblasting is followed with vacuuming or oil-free, dry airblast.
5.0
5.1 General
5.1.1 Preconditioning: To facilitate mixing and application of epoxy coatings, preconditioning of the material to room temperature 24 hours before use is recommended.
5.1.2 Mixing: As with other coating materials, pigmented epoxy coatings must be agitated to redisperse pigments and fillers which may have settled during storage. To ensure proper cure, it is important that the components of the coating are carefully measured and adequately mixed. Thorough mixing will take from 3-5 minutes during which time the side and bottom of the mixing vessel should be scraped with the mixing paddle. Solvents should be added only when recommended by the manufacturer.
5.1.3 Number of Coats: The total dry film thickness desired and the type of coating material will determine the number of coats. A solvent-borne coating will require several coats to achieve the same final thickness as one coat of a 100% solids.
5.2 Methods of Application: Liquid coatings may be applied with brush, roller, air spray or airless spray, and on horizontal surfaces squeegees may be used. Non-sag coatings may be applied by squeegee or trowel. The choice of the method of application is a matter of economics.
Brush
for small areas with corners, edges and odd shapes where masking for spray is not worthwhile
Roller
for large areas where spraying would require extensive masking, ventilation or may present a fire or health hazard
Air spray
where areas to be coated are extensive or irregular and the time required to mask can be offset by a higher production rate
Airless spray
where the high cost of the equipment compared to air spray can be offset by a higher production rate, reduced masking time and overspray losses.
5.3 Primers: Most epoxy coatings are self-priming. In critical applications priming is generally desirable, sometimes even required.
5.3.1 Steel Surfaces: Primers are required to protect steel surfaces when there is a delay between surface preparation and coating application. Coatings applied to primed steel generally have improved impact resistance and adhesion.
5.3.2 Concrete Surfaces: To prevent pinhole formation due to out-gassing, a fast drying primer may be used. A primer may also be used to improve the adhesion of the top coat when the concrete is damp.
5.4 Block Fillers: The texture of cementitious surfaces is sometimes open and porous (paticularly concrete blocks or CMU). To obtain a smooth, pinhole free coating, surface irregularities can be filled with a suitable block filler. For maximum chemical resistance, abrasion resistance and adhesion to both substrate and topcoat, epoxy block fillers are recommended over latex and cement slurry.
5.5 Top Coats: Top coats are applied over primers, block fillers or directly to the substrate when they are self-priming. Since there are many types, it is recommended that the application instructions given on the specific technical bulletin be followed.
5.6 Skid Resistant Coatings
5.6.1 Foot and Light Vehicular Traffic Surface:
Two methods can be used to incorporate grit into a coating for service in areas which receive foot and light vehicular traffic. Where it is desired to build up thickness or develop a high degree of surface aggressiveness, grit is broadcast into the top coating layer to excess. After the coating has cured, the excess grit is removed. The resulting sandpaper-like texture can be moderated and rendered more easily cleanable by a final finish coat applied by roller. Naturally occurring, clean, kiln dried silica sands of 16 to 20 mesh are suitable for this application.
For light duty, where thinner layers or a less aggressive surface is desired in lieu of the broadcast system, a fine aggregate of 60 to 80 mesh can be mixed into the coating material and spread by squeegee as the final coat. Fine silica sands or emery grit may be utilized for this purpose.
5.6.2 Vehicular Traffic Surfaces: For highway applications or other heavy duty service, the coating material is used as a matrix for a chipseal application. Suitable, polish resistant aggregate for chipseal applications include aluminum oxide, emery or calcined bauxite. The aggregate selected should have a hardness on the Mohs' scale of 8 or more and the following gradation:
Standard SieveNo. % Weight Passing
1/4 100
6 97-100
8 55-75
816 0-3

Aggregate should be broadcast evenly over the surface and fall in a vertical direction to avoid displacing the uncured coating. Aggregate should be spread to an excess until the surface appears dry.
The quantity of the aggregate required depends on the thickness of the coating. Typical application rates of 28 to 30 square feet per gallon require 1.3 to 1.5 pounds per square foot of graded aggregate.
After the aggregate has been spread, all traffic should be prohibited until the surfacing material has cured sufficiently to bear traffic. Then, the excess aggregate can be swept up and reused if it is still clean and dry.
5.7 Mastic or Non-Sag Coatings: This system is used where a thick coating is needed to resist abrasion and impact or level irregularities in a vertical surface. The mastic is applied by trowel or squeegee. Sand in the amount recommended by the manufacturer may be blended with the mastic to reduce material cost. Use graded silica sand; washed, kiln dried and bagged. A good "skip" gradation for low void content is a blend by weight of two parts #12 or # 16 mesh to one part #80 or #100 mesh.
6.0 Quality Control
6.1 Wet Film Thickness: Wet film gauges are available to measure film thickness. The measurement is read immediately after the coating is applied (before evaporation takes place), and the final film thickness is computed from the known percentage of solids in the coating material.
6.2 Dry Film Thickness: Dry film gauges are available to measure film thickness of a non-conductive material on a ferrous substrate.
6.3 Adhesion
6.3.1 Scratch Adhesion Test for Steel Substrates:
An instrument with several blades is drawn across the coating making parallel cuts. At 900 to these cuts another set of cuts are made, producing many small squares. Masking tape is then applied over the patchwork and quickly removed. The number of squares removed is an indication of the quality of the adhesion.
6.3.2 Pull-off Test for Concrete or Steel Substrates:
An aluminum or steel "dolly" is cemented to the coating and pulled off with a calibrated tester. This test is generally performed with the aid of an Elcometer adhesion tester (ASTM D4547). The minimum pull-off strength for concrete surfaces is 200 psi. Higher values are obtained for steel substrates. For concrete substrates, failure should occur within the concrete. For steel surfaces, the failure should occur within the coating or in the bond between the steel and the coating. The pull-off strength will depend on the strength of the coating and its thickness. Therefore, technical information for the specific coating should be consulted.
6.4 Pinhole Detection: Pinhole detection devices are available for both steel and concrete surfaces.
6.5 Skid Resistance
6.5.1 Foot Traffic: A common method of measuring skid resistance for foot traffic is the Segler Pendulum Tester. It measures the Kinetic Coefficient of Friction and meets both "Federal Test Method Standard No. 501 a, Method 7121" and "National Bureau of Standards Report BMS 100, 'Relative Slipperiness of Floor and Deck Surfaces'." Typical Kinetic Coefficient of Friction values for floor surfacings range from 0.46 to 0.7; the smaller values represent a more slippery surface.
6.5.2 Vehicular Traffic: Two test methods used to determine the skid resistance of vehicular traffic surfaces are AASHTO T242 and ASTM E247.

Coating Coverage Chart

Applied coating thickness (1000 mils=1") Coverage per U.S. Gallon 100% Solids System
250 mils (1/4 in) 6.4 ft2
187.5 mils (3/16 in) 8.5 ft2
187.5 mils (3/16 in) 8.5 ft2
125 mils (1/8 in) 12.8 ft2
100 mils 16 ft2
62.5 mils (1/16 in) 25.5 ft2
50 mils 32 ft2
31.25 mils 51 ft2
20 mils 80 ft2
15.63 mils (1/64 in) 102 ft2
10 mils 160 ft2
5 mils 320 ft2
1 mil 1600 ft2


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