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Secondary Containment Coating Systems for Industrial Contractors

Article 13 of 24

Polyurethane and Polyurea Containment Systems

Polyurethane and polyurea systems can provide fast return to service, substantial film thickness, toughness, and flexibility. They are not interchangeable, and their success depends on chemical compatibility, substrate condition, proportioning, spray-equipment control, safe handling, and disciplined inspection.

Learning Objectives

After completing this article, you should be able to:

• Distinguish polyurethane, polyurea, and polyurethane-polyurea hybrid systems.
• Explain why flexibility does not automatically establish chemical resistance.
• Recognize the importance of substrate moisture, primer selection, and surface preparation.
• Identify critical plural-component equipment and proportioning controls.
• Understand how rapid gel time affects spray technique, detailing, and inspection.
• Recognize isocyanate hazards and the need for a formal respiratory-protection program.

Three Related but Different Technologies

Polyurethane and polyurea coatings are formed by reactions involving an isocyanate-containing component. The chemistry of the companion component and the resulting reaction help determine whether the material is classified as a polyurethane, polyurea, or hybrid.

• Polyurethane: Generally formed through a reaction between an isocyanate component and a hydroxyl-functional resin component. Cure speed can range from moderate to very fast, depending on formulation.
• Polyurea: Generally formed through a reaction between an isocyanate component and an amine-functional resin component. Many spray-applied systems gel within seconds and require heated plural-component equipment.
• Hybrid: May contain both hydroxyl- and amine-functional components or otherwise combine polyurethane and polyurea characteristics. The word “hybrid” does not define a universal performance level.

Product names and general chemistry labels are not enough for system selection. The contractor must use the manufacturer’s technical data, chemical-resistance information, and written approval for the proposed exposure.

Flexible Does Not Mean Chemically Universal

Polyurethane and polyurea systems are often promoted for elongation, toughness, impact resistance, abrasion resistance, and rapid cure. Those properties can be valuable, but they do not establish resistance to a particular chemical.

Some formulations perform well with water, wastewater, fuels, oils, salts, dilute chemicals, or intermittent splash and spill. Other chemicals can cause swelling, softening, discoloration, loss of tensile properties, permeation, blistering, or adhesion failure.

Chemical compatibility must be evaluated using the exact chemical, concentration, temperature, exposure duration, mixture, and cleaning procedure. Do not select a system based only on elongation, hardness, cure speed, or the word “polyurea.”

Why These Systems Are Considered for Containment

Properly specified polyurethane and polyurea systems may offer:

• Rapid gel and cure
• Fast return to service
• High film build in one or relatively few passes
• Seamless application over large areas
• Flexibility and crack-distribution capability
• Impact, abrasion, and tear resistance
• Application over properly prepared concrete and steel
• Conforming application around curbs, penetrations, and complex details

Each claimed advantage must be confirmed for the specific formulation. A fast-curing material can shorten the shutdown, but its speed also reduces the time available for mixing, atomization, leveling, detailing, and correction.

Aromatic and Aliphatic Systems

Many thick-film polyurethane and polyurea base membranes use aromatic isocyanate chemistry. Aromatic materials can provide strong physical performance but may change color or chalk when exposed to ultraviolet light.

An aliphatic polyurethane or polyurea topcoat may be specified where long-term color and gloss retention are important. The topcoat must be chemically and mechanically compatible with the base membrane and suitable for the expected exposure.

Color change does not always indicate loss of containment performance, but it can affect inspection, appearance, identification markings, and owner acceptance. The specification should distinguish cosmetic weathering from unacceptable degradation.

Rapid Cure Changes the Work

Some spray polyureas gel within seconds. This permits rapid film build and quick walk-on time, but the coating may stop flowing before it can level or release entrained air. The applicator must create uniform thickness through spray technique rather than depending on the material to flow into place.

Overlaps between passes must be controlled. If the spray pattern is uneven or the gun travels too quickly, thin areas may remain. If the gun slows or dwells, excessive thickness can accumulate. Complex details can harden before the applicator has time to correct them.

Fast surface cure also does not prove full chemical-service cure. The material may support foot traffic while it continues developing its final physical and chemical properties. Follow the manufacturer’s stated service-cure requirements.

Surface Preparation Still Controls Adhesion

A thick, flexible membrane can still separate from an improperly prepared substrate. Concrete and steel must be prepared to the profile, cleanliness, strength, moisture, and environmental requirements of the approved system.

Concrete must be sound and free of laitance, curing compounds, oil, grease, chemicals, weak surface paste, dust, and incompatible repairs. Bugholes, voids, cracks, joints, and transitions must be treated before membrane application.

Steel must meet the specified blast-cleanliness and surface-profile requirements. Oil, grease, soluble salts, dust, weld spatter, sharp edges, and fabrication defects must be corrected before priming.

Elongation cannot compensate for contamination or weak substrate. If the membrane is bonded to unsound concrete, the failure may occur within the concrete rather than within the coating.

Moisture and Outgassing

Fast-reacting polyurea is sometimes described as moisture tolerant. This statement must not be interpreted to mean that the system can be applied over wet, contaminated, or moisture-troubled concrete without evaluation.

Moisture can interfere with primer adhesion, contribute to blistering, increase outgassing, or react with certain isocyanate-containing materials. Polyurethane chemistry can be particularly sensitive to uncontrolled moisture during application.

Porous concrete can release air as the substrate warms. Because spray polyurea gels rapidly, escaping air may form bubbles, pinholes, craters, or blisters instead of passing through the film.

Concrete moisture testing, substrate evaluation, primer selection, application timing, and temperature control remain essential. A moisture-tolerant product is not automatically a moisture-mitigation system.

Primer Selection

The primer connects the prepared substrate to a membrane that may gel almost immediately after application. It must be approved for the substrate, membrane, moisture condition, exposure, and installation schedule.

A primer may reduce concrete porosity and outgassing, improve wetting, and provide a uniform bonding surface. On steel, it may improve adhesion and protect the prepared surface before membrane installation.

The primer must be sufficiently cured to receive the membrane but remain within its permitted recoat window. An undercured primer can be disturbed by the hot, high-pressure spray. An overcured or contaminated primer may require cleaning, abrasion, or repriming.

Broadcast aggregate is sometimes specified to provide a mechanical bond or improve traction between layers. Aggregate type, size, coverage, embedment, and removal of loose excess material must follow the approved procedure.

Plural-Component Equipment Is Part of the Process

Many thick-film polyurethane and polyurea systems require heated plural-component spray equipment. The equipment separately heats, meters, pressurizes, and transports the two components before they are combined at or near the spray gun.

Critical equipment functions include:

• Maintaining the specified component ratio
• Conditioning both components to the required temperature
• Providing balanced pressure and stable material delivery
• Maintaining sufficient heated-hose temperature
• Mixing the components completely at the spray gun
• Producing the required atomization and spray pattern
• Detecting or responding to an off-ratio condition

The spray machine is not merely a pump. It is a proportioning and process-control system. Operators must understand how temperature, viscosity, pressure, feed-pump performance, restrictions, worn parts, and material level affect proportioning.

Ratio Verification

An acceptable-looking spray pattern does not prove that the material is on ratio. Off-ratio material may initially appear normal and later remain soft, become brittle, foam, discolor, lose adhesion, or fail under chemical exposure.

Ratio checks should follow the equipment manufacturer’s and material manufacturer’s procedures. The contractor should document:

• Required component ratio
• Actual volume or weight collected from each side
• Component temperatures and pressures
• Date, time, equipment identification, and operator
• Corrective action when results fall outside tolerance

Ratio checks should be performed before production, after equipment maintenance, after a shutdown, when material or settings change, and whenever pressure imbalance or coating abnormality indicates a possible problem.

Component Conditioning and Agitation

Components must be stored and conditioned within the manufacturer’s required temperature range. Cold material can become too viscous for accurate proportioning and complete mixing. Excessive heat can affect viscosity, pressure, reaction rate, and material stability.

Some resin-side components contain pigments, fillers, or additives that can settle. They may require controlled agitation before and during use. Other components must not be agitated or exposed to unnecessary moisture. Follow the product-specific instructions.

Drums and day tanks should remain properly sealed. Desiccant dryers, dry-gas blankets, or other moisture-control devices may be required to protect moisture-sensitive material.

Spray Technique

The applicator should maintain a consistent gun distance, gun angle, travel speed, triggering sequence, and overlap. The spray should strike the surface as close to perpendicular as access permits.

Because the material may gel within seconds, fan shape and pass placement become visible in the finished film. Applicators should build thickness through controlled passes rather than attempting to place the entire specified thickness with one slow, heavy pass.

Edges, coves, penetrations, drains, equipment bases, wall-to-floor transitions, and other details may require a separate application sequence. Broad production spraying should not begin until the crew understands how those locations will be completed without thin spots or excessive buildup.

Overspray that has already gelled can create a rough, poorly bonded surface for later passes. Keep the spray edge active and follow the manufacturer’s requirements for overlaps, tie-ins, and preparation of cured overspray.

Film-Thickness Control

The specified thickness is part of the containment design. Thickness affects continuity, crack distribution, impact resistance, permeation, service life, and holiday-testing requirements.

Fast-setting elastomeric coatings can make wet-film gauges difficult or unsuitable to use. The approved quality-control plan may rely on calibrated spray output, material usage, measured area, witness panels, depth gauges, destructive samples, or dry-film instruments suitable for the substrate and system.

Measure more than the average thickness. A satisfactory average can conceal individual areas below the specified minimum. Corners, vertical transitions, behind penetrations, around equipment bases, and pass overlaps deserve special attention.

Cracks, Joints, and Movement

Elastomeric membranes can tolerate more movement than many rigid coatings, but their capacity is not unlimited. Elongation measured on a free laboratory specimen does not equal the movement capability of a fully bonded field membrane.

Movement concentrated across a narrow crack can impose far greater local strain than the same movement distributed over a wider reinforced detail. Crack width, rate of movement, temperature, membrane thickness, bond width, and chemical exposure all influence performance.

Active cracks and expansion joints require approved details. These may include bond breakers, flexible joint sealants, reinforced strips, transition membranes, or mechanical joint systems. Do not spray directly over a moving joint and assume the membrane’s elongation will protect it.

Tie-Ins, Repairs, and Recoat Windows

Rapid-cure systems can exceed their recoat window quickly. When work stops at the end of a shift or around an obstruction, the termination must be planned so the next application can form a reliable bond.

A cured surface may require cleaning, solvent treatment when specifically approved, mechanical abrasion, a tie coat, or another preparation procedure before recoating. The procedure must identify the width of the prepared overlap and the method used to verify sound adhesion.

Repair materials must be chemically compatible with the original membrane and suitable for the repair size and exposure. A slow-cure hand-applied repair compound should not be substituted merely because it is convenient.

Common Application Defects

• Off-ratio material: May remain soft, become brittle, foam, discolor, or fail chemically.
• Pinholes and bubbles: May result from concrete outgassing, moisture, poor priming, air entrainment, or unsuitable application conditions.
• Poor adhesion: May result from contamination, weak concrete, moisture, incorrect profile, poor primer selection, or an exceeded recoat window.
• Rough or textured finish: May result from incorrect temperature, poor atomization, excessive gun distance, overspray, or material that gels before reaching the surface.
• Thin or missed areas: Often occur at pass edges, corners, penetrations, curbs, vertical transitions, and difficult-to-see locations.
• Excessive thickness: Can occur where the gun slows, changes direction, or repeatedly crosses the same detail.
• Delamination between passes: May result from cured overspray, contamination, or improper tie-in preparation.
• Chemical swelling or softening: May indicate an incorrect system, incomplete cure, excessive temperature, or exposure beyond the approved limit.

Isocyanate Safety

Polyurethane and polyurea components can contain isocyanates. Exposure can affect the respiratory system, skin, and eyes. Sensitization can occur, after which very small exposures may produce a serious response.

High-pressure spray can create airborne material that is not adequately controlled by relying on odor, disposable dust masks, or ordinary ventilation. The employer must perform a hazard assessment and establish appropriate engineering controls, work practices, protective clothing, decontamination procedures, and respiratory protection.

Respirator selection must be made under a compliant respiratory-protection program that includes medical evaluation, fit testing where applicable, training, equipment inspection, cartridge-change procedures when permitted, and program evaluation. Supplied-air respiratory protection may be required by the exposure, application method, product instructions, or site program.

Workers outside the immediate spray area can also be exposed to airborne material. Establish controlled boundaries, ventilation, communication, and reentry requirements before spraying begins.

Field Quality-Control Checklist

• Written chemical-resistance approval matches the exposure.
• Substrate preparation, moisture condition, and primer are accepted.
• Materials are within shelf life and properly conditioned.
• Components and feed systems are protected from contamination and moisture.
• Proportioner ratio, temperatures, pressures, and alarms are verified.
• Ratio checks are documented at required intervals.
• Spray pattern and test application are acceptable before production.
• Required thickness is achieved across both open surfaces and details.
• Cracks, joints, penetrations, coves, and transitions follow approved details.
• Tie-ins and repairs receive the specified preparation.
• No off-ratio material, pinholes, bubbles, voids, or delamination remain.
• Holiday testing is completed when required.
• All repairs are cured and retested.
• Full chemical-service cure is documented before release.

Technical References

Use the editions identified in the contract documents and verify current designations before incorporating standards into a proposal or work plan.

• AMPP — Application of Thick-Film Polyurea and Polyurethane Coatings Using Plural-Component Equipment
• AMPP — Two-Component Thick-Film Polyurea and Polyurea-Polyurethane Hybrid Coatings
• AMPP — Safe Use and Handling of Isocyanate-Containing Polyurethane and Polyurea Coatings
• AMPP — Plural-Component Application for Polyureas and High-Solids Coatings
• AMPP — Design, Installation, and Maintenance of Coating Systems for Concrete Used in Secondary Containment
• OSHA — Isocyanates: Safety and Health Information
• OSHA — Isocyanate Standards and Regulatory Information
• AMPP SP0188: Discontinuity holiday testing of protective coatings and linings.
• Manufacturer documentation: Current technical data sheets, safety data sheets, chemical-resistance guides, processing instructions, equipment settings, detail drawings, repair procedures, and written project-specific recommendations.

Key Takeaways

• Polyurethane, polyurea, and hybrid systems are related but not interchangeable.
• Flexibility and rapid cure do not prove chemical compatibility.
• Moisture-tolerant does not mean suitable for wet or moisture-troubled concrete.
• Surface preparation and primer selection remain critical to adhesion.
• Plural-component equipment must accurately heat, meter, mix, and deliver both components.
• A good spray pattern does not prove that the material is on ratio.
• Rapid gel requires disciplined spray technique and planned tie-ins.
• Active cracks and moving joints require engineered details.
• Isocyanate hazards require formal exposure control and respiratory protection.
• Written manufacturer approval should address the complete exposure and system.

Professional responsibility: This article provides foundational educational information and is not a substitute for the project specification, engineering direction, exposure assessment, regulatory requirements, equipment training, or the manufacturer’s current written recommendation. Polyurethane and polyurea components may contain isocyanates and may create serious inhalation, skin, eye, pressure-injection, and process hazards. Review all current technical data sheets, safety data sheets, equipment manuals, respiratory-protection requirements, and site-safety procedures before beginning work.

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