Polyurethane, Polyurea, and Elastomeric Linings

Why Elastomeric Linings Are Different

Many rigid lining systems form a hard barrier between the substrate and the service environment. Elastomeric linings are designed to retain a degree of flexibility after cure. That flexibility can help a properly selected lining withstand vibration, impact, abrasion, minor substrate movement, and changes in temperature.

Flexibility does not mean that the lining can correct every crack or structural problem. It also does not mean that all elastomeric products provide the same chemical resistance. The contractor must understand the particular material, substrate, service environment, and application equipment before work begins.

Understanding the Three Material Groups

Polyurethane linings

Polyurethane systems are produced through a reaction involving an isocyanate component and a resin component. Their properties can range from relatively hard and abrasion-resistant to soft and flexible. The specific formulation determines chemical resistance, hardness, elongation, weathering resistance, cure speed, and temperature limitations.

Some polyurethane systems provide excellent abrasion, impact, and wear resistance. Others are used as flexible membranes or protective topcoats. Product selection must be based on the actual service rather than the general term polyurethane.

Polyurea linings

Polyurea linings are also commonly produced as two-component reactive systems. Many are applied through heated, high-pressure plural-component equipment. Their extremely rapid reaction can allow high film build and short return-to-service times.

The same fast reaction that makes polyurea productive leaves little time for the material to flow, release air, or correct poor spray technique. Incorrect proportioning, inadequate mixing, improper temperatures, or weak substrate preparation may be locked into the lining almost immediately.

Hybrid and other elastomeric systems

Polyurethane-polyurea hybrids combine characteristics of both resin families. Other elastomeric lining products may include flexible epoxies, rubber-like membranes, or specialized formulations intended for containment, waterproofing, impact, or abrasion service.

The word hybrid does not establish performance. Contractors must review the complete technical data, chemical-resistance information, application instructions, and limitations for the specific product.

Where These Linings Are Used

Polyurethane, polyurea, and elastomeric linings may be used in industrial areas requiring toughness, flexibility, waterproofing, or rapid return to service.

• Secondary containment structures
• Wastewater structures and treatment equipment
• Truck beds, trailers, hoppers, and material-handling equipment
• Concrete floors, decks, ramps, trenches, and sumps
• Tank exteriors and selected tank-interior services
• Steel and concrete process equipment
• Mining, aggregate, and bulk-material equipment
• Roofing and waterproofing applications
• Pipelines, joints, transitions, and irregular surfaces

Not every elastomeric lining is suitable for continuous immersion. Products intended for waterproofing or exterior wear may not withstand stored chemicals, heated liquids, solvents, fuels, or continuous water immersion. Written confirmation of suitability should be obtained from the manufacturer.

Flexibility and Crack Bridging

Elastomeric linings are often selected because they can tolerate more movement than a rigid lining. However, the amount and type of movement matter. A stable hairline crack is different from an active structural crack, expansion joint, moving seam, or unsupported gap.

Active cracks and joints may require engineering evaluation, routing, filling, reinforcement, a flexible joint detail, or another treatment specifically designed for movement. Simply spraying a thicker layer over a moving defect does not guarantee long-term performance.

Chemical Resistance Must Be Verified

A flexible lining can provide excellent abrasion or impact resistance while offering limited resistance to a particular chemical. Acids, alkalis, solvents, fuels, oxidizers, oils, wastewater contaminants, cleaning chemicals, and elevated temperatures can affect different formulations in different ways.

Product selection should consider:

• Every material that may contact the lining
• Normal and maximum chemical concentrations
• Normal and maximum service temperatures
• Continuous immersion, intermittent contact, splash, or vapor exposure
• Cleaning chemicals and cleaning temperatures
• Expected abrasion, impact, pressure, and movement
• Whether outdoor ultraviolet exposure is expected
• Required service life and return-to-service schedule

Chemical-resistance charts are useful screening tools, but final selection should be supported by the manufacturer's written recommendation for the actual service conditions.

Surface Preparation of Steel

Steel must be free of oil, grease, salts, rust, mill scale, previous coatings, dust, and other contamination. Abrasive blasting should produce the specified cleanliness and surface profile.

Sharp edges, weld spatter, rough welds, pits, laminations, and irregular transitions should be corrected or treated as required by the specification. Deep pits may require filling with a compatible material before the full lining system is applied.

The prepared surface must be inspected before priming. If flash rust, condensation, dust, or contamination develops, the surface may require additional preparation. The specified primer should be applied within the permitted time and environmental conditions.

Surface Preparation of Concrete

Concrete must be sound, clean, sufficiently cured, and properly profiled. Remove laitance, curing compounds, oil, grease, existing coatings, weak concrete, and chemical contamination.

Bugholes, voids, cracks, joints, honeycombing, and surface irregularities must be repaired or detailed with materials compatible with the lining system. Filling these defects also helps reduce outgassing and pinhole formation.

Concrete moisture requires special attention. Liquid moisture and moisture vapor can affect primer adhesion, react with certain materials, produce blistering, or create pinholes. Use the moisture evaluation and acceptance criteria required by the project specification and lining manufacturer.

The Importance of the Primer

The primer creates the bond between the prepared substrate and the elastomeric lining. It may also help seal porous concrete and reduce outgassing. A primer should never be selected merely because it is available on the job.

Confirm that the primer is approved for:

• The particular steel, concrete, or other substrate
• The selected lining material
• The expected chemical and immersion service
• The application temperature and moisture conditions
• The required recoat interval

Applying the lining too early may disturb the primer. Applying it after the maximum recoat period may reduce intercoat adhesion. When a recoat window is missed, follow the manufacturer's written repair or surface-conditioning procedure.

Plural-Component Spray Equipment

Many fast-reacting polyurea and polyurethane linings are applied with proportioning equipment that heats, meters, pressurizes, and delivers two components separately to an impingement-mix spray gun. The components do not mix until they enter the gun's mixing chamber.

Successful application depends on the entire equipment system:

• Correct proportioner ratio
• Stable material supply to both proportioning pumps
• Correct primary-heater and heated-hose temperatures
• Balanced component pressures
• Clean inlet screens and properly operating transfer pumps
• Correct hose size, length, and temperature capability
• A clean, correctly sized mixing chamber
• Proper spray-gun operation and maintenance

The applicator must be trained to recognize off-ratio conditions. A pressure imbalance, restricted component, empty supply container, leaking seal, heater problem, or blocked mixing chamber can produce material that appears sprayed but never develops the intended physical properties.

Recognizing Off-Ratio Material

Possible signs of off-ratio or poorly mixed material include:

• Soft, gummy, or greasy areas
• Brittle or unusually hard areas
• Color variation or streaking
• Unusual surface texture
• Material that remains tacky
• Foaming, bubbling, or excessive pinholing
• Unbalanced equipment pressures
• A noticeable change in the spray pattern

Suspected off-ratio material should not be covered and forgotten. Stop application, identify the cause, isolate the affected area, and follow the manufacturer's procedure for removal and repair.

Spray Technique and Film Build

Fast-set materials require a consistent gun distance, gun angle, travel speed, and overlap. The applicator must keep the gun square to the surface and use a planned spray sequence that minimizes thin areas, excessive buildup, rough texture, and trapped overspray.

Corners, penetrations, welds, seams, terminations, and other difficult areas may require stripe treatment or special detailing. Excessive thickness in one pass may create heat, sagging, internal stress, or surface irregularities. Insufficient thickness can leave the lining unable to provide the specified barrier protection.

Measure thickness using the method and frequency required by the specification. Do not assume that a uniform-looking surface has uniform film thickness.

Overspray and Recoat Windows

Polyurea and other fast-set materials may produce dry overspray that does not become fully incorporated into the lining. Spraying from excessive distance, using incorrect temperatures or pressures, or allowing material to drift across a previously applied area can create a weak, textured surface.

Recoat intervals are equally important. Some systems develop a surface that requires cleaning, abrasion, or a special activator before additional material can be applied. Repairs made outside the approved recoat window may separate from the original lining.

Isocyanate and Application Safety

Many polyurethane, polyurea, and hybrid systems contain isocyanate components. Isocyanate exposure can affect the skin, eyes, and respiratory system and may cause sensitization. Once a worker becomes sensitized, later exposure may produce a serious reaction.

• Review the current safety data sheet for every component.
• Establish restricted work and overspray areas.
• Provide adequate engineered ventilation.
• Use the respiratory protection specified by the hazard assessment.
• Maintain a compliant written respiratory-protection program.
• Use chemical-resistant gloves, clothing, and eye protection.
• Prevent skin contact while handling materials and cleaning equipment.
• Follow confined-space procedures when working in tanks or vessels.
• Protect nearby workers from vapor, aerosol, and overspray exposure.

Cartridge respirators are not automatically appropriate for every sprayed isocyanate operation. Respirator selection must be based on the product, exposure evaluation, application conditions, applicable regulations, and the employer's respiratory-protection program.

Inspection and Quality Control

Quality control should begin before material is sprayed and continue through final acceptance. Inspection records should document:

• Product names, batch numbers, shelf life, and storage conditions
• Substrate repairs, surface cleanliness, and surface profile
• Moisture-test results where required
• Primer application and recoat times
• Air, surface, and material temperatures
• Relative humidity and dew point
• Proportioner ratio and operating temperatures
• Component pressures and observed pressure balance
• Applied film thickness
• Visual defects, holidays, repairs, and retesting
• Cure and return-to-service authorization

Holiday detection may be required for conductive substrates or specially designed systems. Test voltage and equipment must be appropriate for the lining type and thickness. Excessive test voltage can damage an otherwise acceptable lining.

Common Causes of Failure

• Selecting a product that is incompatible with the service environment
• Applying over moisture, contamination, dust, or weak concrete
• Using an unapproved or improperly cured primer
• Applying outside the primer or lining recoat window
• Incorrect component temperature or pressure
• Off-ratio proportioning or inadequate mixing
• Excessive overspray or poor spray technique
• Pinholes caused by concrete outgassing
• Insufficient thickness at edges and irregular surfaces
• Attempting to bridge active structural cracks without an approved detail
• Returning the lining to chemical service before it is ready

Contractor Field Checklist

Knowledge Check

Key Takeaway