Pull-Off Adhesion Testing

Coating Adhesion and Bond Strength Testing – Laboratory and On-Site

AECTL provides pull-off adhesion testing of protective coatings, paint systems, linings and other bonded surface systems to assess their tensile bond performance on steel, concrete and other rigid substrates. Testing can be undertaken at our Sydney laboratory or on site using an automatic, electronically controlled hydraulic adhesion tester.

The test applies an increasing tensile load perpendicular to the surface until separation, fracture or a nominated proof condition is reached. The measured pull-off stress is considered together with the location and nature of the fracture. This distinction is important because a numerical result alone does not identify whether the weakest plane was the coating-to-substrate interface, an intercoat boundary, the coating itself, the substrate or the adhesive used to attach the test dolly.

Pull-off testing is therefore useful for coating quality control, specification compliance, coating qualification, condition assessment and investigation of premature peeling, delamination or bond failure.

DeFelsko PosiTest AT-A automatic pull-off adhesion tester. Image source: DeFelsko. For the published page, preferably replace it with an original photograph of AECTL’s own instrument in use.

Applicable Test Standards

Testing can be undertaken in accordance with the applicable Australian or international method, project specification or coating manufacturer’s requirement. Commonly referenced pull-off adhesion standards include:

AS/NZS 1580.408.5 – Paints and related materials – Methods of test – Adhesion – Pull-off test.
ISO 4624 – Paints and varnishes – Pull-off test for adhesion.
ASTM D4541 – Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.
ASTM D7234 – Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers.
ISO 16276-1 – Assessment of, and acceptance criteria for, the adhesion/cohesion of a coating by pull-off testing.

Other test methods may be applicable to particular substrates or bonded systems. The method and test configuration should be confirmed before testing, particularly where the project specification nominates a specific dolly size, test rate, surface isolation procedure or acceptance criterion.

What Is a Pull-Off Adhesion Test?

A loading fixture, commonly referred to as a dolly, is bonded to the coating surface using a suitable high-strength adhesive. After adequate cure of the attachment adhesive, the dolly is connected to the pull-off actuator. Where required by the nominated method or coating configuration, the coating around the dolly is cut or cored to isolate the test area.

The instrument then applies a controlled tensile load normal to the surface. Loading continues until fracture or separation occurs, the equipment reaches its operating limit, or a specified proof load is achieved where the applicable procedure permits proof testing. The result is normally expressed as pull-off stress in MPa, together with the observed fracture mode.

Why the Failure Mode Matters

The recorded MPa value represents the stress at which the weakest part of the tested assembly failed. Correct interpretation therefore requires examination of both the dolly face and the exposed test surface.

Coating-to-Substrate Adhesive Failure

Separation occurs at the interface between the coating system and the substrate. This may indicate that the coating-to-substrate interface governed the measured strength and can be relevant to surface preparation, contamination, coating compatibility or application conditions.

Intercoat Adhesive Failure

Separation occurs between layers of a multi-coat system. The result relates to the strength of that interface rather than necessarily to the bond between the complete coating system and the substrate.

Cohesive Failure Within a Coating Layer

Fracture occurs through a coating layer, with the same coating material remaining on both sides of the fracture. The measured value is then governed by the cohesive strength of the failed layer.

Cohesive Failure Within the Substrate

The substrate fractures before the coating separates. This is particularly important for coated concrete. In such cases, the test demonstrates that the system sustained at least the stress at which the substrate failed; it does not establish a lower coating-to-substrate adhesion value.

Dolly Adhesive / Glue Failure

Failure occurs within or at the adhesive used to attach the dolly. If the attachment adhesive fails before the coating system, the actual coating adhesion or cohesive strength may not have been reached and the result must be interpreted accordingly.

Mixed Failure

More than one fracture mode may be present across the test area. Where required, the approximate proportion of each mode can be recorded to describe the result more accurately.

Automatic Pull-Off Testing Equipment

AECTL uses a DeFelsko PosiTest AT-A Automatic Adhesion Tester. The instrument uses an electronically controlled hydraulic pump to apply smooth, continuous pull-off pressure at a user-selected rate. The automatic loading sequence reduces the potential for variation associated with manual pumping and helps provide a consistent test procedure.

Useful capabilities of the PosiTest AT-A include:

Automatic electronically controlled hydraulic loading.
User-adjustable pull rate, pull limit and hold time.
Self-aligning dolly/actuator arrangement to minimise eccentric loading.
Digital display of pull-off pressure and test parameters.
Recording of maximum pull-off pressure, pull rate, test duration and pressure-versus-time data.
Recording of dolly size, pass/fail status and fracture classification.
Internal memory for large test batches and transfer of results for reporting.
Portable, environmentally sealed construction suitable for laboratory and field use.
Manufacturer-stated ±1% full-scale accuracy with calibration traceable to a national metrology standard.

The platform supports different dolly sizes and stand-off configurations. Smaller dollies extend the measurable range for higher-strength coating systems, whereas larger dollies are suited to lower-strength systems and substrates such as concrete. The selected configuration must match the expected strength, substrate and nominated standard.

Typical Test Procedure

Review the coating system, substrate, nominated standard, specification and acceptance criteria.
Select representative test locations and the appropriate dolly/test configuration.
Prepare the test surface and dolly to obtain a reliable adhesive bond.
Bond the dolly using a suitable adhesive and allow adequate curing time.
Cut or core around the dolly where required by the applicable method or test objective.
Connect the automatic actuator and apply the specified or selected pull rate.
Record the pull-off value and test parameters.
Examine both fracture surfaces and classify the failure mode.
Assess the result against the applicable acceptance requirement and report relevant observations.

Materials and Coating Systems

Subject to the nominated standard and test configuration, pull-off adhesion testing can be applied to:

Protective paint systems on structural steel.
Epoxy and polyurethane coatings.
High-build and multi-coat corrosion protection systems.
Concrete protective coatings and industrial floor coatings.
Tank, vessel and pipeline lining systems.
Water and wastewater infrastructure coatings.
Marine and civil infrastructure coatings.
Resin-based coatings and bonded repair systems.
Coating qualification panels and comparative trial systems.

The method is intended for rigid substrates. Flexible substrates, very thin components, highly irregular surfaces or brittle materials may require additional consideration because substrate deformation, geometry or premature substrate fracture can influence the result.

On-Site Pull-Off Adhesion Testing

The portable automatic system allows AECTL to undertake on-site coating adhesion testing in Sydney, throughout NSW and at project locations across Australia. Field testing is suitable for installed coating systems where laboratory panels would not represent the actual substrate preparation, application conditions or service environment.

Typical field applications include:

Structural steelwork and fabricated steel structures.
Bridges and civil infrastructure.
Tanks, vessels and pipelines.
Water and wastewater assets.
Concrete structures and industrial floors.
Marine, mining and processing facilities.
Newly applied coating systems requiring quality verification.
Existing coatings undergoing condition assessment or failure investigation.

Pull-off testing is normally locally destructive. The dolly and a small area of coating are removed during testing, and repair of the test location may be required after completion.

Laboratory Pull-Off Adhesion Testing

Coated components, representative specimens and test panels can be submitted to AECTL’s Sydney laboratory for controlled testing. Laboratory work is particularly useful for coating qualification, product comparison, quality control, surface preparation trials, application or curing studies and investigation of coating delamination.

Applications

Quality Control and Specification Compliance

Verify whether an applied coating system satisfies a nominated pull-off requirement and document the result against the project specification or manufacturer’s acceptance criterion.

Coating Failure Investigation

Determine whether premature peeling or delamination is associated with the coating-to-substrate interface, an intercoat boundary, the coating itself, substrate fracture or another part of the bonded system.

Surface Preparation Assessment

Compare coating performance following different surface preparation procedures or investigate whether an existing preparation method is producing adequate bond performance.

Coating System Qualification

Compare candidate coating systems, trial panels or application procedures before full-scale use.

Concrete Coating Assessment

Assess the tensile bond behaviour of coatings applied to concrete using the test configuration and method appropriate to the project.

Asset Condition Assessment

Generate quantitative adhesion data to support maintenance, repair, recoating or remediation decisions for existing assets.

More Than a Pull-Off Value

A pull-off result should be interpreted in the context of the coating system and the condition being investigated. Where required, AECTL can combine adhesion testing with other materials and coating assessment techniques to identify why a system is performing satisfactorily or why deterioration has occurred.

Visual coating inspection and defect assessment.
Dry film thickness measurement.
Surface profile and surface preparation assessment.
Holiday/discontinuity detection.
Corrosion examination.
Microscopic examination and coating cross-section assessment.
SEM/EDS analysis of deposits, corrosion products or interfacial contaminants.
FTIR analysis of organic materials and coating-related residues.
Accelerated corrosion and coating performance testing.
Failure analysis and root cause investigation.

For example, an investigation of coating delamination may consider pull-off strength and fracture location together with coating thickness, surface condition, contamination, corrosion morphology, coating cure and substrate preparation. This provides a stronger engineering basis than relying on a single adhesion value.

Test Reporting

Depending on the agreed scope and applicable method, AECTL reporting can include:

Identification of the coating system and substrate.
Applicable standard, specification and test procedure.
Test locations and specimen identification.
Dolly size and test configuration.
Relevant surface and dolly preparation details.
Pull rate and individual pull-off strength results.
Failure mode and approximate mixed-mode distribution where applicable.
Photographic documentation before and after testing.
Comparison with nominated acceptance criteria.
Engineering observations and interpretation where required.

A result is not automatically satisfactory or unsatisfactory because of the MPa value alone. Acceptance should be based on the applicable specification and must take account of the test configuration and the location of fracture.

Why AECTL?

AECTL combines automatic pull-off measurement with materials engineering, coating inspection, corrosion assessment and failure investigation capability. This allows the service to extend beyond simply recording a numerical pull-off value. Where required, the fracture mode, coating condition, substrate behaviour and project specification can be considered together to provide a technically meaningful assessment.

Automatic controlled-rate pull-off testing.
Laboratory and on-site capability.
Testing to recognised Australian and international methods.
Assessment of coatings on steel, concrete and other suitable rigid substrates.
Fracture-mode examination and photographic documentation.
Engineering interpretation and integration with broader coating or failure investigations.

Request Pull-Off Adhesion Testing

For an accurate quotation, please provide the coating type or coating system, substrate material, applicable standard or project specification, acceptance criterion, number and approximate location of tests, and whether testing is required on site or in the laboratory. Relevant coating drawings, specifications and product data sheets are also useful where available.