🔩 Adhesion of coatings by the pull-off test — three methods, stress increasing at up to 1 MPa/s
In short
A dolly is glued to the coating and, after the adhesive has cured, the assembly is subjected to controlled tension perpendicular to the substrate. The test is performed according to PN-EN ISO 4624:2023-11; Methods: Method A — two dollies, rigid and deformable substrates; method B — one dolly, testing from one side, rigid substrates only; method C — dollies of which one is the painted substrate. The procedure comprises 8 steps (about 2 min in total); it is used for: Acceptance of paint coatings on steel structures, bridges and rail rolling stock, Checking the adhesion of coatings on concrete (dollies with a diameter of up to 100 mm), Comparing the adhesion behaviour of different coating systems.
At a glance
- Standard: PN-EN ISO 4624:2023-11
- Category: Materials and NDT
- Procedure steps: 8
- Total time of stages: 2 min
- Methods: Method A — two dollies, rigid and deformable substrates; method B — one dolly, testing from one side, rigid substrates only; method C — dollies of which one is the painted substrate
- Loading rate: Stress perpendicular to the plane of the coating, increasing uniformly, at no more than 1 MPa/s
- Time to failure: Failure of the assembly should occur within 90 s
Overview
The standard specifies three methods for determining adhesion by carrying out a pull-off test on a single coating or on a multi-coat system of paint, varnish or a related product. Method A uses two dollies and is suitable for rigid and deformable substrates; method B consists of testing from one side only with a single dolly and is suitable for rigid substrates only; method C uses dollies of which one acts as the painted substrate.
The methods are useful for comparing the adhesion behaviour of different coatings and are particularly useful for producing relative ratings for a series of panels coated with coatings showing significant differences in adhesion. The test can be applied to a wide range of substrates, with the procedures differing depending on whether the substrate is deformable, such as thin sheet metal, plastics and wood, or rigid, such as thick concrete and thick metal plates. To avoid deformation of the substrate during the tensile test, a sandwich construction is commonly used, and in special cases the coating can be applied directly to the face of the dolly.
The ISO 4624:2023 edition added larger dollies, up to 100 mm, intended for applying the method on concrete, supplemented the requirement for the adhesive with regard to the substrate and the coating material, and extended the information on the single-dolly method. The pull-off adhesion test, alongside the cross-cut method, is a standard element of the acceptance of paint coatings, including in the painting requirements for rail rolling stock.
Method principle
The product or system under test is applied at uniform thickness to flat panels of uniform surface texture. After the coating has dried and cured, dollies are glued to its surface with an adhesive. After the adhesive has cured, the assemblies with the dollies are placed in a suitable tensile testing machine and subjected to a controlled tensile test, measuring the force needed to break the coating–substrate bond. To avoid deformation of the substrate during the test, dollies with a diameter smaller than the 2 cm used for steel substrates can be used, which reduces the force applied. The result of the test is the tensile stress needed to break the weakest interface (adhesive failure) or the weakest component of the assembly (cohesive failure); mixed adhesive-cohesive failures also occur.
Applications
- Acceptance of paint coatings on steel structures, bridges and rail rolling stock
- Checking the adhesion of coatings on concrete (dollies with a diameter of up to 100 mm)
- Comparing the adhesion behaviour of different coating systems
- Assessment of adhesion after local repairs of the coating
- Assessment of the adhesion of liquid coatings on composite substrates and laminates
Key parameters
| Parameter | Value |
|---|---|
| Methods | Method A — two dollies, rigid and deformable substrates; method B — one dolly, testing from one side, rigid substrates only; method C — dollies of which one is the painted substrate |
| Loading rate | Stress perpendicular to the plane of the coating, increasing uniformly, at no more than 1 MPa/s |
| Time to failure | Failure of the assembly should occur within 90 s |
| Dolly diameter | Typically 2 cm for steel substrates; smaller to limit the force on deformable substrates; up to 100 mm for concrete |
| Types of failure | Adhesive (weakest interface), cohesive (weakest component of the assembly) or mixed |
| Substrates | Deformable — thin sheet metal, plastics, wood; rigid — thick concrete, thick metal plates; a sandwich construction is used for deformable ones |
| Measured quantity | The smallest tensile stress needed to detach or rupture the coating in the direction perpendicular to the substrate |
Standard
- Standard number
- PN-EN ISO 4624:2023-11
- Title (PL)
- Farby i lakiery — Próba odrywania do oceny przyczepności
- Title (EN)
- Paints and varnishes — Pull-off test for adhesion
Step-by-step procedure
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Preparation of test specimens
Apply the product or system at uniform thickness to flat panels of uniform surface texture; sampling of the product according to ISO 15528, examination and preparation of samples according to ISO 1513, panels according to ISO 1514.
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Drying and conditioning
Dry and cure the coating in accordance with the product requirements, then condition at a temperature and humidity in accordance with ISO 3270.
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Measurement of coating thickness
Determine the coating thickness in accordance with ISO 2808 and record it; the coating thickness affects the result and is part of the test report.
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Choice of method
Choose method A (two dollies, rigid and deformable substrates), method B (one dolly, rigid substrates only) or method C (one of the dollies as the painted substrate). For deformable substrates provide a sandwich construction so that the substrate does not deform.
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Gluing the dollies
Degrease the faces of the dollies and the coating surface, apply the adhesive and glue the dollies directly to the surface of the coated and cured panel. Select the adhesive in accordance with the requirement of the standard with regard to the substrate and the coating material.
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Curing of the adhesive
Leave the assembly until the adhesive has fully cured under the conditions recommended by its manufacturer, without stressing the joint.
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Pull-off test
Place the assembly in the tensile testing machine and apply the stress perpendicular to the plane of the coated substrate, increasing it at an essentially uniform rate of no more than 1 MPa/s, so that failure of the assembly occurs within 90 s.
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Assessment and report
Read the breaking stress and assess the nature of the failure: adhesive, cohesive or mixed, stating which interface or which component failed and over what proportion of the area. In the report state the method, dolly diameter, adhesive, coating thickness, conditioning conditions, breaking stress and type of failure.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Tensile testing machine | Suited to the chosen procedure, loading perpendicular to the plane of the coating with a uniform increase in stress | — |
| Dollies | Typically 2 cm in diameter for steel; smaller for deformable substrates; up to 100 mm when testing on concrete | — |
| Test panels | Flat panels of uniform surface texture according to ISO 1514 | — |
| Coating thickness gauge | Determination of the coating thickness according to ISO 2808 before the test | — |
| Circular cutter or cutting tool | For cutting the coating around the dolly when the procedure requires it | — |
| Conditioning chamber | Temperature and humidity for conditioning and testing according to ISO 3270 | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Adhesive for the dollies | — | Must meet the requirements of the standard with regard to the substrate and the coating material; must not affect the coating or penetrate it in a way that changes the result. Usually a two-component epoxy or cyanoacrylate adhesive |
| Degreasing solvent | — | For preparing the face of the dolly and the coating surface before gluing |
Health and safety (OHS)
- Sudden rupture of the assembly under load — guard of the working zone, safety glasses, no hands in the axis of tension
- Epoxy and cyanoacrylate adhesives — sensitising, bond skin; gloves, ventilation, work in accordance with the safety data sheet
- Degreasing solvents — flammable and volatile, ventilation, no open flame
- Cutting the coating around the dolly — sharp tools, cut-resistant gloves
- Field testing on structures — work at height, fall protection, securing the elements being pulled off
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 4624:2023-11 — “Paints and varnishes — Pull-off test for adhesion”.
How does this method work?
The product or system under test is applied at uniform thickness to flat panels of uniform surface texture. After the coating has dried and cured, dollies are glued to its surface with an adhesive.
What is the measuring range and accuracy?
Methods: Method A — two dollies, rigid and deformable substrates; method B — one dolly, testing from one side, rigid substrates only; method C — dollies of which one is the painted substrate; Loading rate: Stress perpendicular to the plane of the coating, increasing uniformly, at no more than 1 MPa/s; Time to failure: Failure of the assembly should occur within 90 s; Dolly diameter: Typically 2 cm for steel substrates; smaller to limit the force on deformable substrates; up to 100 mm for concrete.
How long does the test take?
The times given for the individual stages add up to approximately 2 min. The procedure comprises 8 steps.
What equipment is required?
Tensile testing machine, Dollies, Test panels, Coating thickness gauge, Circular cutter or cutting tool, Conditioning chamber.
Where is this test used?
Acceptance of paint coatings on steel structures, bridges and rail rolling stock; Checking the adhesion of coatings on concrete (dollies with a diameter of up to 100 mm); Comparing the adhesion behaviour of different coating systems; Assessment of adhesion after local repairs of the coating; Assessment of the adhesion of liquid coatings on composite substrates and laminates.
What safety precautions apply?
Sudden rupture of the assembly under load — guard of the working zone, safety glasses, no hands in the axis of tension; Epoxy and cyanoacrylate adhesives — sensitising, bond skin; gloves, ventilation, work in accordance with the safety data sheet; Degreasing solvents — flammable and volatile, ventilation, no open flame; Cutting the coating around the dolly — sharp tools, cut-resistant gloves; Field testing on structures — work at height, fall protection, securing the elements being pulled off.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 4624.