Adhesion Testers for Paints and Coatings

Plastic Film Tester | for Paints and Coatings

For coatings to perform satisfactorily, they must adhere to the substrate on which they are applied. A variety of recognized methods can be used to determine how well a coating is bonded to the substrate. Common measuring techniques include mechanical adhesion testing, plastics testing, or using a knife.

Understanding Adhesion in Coatings

Adhesion is the ability of a coating—such as paints and varnishes—to bond to a substrate (known as substrate adhesion) or to adhere to multiple coats in a system (termed inter-coat adhesion). For coatings to perform satisfactorily, they must bond effectively to various substrates including metal, concrete, wood, and more.

Adhesion Test Methods

Pull-off Adhesion Test

A Tensile Pull-Off Adhesion Test quantitatively measures the force required to detach a loading fixture (dolly or stub) from the coating. A portable pull-off adhesion tester applies an increasing load until the dolly is pulled off. The measured force—expressed in mega Pascals (MPa) or pounds per square inch (psi)—indicates the tensile strength of the coating bond. Standards such as ASTM D4541/D7234 and ISO 4624 guide this method.

Push-off Adhesion Test

Similar to the pull-off test, the Push-off Adhesion Test measures the force required to push a dolly (stub) off the surface. A center hole in each dolly allows a pin to be inserted, and manual or hydraulic pressure is applied until the dolly is detached.

Knife Adhesion Test

This subjective test involves cutting an “X” (with a 30° to 45° angle between the legs) into the coating using a utility knife and a metal straightedge. The knife is then used to attempt to lift the coating from the substrate or an underlying layer. ASTM D6677 details this test method.

Tape Adhesion Tests

ASTM D3359 outlines two methods for testing adhesion using pressure-sensitive tape applied over cuts in the film:

Method A (X-cut): For coating systems over 5 mils (125 µm), an X-cut is made through the coating, tape is applied at the intersection, and then removed rapidly to inspect for coating removal.

Method B (Cross-cut): For coatings less than 5 mils (125 µm), a lattice or cross-hatch pattern is cut, and tape is applied and removed. The area is then rated against the ASTM D3359 standard.

Scrape Adhesion Test

The Scrape Adhesion Test utilizes a weighted balance beam with a rounded stylus. The coated panel is pushed beneath the stylus, and increasing weight is applied until the film is scraped loose. The adhesion is reported as the weight (in kilograms) required to remove the film. Depending on the coating and substrate, ASTM standards such as D2197, D2248, D2454, and D5178 may apply.

Additional Adhesion Test Methods

Knife Test

In the Knife Test, a utility knife is used to make an X-cut through the coating down to the substrate. The inspector then attempts to lift the coating from the substrate at the vertex of the cut. This method is highly subjective and its value depends on the inspector’s experience. ASTM D6677 provides the standard for this test.

Tape Test

A more formal version of the Knife Test on metal substrates is the Tape Test. It involves:

X-cut Tape Test: An X-cut is made, tape is applied over the cuts, and then removed rapidly. The coating removal is inspected and rated.

Cross-hatch Tape Test: A cross-hatch pattern is cut (using a special cross-hatch cutter), tape is applied, and then removed. The test area is compared to ASTM D3359 standards.

Pull-Off Adhesion Test (Revisited)

This test, as previously described, involves affixing a dolly to the coating with adhesive. The pull-off force is gradually increased until the dolly detaches. The test is sensitive to device specifications and substrate stiffness, and various dolly sizes (e.g., 20 mm, 50 mm) may be used to suit different bond strengths.

Scrape Test (Revisited)

In a controlled laboratory setting, the Scrape Test uses a balanced-beam scrape-adhesion tester to determine the force required to remove the coating from a smooth, flat surface, following ASTM D2197 guidelines.

Paint Adhesion Tests

Paint adhesion tests evaluate how well a paint or coating adheres to a surface. Common tests include:

Cross-cut Test: A grid pattern is cut into the coating, pressure-sensitive tape is applied and then removed. The adhesion is rated, typically on a 0–5 scale, as per ASTM D3359.

Pull-off Test: A dolly is affixed to the surface and a tensile force is applied until the coating is detached. This method is suitable for on-site testing.

Scrape Adhesion Test: Measures the force required to scrape the coating from the surface.

Burnishing Test: Another method used to evaluate paint adhesion.

Other Adhesion Test Methods and Standards

Additional methods involve tensile test machines where paint is applied to a substrate with a textile strip embedded, or between two substrate sheets. ASTM D2370 covers tests for elongation, tensile strength, and stiffness of organic films. Adhesion of coatings on plastic substrates using aluminum studs is addressed in ASTM D5179.

ASTM D5179 Fixture

The ASTM D5179 fixture is used for instrumented indentation testing (nano- or micro-indentation) to measure the hardness of plastics and elastomers. Its key features include:

Indentation mechanism with a hard probe

Stable base for secure specimen support

Precise load application system

Depth measurement system for assessing indentation depth

This fixture is widely used in quality control and R&D in industries such as automotive, aerospace, and consumer goods.

Additional Standards

Other relevant ASTM standards include:

ASTM D4145: Bending test for determining flexibility and adhesion of coatings on prepainted metallic substrates.

ASTM 4146: Formability and adhesion testing for factory-applied zinc-rich primer/chromate complex coatings on steel.

Hardness tests (pencil hardness, gravelometer, impact tests, mandrel bend) that also note coating chip-off or loss of adhesion.

ASTM Paint Adhesion Testing Methods and Standards

There are three primary paint adhesion tests used to assess the resistance of paints:

Cross-Cut Test: Cuts are made through the coating, tape is applied and removed, and the adhesion is rated based on ASTM D3359.

Scrape Test: Conducted in the laboratory, this test measures the force required to scrape the coating from the substrate (ASTM D2197).

Pull-Off Test: Measures the tensile stress required to remove the coating using a dolly, in accordance with ASTM D4541 and ISO 4624.

These tests quantify the strength of the bond between the coating and substrate or between multiple layers, ensuring that coatings meet quality standards and perform reliably.

결론

From the largest man-made structures to the smallest household appliances, protective and cosmetic coatings are essential. Adhesion testing provides a quantitative assessment of the bond strength between coatings and substrates, helping to detect potential failures and ensure long-term durability. Consistent use of standardized test methods is crucial for maintaining quality and reducing rework costs.

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