Circle method adhesion tester for evaluating the bonding strength of organic coatings

How it works:

The circle method adhesion tester is an instrument that evaluates the bond strength between an organic coating and a substrate by mechanical scratching. The core principle is that a fixed load of a needle is driven by a motor to make a spiral scratch on the coating surface at a constant speed. As the scribe cuts through the coating and gradually deepens, the coating peels off the substrate. The adhesion grade is evaluated by observing the continuity and morphology of the coating peeling, and according to the number of turns of the scratch trajectory. The process simulates the failure behavior of the coating under mechanical stress, and the results are closely related to the cohesion and interfacial bonding strength of the coating.

Test methodology

During the test, the specimen is fixed on the instrument platform, and the appropriate load of the scribe is selected according to the estimated hardness of the coating. After starting the instrument, the scribing needle draws a spiral line from the inside to the outside with a constant pitch on the coating surface. After testing, use a soft-bristled brush to remove loose coating fragments from the scratched area and observe under an illuminated magnifying glass. The adhesion level is usually determined by the number of turns in which the first coating peels off: for example, if the coating begins to peel off on the fifth turn, the adhesion level is credited as level 5. Different national standards have slightly different levels of classification, but the core ideas are the same.

Widely referenced standards include: GB/T 9286 (equivalent to ISO 2409) "Grid test for color paints and varnishes", although this method is often called the "grid method", but its circle method principle and rating ideas are widely used for the evaluation of similar instruments; The evaluation principles described in ASTM D3359, Standard Test Method for Measuring Adhesion by Tape Testing, are also often used as a supplement to the analysis of circle results. These criteria provide a unified framework for quantitative evaluation.

Influencing factors

Adhesion test results of coatings are influenced by a variety of factors. The surface treatment of the substrate (e.g., cleanliness, roughness) is the primary condition for determining the interfacial adhesion. The properties of the coating itself, including curing degree, hardness, flexibility, and cohesion, directly affect its ability to resist shear stress. Test parameters, such as the radius of the scribing needle, application load, and scratch speed, need to be standardized according to the coating characteristics to ensure comparability of results. Environmental conditions such as temperature and humidity can also affect the physical state of the coating, which can interfere with test results.

Grading

After the circle method test, a grading system is usually used for evaluation. The lower the grade number, the stronger the adhesion (i.e., the coating begins to peel off after the outer turn). A typical example of a rating comparison is as follows:

The interpretation of the results should be combined with the peeling morphology of the coating. For example, the peeling of large pieces of coating and fine debris may point to the failure of interfacial bonding and coating cohesion, respectively. This distinction has guiding value for analyzing the failure mechanism and improving the process.

Scope of application:

This method is suitable for organic coatings on a variety of rigid or flexible substrates (such as metal, wood, plastic), including paints, varnishes, powder coatings, etc. It is widely used in industrial production quality control, formula screening in the research and development stage, and coating process optimization. However, this method also has limitations: it is not effective in testing softer or extremely elastic coatings; The test results are subjective and depend on the operator's observation. And it is a destructive test and cannot be used for full inspection of finished products. Therefore, it is often necessary to corroborate the results of other test methods, such as the pull-out method.

Epilogue

As a classic characterization tool, the circle adhesion tester provides an intuitive and relatively fast semi-quantitative means for evaluating the adhesion of organic coatings. Through standardized operation and detailed result analysis, it can effectively reflect the interface performance and overall quality of the coating system. In practical applications, understanding its principles and influencing factors, combined with specific product standards for comprehensive evaluation, is the key to obtaining reliable conclusions and guiding production practices.

References

GB/T 9286-2021, Colored paints and varnishes - Grid test.

ISO 2409:2020, Paints and varnishes — Cross-cut test.

ASTM D3359-22, Standard Test Methods for Rating Adhesion by Tape Test.

Technical Handbook of Coatings and Coatings, Chemical Industry Press.