Application of Cross-cut Adhesion Tester in Multi-coatings
Introduction
In the surface treatment process of many industrial products, multi-coating systems are widely used due to their excellent protective and decorative properties. The bond strength between the coating and the substrate, as well as between the coatings, is the key indicator for evaluating the durability and reliability of the entire coating system. As a classic, intuitive and standardized evaluation method, the corresponding test instruments play an indispensable role in the quality control and research and development of multi-coating systems. This article will discuss the application points, result interpretation and technical advantages of the grid adhesion tester in multi-coating system testing.
Test Principles and Standards
The basic principle of grid testing is to use a cutting tool to create grid-like scratches at specified spacing on the surface of the coating until it penetrates into the substrate. Subsequently, the adhesion level is evaluated by observing the peeling off of the coating in the mesh area using special tape and rapid peeling. It is quantified based on the proportion of area that the coating separates from the substrate or the underlying coating. The process can be summarized by the following relations:
A = (Nd / Nt) × 100%
where A represents the percentage of coating shedding area, Nd is the number of grids that are shed, Nt This is the total number of grids. The smaller the value, the stronger the adhesion.
Corresponding test standards have been formulated for different industries at home and abroad, which have detailed regulations on cutting spacing, tool type, cutting speed and result evaluation methods. Common criteria include, but are not limited to:
| ISO 2409 | Color and varnish – grid test |
| ASTM D3359 | Standard test method for measuring adhesion by tape testing |
| GB/T 9286 | Colored paint and varnish grid test |
The challenge of multi-coating testing
Adhesion testing in multi-coating systems is more complex than that of single-layer coatings. The challenges arise from the following aspects: First, it is necessary to clarify the interface between the top coat and the middle coat, between the middle coat and the primer, or between the primer and the substrate. Secondly, there are differences in the physical properties of different coatings (e.g., hardness, modulus of elasticity, thickness), and uniform cutting parameters may not ideally penetrate all coatings at the same time, or cause irregular kerf edges, affecting judgment. Finally, the chemical compatibility between coatings can also affect the adhesion between layers, and the test needs to be able to sensitively reflect changes in this interface condition.
Instrument Application Points
To address these challenges, the following technical points should be taken into account when evaluating multi-coating systems using a grid adhesion tester:
1. Tool Selection and Calibration:The appropriate size of the cutting blade (e.g., six-edged or single-edge) should be selected based on the total thickness of the coating and the hardness of the hardest coating. The sharpness of the blade edge needs to be calibrated regularly to ensure that each cut penetrates the substrate clearly and straight, avoiding coating squeezing and peeling caused by blade passivation and leading to misjudgment.
2. Cutting Parameter Setting:The cutting spacing is selected according to the total coating thickness and test criteria. Generally, thicker coating systems will have a wider pitch (e.g., 2 mm or 3 mm). The cutting speed should be uniform and constant, and both manual or automatic instruments should ensure that the applied cutting pressure is consistent to guarantee consistent scratch depth.
3. Execution of the testing process:After the grid is finished, use a soft-bristled brush to gently sweep diagonally along the grid to remove loose coating debris. Subsequently, the standard-specified pressure-sensitive tape is applied, ensuring that it is in full contact with the grid area and free of air bubbles. Quickly tear off the tape at an angle close to 60°. The consistency of this series of operations is crucial for comparability of results.
4. Failure Mode Analysis:Observing the layer where the coating is peeled off is key to the analysis of the results. By looking at the sides of the shedding mesh under a microscope, it is possible to determine whether the failure occurs within the layer (coating cohesion failure) or at the interface (attachment failure). This is instructive for diagnosing the design or construction problems of the coating system.
Interpretation and significance of results
The interpretation of the test results is carried out strictly according to the grading table of the selected standard. Typically, grade 0 or 5B represents the best adhesion (completely smooth edge of the cut without a single grid falling off), while grade 5 or 0B represents the worst adhesion. For multi-coating systems, the results should be recorded with both the adhesion level and the observed failure interface location.
| Class (ISO 2409) | Description (Appearance of the cutting area) |
| 0 | The cut edges are completely smooth and the mesh does not fall off |
| 1 | A few flakes fall off at the intersection of the incisions, and the affected area is obviously no more than 5% |
| 2 | Coating peeling at the edge and/or intersection of the incision with more than 5% but no more than 15% of the affected area |
| 3 | The coating partially or completely peels off in large fragments along the edge of the incision, and/or partially or completely peels off in different parts of the lattice, with an area greater than 15% but not more than 35% affected |
| 4 | Large pieces of coating fall off along the edge of the cut, and/or some lattice partially or completely fall off, with an area greater than 35% but not more than 65% affected |
| 5 | Any degree of shedding exceeds level 4 |
The significance of this test is not only to determine the quality qualification, but also to provide data support for the research and development of coating systems and process optimization. For example, by comparing the test results of different pretreatment processes, different interlayer drying times, or different coating formulations, effective ways to improve interlayer adhesion can be found.
Technical advantages and limitations
The application of grid adhesion tester in multi-coating system has obvious advantages: relatively simple and fast operation, low test cost; The results are intuitive and visual, which is convenient for quick judgment on site. The method is highly standardized and the data is highly comparable. However, the method also has certain limitations: it is inherently qualitative or semi-quantitative; For extremely hard or tough coatings, it may be difficult to obtain clear cuts; The test results may be affected by the subjective factors of the operator. Therefore, it is often used in combination with other quantitative testing methods, such as the pull-off method, to obtain a more comprehensive evaluation of adhesion performance.
Conclusion
The grid adhesion tester is an effective tool for evaluating the interlayer and overall adhesion of multi-coating systems. Through standardized instrument operation, refined parameter setting, and in-depth analysis of failure modes, the method can reliably reflect the interface bonding state of the coating system, providing a key technical basis for product quality control, process improvement, and new material research and development. In practical applications, the value of this test method can be maximized by combining specific product standards and requirements, as well as fully understanding its technical characteristics.
References
ISO 2409:2020, Paints and varnishes — Cross-cut test.
ASTM D3359-22, Standard Test Methods for Rating Adhesion by Tape Test.
GB/T 9286-2021, Colored paints and varnishes - Grid test.
W. Brockmann, et al. Adhesion in Multi-layer Systems. Progress in Organic Coatings.
P. A. Sorensen. Evaluation of Coating Adhesion on Substrates.
