Overview
The grid adhesion test is a physical test method widely used in the adhesion performance evaluation of color paints, varnishes and related coating systems. This method uses precise cross-cutting on the coating surface to form a grid pattern, followed by a peeling operation with a specific tape, and finally grades the adhesion failure mode of the coating according to a standard map. Its core value lies in evaluating the bond strength between coatings and substrates, or between layers of composite coatings, providing critical data support for product quality control, process optimization, and durability prediction.
Test Principle:
The basic principle of this method is to use a cutting tool with a specific edge spacing to cut parallel notches perpendicular to each other on the coating, penetrating into the substrate, thus forming a series of square grids with regular dimensions. This artificially creates stress concentration areas. The standard pressure-sensitive tape is then tightly applied to the grid area and quickly peeled off. The adhesion level of the coating can be quantitatively evaluated by observing the peeling of the coating in the mesh area and comparing it with the standard grading map. The higher the adhesion level, the stronger the coating's resistance to peeling.
The main instrument is the scribing adhesion knife (or scriber), and its key component is the cutting blade. The blade is usually made of carbide material to ensure that the edge is sharp and durable, capable of cutting the coating cleanly and avoiding edge fuzzing or crushing damage. According to relevant standards (e.g., ISO 2409, ASTM D3359), the blade spacing (i.e., the grid cell side length) is usually 1mm, 2mm, etc., depending on the total thickness of the coating and the severity of the test.
Test steps
A standardized test process is a prerequisite for reliable, repeatable results. The operation should be carried out in a standard temperature and humidity environment, and the coating should be fully cured.
Step 1,Sample preparation and inspection。 Ensure that the coating surface in the test area is flat, clean, and free of defects. For different substrates (e.g. metal, plastic, wood), it should be clearly stated in the report.
Step 2,Make cuts。 Hand scribing machine perpendicular to the coating surface to cut with uniform pressure and speed. Start by cutting a set of parallel lines in one direction, then rotate about 90° to cut another set of parallel lines to form a cross mesh. The entire coating must be penetrated to the substrate during cutting, which can be confirmed by looking at the bottom of the cut by looking at it under a microscope.
Step 3,Tape adhesion and peeling。 Use a soft-bristled brush or air flow to gently remove debris from cutting. Cover the center of a standard test tape (such as a 25mm wide pressure-sensitive tape) over the grid area and press firmly with your fingers or eraser to ensure that the tape is in full contact with the coating. After waiting for about 90 seconds, hold one end of the tape and quickly and smoothly tear off the tape at an angle of nearly 60° within 0.5 to 1.0 seconds.
Step 4,Result evaluation。 In sufficient light, use a magnifying glass to observe the coating peeling off in the mesh area. The observations were compared to the grading map provided in the standard to determine the adhesion level.
The results are evaluated and graded
At present, the standard grading system widely used internationally divides the results into grades from 0 to 5, with grade 0 representing the best adhesion and grade 5 representing the worst. The specific classification is described in the table below:
| grade | Description (Appearance of the cut area) |
| 0 | The cut edges are completely smooth and the grid does not fall off. |
| 1 | There is a slight peeling of the coating at the cutting intersection, and the affected area is significantly less than 5%. |
| 2 | The coating peels off along the cut edges and/or intersections, affecting 5% to 15% of the affected area. |
| 3 | The coating partially or completely peels off along the cutting edge, and part of the mesh falls off as a whole, affecting 15% to 35% of the affected area. |
| 4 | The coating peels off a large area along the cut edge, and some of the mesh falls off as a whole, affecting 35% to 65% of the affected area. |
| 5 | Peeling is more than grade 4, any shedding greater than 65%. |
The evaluation report should clearly document the test criteria, blade spacing, substrate type, test location, final grade, and any special failure modes (such as coating cohesion failure).
Influencing factors
Test results are affected by a variety of factors and need to be strictly controlled to ensure comparability.
Coating performance factors: The thickness, hardness, elastic modulus and cohesive strength of the coating directly affect the cutting behavior and peeling resistance. Coatings that are too thick or brittle can produce irregular chipping.
Substrate factors: The rigidity, surface roughness and cleanliness of the substrate are the fundamentals that determine the adhesion. Before testing, it is necessary to confirm that the coating is firmly attached.
Operational factors: The cutting pressure and speed, the cutting angle, the pressure and tearing angle and speed of tape pasting must be strictly implemented in accordance with the standards. Blade sharpness is key, and passivated blades can cause the coating to be squeezed rather than cut, affecting the accuracy of the results, so the blades need to be replaced regularly.
Environmental factors: Temperature and humidity may affect the physical state of the coating and the adhesive tape, so it is recommended to operate in a standard laboratory environment.
Scope of application
The grid method is mainly suitable for color paints and varnish coatings with a total thickness of no more than 250 microns, and for thicker coatings or coatings with high elasticity (such as some elastomers), the fork method (X-ray method) or other methods may be required. This method is a qualitative or semi-quantitative rapid screening tool that may require comprehensive analysis in combination with quantitative tests such as pull-out method to study the microscopic mechanism of adhesion failure. Its advantages are simple equipment, quick operation, and small sample damage area, making it ideal for routine quality control on production sites and laboratories.
Conclusion
The grid adhesion test is an effective and practical method to evaluate the adhesion performance of color paint coatings. Through standardized instruments, rigorous operating procedures and unified grading standards, the bonding between the coating and the substrate can be reflected relatively reliably. Understanding its principles, mastering the specification steps, and recognizing the influencing factors is of practical significance for the correct application of the method, interpretation of test data, and then guiding the research and development and process improvement of coating systems. In practical applications, it is recommended to combine product specifications, select appropriate test parameters, and establish internal operating specifications to ensure consistency in results.
References
ISO 2409:2020, Paints and varnishes — Cross-cut test.
ASTM D3359-17, Standard Test Methods for Rating Adhesion by Tape Test.
GB/T 9286-2021, Colored paints and varnishes - Grid test.