Overview
Coating adhesion is one of the key indicators to evaluate the performance of coating systems, which directly affects the durability and protection effect of coatings. In the field of industrial testing, the use of a hundred grid knife and special adhesion test tape for grid testing is a widely used qualitative or semi-quantitative evaluation method. This method intuitively reflects the bond strength between the coating and the substrate or between the coatings through standardized mechanical grid and tape peeling operations, and its operation is simple and the results are intuitive, and it is suitable for coating testing in a variety of non-medical fields, such as automotive coatings, architectural coatings, furniture surface treatment and general industrial anti-corrosion coatings.
Test Principle:
The basic principle of testing is to use a hundred grid cutter to cut a grid scratch at a specified distance on the surface of the coating, penetrating into the substrate. The adhesion test tape is then applied and peeled off quickly, and the adhesion level is evaluated by observing the peeling of the coating in the grid area. The process simulates the coating's behavior under shear stress, and the adhesive force of the tape provides a standardized peel force.
Relevant domestic and foreign technical standards have detailed regulations on this method, such as the international standard ISO 2409, ASTM D3359, and the Chinese national standard GB/T 9286. These standards set unified requirements for key parameters such as cutting spacing, number of cuts, tape type, peeling angle and speed, ensuring comparability and repeatability of test results. Test results are usually expressed as 0 to 5 grades (or corresponding classifications), with grade 0 indicating no shedding and grade 5 indicating severe shedding.
Test instruments
The core instrument is a Baige knife, and its blade spacing is usually 1mm, 2mm, or 3mm, depending on the coating thickness and test standards. The blade should remain sharp, ensuring that the cutting edge is neat and penetrates into the substrate. Adhesion test tapes must meet the standard adhesion range, e.g. a bond force of about (3.5±0.3) N/10mm, and have a uniform adhesive layer. The tape width is usually 25mm. Before testing, ensure that the coating is fully cured and conditioned under standard temperature and humidity conditions.
Operational points
First, clean the coated surface and select a leveled area. Use a hundred grid knife to cut vertically and horizontally with uniform pressure and speed to form a grid pattern. After cutting, gently remove debris with a soft brush. The adhesion test tape is then applied tightly to the grid area, ensuring no air bubbles and applying even pressure. Quickly peel off the tape at an angle close to 180° within 1 to 5 minutes. Finally, under sufficient light, a magnifying glass was used to observe the coating peeling off in the mesh area and compared with the standard map.
During operation, it should be noted that the cutting depth must reach the substrate; The tape should not be left for too long after being pasted; The peeling action should be quick and coherent; The same specimen can be tested multiple times to obtain representative results. Ambient temperature and humidity may affect coating performance and tape stickiness, and it is recommended to operate under standard conditions.
Evaluation of results and analysis of influencing factors
The rating is mainly based on the proportion and morphology of the area where the coating falls off in the grid area. The following table provides a brief description of common ratings:
| grade | Description |
| 0 | The cut edges are completely smooth and do not fall off |
| 1 | Shedding area less than 5% |
| 2 | The shedding area is between 5% and 15% |
| 3 | The shedding area is between 15% and 35% |
| 4 | The shedding area is between 35% and 65% |
| 5 | Shedding area greater than 65% |
Test results are influenced by a variety of factors. The coating's own factors include curing degree, thickness, internal stress, and compatibility with the substrate. Substrate factors involve surface roughness, cleanliness, and pretreatment quality. Operational factors include accuracy of cutting depth and spacing, standardization of tape application and peeling. Environmental factors such as temperature and humidity can also introduce variables. These factors need to be taken into account when analyzing the results.
Scope of application
This method is suitable for coatings with a thickness of less than 250 μm, and for thicker or more elastic coatings, the knife spacing may need to be adjusted or other methods may be used. It is mainly used to quickly compare the adhesion differences of different coating systems or processes, and is an effective means of quality control. However, this method is qualitative or semi-quantitative and cannot provide precise adhesion values. For high adhesion coatings, the degree of discrimination may be insufficient; For brittle coatings, edge peeling may be magnified. Therefore, when quantitative data is required or specific interface strengths are evaluated, quantitative tests such as the pull-apart method are supplemented.
Technological development
With the development of inspection technology, some automated or digital grid and image analysis equipment have begun to be used, which can improve the consistency and efficiency of ratings. However, in most scenarios, manual operation is still the most common method. Practical suggestions include: establishing internal operating specifications and regularly calibrating the blade of the Baige knife; Train operators to ensure consistent techniques; Record the environmental conditions at the time of the test; Combined with other test methods (such as bending test, impact test) for comprehensive performance evaluation. For critical applications, regular adhesion monitoring is recommended at all stages of development and production.
Epilogue
As a classic method for coating adhesion evaluation, Baige Knife plays an important role in quality control in many industrial fields due to its simple and intuitive characteristics. Correctly understanding its principles, strictly following standard operations, interpreting the results and recognizing its limitations are of great significance for accurately evaluating coating performance and optimizing the coating process. In practical application, the test method should be scientifically selected and standardized according to specific product requirements and industry specifications.
References
ISO 2409, Paints and varnishes — Cross-cut test.
ASTM D3359, Standard Test Methods for Rating Adhesion by Tape Test.
GB/T 9286, Colored paints and varnishes - Grid test.