Overview
The circular rolling method is a widely used test method for evaluating the adhesion of colored paint coatings, and its principle is based on the circle method adhesion tester. The method depicts continuous circular line scratches on the surface of the coating through the fixed load and uniform motion of the instrument, and the adhesion level is evaluated according to the shape of the scratch and the peeling condition of the coating. This method is relatively simple to operate, the results are intuitive, and it is suitable for the coating of color paint on the surface of a variety of substrates, and plays an important role in industrial quality control and R&D testing.
Test principle
The core components of the circle adhesion tester include a loading device, a needle scribing and a moving platform. During the test, the test plate coated with the sample is fixed on the moving platform, and the scribing needle is in contact with the coating surface under a fixed vertical load. As the platform moves at a constant speed, the scribing needle marks a circular rolling trajectory on the coating. As the scratch circumference increases, the shear force acting on the coating gradually changes, and the coating may peel off the substrate. By observing the degree of peeling of the coating in the scratched area, the adhesion classification can be performed according to the standard map or description.
The mechanical process of scratch formation can be simplified to a comparison between the shear stress τ applied to the coating and the adhesion at the coating-substrate interface. Peeling occurs when τ exceeds the interfacial adhesion. The radius of curvature and movement speed of the circular roll of the instrument design must comply with the standard specifications to ensure the consistency of the test conditions.
Trial steps
Before the test, the coating test plate should be prepared according to the relevant standards and adjusted under the specified conditions. The test plate is firmly fixed on the tester platform to ensure a flat surface. Depending on the estimated adhesion of the coating or standard requirements, select the appropriate scribe size (usually the radius of curvature of the tip is a specific value) and vertical load. Start the instrument so that the scribing needle is in contact with the coating under load, and the platform drives the test plate to move at a constant speed to complete the circular rolling line scratch. The scratch length should be sufficient to contain the area from the beginning to the area where the coating is visibly peeling. After the test, use a soft brush to gently remove loose paint chips from the scratched area and observe under suitable light.
Result evaluation
Ratings are usually based on comparison with standard rolling line maps or grading based on textual descriptions. The grading is based on the proportion and shape of the area of coating peeling in the scratched area. For example, a common grading system divides adhesion into multiple grades, with higher grades indicating stronger adhesion, i.e., a smaller proportion of peeling area. The continuity of the scratch and the peeling form of the coating (such as flakes, chips, etc.) should be paid attention to when evaluating, which will help to judge the adhesion performance more accurately.
Influencing factors
Coating thickness, substrate surface roughness, curing degree, ambient temperature and humidity, etc. may affect the test results. Before testing, ensure that these parameters meet the requirements of the test standard. The wear of the tip of the scribing needle can change the contact geometry and needs to be checked and replaced regularly. The load selection should match the hardness of the coating, overload may cause scratches on the substrate, and insufficient load may not effectively cause peeling. The lighting conditions should be uniform during observation to avoid shadows or reflections interfering with judgment. For multi-layer coating systems, this method evaluates the adhesion of the entire coating system to the substrate.
Scope of application
The circular rolling method is suitable for uniform application of dry coatings of colored paints, varnishes and related products on hard flat substrates such as metals, wood, plastics, etc. This method provides a relatively fast comparative evaluation of coating adhesion, especially suitable for production process control. However, the result is an ordinal level and not an exact mechanical parameter. Coatings with too strong or too weak adhesion may not be effectively distinguished or produce abnormal scratches within the standard load range. In addition, the method has high requirements for the flatness of the test plate and is not suitable for severely curved or uneven surfaces.
Association with other adhesion testing methods
As one of the adhesion testing methods, the circular rolling method is often supplemented with the pulling method and the grid method. The pull-open method provides quantitative data on adhesion strength, while the grid method is more suitable for evaluating interlayer adhesion on thinner coatings or multi-layer systems. The choice of method depends on the coating system, the purpose of the test and the relevant product standards. The advantage of the circular rolling method is that it has a continuous testing process and can show the response of adhesion to scratch conditions in a single scratch.
Safety and maintenance
When operating the instrument, avoid contact with moving parts of the body. The needle is sharp, so be careful when taking it. The instrument should be placed on a stable table, and the motion guide rail should be cleaned regularly and an appropriate amount of lubricant should be added to ensure smooth operation. When not in use for a long time, the load should be removed and the key components should be treated with anti-rust. Follow the maintenance guidelines provided by the manufacturer for routine maintenance.
Cited Literature
GB/T 1720-1979 Determination method for adhesion of paint film
ASTM D2197-1998 Standard Test Method for Determination of Adhesion of Organic Coatings by Circular Roller
ISO 2409:2020 Coloured paints and varnishes - Grid tests
Wang Jianguo. Coating adhesion test technology. Beijing: Chemical Industry Press, 2010.