Definition
The paint film circle tester is a special inspection equipment used to evaluate the adhesion performance of coatings. The instrument uses a specific geometry of a scribing needle to apply uniform pressure to the surface of the coating and makes a continuous scribing motion, and evaluates the adhesion level of the coating according to the shape of the scratch. This test method is widely used in the fields of coatings, coatings and related materials, and is one of the key means to evaluate the firmness of the bond between coatings and substrates.
Principle
The core principle of the instrument is based on the mechanical behavior of mechanical scratching and coating peeling. During the test, the scribing needle under a fixed load draws a circular trajectory that overlaps sequentially on the surface of the coating at a constant rate. As the scribing radius decreases, the shear stress applied by the scribing needle to the coating continues. When this stress exceeds the bond strength between the coating and the substrate, the coating will peel locally. The coating adhesion grade can be determined by observing the damage of the coating in the scratch trajectory and comparing it with the standard map. The mechanical relationship can be approximately expressed as: τ = F/A, where τ is the interfacial shear stress, F is the vertical load applied by the scribing needle, and A is the effective contact area between the scribe and the coating.
Measurement method
The measurement process is subject to the relevant standard procedures. First, the well-prepared coating sample is fixed flat on the instrument table. Select the scribe with the appropriate load according to the standard and install it. After starting the instrument, the scribing needle draws a series of concentric circle trajectories with decreasing diameter from the outside to the inside driven by the motor. The total number of scratches is usually 6 to 11 turns. Once the test is complete, remove the sample and use a soft-bristled brush to gently remove loose coating debris from the scratched area. Under the specified lighting conditions, the scratched area is observed through a magnifying glass, and the coating peeling morphology is compared with the grading map given in the standard to determine the adhesion grade. Grades are usually divided into grades 1 to 7, with grade 1 indicating the best attachment and grade 7 indicating the worst.
Influencing factors
Test results are influenced by a variety of factors. The coating itself includes resin type, pigment volume concentration, curing degree, and cohesion. Substrate factors involve surface roughness, cleanliness, and chemical properties. Instrument parameters such as the radius of curvature of the scribing needle tip, the accuracy of the applied load, and the speed stability of the scribing motion all have a direct impact on the uniformity of the stress application. Environmental conditions such as temperature and humidity can alter the physical state of the coating, which in turn affects its peeling behavior. Whether the preparation and maintenance of the sample meet the standards is also a prerequisite for ensuring the comparability of results.
Application:
The application of this tester covers a wide range of industrial fields. In automobile manufacturing, it is used to evaluate the adhesion of body electrophoretic paints, topcoats and varnishes. In marine and marine engineering, the bonding performance of anti-corrosion coatings on steel plates is tested. In the field of home appliances and consumer electronics, the durability of shell spray coatings is checked. In the building materials industry, it is used to test the adhesion of architectural coatings to substrates such as concrete and wood. In addition, it is also a routine test item in coating research and development, quality control and inbound and outbound inspection, providing a quantitative basis for product performance.
Selection
Choosing the right instrument requires a combination of aspects. First of all, it is necessary to clarify the standard system followed by the test, such as national standards, international standards or industry standards, which have specific requirements for the load range, scribbling specifications and motion trajectory of the instrument. Secondly, according to the type of sample and the expected accuracy of the daily inspection sample, the load resolution and motion control accuracy of the instrument are considered. The structural durability, ease of operation, and safety design of the instrument are also important considerations. In addition, whether the supplier can provide complete standard accessories, calibration services and technical support is of practical significance to ensure the long-term reliability and reproducibility of the test.