Overview
Coating flexibility testers are specialized devices used to evaluate the ability of coatings to resist cracking or peeling when the substrate is deformed. Its core principle is to quantitatively evaluate the bending adaptability of the coating by bending the coating specimen around the shaft rod of different diameters to observe whether cracks or peeling appear on the coating surface. This index is directly related to the durability and reliability of coated products in practical applications, especially in industries such as machining, automobile manufacturing, furniture and metal components.
Detection principle
The determination process is based on the co-deformation ability of the coating and the substrate under bending stress. The prepared coating specimen is placed on the instrument shaft bar, bent at a uniform speed to a specified angle under specified conditions, and then inspected the coating surface by visual or magnification device. Common test methods include tapered shaft bending, cylindrical shaft bending and T-shaped bending, etc., and the corresponding methods are selected according to different standards. Flexural adaptability is typically expressed as the minimum shaft bar diameter where the coating does not crack, with smaller diameters indicating better coating flexibility.
The stress-strain relationship of the coating during the bending process can be roughly described by the following formula:
σ = E × ε
Among them σ is the stress subjected to by the coating, E is the elastic modulus of the coating, and ε is the strain caused by bending. Coating cracking often occurs when stress exceeds its breaking strength.
Standard specifications
During the testing process, a number of parameters need to be strictly controlled, including shaft rod diameter series, bending speed, ambient temperature and humidity, and specimen thickness. Major domestic and foreign standard systems such as GB/T, ISO, ASTM, etc. have detailed regulations on test conditions to ensure the comparability and repeatability of results. For example, room temperature testing is usually required to be carried out under the conditions of temperature 23±2°C and relative humidity of 50±5%, and the bending speed is generally controlled within 1-2 seconds.
| Shaft rod diameter is commonly used series | 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm |
| Common bending angles | 90 degrees, 180 degrees |
| Typical specimen size | Length ≥ 120mm, width ≤25mm |
| Coating drying requirements | Fully cured according to product standards |
Notes:
Before operation, it is necessary to confirm that the instrument is placed horizontally and the surface of the shaft rod is clean and undamaged. The specimen should be flat and defect-free, with uniform coating thickness and within the standard range. During the test, the specimen coating surface is placed up or down (according to the standard) against the shaft rod to bend the specimen smoothly and evenly. The examination should be carried out in sufficient light and use a magnifying glass if necessary. The same specimen should be tested multiple times to confirm consistency of results.
It should be noted that ambient temperature and humidity have a significant impact on the physical state of the coating, especially for temperature-sensitive coatings. Specimen preparation, curing conditions and test interval should strictly follow the corresponding standards to avoid errors due to operational variations.
Influencing factors
The result evaluation is usually expressed as the minimum shaft bar diameter when the coating is crack-free and non-peeling. If cracks appear after testing all shaft rods, it can be recorded as greater than the maximum test diameter. The main factors affecting the measurement results include coating composition, film formation mechanism, substrate properties, coating thickness and curing conditions. For example, coatings with higher plasticizer content tend to exhibit better flexibility; Coatings that are too thick are prone to cracking due to large internal stresses when bending.
| Coating components: | Resin type, plasticizer, filler ratio |
| Process conditions | Drying temperature, curing time, coating thickness |
| Substrate characteristics | Metal type, surface roughness, pretreatment method |
| Environmental factors | Test temperature, humidity, and aging state of the specimen |
With the advancement of material science, coating flexibility determination technology is developing in the direction of automation and high precision. The new instrument gradually integrates functions such as digital control of bending speed, high-resolution image acquisition, and automatic crack identification to improve testing efficiency and objectivity. At the same time, testing methods for special materials such as elastic coatings and multi-layer composite coatings are also being improved to meet diverse industrial needs. In the future, combined with computer simulation stress distribution analysis, it is expected to reveal the failure mechanism of coatings during bending.
References
GB/T 1731-2020 Determination method for the flexibility of paint films
ISO 1519:2011 Colours and varnishes – Bending tests (cylindrical shafts)
ASTM D522/D522M-2021 Standard Test Method for Mandrel Bending Test for Adhered Organic Coatings
Coating Technology (4th ed.), Chemical Industry Press