Overview
Paint film flexibility is a key performance index to evaluate the ability of coatings to resist cracking or peeling when the substrate is deformed, which directly affects the durability and applicability of coatings. As a classic method for determining the flexibility of paint films, the shaft rod bending method provides a standardized and quantifiable evaluation method for laboratories by simulating the condition of coatings under bending stress. This method is widely used in the performance testing of various coatings, inks and related overlays, and its test results are of great significance for product development, quality control and process optimization.
Test Principle:
The core principle of shaft rod bending method is based on the bending deformation theory in material mechanics. During the test, the test plate coated with paint film is bent on a shaft rod of a specific diameter, so that the paint film is deformed with the substrate. The paint film is subjected to tensile and compressive stresses during the bending process, and the strain size generated inside it is directly related to the diameter of the shaft rod and the thickness of the paint film. When the paint film cannot withstand this strain, cracking or peeling will occur, and its flexibility limit will be judged.
The maximum strain ε on the surface of the paint film during bending can be approximated by the following formula:
ε = t / (2R + t) × 100%
where t represents the thickness of the paint film and R is the radius of the shaft rod. This formula shows that under the same paint film thickness, the smaller the diameter of the shaft rod, the greater the strain generated during bending, and the higher the requirements for the flexibility of the paint film.
Shaft rod specifications
The paint film flexibility tester is usually composed of a set of shaft rods of different diameters, a bending operating device and a fixed fixture. The shaft rods are arranged in a standard diameter sequence to form a progressively increasing bending strength sequence in order to accurately determine the critical diameter of the paint film to pass through or not pass.
| Shaft rod diameter sequence (common specifications) | Corresponding flexural strength |
| 2 mm | High |
| 4 mm | Middle and high |
| 6 mm | Medium |
| 8 mm | Medium low |
| 10 mm | low |
During the test, it usually starts from the large diameter shaft rod, and gradually changes to the small diameter shaft rod for bending until the paint film is damaged, and the minimum shaft rod diameter that does not cause the damage of the paint film is used as the basis for flexibility evaluation.
Testing process
The standard test process includes four main steps: test plate preparation, environmental conditioning, bending operation, and result observation. The test plate usually uses standard-sized tinplate or thin steel plate, which needs to be dried and adjusted under specified conditions after coating. During the test, the paint surface of the test plate is turned up or down (according to the standard), and the shaft rod is bent 180° at a uniform speed within 2-3 seconds, and then the paint film is immediately observed with the naked eye or magnifying glass whether the paint film cracks, peels off, etc.
Attention should be paid to distinguishing between paint film damage and slight texture change. Usually, the minimum shaft rod diameter of the paint film without destruction is recorded based on the consistent test results of at least two test plates, and the flexibility of the paint film is expressed by this diameter value or corresponding grade. If all shaft rods do not cause damage, the flexibility may be reported to be better than the minimum diameter specification.
Influencing factors
The test results are influenced by various factors, including the uniformity of paint film thickness, drying and curing degree, ambient temperature and humidity, and substrate properties. Too thick a paint film often reduces its flexibility, so it is necessary to confirm that the thickness of the paint film meets the relevant standard range before testing. In addition, the speed and uniformity of the bending operation must be tightly controlled to avoid additional stress interference.
To ensure the comparability and accuracy of test results, it is recommended to operate in a standard laboratory environment (typically 23±2°C and 50±5% relative humidity) and regularly calibrate the diameter of the shaft rod to prevent diameter changes due to wear and tear from affecting the test accuracy.
Epilogue
As a paint film flexibility test method with clear principle and easy operation, the shaft rod bending method occupies a solid position in the quality evaluation system of coatings and related industries. By standardizing equipment and processes, this method can effectively simulate the bending stress that paint films may encounter in practical applications, providing a reliable basis for product performance evaluation. An in-depth understanding of its testing principles and influencing factors can help experimenters interpret the test data more accurately, thereby guiding material improvement and application optimization.
References
National standard: paint film flexibility measurement method
International Standard: Paints and varnishes — Bend test (cylindrical mandrel)
Industry Technical Handbook: Coating Performance Test Methods and Instruments