The flexibility of the paint film is one of the key mechanical indicators to evaluate the quality of the coating, which comprehensively reflects the elasticity, plasticity and adhesion of the paint film when it follows the deformation of the substrate. The measurement of this performance has direct implications for predicting the durability, anti-cracking ability and protection effect of the coating in practical applications. Based on relevant standards at home and abroad, this paper systematically sorts out the main measurement methods, technical principles and operation points of paint film flexibility.
Definition and influencing factors of flexibility
According to the term definition of GB/T 1731-2020 "Determination of the Flexibility of Paint Film and Putty Film", flexibility refers to "the ability of the paint film or putty film to deform with its substrate without breaking". This capability is not determined by a single factor, but is closely related to the composition of the coating, the internal structure of the paint film, the density of cross-linking, and the environmental conditions at the time of testing. The results show that ambient humidity has a significant effect on the flexibility test results of elastomeric coatings, and the measured elasticity value is usually higher when the humidity is higher, while the strength decreases. In addition, the properties of the substrate, the thickness of the paint film and the rate of deformation during the test all affect the final decision.
Main measurement method system
At present, the flexibility measurement methods commonly used in laboratories are mainly divided into three categories: shaft rod measurement method, cylindrical shaft bending test method and tapered shaft bending test method. The principle of these three methods is similar, and the flexibility is characterized by the minimum shaft diameter (or maximum degree of bending) of the paint film without cracking or peeling by bending the painted test plate around different diameters, but its scope of application and technical details are different.
Shaft rod determination method (GB/T 1731-2020)
The shaft rod determination method is the most widely used method in China. The core equipment of the method is a flexibility tester, consisting of 7 steel shaft rods of different diameters fixed to the base. According to the provisions of GB/T 1731-2020, the curvature diameters of the shaft rod are 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 10 mm and 15 mm, and the tolerance value is not more than 0.1 mm, and the length of the shaft rod is 35 mm. During the test, the painted tinplate of the specified size (usually 120 mm × 25 mm × 0.2 mm~0.3 mm) is bent on the shaft rod, and the bending process should be completed at a uniform speed within 2 seconds to 3 seconds. The area within 2 mm of the edge of the test plate should be excluded during observation, and a 4x magnifying glass can be used to assist in the judgment. The test results are expressed as the minimum shaft rod diameter (mm) that does not cause damage to the paint film, and the smaller the diameter, the better the flexibility of the paint film.
GB/T 1731-2020 also adds a new method for determining the flexibility of putty films, using semicircular shaft rods (diameter 50 mm and 100 mm), and specifying the requirements for excluding defects within 4 mm of edges. This method integrates and replaces the original GB/T 1731-1993 "Paint Film Flexibility Determination Method" and GB/T 1748-1979 "Putty Film Flexibility Determination Method", which will be implemented from October 1, 2021.
Cylindrical shaft bending test method (GB/T 6742)
The corresponding standard of the cylindrical shaft bending test method is GB/T 6742 "Colored paint and varnish bending test (cylindrical shaft)". This method is suitable for test plates with a thickness of less than 0.3 mm, and the available cylindrical shaft diameters are wider, including 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, and 32 mm. During the test, insert the test plate into the instrument so that the coating surface is facing outward, and then bend the test plate 180° in the instrument to observe whether the paint film cracks or peels off. The advantage of this method is that the whole plate test can be carried out, and there is no need to directly contact the coating during operation, thus eliminating the possible influence of human temperature on the test plate and test results.
Conical shaft bending test method (GB/T 11185)
The tapered shaft bending test method is carried out in accordance with GB/T 11185 "Colored paint and varnish bending test (tapered shaft)". The core equipment is a tapered deflection tester, with a tapered central axis, a length of (203±3) mm, and a continuous change in diameter from about 3.2 mm at the thin end to 38 mm at the thick end. During the test, a test sample with a size of 75 mm × 150 mm is inserted into the instrument so that the test plate is bent against the surface of the tapered shaft. Under sufficient light conditions, the coating is observed for cracking or peeling using the naked eye or a 10x magnifying glass, and the distance from the thin end of the shaft to the last visible crack is measured in millimeters (mm). This method allows for more precise pinning of coating flexibility, providing continuous test results rather than discrete shaft diameter grades.
Comparison of different assay methods
In order to facilitate the selection of the appropriate method according to the sample type and test requirements in actual work, the characteristics of the three main determination methods are summarized in the table below.
| Assay method | Method characteristics and technical points |
|---|---|
| Shaft rod determination method （GB/T 1731） | Seven fixed diameter shaft rods (1-15 mm) are used, and the results are expressed in the minimum pass shaft diameter. It is suitable for paint film and putty film, and the edge area of the test plate needs to be excluded for observation. |
| Cylindrical shaft bending method （GB/T 6742） | Wide range of shaft diameters (2-32 mm) and test plate bending 180°. The coating faces outward and is suitable for thin plates to avoid the influence of the operator's body temperature. |
| Conical shaft bending method （GB/T 11185） | The shaft diameter varies continuously (3.2-38 mm) and measures the distance from the thin end of the cracking position. It can accurately locate the critical point of flexibility and provide quantitative data. |
Test key steps and standardization requirements
To ensure the accuracy and comparability of test results, standardized operating procedures must be strictly followed.
Test plate preparation:The substrate is usually made of tinplate that meets the requirements of GB/T 9271, with a size of 120 mm × 25 mm × (0.20 mm~0.30 mm). The substrate should be treated and painted according to the prescribed method. The painted test plate should be dried and conditioned under standard environmental conditions (temperature 23°C ± 2°C, relative humidity 50% ± 5%) for no less than 16 hours. The thickness of the paint film should be determined according to the method specified in GB/T 13452.2.
Test Operations:The test environment should be maintained under standard condition conditioning. For the shaft rod measurement method, press the test plate paint film upwards on the shaft rod, and use the thumb to bend it evenly to 180° within 2 to 3 seconds, and the bending process should avoid sudden force. For cylindrical shaft and tapered shaft instruments, the handle should be shaken at a uniform speed according to the equipment operating procedures to complete the bending.
Result verdict:After bending, immediately observe whether the paint film cracks or peels off under suitable lighting conditions (refer to GB/T 37356). Areas within a certain distance of the edge of the plate (e.g., 2 mm or 4 mm) should be excluded when observing, as the deformation of these areas may be affected by edge effects. Each batch of specimens should be repeated no less than 3 times to ensure the reliability of the results.
Factors affecting the test:In addition to the test method itself, the film making process and the curing environment can significantly affect the flexibility test results. It is pointed out that the multi-channel film making method can effectively reduce the pores in the paint film, and the existence of pores will reduce the flexibility test value of the elastomeric coating. In addition, the curing humidity has a greater impact on the elasticity data, and the higher the humidity, the higher the elasticity measured, but it is accompanied by a decrease in strength. Therefore, when comparing test results from different samples or batches, it is essential to ensure the consistency of film preparation methods and curing conditions.
Epilogue
The determination of paint film flexibility is an important part of evaluating the mechanical properties of coatings. GB/T 1731-2020, GB/T 6742 and GB/T 11185 together constitute a complete system of flexibility test methods, which apply bending stress to the coating through different forms of shaft rods. Understanding the technical principles, scope of application, and key control points in operation of each method is a prerequisite for reliable test results. In the actual testing work, the most appropriate measurement method should be selected according to the type of coating, the intended use and the requirements of relevant product specifications.
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