Paint film flexibility bending test, which of these 5 instruments is more suitable?

When the paint film is bent by an external force, the comprehensive performance of elasticity, plasticity and adhesion is called flexibility. The flexibility of the coating film is determined by the composition of the coating, and is related to the time and speed of coating deformation during testing. Impact resistance and post-formability are also a reflection of flexibility. The method of measuring flexibility is: through bending the coating film and the substrate at the same time, check the rupture and elongation of the paint film, which also includes the interface between the coating film and the substrate.

The bending test of the paint film is a test of the stretching and stretching of the paint film to the substrate adhesion . Before the test, the sample should be placed at the specified temperature for a period time to allow it to reach an equilibrium state (temperature and relaxation) and bend at a specified speed. ASTM D 1737 stipulates that it should be placed under constant temperature and humidity for 1 hour, and bent at a speed of 1 second, otherwise, the reproducibility of the test data will be poor.

Thick paint films tend to crack more easily than thin paint films. The edge of the curved part of the sample should be observed with a magnifying glass , and should be observed again the next day, sometimes cracking is later.

At present, there are mainly the following five instruments for the measurement of coating flexibility.

Figure 1 Flexibility Tester  1~6—axle, 7—base

①Spindle Tester

The national standard GB 1731-79 " Determination of Paint Film Flexibility" stipulates the use of a shaft rod Tester (see Figure 1). It consists of 6 steel shaft rods with different thicknesses, fixed on the base 7, and the base can fixed on the edge of the test bench with screws. The length of each shaft rod is 35mm, and the radius of curvature is 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 5mm and 7.5mm, respectively. During the test, the painted tinplate plate is bent on shafts of different diameters, and the minimum shaft diameter (mm) that does not cause paint film damage after bending is expressed.

When the paint film is bent on mandrels of different diameters, the relationship between the diameter of the mandrel and the relative elongation of the paint film is shown in Table 1.

Table 1 The relationship between the diameter of the mandrel and the relative elongation of the paint film

The above elongation is calculated under the condition . The calculation formula is as follows:

In the formula, e is the elongation of the inner surface of the paint film, %; e is the elongation of the outer surface of the paint film, %; h is the thickness of the paint film, mm; h2 is the thickness of the bottom plate, mm; r is the radius of the shaft rod, mm.

When other conditions are the same, if the thickness of the paint film (or the thickness of the bottom plate) is increased, the relative elongation of the paint film will also increase accordingly.

② Cylindrical shaft bending Tester

The national standard GB 6742-86 "Paint Film Bending Test (Cylindrical Shaft)" stipulates the use of cylindrical shaft bending test (as shown in Figure 2 ). It is suitable for test panels with a thickness of less than 0.3mm, and the diameters of the shafts are 2mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, 16mm , 20mm, 25mm and 32mm. When testing, insert the test panel with the painted side facing outward, close the instrument smoothly, bend the test panel 180° on the axis, and then observe whether the paint film is cracked or peeled off. The advantage of this method is that the whole panel can be tested, and the palm does not touch the paint film, which eliminates the influence of the human body on the temperature rise of the test panel.

Fig. 2 Cylindrical Shaft Bending Tester  1—shaft, 2—stop bar equivalent to the height of the shaft

③Conical deflection Tester

The national standard GB 11185-89 "Paint Film Bending Test (Conical Shaft)" stipulates the use of a conical bending Tester. Its central axis is a cone with a length of 203mm and a diameter extending from a maximum of 38mm to a minimum of 3.2mm. After inserting and fixing the test sample, turn the upper handle to make the test plate deflect close to the surface of the cone, and observe the minimum diameter (mm) that causes paint film damage, which represents the flexibility of the paint film. The characteristic of this instrument is also that the whole plate test can be used, and the discontinuity of the results with a set of conventional shaft rods is avoided. When the thickness of the paint film is known, the percentage elongation of the paint film can also be obtained.

In addition, there is another standard method for the determination of putty flexibility, which is determined by a flexibility Tester. For specific methods, refer to GB 1748-79 (88) "Determination of Putty Film Flexibility".

④ Cupping testing machine

The cupping test (also called top cup test or dent test) uses a spherical punch at the head of the instrument, which is pushed to the back of the painted test panel at a constant speed to observe whether the front paint film is cracked or peeled off from the substrate. peel off. When the paint film is damaged, the minimum depth of the punch is the cupping index (also known as the Erichsen (Erichsen) number), expressed in mm, which is different from the performance of the impact resistance. The main structure of the cupping testing machine is shown in Figure 3.

Initially, the cupping test was mainly used to determine the strength and deformation properties of sheet metal. If there is a crack in stamping, its indentation depth is the strength of the metal sheet. Testing a paint film on a metal substrate is actually measuring its strength, elasticity and adhesion to the metal while the substrate is elongated . This is a required test item for those coatings that need to be post-formed in the coil coating industry and the can industry , such as coil coatings, can coatings, etc.

Figure 3 Cupping testing machine  1—film punching; 2 — test plate clamp; 3—punch; 4—test plate

According to the provisions of GB 9753-88 "Cupping Test of Paints and Varnishes", when testing the painted sample, the diameter and the test panel should be flat, without deformation, with a thickness of not less than 0.3mm and no Polished steel plate larger than 1.25mm. In the actual measurement, if a tinplate plate with a thickness of less than 0.3mm is used, when the stamping depth reaches 8mm, although the paint film is not damaged or falls off, the tinplate plate of the substrate has been cracked. The cupping test is performed under prescribed standard conditions.

⑤T-shaped bending test

T-bending is widely used in coil coatings. Fold the sample in half with the paint film facing the outside. If there is no crack at the fold , it is 0T, which means that there is no metal plate on the inside of the folded metal. If the paint film is cracked, add another metal plate and then bend it. This time it is 1T without cracking, so do 2T, 3T, etc. The test is affected by temperature and turning speed, which should be strictly controlled and observed after a period of time.