When the cross-hatch method is used to measure adhesion, the coating film with a thickness of up to 250μm can be measured. According to the thickness of the coating, you can choose different cross-cut spacing, generally the coating is less than 60μm, the spacing between hard substrates is 1mm, and the spacing between soft substrates is 2mm; the coating thickness is 60-120μm, the spacing between soft and hard substrates Both are 2mm; the coating thickness is greater than 120μm, and the distance between soft and hard substrates is 3mm. It is stipulated in ISO12944 that the adhesion needs to reach level 1 to be considered qualified; in GB, when the adhesion reaches 1-2, it is considered qualified.
The difference between the circle method and the cross grid is that the area of the part formed by the intersection of the circle is increasing, and the rating examines the location of the undamaged area, while the area of each grid is fixed, and the rating is adopted Damaged area ratio.
(3) Measure adhesion by pulling apart method
The adhesion force measured by the pull-off method refers to the application of a vertical and uniform tensile force on the cemented surface of the sample at a specified rate to determine the force required for the adhesion failure between the coating or the coating and the substrate. Mpa said. This method can not only test the degree of adhesion between the coating and the substrate, but also detect the interlayer adhesion between the coatings; investigate whether the matching of the coating is reasonable, and comprehensively evaluate the overall adhesion effect of the coating. The relevant standards for the pull method test are ISO4624-2004 (the latest version of the standard), ASTMD-4514, GB5210, etc.
The instrument commonly used abroad to measure the pull method is the Elcometer adhesion tester. This instrument is small and can be used for field testing. But sometimes, like Elcometer-106 manual pull tester, because the manual operation is unstable and affect the accuracy of test results, it is no longer used in some industries in some countries. The Elcometer test is to stick an aluminum test slider on the coating. On the layer, a mechanical tensile testing machine with scale is used to pull the pull head off, and the pulling force of the aluminum head is read from the scale of the scale. Generally, three types of failures may be found in the pull test on the metal substrate:
(A) Adhesive failure, that is, the adhesive layer is pulled off from the coating or the test slider or the inside of itself after tensile force, which is considered to be the failure of the adhesive. The adhesion between the coating and the substrate or the adhesion between the coating and the coating all returns to a certain value.
(B) Adhesion failure, that is, the coating and the substrate are separated under tension, this value is the adhesion between the coating and the substrate.
(C) Cohesion failure, that is, the coating itself is broken. This value is used as the value of the adhesion between the layers, and the adhesion between the coating and the substrate exceeds this value. For each kind of coating, there is a prescribed value for the measurement by the pull-open method, which is generally required to be greater than 2Mpa, and epoxy two-component coatings are greater than 4MPA.
It is worth noting that there is a certain gap between the adhesion data measured by the Elcometer tester and the value measured by the tensile testing machine specified by the national standard. After many experiments, the data of Elcometer tester multiplied by 3-3.5 times is close to the value measured by the tensile machine. Therefore, the test data of each test method can only be compared with the same kind and has a certain degree of accuracy. When filling in the test report, the test instrument and method used should also be indicated.
For the adhesion requirements, the adhesion requirements of the coating system (dry film thickness greater than 250μm) in ISO12944-6 are in accordance with the ISO4624 pull method adhesion test, at least 5Mpa. The reference value for the old coating is 2Mpa, if it is lower than 2 Mpa, remove the old coating