Coating Film thickness measurement, see this is enough

The thickness of the paint film refers to the uniformity of the paint and the surface of the object after film formation. In the coating inspection process, the paint film thickness is a very important phase system index. The measurement of certain physical properties of coatings and some test-use performance tests such as durability require the coating to be made into a sample and compared under a certain paint film; Failure to meet the specified requirements will have a great impact on the coating performance. Therefore, how to correctly measure the paint film thickness is an important part of the quality inspection, and it needs to be given due attention.

1 Test method for paint film thickness

1.1 Applicable scope and accuracy of national standards for paint film thickness determination

The current national standard equivalently adopts the international standard IS02808_1974 to formulate "GB/T13452.2-92 Determination of Film Thickness of Paints and Varnishes", which stipulates the measurement method of paint film thickness of paints and related materials, see Table 1.

1.2 Determination of dry film thickness

What is often encountered in practical work is the measurement of dry film thickness. At present, there are many methods and instruments, but each method has certain limitations, and only a few of them can be applied to all types of samples and environments. According to the working principle, it can basically be divided into two categories: magnetic method and mechanical method.

(1) Magnetic method

Depending on the substrate to be tested, it can be measured with magnetic and non-magnetic Thickness Gauges. The magnetic Thickness Gauge mainly uses the principle of electromagnetic field reluctance to measure the thickness of the coating on the steel substrate; the non-magnetic Thickness Gauge uses the principle of eddy current thickness measurement to measure the thickness of the coating on the non-magnetic substrate such as aluminum plate and copper plate. . It should be noted that some paint varieties contain iron red, aluminum powder, etc., which will have a certain impact on the test results. Magnetic method has become the main method of dry film thickness determination. There are magnetic and non-magnetic Thickness Gauges produced at home and abroad, and at the same time continuous improvement, a variety of types and grades of Thickness Gauges have been developed. For example, permanent magnets are used instead of electromagnetic fields to measure the thickness of the dry film on the construction site. Its structure is simple and convenient. Portable, but the accuracy is slightly poor, the power supply is changed to dry batteries or rechargeable batteries, so that the structure is compact, easy to carry, and can still maintain high accuracy. The current Thickness Gauge is mainly digital display type, which can directly read out the data, and is suitable for measuring the thickness of various shapes of surfaces. The models and specifications of commonly used magnetic Thickness Gauges and non-magnetic Thickness Gauges can be found in Table 2.

(2) Mechanical method

使用杠杆千分尺或千分表测定涂膜厚度的方法使用较久,优点是使用时不受底材性质的限制也不受漆膜中导电或导磁颜料的影响,仪器本身精度达±2μm。但只能对较小面积的样板进行测试,为了消除误差,需要多次测量,手续烦琐,不如磁性测厚仪简便。测定漆膜厚度的显微镜法,已被建议为漆膜厚度测定的仲裁方法。其测试原理如图1所示。该法是用一定角度的切割刀具将涂层作一V形缺口直至底材,然后用带有标尺的显微镜测定a`和b’的宽度。标尺的分度己通过校准系数换算成相应的微米数,因此可从显微镜中直接读出漆膜的实际厚度。

( a、 b) 。此法的最大优点是除能测定总漆膜厚度外, 还能测定多层漆系统的每层漆的漆膜厚度, 同时可以在任何底材上进行, 其不足之处是将使漆膜適受局部破坏。

1 . 3湿膜厚度的测定

湿膜的测量需要在漆膜制备后立即进行, 以免由于挥发溶剂的蒸发而使漆膜发生收缩 。 «GB/T13452.2-92»的方法6中规定使用轮规和梳规测定的方法。在美国 ASTM D1212-79中规定用轮规和用 Pfund湿膜计测定的方法。

①轮规  基本上是由3个因盘组成的-一个整体, 外侧两个圆盘同样大小, 中间圆盘是偏心的, 且半径较短,以使3个圆盘的半径方向上, 间隔即为最大 。在圆盘外侧有刻度, 以指示不同问隙的读数。测试时(见图2)须注意仪器需要垂直于被测表面,不能左右晃动,否则将得出不正确的结果;另外仪器在表面上滚动,若是由零开始,则由于湿膜的被挤压而把漆推向前,得出的厚度读数将大于实际湿膜厚度, 使结果产生一 定的误差。

②梳规  一种可随身携带的金属或塑料片,形状为正方形或矩形, 如图3所示。在其4边都切有带不同读数, 每一边的两端都处在同一水平面上, 而中间各齿则距水平面有依次递升的不同问隙。使用时将其垂直接触于试验表面,这样将有一 部分il11被漆膜所沾湿。湿膜厚度为在沾湿的最后一 齿与下一个未沾湿齿之间的读数 。 梳规是一种价格低廉的简便测量仪器, 特别适用于在施工现场使用。

It should be pointed out that due to the surface tension of the coating film on the mirror surface, the measured wet film thickness is slightly different from the actual wet film thickness. The formula is established under the assumption that the two are completely equal. In order to make the results more reliable, a correction factor needs to be introduced, see Figure ASTMD1212-79 for details.

From the practical application point of view of the above three kinds of Film Thickness Gauges, the wheel gauge is more satisfactory. It can be used not only in the laboratory, but also in the field for measurement. It is easy to use and accurate in reading. Although the Pfund wet Film Thickness Gauge is more accurate, the operation and calculation are more cumbersome. Comb gauges are cheap and easy to carry, but have large errors, so they can only be used for rough measurement of wet film thickness on the construction site.

2 Applicable scope and selection method of paint film thickness measurement

2.1 Scope and method

At present, there are various methods and instruments for measuring the thickness of the paint film. The occasion (laboratory or field), the substrate (metal, wood, glass), the surface condition (flat, rough, curved surface) and the paint film should be considered when selecting the method. Film state (dry, wet) and other factors, so that the detection equipment can be used reasonably and the test accuracy can be improved.

2.2 Effect of paint film thickness on paint performance test

In coating performance testing, the thickness of the paint film is the key. Taking mechanical paint as an example, the thickness of the paint film after drying is within the range of (18±3) µm according to the standard. If the paint film thickness is greater than the specified standard, it is measured under constant temperature and humidity conditions. , There will be phenomena such as poor adhesion of the paint film, low hardness, and peeling off of the paint film after impact, which will affect other performance indicators. In addition to the direct impact of paint film thickness on the performance of the paint, we also need to check whether the paint is in a normal state when preparing the paint film, remove the surface crust if it is found, and dilute it to an appropriate viscosity according to the product regulations to make the coating Uniform paint film, avoiding dry orange peel wrinkling and pinholes.

3 Epilogue

The quality of various properties of the paint is closely related to the thickness of the paint film. In particular, the physical and mechanical properties of the paint film are most obviously affected by the thickness. Therefore, when measuring the performance of the coating film, it is necessary to test within the specified thickness range, and it is also confirmed that the thickness is a must-test item. It is hoped that there will be more complete new standards in the future, so that the production and use of coatings can reach a better height and bring greater convenience to people's lives.