Film adhesion detection method and its applicability analysis

I. Introduction

Paint film adhesion refers to the bond strength between the coating material and the substrate, which reflects the adhesion ability of the coating to the surface of the substrate under physical, chemical or mechanical action. This property plays a decisive role in the protective properties and service life of the coating. The stronger the adhesion, the more effective the coating can resist the damage of corrosive factors such as moisture, chemicals, and ultraviolet rays in the external environment, thereby delaying the aging or corrosion of the substrate. If the adhesion of the coating is insufficient, no matter how intact the coating surface is, it will peel off, crack or blister, and the protection will be greatly reduced or even completely ineffective. Therefore, evaluating the adhesion of coatings is a key step to ensure the quality of the coating system, which helps to screen suitable coating materials, optimize the coating process, and provide a scientific basis for engineering construction and quality control, so as to achieve long-term protection of the substrate.

In recent years, paint film adhesion testing methods have been widely studied and applied, covering two categories: on-site rapid testing and laboratory accurate measurement. These methods evaluate the bond strength between the coating and the substrate by physical or mechanical means, and provide an important basis for coating performance evaluation. The following summarizes the characteristics and applicable scenarios of different detection methods as follows:

2. Classification of paint film adhesion testing methods

The test methods for paint film adhesion can be classified in detail according to the different test environments and test principles. Each method has its own unique advantages and application scenarios, and the most suitable detection method can be selected for different needs.

1. Classified according to the test environment

On-site testing:Portable and quick to evaluate

On-site inspection methods are usually convenient, fast, and do not require complex instruments, and are suitable for construction sites, maintenance inspections, and preliminary quality control. These methods are generally performed manually, and can quickly obtain qualitative or quantitative data on coating adhesion without the need for complex laboratory equipment, allowing operators to quickly judge coating quality on site.

Common Methods:

Draw X method:The adhesion of the coating is evaluated by making intersecting cuts on the surface of the coating with a utility knife and then using tape to pull it apart, usually on a scale of "5A" to "0A".

Griddling method:The coating is cut into a tic-tac-toe shape or lattice by scratching and peeled off with adhesive tape, and the adhesion grade is determined according to the peeling condition. It is usually graded on a scale of 0 to 5, with 0 being the best and 5 being the worst.

Pull Method:The adhesion strength of a coating is measured by drawing the coating, usually in the field using a simple drawing instrument.

Laboratory tests:High accuracy for system analysis

Laboratory testing methods typically require high test accuracy and provide detailed quantitative data, making them suitable for applications that require accurate data, such as coating R&D, quality control, and system analysis. Laboratory testing methods often rely on specialized instruments and are complex to provide more in-depth analysis of the failure mode of coating adhesion, adhesion values, and more.

Common Methods:

Pull Method:The strength of adhesion is determined by measuring the required force (MPa) by using a drawing device to pull the coating off the substrate, which is suitable for laboratory studies with high adhesion requirements.

Circle Method:A special instrument is used to carry out a scratch test on the surface of the paint film to evaluate the adhesion of the paint film in the scratch area, and the integrity of the circle directly reflects the adhesion level. System analysis for laboratories.

2. Classification according to test principle

(1) Scratch method(Mechanical Destructive Testing)

The scratch method uses a certain mechanical force to make the tool scratch on the surface of the paint film, and then evaluates the adhesion of the paint film by analyzing the damage of the coating. This method can be used to understand the quality of adhesion by directly observing the failure mode of the coating at different strengths.

Peculiarity:

Mechanical Destructiveness:The scratch method is a destructive test that causes some damage to the coating and is therefore mainly used to evaluate the behavior of the coating under force.

Easy to operate:This method can be done with a simple tool (such as a utility knife) and is quick and easy to operate.

Qualitative Assessment:Adhesion is usually assessed based on the depth of the scratches, the number of cracks, and the distribution of cracks, which is more intuitive, but lacks accurate quantitative data.

(2) Pulling method(Mechanical Testing)

Principle: The drawing method uses a tensile device (such as a tensile machine) to measure the force required for the coating to peel off by physically separating the adhesion between the coating and the substrate, so as to derive the strength of the adhesion (in MPa). The drawing method not only provides numerical data on the adhesion of the coating, but also observes the failure mode of the adhesion (e.g., cohesion failure or adhesion failure) during the testing process.

Peculiarity:

Principles of Mechanics:The pull-out method is based on the principle of mechanical testing, which directly measures the tensile strength between the coating and the substrate, and the results are usually more accurate.

Quantitative data:It can provide a value of coating adhesion accurate to the MPa level, which is suitable for research or quality control with high precision requirements.

Wide range of applications:It is suitable for a variety of coating systems, especially in situations with high strength requirements, such as industrial coatings, automotive coatings, etc.

3. Commonly used paint film adhesion testing methods

(1) X-cut tape test

Principle:Coating adhesion is evaluated by knife cutting and tape pulling.

Test Procedure:

Use the cutter to make a cross line up to the substrate.

Tear and pull after attaching the tape to observe the peeling off of the coating.

Evaluation Criteria:ASTM D3359 Method A 5A~0A rating system.

Scope of application:The dry film thickness > 125 μm.

Advantages and disadvantages:It is easy to operate, but it is highly subjective.

(2) Cross-cut tape test

Principle:Tic-tac-toe grids are cut on the surface of the coating, and the adhesion is evaluated after the tape is applied.

Test Procedure:

• Choose the appropriate cutting spacing according to the thickness of the paint film.

• After attaching the tape, tear and pull to record the damage of the paint film.

• Evaluation criteria: ISO 2409 or GB/T 9286 grade 0~5 rating.

Scope of application:The thickness of the dry film is 0~250μm.

Advantages and disadvantages:The test is reproducible and has a wide range of applications.

(3) Pull-off test

Principle:By applying a pull-out force, the strength of the damage to the adhesion of the coating is measured.

Test Procedure:

• Fix the test with adhesive to test the inverted (dolly).

• A tensile force is applied using a tensile force meter to record the strength of failure (MPa).

• Evaluation standard: ISO 4624 or GB 5210.

Scope of application:Wide range of substrates and coating thicknesses.

Advantages and disadvantages:Accuracy is high, but testing is time-consuming and destructive.

(4) Scratch test

Principle:Scratches on the surface of the paint film using a rotating needle to evaluate the adhesion of the coating.

Test Procedure:

• Fix the template and apply weights.

• Rotate the scratches evenly to assess the damage to the paint film.

Evaluation Criteria:The strength of adhesion is expressed by the number of grades, with grade 1 being the best and grade 7 being the worst.

Scope of application:Laboratory performance evaluation of primers and topcoats.

Advantages and disadvantages:Simple and intuitive, but requires high equipment maintenance.

Fourth, the precautions for paint film adhesion testing

Sample preparation and handling are critical when performing paint film adhesion testing. First of all, the cleanliness of the substrate surface directly affects the adhesion test results of the coating, any oil, dust or other impurities may cause a decrease in the adhesion between the coating and the substrate, so it is essential to thoroughly clean the substrate before testing. Special attention is also needed to ensure the integrity of the coating, as any cracks, bubbles, or uneven coating can affect the accuracy of the test results. Also, make sure that the coating is completely dry and cured before testing to avoid spurious results during testing.

Environmental factors, especially temperature and humidity, also have a significant impact on the test results of film adhesion. At temperatures that are too high or too low, the coating may not fully cure, resulting in inaccurate adhesion testing; When the humidity is too high, the moisture on the surface of the coating may not evaporate in time, which will affect the measurement of adhesion. Therefore, the test should be carried out under stable environmental conditions. The choice of equipment and tools is also crucial, and the accuracy of the tools used, such as knives, tapes, tensile meters, etc., must meet the requirements of the standard. Wear or inaccuracy of any tool can lead to deviations in test data, so regular calibration and maintenance of the equipment is required to ensure standardized operation.