Testing Principles and Methods of Film Friction Coefficient Tester

Test Principle:

The core test principle of the thin film friction coefficient meter is based on the classical law of friction. When two surfaces are in relative motion, the ratio of the tangential force that prevents their motion to the normal force acting perpendicular to the contact surface is defined as the coefficient of friction. For thin film materials, the static and dynamic friction coefficients of their surfaces are usually concerned. The static friction coefficient refers to the ratio of the maximum tangential force to the normal force required to overcome the initial resting state and make the two contact film surfaces begin to slide relatively. The dynamic friction coefficient refers to the ratio of the tangential force and normal force required to maintain the relative sliding of the two contact surfaces at a uniform speed.

The instrument measures the above force values through precision sensors. During testing, two film samples to be tested are superimposed under specific conditions (e.g., a specific surface, a specific pressure). The upper slider starts moving driven by the drive mechanism, and the lower platform remains stationary or in reverse motion. The force sensor connected to the slider records changes in tangential resistance during movement in real time. The normal force is pre-set by the loading device and remains constant. The coefficient of friction μ can be calculated using the following formula:

μ = F_f / F_n

Among them, F_frepresents the measured friction, F_nRepresents the normal pressure applied. The instrument software automatically calculates the static and dynamic friction coefficients by analyzing the eigenvalues on the force-time or force-displacement curves.

Test methodology

To ensure the reliability and comparability of test results, the testing process must follow standardized operating procedures and environmental controls. The main test steps and key points are as follows:

Sample preparation is performed first. Specimens that are sized to meet standard requirements from the film membrane to be tested are typically required to have a flat surface with no creases, stains, or visible defects. The specimen should be conditioned under standard temperature and humidity environment (such as 23±2°C, 50±10% RH) for no less than 4 hours.

The second is instrument calibration and parameter setting. The force sensor and displacement sensor of the instrument need to be calibrated before testing. Key test parameters need to be set according to relevant standards or research purposes, and the range of common parameters is shown in the table below:

During the formal test, one specimen is fixed flat on the horizontal platform and the other is fixed on the bottom of the slider to ensure a tight fit on the contact surface and no bubbles. Start the test, the slider moves at the set speed. The same group of specimens usually needs to be tested multiple times (e.g., more than 5 times) at different positions, and the average value is taken as the final result to eliminate the influence of local inhomogeneity of the material.

When processing data, it is necessary to distinguish between static and dynamic friction coefficients. The static friction peak is usually taken from the maximum force value at the moment of the start of sliding. The dynamic friction coefficient can be taken from the average force value of the uniform speed sliding stage, or the data of the stable section can be statistically calculated. The test report should record in detail the test conditions, sample information, measurements and averages.

Influencing factors

The test results of the coefficient of friction of the film are influenced by various factors, and understanding these factors is crucial for correctly interpreting the data.

The properties of the material itself are the fundamental factor. The roughness and texture of the film surface, the distribution and migration of additives (such as slip agents), and the crystallization state of the polymer can significantly alter the surface friction behavior. For example, the migration of slippers to the surface over time can cause the coefficient of friction to change with the time the film is stored.

The impact of test conditions is equally significant. The increase of normal load will change the true contact area and interface state of the contact surface, which may lead to changes in the friction coefficient. The sliding velocity will affect the frictional heat generation at the interface and the viscoelastic response of the polymer material, thus affecting the dynamic friction coefficient. Ambient temperature and humidity will change the physical properties of the material surface and the adsorption of water film, which will affect the test results.

Operational consistency also needs attention. The cutting direction of the specimen (relative to the film forming direction), the choice of contact surfaces (e.g., corona-treated and non-treated surfaces), the clamping flatness of the specimen, and the static contact time before testing are all variables that need to be tightly controlled.

Applications:

Film friction coefficient testing is valuable in various industrial fields such as packaging, printing, and composite materials. In the packaging industry, it is about the smooth operation of film coils on high-speed automatic filling lines, the opening of the pouches and the stability of the stack. In the printing and lamination process, the appropriate friction coefficient is the prerequisite for ensuring smooth material transportation and avoiding slippage or tensile deformation.

Tests usually refer to widely recognized technical standards at home and abroad, which specify in detail instrument requirements, sample preparation, test procedures and report formats, and provide a unified benchmark for data comparison between different laboratories. Examples of relevant standards are as follows:

In summary, thin film friction coefficient meters quantify the surface friction properties of materials by accurately measuring the force values in relative motion. Standardized testing methods and control of key influencing factors are the basis for obtaining accurate and repeatable data, so as to provide effective technical support for material research and development, process control and product application.

References

1. ASTM International. Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting.

2. International Organization for Standardization. Plastics — Film and sheeting — Determination of the coefficients of friction.

3. National Technical Committee for Standardization of Plastic Products. Plastic film and flake friction coefficient determination method.

4. Relevant materials science and tribology principles textbooks and industry technical manuals.