When laboratory technicians conduct coefficient of friction (COF) testing on materials such as plastic films, they have found that the results are not always consistent and are highly sensitive. Even minor deviations during the testing process can lead to inconsistent results. Through numerous research experiments, we have identified ten factors that affect the accuracy of test results, along with methods to control them. Let’s take a closer look.
1. Slip Agents Migration (Leaching)
Typically, manufacturers add slip agents during production to reduce the friction properties of materials. However, slip agents do not always remain within the material; over time, they gradually migrate to the material’s surface, forming a microscopic lubricating layer. This is often the reason why the coefficient of friction measured by customers after receiving the goods differs from the laboratory test data.
You can improve data consistency by establishing a standardized testing procedure. Before testing, standardize the samples by pre-treating them under specific environmental conditions for a specified period (for example, 24 or 48 hours, depending on your product) to stabilize slip-agent migration. At the same time, clearly document the sample handling process in the test report to minimize variations.
2. Load Cell Sensitivity
Sensors capture real-time changes in force; the higher the sensitivity, the more accurate the data. The coefficient of static friction primarily captures the initial peak force when a material transitions from a stationary to a moving state; sensors with low sensitivity cannot capture this instantaneous peak and often underestimate the coefficient of static friction.
The losses incurred by products with inadequate friction performance far exceed the cost of purchasing high-sensitivity equipment. Manufacturers must select equipment equipped with high-sensitivity sensors to ensure the accuracy of test results. Saicheng’s MXD-02 Coefficient of Friction Tester is equipped with an advanced sensor capable of achieving 0.5 FS precision, maximizing the accuracy of test results and capturing microscopic and instantaneous changes in force.
3. Surface Contamination and Improper Sample Handling
If operators do not follow standard procedures and handle samples directly with their bare hands, oils, moisture, and other substances on the skin are likely to transfer to the sample. This may cause unintended lubrication or act as an adhesive, affecting the measurement of the coefficient of friction. Additionally, the presence of dust or contaminants on the sample surface can also lead to inaccurate coefficient of friction results.
Operators must wear clean, powder-free nitrile or latex gloves when handling samples. Furthermore, before testing film samples, the surface must be cleaned of dust, contaminants, and other impurities to ensure the sample remains clean.
4. Test Slider Dwell Time
If the test slider remains in contact with the sample surface for too long before the test begins, the microscopic contact points between the two materials will undergo plastic deformation and cold welding due to the force of gravity. When the test begins, these changes can result in an excessively high static coefficient of friction. When testing plastic films from the same batch, inconsistencies in dwell time can affect the assessment of product performance stability and consistency.
Establish standardized operating times, such as specifying a dwell time window of 5–10 seconds, within which the operator must initiate the test.
5. Equipment’s Pulling System (Nylon Cord, Steel Wire, Connecting Hooks)
If the nylon cord, steel wire, connecting hooks, etc., are not perfectly level, there is a possibility of increasing or decreasing the vertical normal force, which will cause the coefficient of friction results to drift and affect the interpretation of the results.
Operators must periodically check whether the instrument’s sliding platform is perfectly level and whether the pulling system remains perfectly parallel. The most effective solution is to use a coefficient of friction tester equipped with precision linear guides as the pulling mechanism.
6. Sled Calibration, Flatness, and Rubber Pad Wear
The standard slider weighs 200 g, with a tolerance of up to 5 g. The back of the slider is fitted with a flexible sponge rubber pad. After repeated testing, the sponge rubber will wear down, making it difficult for the slider to apply force evenly when in contact with the test surface. This alters the actual contact area and affects the friction coefficient test results.
Calibrate the slider’s total weight weekly to ensure it remains within the weight specified by the standard. Additionally, establish a strict replacement schedule (for example, every three or six months).
7. Consistency of Equipment Vibration and Speed
Standard methods such as ASTM D1894 require that test equipment maintain a continuous and stable operating speed. If the equipment’s drive system experiences slight fluctuations, this will not only alter the material’s friction characteristics but also affect the load cell’s ability to sense force values, thereby impacting the test results.
First, select equipment equipped with a high-resolution motor to ensure it can provide uniform, continuous linear velocity. During testing, ensure the tester is placed on a stable, vibration-isolated laboratory platform, and ensure the equipment’s drive mechanism remains properly lubricated to prevent internal jamming during testing. The test table and test slider of Saicheng’s MXD-02 Coefficient of Friction Tester have both undergone demagnetization treatment and residual magnetism testing, which effectively reduces testing errors in the system.
8. Ambient Temperature and Relative Humidity
Plastic film materials are inherently highly sensitive to environmental changes. If the ambient temperature rises, the lubricants within the material will migrate more rapidly to the surface, leading to inaccurate measurements. If the ambient humidity increases, hydrophilic materials such as nylon and cellophane will absorb moisture from the air, increasing the microscopic attractive forces between the materials and resulting in an artificially high static coefficient of friction.
Manufacturers must conduct testing in an environment that complies with the requirements of ASTM D1894 and ISO 8295, ensuring that environmental conditions are and relative humidity. They must also monitor and record the test environment conditions in real time.
9. Sample Orientation: MD vs. TD and “Like vs. Unlike” Surfaces
Because of the extrusion and stretching process, plastic films are highly anisotropic (having different physical properties in different directions). Friction is almost always lower when sliding parallel to the polymer chain orientation (Machine Direction, MD) than perpendicular to it (Transverse Direction, TD). Furthermore, co-extruded films have different chemical structures on their corona-treated (print) side versus their untreated (sealant) side.
Explicitly label your test directions and ensure you are consistently testing MD sliding on MD (or TD on TD). In your SOP, clearly state whether the test is treated-to-treated, treated-to-untreated, or film-to-metal (simulating packaging machinery).
10. Elasticity of the Driving Link
Using a monofilament nylon line or string with high elasticity acts like a dampening spring. It stretches, absorbing the initial force spikes and falsely smoothing out the static peak, making it impossible to accurately diagnose stick-slip behaviors.
Replace standard nylon thread with a strictly rigid, non-elastic braided stainless steel wire or a calibrated metal hook system as recommended by ASTM D1894.
These factors indicate that reliable coefficient of friction test results must take into account the sample condition, equipment settings, environmental controls, and operating procedures. Adopting a systematic approach to coefficient of friction testing helps laboratories establish consistent testing protocols and obtain results suitable for material comparison and quality control.
Choosing Saicheng’s COF Tester for Your Lab
Saicheng Instruments’ equipment primarily utilizes advanced, high-precision sensors and is designed and manufactured in accordance with international standards, with the aim of providing customers with accurate, reliable, and compliant test results. We offer two product series for coefficient of friction testing:
MXD-02 Coefficient of Friction Tester
The MXD-02 is a specialized coefficient of friction tester we developed in accordance with the international standards ISO 8295 and ASTM D1894. Both the test table and the test slider have undergone demagnetization and residual magnetism testing to prevent systematic errors. The maximum test speed reaches 800 millimeters per minute, and the speed is infinitely variable.
Specifically designed to measure the static and dynamic coefficients of friction of materials such as plastic films and sheets, rubber, paper, cardboard, woven bags, textiles, composite metal tapes for telecommunications and optical cables, conveyor belts, wood, coatings, brake pads, windshield wipers, footwear materials, and tires during sliding.
The X-810 is a dual-function friction and peel tester. In addition to performing high-precision ASTM D1894 coefficient of friction tests, it can seamlessly switch to a high-precision peel tester to measure the peel strength of products such as medical patches, release paper, and protective films.
It features stepless speed control, with a maximum test speed of 600 mm/min. Additionally, the system’s specialized software provides statistical analysis with superimposed test result curves, presenting test results to users accurately and intuitively.
Conclusion
Proper operating procedures and a high-precision coefficient of friction tester can effectively resolve the issue of inaccurate test results. By understanding the issues and solutions outlined above and using a Saicheng coefficient of friction tester, you can ensure accurate and compliant test results. If you would like to learn more about our products or receive professional assistance, please feel free to contact Saicheng Instrument❗️