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The dynamic coefficient of friction (COF) test measures the resistance to sliding between two surfaces in relative motion, making it essential for evaluating plastic films, packaging materials, and other sheet products. This guide details the complete experimental procedure for determining the dynamic friction coefficient in accordance with international standards such as ASTM D1894 and ISO 8295. Understanding the correct test steps ensures accurate, reproducible results for quality control and material selection.
Technical Overview
The dynamic coefficient of friction (also called the kinetic coefficient of friction, denoted as μd or μk) is a dimensionless value representing the ratio of the frictional force resisting motion between two surfaces to the normal force pressing them together while sliding is in progress. Unlike the static coefficient of friction, which describes the force needed to initiate movement, the dynamic COF characterizes the resistance sustained during continuous relative motion between the test surfaces.
The most widely used instrument for this measurement is the horizontal plane COF tester, which consists of a motor-driven horizontal platform, a sled of standardized mass, and a force-measuring system. The sled is placed on a specimen mounted on the moving platform, and as the platform slides at a constant speed, the force sensor records the frictional resistance. The dynamic COF is calculated by dividing the mean frictional force recorded during steady sliding by the normal force exerted by the sled weight.
This test is particularly critical in the flexible packaging and converting industries, where film-on-film and film-on-metal friction characteristics directly influence machine performance on form-fill-seal lines, bag-making equipment, and automated packaging systems. Principal reference methods include ASTM D1894, ISO 8295, and the Chinese national standard GB/T 10006.
Testing Samples
Plastic Films and Sheeting
Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), and laminated multilayer films represent the most common sample categories. Specimens must be flat, free from creases, wrinkles, and surface contamination, as these defects introduce unacceptable variability into friction measurements.
Paper and Paperboard
Coated and uncoated papers, carton board, and kraft papers used in printing and packaging applications can be evaluated using the same horizontal plane principle, provided the specimen thickness does not interfere with sled travel.
Rubber, Textiles, and Coated Materials
Conveyor belt coverings, elastomeric sheets, coated fabrics, and surface-treated materials may also be tested, although the applicable standards and sled configurations may differ. Accredited third-party laboratories can advise on method suitability for non-standard materials.
Testing Items
- Dynamic (kinetic) coefficient of friction, μd — the ratio of the average frictional force during sustained sliding to the normal force
- Static coefficient of friction, μs — the ratio of the peak force at the onset of motion to the normal force, typically measured in the same test run
- Film-to-film COF — friction between two layers of the same or different film materials
- Film-to-substrate COF — friction between film and other surfaces such as stainless steel, which simulates contact with packaging machine contact plates
Testing Methods
Apparatus and Test Conditions
The test requires a COF tester comprising a horizontal flat bed driven at constant velocity, a sled with a specified mass (commonly 200 g with a contact area of 60 mm × 60 mm or 63.5 mm × 63.5 mm depending on the standard), and a load cell connected to the sled through a thin, flexible filament. Tests are conducted under standard laboratory conditions of 23 ± 2 °C and 50 ± 5 % relative humidity, and specimens must be conditioned in this atmosphere for a minimum of 40 hours prior to testing.
Experimental Procedure
- Step 1 — Specimen preparation: Cut at least three to five pairs of specimens for each test condition. Cut one specimen large enough to cover the moving platform (at least 250 mm in the sliding direction for ISO 8295), and cut the sled specimen to match the sled contact face dimensions.
- Step 2 — Secure the plane specimen: Attach the large specimen to the horizontal platform using double-sided adhesive tape along the edges, ensuring the test surface remains perfectly flat, smooth, and free of tension, wrinkles, or trapped air.
- Step 3 — Mount the sled specimen: Fix the second specimen securely to the underside of the sled, taking care that no adhesive penetrates the test surface and that the material orientation (machine direction versus transverse direction) is consistent with the test plan.
- Step 4 — Connect the force sensor: Link the sled to the load cell through the flexible connector and verify that the connection is horizontal and free of slack before motion begins.
- Step 5 — Initiate the test: Start the platform drive at the specified speed (150 mm/min for ASTM D1894; 100 mm ± 10 mm/min for ISO 8295 and GB/T 10006). The sled initially moves with the platform until the connector becomes taut.
- Step 6 — Record the friction force: Capture the initial peak force, which defines the static COF, then record the force-time curve during steady sliding over the specified travel distance. The average force during this sliding phase defines the dynamic friction force.
- Step 7 — Calculate the coefficients: Compute the static COF as the peak force divided by the sled normal force, and compute the dynamic COF as the mean sliding force divided by the sled normal force.
- Step 8 — Repeat and report: Test a minimum of three specimen pairs for each surface combination, report individual and mean values to two or three decimal places, and include the standard, specimen orientation, sled mass, and sliding speed in the report.
Key Precautions
- Never touch the test surfaces with bare hands; skin oils alter frictional behavior. Handle specimens by the edges or wear powder-free gloves.
- Allow specimens to relax after unrolling, since residual stresses and curl cause uneven contact and unreliable readings.
- Clean the sled and any metal counter-surfaces with a suitable solvent before each series, and verify sled mass calibration periodically.
- Avoid reusing the same contact area for repeated measurements; sliding modifies the surface and changes the coefficient.
- Do not test specimens immediately after production; slip additives migrate over time, so measurements should reflect the material condition at the point of use.
Applications
- Flexible packaging: verifying that films have the COF window required for smooth operation on vertical form-fill-seal and horizontal flow-wrapping machines
- Quality control: monitoring the consistency of slip agents, antiblock additives, and surface treatments during film extrusion and lamination
- Material development: comparing the tribological performance of new resin formulations, coatings, and surface treatments
- Paper and printing: assessing paperboard stackability and feedability in printing and converting lines
- Automotive and industrial: evaluating friction characteristics of rubber sheets, brake components materials, conveyor surfaces, and interior trim laminates
- Medical and pharmaceutical packaging: ensuring consistent machinability of blister foils, Tyvek-type substrates, and sterile barrier films
FAQ
What is the difference between static and dynamic coefficient of friction?
The static COF describes the resistance at the moment motion begins, calculated from the initial peak force, whereas the dynamic COF describes the sustained resistance during continuous sliding, calculated from the average force over the sliding distance. A single test run typically yields both values.
Which standards govern the dynamic COF test?
ASTM D1894, ISO 8295, and GB/T 10006 are the primary standards for plastic film and sheeting. Each specifies sled geometry, sliding speed, travel distance, and calculation methods, so results from different standards are not directly interchangeable.
What COF values are considered acceptable for packaging films?
For films processed on high-speed packaging machinery, a film-to-film dynamic COF between roughly 0.20 and 0.35 is commonly targeted. Values below approximately 0.20 indicate high slip, while values above the target range may cause jamming and inconsistent feeding; exact limits depend on the specific application and end-user specification.
Why do test results sometimes vary between laboratories?
Variations typically arise from differences in specimen conditioning, contact pressure distribution, surface contamination, sliding speed, and the age of the specimens after production. Strict adherence to the specified atmosphere, handling procedures, and calibration requirements minimizes inter-laboratory discrepancies.
How many specimens should be tested?
A minimum of three specimen pairs per surface combination is recommended by most standards, though testing five or more improves statistical confidence. Report both the mean and the individual values to identify outliers caused by surface defects.