Coefficient of Friction Test Condition Setting: A Technical Guide to Experimental Parameters, Standa

Rubber, Elastomers, and Coated Surfaces

Seals, gaskets, conveyor components, soft-touch coatings, and anti-slip coatings are characterized under application-relevant loads and speeds, often against glass, steel, or standardized rubber sliders.

Testing Items

Typical test items defined in a COF test plan include the following:

• Static coefficient of friction (µs) – resistance to the initiation of sliding
• Kinetic coefficient of friction (µk) – resistance during continued sliding
• Film-to-film COF (outer surface to outer surface) for stacking and winding behavior
• Film-to-metal COF (outer surface to stainless steel or standard plane) for packaging machine runnability
• Hot COF (hot slip) at elevated temperatures, typically 35–60 °C, simulating hot-fill or heat-sealing operations
• COF after temperature/humidity aging or storage, to assess slip-agent migration and long-term stability
• Wet or contaminated slip resistance values (pendulum test value or dynamic COF) for flooring safety compliance

Testing Methods

Environmental Conditioning

Condition and test specimens in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618. A minimum conditioning period of 24 hours is typical, extended to 40 hours or longer for hygroscopic materials such as paper, PA, and EVOH films, since moisture absorption measurably alters surface friction. Any deviation from the standard atmosphere must be documented in the test report.

Specimen Preparation and Orientation

Cut specimen pairs for each test: one specimen is fastened flat and wrinkle-free to the base of the sled, and a second, larger specimen is secured to the horizontal plane. For anisotropic materials such as blown or oriented films, prepare specimens in both machine direction (MD) and transverse direction (TD) and report results separately. Handle specimens only by the edges using powder-free gloves or tweezers, never touch the test surfaces, and reject specimens showing dust, wrinkles, gel particles, or coating defects. The plane specimen should be long enough to provide the required travel distance, commonly 200–250 mm.

Normal Load and Sled Geometry

ISO 8295 and GB/T 10006 specify a 200 g sled with a 63.5 mm × 63.5 mm contact face, producing a normal force of approximately 1.96 N over a contact area of 40.3 cm², equivalent to a nominal contact pressure of roughly 0.5 kPa. ASTM D1894 likewise uses a 200 g sled with a 63.5 mm square base. If a heavier sled (for example 500 g or 1 kg) is selected for rigid or high-friction materials to improve force measurement resolution, the modified load must be clearly stated, because COF generally varies with contact pressure. Verify sled mass and force-measuring system calibration on a traceable schedule.

Sliding Speed and Travel Distance

ISO 8295 and GB/T 10006 specify a test speed of 100 mm/min, whereas ASTM D1894 specifies 150 mm/min. Because kinetic friction is speed-dependent, data generated under the two methods are not directly comparable; choose one standard and apply it consistently for all comparative and QC testing. Set a travel distance long enough to establish steady sliding, typically 70–100 mm or more, and compute the kinetic COF from the average force over a defined central measurement window that excludes the initial static peak and the deceleration zone.

Dwell Time Before Motion

Static COF increases with the time the sled rests on the specimen before motion begins. To control this variable, fix the interval between sled placement and the start of movement, for example by triggering the test immediately upon contact or after a standardized 10–30 second dwell, and apply the same interval to every specimen in the series.

Elevated-Temperature (Hot Slip) Testing

When packages are filled or sealed hot, COF should be measured at the process temperature. Condition the sled, specimens, and plane at the set temperature (commonly 35–60 °C) using a temperature-controlled chamber for sufficient equilibration before each run, and report the test temperature prominently, since polyolefin films show a pronounced rise in friction with temperature as slip agents become less effective at the surface.

Data Calculation and Reporting

Calculate µs as the peak initiation force divided by the normal force and µk as the mean steady-state sliding force divided by the normal force. Test a minimum of three to five valid specimen pairs per surface combination per direction, and report the mean, standard deviation, and range, together with the test standard, sled mass, speed, temperature, humidity, sliding directions, and counter-surface description. Any specimen exhibiting blocking, stick-slip instability, or surface damage should be discarded and replaced.

Special Conditions for Flooring Slip Resistance

For hard-surface flooring, slip resistance is measured in the wet state: the pendulum method of EN 13036-4 uses a standardized rubber slider swung over a water-wet surface, while the dynamic COF approach referenced by ANSI A137.1 and ANSI A326.3 uses an automated tribometer with a wetted soft rubber actuator and a widely used acceptance criterion of 0.42. Surfaces must be cleaned with a specified detergent, rinsed, and dried before testing, and slider conditioning procedures must be followed strictly.

Applications

Properly set COF test conditions support quality control and product development across many industries. Flexible packaging converters use film-to-metal kinetic COF targets of approximately 0.20–0.40 to ensure smooth film transport, bag forming, and sealing on high-speed packaging lines; values that are too low cause web slippage and misregistration, while values that are too high cause jams, wrinkling, and drag on forming collars. Film-to-film COF governs stack stability, palletizing behavior, and roll winding tension management.

• Vertical and horizontal form-fill-seal packaging line qualification and film release
• Laminate and printed film processability studies for converters
• Paper and label sheet feeding, stacking, and dispensing performance
• Rubber seal and soft-touch component friction characterization
• Protective film adhesion-liner and electronics component handling studies
• Wet slip resistance compliance for ceramic tile, stone, and vinyl flooring
• Hot-fill and heat-seal process simulation through elevated-temperature COF

Manufacturers submitting samples to accredited third-party laboratories should always specify the governing standard, surface combination, sliding direction, sled mass, speed, and test temperature in their test requests, so that reported values are directly comparable with internal reference data.

FAQ

What is the difference between static and kinetic coefficient of friction?

Static COF is derived from the peak force needed to start sliding between two resting surfaces, while kinetic COF is derived from the average force needed to maintain sliding at constant speed. Static values are almost always equal to or higher than kinetic values and are more sensitive to dwell time, so both must be controlled conditions.

Which standard should I choose for plastic film COF testing?

Use ISO 8295 or GB/T 10006 if your supply chain references international or Chinese national standards (100 mm/min, 200 g sled), and ASTM D1894 if North American specifications apply (150 mm/min, 200 g sled). The two methods use different speeds, so results are indicative rather than directly equivalent.

Why do COF results change between seasons or between laboratories?

Friction is highly sensitive to temperature and humidity. Polyolefin films become measurably more frictional as temperature rises, and hygroscopic materials such as paper, PA, and EVOH films change with moisture content. Consistent conditioning at 23 °C and 50 % RH, with adequate dwell time for equilibration, is essential for reproducible data.

What COF value is suitable for packaging machinery?

For the outer film surface sliding against stainless steel on packaging lines, a kinetic COF of approximately 0.20–0.40 is the commonly accepted working range. The optimum value depends on machine design and line speed, so it is best agreed between film supplier and end user with reference to a fixed test condition set.

Should I test film-to-film or film-to-metal friction?

Both configurations answer different questions. Film-to-metal COF predicts runnability over forming shoulders, guide rails, and sealing jaws, while film-to-film COF predicts stack stability, bag-on-bag slip, and winding behavior. A complete specification normally defines limits for both.

How many specimens should be tested, and in which directions?

Test at least three to five valid specimen pairs for each surface combination, in both machine and transverse directions for oriented materials. Report the mean, standard deviation, and any observed stick-slip behavior, since single-value results cannot demonstrate statistical reliability.

Can COF be measured at elevated temperature?

Yes. Hot slip or hot COF testing is performed in a temperature-controlled chamber, typically between 35 °C and 60 °C, after the sled and specimens have fully equilibrated. This condition setting is valuable for hot-fill products and for films whose slip packages lose effectiveness at process temperatures.