Friction coefficient (COF) testing in accordance with the Chinese national standard GB/T 10006 is the benchmark method for evaluating the slip performance of plastic films and sheeting used in packaging, printing, and converting industries. This technical guide explains the test principle, apparatus, specimen requirements, test procedure, and data treatment of national standard friction coefficient testing, and compares the GB method with ISO 8295 and ASTM D1894. A clear understanding of these requirements helps film producers, converters, and accredited third-party laboratories obtain accurate, repeatable, and comparable static and kinetic friction data.
Technical Overview
The coefficient of friction (COF) is the ratio of the tangential force resisting relative sliding between two contacting surfaces to the normal force pressing them together. For plastic films and sheeting, the COF directly determines how a material will behave on high-speed packaging lines, in bag making, during printing and lamination, and in stacking and handling operations. A film that is too slippery cannot be transported or stacked reliably, while a film with excessive friction causes machine jams, wrinkling, and inconsistent feeding.
In China, the friction coefficient of plastic films and thin sheets is determined using the national standard GB/T 10006, "Plastics — Film and sheeting — Determination of coefficients of friction". The current edition, GB/T 10006-2021, replaced the 1988 edition and is modified in relation to the international standard ISO 8295. The method applies to non-tacky plastic films and thin sheeting, typically with a thickness of up to about 0.5 mm, and it measures two key quantities: the static coefficient of friction at the onset of movement and the kinetic (dynamic) coefficient of friction during steady sliding.
The test is performed on a horizontal-plane friction tester. A lightweight sled faced with one specimen is dragged at constant speed across a flat bed carrying a second specimen, and the force required to initiate and maintain sliding is measured with a load cell. The main standardized parameters are summarized below:
- Sled mass: 200 g (200 g ± 2 g per ISO 8295), producing a normal force of approximately 1.96 N
- Sled contact area: approximately 63 mm × 63 mm (about 40 cm²)
- Sliding speed: 100 mm/min as specified in GB/T 10006 and ISO 8295 (ASTM D1894 uses 150 mm/min)
- Test configuration: film-to-film by default, with film-to-metal or film-to-other-substrate configurations also commonly evaluated
- Conditioning atmosphere: 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with GB/T 2918 / ISO 291
Because slip performance depends strongly on surface chemistry, slip-agent migration, temperature, and contact history, strict control of specimen preparation, conditioning, and machine settings is essential for meaningful results.
Testing Samples
Polyolefin Films (PE, PP)
LDPE, LLDPE, mLLDPE, cast polyethylene (CPE), CPP, BOPP, and blown PP films are the most frequently tested materials. Their COF is largely governed by slip additives such as erucamide and oleamide, which migrate to the film surface over time, so reels are often tested at production release and again after defined storage or aging periods.
Polyester, Polyamide, and Barrier Films
BOPET, PET, BOPA (nylon), PVDC-coated films, EVOH-containing structures, and aluminized films are tested both as monofilms and as components of laminate structures. Polyamide films are hygroscopic, so humidity conditioning is particularly important for reproducible results.
Laminated, Coated, and Functional Films
Printed and laminated packaging structures, heat-sealable films, anti-static films, protective films, release liners, and silicone-coated films can all be evaluated. Each side of the film (inner sealing side and outer printing side) should be characterized separately because their friction behavior is usually different.
Specimen Preparation and Conditioning
Sled specimens are cut to match the sled base (approximately 63 mm × 63 mm) and attached with double-sided adhesive tape without wrinkles or bubbles. Counter-surface specimens are cut large enough to provide a sliding stroke of roughly 70 mm, which is sufficient to obtain a stable kinetic force reading. Specimens must be free of creases, contamination, and fingerprints, handled with gloves or tweezers, and conditioned to equilibrium under the standard atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity, commonly for at least 24 hours for thin films.
Testing Items
The core deliverables of a GB/T 10006 test program are listed below:
- Static coefficient of friction (μs): calculated from the peak force at the moment sliding begins, representing the resistance to starting motion
- Kinetic (dynamic) coefficient of friction (μk or μd): calculated from the average force during steady sliding, representing resistance during continuous movement
- Film-to-film testing: the standard configuration, typically with both sides of the same film evaluated
- Film-to-counter-surface testing: for example, film against stainless steel, which simulates transport over machine guide rails, forming collars, and rollers
- Extended studies: COF after thermal aging, elevated-temperature (hot slip) behavior, or storage-time studies, which go beyond the baseline standard method but are frequently requested for packaging development
The standard explicitly notes that it is not intended for tacky or adhesive surfaces, where cohesive or adhesive failure dominates rather than friction.
Testing Methods
Test Principle
A sled of defined mass, faced with a test specimen, is placed on a horizontal plane carrying a second specimen. The sled is drawn across the plane at constant speed while the friction force is recorded continuously. Because the normal force equals the weight of the sled, the coefficient of friction is simply the measured friction force divided by the normal force. The first force peak gives the static coefficient, and the mean force during uniform sliding gives the kinetic coefficient.
Apparatus
- A rigid, flat horizontal test table on which the counter-surface specimen is secured
- A sled with a mass of 200 g and a contact area of approximately 63 mm × 63 mm
- A drive mechanism providing a constant speed of 100 mm/min with low vibration
- A calibrated force sensor (load cell) with suitable resolution and a data acquisition system
- Double-sided adhesive tape and cleaning materials for specimen mounting
Test Procedure
- Condition the specimens in the standard atmosphere until equilibrium is reached
- Attach the sled specimen to the sled base with double-sided tape, ensuring full contact and no wrinkles
- Fix the counter-surface specimen flat on the test table, parallel to the direction of travel
- Place the sled gently onto the surface, without impact, and start the test within a short, consistent contact time
- Record the force–displacement curve, capturing the initial static peak and the steady-state kinetic force over a sliding distance of about 70 mm
- Repeat the test with fresh specimen pairs, using a minimum of three and preferably five valid determinations
Calculation of Results
The coefficients of friction are calculated as follows:
- μs = Fs,max / FN, where Fs,max is the maximum force at the onset of sliding
- μk = Fk,mean / FN, where Fk,mean is the mean force during steady sliding, excluding the start-up region
- FN = m × g ≈ 1.96 N for a 200 g sled (using g = 9.81 m/s²)
Result Expression and Reporting
The test report presents the arithmetic mean, the standard deviation, and the range of the individual values, together with full identification of the material, film thickness, tested sides, test configuration (film-to-film or film-to-other-surface), conditioning details, sled mass and speed, and any deviations from the standard. Reporting both film sides and the exact surface pairing is essential, because COF values from different configurations are not interchangeable.
Applications
GB/T 10006 friction coefficient data are used throughout the plastics and packaging value chain for material qualification, process optimization, and quality control:
- Flexible packaging machinability: verifying that films run reliably on vertical form-fill-seal (VFFS), horizontal form-fill-seal (HFFS), and pre-made bag lines, where the film must slide smoothly over metal forming collars, rollers, and guides
- Slip agent and masterbatch development: optimizing erucamide, oleamide, and antiblock dosages, and monitoring how COF evolves during reel storage as slip agents migrate
- Printing, lamination, and converting: balancing web tension control, winding quality, and surface treatment effects with downstream friction requirements
- Bag and pouch making: ensuring reliable film transport, sealing, cutting, and stacking behavior
- Protective and release films: tuning friction to combine processability with clean application and removal
- Incoming inspection and dispute resolution: independent verification by accredited third-party laboratories using standardized GB, ISO, or ASTM methods
As general industrial practice, film-to-film and film-to-metal COF values in the range of about 0.2 to 0.3 are considered well suited to automatic packaging, while values above roughly 0.4 increase the risk of jams and values below roughly 0.15 can compromise web control and stacking. The optimal range ultimately depends on the machine design and laminate structure, and should be agreed between the film supplier and the converter.
FAQ
What is the difference between static and kinetic coefficient of friction?
The static coefficient describes the resistance at the instant sliding starts and is derived from the first force peak; it governs whether a film will start moving when a machine pushes or pulls it. The kinetic coefficient describes resistance during continuous sliding and is derived from the average force in the steady-state region; it governs stable transport through the machine. Both values are required for a complete assessment of machinability.
What are the differences between GB/T 10006, ISO 8295, and ASTM D1894?
GB/T 10006 is technically aligned with ISO 8295 and uses a 200 g sled at 100 mm/min on a horizontal plane. ASTM D1894 uses a similar sled (63.5 mm × 63.5 mm) but a higher test speed of 150 mm/min, along with some differences in specimen and calculation details. Because of these differences, results from the different standards correlate well but are not directly interchangeable, and the applied method must always be stated in the report.
Why do friction coefficient results change over time?
Many polyolefin films rely on migratory slip additives. Immediately after production, little slip agent has reached the surface and COF is high; over days or weeks of storage, the additive migrates to the surface and COF decreases, sometimes stabilizing and sometimes drifting again with temperature. This is why COF is often specified and tested at defined times after extrusion.
How do temperature and humidity affect the results?
Higher temperatures generally increase the COF of slip-laden films because the slip agent becomes more soluble in the polymer and partially leaves the surface. Elevated-temperature testing or hot-slip evaluations are therefore useful for summer conditions and heat-sealing operations. Humidity mainly affects hygroscopic materials such as polyamide films, which is why conditioning at 23 °C and 50% RH is mandatory for comparable data.
How many specimens are needed for a valid test?
A minimum of three valid specimen pairs is required, and five is recommended. Specimens that slip off the table, exhibit wrinkling, or show anomalous curves should be discarded and replaced. The mean, standard deviation, and range of the valid determinations are reported.
How can the friction coefficient of a plastic film be adjusted?
Formulators control COF through slip agents (erucamide, oleamide, stearamide), antiblock particles (silica, talc), resin selection, film structure, and surface treatment such as corona treatment. The target must be balanced against heat-seal strength, printability, haze, and food-contact compliance, which is why COF testing is performed alongside these other properties.
Can the test be performed against surfaces other than film?
Yes. Although the standard method is defined for film-to-film contact, film-to-metal and film-to-different-film configurations are widely used to simulate real machine and laminate conditions. In such cases, the counter-surface material, roughness, and test geometry must be clearly documented, since results are only valid for the specific pairing tested.