The kinetic coefficient of friction (COF) test quantifies the sliding behavior of plastic coatings and films, providing critical data for packaging machinability, web handling, and surface quality control. Performed in accordance with internationally recognized standards such as ASTM D1894, ISO 8295, and GB/T 10006, this test measures the tangential force required to sustain motion between a weighted sled and a coated plastic surface. This article explains the test principle, specimen requirements, methodology, typical acceptance values, and practical applications across the plastics and packaging industries.
Technical Overview
The kinetic (dynamic) coefficient of friction is defined as the ratio of the tangential force required to maintain relative sliding motion between two contacting surfaces to the normal force pressing those surfaces together. For plastic coatings, films, and coated flexible substrates, the kinetic COF is one of the most important surface performance indicators because it directly determines how a material will run on high-speed converting and packaging equipment.
The test is typically conducted using the horizontal plane sled method described in ASTM D1894, ISO 8295, and GB/T 10006 (the Chinese national standard technically aligned with ISO 8295). A sled of known mass, with a specimen of the coated plastic material bonded to its base, is drawn at constant velocity across a second specimen resting on a horizontal platform. The force required to sustain uniform sliding is recorded continuously by a load cell, and the kinetic coefficient of friction is calculated as:
μk = Fk / N, where Fk is the mean kinetic friction force and N is the normal force (sled weight, m × g). For a standard 200 g sled, N is approximately 1.96 N.
Because plastic coatings often contain migratory additives such as slip agents (erucamide, oleamide) and antiblock agents, the kinetic COF is not a fixed material constant. It can evolve over storage time as additives bloom to the surface, and it is sensitive to temperature, humidity, surface treatment, and coating cure state. For this reason, kinetic COF testing is routinely performed by accredited third-party laboratories as part of incoming inspection, batch release, and product development programs.
Testing Samples
Coated Plastic Films and Sheeting
Monolayer and multilayer films of PE, PP (CPP, OPP, BOPP), PET, PVC, PA, and EVOH carrying functional surface layers such as heat-seal coatings, lacquers, anti-fog coatings, overprint varnishes, or printable treated surfaces. Both machine direction (MD) and transverse direction (TD) specimens are prepared, since orientation can make friction behavior anisotropic.
Flexible Packaging Laminates
Finished laminate structures used in vertical form-fill-seal (VFFS), horizontal form-fill-seal (HFFS), flow wrapping, pouch making, and thermoforming. The sealing side and the outside (print) side are tested separately, as each face contacts different machine surfaces during processing.
Plastic and Polymeric Coatings on Rigid Substrates
Silicone release coatings on liners, coated paper and paperboard for cups and cartons, coated metal foil, painted plastic panels, soft-touch coatings, and protective film coatings. Testing evaluates the coating surface against itself or against a reference counter-material such as polished stainless steel.
Specimen Conditioning
All specimens are cut flat and free of wrinkles, creases, and fingerprints, and are conditioned to a standard laboratory atmosphere prior to testing, typically 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618. Because freshly wound rolls may have incomplete additive bloom, specimens are frequently tested at defined intervals after production when time-dependent slip development must be characterized.
Testing Items
Core and supplementary test items associated with kinetic friction evaluation of plastic coatings include:
- Kinetic (dynamic) coefficient of friction, coated surface against the same coated surface (film-to-film)
- Kinetic coefficient of friction, coated surface against a standard counter-face such as stainless steel (film-to-metal)
- Static coefficient of friction, captured from the initial force peak in the same test trace for direct comparison
- Hot or elevated-temperature kinetic COF, simulating heat-sealing zones and hot-fill conditions
- COF development over time, evaluating slip agent migration and post-production aging
- COF after abrasion, blocking, or environmental aging for durability assessments
Testing Methods
Horizontal Plane Sled Test Procedure
The standard procedure per ASTM D1894, ISO 8295, and GB/T 10006 is as follows. A plane specimen is secured to a flat, level testing table. A second specimen is attached to the underside of a standard sled. The sled is placed on the plane specimen without impact, allowed a brief dwell period, and then drawn horizontally at a constant speed while the friction force is recorded. Typical parameters are:
- Sled mass: 200 g, giving a normal force of approximately 1.96 N
- Sled base dimensions: 63.5 mm × 63.5 mm per ASTM D1894, with equivalent square-base sleds used under ISO 8295 and GB/T 10006
- Sliding speed: 150 mm/min per ASTM D1894 or 100 mm/min per ISO 8295 and GB/T 10006
- Plane specimen dimensions: sufficiently larger than the sled path to ensure continuous fresh contact over the full sliding distance
Data Reduction and Calculation
The force-time trace shows an initial peak corresponding to the static friction force, followed by a fluctuating plateau representing kinetic friction. The kinetic COF is calculated from the average force over the specified sliding distance, excluding the initial static peak and any end-effects. At least three, and preferably five, specimen pairs are tested for each surface and direction, and the mean, standard deviation, and range are reported along with test direction, temperature, humidity, and conditioning history.
Elevated Temperature and Special Conditions
For heat-sealable coatings and films intended for VFFS packaging lines, kinetic COF can be measured on instruments fitted with a heated table and heated sled, typically in the range of 40 °C to 90 °C. This hot-slip evaluation identifies coatings that become tacky at sealing temperatures, which is a common cause of film jams and inconsistent package formation in production.
Applications
Kinetic friction coefficient testing of plastic coatings supports a wide range of industrial uses:
- Flexible packaging quality control: verifying that films will feed, form, and seal reliably on high-speed packaging machines, with film-to-film kinetic COF targets commonly in the 0.20 to 0.30 range for good machinability
- Formulation development: optimizing slip agent and antiblock loadings in polyolefin films and evaluating additive bloom kinetics over storage time
- Release liner qualification: confirming silicone release coatings provide consistent low-friction, non-blocking performance for labels and adhesive products
- Coated paper and board: assessing the fold, feed, and stacking behavior of coated cartons, cups, and food service board
- Automotive and consumer products: characterizing soft-touch and tactile coatings for feel, grip, and gliding performance
- Printed films: evaluating the effect of inks, varnishes, and primers on downstream converting performance
- Supplier verification and dispute resolution: accredited third-party laboratories provide ISO/IEC 17025-traceable COF data for incoming material inspection and lot acceptance
FAQ
What is the difference between static and kinetic coefficient of friction?
The static COF describes the resistance at the instant sliding begins and is derived from the initial force peak, while the kinetic COF describes the resistance during sustained sliding and is derived from the average force over the sliding plateau. Both values are captured in a single sled test, but the kinetic value is generally the more relevant parameter for continuous web transport on packaging lines.
Which standards govern this test?
The most widely applied methods are ASTM D1894, ISO 8295, GB/T 10006, and DIN ISO 8295. All are based on the same horizontal sled principle but differ slightly in sled geometry, sliding speed, and reporting details, so the specific method used must always be stated in the test report.
What specimen sizes and quantities are required?
A typical requirement is a plane specimen of at least 127 mm × 254 mm for ASTM D1894, plus square sled specimens of approximately 63.5 mm per side, or equivalent sizes under ISO 8295 and GB/T 10006. A minimum of three to five specimen pairs per surface and per direction is recommended for statistically meaningful results.
What kinetic COF values are considered acceptable?
For polyolefin packaging films, film-to-film kinetic COF between 0.20 and 0.30 is generally regarded as optimum for machinability. Values above roughly 0.40 increase the risk of film sticking, jamming, and poor bag opening, while extremely low values below about 0.10 can cause web tracking and traction problems on drive belts.
Why do results sometimes differ between laboratories?
Kinetic COF is highly sensitive to surface state. Differences in conditioning atmosphere, specimen age after production (slip agent bloom), surface contamination, sled alignment, drive speed, and instrument calibration can all shift results. Strict adherence to the specified conditioning requirements and calibration procedures is essential for reproducibility.
Can the test be performed against counter-surfaces other than film?
Yes. Film-to-metal tests against a polished stainless steel plane are common for evaluating performance against packaging machine surfaces, and custom counter-materials can be used when simulating a specific product contact pair. Elevated-temperature testing is also available for hot-slip evaluation.