The sliding coefficient of friction (COF) is one of the most critical surface properties of plastic materials, governing how films, sheets, and molded parts behave during processing, handling, and end-use service. Accurate determination of the static and kinetic friction coefficients of plastics, performed in accordance with internationally recognized standards such as ISO 8295, ASTM D1894, and GB/T 10006, is essential for quality control, material selection, and product development across the packaging, automotive, electronics, and polymer engineering industries. This article provides a comprehensive technical guide to plastic sliding friction coefficient testing, covering test principles, sample requirements, instrumentation, procedures, calculation methods, and typical applications.
Technical Overview
The coefficient of friction (COF) is defined as the ratio of the tangential friction force resisting relative motion between two surfaces to the normal force pressing those surfaces together. In the characterization of plastics, two values are distinguished: the static coefficient of friction (μs), which quantifies the resistance encountered when sliding is initiated, and the kinetic or dynamic coefficient of friction (μk), which quantifies the resistance sustained during sliding at a constant velocity. Both quantities are dimensionless and are most commonly measured using a horizontal-plane, sled-type friction tester.
Friction of polymeric surfaces arises from a combination of molecular adhesion, surface deformation, and plowing effects, making it highly sensitive to formulation and processing history. Additives such as slip agents (for example, erucamide or oleamide), antiblock agents (silica, talc), plasticizers, and surface treatments (corona or flame treatment, printing, lamination) can shift COF values dramatically. Consequently, friction testing is a key quality-control tool for plastic film and sheet manufacturers and for converters who must guarantee machinability on high-speed packaging lines.
- Surface roughness and topography of both mating surfaces
- Polymer formulation, including slip, antiblock, and filler content
- Ambient temperature and relative humidity during testing
- Contact pressure, contact time before sliding, and sliding velocity
- Counter-surface material and its surface condition (for example, polished stainless steel, glass, or a second plastic)
- Stick-slip tendency, which appears as force oscillation during sliding
- Processing history, orientation, and surface treatment of the specimen
For plastic films and sheeting, the internationally recognized test methods are ISO 8295, ASTM D1894, and GB/T 10006 (which is technically aligned with ISO 8295). For bulk plastic materials and engineering components subjected to sustained sliding contact, ISO 6601, GB/T 3960, and ASTM D3702 address the combined evaluation of friction and wear. Accredited third-party laboratories routinely perform coefficient of friction testing to these standards for product certification, supplier qualification, and research purposes.
Testing Samples
Flexible Plastic Films
Flexible films are the most frequently tested category and include polyethylene (LDPE, LLDPE), polypropylene (BOPP, CPP), polyester (PET), polyamide (PA), PVC, cellophane, and multilayer laminates used in food, medical, and industrial packaging. Specimens are typically taken from finished rolls and may be tested in film-to-film or film-to-metal configurations, with the mating side (inner versus outer surface) clearly identified and reported.
Plastic Sheeting and Rigid Flat Materials
Thermoforming sheets, rigid PVC, APET and RPET sheets, polycarbonate glazing, foamed sheets, and other flat plastic products within the thickness range covered by the film and sheeting standards can be evaluated using the same horizontal-plane sled method, provided the material lies flat and does not curl during the test.
Molded Parts and Engineering Components
Bushings, gears, wear pads, clips, guides, and other functional components are assessed under sustained sliding conditions using configurations such as pin-on-disc, ring-on-block, or thrust-washer geometries in accordance with ISO 6601, GB/T 3960, or ASTM D3702. These tests capture friction behavior under realistic loads, speeds, and temperatures.
Counter-Surface Materials
Depending on the test objective, the counter-surface may be a second specimen of the same plastic, a different plastic, polished stainless steel, chrome-plated steel, glass, or anodized aluminum. All counter-surfaces must be clean, dry, and free from scratches, corrosion, dust, and fingerprints, since even minor contamination can alter measured friction values.
Testing Items
The following friction properties are typically determined:
- Static coefficient of friction (μs), representing the resistance at the onset of sliding
- Kinetic (dynamic) coefficient of friction (μk), representing the resistance during steady sliding
- Friction force versus displacement curves, used to detect stick-slip behavior and slip-stick oscillation
- Coefficient of friction of plastic against plastic or plastic against metal mating pairs
- Friction and wear behavior under sustained loaded sliding, including wear rate and mass loss, per ISO 6601, GB/T 3960, and ASTM D3702
- COF evaluation of film inner and outer surfaces, treated and untreated sides, or printed and unprinted regions
Test results are reported as the mean value together with the range or standard deviation, along with full identification of the test configuration, sliding speed, and conditioning atmosphere, because all of these parameters materially influence the outcome.
Testing Methods
Horizontal Plane (Sled) Method — ISO 8295 / ASTM D1894 / GB/T 10006
The standard method for films and sheeting is based on a horizontal-plane apparatus. One specimen is fixed flat onto a horizontal test bed, and a second specimen is attached to the underside of a standardized sled, commonly with a mass of 200 g and a contact area of 63.5 mm × 63.5 mm in ASTM D1894. The sled is drawn across the plane at a constant velocity by a drive mechanism connected through a force transducer, which records the tensile force required to start and to sustain motion. The coefficient of friction is obtained directly from the ratio of the measured friction force to the normal force, which equals the weight of the sled.
Specimen Preparation and Conditioning
Specimens are cut cleanly without burrs or wrinkles; plane specimens are typically about 200 mm × 80 mm, and sled specimens are cut to match the sled contact dimensions. Test surfaces must never be touched by hand, and contamination should be removed only with a dry, lint-free cloth, avoiding solvents that could extract or redistribute surface additives. Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity, typically for a minimum of 16 hours, in accordance with the conditioning requirements referenced by the applicable standard.
Test Procedure
- Mount the plane specimen on the test bed, ensuring it is flat, smooth, and wrinkle-free
- Secure the sled specimen to the sled base using double-sided tape so that no tape is exposed within the contact area
- Allow the specified dwell or contact time between sled and plane before initiating motion
- Drive the sled at the standard speed, typically 100 mm/min per ISO 8295 and GB/T 10006 or 150 mm/min per ASTM D1894
- Record the initial force peak, which defines the static friction force, and the average force during the steady sliding region, which defines the kinetic friction force
- Repeat the measurement on multiple specimens and sliding tracks, using fresh surface areas for each run, and average the results
Calculation and Expression of Results
The static and kinetic coefficients of friction are calculated as μs = Fs/N and μk = Fk/N, where Fs is the maximum force at the initiation of sliding, Fk is the mean force during sliding, and N is the normal force exerted by the sled weight. As orientation matters, the results are reported separately for each surface pairing, for example outer-to-outer, inner-to-inner, or film-to-steel. Typical commercial packaging films exhibit kinetic COF values of roughly 0.2 to 0.4, high-slip films may fall below 0.2, and untreated polyethylene films can exceed 0.7, a level associated with blocking, poor machinability, and handling difficulties.
Sustained Sliding Friction and Wear — ISO 6601 / GB/T 3960 / ASTM D3702
For engineering plastics and tribological components, friction is evaluated under sustained, loaded sliding rather than a single light-load pass. ISO 6601 describes general principles for determining sliding friction and wear of plastics, GB/T 3960 specifies a ring-on-block sliding friction and wear procedure, and ASTM D3702 covers self-lubricated materials using a thrust-washer configuration. In these tests, a defined normal load and sliding velocity are applied for an extended period while the frictional torque or force is continuously recorded, after which wear is quantified gravimetrically or dimensionally. These methods are indispensable for materials such as PTFE-filled compounds, polyacetal, nylon, and other bearing-grade plastics.
Applications
Flexible Packaging
Coefficient of friction testing is fundamental to packaging quality control. Film processors use COF data to ensure reliable transport of films over forming collars and stainless steel chutes on vertical form-fill-seal machines, to guarantee stable stacking of filled bags on pallets, and to balance slip behavior in laminated structures. Both excessively high and excessively low COF values cause production stoppages and product damage.
Automotive and Transportation
Interior trim components, glides, seat adjusters, and functional sliding elements made of plastics are evaluated for friction to prevent squeak and rattle noise, ensure smooth operation, and validate long-term durability of sliding pairs.
Medical and Pharmaceutical Packaging
Sterile barrier materials, blister lidding foils, and pouch films are tested for COF to confirm consistent feeding on packaging lines and to support compliance with packaging validation requirements for terminally sterilized devices.
Polymer Engineering Components
Manufacturers of bearings, gears, rollers, conveyor components, and wear pads rely on sustained sliding friction and wear data to select materials, predict service life, and compare lubricated, self-lubricated, and filled polymer grades.
Material Development and Quality Assurance
COF measurement supports the optimization of slip and antiblock additive levels, the verification of corona and flame treatment effects, the qualification of coatings and surface modifications, and the monitoring of batch-to-batch consistency. Accredited third-party laboratories provide ISO 8295, ASTM D1894, and GB/T 10006 testing services that support supplier declarations, incoming inspection, and regulatory submissions.
FAQ
What is the difference between static and kinetic coefficients of friction?
The static coefficient of friction describes the resistance to the initiation of sliding and is derived from the first force peak, while the kinetic coefficient of friction describes the resistance during continuous sliding and is derived from the average force over the steady sliding region. Static COF is normally equal to or higher than kinetic COF, and a large difference between the two often indicates stick-slip behavior.
Which standards apply to plastic sliding friction coefficient testing?
For plastic films and sheeting, the principal standards are ISO 8295, ASTM D1894, GB/T 10006, and the corresponding Japanese standard JIS K 7125. For bulk plastics and sliding components under sustained contact, ISO 6601, GB/T 3960, and ASTM D3702 apply, while ASTM G115 offers general guidance on measuring and reporting friction coefficients.
What COF values are suitable for packaging films?
For automated form-fill-seal operations, film-to-film kinetic COF values of approximately 0.2 to 0.3 are generally considered optimal. Values that are too low cause bags to slide and topple during stacking, whereas values that are too high lead to film jamming, wrinkling, and machine downtime. The target range is always agreed between the film supplier and the converter based on the specific equipment.
How do temperature and humidity affect friction test results?
COF is strongly temperature dependent because slip additives migrate to the surface at rates that vary with temperature, and polymer surfaces soften as temperature rises. Elevated humidity can increase the COF of certain films, notably polyamide-containing structures. For these reasons, testing must be performed in a controlled atmosphere of 23 °C and 50 % relative humidity, and additional high-temperature testing is recommended for films intended for hot-fill or tropical climates.
Can the test be performed plastic against metal?
Yes. Film-to-metal measurements against polished stainless steel simulate transport over machine chutes and forming shoulders, while molded plastics can be tested against steel counterparts under loaded sliding per ISO 6601 or GB/T 3960 to replicate real tribological contact conditions.
How many specimens are needed for a reliable result?
Although minimum requirements vary between standards, best practice is to test at least three to five specimens per surface pairing, using fresh contact areas for each run, and to report the mean, range, and standard deviation. Larger sample numbers are recommended when stick-slip or high variability is observed.
