Dry friction coefficient testing measures the ratio of friction force to normal force between two solid surfaces in contact without lubrication, and it is a critical quality control parameter for plastic films, flooring, rubber, paper, and coated materials. Because friction values are highly sensitive to temperature, humidity, surface preparation, normal load, and sliding speed, the correct setting of test conditions is essential for obtaining reproducible and comparable results. This guide explains the key condition parameters, applicable international standards, specimen requirements, and measurement methodology for dry coefficient of friction determination.
Technical Overview
The dry friction coefficient (COF) is a dimensionless value defined as the ratio of the tangential friction force resisting relative motion to the normal force pressing two surfaces together. Two values are typically distinguished: the static coefficient of friction, calculated from the force required to initiate sliding, and the kinetic (dynamic) coefficient of friction, calculated from the force required to maintain sliding at constant velocity. Both are routinely determined under dry conditions, meaning no lubricant, water film, or contaminant layer is present at the interface.
Unlike intrinsic material properties such as density or melting point, the friction coefficient is a system property: it depends on the pairing of surfaces, the applied pressure, the sliding speed, the dwell time before motion, and the ambient environment. For this reason, condition setting is not a peripheral detail but the core of a valid test. Small changes in humidity, surface contamination, or sled mass can shift results significantly and lead to false acceptance or rejection of a material batch.
Several recognized standardization organizations publish methods that govern dry COF determination and its condition parameters, including ISO, ASTM, GB, and ANSI. The most widely applied documents include:
- ISO 8295 and GB/T 10006 — determination of coefficients of friction for plastics film and sheeting
- ASTM D1894 — static and kinetic coefficients of friction of plastic film and sheeting
- ASTM D2047 — static coefficient of friction of floor polish-coated surfaces using the James machine under dry conditions
- ISO 15113 — determination of frictional properties of rubber
- ISO 21182 — determination of the coefficient of friction for light conveyor belts
- ASTM D4917 and ASTM D4918 — horizontal plane and inclined plane methods for paper and paperboard
Accredited third-party laboratories and in-house quality departments follow these documents to define every element of the test setup, from specimen conditioning atmospheres to sled geometry and data acquisition windows, so that results remain traceable and comparable across sites and over time.
Testing Samples
Plastic Films and Sheeting
Flexible packaging films, including polyethylene, polypropylene, PET, and multilayer laminates, are the most frequently tested materials for dry COF. Slip agents, antiblocking additives, and surface treatment levels strongly influence the measured values, so specimens must be cut from defined zones of the web and identified by machine direction (MD) and transverse direction (TD). Specimens are typically prepared as flat strips bonded to the test plane and as coverings wrapped around the sled face, ensuring the same film surface is tested against itself or against a specified counter-surface.
Rigid Sheets, Flooring, and Laminates
Ceramic tiles, vinyl flooring, engineered wood, laminates, and coated panels are tested to characterize dry slip resistance and machinability. Specimens are cut to fit the horizontal platform, and the counter-face material, such as a standardized leather, rubber, or elastomer pad, is mounted on the test foot. Surface cleanliness and flatness are controlled because wax residues, release agents, or polishing compounds dramatically alter dry friction values.
Rubber and Elastomeric Products
Conveyor belt covers, seals, grips, footwear outsoles, and industrial rubber goods are evaluated for frictional performance under dry contact. Because rubber friction is highly speed- and load-dependent, condition settings must define the normal force, sliding velocity, contact pressure, and the number of conditioning cycles run before the recorded measurement.
Paper, Paperboard, and Packaging Substrates
Printing papers, carton board, and flexible paper laminates are tested to predict feeding and stacking behavior on packaging lines. Specimens are die-cut to standard dimensions and conditioned to equilibrium, since paper friction is especially sensitive to moisture content in the surrounding atmosphere.
Textiles, Coated Fabrics, and Technical Surfaces
Coated fabrics, synthetic leather, touch surfaces, and interior trim materials are measured against defined counter-materials to assess tactile grip and functional slip behavior in dry service conditions.
Testing Items
A dry friction coefficient test program generally reports the following measured and derived quantities:
- Static coefficient of friction (μs) — the ratio of the initial peak force at the onset of sliding to the applied normal force
- Kinetic coefficient of friction (μk) — the ratio of the average steady-state friction force to the applied normal force
- Force-displacement curves showing the breakaway peak and the plateau region used for kinetic averaging
- Directional results, such as MD versus TD for films, and side A versus side B for laminates
- Statistical summaries including mean, standard deviation, minimum, and maximum across replicate specimens
- Conditioning and test environment records (temperature and relative humidity) accompanying each data set
For quality control purposes, acceptance thresholds are commonly defined separately for static and kinetic values. For example, many packaging film specifications require a kinetic dry COF between 0.20 and 0.35 to balance machinability against slip risk on filling lines.
Testing Methods
Specimen Preparation and Conditioning
All specimens are cut with clean, burr-free edges using gloves or tweezers to prevent transfer of skin oils. Films are bonded flat to the plane and sled using double-sided adhesive, avoiding wrinkles, bubbles, and stretch. Specimens are then conditioned in a standard atmosphere according to ISO 291 or an equivalent standard, typically 23 ± 2 °C and 50 ± 10 % relative humidity, for a minimum period defined by the applicable method, commonly no fewer than 24 to 40 hours depending on material thickness. Conditioning brings surfaces to an equilibrium moisture state so that measured friction reflects the material rather than transient environmental exposure.
Environmental Condition Setting During the Test
The test itself is performed in the same controlled atmosphere in which specimens were conditioned. The laboratory records temperature and relative humidity at the time of measurement and verifies that the apparatus is level and free from vibration. Air currents, direct sunlight, and nearby heat sources are excluded because surface temperature drift alters the friction of polymers. For materials prone to static charging, such as polyolefin films, charge control measures are applied, since accumulated electrostatic force adds an uncontrolled normal load component and inflates apparent friction values.
Normal Load and Sled Configuration
The normal force is generated by a calibrated sled whose mass and contact geometry are fixed by the standard. For plastic film testing under ASTM D1894 and ISO 8295, a sled mass of 200 g with a square contact face on the order of 40 mm to 65 mm per side is typical. The sled must be parallel to the plane so that pressure is distributed uniformly; edge loading, tilted mounting, or debris under the specimen invalidates the run. For flooring and rubber applications, heavier standard loads and specified counter-face pads are used, and the load is documented in the report because polymer friction is pressure-dependent.
Sliding Speed and Travel Distance
The crosshead or table speed is set to the standard-specified value: for example, 150 ± 15 mm/min under ASTM D1894 and 100 ± 10 mm/min under ISO 8295 for films. The travel distance must be long enough to capture a stable kinetic plateau after the breakaway peak, while remaining short enough to avoid frictional heating of the interface. Speed changes during a run, ramp-up transients, and stop-and-restart cycles are excluded from the averaging window.
Dwell Time and Surface Pairing
The interval between applying the sled and starting motion is controlled because static friction for many polymers increases with rest time. Specimens are paired according to the test plan, either like-on-like or against a defined counter-material such as stainless steel, leather, or a standard elastomer. Each surface is used for the number of runs permitted by the method before replacement, since repeated sliding polishes or transfers additives and changes the tribological system.
Instrument Calibration and Verification
Before testing, the load cell is calibrated against certified reference weights, the speed is verified with a traceable timer or encoder, and the flatness of the plane is checked. Force zeroing is performed with the sled disengaged, and the system alignment is confirmed so that only horizontal friction force, not vertical load or cable angle, is measured.
Data Acquisition and Calculation
The static coefficient is computed as the first peak force divided by the normal force, and the kinetic coefficient as the mean force over the defined steady-state displacement interval divided by the normal force. A minimum number of valid replicates, typically three to five per condition and direction, are run, and outliers are treated according to the statistical rules of the standard. The final report lists the individual and mean values, the standard deviation, the full condition settings, and any deviations from the referenced method.
Applications
Condition-controlled dry COF testing supports decision-making across many industries:
- Flexible packaging — verifying slip agent performance so films run reliably on vertical and horizontal form-fill-seal machines without jams or bag stacking failures
- Flooring and building products — demonstrating dry slip characteristics for safety assessments of tiles, laminates, and resilient flooring in commercial and residential spaces
- Rubber and conveying technology — matching belt cover friction to drive and load requirements for light conveyor belts and rollers
- Paper and print converting — predicting sheet feeding, stacking, and transport behavior on high-speed lines
- Automotive and interiors — evaluating grip surfaces, trim materials, and functional sliding components in dry cabin conditions
- Medical and pharmaceutical packaging — confirming that pouches, lids, and films feed correctly through sterile packaging and dosing equipment
- Electronics manufacturing — assessing trays, carriers, and protective films where excessive slip or excessive grip disrupts automated handling
Manufacturers also use dry COF data for incoming material inspection, additive development, process troubleshooting, and comparative benchmarking against competitor or legacy materials.
FAQ
What is the difference between static and kinetic dry friction coefficient?
The static coefficient is calculated from the peak force needed to start sliding from rest, while the kinetic coefficient is calculated from the lower, steadier force needed to maintain sliding. Static values are almost always equal to or higher than kinetic values, and both must be reported with their condition settings to be meaningful.
Why must specimens be conditioned at 23 °C and 50 % relative humidity?
Polymer films, paper, and elastomers absorb or desorb moisture, and ambient temperature softens or stiffens surfaces; both effects change friction. Conditioning in the standard atmosphere defined by ISO 291 or equivalent ensures that measured values reflect the material in a defined equilibrium state and remain comparable between laboratories and over time.
What sled weight and sliding speed are used for plastic film testing?
Under ASTM D1894 a 200 g sled with an approximately 63.5 mm square face is driven at 150 mm/min, while ISO 8295 uses a 200 g sled at 100 mm/min. Because these settings differ, results from the two standards must not be compared directly without documented conversion studies.
How many specimens should be tested?
Most methods require a minimum of three to five valid replicates per surface pairing and direction, reporting the mean and standard deviation. More replicates are recommended for materials with known variability, such as films with migrating slip additives.
How is surface contamination prevented?
Operators handle specimens only with clean gloves or tweezers, avoid breathing directly on test surfaces, keep specimens covered until the moment of testing, and never clean surfaces with solvents unless the standard explicitly permits it, since solvents can extract or redistribute additives and distort the friction system.
Can dry COF results be used to predict wet slip resistance?
No. Dry and wet friction are governed by different mechanisms, and lubricating films can raise or lower friction unpredictably. Wet or contaminated slip resistance must be assessed with dedicated methods and condition settings appropriate to the wet interface.