The dry coefficient of friction (COF) is a critical performance and safety parameter for plastic films, floor coverings, textiles, paper, rubber, and engineered surfaces, directly affecting machinability, slip resistance, and product reliability. This article analyzes the principal dry friction testing methods, including the horizontal sled method, the inclined plane method, and tribometer-based techniques, together with the international standards that govern them. It is intended to help manufacturers, packaging engineers, and quality teams select the appropriate method and interpret results correctly.
Technical Overview
Dry friction, also known as Coulomb friction, is the tangential resisting force generated at the interface between two solid surfaces that are in relative motion or on the verge of motion, without any lubricant present. The coefficient of friction (COF) is defined as the ratio of the friction force to the normal force pressing the two surfaces together. It is a dimensionless value that quantifies how easily one surface slides over another.
Two distinct coefficients are measured in practice. The static coefficient of friction characterizes the force required to initiate sliding, while the kinetic (or dynamic) coefficient of friction characterizes the force required to sustain sliding at a given speed. In almost all material systems, the static coefficient is equal to or higher than the kinetic coefficient, which is why an object is harder to start moving than to keep moving.
An essential point for engineers is that friction is a system property rather than an intrinsic material constant. The measured value depends on the mating pair, surface roughness, cleanliness, temperature, humidity, normal load, sliding velocity, and test geometry. For this reason, dry friction testing must follow standardized methods so that results are repeatable within a laboratory and comparable between accredited third-party laboratories.
- Static COF: friction ratio at the onset of relative motion
- Kinetic COF: friction ratio during sustained sliding
- Dry friction: no lubricant, coating, or interfacial fluid deliberately applied
- System dependence: results are valid only for the tested material pairing and conditions
Testing Samples
Plastic Films and Sheets
Polyethylene, polypropylene, PET, and multilayer laminate films used in flexible packaging are the most frequently tested materials for dry COF. Specimens are typically cut into flat strips, tested film-to-film or film-to-metal, and conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618 before testing.
Floor Covering Materials
Ceramic and porcelain tiles, resilient flooring, laminates, and rubber matting are tested for pedestrian slip resistance. Tiles are cut to standard dimensions, cleaned according to the relevant standard, and bonded to a rigid backing to simulate installed conditions.
Rubber and Elastomer Products
Conveyor belt covers, seals, gaskets, shoe soles, and elastomer sheets are die-cut into pads of defined contact area and mounted on the test sled or fixture.
Paper and Paperboard
Printing grades, packaging board, and carton blanks are tested flat and free of curl, since wrinkles and edge damage strongly distort friction readings.
Textiles and Fabrics
Woven, knitted, and nonwoven fabrics are conditioned and mounted without tension or creasing, typically tested fabric-to-fabric or fabric-to-a-standard counterface.
Metals, Coatings, and Engineering Components
Machined coupons, coated panels, polymers, and composites are prepared with documented surface roughness and cleaning procedures, because surface preparation dominates the measured friction of rigid materials.
Testing Items
Depending on the product and the applicable specification, dry friction evaluation typically includes one or more of the following test items:
- Static coefficient of friction (film-to-film, film-to-metal, or material-to-standard counterface)
- Kinetic (dynamic) coefficient of friction over a defined sliding distance
- COF variation with sliding time or distance, reflecting wear-in and running behavior
- COF after thermal aging, UV exposure, or migration of slip additives
- COF as a function of temperature, humidity, normal load, or sliding speed
- Verification against product acceptance limits, such as minimum slip-resistance thresholds for flooring
Testing Methods
Horizontal Plane (Sled) Method
The sled method is the most widely used approach for flexible materials and is specified in ASTM D1894, ISO 8295, and GB/T 10006 (which is technically aligned with ISO 8295). A sled of defined mass carrying the test specimen is drawn across a second specimen or a reference surface at a constant speed, typically in the range of 100 to 150 mm/min depending on the standard. The force transducer records the peak force at the start of motion and the average force during sliding, from which the static and kinetic coefficients are calculated as the force divided by the sled weight. ASTM D1894 and JIS K 7125 follow the same principle for film and sheeting products.
Inclined Plane (Ramp) Method
In the inclined plane method, a specimen is placed on a plane that is gradually tilted until sliding begins. The tangent of the angle at the onset of sliding equals the static coefficient of friction. The method is simple and well suited to paper and board testing, and it also underpins ramp-based floor slip-resistance classifications, such as the shod ramp test with motor oil that assigns R9 to R13 classes and the barefoot ramp test that assigns A, B, and C classes. The method measures only the static coefficient and is sensitive to operator technique, so strict adherence to the tilting rate is required.
Tribometer-Based Methods
For metals, coatings, ceramics, and composites, laboratory tribometers provide the most informative data. In the pin-on-disk configuration of ASTM G99, a stationary pin is loaded against a rotating disk while a force sensor continuously records friction. The block-on-ring method of ASTM G77 is used for ranking engineering material pairs. ASTM G115 provides general guidance for measuring and reporting friction coefficients across geometries. These methods yield friction traces over time, enabling the study of run-in, steady-state friction, and wear transitions under controlled load, speed, and temperature.
Floor Slip-Resistance Methods
Flooring products are evaluated by dedicated procedures. ASTM C1028 measures the static COF of tile and other surfaces using a weighted sled pulled horizontally with a dynamometer. ASTM D2047 determines the static COF of floor polishes using a standardized drag apparatus. ANSI A326.3 measures the dynamic COF of hard-surface flooring, most commonly wet, using a calibrated tribometer. EN 13893 addresses the dynamic COF of resilient floor coverings for CE marking purposes, with a commonly referenced acceptance value of at least 0.30.
Product-Specific Standards
Additional standards cover specialized products, including ISO 21182 for the coefficient of friction of light conveyor belts, and ASTM D3108 and ASTM D3412 for yarn-to-solid and yarn-to-yarn friction respectively. Where a national or sector specification exists, it should take precedence over generic methods.
Factors Influencing Test Results
Regardless of the method chosen, the following factors must be controlled and reported to ensure valid comparison of results:
- Temperature and relative humidity during conditioning and testing
- Normal load (sled mass) and apparent contact area
- Sliding speed and sliding distance
- Surface cleanliness, roughness, and preparation sequence
- Dwell time before sliding, which strongly affects the static coefficient
- Specimen orientation, especially for anisotropic films and fabrics
Applications
Dry friction coefficient testing supports design, safety, and quality decisions across many industries:
- Flexible packaging: optimizing film slip levels for vertical form-fill-seal and high-speed packaging lines, where excessive COF causes jams and insufficient COF causes unstable pallets
- Flooring and construction: demonstrating pedestrian slip resistance for product certification, specification, and regulatory compliance
- Automotive and rail: evaluating brake friction materials, tires, wipers, and interior sliding components
- Conveying and logistics: selecting belt covers and package surfaces with compatible friction behavior
- Textiles: assessing fabric hand, processability in cutting and sewing, and layer-to-layer slip in automated handling
- Medical and pharmaceutical: controlling the surface friction of tubing, bags, gloves, and device components
- Research and development: ranking materials, coatings, and surface treatments under controlled laboratory conditions
- Quality control: verifying incoming materials and monitoring production consistency against defined COF limits
FAQ
Q: What is the difference between static and kinetic coefficient of friction?
A: The static coefficient describes the resistance at the moment sliding starts and is measured from the peak force at the initiation of motion. The kinetic coefficient describes resistance during continuous sliding and is calculated from the average force while motion is sustained. The static value is generally higher, and both values should be reported for materials intended for dynamic handling.
Q: Which standard should be used for plastic packaging film?
A: ASTM D1894, ISO 8295, and GB/T 10006 are the dominant standards, and all are based on the horizontal sled principle. The choice usually depends on the market or customer specification, and it is important to report the standard used, the sliding speed, the sled mass, and whether the test was film-to-film or film-to-metal.
Q: Can friction coefficients from different test methods be compared directly?
A: No. Because friction is a system property, coefficients obtained with different apparatus, loads, speeds, counterfaces, and environmental conditions are not interchangeable. Comparison is only valid when the same method and conditions are used, which is why test reports must fully document the procedure.
Q: How do temperature and humidity affect the results?
A: Elevated temperature typically lowers the surface friction of polymer films as slip additives migrate to the surface, while humidity changes the surface state of paper, textiles, and some flooring materials. Specimens must be conditioned at standard laboratory atmosphere and tested under the conditions specified in the applicable standard.
Q: How many specimens should be tested?
A: Most standards require a minimum of three to five replicate measurements per material pairing, with the mean and range reported. For product certification or dispute resolution, testing is normally performed by accredited third-party laboratories using calibrated equipment and documented procedures.