The dynamic coefficient of friction under dry conditions quantifies the sliding resistance between two unlubricated surfaces and is a decisive parameter for material selection, machinability, safety, and quality control in the packaging, flooring, polymer, rubber, and automotive industries. This article explains the testing principle, applicable international standards, sample requirements, measurement procedure, data interpretation, and typical applications of kinetic friction determination, providing engineers and quality professionals with a practical reference for obtaining accurate and reproducible results.
Technical Overview
The dynamic coefficient of friction (also called the kinetic coefficient of friction) is defined as the ratio of the tangential friction force required to maintain steady relative sliding between two surfaces to the normal force pressing them together. When the measurement is performed with no lubricant, contaminant film, or interfacial medium between the mating surfaces, it is referred to as friction under dry friction conditions. The parameter is dimensionless and is typically denoted as μd or μk.
During a sliding test, the force signal typically shows an initial peak corresponding to the breakaway (static) force, followed by a lower, fluctuating plateau representing the sustained sliding force. The dynamic coefficient is calculated from this plateau region, while the static coefficient is calculated from the initial peak. Because the dynamic value governs continuous motion behavior, it is the key figure for processes such as film unwinding, product conveying, sliding of packaged goods, and pedestrian gait dynamics.
Dry friction determination is standardized in several international standards, including ASTM D1894 (plastic film and sheeting), ISO 8295 (plastics film and sheeting), GB/T 10006 (Chinese adoption of ISO 8295), ASTM D4917 (uncoated printing and writing paper), ISO 15113 (rubber), and EN 13893 (resilient, laminate, and textile floor coverings). Accredited third-party laboratories routinely perform these tests to support product development, incoming inspection, and regulatory compliance.
Testing Samples
Plastic Films and Flexible Packaging Materials
Polyethylene (PE), polypropylene (PP), PET, PA, and multilayer laminate films are the most frequently tested materials. Dynamic friction directly determines runnability on vertical form-fill-seal machines, horizontal wrappers, and bag-making equipment, where excessive friction causes web tracking faults, stretching, and jams.
Rubber and Elastomeric Products
Seals, gaskets, conveyor belts, tires, gloves, and anti-slip components are evaluated under dry sliding against glass, steel, or counterface polymers to characterize grip, wear interaction, and tactile performance.
Paper and Paperboard
Printing paper, carton board, and packaging papers are tested to ensure reliable feeding in printers, copiers, and cartoning lines, as well as stack stability during transport.
Flooring, Footwear, and Surface Finishes
Resilient flooring, laminates, textiles floor coverings, coatings, and treated surfaces are measured under dry conditions to assess pedestrian slip potential and to demonstrate conformity with safety-related requirements.
Metal, Coated, and Composite Components
Sheet metal with drawing or forming coatings, painted panels, powder-coated parts, and polymer-molded components are characterized to predict assembly behavior, sliding fit, squeak-and-rattle propensity, and wear performance.
Testing Items
Typical test items and reported quantities include:
- Dynamic (kinetic) coefficient of friction under dry conditions, μd
- Static (breakaway) coefficient of friction, μs, as a companion measurement
- Friction force versus sliding distance curves, revealing stick-slip behavior and surface uniformity
- Both-side pairings, for example film-to-film and film-to-metal (stainless steel sled)
- Directional dependence, such as machine direction versus transverse direction for oriented films
- Conditioned tests at specified temperature and humidity, including elevated-temperature measurements where relevant
- Statistical summary of replicate measurements, typically the mean value, standard deviation, and coefficient of variation
Testing Methods
Horizontal Plane (Sled) Method
The most widely used configuration follows the principle of ASTM D1894 and ISO 8295. A flat test specimen is secured on a horizontal stage, and a second specimen is attached to the underside of a sled of defined mass and contact area. The sled is connected to a load cell through a thin, flexible filament, and the stage (or the sled) is driven at a constant speed, commonly in the range of 100 to 150 mm/min depending on the standard. The drive speed, normal force, contact geometry, and sliding distance are held constant throughout the measurement.
Inclined Plane Method
In this approach, a plane carrying one specimen is gradually tilted until a slider on the opposing specimen begins to move. While primarily yielding the static coefficient, the method is sometimes used for screening; the horizontal plane method remains the reference technique for kinetic friction.
Specimen Preparation and Conditioning
Specimens are cut to the dimensions prescribed by the applicable standard, free from creases, folds, dust, fingerprints, and surface contaminants. Both surfaces are conditioned in a standard laboratory atmosphere, typically 23 °C ± 2 °C and 50 % ± 5 % relative humidity per ISO 291 or an equivalent ASTM conditioning practice, for a minimum conditioning period before testing. Handling with clean gloves or tweezers is mandatory because skin oils can markedly alter the measured friction.
Test Procedure and Calculation
The procedure generally consists of the following steps:
- Verify calibration of the load cell, drive speed, and sled mass before testing
- Mount the specimen pair with the defined test surfaces in contact, without entrapped air bubbles or wrinkles
- Apply the sled gently to avoid impact loads, then start the drive at the specified constant velocity
- Record the friction force continuously over the specified sliding distance
- Identify the first peak force as the static force and compute the average plateau force as the dynamic friction force
- Calculate μd as the average dynamic friction force divided by the total normal force (sled weight plus any added load)
- Repeat on multiple specimen pairs and report the statistical summary together with the force-distance trace
Factors Affecting Results
Friction is a system property rather than an intrinsic material constant. Reproducible results require control of surface roughness and cleanliness, temperature and humidity, sliding speed, normal load, contact pressure and geometry, specimen age and storage history, sliding distance and prior run-in, and wear debris generation during the test. Stick-slip oscillation, common with viscoelastic materials at low speeds, should be noted because it affects the plateau force evaluation.
Applications
Determination of the dry dynamic coefficient of friction supports a broad range of industrial purposes:
- Flexible packaging: qualifying films for high-speed filling and wrapping lines, where dynamic friction values of approximately 0.2 to 0.3 or below are commonly targeted for machine-direction runnability
- Quality control: verifying slip agents, antiblock masterbatches, coatings, and surface treatments in production consistency checks
- Consumer and medical products: optimizing grip and donning performance of gloves, syringe plungers, tubing, and device sliding interfaces
- Automotive and electronics: tuning interior trim friction to prevent squeak and rattle, and evaluating sliding components and connectors
- Floor safety: benchmarking dry slip resistance of floor coverings and surface treatments against referenced acceptance levels
- Materials R&D: comparing polymer formulations, textures, lacquers, and surface modification technologies during development
- Supplier agreements and certification: providing objective, standard-based test data for incoming goods inspection and technical datasheets
FAQ
What is the difference between static and dynamic coefficient of friction?
The static coefficient is derived from the force needed to initiate sliding from rest, while the dynamic coefficient is derived from the lower force needed to sustain sliding. Packaging machinery, conveyors, and slip-related safety assessments often rely on the dynamic value because it describes behavior during continuous motion.
Why must the test be performed under dry conditions?
Lubricants, moisture films, dust, and skin oils dramatically change interfacial shear strength. Specifying dry friction conditions isolates the intrinsic surface interaction of the material pair and makes results comparable between laboratories and over time.
What dynamic friction value is considered acceptable?
Acceptance levels depend entirely on the application. High-speed packaging films generally require low dynamic friction for smooth feeding, whereas grip surfaces, flooring, and footware outsoles may require higher values for safety. Acceptance criteria should always be agreed between supplier and purchaser with reference to the test standard.
How many measurements are required?
Standards typically require testing of multiple specimen pairs (for example, three to five or more replicates) with the mean and standard deviation reported, since friction data scatter is inherently larger than for many other mechanical properties.
Can testing be performed at elevated temperature or against a specific counterface?
Yes. Conditioned and elevated-temperature measurements, as well as sliding against specified counterfaces such as stainless steel, glass, or a mating plastic, are common customized test configurations. The report must clearly state the counterface, temperature, humidity, speed, and normal load so that results are correctly interpreted.