The dynamic coefficient of friction is a decisive quality parameter that governs how plastic films, sheeting, and converted packaging materials run on high-speed production, printing, and filling lines. In China, this property is determined using the national standard test method GB/T 10006, a sled-type horizontal plane method that is technically aligned with ISO 8295. This article explains the test principle, apparatus, specimen requirements, operating procedure, and calculation of the dynamic coefficient of friction under the national standard, together with typical applications and practical frequently asked questions.
Technical Overview
Friction is the resisting force generated when two surfaces in contact slide relative to one another. The dynamic coefficient of friction, also known as the kinetic or sliding coefficient of friction, is defined as the ratio between the mean friction force measured during steady sliding and the normal force pressing the two surfaces together. In equation form, it is expressed as the sliding friction force divided by the normal force acting through the contact area.
For plastic films and sheeting, the governing Chinese national standard is GB/T 10006, titled Plastics — Film and Sheeting — Determination of Coefficients of Friction. The current edition, GB/T 10006-2021, supersedes the 1988 version and is closely harmonized with the international method ISO 8295. The standard describes a horizontal plane sled method in which a weighted sled is drawn across the surface of a fixed specimen at a constant speed. The first force peak on the force-displacement curve corresponds to the static friction force, while the average force recorded during the subsequent steady sliding phase is used to calculate the dynamic coefficient of friction.
Because the dynamic coefficient of friction directly reflects how a material behaves once it is already in motion, it is the parameter most relevant to the smooth transport of films over rollers, forming collars, guide plates, and sealing jaws on converting and packaging machinery. International equivalents and related methods include ISO 8295 and ASTM D1894, which follow the same fundamental sled-on-plane principle with minor differences in test speed and sled geometry.
Testing Samples
Plastic Films and Sheeting
The primary scope of GB/T 10006 covers polymeric films and thin sheeting, including polyethylene (PE), polypropylene (BOPP, CPP, IPP), polyester (PET), polyamide (PA), polyvinyl chloride, ethylene-vinyl acetate, and their single-layer, coextruded, or metallized variants. Typical specimen thicknesses range from approximately 20 to 200 micrometers, corresponding to the materials used in flexible packaging, lamination, and bag making.
Laminates, Coated Films, and Printed Surfaces
Multi-layer laminate structures, heat-sealable films, lacquered films, cold-seal coated webs, release-coated liners, and printed or surface-treated films can all be evaluated. Testing may be performed on the sealing side, the printed side, or the outer decorative side, since each surface may carry a different slip characteristic. Corona-treated and slip-agent-modified surfaces are frequently compared against untreated references to verify processing performance.
Film Against Counter Materials
Although film-to-film testing of the same surface is the most common configuration, the method also supports film-to-film combinations of different materials and film-to-substrate configurations. A frequent industrial case is the measurement of a heat-seal layer sliding against polished stainless steel, which simulates the behavior of the film on the forming shoulder and vertical guides of a form-fill-seal packaging machine.
Testing Items
A complete friction evaluation under the national standard typically reports the following items, with the dynamic coefficient of friction as the core deliverable:
- Dynamic (kinetic) coefficient of friction, calculated from the mean sliding friction force at constant speed.
- Static coefficient of friction, derived from the first force peak, usually tested in parallel as a companion result.
- Full friction force versus displacement curves, which reveal stick-slip behavior, surface contamination, and sliding instability.
- Directional comparison of results in the machine direction and transverse direction of the web.
- Elevated-temperature sliding performance, optionally measured with a temperature-controlled chamber to simulate hot-fill or summer transport conditions.
Testing Methods
Apparatus and Standard Conditions
The instrument consists of a rigid horizontal flat bed, a drive mechanism capable of a constant test speed of (100 ± 10) mm/min, and a force-measuring system, typically a load cell, with suitable accuracy and data acquisition. The standard sled has a mass of 200 g and a planar contact area of 40 cm², producing a normal force of approximately 1.96 N. The bottom face of the sled carries a compliant backing so that the specimen conforms uniformly to the counterface. Tests are performed in a controlled laboratory atmosphere of (23 ± 2) °C with relative humidity of approximately 50 %, following the conditioning atmosphere specified in GB/T 2918.
Specimen Preparation and Conditioning
For each test, a pair of specimens is required: a bottom strip that is fixed flat onto the test table, usually with double-sided adhesive, and a matching piece bonded to the underside of the sled. Specimens must be cut cleanly, free of wrinkles, creases, fingerprints, dust, and surface contamination, and the tested surfaces must not be touched or cleaned before measurement. Samples are conditioned to equilibrium in the standard atmosphere before testing. Because slip additives in polyolefin films migrate to the surface over time, specimen age and storage history must be recorded and controlled for meaningful comparison.
Procedure and Calculation
The bottom specimen is secured to the horizontal bed so that its surface is flat and free of bubbles or wrinkles. The prepared sled is then placed gently onto the specimen without impact or lateral dragging. The drive is started, and the sled is drawn across the bottom specimen at (100 ± 10) mm/min while the friction force is recorded continuously. From the force-displacement curve, the maximum force of the first peak is taken as the static friction force, and the average force during the steady sliding portion of the stroke, excluding the initial transition and the end-of-travel region, is taken as the dynamic friction force.
The coefficients are then calculated by dividing the respective force by the normal force exerted by the sled. At least three valid determinations are required for each surface combination and each direction, and five determinations are recommended for improved statistical confidence. The test report states the mean and range of both static and dynamic coefficients, the directions tested, the surfaces in contact, the number of determinations, and any deviations from the standard conditions. When a different counter material such as stainless steel is of interest, the same procedure is followed with the counter material replacing the film on either the bed or the sled face.
Applications
Dynamic coefficient of friction data generated under the national standard method support a wide range of industrial decisions in materials development and quality control:
- Film producers use the test to formulate and verify slip agent and antiblock masterbatch levels in polyethylene and polypropylene films for consistent machinability.
- Converting and flexible packaging operations use the results to qualify films for printing, laminating, bag making, and high-speed form-fill-seal lines, where a typical target range is roughly 0.2 to 0.4 against the same surface.
- Packers and brand owners apply the data for incoming inspection and supplier comparison, ensuring reliable feeding, stacking, and transport of pouches and bags.
- Release liner and label stock manufacturers monitor friction to balance smooth die-cutting and dispensing behavior.
- Automotive, hygiene, and medical packaging applications use the method to evaluate interior films, backsheet materials, and sterile barrier webs under both ambient and elevated temperatures.
- Research and development teams compare surface treatments, coatings, and structures, and use friction curves to diagnose stick-slip phenomena that cause web wandering or machine jams.
FAQ
What is the difference between static and dynamic coefficient of friction?
The static coefficient describes the resistance at the instant sliding begins and is calculated from the first peak of the force curve. The dynamic coefficient describes the resistance during continuous sliding and is calculated from the average force in the steady sliding region. The dynamic value is generally lower than the static value and is the more relevant indicator for materials running on moving equipment.
Which national standard applies to dynamic friction testing of plastic films?
GB/T 10006 is the applicable national standard. The current edition, GB/T 10006-2021, adopts the same sled-type horizontal plane principle as ISO 8295 and replaced the earlier GB/T 10006-1988 edition. For export-oriented specifications, ASTM D1894 is a commonly referenced equivalent, noting that it prescribes a nominal test speed of 150 mm/min compared with 100 mm/min under the GB method, so results from the two methods are not automatically interchangeable.
What coefficient of friction values are acceptable for packaging films?
Industry practice for polyolefin packaging films generally targets a coefficient of roughly 0.2 to 0.4 for the outside surface against itself. Values that are too high cause film sticking, poor feeding, and machine jams, while values that are too low can cause film slippage on rollers, telescoping rolls, and unstable palletized stacks. Each application and packaging machine should define its own specification window.
Why do results differ between laboratories or between repeated tests?
Common causes include differences in temperature and humidity, the state of slip agent migration on the film surface, specimen age and storage history, surface contamination from handling, sled mass and speed calibration, and contact pressure variations. Strict control of conditioning, cleanliness, and instrument calibration is essential for reproducible dynamic coefficient of friction results.
Can the GB method be performed at elevated temperature?
Yes. Many modern friction testers offer heated beds and chambers that allow measurement at elevated temperatures, for example 35 °C to 50 °C, to simulate hot filling, heat sealing environments, or summer logistics conditions. Such results should be reported as non-standard supplementary data alongside the standard-condition values, since the reference test conditions in GB/T 10006 are defined at room temperature.