Coefficient of friction (COF) testing measures the slip characteristics of plastic films, sheets, and molded components, delivering essential data for packaging machinability, handling performance, and tribological design. Testing is performed by accredited third-party laboratories in accordance with recognized standards such as ASTM D1894, ISO 8295, and GB/T 10006. This article explains the test principle, sample requirements, measurement procedures, typical applications, and answers to frequently asked questions.
Technical Overview
The coefficient of friction (COF) is a dimensionless value defined as the ratio of the tangential force required to move one surface across another to the normal force pressing the two surfaces together. Two parameters are measured for plastic materials: the static coefficient of friction (µs), which describes the resistance at the exact onset of sliding, and the kinetic coefficient of friction (µk), which describes the resistance during continuous relative motion. For most polymer pairings the static value is equal to or slightly higher than the kinetic value, and an excessive gap between the two can cause stick-slip behavior, squeaking, and erratic feeding on processing lines.
Because plastics are viscoelastic materials, their frictional response depends not only on the base polymer but also on crystallinity, surface roughness, and the additives present in the formulation. Migratory slip agents (such as erucamide and oleamide), antiblock particles, plasticizers, fillers, coatings, and surface treatments such as corona or flame treatment all influence the measured COF. Temperature, sliding speed, contact pressure, humidity, and specimen age further affect the result, which is why standardized conditioning and test parameters are essential for generating comparable data.
Typical kinetic COF reference ranges for common plastic surfaces include the following:
- PTFE and other fluoropolymers: approximately 0.05 to 0.15
- Polyolefin films containing slip agents (PE, PP): approximately 0.10 to 0.25
- PET and PA films: approximately 0.30 to 0.50
- Untreated polyolefin surfaces: 0.50 and above
- POM (acetal) sliding components: approximately 0.20 to 0.35
Testing Samples
Flexible Films and Laminates
Single-layer and multilayer films used in flexible packaging, including LDPE, LLDPE, CPP, BOPP, BOPET, BOPA, and metallized or coated laminates, represent the largest category of COF test specimens. Films are usually tested in both film-to-film and film-to-metal configurations, with the sliding direction documented as machine direction (MD) or transverse direction (TD).
Rigid and Semi-Rigid Sheets
Extruded or calendered sheets of PVC, PET, PS, PP, and PC used for thermoforming, cards, and graphic applications can be evaluated using the same sled-based principle, provided the material can be mounted flat and free of curvature.
Molded and Machined Components
Gears, bearings, bushings, clips, rollers, and guides made from engineering plastics such as POM, PA, PEEK, PTFE-filled compounds, and internally lubricated grades are tested using plate-on-plate or thrust-washer configurations that better represent bulk material behavior under realistic loads.
Coated, Treated, and Special Surfaces
Silicone-coated release liners, lacquered sealing layers, corona- or flame-treated webs, anti-fog and anti-static coatings, and heat-sealable lacquers are tested to verify that the treatment or coating delivers the intended frictional performance.
Testing Items
Standard COF test programs for plastic materials typically include the following measurements:
- Static coefficient of friction (µs), the ratio measured at the moment sliding initiates
- Kinetic coefficient of friction (µk), the ratio measured during steady-state sliding
- Film-to-film COF, covering same-surface or mating-surface pairings such as inner side against inner side
- Film-to-metal COF, simulating contact with the polished steel or anodized surfaces of packaging machinery
- Directional testing in machine direction (MD) and transverse direction (TD)
- Temperature-dependent COF, for example at 35 to 45 °C to simulate heat-sealing zones or warm climates
- Aging studies tracking COF development over days to weeks after production, relevant for migratory slip agents
Testing Methods
ASTM D1894 (Sled Method for Films and Sheeting)
In the ASTM D1894 procedure, a square specimen (63.5 mm by 63.5 mm) is attached to the underside of a sled of defined mass (standard 200 g, giving a normal force of approximately 1.96 N), and the sled is dragged at a constant speed of 150 mm/min across a second specimen mounted on a rigid flat plane, or across a standard metal surface. The force transducer records the initial peak force (static force) and the average force during sliding (kinetic force). The coefficients are calculated as µs = Fs/N and µk = Fk/N, where N is the normal force.
ISO 8295 and GB/T 10006
ISO 8295 and its Chinese counterpart GB/T 10006 apply the same horizontal sled principle. Results are broadly comparable with ASTM D1894, although the standards differ in details such as sliding speed, specimen dimensions, and the kinetic averaging window. These methods are widely specified for film qualification in Asia and Europe.
ISO 6601 and ASTM D3702 (Bulk Plastics and Sliding Components)
For injection-molded or machined parts, ISO 6601 defines a general pin-on-disc or plate-on-plate tribometry method under controlled load, speed, and temperature, while ASTM D3702 uses a stationary thrust washer against a rotating mating washer to evaluate self-lubricating plastics. These methods also yield wear data, which complements the friction measurement.
Specimen Preparation and Conditioning
Specimens must be free of wrinkles, creases, and contamination and should be cut with clean edges. Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity (per ISO 291 or ASTM D618) for a minimum of 40 hours before testing. Test surfaces must not be touched, wiped, or cleaned with solvents, since skin oils and residues measurably alter the result.
Data Reporting
A compliant test report states the test standard, specimen identification and side (inner or outer, treated or untreated), the surface pairing (film-to-film or film-to-metal), sliding direction, conditioning history, individual and mean static and kinetic COF values, and the test temperature. Most specifications require at least three to five valid replicate measurements per condition.
Applications
COF data directly influence material selection, formulation tuning, and line-speed settings across many industries:
- Flexible packaging: ensuring films run reliably on vertical form-fill-seal and horizontal flow-wrap machines, where kinetic COF below roughly 0.2 to 0.3 is commonly targeted on the machine-contact side
- Bag and pouch production: balancing low inner-surface friction for easy opening with sufficient outer-surface friction for stable stacking and palletizing
- Automotive: controlling the tactile feel and squeak-free performance of interior trim and the friction of sliding components such as seat rails and console parts
- Medical devices: verifying the lubricity of catheter surfaces, tubing, and IV bag films for handling and downstream processing
- Engineering components: qualifying low-friction grades of POM, PA, PEEK, and PTFE composites for gears, bearings, and wear pads
- Converting operations: setting corona treatment levels and slip-agent concentrations so that printing, laminating, and metallizing processes remain stable
- Quality control: monitoring batch-to-batch consistency of slip and antiblock additive dosing during film extrusion
FAQ
What is the difference between static and kinetic coefficient of friction?
The static coefficient of friction (µs) is measured at the instant motion begins, while the kinetic coefficient of friction (µk) is measured while sliding continues at constant speed. In practice, µs reflects how hard it is to start movement, which is relevant to bag opening and roll unwinding, whereas µk reflects how easily a surface keeps moving, which governs machine feeding and conveying behavior.
Which standard should be specified for plastic film COF testing?
ASTM D1894, ISO 8295, and GB/T 10006 are the most widely used horizontal sled methods for films and sheeting and produce broadly comparable values. ISO 6601 or ASTM D3702 are appropriate when molded or self-lubricating components must be evaluated under load. The choice is usually driven by the end market or the customer's technical datasheet.
Why does the COF of polyethylene film change after production?
Slip agents such as erucamide are incorporated in the polymer melt and migrate to the surface over hours to weeks after extrusion. Immediately after production the COF can be relatively high; it typically decreases as the additive blooms to the surface, and it may rise again if excess additive forms a weak boundary layer. COF specifications are therefore often referenced to a defined aging period.
What specimen size and quantity are needed?
For ASTM D1894, the sled specimen is a 63.5 mm square, and the plane specimen is a flat piece long enough to allow the specified sliding travel and slightly wider than the sled, typically around 250 mm long by 130 mm wide. Specimens should be supplied with at least three to five replicates per surface pairing and direction, taken from representative production samples.
Can COF be measured at elevated or low temperatures?
Yes. Accredited third-party laboratories can mount the sled apparatus in a temperature-controlled chamber to measure COF at sub-ambient or elevated temperatures, which matters for packaging lines operating near heat-sealing zones and for components used in cold-chain or under-hood environments. Because polymer friction is strongly temperature-dependent, the test temperature must always be reported.
What COF value indicates a problem on a packaging line?
There is no universal pass or fail limit, but as a practical guideline a kinetic COF above approximately 0.3 on the machine-contact side of a film often causes drag, film stretching, and misregistration on high-speed form-fill-seal equipment, while a very low outer-surface COF can cause slippage during stacking. Acceptance criteria should be agreed between supplier and converter based on the actual equipment in service.