Determination of Speed Effects on the Coefficient of Friction of Packaging Materials

The coefficient of friction (COF) of packaging films and laminates directly governs machinability on high-speed filling, forming, and sealing lines, yet the kinetic COF changes measurably with sliding speed. Determining the speed dependence of COF in accordance with recognized methods such as ISO 8295, ASTM D1894, and GB/T 10006 allows converters and brand owners to align laboratory data with real production conditions. This article explains the principle, instrumentation, test procedure, data evaluation, and practical applications of multi-speed COF determination for packaging materials.

Technical Overview

The coefficient of friction (COF) is the ratio between the tangential force required to slide one surface against another and the normal force pressing them together. For flexible packaging materials, COF is a critical functional property: if friction is too high, films jam, stretch, or wrinkle on vertical or horizontal form-fill-seal machines; if friction is too low, webs slip on drive rollers, pouches slide during stacking, and printed layers smear. Laboratory COF measurement distinguishes the static coefficient (the force needed to initiate sliding) from the kinetic coefficient (the force needed to maintain sliding at a constant velocity).

Because polymeric materials are viscoelastic, the kinetic COF is not a single fixed number but a function of sliding velocity. Surface-active additives such as erucamide and oleamide slip agents, antiblock particles, lacquers, and adhesive residues further modify how friction responds to speed. A film that performs acceptably when tested at the standard laboratory speed of 100 mm/min (ISO 8295) or 150 mm/min (ASTM D1894) may behave very differently at the 0.5–2 m/s surface velocities encountered on modern high-speed packaging lines. Determining the speed influence on COF therefore involves measuring static and kinetic friction across a programmed series of sliding speeds and evaluating the resulting friction–velocity profile.

• ISO 8295 — Plastics; film and sheeting; determination of coefficients of friction
• ASTM D1894 — Standard test method for static and kinetic coefficients of friction of plastic film and sheeting
• GB/T 10006 — Determination of friction coefficients of plastic film and sheeting (Chinese national standard, technically aligned with ISO 8295)
• TAPPI T549 — Coefficients of static and kinetic friction of paper and paperboard (horizontal plane method)

Testing Samples

Polyolefin Films

Single-layer and coextruded LDPE, LLDPE, mLLDPE, CPP, and BOPP films represent the largest sample group. These materials typically contain migratory slip agents whose concentration at the surface evolves over days or weeks after extrusion, so both the absolute COF value and its speed dependence can shift with sample age. Specimens are normally tested in both machine direction (MD) and transverse direction (TD), and in film-on-film as well as film-on-stainless-steel pairings.

Barrier Films and Multilayer Laminates

PET, PA (nylon), metallized films, aluminum foil laminates, and coated or lacquered structures are tested to characterize sealing-side and printing-side friction. Rigid barrier layers and cured adhesives usually exhibit less slip-agent migration but stronger viscoelastic speed sensitivity, and some laminates display pronounced stick-slip at intermediate speeds that only appears when velocity is varied.

Paper, Paperboard, and Other Substrates

Coated and uncoated papers, carton board, and paper-plastic composites are evaluated for feeding, folding, and stacking behavior. Surface smoothness, moisture content, and coatings dominate friction behavior, and dedicated paper friction methods (for example TAPPI T549) are applied alongside plastic film procedures when relevant.

Testing Items

Depending on the material specification and the end-use packaging machine, the following items are determined:

• Static coefficient of friction, µs, at the reference speed of the applicable standard
• Kinetic coefficient of friction, µd, at the reference speed (100 mm/min per ISO 8295 and GB/T 10006; 150 mm/min per ASTM D1894)
• Kinetic COF speed sweep across a programmed series of sliding speeds, commonly 10, 30, 50, 100, 150, 300, and 500 mm/min, extended up to 1000 mm/min for high-speed line simulation
• Friction–velocity profile and derived speed sensitivity index, for example the ratio µd(500 mm/min)/µd(100 mm/min) or the slope of µd versus the logarithm of speed
• Stick-slip amplitude, critical onset speed, and force-trace oscillation characteristics
• Directional comparison (MD versus TD) and pairing comparison (film-on-film versus film-on-metal)
• Optional influence factors studied on the same platform: temperature, conditioning time after production, and contact pressure

Testing Methods

Specimen Preparation and Conditioning

Test pieces are cut flat and wrinkle-free from representative regions of the roll, avoiding damaged edges and printed registration areas. The specimen mounted on the fixed plane is typically a strip at least 80 mm wide and 200 mm long, while the sliding specimen is attached to the base of the sled. Both surfaces must be free of fingerprints, dust, and creases, and only the test surfaces may be handled. Specimens are conditioned in the standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity (ISO 291) for a minimum of 40 hours, or longer where specifications require, because slip-agent bloom and moisture uptake are equilibrium processes.

Apparatus

A horizontal-plane COF tester is used, comprising a flat, rigid test bed, a sled of 200 g total mass carrying a flat pressure foot, a low-inertia drive mechanism with programmable velocity, and a calibrated force-measuring system (load cell) with suitable data acquisition. The instrument must deliver a smooth, constant-velocity traverse with negligible vibration, since stick-slip artifacts can be generated by the machine itself if the drive is not sufficiently stiff. A rigid stainless-steel or glass reference plane is used when film-on-metal friction is required.

Test Procedure

• Mount the plane specimen on the test bed, securing the ends so the surface lies flat without stretching.
• Attach the sled specimen, position the sled on the plane, and allow the specified pre-contact dwell time (commonly 15 seconds to several minutes, as required by the specification) before motion starts, because dwell time affects the static peak.
• Run the traverse at the selected speed over a travel distance sufficient to capture the initial static peak and at least 60–70 mm of steady sliding for kinetic averaging.
• Record the force–displacement curve; the maximum force at the onset of sliding gives the static friction force Fs, and the mean force during steady sliding gives the kinetic friction force Fd.
• Repeat the measurement at each programmed speed on fresh, non-overlapped contact tracks, with a minimum of three (preferably five) valid replicates per speed and per direction.
• Calculate the coefficients as µs = Fs/(m·g) and µd = Fd/(m·g), where m·g is the normal force exerted by the 200 g sled (approximately 1.96 N).

Data Reduction and Expression of Results

For each speed, the mean, standard deviation, and, where applicable, the maximum and minimum kinetic force within the oscillating trace are reported. The speed influence is expressed by plotting µd against sliding speed (often on a logarithmic velocity axis) and by calculating a speed sensitivity parameter. A pronounced negative slope, rising slope, or pronounced stick-slip band identifies films whose laboratory reference COF will not predict performance at production speeds. Test reports state the standard followed, sled mass, dwell time, test speeds, travel distance, specimen age and conditioning, surface pairing, and direction of sliding relative to MD/TD.

Applications

• Verification of incoming film against supplier COF specifications, including confirmation that the specification holds at more than one speed
• Slip-agent masterbatch optimization, dose screening, and maturation studies that track how COF and its speed dependence evolve in the weeks after extrusion
• Predicting runnability on vertical and horizontal form-fill-seal machines, flow wrappers, bagmakers, and pouch lines where surface speeds far exceed standard laboratory speeds
• Diagnosis of field problems such as web drift, film sticking on forming shoulders, inconsistent bag length, and pouch stacking instability
• Comparison of lacquer, coating, and adhesive systems in laminate development, including sealant-layer selection
• Quality-control trending for converters, and generation of technical data supporting material qualification by accredited third-party laboratories and end users

FAQ

Why does the coefficient of friction change with test speed?

Polymers are viscoelastic, so the real contact area and deformation behavior under the sliding interface depend on how long the asperities remain in contact, which is governed by velocity. In lubricated polyolefin films containing migratory slip agents, kinetic COF frequently decreases as speed rises because boundary lubrication becomes more effective; in unlubricated or highly adhesive films, friction may instead rise with speed. Only direct measurement across a speed range reveals which behavior applies to a given material.

What is the difference between static and kinetic COF, and which one depends on speed?

Static COF characterizes the force needed to start sliding, while kinetic COF characterizes sustained sliding. Static COF is inherently a single-event value and is not speed-defined, whereas kinetic COF is velocity-dependent and must always be quoted together with the sliding speed at which it was measured.

What COF range is generally considered processable?

For most polyolefin packaging films, a kinetic COF between about 0.20 and 0.30 (film-on-film, sealing side to sealing side) provides a good balance between controlled transport and reliable feeding. Values below roughly 0.15 can cause web slip and stacking problems, while values above roughly 0.35 increase the risk of jams and film stretching. The acceptable window should nevertheless be confirmed at the speeds relevant to the actual packaging line.

Do laboratory speeds represent real packaging line conditions?

Not fully. Standard methods were designed for repeatability, not line-speed simulation, which is precisely why multi-speed determination is valuable. Extending the sweep to 500 or 1000 mm/min narrows the gap, and combining the speed profile with machine trials gives the most reliable runnability prediction.

What is stick-slip and why does it matter?

Stick-slip is a periodic seizure-and-release pattern visible as oscillation in the force trace. It typically appears within a certain velocity window and causes jerky film feed, inconsistent sealing pressure, and intermittent jams on packaging machines. A speed sweep identifies the critical speeds at which stick-slip occurs so they can be avoided in production.

How should samples be handled to obtain reproducible results?

Specimens must be conditioned to equilibrium at 23 °C and 50% RH, protected from surface contamination, tested on fresh contact tracks, and reported together with their age, because slip-agent migration continues after production. Consistent dwell time, sled mass, sliding direction, and surface pairing are essential for comparing data between laboratories.