The friction coefficient of packaging materials is a critical parameter that determines runnability on high-speed filling, forming, and sealing lines as well as stacking stability during transport and storage. This article provides a comprehensive technical overview of static and kinetic friction coefficient determination for plastic films, paper, foil, and laminates in accordance with ISO 8295, ASTM D1894, and GB/T 10006. It covers testing principles, sample preparation, instrumentation, influencing factors, typical acceptance values, and practical applications in packaging quality control.
Technical Overview
Friction coefficient (COF) measurement quantifies the slip properties of packaging surfaces by determining the ratio of the tangential force resisting relative motion between two contacting surfaces to the normal force pressing them together. For flexible packaging materials, this property directly governs how a film or sheet behaves during unwinding, forming, filling, sealing, folding, and stacking operations. Two distinct values are measured: the static coefficient of friction, which describes the force required to initiate movement, and the kinetic coefficient of friction, which describes the force required to sustain movement at a constant speed.
Most commercial packaging lines operate at high speeds, and materials with unsuitable friction behavior can cause misfeeds, jams, film wandering, wrinkling, inconsistent sealing, and unstable pallet loads. If the coefficient of friction is too low, packages may slide off stacking surfaces or slip out of alignment in the forming collar; if it is too high, films may block on the roll, seize in converting equipment, or generate excessive drag that stalls the machine. For these reasons, friction coefficient determination is a routine quality control test performed by packaging manufacturers, converters, brand owners, and accredited third-party laboratories worldwide.
The internationally recognized test methods are based on the horizontal plane sled principle. The principal standards include ISO 8295 (Plastics – Film and sheeting – Determination of the coefficients of friction), ASTM D1894 (Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting), and GB/T 10006, the Chinese national standard technically aligned with ISO 8295. For paper and paperboard, related slip test methods such as those published by ISO and TAPPI are also applied.
Testing Samples
Plastic Films and Sheets
Single-layer and multilayer polymer films are the most frequently tested materials. These include polyethylene (PE), polypropylene (PP, both cast and oriented CPP/OPP), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), ethylene vinyl alcohol (EVOH) containing structures, and sealant films. Test specimens are typically cut from production rolls in both the machine direction (MD) and transverse direction (TD), because molecular orientation can produce anisotropic friction behavior.
Paper and Paperboard
Kraft paper, coated papers, and folding boxboard are tested to evaluate feeding performance on cartoning and wrapping machines as well as the stability of stacked cartons. Paperboard friction is particularly important for erecting, folding, and gluing operations in carton converting lines.
Aluminum Foil and Metalized Substrates
Bare and lacquered aluminum foils, vacuum-metalized films, and foil-containing laminates are evaluated for friction against themselves and against machine contact surfaces. Foil laminates for blister packaging and pharmaceutical strip packs require controlled slip for reliable feeding through high-speed packaging equipment.
Laminates, Coated Films, and Printed Surfaces
Converted structures such as PET/PE, BOPP/CPP, PA/PE, and paper/Al/PE laminates are tested after lamination, printing, coating, or surface treatment, since inks, adhesives, varnishes, and corona or flame treatment significantly alter surface friction. Aged and heat-treated samples are also tested to study the migration kinetics of slip agents over time.
Testing Items
Typical test items and reported parameters for friction coefficient determination include:
- Static coefficient of friction, film-to-film (same surface against itself)
- Kinetic coefficient of friction, film-to-film
- Static and kinetic coefficients of friction, film-to-standard metal surface (commonly polished stainless steel, simulating packaging machine contact)
- Friction coefficient of sealing or treatment surface versus non-sealing surface
- COF variation after heat aging, to characterize slip agent bloom and migration
- COF at elevated temperatures, relevant to hot-filling and heat-sealing processes
- Directional comparison of MD versus TD slip behavior
- Blocking assessment, where film layers adhere under pressure and temperature
Testing Methods
Test Principle and Instrumentation
The measurement is performed on a dedicated coefficient-of-friction tester consisting of a smooth horizontal test plane, a movable carriage driven at constant speed, and a sled of defined mass and contact area attached to a load cell via a thin, flexible connector. Under ISO 8295 and GB/T 10006, a 200 g sled with a 40 mm × 60 mm contact face is drawn at 100 mm/min (with 150 mm/min also specified for certain conditions), while ASTM D1894 commonly uses a sled providing a 200 g normal force slid at 150 mm/min. The force transducer records the initial peak, corresponding to static friction, and the average plateau force during steady sliding, corresponding to kinetic friction.
Specimen Preparation and Conditioning
Specimens must be free of wrinkles, creases, fingerprints, and surface contamination, as even minor handling artifacts distort results. Samples are conditioned to equilibrium in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a defined period (typically not less than 40 hours) before testing. Because static friction increases with dwell time, the resting interval between sled placement and the start of sliding must be controlled and reported, and the contact time is specified by the applicable standard.
Test Procedure
The general sequence of a test is as follows:
- Cut test specimens of adequate size from the sample, marking surface orientation and the side to be tested (inner/sealing side or outer/print side)
- Secure one specimen flat on the horizontal plane without bubbles or wrinkles
- Mount the second specimen on the underside of the sled using double-sided tape or the standard foam backing
- Place the sled on the specimen and allow the specified dwell time to elapse
- Start the drive and record the force–displacement curve during the sliding stroke
- Read the first maximum force as the static friction force and the mean force over the steady sliding region as the kinetic friction force
- Calculate the coefficient as the measured force divided by the normal force (sled weight), reporting the average of replicate measurements (typically at least three valid runs)
Key Influencing Factors
Accurate and reproducible results depend on strict control of the following variables:
- Slip agent content and migration state: amide slip additives such as erucamide and oleamide bloom to the surface over days or weeks, lowering COF; fresh versus aged film can differ substantially
- Temperature and humidity: higher temperature generally softens the surface and changes COF; elevated-temperature testing may be required for hot-fill or tropical climates
- Surface treatment level: corona and flame treatment raise surface energy and typically increase friction
- Additives, coatings, inks, and varnishes: each converting step modifies the surface and must be reflected in the tested specimen
- Sled mass, contact area, sliding speed, and dwell time: non-standard parameters make results non-comparable between laboratories
- Electrostatic charge, surface contamination, and film gauge variation also contribute to scatter
Applications
Friction coefficient data are used throughout the packaging value chain for material selection, process optimization, and quality assurance:
- Vertical form-fill-seal (VFFS) and horizontal form-fill-seal (HFFS) machine setup, where film-to-metal COF governs feeding over forming collars and drawing belts; a common practical target for film against metal is approximately 0.2 to 0.4 depending on machine design
- Roll-stock converting, printing, and lamination, ensuring films unwind cleanly without telescoping, blocking, or telescopic slip
- Stacking and palletizing studies, where higher film-to-film COF improves load stability of bags, pouches, and overwrapped packs
- Quality control of slip agent masterbatch dosing in film extrusion, detecting under- or over-additivation before shipment
- Shelf-life and aging studies, tracking COF drift during storage and transport simulations
- Pharmaceutical blister and strip pack development, verifying foil and laminate slip for high-speed feeding and sealing
- Comparative benchmarking of supplier materials and failure investigation when line stoppages or pack instability occur
FAQ
What is the difference between static and kinetic coefficient of friction?
The static coefficient describes the resistance at the instant motion begins and is derived from the first peak force, while the kinetic coefficient describes the resistance during steady sliding and is derived from the average sliding force. Static COF is always equal to or greater than kinetic COF, and packaging machinery must overcome the static value to initiate film movement.
Which standards govern friction coefficient testing of packaging materials?
The primary methods are ISO 8295 and GB/T 10006 for plastic films and sheeting, and ASTM D1894 for plastic film and sheeting using the sled method. Paper and board slip behavior may additionally be assessed with ISO and TAPPI-published methods. Test reports should always identify the standard, sled mass, sliding speed, and surface pairing used.
What COF values are acceptable for packaging lines?
There is no single universal limit; acceptance criteria depend on machine type, speed, and pack format. As general guidance, many form-fill-seal operations run reliably with film-to-metal static COF in the range of roughly 0.2 to 0.4, while film-to-film values may be kept near or above 0.3 for stack stability. Individual converters define specifications based on validated machine trials.
Why do friction results change with time after film production?
Slip additives incorporated in polyolefin films migrate to the surface gradually after extrusion. COF measured within hours of production can be significantly higher than COF measured after days or weeks of storage. Testing protocols therefore specify the film age or include aging conditions in the test plan.
Should testing be done film-to-film or film-to-metal?
Both pairings are informative: film-to-film COF relates to winding, stacking, and bag-in-bag behavior, whereas film-to-metal COF against a standard stainless steel surface simulates contact with machine parts. Most comprehensive specifications require both measurements.
Can friction coefficient be tested at elevated temperatures?
Yes. Heated-plane COF instruments allow measurement at controlled temperatures up to typical heat-sealing conditions. This is valuable for hot-filled products, retort applications, and materials used in warm climates, since polymer surfaces soften and COF behavior shifts significantly with temperature.
How can laboratories ensure comparable results?
Interlaboratory comparability requires strict adherence to the selected standard: identical sled mass and geometry, sliding speed, dwell time, conditioning atmosphere, specimen age, surface orientation, and clean handling. Accredited third-party laboratories with calibrated COF testers and documented procedures provide the highest confidence in reported values.