Dynamic Coefficient of Friction (COF) Testing for Packaging Materials

Dynamic coefficient of friction (COF) testing is an essential quality control method for evaluating the slip properties and machinability of packaging materials, including plastic films, laminates, foils, and paperboard. Conducted in accordance with recognized standards such as ISO 8295, ASTM D1894, and GB/T 10006, the test quantifies the frictional forces that govern how packaging webs behave on high-speed converting and filling lines. This article presents a comprehensive technical overview of dynamic COF testing, covering test principles, applicable sample types, testing procedures, industrial applications, and answers to frequently asked questions.

Technical Overview

The coefficient of friction (COF) is a dimensionless value that quantifies the resistance to sliding between two surfaces in contact. It is expressed as the ratio of the tangential frictional force to the normal force pressing the surfaces together. Two distinct parameters are measured: the static coefficient of friction (μs), which describes the force required to initiate movement, and the dynamic (kinetic) coefficient of friction (μd), which describes the force required to maintain continuous sliding motion at a constant speed. Dynamic COF testing focuses on the latter and is the more relevant parameter for predicting the behavior of packaging materials during sustained transport through processing machinery.

For packaging materials, dynamic COF is one of the most important indicators of machinability and runnability:

• On vertical and horizontal form-fill-seal (FFS) machines, an excessively high COF causes film drag, inconsistent feeding, web wrinkles, and machine jams, while an excessively low COF causes slippage on rollers and unstable package stacking after filling.
• During winding, slitting, printing, and lamination, controlled friction ensures stable web tension and clean roll formation.
• For palletized goods, appropriate friction between outer package surfaces contributes to stack stability during storage and distribution.

In industrial practice, most converting and filling lines require a film-to-film or film-to-metal dynamic COF in the approximate range of 0.2 to 0.4. The principal international and national standards governing this test include ISO 8295, ASTM D1894, and GB/T 10006 for plastic films, and TAPPI T549 for paper-based materials. Testing is routinely performed by manufacturers' quality control laboratories and by accredited third-party laboratories for verification and dispute resolution.

Testing Samples

Dynamic COF testing is applicable to virtually all flexible and semi-rigid packaging materials supplied as films, sheets, laminates, or webs. The most common sample categories are described below.

Plastic Monolayer Films

Single-layer polyolefin and barrier films, including LDPE, LLDPE, HDPE, CPP, IPP, BOPP, BOPET (PET), BOPA (PA), PVC, and EVOH-based films. These materials frequently contain slip agents (such as erucamide or oleamide) and antiblock additives whose concentration and migration behavior directly control the measured COF, making friction testing a key tool for additive and formulation control.

Multilayer Laminates and Composite Structures

Typical converting structures such as PET/AL/PE, BOPP/CPP, PET/PE, PA/PE, and metallized laminates. Because each layer and each surface (inner sealant side versus outer print side) exhibits different frictional behavior, COF is evaluated on the specific surfaces that will contact machine parts or other packages.

Paper, Paperboard, and Coated Papers

Folding carton board, kraft papers, coated and varnished papers, and release liners. For these substrates, friction testing is typically performed using the horizontal plane method described in TAPPI T549 or comparable standards, and the results correlate with feeding performance on cartoning machines and stacking stability.

Aluminum Foil and Metallized Films

Bare and converted aluminum foils, metallized BOPP and PET films, and foil-polymer laminates. Metallic and metallized surfaces present distinctive friction characteristics, particularly after coating, laminating, printing, or embossing operations.

Other Packaging Substrates

Flash-spun polyolefin nonwovens for medical packaging, elastomeric and heat-shrink films, release films, adhesive-coated webs (tested with release liners in place), and rubber sheets used in sealing and gasket applications.

Testing Items

The scope of dynamic COF testing typically includes the following measurable items and test configurations:

• Dynamic coefficient of friction (μd), the primary reported result for sustained sliding.
• Static coefficient of friction (μs), usually captured in the same test run from the initial peak force.
• Film-to-film COF, with the same material tested against itself on the surfaces that will contact in the finished package or roll.
• Film-to-metal COF, against a standard polished stainless steel plane, simulating contact with machine guide plates and forming shoulders.
• Asymmetric surface combinations, such as inner-to-inner, outer-to-outer, or film against other specified counter-materials.
• Aged or conditioned COF, evaluating changes after heat exposure, storage time (slip agent migration), or environmental conditioning.

Testing Methods

Test Principle and Apparatus

The universally applied method is the horizontal plane (sled) method. A standard test sled, typically a 63.5 mm by 63.5 mm block with a total mass of 200 g, is covered with the specimen film and rested on a second specimen mounted on a rigid horizontal plane. A drive mechanism moves either the plane or the sled at a constant speed, and a calibrated load cell records the frictional force throughout the sliding process. Modern COF testers integrate precision drive systems, force sensors, and software that automatically distinguishes the static peak from the mean dynamic force and calculates both coefficients.

Specimen Preparation and Conditioning

Specimens are cut flat and free of wrinkles, creases, and visible defects, with dimensions matching the apparatus requirements (the stationary specimen typically measuring at least 80 mm in width and 200 mm in length). The sliding surfaces must not be touched or contaminated during handling. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291, ASTM D618, or equivalent conditioning standards. Because slip additives migrate over time, specimens from finished packages or aged rolls may additionally be tested after defined storage intervals to assess friction stability.

Test Procedure

• Secure the first specimen to the horizontal plane, ensuring full, bubble-free contact without stretching the film.
• Mount the second specimen on the underside of the standard sled.
• Allow the system to equilibrate, then start the drive at the specified speed: 100 mm/min per ISO 8295 and GB/T 10006, or 150 mm/min per ASTM D1894.
• Record the force-displacement curve over the prescribed sliding distance; the first peak corresponds to static friction, and the average force during sustained sliding corresponds to dynamic friction.
• Repeat the test on multiple specimen pairs (commonly three to five determinations) and report both individual and mean values.

Calculation and Result Evaluation

The coefficients of friction are calculated from the measured forces and the normal force applied by the sled weight: μ = F / N, where F is the measured frictional force in newtons and N is the normal force in newtons, equal to the sled weight. The initial peak force yields the static coefficient μs, and the mean force during steady sliding yields the dynamic coefficient μd. Results are reported to two decimal places together with the test standard, sliding speed, sled mass, surface combination, conditioning details, and specimen orientation (machine direction versus transverse direction), since COF can differ significantly with direction in oriented films.

Applications

Dynamic COF testing supports quality assurance, process optimization, and compliance verification across the packaging value chain:

• Film producers use COF data to control slip-agent and antiblock masterbatch formulations and to release consistent, machinable products.
• Converters verify that printed, laminated, or metallized webs remain within the friction window required for slitting, print registration, and lamination.
• Food and beverage packagers validate film behavior on vertical form-fill-seal, horizontal flow-wrap, and sachet machines before full-scale production.
• Pharmaceutical and medical device manufacturers confirm the frictional compatibility of blister lidding foils, laminate pouches, and sterile barrier materials with packaging equipment.
• Brand owners and packaging engineers use COF data for stacking studies, pallet-load stability assessments, and troubleshooting of line stoppages and web breaks.
• Accredited third-party laboratories perform COF testing for incoming material inspection, supplier qualification, comparative benchmarking, and failure analysis.

FAQ

What is the difference between static and dynamic coefficient of friction?

The static coefficient of friction characterizes the resistance to the start of sliding between two surfaces, while the dynamic coefficient characterizes the resistance during continuous sliding. In packaging, static COF relates to initial film separation and bag opening, whereas dynamic COF governs sustained web transport through machine rollers, forming collars, and sealing jaws. Both values are captured in a single ISO 8295 or ASTM D1894 test run.

Which standards are used for packaging material COF testing?

The primary standards are ISO 8295 for plastic films and sheeting, ASTM D1894 for static and kinetic COF of plastic film and sheeting, GB/T 10006 as the Chinese national standard aligned with ISO 8295, and TAPPI T549 for paper and paperboard. All employ the horizontal plane sled method with a 200 g standard sled.

What COF value is suitable for packaging production lines?

Most form-fill-seal and flow-wrapping applications run reliably when the dynamic COF falls between approximately 0.2 and 0.4. Values below this range risk web slippage and unstable stacks; values above it increase drag, tension fluctuations, and jam frequency. The exact acceptance window is defined by each packager based on machine design and line speed.

How do slip agents and antiblock additives affect COF results?

Amide slip agents migrate to the film surface and lubricate the interface, lowering COF over time, while inorganic antiblock particles create micro-roughness that reduces real contact area. The measured COF therefore depends on additive loading, film thickness, storage time, and temperature. COF testing is the standard method for monitoring and optimizing these effects.

Do temperature and humidity influence the test result?

Yes. Higher temperatures accelerate slip-agent migration and soften many polymers, typically altering COF, while high humidity can affect hygroscopic films such as polyamide and paper-based materials. For this reason, conditioning in the standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity is mandatory, and additional high-temperature COF evaluation may be performed when films will run on hot-filling or heat-sealing equipment.

How many specimens are required, and how are results reported?

Most protocols require at least three to five valid determinations per surface combination, with the mean value reported. A complete report identifies the test standard, sled mass and speed, specimen orientation, surface pairing, conditioning history, and individual and average static and dynamic COF values.

Can COF testing be performed on finished packages?

Yes. Specimens can be cut from finished bags, pouches, or laminated roll stock to verify that converting processes such as printing, coating, and lamination have not shifted friction outside the acceptable window. This is a common incoming-inspection and field-failure investigation practice performed by both in-house and accredited third-party laboratories.