Dynamic Coefficient of Friction Testing for Packaging Coatings

Dynamic coefficient of friction (COF) testing evaluates how coated packaging surfaces slide against themselves or other materials during processing, filling, stacking, and transport. Reliable COF data is critical for ensuring smooth machinability on high-speed packaging lines and stable load handling throughout the supply chain. This article explains the test principles, applicable standards such as ISO 8295, ASTM D1894, and GB/T 10006, common sample types, testing procedures, and typical industrial applications.

Technical Overview

The dynamic coefficient of friction (also called kinetic COF) is the ratio of the force required to maintain sliding motion between two surfaces to the normal force pressing them together. For coated packaging materials, this property determines how a surface behaves when it rubs against machine guides, rollers, other packages, or pallet surfaces during continuous movement.

Coatings applied to paper, paperboard, plastic films, and foils—such as varnishes, lacquers, barrier layers, heat-seal coatings, and anti-slip treatments—significantly alter the surface energy and texture of the substrate. As a result, the COF of a coated surface can differ substantially from that of the uncoated base material. If the dynamic COF is too low, packages may slide, topple, or shift during stacking and transport. If it is too high, materials may jam, misfeed, or stall on filling, forming, wrapping, and printing lines.

Dynamic COF testing therefore serves as a key quality control and design verification tool. In general, plastic films intended for form-fill-seal operations are engineered to a dynamic COF of approximately 0.20 to 0.40, while coated paperboard for cartoning and stacking often targets higher values to maintain load stability. Accredited third-party laboratories routinely perform these tests according to internationally recognized standards to support supplier qualification, incoming inspection, and product development.

Testing Samples

Coated Paper and Paperboard

This category includes folding carton board with UV or water-based varnishes, printed and lacquered surfaces for consumer goods packaging, and paperboard with barrier coatings for grease or moisture resistance. Both the coated outer surface and the reverse side can be evaluated to assess stacking and feeding behavior.

Coated Polymer Films

Flexible packaging films frequently carry functional coatings such as PVDC, PVOH, acrylic, or inorganic oxide layers (for example, silicon oxide or aluminum oxide vacuum-deposited coatings). Because these coatings can mask or interact with migratory slip agents in the film, direct COF measurement of the coated surface is essential.

Coated Foils and Laminates

Lacquered aluminum foils, coated foil laminates for blister packs, and multi-layer flexible laminates with printed or sealing-side coatings are tested to confirm that converting and packaging line speeds can be maintained without material slip or blocking issues.

Specialty Coated Materials

Heat-seal coatings, cold-seal coatings, release coatings, anti-block coatings, and anti-slip coatings each have distinct friction requirements. Testing verifies that these functional layers deliver the intended slip behavior after curing, rewinding, and storage.

Testing Items

The primary test parameter is the dynamic coefficient of friction, which reflects sustained sliding behavior. Depending on the specification and application, testing programs may also include the following related items:

• Dynamic (kinetic) coefficient of friction of the coated surface against itself
• Static coefficient of friction, measured simultaneously with the dynamic value in most instrument configurations
• COF of the coated surface against the uncoated reverse side of the material
• COF of the coated surface against stainless steel or other machine-contact surfaces
• COF after conditioning, accelerated aging, or storage, to evaluate the stability of migratory slip agents and the curing state of the coating
• Blocking resistance evaluations, which are often performed alongside COF studies on wound rolls

Testing Methods

The most widely used technique is the horizontal plane sled method, described in ISO 8295, ASTM D1894, and the Chinese national standard GB/T 10006 (which is technically aligned with ISO 8295). The general procedure is as follows:

• Specimens are die-cut from representative areas of the coated material, avoiding wrinkles, folds, and printed registration marks that could distort results.
• Specimens are conditioned in a standard laboratory atmosphere, typically 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618, for a defined period before testing.
• One specimen is fixed flat to a horizontal test table, and another is attached to the underside of a standardized sled, commonly with a 200 g mass, ensuring full contact between the two surfaces.
• The sled is drawn across the table at a constant speed—typically 100 mm/min per ISO 8295 and GB/T 10006, or 150 mm/min per ASTM D1894—while a load cell records the friction force.
• The static COF is calculated from the initial force peak, and the dynamic COF is calculated from the average force recorded during the steady sliding phase, using the relationship μ = F / N, where F is the measured friction force and N is the normal force (the sled weight).
• Multiple replicate measurements are performed, and both the static and dynamic COF values are reported, usually to two decimal places, together with the test standard, environment, sled mass, and surface pairing.

For paper-based materials, an inclined plane (ramp) method may also be used to determine the static coefficient of friction; however, because this approach measures the angle at which sliding initiates, it is not suitable for determining the dynamic COF and is typically supplementary. Testing directions should be considered as well: when a coating exhibits directional surface features, such as those from gravure or metering application, COF can differ between the machine direction and transverse direction, and both orientations should be characterized.

Applications

Dynamic COF test results for coated packaging materials are applied across the entire packaging value chain:

• Form-fill-seal operations: verifying that coated films feed, form, and draw down reliably on vertical and horizontal machines at production speeds
• Cartoning and erecting lines: confirming that coated carton blanks separate, erect, and glide through folding and gluing stations without double-feeds or jams
• Labeling and wrapping: ensuring coated labels and wrappers move consistently through applicators and shrink tunnels
• Palletizing and distribution: validating sufficient friction between coated outer surfaces to maintain stack stability during warehousing and transport
• Quality control in converting: monitoring coating application weight, cure completeness, and slip-agent performance from batch to batch
• Research and development: comparing coating formulations, substrate treatments, and surface structures to engineer targeted slip performance
• Supplier qualification and incoming inspection: using accredited third-party laboratory data to establish and verify material specifications between converters, brand owners, and film or coating suppliers

FAQ

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

Static COF describes the resistance to the start of sliding between two surfaces at rest, calculated from the initial force peak. Dynamic COF describes the resistance to continued sliding once motion is underway, calculated from the average force during steady movement. Packaging machinery performance depends primarily on the dynamic value, while stacking and pallet stability also depend heavily on the static value.

What dynamic COF value is considered acceptable?

There is no single universal target; acceptance limits depend on the substrate, coating, and equipment. As a general industry guideline, coated flexible films for form-fill-seal applications commonly target a dynamic COF of 0.20 to 0.40, while coated paperboard intended for stable pallet stacking is often specified at higher values. End-users typically define limits based on validated machine trials.

Why do coatings change the friction behavior of packaging materials?

Coatings modify surface roughness and surface energy, and some interact with or block migratory slip additives in the base film. Incomplete curing of varnishes or lacquers can also leave a tacky surface with abnormally high COF. Direct measurement of the finished coated surface is the only reliable way to confirm the final slip performance.

Do temperature and humidity affect the test results?

Yes. Coated papers and hygroscopic coatings in particular absorb moisture at elevated humidity, which can soften surfaces and change friction. Cellulose-based and certain coated films may show measurably different COF values at different climatic conditions, which is why specimens must be conditioned in a controlled atmosphere before testing.

Which standard should be selected for testing?

ISO 8295 and GB/T 10006 are the preferred methods for plastic films and coated flexible materials and use a test speed of 100 mm/min. ASTM D1894 is widely used in North American supply chains and specifies a test speed of 150 mm/min. Because values obtained under different standards may not be directly comparable, the specification and customer agreement should always identify the governing standard.

Can the COF of a coated material change over time?

Yes. Migratory slip agents in polymer films continue to bloom to the surface during storage, and some coatings continue to cure or undergo surface changes after conversion. It is common practice to test coated materials both immediately after production and after a defined aging period, and to store wound rolls under controlled conditions to prevent blocking before use.