Kinetic coefficient of friction (COF) testing measures the sliding friction behavior of plastic films and is a critical quality control parameter for flexible packaging machinability. This technical article explains the testing principles, applicable standards, sample types, test procedures, and industrial applications of dynamic friction coefficient measurement for plastic films and sheets.
Technical Overview
The kinetic coefficient of friction (also called the dynamic coefficient of friction, μk) of a plastic film is the ratio of the friction force resisting relative sliding motion between the film surface and a counter-surface to the normal force pressing the two surfaces together, measured once steady sliding has been established. In a typical test, a weighted sled carrying the film specimen is dragged across a second film surface at a constant speed, and the average force required to maintain sliding is divided by the sled weight to obtain μk = Fk / N, where N is approximately 1.96 N for a standard 200 g sled.
For flexible packaging converters and film producers, kinetic COF is one of the most important machinability indicators. If the value is too high, films jam, misfeed, or wrinkle on vertical or horizontal form-fill-seal machines and generate excessive web tension. If the value is too low, films slip on transport belts, stacks become unstable, and bag opening becomes difficult. Slip performance is typically engineered through migratory slip agents (such as erucamide and oleamide) and antiblock particles, while corona treatment, printing, and lamination tend to raise friction.
Key factors that influence the measured kinetic COF include:
- Polymer type, film formulation, and slip or antiblock additive content
- Surface treatment (corona, flame, plasma) and printing or coating layers
- Lamination structure and layer distribution
- Temperature, relative humidity, contact pressure, and sliding speed
- Storage and aging time, as slip agents migrate to the film surface over days or weeks
Testing Samples
Polyolefin Films
LDPE, LLDPE, mLLDPE, CPP (cast polypropylene), IPP, and BOPP films are the most frequently tested materials. These films rely on migratory slip additives, so both routine production quality control and aging studies commonly include kinetic COF measurement of the sealing (inner) surface.
Polyester and Polyamide Films
BOPET and BOPA films naturally exhibit relatively high friction. Kinetic COF testing is performed film-to-film and film-to-metal to evaluate treated, metallized, or coated surfaces and to confirm suitability for high-speed winding and converting operations.
Laminated, Coated, and Printed Films
Multilayer laminates containing aluminum foil or metallized PET, as well as films with PVDC, EVOH, or acrylic coatings and printed surfaces, are tested to quantify how inks, lacquers, and adhesives alter sliding behavior compared with the base film.
Specialty Films
Release films, heat-sealable films, protective films, POF shrink films, and agricultural films may be tested under standard conditions or after defined conditioning, aging, or elevated-temperature exposure to reflect real service environments.
Testing Items
Typical test items offered for plastic film friction evaluation include:
- Kinetic (dynamic) coefficient of friction, film-to-film configuration
- Kinetic coefficient of friction, film-to-metal (stainless steel) or other substrate configuration
- Static coefficient of friction (initiation peak) reported alongside the kinetic value
- Differentiated testing of the inner (sealing) surface and outer (machine-contact) surface
- Kinetic COF after conditioning, storage aging, or elevated-temperature exposure
Test reports normally state the standard applied, sled mass and speed, environmental conditions, individual and mean values of static and kinetic COF, and the number of specimens tested.
Testing Methods
Applicable Standards
- ASTM D1894: Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting
- ISO 8295: Plastics — Film and sheeting — Determination of the coefficients of friction
- GB/T 10006: Chinese national standard for determining the coefficients of friction of plastic film and sheeting, technically aligned with ISO 8295
Apparatus and Principle
A horizontal flat-bed COF tester is used, consisting of a fixed or driven plane, a sled of 200 g providing a normal force of approximately 1.96 N, a calibrated load cell, and a drive mechanism that produces constant relative motion between the sled and the plane. One film specimen is secured flat on the plane, and a second specimen is attached to the underside of the sled, with the test surfaces facing each other for film-to-film measurement or the film facing a polished stainless-steel plane for film-to-metal measurement.
Test Procedure
- Condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618 until moisture and temperature equilibrium is reached
- Cut specimens to the dimensions required by the selected standard and handle them carefully without touching the test areas
- Mount the plane specimen and attach the sled specimen, ensuring intimate, wrinkle-free contact
- Run the test at the specified constant sliding speed: 150 mm/min for ASTM D1894 or 100 mm/min for ISO 8295 and GB/T 10006
- Record the force-displacement curve: the initial peak represents the static friction force, and the average force during sustained sliding represents the kinetic friction force
- Calculate μ = F / N for each specimen, test at least three to five specimen pairs, and report the mean values and scatter
Interpretation and Typical Acceptance Criteria
Common industry practice specifies a film-to-film kinetic COF between approximately 0.2 and 0.4 for automatic packaging applications: values below about 0.2 may cause slippage in stacks and transport systems, while values above about 0.4 often lead to feeding problems and excessive drag on packaging machines. Acceptance limits vary by end use, film structure, and customer specification, so the agreed purchase specification always prevails. Because migratory slip agents need time to bloom to the surface, polyolefin films are frequently tested after a defined post-extrusion aging interval, and elevated-temperature testing can reveal loss of slip performance during heat exposure.
Applications
Kinetic COF testing of plastic films supports quality assurance and product development across the flexible packaging supply chain:
- Routine quality control on blown, cast, and BOPP film production lines
- Validation of slip agent and antiblock masterbatch dosage and migration behavior
- Incoming inspection by converters before printing, lamination, slitting, and bag making
- Troubleshooting of packaging line problems such as film jams, misfeeds, and unstable web tension
- Research and development of surface treatments, coatings, and multilayer structures
- Independent verification of compliance with customer or buyer specifications by accredited third-party laboratories
FAQ
What is the difference between static and kinetic coefficient of friction?
The static coefficient of friction characterizes the force needed to initiate sliding between two surfaces, corresponding to the initial peak of the force-displacement curve. The kinetic coefficient of friction characterizes the force needed to maintain sliding at constant speed, calculated from the average force during the sustained sliding phase. Both values are normally reported together because packaging machinery experiences both start-up and continuous motion.
What kinetic COF range is suitable for automatic packaging lines?
A film-to-film kinetic COF of roughly 0.2 to 0.4 is widely regarded as a practical working window for high-speed automatic packaging, with the optimal value depending on the machine design and application. Exact limits should be established jointly between the film supplier and the end user based on actual line trials.
Which standard should be used, ASTM D1894 or ISO 8295?
Both standards are internationally accepted and technically equivalent in principle. They differ in details such as the specified sliding speed, 150 mm/min for ASTM D1894 versus 100 mm/min for ISO 8295, so results may not be directly interchangeable. The choice is usually dictated by the target market or the customer's specification; GB/T 10006 is the standard commonly referenced in the Chinese market and follows the ISO approach.
Why does the COF of polyolefin films change over time?
Migratory slip additives incorporated in the resin gradually diffuse and bloom to the film surface after extrusion. As a result, the measured kinetic COF typically decreases during the first days or weeks of storage and then stabilizes, with the rate accelerated by higher temperatures. Testing after a defined aging interval, such as seven to fourteen days at room temperature, provides more representative and reproducible results.
Can both sides of a film be tested separately?
Yes. The inner sealing surface is normally evaluated in a film-to-film configuration to predict bag making and stacking behavior, while the outer surface is often evaluated in a film-to-metal configuration to predict behavior against machine guides, forming collars, and transport rollers.