Coefficient of Friction Testing for Plastic Films and Flexible Packaging
Plastic films and flexible packaging materials must maintain precise friction characteristics to run reliably on high-speed form-fill-seal equipment and to open cleanly in the hands of end users. Even a small shift in static or kinetic COF — caused by a change in resin grade, slip additive concentration, or surface treatment — can stall a VFFS line, cause bag jams, or make pouches impossible to peel apart. The MXD-02A Coefficient of Friction Tester measures both static and kinetic COF to ASTM D1894 and ISO 8295, giving film manufacturers and converters the data they need to control slip performance from raw resin to finished roll.
Quick Answer
COF testing for plastic films measures the resistance to sliding between film-to-film or film-to-metal surfaces under standardized sled load and speed conditions. ASTM D1894 uses a 200 g sled at 150 mm/min; ISO 8295 uses a 100 g sled at 100 mm/min. Static COF captures the initial breakaway force, while kinetic COF reflects the running friction during packaging machine operation.
Why COF Matters for Plastic Films
On a vertical form-fill-seal or horizontal FFS machine, the film must slide through guides, over forming tubes, and across sealing jaws without sticking or skidding. Too high a kinetic COF and the film drags, stretching or tearing. Too low a static COF and stacked bags slide off pallets during transit, causing warehouse damage. Film-to-film COF also governs bag-mouth opening — consumers need enough grip to separate the layers, but not so much friction that the bag tears. Flexible packaging specifications routinely define acceptable COF windows of ±0.05 to ensure consistent machinability, and incoming quality control labs run ASTM D1894 on every roll to verify that the converter's slip package is performing as designed.
ASTM D1894 Sled Method for Films
ASTM D1894 is the dominant standard for plastic film COF testing in North America and globally. The method places a 200 g sled wrapped with the test film on a flat, stationary surface — either another film specimen or a polished metal plate — and pulls it at 150 mm/min. The MXD-02A records the peak force as static COF and the average force over a defined travel distance as kinetic COF. The 200 mm/min test stroke is sufficient to capture the steady-state kinetic plateau. Conditioning at 23°C and 50% RH for at least 40 hours is specified; the MXD-02A's software timestamps every test and logs ambient conditions for traceability. Both film-on-film and film-on-metal substrate configurations are supported with the standard sled and surface clamp accessories included with the instrument.
Common Film Types and Their COF Ranges
Different film chemistries and surface treatments produce markedly different friction profiles. Untreated BOPP typically shows static COF of 0.45–0.65 against itself, which is too high for most packaging lines. Slip-treated BOPP targets static 0.20–0.35, kinetic 0.15–0.30. Cast PE and blown PE films with erucamide slip additive typically measure static 0.15–0.30, kinetic 0.10–0.25 under D1894. Biaxially oriented PET (BOPET) is inherently lower in friction, often 0.15–0.25 static on metallized surfaces. Nylon films used in cook-in or vacuum packaging tend toward higher COF (0.40–0.55 static) due to hygroscopicity, which is why conditioning time is critical before testing. Laminated structures can shift the final COF significantly relative to the component films, making finished-roll testing essential.
Film-to-Film vs Film-to-Metal Testing
Two substrate configurations are commonly required in film QC. Film-to-film testing uses the same film on the sled and the platen, simulating stack-to-stack contact in a pallet or magazine feed. Film-to-metal testing wraps film on the sled and slides it across the polished stainless steel platen, simulating contact with forming tubes, guide rails, and sealing jaws. Both configurations use the same 200 g sled at 150 mm/min under D1894. The MXD-02A supports rapid configuration switching: the steel platen and the film-clamp platen are interchangeable without tools. Laboratories that need to report both configurations in a single test run can use the dual-clamp accessory to test two specimens sequentially with identical load and speed parameters.
Slip Additives and COF Control
Slip additives — typically erucamide, oleamide, or stearamide — bloom to the film surface during extrusion and cooling, lowering COF. Their effectiveness depends on additive loading (typically 500–2,000 ppm), resin compatibility, processing temperature, and time since extrusion. Freshly extruded film often shows higher COF than film that has been aged 24–72 hours, which is why many specification sheets define COF after conditioning. Corona treatment, used to improve ink adhesion, temporarily raises surface energy and can increase COF by 0.10–0.20 units, sometimes causing converter problems if the film is not retested after treating. The MXD-02A allows labs to run aging studies — testing the same lot at 0, 24, 48, and 72 hours post-extrusion — by storing and recalling test programs with fixed parameters, ensuring all data points are directly comparable.
Selecting the Right MXD-02A Configuration for Film Testing
For most plastic film and flexible packaging applications, the standard MXD-02A with the 0–5 N load cell and the ASTM D1894 200 g sled kit is the correct configuration. The 5 N range provides resolution adequate for measuring COF values as low as 0.05 across the full 0–200 g sled weight range. The ISO 8295 100 g sled is included for laboratories that also supply to European customers requiring ISO certification. The stainless steel platen (included) covers film-to-metal tests; the film clamp platen enables film-to-film tests. For coextruded barrier films or foil laminates where surface texture varies significantly, the extended 50 mm travel stroke captures enough data to average out texture variations in the kinetic COF reading. Contact KHT to discuss whether the optional 10 N load cell is warranted for thick or coextruded multilayer films with high interply friction.
Frequently Asked Questions
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