Coefficient of Friction of Plastic Film: What Affects the Number and What It Should Be

Why the Same Film Can Give You Different COF Numbers

Start with the surface-level explanation: different films have different COF values. That much is obvious. But the same film, same resin, same extrusion run, can produce different COF readings depending on which surface you pair it with, how long ago it was extruded, and what the lab conditions look like on test day.

COF is a system property, not a single-surface property. The standard formulas confirm this: under ISO 8295, static COF equals the force to initiate sliding (Fs) divided by the standard normal force (FP = 1.96 N), and kinetic COF equals the force during sustained sliding (FD) divided by FP. Change the opposing surface and both forces change.

How large is the pairing effect? Lab data comparing two different material pairings on the same friction tester produced kinetic COF values of 0.639 versus 0.797 and static COF values of 0.686 versus 0.926 — a 25% kinetic and 35% static difference from material pairing alone.

Static COF is the force needed to start sliding; kinetic COF is the force to keep sliding. Static is almost always the higher number. Both matter on a packaging line. Static COF governs whether a pouch stays put in a stack; kinetic COF governs whether film feeds smoothly through guide rollers.

For application-specific guidance on testing film pairings, see friction testing for plastic films and flexible packaging.

How Polymer Type Sets the COF Baseline

Not all polymers start from the same baseline. Surface energy, crystallinity, and the typical additive package all differ by polymer family, and so does the COF behavior.

PE Films (LDPE and LLDPE)

Polyethylene films tend toward naturally high COF — typically in the 0.3 to 0.5 range for untreated, additive-free film. That is too high for most packaging machinery, which is why nearly all commercial PE films contain slip agents such as erucamide or oleamide. These fatty acid amides migrate to the film surface over time, forming a lubricating layer that pulls COF down.

For vertical form fill seal (VFFS) packaging, the target COF ranges are well established: inside COF of 0.200 to 0.300 and outside COF of 0.300 to 0.400, with an ideal difference of about 0.100 between the two faces. The inside surface needs to be slipperier so the film slides over the forming collar without dragging.

BOPP Films

BOPP film has a lower inherent COF than PE because the biaxial orientation process creates a smoother, denser surface. But that advantage gets complicated fast. Corona treatment, applied to improve ink adhesion and lamination bond strength, raises surface energy and COF along with it. A BOPP film optimized for printability may run at a measurably higher COF than the same base film without corona treatment.

What does that mean in practice? If a printer receives BOPP that was corona-treated three weeks ago versus three days ago, the COF can differ noticeably because corona treatment effects decay over time.

PET and Nylon Films

PET (polyester) and nylon films have higher surface energy than polyolefins, which generally means higher baseline COF. These films are often used in laminated structures where the inner and outer faces serve different functions — the outer face may need printability while the inner face needs heat-seal compatibility. COF requirements for each face can differ, and the additive strategies reflect that. Anti-block particles, silicone coatings, or specialized slip agents replace the erucamide packages common in PE.

COF measurement matters most when the film goes through further processing on packaging or printing machinery. Multi-layer structures multiply the number of surface pairings that need testing.

The Variables That Shift COF After Production

A film's COF on the day it leaves the extruder is not the same COF it will have on the day it runs through a packaging line. Several factors drive the change.

Slip Agent Migration

Slip agent migration is the single largest post-production variable for PE films. Erucamide and oleamide are compounded into the resin at extrusion but take time to bloom to the film surface. Over the first 48 to 72 hours, COF can drop substantially as the slip layer builds up. Films tested immediately after extrusion will show higher COF than the same films tested three days later. Production schedules that do not account for this migration window risk rejecting film that would have been in-spec by the time it reached the packaging line.

Temperature and Humidity

ISO 8295 acknowledges the temperature variable by specifying a test range from -20 C to +50 C with temperature chambers. Higher temperatures generally soften polymer surfaces, which can lower COF by increasing the real contact area. Results generated at a 23 C lab standard may not predict behavior on a plant floor running at 35 C.

Humidity affects COF through surface moisture absorption, particularly on nylon films, which are hygroscopic. A nylon film conditioned at 50% relative humidity will behave differently from one tested at 20% RH.

Surface Treatment Decay

Corona, plasma, and flame treatments raise surface energy and COF, but the effect is not permanent. Surface energy decays over days to weeks depending on storage conditions. Films treated for printing or lamination should be tested close to the time of use, not only at the time of treatment.

How to Get Reliable COF Data — Test Setup That Matters

The sled-on-plane procedure is straightforward in concept: one film strip sits flat, another wraps a weighted sled, and the sled is pulled across at a controlled speed. The force to start sliding gives static COF; the force to maintain sliding gives kinetic COF. But the details of specimen preparation and conditioning determine whether your data means anything beyond the single test you just ran.

How many specimens are enough? ISO 8295 requires a minimum of three specimen pairs. That is a floor, not a target. Films with high variability — common in blown PE — may need more replicates before the average stabilizes.

Specimen conditioning is where most lab-to-lab disagreement originates. Testing film straight off the roll captures the COF at that moment — including whatever slip agent migration state exists. Testing after 24 hours of conditioning at standard atmosphere (typically 23 C, 50% RH) captures a different state. Both numbers are valid, but they answer different questions. The test report must specify conditioning time, or the result cannot be compared to anyone else's data.

Film orientation matters too. The machine direction and transverse direction of a film can produce different COF values, particularly in oriented films like BOPP. The test report should state which direction was tested.

COF Test Report Checklist

Before signing off on a COF result, confirm the report includes all of these:

• Film type, grade, and supplier lot number
• Which face was tested (inside, outside, or both)
• Material pairing (film-to-film, film-to-metal, or film-to-specific substrate)
• Conditioning time and environment (temperature, relative humidity)
• Hours since extrusion (for PE films with slip agents)
• Test standard used (ASTM D1894, ISO 8295, or other)
• Number of specimen pairs tested
• Machine direction or transverse direction
• Static COF and kinetic COF reported separately

Any result missing one of these items cannot be meaningfully compared to another lab's data.

Key Takeaways

• COF is a system property. It depends on the material pairing, the environment, and the film's age, not on the film alone.
• Different polymers start at different baselines: PE runs higher than BOPP, and additive packages do most of the tuning.
• Slip agent migration means day-of-extrusion COF does not equal day-of-use COF. Allow 48 to 72 hours before drawing conclusions.
• Test conditions (temperature, humidity, conditioning time, specimen orientation) must be controlled and reported, or results cannot be compared across labs.
• Match your test protocol to your end-use conditions: specify conditioning time, test temperature, and film direction in every report.