ASTM D1894 Coefficient of Friction Testing of Plastic Film and Sheeting

What the test does

A sled of specified mass, faced with one test material, is placed on a horizontal plane faced with the other. The standard sled is 200 g on a 63.5 mm (2.5 in.) square face, so the normal force pressing the two surfaces together is 1.96 N. The sled is drawn along the plane at 150 ± 30 mm/min (roughly 6 ± 1.2 in./min) through a low-friction pulley or a horizontal load train, and the force needed to move it is recorded continuously. Both coefficients are that force divided by the 1.96 N: a drag of 0.5 N is a coefficient of 0.26. The peak needed to start it moving gives the static coefficient; the steadier force needed to keep it moving gives the kinetic coefficient.

What it measures, and why it matters

Two numbers come out: the static coefficient of friction, which governs whether a stack of film will start to slide, and the kinetic coefficient, which governs how it behaves once moving. For packaging film these decide whether a bag opens on a form-fill-seal line, whether a stack of sheets destacks one at a time, and whether a palletised load stays put. Slip additives migrate to the surface over days, so the figures also track how far a film has bloomed since extrusion.

Both coefficients are dimensionless ratios, which makes them look more transferable than they are. They describe one pair of surfaces at one normal load and one speed, and changing any of the three changes the answer — often substantially, because polymer friction is not well described by the classical model in which the coefficient is independent of load and area. That is the practical reason a specification has to name the sled mass and the speed as well as the method.

Two surfaces, not one material

Friction is not a property of a material. It is a property of a PAIR of surfaces at a given load and speed, which is why both faces are specified and both are recorded.

Sled face

One test material, wrinkle-free

Plane face

The other, or a specified reference surface

Which side faces which

RecordedA coextruded film is rarely symmetrical, so film-to-film results depend on which surfaces meet.

Handling

By the edges onlyA fingerprint changes the answer, and not subtly.

Time since manufacture

Worth recordingDakSlip additive migrates to the surface over days. Comparing a film tested fresh with one tested a month later confuses blooming with formulation.

Test speed and geometry

Sled

200 g on a 63.5 mm (2.5 in.) square faceIt sets the normal force, which is the divisor in both coefficients.

Normal force

1.96 NDak200 g under gravity. A measured drag of 0.5 N is therefore a coefficient of 0.26.

Speed

150 ± 30 mm/min (roughly 6 ± 1.2 in./min)Constant through the traverse. On a 130 mm plane that is about 50 s of sliding.

Pull direction

HORIZONTALA pull line that is not level adds or subtracts a component of the sled's weight, and the error is silent.

Data capture

Fast enough for the static peakDakThe break-free peak is brief. Slow capture rounds it off and under-reports the static coefficient.

What the machine must be capable of

The forces are very small — often well under 10 N — so a low-capacity load cell with clean resolution at the bottom of its range matters far more than frame capacity. The plane must be level and the pull horizontal, or a component of the sled's weight is added to the friction force. The 150 mm/min traverse has to be genuinely constant, and the data capture fast enough to resolve the static peak, which is brief — at that speed a 130 mm plane is crossed in about 50 s, and the break-free peak occupies a small fraction of a second of it.

The whole measurement lives at the bottom of the load range, so the load cell is chosen for its resolution rather than its capacity. The plane must be genuinely level, since even a small tilt adds a component of the sled's weight to the measured force and biases every result in the same direction. Where a pulley is used it has to run freely, because its own friction is added to the film's and cannot be separated afterwards.

How the test runs

1. 01Condition both specimens in the standard atmosphere.
2. 02Handle the film by the edges and note which surfaces will meet.
3. 03Face the sled and the plane, smoothing out wrinkles without stretching.
4. 04Check the plane is level and the pull line horizontal.
5. 05Set the low constant speed and a data rate fast enough to catch the peak.
6. 06Start the pull and record force continuously.
7. 07Take the break-free peak as the static value.
8. 08Average the steady sliding portion for the kinetic value.
9. 09Record which surfaces were in contact and the time since manufacture.

Watch the test

Calculations

What the report has to contain

• Reference to ASTM D1894
• Both materials, and WHICH SURFACES were in contact
• Sled mass and contact area
• Test speed
• Conditioning atmosphere and time since manufacture
• Static coefficient of friction
• Kinetic coefficient of friction
• Number of specimens, mean and standard deviation

What goes wrong in practice

Contaminated surfaces are the commonest fault: a fingerprint changes the answer, and the effect is not subtle. A pull line that is not horizontal adds or subtracts weight. Slow data capture rounds off the static peak and under-reports it. And comparing a film tested a day after extrusion with one tested a month later confuses additive migration with a formulation difference.

Beyond contamination, two errors are common. A sled faced with film that has been stretched during application reads differently from one applied relaxed, because the surface texture changes under tension. And running successive tests over the same track on the plane polishes it — the second and third results drift downward for a reason that has nothing to do with the material, which is why a fresh track is used for each specimen.