ASTM D1894 Coefficient of Friction Test: Step-by-Step Procedure for Film and Sheeting

What ASTM D1894 Actually Measures (and What It Does Not)

ASTM D1894 quantifies two values. Static COF is the force required to initiate sliding motion between two surfaces. Kinetic COF is the force required to sustain that motion at constant speed. Both are dimensionless ratios --- force divided by the normal force (sled weight) --- and both come from a single test run on the same force-displacement curve.

The standard does not cover rubber, textiles, or paper. It does not address surface energy, adhesion, or peel strength. And it draws a hard line at 0.254 mm thickness: materials below that threshold are classified as film; materials above it are sheeting. This distinction is frequently confused --- at least one major testing laboratory has published the threshold reversed on its website.

So what changes between testing a 50-micron stretch wrap and a 1-mm rigid blister? Nearly everything downstream.

Equipment and Sled Configuration --- Film vs Sheeting

Four components make up the test apparatus: a horizontal plane (the test surface), a weighted sled, a flexible connector (monofilament line or thin wire), and a drive mechanism that pulls at constant speed. A load cell mounted in line with the pull direction records force throughout the test.

Sled mass is fixed at 200 g, with a 63.5 x 63.5 mm body. Where film and sheeting diverge is the specimen attachment area on the sled face. For film, the specimen wraps around the sled and is secured on the back, creating a 120 mm square contact area. For sheeting, the rigid specimen is cut to match the sled face at 63.5 mm square and attached directly.

If your material is thicker than 0.254 mm, you are in sheeting territory --- different specimen size, different sled face. Verify your classification before cutting a single specimen.

Specimen Preparation and Conditioning

Specimen preparation is the single largest source of irreproducible COF data, yet most references treat it as a footnote. The standard specifies exact dimensions, strict conditioning, and contamination controls that directly affect measurement repeatability.

Cutting. For film, cut a base surface specimen of 250 x 130 mm and a sled specimen of 120 mm square. For sheeting, cut a base specimen of 254 x 127 mm (10 x 5 in.) and a sled specimen of 63.5 mm square (2.5 in. square). Use a sharp blade or die cutter. Ragged edges introduce debris that contaminates the test surface.

Direction marking. Before cutting, mark the machine direction (MD) and transverse direction (TD) on the parent sheet. Oriented films can show COF differences of 0.1 or more between directions. Losing track of orientation after cutting means the data cannot be traced back to process conditions.

Conditioning. All specimens must be conditioned at 23 +/- 2 degrees C and 50 +/- 5% relative humidity for a minimum of 40 hours. No shortcuts. Moisture content and temperature affect surface slip additives that migrate to the film surface, and equilibration takes time.

Handling. Wear clean, lint-free gloves. Handle specimens by edges only. Fingerprint oils alter COF readings measurably. Do not stack specimens face-to-face --- interleave with clean release paper.

Before you run a single test, confirm your specimens have completed the full 40-hour conditioning window. Rushed conditioning is the most common source of irreproducible COF data across packaging laboratories.

Running the Test --- Speed Control, Sliding Distance, and Direction

Mount the base specimen flat on the horizontal test surface. No wrinkles, no bubbles, no tape overlap on the test area. Secure the edges so the base does not shift during the pull.

Attach the sled specimen to the sled. For film, wrap the specimen around the sled with the test face outward, securing excess material on the back with tape that does not contact the test area. For sheeting, adhere the rigid specimen flush to the sled face.

Place the sled on the base specimen. Connect the pull cord from the sled to the load cell. The cord must be horizontal and aligned with the pull direction --- any vertical angle adds a lifting component that reduces apparent friction.

Start the drive mechanism. The crosshead speed is 150 +/- 30 mm/min (6.0 +/- 1.2 in./min). The sled must travel a minimum of 130 mm after initial contact with the base surface. Speed must remain constant; acceleration artifacts corrupt the static peak reading.

For oriented films, test in both machine direction and transverse direction. That doubles your specimen count. The standard requires a minimum of 5 valid result sets per direction, meaning 10 total specimens minimum for an oriented film tested in both MD and TD.

What happens when the force trace shows repeated spikes instead of a clean plateau? That sawtooth pattern is stick-slip --- and it requires switching to the rigid linkage method specified in the standard. The flexible connector amplifies stick-slip oscillation; a rigid linkage dampens it, producing a usable kinetic reading.

Reading the Force Curve and Calculating COF

The load cell produces a force-displacement curve. Two values come from this curve.

Static COF corresponds to the initial peak --- the highest force recorded as the sled begins to move. This peak typically occurs within the first few millimeters of displacement. Kinetic COF corresponds to the average force measured over the remaining sliding distance after the initial peak, across the plateau region of the curve.

The formula is straightforward:

COF = F / (m x g)

Where F is force (in Newtons), m is sled mass (0.200 kg), and g is gravitational acceleration (9.81 m/s squared). The sled weight in Newtons is 0.200 x 9.81 = 1.962 N.

Worked example. A test on LDPE film yields a static peak force of 0.85 N and an average kinetic force of 0.69 N.

Both values fall within the typical LDPE film-to-film range of 0.3 to 0.5, which serves as a useful sanity check. Values outside 0.1 to 1.0 for polymer-on-polymer contact generally indicate a measurement error --- wrong force units, incorrect sled mass entry, or a contaminated specimen.

Run a sanity check after every calculation: if your COF falls outside 0.1 to 1.0 for polymer-on-polymer, recheck your force units and sled weight before recording the result.

Reporting Requirements and Common Mistakes

ASTM D1894 mandates specific reporting elements in every test report. Missing a required field can invalidate results during an audit or customer qualification review.

Required report elements:

• Material identification (type, grade, manufacturer, lot number)
• Specimen dimensions and thickness
• Conditioning parameters (temperature, humidity, duration)
• Test speed
• Number of specimens tested
• Individual static and kinetic COF values for each specimen
• Mean static and kinetic COF
• Standard deviation

A minimum of 5 valid results must be included. Discard any result where the specimen wrinkled, slipped, or showed visible contamination --- but document the discard and reason.

Common mistakes that invalidate data:

If your standard deviation exceeds 10% of the mean, do not simply add more replicates. Go back and check your specimen preparation. High scatter almost always traces to a preparation variable, not to material variability.

ASTM D1894 Specimen Preparation Checklist

Use this checklist directly in your lab. It covers both film and sheeting paths from material receipt through pre-test verification.

Material Classification

• [ ] Measure material thickness
• [ ] Classify: Film (< 0.254 mm) or Sheeting (> 0.254 mm)
• [ ] Select correct specimen dimensions based on classification

Cutting

• [ ] Mark machine direction (MD) and transverse direction (TD) on parent sheet before cutting
• [ ] Film base specimen: 250 x 130 mm
• [ ] Film sled specimen: 120 mm square
• [ ] Sheeting base specimen: 254 x 127 mm
• [ ] Sheeting sled specimen: 63.5 mm square
• [ ] Use sharp blade or die --- no ragged edges
• [ ] Cut minimum 5 specimen sets per test direction

Conditioning

• [ ] Place specimens in controlled environment: 23 +/- 2 degrees C, 50 +/- 5% RH
• [ ] Record start time of conditioning
• [ ] Minimum duration: 40 hours
• [ ] Do not stack test faces together --- interleave with release paper

Handling and Mounting

• [ ] Wear clean, lint-free gloves
• [ ] Handle by edges only --- no contact with test surfaces
• [ ] Mount base specimen flat, wrinkle-free, edges secured
• [ ] Film sled: wrap specimen around sled, test face out, secure on back
• [ ] Sheeting sled: adhere specimen flush to 63.5 mm sled face
• [ ] Verify no tape overlaps test contact area

Pre-Test Verification

• [ ] Confirm sled mass: 200 g
• [ ] Confirm pull cord is horizontal and aligned
• [ ] Confirm crosshead speed set to 150 mm/min
• [ ] Confirm load cell range appropriate for expected forces
• [ ] Record specimen orientation (MD or TD) for each test