COF of PET Film: Static and Kinetic Coefficient of Friction
The COF of PET film describes resistance when a defined surface starts and continues sliding against another surface. This reference applies to uncoated, coated, treated and PET-based protective film used in unwinding, slitting, laminating and die cutting. Each result must be read with its tested sides, method, load and conditioning history. Equipment compatibility, adhesive behavior and removal performance still require sample testing on the actual material and process.
Before Two COF Numbers Are Compared
This technical reference covers PET base film, coated PET, release carriers and protective-film backings when static or kinetic friction data are needed for quality control. It does not replace an adhesive specification, optical report or line-troubleshooting study. Peel adhesion, tensile strength, elongation, haze and residue observation require separate tests.
Which Layer Was Actually Sliding?
A PET protective structure may contain a base film, pressure-sensitive adhesive, optional outer treatment and release liner. Report the actual interface as A/A, A/B, B/B, film/metal or film/glass, together with MD or TD direction. Film-to-film coefficient of friction is not interchangeable with another pairing. PET functional film grades explain surface-treatment differences, while PET release liner film still needs an independent release-force test.
The Reportable COF Data Set
Item | Typical Value / Reference Range | Test Method or Condition | Notes |
Static COF | Grade-dependent / confirmed | ASTM D1894-24 or ISO 8295 | Initial peak, n-value, variation |
Kinetic COF | Grade-dependent / confirmed average | Approved pairing, load and speed | Averaging interval, stick-slip |
Surface pairing | A/A, A/B, B/B, film/metal, film/glass | Identify both surfaces | Not interchangeable |
Direction | MD, TD or travel direction | Mark before cutting | Relevant for asymmetric grades |
Film structure | PET base, optional treatment, adhesive or liner | Grade and surface ID | Identify sled-contact layer |
Thickness | Confirmed value and tolerance | Calibrated method | Identity and stiffness context |
Data status | Typical, reference or confirmed | State source and grade | No universal PET range |
Read the Trace, Not Only the Result
Static COF comes from the initial force peak; kinetic COF is the average force during a defined sliding interval. Keep the trace with the result. A high peak followed by a lower plateau may require a start-stop trial. Repeated peaks can indicate stick-slip, uneven treatment, contamination or specimen mounting error. An irregular baseline should be retested before approval.
Method Conditions That Can Change the Answer
ASTM D1894-24 and ISO 8295 both address starting and sliding friction, but their test arrangements are not identical. Report the method, sled load, speed, sliding distance, contact time, temperature, humidity, conditioning time and specimen count. Results from different methods should not be treated as interchangeable specifications.
Test Item | What It Checks | Suggested Method or Reference | Why It Matters | When To Request It |
Static / kinetic COF | Starting and sliding resistance | ASTM D1894-24 or ISO 8295 | One defined interface | New grade, coating or contact |
Thickness | Gauge and tolerance | Calibrated method | Sample identity | New or disputed construction |
Peel adhesion | Adhesive removal force | ASTM D3330/D3330M or equivalent | Separates adhesion from friction | Protective film approval |
Tensile / elongation | Strength and deformation | ASTM D882 or equivalent | Tearing assessment | Thin film or high tension |
Haze / transmittance | Optical scattering and transmission | ASTM D1003 or equivalent | Optical change | Glass or polished surfaces |
Timed removal | Adhesion build and surface response | 24 h / 72 h / 7 days internal method | Edge lift and trace | Extended dwell or heat |
Where Laboratory Contact Differs from Production Contact
Glass, polished metal, painted, coated, rough, plastic and low-energy surfaces create different contact conditions. Evaluate friction with line speed, web tension, roller pressure and equipment setting. Load weight, cargo shape and transport distance may also affect roll or sheet movement. Custom converted PET film trials should confirm winding direction, edge quality and machine travel. Corona-treated PET film requires separate surface-energy confirmation because dyne level and COF are different properties.
Protective-Film Checks That COF Cannot Replace
COF measures sliding resistance, not adhesive peel force, tack, release force or clean-removal behavior. Peel adhesion requires a defined substrate, angle and speed. Tensile strength and elongation support tearing and deformation checks; haze and light transmittance support optical review. These values should remain outside the friction result table.
A Controlled Approval Sequence on the Actual Surface
Identify grade, batch, tested side, thickness, direction, coating condition and sampling position. Condition the specimens and record temperature and humidity. Apply a representative sample to the actual surface with a controlled roller or documented operator method. After 24 h, 72 h and 7 days, inspect peel behavior, edge lifting, adhesive trace, surface shadow, gloss change, bubbles, tearing and residue. Add sunlight or heat exposure when relevant, then complete a trial run.
Failure Signatures and the Next Verification
Data Point | If Too Low | If Too High | Risk in Application | Check Before Full Use |
Static COF | Initial movement | Start-up resistance | Shift or feeding shock | Start-stop trial |
Kinetic COF | Continued slip | Drag or stick-slip | Telescoping or wrinkles | Continuous run |
Static/kinetic gap | Weak distinction | Large initial peak | Intermittent feed | Review force trace |
Surface pairing | More slip | More drag | Lab/machine mismatch | Retest actual contact |
Thickness / stiffness | Easy deformation | Low conformity | Creasing or bridging | Tension/roller trial |
Peel adhesion | Edge lift | Adhesion build | Trace, tearing, gloss change | Timed peel checks |
Treatment uniformity | Local slip | Local drag | Batch inconsistency | Both sides / roll positions |
Storage History Belongs in the Test Record
Store rolls dry, away from sunlight, excessive heat and concentrated side pressure. Humidity, storage time, contamination, additive migration, damaged wrapping and poor core support may change surface or roll behavior. Record transport vibration and heat history, and retest when the current lot has a different storage history from the approved sample.
Technical Records Connected to This Test
Surface-side identification, coating records, roll-position sampling and retained-sample codes should accompany the result. Coating, slitting and inspection capabilities connect the specimen to the converted roll or sheet. Functional-film records describe treatment, release-film records describe separation, and conversion records describe dimensions and winding; this record remains focused on the COF of PET film.
FAQ
Why can the COF of PET film differ between laboratories?
Sled load, specimen size, speed, contact time, conditioning, surface pairing and result-processing rules can change the answer. Compare results only when the method and conditions are equivalent.
Does lower COF always improve machine feeding?
No. Lower friction may reduce drag but increase layer movement or telescoping. Higher friction may improve grip but increase start-up load, wrinkles or stick-slip. Confirm performance on the actual equipment.
Can COF confirm clean removal of protective film?
No. COF describes sliding resistance. Peel adhesion, dwell behavior, edge lifting, adhesive trace, gloss change, tearing and residue observation require separate checks on the actual substrate.
Can storage conditions change the measured result?
Yes. Temperature, humidity, sunlight, storage time, roll pressure, contamination and additive migration may alter the surface. Retest material when its history differs from the approved sample.
