The coefficient of friction (COF) is one of the main physical indexes of plastic film or sheeting. It describes the ratio of the force of friction to the normal force pressing two surfaces together when the film slides over itself or another substance under specified conditions:
μ = F / N
where F is the friction force measured by the tester and N is the normal force (typically the weight of the sled).
Two values are distinguished in every professional COF test:
In practice μk is slightly lower than μs, and both must stay inside the window that your converting and packing equipment can tolerate.
Flexible packaging film travels through printing, laminating, slitting, bag-making, filling and palletizing lines — always in sliding contact with itself or with machine parts. If the coefficient of friction drifts outside the proper range, production problems follow immediately.
COF too high:
COF too low:
Therefore it is of great significance for both film manufacturers and packaging users to control the coefficient of friction of flexible packaging roll film within an appropriate range — and COF testing is the only objective way to do that before the film reaches the production line.
The friction coefficient of flexible plastic packaging is mainly affected by:
| Factor | Effect on COF |
|---|---|
| Slip agents (e.g. erucamide, oleamide) | Migrate to the film surface and lubricate it; higher loading → lower COF |
| Anti-blocking agents (silica etc.) | Create micro-roughness; higher loading → higher COF, prevents layers sticking |
| Stiffness and hardness of the material | Stiffer, harder films conform less to the counter-surface and slide more easily → lower COF |
| Surface texture | Polished vs matte surfaces differ; corona treatment and printing raise COF |
| Temperature | Slip agents migrate faster at elevated temperature; COF measured at 23 °C may not reflect hot-filling or summer warehouse conditions |
Manufacturers adjust the COF of films by modifying these factors according to COF test results — which is exactly why a repeatable, standard-based test method matters.
Three method standards dominate COF testing of plastic films. They are technically equivalent in principle (a sled is dragged across a plane at constant speed); the differences are in details:
| Parameter | ASTM D1894 | ISO 8295 | GB 10006 |
|---|---|---|---|
| Full title | Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting | Plastics — Film and sheeting — Determination of coefficients of friction | Determination of coefficients of friction of plastic film and sheeting |
| Sled | 200 g, 63.5 mm × 63.5 mm (2.5 in square) | 200 g, 60 mm × 60 mm | 200 g, 63 mm × 63 mm |
| Test speed | 150 mm/min | 100 mm/min | 100 mm/min ± 10% |
| Conditioning | 23 °C / 50% RH | 23 °C / 50% RH | 23 °C / 50% RH |
| Typical use | Export markets, US customers | Europe and international trade | Chinese domestic market, GB/T 13519 and related product standards |
For domestic trade in China, the test is generally based on GB 10006; for export orders, ASTM D1894 is the most frequently requested method. A modern COF tester can run all three methods — only the speed parameter differs.
The same material shows different coefficients of friction on different friction surface combinations. To truly reflect how the film will behave in each stage of use, select the appropriate surface pair before testing.
Throughout the whole use process of flexible packaging materials — rolling, bag making, filling, heat sealing — friction occurs between:
Abnormal COF on different combinations causes different failures:
A practical comparison of the friction coefficients of different surfaces of the same packaging material is illustrated in this test case.
The procedure below follows ASTM D1894 (GB 10006 is run identically at 100 mm/min), using the LTCOF-05A or LTMXS-06 coefficient of friction tester:
Four operating precautions that decide whether your results are repeatable:
The following reference ranges are collected from converter practice; exact acceptance limits should always be agreed between supplier and user (and GB/T 13519 for PE shrink film explicitly leaves the index to negotiation):
| Film type | Typical outer-surface μs | Typical outer-surface μk | Notes |
|---|---|---|---|
| BOPP (plain, untreated side) | 0.25 – 0.35 | 0.20 – 0.30 | Slip level tuned by erucamide dosage |
| BOPP heat-sealable | 0.30 – 0.45 | 0.25 – 0.35 | Seal layer raises COF |
| PET film | 0.40 – 0.55 | 0.35 – 0.50 | Relatively high, needs slip agent in laminate |
| LDPE / LLDPE film | 0.20 – 0.40 | 0.15 – 0.35 | Very slippery films risk unstable stacking |
| CPP | 0.25 – 0.35 | 0.20 – 0.30 | Common in food laminates |
| PE shrink film (GB/T 13519-2016) | by agreement | by agreement | Standard requires the test but sets no fixed limit |
For polyethylene shrink film specifically, the friction coefficient was added to the "other properties" checklist when GB/T 13519-1992 was replaced by GB/T 13519-2016 — see the dedicated PE shrink film COF test guide.
Link Testing Instruments develops and manufactures coefficient of friction testers for both QC labs and production floors:
| Model | Best for | Features |
|---|---|---|
| LTCOF-05A | Routine film QC per ASTM D1894 / GB 10006 | Automatic test run, direct μs/μk readout |
| LTMXS-06 | Wider material range | Also suitable for paper, woven and non-woven fabric, woven bags, rubber, printed matter, aluminum foil and metal products |
Both instruments offer high test accuracy and intelligent operation. Why a dedicated COF tester belongs in every packaging QC program is explained in Why use a coefficient of friction tester for packaging material QC?
Q1: What is a good coefficient of friction for packaging film?
There is no single magic number. For most flexible packaging intended for machine filling, an outer-surface static COF around 0.2–0.4 gives a workable balance between smooth machine travel and stable stacking. The exact window depends on your equipment and must be fixed by supplier–user agreement — which presupposes you actually test it.
Q2: Static or kinetic COF — which one matters more?
Both, at different moments. Static COF governs film start-up on the machine (roll unwinding, first pull), kinetic COF governs steady running through bag-making and filling. A film can pass a static spec yet jam a machine because its kinetic COF drifts at speed.
Q3: How does temperature affect the COF of film?
Slip agents migrate to the surface faster at elevated temperature, so COF typically drops as temperature rises — a film that runs perfectly in a 23 °C lab may behave differently in a hot workshop or in hot-filling. Cigarette packaging film is the classic case; keeping it slippery under high temperature requires dedicated control, as described in this high-temperature slipperiness guide.
Q4: How often should COF be tested?
At minimum, on every incoming batch of roll film and after any change of resin, slip-agent dosage or surface treatment. Film stored for months should be re-tested before use, because slip-agent migration changes the surface over time.
Q5: Which surface combination should I test?
Test the combination that matches the failure you want to prevent: inner-to-inner for stacking and bag opening, outer-to-outer for pallet stability, film-to-bench for machine travel. See the surface-by-surface comparison case for measured differences on one material.
The same COF test method extends to many specific materials and industries.
Packaging applications:
Other materials:
Link Testing Instruments Co., Ltd is a leading supplier of packaging testing instruments headquartered in Jinan, China. The product portfolio covers permeability testers, sealing and leakage testers, heat seal testers, COF testers and mechanical property testers for flexible packaging, medical packaging, food packaging and container industries. For more details about coefficient of friction testers, please visit www.linktesting.org or contact our engineering team.
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