Determination of the Coefficient of Friction of Plastic Films: Testing Methods, Standards and Applic

The coefficient of friction (COF) of plastic films is a critical parameter that determines how a material will perform on high-speed packaging lines, in bag-making machines, and during stacking and handling. Professional COF testing in accordance with ASTM D1894, ISO 8295 and GB/T 10006 measures both static and kinetic friction values to ensure films slip, track and seal exactly as intended. This article provides a comprehensive technical overview of friction coefficient determination for plastic films, covering sample types, test procedures, influencing factors and typical industry applications.

Technical Overview

The coefficient of friction (COF) is a dimensionless value that quantifies the resistance to sliding between two surfaces in contact. For plastic films, it is one of the most important indicators of machinability and end-use performance. The static coefficient of friction represents the ratio of the force required to initiate sliding motion to the force pressing the two surfaces together (the normal force), while the kinetic coefficient of friction represents the ratio of the force required to maintain sliding at a constant velocity to the normal force.

In flexible packaging and film converting, an improperly controlled friction coefficient leads directly to production problems. A film with an excessively high COF will drag on forming collars, rollers and guide plates of form-fill-seal (FFS) machines, causing film stretching, jamming and inconsistent bag dimensions. A film with an excessively low COF may slip uncontrollably on drive rollers, cause misregistration during printing, or result in unstable stacks of finished bags that slide apart during palletizing. Accurate COF measurement therefore forms an essential part of incoming material inspection, process control and final product release for film manufacturers, converters and packaging end users, and is routinely performed by accredited third-party laboratories.

Three standards dominate friction testing of plastic films worldwide:

• ASTM D1894 – Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, widely used in North America and international trade.
• ISO 8295 – Plastics – Film and sheeting – Determination of the coefficients of friction, the principal international method.
• GB/T 10006 – the Chinese national standard, technically aligned with ISO 8295 and commonly specified in the domestic packaging supply chain.

Testing Samples

Friction coefficient testing is applicable to virtually all flexible film materials used in packaging, industrial and technical applications. Typical sample categories include:

Polyolefin Films

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE) blown and cast films, polypropylene cast films (CPP) and biaxially oriented polypropylene (BOPP) films. These materials commonly contain migratory slip agents such as erucamide or oleamide, whose surface concentration — and therefore the measured COF — changes with time, temperature and winding history.

Polyester and Polyamide Films

Biaxially oriented polyester (BOPET/BOPET) and biaxially oriented polyamide (BOPA) films used in laminations for food and pharmaceutical packaging. These films are frequently corona- or flame-treated to raise surface energy, a process that also modifies the frictional behavior of the surface.

Laminated, Coated and Metallized Films

Multi-layer laminates produced by adhesive or extrusion lamination, films coated with heat-seal lacquers, anti-fog agents or barrier coatings (such as SiOx or AlOx), vacuum-metallized films, and films with antistatic or release coatings. For laminates, the COF of each functional surface can be tested independently.

Specialty and Technical Films

Release films, photovoltaic encapsulant films, protective films, agricultural films, shrink films and stretch films, where friction properties affect unwinding, winding and downstream processing performance.

Testing Items

A standard friction coefficient determination program for plastic films typically includes the following measured and derived quantities:

• Static coefficient of friction (μs) – the peak force at the moment sliding begins, divided by the sled weight (normal force). This value governs film start-up behavior on packaging machines and the stability of stacked packages.
• Kinetic coefficient of friction (μk) – the average force required to sustain sliding, divided by the normal force. This value governs continuous film tracking and machine speed capability.
• Film-to-film friction – measured with film on both the sled and the test bed, representing inner-to-inner or inner-to-printed surface contact, as specified by the customer.
• Film-to-substrate friction – measured with a film-covered sled sliding on a standard reference surface such as polished stainless steel, simulating contact with machine parts.
• Multi-surface characterization – distinguishing the treated side, untreated side, sealed side and printed side of a film, since slip agent migration and surface treatment produce surface-dependent results.

Reported results normally include individual values, mean values, standard deviation and a clear statement of the test direction (machine direction / transverse direction), test surfaces and conditioning history.

Testing Methods

Test Principle and Apparatus

All three major standards employ the horizontal sled method. A standard sled with a defined mass and contact area — commonly 200 g with a 63 mm × 63 mm base — is placed on a film specimen secured to a flat, rigid test bed. The sled is either itself covered with a second film specimen (film-to-film test) or left bare as a reference metal surface (film-to-metal test). A drive mechanism moves the test bed at a constant velocity while a precision load cell attached to the sled records the frictional force. A typical crosshead speed is 150 ± 30 mm/min per ASTM D1894 or 100 ± 10 mm/min per ISO 8295.

Specimen Preparation and Conditioning

Film specimens are cut flat and free of wrinkles, folds and visible defects, with dimensions adequate to cover the test bed and sled surfaces. Because friction is highly sensitive to surface condition, specimens must be handled only by their edges and contact surfaces must not be touched. Prior to testing, specimens are conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 16 hours, unless otherwise specified. Special care is required with slip-agent-containing films, since surface COF continues to evolve for days after production; testing is therefore often scheduled at defined intervals (for example, 24 hours, 7 days and 28 days after extrusion).

Test Procedure

• Secure the test-bed specimen to the horizontal plane using double-sided tape, ensuring the test surface remains completely free of adhesive contamination.
• Attach the sled specimen smoothly to the underside of the sled, excluding air bubbles and wrinkles.
• Allow a defined dwell time (commonly 5 to 15 seconds) for the surfaces to reach intimate contact under the sled weight.
• Initiate relative motion and record the force trace: the initial static peak and the subsequent steady-state kinetic force.
• Calculate the static coefficient of friction as the peak initiating force divided by the normal force, and the kinetic coefficient of friction as the mean dynamic force (usually averaged over a defined sliding distance, excluding the initial 10 % and final 10 % of the trace) divided by the normal force.
• Repeat on a minimum of three to five specimen pairs and report mean values together with individual results.

Factors Influencing Test Results

Friction coefficient values are strongly influenced by test conditions, and consistent control of the following variables is essential for comparable data:

• Temperature and humidity – COF generally increases at elevated temperature; some polyolefin films show a sharp rise above 35–40 °C, which is why hot-tack and climatic-chamber COF testing is often added for tropical markets and heat-generating filling lines.
• Slip agent level and migration state – amide slip agents migrate to the surface over time; both under-migrated and bloomed surfaces give non-representative values.
• Surface treatment level – corona treatment increases surface polarity and typically raises the film-to-film COF.
• Contact pressure and dwell time – longer dwell increases static friction, particularly for tacky or soft films.
• Surface contamination – fingerprints, transfer additives, retained solvent or process oils materially alter results.
• Test direction and specimen history – machine direction versus transverse direction orientation, winding tension and prior abrasion can all shift measured values.

Applications

Friction coefficient data directly support decision-making across the flexible packaging value chain:

• Vertical and horizontal form-fill-seal packaging – verifying that films run reliably on high-speed VFFS and HFFS machines, where kinetic COF against metal surfaces in the range of approximately 0.2 to 0.4 is typically targeted.
• Bag and pouch making – controlling film tracking through folding boxes and sealing jaws and ensuring stable bag stacking after filling.
• Printing and lamination – confirming web handling behavior on press and laminator rollers and preventing misregistration.
• Slip agent formulation development – tracking COF evolution as a function of slip and antiblock masterbatch concentration, film thickness and storage time.
• Release and protective film engineering – balancing unwind force and clean release performance.
• Quality control and supplier qualification – routine COF verification of incoming film lots and certification testing by accredited third-party laboratories for export shipments and customer specifications.

FAQ

What is a good coefficient of friction value for packaging films?

There is no universal target, but for automated polyolefin packaging lines a kinetic film-to-film COF of roughly 0.2 to 0.3 is widely regarded as an optimum operating window. Values below about 0.1 can cause slippage on drive rollers and unstable palletizing, while values above about 0.5 typically cause film drag, stretching and machine jams. Precise limits should always be agreed between film supplier and converter based on the specific machinery.

Why do COF results for the same film change over time?

Most polyolefin films contain migratory slip agents that diffuse from the film bulk to the surface during the first days and weeks after production. Immediately after extrusion the COF is high; it drops as slip agent accumulates at the surface, and it may drift again with continued aging, elevated storage temperature or abrasion. This is why specifications often define the conditioning period before testing.

Static and kinetic COF — which one matters more?

Both matter, but for different failure modes. The static COF governs whether a film starts moving when the machine pulls it and whether stacked bags remain stable. The kinetic COF governs continuous run performance at speed. On packaging lines a large gap between static and kinetic values can produce stick-slip (jerky film motion), which is equally undesirable.

What is the difference between ASTM D1894 and ISO 8295 testing?

The two methods are conceptually identical horizontal sled tests, but they differ in specified sliding speed (150 ± 30 mm/min for ASTM D1894 versus 100 ± 10 mm/min for ISO 8295), sled construction details and reporting conventions. Because these differences can shift results slightly, the standard used must always be stated on the test report, and comparisons should only be made between results obtained under the same method.

Can friction testing be performed at elevated temperature or on the printed surface?

Yes. Climatic-chamber COF testing at temperatures up to 60 °C or higher is commonly requested for films destined for hot climates or heat-generating filling operations, and surface-specific testing — inner, outer, treated, untreated, sealed or printed surfaces — is routinely performed by accredited third-party laboratories to fully characterize how a laminate will behave in real converting and packaging conditions.