Dry Coefficient of Friction (COF) Testing for Plastic Films

Dry coefficient of friction (COF) testing measures the slip properties of plastic films and is one of the most critical quality-control indicators for flexible packaging materials. By quantifying both static and kinetic friction behavior in accordance with recognized standards such as ISO 8295 and ASTM D1894, manufacturers can ensure their films run reliably on high-speed packaging lines. This article provides a comprehensive overview of dry COF testing for plastic films, covering technical principles, sample types, test items, methods, applications, and frequently asked questions.

Technical Overview

The coefficient of friction (COF) is defined as the ratio between the tangential force required to slide one surface across another and the normal force pressing the two surfaces together. Because it is a ratio of two forces, the COF is a dimensionless quantity. In the context of plastic films, dry COF testing refers to measurements performed without any lubricant, oil, or other interfacial medium under controlled laboratory conditions, which reflects how films actually behave during most converting and packaging operations.

Two distinct values are measured in dry COF testing. The static coefficient of friction characterizes the force required to initiate sliding between two surfaces, and it governs phenomena such as bag opening resistance, package stacking stability, and machine start-up behavior. The kinetic (dynamic) coefficient of friction characterizes the force required to maintain sliding at a constant speed, and it determines how smoothly a film travels continuously through forming collars, sealing jaws, and guide rails on form-fill-seal equipment. The kinetic value is typically equal to or lower than the static value.

The frictional behavior of plastic films is strongly influenced by formulation and surface chemistry. Polyolefin films in their natural state can exhibit very high tackiness, with COF values above 0.6 or even approaching non-slip conditions. To achieve processable surfaces, film producers incorporate slip agents (migratory amides such as erucamide and oleamide) and antiblock particles (silica, talc, or calcium carbonate) into the resin. Corona and plasma treatments, film thickness, winding tension, storage time, and ambient temperature all further affect the measured COF, which is why standardized dry COF testing is essential for consistent product quality.

In general industry practice, kinetic film-to-film COF values in the range of approximately 0.2 to 0.4 are considered suitable for high-speed automatic packaging, while values that are too low can cause film slippage on driven rollers and unstable package stacking. Accredited third-party laboratories routinely perform dry COF testing to support product development, incoming inspection, and conformity verification against international standards.

Testing Samples

Polyolefin Films

Polyolefin films represent the largest category of samples submitted for dry COF testing. These include LDPE, LLDPE, metallocene LLDPE, and HDPE blown or cast films, as well as CPP (cast polypropylene), IPP, and BOPP (biaxially oriented polypropylene) films. Because slip and antiblock additive levels directly determine the friction performance of these materials, COF testing is the primary quality-control method for verifying additive formulations and masterbatch dosing accuracy.

Polyester and Polyamide Films

Biaxially oriented polyester (BOPET) and biaxially oriented polyamide (BOPA) films are frequently tested for both film-to-film and film-to-metal friction behavior. These films are commonly used as stiffening or barrier layers in laminates, and their surface friction affects lamination processing, metallization quality, and downstream converting performance.

Laminated, Coextruded, and Coated Films

Multilayer laminate structures, coextruded films, vacuum-metallized films, and barrier-coated films (such as those with AlOx, SiOx, or PVDC coatings) are also tested. For these samples, the sealing side and the printing or laminate side typically exhibit different friction characteristics, so each functional surface is evaluated separately according to the intended application.

Sample Preparation and Conditioning

Test specimens must be flat, free from creases, wrinkles, and folds, and must not be touched by bare hands in the test area, since skin oils and contamination significantly alter friction values. Specimens are cut to the dimensions required by the applicable standard, with the machine direction (MD) and transverse direction (CD) clearly identified. Prior to testing, samples are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity, in accordance with ISO 291 or ASTM D618, typically for a minimum of 40 hours, because both temperature and humidity measurably influence test results.

Testing Items

Dry COF testing of plastic films typically encompasses the following test items, selected according to the end-use requirements of the material:

• Static coefficient of friction, film against film (identical or dissimilar films)
• Kinetic (dynamic) coefficient of friction, film against film
• Static and kinetic coefficient of friction, film against metal (commonly a stainless-steel or standard test-plane surface), simulating contact with packaging machine guides and conveyors
• COF testing of the inner (sealing) surface and the outer surface separately, since each surface performs a different function in the package
• COF measurement in machine direction and transverse direction where orientation effects are relevant
• Aging and slip-migration studies, in which COF is monitored over days or weeks to evaluate how migratory slip agents bloom to the film surface during storage
• Elevated-temperature COF testing as an optional investigation for films used in heat-sensitive or tropical climatic environments

Testing Methods

The internationally recognized methods for dry COF determination of plastic films are ISO 8295 (Plastics — Film and sheeting — Determination of coefficients of friction), ASTM D1894 (Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting), and the corresponding Chinese national standard GB/T 10006. All of these methods are based on the same fundamental principle, executed on a horizontal plane friction tester equipped with a precision load cell and a movable drive carriage.

In the standard test configuration, one specimen is secured flat onto the horizontal test plane, and a second specimen is attached to the underside of a standardized sled with a compliant rubber backing. The sled is then drawn across the plane at a constant speed while the instrument continuously records the drag force. The coefficient of friction is calculated as the measured friction force divided by the normal force, which is equal to the weight of the sled.

The typical test procedure consists of the following steps:

• Condition the specimens in the standard atmosphere and verify that the instrument is calibrated and level
• Mount the plane specimen securely, ensuring the test surface is smooth, flat, and free of contamination
• Attach the sled specimen, taking care to handle the film only by its edges
• Place the sled gently on the plane specimen and allow the prescribed brief dwell period before motion begins
• Drive the sled at the constant speed specified by the standard over the defined travel distance, typically 100 to 150 mm, while recording the force-displacement curve
• Determine the static peak force at the onset of sliding and the average kinetic force during the steady sliding region
• Calculate the static COF and kinetic COF by dividing the respective forces by the normal force (approximately 1.96 N for a 200 g sled)

Key methodological parameters differ slightly between standards: ASTM D1894 specifies a 200 g sled drawn at 150 mm/min, whereas ISO 8295 and GB/T 10006 specify a 200 g sled drawn at 100 mm/min. Because of these differences, the standard used must always be stated in the test report, along with the test surface pairing, specimen orientation, test side, conditioning parameters, and individual and mean COF values reported to two decimal places. Multiple replicates, commonly at least three per condition, are performed to ensure statistical reliability.

Applications

Dry COF test results are used throughout the plastic film and flexible packaging value chain for a wide range of practical purposes:

• Packaging machine compatibility: verifying that films will run smoothly and without jamming, slipping, or oversticking on vertical and horizontal form-fill-seal lines
• Bag and pouch manufacturing: confirming that bag-making, gusseting, and bottom sealing operations achieve correct film transport and easy bag opening for consumers
• Slitting and rewinding operations: evaluating roll quality and preventing telescoping, blocking, and film-to-film adhesion during winding and storage
• Stacking and logistics: ensuring finished packages remain stable on pallets and conveyors without sliding
• Formulation development: optimizing slip agent, antiblock, and masterbatch concentrations in new film structures
• Incoming material inspection and supplier qualification: providing objective acceptance criteria for film purchases
• Barrier and specialty films: quality control of metallized, coated, label, protective, and technical films where surface friction affects downstream processing
• Regulatory and customer conformity: demonstrating compliance with material specifications referenced to ISO 8295, ASTM D1894, or GB/T 10006

FAQ

What is the difference between static and kinetic coefficient of friction?

The static COF describes the resistance at the moment sliding begins, calculated from the peak force at the start of sled movement. The kinetic COF describes the resistance during continuous sliding, calculated from the average force over the steady sliding portion of the test curve. Static values relate to start-up, stacking, and bag-opening behavior, while kinetic values relate to continuous machine operation at production speed.

What COF range is suitable for automatic packaging lines?

Although specifications vary by application and equipment, kinetic film-to-film COF values between roughly 0.2 and 0.4 are widely accepted as the processable window for high-speed form-fill-seal operations. Values that are too high cause film drag, wrinkling, and machine jams, while values that are too low can cause film slippage on driven rollers and unstable package stacking.

Should testing be performed film-to-film or film-to-metal?

Both pairings are relevant. Film-to-film testing reflects interactions between film layers during winding, stacking, and inside finished packages, while film-to-metal testing simulates contact between the film outer surface and the stainless-steel guides, collars, and conveyors of packaging machinery. Many specifications require both configurations, testing each functional surface of the film separately.

Why do COF values change during film storage?

Migratory slip agents added to polyolefin resins gradually diffuse and bloom to the film surface over time, so friction values measured immediately after extrusion can differ significantly from values measured after several days or weeks of storage. Aging studies that track COF over defined storage intervals are therefore an important part of film qualification.

How do temperature and humidity affect test results?

COF is temperature-sensitive; polyolefin films generally show increasing friction at elevated temperatures, which is one reason testing is performed under strictly controlled conditions of 23 ± 2 °C and 50 ± 5 % relative humidity. Deviations from these conditions should be reported, and elevated-temperature testing can be performed when the end-use environment justifies it.

How should specimens be handled before testing?

Specimens must be cut flat without creases, handled only by the edges or with clean gloves, and conditioned in the standard laboratory atmosphere before measurement. Contamination from skin oils, dust, or prior contact with adhesives and coatings will distort results, so careful specimen preparation is essential for reproducible COF data.