Dry Friction Coefficient Testing: Principles, Standards, and Applications

Dry friction coefficient testing measures the slip resistance between two solid surfaces under unlubricated conditions, providing essential performance data for plastic films, paper, rubber, textiles, and other engineered materials. By determining both static and kinetic coefficients of friction in accordance with internationally recognized standards such as ASTM D1894, ISO 8295, and GB/T 10006, manufacturers can optimize packaging machinability, material handling, and end-use product performance. Accredited third-party laboratories routinely perform these tests to support quality control, product development, and regulatory compliance.

Technical Overview

Dry friction coefficient testing quantifies the resistance to sliding between two solid surfaces when no lubricant or intermediate fluid is present at the interface. The coefficient of friction (COF) is a dimensionless value defined as the ratio of the tangential friction force (F) resisting relative motion to the normal force (N) pressing the two surfaces together, expressed as μ = F/N. Because friction is a property of a material pair rather than of a single material, test results must always be reported together with the counter-surface used during measurement.

Two distinct values are typically determined in a dry friction test program:

• Static coefficient of friction (μs): the ratio measured at the instant sliding begins, representing the force required to initiate movement between the two surfaces.

• Kinetic coefficient of friction (μk): the ratio measured during sustained sliding at a constant speed, representing the force required to maintain movement.

The static value is generally equal to or higher than the kinetic value. For polymeric films, the difference between the two values, and their absolute magnitudes, are strongly influenced by surface additives such as slip agents and antiblocking compounds, surface roughness, contact pressure, temperature, and humidity. In industrial practice, a low COF indicates easy sliding, which is desirable for films traveling over forming collars and stainless steel guides on packaging machines, while a high COF provides grip and stacking stability for bags, cartons, and finished goods.

Testing Samples

Plastic Films and Sheeting

Single-layer and multilayer films including polyethylene (PE), polypropylene (PP), biaxially oriented polypropylene (BOPP), cast polypropylene (CPP), polyethylene terephthalate (PET), polyamide (PA), and PVC are the most frequently tested materials. Both film-to-film and film-to-metal configurations are evaluated to simulate real-world converting and packaging conditions.

Paper and Paperboard

Printing papers, kraft papers, carton board, and label stocks are tested to predict feeding behavior on printing presses, die-cutters, and automatic packaging lines, as well as stack stability in downstream handling.

Rubber and Elastomeric Materials

Seals, gaskets, conveyor belts, tubing, and footwear outsoles are evaluated for friction against standardized counter-surfaces such as stainless steel and glass to assess grip, sealing performance, and slip resistance.

Textiles and Nonwovens

Nonwoven hygiene materials, interlinings, and technical fabrics are tested to characterize handling behavior during converting and layer-to-layer slip in finished products.

Coated, Metallized, and Laminated Materials

Surface-treated, lacquered, metallized, and adhesive-coated substrates are tested to verify that coatings and converting processes have not altered the intended frictional behavior of the material.

Testing Items

Typical dry friction coefficient test items offered by accredited third-party laboratories include:

• Static coefficient of friction, dry, film-to-film
• Kinetic coefficient of friction, dry, film-to-film
• Static and kinetic coefficient of friction, film-to-stainless steel or other counter-substrate
• Coefficient of friction of paper and paperboard, dry, horizontal plane method
• Coefficient of friction of rubber against standard counter-surfaces
• Coefficient of friction after thermal aging, surface treatment, or storage conditioning
• Coefficient of friction at elevated or reduced test temperatures for process simulation

Testing Methods

ASTM D1894

ASTM D1894, Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, is the most widely referenced method in North America. A sled of defined mass and contact area is placed on the specimen attached to a horizontal plane, and the sled is drawn at a constant speed of 150 mm/min while the force is recorded. The peak force divided by the sled weight yields the static COF, and the average force during uniform sliding yields the kinetic COF.

ISO 8295 and GB/T 10006

ISO 8295, Plastics — Film and sheeting — Determination of the coefficients of friction, and its Chinese national equivalent GB/T 10006 follow a similar horizontal-plane principle using a 200 g standard sled drawn at 100 mm/min. These methods are the dominant references for packaging film specifications in Asia and Europe, and they permit both film-to-film and film-to-other-substrate measurements.

ISO 15359 for Paper and Board

ISO 15359 specifies the horizontal plane method for determining the static and kinetic coefficients of friction of paper and board, using a defined sled on a flat specimen table. This method is applied to printing and converting grades where sheet feeding and stack stability are critical.

ISO 15113 for Rubber

ISO 15113 describes a driven-roller or planar method for determining the frictional properties of rubber against various counter-surfaces, used for seals, belting, and other elastomeric components.

Specimen Conditioning

Because friction is sensitive to surface contamination and ambient conditions, specimens are conditioned to equilibrium in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618 prior to testing. Test surfaces must be kept free of fingerprints, dust, and transferred additives, and testing should be performed in the same controlled environment.

Applications

Dry friction coefficient data directly support engineering decisions and quality assurance across many industries:

• Flexible packaging: verifying that films run reliably on vertical and horizontal form-fill-seal machines at high line speeds without jamming or excessive drag.
• Converting and printing: confirming feeder performance, web tracking, and winding behavior for films, papers, and laminates.
• Bag and carton handling: achieving sufficient stack and pallet stability through appropriately high friction on outer package surfaces.
• Medical and pharmaceutical packaging: controlling slip and peel behavior of sterile barrier films, Tyvek-type webs, and lidding materials.
• Automotive and consumer goods: qualifying interior trim, grip components, and elastomeric parts for tactile and functional friction requirements.
• Research and development: benchmarking slip masterbatch levels, surface treatment effects, and coating formulations against performance targets.
• Incoming inspection and supplier qualification: enforcing friction limits in purchase specifications and monitoring batch-to-batch consistency.

FAQ

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

The static coefficient of friction describes the resistance at the moment motion starts, while the kinetic coefficient of friction describes the resistance during continuous sliding. A large gap between the two values can cause stick-slip behavior, which appears as jerky film movement on packaging machines.

What friction values are considered acceptable for packaging films?

For high-speed form-fill-seal applications, a kinetic film-to-film coefficient of friction in the range of roughly 0.20 to 0.30 is commonly targeted, while film-to-metal values are often kept below approximately 0.30 for smooth transport over metal guides. Acceptance limits are ultimately defined by the converter, the packaging machine design, and the relevant product specification.

Why is the test described as "dry" friction testing?

The term dry indicates that no lubricant, oil, or liquid film is applied at the contact interface. This distinguishes the method from lubricated tribology tests used for metals and machine elements and from wet rubbing tests used for color fastness evaluation of textiles.

How are specimens prepared for testing?

Specimens are cut to the dimensions required by the applicable standard, taken from representative areas of the sample away from edges and defects, and conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity before measurement. Contact surfaces are handled carefully to avoid contamination, and testing direction is recorded because machine-direction and transverse-direction friction can differ.

What is the difference between film-to-film and film-to-metal testing?

Film-to-film testing simulates film layers sliding over each other, as in a roll or a bag stack, while film-to-metal testing simulates film contact with stainless steel or other machine surfaces on packaging equipment. Both configurations are frequently required in a complete friction specification.

Can friction change over time or with temperature?

Yes. Slip additives in polyolefin films migrate to the surface gradually, so friction measured shortly after production can differ from friction measured weeks later. Elevated temperature accelerates migration and softens surfaces, which is why some specifications require testing after defined aging periods or at controlled elevated temperatures.

How should a testing provider be selected?

Look for accredited third-party laboratories that operate friction instruments under ISO/IEC 17025 quality systems, follow the exact method edition required by your specification, and report results with the counter-surface, test direction, conditioning history, and instrument parameters clearly identified.