PET film coefficient of friction (COF) testing is a critical quality control procedure that measures the slip properties of polyethylene terephthalate films used in packaging, electronics, and industrial applications. This technical article explains the testing principles, applicable standards such as ASTM D1894 and ISO 8295, sample preparation requirements, test procedures, and typical COF values required for reliable high-speed packaging machinability.
Technical Overview
Polyethylene terephthalate (PET) film, particularly biaxially oriented PET (BOPET), is one of the most widely used flexible substrate materials in the packaging, printing, electronics, and converting industries due to its excellent tensile strength, dimensional stability, optical clarity, and barrier performance. However, the surface slip behavior of PET film directly determines how efficiently the material can be processed on high-speed packaging and converting equipment. The coefficient of friction (COF) is the quantitative measure of this slip behavior, defined as the ratio of the frictional force resisting relative motion between two surfaces to the normal force pressing them together.
Two distinct friction parameters are evaluated during testing: the static coefficient of friction, which represents the resistance at the moment motion begins, and the kinetic coefficient of friction, which represents the resistance during continuous sliding at constant velocity. Untreated plain PET film typically exhibits a film-to-film COF in the range of 0.4 to 0.7, which is too high for smooth machine operation. Film manufacturers therefore incorporate slip additives, antiblock agents, or surface coatings to reduce the COF to an optimal processing window of approximately 0.20 to 0.35 for film-to-film contact. Accurate COF measurement ensures that PET film will feed, form, fill, and seal reliably on vertical form-fill-seal (VFFS) and horizontal form-fill-seal (HFFS) packaging lines without jamming, telescoping of rolls, or inconsistent web tracking.
Testing Samples
PET film coefficient of friction testing can be performed on a wide variety of film types and surface conditions. Proper sampling across the film web, including both edges and the center, is essential because additive distribution and surface treatment levels can vary across the width of the film.
Plain BOPET Film
Uncoated, biaxially oriented PET film in typical thicknesses ranging from 12 μm to 250 μm. Both the inner (treated) and outer (untreated) surfaces may be evaluated separately, as corona or flame treatment significantly alters surface energy and friction behavior.
Slip-Modified and Antiblock PET Film
Coated and Metallized PET Film
PET films with functional coatings, including heat-sealable coatings, PVDC barrier coatings, silicone release coatings, and vacuum-deposited aluminum metallized layers, exhibit friction behavior that differs substantially from the base film. Each coated surface must be characterized independently.
Laminated Structures Containing PET Layers
Multilayer laminates such as PET/PE, PET/Al/PE, and PET/BOPP used in flexible packaging require COF verification on the sealing side and the outer print side to guarantee machinability after lamination and printing processes.
Testing Items
A comprehensive PET film friction evaluation program typically includes the following test items:
- Static coefficient of friction, film-to-film, same-surface contact (outer-to-outer or inner-to-inner)
- Kinetic coefficient of friction, film-to-film, same-surface contact
- Static and kinetic coefficient of friction, film-to-metal (stainless steel or chrome-plated surface)
- Coefficient of friction of both film surfaces (treated side versus untreated side)
- Hot coefficient of friction (hot slip) measured at elevated temperatures of 40°C to 95°C to simulate heat-sealing conditions
- Coefficient of friction after aging or storage to track slip agent migration over time
- Coefficient of friction under varying environmental conditions of temperature and relative humidity
Testing Methods
The internationally recognized test methods for PET film coefficient of friction measurement are ASTM D1894, ISO 8295, and GB/T 10006. All three methods employ the same fundamental principle: a standardized sled wrapped with the test film is dragged across a second film specimen mounted on a rigid horizontal platform, and the frictional force is recorded by a precision load cell.
Instrumentation and Test Setup
A typical coefficient of friction tester consists of a horizontal test table, a motorized drive carriage moving at constant velocity, a sled of defined mass, and a force measuring system with data acquisition software. The apparatus may be configured with a temperature-controlled chamber for hot slip testing.
Specimen Preparation and Conditioning
Specimens must be free of wrinkles, creases, fingerprints, and surface contamination. Gloves should be worn during handling to prevent transfer of skin oils, which can dramatically alter friction results. All specimens are conditioned in a standard laboratory atmosphere of 23°C ± 2°C and 50% ± 5% relative humidity for a minimum of 24 hours prior to testing, in accordance with ASTM D618 or ISO 291.
Test Procedure According to ASTM D1894
- Cut a plane specimen of sufficient size and secure it flat on the horizontal test table, ensuring full contact without wrinkles.
- Mount a specimen of the same film on the bottom face of the standard 200 g sled using double-sided tape, with the test surface facing outward.
- Position the sled on the plane specimen and allow the system to stabilize for a brief settling period.
- Start the drive mechanism, which pulls the sled at a constant velocity of 150 mm/min (6 inches per minute).
- Record the initial force peak, which defines the static COF, and the average force during uniform sliding, which defines the kinetic COF.
- Calculate the coefficient of friction as the measured force divided by the normal force (total weight of the sled): μ = F/W.
- Perform a minimum of three to five replicate tests and report the average value for both static and kinetic coefficients.
Test Conditions Under ISO 8295 and GB/T 10006
ISO 8295 and its Chinese national equivalent GB/T 10006 specify a 200 g sled with a test velocity of 100 mm/min, tested at 23°C and 50% relative humidity. Results obtained under different standards should not be directly compared without acknowledging the difference in sliding velocity, which can measurably influence the kinetic friction value.
Applications
Coefficient of friction data for PET film is essential across many industries and process stages:
- Flexible packaging: verifying machinability of PET-based laminates on VFFS and HFFS packaging machines running at speeds exceeding 60 packs per minute.
- Film manufacturing: quality control release testing for every production lot, ensuring consistent slip additive dosing and surface treatment.
- Printing and laminating: confirming that film surfaces will pass through rollers, tension zones, and nip points without blocking or sticking.
- Electronics: evaluating slip behavior of PET films used in flexible printed circuits, capacitor dielectrics, and insulation layers.
- Photovoltaic modules: characterizing the friction of PET-based backsheet films during module lamination and handling.
- Release liners: assessing silicone-coated PET release performance and processability in label and adhesive converting.
- Product development: comparing candidate surface treatments, coating formulations, and additive packages during new film grade development.
FAQ
What is the ideal coefficient of friction value for PET film in packaging applications?
For reliable operation on high-speed form-fill-seal equipment, most packaging engineers specify a film-to-film kinetic COF between 0.20 and 0.35. Values below 0.15 may cause film slippage on drive belts and inconsistent web tension, while values above 0.40 increase the risk of jams and dragging against metal machine parts.
Why is it necessary to measure both static and kinetic coefficient of friction?
The static COF governs the force required to initiate motion, which is critical during film startup, splicing, and intermittent machine motions, while the kinetic COF governs continuous sliding behavior during steady-state machine operation. Both values must fall within specification for trouble-free performance.
How does temperature affect the COF of PET film?
Elevated temperatures generally increase the COF of PET film because the film softens slightly and migratory slip agents redistribute within the polymer matrix. Hot slip testing at sealing temperatures is therefore recommended for films used in heat-sealing applications to ensure the film does not stick to sealing jaws.
Why do COF results sometimes change over storage time?
Migratory slip additives continuously diffuse toward the film surface, so the COF measured immediately after production may differ from the value measured after several weeks of storage. Testing should be performed after the additive system has reached migration equilibrium, and aging studies are recommended for quality assurance.
How many tests should be performed for a valid COF measurement?
A minimum of three replicates per surface is standard, with five or more replicates recommended for specification testing or referee purposes. Fresh contact areas must be used for each replicate because repeated sliding over the same track alters the surface and produces misleadingly low results.
Why Choose ZKGX?
- State-of-the-art analytical equipment
- Highly qualified scientific team
- Fast turnaround time
- Competitive pricing