Static friction coefficient testing measures the resistance of a plastic film surface to the initiation of sliding, a parameter that directly determines machinability on packaging lines and the stacking stability of finished packages. The test is routinely performed by accredited third-party laboratories in accordance with ISO 8295, ASTM D1894, and GB/T 10006. This article explains the technical principles, sample requirements, test procedures, calculation methods, and practical applications of static friction coefficient testing for plastic films.
Technical Overview
The static coefficient of friction (denoted as μs or COF-S) is defined as the ratio of the maximum force required to initiate sliding between two contact surfaces to the normal force pressing them together. For plastic films, this value is one of the most important indicators of surface slip performance. During packaging operations such as vertical form-fill-seal (VFFS) and horizontal form-fill-seal (HFFS) processes, the film must slide smoothly over metal forming collars, guide rollers, and platens. A static coefficient of friction that is too high causes film jamming, web tracking deviations, and machine downtime, while a value that is too low can lead to film slippage on rollers, unstable winding, and poor stack stability of finished bags.
Plastic films are inherently tacky materials because polymer chains at the surface tend to adhere to contacting surfaces. To control friction, film manufacturers incorporate slip agents (typically migratory fatty acid amides such as erucamide and oleamide) and antiblock agents (such as silica or talc) into the resin. Because slip agents migrate to the film surface over time and are affected by temperature, the measured static coefficient of friction is not a fixed material constant; it changes with storage time, temperature, humidity, surface treatment, and winding tension. For this reason, friction testing must be carried out under strictly controlled conditions.
The most widely applied test standards for plastic film friction include ISO 8295 (Plastics — Film and sheeting — Determination of the coefficients of friction), ASTM D1894 (Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting), and GB/T 10006, which is technically aligned with ISO 8295. These standards all use the horizontal plane sled method as the fundamental test principle.
- Machine-direction (MD) and transverse-direction (TD) sliding behavior may differ and are evaluated separately.
- Typical target values for automatic packaging applications fall in the range of approximately 0.2 to 0.4 for the inner (sealing) surface.
- Values above approximately 0.5 generally indicate poor machinability, while values below approximately 0.15 may cause slippage and stacking problems.
- Test results are strongly influenced by conditioning atmosphere, contact time between surfaces, and testing temperature.
Testing Samples
Polyolefin Films
Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and cast polypropylene (CPP) films are the most frequently tested sample types because they rely heavily on migratory slip additives. Blown polyethylene films for bags, liners, and lamination sealant layers are commonly evaluated for both film-to-film and film-to-metal friction behavior.
Biaxially Oriented Films
Biaxially oriented polypropylene (BOPP), biaxially oriented polyester (BOPET), and biaxially oriented polyamide (BOPA) films are tested as supplied or after surface treatment, printing, or coating. These stiff films are widely used as the outer web of laminates, where consistent friction against machine surfaces is essential for high-speed converting and packaging.
Laminated, Metallized, and Printed Films
Multilayer laminates, metallized films, and printed or lacquered films are tested to verify that converting processes have not altered slip performance. Inks, adhesives, and top coatings frequently raise the coefficient of friction, so finished laminate structures must be re-evaluated before release for packaging machine use.
Sample Preparation Requirements
Specimens must be representative, free from wrinkles, creases, folds, and surface contamination, and must not be touched on the test area during cutting and handling. Typical practice is to cut one specimen large enough to fully cover the horizontal plane and a second specimen to cover the base of the sled, with the test surfaces facing each other. Samples are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity, generally for a minimum of 40 hours in accordance with ASTM D618 or the equivalent conditioning requirements referenced in ISO 8295 and ISO 291.
Testing Items
The primary testing item is the static coefficient of friction (μs), which characterizes the resistance at the moment sliding begins. Because most instruments record the full force-time curve, the kinetic coefficient of friction (μk) is normally obtained in the same test run and reported together with the static value.
- Static coefficient of friction (μs), film-to-film, inner surface to inner surface.
- Static coefficient of friction (μs), film-to-film, outer surface to outer surface.
- Static coefficient of friction (μs), film-to-metal or film-to-other substrate.
- Kinetic coefficient of friction (μk) measured concurrently during sustained sliding.
- Evaluation of friction in machine direction and transverse direction as required.
- Optional evaluation of friction after aging, heat exposure, or surface treatment to study slip agent migration behavior.
Testing Methods
Horizontal Plane Sled Method (Principle)
The test is performed on a coefficient-of-friction tester consisting of a fixed horizontal plane, a drive mechanism that moves the plane (or pulls the sled) at a constant speed, and a force-measuring system attached to the sled through a thin flexible connector. A standard sled with a mass of 200 g and a flat square sliding base is used. One specimen is fastened to the plane and another to the sled base so that the two film surfaces of interest are in contact. As relative motion begins, the force transducer records the initial force peak, which corresponds to static friction, followed by the force plateau during sliding, which corresponds to kinetic friction.
Test Conditions and Parameters
Testing is conducted at the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity. ISO 8295 and GB/T 10006 specify a sliding speed of 100 mm/min, while ASTM D1894 specifies 150 mm/min. The contact time between the sled and the plane before motion starts is standardized because static friction increases with dwell time for many film surfaces. When comparing results, the same standard, sled mass, speed, and dwell time must be used.
Typical Test Procedure
- Condition the specimens in the standard atmosphere until equilibrium is reached.
- Cut specimens to size, ensuring the test surfaces remain untouched and uncontaminated.
- Secure one specimen flat on the horizontal plane, free from bubbles and wrinkles.
- Attach the second specimen to the base of the 200 g sled with the test surface facing outward.
- Place the sled on the specimen on the plane and connect the sled to the force sensor.
- Allow the specified dwell time to elapse, then start the drive at the specified constant speed.
- Record the maximum force at the onset of sliding and the average force during steady sliding.
- Repeat the test on multiple specimen pairs (typically at least three to five determinations) and report the arithmetic mean.
Calculation of Results
The static coefficient of friction is calculated as μs = Fs / W, where Fs is the maximum force recorded at the initiation of sliding and W is the normal force equal to the weight of the sled (for a 200 g sled, W is approximately 1.96 N). The kinetic coefficient of friction is calculated as μk = Fk / W, where Fk is the average force during sustained sliding. Results are reported separately for each surface combination and sliding direction, together with the test standard used, conditioning details, and number of determinations.
Special Considerations
For films containing migratory slip agents, the measured static coefficient can decrease significantly within days or weeks after extrusion as the additive blooms to the surface, and it can increase again at elevated temperatures where the slip agent redissolves into the polymer. Testing immediately after production, after controlled aging, or at elevated temperatures may therefore be specified to simulate conditions on hot packaging equipment. Corona or flame treatment, printing, and lamination all modify the surface and must be taken into account when sampling.
Applications
Static friction coefficient testing is applied throughout the flexible packaging value chain, from resin and film producers to converters, brand owners, and accredited third-party laboratories performing quality verification and incoming inspection. The data support formulation development, process control, complaint investigation, and packaging line commissioning.
- Machinability qualification of films for VFFS, HFFS, flow-wrap, and bag-making machines.
- Quality control of slip and antiblock masterbatch dosage in film extrusion plants.
- Incoming inspection of purchased films by converters and packaging end users.
- Evaluation of the effects of printing, laminating, and surface treatment on slip performance.
- Aging studies to track slip agent migration and shelf-life stability of film surface properties.
- Support for food, pharmaceutical, and consumer goods packaging development where consistent feeding and stacking behavior is required.
FAQ
Question: What is the difference between the static and kinetic coefficients of friction?
Answer: The static coefficient describes the resistance at the exact moment sliding starts and is calculated from the first force peak, while the kinetic coefficient describes the resistance during continuous sliding and is calculated from the average sliding force. For most plastic films, the static value is slightly higher than the kinetic value, and it is the static value that governs whether a film will stall when a packaging machine starts to pull the web.
Question: Which standards govern plastic film friction testing?
Answer: The principal standards are ISO 8295, ASTM D1894, and GB/T 10006. ISO 8295 and GB/T 10006 use a sliding speed of 100 mm/min, whereas ASTM D1894 uses 150 mm/min; results from different standards should not be compared directly.
Question: What static coefficient of friction value is suitable for automatic packaging?
Answer: For the sealing side of films used on high-speed packaging machines, a film-to-film static coefficient of roughly 0.2 to 0.3 is commonly targeted, with an acceptable working range of approximately 0.2 to 0.4. Values above 0.5 typically cause film jamming and inconsistent feeding, while very low values can cause slippage and unstable bag stacking.
Question: Why do friction results on the same film change over time?
Answer: Migratory slip agents continuously diffuse from the film interior to the surface and can also redissolve at elevated temperatures. Immediately after extrusion the film may show a higher coefficient, which then decreases as slip agent blooms. This is why testing schedules and storage conditions are defined when evaluating film friction.
Question: Can the test be performed between a film and a metal surface?
Answer: Yes. The sled method allows film-to-film as well as film-to-metal or film-to-other substrate configurations. Film-to-metal testing simulates the film sliding over stainless steel forming collars and guide plates on packaging machinery and is often requested alongside film-to-film measurement.
Question: How should samples be handled before testing?
Answer: Specimens must be cut cleanly without creasing, the test surfaces must never be touched by hand, and samples must be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity before measurement. Contamination, fingerprints, dust, and static charging of the film surface can all distort the measured coefficient.