Coefficient of Friction Testing for Packaging Materials: Methods, Standards, Applications & Troubleshooting

Friction affects virtually everything. In practical applications such as processing, transportation, storage, and use, even small changes in surface friction can cause slippage, sticking, misalignment, and operational problems. Coefficient of Friction Testing for Packaging Materials helps ensure reliable performance in real-world applications.

What Is Coefficient of Friction Testing?

A coefficient of friction test is a standardized method for measuring the friction characteristics between two contacting surfaces. It quantifies the frictional resistance generated during initial sliding and continuous motion between two material surfaces, and the resulting data reflects how easily or difficultly the surfaces move relative to each other.

In the field of packaging materials, coefficient of friction testing is widely used to evaluate the friction properties of materials such as films, paper and cardboard, labels and label materials, pharmaceutical packaging, and medical device packaging. It provides objective data to assess a material’s suitability for processing, transportation, storage, and end-use, as well as the stability of production processes.

Standard coefficient of friction testing is typically used to determine two key characteristics:

Static coefficient of friction: The resistance that must be overcome for a material to transition from a stationary state to a moving state.
Kinetic coefficient of friction: The frictional resistance experienced during sustained sliding of a material.

The static and kinetic coefficients of friction reflect the frictional behavior of a material during the initiation and sustained sliding phases, respectively, providing data support for evaluating material performance and the stability of production processes.

Static vs. Kinetic Coefficient of Friction

Generally speaking, the static coefficient of friction is higher than the kinetic coefficient of friction. This is because it typically requires a greater force to move a material from a stationary state to a moving state. However, it is important to pay attention to the difference between the two; if this difference is too large, it can easily lead to stick-slip behavior, resulting in unstable material feeding, printing misalignment, and other issues.

Does a lower coefficient of friction always mean better performance? Not necessarily. An excessively high coefficient of friction may cause jamming or feeding difficulties, while an excessively low coefficient of friction can lead to slippage or misalignment. A reasonable balance between the static and kinetic coefficients of friction should be achieved based on the production process and specific application scenarios.

For a deeper explanation of Static COF vs. Kinetic COF, including how to interpret the two values and what they mean in real packaging applications, read our detailed guide: Static vs. Kinetic Coefficient of Friction.

Why COF Testing Matters for Packaging and Materials?

During production, storage, transportation, and final use, materials constantly come into contact with other surfaces and generate friction. Instability in friction properties can affect the continuity of production processes and the quality of processing; it can lead to packaging slipping, falling off, or collapsing during transportation and storage; and it can also impact the handling and user experience of the product during actual use. Thus, friction behavior affects the entire lifecycle of a material.

COF testing can help manufacturers with:

Material Development: Compare substrates, coatings, additives, and surface treatment processes.
Supplier Qualification: Evaluate friction characteristics and consistency among different suppliers.
Quality Control: Monitor variations in the coefficient of friction (COF) across different production batches.
Process Troubleshooting: Identify friction-related issues such as feed, slippage, tracking, or stacking.
Production Optimization: Establish appropriate coefficient of friction (COF) ranges for specific materials and processes.

For a broader discussion of the role of COF in flexible packaging and manufacturing, see Why Coefficient of Friction Testing Matters in Flexible Packaging and Manufacturing.

How Does COF Testing Work?

1. Prepare the Sample

Cut the material to standardized dimensions. Ensure the sample surface is clean and free of contamination. Pretreat the sample under controlled temperature and humidity conditions.

2. Mount the Sample and Slider

Place the sample on the test platform, ensuring the test surface is facing up. Mount another test sample onto a standardized slider, then place the prepared slider onto the sample surface.

3. Start the Test

Start the equipment; the slider moves across the sample at a controlled speed. The test system records the friction force throughout the entire test process. The initial peak generally represents the force required to overcome static friction, while the force during sliding represents kinetic friction.

4. Calculate the COF

The coefficient of friction can be calculated using the measured friction force and the weight of the slider(The weight of the standard slider is 200 g.).

Common COF Testing Standards

There is a wide variety of packaging materials, and there is no single testing standard that applies to all of them. Typically, the appropriate testing standard must be selected based on the type of material being tested and the type of friction involved.

ASTM D1894 and ISO 8295 are primarily used to measure the coefficients of friction for plastic films and sheets. Both standards specify test methods for measuring the static and kinetic coefficients of friction for these materials; however, the experimental conditions are not entirely consistent between the two. Differences can include test speed, specimen conditioning, sled requirements, and other procedural details. Therefore, test results from these two standards cannot be directly compared.

For paper and paperboard, ISO 15359 specifies a horizontal-plane method for testing both the static and kinetic coefficients of friction. TAPPI T815 provides an inclined-plane method for measuring the static coefficient of friction of packaging materials such as shipping bag paper, corrugated fiberboard, and solid fiberboard. For uncoated writing and printing paper, TAPPI T549 uses a horizontal-plane method to determine both static and kinetic coefficients of friction (COF). TAPPI specifically distinguishes this method from T 815, which is intended for packaging materials.

Learn more about ASTM D1894 and ISO 8095:

ISO 8295 COF Testing for Plastic Film and Sheeting

ASTM D1894 Standard for Coefficient of Friction Testing of Plastic Films and Sheets.

COF Testing Applications

Materials exhibit different friction characteristics depending on their application. Friction coefficient testing evaluates whether these characteristics meet production and real-world requirements. COF data supports material selection, performance comparison, process optimization, and quality control, while helping engineers analyze friction-related issues in feeding, conveying, positioning, forming, sealing, stacking, and transportation.

Plastic Films & Flexible Packaging

During high-speed unwinding, slitting, and lamination, film must strike a balance between smooth conveyance and positioning stability. Abnormal frictional resistance between layers or between the material and the guide rollers can lead to excessive stretching (neck-in), tension fluctuations, end-of-roll misalignment, or forming deviations.

Learn More: Plastic Film COF Testing in Packaging Applications

Food Packaging Films

On high-speed vertical and horizontal FFS packaging equipment, film must continuously pass through the forming chamber and heat-sealing zone. Slip agent migration, surface treatments, and localized high temperatures from heat-sealing blades can all alter the film’s COF, causing jams in the forming channel or wrinkles in the seal that lead to air leaks.

Learn More: COF Testing for Food Packaging Films

Paper & Paperboard Packaging

Interface friction during high-speed sheet separation, conveying, folding, and box gluing directly affects paper positioning. Improper COF can easily cause double-sheet feeding, scratches on the printed surface, and slippage during box gluing; during warehousing and transportation, insufficient surface friction on cardboard boxes can lead to pallet stack collapse.

Learn More: Paper and Paperboard COF Testing

Pharmaceutical Packaging

Pharmaceutical packaging involves the coordinated advancement of medicinal aluminum foil, plastic film, paper, and multilayer composite films along the same track. Fluctuations in friction between different material combinations and the equipment’s metal channels can cause jamming of aluminum-plastic blister packs, cracking during cold-stamping of aluminum, uneven depth, or sealing misalignment.

Learn More: COF Testing for Pharmaceutical Packaging

Medical Packaging

Sterile pouches (such as Tyvek® composite pouches) and rigid trays are highly sensitive to interfacial resistance during automated filling and sealing processes. Excessive frictional resistance can cause material snagging and wrinkling in the sealing zone (leading to channel leaks), while material abrasion may generate particulate contamination in cleanroom environments.

Learn More: COF Testing in Medical Packaging

Labels and Label Materials

During production and use, label materials experience friction with face stock, release liners, rollers, peel plates, conveyor components, and the surface of the final packaging. In practical applications, issues such as web misalignment, web displacement, unstable release liner separation, abnormal label conveyance, label overlap, or label positioning deviations may occur.

Learn More: COF Testing for Labels & Label Materials

Factors That Affect COF Test Results

COF test results can be affected by both the material and the testing conditions. Key factors include:

Slip Agent Migration: Migration of slip agents to the material surface can change friction over time.
Load Cell Sensitivity: Sensor sensitivity affects the ability to capture the initial peak force used to determine static COF.
Surface Contamination & Sample Handling: Dust, oil, moisture, and improper handling can alter the measured friction.
Test Slider Dwell Time: Different contact times before testing can change the initial friction between the surfaces.
Pulling System Alignment: An uneven or misaligned pulling system can change the applied force and affect results.
Sled Condition: Slider weight, flatness, and rubber pad wear can influence the actual contact condition.
Equipment Vibration & Speed: Unstable movement or test speed can introduce measurement variations.
Temperature & Humidity: Environmental conditions can change material and surface properties, particularly for films.

For a detailed discussion of these factors and their control methods, see 10 Critical Factors Affecting Coefficient of Friction Testing Results.

Common COF Testing Problems and Troubleshooting

COF testing problems may result from the sample, test conditions, equipment, or operating procedure. Common issues include:

Unstable or Fluctuating Results: Check sample consistency, surface condition, environmental conditions, and equipment stability.
Static COF Results Too High or Too Low: Inspect the sled, contact surfaces, load cell, and pulling system, and verify the test setup.
Large Variation Between Repeated Tests: Check sample preparation, testing direction, slider condition, and test parameters.
Abnormal Initial Peak: Check the load cell response, pulling connection, sled movement, and alignment.
Results Change Over Time: Consider slip agent migration, temperature and humidity changes, and differences in sample conditioning.

For detailed troubleshooting methods and solutions, see Plastic Film COF Testing Errors: Troubleshooting & Operation Guide.

How to Choose a COF Tester?

Choosing a COF tester should be based on the material, testing standard, friction interfaces, and application requirements. Key considerations include whether the instrument can measure both static and kinetic COF, whether its test parameters meet the applicable standards, and whether the load, speed, sled, and sample size are suitable for the materials being tested. For laboratories testing different packaging materials, ease of operation, measurement repeatability, calibration, and data recording should also be considered.

For a step-by-step selection guide, see How to Choose a Coefficient of Friction Tester: 7-Step Selection Process.

COF Tester vs. Universal Testing Machine

A coefficient of friction tester is an instrument specifically designed to measure the frictional behavior of material surfaces. It typically uses relatively small test loads and is primarily used to test the static and kinetic coefficients of friction for plastic films, paper, rubber, labels, and other flexible materials. A universal tensile testing machine, on the other hand, is a multifunctional material testing device primarily used for mechanical property tests such as tensile, compression, peel, and puncture tests, and typically features a wider load range.

Universal testing machines can perform friction tests by equipping them with specialized fixtures and test programs; however, the operating procedures tend to be relatively complex. If you only need to measure the coefficient of friction, a dedicated coefficient of friction tester is generally easier to operate and offers higher testing efficiency.

For a detailed comparison and selection guidance, see COF Tester vs. Universal Testing Machine (UTM): How to Choose the Right Instrument for Your Lab.

Conclusion

Friction coefficient testing provides objective data on friction behavior in actual production. By correlating the test results with real-world production issues, manufacturers can adjust processing conditions, improve quality control, and ensure that the material’s friction properties meet the requirements of the production process and actual application scenarios.