📋 Table of Contents
- What is Coefficient of Friction (COF)?
- Static COF vs Kinetic COF — The Critical Difference
- Why COF is Critical for Packaging Machinery Performance
- Slip Additives: How Films Get Their Friction Properties
- The Blooming Phenomenon — Why COF Changes After Extrusion
- ASTM D1894: The Standard Flat Sled Test Method
- TAPPI T-815: The Inclined Plane Method
- Test Configurations — Film-to-Film vs Film-to-Metal
- Step-by-Step: Performing the COF Test (ASTM D1894)
- COF Reference Values for Common Packaging Films
- The COF Tester — Specifications
- Diagnosing High COF Problems
- Diagnosing Low COF Problems
- Tips for Accurate and Reproducible COF Results
- Frequently Asked Questions
One number — typically between 0.1 and 0.8 — determines whether your film runs smoothly through a 600-bag-per-minute packaging machine or jams it. COF (Coefficient of Friction) is the most operational quality parameter in flexible packaging. This guide covers the physics, the chemistry of slip additives, both test methods (ASTM D1894 and TAPPI T-815), and how to diagnose COF problems in production.
COF is deceptively simple to measure — yet surprisingly complex to control. It is influenced by additive chemistry, extrusion temperature, cooling rate, storage conditions, humidity, and testing time. Understanding all these factors is the key to producing consistent film that performs reliably on your customer's packaging equipment.
What is Coefficient of Friction (COF)?
COF is a dimensionless ratio derived from Amontons' laws of friction (1699): the friction force between two surfaces is proportional to the normal force pressing them together, and independent of the contact area.
µ (COF) = F (friction force) / N (normal force)
µ
COF — dimensionless ratio
F
Friction force (Newtons)
N
Normal force = sled weight × g
For the ASTM D1894 standard sled: N = 0.200 kg × 9.81 m/s² = 1.962 N. So COF = measured force (N) / 1.962. A force of 0.392 N gives COF = 0.20. A force of 0.785 N gives COF = 0.40.
Static COF vs Kinetic COF — The Critical Difference
Every COF test produces two values — and both matter for different reasons:
⏸ Static COF (µs)
▸ The COF at the instant movement begins
▸ Always higher than kinetic COF
▸ Determined by the peak force on the chart
▸ Governs: film start-up on packaging machines
▸ Critical for: bag feeding, film transport initiation
▸ Formula: µs = Peak force (N) / 1.962 N
▶️ Kinetic COF (µk)
▸ The COF during sustained sliding motion
▸ Always lower than static COF
▸ Determined by the mean force during travel
▸ Governs: steady-state film transport speed
▸ Critical for: film unwinding, slitting, converting
▸ Formula: µk = Mean sliding force (N) / 1.962 N
The stick-slip phenomenon
When the ratio of static to kinetic COF is high (µs/µk > 1.5), films exhibit stick-slip behaviour — the film hesitates, jerks, then slides, then hesitates again. On high-speed packaging machinery, stick-slip causes film tension fluctuations, registration errors, seal misalignment, and in severe cases, film tears or machine jams. Achieving a smooth, uniform COF with µs/µk ratio close to 1.0 is the goal for high-speed packaging films.
Why COF is Critical for Packaging Machinery Performance
Automatic packaging machinery — form-fill-seal machines, horizontal flow wrappers, vertical VFFS machines, pouch making machines — all depend on precise film friction control. Here is what goes wrong at each extreme:
| COF Range | Classification | Consequences | Corrective Action |
|---|---|---|---|
| < 0.10 | Too slippery | Film rolls telescope on rewind mandrel; bags slide off pallets; film slips on guide rolls causing mistrack; print register fails | Reduce slip additive; increase anti-block; check storage temperature |
| 0.10 – 0.15 | Slightly low | Acceptable for some applications; pallet stability marginal; check with end-user machinery | Minor additive adjustment usually sufficient |
| 0.15 – 0.40 | Optimal | Smooth machine running; good pallet stack stability; reliable bag feeding; consistent seal alignment | Target zone for most automatic packaging films |
| 0.40 – 0.55 | Slightly high | Film may slow in guides; some hesitation at start; acceptable for manual or semi-auto packaging | Increase slip additive; check conditioning time |
| > 0.55 | Too high — critical | Film jams in automatic packaging machines; bags stick together; film marks products; seal bars burn film due to drag | Immediate investigation — slip additive deficiency or bloom failure |
Slip Additives: How Films Get Their Friction Properties
Raw polyethylene and polypropylene have naturally high COF (0.5–0.8) due to molecular-scale adhesion between polymer surfaces. Slip additives are added to reduce this to the 0.15–0.40 target range for packaging applications.
Primary slip additives — fatty acid amides
| Additive | Chemistry | Typical Loading | Characteristics | Notes |
|---|---|---|---|---|
| Erucamide | Cis-13-docosenamide; from rapeseed oil | 0.05 – 0.20% | Long-chain: slow, persistent bloom; preferred for LDPE and LLDPE blown film; most widely used slip additive in packaging | COF stabilises at 24–48h; low odour |
| Oleamide | Cis-9-octadecenamide; from tall oil | 0.05 – 0.15% | Shorter chain: fast bloom but less persistent; gives lower initial COF; tends to migrate faster | COF may increase again after 2–4 weeks at ambient temperature |
| Behenamide | Docosanamide; saturated | 0.05 – 0.20% | Very slow bloom; excellent for high-temperature applications (PP at 230°C extrusion); minimal odour | Used in PP and HDPE; 72–120h to achieve target COF |
| Stearamide | Octadecanamide; saturated | 0.05 – 0.15% | Very fast bloom; high-activity but short-lived; often used in combination blends | Rapid initial drop in COF then gradual recovery |
Anti-block additives — the counterbalance
Anti-block additives (typically amorphous silica or talc at 0.1–0.5%) create microscale surface roughness that prevents adjacent film layers from cold-welding together during winding and storage. However, anti-block agents increase COF — they act in opposition to slip additives. Film formulation is a balance between slip (for friction reduction) and anti-block (for layer separation). Changing either without considering the other can shift COF outside specification.
The Blooming Phenomenon — Why COF Changes After Extrusion
The most common source of confusion in COF testing is the time-dependent change in COF after film extrusion — called blooming. Understanding it is essential for consistent QC.
COF vs Time After Extrusion — Typical Profile for LDPE with Erucamide
0h (fresh)
0.65
Still hot — no bloom
6h
0.45
Additive starting to bloom
24h
0.22
Near target — test here
72h
0.18
Stable — specification range
2 weeks
0.16
Equilibrium — fully bloomed
Why blooming happens: The slip additive (e.g. erucamide) is thermodynamically incompatible with the PE matrix at room temperature. After the film cools, the additive molecules gradually diffuse from the bulk polymer to the film surface — driven by the lower energy state at the air-polymer interface. This surface layer of slip additive molecules acts as a boundary lubricant, reducing surface adhesion and therefore COF.
Practical implications of blooming for quality control
▸Always condition at 23°C ± 2°C / 50% RH for 24 hours before testing — this gives a reproducible reference point during the bloom curve
▸Use the same conditioning time for all specimens being compared — even 1 hour difference can cause 0.05–0.10 COF variation
▸Warn customers about initial high COF — fresh film (< 6h) may not run on their equipment; specify a minimum 24h curing time before use
▸Temperature during storage matters — film stored at 35°C blooms faster than film at 15°C; summer vs winter storage can shift final COF
▸Test COF from the middle of the roll, not the outer wraps — outer wraps may be more bloomed due to longer time since extrusion
ASTM D1894: The Standard Flat Sled Test Method
ASTM D1894 (Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting) is the universally accepted method for COF testing of packaging films. Its equivalent standards are ISO 8295 (international) and IS 9738 (Indian).
Test geometry and parameters
Reading the COF force-displacement chart
📈
Initial peak
Force rises rapidly as sled starts to move — this peak is the static friction force
📉
Drop to plateau
Force drops after the peak as sliding begins — now kinetic friction dominates
➡️
Sustained plateau
Relatively flat region = kinetic friction force. Mean of this region = kinetic friction force
TAPPI T-815: The Inclined Plane Method
TAPPI T-815 (Coefficient of Static Friction of Corrugated and Solid Fiberboard / Plastic Film) uses an inclined plane rather than a horizontal pull. The inclined plane method is particularly useful for:
▸Very slippery films (COF < 0.12) where the horizontal method gives unstable, noisy readings
▸Very rough or embossed surfaces where the sled rocks on texture peaks
▸Testing COF between different material pairs — e.g. film vs corrugated board
▸Applications where the angle of inclination directly translates to a pallet stability angle
▸Films used in bag-in-box, carton liners, and stacking applications
Test principle: A block (typically 500 g) covered with test material is placed on an inclined surface covered with the second material. The inclined plane angle is gradually increased at a controlled rate until the block begins to slide. The angle at which sliding begins is the slip angle (θ). Static COF = tan(θ).
µs = tan(θ) = sin(θ) / cos(θ)
Example: Slip angle θ = 11.3° → µs = tan(11.3°) = 0.20
Test Configurations — Film-to-Film vs Film-to-Metal
ASTM D1894 tests four surface combinations — the test report must always specify which configuration was used, as results differ significantly between them:
| Configuration | Description | When to Use |
|---|---|---|
| Film-to-Film (inner vs inner) | Most common — measures slip between packaging film surfaces as they contact each other in a bag or pouch | Automatic packaging machine bag-to-bag friction; pallet stack stability |
| Film-to-Film (outer vs outer) | Outer surface friction — for symmetrical films or where orientation matters | Wrap films where outer surfaces contact adjacent packages |
| Film-to-Metal | Film surface vs polished steel plate — simulates friction against machine guide rails, forming tubes, seal bars | VFFS forming tube friction; horizontal wrapper guide friction |
| Film-to-Film (inner vs outer) | Cross-surface test for asymmetric films (e.g. coextruded with different inner/outer layers) | Bags where the inner of one contacts the outer of another in a stack |
Step-by-Step: Performing the COF Test (ASTM D1894)
Condition specimens — critical first step
Condition all film specimens at 23°C ± 2°C and 50% ± 5% relative humidity for exactly 24 hours (or the specified time per the quality specification). This is the single most important step — all subsequent variability in COF results from non-standard conditioning. Handle specimens only by the edges; fingerprints contaminate the test surface and increase measured COF.
Cut specimens to size
Cut the plate specimen (flat surface below the sled) to the required dimensions — typically 250 mm × 130 mm or larger. Cut the sled specimen to wrap around the sled base (63.5 mm × 63.5 mm) without overlap. Mark the machine direction (MD) on each specimen so the pull direction can be aligned correctly.
Mount the plate specimen
Secure the plate specimen flat on the horizontal testing surface of the COF tester. Ensure it is completely flat, wrinkle-free, and firmly held. Any wrinkle creates a raised contact point that dramatically increases measured COF. The film surface to be tested must face upward.
Wrap and mount the sled
Wrap the sled specimen smoothly around the 200 g sled base. Secure with tape on the top face only — never on the test surface. The sled face must be smooth and wrinkle-free. Set the sled on the plate specimen gently — do not drop or drag. Connect the flexible pull wire horizontally from the sled to the force gauge.
Align the pull direction
Ensure the pull wire is horizontal and perfectly parallel to the test surface. Any upward or downward angle in the pull wire changes the effective normal force and invalidates the result. The pull direction should match the machine direction (MD) of the film — the direction in which the film runs in the packaging machine.
Start the test at 150 mm/min
Start the motorised pull at 150 mm/min. Record force continuously throughout the 130 mm travel. The force recording shows: a rising peak (static friction initiation) followed by a plateau (kinetic friction during sliding).
Calculate and record results
Static COF (µs) = Peak force (N) / 1.962 N. Kinetic COF (µk) = Mean force during plateau (N) / 1.962 N. Run at least 5 specimens and calculate the mean and standard deviation. If standard deviation > 0.02, investigate — excessive variability suggests non-uniform conditioning or specimen preparation errors.
Report complete test details
Report: Static COF (µs) and Kinetic COF (µk) to 3 decimal places, mean of 5+ specimens; test surface configuration (film-to-film inner, film-to-metal, etc.); film direction (MD/TD); conditioning (23°C / 50% RH / 24h); test standard (ASTM D1894); date, operator, film lot number.
🔗 Related Products:
- → COF Tester / Inclined Plane Tester — ASTM D1894 · TAPPI T-815 · CE & ISO certified — both flat and inclined plane methods
COF Reference Values for Common Packaging Films
The following values are typical for conditioned specimens (23°C / 50% RH / 24h), film-to-film configuration (inner vs inner), machine direction pull:
| Film / Configuration | Static COF (µs) | Kinetic COF (µk) | Notes |
|---|---|---|---|
| LDPE blown film — no slip additive | 0.55 – 0.80 | 0.45 – 0.70 | Natural PE adhesion; not suitable for automatic packaging |
| LDPE blown film — with erucamide | 0.15 – 0.30 | 0.10 – 0.25 | Standard packaging film; target range for most applications |
| LLDPE blown film — with slip | 0.20 – 0.35 | 0.15 – 0.30 | Higher base COF than LDPE; requires more slip additive |
| HDPE blown film (bags) | 0.25 – 0.50 | 0.20 – 0.40 | Stiffer film; somewhat higher COF than LDPE |
| BOPP (biaxially oriented PP) | 0.25 – 0.45 | 0.20 – 0.35 | Smooth surface; surface coatings affect COF significantly |
| Cast PP (CPP) — sealant layer | 0.30 – 0.55 | 0.25 – 0.45 | Higher COF vs BOPP; inner sealant surface |
| PET (polyester) film | 0.20 – 0.40 | 0.15 – 0.30 | Smooth, consistent; print surface vs sealant surface differ |
| Nylon (PA 6) film | 0.30 – 0.55 | 0.25 – 0.45 | Higher COF; moisture-sensitive — humidity affects results |
| Metalized PET / metalized BOPP | 0.15 – 0.30 | 0.10 – 0.25 | Very smooth metal surface gives low COF |
| PE/PA/PE co-extruded pouches | 0.25 – 0.45 | 0.20 – 0.35 | Outer PE layer; blend of PE and PA properties |
| LDPE film-to-steel (machine guide) | 0.30 – 0.60 | 0.25 – 0.50 | Higher than film-to-film; depends on steel surface finish |
The COF Tester — Specifications
The COF Tester from International Equipments performs both the ASTM D1894 flat sled method and the TAPPI T-815 inclined plane method on a single instrument:
Diagnosing High COF Problems
📉 Insufficient slip additive concentration
Most common cause. Verify the slip additive loading in the compound specification. Check masterbatch letdown ratio at the extruder. Measure additive content by extraction and GC analysis if available. Increase erucamide or oleamide loading by 0.02–0.05% and retest after 24h.
⏰ Testing too soon after extrusion (bloom not complete)
Film tested < 12 hours after extrusion will show high COF because slip additive has not yet bloomed. Condition for 24h minimum before testing. Tag rolls with production timestamp and enforce minimum hold time before dispatch.
⚡ Excessive corona treatment
Surface corona treatment (used to improve ink adhesion) oxidises the film surface, increasing surface energy and adhesion — raising COF. Check corona power settings. Corona effect on COF typically decreases over 48–72h as the surface partially de-activates. Test after the appropriate decay time.
🌡️ Low extrusion melt temperature
At lower melt temperatures, slip additive mobility in the polymer melt is reduced, slowing migration to the surface. Increasing barrel temperature by 10–15°C often improves bloom rate. Check actual melt temperature (not just set temperature) using a melt thermocouple.
💧 High anti-block agent loading
Silica anti-block creates roughness that mechanically interocks film surfaces. If anti-block loading has been increased (e.g. to prevent blocking in summer), it raises COF. Reduce anti-block by 0.05–0.10% and retest. Consider switching to a finer particle size silica which gives less COF increase per unit loading.
Diagnosing Low COF Problems
🎿 Excessive slip additive loading
Excessive erucamide (> 0.25%) causes COF to drop below 0.10 — film becomes unmanageable. Film rolls telescope; bags slide off pallets; film misregisters. Reduce additive loading gradually (0.02% steps), retesting at 24h after each change.
🌡️ High storage temperature accelerating bloom
Film stored in summer warehouse conditions (35–40°C) blooms much faster and further than film at 23°C. COF may be 0.10–0.15 lower than expected. Improve warehouse air-conditioning or test samples taken from the centre of the roll where bloom is more moderate.
🔄 Old film / long-term storage
Film stored more than 4–6 weeks at ambient temperature eventually reaches maximum bloom — COF stabilises but at a lower value than fresh-to-24h measurements. Implement FIFO stock rotation. Test COF at point of use, not just at manufacture.
💧 High humidity during testing
Humidity above 60% RH increases the blooming rate of fatty acid amides significantly. If the testing area is not humidity-controlled, summer test results can be 0.05–0.10 lower than winter results for the same film. Install humidity control or correct results for humidity deviation.
Tips for Accurate and Reproducible COF Results
- 1Control conditioning time precisely. Set a 24-hour conditioning timer for every batch. Log the conditioning start time on each specimen envelope. Even 2–3 hours' difference in conditioning time can cause 0.05 COF variation — more than most film specifications allow.
- 2Handle specimens only by the edges. Finger oils contain fatty acids that act like slip additives — a single fingerprint in the test area can reduce local COF by 0.05–0.10, creating anomalous low readings in that specimen.
- 3Keep the testing surface and sled scrupulously clean. Clean the flat plate and sled base with isopropanol before each test session. Residual slip additive from previous tests accumulates and biases subsequent results.
- 4Always specify and match MD direction. COF in the machine direction (MD) can differ from the transverse direction (TD) by 0.05–0.15 in oriented films (BOPP, PET). Always test in the direction relevant to the packaging machine — typically MD.
- 5Test 5 specimens minimum and discard obvious outliers. A single anomalous specimen (wrinkle, fingerprint, dust) can skew the mean. With 5 specimens, one outlier can be discarded if it is more than 2 standard deviations from the mean.
- 6Control laboratory temperature and humidity. COF is sensitive to both. A 5°C temperature increase can shift COF by 0.03–0.05 for PE films; a 20% RH increase can shift by 0.03–0.08. Test in a conditioned room (23°C / 50% RH) or correct for measured conditions.
- 7Test both film-to-film and film-to-metal if your customer uses specific metal guide materials. Film-to-metal COF can be significantly different from film-to-film COF — and the machine contact surfaces matter as much as the bag-to-bag contact.
- 8Check the sled weight regularly. The 200 g sled must be within ± 0.5 g. Wear, contamination, or damage to the sled changes the normal force and shifts all COF results systematically. Verify sled weight weekly on a calibrated balance.
Key Takeaways
- ✓COF (Coefficient of Friction) = Friction Force / Normal Force. Static COF governs film start-up; kinetic COF governs sustained transport. Both must be measured and reported.
- ✓Target COF for automatic packaging films: static 0.15–0.40; kinetic 0.10–0.30. Below 0.10 = too slippery (instability); above 0.50 = too high (machine jams).
- ✓ASTM D1894 (flat sled, 200 g, 150 mm/min) is the primary standard. TAPPI T-815 (inclined plane) is preferred for very smooth or very rough surfaces. Both are performed on the same COF Tester from International Equipments.
- ✓COF changes after extrusion due to slip additive blooming — typically decreasing from 0.60+ (fresh) to 0.15–0.25 (24–72h). Always condition at 23°C / 50% RH for 24h before testing for reproducible results.
- ✓High COF (> 0.50): check slip additive loading, test too soon after extrusion, corona treatment level, and anti-block loading. Low COF (< 0.10): check for additive overdose, high storage temperature, or old film.
- ✓The COF Tester from International Equipments performs both ASTM D1894 and TAPPI T-815 on a single instrument — CE and ISO certified, with PC data output for force-displacement charts.
Frequently Asked Questions
Common questions about COF testing, ASTM D1894, slip additives, and COF tester selection.