Slip Additive Migration and When to Test PE Film COF

What is slip additive migration in PE film?

Erucamide is a migrating slip additive. It is mixed into the polymer during extrusion and then moves to the film surface over time, where it changes the surface properties. The drop in COF depends on how much erucamide is actually on the surface.

Both main COF standards warn about this. ISO 8295 says slip additives have varying compatibility with the film, may bloom or exude to the surface, and that results must be related to the age of the film. ASTM D1894 also notes that slip additives in films such as polyethylene bloom to the surface.

Migration can also go wrong in either direction. Too little additive at the surface leaves COF high, while an excess can cause its own problems, as the temperature section below shows.

How does time after extrusion change COF?

ASTM D1894 states that the blooming of many slip additives is time dependent. It adds that comparing films produced at different times can be meaningless, unless that effect is what you want to study.

Published work gives a sense of the timescale. In a study of a three-layer LDPE film with about 3,890 ppm erucamide, COF values after 72 hours at room temperature matched those of samples stored at 50 °C.

The effect can also continue for months. A study of LLDPE films with 1,000 or 2,500 ppm erucamide and stable-COF additives found them still suitable for low-COF uses after 360 days at 23 °C and 50% RH.

Other migrating additives can interfere. In the same LLDPE study, an antistatic additive did not change COF in the first 14 days after extrusion. After that, COF values, mainly kinetic, could increase.

How does temperature change slip additive migration?

Heat speeds migration up. In the LDPE study, COF fell quickly as storage temperature increased.

Faster is not always better. The film stored at 50 °C carried visible excess erucamide crystals on its surface, which raised the film's initial sealing temperature. The authors noted that any rise in sealing temperature may be related to excess erucamide at the surface.

Temperature is also a tool for shelf-life work. The LLDPE study found that COF evaluation could be accelerated by storage at 40 °C for 120 days or 60 °C for 28 days, giving faster estimates of useful life.

Want to see how your film's COF moves with age? Tell us your test requirements. We source similar samples, run the test and send you the video. No need to ship samples.

When should you test PE film COF?

Pick test points that answer a specific question, and record the film age at each one. These points follow from the time and temperature effects above.

• Soon after extrusion, as a process check, knowing COF may still fall
• After the migration period your own data shows for that film
• Before shipment and again at goods receipt, when the film is older
• After warm storage or transport, when excess bloom is possible
• Before a packaging run, to confirm the film still slides as needed

Use fresh specimens for each measurement. For film run against metal or another material, ISO 8295 warns that repeat runs on one specimen may abrade it, and that slip agent transfer must be considered.

For how polymer type, surface treatment and conditioning affect results, see our guide to the coefficient of friction of plastic film.

What should a COF report record for slip films?

A slip film result needs its context written next to it. Without these fields, two results for the same film cannot be compared.

• Extrusion date and film age at test
• Storage temperature and humidity since extrusion
• Which film surface was tested, as ISO 8295 requires in the report
• The test method, since ASTM D1894 and ISO 8295 results are not directly comparable

Our MXD-02A coefficient of friction tester runs ASTM D1894 and ISO 8295 sled tests, with a 5 N force range and force accuracy of 0.5 % FS.

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Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].