Reduced Bloom in Extruded PP Film

I recently received this from an extruder manufacturing PP film…

We are processing a 1.1mil 3-layer PP film with a random copolymer.

Initially, we were processing at 210ºC with a kinetic COF (coefficient of friction) = 0.5 initially, and 0.2 after two weeks.

Currently, if we process at 210ºC, we get a COF of 0.8, so we must process at 225 to get a COF of 0.5.

Do you know why we are getting this increase in COF?
note: After further questioning, it was discovered that the quantities of slip, AB, and MB anti-block additives have not changed. Also, they do not currently collect any MFI (melt flow index) or viscosity data from the supplier.
My Response
It is very likely that your material supplier has changed material characteristics on you. The efficiency of your additives relies on their ability to ‘bloom’ or migrate to the surface. Additionally… it appears that overall morphology of the polymer, including crystallinity, has changed since the material no longer exhibits the two week drop in COF as it once did.
If a significant change in either molecular weight, or molecular weight distribution has occurred, then it will change the COF of the base polymer as well as the additive’s ability to ‘bloom’ or migrate to the surface.
You should require the supplier to provide certification for each lot that you receive. This should include some basic data including the Melt Flow Index.
note: the same effect occurs with injection molding with molded-in lubricants and internal release agents.
Additional Thoughts
You should consider performing some basic material tests at your facility… this should include the MFI (Melt Flow Index), but may also include the DSC (Differential Scanning Calorimeter) and capillary rheometer.
Some companies actually purchase a benchtop extrusion line to test and understand the processing characteristics of incoming material lots prior to actual production.

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