I recently received the following two questions by e-mail on COF of laminates and thought you might be interested.
1. A foil/paper laminate roll is kept in an AC room for three months, but after three months, the non-laminated side of foil shows drop in treatment level. Can you explain the reason for such drop in treatment level of foil?
I would expect that migratory additives are transferring from the paper to the foil surface. This would be typical. Also there may be some oxidation of the foil changing the chemistry of the surface. At any rate, to fully understand the changes you would have to measure the chemical changes taking place at the aluminum surface. Specifically you would be looking for organic contamination or a change in the aluminum oxide composition or hydrolysis
2. Whether the COF (coefficient of friction) of a PET / Polyethylene laminate drops after 5 days after lamination? If the COF of laminate is 0.405 and then drops to 0.250, Is it possible and if yes then why?
Here again we have a slip migration occurring. Typically the lamination COF is high after lamination as the surfaces are free of additive (migratory slip additives such as erucamide of Stearamide etc) due to the high melt temperatures in processing.
As the sample cools the slip chemicals in the bulk diffuse to the film surface and create a layer of the migratory additives at the surface. The slip materials are low molecular weight and “contaminate” the surface with a grease like coating. This then lubricates the surface lowering the COF.
Slip migration is very sensitive to heating and cooling during processing and storage and In North America the migration is very difficult to control. This will be true for any region where there are distinct changes in average temperatures between winter and summer. In these variable climate regions, for many films using a migratory slip system it is necessary to have a summer and winter formulation. In general less slip is needed in summer due to the fast migration at summer temperatures and more is needed in winter so that a sufficient amount comes to the surface. Of course the amount needed in the winter is generally too much for the summer and the formulation slip levels have to be watched closely to prevent problems from too much surface slip (poor adhesion generally in lamination bond and or ink).
So for the migratory slip technology the two most difficult questions to be answered are; first, is it summer yet, and second, is it winter yet?