## Polymer Surface Energy vs. Coefficient of Friction (COF)

Question: Our customer is concerned with controlling their finished product’s COF, but we are using the dyne test to monitor the printing of their plastic parts. What is the relationship between the two? Can one measurement be used to predict the other?

Answer: There is a relationship between surface energy and coefficient of friction (COF): higher COF, like higher surface energy (greater wettability), tends to correlate positively with better adhesion(1). Similarly, higher COF tends to correlate to higher surface energy, as shown in the following link:

se-vs-cof (2)

Regression line shown in chart: Surface energy = 34.8 + 65.6 x COF;
or, COF = 0.26 + .011 x Surface energy

The correlation coefficient between these two variables is a very respectable +0.84. Nevertheless, for the two data points furthest from the regression line (PP and PVC), if we were to use COF to predict surface energy, we’d be off by more than 10 and 8 dynes/cm, respectively. A similar analysis of five polymers tested for COF at varying loads showed similar correlation coefficients with surface energy of approximately +0.70 to +0.75(3).

Despite these strong correlations with surface energy, COF is generally more dependent on tribological factors (e.g., consider the greater sliding resistance of coarsely sanded surfaces vs. finely sanded ones, despite the fact that the surface energies of the two will be very similar). And, the wettability of most polymers is determined more by the degree of polarity at the surface — this is a primary reason why corona, flame, and plasma treatments are all effective.

Increasing the polarity of the surface will, by itself, increase COF, as there will be greater interfacial attraction between surfaces of higher polarity. The classic example of this is blocking in rolls of corona-treated film, where the release resistance of the web increases with higher treat levels. In this case, blocking is likely caused primarily by the polar attraction between the surfaces, and secondarily by static charges.

But corona treatment also changes the morphology of the film’s surface, so it also has a tribological effect that would contribute to changes in sliding resistance, which is a typical way to measure COF. Depending on the molecular chain structure of the polymer, and how it is altered by surface treatment, this effect could be either positive or negative. I would guess that variations in treater gap, temperature, ozone concentration, polymer type, and other factors could all contribute to this rather complicated scenario. A smoothly stippled treated surface may have a lower COF, whereas one with sharper “edges” may have a higher COF.

Two predominant methods are employed to combat blocking and other film-handling problems — anti-blocking agents and slip additives. The two methods are functionally quite different: anti-blocking agents protrude slightly from the surface of the film, drastically reducing the actual interfacial area. This results in a commensurate reduction in COF. Conversely, slip additives, as they migrate to the surface, produce a lubricious film which reduces COF. In terms of surface energy, slip additives cause a significant (and sometimes problematic!) decrease, whereas anti-blocking agents exhibit a far less pronounced effect. Other types of surface-blooming additives will likely have effects somewhere in between.

Based on the number of variables involved, I would not recommend trying to use either measurement as a predictor of the other, even though overall a strong correlation undoubtedly prevails. Introducing additives, various surface treatments, and other process variables will only increase the scatter, and decrease the degree of correlation.

By way of a summary, years ago a European business developed an online surface treatment monitoring device that used COF as a proxy for treatment level. The concept was that if a baseline COF could be measured upstream from the treater, the change in COF from that baseline to a downstream post-treat measurement would provide an excellent proxy for surface energy change. Unfortunately, the concept apparently fell short in real-world testing: I don’t believe that a single unit was sold. This was probably due to a combination of high capital cost and the litany of process and material interactions this article has briefly discussed.

References:

1) N. Maeda, N. Chen, M. Tirrell, and J.N. Israelachvili, “Adhesion and friction mechanisms of polymer-on-polymer surfaces,” Science, 297, (2002), 379-382.

2) Based on data from V.R. Sastri, Plastics in Medical Devices: Properties, Requirements, and Applications, Elsevier, 2010, p. 57.

3) L.-H. Lee, “Effect of surface energies on polymer friction and wear,” in Advances in Polymer Friction and Wear, Vol. 1, Plenum Press, 1974.