Question: We recently did a dyne test on epoxy-coated steel wire. The lowest level test marker we had was 34 dynes/cm, and it beaded up instantly. Your polymer charts state that epoxies should run between about 42 and 52 dynes/cm. What gives?
Answer: The polymer chart for epoxies (http://www.accudynetest.com/polymer_surface_data/epoxy.pdf) shows data for unmodified epoxy resins. In many cases, these resins are modified to increase hydrophobic properties, making them more weather-resistant. These epoxy blends, once cured, will have lower surface energies than unmodified epoxies. There is a very good chance that the resin you are using is modified with a low energy additive. I’ll briefly discuss two examples pulled from the literature.
a) Epoxy modified with fluorinated poly (aryl ether ketone) (PETK): PETK, like other fluorinated polymers, has a very low surface energy. A study published in 2010(1) demonstrated that higher concentrations of PETK did, in fact, significantly reduce the surface energy of the epoxy blend. One interesting result from this study was that resins cured at low temperatures (30° C) demonstrated markedly greater reductions in surface energy than did those cured at higher temperatures (80° C). The reason for this is that at the higher cure temperature, phase separation is less pronounced because cross-linking occurred more rapidly, impeding the ability of the PETK to bloom to the surface.
b) Epoxy modified with polysiloxane showed similar results(2). For example, for material processed in a silicone mold, unmodified epoxy showed a contact angle with water of 68° (roughly 39 dynes/cm), whereas blends containing polysiloxane showed contact angles of up to nearly 100° (roughly 28 dynes/cm). Undoubtedly the cure in a silicone mold also influenced both readings, as surface-to-surface transfer within the mold would result in traces of silicone – another low energy solid – on the surface of the cured resin.
Please keep in mind that, in general, lower surface energy components will tend to move towards the surface during curing, so the decrease in surface energy can be quite dramatic even at low concentrations of additives.
References:
1) W. Brostow, M. Dutta, and P. Rusek, “Modified epoxy coatings on mild steel: Tribology and surface energy,” European Polymer J., 46 (2010), 2181-2189.
2) Z.-x. Huang, Y. Huang, and Y.-z. Yu, “Modification of epoxy resin with polysiloxane bearing pendant quarternary ammonium groups,” Chinese J. of Polymer Science, 20 (2002), 537-541.