Formulation Efforts to Improve the Performance of Military Coatings
Felicia Levine, John J. La Scala, and John A. Escarsega. Army Research Laboratory, Building 4600, Aberdeen Proving Ground, MD 21005
The military has a need for coating systems with improved durability and decreased permeability to chemicals. In an effort to improve these properties, ARL has formulated water-dispersible polyurethane topcoats using small amounts of fluorinated polyol component and through changing the isocyanate to hydroxyl indexing. Increasing the isocyanate to hydroxyl ratio had no effect on Tg of clear coats, but increased the cross-link density as measured by the rubbery modulus through dynamic mechanical analysis. Permeability and solubility measurements correlated well with the cross-link density, as both decreased as isocyanate to hydroxyl indexing was increased. Fluorinated additives produced little effect on thermo-mechanical properties, including Tg, cross-link density, and modulus. X-Ray photoelectron spectroscopy shows that the fluorine additive migrated to the surface and produced fluorine surface concentration 10-90 times that of the bulk. Yet, water contact angle was reduced at low fluorinated polyol content because the ethoxylation of the fluorinated additive to increase its water dispersibility also decreased the contact angle. Phase contrast microscopy showed that the morphology of these films went from being uniform to having distinct aggregates that increased in size and number as fluorinated polyol fraction increased. Permeability and solubility testing is underway to determine whether these changes in morphology affect these properties. Fluorinated additives are supposed to improve weatherability of coatings systems, but color and gloss retention was slightly worse for the fluorinated polyols than the baseline systems.