Properties of clear polyurethane films modified with a fluoropolymer emulsion

Abstract

An emulsion of a fluoroethylene vinyl ether polyol, Lumiflon FE-4400, was incorporated into clearcoats of two-part water-based military polyurethane topcoats. This additive was mixed with the aqueous dispersion polyol component. The polyol component was then mixed with the isocyanate component with a high speed disperser to properly mix the reactants. Films were prepared by drawing down the dispersion onto glass substrates using a 4-mil stainless steel Bird applicator. After drying at ambient conditions the films were removed from the glass for analysis. As measured using X-ray photoelectron spectroscopy, Lumiflon FE-4400 segregated to the surface providing significant fluorine enrichment at the air interface, as high as 90 times the bulk concentration. Although increasing the concentration of Lumiflon FE-4400 increased the surface concentration, the ratio of surface concentration to the bulk concentration decreased. As expected, water contact angle increased approximately in a linear fashion with the surface fluorine concentration. At 1–5 wt% added Lumiflon FE-4400, the physical and mechanical properties were not significantly affected. However, addition of Lumiflon FE-4400 at or above 10% of polyol fraction significantly reduced the glass transition temperature, modulus, crosslink density, strength, toughness and weathering resistance. 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, likely explaining the reduction in film properties. Accelerated weathering of pigmented coatings was also performed. Although fluorinated resins typically improve weathering resistance of coatings, in this study, increasing the Lumiflon FE-4400 content decreased the weathering resistance as a result of the ethoxylation of the fluorinated additive necessary to produce an aqueous fluoropolymer emulsion.