Interfacial studies of crosslinked polyurethanes .1. Quantitative and structural aspects of crosslinking near film-air and film-substrate interfaces in solvent-borne polyurethanes

One of the reactions leading to the formation of polyurethane (PU cross linked networks is the reaction of NCO and Cii functionalities. In this study, we examined how crosslinking reactions of hexamethylene diisocyanate isocyanurate and polyacrylate near the film-air (F-A) and film-substrate (F-S) interfaces in urethane coatings may effect crosslink density as well as other network properties, While at the initial stages of the crosslinking reactions, solvent evaporation competes with the urethane network formation and isocyante consumption changes at various depths from the F-A and F-S interfaces. Quantitative analysis of the NCO consumption of a function of depth showed that the NCO concentrations change From 2.35 X 10(-5) to 2.09 X 10(-5) M, while going from 0.27 to 1.14 mu m. During reaction times not exceeding two to three hours, the NCO consumption of the F-A and F-S interfaces consumed more rapidly. Allow relative humidity conditions, excessive amounts of unreacted NCO exists ai both the F-a and F-S interfaces, However, at the extended reaction times, NCO concentration levels at the F-S are greater than at the F-A interface, and the NCO concentration differences can be as high as 3 X 10(-5) M. In this study we also examined how crosslinking reactions of hexamethylene diisocyanate (HDI) isocyanurate and polyacrylate near the film-substrate F-S) interfaces in urethane films may affect orientation and distribution of urethane functionalities. It appears that at initial stages of the crosslinking reactions, driving forces resulting from interfacial tension effects compete with the urethane network formation. Orientation of the H-bonded urethane carbonyl functionalities change as a function of distance from the FS interfaces, ranging from 0.35 mu m to 1.51 mu m into a coating. Molecular migration and rearrangements near the F-S interface result from interfacial tensions between PU and substrates.