Integrated approach to studying the development and final network properties of urethane acrylate coatings

An integrated approach, involving the use of a number of analytical techniques, was used to study polymer coating network development during photopolymerization of a series of fast reacting, low modulus coating formulations containing urethane/acrylate oligomers and acrylic comonomers. Real-time UV rheology measurements, to assess the development of viscoelastic properties, and real-time FTIR, to assess the disappearance of acrylic groups, were used to study the early stages of the photocuring reaction up to the gel point. These results showed that the development of the cross-linked network and the rate at which acrylate double bonds reacted were not necessarily directly proportional as might be expected and were dependent upon the structures of the urethane/acrylate oligomer and comonomers comprising a particular coating. Samples of partially polymerized coatings were also prepared having degrees of acrylate conversion ranging from similar to 50% through full conversion. Analysis of the amount of extractable material and the molecular weight distribution of soluble oligomeric material as a function of degree of conversion also showed a dependence upon the coating formulation components. In some cases the extraction results for intermediate cure level samples could be correlated to the reactivity trends observed in the early stages of the curing reaction. We also made use of solid-state NMR H-1 T-2 relaxation measurements to assess both partially and fully reacted films. The rate of magnetization decay correlated well with both the developing level of cross-link density as the degree of polymerization increased and also with the theoretical cross-link densities expected for the fully cured networks based on the structure of the coating formulation components. The latter also correlated well with modulus measurements made on fully cured coating film samples.