Elastic recovery and creep properties of waterborne two-component polyurethanes investigated by micro-indentation

Abstract

Polyurethane (PUR) films were obtained by casting waterborne formulation of polyhydroxylated polyacrylate resin and hexamethylene 1,6-diisocyanate trimer hardener. Conversion of the polyaddition reaction was followed by FTIR spectroscopy and appears independent of the NCO/OH stoichiometric ratio between HDI trimer NCO groups and polyhydroxylated polyacrylate OH groups (resin), but thermally activated. The crosslinked networks were investigated by swelling experiments and dynamic mechanical analysis. Calculated values of the average mass between crosslinks allowed explaining the evolutions of the conservation modulus and loss factor with NCO/OH stoichiometric ratio. Elastic recovery and creep behavior of the PUR films were investigated by micro-indentation. A four-element viscoelastic model was used to fit the indentation depth evolution during micro-indentation creep experiments. Results show that creep instantaneous elasticity is fully controlled by the network elasticity and thus by the NCO/OH stoichiometric ratio and that the ability of the network to dissipate energy remains high for non-continuous (NCO/OH ratio<1) networks. On the other hand, for high NCO/OH ratios, it was shown that hard PUR networks exhibit creep properties and significant retardation times. Finally the evolution of indentation springback factor vs. indentation creep factor was determined, showing that instantaneous elastic recovery behavior increases as creep behavior decreases.