The location of adsorbed water in pigmented epoxy-amine coatings

Pigmentation can exacerbate problematic water uptake into protective paints, leading to detrimental effects such delamination, swelling and plasticization. Potential underlying mechanisms include poor adhesion between the pigment and binder, the formation of hygroscopic interphase regions close to the pigment particles, and internal stress. Here, we demonstrate the use of nanoscale chemical mapping using AFM-IR and nanothermal analysis to investigate the contribution of these mechanisms directly for model coatings based on diglycidyl ether of bisphenol-A (DGEBA) cross-linked with triethylenetetramine (TETA). Whilst moisture uptake increases in the presence of a microscale silica matting agent, no chemical interphase could be detected close to the wax-coated particles. Instead, nanothermal analysis reveals attenuated thermal expansion of the polymeric binder and raised Tg values, supporting favourable polymer-pigment interaction and an increase in internal stress in the polymeric network. This is confirmed by mapping water distributions directly using AFM-IR, where no accumulation at the intact pigment-polymer interface/interphase is detected. Finally, further evidence is provided by bulk thermal analysis, where increased plasticization and desorption enthalpies demonstrate that water sorption into the bulk polymer binder increases as a function of added pigment volume concentrations.