Effect of wet vs. dry testing on the mechanical properties of hydrophilic self-etching primer polymers

Self-etching primers and adhesives contain very hydrophilic methacrylate monomers that result in high water sorptions by their polymers. Water sorption plasticizes the polymers and lowers their mechanical properties. The purpose of this work was to rank the hydrophilicity of a series of acidic primers by their Hoy's solubility parameters (delta) to determine if there was a significant relationship between the delta of polymers and their mechanical properties. A series of six acidic primer blends containing a fixed concentration of phenyl-P but variable amounts of 2-hydroxyethyl methacrylate (HEMA), 2,2 bi[4-(2-hydroxy-3-methacryloyloxy)propane (BisGMA), and triethylene-glycol dimethacrylate (TEGDMA) was formulated and their Hoy's solubility parameters calculated. The polymers were cast into small 'I' beams and light-cured. The modulus of elasticity (E) and ultimate tensile strength (UTS) were measured in dry polymers and after immersion in water for 24 h. The results showed significant correlations between E and UTS under dry or wet conditions. Both E and UTS fell significantly when the specimens were immersed in water. After water immersion, the E and UTS showed significant correlations with Hoy's delta(p) values. Both E and UTS correlated significantly with the BisGMA concentration of the polymers, either wet or dry. The percentage changes in E or UTS were significantly correlated with the water sorption of the polymers.