Effect of acidic monomer concentration on the dentin bond stability of self-etch adhesives

This study evaluated the effect of the concentration of acidic functional monomer on the dentin bond stability of a model two-step, self-etch adhesive system. Six self-etch primers were formulated using hydroxyethyl methacrylate (HEMA), 1,3-glycerol dimethacrylate phosphate (GDMA-P), ethanol and water. Different mass concentrations of GDMA-P were tested: 0, 15, 30, 50, 70 or 100% (primers labeled P0–100). The pH of the solutions was measured. The bonding resin was composed of (di)methacrylates. Bond strength to bovine dentin was assessed through a microtensile bond test. The beam specimens were stored in distilled water, at 37 °C, for 24 h, 6 months or 1 year. Data were statistically analyzed and failure modes classified under magnification. The increase in acidic monomer concentration was associated with an exponential decrease in pH (R2=0.999; P<0.001). All specimens debonded prematurely for the primers P0, P70 and P100. After 24 h, the bond strengths for P50>P30=P15. After 6 months and 1 year, P50=P30>P15. The bond strength after 6 months was similar to 24 h for P15 and P50, but significantly lower after 1 year. P30 showed no differences in bond strength over the 1-year storage period. A predominance of mixed failures was detected for all primers at 24 h. After 6 months, P30 and P50 showed a predominance of adhesive failures. After 1 year, the predominant failure mode for all primers was cohesive within dentin. In conclusion, a mass fraction of 50% of phosphate monomer is a limit to be added to self-etch primers; a more stable longevity of the bonds was obtained with the primer with 30% of phosphate methacrylate.

► Dental self-etch primers with varied concentration of acidic monomer were tested.
► Increased incorporation of acidic monomer caused exponential decrease in pH.
► A 50% mass of acidic monomer is a limit to be added to self-etch primers.
► More stable longevity of dentin bonds was obtained using 30% of acidic monomer.