Does the bond decrease over time and why?

In laboratory testing, bond strengths of some bonding systems may decrease as a result of thermal cycling; however, some bonding systems actually show increased bond strength with thermal cycling. Clinically, some research suggests that total-etch bonding agents may experience decreased bond strength as a result of water penetration (water trees) and also enzymatic degradation.

When using a self etched system it is important to to rub the liquid thoroghly onto the enamel to get a good bond.