Bond strength of self-etching adhesives pressure that stimulates the penetration of oral flu- bonding agents.33 Also, the water sorption and sol- ids into the margins. If the temperature increases, ubility of composites as well as adhesives increase the LTEC, pressure, and µL also increase.24, 25 On progressively with time.34 Furthermore, the etching the other hand, it has been advocated that the resin procedure can subject collagen fibers to degrada- composites present a very slow rate of thermal dif- tion, which can also be produced by the activation of fusion.26 Clinically, the short duration of thermo- host-derived matrix metalloproteinases (MMPs).35 cycles does not cause a dimensional change of the Despite the reduction in the µTBS with time, the material and might not affect the microleakage.26, 27 mean µTBS values observed for ASB2 were higher The effect of mechanical stresses is related than the mean values seen for the two self-etching to the differing elastic modules of dentine (18-20 adhesives. GPa), enamel (80 GPa), and resin (16 GPa). It has The outcome of composite restorations is based also been claimed that the combination of thermal on the adhesive interface integrity, and several changes and occlusal forces comprises approxi- polymers have been created to promote more re- mately 95 % of the stresses applied to posterior sistance over time.2 In oral conditions, the water restorations.28 The consequence of the mechanical environment, thermal changes, and fatigue deter- stress is usually the increase of gaps in the axial mine the progressive degradation of adhesive inter- wall or in the enamel or dentin margins.29, 30 Accord- face components, increasing the potential failure of ing to the results of the present study, the percent- restorations.1 The degree of resistance to degrada- age of gap-free axial interfaces was reduced after tion can also be related to the solvent added to the the thermo-mechanical treatment (Figure 1), irre- adhesive.36 Water is a highly polar solvent of Adper spective of the adhesive applied. On the other hand, Prompt L-Pop which presents a high dielectric con- statistically equivalent gap widths were observed stant, which allows the strong hydrogen bridges to among the experimental groups (Table 3). Thus, the be formed and to expand the collagen matrix.3 Un- second hypothesis was not accepted, as no differ- fortunately, high ebullition temperature, low vapor ence was noticed regarding the axial gap width. pressure, and oral cavity humidity makes its evapo- The third hypothesis, that the bond strength to ration more difficult.37 Self-etching adhesives re- dentin of self-etching adhesives would be similar quire a higher volume of solvent to keep the hydro- to that exhibited by a conventional etch-and-rinse philic and hydrophobic monomers together and to adhesive, was not accepted. Both the immediate contribute to the ionization process for self-etching and the 6-month µTBS mean values found when activity. Thus, these systems behave as semi-per- the self-etching adhesives were used demonstrat- meable membranes that can lose their properties ed significantly inferior results. When the conven- through the separation of components.12 The main tional etch-and-rinse adhesive system was applied component of Adper Prompt L-Pop is HEMA-phos- to dentin, significantly higher values were obtained phate. HEMA phosphate and phosphoric acid are (P<.05). Therefore, after 6 months, a significant re- comparably aggressive in terms of etching poten- duction was observed for the ASB2 adhesive (P<.05). tial.38 However, HEMA-phosphate exhibited a lower On the other hand, despite the lower mean values, apatite-dissolving capacity than H3PO4.39 The reason the bonding to dentin observed after 6 months was for this factor is probably because HEMA-phosphate statistically equivalent for both ASE and APL-P is considered an unstable molecule40 and because compared to the immediate values (P>.05). of its high molecular weight (322 g/mol), which The results expressed in the present study were gives it a lower molar concentration. Although wa- consistent with the findings of other authors who ter is required for ionization of acidic monomers, found that the efficacy of the conventional etch-and- residual water also lowers their concentration.41 In rinse adhesive system decreased with time when water, HEMA-phosphate component degrades into it was submitted to an artificial aging.7, 31, 32 Water HEMA and phosphoric acid molecules. This prob- molecules degrade the dentin/restoration interface ably results in a resin-sparse, less highly polymer- by hydrolysis, as a plasticizer agent or by increasing ized region along the base of the hybrid layers.19 the coefficient of thermal expansion.22 The conse- Despite its low pH (0.8), which can effectively seal quences are the lixiviation of matrix molecules such enamel margins in Class V restorations,42, 43 the in- as diluents, additives, plasticizers, initiators, and stability of Adper Prompt L-Pop can form irregular 174 European Journal of Dentistry