Effect of evaporation of primer components on ultimate tensile strengths of primer–adhesive mixture
Introduction
In the last decade, the effect of mechanical properties of adhesive resins on resin–dentin bonding have received enhanced attention. Theoretically, superior mechanical properties of adhesive resins must result in improved bonding effectiveness [1]. Actually, some self-etch adhesives have been reported to exhibit similar dentin bond strengths, that also closely correlated to the fracture strength of the respective adhesive resins [2]. Another study concluded that the adhesive resin should achieve a sufficient level of physico-mechanical properties prior to application of the restorative composite, this to the immediate benefit of marginal adaptation [3].
Most studies that investigated the relationship between bond and fracture strength have looked solely to pure adhesive resin solutions. However, structural interface characterization, using confocal fluorescence microscopy, has shown that the adhesive resin is actually mixed with the primer applied beforehand in the case of multi-step etch&rinse and self-etch adhesives [4], [5]. Such mixture of adhesive resin with primer has been reported to decrease the strength of the adhesive resin [6]. By adding the primer to the adhesive resin a partially inhibited free-radical polymerization could explain this decrease in fracture strength [6], [7]. However, it is currently not known to what extent the fracture strength of resin is decreased by the application of a primer that contains hydrophilic monomers dissolved in solvents, such as acetone, ethanol, water, or a combination of these. During the short application time and subsequent air-drying of the primer, as much solvent as possible should be removed through evaporation, since residual solvent may inhibit polymerization and/or weaken the fracture strength. In this respect, insufficient air-drying of primers has been reported to result in lower bonding performance [7] and increased microleakage [8]. Hence, it is of crucial interest to determine the effect of evaporation of primer components on the fracture strength of primer–adhesive mixtures.
The purpose of the present study was to evaluate the effect of evaporation of primer components on the ultimate micro-tensile strength (UTS) of primer–adhesive mixtures. The null hypothesis tested was that primer components evaporate sufficiently, not affecting the UTS of the primer–adhesive mixture.
Section snippets
Products used
Two adhesives representing either the category of three-step etch&rinse adhesives (OptiBond FL, Kerr, Orange, CA, USA) or the category of two-step self-etch adhesives (Clearfil SE, Kuraray, Osaka, Japan) were selected (Table 1). Both adhesives presented with a fairly favorable bonding effectiveness in independent laboratory studies [9], [10], [11], [12], [13], [14].
Evaporation of primer components by storage in 37 °C dry oven
A drop of primer from Optibond FL or Clearfil SE was placed on a glass-plate (glass is chemically stable and will not lose weight
Evaporation of primer components by storage in a 37 °C dry oven
The degree of evaporation of the primer components by storage in a 37 °C dry oven with time is graphically presented in Fig. 1. The primer weight of OptiBond FL and Clearfil SE decreased rapidly in the first 5-min and 10-min time intervals, respectively. Then, the loss of weight gradually decreased to reach a linear weight decrease with time (line A for OptiBond FL and line B for Clearfil SE, see Fig. 1). The lines A and B were plotted connecting the 20-, 30-, 60- and 180-min weight loss values
Discussion
While OptiBond FL belongs to the class of etch&rinse adhesives (three-step), Clearfil SE is categorized as a two-step self-etch adhesive [13]. The function of the primer is obviously different for the two categories [15]. Regarding OptiBond FL, the primer contains monomers with hydrophilic properties (such as HEMA, GPDM and PAMM) that have an affinity for the exposed collagen fibril arrangement and with hydrophobic properties for co-polymerization with the adhesive resin. The monomers are
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