Effects of resin hydrophilicity on water sorption and changes in modulus of elasticity

Abstract

As acidic monomers of self-etching adhesives are incorporated into dental adhesives at high concentrations, the adhesive becomes more hydrophilic. Water sorption by polymers causes plasticization and lowers mechanical properties. The purpose of this study was to compare the water sorption and modulus of elasticity (E) of five experimental neat resins (EX) of increasing hydrophilicity, as ranked by their Hoy's solubility parameters and five commercial resins.

Methods

After measuring the initial modulus of all resin disks by biaxial flexure, half the specimens were stored in hexadecane and the rest were stored in water. Repeated measurements of stiffness were made for 3 days. Water sorption and solubility measurements were made in a parallel experiment.

Results

None of the specimens stored in oil showed any significant decrease in modulus. All resins stored in water exhibited a time-dependent decrease in modulus that was proportional to their degree of water sorption. Water sorption of EX was proportional to Hoy's solubility parameter for polar forces (${\delta }_{\mathrm{p}}$) with increasing polarity resulting in higher sorption. The least hydrophilic resin absorbed 0.55 wt% water and showed a 15% decrease in modulus after 3 days. The most hydrophilic experimental resin absorbed 12.8 wt% water and showed a 73% modulus decrease during the same period. The commercial resins absorbed between 5% and 12% water that was associated with a 19–42% reduction in modulus over 3 days.

Introduction

As dentine adhesives increase in hydrophilicity, they can absorb considerably more water [1], [2] than the original enamel adhesives or pit-and-fissure sealants that are composed of comparatively more hydrophobic resins [3]. Because of small molecular size and high molar concentration of water, it can penetrate into nanometer-size free volume spaces between polymer chains [4], [5], or cluster around functional groups that are capable of hydrogen bonding [6], [7]. The major effect of water on polymer matrices is a depression of glass-transition temperature (T_g), that results in a decrease in thermal stability and polymer plasticization. These changes occur by different mechanisms, depending on the level of interaction of sorbed water molecules with the polymer matrix. Water sorption may deteriorate polymer mechanical properties, such as the modulus of elasticity, yield strength and produce changes in yield/deformation mechanisms. Sorbed water may also result in hygrothermal degradation during aging (such as the formation of swelling stresses, microcrack and craze formation), degradation of the matrix/fiber or matrix/filler interfaces, and polymer chain scission through hydrolytic cleavage [8], [9], [10]. It is thus anticipated that if the adhesive layer coupling resin composite to hybridized dentine becomes less stiff over time due to water sorption, it may adversely affect stress distribution across the bonded interface possibly resulting in debonding under repeated loading.

Water sorption within polymer matrices created by contemporary hydrophilic dentine adhesives is not always uniform. Using ammoniacal silver nitrate to trace the distribution of water sorbed, Tay et al. have shown both uniform and non-uniform water uptake into commercial adhesive resins [11], [12], [13]. Uniform absorption was seen as isolated individual silver grains, while the non-uniform type formed linear, branched water-filled channels [12]. When dentine bonded with adhesive resins was stored in water for 12 months, the distribution of absorbed water increased dramatically [13]. Clearly, water sorption by dental resins and the hybrid layer is more complex than expected [14].

Data on the effect of water uptake on the mechanical properties of polymers has been based mostly on epoxy-based systems. However, little is known of the water sorption characteristics in hydrophilic methacrylate-based resins that are commercially employed as adhesives for bonding to hydrophilic substrates such as dentine. Hydrophilic and/or ionic resin monomers are incorporated in most contemporary dentine adhesives to enable them to bond to intrinsically wet dentine substrates. In order for self-etching primers and adhesives to diffuse through smear layers and demineralize the underlying dentine, they are rendered more acidic by increasing the concentration of ionic or acidic monomers. Moreover, the correlation between resin hydrophilicity and water sorption has not been convincing in previous studies as a systematic method for calculating the polarity of the resin systems has not been utilized [5], [10], [15], [16].

The purpose of this work was to study the effects of water sorption on the elastic modulus of a series of unfilled resins of known composition as well as dental resins sold commercially. These resins included both hydrophobic and hydrophilic monomers, from which the relative hydrophilicity could be determined and expressed in terms of Hoy's solubility parameters. It was hypothesized that resins having higher Hoy's solubility parameter for polar forces result in (1) higher water sorption values and (2) decreased elastic modulus.