Chemical Softening and Wear of Dental Composites

Abstract

The purpose of this work was to determine the influence of chemical food-simulating liquids on the wear of various commercial dental composite restoratives. In many cases, pre-conditioning the restoratives in these liquids for one week produced swelling of the polymer matrix and considerable surface damage. The resulting degradation reduced the hardness and enhanced the wear as measured by a pin-and-disc apparatus. Four kinds of commercial composites were investigated: a conventional quartz-filled, a strontium-glass-filled, a visible-light-activated, and a microfilled composite. The liquids employed were heptane and several aqueous solutions of ethanol with solubility parameters, delta, ranging from delta = 1.5 to 4.8 X 10(4) J1/2m-3/2. With all restoratives, the decline in hardness during pre-conditioning maximized at about delta = 3 X 10(4), which corresponds to a 75% ethanol solution. The wear behavior was considerably more complicated and variable, as discussed in the text. For the most part, the increase in wear rate from pre-conditioning corresponded to the fall in hardness. A notable exception was for the strontium-glass-filled composite pre-conditioned in pure water. Here the wear was enhanced considerably, with no decrease in hardness. In this case, the degradation mode is assumed to be different from the others in that it is attributed to stress corrosion of the glass filler.

Surface and bulk properties of dental resin- composites after solvent storage

“…The effect of water on VHN of the investigated material can be explained by the difference in the solubility parameter of the water (48.0 ␦/MPa 1/2 ) compared to the resin-based materials. These results were in agreement with other studies comparing different solvents effect on resin-based composites [11,26,37,45,46]. Organic solvents, as Ethanol and MEK, showed more detrimental effect on the mechanical properties compared to water storage.…”

“…Both solvents have solubility parameters that closely match that of the resin, (e.g. PMMA = 18.6, as a close approximation to dimethacrylates), 26.2 ␦/MPa 1/2 and 19.3 ␦/MPa 1/2 , respectively [11,12,18,35,37]. The closer the solubility parameter of a solvent to a substance, the stronger the softening effect of the solvent is [12,37].…”

“…Additionally, micro-hardness is considered as a strong indication to the degree of conversion of the material. Accordingly, measuring the surface microhardness could be a method of determining the longevity and the service life of the direct resin-based restorations [11,23,26,32,33]. Top surface hardness was measured as it received the maximum irradiance from the light curing unit and to avoid the attenuation of the light by passing through the bulk of the material [34].…”

“…Degradation of resin-composite restorations in the oral environment can be simulated by the use of different food simulating solvents which are known to cause different effects on the mechanical properties of the restoration and its longevity [4,[8][9][10]. Similar polarity of a particular solvent and a substance will tend to make them mutually soluble, different polarity on the other hand will make the solubility difficult [11,12]. The effect of solvents on resin-composites is determined by their solubility parameter and the nature of their monomer.…”

“…It provides a clear numerical way of predicting the extent of interaction between materials. Solubility parameter is expressed as ␦/MPa 1/2 [11,12]. Water is known to be used as an aging media in several studies [10,[13][14][15], ethanol and Methyl ethyl ketone (MEK) are known to represent food simulating solvents and cause an extreme dietary effect.…”