To investigate the surface micro-hardness and the diametral tensile strength (DTS) of bulk-fill and conventional resin-composites after storage in food simulating solvents.
Eight materials were investigated. For the micro-hardness measurement, Teflon mould with an internal dimensions of 10 mm and 2 mm ( n = 15). For the DTS measurement, Split stainless steel moulds were used to make disk-shaped specimens of 6 mm diameter and 2 mm thickness ( n = 15). Materials were subdivided in to three groups (water, 75% ethanol/water and MEK). Micro-hardness measurements were made under a load of 300 gm with a dwell time of 15 s at 7, 30, and 90ds after storage. DTS was measured after 30ds at a cross head speed of 0.5 mm/min.
The storage time and type of solvent had a significant influence on the micro-hardness. MEK showed more drastic reduction in the material micro-hardness with an exception of G-aenial universal flo (GA-F) which showed similar results in water/ethanol and MEK. DTS values of materials stored in water ranged from 48.7 MPa for the GA-F and 30.6 MPa for Ever X posterior (EXP). Generally, the results are observed to decrease with increasing solvent power, except for GA-F.
Bulk-fill materials showed no superior results compared with the other materials. For the bulk-fill materials that are designed to be used as a base, their penetration by the solvents may be shielded and thus the changes observed in this study may not be of clinical importance.
The continuous development of resin-composite materials aims for long lasting aesthetic dental restorations. There is a large range of available resin-composites, generalisation about their behaviour and performance should be made cautiously, many resin-composite materials still have their own shortcoming in clinical performance . Resin-composite restorations must survive an aggressive environment that varies from patient to patient as do the masticatory forces, occlusal habits, abrasive foods, chemically active foods and liquids, temperature fluctuations, humidity variations, bacterial products, and salivary enzymes. These factors, separately or collectively, determine the longevity of the restoration . From a material aspect, the performance of the resin-composite restoration depends on several factors including the monomer system, the filler type, filler loading and the extent of cure .
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 . 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 . The effect of solvents on resin-composites is determined by their solubility parameter and the nature of their monomer. The solubility parameter is important in terms of its similarity to that of other substance .
The solubility parameter is defined as the square root of the cohesive energy density of the solvent; it is a mean of predicting the ability of a solvent to dissolve a substance (Eq. 5-1). It provides a clear numerical way of predicting the extent of interaction between materials. Solubility parameter is expressed as δ/MPa 1/2 . Water is known to be used as an aging media in several studies , ethanol and Methyl ethyl ketone (MEK) are known to represent food simulating solvents and cause an extreme dietary effect.
δ = c = Δ H − R T V m 1 / 2