Hardness and Fracture Toughness of Photo-cured Resin Composite After Solvent Storage

Objectives: To investigate the surface micro-hardness (VHN) and the fracture toughness (KIC) of Conventional and bulk-fill resin-composites after solvent storage.
Methods: Seven resin composite materials were examined: three bulk-fill (Ever X™,Filtek bulk Fill™, Filtek bulk one™), two packable nano-hybrid (Filtek XTE™, NovoPro Fill™) and two flowable nano-hybrid resin-composites were studied (Filtek Flow XTE™, NovoPro Flow™). For the micro-hardness measurement, materials were placed in a mould between two glass plates and cured from both sides for 20s by LED source. Specimens of each composite material were assigned in two groups (n=3), distilled water, 75% ethanol/water. Materials were evaluated post-irradiation at 1 h, 1 day and 30 days after storage at 37 ± 1°C. Vickers hardness was measured under a load of 300g for 15s. For the fracture toughness measurement, 16 Single edged-notched (SEN) specimens of each material type were prepared in a Teflon-lined brass mould. Specimens of each composite material were assigned to two groups (n=8) according to the period of water storage: 1 and 7 days at 37°C. The K_IC for each specimen was tested using three-point bending. Utilizing scanning electron microscopy (SEM) the fractured surfaces of fibre reinforced composite were examined to establish the relationship between fracture toughness and microstructures.

Results: The storage time and type of solvent have a significant influence on the micro-hardness. 75% ethanol/water showed more reduction in the material micro-hardness. The KIC values ranged from 1.09 to 2.54 MPa, EverX showed the highest K_IC while NovoPro Flow showed the lowest values.
Conclusions: 75% ethanol/water caused the greatest softening (reduction in VHN) for most of the composites examined. A highly filled packable nano-hybrid composite Filtek XTE had the highest VHN over time, regardless of the storage conditions. While for the KIC evaluation water storage of resin composites showed no significant change in the K_IC values, were EverX showed the highest values regardless of the storage time.