The free-radical polymerisation reactions of photoactive resins used in dentistry are inhibited by oxygen. There is a need to better understand the effects oxygen inhibition on critical properties of dental resins and resin composites which may compromise the success of dental resin composite restorations. This investigation aims to examine the formation of the oxygen inhibition layer (OIL) on experimental and commercial methacrylate and epoxy resin based materials and its effect on incremental bond strengths and stain resistance. Experimental BisGMA and TEGDMA comonomer resin mixtures loaded with either barium- or strontium-silicate glass fillers were fabricated. Commercially available dental resin composite restorative materials, including a novel epoxy-based system were also examined. The OIL of each material type was measured using a novel optical depth of focus technique, the integrity of resin composite layers and stain resistance was assessed by shear bond strength and colorimeter testing, respectively. An OIL was measurable for all materials polymerised in atmospheric air including the epoxy-based system Filtek™ Silorane. Different filler particle types complicate OIL formation for similar BisGMA/TEGDMA comonomer mixtures. The OIL had a detrimental effect on stain resistance, where an increased OIL thickness resulted in increased stain susceptibility. From the present findings it was suggested that incremental bond strength between composite layers is not wholly reliant on surface inhibition.
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