Oxygen inhibition layer of composite resins: effects of layer thickness and surface layer treatment on the interlayer bond strength

An oxygen inhibition layer develops on surfaces exposed to air during polymerization of particulate filling composite. This study assessed the thickness of the oxygen inhibition layer of short‐fiber‐reinforced composite in comparison with conventional particulate filling composites. The effect of an oxygen inhibition layer on the shear bond strength of incrementally placed particulate filling composite layers was also evaluated. Four different restorative composites were selected: everX Posterior (a short‐fiber‐reinforced composite), Z250, SupremeXT, and Silorane. All composites were evaluated regarding the thickness of the oxygen inhibition layer and for shear bond strength. An equal amount of each composite was polymerized in air between two glass plates and the thickness of the oxygen inhibition layer was measured using a stereomicroscope. Cylindrical‐shaped specimens were prepared for measurement of shear bond strength by placing incrementally two layers of the same composite material. Before applying the second composite layer, the first increment's bonding site was treated as follows: grinding with 1,000‐grit silicon‐carbide (SiC) abrasive paper, or treatment with ethanol or with water‐spray. The inhibition depth was lowest (11.6 μm) for water‐sprayed Silorane and greatest (22.9 μm) for the water‐sprayed short‐fiber‐reinforced composite. The shear bond strength ranged from 5.8 MPa (ground Silorane) to 36.4 MPa (water‐sprayed SupremeXT). The presence of an oxygen inhibition layer enhanced the interlayer shear bond strength of all investigated materials, but its absence resulted in cohesive and mixed failures only with the short‐fiber‐reinforced composite. Thus, more durable adhesion with short‐fiber‐reinforced composite is expected.