Objective: Strong interactions between the resin matrix and the filler are essential for a dental composite to attain excellent physical properties. A gap present at their interface will undermine their interactions. This work investigated the effect of polymerization volumetric shrinkage on the formation of gaps.
Methods: A bis-GMA and TEGDMA blend plus photoinitiators was used as the resin matrix, into which 10 or 20 wt% non-silane-treated or silane-treated powders were added to make a lightly filled composite. The translucent composite was placed into a cavity (f6.4mm and 2.0mm deep) created by an aluminum ring and a polyester strip. The wall of aluminum ring was treated either with a silicone releasing agent for unrestricted polymerization shrinkage, or with a dental bonding agent for restricted shrinkage. After photocure, the formation of gap was assessed with light attenuation through the 2.0 mm thick sample disk. A lower intensity attenuated by the specimen suggested more gaps at the interface since more light was scattered by the mismatched refractive indices. SEM was used to verify the presence of gaps.
Results:
Measured light intensity (mW/cm2, n=4), attenuated from initial 500 mW/cm2 and analyzed with the t-test.
Filler (wt%) | Non-silane-treated | Silane-treated | ||
Unrestricted | Restricted | Unrestricted | Restricted | |
Spherical 1 (20) | 64(3) a | 54(1)b | 75(6) a | 72(2) a |
Spherical 2 (10) | 118(6) a | 91(6) b | 147(4) a | 110(10) b |
Irregular 1 (10) | 206(5) a | 185(2) b | 187(1) a | 178(5) a |
Irregular 2 (10) | 202(5) a | 192(2) b | 208(4) a | 208(2) a |
Irregular 3 (10) | 101(5) a | 87(3) b | - | - |
Conclusion: Restricted volumetric shrinkage created gaps between filler and resin matrix while free shrinkage did not. Good silane treatment on filler can prevent the gap formation by strengthening the filler-resin interaction.