The effect of filler and silane content on conversion of resin-based composite

Objective. This study examines the influence of filler loading and silane content on the conversion of photoactivated, resin-based composites as determined using Fourier transform infrared spectroscopy (FTIR). Method. Zirconia/silica filler was processed with a silane coupling agent (γ-methacryloxypropyltrimethoxysilane) to achieve a range of silane-to-filler compositions. Treated fillers were compounded with a photoactivated BisGMA/TEGDMA resin to yield a series of pastes all containing 72 wt% total solids. Diffuse reflectance FTIR was used to characterize methacrylate unsaturation of the silane on the filler particles while paste conversion was determined using transmission FTIR. A resin matrix conversion was determined by adjusting the paste conversion for silane unsaturation. Two additional series of pastes were compounded to achieve variable filler-to-resin ratios using non-treated filler and filler processed with 8 wt% silane. Paste conversion was determined using transmission FTIR. Results. Pastes compounded from fillers containing variable percent silane demonstrated linearly decreasing conversion (R2=0.986) with increasing silane content and ranged from 52.7 to 62.8%. Adjusting paste conversion for silane unsaturation yielded similar resin matrix conversion for all cured pastes (65.1±0.8%). Pastes compounded with increasing filler-to-resin ratios had progressively decreasing conversion. Correcting for silane unsaturation suggests this effect was independent of whether the filler was silane-treated or not. Significance. Composite resin matrix conversion as determined via FTIR can be underestimated by the presence of unreacted methacrylate from silane on the filler. A corrected resin matrix conversion can be estimated by adjusting for silane unsaturation. Additionally, increasing filler-to-resin ratio progressively decreases conversion independent of the presence of silane on the filler.