Thermal Pretreatment and Characterization of Silica Dental Fillers

Background: Surface silanol groups play a critical role in engineering filler-resin interfaces and eventually composite properties. Silica materials synthesized by different chemical methods yield materials with different surface silanol concentrations, and therefore different reactivity with silanes.

Objective: To optimize silanization of monodisperse silica fillers as part of our study of ordered composites, we need to estimate silanol content following thermal pretreatments that eliminate any organic impurities.

Methods: Stober process synthesized silica (Alfa Aesar A-05) and two other composite filler materials (Nalco N-2329, Esstech E-V258) were selected for 10 pretreatments each involving a combination of decontamination, heating (200~800ºC), and/or rehydroxylation. Particles were decontaminated by washing in a boiling sodium peroxodisulfate solution (5wt%), followed by ultrasonic rinsing with acetone. Rehydroxylation was carried out by boiling particles overnight in de-ionized water. The particles were characterized using TGA, SEM, FTIR, and pycnometry.

Results: Both A-05 (~500nm) and N-2329 (~75nm) were spherical, with A-05 having better monodispersity (~6%). E-V258 (~700nm) particles were irregular with a broad size distribution, and fused at 800ºC. Heating all materials at 200ºC removed physically adsorbed water (dehydration). Subsequently, the TGA weight loss between 200~1000ºC was 6.4% (A-05), 1.2% (N-2329), and 1.1% (E-V258), respectively. Weight loss for A-05 after heating above 200ºC was attributed to removal of H-bonded and/or isolated silanol groups (dehydroxylation), internal water, and organic components. Weight loss was irreversible even after rehydroxylation.

Conclusions: Only A-05 had measurable organic content. The higher the treatment temperature, the lower the estimate of silanol content. For the A-05 silica, treatment at 600oC followed by rehydroxylation yields particles free of organic content and with surface silanols for silanization.

Acknowledgement: This study is supported by NIH R21DE018330. We thank Esstech and Nalco for providing materials.