Objectives: To evaluate the effects of filler pretreatments on the silanization of three submicron silica fillers.
Methods: Ten pretreatments involving combinations of peroxodisulfate cleaning, heating (200~800ºC), and/or boiling (overnight) in water were carried out on monodisperse silica particles (Alfa Aesar A-05 and Nalco N-2329; both spherical) and an irregularly shaped dental filler (Esstech E-V258, ground glass). Each material was then stirred (2hr) in a methacryloxypropyl trimethoxysilane (MPS) solution (1wt% in acetone), dried under vacuum (~25°C, overnight), heated at atmospheric pressure (110°C, 2hr), washed three times with methanol, and again dried under vacuum (overnight). The particles were characterized using TGA, SEM, FTIR, and elemental analysis (combustion).
Results: The Stöber process synthesized A-05 (~500nm) was found to contain considerable organic content (~2.2wt% carbon), possibly due to unhydrolyzed ethoxyl groups. The carbon contents for N-2329 (~75nm, synthesized from sodium silicate) and E-V258 (~700nm) were minimal (both ~0.1wt%). The difference (ΔW) of TGA weight losses (200~1000°C) from the same material before and after silanization was used to quantify the chemical attachment of MPS. For A-05, cleaning and rehydroxylation/heating (up to 400°C) appeared to enhance silane attachment, with ΔW increasing from 0.2wt% to 0.3~0.6wt%. After heat treatment at higher temperature (600 or 800°C), the silane attachment decreased, with a weight loss less than 0.2wt% even after overnight rehydroxylation.
Conclusions: The optimal pretreatment condition for A-05 silanization includes peroxodisulfate cleaning and heating at up to 400°C, which eliminates most organic impurities. The effect of pretreatment is minimal for N-2329 and E-V258.
Acknowledgement: Support from NIH R21DE018330 is appreciated. We thank Esstech and Nalco for providing materials.