Silica Coating of Nonsilicate Nanoparticles for Resin-Based Composite Materials

Abstract

This study was designed to develop and characterize a silica-coating method for crystalline nonsilicate ceramic nanoparticles (Al₂O₃, TiO₂, and ZrO₂). The hypothesis was that the coated nonsilicate nanoparticles would stably reinforce a polymeric matrix due to effective silanation. Silica coating was applied via a sol-gel method, with tetraethyl orthosilicate as a silica precursor, followed by heat treatment. The chemical and microstructural characteristics of the nanopowders were evaluated before and after silica coating through x-ray diffraction, BET (Brunauer-Emmett-Teller), energy-dispersive x-ray spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy analyses. Coated and noncoated nanoparticles were silanated before preparation of hybrid composites, which contained glass microparticles in addition to the nanoparticles. The composites were mechanically tested in 4-point bending mode after aging (10,000 thermal cycles). Results of all chemical and microstructural analyses confirmed the successful obtaining of silica-coated nanoparticles. Two distinct aspects were observed depending on the type of nanoparticle tested: 1) formation of a silica shell on the surface of the particles and 2) nanoparticle clusters embedded into a silica matrix. The aged hybrid composites formulated with the coated nanoparticles showed improved flexural strength (10% to 30% higher) and work of fracture (35% to 40% higher) as compared with composites formulated with noncoated nanoparticles. The tested hypothesis was confirmed: silanated silica-coated nonsilicate nanoparticles yielded stable reinforcement of dimethacrylate polymeric matrix due to effective silanation. The silica-coating method presented here is a versatile and promising novel strategy for the use of crystalline nonsilicate ceramics as a reinforcing phase of polymeric composite biomaterials.

Keywords: CAD-CAM; microscopy; nanotechnology; polymers; silane; surface properties.

Figure 1.

Representative illustration of the interaction among self-assembled crosslinked siloxane layers formed over the nanoparticles modified or not with the silica-coating method. (a) The trialkoxysilane function of the organosilane cannot chemically bond to the nanoparticles due to the absence of silica; thus, only physical interactions with the surfaces are formed. A siloxane layer is deposited around the nanoparticles by crosslinking among the silane molecules, but the coupling with the nanoparticle is not effective or stable. (b) The presence of a silica layer around the nanoparticles, deposited by the method proposed here, enables effective and stable silanation by formation of siloxane covalent bonds with the now silica-rich surfaces. The methacrylate group on the other end of the organosilane molecules makes the fillers compatible with the polymeric matrix.

Figure 2.

Transmission electron microscopy images of the nanopowders: (a) Silica-coated zirconia particles (×1.2M magnification). The white arrows point to zirconia nanoparticles (darker area at the center) surrounded by a silica layer (grayish area). The thickness of the silica layer seems to vary according to the diameter of the zirconia particles, which is visible when the 2 examples indicated by the white arrows are compared. (b) Silica-coated alumina particles (×1M magnification). Discrete nanoparticles much smaller than the other particles are observed. The blue arrow points to an example of a coated particle. (c) Silica-coated titania particles (×100k magnification). Most particles are seen embedded into clusters with a silica matrix (hollow black arrows), though some coated particles are also present (green arrows). (d) Silica nanoparticles (×300k magnification) were detected, most likely as a result of a secondary phase formation during heat treatment of tetraethylorthosilicate molecules that were not bound to the nanopowders in the solution used for the silica-coating method. This figure is available in color online at http://jdr.sagepub.com.

Figure 3.

Field emission scanning electron microscopy images of the nanopowders. Imaging conditions: electron beam set at 1 kV and 7 pA, sample distance 3 mm, and imaged in a field width of 2 μm—equivalent to approximately ×180k magnification: (a) Image of the nanopowders as received. Alumina (Al) particles are visibly smaller than the other particles. (b) Image of the nanopowders after silica coating. Most of the titania (Ti) particles are embedded into clusters with a silica matrix, which does not seem to happen with the other particles. All particles became highly unstable after silica coating (c), where some areas in the field (red arrows) show the particles fused as a reaction to the energy of the microscope beam. This is clearly seen for the silica-coated zirconia image, where the fusing started near the reduced focus area in the center of the field. (d) The uncontrolled fusing of particles, caused by the energy of the beam and taking over the entire field, thereby generating a 3-dimensional porous structure. This figure is available in color online at http://jdr.sagepub.com.