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Improved Filler-Matrix Coupling in Resin Composites
1 Department of Biomaterials, Okayama University Graduate Schools of Medicine and Dentistry, 2-5-1 Shikata-cho, Okayama 700-8525, Japan; Correspondence: *corresponding author, bart.vanmeerbeek{at}med.kuleuven.ac.be
Ineffective silane coupling between filler and matrix within dental composites is prone to accelerated in vivo degradation. In this study, we examined to what degree a procedure involving chemical decontamination of filler prior to silanization could improve the filler-matrix bonding, and thus the physico-mechanical properties, of composites. X-ray photoelectron spectroscopy revealed that filler-matrix coupling largely depended upon siloxane bridge (Si-O-Si) formation between the silica surface and the silane molecule, rather than on intermolecular bonding between adjacent silane molecules. Pre-silanization decontamination based upon boiling silica in 0.05-5% sodium peroxodisulfate, followed by ultrasonic rinsing in acetone, most effectively decontaminated filler. Consequently, it significantly improved the bonding of silane molecules to silanol groups at the silica surface. Experimental composites produced following pre-silanization decontamination of filler revealed a diametral tensile strength that was resistant to degradation by thermocycling.
Key Words: silanization • filler • XPS • decontamination • resin composites
In light of the current trend toward "minimally invasive dentistry" (Degrange and Roulet, 1997) and in response to the growing patient demand for improved esthetics, resin composites are the materials of choice for the restoration of anterior teeth. Since they have greatly improved over the years, even in stress-bearing posterior areas, composites are increasingly being used. However, little evidence is available to support the direct use of composites as cuspal replacement materials or in areas under heavy occlusal function (Xu et al., 1999; Ferracane, 2000). Consequently, their overall use in the posterior region can still not be recommended. Major technical parameters that determine the longevity of direct composite restorations are: material strength or resistance to fracture, fatigue resistance or resistance to degradation upon cyclic loading, and wear resistance. Among others, one factor that contributes largely to these physico-mechanical properties is the silane coupling at filler-matrix interfaces (Söderholm and Shang, 1993). Effective coupling between resin matrix and glass filler has been reported to slow degradation processes (Broutman and Sahu, 1971; Brown, 1980), to protect the filler surface against fracture (Mohsen and Craig, 1995), and also to improve distribution and stress transmission from the flexible resin matrix to the stiffer and stronger inorganic filler particles (Calais and Söderholm, 1998). Although numerous studies have reported on silane coupling agents (Söderholm, 1984; Söderholm and Shang, 1993; Kim et al., 1994; Vallittu, 1997; Nihei et al., 2000), very few directly attempted to improve the interfacial silane-glass coupling. For the latter purpose, diverse methods to decontaminate glass filler prior to silanization have been tested (Shirai et al., 2000). That study revealed that SiO2, boiled with a 5% sodium peroxodisulfate aqueous solution for 15 min, followed by ultrasonic rinsing with acetone for 30 min, was most effective among 18 glass-decontamination methods tested. Moreover, nano-indentation measurements confirmed that the above-mentioned method did not weaken filler integrity (Shirai et al., 2000). In a continuation of that study, we used x-ray photoelectron spectroscopy, first, to optimize the formulation of the decontamination solution to the lowest still-effective concentration of sodium peroxydisulfate, and second, to evaluate, through chemical interfacial characterization of the silane-filler coupling, whether this pre-silanization decontamination method actually improved the effectiveness of silanization and consequently the physico-mechanical properties of composites.
Optimization of the Pre-silanization Decontamination Formulation SiO2 plates (Lot. No. SK-4303, Sumikinsekiei, Tokyo, Japan) were boiled in 0.001, 0.01, 0.05, 0.1, 0.5, 1, 5, and 10 w/w% sodium peroxodisulfate (Na2S2O8, Katayama, Osaka, Japan) solutions for 15 min, followed by ultrasonic rinsing with acetone for 30 min, so that we could determine the most effective concentration for surface decontamination. SiO2 plates were first used, since they allowed for more standardized and detailed quantification of surface decontamination efficacy. Then, both the lowest and highest concentrations were selected among the most effective solutions, so that we could confirm its decontamination efficacy on SiO2 filler (filler = 4 µm, irregular; GC, Tokyo, Japan).
Determination of Silanization Efficacy
For silanization, two silane molecules,
X-ray Photoelectron Spectroscopy (XPS)
Diametral Tensile Strength Measurement
XPS of non-decontaminated SiO2 detected C originating from contamination as well as Si and O as basic constituents (Fig. 1a  ). Decontamination of SiO2 through boiling in sodium peroxodisulfate for 15 min followed by ultrasonic rinsing with acetone for 30 min significantly decreased the intensity of the C 1s peak (Figs. 1b-1e). Most effective decontamination was achieved with 0.05-5% sodium peroxodisulfate, by which C fell below the detection limit (Figs. 1c, 1d).
When decontaminated SiO2 was treated with  -MPTS, the C 1s peak re-appeared (Fig. 2a). It therefore must be attributed to -MPTS that, despite ultrasonic rinsing, remained attached to the SiO2 surface. Likewise, decontaminated SiO2 treated with TTDCS resulted in an intense F 1s peak, indicating that TTDCS strongly attached to the SiO2 surface and resisted ultrasonic rinsing (Fig. 2b). However, when TTDCS was applied to non-decontaminated SiO2, the F 1s peak was substantially less intense, indicating that less silanization was achieved (Fig. 2c).
XPS wide-scan spectra of Si (Fig. 3a ) that was decontaminated by means of sodium peroxodisulfate/acetone (Fig. 3b), then dipped in a 10% hydrofluoric acid solution, then rinsed well with ultrapure water, revealed only Si peaks, indicating that any organic contamination and oxidation deposited on the Si surface were effectively removed, and a silicon single-crystal surface was exposed (Fig. 3c). With either -MPTS or TTDCS, contamination-free bare Si could not be silanized, since the resultant spectra did not differ from that of unsilanized, contamination-free bare Si (Figs. 3d and 3e compared with Fig. 3c).
Measurement of diametral tensile strength revealed that the immediate (no thermocycling) diametral tensile strength of the filler-decontaminated composite (43.1 ± 4.2 MPa) was not significantly different (p < 0.05) from that of the control composite (40.9 ± 6.2 MPa). However, thermocycling for 5000 and 10,000 cycles resulted in diametral tensile strengths of, respectively, 40.4 ± 5.2 and 38.5 ± 4.2 MPa for the experimental composite, readings which were significantly higher than those of the thermocycled control material, being 36.1 ± 5.5 and 28.8 ± 4.0 MPa. In addition, the average diametral tensile strengths of the experimental composite thermocycled 5000 and 10000 times were not significantly different from the diametral tensile strength of the non-thermocycled control composite.
Dental composites essentially consist of an organic resin matrix loaded with a finely dispersed glass or silica filler that is bonded to the matrix polymer through a silane coupling agent. During fabrication of resin composites, the silanization process is a critical step that, to a large degree, determines the physico-mechanical properties. So far, little attention has been paid to contamination adsorbed to the filler surface that may harm subsequent adequate wetting and bonding of the silane coupling agent during silanization procedures. As a result, structural defects that may lead to micro-cracks will occur at the filler-matrix interface (Griffith, 1920); these in turn may initiate crack growth along the filler-matrix coupling during cyclic loading. This was the major reason why we thought that pre-silanization decontamination might be an effective way to optimize filler-matrix coupling. This study revealed that a methodology based upon boiling filler in a sodium peroxodisulfate solution at a concentration as low as 0.05% for 15 min, followed by ultrasonic rinsing with acetone for 30 min, decreased the carbon contamination of the SiO2 surface below the detection limit of XPS. This method was more effective than common decontamination methods used, for example, to clean silicon wafers in the semi-conductor industry or glass in chemical laboratories (Shirai et al., 2000). Of direct importance to the production process of resin composites is that this method is not time-consuming, it is simple, safe, and consequently relatively inexpensive (Shirai et al., 2000).
To assess silanization efficacy, we used a fluoride-tagged silane coupling agent (TTDCS) in addition to the
XPS revealed that, upon TTDCS silanization, fluoride was detected on decontaminated SiO2 (Fig. 2b
When glass filler was silanized with -MPTS, multiple layers of silane molecules were reported to be attached to the filler surface, forming a distinct film around the filler particles (Söderholm and Shang, 1993). From XPS of Si treated with -MPTS, the carbon peak that should originate from -MPTS was hardly detected. This means that the deposition of a multi-layer film highly depends upon siloxane bridge (Si-O-Si) formation between the filler surface and the silane molecules, and/or that if the silane molecules mutually interacted to form a multi-layer film, it was unstable and could easily be removed from the surface. In other words, silane coupling depends largely upon the formation of a siloxane bridge between the filler surface and the silane molecule, whereas intermolecular bonds between adjacent silane molecules are less contributory. Consequently, to optimize silanization of glass filler as much as possible, one should increase the number of siloxane bridges between filler surface and silane molecules.
The atomic ratio C/Si for TTDCS after silanization is 10. That for It is concluded that pre-silanization decontamination of silica filler improves the silanization efficacy so that filler will be better retained physico-mechanically within the composite matrix. This benefit can be achieved by the use of a rather simple methodology based upon boiling filler in a sodium peroxodisulfate solution in a concentration as low as 0.05% for 15 min, followed by ultrasonic rinsing with acetone for 30 min. This method was demonstrated to improve directly the physico-mechanical properties of composites thanks to an improved and more hydrolysis-resistant filler-matrix coupling.
This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (No. 08771789 and No. 12771182). The authors thank GC for the fabrication of the experimental composites. Received for publication May 14, 2001. Revision received February 5, 2002. Accepted for publication February 7, 2002.
Journal of Dental Research, Vol. 81, No. 4,
270-273 (2002)
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ABSTRACT
INTRODUCTION
cm), following, in part, a protocol described by Takahagi et al. (1990). 
monochromatic x-ray source. Wide- and narrow-scan spectra were acquired at a pass energy of 80 and 40 eV, respectively. 
