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J Dent Res 81(4): 270-273, 2002
© 2002 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Improved Filler-Matrix Coupling in Resin Composites

Y. Yoshida1, K. Shirai2, Y. Nakayama3, M. Itoh2, M. Okazaki4, H. Shintani2, S. Inoue5, P. Lambrechts6, G. Vanherle6, and B. Van Meerbeek6,*

1 Department of Biomaterials, Okayama University Graduate Schools of Medicine and Dentistry, 2-5-1 Shikata-cho, Okayama 700-8525, Japan;
2 Department of Operative Dentistry, Hiroshima University Faculty of Dentistry, 1-2-3 Kasumi, Minami-Ku, Hiroshima 734-8553, Japan;
3 Surface Science Laboratory, Toray Research Centre Inc., Sonoyama 3-3-7, Otsu, Shiga 520-8567, Japan;
4 Department of Biomaterials Science, Hiroshima University Faculty of Dentistry, 1-2-3 Kasumi, Minami-Ku, Hiroshima 734-8553, Japan;
5 Department of Oral Health Science, Hokkaido University Graduate School of Dental Medicine, Kita 13 Nishi 7, Kita-ku, Sapporo 060-8586, Japan; and
6 Leuven BIOMAT Research Cluster-Department of Conservative Dentistry, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, B-3000, Leuven, Belgium;

*corresponding author, bart.vanmeerbeek{at}med.kuleuven.ac.be


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
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
A silicon (100) wafer (Lot. No. 780106687, JEOL DATUM, Tokyo, Japan) with a SiO2 layer of 100 ± 10 nm on one side was used for characterization of the effects of silanization. First, to obtain a contamination-free SiO2 surface, we boiled the wafer in a 5% sodium peroxodisulfate (Na2S2O8) solution for 15 min, followed by ultrasonic rinsing with acetone for 30 min. As control, we obtained a contamination-free bare Si (the other side of the wafer) surface using the above-mentioned decontamination method, followed by successively dipping the Si in a 10% hydrofluoric acid (HF) solution to remove the inherent oxide layer, then rinsing with ultrapure water (milli-Q water: > 18 M{Omega}cm), following, in part, a protocol described by Takahagi et al. (1990).

For silanization, two silane molecules, {gamma}-methacryloxypropyl trimethoxysilane ({gamma}-MPTS, C10H20O5Si, Lot. No. 5J87, Toshiba Silicon, Tokyo, Japan) and tridecafluoro-1,1,2,2-tetrahydrooctyl dimethylchlorosilane (TTDCS, C10F13H10SiCl, Lot. No. 95F-0930, Chisso, Tokyo, Japan) were used. {gamma}-MPTS is the filler-matrix coupling agent most commonly utilized by dental composite manufacturers and is expected to bond chemically to the filler surface and to co-polymerize with the methacrylic polymer matrix. The fluoride in TTDCS was used as a label for quantification of the effect of filler decontamination on silanization efficiency. For {gamma}-MPTS, SiO2 and Si were immersed in a 2% {gamma}-MPTS solution at room temperature for 2 hrs, then heated successively at 70°C for 1 hr and at 110°C for 3 hrs, followed by ultrasonic rinsing twice with ultrapure water for 5 min. For TTDCS, SiO2 and Si were treated with 2 x 10-3 M TTDCS in a 2:3 chloroform/carbon tetrachloride (CHCl3/CCl4) solution at room temperature for 2 hrs, followed by ultrasonic rinsing twice with chloroform (CHCl3) for 5 min.

X-ray Photoelectron Spectroscopy (XPS)
The surfaces of SiO2 and Si were chemically analyzed by XPS (AXIS-HS, Kratos, Manchester, UK) with an Al-K{alpha} monochromatic x-ray source. Wide- and narrow-scan spectra were acquired at a pass energy of 80 and 40 eV, respectively.

Diametral Tensile Strength Measurement
The effect of pre-silanization decontamination on physico-mechanical properties was determined by the measurement of diametral tensile strength. An experimental resin composite (SiO2 filler within a 70/30 wt% Bis-GMA/TEGDMA matrix; GC) was fabricated with the silica filler particles decontaminated prior to silanization. An identically composed composite without filler decontamination served as control. Forty-five cylindrical specimens of the experimental and control material were cured in a Teflon mold (diameter = 6 mm; height = 3 mm) for 1 min by means of an Optilux light-curing unit (Demetron/Kerr, Danbury, CT, USA). After removal from the mold, the specimens were cured for another min from the bottom side. All samples were then stored in distilled water at 37°C for 1 day, after which 15 samples of each material were thermocycled (30-second immersion alternatively in a 5 and 55°C water bath with a cycle time of 71 sec) during 5000 cycles, and another 15 samples during 10,000 cycles. The remaining 15 samples of the experimental and control composite were not thermocycled. The diametral tensile strengths of all 90 samples were then determined by means of an Autograph AGS-5kNG (Shimadzu, Kyoto, Japan) material tester at a cross-head speed of 1 mm/min. We used Student's t test to determine statistical differences at a significance level of 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
XPS of non-decontaminated SiO2 detected C originating from contamination as well as Si and O as basic constituents (Fig. 1aGo). 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-1eGo). Most effective decontamination was achieved with 0.05-5% sodium peroxodisulfate, by which C fell below the detection limit (Figs. 1c, 1dGo).



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  		Figure 1. XPS wide-scan spectra of un-decontaminated SiO2 in (a), SiO2 boiled in 0.01 (b), 0.05 (c), 5 (d), and 10% (e) sodium peroxodisulfate solutions for 15 min, followed by ultrasonic rinsing with acetone for 30 min.

 
When decontaminated SiO2 was treated with {gamma}-MPTS, the C 1s peak re-appeared (Fig. 2aGo). It therefore must be attributed to {gamma}-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. 2bGo). However, when TTDCS was applied to non-decontaminated SiO2, the F 1s peak was substantially less intense, indicating that less silanization was achieved (Fig. 2cGo).



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  		Figure 2. XPS wide-scan spectra of SiO2 decontaminated with 5% sodium peroxodisulfate solution and acetone, then treated with {gamma}-MPTS, followed by ultrasonic rinsing twice in ultrapure water for 5 min in (a), SiO2 decontaminated with 5% sodium peroxodisulfate solution and acetone, then treated with TTDCS, followed by ultrasonic rinsing twice in chloroform for 5 min in (b), and SiO2 treated with TTDCS without pre-silanization decontamination in (c).

 
XPS wide-scan spectra of Si (Fig. 3aGo) that was decontaminated by means of sodium peroxodisulfate/acetone (Fig. 3bGo), 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. 3cGo). With either {gamma}-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 3eGo compared with Fig. 3cGo).



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  		Figure 3. XPS wide-scan spectrum of untreated Si in (a), Si decontaminated with 5% sodium peroxodisulfate solution and acetone in (b), Si decontaminated with 5% sodium peroxodisulfate solution and acetone, then dipped in a 10% hydrofluoric acid solution to remove the oxidation layer, followed by rinsing with ultrapure water in (c), contamination-free bare Si treated with {gamma}-MPTS in (d), and contamination-free bare Si treated with TTDCS in (e).

 
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.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
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 {gamma}-MPTS that is most commonly used by dental manufacturers to couple the filler to the matrix phase of composites. The results of both silanization methods allowed us not only to evaluate if pre-silanization decontamination actually improved the silanization effectiveness of both coupling agents, but also to elucidate the underlying mechanism of the interfacial interaction of the coupling agents with the SiO2 surface. Considering the high-resolution chemical surface analysis capability needed, XPS was perfectly suited for this purpose (Briggs and Seah, 1990).

XPS revealed that, upon TTDCS silanization, fluoride was detected on decontaminated SiO2 (Fig. 2bGo), but not on contamination-free bare Si (Fig. 3eGo). In addition, chloride was not detected on TTDCS-silanized decontaminated SiO2 (Fig. 2bGo). If unreacted TTDCS residue had remained on SiO2, the expected ratio of fluoride to chloride would have been 13 to 1 following the chemical formulation of TTDCS. Consequently, it is evident that TTDCS chemically bonded to silanol groups (Si-OH) at the surface of SiO2 with expulsion of HCl, but not to silyl groups (Si-H) at the surface of Si (Figs. 4a, 4bGo). Likewise, when {gamma}-MPTS was applied to decontaminated SiO2, a carbon peak appeared that should be attributed to {gamma}-MPTS bonded to SiO2 (Fig. 2aGo). Since carbon was not detected when {gamma}-MPTS was applied to contamination-free bare Si, it can be concluded that {gamma}-MPTS chemically bonded to silanol groups at the SiO2 surface with expulsion of methanol (CH3OH), but not to silyl groups at the Si surface (Figs. 4c, 4dGo).



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  		Figure 4. Schematic presentation of the chemical interaction between TTDCS and Si in (a), TTDCS and SiO2 in (b), {gamma}-MPTS and Si in (c), and {gamma}–MPTS and SiO2 in (d), respectively.

 
When glass filler was silanized with {gamma}-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 {gamma}-MPTS, the carbon peak that should originate from {gamma}-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 {gamma}-MPTS should be 7 because of the elimination of three methoxy groups. Consequently, the relative intensity of C 1s vs. Si 2p in {gamma}-MPTS should be smaller than that for TTDCS so far as one silane molecule reacted with one silanol group on the surface. However, the C 1s intensity for {gamma}-MPTS is much larger than that for TTDCS (Figs. 2a, 2bGo). This suggests that a polymerization of {gamma}-MPTS occurred and enhanced the intensity of C 1s for {gamma}-MPTS. Since TTDCS is a mono-functional fluoride-tagged reagent, one TTDCS molecule could react only with one silanol group (Si-OH) on the surface, which thus can be applied for quantification of silanol groups at the filler surface. XPS wide-scan spectra in Figs. 2b and 2cGo showed that the F 1s peak of SiO2 that was successively decontaminated and treated with TTDCS was considerably larger than when non-decontaminated SiO2 was silanized with TTDCS. Consequently, appropriate pre-silanization decontamination of glass filler by sodium peroxodisulfate/acetone considerably improved silanization efficacy. Moreover, the measurement of the diametral tensile strength demonstrated that this filler decontamination methodology also improved the resistance against degradation by thermocycling.

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.


   ACKNOWLEDGMENTS
 
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 May 14, 2001; Last revision February 5, 2002; Accepted February 7, 2002


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Briggs D, Seah MP (1990). Practical surface analysis. Vol. 1. Auger and x-ray photoelectron spectroscopy. 2nd ed. New York: John Wiley & Sons.

Broutman LJ, Sahu S (1971). The effect of interfacial bonding on the toughness of glass filled polymers. Mater Sci Eng 8:98–107.

Brown SK (1980). Mechanics of fracture in filled thermosetting resins. Br Polymer J 12:24–30.

Calais JG, Söderholm KJM (1998). Influence of filler type and water exposure on flexural strength of experimental composite resin. J Dent Res 67:836–840.[Abstract/Free Full Text]

Degrange M, Roulet JF (1997). Minimally invasive restorations with bonding. Chicago: Quintessence Publishing.

Ferracane JL (2000). Status of research on new fillers and new resins for dental composites. In: Proceedings of the 3rd International Kuraray Symposium, Advanced Adhesive Dentistry, Granada, Spain, December 3-4 1999. Tagami J, Toledano M, Prati C, editors. Cirimido (Como), Italy: Grafice Erredue, pp. 3-29.

Griffith AA (1920). The phenomena of rupture and flow in solids. Phil Trans R SocCCXXI:163–198.

Kim KH, Park JH, Imai Y, Kishi T (1994). Microfracture mechanisms of dental resin composites containing spherically-shaped filler particles. J Dent Res 73:499–504.[Abstract/Free Full Text]

Mohsen NM, Craig RG (1995). Effect of silanation of fillers on their dispersability by monomer systems. J Oral Rehabil 22:183–189.[Medline]

Nihei T, Kurata S, Yamanaka H, Kurosaka N, Kondo Y, Yoshino N, et al. (2000). Improvement of long term water resistance of silane coupling agent layer modified with polyfluoroalkyltrimethoxysilane/3-methacryloxypropyltrimethoxysilane mixture. Jpn J Dent Mater 19:495–501.

Shirai K, Yoshida Y, Nakayama Y, Fujitani M, Shintani H, Wakasa K, et al. (2000). Assessment of decontamination methods as pretreatment of silanization of composite glass fillers. J Biomed Mater Res (Appl Biomater) 53:204–210.[Medline]

Söderholm KJM (1984). Influence of silane treatment and filler fraction on thermal expansion of composite resin. J Dent Res 63:1321–1326.[Abstract/Free Full Text]

Söderholm KJM, Shang SW (1993). Molecular orientation of silane at the surface of colloidal silica. J Dent Res 72:1050–1054.[Abstract/Free Full Text]

Takahagi T, Ishitani A, Kuroda H, Nagasawa Y, Ito H, Wakao S (1990). Control of chemical reactivity of a silicon single-crystal surface using the chemical modification technique. J Appl Phys 68:2187–2191.

Vallittu PK (1997). Curing of a silane coupling agent and its effect on the transverse strength of autopolymerizing polymethylmethacrylate-glass fibre composite. J Oral Rehabil 24:124–130.[Medline]

Xu HHK, Martin TA, Antonucci JM, Eichmiller FC (1999). Ceramic whisker reinforcement of dental resin composites. J Dent Res 78:706–712.[Abstract/Free Full Text]





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