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Enhanced Hydrolytic Stability of Dental Composites by Use of Fluoroalkyltrimethoxysilanes
1 Department of Operative Dentistry and Endodontics and Correspondence: * corresponding author, niheitom{at}kdcnet.ac.jp
The hydrolytic stability of a group of experimental composite materials was evaluated. Seven distinct composites were formed by the mixing of a resin monomer mixture with silica filler that had been pre-treated with one of 7 different ethanol solutions. In one case, the filler was treated with an ethanol solution that contained only 3-methacryloyloxypropyltrimethoxysilane. In 5 cases, it was treated with solution containing a mixture of 3-methacryloyloxypropyltrimethoxysilane and one of the following hydrophobic fluoroalkyltrimethoxysilanes: trifluoropropyl-, nonafluorohexyl-, tridecafluorooctyl-, heptadecafluorodecyl-, and henicosafluorododecyl-trimethoxysilane. The tensile strength, after being immersed in water for 1800 days, of 2 of the experimental composites, whose pre-treatment regimen had included a fluoroalkyltrimethoxysilane, was significantly higher than that of the composite whose pre-treatment regimen had not included a fluoroalkyltrimethoxysilane. Moreover, there was no significant difference between the tensile strength of fresh samples of these 2 composites and the tensile strength of identically produced samples that had remained under water for 1800 days or that had been subjected to 30,000 cycles of thermal stress.
Key Words: 3-methacryloyloxypropyltrimethoxysilane • fluoroalkylsilane • resin composite • tensile strength • hydrolytic stability
Research has well established that the mechanical properties of polymeric composite materials crucially depend upon the condition of the interface between surfaces of the inorganic filler particles and the polymerized organic resin in which the filler particles are embedded. The state of the art of restorative dentistry, therefore, advances whenever a composite is developed in which this interface is stronger or more long-lasting, because the new composite will better resist the destructive environment that exists inside the human oral cavity. Temperature fluctuations, high humidity, and the high occlusal bite forces that are applied during mastication and nocturnal bruxing all contribute to this harsh environment. An important advance took place when it was discovered that silane-coupling agents promote adhesion not only between mineral fillers and organic matrix resins in resin composites, but also prime ceramic surfaces for better adhesion to a variety of bonding agents. Even so, the mechanical properties of a resin composite restoration change over the long term, owing to hydrolysis of the coupling layer at the interface between the matrix resin and the inorganic filler particles (Schrader and Block, 1971; Söderholm, 1981). Söderholm and colleagues discovered that if filler particles are pre-treated with hydrophobic silanes, the resulting composites are more durable, because the coupling layer is more resistant to the hydrolytic attack of absorbed water molecules (Söderholm et al., 1984). Kurata and Yamazaki (1993), Yamanaka et al. (1996), and Nihei et al. (2000), in studies of composite materials on glass surfaces, have all shown that siloxane structures modified with one of a variety of hydrophobic polyfluoroalkyltrimethoxysilanes are more resistant to hydrolysis than are unmodified siloxanes. They have also shown that the tensile strength of the bond between ordinary resin and a glass surface treated with a mixture of 3-methacryloyloxypropyltrimethoxysilane (3-MPS) and polyfluoroalkyltrimethoxysilane is significantly higher than the bond between the same resin and a glass surface treated with 3-MPS alone, and they have demonstrated that these same materials remain hydrophobic even after being stored in water for 720 days and that their tensile strengths are not significantly less even after 28,000 cycles of thermal stress. In contrast, a hydrophilic siloxane layer is produced at the organic-inorganic interface when composites are made with a filler treated only with 3-MPS, and the siloxane bonds in this layer are gradually broken by the hydrolytic action of water molecules absorbed by the resin in composites that are immersed in water. As more of these chemical bonds are hydrolyzed, cracks develop between filler-particle surfaces and the matrix resin, and the mechanical strengths of the composites decreases. Research results obtained thus far strongly suggest that the strength and durability of composite resins depend upon the quality of the hydrophobic siloxane layer at the organic-inorganic interface. The goal is to modify the surfaces of filler particles by pre-treating them with chemical mixtures that produce a hydrophobic, water-tight barrier that protects the filler particles against hydrolytic leaching. This is done by pre-treating filler with a combination of 3-MPS and one of the hydrophobic silanes. The present study has been undertaken to identify those pre-treatment mixtures that produce resin composites with the highest tensile strength and the best long-term resistance to hydrolytic attack.
Preparing the Experimental Composites The chemical formula and the codes of the silanes used are listed in the Table  . The hydrophobic silane coupling agents are trifluoropropyl- (1F), nonafluorohexyl- (4F), tridecafluorooctyl- (6F), heptadecafluorodecyl- (8F), and henicosafluorododecyl-trimethoxysilane (10F). The ratio of hydrophobic silanes to 3-MPS used for the silica filler treatment is 40 wt% at 1F, 20 wt% at 4F, 15 wt% at 6F, 10 wt% at 8F, and 5 wt% at 10F, respectively. It was based on the literature reported by Yamanaka et al. (1996). Ethanolic solutions containing 3 wt% of each silane mixture were prepared. Two types of silica fillers, having an average particle diameter of 0.04 µm (spherical type) and 3.0 µm (crushed type), respectively, were mixed at a weight ratio of 1:15. The surfaces of the mixed filler were modified with the ethanol solution of silane mixture at room temperature for 7 days. The mass of the silane mixture added was, in each case, 3 wt% for the filler. After the solvent had been allowed to evaporate at room temperature for 7 days, the modified filler was heated in an oven at 120°C for 2 hrs. We prepared the resin monomer mixture used by mixing equal amounts of Bis-GMA and TEGDMA, dl-camphorquinone (1 wt%) as photo initiator, and 2-(dimethylamino)ethylmethacrylate (2 wt%) as accelerator. We prepared the light-cured experimental composites by mixing 20 wt% of the monomer mixture and 80 wt% of silane-modified fillers. To provide a control for the silane treatment, we also prepared the composite containing unmodified fillers treated with pure ethanol using the same procedures.
Tensile Strength and Water-absorption Test For determination of the direct tensile strength and water adsorption, respectively, dumb-bell-shaped (25 x 2 x 2 mm) samples and disk-shaped (20 mm diameter x 1 mm thick) samples of the prepared composites or the resin of monomer mixture were placed in the stainless steel mold. The mold was covered with a flat glass slide, and the composites or resin monomer mixture was polymerized by irradiation, twice from the top and lower sides, for 60 sec (Optilux 400; Demetron, Danbury, CT, USA). After polymerization, the dumb-bell-shaped specimens were polished with #600, #1000, and #1500 silicon carbide water-proof paper under running water, before being finally stored in distilled water for 1, 90, 180, 360, and 1800 days at 37°C or thermally stressed between 4°C and 60°C water baths for 30,000 cycles of 1 min each (thermal stress). Tensile strength was measured with a universal mechanical testing machine (AGS-500; Shimadzu, Kyoto, Japan) at a cross-head speed of 0.5 mm/min. Five specimens were tested for each silane mixture and for each storage condition. A commercially available resin composite, Photo Clearfil A (PCA; Kuraray, Okayama, Japan), was prepared according to the manufacturer's instructions and tested for tensile strength. The composition of PCA is similar to that of our experimental composites. The water absorption test was conducted according to American Dental Association (ADA) specification No. 27 for direct-filling resins (1977). After the conditioned weight of the disk specimens was determined, they were immersed in distilled water at 37°C for either 1, 3, 7, 14, 21, 28, 60, or 90 days (Pearson, 1979). Each specimen was weighed immediately after being removed from the water bath. Each disk weighed within 0.01 mg/cm2 of the average value for the 3 disks in each group.
Contact Angles of the Modified Glass Surface
Tensile Strength The average values of the direct tensile strength of all investigated materials are listed in Fig. 1 . The strength of the matrix resin sample showed a significant decrease after 90 days of water immersion or thermal stress of 30,000 cycles compared with that after one day in water (p < 0.05). The strength of the composite containing unmodified filler was significantly less than that of resin material for all storage periods (p < 0.05). The tensile strength of the composite containing filler treated with 3-MPS alone (3-MPS composite) was approximately 50 MPa after one day in water. The strength of 3-MPS composite after 1800 days in water or thermal stress was 32 or 33 MPa, respectively. These values were significantly lower compared with the strength measured after one day in water (p < 0.05). On the other hand, the tensile strengths of the composites containing filler modified with 1F/3-MPS or 4F/3-MPS were not significantly lower after 1800 days of storage (38 MPa, 54 MPa) or thermal stress (42 MPa, 55 MPa). There were no significant differences in the strength between the 3-MPS composite and the 6F/3-MPS, 8F/3-MPS, and 10F/3-MPS composites for all storage periods. The PCA samples were significantly less durable than the 4F/3-MPS composite after 1800 days in water or thermal stress (p < 0.05).
Water Absorption The values of water absorption for the composites containing silane-modified filler, for that with unmodified filler, and for the matrix resin without filler are shown in Fig. 2 . The value of water absorption of the matrix resin was approximately 1.4 mg/cm2 after one day in water. It reached a maximum toward the end of the storage period used in this study. By the end of 90 days' storage in water, the values of water absorption for the composites containing various silane-modified fillers and for the PCA composite were significantly less than those of the matrix-resin samples or composites containing unmodified filler (p < 0.05). There were no significant differences in the water absorption of 3-MPS composites, the various silane-mixture composites, and the PCA composite. The water absorption value of each composite containing a silane-treated filler and the PCA composite was less than approximately 0.7 mg/cm2 at the end of 7 days' storage. Therefore, all experimental composites except the composite containing unmodified filler are classified as Type II on the basis of ADA specification No. 27 for direct-filling resins.
Contact Angles of the Modified Glass Surface The contact angles formed at the junction of the resin monomer and the various experimental glass surfaces (modified with one of the silane mixtures) are shown in Fig. 3 . The contact angle for the monomer and the glass surface treated with 3-MPS alone was approximately 27°. The contact angle between resin and glass surface reached a minimum value when the glass surface was treated with 20 wt% silane mixtures for 1F/3-MPS, 4F/3-MPS, 6F/3-MPS, and 8F/3-MPS. All of these minima were significantly less than the minimum measured for glass surfaces treated with 3-MPS alone (p < 0.05).
Fujishima (1988) studied the effects of silane coupling agents on bonding at the interface between filler and matrix resin. They suggested that the decrease in tensile strength of composite resins after storage in water was caused by some kind of degradation of the chemical bonding at the filler-matrix interface and by softening of the matrix resin itself due to water absorption. Composites containing unmodified filler are weakest, since the chemical interaction between the inorganic filler and the organic matrix resin is weak, either Van der Waals' force or hydrogen bonding. Bonds are much stronger, with the concomitant improvement in the mechanical properties, for resin composites containing silane-modified fillers (Arikawa et al., 1995). The tensile strength of the 3-MPS composite after 1800 days' storage in water or after being thermally stressed is significantly less than after one day's water immersion. The alkoxy groups of 3-MPS hydrolyze very slowly in water because of the longer organic group, even in homogeneous solution in water-miscible solvents (Kurata and Yamazaki, 1992). Complete hydrolysis of alkoxy groups in the silane-coupling agent is very difficult. An additional difficulty is that the silanol groups produced by silane hydrolysis condense perfectly, so that the silanol groups either attach to the silica surface or to each other. Therefore, both unreacted alkoxy and silanol groups may exist in the organo-siloxane layer, interfering with chemical bonding between the filler and matrix resin. Furthermore, the silanol groups present at the interface absorb water, causing the resin to expand, probably increasing the inner stress between the filler and the matrix resin. As a result, the strength of the composite may decrease as a function of the length of time it is immersed in water. We hypothesize that polyfluoroalkyltrimethoxysilanes, which are strongly water- and oil-repellent (Teranaka et. al., 1994), promote the sealing of filler surfaces, and will thus produce more durable resin composites.
Söderholm et al.(1984) and Montes-G. and Draughn (1986) explained that the change in mechanical properties of resin composites after thermal stress was due to the inner stress caused (1) by a large difference between the coefficient of thermal expansion of the filler and that of the resin, and (2) by water sorption of the resin. However, others speculated that the decrease in the strengths of resin composites after prolonged water immersion could be attributed to water sorption by material at the interface between filler particles and matrix, rather than to differences in the coefficients of thermal expansion. Because the tensile strengths of two composites (those containing fillers treated with 1F/3-MPS and 4F/3-MPS) showed higher stability than did the others after prolonged water immersion, and because they were significantly higher than that of the commercial composite PCA, it was thought that the fluorocarbon chain in the 1F/3-MPS and 4F/3-MPS coupling layers protected the coupling layers from water while at the same time allowing full play of the 3-MPS coupling effect. The slight decrease in the strength of the 4F/3-MPS composite after water immersion may be due to water absorption by the matrix resin rather than to the destruction of the siloxane bonds between the filler and the coupling layer. On the other hand, the strength of 6F/3-MPS, 8F/3-MPS, and 10F/3-MPS composites decreased significantly after water storage or thermal stress. The cause of this decrease may be due to poor wettability of the filler surfaces modified with 6F/3-MPS, 8F/3-MPS, and 10F/3-MPS (Fig. 3 The water absorption values of the composites containing filler treated with silane mixture were similar to those of the 3-MPS and PCA composites. The water absorption value of the matrix resin without filler was significantly higher than those of all experimental composites after 90 days' water storage. Water absorption of the experimental composites might be dependent on the volume ratio of the matrix resin in the composites. The functional groups of the cured matrix resin are the hydroxy group and the ether and ester bonds, all of which possess a relatively high affinity to water. Kalachandra and Kusy (1991) observed that the mass of water absorbed into composites was less for hydrophobic matrix monomer, and that it was affected by the cross-linking density of matrix resin and filler content in the resin composites. Water absorption of the composite with unmodified filler was significantly different from that of the other modified filler for storage periods up to 21 days. Hence, we suggest that water was drawn not only into the matrix resin but also into the layer at the interface between filler and matrix resin. Nishiyama et al.(1995) concluded that the silane coupling layer prevents water from penetrating the layer at the interface. Based on the results of this study, we suggest that the water resistance of the composites containing the silane-treated filler is due to a water-shielding effect of the coupling layer containing fluoroalkyl groups and is not due to a drier matrix resin arising from diffusion of fluoroalkyl silanes into the resin.
The minimum contact angles were displayed at a concentration of 20 wt% for both 1F and 4F in the mixed silane (Fig. 3
This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan (Nos. 09771657 and 13470406), and was performed in Kanagawa Dental College, Research Center of Advanced Technology for Craniomandibular Function, and supported by grants-in-aid for Bioventure Research from the Japan Ministry of Education, Science, and Culture. A preliminary report was presented at the 77th General Session & Exhibition of the IADR, March 11, 1999, Vancouver, BC, Canada. Received for publication September 5, 2001. Revision received April 23, 2002. Accepted for publication May 15, 2002.
Journal of Dental Research, Vol. 81, No. 7,
482-486 (2002)
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