Three-body-wear resistance of the experimental composites containing filler treated with hydrophobic silane coupling agents

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 760–764

available at www.sciencedirect.com

journal homepage: www.intl.elsevierhealth.com/journals/dema

Three-body-wear resistance of the experimental composites containing filler treated with hydrophobic silane coupling agents Tomotaro Nihei a , Alp Dabanoglu b , Toshio Teranaka a,∗ , Shigeaki Kurata c , Katsura Ohashi a , Yukishige Kondo d , Norio Yoshino d , Reinhard Hickel b , Karl-Heinz Kunzelmann b a

Department of Oral Medicine, Kanagawa Dental College, 82 Inaoaka-cho, Yokosuka, Kanagawa 238-8580, Japan Division of Restorative Dentistry, Dental School of Ludwig-Maximilians-University, Munich, Germany c Department of Dental Materials and Devices, Kanagawa Dental College, Kanagawa, Japan d Department of Industrial Chemistry, Tokyo University of Science, Tokyo, Japan b

a r t i c l e

i n f o

a b s t r a c t

Article history:

This

Received 22 June 2007

fillers which were modified with a novel hydrophobic silane coupling agent. The

Received in revised form

novel silane coupling agent containing hydrophobic phenyl group 3-(3-methoxy-

25 August 2007

4-methacryloyloxyphenyl)propyltrimethoxysilane

Accepted 11 September 2007

experimental light-cure hybrid composites containing 85 wt% of filler modified with

paper

evaluated

the

wear

resistance

of

resin

(p-MBS)

composite

was

materials

synthesized.

with

The

this silane were formulated. Twelve specimens were prepared for the three-body-wear test with the ACTA machine and the collected data were analyzed statistically using a one-way Keywords:

ANOVA and Tukey’s multiple comparison test as the post hoc test.

Dental materials

The wear of the composites containing fillers treated with p-MBS was significantly

Composite resin

lower compared with the composite materials containing fillers pretreated with 3-

Wear resistance

methacryloyloxypropyltrimethoxysilane or the commercially composites (AP-X and ELS

Silane coupling agent

extra low shrinkage) after a wear test for 200,000 cycles (p < 0.05). It is suggested that the

Three-body-wear

resin composites containing fillers modified with the novel hydrophobic silane has high wear resistant, because of the coupling layers treated with this silane had an excellent affinity with the base resin and formed a highly hydrophobic layer on the filler surface. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The mechanical properties of polymeric composite materials crucially depend upon the condition of the interface between the surfaces of the inorganic filler particles and the polymerized organic resin in which the filler particles are embedded. The state of restorative dentistry, therefore, advances whenever a composite is developed in which this interface is

∗

stronger or more long-lasting. It is widely accepted that the cause of degradation of composite materials in the oral cavity is hydrolysis of the silane coupling agent between the resin matrix and the filler interface [1–6]. Yamanaka et al. [7] and Nihei et al. [8] studied the improvement of the coupling effect on a glass surface by adding several hydrophobic poly(fluoro)alkyltrimethoxysilane at various ratios to 3-methacryloyloxypropyltrimethoxysilane (3-MPS).

Corresponding author. Tel.: +81 46 822 8854; fax: +81 46 822 8853. E-mail address: [email protected] (T. Teranaka). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.09.001

761

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 760–764

The applied hydrophobic silanes were trifluoropropyl(1F), nonafluorohexyl(4F), tridecafluorooctyl(6F), heptadecafluorodecanyl- (8F), and henicsafluorododecyltrimethoxysilane (10F). As a result, the tensile bond strength of bis-GMA-based resin composites to the glass surface modified with the mixture of 3-MPS and 20–40 wt% of 1F or 4F and 5 wt% of 8F or 10F was significantly higher when compared with that of the 3-MPS alone specimen. The specimen treated with the mixture of 3-MPS and 20–40 wt% of 1F or 4F also demonstrated stable bond strength, excellent hydrophobicity, and thermal resistance. Nihei et al. [9] also reported that the tensile strength of the experimental composites containing fillers treated with the mixture of 3-MPS and 20 wt% of 4F was not significantly decrease after water immersion for 1800 days and thermal stressed for 30,000 cycles. Recently, Okada et al. [10] has successfully synthesized several novel hydrophobic silane coupling agents having a polymerizable group. The authors evaluated the tensile bond strength of resin composite to the glass surface modified with those silanes. As a result, the tensile bond strength of the glass surface modified with 3-(3-methoxy-4methacryloyloxyphenyl)propyltrimethoxysilane (p-MBS) was significantly high compared with that of 3-MPS after the thermal stress, and showed no significant change after water immersion or thermal cycling [11,12]. The composite containing fillers treated with the hydrophobic silane could inhibit the diffusion into the matrix and prevent the plastisizing effect of water, and these composites are also more wear resistant than those with hydrophilic treatment of the fillers. The present study investigated the wear resistance of the experimental composites containing fillers treated with the novel hydrophobic silane coupling agent with the three-bodywear test which was developed at the Academisch Centrum Tandheelkunde Amsterdam and which is usually referred to as the ACAT wear machine.

2.

Materials and methods

The chemical formulae and the codes of the silanes used are listed in Table 1. The hydrophobic silane coupling agents are nonafluorohexyl- (4F), heptadecafluorodecyltrimethoxysilane (8F), and phenyl group silane containing double bond in a molecule (p-MBS). The ration of fluoroalkyl silanes to 3-MPS used for the silica filler treatment is 20 wt% at 4F, 10 wt% at 8F, respectively (4F/3-MPS, 8F/3-MPS). The p-MBS was also used alone. It was based on the literature

Table 2 – Component of the experimental composites

Table 1 – Used silane coupling agents and codes Formula

C4 F9 (CH2 )2 Si(OCH3 )3 C8 F17 (CH2 )2 Si(OCH3 )3

reported by Nihei et al. [9]. The component of the experimental composites is listed in Table 2. Ethanolic solutions containing 3 wt% of each silane mixture was prepared. Two types of silica fillers, having an average particle diameter of 0.04 ␮m (spherical type) and 3.0 ␮m (crushed type), respectively, were mixed at a weight ratio of 1:15. The surfaces of the mixed fillers were modified with the ethanol solution of a 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 fillers were heated in an oven at 100 ◦ C for 2 h. 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 15 wt% of the monomer mixture and 85 wt% of silanemodified 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 (filler contented 80 wt%). Dimensionally standardized experimental composite specimens were made in rectangular recesses (10 mm length × 7 mm width × 5 mm depth) of customized silicone moulds. To save test materials, a 3 mm thick layer of resin composite (Clearfil AP-X, Lot 0456AA, Kuraray, Okayama, Japan) was placed at the bottom of each mould and light cured (Elipar Trilight, 3M-ESPE, USA) for 20 s. The another 2 mm thick layer of experimental composite material was placed and polymerized with a Dentacolor XS photo-curing unit (Heraeus Kulzer, Germany) for 3 min. The Dentacolor XS photo-curing unit was used to ensure a uniform curing of the whole surface the samples at the same time. Composite excess was removed from the samples by low speed electrical hand-pieces with diamond discs. The composite samples were place using a luting composite (Bistite II, Lot A578Y5, Tokuyama Dental, Tokyo, Japan) in a cylindrical metal sample-holder with 20 compartments. To obtain a round sample wheel, the surface was ground by cylindrical abrasive discs (Table 3) (EMS Winter & Sohn GmbH & Co., D-Norderstedt, Germany) by the ACTA wear machine (ACTA 3, Willytec GmbH, Munich). The grinding procedure was carried out under water using grinding wheels from number 4 to 1. During the grinding procedure, the speed of the grinding wheel was 206 rpm and sample-wheel rotated at 60 rpm. A total of 500 cycles was performed for each of the four different grinding wheels. The axes of both

Code

Unfilled resin

3-MPS

Initiator Accelerator

4F 8F

Filler Coupling agent

p-MBS Filler content

Bis-GMA + TEGDMA (1:1) dl-Camphorquinone (1.0 wt%) 2-(Dimethylamino)ethylmethacrylate (2.0 wt%) Spherical filler (0.04 ␮m) + crushed Ba filler (3.0 ␮m) (1:15) 3-MPS, 4F/3-MPS (20:80), 8F/3-MPS (10:90), p-MBS 80–85 wt%

762

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 760–764

Table 3 – Specifications of grinding abrasive wheel No. 4 3 2 1

Lot number

Mean particles size (␮m)

1A1-20-15-2-6/D126/K-Plus/C100 1A1-20-15-2-6/D91/K-Plus/C100 1A1-20-15-2-6/D64/K-Plus/C100 1A1-20-15-2-6/D25/K-Plus/C100

126 91 64 25

Procedure Coarse processing Coarse processing finer unevenness Smoothing through the surface Polishing

Product by Ems Winter & Sohn GmbH & Co.

wheels rotated in the same direction during this grinding process. During the wear simulation, an antagonist wheel was used which can be considered as the antagonist. The sample-wheel was the agonist and a millet seed suspension was used as the abrasive. The abrasive slurry was in a bowl, which contains 220 ml distilled water, and 150 g ground millet seed. 150 g millet was prepared in three parts, 50 g each, in one by rotating blade grinder for exactly 5 s. The rotation speed of the sample-holding wheel was held at 1 Hz to simulate the chewing frequency. The samples were subjected to 200,000 cycles of three-body-wear using a contact force of 15 N between the opposing surfaces. The suspension was renewed after every 50,000 cycles to avoid a waxy layer on the surface of the wheel. After the experiment, the sample holder wheel was removed from the ACTA wear machine and was cleaned under water. There were two lines on the composite samples. The surface between these lines determines the amount of wear of the composites. Unworn areas, which are situated outside of these lines, were taken as a reference. The amount of wear was measured with a 3D scanner (Laserscan 3D Pro, Willytec GmbH, Munich, Germany) [13]. The obtained data were evaluated and the mean wear value of each material was calculated with the Match3D software. For this measurement the actual 3D data of the worn surface are subtracted from a plan, which was calculated by interpolation between the two adjacent unworn surface planes. All differences were summarized and divided by the number of pixels of the worn surface, which results in the mean wear per sample. Twelve specimens were tested for each silane. The commercially available resin composites Clearfil AP-X (APX, Shade XL, Lot 00458, Kuraray, Okayama, Japan), ELS extra low shrinkage (ELS, Shade A2, Lot 7104, Saremco, Rebstein,

Table 4 – Used materials of this study Composite Experimental composites 3-MPS 4F/3-MPS 8F/3-MPS p-MBS Unmodified Commercially composites Clearfil AP-X (Kuraray Medical Co., Japan) ELS extra low shrinkage (Saremco Dental, Switzerland)

Lot KM05081 KM05082 KM05083 KM05084 KM05085

Code 3-MPS 4F/3-MPS 8F/3-MPS p-MBS Unmodified

00458 (shade: XL)

AP-X

7104 (shade: A2)

ELS

Table 5 – Wear loss values of after ACTA wear test (200,000 cycles) Code

Mean wear loss (␮m)

4F/3-MPS p-MBS 3-MPS AP-X 8F/3-MPS ELS Unmodified

32.3 a 33.8 a 36.0 a,b 39.1 b 47.3 c 48.0 c 93.9 d

Statistically analyzed with one-way ANOVA and post hoc Tukey’s multiple comparison tests (n = 12). Same letters indicate no significant differences (Tukey’s test, ˛ = 0.05).

Switzerland), were also prepared according to the manufacturer’s instructions (Table 4). Obtained data were statistically analyzed with a one-way analysis of variance (ONEWAY). Tukey’s multiple comparison tests was used as the post hoc test to identify statistically homogenous subsets (˛ = 0.05, 12.0 version software, SPSS Inc., Chicago, IL).

3.

Results

Mean values of wear and standard deviation of the evaluated experimental composites is listed in Table 5. Mean wear loss values of the composite containing filler treated with 3-MPS alone (3-MPS composite) was 36.0 ␮m after the 200,000 cycles wear test. On the other hand, mean wear loss values of the composites containing fillers modified with 4F/3-MPS and p-MBS were significantly lower when compared with the 3-MPS composite (32.3 ␮m, 33.8 ␮m, p < 0.05). The commercial composites AP-X and ELS had significantly higher wear values than the 4F–3-MPS and p-MBS composites (39.1 ␮m, 48.0 ␮m, p < 0.05). There were not significant differences in the wear values between the 8F/3-MPS composite and ELS after 200,000 cycles (47.3 ␮m). The highest wear values had the composite containing unmodified fillers (93.9 ␮m), which was significantly higher than all other composites (p < 0.05).

4.

Discussion

In general silane coupling agents are widely used to improve bonding of the organic and inorganic materials. The typical silane coupling agent used as a filler surface modifier agent for dental composites was 3-MPS, which formed the siloxane layer on the filler surface by hydrolysis of the silane. However, the siloxane bonds between the silica filler and

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 760–764

the siloxane or in the siloxane layer are hydrolyzed after long-term water immersion [14]. As a result, the mechanical property of the resin composite decreased. For the resistance against this destruction, the hydrophobic type silane coupling agents of the phenyl group silane and the poly(fluoro)alkyltrimethoxysilanes with water and oil repellency were used. In this study, the three-body-wear resistance of the experimental composites containing fillers treated with the novel hydrophobic silane coupling agent has been used to investigate the adhesion between resin matrix and modified filler. The wear resistance of dental composite materials is one of the important factors for the functionality of composites restorations in the oral cavity. The wear of dental composite material has been explained in term of the effects of the amount of filler, the nature of the matrix or the silane coupling agent [15–18]. The wear mechanism has four modes that are used in the classification of wear (adhesive wear, abrasive wear, surface fatigue wear and corrosive wear) [19,20]. The ACTA in vitro wear testing device has been shown to be predictive of the clinical performance [21–24]. Categorized as a three-body-wear system, the in vitro data correlate well with values obtained from clinical measurements [25]. De Gee and Pallaw [19] has developed this three-body-wear testing machine. Currently, the three-body-wear test with the ACTA wear machine is known as a classic method for in vitro wear evaluation [26–30]. The mean wear value of the composite containing unmodified fillers was the highest compared with all other composites. The 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 filler [31]. 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 [32]. The 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 wear loss values of the composite may increase as a function of the length of time it is immersed in water. We hypothesize that polyfluoroalkyltrimethoxysilanes, which are strongly water- and oil-repellent [33], promote the sealing of the filler surface, and will thus produce more durable resin composites. The lower wear loss values of the 4F/3-MPS composite after ACTA wear test may be due to water absorption by the matrix resin rather than to the destruction of the siloxane bonds

763

between the filler and the coupling layer. On the other hand, the wear loss of the 8F/3-MPS composite was significantly higher when compared with the 4F/3-MPS composite. The cause of this increase may be the worse wettability of the filler surfaces modified with 8F/3-MPS. This result shows a similar tendency as the tensile strength of composites containing filler treated with same mixture of silanes [9]. Plueddemann and Pape [34] concluded that many composites are stronger and resist water better when an appropriate silane mixture, rather than a single silane, is used. Karata and Yamazaki [35] and Craig and Dootz [36] also showed that fluorinated hydrophobic silanes increase the hydrolytic stability of the interface between filler and matrix resin, even though they do not contain C C bonds capable of reacting with the matrix monomer. The wear loss value of the composite containing fillers modified with p-MBS as a novel silane coupling agent was significantly lower compared with the 3-MPS composite. In general, the phenyl silane is used as the purpose of the improvement in the waterproof, for containing the benzene ring [37]. Especially, used the phenyl silane by substitution at p-position in this study have properties of high waterproof and unresolved intramolecular. The some authors also demonstrated that this silane increase the water resistance, because the materials remain hydrophobic even after being stored in water for 360 days and that the tensile bond strength is not significantly less even after 10,000 cycles of thermal stress [11,12]. The wear of the composites containing fillers treated with 4F/3-MPS or p-MBS is 67–86% of that of the commercially composites. It is thought that the fluorocarbon chain in the 4F/3-MPS coupling layers protected the coupling layers from wear stress. The molecular structure of the p-MBS coupling agent has the organo functional group and the hydrolytic group on symmetrically with benzene ring is also same coupling effect as the 3-MPS (Table 1). Furthermore, the contact angles of the glass plate modified with these silanes showed high wettability in the case of the matrix monomer [9,11]. Based on the results of this study, we suggest that the wear resistance of the resin-based composites containing the silane-treated filler is due to a wear stress shielding effect of the coupling layer containing fluoroalkyl or phenyl group and is not due to a drier matrix resin arising from diffusion of fluoroalkyl or phenyl silanes into the resin. Therefore, composite wear is not only correlated with the degree of conversion and the filler system of a composite, but it is also depending on the method of filler silanization.

Acknowledgments 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. 15592077, 16390548, 16791158, and 16791166). The authors would like to express their appreciation to Kuraray Medical Corporation for providing the experimental composites. A preliminary report was presented at the 84th General Session & Exhibition of the IADR, June 28, 2006, Brisbane, Australia.

764

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 760–764

references [18] [1] Schrader ME, Block A. Tracer study of kinetics and mechanism of hydrolytically induced interfacial failure. J Polym Sci Part C 1971;30:281–91. ¨ [2] Soderholm KJ. Degradation of glass filler in experiment composites. J Dent Res 1981;60:1867–75. ¨ [3] Soderholm KJ, Zigan M, Ragan M, Fischlschweiger W, Bergman M. Hydrolytic degradation of dental composites. J Dent Res 1984;63:1248–54. [4] Pillar RM, Smith DC, Maric B. Fracture toughness of dental composite determined using short-rod fracture toughness test. J Dent Res 1986;65:1308–14. ¨ [5] Calais JG, Soderholm KJ. Influence of filler type and water exposure on flexural strength of experimental composite resin. J Dent Res 1988;67:836–40. [6] Ferracane JL, Marker VA. Solvent degradation and reduced fracture toughness in aged composite. J Dent Res 1992;71:13–9. [7] Yamanaka H, Teranaka T, Kurata S, Yoshino N. Improvement of bond strength and water resistance of silane coupling agent containing poly(fluoro)alkyltrimethoxysilane. In: Joint Canada–Japan Workshop on Composites. 1996. p. 229–32. [8] Nihei T, Kurata S, Yamanaka H, Kurosaka N, Kondo Y, Yoshino N, et al. Improvement of long term water resistance of silane coupling agent layer modified with polyfluoroalkyltrimethoxysilane/3-methacryloyloxypropyltrimethoxysilane mixture. Jpn J Dent Mater 2000;19:495–501. [9] Nihei T, Kurata S, Kondo Y, Umemoto K, Yoshino N, Teranaka T. Enhanced hydrolytic stability of dental composites by use of fluoroalkyltrimethoxysilanes. J Dent Res 2002;81:482–6. [10] Okada H, Kondo Y, Yoshino N. Synthesis of aromatic silane coupling agents having a polymerizable group. Mater Technol 2001;19:197–202. [11] Nihei T, Kurata S, Ohashi K, Kondo Y, Umemoto K, Yoshino N, et al. Water resistance of novel silane coupling agents having aromatic group substituted a reactive double bond. Jpn J Dent Mater 2005;24:1–8. [12] Ohashi K, Nihei T, Kurata S, Kondo Y, Umemoto K, Yoshino N, et al. Adhesion of resin to ceramic surface modified with phenyl group silane coupling agents containing a double bond. Jpn J Dent Mater 2005;24:247–52. [13] Mehl A, Gloger W, Kunzelmann KH, Hickel R. A new optical 3D device for the detection of wear. J Dent Res 1997;76:1799–807. [14] Oysaed H, Ruyter IE. Composites for use in posterior teeth: mechanical properties under dry and wet conditions. J Biomed Mater Res 1986;20:261–71. [15] Montes-G GM, Draughn RA. In vitro surface degradation of composites by wear and thermal cycling. Dent Mater 1986;2:193–7. [16] Frazier KB, Sarrett DC. Wear resistance of dual-cures resin luting agents. Am J Dent 1995;8:161–4. [17] Venhowen BMA, De Gee AJ, Werner A, Davidson CL. Influence of filler parameters on the mechanical adherence

[19] [20]

[21]

[22]

[23] [24] [25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36] [37]

of dental restorative resin composites. Biomaterials 1996;17:735–40. ¨ Soderholm KJ, Richards ND. Wear resistance of composites: a solved problem? Gen Dent 1998;46:256–64. De Gee AJ, Pallaw P. Occlusal wear simulation with ACTA wear machine. J Dent 1994;22(Suppl 1):S21–7. Mair LH, Stolarski TA, Vowles RW, Lloyd CH. Wear: mechanisms, manifestations and measurement. Report of a workshop. J Dent 1996;24:141–8. Lutz F, Phillips RW, Roulet JF, Setcos JC. In vivo and in vitro wear of potential posterior composites. J Dent Res 1984;63:914–20. Embong A, Glyn Jones J, Harrison A. The wear effects of selected composites on restorative materials and enamel. Dent Mater 1987;3:236–40. Gallegos LI, Nicholls JI. In vitro two-body wear of three veneering resins. J Prosthet Dent 1988;60:172–8. Leinfelder KF, Beaudreau RW, Mazer RB. An in vitro device for predicting clinical wear. Quint Int 1989;20:755–61. ¨ Sarrett DC, Soderholm KJ, Batich CD. Water and abrasive effect on three-body wear of composites. J Dent Res 1991;70:1074–81. Graf K, Johnson GH, Mehl A, Rammelsberg P. The influence of dental alloys on three-body wear of human enamel and dentin in an inlay-like situation. Oper Dent 2002;27:167–74. Sunnegardh-Gronberg K, Peutzfeldt A, van Dijken JW. Hardness and in vitro wear of a novel ceramic restorative cement. Eur J Oral Sci 2002;110:175–8. Say EC, Civelek A, Nobecourt A, Ersoy M, Guleryuz C. Wear and microhardness of different resin composite materials. Oper Dent 2003;28:628–34. ¨ Kunzelmann KH, Burkle V, Bauer C. Two-body and three-body wear of glass ionomer cements. Int J Paediatr Dent 2003;13:434–40. ¨ Schwartz JI, Soderholm KJ. Effect of filler size, water, and alcohol on hardness and laboratory wear of dental composites. Acta Odontol Scand 2004;62:102–6. Arikawa H, Kuwahata H, Seki H, Kanie T, Fujii K, Inoue K. Deterioration of mechanical properties of composite resin. Dent Mater J 1995;14:78–83. Kurata S, Yamazaki N. Effect of silane coupling agents with various organofunctional and hydrolyzable group on silicon and its water-resistance. Jpn J Dent Mater 1992;11:916–21. Teranaka T, Iwamoto T, Yoshino N. Application of newly developed surface modifier to dental material. Bull Kanagawa Dent Coll 1994;22:151–5. Plueddemann EP, Pape PG. The use of mixed silane coupling agents. In: Proceedings of the 40th Annual Conference, Reinforced Plastics/Composites Institute, Session 17-F. The Society of the Plastics Industry Inc.; 1985. p. 1–4. Karata S, Yamazaki N. Effect of silane coupling agents with a bisfunctional hydrolyzable group. Dent Mater J 1993;12:127–35. Craig RG, Dootz ER. Effect of mixed silanes on the hydrolytic stability of composites. J Oral Rehabil 1996;23:751–6. Morikawa A, Iyoku Y, Kakimoto M, Imai Y. Preparation on new polyimide-silica hybrid materials via the sol–gel process. J Mater Chem 1992;2:679–90.