- Email: [email protected]
resin interfacial shear strengths
Dental Materials 19 (2003) 441–448 www.elsevier.com/locate/dental
Silane treatment effects on glass/resin interfacial shear strengths Subir Debnatha, Stephanie L. Wundera, John I. McCoolb, George R. Baranc,* a Department of Chemistry, Temple University, Philadelphia, PA 19122, USA Department of Industrial Engineering, Penn State Great Valley, Malvern, PA 19355, USA c Center for Bioengineering and Biomaterials, College of Engineering, Temple University, 1947 North 12th Street, Philadelphia, PA 19122, USA b
Received 18 January 2002; revised 26 June 2002; accepted 18 July 2002
Abstract Objectives. Methacrylic resin-based dental composites normally use a bifunctional silane coupling agent with an intermediary carbon connecting segment to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pull-out tests were used to measure the interfacial bond strength at the fiber– matrix interface. Methods. Glass fibers (approximately 30 mm diameter, 8 £ 1022 m length, MoSci) were silanated using various concentrations (1, 5 and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyltrimethoxy-silane (GPS) in acetone (99.8%). Rubber (poly(butadiene/acrylonitrile), amine terminated, Mw 5500) molecules were also attached to the fiber surface via GPS molecules. The resin was comprised of a 60/40 mixture of Bis-phenol-A bis-(2-hydroxypropyl)-methacrylate (BisGMA) and tri (ethylene glycol) dimethacrylate (TEGDMA). A bead of resin approximately 2– 4 £ 1023 m in embedded length was placed on the treated fibers and light cured. The load required to pull the fiber out of the resin was converted to shear bond strength. Results. Interfacial shear strengths were greater for silanated specimens compared with unsilanated, and for MPS compared with GPS. The same set of samples soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month showed a decrease in properties. Significance. A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples. q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Dental materials; Composites; Silane; Shear strength; Resin; Bonding
1. Introduction Inorganic fillers in dental composites are typically coated with silanes in order to improve the bond to the resin matrix and increase the service life of the composite [1]; an additional benefit is the improved dispersability of silanated fillers in matrix monomers [2]. Composites with silanated fillers possess superior mechanical properties and wear resistance, and increased resistance to water sorption [3] when compared with composites containing non-silanated fillers. The stronger the filler – resin interface, the greater the improvement in static [4], impact [5], and fatigue properties [6]. In dental composites, filler morphology may be spherical, irregular or fibrous. Spherical and irregularly shaped fillers are dispersed randomly, while in fiberreinforced composites, the filler is arranged in a unidirectional manner in order to best take advantage of * Corresponding author. Tel.: þ 1-215-204-8824; fax: þ1-215-204-4956. E-mail address: [email protected] (G.R. Baran).
the strengthening effect, although the position of the fibers also plays a role in the degree of strengthening [7]. Numerous methods have been developed for evaluating the quality of the filler – resin interface, and these have been recently reviewed [8]. A common technique is the embedded single fiber (fragmentation) test [9], though the test requires knowledge of the fiber tensile strength at the critical length of the fiber fragment, i.e. an additional test is required to obtain that property [10]. The shear strength of the interface may also be determined by a fiber pull-out test [11], but the fiber lengths must be kept small to ensure that fibers do not break prior to interface failure; the short fiber specimens required pose experimental difficulties. The microbond test was developed for fibers with small diameters, and relies on the ability to displace a small resin droplet that has been cured around a fiber [12]. A criticism made of all these tests is that the complicated stress states surrounding the fibers do not permit measurement of fiber –matrix adhesion [13,14], that residual axial stresses may comprise a significant portion of the measured shear
0109-5641/03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0109-5641(02)00089-1
442
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Table 1 Reagents Characteristics
Silica
Silanes
Resin components
Supplier
Chemical structure
Surface area
Particle size
A380 fumed silica Ground glass E-glass fibers (52–58% SiO2, 11–17% Al2O3, 6–7% B2O3, 18–25% CaO, 0.5–1% MgO, ,0.5% Fe2O3, ,0.1% Na2O, K2O)
380 m2/g
7 nm 3.6 mm 30 mm (diameter) 8 cm (length)
Acronym
Function
3-Methacryloxypropyl-trimethoxysilane Glycidoxypropyltrimethoxy-silane [Poly(butadiene/acrylonitrile)] amine terminated, Mw ¼ 5500 g/mol, 20% 1,2 vinyl content Pyridine (99.9%)
MPS GPS Rubber
Methylmethacrylate Benzoyl peroxide Dimethyl para toluidine bis-Phenol-A bis-(2-hydroxypropyl)methacrylate Tri(ethylene glycol) dimethacrylate Camphorquinone Tertiary amine, dimethylaminoethyl methacrylate Hydroquinone monomethylether
MMA BPO DMPT BisGMA TEGDMA CQ DMAEMA HMME
strength [15] and that the interfacial shear stress is affected by residual thermal stresses and the roughness of the interface [16]. Dynamic mechanical analysis (DMA) of composites may also provide information about the strength of the filler – resin interface: the feature most sensitive to the interface is the height of the main loss tangent peak (tan d at the glass transition temperature [3], though the flexural storage modulus E0 is also affected [17]. The structural composites literature contains a number of reports on the strength of the filler–matrix interface, but we are aware of only limited information regarding interfacial shear strengths in dental composite systems [18]. The effectiveness of silanation protocols has most often been indirectly assessed, usually by subjecting composites to soaking or boiling water treatments, then measuring the strength of the composite [19,20]. The implied assumption has been that weaker, or more readily degraded interfaces, will result in lower composite strengths. In this study, we employ the microbond test to evaluate the shear strength of the interface between glass fibers and a BISGMA/TEGDMA matrix following fiber surface treatment using 3-methacryloxypropyl-trimethoxysilane (MPS), and an adduct composed of glycidoxypropyltrimethoxy-silane (GPS) and a (poly(butadiene/acrylonitrile), amine terminated rubber, before and after soaking in a 50:50 (v/v) methanol–water mixture. The soaking was carried out because there is a concern that water sorption could influence the stability of the fiber–matrix interface and therefore affect the strength of polymer–fiber composites [21].
Degussa MoSci MoSci
Aldrich Aldrich Polysciences Catalyst
Aldrich
Monomer Initiator Accelerator Monomer Monomer Initiator Promoter Inhibitor
Aldrich Aldrich Aldrich Esstech Esstech Esstech Esstech Esstech
2. Materials and methods 2.1. Chemicals The chemicals used and their suppliers are listed in Table 1. All materials were used as received except for the 3.6 mm glass beads, which were washed with distilled water and evacuated at room temperature. The solvents ethanol (95%), acetone (99.8%), methanol (95%), chloroform (100%), were obtained from Aldrich and used without further purification. 2.2. Preparation of the resin Sixty grams of BisGMA was weighed into a Teflon beaker and mixed with 40 g of TEGDMA (in the dark). This mixture was stirred manually with a glass rod until a uniform consistency was achieved, and then it was allowed to rest for a day. After addition of 5 £ 1024 kg photosensitizer (CQ), 5 £ 1024 kg reducing agent (DMAEMA) and 8.5 £ 1023 kg HQ, it was stirred overnight in the dark using a magnetic stirrer. The homogeneous mixture was then bottled and stored for future use. 2.3. Silanization using acetone as solvent MPS (or GPS) was dissolved in different volumes of acetone to obtain silane concentrations of 1, 5 and 10% (w/w) for MPS and 5% for GPS. The fibers were silanated by agitation for 2 h in these solutions and were then
S. Debnath et al. / Dental Materials 19 (2003) 441–448
443
separated using a centrifuge run at 10,000 rpm. The silanated fibers were dried at room temperature (25 8C) under vacuum overnight and then additionally at 110 8C for 2 h at atmospheric pressure. The fibers were further washed three times with methanol, then dried in vacuum overnight.
chloroform, and evacuated again at room temperature overnight. The 3.6 mm beads were coated using the same silanation and rubber coating procedures as for the fibers.
2.4. Rubber coating
In addition, A380 fumed silica silanated with a rubber coating applied using Method II was used as a filler for a polymethylmethacrylate matrix prepared as in Ref. [20], in order to determine whether the unsaturated double bonds in the rubber molecules were incorporated into the growing PMMA chains. The rubber coated A380 fumed silica (0.5 £ 10 24 kg) was added to a vial containing 8.488 £ 1024 kg MMA monomer and 0.09 (1%) DMPT and sonicated for 30 min. This was followed by addition of 3.6 £ 1024 kg (4%) BPO. The composites were cured at room temperature for 48 h and additionally at 60 8C for 6 h. The composite thus prepared was dissolved in 100 ml acetone. The silica filler was separated using a centrifuge at 9000 rpm for 20 min, resuspended in 30 ml acetone and further washed following the same procedure until achieving constant TGA weight loss. The washed silica was dried under vacuum for 12 h before each TGA analysis.
The fibers were coated with varying amounts of rubber using several procedures. Chemical attachment was achieved by first reacting the silane with the silica, followed by reaction with rubber (Method I) or by first reacting the silane with the rubber, followed by reaction with the silica (Method II). The silane was bifunctional, with one end (CH3O) reactive towards the silica and the other end (epoxy) reactive towards the amine-terminated rubber. Similarly, the rubber was bifunctional, containing unsaturated CyC bonds that could be incorporated into the polymerizing methacrylate resin. As a control, the rubber was also nonspecifically adsorbed onto the silica (control rubber). 2.5. Rubber coating—Method I The fibers were first silanated using a 1% GPS solution in ethanol. The silanation procedure used was the same as that described for silanation using MPS/GPS. A solution consisting of 9.5 £ 1024 g rubber dissolved in 6.5 ml chloroform was used to disperse the washed, dried silanated fibers. After addition of 1 ml pyridine as catalyst for the reaction of the epoxy group of the silane with the amino end groups of the rubber molecules, the mixture was stirred for 4 h. The fibers were evacuated at room temperature (25 8C) overnight after separation by centrifugation (10,000 rpm) from the silane solution. The fibers were then washed twice with chloroform and evacuated at room temperature (25 8C) overnight. The dry fibers thus prepared were directly used to make the shear strength specimens described below. 2.6. Rubber coating—Method II In this method, 9.5 £ 1024 kg rubber, 45 ml GPS and 1 ml pyridine were reacted in 6.5 ml chloroform for 2 days at room temperature, to directly form a silane –rubber adduct. Fibers were added to the chloroform, GPS-rubber adduct solution and agitated for 4 h. The fibers were separated using a centrifuge (10,000 rpm) followed by evacuation at room temperature (25 8C) overnight. They were then washed twice with chloroform and evacuated at room temperature (25 8C) overnight. 2.7. Rubber coating—control The fibers were stirred in a solution of 9.5 £ 1024 kg rubber, 6.5 ml chloroform and 1 ml pyridine for 4 h. The fibers were separated by centrifugation, evacuated at room temperature (25 8C) overnight, washed twice with
2.8. Covalent attachment of rubber to resin
2.9. Determination of coating thickness Thermogravimetric analysis (TGA) methods, on a TA Instruments Hi-Res 2950 Thermogravimetric Analyzer using a ramp rate of 10 8C/min, were used to quantify the amount of silane or rubber molecules attached to the silica (3.6 mm) bead surface. Samples were heated from 25 to 800 8C and the amount of surface attached material was calculated from the amount of weight loss over this temperature interval. It was assumed that results found for the 3.6 mm beads were the same as for the fibers. It was not possible to obtain direct results for the latter due to their low surface area. The amount of silane/rubber molecule coverage on the beads was calculated using the TGA weight loss and assuming the formation of a uniform and spherical layer of coated molecules on the glass beads. The same assumption was also used for the calculation of their layer thickness. Using the formula TGA weight loss ð%Þ ¼
Weight of adsorbate Weight of adsorbate þ weight of 3:6 mm beads
¼
4=3p½R3 2 r 3 rA 4=3p½R3 2 r 3 rA þ 4=3pr 3 rSilica
ð1Þ
where rA is the density of adsorbate (either silane (1 kg/l) or rubber (0.956 kg/l)), rSilica the density of 3.6 mm beads (2.5 kg/l) and R ¼ r þ d (as defined in Fig. 1), and the measured TGE weight losses, an estimate was made for d.
444
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Fig. 1. Schematic diagram for 3.6 mm silane/rubber-coated glass bead.
2.10. Raman spectroscopy A380 fumed silica was coated using the same silanation and rubber coating procedures as for the fibers, in order to be able to determine spectroscopically whether the silane and rubber were attached to the silica surface. Raman spectra were recorded using a JASCO Ventuno 21 microRaman spectrometer, with 12 MW illumination at the sample of 514.5 nm light from an argon ion laser. A Princeton Instruments, Inc., liquid nitrogen cooled Andor 1100 pixel CCD detector, and 1200 lines/mm gratings were used, resulting in a resolution of 2 cm21. 2.11. Preparation and testing of microbond shear strength samples All the silane and rubber treated fibers were stored in a dessicator. The method followed to make the shear strength samples was similar for all the fibers. A drop of the resin mixture was first placed on forceps. Fine resin beads of about 2– 4 £ 1023 m embedded length were placed on the fibers by holding the dry fiber at one end and pulling it through the resin droplet. The beads formed spontaneously and care was taken to ensure a spacing distance between them such that several specimens could be prepared along one fiber length. The fiber was cured in a light-curing oven (Dentsply/York division model TCU-II, 115 V, 600 W) for 4 min, then placed in petri dishes overnight. One end of the fiber was fixed onto a piece of cardboard using (5 min) epoxy, and allowed to completely cure overnight. The cardboard end of the sample was inserted in the top grip of an Instron tensile tester (model 1122). The fixed bottom grip (Fig. 2) consisted of a specially made device that had two glass slides that could be moved horizontally. The upper grip was used to position the bead just below the slides, which were closed until they just touched the outer surface of the fiber. The load was measured at a crosshead speed of 1 £ 1023 m/min and testing was complete when the bead had been pulled from the fiber or the fiber failed. To assess the effect of solvent-induced silane degradation on interface strength, half of the selected microbond shear strength samples were soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month. These soaked samples were tested using the same method mentioned above for the unsoaked samples.
Fig. 2. Schematic diagram of the bottom grip used for pullout tests.
The peak load from the load versus displacement curve was recorded and used to calculate the interfacial shear strength from the following equation
t ¼ ðF=pdlÞ
ð2Þ
where t is the interfacial shear stress at failure, F the peak debonding load, d the fiber diameter and l is the embedded length of the resin bead. The interfacial shear strength was obtained for beads cured: (i) on uncoated fibers, whose hydrophilic surface contains silanols; (ii) on the fibers coated with three concentrations of MPS, for which there exists coupling between the silane and matrix; (iii) on fibers coated with GPS, for which there is a chemical attachment to the silane, but no coupling to the matrix; (iv) on fibers coated nonspecifically with the rubber, for which there is no chemical attachment to the fiber, but for which there is the possibility of reaction between the rubber and the matrix; and (v) on fibers coated with silane and rubber for which there is chemical attachment to the silica, a rubbery interface and chemical reactivity between the rubber and the matrix. 2.12. Statistical analysis The sample sizes for the unsoaked specimens ranged from 13 to 20. The data were analyzed using a one-way analysis of variance (ANOVA). Sample sizes for soaked specimens ranged from 11 to 21. Individual two sided t-tests were conducted for differences between the dry and soaked condition for the control and for each of the five treatments that had been tested in both the soaked and dry states. 3. Results The TGA results for all the samples, as well as thicknesses calculated according to Eq. (1), are presented in Table 2. As shown in Fig. 3, an increase of 25.88% in the TGA weight loss was observed due to grafting of PMMA chains onto the rubber coated A380. These results indicate that double bonds in the rubber are available for crosslinking with the methacrylate moieties from the resin matrix, thus enhancing filler – matrix coupling. Raman spectra between 1000 and 3200 cm21 of A380 fumed silica silanated with GPS, the rubber attached via
S. Debnath et al. / Dental Materials 19 (2003) 441–448
445
Table 2 Change in absorbate (GPS, MPS or rubber) layer thickness with various coating methods Type of treatment
TGA weight loss (%)
Coverage (number of of adsorbate molecules/nm2 silica surface)
Layer thickness d (nm)
1% MPS 5% GPS 5% MPS 10% MPS Control rubber Direct rubber Reverse rubber
0.05 0.15 0.11 0.13 0.08 0.21 1.28
1.93 4.67 4.82 5.85 0.15 0.29 1.85
0.7 1.5 1.6 1.9 1.3 3.3 20.1
Method II, and the neat GPS and rubber are shown in Fig. 4. Bands characteristic of the GPS, in particular the CH stretching and bending vibrations, are observed on the silanated beads. In addition, the double bond and cyano groups from the rubber, at 1660 and 2225 cm21, respectively, are also observed on the rubber-coated beads. When the rubber is applied using Method I, these bands are also observed, but with decreased intensity. Since the molar ratio of the mass of the rubber to that of the GPS is , 20 £ , and the GPS bands are not uniquely different from the rubber, the GPS was not observed in the presence of the rubber. A typical sample load – displacement ðP – LÞ curve for an unsoaked 5% MPS specimen is shown in Fig. 5. The results of the bond strength tests, calculated using Eq. (2), are presented in Table 3 for both soaked and unsoaked samples. The mean strengths analyzed using a one-way ANOVA were shown to differ significantly among the treatments ðp , 0:001Þ: A plot of the residuals confirmed approximate normality. Tukey’s multiple comparison tests using a 5% experiment wise error rate then showed that one surface treatment (5% MPS) had significantly higher mean shear strength than all others. Every sample mean, but one was higher than the control. The mean strength for the control determined using individual
Fig. 3. TGA curves for testing covalent attachment of rubber to resin.
two sided t-tests was lower in the soaked condition, but not significantly so ðp ¼ 0:232Þ: For the surface treated specimens, soaking resulted in a significant lowering of the mean strength with p , 0:001 for four of the treatments and p ¼ 0:009 for the fifth. Again, a one-way ANOVA confirmed differences in mean strength among the soaked specimens. Although the 5% MPS treatment again had the highest sample average among the soaked specimens, Tukey’s multiple comparison test could not confirm that it was superior to the soaked control. It was shown to be significantly stronger than two of the other treatments.
4. Discussion 4.1. Unsoaked fibers Pluedemann has summarized the various theories of bonding via coupling agents [22]. The covalent, chemical bonding theory may be substantiated through spectroscopic
Fig. 4. Raman spectra of rubber and GPS only, and when coated on A380 silica.
446
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Fig. 5. Typical load –displacement curve for unsoaked 5% MPS specimen.
evidence, and we have described such evidence in a prior publication [23]. The wetting theory suggests that good bonding is achieved when the resin is able to completely wet the silanated glass surface, but published data suggest that non-reactive silanes which provide good wettability do not increase the mechanical properties of composites. In our case, all coupling agents used were reactive with the matrix. Finally, it has been suggested that the coupling agent modifies the matrix polymer morphology, either weakening (deformable layer theory) or strengthening the matrix (restrained layer theory). Data in support of these latter explanations may be gathered by nanoindentation methods, but are not yet available for consideration in the study described here. The interfacial bond strength is expected to be dependent on the strengths of the interactions (van der Waals, Hbonding, covalent) between the species comprising the interfacial layer. In the case of the unsoaked fibers, interfacial bond strength is poorest, and similar for the ‘as is’ silica, and for silica silanated with 5% GPS, and coating with rubber applied as a control. In these cases, at least one
of the surfaces has no covalent linkage, either to the silica (control rubber) or to the matrix (as is and 5% GPS). The average value obtained here for the as is silica is slightly lower than that found by McDonough, et al., but within their standard deviation [18]. The use of MPS coated silica increases interfacial bond strength, indicating that the silane molecules do act as coupling agents, chemically reacting both with the silica (via the methoxy end) and the matrix (via the methacrylate end). The greatest effect was observed at 5% MPS coverage, and the value obtained was lower than that found by McDonough et al., but also within their standard deviation [18]. Our previous studies using MPS silanated OX50 and polymethylmethacrylate (PMMA) have shown that as the amount of silane in the reaction solution was increased, higher amounts of silane were attached onto the silica surface [23]. However, the efficiency of grafting (to PMMA chains) was greatest for the silica silanated using a 1% MPS silanization solution and was attributed to monolayer formation of MPS; use of a 5% MPS silanization solution resulted in coverage that consisted of bi- or multilayer structures in which some of the MPS was not
Table 3 Average interfacial shear bond strengths (MPa) of treated glass fibers before and after soaking. Sets of treatments whose strengths do not differ are underscored by a common line
S. Debnath et al. / Dental Materials 19 (2003) 441–448
covalently attached to the silica and could therefore subsequently diffuse into the surrounding matrix. In the present study, the 5% MPS treated sample exhibited the highest interfacial strength. We attribute the decrease in interfacial strength for the 10% MPS sample to the same effect, namely, the formation of a silane multi-layer structure on the fibers. Due to the large difference in surface areas and surface properties between the A380 fumed silica and 3.6 mm beads, conditions to achieve monolayer coverage are not expected to be the same. In the multi-layer structure, the number of accessible methacrylate groups accessible for cross-linking with the resin is reduced [23]. Miller et al. [12] have also reported the apparent lack of effectiveness resulting from over treatment with silane compounds, which they attributed to cohesive failure within the thicker silane coatings. Similar results were reported for silanated glass fibers by Yue and Quek [24], who suggested the existence of a critical thickness for the silane layer on the treated fiber. 4.2. Effect of rubber The TGA data indicate that attaching rubber via Method II substantially increases the amount of rubber on the silica compared with Method I, and that interfacial strength is greater with increased rubber coverage. The effect of the interfacial rubber layer is greater in the case of Method II compared with Method I, and its effect is similar to that of the 5% MPS sample. Although we hope that the rubbery coating will increase composite toughness, this interface had only a minimal effect on interfacial strength, suggesting that the number of covalent attachments between the silane/matrix and the silane/rubber/matrix are similar. The increase in strength with the number of attached rubber molecules on the surface may reflect the number of covalent attachments between the rubber and the matrix. The ability of the rubber to covalently attach to the matrix was confirmed by the TGA weight loss measurements using MMA monomer. The ability of the rubber to react with the MMA was due to the 20% 1,2 vinyl content of the butadiene in the rubber, as has also been observed for other methacrylate monomers polymerized in a pure polybutadiene matrix [25]. The results of polymerizing the rubber in the presence of resin (in the absence of filler) showed that even at a 10/90% rubber/resin ratio, the system did not gel, and still appeared homogeneous. This raises the possibility that in composites next to the silica filler particle, there is not much cross-linking in the rubber/matrix interphase region. 4.3. Effect of soaking Hydrolytic degradation of silane – silica bonds has been previously described: Plueddemann proposed a ‘reversible hydrolizable bond mechanism’ where hydrolysis and reformation of polymeric SiOSi bonds of a silane coupling
447
agent on a glass surface are considered as a dynamic equilibrium [22]. Along similar lines, Ishida and Koenig [26] reported reduction in the amount of silane on glass fibers during soaking in hot water due to the hydrolytic degradation of the former. The siloxane bonds formed by hydrolysis of silanes could be partially reformed back to silanols. Soaking decreases the interfacial shear strength for all the samples as has been previously observed [2,18]. Similarly, the flexural strength of fiber-reinforced composites also decreases following soaking in water [27]. There is a greater similarity in interfacial shear strength values among the soaked specimens than among the unsoaked specimens, with values comparable to the as is specimens. McDonough et al., observed this same effect for as is and MPS silanated fibers, but interfacial shear strengths were maintained for fibers coated with a hydrophobic silane, following a 24 h soaking procedure [18]. The interfacial strengths after soaking in this study (1 month) remain greatest for the 5% MPS and rubber coated (Method II) specimens. The values of the as is, 5% GPS and control rubber treated samples remain similar before and after soaking, which can be attributed to the lack of covalent bonding with the resin matrix. The 10% MPS treated samples showed the largest (, 50%) drop in properties before and after soaking. This may be due to a silane multilayer structure, which debonds after soaking due to hydrolysis. There is a positive correlation between the amount of silane on the filler surface and the magnitude of the property loss after soaking. The percent property drop in the case of soaked rubber treated samples is lower compared with 5 and 10% MPS samples. Acknowledgements This investigation was supported by Research Grant R 01 DE09530 from the National Institute for Dental and Craniofacial Research, Bethesda, Maryland, USA. The contribution of resin materials by ESSTECH is also gratefully acknowledged. One of us (S.D.) also thanks Temple University for the David Swern scholarship. References [1] Chen TM, Brauer GM. Solvent effects on bonding organo-silane to silica surfaces. J Dent Res 1982;61:1439–43. [2] Mohsen NM, Craig RG. Effect of silanation of fillers on their dispersability by monomer systems. J Oral Rehab 1995;22:183 –9. [3] Wang J-W, Ploehn H. Dynamic mechanical analysis of the effect of water on glass bead–epoxy composites. J Appl Polym Sci 1996;59: 345–57. [4] Zhao F, Takeda N. Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part I: experimental results. Composites, Part A 2000; 31:1203–14. [5] Kessler A, Bleddzki A. Correlation between interphase-relevant tests and the impact– damage resistance of glass/epoxy laminates with
448
[6]
[7]
[8] [9] [10]
[11] [12]
[13]
[14] [15] [16]
S. Debnath et al. / Dental Materials 19 (2003) 441–448 different fibre surface treatments. Compos Sci Technol 2000;60: 125–30. Keusch S, Queck H, Gillespie K. Influence of glass fibre/epoxy resin interface on static mechanical properties of unidirectional composites and on fatigue performance of cross ply composites. Composites, Part A 1998;29:701 –5. Nohrstrom TJ, Vallittu PK, Yli-Urpo A. The effect of position and quantity of glass fibers on the fracture resistance of provisional fixed partial dentures. Int J Prosthodont 2000;13:72–8. Pitkethly M. A round-robin programme on interfacial test methods. Compos Sci Technol 1993;48:205– 14. DiBenedetto A, Lex P. Evaluation of surface treatments for glass fibers in composite materials. Polym Engng Sci 1989;29:543–55. Zinck P, Pays M, Resakhanlou R, Gerard J. Mechanical characterisation of glass fibres as an indirect analysis of the effect of surface treatment. J Mater Sci 1999;34:2121–33. Piggott M. Why interface testing by single-fibre methods can be misleading. Compos Sci Technol 1997;57:965– 74. Miller B, Muri P, Rebenfield L. A microbond method for determination of the shear strength of a fiber/resin interface. Compos Sci Technol 1987;28:17–32. Woerdeman D, Amouroux D, Ponsinet V, Jandeau G, Hervet H, Leger L. Characterization of glass –epoxy adhesion using JKR methods and atomic force microscopy. Composites, Part A 1999;30: 95–109. Pisanova E, Shandarov S, Mader E. How can adhesion be determined from micromechanical tests? Composites, Part A 2001;32:425– 34. Kerans R, Parthasarathy T. Theoretical analysis of the fiber pullout and pushout tests. J Am Ceram Soc 1991;74:1585– 96. Chai Y, Mai Y-W. New analysis on the fiber push-out problem with interface roughness and thermal residual stresses. J Mater Sci 2001; 36:2095–104.
[17] Keusch S, Haessler R. Influence of surface treatment of glass fibres on the dynamic mechanical properties of epoxy resin composites. Composites, Part A 1999;30:997 –1002. [18] McDonough W, Antonucci J, Dunkers J. Interfacial shear strengths of dental resin– glass fibers by the microbond test. Dent Mater 2001;17: 492 –8. [19] Craig RG, Dootz E. Effect of mixed silanes on the hydrolytic stability of composites. J Oral Rehab 1996;23:751–6. [20] Mohsen N, Craig RG. Hydrolytic stability of silanated zirconia– silica–urethane dimethacrylate composites. J Oral Rehab 1995;22: 213 –20. [21] Miettinen VM, Narva K, Vallittu PK. Water sorption, solubility, and post-curing of glass fibre reinforced polymers. Biomaterials 1999;20: 1187–94. [22] Pluedemann E. Silane coupling agents. New York: Plenum Press; 1982. [23] Liu Q, Ding J, Debnath S, Wunder S, Baran G. Filler–coupling agent– matrix interactions in silica/polymethylmethacrylate composites. J Biomed Mater Res 2001;57:384–93. [24] Yue C, Quek M. The interfacial properties of fibrous composites part III effect of the thickness of the silane coupling agent. J Mater Sci 1994;29:2487–90. [25] Mateo JL, Calvo M, Bosch P. Photoinitiated polymerization of methacrylic monomers in a polybutadiene matrix (PB): kinetic, mechanistic, and structural aspects. J Polym Sci, Part A: Polym Chem 2001;39:2444–53. [26] Ishida H, Koenig J. Effect of hydrolysis and drying on the siloxane bonds of a silane coupling agent deposited on E-glass fibers. J Polym Sci: Polym Phys Ed 1980;18:233–7. [27] Vallittu PK. Effect of 180 weeks water storage on the flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthod 2000;13:334–9.
Silane treatment effects on glass/resin interfacial shear strengths Subir Debnatha, Stephanie L. Wundera, John I. McCoolb, George R. Baranc,* a Department of Chemistry, Temple University, Philadelphia, PA 19122, USA Department of Industrial Engineering, Penn State Great Valley, Malvern, PA 19355, USA c Center for Bioengineering and Biomaterials, College of Engineering, Temple University, 1947 North 12th Street, Philadelphia, PA 19122, USA b
Received 18 January 2002; revised 26 June 2002; accepted 18 July 2002
Abstract Objectives. Methacrylic resin-based dental composites normally use a bifunctional silane coupling agent with an intermediary carbon connecting segment to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pull-out tests were used to measure the interfacial bond strength at the fiber– matrix interface. Methods. Glass fibers (approximately 30 mm diameter, 8 £ 1022 m length, MoSci) were silanated using various concentrations (1, 5 and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyltrimethoxy-silane (GPS) in acetone (99.8%). Rubber (poly(butadiene/acrylonitrile), amine terminated, Mw 5500) molecules were also attached to the fiber surface via GPS molecules. The resin was comprised of a 60/40 mixture of Bis-phenol-A bis-(2-hydroxypropyl)-methacrylate (BisGMA) and tri (ethylene glycol) dimethacrylate (TEGDMA). A bead of resin approximately 2– 4 £ 1023 m in embedded length was placed on the treated fibers and light cured. The load required to pull the fiber out of the resin was converted to shear bond strength. Results. Interfacial shear strengths were greater for silanated specimens compared with unsilanated, and for MPS compared with GPS. The same set of samples soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month showed a decrease in properties. Significance. A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples. q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Dental materials; Composites; Silane; Shear strength; Resin; Bonding
1. Introduction Inorganic fillers in dental composites are typically coated with silanes in order to improve the bond to the resin matrix and increase the service life of the composite [1]; an additional benefit is the improved dispersability of silanated fillers in matrix monomers [2]. Composites with silanated fillers possess superior mechanical properties and wear resistance, and increased resistance to water sorption [3] when compared with composites containing non-silanated fillers. The stronger the filler – resin interface, the greater the improvement in static [4], impact [5], and fatigue properties [6]. In dental composites, filler morphology may be spherical, irregular or fibrous. Spherical and irregularly shaped fillers are dispersed randomly, while in fiberreinforced composites, the filler is arranged in a unidirectional manner in order to best take advantage of * Corresponding author. Tel.: þ 1-215-204-8824; fax: þ1-215-204-4956. E-mail address: [email protected] (G.R. Baran).
the strengthening effect, although the position of the fibers also plays a role in the degree of strengthening [7]. Numerous methods have been developed for evaluating the quality of the filler – resin interface, and these have been recently reviewed [8]. A common technique is the embedded single fiber (fragmentation) test [9], though the test requires knowledge of the fiber tensile strength at the critical length of the fiber fragment, i.e. an additional test is required to obtain that property [10]. The shear strength of the interface may also be determined by a fiber pull-out test [11], but the fiber lengths must be kept small to ensure that fibers do not break prior to interface failure; the short fiber specimens required pose experimental difficulties. The microbond test was developed for fibers with small diameters, and relies on the ability to displace a small resin droplet that has been cured around a fiber [12]. A criticism made of all these tests is that the complicated stress states surrounding the fibers do not permit measurement of fiber –matrix adhesion [13,14], that residual axial stresses may comprise a significant portion of the measured shear
0109-5641/03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0109-5641(02)00089-1
442
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Table 1 Reagents Characteristics
Silica
Silanes
Resin components
Supplier
Chemical structure
Surface area
Particle size
A380 fumed silica Ground glass E-glass fibers (52–58% SiO2, 11–17% Al2O3, 6–7% B2O3, 18–25% CaO, 0.5–1% MgO, ,0.5% Fe2O3, ,0.1% Na2O, K2O)
380 m2/g
7 nm 3.6 mm 30 mm (diameter) 8 cm (length)
Acronym
Function
3-Methacryloxypropyl-trimethoxysilane Glycidoxypropyltrimethoxy-silane [Poly(butadiene/acrylonitrile)] amine terminated, Mw ¼ 5500 g/mol, 20% 1,2 vinyl content Pyridine (99.9%)
MPS GPS Rubber
Methylmethacrylate Benzoyl peroxide Dimethyl para toluidine bis-Phenol-A bis-(2-hydroxypropyl)methacrylate Tri(ethylene glycol) dimethacrylate Camphorquinone Tertiary amine, dimethylaminoethyl methacrylate Hydroquinone monomethylether
MMA BPO DMPT BisGMA TEGDMA CQ DMAEMA HMME
strength [15] and that the interfacial shear stress is affected by residual thermal stresses and the roughness of the interface [16]. Dynamic mechanical analysis (DMA) of composites may also provide information about the strength of the filler – resin interface: the feature most sensitive to the interface is the height of the main loss tangent peak (tan d at the glass transition temperature [3], though the flexural storage modulus E0 is also affected [17]. The structural composites literature contains a number of reports on the strength of the filler–matrix interface, but we are aware of only limited information regarding interfacial shear strengths in dental composite systems [18]. The effectiveness of silanation protocols has most often been indirectly assessed, usually by subjecting composites to soaking or boiling water treatments, then measuring the strength of the composite [19,20]. The implied assumption has been that weaker, or more readily degraded interfaces, will result in lower composite strengths. In this study, we employ the microbond test to evaluate the shear strength of the interface between glass fibers and a BISGMA/TEGDMA matrix following fiber surface treatment using 3-methacryloxypropyl-trimethoxysilane (MPS), and an adduct composed of glycidoxypropyltrimethoxy-silane (GPS) and a (poly(butadiene/acrylonitrile), amine terminated rubber, before and after soaking in a 50:50 (v/v) methanol–water mixture. The soaking was carried out because there is a concern that water sorption could influence the stability of the fiber–matrix interface and therefore affect the strength of polymer–fiber composites [21].
Degussa MoSci MoSci
Aldrich Aldrich Polysciences Catalyst
Aldrich
Monomer Initiator Accelerator Monomer Monomer Initiator Promoter Inhibitor
Aldrich Aldrich Aldrich Esstech Esstech Esstech Esstech Esstech
2. Materials and methods 2.1. Chemicals The chemicals used and their suppliers are listed in Table 1. All materials were used as received except for the 3.6 mm glass beads, which were washed with distilled water and evacuated at room temperature. The solvents ethanol (95%), acetone (99.8%), methanol (95%), chloroform (100%), were obtained from Aldrich and used without further purification. 2.2. Preparation of the resin Sixty grams of BisGMA was weighed into a Teflon beaker and mixed with 40 g of TEGDMA (in the dark). This mixture was stirred manually with a glass rod until a uniform consistency was achieved, and then it was allowed to rest for a day. After addition of 5 £ 1024 kg photosensitizer (CQ), 5 £ 1024 kg reducing agent (DMAEMA) and 8.5 £ 1023 kg HQ, it was stirred overnight in the dark using a magnetic stirrer. The homogeneous mixture was then bottled and stored for future use. 2.3. Silanization using acetone as solvent MPS (or GPS) was dissolved in different volumes of acetone to obtain silane concentrations of 1, 5 and 10% (w/w) for MPS and 5% for GPS. The fibers were silanated by agitation for 2 h in these solutions and were then
S. Debnath et al. / Dental Materials 19 (2003) 441–448
443
separated using a centrifuge run at 10,000 rpm. The silanated fibers were dried at room temperature (25 8C) under vacuum overnight and then additionally at 110 8C for 2 h at atmospheric pressure. The fibers were further washed three times with methanol, then dried in vacuum overnight.
chloroform, and evacuated again at room temperature overnight. The 3.6 mm beads were coated using the same silanation and rubber coating procedures as for the fibers.
2.4. Rubber coating
In addition, A380 fumed silica silanated with a rubber coating applied using Method II was used as a filler for a polymethylmethacrylate matrix prepared as in Ref. [20], in order to determine whether the unsaturated double bonds in the rubber molecules were incorporated into the growing PMMA chains. The rubber coated A380 fumed silica (0.5 £ 10 24 kg) was added to a vial containing 8.488 £ 1024 kg MMA monomer and 0.09 (1%) DMPT and sonicated for 30 min. This was followed by addition of 3.6 £ 1024 kg (4%) BPO. The composites were cured at room temperature for 48 h and additionally at 60 8C for 6 h. The composite thus prepared was dissolved in 100 ml acetone. The silica filler was separated using a centrifuge at 9000 rpm for 20 min, resuspended in 30 ml acetone and further washed following the same procedure until achieving constant TGA weight loss. The washed silica was dried under vacuum for 12 h before each TGA analysis.
The fibers were coated with varying amounts of rubber using several procedures. Chemical attachment was achieved by first reacting the silane with the silica, followed by reaction with rubber (Method I) or by first reacting the silane with the rubber, followed by reaction with the silica (Method II). The silane was bifunctional, with one end (CH3O) reactive towards the silica and the other end (epoxy) reactive towards the amine-terminated rubber. Similarly, the rubber was bifunctional, containing unsaturated CyC bonds that could be incorporated into the polymerizing methacrylate resin. As a control, the rubber was also nonspecifically adsorbed onto the silica (control rubber). 2.5. Rubber coating—Method I The fibers were first silanated using a 1% GPS solution in ethanol. The silanation procedure used was the same as that described for silanation using MPS/GPS. A solution consisting of 9.5 £ 1024 g rubber dissolved in 6.5 ml chloroform was used to disperse the washed, dried silanated fibers. After addition of 1 ml pyridine as catalyst for the reaction of the epoxy group of the silane with the amino end groups of the rubber molecules, the mixture was stirred for 4 h. The fibers were evacuated at room temperature (25 8C) overnight after separation by centrifugation (10,000 rpm) from the silane solution. The fibers were then washed twice with chloroform and evacuated at room temperature (25 8C) overnight. The dry fibers thus prepared were directly used to make the shear strength specimens described below. 2.6. Rubber coating—Method II In this method, 9.5 £ 1024 kg rubber, 45 ml GPS and 1 ml pyridine were reacted in 6.5 ml chloroform for 2 days at room temperature, to directly form a silane –rubber adduct. Fibers were added to the chloroform, GPS-rubber adduct solution and agitated for 4 h. The fibers were separated using a centrifuge (10,000 rpm) followed by evacuation at room temperature (25 8C) overnight. They were then washed twice with chloroform and evacuated at room temperature (25 8C) overnight. 2.7. Rubber coating—control The fibers were stirred in a solution of 9.5 £ 1024 kg rubber, 6.5 ml chloroform and 1 ml pyridine for 4 h. The fibers were separated by centrifugation, evacuated at room temperature (25 8C) overnight, washed twice with
2.8. Covalent attachment of rubber to resin
2.9. Determination of coating thickness Thermogravimetric analysis (TGA) methods, on a TA Instruments Hi-Res 2950 Thermogravimetric Analyzer using a ramp rate of 10 8C/min, were used to quantify the amount of silane or rubber molecules attached to the silica (3.6 mm) bead surface. Samples were heated from 25 to 800 8C and the amount of surface attached material was calculated from the amount of weight loss over this temperature interval. It was assumed that results found for the 3.6 mm beads were the same as for the fibers. It was not possible to obtain direct results for the latter due to their low surface area. The amount of silane/rubber molecule coverage on the beads was calculated using the TGA weight loss and assuming the formation of a uniform and spherical layer of coated molecules on the glass beads. The same assumption was also used for the calculation of their layer thickness. Using the formula TGA weight loss ð%Þ ¼
Weight of adsorbate Weight of adsorbate þ weight of 3:6 mm beads
¼
4=3p½R3 2 r 3 rA 4=3p½R3 2 r 3 rA þ 4=3pr 3 rSilica
ð1Þ
where rA is the density of adsorbate (either silane (1 kg/l) or rubber (0.956 kg/l)), rSilica the density of 3.6 mm beads (2.5 kg/l) and R ¼ r þ d (as defined in Fig. 1), and the measured TGE weight losses, an estimate was made for d.
444
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Fig. 1. Schematic diagram for 3.6 mm silane/rubber-coated glass bead.
2.10. Raman spectroscopy A380 fumed silica was coated using the same silanation and rubber coating procedures as for the fibers, in order to be able to determine spectroscopically whether the silane and rubber were attached to the silica surface. Raman spectra were recorded using a JASCO Ventuno 21 microRaman spectrometer, with 12 MW illumination at the sample of 514.5 nm light from an argon ion laser. A Princeton Instruments, Inc., liquid nitrogen cooled Andor 1100 pixel CCD detector, and 1200 lines/mm gratings were used, resulting in a resolution of 2 cm21. 2.11. Preparation and testing of microbond shear strength samples All the silane and rubber treated fibers were stored in a dessicator. The method followed to make the shear strength samples was similar for all the fibers. A drop of the resin mixture was first placed on forceps. Fine resin beads of about 2– 4 £ 1023 m embedded length were placed on the fibers by holding the dry fiber at one end and pulling it through the resin droplet. The beads formed spontaneously and care was taken to ensure a spacing distance between them such that several specimens could be prepared along one fiber length. The fiber was cured in a light-curing oven (Dentsply/York division model TCU-II, 115 V, 600 W) for 4 min, then placed in petri dishes overnight. One end of the fiber was fixed onto a piece of cardboard using (5 min) epoxy, and allowed to completely cure overnight. The cardboard end of the sample was inserted in the top grip of an Instron tensile tester (model 1122). The fixed bottom grip (Fig. 2) consisted of a specially made device that had two glass slides that could be moved horizontally. The upper grip was used to position the bead just below the slides, which were closed until they just touched the outer surface of the fiber. The load was measured at a crosshead speed of 1 £ 1023 m/min and testing was complete when the bead had been pulled from the fiber or the fiber failed. To assess the effect of solvent-induced silane degradation on interface strength, half of the selected microbond shear strength samples were soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month. These soaked samples were tested using the same method mentioned above for the unsoaked samples.
Fig. 2. Schematic diagram of the bottom grip used for pullout tests.
The peak load from the load versus displacement curve was recorded and used to calculate the interfacial shear strength from the following equation
t ¼ ðF=pdlÞ
ð2Þ
where t is the interfacial shear stress at failure, F the peak debonding load, d the fiber diameter and l is the embedded length of the resin bead. The interfacial shear strength was obtained for beads cured: (i) on uncoated fibers, whose hydrophilic surface contains silanols; (ii) on the fibers coated with three concentrations of MPS, for which there exists coupling between the silane and matrix; (iii) on fibers coated with GPS, for which there is a chemical attachment to the silane, but no coupling to the matrix; (iv) on fibers coated nonspecifically with the rubber, for which there is no chemical attachment to the fiber, but for which there is the possibility of reaction between the rubber and the matrix; and (v) on fibers coated with silane and rubber for which there is chemical attachment to the silica, a rubbery interface and chemical reactivity between the rubber and the matrix. 2.12. Statistical analysis The sample sizes for the unsoaked specimens ranged from 13 to 20. The data were analyzed using a one-way analysis of variance (ANOVA). Sample sizes for soaked specimens ranged from 11 to 21. Individual two sided t-tests were conducted for differences between the dry and soaked condition for the control and for each of the five treatments that had been tested in both the soaked and dry states. 3. Results The TGA results for all the samples, as well as thicknesses calculated according to Eq. (1), are presented in Table 2. As shown in Fig. 3, an increase of 25.88% in the TGA weight loss was observed due to grafting of PMMA chains onto the rubber coated A380. These results indicate that double bonds in the rubber are available for crosslinking with the methacrylate moieties from the resin matrix, thus enhancing filler – matrix coupling. Raman spectra between 1000 and 3200 cm21 of A380 fumed silica silanated with GPS, the rubber attached via
S. Debnath et al. / Dental Materials 19 (2003) 441–448
445
Table 2 Change in absorbate (GPS, MPS or rubber) layer thickness with various coating methods Type of treatment
TGA weight loss (%)
Coverage (number of of adsorbate molecules/nm2 silica surface)
Layer thickness d (nm)
1% MPS 5% GPS 5% MPS 10% MPS Control rubber Direct rubber Reverse rubber
0.05 0.15 0.11 0.13 0.08 0.21 1.28
1.93 4.67 4.82 5.85 0.15 0.29 1.85
0.7 1.5 1.6 1.9 1.3 3.3 20.1
Method II, and the neat GPS and rubber are shown in Fig. 4. Bands characteristic of the GPS, in particular the CH stretching and bending vibrations, are observed on the silanated beads. In addition, the double bond and cyano groups from the rubber, at 1660 and 2225 cm21, respectively, are also observed on the rubber-coated beads. When the rubber is applied using Method I, these bands are also observed, but with decreased intensity. Since the molar ratio of the mass of the rubber to that of the GPS is , 20 £ , and the GPS bands are not uniquely different from the rubber, the GPS was not observed in the presence of the rubber. A typical sample load – displacement ðP – LÞ curve for an unsoaked 5% MPS specimen is shown in Fig. 5. The results of the bond strength tests, calculated using Eq. (2), are presented in Table 3 for both soaked and unsoaked samples. The mean strengths analyzed using a one-way ANOVA were shown to differ significantly among the treatments ðp , 0:001Þ: A plot of the residuals confirmed approximate normality. Tukey’s multiple comparison tests using a 5% experiment wise error rate then showed that one surface treatment (5% MPS) had significantly higher mean shear strength than all others. Every sample mean, but one was higher than the control. The mean strength for the control determined using individual
Fig. 3. TGA curves for testing covalent attachment of rubber to resin.
two sided t-tests was lower in the soaked condition, but not significantly so ðp ¼ 0:232Þ: For the surface treated specimens, soaking resulted in a significant lowering of the mean strength with p , 0:001 for four of the treatments and p ¼ 0:009 for the fifth. Again, a one-way ANOVA confirmed differences in mean strength among the soaked specimens. Although the 5% MPS treatment again had the highest sample average among the soaked specimens, Tukey’s multiple comparison test could not confirm that it was superior to the soaked control. It was shown to be significantly stronger than two of the other treatments.
4. Discussion 4.1. Unsoaked fibers Pluedemann has summarized the various theories of bonding via coupling agents [22]. The covalent, chemical bonding theory may be substantiated through spectroscopic
Fig. 4. Raman spectra of rubber and GPS only, and when coated on A380 silica.
446
S. Debnath et al. / Dental Materials 19 (2003) 441–448
Fig. 5. Typical load –displacement curve for unsoaked 5% MPS specimen.
evidence, and we have described such evidence in a prior publication [23]. The wetting theory suggests that good bonding is achieved when the resin is able to completely wet the silanated glass surface, but published data suggest that non-reactive silanes which provide good wettability do not increase the mechanical properties of composites. In our case, all coupling agents used were reactive with the matrix. Finally, it has been suggested that the coupling agent modifies the matrix polymer morphology, either weakening (deformable layer theory) or strengthening the matrix (restrained layer theory). Data in support of these latter explanations may be gathered by nanoindentation methods, but are not yet available for consideration in the study described here. The interfacial bond strength is expected to be dependent on the strengths of the interactions (van der Waals, Hbonding, covalent) between the species comprising the interfacial layer. In the case of the unsoaked fibers, interfacial bond strength is poorest, and similar for the ‘as is’ silica, and for silica silanated with 5% GPS, and coating with rubber applied as a control. In these cases, at least one
of the surfaces has no covalent linkage, either to the silica (control rubber) or to the matrix (as is and 5% GPS). The average value obtained here for the as is silica is slightly lower than that found by McDonough, et al., but within their standard deviation [18]. The use of MPS coated silica increases interfacial bond strength, indicating that the silane molecules do act as coupling agents, chemically reacting both with the silica (via the methoxy end) and the matrix (via the methacrylate end). The greatest effect was observed at 5% MPS coverage, and the value obtained was lower than that found by McDonough et al., but also within their standard deviation [18]. Our previous studies using MPS silanated OX50 and polymethylmethacrylate (PMMA) have shown that as the amount of silane in the reaction solution was increased, higher amounts of silane were attached onto the silica surface [23]. However, the efficiency of grafting (to PMMA chains) was greatest for the silica silanated using a 1% MPS silanization solution and was attributed to monolayer formation of MPS; use of a 5% MPS silanization solution resulted in coverage that consisted of bi- or multilayer structures in which some of the MPS was not
Table 3 Average interfacial shear bond strengths (MPa) of treated glass fibers before and after soaking. Sets of treatments whose strengths do not differ are underscored by a common line
S. Debnath et al. / Dental Materials 19 (2003) 441–448
covalently attached to the silica and could therefore subsequently diffuse into the surrounding matrix. In the present study, the 5% MPS treated sample exhibited the highest interfacial strength. We attribute the decrease in interfacial strength for the 10% MPS sample to the same effect, namely, the formation of a silane multi-layer structure on the fibers. Due to the large difference in surface areas and surface properties between the A380 fumed silica and 3.6 mm beads, conditions to achieve monolayer coverage are not expected to be the same. In the multi-layer structure, the number of accessible methacrylate groups accessible for cross-linking with the resin is reduced [23]. Miller et al. [12] have also reported the apparent lack of effectiveness resulting from over treatment with silane compounds, which they attributed to cohesive failure within the thicker silane coatings. Similar results were reported for silanated glass fibers by Yue and Quek [24], who suggested the existence of a critical thickness for the silane layer on the treated fiber. 4.2. Effect of rubber The TGA data indicate that attaching rubber via Method II substantially increases the amount of rubber on the silica compared with Method I, and that interfacial strength is greater with increased rubber coverage. The effect of the interfacial rubber layer is greater in the case of Method II compared with Method I, and its effect is similar to that of the 5% MPS sample. Although we hope that the rubbery coating will increase composite toughness, this interface had only a minimal effect on interfacial strength, suggesting that the number of covalent attachments between the silane/matrix and the silane/rubber/matrix are similar. The increase in strength with the number of attached rubber molecules on the surface may reflect the number of covalent attachments between the rubber and the matrix. The ability of the rubber to covalently attach to the matrix was confirmed by the TGA weight loss measurements using MMA monomer. The ability of the rubber to react with the MMA was due to the 20% 1,2 vinyl content of the butadiene in the rubber, as has also been observed for other methacrylate monomers polymerized in a pure polybutadiene matrix [25]. The results of polymerizing the rubber in the presence of resin (in the absence of filler) showed that even at a 10/90% rubber/resin ratio, the system did not gel, and still appeared homogeneous. This raises the possibility that in composites next to the silica filler particle, there is not much cross-linking in the rubber/matrix interphase region. 4.3. Effect of soaking Hydrolytic degradation of silane – silica bonds has been previously described: Plueddemann proposed a ‘reversible hydrolizable bond mechanism’ where hydrolysis and reformation of polymeric SiOSi bonds of a silane coupling
447
agent on a glass surface are considered as a dynamic equilibrium [22]. Along similar lines, Ishida and Koenig [26] reported reduction in the amount of silane on glass fibers during soaking in hot water due to the hydrolytic degradation of the former. The siloxane bonds formed by hydrolysis of silanes could be partially reformed back to silanols. Soaking decreases the interfacial shear strength for all the samples as has been previously observed [2,18]. Similarly, the flexural strength of fiber-reinforced composites also decreases following soaking in water [27]. There is a greater similarity in interfacial shear strength values among the soaked specimens than among the unsoaked specimens, with values comparable to the as is specimens. McDonough et al., observed this same effect for as is and MPS silanated fibers, but interfacial shear strengths were maintained for fibers coated with a hydrophobic silane, following a 24 h soaking procedure [18]. The interfacial strengths after soaking in this study (1 month) remain greatest for the 5% MPS and rubber coated (Method II) specimens. The values of the as is, 5% GPS and control rubber treated samples remain similar before and after soaking, which can be attributed to the lack of covalent bonding with the resin matrix. The 10% MPS treated samples showed the largest (, 50%) drop in properties before and after soaking. This may be due to a silane multilayer structure, which debonds after soaking due to hydrolysis. There is a positive correlation between the amount of silane on the filler surface and the magnitude of the property loss after soaking. The percent property drop in the case of soaked rubber treated samples is lower compared with 5 and 10% MPS samples. Acknowledgements This investigation was supported by Research Grant R 01 DE09530 from the National Institute for Dental and Craniofacial Research, Bethesda, Maryland, USA. The contribution of resin materials by ESSTECH is also gratefully acknowledged. One of us (S.D.) also thanks Temple University for the David Swern scholarship. References [1] Chen TM, Brauer GM. Solvent effects on bonding organo-silane to silica surfaces. J Dent Res 1982;61:1439–43. [2] Mohsen NM, Craig RG. Effect of silanation of fillers on their dispersability by monomer systems. J Oral Rehab 1995;22:183 –9. [3] Wang J-W, Ploehn H. Dynamic mechanical analysis of the effect of water on glass bead–epoxy composites. J Appl Polym Sci 1996;59: 345–57. [4] Zhao F, Takeda N. Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part I: experimental results. Composites, Part A 2000; 31:1203–14. [5] Kessler A, Bleddzki A. Correlation between interphase-relevant tests and the impact– damage resistance of glass/epoxy laminates with
448
[6]
[7]
[8] [9] [10]
[11] [12]
[13]
[14] [15] [16]
S. Debnath et al. / Dental Materials 19 (2003) 441–448 different fibre surface treatments. Compos Sci Technol 2000;60: 125–30. Keusch S, Queck H, Gillespie K. Influence of glass fibre/epoxy resin interface on static mechanical properties of unidirectional composites and on fatigue performance of cross ply composites. Composites, Part A 1998;29:701 –5. Nohrstrom TJ, Vallittu PK, Yli-Urpo A. The effect of position and quantity of glass fibers on the fracture resistance of provisional fixed partial dentures. Int J Prosthodont 2000;13:72–8. Pitkethly M. A round-robin programme on interfacial test methods. Compos Sci Technol 1993;48:205– 14. DiBenedetto A, Lex P. Evaluation of surface treatments for glass fibers in composite materials. Polym Engng Sci 1989;29:543–55. Zinck P, Pays M, Resakhanlou R, Gerard J. Mechanical characterisation of glass fibres as an indirect analysis of the effect of surface treatment. J Mater Sci 1999;34:2121–33. Piggott M. Why interface testing by single-fibre methods can be misleading. Compos Sci Technol 1997;57:965– 74. Miller B, Muri P, Rebenfield L. A microbond method for determination of the shear strength of a fiber/resin interface. Compos Sci Technol 1987;28:17–32. Woerdeman D, Amouroux D, Ponsinet V, Jandeau G, Hervet H, Leger L. Characterization of glass –epoxy adhesion using JKR methods and atomic force microscopy. Composites, Part A 1999;30: 95–109. Pisanova E, Shandarov S, Mader E. How can adhesion be determined from micromechanical tests? Composites, Part A 2001;32:425– 34. Kerans R, Parthasarathy T. Theoretical analysis of the fiber pullout and pushout tests. J Am Ceram Soc 1991;74:1585– 96. Chai Y, Mai Y-W. New analysis on the fiber push-out problem with interface roughness and thermal residual stresses. J Mater Sci 2001; 36:2095–104.
[17] Keusch S, Haessler R. Influence of surface treatment of glass fibres on the dynamic mechanical properties of epoxy resin composites. Composites, Part A 1999;30:997 –1002. [18] McDonough W, Antonucci J, Dunkers J. Interfacial shear strengths of dental resin– glass fibers by the microbond test. Dent Mater 2001;17: 492 –8. [19] Craig RG, Dootz E. Effect of mixed silanes on the hydrolytic stability of composites. J Oral Rehab 1996;23:751–6. [20] Mohsen N, Craig RG. Hydrolytic stability of silanated zirconia– silica–urethane dimethacrylate composites. J Oral Rehab 1995;22: 213 –20. [21] Miettinen VM, Narva K, Vallittu PK. Water sorption, solubility, and post-curing of glass fibre reinforced polymers. Biomaterials 1999;20: 1187–94. [22] Pluedemann E. Silane coupling agents. New York: Plenum Press; 1982. [23] Liu Q, Ding J, Debnath S, Wunder S, Baran G. Filler–coupling agent– matrix interactions in silica/polymethylmethacrylate composites. J Biomed Mater Res 2001;57:384–93. [24] Yue C, Quek M. The interfacial properties of fibrous composites part III effect of the thickness of the silane coupling agent. J Mater Sci 1994;29:2487–90. [25] Mateo JL, Calvo M, Bosch P. Photoinitiated polymerization of methacrylic monomers in a polybutadiene matrix (PB): kinetic, mechanistic, and structural aspects. J Polym Sci, Part A: Polym Chem 2001;39:2444–53. [26] Ishida H, Koenig J. Effect of hydrolysis and drying on the siloxane bonds of a silane coupling agent deposited on E-glass fibers. J Polym Sci: Polym Phys Ed 1980;18:233–7. [27] Vallittu PK. Effect of 180 weeks water storage on the flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthod 2000;13:334–9.