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Effect of silane coupling agents on the wear resistance of polymer-nanoporous silica gel dental composites
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering C 5 (1997) 15-22
12
Effect of silane coupling agents on the wear resistance of polymer-nanoporous silica gel dental composites Jiazhong Luo a, Robert Seghi b, John Lannutti a,, The Ohio State University, Depclrtment of Materials Science and Engineering, Columbus, OH 43210, USA b The Ohio State Universio,, College of DenfistJy, Columbus, 0H43210, USA Received 15 May 1996; revised 25 October 1996
Abstract
Particle pre-treatment with a silane coupling agent, y-methacryloxypropyltrimethoxysilane, increased the wear rate of triethylglycoldimethacrylate (TEGDMA) composites reinforced with HF-catalyzed nanoporous silica gel. The percentage filler porosity appeared to exert the greatest control on the wear properties. Increasing silanation led to a decrease in the surface area and total pore volume, resulting in a linear increase in the wear rate. It is believed that decreasing penetration of crosslinked TEGDMA into the gel particles contained within the composite caused a transition from plastic deformation to brittle fracture during wear. In addition, the presence of the coupling agent had no net effect on wear property preservation. © 1997 Elsevier Science S.A. Ke)words: Nanostructured composite; Silane coupling agent; Silica sol-gel
1. Introduction Dental ceramics show good optical, chemical and biological compatibility with the oral environment. However, they require complex fabrication methods and their clinical predictability is limited by their inherent brittle nature. Pure polymers (epoxy [1] and acrylic [2] resins), on the other hand, can be easily and rapidly processed but exhibit poor color stability and low strength. Composite materials, which integrate both polymeric and ceramic phases, exhibit good optical compatibility, can be easily, rapidly and inexpensively processed and possess intermediate properties. Biomimetics in materials science is very relevant to these types of dental restorative materials. The direct analog, the human tooth, is mainly composed of hydroxyapatite ((Calo(PO4)6(OH)2). Enamel (the harder outer coating protecting teeth) and dentine (the softer core) are made up of 92 and 48 rot.% respectively of this mineral [3]. The remaining organic component present between the nanometer-sized mineral is protein. This type of nanostructured inorganic-organic biocomposite contains specific structures which impart optimal mechanical properties and durability [4]. Current dental composites are far from ideal copies of the natural form. Formation begins by pre-treating inorganic fill* Corresponding author, 0928-4931/97/$17.00 © 1997 Elsevier Science S.A, All rights reserved PHS0928-493 1(96)00155-5
ers/powders with silane coupling agents. These are then mixed with organic monomer to make a paste. The presence of an initiator allows the production of a solid dental composite after curing the paste by exposure to light or heat [5,6]. Silane coupling agents are thought to play a key role in enhancing the adhesion of the interface between the inorganic powder and the organic polymer [7,8]i Almost all currently employed inorganic fillers are non-porous solid structures. However, inadequate wear resistance limits their use in posterior restorations. In this paper, we report the use of nanoporous inorganic gels as reinforcing agents. The nanometer-sized porosity improves phase intermingling allowing better simulation of nanostructured enamel and thus improved wear resistance. The desired nanostructure is obtained when organic monomer fills these pores and is polymerized in situ to allow better simulation of the interpenetrating network of natural protein. These synthetic composites show large interfacial phase connection which results in better load transfer. Although silane coupling agents have been used extensively in inorganic-organic composites, we have found that their application in nanoporous filler structures results in a negative effect on the composite wear resistance. This provides important information which will govern the technological transition from the macroscopic or microscopic level currently used to the nanometric form often used by nature:
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J. Luo et al. /Materials Science and Engineering C 5 (1997) 15-22
2. Experimental details
for subsequent incorporation/polymerization into the polymer matrix. The standard procedure involved dissolving this liquid into the solvent with stirring to produce a 2 vol.% solution. After allowing 5 rain for hydrolysis and silanol formation, dried gel particles were slowly stirred into this solution. After filtration, the powders were washed three times using ethanol or acetone to desorb physically adsorbed silane. The resulting silylated powder was carefully dried at 75 °C and stored under vacuum over desiccant. A variation on this procedure involved the use of a 95% ethanol solution with an acetic acid adjusted pH value of 4.5 as the reaction medium. Absolute ethanol was used to triply wash the powder after a reaction time of 15 min. Two other treatments involved the addition of 2 vol.% n-propylamine catalyst to unmodified 95% eth~mol and cyclohexane [ 11,12]. A reaction time of 2 h was followed by triple washing with acetone. Unsilanated powders were also used to establish a baseline. Fourier transkbnn infrared (FT-tR) spectra (Perkin Elmer 16PC) were obtained from KBr pellets containing approximately 2 wt.% of the silylated powders. Multipoint nitrogen BET (Quanta Chrome Corporation, model AS- 1) was used
2.1. Silica gel synthesis [9,10] To produce controllable levels of porosity and pore size, we prepared silica gels by acid-catalyzed hydrolysis and condensation of silicon alkoxide+ Pure ethanol (McCormick Distilling Co., Inc.) and water were mixed with hydrofluoric acid (Fisher Scientific, 49%). Tetraethylorthosilicate (TEOS. Aldrich Chemical Company, 98%) was poured slowly into these solvents with stirring to yield a molar ratio of TEOS : ethanol : water : catalyst of 1 : 3 : 4 : 0.05. After gelation and dehydration, the resulting dried solid was lightly crushed in a ball mill and sieved producing a gaussian distribution of particle sizes averaging 10.0 ~xm in diameter. The gel powder was dried at t80 °C for 16 h prior to silanation.
2.2. Silane coupling [11,12] The coupling agent used was y-methacryloxypropyltrimethoxysilane (Dow Coming Corporation, Z6030). This contains a carbon-carbon double bond believed to be suitable
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Wavenumber (cm- 1) Fig. 1. FF-IR spectra of untreated silica gel (NC) and sitanated derivatives with 3,-methacryloxypropyltrimethoxysilane. The spectra are labeled as follows: untreated gel, no coupling (NC); silanated for 15 min using 95% ethanol solution at pH 4.5, coupled with ethanol and pH modification CCEP); coupled in ethanol + n-propylamine for 2 h (CEN) ; coupled in cyclohexane + n-propylamine for 2 h (CCN).
J. Luo et al./'Materials Science and Engineering C 5 (1997) 15-22
to characterize the porosity, surface area and pore size distribution of the gel powders.
merization follows standard procedures for dental composite fabrication [13,14]. An ongoing study demonstrates that there is no significant net difference between chemically initiated and light-initiated polymerization [ 15].
2.3. Composite preparation
2.4. T~t,o-bodyabrasion
Approximately 34 wt.% of each of the gel powders was added to monomer solution under vacuum (OXY Dental Product, Inc.) and vibration (E & D Dental Manufacturing Co., model 007125) to form a clear paste. The monomer solution consisted of triethylglycol-dimethacrylate (TEGDMA), I% of camphorquinone as photoinitiator and 1% of 2-(diethylamino)ethylmethacrylate as accelerator. The paste was introduced into cylindrical glass molds (diameter, 3.5 mm; length, approximately 8 ram). Polymerization took place using a 3 min exposure to a high intensity visible light source (COE-LITE, model 400, Imperial Chemical Industries PLC). Subsequent heating to 100 °C for 10 min ensured maximum polymerization. Ten specimens of each of the four composite materials were formed. This photopoly-
Five specimens were stored in water at 37 °C and five at 65 °C for 1 week. Relative abrasion rates were determined using a two-body pin-on-disk-type apparatus designed to produce continuous sliding contact between the sample cylinders and a 35 ~m diamond disk. An LVDT recorded the change in length on a personal computer. Pre-ground specimens were held in contact with the abrasive surface using a continuous 1.5 N load while they traced a circular orbiting pattern. The contacting surfaces were continuously flushed by constantly flowing tap water. The length of the sample was recorded at 2 s intervals for the entire test period by an electronic micrometer having an accuracy of 4
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CEN CCN PPM ENM HXR (C) Sample Fig. 2. (a) Relative wear resistances of I-IF-catalyzed gel-polymer composites with the same (33.88 wt.%) maximum filler content, These are compared with those of a light-cured pure polymer (PPM), a commercial composite (HXR) and human enamel (ENM), (b) Durability of the gel-based composite wear resistance after storage in water at room temperature and at 65 °C for 1 week. Heat-cured pure polymer, a commercial product and enamel are included as controls, (c) The corresponding total change in wear rate.
18
J. Luo et al,/ Materials Science and Engineering C 5 (1997) 15-22
0.0005 mm. The number of revolutions traveled was also recorded. The change in length vs. the number of revolutions was recorded followed by linear regression analysis. The slope of the regression line represents the rate of abrasion ( micrometer per revolution). Human enamel and a commercial dental composite (Herculite XR, Kerr) were used as positive controls. The results were plotted using error bars of 2o- in height to show that they were reproducible. 2.5. Electron microscopy
Scanning electron microscopy (SEM, Phillips XL-30) was employed to examine the wear surfaces. For specimen preparation, the worn surface was washed with ethanol and stored at 75 °C until dry. It was then coated with a thin layer of A u Pd prior to examination at 15 keV. Transmission electron microscopy (TEM, Phillips CM200) was used to examine the organic-inorganic interface. TEM specimens of approximately 90-100 nm in thickness were cut using a diamond knife microtome (Reichert, Ultracut E). A thin layer of carbon was applied before examination in the microscope at 200 keV. 1.00
3. Resultsand discussion 3.1. Effect o f coupling on the w e a r resistance
Fig. 1 shows the FT-IR reflectance spectra of the unsilanated and silanated silica gel powders. The spectra are labeled as follows: untreated gel, no coupling (NC): silanated for 15 rain using 95% ethanol solution at pH 4.5, coupled with ethanol and pH modification (CEP); silanated in ethanol + npropylamine catalyst for 2 h, coupled with ethanol and npropylamine (CEN); silanated in cyclohexane + n-propylamine for 2 h, coupled with cyctohexane and n-propytamine (CCN). NC shows a large IR peak between 1000 and 1400 c m - t corresponding to Si-O-Si bonds. As these are stable and plentiful in the bu]k gel, this peak was selected as a reference. Surface hydroxyl groups ( - O H ) are identified by the stretching vibration between 3400 and 3500 c m - 1. The peak at 1642 cm - ~represents surface - S i - O - H . No significant C - H vibrations were detected at 3000 c m - 1, indicating that the procedure had reduced the amount of chemisorbed ethanol to levels below detectability. The various silanated particles show a relative decrease in the strength of the two O - H related frequencies paralleling
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Fig. 3. Pore characterization of untreated and coupled powders. (a) Cumuiative pore volume vs. pore radius ( Vpvs. log R). (b) Pore size distribution (du(Iog R) vs. pore radius R).
Fig, 4. SEM images of the wear surface of an unsilanated silica-derived composite containing 33.88 wt.% filler: (a) 6000×; (b) 24000×. This represents the extreme case of plastic deformation.
I . L t m er al. / Materials Science and Engineering C 5 (1.!997) 15-22
an increase in the peaks at 1724, 1650 and 3000 cm-1. The latter represent C=O, C = C and C - H bond stretches respectively, and are associated with the presence of the coupling agent on the particle surface. Based on this information, Fig. 1 depicts the spectra in order of increased coupling (CCN > CEN > CEP). The different reaction conditions are apparently responsible for the expected variation in coupling. This agrees with both Kaas and Kardos [7] and Chen and Brauer [ 11 ] who determined that n-propylamine can act as a catalyst during silanation. The degree of coupling on a colloidal silica surface was shown to vary with both the type of catalyst and the solvent. The differences in coupling found in our investigation are consistent with these previous results. The mean composite wear rates are compared with those of human enamel, pure polymerized T E G D M A and a commercial dental composite (Herculite XR) in Fig. 2(a). The commercial composite is composed of a bis-GMA/ T E G D M A polymer and 78 wt.% of a mixture of 0.6 tzm barium silicate glass and 0.04 p.m colloidal silica as fillers. For the experimental materials, the wear rate increases with the degree of coupling. The uncoupled composites show good wear resistance on par with that of the commercial product.
Fig. 5. Worn CEP-derived composite containing 33.88 wt.% filler: (a) 6000 × : (b) 24 000 ×. The wear debris undergoes a distinct ificreasein size in comparison with the composite produced from uncoupled filler.
19
This plot clearly shows that coupling has a negative effect on the wear resistance of these specific nanoporous composites. The uncoupled experimental composite shows the lowest wear rate. The composite with the highest degree of coupling shows the highest wear rate. In Fig. 2(b), the shaded bars indicate the mean wear rate of the materials after 1 week's storage in water at 65 °C. The mean wear rates of all the experimental composites have increased in comparison with those stored in water at 37 °C over the same time period. However, the change in wear rate (Fig. 2 ( c ) ) of the four composites is similar to that of the pure T E G D M A polymer. The observed wear rate increase suggests an effect caused by polymer phase hydrolysis rather than inorganic-organic decoupling. Hydrolysis of ester group side chains or ether bonding of main chains could be involved [ 16-18 ]. The presence of coupling agent does not obviously benefit wear resistance preservation. This is probably caused by good initial penetration of the porous gels by the monomer; this is not improved by surface coupling.
3.2. Effect of coupling on porosity Multipoint desorption BET analysis of the gel particles before and after silanization is shown in Fig. 3 (a) and reveals a clear relationship between the cumulative porosity and the
Fig. 6. Worn CEN-derived composite containing 33.88 wt.% filler: (a) 6000 X ; (b) 24 000 ×. The wear debris continues to increase in size.
20
J. Luo et al. ~Materials Science and Engineering C 5 (1997) 15-22
pore radii. Without silanization, the porosity is constant at 0.8121 cm 3 g - i. After silanization, however, it decreases to 0.7310, 0.6164 and 0.1776 cm 3 g - ~for CEP, CEN and CCN respectively. The largest decrease is 78.13%. Similarly, the surface area decreases by 9.63%, 52.9% and 79.10% for CEP, CEN and CCN respectively. With the exception of CEN, none of the coupling methods change the pore size distribution significantly. Fig. 3(b) shows that the average pore radii of NC, CEP and CCN are 26.7, 26.6 and 27,9 .A respectively. CEN is shifted to 43.17 ]~. These data suggest that, in CEP and CCN, the coupling agent blocks/fills the pores uniformly without substantially changing the average unblocked pore radius. For CEN, this is biased towards the smaller pores and the derivative representation results in an apparent population of larger pores. Silane multilayers may be involved. The toss of the smallest pores is responsible for the substantial decrease in area even though the net porosity decrease is not large. Linear regression analysis indicates a high degree of correlation ( r = 0.934) between the pore volume and wear rate. An increase in accessible pore volume clearly results in a decrease in wear rate. Within the range of particles studied, the pore size apparently has a much tess direct effect on the wear rate. In these comparisons, some caution is necessary. BET measurements involve a surface of a different character than that penetrated by the monomer. The passage of monomer into a pore in the solution state is likely to be impeded more strongly by surface-attached coupling agent when that agent has a specific radius of gyration. The adsorption of N2 monolayers at low temperatures must be less restricted by the solidified coupling agent. Regardless, a clear correlation between wear and the measured pore volume is evident. Due to the complexity involved in the use of a biphasic (coupling agent followed by monomer) penetrant, the overall composite density cannot be used to determine the degree of monomer penetration. The blocking effect of the coupling agent can either partially or completely fill a given pore. To demonstrate this, all composites were cryomilled to a fine powder and used to estimate the residual porosity. This was consistently in the 1 0 - 3 c m 3 g-1 region. We can conclude that penetration by silane and monomer is well over 95% complete and that density differences between these systems are not significant. 3.3. S E M evaluation o f worn surfaces
SEM characterization of the surfaces produced by the wear experiment was carried out to understand better the resulting wear data. Figs. 4-7 show the worn surfaces of the composites containing the same loading percentage (33.88 wt.%) of silica gel. The wear surface of the most resistant uncoupled composite (Fig. 4(a) and ( b ) ) is considerably smoother than previously observed. The higher magnification image ~Fig. 4 ( b ) ) suggests the formation of a "smear" layer apparently consisting of submicrometer wear debris. With
Fig. 7. Worn CCN-based composite containing 33.88 wt.Ck filler: (a) 6000 ×; (b) 24 000 ×. This surfaceshows the greatest evidenceof brittle fracture and pullout. coupling, a gradual transition (Figs. 5-7) from this plastic, smear-type wear to brittle, pullout-type wear occurs. In the micrographs, this is apparent at low magnifications as a change in the net size of the objects occupying the wear surface. The latter behavior is especially obvious in the more strongly coupled CEN- and CCN-derived composites. The filter displays an increased tendency for pullout as coupling increases. The extremely porous nature of the fillers [ 9] can be combined with these observations to form the following hypothesis: the presence of coupling agent in or on the porous structure decreases subsequent penetration by TEGDMA; without coupling, TEGDMA impregnates the nanometerscale pores more efficiently; as this penetration becomes more limited/uneven, tile gel fractures in an increasingly brittle fashion; the apparent debris size increases together with the wear rate, 3.4. T E M results
Fig. 8 contains TEM images of the composites from uncoupled powder (NC) and from the most coupled powder (CCN). Fig. 8(a) and (c) show the lowest magnification images (5000 × ) for the two composites. Brittle fracture is
J. Luo et al. / M a t e r i a l s Science and Engineering C 5 (1997) ] 5 - 2 2
21
Fig. 8. TEM images of the composites from the uncoupled (a, b) and most coupled (c, d) powder; (a, c) 5000 × ; (b, d) 200 000 ×. The highest wear rate composite shows significant brittle fracture.
clearly more pronounced in the silanated gel during microtoming than in the uncoupled gel composite. The absence of such fracture in the NC-derived composite is probably caused by substantial TEGDMA penetration. The resulting impregnated filler particles are more likely to exhibit plastic deformation, known to be significant in preventing tooth failure [ 12]. In contrast, the most coupled particles will be the least penetrated by TEGDMA and produce a composite more subject to brittle fracture. At higher magnifications (200 000X in Fig. 8(b) and (d)), limited atomic contrast prevents us from resolving the pores clearly. However, the white spots in the black (silica) phase on the higher magnification image for the uncoupled get suggest the presence of polymer. In a sense, the microtoming operation mimics the wear process: as the diamond knife moves through the coupled composite, the brittleness of the gel phase results in significant crack propagation and the loss of significant areas (Fig. 8 (c)) of the composite. In contrast, the uncoupled composite experiences little of this brittle fracture (Fig. 8(a)) and no material loss.
3.5. Wear in nanostructured composites
Conventional wisdom states that coupling agents improve the adhesion between a polymeric phase and a dense inorganic phase, leading to better load transfer and increased wear resistance. For some nanoporous particles, however, coupling clearly has a negative effect on the wear resistance (Fig. 2). Fig. 9 shows a schematic representation of the polymer-gel coupling. The silica gel (light phase) is depicted as having a regular array of fine pores. In the uncoupled (NC) case, these pores are shown to be fully penetrated by the monomer. It has been reported that acryIate monomers are readiIy adsorbed onto silica gel [ 19], thus improving wetting and penetration. Extensive coupling either at or below the surface eventually results in areas unpenetrated by polymerized TEGDMA exhibiting more of the brittle nature of unreinforced gel. This allows for greater in-depth crack propagation and larger wear debris. The nature of this blockage depends on the coupling process itself. Greater coupling efficiency (CCN) appears to block/fill all pore sizes evenly. An intermediate degree of coupling (CEN) is apparently specific to small pores.
22
J. Luo et al. /Materials Science and Engineering C 5 (1997) 15-22
porous silica gel
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l=ig, 9. Schematic illustration of the negative effect of coupling agents on the wear resistance. Specific volumes of gel are not interpenetratedby crosslinked TEGDMA ("polymer") due to the blocking/filling effect of the coupling agent (represented by small black lines bridging specific pore diameters). These uninterpenetratedareas have the brittle nature of unreinforced gel and allow for greater in-depth crack propagation. Regardless of the differences in available pore size, a high degree of correlation exists between the wear resistance and the total available pore volume.
,1. Conclusions Nanoporous silica gel-derived composites were used to mimic the wear resistance o f natural tooth enamel. Contrary to conventional wisdom, silane coupling agents were found to diminish the wear resistance of crosslinked T E G D M A based composites reinforced with nanoporous particles. W e a r rates increased as the degree of coupling increased. An increased wear resistance was associated with the formation of a flat, smooth surface apparently consisting of a layer of fine wear debris. A low wear resistance was associated with an irregular surface showing considerable evidence of brittle fracture. The baseline, uncoupled gel composite had a wear resistance comparable with that of a commercial product. The available filler porosity appeared to determine the wear properties. Use of a coupling agent was found to decrease the pore volume. As the coupling efficiency was increased, the specific surface area and the total pore volume detectable by BET analysis decreased. The reduction in accessible pore volume probably led to a decrease in the pore penetration of crosslinked TEGDlVIA in the final composite. Brittle fracture then replaced plastic deformation as the dominant mode of surface failure. The wear resistance varied linearly with the changes in the accessible porosity.
F o r these types of nanocomposite, the application of a coupling agent resulted in no detectable improvement in the preservation of the wear properties. This was probably caused by good initial penetration of the original porous gels. The macroscopic or microscopic level at which coupling agents are normally of benefit does not improve the wear resistance of this composite [orm. On the basis of these results, the current study shows that nanoporous, unsilanated silica gel fillers can be combined with a thermoset to produce wear resistances similar to enamel. The associated principles involving nanoporous structures are worthy of further exploration in the areas of biomimetics and in the fabrication of improved inorganicorganic materials for biomedical applications.
Acknowledgements This work was supported by the National Institute of Dental Research (project number N I H / N I D R R 0 1 - D E l l 3 0 6 -
01). References [ I ] R.L. Bowen, J. Dental Res., 35 (1956) 360-369. [2] R.L. Bowen, J. Dental Res., 37 (1958) 90. [3] J.E. Mark and P.D. Calvert, Mater. Sci. Eng. C, 1 (1994) 159-173. [4] L.F. Francis K.J. Vaidya, H.Y. Huang and W.D. Wolf., Mate1. ScL Eng. C, 3 (1995) 63-74. [5] R.L. Bowen, US Patent 3 194 784, 1965. [6] G. Vanherle and D.C. Smith (eds.), Posterior Composite Resin Dental Restorative Materials, Peter Szulc Publishing Co., 1985. 7] R.L. Kaas and J.L. Kardos, Polym. Eng. Sci., 1] ( 1971 ) I 1-18. [8] D.E. Leyden (ed.), Silanes, Surfaces and Interfaces, Gordon and Breach, New York, 1985. [9] EJ. Pope and J.D. Mackenzie,J. Non-Cr)'st. Solids, 87 (1986) 185198. [ 10] L.C. Klein and G.J. Garvey, J. Non-Cryst. Solids, 48 (1982) 97-104. [ 11] T.M. Chen and G.M. Brauer, J. Dental Res., 61 (12) (1982) 143% 1443. [12] C.P. Lin and W.H. Douglas, J. Dental Res., 73 (5) (1994) 1072I078. [ 13 ] J.M. Power, L.T. Smith, M. Eldiwanyand G.D.Ladd,Am. Z Dentisto,, 6 (5) (1993) 232-234. [ 14] S. Hirabayashi, J.A. Hood and T. Hirasawa, Dental Mater. J., 12 (2) (1993) 159-170. [15] J. Luo, R. Seghi and J. Lannutti, Dental Mater. J., submitted for publication. [ I6] T. Nambu, C. Watanabe and Y. Tani, DentalMater. J., 10 (2) ( 1991) 138-148. [ t7] W. Wu and J.E. McK.inney,J. Dental Res., 61 (10) (1982) 11801t83. [ 18] I.B. Larsen, L.B. Larsen, M. Frued and E.C. Murksgaard, J. Dental Res., 71 i l l ) (1992) i851-1853. [i9] D.L. Allara, Z. Wang and C.G. Pantano, J. Non-C~3"st. Solids. 120 (1990) 93-101.
Materials Science and Engineering C 5 (1997) 15-22
12
Effect of silane coupling agents on the wear resistance of polymer-nanoporous silica gel dental composites Jiazhong Luo a, Robert Seghi b, John Lannutti a,, The Ohio State University, Depclrtment of Materials Science and Engineering, Columbus, OH 43210, USA b The Ohio State Universio,, College of DenfistJy, Columbus, 0H43210, USA Received 15 May 1996; revised 25 October 1996
Abstract
Particle pre-treatment with a silane coupling agent, y-methacryloxypropyltrimethoxysilane, increased the wear rate of triethylglycoldimethacrylate (TEGDMA) composites reinforced with HF-catalyzed nanoporous silica gel. The percentage filler porosity appeared to exert the greatest control on the wear properties. Increasing silanation led to a decrease in the surface area and total pore volume, resulting in a linear increase in the wear rate. It is believed that decreasing penetration of crosslinked TEGDMA into the gel particles contained within the composite caused a transition from plastic deformation to brittle fracture during wear. In addition, the presence of the coupling agent had no net effect on wear property preservation. © 1997 Elsevier Science S.A. Ke)words: Nanostructured composite; Silane coupling agent; Silica sol-gel
1. Introduction Dental ceramics show good optical, chemical and biological compatibility with the oral environment. However, they require complex fabrication methods and their clinical predictability is limited by their inherent brittle nature. Pure polymers (epoxy [1] and acrylic [2] resins), on the other hand, can be easily and rapidly processed but exhibit poor color stability and low strength. Composite materials, which integrate both polymeric and ceramic phases, exhibit good optical compatibility, can be easily, rapidly and inexpensively processed and possess intermediate properties. Biomimetics in materials science is very relevant to these types of dental restorative materials. The direct analog, the human tooth, is mainly composed of hydroxyapatite ((Calo(PO4)6(OH)2). Enamel (the harder outer coating protecting teeth) and dentine (the softer core) are made up of 92 and 48 rot.% respectively of this mineral [3]. The remaining organic component present between the nanometer-sized mineral is protein. This type of nanostructured inorganic-organic biocomposite contains specific structures which impart optimal mechanical properties and durability [4]. Current dental composites are far from ideal copies of the natural form. Formation begins by pre-treating inorganic fill* Corresponding author, 0928-4931/97/$17.00 © 1997 Elsevier Science S.A, All rights reserved PHS0928-493 1(96)00155-5
ers/powders with silane coupling agents. These are then mixed with organic monomer to make a paste. The presence of an initiator allows the production of a solid dental composite after curing the paste by exposure to light or heat [5,6]. Silane coupling agents are thought to play a key role in enhancing the adhesion of the interface between the inorganic powder and the organic polymer [7,8]i Almost all currently employed inorganic fillers are non-porous solid structures. However, inadequate wear resistance limits their use in posterior restorations. In this paper, we report the use of nanoporous inorganic gels as reinforcing agents. The nanometer-sized porosity improves phase intermingling allowing better simulation of nanostructured enamel and thus improved wear resistance. The desired nanostructure is obtained when organic monomer fills these pores and is polymerized in situ to allow better simulation of the interpenetrating network of natural protein. These synthetic composites show large interfacial phase connection which results in better load transfer. Although silane coupling agents have been used extensively in inorganic-organic composites, we have found that their application in nanoporous filler structures results in a negative effect on the composite wear resistance. This provides important information which will govern the technological transition from the macroscopic or microscopic level currently used to the nanometric form often used by nature:
16
J. Luo et al. /Materials Science and Engineering C 5 (1997) 15-22
2. Experimental details
for subsequent incorporation/polymerization into the polymer matrix. The standard procedure involved dissolving this liquid into the solvent with stirring to produce a 2 vol.% solution. After allowing 5 rain for hydrolysis and silanol formation, dried gel particles were slowly stirred into this solution. After filtration, the powders were washed three times using ethanol or acetone to desorb physically adsorbed silane. The resulting silylated powder was carefully dried at 75 °C and stored under vacuum over desiccant. A variation on this procedure involved the use of a 95% ethanol solution with an acetic acid adjusted pH value of 4.5 as the reaction medium. Absolute ethanol was used to triply wash the powder after a reaction time of 15 min. Two other treatments involved the addition of 2 vol.% n-propylamine catalyst to unmodified 95% eth~mol and cyclohexane [ 11,12]. A reaction time of 2 h was followed by triple washing with acetone. Unsilanated powders were also used to establish a baseline. Fourier transkbnn infrared (FT-tR) spectra (Perkin Elmer 16PC) were obtained from KBr pellets containing approximately 2 wt.% of the silylated powders. Multipoint nitrogen BET (Quanta Chrome Corporation, model AS- 1) was used
2.1. Silica gel synthesis [9,10] To produce controllable levels of porosity and pore size, we prepared silica gels by acid-catalyzed hydrolysis and condensation of silicon alkoxide+ Pure ethanol (McCormick Distilling Co., Inc.) and water were mixed with hydrofluoric acid (Fisher Scientific, 49%). Tetraethylorthosilicate (TEOS. Aldrich Chemical Company, 98%) was poured slowly into these solvents with stirring to yield a molar ratio of TEOS : ethanol : water : catalyst of 1 : 3 : 4 : 0.05. After gelation and dehydration, the resulting dried solid was lightly crushed in a ball mill and sieved producing a gaussian distribution of particle sizes averaging 10.0 ~xm in diameter. The gel powder was dried at t80 °C for 16 h prior to silanation.
2.2. Silane coupling [11,12] The coupling agent used was y-methacryloxypropyltrimethoxysilane (Dow Coming Corporation, Z6030). This contains a carbon-carbon double bond believed to be suitable
0 "O
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Wavenumber (cm- 1) Fig. 1. FF-IR spectra of untreated silica gel (NC) and sitanated derivatives with 3,-methacryloxypropyltrimethoxysilane. The spectra are labeled as follows: untreated gel, no coupling (NC); silanated for 15 min using 95% ethanol solution at pH 4.5, coupled with ethanol and pH modification CCEP); coupled in ethanol + n-propylamine for 2 h (CEN) ; coupled in cyclohexane + n-propylamine for 2 h (CCN).
J. Luo et al./'Materials Science and Engineering C 5 (1997) 15-22
to characterize the porosity, surface area and pore size distribution of the gel powders.
merization follows standard procedures for dental composite fabrication [13,14]. An ongoing study demonstrates that there is no significant net difference between chemically initiated and light-initiated polymerization [ 15].
2.3. Composite preparation
2.4. T~t,o-bodyabrasion
Approximately 34 wt.% of each of the gel powders was added to monomer solution under vacuum (OXY Dental Product, Inc.) and vibration (E & D Dental Manufacturing Co., model 007125) to form a clear paste. The monomer solution consisted of triethylglycol-dimethacrylate (TEGDMA), I% of camphorquinone as photoinitiator and 1% of 2-(diethylamino)ethylmethacrylate as accelerator. The paste was introduced into cylindrical glass molds (diameter, 3.5 mm; length, approximately 8 ram). Polymerization took place using a 3 min exposure to a high intensity visible light source (COE-LITE, model 400, Imperial Chemical Industries PLC). Subsequent heating to 100 °C for 10 min ensured maximum polymerization. Ten specimens of each of the four composite materials were formed. This photopoly-
Five specimens were stored in water at 37 °C and five at 65 °C for 1 week. Relative abrasion rates were determined using a two-body pin-on-disk-type apparatus designed to produce continuous sliding contact between the sample cylinders and a 35 ~m diamond disk. An LVDT recorded the change in length on a personal computer. Pre-ground specimens were held in contact with the abrasive surface using a continuous 1.5 N load while they traced a circular orbiting pattern. The contacting surfaces were continuously flushed by constantly flowing tap water. The length of the sample was recorded at 2 s intervals for the entire test period by an electronic micrometer having an accuracy of 4
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CEN CCN PPM ENM HXR (C) Sample Fig. 2. (a) Relative wear resistances of I-IF-catalyzed gel-polymer composites with the same (33.88 wt.%) maximum filler content, These are compared with those of a light-cured pure polymer (PPM), a commercial composite (HXR) and human enamel (ENM), (b) Durability of the gel-based composite wear resistance after storage in water at room temperature and at 65 °C for 1 week. Heat-cured pure polymer, a commercial product and enamel are included as controls, (c) The corresponding total change in wear rate.
18
J. Luo et al,/ Materials Science and Engineering C 5 (1997) 15-22
0.0005 mm. The number of revolutions traveled was also recorded. The change in length vs. the number of revolutions was recorded followed by linear regression analysis. The slope of the regression line represents the rate of abrasion ( micrometer per revolution). Human enamel and a commercial dental composite (Herculite XR, Kerr) were used as positive controls. The results were plotted using error bars of 2o- in height to show that they were reproducible. 2.5. Electron microscopy
Scanning electron microscopy (SEM, Phillips XL-30) was employed to examine the wear surfaces. For specimen preparation, the worn surface was washed with ethanol and stored at 75 °C until dry. It was then coated with a thin layer of A u Pd prior to examination at 15 keV. Transmission electron microscopy (TEM, Phillips CM200) was used to examine the organic-inorganic interface. TEM specimens of approximately 90-100 nm in thickness were cut using a diamond knife microtome (Reichert, Ultracut E). A thin layer of carbon was applied before examination in the microscope at 200 keV. 1.00
3. Resultsand discussion 3.1. Effect o f coupling on the w e a r resistance
Fig. 1 shows the FT-IR reflectance spectra of the unsilanated and silanated silica gel powders. The spectra are labeled as follows: untreated gel, no coupling (NC): silanated for 15 rain using 95% ethanol solution at pH 4.5, coupled with ethanol and pH modification (CEP); silanated in ethanol + npropylamine catalyst for 2 h, coupled with ethanol and npropylamine (CEN); silanated in cyclohexane + n-propylamine for 2 h, coupled with cyctohexane and n-propytamine (CCN). NC shows a large IR peak between 1000 and 1400 c m - t corresponding to Si-O-Si bonds. As these are stable and plentiful in the bu]k gel, this peak was selected as a reference. Surface hydroxyl groups ( - O H ) are identified by the stretching vibration between 3400 and 3500 c m - 1. The peak at 1642 cm - ~represents surface - S i - O - H . No significant C - H vibrations were detected at 3000 c m - 1, indicating that the procedure had reduced the amount of chemisorbed ethanol to levels below detectability. The various silanated particles show a relative decrease in the strength of the two O - H related frequencies paralleling
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Fig. 3. Pore characterization of untreated and coupled powders. (a) Cumuiative pore volume vs. pore radius ( Vpvs. log R). (b) Pore size distribution (du(Iog R) vs. pore radius R).
Fig, 4. SEM images of the wear surface of an unsilanated silica-derived composite containing 33.88 wt.% filler: (a) 6000×; (b) 24000×. This represents the extreme case of plastic deformation.
I . L t m er al. / Materials Science and Engineering C 5 (1.!997) 15-22
an increase in the peaks at 1724, 1650 and 3000 cm-1. The latter represent C=O, C = C and C - H bond stretches respectively, and are associated with the presence of the coupling agent on the particle surface. Based on this information, Fig. 1 depicts the spectra in order of increased coupling (CCN > CEN > CEP). The different reaction conditions are apparently responsible for the expected variation in coupling. This agrees with both Kaas and Kardos [7] and Chen and Brauer [ 11 ] who determined that n-propylamine can act as a catalyst during silanation. The degree of coupling on a colloidal silica surface was shown to vary with both the type of catalyst and the solvent. The differences in coupling found in our investigation are consistent with these previous results. The mean composite wear rates are compared with those of human enamel, pure polymerized T E G D M A and a commercial dental composite (Herculite XR) in Fig. 2(a). The commercial composite is composed of a bis-GMA/ T E G D M A polymer and 78 wt.% of a mixture of 0.6 tzm barium silicate glass and 0.04 p.m colloidal silica as fillers. For the experimental materials, the wear rate increases with the degree of coupling. The uncoupled composites show good wear resistance on par with that of the commercial product.
Fig. 5. Worn CEP-derived composite containing 33.88 wt.% filler: (a) 6000 × : (b) 24 000 ×. The wear debris undergoes a distinct ificreasein size in comparison with the composite produced from uncoupled filler.
19
This plot clearly shows that coupling has a negative effect on the wear resistance of these specific nanoporous composites. The uncoupled experimental composite shows the lowest wear rate. The composite with the highest degree of coupling shows the highest wear rate. In Fig. 2(b), the shaded bars indicate the mean wear rate of the materials after 1 week's storage in water at 65 °C. The mean wear rates of all the experimental composites have increased in comparison with those stored in water at 37 °C over the same time period. However, the change in wear rate (Fig. 2 ( c ) ) of the four composites is similar to that of the pure T E G D M A polymer. The observed wear rate increase suggests an effect caused by polymer phase hydrolysis rather than inorganic-organic decoupling. Hydrolysis of ester group side chains or ether bonding of main chains could be involved [ 16-18 ]. The presence of coupling agent does not obviously benefit wear resistance preservation. This is probably caused by good initial penetration of the porous gels by the monomer; this is not improved by surface coupling.
3.2. Effect of coupling on porosity Multipoint desorption BET analysis of the gel particles before and after silanization is shown in Fig. 3 (a) and reveals a clear relationship between the cumulative porosity and the
Fig. 6. Worn CEN-derived composite containing 33.88 wt.% filler: (a) 6000 X ; (b) 24 000 ×. The wear debris continues to increase in size.
20
J. Luo et al. ~Materials Science and Engineering C 5 (1997) 15-22
pore radii. Without silanization, the porosity is constant at 0.8121 cm 3 g - i. After silanization, however, it decreases to 0.7310, 0.6164 and 0.1776 cm 3 g - ~for CEP, CEN and CCN respectively. The largest decrease is 78.13%. Similarly, the surface area decreases by 9.63%, 52.9% and 79.10% for CEP, CEN and CCN respectively. With the exception of CEN, none of the coupling methods change the pore size distribution significantly. Fig. 3(b) shows that the average pore radii of NC, CEP and CCN are 26.7, 26.6 and 27,9 .A respectively. CEN is shifted to 43.17 ]~. These data suggest that, in CEP and CCN, the coupling agent blocks/fills the pores uniformly without substantially changing the average unblocked pore radius. For CEN, this is biased towards the smaller pores and the derivative representation results in an apparent population of larger pores. Silane multilayers may be involved. The toss of the smallest pores is responsible for the substantial decrease in area even though the net porosity decrease is not large. Linear regression analysis indicates a high degree of correlation ( r = 0.934) between the pore volume and wear rate. An increase in accessible pore volume clearly results in a decrease in wear rate. Within the range of particles studied, the pore size apparently has a much tess direct effect on the wear rate. In these comparisons, some caution is necessary. BET measurements involve a surface of a different character than that penetrated by the monomer. The passage of monomer into a pore in the solution state is likely to be impeded more strongly by surface-attached coupling agent when that agent has a specific radius of gyration. The adsorption of N2 monolayers at low temperatures must be less restricted by the solidified coupling agent. Regardless, a clear correlation between wear and the measured pore volume is evident. Due to the complexity involved in the use of a biphasic (coupling agent followed by monomer) penetrant, the overall composite density cannot be used to determine the degree of monomer penetration. The blocking effect of the coupling agent can either partially or completely fill a given pore. To demonstrate this, all composites were cryomilled to a fine powder and used to estimate the residual porosity. This was consistently in the 1 0 - 3 c m 3 g-1 region. We can conclude that penetration by silane and monomer is well over 95% complete and that density differences between these systems are not significant. 3.3. S E M evaluation o f worn surfaces
SEM characterization of the surfaces produced by the wear experiment was carried out to understand better the resulting wear data. Figs. 4-7 show the worn surfaces of the composites containing the same loading percentage (33.88 wt.%) of silica gel. The wear surface of the most resistant uncoupled composite (Fig. 4(a) and ( b ) ) is considerably smoother than previously observed. The higher magnification image ~Fig. 4 ( b ) ) suggests the formation of a "smear" layer apparently consisting of submicrometer wear debris. With
Fig. 7. Worn CCN-based composite containing 33.88 wt.Ck filler: (a) 6000 ×; (b) 24 000 ×. This surfaceshows the greatest evidenceof brittle fracture and pullout. coupling, a gradual transition (Figs. 5-7) from this plastic, smear-type wear to brittle, pullout-type wear occurs. In the micrographs, this is apparent at low magnifications as a change in the net size of the objects occupying the wear surface. The latter behavior is especially obvious in the more strongly coupled CEN- and CCN-derived composites. The filter displays an increased tendency for pullout as coupling increases. The extremely porous nature of the fillers [ 9] can be combined with these observations to form the following hypothesis: the presence of coupling agent in or on the porous structure decreases subsequent penetration by TEGDMA; without coupling, TEGDMA impregnates the nanometerscale pores more efficiently; as this penetration becomes more limited/uneven, tile gel fractures in an increasingly brittle fashion; the apparent debris size increases together with the wear rate, 3.4. T E M results
Fig. 8 contains TEM images of the composites from uncoupled powder (NC) and from the most coupled powder (CCN). Fig. 8(a) and (c) show the lowest magnification images (5000 × ) for the two composites. Brittle fracture is
J. Luo et al. / M a t e r i a l s Science and Engineering C 5 (1997) ] 5 - 2 2
21
Fig. 8. TEM images of the composites from the uncoupled (a, b) and most coupled (c, d) powder; (a, c) 5000 × ; (b, d) 200 000 ×. The highest wear rate composite shows significant brittle fracture.
clearly more pronounced in the silanated gel during microtoming than in the uncoupled gel composite. The absence of such fracture in the NC-derived composite is probably caused by substantial TEGDMA penetration. The resulting impregnated filler particles are more likely to exhibit plastic deformation, known to be significant in preventing tooth failure [ 12]. In contrast, the most coupled particles will be the least penetrated by TEGDMA and produce a composite more subject to brittle fracture. At higher magnifications (200 000X in Fig. 8(b) and (d)), limited atomic contrast prevents us from resolving the pores clearly. However, the white spots in the black (silica) phase on the higher magnification image for the uncoupled get suggest the presence of polymer. In a sense, the microtoming operation mimics the wear process: as the diamond knife moves through the coupled composite, the brittleness of the gel phase results in significant crack propagation and the loss of significant areas (Fig. 8 (c)) of the composite. In contrast, the uncoupled composite experiences little of this brittle fracture (Fig. 8(a)) and no material loss.
3.5. Wear in nanostructured composites
Conventional wisdom states that coupling agents improve the adhesion between a polymeric phase and a dense inorganic phase, leading to better load transfer and increased wear resistance. For some nanoporous particles, however, coupling clearly has a negative effect on the wear resistance (Fig. 2). Fig. 9 shows a schematic representation of the polymer-gel coupling. The silica gel (light phase) is depicted as having a regular array of fine pores. In the uncoupled (NC) case, these pores are shown to be fully penetrated by the monomer. It has been reported that acryIate monomers are readiIy adsorbed onto silica gel [ 19], thus improving wetting and penetration. Extensive coupling either at or below the surface eventually results in areas unpenetrated by polymerized TEGDMA exhibiting more of the brittle nature of unreinforced gel. This allows for greater in-depth crack propagation and larger wear debris. The nature of this blockage depends on the coupling process itself. Greater coupling efficiency (CCN) appears to block/fill all pore sizes evenly. An intermediate degree of coupling (CEN) is apparently specific to small pores.
22
J. Luo et al. /Materials Science and Engineering C 5 (1997) 15-22
porous silica gel
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penetration polymerization~
l=ig, 9. Schematic illustration of the negative effect of coupling agents on the wear resistance. Specific volumes of gel are not interpenetratedby crosslinked TEGDMA ("polymer") due to the blocking/filling effect of the coupling agent (represented by small black lines bridging specific pore diameters). These uninterpenetratedareas have the brittle nature of unreinforced gel and allow for greater in-depth crack propagation. Regardless of the differences in available pore size, a high degree of correlation exists between the wear resistance and the total available pore volume.
,1. Conclusions Nanoporous silica gel-derived composites were used to mimic the wear resistance o f natural tooth enamel. Contrary to conventional wisdom, silane coupling agents were found to diminish the wear resistance of crosslinked T E G D M A based composites reinforced with nanoporous particles. W e a r rates increased as the degree of coupling increased. An increased wear resistance was associated with the formation of a flat, smooth surface apparently consisting of a layer of fine wear debris. A low wear resistance was associated with an irregular surface showing considerable evidence of brittle fracture. The baseline, uncoupled gel composite had a wear resistance comparable with that of a commercial product. The available filler porosity appeared to determine the wear properties. Use of a coupling agent was found to decrease the pore volume. As the coupling efficiency was increased, the specific surface area and the total pore volume detectable by BET analysis decreased. The reduction in accessible pore volume probably led to a decrease in the pore penetration of crosslinked TEGDlVIA in the final composite. Brittle fracture then replaced plastic deformation as the dominant mode of surface failure. The wear resistance varied linearly with the changes in the accessible porosity.
F o r these types of nanocomposite, the application of a coupling agent resulted in no detectable improvement in the preservation of the wear properties. This was probably caused by good initial penetration of the original porous gels. The macroscopic or microscopic level at which coupling agents are normally of benefit does not improve the wear resistance of this composite [orm. On the basis of these results, the current study shows that nanoporous, unsilanated silica gel fillers can be combined with a thermoset to produce wear resistances similar to enamel. The associated principles involving nanoporous structures are worthy of further exploration in the areas of biomimetics and in the fabrication of improved inorganicorganic materials for biomedical applications.
Acknowledgements This work was supported by the National Institute of Dental Research (project number N I H / N I D R R 0 1 - D E l l 3 0 6 -
01). References [ I ] R.L. Bowen, J. Dental Res., 35 (1956) 360-369. [2] R.L. Bowen, J. Dental Res., 37 (1958) 90. [3] J.E. Mark and P.D. Calvert, Mater. Sci. Eng. C, 1 (1994) 159-173. [4] L.F. Francis K.J. Vaidya, H.Y. Huang and W.D. Wolf., Mate1. ScL Eng. C, 3 (1995) 63-74. [5] R.L. Bowen, US Patent 3 194 784, 1965. [6] G. Vanherle and D.C. Smith (eds.), Posterior Composite Resin Dental Restorative Materials, Peter Szulc Publishing Co., 1985. 7] R.L. Kaas and J.L. Kardos, Polym. Eng. Sci., 1] ( 1971 ) I 1-18. [8] D.E. Leyden (ed.), Silanes, Surfaces and Interfaces, Gordon and Breach, New York, 1985. [9] EJ. Pope and J.D. Mackenzie,J. Non-Cr)'st. Solids, 87 (1986) 185198. [ 10] L.C. Klein and G.J. Garvey, J. Non-Cryst. Solids, 48 (1982) 97-104. [ 11] T.M. Chen and G.M. Brauer, J. Dental Res., 61 (12) (1982) 143% 1443. [12] C.P. Lin and W.H. Douglas, J. Dental Res., 73 (5) (1994) 1072I078. [ 13 ] J.M. Power, L.T. Smith, M. Eldiwanyand G.D.Ladd,Am. Z Dentisto,, 6 (5) (1993) 232-234. [ 14] S. Hirabayashi, J.A. Hood and T. Hirasawa, Dental Mater. J., 12 (2) (1993) 159-170. [15] J. Luo, R. Seghi and J. Lannutti, Dental Mater. J., submitted for publication. [ I6] T. Nambu, C. Watanabe and Y. Tani, DentalMater. J., 10 (2) ( 1991) 138-148. [ t7] W. Wu and J.E. McK.inney,J. Dental Res., 61 (10) (1982) 11801t83. [ 18] I.B. Larsen, L.B. Larsen, M. Frued and E.C. Murksgaard, J. Dental Res., 71 i l l ) (1992) i851-1853. [i9] D.L. Allara, Z. Wang and C.G. Pantano, J. Non-C~3"st. Solids. 120 (1990) 93-101.