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Tetrahydrofuran as alternative solvent in dental adhesive systems Silvia T. Fontes a,∗ , Fabrício A. Ogliari b , Giana S. Lima a , Márcia Bueno a , Luis Felipe J. Schneider c , Evandro Piva a a b c
School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil Angelus - Science and Technology, Londrina, PR, Brazil School of Dentistry, University of Passo Fundo, Passo Fundo, RS, Brazil
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To evaluate the influence of tetrahydrofuran (THF) on the resin-to-dentin microten-
Received 11 February 2009
sile bond strength (TBS) after water storage, for 24 h and 6 months, and to compare its
Received in revised form
behavior with that of traditional solvents.
13 June 2009
Methods. Seven versions of monomer/solvent mixtures (primers) were prepared using the
Accepted 22 June 2009
following solvent and water combinations: (1) THF, (2) acetone, (3) ethanol, (4) water, (5) THF/water, (6) acetone/water and (7) ethanol/water. An experimental adhesive resin was also synthesized to compare adhesive systems with the different primers. Forty-two
Keywords:
bovine incisors, randomly separated into seven groups, had their superficial coronal dentin
Adhesive
exposed. After acid-etching and rinsing, the excess water was removed from the surface
Bond strength
with absorbent paper. Each experimental primer was applied with agitation (30 s) followed
Dentin
by a mild air stream (10 s). The experimental adhesive resin was applied and light-activated
Solvent
(20 s). Resin composite restorations were constructed incrementally. Restored teeth were
Storage
stored in distilled water at 37 ◦ C (24 h) and sectioned to obtain sticks with an area of 0.5 mm2 . Half the specimens were subjected to the TBS test immediately after being cut and the other half were tested after 6 months of water storage. Data (MPa) were analyzed by two-way ANOVA (solvent type and storage time as factors) and Tukey–Kramer’s test at ˛ = 0.05. Results. Factors and interaction showed a statistical effect. After 6 months storage, acetone groups and primers containing THF showed similar TBS to initial means. Significance. THF seems to be a promising solvent for use in dental adhesive systems, maintaining bond strength on dentin substrate after storage. © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A wide range of dental adhesive systems contain hydrophilic resin monomers dissolved in volatile solvents, such as acetone, ethanol, water or a combination of them [1]. The main
purposes of including such elements in etch-and-rinse adhesive formulations are to improve the dentin hybridization [2] and the net expansion of dried demineralized dentin matrix [3–7]. Because of the low molecular weight and high diffusion capacity, the solvents also decrease the viscosity and increase
∗ Corresponding author at: Centro de Controle e Desenvolvimento de Biomateriais (CDC-Bio), Faculdade de Odontologia, Universidade Federal de Pelotas (UFPel), Rua Gonc¸alves Chaves 457, CEP 96015-560, Pelotas, RS, Brazil. Tel.: +55 53 32226690; fax: +55 53 32226690. E-mail address:
[email protected] (S.T. Fontes). 0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2009.06.021
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the hydrophilicity of formulations [1,3,8]. Indeed, organic solvents help the displacement of residual water that should be completely eliminated from the bonding surfaces prior to curing [9–12]. The knowledge of chemical and physical properties of adhesive components is indispensable to understand or even predict their behavior. Some of the main characteristics of solvents are the boiling temperature, vapor pressure, polarity and hydrogen bonding (H-bonding) capacity [1]. The first two properties aid evaporation of the solvents after application [13,14], but volatility may also influence the shelf life of adhesives [15]. The H-bonding ability is desirable, because in this way, solvents can simultaneously remove water from the demineralized dentin matrix and preserve the collagen interfibrillar spaces [3–7]. Additionally, their polarity determines the solubility characteristics of molecules [1]. Another important aspect is that increased concentrations of hydrophilic components have to be used in primer and/or adhesive resin formulations to enhance the ability to penetrate into the demineralized dentin [2]. Unfortunately, solvents may impair the satisfactory degree of conversion of adhesive systems [16] and these products are prone to water sorption, which adversely affects the stability of resin–dentin interfaces [17–20]. Thus, bonding procedures should ideally be done in the absence of water and using hydrophobic resins that are expected to be more stable over time [2,21,22]. As residual solvent and water are considered degradation promoting factors [19,22], the role of the solvent type in eliminating them from the substrate has been recognized [14,23,24]. Moreover, prolonged air-drying [11,14,23,25] and prolonged application times [13] have been proposed to optimize the elimination of organic solvents and water. However, there are very few proposals with regard to alternative solvents. Tetrahydrofuran (THF) is a heterocyclic organic and polar aprotic solvent that dissolves a wide range of polar and nonpolar components [26]. These features may justify the use of THF in dental adhesives that combine hydrophobic and hydrophilic molecules [1]. Moreover, the volatile character of THF implies that this solvent might easily be removed after application [13]. Although THF might present the aforementioned advantages, little information about THF has been found to-date. Consequently, this study evaluated the influence of THF – a hypothetical alternative solvent for use in the primer formulation of an experimental adhesive system – on the resin-to-dentin microtensile bond strength (TBS) after 24 h and 6 months of water storage. Comparisons were made with traditional solvents. The null hypothesis tested was that the solvent type would not affect the TBS, irrespective of the storage period.
2.
Materials and methods
2.1.
Material preparation
Seven versions of solvent-based experimental primers were used in this study. The blends were formulated through the intensive mixture of 50 wt% 2-hydroxyethyl methacrylate (HEMA) (Aldrich, St. Louis, MO, USA), 10 wt% phosphate
Fig. 1 – Molecular formula of different solvents used in the experimental primers composition.
monomer [27] and 40 wt% solvent (Labsynth, Diadema, SP, Brazil), as described in Table 1. The concentrations of experimental primers were based on a pilot study, which evaluated different ratios of monomer/solvent and indicated that a solvent concentration of 40 wt% had the best performance for the monomer blend used (unpublished data). These organic solvents (Fig. 1) were used as they were received from the manufacturers, without any additional treatment. An experimental adhesive resin (AD-50, CDC-Bio, Pelotas, RS, Brazil) was also prepared by the addition of methacrylates monomers: bisphenol A glycidyl methacrylate (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), HEMA, photoinitiators and stabilizers. All the weight measurements were carried out isothermally at 23 ◦ C (±1) using an analytical balance (AG 200, Gehaka, São Paulo, SP, Brazil). The freshly prepared blends were ultra-sonicated for 15 min, to ensure homogeneity, and used within 24 h after the preparation.
2.2.
Specimen preparation
Forty-two extracted bovine incisors, disinfected in 0.5% chloramine-T and used within 3 months after extraction, were chosen for this study. The teeth were randomly separated into seven experimental groups, assigning six teeth to each group. The superficial coronal dentin was exposed by trimming the buccal enamel and polished with 600 grit silicon-carbide paper applied for 60 s to create a standardized smear layer. All the bonding procedures were carried out by a single operator, at room temperature of 23 ◦ C (±1) and relative humidity of 70% (±5). After acid-etching with 35% phosphoric acid for 15 s, the surfaces were rinsed with distilled water for 30 s. The tooth surface was air dried for 30 s with oil-free compressed air and rewetted with 2.5 l of water by means of a micropipette (Micropipette, Pipetman, Gilson, NY, USA). The excess water was removed with absorbent paper kept in contact with the surface for 10 s. According to the group, one coat of the solvent-based experimental primer was applied to the dentin with agitation for 30 s using a brush. A mild air stream (80 pounds) was used to help solvent evaporation for 10 s at a distance of 10 cm. One coat of an experimental adhesive resin was then applied and light-activated for 20 s (LED Radii, SDI, Bayswater, Victoria, Australia) at 1400 mW/cm2 .
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Resin composite restoration was constructed on the treated surface with four incremental layers of approximately 1.0 mm in each increment (Charisma, Heraeus Kulzer, Hanau, Hesse, Germany). Each layer of composite was light-activated using the same treatment described above.
Cary, NC) using two-way analyses of variance (ANOVA) and Tukey–Kramer’s multiple comparisons test. The factors investigated were solvent and storage time. The level of confidence was established at ˛ = 0.05.
2.4. 2.3.
Microtensile bond strength test
After storage in distilled water, at 37 ◦ C for 24 h, the restored teeth were sectioned in both the mesio-distal and incisocervical directions across the bonded interface, with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA) at 400 rpm to obtain sticks with a cross-sectional area of approximately 0.5 mm2 . The cross-sectional area of each stick was measured with a digital caliper (DC 500, Mitutoyo, Suzano, SP, Brazil) and recorded for subsequent TBS calculation. Approximately six sticks were produced from each tooth (eighteen sticks per group). Half was tested immediately after the cutting procedure, while the other half was tested after 6 months of water storage at 37 ◦ C. Each stick was individually attached to a specific device for microtensile testing with cyanoacrylate resin (Loctite, Henkel, Diadema, SP, Brazil). Specimens were subjected to tensile forces in a universal testing machine (DL 500, Emic, São José dos Pinhais, PR, Brazil) at a cross-head speed of 1.0 mm/min. TBS values, in mega-Pascal (MPa), were obtained by dividing the force, in Newton (N), by the calculated area, in mm2 . Data were submitted to statistical analysis using SAS statistical software (SAS, JMP version 9.1.3, SAS Inst Inc.,
Failure pattern analyses
In each specimen, the half corresponding to dentin was removed from the device for microtensile testing and evaluated at 500× magnification under in a light microscope (FM 700, Futuretech, Kawasaki, Tokyo, Japan). The failure patterns were classified as: cohesive in dentin (failure exclusively within dentin), cohesive in resin (failure exclusively within resin), adhesive (failure at the resin–dentin interface), mixed (failure at the resin–dentin interface that simultaneously included cohesive failure of the neighboring substrates) or prematurely debonded (failure during specimen preparation or water storage).
3.
Results
3.1. Data were transformed into ranks to fit the requirements of normality and equality of variances necessary to run parametric tests The mean values and the standard deviations of resin-todentin microtensile bond strength are expressed in Table 2. Two-way analysis of variance (solvent type and storage
Table 1 – Composition of the experimental primers used in present study. Groups
THF Acetone Ethanol Water THF/water Acetone/water Ethanol/water
Reagent (wt%) HEMA
Phosphate monomer
THF
Acetone
Ethanol
Water
Total
50 50 50 50 50 50 50
10 10 10 10 10 10 10
40 0 0 0 20 0 0
0 40 0 0 0 20 0
0 0 40 0 0 0 20
0 0 0 40 20 20 20
100 100 100 100 100 100 100
HEMA: 2-hydroxylethyl methacrylate; THF: tetrahydrofuran.
Table 2 – Microtensile bond strength means values in MPa (±standard deviation) according to storage times and solvent types/combinations. Groups
Storage period 24 h
THF Acetone Ethanol Water THF/water Acetone/water Ethanol/water
A
55.3 56.0A 61.1A 42.0A 59.3A 56.5A 58.5A
(13.2) (14.3) (11.4) (12.1) (16.6) (19.6) (14.0)
6 months ab ab a b a ab a
A
55.9 (9.7) 46.5(12.8) B 36.9 (19.3) B 17.5(11.5) A 44.0 (14.3) B 31.3 (17.5) B 13.2 (5.8) A
a ab bc de ac cd e
Abbreviation: THF: tetrahydrofuran. Different superscript capital letters indicate statistically significant differences in rows. Different lowercase letters indicate statistically significant differences in columns (p < 0.05).
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Fig. 2 – Failure pattern distribution of different groups tested.
time) showed a significant effect for the factor solvent type (p = 0.002), storage time (p < 0.001) and for interaction between them (p < 0.001). After 24 h TBS means for solvent-based primers showed means similar to those of their corresponding formulations containing water. According to comparisons after 6 months storage, THF group showed statistically higher means than those of ethanol, acetone/water, ethanol/water and water (p < 0.05). After 6 months of water storage, the TBS means of groups THF, acetone and THF/water primer were similar to means at 24 h, while other groups showed significantly lower means after storage (p < 0.05). Adhesive failures and prematurely debonded sticks were found predominantly in groups with low TBS. Sixteen samples failed cohesively in resin composite. There was no premature specimen debonding during preparation when experiments were carried out after 24 h. On the other hand, there were eight premature failures after storage in water for 6 months. The overall fracture mode distribution is expressed in Fig. 2.
4.
Discussion
Recent studies have demonstrated that bond strength is dependent on the dental adhesive system formulation [24,28,29]. However, many are the components that influence the behavior of materials when commercially available adhesives are used for comparisons and manufacturers are usually reluctant to reveal the complete information of their products [1]. The present study used experimental adhesive systems that only differed as regards the types of solvent in their primer composition. The HEMA component of these adhesive systems gives them a hydrophilic character that is very important in wet bonding. On the other hand, the deleterious effects are known, because of its susceptibility to hydrolytic degradation [2]. Thus in this study THF, an aprotic solvent capable of dissolving one or more substances (such as monomers), was combined with HEMA to determine the performance of THFbased primer in comparison with some other solvents usually found in dental products (such as acetone, ethanol, water or combinations of these).
According to the methodology used in the present investigation, the dentin moisture and primer solution application protocols were adopted equally for all experimental adhesives tested, irrespective of the solvent type. Although currently available etch-and-rinse adhesive systems require a moist dentin surface for application, it is known that the amount of water necessary to maximize the bond strength varies according to the agent [4,24,29]. Although different artifices, such as prolonged application times and an air-dry stream, can be used to improve the removal of residual water and organic solvents [13], it has been reported that complete elimination is difficult to achieve and is also dependent on the solvent type used in the formulation [14,23,24]. According to the current findings, the hypothesis tested was rejected, since it was demonstrated that both solvent type and storage time are significant factors. Although similar values were verified after 24 h storage for THF, acetone and ethanol-based primers, it was demonstrated that these values were maintained after 6 months, only when the THF-based primers was used. Additionally, the bond strength values suffered a significant reduction over time (p < 0.05) for all the groups when water was added to the formulations. Most dental adhesive systems currently available on the market show favorable immediate bonding, but the stability of the bonded interfaces still causes major concerns [22]. The present data represent the effect of solvent choice on microtensile bond strength of dental adhesives. Considering bond strength and further information on boiling temperature and vapor pressure [1,15], the authors speculated that there might be dependence between the mechanical properties of the bonded areas, and the volatile characteristics of the solvents. Consequently, ethanol (43.9 mmHg of vapor pressure) and water-based (17.5 mmHg of vapor pressure) primers probably present lower vapor pressure and greater difficulty in displacing the remaining solvent and water than the others do. Additionally, solvents with high H-bonding capacity, such as ethanol and water, interact with collagen peptides, preventing the collapse of the dried demineralized dentin matrix [3–5], but also increase the percentage of retained solvent [18,23]. For these reasons, these groups could perhaps achieve similar means after long-term storage, if a less moist dentin surface had been used at the time of application [24,29]. It has been recognized that water sorption of dental adhesive
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resins is a concern, mainly when hydrophilic components are added to formulations [2,21]. The hydrolysis of resin–dentin bonds involves degradation of the hybrid layer [22]. Therefore, storage time can be deleterious to the mechanical properties of some of dental bonding agents, compromising the durability of resin–dentin bonds [17–20]. As previously mentioned, the H-bonding capacity of a solvent has been shown to be an important property in the removal of water and re-expansion of the shrunken demineralized collagen network after dehydration [3–7]. However, this feature can also facilitate the attraction between polymer and solvent, increasing the retention of solvent and water [18,23]. By the presence of an ether-group (–O–), to form lower hydrogen bonds than alcohols (–OH), and having a highly volatile character (173 mmHg of vapor pressure) similar to that of acetone (184 mmHg of vapor pressure), THF demonstrated a good water-removing capacity and enhanced evaporation of the humidity left in dentin. These findings could explain the performance of the groups that had no water in their formulation, after long-term storage, considering that all the adhesive procedures were accomplished by the wet bonding technique. The authors also explain the advantage of THF over the other solvent types, because of its capacity for reacting as a chelating agent, contributing to adhesion [30]. Adhesive failures and premature debonding were both associated with lower TBS values, especially when water was used in the formulation. As regards the bond strength, this seemed to rely on the quality of the hybrid layer [31], in which poor-hybridized dentin had worse interfacial mechanical properties to resist the degradation processes. These findings agree with previous studies that demonstrated that both organic solvent and water entrapped within the adhesive resin severely compromise the bond strength and affect the structural integrity of the hybrid layer [17,20]. Moreover, the lowest TBS values presented by the water-based solvents in the current investigation indicate that the excess water is probably more deleterious to this type of solvent [24]. Considering the potential and advantages of THF and scarce information available in the literature, further studies such as cytotoxicity tests and long-term evaluations are necessary for better understanding of its use as a solvent in dental adhesives.
5.
Conclusions
Within the limits of this investigation, it was concluded that THF appears to be a promising solvent for use in dental adhesive systems due to its bond strength stability over time. The null hypothesis was rejected because it was demonstrated that the solvent type and storage period directly affected the bond strength.
Acknowledgements The authors are grateful to the Brazilian National Research Council (CNPq) for scholarship and Grants 478731/2007-8 and 555799/2006-9 as financial support.
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