Ó 2014 Eur J Oral Sci

Eur J Oral Sci 2015; 123: 46–52 DOI: 10.1111/eos.12164 Printed in Singapore. All rights reserved

European Journal of Oral Sciences

Influence of methyl mercaptan on the repair bond strength of composites fabricated using self-etch adhesives

Miho Yokokawa, Akitomo Rikuta, Akimasa Tsujimoto, Kenji Tsuchiya, Syo Shibasaki, Saki Matsuyoshi, Masashi Miyazaki Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan

Yokokawa M, Rikuta A, Tsujimoto A, Tsuchiya K, Shibasaki S, Matsuyoshi S, Miyazaki M. Influence of methyl mercaptan on the repair bond strength of composites fabricated using self-etch adhesives. Eur J Oral Sci 2015; 123: 46–52 © 2014 Eur J Oral Sci The influence of methyl mercaptan on the repair bond strength of composites fabricated using self-etch adhesives was investigated. The surface free-energies were determined by measuring the contact angles of test liquids placed on composites that had been immersed in different concentrations of methyl mercaptan (0.01, 0.1, and 1.0 M). To determine the repair bond strength, self-etch adhesives were applied to the aged composite, and then newly added composites were condensed. Ten samples of each specimen were subjected to shear testing at a crosshead speed of 1.0 mm min1. Samples were analyzed using two-way ANOVA followed by Tukey’s honestly significant difference (HSD) test. Although the dispersion force of the composites remained relatively constant, their polar force increased slightly as the concentration of methyl mercaptan increased. The hydrogen-bonding forces were significantly higher after immersion in 1.0 M methyl mercaptan, leading to higher surface-free energies. However, the repair bond strengths for the repair restorations prepared from composites immersed in 1.0 M methyl mercaptan were significantly lower than for those immersed in 0.01 and 0.10 M methyl mercaptan. Considering the results of this study, it can be concluded that the repair bond strengths of both the aged and newly added composites were affected by immersion in methyl mercaptan solutions.

Minimal intervention is the main concept in modern dentistry that has driven the development of adhesive technologies for the restoration of decayed tooth structures resulting from carious lesions (1). When a composite restoration fails because of discoloration and wear of the material, the repair or replacement of the restoration is required. Total replacement of the restoration may lead to significant loss of sound tooth structure because it is difficult to recognize the interface between the tooth-colored restoration and the cavity wall, which mainly consists of dentin. Preservation of a sound tooth structure is desirable in terms of reduction of harmful effects to the pulpal tissue and the time required for the clinical restoration of the cavity (2). Dental restoration repair is thought to be an ideal treatment given the minimal intervention required, and the advantages of restoration repair over total removal of restorations have been recognized by dental practitioners (3). The repair bond strengths of newly added composites to aged composites have been evaluated for different surface-treatment methods, such as roughening with diamond points, airborne particle abrasion, phosphoric

Dr Masashi Miyazaki, Department of Operative Dentistry, Nihon University School of Dentistry, 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan E-mail: [email protected] Key words: light-cured composite; methyl mercaptan; repair bond strength; repair restoration; surface free-energy Accepted for publication December 2014

acid etching, and the application of silane coupling agents (4, 5). Whilst surface roughness promotes micromechanical retention to the aged composite restorations, intermediate materials, such as adhesive resin or silane coupling agents, are required to establish surface wetting and thus formation of a durable bond between the old and the newly added composites (6, 7). Currently, many kinds of self-etch adhesives are available for use in clinical situations. Recently developed singlestep self-etch adhesives combine the functions of etching, priming, and bonding (8), thereby reducing the risk of technical errors during restorations. The self-etch adhesives are applied to the restorative surface before placement of composites; this may ensure adhesion by improving the wettability of the aged composite surface by the resin components. The efficacy of repair of aged composite is dependent on the adhesive used, and it has been reported that self-etching adhesives can be used to repair aged composites (9). Chemical interaction of self-etch adhesives is related to acidic functional monomers, which may interact with the inorganic components of the composites, creating a continuous flow between the aged composite and the adhesive.

Repair bond strength of resin composites

The wettability of the conditioned adherent surface by the adhesive resin is important for bonding of resin composites, regardless of the mechanism of adhesion (chemical, micromechanical interlocking, or a combination of both) (10). The repair bond strength of composites depends on several factors, including the surface treatment of the aged composite, the ability of the adhesive to wet the aged composite surface, and the polymerization reaction involved in the formation of the newly added composite (11). Measurement of the contact angle of an aged composite surface can provide information about its surface free-energy, which is related to its bonding characteristics (12). In addition, before performing repair restoration, the possible degradation of the composite restoration as a result of long-term service in an oral environment must be considered. The restoratives are exposed to complex conditions, including saliva, bacteria, enzymes, and different types of compounds from dietary materials. Diffusion of water into the polymer network and its boundaries with filler particles is thought to be related to the hydrolytic degradation of the material itself (13). Moreover, the chemical breakdown of composites occurs in the presence of salivary-like enzymes and is prolonged by the subsequent biological effects of the generated by-products on the surrounding bacteria (14). Furthermore, some oral microorganisms are capable of producing volatile sulfur compounds – including primarily hydrogen sulfide, dimethyl sulfide, and methyl mercaptan – that are responsible for oral malodor (15). Volatile sulfur compounds may be diffused into composite restorations with oral fluids such as saliva, and these compounds may also affect the bond between aged and newly added composites during repair restorations (16). Methyl mercaptan, for example, can act as a chain-transfer agent in free-radical polymerizations, leading to the formation of products with low molecular weights (17). The propagation of the polymer chain is halted by transferring an atom to the radical located at the end of the polymer chain during polymerization. Therefore, methyl mercaptan present in the mouth has the potential to negatively affect polymerization when repair restorations are performed (16). Analysis of the surface free-energies of resin composites as models for

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repair restorations, before and after exposure to methyl mercaptan, may thus provide more insight into the evolution of repair bond strengths. Consequently, in the present study, the influence of methyl mercaptan on the surface free-energies and repair bond strengths of light-cured composites fabricated using self-etch adhesives was investigated. It was hypothesized that the repair bond strengths of composites immersed in methyl mercaptan solutions would be the same as those of the control specimens.

Material and methods Restorative materials Information on the composites used in this study, Estelite Σ Quick (EQ) (shade A2; Tokuyama Dental, Tokyo, Japan) and Beautifil II (BF) (shade A2; Shofu, Kyoto, Japan), is provided in Table 1. Adhesive systems were used in combination with the manufacturers’ resin composites. A curing unit (Optilux 501; Kerr, Orange, CA, USA) was used to cure the composites, and the light intensity (800 mW cm2) of the curing unit was confirmed using a dental radiometer (Model 100; Kerr) before fabrication of the specimens. Surface free-energy During irradiation, the composites were placed in a Teflon mold (6 mm diameter; 2 mm height) that was positioned on a glass slide with a white-filter backing. The exit window of the curing unit was then placed against the glass plate at the center of the specimen, and the sample was exposed to light irradiation for 40 s. The final finish was accomplished by grinding the surface with wet #2,000-grit silicon carbide (SiC) paper, followed by washing and drying with oil-free compressed air. Three different concentrations of methyl mercaptan (0.01, 0.10, and 1.0 M) were prepared from a standard solution of methyl mercaptan (~15% in water, lot. KWG5743; Wako Pure Chemical Industry, Tokyo, Japan). The specimens were then randomly divided into four immersion groups consisting of the three different concentrations of methyl mercaptan and distilled water as the control group. All specimens were aged for 4 weeks in their respective solutions at 37°C.

Table 1 Materials used in this study Code

Composite (lot no.)

Main components

Adhesive (lot no.)

Main components

EQ

Estelite Σ Quick (J022)

Bond Force (084)

BF

Beautifil II (110813)

Bis-GMA, TEGDMA, silica-zirconia filler, RAP initiator (0.2 lm spherical filler, 82 wt%) Bis-GMA, TEGDMA almino-silicate-glass (0.01–4.0 lm S-PRG fillers, 83 wt%)

Phosphoric acid monomer, bis-GMA, HEMA, TEGDMA, isopropanol, water, camphorquinone, dibutyl hydroxytoluene 4-MET, 6-MHPAc, bis-GMA, TEGDMA, acetone, water

BeautiBond (081010)

Manufacturer Tokuyama Dental

Shofu

4-MET, 4-methacryloyloxyethyltrimellitate; 6-MHPAc, 6-methacryloyloxyhexyl phosphonoacetate; bis-GMA, 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)propane; HEMA, 2-hydroxyethyl methacrylate; S-PRG filler, surface prereacted glass-ionomer filler; TEGDMA, triethyleneglycol dimethacrylate.

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Yokokawa et al.

The surfaces of the composites were washed with distilled water and air dried with oil-free compressed air. The surface free-energy of each sample was then determined by measuring the contact angles for three test liquids: 1-bromonaphthalene, diiodomethane, and distilled water (Table 2) (18). A contact angle meter (Drop Master DM500; Kyowa Interface Science, Saitama, Japan) fitted with a charge-coupled device camera was used for automatic measurement of the contact angles (Fig. 1). For each test liquid, the equilibrium contact angles (h) for five samples of aged composites were measured using the sessile drop method at 23  1°C immediately after the test liquid was applied. The surface free-energy parameters for the solids were then determined according to the fundamental concepts of wetting (19). The Young–Dupre equation describes the work of adhesion for a solid (S) and a liquid (L) that are in contact (WSL), the interfacial freeenergy between the solid and liquid (cSL), and the surface free-energy of the liquid and the solid (cL and cS, respectively) as follows: WSL ¼ cL þ cS  cSL ¼ cL ð1 þ cos hÞ

Scanning electron microscopy

Using the extended Fowkes equation, cSL can be expressed as: cSL ¼ cL þ cS  2 cdL cdS

1=2

2 cpL cpS

1=2

in place on the aged composite surfaces. The new resin composites added were the same brand and color as the aged composites. The mold was firmly attached to the double-sided adhesive tape on each sample, and then the same brand of composite used for each aged sample was condensed into the mold and cured for 40 s. The finished specimens were transferred to distilled water and stored at 37°C for 24 h. Ten specimens per group were tested using a shear knife-edge testing apparatus in a universal testing machine (Type 4204; Instron, Canton, MA, USA) at a crosshead speed of 1.0 mm min1. The shear bond-strength values were calculated from the peak load at failure divided by the specimen surface area. After testing, the specimens were examined under an optical microscope (SZH-131; Olympus, Tokyo, Japan) at a magnification of 910 to define the location of the bond failure. The failure type was determined to be adhesive if it occurred between the composites and cohesive if it occurred within the composites.

2 chL chS

1=2

cL ¼ cdL þ cpL þ chL ; cS ¼ cdS þ cpS þ chS ; where cdL , cpL , and chL are the components of the surface free-energy (c) arising from the dispersion force, the polar (permanent and induced) force, and the hydrogen-bonding force, respectively. The surface free-energy parameters were calculated on the basis of these equations using the software FAMAS (Kyowa Interface Science). Repair bond strength Adherent surfaces were prepared in the same manner as those used for determination of contact angles. After washing with distilled water and air drying with oil-free compressed air, a piece of double-sided adhesive tape with a 4-mm-diameter hole was then attached to each composite surface. For the EQ composites, Bond Force was applied on the aged composites and left for 20 s. For the BF samples, BeautiBond was applied on the aged composites and left for 10 s. After gently blowing with air for 5 s, each composite sample was irradiated with light for 10 s. Next, a Duracon mold (4 mm diameter; 2 mm height) was used to shape the newly formed composites and hold them

Bonded specimens from each group of samples (n = 5) were stored in distilled water at 37°C for 24 h and were then embedded in a self-curing epoxy resin. These embedded specimens were sectioned perpendicularly and then the surfaces of the cut halves were polished successively using SiC paper with grit sizes of 600, 1,200, and 4,000 lm (Ecomet 4/Automet 2; Buehler, Lake Bluff, IL, USA). The surface was finally polished with a soft cloth using diamond paste with a grit size of 1.0 lm. These surfaces were coated with a thin film of gold in a vacuum evaporator (Quick Coater Type SC-701; Sanyu Denshi, Tokyo, Japan) and observed using scanning electron microscopy (ERA 8800FE; Elionix, Tokyo, Japan). Statistical analysis Data were analyzed using two-way ANOVA followed by Tukey’s honestly significant difference (HSD) test (a = 0.05). All statistical analyses were performed using the statistical software package SIGMA STAT Ver. 3.1 (SPSS, Chicago, IL, USA).

Results The surface free-energies, and their components, of the aged composite surface are shown in Table 3. For all composite surfaces, the cdS values remained relatively constant at 39.1–39.9 mN m1. The cpS component values increased slightly with an increase in the

Table 2 Surface free-energies and component values for the three test liquids Liquid 1-Bromonaphthalene Diiodomethane Distilled water

Lot no.

Manufacturer

ALH4513 ALL2310 –

Wako Pure Chemical Industries Wako Pure Chemical Industries –

cdL

cpL

chL

cL

44.4 46.8 29.1

0.2 4.0 1.3

0.0 0.0 42.4

44.6 50.8 72.8

Values are given as mN m1. cL, Total free energy of liquid; cdL , dispersion force; chL , hydrogen-bonding force; cpL , polar force.

Repair bond strength of resin composites A

B

Fig. 1. Drop Master DM500 apparatus fitted with a chargecoupled device camera (A) for automatic measurements of the contact angles (B).

concentration of the methyl mercaptan solution, but the differences were not significant. However, for both composites, the chS values obtained following immersion in 1.0 M methyl mercaptan were significantly higher compared with those obtained following immersion in 0.01 and 0.10 M methyl mercaptan, leading to higher surface free-energies. The repair bond strengths of repair restorations, with and without immersion of the aged composite in methyl mercaptan, are presented in Table 4. The twoway ANOVA results indicated that both the concentration of the methyl mercaptan immersion solution and the type of composite affected the repair bond strength (P < 0.001). Significant interaction was observed

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between the immersion solutions and composites (P < 0.001). The repair bond strengths for the composites immersed in 1.0 M methyl mercaptan were significantly lower than for those immersed in the 0.01 and 0.10 M methyl mercaptan solutions, for both composites. In addition, while significantly lower repair bond strengths compared with the control were observed for the EQ samples immersed in the 0.01 and 0.1 M methyl mercaptan solutions, no significant differences were found for the BF composites. Interestingly, for the control and for composites immersed in 0.01 and 0.1 M methyl mercaptan solutions, the predominant mode of fracture was cohesive failure in the aged composite. In contrast, the predominant mode of failure for the composites immersed in a 1.0 M methyl mercaptan solution was adhesive failure. Scanning electron microscopy images of the aged composite and newly added composite interface are shown in Fig. 2. Although the thickness of each of the adhesive resins was different for each group of composites, the composite-to-composite interfaces for the control and the composites immersed in a 0.1 M solution of methyl mercaptan exhibited good adhesion, without any gaps. On the other hand, gaps between the adhesive resin and aged composites were observed for both materials when the samples were immersed in a 1.0 M solution of methyl mercaptan.

Discussion Providing an optimal bonding interface between aged and newly added composites is necessary to achieve successful repair restorations. During long-term service in the oral environment, composite restorations absorb water, and any remaining double-bond activity is diminished. To improve the bonding between aged and newly added composites, surface roughness of the aged composite is required to create mechanical interactions. Silane coupling agents enhance the wetting of the surface for chemical bonding and are thought to infiltrate readily into the entire surface of aged composites (20). In the present study, the effect of applying self-etch adhesives on aged composites immersed in methyl

Table 3 Surface free-energies of aged composites immersed in various concentrations of methyl mercaptan

Values are given in as mean mN m1 (SD). n = 5. Values connected by vertical lines indicate no significant difference (P > 0.05). cS, Total free energy of a solid; cdS , dispersion force; chS , hydrogen-bonding force; cpS , polar force; BF, Beautifil II; EQ, Estelite Σ Quick. *Distilled water.

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Yokokawa et al. Table 4 Influence of methyl mercaptan on the repair bond strength between aged and newly added composites. Methyl mercaptan concentration

Code

Control*

0.01 M

0.1 M

1.0 M

EQ Failure mode BF Failure mode

16.7 (0.9)a 0/0/10 15.8 (2.1)d 0/0/10

10.6 (1.8)b 1/2/8 14.2 (1.8)d 0/1/9

9.8 (3.2)b 2/1/7 14.5 (1.5)d 0/1/9

4.5 (0.9)c 10/0/0 7.0 (1.7)e 10/0/0

Values are given as mean MPa (SD). n = 10. Values with the same superscript letters are not significantly different (P > 0.05). Failure mode is given as: adhesive failure/cohesive failure in adhesive/cohesive failure in aged composite. BF, Beautifil II; EQ, Estelite Σ Quick. *Distilled water.

Fig. 2. Scanning electron microscopy images of the aged and newly added composite interfaces for Estelite Σ Quick (EQ) and Beautifil II (BF). Although the thicknesses of the adhesive resins were different for each group, the composite-tocomposite interfaces for the control and the composites immersed in 0.1 M methyl mercaptan exhibited good adaptation without any gap formation. On the other hand, gaps between the adhesive resin and aged composite were observed for both composites immersed in 1.0 M methyl mercaptan.

mercaptan solutions was evaluated. Therefore, to focus on the effect of the application of the self-etch adhesives on the repair bond performance, the surfaces of the aged composites were not roughened and silane coupling agents were not employed. Achieving optimal wettability for spreading an adhesive material over the entire surface of an aged composite is necessary to establish adhesion to a newly added composite (21). The factors that affect the wetting of a solid by a liquid include the surface freeenergy of the solid and the surface tension of the liquid (22). The surface free-energy is thought to be a useful indicator of bonding and to reflect the interaction between an aged composite and a self-etch adhesive (23). The surface free-energy of a solid (cS) generally has three components: dispersion (cdS ), polar (cpS ), and hydrogen (chS ) bonding (19). Changes in the surface free-energy are expressed as the sum of the geometrical means of the components. Contact angle data for the following three types of liquids were thus used to calculate the surface free-energy of the composite samples: a purely non-polar liquid (1-bromonaphthalene), a polar liquid (diiodomethane), and a hydrogen-bonding liquid (distilled water) (24).

The cdS and cpS values of the composites were not notably affected by immersion in the methyl mercaptan solutions. The cpS values for all specimens were relatively low (4.2–7.0 mN m1). The cpS value involves polar interactions, or non-dispersion forces, and refers to hydrophilic interactions. The composite surfaces thus appeared to be mainly hydrophobic in character, resulting in lower-polarity surfaces. On the contrary, the specimens immersed in the 1.0 M methyl mercaptan solution had significantly higher cS values relative to those of the control and the samples immersed in a 0.01 M methyl mercaptan solution. Methyl mercaptan is used as a chain-transfer agent because of the low energy of the S-H bond and the high reactivity of the generated thiyl radicals (25). The presence of methyl mercaptan with its S-H bond may therefore lead to higher chS values of the aged composite surfaces. Indeed, immersion in higher methyl mercaptan solutions with higher concentrations resulted in significant increases in the chS values, leading to significantly higher cS values. Therefore, changes in the surface characteristics, most notably the increasing surface density of methyl mercaptan, may explain the observed increases in the cS values. The strength of the bond between an aged and a newly added composite depends on several factors, among which are the characteristics of the adherend surface and the ability of the adhesive to wet it (26). The wetting properties of the adherend surface by adhesives are indicated by the surface free-energies related to the bonding characteristics of solids (27). Despite the significant increase in the cS values, the repair bond strengths for the aged composites immersed in a solution of 1.0 M methyl mercaptan were significantly lower than those for the composites immersed in lower concentrations of methyl mercaptan. After application of the self-etch adhesives on the aged composites, the resin monomers may react with the composites and penetrate into the polymer network. After penetration over the entire surface of the aged composite, the adhesive resin should polymerize. Theoretically, there should be a significant correlation between the mechanical properties of the adhesive resin and bond strength values (28). The strength of a cured adhesive resin is dependent on the composition, degree

Repair bond strength of resin composites

of conversion, and length of the polymer chains. Methyl mercaptan remaining on the surface of an aged composite may alter the mechanical properties of the adhesive resin by reduction of the polymer chain lengths through chain transfer, leading to a lower repair bond strength (29). Indeed, scanning electron microscopy images of the repair restorations revealed the formation of gaps at the interfaces between the newly added composite and the aged composite for the samples immersed in higher concentrations of methyl mercaptan. A reduction of the mechanical properties of the adhesive resins in these samples following exposure to a higher concentration of methyl mercaptan may have resulted in the formation of these gaps between the aged composites and the applied adhesive resins. Note that the decreasing trend in bond strength was different for the EQ and BF composites. For the EQ composite, the repair bond strengths of those immersed in the 0.01 and 0.1 M methyl mercaptan solutions were significantly lower than that immersed in the control, whereas no significant differences were observed for the corresponding BF composites. Each self-etch adhesive contains a certain number of functional monomers, and the polar nature of the functional monomers may contribute to bonding with the inorganic filler particles in the aged composites. The differences in the adhesive performance of the functional monomers can be explained by considering their chemical structures (30). The differences in the sizes and types of filler particles are also important factors that may affect the reactivity of the functional monomers. The EQ composite contains a silica–zirconia spherical filler with a diameter of 0.2 lm with their pre-polymerized fillers, whilst the BF composite contains surface prereacted glass-ionomer (SPRG) fillers with an average particle size of 0.8 lm (0.01–4.0 lm). The S-PRG filler particles are prepared via an acid–base (glass-ionomer) reaction between fluoroaluminosilicate glass (SiO2, 3Al2O3•2SiO2, B2O3, Na3AlF6, SrF2, SrCO3) and polyacrylic acid in the presence of water to form a stable glass-ionomer phase on the surface of the multifunctional glass particles (31), thus maintaining their ability to react with acidic functional monomers. The filler particles in the BF composite were much larger than the filler particles in the EQ composite, and accordingly may have more sites for reaction with functional monomers. Therefore, even with residual methyl mercaptan on the resin matrix of the aged composite surface, the S-PRG filler particles may still have provided sites for bonding of the aged composite to the newly added composite via the applied self-etch adhesive. This study indicated that the repair bond strengths of aged composites immersed in methyl mercaptan solutions bonded to newly added composites utilizing self-etch adhesives were lower than those of the control composites (non-immersed). Therefore, it can be concluded that methyl mercaptan remained on the surfaces of the composites, and the S-H groups in the methyl mercaptan acted as chain-transfer agents, resulting in a reduction of the polymerization reaction of the adhesive resins. Further research is needed to

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confirm that these findings are consistent with clinical performance. Acknowledgements – This work was supported in part by a Grantin-Aid for Scientific Research (C) 23592810 and a Grant-in-Aid for Young Scientists (B) 23792186 from the Japan Society for the Promotion of Science. This project was also supported in part by the Sato Fund and by a grant from the Dental Research Center of the Nihon University School of Dentistry, Japan. Conflicts of interest – The authors declare no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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Influence of methyl mercaptan on the repair bond strength of composites fabricated using self-etch adhesives.

The influence of methyl mercaptan on the repair bond strength of composites fabricated using self-etch adhesives was investigated. The surface free-en...
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