Relationship between mechanical properties of one-step self-etch adhesives and water sorption

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

available at www.sciencedirect.com

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

Relationship between mechanical properties of one-step self-etch adhesives and water sorption Keiichi Hosaka a,∗ , Masatoshi Nakajima a , Masahiro Takahashi a , Shima Itoh a , Masaomi Ikeda b , Junji Tagami a,c , David H. Pashley d a

Cariology and Operative Dentistry, Department of Restorative Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan b School of Dental Technology, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan c Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University Japan d Department of Oral Biology, School of Dentistry, Medical College of Georgia, 1120 15th Street, CL2112 Augusta, GA, 30907, USA

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. The purpose of this study was to evaluate the relationship between changes in

Received 29 January 2009

the modulus of elasticity and ultimate tensile strength of one-step self-etch adhesives, and

Received in revised form

their degree of water sorption.

30 July 2009

Methods. Five one-step self-etch adhesives, Xeno IV (Dentsply Caulk), G Bond (GC Corp.),

Accepted 9 December 2009

Clearfil S3 Bond (Kuraray Medical Inc.), Bond Force (Tokuyama Dental Corp.), and One-Up Bond F Plus (Tokuyama Dental Corp.) were used. Ten dumbelled-shaped polymers of each adhesive were used to obtain the modulus of elasticity by the three-point flexural bending

Keywords:

test and the ultimate tensile strength by microtensile testing. The modulus of elasticity and

One-step self-etch adhesives

the ultimate tensile strength were measured in both dry and wet conditions before/after

Polymers

immersion in water for 24 h. Water sorption was measured, using a modification of the

Modulus of elasticity

ISO-4049 standard. Each result of the modulus of elasticity and ultimate tensile strength

Ultimate tensile strength

was statistically analyzed using a two-way ANOVA and the result of water sorption was

Water sorption

statistically analyzed using a one-way ANOVA. Regression analyses were used to determine the correlations between the modulus of elasticity and the ultimate tensile strength in dry or wet states, and also the percent decrease in these properties before/after immersion of water vs. water sorption. Results. In the dry state, the moduli of elasticity of the five adhesive polymers varied from 948 to 1530 MPa, while the ultimate tensile strengths varied from 24.4 to 61.5 MPa. The wet specimens gave much lower moduli of elasticity (from 584 to 1073 MPa) and ultimate tensile strengths (from 16.5 to 35.0 MPa). Water sorption varied from 32.1 to 105.8 g mm−3 . Significance. The moduli of elasticity and ultimate tensile strengths of the adhesives fell significantly after water-storage. Water sorption depended on the constituents of the adhesive systems. The percent decreases in the ultimate tensile strengths of the adhesives were related to water sorption, while the percent reductions in the moduli of elasticity of the adhesives were not related to water sorption. © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗

Corresponding author. Tel.: +81 3 5803 5483; fax: +81 3 5803 0195. E-mail address: [email protected] (K. Hosaka). 0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2009.12.007

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

1.

Introduction

Recently, the clinical use of one-step self-etch adhesives has increased. The products contain water and hydrophilic monomers as ingredients in order to promote effective acidetching and resin–dentin bonding. Consequently, they are intrinsically hydrophilic owing to the presence of acidic and highly polar functional groups substituted on methacrylates. These hydrophilic adhesives tend to rapidly absorb water [1–3], which results in polymer swelling, plasticizing [2,4], and weakening of the polymer network [4,5]. It is possible that these changes would make resin–dentin interface created by hydrophilic one-step self-etch adhesives much unstable over time. Since the water sorption of adhesives within hybrid layer could affect the long-term durability of resin–dentin bond, the influence of water sorption on the mechanical properties of resins must be understood [6–8]. It has been shown that water sorption into adhesive polymers is related to the hydrophilicity of adhesives [2,4,9,10]. According to these reports, it seems that the more hydrophilic the adhesives are, the more water their polymers absorb. It has also been reported that water sorption by hydrophilic resins contributes to the commonly observed decrease in their mechanical properties [4,11]. It is well known that hydrophilic constituents such as 2-hydroxyethyl methacrylate (HEMA) increases water sorption [12]. The acidic in carboxylated or phosphate-derivatized methacrylates, which are polar, would increase the water sorption and tend to increase initial bond strength to dentin [13]. The hydrophobic nature of constituent monomer in adhesives, such as bis-GMA, MMA, would also be a major factor in decreasing water sorption [14]. Braden and Clarke [15] and Mes¸e et al. [16] reported lower water sorption in resins that contain higher filler volumes since such resins contain less for water sorption. Although more and more one-step self-etch adhesives have been marketed, the relationship between water sorption and the mechanical properties of one-step self-etch adhesives has not been well understood. The purposes of this study were to evaluate the modulus of elasticity and the ultimate tensile strength of five contemporary one-step self-etch adhesives after polymerization, while dry or after water sorption, and correlate these changes to the amount of water sorption that occurs after immersion in water. Finally, the quantitative relationship between the moduli of elasticity and the ultimate tensile strength of five one-step adhesives and water sorption was examined. The null hypotheses tested were that water sorption does not decrease the moduli of elasticity or the ultimate tensile strength of one-step self-etch adhesives; and there is no relationship between reduction in these mechanical properties after immersion of water and the water sorption values of one-step self-etch adhesives.

2.

361

Materials and methods

Five commercial one-step self-etch adhesives, Xeno IV (XE; Dentsply Caulk; DE, USA), G Bond (G; GC Co.; Tokyo, Japan),

Fig. 1 – Schematic of the shape of the specimens used to measure the modulus of elasticity (E) and the ultimate tensile strength (UTS) of dry vs. wet polymer specimens.

Clearfil S3 Bond (S3; Kuraray Medical Inc.; Tokyo, Japan), Bond Force (BF; Tokuyama Dental Corp.; Tokyo, Japan), and One-Up Bond F Plus (OBF; Tokuyama Dental Corp.; Tokyo, Japan) were used in this study (Table 1).

2.1.

Specimen preparation

One milliliter of each adhesive was placed in a tared, wide, round and flat container (9.0 cm in diameter). The initial weight of the solvated adhesives measured on an analytical balance to the nearest 0.1 mg. In subdued light, the solvents of each adhesive were evaporated with a 3-way dental airsyringe for 10 min at a distance of 15 cm at air pressure of 3.8 kgf/cm2 until the container stopped losing weight [11]. When the mixture reached a constant mass, volatile solvent evaporation was assumed to be complete. After the evaporation of the solvents, the adhesives were poured into dumbbell-shaped (10 mm long × 0.5 mm thick, Fig. 1) silicone molds with a gauge length of 5 mm [11] for the three-point flexural bending test and the microtensile test, and roundshaped silicone molds (8.0 mm in diameter and 1.5 mm in thick) for the water sorption measurement. These molds were positioned on glass slabs and covered by a transparent thin Mylar film and another glass slab. The adhesive in these molds was irradiated for 180 s with a light-curing unit (XL3000, 3M ESPE, St. Paul, MN, USA) with a light output > 600 mW cm−2 . Next, the glass slab and Mylar film were carefully peeled off. Twenty dumbbell-shaped polymerized adhesives and 10 round-shaped resin disks for each adhesive were prepared. After polymerization, all specimens (dumbbell-shaped specimens and resin disks) were stored in a container filled with anhydrous calcium sulfate (CaSO4 ) for 24 h to ensure dryness for measurement of the initial mass of such specimens.

362

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

Table 1 – Chemical composition of the materials tested in the study. Material Xeno IV (XE; Dentsply Caulk) G Bond (G; GC corp.) Clearfil S3 Bond (S3; Kuraray Medical) Bond Force (BF; Tokuyama) One-Up Bond F Plus (OBF; Tokuyama Corp.)

Composition UDMA, PENTA, water, acetone, mono-, di-, and trimethacrylate resins, cetylamine hydrofluoride, photo-initiator 4-MET, phosphate ester monomer, UDMA, acetone, water, micro-filler, photo-initiator 10-MDP, HEMA, bis-GMA, water, ethanol, silanated colloidal silica, CQ Methacryloyloxyalkyl acid phosphate, HEMA, bis-GMA, TEGDMA, Water, isopropyl alcohol, Glass Filler, CQ Adhesive A: MAC-10, MMA, HEMA, water, coumarin dye, metacryloyloxyalkyl acid phosphate Adhesive B: multifuntional methacrylic monomer, fluoraluminosilicate glass, photo-initiator (arylborate catalyst)

Abbreviations—UDMA: urethane dimethacrylate; PENTA: dipentaerythritol penta-acrylate phosphate; 4-MET: 4-methacryloxyethyl trimellitic acid; 10-MDP: methacryloloxydecyl dihydrogenphosphate; HEMA: 2-hydroxyethyl methacrylate; bis-GMA: bis-phenol A diglydidylmethacrylate; CQ: camphoroquinone; MAC-10: 11-methacryloyloxy-1,1-undecanedicarboxyric acid; MMA: methyl methacrylate; MMA: methylmethacrylate; TEGDMA: triethylene glycol dimethacrylate

2.2. Measurement of three-point flexural bending test and microtensile test After storage for 24 h in the dry condition, twenty dumbbellshaped specimens of adhesives were divided into two groups. Dry specimens (n = 10) were stored in a dry condition for an additional 24 h (dry-group). The rest of the specimens (n = 10) were immersed in distilled water for an additional 24 h (wetgroup). All dumbbell-shaped specimens were subjected to three-point flexural bending test for measuring modulus of elasticity (E). For wet specimens, the modulus of elasticity was measured after the water around was blot-dried. Threepoint flexural bending test was performed with a miniature three-point bending aluminum device consisting of a supporting base with a 5 mm span and a loading piston. Three-point flexure was measured by centrally loading the polymer specimens using a material testing machine (Vitrodyne V 1000; John Chatillon & Sons, Greensboro, NC, USA) and a displacement rate of 0.6 mm min−1 , sufficient to induce a 3% strain. The compressive force necessary to induce a 3% strain in resin was measured with either a 2.5 N (for dry specimens) or 1 N (for wet specimens) load cell (Transducer Techniques, Temcula, CA, USA). Load–displacement values were converted to stress and strain. The modulus of elasticity values was calculated as the slope of the linear portion of stress–strain curve from the following formula.

E=

FL3 4Dbh3

(1)

where F is the force (N), L is the span length (5.0 mm), b is the width of test specimens (1.0 mm ± 0.1 mm), D is the vertical deflection (mm) of the specimen, and h is the thickness (0.5 mm). Modulus of elasticity was expressed in MPa. The strain (ε) produced a three-point bending, was calculated as: ε=

6hd L

(2)

where h is the thickness of the beam (mm), d is displacement of the beam (mm), L is the span length of the beam between the supports (5 mm), ε is strain %.

After the three-point flexural test was completed, both dry and wet specimens were subjected to tensile stress for measuring ultimate tensile strength. The dumbbell-shaped specimens were attached to the tensile testing jig of universal testing machine (Vitrodyne V 1000) with a cyanoacrylate adhesive (Zapit, Dental Ventures of America, Corona, CA, USA) and pulled to failure at a cross-head speed of 0.6 mm min−1 . The ultimate tensile strength of the adhesive was calculated as: UTS =

F A

where F is the tensile force at failure (N), and A is the crosssectional area of the specimen (mm2 ). The ultimate tensile strength (N/mm2 ) was expressed in MPa. Differences in the moduli of elasticity and the ultimate tensile strengths of the specimens between with and without immersion of specimens in water were calculated as percent decreases of those properties (%E, %UTS). %E = −

Ewet − Edry

%UTS = −

Edry

× 100

UTSwet − UTSdry UTSdry

× 100

Ewet is the modulus of elasticity of wet specimens. Edry is the modulus of elasticity of dry specimens. UTSdry is the ultimate tensile strength of dry specimens. UTSwet is the ultimate tensile strength of wet specimens.

2.3.

Water sorption

Water sorption was determined according to the ISO specification 4049 (7-2000), except for specimens’ dimensions and period of water immersion. Resin disks were used in this test. Immediately after polymerization and storage for 24 h in the dry state, the specimens were weighed until a constant mass (M1 ) was obtained. Thickness (1.5 mm) and diameter (8 mm) of the specimens were measured using a digital caliper, rounded to the nearest 0.01 mm, and these measurements were used to calculate the volume (V) of each specimen (in mm3 ). The specimens were individually immersed in distilled water at 37 ◦ C for the water sorption. After 24 h, the resin disks were

363

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

Table 2 – Modulus of elasticity and ultimate tensile strength in dry and wet polymers of adhesives and water sorption of one-step self-etch adhesive polymers. Modulus of elasticity (MPa)

Ultimate tensile strength (MPa)

Water sorption (␮g/mm3 )

Adhesives

Group

Xeno IV

Dry Wet

1466.8 (323.5)A 1072.8 (232.7)a

53.5 (8.8)A 35.0 (6.5)a

46.4 (8.6)P

G Bond

Dry Wet

948.1 (251.9)B 620.4 (153.1)b

24.4 (3.6)B 17.3 (3.9)b

32.1 (3.7)Q

S3 Bond

Dry Wet

1218.5 (445.8)C 594.2 (79.9)b

44.5 (5.3)C 16.5 (0.9)b

78.8 (14.5)R

Bond Force

Dry Wet

1530.7 (215.4)D 914.8 (140.9)c

61.5 (8.5)D 26.7 (4.1)c

98.8 (3.7)S

One-Up Bond F Plus

Dry Wet

1364.2 (163.8)C 583.8 (149.9)b

43.4 (7.4)C 18.0 (2.8)b

105.8 (10.3)S

Values are mean ± SD. Different superscript letters indicate statistically significant differences in the same column (p < 0.05). Upper case letters refer to dry specimens and lower case letters refer to wet specimens. Superscript letters P–S refer to water sorption data.

gently wiped with absorbent paper, weighed, and restored in distilled water. Water sorption (WS) was calculated using the following formula: WS =

M2 − M1 V

where M1 is the initial dry constant mass (␮g) before water immersion; M2 is the constant wet mass (␮g) after water immersion; and V is the specimen volume in mm3 .

2.4.

Statistic analyses

Each of the moduli of elasticity and ultimate tensile strengths data were analyzed using a two-way ANOVA to test the effect of the adhesive system and the experimental condition (state of hydration—dry or wet). The interaction between the two factors was also analyzed. The Bonferroni test was used for multiple comparisons at the 95% level of confidence. The water sorption data were analyzed by using a one-way ANOVA to determine if there were any statistically significant differences among the adhesives. The Bonferroni test was used for post hoc multiple comparisons at the 95% level of confidence. Regression analyses were used to determine the correlation between the moduli of elasticity values and the ultimate tensile strength values in the dry or wet conditions, and the correlations between both percent decrease of moduli of elasticity (%E) vs. water sorption, and the percent decrease in the ultimate tensile strength (%UTS) vs. water sorption.

3.

Results

A summary of the modulus of elasticity of dry vs. wet of onestep self-etch adhesives is shown in Table 2. The distribution of moduli of elasticity of the one-step polymerized adhesives ranged from 948 MPa for G Bond to 1531 MPa for Bond Force in dry specimens. The specimens tested after immersion in water for 24 h gave the much lower moduli of elasticity values, varying from 584 MPa for One-Up Bond F to 1073 MPa for Xeno IV. Two-way ANOVA revealed that the moduli of elasticity were significantly influenced both by the immersion in water for 24 h (p < 0.001) and by the adhesive system (p < 0.001). There was a significant interaction between two independent

variables (p = 0.024). All adhesive materials were significantly different except for Clearfil S3 Bond and One-Up Bond F in the dry condition. After water sorption, there were no significant differences between Clearfil S3 , G Bond or One-Up Bond F, although there were all significantly lower (p < 0.05) than Xeno IV and Bond Force. The ultimate tensile strengths of dry vs. wet adhesives are shown in Table 2. Two-way ANOVA revealed that the ultimate tensile strength was significantly influenced both by the immersion in water for 24 h (p < 0.001) and by the adhesive system (p < 0.001). The highest ultimate tensile strength values in dry specimens were seen in Bond Force > Xeno IV > Clearfil S3 Bond = One-Up Bond F > G Bond. In wet specimens, the highest ultimate tensile strength values in dry specimens were seen in Xeno IV > Bond Force > One-Up Bond F = G Bond = Clearfil S3 Bond. There was a significant interaction between two independent variables (p < 0.001). There were significant difference (p < 0.05) in the ultimate tensile strength between adhesive materials, except for Clearfil S3 Bond and One-Up Bond F in both dry and wet conditions (Table 2). When the ultimate tensile strength of the adhesives were plotted against their moduli of elasticity, a highly significant relationship was found (Fig. 2, R2 = 0.94, p < 0.05 for dry specimens and R2 = 0.98, p < 0.005 for wet specimens). The water sorption values of all five adhesives polymers were summarized in Table 2. The one-way ANOVA revealed that water sorption was influenced by the adhesive material (p < 0.001). There were significant differences in water sorption between every adhesive except for Clearfil S3 Bond and One-Up Bond F that were not significantly different (p > 0.05). Regression analyses of the percent change in moduli of elasticity vs. water sorption were not significant (R2 = 0.59, p > 0.05, Fig. 3A), while there was highly significant correlation between the percent change in the ultimate tensile strength and water sorption (R2 = 0.90, p < 0.01, Fig. 3B).

4.

Discussion

The results of this study require rejection of the null hypothesis that water sorption does not decrease modulus of elasticity and the ultimate tensile strength of one-step self-etch adhe-

364

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

Fig. 2 – Plot of the ultimate tensile strength of the polymers vs. their modulus of elasticity under dry (round symbol) or wet (square symbol) conditions. Note the high R2 values of 0.94–0.98.

sives. There were large reductions in the modulus of elasticity (i.e. 1.4–2.3 fold) and their corresponding ultimate tensile strengths (1.4–2.4 fold) (Table 2) between dry and wet specimens of all adhesives. These findings were in agreement with the previous studies using experimental adhesives [5,11]. Water can penetrate into nano-meter spaces between polymer chains [17,18] or cluster around polar functional groups that are capable of hydrogen bonding [19] due to their small molecular size and high molar concentration. Water absorbed into resin polymers has been demonstrated to weaken the threedimensional polymer chain network [20], and to decrease glass transition temperatures [21] and stiffness of polymers [4,11]. Consequently, resin polymers become swollen and plasticized [4,20,22–25] and their mechanical properties are lowered [5,11]. Additionally, regression analysis indicated a highly significant correlation between the moduli of elasticity and the ultimate tensile strengths of the contemporary one-step adhesives in dry or wet condition (Fig. 3, R2 = 0.94, p < 0.01 for dry specimens; R2 = 0.98, p < 0.005 for wet specimens) in this study. These results were also in agreement with the results of experimental resins in our previous study [11]. Measurements of the moduli of elasticity of elastic materials measures their stiffness with their elastic range. On the other hand, ultimate tensile strength is the maximum stress at which a material undergoes cohesive failure. These two properties do not depend on the size of the tested specimens. The stiffness and maximum tensile strength of same adhesive provide information on the elastic vs. cohesive mechanical properties. Takahashi et al. reported that there was a positive relationship between the ultimate tensile strength of adhesives and microtensile bond strength [26]. The results indicated that water sorption differed among the adhesive systems (Table 2). The higher water sorption values were obtained in HEMA-containing adhesives, they included, Clearfil S3 Bond (S3) (78.8 ␮g mm−3 ), Bond Force (BF) (98.8 ␮g mm−3 ) and One-Up Bond F Plus (OBF) (105.8 ␮g mm−3 ), compared to the HEMA-free adhesives, Xeno IV (XE) (46.4 ␮g mm−3 ) and G Bond (G) (32.1 ␮g mm−3 ). HEMA

Fig. 3 – Correlations between the reduction of modulus of elasticity vs. water sorption of the polymers. (A) Plot of the percent decrease of modulus of elasticity of the polymers vs. their water sorption. (B) Plot of the percent decrease of ultimate tensile strength of the polymers vs. their water sorption.

has been frequently added to bonding resins for its positive influence on the bond strength to dentin [13]. As HEMA serves as a solvent for less miscible dimethacrylate monomers and generally improves miscibility of hydrophobic and hydrophilic components in adhesive blends [27–29], it facilitates the solubility of hydrophobic monomers in adhesives and prevents phase separation [28,30]. However, HEMA increases the hydrophilicity of adhesives, resulting in increase of water sorption of adhesive copolymers [4,10]. As HEMA lowers the vapor pressure of water, high amount of HEMA in adhesives may hinder good solvent evaporation from adhesive solutions [31]. Since HEMA-containing adhesives contain some amount of water or ethanol after polymerization, which further increases the hydrophilicity of polymers, it is suspected that HEMA-containing adhesive polymers promote additional water sorption. Although HEMA-containing systems absorbed more water than the HEMA-free systems in this study, regression analysis indicated that water sorption could only predict 59% of the decrease in the moduli of elas-

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

ticity of the adhesives (Fig. 3A). This suggests that there are additional influences such as other polar constituents, microor nano-fillers, hydrophilic and hydrophobic monomers of adhesive systems responsible for water sorption. Even if the hydrophobic nature of constituent monomer in adhesives, such as triethylene glycol dimethacrylate and methacryloyloxyalkyl acid phosphate, would also be a major factor in water sorption [14], their polymers also can absorb water [32]. Moreover, it is reported that filler content influence water sorption, that is, less water sorption occurs in resins that contain higher filler volumes [15,16]. HEMA-containing systems, Clearfil S3 Bond, One-Up Bond F Plus and Bond Force, exhibited relatively larger percent decreases in their moduli of elasticity of 40–57% and percent decrease of their ultimate tensile strength of 57–63%, compared to HEMA-free systems, Xeno IV and G bond that showed percent decreases in their moduli of elasticity of 27 and 35%, respectively. Those same adhesives showed smaller percent decreases in their ultimate tensile strength of 29 and 35%, respectively. Before polymerization, HEMA-containing blends can form hydrogels with water. Even after polymerization of HEMA, poly-HEMA attracts water [4] and promotes polyHEMA hydrogels which weakens the mechanical strength of the HEMA-containing adhesive resin itself [8,21]. Paul et al. [5] reported that the modulus of elasticity of the HEMA-rich polymers was so low that they became elastomers instead of polymers. Even after polymerization, HEMA-containing polymers attract water [4]. HEMA is also a powerful allergen [33,34]. Thus, for many reasons, HEMA-free adhesive systems have been introduced in order to reduce the negative influences of HEMA. In addition to HEMA or other functional monomers, One-Up Bond F Plus contains 5–20% MMA as a monomer. Although poly(methyl methacrylate) (PMMA) is known to absorb water [35,36]. It produces lower polymer stiffness [37] because it only forms linear polymers that cannot cross-link polymer chains. One-Up Bond F Plus also contains water that is hydrogen bonded to glass ionomer fills and other constituents of the adhesives. This might be the reason why that adhesive showed the largest water sorption and the largest reductions in modulus of elasticity and ultimate tensile strength when measured before and after water sorption. Water, a necessary solvent for ionizing self-etching monomer, is not miscible with many constituents of selfetching adhesive and hence is combined with acetone used in Xeno IV and G Bond, or with alcohol in Clearfil S3 Bond, One-Up Bond F Plus and Bond Force. Recent work indicates both acetone and ethanol evaporate faster than water because they have higher vapor pressures. Their evaporation increases the concentration of monomers in the adhesives to between 2 and 3.9 mol/L [38] which lowers the vapor pressure of the remaining residual solvents, especially water, making it more difficult to evaporate residual water [28,38]. It has reported that “water sorption” depends on the type of solvents in resin blends [16]. Generally, residual solvents are displaced by water during water sorption. The presence of residual water in comonomer mixtures increases the free volume of polymers and can promote water sorption even after its evaporation prior to water immersion. These factors could not be studied in the current study because the quantitative concentrations of the constituents of these commercial adhesives (i.e. acetone,

365

ethanol, water content, percent of HEMA, or acidic monomers vs. dimethacrylates) are proprietary secrets. G bond contains 35–45% acetone as a solvent, theoretically, resulting in a better evaporation of water due to the “azeotrope effect” [39,40] or “water-chasing effect” [41] of acetone. The presence of water droplets in the adhesive argues against acetone being very effective in promoting water evaporation. Moreover, for G Bond, a strong air-drying procedure is clinically recommended in order to eliminate residual droplets. Consequently, the manufacturer’s position is that strong, aggressive air-drying of G Bond just prior to polymerization should increase the mechanical properties of the adhesive layer. However, it is possible those procedures might over thin the adhesive layer that could become fully saturated with oxygen [42] and that result in destroy-free radicals generated during light-curing, resulting in incomplete resin polymerization. Aggressive air-drying may also evaporate water from dentinal fluid [43] so rapidly that it causes aspiration of odontoblasts into dentinal tubules [44]. There are many surfaces in complex cavity preparations where aggressive air-drying is not possible. Thus, the manufacture’s recommendations for their bonding technique seem to introduce technique-sensitivity into a “simplified adhesive system”. In order to evaluate the relationship between reduction in modulus of elasticity and ultimate tensile strength of the adhesives and water sorption, we performed regression analysis between these values and water sorption. Highly significant R2 values were obtained (R2 = 0.90) between the percent reduction in ultimate tensile strength and water sorption (Fig. 3B), meaning that the equation predict 90% of the experimental results. These results require rejection of the second null hypothesis that there is no correlation between moduli of elasticity and ultimate tensile strength, and water sorption values of one-step self-etch adhesives. It was suggested that water sorption strongly affected the decrease of ultimate tensile strength. However, the correlation between the percent decrease in moduli of elasticity and water sorption did not quite reach statistical significance (R2 = 0.59, p > 0.05, Fig. 3A) and could only account for 59% of the experimental data. These results might have been influenced by the difference in the thickness of the specimens used for measuring moduli of elasticity and water sorption (i.e. dumbbell-shaped specimens and resin disks). Since water sorption is diffusionlimited, the time necessary to diffuse 0.25 mm from each surface of the dumbbell-shaped specimens may have occurred within one day, whereas the 1.5 mm thick disks may not have been completely saturated with water. It is expected that further research using smaller specimens and more adhesives might reveal positive correlation between water sorption and decrease in the modulus of elasticity of adhesives. The water sorption and solubility values of bonding agents have been reported to be much higher than resin composite filling materials [45]. Water sorption and solubility of polymerized adhesives in a hybrid layer and in the overlying adhesive layer could lead not only to marginal discoloration, but also to decreased mechanical properties of the resin–dentin interface and possibly to compromised restoration longevity. In this study, the moduli of elasticity and ultimate tensile strengths of adhesives were measured after immersion in water for 24 h. Reis et al. [46] reported that water uptake (i.e. water sorption

366

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

and solubility) of one-step self-etch adhesives continued to increase over 24 h. Although the specimens used in this study were small, the percent decreases of mechanical properties could be even higher over longer time period. Since it has been confirmed in this study that the more the adhesive absorbed water, the more the ultimate tensile strength of the adhesives decreased, it is speculated that ultimate tensile strength of one-step adhesives would decrease and affect the stability of resin–dentin bonding over time after the adhesive absorb water for 24 h. Also, the solubility of these polymers would be expected to increase even after water sorption reaches a plateau [46]. Both of the processes could lower the mechanical properties of polymerized adhesives. We propose that the time-dependent reductions of resin–dentin bond strength created by one-step self-etch adhesives occur as a result of water sorption-induced decreases in the mechanical properties of resins. It is anticipated that the water sorption and subsequent reduction of mechanical properties may result in poor load transfer across the bonded interface over time. If the adhesive layer coupling resin composite to hybridized dentin becomes less stiff and/or weaker over time, it may adversely affect stress distribution across the bonded interface possibly resulting in plastic deformation or debonding under repeated loading. However, it must be mentioned that the rate of reduction of these mechanical properties reported in this in vitro study would be expected to occur more slowly in clinical resin-bonded interfaces that are protected from water by the underlying dentin and the overlying resin composite that would greatly restrict free water diffusion. Clearly, the long-term studies on the durability of resin–dentin bonds are needed to evaluate the longevity of bonds created by one-step self-etch adhesives.

5.

Conclusion

Within the limitations of the present study, the following conclusion can be drawn:

1. Water sorption for 24 h lowered the moduli of elasticity and ultimate tensile strengths of all five one-step adhesives. 2. Water sorption of one-step self-etch adhesives depended, in part, on the HEMA-concentration and other hydrophilic constituents of the adhesive systems. 3. The reduction in ultimate tensile strength (% decrease of UTS) of one-step self-etch adhesives was significantly correlated with water sorption values.

Acknowledgements This study was supported by two grants, Grant-in-Aid for Young Scientists (B) (#20791382) from Ministry of Education, Culture, Sports, Science and Technology of Japan and the grant from Ministry of Education, Culture, Sports, Science and Technology of Japan, and Global Center of Excellence (GCOE) Program, “International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University”.

references

[1] Unemori M, Matsuya Y, Matsuya S, Akashi A, Akamine A. Water absorption of poly(methyl methacrylate) containing 4-methacryloxyethyl trimellitic anhydride. Biomaterials 2003;24:1381–7. [2] Malacarne J, Carvalho RM, De Goes MF, Svizero N, Pashley DH, Tay FR, et al. Water sorption/solubility of dental adhesive resins. Dent Mater 2006;22:973–80. [3] Yiu CK, King NM, Carrilho MR, Sauro S, Rueggeberg FA, Prati C, et al. Effect of resin hydrophilicity and temperature on water sorption of dental adhesive resins. Biomaterials 2006;27:1695–703. [4] Ito S, Hashimoto M, Wadgaonkar B, Svizero N, Carvalho RM, Yiu C, et al. Effects of resin hydrophilicity on water sorption and changes in modulus of elasticity. Biomaterials 2005;26:6449–59. [5] Paul SJ, Leach M, Rueggeberg FA, Pashley DH. Effect of water content on the physical properties of model dentine primer and bonding resins. J Dent 1999;27:209–14. [6] Inoue S, Koshiro K, Yoshida Y, De Munck J, Nagakane K, Suzuki K, et al. Hydrolytic stability of self-etch adhesives bonded to dentin. J Dent Res 2005;84:1160–4. [7] Hashimoto M, Fujita S, Kaga M, Yawaka Y. In vitro durability of one-bottle resin adhesives bonded to dentin. Dent Mater J 2007;26:677–86. [8] Torkabadi S, Nakajima M, Ikeda M, Foxton RM, Tagami J. Bonding durability of HEMA-free and HEMA-containing one-step adhesives to dentine surrounded by bonded enamel. J Dent 2008;36:80–6. [9] Yiu CKY, Pashley EL, Hiraishi N, King NM, Goracci C, Ferrari M, et al. Solvent and water retention in dental adhesive blends after evaporation. Biomaterials 2005;26:6863–72. [10] Nishitani Y, Yoshiyama M, Hosaka K, Tagami J, Donnelly A, Carrilho M, et al. Use of Hoy’s solubility parameters to predict water sorption/solubility of experimental primers and adhesives. Eur J Oral Sci 2007;115:81–6. [11] Hosaka K, Tagami J, Nishitani Y, Yoshiyama M, Carrilho M, Tay FR, et al. Effect of wet vs. dry testing on the mechanical properties of hydrophilic self-etching primer. Eur J Oral Sci 2007;115:239–45. [12] Burrow MF, Inokoshi S, Tagami J. Water sorption of several bonding resins. Am J Dent 1999;12:295–8. [13] Nakaoki Y, Nikaido T, Pereira PN, Inokoshi S, Tagami J. Dimensional changes of demineralized dentin treated with HEMA primers. Dent Mater 2000;16:441–7. [14] Oysaed H, Ruyter IE. Water sorption and filler characteristics of composites for use in posterior teeth. J Dent Res 1986;65:1315–8. [15] Braden M, Clarke RL. Water absorption characteristics of dental microfine composite filling materials. Biomaterials 1984;5:369–72. [16] Mes¸e A, Burrow MF, Tyas MJ. Sorption and solubility of luting cements in different solutions. Dental Mater J 2008;27:702–9. [17] Soles CL, Chang FT, Bolan BA, Hristov HA, Gidley DW, Yee AF. Contribution of the nanovoid structure to the moisture absorption properties of epoxy resins. J Polym Sci 1998;36:3035–48. [18] Soles CL, Yee AF. A discussion of the molecular mechanisms of moisture transport in epoxy resins. J Polym Sci 2000;38:792–802. [19] Van Landingham MR, Eduljee RF, Gillespie JW. Moisture diffusion in epoxy systems. J Appl Polym Sci 1999;71:787–98. [20] Tay FR, Pashley DH, Yoshiyama M. Two modes of nanoleakage expression in single-step adhesives. J Dent Res 2002;81:472–6.

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 360–367

[21] Yiu CK, King NM, Pashley DH, Suh BI, Carvalho RM, Carrilho MR, et al. Effect of resin hydrophilicity and water storage on resin strength. Biomaterials 2004;25: 5789–96. [22] Montes GGM, Draughn RA. Slow crack propagation in composite restorative materials. J Biomed Mater Res 1987;21:629–42. [23] Ping ZH, Nguyen QT, Chen SM, Zhou JQ, Ding JD. States of water in different hydrophilic polymers—DSC and FTIR studies. Polymer 2001;42:8461–7. [24] Sideridou ID, Achilias DS, Karabela MM. Sorption kinetics of ethanol/water solution by dimethacrylate-based dental resins and resin composites. J Biomed Mater Res 2007;81:207–18. [25] McCabe JF, Rusby S. Water absorption, dimensional changes and radial pressure in resin matrix dental restorative materials. Biomaterials 2004;25:4001–7. [26] Takahashi A, Sato Y, Uno S, Pereira PN, Sano H. Effects of mechanical properties of adhesive resins on bond strength to dentin. Dent Mater 2002;18:263–8. [27] Toledano M, Osorio R, de Leonardi G, Rosales-Leal JI, Ceballos L, Cabrerizo-Vilchez MA. Influence of self-etching primer on resin adhesion to enamel and dentin. Am J Dent 2001;14:205–10. [28] Van Landuyt KL, De Munck J, Snauwaert J, Coutinho E, Poitevin A, Yoshida Y, et al. Monomer-solvent phase separation in one-step self-etching adhesives. J Dent Res 2005;84:183–8. [29] Monszner N, Salz U, Zimmermann J. Chemical aspects of self-etching enamel-dentin adhesives: a systematic review. Dent Mater 2005;21:895–910. [30] Spencer P, Wang Y. Adhesive phase separation at the dentin interface under wet bonding conditions. J Biomed Mater Res 2002;62:447–56. [31] Pashley EL, Zhang Y, Lockwood P, Rueggeberg F, Pashley DH. Effects of HEMA on water evaporation from water–HEMA mixtures. Dent Mater 1998;14:6–10. [32] Sideridou ID, Karabela MM, Vouvoudi ECh. Volumetric dimensional changes of dental light-cured dimethacrylate resins after sorption of water or ethanol. Dent Mater 2008;24:1131–6. [33] Jontell M, Hanks CT, Bratel J, Bergenholtz G. Effects of unpolymerized resin components on the function of

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41] [42]

[43]

[44]

[45]

[46]

367

accessory cells derived from the rat incisor pulp. J Dent Res 1995;74:1162–7. Paranjpe A, Bordador LC, Wang MY, Hume WR, Jewett A. Resin monomer 2-hydroxyethyl methacrylate (HEMA) is a potent inducer of apoptotic cell death in human and mouse cells. J Dent Res 2005;84:172–7. Rose EE, Lal J, Green R, Cornell J. Direct resin filling materials: coefficient of thermal expansion and water sorption of polymethyl methacrylate. J Dent Res 1955;34:589–96. Miettinen VM, Vallittu PK, Docent DT. Water sorption and solubility of glass fiber-reinforced denture polymethyl methacrylate resin. J Prosthet Dent 1997;77:531–4. Kurata S, Morishita K, Shimoyama K, Umemoto K. Basic study on the application of novel functional monomers to a denture base resin. Dent Mater J 2008;27:273–7. Cadenaro M, Breschi L, Rueggeberg FA, Suchko M, Grodin E, Agee K, et al. Effects of residual ethanol on the rate and degree of conversion of five experimental resins. Dent Mater 2009;25:621–8. Morrison RT, Boyd RN. Organic chemistry. Boston: Allyn and Bacon; 1973. Moszner N, Salz U, Zimmermann J. Chemical aspects of selfetching enamel–dentin adhesives: a systematic review. Dent Mater 2005;21(10):895–910. Jacobsen T, Söderholm KJ. Some effects of water on dentin bonding. Dent Mater 1995;11(2):132–6. Rueggegberg FA, Margeson DH. The effect of oxygen inhibition on an unfilled/filled composite system. J Dent Res 1990;69:1652–8. Hashimoto M, Ito S, Tay FR, Svizero NR, Sano H, Kaga M, et al. Fluid movement across the resin–dentin interface during and after bonding. J Dent Res 2004;83:843–8. Lilja J, Nordenvall K-J, Brännström M. Dentin sensitivity, odontoblasts and nerves under desiccated or infected experimental cavities. Swed Dent J 1982;6:93–103. Mortier E, GerdolleDA, Jacquot B, Panighi MM. Importance of water sorption and solubility studies for couple bonding agent—resin-based filling material. Oper Dent 2004;29:669–76. Reis AF, Giannini M, Pereira PN. Influence of water-storage time on the sorption and solubility behavior of current adhesives and primer/adhesive mixtures. Oper Dent 2007;32:53–9.