The purpose of this study was to evaluate the relationship between changes in the modulus of elasticity and ultimate tensile strength of one-step self-etch adhesives, and their degree of water sorption.
Five one-step self-etch adhesives, Xeno IV (Dentsply Caulk), G Bond (GC Corp.), 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 test and the ultimate tensile strength by microtensile testing. The modulus of elasticity and the ultimate tensile strength were measured in both dry and wet conditions before/after immersion in water for 24 h. Water sorption was measured, using a modification of the ISO-4049 standard. Each result of the modulus of elasticity and ultimate tensile strength was statistically analyzed using a two-way ANOVA and the result of water sorption was 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.
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 .
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.
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 acid-etching 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 , which results in polymer swelling, plasticizing , and weakening of the polymer network . 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 .
It has been shown that water sorption into adhesive polymers is related to the hydrophilicity of adhesives . 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 .
It is well known that hydrophilic constituents such as 2-hydroxyethyl methacrylate (HEMA) increases water sorption . 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 . The hydrophobic nature of constituent monomer in adhesives, such as bis-GMA, MMA, would also be a major factor in decreasing water sorption . Braden and Clarke and Meşe et al. 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.
Materials and methods
Five commercial one-step self-etch adhesives, Xeno IV (XE; Dentsply Caulk; DE, USA), G Bond (G; GC Co.; Tokyo, Japan), Clearfil S 3 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 ).
|Xeno IV (XE; Dentsply Caulk)||UDMA, PENTA, water, acetone, mono-, di-, and trimethacrylate resins, cetylamine hydrofluoride, photo-initiator|
|G Bond (G; GC corp.)||4-MET, phosphate ester monomer, UDMA, acetone, water, micro-filler, photo-initiator|
|Clearfil S 3 Bond (S3; Kuraray Medical)||10-MDP, HEMA, bis-GMA, water, ethanol, silanated colloidal silica, CQ|
|Bond Force (BF; Tokuyama)||Methacryloyloxyalkyl acid phosphate, HEMA, bis-GMA, TEGDMA, Water, isopropyl alcohol, Glass Filler, CQ|
|One-Up Bond F Plus||Adhesive A: MAC-10, MMA, HEMA, water, coumarin dye, metacryloyloxyalkyl acid phosphate|
|(OBF; Tokuyama Corp.)||Adhesive B: multifuntional methacrylic monomer, fluoraluminosilicate glass, photo-initiator (arylborate catalyst)|
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 air-syringe for 10 min at a distance of 15 cm at air pressure of 3.8 kgf/cm 2 until the container stopped losing weight . 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 for the three-point flexural bending test and the microtensile test, and round-shaped 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 (CaSO 4 ) for 24 h to ensure dryness for measurement of the initial mass of such specimens.
Measurement of three-point flexural bending test and microtensile test
After storage for 24 h in the dry condition, twenty dumbbell-shaped 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 (wet-group). 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. Three-point 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.
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:
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:
where F is the tensile force at failure (N), and A is the cross-sectional area of the specimen (mm 2 ).
The ultimate tensile strength (N/mm 2 ) 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 (%Δ E , %ΔUTS).