Factors of glass-ionomer cements influencing the bond strength to resin composites

Factors .of glass-ionomer cements tnfluenctng the bond strength to resin composites K. Hinoura* H, Suzuki J. Yoshimura H. Onose

R

Department of Operative Dentistry Nihon University School of Dentistry 1-8-13, Kanda-Surugadai Chiyoda-ku Tokyo, 101, Japan Received May 12, 1989 Accepted August 11, 1989 *To whom correspondence and reprint requests should be addressed Dent Mater 6:94-98, April, 1990

Abstract-The bond strength between composites and various particle sizes of glass-ionomer cetnents was investigated. The best bond strength was obtained after use of a small-particle cement and with the highest powder-to-liquid ratio employed for mixing the cement. In addition, use of a small-particle cement and the highest powder:liquid ratio produced cements with significantly stronger tensile strengths. Failure usually occurred within the cement. Consequently, the recorded bond strength actually reflected the tensile strength of the relevant cement.

94 HINOURA

esin composites are widely used as r e s t o r a t i v e m a t e r i a l s because of their physical properties and e s t h e t i c s . H o w e v e r , composites might not be mild to the pulp (Wennberg et al., 1983; MjSr and Wennberg, 1985) and may permit marginal leakage, especially around the celwical margin (Br~innstrSm and Nyborg, 1973; Hembree, 1986; Hinoura et al., 1988). On the other hand, glass-ionomer cements have certain characteristics that are attractive to the dentist. For example, they bond adhesively to both enamel and dentin (Powis et al., 1982; Hinoura et al., 1986), release fluoride ions over a prolonged period of time (Swartz et al., 1984), are biocompatible (Kawahara et al., 1979; Pameijer et al., 1981), and have approximately the same coefficient of thermal expansion as enamel. However, such cements usually have s o m e w h a t inferior e s t h e t i c s and abrasion resistance as compared with resin composite. This has led to the development of the so-called "sandwich technique" in which the cement is acid-etched to permit the overlying composite to bond to it, thus taldng advantage of certain qualities of each material (McLean et al., 1985; Hinoura et al., 1987a). This procedure takes advantage of the adhesive properties and biocompatibility of the glass-ionomer cement and the superior surface and esthetics of the composite. It has been reported that etching the surface of glass-ionomer cement markedly increased the bond strength to the bonding a g e n t / c o m p o s i t e (Sneed and Looper, 1985; Hinoura et al., 1987a). Properly treated glassionomer s u r f a c e s r e s u l t in bond strengths to composites and theh" bonding agents comparable to the strengths reported between glassionomer cements and dentin. The matrix of the hardened glass-ionomer cement dissolves in acid, re-

et

al./RESIN COMPOSITE BOND TO GLASS-IONOMER CEMENT

sulting in a rough and porous sm'face. The bonding agent then penetrates the surface irregularities and hardens. There are many factors which influence the bond strength values. A previous study reported that the best bond strength was obtained when the lowest-viscosity bonding agent was used (Hinoura et al., 1987b). The purpose of this study was to evaluate the effects of various particle sizes of glass-ionomer cement and the powder-to-liquid ratio on the bond strength between glass-ionomer cement and resin composites. MATERIALS AND METHODS The bond strength was determined by subjecting cylinders of pairs of the materials to a tensile-type stress. The mold and technique used for these experiments have been described by Hinoura et al. (1987a). A two-part Teflon mold, 12 mm high and 4 mm in diameter, was used to form and hold the cement and the resin. A 45degree chamfer was cut at one end of each half of the mold to retain the materials during loading, as shown in Fig. 1. Three experimental glass-ionomer cements were employed (Table 1). The particle sizes of the cements were 2.3 ~m, 2.7 ~m, and 3.2 ~m. The glass-ionomer cements were mixed on a paper pad, inserted into the mold, and placed against a fiat glass plate on the end opposite the chamfer. After the glass-ionomer cement had set for seven min at 23 +_ 1°C, the flat surface of the cement was etched with 37% phosphoric acid for 60 s, washed with tap water for 20 s, and air-dried. To determine the effect of the powder-to-liquid ratio, we used small-particle cement. The o t h e r mold half was then aligned, filled with cement, and held in position with plastic electrical tape. Five composites were employed (Table 2). For measuring the effects of

II

]

•

~

tensile strength itself

50

C

8 *

tensile bond strength to Silux

kg/cm 2

A -'r---

I

40

30

20

D 10

A

4mm

small

medium

large particle size

Fig. 2. Effect of the particle sizes on the tensile bend strength to Silux and the tensile strength of cement.

Fig. 1. Cross-section of the apparatus used to measure the tensile bond strength. (A) Threaded cap with attached wires for insertion in grip of the testing machine. (B) Two-component mold with a 30° chamfer. (C) Glass-ionomer cement. (D) Composite.

TABLE 1

particle sizes and powder-to-liquid ratio, we used Silux and its respective bonding agent, Scotchbond. The bonding agents were placed with a small sponge over the cement and light-cm-ed for 20 s with Optilux (3M). The composites were then added in three increments, each of which was cured for 30 s. The finished specimens were allowed to set for one hour, and then stored in distilled water at 37°C. After 24 hours, the elhctl"ical tapes were r e m o v e d , and t h e molds

EXPERIMENTALGLASS-IONOMERCEMENTS EVALUATED Average Particle Mixing Ratio Size Powder/Liquid Small-particle size 2.3 ~m 1.45 g/1.0 g Medium-particle size 2.7 ~m 1.45 g/1.0 g Large-particle size 3.2 #m 1:45 g/1.O g Manufacturer: GO, Japan

screwed into the threaded caps that attached them to the testing machine. Each test group consisted of 10 specimens. Tensile bond strengths were determined in an Instron Testing Machine (Instron, Canton, MA) at a cross-head speed of 0.5 mm p e r min.So that the cohesive strength of glass-ionomer cements could be determined, the tensile strength of 24hour-old specimens was determined by the application of a tensile load to cylinders of the materials. The mold

TABLE 2

COMPOSITESAND BONDINGAGENTS USED Composite Silux P-30 Prisma-fil Ful-fil Occlusin

Bonding Agent Scotchbond Scotchbond Prisma bond Prisma bond ICl bond

Manufacturer 3M Co. 3M Co. L.D. Caulk L.D. Caulk ICl

Batch No. 6L3 6P3 022855 081782 LH06

Dental Materials~April 1990 95

~

tensile bond strength to Silux

~

tensile strength itself

face profile tracings of etched experimental cements with the diamond stylus traveling at 0.8 mm/s. The etched surfaces were also examined by scanning electron microscopy. RESULTS

kg/cm 2 50

40

30

20

10

1.30

1.45

1.60

powder to liquid ratio Fig. 3. Effect of the powder-to-liquid ratio on the bond strength to Silux and the tensile strength of cement.

and technique used for these experiments have been described by Phillips et al. (1968). The tensile strengths were determined by use of a mechanical testing machine (Instron,

Canton, MA) operating at a crosshead speed of 0.5 mm p e r min. A Surfcom (Tokyo Seimitsu, 110B) was used to obtain the a v e r a g e roughness measurements and sur-

TABLE 3

TABLE 4

EFFECTOF THE PARTICLESIZES ON THE TENSILE BOND STRENGTHTO SILUX AND THE TENSILE STRENGTHOF CEMENT Bond Tensile Particle Strength Strength Size (kg/crn2) (kg/crn2) Small 42.6 (11.14) 37.2 (6.23) Medium 28.4 (6.31) I 29.5 (6.26) I Large 31.1 (6.49) I 29.5 (5.96) I ():SD. Values connected by vertical lines are not significantly different (p > 0.05).

EFFECTOF THE POWDER-TO-LIQUIDRATIO ON THE BOND STRENGTHTO SILUX AND THE TENSILE STRENGTHOF CEMENT Bond Tensile P/L Strength Strength Ratio (kg/cm2) (kg/cm2)

Table 3 and Fig. 2 show the tensile bond s t r e n g t h b e t w e e n Silux/ Scotchbond and three different-particle-sized experimental glass-iono m e r c e m e n t s and the tensile strength of the experimental cemerits. The results were analyzed by analysis of variance (ANOVA) followed by a Student's t test with critical values at 0.05. Values connected by vertical lines are not significantly different (p > 0.05). The best bond strength (42.6 kg/cme) was obtained with the small-particle-sized cement, followed by the large-particlesized cement and then the mediumparticle-sized cement, which was the lowest. There was no significant difference between the bond strengths achieved by the medium-particle and the large-particle cements. Failures usually occurred within the cement. The small-particle cement exhibited a significantly stronger tensile strength, followed by the mediumparticle cement and then the largeparticle cement, which was the weakest. The tensile strengths of the medium-particle and large-particle cements were not significantly different (Table 3). The effect of the powder-to-liquid ratio of small-particle cement on the bond strength between small-particle cement and Silux is shown in Table 4 and Fig. 3. A P/L ratio of 1.60 produced cements with significantly higher tensile bond strengths and tensile strengths. The bond strength and tensile strength values obtained TABLE 5

1.30

37.0 (5.77) I'

35.8 (8.43)1

1.45 42.6 (11.14) I 37.2 (6.23) I 1.60 54.5 (9.95) 49.2 (5.38) ( ): SD. Values connected by vertical lines are not significantly different (p > 0.05).

96 HINOURA et alJRESIN COMPOSITE BOND TO GLASS-IONOMER CEMENT

BOND STRENGTHBETWEENSMALLPARTICLE CEMENTAND FIVE COMPOSITES Bond

Strength (kg/cm2) 42.6 (11.14) 43.1 (8.25) 42.8 (8.57) 49.4 (10.36) 46.3 (8.59)

Composite Silux e-30 Prisma-fil Ful-fil Occlusin ():SD. Values connected by vertical lines are not significantly different (p > 0.05).

after use of P/L ratios of 1.30 and 1.45 were lower and not statistically different. The tensile bond s t r e n g t h s between small-particle cement and five composites are listed in Table 5. The best bond strength was obtained with Ful-fifl, but there was no significant difference among the five. The arithmetic average roughness values for the etched experimental cements are recorded in Table 6. The surface of the large-particle cement was significantly rougher than that of the others. The profile tracings of the etched surfaces are shown in Fig. 4. The large-particle cement showed considerably rougher surfaces than did the others. Scanning electron micrographs of the etched surfaces are shown in Fig. 5. DISCUSSION

It is interesting to note that the best bond strength was obtained with the small-particle cement which exhibited a significantly higher tensile strength. The tensile strengths of experimental glass-ionomer cements and the surface roughness of etched cements showed considerable variation. Mowery et al. (1987) compared the effect of the surface roughness on the bond strength between dentin and composites, and reported that increased surface roughness significantly increased the bond strength. Since the surface of the etched smallparticle cement was not rougher than that of the other etched cements, the tensile strength of the cement itself reflects the bond strength. Surface roughness was not a significant factor. There would appear to be a marginal increase in the bond strength when compared with the strength of the cements alone, but this is not statistically significant. It has been suggested that there may be chemical as well as mechanical bonds between etched glass-ionomer cement and composites, but in this study,

small particle

medium particle

I

l large particle

..-1

4,O01Wm

Fig. 4. Surface roughnessof etched experimentalglass-ionomercement.

TABLE 6

SURFACEROUGHNESSOF ETCHED EXPERIMENTALGLASS-IONOMERCEMENTS Small particle Medium particle Large particle

Ra (~m) 1.82 2.08 3.20

SD 0.28 0.22 0.50

Fig. 5. SEM photomicrographsof etchedsurfacesof experimentalglass-ionomercements.

Dental Materials~April 1990 97

cohesive failure of the cement generally occurred; thus, it is unlikely that there was any enhancement from a chemical element in the bond. It is interesting to note that significantly higher tensile strengths were obtained with the highest P/L ratio. The tensile strength of the cement could be the most important factor for achieving a successful bond, and this varies with the powder-toliquid ratio. Any reduction in the amount of powder incorporated would result in a reduction in both the tensile s t r e n g t h and tensile bond strength. There are many factors which can influence the bond strength values, such as wettability, viscosity, and contact angle. Another factor is the presence or absence of any chemical bonding mechanism between the bonding agent and the cement. Hinoura et al. (1987b) reported that the consistency of a resin was the most important factor influencing bond strength between composites and glass-ionomer cement, and that the best bond strength was obtained when the lowest-viscosity bonding agent was used. In the present study, no significant difference was found among five composites on the bond strength to the small-particle cement. Scotchbond and Prisma-bond are both low-viscosity b o n d i n g agents. Consequently, the bonding agent was not an effective factor in the present study, since acid-etching of the cement was always followed

by application of a low-viscosity bonding agent. Failures usually occun'ed cohesively in the cement; thus, the tensile strength of the cement itself is of primary importance.

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