Importance of Polymerization Initiator Systems and Interfacial Initiation of Polymerization in Adhesive Bonding of Resin to Dentin Y. IMAI, Y. KADOMA, K. KOJIMA, T. AKIMOTO, K. IKAKURA, and T. OHTA Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, 2-3-10 Surugadai, Kanda, Chiyoda-ku, Tokyo 101, Japan
Although various adhesive resins for dentin have been developed and used clinically, most attention has been directed to adhesion-promoting monomers and pre-treatment agents. The role of polymerization initiator systems in bonding has been overlooked. The purpose of this work was to study the role of initiators from the viewpoint of interfacial initiation of polymerization in dentin bonding. The bond strength between dentin and methyl methacrylate resin was significantly improved by a possible interfacial initiation with (1) the combination of ferric chloride, adsorbed onto dentin, and oxidized tri-n-butylborane (TBBO) and (2) the addition of tertiary butyl peroxymaleic acid (containing a carboxylic acid group, which has an affinity with dentin) to chemically- or light-activated initiator systems. J Dent Res 70(7):1088-1091, July, 1991
Introduction. Various adhesive resins for dentin have been developed and used clinically. Recently, more attention has been directed to adhesion-promoting monomers and pre-treatment agents in the research and development of dental adhesive systems. Little attention has been paid to the polymerization initiator system and its effect on the initiation site and the direction of polymerization shrinkage, which in turn may affect bonding. Some polymerization initiator systems seem to play a key role in bonding systems showing strong adhesion to dentin, although their role has not been adequately explained. Even if adhesionpromoting monomers exist at the dentin interface, strong adhesion may not be obtained without suitable polymerization initiators. Initiation of polymerization at the interface is advantageous in the bonding of resins to dentin. The concept of polymerization by interfacial initiation has previously been documented and is found in studies by Masuhara (1969), Misra et al. (1975), Misra and Bowen (1977), Antonucci and Bowen (1977), Bowen et al. (1984, 1987), and Misra and Johnston (1987). The purpose of the present work was to investigate the role of the initiator system from the viewpoint of interfacial initiation of polymerization in dentin bonding. We studied the effects of ferric ion, adsorbed onto etched dentin, on methyl methacrylate/poly(methyl methacrylate) (MMA/PMMA) resin (with and without 4-META) initiated with oxidized tri-n-butylborane (TBBO), and the effects of tertiary butyl peroxymaleic acid (MA) (which contains a carboxylic acid group having an affinity with dentin) as an additive to chemicallyor light-activated initiator systems.
Materials and methods. Materials. - 1,3,5-Trimethyl-2-thiobarbituric acid (TMTB)
was synthesized from diethyl methylmalonate and dimethyl-
thiourea in essentially the same manner as the preparation of 1,3,5-trimethylbarbituric acid (Bredereck et al., 1966). Sodium sulfinates of 4-n-butylbenzene and 4-n-octylbenzene were prepared in a manner similar to the preparation of sodium toluenesulfinate (Whitmore and Hamilton, 1967). Tertiary butyl peroxymaleic acid (MA) was supplied from Nippon Oil & Fats Co., Tokyo, Japan. All other compounds were obtained commercially. The chemical structures of key compounds used are shown in Fig. 1. Measurement of curing time. -The resin pastes, mixed as described below, were placed at room temperature in a polyethylene cup (8 mm inside diameter, and 8 mm high), with a central hole for insertion of the tip of a thermocouple. A sheet of cellophane was placed on top of the cup. Curing time was recorded as the time when the peak temperature was reached. Measurement of bond strength between dentin and experimental resins with initiator systems A, B, and C. -Bovine anterior teeth were wet-sectioned into two parts with a diamond saw (Isomet, Buehler Ltd., Evanston, IL) so as to expose sufficient dentin surface for testing. The dentin surface of the labial side was used for adhesion testing without any further finishing. The dentin surface was pre-treated for 30 s with 10% aqueous citric acid containing 3% cupric chloride, washed with water, dried with an air syringe, and taped with adhesive tape having a 5-mm-diameter hole to control the surface area for adhesion. The above solution has been used as a standard pretreatment agent-along with 10% aqueous citric acid containing 3% ferric chloride-in our laboratory (Kojima et aL, 1982). The pre-treated dentin specimens were then bonded with acrylic resin cements, by use of three types of curing systems, as described below. All the bonded specimens were immersed in distilled water at 370C for 24 h and then tested with a tensile testing machine (Shimadzu Co., Kyoto, Japan) at a cross-head speed of 2 mm/min. Five specimens were tested in each set. Duncan's multiple-range test was used for statistical analysis. (1) Bonding with chemically-initiated system A. -MMA (0.1 g) containing 0.008% cupric acetylacetonate and 0.08% methacryloylcholine chloride was mixed with 0.1 g of PMMA powder containing 2% TMTB and various amounts of MA. Free radicals generated from the initiator system seem to induce decomposition of MA, which leads to polymerization of MMA (Bredereck et al., 1966). The mixed resin paste was applied to the dentin specimen, and an 8-mm-diameter acrylic rod was fixed perpendicularly in the resin cement. The specimen was kept for 30 min in air at room temperature and then immersed in water. (2) Bonding with chemically-initiated system B. -PMMA
0
CH3C-O-0-C-CH=CHCOH
CH, t -butl peroxymaleic acid ( MA )
Received for publication September 7, 1990 Accepted for publication March 4, 1991
1088
CH3
o
CH
,C- N
CH;-CH
,C-N.
on
C=S
'CH,
Rig SONa R = H, CH,
n-
CHN, n - C.H,
1.3.5 -timethyl -
sodium alkytbenzene
2 - thiobarbituric
suffiroates
acid
( TMTB )
Fig. 1-The chemical structures of the key compounds used.
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Vol. 70 No. 7
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ROLE OF INITIATOR IN ADHESION OF RESIN TO DENTIN
powder (0.1 g) was mixed into MMA (0.12 g) containing 1.24 mol% of sodium aryl sulfinate (Yamauchi, 1986) and MA. After the resin paste was applied to the dentin specimen, an acrylic plate with a diameter of 8 mm and a thickness of 3 mm was placed on the resin cement. After immersion in water, an acrylic rod was attached to the acrylic plate with 2-cyanoacrylate adhesive before tensile testing. (3) Bonding with photo-initiated system C. -PMMA powder (0.1 g) containing 1% TMTB was mixed into MMA (0.1 g) containing 1% camphorquinone and various amounts of MA (Kadoma and Imai, 1989). After the resin paste was applied to the dentin specimen, a transparent 3-mm-thick acrylic plate was placed on the paste, allowed to stand for two min, and then irradiated for three min with a visible-light unit (Translux, Kulzer & Co., Germany). The specimen obtained was immersed in water immediately after the irradiation. An acrylic rod was attached to the plate as described above. Measurement of bond strength between dentin and 4-META! MMA-TBBO resin. -The testing method was essentially the same as that described in the preceding section, but with the following modifications. The dentin surfaces, pre-treated with the solutions (Table 1), were brushed with 0.01-1.0% aqueous ferric chloride, allowed to stand for 30 s, and then dried. These dentin specimens were then bonded to an acrylic rod with 4META/MMA-TBBO resin (Super-Bond C&B, Sun Medical Co., Kyoto, Japan) in the manner described in paragraph (1) above.
Results. The effect of ferric chloride post-treatment of acid-etched dentin on the bond strength is summarized in Table 1. No significant difference (p>0.01) in the bond strength was observed between the post-treatment with an appropriate concentration of ferric chloride and the recommended treatment with a mixed solution of 10% citric acid and 3% ferric chloride. TABLE 1
EFFECT OF FERRIC CHLORIDE POST-TREATMENT ON ADHESION OF 4-META/MMA-TBBO RESIN TO DENTIN Concentration Adhesive Strength Pre-treatment of Ferric (MPa) Solution Time (s) Chloride (%)b Meanc S.D. 10% Citric acid-3% FeCl3 (Washed) 12.2 11.6 10% Citric acid-3% FeCl3 (Washed) 10.3d 3.2 10% Citric acid 60 1.0 2.1 _ 0.4 60 0.3 3.2 2.1 60 0.1 9.5 1-1_ 1.2 30 0.075 14.2 3.0 60 0.075 10.0 _ 2.5 60 0.05 9.2 2.0 _ 60 0.025 8.2 _ 1.5 60 0.01 5.8 i 2.2 30 0 2.2 0.6 60 0 2.6 0.7 40% Phosphoric acid 10 0.075 9.6 _ 3.6 10 0 2.4 0.9 i 30 0.075 7.1 2.5 30 0 1.4 0.6 15% EDTA 60 0.075 10.3 _ 4.6 60 0 2.0 _ 0.9 aWashed and dried after pre-treatment. bThe washed specimens were brushed with the solutions and dried after 30 s. cVertical lines join values that were not significantly different at the p>0.01 level. dMMA-TBBO resin without 4-META.
Maximum bond strengths of about 10 MPa were usually obtained by a post-treatment with an appropriate ferric chloride solution, and these were independent of pre-treatment, whether with citric acid, phosphoric acid, or ethylenediaminetetraacetic acid. The results for curing times and the bond strengths obtained with different types of initiator systems are presented in Tables 2, 3, and 4. In the experiment that used the chemically-activated initiator system A-consisting of TMTB, cupric acetylacetonate, methacryloylcholine chloride, and MA-the adhesive strength was 4.4 MPa without MA. Addition of MA increased the bond strength significantly, to 8.6-9.2 MPa (Table 2). These values were comparable with those obtained with the TBBO/ferric ion system (Table 1). Curing time increased gradually with the increase in the amount of MA. In the second chemically-activated initiator system B, composed of sodium alkylbenzenesulfinates and MA, the bond strength ranged between 4.4 and 12.0 MPa (Table 3). Toluenesulfinate and benzenesulfinate were most effective and yielded a bond strength of 11.2-12.0 MPa. Butyl- and octylbenzenesulfinates were not as effective. MA or the sulfinates alone did not initiate fast polymerization of MMA at room temperature. Curing time and solubility in water at 220C decreased with an increase in the number of carbon atoms of the alkyl chain attached to the benzene ring. In the visible-light-activated initiator system C, consisting of camphorquinone, TMTB, and MA, the bond strength was 3.2 MPa without the peroxyester. Addition of MA almost tripled the strength, and values of 9.4 to 13.4 MPa were obtained. Curing times were 160 s and 100 s without and with MA, respectively.
Discussion. A 4-META/MMA resin initiated with oxidized tri-n-butylborane (TBBO), known commercially as Super-Bond, gives an excellent adhesion to moist dentin when the dentin is pretreated with citric acid solution containing ferric chloride (Nakabayashi et al., 1982). In 4-META/MMA resin, the role of 4-META as an adhesion-promoting monomer has limitations (Nakabayashi, 1985; Misra, 1989), and the combination of TBBO and ferric ion may play a crucial role. When dentin was treated with citric acid or phosphoric acid, the adhesive bond strength of MMA/TBBO resin was 5-6 MPa in the absence or presence of 4-META (Nakabayashi et al., 1982). Therefore, no specific effect of 4-META in increasing bond strength was noted. On the other hand, when the dentin TABLE 2
ADHESION OF MMA/PMMA RESIN TO DENTIN WITH CHEMICALLY-ACIIVATED INITIATOR SYSTEM Aa MA in PMMA Curing Powder Adhesive Strength (MPa) Time Meanb (Wt%) (min) S.D. 0 3.0 4.4] 3.4 1 2 4 6 8
6.4 9.2 2.6 7.0 8.2- _ 1.3 8.6 8.6 2.4 8.5 8.9 _ 3.9 9.5 6.8 _i 2.5 aContaining 0.008% cupric acetylacetonate and 0.08% methacryloylcholine chloride in MMA, and 2% trimethylthiobarbituric acid and tertbutyl peroxymaleic acid (MA) in PMMA. bVertical lines join values that were not significantly different at the p>0.01 level.
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IMAI et al.
J Dent Res July 1991
TABLE 3 ADHESION OF MMA/PMMA RESIN TO DENTIN BY CHEMICALLY-ACTIVATED INITIATOR SYSTEM Ba Sodium Solubility Curing Adhesive Strength (MPa) Time Sulfinate in Water Meanb S.D. (bonded with) (w/W)% (mm) Benzene 32.8 6.4 11.2 7 5.6 Toluene 14.9 5.5 12.0 u 1.1 4.0 4.4 7 n-Butylbenzene 5.0 3.4 4.4 0.23 4.7 i 1.5 n-Octylbenzene aContaining 1.24 mol% sodium sulfinates and 1.24 mol% tert-butyl peroxymaleic acid in MMA. 'Vertical lines join values that were not significantly different at the p>0.01 level.
TABLE 4 ADHESION OF MMA/PMMA RESIN TO DENTIN BY VISIBLELIGHT-ACTIVATED INITIATOR SYSTEM Ca MA Curing in MMA Time Adhesive Strength (MPa) Meanb S.D. (wt%) (s) 0 160 3.2 2.2 1 100 9.4 2.2 3 100 9.93.3 5 100 13.4 i 3.6 aContaining 1% trimethylthiobarbituric acid in PMMA powder, and 1% camphorquinone and tert-butyl peroxymaleic acid (MA) in MMA. bVertical lines join values that were not significantly different at the p>0.01 level.
was pre-treated with aqueous citric acid containing ferric chloride, the bond strength of the resin increased remarkably to 12-18 MPa. Nakabayashi (1985) suggested that the ferric chloride suppressed denaturation of dentin collagen during the demineralization process with citric acid, since post-treatment with ferric chloride on acid-etched dentin did not improve adhesion. From his explanation, however, it is not possible to understand why ferric chloride is so effective. Therefore, we studied the role of the ferric ion in adhesion. Polymerization of methyl methacrylate (MMA) initiated with TBBO in the presence of collagen powder, collagen sheets, or decalcified dentin slices treated with ferric ion was investigated (Kadoma and Imai, 1988; Akimoto et al., 1990). A remarkable effect of collagen and ferric ion in accelerating polymerization of MMA was observed. The combination of collagen and ferric or cupric ions was most effective in promoting the polymerization; the presence of 4-META in MMA had an additional effect. These experimental results led us to the hypothesis that the ferric ions adsorbed onto dentin collagen may be involved in promoting polymerization of MMA, and thus may influence bond strength of MMA/TBBO resin to dentin. On the basis of our hypothesis, a post-treatment with ferric chloride on acid-etched dentin could be effective, contrary to Nakabayashi's (1985) suggestion. Therefore, we studied the post-treatment effect of ferric chloride on acid-etched dentin. Table 1 shows that the post-treatment was as effective as the treatment with citric acid containing ferric chloride, provided that an optimal concentration of ferric chloride was used. This result is reasonable because ferric chloride acts both as an inhibitor and as an accelerator of polymerization, depending on the concentration (Kadoma et al., 1983). An excess amount of the iron inhibits polymerization of MMA, and hence, results in low bond strength. Fast polymerization will be initiated by radicals from TBBO and is accelerated by ferric ions adsorbed onto dentin (Kadoma and Imai, 1988). Rapid initiation of polymerization at the dentin-resin interface is beneficial for increasing the bond strength at the interface, because polymerization will proceed outward from the interface, and the undesirable effect of polymerization shrinkage can be minimized. The strain generated by polymerization shrinkage will be shifted to the adhesive resin layer itself, and the stress will be induced in the adhesive resin. Consequently, when tensile force is applied, cohesive failure will occur within the resin layer, rather than at the interface. Moreover, the fact that the molecular weight of PMMA polymerized within or near the collagen was high (Akimoto et al., 1990) is favorable for reinforcing the interface. We extended our hypothesis to a general concept of interfacial initiation of polymerization for obtaining strong adhesion and prevention of contraction-gap formation between dentin
and resin. The rationale of the concept will be clear by a comparison of the resins that use different types of initiators. Fig. 2 shows diagrammatically the early distribution of free radicals and the direction of shrinkage accompanied by polymerization. When a chemically-activated initiation system such as BPO/ amine is used, shrinkage should occur toward the center of the resin, and a gap would be formed between dentin and the resin. In the light-activated system, initiation of polymerization should occur from near the resin surface, shrinkage should occur toward the external surface of the resin, and the gap would be generated at the bottom of the cavity wall. In contrast to these non-interfacial initiations, the direction of shrinkage in the interfacial initiation system should be toward the cavity wall, and therefore, no gap would be formed between the dentin and the resin. There seem to be two approaches for our concept to become a reality: (1) utilization of metal ions such as ferric or cupric ion adsorbed onto dentin as a component of redox-type initiators and (2) use of initiators having high affinity with dentin. High affinity with dentin can be expected from initiators containing functional groups exhibiting affinity for dentin. The first example has already been described. As a second example of initiators, we explored the use of a peroxyester containing a carboxylic acid group. A tertiary butyl peroxyester derivative of maleic acid (MA) was selected for this experiment. This compound (MA) has been studied by Antonucci et al. (1979) as a component of oxidation-reduction types of initiator systems for dental resins. The peroxyester alone was not effective because it is stable at room temperature, and therefore, it was necessary to induce its decomposition by combining it with other initiator systems capable of generating free radicals at low temperature. We tried three types of initiator systems: A, B, and C. In all these systems, addition of the peroxyester improved the bond strength, although the mechanism of its effectiveness was not entirely explained either by affinity of MA for dentin or by its contribution to an interfacial initiation. Cupric ions adsorbed onto dentin from the pre-treatment solution may also contribute to promotion of interfacial initiation of polymerization by the formation of a redox system with MA, as may be inferred from the study of Antonucci et al. (1979). However, the real role of MA in the bonding mechanism remains to be investigated. When the resin-dentin bond broke at higher than 8 MPa, the adhesive resin layer usually fractured cohesively, and interfacial fracture did not occur. Therefore, if the mechanical strength of the adhesive layer is improved, an increase in the adhesive bond strength can be expected. In system B, sodium alkylbenzenesulfinates initiate polymerization in conjunction with MA. The sulfinate salts with higher hydrophilicity, as exhibited by their solubility in water
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ROLE OF INITIATOR IN ADHESION OF RESIN TO DENTIN
Vol. 70 No. 7 Interfacial
Non-interfacial initiation
initiation Chemically activated
(BPO/Amine)
BOWEN, R.L.; COBB, E.N.; and MISRA, D.N. (1984): Adhesive Bonding by Surface Initiation of Polymerization, Ind Eng Chem Prod Res Dev 23:78-81. BOWEN, R.L.; TUNG, M.S.; BLOSSER, R.L.; and ASMUSSEN,
Lightactivated
(CQ/Amine)
'. . . *. - ... ..
.
0
I
i -.
.ff f F7
M
Dentin
r
Resin
Gap generated by shrinkage
1091
Free radical
Direction of shrinkage
Fig. 2-Early distribution of free radicals and direction of shrinkage accompanied by polymerization.
(Table 3), are more favorable for adhesion. Sodium benzenesulfinate used in Clearfil bonding agent (Kuraray Co., Osaka, Japan) appears to play a similar role because only poor bond strength has been obtained in the benzoylperoxide/amine system without this key compound (Yamauchi, 1986). In this study, we demonstrated that an interfacial initiation of polymerization is beneficial for strong adhesion between dentin and restorative resins to be obtained. Regarding interfacial initiation, Bowen et al. (1984) referred to surface initiation of polymerization to explain the strong adhesion between dentin and their resin composite bonding system, but they subsequently suggested (Bowen et al., 1987) that components other than ferric ion may be involved in the interfacial initiation of polymerization. REFERENCES
AKIMOTO, T.; KADOMA, Y.; and IMAI, Y. (1990): Study on Adhesion Mechanism of MMA-TBBO Resin to Dentin - A Model Experiment Using Decalcified Dentin, i Jpn Dent Mater 9:320325. ANTONUCCI, J.M. and BOWEN, R.L. (1977): Adhesive Bonding of Various Materials to Hard Tooth Tissues: XIII. Synthesis of a Polyfunctional Surface-active Amine Accelerator, J Dent Res 56:937-942. ANTONUCCI, J.M.; GRAMS, C.L.; and TERMINI, D.J. (1979): New Initiator Systems for Dental Resins Based on Ascorbic Acid, J Dent Res 58:1887-1899. BLOSSER, R.L. and BOWEN, R.L. (1988): Effects of Purified Ferric Oxalate/Nitric Acid Solutions as a Pretreatment for the NTGGMA and PMDM Bonding System, Dent Mater 4:225-231.
E. (1987): Dentine and Enamel Bonding Agents, Int Dent J 37:158161. BREDERECK, V.H.; FOHLISCH, B.; and FRANZ, R. (1966): Uber CH-aktive Polymerisationsinitiatoren XIII. Mitt. Polymerisationen und Polymerisationsinitiatoren, Makromol Chem 92:70-90. KADOMA, Y. and IMAI, Y. (1988): Effect of Ferric Salts on Polymerization of MMA by TBBO in the Presence of a Collagen Sheet - A Model to Study the Mechanism of Adhesion of MMA Resin to Dentin, J Jpn Dent Mater 7:817-823. KADOMA, Y. and IMAI, Y. (1989): Studies on Visible-Light Initiator System Using Thiobarbituric Acid Derivatives, J Jpn Dent Mater 8:533-538. KADOMA, Y.; KOJIMA, K.; and MASUHARA, E. (1983): Studies on Dental Self-curing Resins (25). Effect of Ferric Chloride or Cupric Chloride on the Polymerization of Methyl Methacrylate, J Jpn Dent Mater 2:495-503. KOJIMA, K.; KADOMA, Y.; and MASUHARA, E. (1982): Studies on Dental Fluoride Releasing Polymers (Part 3). Properties of Methacryloyl Fluoride-Methyl Methacrylate Copolymers as Dental Materials, J Jpn Dent Mater 1: 131-137. MASUHARA, E. (1969): Uber die Chemie eines Neuen Haftfahigen Kunststoff-Fiillungsmaterials, Dtsch Zahndrztl Z 24:620-628. MISRA, D.N. (1985): Adsorption of Zirconyl Salts and Their Acids on Hydroxyapatite: Use of the Salts as Coupling Agents to Dental Polymer Composites, J Dent Res 64:1405-1408.
MISRA, D.N. (1989): Adsorption of 4-Methacryloxyethyl Trimellitate Anhydride (4-META) on Hydroxyapatite and its Role in Composite Bonding, J Dent Res 68:42-47. MISRA, D.N. and BOWEN, R.L. (1977): Adhesive Bonding of Various Materials on Hard Tooth Tissues. XII. Adsorption of N-(2hydroxy-3-methacryloxypropyl)-N-phenylglycine(NPG-GMA) on Hydroxyapatite, J Coll Interfac Sci 61:14-20. MISRA, D.N.; BOWEN, R.L.; and WALLACE, B.M. (1975): Adhesive Bonding of Various Materials to Hard Tooth Tissues. VIII. Nickel and Copper Ions on Hydroxyapatite; Role of Ion Exchange
and Surface Nucleation, J Coll Interfac Sci 51:36-43.
MISRA, D.N. and JOHNSTON, A.D. (1987): Adsorption of N-phenylglycine on Hydroxyapatite: Role in the Bonding Procedure of a
Restorative Resin to Dentin, J Biomed Mater Res 21:1329-1339.
NAKABAYASHI, N. (1985): Bonding of Restorative Materials to Dentine: The Present Status in Japan, Int Dent J 35:145-154. NAKABAYASHI, N.; KOJIMA, K.; and MASUHARA, E. (1982): Studies on Dental Self-Curing Resins (24). Adhesion to Dentin by Mechanical Interlocking, JJpn Dent Mater 1:74-77. WHITMORE, F.C. and HAMILTON, F.H. (1967): Sodium p-toluenesulfinate. In: Organic Syntheses, Vol. 1, H. Gilman and A.H. Blatt, Eds., New York: John Wiley & Sons, Inc., pp. 492-494. YAMAUCHI, J. (1986): Study of Dental Adhesive Resin Containing Phosphoric Acid Methacrylate Monomer, J Jpn Dent Mater 5:144154.
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Although various adhesive resins for dentin have been developed and used clinically, most attention has been directed to adhesion-promoting monomers and pre-treatment agents. The role of polymerization initiator systems in bonding has been overlooked. The purpose of this work was to study the role of initiators from the viewpoint of interfacial initiation of polymerization in dentin bonding. The bond strength between dentin and methyl methacrylate resin was significantly improved by a possible interfacial initiation with (1) the combination of ferric chloride, adsorbed onto dentin, and oxidized tri-n-butylborane (TBBO) and (2) the addition of tertiary butyl peroxymaleic acid (containing a carboxylic acid group, which has an affinity with dentin) to chemically- or light-activated initiator systems. J Dent Res 70(7):1088-1091, July, 1991
Introduction. Various adhesive resins for dentin have been developed and used clinically. Recently, more attention has been directed to adhesion-promoting monomers and pre-treatment agents in the research and development of dental adhesive systems. Little attention has been paid to the polymerization initiator system and its effect on the initiation site and the direction of polymerization shrinkage, which in turn may affect bonding. Some polymerization initiator systems seem to play a key role in bonding systems showing strong adhesion to dentin, although their role has not been adequately explained. Even if adhesionpromoting monomers exist at the dentin interface, strong adhesion may not be obtained without suitable polymerization initiators. Initiation of polymerization at the interface is advantageous in the bonding of resins to dentin. The concept of polymerization by interfacial initiation has previously been documented and is found in studies by Masuhara (1969), Misra et al. (1975), Misra and Bowen (1977), Antonucci and Bowen (1977), Bowen et al. (1984, 1987), and Misra and Johnston (1987). The purpose of the present work was to investigate the role of the initiator system from the viewpoint of interfacial initiation of polymerization in dentin bonding. We studied the effects of ferric ion, adsorbed onto etched dentin, on methyl methacrylate/poly(methyl methacrylate) (MMA/PMMA) resin (with and without 4-META) initiated with oxidized tri-n-butylborane (TBBO), and the effects of tertiary butyl peroxymaleic acid (MA) (which contains a carboxylic acid group having an affinity with dentin) as an additive to chemicallyor light-activated initiator systems.
Materials and methods. Materials. - 1,3,5-Trimethyl-2-thiobarbituric acid (TMTB)
was synthesized from diethyl methylmalonate and dimethyl-
thiourea in essentially the same manner as the preparation of 1,3,5-trimethylbarbituric acid (Bredereck et al., 1966). Sodium sulfinates of 4-n-butylbenzene and 4-n-octylbenzene were prepared in a manner similar to the preparation of sodium toluenesulfinate (Whitmore and Hamilton, 1967). Tertiary butyl peroxymaleic acid (MA) was supplied from Nippon Oil & Fats Co., Tokyo, Japan. All other compounds were obtained commercially. The chemical structures of key compounds used are shown in Fig. 1. Measurement of curing time. -The resin pastes, mixed as described below, were placed at room temperature in a polyethylene cup (8 mm inside diameter, and 8 mm high), with a central hole for insertion of the tip of a thermocouple. A sheet of cellophane was placed on top of the cup. Curing time was recorded as the time when the peak temperature was reached. Measurement of bond strength between dentin and experimental resins with initiator systems A, B, and C. -Bovine anterior teeth were wet-sectioned into two parts with a diamond saw (Isomet, Buehler Ltd., Evanston, IL) so as to expose sufficient dentin surface for testing. The dentin surface of the labial side was used for adhesion testing without any further finishing. The dentin surface was pre-treated for 30 s with 10% aqueous citric acid containing 3% cupric chloride, washed with water, dried with an air syringe, and taped with adhesive tape having a 5-mm-diameter hole to control the surface area for adhesion. The above solution has been used as a standard pretreatment agent-along with 10% aqueous citric acid containing 3% ferric chloride-in our laboratory (Kojima et aL, 1982). The pre-treated dentin specimens were then bonded with acrylic resin cements, by use of three types of curing systems, as described below. All the bonded specimens were immersed in distilled water at 370C for 24 h and then tested with a tensile testing machine (Shimadzu Co., Kyoto, Japan) at a cross-head speed of 2 mm/min. Five specimens were tested in each set. Duncan's multiple-range test was used for statistical analysis. (1) Bonding with chemically-initiated system A. -MMA (0.1 g) containing 0.008% cupric acetylacetonate and 0.08% methacryloylcholine chloride was mixed with 0.1 g of PMMA powder containing 2% TMTB and various amounts of MA. Free radicals generated from the initiator system seem to induce decomposition of MA, which leads to polymerization of MMA (Bredereck et al., 1966). The mixed resin paste was applied to the dentin specimen, and an 8-mm-diameter acrylic rod was fixed perpendicularly in the resin cement. The specimen was kept for 30 min in air at room temperature and then immersed in water. (2) Bonding with chemically-initiated system B. -PMMA
0
CH3C-O-0-C-CH=CHCOH
CH, t -butl peroxymaleic acid ( MA )
Received for publication September 7, 1990 Accepted for publication March 4, 1991
1088
CH3
o
CH
,C- N
CH;-CH
,C-N.
on
C=S
'CH,
Rig SONa R = H, CH,
n-
CHN, n - C.H,
1.3.5 -timethyl -
sodium alkytbenzene
2 - thiobarbituric
suffiroates
acid
( TMTB )
Fig. 1-The chemical structures of the key compounds used.
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Vol. 70 No. 7
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ROLE OF INITIATOR IN ADHESION OF RESIN TO DENTIN
powder (0.1 g) was mixed into MMA (0.12 g) containing 1.24 mol% of sodium aryl sulfinate (Yamauchi, 1986) and MA. After the resin paste was applied to the dentin specimen, an acrylic plate with a diameter of 8 mm and a thickness of 3 mm was placed on the resin cement. After immersion in water, an acrylic rod was attached to the acrylic plate with 2-cyanoacrylate adhesive before tensile testing. (3) Bonding with photo-initiated system C. -PMMA powder (0.1 g) containing 1% TMTB was mixed into MMA (0.1 g) containing 1% camphorquinone and various amounts of MA (Kadoma and Imai, 1989). After the resin paste was applied to the dentin specimen, a transparent 3-mm-thick acrylic plate was placed on the paste, allowed to stand for two min, and then irradiated for three min with a visible-light unit (Translux, Kulzer & Co., Germany). The specimen obtained was immersed in water immediately after the irradiation. An acrylic rod was attached to the plate as described above. Measurement of bond strength between dentin and 4-META! MMA-TBBO resin. -The testing method was essentially the same as that described in the preceding section, but with the following modifications. The dentin surfaces, pre-treated with the solutions (Table 1), were brushed with 0.01-1.0% aqueous ferric chloride, allowed to stand for 30 s, and then dried. These dentin specimens were then bonded to an acrylic rod with 4META/MMA-TBBO resin (Super-Bond C&B, Sun Medical Co., Kyoto, Japan) in the manner described in paragraph (1) above.
Results. The effect of ferric chloride post-treatment of acid-etched dentin on the bond strength is summarized in Table 1. No significant difference (p>0.01) in the bond strength was observed between the post-treatment with an appropriate concentration of ferric chloride and the recommended treatment with a mixed solution of 10% citric acid and 3% ferric chloride. TABLE 1
EFFECT OF FERRIC CHLORIDE POST-TREATMENT ON ADHESION OF 4-META/MMA-TBBO RESIN TO DENTIN Concentration Adhesive Strength Pre-treatment of Ferric (MPa) Solution Time (s) Chloride (%)b Meanc S.D. 10% Citric acid-3% FeCl3 (Washed) 12.2 11.6 10% Citric acid-3% FeCl3 (Washed) 10.3d 3.2 10% Citric acid 60 1.0 2.1 _ 0.4 60 0.3 3.2 2.1 60 0.1 9.5 1-1_ 1.2 30 0.075 14.2 3.0 60 0.075 10.0 _ 2.5 60 0.05 9.2 2.0 _ 60 0.025 8.2 _ 1.5 60 0.01 5.8 i 2.2 30 0 2.2 0.6 60 0 2.6 0.7 40% Phosphoric acid 10 0.075 9.6 _ 3.6 10 0 2.4 0.9 i 30 0.075 7.1 2.5 30 0 1.4 0.6 15% EDTA 60 0.075 10.3 _ 4.6 60 0 2.0 _ 0.9 aWashed and dried after pre-treatment. bThe washed specimens were brushed with the solutions and dried after 30 s. cVertical lines join values that were not significantly different at the p>0.01 level. dMMA-TBBO resin without 4-META.
Maximum bond strengths of about 10 MPa were usually obtained by a post-treatment with an appropriate ferric chloride solution, and these were independent of pre-treatment, whether with citric acid, phosphoric acid, or ethylenediaminetetraacetic acid. The results for curing times and the bond strengths obtained with different types of initiator systems are presented in Tables 2, 3, and 4. In the experiment that used the chemically-activated initiator system A-consisting of TMTB, cupric acetylacetonate, methacryloylcholine chloride, and MA-the adhesive strength was 4.4 MPa without MA. Addition of MA increased the bond strength significantly, to 8.6-9.2 MPa (Table 2). These values were comparable with those obtained with the TBBO/ferric ion system (Table 1). Curing time increased gradually with the increase in the amount of MA. In the second chemically-activated initiator system B, composed of sodium alkylbenzenesulfinates and MA, the bond strength ranged between 4.4 and 12.0 MPa (Table 3). Toluenesulfinate and benzenesulfinate were most effective and yielded a bond strength of 11.2-12.0 MPa. Butyl- and octylbenzenesulfinates were not as effective. MA or the sulfinates alone did not initiate fast polymerization of MMA at room temperature. Curing time and solubility in water at 220C decreased with an increase in the number of carbon atoms of the alkyl chain attached to the benzene ring. In the visible-light-activated initiator system C, consisting of camphorquinone, TMTB, and MA, the bond strength was 3.2 MPa without the peroxyester. Addition of MA almost tripled the strength, and values of 9.4 to 13.4 MPa were obtained. Curing times were 160 s and 100 s without and with MA, respectively.
Discussion. A 4-META/MMA resin initiated with oxidized tri-n-butylborane (TBBO), known commercially as Super-Bond, gives an excellent adhesion to moist dentin when the dentin is pretreated with citric acid solution containing ferric chloride (Nakabayashi et al., 1982). In 4-META/MMA resin, the role of 4-META as an adhesion-promoting monomer has limitations (Nakabayashi, 1985; Misra, 1989), and the combination of TBBO and ferric ion may play a crucial role. When dentin was treated with citric acid or phosphoric acid, the adhesive bond strength of MMA/TBBO resin was 5-6 MPa in the absence or presence of 4-META (Nakabayashi et al., 1982). Therefore, no specific effect of 4-META in increasing bond strength was noted. On the other hand, when the dentin TABLE 2
ADHESION OF MMA/PMMA RESIN TO DENTIN WITH CHEMICALLY-ACIIVATED INITIATOR SYSTEM Aa MA in PMMA Curing Powder Adhesive Strength (MPa) Time Meanb (Wt%) (min) S.D. 0 3.0 4.4] 3.4 1 2 4 6 8
6.4 9.2 2.6 7.0 8.2- _ 1.3 8.6 8.6 2.4 8.5 8.9 _ 3.9 9.5 6.8 _i 2.5 aContaining 0.008% cupric acetylacetonate and 0.08% methacryloylcholine chloride in MMA, and 2% trimethylthiobarbituric acid and tertbutyl peroxymaleic acid (MA) in PMMA. bVertical lines join values that were not significantly different at the p>0.01 level.
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J Dent Res July 1991
TABLE 3 ADHESION OF MMA/PMMA RESIN TO DENTIN BY CHEMICALLY-ACTIVATED INITIATOR SYSTEM Ba Sodium Solubility Curing Adhesive Strength (MPa) Time Sulfinate in Water Meanb S.D. (bonded with) (w/W)% (mm) Benzene 32.8 6.4 11.2 7 5.6 Toluene 14.9 5.5 12.0 u 1.1 4.0 4.4 7 n-Butylbenzene 5.0 3.4 4.4 0.23 4.7 i 1.5 n-Octylbenzene aContaining 1.24 mol% sodium sulfinates and 1.24 mol% tert-butyl peroxymaleic acid in MMA. 'Vertical lines join values that were not significantly different at the p>0.01 level.
TABLE 4 ADHESION OF MMA/PMMA RESIN TO DENTIN BY VISIBLELIGHT-ACTIVATED INITIATOR SYSTEM Ca MA Curing in MMA Time Adhesive Strength (MPa) Meanb S.D. (wt%) (s) 0 160 3.2 2.2 1 100 9.4 2.2 3 100 9.93.3 5 100 13.4 i 3.6 aContaining 1% trimethylthiobarbituric acid in PMMA powder, and 1% camphorquinone and tert-butyl peroxymaleic acid (MA) in MMA. bVertical lines join values that were not significantly different at the p>0.01 level.
was pre-treated with aqueous citric acid containing ferric chloride, the bond strength of the resin increased remarkably to 12-18 MPa. Nakabayashi (1985) suggested that the ferric chloride suppressed denaturation of dentin collagen during the demineralization process with citric acid, since post-treatment with ferric chloride on acid-etched dentin did not improve adhesion. From his explanation, however, it is not possible to understand why ferric chloride is so effective. Therefore, we studied the role of the ferric ion in adhesion. Polymerization of methyl methacrylate (MMA) initiated with TBBO in the presence of collagen powder, collagen sheets, or decalcified dentin slices treated with ferric ion was investigated (Kadoma and Imai, 1988; Akimoto et al., 1990). A remarkable effect of collagen and ferric ion in accelerating polymerization of MMA was observed. The combination of collagen and ferric or cupric ions was most effective in promoting the polymerization; the presence of 4-META in MMA had an additional effect. These experimental results led us to the hypothesis that the ferric ions adsorbed onto dentin collagen may be involved in promoting polymerization of MMA, and thus may influence bond strength of MMA/TBBO resin to dentin. On the basis of our hypothesis, a post-treatment with ferric chloride on acid-etched dentin could be effective, contrary to Nakabayashi's (1985) suggestion. Therefore, we studied the post-treatment effect of ferric chloride on acid-etched dentin. Table 1 shows that the post-treatment was as effective as the treatment with citric acid containing ferric chloride, provided that an optimal concentration of ferric chloride was used. This result is reasonable because ferric chloride acts both as an inhibitor and as an accelerator of polymerization, depending on the concentration (Kadoma et al., 1983). An excess amount of the iron inhibits polymerization of MMA, and hence, results in low bond strength. Fast polymerization will be initiated by radicals from TBBO and is accelerated by ferric ions adsorbed onto dentin (Kadoma and Imai, 1988). Rapid initiation of polymerization at the dentin-resin interface is beneficial for increasing the bond strength at the interface, because polymerization will proceed outward from the interface, and the undesirable effect of polymerization shrinkage can be minimized. The strain generated by polymerization shrinkage will be shifted to the adhesive resin layer itself, and the stress will be induced in the adhesive resin. Consequently, when tensile force is applied, cohesive failure will occur within the resin layer, rather than at the interface. Moreover, the fact that the molecular weight of PMMA polymerized within or near the collagen was high (Akimoto et al., 1990) is favorable for reinforcing the interface. We extended our hypothesis to a general concept of interfacial initiation of polymerization for obtaining strong adhesion and prevention of contraction-gap formation between dentin
and resin. The rationale of the concept will be clear by a comparison of the resins that use different types of initiators. Fig. 2 shows diagrammatically the early distribution of free radicals and the direction of shrinkage accompanied by polymerization. When a chemically-activated initiation system such as BPO/ amine is used, shrinkage should occur toward the center of the resin, and a gap would be formed between dentin and the resin. In the light-activated system, initiation of polymerization should occur from near the resin surface, shrinkage should occur toward the external surface of the resin, and the gap would be generated at the bottom of the cavity wall. In contrast to these non-interfacial initiations, the direction of shrinkage in the interfacial initiation system should be toward the cavity wall, and therefore, no gap would be formed between the dentin and the resin. There seem to be two approaches for our concept to become a reality: (1) utilization of metal ions such as ferric or cupric ion adsorbed onto dentin as a component of redox-type initiators and (2) use of initiators having high affinity with dentin. High affinity with dentin can be expected from initiators containing functional groups exhibiting affinity for dentin. The first example has already been described. As a second example of initiators, we explored the use of a peroxyester containing a carboxylic acid group. A tertiary butyl peroxyester derivative of maleic acid (MA) was selected for this experiment. This compound (MA) has been studied by Antonucci et al. (1979) as a component of oxidation-reduction types of initiator systems for dental resins. The peroxyester alone was not effective because it is stable at room temperature, and therefore, it was necessary to induce its decomposition by combining it with other initiator systems capable of generating free radicals at low temperature. We tried three types of initiator systems: A, B, and C. In all these systems, addition of the peroxyester improved the bond strength, although the mechanism of its effectiveness was not entirely explained either by affinity of MA for dentin or by its contribution to an interfacial initiation. Cupric ions adsorbed onto dentin from the pre-treatment solution may also contribute to promotion of interfacial initiation of polymerization by the formation of a redox system with MA, as may be inferred from the study of Antonucci et al. (1979). However, the real role of MA in the bonding mechanism remains to be investigated. When the resin-dentin bond broke at higher than 8 MPa, the adhesive resin layer usually fractured cohesively, and interfacial fracture did not occur. Therefore, if the mechanical strength of the adhesive layer is improved, an increase in the adhesive bond strength can be expected. In system B, sodium alkylbenzenesulfinates initiate polymerization in conjunction with MA. The sulfinate salts with higher hydrophilicity, as exhibited by their solubility in water
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ROLE OF INITIATOR IN ADHESION OF RESIN TO DENTIN
Vol. 70 No. 7 Interfacial
Non-interfacial initiation
initiation Chemically activated
(BPO/Amine)
BOWEN, R.L.; COBB, E.N.; and MISRA, D.N. (1984): Adhesive Bonding by Surface Initiation of Polymerization, Ind Eng Chem Prod Res Dev 23:78-81. BOWEN, R.L.; TUNG, M.S.; BLOSSER, R.L.; and ASMUSSEN,
Lightactivated
(CQ/Amine)
'. . . *. - ... ..
.
0
I
i -.
.ff f F7
M
Dentin
r
Resin
Gap generated by shrinkage
1091
Free radical
Direction of shrinkage
Fig. 2-Early distribution of free radicals and direction of shrinkage accompanied by polymerization.
(Table 3), are more favorable for adhesion. Sodium benzenesulfinate used in Clearfil bonding agent (Kuraray Co., Osaka, Japan) appears to play a similar role because only poor bond strength has been obtained in the benzoylperoxide/amine system without this key compound (Yamauchi, 1986). In this study, we demonstrated that an interfacial initiation of polymerization is beneficial for strong adhesion between dentin and restorative resins to be obtained. Regarding interfacial initiation, Bowen et al. (1984) referred to surface initiation of polymerization to explain the strong adhesion between dentin and their resin composite bonding system, but they subsequently suggested (Bowen et al., 1987) that components other than ferric ion may be involved in the interfacial initiation of polymerization. REFERENCES
AKIMOTO, T.; KADOMA, Y.; and IMAI, Y. (1990): Study on Adhesion Mechanism of MMA-TBBO Resin to Dentin - A Model Experiment Using Decalcified Dentin, i Jpn Dent Mater 9:320325. ANTONUCCI, J.M. and BOWEN, R.L. (1977): Adhesive Bonding of Various Materials to Hard Tooth Tissues: XIII. Synthesis of a Polyfunctional Surface-active Amine Accelerator, J Dent Res 56:937-942. ANTONUCCI, J.M.; GRAMS, C.L.; and TERMINI, D.J. (1979): New Initiator Systems for Dental Resins Based on Ascorbic Acid, J Dent Res 58:1887-1899. BLOSSER, R.L. and BOWEN, R.L. (1988): Effects of Purified Ferric Oxalate/Nitric Acid Solutions as a Pretreatment for the NTGGMA and PMDM Bonding System, Dent Mater 4:225-231.
E. (1987): Dentine and Enamel Bonding Agents, Int Dent J 37:158161. BREDERECK, V.H.; FOHLISCH, B.; and FRANZ, R. (1966): Uber CH-aktive Polymerisationsinitiatoren XIII. Mitt. Polymerisationen und Polymerisationsinitiatoren, Makromol Chem 92:70-90. KADOMA, Y. and IMAI, Y. (1988): Effect of Ferric Salts on Polymerization of MMA by TBBO in the Presence of a Collagen Sheet - A Model to Study the Mechanism of Adhesion of MMA Resin to Dentin, J Jpn Dent Mater 7:817-823. KADOMA, Y. and IMAI, Y. (1989): Studies on Visible-Light Initiator System Using Thiobarbituric Acid Derivatives, J Jpn Dent Mater 8:533-538. KADOMA, Y.; KOJIMA, K.; and MASUHARA, E. (1983): Studies on Dental Self-curing Resins (25). Effect of Ferric Chloride or Cupric Chloride on the Polymerization of Methyl Methacrylate, J Jpn Dent Mater 2:495-503. KOJIMA, K.; KADOMA, Y.; and MASUHARA, E. (1982): Studies on Dental Fluoride Releasing Polymers (Part 3). Properties of Methacryloyl Fluoride-Methyl Methacrylate Copolymers as Dental Materials, J Jpn Dent Mater 1: 131-137. MASUHARA, E. (1969): Uber die Chemie eines Neuen Haftfahigen Kunststoff-Fiillungsmaterials, Dtsch Zahndrztl Z 24:620-628. MISRA, D.N. (1985): Adsorption of Zirconyl Salts and Their Acids on Hydroxyapatite: Use of the Salts as Coupling Agents to Dental Polymer Composites, J Dent Res 64:1405-1408.
MISRA, D.N. (1989): Adsorption of 4-Methacryloxyethyl Trimellitate Anhydride (4-META) on Hydroxyapatite and its Role in Composite Bonding, J Dent Res 68:42-47. MISRA, D.N. and BOWEN, R.L. (1977): Adhesive Bonding of Various Materials on Hard Tooth Tissues. XII. Adsorption of N-(2hydroxy-3-methacryloxypropyl)-N-phenylglycine(NPG-GMA) on Hydroxyapatite, J Coll Interfac Sci 61:14-20. MISRA, D.N.; BOWEN, R.L.; and WALLACE, B.M. (1975): Adhesive Bonding of Various Materials to Hard Tooth Tissues. VIII. Nickel and Copper Ions on Hydroxyapatite; Role of Ion Exchange
and Surface Nucleation, J Coll Interfac Sci 51:36-43.
MISRA, D.N. and JOHNSTON, A.D. (1987): Adsorption of N-phenylglycine on Hydroxyapatite: Role in the Bonding Procedure of a
Restorative Resin to Dentin, J Biomed Mater Res 21:1329-1339.
NAKABAYASHI, N. (1985): Bonding of Restorative Materials to Dentine: The Present Status in Japan, Int Dent J 35:145-154. NAKABAYASHI, N.; KOJIMA, K.; and MASUHARA, E. (1982): Studies on Dental Self-Curing Resins (24). Adhesion to Dentin by Mechanical Interlocking, JJpn Dent Mater 1:74-77. WHITMORE, F.C. and HAMILTON, F.H. (1967): Sodium p-toluenesulfinate. In: Organic Syntheses, Vol. 1, H. Gilman and A.H. Blatt, Eds., New York: John Wiley & Sons, Inc., pp. 492-494. YAMAUCHI, J. (1986): Study of Dental Adhesive Resin Containing Phosphoric Acid Methacrylate Monomer, J Jpn Dent Mater 5:144154.
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