Dental Materials Journal 15(2): 132-143, 1996Original paper

Effect of 4-Acryloxyethyltrimellitic Acid in a Self-etching Primer

on Bonding to Ground Dentin

Kunio IKEMURA, Yoshiaki KOURO and Takeshi ENDO1

Department of Research and Development, Shofu Inc.,

11, Kamitakamatsu-cho, Fukuine, Higashiyama-ku, Kyoto 605, Japan'Research Laboratory of Resources Utilization

, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226, Japan

Received July 29, 1996/Accepted October 26, 1996

To develop a self-etching primer for ground dentin, 4-acryloxyethyltrimellitic acid (4-AET) was newly synthesized, and the effect of concentrations varying from 0 (control) to 56.5wt% 4-AET in the water/HEMA primer on bonding to the dentin was investigated. Bond strength to the dentin was significantly affected by the inclusion of 4-AET in the primer which also contained N, N-di(hydroxyethyl)-p-toluidine (DEPT) when compared with the control (0% 4-AET) (p<0.01). The optimum 4-AET-concentration and the mean bond strength (SD) were found to be 6.5wt%: 24.2 (3.6)MPa, 37.4wt%: 25.3 (4.4)MPa, 47.2wt%: 26.9 (11.6)MPa and 54.4wt%: 29.7 (12.9)MPa. The role of DEPT in the 4-AET/HEMA primer was assessed, and the optimum DEPT-concentration was found to be 0.154 and 0.307mol%. Regarding the hypothetical bonding mechanism to dentin, it was thought that the ionized 4-AET in water/HEMA would

penetrate into dentin substrates, and DEPT as an accelerator in situ would facilitate photo-polymerization at the dentin-resin interface, and result in increased bond strength to ground dentin.

Key words: 4-AET, Self-etching primer, Shear bond strength

INTRODUCTION

The so-called acid etching technique for bonding adhesive resin to enamel has been well

established clinically since Buonocore et al.1). However, bonding to dentin is more problem-

atic due to the structure and nature of dentinal tissue, and the smear layer on superficial

ground teeth. Inadequate bonding to dentinal tissues can have serious consequences, leading to marginal leakage and discoloration, restorative failure, and ultimately secondary caries. The development of a bonding system that will provide good bond strength to both enamel

and dentin is, therefore, an important goal in the development of dental resin materials. Accordingly, there has been a continuous search to achieve a strong and permanent bond to

all hard tissues of the teeth. Recently, much interest has been focused on bonding to dentin.

To achieve a strong bond to dentin, adhesive resin systems having several adhesive promot-

ing monomers have been studied by several researchers in recent years2-10), and bonding

strength to dentin has been significantly improved.

Most contemporary effort in the studies on dentin bonding is concerned with an aqueous

primer composition that do not require an acid etching agent for hard tissues. Suzuki et al.11) reported that the aqueous solutions comprised methacrylates of amino acids,

2-hydroxyethylmethacrylate (HEMA) and water could improve bonding adhesive to dentin.

Aasen et at.12) invented a primer composition comprising water, a water soluble film former

IKEMURA et al. 133

and an acid. Watanabe13), Chigira et al.14) and Watanabe et al.15) proposed a self-etching

primer comprising 2-methacryloxyethyl phenyl hydrogen phosphate (Phenyl-P) for ground dentin. Itou et al.16) more recently reported an aqueous primer comprising N-(meth) acryloyl

asparatic acid that promoted bond strength without unacceptable damage to dentin.

During our development of a self-etching primer for ground dentin, a new advanced

primer composition comprising a curing agent, acidic monomers, HEMA and water was invented17), and a light-cured dentin bonding system, Imperva Bond, was recently devel-

oped18). Following this, a new fluoride-releasing 2-step bonding resin has more recently

been developed19). In the present study, a new adhesive promoting monomer, 4-AET, was

synthesized, characterized and evaluated in its bonding performance to dentin in water/

HEMA primer. Various adhesive promoting monomers having both hydrophilic and hydro-

phobic groups in their molecular structures have been synthesized and evaluated for their bonding to dentin2-10), i.e. N-phenylglycine glycidylmethacrylate (NPG-GMA)2), 4-meth-

acryloxyethyltrimellitate anhydride (4-META)3-5), Phenyl-P6,7), 10-methacryloxydecyl

hydrogen phosphate (MDP)8), and an adduct of pyromellitic acid dianhydride and HEMA in

1:2 molar ratio (PMDM)9,10). These monomers have been utilized in commercially avail-

able dentin bonding systems. While the evaluation of adhesive monomers with application

to dentin bonding is well known, the only examples involving these developments have been

concerned with bonding resins or aqueous primer which did not comprise a curing agent, and

little attention has been paid to evaluate the effect of these monomers in combination with

a curing agent, in the presence of water and HEMA, for bonding to ground dentin. In order

to develop a self-etching primer for ground dentin, the purpose of this study was, therefore,

to evaluate not only the effect of the inclusion of 4-AET, but also the role of DEPT as a

curing agent (or as accelerator) in the water/HEMA primer on bonding to ground dentin

which did not pretreated with an acid etching agent.

MATERIALS AND METHODS

Syntheses of 4-AETA and 4-AET

The syntheses of these monomers were carried out in a method similar to that of 4-META

and 4-methacryloxyethyltrimellitic acid (4-MET) in a previous paper20). The adhesive

promoting monomer of 4-AET was synthesized by the hydrolysis of 4-acryloxyethyl-trimellitate anhydride (4-AETA) synthesized from anhydrous trimellitic acid chloride* and

2-hydroxyethylacrylate (2-HEA)* by esterification. After the esterification, a light yellow

crystal as 4-AETA was obtained in 38.7% yield. After the hydrolysis of 4-AETA, the

reaction product was recrystallized from n-hexane and a white needle crystal as 4-AET was

obtained in 61.2% yield. It was determined that an absorption of the carboxylic group (1695

cm-1) had been observed in displacement of acid anhydride group (1780, 1855cm-1) in infrared

spectra (IR)**. The synthesized 4-AET and 4-AETA were identified by IR spectra, nuclear

magnetic resonance spectra (1H-NMR)# and an elemental analysis. The syntheses of 4-AET

*Wako Pure Chemical Industries , Ltd., Osaka, Japan**FT-300

, Horiba, Ltd., Kyoto, Japan#PMX60SI, JEOL Co., Ltd., Tokyo, Japan

134 EFFECT OF 4-AET ON DENTIN BONDING

Fig. 1 Syntheses of 4-AETA and 4-AET. 4-AETA was synthesized from anhy-

drous trimellitic acid chloride and 2-HEA by esterification. 4-AET was

synthesized by the hydrolysis of 4-AETA.

and 4-AETA are given in Fig. 1.

Preparation of reagents

1. Dimethacryloxyethyl-2, 2, 4-trimethylhexamethylene diurethane (TMDI-HEMA)

TMDI-HEMA as a hydrophobic urethane dimethacrylate (UDMA), was synthesized by an

addition reaction of 2, 2, 4-trimethylhexamethylene diisocyanate (TMDI)* and HEMA* in

1:2 molar ratio, according to the method previously reported21). After the reaction, the

absorption of vN=C=O (2380cm-1) disappeared on the IR spectra, and a translucent viscous

liquid was quantitatively obtained. TMDI-HEMA: ƒÅ 23•Ž: 5800-5950cp, IR spectra (cm-1):

1563, 1630, 1710, UVTHF (nm): 230, NCO22) (%): 0.077, OH value23) (%): 3.94 (calc. 3.47).

2. (6-methacryloxy) hexyl-3-phsphonopropyonate (MHPP)

MHPP, CH2=C(CH3)COO(CH2)6OOCCH2CH2-PO(OH)2, was synthesized by the ester-

ification of 2-carboxyethylphosphoric acid and 6-hydroxyhexylmethacrylate prepared from

1, 6-hexanediol* and methacrylic acid chloride* in 1:1 molar ratio by the esterification,

according to the method previously reported24).

3. N-phenylglycine glycidylmethacrylate (NPG-GMA)

NPG-GMA, CH2=C(CH3)COOCH2CH(OH)CH2N(C6H4)-CH2COOH, was synthesized by an

addition reaction of N-phenylglycine* and glycidylmethacrylate*, according to Bowen's

method previously reported2).

Preparation of experimental primer solutions and a light-cured bonding resin

Thirty-five experimental primer solutions were prepared from 4-AET, HEMA*, acidic

monomers, maleic acid*, distilled water* and DEPT* according to the formulation shown in

Tables 1-4. To determine the effect of water/HEMA concentration, 7 parts by weight

IKEMURA et al. 135

(6.5wt%) of 4-AET and 0.307mol% DEPT relative to the mixture of water and HEMA were incorporated with the mixture of water/HEMA (Table 1). To assess the effect of the

concentration varying from 0 (control) to 56.5wt% of 4-AET, 0.307mol% DEPT relative to

the mixture of water (40 parts by weight)/HEMA (60 parts by weight) was incorporated with

the mixture (Table 2). To evaluate the effect of acidic compounds, 0.307mol% DEPT

relative to the mixture of water (40 parts by weight)/HEMA (60 parts by weight) was

incorporated with the mixture (Table 3). In Table 3, the concentration of 1wt% NPG-GMA

was used within its solubility in water/HEMA mixture. To evaluate the effect of the

concentration of DEPT, varying from 0 to 15.4mol% (0-30 parts by weight) DETP, which

denoted mol% relative to the mixture of water (40 parts by weight)/HEMA (60 parts by

weight), and 7 parts by weight (6.5wt%) of 4-AET were incorporated with the mixture of

water and HEMA (Table 4).

An experimental light-cured bonding resin was prepared by mixing 54wt% TMDI-

HEMA, 34wt% triethyleneglycol dimethacrylate (TEGDMA)##, 2wt% ethyleneglycol dimeth-

acrylate (EGDMA)@, 3.5wt% HEMA, 4.5wt% 4-AET, 0.95wt% DL-camphorquinone (CQ)*,

1.0wt% N, N-dimethylaminoethyl methacrylate (DMAEM)* and 0.05wt% butylated hydroxy-

toluene (BHT)*.

Measurement of shear bond strength

For the evaluation of experimental primers on the bonding test for ground dentin embedded

in epoxy resin, freshly extracted bovine incisors were embedded in epoxy resin, then flat-

ground into dentin using 600-grit SiC abrasive paper under running water, and then air-dried

(ground dentin). The dentin was then treated with the primer in a rubbing manner for 60s

with a small sponge pellet, then dried with oil-free compressed air for 10s. After drying, a

double-faced adhesive tape with a 4.0mm diameter hole was fixed on the dentin surface, and

an experimental bonding resin was applied, and then cured with visible light@@ for 30s. A

cylindrical Teflon mold with 4.0mm inner diameter and 2.0mm in height was fixed onto the

dentin and a light-cured resin composite$ was placed in the mold and light-cured for 30s.

After removal from the mold, the specimens (n=7 in each group) were immersed in water at

37•}2•Ž. After 24h, shear bond strength was measured using a mechanical testing

machine$$, at a cross-head speed of 1mm/min. All tests described above were carried out at

a room temperature of 23•}2•Ž. Fractured surfaces of specimens were examined under a

microscope and cohesive failures in dentin were recorded. The mean and standard devia-

tions for load at failure were calculated and the results were subjected to a One-Way

Analysis of Variance (ANOVA), followed by Nownam-Keuls multiple comparison test.

##NK-ester 3G , Shin-Nakamura Chemical Co. Ltd., Wakayama, Japan@NK-ester IG , Shin-Nakamura Chemical Co. Ltd., Wakayama, Japan

@@Grip Light II, Shofu Inc., Kyoto, Japan$LITE-FIL IIA, A2, Shofu Inc., Kyoto, Japan$$Autograph AG 5000B, Shimadzu Corp., Kyoto, Japan

136 EFFECT OF 4-AET ON DENTIN BONDING

RESULTS

Identification of 4-AETA and 4-AET

Satisfactory IR spectra, 1H-NMR spectra and an elemental analysis (only for 4-AET),

together with melting point, were obtained for 4-AETA and 4-AET, and those for new

compounds are shown below. 4-AETA: a light-yellow crystal, mp 43.0-45.0•Ž, IR spectra

(cm-1): 1780, 1855 (C=O, from acid anhydride), 1710, 1720 (C=O, from ester). 1H-NMR

spectra (ppm): 4.53 (4H, -CH2CH2-), 5.83, 6.20 (3H, CH2=CH-), 8.03, 8.53, 8.55 (3H, aromatic).

4-AET: a white needle crystal, mp 119.5-121.0•Ž, IR spectra (cm-1): 1695 (C=O, from

carboxylic acid), 1H-NMR spectra (ppm): 4.53 (4H, -CH2CH2-), 5.83, 6.20 (3H, CH2=CH-),

8.03, 8.53, 8.55 (3H, aromatic). Anal. Calced for C14H12O8: C, 54.54; H, 3.90. Found:

C, 54.25; H, 3.94.

Shear bond strength to ground dentinData with respect to primer compositions and shear bond strength, standard deviation (SD)

and coefficients of variation [V(%)], together with the percent of cohesive failure in dentin

[D(%)] are given in Tables 1 to 4.The effect of the concentration of water/HEMA in the self-etching primer comprising

4-AET and DEPT on bonding to ground dentin was assessed, and together with the bond

strength to dentin are given in Table 1. The coefficient of variation shows a preferable range from 14.4 to 27.2%. It is apparent that high values of bond strength of 21.2 (3.8)MPa and 24.2 (3.6)MPa are obtainable in the concentration of water/HEMA of 50/50 (parts by weight) [=46.5/46.5(wt%)] and 40/60 (parts by weight) [=37.2/55.8(wt%)], respectively, and most fractured surfaces of specimens showed a cohesive failure in dentin (D=85.7%). Statistical analysis (ANOVA) indicated that these bond strengths were significantly higher than the data obtained by using other HEMA-concentrations (p<0.01). The optimum

Table 1 The effect of water/HEMA concentration in primer comprising 4-AET1 on shear bond strength to dentin

n=7, 1The content of 4-AET in the Table denotes 7 parts by weight (6.5wt%), and the content of DEPT in the Table denotes 0.307mol% relative to the mixture of water and HEMA. 2Standard deviation, 3Coefficient of variation, 4Percent of cohesive failure in dentin, 5Means with small letters were not significantly different at the level of 0.01.

IKEMURA et al. 137

HEMA-concentration in the primer which also contained 4-AET indicates 46.5 and 55.8wt%.

The experimental self-etching primers containing from 0 (control) to 56.5wt% 4-AET

in the presence of HEMA, water and DEPT were prepared, and the effect of inclusion of

4-AET in the primer on bonding to dentin was investigated. Data of the bond strength of

the 4-AET-primers to ground dentin are shown in Table 2. The coefficient of variation

shows a range from 14.9 to 60.5%. As apparent from Table 2, the bond strengths to dentin

are affected by the inclusion of 4-AET in the primer. The effective concentration of 4-AET

in the primer and the corresponding mean shear bond strength (SD), particularly over 19

MPa, are 2.4wt%: 19.5 (6.2)MPa, 4.7wt%: 19.6 (5.3)MPa, 6.5wt%: 24.2 (3.6)MPa, 37.4

wt%: 25.3 (4.4)MPa, 47.2wt%: 26.9 (11.6)MPa, 50.0wt%: 21.2 (8.6)MPa and 54.4wt%:

29.7 (12.9)MPa, respectively, and most fractured surfaces of specimens showed a cohesive

failure in dentin (D=42.9•`100%). Statistical analysis indicated that in the above 4-AET-

concentrations, there was no significant difference between those bonding data, however,

there was a significant difference in bond strength to dentin when compared with the control

(p<0.01). The optimum 4-AET-concentration indicates 6.5wt%, 37.4wt%, 47.2wt% and

54.4wt%.

Table 3 presents the effect of acidic compounds in the primer on shear bond strength to

ground dentin. MHPP, NPG-GMA and 4-AET were used, together with maleic acid (MA).

As apparent from Table 3, the bond strength to dentin using primer containing either 4-AET

or MHPP shows significantly higher strength than either NPG-GMA or MA (p<0.01).

Effect of DEPT content in the primer which also contained 6.5wt% 4-AET on bonding

to dentin was further investigated. Table 4 shows the effect of increasing the DEPT content

Table 2 Effect of the concentration of 4-AET in water/HEMA primer1 on shear bond strength to dentin

n=7, 1The content of DEPT in the Table denotes 0.307mol% relative to the mixture of water (40

parts by weight)/HEMA (60 parts by weight), 2-4Denotes with the same definition as described in Table 1, 5Means with small letters were not significantly different at the level of 0.01.

138 EFFECT OF 4-AET ON DENTIN BONDING

Table 3 Effect of acidic compounds in the primer1 on shear bond strength to dentin

n=7, 1The content of DEPT in the Table denotes 0.307mol% relative to the mixture of water (40

parts by weight)/HEMA (60 parts by weight), 2-4Denotes with the same as definition as described in Table 1, 5(6-methacryloxy) hexyl-3-phosphono-propyonate: CH2=C(CH3)COO(CH2)6OOCCH2CH2-PO(OH)2, 6N-phenylglycine glycidylmeth-acrylate: CH2=C(CH3)COOCH2CH(OH)CH2N(C6H4)-CH2COOH, 7Maleic acid, 8Means with small letters were not significantly different at the level of 0.01.

Table 4 Effect of the concentration of DEPT in the primer1 on shear bond strength to dentin

n=7, 1The content of 4-AET in the Table denotes 7 parts by weight (6.5wt%), 2The numerals in the parentheses in the Table denotes mol% relative to the mixture of water (40 parts by weight)/HEMA (60 parts by weight), 3-5Denotes with the same definition as described in Table 1. 6Means with small letters were not significantly different at the level of 0.01.

in the 4-AET/HEMA primer on the bond strength to dentin. The coefficient of variation

shows a preferable range from 14.9 to 41.7%. As apparent from Table 4, bond strengths to

dentin are affected by the inclusion of DEPT in the 4-AET/HEMA primer. Analysis

indicated that where the primer contained 0.154 and 0.307mol% DEPT, a significantly

increased bond strength to dentin was shown when compared with the other DEPT-concen-

trations (p<0.01). The optimum concentration of DEPT and the shear bond strength

indicate 0.154mol%: 21.4 (4.3)MPa and 0.307mol%: 24.2 (3.2)MPa.

DISCUSSION

A new adhesive promoting monomer, 4-AET, was synthesized, characterized by IR, 'H-NMR and elemental analysis

, and was evaluated in its bonding performance to dentin in

an aqueous primer. Nakabayashi et al.5) reported that a monomer having both hydrophilic

IKEMURA et al. 139

and hydrophobic groups in its molecular structure, i.e. 4-META, exhibited good adhesion to

dentin. The chemical structure of 4-AET is similar to 4-MET which was synthesized from

4-META by hydrolysis. Various adhesive promoting monomers have been previously

synthesized and evaluated for their bonding effects to dentin2-10). In these studies, however,

these monomers were incorporated with hydrophobic monomers or acetone as a bonding resin which did not contain water. Although the adhesive monomers have acidic groups in

their molecular structure, they exhibit poor ionization in hydrophobic environments. Since

the adhesive monomers in bonding resin have not proved consistently successful in hydro-

phobic resins, it was thought advisable to investigate the effect of ionized acidic monomer in an aqueous primer solution for bonding to dentin. Nakabayashi et al.3) reported that

so-called 4-META/MMA-TBB-O resin has made a remarkable progress on bonding to

dentin etched with a 10% citric acid/3% ferric chloride solution called 10-3 solution. It was

considered that 4-META was hydrolyzed to form ionized 4-MET in the presence of water,

TBB-O in situ could be, then, initiated at the dentin-resin interface. It was thought that

both the ionized acidic monomer and water soluble initiator system performed an important

role on bonding to dentin. Although the evaluation of adhesive monomers with application

to dentin bonding is well known, the only examples involving these developments have been

concerned with bonding resins or aqueous primers which did not contain a curing agent, and

little attention has been paid to the effect of these monomers in combination with a curing

agent, in the presence of water and HEMA, for bonding to ground dentin.

In order to develop a self-etching primer for ground dentin, the present study was,

therefore, undertaken to evaluate the effect, not only of the inclusion of 4-AET, but also of

DEPT as a curing agent in water/HEMA primer on bond strength to dentin. To evaluate

the influence of dentin substrate on the bonding methodology, the bovine teeth embedded in

epoxy resin (embedded bovine teeth) was compared with freshly extracted bovine teeth with

dental pulp (fresh bovine teeth)25). It was found that shear bond strength to the embedded bovine teeth showed 20.5MPa and tensile bond strength to the fresh bovine teeth showed 18.6

MPa25), and confirmed that there was no significant difference between the figures. In the

present study, therefore, the bovine teeth embedded in epoxy resin were used for shear bond testing.

Although 4-AET was slightly soluble in water, it was found that the solubility of 4-AET

was markedly increased in the presence of HEMA. It was considered that a determination

of water/HEMA-concentration was of importance in this study. The optimum concentra-

tion of water/HEMA in the primer which also comprised 6.8wt% 4-AET was found to be

46.5/46.5(wt%) and 37.2/55.8(wt%), and the mean shear bond strength (SD) indicated 21.2

(3.8)MPa and 24.2 (3.5)MPa (Table 1). The optimum HEMA-concentration determined in this study was not concordant with the concentration previously reported by Munksgaad

et al.26,27), where the bond strength was highly developed in the HEMA-concentration, with

a maximum at 35%, and nearly independent of glutaraldehyde (GA)-concentration when

greater than 3%, and the highest mean bond strength was about 18MPa. Although the HEMA-concentration determined was 35%, the development of the primer was not extended

beyond the mixture of HEMA and GA. Itoh et al.28) reported that 35vol% HEMA solution

without aldehydes was equally effective as a dentin primer when priming was followed by a

140 EFFECT OF 4-AET ON DENTIN BONDING

bonding resin containing MDP. Hayakawa et al.29) reported that a tensile bond strength to

dentin of 19.0MPa was obtained by a mixed primer (HEMA-concentration: 35%) treatment

of solution A ( 70% HEMA+6% MTYA) and solution B (2% GA) after treatment with 10%

citric acid. It seems reasonable that the carbon-nitrogen double bond of Shiff base (-N=CH-),

produced by the reaction of the aldehyde group with active nitrogen groups in both collagen and MTYA molecules, will produce the formation of a cross-linking bond between collagen

and MTYA at the acidic surface of dentin. However, in general organic chemistry, it is

known that the cross-linked Shiff base is easily hydrolyzed to aldehyde and amine in water30).

While the optimum HEMA-concentration was indicated to be 35% in these studies, the HEMA-concentration was found to be 46.5wt% and 55.8wt% in the present study, which

suggests that the concentration was influenced by the inclusion of 4-AET in the primer.Although increasing interest is being shown in the use of GA, as a key ingredient in the

primer, an acidic monomer of 4-AET, instead of this GA, was incorporated with the mixture of water/HEMA at a predetermined concentration in the present study. To assess the effect

of 4-AET, experimental primers comprising varying amounts from 0 (as control) to 56.4wt%

4-AET, HEMA, DEPT and water were prepared, and the bond strength to ground dentin was

measured. From the data in Table 2, it was made clear that the mean shear bond strength

to ground dentin was significantly affected by the inclusion of 4-AET in the primer which

also contained DEPT, when compared with the control (0% 4-AET) (p<0.01). The effective

concentration of 4-AET and the corresponding mean bond strength were found to be 6.5

wt%: 24.2 (3.6)MPa, 37.4wt%: 25.3 (4.4)MPa, 47.2wt%: 26.9 (11.6)MPa and 54.4wt%:

29.7 (12.9)MPa, respectively. These figures showed that the primer comprising 4-AET

indicated about 3.5 to 5.3 times higher bond strength to dentin than that of the control. A

particularly high value of mean bond strength (29.7MPa) was obtained by 54.4wt% 4-AET, and one bond test specimen which showed the maximum bond strength of 46.4MPa with a

cohesive failure in dentin (D), and which was very close to the tensile strength (51.7MPa) of

natural dentin31). It was found that the 4-AET with high solubility in water/HEMA showed

a noteworthy effect on bonding to ground dentin, and performed an important role in the self-

etching primer.

To evaluate the effect of the hydrophilic group and methacryloyl group in the structure

of an acid compound, the effect of acidic compounds in the primer on the bond strength to

ground dentin was also investigated. It was apparent that the bond strength to dentin using a primer comprising 4-AET having divalent -(COOH)2 and MHPP having a phosphonic acid

group showed higher bond strength than NPG-GMA as amino acid derivative or MA which had no methacryloyl group (Table 3). The effective bonding strength of MHPP showed a

similar tendency to the data of Phenyl-P in a previous paper13), but in the contrast, the low

value of bond strength obtained by NPG-GMA was caused by its low solubility in water/

HEMA. In the case of MA, it was thought that the decreased bond strength was caused by

the molecular structure without the (meth) acryloyl group. The data in Table 3 suggested

that both solubility and (meth) acryloyl group of acidic compounds were important in the

effective bonding to ground dentin.In the present study, the role of DEPT as a curing agent in the self-etching primer was

further investigated. With respect to the influence of amino compounds in an aqueous

IKEMURA et al. 141

primer in previous papers, Chen et al.32) reported that one disadvantage encountered with N-toluylglycine glycidylmethacrylate (NTG-GMA), having a carboxylic group, or a-N-arylamino acid primer that contains a methacrylate group was the tendency toward

premature polymerization during its synthesis and storage. Antonucci et al.33) and Schuma-cher et al.34) reported that this premature polymerization was a generation of free radicals

produced via the interaction of the tertiary aryl amine group with the carboxylic group of NTG-GMA. It is known that aromatic acid anhydride reacts with aromatic amines to form a yellowish charge-transfer complex (CT-complex)9,35). Although a likely cause of this

premature polymerization was generated during storage of the primer comprising 4-AET and DEPT, it is different from the case of NTG-GMA, where it can be separated into two component primers for utilization. The primer has the ability to form CT-complex. Thus far the role of DEPT as curing agent in a primer solution has not been clarified. The present

study was, therefore, undertaken to determine the role of DEPT in the 4-AET/HEMA/water

primer, essentially. The effect of the concentration of DEPT in the primer on bonding to ground dentin was evaluated. The bond strengths to dentin were affected by the inclusion of DEPT in the water/4-AET/HEMA primer (Table 4). The optimum concentration of DEPT in the primer and corresponding shear bond strength was found to be 0.154mol%: 21.4

(4.3)MPa and 0.307mol%: 24.2 (3.6)MPa. It was found that DEPT as curing agent with 4-AET/HEMA primer performed an important role in effective bonding to ground dentin.

A mechanism of bonding to dentin with a hybrid layer at the dentin-resin interface is now accepted in morphological aspects since the proposal by Nakabayashi4). The hybrid layer was analyzed using an electron probe microanalyzer (EPMA)36). While bonding mechanisms to dentin has usually been studied on a dentin-resin hybrid layer in a

morphological aspect, in the present study, a hypothetically bonding mechanism of the 4-AET/HEMA primer to ground dentin can be considered from its wettability and reactivity

of monomers on the principle of adhesion in chemical aspects. It was considered that the ionization of an acidic group of 4-AET led to sufficient chemical interaction with dentinal tissue, and resulted in good wettability of bonding resin to dentin at the adhesive interface. Thus far the adhesive promoting monomers have dealt almost exclusively with methacrylic ester derivatives. Itou et al.16) more recently reported that N-acryloyl asparatic acid showed effective bond strength to dentin when compared with N-methacrloyl asparatic acid. In general polymer chemistry, it is known that the polymerizing reaction of acrylic ester derivatives (CH2=CH-COO-R) have much faster reactivity than that of methacrylic ester derivatives [CH2=C(CH3)-COO-R]37). An acrylic ester of 4-AET used in this study has a high reactivity of polymerization and is highly soluble in water/HEMA solution, and indicated remarkable bonding to ground dentin. With respect to the hypothetical bonding mechanism to dentin, it was thought that the ionized 4-AET in water/HEMA would

penetrate into dentin substrates, and DEPT as a curing agent (or as accelerator) in situ would facilitate photo-polymerization at the dentin-resin interface, and resulted in increased shear

bond strength to ground dentin. In order to determine the chemical proof for this hypothesis, it is necessary to investigate further the scope and bonding mechanism of the self-etching

primer for dentin used in this study.

142 EFFECT OF 4-AET ON DENTIN BONDING

CONCLUSION

A conclusion of the noteworthy aspects of this study is as follows:

1. The optimum concentration of water/HEMA in the primer was found to be 46.5/46.5 (wt%)

and 37.2/55.8 (wt%), suggesting that the concentration was influenced by the inclusion of

4-AET in the primer.

2. Shear bond strength to dentin was significantly affected by the inclusion of 4-AET in the

primer when compared with the control (0% 4-AET) (p<0.01), and the effective concentra-tion of 4-AET and the mean shear bond strength were found to be 6.5wt%: 24.2 (3.6)MPa,

37.4wt%: 25.3 (4.4)MPa, 47.2wt%: 26.9 (11.6)MPa and 54.4wt%: 29.7 (12.9)MPa.

3. A favorable content of DEPT in the 4-AET/HEMA primer increased bond strength to

ground dentin, and the optimum concentration of DEPT in the primer was found to be 0.154 and 0.307mol%.4. Regarding the hypothetical bonding mechanism to dentin, it was thought that the ionized

4-AET in water/HEMA would penetrate into dentin substrates, and DEPT as an accelerator

in situ would facilitate photo-polymerization at the dentin-resin interface, and resulted in

increased shear bond strength to ground dentin.

REFERENCES

1) Buonocore, M.G., Wilieman, W. and Brudevold, F.: Simple method of increasing the adhesion of acrylic materials to enamel surfaces, J Dent Res 34: 849-853, 1955.

2) Bowen, R.L.: Adhesive bonding of various materials to hard tooth tissues. II. Bonding to dentin promoted by a surface-active monomer, J Dent Res 44: 895-902, 1965.

3) Nakabayashi, N., Takeyama, M., Kojima, K. and Masuhara, E.: Studies on dental self-curing resins (19)-Adhesion of 4-META/MMA-TBB resin to pretreated dentin, J Japan Soc Dent Appar Mat 23: 29-33, 1982. (in Japanese)

4) Nakabayashi, N.: Bonding of restorative materials to dentin: the present status in Japan, Int Dent J 35(2): 145-154, 1985.

5) Nakabayashi, N.: Adhesion promoting monomers to tooth substrates. Hybridization of tissue and polymer, Yukigousei Kagaku Kyoukaisi 42(11): 1031-1040, 1984. (in Japanese)

6) Yamauchi, J., Nakabayashi, N. and Masuhara, E.: Adhesive agents for hard tissue containing phosphoric acid monomers, ACS polymer preprints 20: 594-595, 1979.

7) Yamauchi, J.: Study on dental adhesive resin containing phosphoric acid methacrylate monomer,

J J Dent Mater 5(1): 144-154, 1989. (in Japanese)8) Omura, I. and Yamauchi, J.: Correlation between molecular structure of adhesive monomer and

adhesive property.: 1st Cong Int Dent Mater: 356, 1989.9) Bowen, R.L., Cobb, E.N. and Rapson, J.E.: Adhesive bonding of various materials to hard tooth

tissues: Improvement in bond strength to dentin, J Dent Res 61(9): 1070-1076, 1982.10) Bowen, R.L.: Method for obtaining strong adhesive bonding of composite to dentin, enamel and other

substances, US Pat, No.4,521,550, 1985.11) Suzuki, K., Takahashi, M. and Nakai, H.: Adhesion of tooth substance-Treatment of dentinal

surface by aqueous solution of amino acid derivatives-HEMA-, J Okayama Dent Soc 12(2): 257-264, 1993. (in Japanese)

12) Aasen, S.M. and Oxman, J.D.: Method for priming hard tissue, US Pat. No.4,719,149, 1988.13) Watanabe, I.: Photocure bonding agents to ground dentin, J J Dent Mater 11(6): 955-973, 1992. (in

Japanese)14) Chigira, H., Yukitani, W., Hasegawa, T., Manabe, A., Itoh, K., Hayakawa, T., Debari, K., Wakumoto,

IKEMURA et al. 143

S. and Hisamitsu, H.: Self-etching dentin primers containing Phenyl-P, J Dent Res 73, 1088-1095, 1994.

15) Watanabe, I., Nakabayashi, N. and Pashley, D.H.: Bonding to ground dentin by a Phenyl-P self-etching primer, J Dent Res 73: 1212-1220, 1994.

16) Itou, K., Torii, Y., Suzuki, K., Nakai, H. and Inoue, K.: Priming effect of amino acid derivatives on adhesion to dentin, J Dent Res 75, Special issue: 394, 1996.

17) Ikemura, K. and Kouro, Y.: Primer composition, US Pat. No.5,264,513, 1993.18) Ikemura, K., Tachidokoro, H. and Kouro.Y.,: Development of a super adhesive bonding system

Imperva Bond: Its adhesive ability and marginal sealing to teeth with/or without acid etching

pretreatment, J J Dent Mater 11, Special issue: 254-256, 1992. (in Japanese)19) Ikemura, K. and Kouro, Y.: Development of a new fluoride-releasing 2-step bonding resin, J Dent Res

74, Special issue: 430, 1995.20) Takeyama, M., Kashibuti, S., Nakabayashi, N. and Masuhara, E.: Studies on dental self-curing resins

(17), Adhesion of PMMA with bovine enamel or dental alloys, J Japan Soc Dent Appar Mat 19(47), 179-185, 1978. (in Japanese)

21) Kawaguchi, M., Fukushima, T. and Horibe, T.: Mechanical properties and cure depth of UDMA- based composite resins, J J Dent Mater 9(2): 265-270, 1990. (in Japanese)

22) Sorenson, W.R. and Campbel, T.W.: Experimental methodology of polymer syntheses, Tokyo Kagaku Doujin Co., Tokyo, 1969, pp. 122. (Hoshino et al: in Japanese)

23) Lee, H. and Neville, K.: Handbook of epoxy resins, McGraw-Hill Book Co., New York, 1967, pp. 4-22.

24) Ikemura, K., Kouro, Y. and Urabe, S.: (Meth) acryllic acid derivatives, JP-LO, Hei 3-294286, 1991. (in Japanese)

25) Shofu Inc., A new advanced dentin bonding system, Imperva Bond, and it's clinical use, Technical manual, Shofu Inc., Kyoto, 1992, pp. 15-22. (in Japanese)

26) Munksgaard, E.C. and Asmussen, E.: Bond strength between dentin and restorative resins, Mediated by mixtures of HEMA and glutaraldehyde, J Dent Res 63(8): 1087-1089, 1984.

27) Asmussen, E. and Munksgaad, E.C.: Bonding of restorative resins to dentin promoted by aqueous mixture of aldehyde and active monomers, Int Dent J 35(2): 160-165, 1985.

28) Itoh, K., Hashimoto, K. and Wakumoto, S.: Effect of GLUMA concentration on adhesion to dentin,

Japan J Conserv Dent 28: 895-901, 1985. (in Japanese)29) Hayakawa, T., Endo, H., Hara, T., Fukai, K. and Horie, K.: Studies on adhesion to tooth substrate.

IV. Adhesion of MMA/TBB-O resin to dentin improved by 1-35 (MTYA), Dent Mater J 7(1): 19-23, 1988.

30) Yukawa, Y., Hanafusa, T., Mukaiyama, T. and Yoshimura, J.: Cram's Organic Chemistry 1st ed, Hayakawa Publishing Co., Tokyo, 1968. pp. 379-380 (in Japanese), Original English language edition, Cram, D.J. and Hammond, G.S.: Organic chemistry 2nd ed, McGraw-Hill Book Co., New York.

31) Craig, R.G.: Restorative dental materials, 9th ed., C.V. Mosby, St. Louis, 1993, pp. 68-69.32) Chen, R.S. and Bowen, R.L.: The role of N-phenylglycine in a dental adhesive, J Adhesion Sci

Technol 3: 49-54, 1989.33) Antonoucci, J.M., Stansbury, J.W. and Farahani, M.: Polymerization of dental resins via acid-amine

interactions, J Dent Res 71, Special issue: 239, 1992.34) Schumacher, G.E., Eichmiller, F.C. and Antonucci, J.M.: Effects of surface-active resins on dentin/

composite bonds., Dent Mater 8: 278-282, 1992.

35) Nakamura, M.: Adhesive self-curing acrylic resin-Composition of 4-META bonding agent-, J J Dent Mater 4(6): 762-691, 1985. (In Japanese)

36) Fukushima, T. and Horibe, T.: Line analysis of interface layer on dentin by neans of electron-probe microanalysis, J Bio Mater Res 25: 129-140, 1991.

37) Ohtsu, T.: Chemistry of polymer syntheses, Kagaku Doujin Co., Tokyo, 1994, pp. 71-1321. (in Japanese)

251

4-ア ク リロキ シエ チ ル トリメ リッ ト酸 を含 むセ ル フエ ッチ ング プ ライ マー の

研削象牙質への接着効果

池村邦夫,紅 露良明,遠 藤 剛1

株式会社松風研究開発部1東京工業大学資源化学研究所

研 削 象 牙質 へ のセ ル フエ ッチ ン グプ ラ イマ ー の 開発 の

た め,4-ア ク リロキ シエ チ ル ト リメ リ ッ ト酸(4-AET)

を新 規 に合成 し,研 削 象 牙質 へ の 剪 断接 着 強 さ にお け る,

プ ラ イマ ー 中4-AETの 配 合効 果 お よ びN, N-ジ(ヒ ド

ロキ シエ チ ル)-P-ト ル イ ジ ン(DEPT)の 硬 化 剤 と して

の役 割 を検 討 した結 果,象 牙 質 へ の接 着 強 さ は4-AET

配 合 に よ り 顕 著 に 影 響 を 受 け,コ ン ト ロ ー ル

(0% 4-AET)に 対 し有意 に高 い こ とが 明 らか とな っ た

(p>0.01). 4-AET最 適 濃 度 と平 均 接 着 強 さ は6.8

wt%: 24.2 (3.6)MPaお よ び37.4wt%～54.4wt%:

21.2～29.7 (12.9)MPaを 示 す こ とが 見 出 され た.一 方,

4-AET/HEMA系 プ ライ マ ー中DEPTの 役 割 を検 索 し

た 結果,DEPTの 最 適 濃 度 はO.307mol%で あ った.そ の

接 着機 構 は,水/HEMA中 で イ オ ン化 した4-AETが 象

牙 質組 織 内へ 浸透 し,さ らにDEPTが 接 着界 面 付 近 の レ

ジ ンの光 重 合 を促 進 す る こ とに よ り,研 削 象牙 質 に対 す

る接着 強 さが 向上 した こ とが 考 え られ た.