International Journal of Adhesion & Adhesives 31 (2011) 571–574

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International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ijadhadh

Effect of acidic monomer concentration on the dentin bond stability ofself-etch adhesives

Fernanda B. Leal a, Francine C. Madruga a, Emılia P. Prochnow a, Giana S. Lima b, Fabrıcio A. Ogliari c,Evandro Piva a, Rafael R. Moraes a,n

a School of Dentistry, Federal University of Pelotas, Rua Gonc-alves Chaves 457, 96015-560 Pelotas-RS, Brazilb School of Dentistry, University of Varzea Grande, Av. Dom Orlando Chaves 2655, 78118-900 Varzea Grande-MT, Brazilc Materials Engineering School, Federal University of Pelotas, Rua Felix da Cunha 809, 96010-000 Pelotas-RS, Brazil

a r t i c l e i n f o

Article history:

Accepted 18 May 2011This study evaluated the effect of the concentration of acidic functional monomer on the dentin bond

stability of a model two-step, self-etch adhesive system. Six self-etch primers were formulated using

Available online 1 June 2011

Keywords:

Adhesion by mechanical interlocking

Aging

Primers and coupling agents

Dental adhesives

96/$ - see front matter & 2011 Elsevier Ltd. A

016/j.ijadhadh.2011.05.007

esponding author. Tel./fax: þ55 53 3222 669

ail address: moraesrr@gmail.com (R.R. Morae

a b s t r a c t

hydroxyethyl methacrylate (HEMA), 1,3-glycerol dimethacrylate phosphate (GDMA-P), ethanol and

water. Different mass concentrations of GDMA-P were tested: 0, 15, 30, 50, 70 or 100% (primers labeled

P0–100). The pH of the solutions was measured. The bonding resin was composed of (di)methacrylates.

Bond strength to bovine dentin was assessed through a microtensile bond test. The beam specimens

were stored in distilled water, at 37 1C, for 24 h, 6 months or 1 year. Data were statistically analyzed

and failure modes classified under magnification. The increase in acidic monomer concentration was

associated with an exponential decrease in pH (R2¼0.999; Po0.001). All specimens debonded

prematurely for the primers P0, P70 and P100. After 24 h, the bond strengths for P504P30¼P15. After

6 months and 1 year, P50¼P304P15. The bond strength after 6 months was similar to 24 h for P15 and

P50, but significantly lower after 1 year. P30 showed no differences in bond strength over the 1-year

storage period. A predominance of mixed failures was detected for all primers at 24 h. After 6 months,

P30 and P50 showed a predominance of adhesive failures. After 1 year, the predominant failure mode for

all primers was cohesive within dentin. In conclusion, a mass fraction of 50% of phosphate monomer is

a limit to be added to self-etch primers; a more stable longevity of the bonds was obtained with the

primer with 30% of phosphate methacrylate.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Self-etch adhesives (SEAs) are increasingly being used indentistry. The self-etch approach reduces the risk of post-opera-tive sensitivity by reducing the discrepancy between the depthsof dentin demineralization and monomer infiltration usuallyassociated with etch-and-rinse systems. As self-etch systems areapplied to the dry substrate they also eliminate the step ofcontrolling the dentin moisture for proper bonding, whichreduces the technique-sensitivity of bonding agents.

Self-etch primers are made up of acidic functional monomers,hydrophilic monomers, water and ethanol. The acidic monomersare responsible for demineralization; water is required to pro-mote ionization of these functional monomers and enable theetching effect [1,2]. Previous studies have reported on the impactof water concentration on the strength of bonding and etching

ll rights reserved.

0.

s).

aggressiveness of self-etch primers applied to enamel and dentin[1–3]. Ethanol, on the other hand, is added to aid in removing theexcess water after the primer is applied. Dentin is a naturallymoist tissue; therefore, hydrophilic monomers are necessary toimprove the infiltration of the solution into the demineralizedsubstrate [4].

Apart from the effect of water concentration, little is knownregarding the effect of the concentration of acidic and conven-tional methacrylate monomers on the bonding mechanism todentin. Ogliari et al. [5] reported that the concentration of acidicmonomer has an impact on the immediate bond strength todentin; however, the effect of the functional monomer content onthe longevity of the bonds is unknown. Increasing the concentra-tion of acidic monomers may increase the etching aggressivenessof the solution. However, increased aggressiveness may notalways generate higher bond strengths [3], especially consideringthe long-term stability of the etched substrate.

The purpose of this study was to evaluate the effect of theconcentration of acidic functional methacrylate on the 1-yeardentin bond stability of a model two-step, self-etch adhesive

F.B. Leal et al. / International Journal of Adhesion & Adhesives 31 (2011) 571–574572

system. The null-hypothesis tested was that the acidic monomerconcentration would have no significant influence on the degra-dation of the dentin bonds.

2. Materials and methods

2.1. Synthesis of 1,3-glycerol dimethacrylate phosphate (GDMA-P)

The synthesis of the monomers followed similar proceduresdescribed previously [1,3,5]. In summary, a round-bottomedvessel at 0 1C was charged with methylene chloride. Phosphoruspentoxide was added; the slurry was vigorously stirred with amagnetic bar. With an addition funnel, the monomer 1,3-glyceroldimethacrylate (GDMA) from Esstech Inc. (Essington, PA, USA),dissolved in methylene chloride, was added slowly for 2 h; thereaction proceeded at room temperature for 24 h. The productwas filtered; butylated hydroxytoluene was added as a radical

Fig. 1. Molecular structure of the synthesized acidic monomer GDMA-P. Product

is an equimolar mixture of mono and di-ester phosphates containing two or four

methacrylate groups, respectively.

Fig. 2. Fourier transform mid-infrared spectra (32 scans, 4 cm�1 resolution) of the

scavenger, and the methylene chloride was removed underdistillation using a rotary evaporator. The final product was anequimolar mixture of mono and di-ester phosphates containingtwo or four methacrylate groups, respectively. The reactionproduct was then used for formulating the experimental self-etchprimers. The molecular structure of the monomers is shown inFig. 1. Fig. 2 shows the Fourier transform mid-infrared spectra ofthe starting reagent and synthesized phosphate monomer. Themain evidence of the successful synthesis was the disappearanceof the band corresponding to the hydroxyl from GDMA in therange between 3300 and 3600 cm�1.

2.2. Formulation of the self-etch primers

Six experimental self-etch primers were formulated usinghydroxyethyl methacrylate (HEMA), GDMA-P, ethanol and/ordistilled water, as described in Table 1. GDMA-P was addedserially replacing HEMA, while a 30% mass of solvents (ethanoland water) was kept constant. The primers were labeled as P0 toP–100, according to the concentration of the acidic monomer. ThepH of the primers was measured using a pHmeter (An2000;Analion, Ribeir~ao Preto, SP, Brazil). An experimental unfilled resinmade up of crosslinking methacrylates, hydrophilic methacrylate,photoinitiators and stabilizers was used as bonding resin.

2.3. Preparation of specimens

Thirty extracted bovine incisors were used. The buccal faceswere wet-ground to create a flat surface in medium dentin. Thedentin surface was wet-polished with 600-grit SiC papers for1 min to standardize the smear layer. The specimens wererandomly separated into six groups according to the primer used.After thorough rinsing water was removed with absorbent paper,which left the surface visibly wet. The prepared dentin surfaceswere vigorously etched with the primers for 30 s using a micro-brush and air-dried for 10 s. One coat of the unfilled resin wasthen applied and photoactivated for 20 s using a light-emitting

starting reagent (GDMA) and the synthesized phosphate monomer (GDMA-P).

Table 1Composition of the experimental self-etch primers (mass %).

P0 P15 P30 P50 P70 P100

HEMA 70 55 40 20 0 0

GDMA-P 0 15 30 50 70 100

Ethanol 15 15 15 15 15 0

Water 15 15 15 15 15 0

pH 4.6 1.3 1.1 0.9 0.8 a

a The pH could not be measured as the material contained no water.

Table 2Means (SD) for bond strength (MPa)a.

24 hours 6 months 1 year

P15 41.8 (11.6)A,b 37.6 (10.7)AB,b 29.2 (13.4)B,b

P30 47.5 (6.4)A,b 50.7 (12.9)A,a 44.7 (15.1)A,a

P50 58.8 (13.1)A,a 53.3 (10.3)AB,a 44.5 (12.0)B,a

Distinct capital letters in the same row indicate differences for the storage period.

Distinct lowercase letters in the same column indicate differences for concentra-

tion of acidic monomer.

a It was not possible to test any specimen for primers P0, P70 and P100 as all

beams debonded prematurely during sectioning.

F.B. Leal et al. / International Journal of Adhesion & Adhesives 31 (2011) 571–574 573

diode at 800 m W/cm2 irradiance (Radii; SDI, Bayswater, Victoria,Australia).

A composite restoration was built-up on each dental surfaceusing 2 mm increments of a resin composite (Charisma; HeraeusKulzer, Hanau, Germany); each increment was photoactivated for20 s. After storage in distilled water at 37 1C for 24 h, the speci-mens were sectioned perpendicular to the bonded interface witha refrigerated low-speed diamond saw into beams with cross-sectional area of approximately 0.5 mm2. For each tooth 9 beamswere obtained and randomly separated into three storage times(24 h, 6 months or 1 year). For each storage time, 15 beams werestored in distilled water at 37 1C; the storage medium waschanged every other week.

2.4. Bond strength testing

After each storage period the specimens were fixed to the gripsof a microtensile device, and were tested in tension on amechanical testing machine (DL500; EMIC, S~ao Jose dos Pinhais,PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. Bondstrength values were calculated in MPa. Data were submittedto two-way ANOVA (acidic monomer concentration vs. storageperiod). All pairwise multiple comparison procedures wereperformed by the Fisher’s LSD method (Po0.05). After testingthe fractured specimens were carefully removed from the testingdevice and analyzed under optical microscopy at 100 and 500�magnifications. The modes of failure were classified as adhesivefailure, cohesive failure within adhesive resin, cohesive failurewithin dentin or mixed failure.

Fig. 4. Distribution of failure modes for all groups (15, 30 and 50% are acidic

monomer concentrations).

3. Results

As shown in Table 1, the addition of GDMA-P had a directeffect on the pH of the primers. Fig. 3 depicts a non-linearregression plot with pH as dependent variable, which showedthat the increased incorporation of acidic monomer was asso-ciated with an exponential decrease in pH (R2

¼0.999; Po0.001).Table 2 presents the results for bond strength. The statisticalanalysis showed the factors ‘monomer concentration’ and ‘storageperiod’ were both significant (Po0.001), whereas the interactionbetween these two factors was not significant (P¼0.4). It was notpossible to test any specimen obtained using the primers P0, P70

and P100 as all beams debonded prematurely during the

Fig. 3. Non-linear regression plot with pH of the experimental self-etch primer as

dependent variable. Model was significant thus showing an exponential decrease

in pH associated with the increased concentration of acidic monomer.

sectioning procedures. Comparing the primers at 24 h, P50

showed significantly higher bond strength than P15 and P30

(Pr0.014). After 6 months and 1 year, P30 and P50 showed similarresults (PZ0.59), and both presented significantly higher bondstrengths than P15 (Pr0.006).

Regarding the storage periods the bond strength after 6 monthswas similar to that of 24 h for P15 and P50 (PZ0.269); for bothprimers the bond strength after 1 year was significantly lowercompared with that of 24 h (Pr0.007). P30, in contrast, showedno significant differences in bond strength over the 1-year storageperiod. Results for failure modes are shown in Fig. 4. A predomi-nance of mixed failures was detected for all primers at 24 h. After6 months while P15 still presented a predominance of mixedfailures, P30 and P50 showed a predominance of adhesive failures.After 1 year the predominant failure mode for all primers wascohesive within dentin.

4. Discussion

As the acidic monomer concentration had a significant effecton the longevity of the bonds the null-hypothesis tested wasrejected. The immediate bond strength was also affected by theconcentration of acidic monomer: P15 and P30 showed lower bondstrength compared with P50. This result is probably related tothe lower concentration of phosphate monomers rendering theprimers’ lower ability to demineralize and infiltrate the substrate.At least theoretically, the higher the content of GDMA-P, the greaterits potential to dissociate into its ionic form and provide protons fordemineralization [6]. The present findings indicate, however, thatthere is a limit to the amount of acidic monomers that can be addedto self-etch primers to prevent impairing their bonding capacity.

F.B. Leal et al. / International Journal of Adhesion & Adhesives 31 (2011) 571–574574

As expected, all beams obtained using the primer with noGDMA-P content debonded prematurely. The fact that all speci-mens obtained using the primers with 70 and 100% of GDMA-Palso failed prematurely is interesting. This finding indicates that amass of at least 20% of HEMA in the primer was needed for theself-etch adhesive to bond to dentin effectively. Van Landuyt et al.[7] reported that small amounts of HEMA improved the bondingefficacy of an one-step, self-etch adhesive. The effect of HEMA isattributed to its polar properties and small dimensions, enhancingthe wetting properties of the adhesive and penetration efficacyinto the dentin [7,8], and facilitating subsequent penetration ofthe bonding resin that copolymerizes with the primer.

The adverse effect of high concentrations of GDMA-P mightalso be related to the high concentration of acidic groups inter-fering with the polymerization mechanism of the adhesive resin.Acidic monomers may exert a detrimental effect on both the rateand extent of co-polymerization with unmodified methacrylates[9,10]. This event is explained by a deactivating effect of the acidicgroups on free radicals, as well as a neutralization effect of theamine coinitiator. Radicals terminated by an acid group are stableand hence less reactive than free radicals derived from unmodi-fied monomers, reducing the readiness with which the polymer-ization occurs [9]. For the primer P100 another detrimental effectis the absence of water. Although previous studies suggest thewater deriving from the underlying dentin may enable ionizationof the acidic monomers [1–3], the amount of water accessible inthe conditions of the present study was probably not enough toallow this effect.

When observing the degradation patterns over the course oftime, the primers P15 and P50 were less stable than the primer P30.This result, in addition to the lower immediate dentin bondstrength of P15, indicates that a concentration of 15% of acidicmonomer is not enough to demineralize the dentin and obtain along-lasting bond. For P50, despite its higher immediate bondstrength, the hydrolytic degradation was more pronounced com-pared with P30. This result might be explained by a probable morehomogeneous hybridization obtained with the primer containing30% of GDMA-P. The high content of GDMA-P in P50 may render thissolution a great capacity to demineralize the surface; this effect,however, could make it more difficult for the bonding resinto completely infiltrate the irregularities created on the dentin.Non-infiltrated areas may allow water to access the polymer thusenhancing the degradation of the bonding assembly during storage.

The failure analysis provided good evidence regarding theinfluence of the concentration of acidic monomer on the longevityof the bonds: a shift in the failure modes was noticed during theaging period. In the 24 h evaluation a predominance of mixedfailures was detected. After 6 months, adhesive failures werepredominant for P30 and P50. This indicates the storage in waterwas able to interfere with the failure mode but not with the bondstrength values; none of the primers showed differences in bondstrength between the times 24 h and 6 months. This findingreinforces the need to associate bond strength data with failureanalysis, while also indicates a hydrolytic effect of the polymerswas occurring. For P15 the 6-month storage did not affect thefailure modes, probably because a less conditioned surface wasformed; hence not many areas were available for water accessingthe bonding assembly during storage.

After 1 year, on the other hand, a predominance of cohesivefailures within the dentin substrate was detected. In addition tothe hydrolysis of the adhesive polymer, it has been shown thatthe collagen component of the dentin can also suffer fromhydrolytic degradation over the course of time [11]. This eventhas been associated with activation of dentin intrinsic matrixmetalloproteinases (MMPs) [12]. In addition, some self-etch

systems produce continuous in-depth demineralization of dentinafter polymerization [13], creating an etched, non-infiltrated layeralong the base of the hybrid layer that is prone to collagendegradation by the MMPs. This can be linked to the shift tocohesive failures within dentin after 1 year. Although one couldexpect this effect to be more pronounced for primers withhigher contents of acidic monomers, all primers showed the samebehavior.

The present results indicate that the content of acidic mono-mer affects both the immediate bond strength and the bondstrength after water storage of two-step, self-etch systems. Thefailure analysis provided evidence that a hydrolytic effect wastaking place during the whole time the specimens were stored inwater; however, the bond strength test by itself was unable todetect significant changes after 6 months. The primer with 30% ofGDMA-P achieved a more stable bond than the others primers.Under clinical conditions, the use of self-etch adhesives withoptimal concentration of acidic and conventional methacrylatescould improve the clinical service of restorations [14].

5. Conclusion

The increasing incorporation of acidic monomer is associatedwith an exponential decrease in pH of self-etch primers. A massfraction of 50% of phosphate monomer is a limit to be added. Theimmediate bond strength was higher for the primer with 50% ofacidic monomer, but a more stable longevity of the bonds over a1-year storage period was obtained with the primer with 30% ofphosphate methacrylate.

References

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[11] Osorio R, Osorio E, Aguilera FS, Tay FR, Pinto A, Toledano M. Influence ofapplication parameters on bond strength of an ‘‘all in one’’ water-based self-etching primer/adhesive after 6 and 12 months of water aging. Odontology2010;98:117–25.

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