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Polypropylene glycol phosphate methacrylate as an alternative acid-functional monomer on self-etching adhesives
journal of dentistry 43 (2015) 94–102
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Polypropylene glycol phosphate methacrylate as an alternative acid-functional monomer on self-etching adhesives Eliseu Aldrighi Mu¨nchow a,*, Adriana Fernandes da Silva b,c, Giana da Silveira Lima b,c, Tais Wulff b, Marı´lia Barbosa b, Fabrı´cio Aulo Ogliari b,d, Evandro Piva b,c a
Graduate Program in Dentistry, School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil Developmental and Control Center of Biomaterials, Federal University of Pelotas, Pelotas, RS, Brazil c Department of Operative Dentistry, School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil d Department of Polymeric Materials, School of Materials Engineering, Federal University of Pelotas, Pelotas, RS, Brazil b
article info
abstract
Article history:
Objectives: The aim of this study was to synthesize an alternative acidic monomer (poly-
Received 27 July 2014
propylene glycol phosphate methacrylate – Poly-P) to constitute experimental two-step self-
Received in revised form
etch adhesive systems and also to evaluate its influence on the pH and microshear bond
31 October 2014
strength (mSBS) to enamel.
Accepted 10 November 2014
Methods: Primers containing Poly-P (10, 15, 30 or 50 wt.%), HEMA, ethanol, and water were prepared and allocated in subgroups according to a buffered or non-buffered pH. One experimental control and one commercial (ClearfillTM SE Bond) references were used. mSBS
Keywords:
protocol was performed at human enamel, followed by mechanical testing. Scanning
Dental adhesive
electron microscopy (SEM) was performed after each primer application. Data was analysed
Micro-shear bond strength
by one-way Kruskal–Wallis and Student–Newman–Keuls tests ( p < 0.05) and by linear
Self-etching
regression predictive models.
Acidic monomer
Results: As greater the Poly-P content, the lower the primer’s pH. Buffered groups showed
pH
lower mSBS values than non-buffered groups. Groups with Poly-P content equal or higher
Scanning electron microscopy
than 30 wt.% showed similar mSBS results when compared to the controls. SEM images demonstrated that primers with high Poly-P content etched enamel with prisms exposure. Conclusions: The pH of the primer was directly influenced by the concentration of acidic monomer, which directly affected the adhesion to enamel. Both the acidity and the type of acid-functional monomer present in the adhesive influenced the bond strength results. Clinical significance: Poly-P synthesis was easy and effective, and considering the good bond strength results obtained, this acid-functional monomer may be potentially used in the formulation of self-etch dental adhesive systems. # 2014 Elsevier Ltd. All rights reserved.
* Corresponding author at: Rua Gonc¸alves Chaves, 457 – Developmental and Controlling Center of Biomaterials (CDC-Bio), Pelotas, RS 96015-560, Brazil. Tel.: +55 53 32226690; fax: +55 5332226690. E-mail address: [email protected] (E.A. Mu¨nchow). http://dx.doi.org/10.1016/j.jdent.2014.11.005 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
journal of dentistry 43 (2015) 94–102
1.
Introduction
Dental adhesive systems have already demonstrated efficacy and satisfactoriness in bonding to both enamel and dentine substrates,1–5 although their bonding ability is strongly influenced by the type of material and/or adhesion strategy used.5 While an adequate bonding to dentine is completely possible to be achieved with both etch-and-rinse or self-etch adhesives, at enamel, an etch-and-rinse approach using phosphoric acid remains the choice of preference.5–7 According to Van Meerbeek et al.,5 this is mainly because micromechanical interlocking is still the best strategy to bond to enamel. However, self-etch adhesives have less acidic composition when compared to phosphoric acid, thus reducing their potential to demineralise the full-mineral phase of enamel and consequently to create micro-retentive porosities.3 Considering their initial acidity, self-etch adhesives can be classified as ‘‘strong’’ (pH 1), ‘‘intermediately strong’’ (pH between 1 and 2), ‘‘mild’’ (pH around 2), or ‘‘ultra-mild’’ (pH > 2.5) adhesives.5,8 The type of acidic monomer incorporated into the adhesive formulation influences on the material’s acidity potential. The most common acid-functional monomers present in self-etch dental materials (e.g., adhesive systems, resin cements) are carboxylic- or phosphate-based, which are normally obtained by the reaction between (meth)acrylates and molecules containing carboxylic acid or phosphoric acid groups, respectively.5,9 For many years the acidity of self-etch adhesives was considered the main factor related to their interaction with tooth substrates.10 Notwithstanding, Yoshida et al.11 and Yoshioka et al.,12 demonstrated that acids may not interact with the substrate only by an acid-dependent mechanism, but mainly by a process called ‘‘Adhesion-Decalcification concept’’ (AD-concept), which can be separated in two phases: first any acid molecule chemically bond to hydroxyapatite, forming an ionic interaction with the substrate (calcium salt) and thus an electro-neutral surface; next, depending on the stability of the calcium salt formed, the acid may remain bonded (adhesion) or de-bond (decalcification) from the surface. Generally, ‘‘strong’’ self-etch adhesives are more prone to decalcify the substrate when compared to less acidic adhesives.5 Even though the AD-concept reasonably explains the main mechanism involved in bonding to tooth substrates by application of acidic solutions, self-etch adhesives still fail in promoting satisfactory and long-durable bond strength to enamel.2,3,6,7 This is mainly due to their limited potential in creating the micro-mechanical interlocking between resin monomers and mineralized enamel.13 Moreover, today the use of ‘‘strong’’ self-etch adhesives has been discouraged since the ‘‘mild’’ or ‘‘ultra-mild’’ materials would be less aggressive to dental tissues.5 On the other hand, the latter adhesives are less effective in etching enamel, thus leading to poor hybridization.5 This fact rises back to the question that at least for enamel the acidity of the self-etch adhesive may be an important factor related to its bonding ability. Although some previous studies have already investigated the influence of the acidity of self-etch adhesives on their bonding ability to tooth substrates,14–16 the literature lacks in
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studies where this factor (acidity) was properly controlled and isolated from compositional variables of different commercial products. Hence, the aim of this study was first to synthesize an alternative acid-functional monomer for constitute an experimental two-step self-etch adhesive system, and second to evaluate the influence of the pH of self-etch primers containing different content of acidic monomer on the bond strength to enamel. The hypotheses formulated were as follow: (1) as higher the concentration of acidic monomer, the greater the primer acidity; and (2) as higher the primer acidity, the greater the bond strength to enamel.
2.
Materials and methods
2.1.
Reagents
Methylene chloride was obtained from Synth (Diadema, Sa˜o Paulo, SP, Brazil). Phosphorous pentoxide and ethanol were purchased from Vetec (Duque de Caxias, RJ, Brazil). Poly (propylene glycol) monomethacrylate, 2-hydroxyethyl methacrylate (HEMA), bisphenol A glicidyl dimethacrylate (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), and camphorquinone (CQ) were obtained from Esstech (Essignton, PA, USA). 2,6-di-tertbutyl-4-methyl phenol (BHT) was purchased from Merck Millipore Brasil (Porto Alegre, RS, Brazil). Ethyl 4(dimethylamino)benzoate (EDAB) was obtained from Fluka (Milwaukee, WI, USA). All the reagents were used as received, without further purification.
2.2.
Synthesis of the acid-functional phosphate monomer
The phosphate monomer was synthesized by the cyclohexane azeotrope method, using a 100 mL round-bottom vessel and ice bath of 50 mL of methylene chloride, as described in a previous study.17 Briefly, phosphorous pentoxide (4.82 mmol) was added and the slurry was vigorously agitated with a stirring magnetic bar. In the sequence, poly (propylene glycol) monomethacrylate (29 mmol) was slowly and continuously added during 1 h using an addition funnel. The ice bath was then removed and the reaction was conducted at room temperature for 24 h. The product was filtered and BHT (6 mg) was added, and the solvent was evaporated in the rotavapor (Model MA 055, Marconi – Equipamentos para laborato´rio, Piracicaba, SP, Brazil). Finally, the product obtained (hereafter referred to as ‘‘Poly-P’’) was verified by Fourier transform infrared spectroscopy (FTIR) analysis.
2.3. Formulation of experimental two-step self-etch adhesive systems and pH measurement Two-step self-etch adhesive systems were prepared in the study. Four experimental self-etch primers were formulated mixing Poly-P, HEMA, ethanol, and water. The primers differed in the concentration of Poly-P (10, 15, 30, or 50 wt.%) and HEMA (50, 45, 30, or 10 wt.%), while the solvents were used at the same concentration (20:20 w/w), as shown in Table 1. Each primer was then equally divided into two flasks: one received sodium hydroxide to titrate the solution pH in 2 (buffered
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Table 1 – Self-etch primers composition. Primers
P10b P15b P30b P50b
and and and and
2.5. Microshear bond strength test and fracture mode analysis
Composition (wt.%)
P10nb P15nb P30nb P50nb
Poly-P
HEMA
Ethanol
Water
10 15 30 50
50 45 30 10
20 20 20 20
20 20 20 20
‘‘b’’, self-primer with buffered pH; ‘‘nb’’, self-primer with nonbuffered pH. Poly-P, equimolar mixture of polypropylene glycol phosphate methacrylate/polypropylene glycol phosphate dimethacrylate; HEMA, 2-hydroxyethyl methacrylate.
group, ‘‘b’’), whereas the other maintained the solution intact (non-buffered group, ‘‘nb’’). The pH was measured in triplicate using a digital pHmeter (Analion, FM 608, Brazil). A referential control primer (PC) was also formulated mixing 30 wt.% of methacryloyloxyethyl dihydrogen phosphate/bis(methacryloyloxyethyl) hydrogen phosphate (MEP/Bis-MEP), 30 wt.% of HEMA, 20 wt.% of ethanol, and 20 wt.% of water, as previously described.18 The PC pH was also measured with the pHmeter (n = 3) and the mean was 1.5. In addition, a resin bond was formulated mixing 50 wt.% of Bis-GMA, 25 wt.% of TEGDMA, 25 wt.% of HEMA, 0.4 wt.% of CQ, and 0.8 wt.% of EDAB, which was used as a universal resin bond with all experimental selfprimers. ClearfilTM SE Bond (CLSE) (Kuraray Medical Inc, Tokyo, Japan) was used as a commercial reference material, which pH was 2.0.
2.4.
Specimen preparation
Sixty freshly extracted sound human molars were selected following approval by the local Institutional Review Board Committee (protocol No. 078/2009). The teeth were first stored in 0.5% chloramine T solution for 7 days and then transferred to distilled water and kept frozen until their use. Ten of these teeth were separated for scanning electron microscopy analysis. The roots of 50 teeth were removed and each crown was mesiodistally sectioned (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA), totalizing 100 halves (n = 10), which were embedded in acrylic resin with the buccal/lingual surfaces exposed. Next, the surfaces were grounded in wet 600-grit silicon carbide paper in order to remove the aprismatic enamel layer and to standardize the surface. In the sequence, the adhesive systems were applied: one coat of the respective selfprimer (experimental or PC), which were applied for 20 s and gently air-dried for 10 s, and one coat of the resin bond, which was applied for 10 s. CLSE was applied following the manufacturer instructions. To delimitate the bond testing area, a silicon matrix (array) with two orifices of 1.5 mm in diameter was positioned over the enamel surface,19 followed by light-activation for 20 s in each orifice with a light-emitting diode light-curing unit (Radii SDI, Bayswater, VIC, Australia). Then, an increment of resin composite (Filtek Z-250, 3M ESPE, St. Paul, MN, USA) was placed into the orifices and lightactivated for 40 s. Finally, the matrix was gently removed, resulting in specimens with two cylindrical restorations at the enamel surface, which were stored in distilled water at 37 8C for 24 h.
The specimens were positioned in a universal testing machine (DL500, EMIC, Sa˜o Jose´ dos Pinhais, PR, Brazil). A thin wire was looped around the resin composite specimens, which were then submitted to shear bond strength test at a crosshead speed of 1 mm/min and the results were expressed in MPa. Each enamel surface was then examined at 20 magnification under a light stereomicroscope for the failure mode analysis, which were classified as adhesive (on the adhesive interface), cohesive in enamel, cohesive in resin adhesive, or mixed.
2.6.
Scanning electron microscopy (SEM) evaluation
Ten teeth had their roots and part of the crown area cut off, remaining only the buccal surface. Each sample was grounded in wet 320-, 600-, and 1500-grit silicon carbide papers, followed by polishing with felt disc for 5 min, cleansing in ultrasound, and drying procedure. Each sample was then treated with one of the 10 self-primers investigated in this study: 8 experimental (4 with buffered pH and 4 with non-buffered pH), 1 control reference, and 1 commercial reference. All primers were applied as previously described. Next, the surfaces were dehydrated in ascending ethanol concentrations (70, 80, 90, and 100%) for 15 min each, dried for 15 s with an air-spray, and kept in contact with a filter paper at room temperature for 24 h. After dehydration process, the surfaces were mounted on aluminium stubs and sputter-coated with gold/palladium for SEM evaluation using a scanning electron microscope (SSX550, Shimadzu, Tokyo, Japan). The etching pattern and enamel surface characteristics were evaluated qualitatively according to the images obtained.
2.7.
Statistical analysis
The microshear bond strength data obtained in this study was analysed with the statistical programme SigmaPlot version 12 (Systat Software Inc., San Jose, CA, USA) using Kruskal–Wallis one way analysis of variance on ranks and the Student– Newman–Keuls test for multiple comparison (a = 5%). Two linear regression models were used: one to analyse the correlation between the acidity of self-primers (pH) and Poly-P concentration, and the other to analyse the correlation between the acidity of self-primers (pH) and enamel bond strength results.
3.
Results
3.1.
Synthesis of the acid-functional phosphate monomer
The phosphate monomer was successfully synthesized, yielding approximately 100%. The reaction resulted in an equimolar mixture of monoester (polypropylene glycol phosphate methacrylate) and diester (polypropylene glycol phosphate dimethacrylate) monomers, which molecular structures are illustrated in Fig. 1. The FTIR spectra of the starting reagent (poly (propylene glycol) monomethacrylate) and the final
journal of dentistry 43 (2015) 94–102
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Fig. 1 – Molecular structure, molecular formula (MF), and molar mass (MM) of the (1) monoester ‘‘polypropylene glycol phosphate methacrylate’’ and of the (2) diester ‘‘polypropylene glycol phosphate dimethacrylate’’ synthesized in the study.
product (Poly-P) can be observed in Fig. 2. Accidental polymerization was not detected during the reaction time or in the purification procedure.
3.2.
pH vs. Poly-P concentration
The linear regression model showed an inverse relationship between Poly-P concentration and the pH of the non-buffered primers (R2 = 0.987) (Fig. 3a). As higher the Poly-P concentration, the lower the pH value obtained ( p = 0.007), with P10nb, P15nb, P30nb, and P50nb showing, respectively, pH values equal to 1.7, 1.6, 1.4, and 1.2 (Table 2).
3.3. Microshear bond strength to enamel and fracture mode analysis Medians of the microshear bond strength data are presented in Table 2. The median values obtained with the reference
materials (PC and CLSE) and with the non-buffered adhesive systems containing high Poly-P concentration (30 and 50 wt.%) were similar to each other, but these four groups showed statistically higher median values than other groups ( p < 0.001). The non-buffered adhesives containing low Poly-P content (10 and 15 wt.%) were similar. The statistically lowest microshear bond strength was found in group P30b ( p < 0.001). The linear regression model showed an inverse relationship between microshear bond strength and pH of the experimental self-primers containing Poly-P (R2 = 0.836) (Fig. 3b), and the lower the pH value, the higher the bond strength obtained ( p = 0.001). Table 2 also shows the percentage of the fracture modes obtained in the study, which were adhesive or mixed patterns. The experimental buffered adhesives presented predominance of adhesive failures, whereas the non-buffered and reference adhesives showed higher percentage of mixed failures than adhesive failures.
Fig. 2 – FTIR spectra of starting reagent (poly (propylene glycol) monomethacrylate) and the final product polypropylene glycol phosphate methacrylate plus polypropylene glycol phosphate dimethacrylate (Poly-P) in an equimolar mixture.
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journal of dentistry 43 (2015) 94–102
Fig. 3 – (a) Linear regression predictive model that shows a significant relationship between Poly-P content and the pH of the experimental non-buffered primers (R2 = 0.987; p = 0.007); and (b) linear regression predictive model that demonstrates a significant relationship between microshear bond strength and pH of the experimental primers (R2 = 0.836; p = 0.001).
3.4.
SEM analysis
The etching pattern obtained with the application of each experimental and reference self-primers are illustrated in Fig. 4. The enamel prisms cores were exposed only with the application of self-primers containing 30 or 50 wt.% of Poly-P (Fig. 4e–h), although this effect was more evident after application of the non-buffered primers (Fig. 4f and h). The primers with low Poly-P content did not modify the enamel topography (Fig. 4a–d), although the application of P15nb (Fig. 4d) resulted in a slightly more irregular surface than the others. The control reference (PC) slightly exposed the enamel prisms cores, and the commercial reference (CLSE) did not expose prisms core, but clearly produced a rough and irregular topography.
4.
Discussion
The first purpose of this study was to synthesize an alternative acid-functional monomer to constitute a two-step self-etch adhesive system. This synthesis was aimed because a satisfactory resin–enamel bond has not been yet completely achieved when using self-etch adhesives, even with the existence of various acidic monomers available for this purpose and in different materials. Moreover, according to Van Meerbeek et al.,5 adhesive dentistry still needs efficient acid-functional monomers able to produce good and stable chemical bonding to tooth substrates. Most of the acidic monomers used in self-etch formulations are hydrophilic in
nature, once they are usually constituted of carboxylic or phosphoric functional groups.9 Nonetheless, enamel is a hydrophobic substrate, thus requiring treatment with solutions the less hydrophilic as possible. The phosphate monomer synthesized in the present study was intended due to its long carbon chain molecule and low content of polar groups (Fig. 1), which is believed to confer moderate hydrophobicity to the final material, and consequently, better chemical compatibility with enamel. A phosphate monomer rather than carboxylic was chosen because according to Yoshida et al.,20 the latter may weakly interact with hydroxyapatite minerals when compared to phosphate-based monomers. The reaction between phosphorous pentoxide and poly (propylene glycol) monomethacrylate was successful since a clear consumption of hydroxyl groups at peak 3600–3300 cm 1 as well as the appearance of absorptions for phosphorous compounds (phosphate esters at peaks 1299–1250 cm 1; P–OH group at peak 1040–910 cm 1; and P–O–C (aliphatic) group at peak 830–740 cm 1) could be observed in the final product molecule (Fig. 2). Indeed, the synthesis was performed at room temperature, without catalysts, and providing high yields and easy purification. The first study hypothesis was that the higher the concentration of acidic monomer, the greater the acidity of the self-primer. According to Fig. 3a, this hypothesis can be fully accepted, since the increase in the Poly-P content from 10 to 50 wt.% progressively reduced the pH of the primer ( p = 0.007), showing an excellent adjust to the linear regression predictive model (R2 = 0.986) used. This result corroborates with a previous study,21 and may be explained because
Table 2 – pH value of the experimental/referential self-primers evaluated in this study, microshear bond strength (mSBS, in MPA) median values, and ratio of adhesive/mixed fracture modes (AD/MX). Primersa pH mSBSb AD/MX a
P10b
P15b
P30b
P50b
P10nb
P15nb
P30nb
P50nb
PC
CLSE
2.0 10.1 C 40/60
2.0 9.6 C 55/45
2.0 7.5 D 70/30
2.0 9.9 C 65/35
1.7 11.0 B 50/50
1.6 11.5 B 40/60
1.4 16.1 A 10/90
1.2 15.3 A 10/90
1.5 19.6 A 15/85
2.0 18.6 A 20/80
Subscript numbers next to the Primer (P) indicate the concentration of Poly-P. PC, control reference; CLSE, ClearfilTM SE Bond (commercial reference); ‘‘b’’, primer with buffered pH; ‘‘nb’’, primer with non-buffered pH. b Different letters after median values indicate statistically significant differences among the adhesive systems evaluated ( p < 0.05).
journal of dentistry 43 (2015) 94–102
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Fig. 4 – SEM images of the etching ability of the self-primers (experimental and referential ones). P10b (a) – the surface is rough, with grooves (white arrows) resulted from the SiC abrasion procedure; the primer application did not enhance that roughness; P10nb (b); P15b (c) – the surface is rough as image ‘‘a’’; P15nb (d) – a rougher surface is achieved, but without exposing the enamel rods; P30b (e) – the enamel rods are partially exposed (black arrows), and the SiC abrasion grooves are
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journal of dentistry 43 (2015) 94–102
the higher the content of Poly-P, the greater the potential to dissociate into its ionic form and generate protons, which in turn makes the medium acidic.10 In addition, the content of water, which is essential to ionize acidic molecules,22 has been previously demonstrated to affect the bonding performance of self-etch adhesive systems.18 Considering this, the present study used the same concentration of water in the preparation of all experimental materials (Table 1). The second purpose of the present study was to evaluate the influence of the pH of self-etch primers containing different content of acidic monomer on the bond strength to enamel, testing the hypothesis that the higher the acidity of the self-primer, the greater the resin–enamel bond strength. To properly investigate this hypothesis, each primer formulated was divided into two identical parts, in which one the pH of the solution was maintained intact (non-buffered group), and in the other the pH was adjusted to an exact value (pH 2, buffered group). Following this study design, only the pH of the materials was truly different, thus allowing high control and reduction of variables. According to Fig. 3b, the lower the pH of the self-primer, the higher the bond strength results ( p = 0.001), showing a satisfactory adjust to the linear regression predictive model (R2 = 0.836) used. Therefore, the second hypothesis was also totally accepted. It is well known that bonding to enamel is an aciddependent process.23 Nonetheless, the acidity of self-etch primers has been disregarded as the major factor involved in the interaction between adhesives and dental substrates.11,12,24 Nevertheless, and from the best of our knowledge, this is the first study in which the pH/acidity of experimental self-etch primers was thoroughly controlled from compositional variables. Consequently, the present findings may be potentially used to add scientific information to this research field. Considering only the Poly-P-based adhesives, it can be observed that their acidity played a significant role on their bonding ability (Table 2), with all the non-buffered adhesives producing higher bond strength when compared to their buffered counterparts. Furthermore, within the non-buffered adhesives, the most acidic ones (P30nb and P50nb) resulted in higher bond strength than the less acidic groups (P10nb and P15nb). Considering that pH is the negative decadic logarithm of the hydrogen ion concentration, small reductions in pH values may represent a substantial increment in acidity.25 In fact, the difference between pH 1.2 (P50nb) and pH 1.7 (P10nb) correspond to a 5-fold increase in acidity, thus meaning that the former self-primer was five times more acidic than the latter. Therefore, all the aforementioned strongly suggests that the acidity of self-etch adhesives influence on the bond strength to enamel, at least when the performance of materials containing the same composition/ type of acidic monomer is compared between each other. The present bond strength findings are in accordance with the SEM images obtained (Fig. 4). The primers with low Poly-P content (10 and 15 wt.%), and regardless of their pH, did not
modify the enamel etching pattern, thus maintaining the same surface topography achieved during the finishing process, which was a slight grooving roughness (Fig. 4a–d). Nonetheless, the bond strength results were significantly improved after application of the non-buffered primers. In fact, after a careful analysis of the prime-etched surfaces, the non-buffered primers created a slightly more irregular surface when compared to their buffered counterparts (Fig. 4b and d). This may reinforce the positive role of the acidity on the bonding process to enamel. With regards to the more concentrated Poly-P-based primers (30 and 50 wt.%), both the buffered and the non-buffered solutions created a Type 1 enamel etching pattern (i.e., the same obtained with the application of phosphoric acid during the etch-and-rinse approach), although the non-buffered self-primers were clearly more effectives in etching the surface (Fig. 4e–h). In addition, the non-buffered self-primers resulted in higher bond strength results when compared to their buffered counterparts (Table 2). Taking these findings together, once again the acidity of the self-primer seemed to be positively related to enhanced resin–enamel bond strength. The bond strength performance of the Poly-P-based adhesive systems was also compared to internal (control reference, PC) and external (commercial reference, CLSE) materials. The PC is an effective self-etch primer previously prepared and used by our team, which has demonstrated excellent bonding ability to both enamel26 and dentine.18 It is constituted of an equimolar mixture of MEP/Bis-MEP as acidic monomers, which in a concentration of 30 wt.% (pH 1.5) resulted in the highest mSBS median value (Table 2). Differently, CLSE is a classical two-step self-etch adhesive system widely used in clinical and in vitro evaluations. It is constituted of 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which is a known phosphate acidic monomer able to interact chemically with hydroxyapatite, creating strong and stable bond strength to dental substrates.5,20 In the current study, CLSE performed as the expected, showing high bond strength to enamel (Table 2). Comparing the bonding performance of all the adhesive systems investigated among each other, it is possible to verify that adhesives containing 30 or 50 wt.% of Poly-P (but with non-buffered pH) performed similarly to the references (Table 2), suggesting that Poly-P may be potentially used as acidic monomer of self-etch formulations. Surprisingly, another important finding of the present study is that irrespective of showing similar bonding performance, the distinct adhesive formulations investigated (P30nb, P50nb, PC, and CLSE) showed different initial pH values (i.e., different acidity), as well as different etching ability (Table 2 and Fig. 4f, h–j). These results suggest that when different materials are compared to each other the acidity may not be considered as the major factor affecting their bonding ability, but probably their composition (i.e., type of acidic component used to etch the substrate), corroborating with the so called
still present (white arrow); P30nb (f) – the enamel rods are clearly exposed (Type 1 etching pattern – black arrows); P50b (g) – the Type 1 pattern is present (black arrow), but it is less evidenced; P50nb (h) – the Type 1 pattern is still present (black arrows); PC (i) – the Type 1 pattern is present (black arrow), but it is less evidenced; CLSE (j) – the enamel rods are not exposed, but a rougher surface is predominantly seen.
journal of dentistry 43 (2015) 94–102
‘A–D concept’.11,12,24 Indeed, when the SEM images are analysed together, it is possible to observe that high bond strength results have not exclusively occurred in a Type 1 enamel etching pattern, with prisms rods exposure (Fig. 4). Notwithstanding, it is once again impossible not to point out that depending on the adhesive composition (e.g., within the Poly-P-based adhesives), the higher the prisms rod exposure, the higher the resin–enamel bond strength, probably due to the greater micro-mechanical interlocking with the substrate.5 Poly-P worked well as an acid-functional monomer in the self-etch formulations presented in this study. The selfprimers containing low Poly-P content (10 and 15 wt.%) demonstrated low mSBS median values (Table 2), in part due to their lower ability to etch the enamel surface, but also probably due to their high content of HEMA (higher than 45 wt.%), which was revealed to dramatically increase the material’s hydrophylicity.27–29 According to Van Landuyt et al.,30 HEMA concentrations beyond 10 wt.% did not have any supplementary advantageous effects on the bond strength to enamel/dentine by using self-etch adhesives. Considering that enamel is more receptive to hydrophobic components, the high hydrophylic nature of the P10b, P10nb, P15b, and P15nb HEMA-rich primers (Table 1) may have contributed to poor hybridization of enamel, fact that corroborates with the fracture mode patterns obtained (Table 2), where the primers with low Poly-P content produced a predominance of adhesive failures. However, mixed failures have frequently occurred in groups showing high bond strength median values, which can be explained because since enamel is tougher than the underlying dentine and the enamel–dentine junction, crack propagation is allowed to occur through the substrate, producing the mixed/cohesive failures seen in the study.31 Despite of all the findings described, Poly-P molecule is characterized by a long-chain polymer structure constituted by oxy-ethylene units (Fig. 1), which confers higher hydrophobicity than other routinely acidic molecules present in self-etch adhesive formulations. This characteristic may be advantageous for bonding purposes to enamel, since a potential strengthening of the resin–enamel bond interface against hydrolytic degradation may be produced. Considering this aspect, studies evaluating the long-term bond strength of adhesive systems containing Poly-P are necessary. In conclusion, the acidity of self-etch adhesive systems seemed to be a determinant factor affecting the resin–enamel bond strength only when materials with the same composition were investigated; otherwise, the bonding ability of adhesives with different compositions seemed to be more related to the type of acidic monomer presented in their composition. Poly-P was successfully synthesized in the study, being a potential acid-functional monomer to be used in the formulation of self-etch dental adhesive systems.
Acknowledgements Authors are grateful to Brazilian National Council for Scientific and Technological Development (CNPq) for financial support (# 308087/2011-9), to CAPES for scholarship, and to Esstech Inc. for reagents donation.
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references
1. Marchesi G, Frassetto A, Mazzoni A, Apolonio F, Diolosa M, Cadenaro M, et al. Adhesive performance of a multi-mode adhesive system: 1-year in vitro study. Journal of Dentistry 2014;42:603–12. 2. Ozer F, Blatz MB. Self-etch and etch-and-rinse adhesive systems in clinical dentistry. Compendium of Continuing Education in Dentistry 2013;34:12–4. 16, 18; quiz 20, 30. 3. Pashley DH, Tay FR, Breschi L, Tjaderhane L, Carvalho RM, Carrilho M, et al. State of the art etch-and-rinse adhesives. Dental Materials 2011;27:1–16. 4. van Dijken JW, Sunnegardh-Gronberg K, Lindberg A. Clinical long-term retention of etch-and-rinse and self-etch adhesive systems in non-carious cervical lesions. A 13 years evaluation. Dental Materials 2007;23:1101–7. 5. Van Meerbeek B, Yoshihara K, Yoshida Y, Mine A, De Munck J, Van Landuyt KL. State of the art of self-etch adhesives. Dental Materials 2011;27:17–28. 6. Erickson RL, Barkmeier WW, Kimmes NS. Bond strength of self-etch adhesives to pre-etched enamel. Dental Materials 2009;25:1187–94. 7. Rotta M, Bresciani P, Moura SK, Grande RH, Hilgert LA, Baratieri LN, et al. Effects of phosphoric acid pretreatment and substitution of bonding resin on bonding effectiveness of self-etching systems to enamel. Journal of Adhesive Dentistry 2007;9:537–45. 8. Koshiro K, Sidhu SK, Inoue S, Ikeda T, Sano H. New concept of resin–dentin interfacial adhesion: the nanointeraction zone. Journal of Biomedical Materials Research Part B Applied Biomaterials 2006;77:401–8. 9. Ferracane JL, Stansbury JW, Burke FJ. Self-adhesive resin cements – chemistry, properties and clinical considerations. Journal of Oral Rehabilitation 2011;38:295–314. 10. Salz U, Mucke A, Zimmermann J, Tay FR, Pashley DH. pKa value and buffering capacity of acidic monomers commonly used in self-etching primers. Journal of Adhesive Dentistry 2006;8:143–50. 11. Yoshida Y, Van Meerbeek B, Nakayama Y, Yoshioka M, Snauwaert J, Abe Y, et al. Adhesion to and decalcification of hydroxyapatite by carboxylic acids. Journal of Dental Research 2001;80:1565–9. 12. Yoshioka M, Yoshida Y, Inoue S, Lambrechts P, Vanherle G, Nomura Y, et al. Adhesion/decalcification mechanisms of acid interactions with human hard tissues. Journal of Biomedical Materials Research 2002;59:56–62. 13. De Munck J, Van Landuyt K, Peumans M, Poitevin A, Lambrechts P, Braem M, et al. A critical review of the durability of adhesion to tooth tissue: methods and results. Journal of Dental Research 2005;84:118–32. 14. Kenshima S, Reis A, Uceda-Gomez N, Tancredo Lde L, Filho LE, Nogueira FN, et al. Effect of smear layer thickness and pH of self-etching adhesive systems on the bond strength and gap formation to dentin. Journal of Adhesive Dentistry 2005;7:117–26. 15. Moura SK, Pelizzaro A, Dal Bianco K, de Goes MF, Loguercio AD, Reis A, et al. Does the acidity of self-etching primers affect bond strength and surface morphology of enamel? Journal of Adhesive Dentistry 2006;8:75–83. 16. Moura SK, Reis A, Pelizzaro A, Dal-Bianco K, Loguercio AD, Arana-Chavez VE, et al. Bond strength and morphology of enamel using self-etching adhesive systems with different acidities. Journal of Applied Oral Sciences 2009;17:315–25. 17. Ogliari FA, da Silva Ede O, Lima Gda S, Madruga FC, Henn S, Bueno M, et al. Synthesis of phosphate monomers and bonding to dentin: esterification methods and use of phosphorus pentoxide. Journal of Dentistry 2008;36:171–7.
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18. Lima Gda S, Ogliari FA, da Silva EO, Ely C, Demarco FF, Carreno NL, et al. Influence of water concentration in an experimental self-etching primer on the bond strength to dentin. Journal of Adhesive Dentistry 2008;10:167–72. 19. Mu¨nchow EA, Bossardi M, Priebe TC, Valente LL, Zanchi CH, Ogliari FA, et al. Microtensile versus microshear bond strength between dental adhesives and the dentin substrate. International Journal of Adhesion and Adhesives 2013;46:95–9. 20. Yoshida Y, Nagakane K, Fukuda R, Nakayama Y, Okazaki M, Shintani H, et al. Comparative study on adhesive performance of functional monomers. Journal of Dental Research 2004;83:454–8. 21. Leal FB, Madruga FC, Prochnow EP, Lima GS, Ogliari FA, Piva E, et al. Effect of acidic monomer concentration on the dentin bond stability of self-etch adhesives. International Journal of Adhesion and Adhesives 2011;31:571–4. 22. Hiraishi N, Nishiyama N, Ikemura K, Yau JY, King NM, Tagami J, et al. Water concentration in self-etching primers affects their aggressiveness and bonding efficacy to dentin. Journal of Dental Research 2005;84:653–8. 23. Zanchia CH, D’Avila OP, Rodrigues-Junior SA, Burnett Jr LH, Demarco FF, Pinto MB. Effect of additional acid etching on bond strength and structural reliability of adhesive systems applied to caries-affected dentin. Journal of Adhesive Dentistry 2010;12:109–15. 24. Dieng-Sarr F, Sharrock P, Dabsie F, Gregoire G. Modifications of the organic and mineral fractions of dental tissues
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following conditioning by self-etching adhesives. Journal of Dentistry 2011;39:141–7. Kaaden C, Powers JM, Friedl KH, Schmalz G. Bond strength of self-etching adhesives to dental hard tissues. Clinical Oral Investigations 2002;6:155–60. Lima GS, Ogliari FA, Moraes RR, Mattos ES, Silva AF, Carren˜o NLV, et al. Water content in self-etching primers affects their aggressiveness and strength of bonding to ground enamel. The Journal of Adhesion 2010;86:939–52. Collares FM, Ogliari FA, Zanchi CH, Petzhold CL, Piva E, Samuel SM. Influence of 2-hydroxyethyl methacrylate concentration on polymer network of adhesive resin. Journal of Adhesive Dentistry 2011;13:125–9. Munchow EA, Zanchi CH, Ogliari FA, Silva MG, Oliveira IR, Piva E. Replacing HEMA with alternative dimethacrylates in dental adhesive systems: evaluation of polymerization kinetics and physicochemical properties. Journal of Adhesive Dentistry 2014;16:221–8. Takahashi M, Nakajima M, Hosaka K, Ikeda M, Foxton RM, Tagami J. Long-term evaluation of water sorption and ultimate tensile strength of HEMA-containing/-free onestep self-etch adhesives. Journal of Dentistry 2011;39:506–12. Van Landuyt KL, Snauwaert J, Peumans M, De Munck J, Lambrechts P, Van Meerbeek B. The role of HEMA in onestep self-etch adhesives. Dental Material 2008;24:1412–9. Bechtle S, Habelitz S, Klocke A, Fett T, Schneider GA. The fracture behaviour of dental enamel. Biomaterials 2010;31: 375–84.
Available online at www.sciencedirect.com
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Polypropylene glycol phosphate methacrylate as an alternative acid-functional monomer on self-etching adhesives Eliseu Aldrighi Mu¨nchow a,*, Adriana Fernandes da Silva b,c, Giana da Silveira Lima b,c, Tais Wulff b, Marı´lia Barbosa b, Fabrı´cio Aulo Ogliari b,d, Evandro Piva b,c a
Graduate Program in Dentistry, School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil Developmental and Control Center of Biomaterials, Federal University of Pelotas, Pelotas, RS, Brazil c Department of Operative Dentistry, School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil d Department of Polymeric Materials, School of Materials Engineering, Federal University of Pelotas, Pelotas, RS, Brazil b
article info
abstract
Article history:
Objectives: The aim of this study was to synthesize an alternative acidic monomer (poly-
Received 27 July 2014
propylene glycol phosphate methacrylate – Poly-P) to constitute experimental two-step self-
Received in revised form
etch adhesive systems and also to evaluate its influence on the pH and microshear bond
31 October 2014
strength (mSBS) to enamel.
Accepted 10 November 2014
Methods: Primers containing Poly-P (10, 15, 30 or 50 wt.%), HEMA, ethanol, and water were prepared and allocated in subgroups according to a buffered or non-buffered pH. One experimental control and one commercial (ClearfillTM SE Bond) references were used. mSBS
Keywords:
protocol was performed at human enamel, followed by mechanical testing. Scanning
Dental adhesive
electron microscopy (SEM) was performed after each primer application. Data was analysed
Micro-shear bond strength
by one-way Kruskal–Wallis and Student–Newman–Keuls tests ( p < 0.05) and by linear
Self-etching
regression predictive models.
Acidic monomer
Results: As greater the Poly-P content, the lower the primer’s pH. Buffered groups showed
pH
lower mSBS values than non-buffered groups. Groups with Poly-P content equal or higher
Scanning electron microscopy
than 30 wt.% showed similar mSBS results when compared to the controls. SEM images demonstrated that primers with high Poly-P content etched enamel with prisms exposure. Conclusions: The pH of the primer was directly influenced by the concentration of acidic monomer, which directly affected the adhesion to enamel. Both the acidity and the type of acid-functional monomer present in the adhesive influenced the bond strength results. Clinical significance: Poly-P synthesis was easy and effective, and considering the good bond strength results obtained, this acid-functional monomer may be potentially used in the formulation of self-etch dental adhesive systems. # 2014 Elsevier Ltd. All rights reserved.
* Corresponding author at: Rua Gonc¸alves Chaves, 457 – Developmental and Controlling Center of Biomaterials (CDC-Bio), Pelotas, RS 96015-560, Brazil. Tel.: +55 53 32226690; fax: +55 5332226690. E-mail address: [email protected] (E.A. Mu¨nchow). http://dx.doi.org/10.1016/j.jdent.2014.11.005 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
journal of dentistry 43 (2015) 94–102
1.
Introduction
Dental adhesive systems have already demonstrated efficacy and satisfactoriness in bonding to both enamel and dentine substrates,1–5 although their bonding ability is strongly influenced by the type of material and/or adhesion strategy used.5 While an adequate bonding to dentine is completely possible to be achieved with both etch-and-rinse or self-etch adhesives, at enamel, an etch-and-rinse approach using phosphoric acid remains the choice of preference.5–7 According to Van Meerbeek et al.,5 this is mainly because micromechanical interlocking is still the best strategy to bond to enamel. However, self-etch adhesives have less acidic composition when compared to phosphoric acid, thus reducing their potential to demineralise the full-mineral phase of enamel and consequently to create micro-retentive porosities.3 Considering their initial acidity, self-etch adhesives can be classified as ‘‘strong’’ (pH 1), ‘‘intermediately strong’’ (pH between 1 and 2), ‘‘mild’’ (pH around 2), or ‘‘ultra-mild’’ (pH > 2.5) adhesives.5,8 The type of acidic monomer incorporated into the adhesive formulation influences on the material’s acidity potential. The most common acid-functional monomers present in self-etch dental materials (e.g., adhesive systems, resin cements) are carboxylic- or phosphate-based, which are normally obtained by the reaction between (meth)acrylates and molecules containing carboxylic acid or phosphoric acid groups, respectively.5,9 For many years the acidity of self-etch adhesives was considered the main factor related to their interaction with tooth substrates.10 Notwithstanding, Yoshida et al.11 and Yoshioka et al.,12 demonstrated that acids may not interact with the substrate only by an acid-dependent mechanism, but mainly by a process called ‘‘Adhesion-Decalcification concept’’ (AD-concept), which can be separated in two phases: first any acid molecule chemically bond to hydroxyapatite, forming an ionic interaction with the substrate (calcium salt) and thus an electro-neutral surface; next, depending on the stability of the calcium salt formed, the acid may remain bonded (adhesion) or de-bond (decalcification) from the surface. Generally, ‘‘strong’’ self-etch adhesives are more prone to decalcify the substrate when compared to less acidic adhesives.5 Even though the AD-concept reasonably explains the main mechanism involved in bonding to tooth substrates by application of acidic solutions, self-etch adhesives still fail in promoting satisfactory and long-durable bond strength to enamel.2,3,6,7 This is mainly due to their limited potential in creating the micro-mechanical interlocking between resin monomers and mineralized enamel.13 Moreover, today the use of ‘‘strong’’ self-etch adhesives has been discouraged since the ‘‘mild’’ or ‘‘ultra-mild’’ materials would be less aggressive to dental tissues.5 On the other hand, the latter adhesives are less effective in etching enamel, thus leading to poor hybridization.5 This fact rises back to the question that at least for enamel the acidity of the self-etch adhesive may be an important factor related to its bonding ability. Although some previous studies have already investigated the influence of the acidity of self-etch adhesives on their bonding ability to tooth substrates,14–16 the literature lacks in
95
studies where this factor (acidity) was properly controlled and isolated from compositional variables of different commercial products. Hence, the aim of this study was first to synthesize an alternative acid-functional monomer for constitute an experimental two-step self-etch adhesive system, and second to evaluate the influence of the pH of self-etch primers containing different content of acidic monomer on the bond strength to enamel. The hypotheses formulated were as follow: (1) as higher the concentration of acidic monomer, the greater the primer acidity; and (2) as higher the primer acidity, the greater the bond strength to enamel.
2.
Materials and methods
2.1.
Reagents
Methylene chloride was obtained from Synth (Diadema, Sa˜o Paulo, SP, Brazil). Phosphorous pentoxide and ethanol were purchased from Vetec (Duque de Caxias, RJ, Brazil). Poly (propylene glycol) monomethacrylate, 2-hydroxyethyl methacrylate (HEMA), bisphenol A glicidyl dimethacrylate (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), and camphorquinone (CQ) were obtained from Esstech (Essignton, PA, USA). 2,6-di-tertbutyl-4-methyl phenol (BHT) was purchased from Merck Millipore Brasil (Porto Alegre, RS, Brazil). Ethyl 4(dimethylamino)benzoate (EDAB) was obtained from Fluka (Milwaukee, WI, USA). All the reagents were used as received, without further purification.
2.2.
Synthesis of the acid-functional phosphate monomer
The phosphate monomer was synthesized by the cyclohexane azeotrope method, using a 100 mL round-bottom vessel and ice bath of 50 mL of methylene chloride, as described in a previous study.17 Briefly, phosphorous pentoxide (4.82 mmol) was added and the slurry was vigorously agitated with a stirring magnetic bar. In the sequence, poly (propylene glycol) monomethacrylate (29 mmol) was slowly and continuously added during 1 h using an addition funnel. The ice bath was then removed and the reaction was conducted at room temperature for 24 h. The product was filtered and BHT (6 mg) was added, and the solvent was evaporated in the rotavapor (Model MA 055, Marconi – Equipamentos para laborato´rio, Piracicaba, SP, Brazil). Finally, the product obtained (hereafter referred to as ‘‘Poly-P’’) was verified by Fourier transform infrared spectroscopy (FTIR) analysis.
2.3. Formulation of experimental two-step self-etch adhesive systems and pH measurement Two-step self-etch adhesive systems were prepared in the study. Four experimental self-etch primers were formulated mixing Poly-P, HEMA, ethanol, and water. The primers differed in the concentration of Poly-P (10, 15, 30, or 50 wt.%) and HEMA (50, 45, 30, or 10 wt.%), while the solvents were used at the same concentration (20:20 w/w), as shown in Table 1. Each primer was then equally divided into two flasks: one received sodium hydroxide to titrate the solution pH in 2 (buffered
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Table 1 – Self-etch primers composition. Primers
P10b P15b P30b P50b
and and and and
2.5. Microshear bond strength test and fracture mode analysis
Composition (wt.%)
P10nb P15nb P30nb P50nb
Poly-P
HEMA
Ethanol
Water
10 15 30 50
50 45 30 10
20 20 20 20
20 20 20 20
‘‘b’’, self-primer with buffered pH; ‘‘nb’’, self-primer with nonbuffered pH. Poly-P, equimolar mixture of polypropylene glycol phosphate methacrylate/polypropylene glycol phosphate dimethacrylate; HEMA, 2-hydroxyethyl methacrylate.
group, ‘‘b’’), whereas the other maintained the solution intact (non-buffered group, ‘‘nb’’). The pH was measured in triplicate using a digital pHmeter (Analion, FM 608, Brazil). A referential control primer (PC) was also formulated mixing 30 wt.% of methacryloyloxyethyl dihydrogen phosphate/bis(methacryloyloxyethyl) hydrogen phosphate (MEP/Bis-MEP), 30 wt.% of HEMA, 20 wt.% of ethanol, and 20 wt.% of water, as previously described.18 The PC pH was also measured with the pHmeter (n = 3) and the mean was 1.5. In addition, a resin bond was formulated mixing 50 wt.% of Bis-GMA, 25 wt.% of TEGDMA, 25 wt.% of HEMA, 0.4 wt.% of CQ, and 0.8 wt.% of EDAB, which was used as a universal resin bond with all experimental selfprimers. ClearfilTM SE Bond (CLSE) (Kuraray Medical Inc, Tokyo, Japan) was used as a commercial reference material, which pH was 2.0.
2.4.
Specimen preparation
Sixty freshly extracted sound human molars were selected following approval by the local Institutional Review Board Committee (protocol No. 078/2009). The teeth were first stored in 0.5% chloramine T solution for 7 days and then transferred to distilled water and kept frozen until their use. Ten of these teeth were separated for scanning electron microscopy analysis. The roots of 50 teeth were removed and each crown was mesiodistally sectioned (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA), totalizing 100 halves (n = 10), which were embedded in acrylic resin with the buccal/lingual surfaces exposed. Next, the surfaces were grounded in wet 600-grit silicon carbide paper in order to remove the aprismatic enamel layer and to standardize the surface. In the sequence, the adhesive systems were applied: one coat of the respective selfprimer (experimental or PC), which were applied for 20 s and gently air-dried for 10 s, and one coat of the resin bond, which was applied for 10 s. CLSE was applied following the manufacturer instructions. To delimitate the bond testing area, a silicon matrix (array) with two orifices of 1.5 mm in diameter was positioned over the enamel surface,19 followed by light-activation for 20 s in each orifice with a light-emitting diode light-curing unit (Radii SDI, Bayswater, VIC, Australia). Then, an increment of resin composite (Filtek Z-250, 3M ESPE, St. Paul, MN, USA) was placed into the orifices and lightactivated for 40 s. Finally, the matrix was gently removed, resulting in specimens with two cylindrical restorations at the enamel surface, which were stored in distilled water at 37 8C for 24 h.
The specimens were positioned in a universal testing machine (DL500, EMIC, Sa˜o Jose´ dos Pinhais, PR, Brazil). A thin wire was looped around the resin composite specimens, which were then submitted to shear bond strength test at a crosshead speed of 1 mm/min and the results were expressed in MPa. Each enamel surface was then examined at 20 magnification under a light stereomicroscope for the failure mode analysis, which were classified as adhesive (on the adhesive interface), cohesive in enamel, cohesive in resin adhesive, or mixed.
2.6.
Scanning electron microscopy (SEM) evaluation
Ten teeth had their roots and part of the crown area cut off, remaining only the buccal surface. Each sample was grounded in wet 320-, 600-, and 1500-grit silicon carbide papers, followed by polishing with felt disc for 5 min, cleansing in ultrasound, and drying procedure. Each sample was then treated with one of the 10 self-primers investigated in this study: 8 experimental (4 with buffered pH and 4 with non-buffered pH), 1 control reference, and 1 commercial reference. All primers were applied as previously described. Next, the surfaces were dehydrated in ascending ethanol concentrations (70, 80, 90, and 100%) for 15 min each, dried for 15 s with an air-spray, and kept in contact with a filter paper at room temperature for 24 h. After dehydration process, the surfaces were mounted on aluminium stubs and sputter-coated with gold/palladium for SEM evaluation using a scanning electron microscope (SSX550, Shimadzu, Tokyo, Japan). The etching pattern and enamel surface characteristics were evaluated qualitatively according to the images obtained.
2.7.
Statistical analysis
The microshear bond strength data obtained in this study was analysed with the statistical programme SigmaPlot version 12 (Systat Software Inc., San Jose, CA, USA) using Kruskal–Wallis one way analysis of variance on ranks and the Student– Newman–Keuls test for multiple comparison (a = 5%). Two linear regression models were used: one to analyse the correlation between the acidity of self-primers (pH) and Poly-P concentration, and the other to analyse the correlation between the acidity of self-primers (pH) and enamel bond strength results.
3.
Results
3.1.
Synthesis of the acid-functional phosphate monomer
The phosphate monomer was successfully synthesized, yielding approximately 100%. The reaction resulted in an equimolar mixture of monoester (polypropylene glycol phosphate methacrylate) and diester (polypropylene glycol phosphate dimethacrylate) monomers, which molecular structures are illustrated in Fig. 1. The FTIR spectra of the starting reagent (poly (propylene glycol) monomethacrylate) and the final
journal of dentistry 43 (2015) 94–102
97
Fig. 1 – Molecular structure, molecular formula (MF), and molar mass (MM) of the (1) monoester ‘‘polypropylene glycol phosphate methacrylate’’ and of the (2) diester ‘‘polypropylene glycol phosphate dimethacrylate’’ synthesized in the study.
product (Poly-P) can be observed in Fig. 2. Accidental polymerization was not detected during the reaction time or in the purification procedure.
3.2.
pH vs. Poly-P concentration
The linear regression model showed an inverse relationship between Poly-P concentration and the pH of the non-buffered primers (R2 = 0.987) (Fig. 3a). As higher the Poly-P concentration, the lower the pH value obtained ( p = 0.007), with P10nb, P15nb, P30nb, and P50nb showing, respectively, pH values equal to 1.7, 1.6, 1.4, and 1.2 (Table 2).
3.3. Microshear bond strength to enamel and fracture mode analysis Medians of the microshear bond strength data are presented in Table 2. The median values obtained with the reference
materials (PC and CLSE) and with the non-buffered adhesive systems containing high Poly-P concentration (30 and 50 wt.%) were similar to each other, but these four groups showed statistically higher median values than other groups ( p < 0.001). The non-buffered adhesives containing low Poly-P content (10 and 15 wt.%) were similar. The statistically lowest microshear bond strength was found in group P30b ( p < 0.001). The linear regression model showed an inverse relationship between microshear bond strength and pH of the experimental self-primers containing Poly-P (R2 = 0.836) (Fig. 3b), and the lower the pH value, the higher the bond strength obtained ( p = 0.001). Table 2 also shows the percentage of the fracture modes obtained in the study, which were adhesive or mixed patterns. The experimental buffered adhesives presented predominance of adhesive failures, whereas the non-buffered and reference adhesives showed higher percentage of mixed failures than adhesive failures.
Fig. 2 – FTIR spectra of starting reagent (poly (propylene glycol) monomethacrylate) and the final product polypropylene glycol phosphate methacrylate plus polypropylene glycol phosphate dimethacrylate (Poly-P) in an equimolar mixture.
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Fig. 3 – (a) Linear regression predictive model that shows a significant relationship between Poly-P content and the pH of the experimental non-buffered primers (R2 = 0.987; p = 0.007); and (b) linear regression predictive model that demonstrates a significant relationship between microshear bond strength and pH of the experimental primers (R2 = 0.836; p = 0.001).
3.4.
SEM analysis
The etching pattern obtained with the application of each experimental and reference self-primers are illustrated in Fig. 4. The enamel prisms cores were exposed only with the application of self-primers containing 30 or 50 wt.% of Poly-P (Fig. 4e–h), although this effect was more evident after application of the non-buffered primers (Fig. 4f and h). The primers with low Poly-P content did not modify the enamel topography (Fig. 4a–d), although the application of P15nb (Fig. 4d) resulted in a slightly more irregular surface than the others. The control reference (PC) slightly exposed the enamel prisms cores, and the commercial reference (CLSE) did not expose prisms core, but clearly produced a rough and irregular topography.
4.
Discussion
The first purpose of this study was to synthesize an alternative acid-functional monomer to constitute a two-step self-etch adhesive system. This synthesis was aimed because a satisfactory resin–enamel bond has not been yet completely achieved when using self-etch adhesives, even with the existence of various acidic monomers available for this purpose and in different materials. Moreover, according to Van Meerbeek et al.,5 adhesive dentistry still needs efficient acid-functional monomers able to produce good and stable chemical bonding to tooth substrates. Most of the acidic monomers used in self-etch formulations are hydrophilic in
nature, once they are usually constituted of carboxylic or phosphoric functional groups.9 Nonetheless, enamel is a hydrophobic substrate, thus requiring treatment with solutions the less hydrophilic as possible. The phosphate monomer synthesized in the present study was intended due to its long carbon chain molecule and low content of polar groups (Fig. 1), which is believed to confer moderate hydrophobicity to the final material, and consequently, better chemical compatibility with enamel. A phosphate monomer rather than carboxylic was chosen because according to Yoshida et al.,20 the latter may weakly interact with hydroxyapatite minerals when compared to phosphate-based monomers. The reaction between phosphorous pentoxide and poly (propylene glycol) monomethacrylate was successful since a clear consumption of hydroxyl groups at peak 3600–3300 cm 1 as well as the appearance of absorptions for phosphorous compounds (phosphate esters at peaks 1299–1250 cm 1; P–OH group at peak 1040–910 cm 1; and P–O–C (aliphatic) group at peak 830–740 cm 1) could be observed in the final product molecule (Fig. 2). Indeed, the synthesis was performed at room temperature, without catalysts, and providing high yields and easy purification. The first study hypothesis was that the higher the concentration of acidic monomer, the greater the acidity of the self-primer. According to Fig. 3a, this hypothesis can be fully accepted, since the increase in the Poly-P content from 10 to 50 wt.% progressively reduced the pH of the primer ( p = 0.007), showing an excellent adjust to the linear regression predictive model (R2 = 0.986) used. This result corroborates with a previous study,21 and may be explained because
Table 2 – pH value of the experimental/referential self-primers evaluated in this study, microshear bond strength (mSBS, in MPA) median values, and ratio of adhesive/mixed fracture modes (AD/MX). Primersa pH mSBSb AD/MX a
P10b
P15b
P30b
P50b
P10nb
P15nb
P30nb
P50nb
PC
CLSE
2.0 10.1 C 40/60
2.0 9.6 C 55/45
2.0 7.5 D 70/30
2.0 9.9 C 65/35
1.7 11.0 B 50/50
1.6 11.5 B 40/60
1.4 16.1 A 10/90
1.2 15.3 A 10/90
1.5 19.6 A 15/85
2.0 18.6 A 20/80
Subscript numbers next to the Primer (P) indicate the concentration of Poly-P. PC, control reference; CLSE, ClearfilTM SE Bond (commercial reference); ‘‘b’’, primer with buffered pH; ‘‘nb’’, primer with non-buffered pH. b Different letters after median values indicate statistically significant differences among the adhesive systems evaluated ( p < 0.05).
journal of dentistry 43 (2015) 94–102
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Fig. 4 – SEM images of the etching ability of the self-primers (experimental and referential ones). P10b (a) – the surface is rough, with grooves (white arrows) resulted from the SiC abrasion procedure; the primer application did not enhance that roughness; P10nb (b); P15b (c) – the surface is rough as image ‘‘a’’; P15nb (d) – a rougher surface is achieved, but without exposing the enamel rods; P30b (e) – the enamel rods are partially exposed (black arrows), and the SiC abrasion grooves are
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the higher the content of Poly-P, the greater the potential to dissociate into its ionic form and generate protons, which in turn makes the medium acidic.10 In addition, the content of water, which is essential to ionize acidic molecules,22 has been previously demonstrated to affect the bonding performance of self-etch adhesive systems.18 Considering this, the present study used the same concentration of water in the preparation of all experimental materials (Table 1). The second purpose of the present study was to evaluate the influence of the pH of self-etch primers containing different content of acidic monomer on the bond strength to enamel, testing the hypothesis that the higher the acidity of the self-primer, the greater the resin–enamel bond strength. To properly investigate this hypothesis, each primer formulated was divided into two identical parts, in which one the pH of the solution was maintained intact (non-buffered group), and in the other the pH was adjusted to an exact value (pH 2, buffered group). Following this study design, only the pH of the materials was truly different, thus allowing high control and reduction of variables. According to Fig. 3b, the lower the pH of the self-primer, the higher the bond strength results ( p = 0.001), showing a satisfactory adjust to the linear regression predictive model (R2 = 0.836) used. Therefore, the second hypothesis was also totally accepted. It is well known that bonding to enamel is an aciddependent process.23 Nonetheless, the acidity of self-etch primers has been disregarded as the major factor involved in the interaction between adhesives and dental substrates.11,12,24 Nevertheless, and from the best of our knowledge, this is the first study in which the pH/acidity of experimental self-etch primers was thoroughly controlled from compositional variables. Consequently, the present findings may be potentially used to add scientific information to this research field. Considering only the Poly-P-based adhesives, it can be observed that their acidity played a significant role on their bonding ability (Table 2), with all the non-buffered adhesives producing higher bond strength when compared to their buffered counterparts. Furthermore, within the non-buffered adhesives, the most acidic ones (P30nb and P50nb) resulted in higher bond strength than the less acidic groups (P10nb and P15nb). Considering that pH is the negative decadic logarithm of the hydrogen ion concentration, small reductions in pH values may represent a substantial increment in acidity.25 In fact, the difference between pH 1.2 (P50nb) and pH 1.7 (P10nb) correspond to a 5-fold increase in acidity, thus meaning that the former self-primer was five times more acidic than the latter. Therefore, all the aforementioned strongly suggests that the acidity of self-etch adhesives influence on the bond strength to enamel, at least when the performance of materials containing the same composition/ type of acidic monomer is compared between each other. The present bond strength findings are in accordance with the SEM images obtained (Fig. 4). The primers with low Poly-P content (10 and 15 wt.%), and regardless of their pH, did not
modify the enamel etching pattern, thus maintaining the same surface topography achieved during the finishing process, which was a slight grooving roughness (Fig. 4a–d). Nonetheless, the bond strength results were significantly improved after application of the non-buffered primers. In fact, after a careful analysis of the prime-etched surfaces, the non-buffered primers created a slightly more irregular surface when compared to their buffered counterparts (Fig. 4b and d). This may reinforce the positive role of the acidity on the bonding process to enamel. With regards to the more concentrated Poly-P-based primers (30 and 50 wt.%), both the buffered and the non-buffered solutions created a Type 1 enamel etching pattern (i.e., the same obtained with the application of phosphoric acid during the etch-and-rinse approach), although the non-buffered self-primers were clearly more effectives in etching the surface (Fig. 4e–h). In addition, the non-buffered self-primers resulted in higher bond strength results when compared to their buffered counterparts (Table 2). Taking these findings together, once again the acidity of the self-primer seemed to be positively related to enhanced resin–enamel bond strength. The bond strength performance of the Poly-P-based adhesive systems was also compared to internal (control reference, PC) and external (commercial reference, CLSE) materials. The PC is an effective self-etch primer previously prepared and used by our team, which has demonstrated excellent bonding ability to both enamel26 and dentine.18 It is constituted of an equimolar mixture of MEP/Bis-MEP as acidic monomers, which in a concentration of 30 wt.% (pH 1.5) resulted in the highest mSBS median value (Table 2). Differently, CLSE is a classical two-step self-etch adhesive system widely used in clinical and in vitro evaluations. It is constituted of 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which is a known phosphate acidic monomer able to interact chemically with hydroxyapatite, creating strong and stable bond strength to dental substrates.5,20 In the current study, CLSE performed as the expected, showing high bond strength to enamel (Table 2). Comparing the bonding performance of all the adhesive systems investigated among each other, it is possible to verify that adhesives containing 30 or 50 wt.% of Poly-P (but with non-buffered pH) performed similarly to the references (Table 2), suggesting that Poly-P may be potentially used as acidic monomer of self-etch formulations. Surprisingly, another important finding of the present study is that irrespective of showing similar bonding performance, the distinct adhesive formulations investigated (P30nb, P50nb, PC, and CLSE) showed different initial pH values (i.e., different acidity), as well as different etching ability (Table 2 and Fig. 4f, h–j). These results suggest that when different materials are compared to each other the acidity may not be considered as the major factor affecting their bonding ability, but probably their composition (i.e., type of acidic component used to etch the substrate), corroborating with the so called
still present (white arrow); P30nb (f) – the enamel rods are clearly exposed (Type 1 etching pattern – black arrows); P50b (g) – the Type 1 pattern is present (black arrow), but it is less evidenced; P50nb (h) – the Type 1 pattern is still present (black arrows); PC (i) – the Type 1 pattern is present (black arrow), but it is less evidenced; CLSE (j) – the enamel rods are not exposed, but a rougher surface is predominantly seen.
journal of dentistry 43 (2015) 94–102
‘A–D concept’.11,12,24 Indeed, when the SEM images are analysed together, it is possible to observe that high bond strength results have not exclusively occurred in a Type 1 enamel etching pattern, with prisms rods exposure (Fig. 4). Notwithstanding, it is once again impossible not to point out that depending on the adhesive composition (e.g., within the Poly-P-based adhesives), the higher the prisms rod exposure, the higher the resin–enamel bond strength, probably due to the greater micro-mechanical interlocking with the substrate.5 Poly-P worked well as an acid-functional monomer in the self-etch formulations presented in this study. The selfprimers containing low Poly-P content (10 and 15 wt.%) demonstrated low mSBS median values (Table 2), in part due to their lower ability to etch the enamel surface, but also probably due to their high content of HEMA (higher than 45 wt.%), which was revealed to dramatically increase the material’s hydrophylicity.27–29 According to Van Landuyt et al.,30 HEMA concentrations beyond 10 wt.% did not have any supplementary advantageous effects on the bond strength to enamel/dentine by using self-etch adhesives. Considering that enamel is more receptive to hydrophobic components, the high hydrophylic nature of the P10b, P10nb, P15b, and P15nb HEMA-rich primers (Table 1) may have contributed to poor hybridization of enamel, fact that corroborates with the fracture mode patterns obtained (Table 2), where the primers with low Poly-P content produced a predominance of adhesive failures. However, mixed failures have frequently occurred in groups showing high bond strength median values, which can be explained because since enamel is tougher than the underlying dentine and the enamel–dentine junction, crack propagation is allowed to occur through the substrate, producing the mixed/cohesive failures seen in the study.31 Despite of all the findings described, Poly-P molecule is characterized by a long-chain polymer structure constituted by oxy-ethylene units (Fig. 1), which confers higher hydrophobicity than other routinely acidic molecules present in self-etch adhesive formulations. This characteristic may be advantageous for bonding purposes to enamel, since a potential strengthening of the resin–enamel bond interface against hydrolytic degradation may be produced. Considering this aspect, studies evaluating the long-term bond strength of adhesive systems containing Poly-P are necessary. In conclusion, the acidity of self-etch adhesive systems seemed to be a determinant factor affecting the resin–enamel bond strength only when materials with the same composition were investigated; otherwise, the bonding ability of adhesives with different compositions seemed to be more related to the type of acidic monomer presented in their composition. Poly-P was successfully synthesized in the study, being a potential acid-functional monomer to be used in the formulation of self-etch dental adhesive systems.
Acknowledgements Authors are grateful to Brazilian National Council for Scientific and Technological Development (CNPq) for financial support (# 308087/2011-9), to CAPES for scholarship, and to Esstech Inc. for reagents donation.
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