Resin zirconia bonding promotion with some novel coupling agents

Abstract

Objectives

To evaluate and compare three novel coupling agents: 2-hydroxyethyl methacrylate, itaconic acid and oleic acid to two silane coupling agents, one commercial silane product and 3-acryloxypropyltrimethoxysilane on the bond durability of resin composite to zirconia.

Methods

Zirconia samples were silica-coated by air abrasion and each of the five coupling agents was then applied to give five test groups. Resin composite stubs were bonded onto the conditioned zirconia surfaces. The samples were stored: dry storage, 30 days in water and thermocycled to give a total of fifteen test groups. The shear bond strengths were determined using a universal testing machine and data analyzed by two-way ANOVA and Tukey HSD ( p < 0.05) with shear bond strength as dependent variable and storage condition and primers as independent variables. The bond formation of the five coupling agents to zirconia was examined by X-ray photoelectron spectroscopy (XPS).

Results

Two-way ANOVA analysis showed that there was a significant difference for different primers ( p < 0.05) and different storage conditions ( p < 0.05) on the shear bond strength values measured. XPS analysis showed a shift in binding energy for O 1s after priming with the five coupling agents which revealed different bond formations related to the functional groups of the coupling agents.

Significance

The shear bond strength values measured for all coupling agents after water storage and thermocycling exceed the minimum shear bond strength value of 5 MPa set by ISO. The silane coupling agent, 3-acryloxypropyltrimethoxysilane, showed the highest bond strength of the three storage conditions.

Introduction

Zirconia is used as a biomaterial because of superior mechanical properties, chemical inertness and biocompatibility . Normally, zirconia is doped with a small amount of yttria (Y 2 O 3 ) to form yttria tetragonal zirconia polycrystals (TZP) which increases the fracture toughness, flexural strength and wear resistance . Yttria tetragonal zirconia polycrystals are widely used in dentistry as root canal posts, orthodontic brackets, dental implant abutments and all-ceramic restorations . Inertness of zirconia has made resin to zirconia bonding challenging. Tribochemical silica-coating using a Rocatec system followed by silanization has been suggested as a pre-treatment before cementation . Other zirconia surface treatment methods have been reported, these include: chemical treatments, selective infiltration etching, laser irradiation, nano-structured alumina coating and chemical vapor deposition . All of these treatment methods, with different surface conditioning mechanisms, activate the zirconia surfaces for bonding to resin composite.

A coupling agent has two different functional groups so as to connect dissimilar materials, such as metals to polymers. Silane coupling agents are widely used to promote adhesion of dental restorations to tooth tissue . Due to the large variety of silane coupling agents with different functional groups, numerous in vitro studies of experimental silane coupling agents in resin zirconia bonding have been investigated . 3-Acryloxypropyltrimethoxysilane ( Fig. 1 ) is a promising coupling agent with in vitro results comparable to other silane coupling agents under dry and artificial aging conditions . Coupling agents such as phosphates, e.g. 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and zirconates have also been investigated for resin to zirconia bonding with the former gaining popularity as an alternative to silane coupling agents because of enhanced bonding and hydrolytic stability . However, some studies report that the bond durability under artificial aging of resin to zirconia primed with silane coupling agents is higher than that primed with phosphate coupling agents .

Fig. 1
Molecular structures of the five coupling agents. Key: (I) 3-methacryloxypropyltrimethoxysilane, (II) 3-acryloxypropyltrimethoxysilane, (III) 2-hydroxyethyl methacrylate, (IV) itaconic acid and (V) oleic acid. The functional groups which react to resin composite and silica-coated zirconia are highlighted.

The three novel coupling agents for resin zirconia bonding investigated in this study were: 2-hydroxylethyl methacrylate, itaconic acid and oleic acid. 2-Hydroxylethyl methacrylate, contains a >C C< and OH group and itaconic acid and oleic acid, contain >C C< and COOH groups ( Fig. 1 ). These coupling agents have many applications in industry and medicine . 2-Hydroxylethyl methacrylate has been used for the surface modification of a polysulfone membrane for treatment of oily wastewater, contact lens applications and synthesis of macroporous hydrogels for adsorption of proteins for biomedical applications. Itaconic acid is added to vinylidene chloride coatings to improve adhesion to paper and cellophane. The reaction of itaconic acid with amines forms N-substituted pyrrolidones which can be used as thickeners in lubricating grease, shampoos, detergents, pharmaceuticals and herbicides. In medicine, esters of partly substituted itaconic acid have anti-inflammatory and analgesic properties. Oleic acid, n -octadecan-9-enoic acid, is a mono-unsaturated omega-9 fatty acid which can be found in olive oil. It has been used as a drug delivery vehicle for the medical management of keloid and hypertrophic scaring. It is also used as a protective coating on mild steel against corrosion, solvent attack and as an environmentally friendly biolubricant. In addition, 2-hydroxylethyl methacrylate has been used successfully in resin dentin bonding . Other coupling agents with the functional group ( COOH) are also found in dentin adhesives, these include 4-methacryloxyethyl trimellitic acid and 11-methyacryloyloxy-1,1′-undecanedicarboxylic acid .

The aim of this in vitro study was to evaluate and compare the bond durability of the three novel coupling agents, 2-hydroxylethyl methacrylate, itaconic acid and oleic acid, to two silane coupling agents, one commercial dental silane product and one experimental silane coupling agent, 3-acryloxypropyltrimethoxysilane, for resin composite to zirconia bonding under different storage conditions. The hypothesis was that there is no difference in bond durability between the three novel coupling agents and the two silane coupling agents under different storage conditions.

Materials and methods

Zirconia blocks (Lava, 3 M ESPE, Seefeld, Germany) were cut into blocks of 16 mm × 15 mm × 3 mm and embedded in cylindrical plastic molds filled with poly(methylmethacrylate) resin. Five test groups of resin composite were bonded to silica-coated and primed zirconia and investigated under three different storage conditions, giving a total of fifteen experimental groups of randomly assigned samples. Each experimental group consisted of 15 resin composite stubs for bond strength measurement. The 3 M ESPE Sil silane is a pre-hydrolyzed dental silane product of 3-methacryloxypropyltrimethoxysilane ( Fig. 1 ), at a silane content of “<3 vol.%”, indicated for silica-coated metallic and ceramic indirect restorations .

Preparation of silica-coated zirconia samples

The zirconia sample surfaces were polished with a 400-grit silicon carbide paper under running deionized water, then cleaned ultrasonically for 10 min in deionized water and rinsed with deionized water. They were allowed to dry in air at room temperature for 30 min. Silica-coating of the polished zirconia specimens was performed using Rocatec Sand Plus (110 μm in size of silica-coated alumina particles, 3 M ESPE, Seefeld, Germany) at a constant pressure of 280 kPa for 30 s/cm 2 and at a perpendicular distance of 10 mm . The samples were then cleansed in an ultra-sonic bath in 70% ethanol for 10 min and then rinsed with 70% ethanol. They were allowed to air-dry at room temperature for 30 min.

Preparation of primer solutions and primer coating on zirconia surface

A silane solution of 1.0 vol.% of 3-acryloxypropyltrimethoxysilane (95%, Gelest, Morrisville, PA, USA) in a solvent mixture of 95.0 vol.% absolute ethanol (99.8%, Riedel-de Haën, Seelze, Germany) and 5.0 vol.% deionized water was prepared. The pH of the solvent mixture was adjusted to 4.0 by 1 M acetic acid. The silane solution was then allowed to hydrolyze for 1 h .

Solutions of 1.0 vol.% 2-hydroxyethyl methacrylate (98%, Sigma, St. Louis, MO, USA), itaconic acid (BDH, PA, USA) and oleic acid (92%, BDH, PA, USA) were all prepared in a solvent mixture of 95.0 vol.% acetone (99.9%, VWR International SAS) and 5.0 vol.% deionized water. The pH of the solvent mixture was adjusted to 4.0 by 1 M acetic acid. These three coupling agents do not require hydrolysis. The five primer solutions were applied onto the silica-coated zirconia surfaces with one coating using a new fine brush each time. This was allowed to dry and react for 5 min, before the next bonding step.

Bonding of resin composite to silica-coated and primed zirconia

Rely X Unicem Aplicap resin composite (3 M ESPE, Seefeld, Germany) was activated and mixed according to manufacturers’ instructions and manipulated using dental hand instrument. The resin composite was carefully packed into a polyethylene mold of 3.7 mm in diameter and 4.0 mm in height which was positioned on the primed test zirconia surface. After this, it was light cured for 40 s (Lunar Curing Light, Benlioglu Dental Inc., Ankara, Turkey) and the mold was removed carefully after curing, by pressing the resin stub perpendicularly with the same instrument. The zirconia samples primed with the five coupling agents were randomly divided into three groups. The first group was kept in a desiccator at room temperature for 24 h prior to bond strength testing to obtain the initial bond strength. The second group was stored in deionized water for 30 days for artificial aging in sealed polyethylene containers and kept in an incubator at a constant temperature of 37.0 ± 0.1 °C. The third group was subjected to thermocycling for 6000 cycles between 5.0 ± 0.5 °C and 55.0 ± 0.5 °C with 20 s in dwell time in each deionized water bath.

Surface roughness measurement

Seven selected zirconia samples were prepared for surface roughness measurement. The sample preparation with different surface treatments is shown in Table 1 . The average surface roughness, R a , was measured using an electro-mechanical profilometer (Surtronic 3+, Taylor Hobson, Leicester, England). Three parallel readings were taken at different randomly selected regions on each specimen surface. The surface roughness was then reported as mean R a ± SD.

Table 1
Various surface treatments on zirconia for surface roughness measurement and XPS analysis.
Sample Surface treatment conditions
A (i) Polishing, (ii) rinsing
B (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing
C (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 3M ESPE Sil silane primer
D (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 1 vol.% ACPS
E (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) primed with 1 vol.% HEMA
F (i) Polishing, (ii) Rinsing, (iii) Sandblasting, (iv) Rinsing, (v) Primed with 1 vol.% IA
G (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) primed with 1 vol.% OA
Note : (1) All sample surfaces were polished with 400-grit silicon carbide paper. (2) After polishing and sandblasting, all samples were rinsed in 70% ethanol in the ultra-sonic bath for 10 min and air-dried. (3) Samples B–G were sand-blasted (30 s/cm 2 ) using Rocatec Plus sand (110-μm silica-coated alumina) on the surface of zirconia at pressure of 280 kPa. Key: ACPS = 3-acryloxypropyltrimethoxysilane, HEMA = 2-hydroxyethyl methacrylate, IA = itaconic acid, OA = oleic acid.

Shear bond strength testing

The samples were mounted in a jig on a universal testing machine (Instron, Model 1185, Norwood, MA). A constant load of 1000 N was applied at a cross-head speed of 1.0 mm/min until fracture occurred. The shear bond strengths were calculated by dividing the maximum fracture loading with the circular area of the resin stub. The mode of failure of the zirconia samples was assessed after shear bond strength testing: when the resin composite stub remaining was less than 1/3, the failure mode was assigned as ‘adhesive’ and when the remaining was more than 1/3 but less than 2/3, it was assigned as ‘mixed’. When the amount remaining was more than 2/3, it was assigned as ‘cohesive’ failure .

Scanning electron microscopy (SEM)

Representative and selected zirconia samples after shear bonding test were analyzed by SEM (XL30CP, Philips Electron Optics, Eindhoven, The Netherlands). The operational voltage was 10 kV and the vacuum pressure for measurement was 3.5 × 10 −5 Pa.

Statistical analysis

The mean shear bond strength of each test group was analyzed using a two-way ANOVA ( p = 0.05) with the shear bond strength as the dependent valuable and types of primers and storage conditions as the independent valuables (StatPlus 2009 Professional, Analyst Soft Inc., Vancouver, Canada). Tukey HSD test ( p = 0.05) was used to compare the means if there was a significant difference. Furthermore, the mean shear bond strength values were analyzed by using a one-way ANOVA under different coupling agents and different storage conditions respectively.

XPS analysis

The chemical composition of the surfaces of the samples after different surface treatments ( Table 1 ) was examined by X-ray photoelectron spectroscopy using a Perkin-Elmer PHI 5400 spectrometer, with a mean radius of 140 mm and equipped with a resistive anode detector. The ionization source used was Mg K α radiation ( = 1253.6 eV) from a twin-anode X-ray tube. Broad-range survey scans, at a pass energy of 89.45 eV and an entrance slit width of 4.0 mm, were performed to determine atomic concentration. High-resolution narrow-range scans were also performed at a pass energy of 37.75 eV for selected specific photolines (Zr 3d , Si 2p , O 1s ) to determine the chemical shifts. The chamber base pressure was maintained at about 8 × 10 −8 Pa and the X-ray tube was operated at 200 W. The C 1s photopeaks were used to calibrate the binding energy scale of the high-resolution spectra for chemical shift measurements. The peak composition and energy positions were then determined using the least-squares curve-fitting technique with the Igor Pro analysis environment in the SPANCF macro package .

Materials and methods

Zirconia blocks (Lava, 3 M ESPE, Seefeld, Germany) were cut into blocks of 16 mm × 15 mm × 3 mm and embedded in cylindrical plastic molds filled with poly(methylmethacrylate) resin. Five test groups of resin composite were bonded to silica-coated and primed zirconia and investigated under three different storage conditions, giving a total of fifteen experimental groups of randomly assigned samples. Each experimental group consisted of 15 resin composite stubs for bond strength measurement. The 3 M ESPE Sil silane is a pre-hydrolyzed dental silane product of 3-methacryloxypropyltrimethoxysilane ( Fig. 1 ), at a silane content of “<3 vol.%”, indicated for silica-coated metallic and ceramic indirect restorations .

Preparation of silica-coated zirconia samples

The zirconia sample surfaces were polished with a 400-grit silicon carbide paper under running deionized water, then cleaned ultrasonically for 10 min in deionized water and rinsed with deionized water. They were allowed to dry in air at room temperature for 30 min. Silica-coating of the polished zirconia specimens was performed using Rocatec Sand Plus (110 μm in size of silica-coated alumina particles, 3 M ESPE, Seefeld, Germany) at a constant pressure of 280 kPa for 30 s/cm 2 and at a perpendicular distance of 10 mm . The samples were then cleansed in an ultra-sonic bath in 70% ethanol for 10 min and then rinsed with 70% ethanol. They were allowed to air-dry at room temperature for 30 min.

Preparation of primer solutions and primer coating on zirconia surface

A silane solution of 1.0 vol.% of 3-acryloxypropyltrimethoxysilane (95%, Gelest, Morrisville, PA, USA) in a solvent mixture of 95.0 vol.% absolute ethanol (99.8%, Riedel-de Haën, Seelze, Germany) and 5.0 vol.% deionized water was prepared. The pH of the solvent mixture was adjusted to 4.0 by 1 M acetic acid. The silane solution was then allowed to hydrolyze for 1 h .

Solutions of 1.0 vol.% 2-hydroxyethyl methacrylate (98%, Sigma, St. Louis, MO, USA), itaconic acid (BDH, PA, USA) and oleic acid (92%, BDH, PA, USA) were all prepared in a solvent mixture of 95.0 vol.% acetone (99.9%, VWR International SAS) and 5.0 vol.% deionized water. The pH of the solvent mixture was adjusted to 4.0 by 1 M acetic acid. These three coupling agents do not require hydrolysis. The five primer solutions were applied onto the silica-coated zirconia surfaces with one coating using a new fine brush each time. This was allowed to dry and react for 5 min, before the next bonding step.

Bonding of resin composite to silica-coated and primed zirconia

Rely X Unicem Aplicap resin composite (3 M ESPE, Seefeld, Germany) was activated and mixed according to manufacturers’ instructions and manipulated using dental hand instrument. The resin composite was carefully packed into a polyethylene mold of 3.7 mm in diameter and 4.0 mm in height which was positioned on the primed test zirconia surface. After this, it was light cured for 40 s (Lunar Curing Light, Benlioglu Dental Inc., Ankara, Turkey) and the mold was removed carefully after curing, by pressing the resin stub perpendicularly with the same instrument. The zirconia samples primed with the five coupling agents were randomly divided into three groups. The first group was kept in a desiccator at room temperature for 24 h prior to bond strength testing to obtain the initial bond strength. The second group was stored in deionized water for 30 days for artificial aging in sealed polyethylene containers and kept in an incubator at a constant temperature of 37.0 ± 0.1 °C. The third group was subjected to thermocycling for 6000 cycles between 5.0 ± 0.5 °C and 55.0 ± 0.5 °C with 20 s in dwell time in each deionized water bath.

Surface roughness measurement

Seven selected zirconia samples were prepared for surface roughness measurement. The sample preparation with different surface treatments is shown in Table 1 . The average surface roughness, R a , was measured using an electro-mechanical profilometer (Surtronic 3+, Taylor Hobson, Leicester, England). Three parallel readings were taken at different randomly selected regions on each specimen surface. The surface roughness was then reported as mean R a ± SD.

Table 1
Various surface treatments on zirconia for surface roughness measurement and XPS analysis.
Sample Surface treatment conditions
A (i) Polishing, (ii) rinsing
B (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing
C (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 3M ESPE Sil silane primer
D (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 1 vol.% ACPS
E (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) primed with 1 vol.% HEMA
F (i) Polishing, (ii) Rinsing, (iii) Sandblasting, (iv) Rinsing, (v) Primed with 1 vol.% IA
G (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) primed with 1 vol.% OA
Note : (1) All sample surfaces were polished with 400-grit silicon carbide paper. (2) After polishing and sandblasting, all samples were rinsed in 70% ethanol in the ultra-sonic bath for 10 min and air-dried. (3) Samples B–G were sand-blasted (30 s/cm 2 ) using Rocatec Plus sand (110-μm silica-coated alumina) on the surface of zirconia at pressure of 280 kPa. Key: ACPS = 3-acryloxypropyltrimethoxysilane, HEMA = 2-hydroxyethyl methacrylate, IA = itaconic acid, OA = oleic acid.

Shear bond strength testing

The samples were mounted in a jig on a universal testing machine (Instron, Model 1185, Norwood, MA). A constant load of 1000 N was applied at a cross-head speed of 1.0 mm/min until fracture occurred. The shear bond strengths were calculated by dividing the maximum fracture loading with the circular area of the resin stub. The mode of failure of the zirconia samples was assessed after shear bond strength testing: when the resin composite stub remaining was less than 1/3, the failure mode was assigned as ‘adhesive’ and when the remaining was more than 1/3 but less than 2/3, it was assigned as ‘mixed’. When the amount remaining was more than 2/3, it was assigned as ‘cohesive’ failure .

Scanning electron microscopy (SEM)

Representative and selected zirconia samples after shear bonding test were analyzed by SEM (XL30CP, Philips Electron Optics, Eindhoven, The Netherlands). The operational voltage was 10 kV and the vacuum pressure for measurement was 3.5 × 10 −5 Pa.

Statistical analysis

The mean shear bond strength of each test group was analyzed using a two-way ANOVA ( p = 0.05) with the shear bond strength as the dependent valuable and types of primers and storage conditions as the independent valuables (StatPlus 2009 Professional, Analyst Soft Inc., Vancouver, Canada). Tukey HSD test ( p = 0.05) was used to compare the means if there was a significant difference. Furthermore, the mean shear bond strength values were analyzed by using a one-way ANOVA under different coupling agents and different storage conditions respectively.

XPS analysis

The chemical composition of the surfaces of the samples after different surface treatments ( Table 1 ) was examined by X-ray photoelectron spectroscopy using a Perkin-Elmer PHI 5400 spectrometer, with a mean radius of 140 mm and equipped with a resistive anode detector. The ionization source used was Mg K α radiation ( = 1253.6 eV) from a twin-anode X-ray tube. Broad-range survey scans, at a pass energy of 89.45 eV and an entrance slit width of 4.0 mm, were performed to determine atomic concentration. High-resolution narrow-range scans were also performed at a pass energy of 37.75 eV for selected specific photolines (Zr 3d , Si 2p , O 1s ) to determine the chemical shifts. The chamber base pressure was maintained at about 8 × 10 −8 Pa and the X-ray tube was operated at 200 W. The C 1s photopeaks were used to calibrate the binding energy scale of the high-resolution spectra for chemical shift measurements. The peak composition and energy positions were then determined using the least-squares curve-fitting technique with the Igor Pro analysis environment in the SPANCF macro package .

Results

Shear bond strength testing

The results of the mean shear bond strengths for all test groups are shown in Table 2 . A two-way ANOVA analysis showed that there were significant differences for different storage conditions ( p < 0.001) and different primers ( p < 0.001) used on the shear bond strength. There was a significant interaction between storage condition and the five coupling agents ( p < 0.005). The decrease in bond strengths between the dry groups and 30 d water storage was significant for 2-hydroxyethyl methacrylate ( p < 0.05) and oleic acid ( p < 0.05). There was no significant decrease in bond strengths for 3 M ESPE Sil silane ( p > 0.1), 3-acryloxypropyltrimethoxysilane ( p > 0.05) and itaconic acid ( p > 0.1) between the dry groups and the water storage groups. For the thermocycling groups, a significant difference was found for the shear bond strengths when compared to the dry groups for 3 M ESPE Sil ( p < 0.001), 2-hydroxyethyl methacrylate ( p < 0.001), itaconic acid ( p < 0.005) and oleic acid ( p < 0.001). No significant difference was found for 3-acryloxypropyltrimethoxysilane ( p > 0.1). There were significant differences in shear bond strengths for the five coupling agents under different storage conditions: (i) dry ( p < 0.001), (ii) water storage ( p < 0.001) and (iii) thermocycling ( p < 0.0001). The highest overall bond strengths were obtained with 1 vol.% 3-acryloxypropyltrimethoxysilane in: dry (13.5 MPa); water storage (14.6 MPa); and thermocycled (14.5 MPa) conditions.

Table 2
Means and standard deviation of shear bond strength for various coupling agents under different storage conditions. Different uppercase letters in the same row means the groups are significant different. Different lowercase letters in the same column means the groups are significant different ( p < 0.05).
Coupling agents Mean shear bond strength (SD)/MPa
Dry 30-Day water storage Thermocycling
3M ESPE Sil 12.9(1.9)Aa
12.9 ( 1.9 ) a A
12.1(2.9)Ab
12.1 ( 2.9 ) b A
6.7(1.4)Bb
6.7 ( 1.4 ) b B
3-Acryloxypropyltrimethoxysilane 13.5(2.1)Aa,c
13.5 ( 2.1 ) a,c A
14.6(1.1)Aa
14.6 ( 1.1 ) a A
14.5(2.2)Aa
14.5 ( 2.2 ) a A
2-Hydroxyethyl methacrylate 11.4(1.9)Aa,b
11.4 ( 1.9 ) a,b A
7.9(1.6)Bc
7.9 ( 1.6 ) c B
6.7(0.6)Bb
6.7 ( 0.6 ) b B
Itaconic acid 10.7(3.2)Aa,b
10.7 ( 3.2 ) a,b A
10.8(1.8)Ab
10.8 ( 1.8 ) b A
5.9(1.9)Bb
5.9 ( 1.9 ) b B
Oleic acid 9.6(2.0)Ab
9.6 ( 2.0 ) b A
7.4(2.6)Bc
7.4 ( 2.6 ) c B
5.5(0.6)Cb
5.5 ( 0.6 ) b C
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Resin zirconia bonding promotion with some novel coupling agents
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