Resin zirconia bonding promotion with some novel coupling agents

Resin zirconia bonding promotion with some novel coupling agents

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d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

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Resin zirconia bonding promotion with some novel coupling agents Christie Ying Kei Lung a , Michael G. Botelho b , Markku Heinonen c , Jukka P. Matinlinna a,∗ a b c

Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, Special Administrative Region P.R. China Oral Rehabilitation, Faculty of Dentistry, The University of Hong Kong, Special Administrative Region P.R. China Department of Physics and Astronomy, Faculty of Mathematics and Natural Sciences, The University of Turku, Turku, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To evaluate and compare three novel coupling agents: 2-hydroxyethyl methacry-

Received 30 November 2011

late, itaconic acid and oleic acid to two silane coupling agents, one commercial silane

Received in revised form

product and 3-acryloxypropyltrimethoxysilane on the bond durability of resin composite

22 February 2012

to zirconia.

Accepted 16 April 2012

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

Resin composite

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

Coupling agent

examined by X-ray photoelectron spectroscopy (XPS).

Bond strength

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 O1s 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. © 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.



Zirconia is used as a biomaterial because of superior mechanical properties, chemical inertness and biocompatibility [1]. Normally, zirconia is doped with a small amount of yttria

(Y2 O3 ) to form yttria tetragonal zirconia polycrystals (TZP) which increases the fracture toughness, flexural strength and wear resistance [2]. Yttria tetragonal zirconia polycrystals are widely used in dentistry as root canal posts, orthodontic brackets, dental implant abutments and all-ceramic restorations [3]. Inertness of zirconia has made resin to zirconia

Corresponding author at: Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, 4/F, Prince Philip Dental Hospital, 34 Hospital Road, Sai Ying Pun, Hong Kong Special Administrative Region P.R. China. Tel.: +852 2859 0380; fax: +852 2548 9464. E-mail addresses: [email protected] (C.Y.K. Lung), [email protected] (M.G. Botelho), [email protected]fi (M. Heinonen), [email protected] (J.P. Matinlinna). 0109-5641/$ – see front matter © 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.


d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

bonding challenging. Tribochemical silica-coating using a Rocatec system followed by silanization has been suggested as a pre-treatment before cementation [4]. 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 [5–11]. 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 [6]. 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 [12–15]. 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 [13,15]. Coupling agents such as phosphates, e.g. 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and zirconates have also been investigated for resin to zirconia bonding [16,17] with the former gaining popularity as an alternative to silane coupling agents because of enhanced bonding and hydrolytic stability [18]. 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 [19,20]. 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 [21–28]. 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 [29–31]. 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 [32,33]. 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 [4,6].


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 ␮m in size of silica-coated alumina particles, 3 M ESPE, Seefeld, Germany) at a constant pressure of 280 kPa for 30 s/cm2 and at a perpendicular distance of 10 mm [34]. 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.

2.2. Preparation of primer solutions and primer coating on zirconia surface A silane solution of 1.0 vol.% of 3acryloxypropyltrimethoxysilane (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 [12]. 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.


d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

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.

Bonding of resin composite to silica-coated and 2.3. 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, Ra , 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 Ra ± SD.

Table 1 – Various surface treatments on zirconia for surface roughness measurement and XPS analysis. Sample A B C D E F G

Surface treatment conditions (i) Polishing, (ii) rinsing (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 3M ESPE Sil silane primer (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) silanized with 1 vol.% ACPS (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsing, (v) primed with 1 vol.% HEMA (i) Polishing, (ii) Rinsing, (iii) Sandblasting, (iv) Rinsing, (v) Primed with 1 vol.% IA (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/cm2 ) using Rocatec Plus sand (110-␮m 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,


d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

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

3M ESPE Sil 3-Acryloxypropyltrimethoxysilane 2-Hydroxyethyl methacrylate Itaconic acid Oleic acid

30-Day water storage A


12.9(1.9)a A 13.5(2.1)a,c A 11.4(1.9)a,b A 10.7(3.2)a,b A 9.6(2.0)b

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 [35].

12.1(2.9)b A 14.6(1.1)a B 7.9(1.6)c A 10.8(1.8)b B 7.4(2.6)c

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 Xray 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␣ radiation (h = 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 (Zr3d , Si2p , O1s ) 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 C1s 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 [36].


6.7(1.4)b A 14.5(2.2)a B 6.7(0.6)b B 5.9(1.9)b C 5.5(0.6)b

Table 3 – Mean surface average roughness, Ra , measurement of zirconia surfaces with various treatments. Different letters means that the groups are significant different (p < 0.05). Sample




Mean Ra /␮m ± SD 0.95 1.17 1.78 1.19 1.35 1.39 1.41




Shear bond strength testing

± ± ± ± ± ± ±

0.04a 0.06a,c 0.02b 0.08a,c 0.14c 0.15c 0.06c

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 2hydroxyethyl 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), 2hydroxyethyl 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.


Surface topographic analysis

The surface roughness of zirconia surfaces measured for samples A–G after the various treatments are presented in Table 3. A significant difference in surface roughness between

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872


Fig. 2 – SEM micrographs (20×) for resin composite residue left on the zirconia surface after shear bond testing primed with: (i) 3 M ESPE Sil silane (adhesive failure), (ii) 3-acryloxypropyltrimethoxysilane (adhesive failure), (iii) 2-hydroxyethyl methacrylate (adhesive failure), (iv) itaconia acid (adhesive failure) and (v) oleic acid (adhesive failure) in dry condition.

polishing and sandblasting (p < 0.05) was found. A significant difference was found between samples C, D, E, F and G with different primer coatings (p < 0.05). There was also a significant difference between samples B, C, D, E, F and G (p < 0.001). Some SEM micrographs were selected to illustrate the types of failure for the five coupling agents investigated in this study (Fig. 2). The mode of failure assessment for zirconia samples

after shear bond testing is shown in Table 4. The predominant mode of failure for all the zirconia samples was adhesive.


XPS analysis

The atomic concentrations after various surface treatments are shown in Table 5. For samples B–G after surface treatment, the silicon and oxygen content was increased presumably


d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

Table 4 – Percentage of mode of failure of all zirconia samples after bonding test. Key: ACPS = 3-acryloxypropyltrimethoxysilane, HEMA = 2-hydroxyethyl methacrylate. Coupling agents

Storage condition

Mode of failure Adhesive




Dry 30-Day water storage Thermocycling

15 (100%) 11 (73.3%) 15 (100%)

0 2 (13.3%) 0

0 2 (13.3%) 0


Dry 30-Day water storage Thermocycling

15 (100%) 2 (13.3%) 14 (93.3%)

0 10 (66.7%) 1 (6.7%)

0 3 (20%) 0


Dry 30-Day water storage Thermocycling

15 (100%) 14 (93.3%) 15 (100%)

0 1 (6.7%) 0

0 0 0

Itaconic acid

Dry 30-Day water storage Thermocycling

12 (80%) 7 (46.7%) 15 (100%)

3 (20%) 4 (26.7%) 0

0 4 (26.7%) 0

Oleic acid

Dry 30-Day water storage Thermocycling

15 (100%) 13 (86.7%) 15 (100%)

0 2 (13.3%) 0

0 0 0

Table 5 – Atomic concentration of zirconia samples after different surface treatments. Sample


Atomic concentration/% C1s





62.2 16.8 18.8 23.0 32.2 31.5 30.5

24.1 53.6 52.8 50.2 43.8 45.4 44.0

3.8 18.2 20.4 18.4 14.9 17.2 17.8

8.8 5.5 3.7 5.1 5.2 3.3 3.7

– 5.9 4.3 3.3 3.9 2.6 4.0

from sandblasting and primer coating and Al was detected which came from silica-coated alumina particles used for sandblasting. The XPS spectra of Zr3d , Si2p , and O1s for samples A–G are shown in Fig. 3. For sample A, the Si2p peak binding energy at 101.9 eV corresponds to Si–C from polishing with silicon carbide (SiC) paper [37]. There is a shift of peak position for binding energy of O1s after various surface treatments, i.e. sandblasting and primer coating. There are no observable changes in binding energies for Zr3d (samples A–G) and Si2p (samples B–G) after sandblasting and primer coating.



Silane coupling agents, after activation by hydrolysis, form silanols ( Si OH) which are known to react readily with surface hydroxyl groups on the silica-coated zirconia surface to form siloxane ( Si O Si ) linkages. For 2-hydroxyethyl methacrylate, the reaction of the COH group with surface hydroxyl groups of silica-coated zirconia forms silyl ether ( Si O C ) linkages. For itaconic and oleic acids, the reaction of the COOH group with surface hydroxyl group of silica) linkages. Given coated zirconia forms silyl ester ( this, all coupling agents containing >C C< bonds can react

with the >C C< bond in the resin composite used in this study [38–40]. Fig. 4 shows the XPS spectra of O1s of samples A–G. The binding energy shift to higher values for samples B–G after various surface treatments indicates different chemical states of oxygen present. The peaks can be resolved into individual components by curve fitting. The O1s peaks are assigned using the ‘Principle of Electronegativity Equalization’ [41] since different elements have different electronegativity. The first peaks would be assigned as ZrO2 , the second peak would be assigned as Zr O Si since Si is more electronegative than Zr and the third peak would be assigned as Si O Si. The other O1s peaks are assigned accordingly [42–44]. The reported O1s binding energy values are close to the literature values [45,46]. As expected, the surface roughness Ra is increased after sandblasting (Table 3) with a more irregular surface produced by impact of high energy silica-coated alumina particles onto the surface, this may enhance mechanical retention for bonding [47]. SEM images (Fig. 2) for the five coupling agents show different amounts of resin composite residue left after the shear bond strength test. All of them show an adhesive mode of failure which indicates the interfacial failure occurred between the substrate, i.e. resin composite, silica layer or zirconia and the adhesive layer. Extensive in vitro studies investigating the effects of various surface pretreatments and different silane coupling agents on resin-composite to zirconia bonding show bond strength

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Fig. 3 – XPS spectra of Zr3d , Si2p , and O1s of samples A–G (Table 1) with different surface treatments.

degradation over time under artificial aging [12–15,48–57]. However, some reports showed no decrease in bond strengths after artificial aging test [13,50]. It is suggested that under water immersion, water diffuses into the interfacial layer of resin composite and zirconia and causes hydrolytic degradation of the bonding [58]. Thermocycling combines the effects of hydrolytic degradation and thermal irradiation. The difference in linear coefficient of thermal expansion of resin composite and zirconia would induce thermal stress at the interfacial layer which causes the breaking of the bond [15]. In this study, we found the bond strength values for both silane coupling agents under artificial aging were not significantly lower than for dry conditions except the thermocycling group for the commercial silane (Table 2). Senyilmaz et al. [50], using the same resin composite, silane and surface pretreatment of zirconia, reported there was no significant decrease of bond strength measured after 24 h water immersion and thermocycling for 6000 cycles. Likewise, Matinlinna and Lassila [13], using identical silane and resin composite, reported no significant decrease of bond strength after thermocycling. Heikkinen et al. [15], using the same silanes but a different resin composite, observed a significant decrease of bond strength after thermocycling. Matinlinna et al. [12] also

investigated five silane coupling agents and found a significant difference in shear bond strength after thermocycling. Valandro et al. [49] reported a significant decrease of bond strength after 150 days of water immersion with and without thermocycling for 12,000 cycles. These different findings may be due to different resin composites and silane coupling agents used, different surface pre-treatments and testing conditions of in vitro artificial aging. Other possible reasons may be due to the continued polymerization of un-reacted resin composite with the un-reacted silane monomers during artificial aging which increases the bond formation [12]. For the three coupling agents tested, 2-hydroxyethyl methacrylate, itaconic acid and oleic acid, the bond strengths measured under artificial aging are significantly lower than the dry condition except for the water storage group of itaconic acid (Table 2). The bond linkages formed from the three novel coupling agents, like silane coupling agents, degrade over time under artificial aging. However, the bond strength values measured for the three coupling agents under artificial aging tests are still higher than the minimum acceptable shear bond strength value of 5 MPa set by ISO standard 10477 [59]. Despite of the relatively moderate bond strength results, these coupling agents could be an alternative to the silane


d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872

Fig. 4 – Curve fitting of XPS O1s spectra of various surface treatments for samples A–G.

coupling agents because: (i) no hydrolysis is required (they can be used after preparation), (ii) they are relatively cheap and, (iii) have a longer shelf life (no self-condensation). Commercial single-bottle silanes are pre-hydrolyzed and produce varying bond strengths [60]. Once hydrolyzed, the silane monomers would react with one another to form higher molecular weight oligomers which would decrease the bond strength between resin composite and zirconia over time [61]. Further study of other coupling agents on bonding resin composite to dental ceramics, metal and metal alloys will be carried out, together with various surface conditioning methods. The hypothesis set was not met: there were differences in bond durability between the novel coupling agents and the two silane coupling agents.

shelf-life and hydrolysis time, these novel coupling agents might have the potential for use in bonding less stress bearing orthodontic attachments.




The resin composite to zirconia bonding of these three novel coupling agents showed bond degradation over time under artificial aging. However, the bond strength values measured are still higher than the minimum acceptable shear bond strength value set by ISO standard. With the advantages over conventional silane coupling agents in terms of costs,

Acknowledgments This work was financially supported from the research grants of the Faculty of Dentistry, The University of Hong Kong. The authors wish to thank 3 M ESPE for generously providing resin composite and the dental silane coupling agent to our study. Gelest Inc., is acknowledged for providing the 3acryloxypropyltrimethoxysilane.

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