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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

Available online at www.sciencedirect.com

jo u rn al hom epa ge : www.int l .e lsev ierhea l th .com/ journa ls /dema

esin zirconia bonding promotion with some novel couplinggents

hristie Ying Kei Lunga, Michael G. Botelhob, Markku Heinonenc, Jukka P. Matinlinnaa,∗

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

r t i c l e i n f o

rticle history:

eceived 30 November 2011

eceived in revised form

2 February 2012

ccepted 16 April 2012

eywords:

esin composite

irconia

dhesion

oupling agent

ond strength

a b s t r a c t

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

late, 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 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

stora

emy

wear resistance [2]. Yttria tetragonal zirconia polycrystals are

strength of the three

© 2012 Acad

. Introduction

irconia is used as a biomaterial because of superior mechan-cal properties, chemical inertness and biocompatibility [1].ormally, zirconia is doped with a small amount of yttria

∗ Corresponding author at: Dental Materials Science, Faculty of Dentistr4 Hospital Road, Sai Ying Pun, Hong Kong Special Administrative Regi

E-mail addresses: yklung@graduate.hku.hk (C.Y.K. Lung), botelhopmat@hku.hk (J.P. Matinlinna).109-5641/$ – see front matter © 2012 Academy of Dental Materials. Puttp://dx.doi.org/10.1016/j.dental.2012.04.023

ge conditions.

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

(Y2O3) to form yttria tetragonal zirconia polycrystals (TZP)which increases the fracture toughness, flexural strength and

y, The University of Hong Kong, 4/F, Prince Philip Dental Hospital,on P.R. China. Tel.: +852 2859 0380; fax: +852 2548 9464.@hkucc.hku.hk (M.G. Botelho), mahehe@utu.fi (M. Heinonen),

widely used in dentistry as root canal posts, orthodonticbrackets, dental implant abutments and all-ceramic restora-tions [3]. Inertness of zirconia has made resin to zirconia

blished by Elsevier Ltd. All rights reserved.

l s 2

864 d e n t a l m a t e r i a

bonding challenging. Tribochemical silica-coating using aRocatec system followed by silanization has been suggestedas a pre-treatment before cementation [4]. Other zirconia sur-face treatment methods have been reported, these include:chemical treatments, selective infiltration etching, laser irra-diation, nano-structured alumina coating and chemical vapordeposition [5–11]. All of these treatment methods, with differ-ent surface conditioning mechanisms, activate the zirconiasurfaces for bonding to resin composite.

A coupling agent has two different functional groups soas to connect dissimilar materials, such as metals to poly-mers. Silane coupling agents are widely used to promoteadhesion of dental restorations to tooth tissue [6]. Due to thelarge variety of silane coupling agents with different func-tional groups, numerous in vitro studies of experimental silanecoupling agents in resin zirconia bonding have been investi-gated [12–15]. 3-Acryloxypropyltrimethoxysilane (Fig. 1) is apromising coupling agent with in vitro results comparable toother silane coupling agents under dry and artificial agingconditions [13,15]. Coupling agents such as phosphates, e.g.10-methacryloyloxydecyl dihydrogen phosphate (MDP) andzirconates have also been investigated for resin to zirconiabonding [16,17] with the former gaining popularity as an alter-native to silane coupling agents because of enhanced bondingand hydrolytic stability [18]. However, some studies report thatthe bond durability under artificial aging of resin to zirconiaprimed with silane coupling agents is higher than that primedwith phosphate coupling agents [19,20].

The three novel coupling agents for resin zirconia bondinginvestigated 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 oleicacid, contain >C C< and COOH groups (Fig. 1). These cou-pling agents have many applications in industry and medicine[21–28]. 2-Hydroxylethyl methacrylate has been used for thesurface modification of a polysulfone membrane for treatmentof oily wastewater, contact lens applications and synthesis ofmacroporous hydrogels for adsorption of proteins for biomed-ical applications. Itaconic acid is added to vinylidene chloridecoatings to improve adhesion to paper and cellophane. Thereaction of itaconic acid with amines forms N-substitutedpyrrolidones which can be used as thickeners in lubricatinggrease, shampoos, detergents, pharmaceuticals and herbi-cides. In medicine, esters of partly substituted itaconic acidhave anti-inflammatory and analgesic properties. Oleic acid,n-octadecan-9-enoic acid, is a mono-unsaturated omega-9fatty acid which can be found in olive oil. It has been usedas a drug delivery vehicle for the medical management ofkeloid and hypertrophic scaring. It is also used as a protec-tive coating on mild steel against corrosion, solvent attackand as an environmentally friendly biolubricant. In addition,2-hydroxylethyl methacrylate has been used successfully inresin dentin bonding [29–31]. Other coupling agents with thefunctional group ( COOH) are also found in dentin adhe-sives, these include 4-methacryloxyethyl trimellitic acid and11-methyacryloyloxy-1,1′-undecanedicarboxylic acid [32,33].

The aim of this in vitro study was to evaluate andcompare the bond durability of the three novel couplingagents, 2-hydroxylethyl methacrylate, itaconic acid andoleic acid, to two silane coupling agents, one commercial

8 ( 2 0 1 2 ) 863–872

dental silane product and one experimental silane couplingagent, 3-acryloxypropyltrimethoxysilane, for resin compositeto zirconia bonding under different storage conditions. Thehypothesis was that there is no difference in bond durabilitybetween the three novel coupling agents and the two silanecoupling agents under different storage conditions.

2. Materials and methods

Zirconia blocks (Lava, 3 M ESPE, Seefeld, Germany) werecut into blocks of 16 mm × 15 mm × 3 mm and embedded incylindrical plastic molds filled with poly(methylmethacrylate)resin. Five test groups of resin composite were bonded tosilica-coated and primed zirconia and investigated underthree different storage conditions, giving a total of fif-teen experimental groups of randomly assigned samples.Each experimental group consisted of 15 resin compos-ite stubs for bond strength measurement. The 3 M ESPESil silane is a pre-hydrolyzed dental silane product of3-methacryloxypropyltrimethoxysilane (Fig. 1), at a silanecontent of “<3 vol.%”, indicated for silica-coated metallic andceramic indirect restorations [4,6].

2.1. Preparation of silica-coated zirconia samples

The zirconia sample surfaces were polished with a 400-gritsilicon carbide paper under running deionized water, thencleaned ultrasonically for 10 min in deionized water andrinsed with deionized water. They were allowed to dry inair at room temperature for 30 min. Silica-coating of the pol-ished zirconia specimens was performed using Rocatec SandPlus (110 �m in size of silica-coated alumina particles, 3 MESPE, Seefeld, Germany) at a constant pressure of 280 kPa for30 s/cm2 and at a perpendicular distance of 10 mm [34]. Thesamples were then cleansed in an ultra-sonic bath in 70%ethanol for 10 min and then rinsed with 70% ethanol. Theywere allowed to air-dry at room temperature for 30 min.

2.2. Preparation of primer solutions and primercoating 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.% deion-ized water was prepared. The pH of the solvent mixture wasadjusted to 4.0 by 1 M acetic acid. The silane solution wasthen 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) andoleic acid (92%, BDH, PA, USA) were all prepared in a solventmixture of 95.0 vol.% acetone (99.9%, VWR International SAS)and 5.0 vol.% deionized water. The pH of the solvent mixturewas adjusted to 4.0 by 1 M acetic acid. These three coupling

agents do not require hydrolysis. The five primer solutionswere applied onto the silica-coated zirconia surfaces with onecoating using a new fine brush each time. This was allowed todry 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 ) 863–872 865

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 functionalg onia are highlighted.

2p

RGfipwAlclwgpiicoc2

2

SfdaeLfT

Table 1 – Various surface treatments on zirconia forsurface roughness measurement and XPS analysis.

Sample Surface treatment conditions

A (i) Polishing, (ii) rinsingB (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv) rinsingC (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv)

rinsing, (v) silanized with 3M ESPE Sil silane primerD (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv)

rinsing, (v) silanized with 1 vol.% ACPSE (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv)

rinsing, (v) primed with 1 vol.% HEMAF (i) Polishing, (ii) Rinsing, (iii) Sandblasting, (iv)

Rinsing, (v) Primed with 1 vol.% IAG (i) Polishing, (ii) rinsing, (iii) sandblasting, (iv)

rinsing, (v) primed with 1 vol.% OA

Note: (1) All sample surfaces were polished with 400-grit sil-icon carbide paper. (2) After polishing and sandblasting, allsamples were rinsed in 70% ethanol in the ultra-sonic bathfor 10 min and air-dried. (3) Samples B–G were sand-blasted(30 s/cm2) using Rocatec Plus sand (110-�m silica-coated alu-mina) on the surface of zirconia at pressure of 280 kPa. Key:

roups which react to resin composite and silica-coated zirc

.3. Bonding of resin composite to silica-coated andrimed zirconia

ely X Unicem Aplicap resin composite (3 M ESPE, Seefeld,ermany) was activated and mixed according to manu-

acturers’ instructions and manipulated using dental handnstrument. The resin composite was carefully packed into aolyethylene mold of 3.7 mm in diameter and 4.0 mm in heighthich was positioned on the primed test zirconia surface.fter this, it was light cured for 40 s (Lunar Curing Light, Ben-

ioglu Dental Inc., Ankara, Turkey) and the mold was removedarefully after curing, by pressing the resin stub perpendicu-arly with the same instrument. The zirconia samples primedith the five coupling agents were randomly divided into three

roups. The first group was kept in a desiccator at room tem-erature for 24 h prior to bond strength testing to obtain the

nitial bond strength. The second group was stored in deion-zed water for 30 days for artificial aging in sealed polyethyleneontainers and kept in an incubator at a constant temperaturef 37.0 ± 0.1 ◦C. The third group was subjected to thermocy-ling for 6000 cycles between 5.0 ± 0.5 ◦C and 55.0 ± 0.5 ◦C with0 s in dwell time in each deionized water bath.

.4. Surface roughness measurement

even selected zirconia samples were prepared for sur-ace roughness measurement. The sample preparation withifferent surface treatments is shown in Table 1. Theverage surface roughness, Ra, was measured using an

lectro-mechanical profilometer (Surtronic 3+, Taylor Hobson,eicester, England). Three parallel readings were taken at dif-erent randomly selected regions on each specimen surface.he surface roughness was then reported as mean Ra ± SD.

ACPS = 3-acryloxypropyltrimethoxysilane, HEMA = 2-hydroxyethylmethacrylate, IA = itaconic acid, OA = oleic acid.

2.5. Shear bond strength testing

The samples were mounted in a jig on a universal testingmachine (Instron, Model 1185, Norwood, MA). A constant loadof 1000 N was applied at a cross-head speed of 1.0 mm/minuntil fracture occurred. The shear bond strengths were cal-culated by dividing the maximum fracture loading with the

circular area of the resin stub. The mode of failure of the zir-conia samples was assessed after shear bond strength testing:when the resin composite stub remaining was less than 1/3,

866 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 storageconditions. Different uppercase letters in the same row means the groups are significant different. Different lowercaseletters 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 12.1(2.9)Ab 6.7(1.4)Bb3-Acryloxypropyltrimethoxysilane 13.5(2.1)Aa,c 14.6(1.1)Aa 14.5(2.2)Aa

A 7.9(1.6)Bc 6.7(0.6)Bb10.8(1.8)Ab 5.9(1.9)Bb7.4(2.6)Bc 5.5(0.6)Cb

Table 3 – Mean surface average roughness, Ra,measurement of zirconia surfaces with varioustreatments. Different letters means that the groups aresignificant different (p < 0.05).

Sample Mean Ra/�m ± SD

A 0.95 ± 0.04a

B 1.17 ± 0.06a,c

C 1.78 ± 0.02b

D 1.19 ± 0.08a,c

E 1.35 ± 0.14c

2-Hydroxyethyl methacrylate 11.4(1.9)a,b

Itaconic acid 10.7(3.2)Aa,b

Oleic acid 9.6(2.0)Ab

the failure mode was assigned as ‘adhesive’ and when theremaining was more than 1/3 but less than 2/3, it was assignedas ‘mixed’. When the amount remaining was more than 2/3,it was assigned as ‘cohesive’ failure [35].

2.6. Scanning electron microscopy (SEM)

Representative and selected zirconia samples after shearbonding test were analyzed by SEM (XL30CP, Philips ElectronOptics, Eindhoven, The Netherlands). The operational voltagewas 10 kV and the vacuum pressure for measurement was3.5 × 10−5 Pa.

2.7. Statistical analysis

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

2.8. XPS analysis

The chemical composition of the surfaces of the samples afterdifferent surface treatments (Table 1) was examined by X-ray photoelectron spectroscopy using a Perkin-Elmer PHI 5400spectrometer, with a mean radius of 140 mm and equippedwith a resistive anode detector. The ionization source usedwas Mg K� radiation (h� = 1253.6 eV) from a twin-anode X-raytube. Broad-range survey scans, at a pass energy of 89.45 eVand an entrance slit width of 4.0 mm, were performed to deter-mine atomic concentration. High-resolution narrow-rangescans were also performed at a pass energy of 37.75 eV forselected specific photolines (Zr3d, Si2p, O1s) to determine thechemical shifts. The chamber base pressure was maintainedat about 8 × 10−8 Pa and the X-ray tube was operated at 200 W.The C1s photopeaks were used to calibrate the binding energyscale of the high-resolution spectra for chemical shift mea-

surements. The peak composition and energy positions werethen determined using the least-squares curve-fitting tech-nique with the Igor Pro analysis environment in the SPANCFmacro package [36].

F 1.39 ± 0.15c

G 1.41 ± 0.06c

3. Results

3.1. Shear bond strength testing

The results of the mean shear bond strengths for all testgroups are shown in Table 2. A two-way ANOVA analysisshowed that there were significant differences for differentstorage conditions (p < 0.001) and different primers (p < 0.001)used on the shear bond strength. There was a significantinteraction between storage condition and the five couplingagents (p < 0.005). The decrease in bond strengths betweenthe 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 for3 M ESPE Sil silane (p > 0.1), 3-acryloxypropyltrimethoxysilane(p > 0.05) and itaconic acid (p > 0.1) between the dry groups andthe water storage groups. For the thermocycling groups, a sig-nificant difference was found for the shear bond strengthswhen 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 foundfor 3-acryloxypropyltrimethoxysilane (p > 0.1). There were sig-nificant differences in shear bond strengths for the fivecoupling agents under different storage conditions: (i) dry(p < 0.001), (ii) water storage (p < 0.001) and (iii) thermocy-cling (p < 0.0001). The highest overall bond strengths wereobtained with 1 vol.% 3-acryloxypropyltrimethoxysilane in:dry (13.5 MPa); water storage (14.6 MPa); and thermocycled(14.5 MPa) conditions.

3.2. Surface topographic analysis

The surface roughness of zirconia surfaces measured forsamples A–G after the various treatments are presented inTable 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 ) 863–872 867

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-hydroxyethylmethacrylate (adhesive failure), (iv) itaconia acid (adhesive failure) and (v) oleic acid (adhesive failure) in dry condition.

pcGs(

o(

olishing and sandblasting (p < 0.05) was found. A signifi-ant difference was found between samples C, D, E, F and

with different primer coatings (p < 0.05). There was also aignificant difference between samples B, C, D, E, F and G

p < 0.001).

Some SEM micrographs were selected to illustrate the typesf 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 predominantmode of failure for all the zirconia samples was adhesive.

3.3. XPS analysis

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

868 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 Mixed Cohesive

3M ESPE Sil Dry 15 (100%) 0 030-Day water storage 11 (73.3%) 2 (13.3%) 2 (13.3%)Thermocycling 15 (100%) 0 0

ACPS Dry 15 (100%) 0 030-Day water storage 2 (13.3%) 10 (66.7%) 3 (20%)Thermocycling 14 (93.3%) 1 (6.7%) 0

HEMA Dry 15 (100%) 0 030-Day water storage 14 (93.3%) 1 (6.7%) 0Thermocycling 15 (100%) 0 0

Itaconic acid Dry 12 (80%) 3 (20%) 030-Day water storage 7 (46.7%) 4 (26.7%) 4 (26.7%)Thermocycling 15 (100%) 0 0

Oleic acid Dry 15 (100%) 0 030-Day water storage 13 (86.7%) 2 (13.3%) 0Thermocycling 15 (100%) 0 0

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

Sample Atomic concentration/%

C1s O1s Si2p Zr3d Al2p

A 62.2 24.1 3.8 8.8 –B 16.8 53.6 18.2 5.5 5.9C 18.8 52.8 20.4 3.7 4.3D 23.0 50.2 18.4 5.1 3.3

E 32.2 43.8

F 31.5 45.4

G 30.5 44.0

from sandblasting and primer coating and Al was detectedwhich came from silica-coated alumina particles used forsandblasting.

The XPS spectra of Zr3d, Si2p, and O1s for samples A–G areshown in Fig. 3. For sample A, the Si2p peak binding energyat 101.9 eV corresponds to Si–C from polishing with siliconcarbide (SiC) paper [37]. There is a shift of peak position forbinding energy of O1s after various surface treatments, i.e.sandblasting and primer coating. There are no observablechanges in binding energies for Zr3d (samples A–G) and Si2p

(samples B–G) after sandblasting and primer coating.

4. Discussion

Silane coupling agents, after activation by hydrolysis, formsilanols ( Si OH) which are known to react readily with sur-face hydroxyl groups on the silica-coated zirconia surfaceto form siloxane ( Si O Si ) linkages. For 2-hydroxyethylmethacrylate, the reaction of the COH group with surfacehydroxyl 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-

coated zirconia forms silyl ester ( ) linkages. Giventhis, all coupling agents containing >C C< bonds can react

14.9 5.2 3.917.2 3.3 2.617.8 3.7 4.0

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. Thebinding energy shift to higher values for samples B–G aftervarious surface treatments indicates different chemical statesof oxygen present. The peaks can be resolved into individ-ual components by curve fitting. The O1s peaks are assignedusing the ‘Principle of Electronegativity Equalization’ [41] sincedifferent elements have different electronegativity. The firstpeaks would be assigned as ZrO2, the second peak would beassigned as Zr O Si since Si is more electronegative than Zrand the third peak would be assigned as Si O Si. The otherO1s 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 producedby impact of high energy silica-coated alumina particles ontothe surface, this may enhance mechanical retention for bond-ing [47]. SEM images (Fig. 2) for the five coupling agentsshow different amounts of resin composite residue left afterthe shear bond strength test. All of them show an adhesivemode of failure which indicates the interfacial failure occurredbetween the substrate, i.e. resin composite, silica layer or zir-

conia and the adhesive layer.

Extensive in vitro studies investigating the effects of varioussurface pretreatments and different silane coupling agentson resin-composite to zirconia bonding show bond strength

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 863–872 869

ples

dHawrtofci

scg[pdsarmdb

Fig. 3 – XPS spectra of Zr3d, Si2p, and O1s of sam

egradation over time under artificial aging [12–15,48–57].owever, some reports showed no decrease in bond strengthsfter artificial aging test [13,50]. It is suggested that underater immersion, water diffuses into the interfacial layer of

esin composite and zirconia and causes hydrolytic degrada-ion of the bonding [58]. Thermocycling combines the effectsf hydrolytic degradation and thermal irradiation. The dif-erence in linear coefficient of thermal expansion of resinomposite and zirconia would induce thermal stress at thenterfacial layer which causes the breaking of the bond [15].

In this study, we found the bond strength values for bothilane coupling agents under artificial aging were not signifi-antly lower than for dry conditions except the thermocyclingroup for the commercial silane (Table 2). Senyilmaz et al.50], using the same resin composite, silane and surfaceretreatment of zirconia, reported there was no significantecrease of bond strength measured after 24 h water immer-ion and thermocycling for 6000 cycles. Likewise, Matinlinnand Lassila [13], using identical silane and resin composite,

eported no significant decrease of bond strength after ther-

ocycling. Heikkinen et al. [15], using the same silanes but aifferent resin composite, observed a significant decrease ofond strength after thermocycling. Matinlinna et al. [12] also

A–G (Table 1) with different surface treatments.

investigated five silane coupling agents and found a signifi-cant difference in shear bond strength after thermocycling.Valandro et al. [49] reported a significant decrease of bondstrength after 150 days of water immersion with and with-out thermocycling for 12,000 cycles. These different findingsmay be due to different resin composites and silane couplingagents used, different surface pre-treatments and testing con-ditions of in vitro artificial aging. Other possible reasons may bedue to the continued polymerization of un-reacted resin com-posite with the un-reacted silane monomers during artificialaging which increases the bond formation [12].

For the three coupling agents tested, 2-hydroxyethylmethacrylate, itaconic acid and oleic acid, the bond strengthsmeasured under artificial aging are significantly lower thanthe dry condition except for the water storage group of ita-conic acid (Table 2). The bond linkages formed from the threenovel coupling agents, like silane coupling agents, degradeover time under artificial aging. However, the bond strengthvalues measured for the three coupling agents under artifi-

cial aging tests are still higher than the minimum acceptableshear 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

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

vari

r

Fig. 4 – Curve fitting of XPS O1s spectra of

coupling agents because: (i) no hydrolysis is required (they canbe used after preparation), (ii) they are relatively cheap and,(iii) have a longer shelf life (no self-condensation). Commercialsingle-bottle silanes are pre-hydrolyzed and produce varyingbond strengths [60]. Once hydrolyzed, the silane monomerswould react with one another to form higher molecular weightoligomers which would decrease the bond strength betweenresin composite and zirconia over time [61].

Further study of other coupling agents on bonding resincomposite to dental ceramics, metal and metal alloys will becarried out, together with various surface conditioning meth-ods. The hypothesis set was not met: there were differencesin bond durability between the novel coupling agents and thetwo silane coupling agents.

5. Conclusions

The resin composite to zirconia bonding of these three novelcoupling agents showed bond degradation over time under

artificial aging. However, the bond strength values measuredare still higher than the minimum acceptable shear bondstrength value set by ISO standard. With the advantagesover conventional silane coupling agents in terms of costs,

ous surface treatments for samples A–G.

shelf-life and hydrolysis time, these novel coupling agentsmight have the potential for use in bonding less stress bearingorthodontic attachments.

Acknowledgments

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

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