Effectiveness and stability of silane coupling agent incorporated in 'universal' adhesives

Pleasecitethisarticleinpressas:YoshiharaK,etal.Effectivenessandstabilityofsilanecouplingagentincorporatedin‘universal’adhesives.

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Effectiveness

and

stability

of

silane

coupling

agent

incorporated

in

‘universal’

adhesives

Kumiko

Yoshihara

_,

_Noriyuki

_Nagaoka

_,

_Akinari

_Sonoda

_,

Yukinori

Maruo

_,

_Yoji

_Makita

_,

_Takumi

_Okihara

_,

_Masao

_Irie

_,

Yasuhiro

Yoshida

_,

_Bart

_Van

_Meerbeek

a_{CenterforInnovativeClinicalMedicine,OkayamaUniversityHospital,2-5-1Shikata-cho,Kita-ku,} Okayama700-8558,Japan

b_{AdvancedResearchCenterforOralandCraniofacialSciences,OkayamaUniversityDentalSchool,} 2-5-1Shikata-cho,Kita-ku,Okayama700-8558,Japan

c_{HealthResearchInstitute,NationalInstituteofAdvancedIndustrialScienceandTechnology(AIST),} 2217-14Hayashi-cho,Takamatsu,Kagawa761-0395,Japan

d_{DepartmentofOcclusionandRemovableProsthodontics,OkayamaUniversityHospital,2-5-1Shikata-cho,} Kita-ku,Okayama700-8558,Japan

e_{DivisionofAppliedChemistry,GraduateSchoolofNaturalScienceandTechnology,OkayamaUniversity,} 3-1-1Tsushima-Naka,Kita-ku,Okayama700-8530,Japan

f_{DepartmentofBiomaterials,GraduateSchoolofMedicine,DentistryandPhamaceuticalSciences,} OkayamaUniversity,2-5-1Shikata-cho,Kita-ku,Okayama700-8558,Japan

g_{DepartmentofBiomaterialsandBioengineering,GraduateSchoolofDentalMedicine,HokkaidoUniversity,Kita13,} Nishi7,Kita-ku,Sapporo,Hokkaido,060-8586,Japan

h_{KULeuven(UniversityofLeuven),DepartmentofOralHealthSciences,BIOMAT&UniversityHospitalsLeuven,} Dentistry,Kapucijnenvoer7blokabus7001,B-3000Leuven,Belgium

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Received23November2015 Receivedinrevisedform 12May2016 Accepted4July2016 Availableonlinexxx Keywords: Silane Couplingagent Universaladhesive NMR FTIR Bondstrength

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Objective.Forbondingindirectrestorations,some‘universal’adhesivesincorporateasilane couplingagenttochemicallybondtoglass-richceramicssothataseparateceramicprimer isclaimedtobenolongerneeded.Withthiswork,weinvestigatedtheeffectiveness/stability ofthesilanecouplingfunctionofthesilanecontainingexperimentallypreparedadhesives andScotchbondUniversal(3MESPE).

Methods and materials. Experimental adhesives consisted of Scotchbond Universal and the silane-free Clearfil S3 ND Quick (Kuraray Noritake) mixed with Clearfil Porcelain BondActivator(KurarayNoritake)andthetwoadhesivestowhich␥ -methacryloxypropyl-trimethoxysilane(␥-MPTS)wasadded.Shearbondstrengthwasmeasuredontosilica-glass plates; theadhesiveformulationswereanalyzed usingfouriertransforminfrared spec-troscopy(FTIR)and13Cnuclearmagneticresonance(NMR).Inaddition,shearbondstrength ontoCAD-CAMcompositeblockswasmeasuredwithoutandafterthermo-cyclingageing. Results.AsignificantlyhigherbondstrengthwasrecordedwhenClearfilPorcelainBond Acti-vatorwasfreshlymixedwiththeadhesive.Likewise,theexperimentaladhesives,towhich

∗ _{Correspondingauthor.}

E-mailaddresses:k-yoshi@md.okayama-u.ac.jp(K.Yoshihara),nagaoka@okayama-u.ac.jp(N.Nagaoka),a.sonoda@aist.go.jp

(A.Sonoda),ykmar@md.okayama-u.ac.jp(Y.Maruo),y-makita@aist.go.jp(Y.Makita),okihara@cc.okayama-u.ac.jp(T.Okihara),

mirie@md.okayama-u.ac.jp(M.Irie),yasuhiro@den.hokudai.ac.jp(Y.Yoshida),bart.vanmeerbeek@med.kuleuven.be(B.VanMeerbeek).

http://dx.doi.org/10.1016/j.dental.2016.07.002

␥-MPTSwasadded,revealedasignificantlyhigherbondstrength,butonlywhenthe adhe-sivewasappliedimmediatelyaftermixing;delayedapplicationresultedinasignificantly lowerbondstrength.FTIRand13_{CNMRrevealedhydrolysisanddehydrationcondensation}

toprogresswiththetimeafter␥-MPTSwasmixedwiththetwoadhesives.After thermo-cycling,thebondstrengthontoCAD-CAMcompositeblocksremainedstableonlyforthe twoadhesiveswithwhichClearfilPorcelainBondActivatorwasmixed.

Significance.Onlythesilanecouplingeffectoffreshlypreparedsilanecontainingadhesives waseffective.Clinically,theuseofaseparatesilaneprimerorsilanefreshlymixedwiththe adhesiveremainsrecommendedtobondglass-richceramics.

©2016TheAcademyofDentalMaterials.PublishedbyElsevierLtd.Allrightsreserved.

1.

Introduction

In restorative dentistry, ceramics meet best the patient’s demandforaesthetics.Glass-richdental ceramics,such as feldsparceramics,thediversekindsofglass-ceramics (includ-ing monolithic lithium disilicate ceramics) and some new polymer-infiltrated ceramics, are least invasive to restore teeth, as they can be fully bonded to the remaining sound tooth tissue using composite cements. At the restoration site, the bonding protocol of preference to adhesivelylutethese‘etchable’ceramicsconsistsof hydroflu-oric acid (HF) etching to provide micro-retention to the cement that by silanization also chemically bonds to the ceramic.Asilane functional monomer, likethe most com-monly in dentistry used methacrylate silane monomer (␥-methacryloyloxypropyltrimethoxysilaneor␥-MTPS), basi-cally possesses a methacrylate end to co-polymerize with theadhesiveand/orcompositecementandtheactualsilane grouptocovalentlybondtotheceramicglassphase.Besides needed forbonding toceramic, silane couplingagents are also often used as part ofa restoration-repair protocol to intra-orallyrepairbothceramicand compositerestorations

[1]. Silaneprimersare mostlywater-free solutions[2]; one-bottleandtwo-bottlesilaneprimersexist.Silaneprimersthat containnon-hydrolyzedsilane,are mostoftendissolved in ethanolinonebottlethatneedstobeactivatedandhydrolyzed bymixingitwithanaqueousaceticacidsolutionoranacidic adhesive in the other bottle [2]. Both water and lower pH cause silane to hydrolyse. The latest generation of silane primerscontainsilanemostlydissolvedinawater-freeand onlymildlyacidicsolution[2],sothatithasarelativelylong shelflife.Akindofmulti-purposeceramic/metalprimersare alsowater-freesolutionsandcontainbesidesasilane func-tionalmonomerotherfunctionalmonomersinordernotonly tobond‘glass-rich’(requiringsilane)and‘glass-poor’ceramics likezirconia(requiring10-methacryloyloxydecyldihydrogen phosphateor10-MDP),butalsotobondpreciousmetalalloys (requiringa monomerwithone endcarrying a thiol ( SH) group).Mostrecently,so-called‘universal’adhesivesenable thedentistnotonlytooptforan‘etch-and-rinse’or‘self-etch’ bondingprotocol,buttheycanalsobeemployedforbothdirect andindirect indications[3–5].Amongsuchuniversal adhe-sives,someadhesivesalsoincorporateasilanecouplingagent, havingbeenclaimedtoprovidetheadhesivedirectchemical bondingpotentialtoglass-richceramicswithouttheneedof aseparateceramicprimer[6].

Itisgenerallywellknownthatwater-containingandacidic, single-bottle, pre-hydrolyzedsilane couplingagents havea relatively shortshelflife[2].Inlightofthisknowledgeand because independentresearch dataare insufficiently avail-able, we hereby investigated the silanization potential of a universaladhesive incorporatinga silanecouplingagent. BondstrengthtoglassplatesandCAD-CAMcompositeblocks was measured, as well as the adhesive formulations were chemically characterized using Fourier transform infrared spectrometry(FTIR)andnuclearmagneticresonance(NMR). Besides the commercially available ‘universal’ and silane-containing adhesive Scotchbond Universal (3M ESPE), we alsoinvestigatedthecommerciallyavailablesilane-free adhe-sive Clearfil S3 Bond ND Quick (Kuraray Noritake Dental, Tokyo,Japan),aswellasexperimentaladhesivestowhich␥ -methacryloxyproyltrimethoxysilane(␥-MPTS)wasadded.The nullhypothesestestedwerethatanadhesiveincorporatinga silanecouplingagentwasmoreeffectiveintermsofbonding effectivenessthananadhesivethatdoesnotcontainsilane (1)andthattheadditionofsilanetotheadhesiveformulation improvedbondstrength(2)andremainedeffectivewithtime (3).

2.

Materials

and

methods

2.1. Shearbondstrengthontoglassplates

Silica-glassplates(Shin-EtsuQuartzProducts,Tokyo,Japan) with a smooth surface and thus low potential for micro-mechanical interlocking were employedto represent glass-richceramics;theplateswere10mm×10mmwideand3mm thick. Six different adhesive protocols using the universal and silane-containing adhesive Scotchbond Universal (3M ESPE)andthesilane-freeadhesiveClearfilS3BondNDQuick (Kuraray Noritake)were testedasfollows:(1)applicationof theadhesive‘assuch’;(2)immediateapplicationofa1:1drop mixture of the adhesive with Clearfil PorcelainBond Acti-vator (Kuraray Noritake);(3) ‘immediate’ application ofthe adhesive towhich 2wt% ␥-MPTS(Sigma–Aldrich, St.Louis, MO, USA)wasadded; (4) ‘1day’ delayedapplication ofthe 2wt%␥-MTPS-containingadhesive;(5)‘3days’delayed appli-cationofthe2wt%␥-MTPS-containingadhesive;(6)‘7days’ delayed application of the 2wt% ␥-MTPS-containing adhe-sive.ClearfilPorcelainBondActivator(KurarayNoritake)isa water-free silanecouplingagent.Weused2wt%␥-MTPSas thisconcentrationcorrespondedtothesilaneconcentration

Pleasecitethisarticleinpressas:YoshiharaK,etal.Effectivenessandstabilityofsilanecouplingagentincorporatedin‘universal’adhesives. mostcommonwithincommercialsilaneprimers,astheywere

listedbyLungandMatinlinna[2].Toachieveahomogeneous mixture,the2wt%␥-MTPSwasaddedtotheadhesivesusinga magneticstirrer.Thesilica-glassplatesweretreatedfollowing theabovementionedexperimentalprotocols,uponwhichthey wereair-dried.Zirconiacylinderblocks(Tosoh,Tokyo,Japan) witha3.6-mmdiameterweresandblastedusingShofuHigh Blaster(Shofu, Kyoto, Japan)with 50-␮malumina particles (Shofu),followedbysilanizationusingClearfilCeramicprimer (KurarayNoritake);theywereeventuallylutedontotheglass platesusingthecompositecementClearfilEstheticCement (KurarayNoritake).Thecementwaslight-curedfor40sfrom twoopposingdirections(totalingtoa80-scuringtime)using G-LightPrimaIIPlus (lightirradianceof2800mW/cm2_;GC, Tokyo,Japan);thespecimenswerenextstoredin37◦Cwater for24h priorto being subjected to a shearbond-strength test.Perexperimentalgroup,tenspecimenswere prepared. Thespecimensweremountedintoatestingmachine(AG-X, Shimadzu,Kyoto,Japan)and subjectedtoshearstress ata crossheadspeedof0.5mm/min.Allfracturedspecimenswere analyzedutilizingalightmicroscope(SMZ-10,Nikon,Tokyo, Japan)at4×magnificationtodeterminethemodeoffracture. Forstatisticalcomparisonsofthedata,theScheffé’stestwas appliedwithp<0.05consideredstatisticallysignificant.

2.2. FTIRchemicalanalysisoftheadhesive formulations

ThecommercialadhesivesScotchbondUniversalandClearfil S3Bond NDQuick, and the experimental adhesive formu-lations consisting of the commercial adhesives to which 2wt%␥-MTPSwasmixed,wereanalyzedusingaShimadzu IRAffinity-1FTIRSpectrophotometer(Shimadzu,Kyoto,Japan) withaKBrplate(Jasco,Tokyo,Japan)intransmissionmode.As control,weuseda2wt%␥-MTPSin98%ethanolsolutionand thesamesolutionmixedwithasolutionof2wt%aceticacid (Sigma–Aldrich)in70wt%ethanoland28wt%water.TheFTIR spectrawererecordedupon256successivescansanda spec-tralresolutionof4cm−1_{. Eachsamplewasmeasured three} times.

2.3. NMRchemicalanalysisoftheadhesive formulations

HalfofthesamplesofScotchbondUniversalandClearfilS3 BondNDQuick,towhich4wt%␥-MTPSwasadded,were ana-lyzedimmediatelyaftermixing,whiletheotherswere kept for1daybeforebeinganalyzed.Ascontrol,Scotchbond Uni-versaland ClearfilS3 BondNDQuick were alsomeasured. Before measurement, the same amount of d-ethanol was addedtothesamples,afterwhichtheywerepouredintoNMR testglasstubeswitha5-mmdiameterandan8-inchlength (Wilmad,Buena,NewJersey).AnNMRspectrometer(UNITY INOVA400NB,Varian Japan,Tokyo, Japan)wasemployed to acquire13_{CNMRspectraat100.58MHzinCD}

3CD2OD.13CNMR spectrawerereferencedattheinternalCD3peak(ı=17ppm) ofd-ethanol.Twosamplesofeachadhesiveformulationwere

analyzedbyNMR.

2.4. ShearbondstrengthontoCAD-CAMcomposite blocks

CAD-CAMcompositeblocks(shadeA3-LT,size14L;Lava Ulti-mate,3MESPE) werecuttodiscs witha1.5-mm thickness. Thesurfacewaspolishedusing15-␮mdiamondlappingfilm (Struers,BallerupDenmark)inordertoreducethepotentialfor mechanicalmicro-retention;thiswasfollowedbyeitherone ofthefollowingsurfacetreatments: (1)Scotchbond Univer-sal(incorporatingsilane)applied‘assuch’;(2)ClearfilS3Bond NDQuick(silane-free)applied‘assuch’;(3)1:1dropmixture ofScotchbondUniversalwithClearfilPorcelainBond Activa-tor; (4)1:1dropmixtureofClearfilS3BondNDQuickwith ClearfilPorcelainBondActivator.Zirconiacylinders,prepared inthesamemannerasforthebond-strengthmeasurements ontoglassplates,werelutedontoCAD-CAMcompositeblocks usingClearfilEstheticCementandlight-curedfor40sfrom twoopposingdirections(totalingtoa80-scuringtime)using G-LightPrimaIIPlus.Perexperimentalgroup,20specimens wereprepared;allspecimensweresubjectedtoashear bond-strengthtestingprotocol,likewiseasdescribedabove,thisfor halfofthespecimensafter24-hstorageinwaterat37◦C,while theother halfwere artificiallyagedthroughthermo-cycling (60sofimmersion,alternatively,ina5and55◦Cwaterbath) during15,000timespriortobond-strengthmeasurement.For statisticalcomparisonsofdata,two-wayANOVAfollowedby Tukey’spost-hoctests(˛<0.05)wasusedwithp<0.05 consid-eredstatisticallysignificant.

3.

Results

3.1. Shearbondstrengthontoglassplates(Fig.1)

Nodifference inbondstrength ontoglass plateswas mea-suredforthesilane-containingScotchbondUniversalandthe silane-freeClearfilS3BondNDQuickwhenapplied‘assuch’. When bothadhesiveswere beforehand mixedwithClearfil PorcelainBondActivator,asignificantlyhigherbondstrength was recorded,againwithout significant differencebetween the twoadhesives.Asignificantlyhigher bondstrength(as comparedtothatfortheapplicationoftheadhesives‘assuch’) wasalsomeasuredwhen␥-MTPSwasaddedtotheadhesives andthe bondstrengthwasmeasuredimmediately;delayed applicationafter1,3or7daysresultedinasignificantlylower bondstrength;nodifferenceinbondstrengthwasmeasured betweenallthe delayedapplicationsandbetweenboththe two␥-MTPS-addedadhesives.

3.2. FTIRchemicalanalysisoftheadhesive formulations(Fig.2a–c)

FTIR revealed a strong peak at 2835cm−1 _{that should be} assignedtotheC Hstretchof Si O CH3,thereby represent-ing␥-MTPS(Fig.2a,b).Whenaceticacidwasaddedto␥-MTPS, thepeakat2835cm−1_{wasnolongerdetected,aswellitwas} notdetectedforthetwoadhesivesScotchbondUniversaland ClearfilS3BondNDQuickwithandwithout␥-MTPS(Fig.2b). NoclearSi Osignals were detectedin the1100–850cm−1 region(Fig.2c).

Fig.1–Shearbondstrength(SBS)ontoglassplatesoftheadhesivesScotchbondUniversal(3MESPE)andClearfilS3ND Quick(KurarayNoritakeDental)applied‘assuch’andaccordingtotheirmodifiedversions.Barsdenotethemeanbond strengthandthewhiskersdefinethestandarddeviation.Insidethebars,themeanSBSvalueandthestandarddeviation areindicated.Meanswiththesameletterarenotsignificantlydifferent.

3.3. 13_{CNMRchemicalanalysisoftheadhesive}

formulations(Fig.3)

13_{CNMRrevealeddiversespeaksthatshouldbeascribedtothe} specificcompositionofboththeadhesivesScotchbond Univer-salandClearfilS3BondNDQuick.When␥-MTPSwasadded toeitheroftheadhesives,mostcharacteristicwerethestrong peaksat8.6and8.8ppmthatshouldbeattributedtosilanol

[7].Thesepeaksweredetectedforboththeadhesiveswhen ␥-MTPS was addedand when the solutions were analyzed immediately;theywerehowevernolongerdetectedwhenthe solutionswere‘1day’old.

3.4. ShearbondstrengthontoCAD-CAMcomposite blocks(Fig.4)

Nodifferencein24-hbondstrengthwasmeasuredforboththe adhesivesapplied‘assuch’orfortheir1:1dropmixtureswith ClearfilPorcelainBondActivator.After15,000times thermo-cycles,the bondstrength ofthe twoadhesives applied‘as such’droppedsignificantly,whilenotfortheir1:1drop mix-tureswithClearfilPorcelainBondActivator.

4.

Discussion

Themainobjectiveofthisstudywastoevaluatethe effective-nessandstabilityofasilanecouplingagentincorporatedin anuniversaladhesive.Therefore,theshearbondstrengthofa silane-containinguniversaladhesiveandasilane-free adhe-sivewas measuredonto twomaterialkinds,being smooth

glass plates,which represent glass-richceramic,and CAD-CAMcompositeblocks,forwhichsilanizationisalsoindicated aspartofthebondingprotocol.Nodifferenceinbondstrength betweenthesilane-containingandsilane-freeadhesiveonto both material kinds was measured, by which hypothesis (1) wasrejected.Adding silanetothe adhesiveformulation improved the bond strength, by which hypothesis (2) was accepted.Thisincreasedbondingeffectivenesswashowever lostforthedelayedapplications,bywhichhypothesis(3)was rejected.

ThereactionmechanismofsilanizationisdetailedinFig.5. To activate a silane bifunctional monomer like ␥-MTPS, it shouldreactwithwater,bywhichithydrolysestosilanol.As silane hydrolysisisslowinwater, acetic acidiscommonly used as reaction catalyst, as was also done in this study. Thissilanoladsorbsandchemicallybondstoglass.However, uponhydrolysissilanemayundergodehydration condensa-tion,thereby forminganoligomerthatno longercanbond toglass[2,8].Therateofoligomerformationdependsonthe structureofthesilanecouplingagent,itspH,thesolventkind andtheenvironmentaltemperature[2,9,10].

FTIR revealed aclear peak at2835cm−1 that should be attributed to Si O CH3 of ␥-MTPS, which was no longer detected when acetic acid was added. This indicates that silanehydrolysesimmediatelyuponexposuretoaceticacid. No peak representing Si O CH3 was detectedfor the two adhesivesinvestigated,thiswithandwithout␥-MTPS.These results demonstrate that any non-hydrolysed ␥-MTPS was present in both the commercial and the experimental ␥ -MTPS-added adhesive formulations. Hooshmandet al. [11]

Pleasecitethisarticleinpressas:YoshiharaK,etal.Effectivenessandstabilityofsilanecouplingagentincorporatedin‘universal’adhesives. Fig.2–FTIRspectraoftheadhesivesScotchbondUniversal(3MESPE)andClearfilS3NDQuick(KurarayNoritake)withand without␥-MTPSadded.Ascontrol,weuseda2wt%␥-MTPSin98%ethanolsolutionandthesamesolutionmixedwitha solutionof2wt%aceticacid(Sigma–Aldrich)in70wt%ethanoland28wt%water.(a)FTIRfull3500–800cm−1_spectrum;(b)

FTIR3500–2000cm−1_{spectrum;(c)FTIR1100–800cm}−1_spectrum.

compositionofthesilanesolutiontestedintheirstudywas 2.5%␥-MTPSand2.5%aceticacidin95%ethanol(nowater).

Inordertoconfirmthestatusofsilane couplingagents, FTIR ismostcommonlyused [11–13]. FTIR didnotsuit for our study,since peaks representing silanol or dehydration

condensationcouldnotbe(clearly)detectedbecauseofthe multitudeofoverlappingpeaksthatshouldbeassociatedwith thecomplexcompositionofthetwoadhesivesinvestigated andalsoduetopresenceofwaterandsilane-treatedfillerin the adhesives.AlthoughSi Osignals shouldappearinthe

Fig.3–13_{CNMRspectraoftheadhesivesScotchbondUniversal(3MESPE)andClearfilS3NDQuick(KurarayNoritake}

Dental)withandwithout␥-MTPSaddedandanalysed‘immediately’or‘1day’aftermixing.

1100–850cm−1 _{regionafterhydrolysisandsiloxane-network} formation, theycould however notbe detectedbecause of overlappingpeaksrepresentingsilane-treatedfiller.

NMR has also been used to analyze silane coupling agents[7,10]. In this study,13_{C NMR spectra revealed that}

dehydrationcondensationprogressedwiththetimeafter␥ -MTPS was mixed with the two adhesives. A multitude of significant peaks were disclosed by 13_{C NMR spectra that} againshouldbeattributedtothecomplexcompositionofthe adhesiveformulations[14].Nevertheless,twocharacteristics

Fig.4–ShearbondstrengthontoCAD-CAMcompositeblocksat24handafter15,000thermo-cyclesoftheadhesives ScotchbondUniversal(3MESPE)andClearfilS3NDQuick(KurarayNoritakeDental),whentheywereapplied‘assuch’,and oftheir1:1dropmixtureswithClearfilPorcelainBondActivator(KurarayNoritakeDental).Barsdenotethemeanbond strengthandthewhiskersdefinethestandarddeviation.Insidethebars,themeanSBSvalueandthestandarddeviation areindicated.Meanswiththesameletterarenotsignificantlydifferent.

Pleasecitethisarticleinpressas:YoshiharaK,etal.Effectivenessandstabilityofsilanecouplingagentincorporatedin‘universal’adhesives. Fig.5–Schematicdiagramofthechemicalinteractionof

␥-MTPSwithglass-richmaterials,involvinghydrolysisto silanoluponexposuretowaterandleadingtoeither adsorptionandbondingtoglassortodehydration condensation,bywhichitcannolongerbondtoglass.

peaksataround8.5–9.0ppmshouldbeascribedtosilanoland were detected when the solutions were analyzed immedi-atelyafter␥-MTPSwasadded;the‘1-day’delayedanalysisdid notrevealthetwosilanolpeaks.NMRhasa concentration-dependent sensitivity. Therefore, we additionally analysed ScotchbondUniversal to which 10wt% ␥-MPTS was added (Fig. 6). Silanol-characteristic peaks were only detected by

NMRforthefreshlyprepared,‘immediately’measured adhe-sive,whilethesepeakswerenolongerdetectable3dayslater. Using NMR,Nishiyamaetal.[7] coulddetect peaksthat shouldbeattributedtodehydrationcondensationof␥-MTPS; we, however, could not detect any peaks characteristicfor dehydrationcondensation,thisusingnotonly13_CNMRbut also 1_{H NMR (data not shown). This must most likely be} attributedtoavaryingstatusofself-condensation,tothefact thattheconcentrationof␥-MTPSwasnothighandtoother components of the adhesives that masked condensation-specificpeaks.AlthoughScotchbondUniversalisclaimedto incorporatesilane[15],13_{CNMRhowevercouldnotdisclose} peaksrepresentingsilanol.

OurFTIRandNMRdatafullyconfirmedthebondstrength data onto glass plates. Only adhesive solutions applied immediatelyafter␥-MTPSadditionpromotedbonding.This confirmsthat␥-MTPSwasimmediatelyactivated(Fig.5)and contributed to the ‘immediate’ bondstrength by chemical interaction.Thedroppedbondstrengthupondelayed applica-tiondemonstratedthatsilanolwasnotstableintheadhesive solutionand wasinactivatedalready after1day.3MESPE’s USPatent4673354A[15]explainedthatsilanolcanbestable insomewater/alcoholconcentrationsatapHaround4.6.The pHofScotchbondUniversalwasdocumentedtobe2.7[6].This lowpHmaythushavepromotedhydrolysisanddehydration condensation.

As silanehasalso been claimedtopromote bondingto CAD-CAMcompositeblocks,bondstrengthtoLavaUltimate wasalsomeasured.Asignificantdropinbondstrengthwas measuredforbothadhesivestestedafterartificialageing,but onlywhensilanewasnotadded. Thisfinding confirmsthe needfor‘fresh’silanizationaswellastheinsufficient effec-tivenessofsilaneincorporatedintheuniversaladhesive.The indication‘crown’using LavaUltimateisno longer recom-mended by3M ESPE,because crowns were documentedto

Fig.6–13_{CNMRspectraofScotchbondUniversal(3MESPE)withoutandwith10wt%␥-MTPSadded;thelatterNMRspectra}

de-bondatahigherthanoriginallyanticipatedrate.AsLava Ultimatecompositeblocksarerecommendedtobelutedusing the compositecement RelyXUltimate (3M ESPE) in combi-nation withScotchbond Universal,the in this study found insufficienteffectivenessofsilaneincorporatedinScotchbond Universalmay havecontributed tothe higher incidenceof crownde-bonding.

Similar in vitro data that a silane-containing universal adhesivewaslesseffectiveand/orstable,havealreadybeen reportedfor composite[16]and lithium disilicateceramics

[17–19]. In order to bond to glass-rich ceramics, HF etch-ingfollowedbysilanizationisthestandardbondingprotocol

[8,20].Auniversal adhesivelikeScotchbondUniversalmust bebeneficialas surfacewettingagentandmust,thanksto its lowviscosity, beabletofloweffectively into the micro-etch pitsatthe etchedceramicsurface. Althoughahigher initialbondstrengthmaybeobtained,thebondstrengthmay decreaseonthelongtermbecausestablechemicalbondingis lacking.

Inthis study,ashearbond-strengthtestwas appliedto measure the bond strength to the silica-glass plates and CAD-CAMcompositeblocks.Weoptedforthesimplest bond-strength testmainly becauseofthe high brittlenessofthe silica-glassplates,whichotherwisewouldmoreeasilyhave fracturedwithintheplateratherthanattheinterfacewhenfor instanceamicro-tensilebondstrengthapproachwouldhave beenused.Thelattercouldhavebeenmoresuitedtomeasure thebondingeffectivenesstotheCAD-CAMcompositeblocks, butwealsooptedfortheshearbond-strengthtestinorder tobeconsistentfortestmethodology.Althoughthe discrimi-nativepowerofashearbond-strengthtestislowerthanthat ofamicro-tensilebond-strengthtest[21],thedataprovided andthesignificantdifferencesfoundwereofthatorderthat definitiveconclusionscouldbedrawn.

5.

Conclusion

Itisconcludedthatthe silane-couplingeffectinthetested silane-containing universal adhesive did not appear very effectiveandstable,mostlikelybecausetheacidicsolution promoted dehydration condensation. Clinically, the use of a separatesilane primer (or silane freshly mixed withthe adhesive)remains recommendedtoachieve enough silane-couplingeffectonetchableceramics.

Acknowledgements

ThecurrentresearchwassupportedbyaJSPSKAKENHIGrant Numbers JP26893156,16K2045508. Theauthors declareno potentialconflictsofinterestwithrespecttotheauthorship and/orpublicationofthisarticle.

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