Photocrosslinking of functionalized rubbers, 8. The thiol-polybutadiene system

Macromol. Chem. Phys.200,1965–1974 (1999) 1965

Photocrosslinking of functionalized rubbers, 8a)

The thiol-polybutadiene system

Christian Decker*, Trieu´ Nguyen Thi Viet

Departement de Photochimie (UMR – CNRS N87525), Ecole Nationale Supe´rieure de Chimie de Mulhouse– Universitede Haute Alsace 3, rue A. Werner – 68200 Mulhouse, France

(Received: October 6, 1998; revised: January 13, 1999)

SUMMARY: The light-induced crosslinking of polystyrene-block-polybutadiene-block-polystyrene (SBS)was carried out in the presence of a trifunctional thiol and a phosphine oxide photoinitiator. The curing pro-cess was followed by infrared spectroscopy, insolubilization and hardness measurements, and was shown toproceed extensively within less than 1 s under intense illumination in air at ambient temperature. The relativereactivity of the thiyl radicals toward the polybutadiene vinyl and butene double bonds was determined, aswell as the competition between homo- and copolymerization for the propagating alkyl radical. An increaseof the SBS vinyl content from 8 to 59% has little effect on the crosslinking process, because it enhancesmainly intramolecular reactions. The thiol/ene polymerization is much more effective to crosslink thermopla-stic SBS elastomers than the copolymerization of the polybutadiene double bonds with a diacrylate mono-mer.

IntroductionThe crosslinking of vinyl-functionalized polymers can bereadily achieved at ambient temperature by UV-irradia-tion in the presence of a free radical-type photoinitia-tor1–4). Most of these studies were performed on styrene-butadiene-styrene (SBS) block copolymers to make suchthermoplastic elastomers more resistant to temperatureand organic solvents by creating covalent bonds betweenthe polybutadiene chains. In a previous paper of this ser-ies5), we have shown that a styrene-butadiene rubber hav-ing only 8% of its unsaturations as pendent vinyl doublebonds can be made totally insoluble within a few secondsupon UV exposure in the presence of air. The addition ofa diacrylate monomer, which copolymerizes with thevinyl and butene double bonds, was found to acceleratemarkedly the crosslinking process, but at the expense ofthe heat resistance of the plasticized rubber. This highlyreactive photosensitive material is therefore ill-suited forhot-melt adhesive applications or flexographic printingplates requiring processing at 1508C.

In the present study, another approach has been used toachieve a rapid photocrosslinking of SBS rubber. It isbased on the photoinduced addition of a thiol (RSH) ontoan olefinic double bond, and it requires incorporation of amultifunctional thiol into the polyene elastomer. A greatnumber of kinetic and mechanistic studies have beendevoted to thiol-ene photopolymerization, a topic whichhas been recently reviewed by Jacobine in a comprehen-sive monography6). The chain reaction proceeds by a step

growth addition mechanism which is propagated by achain transfer reaction involving thiyl radicals (RS9)7, 8).The overall process can be formally represented as fol-lows:

Crosslinking requires the association of a diene orpolyene and of a multifunctional thiol. Because peroxyradicals formed by the O2 scavenging of alkyl radicals arealso capable of abstracting hydrogen atoms from thethiol, UV-curable thiol-ene systems are much less sensi-tive to air inhibition9) than conventional radical-inducedpolymerization10):

Macromol. Chem. Phys.200, No. 8 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/0808–1965$17.50+.50/0

a) Part 7: cf. ref.5)

1966 C. Decker, T. NguyenThi Viet

There are only a few reports in the literature on thephotocrosslinking of thiol-SBS system11,12), with litt leinformation on thereaction kinetics.Like in our previousstudy5), the main objective of this work wasto determinehow fast and how extensively styrene-butadienerubberscould be crosslinked by UV-irradiation at ambienttem-perature in the presenceof a thiol monomer. Specialattention has been given to the influence on the curekinetics of the threekey components:the photoinitiator,the thiol monomerandthe SBSvinyl double bonds.Theprogress of the crosslinking reactionhas beenfollowedthroughtheinsolubilization andhardeningof theUV-irra-diated sample,and by monitoring the thiol and vinylgroupsdisappearanceby IR spectroscopy.

Experimental part

Materials

Two types of polystyrene-block-polybutadiene-block-poly-styrene(SBS), both from SHELL, have beenused in thisstudy. They differ only by their content of pendentvinylgroups:8% of the total double bond content for the low-vinyl polymer (LV-SBS),and 59% for the high-vinyl poly-mer (HV-SBS). The 2-butenylene units (1CH21CH2CH1CH21) locatedon the polybutadienebackbonerepre-sent92% and41% of the unsaturationcontent,respectively.In someexperiments,a paraffinic oil (Nujol from Aldrich)was added to the formulation to increasethe molecularmobility in the polymer film. A trifunctional thiol wasselected as crosslinking agent, trimethylolpropane tris-(3-mercaptopropionate)(TRIS) from Evans Chemetics.Itwas addedto a toluenesolution of SBS at a concentrationbetween3 and20 wt.-% of SBS.

Different types of photoinitiatorswere introducedin thepolymer, at a typical concentrationof 2 wt.-% SBS,namelybenzophenone(BZP from Aldrich), isopropylthioxanthone(QuantacureITX from BioSynthetics),4,49-diphenoxybenzo-phenone(DPB from SNPE) and 2,4,6-trimethylbenzoyl(di-phenyl)phosphine oxide (Lucirin TPO from BASF).Scheme1 showsthe chemicalformulasof the variouscom-poundsusedin this study.

In a typical UV-curingexperiment,20lm thick films werecastfrom a toluenesolutioncontainingthe SBSrubber, thethiol monomerandthephotoinitiatorontoeithera KBr crys-tal for infrared analysisor a glassplate for insolubilizationand hardnessmeasurements.Sampleswere exposedto theradiationof a 80 W/cm mediumpressuremercury lamp, inthe presenceof air, at a passingspeedof 60 m/min, whichcorrespondsto an exposuredurationof 0.1 s per pass.Themaximum light intensity at the sampleposition was meas-uredby radiometry(IL-390 light bug) to be 600mW N cm–2

in theUV range.

Analysis

The kinetics of the light-inducedcrosslinkingof the rubberfilm was studied quantitatively by FTIR spectroscopy, by

following the decreaseuponUV exposureof the absorptionbands characteristicof the vinyl double bond at 910.8,1637.8and1827 cm–1, andof the but-2-enedoublebondat964.7 cm–1. The disappearanceof the thiol monomeruponUV-exposurewasfollowed throughthe IR absorbanceof theSH group at 2569 cm–1. The degreeof conversion(x) wascalculatedfrom theratio of thecorrespondingIR absorbancebeforeandafterUV exposure(A0 andAt) by usingthefollow-ing equation:

x (%) = [1 – (At /A0)] N 100

This valuewasnot correctedfor shrinkage,asit wasfoundto accountfor less than 2%, basedon the variation of theC1H peakat 2960cm–1. Thelossof thephotoinitiatorin theUV-irradiated samplewas followed quantitatively by UV-spectroscopy.

The gel fraction and the degreeof swelling of the irra-diated polymer were determinedby soakingthe sampleintoluenefor onedayat roomtemperature.Theinsolublepoly-mer wasrecoveredby filtration anddried at 708C to a con-stant weight. The hardnessof the coating was evaluatedbeforeandafter irradiationby monitoringthedampingof theoscillationsof a pendulum(Persozhardness).The hardnesswas shownto be strongly dependenton the glasstransitiontemperature13), with Persozvalues ranging typically from30 s for soft elastomericmaterials to 300 s for hard andglassypolymers.The thermalstability of the variousformu-

Scheme1: Chemical formulasof compoundsused

Photocrosslinking of functionalizedrubbers,8 1967

lations was testedby heatingthe uncuredsamplefor up to1 h at 1508C, the processingtemperaturefor hot-meltadhe-siveapplications.

ResultsThe thiol-enepolymerization is known to proceedmoreefficiently with olefins in which the double bond islocated at the end of the monomer molecule (R1CH2CH2) rather than in its backbone (R1CH2CH1R)6).Consequently, a faster crosslinking wasexpectedto occurwith SBS rubberscontaining a high content of pendentvinyl doublebonds arising from the 1–2 polymerizationof butadiene. Our first UV-curing experiments werethereforeconductedwith a speciallydesignedSBSsam-ple whereasmanyas59%of thepolybutadieneunsatura-tionswerevinyl double bonds(HV-SBS).

Influenceof thephotoinitiatoron thephotocrosslinkingof thethiol-SBSsystem

The photoinitiator (PI) used in UV-curing applicationscanbeclassified into two majorcategories,dependingontheway thefreeradicalsaregenerated:– by photocleavageof thePI excitedstates

– throughhydrogenabstractionby thePI excitedstates

As thiols exhibit a strong hydrogendonor character, thesecondtype of photoinitiator is oftenusedin thethiol-enephotopolymerization. The benzoyl radical produced bythe photocleavagereactionis also capable to abstractahydrogenatomfrom thethiol to generate thepropagatingthiyl radical6):

Amongthedifferenttypes of photoinitiatorsexamined,the following four were found to give the best perfor-mance: benzophenone (BZP), isopropylthioxanthone(ITX), diphenoxybenzophenone (DPB) and 2,4,6-tri-methylbenzoyl(diphenyl)phosphineoxide(Lucirin, TPO).Fig. 1 shows how the degreeof conversion of the thiolmonomervaries with the exposure time for a HV-SBSrubber containing 2 wt.-% photoinitiator and 20 wt.-%TRIS, which corresponds to a double bond/SH molar

ratio of 8.3. The phosphineoxide provedto be the mostefficient initiator and was therefore usedin our furtherstudies. Onemay still notice that TPO is 10 times moreefficient than benzophenone in the absence of thiol(Fig. 1 of ref.5)), but only 1.6 times in the presenceofthiol (Tab.1).

It is quite remarkable that the polymerization of thethiol proceedsso rapidly in the solid state,reaching 50%conversion within 0.5 s exposure. The slowing downobserved upon further irradiation was attributed to twofactors:(i) a fastconsumptionof thephotoinitiator andofthe thiol and(ii) molecular mobility restrictions resultingfrom thebuild up of thetridimensional polymernetwork.

A similar ranking of the photoinitiators was found byfollowing the disappearanceof the vinyl double bond ofSBS (Fig. 1). Here again, the reaction slows down after0.5 s andthe vinyl contentis leveling off at around60%of its original value after 3 s. At that final stageof thepolymerization, the absolute amount of vinyl groupswhich have disappearedis about twice that of the SHgroupsconsumed.This result suggests that a significantamount of vinyl doublebonds undergoeshomopolymeri-zation, besides their copolymerization with the trifunc-tional thiol. Both processeswill lead to crosslinking andtherefore insolubilization of the polymers, if the vinyldouble bonds are located on differentSBS chains.Fig. 2shows the insolubilization profiles of the HV-SBS/TRISsystemexposed to UV-light in the presenceof 2 wt.-%photoinitiator. With the most efficient initiator (LucirinTPO), 80%insolubilization wasachievedwithin 0.1 s.Atthat stageof the reaction, 5% of the vinyl doublebondshavepolymerized,i. e.,70groupsperSBSchain.As inso-lubilization requiresthebuild up of only a few crosslinks(at least3 per chain),we areleft to conclude,like in ourpreviousstudy on the UV-curing of the sameSBS sam-

Fig. 1. Influence of the photoinitiator (2 wt.-%) on the photo-crosslinking of the HV-SBS/Tris system. [Tris] = 20 wt.-%.Light intensity:600mW N cm–2


ple5), that the polymerization involves mainly neighbor-ing vinyl groups located on the same polybutadienechain. Such an intramolecularprocessseemsfeasibleinconsideration of thelargenumberof pendentvinyl groupsin HV-SBS(6 for every10 monomerunits).Lucirin TPOproved to be also the best photoinitiator for reachingrapidly a high crosslinkdensity, asshown by theswellingprofilesof Fig. 2. A swelling ratio of 5 wasmeasuredforthe sample UV-irradiatedduring 0.1 s in the presenceofthis phosphineoxide,comparedto valuesrangingfrom 7to 9 with the other photoinitiators. Thesevaluesare lessthanhalf thosemeasured in UV-curedSBS5), thus show-ing thata tighter polymernetworkhasbeenformeduponUV-irradiation of the thiol-SBS system.A performanceanalysisof thephotoinitiatorsstudied in UV-curingof theTRIS/HV-SBS combination (20/80 weight ratio) isreportedin Tab.1. The two setsof results(rate of poly-merization and insolubilization data) correlateperfectlywell for the4 compounds.

Influenceof theSBSvinyl content

The photoinduced thiol-ene polymerization of two SBSsampleshaving very different vinyl group contents (59

and8%) hasbeenstudiedin orderto evaluatetherelativereactivity of vinyl andbutenedouble bonds,andto quan-tify the competition betweenthe homopolymerization ofvinyl double bonds and the transfer copolymerizationwith the trifunctional thiol. The overall crosslinking pro-cesscanberepresentedby thefollowing reactionscheme.

Thiyl radicalsare alsoproduced by hydrogenabstrac-tion on the thiol by any of the secondary free radicalsformedby theaddition process, asshown in the introduc-tion reactionscheme. While vinyl groups undergo bothhomopolymerizationandcopolymerizationwith thethiol,the butenedouble bondsareonly reactingwith the thiylradicals.Indeed, homopolymerizationof thebutenegroupwas not observedupon UV exposure of SBS in theabsenceof thiol5).

Crosslinking resultsfrom both thecopolymerization ofthe butadiene double bonds with the trifunctional thiolandthehomopolymerization of thependentvinyl groups.The structureof the polymer network obtainedis repre-sentedschematically in Fig. 3.

Photocrosslinkingof HV-SBSrubber

The SBS samplecontaining 59% pendentvinyl doublebonds hasbeenexposed to UV-radiation in the presenceof the trifunctional thiol ([TRIS] = 20 wt.-%) andLucirinTPO (2 wt.-%). Infraredspectrawererecordedafter var-

Tab.1. Influenceof thephotoinitiator on thephotocrosslinking kineticsof thethiol/HV-SBSsystem.[Tris] = 20wt.-%. Light inten-sity: 600mW N cm–2

Photoinitiator(2 wt.-%) Rs

molNkgÿ1Nsÿ1

a� Rv

molNkgÿ1Nsÿ1

b� Gel in %c) Swellingratioc)

Benzophenone(BZP) 1.5 2.8 64 9Isopropylthioxanthone(ITX) 1.8 3.0 69 8Diphenoxybenzophenone(DPB) 2.0 3.3 78 7Lucirin (TPO) 2.4 3.6 83 5

a) Initial rate of thiol consumtion.b) Initial rate of vinyl consumption.c) After 0.1sUV exposure.

Fig. 2. Influenceof the photoinitiator (2 wt.-%) on the photo-crosslinking of the HV-SBS/Tris system. [Tris] = 20 wt.-%.Light intensity: 600mW N cm–2


ious exposure times(up to 2 s) to monitor the disappear-ance of the three functional groups: thiol, vinyl andbutene.The butene double bond was found to be muchlessreactivethan the vinyl double bond in the ene-thiolpolymerization. Its initial rate of copolymerization withthe thiol Rb wasmeasuredto be 0.1 mol N kg–1

N s–1, com-paredto 2.3 mol N kg–1

N s–1 for thevinyl copolymerizationrate(Rv)copo(Tab.2).

Therelativereactivity of thetwo typesof double bondstowardthe trifunctional thiol canbe determinedfrom theratio of the rateconstantskv andkb of the addition reac-tion of the thiyl radical to the vinyl and butenedoublebonds,respectively.

(Rv)copo� kv [RS9] [vinyl] (1)

Rb � kb [RS9] [butene] (2)

kv

kb�

�Rv�copo

RbN

�butene��vinyl�

� 16 �3�

The higher reactivity of the vinyl double bond towardthiols is in goodagreementwith previous structure-reac-

tivity studiesof the addition of thiols on olefins14). Forinstance,a-methylstyrene was found to be 25 times asreactive toward thiol glycolateasa,b-dimethylstyrene15).Theenhancedreactivity of thevinyl groupcanbeattribu-ted to an increasein electrondensity of the olefin whichis reactingwith a relatively electrophilic thiyl radical16).An additional factor in HV-SBSis the poor accessibilityof thebackbone butenedouble bondsby thiyl radicals.

From the data shown in Fig. 1, one can evaluate thecompetition which exists for the vinyl radical betweenhomopolymerizationandcopolymerizationwith thethiol.

The rate equation for the thiol consumption can bewritten as:

Rs = ks [P9] [RSH] (4)

whereP9 are the alkyl radicalsproduced by the additionreaction of thethiyl radicalto thedoublebond (consump-tion of the thiol by reaction with the photoinitiator wasneglected).Vinyl groups are consumed by reactionwithboth P9 radicalsandthiyl radicals,which leadsto the rateequation:

Rv = kp [P9] [vinyl] + kv [RS9] [vinyl] (5)

As the thiyl free radical is assumedto reactonly withthe SBSdoublebonds to generatea copolymerunit, onecanwrite:

ks [P9] [RSH] = [RS9] (kv [vinyl] + kb [butene]) (6)

By neglecting kb [butene] which is small in regardof kv

[vinyl] (a 1/23 ratio), the rate equation of the vinyl con-sumption becomes:

Rv = kp [P9] [vinyl] + ks[P9] [RSH] (7)

The competition between homopolymerization andcopolymerization can then be quantified from the ratioRv/Rs:

Rv

Rs

�

kp�vinyl� � ks�RSH�ks�RSH�

�8�

Fig. 3. Chemical structureof thephotocuredSBS/Tris polymernetwork

Tab.2. Performance analysisof the photocrosslinking of styr-ene-butadienerubbers LV-SBSandHV-SBSin thepresenceof atrifunctional thiol (20 wt.-%). Photoinitiator:[Lucirin TPO] = 2wt.-%

LV-SBS HV-SBS

Vinyl contentin % 8 59Ene/thiolmolarratio 8.4 8.4

Initial lossrate in mol N kg–1N s–1:

Thiol Rs 1.8 2.4Vinyl Rv 0.83 3.6

(Rv)homo 0.06 1.3(Rv)copo 0.77 2.3

Butene Rb 0.9 0.1

Relativereactivity:Vinyl groupcopo/homopolymerization:ks/kp – 10Copolymerizationvinyl/butene:kv/kb 10 16

Insolubilizationrate in % N s–1 80 90Swellingdegree(after 0.1s) 6 5


which leads to

kp

ks

�

Rv

Rs

ÿ 1

� ��RSH��vinyl�

� ��9�

In the photocrosslinking of HV-SBS, the followingvalueswere obtained: Rv = 3.6 mol N kg–1

N s–1, Rs = 2.4mol N kg–1

N s–1 and [RSH]/[vinyl] = 0.2. The ratio ks/kp

was calculated from Eq. (9) to be as high as 10, whichmeansthat the propagating alkyl radicalsP9 are10 timesasreactivetowardthe thiol groupasthey are towardthevinyl doublebond, in agreement with previousobserva-tions6). It shouldbe mentionedthat the valueof the vinylhomopolymerization rate in the thiol-SBSsystem, calcu-latedfrom the equation Rhomo = Rv–Rs, is quite similar tothe rate of polymerization of vinyl groupsin neat SBSphotocrosslinkedunderthesameconditions5), 1.2 and1.0mol N kg–1

N s–1, respectively.The network formation is not only making the rubber

insolublein organicsolvents,butit alsoincreasestheShearAdhesionFailure Temperature (SAFT) from 80 to 1608C,andthe hardnessof the UV-exposed polymer aswell, asshownin Fig. 4. Becausehardening occurs mainly uponprolongedUV irradiation,for upto 2 s,it is still possible toobtainasoft andinsolublepolymerbyshortening theexpo-suretime downto 0.3 s.Anotherway to geta crosslinkedpolymershowinga strong elastomericcharacterfor adhe-siveor flexographicapplicationsis by introducing aplasti-cizerin thethiol-SBSformulations,suchasNujol,aparaffi-nic oil. At aweightconcentrationof 35%,insolubilizationof SBSwasfoundtoproceedasfastasfor theunplasticizedsample,but the crosslinked polymer remained soft uponUVexposurefor upto2s,asshownin Fig. 4.

Photocrosslinking of LV-SBSrubber

The samestudywascarried out with a SBSsample con-taining only 8% vinyl group (LV-SBS), a valuewhich is

more typical of commercial styrene-butadiene thermo-plastic elastomers.In thepresenceof 20 wt.-% TRIS, theinsolubilization, swelling and hardeningprofiles (Fig. 5)were found to be very similar to thoseobtainedwith theHV-SBS sample. The same behavior was previouslyobserved in the photocrosslinking of neat SBS copoly-mers5). This surprising result suggests that, in HV-SBS, asubstantial fraction of thevinyl groups disappearthroughintramolecularprocesses involving neighboring pendentdouble bonds,without any net effect on the insolubiliza-tion andhardening.By contrast, in LV-SBSsuchintramo-lecular reactionsare lesslikely to occur, given the aver-age distanceseparating the pendent vinyl doublebonds(12 monomerunits). Crosslinkingwill therefore proceedreadily through intermolecular reactions betweenfunc-tional groups locatedon differentpolymer chains.

The initial rate of thiol consumption in LV-SBS (Rs =1.8 mol N kg–1

N s–1) wasfound to besuperiorto the initialrate of vinyl consumption (Rv = 0.83 mol N kg–1

N s–1),most probably becausethe thiyl radicalsreact with thebackbonebutenedoublebonds,which arepresentin lar-ger amounts and are more accessible than in HV-SBS.Fromthesedataandby usingtheks/kp ratio determinedintheprevioussection,onecanevaluatetherelativereactiv-ity of the thiyl radicalstoward the vinyl andbutenedou-ble bondsof polybutadienein LV-SBS.

Vinyl groupsaredisappearingby both homopolymeri-zation and copolymerization with the thiol, so that onecanwrite:

Rv = (Rv)homo + (Rv)copo (10)

The contribution of eachprocesscan be evaluatedbytheequation

�Rv�copo

�Rv�homo

�

ks�RSH�kp�vinyl�

�11�

Fig. 4. Hardeningof the HV-SBS/Tris systemuponUV expo-sure.[Tris] = 20wt.-%; [Lucirin TPO]= 2 wt.-%

Fig. 5. Photocrosslinkingof the LV-SBS/Tris system.[Tris] =20 wt.-%; [Lucirin TPO]= 2 wt.-%


From the value of the ratio ks/kp = 10, evaluated pre-viously, and the thiol/vinyl molar ratio (1.5), one candetermine the rate of copolymerization of the vinylgroup:

(Rv)copo= 0.933Rv

= 0.77mol N kg–1N s–1

As the thiol is consumedby reaction with both thevinyl andbutenedouble bonds,onecanwrite:

Rs = (Rv)copo+ (Rb)copo (12)

Butenedoublebondsdo not undergo significant homo-polymerization upon UV exposure of LV-SBSin the pre-senceof Lucirin TPO, so that (Rb)copo X Rb. In the pre-senceof 20 wt.-% TRIS, they disappearat an initial rateof 0.9 mol N kg–1

N s–1, a valuein good agreement with thatcalculatedfrom Eq. (12):

Rb = 1.8–0.77= 1.03mol N kg–1N s–1

The relative reactivity of thiyl radicals toward vinylandbutenedoublebondsin LV-SBSwascalculatedfromtheratio of thetwo additionrateconstants:

kv

kb�

�Rv�copo

Rb6

�butene��vinyl�

�13�

�

0:770:9

611:7 � 10

This value confirms that thiyl radicalsaremuchmorereactive toward the pendentvinyl double bonds thantoward the backbone butene double bonds. The largervalue found in HV-SBS (kv/kb = 17) may result from adecreaseof kb causedby themore difficult accessby thiylradicalsof thebutenedoublebondssqueezedbetweenthemanypendent vinyl groups.

In Tab.2, we havesummarized our kinetic dataon thephotocrosslinking of thetwo typesof SBSrubberstudied,in the presenceof 20 wt.-% TRIS and 2 wt.-% LucirinTPO.

Influenceof thethiol concentration

All the photocrosslinking experiments reported so farhavebeencarriedout with SBS samplescontaining thetrifunctional thiol at a weight concentration of 20%,which corresponds to a double bond/SHmolar ratio of8.4. As insolubilization of polymers requiresthe creationof only a few bridgesbetweenthe polymer chains, wehavereducedthe content of TRIS down to 10 wt.-% and5 wt.-%, i. e., ene/thiol ratios of 19 and 40, respectively.Fig. 6 showsthe vinyl lossand thiol conversion profiles

for the two HV-SBSsamplesexposedto UV radiation forup to 2 s in the presenceof 1 wt.-% Lucirin TPO. Thecrosslinkingreaction appearsto proceedasefficiently asat the higher thiol concentration, thus leading to a rapidinsolubilization of the SBS rubber upon UV irradiation(Fig. 7). A 0.1 s exposureprovedto besufficient to get a90%insoluble polymershowing nearlythesamevalue ofthe swelling ratio (6) asthe polymer photocured with 20wt.-% TRIS (swelling ratio of 5).

Lowering the thiol concentrationdown to 5 wt.-% hasthereforeno detrimental effect on thecurekineticsandonthecrosslinking extent of theSBSrubber. Besidesreduc-ing thecostof theformulation, it will alsolessensomeofthe disadvantagesassociated with the useof sulfur com-pounds, in particular the bad smell. In this respect, itshould beemphasizedthat thephotocuredpolymershows

Fig. 6. Influence of the thiol content on the photocrosslinkingof theHV-SBS/Tris system. [Lucirin TPO]= 1 wt.-%.Thiol con-version(0), vinyl loss(h)

Fig. 7. Influence of the thiol content on the photocrosslinkingof the HV-SBS/Tris system.[Lucirin TPO] = 1 wt.-%. Gel frac-tion (0, h); swellingratio (9, H)


no unpleasantodor anymore, becauseessentially all thethiol groupshavereactedand are chemically bonded tothe polymer network. The few unreacted SH groups arestill linked to the polymer becauseof the trifunctionalcharacter of themercaptanused.

Very similar resultshave beenobtainedwith the LV-SBS sampleUV-irradiated in the presenceof 3 wt.-%TRIS and1 wt.-% Lucirin TPO. Hereagaincrosslinkingof this thermoplastic elastomerwas found to proceedreadily, to yield a tight tridimensional polymer network.Fig. 8 shows thepolymerization,insolubilization, harden-ing and swelling profiles of the UV-exposed sample.Insolubilization wasachievedwithin 0.1 s at a vinyl con-sumption of 5%, i. e., 0.05 mol/kg SBS, which corre-spondsto 8 bridgesper polybutadienechain.Nearly thesameamountof crosslinks was produced by reaction ofthe thiol with thebutenedoublebonds,which leadsto anaveragemolecular weightbetweencrosslinksof 7000.

The drastic effect of the trifunctional thiol on thephotocrosslinking of SBS is illustrated in Fig. 9, whichshowsthe variation of the sol fraction and the hardnesswith exposuretime, for both systems. Insolubilizationrequired20 times lessenergy than in the thiol-free SBS,with formation of a much harder crosslinked polymer.Moreover, the sharpdrop of the swelling ratio of UV-curedSBS, from 25 to 4 in the presenceof TRIS, con-firms that a much more efficient crosslinking processistakingplacein thethiol/polybutadiene system.

In the presenceof 5 wt.-% TRIS, insolubilization wasfound to occurnot only muchfaster thanin the neatrub-ber, but evenfasterthanin an SBSsample containing 20wt.-% of a very reactive diacrylatemonomer5), asshownin Fig. 10. This result was unexpected, basedon themonomerfunctionality: f = 3 for the thiol and f = 4 for

thediacrylate.Indeed,a linear polymer will beformedbyusing either a monoacrylate (f = 2) or a difunctional thiol(f = 2) associatedto a diene, becausethelatterchainreac-tion proceeds by a stepgrowthaddition.If the thiol com-pound contains only 1 SH group, therewill be no poly-merizationandno crosslinking, but simply anaddition ofthe thiol to the SBSdouble bond,asshown by the reac-tion schemegivenin theintroduction section.

Consequently, the copolymerization of a diacrylate (ora telechelic acrylate)with theSBSdoublebondswil l gen-erate 4 branch points,insteadof only 3 in the caseof thetrifunctional thiol. The lower crosslink density actuallyfound in the SBS/acrylate system(swelling ratio of 12)wasattributed to the competitive homopolymerization of

Fig. 8. Photocrosslinking of LV-SBS in the presence of 3wt.-% of Tris. [Lucirin TPO] = 1 wt.-%. Vinyl loss(0); gel frac-tion (f); swelling ratio (f) andhardness(h) Fig. 9. Influence of a trifunctional thiol (3 wt.-%) on the

photocrosslinkingof LV-SBS.[Lucirin TPO] = 1 wt.-%. Solublefraction (0, 9) ; hardness(h, H)

Fig. 10. Photocrosslinking of HV-SBS, neat (f), in the pre-sence of a diacrylate(20 wt.-%) (h), or of a trifunctional thiol(5 wt.-%) (0). [Lucirin TPO]= 1 wt.-%


the acrylate double bonds, which doesnot generate anyadditionalcrosslinks betweenSBSchains.

Heatresistanceof UV-curableSBS

If thesephotocurable thermoplasticelastomers are to beusedashot-melt adhesives,they must exhibit a fair heatresistanceat the processing temperature,usually 1508C.A 20lm thick film of HV-SBScontaining5 wt.-%, TRISand1 wt.-% Lucirin TPO washeatedat 1508C for up to30min. The thiol content wasfound to drop rapidly dur-ing this thermaltreatment, 50%lossafter5 min, asshownin Fig. 11. This lossis mainly dueto volatilizationof theliquid TRIS out of thethin film, ratherthanto a chemicalreaction, asindicatedby thefollowing threeobservations:– the thiol disappears asrapidly in anSBSfilm contain-

ing no photoinitiator,– thethiol lossis much reducedin thick SBSsamples,– gelation of SBSdoesnot occurduring the first 5 min

of heatingwhen half of theTRIS is lost.

After 10min heating in the dark, onethird of the sam-ple has become insoluble in toluene (Fig. 12), whichmakesthis very reactive systemunfit for hot-melt adhe-sive applications. In the absenceof photoinitiator, gela-tion startedto proceed5 min later, but insolubilizationdid follow a similar profile as in the presenceof LucirinTPO,asshown in Fig. 12. In a previousstudy, we foundneatSBSto bestable for over20 min at 1508C5). Reduc-ing the thiol content and adding an antioxidant mightthereforehelp in improving the heat resistance of thephotocurablethiol/SBSformulation.

ConclusionThe light-induced thiol/ene polymerization is a veryeffective method to crosslink rapidly styrene-butadieneblock copolymers at ambienttemperature.Under intenseillumination, the curing reactionproceedswithin a frac-tion of a secondby a stepgrowth addition mechanismtoyield an insoluble polymer network. Both the buteneandthe vinyl double bonds of the polybutadiene chain areattacked by the thiyl radicals,which areat least10 timesmore reactive towardthependent vinyl groups.It is how-ever not necessary to increasethe vinyl content of SBSabove 8% becauseit will mainly favor intramolecularhomo- andcopolymerization reactions betweenneighbor-ing vinyl double bonds and will not accelerate the net-work formation. This resultis important from aneconom-ical point of view because it makes this UV-technologydirectly applicable to commercial SBS rubbers whichhave generally a low vinyl content. Another interestingfeature is that the thiol contentcanbereduceddownto 5wt.-%, withoutdetrimentaleffect on thecurespeed.

The main applicationsof this photocurable rubberareexpectedto be found in industrial sectors wherespatialcontrol and cure speedare a major concern, suchas forthe manufacturing of protective coatings, adhesives,photoresistsand printing plates. Further studieson thephotocrosslinking of the thiol/SBS combination are inprogress,aimingat loweringboth thethiol andthephoto-initiator content,to make this systemevenmore econom-ically attractive, moreresistant to heat,and to extenditsfield of applications to thick samples and compositematerials.

Acknowledgement: The authors wish to thank SHELL-Research (Belgium)for a researchgrant.

Fig. 11. Lossof thiol groupuponheating at 1508C of theHV-SBS/Tris system. [Tris] = 5 wt.-%; [Lucirin TPO] = 1 wt.-% (0)or 0%(h)

Fig. 12. Insolubilization of HV-SBS upon heatingat 1508C.[Tris] = 5 wt.-%; [Lucirin TPO] = 1 wt.-% (0) or 0% (h); neatSBS(f)


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Photocrosslinking of functionalized rubbers, 8. The thiol-polybutadiene system

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