Photocrosslinking of functionalized rubbers, 7a

Styrene-butadiene block copolymers

Christian Decker*, Trieu Nguyen Thi Viet

Laboratoire de Photochimie Ge´nerale (URA N8431-CNRS),Ecole Nationale Supe´rieure de Chimie de Mulhouse – Universite´ de Haute Alsace,3, rue Werner – 68200 Mulhouse; France

(Received: May 23, 1998; revised: July 13, 1998)

SUMMARY: The photocrosslinking of polystyrene-block-polybutadiene-block-polystyrene (SBS) was stu-died by means of infrared spectroscopy, by monitoring the disappearance of the pendent vinyl double bonds,which was shown to proceed within a few seconds upon UV exposure in the presence of an acylphosphineoxide photoinitiator. Complete insolubilization requires the reaction of 17 double bonds per polymer chainfor an SBS sample containing 8% vinyl groups. An increase of the vinyl content has little effect on the cross-linking process, because it enhances mainly intramolecular reactions. A paraffinic oil proved to be an effec-tive plasticizer for the photocrosslinked elastomer. The addition of a telechelic acrylate oligomer causes asubstantial increase of both the reaction rate and the final degree of conversion of the SBS double bonds. Thelight-induced copolymerization of the vinyl and butene double bonds with the acrylate double bonds leads tothe formation of a hard and flexible polymer material within a fraction of a second.

IntroductionLight-induced polymerization is one of the most effectivemethods to generate tridimensional polymer networks,because of the high initiation rates reached under intenseillumination1–4). In most UV-curing applications, a sol-vent-free liquid resin is converted quasi-instantly into ahighly crosslinked polymer, selectively in the exposedareas, to produce protective coatings, quick-setting adhe-sives or high-resolution relief images. The same photo-chemical process has been successfully used to crosslinksolid polymers bearing polymerizable functional groupson their backbone chain5), e.g. cinnamates6), chalcones7),epoxides8, 9) or acrylates10). A distinct advantage of photo-initiation is to afford a precise temporal control of thechemical process. The crosslinking reaction will startimmediately, as soon as light is shed on the sample, and itcan be stopped at any time by switching off the UV lamp.Moreover, the reaction rate can be varied in a large range,simply by changing the intensity of the UV-beam.

In the previous articles of this series, we have shownthat epoxy11, 12) or acrylate13) functionalized polyisoprenecan be “photovulcanized” within seconds at ambient tem-perature by UV light in the presence of a cationic or radi-cal-type photoinitiator, respectively. Under the same irra-diation conditions, polyisoprene (natural rubber) did notundergo any crosslinking, because of the low reactivity ofthe amylene double bond. This is not the case for polybu-tadiene which contains the more reactive butene-2 and

vinyl double bonds formed by 1–4 and 1–2 polymeriza-tion of butadiene, respectively. There are, however, onlya few reports in the literature on the photocrosslinking ofpolybutadiene-based rubbers14–18). Most of these studieshave been performed on styrene-butadiene-styrene (SBS)tri-block copolymers19), known under the tradenameKRATON, which are commonly used in pressure-sensi-tive and hot-melt adhesive applications20). These copoly-mers, which have a two-phase morphology, combine theproperties of elastomers and thermoplastic materials, thestyrenic domains acting as physical crosslinks below theTg. Photocuring proved to be an effective method tomake these thermoplastic elastomers more resistant tosolvent and temperature, while improving greatly theadhesive strength14). By creating covalent bonds withinthe elastomeric phase, UV-curing is reinforcing irreversi-bly the already existing physical network.

The main objective of the present study was to deter-mine how fast and how extensively styrene-butadienerubbers can be crosslinked by UV irradiation at ambienttemperature. We have examined in particular the influ-ence of the photoinitiator and of the polybutadiene vinylcontent on the kinetics of the crosslinking process and onsome of the properties of the UV-cured polymer network.An attempt has been made to overcome the rate limita-tions inherent to solid state reactions by incorporating inthe starting material a plasticizer or an acrylate monomer.The photooxidation process which is known to occur

Macromol. Chem. Phys.200, No. 2 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/0202–0358$17.50+.50/0

a Part 6: cf. ref.12)

358 Macromol. Chem. Phys.200,358–367 (1999)

Photocrosslinking of functionalizedrubbers,7 359

uponUV irradiationof this typeof rubberin thepresenceof air21) wasminimizedby therapidconsumptionof oxy-genduring theshortexposure.

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 butene-2double bondslocatedon thepolybutadienebackbonerepresent92%and41%of theunsa-turationcontent,respectively. In someexperiments,a paraf-finic oil (Nujol from Aldrich) wasaddedto the formulationto increasethemolecularmobility in thepolymerfilm. Sucha plasticizingeffect wasalsoachievedby usinga diacrylatemonomer(Ebecryl 150 from UCB) which, in addition,wasexpectedto act as a crosslinkingagentby copolymerizingwith thepolybutadienedoublebonds.

Different types of photoinitiatorswere introducedin thepolymer, at a typical concentrationof 3 wt.-% SBS,namelybenzophenone(BZP from Aldrich), 2,6-bis(azidobenzyli-dene)-4-methylcyclohexanone(bisazidefrom Aldrich), 2,2-dimethoxy-2-phenylacetophenone(Irgacure651 from Ciba),1-benzoyl-1-hydroxycyclohexane(Irgacure184 from Ciba)and2,4,6-trimethylbenzoyldiphenylphosphineoxide(LucirinTPOfrom BASF). 20 lm thick films werecastfrom toluenesolution of SBS and the photoinitiator onto either a KBrcrystal for infraredanalysisor a glassplate for insolubiliza-tion and hardnessmeasurements.Sampleswere exposedtothe radiationof a 80 W/cm mediumpressuremercurylamp,in the presenceof air, at a passingspeedof 50 m/min. Themaximum light intensity at the sampleposition was mea-suredby radiometry(IL-390 light bug)to be600mW cm–2.

Analysis

The kinetics of the light-inducedcrosslinkingof the rubberfilm was studied quantitatively by FTIR spectroscopy, byfollowing the decreaseuponUV exposureof the absorptionbands characteristicof the vinyl double bond at 910.8,1637.8and 1827 cm–1, and of the butene-2doublebond at964.7 cm–1. In formulationscontainingthe acrylatemono-mer, the disappearanceof this doublebond uponUV-expo-sure was followed at 812 cm–1 (CH22CH twisting). Thedegreeof conversion(x) wascalculatedfrom theratio of thecorrespondingIR absorbancebeforeand after UV exposure(A0 andAt) by usingthefollowing equation:

x(%) = [1 – (At /A0)] N 100

This valuewasnot correctedfor shrinkage,asit wasfoundto accountfor less than 5%, basedon the variation of theC1H peakat 2960cm–1. Thelossof thephotoinitiatorin theUV-irradiated sample was followed quantitatively by UVspectroscopy. The gel fraction andthe degreeof swelling of

the irradiatedpolymerweredeterminedby soakingthe sam-ple in toluenefor onedayat roomtemperature.Theinsolublepolymer was recoveredby filtration and dried at 708C to aconstantweight. The hardnessof the coatingwasevaluatedbeforeandafter irradiationby monitoringthedampingof theoscillationsof a pendulum(Persozhardness).The hardnesswas shownto be strongly dependenton the glasstransitiontemperature22), with Persozvalues ranging typically from30 s for soft elastomericmaterials to 300s for hard andglassy polymers. The physical significance of pendulumhardnesshasbeenreviewedby Sato23) who consideredit asareliable measureof the viscoelasticpropertiesof a polymercoating.The adhesionof the UV-curedcoatingon glassorother substrateswas evaluatedby using the standardcross-hatchadhesivetapetest.The thermalstability of the variousformulations was testedby heatingthe uncuredsampleforup to 1 h at 1508C, the processingtemperaturefor hot-meltadhesiveapplications.

ResultsThephotoinitiator (PI) is a key factor for achievinga fastandextensivecrosslinkingby means of UV light. Indeed,both SBS samples were found to remain soluble whenthey were exposedto UV radiation in the absence of aphotoinitiator, for dosesup to 5 J N cm–2. It wasthereforeimportant to select the most efficient photoinitiator forthegivenSBSrubber, andto determine its optimumcon-centration.Therole of thependentvinyl double bond wasalso investigated to know whether it is necessary toincreaseits content to form readily a 3D polymer net-work. Finally, the effect of an acrylate crosslinker on thereaction kinetics wasexamined, in anattemptto enhancethe reactivity of both the vinyl and the butene doublebondsthroughcopolymerization.

Influenceof theradical photoinitiator

Most of the radical-type photoinitiatorsusedin UV-cur-ablesystems consistof aromaticketoneswhich undergo acarbon-carbonbondcleavagewhenexposedto UV radia-tion:

Variousfactorsgovern the initiation efficiencyof thesecompounds: their intrinsic absorbance of the incidentlight, the lifetime of the excitedstates,the rateof photo-lysis, theefficiency of thecleavagereaction andthereac-tivity of theradicalfragments.

Insolubilization profiles were determinedfor the twoSBS samples UV-irradiated in the presenceof differenttypesof radical photoinitiators. Fig. 1 shows somechar-acteristic curvesobtainedby plotting thegel fraction ver-

360 C. Decker, T. NguyenThi Viet

sustheexposuretime for five photoinitiatorsin HV-SBS,at a weight concentrationof 3%. They canbe classedinthefollowing orderof increasingefficiency:

Bisazidea benzophenonea Irgacure651a Irgacure184a Lucirin TPO

The bisazide, commonly used as photoinitiator tocrosslink cyclized polyisoprene,appears to be the leastefficient of the five initiators tested.The best perfor-mancewasreached by using the phosphineoxide initia-tor, nearlyall the polymer becoming insoluble after a 1 sexposure. Thesameclassification wasobtainedwhenthecrosslinking processwasfollowedthrough the lossof thevinyl double bond or the increaseof the rubberhardness.Consequently, this photoinitiator, Lucirin TPO, wasselectedfor all of our furtherstudies.

Kineticsof thecrosslinkingreaction

IR analysisshows thatonly minor chemical modificationsare taking place in SBS upon UV exposure, mainly asmall decreaseof the three peaks of the vinyl doublebondat 910.8,1637.8and1827 cm–1. After 1 s, only 4%of the pendentvinyl groups of HV-SBS were found tohavereacted, whereasthe concentrationof the backbonedouble bondsabsorbing at 964.7 cm–1 remained essen-tially unchanged. It can be seenin Fig. 2 that the SBSvinyl contentdropsrapidly at the very beginningof theUV irradiation andlevels off after 3 s to a constant value(95% of the original content). This apparent lossof reac-tivity of theremaining vinyl doublebondscanbeattribu-ted to therapid photolysisof thephotoinitiator which hasessentially disappeared after a 2-second exposure (seebelow). Mobility restrictions of the reactive sites in thenetwork under formation are also likely to contribute totheprematurestopof thepolymerization.

The photoinitiated polymerization of vinyl groupslocatedon differentpolybutadienechains leadsto thefor-mation of a tridimensional polymer network andaccountsfor the observed insolubilization of the irradiated rubber(Fig. 3). Thepolymernetwork presents,however, a fairlyloosestructure,asindicated by its high degreeof swelling(Psolvent/Ppolymer= 20). A plot of the gel fraction versusthedegreeof conversion (Fig. 4) shows that insolubilizationis achieved once 5% of the vinyl double bonds havereacted, which corresponds to 62 pendentgroups perpolybutadiene chain. Theoretically, only three bridgesneedto be formed per polymer chain to obtain a cross-linkedmaterial of infinite molecular weight. Thefact thatinsolubilization of SBS requiresthe reactionof a muchlarger numberof vinyl double bondsstrongly suggeststhat, in addition to the intermolecularreaction, an intra-molecularpolymerizationprocessis takingplace betweenvinyl double bonds located on the samepolybutadienechain. It should lead to the formation of cyclic struc-tures24) along the polybutadiene chain, which would

Fig. 1. Influenceof the photoinitiator (3 wt.-%) on the insolu-bilization profile of a 20 lm thick HV-SBSfilm exposedto UVradiation(I = 600mW N cm–2)

Fig. 2. Photocrosslinking of a SBS rubber with a high vinylcontent (59%)in thepresenceof Lucirin TPO(3 wt.-%)

Fig. 3. Insolubilization profile of HV-SBS exposed to UVradiationin thepresenceof Lucirin TPO(3 wt.-%)

Photocrosslinking of functionalizedrubbers,7 361

thereforeremainsoluble.This is feasiblebecauseof thehigh vinyl contentof the HV-SBS sample (59% of thetotal number of double bonds). However, becausethepolybutadienechaincontainsalsocis and trans 2-buteneunits, the cyclisation processwill propagate along thepolymerchainover only a few adjacentvinyl units. Thetwo inter- and intramolecular polymerization processesarerepresentedformally in Scheme1.

As crosslinking proceeds upon UV exposure, theamountof boundedelastic chainsbecomeshigherandthehardnessincreases, as shown in Fig. 2. The UV-curedpolymerremains still soft (Persoz value of 80 s),which isimportant for adhesive applicationsto maintain the tackandwetting requiredto achievea good adhesion.

The outer polystyrenesegments of the block copoly-mer, which arephaseseparated,arenot expectedto parti-cipate in the crosslinking process,exceptby a possiblechain transfer reactionif some propagating radicalssuc-ceedin abstracting a labile hydrogenon the PSchain atthe interphase. Increasing the PSchainlength shouldnotaffect insolubilization, which is ensuredthrough thepoly-butadiene crosslinks, but it wil l increasethe degreeofswellingby loweringthecrosslinkdensity.

It shouldbenotedthat a fraction of thevinyl groups islikely to disappear by a photooxidation process21,25,26).While no significant changesin the hydroxyl and etherregion of the IR spectrum could be detected after theshortUV exposure, therewasindeeda small but distinctincreaseof a carbonyl bandat 1694 cm–1 during the first0.5 s of irradiation. Its absorbance wasfound to level offat a constant valueof 0.015upon further exposure, prob-ably becausemost of theoxygendissolved in thefilm hasthen beenconsumed by reaction with the initiator radi-cals.The concentrationof the carbonyl groupwascalcu-latedto be on the orderof 2610–2 mol N kg–1, which cor-respondsto lessthan10%of thetotal amountof thevinylgroups lost. The photoinitiated polymerization appearsthus asthe main processresponsible for the vinyl doublebond consumption during the UV irradiation in the pre-sence of air. Under O2-free conditions, the vinyl groupspolymerizeonly slightly faster thanin air (Fig. 2), whichshows that O2 inhibition is not very important in thesesolid films exposedto intenseradiation.

Influenceof thephotoinitiatorconcentration

In light-inducedreactions,the photoinitiator concentra-tion [PI] controls the rate of initiation (r i) which obeysthefollowing equation:

r i � Ui I0�1ÿ eÿel�PI��

whereUi is the initiation quantum yield, I0 the incidentlight intensity, e the molar extinction coefficient of PI,and l the thicknessof the sample. Becauseof the limit edpenetrationof the incident radiation in the UV-absorbingmedium, crosslinking within the SBSsample wil l followa surfaceto depthgradient which is directly dependenton[PI]. Consequently, an increasein the PI concentrationwil l bothacceleratethecrosslinking reaction andsteepenthe curedepth gradient in the irradiatedsample.Depend-ing on the considered application, the best compromisemust be found betweencure speedand cure depth, thetwo extremes being either a uniform but slow deep-through cure for low absorbingsamples(low [PI]), or arapid but differential through-cure for highly absorbingsample(high [PI]).

Fig. 5 showsthe insolubilization profiles of a 20 lmthick HV-SBSfilm containing 1 to 5 wt.-% Lucirin TPOexposedto UV radiation for up to 3 s. The rate increasewith the PI concentration is particularly important forvalues up to 2 wt.-%. At higher [PI], the acceleratingeffect becomeslesspronounced, becauseof the reducedpenetrationof the incident light in the deep-lyinglayers(inner filter effect).

An increaseof the PI concentrationleadsalso to theformation of a tighter polymer network, becauseof theincreased number of initiating radicals.Theswellingratio

Fig. 4. Dependenceof thegel fractionandhardnessof aphoto-crosslinkedHV-SBSon thedegreeof vinyl conversion

Scheme1:

362 C. Decker, T. NguyenThi Viet

of UV-curedHV-SBS was found to drop from 55 to 25when [TPO] was increased from 1 to 5 wt.-%. Fig. 6showsthevariationof thegel fraction andof theswellingratio with the photoinitiator concentration, for a 20 lmthick sample exposedfor 1 s to UV radiation.It shouldbementioned that, when the TPO concentration wasincreasedabove6 wt.-%, the overall cure extentstartedto decreasebecauseof incompletecrosslinkingof thebot-tom layer. The [PI] value wheremaximum efficiency isreached dependsdirectly on the samplethickness,e.g. ata 10 timesaslow PI concentration for a 10 timesasthickfilm.

A similar behavior was observed by following thehardnessincreaseupon UV exposure (Fig. 7), and thevinyl lossaswell. The variation of thesetwo parameterswith the TPO concentration is shown in Fig. 8. Hereagain,maximumefficiency wasreached for a photoinitia-tor concentration of about 5 wt.-%. For a 20 lm thick

film , a TPOconcentration of 2 wt.-% appearsto give thebest overall performance,with respect to deep-throughcure, insolubilization, network density and moderatehardnessincrease.

It shouldbeemphasizedthat thedepth of cure gradientdueto thePI light absorbance is progressively smoothingoff uponUV irradiation,becausethephotoinitiator is con-verted in non-absorbing photoproducts. Fig. 9 shows theexponential decay of Lucirin TPO, monitored by UVspectroscopy at 380nm, upon UV exposure in a SBSmatrix at different concentrations. Such a first-orderkinetic law wasexpected for a direct photolysis process,where the reaction rate depends primarily on the prob-ability of a chromophore moleculeto be hit by a photonandis thereforeproportional to thechromophore concen-tration, throughout the reaction. From the slope of thestraight line obtainedin thesemi-log plot, ln [PI]t /[PI]0 =–k N t, the decayrateconstantk wascalculated.Its value

Fig. 5. Influence of the photoinitiator concentration (LucirinTPO)on theinsolubilizationprofilesof HV-SBSuponUV expo-sure

Fig. 6. Dependenceof thegel fractionandswelling ratio of thephotocrosslinkedHV-SBS on the photoinitiator concentration.UV exposure:1 s

Fig. 7. Influence of the photoinitiator concentration(LucirinTPO)on thehardeningof HV-SBSupon UV exposure

Fig. 8. Dependenceof thevinyl lossandthehardnessof photo-crosslinked HV-SBS on the photoinitiator concentration.UVexposure:1 s

Photocrosslinking of functionalizedrubbers,7 363

wasfound to drop from 4.3 s–1 to 2.1 s–1 when [PI]0 wasincreasedfrom 1 to 5%, thusshowing the importance oftheradiation inner filter effect27).

One of the main consequences of this photobleachingprocessis to makethe top layer of the irradiatedsamplemore transparent to UV light, which cantherefore pene-tratedeeperinto thesample,thus inducing a frontal poly-merization28). Up to afew millimeter thick specimenshavebeencrosslinked within lessthan1 min by this UV tech-nology.

Influenceof thevinyl contentof SBS

The photocrosslinking experimentsreported so far haveall beencarriedout on a SBSsamplewith a high content(59%) of pendentvinyl doublebonds,which are knownto be more reactive than the in-chain butene doublebonds.As SBSrubbersusually contain a lower amount ofpendentvinyl groups, we have repeatedsomeof theseexperiments with a SBS samplewhere only 8% of theunsaturations are vinyl double bonds(LV-SBS). Surpris-ingly, the crosslinking of this rubberwas found to pro-ceednearly as fast upon UV exposure as that containing59%vinyl group.

Fig. 10showsthedecreaseof thevinyl doublebond in a20lm thick LV-SBSfilm containing3 wt.-% Lucirin TPOexposedfor up to 3 s to UV-radiation. Althoughthevinyldoublebondsseemto disappearfasterin LV-SBSthaninHV-SBS(Fig. 10and2), this is not thecase.Thismislead-ing resultis dueto thechosenrepresentation(relativelossof vinyl group versusexposure time). The initial rate ofpolymerization of thevinyl group,expressed in mol N kg–

1 N s–1, is still 3 times greaterin the samplecontaining alargeamount of vinyl double bonds(1 mol N kg–1 N s–1 ver-sus0.3 mol N kg–1 N s–1). After 0.5s anda 10% vinyl loss,the reaction is slowing down, most probably because ofthe fast consumption of the photoinitiator. At the same

time, only a slight decrease(2%) of the butene doublebond at 967cm–1 wasfoundto occuruponUV irradiation,very much like in HL-SBS.Crosslinking was character-izedby both anincreaseof thepolymerhardness(Fig. 10)and a nearly complete insolubilization of the irradiatedsample(Fig. 11). Although the initial rateof gelation waslower in LV-SBSthanin HV-SBS, thecrosslinkdensityofthe polymer network was similar for both samples, asshown by thevaluesof theswellingratio:27and25after a2 sexposurefor LV-SBSandHV-SBS, respectively.

All theseresults, which aresummarizedin Tab.1, indi-catethat it is not necessaryto increasethe vinyl contentto achieve a fastphotocrosslinking of SBSrubber. In LV-SBS,gelation occurswhenabout10%of thevinyl groupshave reacted, which correspondsto 17 doublebonds perpolybutadienechain. An increaseof the vinyl content ofSBS wil l mainly favor the intramolecularpolymerizationbetween neighboring pendent double bonds, withoutmuch effect on thecrosslinkingprocess.

Fig. 9. Disappearanceof the photoinitiator (Lucirin TPO)uponUV irradiationof HV-SBS

Fig. 10. Lossof the polybutadienedoublebondsandhardnessincreasein a UV-irradiatedSBSrubberwith a low vinyl content(8%). Photoinitiator:[Lucirin TPO]= 3 wt.-%

Fig. 11. Insolubilization profile of a LV-SBSrubberexposedtoUV-radiationin thepresenceof Lucirin TPO(3 wt.-%)

364 C. Decker, T. NguyenThi Viet

Influenceof plasticizers

Like anyreactioncarriedout in thesolid state,thephoto-crosslinking of SBSis hamperedby the restricted mobi-lity of thereactive sites(initiator andpolymer radicals,aswell asvinyl double bonds).The addition of a plasticizerlike Nujol, a parrafinic oil, wasthus expectedto acceler-ate the polymerization process. Suchan effect wasactu-ally observedonly for Nujol concentrationsabove20%,probablybecauseof theelastomeric characterof theSBSmatrix which providesalreadyenoughmolecular mobi-lity in theneatpolymer. For a 50/50mixtureby weightofLV-SBSandNujol, thevinyl double bondswerefoundtodisappearalmosttwicefasterthanin theneatSBSsample(Fig. 12). Insolubilization was accelerated in the same

manner (Fig. 13), but the photocrosslinked polymerremained very soft becauseof thepresenceof theplastici-zeroccludedin thepolymer network (Fig. 12).

It shouldbe mentionedthat the gel fraction of the UV-curedpolymer, which reachescloseto 100% in 2 s, wascalculated on the basisof the SBS content, i. e. 50% ofthe irradiated samplehad becomeinsoluble in toluene.Interestingly, the degreeof swelling of the polymer net-work wascut by half whenLV-SBSwasUV-cured in thepresenceof Nujol (Fig. 13), thusshowing that the cross-

link densityhasincreased. This result is in good agree-ment with theobserveddoubling of thevinyl polymeriza-tion rateandsuggeststhattheplasticizer is favoringinter-molecular polymerization by increasing the molecularmobility of theSBSchains.

Similar results wereobtainedwhenNujol wasaddedtothe SBS samplecontaining 59% pendentvinyl groups.The effect of the plasticizer concentration on the vinylconversion,insolubilization andhardnessis illustratedinFig. 14 for a HV-SBS sampleexposed to UV radiationduring 1 s. For both SBSrubbers, a Nujol concentrationof 30 wt.-% appears to be an optimum value to achievean efficient crosslinkingandobtaina polymer showing astrong elastomeric character and excellent adhesion onvarious substrates(glass, metals, plastics). It should benoticed that the plasticized SBS films remain perfectlyclear, up to a Nujol concentration of 70%, thus showingthegood compatibility of thesetwo compounds.

Tab.1. Performanceanalysis of SBSphotocrosslinking

Thermoplastic elastomer LV-SBS HV-SBSInitial vinyl contentin % 8 59

Rateof vinyl loss% sÿ1

mol N kgÿ1 N sÿ1

�25 100.3 1.0

Rateof gelation (% s–1) 150 240Rateof hardness increase 60 80Swellingratio after2 s 27 25

Fig. 12. Photocrosslinkingof a blendof LV-SBSrubberandaparaffinic oil ([Nujol] = 50wt.-%)

Fig. 13. Insolubilization profile of a 50/50 LV-SBS/Nujolblendexposedto UV irradiationin thepresenceof Lucirin TPO(3 wt.-%). - - - -: neatLV-SBS

Fig. 14. Influence of the plasticizer content (Nujol) on thecharacteristicsof a 1 sUV-irradiatedHV-SBSrubber

Photocrosslinking of functionalizedrubbers,7 365

Influenceof anacrylatecrosslinker

In an attempt to further increasethe crosslinking effi-ciency, an acrylatemonomer, wasintroducedin the SBSrubber, togetherwith thephotoinitiator (Lucirin TPO).At20%by weightof a diacrylatebisphenol A telechelic oli-gomer (Ebecryl 150), both the pendentvinyl and thebackbonebutenedoublebond were foundto reactrapidlyuponUV exposure,asshownby Fig. 15.A 0.1s exposureproved to be sufficient to induce the polymerization of

14% of the vinyl groupsand6% of the butene-2 doublebonds,comparedto 1% and0% in neatHV-SBS,respec-tively. At the sametime, 70% of the acrylategroupshadpolymerized,asshown by thesharpdropof thecharacter-istic absorption peak at 812 cm–1. This result stronglysuggests that an effective copolymerization is takingplace betweenthe acrylatedouble bonds and the vinyldouble bonds. A strong correlation was found to existbetweenthe rate of polymerizationof the polybutadieneunsaturation andthatof theacrylatedoublebonds,duringthewholereaction time.Thesamebehavior wasobservedwith theLV-SBSsample.Fig. 16 showsschematically thestructure of the net-polybutadiene-i-diacrylate formedupon UV irradiation of the SBS-diacrylate mixture. Asexpectedfrom thekinetic profilesof Fig. 15, insolubiliza-tion of theSBSrubberis rapidly taking placewith forma-tion of a relatively tight polymer network(Fig. 17). Aftera 1 s UV exposure, the swelling ratio of HV-SBS wasmeasuredto be 12, compared to 32 for the neat rubber.The additional crosslinks generated by the diacrylatemonomerare causing a substantial increaseof the poly-mer hardness(Persoz values of 200 s). Becauseof theelastomeric character of the polybutadiene chains, theUV-curedfilm still maintainsa high flexibility andpassessuccessfully the severezero-T-bend test. These highly

reactive UV-curablesystems arethereforemoresuitedforprotective coating than for adhesive applications, spe-cially on flexible substrates.

Thermalstability of theUV-curableformulation

When these UV-curable rubbers are used as hot-meltadhesivesor asflexographicprinting plates,the formula-tion is processedat temperaturesup to 1508C19,20). It wastherefore important to evaluate the thermal stability oftheSBSrubbercontainingthephotoinitiator andtheaddi-tive (Nujol or diacrylate). The photoinitiator selected(Lucirin TPO) was found to be relatively stableat thattemperature,its concentration in the sampledroppingbyonly 20%aftera 30 min heating at 1508C in thepresenceof air (Fig. 18). TheUV-curing performanceswere there-fore not very muchaffectedby this thermal treatment.

Fig. 15. Polymerizationof theacrylate andpolybutadienedou-ble bonds,upon UV exposure of a 20/80 blend of diacrylatemonomer(Ebecryl 150)andHV-SBSrubber. [Lucirin TPO] = 3wt.-%

Fig. 16. Structure of polymer network formed by photopoly-merizationof acrylate andpolybutadienedoublebonds

Fig. 17. Insolubilization profile of HV-SBS/Ebecryl 150 blend(80/20 by weight) exposedto UV radiation in the presence ofLucirin TPO(3 wt.-%). - - - -: neatHV-SBS

366 C. Decker, T. NguyenThi Viet

Theconcentrationof vinyl doublebondsremainedessen-tially unchanged during the first 20min of heating,andstartedto decreaseupon further treatment,as shown inFig. 18 for theLV-SBSsample.Thebutenedoublebondsproved to be somewhat more resistant to this thermaltreatment,as expected from their lower reactivity. Theloss of doublebondsis accompanied by polymerizationand oxidation reactions which both occur after a certaininduction period. The SBS rubber started to becomepartly insolubleafter a 20 min heating(Fig. 19), the gelfraction reaching 50% after 40 min, with formation of arelatively tight polymer network (swelling ratio of 10).Dif ferent types of functional groups (alcohol, ketone,ether),resultingfrom thermooxidation were detected byinfrared spectroscopy for heating periods exceeding20min, as shown in Fig. 20. Similar resultshave beenobtainedwith theSBSsamplecontaining59%vinyl dou-ble bonds (HV-SBS) which provedto be nearly asresis-

tant to heatasLV-SBS. If necessary, the thermal stabilityof theUV-curable rubbercanbeincreasedby theadditionof a phenolic antioxidant, like Irganox1010from Ciba15).

Interestingly, the addition of paraffinic oil (Nujol) wasfound to improve the heat resistanceof the SBS rubber(Fig. 19).This is not thecase of thevery reactivediacryl-ate monomer(Ebecryl 150) which promotesthe thermalcuring, as shown in Fig. 21. After a 20 min heating at1508C in air, 30% of both the buteneand vinyl doublebondshadpolymerized,andasmany as70%of theacryl-ate double bonds,with formation of a totally insolublepolymer. The decreasedstability of the butene doublebond in the presenceof an acrylatemonomeris an argu-ment in favor of its copolymerization.

The heatresistanceof SBS rubber containing a phos-phine oxide initiator is strongenoughto allow this UV-curable systemto be usedin hot-melt adhesiveapplica-tions. This is not true for the much more reactiveacryl-

Fig. 18. Heatresistanceat 1508C of a LV-SBSrubber contain-ing 3 wt.-% Lucirin TPO

Fig. 19. Insolubilization of LV-SBSuponheating at 1508C inthepresenceof Lucirin TPO(3 wt.-%)

Fig. 20. Oxidationproducts formeduponheating at 1508C ofa LV-SBSrubbercontaining3 wt.-% Lucirin TPO

Fig. 21. Lossof acrylate andpolybutadiene doublebondsuponheating at 1508C of a LV-SBS rubber containing 3 wt.-%Lucirin TPO

Photocrosslinking of functionalizedrubbers,7 367

ate-loaded SBS formulation which is bestsuitedfor theproduction of fast-drying protective coatings showingexcellent adhesionandimpactresistance.

ConclusionPolystyrene-block-polybutadiene-block-polystyrene canbereadily crosslinkedat ambienttemperatureby UV irra-diation in the presenceof a phosphineoxide photoinitia-tor. The radical-induced polymerization of the pendentvinyl groupsoccurswithin seconds to yield an insolubleelastomeric material. The reaction kinetics canbe finelycontrolledthroughthephotoinitiatorconcentrationwhichgovernsalso the cure depthprofile. Increasing the vinyldoublebond contentof thepolybutadienechainfrom 8 to59%haslitt le effect on both thecurespeedandthecross-link density of the polymer network, as it mainly favorsintramolecular reactions between neighboring pendentdoublebonds.The crosslinkingreaction canbemarkedlyacceleratedby the addition of a telechelicacrylateoligo-mer which promotesan effective copolymerization withthepolybutadienedoublebonds,but at theexpense of theformulation heatresistance.

The photocrosslinking technology offers a numberofadvantages29), suchas cure speed,low energy consump-tion, ambient temperatureoperations,no emissionof sol-vent and spatio-temporal control of the initiation step.Becauseof its processfacility andefficiency, it hasbeensuccessfully appliedto crosslinka commercial-type ther-moplastic elastomerandmodify, selectively in the illumi -nated areas,its physico-chemical characteristics. SuchUV-curable rubbersareparticularly well suitedto produ-cing fast-drying coatings,hot-meltadhesivesandflexibleprinting plates.

Acknowledgement: The authors wish to thank SHELLRESEARCH(Louvain la Neuve– Belgium) for a researchgrant.

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