Electron beam irradiation of epoxidized natural rubber

Chantara Thevy Ratnam a,*, M. Nasir b, A. Baharin b, Khairul Zaman a

a Malaysian Institute for Nuclear Technology Research (MINT), Bangi, 43000 Kajang, Malaysiab School of Industrial Technology, University Science Malaysia, 11800 Penang, Malaysia

Received 28 March 2000; received in revised form 13 June 2000

Abstract

The e�ect of irradiation on ENR50 was studied with particular attention to irradiation-induced crosslinking. ENR50

was irradiated by using a 3.0 MeV electron beam machine with doses ranging from 20 to 200 kGy. The in¯uence of

several additives such as TMPTA, Irganox1010 and tribasic lead sulfate on irrradiation-induced crosslinking of ENR50

was investigated. The gel fraction, Ts, M100, Eb, hardness and Tg were used to follow the irradiation-induced

crosslinking of the rubber. Results revealed that the increase in gel fraction upon irradiation of pure ENR50 could be

associated with both irradiation-induced crosslinking as well as ring opening side chain reaction of oxirane group. The

observation on the trend shown in the properties studied con®rmed that TMPTA is e�cient in enhancing the irradi-

ation-induced crosslinking of ENR50. The addition of Irganox1010 and tribasic lead sulfate found to inhibit irradia-

tion-induced crosslinking of ENR50 to considerable extent. This in¯uence of stabilizing additives in particular,

antioxidant, was observed to be more prominent at lower doses. The importance of TMPTA in preventing ring opening

side chain reactions as well as the role of the stabilizing additives in introducing free chain ends are also discussed.

However studies did not reveal the exact nature of irradiation-induced reactions involved in ENR. Ó 2000 Elsevier

Science B.V. All rights reserved.

Keywords: Irradiation; Irradiation-induced; Crosslinking; Reactions; ENR50

1. Introduction

Epoxidized natural rubber has been introducedin the last decade as a novelty rubber which isknown to possess excellent properties like oil re-sistance, gas impermeability, good wet grip andhigh damping characteristics [1±5]. In recent years

its potential as a substitute to many syntheticelastomers is a subject of investigation. Variousstudies on the physical properties of ENR havebeen reported earlier. These include mechanical [67], scorch [8] and blends of ENR and PVC [9±13].A recent study by Ratnam and Zaman [14]showed the potential for modi®cation of PVC/ENR blend properties by electron beam irradia-tion. More detailed studies on stabilization andirradiation crosslinking of PVC/ENR blends re-vealed that addition of tribasic lead sulfate, Irga-nox1010 and TMPTA to PVC/ENR blends

Nuclear Instruments and Methods in Physics Research B 171 (2000) 455±464

www.elsevier.nl/locate/nimb

* Corresponding author. Tel.: +60-3-8250510; fax: +60-3-

8202968.

E-mail address: chantara@mint.gov.my (C.T. Ratnam).

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 3 0 1 - 3

improved the blend properties upon irradiation[15±17]. To date, no information has been pub-lished on the e�ect of high-energy irradiation orirradiation-crosslinking of ENR to the bestknowledge of the authors. In view of the in-creasing interest in the rubber, it is thus the aim ofthis paper to describe some of our ®ndings on theirradiation-crosslinking of ENR50. In order togain further understanding on the possible en-hancement of ENR by irradiation, e�ects of sev-eral additives namely; an acrylic crosslinkingagent (TMPTA), a phenolic antioxidant (Irga-nox1010) and a lead stabilizer (tribasic lead sul-fate) were also studied. These additives werechosen based on our previous ®ndings on PVC/ENR blend systems [15±17].

2. Experimental

2.1. Materials

1. Epoxidized natural rubber, grade ``ENR50''(Batch No: Epoxyprene 50 B423/5/98/77) with50% epoxidation level was purchased fromGuthrie Polymer, Malaysia.

2. Trimethylolpropane triacrylate (TMPTA) isproduct of UCB Asia Paci®c, Malaysia.

3. Irganox1010 was obtained from Ciba SpecialityChemicals, Singapore.

4. Tribasic lead sulfate (TS-100M) was purchasedfrom Lonover Scienti®c suppliers Ltd., London.

2.2. Methods

About 200� 0:5 g of ENR50 was milled into a¯at sheet on a laboratory two-roll mill at room

temperature. The required amount of additives,which was determined from previous studies [15±17] based on PVC/ENR blend system, were spreadon the sheet. The sheet was then carefully rolledup. The sample was homogenized by milling. Itwas weighed to ensure that there was no excessiveloss of ingredients during the milling. The fullrecipes are given in Table 1.

The ENR compounds obtained above werecompression molded into uniformly ¯at sheets of140 mm� 140 mm� 2 mm under a pressure of150 kg cmÿ2 at 100°C for 4 min. The sheets wereimmediately cooled between two plates of a coldpress at 25°C. Dumbbell shaped test pieces (75 mmof overall length and 4 mm width of the narrowparallel portion) were cut from these sheets in ac-cordance to BS6746.

2.3. Irradiation

The molded sheets and dumbbell test pieceswere irradiated using a 3 MeV electron beam ac-celerator Model EPS 3000 (Cockroft Walton) in adose range of 0±200 kGy. The acceleration energy,beam current and dose rate were 2 MeV, 2 mA and0.78 kGy/s, respectively. The doses are estimatedbased on machine parameters. The samples wereirradiated in the presence of air at room temper-ature.

Under the condition employed in this work thedose uniformity ratio, DUR, of the samples wasfound as 1� 0:05. Cellulose tri-acetate (CTA) ®lmwas used to monitor the absorbed dose during ir-radiation. The CTA ®lms employed were of thetype FTR-125, supplied by Fuji Film, Japan. TheDUR was calculated with the following equation:

DUR � dose maximum=dose minimum: �1�

Table 1

Radiation crosslinking formulation used for ENR50

Composition Control ER1 ER2 ER3 ER4

ENR50 100 100 100 100 100

TMPTA ± 4 ± ± 4

IRGA1010 ± ± 1 ± 1

Tribasic lead sulfate ± ± ± 4 4

456 C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464

2.4. Gel fraction

The samples were extracted using boiling tet-rahydrofuran, THF for 24 h and the extractedsamples were dried to constant weight. The gelfraction was calculated as

gel fraction � �W =W0� � 100; �2�

where W and W0 are the weight of the driedsample after extraction and the weight of thesample before extraction, respectively. At least ®vemeasurements were taken on each sample and theaverage and standard deviation values were cal-culated.

2.5. Measurement of tensile properties

The tensile strength (Ts), modulus at 100%elongation (M100) and elongation at break (Eb)were measured on a Toyoseiki model Strograph-RI in accordance to BS6746. Altogether eightsamples were used for tensile property measure-ments and an average of six results (the maximumand minimum data being discarded) was taken asthe resultant value. The standard deviation valueswere also calculated.

2.6. Hardness

The shore A hardness test was carried out ac-cording to ASTM D2240-89 using Zwick 7206Hardness Tester. At least ®ve measurements weretaken on each sample and the average and stan-dard deviation values were calculated.

2.7. Glass transition temperature (Tg)

In order to determine the Tg, a Dynamic Me-chanical Analysis was performed using Perkin El-mer DMA-7e. in the temperature/time scan modewith a parallel plate. The measurements werecarried out at a heating rate of 10°C/min over atemperature range of )100±50°C and a frequencyof 1 Hz.

3. Results and discussion

3.1. Gel fraction

Radiation e�ects on polymers have been in-vestigated by many authors [18,19] over the lastfew decades. Among other e�ects, high-energyradiation could cause crosslinking and degrada-tion reactions in polymeric materials. These reac-tions are reported to follow a free radicalmechanism [20,21]. It is well known that ¯exiblepolymers such as natural rubber predominantlycrosslink upon irradiation [22±24]. In general, theyield of irradiation-induced crosslinking can beestimated from gel fraction determination [25].The variations of gel content with irradiation dosefor pure ENR and for compound containing var-ious additives are shown in Fig. 1.

Looking at the gel fractions results before irra-diation, the pure ENR50 exhibits higher gel frac-tion compared to the compound with additives.This observation is believed to be attributed by thestructural changes in ENR during milling alongwith the thermal e�ect during molding of the rub-ber which could lead to the formation of ethercrosslinks and other epoxy ring opened products.A report by Kumar et al. [26] indicates that theconcentration of epoxide ring in ENR decreaseswhereas furan ring concentration increases with thetime of milling. These authors also anticipated thatfree radical mechanims occur during milling. Inaddition, it was reiterated that the increase in gelfraction of ENR, is associated with the interchaininteractions by H-bonding through ring openedproducts of ENR such as hydroxyl group, hydro-furan and carbonyl group formations and also byether type crosslink formations [27]. In a study onthermo-oxidative behavior of ENR, the increase ingel with temperature is well elucidated [28].

On the other hand, the decrease in gel fraction ofENR with the addition of Irganox1010 and tribasiclead sulfate could be due to the stabilization of therubber by the additives as well as the radical scav-enging e�ect of the additives. The incorporation ofTMPTA was found to have improved gel fractioncompared to ER2, ER3 and ER4 indicatingthat TMPTA has reacted with ENR to formcrosslinks even before exposure to irradiation. The

C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464 457

introduction of an acrylate was found to contributeto a reduction in ring opening side reactions inENR [16,29], thus accounting for the lower gelfraction observed for ER1 compared to the control.

The general rise in gel fraction with irradiationdose suggests that ring opening of epoxy andcrosslinking may be the prevailing irradiation-in-duced reactions compared to chain scissioning.Fig. 1 also presents the in¯uence of the additiveson the gel fraction of the rubber upon irradiation.Apparently, the dose required to achieve 70% gelfraction increases in the following order:

ER1 < ER4 < control < ER3 < ER2:

Obviously, the gelation is induced at lower dosewhen TMPTA is present in the compound. Suchan observation is expected as the TMPTA em-ployed in this study is well known as reactive ad-ditive which forms crosslink bridges by anirradiation-induced free radical mechanism[30,31]. The interaction of multifunctional acry-lates (MFA) with growing grafted chain radicals iswell described by Dworjanyn et al. [32]. On com-paring the dose to achieve 70% gel fraction be-tween ER2, ER3 and control, it is obvious that theaddition of Irganox1010 and tribasic lead sulfateinhibits irradiation-induced crosslinking in ENR.It has been reported that the most noticeable e�ectof the addition of antioxidants to polymers is the

deactivation of the free radicals induced by theirradiation which otherwise could react to formcrosslinks [33]. In fact, it was found that the ra-diaolysis of polyethylene and other polyole®nscontaining one or more of the commercially in-teresting antioxidants often lead to signi®cant in-hibition of irradiation-induced crosslinking [34].The inhibition in irradiation-induced crosslinkingwith the addition of tribasic lead sulfate is in per-fect agreement with Mendibal et al. [25] who re-ported that in the presence of crosslinking agent,addition of dioctyl tin oxide stabilizer to PVC-based formulations reduced the amount of gelobtained upon irradiation. Similar observationswere also noted by Ramesh and De [35] with theaddition of tribasic lead sulfate in their work onself-crosslinkable PVC/ENR blends. In contrastthe gel fraction of ER3 was found to exceed the gelfraction of the control at doses above 100 kGy.This could indicate that the stabilizer is beingconsumed as a result of the oxidation chemistryensuing during irradiation resulting in reducedradical scavenging ability at higher doses.

3.2. Tensile strength

Fig. 2 depicts the e�ects of irradiation on thetensile strength of ENR50 formulated with variousadditives. In agreement with the gel fraction, ad-

Fig. 1. E�ect of irradiation on gel fraction of ENR50.

458 C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464

dition of TMPTA rendered highest Ts values fol-lowed by a reduction in Ts with the addition ofstabilizing additives. This trend of results rea�rmsthe e�ciency of TMPTA as a crosslinking agentand the radical scavenging ability of the additivesused. The higher Ts achieved at the 70% gel frac-tion for ER1 compared to that of control, ER2,ER3 and ER4 could re¯ect the increased inter-molecular crosslinks formed upon irradiation withthe presence of TMPTA. ENR contains both ep-oxide and unsaturated sites.

The presence of free radicals in irradiatedpolymers and reactions involving radicals havebeen amply demonstrated [23]. Two polymericradicals are expected to contribute greatly to theoverall crosslinking yield in unsaturated elastom-ers [24]. The acceleration of crosslinking processwith the presence of unsaturated group for acry-lates like TMPTA have been described elsewhere[32]. Despite the residual double bond in ENR, theepoxide groups provide alternative sites forcrosslinking with polyfunctional chemicals [36,4]and acrylates [29]. This could also account for highintermolecular crosslinks. However, because theENR50 chains contain such a great number ofneighboring epoxy groups, intramolecular reac-tions are also likely to proceed by ring opening ofepoxy. This will lead to the formation of THF

structures along with the ether bond loop [29]. Theimportance of such ring opening products of ENRin increasing the gel fraction is described by vari-ous authors [29,37]. An ENR50 vulcanizate inwhich approximately 15% of the epoxide groupshad undergone ring opening had a lower Ts [37].Another salient point is, that the introduction ofthe acrylate group, by interrupting the sequence ofepoxy groups along the chain, was expected tomake intramolecular ring opening reaction lesslikely to occur thus more intermolecular crosslinkscould have resulted. Similar observations were alsonoted by Decker et al. [29] in their studies on``Photo crosslinking of Liquid ENR''.

Furthermore, at doses corresponding to 70%gel fraction, the Ts for the ER2, ER3 and ER4exhibit lower values than the control. This trend ofresults is believed to be associated with the pres-ence of more chain ends in the rubber network.The modulus measurements were expected torender further evidence of the existence on theabove chain ends.

3.3. Modulus

The e�ects of additives on the modulus ofENR50 with increasing irradiation dose is illus-trated in Fig. 3. The modulus is found to exhibit

Fig. 2. E�ect of irradiation on Ts of ENR50.

C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464 459

similar trends as Ts. The modulus depends directlyon the number of closed loops in the network, orin other words, a perfect network [38]. The e�ectsof radical scavenging stabilizers, such as antioxi-dants which resulted in a network with more chainends is well illustrated by Lyons [34]. The chainends in the network which may be formed uponirradiation and retained in the gel, will not a�ectthe measured gel fraction but would reduce themodulus. Thus, at doses corresponding to 70% gelfraction the lower modulus observed for theENR50, with the existence of stabilizing additivesimplies that addition of such additives resulted in anetwork with more chain scission. The highestmodulus shown by ER1 again supports our pre-vious observation that TMPTA is e�ective in ac-celerating the irradiation-induced crosslinking inENR50. The marginal increase in M100 in thecontrol sample with irradiation dose compared tothe ER2 and ER3 further suggest that upon irra-diation of the pure ENR, the ring opening sidechain reaction may prevail resulting in formationof a less perfect network.

3.4. Elongation at break (Eb)

Plots of the Eb versus irradiation dose forENR50 with and without additives are shown in

Fig. 4. Generally increasing the irradiation dosecauses a substantial reduction in the elongation atthe break of the rubber. Such decline is expectedsince the rubber becomes increasingly brittle as aconsequence of the increase in number of cross-links as the absorbed dose increases. Therefore, thereduction in Eb could be attributed to the reducedsegmental mobility of the rubber chains whichresulted from the increasing crosslinking of therubber. In other words the onset of crosslinkingreduces the ability of the rubber chain to plasti-cally deform hence a progressive reduction in Ebwith increasing dose is noted.

However, looking at the plot for the pureENR50 the trend is not the same. Here, at lowabsorbed dose range (20 kGy) the Eb of the con-trol is essentially constant. A slight increase in Ebin the range of 20±150 kGy is observed. This trendof results could be associated with the moregradual formation of irradiation-induced cross-linking in the absence of additives which allows therubber to undergo strain-induced crystallization.This is similar to the sulfur vulcanized ENR50which was found to be capable of undergoingstrain-induced crystallization as above [39]. On thecontrary, the opposite situation which is observedat doses above 150 kGy explains the furthercrosslinking of the ENR50 could have introduced

Fig. 3. E�ect of irradiation on M100 of ENR50.

460 C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464

irregularity to the rubber chain, thus eliminatingthe strain-induced crystallization. A similar pat-tern of results was also observed by Sanita et al.[40] while working on rubber dominant irradiatedPE/NR blends in the absence of curatives.

Interestingly, the initial Eb of ENR50 is ob-served to be high for samples which were formu-lated with stabilizing additives. This observationcould be attributed to the lubricating action of theadditives coupled with the low gel fraction, whichresulted from the reduced ring opening reactions.Subsequently, a relatively high Eb of ER3 even at200 kGy indicates that tribasic lead sulfate resultedin soft and rubbery ENR50. The drastic decline inEb observed for ER4 from 0 to 50 kGy explainsthe severe embrittlement of the rubber due to ex-cessive crosslinks formed with the presence ofTMPTA upon consumption of the added stabi-lizers.

3.5. Hardness

The irradiation-induced crosslinking in poly-mers is often re¯ected by the hardness of the ir-radiated sample [41]. The variation in hardness ofthe rubber with irradiation upon addition ofTMPTA and stabilizing additives is shown inFig. 5. At lower doses (below 100 kGy) the in-

crease hardness of all samples was found to ex-hibit similar trends as the gel fraction, M100 andTs. At above 100 kGy, a steady state in hardnessof ER4 exceeding the hardness of ER1 was indi-cated in Fig. 5. This behavior is again a proof ofthe consumption of the Irganox1010 at higherdoses.

3.6. Glass transition temperature, Tg

Structural changes in ENR is often related tochanges in Tg. Studies have shown that vulcani-zation of ENR with dibasic carboxylic acids isassociated with a signi®cant increase in Tg of thematerial [42]. Thus, the Tg values for ENR50 withthe presence of additives were measured andcompared with the control before and after irra-diation at 150 kGy.

Here, the Tg values were taken as temperaturethat correspond to the tangent d peak since theycoincide with Tg values determined by othermethods, for example free volume studies [43].Table 2 shows the changes in Tg of the ENR50compound upon irradiation at 150 kGy. A shift inTg towards higher values with irradiation and afurther increase in Tg with the presence ofTMPTA clearly implies that the elevated Tg uponirradiation could be due to radiation-induced

Fig. 4. E�ect of irradiation on Eb of ENR50.

C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464 461

crosslinking. Such e�ect whereby shifts in Tg tohigher temperature with increasing crosslinking ofepoxy compound (Epon 828) were noted has beenreported previously [44].

At 0 kGy, ER2, ER3 and ER4 do not exhibitsigni®cant di�erence in Tg as compared to control.However, the higher Tg observed for ER1 com-pared to control prior to irradiation supports ourprevious results in which TMPTA was found toaccelerate crosslinking even before irradiation.Looking from another perspective, the lower Tgobtained for control compared to ER1 furthersuggests that the higher gel fraction noted forcontrol at 0 kGy is attributed to the ring openingside chain reaction which was prevailing duringprocessing. Indeed, it is likely that crosslinkedstructures in ENR would have a greater contri-

bution to an increase in Tg than the analogues ringopening side chain structures.

The drastic increase in Tg of ER1 and ER4upon irradiation at 150 kGy con®rms again theacceleration of irradiation-induced crosslinking ofthe rubber by TMPTA as progressive main chainmodi®cation of polymer invariably leads to anincrease in the values of Tg because of loweredrotational freedom of the modi®ed segments. It isalso noteworthy that the control shows only slightincrease in Tg upon irradiation. This again sup-ports our initial assumption that more ring open-ing side chain reactions exist upon irradiation ofENR50 with the absence of TMPTA.

It can be seen in Table 2 that the increase in Tgis almost in similar sequence with the enhancementof mechanical properties. The increase in Tg wouldcertainly tend to cause reduction in strain atfracture, indicating a close resemblance with Ebresults. Accordingly, in the case of ER2 and ER3the increase in Tg is less marked at 150 kGy. Thiscould be related to several factors such as lower gelfraction obtained and existence of chain ends. Thelubricating action of the excessive amount oftribasic lead sulfate could also account for thesmaller Tg value obtained for ER3 as compared toER2. On the other hand, the almost similar valuesof Tg exhibited by ER1 and ER4 upon irradiation

Fig. 5. E�ect of irradiation on hardness of ENR50.

Table 2

Changes in Tg of ENR50 upon irradiation

Formulation 0 kGy (°C) 150 kGy (°C)

Control )14.845 )12.754

ER1 )11.940 )6.506

ER2 )15.132 )10.633

ER3 )15.794 )13.675

ER4 )15.349 )8.507

462 C.T. Ratnam et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 455±464

at 150 kGy explains the severe enhancement inirradiation-induced crosslinking upon consump-tion of the antioxidant with the coexistence ofTMPTA.

4. Conclusions

The electron beam irradiation of ENR50 isfound to be an e�cient method to producecrosslinked ENR50. This study reveals thataddition of crosslinking agent such as TMPTAcan provide excellent enhancement in irradiation-induced crosslinking in ENR50. Results obtainedalso indicate that the degree of irradiation-in-duced crosslinking as well as the acceleration ofcrosslinking reduces with the addition of stabiliz-ing additives. On comparing the mechanicalproperties of the various formulations at 70% gelfraction, it was evident that the additives playmajor roles in controlling the type of radiation-induced reaction in ENR50, presumably, ringopening side chain reactions, crosslinking or for-mation of chain ends. The deviations from theobserved trends of properties at higher doses canbe attributed to the consumption of the stabilizingadditives, Irganox1010, in particular. This obser-vation was further supported by the resultsobtained on Tg.

Acknowledgements

The authors wish to thank ALURTRON sta�of MINT for the assistance during the irradiation.

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