Materials Chemistry and Physics 93 (2005) 154–158

A comparative study of gamma irradiation of poly(ethylene-co-vinylacetate) and poly(ethylene-co-vinyl acetate)/carbon black mixture

Murat Sen∗, Mehmet CopurogluHacettepe University, Department of Chemistry, Polymer Chemistry Division, 06532 Beytepe, Ankara, Turkey

Received 18 October 2004; received in revised form 7 December 2004; accepted 2 March 2005

Abstract

In this comparative study, the effect of gamma rays on poly(ethylene-co-vinyl acetate) (EVA) and poly(ethylene-co-vinyl acetate)/carbonblack mixture (EVA/CB) was investigated. EVA, containing 13% vinyl acetate (VA), and EVA/CB, containing 13% VA and 1% carbon black(CB), were irradiated with gamma rays at ambient conditions up to 400 kGy. Sol–gel analyses were made to determine the percentage gelationof both virgin and irradiated samples. FT-IR measurements were performed to follow the chemical changes, which took place in the samplesduring irradiation. Dynamic and isothermal thermogravimetry studies were performed for determination of the thermal stabilities of virginand irradiated samples.

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Sol–gel analysis results showed that both EVA and EVA/CB have tendencies to form a gel under gamma irradiation. As a reR measurements, some oxidation products were observed in EVA and EVA/CB upon gamma irradiation. Thermal analysis exxhibited that the overall thermal stabilities of EVA and EVA/CB do not change, whereas, the amount of volatile products, formeamma irradiation, increase with irradiation dose both in EVA and EVA/CB.2005 Elsevier B.V. All rights reserved.

eywords:Poly(ethylene-co-vinyl acetate); Carbon black; Gamma irradiation

. Introduction

Restricted properties and limited use of homopoly-ers alone, has given rise to exploration of composites,

opolymers, blends, etc. Copolymers such as poly(ethylene-o-vinyl acetate) (EVA), poly(ethylene-co-butyl acrylate),oly(ethylene-co-ethyl acrylate) (EEA) have wide range ofsages in different industries. Among the numerous ethyleneopolymers, due to its wide range of properties depending onts vinyl acetate content, EVA has become one of the mostseful copolymers in the transportation industry as an insu-

ator, in the electric industry as a cable insulator, in the shoendustry as soles, and in many other industries as a hot meltdhesive, a coating, etc. Several works looked at the influencef gamma rays on polymers.

Black and Charlesby[1] performed one of the earliesttudies on gamma irradiation of polymers. They showed

∗ Corresponding author. Tel.: +90 312 2977989; fax: +90 312 2977989.E-mail address:msen@hacettepe.edu.tr (M. S¸en).

the changes in chemical structure and physical propertpolyethylene upon irradiation with gamma or X-rays. Fortion of crosslinks, main chain fracture, and unsaturationamong these changes. These experiments were donethe presence of oxygen.

In a study, carried out by Geuskens et al.[2], the influ-ences of gamma rays and UV light on poly(vinyl acet(PVA) were investigated and compared. They estimateproducts of radiolysis and photolysis of PVA. They also pposed some mechanisms for chemical changes underduring radiolysis and photolysis of PVA.

Lacoste and Carlsson[3] undertook a study about gammphoto-, and thermally initiated oxidation of linear low denpolyethylene, and compared the detailed oxidation prodfor different initialization types. They observed commonidation products, such as hydroperoxide, ketone, vinylester groups, for all types of initialization but with quanttive differences.

A study on thermal degradation of electron beam cEVA was performed by Dutta et al.[4] They explained th

254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2005.03.005

M. Sen, M. Copuroglu / Materials Chemistry and Physics 93 (2005) 154–158 155

decomposition mechanisms and showed the effect of electronbeam on the thermal stability of EVA.

In order to investigate the effect of gamma rays on thethermal and mechanical stabilities EVA and EEA, S¸en andGuven[5] carried out a comparative study and tried to finda correlation between the thermal and mechanical stabilitiesof copolymers.

In our previous studies[6,7], we investigated the ac-celerated thermal and UV ageing characteristics of EVAand poly(ethylene-co-vinyl acetate)/carbon black mixture(EVA/CB), since EVA/CB, with 13% VA and 1% CB, is awidely used material in particular by Turkish State Railways(TCDD) due to its elastic structure and insulation property.After these studies, we concluded that EVA is a material that issusceptible to both heat and UV light; whereas EVA, contain-ing 1% CB, is a durable material against heat, but vulnerableto UV light.

In this comparative study; the effect of gamma rays onEVA (13% VA) and EVA/CB (13% VA and 1% CB) was in-vestigated. It is thought to be important; because, any possiblechange in these materials’ chemical and physical propertiescould affect their industrial importance. It has been planned,as a further study, that whether gamma irradiation has an im-proving effect on the weathering characteristics of EVA andEVA/CB.

2

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2

2the

g per-f r andw tagesw ing

equation:

% Gel= m

m0× 100

wherem0 andmare the masses of a sample before and afterextraction, respectively.

2.3.2. FT-IR studiesFT-IR studies were carried out by means of Nicolet 520

model spectrometer. Samples were pressed in a hotplate atabout 100◦C for 10 s in order to obtain film forms. Samples,crosslinked to a high extent, were scraped to get tiny particlesand then were mixed with KBr.

2.3.3. Thermogravimetric analyses (TGA)Thermogravimetric analyses were performed by utiliz-

ing Du Pont Instruments-Thermal Analyzer, Model 951. Dy-namic thermogravimetric studies were carried out under ni-trogen atmosphere; and 10◦C min−1 heating rate was used.Dynamic thermogravimetric study results indicated the ther-mal stabilities of virgin and irradiated samples. Isothermalthermogravimetric studies were performed at 350◦C, andused to determine the percentages of volatile degradationproducts of virgin and irradiated samples.

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tureo la-t ofg /CBi edt lentb om-b g ir-rS les,

. Experimental

.1. Materials

EVA, containing 13% VA and of density 0.9288 kg l−1,as supplied by Elf-Atochem Co. in the form of granuVA/CB plates with contents of 13% VA and 1% CB webtained from Panel Co., Inc., Turkey. EVA, used inreparation of this EVA/CB mixture, obtained from Etochem Co.; whereas masterbatch (PE Black 99209)iba Co., Italy, in the form of 50% dispersion of CB, tyRF, in LDPE. Xylene, used as a solvent for EVAVA/CB, for determination of gelation, was obtained frerck.

.2. Irradiation of materials

EVA granules, and EVA/CB specimens of dimensi.4 mm× 3.9 mm with 1.9 mm thickness were irradiated wamma rays at ambient conditions up to 400 kGy.

.3. Analyses

.3.1. Determination of percentage gelationsFor investigation of the influence of gamma rays on

elations of EVA and EVA/CB, sol–gel analyses wereormed. Xylene was used as a solvent in soxhlet extractoas fluxed through each sample for 14 h. Gel percenere calculated gravimetrically according to the follow

. Results and discussion

.1. Sol–gel analyses

Before investigation of chemical changes in the strucf EVA and EVA/CB, the effect of gamma rays on the ge

ion of EVA and EVA/CB was investigated. The effectamma rays on the percentage gelations of EVA and EVA

s given inFig. 1. Gelation in polymers is generally referro as crosslinking of macromolecules by means of covaonds. Crosslinking might be formed most likely due to cinations of the macromolecular radicals formed durinadiation, as explained in the literature for polyolefins[8].ince the main chains of EVA and EVA/CB macromolecu

Fig. 1. Variation of percentage gelation with irradiation dose.

156 M. Sen, M. Copuroglu / Materials Chemistry and Physics 93 (2005) 154–158

used in this study, resemble those of polyethylene (PE), itshows similar behaviour to what PE does. An explanationproposed by Geuskens et al.[2] for PVA, might also be validfor EVA and EVA/CB.Fig. 1clearly shows that the gelationoccurred to a great extent, beginning from the initial stages,upon irradiation with gamma rays, in EVA and EVA/CB.This is due to high energy and low selectivity of gammarays, which are capable to be absorbed even by CC and/orC H bonds. Thus, formed radicals lead to formation of highamount of crosslinks in both EVA and EVA/CB. % Gelationof EVA/CB, however, is more significant than that of EVA atall doses. This may be due to contribution of CB to the for-mation of crosslinks by using its oxygen-containing groups,under gamma irradiation.

In order to determine the crosslink and the chainscission reaction yields occurring during irradiation ofboth EVA and EVA/CB, the usual Charlesby–Pinnerequation was used. Charlesby–Pinner equation[9](s+

√s=po/qo + 2/(qou2,0D)), has been used by many

researchers so far for simultaneous determination of thecrosslinking and the chain scission reaction yields of poly-mers being irradiated by ionizing radiation. In this equation,s is the sol fraction,po is the chain scission yield, averagenumber of main chain scissions per monomer unit and perunit dose,qo is crosslinking yield, proportion of monomerunits crosslinked per unit dose,u is initial weight,ad

hta as9 al-c ipro-c( rE pta

da .80,r Bd id-u oref on,s

3

auseC andsa FT-I oseo er upc n FT-I on ofe placeo ectra

Fig. 2. FT-IR spectra, in the range of 1950–1550 cm−1, of virgin and irra-diated EVA. Numbers on the curves represent irradiation dose (kGy).

have a high signal to noise ratio (except unirradiated ones)because of the difficulty in mixing the crosslinked sampleswith KBr. An outstanding feature of these spectra is the rapiddevelopment of wide peaks at 1655 and 1630 cm−1, whichcan be attributed to the conjugate dienes. This interpretationis consistent with the explanation of very high rates of for-mation of radicals; because, after crosslinking (either intra orinter) of some radicals, other adjacent radicals which are ster-ically hindered to form crosslinks, combine with each other.This formation was observed both in EVA and EVA/CB,even at the irradiation dose of 25 kGy. The shoulder near1715 cm−1, appeared both in EVA and EVA/CB upon irra-diation, can be attributed to the formation of ketone species.Further evaluation is not possible due to low signal to noiseratio.

Fd y).

2,0verage degree of polymerization, andD is the irradiationose.

In this equation,u2,0 was calculated as the ratio of weigverage molecular weight (obtained from Elf-Atochem5,000 g mol−1) to average weight of a monomer unit (culated as 30.8), and was found as 3084. Plots of the recal of irradiation dose, 1/D versuss+

√s; yield straight lines

with a regression coefficient,r = 0.975 for EVA and 0.922 foVA/CB) andpo/qo andqo were calculated from the intercend the slope of the lines, respectively.

As a result of calculations,po andqo (per kGy) were founs 1.65 and 1.75, respectively, for EVA, and 4.92 and 5espectively, for EVA/CB. Although the values for EVA/Ciffer from those for EVA, when they are evaluated indivally, one can conclude that the crosslink formation is m

avorable for both EVA and EVA/CB than chain scissiincepo/qo = 0.941 for EVA and 0.848 for EVA/CB.

.2. FT-IR studies

EVA and EVA/CB have almost the same spectra; becB itself has no remarkable characteristic absorption bnd thus, does not influence the spectrum of EVA. The

R spectra of virgin and gamma irradiated EVA and thf EVA/CB are given inFigs. 2 and 3, respectively, in thange of 1950–1550 cm−1; because main functional grohanges were observed in this region. Samples used iR measurements were obtained from the surface regiach granule or specimen, since chemical changes taken the surface of the samples, to a greater extent. Sp

ig. 3. FT-IR spectra, in the range of 1950–1550 cm−1, of virgin and irra-iated EVA/CB. Numbers on the curves represent irradiation dose (kG

M. Sen, M. Copuroglu / Materials Chemistry and Physics 93 (2005) 154–158 157

Fig. 4. Dynamic thermograms of irradiated EVA in nitrogen atmosphere.Numbers on the curves indicate the irradiation dose in kGy.

3.3. Influence of gamma irradiation on the thermalstabilities of EVA and EVA/CB

Figs. 4 and 5show the dynamic TGA thermograms ofvirgin and irradiated EVA and EVA/CB, in nitrogen atmo-sphere, respectively. As shown inFigs. 4 and 5, virgin EVAand EVA/CB show the typical step degradation profile withthe initial stage involving acetic acid evolution and the secondinvolving main chain degradation[10,11]. Under high-energyradiation, radicals are formed at high concentrations in closeproximity to one another so that second-order crosslinking re-actions are favored compared with first-order chain scissions[8]. This explanation seems to be true in the case of EVA andEVA/CB, because typical thermal degradation route does notchange significantly upon irradiation with gamma rays forboth EVA and EVA/CB. It can be said that both chain scissionand crosslinking take place simultaneously during irradiationof EVA and EVA/CB with gamma rays. These results are alsoconsistent with crosslinking and chain scission yield values.

Degradation activation energies of EVA and EVA/CB werecalculated by using Freeman–Carroll equation and the dataobtained from derivatives of dynamic thermograms of gammairradiated EVA and EVA/CB were used in the calculations

F here.N

Table 1Variation of the degradation activation energies of EVA and EVA/CB withirradiation

Degradation activation energy (kJ mol−1)

0 kGy 25 kGy 50 kGy 100 kGy 200 kGy 400 kGy

EVA 282 294 284 277 309 277EVA/CB 295 278 277 268 277 385

[12]:

�log(d%/dT )

�log(1− c)= n − E

2.3Rx

�(1/T )

�log(1− c)

whereT is the temperature in Kelvin,n is the order of reaction,R is the gas constant which has a value of 8.314 J mol−1 K−1,E is the degradation activation energy.c is the conversion ratioand is equal to (m0 −m)/m0, wherem0 is the initial mass andm is the mass at any time.

When �(1/T)/�log(1− c) versus �log(d%/dT)/�log(1− c) is plotted, the slope of this plot gives thedegradation activation energy and the intersection of ordi-nate indicates the order of reaction. Thec andT values wereobtained from derivatives of thermograms. The effect ofgamma irradiation on the degradation activation energies ofEVA and EVA/CB is shown inTable 1.

Consistent with the results of dynamic thermograms,degradation activation energies of EVA and EVA/CB do notchange to remarkable extent until 400 kGy. According tothese results, it can be said that the total effects of crosslink-ing, chain scission and oxidation make no difference in ther-mal stability.

As can be seen fromFigs. 4 and 5, due to overlapping offirst and second degradation steps, it is not easy to interpretand determine the amounts of readily decomposing groupsfrom dynamic TGA thermograms. For detailed analysis of thefi werep -t ingg VAa e

F here.N

ig. 5. Dynamic thermograms of irradiated EVA/CB in nitrogen atmospumbers on the curves indicate the irradiation dose in kGy.

rst stage of the degradation, isothermal TGA analyseserformed in nitrogen atmosphere at 350◦C. This tempera

ure is sufficient to break down all kinds of oxygen-containroups. Isothermal thermograms of virgin and irradiated End EVA/CB are given inFigs. 6 and 7, respectively. Th

ig. 6. Isothermal thermograms of irradiated EVA in nitrogen atmospumbers on the curves indicate the irradiation dose in kGy.

158 M. Sen, M. Copuroglu / Materials Chemistry and Physics 93 (2005) 154–158

Fig. 7. Isothermal thermograms of irradiated EVA/CB in nitrogen atmo-sphere. Numbers on the curves indicate the irradiation dose in kGy.

plateaus in these figures clearly indicate that all the oxygen-containing volatile groups are eliminated from polymer atthe end of 70 min. As can be seen from these figures, thereis a sharp increase in the amount of volatile products whenEVA and EVA/CB were irradiated up to 25 kGy. After thatdose, a relation between the irradiation dose and the amountof volatile products could not be obtained for EVA. On theother hand, after that sharp increase, a slight and continuousdecrease was observed in EVA/CB with dose. Besides, theamount of these products is lower in EVA/CB than in EVA ateach dose except 0 kGy. Like the results of FT-IR, the resultsof isothermal TGA analysis confirm the formation of oxida-tion both in EVA and EVA/CB, during gamma irradiation.

4. Conclusions

In order to investigate the effect of gamma rays on EVAand EVA/CB, samples were subjected to gamma rays at am-bient conditions up to 400 kGy. Sol–gel analyses showed thatboth EVA and EVA/CB tend to crosslink when irradiated withgamma rays. FT-IR spectra of virgin and irradiated EVA and

EVA/CB showed that, some chemical changes took place inEVA and EVA/CB during irradiation (such as the formationof diene, ketone, etc.). Dynamic thermograms, carried out innitrogen atmosphere, indicated that the thermal stabilities ofEVA and EVA/CB do not change during irradiation. Isother-mal TGA analyses, performed in nitrogen atmosphere andat 350◦C, showed that the concentration of volatile productsincreases rapidly at the initial stages of irradiation both inEVA and EVA/CB. As a conclusion, all these studies showthat, oxidation, crosslinking and chain scission are the mainconsequences of gamma irradiation of EVA and EVA/CB.Moreover, the manner of changes is identical both in EVAand EVA/CB.

Acknowledgement

The authors would like to thank Prof. Dr. Olgun Guven forthe laboratory facilities he supplied and for his encouragingand constructive advice during this study.

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