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

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1 Materials Chemistry and Physics 93 (2005) A comparative study of gamma irradiation of poly(ethylene-co-vinyl acetate) and poly(ethylene-co-vinyl acetate)/carbon black mixture Murat Şen, Mehmet Çopuroğlu Hacettepe University, Department of Chemistry, Polymer Chemistry Division, 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)/carbon black 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 gelation of both virgin and irradiated samples. FT-IR measurements were performed to follow the chemical changes, which took place in the samples during irradiation. Dynamic and isothermal thermogravimetry studies were performed for determination of the thermal stabilities of virgin and irradiated samples. Sol gel analysis results showed that both EVA and EVA/CB have tendencies to form a gel under gamma irradiation. As a result of FT- IR measurements, some oxidation products were observed in EVA and EVA/CB upon gamma irradiation. Thermal analysis experiments exhibited that the overall thermal stabilities of EVA and EVA/CB do not change, whereas, the amount of volatile products, formed during gamma irradiation, increase with irradiation dose both in EVA and EVA/CB Elsevier B.V. All rights reserved. Keywords: Poly(ethylene-co-vinyl acetate); Carbon black; Gamma irradiation 1. Introduction Restricted properties and limited use of homopolymers alone, has given rise to exploration of composites, copolymers, blends, etc. Copolymers such as poly(ethyleneco-vinyl acetate) (EVA), poly(ethylene-co-butyl acrylate), poly(ethylene-co-ethyl acrylate) (EEA) have wide range of usages in different industries. Among the numerous ethylene copolymers, due to its wide range of properties depending on its vinyl acetate content, EVA has become one of the most useful copolymers in the transportation industry as an insulator, in the electric industry as a cable insulator, in the shoe industry as soles, and in many other industries as a hot melt adhesive, a coating, etc. Several works looked at the influence of gamma rays on polymers. Black and Charlesby [1] performed one of the earliest studies on gamma irradiation of polymers. They showed Corresponding author. Tel.: ; fax: address: msen@hacettepe.edu.tr (M. Şen). the changes in chemical structure and physical properties of polyethylene upon irradiation with gamma or X-rays. Formation of crosslinks, main chain fracture, and unsaturation were among these changes. These experiments were done also in the presence of oxygen. In a study, carried out by Geuskens et al. [2], the influences of gamma rays and UV light on poly(vinyl acetate) (PVA) were investigated and compared. They estimated the products of radiolysis and photolysis of PVA. They also proposed some mechanisms for chemical changes undergoing during radiolysis and photolysis of PVA. Lacoste and Carlsson [3] undertook a study about gamma-, photo-, and thermally initiated oxidation of linear low density polyethylene, and compared the detailed oxidation products for different initialization types. They observed common oxidation products, such as hydroperoxide, ketone, vinyl and ester groups, for all types of initialization but with quantitative differences. A study on thermal degradation of electron beam cured EVA was performed by Dutta et al. [4] They explained the /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 M. Şen, M. Çopuroğlu/ Materials Chemistry and Physics 93 (2005) decomposition mechanisms and showed the effect of electron beam on the thermal stability of EVA. In order to investigate the effect of gamma rays on the thermal and mechanical stabilities EVA and EEA, Şen and Güven [5] carried out a comparative study and tried to find a correlation between the thermal and mechanical stabilities of copolymers. In our previous studies [6,7], we investigated the accelerated thermal and UV ageing characteristics of EVA and poly(ethylene-co-vinyl acetate)/carbon black mixture (EVA/CB), since EVA/CB, with 13% VA and 1% CB, is a widely 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 is susceptible to both heat and UV light; whereas EVA, containing 1% CB, is a durable material against heat, but vulnerable to UV light. In this comparative study; the effect of gamma rays on EVA (13% VA) and EVA/CB (13% VA and 1% CB) was investigated. It is thought to be important; because, any possible change in these materials chemical and physical properties could affect their industrial importance. It has been planned, as a further study, that whether gamma irradiation has an improving effect on the weathering characteristics of EVA and EVA/CB. 2. Experimental 2.1. Materials EVA, containing 13% VA and of density kg l 1, was supplied by Elf-Atochem Co. in the form of granules. EVA/CB plates with contents of 13% VA and 1% CB were obtained from Panel Co., Inc., Turkey. EVA, used in the preparation of this EVA/CB mixture, obtained from Elf- Atochem Co.; whereas masterbatch (PE Black 99209) from Viba Co., Italy, in the form of 50% dispersion of CB, type SRF, in LDPE. Xylene, used as a solvent for EVA and EVA/CB, for determination of gelation, was obtained from Merck. equation: % Gel = m m where m 0 and m are the masses of a sample before and after extraction, respectively FT-IR studies FT-IR studies were carried out by means of Nicolet 520 model spectrometer. Samples were pressed in a hotplate at about 100 C for 10 s in order to obtain film forms. Samples, crosslinked to a high extent, were scraped to get tiny particles and then were mixed with KBr Thermogravimetric analyses (TGA) Thermogravimetric analyses were performed by utilizing Du Pont Instruments-Thermal Analyzer, Model 951. Dynamic thermogravimetric studies were carried out under nitrogen atmosphere; and 10 C min 1 heating rate was used. Dynamic thermogravimetric study results indicated the thermal stabilities of virgin and irradiated samples. Isothermal thermogravimetric studies were performed at 350 C, and used to determine the percentages of volatile degradation products of virgin and irradiated samples. 3. Results and discussion 3.1. Sol gel analyses Before investigation of chemical changes in the structure of EVA and EVA/CB, the effect of gamma rays on the gelation of EVA and EVA/CB was investigated. The effect of gamma rays on the percentage gelations of EVA and EVA/CB is given in Fig. 1. Gelation in polymers is generally referred to as crosslinking of macromolecules by means of covalent bonds. Crosslinking might be formed most likely due to combinations of the macromolecular radicals formed during irradiation, as explained in the literature for polyolefins [8]. Since the main chains of EVA and EVA/CB macromolecules, 2.2. Irradiation of materials EVA granules, and EVA/CB specimens of dimensions 3.4 mm 3.9 mm with 1.9 mm thickness were irradiated with gamma rays at ambient conditions up to 400 kgy Analyses Determination of percentage gelations For investigation of the influence of gamma rays on the gelations of EVA and EVA/CB, sol gel analyses were performed. Xylene was used as a solvent in soxhlet extractor and was fluxed through each sample for 14 h. Gel percentages were calculated gravimetrically according to the following Fig. 1. Variation of percentage gelation with irradiation dose.

3 156 M. Şen, M. Çopuroğlu/ Materials Chemistry and Physics 93 (2005) used in this study, resemble those of polyethylene (PE), it shows similar behaviour to what PE does. An explanation proposed by Geuskens et al. [2] for PVA, might also be valid for EVA and EVA/CB. Fig. 1 clearly shows that the gelation occurred 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 gamma rays, which are capable to be absorbed even by C C and/or C H bonds. Thus, formed radicals lead to formation of high amount of crosslinks in both EVA and EVA/CB. % Gelation of EVA/CB, however, is more significant than that of EVA at all doses. This may be due to contribution of CB to the formation of crosslinks by using its oxygen-containing groups, under gamma irradiation. In order to determine the crosslink and the chain scission reaction yields occurring during irradiation of both EVA and EVA/CB, the usual Charlesby Pinner equation was used. Charlesby Pinner equation [9] (s + s = p o /q o + 2/(q o u 2,0 D)), has been used by many researchers so far for simultaneous determination of the crosslinking and the chain scission reaction yields of polymers being irradiated by ionizing radiation. In this equation, s is the sol fraction, p o is the chain scission yield, average number of main chain scissions per monomer unit and per unit dose, q o is crosslinking yield, proportion of monomer units crosslinked per unit dose, u 2,0 is initial weight, average degree of polymerization, and D is the irradiation dose. In this equation, u 2,0 was calculated as the ratio of weight average molecular weight (obtained from Elf-Atochem as 95,000 g mol 1 ) to average weight of a monomer unit (calculated as 30.8), and was found as Plots of the reciprocal of irradiation dose, 1/D versus s + s; yield straight lines (with a regression coefficient, r = for EVA and for EVA/CB) and p o /q o and q o were calculated from the intercept and the slope of the lines, respectively. As a result of calculations, p o and q o (per kgy) were found as 1.65 and 1.75, respectively, for EVA, and 4.92 and 5.80, respectively, for EVA/CB. Although the values for EVA/CB differ from those for EVA, when they are evaluated individually, one can conclude that the crosslink formation is more favorable for both EVA and EVA/CB than chain scission, since p o /q o = for EVA and for EVA/CB. Fig. 2. FT-IR spectra, in the range of cm 1, of virgin and irradiated 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 samples with KBr. An outstanding feature of these spectra is the rapid development of wide peaks at 1655 and 1630 cm 1, which can be attributed to the conjugate dienes. This interpretation is consistent with the explanation of very high rates of formation of radicals; because, after crosslinking (either intra or inter) of some radicals, other adjacent radicals which are sterically 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 near 1715 cm 1, appeared both in EVA and EVA/CB upon irradiation, can be attributed to the formation of ketone species. Further evaluation is not possible due to low signal to noise ratio FT-IR studies EVA and EVA/CB have almost the same spectra; because CB itself has no remarkable characteristic absorption bands and thus, does not influence the spectrum of EVA. The FT- IR spectra of virgin and gamma irradiated EVA and those of EVA/CB are given in Figs. 2 and 3, respectively, in the range of cm 1 ; because main functional group changes were observed in this region. Samples used in FT- IR measurements were obtained from the surface region of each granule or specimen, since chemical changes take place on the surface of the samples, to a greater extent. Spectra Fig. 3. FT-IR spectra, in the range of cm 1, of virgin and irradiated EVA/CB. Numbers on the curves represent irradiation dose (kgy).

4 M. Şen, M. Çopuroğlu/ Materials Chemistry and Physics 93 (2005) Table 1 Variation of the degradation activation energies of EVA and EVA/CB with irradiation Degradation activation energy (kj mol 1 ) 0 kgy 25 kgy 50 kgy 100 kgy 200 kgy 400 kgy EVA EVA/CB Fig. 4. Dynamic thermograms of irradiated EVA in nitrogen atmosphere Influence of gamma irradiation on the thermal stabilities of EVA and EVA/CB Figs. 4 and 5 show the dynamic TGA thermograms of virgin and irradiated EVA and EVA/CB, in nitrogen atmosphere, respectively. As shown in Figs. 4 and 5, virgin EVA and EVA/CB show the typical step degradation profile with the initial stage involving acetic acid evolution and the second involving main chain degradation [10,11]. Under high-energy radiation, radicals are formed at high concentrations in close proximity to one another so that second-order crosslinking reactions are favored compared with first-order chain scissions [8]. This explanation seems to be true in the case of EVA and EVA/CB, because typical thermal degradation route does not change significantly upon irradiation with gamma rays for both EVA and EVA/CB. It can be said that both chain scission and crosslinking take place simultaneously during irradiation of EVA and EVA/CB with gamma rays. These results are also consistent with crosslinking and chain scission yield values. Degradation activation energies of EVA and EVA/CB were calculated by using Freeman Carroll equation and the data obtained from derivatives of dynamic thermograms of gamma irradiated EVA and EVA/CB were used in the calculations [12]: log(d%/dt ) log(1 c) = n E 2.3R x (1/T) log(1 c) where T is the temperature in Kelvin, n is the order of reaction, R is the gas constant which has a value of J mol 1 K 1, E is the degradation activation energy. c is the conversion ratio and is equal to (m 0 m)/m 0, where m 0 is the initial mass and m 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 the degradation activation energy and the intersection of ordinate indicates the order of reaction. The c and T values were obtained from derivatives of thermograms. The effect of gamma irradiation on the degradation activation energies of EVA and EVA/CB is shown in Table 1. Consistent with the results of dynamic thermograms, degradation activation energies of EVA and EVA/CB do not change to remarkable extent until 400 kgy. According to these results, it can be said that the total effects of crosslinking, chain scission and oxidation make no difference in thermal stability. As can be seen from Figs. 4 and 5, due to overlapping of first and second degradation steps, it is not easy to interpret and determine the amounts of readily decomposing groups from dynamic TGA thermograms. For detailed analysis of the first stage of the degradation, isothermal TGA analyses were performed in nitrogen atmosphere at 350 C. This temperature is sufficient to break down all kinds of oxygen-containing groups. Isothermal thermograms of virgin and irradiated EVA and EVA/CB are given in Figs. 6 and 7, respectively. The Fig. 5. Dynamic thermograms of irradiated EVA/CB in nitrogen atmosphere. Fig. 6. Isothermal thermograms of irradiated EVA in nitrogen atmosphere.

5 158 M. Şen, M. Çopuroğlu/ Materials Chemistry and Physics 93 (2005) EVA/CB showed that, some chemical changes took place in EVA and EVA/CB during irradiation (such as the formation of diene, ketone, etc.). Dynamic thermograms, carried out in nitrogen atmosphere, indicated that the thermal stabilities of EVA and EVA/CB do not change during irradiation. Isothermal TGA analyses, performed in nitrogen atmosphere and at 350 C, showed that the concentration of volatile products increases rapidly at the initial stages of irradiation both in EVA and EVA/CB. As a conclusion, all these studies show that, oxidation, crosslinking and chain scission are the main consequences of gamma irradiation of EVA and EVA/CB. Moreover, the manner of changes is identical both in EVA and EVA/CB. Fig. 7. Isothermal thermograms of irradiated EVA/CB in nitrogen atmosphere. plateaus in these figures clearly indicate that all the oxygencontaining volatile groups are eliminated from polymer at the end of 70 min. As can be seen from these figures, there is a sharp increase in the amount of volatile products when EVA and EVA/CB were irradiated up to 25 kgy. After that dose, a relation between the irradiation dose and the amount of volatile products could not be obtained for EVA. On the other hand, after that sharp increase, a slight and continuous decrease was observed in EVA/CB with dose. Besides, the amount of these products is lower in EVA/CB than in EVA at each dose except 0 kgy. Like the results of FT-IR, the results of isothermal TGA analysis confirm the formation of oxidation both in EVA and EVA/CB, during gamma irradiation. 4. Conclusions In order to investigate the effect of gamma rays on EVA and EVA/CB, samples were subjected to gamma rays at ambient conditions up to 400 kgy. Sol gel analyses showed that both EVA and EVA/CB tend to crosslink when irradiated with gamma rays. FT-IR spectra of virgin and irradiated EVA and Acknowledgement The authors would like to thank Prof. Dr. Olgun Güven for the laboratory facilities he supplied and for his encouraging and constructive advice during this study. References [1] R.M. Black, A. Charlesby, The Int. J. Appl. Radiat. Isotopes 7 (1959) 127. [2] G. Geuskens, M. Borsu, C. David, Eur. Polym. J. 8 (1972) 883. [3] J. Lacoste, D.J. Carlsson, J. Polym. Sci. A: Polym. Chem. 30 (1992) 493. [4] S.K. Dutta, A.K. Bhowmick, P.G. Mukunda, T.K. Chaki, Polym. Degrad. Stability 50 (1995) 75. [5] M. Şen, O. Güven, Radiat. Phys. Chem. 46 (1995) 871. [6] M. Çopuroğlu, M. Şen, Polym. Adv. Technol. 15 (2004) 393. [7] M. Çopuroğlu, M. Şen, Polym. Adv. Technol. 16 (2005) 61. [8] N. Grassie, G. Scott, Polymer Degradation and Stabilisation, Cambridge University Press, Cambridge, [9] A. Charlesby, Atomic Radiation and Polymers, WNT, Warsaw, [10] N.S. Allen, M. Edge, M. Rodriguez, C.M. Liauw, E. Fontan, Polym. Degrad. Stability 71 (2001) 1. [11] B.J. McGrattan, Appl. Spectrosc. 48 (12) (1994) [12] E.S. Freeman, B. Carroll, J. Phys. Chem. 62 (1958) 394.