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Stabilization effects of naringenin and caffeic acid on γ-irradiatedEPDM
Radiation Physics and Chemistry 84 (2013) 35–38
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Stabilization effects of naringenin and caffeic acid on g-irradiated EPDM T. Zaharescu a,n, S. Jipa b, A. Mantsch a, D. Henderson c a b c
Institute for Electrical Engineering (INCDIE, ICPE CA), 313 Splaiul Unirii, PO Box 149, Bucharest 030138, Romania ‘‘Valachia’’ University, Faculty of Sciences, 18-22 Unirii Av., Tˆ argovis- te 130082, Romania Trinity College, Hartford, CT 06106, USA
a r t i c l e i n f o
abstract
Article history: Received 1 January 2011 Accepted 1 January 2012 Available online 6 July 2012
The stabilization of ethylene-propylene diene rubber (EPDM) with naringenin and caffeic acid is studied. The selected concentrations were 0.25, 0.50 and 1 phr. The degradation was performed by g–irradiation. The protective effect of these antioxidants was investigated by isothermal chemiluminescence at 170 1C and FTIR spectroscopy. The synergetic action of these compounds and metallic selenium was also revealed. The exceptional contribution provided by these phenolic stabilizers is characterized by three kinetic parameters: initial CL intensity, oxidation induction time and maximum period of degradation. The radiation stability of stabilized EPDM is efficiently depicted by induction periods which are the minimum 6 times longer for unirradiated samples and 2–50 times longer for 50 kGy-irradiated specimens than pristine EPDM. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Stabilization EPDM Naringenin Caffeic acid Selenium
1. Introduction The durability of materials is a problem of the most importance which characterizes the product life time. The practical procedure for the improvement in polymer stability is the addition of suitable compounds with antioxidant properties. The chemical industry produces several types of oxidation inhibitors, the most amount being covered by phenolic structures. There were performed several studies on the oxidation protection of synthesis phenols, whose stabilization mechanism leads to quinone structures (Pospı´sˇ il and Neˇspu˚rek, 1995). Because this kind of intermediates is dangerous for people health, the best alternatives are the natural extracts and their purified components. The prevention of oxidation in polymers can be efficiently ensured by natural products. It was previously investigated (Kim et al., 2008; Popa et al., 2010; Zaharescu et al., 2010) because ionizing radiation causes oxidative stress. The modifications induced by ionizing radiation in various components of natural extracts were studied for the explanation of structural changes in these compounds (Breitfellner et al., 2003; Soriani et al., 2005; Nagy et al., 2008; Jipa et al., 2009). The correlation between the antioxidant efficiency and the stabilizer structure is essential for the selection of natural extracts because of the involvement in environmental durability,
n Corresponding author at: Institute for Electrical Engineering (INCDIE, ICPE CA), 313 Splaiul Unirii, PO Box 149, Bucharest 030138, Romania. Tel.: þ 40 21 346 7235; fax: þ 40 21 3468299. E-mail address: [email protected] (T. Zaharescu).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.06.050
especially in biological surroundings. The inhibition of radiation degradation represents a great benefit of human health protection (Prasad et al, 2004; Maris- et al., 2010) and in the oxidation of polymer materials (Ho et al., 2000; Jipa et al., 2005; Zaharescu et al., 2009). Numerous studies have concerned on the radiation protection induced by natural extracts (Emerit et al., 1995; Kyo et al., 2001; Goel, 2002; Kumar et al., 2003; Samarth and Kumar, 2003; Jipa et al., 2004; Cr˘aciunescu et al., 2007), which proves the efficiency of plant extracts on the radical scavenging in the cases of the fast generation of free radicals, like the irradiation with high energy beams. Starting from the encouraging results obtained for the thermal protection activity provided by naringenin (Maris- et al., 2008) and caffeic acid (Jipa et al., 2008), this paper presents the tremendous stabilization effect of two flavonoides: naringenine and caffeic acid, which are the main components of natural extracts from herbs and spices. This investigation is a proposition for the use of these natural stabilizers for manufacture of ecological products as faithful alternative to the stabilization of polymers with synthesis compounds.
2. Experimental Ethylene-propylene-diene terpolymer (TERPIT Cs produced by ARPECHIM, Romania) having 3.5% ethylidene norbornene was studied as received. After dissolution of a certain mass of polymer in CHCl3 (pro analysis), the supernatant was separated by filtration. Then, the purified polymer material was precipitated by pouring cold ethanol into the pristine solution.
36
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
The reagent powders of naringenin (Sigma-Aldrich, USA), caffeic acid (Sigma-Aldrich, USA) and metallic selenium (Merck, Germany) were separately added to EPDM solution under vigorous homogenization according with the suitable amount for selected concentrations, the homogeneity being controlled by the previous chemiluminescence investigation of oxidative degradation on three similar samples. LDPE samples containing three concentrations of stabilizers (0.25, 0.50 and 1% w/w) were prepared. Additionally, other two stabilized EPDM systems were obtained by simultaneous adding metallic selenium and antioxidant at the concentration of 0.25%. For references, pristine elastomer was also investigated. The molecular structures of additives are illustrated in Fig. 1. The exposure to g–radiation was carried out in a 137Cs GAMMATOR M-38-2 installation. The dose rate was 0.4 kGy/h. The radiation treatment was performed at room temperature. The determination of thermal stability for neat and modified polyethylene samples was run applying isothermal chemiluminescence procedure (Jipa et al., 1999). Al these experiments were carried out in air as oxidant environment at 170 1C. The evaluation of oxidation state of polymer samples was carried out by FTIR spectroscopy (JASCO spectrometer—Japan, model 4200) on films of 100 mm.
3. Results and discussion
weakness determines the oxidation prevention activity and the performances of materials in which they are added. The accelerated oxidative degradation in polyolefins caused by ionizing radiation starts with the accumulation of primary hydrocarbon radicals which are converted into oxygenated products as the result of autocatalytic process. The increase in the concentration of various oxygenated structures depends on the irradiation dose and the radiation resistance of polymer formulation ˇ ´ erov et al., 2008). The detailed (Zaharescu et al., 1999; Sec mechanism of the oxidative degradation of ethylene-propylene diene terpolymer was reported (Zaharescu et al., 1999; Gamlin, et al., 2003; Rivaton et al., 2005). The analysis of FTIR spectra recorded for studied EPDM formulations reveals the stabilization potential of naringenin and caffeic acid over a large range of low and medium dose (Fig. 2). At 150 kGy, when g–radiation affects significantly polymer matrix, additives are also degraded and the transmission values attain relative similar values. However, it can be noted that these natural antioxidants are capable to delay oxidation during irradiation for doses exceeding sterilization doses. In Fig. 3 the evident action of additives is pointed out by the blocking oxidation process over very long period in comparison with pristine material. Their efficiency is remarkable proved by the extended oxidation induction times. For the additive concentration of 0.25%, the thermal stability of ethylene-propylene diene terpolymer characterized by oxidation induction time is 6.5 and 8.5 times higher for EPDM/naringenin and, EPDM/caffeic acid, respectively in comparison with free-additive material.
The addition of naringenin and caffeic acid is a proper manner for the improvement of material durability provided by unhazardous compounds. The polyhenolic structure of flavonoides is the basis for their antioxidant activity, which explains the beneficial consequences of oxidative degradation inhibition. The ability of radical scavenging exhibited by natural antioxidants is provided by the mobility of phenolic protons, which are related to the positions of C5, C6 and C7 (Maris- et al., 2008). Their bond
Fig. 1. Molecular structures of used additives.
Fig. 3. Chemiluminescence curves of neat EPDM in air at 170 1C.
Fig. 2. FTIR spectra of irradiated EPDM. (a) pristine; (b) modified with naringenin 0.25%; (c) modified with caffeic acid 0.25%.
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
It was early reported (Davenas et al., 2003) that ethylenepropylene diene terpolymer is sensitive to the action of high energy radiation. This polymer free of any antioxidant exhibits a satisfactory thermal resistance when it is not irradiated (Fig. 4). By g–exposure the thermal resistance of this material is diminished, because the presence of diene causes the easy attack of oxygen followed by the accelerated formation of peroxyl radicals that initiate the autocatalytic degradation of elastomers. The effect of some commercial phenolic antioxidants on the oxidative
Fig. 4. The time dependency of CL intensity for unirradiated EPDM under various state of modification at 170 1C in air. (square) pristine EPDM; (circle) Se; (rhombus) naringenin; (triungle) caffeic acid. Additive concentration: 0.25% (w/w).
37
protection of EPDM was reported (Zaharescu, et al., 1999) according with their efficiency in the degradation of EPDM. The radiation degradation of g–irradiated EPDM samples is delayed by the two selected flavonoids. Fig. 5 illustrates the noticeable contribution of naringenin to the increased stabilization of g–irradiated EPDM. The synthetic picture of the stability improvement brought about by naringenin and caffeic acid can be found presented in Table 1 The simultaneous addition of metallic selenium increases further the thermal and radiation stability of elastomers substrate. If unirradiated samples exhibit the values of induction oxidation time as the simple sum of inductions corresponding to individual additives, irradiated samples containing the pair of additives (naringeninþSe) and (caffeic acidþ Se) present synergistic effect (Fig. 6). The stabilization of g–irradiated EPDM is also depicted by the other calculated parameters: initial CL emission, I0 and maximum oxidation times, tmax, which vary according with the level of stabilization. The oxidation rate decreases as the concentration of additive enhances. If naringenin-modified and irradiated EPDM samples present sharp increase in CL emission intensity, caffeic acid-modified and irradiated EPDM samples present smoother augmentation in CL intensity. These features are related to structural characteristics, which prove the higher radiation stability of naringenin relative to caffeic acid. The efficiency of metallic selenium in the oxidation prevention of EPDM is placed between the efficiencies of naringenin and caffeic acid as it was demonstrated earlier (Jipa et al., 2000, 2009). The prominent long periods of stabilization determined at the oxidation of EPDM in the presence of 1% naringenin, caffeic acid and selenium certify the possibility of their addition in other polyolefins which are used in various applications involving human handling, especially in the manufacture of toys, ecological packaging, commodities.
4. Conclusion
Fig. 5. CL curves recorded for EPDM stabilized with different concentrations of naringenin.
The protection to degradation of g-irradiated ethylene-propylene diene terpolymer promoted by naringenin, caffeic acid and metallic selenium is obtained even at high exposure dose (100 kGy). The polyphenolic antioxidants obtained from plants are suitable alternative for delaying the start of oxidation, being demonstrated that their presence extends the oxidation times of several orders of magnitude. Even though these organic structures are susceptible to degradation, their intermediates sustain further protection, so that material preserves its stability for long time. The synergistic effect of naringenin/Se and caffeic acid/Se couples
Table 1 Some kinetic parameters calculated for EPDM 1inder different states of stabilization. Sample
EPDM control 0.25% 0.50% EPDM þNaringenin 1.00% 0.25% þ0.25% Se 0.25% 0.50% EPDM þCaffeic acid 1.00% 0.25% þ0.25% Se 0.25% EPDM þSe 0.50% 1.00%
0 kGy
50 kGy
100 kGy
I0 106 (Hz/g)
ti (min)
tmax (min)
Vox (Hz/g min)
I0106 (Hz/g)
ti (min)
tmax (min)
Vox (Hz/g min)
I0 106 (Hz/g)
2.50 0.35 0.61 0.63
196 1248 1762 2600
298 1319 2132 3125
424286 421277 108152 48667
10.72 17.56 19.45 22.78
5 55 236 280
26 397 565 984
1041429 237073 136129 40256
0.59
1933
2233
72526
21.16
2098
0.69 0.90 1.09
1673 2051 2304
1814 2159 2667
153696 170952 172615
8.01 4.03 3.62
10 789 1381
2817 119 912 1760
0.51
2546
2650
177400
5.67
785
0.67 0.59 0.50
862 1410 2075
1076 1572 2148
236486 163137 177111
4.02 2.16 0.44
400 1179 1919
1104 445 1515 3251
ti (min)
tmax (min)
Vox (Hz/g min)
34.33 37.17 27.04 23.35
2 5 138 205
41 56 485 975
8576
37.30
676
1519
27857
298125 151250 102500
28.59 21.35 31.51
8 503 865
35 966 1329
623333 66521 168491
147073
38.86
586
733
173871
976667 665512 5817
30.53 31.34 12.02
141 803 2599
872 2258 4994
36111 5995 1692
1032000 360000 110606 35026
38
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
Fig. 6. Modification CL emission during thermal degradation of EPDM stabilized with naringenin þSe (square) and caffeic acid þ Se (circle).
was pointed out at 50 and 100 kGy. It demonstrates the cooperative activity of natural extracts and metallic selenium in the delaying of oxidation and in the diminishing of oxidation rate on the propagation stage. References Breitfellner, F., Solar, S., Sontag, G., 2003. Radiation induced chemical changes of phenolic compounds in strawberries. Radiat. Phys. Chem. 67, 497–499. Cr˘aciunescu, O., S- tefan, L.M., B˘alan, M., Moldovan, L., 2007. Evaluation of the radioprotective activity of new green tea-collagen materials. J. Optoelectr. Adv. Mater. 9, 2602–2607. Davenas, J., Stevenson, I., Celette, N., Vigier, G., David, L., 2003. Influence of the molecular modifications on the properties of EPDM elastomers under irradiation. Nucl. Instrum. Meth. Phys. Res. B 208, 461–465. Emerit, I., Oganesian, N., Sarkisian, T., Arutyunyan, R., Pogosian, A., Asrian, K., Levy, A., Cernjavski, L., 1995. Clastogenic factors in the plasma of Chernobyl accident recovery workers: anticlastogenic effect of Ginkgo biloba extract. Radiat. Res. 144, 198–205. Gamlin, C.D., Dutta, N.K., Choudhury, N.R., 2003. Mechanism and kinetics of the isothermal thermodegradation of ethylene-propylene-diene (EPDM) elastomers. Polym. Degrad. Stab. 80, 525–531. Goel, H.C., Prasad, J., Singh, S., Sagar, R.K., Kumar, P.I., Sinha, A.K., 2002. Radioprotection by a herbal preparation of Hippophae rhamnoides, RH-3, against whole body lethal irradiation in mice. Phytomedicine 9, 15–25. Ho, C.T., Wang, M., Wei, G.J., Huang, T.C., Huang, M.T., 2000. Chemistry and antioxidative factors in rosemary and sage. Biofactors 13, 161–166. Jipa, S., Setnescu, R., Setnescu, T., Zaharescu, T., 2000. Efficiency assessment of additives in thermal degradation of i-PP by chemiluminescence. Part II. Selenium. Polym. Degrad. Stabil. 68, 165–169. Jipa, S., Zaharescu, T., Gorghiu, L.M., Dumitrescu, C., Setnescu, R., Brites, M.J.S., Esteves, M.A., Gigante, B., 2004. LDPE protected by secondary amines of dehydroabietic methyl ester derivatives. Polym. Testing 23, 911–917.
Jipa, S., Zaharescu, T., Kappel, W., Dumitrescu, C., Maris-, A.M., Mantsch, A., Lungulescu, M., 2008. Scavenger capacitz of natural phenols in some selected labiatae herbs. Optoelectr. Adv. Mater. Rapid Commun. 2, 669–673. Jipa, S., Zaharescu, T., Kappel, W., D˘anet¸, A.F., Popa, C.V., Bumbac, M., Gorghiu, L.M., Maris-, A.M., 2009. The effects of g-irradiation on the antioxidant activity of rosemary extract. Optoelectr. Adv. Mater. Rapid Commun. 3, 1315–1320. Jipa, S., Zaharescu, T., Setnescu, R., Gorghiu, L.M., Dumitrescu, C., Santos, C., Silva, A.M.G., Gigante, B., 2005. Kinetic approach on stabilization of LDPE in the presence of carnosic acid and related compounds. I. Thermal investigation. J. Appl. Polym Sci. 95, 1571–1577. Jipa, S., Zaharescu, T., Setnescu, R., Setnescu, T., Brites, M.J.S., Silva, A.M.G., Marcelo-Curto, A.J., Gigante, B., 1999. Chemiluminescence study of thermal and photostability of polyethylene. Polym. Int. 48, 414–420. Kim, J.-W., Lee, B.C., Lee, J.-H., Nam, K.-C., Lee, S.-C., 2008. Effect of electron-beam irradiation on the antioxidant activity of extracts from Citrus unshiu pomaces. Radiat. Phys. Chem. 77, 87–91. Kumar, M., Sharma, M.K., Saxena, P.S., Kumar, A., 2003. Radioprotective Effect of Panax ginseng on the phosphatases and lipid peroxidation level in testes of Swiss albino Mice. Biol. Pharm. Bull. 26, 308–312. Kyo, E., Uda, N., Kasuga, S., Itakura, Y., 2001. Immunomodulatory effects of aged garlic extract. J. Nutr. 131, 1075S–1079S. Maris-, M., Maris-, D.A., Jipa, S., Zaharescu, T., Gorghiu, L.M., 2010. Radio-protective potential of rosemary (Rosmarinus officinalis) against effects of ionizing radiation. Rev. Chim. 61, 235–237. Maris-, M., Maris-, D.A., Jipa, S., Zaharescu, T., Kappel, W., Mantsch, A., Lungulescu, M., 2008. Estimation of the antioxidant activity of flavonoids by isothermal chemiluminescence method. J. Optoelectr. Adv. Mater. 10, 3364–3367. Nagy, T.O., Ledolter, K., Solar, S., 2008. Oxidation of naringenin by gammaradiation. Radiat. Phys. Chem. 77, 728–733. Popa, H.C., Jipa, S., Zaharescu, T., Kefalas, P., 2010. Kinetic approach of natural antioxidant depletion during thermal oxidation. Optoelectr. Adv. Mater., Rapid Commun. 4, 223–227. Pospı´sˇ il, J., Neˇspu˚rek, S., 1995. Chain-breaking stabilizers in polymers: the current status. Polym. Degrad. Stab. 49, 99–110. Prasad, K.N., Cole, W.C., Haase, G.M., 2004. Radiation protection in humans: extending the concept of as low as reasonably achievable (ALARA) from dose to biological damage. Brit. J. Radiol. 77, 97–99. Rivaton, A., Gardette, J.-L., Mailhot, B., Morlat-Therlas, S., 2005. Basic aspects of polymer degradation. Macromol. Symp. 225, 129–146. Samarth, R.M., Kumar, A., 2003. Radioprotection of Swiss albino mice by plant extract Mentha piperita (Linn.). J. Radiat. Res. 44, 101–109. Soriani, R.R., Satomi, L.C., Pinto, T.A., 2005. Effects of ionizing radiation in ginkgo and guarana. Radiat. Phys. Chem. 73, 239–242. ˇ ´ erov, B., Marinovic´-Cincovic´, M., Popovic´, S., Nedic´, Z., Popovic´ Kacarevic´, Z., Sec 2008. Characterization of gamma irradiated ethylene–norbornene copolymer using FTIR, UV–vis and DSC techniques. Polym. Bull. 60, 313–322. Zaharescu, T.M., Giurginca, G., Jipa, S., 1999. Radiochemical oxidation of ethylenepropylene elastomers in the presence of some phenolic antioxidants. Polym. Degrad. Stab. 63, 245–251. Zaharescu, T., Jipa, S., Maris-, D.A., Maris-, M., Kappel, W., 2009. Effects of rosemary extract on the radiation stability of UHMWPE. e-Polymers, no. 149. Zaharescu, T., Jipa, S., Henderson, D., Kappel, W., Maris-, D.A., Maris-, M., 2010. Thermal and radiation resistance of stabilized LDPE. Radiat. Phys. Chem. 79, 375–378.
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Stabilization effects of naringenin and caffeic acid on g-irradiated EPDM T. Zaharescu a,n, S. Jipa b, A. Mantsch a, D. Henderson c a b c
Institute for Electrical Engineering (INCDIE, ICPE CA), 313 Splaiul Unirii, PO Box 149, Bucharest 030138, Romania ‘‘Valachia’’ University, Faculty of Sciences, 18-22 Unirii Av., Tˆ argovis- te 130082, Romania Trinity College, Hartford, CT 06106, USA
a r t i c l e i n f o
abstract
Article history: Received 1 January 2011 Accepted 1 January 2012 Available online 6 July 2012
The stabilization of ethylene-propylene diene rubber (EPDM) with naringenin and caffeic acid is studied. The selected concentrations were 0.25, 0.50 and 1 phr. The degradation was performed by g–irradiation. The protective effect of these antioxidants was investigated by isothermal chemiluminescence at 170 1C and FTIR spectroscopy. The synergetic action of these compounds and metallic selenium was also revealed. The exceptional contribution provided by these phenolic stabilizers is characterized by three kinetic parameters: initial CL intensity, oxidation induction time and maximum period of degradation. The radiation stability of stabilized EPDM is efficiently depicted by induction periods which are the minimum 6 times longer for unirradiated samples and 2–50 times longer for 50 kGy-irradiated specimens than pristine EPDM. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Stabilization EPDM Naringenin Caffeic acid Selenium
1. Introduction The durability of materials is a problem of the most importance which characterizes the product life time. The practical procedure for the improvement in polymer stability is the addition of suitable compounds with antioxidant properties. The chemical industry produces several types of oxidation inhibitors, the most amount being covered by phenolic structures. There were performed several studies on the oxidation protection of synthesis phenols, whose stabilization mechanism leads to quinone structures (Pospı´sˇ il and Neˇspu˚rek, 1995). Because this kind of intermediates is dangerous for people health, the best alternatives are the natural extracts and their purified components. The prevention of oxidation in polymers can be efficiently ensured by natural products. It was previously investigated (Kim et al., 2008; Popa et al., 2010; Zaharescu et al., 2010) because ionizing radiation causes oxidative stress. The modifications induced by ionizing radiation in various components of natural extracts were studied for the explanation of structural changes in these compounds (Breitfellner et al., 2003; Soriani et al., 2005; Nagy et al., 2008; Jipa et al., 2009). The correlation between the antioxidant efficiency and the stabilizer structure is essential for the selection of natural extracts because of the involvement in environmental durability,
n Corresponding author at: Institute for Electrical Engineering (INCDIE, ICPE CA), 313 Splaiul Unirii, PO Box 149, Bucharest 030138, Romania. Tel.: þ 40 21 346 7235; fax: þ 40 21 3468299. E-mail address: [email protected] (T. Zaharescu).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.06.050
especially in biological surroundings. The inhibition of radiation degradation represents a great benefit of human health protection (Prasad et al, 2004; Maris- et al., 2010) and in the oxidation of polymer materials (Ho et al., 2000; Jipa et al., 2005; Zaharescu et al., 2009). Numerous studies have concerned on the radiation protection induced by natural extracts (Emerit et al., 1995; Kyo et al., 2001; Goel, 2002; Kumar et al., 2003; Samarth and Kumar, 2003; Jipa et al., 2004; Cr˘aciunescu et al., 2007), which proves the efficiency of plant extracts on the radical scavenging in the cases of the fast generation of free radicals, like the irradiation with high energy beams. Starting from the encouraging results obtained for the thermal protection activity provided by naringenin (Maris- et al., 2008) and caffeic acid (Jipa et al., 2008), this paper presents the tremendous stabilization effect of two flavonoides: naringenine and caffeic acid, which are the main components of natural extracts from herbs and spices. This investigation is a proposition for the use of these natural stabilizers for manufacture of ecological products as faithful alternative to the stabilization of polymers with synthesis compounds.
2. Experimental Ethylene-propylene-diene terpolymer (TERPIT Cs produced by ARPECHIM, Romania) having 3.5% ethylidene norbornene was studied as received. After dissolution of a certain mass of polymer in CHCl3 (pro analysis), the supernatant was separated by filtration. Then, the purified polymer material was precipitated by pouring cold ethanol into the pristine solution.
36
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
The reagent powders of naringenin (Sigma-Aldrich, USA), caffeic acid (Sigma-Aldrich, USA) and metallic selenium (Merck, Germany) were separately added to EPDM solution under vigorous homogenization according with the suitable amount for selected concentrations, the homogeneity being controlled by the previous chemiluminescence investigation of oxidative degradation on three similar samples. LDPE samples containing three concentrations of stabilizers (0.25, 0.50 and 1% w/w) were prepared. Additionally, other two stabilized EPDM systems were obtained by simultaneous adding metallic selenium and antioxidant at the concentration of 0.25%. For references, pristine elastomer was also investigated. The molecular structures of additives are illustrated in Fig. 1. The exposure to g–radiation was carried out in a 137Cs GAMMATOR M-38-2 installation. The dose rate was 0.4 kGy/h. The radiation treatment was performed at room temperature. The determination of thermal stability for neat and modified polyethylene samples was run applying isothermal chemiluminescence procedure (Jipa et al., 1999). Al these experiments were carried out in air as oxidant environment at 170 1C. The evaluation of oxidation state of polymer samples was carried out by FTIR spectroscopy (JASCO spectrometer—Japan, model 4200) on films of 100 mm.
3. Results and discussion
weakness determines the oxidation prevention activity and the performances of materials in which they are added. The accelerated oxidative degradation in polyolefins caused by ionizing radiation starts with the accumulation of primary hydrocarbon radicals which are converted into oxygenated products as the result of autocatalytic process. The increase in the concentration of various oxygenated structures depends on the irradiation dose and the radiation resistance of polymer formulation ˇ ´ erov et al., 2008). The detailed (Zaharescu et al., 1999; Sec mechanism of the oxidative degradation of ethylene-propylene diene terpolymer was reported (Zaharescu et al., 1999; Gamlin, et al., 2003; Rivaton et al., 2005). The analysis of FTIR spectra recorded for studied EPDM formulations reveals the stabilization potential of naringenin and caffeic acid over a large range of low and medium dose (Fig. 2). At 150 kGy, when g–radiation affects significantly polymer matrix, additives are also degraded and the transmission values attain relative similar values. However, it can be noted that these natural antioxidants are capable to delay oxidation during irradiation for doses exceeding sterilization doses. In Fig. 3 the evident action of additives is pointed out by the blocking oxidation process over very long period in comparison with pristine material. Their efficiency is remarkable proved by the extended oxidation induction times. For the additive concentration of 0.25%, the thermal stability of ethylene-propylene diene terpolymer characterized by oxidation induction time is 6.5 and 8.5 times higher for EPDM/naringenin and, EPDM/caffeic acid, respectively in comparison with free-additive material.
The addition of naringenin and caffeic acid is a proper manner for the improvement of material durability provided by unhazardous compounds. The polyhenolic structure of flavonoides is the basis for their antioxidant activity, which explains the beneficial consequences of oxidative degradation inhibition. The ability of radical scavenging exhibited by natural antioxidants is provided by the mobility of phenolic protons, which are related to the positions of C5, C6 and C7 (Maris- et al., 2008). Their bond
Fig. 1. Molecular structures of used additives.
Fig. 3. Chemiluminescence curves of neat EPDM in air at 170 1C.
Fig. 2. FTIR spectra of irradiated EPDM. (a) pristine; (b) modified with naringenin 0.25%; (c) modified with caffeic acid 0.25%.
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
It was early reported (Davenas et al., 2003) that ethylenepropylene diene terpolymer is sensitive to the action of high energy radiation. This polymer free of any antioxidant exhibits a satisfactory thermal resistance when it is not irradiated (Fig. 4). By g–exposure the thermal resistance of this material is diminished, because the presence of diene causes the easy attack of oxygen followed by the accelerated formation of peroxyl radicals that initiate the autocatalytic degradation of elastomers. The effect of some commercial phenolic antioxidants on the oxidative
Fig. 4. The time dependency of CL intensity for unirradiated EPDM under various state of modification at 170 1C in air. (square) pristine EPDM; (circle) Se; (rhombus) naringenin; (triungle) caffeic acid. Additive concentration: 0.25% (w/w).
37
protection of EPDM was reported (Zaharescu, et al., 1999) according with their efficiency in the degradation of EPDM. The radiation degradation of g–irradiated EPDM samples is delayed by the two selected flavonoids. Fig. 5 illustrates the noticeable contribution of naringenin to the increased stabilization of g–irradiated EPDM. The synthetic picture of the stability improvement brought about by naringenin and caffeic acid can be found presented in Table 1 The simultaneous addition of metallic selenium increases further the thermal and radiation stability of elastomers substrate. If unirradiated samples exhibit the values of induction oxidation time as the simple sum of inductions corresponding to individual additives, irradiated samples containing the pair of additives (naringeninþSe) and (caffeic acidþ Se) present synergistic effect (Fig. 6). The stabilization of g–irradiated EPDM is also depicted by the other calculated parameters: initial CL emission, I0 and maximum oxidation times, tmax, which vary according with the level of stabilization. The oxidation rate decreases as the concentration of additive enhances. If naringenin-modified and irradiated EPDM samples present sharp increase in CL emission intensity, caffeic acid-modified and irradiated EPDM samples present smoother augmentation in CL intensity. These features are related to structural characteristics, which prove the higher radiation stability of naringenin relative to caffeic acid. The efficiency of metallic selenium in the oxidation prevention of EPDM is placed between the efficiencies of naringenin and caffeic acid as it was demonstrated earlier (Jipa et al., 2000, 2009). The prominent long periods of stabilization determined at the oxidation of EPDM in the presence of 1% naringenin, caffeic acid and selenium certify the possibility of their addition in other polyolefins which are used in various applications involving human handling, especially in the manufacture of toys, ecological packaging, commodities.
4. Conclusion
Fig. 5. CL curves recorded for EPDM stabilized with different concentrations of naringenin.
The protection to degradation of g-irradiated ethylene-propylene diene terpolymer promoted by naringenin, caffeic acid and metallic selenium is obtained even at high exposure dose (100 kGy). The polyphenolic antioxidants obtained from plants are suitable alternative for delaying the start of oxidation, being demonstrated that their presence extends the oxidation times of several orders of magnitude. Even though these organic structures are susceptible to degradation, their intermediates sustain further protection, so that material preserves its stability for long time. The synergistic effect of naringenin/Se and caffeic acid/Se couples
Table 1 Some kinetic parameters calculated for EPDM 1inder different states of stabilization. Sample
EPDM control 0.25% 0.50% EPDM þNaringenin 1.00% 0.25% þ0.25% Se 0.25% 0.50% EPDM þCaffeic acid 1.00% 0.25% þ0.25% Se 0.25% EPDM þSe 0.50% 1.00%
0 kGy
50 kGy
100 kGy
I0 106 (Hz/g)
ti (min)
tmax (min)
Vox (Hz/g min)
I0106 (Hz/g)
ti (min)
tmax (min)
Vox (Hz/g min)
I0 106 (Hz/g)
2.50 0.35 0.61 0.63
196 1248 1762 2600
298 1319 2132 3125
424286 421277 108152 48667
10.72 17.56 19.45 22.78
5 55 236 280
26 397 565 984
1041429 237073 136129 40256
0.59
1933
2233
72526
21.16
2098
0.69 0.90 1.09
1673 2051 2304
1814 2159 2667
153696 170952 172615
8.01 4.03 3.62
10 789 1381
2817 119 912 1760
0.51
2546
2650
177400
5.67
785
0.67 0.59 0.50
862 1410 2075
1076 1572 2148
236486 163137 177111
4.02 2.16 0.44
400 1179 1919
1104 445 1515 3251
ti (min)
tmax (min)
Vox (Hz/g min)
34.33 37.17 27.04 23.35
2 5 138 205
41 56 485 975
8576
37.30
676
1519
27857
298125 151250 102500
28.59 21.35 31.51
8 503 865
35 966 1329
623333 66521 168491
147073
38.86
586
733
173871
976667 665512 5817
30.53 31.34 12.02
141 803 2599
872 2258 4994
36111 5995 1692
1032000 360000 110606 35026
38
T. Zaharescu et al. / Radiation Physics and Chemistry 84 (2013) 35–38
Fig. 6. Modification CL emission during thermal degradation of EPDM stabilized with naringenin þSe (square) and caffeic acid þ Se (circle).
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