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Influence of high doses γ-irradiation on oxygen permeability of linear low-density polyethylene and cast polypropylene films
Radiation Physics and Chemistry 97 (2014) 304–312
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Influence of high doses γ-irradiation on oxygen permeability of linear low-density polyethylene and cast polypropylene films Damir Klepac a, Mario Ščetar b, Goran Baranović c, Kata Galić b, Srećko Valić a,c,n a
Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Braće Branchetta 20, HR-51000 Rijeka, Croatia Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia c Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia b
H I G H L I G H T S
High doses γ-irradiation reduces oxygen permeability in PE and PP films. Increase of crystallinity and decrease of melting point in PP film induced by radiation. Radiation induced radical formation in PE and PP and their decay monitored by ESR.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 October 2013 Accepted 5 December 2013 Available online 17 December 2013
Linear low density polyethylene (PE-LLD) and cast polypropylene (PPcast) films were irradiated in a 60 Co γ-source. The total irradiation dose varied from 0 kGy (unirradiated samples) to 200 kGy. Oxygen transport was investigated by a manometric method and the structural changes were studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Free radicals decay as a function of time was monitored by electron spin resonance (ESR) spectroscopy. The results show that the γ-irradiation reduces oxygen permeability coefficient in both films. The reduction was associated with an increase in crystallinity. DSC thermograms revealed a decrease in PPcast melting point with increasing irradiation dose, indicating higher degradation compared to PE-LLD. The observed peak in FTIR spectra for both samples at 1716 cm 1 corresponds to the stretching of the carbonyl and carboxylic groups which arise from the reaction of oxygen with the free radicals produced in the polymer matrix as a result of irradiation. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Polypropylene Gamma-irradiation Oxygen permeability ESR FTIR
1. Introduction Linear low-density polyethylene (PE-LLD) and cast polypropylene (PPcast) are the most widely used polymeric materials in the food packaging industry, mainly because of their special physical properties such as high barrier resistance against water vapour, high tear strength and toughness, excellent environmental stress cracking resistance and improved processability compared to conventional polyethylene and polypropylene (Galić et al., 2011; Guillard et al., 2010). However, these films also have some disadvantages. Their intrinsic properties can be affected by an application of relatively small external mechanical force or by high energy irradiation. The deformation can occur during transportation and handling of food products, while an effect of high energy radiation n Corresponding author at: Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Braće Branchetta 20 HR-51000 Rijeka Croatia. Tel.: þ 385 51 651134; fax: þ 385 51 678895. E-mail address: [email protected] (S. Valić).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.12.005
is encountered during food sterilization process. An influence of uniaxial deformation applied on PE-LLD film was investigated recently (Klepac et al., 2013). Although one can imagine that the effect of deformation could increase the gas permeability, previous results show an opposite effect (Compañ et al., 1996; Holden et al., 1985; Klepac et al., 2013; Villaluenga and Seoane, 1998). In fact, it was found that the oxygen permeability (P) and diffusion (D) coefficients of PE-LLD film decrease after an application of uniaxial deformation (Klepac et al., 2013). This study is focused on the changes in structural and barrier properties of PE-LLD and PPcast films provoked by γ-irradiation. Previous investigations of PE-LD and PP films exposed to high energy radiation did not show any significant difference in the permeability of CO2 and O2 (Goulas et al., 2002, 2003). Moreover, no important changes in the structure of investigated materials were reported. These investigations were performed using relatively low radiation doses (up to 10 kGy), since such doses are usually applied during the food sterilization. However, certain countries permit higher doses for the sterilization of some spices,
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seeds, shelf-stable meat products and also for food for immunocompromised hospital patients (WHO, 1999). The main goal of this study is to determine the influence of high γ-irradiation doses on barrier properties of PE-LLD and PPcast films. For this purpose, the doses up to 200 kGy were used. Thermal and structural effects were investigated by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), while the barrier properties were measured by the manometric method. The decay of free radicals formed during irradiation process was monitored by electron spin resonance (ESR) spectroscopy in a function of time.
2. Experimental 2.1. Materials The samples used in this study were linear low-density polyethylene (PE-LLD) and cast polypropylene (PPcast) films. PE-LLD film, a copolymer ethylene-co-butene, with a density of 0.922 g/cm3, 31% crystallinity, 50 μm thickness and melting point at 113.4 1C was used. PPcast film had a thickness of 30 μm, 43% crystallinity and melting point at 155.7 1C. Both films were prepared by an extrusion process and obtained from Aluflexpack d.o.o., Umag, Croatia.
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were performed at a heating rate of 10 K/min in a nitrogen atmosphere on a Mettler-Toledo DSC822e differential scanning calorimeter calibrated with indium. DSC curves were recorded using 10 mg samples in the temperature range from 298 to 423 K for PE-LLD and from 298 to 473 K for PPcast samples. The degrees of crystallinity (χc) were calculated by dividing the heats of fusion of the samples by the heats of fusion for 100% crystalline samples, taken to be 293 J/g for polyethylene and 190 J/g for polypropylene according to Brandrup (Brandrup, 1989). 2.5. Electron spin resonance (ESR) ESR measurements were performed on a Varian E-109 spectrometer operating at 9.27 GHz, equipped with a Bruker ER 041 XG microwave bridge. Spectra were recorded at a room temperature immediately after irradiation and as a function of time after irradiation. Spectroscopic parameters were modulation amplitude 1.1 G and magnetic field sweep 100 G or 160 G, and the microwave field power was varied from 1 mW to 10 mW, depending on signal intensity. EW (EPRWare) Scientific Software Service program was used for data accumulation and manipulation. The number of accumulations varied from 2 to 5, depending on signal to noise ratio. 2.6. Fourier transform infrared spectroscopy (FTIR).
2.2. Sample irradiation PE-LLD and PPcast samples were irradiated in a 60Co γ-source in the presence of air at room temperature with a dose rate of 11.1 kGy/h. The total irradiation doses were 0 kGy (unirradiated samples), 50 kGy, 100 kGy, 150 kGy and 200 kGy. 2.3. Oxygen permeability measurements Oxygen permeability measurements were performed using a manometric method, on a permeability testing appliance, Type GDP-C (Anonymous, 1993). The increase in pressure during the test period is evaluated and displayed by an external computer. Using the Method A, suitable for monofilms, it was possible to determine the permeability (P), solubility (S) and diffusion (D) coefficients. The solubility and diffusion coefficients were calculated from the time lag (tL) values and known sample thicknesses. Data were recorded and evaluated by a personal computer (PC) connected to the GDP-C with a serial interface. 2.4. Differential scanning calorimetry (DSC) DSC analysis was used to determine the melting points and heats of fusion of the PE-LLD and PPcast samples. Measurements
IR spectra of unirradiated and irradiated PE-LLD and PPcast films were recorded on FTIR spectrometer ABB Bomem MB102. Before each measurement, samples were cleaned and cut to the appropriate size. Measurements of free standing films were performed at room temperature in wave number range from 400 cm 1 to 4000 cm 1 in transmission mode. All spectra were recorded with the resolution of 4 cm 1 and 60 accumulations per spectrum. Interference fringes due to the micrometre thickness of the polymer films are clearly seen (around 500 cm 1 in Fig. 4 and in a broad interval around 2000 cm 1 in Fig. 5) but presented no obstacle in observing the spectral changes induced by irradiation.
3. Results and discussion 3.1. Effect of γ-irradiation on O2 permeability Figs. 1, 2 and 3 show the change in permeability (P), diffusion (D) and solubility (S) coefficients, respectively, for both PE-LLD and PPcast films irradiated with various doses. It is evident that the permeability coefficient decreases for both films with an increase in irradiation dose (Fig. 1). The decrease in P value measured for 200 kGy is about 8% for PE-LLD and 19% for PPcast film. The main
Fig. 1. Oxygen permeability coefficient (P) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
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Fig. 2. Oxygen diffusion coefficient (D) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
Fig. 3. Oxygen solubility coefficient (S) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
reason for such decrease in P value for PE-LLD is related to decrease in the diffusion coefficient (D) (Fig. 2a). The solubility coefficient (Fig. 3) showed opposite behaviour to diffusion (Fig. 2). Thus, in the case of PE-LLD film O2 permeability decrease is due to O2 diffusivity decrease, while for PPcast it is due to decreased gas solubility. Goulas and coworkers (Goulas et al., 2002) studied permeability of oxygen, carbon dioxide and water vapour through PE-LD and PP films and concluded that doses below 30 kGy did not induce any statistically significant changes in the permeability. A decrease in oxygen permeability observed after an application of higher doses is also reported by Kanitz and Huang (1970). They investigated nitrogen permeability of polyethylene samples irradiated with high doses (up to 900 kGy) and observed a decrease in P and D values and an increase in S value with increasing irradiation dose. These observations are attributed to the changes in polymer structure caused by irradiation. 3.2. Structural changes induced by γ-irradiation Material degradation was monitored by Fourier transform infrared (FTIR) spectroscopy. A strong absorption in the range from 3000 to 2800 cm 1 can be observed for PE-LLD film (Fig. 4a). This range is characteristic for the symmetric and antisymmetric stretching of C H bonds in methyl and methylene groups. Bands detected at 1464 cm 1 and 719 cm 1 are ascribed to deformation of methylene groups while that at 1377 cm 1 is due to symmetric deformation of methyl groups (Gulmine et al., 2002). Bands at 1464 cm 1 and 719 cm 1 split in the presence of crystalline phase resulting in the appearance of bands at 1470 cm 1 and 731 cm 1 respectively. Polypropylene films show strong absorption in the range from 2974 cm 1 to 2838 cm 1 and also at 1377 cm 1 (Fig. 5a). These regions contain the bands that correspond to the deformation of methyl groups which partially overlap with the
deformation of methylene groups. Therefore, the scissoring band of methylene groups which normally occurs at nearly constant position near 1465 cm 1 is shifted to 1458 cm 1, due to the antisymmetrical bending vibrations of methyl groups. An appearance of a narrow vibrational band at 1717 cm 1 and the increase in its intensity with increasing irradiation dose can be clearly seen in the spectrum of irradiated samples. However, this band does not appear in the spectrum of unirradiated sample since it corresponds to the stretching of CQO bonds in carbonyl groups formed during the irradiation, as a consequence of sample degradation process (Fig. 4b). This is in agreement with previous investigations (Riganakos et al., 1999) which demonstrated the formation of degradation products like aldehydes and ketones in PE-LLD films irradiated with 100 kGy. Rojas de Gante and Pascat found that the doses above 100 kGy also caused increase in the number of double bonds along the main chain, production of final degradation products and the evolution of carbon dioxide (Rojas de Gante and Pascat, 1990). The same effect of radiation can be observed with PPcast film (Fig. 5). The intensity of the absorption band at 1717 cm 1 in irradiated samples of both, PE-LLD and PPcast films (Figs. 4 and 5) is related to the oxidative degradation of material. This intensity increases in both types of films with an increase in the irradiation dose (Figs. 4 and 5b). A direct measure of material degradation is the carbonyl index (CI). Its dependence on the irradiation dose is shown in Fig. 6. An increase in CI value with increasing the irradiation dose is observed for both PE-LLD and PPcast samples. It is known that degradation reactions in semicrystalline polymers take place mostly in amorphous regions although physical factors like size, arrangement and distribution of crystallites also play an important role. The degradation kinetics of polymers depends generally on oxygen accessibility. The rate of oxidation decreases with decreasing oxygen diffusion due to the crystalline phase growth accompanied with local orientation of chain segments.
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Fig. 4. FTIR spectra of PE-LLD films irradiated with different doses in the range (a) from 400 cm 1 to 4000 cm 1 and (b) of carbonyl group stretching.
Fig. 5. FTIR spectra of PPcast films irradiated with different doses in the range (a) from 500 cm-1 to 3500 cm 1 and (b) of carbonyl group stretching.
The number of radicals developed during irradiation for different doses was measured by electron spin resonance (ESR) spectroscopy. Fig. 7 shows ESR spectra of PE-LLD films irradiated with various doses measured in a function of time passed immediately after irradiation. All spectra, except that of the sample irradiated with 50 kGy, have similar composite spectral shapes, indicating the same structure and composition of radicals formed by irradiation. It should be pointed out that the ESR signal of PE-LLD irradiated with 50 kGy after 6 days became extremely weak (at the limit of detection). Spectra obtained
with the sample irradiated with 200 kGy show the highest signal intensity and the best signal-to-noise ratio. Signal intensities, measured immediately after the irradiation, are presented in Table 1. Spectra of PPcast film (Fig. 8) contrary to those of PE-LLD, show higher intensities and no significant difference in the spectral shape between the samples irradiated with various doses was observed. A decrease in the ESR signal intensity in a function of time after irradiation was observed for both types of samples. Time dependent changes in the signal intensity, which are directly related to the number of radicals, are presented in Figs. 9 and 10.
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Fig. 6. Carbonyl index (CI) for (a) PE-LLD and (b) PPcast film in a function of irradiation dose.
Fig. 7. ESR spectra of PE-LLD films irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Table 1 ESR signal intensities of PE-LLD and PPcast films irradiated with different doses, measured immediately after finishing the irradiation process. Dose/kGy
50 100 150 200
Signal intensity/a.u. PE-LLD
PPcast
2483 1887 2240 12,188
39,888 72,244 74,030 195,490
An increase in the initial signal intensity (for t¼0) can be observed for both samples with increasing irradiation dose. However, this intensity is much higher for PPcast film (Fig. 10) when compared with PE-LLD (Fig. 9) irradiated with the same corresponding dose. This undoubtedly indicates that the irradiation of PPcast leads to the formation of much higher number of radicals
than the irradiation of PE-LLD. Furthermore, the signal loss, particularly for lower doses, is much faster in PE-LLD than in PPcast. Therefore, the time dependence of signal intensity for PELLD irradiated with 50 kGy is not given in Fig. 9. This might be due to the higher number of radicals and their mutual recombination. For higher doses (200 kGy), the radical recombination rate is similar for both samples and the signal intensity reaches its minimal value after approximately 20 days. Degrees of crystallinity for irradiated PE-LLD and PPcast films are determined by DSC and the results are shown in Fig. 11. An increase in the amount of crystalline phase fraction with increasing irradiation dose is evident for both samples. For the dose of 200 kGy, this increase is about 12% for PE-LLD and 22% for PPcast, when compared with unirradiated samples. The increased crystallinity (Fig. 11) is directly related to gas permeability (Fig. 1). Thus, the higher the crystalline content, the lower the permeability. Apart from the crystallinity, free volume and orientation are closely linked to semicrystalline polymers (PP, PE) permeability (Wang and Porter, 1984).
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Fig. 8. ESR spectra of PPcast films irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Fig. 9. ESR signal intensity of PE-LLD irradiated with (a) 100 kGy, (b) 150 kGy and (c) 200 kGy, measured in a function of time.
DSC was also used to investigate the influence of irradiation dose on the melting point (Fig. 12). For PE-LLD (Fig. 12a) the melting point remains more or less constant (in the limit of experimental error) regardless of the
applied irradiation dose. Contrarily, for PPcast (Fig. 12b) it continuously decreases with an increase in the irradiation dose from 156.5 1C for unirradiated sample to 145.9 1C for the sample irradiated with 200 kGy. This is probably due to the chain scission.
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Fig. 10. ESR signal intensity of PPcast irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Fig. 11. Degree of crystallinity (χc) in (a) PE-LLD and (b) PPcast films as a function of irradiation dose.
Fig. 12. Melting point (Tm) of (a) PE-LLD and (b) PPcast samples as a function of irradiation dose.
Material degradation caused by irradiation is evident not only through the formation of free radicals, but also through the formation of volatile and non-volatile radiolitic products of low molecular
mass and hydrogen production followed by an increase of unsaturated covalent bonds, polymer chains scission and a decrease in molecular mass (Brody and Marsh, 1997; Goulas et al., 2003).
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The presence of oxygen from atmosphere favours the degradation process due to reactions with free radicals formed during irradiation (Gillen et al., 1993; Goldman et al., 1996; Goulas et al., 2003). An influence of the dose rate on the effect of degradation was also observed. Tidjani and Wilkie have found that the degradation is more severe at lower dose rates (Tidjani and Wilkie, 2006). They hypothesized that, at high dose rates, one should expect an excess production of alkyl radicals that steadily combines before they can be reached by oxygen molecules to propagate the oxidation and thus favour crosslinking over oxidation. On the other hand, macroradicals generated at a low dose rate combine easily with oxygen, leading to oxidation products. Therefore, it is reasonable to expect much higher formation of oxidation products at lower dose rates. In contrast to other packaging materials such as poly(ethylene terephthalate) (PET), polyamide (PA) or polystyrene (PS), polyolefin materials (PE and PP) showed an increase of low volatile compounds after irradiation with 44 kGy due to an oxidative decomposition of the polymer, oligomers and additives (Demertzis et al., 1999). As mentioned above, the oxidative degradation was measured by carbonyl index (CI). Its value was determined for PE-LLD from the intensity ratio of carbonyl band at 1717 cm 1 and reference band at 1470 cm 1 while CI for PPcast film was calculated using the reference band at 1458 cm 1. It is evident from Fig. 6 that CI in both types of films increases linearly with increasing irradiation dose. It can be also seen that CI values for PPcast are higher than for PE-LLD. This again indicated the higher degradation of PPcast films when compared to PE-LLD film, as also confirmed by DSC. This point will be discussed later. It was also observed that PPcast films became extremely brittle when exposed to higher irradiation doses. Mechanical measurements performed on PPcast samples irradiated with various doses showed severe degradation at 60 kGy indicated by a decrease in elongation at break by almost 93% when compared to unirradiated sample (Goulas et al., 2004). The lower radiation resistance of PPcast films in comparison with the PE-LLD films may be attributed to the backbone structure of PP. Because of the presence of tertiary carbon in the main chain, irradiation of PP produces tertiary radicals. Such radicals, due to the steric hindrance, react with oxygen and produce peroxides or peroxyl radicals, which in turn, acting as strong oxidants, attack PP molecules (Bernstein et al., 2007; Bourges et al., 1992; Goulas et al., 2004). ESR results contribute to clarify the mechanism of oxidative degradation of PE-LLD and PPcast films. Signal intensity was calculated by double integration of ESR spectra and the results are shown in Table 1. It should be noted here that the signal intensity is proportional to the concentration (i.e. to the number) of radicals in the sample. It is easy to see that the number of radicals formed using the same doses is much higher for PPcast when compared to PE-LLD film, irrespectively of irradiation dose. This result could be a consequence of higher reactivity of tertiary carbon atoms in polypropylene (Goulas et al., 2004). Since the irradiation has an effect on both crystalline and amorphous phase, the mobility of formed free radicals is different. Depending on their location and mobility, radicals may recombine and this recombination leads to the crosslinking, branching and formation of double bonds (Premnath et al., 1996). The characteristic shape of ESR spectra indicates that the irradiation of PE-LLD and PPcast films in the presence of oxygen causes mainly the formation of peroxide radicals (Davis et al., 1973; Williams, 1991). The specific shape of ESR spectrum of PE-LLD irradiated with 50 kGy is different from spectra of samples irradiated with higher doses, probably due to the presence of alkyl radicals (Williams, 1991) and therefore, spectra observed for samples irradiated with different doses cannot be compared directly. The radicals formed in PE-LLD using the dose of 50 kGy, in spite of their higher concentration than in PE-LLD samples irradiated with higher doses (except
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200 kGy), Table 1, disappeared after 6 days, as illustrated in Fig. 7a. Contrarily, ESR spectra of samples irradiated with higher doses indicate an existence of radicals even after 48 days (Fig. 7). However, the concentration of radicals remained after this time is much lower for both PE-LLD and PPcast. These radicals are probably placed in the crystalline regions or in the interphase between crystalline and amorphous regions where, due to their lower mobility, the recombination is less probable. A decrease in the signal intensity in a function of time for both samples, regardless of irradiation dose, can be easily observed (Figs. 9 and 10). The highest lowering of the signal is detected for the highest irradiation dose (200 kGy). The recombination of radicals formed by γ-irradiation results in the formation of carbonyl compounds (Gates et al., 1979; Premnath et al., 1996). Carbonyl groups, due to their polarity, probably influence the permeability and diffusion coefficients in both types of samples (Figs. 1 and 2). Irradiation of samples in the absence of oxygen leads to an increase in the crosslink density, but at the same time decreases the amount of crystalline phase (Andjelić and Richard, 2001; Deng and Shalaby, 2001; Shinde and Salovey, 1985). Contrarily, the irradiation with low and medium doses performed in the presence of air causes an increase in crystallinity (Deng and Shalaby, 2001; Kang and Nho, 2001; Shinde and Salovey, 1985), which is also the case of investigated samples (Fig. 11). An increase in the degree of crystallinity is evident for both samples. However, this phenomenon is more pronounced in the case of PPcast when compared with PE-LLD. It is possible that the material degradation caused by high energy radiation produces chain fragments which can be reorganized in a way that they are partially included in the crystalline phase. The newly formed crystals contribute to the decrease in oxygen permeability, as in the case of analysed samples (Fig. 1). Additional result of the irradiation process in the presence of air is a decrease in molecular mass (Gorelik et al., 1993). Based on the results obtained, it can be concluded that the irradiation of PPcast films mostly results in the chain scission accompanied with more pronounced oxidation and a higher degree of crystallinity, while in the case of PE-LLD films the crosslinking dominates.
4. Conclusions The influence of high doses γ-irradiation on oxygen permeability through the thin polymer films (PE-LLD and PPcast) is investigated using various methods. The main result of this study indicates that γ-irradiation causes a decrease in the permeability coefficient in both types of films. Since the irradiation was performed in the presence of air, free radicals generated by γ-irradiation react with oxygen. The oxidation process leads to the formation of carbonyl and carboxyl groups accompanied with changes in the polarity of the chains. This probably contributes to the changes in the diffusion and permeability of nonpolar gases, like oxygen. Additionally, material degradation which occurs during the irradiation and results in the chain scission, contributes to the formation of chain fragments. This leads to structural reorganization and makes possible the incorporation of some fragments in the crystalline phase. An increase in the degree of crystallinity also contributes to the decrease in the oxygen permeability in both films. The fact that PPcast films became extremely brittle when irradiated with higher doses must be taken into account when it is used as a packaging material. However, this is not the case with PE-LLD. The irradiation of PPcast films generally results in the chain scission accompanied with more pronounced oxidation and a higher degree of crystallinity, while in the case of PE-LLD films the crosslinking dominates.
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Influence of high doses γ-irradiation on oxygen permeability of linear low-density polyethylene and cast polypropylene films Damir Klepac a, Mario Ščetar b, Goran Baranović c, Kata Galić b, Srećko Valić a,c,n a
Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Braće Branchetta 20, HR-51000 Rijeka, Croatia Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia c Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia b
H I G H L I G H T S
High doses γ-irradiation reduces oxygen permeability in PE and PP films. Increase of crystallinity and decrease of melting point in PP film induced by radiation. Radiation induced radical formation in PE and PP and their decay monitored by ESR.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 October 2013 Accepted 5 December 2013 Available online 17 December 2013
Linear low density polyethylene (PE-LLD) and cast polypropylene (PPcast) films were irradiated in a 60 Co γ-source. The total irradiation dose varied from 0 kGy (unirradiated samples) to 200 kGy. Oxygen transport was investigated by a manometric method and the structural changes were studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Free radicals decay as a function of time was monitored by electron spin resonance (ESR) spectroscopy. The results show that the γ-irradiation reduces oxygen permeability coefficient in both films. The reduction was associated with an increase in crystallinity. DSC thermograms revealed a decrease in PPcast melting point with increasing irradiation dose, indicating higher degradation compared to PE-LLD. The observed peak in FTIR spectra for both samples at 1716 cm 1 corresponds to the stretching of the carbonyl and carboxylic groups which arise from the reaction of oxygen with the free radicals produced in the polymer matrix as a result of irradiation. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Polypropylene Gamma-irradiation Oxygen permeability ESR FTIR
1. Introduction Linear low-density polyethylene (PE-LLD) and cast polypropylene (PPcast) are the most widely used polymeric materials in the food packaging industry, mainly because of their special physical properties such as high barrier resistance against water vapour, high tear strength and toughness, excellent environmental stress cracking resistance and improved processability compared to conventional polyethylene and polypropylene (Galić et al., 2011; Guillard et al., 2010). However, these films also have some disadvantages. Their intrinsic properties can be affected by an application of relatively small external mechanical force or by high energy irradiation. The deformation can occur during transportation and handling of food products, while an effect of high energy radiation n Corresponding author at: Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Braće Branchetta 20 HR-51000 Rijeka Croatia. Tel.: þ 385 51 651134; fax: þ 385 51 678895. E-mail address: [email protected] (S. Valić).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.12.005
is encountered during food sterilization process. An influence of uniaxial deformation applied on PE-LLD film was investigated recently (Klepac et al., 2013). Although one can imagine that the effect of deformation could increase the gas permeability, previous results show an opposite effect (Compañ et al., 1996; Holden et al., 1985; Klepac et al., 2013; Villaluenga and Seoane, 1998). In fact, it was found that the oxygen permeability (P) and diffusion (D) coefficients of PE-LLD film decrease after an application of uniaxial deformation (Klepac et al., 2013). This study is focused on the changes in structural and barrier properties of PE-LLD and PPcast films provoked by γ-irradiation. Previous investigations of PE-LD and PP films exposed to high energy radiation did not show any significant difference in the permeability of CO2 and O2 (Goulas et al., 2002, 2003). Moreover, no important changes in the structure of investigated materials were reported. These investigations were performed using relatively low radiation doses (up to 10 kGy), since such doses are usually applied during the food sterilization. However, certain countries permit higher doses for the sterilization of some spices,
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seeds, shelf-stable meat products and also for food for immunocompromised hospital patients (WHO, 1999). The main goal of this study is to determine the influence of high γ-irradiation doses on barrier properties of PE-LLD and PPcast films. For this purpose, the doses up to 200 kGy were used. Thermal and structural effects were investigated by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), while the barrier properties were measured by the manometric method. The decay of free radicals formed during irradiation process was monitored by electron spin resonance (ESR) spectroscopy in a function of time.
2. Experimental 2.1. Materials The samples used in this study were linear low-density polyethylene (PE-LLD) and cast polypropylene (PPcast) films. PE-LLD film, a copolymer ethylene-co-butene, with a density of 0.922 g/cm3, 31% crystallinity, 50 μm thickness and melting point at 113.4 1C was used. PPcast film had a thickness of 30 μm, 43% crystallinity and melting point at 155.7 1C. Both films were prepared by an extrusion process and obtained from Aluflexpack d.o.o., Umag, Croatia.
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were performed at a heating rate of 10 K/min in a nitrogen atmosphere on a Mettler-Toledo DSC822e differential scanning calorimeter calibrated with indium. DSC curves were recorded using 10 mg samples in the temperature range from 298 to 423 K for PE-LLD and from 298 to 473 K for PPcast samples. The degrees of crystallinity (χc) were calculated by dividing the heats of fusion of the samples by the heats of fusion for 100% crystalline samples, taken to be 293 J/g for polyethylene and 190 J/g for polypropylene according to Brandrup (Brandrup, 1989). 2.5. Electron spin resonance (ESR) ESR measurements were performed on a Varian E-109 spectrometer operating at 9.27 GHz, equipped with a Bruker ER 041 XG microwave bridge. Spectra were recorded at a room temperature immediately after irradiation and as a function of time after irradiation. Spectroscopic parameters were modulation amplitude 1.1 G and magnetic field sweep 100 G or 160 G, and the microwave field power was varied from 1 mW to 10 mW, depending on signal intensity. EW (EPRWare) Scientific Software Service program was used for data accumulation and manipulation. The number of accumulations varied from 2 to 5, depending on signal to noise ratio. 2.6. Fourier transform infrared spectroscopy (FTIR).
2.2. Sample irradiation PE-LLD and PPcast samples were irradiated in a 60Co γ-source in the presence of air at room temperature with a dose rate of 11.1 kGy/h. The total irradiation doses were 0 kGy (unirradiated samples), 50 kGy, 100 kGy, 150 kGy and 200 kGy. 2.3. Oxygen permeability measurements Oxygen permeability measurements were performed using a manometric method, on a permeability testing appliance, Type GDP-C (Anonymous, 1993). The increase in pressure during the test period is evaluated and displayed by an external computer. Using the Method A, suitable for monofilms, it was possible to determine the permeability (P), solubility (S) and diffusion (D) coefficients. The solubility and diffusion coefficients were calculated from the time lag (tL) values and known sample thicknesses. Data were recorded and evaluated by a personal computer (PC) connected to the GDP-C with a serial interface. 2.4. Differential scanning calorimetry (DSC) DSC analysis was used to determine the melting points and heats of fusion of the PE-LLD and PPcast samples. Measurements
IR spectra of unirradiated and irradiated PE-LLD and PPcast films were recorded on FTIR spectrometer ABB Bomem MB102. Before each measurement, samples were cleaned and cut to the appropriate size. Measurements of free standing films were performed at room temperature in wave number range from 400 cm 1 to 4000 cm 1 in transmission mode. All spectra were recorded with the resolution of 4 cm 1 and 60 accumulations per spectrum. Interference fringes due to the micrometre thickness of the polymer films are clearly seen (around 500 cm 1 in Fig. 4 and in a broad interval around 2000 cm 1 in Fig. 5) but presented no obstacle in observing the spectral changes induced by irradiation.
3. Results and discussion 3.1. Effect of γ-irradiation on O2 permeability Figs. 1, 2 and 3 show the change in permeability (P), diffusion (D) and solubility (S) coefficients, respectively, for both PE-LLD and PPcast films irradiated with various doses. It is evident that the permeability coefficient decreases for both films with an increase in irradiation dose (Fig. 1). The decrease in P value measured for 200 kGy is about 8% for PE-LLD and 19% for PPcast film. The main
Fig. 1. Oxygen permeability coefficient (P) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
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Fig. 2. Oxygen diffusion coefficient (D) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
Fig. 3. Oxygen solubility coefficient (S) through (a) PE-LLD and (b) PPcast films in a function of irradiation dose.
reason for such decrease in P value for PE-LLD is related to decrease in the diffusion coefficient (D) (Fig. 2a). The solubility coefficient (Fig. 3) showed opposite behaviour to diffusion (Fig. 2). Thus, in the case of PE-LLD film O2 permeability decrease is due to O2 diffusivity decrease, while for PPcast it is due to decreased gas solubility. Goulas and coworkers (Goulas et al., 2002) studied permeability of oxygen, carbon dioxide and water vapour through PE-LD and PP films and concluded that doses below 30 kGy did not induce any statistically significant changes in the permeability. A decrease in oxygen permeability observed after an application of higher doses is also reported by Kanitz and Huang (1970). They investigated nitrogen permeability of polyethylene samples irradiated with high doses (up to 900 kGy) and observed a decrease in P and D values and an increase in S value with increasing irradiation dose. These observations are attributed to the changes in polymer structure caused by irradiation. 3.2. Structural changes induced by γ-irradiation Material degradation was monitored by Fourier transform infrared (FTIR) spectroscopy. A strong absorption in the range from 3000 to 2800 cm 1 can be observed for PE-LLD film (Fig. 4a). This range is characteristic for the symmetric and antisymmetric stretching of C H bonds in methyl and methylene groups. Bands detected at 1464 cm 1 and 719 cm 1 are ascribed to deformation of methylene groups while that at 1377 cm 1 is due to symmetric deformation of methyl groups (Gulmine et al., 2002). Bands at 1464 cm 1 and 719 cm 1 split in the presence of crystalline phase resulting in the appearance of bands at 1470 cm 1 and 731 cm 1 respectively. Polypropylene films show strong absorption in the range from 2974 cm 1 to 2838 cm 1 and also at 1377 cm 1 (Fig. 5a). These regions contain the bands that correspond to the deformation of methyl groups which partially overlap with the
deformation of methylene groups. Therefore, the scissoring band of methylene groups which normally occurs at nearly constant position near 1465 cm 1 is shifted to 1458 cm 1, due to the antisymmetrical bending vibrations of methyl groups. An appearance of a narrow vibrational band at 1717 cm 1 and the increase in its intensity with increasing irradiation dose can be clearly seen in the spectrum of irradiated samples. However, this band does not appear in the spectrum of unirradiated sample since it corresponds to the stretching of CQO bonds in carbonyl groups formed during the irradiation, as a consequence of sample degradation process (Fig. 4b). This is in agreement with previous investigations (Riganakos et al., 1999) which demonstrated the formation of degradation products like aldehydes and ketones in PE-LLD films irradiated with 100 kGy. Rojas de Gante and Pascat found that the doses above 100 kGy also caused increase in the number of double bonds along the main chain, production of final degradation products and the evolution of carbon dioxide (Rojas de Gante and Pascat, 1990). The same effect of radiation can be observed with PPcast film (Fig. 5). The intensity of the absorption band at 1717 cm 1 in irradiated samples of both, PE-LLD and PPcast films (Figs. 4 and 5) is related to the oxidative degradation of material. This intensity increases in both types of films with an increase in the irradiation dose (Figs. 4 and 5b). A direct measure of material degradation is the carbonyl index (CI). Its dependence on the irradiation dose is shown in Fig. 6. An increase in CI value with increasing the irradiation dose is observed for both PE-LLD and PPcast samples. It is known that degradation reactions in semicrystalline polymers take place mostly in amorphous regions although physical factors like size, arrangement and distribution of crystallites also play an important role. The degradation kinetics of polymers depends generally on oxygen accessibility. The rate of oxidation decreases with decreasing oxygen diffusion due to the crystalline phase growth accompanied with local orientation of chain segments.
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Fig. 4. FTIR spectra of PE-LLD films irradiated with different doses in the range (a) from 400 cm 1 to 4000 cm 1 and (b) of carbonyl group stretching.
Fig. 5. FTIR spectra of PPcast films irradiated with different doses in the range (a) from 500 cm-1 to 3500 cm 1 and (b) of carbonyl group stretching.
The number of radicals developed during irradiation for different doses was measured by electron spin resonance (ESR) spectroscopy. Fig. 7 shows ESR spectra of PE-LLD films irradiated with various doses measured in a function of time passed immediately after irradiation. All spectra, except that of the sample irradiated with 50 kGy, have similar composite spectral shapes, indicating the same structure and composition of radicals formed by irradiation. It should be pointed out that the ESR signal of PE-LLD irradiated with 50 kGy after 6 days became extremely weak (at the limit of detection). Spectra obtained
with the sample irradiated with 200 kGy show the highest signal intensity and the best signal-to-noise ratio. Signal intensities, measured immediately after the irradiation, are presented in Table 1. Spectra of PPcast film (Fig. 8) contrary to those of PE-LLD, show higher intensities and no significant difference in the spectral shape between the samples irradiated with various doses was observed. A decrease in the ESR signal intensity in a function of time after irradiation was observed for both types of samples. Time dependent changes in the signal intensity, which are directly related to the number of radicals, are presented in Figs. 9 and 10.
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Fig. 6. Carbonyl index (CI) for (a) PE-LLD and (b) PPcast film in a function of irradiation dose.
Fig. 7. ESR spectra of PE-LLD films irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Table 1 ESR signal intensities of PE-LLD and PPcast films irradiated with different doses, measured immediately after finishing the irradiation process. Dose/kGy
50 100 150 200
Signal intensity/a.u. PE-LLD
PPcast
2483 1887 2240 12,188
39,888 72,244 74,030 195,490
An increase in the initial signal intensity (for t¼0) can be observed for both samples with increasing irradiation dose. However, this intensity is much higher for PPcast film (Fig. 10) when compared with PE-LLD (Fig. 9) irradiated with the same corresponding dose. This undoubtedly indicates that the irradiation of PPcast leads to the formation of much higher number of radicals
than the irradiation of PE-LLD. Furthermore, the signal loss, particularly for lower doses, is much faster in PE-LLD than in PPcast. Therefore, the time dependence of signal intensity for PELLD irradiated with 50 kGy is not given in Fig. 9. This might be due to the higher number of radicals and their mutual recombination. For higher doses (200 kGy), the radical recombination rate is similar for both samples and the signal intensity reaches its minimal value after approximately 20 days. Degrees of crystallinity for irradiated PE-LLD and PPcast films are determined by DSC and the results are shown in Fig. 11. An increase in the amount of crystalline phase fraction with increasing irradiation dose is evident for both samples. For the dose of 200 kGy, this increase is about 12% for PE-LLD and 22% for PPcast, when compared with unirradiated samples. The increased crystallinity (Fig. 11) is directly related to gas permeability (Fig. 1). Thus, the higher the crystalline content, the lower the permeability. Apart from the crystallinity, free volume and orientation are closely linked to semicrystalline polymers (PP, PE) permeability (Wang and Porter, 1984).
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Fig. 8. ESR spectra of PPcast films irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Fig. 9. ESR signal intensity of PE-LLD irradiated with (a) 100 kGy, (b) 150 kGy and (c) 200 kGy, measured in a function of time.
DSC was also used to investigate the influence of irradiation dose on the melting point (Fig. 12). For PE-LLD (Fig. 12a) the melting point remains more or less constant (in the limit of experimental error) regardless of the
applied irradiation dose. Contrarily, for PPcast (Fig. 12b) it continuously decreases with an increase in the irradiation dose from 156.5 1C for unirradiated sample to 145.9 1C for the sample irradiated with 200 kGy. This is probably due to the chain scission.
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Fig. 10. ESR signal intensity of PPcast irradiated with (a) 50 kGy, (b) 100 kGy, (c) 150 kGy and (d) 200 kGy measured in a function of time.
Fig. 11. Degree of crystallinity (χc) in (a) PE-LLD and (b) PPcast films as a function of irradiation dose.
Fig. 12. Melting point (Tm) of (a) PE-LLD and (b) PPcast samples as a function of irradiation dose.
Material degradation caused by irradiation is evident not only through the formation of free radicals, but also through the formation of volatile and non-volatile radiolitic products of low molecular
mass and hydrogen production followed by an increase of unsaturated covalent bonds, polymer chains scission and a decrease in molecular mass (Brody and Marsh, 1997; Goulas et al., 2003).
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The presence of oxygen from atmosphere favours the degradation process due to reactions with free radicals formed during irradiation (Gillen et al., 1993; Goldman et al., 1996; Goulas et al., 2003). An influence of the dose rate on the effect of degradation was also observed. Tidjani and Wilkie have found that the degradation is more severe at lower dose rates (Tidjani and Wilkie, 2006). They hypothesized that, at high dose rates, one should expect an excess production of alkyl radicals that steadily combines before they can be reached by oxygen molecules to propagate the oxidation and thus favour crosslinking over oxidation. On the other hand, macroradicals generated at a low dose rate combine easily with oxygen, leading to oxidation products. Therefore, it is reasonable to expect much higher formation of oxidation products at lower dose rates. In contrast to other packaging materials such as poly(ethylene terephthalate) (PET), polyamide (PA) or polystyrene (PS), polyolefin materials (PE and PP) showed an increase of low volatile compounds after irradiation with 44 kGy due to an oxidative decomposition of the polymer, oligomers and additives (Demertzis et al., 1999). As mentioned above, the oxidative degradation was measured by carbonyl index (CI). Its value was determined for PE-LLD from the intensity ratio of carbonyl band at 1717 cm 1 and reference band at 1470 cm 1 while CI for PPcast film was calculated using the reference band at 1458 cm 1. It is evident from Fig. 6 that CI in both types of films increases linearly with increasing irradiation dose. It can be also seen that CI values for PPcast are higher than for PE-LLD. This again indicated the higher degradation of PPcast films when compared to PE-LLD film, as also confirmed by DSC. This point will be discussed later. It was also observed that PPcast films became extremely brittle when exposed to higher irradiation doses. Mechanical measurements performed on PPcast samples irradiated with various doses showed severe degradation at 60 kGy indicated by a decrease in elongation at break by almost 93% when compared to unirradiated sample (Goulas et al., 2004). The lower radiation resistance of PPcast films in comparison with the PE-LLD films may be attributed to the backbone structure of PP. Because of the presence of tertiary carbon in the main chain, irradiation of PP produces tertiary radicals. Such radicals, due to the steric hindrance, react with oxygen and produce peroxides or peroxyl radicals, which in turn, acting as strong oxidants, attack PP molecules (Bernstein et al., 2007; Bourges et al., 1992; Goulas et al., 2004). ESR results contribute to clarify the mechanism of oxidative degradation of PE-LLD and PPcast films. Signal intensity was calculated by double integration of ESR spectra and the results are shown in Table 1. It should be noted here that the signal intensity is proportional to the concentration (i.e. to the number) of radicals in the sample. It is easy to see that the number of radicals formed using the same doses is much higher for PPcast when compared to PE-LLD film, irrespectively of irradiation dose. This result could be a consequence of higher reactivity of tertiary carbon atoms in polypropylene (Goulas et al., 2004). Since the irradiation has an effect on both crystalline and amorphous phase, the mobility of formed free radicals is different. Depending on their location and mobility, radicals may recombine and this recombination leads to the crosslinking, branching and formation of double bonds (Premnath et al., 1996). The characteristic shape of ESR spectra indicates that the irradiation of PE-LLD and PPcast films in the presence of oxygen causes mainly the formation of peroxide radicals (Davis et al., 1973; Williams, 1991). The specific shape of ESR spectrum of PE-LLD irradiated with 50 kGy is different from spectra of samples irradiated with higher doses, probably due to the presence of alkyl radicals (Williams, 1991) and therefore, spectra observed for samples irradiated with different doses cannot be compared directly. The radicals formed in PE-LLD using the dose of 50 kGy, in spite of their higher concentration than in PE-LLD samples irradiated with higher doses (except
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200 kGy), Table 1, disappeared after 6 days, as illustrated in Fig. 7a. Contrarily, ESR spectra of samples irradiated with higher doses indicate an existence of radicals even after 48 days (Fig. 7). However, the concentration of radicals remained after this time is much lower for both PE-LLD and PPcast. These radicals are probably placed in the crystalline regions or in the interphase between crystalline and amorphous regions where, due to their lower mobility, the recombination is less probable. A decrease in the signal intensity in a function of time for both samples, regardless of irradiation dose, can be easily observed (Figs. 9 and 10). The highest lowering of the signal is detected for the highest irradiation dose (200 kGy). The recombination of radicals formed by γ-irradiation results in the formation of carbonyl compounds (Gates et al., 1979; Premnath et al., 1996). Carbonyl groups, due to their polarity, probably influence the permeability and diffusion coefficients in both types of samples (Figs. 1 and 2). Irradiation of samples in the absence of oxygen leads to an increase in the crosslink density, but at the same time decreases the amount of crystalline phase (Andjelić and Richard, 2001; Deng and Shalaby, 2001; Shinde and Salovey, 1985). Contrarily, the irradiation with low and medium doses performed in the presence of air causes an increase in crystallinity (Deng and Shalaby, 2001; Kang and Nho, 2001; Shinde and Salovey, 1985), which is also the case of investigated samples (Fig. 11). An increase in the degree of crystallinity is evident for both samples. However, this phenomenon is more pronounced in the case of PPcast when compared with PE-LLD. It is possible that the material degradation caused by high energy radiation produces chain fragments which can be reorganized in a way that they are partially included in the crystalline phase. The newly formed crystals contribute to the decrease in oxygen permeability, as in the case of analysed samples (Fig. 1). Additional result of the irradiation process in the presence of air is a decrease in molecular mass (Gorelik et al., 1993). Based on the results obtained, it can be concluded that the irradiation of PPcast films mostly results in the chain scission accompanied with more pronounced oxidation and a higher degree of crystallinity, while in the case of PE-LLD films the crosslinking dominates.
4. Conclusions The influence of high doses γ-irradiation on oxygen permeability through the thin polymer films (PE-LLD and PPcast) is investigated using various methods. The main result of this study indicates that γ-irradiation causes a decrease in the permeability coefficient in both types of films. Since the irradiation was performed in the presence of air, free radicals generated by γ-irradiation react with oxygen. The oxidation process leads to the formation of carbonyl and carboxyl groups accompanied with changes in the polarity of the chains. This probably contributes to the changes in the diffusion and permeability of nonpolar gases, like oxygen. Additionally, material degradation which occurs during the irradiation and results in the chain scission, contributes to the formation of chain fragments. This leads to structural reorganization and makes possible the incorporation of some fragments in the crystalline phase. An increase in the degree of crystallinity also contributes to the decrease in the oxygen permeability in both films. The fact that PPcast films became extremely brittle when irradiated with higher doses must be taken into account when it is used as a packaging material. However, this is not the case with PE-LLD. The irradiation of PPcast films generally results in the chain scission accompanied with more pronounced oxidation and a higher degree of crystallinity, while in the case of PE-LLD films the crosslinking dominates.
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