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Influence of the e-beam irradiation and photo-oxidation aging on the structure and properties of LDPE-OMMT nanocomposite films
Radiation Physics and Chemistry 81 (2012) 432–436
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Influence of the e-beam irradiation and photo-oxidation aging on the structure and properties of LDPE-OMMT nanocomposite films N.Tz. Dintcheva a,n, S. Alessi b, R. Arrigo b, G. Przybytniak c, G. Spadaro b a
Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy Dipartimento di Ingegneria Chimica, Gestionale, Informatica, Meccanica, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy c Institute of Nuclear Chemistry and Technology, Department of Radiation Chemistry and Technology, Laboratory of Radiation Modified Polymers, ul. Dorodna 16, 03-195 Warszawa, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2011 Accepted 14 December 2011 Available online 30 December 2011
In this paper two systems, polyethylene (LDPE) and polyethylene/commercial organo-modified montmorillonite (LDPE/OMMT) nanocomposite, were subjected to e-beam irradiation at different doses and both the molecular modifications and mechanical properties have been investigated through solubility, FT-IR, calorimetric and tensile tests. Moreover, in some of the irradiated systems photooxidation aging was performed and its effects were studied. The results show an enhancement with irradiation of the positive effect of the nano-filler loading, related to the increase of the mechanical properties, due to the increase of the nano-filler polymer interaction. Nevertheless calorimetric and FT-IR data indicate that the well known reduction of LDPE/OMMT nanocomposite resistance to photo-oxidation ageing, with respect to LDPE, is amplified by ionizing radiation. Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved.
Keywords: LDPE-OMMT nanocomposite film e-beam irradiation Photo-oxidation ageing Mechanical properties Carbonyl index Molecular modification
1. Introduction The silicate based polymer nano-composites had a great interest in the last decade because their properties and performance are superior than that the pristine polymeric matrix and/or the traditional composite materials (Sinha Ray and Okamoto, 2003; Pavlidou and Papaspyrides, 2008; Alexandre and Dubois, 2000; La Mantia et al., 2008; LeBaron et al., 1999). The nano-fillers have at least one dimension in nano-metric scale and, as it is well known in the literature, the nanocomposites show significantly improved mechanical, thermal, optical and physical properties. In particular high values of elastic modulus without loss in ductility is obtained. Furthermore the very small dimensions of the nano-particles lead to good dispersion and transparency, and to a significant improvement of the barrier properties. From a thermal point of view, the nano-filler presence leads to an improved thermal and flame stabilities. The significant enhancement of the nano-composites’s properties can be obtained by nano-filler loading less than 10 wt%, while in order to obtain similar properties change of a micro-composites, about 30–50 wt% of micro-filler is requested (Burmistr et al., 2005; Shi et al., 2009; Kiliaris and Papaspyrides, 2010; Gilman, 1999).
n
Corresponding author. Tel.: þ3909123863704; fax: þ 390917025020. E-mail address: [email protected] (N.Tz. Dintcheva).
g-ray, x-ray or e-beam irradiation of the polymeric materials causes different phenomena, such as chain branching up to crosslinking and chain scission with molecular degradation. The crosslinking and degradation processes are two co-current phenomena and the predominant effect should be evaluated considering the treatment conditions and the intrinsic polymer properties (Spinks and Woods, 1990; Singh and Silverman, 1992). Few papers are available in the actual literature about the ionizing radiation treatment of nano-filled polymer based nano-composites (Lu et al., 2005; Gad, 2009; Jiao et al., 2006). In particular, Lu et al. (2005) reported an interesting study about the mechanical and morphological variations of the HDPE/EVA blend without and with clays upon g-irradiation. As widely reported in the scientific literature (Fornes et al., 2003; Qin et al., 2003; Morlat-Therias et al., 2005; La Mantia et al., 2006; Dintcheva et al., 2009) the clay adding to the polymer matrix leads to a significant reduction of the photo-oxidation resistance with respect to the unfilled polymeric matrix. Different reasons for the accelerated photo-oxidation degradation are reported in the papers, in particular, the pro-degradative effect of the present iron ions, the degradation and the subsequent radical formation of the organo-modifier, the formation of the catalytic acidic sites on the layers and the entrapment of the polar antioxidant into the clay galleries. In this paper both polyethylene and nanocomposite films based on polyethylene and commercial organo-modified montmorillonite were irradiated at different doses and their photo-oxidation
0969-806X/$ - see front matter Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.12.018
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
2. Experimental The materials used in this work were as follows: – a film grade Low Density Polyethylene (Riblene& FC30), LDPE, supplied by Polimeri Europa; density r ¼0.922 g/cm3 at 23 1C and MFI190 1C/2.16 kg of 0.27 g/10 min. LDPE samples do not contain any kind of antioxidant; – an organo-modified montmorillonite commercialized by Southern Clay Products (U.S.A.) as ‘‘Cloisites 15 A’’ (further indicated as ‘‘OMMT’’; modified with dimethyldihydrogenated tallow-quaternary ammonium chloride –2M2HT– quat surfactant; quat concentration¼125 meq/100 g clay; d001 ¼3.15 nm; density¼1.66 g/cm3). Hydrogenated tallow of OMMT is a blend of saturated n-alkyl groups with approximate composition: 65% C18; 30% C16; 5% C14. A modular co-rotating twin screw extruder, OMC (Italy), has been used to prepare the filled sample. In order to have the same thermal history, the unfilled matrix was subjected to the same extrusion. The temperature profile was 120–130–140–150–160– 170–180 1C and the rotational speed 200 rpm. The extruded material has been immediately cooled and ground on line to obtain pellets for the further processing and characterizations. The films were obtained by a single screw extruder equipped with a film blowing head and with a Brabender film blowing unit. The head temperature was 180 1C and the screw speed was 60 rpm. The film thickness was about 60 mm for all the samples. The e-beam irradiation of the unfilled LDPE and of OMMT-filled LDPE films has been carried out by the LAE 10 MeV linear, pulsed accelerator located in the laboratory of the ICHTJ (Institute of Nuclear Chemistry and Technology) of Warsaw, Poland. The samples have been placed in the horizontal position in the front of the pulsed beam and the total doses were obtained by multipass exposure. The irradiation conditions were pulse current 470 mA, pulse frequency 400 Hz and dose/pass 25 kGy. Samples at 25, 50, 75, 100, 125, 150, 200 and 250 kGy of total doses have been irradiated. The unfilled LDPE and the composite sheets, irradiated at 0, 50 and 150 kGy, were exposed to accelerated weathering in a Q-UV (U.S.A.) chamber, containing eight UVB-313 (Q-Labs Corp., U.S.A) lamps. The exposure cycle conditions were as follows: 8 h of light at T¼55 1C followed by 4 h condensation at T¼35 1C. The measured photon flux was 2 mW/cm2. FT-IR spectra were measured by a Spectrum One spectrometer by Perkin-Elmer, using the Spectrum software. Spectra were obtained through 32 scans with a 4 cm 1 resolution. Carbonyl (peak measured between 1675 and 1825 cm 1) and hydroxyl (peak measured between 3200 and 3600 cm 1), indices were determined from peak absorption area (peak area absorbance of the group compared to that of a reference area measured between 1979 and 2110 cm 1; reference peak corresponds to the C–H vibrations and it does not change during ageing), as follows: Carbonyl Index¼Absorbance (1675–1825 cm 1)/Absorbance (1979–2110 cm 1)
Hydroxyl Index ¼Absorbance (3200–3600 cm 1)/Absorbance (1979–2110 cm 1) Measurements were obtained from the average of triplicate samples. Tensile properties were determined at both room temperature and humidity, using an Instron (U.S.A.) dynamometer mod. 3365, according to ASTM test method D882. The specimens were cut out from films and tested in the machine direction. The modulus was measured at the speed of 1 mm/min. When the deformation was about 10%, the speed was increased up to 100 mm/min until break. The data reported are the average values (with the related error bars) obtained through ten tests per sample. Calorimetric analysis was performed by a Perkin Elmer Precisely Jade DSC in order to evaluate the melting enthalpy values of each sample. The analysis was carried out according to the following program: heating from 30 1C to 160 1C at 5 1C/min, holding at 160 1C for 1 min, cooling from 160 1C to 30 1C at 5 1C/min and heating from 30 1C to 160 1C at 5 1C/min. The melting enthalpy data have been calculated for the second heating run and with respect to the soluble weight fractions of each sample. Extraction tests were performed by Soxhlet extractor using p-xylene as solvent. Approximately 0.3 g of any sample was exposed to refluxing in p-xylene close to its boiling point. Extraction time was about 72 h. Wide-angle X-ray analyses (WAXD) were performed at room temperature in the reflection mode on a Siemens D-500 X-ray diffractometer with Cu Ka radiation of wavelength of 0.1542 nm. A scanning rate of 10 1C min 1 was used. The distances d001 between the silicate layers of the clay in the nanocomposite blends was evaluated using the Bragg’s condition d001 ¼ nl/(2 sin y), where l is the wavelength, y is the angle of incidence of X-ray beam and n is an integer.
3. Results and discussion The molecular modifications induced by irradiation have been investigated through solvent extraction tests and FT-IR analysis. In Fig. 1 insoluble fractions as function of the dose are reported. It can be observed that, for LDPE/OMMT irradiated
50
LDPE LDPE/OMMT 40 Insoluble franction, %
aging was studied. The effect of the structural variations, due to e-beam irradiation and photo-oxidation, on the mechanical behavior of nano-filled films was investigated. The interest toward this study is justified by the possibility to use ionizing radiation for sterilization in food packaging. It can be important to have information about the different response of clay filled and unfilled samples after both e-beam irradiation and further photo oxidation processes.
433
30
20
10
0 0
50
100 150 Dose, kGy
200
250
Fig. 1. Insoluble fraction of LDPE and LDPE/OMMT films as a function of the dose.
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
samples, the values are always lower than those of the corresponding irradiated unfilled LDPE samples. This result indicates that the e-beam induced cross-linking of LDPE is higher than LDPE-OMMT systems, according to the expected effect of an inert filler on the irradiation of polymeric systems. The occurrence of oxidative phenomena during irradiation has been investigated through FTIR analysis. The corresponding spectra are reported in Fig. 2. The spectra of LDPE, Fig. 2a, show a gradual band rise at 1717 cm 1 due to the formation of the carbonyl group. Furthermore, the data correspondent to LDPE/OMMT, Fig. 2b, show a similar formation of the same oxidation products. Moreover a peak at 1740 cm 1, related to some impurities present in the structure of the pristine OMMT, is evidenced. For these samples the area of the carbonyl group has been calculated eliminating the contribution of the impurities of the pristine OMMT, by mathematical deconvolution of the peaks in the carbonyl range. The area of the carbonyl group as a function of the dose is reported in Fig. 3, showing that the carbonyl concentration is very similar for both unfilled and nanofilled systems, thus indicating comparable oxidation levels. On the basis of FTIR spectra, it is possible to conclude that the concentration of oxidized products is very low, according to the very high dose rate (Singh and Silverman, 1992; Clegg and Collyer, 1991). Furthermore, due to OMMT presence, some additional reactive sites come from the Hoffman elimination of the organomodifier of the clay nano-particles, as already reported in literature (Fornes et al., 2003; Qin et al., 2003; Dintcheva et al., 2009). The Hoffman elimination starts during the processing, compounding and film-blowing, and leads to the formation of a-olefins, amines
Fig. 2. FT-IR spectra, i.e. carbonyl range, of LDPE (a) and LDPE/OMMT (b) films as a function of the dose.
2 LDPE LDPE/OMMT 1.5 Carbonyl area
434
1
0.5
0 0
50
100 150 Dose, kGy
200
250
Fig. 3. Carbonyl area of LDPE and LDPE/OMMT films as a function of the dose.
Table 1 Mechanical properties of LDPE and OMMT filled LDPE films and interlayer distances of pristine clay and of LDPE/OMMT sample. Samples
E (Mpa)
TS (Mpa)
EB (%)
Interlayer distance (nm)
LDPE LDPE/OMMT OMMT
140 175 –
16 17 –
450 455 –
– 3.30 3.15
and secondary degradation products. FT-IR results indicate that the additional amount of the reactive sites does not imply the formation of supplementary oxidation products. In Table 1 the mechanical behavior of both LDPE and LDPE-clay unirradiated systems are reported. It can be observed that the OMMT loading causes a significant increase of the elastic modulus, while the properties at break remain almost unchanged, according to the well known improvement of the matrix rigidity without losses in ductility caused by clay loading (Sinha Ray and Okamoto, 2003; Pavlidou and Papaspyrides, 2008; Alexandre and Dubois, 2000). In the same Table 1 (last column), the calculated interlayer distance between the layers of nanoparticles in the pristine OMMT and LDPE/OMMT film are reported. The slight increase of interlayer distance for LDPE/OMMT sample, with respect to the pristine OMMT, suggests the formation of predominantly intercalated clay morphology upon processing. In Fig. 4 the effect of irradiation on mechanical properties of both filled and unfilled systems as function of the dose are reported. Data are shown as dimensionless values, with reference to those of the corresponding unirradiated samples, already reported in Table 1. It can be observed that the elastic modulus increases and the properties at break decrease with the dose. These effects are enhanced by the presence of OMMT, since in all the investigated dose range, with respect to LDPE, LDPE/OMMT composites show higher increase of the elastic modulus and higher decrease of both tensile strength and elongation at break. This mechanical behavior is not in agreement with the solubility data, which show a less efficient cross-linking on LDPE/OMMT composites with respect to LDPE, see Fig. 1. The observed influence of the clay on mechanical behavior of irradiated composites can be explained by the fact that irradiation
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
50 E-LDPE E-LDPE/OMMT TS-LDPE TS-LDPE/OMMT EB-LDPE EB-LDPE/OMMT
1.5
Hydroxyl Index
Dimensionless mechnical properties
2
1
0.5
LDPE-0kGy LDPE-50kGy LDPE-150kGy LDPE/OMMT-0kGy LDPE/OMMT-50kGy LDPE/OMMT-150kGy
25
0 0
50
100 150 Dose, kGy
200
250
Fig. 4. Dimensionless mechanical properties of LDPE and LDPE/OMMT films as a function of the dose.
0
50
100 150 200 Photo-oxidation time, h
250
300
Fig. 6. Hydroxyl Index of LDPE and LDPE/OMMT films as a function of the photooxidation time (the inset reports the hydroxyl index at low ageing times of the same samples).
200
70 LDPE-0kGy LDPE-50kGy LDPE-150kGy LDPE/OMMT-0kGy LDPE/OMMT-50kGy LDPE/OMMT-150kGy
not aged aged
60 Insoluble fraction, %
150 Carbonyl Index
435
100
50
58 47
46
50
42 38
40
35
32 30
25
23
17
20 10 0.3
0.2
causes chemical interactions between the clay and the polymer chains, enhancing the rigidity of the composite. Some chosen LDPE and LDPE/OMMT e-beam irradiated samples were subjected to accelerated photo-oxidation aging. The carbonyl and hydroxyl indices of the investigated samples, as function of the accelerated photo-oxidation time, are reported in Figs. 5 and 6, respectively. For all aged samples, both carbonyl and hydroxyl indices increase with the photo oxidation time. The data relative to LDPE/OMMT composites are higher than the corresponding data relative to LDPE, thus indicating a reduction of the photo-oxidation resistance due to the presence of OMMT. As for the effect of irradiation, a decrease of the photo-oxidation resistance increasing the dose can be observed. In the insets of Figs. 5 and 6, the carbonyl and hydroxyl indices at the low ageing times range are also reported. It can be observed
50 T1 M M
/O LD PE
/O
M
M M
M T-
50
kG
kG
y
y
y T0
kG
y kG
0
50 PE /O
LD PE
Fig. 5. Carbonyl Index of LDPE and LDPE/OMMT films as a function of the photooxidation time (the inset reports the carbonyl index at low ageing times of the same samples).
LD
300
PE -1
250
LD
100 150 200 Photo-oxidation time, h
PE -5
50
LD
0
LD PE
-0
0
kG
kG
y
y
0
Fig. 7. Insoluble fraction of the e-beam treated LDPE and LDPE/OMMT films (0, 50 and 150 kGy) before and after aging (at 264 h).
that the induction photo-oxidation time, i.e. the time at which the carbonyl and hydroxyl index curves change the slope, is significantly reduced for LDPE/OMMT with respect to LDPE. In particular the estimated induction photo-oxidation time of the investigated LDPE samples is in the range 50–72 h, while, the estimated time of the filled samples is reduced to about 25 h. A similar effect can also be observed on increasing the dose. These results confirm the before mentioned effect of both OMMT loading and of the dose on the photo oxidation resistance. In Fig. 7 the insoluble fractions for all the aged and not aged samples are reported. Upon the photo-oxidation the insoluble fractions of the LDPE/OMMT samples are higher than the unfilled LDPE samples at the same dose. Moreover, the differences between the not aged and the photo-oxidized samples, at the
436
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
300 not aged aged
250
217
215 201
192
200 ΔH, J/g
176 160
167
142
142 111
103
100
References
TM
M LD
PE
/O M
/O M PE LD
kG y
50 T-
TM /O M
PE
15 0
kG y
kG y 0
kG y -1 50
PE LD
LD
LD
PE
PE
-0
-5 0
kG y
kG y
0
LD
solubility data, showing a less efficient cross-linking on LDPE/ OMMT composites with respect to LDPE, is interpreted assuming that irradiation favours chemical interactions between the clay and the polymer chains, enhancing the rigidity of the polymer matrix. Results on aged samples indicate a synergic effect of both OMMT loading and ionizing radiation on the reduction of photooxidation resistance of polyethylene films.
Fig. 8. Fusion enthalpies (estimated by second heating) of the e-beam treated LDPE and LDPE/OMMT films (0, 50 and 150 kGy) before and after aging (at 264 h).
same dose, are more pronounced as consequence of the OMMT presence. These data can be explained considering that the photooxidation process causes the formation of insoluble fractions through the intermediate peroxides groups formed during ageing (La Mantia and Dintcheva, 2004, 2005). These molecular effects are enhanced by the OMMT presence, further confirming what was before discussed about the radiation resistance of the materials. Enthalpy data are shown in Fig. 8 for the same systems. It can be observed that the ability to crystallize from the melt increases with the increase of the ageing time. Considering that these values are calculated with respect to the soluble fractions, they indicate that photo-oxidation causes a molecular weight decrease of the soluble parts, in agreement with FT-IR results, thus favouring the crystallization from the melt. All obtained results suggest that the OMMT loading plays a significant role in the photo-oxidation behavior of the films also upon e-beam treatment and the synergic effect of the OMMT loading and e-beam irradiation leads to a more marked photooxidative degradation.
4. Summary In this paper both polyethylene (LDPE) and polyethylene/ commercial organo-modified montmorillonite nanocomposite films (LDPE/OMMT) were subjected to e-beam irradiation at various doses and their photo-oxidation aging was studied. The effect of the dose on the structure and on the mechanical behavior of LDPE and nano-filled films was investigated through solubility, FTIR, calorimetric and tensile tests. The LDPE/OMMT composites show higher increase of the elastic modulus and higher decrease of both tensile strength and elongation at break than the LDPE. This mechanical behavior, not explained by the
Alexandre, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. R: Rep. 28, 1–63. Burmistr, M.V., Sukhyy, K.M., Shilov, V.V., Pissis, P., Spanoudaki, A., Sukha, I.V., Tomilo, V.I., Gomza, Y.P., 2005. Synthesis, structure, thermal and mechanical properties of nanocomposites based on linear polymers and layered silicates modified by polymeric quaternary ammonium salts (ionenes). Polymer 46, 12226–12232. Clegg, D.W., Collyer, A.A., 1991. Irradiation Effects on Polymers. Elsevier Science Publishers Ltd., London. Dintcheva, N.Tz., Al-Malaika, S., La Mantia, F.P., 2009. Effect of extrusion and photo-oxidation on polyethylene/clay nanocomposites. Polym. Degrad. Stabil. 94, 1571–1588. Fornes, T.D., Yoon, P.J., Paul, D.R., 2003. Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites. Polymer 44, 7545–7556. Gad, Y.H., 2009. Improving the properties of poly(ethylene-co-vinyl acetate)/clay composite by using electron beam irradiation. Nucl. Instrum. Methods Phys. Res. B 267, 3528–3534. Gilman, J.W., 1999. Flammability and thermal stability studies of polymer layeredsilicate (clay) nanocomposites. Appl. Clay Sci. 15, 31–49. Jiao, C., Wang, Z., Chen, X., Yu, B., Hu, Y., 2006. Irradiation crosslinking and halogen-free flame retardation of EVA using hydrotalcite and red phosphorus. Radiat. Phys. Chem. 75, 557–563. Kiliaris, P., Papaspyrides, C.D., 2010. Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog. Polym. Sci. 35, 902–958. La Mantia, F.P., Dintcheva, N.T., Malatesta, V., Pagani, F., 2006. Improvement of photo-stability of LLDPE-based nanocomposites. Polym. Degrad. Stabil. 91, 3208–3213. La Mantia, F.P., Dintcheva, N.T., Scaffaro, R., Marino, R., 2008. Morphology and properties of polyethylene/clay nanocomposite drawn fibers. Macromol. Mater. Eng. 293, 83–91. La Mantia, F.P., Dintcheva, N.T., 2005. Processability and properties of re-graded photo-oxidized post-consumer greenhouse films. Macromol. Mater. Eng. 290, 970–975. La Mantia, F.P., Dintcheva, N.T., 2004. Time-carbonyl groups equivalence in photooxidative aging of virgin/recycled polymer blends. Plast. Rubber Compos. 33, 184–186. LeBaron, P.C., Wang, Z., Pinnavaia, T.J., 1999. Polymer-layered silicate nanocomposites: an overview. Appl. Clay Sci. 15, 11–29. Lu, H., Hu, Y., Kong, Q., Chen, Z., Fan, W., 2005. Gamma irradiation of high density poly(ethylene)/ethylene-vinyl acetate/clay nanocomposites: possible mechanism of the influence of clay on irradiated nanocomposites. Polym. Adv. Technol. 16, 688–692. Morlat-Therias, S., Mailhot, B., Gardette, J.L., Da Silva, C., Haidar, B., Vidal, A., 2005. Photooxidation of ethylene-propylene-diene/montmorillonite nanocomposites. Polym. Degrad. Stabil. 90, 78–85. Pavlidou, S., Papaspyrides, C.D., 2008. A review on polymer-layered silicate nanocomposites. Prog. Polym. Sci. 33, 1119–1198. Qin, H., Zhao, C., Zhang, Z., Chen, G., Yang, M., 2003. Photo-oxidative degradation of polyethylene/montmorillonite nanocomposite. Polym. Degrad. Stabil. 81, 497–500. Shi, Y., Kashiwagi, T., Walters, R.N., Gilman, J.W., Lyon, R.E., Sogah, D.Y., 2009. Ethylene vinyl acetate/layered silicate nanocomposites prepared by a surfactant-free method enhanced flame retardant and mechanical properties. Polymer 50, 3478–3487. Singh, A., Silverman, J., 1992. Radiation Processing of Polymers. Hanser Publishers, Munich. Sinha Ray, S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. Polym. Sci. 28, 1539–1641. Spinks, J.W.T., Woods, R.J., 1990. Introduction to Radiation Chemistry. Wiley Interscience, New York.
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Influence of the e-beam irradiation and photo-oxidation aging on the structure and properties of LDPE-OMMT nanocomposite films N.Tz. Dintcheva a,n, S. Alessi b, R. Arrigo b, G. Przybytniak c, G. Spadaro b a
Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy Dipartimento di Ingegneria Chimica, Gestionale, Informatica, Meccanica, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy c Institute of Nuclear Chemistry and Technology, Department of Radiation Chemistry and Technology, Laboratory of Radiation Modified Polymers, ul. Dorodna 16, 03-195 Warszawa, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2011 Accepted 14 December 2011 Available online 30 December 2011
In this paper two systems, polyethylene (LDPE) and polyethylene/commercial organo-modified montmorillonite (LDPE/OMMT) nanocomposite, were subjected to e-beam irradiation at different doses and both the molecular modifications and mechanical properties have been investigated through solubility, FT-IR, calorimetric and tensile tests. Moreover, in some of the irradiated systems photooxidation aging was performed and its effects were studied. The results show an enhancement with irradiation of the positive effect of the nano-filler loading, related to the increase of the mechanical properties, due to the increase of the nano-filler polymer interaction. Nevertheless calorimetric and FT-IR data indicate that the well known reduction of LDPE/OMMT nanocomposite resistance to photo-oxidation ageing, with respect to LDPE, is amplified by ionizing radiation. Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved.
Keywords: LDPE-OMMT nanocomposite film e-beam irradiation Photo-oxidation ageing Mechanical properties Carbonyl index Molecular modification
1. Introduction The silicate based polymer nano-composites had a great interest in the last decade because their properties and performance are superior than that the pristine polymeric matrix and/or the traditional composite materials (Sinha Ray and Okamoto, 2003; Pavlidou and Papaspyrides, 2008; Alexandre and Dubois, 2000; La Mantia et al., 2008; LeBaron et al., 1999). The nano-fillers have at least one dimension in nano-metric scale and, as it is well known in the literature, the nanocomposites show significantly improved mechanical, thermal, optical and physical properties. In particular high values of elastic modulus without loss in ductility is obtained. Furthermore the very small dimensions of the nano-particles lead to good dispersion and transparency, and to a significant improvement of the barrier properties. From a thermal point of view, the nano-filler presence leads to an improved thermal and flame stabilities. The significant enhancement of the nano-composites’s properties can be obtained by nano-filler loading less than 10 wt%, while in order to obtain similar properties change of a micro-composites, about 30–50 wt% of micro-filler is requested (Burmistr et al., 2005; Shi et al., 2009; Kiliaris and Papaspyrides, 2010; Gilman, 1999).
n
Corresponding author. Tel.: þ3909123863704; fax: þ 390917025020. E-mail address: [email protected] (N.Tz. Dintcheva).
g-ray, x-ray or e-beam irradiation of the polymeric materials causes different phenomena, such as chain branching up to crosslinking and chain scission with molecular degradation. The crosslinking and degradation processes are two co-current phenomena and the predominant effect should be evaluated considering the treatment conditions and the intrinsic polymer properties (Spinks and Woods, 1990; Singh and Silverman, 1992). Few papers are available in the actual literature about the ionizing radiation treatment of nano-filled polymer based nano-composites (Lu et al., 2005; Gad, 2009; Jiao et al., 2006). In particular, Lu et al. (2005) reported an interesting study about the mechanical and morphological variations of the HDPE/EVA blend without and with clays upon g-irradiation. As widely reported in the scientific literature (Fornes et al., 2003; Qin et al., 2003; Morlat-Therias et al., 2005; La Mantia et al., 2006; Dintcheva et al., 2009) the clay adding to the polymer matrix leads to a significant reduction of the photo-oxidation resistance with respect to the unfilled polymeric matrix. Different reasons for the accelerated photo-oxidation degradation are reported in the papers, in particular, the pro-degradative effect of the present iron ions, the degradation and the subsequent radical formation of the organo-modifier, the formation of the catalytic acidic sites on the layers and the entrapment of the polar antioxidant into the clay galleries. In this paper both polyethylene and nanocomposite films based on polyethylene and commercial organo-modified montmorillonite were irradiated at different doses and their photo-oxidation
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N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
2. Experimental The materials used in this work were as follows: – a film grade Low Density Polyethylene (Riblene& FC30), LDPE, supplied by Polimeri Europa; density r ¼0.922 g/cm3 at 23 1C and MFI190 1C/2.16 kg of 0.27 g/10 min. LDPE samples do not contain any kind of antioxidant; – an organo-modified montmorillonite commercialized by Southern Clay Products (U.S.A.) as ‘‘Cloisites 15 A’’ (further indicated as ‘‘OMMT’’; modified with dimethyldihydrogenated tallow-quaternary ammonium chloride –2M2HT– quat surfactant; quat concentration¼125 meq/100 g clay; d001 ¼3.15 nm; density¼1.66 g/cm3). Hydrogenated tallow of OMMT is a blend of saturated n-alkyl groups with approximate composition: 65% C18; 30% C16; 5% C14. A modular co-rotating twin screw extruder, OMC (Italy), has been used to prepare the filled sample. In order to have the same thermal history, the unfilled matrix was subjected to the same extrusion. The temperature profile was 120–130–140–150–160– 170–180 1C and the rotational speed 200 rpm. The extruded material has been immediately cooled and ground on line to obtain pellets for the further processing and characterizations. The films were obtained by a single screw extruder equipped with a film blowing head and with a Brabender film blowing unit. The head temperature was 180 1C and the screw speed was 60 rpm. The film thickness was about 60 mm for all the samples. The e-beam irradiation of the unfilled LDPE and of OMMT-filled LDPE films has been carried out by the LAE 10 MeV linear, pulsed accelerator located in the laboratory of the ICHTJ (Institute of Nuclear Chemistry and Technology) of Warsaw, Poland. The samples have been placed in the horizontal position in the front of the pulsed beam and the total doses were obtained by multipass exposure. The irradiation conditions were pulse current 470 mA, pulse frequency 400 Hz and dose/pass 25 kGy. Samples at 25, 50, 75, 100, 125, 150, 200 and 250 kGy of total doses have been irradiated. The unfilled LDPE and the composite sheets, irradiated at 0, 50 and 150 kGy, were exposed to accelerated weathering in a Q-UV (U.S.A.) chamber, containing eight UVB-313 (Q-Labs Corp., U.S.A) lamps. The exposure cycle conditions were as follows: 8 h of light at T¼55 1C followed by 4 h condensation at T¼35 1C. The measured photon flux was 2 mW/cm2. FT-IR spectra were measured by a Spectrum One spectrometer by Perkin-Elmer, using the Spectrum software. Spectra were obtained through 32 scans with a 4 cm 1 resolution. Carbonyl (peak measured between 1675 and 1825 cm 1) and hydroxyl (peak measured between 3200 and 3600 cm 1), indices were determined from peak absorption area (peak area absorbance of the group compared to that of a reference area measured between 1979 and 2110 cm 1; reference peak corresponds to the C–H vibrations and it does not change during ageing), as follows: Carbonyl Index¼Absorbance (1675–1825 cm 1)/Absorbance (1979–2110 cm 1)
Hydroxyl Index ¼Absorbance (3200–3600 cm 1)/Absorbance (1979–2110 cm 1) Measurements were obtained from the average of triplicate samples. Tensile properties were determined at both room temperature and humidity, using an Instron (U.S.A.) dynamometer mod. 3365, according to ASTM test method D882. The specimens were cut out from films and tested in the machine direction. The modulus was measured at the speed of 1 mm/min. When the deformation was about 10%, the speed was increased up to 100 mm/min until break. The data reported are the average values (with the related error bars) obtained through ten tests per sample. Calorimetric analysis was performed by a Perkin Elmer Precisely Jade DSC in order to evaluate the melting enthalpy values of each sample. The analysis was carried out according to the following program: heating from 30 1C to 160 1C at 5 1C/min, holding at 160 1C for 1 min, cooling from 160 1C to 30 1C at 5 1C/min and heating from 30 1C to 160 1C at 5 1C/min. The melting enthalpy data have been calculated for the second heating run and with respect to the soluble weight fractions of each sample. Extraction tests were performed by Soxhlet extractor using p-xylene as solvent. Approximately 0.3 g of any sample was exposed to refluxing in p-xylene close to its boiling point. Extraction time was about 72 h. Wide-angle X-ray analyses (WAXD) were performed at room temperature in the reflection mode on a Siemens D-500 X-ray diffractometer with Cu Ka radiation of wavelength of 0.1542 nm. A scanning rate of 10 1C min 1 was used. The distances d001 between the silicate layers of the clay in the nanocomposite blends was evaluated using the Bragg’s condition d001 ¼ nl/(2 sin y), where l is the wavelength, y is the angle of incidence of X-ray beam and n is an integer.
3. Results and discussion The molecular modifications induced by irradiation have been investigated through solvent extraction tests and FT-IR analysis. In Fig. 1 insoluble fractions as function of the dose are reported. It can be observed that, for LDPE/OMMT irradiated
50
LDPE LDPE/OMMT 40 Insoluble franction, %
aging was studied. The effect of the structural variations, due to e-beam irradiation and photo-oxidation, on the mechanical behavior of nano-filled films was investigated. The interest toward this study is justified by the possibility to use ionizing radiation for sterilization in food packaging. It can be important to have information about the different response of clay filled and unfilled samples after both e-beam irradiation and further photo oxidation processes.
433
30
20
10
0 0
50
100 150 Dose, kGy
200
250
Fig. 1. Insoluble fraction of LDPE and LDPE/OMMT films as a function of the dose.
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
samples, the values are always lower than those of the corresponding irradiated unfilled LDPE samples. This result indicates that the e-beam induced cross-linking of LDPE is higher than LDPE-OMMT systems, according to the expected effect of an inert filler on the irradiation of polymeric systems. The occurrence of oxidative phenomena during irradiation has been investigated through FTIR analysis. The corresponding spectra are reported in Fig. 2. The spectra of LDPE, Fig. 2a, show a gradual band rise at 1717 cm 1 due to the formation of the carbonyl group. Furthermore, the data correspondent to LDPE/OMMT, Fig. 2b, show a similar formation of the same oxidation products. Moreover a peak at 1740 cm 1, related to some impurities present in the structure of the pristine OMMT, is evidenced. For these samples the area of the carbonyl group has been calculated eliminating the contribution of the impurities of the pristine OMMT, by mathematical deconvolution of the peaks in the carbonyl range. The area of the carbonyl group as a function of the dose is reported in Fig. 3, showing that the carbonyl concentration is very similar for both unfilled and nanofilled systems, thus indicating comparable oxidation levels. On the basis of FTIR spectra, it is possible to conclude that the concentration of oxidized products is very low, according to the very high dose rate (Singh and Silverman, 1992; Clegg and Collyer, 1991). Furthermore, due to OMMT presence, some additional reactive sites come from the Hoffman elimination of the organomodifier of the clay nano-particles, as already reported in literature (Fornes et al., 2003; Qin et al., 2003; Dintcheva et al., 2009). The Hoffman elimination starts during the processing, compounding and film-blowing, and leads to the formation of a-olefins, amines
Fig. 2. FT-IR spectra, i.e. carbonyl range, of LDPE (a) and LDPE/OMMT (b) films as a function of the dose.
2 LDPE LDPE/OMMT 1.5 Carbonyl area
434
1
0.5
0 0
50
100 150 Dose, kGy
200
250
Fig. 3. Carbonyl area of LDPE and LDPE/OMMT films as a function of the dose.
Table 1 Mechanical properties of LDPE and OMMT filled LDPE films and interlayer distances of pristine clay and of LDPE/OMMT sample. Samples
E (Mpa)
TS (Mpa)
EB (%)
Interlayer distance (nm)
LDPE LDPE/OMMT OMMT
140 175 –
16 17 –
450 455 –
– 3.30 3.15
and secondary degradation products. FT-IR results indicate that the additional amount of the reactive sites does not imply the formation of supplementary oxidation products. In Table 1 the mechanical behavior of both LDPE and LDPE-clay unirradiated systems are reported. It can be observed that the OMMT loading causes a significant increase of the elastic modulus, while the properties at break remain almost unchanged, according to the well known improvement of the matrix rigidity without losses in ductility caused by clay loading (Sinha Ray and Okamoto, 2003; Pavlidou and Papaspyrides, 2008; Alexandre and Dubois, 2000). In the same Table 1 (last column), the calculated interlayer distance between the layers of nanoparticles in the pristine OMMT and LDPE/OMMT film are reported. The slight increase of interlayer distance for LDPE/OMMT sample, with respect to the pristine OMMT, suggests the formation of predominantly intercalated clay morphology upon processing. In Fig. 4 the effect of irradiation on mechanical properties of both filled and unfilled systems as function of the dose are reported. Data are shown as dimensionless values, with reference to those of the corresponding unirradiated samples, already reported in Table 1. It can be observed that the elastic modulus increases and the properties at break decrease with the dose. These effects are enhanced by the presence of OMMT, since in all the investigated dose range, with respect to LDPE, LDPE/OMMT composites show higher increase of the elastic modulus and higher decrease of both tensile strength and elongation at break. This mechanical behavior is not in agreement with the solubility data, which show a less efficient cross-linking on LDPE/OMMT composites with respect to LDPE, see Fig. 1. The observed influence of the clay on mechanical behavior of irradiated composites can be explained by the fact that irradiation
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
50 E-LDPE E-LDPE/OMMT TS-LDPE TS-LDPE/OMMT EB-LDPE EB-LDPE/OMMT
1.5
Hydroxyl Index
Dimensionless mechnical properties
2
1
0.5
LDPE-0kGy LDPE-50kGy LDPE-150kGy LDPE/OMMT-0kGy LDPE/OMMT-50kGy LDPE/OMMT-150kGy
25
0 0
50
100 150 Dose, kGy
200
250
Fig. 4. Dimensionless mechanical properties of LDPE and LDPE/OMMT films as a function of the dose.
0
50
100 150 200 Photo-oxidation time, h
250
300
Fig. 6. Hydroxyl Index of LDPE and LDPE/OMMT films as a function of the photooxidation time (the inset reports the hydroxyl index at low ageing times of the same samples).
200
70 LDPE-0kGy LDPE-50kGy LDPE-150kGy LDPE/OMMT-0kGy LDPE/OMMT-50kGy LDPE/OMMT-150kGy
not aged aged
60 Insoluble fraction, %
150 Carbonyl Index
435
100
50
58 47
46
50
42 38
40
35
32 30
25
23
17
20 10 0.3
0.2
causes chemical interactions between the clay and the polymer chains, enhancing the rigidity of the composite. Some chosen LDPE and LDPE/OMMT e-beam irradiated samples were subjected to accelerated photo-oxidation aging. The carbonyl and hydroxyl indices of the investigated samples, as function of the accelerated photo-oxidation time, are reported in Figs. 5 and 6, respectively. For all aged samples, both carbonyl and hydroxyl indices increase with the photo oxidation time. The data relative to LDPE/OMMT composites are higher than the corresponding data relative to LDPE, thus indicating a reduction of the photo-oxidation resistance due to the presence of OMMT. As for the effect of irradiation, a decrease of the photo-oxidation resistance increasing the dose can be observed. In the insets of Figs. 5 and 6, the carbonyl and hydroxyl indices at the low ageing times range are also reported. It can be observed
50 T1 M M
/O LD PE
/O
M
M M
M T-
50
kG
kG
y
y
y T0
kG
y kG
0
50 PE /O
LD PE
Fig. 5. Carbonyl Index of LDPE and LDPE/OMMT films as a function of the photooxidation time (the inset reports the carbonyl index at low ageing times of the same samples).
LD
300
PE -1
250
LD
100 150 200 Photo-oxidation time, h
PE -5
50
LD
0
LD PE
-0
0
kG
kG
y
y
0
Fig. 7. Insoluble fraction of the e-beam treated LDPE and LDPE/OMMT films (0, 50 and 150 kGy) before and after aging (at 264 h).
that the induction photo-oxidation time, i.e. the time at which the carbonyl and hydroxyl index curves change the slope, is significantly reduced for LDPE/OMMT with respect to LDPE. In particular the estimated induction photo-oxidation time of the investigated LDPE samples is in the range 50–72 h, while, the estimated time of the filled samples is reduced to about 25 h. A similar effect can also be observed on increasing the dose. These results confirm the before mentioned effect of both OMMT loading and of the dose on the photo oxidation resistance. In Fig. 7 the insoluble fractions for all the aged and not aged samples are reported. Upon the photo-oxidation the insoluble fractions of the LDPE/OMMT samples are higher than the unfilled LDPE samples at the same dose. Moreover, the differences between the not aged and the photo-oxidized samples, at the
436
N.Tz. Dintcheva et al. / Radiation Physics and Chemistry 81 (2012) 432–436
300 not aged aged
250
217
215 201
192
200 ΔH, J/g
176 160
167
142
142 111
103
100
References
TM
M LD
PE
/O M
/O M PE LD
kG y
50 T-
TM /O M
PE
15 0
kG y
kG y 0
kG y -1 50
PE LD
LD
LD
PE
PE
-0
-5 0
kG y
kG y
0
LD
solubility data, showing a less efficient cross-linking on LDPE/ OMMT composites with respect to LDPE, is interpreted assuming that irradiation favours chemical interactions between the clay and the polymer chains, enhancing the rigidity of the polymer matrix. Results on aged samples indicate a synergic effect of both OMMT loading and ionizing radiation on the reduction of photooxidation resistance of polyethylene films.
Fig. 8. Fusion enthalpies (estimated by second heating) of the e-beam treated LDPE and LDPE/OMMT films (0, 50 and 150 kGy) before and after aging (at 264 h).
same dose, are more pronounced as consequence of the OMMT presence. These data can be explained considering that the photooxidation process causes the formation of insoluble fractions through the intermediate peroxides groups formed during ageing (La Mantia and Dintcheva, 2004, 2005). These molecular effects are enhanced by the OMMT presence, further confirming what was before discussed about the radiation resistance of the materials. Enthalpy data are shown in Fig. 8 for the same systems. It can be observed that the ability to crystallize from the melt increases with the increase of the ageing time. Considering that these values are calculated with respect to the soluble fractions, they indicate that photo-oxidation causes a molecular weight decrease of the soluble parts, in agreement with FT-IR results, thus favouring the crystallization from the melt. All obtained results suggest that the OMMT loading plays a significant role in the photo-oxidation behavior of the films also upon e-beam treatment and the synergic effect of the OMMT loading and e-beam irradiation leads to a more marked photooxidative degradation.
4. Summary In this paper both polyethylene (LDPE) and polyethylene/ commercial organo-modified montmorillonite nanocomposite films (LDPE/OMMT) were subjected to e-beam irradiation at various doses and their photo-oxidation aging was studied. The effect of the dose on the structure and on the mechanical behavior of LDPE and nano-filled films was investigated through solubility, FTIR, calorimetric and tensile tests. The LDPE/OMMT composites show higher increase of the elastic modulus and higher decrease of both tensile strength and elongation at break than the LDPE. This mechanical behavior, not explained by the
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