- Email: [email protected]
clay nanocomposites
ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 1698–1702 www.elsevier.com/locate/radphyschem
Electron beam crosslinking of poly(ethylene-co-vinyl acetate)/clay nanocomposites Jamaliah Sharifa,, Khairul Zaman Mohd Dahlana, Wan Md Zin Wan Yunusb a
Malaysian Institute for Nuclear Technology Research (MINT), Bangi, 43000 Kajang, Malaysia Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia
b
Received 26 December 2006; accepted 2 February 2007
Abstract Effect of electron beam irradiation on the thermal and mechanical properties of poly(ethylene-co-vinyl acetate) (EVA)/clay nanocomposites prepared by melt blending method has been investigated. The hot set test results show that elongation at high temperature under static load decreased with the increase of irradiation dose. The tensile modulus increased continuously with increasing dose. While the tensile strength increased up to 100 kGy, it decreased with further increase in dose. The elongation at break decreased continuously with increasing dose. Thermogravimetric analysis showed that thermal stability of the EVA/clay nanocomposites improved with increasing dose. The improvement in the mechanical and thermal properties is attributed to the formation of radiation-induced crosslinking as evidenced by the gel content results. r 2007 Elsevier Ltd. All rights reserved. Keywords: EVA; Clay; Nanocomposites; Electron beam; Crosslinking
1. Introduction Poly(ethylene-co-vinyl acetate) (EVA) is widely used as a matrix for nanocomposites with organically modified layered clay. This is due to the presence of polar vinyl acetate groups all along the chains that improve the ability of the copolymer to intercalate into organo-modified montmorillonites (Alexander and Dubois, 2000). The formation of intercalated or exfoliated nanocomposites with fine dispersion of the clay layers in the EVA matrix leads to an improvement in the thermal properties (Zanetti et al., 2001; Riva et al., 2002) and flame retardant properties (Duquesne et al., 2003). The polarity of EVA and the basal spacing of the organoclay are of importance to the morphology and properties of EVA/clay nanocomposites (Zhang et al., 2003; Li et al., 2003). EVA is also widely used in radiation-processing technology for the production of wire and cable, heat shrinkable tube and many other products. EVA was chosen due to its ability to be compounded with large amount of additives and its Corresponding author. Tel.: +60 389250510; fax: +60 389282963.
E-mail address: [email protected] (J. Sharif). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.091
readiness to form crosslinking network at low irradiation dose (Burns, 1979). Electron beam-induced crosslinking at ambient temperature is much more stable than the crosslinking brought about by conventional vulcanizates. The conventional curing of cables includes mixing curative with base polymer and moulding at high temperature, which sometimes affects the thermal stability. Therefore, radiation crosslinking is a very useful technique to improve the thermal stability, stress crack resistance, solvent resistance and toughness of polymeric materials (Fisher, 1969). Literature search shows no report on radiation effects on EVA/clay nanocomposites. In this work we have prepared EVA-based nanocomposite through melt compounding and exposed them to electron beam irradiation at various doses. The mechanical and thermal property changes were determined and the results are presented in this report. 2. Experimental 2.1. Materials Ethylene vinyl acetate with about 18 wt% vinyl acetate content, MFI ¼ 1.66 g/10 min (at 190 1C and 2.16 kg load)
ARTICLE IN PRESS J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
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and density of 0.942 g/mL was bought from Asia Polymer Corporation, Taiwan. Sodium montmorillonite (Kunipia F) with a cation exchange capacity of 119 meq/100 g was purchased from Kunimine Ind. Co., Japan. Dimethyl dihydrogenated tallow ammonium-modified montmorillonite (Cloisite 20A) was bought from Southern Clay Products Inc., Gonzales, TX. Irganox 1010 was obtained from Ciba Geigy, Malaysia.
strength, about 3% for the modulus at 300% elongation and 5% for elongation at break. The thermogravimetric analysis was carried out using a Perkin Elmer TGA7. About 10 mg samples were heated from 30 to 700 1C at a scan rate of 10 1C/min in a dynamic nitrogen atmosphere with a flow rate of 20 ml/min.
3. Nanocomposite preparations
6.1. Degree of crosslinking
The EVA/clay nanocomposite was prepared using a twin-screw compounder (Sino). The EVA pellets, organoclay (C20A) and antioxidant were mixed together and charged into the mixing chamber through a main feeder, which is heated at 140 and 120 1C, respectively. EVA compounded with antioxidant only was also prepared for comparison purposes. Samples of the nanocomposites were then compression moulded in an electrically heated hydraulic press. Hotpress procedure involved preheating at 130 1C for 4 min followed by compression at 110 kg/cm2 for 4 min at the same temperature and subsequently cooling at 20 1C under the same pressure for 4 min.
The relationship between the gel content and irradiation dose of EVA-based nanocomposite containing 5 phr Cloisite 20A (EVA/C20A) is shown in Fig. 1. It can be seen from the figure that the gel content increases rapidly with the increase of irradiation dose up to 100 kGy. Further increase in irradiation dose increases the gel content only marginally. It is also observed that the gel content of EVA/C20A is slightly higher than that of EVA compound without the organoclay. This is probably due to the presence of dimethyl dihydrogenated tallow quaternary ammonium-modified montmorillonite (Cloisite 20A), which seems to increase the formation of radiation-induced crosslinking network in the nanocomposite.
4. Irradiation of samples
6.2. Hot set test
Irradiation was carried out in air using a Cockroft Walton-type electron beam accelerator (Nissin High Voltage, Japan) at an acceleration voltage of 2 MeV and beam current of 5 mA. The distance of the sample from the scan horn was 20 cm and the conveyer speed was set at 0.94 m/min. The dose rate was 50 kGy/pass. The samples were irradiated at doses ranging from 50 to 250 kGy.
Hot set test provides a quick check on crosslinking of samples and can be correlated with the gel fraction. Hot set test was carried out and the result is shown in Table 1. It can be seen from the table that unirradiated EVA and EVA/5C20A samples break immediately when placed in the hot oven. This is because under elevated temperature of 200 1C and load of 20 N/cm2 the unirradiated sample was melted and easily deformed by the load on the samples. The samples irradiated at 50 kGy failed the test at a longer time. This is because the level of crosslinking of about 29% (Fig. 1) is not enough to prevent the polymer from deformation by the load. It is also noted that the presence
The gel fraction of the samples was determined by boiling the samples in xylene for 24 h. The extracted samples were washed with xylene and soaked in methanol for about 30 min and then vacuum dried to constant weight for 16 h. The gel fraction was calculated as the ratio of weight of the dried sample after extraction to the initial weight of the sample before extraction. Hot set test was done to measure the elongation of samples for static load at a fixed temperature. Measurements were carried out in an oven at 200 1C with normal air conditions. The sample loading was 20 N/cm2. The hot elongation was determined after the sample was heated for 15 min. Tensile tests were carried out according to ASTM D417 using an Instron model 4301 testing machine. The dumbbell-shaped specimens were extended at a crosshead speed of 100 mm/min. The tensile property values reported represent an average of the results from tests run on five specimens. Standard deviations were 2–5% for tensile
90 80 70 Gel content (%)
5. Characterization of samples
6. Results and discussion
60 50 40 30 20 10
EVA
EVA/5C20A
0 0
50
100
150
200
250
Irradiation dose (kGy) Fig. 1. Relationship between gel fraction and irradiation dose of the samples.
ARTICLE IN PRESS Table 1 Hot set test results at 200 1C under 20 N/cm2 load for 15 min Irradiation dose (kGy)
0 50 100 150 200 250
Elongation at 120 1C (%) EVA
EVA/C20A
Break immediately Break 14 s 220 84 52 40
Break immediately Break 28 s 200 79 47 37
Modulusat 300% elongation (MPa)
J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
1700
9
8
7
6 EVA 5 0
6.3. Tensile properties The modulus at 300% elongation of EVA and EVA/ C20A nanocomposites is shown in Fig. 2. It can be seen that the tensile modulus of both EVA and EVA/C20A increases continuously with the increase of irradiation dose up to 250 kGy. The increase in tensile modulus with irradiation dose can be related to the increased formation of radiation-induced crosslinking network as evidenced by the gel-content result shown in Fig. 1. The higher tensile modulus shown by EVA/C20A nanocomposite compared to that of EVA is due to the dispersion of the silicate layers in the EVA that provide the reinforcement effect to the polymer matrix therefore increasing the tensile modulus. The improvement in elastic modulus has been reported by several researchers and it is attributed to the exfoliation and good dispersion of nanosized clay that restricts the mobility of polymer chain under loading as well as good interfacial adhesion between the clay layers and the epoxy as reported by Yasmin et al. (2003) and Pozsgay et al. (2004). Fig. 3 shows the variation of tensile strength with irradiation dose of the samples. It is found that before irradiation the tensile strength of both EVA and EVA/ C20A is about the same but after irradiation the tensile
100
200
300
Irradiation dose (kGy) Fig. 2. Relationship between modulus at 300% elongation with irradiation dose.
36 34 Tensile strength (MPa)
of C20A has delayed the breaking time of the samples. However, at 100 kGy and above the samples elongated to a certain length and remained at that length until the 15 min testing time was completed. This is due to the increased formation of radiation-induced crosslinking as evident by the gel content results shown in Fig. 1. It is also found from the table that the presence of organoclay decreases slightly the hot elongation. This is probably due to the higher crosslinking and shielding effect provided by the clay layers to the confine polymer chains. Weisner (1991) reported that different polymer structures require different levels of crosslinking for certain reduction of deformation caused by the load in the hot set test. Formation of adequate radiation crosslinking network gives the polymer better thermal stability at high temperature. Table 1 also shows that elongation value for EVA and EVA/C20A was noted at irradiation dose of 100 kGy, which is equivalent to the gel content of about 60%.
EVA/5C20A
32 30 28 26 24 22
EVA
EVA/5C20A
20 0
50
100
150
200
250
Irradiation dose (kGy) Fig. 3. Relationship between tensile strength and irradiation dose.
strength of the EVA/C20A nanocomposites increases much more compared to that of the unfilled EVA. This is probably due to the formation of crosslinking, which improves the reinforcement effect of the silicate layers to the EVA matrix. The tensile strength is increased with the increase of irradiation dose up to 100 kGy and decreases with further increase of dose up to 250 kGy. The increase in tensile strength up to 100 kGy is probably due to the fact that a large network structure is formed up to 100 kGy radiation dose. However, at higher irradiation dose these network structures become smaller because the process of the radiation-induced crosslinking continues to form between already crosslinked macromolecule as reported by Mateev and Karageorgiev (1998). It is generally known that tensile strength and elongation at break depend on the degree of the strain-induced crystallization, which in turn depends on the polymer chain length and the degree of crosslinking. Longer polymer chains can exhibit higher strain-induced crystallization and better strength properties compared to shorter ones (Datta et al., 1996). Crosslinking of the polymer chain reduces the tendency of chain slippage during straining leading to an increase in the strain-induced
ARTICLE IN PRESS J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
Decomposition temperature (°C)
Elongation at break (%)
700 650 600 550 500 450 400 350
EVA
EVA/5C20A
300 0
50
100
150
200
250
300
535 530 525 520 515 510 505
EVA
crystallization. However, too much crosslinking may also lower the strain-induced crystallization process. From the results, it can be concluded that EB radiation help increase the tensile strength of the nanocomposite. The elongation at break of all EVA compounds decreases with the increase of radiation dose as shown in Fig. 4. This is attributed to the formation of radiation crosslinking network in the polymer matrix, which restricted the mobility of the molecular chain during drawing (Lyons, 1992).
6.4. Thermogravimetric study The thermal stability of EVA/C20A nanocomposite irradiated at 50 to 250 kGy was determined using thermogravimetric analysis method and the results are plotted in Fig. 5. The nanocomposite decomposes in two stages. The first stage is due to the elimination of acetic acid and formation of double bonds, which usually occurs between 300 and 420 1C. The second degradation step involves the ethylene backbone chain, which occurs between 420 and 550 1C and leads to complete polymer volatilization. It can be seen from Fig. 5 that the thermal stability of the EVA/C20A nanocomposites improves after irradiation with electron beam. The decomposition temperature of the nanocomposite samples increases remarkably with the increase of irradiation dose from 50 to 250 kGy. The increase is higher compare to that of the EVA without the clay. The improvement in thermal stability is due to the formation of more compact three-dimensional crosslinking networks which is more stable against formation of gaseous products on heating (Krupa, 2001). In addition, the marked improvement in thermal stability of the EVA/C20A nanocomposite is probably due to presence of silicate layers that act as barriers to minimize the permeability of volatile degradation product out from the material and also decrease the oxygen diffusion from the gas phase into the polymer matrix (Agag et al., 2001).
EVA/5C20A
500 0
50
100
150
200
250
300
Irradiation dose (kGy)
Irradiation dose (kGy) Fig. 4. Relationship between elongation at break and irradiation dose.
1701
Fig. 5. Relationship between decomposition temperature and irradiation dose.
7. Conclusion The study has demonstrated that it is possible to crosslink the EVA/C20A nanocomposites with electron beam irradiation. The presence of C20A did not interfere with the formation of radiation-induced crosslinking network in the polymer matrix. The tensile modulus increased continuously and the elongation at break decreased continuously with the increase of irradiation dose. The tensile strength increased and maximized at 100 kGy. The decomposition temperature of the nanocomposite also increases remarkably with the increase of irradiation dose. The improvement in the tensile strength and decomposition temperature of the EVA/C20A nanocomposite with increase dose is due to the increased formation of radiation-induced crosslinking network in the nanocomposite. Acknowledgment We thank the staff from Electron Beam facility of Malaysian Institute for Nuclear Technology Research (MINT) for their assistance in irradiation work of the samples. Thanks also due to Mr. Siva for his help in preparing the nanocomposite with twin-screw compounder. References Agag, T., Koga, T., Takeichi, T., 2001. Studies on thermal and mechanical properties of polyimide–clay nanocomposites. Polymer 42, 3399–3408. Alexander, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. R 28 (1), 1–63. Burns, N.M., 1979. The radiation crosslinking of ethylene copolymers. Radiat. Phys. Chem. 14, 797–808. Datta, S.K., Chaki, T.K., Khastgir, D., 1996. Effect of electron beam radiation on mechanical and electrical properties of poly(ethylene-covinyl acetate). Die Angew. Makromol. Chem. 238, 105–117. Duquesne, S., Jama, C., Le- Bras, M., Delobel, R., Recourt, P., Gloaguen, J.M., 2003. Elaboration of EVA–nanoclay systems-characterization, thermal behavior and fire performance. Compos. Sci. Technol. 23, 1141–1148.
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Fisher, J., 1969. Effect of gamma radiation on ethylene vinyl acetate copolymers. 27th SPE ANTEC, pp. 556–566. Krupa, I., Luyt, A.S., 2001. Thermal and mechanical properties of LLDPE crosslinked with gamma radiation. Polym. Degrad. Stab. 71, 361–366. Li, X., Ha, C.S., 2003. Nanostructured of EVA/organoclay nanocomposites: effects of kinds of organoclays and grafting of maleic anhydride onto EVA. J. Appl. Polym. Sci. 87 (12), 1901–1909. Lyons, B.J., 1992. The physical and chemical effects of ionizing radiation on polyethylene containing a phenolic scavenger and/or other additives. Radiat. Phys. Chem. 40 (6), 489–499. Mateev, M., Karageorgiev, S., 1998. The effect of electron beam irradiation and content of EVA upon the gel-forming processes in LDPE-EVA films. Radiat. Phys. Chem. 51, 205–206. Pozsgay, A., Frater, T., Szazdi, L., Muller, P., Sajo, I., Pukanszky, B., 2004. Gallery structure and exfoliation of organophilized
montmorillonite: effect on composite properties. Eur. Polym. J. 40, 27–36. Riva, A., Zanetti, M., Braglia, M., Camiro, G., Falqui, L., 2002. Thermal degradation and rheological bahaviour of EVA/montmorillonite nanocomposites. Polym. Degrad. Stab. 77 (2), 299–304. Wiesner, L., 1991. Effects of radiation on polyethylene and other polyolefins in the presence of oxygen. Radiat. Phys. Chem. 37 (1), 77–81. Yasmin, A., Abot, J.L., Daniel, I.M., 2003. Processing of epoxy/clay nanocomposites by shear mixing. Scripta Mates. 49, 81–86. Zanetti, M., Camiro, G., Thomann, R., Mullhaupt, R., 2001. Synthesis and thermal behaviour of layered silicate-EVA nanocomposites. Polymer 42, 4501–4507. Zhang, W., Chen, D., Zhao, Q., Fang, Y., 2003. Effects of different kinds of clay and different vinyl acetate content on the morphology and properties of EVA/clay nanocomposites. Polymer 44 (1), 7953–7961.
Radiation Physics and Chemistry 76 (2007) 1698–1702 www.elsevier.com/locate/radphyschem
Electron beam crosslinking of poly(ethylene-co-vinyl acetate)/clay nanocomposites Jamaliah Sharifa,, Khairul Zaman Mohd Dahlana, Wan Md Zin Wan Yunusb a
Malaysian Institute for Nuclear Technology Research (MINT), Bangi, 43000 Kajang, Malaysia Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia
b
Received 26 December 2006; accepted 2 February 2007
Abstract Effect of electron beam irradiation on the thermal and mechanical properties of poly(ethylene-co-vinyl acetate) (EVA)/clay nanocomposites prepared by melt blending method has been investigated. The hot set test results show that elongation at high temperature under static load decreased with the increase of irradiation dose. The tensile modulus increased continuously with increasing dose. While the tensile strength increased up to 100 kGy, it decreased with further increase in dose. The elongation at break decreased continuously with increasing dose. Thermogravimetric analysis showed that thermal stability of the EVA/clay nanocomposites improved with increasing dose. The improvement in the mechanical and thermal properties is attributed to the formation of radiation-induced crosslinking as evidenced by the gel content results. r 2007 Elsevier Ltd. All rights reserved. Keywords: EVA; Clay; Nanocomposites; Electron beam; Crosslinking
1. Introduction Poly(ethylene-co-vinyl acetate) (EVA) is widely used as a matrix for nanocomposites with organically modified layered clay. This is due to the presence of polar vinyl acetate groups all along the chains that improve the ability of the copolymer to intercalate into organo-modified montmorillonites (Alexander and Dubois, 2000). The formation of intercalated or exfoliated nanocomposites with fine dispersion of the clay layers in the EVA matrix leads to an improvement in the thermal properties (Zanetti et al., 2001; Riva et al., 2002) and flame retardant properties (Duquesne et al., 2003). The polarity of EVA and the basal spacing of the organoclay are of importance to the morphology and properties of EVA/clay nanocomposites (Zhang et al., 2003; Li et al., 2003). EVA is also widely used in radiation-processing technology for the production of wire and cable, heat shrinkable tube and many other products. EVA was chosen due to its ability to be compounded with large amount of additives and its Corresponding author. Tel.: +60 389250510; fax: +60 389282963.
E-mail address: [email protected] (J. Sharif). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.091
readiness to form crosslinking network at low irradiation dose (Burns, 1979). Electron beam-induced crosslinking at ambient temperature is much more stable than the crosslinking brought about by conventional vulcanizates. The conventional curing of cables includes mixing curative with base polymer and moulding at high temperature, which sometimes affects the thermal stability. Therefore, radiation crosslinking is a very useful technique to improve the thermal stability, stress crack resistance, solvent resistance and toughness of polymeric materials (Fisher, 1969). Literature search shows no report on radiation effects on EVA/clay nanocomposites. In this work we have prepared EVA-based nanocomposite through melt compounding and exposed them to electron beam irradiation at various doses. The mechanical and thermal property changes were determined and the results are presented in this report. 2. Experimental 2.1. Materials Ethylene vinyl acetate with about 18 wt% vinyl acetate content, MFI ¼ 1.66 g/10 min (at 190 1C and 2.16 kg load)
ARTICLE IN PRESS J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
1699
and density of 0.942 g/mL was bought from Asia Polymer Corporation, Taiwan. Sodium montmorillonite (Kunipia F) with a cation exchange capacity of 119 meq/100 g was purchased from Kunimine Ind. Co., Japan. Dimethyl dihydrogenated tallow ammonium-modified montmorillonite (Cloisite 20A) was bought from Southern Clay Products Inc., Gonzales, TX. Irganox 1010 was obtained from Ciba Geigy, Malaysia.
strength, about 3% for the modulus at 300% elongation and 5% for elongation at break. The thermogravimetric analysis was carried out using a Perkin Elmer TGA7. About 10 mg samples were heated from 30 to 700 1C at a scan rate of 10 1C/min in a dynamic nitrogen atmosphere with a flow rate of 20 ml/min.
3. Nanocomposite preparations
6.1. Degree of crosslinking
The EVA/clay nanocomposite was prepared using a twin-screw compounder (Sino). The EVA pellets, organoclay (C20A) and antioxidant were mixed together and charged into the mixing chamber through a main feeder, which is heated at 140 and 120 1C, respectively. EVA compounded with antioxidant only was also prepared for comparison purposes. Samples of the nanocomposites were then compression moulded in an electrically heated hydraulic press. Hotpress procedure involved preheating at 130 1C for 4 min followed by compression at 110 kg/cm2 for 4 min at the same temperature and subsequently cooling at 20 1C under the same pressure for 4 min.
The relationship between the gel content and irradiation dose of EVA-based nanocomposite containing 5 phr Cloisite 20A (EVA/C20A) is shown in Fig. 1. It can be seen from the figure that the gel content increases rapidly with the increase of irradiation dose up to 100 kGy. Further increase in irradiation dose increases the gel content only marginally. It is also observed that the gel content of EVA/C20A is slightly higher than that of EVA compound without the organoclay. This is probably due to the presence of dimethyl dihydrogenated tallow quaternary ammonium-modified montmorillonite (Cloisite 20A), which seems to increase the formation of radiation-induced crosslinking network in the nanocomposite.
4. Irradiation of samples
6.2. Hot set test
Irradiation was carried out in air using a Cockroft Walton-type electron beam accelerator (Nissin High Voltage, Japan) at an acceleration voltage of 2 MeV and beam current of 5 mA. The distance of the sample from the scan horn was 20 cm and the conveyer speed was set at 0.94 m/min. The dose rate was 50 kGy/pass. The samples were irradiated at doses ranging from 50 to 250 kGy.
Hot set test provides a quick check on crosslinking of samples and can be correlated with the gel fraction. Hot set test was carried out and the result is shown in Table 1. It can be seen from the table that unirradiated EVA and EVA/5C20A samples break immediately when placed in the hot oven. This is because under elevated temperature of 200 1C and load of 20 N/cm2 the unirradiated sample was melted and easily deformed by the load on the samples. The samples irradiated at 50 kGy failed the test at a longer time. This is because the level of crosslinking of about 29% (Fig. 1) is not enough to prevent the polymer from deformation by the load. It is also noted that the presence
The gel fraction of the samples was determined by boiling the samples in xylene for 24 h. The extracted samples were washed with xylene and soaked in methanol for about 30 min and then vacuum dried to constant weight for 16 h. The gel fraction was calculated as the ratio of weight of the dried sample after extraction to the initial weight of the sample before extraction. Hot set test was done to measure the elongation of samples for static load at a fixed temperature. Measurements were carried out in an oven at 200 1C with normal air conditions. The sample loading was 20 N/cm2. The hot elongation was determined after the sample was heated for 15 min. Tensile tests were carried out according to ASTM D417 using an Instron model 4301 testing machine. The dumbbell-shaped specimens were extended at a crosshead speed of 100 mm/min. The tensile property values reported represent an average of the results from tests run on five specimens. Standard deviations were 2–5% for tensile
90 80 70 Gel content (%)
5. Characterization of samples
6. Results and discussion
60 50 40 30 20 10
EVA
EVA/5C20A
0 0
50
100
150
200
250
Irradiation dose (kGy) Fig. 1. Relationship between gel fraction and irradiation dose of the samples.
ARTICLE IN PRESS Table 1 Hot set test results at 200 1C under 20 N/cm2 load for 15 min Irradiation dose (kGy)
0 50 100 150 200 250
Elongation at 120 1C (%) EVA
EVA/C20A
Break immediately Break 14 s 220 84 52 40
Break immediately Break 28 s 200 79 47 37
Modulusat 300% elongation (MPa)
J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
1700
9
8
7
6 EVA 5 0
6.3. Tensile properties The modulus at 300% elongation of EVA and EVA/ C20A nanocomposites is shown in Fig. 2. It can be seen that the tensile modulus of both EVA and EVA/C20A increases continuously with the increase of irradiation dose up to 250 kGy. The increase in tensile modulus with irradiation dose can be related to the increased formation of radiation-induced crosslinking network as evidenced by the gel-content result shown in Fig. 1. The higher tensile modulus shown by EVA/C20A nanocomposite compared to that of EVA is due to the dispersion of the silicate layers in the EVA that provide the reinforcement effect to the polymer matrix therefore increasing the tensile modulus. The improvement in elastic modulus has been reported by several researchers and it is attributed to the exfoliation and good dispersion of nanosized clay that restricts the mobility of polymer chain under loading as well as good interfacial adhesion between the clay layers and the epoxy as reported by Yasmin et al. (2003) and Pozsgay et al. (2004). Fig. 3 shows the variation of tensile strength with irradiation dose of the samples. It is found that before irradiation the tensile strength of both EVA and EVA/ C20A is about the same but after irradiation the tensile
100
200
300
Irradiation dose (kGy) Fig. 2. Relationship between modulus at 300% elongation with irradiation dose.
36 34 Tensile strength (MPa)
of C20A has delayed the breaking time of the samples. However, at 100 kGy and above the samples elongated to a certain length and remained at that length until the 15 min testing time was completed. This is due to the increased formation of radiation-induced crosslinking as evident by the gel content results shown in Fig. 1. It is also found from the table that the presence of organoclay decreases slightly the hot elongation. This is probably due to the higher crosslinking and shielding effect provided by the clay layers to the confine polymer chains. Weisner (1991) reported that different polymer structures require different levels of crosslinking for certain reduction of deformation caused by the load in the hot set test. Formation of adequate radiation crosslinking network gives the polymer better thermal stability at high temperature. Table 1 also shows that elongation value for EVA and EVA/C20A was noted at irradiation dose of 100 kGy, which is equivalent to the gel content of about 60%.
EVA/5C20A
32 30 28 26 24 22
EVA
EVA/5C20A
20 0
50
100
150
200
250
Irradiation dose (kGy) Fig. 3. Relationship between tensile strength and irradiation dose.
strength of the EVA/C20A nanocomposites increases much more compared to that of the unfilled EVA. This is probably due to the formation of crosslinking, which improves the reinforcement effect of the silicate layers to the EVA matrix. The tensile strength is increased with the increase of irradiation dose up to 100 kGy and decreases with further increase of dose up to 250 kGy. The increase in tensile strength up to 100 kGy is probably due to the fact that a large network structure is formed up to 100 kGy radiation dose. However, at higher irradiation dose these network structures become smaller because the process of the radiation-induced crosslinking continues to form between already crosslinked macromolecule as reported by Mateev and Karageorgiev (1998). It is generally known that tensile strength and elongation at break depend on the degree of the strain-induced crystallization, which in turn depends on the polymer chain length and the degree of crosslinking. Longer polymer chains can exhibit higher strain-induced crystallization and better strength properties compared to shorter ones (Datta et al., 1996). Crosslinking of the polymer chain reduces the tendency of chain slippage during straining leading to an increase in the strain-induced
ARTICLE IN PRESS J. Sharif et al. / Radiation Physics and Chemistry 76 (2007) 1698–1702
Decomposition temperature (°C)
Elongation at break (%)
700 650 600 550 500 450 400 350
EVA
EVA/5C20A
300 0
50
100
150
200
250
300
535 530 525 520 515 510 505
EVA
crystallization. However, too much crosslinking may also lower the strain-induced crystallization process. From the results, it can be concluded that EB radiation help increase the tensile strength of the nanocomposite. The elongation at break of all EVA compounds decreases with the increase of radiation dose as shown in Fig. 4. This is attributed to the formation of radiation crosslinking network in the polymer matrix, which restricted the mobility of the molecular chain during drawing (Lyons, 1992).
6.4. Thermogravimetric study The thermal stability of EVA/C20A nanocomposite irradiated at 50 to 250 kGy was determined using thermogravimetric analysis method and the results are plotted in Fig. 5. The nanocomposite decomposes in two stages. The first stage is due to the elimination of acetic acid and formation of double bonds, which usually occurs between 300 and 420 1C. The second degradation step involves the ethylene backbone chain, which occurs between 420 and 550 1C and leads to complete polymer volatilization. It can be seen from Fig. 5 that the thermal stability of the EVA/C20A nanocomposites improves after irradiation with electron beam. The decomposition temperature of the nanocomposite samples increases remarkably with the increase of irradiation dose from 50 to 250 kGy. The increase is higher compare to that of the EVA without the clay. The improvement in thermal stability is due to the formation of more compact three-dimensional crosslinking networks which is more stable against formation of gaseous products on heating (Krupa, 2001). In addition, the marked improvement in thermal stability of the EVA/C20A nanocomposite is probably due to presence of silicate layers that act as barriers to minimize the permeability of volatile degradation product out from the material and also decrease the oxygen diffusion from the gas phase into the polymer matrix (Agag et al., 2001).
EVA/5C20A
500 0
50
100
150
200
250
300
Irradiation dose (kGy)
Irradiation dose (kGy) Fig. 4. Relationship between elongation at break and irradiation dose.
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Fig. 5. Relationship between decomposition temperature and irradiation dose.
7. Conclusion The study has demonstrated that it is possible to crosslink the EVA/C20A nanocomposites with electron beam irradiation. The presence of C20A did not interfere with the formation of radiation-induced crosslinking network in the polymer matrix. The tensile modulus increased continuously and the elongation at break decreased continuously with the increase of irradiation dose. The tensile strength increased and maximized at 100 kGy. The decomposition temperature of the nanocomposite also increases remarkably with the increase of irradiation dose. The improvement in the tensile strength and decomposition temperature of the EVA/C20A nanocomposite with increase dose is due to the increased formation of radiation-induced crosslinking network in the nanocomposite. Acknowledgment We thank the staff from Electron Beam facility of Malaysian Institute for Nuclear Technology Research (MINT) for their assistance in irradiation work of the samples. Thanks also due to Mr. Siva for his help in preparing the nanocomposite with twin-screw compounder. References Agag, T., Koga, T., Takeichi, T., 2001. Studies on thermal and mechanical properties of polyimide–clay nanocomposites. Polymer 42, 3399–3408. Alexander, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. R 28 (1), 1–63. Burns, N.M., 1979. The radiation crosslinking of ethylene copolymers. Radiat. Phys. Chem. 14, 797–808. Datta, S.K., Chaki, T.K., Khastgir, D., 1996. Effect of electron beam radiation on mechanical and electrical properties of poly(ethylene-covinyl acetate). Die Angew. Makromol. Chem. 238, 105–117. Duquesne, S., Jama, C., Le- Bras, M., Delobel, R., Recourt, P., Gloaguen, J.M., 2003. Elaboration of EVA–nanoclay systems-characterization, thermal behavior and fire performance. Compos. Sci. Technol. 23, 1141–1148.
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