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Effect of gamma irradiation on ethylene propylene diene terpolymer rubber composites
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 111–116 www.elsevier.com/locate/nimb
Effect of gamma irradiation on ethylene propylene diene terpolymer rubber composites M.M. Abou Zeid a,*, S.T. Rabie b, A.A. Nada b, A.M. Khalil b, R.H. Hilal c a
Radiation Chemistry Department, National Center for Radiation Research and Technology (NCRRT), Nasr City, Cairo, Egypt b Photochemistry Department, National Research Center (NRC), Dokki, Giza, Egypt c Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Received 22 June 2007; received in revised form 8 October 2007 Available online 17 November 2007
Abstract Composites of ethylene propylene dine terpolymer rubber (EPDM), high density polyethylene (HDPE) and ground tire rubber powder (GTR) at different ratios were subjected to gamma irradiation at various doses up to 250 kGy. The physical, mechanical and thermal properties were investigated as a function of irradiation dose and blend composition. Gamma irradiation led to a significant improvement in the properties for all blend compositions. The results indicate that the improvement in properties is inversely proportional to the substituted ratio of GTR, attributed to the development of an interfacial adhesion between GTR and blend components. The results were confirmed by examining the fracture surfaces by scanning electron microscopy. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.25.Hk; 61.25.Hp Keywords: Gamma radiation; Rubber composites
1. Introduction Recycling of waste tire rubber is a worldwide environmental and economic problem facing the rubber industry. Recycled polymers often show low mechanical properties and are not suitable for use. Ionizing radiation offers possibilities of recycling polymers due to the ability to cause cross-linking and/or chain scission of a wide range of materials without introducing any chemical initiators or the need to dissolve them. This avoids phase separation and eventually leads to improved mechanical properties. The use of ground tire rubber in the preparation of thermoplastic/elastomer blends can provide new product at a lower cost. Though the application of radiation for rubber recycling is not widespread, gamma irradiation of EPDM/polypropylene (PP) of different ratios, utilizing waste PP, was shown *
Corresponding author. Tel.: +20 2 417 400 2; fax: +20 2 417 456 3. E-mail address: [email protected] (M.M. Abou Zeid).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.037
to increase gel content and improve mechanical properties [1,2]. There are studies in which recycled reinforced polymer systems were prepared using PP from reprocessed car bumpers [3,4]. Significant improvement upon electron beam irradiation with even low doses was noted. Irradiation to a dose of about 70 kGy significantly increased the plasticity of crumb rubber, and greatly improved the mold ability of mixtures containing virgin rubber incorporated with recycled crumb [5,6]. Other mechanical properties are diminished at this dose. Rubber stocks that were blended with recycled and irradiated butyl crumb were reported to have some enhanced characteristics, including shortened vulcanization period and anti-tearing properties [7]. Gamma irradiation achieving in situ compatibility of recycled HDPE/GTR blends led to an improvement of mechanical properties [8]. The effect of gamma irradiation on polymer blends based on EPDM/HDPE was investigated [9]. Some investigations examined the incorporation of ground rubber
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powder (GRP) and devulcanized rubber waste as filler and a part of rubber in virgin natural rubber (NR) [10]. The GRP and the devulcanized rubber, which were generated from passenger car and light truck tires, showed useful mechanical properties in the new rubber compounds, while the compounds that contained devulcanized rubber showed better mechanical properties than those with GRP [11]. The improvement of tensile strength, tear strength and elongation at break of EPDM rubber was observed when the waste EPDM powder was added, due to the reinforcement of the carbon containing waste EPDM powder [12,13]. Studies were made on using a ground tire rubber (GTR) fraction to produce thermoplastic elastomers composed of PE, fresh rubber and GTR with and without dynamic curing [14–16]. This paper deals with the use of gamma irradiation to vulcanized thermoplastic/elastomers blends containing GTR to evaluate the possibility of recycling GTR and to obtain a product with proper properties. The EPDM component in a EPDM/HDPE blend was substituted by different ratios of GTR, while the ratio of HDPE was kept constant. The physical and mechanical properties of EPDM/GTR/HDPE blends were studied before and after gamma irradiation. The morphological properties were examined by scanning electron microscope (SEM). 2. Experimental 2.1. Materials EPDM rubber of Vistalon 5600 type is characterized by an ethylene content of 60 wt%, a specific gravity of 0.86 g/ cm3 and Mooney viscosity ML (1 + 8) at 127 °C of 48–52. HDPE is characterized by having a density of 0.96 g/cm3, melt temperature of 125–150 °C and melt flow index of 2 g/10 min. Ground tire rubber (GTR) waste of particle size 80 mesh (150 lm) was used. The blends used in this study also contain additives such as zinc oxide as an accelerator and stearic acid as an activator.
2.4. Measurements Measurements of mechanical properties were carried out on dumbbell-shaped samples of 4 mm width and 50 mm length. Tensile strength (TS), elongation at break (Eb%) and modulus at 100% elongation (M100) were measured using a universal tensile testing machine. For hardness measurements, samples of at least 0.12 mm in thickness with a flat surface were fabricated. Measurement was carried out according to ASTM D2240 using a durometer. The unit of hardness is expressed in Shore A. The gel content was obtained by extracting the samples (W0) in toluene using a Soxhlet system for 24 h. The extracted samples were dried in a vacuum oven at 60 °C to a constant weight (W1). The gel content (%) is equal to (W1/W2) 100. Toluene was also used as a swelling solvent. The degree of swelling in toluene for 24 h at room temperature was determined from the swelling ratio, equal to (W2 W1)/W1, where W1 is the original weight and W2 is the weight of the swelled sample. A SEM was used to examine the structure morphology of rubber blends. Thermo gravimetric analysis (TGA) was carried out with samples of (0.98–1.5 mg) encapsulated in aluminum pans and heated from 50 °C to 600 °C at 10 °C/min under a nitrogen atmosphere. 3. Results and discussion 3.1. Gel content Fig. 1 shows the effect of irradiation dose on the gel content of a EPDM/HDPE blend and blends of EPDM/GTR/ HDPE. The gel contents increase steadily with increasing
2.2. Preparation of EPDM/GTR/HDPE blends Accurately weighed ingredients were carefully mixed together. The thermoplastic HDPE was first added and then the elastomer EPDM as well as the GTR and other ingredients. After mixing, the samples were passed through a two-roller mill to obtain sheets of the blend. Sheets of 1 mm in thickness were then obtained by compression molding between Holland cloth in clear and polished molds of an electrically heated hydraulic press. 2.3. Gamma irradiation Blend samples were gamma irradiated to the required dose using a Cobalt-60 source with a dose rate of about 6.5 kGy/h. The irradiation process was carried out under atmospheric conditions for doses ranging from 50 to 250 kGy.
Fig. 1. Effect of irradiation dose on gel content % of EPDM/HDPE blends with different contents of GTR.
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irradiation dose, depending on blend composition. This indicates cross-linking induced by gamma irradiation. [17]. However, at a constant irradiation dose the gel content decreased steadily with increase in GTR content. The partial substitute of virgin EPDM by GTR up to 33% causes approximately equal drop in gel content as the EPDM/HDPE blend. The decrease in gel content of EPDM/HDPE caused by the GTR may be explained by the non-compatibility of GTR and phase separation. The particles of GTR separate the molecules of EPDM and HDPE and hence retard the formation of cross-linking. Fig. 2 shows the effect of dose on the swelling number (SN) for blends of different compositions. The SN value decreases with dose. Also, the higher the ratio of GTR, the higher the values of SN, indicating the presence of more spaces in the polymeric network due to the incomplete cross-linking of GTR. 3.2. Mechanical properties One of the clearest effects of irradiation of thermoplastic/elastomer blends due to their cross-linking and degradation is the change in tensile strength (TS). Fig. 3 shows the effect of dose on the tensile strength of blends of different compositions. The TS values increase with dose up 150 kGy and then decrease with dose up to 250 kGy. Also, at any dose the TS values decrease with increasing GTR ratio in the blend, due to the cross-linking of the EPDM and HDPE components. The decrease in TS observed at a higher dose is due to the occurrence of oxidation degradation. The tensile strength of the EPDM/HDPE blend at 150 kGy decreased by 4%, 13% and 22% when 25%,
Fig. 2. Effect of irradiation dose on swelling number of EPDM/HDPE blends with different contents of GTR.
Fig. 3. Effect of irradiation dose on tensile strength of EPDM/HDPE blends with different contents of GTR.
33% and 50% of the EPDM was substituted by GTR, respectively. This indicates that the decrease of tensile strength is not systematic with the substituted ratio of EPDM by GTR. Therefore, it may be concluded that the presence of GTR reduces the cross-linking of EPDM and HDPE and cannot protect the blend from deterioration, particularly at higher doses and acts as filler [18]. Fig. 4 shows the effect of irradiation dose on the elongation at the break of blends of different compositions. Generally it decreases with dose for all blends. As the dose increases, more cross-linking is produced in the sample, which prevents the structural reorganization during drawing and brings about a decrease in internal chain mobility and elongation [19]. Also, at any dose, the elongation at break decreases with increasing ratio of GTR in the blend. The amorphous part of HDPE is characterized by being poorly branched and hence the stiffness of the blends as a whole would be increased, leading to a considerable decrease in elongation at break. Under practical or engineering conditions of applications, polymeric based materials such as blends are not stretched until they undergo rupture. Therefore, despite its importance as a mechanical property, the tensile strength measurements are not appropriate. The property that measures the resistance to a limited strain deformation of polymeric materials under practical applications is the tensile modulus at 100% elongation. Fig. 5 shows the effect of dose on the modulus at 100% elongation M100 of blends at different compositions. The value of M100 increases linearly with dose regardless of blend composition. This may be attributed to increased cross-linking induced by irradiation as well as the presence of HDPE in the blend, which makes the polymer blends rigid enough
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Fig. 4. Effect of irradiation dose on elongation at break % of EPDM/ HDPE blends with different contents of GTR.
Fig. 6. Effect of irradiation dose on hardness of EPDM/HDPE blends with different contents of GTR.
3.3. Thermogravimetric analysis Fig. 7 shows the TGA thermogram of EPDM/GTR/ HDPE at different compositions at a dose of 150 kGy. The temperature at which different weight loss occurs for these blends is shown in Table 1. Up to 300 °C, all blends are thermally stable with no difference in weight loss. The major thermal decomposition occurs within the temperature range from 300–425 °C. In this regard, blends contain-
Fig. 5. Effect of irradiation dose on modulus at 100% elongation of EPDM/HDPE blends with different contents of GTR.
and thus the modulus values increased. Modulus values decrease with an increase of GTR content in the blends. Radiation induced cross-linking in polymer materials should be reflected as an increase in hardness. Fig. 6 shows the blend composition dependence of hardness of EPDM/ GTR/HDPE on dose. The values of hardness of all blends increase slightly with dose up to 250 kGy. Also, the hardness values decrease with increasing GTR content. The high degree of crystallinity of HDPE affects the hardness to a high extent [20].
Fig. 7. TGA curves of EPDM/HDPE blends with different contents of GTR at a dose of 150 kGy.
M.M. Abou Zeid et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 111–116
115
Table 1 Temperatures at different weight loss for EPDM/HDPE blends with different ratios of GTR irradiated at a dose of 150 kGy EPDM/HDPE Composition (%)
Decomposition temperatures at different weight loss (°C) 10%
20%
40%
EPDM/HDPE (100/50) EPDM/GTR/HDPE (75/25/50) EPDM/GTR/HDPE (67/33/50) EPDM/GTR/HDPE (50/50/50)
321 373 371 401
339 425 420 433
357 455 441 451
ing GTR possess higher thermal stability than EPDM/ HDPE blend. From Table 1 the temperatures of the maximum rate of reaction (Tmax), taken from the TGA thermogram, increase with increasing ratio of GTR up to 33%, then decrease at higher ratio. In general, the higher thermal stability of the composition containing GTR compared to the EPDM/HDPE blend is due to the oxidation degradation of the GTR and the formation of carbonyl groups with higher dissociation energy than CH groups.
Tmax (°C) 461 539 507 477
3.4. Scanning electron microscopy SEMs of blends of different compositions are shown in Fig. 8. EPDM and HDPE are non-compatible polymers. However, the effect of irradiation is clear. The surface is homogeneous, smooth and exhibits no indication of phase separation due to the occurrence of cross-linking shown in Fig. 8(a). The appearance of white particles across the SEMs shown in Fig. 8(b) indicates the non-compatibility between EPDM, HDPE and GTR. Meanwhile, the presence of GTR does not affect the cross-linked polymer matrix. These features increased with the ratio of GTR which may explain the decrease in tensile and hardness properties associated with the introduction of GTR [21]. 4. Conclusions This work was undertaken to study the effects of irradiation on the physical and mechanical properties of blends of different compositions. The following conclusions may be made: First, irradiation of all the blends improved to a great extent the physical and mechanical properties to a large extent. In this regard, the gel content increased with dose, the degree of swelling of the vulcanized blends is reduced with dose and increased with GTR content due to incomplete cross-linking of GTR. Second, the results of tensile strength, modulus at 100% elongation, elongation break and hardness showed a good agreement with gel content results. Third, the vulcanized blends with low ratio of GTR exhibit higher thermal stability than the EPDM/HDPE blend upon irradiation due to the occurrence of oxidation degradation of the large surface area of the GTR powder. The optimum ratio for the blends is EPDM/GTR/HDPE (75/25/50) at 150 kGy dose. This ratio possesses the best mechanical and thermal properties, so it may be useful for some industrial applications. References
Fig. 8. SEM micrographs of the fractured surface of (a) EPDM/HDPE (100/50) and (b) EPDM/GTR/HDPE (75/25/50), both irradiated at 150 kGy.
[1] T. Zaharescu, M. Chipara, M. Postolache, Polym. Degrad. Stabil. 66 (1999) 5. [2] T. Zaharescu, R. Setnescu, S. Jipa, T. Setnescu, J. Appl. Polym. Sci. 77 (2000) 982. [3] T. Czvikovszky, H. Hargitai, Nucl. Instr. and Meth. B 131 (1997) 300. [4] T. Czvikovszky, H. Hargitai, Radiat. Phys. Chem. 55 (1999) 727.
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[5] J. Yang, Radiation recycling of butyl rubber waste, in: W.J. Cooper, R.D. Curry, K.E. O’Shea (Eds.), Environmental Application of Ionizing Radiation, Wiley, New York, 1998, p. 601. [6] J. Yang, W. Liu, Technological development of irradiation reclamation of butyl rubber waste, Presented at IAEA Consultants Meeting, Saclay, France, September 19–21 2000. [7] T. Zaharescu, C. Cazac, S. Jipa, R. Setnescu, Nucl. Instr. and Meth. B (2001) 185. [8] R. Sonnier, E. Leroy, L. Clerc, A. Bergeret, J.M. Lopez-Cuesta, Polm. Degrad. Stabil. 91 (2006) 2375. [9] J. Shaojin, Z. Zhicheng, W. Zhengzhou, Z. Xianfeng, D. Zhiwen, Polym. Int. 54 (2005) 320. [10] A.A. Yehia, M.A. Mull, M.N. Ismail, Y.A. Hefny, E.M. Abdel-Bary, J. Appl. Polym. Sci. 93 (2004) 30. [11] L. Shuyan, L. Johanna, H. Kalle, Polym. Eng. Sci. 45 (2005) 1239. [12] C. Jacob, P.P. De, A.K. Bhowmick, S.K. De, J. Appl. Polym. Sci. 82 (2001) 3293.
[13] C. Jacob, A.K. Bhattacharya, A.A. Bhowmick, P.P. De, S.K. De, J. Appl. Polym. Sci. 87 (2000) 2204. [14] S. RudheshKumar, I. Fuhrmann, Karger-Kocsis, J. Polym. Degrad. Stabil. 76 (2002) 137. [15] S. Rudheshkumar, Karger-Kocsis, J. Plast. Rub. Compos. 31 (2002) 99. [16] A.K. Naskar, A.K. Bhowmick, S.K. De, Polym. Eng. Sci. 41 (2001) 1087. [17] I. Banik, A.K. Bhowmick, Radiat. Phys. Chem. 58 (2000) 293. [18] R. Scaffaro, Tzankova Dintcheva, M.A. Nocilla, F.P. La Mantia, Polym. Degrad. Stabil. 90 (2005) 281. [19] J. De Boer, L. Pennings, Colloid Polym. Sci. 261 (1983) 750. [20] J. Silverman, A. Singh, Radiation Processing of Polymers, J. Hanser publishers, New York, USA, 1992, p. 15. [21] O.P. Grigoryeva, A.M. Fainleib, A.L. Tolstov, O.M. Starostenko, E. Lievana, J. Appl. Poly. Sci. 95 (2005) 659.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 111–116 www.elsevier.com/locate/nimb
Effect of gamma irradiation on ethylene propylene diene terpolymer rubber composites M.M. Abou Zeid a,*, S.T. Rabie b, A.A. Nada b, A.M. Khalil b, R.H. Hilal c a
Radiation Chemistry Department, National Center for Radiation Research and Technology (NCRRT), Nasr City, Cairo, Egypt b Photochemistry Department, National Research Center (NRC), Dokki, Giza, Egypt c Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt Received 22 June 2007; received in revised form 8 October 2007 Available online 17 November 2007
Abstract Composites of ethylene propylene dine terpolymer rubber (EPDM), high density polyethylene (HDPE) and ground tire rubber powder (GTR) at different ratios were subjected to gamma irradiation at various doses up to 250 kGy. The physical, mechanical and thermal properties were investigated as a function of irradiation dose and blend composition. Gamma irradiation led to a significant improvement in the properties for all blend compositions. The results indicate that the improvement in properties is inversely proportional to the substituted ratio of GTR, attributed to the development of an interfacial adhesion between GTR and blend components. The results were confirmed by examining the fracture surfaces by scanning electron microscopy. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.25.Hk; 61.25.Hp Keywords: Gamma radiation; Rubber composites
1. Introduction Recycling of waste tire rubber is a worldwide environmental and economic problem facing the rubber industry. Recycled polymers often show low mechanical properties and are not suitable for use. Ionizing radiation offers possibilities of recycling polymers due to the ability to cause cross-linking and/or chain scission of a wide range of materials without introducing any chemical initiators or the need to dissolve them. This avoids phase separation and eventually leads to improved mechanical properties. The use of ground tire rubber in the preparation of thermoplastic/elastomer blends can provide new product at a lower cost. Though the application of radiation for rubber recycling is not widespread, gamma irradiation of EPDM/polypropylene (PP) of different ratios, utilizing waste PP, was shown *
Corresponding author. Tel.: +20 2 417 400 2; fax: +20 2 417 456 3. E-mail address: [email protected] (M.M. Abou Zeid).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.037
to increase gel content and improve mechanical properties [1,2]. There are studies in which recycled reinforced polymer systems were prepared using PP from reprocessed car bumpers [3,4]. Significant improvement upon electron beam irradiation with even low doses was noted. Irradiation to a dose of about 70 kGy significantly increased the plasticity of crumb rubber, and greatly improved the mold ability of mixtures containing virgin rubber incorporated with recycled crumb [5,6]. Other mechanical properties are diminished at this dose. Rubber stocks that were blended with recycled and irradiated butyl crumb were reported to have some enhanced characteristics, including shortened vulcanization period and anti-tearing properties [7]. Gamma irradiation achieving in situ compatibility of recycled HDPE/GTR blends led to an improvement of mechanical properties [8]. The effect of gamma irradiation on polymer blends based on EPDM/HDPE was investigated [9]. Some investigations examined the incorporation of ground rubber
112
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powder (GRP) and devulcanized rubber waste as filler and a part of rubber in virgin natural rubber (NR) [10]. The GRP and the devulcanized rubber, which were generated from passenger car and light truck tires, showed useful mechanical properties in the new rubber compounds, while the compounds that contained devulcanized rubber showed better mechanical properties than those with GRP [11]. The improvement of tensile strength, tear strength and elongation at break of EPDM rubber was observed when the waste EPDM powder was added, due to the reinforcement of the carbon containing waste EPDM powder [12,13]. Studies were made on using a ground tire rubber (GTR) fraction to produce thermoplastic elastomers composed of PE, fresh rubber and GTR with and without dynamic curing [14–16]. This paper deals with the use of gamma irradiation to vulcanized thermoplastic/elastomers blends containing GTR to evaluate the possibility of recycling GTR and to obtain a product with proper properties. The EPDM component in a EPDM/HDPE blend was substituted by different ratios of GTR, while the ratio of HDPE was kept constant. The physical and mechanical properties of EPDM/GTR/HDPE blends were studied before and after gamma irradiation. The morphological properties were examined by scanning electron microscope (SEM). 2. Experimental 2.1. Materials EPDM rubber of Vistalon 5600 type is characterized by an ethylene content of 60 wt%, a specific gravity of 0.86 g/ cm3 and Mooney viscosity ML (1 + 8) at 127 °C of 48–52. HDPE is characterized by having a density of 0.96 g/cm3, melt temperature of 125–150 °C and melt flow index of 2 g/10 min. Ground tire rubber (GTR) waste of particle size 80 mesh (150 lm) was used. The blends used in this study also contain additives such as zinc oxide as an accelerator and stearic acid as an activator.
2.4. Measurements Measurements of mechanical properties were carried out on dumbbell-shaped samples of 4 mm width and 50 mm length. Tensile strength (TS), elongation at break (Eb%) and modulus at 100% elongation (M100) were measured using a universal tensile testing machine. For hardness measurements, samples of at least 0.12 mm in thickness with a flat surface were fabricated. Measurement was carried out according to ASTM D2240 using a durometer. The unit of hardness is expressed in Shore A. The gel content was obtained by extracting the samples (W0) in toluene using a Soxhlet system for 24 h. The extracted samples were dried in a vacuum oven at 60 °C to a constant weight (W1). The gel content (%) is equal to (W1/W2) 100. Toluene was also used as a swelling solvent. The degree of swelling in toluene for 24 h at room temperature was determined from the swelling ratio, equal to (W2 W1)/W1, where W1 is the original weight and W2 is the weight of the swelled sample. A SEM was used to examine the structure morphology of rubber blends. Thermo gravimetric analysis (TGA) was carried out with samples of (0.98–1.5 mg) encapsulated in aluminum pans and heated from 50 °C to 600 °C at 10 °C/min under a nitrogen atmosphere. 3. Results and discussion 3.1. Gel content Fig. 1 shows the effect of irradiation dose on the gel content of a EPDM/HDPE blend and blends of EPDM/GTR/ HDPE. The gel contents increase steadily with increasing
2.2. Preparation of EPDM/GTR/HDPE blends Accurately weighed ingredients were carefully mixed together. The thermoplastic HDPE was first added and then the elastomer EPDM as well as the GTR and other ingredients. After mixing, the samples were passed through a two-roller mill to obtain sheets of the blend. Sheets of 1 mm in thickness were then obtained by compression molding between Holland cloth in clear and polished molds of an electrically heated hydraulic press. 2.3. Gamma irradiation Blend samples were gamma irradiated to the required dose using a Cobalt-60 source with a dose rate of about 6.5 kGy/h. The irradiation process was carried out under atmospheric conditions for doses ranging from 50 to 250 kGy.
Fig. 1. Effect of irradiation dose on gel content % of EPDM/HDPE blends with different contents of GTR.
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irradiation dose, depending on blend composition. This indicates cross-linking induced by gamma irradiation. [17]. However, at a constant irradiation dose the gel content decreased steadily with increase in GTR content. The partial substitute of virgin EPDM by GTR up to 33% causes approximately equal drop in gel content as the EPDM/HDPE blend. The decrease in gel content of EPDM/HDPE caused by the GTR may be explained by the non-compatibility of GTR and phase separation. The particles of GTR separate the molecules of EPDM and HDPE and hence retard the formation of cross-linking. Fig. 2 shows the effect of dose on the swelling number (SN) for blends of different compositions. The SN value decreases with dose. Also, the higher the ratio of GTR, the higher the values of SN, indicating the presence of more spaces in the polymeric network due to the incomplete cross-linking of GTR. 3.2. Mechanical properties One of the clearest effects of irradiation of thermoplastic/elastomer blends due to their cross-linking and degradation is the change in tensile strength (TS). Fig. 3 shows the effect of dose on the tensile strength of blends of different compositions. The TS values increase with dose up 150 kGy and then decrease with dose up to 250 kGy. Also, at any dose the TS values decrease with increasing GTR ratio in the blend, due to the cross-linking of the EPDM and HDPE components. The decrease in TS observed at a higher dose is due to the occurrence of oxidation degradation. The tensile strength of the EPDM/HDPE blend at 150 kGy decreased by 4%, 13% and 22% when 25%,
Fig. 2. Effect of irradiation dose on swelling number of EPDM/HDPE blends with different contents of GTR.
Fig. 3. Effect of irradiation dose on tensile strength of EPDM/HDPE blends with different contents of GTR.
33% and 50% of the EPDM was substituted by GTR, respectively. This indicates that the decrease of tensile strength is not systematic with the substituted ratio of EPDM by GTR. Therefore, it may be concluded that the presence of GTR reduces the cross-linking of EPDM and HDPE and cannot protect the blend from deterioration, particularly at higher doses and acts as filler [18]. Fig. 4 shows the effect of irradiation dose on the elongation at the break of blends of different compositions. Generally it decreases with dose for all blends. As the dose increases, more cross-linking is produced in the sample, which prevents the structural reorganization during drawing and brings about a decrease in internal chain mobility and elongation [19]. Also, at any dose, the elongation at break decreases with increasing ratio of GTR in the blend. The amorphous part of HDPE is characterized by being poorly branched and hence the stiffness of the blends as a whole would be increased, leading to a considerable decrease in elongation at break. Under practical or engineering conditions of applications, polymeric based materials such as blends are not stretched until they undergo rupture. Therefore, despite its importance as a mechanical property, the tensile strength measurements are not appropriate. The property that measures the resistance to a limited strain deformation of polymeric materials under practical applications is the tensile modulus at 100% elongation. Fig. 5 shows the effect of dose on the modulus at 100% elongation M100 of blends at different compositions. The value of M100 increases linearly with dose regardless of blend composition. This may be attributed to increased cross-linking induced by irradiation as well as the presence of HDPE in the blend, which makes the polymer blends rigid enough
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Fig. 4. Effect of irradiation dose on elongation at break % of EPDM/ HDPE blends with different contents of GTR.
Fig. 6. Effect of irradiation dose on hardness of EPDM/HDPE blends with different contents of GTR.
3.3. Thermogravimetric analysis Fig. 7 shows the TGA thermogram of EPDM/GTR/ HDPE at different compositions at a dose of 150 kGy. The temperature at which different weight loss occurs for these blends is shown in Table 1. Up to 300 °C, all blends are thermally stable with no difference in weight loss. The major thermal decomposition occurs within the temperature range from 300–425 °C. In this regard, blends contain-
Fig. 5. Effect of irradiation dose on modulus at 100% elongation of EPDM/HDPE blends with different contents of GTR.
and thus the modulus values increased. Modulus values decrease with an increase of GTR content in the blends. Radiation induced cross-linking in polymer materials should be reflected as an increase in hardness. Fig. 6 shows the blend composition dependence of hardness of EPDM/ GTR/HDPE on dose. The values of hardness of all blends increase slightly with dose up to 250 kGy. Also, the hardness values decrease with increasing GTR content. The high degree of crystallinity of HDPE affects the hardness to a high extent [20].
Fig. 7. TGA curves of EPDM/HDPE blends with different contents of GTR at a dose of 150 kGy.
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Table 1 Temperatures at different weight loss for EPDM/HDPE blends with different ratios of GTR irradiated at a dose of 150 kGy EPDM/HDPE Composition (%)
Decomposition temperatures at different weight loss (°C) 10%
20%
40%
EPDM/HDPE (100/50) EPDM/GTR/HDPE (75/25/50) EPDM/GTR/HDPE (67/33/50) EPDM/GTR/HDPE (50/50/50)
321 373 371 401
339 425 420 433
357 455 441 451
ing GTR possess higher thermal stability than EPDM/ HDPE blend. From Table 1 the temperatures of the maximum rate of reaction (Tmax), taken from the TGA thermogram, increase with increasing ratio of GTR up to 33%, then decrease at higher ratio. In general, the higher thermal stability of the composition containing GTR compared to the EPDM/HDPE blend is due to the oxidation degradation of the GTR and the formation of carbonyl groups with higher dissociation energy than CH groups.
Tmax (°C) 461 539 507 477
3.4. Scanning electron microscopy SEMs of blends of different compositions are shown in Fig. 8. EPDM and HDPE are non-compatible polymers. However, the effect of irradiation is clear. The surface is homogeneous, smooth and exhibits no indication of phase separation due to the occurrence of cross-linking shown in Fig. 8(a). The appearance of white particles across the SEMs shown in Fig. 8(b) indicates the non-compatibility between EPDM, HDPE and GTR. Meanwhile, the presence of GTR does not affect the cross-linked polymer matrix. These features increased with the ratio of GTR which may explain the decrease in tensile and hardness properties associated with the introduction of GTR [21]. 4. Conclusions This work was undertaken to study the effects of irradiation on the physical and mechanical properties of blends of different compositions. The following conclusions may be made: First, irradiation of all the blends improved to a great extent the physical and mechanical properties to a large extent. In this regard, the gel content increased with dose, the degree of swelling of the vulcanized blends is reduced with dose and increased with GTR content due to incomplete cross-linking of GTR. Second, the results of tensile strength, modulus at 100% elongation, elongation break and hardness showed a good agreement with gel content results. Third, the vulcanized blends with low ratio of GTR exhibit higher thermal stability than the EPDM/HDPE blend upon irradiation due to the occurrence of oxidation degradation of the large surface area of the GTR powder. The optimum ratio for the blends is EPDM/GTR/HDPE (75/25/50) at 150 kGy dose. This ratio possesses the best mechanical and thermal properties, so it may be useful for some industrial applications. References
Fig. 8. SEM micrographs of the fractured surface of (a) EPDM/HDPE (100/50) and (b) EPDM/GTR/HDPE (75/25/50), both irradiated at 150 kGy.
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