- Email: [email protected]
Effect of electron beam irradiation and degree of boric acid loading on the properties of styrene-butadiene rubber
Reactive & Functional Polymers 69 (2009) 229–233
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Effect of electron beam irradiation and degree of boric acid loading on the properties of styrene-butadiene rubber N.A. Shaltout * Radiation Chemistry Department, National Center for Radiation Research and Technology, 21830 NCRRT UN Telex, AEA, Nasr City, P.O. Box 29, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 20 September 2008 Received in revised form 20 December 2008 Accepted 26 December 2008 Available online 9 January 2009 Keywords: Styrene-butadiene rubber Boric acid Mechanical Thermal Electrical properties
a b s t r a c t Synthetic and totally amorphous styrene-butadiene rubber (SBR) has been loaded with varying contents of boric acid. Vulcanization of prepared composites as well as of unloaded rubber has been induced by ionizing radiation of accelerated electron beam of varying doses up to 250 kGy. Evaluation of prepared composite subjected to this range of irradiation has been followed up through the measurement of mechanical, physical, electrical and thermal properties of vulcanized composites. Mechanical properties, namely tensile strength (TS) and Young’s modulus were found to increase, whereas elongation at break (Eb) and permanent set (PS) were found to decrease with the increase in degree of boric acid loading as well as irradiation dose. On the other hand, physical properties, namely the gel content, have increased whereas the swelling number has decreased. Moreover, increase in the decomposition temperature has been attained. Also, limited increase in electrical conductivity has taken place. Data obtained indicate enhancement in thermal as well as in physico-mechanical properties of prepared composites. Moreover, 60 phr of boric acid has attained good mechanical properties. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Inorganic fillers are used for the loading of elastomers and may act in one way or the another, as an active additive or it may be considered as merely as inactive additive. CaCO3 is an example of the latter type of fillers as it is mainly used for increasing the amount of product rubber composite without affecting the properties of the elastomeric phase [1]. Active inorganic fillers such as carbon blacks and silica, may on the other hand, participate in upgrading certain properties of the elastomeric phase in the composite such as its physical, thermal or electrical properties [2]. Such active fillers may also act as protectors for the rubber phase from the degrading effect of ionizing radiation, such as gamma rays or accelerated electrons. In such a case, they perform their function through attenuation or absorption of the energy of ionizing radiation, which may be either charged or uncharged. It has been found in the case of uncharged radiations, such as gamma and X-rays, that the elements of high density and high atomic number, such as iron or preferably lead are the most suitable materials to act in this respect. In the case of charged radiations such as beta-rays or accelerated electrons, the use of such metals is not suitable due to the production of braking radiation, i.e., bremsstrahlung that is
* Fax: +20 2 22749298. E-mail address: [email protected] 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.12.025
produced during the deceleration of such charged particles in absorbing the elements of high atomic number [3]. To avoid such penetrating braking radiation, the elements of suitably high density and specifically low atomic number have been reported to be suitable as absorbing materials. Elements such as boron or hydrogen and their combinations would then act effectively in this respect. Boric acid H3PO4 is an example of such combination. Moreover, composites of boron-based compounds loaded to different rubbers, such as ethylene propylene diene rubber (EPDM), low density polyethylene (LDPE) and boron carbide (B4C) [4] as well as composites of natural rubber (NR)/H3BO4 [5], have proved to be efficient shields for thermal neutrons. It would be appropriate, however, for such composites to posses reasonably good physicochemical, thermal and electrical properties after being subjected to relatively high cumulative radiation doses. One of the most widely used synthetic rubbers is styrene-butadiene rubber (SBR). This polymer is mainly used in blends in the manufacture of tires due to its good crack resistance, wet grip and weather resistance [6]. However, other important industrial applications as membranes, wires and cables are included in the uses of this material [7–9]. In the present investigation, composites of SBR having varying contents of boric acid have been prepared. The effect of cumulative effect of electron-beam irradiation as well as the content of boric acid on physico-mechanical, thermal and electrical properties has been followed up.
230
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
2. Materials and methods
3. Results and discussion
2.1. Materials
3.1. Mechanical properties
A commercial grade styrene-butadiene rubber (SBR-1502) with 23.6% styrene content was used as the matrix polymer. The boric acid loading was varied up to 100 parts per 100 parts of rubber (phr) by weight. The recipe of this study also contained other additives, namely ZnO, stearic acid and tetrene. The first two additives act as accelerators as well as activators and their contents were 5 phr and 1 phr, respectively. On the other hand, Tetrene acts as antioxidant and was added by 1 phr. Solvent and other chemicals used were of commercial grade, and were used as received.
The mechanical properties of blank SBR samples and their composites with different contents of boric acid are given in Figs. 1–3. These mechanical properties of SBR samples containing different contents of boric acid are expected to be a function of irradiation dose as well as the content of boric acid. A comparison between blank SBR samples and boric acid loaded SBR is interesting to detect the role played by the boric acid and its content in the composites obtained at all irradiation doses. The data obtained for the variation of tensile strength as a function of irradiation dose are depicted in Fig. 1. It can be seen that the TS value for loaded but unirradiated composites have changed but slightly with respect to the value of unloaded and unirradiated sample. On the other hand, the tensile strength values of blank SBR samples and these loaded with different contents of boric acid increased with increasing the irradiation dose up to 150 kGy and then decreased. The increase in the tensile strength values with irradiation dose may be attributed to induced crosslinking by irradiation. Also, it is very important to notice that the TS values at the same irradiation dose increased with increasing the content of boric acid up to 60 phr and then decreased. The increase in TS of composites up to degree of loading of 60 phr of boric acid may be attributed to the participation of the latter as a crosslinking agent [10]. On loading the samples with 80–100 phr of boric acid, it would be expected that the matrix of the composite obtained is no longer composed of continuous polymeric phase as it may occur that boric acid particles would adhere to one another and hence proper wetting of its particles by elastomer has not taken place. Under such circumstances, the polymeric composite would lose much of its inherent elasticity [11]. Also, the decrease in TS values at higher doses than 150 kGy may be due to the degradation of blank SBR as well as the decrease in the orientation processes of the polymeric matrix that may occur at the high levels of crosslinking. From Fig. 2, it can be seen that Young’s modulus value has increased considerably, reaching its maximum value at 150 kGy, and then decreased up to 250 kGy. This behavior indicates that the contribution of induced crosslinking by accelerated electron
2.2. Sample preparation SBR, ZnO, stearic acid, tetrene and boric acid were mixed on a rubber mill (300 470 mm) with a gear ratio of 1.14:1 at 80 °C. The compound was pressed to a 1 mm thick sheet using a hot press at 150 °C for 20 min and under 16 MPa pressure. Electron beam irradiation was carried out in the atmosphere using an electron beam accelerator (1.5 MeV, 25 kW) at NCRRT, Cairo. 2.3. Physical properties 2.3.1. Gel fraction Gel fraction, expressed as the fraction of insoluble weight, was obtained by extracting soluble part in benzene using Soxhlet for 24 hr at room temperature, and drying insoluble part completely in vacuum oven at 50 °C. The gel fraction % was calculated according to the following equation:
Gel fraction ¼ W 1 =W 0 where W0 is the initial weight before extraction and W1 is the final weight after extraction. 2.3.2. Swelling number Degree of equilibrium swelling in benzene for 24 h at room temperature was calculated using the following equation:
Swelling number ¼ ðW 3 W 2 Þ=W 2 where W 2 is the initial weight before swelling and W3 is the final weight after equilibrium swelling. 2.4. Mechanical properties Mechanical properties measurements were carried out on dumbbell-shaped specimens of 4 mm width and 50 mm length. Tensile strength (TS), Young’s modulus at 50% elongation (M50), elongation at break percentage (Eb) and permanent set (PS) have been measured using a universal testing machine. The given results are the mean value of three separate specimens. The error in these measurements is 5%. 2.5. Thermal analysis Thermal analysis was carried out using thermal gravimetric analysis (TGA) apparatus namely Shimadzu TGA-50, whereby the samples of (0.98–1.5 mg) were encapsulated in aluminum pans and heated from 50 to 455 °C at 10 °C/min (under N2 atmosphere). 2.6. Electrical conductivity An electrometer was used for measuring the volume resistivity. The samples thickness was 0.07–0.4 cm; cylindrical in shape with 6 cm2 area.
Fig. 1. Effect of irradiation dose on the tensile strength of SBR loaded with different contents of boric acid.
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
Fig. 2. Effect of irradiation dose on the tensile modulus of SBR loaded with different contents of boric acid.
irradiation to the value of Young’s modulus is of high extent. Similarly, the value of the Young’s modulus increased with increasing the content of boric acid up to 60 phr and then decreased with increasing its content up to 100 phr. Similar reasons, as in the case of tensile strength results, may be also anticipated here for the behavior of the Young’s modulus. Moreover, it can be seen that Young’s modulus values, as in the case of TS values, for unirradiated samples have not changed with increasing the degree of loading with boric acid. This behavior may be taken as an indication that boric acid filler exists mainly as separate particles surrounded by the rubber and not in the form of separate single phase as in the case of, for example, saturated HAF carbon black filler [12–14]. On the other hand, it may be observed that the value of Young’s modulus at 150 kGy has increased from 20 MPa for unloaded rubber to 40 MPa for loaded SBR with 60 phr of boric acid. Here again, and for the same irradiation dose, as the content of boric acid increases up to 60 phr the Young’s modulus values increase. This increase as mentioned before, may be due to the high adhe-
Fig. 3. Effect of irradiation dose on the elongation at break of SBR loaded with different contents of boric acid.
231
sion between the matrix of SBR rubber and boric acid which becomes less as the content of boric acid increases up to 100 phr hence the decrease in the values of Young’s modulus was expected to be attained as in TS measurements. Fig. 3 illustrates the variation of the percent elongation at rupture, Eb as a function of irradiation dose. It may be observed that the values of Eb for the blank SBR as well as its composites have sharply decreased up to 50 kGy and then slightly decreased with increasing the radiation dose. This behavior may be ascribed to the induced crosslinking by irradiation. On the other hand, the values of Eb decrease systematically with increasing the extent of loading with the boric acid, for unirradiated composites and then slightly decreased with irradiation dose. The same trend was observed in the permanent set values as shown in Fig. 4, namely, the values of PS decrease almost semi-linearly with increasing the irradiation dose for all compositions. Moreover, the values of PS, at the same irradiation dose, decrease with increasing the content of boric acid in the composite. However, the range over which the values of PS changes over the whole irradiation range, for the same composition is relatively limited and thereby indicating that the resilience property of the prepared composite did not change effectively. From the above data, it is important to mention that SBR cannot be used without additives. 3.2. Swelling measurements Whereas the mechanical properties would account for the crosslinking and degradation processes that have taken place simultaneously on irradiation, the physical properties are affiliated only with crosslinking processes. Therefore, the latter processes, namely gel fraction and swelling number, were followed up as a function of irradiation dose for the blank SBR as well as loaded elastomer. Swelling of blank SBR and its composites with varying contents of boric acid up to 100 phr was carried out in benzene. The evaluation of equilibrium swelling is required to determine the degree of crosslinking and also the resistivity of rubber dissolution in benzene. The variation of the swelling number, SN as a function of irradiation dose for unloaded SBR as well as its composites of varying degrees of boric acid loading, is shown in Fig. 5. It may be observed
Fig. 4. Effect of irradiation dose on the permanent set of SBR loaded with different contents of boric acid.
232
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
that an appreciable decrease in SN values has been attained in the case of the straight radiation vulcanization of blank SBR on increasing the radiation dose from 50 to 250 kGy. Moreover, it may be observed that SN values of loaded samples have decreased, almost in a similar manner as those decreased for blank ones with increasing the dose of irradiation. These data indicate clearly that both the degree of boric acid loading and the irradiation dose have contributed to the degree of induced crosslinking of prepared composites. 3.3. Gel fraction measurements The data illustrated in Fig. 6 for the variation of the gel fraction GF, as a function of irradiation dose for blank SBR as well as its composites, offer confirmation to those of SN discussed before. It may be observed that the values of GF increase with the increase in the irradiation dose up to 250 kGy. The values of GF increase with increasing the extent of loading with boric acid at all irradiation doses. At the highest dose of irradiation, namely 250 kGy, the GF values varied between GF = 0.97 for the highest degree of loading and GF = 0.92 for the lowest degree of loading, whereas this value was only GF = 0.84 in the case of blank SBR. Moreover, the data shown in Fig. 6 show that blank SBR did not undergo crosslinking before a dose of 50 kGy. Apparently, the benzyl radical formed on irradiation of SBR with its expected reduced activity and increased half-life due to its resonance stabilization has been formed at such low extent that was not sufficient to undergo crosslinking. Also, the relatively low maximum value of GF for blank SBR accounts, as expected, for its radiation resistance property and the necessity of using boric acid for upgrading its mechanical properties of its radiation vulcanized end product. 3.4. Thermal stability Thermal analysis has been applied for measuring the thermal resistance of polymers [15,16]. The purpose of the present investigation is to estimate the stabilizing effectiveness of blank SBR rubber and reinforced SBR with different contents of boric acid up to 100 phr and irradiated to a dose up to 250 kGy. The thermo-gravimetric curves were recorded in air. The decomposition temperature (ti) is the initial temperature at the beginning of weight loss was determined for unreinforced samples and those reinforced
Fig. 5. Effect of irradiation dose on the swelling number of SBR loaded with different contents of boric acid.
Fig. 6. Effect of irradiation dose on the gel fraction of SBR loaded with different contents of boric acid.
with different contents of boric acid up to 100 phr. Decomposition temperature (ti) as a function of irradiation dose up to 250 kGy is shown in Fig. 7. It can be seen that ti for all the samples increases with increasing the irradiation dose till it reaches its maximum value at 200 kGy and then decreases. Also, it can be seen is that ti increases with increasing the content of boric acid up to 60 phr and then decreases. It is interesting to observe that the variation of thermal stability values of the samples as a function of irradiation dose matched with that of TS values as shown in Fig. 1. Besides, it may be said that the content of boric acid at 60 phr has a good effect on the thermal stability of prepared composites. Here again, it may be concluded that the factors affecting the role played by the degree of crosslinking in the case of mechanical properties have also contributed to the obtained behavior of thermal stability measurements.
Fig. 7. Effect of irradiation dose on the decomposition temperature of SBR loaded with different contents of boric acid.
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
233
on the obtained results of mechanical, thermal and physicomechanical properties, it can be concluded that the processing of SBR composites at a dose of 150 kGy of accelerated electron provides a noticeable properties that are recommended for industrial applications. 4. Conclusions From the results discussed above for the radiation vulcanized SBR composites, the following observations can be made:
Fig. 8. Effect of irradiation dose on the electrical conductivity of SBR loaded with different contents of boric acid.
3.5. Electrical conductivity Polymeric materials that are known to behave as insulators under normal conditions have been found to show either temporary or permanent electrical conductivity behavior when subjected to ionizing radiation. Accordingly, it has been anticipated that the temporary effect may be due to nonpermanent changes such as polarization and orientation ones, whereas the latter cases may be attributed to stable chemical changes encountered in the polymer structure due to irradiation [17]. This is the case of pure polymeric materials. Loading of such material with different additives would certainly affect its electrical behavior and hence it has been found appropriate to follow the electrical conductivity character of blank SBR and its composites with different contents of boric acid as a function of electron beam irradiation dose. Fig. 8 illustrates the variation of electrical conductivity as a function of irradiation dose for blank and SBR composites. Practically, it may be regarded that electrical conductivity values for such composites did not change appreciably with radiation dose as well as the content of boric acid except in the case of composite loaded with boric acid of 40 phr and that all the values of electrical conductivity measured lie within the non-conductive range. Based
Appreciable upgrading of properties of vulcanized composites has been attained with respect to the properties of vulcanized blank rubber. The composite SBR/boric acid 100/60, as a suitable reference with respect to commercially available rubber masterbatches, has attained a TS value of 3.3 MPa, Young’s modulus value of 42 MPa, Eb value of 80%, when irradiated with 150 kGy. Also, the physical properties were enhanced to a great extent. The decomposition temperature for the same composite was found to increase from 410 to 440 °C at 200 kGy. This composite may be used in many industrial applications as flame retardent material or shielding materials for thermal neutrons. References [1] N.A. Shaltout, M.M. Abou Zeid, M.H. Monstafa, A.A. ElMiligy, Arab J. Nucl. Sci. Appl. 38 (2005) 1. [2] N.A. Shaltout, M.M. Abou Zeid, M.H. Monstafa, A.A. ElMiligy, Arab J. Nucl. Sci. Appl. 36 (2003) 2. [3] H.F. Mark, Encyclopedia of Polymer Science and Technology, third ed., vol. 12, John Wiley and Sons Inc., USA, 2005. [4] M.M. Abdel-Aziz, S.E. Gwarly, A.S. Makarvious, A. El-sayed Abdo, Polym. Degrad. Stabil. 50 (1995) 235. [5] S.E. Gwaily, H.H. Hassan, M.M. Badawy, M. Madani, Polym. Test. 21 (2002) 513. [6] J.N. Henderson, Rubber Technology, Van Nostrand Reinhold, New York, 1987. [7] S.C. George, K.N. Ninan, S. Thomas, Polym. Polym. Compos. 7 (1999) 343. [8] S. Ray, S.K. Ray, J. Membrane Sci. 2 (2006) 132. [9] Y. Wang, D. Wen, Smart Mater. Struct. 14 (2005) 941. [10] H. Hsiung Wong, T. Wei Shy, R. Mei-Shan Hui, J. Appl. Polym. Sci. 74 (1999) 3064. [11] N.A. Shaltout, M.M. Abou Zeid, M.M. Abdel Aziz, A.A. El Miligy, J. Elast. Plast. 32 (2000) 86. [12] R.W. Sambrook, Rubber Chem. Technol. 44 (1971) 728. [13] C.W. Sweitzer, Rubber Age 39 (1961) 269. [14] N.A. Shaltout, M.M. Abou Zeid, M.ll. Moustafa, A.A. EI Miligy Arab, J. Nucl. Sci. Appl. 35 (1) (2002) 25. [15] M. Nucha, J. Polym. Sci., symposium no. 57 (1976) 25. [16] M. Nucha, M. Kry, S.Z. Eski, J. Polym. Sci., symposium no. 42 (1973) 1313. [17] E.G. Linderand, P. Rappaport, Phys. Rev. v91 (1953) 202.
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Effect of electron beam irradiation and degree of boric acid loading on the properties of styrene-butadiene rubber N.A. Shaltout * Radiation Chemistry Department, National Center for Radiation Research and Technology, 21830 NCRRT UN Telex, AEA, Nasr City, P.O. Box 29, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 20 September 2008 Received in revised form 20 December 2008 Accepted 26 December 2008 Available online 9 January 2009 Keywords: Styrene-butadiene rubber Boric acid Mechanical Thermal Electrical properties
a b s t r a c t Synthetic and totally amorphous styrene-butadiene rubber (SBR) has been loaded with varying contents of boric acid. Vulcanization of prepared composites as well as of unloaded rubber has been induced by ionizing radiation of accelerated electron beam of varying doses up to 250 kGy. Evaluation of prepared composite subjected to this range of irradiation has been followed up through the measurement of mechanical, physical, electrical and thermal properties of vulcanized composites. Mechanical properties, namely tensile strength (TS) and Young’s modulus were found to increase, whereas elongation at break (Eb) and permanent set (PS) were found to decrease with the increase in degree of boric acid loading as well as irradiation dose. On the other hand, physical properties, namely the gel content, have increased whereas the swelling number has decreased. Moreover, increase in the decomposition temperature has been attained. Also, limited increase in electrical conductivity has taken place. Data obtained indicate enhancement in thermal as well as in physico-mechanical properties of prepared composites. Moreover, 60 phr of boric acid has attained good mechanical properties. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Inorganic fillers are used for the loading of elastomers and may act in one way or the another, as an active additive or it may be considered as merely as inactive additive. CaCO3 is an example of the latter type of fillers as it is mainly used for increasing the amount of product rubber composite without affecting the properties of the elastomeric phase [1]. Active inorganic fillers such as carbon blacks and silica, may on the other hand, participate in upgrading certain properties of the elastomeric phase in the composite such as its physical, thermal or electrical properties [2]. Such active fillers may also act as protectors for the rubber phase from the degrading effect of ionizing radiation, such as gamma rays or accelerated electrons. In such a case, they perform their function through attenuation or absorption of the energy of ionizing radiation, which may be either charged or uncharged. It has been found in the case of uncharged radiations, such as gamma and X-rays, that the elements of high density and high atomic number, such as iron or preferably lead are the most suitable materials to act in this respect. In the case of charged radiations such as beta-rays or accelerated electrons, the use of such metals is not suitable due to the production of braking radiation, i.e., bremsstrahlung that is
* Fax: +20 2 22749298. E-mail address: [email protected] 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.12.025
produced during the deceleration of such charged particles in absorbing the elements of high atomic number [3]. To avoid such penetrating braking radiation, the elements of suitably high density and specifically low atomic number have been reported to be suitable as absorbing materials. Elements such as boron or hydrogen and their combinations would then act effectively in this respect. Boric acid H3PO4 is an example of such combination. Moreover, composites of boron-based compounds loaded to different rubbers, such as ethylene propylene diene rubber (EPDM), low density polyethylene (LDPE) and boron carbide (B4C) [4] as well as composites of natural rubber (NR)/H3BO4 [5], have proved to be efficient shields for thermal neutrons. It would be appropriate, however, for such composites to posses reasonably good physicochemical, thermal and electrical properties after being subjected to relatively high cumulative radiation doses. One of the most widely used synthetic rubbers is styrene-butadiene rubber (SBR). This polymer is mainly used in blends in the manufacture of tires due to its good crack resistance, wet grip and weather resistance [6]. However, other important industrial applications as membranes, wires and cables are included in the uses of this material [7–9]. In the present investigation, composites of SBR having varying contents of boric acid have been prepared. The effect of cumulative effect of electron-beam irradiation as well as the content of boric acid on physico-mechanical, thermal and electrical properties has been followed up.
230
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
2. Materials and methods
3. Results and discussion
2.1. Materials
3.1. Mechanical properties
A commercial grade styrene-butadiene rubber (SBR-1502) with 23.6% styrene content was used as the matrix polymer. The boric acid loading was varied up to 100 parts per 100 parts of rubber (phr) by weight. The recipe of this study also contained other additives, namely ZnO, stearic acid and tetrene. The first two additives act as accelerators as well as activators and their contents were 5 phr and 1 phr, respectively. On the other hand, Tetrene acts as antioxidant and was added by 1 phr. Solvent and other chemicals used were of commercial grade, and were used as received.
The mechanical properties of blank SBR samples and their composites with different contents of boric acid are given in Figs. 1–3. These mechanical properties of SBR samples containing different contents of boric acid are expected to be a function of irradiation dose as well as the content of boric acid. A comparison between blank SBR samples and boric acid loaded SBR is interesting to detect the role played by the boric acid and its content in the composites obtained at all irradiation doses. The data obtained for the variation of tensile strength as a function of irradiation dose are depicted in Fig. 1. It can be seen that the TS value for loaded but unirradiated composites have changed but slightly with respect to the value of unloaded and unirradiated sample. On the other hand, the tensile strength values of blank SBR samples and these loaded with different contents of boric acid increased with increasing the irradiation dose up to 150 kGy and then decreased. The increase in the tensile strength values with irradiation dose may be attributed to induced crosslinking by irradiation. Also, it is very important to notice that the TS values at the same irradiation dose increased with increasing the content of boric acid up to 60 phr and then decreased. The increase in TS of composites up to degree of loading of 60 phr of boric acid may be attributed to the participation of the latter as a crosslinking agent [10]. On loading the samples with 80–100 phr of boric acid, it would be expected that the matrix of the composite obtained is no longer composed of continuous polymeric phase as it may occur that boric acid particles would adhere to one another and hence proper wetting of its particles by elastomer has not taken place. Under such circumstances, the polymeric composite would lose much of its inherent elasticity [11]. Also, the decrease in TS values at higher doses than 150 kGy may be due to the degradation of blank SBR as well as the decrease in the orientation processes of the polymeric matrix that may occur at the high levels of crosslinking. From Fig. 2, it can be seen that Young’s modulus value has increased considerably, reaching its maximum value at 150 kGy, and then decreased up to 250 kGy. This behavior indicates that the contribution of induced crosslinking by accelerated electron
2.2. Sample preparation SBR, ZnO, stearic acid, tetrene and boric acid were mixed on a rubber mill (300 470 mm) with a gear ratio of 1.14:1 at 80 °C. The compound was pressed to a 1 mm thick sheet using a hot press at 150 °C for 20 min and under 16 MPa pressure. Electron beam irradiation was carried out in the atmosphere using an electron beam accelerator (1.5 MeV, 25 kW) at NCRRT, Cairo. 2.3. Physical properties 2.3.1. Gel fraction Gel fraction, expressed as the fraction of insoluble weight, was obtained by extracting soluble part in benzene using Soxhlet for 24 hr at room temperature, and drying insoluble part completely in vacuum oven at 50 °C. The gel fraction % was calculated according to the following equation:
Gel fraction ¼ W 1 =W 0 where W0 is the initial weight before extraction and W1 is the final weight after extraction. 2.3.2. Swelling number Degree of equilibrium swelling in benzene for 24 h at room temperature was calculated using the following equation:
Swelling number ¼ ðW 3 W 2 Þ=W 2 where W 2 is the initial weight before swelling and W3 is the final weight after equilibrium swelling. 2.4. Mechanical properties Mechanical properties measurements were carried out on dumbbell-shaped specimens of 4 mm width and 50 mm length. Tensile strength (TS), Young’s modulus at 50% elongation (M50), elongation at break percentage (Eb) and permanent set (PS) have been measured using a universal testing machine. The given results are the mean value of three separate specimens. The error in these measurements is 5%. 2.5. Thermal analysis Thermal analysis was carried out using thermal gravimetric analysis (TGA) apparatus namely Shimadzu TGA-50, whereby the samples of (0.98–1.5 mg) were encapsulated in aluminum pans and heated from 50 to 455 °C at 10 °C/min (under N2 atmosphere). 2.6. Electrical conductivity An electrometer was used for measuring the volume resistivity. The samples thickness was 0.07–0.4 cm; cylindrical in shape with 6 cm2 area.
Fig. 1. Effect of irradiation dose on the tensile strength of SBR loaded with different contents of boric acid.
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
Fig. 2. Effect of irradiation dose on the tensile modulus of SBR loaded with different contents of boric acid.
irradiation to the value of Young’s modulus is of high extent. Similarly, the value of the Young’s modulus increased with increasing the content of boric acid up to 60 phr and then decreased with increasing its content up to 100 phr. Similar reasons, as in the case of tensile strength results, may be also anticipated here for the behavior of the Young’s modulus. Moreover, it can be seen that Young’s modulus values, as in the case of TS values, for unirradiated samples have not changed with increasing the degree of loading with boric acid. This behavior may be taken as an indication that boric acid filler exists mainly as separate particles surrounded by the rubber and not in the form of separate single phase as in the case of, for example, saturated HAF carbon black filler [12–14]. On the other hand, it may be observed that the value of Young’s modulus at 150 kGy has increased from 20 MPa for unloaded rubber to 40 MPa for loaded SBR with 60 phr of boric acid. Here again, and for the same irradiation dose, as the content of boric acid increases up to 60 phr the Young’s modulus values increase. This increase as mentioned before, may be due to the high adhe-
Fig. 3. Effect of irradiation dose on the elongation at break of SBR loaded with different contents of boric acid.
231
sion between the matrix of SBR rubber and boric acid which becomes less as the content of boric acid increases up to 100 phr hence the decrease in the values of Young’s modulus was expected to be attained as in TS measurements. Fig. 3 illustrates the variation of the percent elongation at rupture, Eb as a function of irradiation dose. It may be observed that the values of Eb for the blank SBR as well as its composites have sharply decreased up to 50 kGy and then slightly decreased with increasing the radiation dose. This behavior may be ascribed to the induced crosslinking by irradiation. On the other hand, the values of Eb decrease systematically with increasing the extent of loading with the boric acid, for unirradiated composites and then slightly decreased with irradiation dose. The same trend was observed in the permanent set values as shown in Fig. 4, namely, the values of PS decrease almost semi-linearly with increasing the irradiation dose for all compositions. Moreover, the values of PS, at the same irradiation dose, decrease with increasing the content of boric acid in the composite. However, the range over which the values of PS changes over the whole irradiation range, for the same composition is relatively limited and thereby indicating that the resilience property of the prepared composite did not change effectively. From the above data, it is important to mention that SBR cannot be used without additives. 3.2. Swelling measurements Whereas the mechanical properties would account for the crosslinking and degradation processes that have taken place simultaneously on irradiation, the physical properties are affiliated only with crosslinking processes. Therefore, the latter processes, namely gel fraction and swelling number, were followed up as a function of irradiation dose for the blank SBR as well as loaded elastomer. Swelling of blank SBR and its composites with varying contents of boric acid up to 100 phr was carried out in benzene. The evaluation of equilibrium swelling is required to determine the degree of crosslinking and also the resistivity of rubber dissolution in benzene. The variation of the swelling number, SN as a function of irradiation dose for unloaded SBR as well as its composites of varying degrees of boric acid loading, is shown in Fig. 5. It may be observed
Fig. 4. Effect of irradiation dose on the permanent set of SBR loaded with different contents of boric acid.
232
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
that an appreciable decrease in SN values has been attained in the case of the straight radiation vulcanization of blank SBR on increasing the radiation dose from 50 to 250 kGy. Moreover, it may be observed that SN values of loaded samples have decreased, almost in a similar manner as those decreased for blank ones with increasing the dose of irradiation. These data indicate clearly that both the degree of boric acid loading and the irradiation dose have contributed to the degree of induced crosslinking of prepared composites. 3.3. Gel fraction measurements The data illustrated in Fig. 6 for the variation of the gel fraction GF, as a function of irradiation dose for blank SBR as well as its composites, offer confirmation to those of SN discussed before. It may be observed that the values of GF increase with the increase in the irradiation dose up to 250 kGy. The values of GF increase with increasing the extent of loading with boric acid at all irradiation doses. At the highest dose of irradiation, namely 250 kGy, the GF values varied between GF = 0.97 for the highest degree of loading and GF = 0.92 for the lowest degree of loading, whereas this value was only GF = 0.84 in the case of blank SBR. Moreover, the data shown in Fig. 6 show that blank SBR did not undergo crosslinking before a dose of 50 kGy. Apparently, the benzyl radical formed on irradiation of SBR with its expected reduced activity and increased half-life due to its resonance stabilization has been formed at such low extent that was not sufficient to undergo crosslinking. Also, the relatively low maximum value of GF for blank SBR accounts, as expected, for its radiation resistance property and the necessity of using boric acid for upgrading its mechanical properties of its radiation vulcanized end product. 3.4. Thermal stability Thermal analysis has been applied for measuring the thermal resistance of polymers [15,16]. The purpose of the present investigation is to estimate the stabilizing effectiveness of blank SBR rubber and reinforced SBR with different contents of boric acid up to 100 phr and irradiated to a dose up to 250 kGy. The thermo-gravimetric curves were recorded in air. The decomposition temperature (ti) is the initial temperature at the beginning of weight loss was determined for unreinforced samples and those reinforced
Fig. 5. Effect of irradiation dose on the swelling number of SBR loaded with different contents of boric acid.
Fig. 6. Effect of irradiation dose on the gel fraction of SBR loaded with different contents of boric acid.
with different contents of boric acid up to 100 phr. Decomposition temperature (ti) as a function of irradiation dose up to 250 kGy is shown in Fig. 7. It can be seen that ti for all the samples increases with increasing the irradiation dose till it reaches its maximum value at 200 kGy and then decreases. Also, it can be seen is that ti increases with increasing the content of boric acid up to 60 phr and then decreases. It is interesting to observe that the variation of thermal stability values of the samples as a function of irradiation dose matched with that of TS values as shown in Fig. 1. Besides, it may be said that the content of boric acid at 60 phr has a good effect on the thermal stability of prepared composites. Here again, it may be concluded that the factors affecting the role played by the degree of crosslinking in the case of mechanical properties have also contributed to the obtained behavior of thermal stability measurements.
Fig. 7. Effect of irradiation dose on the decomposition temperature of SBR loaded with different contents of boric acid.
N.A. Shaltout / Reactive & Functional Polymers 69 (2009) 229–233
233
on the obtained results of mechanical, thermal and physicomechanical properties, it can be concluded that the processing of SBR composites at a dose of 150 kGy of accelerated electron provides a noticeable properties that are recommended for industrial applications. 4. Conclusions From the results discussed above for the radiation vulcanized SBR composites, the following observations can be made:
Fig. 8. Effect of irradiation dose on the electrical conductivity of SBR loaded with different contents of boric acid.
3.5. Electrical conductivity Polymeric materials that are known to behave as insulators under normal conditions have been found to show either temporary or permanent electrical conductivity behavior when subjected to ionizing radiation. Accordingly, it has been anticipated that the temporary effect may be due to nonpermanent changes such as polarization and orientation ones, whereas the latter cases may be attributed to stable chemical changes encountered in the polymer structure due to irradiation [17]. This is the case of pure polymeric materials. Loading of such material with different additives would certainly affect its electrical behavior and hence it has been found appropriate to follow the electrical conductivity character of blank SBR and its composites with different contents of boric acid as a function of electron beam irradiation dose. Fig. 8 illustrates the variation of electrical conductivity as a function of irradiation dose for blank and SBR composites. Practically, it may be regarded that electrical conductivity values for such composites did not change appreciably with radiation dose as well as the content of boric acid except in the case of composite loaded with boric acid of 40 phr and that all the values of electrical conductivity measured lie within the non-conductive range. Based
Appreciable upgrading of properties of vulcanized composites has been attained with respect to the properties of vulcanized blank rubber. The composite SBR/boric acid 100/60, as a suitable reference with respect to commercially available rubber masterbatches, has attained a TS value of 3.3 MPa, Young’s modulus value of 42 MPa, Eb value of 80%, when irradiated with 150 kGy. Also, the physical properties were enhanced to a great extent. The decomposition temperature for the same composite was found to increase from 410 to 440 °C at 200 kGy. This composite may be used in many industrial applications as flame retardent material or shielding materials for thermal neutrons. References [1] N.A. Shaltout, M.M. Abou Zeid, M.H. Monstafa, A.A. ElMiligy, Arab J. Nucl. Sci. Appl. 38 (2005) 1. [2] N.A. Shaltout, M.M. Abou Zeid, M.H. Monstafa, A.A. ElMiligy, Arab J. Nucl. Sci. Appl. 36 (2003) 2. [3] H.F. Mark, Encyclopedia of Polymer Science and Technology, third ed., vol. 12, John Wiley and Sons Inc., USA, 2005. [4] M.M. Abdel-Aziz, S.E. Gwarly, A.S. Makarvious, A. El-sayed Abdo, Polym. Degrad. Stabil. 50 (1995) 235. [5] S.E. Gwaily, H.H. Hassan, M.M. Badawy, M. Madani, Polym. Test. 21 (2002) 513. [6] J.N. Henderson, Rubber Technology, Van Nostrand Reinhold, New York, 1987. [7] S.C. George, K.N. Ninan, S. Thomas, Polym. Polym. Compos. 7 (1999) 343. [8] S. Ray, S.K. Ray, J. Membrane Sci. 2 (2006) 132. [9] Y. Wang, D. Wen, Smart Mater. Struct. 14 (2005) 941. [10] H. Hsiung Wong, T. Wei Shy, R. Mei-Shan Hui, J. Appl. Polym. Sci. 74 (1999) 3064. [11] N.A. Shaltout, M.M. Abou Zeid, M.M. Abdel Aziz, A.A. El Miligy, J. Elast. Plast. 32 (2000) 86. [12] R.W. Sambrook, Rubber Chem. Technol. 44 (1971) 728. [13] C.W. Sweitzer, Rubber Age 39 (1961) 269. [14] N.A. Shaltout, M.M. Abou Zeid, M.ll. Moustafa, A.A. EI Miligy Arab, J. Nucl. Sci. Appl. 35 (1) (2002) 25. [15] M. Nucha, J. Polym. Sci., symposium no. 57 (1976) 25. [16] M. Nucha, M. Kry, S.Z. Eski, J. Polym. Sci., symposium no. 42 (1973) 1313. [17] E.G. Linderand, P. Rappaport, Phys. Rev. v91 (1953) 202.