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natural rubber blends
Polymer 45 (2004) 4909–4916 www.elsevier.com/locate/polymer
Oil and thermal aging resistance in compatibilized and thermally stabilized chlorinated polyethylene/natural rubber blends Chakrit Sirisinhaa,b,*, Pongdhorn Saeouic, Jantagarn Guaysomboona b
a Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Rd., Bangkok 10400, Thailand Rubber Research Unit, Faculty of Science, Mahidol University, Salaya Campus, Phutthamonthon 4 Rd., Salaya, Nakhon Pathom 73170, Thailand c The National Metal and Materials Technology Center (MTEC), 114 Thailand Science Park Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120, Thailand
Received 6 January 2004; received in revised form 28 April 2004; accepted 7 May 2004 Available online 25 May 2004
Abstract Influences of EPDM-g-MA as a compatibilizer and a phenolic antioxidant on oil and thermal aging resistance in 50/50 CPE/NR blends were investigated. It has been found that EPDM-g-MA could decrease phase size of the blend system, indicating compatibilizing effect. The optimal concentration of EPDM-g-MA is 1 phr. Beyond this concentration, phase size starts to increase. The addition of phenolic antioxidant apparently decreases the phase size in blends. This is probably due to the improvement in a thermal stabilization of NR phase in blends provided by the antioxidant, which leads to a reduction in phase coalescence during blending. In addition, the results of oil and thermal aging resistance are in good agreement with the morphological results, indicating that the oil resistance and thermal aging properties based on relative tensile strength in the 50/50 CPE/NR blends are strongly controlled by the size of the NR dispersed phase in CPE matrix. The smaller the dispersed phase size, the higher the resistance to oil and thermal aging. q 2004 Elsevier Ltd. All rights reserved. Keywords: Oil resistance; Thermal resistance; Chlorinated polyethylene/natural rubber blends
1. Introduction Chlorinated polyethylene elastomer (CPE) has been produced by the introduction of chlorine atoms onto the polyethylene backbone in order to reduce the ability of crystallization of polyethylene. In addition, the enhancement in resistance to hydrocarbon oil, heat and weathering is also achieved. To gain desired properties of the final products, CPE has been blended with many polymers including polyvinyl chloride (PVC) [1 –4], styrene-acrylonitrile (SAN) [5,6] and polyurethane (PU) [7,8]. Compared to natural rubber (NR), CPE is relatively expensive and therefore the blend of CPE/NR is one of methods to reduce the production cost of the final products requiring CPE properties. Generally, mechanical properties of polymer blends are governed by many factors including nature of polymers, blend composition [9 –14], blending conditions [15 – 17] * Corresponding author. Tel.: þ 661-8072-742; fax: þ 662-2458-332. E-mail address: [email protected] (C. Sirisinha). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.05.015
and interfacial adhesion [18 – 24]. In CPE/NR blends, the changes in oil and thermal aging resistance as functions of blend composition and mixing condition have previously been reported [12,17]. It has been found that phase size of NR dispersed in CPE matrix plays significant role in oil and thermal aging resistance, i.e. the smaller the phase size, the higher the resistance to oil and thermal aging. The present study aims to extend the previous work by focusing on the influence of maleated ethylene propylene diene rubber (EPDM-g-MA) as a compatibilizer and a phenol-based antioxidant, namely, 2,20 -dicyclopentyl-bis (4-methyl-6tert-butyl-phenol), on changes in phase morphology as well as oil and thermal aging resistance in CPE/NR blends.
2. Experimental 2.1. Materials The materials used in the present study are summarized in Table 1.
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Table 1 Materials used in the present study Chemical name
Grade/Supplier
Chlorinated polyethylene (CPE)
Tyrin 3615P (36% Chlorine content)/Dupont Dow Elastomer Co., Ltd., USA STR5, Thailand (Mooney viscosity at 100 8C ¼ 72) ROYALTUF 490, Uniroyal Co., Inc., USA 22 CP 46, Lowi Co., Ltd., Thailand Percumyl D/Chemmin Co., Ltd., Thailand BecThai Co., Ltd., Thailand
Natural rubber (NR) Maleic anhydride grafted ethylene propylene diene rubber (EPDM-g-MA) 2,20 -dicyclopentyl-bis (4-methyl-6-tert-butyl-phenol) Dicumyl peroxide (DCP) (98% active DCP) Osmium tetroxide (OsO4)
2.2. Mixing and vulcanization procedures The compound ingredients as shown in Table 2 were mixed in a laboratory-scale Haake Rheomix 90 with a mixing temperature, a rotor speed and a fill factor of 145 8C, 45 and 0.65 rpm, respectively. Notably, the mixing conditions were acquired from our previous work [17]. NR was first masticated using a two-roll mill before being used in blending process in order to reduce an initial Mooney viscosity from 72 to 36 Mooney units. Masticated NR was charged to the mixer and masticated for 1 min and, thereafter, CPE was added and mixed further for 4 min. Then, EPDM-g-MA was charged to the mixer, and the mixing was carried on for further 4 min. Finally DCP was added and mixed for 1 min before discharging. The mixes were sheeted on the cold mill and kept at room temperature for 24 h before testing. For the mixes with antioxidant, the antioxidant was initially premixed with NR during mastication on two-roll mill. To vulcanize the blends, the mixes were compression molded using a hydraulic hot press (Wabash Genesis Series model G30H) at 155 8C, under pressure of 15 MPa. The vulcanization times were calculated from a decomposition half-life of dicumyl peroxide (DCP). In the present study, at the cure temperature of 155 8C, the decomposition half-life of DCP is approximately 5.5 min. Consequently, to achieve approximately 97% cure, the cure time of about 30 min was used. 2.3. Mooney viscosity measurement Mooney viscometer (Monsanto model 1500) with a large Table 2 Compound formulation used Ingredients
Part per hundred of rubber (phr)
CPE NR DCP EPDM-g-MA Antioxidant (22 CP 46)
50 50 1.0 0.0, 1.0a, 3.0a, 5.0a, 7.0a 0.0, 1.0b
a b
Used in the study of compatibilizer effect. Used in the study of antioxidant effect.
rotor at the test temperature of 100 8C was utilized for measuring Mooney viscosity, according to ASTM D164687 and reported in Mooney Unit (MU). At least 5 samples were used for a measurement. 2.4. Morphological study Morphological study was carried out using the JEOL JSM-5800LV scanning electron microscope with 15 kV accelerating voltage with complementary surface preparation techniques via OsO4 staining to improve the phase contrast. Measurement of the number-average diameter of the dispersed phase was accomplished by the use of Image Pro Plus software developed by Media Cybernetics Co. 2.5. Oil and thermal aging resistance measurement According to the previous work [12,16,17], the dumbbell-shape (punched out using Die C-ASTM D412-92) test specimens were immersed in oil at room temperature for 70 h. The excess oil on the specimen surfaces was wiped-off with acetone. Changes in tensile strength of specimens after oil immersion were used to determine oil resistance, as shown in Eq. (1). In this study, the relative tensile strength (TSrel), calculated from the ratio of tensile strength after to that before oil immersion, was used in order to eliminate the mastication effect probably taking place during blending process. Tensile strength were measured using an Instron 4301 tensile tester with a crosshead speed of 500 mm/min and a full-scale load cell of 1 kN. TSrel ¼
TSafter TSbefore
ð1Þ
where TSbefore and TSafter are tensile strength of specimens before and after oil immersion, respectively. For the determination of thermal aging properties, the specimens were placed in an oven equipped with air circulating system at the test temperature of 100 8C for 24 h, according to ASTM D573. The aged specimens were then measured for tensile properties. Similar to the measurement of oil resistance, the changes in tensile strength after thermal aging were used to determine thermal aging resistance.
C. Sirisinha et al. / Polymer 45 (2004) 4909–4916
3. Results and discussion 3.1. Effects of EPDM-g-MA 3.1.1. Effect on phase morphology As one might expect, blends of CPE and NR are incompatible, which is undoubtedly attributed to a large polarity difference, and thus the interfacial tension. To improve blend compatibility, the graft copolymer with segments, that are chemically identical or similar to CPE and NR components in a blend of CPE/NR known as a compatibilizer, is added. A suitably selected compatibilizer will, in theory, locate at the interface between the two phases, leading to a reduction in interfacial tension, an improvement in interfacial adhesion, and a decrease in phase coalescence processes. The degree of improvement depends strongly on the characteristics of the graft copolymer selected. In the present study, maleic anhydride grafted ethylene –propylene diene rubber (EPDM-g-MA) was used as a compatibilizer in blends because it has been reported that the EPDM-g-MA could successfully provide the compatibilizing effect in blends between polar and nonpolar polymers [24 – 26]. It is believed that the EPDM segments with maleic anhydride groups would interact with the chlorine atoms of CPE, promoting good compatibility between CPE and NE phases in blends. The influence of EPDM-g-MA concentration on size of the NR dispersed phase is shown in Figs. 1 and 2. It is evident that, with the addition of 1-phr EPDM-g-MA as a compatibilizer, the phase size decreases obviously. Compatibilization by the addition of graft copolymers provides a reduction in phase size and an improvement in morphological stability by lowering the interfacial tension between the phases, and by introducing a steric hindrance to phase coalescence [27 –30]. Unexpectedly, the addition of EPDMg-MA greater than 1 phr appears to increase NR phase size, although the phase size is still smaller than that in the uncompatibilized blend. Similar result trends have extensively been reported elsewhere [27,31 –33], and it has been proposed that, with the addition of EPDM-g-MA greater than 1 phr, the formation of micelles of EPDM-g-MA in the continuous CPE matrix might take place, and thereafter some of the EPDM-g-MA already being at the interfaces between NR and CPE would leave the interfaces and coalesce with the EPDM-g-MA micelles. This leads to a reduction in magnitude of compatibilizing effect and thus an increase in domain size. In other words, the 1 phr of EPDMg-MA is believed to be sufficient for fully occupying the interfaces between NR and CPE. As a result, an excess of EPDM-g-MA remains in the bulk and no longer contributes to the reduction of interfacial tension. 3.1.2. Effect on oil resistance As mentioned in the previous work [12,16,17], the relative tensile strength is successfully used as an indicator
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for determining oil resistance. The higher the relative tensile strength, the higher the oil resistance. Fig. 3 and Table 3 illustrates the relative tensile strength of blends as a function of EPDM-g-MA concentration. It is evident that the relative tensile strength of the blend increases significantly with the addition of up to 1 phr EPDM-g-MA, and subsequently decreases progressively. The results are clearly in good agreement with the phase morphological results, i.e. the smaller the phase size, the higher the oil resistance. Similar results have been reported elsewhere [16,17]. Fig. 4 illustrates the proposed explanation model of dispersed phase size effect on oil resistance. Compared to NR, CPE possesses excellent resistance to hydrocarbon liquids. Thus, when the blends are immersed in oil, NR dispersed phase swells markedly leading to low resistance to failure of the blends. In the case of small dispersed phase size of NR, the large surface area of the small dispersed phase of NR is surrounded by CPE phase, possessing high resistance to oil. Thus, oil swelling occurring mainly within small NR phase will effectively be stopped by the surrounding CPE, resulting in high value of relative tensile strength. By contrast, a large degree of swelling in a large NR dispersed phase would be stopped ineffectively by CPE due to a small surface area of NR dispersed phase surrounded by CPE. This leads to low resistance to failure and thus low relative tensile strength. 3.1.3. Effect on thermal aging properties According to the previous work [17], the relative tensile strength, (i.e. the ratios of tensile strength after thermal aging to that before thermal aging) was used as an indicator of the thermal aging resistance, in order to minimize the mastication and degree of crosslink effects that might occur during blending or curing, which could interfere the results of thermal aging tests. After thermal aging at 100 8C for 24 h, the relative tensile strength of the blends was measured as a function of EPDM-g-MA concentration, and the results obtained are shown in Fig. 5 and Table 3. It can be seen that the relative tensile strength significantly increases with the addition of EPDM-g-MA up to 1 phr, and then decreases. The result agrees very well with the morphological change caused by the addition of EPDM-gMA, i.e. the smaller the phase size, the higher the thermal aging resistance. 3.2. Thermal stabilization Since the natural rubber (NR) possesses high levels of unsaturation in the polymer backbone, the degradation of NR is promoted by the exposure of NR to high temperature, oxygen, ozone, ultraviolet light, moisture, radiation and chemical agents. Multiple exposures, such as a combination of moisture and heat or oxygen and light, can often result in accelerated deterioration. In high shear mixing processes and under the influence of small amounts of oxygen, a scission of polymer chains could take place. Oxidative
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Fig. 1. Scanning electron micrographs of blends with various contents of EPDM-g-MA: (a) 0 phr; (b) 1 phr; (c) 3 phr; (d) 5 phr; (e) 7 phr.
Table 3 Actual values of tensile strength (MPa) of blends with EPDM-g-MA Amount of EPDM-g-MA
Control
After oil immersion
After thermal aging
0 1 3 5 7
3.48 ^ 0.55 4.94 ^ 0.53 4.75 ^ 0.42 4.41 ^ 0.58 3.95 ^ 0.59
1.53 ^ 0.20 2.95 ^ 0.22 2.66 ^ 0.09 2.51 ^ 0.17 2.23 ^ 0.17
1.50 ^ 0.47 3.14 ^ 0.46 2.73 ^ 0.32 2.45 ^ 0.33 1.96 ^ 0.29
C. Sirisinha et al. / Polymer 45 (2004) 4909–4916
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Fig. 2. Relationship between a number-average diameter ðDn Þ of the dispersed phase and EPDM-g-MA concentrations.
degradation is enhanced at elevated temperatures during the processing of the polymer. The addition of antioxidants is acknowledged as the most convenient and effective way to reduce the thermal oxidation of polymers. The main objective of this part aims to investigate how the antioxidant would help improving the thermal stabilization of NR phase in CPE/NR blends. The phenolic antioxidant (22 CP 46) is used, and its effect on properties and morphology are illustrated. 3.2.1. Effect on phase morphology The influence of antioxidant on a number-average domain diameter of NR dispersed phase in CPE matrix is
shown in Figs. 6 and 7. It is clear that the dispersed phase size of NR decreases after the addition of antioxidant. The explanation is proposed based on the elasticity of NR phase in CPE matrix. It is known that NR is prone to the thermal oxidation leading to the molecular chain-scission and thus a reduction in elasticity. As a consequence, the phase coalescence of NR dispersed phase could occur readily, resulting in a relatively large phase size in blends. As the antioxidant is added, the degree of thermal degradation of NR phase is relatively low and therefore the elasticity of NR phase is relatively high which could efficiently hinder the phase coalescence, leading to a relatively small phase size of NR in CPE matrix. The explanation is supported by the
Fig. 3. Relationships among oil resistance, EPDM-g-MA concentrations and phase size of CPE/NR blends.
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Fig. 4. Proposed model of the blends with different sizes of the non-polar dispersed phase in polar matrix: large phase size (a); small phase size (b).
Fig. 6. Scanning electron micrographs of CPE/NR blend: (a) without antioxidant; (b) with antioxidant.
Fig. 5. Relationships among thermal aging resistance, EPDM-g-MA concentrations and dispersed phase size of the blends.
C. Sirisinha et al. / Polymer 45 (2004) 4909–4916
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Fig. 8. Oil resistance of blends with and without antioxidant.
Fig. 7. Number-average diameter ðDn Þ of the dispersed phase in blends with and without antioxidant.
results of mixing torque and molecular weight of NR, as shown in Table 4. It is clear that the molecular weight and mixing torque of NR with antioxidant are significantly higher than those of NR without antioxidant. The results obtained, thus, confirm that there is the oxidative degradation occurring in the mixer leading to a decrease in molecular weight of NR during processing at high temperature. The addition of antioxidant could retard the thermal degradation of NR phase in the blends, leading to an improvement in NR elasticity, and thus, a reduction in phase coalescence of the blends. 3.2.2. Effect on oil resistance The effect of antioxidant on oil resistance (via the use of relative tensile strength) is shown in Fig. 8 and Table 5, indicating that the oil resistance increases when the antioxidant is added. The result is corresponding to the morphological result, as previously illustrated in Figs. 6 and 7, i.e. phase size of blend with antioxidant is smaller than that without antioxidant. As discussed earlier, the smaller the phase size, the higher the oil resistance. 3.2.3. Effect on thermal aging properties Fig. 9 and Table 5 reveals the thermal aging resistance affected by the addition of antioxidant. It is obvious that the Table 4 Effect of antioxidant (AO) on mixing torque and molecular weight of NR Compound
Torque (Nm)
n M ( £ 105)
w M ( £ 106)
v M ( £ 106)
Polydispersity
NR NR þ AO
44.2 59.5
6.03 10.76
4.47 4.55
2.91 3.28
7.41 4.22
n ; number-average molecuDetermined from a capillary viscometer. M v ; viscosity-average w ; weight-average molecular weight; M lar weight; M w =M n: molecular weight; polydispersity, M
relative tensile strength, as an indicator of the thermal aging resistance, is higher in the blend with antioxidant. The result suggests that antioxidant added to the blend is efficient in protecting NR phase in blends against the thermal degradation, leading to an improvement in a bulk thermal aging resistance. In addition, the result can again be explained by the phase morphology of blend, as illustrated in Figs. 6 and 7. It is thus logical to state that the reduction in thermal degradation of NR phase by the addition of antioxidant, resulting in a decrease in phase size of the blends, plays significant role in thermal aging resistance, i.e. the smaller the dispersed phase size, the higher the relative tensile strength and thus the thermal aging resistance.
4. Conclusions Influences of EPDM-g-MA as a compatibilizer and a phenolic antioxidant on oil and thermal aging resistance in 50/50 CPE/NR blends were investigated. In the case of compatibilizing effect, it has been found that EPDM-g-MA could function as a compatibilizer in the blend system studied. The optimal concentration of EPDM-g-MA is 1 phr. Beyond this concentration, phase size starts to increase. As for the thermal stabilization of the blends, the results reveal that the addition of phenolic antioxidant strongly affects the phase morphology in the way that the phase size decreases. This is due to the fact that the thermal stabilization of NR phase in blends provided by the antioxidant is improved, resulting in the relatively high molecular weight, and thus increases the elasticity of NR Table 5 Actual values of tensile strength (MPa) of blends with and without antioxidant (AO) Blend
Control
After oil immersion
After thermal aging
Without AO With AO
5.95 ^ 0.98 8.35 ^ 0.53
2.81 ^ 0.61 4.97 ^ 0.53
2.67 ^ 0.57 4.50 ^ 0.11
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Fig. 9. Thermal aging resistance of blends with and without antioxidant.
phase in blends, which reduces the phase coalescence during blending. In addition, the results of oil and thermal aging resistance are in good agreement with the morphological results, indicating that the oil and thermal aging resistance based on relative tensile strength in the 50/50 CPE/NR blends are controlled significantly by the size of the NR dispersed phase in CPE matrix. The smaller the dispersed phase size, the higher the resistance to oil and thermal aging.
Acknowledgements The authors would like to express their gratitude to the Thailand Graduate Institute of Science and Technology (TGIST), National Science and Technology Development Agency (NSTDA) as well as the Thailand Research Fund (TRF) for the financial support of this research.
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Oil and thermal aging resistance in compatibilized and thermally stabilized chlorinated polyethylene/natural rubber blends Chakrit Sirisinhaa,b,*, Pongdhorn Saeouic, Jantagarn Guaysomboona b
a Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Rd., Bangkok 10400, Thailand Rubber Research Unit, Faculty of Science, Mahidol University, Salaya Campus, Phutthamonthon 4 Rd., Salaya, Nakhon Pathom 73170, Thailand c The National Metal and Materials Technology Center (MTEC), 114 Thailand Science Park Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120, Thailand
Received 6 January 2004; received in revised form 28 April 2004; accepted 7 May 2004 Available online 25 May 2004
Abstract Influences of EPDM-g-MA as a compatibilizer and a phenolic antioxidant on oil and thermal aging resistance in 50/50 CPE/NR blends were investigated. It has been found that EPDM-g-MA could decrease phase size of the blend system, indicating compatibilizing effect. The optimal concentration of EPDM-g-MA is 1 phr. Beyond this concentration, phase size starts to increase. The addition of phenolic antioxidant apparently decreases the phase size in blends. This is probably due to the improvement in a thermal stabilization of NR phase in blends provided by the antioxidant, which leads to a reduction in phase coalescence during blending. In addition, the results of oil and thermal aging resistance are in good agreement with the morphological results, indicating that the oil resistance and thermal aging properties based on relative tensile strength in the 50/50 CPE/NR blends are strongly controlled by the size of the NR dispersed phase in CPE matrix. The smaller the dispersed phase size, the higher the resistance to oil and thermal aging. q 2004 Elsevier Ltd. All rights reserved. Keywords: Oil resistance; Thermal resistance; Chlorinated polyethylene/natural rubber blends
1. Introduction Chlorinated polyethylene elastomer (CPE) has been produced by the introduction of chlorine atoms onto the polyethylene backbone in order to reduce the ability of crystallization of polyethylene. In addition, the enhancement in resistance to hydrocarbon oil, heat and weathering is also achieved. To gain desired properties of the final products, CPE has been blended with many polymers including polyvinyl chloride (PVC) [1 –4], styrene-acrylonitrile (SAN) [5,6] and polyurethane (PU) [7,8]. Compared to natural rubber (NR), CPE is relatively expensive and therefore the blend of CPE/NR is one of methods to reduce the production cost of the final products requiring CPE properties. Generally, mechanical properties of polymer blends are governed by many factors including nature of polymers, blend composition [9 –14], blending conditions [15 – 17] * Corresponding author. Tel.: þ 661-8072-742; fax: þ 662-2458-332. E-mail address: [email protected] (C. Sirisinha). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.05.015
and interfacial adhesion [18 – 24]. In CPE/NR blends, the changes in oil and thermal aging resistance as functions of blend composition and mixing condition have previously been reported [12,17]. It has been found that phase size of NR dispersed in CPE matrix plays significant role in oil and thermal aging resistance, i.e. the smaller the phase size, the higher the resistance to oil and thermal aging. The present study aims to extend the previous work by focusing on the influence of maleated ethylene propylene diene rubber (EPDM-g-MA) as a compatibilizer and a phenol-based antioxidant, namely, 2,20 -dicyclopentyl-bis (4-methyl-6tert-butyl-phenol), on changes in phase morphology as well as oil and thermal aging resistance in CPE/NR blends.
2. Experimental 2.1. Materials The materials used in the present study are summarized in Table 1.
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Table 1 Materials used in the present study Chemical name
Grade/Supplier
Chlorinated polyethylene (CPE)
Tyrin 3615P (36% Chlorine content)/Dupont Dow Elastomer Co., Ltd., USA STR5, Thailand (Mooney viscosity at 100 8C ¼ 72) ROYALTUF 490, Uniroyal Co., Inc., USA 22 CP 46, Lowi Co., Ltd., Thailand Percumyl D/Chemmin Co., Ltd., Thailand BecThai Co., Ltd., Thailand
Natural rubber (NR) Maleic anhydride grafted ethylene propylene diene rubber (EPDM-g-MA) 2,20 -dicyclopentyl-bis (4-methyl-6-tert-butyl-phenol) Dicumyl peroxide (DCP) (98% active DCP) Osmium tetroxide (OsO4)
2.2. Mixing and vulcanization procedures The compound ingredients as shown in Table 2 were mixed in a laboratory-scale Haake Rheomix 90 with a mixing temperature, a rotor speed and a fill factor of 145 8C, 45 and 0.65 rpm, respectively. Notably, the mixing conditions were acquired from our previous work [17]. NR was first masticated using a two-roll mill before being used in blending process in order to reduce an initial Mooney viscosity from 72 to 36 Mooney units. Masticated NR was charged to the mixer and masticated for 1 min and, thereafter, CPE was added and mixed further for 4 min. Then, EPDM-g-MA was charged to the mixer, and the mixing was carried on for further 4 min. Finally DCP was added and mixed for 1 min before discharging. The mixes were sheeted on the cold mill and kept at room temperature for 24 h before testing. For the mixes with antioxidant, the antioxidant was initially premixed with NR during mastication on two-roll mill. To vulcanize the blends, the mixes were compression molded using a hydraulic hot press (Wabash Genesis Series model G30H) at 155 8C, under pressure of 15 MPa. The vulcanization times were calculated from a decomposition half-life of dicumyl peroxide (DCP). In the present study, at the cure temperature of 155 8C, the decomposition half-life of DCP is approximately 5.5 min. Consequently, to achieve approximately 97% cure, the cure time of about 30 min was used. 2.3. Mooney viscosity measurement Mooney viscometer (Monsanto model 1500) with a large Table 2 Compound formulation used Ingredients
Part per hundred of rubber (phr)
CPE NR DCP EPDM-g-MA Antioxidant (22 CP 46)
50 50 1.0 0.0, 1.0a, 3.0a, 5.0a, 7.0a 0.0, 1.0b
a b
Used in the study of compatibilizer effect. Used in the study of antioxidant effect.
rotor at the test temperature of 100 8C was utilized for measuring Mooney viscosity, according to ASTM D164687 and reported in Mooney Unit (MU). At least 5 samples were used for a measurement. 2.4. Morphological study Morphological study was carried out using the JEOL JSM-5800LV scanning electron microscope with 15 kV accelerating voltage with complementary surface preparation techniques via OsO4 staining to improve the phase contrast. Measurement of the number-average diameter of the dispersed phase was accomplished by the use of Image Pro Plus software developed by Media Cybernetics Co. 2.5. Oil and thermal aging resistance measurement According to the previous work [12,16,17], the dumbbell-shape (punched out using Die C-ASTM D412-92) test specimens were immersed in oil at room temperature for 70 h. The excess oil on the specimen surfaces was wiped-off with acetone. Changes in tensile strength of specimens after oil immersion were used to determine oil resistance, as shown in Eq. (1). In this study, the relative tensile strength (TSrel), calculated from the ratio of tensile strength after to that before oil immersion, was used in order to eliminate the mastication effect probably taking place during blending process. Tensile strength were measured using an Instron 4301 tensile tester with a crosshead speed of 500 mm/min and a full-scale load cell of 1 kN. TSrel ¼
TSafter TSbefore
ð1Þ
where TSbefore and TSafter are tensile strength of specimens before and after oil immersion, respectively. For the determination of thermal aging properties, the specimens were placed in an oven equipped with air circulating system at the test temperature of 100 8C for 24 h, according to ASTM D573. The aged specimens were then measured for tensile properties. Similar to the measurement of oil resistance, the changes in tensile strength after thermal aging were used to determine thermal aging resistance.
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3. Results and discussion 3.1. Effects of EPDM-g-MA 3.1.1. Effect on phase morphology As one might expect, blends of CPE and NR are incompatible, which is undoubtedly attributed to a large polarity difference, and thus the interfacial tension. To improve blend compatibility, the graft copolymer with segments, that are chemically identical or similar to CPE and NR components in a blend of CPE/NR known as a compatibilizer, is added. A suitably selected compatibilizer will, in theory, locate at the interface between the two phases, leading to a reduction in interfacial tension, an improvement in interfacial adhesion, and a decrease in phase coalescence processes. The degree of improvement depends strongly on the characteristics of the graft copolymer selected. In the present study, maleic anhydride grafted ethylene –propylene diene rubber (EPDM-g-MA) was used as a compatibilizer in blends because it has been reported that the EPDM-g-MA could successfully provide the compatibilizing effect in blends between polar and nonpolar polymers [24 – 26]. It is believed that the EPDM segments with maleic anhydride groups would interact with the chlorine atoms of CPE, promoting good compatibility between CPE and NE phases in blends. The influence of EPDM-g-MA concentration on size of the NR dispersed phase is shown in Figs. 1 and 2. It is evident that, with the addition of 1-phr EPDM-g-MA as a compatibilizer, the phase size decreases obviously. Compatibilization by the addition of graft copolymers provides a reduction in phase size and an improvement in morphological stability by lowering the interfacial tension between the phases, and by introducing a steric hindrance to phase coalescence [27 –30]. Unexpectedly, the addition of EPDMg-MA greater than 1 phr appears to increase NR phase size, although the phase size is still smaller than that in the uncompatibilized blend. Similar result trends have extensively been reported elsewhere [27,31 –33], and it has been proposed that, with the addition of EPDM-g-MA greater than 1 phr, the formation of micelles of EPDM-g-MA in the continuous CPE matrix might take place, and thereafter some of the EPDM-g-MA already being at the interfaces between NR and CPE would leave the interfaces and coalesce with the EPDM-g-MA micelles. This leads to a reduction in magnitude of compatibilizing effect and thus an increase in domain size. In other words, the 1 phr of EPDMg-MA is believed to be sufficient for fully occupying the interfaces between NR and CPE. As a result, an excess of EPDM-g-MA remains in the bulk and no longer contributes to the reduction of interfacial tension. 3.1.2. Effect on oil resistance As mentioned in the previous work [12,16,17], the relative tensile strength is successfully used as an indicator
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for determining oil resistance. The higher the relative tensile strength, the higher the oil resistance. Fig. 3 and Table 3 illustrates the relative tensile strength of blends as a function of EPDM-g-MA concentration. It is evident that the relative tensile strength of the blend increases significantly with the addition of up to 1 phr EPDM-g-MA, and subsequently decreases progressively. The results are clearly in good agreement with the phase morphological results, i.e. the smaller the phase size, the higher the oil resistance. Similar results have been reported elsewhere [16,17]. Fig. 4 illustrates the proposed explanation model of dispersed phase size effect on oil resistance. Compared to NR, CPE possesses excellent resistance to hydrocarbon liquids. Thus, when the blends are immersed in oil, NR dispersed phase swells markedly leading to low resistance to failure of the blends. In the case of small dispersed phase size of NR, the large surface area of the small dispersed phase of NR is surrounded by CPE phase, possessing high resistance to oil. Thus, oil swelling occurring mainly within small NR phase will effectively be stopped by the surrounding CPE, resulting in high value of relative tensile strength. By contrast, a large degree of swelling in a large NR dispersed phase would be stopped ineffectively by CPE due to a small surface area of NR dispersed phase surrounded by CPE. This leads to low resistance to failure and thus low relative tensile strength. 3.1.3. Effect on thermal aging properties According to the previous work [17], the relative tensile strength, (i.e. the ratios of tensile strength after thermal aging to that before thermal aging) was used as an indicator of the thermal aging resistance, in order to minimize the mastication and degree of crosslink effects that might occur during blending or curing, which could interfere the results of thermal aging tests. After thermal aging at 100 8C for 24 h, the relative tensile strength of the blends was measured as a function of EPDM-g-MA concentration, and the results obtained are shown in Fig. 5 and Table 3. It can be seen that the relative tensile strength significantly increases with the addition of EPDM-g-MA up to 1 phr, and then decreases. The result agrees very well with the morphological change caused by the addition of EPDM-gMA, i.e. the smaller the phase size, the higher the thermal aging resistance. 3.2. Thermal stabilization Since the natural rubber (NR) possesses high levels of unsaturation in the polymer backbone, the degradation of NR is promoted by the exposure of NR to high temperature, oxygen, ozone, ultraviolet light, moisture, radiation and chemical agents. Multiple exposures, such as a combination of moisture and heat or oxygen and light, can often result in accelerated deterioration. In high shear mixing processes and under the influence of small amounts of oxygen, a scission of polymer chains could take place. Oxidative
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Fig. 1. Scanning electron micrographs of blends with various contents of EPDM-g-MA: (a) 0 phr; (b) 1 phr; (c) 3 phr; (d) 5 phr; (e) 7 phr.
Table 3 Actual values of tensile strength (MPa) of blends with EPDM-g-MA Amount of EPDM-g-MA
Control
After oil immersion
After thermal aging
0 1 3 5 7
3.48 ^ 0.55 4.94 ^ 0.53 4.75 ^ 0.42 4.41 ^ 0.58 3.95 ^ 0.59
1.53 ^ 0.20 2.95 ^ 0.22 2.66 ^ 0.09 2.51 ^ 0.17 2.23 ^ 0.17
1.50 ^ 0.47 3.14 ^ 0.46 2.73 ^ 0.32 2.45 ^ 0.33 1.96 ^ 0.29
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Fig. 2. Relationship between a number-average diameter ðDn Þ of the dispersed phase and EPDM-g-MA concentrations.
degradation is enhanced at elevated temperatures during the processing of the polymer. The addition of antioxidants is acknowledged as the most convenient and effective way to reduce the thermal oxidation of polymers. The main objective of this part aims to investigate how the antioxidant would help improving the thermal stabilization of NR phase in CPE/NR blends. The phenolic antioxidant (22 CP 46) is used, and its effect on properties and morphology are illustrated. 3.2.1. Effect on phase morphology The influence of antioxidant on a number-average domain diameter of NR dispersed phase in CPE matrix is
shown in Figs. 6 and 7. It is clear that the dispersed phase size of NR decreases after the addition of antioxidant. The explanation is proposed based on the elasticity of NR phase in CPE matrix. It is known that NR is prone to the thermal oxidation leading to the molecular chain-scission and thus a reduction in elasticity. As a consequence, the phase coalescence of NR dispersed phase could occur readily, resulting in a relatively large phase size in blends. As the antioxidant is added, the degree of thermal degradation of NR phase is relatively low and therefore the elasticity of NR phase is relatively high which could efficiently hinder the phase coalescence, leading to a relatively small phase size of NR in CPE matrix. The explanation is supported by the
Fig. 3. Relationships among oil resistance, EPDM-g-MA concentrations and phase size of CPE/NR blends.
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Fig. 4. Proposed model of the blends with different sizes of the non-polar dispersed phase in polar matrix: large phase size (a); small phase size (b).
Fig. 6. Scanning electron micrographs of CPE/NR blend: (a) without antioxidant; (b) with antioxidant.
Fig. 5. Relationships among thermal aging resistance, EPDM-g-MA concentrations and dispersed phase size of the blends.
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Fig. 8. Oil resistance of blends with and without antioxidant.
Fig. 7. Number-average diameter ðDn Þ of the dispersed phase in blends with and without antioxidant.
results of mixing torque and molecular weight of NR, as shown in Table 4. It is clear that the molecular weight and mixing torque of NR with antioxidant are significantly higher than those of NR without antioxidant. The results obtained, thus, confirm that there is the oxidative degradation occurring in the mixer leading to a decrease in molecular weight of NR during processing at high temperature. The addition of antioxidant could retard the thermal degradation of NR phase in the blends, leading to an improvement in NR elasticity, and thus, a reduction in phase coalescence of the blends. 3.2.2. Effect on oil resistance The effect of antioxidant on oil resistance (via the use of relative tensile strength) is shown in Fig. 8 and Table 5, indicating that the oil resistance increases when the antioxidant is added. The result is corresponding to the morphological result, as previously illustrated in Figs. 6 and 7, i.e. phase size of blend with antioxidant is smaller than that without antioxidant. As discussed earlier, the smaller the phase size, the higher the oil resistance. 3.2.3. Effect on thermal aging properties Fig. 9 and Table 5 reveals the thermal aging resistance affected by the addition of antioxidant. It is obvious that the Table 4 Effect of antioxidant (AO) on mixing torque and molecular weight of NR Compound
Torque (Nm)
n M ( £ 105)
w M ( £ 106)
v M ( £ 106)
Polydispersity
NR NR þ AO
44.2 59.5
6.03 10.76
4.47 4.55
2.91 3.28
7.41 4.22
n ; number-average molecuDetermined from a capillary viscometer. M v ; viscosity-average w ; weight-average molecular weight; M lar weight; M w =M n: molecular weight; polydispersity, M
relative tensile strength, as an indicator of the thermal aging resistance, is higher in the blend with antioxidant. The result suggests that antioxidant added to the blend is efficient in protecting NR phase in blends against the thermal degradation, leading to an improvement in a bulk thermal aging resistance. In addition, the result can again be explained by the phase morphology of blend, as illustrated in Figs. 6 and 7. It is thus logical to state that the reduction in thermal degradation of NR phase by the addition of antioxidant, resulting in a decrease in phase size of the blends, plays significant role in thermal aging resistance, i.e. the smaller the dispersed phase size, the higher the relative tensile strength and thus the thermal aging resistance.
4. Conclusions Influences of EPDM-g-MA as a compatibilizer and a phenolic antioxidant on oil and thermal aging resistance in 50/50 CPE/NR blends were investigated. In the case of compatibilizing effect, it has been found that EPDM-g-MA could function as a compatibilizer in the blend system studied. The optimal concentration of EPDM-g-MA is 1 phr. Beyond this concentration, phase size starts to increase. As for the thermal stabilization of the blends, the results reveal that the addition of phenolic antioxidant strongly affects the phase morphology in the way that the phase size decreases. This is due to the fact that the thermal stabilization of NR phase in blends provided by the antioxidant is improved, resulting in the relatively high molecular weight, and thus increases the elasticity of NR Table 5 Actual values of tensile strength (MPa) of blends with and without antioxidant (AO) Blend
Control
After oil immersion
After thermal aging
Without AO With AO
5.95 ^ 0.98 8.35 ^ 0.53
2.81 ^ 0.61 4.97 ^ 0.53
2.67 ^ 0.57 4.50 ^ 0.11
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Fig. 9. Thermal aging resistance of blends with and without antioxidant.
phase in blends, which reduces the phase coalescence during blending. In addition, the results of oil and thermal aging resistance are in good agreement with the morphological results, indicating that the oil and thermal aging resistance based on relative tensile strength in the 50/50 CPE/NR blends are controlled significantly by the size of the NR dispersed phase in CPE matrix. The smaller the dispersed phase size, the higher the resistance to oil and thermal aging.
Acknowledgements The authors would like to express their gratitude to the Thailand Graduate Institute of Science and Technology (TGIST), National Science and Technology Development Agency (NSTDA) as well as the Thailand Research Fund (TRF) for the financial support of this research.
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