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recycled ethylene–propylene–diene rubber blends by means of introducing pre-vulcanised ethylene–propylene–diene rubber and electron beam irradiation
Accepted Manuscript Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-Propylene-Diene Rubber Blends by Means of Introducing Pre-vulcanised EthylenePropylene-Diene Rubber and Electron Beam Irradiation H. Nabil, H. Ismail PII: DOI: Reference:
S0261-3069(13)01153-9 http://dx.doi.org/10.1016/j.matdes.2013.12.020 JMAD 6108
To appear in:
Materials and Design
Received Date: Accepted Date:
3 October 2013 9 December 2013
Please cite this article as: Nabil, H., Ismail, H., Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-Propylene-Diene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-Propylene-Diene Rubber and Electron Beam Irradiation, Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes. 2013.12.020
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Title Page
Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-PropyleneDiene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-PropyleneDiene Rubber and Electron Beam Irradiation
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Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-PropyleneDiene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-PropyleneDiene Rubber and Electron Beam Irradiation
H. Nabil and H. Ismail* School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia
Abstract Most rubber materials are subjected to oxidation. The rate of oxidation depends on the type of rubber, processing method, and end-use conditions. The oxidation of rubber can result in the loss of physical properties, such as tensile strength, elongation, and flexibility. Hence, the service life is determined by oxidation stability. Thermal properties are relevant to the potential use of polymeric materials in many consumer oriented applications. Thermooxidative ageing and thermogravimetric analysis (TGA) have been proven to be successful techniques in determining the thermal stability of polymers and polymer blends. In this article, preparation of a series of natural rubber/recycled ethylene-propylene-diene rubber (NR/R-EPDM) blends is described. Processing of the blends, by means of introducing prevulcanised EPDM and electron beam (EB) irradiation, was carried out. Two thermal analysis methods, namely thermo-oxidative ageing and thermogravimetric analysis, were conducted. The results indicated that pre-vulcanising EPDM for 1.45 mins (ts - 2) is sufficient to gain the optimum retained tensile and elongation at break. It was simultaneously observed that the introduction of pre-vulcanised EPDM increased decomposition temperature and activation energy by showing optimum values at a pre-vulcanising time of 3.45 mins (ts). In the latter study, the retained properties increased after EB irradiation. The results can be verified by the 2
thermal decomposition temperature and their activation energy. The obtained TG profiles and the calculated kinetic parameters indicated that introducing EB irradiation into the blends enhanced their thermal stability. The thermal stability of the blends, processed by these two means, is significantly enhanced; irrespective of pre-vulcanising time or irradiation dose.
Keywords; Natural Rubber; Recycled Ethylene-Propylene-Diene Rubber; Electron Beam Irradiation; Thermal Stability; Vulcanisation
*
Address correspondence to H. Ismail, Tel: +604-5996113, Fax: +604-5941011, E-mail:
[email protected]
1. Introduction The massive use of rubber materials in our daily lives is driven by a remarkable combination of their properties, low mass, and ease of processing. However, rubbers are also known for their relatively low thermal stability; compared to metals or ceramics. Consequently, enhancing the thermal stability of rubbers is a major challenge for further extending their applications [1, 2]. Blending two or more rubbers is a versatile way of developing new materials with a desirable combination of properties [3, 4]. Natural Rubber (NR) and its blend compounds have been extensively studied due to their superior performance in tire applications. Due to its highly unsaturated polymeric backbone, natural rubber is prone to deterioration from ozone attack. In general, improvement in the poor ozone resistance of NR can be achieved by blending with low-unsaturated rubbers, such as ethylene–propylene–diene rubber (EPDM). EPDM is obtained by polymerising ethylene and propylene with a small amount of a non-conjugated diene; which usually imparts good resistance to ageing, weathering, and chemical resistance [5, 6].
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Due to legislative pressures to recycle and reduce scrap waste, the disposal of rubber waste products is of great concern, because landfill disposal and transportation costs are expected to increase [7, 8]. Considering the economic and environmental advantages, recycling is one of the best options. A number of possible applications of various forms of waste rubber in numerous disciplines have been studied and reported [7-9]. Recycled rubber can be generalised as any rubber waste that has been converted into an economically useful form, such as reclaimed rubber, ground rubber, or reprocessed synthetic rubber [7-9]. The blending of natural rubber with recycled ethylene-propylene-diene rubber (NR/R-EPDM) and its thermal characteristics has been studied previously [4, 10]. Even though greater thermal resistance can be achieved through increasing the weight ratio of R-EPDM into the blends, satisfactory thermal stability is difficult to attain, due to the cure incompatibility between NR and R-EPDM. The differences in unsaturated levels lead to a cure mismatch; especially in sulphur vulcanisation systems.
Several attempts have been made to overcome the cure mismatch in the blends of virgin EPDM with diene rubbers; as well as getting better filler and curative distribution in such blends to optimise the blend’s properties, such as two-stage vulcanisations [11, 12] and the cross-linking of the rubbers by means of electron beam irradiation [13-15]. The first method involves with pre-vulcanising one certain matrix before blending it with another matrix (known as reactive blending); and by applying this method, the pre-vulcanised matrix is blended well with the filler and accelerators for a period of time, and its chains have a certain probability to contact the filler and pre-cross-linking. Therefore, more homogenous crosslinking distribution and cure compatibility in the blends is observed. Cross-linking of the polymers, by means of electron beam irradiation, has also been strongly developed in the use of cross-linking agents [13-15]. In the latter method, an irradiation process presents the main
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advantages of being carrying out quickly under well-controlled conditions, particularly at room temperature, and leads to final products that exhibit good mechanical characteristics. To the best of our understanding, no works have been done so far on enhancing the thermal stability of NR/R-EPDM blends by means of applying two-stage vulcanisations and EB irradiation. This article presents the results of assessing the effect of processing means on the enhancement of the thermal stability of NR/R-EPDM blends. Two methods of thermal analysis, namely thermo-oxidative ageing and thermogravimetric analysis, were conducted in this study.
2. Experimental Details 2.1 Materials Natural rubber, SMR 5L grade, was supplied by the Rubber Research Institute of Malaysia (RRIM). Recycled ethylene-propylene-diene rubber (R-EPDM), which was obtained from gaskets and O-ring products, was supplied by Zarm Scientific (M) Sdn. Bhd., Penang, Malaysia. R-EPDM was ground into powder form (to give particle sizes of around 10 – 200 µm prior to blending) using a Table Type Pulverising Machine from Rong Tsong Precision Technology Co. Ltd. R-EPDM was in an irregular, rough shape and a typical particle in an aggregate that was broken by a mechanical crusher. The specific gravity of the powdered recycled EPDM was found to be 1.06 g/cm3. The carbon black content in the R-EPDM was around 29.33%. The physical characteristics of R-EPDM were reported in our previous study [4]. The N330 grade carbon black was supplied by Malayan Carbon (M) Ltd., Malaysia. The trimethylolpropane triacrylate (TMPTA) was purchased from UCB Asia Pacific Ltd., Malaysia. Other compounding ingredients, such as zinc oxide, stearic acid, N-cyclohexylbenzothiazyl-sulfenamide (CBS) sulphur, were purchased from Bayer (M) Ltd., and were used as received.
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2.2 Compounding, Curing Characteristics, and Vulcanisation The formulations used for blending are presented in Table 1. The compounding of natural rubber, EPDM, R-EPDM with accelerators, and other additives, such as zinc oxide, stearic acid, and sulphur, were done on a HAAKE rheomix and laboratory-sized (160 mm x 320 mm) two-roll-mill, model XK-160. A semi-efficient sulphur vulcanisation system (semi-EV) with the ratio of 70/30 (phr/phr) of NR/R-EPDM was processed as control and EB irradiation. Meanwhile, a blend ratio of 70/20/10 (phr/phr/phr) of NR/R-EPDM/EPDM was prepared for carrying out the blends processed by means of introducing pre-vulcanised EPDM. Figures 1 and 2 illustrate schematics representing the preparation of NR/R-EPDM blends, processed by means of introducing pre-vulcanised EPDM and electron beam irradiation. This diagram is further simplified and summarised in Table 2.
The preparation of blends processed by means of introducing pre-vulcanised EPDM has been thoroughly described in our previous work [16]. All the ingredients given in Table 1, except for natural rubber and carbon black, were prepared in a HAAKE rheomix. The mixer was operated with a rotor speed of 60 rpm and an initial temperature of 50 oC. The virgin EPDM was first masticated for 1 min, followed by the incorporation of R-EPDM for a further 1 min, and then ZnO and stearic acid were added for another 1 min. After 3 mins of mixing, CBS and sulphur were introduced and the mixing continued for another 2 mins. After dumping, the compound was later sheeted onto the two-roll-mill. A sample of the respective compound was tested for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000). The compounds were subsequently pre-vulcanised in a compression moulding machine at 150oC; according to a designated time taken from the MDR 2000, as shown in Table 3. The resulting compound (R1 compound) was immediately removed from the mould and left to cool at room temperature. The R1 compound was later blended with natural rubber
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and carbon black (R2 compound) on the two-roll-mill to a final NR/R-EPDM/EPDM ratio blend of 70/20/10 (phr/phr/phr). Finally, the blended compound was tested again for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000) and compressmoulded using a laboratory hot-press; based on the respective curing times.
For the latter blends, the blended compound was prepared by mixing the entire amount of additives, as well as the natural rubber and R-EPDM, which were prepared on a laboratorysized two-roll-mill (model XK-160) at ambient temperature. The resulting blends were later tested for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000). The compounds were subsequently compression-moulded using a stainless steel mould at 150°C with a pressure of 10 MPa using a laboratory hot-press, based on the respective curing times. The moulded sheets and dumbbell test pieces were irradiated using a 3MeV electron beam accelerator (NHV EPS-3000) at a dose range of 0-200 kGy. The acceleration energy, beam current, and dose rate, were 2MeV, 5mA, and 50kGy/pass, respectively.
2.3 Measurement of Tensile Properties Dumbbell-shaped samples were cut from the moulded sheets according to ASTM: D41206ae2. Tensile tests were performed at a cross-head speed of 500 mm/min. Tensile tests were carried out with an Instron 3366 universal tensile machine to determine tensile properties, in terms of tensile strength, stress at 100% strain (M100), and elongation at break. The hardness measurements of the samples were done according to ASTM: D2240-05(2010), using a type Shore A manual durometer.
2.4 Thermo-oxidative Ageing
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To determine thermal ageing properties, the dumbbell-shaped specimens were placed in an oven equipped with an air circulating system at a test temperature of 100 oC for 48 hours, according to ASTM: D573-04(2010). The aged specimens were then measured for tensile properties. Changes in stress at 100% strain, tensile strength, elongation at break, and hardness after thermal ageing, were used to determine thermal ageing resistance. The retention in each property was evaluated according to Eq. (1).
Retention (%) =
Value after ageing × 100 Value before ageing
(1)
2.5 Thermogravimetric Analysis (TGA) Thermogravimetric analysis of the blends, by means of introducing pre-vulcanised EPDM and EB irradiation, was carried out with a Perkins-Elmer Pyris 6 TGA analyser. The sample was scanned from 30 to 600oC in a nitrogen air flow of 30 ml/min, with a heating rate of 20oC /min.
2.6 Activation Energy of the Degradation Process The degradation activation energies of the blends were determined by applying the CoatsRedfern’s method as follows [17];
log[
− log(1 − α ) 2
T
] = log[(
AR 1 − 2 RT E )×( )] − βE E 2.303RT
(2)
Where, α is the fractional mass loss at time t, T is the absolute temperature, A is the preexponential factor, R is the universal gas const, β is the heating rate, and E is the activation
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energy. A plot of log[- log(1- α)/T2] as a function of 1/T gives a straight line with a slope equal to E/2.303R and the y-intercept is log[(AR)/(βE)(1-2RT/E)].
3. Results and Discussion 3.1 Effect of Introducing Pre-vulcanised EPDM 3.1.1 Thermo-Oxidative Ageing Figure 3 illustrates the possible R1 compound mechanism after being pre-vulcanised according to respective scorch time and processing using a conventional two-roll-mill. Our previous work [16] disclosed that CBS fragment was initially bound to the positions provided on the third monomer ethylidene norbornene of the virgin EPDM, resulting in a rubber bound intermediate. After R1 compound were initially pre-vulcanised, virgin EPDM with pendant CBS fragments resulted in a restriction of the curative migration from the virgin EPDM phase towards a higher unsaturated natural rubber level, which consequently enhanced the crosslink distribution. By applying this method, co-crosslinking between virgin EPDM and natural rubber phases are presented, as schematically displayed in the bottom part of Figure 3.
Tensile strength and elongation at break of un-aged and aged blends, based on various prevulcanising times, are shown in Figures 4 and 5. The tensile strength of un-aged blends increased by increasing the pre-vulcanising time of the R1 compound up to 3.45 mins (ts) and then reduced. Significantly, by introducing pre-vulcanised EPDM, the tensile strength had improved. The pre-vulcanising of the R1 compound caused to reduce the curative migration from the EPDM phase towards the higher unsaturated natural rubber phases, which consequently improved the cross-link distribution. High tensile strength of such reactive processing blend’s vulcanisates must again be the result of a better homogeneity of either cross-link distribution or the carbon black distribution or both [11, 12]. It is interesting to
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highlight that even though unaged tensile strength, beyond 3.45 mins (ts) pre-vulcanising time of R1 compound, dropped slightly; it was still higher than the simple processing blends.
Elongation at break indicates the ductility, elasticity, and flexibility of the vulcanisates [18]. Elongation at break of the blends processed by means of introducing pre-vulcanised EPDM was higher than simple blend irrespective of pre-vulcanising time. This is due to better filler distribution, cure matching, and higher cross-linking distribution of the blends. It was also found that the elongation at break decreased with an increasing pre-vulcanising time of the R1 compounds. When the cross-link density was too high, the average molar mass of the rubber chain between two successive cross-link points decreased, and the mobility of the chain segment was restricted; which consequently limited the orientation of the network chain. This restriction reduced the number of effective network chains, resulting in a decreased elongation [19]. Another probable reason was the reduction of the deformability of the blends. Since the stiffness of the blends increased towards the pre-vulcanising time of the R1 compound (as discussed in the latter section), this might reduce the flexibility of the blends as well.
Stress at 100% strain of unaged and aged blends is listed in Table 4. It was observed that the stress at 100% strain of un-aged blends increased towards the pre-vulcanising time of the R1 compound up to 4.45 mins (ts + 1) and then decreased. Tensile modulus basically indicates the stiffness (or hardness) of the vulcanisates upon stretching. The increment of modulus, as a function of pre-vulcanising time, is due to an increasing of degree of cross-linking; which takes place after the pre-vulcanising step, and also more homogeneous carbon black distribution in the blends. However, the modulus of the blends drops slightly as the prevulcanising time approaches 5.45 mins (ts + 2) of the R1 compound. By pre-vulcanising R1
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compound beyond 5.45 mins (ts + 2), the blend remains visually heterogeneous during the blending with the natural rubber on the two-roll-mill; thus indicating that the processability of R1 compound is comparatively less. Small particles of pre-vulcanised R1 compound were dispersed in the blends for 5.45 mins (ts + 2), eventually lead to low modulus in the blends. In addition, the overall modulus of reactively processing blends was lower than simple processing blend. As stated earlier, the blends at various pre-vulcanising times were prepared using an internal mixer. Thus, the shearing coefficient was higher in comparison to the tworoll-mill, resulting in less stiffness in the blends. On the contrary, the hardness property (see Table 4) of the blends processed by means of pre-vulcanised EPDM showed no difference. Since the compound formulation was unchanged, the blends therefore exhibited similar hardness results.
In addition, the tensile strength and elongation at break of all aged samples reduced upon thermal ageing. Reduction in tensile strength and elongation at break could be attributed to rubber oxidation, which resulted in chain scissions under thermal exposure. Scission of the larger molecular chains increased the number of shorter chains of the respective rubber, which led to fewer entanglements and reduce the ability of the strain-crystallisation process, thereby decreased the tensile strength, as well as elongation at break. The retained property percentage e.g., the retained elongation, retained tensile strength, retained tensile modulus, and the retained hardness property after thermal ageing, was used to evaluate the thermal stability of the rubbers. The higher the retained properties obtained, the greater is the thermal stability. It can be observed that the retained tensile strength and elongation at break were obtained similarly. The blends showed optimum retained tensile and elongation at break at 1.45 mins (ts – 2) of pre-vulcanising time. By introducing pre-vulcanised EPDM, the ageing resistance of the blends is clearly observed to be as a result of a good cross-link distribution
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between the rubber phases. Another probable reason might be due to a better carbon black distribution in the blends. The R-EPDM used in this study was ground and mostly crosslinked, the cross-linked EPDM therefore consisted of higher viscosity than natural rubber. In this case, carbon black was mostly located in the natural rubber phase and was especially concentrated at the interface of the two polymers, like an envelope formation. Thus, it was found that carbon black distributed unevenly in the blends. By introducing pre-vulcanised EPDM, the uncured virgin EPDM blended well with carbon black for a period of time, where virgin EPDM chains had a certain area to contact with carbon black and be entangled or trapped in the void of the carbon black aggregates. As a consequence, the migration of carbon black to natural rubber reduced, more homogenous carbon black distribution in the blends was observed and therefore enhanced the ageing resistance to the blends. As mentioned previously, the blends showed optimum retained tensile and elongation at break at 1.45 mins (ts – 2) of pre-vulcanising time. This suggests that the ageing resistance was most pronounced at this stage. Pre-vulcanising the R1 compound for 1.45 mins (ts – 2) is sufficient to gain good thermal ageing properties.
3.1.2 Thermogravimetric Analysis (TGA) The thermal decomposition behaviour and derivative weight thermograms of NR/R-EPDM blends, processed by means of introducing pre-vulcanised EPDM, are shown in Figures 6 and 7, respectively. The decomposition temperature at various weight losses, stages, and char residues is also summarised in Table 5. Two regions of degradation of the blends were clearly observed. Here, an initial minor weight loss at around 180 - 200oC was caused by the presence of a volatile matter, such as stearic acid, as well as the adsorbed water at around 300oC [20]. The first step of degradation of the blends began at about 330oC and was completed at around 450oC. The second stage of degradation occurred in the region 450 – 12
520oC. The former stage of decomposition was caused by the degradation of natural rubber (polyisoprene) segments, which corresponded to a major peak being observed from the DTG curve (Figure 7). The degradation of natural rubber segments is sensitive to the presence of an oxidised structure, as well as the depletion of sulfidic cross-linking in the natural rubber. The latter degradation, which can be observed from the DTG curve’s small peak area, was attributed to the scission of cross-linked R-EPDM and conjugated polyene remaining after the first stage of degradation [21].
Notably, the decomposition temperature at 10%, 30%, 50%, and 60% weight loss of the blends, was influenced by the pre-vulcanising time. It was observed that the decomposition temperature clearly differed from the earlier stages of decomposition. Introducing prevulcanised EPDM into the blends greatly affected their thermal stability. The decomposition temperature of the blends, processed by means of introducing pre-vulcanised EPDM, showed further significance at the decomposition temperatures above 50% of weight loss. When considering only the blends, introducing pre-vulcanised EPDM gave a distinctively better thermal stability than the simple blend alone; thus confirming the enhancement of crosslinking and/or the filler distribution within the blends. Table 5 also illustrates the decomposition temperature at different stages and char residues of the blends derived from TG profiles. The decomposition temperature at the first maximum peak (Tmax I) represents the thermal decomposition of natural rubber phase, whereas the decomposition temperature at the second maximum peak (Tmax II) corresponds to the scission of the cross-linked R-EPDM. Significantly, applying pre-vulcanised EPDM enhanced thermal stability by shifting the decomposition temperature upwards. This is simply due to the blends consisting of a high cross-linked distribution; and therefore, a higher temperature is required to cleave the linkages. It can also be seen that the decomposition temperature at Tmax 13
I
showed no
difference. Since the natural rubber phase is unmodified, this resulted in the decomposition temperature remaining unchanged. It was similarly observed that after introducing prevulcanised EPDM into the blends, the decomposition temperature exhibited an optimum value at 3.45 mins (ts) of pre-vulcanising time. The inducement of co-crosslinking between pre-vulcanised EPDM and NR in the blends is most pronounced during this stage thereby increasing the resistance for bond cleavage at a pre-vulcanising time of 3.45 mins (ts). Furthermore, the char residue of all blends was not influenced by introducing pre-vulcanised EPDM. This is simply due to the fact that the amount of filler and blend ratios was constant; thus keeping the char residue mostly unchanged. According to Chakraborty et al. [22], the amount of char is very much dependent on the type and amount of filler. The difference in filler amount mainly concerns the char formation.
3.1.3 Activation Energy of Degradation Process Since thermal stability is related to both the initial temperature and the rate of degradation of polymers, the determination of kinetic parameters associated with the degradation process is an interesting topic. Thermal analysis can also be used for the durability assessment and lifetime prediction of a product. Such study is very important for improvement of the product’s service performance [23]. The different stages of degradation can be conveniently identified from different slopes. Coats-Redfern plots, for NR/R-EPDM blends processed by means of introducing pre-vulcanised blends, are presented in Figure 8. The activation energies obtained by this approach are summarised in Table 5.
From the DTG curves, it was observed that the blends decomposed in two stages, which can be classified as the degradation of natural rubber phase (Stage I or EI) and EPDM or REPDM (Stage II or EII). The first stage of degradation (330 – 450oC) involved activation
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energies of 68.91 kJ/mol, 69.03 kJ/mol, 69.85 kJ/mol, 71.59 kJ/mol, 69.74 kJ/mol, and 69.25 kJ/mol of the blends, without introducing pre-vulcanised EPDM, pre-vulcanising times of the R1 compound for 1.45 mins (ts – 2), 2.45 mins (ts – 1), 3.45 mins (ts), 4.45 mins (ts + 1), and 5.45 mins (ts + 2), consecutively. During the second stage of decomposition (450 – 520 oC), the calculated energies were 23.36 kJ/mol, 23.35 kJ/mol, 24.69 kJ/mol, 25.62 kJ/mol, 24.31 kJ/mol, and 24.18 kJ/mol, concerning the blends without introduction of pre-vulcanised EPDM, pre-vulcanising times of the R1 compound for 1.45 mins (ts – 2), 2.45 mins (ts – 1), 3.45 mins (ts), 4.45 mins (ts + 1), and 5.45 mins (ts + 2), respectively. Activation energy is an indication of the energy required to decompose the molecular network. Here, it was observed that the activation energy obtained from the blends processed by means of introducing prevulcanised EPDM was higher than the simple blend, suggesting that higher cross-linking distribution was achieved in the blends. Kader and Bhowmick [19] exemplified the relationship between activation energy of degradation and the cross-linking of rubber vulcanisates. It was ascribed that the inducement of cross-linking was able to increase the resistance for bond cleavage or depletion at the degradation temperature. It was also found that pre-vulcanising the R1 compound for 3.45 is sufficient to gain the highest thermal stability; particularly at a higher temperature. The higher activation energy observed when applying this method was clearly successful at improving the thermal stability of the blends; irrespective of the pre-vulcanising time of the R1 compound.
3.2 Effect of Electron Beam Irradiation 3.2.1 Thermo-Oxidative Ageing Figure 9 shows the tensile strength of un-aged and aged NR/R-EPDM blends processed by means of introducing EB irradiation. It was observed that the tensile strength of un-aged blends exhibited optimum strength at irradiation dose of 50 kGy, which then reduced
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continuously upon irradiation. According to Akiba and Hashims [24], the electron beam initiates radical reactions, resulting in the generation of intermolecular and intermolecular -CC- bonds in rubbers; as was the case with peroxide cross-linking. Peroxide vulcanisation is a thermally initiated event with the cure temperature. However, electron beam irradiation is performed at room temperature and results in significantly less energy applied to the process. Therefore, the idealised model representing the network formation, as described by Henning [25], is highly suggested (see Figure 10). In this study, the cross-linking promoter (i.e., TMPTA) generally makes efficient use of the radicals; whether acting to suppress nonnetwork forming side reactions during cure [26, 27] or to generate additional cross-links [28]. The possibility of the production of a cross-linking promoter to form a higher modulus fillerlike domain of thermoset cross-linking promoter, together with the formation of co-crosslinks of -Sx- and -C-C- after EB irradiation, may result in an increment to the extent of crosslinking. However, more cross-linking formed in the blend sites, with increasing irradiation doses, restrained the chains from structural rearrangement during elongation and reduced the tensile strength. Rooj et al. [29], suggested that when the cross-link density is too high, a rubber is deformed externally; part of the applied energy is stored elastically in the chains and is available as a driving force for fracture. The remaining energy is dissipated through molecular motions into heat, and as such, is made inaccessible to break further chains. At high cross-link levels, chain motions become restricted and the tight network is incapable of dissipating much energy; resulting in a decrease of tensile strength.
As shown in Figure 11, the elongation at break of irradiated blends reduced continuously upon EB irradiation. This behaviour was most likely due to the breakdown of the tie-chain molecules and entanglements, which consequently decreased the ductility of the polymer. As mentioned in the preceding section, the presence of TMPTA played a major role in the
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irradiation-induced cross-linking of the blend. The rubber chains were locked by the crosslinking after irradiation, which consequently resisted the mobility of the rubber molecules. The phenomenon of reduction of elongation at break for irradiated rubber is in agreement with various EB irradiation studies [30, 31]. Stress at 100% strain (M100) and hardness results of the blends, are tabulated in Table 6. Modulus and hardness are indirectly related to stiffness and rigidity; and therefore, reflect upon the cross-linking of the vulcanisate. The enhancement in M100 and hardness after EB irradiation is due to an increase in cross-linking. The occurrence of irradiation-induced cross-linking is believed to be responsible for the increases in tensile modulus and hardness property.
Furthermore, tensile strength and elongation at break of all aged samples reduced upon thermal ageing. Reduction in tensile strength and elongation at break could be attributed to the oxidation of the polymer, which resulted in either chain cleavage leading to a drop in molecular weight or crosslinking. It was observed that the retained tensile strength at an irradiation dose of 50kGy dropped slightly before increasing gradually towards the irradiation doses. EB irradiation of the blends results in the formation of a three-dimensional network structure through the union of in-situ-generated macroradicals. Here, polyfunctional monomers (i.e., TMPTA) are mixed with the rubber matrix to achieve cross-linking at a reduced irradiation level, in order to minimise the deterioration of the base polymers [32]. The free radicals, which can be generated on the carbon atoms of the rubbers, may react with each other or initiate grafting with Polyfunctional monomers, followed by cross-linking. Similarly, the blends may also undergo cross-linking and grafting through radical formation when exposed to irradiation. Therefore, the cross-linking is enhanced by introducing EB irradiation. Generally, the radical termination, after thermal ageing of the rubber, is involved with the abstraction of hydrogen atoms from an allylic position on the rubber molecule.
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However, this process already took place after EB irradiation. Therefore, the radical termination in the bulk polymer no longer exists. Hence, upon thermal ageing, the modulus and tensile strength of the blends reduced significantly compared to the control blend; particularly at a lower dose, but increased slightly after a certain EB irradiation dose. On the contrary, the hardness result, which is generally an opposite phenomenon to the tensile modulus, exhibits higher retained properties than the control blend after EB irradiation. This suggests that EB irradiation has no effect on static testing (i.e., non-destructive testing), such as the hardness property. Elongation at break also showed higher retained properties towards irradiation dosages. This means that applying EB irradiation to the blends results in an enhanced flexibility or elasticity, and therefore increases the elongation at break.
3.2.2 Thermogravimetric Analysis (TGA) The thermal decomposition curves (TG and DTG) are depicted in Figures 12 and 13. The decomposition temperature at various weight losses, stages, and char residues is listed in Table 7. Since the blend is composed of two rubber phases, similar regions of degradation were observed. The first step of degradation of the blends (330 – 450oC) was caused by the degradation of natural rubber (polyisoprene) segments and the second stage of degradation, which occurred in the region 450 – 520 oC, was attributed to the scission of cross-linked REPDM and conjugated polyene that remained after the first stage of degradation [23]. It is interesting to note that the thermal stability of the blends increased after EB irradiation. The decomposition temperature at various weight losses clearly shows that by introducing EB irradiation, thermal stability is greatly enhanced. The enhancement in thermal stability of the irradiated blends is due to the formation of more compact three dimensional cross-linked networks, which is more stable against the formation of the degradation. A similar observation was found for the decomposition temperature at different stages. The
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decomposition temperature shifted to a higher temperature towards irradiation doses. This clearly verifies that by applying the EB irradiation, thermal stability is significantly enhanced, due to the formation of additional cross-linking of the blends. It is suggested that a higher temperature is needed to decompose the cross-links or linkages. In the case of char residue, it showed no difference among the blends; as similarly observed and previously discussed in the study of introducing pre-vulcanised EPDM.
3.2.3 Activation Energy of the Degradation Process The different stages of degradation can be conveniently identified from the differing slopes. Coats-Redfern plots for NR/R-EPDM blends, processed by means of introducing electron beam irradiation, are shown in Figure 14. The activation energies obtained using this approach are summarised in Table 7. It can be observed that the blends are decomposed in two stages, which can be classified from the degradations of natural rubber phase (Stage I) and R-EPDM (Stage II). The first stage of degradation (330 – 450 oC) involved activation energies of 69.24kJ/mol, 70.91kJ/mol, 71.89kJ/mol, 72.04kJ/mol, and 72.06kJ/mol, in the neat blends and irradiated blends at irradiation doses of 50kGy, 100kGy, 150kGy, and 200kGy, consecutively. In the second stage decomposition (450 – 520oC), the calculated energies were 20.33kJ/mol, 25.56kJ/mol, 25.91kJ/mol, 26.15kJ/mol, and 26.31kJ/mol, in the control blend and irradiated blends at irradiation dosages of 50kGy, 100kGy, 150kGy, and 200kGy, respectively. This shows that the increase in irradiation dose makes the system more resistant to thermal degradation and the blend at an irradiation dose of 200kGy is the most stable of all.
19
4. Conclusions The accomplishment of enhancing the thermal stability of NR/R-EPDM blends is a major issue in this study. The enhancement of the thermal resistance of NR/R-EPDM can be achieved by introducing pre-vulcanised EPDM and EB irradiation. In the first study, the introduction of pre-vulcanised EPDM into the blends greatly influenced thermal resistance, whereby the pre-vulcanised EPDM for 1.45 mins (ts – 2) to 3.45 mins (ts) of pre-vulcanising time, revealed good thermal resistance by showing higher retained tensile properties, as well as shifting the initial decomposition to a higher temperature (as confirmed by the TGA profiles). Furthermore, introducing EB irradiation to NR/R-EPDM also enhanced the thermal stability of the blends, by showing similar findings as seen in the first study. However, it is interesting to highlight that the decomposition temperature, as well as the activation energy, was found to increase towards the irradiation dose. This is associated with the greater crosslinking observed, due to the formation of irradiation-induced-crosslinks after EB irradiation of the blends. From the overall properties, pre-vulcanising of the R1 compound for 3.45 mins (ts) and an EB irradiation dose of 100 kGy, are highly recommended for NR/R-EPDM, especially when high strength and thermal stability of the blends are of major concern.
Acknowledgements H. Nabil gratefully acknowledges the personally financial support under Graduate Assistant Scheme and Postgraduate Research Grant Scheme (PRGS) provided by Institute of Postgraduate
Studies
(IPS),
Universiti
Sains
Malaysia
(Account
Number:
1001/PBAHAN/8045014). Also, the research grant of Exploratory Research Grant Scheme (ERGS),
Ministry of
Higher
Education
203/PBAHAN/6730066) for the great support.
20
(MOHE),
Malaysia
(Account
Number:
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[15] Noriman N, Ismail H. The effects of electron beam irradiation on the thermal properties, fatigue life and natural weathering of styrene butadiene rubber/recycled acrylonitrile– butadiene rubber blends. Mater Design 2011;32:3336-3346. [16] Nabil H, Ismail H, Azura AR. Effects of virgin ethylene–propylene–diene–monomer and its preheating time on the properties of natural rubber/recycled ethylene–propylene–diene– monomer blends. Mater Design 2013;50:27-37. [16] Coats A, Redfern J. Kinetic parameters from thermogravimetric data. Nature. 1964. [18] Nabil H, Ismail H, Azura AR. Recycled polyethylene terephthalate filled natural rubber compounds: Effects of filler loading and types of matrix. J Elas Plas 2011;43:429-449 [19] Kader MA, Bhowmick AK. Thermal ageing, degradation and swelling of acrylate rubber, fluororubber and their blends containing polyfunctional acrylates. Polym Degrad Stab 2003;79:283-295. [20] Tomer N, Delor-Jestin F, Singh R, Lacoste J. Cross-linking assessment after accelerated ageing of ethylene propylene diene monomer rubber. Polym Degrad Stab 2007;92:457-463. [21] Saha Deuri A, Bhowmick AK, Ghosh R, John B, Sriram T, De S. Thermal and ablative properties of rocket insulator compound based on EPDM. Polym Degrad Stab 1988;21:21-28. [22] Chakraborty S, Roy P, Pathak A, Debnath M, Dasgupta S, Mukhopadhyay R, Bandyopadhyay S. Composition analysis of carbon black-filled polychloroprene rubber compound by thermooxidative degradation of the compound. J Elast Plast 2011;43:499–508. [23] He W, Jiang YY, Luyt AS, Ocaya RO, Ge TJ. Synthesis and degradation kinetics of a novel polyester containing bithiazole rings. Thermochim Acta 2011;525:9-15. [24] Akiba M, Hashim A. Vulcanization and crosslinking in elastomers. Progress in Polym Sci 1997;22:475-521. [25] Henning, SK. Proceedings of the 56th International Wire and Cable Symposium, Lake Buena Vista, Florida, U.S.A., 2007, November 11-14, 2007, 537-593.
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[26] García‐Quesada J, Gilbert M, Peroxide crosslinking of unplasticized poly (vinyl
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[30] Senna MM, Hossam FM, El‐Naggar AWM. Compatibilization of low‐density
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[31] Ratnam CT, Nasir M, Baharin A, Zaman K, Evidence of irradiation‐induced crosslinking
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Captions Tables Table 1 Formulations of the blends. Table 2 Preparation of simple blend and the blends processed by means of introducing prevulcanised EPDM. Table 3 Curing characteristics of the blends (R1 compound) and its pre-vulcanising time prerequisite to blend using two-roll-mill. Table 4 Stress at 100% strain and a hardness result of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Table 5 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Table 6 Stress at 100% strain and hardness results of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Table 7 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figures Figure 1 Schematic illustration representing the preparation of NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 2 Schematic illustration representing the preparation of NR/R-EPDM blends processed by means of electron beam irradiation. Figure 3 Possible mechanism of the blends with respect to R1 and R2 compounds [16]. Figure 4 Tensile strength of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 5 Elongation at break of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 6 TG curves of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 7 DTG curves of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 8 Coats-Redfern plot of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 9 Tensile strength of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 10 Idealised network from EB irradiation curing in the presence of cross-linking promoter (TMPTA). Cross-links can be derived from (A) rubber radicals, (B) -Sx- linkages, (C) mixed -Sx- and -C-C- linkages, (D) cross-linking promoter forming cross-links, and (E) 26
thermoset domains of cross-linking promoter grafted to polymer chains (adapted from Henning [25]). Figure 11 Elongation at break of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 12 TG curves of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 13 DTG curves carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 14 Coats-Redfern plot of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation.
27
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Table 1 Formulations of the blends. Materials Control (phr) SMR L EPDM R-EPDM ZnO Stearic acid CBS Sulphur N330 TMPTA EB Irradiation (kGy)
70.0 30.0 5.0 2.0 1.2 1.8 30.0 -
Introduction of Pre-vulcanised EPDM (phr) 70.0 10.0 20.0 5.0 2.0 1.2 1.8 30.0 -
Electron Beam Irradiation (phr) 70.0 30.0 5.0 2.0 1.2 1.8 30.0 3.0 Variable
SMR L = Standard Malaysian Rubber L EPDM = Virgin Ethylene-Propylene-Diene Rubber. R-EPDM = Recycled Ethylene-Propylene-Diene Rubber. ZnO = Zinc Oxide. CBS = N-Cyclohexyl-Benzothiazyl-Sulphenamide TMPTA = Trimethylolpropane-triacrylate
Table 2 Preparation of simple blend and the blends processed by means of introducing prevulcanised EPDM. Additives used with respect to Control or Non-preheated Introduction of Pre-vulcanised EPDM Compound Two-Roll-Mill HAAKE Two-Roll-Mill at ambient at ambient temperature at 50 oC (R1 Compound) temperature (R2 Compound) SMR L EPDM R-EPDM ZnO Stearic acid CBS Sulphur N330 TMPTA
EPDM R-EPDM ZnO Stearic acid CBS Sulphur
28
SMR L N330
Table 3 Curing characteristics of the blends (R1 compound) and its pre-vulcanizing time prerequisite to blend using two-roll-mill [16]. R1 Compound Cure characteristics Sample code Pre-vulcanizing time (-/+ to ts2, min) MH 16.29 ts – 2 1.45 MH – ML 13.97 ts – 1 2.45 ts2 3.45 ts 3.45 tc90 23.07 ts + 1 4.45 CRI 5.10 ts + 2 5.45 ML = Elastic minimum torque (dN.m) MH = Elastic maximum torque (dN.m) MH – ML = Torque difference (dN.m) ts2 = Scorch time (min) tc90 = Curing time (min) CRI = Cure rate index (CRI)
Table 4 Stress at 100% elongation and a hardness result of carbon black filled blends processed by means of introducing pre-vulcanised EPDM. Sample M100 (MPa) Hardness (Shore A) Code Un-aged Aged Retention Un-aged Aged (%) 0 1.96 ± 0.06 2.50 ± 0.10 127.74 65 ± 1.01 65 ± 0.67 ts - 2 1.60 ± 0.13 2.50 ± 0.02 156.69 64 ± 0.82 67 ± 0.85 ts - 1 1.65 ± 0.25 2.22 ± 0.01 134.94 65 ± 0.89 67 ± 0.62 ts 1.92 ± 0.09 2.05 ± 0.03 106.78 64 ± 0.62 67 ± 0.83 ts + 1 2.05 ± 0.33 2.17 ± 0.09 105.92 64 ± 0.89 66 ± 0.70 ts + 2 1.74 ± 0.02 2.04 ± 0.04 117.25 63 ± 1.29 66 ± 1.04
NR/R-EPDM
Retention (%) 100.00 104.69 103.08 104.69 103.12 104.76
Table 5 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Activation Sample The decomposition temperature (oC) energy CR codes (kJ/mol) (%) T-10% T-30% T-50% T-60% Tmax I Tmax II EI EII 0 369.06 405.41 452.57 484.68 400 487 68.91 23.36 31.4 ts - 2 373.08 406.73 455.93 493.97 400 500 69.03 23.35 31.4 ts - 1 373.79 407.36 457.43 494.30 400 500 69.85 24.69 31.4 ts 374.89 407.45 457.83 495.47 400 501 71.59 25.62 31.6 ts + 1 374.12 407.40 455.15 494.44 400 500 69.74 24.31 31.5 ts + 2 372.84 406.26 454.76 493.77 400 499 69.25 24.18 31.6
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Table 6 Stress at 100% elongation and hardness results of carbon black filled blends processed by means of electron beam irradiation. Sample M100 (MPa) Hardness (Shore A) Code Un-aged Aged Retention Un-aged Aged (%) Control 2.11 ± 0.04 5.83 ± 0.08 276.27 65 ± 0.44 67 ± 0.44 50 kGy 2.32 ± 0.12 3.67 ± 0.11 158.58 68 ± 0.44 72 ± 1.26 100 kGy 2.76 ± 0.07 3.71 ± 0.10 134.46 69 ± 0.53 73 ± 0.50 150 kGy 2.94 ± 0.10 3.82 ± 0.15 130.07 70 ± 0.53 74 ± 0.50 200 kGy 3.21 ± 0.06 3.94 ± 0.28 122.66 72 ± 0.53 75 ± 0.55
NR/R-EPDM
Retention (%) 103.08 105.88 105.79 105.71 104.17
Table 7 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Activation Sample The decomposition temperature (oC) energy CR codes (kJ/mol) (%) T-10% T-30% T-50% T-60% Tmax I Tmax II EI EII Control 367.18 403.58 446.97 489.75 399 487 69.24 20.23 33.6 50 kGy 373.43 409.31 456.92 492.50 401 496 70.91 25.56 33.8 100 kGy 374.76 410.19 457.97 493.93 402 498 71.89 25.91 33.7 150 kGy 375.85 410.32 458.86 495.05 402 498 72.04 26.15 33.2 200 kGy 377.05 411.73 460.17 496.73 403 499 72.06 26.31 33.7
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Highlights New route of processing was introduced to optimise the thermal stability of NR/REPDM blends. Pre-vulcanised EPDM and EB irradiation were introduced into NR/R-EPDM blends. Thermal stability is obviously enhanced by applying these two techniques. Applying new route of processing methods is clearly successful to NR/R-EPDM blends.
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S0261-3069(13)01153-9 http://dx.doi.org/10.1016/j.matdes.2013.12.020 JMAD 6108
To appear in:
Materials and Design
Received Date: Accepted Date:
3 October 2013 9 December 2013
Please cite this article as: Nabil, H., Ismail, H., Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-Propylene-Diene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-Propylene-Diene Rubber and Electron Beam Irradiation, Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes. 2013.12.020
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Title Page
Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-PropyleneDiene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-PropyleneDiene Rubber and Electron Beam Irradiation
1
Enhancing the Thermal Stability of Natural Rubber/Recycled Ethylene-PropyleneDiene Rubber Blends by Means of Introducing Pre-vulcanised Ethylene-PropyleneDiene Rubber and Electron Beam Irradiation
H. Nabil and H. Ismail* School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia
Abstract Most rubber materials are subjected to oxidation. The rate of oxidation depends on the type of rubber, processing method, and end-use conditions. The oxidation of rubber can result in the loss of physical properties, such as tensile strength, elongation, and flexibility. Hence, the service life is determined by oxidation stability. Thermal properties are relevant to the potential use of polymeric materials in many consumer oriented applications. Thermooxidative ageing and thermogravimetric analysis (TGA) have been proven to be successful techniques in determining the thermal stability of polymers and polymer blends. In this article, preparation of a series of natural rubber/recycled ethylene-propylene-diene rubber (NR/R-EPDM) blends is described. Processing of the blends, by means of introducing prevulcanised EPDM and electron beam (EB) irradiation, was carried out. Two thermal analysis methods, namely thermo-oxidative ageing and thermogravimetric analysis, were conducted. The results indicated that pre-vulcanising EPDM for 1.45 mins (ts - 2) is sufficient to gain the optimum retained tensile and elongation at break. It was simultaneously observed that the introduction of pre-vulcanised EPDM increased decomposition temperature and activation energy by showing optimum values at a pre-vulcanising time of 3.45 mins (ts). In the latter study, the retained properties increased after EB irradiation. The results can be verified by the 2
thermal decomposition temperature and their activation energy. The obtained TG profiles and the calculated kinetic parameters indicated that introducing EB irradiation into the blends enhanced their thermal stability. The thermal stability of the blends, processed by these two means, is significantly enhanced; irrespective of pre-vulcanising time or irradiation dose.
Keywords; Natural Rubber; Recycled Ethylene-Propylene-Diene Rubber; Electron Beam Irradiation; Thermal Stability; Vulcanisation
*
Address correspondence to H. Ismail, Tel: +604-5996113, Fax: +604-5941011, E-mail:
[email protected]
1. Introduction The massive use of rubber materials in our daily lives is driven by a remarkable combination of their properties, low mass, and ease of processing. However, rubbers are also known for their relatively low thermal stability; compared to metals or ceramics. Consequently, enhancing the thermal stability of rubbers is a major challenge for further extending their applications [1, 2]. Blending two or more rubbers is a versatile way of developing new materials with a desirable combination of properties [3, 4]. Natural Rubber (NR) and its blend compounds have been extensively studied due to their superior performance in tire applications. Due to its highly unsaturated polymeric backbone, natural rubber is prone to deterioration from ozone attack. In general, improvement in the poor ozone resistance of NR can be achieved by blending with low-unsaturated rubbers, such as ethylene–propylene–diene rubber (EPDM). EPDM is obtained by polymerising ethylene and propylene with a small amount of a non-conjugated diene; which usually imparts good resistance to ageing, weathering, and chemical resistance [5, 6].
3
Due to legislative pressures to recycle and reduce scrap waste, the disposal of rubber waste products is of great concern, because landfill disposal and transportation costs are expected to increase [7, 8]. Considering the economic and environmental advantages, recycling is one of the best options. A number of possible applications of various forms of waste rubber in numerous disciplines have been studied and reported [7-9]. Recycled rubber can be generalised as any rubber waste that has been converted into an economically useful form, such as reclaimed rubber, ground rubber, or reprocessed synthetic rubber [7-9]. The blending of natural rubber with recycled ethylene-propylene-diene rubber (NR/R-EPDM) and its thermal characteristics has been studied previously [4, 10]. Even though greater thermal resistance can be achieved through increasing the weight ratio of R-EPDM into the blends, satisfactory thermal stability is difficult to attain, due to the cure incompatibility between NR and R-EPDM. The differences in unsaturated levels lead to a cure mismatch; especially in sulphur vulcanisation systems.
Several attempts have been made to overcome the cure mismatch in the blends of virgin EPDM with diene rubbers; as well as getting better filler and curative distribution in such blends to optimise the blend’s properties, such as two-stage vulcanisations [11, 12] and the cross-linking of the rubbers by means of electron beam irradiation [13-15]. The first method involves with pre-vulcanising one certain matrix before blending it with another matrix (known as reactive blending); and by applying this method, the pre-vulcanised matrix is blended well with the filler and accelerators for a period of time, and its chains have a certain probability to contact the filler and pre-cross-linking. Therefore, more homogenous crosslinking distribution and cure compatibility in the blends is observed. Cross-linking of the polymers, by means of electron beam irradiation, has also been strongly developed in the use of cross-linking agents [13-15]. In the latter method, an irradiation process presents the main
4
advantages of being carrying out quickly under well-controlled conditions, particularly at room temperature, and leads to final products that exhibit good mechanical characteristics. To the best of our understanding, no works have been done so far on enhancing the thermal stability of NR/R-EPDM blends by means of applying two-stage vulcanisations and EB irradiation. This article presents the results of assessing the effect of processing means on the enhancement of the thermal stability of NR/R-EPDM blends. Two methods of thermal analysis, namely thermo-oxidative ageing and thermogravimetric analysis, were conducted in this study.
2. Experimental Details 2.1 Materials Natural rubber, SMR 5L grade, was supplied by the Rubber Research Institute of Malaysia (RRIM). Recycled ethylene-propylene-diene rubber (R-EPDM), which was obtained from gaskets and O-ring products, was supplied by Zarm Scientific (M) Sdn. Bhd., Penang, Malaysia. R-EPDM was ground into powder form (to give particle sizes of around 10 – 200 µm prior to blending) using a Table Type Pulverising Machine from Rong Tsong Precision Technology Co. Ltd. R-EPDM was in an irregular, rough shape and a typical particle in an aggregate that was broken by a mechanical crusher. The specific gravity of the powdered recycled EPDM was found to be 1.06 g/cm3. The carbon black content in the R-EPDM was around 29.33%. The physical characteristics of R-EPDM were reported in our previous study [4]. The N330 grade carbon black was supplied by Malayan Carbon (M) Ltd., Malaysia. The trimethylolpropane triacrylate (TMPTA) was purchased from UCB Asia Pacific Ltd., Malaysia. Other compounding ingredients, such as zinc oxide, stearic acid, N-cyclohexylbenzothiazyl-sulfenamide (CBS) sulphur, were purchased from Bayer (M) Ltd., and were used as received.
5
2.2 Compounding, Curing Characteristics, and Vulcanisation The formulations used for blending are presented in Table 1. The compounding of natural rubber, EPDM, R-EPDM with accelerators, and other additives, such as zinc oxide, stearic acid, and sulphur, were done on a HAAKE rheomix and laboratory-sized (160 mm x 320 mm) two-roll-mill, model XK-160. A semi-efficient sulphur vulcanisation system (semi-EV) with the ratio of 70/30 (phr/phr) of NR/R-EPDM was processed as control and EB irradiation. Meanwhile, a blend ratio of 70/20/10 (phr/phr/phr) of NR/R-EPDM/EPDM was prepared for carrying out the blends processed by means of introducing pre-vulcanised EPDM. Figures 1 and 2 illustrate schematics representing the preparation of NR/R-EPDM blends, processed by means of introducing pre-vulcanised EPDM and electron beam irradiation. This diagram is further simplified and summarised in Table 2.
The preparation of blends processed by means of introducing pre-vulcanised EPDM has been thoroughly described in our previous work [16]. All the ingredients given in Table 1, except for natural rubber and carbon black, were prepared in a HAAKE rheomix. The mixer was operated with a rotor speed of 60 rpm and an initial temperature of 50 oC. The virgin EPDM was first masticated for 1 min, followed by the incorporation of R-EPDM for a further 1 min, and then ZnO and stearic acid were added for another 1 min. After 3 mins of mixing, CBS and sulphur were introduced and the mixing continued for another 2 mins. After dumping, the compound was later sheeted onto the two-roll-mill. A sample of the respective compound was tested for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000). The compounds were subsequently pre-vulcanised in a compression moulding machine at 150oC; according to a designated time taken from the MDR 2000, as shown in Table 3. The resulting compound (R1 compound) was immediately removed from the mould and left to cool at room temperature. The R1 compound was later blended with natural rubber
6
and carbon black (R2 compound) on the two-roll-mill to a final NR/R-EPDM/EPDM ratio blend of 70/20/10 (phr/phr/phr). Finally, the blended compound was tested again for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000) and compressmoulded using a laboratory hot-press; based on the respective curing times.
For the latter blends, the blended compound was prepared by mixing the entire amount of additives, as well as the natural rubber and R-EPDM, which were prepared on a laboratorysized two-roll-mill (model XK-160) at ambient temperature. The resulting blends were later tested for its curing characteristics using a Monsanto Moving Die Rheometer (MDR 2000). The compounds were subsequently compression-moulded using a stainless steel mould at 150°C with a pressure of 10 MPa using a laboratory hot-press, based on the respective curing times. The moulded sheets and dumbbell test pieces were irradiated using a 3MeV electron beam accelerator (NHV EPS-3000) at a dose range of 0-200 kGy. The acceleration energy, beam current, and dose rate, were 2MeV, 5mA, and 50kGy/pass, respectively.
2.3 Measurement of Tensile Properties Dumbbell-shaped samples were cut from the moulded sheets according to ASTM: D41206ae2. Tensile tests were performed at a cross-head speed of 500 mm/min. Tensile tests were carried out with an Instron 3366 universal tensile machine to determine tensile properties, in terms of tensile strength, stress at 100% strain (M100), and elongation at break. The hardness measurements of the samples were done according to ASTM: D2240-05(2010), using a type Shore A manual durometer.
2.4 Thermo-oxidative Ageing
7
To determine thermal ageing properties, the dumbbell-shaped specimens were placed in an oven equipped with an air circulating system at a test temperature of 100 oC for 48 hours, according to ASTM: D573-04(2010). The aged specimens were then measured for tensile properties. Changes in stress at 100% strain, tensile strength, elongation at break, and hardness after thermal ageing, were used to determine thermal ageing resistance. The retention in each property was evaluated according to Eq. (1).
Retention (%) =
Value after ageing × 100 Value before ageing
(1)
2.5 Thermogravimetric Analysis (TGA) Thermogravimetric analysis of the blends, by means of introducing pre-vulcanised EPDM and EB irradiation, was carried out with a Perkins-Elmer Pyris 6 TGA analyser. The sample was scanned from 30 to 600oC in a nitrogen air flow of 30 ml/min, with a heating rate of 20oC /min.
2.6 Activation Energy of the Degradation Process The degradation activation energies of the blends were determined by applying the CoatsRedfern’s method as follows [17];
log[
− log(1 − α ) 2
T
] = log[(
AR 1 − 2 RT E )×( )] − βE E 2.303RT
(2)
Where, α is the fractional mass loss at time t, T is the absolute temperature, A is the preexponential factor, R is the universal gas const, β is the heating rate, and E is the activation
8
energy. A plot of log[- log(1- α)/T2] as a function of 1/T gives a straight line with a slope equal to E/2.303R and the y-intercept is log[(AR)/(βE)(1-2RT/E)].
3. Results and Discussion 3.1 Effect of Introducing Pre-vulcanised EPDM 3.1.1 Thermo-Oxidative Ageing Figure 3 illustrates the possible R1 compound mechanism after being pre-vulcanised according to respective scorch time and processing using a conventional two-roll-mill. Our previous work [16] disclosed that CBS fragment was initially bound to the positions provided on the third monomer ethylidene norbornene of the virgin EPDM, resulting in a rubber bound intermediate. After R1 compound were initially pre-vulcanised, virgin EPDM with pendant CBS fragments resulted in a restriction of the curative migration from the virgin EPDM phase towards a higher unsaturated natural rubber level, which consequently enhanced the crosslink distribution. By applying this method, co-crosslinking between virgin EPDM and natural rubber phases are presented, as schematically displayed in the bottom part of Figure 3.
Tensile strength and elongation at break of un-aged and aged blends, based on various prevulcanising times, are shown in Figures 4 and 5. The tensile strength of un-aged blends increased by increasing the pre-vulcanising time of the R1 compound up to 3.45 mins (ts) and then reduced. Significantly, by introducing pre-vulcanised EPDM, the tensile strength had improved. The pre-vulcanising of the R1 compound caused to reduce the curative migration from the EPDM phase towards the higher unsaturated natural rubber phases, which consequently improved the cross-link distribution. High tensile strength of such reactive processing blend’s vulcanisates must again be the result of a better homogeneity of either cross-link distribution or the carbon black distribution or both [11, 12]. It is interesting to
9
highlight that even though unaged tensile strength, beyond 3.45 mins (ts) pre-vulcanising time of R1 compound, dropped slightly; it was still higher than the simple processing blends.
Elongation at break indicates the ductility, elasticity, and flexibility of the vulcanisates [18]. Elongation at break of the blends processed by means of introducing pre-vulcanised EPDM was higher than simple blend irrespective of pre-vulcanising time. This is due to better filler distribution, cure matching, and higher cross-linking distribution of the blends. It was also found that the elongation at break decreased with an increasing pre-vulcanising time of the R1 compounds. When the cross-link density was too high, the average molar mass of the rubber chain between two successive cross-link points decreased, and the mobility of the chain segment was restricted; which consequently limited the orientation of the network chain. This restriction reduced the number of effective network chains, resulting in a decreased elongation [19]. Another probable reason was the reduction of the deformability of the blends. Since the stiffness of the blends increased towards the pre-vulcanising time of the R1 compound (as discussed in the latter section), this might reduce the flexibility of the blends as well.
Stress at 100% strain of unaged and aged blends is listed in Table 4. It was observed that the stress at 100% strain of un-aged blends increased towards the pre-vulcanising time of the R1 compound up to 4.45 mins (ts + 1) and then decreased. Tensile modulus basically indicates the stiffness (or hardness) of the vulcanisates upon stretching. The increment of modulus, as a function of pre-vulcanising time, is due to an increasing of degree of cross-linking; which takes place after the pre-vulcanising step, and also more homogeneous carbon black distribution in the blends. However, the modulus of the blends drops slightly as the prevulcanising time approaches 5.45 mins (ts + 2) of the R1 compound. By pre-vulcanising R1
10
compound beyond 5.45 mins (ts + 2), the blend remains visually heterogeneous during the blending with the natural rubber on the two-roll-mill; thus indicating that the processability of R1 compound is comparatively less. Small particles of pre-vulcanised R1 compound were dispersed in the blends for 5.45 mins (ts + 2), eventually lead to low modulus in the blends. In addition, the overall modulus of reactively processing blends was lower than simple processing blend. As stated earlier, the blends at various pre-vulcanising times were prepared using an internal mixer. Thus, the shearing coefficient was higher in comparison to the tworoll-mill, resulting in less stiffness in the blends. On the contrary, the hardness property (see Table 4) of the blends processed by means of pre-vulcanised EPDM showed no difference. Since the compound formulation was unchanged, the blends therefore exhibited similar hardness results.
In addition, the tensile strength and elongation at break of all aged samples reduced upon thermal ageing. Reduction in tensile strength and elongation at break could be attributed to rubber oxidation, which resulted in chain scissions under thermal exposure. Scission of the larger molecular chains increased the number of shorter chains of the respective rubber, which led to fewer entanglements and reduce the ability of the strain-crystallisation process, thereby decreased the tensile strength, as well as elongation at break. The retained property percentage e.g., the retained elongation, retained tensile strength, retained tensile modulus, and the retained hardness property after thermal ageing, was used to evaluate the thermal stability of the rubbers. The higher the retained properties obtained, the greater is the thermal stability. It can be observed that the retained tensile strength and elongation at break were obtained similarly. The blends showed optimum retained tensile and elongation at break at 1.45 mins (ts – 2) of pre-vulcanising time. By introducing pre-vulcanised EPDM, the ageing resistance of the blends is clearly observed to be as a result of a good cross-link distribution
11
between the rubber phases. Another probable reason might be due to a better carbon black distribution in the blends. The R-EPDM used in this study was ground and mostly crosslinked, the cross-linked EPDM therefore consisted of higher viscosity than natural rubber. In this case, carbon black was mostly located in the natural rubber phase and was especially concentrated at the interface of the two polymers, like an envelope formation. Thus, it was found that carbon black distributed unevenly in the blends. By introducing pre-vulcanised EPDM, the uncured virgin EPDM blended well with carbon black for a period of time, where virgin EPDM chains had a certain area to contact with carbon black and be entangled or trapped in the void of the carbon black aggregates. As a consequence, the migration of carbon black to natural rubber reduced, more homogenous carbon black distribution in the blends was observed and therefore enhanced the ageing resistance to the blends. As mentioned previously, the blends showed optimum retained tensile and elongation at break at 1.45 mins (ts – 2) of pre-vulcanising time. This suggests that the ageing resistance was most pronounced at this stage. Pre-vulcanising the R1 compound for 1.45 mins (ts – 2) is sufficient to gain good thermal ageing properties.
3.1.2 Thermogravimetric Analysis (TGA) The thermal decomposition behaviour and derivative weight thermograms of NR/R-EPDM blends, processed by means of introducing pre-vulcanised EPDM, are shown in Figures 6 and 7, respectively. The decomposition temperature at various weight losses, stages, and char residues is also summarised in Table 5. Two regions of degradation of the blends were clearly observed. Here, an initial minor weight loss at around 180 - 200oC was caused by the presence of a volatile matter, such as stearic acid, as well as the adsorbed water at around 300oC [20]. The first step of degradation of the blends began at about 330oC and was completed at around 450oC. The second stage of degradation occurred in the region 450 – 12
520oC. The former stage of decomposition was caused by the degradation of natural rubber (polyisoprene) segments, which corresponded to a major peak being observed from the DTG curve (Figure 7). The degradation of natural rubber segments is sensitive to the presence of an oxidised structure, as well as the depletion of sulfidic cross-linking in the natural rubber. The latter degradation, which can be observed from the DTG curve’s small peak area, was attributed to the scission of cross-linked R-EPDM and conjugated polyene remaining after the first stage of degradation [21].
Notably, the decomposition temperature at 10%, 30%, 50%, and 60% weight loss of the blends, was influenced by the pre-vulcanising time. It was observed that the decomposition temperature clearly differed from the earlier stages of decomposition. Introducing prevulcanised EPDM into the blends greatly affected their thermal stability. The decomposition temperature of the blends, processed by means of introducing pre-vulcanised EPDM, showed further significance at the decomposition temperatures above 50% of weight loss. When considering only the blends, introducing pre-vulcanised EPDM gave a distinctively better thermal stability than the simple blend alone; thus confirming the enhancement of crosslinking and/or the filler distribution within the blends. Table 5 also illustrates the decomposition temperature at different stages and char residues of the blends derived from TG profiles. The decomposition temperature at the first maximum peak (Tmax I) represents the thermal decomposition of natural rubber phase, whereas the decomposition temperature at the second maximum peak (Tmax II) corresponds to the scission of the cross-linked R-EPDM. Significantly, applying pre-vulcanised EPDM enhanced thermal stability by shifting the decomposition temperature upwards. This is simply due to the blends consisting of a high cross-linked distribution; and therefore, a higher temperature is required to cleave the linkages. It can also be seen that the decomposition temperature at Tmax 13
I
showed no
difference. Since the natural rubber phase is unmodified, this resulted in the decomposition temperature remaining unchanged. It was similarly observed that after introducing prevulcanised EPDM into the blends, the decomposition temperature exhibited an optimum value at 3.45 mins (ts) of pre-vulcanising time. The inducement of co-crosslinking between pre-vulcanised EPDM and NR in the blends is most pronounced during this stage thereby increasing the resistance for bond cleavage at a pre-vulcanising time of 3.45 mins (ts). Furthermore, the char residue of all blends was not influenced by introducing pre-vulcanised EPDM. This is simply due to the fact that the amount of filler and blend ratios was constant; thus keeping the char residue mostly unchanged. According to Chakraborty et al. [22], the amount of char is very much dependent on the type and amount of filler. The difference in filler amount mainly concerns the char formation.
3.1.3 Activation Energy of Degradation Process Since thermal stability is related to both the initial temperature and the rate of degradation of polymers, the determination of kinetic parameters associated with the degradation process is an interesting topic. Thermal analysis can also be used for the durability assessment and lifetime prediction of a product. Such study is very important for improvement of the product’s service performance [23]. The different stages of degradation can be conveniently identified from different slopes. Coats-Redfern plots, for NR/R-EPDM blends processed by means of introducing pre-vulcanised blends, are presented in Figure 8. The activation energies obtained by this approach are summarised in Table 5.
From the DTG curves, it was observed that the blends decomposed in two stages, which can be classified as the degradation of natural rubber phase (Stage I or EI) and EPDM or REPDM (Stage II or EII). The first stage of degradation (330 – 450oC) involved activation
14
energies of 68.91 kJ/mol, 69.03 kJ/mol, 69.85 kJ/mol, 71.59 kJ/mol, 69.74 kJ/mol, and 69.25 kJ/mol of the blends, without introducing pre-vulcanised EPDM, pre-vulcanising times of the R1 compound for 1.45 mins (ts – 2), 2.45 mins (ts – 1), 3.45 mins (ts), 4.45 mins (ts + 1), and 5.45 mins (ts + 2), consecutively. During the second stage of decomposition (450 – 520 oC), the calculated energies were 23.36 kJ/mol, 23.35 kJ/mol, 24.69 kJ/mol, 25.62 kJ/mol, 24.31 kJ/mol, and 24.18 kJ/mol, concerning the blends without introduction of pre-vulcanised EPDM, pre-vulcanising times of the R1 compound for 1.45 mins (ts – 2), 2.45 mins (ts – 1), 3.45 mins (ts), 4.45 mins (ts + 1), and 5.45 mins (ts + 2), respectively. Activation energy is an indication of the energy required to decompose the molecular network. Here, it was observed that the activation energy obtained from the blends processed by means of introducing prevulcanised EPDM was higher than the simple blend, suggesting that higher cross-linking distribution was achieved in the blends. Kader and Bhowmick [19] exemplified the relationship between activation energy of degradation and the cross-linking of rubber vulcanisates. It was ascribed that the inducement of cross-linking was able to increase the resistance for bond cleavage or depletion at the degradation temperature. It was also found that pre-vulcanising the R1 compound for 3.45 is sufficient to gain the highest thermal stability; particularly at a higher temperature. The higher activation energy observed when applying this method was clearly successful at improving the thermal stability of the blends; irrespective of the pre-vulcanising time of the R1 compound.
3.2 Effect of Electron Beam Irradiation 3.2.1 Thermo-Oxidative Ageing Figure 9 shows the tensile strength of un-aged and aged NR/R-EPDM blends processed by means of introducing EB irradiation. It was observed that the tensile strength of un-aged blends exhibited optimum strength at irradiation dose of 50 kGy, which then reduced
15
continuously upon irradiation. According to Akiba and Hashims [24], the electron beam initiates radical reactions, resulting in the generation of intermolecular and intermolecular -CC- bonds in rubbers; as was the case with peroxide cross-linking. Peroxide vulcanisation is a thermally initiated event with the cure temperature. However, electron beam irradiation is performed at room temperature and results in significantly less energy applied to the process. Therefore, the idealised model representing the network formation, as described by Henning [25], is highly suggested (see Figure 10). In this study, the cross-linking promoter (i.e., TMPTA) generally makes efficient use of the radicals; whether acting to suppress nonnetwork forming side reactions during cure [26, 27] or to generate additional cross-links [28]. The possibility of the production of a cross-linking promoter to form a higher modulus fillerlike domain of thermoset cross-linking promoter, together with the formation of co-crosslinks of -Sx- and -C-C- after EB irradiation, may result in an increment to the extent of crosslinking. However, more cross-linking formed in the blend sites, with increasing irradiation doses, restrained the chains from structural rearrangement during elongation and reduced the tensile strength. Rooj et al. [29], suggested that when the cross-link density is too high, a rubber is deformed externally; part of the applied energy is stored elastically in the chains and is available as a driving force for fracture. The remaining energy is dissipated through molecular motions into heat, and as such, is made inaccessible to break further chains. At high cross-link levels, chain motions become restricted and the tight network is incapable of dissipating much energy; resulting in a decrease of tensile strength.
As shown in Figure 11, the elongation at break of irradiated blends reduced continuously upon EB irradiation. This behaviour was most likely due to the breakdown of the tie-chain molecules and entanglements, which consequently decreased the ductility of the polymer. As mentioned in the preceding section, the presence of TMPTA played a major role in the
16
irradiation-induced cross-linking of the blend. The rubber chains were locked by the crosslinking after irradiation, which consequently resisted the mobility of the rubber molecules. The phenomenon of reduction of elongation at break for irradiated rubber is in agreement with various EB irradiation studies [30, 31]. Stress at 100% strain (M100) and hardness results of the blends, are tabulated in Table 6. Modulus and hardness are indirectly related to stiffness and rigidity; and therefore, reflect upon the cross-linking of the vulcanisate. The enhancement in M100 and hardness after EB irradiation is due to an increase in cross-linking. The occurrence of irradiation-induced cross-linking is believed to be responsible for the increases in tensile modulus and hardness property.
Furthermore, tensile strength and elongation at break of all aged samples reduced upon thermal ageing. Reduction in tensile strength and elongation at break could be attributed to the oxidation of the polymer, which resulted in either chain cleavage leading to a drop in molecular weight or crosslinking. It was observed that the retained tensile strength at an irradiation dose of 50kGy dropped slightly before increasing gradually towards the irradiation doses. EB irradiation of the blends results in the formation of a three-dimensional network structure through the union of in-situ-generated macroradicals. Here, polyfunctional monomers (i.e., TMPTA) are mixed with the rubber matrix to achieve cross-linking at a reduced irradiation level, in order to minimise the deterioration of the base polymers [32]. The free radicals, which can be generated on the carbon atoms of the rubbers, may react with each other or initiate grafting with Polyfunctional monomers, followed by cross-linking. Similarly, the blends may also undergo cross-linking and grafting through radical formation when exposed to irradiation. Therefore, the cross-linking is enhanced by introducing EB irradiation. Generally, the radical termination, after thermal ageing of the rubber, is involved with the abstraction of hydrogen atoms from an allylic position on the rubber molecule.
17
However, this process already took place after EB irradiation. Therefore, the radical termination in the bulk polymer no longer exists. Hence, upon thermal ageing, the modulus and tensile strength of the blends reduced significantly compared to the control blend; particularly at a lower dose, but increased slightly after a certain EB irradiation dose. On the contrary, the hardness result, which is generally an opposite phenomenon to the tensile modulus, exhibits higher retained properties than the control blend after EB irradiation. This suggests that EB irradiation has no effect on static testing (i.e., non-destructive testing), such as the hardness property. Elongation at break also showed higher retained properties towards irradiation dosages. This means that applying EB irradiation to the blends results in an enhanced flexibility or elasticity, and therefore increases the elongation at break.
3.2.2 Thermogravimetric Analysis (TGA) The thermal decomposition curves (TG and DTG) are depicted in Figures 12 and 13. The decomposition temperature at various weight losses, stages, and char residues is listed in Table 7. Since the blend is composed of two rubber phases, similar regions of degradation were observed. The first step of degradation of the blends (330 – 450oC) was caused by the degradation of natural rubber (polyisoprene) segments and the second stage of degradation, which occurred in the region 450 – 520 oC, was attributed to the scission of cross-linked REPDM and conjugated polyene that remained after the first stage of degradation [23]. It is interesting to note that the thermal stability of the blends increased after EB irradiation. The decomposition temperature at various weight losses clearly shows that by introducing EB irradiation, thermal stability is greatly enhanced. The enhancement in thermal stability of the irradiated blends is due to the formation of more compact three dimensional cross-linked networks, which is more stable against the formation of the degradation. A similar observation was found for the decomposition temperature at different stages. The
18
decomposition temperature shifted to a higher temperature towards irradiation doses. This clearly verifies that by applying the EB irradiation, thermal stability is significantly enhanced, due to the formation of additional cross-linking of the blends. It is suggested that a higher temperature is needed to decompose the cross-links or linkages. In the case of char residue, it showed no difference among the blends; as similarly observed and previously discussed in the study of introducing pre-vulcanised EPDM.
3.2.3 Activation Energy of the Degradation Process The different stages of degradation can be conveniently identified from the differing slopes. Coats-Redfern plots for NR/R-EPDM blends, processed by means of introducing electron beam irradiation, are shown in Figure 14. The activation energies obtained using this approach are summarised in Table 7. It can be observed that the blends are decomposed in two stages, which can be classified from the degradations of natural rubber phase (Stage I) and R-EPDM (Stage II). The first stage of degradation (330 – 450 oC) involved activation energies of 69.24kJ/mol, 70.91kJ/mol, 71.89kJ/mol, 72.04kJ/mol, and 72.06kJ/mol, in the neat blends and irradiated blends at irradiation doses of 50kGy, 100kGy, 150kGy, and 200kGy, consecutively. In the second stage decomposition (450 – 520oC), the calculated energies were 20.33kJ/mol, 25.56kJ/mol, 25.91kJ/mol, 26.15kJ/mol, and 26.31kJ/mol, in the control blend and irradiated blends at irradiation dosages of 50kGy, 100kGy, 150kGy, and 200kGy, respectively. This shows that the increase in irradiation dose makes the system more resistant to thermal degradation and the blend at an irradiation dose of 200kGy is the most stable of all.
19
4. Conclusions The accomplishment of enhancing the thermal stability of NR/R-EPDM blends is a major issue in this study. The enhancement of the thermal resistance of NR/R-EPDM can be achieved by introducing pre-vulcanised EPDM and EB irradiation. In the first study, the introduction of pre-vulcanised EPDM into the blends greatly influenced thermal resistance, whereby the pre-vulcanised EPDM for 1.45 mins (ts – 2) to 3.45 mins (ts) of pre-vulcanising time, revealed good thermal resistance by showing higher retained tensile properties, as well as shifting the initial decomposition to a higher temperature (as confirmed by the TGA profiles). Furthermore, introducing EB irradiation to NR/R-EPDM also enhanced the thermal stability of the blends, by showing similar findings as seen in the first study. However, it is interesting to highlight that the decomposition temperature, as well as the activation energy, was found to increase towards the irradiation dose. This is associated with the greater crosslinking observed, due to the formation of irradiation-induced-crosslinks after EB irradiation of the blends. From the overall properties, pre-vulcanising of the R1 compound for 3.45 mins (ts) and an EB irradiation dose of 100 kGy, are highly recommended for NR/R-EPDM, especially when high strength and thermal stability of the blends are of major concern.
Acknowledgements H. Nabil gratefully acknowledges the personally financial support under Graduate Assistant Scheme and Postgraduate Research Grant Scheme (PRGS) provided by Institute of Postgraduate
Studies
(IPS),
Universiti
Sains
Malaysia
(Account
Number:
1001/PBAHAN/8045014). Also, the research grant of Exploratory Research Grant Scheme (ERGS),
Ministry of
Higher
Education
203/PBAHAN/6730066) for the great support.
20
(MOHE),
Malaysia
(Account
Number:
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Captions Tables Table 1 Formulations of the blends. Table 2 Preparation of simple blend and the blends processed by means of introducing prevulcanised EPDM. Table 3 Curing characteristics of the blends (R1 compound) and its pre-vulcanising time prerequisite to blend using two-roll-mill. Table 4 Stress at 100% strain and a hardness result of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Table 5 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Table 6 Stress at 100% strain and hardness results of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Table 7 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figures Figure 1 Schematic illustration representing the preparation of NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 2 Schematic illustration representing the preparation of NR/R-EPDM blends processed by means of electron beam irradiation. Figure 3 Possible mechanism of the blends with respect to R1 and R2 compounds [16]. Figure 4 Tensile strength of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 5 Elongation at break of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 6 TG curves of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 7 DTG curves of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 8 Coats-Redfern plot of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Figure 9 Tensile strength of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 10 Idealised network from EB irradiation curing in the presence of cross-linking promoter (TMPTA). Cross-links can be derived from (A) rubber radicals, (B) -Sx- linkages, (C) mixed -Sx- and -C-C- linkages, (D) cross-linking promoter forming cross-links, and (E) 26
thermoset domains of cross-linking promoter grafted to polymer chains (adapted from Henning [25]). Figure 11 Elongation at break of un-aged and aged carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 12 TG curves of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 13 DTG curves carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Figure 14 Coats-Redfern plot of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation.
27
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Table 1 Formulations of the blends. Materials Control (phr) SMR L EPDM R-EPDM ZnO Stearic acid CBS Sulphur N330 TMPTA EB Irradiation (kGy)
70.0 30.0 5.0 2.0 1.2 1.8 30.0 -
Introduction of Pre-vulcanised EPDM (phr) 70.0 10.0 20.0 5.0 2.0 1.2 1.8 30.0 -
Electron Beam Irradiation (phr) 70.0 30.0 5.0 2.0 1.2 1.8 30.0 3.0 Variable
SMR L = Standard Malaysian Rubber L EPDM = Virgin Ethylene-Propylene-Diene Rubber. R-EPDM = Recycled Ethylene-Propylene-Diene Rubber. ZnO = Zinc Oxide. CBS = N-Cyclohexyl-Benzothiazyl-Sulphenamide TMPTA = Trimethylolpropane-triacrylate
Table 2 Preparation of simple blend and the blends processed by means of introducing prevulcanised EPDM. Additives used with respect to Control or Non-preheated Introduction of Pre-vulcanised EPDM Compound Two-Roll-Mill HAAKE Two-Roll-Mill at ambient at ambient temperature at 50 oC (R1 Compound) temperature (R2 Compound) SMR L EPDM R-EPDM ZnO Stearic acid CBS Sulphur N330 TMPTA
EPDM R-EPDM ZnO Stearic acid CBS Sulphur
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SMR L N330
Table 3 Curing characteristics of the blends (R1 compound) and its pre-vulcanizing time prerequisite to blend using two-roll-mill [16]. R1 Compound Cure characteristics Sample code Pre-vulcanizing time (-/+ to ts2, min) MH 16.29 ts – 2 1.45 MH – ML 13.97 ts – 1 2.45 ts2 3.45 ts 3.45 tc90 23.07 ts + 1 4.45 CRI 5.10 ts + 2 5.45 ML = Elastic minimum torque (dN.m) MH = Elastic maximum torque (dN.m) MH – ML = Torque difference (dN.m) ts2 = Scorch time (min) tc90 = Curing time (min) CRI = Cure rate index (CRI)
Table 4 Stress at 100% elongation and a hardness result of carbon black filled blends processed by means of introducing pre-vulcanised EPDM. Sample M100 (MPa) Hardness (Shore A) Code Un-aged Aged Retention Un-aged Aged (%) 0 1.96 ± 0.06 2.50 ± 0.10 127.74 65 ± 1.01 65 ± 0.67 ts - 2 1.60 ± 0.13 2.50 ± 0.02 156.69 64 ± 0.82 67 ± 0.85 ts - 1 1.65 ± 0.25 2.22 ± 0.01 134.94 65 ± 0.89 67 ± 0.62 ts 1.92 ± 0.09 2.05 ± 0.03 106.78 64 ± 0.62 67 ± 0.83 ts + 1 2.05 ± 0.33 2.17 ± 0.09 105.92 64 ± 0.89 66 ± 0.70 ts + 2 1.74 ± 0.02 2.04 ± 0.04 117.25 63 ± 1.29 66 ± 1.04
NR/R-EPDM
Retention (%) 100.00 104.69 103.08 104.69 103.12 104.76
Table 5 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of introducing pre-vulcanised EPDM. Activation Sample The decomposition temperature (oC) energy CR codes (kJ/mol) (%) T-10% T-30% T-50% T-60% Tmax I Tmax II EI EII 0 369.06 405.41 452.57 484.68 400 487 68.91 23.36 31.4 ts - 2 373.08 406.73 455.93 493.97 400 500 69.03 23.35 31.4 ts - 1 373.79 407.36 457.43 494.30 400 500 69.85 24.69 31.4 ts 374.89 407.45 457.83 495.47 400 501 71.59 25.62 31.6 ts + 1 374.12 407.40 455.15 494.44 400 500 69.74 24.31 31.5 ts + 2 372.84 406.26 454.76 493.77 400 499 69.25 24.18 31.6
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Table 6 Stress at 100% elongation and hardness results of carbon black filled blends processed by means of electron beam irradiation. Sample M100 (MPa) Hardness (Shore A) Code Un-aged Aged Retention Un-aged Aged (%) Control 2.11 ± 0.04 5.83 ± 0.08 276.27 65 ± 0.44 67 ± 0.44 50 kGy 2.32 ± 0.12 3.67 ± 0.11 158.58 68 ± 0.44 72 ± 1.26 100 kGy 2.76 ± 0.07 3.71 ± 0.10 134.46 69 ± 0.53 73 ± 0.50 150 kGy 2.94 ± 0.10 3.82 ± 0.15 130.07 70 ± 0.53 74 ± 0.50 200 kGy 3.21 ± 0.06 3.94 ± 0.28 122.66 72 ± 0.53 75 ± 0.55
NR/R-EPDM
Retention (%) 103.08 105.88 105.79 105.71 104.17
Table 7 The decomposition temperature at various weight losses and stages, activation energy calculated from Coats-Redferm’s equation and char residue (CR) of carbon black filled NR/R-EPDM blends processed by means of electron beam irradiation. Activation Sample The decomposition temperature (oC) energy CR codes (kJ/mol) (%) T-10% T-30% T-50% T-60% Tmax I Tmax II EI EII Control 367.18 403.58 446.97 489.75 399 487 69.24 20.23 33.6 50 kGy 373.43 409.31 456.92 492.50 401 496 70.91 25.56 33.8 100 kGy 374.76 410.19 457.97 493.93 402 498 71.89 25.91 33.7 150 kGy 375.85 410.32 458.86 495.05 402 498 72.04 26.15 33.2 200 kGy 377.05 411.73 460.17 496.73 403 499 72.06 26.31 33.7
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Highlights New route of processing was introduced to optimise the thermal stability of NR/REPDM blends. Pre-vulcanised EPDM and EB irradiation were introduced into NR/R-EPDM blends. Thermal stability is obviously enhanced by applying these two techniques. Applying new route of processing methods is clearly successful to NR/R-EPDM blends.
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