Article pubs.acs.org/IECR
Nanostructured Thermoplastic Vulcanizates by Selectively CrossLinking a Thermoplastic Blend with Similar Chemical Structures Yanchun Tang, Kai Lu, Xiaojun Cao,* and Yongjin Li* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou 310036, China S Supporting Information *
ABSTRACT: Ethylene vinyl acetate rubber (vinyl acetate (VA) content = 50 wt %) (EVM) and ethylene vinyl acetate copolymer (VA content < 50 wt %) (EVA) are polymers with a very similar chemical structure. In this study, a novel thermoplastic vulcanizate (TPV) based on EVM/EVA28 (VA content = 28 wt %) blend has been successfully fabricated by dynamic vulcanization due to the selective cross-linking of EVM. The morphologies and properties of the TPVs have been investigated. It was found that the cross-linked EVM phase and the thermoplastic EVA28 phase form a perfect cocontinuous structure with the rubber phase size of about 100 nm. The fabricated TPV exhibits not only excellent stretchability (>900% elongation at break), nice elasticity (only about 19% remnant strain at 100% stretching), and good flexibility but also superior oil resistance.
1. INTRODUCTION A thermoplastic elastomer (TPE) is defined as a polymeric material with properties and functional performance similar to those of a conventional vulcanized rubber; still, it can be processed in a molten state as a thermoplastic polymer.1 Thermoplastic vulcanizate (TPV) is a very special type of TPE produced via dynamic vulcanization of blends consisting of a rubber and a thermoplastic polymer.2,3 Dynamic vulcanization is the procedure where curing agents are used to selectively cross-link the elastomeric phase during its mixing with molten plastics. The process always needs to be carried out under high shear and above the melting temperature of the thermoplastic component and at the processing conditions to activate and pursue the process of vulcanization of the rubber component.4 Morphologically, the resulting TPVs are characterized by the presence of finely dispersed cross-linked rubber particles (or continuous phase) distributed in a continuous thermoplastic matrix.5 Compared with the blends comprised of uncured components, the rubber phases of such blends are threedimensionally cross-linked within a micrometer (or submicrometer) phase size. Therefore, the physical properties of the obtained TPVs, such as mechanical, permanent set, elastic recovery, and resistance to effect by fluids, are remarkably improved as compared with the simple blends without dynamic vulcanization.1,6 Because of their unique characteristics, TPVs play a very important role in applications of automotives, buildings and construction, wires and cables, etc.7 A literature survey indicates that TPVs are comprised of the fastest growing thermoplastic elastomer market with a global annual growth rate of about 15% in the past decade.8 Thermoplastic vulcanizates (TPVs) based on polypropylene (PP) and ethylene−propylene−diene monomer (EPDM) have gained considerable attention because of a combination of rubbery properties along with their thermoplastic nature as well as their simple preparation method. PP and EPDM are highly compatible, but they form a phase-separated structure upon © 2013 American Chemical Society
melt blending. The mechanical properties of the TPVs depend not only on the properties of the individual component in the blend but also on the final phase morphology in the materials.9 Unfortunately, the poor oil resistance of the TPV from the EPDM/PP system always keeps itself from applications requiring resistance to oil and hydrocarbon solvents. On the other hand, the products, such as acrylate rubber (ACM) and hydrogenated NBR (HNBR), have good oil resistance compared to the polyolefin rubbers, but their relative poor mechanical properties and high cost are often prohibitive for large-scale applications.10 Ethylene vinyl acetate copolymer (EVA) is a kind of thermoplastic resin with different percentages of vinyl acetate (VA) in the molecular chains. A certain amount of polar groups in the molecular chains can offer a superior property of oil resistance. Moreover, EVA also possesses the merit of excellent aging resistance, weather resistance, melt processability, and mechanical properties.11 For its special molecular configuration and excellent characteristics, EVA is very extensively used, especially in production of cables, piping, adhesive tape, etc.12 In the past decades, TPVs based on polyethylene (PE)/EVA systems have been widely investigated, such as ethylene vinyl acetate (EVA) with linear low-density polyethylene (LLDPE),13 low-density polyethylene (LDPE),14 and highdensity polyethylene (HDPE),15 etc. However, the obtained TPVs from these blends are far from the real application because of the poor compatibility in blending rubber and thermoplastic materials. Compatibilization of incompatible polymer blends by use of preformed or in-situ-formed block (or graft) copolymers is an efficient route to decrease the size of the dispersed phase and improve adhesion between the phases, Received: Revised: Accepted: Published: 12613
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were then hot pressed at 180 °C to sheets. Obtained sheets were used for further characterization and property measurements. It should be mentioned that we only presented the main results for the EVM/EVA (50/50) samples in this paper. TPVs were also successfully achieved for other EVM/EVA blends with a weight ratio range from 50/50 to 70/30. 2.3. Structural Characterization. Dynamic mechanical analyses (DMA) of the samples were carried out on DMA Q800 (TA Instrument). The experiment was performed in tension mode at a frequency of 5.0 Hz and a heating rate of 3 °C/min as a function of temperature from −50 to 80 °C. Differential scanning calorimetry (DSC) measurements were carried out from DSC Q2000 (TA Instrument) in an inert atmosphere of nitrogen at a heating or cooling rate of 10 °C/ min with a temperature range from −60 to 200 °C. The morphological features of unvulcanized EVM/EVA blends were analyzed with a scanning electron microscope (SEM) (Hitachi S-4800). Samples were fractured after freezing for 10−15 min in liquid nitrogen and sputtered with gold. The phase structure of unvulcanized and dynamically vulcanized EVM/EVA28 blends was observed by transmission electron microscopy (TEM) (Hitachi HT-6600). Samples were ultramicrotomed at −80 °C to a section with a thickness of about 60−80 nm. Sections were then stained using ruthenium tetroxide (RuO4). Rheological characterization of the systems was carried out using an Physica rheometer MCR301(Anton Paar Instrument) disposed with parallel plates with a diameter of 25 mm. Dynamic frequency sweep tests were executed at a constant strain of 2.5% and a frequency range from 0.1 to 600 rad/s. The temperature for testing was 180 °C. Tensile tests were carried out according to ASTM D412 using a universal tensile testing machine (Instron 5300) at a constant cross-head speed of 20 mm/min. Samples were a dumbbell shape punched out from the molded sheets. Strain recovery tests were carried out using dumbbell specimens on a universal tensile testing machine (Instron 5300) at a cross-head speed of 20 mm/min. Specimens were extended to certain elongation and returned at the same speed as that when stretching until zero stress was reached. The oil resistance of the samples was determined by immersing the specimens in ASTM #2 oil (Calumet Lubricants Co.) at room temperature for 24 h, according to ASTM D46106. Reprocessing characteristics were measured by repeatedly molding the samples at the same processing conditions of their preparation, followed by measuring their mechanical properties. Experiments were carried out for three processing cycle for each sample.
where the role of the block copolymer is mainly inhibition of the coalescence of droplets in shear-induced collisions.16 Borah and Chaki observed that addition of LLDPE-g-MA in LLDPE/ EVA blends reduced the size of the dispersed phase and improved the processability and toughness of the blends.17 Moly et al. reported that compatibilization improved the modulus of the LLDPE/EVA blends, which is due to the fine dispersion of EVA domains in the LLDPE matrix, providing increased interfacial interaction.18 However, since break up of the particles rather than coalescence is the limiting factor for the minimum attainable rubber particle size in the case of dynamic vulcanization, block copolymers are generally not very effective in refining the morphology of TPVs.19 In this study, TPVs based on two EVAs with different VA contents, ethylene vinyl acetate rubber (VA content = 50 wt %) (EVM) and ethylene vinyl acetate copolymer (VA content = 28 wt %) (EVA28), were prepared by dynamic vulcanization for the first time. Both EVM and EVA have low-temperature flexibility, melt processability, mechanical properties, and outstanding oil resistance.11 We consider that dynamically vulcanizing the blends of EVM and EVA is a unique and ingenious approach for development of oil resistance and melt processable materials. Compared with the PE/EVA TPVs, EVM and EVA are polymers with a similar chemical structure of great compatibility. Therefore, there is low interfacial tension between the blend components, which results in a finer and more stable morphology, better adhesion between the components, and, consequently, better properties of the final product. In addition, the cross-linking agent can selectively cross-link the EVM rubber phase for the reason that EVM with higher percentages of vinyl acetate (VA) than EVA28, which results in desirable morphology where three-dimensionally cross-linked EVM phase dispersed in the thermoplastic EVA28 matrix. Therefore, the selectively cross-linked EVM phase with a small size in TPVs gives elastic and mechanical properties, while the non-cross-linked EVA thermoplastic matrix in TPVs provides strength, processability, and recycleability. More important, the excellent oil resistance of both EVM and EVA leads to the superior oil resistance of the prepared TPVs.
2. EXPERIMENTAL SECTION 2.1. Materials. EVM sample used in this study was LEVApren500 (VA = 50 wt %, Lanxess Deutschland GmbH, Germany). EVA9, EVA28, and EVA40 were ELVAX 750 (VA = 9 wt %, DuPont Co., America), ELVAX 260 (VA = 28 wt %, DuPont Co., America), and ELVAX 40L-03 (VA=40 wt %, DuPont Co., America), respectively. The admixture of dicumyl peroxide (DCP) was produced by Shanghai Fangreda Chemical Co., Ltd., China. All polymers were dried in a vacuum oven at 30 °C for at least 12 h. 2.2. Sample Preparation. Blends without adding DCP were prepared using a HAAKE PolyLab (Thermo Fisher Scientific). Mixing was carried out with batch mixers of a constant rotor (camtype) at a speed of 50 rpm at 160 °C for 5 min. The dynamically vulcanized EVM/EVA TPVs were produced via a two-step mixing process. In the first step, preblends containing EVM and DCP were compounded at 70 °C, 50 rpm in the HAAKE batch mixer. After 5 min of mixing time, the preblends were removed from the mixer. In the second step, the TPV compounds were prepared by melt mixing the EVM preblends with the EVA resin in the same mixer. The mixer temperature was kept at 160 °C with a speed of 50 rpm. The EVM/EVA weight ratio was 50/50. All samples
3. RESULTS AND DISCUSSION 3.1. Miscibility and Morphology of EVM/EVA Blends without Dynamic Vulcanization. It is generally accepted that miscibility between the components in TPVs is critically important for high-performance TPVs. Basically, the components should be phase separated due to the multiphase structure of the TPVs, but at the same time good compatibility between the components is also perquisite for the TPVs with satisfied physical properties.2,3 Although EVAs with different VA contents have similar chemical structure, the VA contents affect the properties of the copolymer significantly.20,21 Therefore, the blends of EVM/EVA show different compatibilities dependent upon VA contents in the EVA components. 12614
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In order to select the good EVA matrix to make a blend with EVM for dynamic vulcanization, three types of EVAs with different VA contents have been blended with EVM. Figure 1 compares the morphologies of EVM/EVA (50/50) blends with different EVAs as indicated VA contents. In the pictures, the dark regions represent the EVM phase and the light regions are the EVA phase because the hard EVA phase induces more
secondary electrons than the soft EVM phase. It is obvious that VA content in EVA affects the miscibility of EVA/EVM significantly. The EVM/EVA9 blend (Figure 1a) shows a very coarse sea−island structure where the EVM is dispersed in EVA matrix and the domains have a size of 10−20 μm. DMA investigations indicate two separated glass-transition temperatures for the blend (Figure S1, Supporting Information). This means that the EVA with 9% VA is totally immiscible with EVM, so such blend is infeasible for the following dynamic vulcanization. For the EVM/EVA28 blend (Figure 1b), it shows a much finer cocontinuous morphology with a characteristic length scale in the order of 1−3 μm, which is just located in the region for making high-performance TPVs.22 On the other hand, the EVM/EVA40 blend is homogeneous with no phase separation, as shown in Figure 1c. At the same time, only one narrow molecular chain relaxation was observed from DMA analysis (Figure S2, Supporting Information). These results indicate that EVM and EVA40 are thermodynamically miscible and form a one-phase mixture. The dynamic vulcanization of a homogeneous blend may lead to the 3dimensional cross-linked networks, and such materials cannot be melt processed. On the basis of the morphological characterization and DMA analysis of the EVA/EVM blends, it is concluded that the EVM/EVA28 blend is a good candidate for dynamic vulcanization. Figure 2 shows plots of the dynamic loss (tan δ) by DMA as a function of temperature for neat EVM, neat EVA28, and the
Figure 2. tan δ as a function of temperature for neat EVM, neat EVA28, and the EVM/EVA28 (50/50) blend by DMA analysis.
EVM/EVA28 (50/50) blend. The Tg of neat EVM is at −30 °C, and that of neat EVA 28 is at −15 °C. The EVM/EVA28 (50/50) blend shows only one relaxation peak at about −18 °C, but the relaxation peak of the blend is broadening. As the SEM measurements have shown the phase-separated structure for the EVM/EVA28 blends, the wider single-relaxation peak can be attributed to overlapping of the molecular relaxation of the components.23 The intermediate relaxation peak temperature of the blend originates from the each other shifting of the relaxation peaks of the components due to the compatibility. DMA investigation again means that EVM and EVA28 are phase separated but highly compatible. Therefore, dynamic vulcanization was carried out on the EVM/EVA28 blends in the following sections. 3.2. Selective Cross-Linking of EVM in the EVM/EVA28 Blends and Morphological Analysis of the TPVs. Curing Characteristics under Melt-Processing Conditions. Torque
Figure 1. SEM images of blends based on (a) EVM/EVA9, (b) EVM/ EVA28, and (c) EVM/EVA40 at a constant ratio of 50/50. 12615
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linking processes of the two components (EVM and EVA28) in the EVM/EVA28 (50/50) blends, selective cross-linking of EVM rubber phase can occur during dynamic vulcanization within the appropriate processing time scale. The cross-link mechanism of EVA by peroxides has been well investigated, and the cross-link scheme is shown in Scheme 1.25 Obviously, EVM with more VA content is easily cross-linked by the radical due to the high reactivity of hydrogen in the methyl groups of VA segments. Moreover, in order to ensure selective cross-linking of EVM during dynamic vulcanization, a two-step mixing procedure was used. EVM was first premixed with DCP, followed by dynamic vulcanization with EVA. Figure 4 shows torque curves during mixing for the EVM/ EVA28 compounds at different DCP loadings. It is observed
curves as a function of processing time for pure EVA28, pure EVM, and EVM/EVA28 (50/50) blends in the presence of 0.2 wt % DCP at 160 °C with a screw rotation speed of 50 rpm are illustrated in Figure 3. All samples show similar torque changing
Figure 3. Vulcanization curves of (a) pure EVA28, (b) pure EVM, and (c) the EVM/EVA28 (50/50) blends containing 0.2 wt % DCP.
behaviors. Torque decreases rapidly at the initial stage due to melting of the solid pellets, followed by increasing torque because of the chemical cross-linking by the DCP. Finally, the mixing torque levels off, which indicates finishing of the crosslinking reactions under the processing conditions. However, differences lie in the vulcanization speed of each sample. The characteristic times along with the corresponding torque are summarized in Table 1. t2 and t90 have been used to evaluate
Figure 4. Vulcanization curves of the EVM/EVA28 (50/50) TPVs with addition of the indicated DCP content.
Table 1. Rheometer Output Data of Pure EVM, Pure EVA28, and EVM/EVA28 (50/50) Blends Containing 0.2 wt % DCP
pure EVM pure EVA28 EVM/EVA28 (50:50 parts)
t2 (min)
t90 (min)
vulcanization time (min)
1.27 2.54 1.93
2.32 4.14 3.41
1.05 1.60 1.48
that the final torque increases with increasing DCP concentrations, which means that higher cross-linking density is achieved at higher DCP concentration. The increased crosslinking density originates from a high density of cumyloxy or methyl radicals by DCP. On the other hand, it can also be observed that the cross-linking rate increases with increasing DCP loadings. Note that the dynamically vulcanized samples even at high DCP loadings show good melt processability, and they can be melt molded into smooth TPV sheets by the normal melt press molding method, as shown in Figure 5. However, too much cross-linking agent leads to the high viscosity of the TPVs and thus the difficulty in the processability, as the TPVs containing 0.25 wt % DCP. Morphological Investigations on the Prepared TPVs. Figure 6 shows TEM images of the EVM/EVA28 (50/50)
the cross-link speed for rubbers according to ASTM 2084-07. They are defined as 2% vulcanization time and 90% vulcanization time, respectively, from the torque curves during cross-linking.24 From Table 1, it is observed that the vulcanization time of pure EVA28 is much longer than that of pure EVM. Since there is a large time lag between cross-
Scheme 1. Cross-Linking Mechanism of EVA Initiated by the Peroxide
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Figure 5. Digital photo of the dumbbell-shaped TPV samples of the EVM/EVA28 (50/50) TPVs containing (C) 0.1, (D) 0.15, and (E) 0.2 wt % DCP.
nonvulcanized and dynamically vulcanized samples with different amounts of DCP. EVM is observed as the dark phase, and EVA28 is observed as the bright phase because EVM is more easily stained by RuO4 than EVA28. Figure 1b shows the cocontinuous structure for the nonvulcanized sample. Magnification of TEM observation is higher than that of SEM, so an image with irregular phase structure in TEM (Figure 6a) was obtained. It is seen that the cocontinuous phase size in Figure 6a is 1−3 μm phase size, which is also consistent with the results from SEM. A much finer cocontinuous morphology with a rubber phase average size of about 100 nm was obtained with 0.1 wt % DCP after dynamic vulcanization, as shown in Figure 6b. The drastic changing of the morphology indicates that the selective cross-linked EVM rubber phase occurred during dynamic vulcanization. Different from the dynamically vulcanized blends with 0.1 wt % DCP, the TPV with 0.2 wt % DCP (Figure 6c) has larger and irregular rubber particle domains with a characteristic length scale about 500 nm. The morphology in TPVs is governed by several parameters, including blend composition, viscosity ratio, shear force, and interfacial interaction between the two polymer phases.26 We considered that the unique nanostructures of the TPV materials originated from both the compatibility and the viscosity change during dynamic vulcanization. A similar chemical structure for EVM and EVA results in a very low interfacial tension between the EVA and the EVM phase. The possibility of reaction between EVA 28 and EVM during dynamic vulcanization should also be noted. Formation of certain copolymers leads to a reduction of the interfacial tension, so the fine morphology of the TPVs can be achieved. At the same time, with addition of a curative, the viscosity ratio plays a major role in the morphology evolution (less viscous phase encapsulates the more viscous phase).27 The higher viscosity of the cross-linked EVM leads to an increased EVM-to-EVA viscosity ratio, which limits break up of the EVM phase into small droplets.28,29 On the other hand, the changed viscosity ratio under dynamic shear leads to the smaller EVM phase size. Comparing the morphology of TPVs with different DCP contents, the size of the rubber phase increases with increasing DCP concentrations. It may be attributed to a too high rubber viscosity by the increased cross-linking density of the rubber phase. It should be mentioned that the dynamically vulcanized EVM/EVA28 blend shows the perfect fine nanostructures as a TPV. Thermoplastic elastomers from the block copolymers
Figure 6. TEM images of the EVM/EVA28 (50/50) (a) nonvulcanized blend and dynamically vulcanized blends with (b) 0.1 and (c) 0.2 wt % DCP blends.
containing the soft and hard segments are usually microphase separated into the fine nanostructures. However, very few nanostructured TPVs have been reported so far. The EVA28 matrix provides the thermoplastic properties of the TPV, while the cross-linked EVM rubber phase offers the stretchability, flexibility, and elasticity. More important, the nanometer dispersion (or phase structure) of the cross-linked rubber indicates the low interfacial tension and nice adhesion between the rubber and the plastic phases, which has been pursued longterm for TPV materials by dynamic vulcanization.30 In fact, 12617
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commercial TPVs based on blends of PP and ethylene− propylene−diene (EPDM) rubber usually have rubber particle sizes in the micrometer range.31 The TPVs from EVM/EVA28 blends have much smaller rubber dispersions than that from PP/EPDM blends. This indicates the superior mechanical properties of EVM/EVA28 TPV. Differential Scanning Calorimetry. DSC results of TPVs having different amounts of DCP are shown in Figure 7. The
Figure 7. DSC heating curves of the EVM/EVA28 (50/50) blends with addition of the indicated DCP content.
heating curve of the nonvulcanized EVM/EVA28 blend shows a broad shoulder followed by one peak at about 74 °C. Obviously, the sharp melting peak originates from the crystals by the crystallizable linear ethylene chain segments in EVA.32 However, for the dynamic vulcanized samples, no sharp melting peak was observed. It is considered two factors accounting for the changing of the melting behavior in the TPVs. On the one hand, refinement of the EVM phase induces a significantly increased interface between the rubber and the plastic phases, which impedes crystallization of the EVA28 matrix. On the other hand, it is inevitable that slight cross-links occurred in the EVA matrix during dynamic vulcanization, which is also not beneficial to crystallization of EVA. Therefore, a crystal with a regular structure hardly formed in the dynamic vulcanized samples, and only the broad melting peak was observed. 3.3. Properties of the EVM/EVA TPV. Rheological Behavior. The results of complex viscosity as a function of angular frequency obtained for the dynamically vulcanized samples are shown in Figure 8a. These results clearly indicate that all blends behave with shear-thinning behavior. This means that all samples have melt-processing characteristics. It can also be found that the complex viscosity of dynamically vulcanized blends is higher than the nonvulcanized blend, which is attributed to cross-linking of the EVM phase in the blends. Moreover, the nonvulcanized sample shows a more pseudoNewtonian behavior in low ω regime than the vulcanized samples, which is again attributed to three-dimensional networks in the EVM phase by dynamic vulcanization. The degree of cross-linking increases with increasing DCP dose, so the TPV shows higher complex viscosity with high DCP loading. Figure 8b shows the results of storage modulus versus angular frequency for blends having different DCP contents. The storage modulus of nonvulcanized blend is lower than that of the vulcanized blends in the whole ω regime. Meanwhile, in high ω regime, corresponding to movement within a small time scale, the storage modulus of the TPVs is almost the same as
Figure 8. (a) Complex viscosity as a function of frequency in the EVM/EVA28 (50/50) blends with addition of the indicated DCP content. (b) Storage modulus as a function of frequency in the EVM/ EVA28 (50/50) blends with addition of the indicated DCP content.
that of the nonvulcanized blend. The changes in complex viscosity and G′ can be ascribed to the cross-linking as well as the variation in morphology. It is expected that the change in morphology has a greater contribution since the viscosity of the TPVs at lower DCP content (0.1%) at high frequency is identical to the physical blend. Dynamic Mechanical Analysis. Figure 9a shows the storage modulus curves for the EVM/EVA28 (50/50) blends for different DCP contents. The EVM/EVA28 blend without DCP shows a lower storage modulus than the cross-linked samples over the whole temperature range investigated, especially in the high-temperature region (>60 °C), indicating dynamic vulcanization induces the improved mechanical properties and thermal resistance as well. tan δ versus temperature curves for EVM/EVA28 (50/50) blends having different DCP contents are shown in Figures 9b. Although all samples are phase separated (as shown in TEM image), only one relaxation peak is observed. Compared with the nonvulcanized blend, the peak of tan δ curves shifted to higher temperature for the reason that the cross-links restrict the molecular mobility of the polymer chains. Tensile Properties. Figure 10 shows the stress−strain curves for EVM/EVA28 (50/50) blends with various DCP contents. Obviously, dynamic vulcanization results in significant changes in the deformation behavior upon tensile deformation. The EVM/EVA28 blend without dynamic vulcanization is very soft and highly stretched with elongation at break as large as 1200%. Cross-linking of EVM phase occurring during melt processing leads to a drastic improvement in the modulus but slight decrease in the elongation at break. However, the TPV with 0.2 12618
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Table 2. Mechanical Properties of the EVM/EVA28 (50/50) Blends with Different DCP Contents
elongation at break/% tensile set at 100% deformation/MPa tensile set at 200% deformation/MPa tensile set at 300% deformation/MPa remnant strain of 100% elongation/% remnant strain of 200% elongation/% remnant strain of 300% elongation/%
nonvulcanized blend
vulcanized blend with 0.1 wt % DCP
vulcanized blend with 0.2 wt % DCP
1388.9
918.71
560.00
1.59
2.02
2.58
1.99
3.25
4.28
2.40
4.42
5.88
0.20
0.19
0.20
0.30
0.24
0.28
0.42
0.31
0.36
Figure 9. (a) Storage modulus as a function of temperature for the EVM/EVA28 (50/50) blends with addition of the indicated DCP content, and (b) tan δ as a function of temperature for the EVM/ EVA28 (50/50) blends with addition of the indicated DCP content by DMA analysis.
The tensile properties of TPVs were reported to be strongly dependent upon the rubber particle size.34,35 Elasticity. During the stretching process of the TPV, most of the thermoplastics phase acts as glue between the rubber particles, which are deformed. Only a small fraction of the thermoplastics is irreversibly deformed. During the recovery process, this thermoplastics fraction is partially pulled back by the recovery of the rubber particles.25 Therefore, the cocontinuous phase morphology with the elastomeric crosslinked EVM domains enable the dynamically vulcanized blends to elastically recover from a highly deformed state. Strain recovery curves for the EVM/EVA28 (50/50) TPV with 0.1 wt % DCP at different stretchings are shown in Figure 11. Table 2
Figure 10. Strain−stress curves for the EVM/EVA28 (50/50) blends with addition of the indicated DCP content.
Figure 11. Strain recovery curves for the EVM/EVA28 (50/50) TPVs with 0.1 wt % DCP at the indicated stretching.
wt % DCP still shows elongation at the break of more than 550%, which is a really high value for a TPV and is even higher than the elongation at the break of the commercialized PP/ EPDM TPVs.33 Elongation at break, tensile set at break, and tension set at 100%, 200%, and 300% elongation data for EVM/EVA28 (50/50) blends at different concentrations of DCP are summarized in Table 2. The fact that the tensile strength increases with increasing DCP loadings can be attributed to the increased cross-linking density and also the changing in the phase structure with variation of DCP contents.
summarizes the tensile retention data for the EVM/EVA28 (50/50) blends at different concentrations of DCP. The remnant strain is as low as 19% percent after 100% stretching for the EVM/EVA28 vulcanized sample with 0.1 wt % DCP, indicating excellent elastic recovery of the TPV. Oil Resistance. Oil resistance is one of the critical properties for the elastomeric materials. A comparison of the tensile properties of the TPV with 0.1 wt % DCP before and after immersion in ASTM #2 oil at room temperature for 24 h has been carried out. The changes in the physical properties after 12619
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Table 3. Comparison of Physical Properties of the EVM/EVA28 (50/50, 0.1 wt % DCP) TPV before and after Oil Aging at Room Temperature for 24 h rate of change/%
elongation at break
tensile set at break
mass
volume
thickness
width
length
−44.15
−31.0
1.36
0.40
1.42
0.31
0.50
resistant elastomer to be used in various fields, such as the cables, seal rings, etc.
immersing are shown in Table 3. One can see from Table 3 that the TPV is very stable in both dimension and mass when exposed in oil environments, indicating the excellent oil resistance of the TPV. The volume variation is only 0.4% after immersion in the oil. In contrast, the degree of swelling is as high as 157−263% for the commercialized PP/EPDM TPV.36 The excellent oil resistance is obviously due to the high VA content in both the matrix and the rubber domains. Note should also be paid to the mechanical properties of the TPV after oil immersion. Oil immersion increases the modulus of the TPV slightly but decreases elongation at break greatly (as shown in Figure S3, Supporting Information). However, such change does not have a high impact on application of the rubber materials. The oil-resistant results shown here mean that the EVM/EVA28 TPV obtained is superior in oil resistance and can be used as an oil-resistant elastomer. Reprocessability Studies. The obvious advantage of TPVs over conventional thermosetting rubbers is that TPVs can be reprocessed by all common equipment for plastic processing, such as extruders and injection molders, without significantly changing the physical properties.37 To investigate the thermoplastic elastomeric behavior, the reprocessability of the EVM/ EVA28 TPV was carried out by repeating the melt process of the samples. We can still obtain very nice TPV sheets, even reprocessing many times. The mechanical properties of the sheet samples were evaluated after each cycle and compared with that of the original specimens. The strain−stress curves and corresponding results are shown in Figure S4 and Table S1, Supporting Information, respectively. We found that the reprocessed samples have typical mechanical performance as an elastomer with very high elongation at break, low tensile modulus, and high tensile stress. Moreover, it can also be seen that the reprocessing of the samples induces a slight increase in the modulus and tensile strength but slightly decreases in elongation break. This can be attributed to the increased crosslinking density which occurred during high-temperature reprocessing. Obviously, the fabricated EVM/EVA28 TPV shows excellent reprocessability with high stability in the mechanical properties.
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ASSOCIATED CONTENT
S Supporting Information *
Strain−stress curves after oil immersion, strain−stress curves after reprocessing cycle, table of the mechanical properties after the reprocessing cycle. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 86-571-2886-7026. Fax: 86-571-2886-7899. E-mail:
[email protected],
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (21074029, 51173036), Zhejiang Provincial Natural Science Foundation of China (R4110021), and PCSIRT (IRT 1231).
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REFERENCES
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4. CONCLUSIONS In this work, a novel nanostructured thermoplastic vulcanizate based on two EVAs with different VA contents, ethylene vinyl acetate rubber (VA content = 50 wt %) (EVM) and ethylene vinyl acetate copolymer (VA content = 28 wt %) (EVA28), have been successfully prepared by dynamic vulcanization for the first time. EVA rubber was selectively cross-linked during dynamic vulcanization, and the cross-linked EVA rubber formed a perfect cocontinuous structure with the non-crosslinked EVA plastic in the size of about hundred nanometers. Therefore, the ideal morphology of a high-performance TPV was achieved by dynamic vulcanization. The novel thermoplastic elastomers, based on either 50/50 parts or other ratios of EVM/EVA28, all exhibit superior physical properties including very long elongation at break, excellent elasticity, outstanding oil resistance, and reprocessability. We expect that such TPV material serves as a new high-performance oil12620
dx.doi.org/10.1021/ie401853k | Ind. Eng. Chem. Res. 2013, 52, 12613−12621
Industrial & Engineering Chemistry Research
Article
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