Novel heat and oil-resistant thermoplastic vulcanizates based on ethylene-vinyl acetate rubber/poly(vinylidene fluoride)

Abstract

Thermoplastic vulcanizates (TPVs) combine the excellent elasticity of conventional vulcanized rubbers and the easy processability and recyclability of thermoplastics. In this study, we successfully prepared novel heat and oil-resistant TPVs based on ethylene-vinyl acetate rubber (EVM) and poly(vinylidene fluoride) (PVDF) by dynamic vulcanization (DV). The phase morphology, morphological evolution, and the properties of the EVM/PVDF TPVs were studied, and the microstructure–property relationship during DV was revealed. Interestingly, a large number of EVM rubber nanoparticles are observed in the EVM/PVDF TPVs during DV, and these nanoparticles assemble into oriented EVM fibers and EVM bundles during DV. As the DV further proceeds, the oriented EVM fibers and EVM bundles agglomerate with one another more densely and further assemble into EVM spherulites, leading to an increase in the thickness of the PVDF ligaments and the deterioration of the rubber network. More importantly, our EVM/PVDF TPVs show good mechanical properties, high elasticity, good processability, excellent heat-and oil resistance, and good recyclability. This study provides guidance for the preparation of new TPVs that can replace the traditional thermosetting rubbers in automotive and oil pipeline areas.

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1 Introduction

Thermoplastic vulcanizates (TPVs), consisting of selective crosslinked rubbers as a dispersed phase throughout a continuous thermoplastic matrix under intensive mixing, are a special kind of high performance thermoplastic elastomer (TPE) prepared by dynamic vulcanization (DV).^1,2 Because of their unique structure, TPVs combine the excellent elasticity of conventional vulcanized rubbers and the good melt processability and easy recyclability of thermoplastics.^3–5 Thus, TPVs have attracted considerable attention as “green” polymers to replace the unrecyclable petroleum-based thermoset rubbers and have become one of the fastest growing rubbers in recent years. They have been widely used in the automotive industry, building and construction, and electronics, etc.^6–8 Currently, more and more studies have focused on the development of various kinds of TPVs because of resource saving and environmental protection requirements.

The most important properties of TPVs include their mechanical, elastic and rheological properties, all of which are dominated by the microstructure of the TPVs, i.e., the content and crosslinking degree of the rubber phase, the size and size distribution of the rubber phase and the corresponding rubber network structure.^9–11 Usually, a high-content (60 to 80 wt%) of the rubber phase is required to achieve good elasticity in TPVs. In this case, a continuous rubber phase in the premix before DV is usually obtained. Nevertheless, a continuous plastic phase and dispersed rubber phase are required to achieve easy processability and recyclability of TPVs. Therefore, one of the keys to preparing TPVs is the phase inversion of the rubber phase from a continuous phase (in premix) to a dispersed phase (in TPVs). On the other hand, the size of the dispersed rubber phase plays a key role in the mechanical properties and elasticity of TPVs. Usually, a smaller size of the rubber phase facilitates the preparation of TPVs with better mechanical properties and elasticity. Therefore, a plastic phase and rubber phase with a good compatibility are a better choice for the preparation of TPVs because of the formation of smaller rubber particles.^12,13

The microstructure and morphological evolution of TPVs during DV have attracted attention.^12,14 Many previous studies reported that the rubber phase was elongated and broken into rubber microparticles with diameters of 0.5–3.0 μm dispersed in the matrix, leading to a phase inversion.^1,15–17 Recently, we revealed that the dispersed ethylene propylene diene rubber (EPDM) microparticles were actually agglomerates of EPDM nanoparticles through theoretical calculation and experimental verification.¹⁸ The phase inversion of the EPDM/polypropylene (PP) blend during DV was dominated by the formation and agglomeration of rubber nanoparticles.^14,18 We also found that TPVs based on different blend systems, such as bromo-isobutylene-isoprene rubber (BIIR)/PP and BIIR/polyamide 12 (PA12), showed different mechanisms for morphological evolution and structure–property relationships than that of the EPDM/PP TPVs.^12–14

The first commercialized TPV product was the EPDM/PP TPV developed in 1981 by Monsanto (ExxonMobil AES).¹⁵ Until now, EPDM/PP TPVs are still the most well-known TPV products, and they have been commercially used in various fields such as the automobile, building and electronic industries. However, the poor oil resistance of EPDM/PP TPVs restricts their application in fields requiring a resistance to oil, such as automotive and oil pipeline fields.¹⁹ In recent years, heat and oil resistant TPVs, such as polydimethylsiloxane (PDMS)/PA12 TPVs, ethylene acrylic elastomer (AEM)/PA12 TPVs, and carboxylated acrylonitrile butadiene rubber (XNBR)/PA12 TPVs, have attracted considerable attention.^19–22

In this study, we developed novel heat and oil resistance TPVs based on ethylene-vinyl acetate rubber (EVM) and poly(vinylidene fluoride) (PVDF). EVM with a vinyl acetate content of 40–80 wt% is a special rubber widely used for heat and oil-resistant materials because of its excellent mechanical properties, high oil resistance and high aging resistance.²³ PVDF is a technologically important polymer because of its good mechanical properties, thermal stability, and excellent heat and oil resistance. More importantly, EVM and PVDF show excellent compatibility, facilitating the achievement of a smaller size rubber phase dispersed in a plastic phase, which results in better mechanical properties and elasticity.^24,25 Thus, in this study, we attempted to prepare new EVM/PVDF TPVs with fine phase morphology, good mechanical properties, and excellent heat and oil resistance that can replace the traditional thermosetting rubbers in automotive and oil pipeline areas. Second, we aimed to reveal the mechanism of the morphological evolution of the TPVs and the microstructure–property relationship of the EVM/PVDF TPVs during DV. As far as we know, the morphological evolution and the microstructure–property relationship of heat and oil resistance TPVs during DV has not yet been reported.

2 Experimental

2.1 Materials

PVDF (Kynar®720) was supplied by Arkema (France). EVM (Levapren® 700HV) with a vinyl acetate content of 70 ± 1.5 wt% and a Mooney viscosity ML (1 + 4) at 100 °C of 27 ± 4 was supplied by Lanxess (Germany). Rhenogran PCD-50/EVA, which was used as an anti-hydrolysis agent, was supplied by Rhein Chemie (Germany). Dicumyl peroxide (DCP) (98%) was obtained from Akzo Nobel Polymer Chemicals. 1,3,5-Tri-2-propenyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TAIC) (70%) was supplied by Huaxing (Suqian, China) Chemical Co., Ltd. A commercially available pentaerythritol tetrakys 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate (1010) was used as an antioxidant.

2.2 Preparation of the samples

EVM/PVDF TPVs were prepared in a Haake Rheomix 600 OS internal mixer (Thermo Fisher Scientific, USA) equipped with two counter-rotating rotors. First, EVM and PVDF (65/35 wt%) were melt blended in the Haake Rheomix at 180 °C and 80 rpm for 5 min; meanwhile, the antioxidant was added to prevent aging of the plastic and rubber. Subsequently, the cooled-down homogeneous blend was transferred to a two-roll mill at ambient temperature, and the crosslinking agents and the co-agent were added. The premix was then fed into the Haake Rheomix at 180 °C and a rotor speed of 80 rpm. The composition of the premix is given in Table 1.

Table 1 Composition of the premix

Ingredient	Content of the ingredients (phr)
EVM	100
PVDF	53.8
PCD-50	6
1010	1
DCP	1
TAIC	1.3

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To investigate the microstructure and properties during the dynamic vulcanization process, six samples (designated as samples A–F) were taken at different mixing times according to the torque–time curve and were immediately cooled in liquid nitrogen to stop the crosslinking reaction and avoid further morphological changes.

2.3 Characterization

2.3.1 Volume swell ratio measurements. The reciprocal volume swell ratios (1/Q) of samples A to F were measured by the equilibrium swelling method to give a qualitative indication of the crosslinking degree of the EVM phase, which has been described in our previous studies.¹⁴ A high 1/Q value represents a high crosslinking degree.

2.3.2 Morphology studies. Nanoscope IIIa peak force tapping atom force microscope (PF-AFM) (Bruker, Germany) was used to investigate the morphologies of samples A to F. Before morphological observation, the samples were polished using a cryo-ultramicrotome (Leica EM UC7, Germany) equipped with a glass knife at −90 °C. The distribution of the rubber particle size was determined with the Image-Pro Plus 4.5 software.

2.3.3 Disintegration experiments. Disintegration experiments were carried out by immersing approximately 30 mg of each sample in hot N,N-dimethylformamide (DMF) at 145 °C to soften and dissolve PVDF in DMF for 80 h or less, and photographs of the final state of the samples after immersion were taken. As the DMF can only dissolve PVDF, the results can be used to indicate the occurrence of phase inversion during DV. In order to observe the size and shape of the individual rubber particles, a suspension of the totally dissolved sample F was dropped onto a mica slice, and the mica slice was observed under the PF-AFM. The size of the crosslinked rubber particles was determined with the Image-Pro Plus 4.5 software.^12,18

2.3.4 Rubber process analysis (RPA). The rubber process analyzer (RPA 2000, Alpha Technologies, USA) was used to measure the rubber networks of samples C to F.^12,14 The strain scan conditions were 200 °C, 0.2 Hz, and a strain range of 1–400%. Before the analysis, the samples were preheated at 200 °C for 5 min.

2.3.5 Surface tension and interfacial tension measurements. An optical contact angle meter (Kruss DSA 100, Germany) was used to measure the static contact angles (θ) between a sample (EVM or PVDF) film and a drop of liquid (water and ethylene glycol) at ambient temperature.²⁶ The EVM films were statically vulcanized with different curing times of 0 min, 2 min, 5 min, 10 min and 20 min to reflect the various crosslinking degrees of the rubber phases in the EVM/PVDF TPVs.

The surface tension (γ) and its associated dispersion component (γ^d) and polar component (γ^p) were calculated according to the Ownes–Wendt–Rabel–Kaelble (OWRK) method and Young's equation.²⁷ The interfacial tension (γ₁₂) between the EVM and PVDF was calculated by the famous harmonic mean eqn.²⁸ Moreover, the adhesion work ω_a between the two phases was obtained from eqn (1):

ω_a = γ₁ + γ₂ − γ₁₂ (1)

2.3.6 Dynamic mechanical analysis (DMA). Dynamic mechanical analysis was carried out with a Dynamic Mechanical Analyzer (Metravib, VA3000) with a tension mode. Dynamic loss (tan δ) was determined at a frequency of 1 Hz and a heating rate of 3 °C min⁻¹ from −100 to 100 °C.

2.3.7 Rheological measurements. The rheological characterization of samples C to F was investigated using a capillary rheometer (model RH 2000, Malvern instruments Ltd.) at 200 °C under the single-bore experiment mode and a shear rate range from 20 s⁻¹ to 1000 s⁻¹. The L/D ratio of the capillary was 16/1 and 0, respectively. Each flow curve was a result from data collected at eight different shear rates during the experiment.

2.3.8 Mechanical properties. The tensile testing of the TPVs (samples C to F) was measured according to ASTM D412 and the elasticity of the TPVs (samples C to F) was studied by strain recovery test at 200 mm min⁻¹.¹² Strain recovery tests were first extended to 50% elongation, and then the tensile force was zero; the residual strain is defined as the permanent set. The hysteresis loss at 50% elongation was calculated by subtracting the area under the force–retraction curve from the area under the stress–strain curve.^12,29 For each measurement, six to nine samples were tested at ambient temperature.

2.3.9 Oil resistance. The oil resistance of the samples was determined by immersing the samples in IRM903 oil (Changxing Philippine refined petroleum products co., Ltd., China) at 125 °C for 72 h, according to GB/T 1690-2010.

The mechanical properties after the oil resistance test were measured according to ASTM D 412, which has been mentioned. Rectangular test samples of 25 × 25 × 2 mm³ were used to test the change in the mass and volume. The weights of the samples were measured before and after the oil resistance experiment, and the percent changes of mass and volume were calculated following eqn (2) and (3):

where Δm is the change in mass (%), m₁ is the initial mass of the specimen in air before the oil resistance experiment (g), and m₂ is the mass of the specimen in the air after the oil resistance experiment (g).where ΔV is the change in volume (%), m₃ is the mass of the specimen in water before the oil resistance experiment (g) and m₄ is the mass of the specimen in water after the oil resistance experiment (g).

2.3.10 Recyclability study. One of the major advantages of TPVs is their ability to be recycled without a significant deterioration in the mechanical properties. In order to test the recyclability of the EVM/PVDF TPVs, the samples were cut into small pieces, remixed in the Haake Rheomix at 180 °C for 2 min and sheeted out at 200 °C for 10 min under a pressure of 15 MPa to form tensile sheets. This process was recorded as 1 time. Dumbbell-shaped samples were cut off from the sheets to test the tensile strength and tensile recovery. For each measurement, six to nine samples were tested at ambient temperature. The recycling was repeated four times.

3 Results and discussion

3.1 Variations in the crosslinking degree of the EVM rubber phase during DV

The crosslinking degree of the rubber phase plays a significant role in the morphology development of TPVs during DV. To investigate the mechanism for the morphological evolution of the EVM/PVDF TPVs, we measured the development of the crosslinking degree in the rubber phase during DV. Six samples (from A to F) were chosen based on the torque–time curve. These samples were frozen in liquid nitrogen immediately to prevent a change in morphology before morphology observations using the PF-AFM. The variations in the torque and temperature as a function of mixing time during DV of the EVM/PVDF blend in the Haake Rheomix are shown in Fig. 1. At the initial stage, the torque decreases to a minimum at point A with an increase in the mixing time because of melting of the blend, and the torque rises dramatically until it reaches peak B because of the rapid crosslinking of the rubber phase. Then, the torque largely decreases until point C because of the phase inversion. The torque is constant from point D until the end of the DV because of the breakage and aggregation of the crosslinked rubber particles.

Fig. 1 Variations in the torque, temperature, and reciprocal volume swell ratio 1/Q with mixing time during DV for EVM/PVDF blends in Haake Rheomix.

The variations in the crosslinking degree of the rubber phase in the dynamically vulcanized EVM/PVDF blends were represented by the reciprocal volume swell ratio (1/Q) of the six samples collected at points A to F, and the results are summarized in Fig. 1.^12,30 The crosslinking degree of the rubber phase increases rapidly at the early stage of DV (A to C), increases slightly from C to D, and then reaches a plateau in the late stage of DV (D to F), indicating that the crosslinking of the rubber phase mainly occurs in the early stage of DV in the EVM/PVDF TPVs. The variations in the crosslinking degree during DV are similar to that seen in dynamically vulcanized EPDM/PP blends and BIIR/PP blends.^12,14

3.2 Phase morphology and morphological evolution of EVM/PVDF TPVs during DV

To study the morphology evolution of EVM/PVDF TPVs during DV, the phase morphologies of samples A to F collected at different mixing times were obtained using PF-AFM, as shown in Fig. 2. At the initial stage of DV (sample A), PVDF is dispersed in the EVM matrix with an elongated fiber structure (see Fig. 2(a)) because of the much lower content of the PVDF phase than the EVM phase (see Table 1). A co-continuous phase structure is observed (see Fig. 2(b)) in sample B, which is ascribed to the coalescence of the rubber phase at low crosslinking degrees and the coalescence of the PVDF phase caused by a decrease in the viscosity at higher temperatures (see Fig. 1). With the increase in the degree of crosslinking, the EVM rubber phase forms oriented fibers (see Fig. 2(c)) again. These EVM fibers assembled into EVM bundles or EVM spherulites. In this case, the continuous EVM rubber phase transforms into a dispersed phase because the highly crosslinked rubber particles cannot coalesce, indicating the occurrence of a phase inversion, which was demonstrated by the disintegration experiment (see below Fig. 4). As the DV proceeds, the EVM fibers more densely agglomerate with one another and form a large number of EVM spherulites (see Fig. 2(d)–(f)). The size of the EVM spherulites and the thickness of the PVDF ligaments increase with an increase in the DV time, leading to the deterioration of the rubber network structure. Meanwhile, the degree of orientation of the EVM fibers decreases with an increase in the DV time.

Fig. 2 AFM micrographs of the EVM/PVDF (65/35) samples (the darker regions represent the EVM phase and the lighter represent the PVDF phase): (a) sample A; (b) sample B; (c) sample C; (d) sample D; (e) sample E; (f) sample F; (c′) sample C; (d′) sample D; (e′) sample E; (f′) sample F. The scale bar of (a)–(f) is 500 nm, and the scale bar of (c′)–(f′) is 200 nm.

To see the morphology evolution of the EVM/PVDF TPVs during DV, the AFM images with a larger magnification for samples C to F were observed, as shown in Fig. 2(c′)–(f′). As expected, a large number of EVM rubber nanoparticles were observed in all the EVM/PVDF TPVs (samples C to F) and were similar to those observed in the crosslinked EPDM/PP and BIIR/PP blends. Interestingly, the EVM rubber nanoparticles were self-assembled into EVM fibers in the EVM/PVDF TPVs instead of the rubber spherulites observed in the EPDM/PP and BIIR/PP TPVs. As far as we know, this special fibrous phase morphology has not yet been reported in TPVs systems. On the other hand, the formation and self-assembly of rubber nanoparticles into EVM fibers lead to the occurrence of a phase inversion in the TPVs.

To further confirm the formation of rubber nanoparticles in the EVM/PVDF TPVs, the continuous PVDF phase in these TPV samples was dissolved and washed with N,N-dimethylformamide (DMF) to observe the size and morphology of the single crosslinked EVM particles. A typical AFM image of the dissolved samples dropped on a clean mica slice is shown in Fig. 3(a), and the corresponding diameter distribution of the rubber particles is summarized in Fig. 3(b). Obviously, the diameters of most of the rubber particles are in the range of 40 to 80 nm, demonstrating the formation of rubber nanoparticles in the EVM/PVDF TPVs.

Fig. 3 (a) AFM micrograph of dissolved sample F (the darker regions represent the rubber phase and the lighter regions represent the mica slice substrate) and (b) the corresponding diameter distribution of the rubber nanoparticles.

The dissolving experiments of samples A to F were carried out to further confirm the phase morphology and morphological evolution of the crosslinked EVM/PVDF blends because only the PVDF phase dissolves in DMF and the crosslinked EVM phase only swells in DMF. The photographs of the samples immersed in DMF at 145 °C for 80 h or less are shown in Fig. 4. Fig. 4(a) shows that sample A does not disintegrate in DMF even after 80 h, indicating that the EVM rubber phase is the continuous phase. Fig. 4(b) shows that sample B partially disintegrates into small pieces and sinks to the bottom, indicating a partial EVM rubber phase was inversed into a dispersed phase. Fig. 4(c) shows that samples C, D, E and F completely disintegrate in DMF after 70 h, 65 h, 55 h and 50 h, respectively, indicating that the phase inversion of the EVM rubber from a continuous phase to a dispersed phase is complete in these samples. These results demonstrate that the high content of the crosslinked EVM phase is dispersed in the low content of the PVDF continuous phase in samples C, D, E and F, which agrees well with the AFM results.

Fig. 4 Final states of samples immersed in DMF at 145 °C for 80 h or less: (a) sample A, not disintegrated; (b) sample B, partially disintegrated; (c) samples C to F, totally disintegrated.

RPA was used to confirm the variations in the rubber network of samples C to F, and the results are shown in Fig. 5. As previously reported, the dispersed rubber phase with a high crosslinking degree and elastic modulus can be regarded as a filler dispersed in a PVDF matrix.^12,14 The difference between the maximum and minimum storage modulus (G′) is a measurement of the filler network. A higher ΔG′ implies a stronger filler network. The variation of G′ as a function of strain from samples C to F are shown in Fig. 5. It can be seen that ΔG′ decreases during the DV process from samples C to F, indicating that the rubber network is deteriorating. The result is consistent with the decrease in the disintegration time from samples C to F reported in Section 3.2. The deterioration of the rubber network from sample C to F is mainly attributed to the increase in the size of the rubber agglomerates caused by the agglomeration of EVM single nanoparticles and EVM fibers.

Fig. 5 Storage modulus (G′) versus strain with a frequency of 0.2 Hz from samples C to F.

3.3 Properties of the EVM/PVDF TPVs

The special microstructure has significant effects on the properties of the EVM/PVDF TPVs, including the rheological, elastic and mechanical properties, and recyclability. Specifically, the morphology of the rubber phase, the rubber network structure, and the thickness of the plastic ligaments dominate the properties of the EVM/PVDF TPVs. We carefully studied the properties and the relationship between the microstructure and properties in the TPVs (samples C to F), and the results are shown in Fig. 6–9.

Fig. 6 Shear viscosity (η) as a function of shear rate for DV from samples C to F.

Fig. 7 (a) Stretching recovery curves and (b) variations in the permanent set and hysteresis loss from samples C to F.

Fig. 8 (a) Stress–strain curves and (b) variations of tensile stress, elongation at break and set at break for samples C to F.

Fig. 9 Tensile stress and elongation at break for sample F as a function of the recycle times.

The rheological property, which is dominated by the continuous plastic matrix, plays an important role in the melt processibility, recyclability and production efficiency of TPVs.¹² A low shear viscosity represents easy processibility, easy recyclability, and high production efficiency for TPVs. The rheological property of samples C to F was studied by using a capillary rheometer, and the results are shown in Fig. 6. The shear viscosity at a given frequency decreased from C to F, indicating improvement in the melt processibility of the TPVs with an increase in the DV time. This is ascribed to an increase in the thickness of the PVDF ligaments and the deterioration of the rubber network during DV due to the agglomeration of the rubber fibers, which was demonstrated by the disintegration tests and RPA results (see Section 3.2). The viscosities of all the samples decreased significantly with an increase in the shear rate, indicating the pseudoplastic nature of these TPVs. Importantly, all of these samples show easy processibility, facilitating the industrial applications of the EVM/PVDF TPVs.

The tensile recovery tests of samples C to F were carried out to investigate the elasticity of the samples, and the results are shown in Fig. 7(a). Hysteresis loss and permanent set are two important parameters that indicate the elasticity of elastomers, and they were summarized on the basis of the tensile recovery curves, as shown in Fig. 7(b). Low permanent set and hysteresis loss represent a high elasticity.^31,32 We can see that all the EVM/PVDF TPVs (samples C to F) exhibit good elasticity, as demonstrated by the low permanent set (11% to 17%) of the EVM/PVDF TPV samples, according to ASTM D1566-07a. The good elasticity of these TPV samples was caused by the high content and high crosslinking degree of the EVM phase. The good elasticity of these EVM/PVDF TPVs is a prerequisite for the production of TPVs with low hardness and high dynamic fatigue-resistance. Both the permanent sets and the hysteresis losses increase from samples C to F, indicating a decrease in the elasticity of the EVM/PVDF TPVs with increasing DV time. The slight decrease in elasticity from samples C to F is attributed to a slight deterioration in the rubber network caused by the agglomeration of single rubber nanoparticles and rubber fibers, as demonstrated in Section 3.2.

The stress–strain curves of samples C to F are shown in Fig. 8(a), and the corresponding tensile stress, elastic modulus and the elongation at break of samples C to F are shown in Fig. 8(b). Both the tensile strength and the elongation at break increased significantly from samples C to F, whereas the elastic modulus decreased slightly. The reason is that the oriented rubber fibers tend to agglomerate with increasing DV time (see Section 3.2), leading to a decrease in the oriented degree of the rubber fibers. Thus, sample C with a higher orientation degree shows a lower elongation at break but higher elastic modulus, whereas sample E and F show higher elongations at break but lower elastic modulus values. The tensile strength reaches nearly 20 MPa, and the elongation at break is greater than 400% for samples E and F, suggesting the excellent mechanical strength and toughness of the EVM/PVDF TPVs.

Currently, recyclability of materials has attracted more attention because of the requirements of environmental protection and resource saving. Thus, the recyclability of the EVM/PVDF TPVs (sample F) was investigated, and the change in the tensile stress and the elongation at break before and after recycling 0 to 4 times are shown in Fig. 9. We can see that both the tensile strength and the elongation at break of the EVM/PVDF TPVs after recycling for four times were only slightly changed, indicating that the mechanical properties of the TPVs are still good. Therefore, our EVM/PVDF TPVs showed good recyclability.

We successfully prepared EVM/PVDF TPVs with good mechanical properties, good elasticity, easy processibility and recyclability. In addition, the agglomerates were agglomerates of oriented rubber fibers (which were form by single rubber nanoparticles), the interfacial phase thickness increased, the mechanical and rheological properties increased, and the elasticity decreased during DV of the EVM/PVDF TPVs, indicating a structure–property relationship unlike the TPVs we had previously reported based on EPDM/PP and BIIR/PP, which also had the single nanoparticles.^12,14

3.4 Oil resistance

The oil resistance of the EVM, PVDF and EVM/PVDF TPVs was measured by immersing the samples in IRM 903 oil at 125 °C for 72 h. The changes in the parameters including mechanical properties, mass and volume of these samples after the immersing experiments are shown in Table 2. We can see that all the parameters of pure EVM change significantly after the immersing experiment. Thus, despite the high VA content in EVM, the oil resistance of EVM is still not high enough. The changes in the tensile strength, elongation at break, mass and volume of all the EVM/PVDF TPVs are much lower than that of the EVM but higher than that of the PVDF. Specifically, the changes in the mass and volume of all the TPVs were below 14% and 19%, respectively, and the changes in the tensile strength and elongation at break for all the TPVs were below −26% and −19%, respectively. Compared with other TPV systems in previous studies, the changes in the mechanical properties, mass and volume of the EVM/PVDF TPVs after an oil resistance experiment at a higher temperature (125 °C) were smaller, indicating excellent oil resistance of the EVM/PVDF TPVs at a high temperature.^7,22,23 The excellent oil resistance facilitates the wider application of TPVs, especially in the automotive and oil pipeline fields. The excellent oil resistance of the EVM/PVDF TPVs is mainly attributed to the PVDF matrix, which has a stronger polarity and higher crystallinity, as the temperature of the oil resistance experiment is far below the melting temperature of PVDF. Thus, it is very difficult for the oil permeate into the TPVs. Here, it should be noted that the changes in the tensile strength and elongation at break for samples E and F were smaller than that of samples C and D. This is because the phase morphology of the rubber in samples C and D is an oriented fibrous shape, which is metstable, whereas the phase morphology of the rubber in samples E and F is a spherulite shape formed by the assembly of fibrous EVM, which is more stable than that of samples C and D.

Table 2 Changes in tensile, elongation at break, mass and volume of the EVM, PVDF and TPVs before and after the oil resistant experiment (72 h@125 °C)

Sample	Change of tensile strength%	Change of elongation at break%	Change of mass%	Change of volume%
EVM	−42 ± 2	−17 ± 1	32.0 ± 0.1	37.3 ± 0.3
PVDF	—	—	0.1 ± 0.01	1.0 ± 0.02
Sample C	−25.6 ± 0.2	−18.2 ± 0.2	14.0 ± 0.3	18.5 ± 0.1
Sample D	−23.1 ± 0.2	−17.1 ± 0.3	13.7 ± 0.1	18.4 ± 0.2
Sample E	−21.9 ± 0.3	−10.2 ± 0.3	13.6 ± 0.1	18.7 ± 0.1
Sample F	−22.0 ± 0.1	−13.4 ± 0.2	13.7 ± 0.1	18.2 ± 0.2

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4 Conclusions

In summary, we have successfully fabricated novel heat and oil-resistant TPVs based on EVM and PVDF and carefully studied the microstructure and morphological evolution during DV and the properties of the TPVs. The results indicate that a large number of EVM rubber nanoparticles are formed in the EVM/PVDF TPVs during DV owing to the good compatibility between EVM and PVDF. These nanoparticles assemble into oriented EVM fibers and EVM bundles under the shearing and elongation effect during DV. The oriented EVM fibers and EVM bundles agglomerate with one another more densely and further assemble into EVM spherulites with a further increase in the DV time, resulting in an increase in the thickness of the PVDF ligaments and the deterioration of the rubber network. The phase inversion of the dynamically vulcanized EVM/PVDF blends during DV is dominated by the formation and agglomeration of rubber nanoparticles, which is consistent with those observed in the EPDM/PP and BIIR/PP TPVs. In addition, the as-prepared EVM/PVDF TPVs exhibit good processibility, high elasticity, good mechanical properties, excellent heat-and oil resistant properties, and good recyclability, facilitating the application of the EVM/PVDF TPVs in automotive and oil pipeline areas.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51525301 and 51521062) and the National Basic Research Program of China (Grant No. 2011CB606003) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19335h

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