Microstructure and properties of bromo-isobutylene–isoprene rubber/polyamide 12 thermoplastic vulcanizate toward recyclable inner liners for green tires

Abstract

We successfully prepared bromo-isobutylene–isoprene rubber (BIIR)/polyamide 12 (PA 12) thermoplastic vulcanizate (TPV) by dynamic vulcanization (DV), and studied the microstructure and properties of BIIR/PA 12 TPV toward recyclable inner liners for green tires, to reduce fuel consumption and carbon emissions. The as-prepared BIIR/PA 12 TPV exhibits good mechanical properties, good elasticity, easy processability and good gas barrier properties. More importantly, our BIIR/PA 12 TPV still exhibits good properties after recycling several times. The mechanism for the formation of the phase morphology and morphological evolution during dynamic vulcanization, and the microstructure–property relationship of BIIR/PA 12 TPV were thoroughly studied to provide guidance for the preparation of high-performance BIIR/PA 12 TPVs toward tire inner liners. As the DV proceeded, the size of the dispersed BIIR particles and the thickness of the PA 12 ligaments decreased, leading to the strengthening of the rubber network. Although the rheological properties slightly deteriorated, the elasticity, the mechanical properties and the gas barrier properties of the TPVs were obviously improved as the DV proceeded.

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1. Introduction

Thermoplastic vulcanizates (TPVs), as a special class of thermoplastic elastomers (TPEs), combine the good elasticity of traditional thermoset crosslinked rubbers and the good melt processability and recyclability of thermoplastics.^1–4 The unique properties of TPVs depend on their microstructure, formed by a special kind of polymer reactive blending technique, dynamic vulcanization (DV).^4,5 During DV, a high-content rubber phase is selectively crosslinked at high temperature and broken up simultaneously into a dispersed phase in a low-content of thermoplastic continuous phase under shear and mixing.^1,6 Because of the requirements of resource saving, environmental protection and sustainable development, TPVs have attracted much attention as typical “green” polymers to replace the unrecyclable petroleum-based thermoset rubbers and have been widely used in industries such as automobile, construction, and electronics.^7,8

Usually, the most important properties of TPVs are the rheological property, the elasticity and the mechanical property, all of which depends on the microstructure of TPVs. Specifically, the content and crosslinking degree of the rubber phase,⁹ the size and size distribution of the rubber phase¹⁰ and corresponding rubber network structure,¹¹ the thickness of the plastic ligaments¹² and the compatibility between plastic and rubber^13–15 play key roles in the properties of TPVs. Technically, a high-content (60–80 wt%) rubber phase is needed to ensure the good elasticity of TPVs, leading to a continuous rubber phase in premix before DV. However, a continuous plastic phase and dispersed rubber phase is required to obtain good melt processability. Thus, the phase inversion of rubber phase from continuous phase (in premix) to dispersed phase (in TPV) is the key to obtain TPVs.^16–18 Therefore, the phase morphology and morphological evolution of TPVs during DV have attracted much attention.^19–23 Most previous studies on TPVs were focused on ethylene-propylene-diene terpolymer (EPDM)/polypropylene (PP) TPV^24–26 because the EPDM/PP TPV is the most widely-used industrialized TPV products nowadays.

Isobutylene–isoprene rubber (IIR) and polyamide (PA) are both well known for their good barrier property as homopolymers.²⁷ TPVs prepared by these two homopolymers find application in tire inner liners because IIR/PA TPVs exhibit good barrier property. Tire inner liners prepared by IIR/PA TPVs are expected to present lower inflation pressure retention,²⁸ and thus can reduce fuel consumption and carbon emission of automobiles. Compared with tire inner liners prepared by traditional thermoset IIR, tire inner liners prepared by IIR/PA TPVs can be much thinner/lighter because of their much better gas barrier property. Meanwhile, tire inner liners prepared by IIR/PA TPVs show better durability and can survive for longer service life, as claimed by Exxonmobil company.²⁹ Predictably, IIR/PA TPVs instead of traditional thermoset IIR are promising as inner liners for green tire because of high-gas-barrier and light-weight. A few previous studies^30–32 and many patents^33–35 have been focused on the preparation, formula and mechanical property of IIR/PA TPVs used for gas-barrier tire inner liner. Nevertheless, to the best of our knowledge, studies on the phase morphology, morphological evolution during DV and the corresponding mechanism, the microstructure–property relationship, and the recyclability of IIR/PA TPVs toward the tire inner liners have not been reported yet.

Herein, we focused our research on the preparation, microstructure and properties, especially the gas barrier property and the recyclability of bromo-isobutylene–isoprene rubber (BIIR)/polyamide (PA 12) TPV toward the high-performance tire inner liners. We deeply studied the phase morphology and morphological evolution of BIIR/PA TPV during DV. The variation of the crosslinking degree of the rubber phase, the phase inversion, the size of the dispersed rubber phase and the rubber network structure during DV were studied. The properties including the mechanical property, elasticity, rheological property, gas barrier property and the recyclability required for tire inner liners were studied. The mechanism for the formation of phase morphology and the microstructure–property relationship of the BIIR/PA 12 TPV were discussed to provide guidance for the preparation of high-performance BIIR/PA 12 TPV for its application in tire inner liners.

2. Experimental

2.1. Materials

BIIR 2030 with Br content of 1.8 ± 0.2 wt% was supplied by Lanxess, Germany. PA 12 (3030 JI5) with melting temperature of 173 °C was supplied by UBE, Japan. PP HP500D was supplied by Basell, Thailand and EPDM 3092 was supplied by Mitsui Chemicals, Japan, which were the same with that in EPDM/PP TPV as we previously reported.²⁰ Pentaerythritol tetrakis 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1010) was used as an antioxidant, and ZnO and N,N-meta-phenylene bis maleimide (HVA-2) were used as crosslinking agents. All of the chemicals used were commercial products.

2.2. Sample preparation

The BIIR/PA 12 TPV was prepared in a Haake Rheomix 600 OS internal mixer (Thermo Fisher Scientific, USA) equipped with two counter-rotating rotors. PA 12 was dried in a vacuum oven at 60 °C for 12 h before used and the BIIR/PA 12 premix was prepared with the same method as we described in our previous study:²⁰ BIIR and PA 12 were first melt-blended with a mass ratio of 65/35 and then the curing agents were added into the cooled-down premix on an open mill, resulting in a continuous BIIR phase because of the high content of BIIR. The blend was dynamically vulcanized in the Haake Rheomix at 180 °C with a rotor speed of 70 rpm. Six samples (designated as A, B, C, D, E and F) with various degrees of vulcanization were selected at various DV time according to the torque–time curve.

The unvulcanized BIIR/PA 12 blend and unvulcanized EPDM/PP blend both are also prepared in the Haake Rheomix at 180 °C with a rotor speed of 70 rpm. The blend was cooled down immediately by liquid nitrogen. The mass ratio of rubber/plastic is 30/70.

The BIIR blend was prepared on an open mill at room temperature. The compositions of the BIIR/PA 12 TPV, the unvulcanized BIIR/PA 12 blend, the unvulcanized EPDM/PP blend and the BIIR blend are shown in Table 1.

Table 1 Compositions of BIIR/PA 12 TPV, unvulcanized BIIR/PA 12 blend, unvulcanized EPDM/PP blend and BIIR blend

Ingredients	BIIR/PA 12 TPV	Unvulcanized BIIR/PA 12 blend	Unvulcanized EPDM/PP blend	BIIR blend
BIIR	100	30	0	100
PA 12	54	70	0	0
EPDM	0	0	30	0
PP	0	0	70	0
1010	1	1	1	0
ZnO	1.2	0	0	1.2
HVA-2	2.2	0	0	2.2

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2.3. Characterization

2.3.1. Volume swell ratio measurements. The reciprocal volume swell ratio (1/Q) was measured to characterize the crosslinking degree³⁶ and high 1/Q value refers to high crosslinking degree. The volume swell ratios of samples A to F were measured by using the method described in our previous study and calculated according to eqn (1):²¹where V_p and V are the volumes of the sample before and after swelling, respectively; m and m_d are the mass of the swelled sample and that of the dried sample, respectively; and ρ_d and ρ_s are the densities of the dried polymer sample and the solvent, respectively.

2.3.2. Morphology studies. The morphologies of samples A to F were observed under a Nanoscope IIIa peak force tapping atom force microscope (PF-AFM) (Bruker, Germany). The samples were first polished by using a cryo-ultramicrotome (Leica EM UC7, Germany) equipped with a glass knife at −100 °C and the particle size (distribution) was determined with Image-Pro Plus 4.5 software based on the AFM graphs. For each sample, more than 50 particles in at least 5 different AFM pictures was examined to obtain statistically meaningful results and the diameter of an ellipse-like particles is calculated as the average value of the minor and major axes. The number-averaged diameter (d_n), the volume-averaged diameter (d_v), the polydispersity index (PDI) and the interparticle distance (ID_poly) were calculated as we described in our previous study.^10,37

The morphology of unvulcanized BIIR/PA 12 blend was observed under an S-4700 scanning electron microscope (SEM) (Hitachi, Japan). The sample was first polished and then immersed into cyclohexane at room temperature for 30 min in order to etch the BIIR. After that, the sample was coated with a thin layer of gold before observation.

The morphology of unvulcanized EPDM/PP blend was observed under an H-800-I transmission electron microscopy (TEM) (Hitachi, Japan). The sample was first cryo-microtomed into 100 nm-thick sections at −130 °C and then vapor stained with ruthenium tetroxide for 25 min.

2.3.3. Disintegration tests. Disintegration tests were carried out to investigate the phase inversion during DV. Samples with a mass of about 30 mg each and similar shapes were immersed in hexafluoroisopropanol (HFIP) at room temperature for 24 h or less, and photographs of the final states of the samples after immersion were taken. As PA 12 can be dissolved in hexafluoroisopropanol (HFIP) while BIIR can't, the totally disintegration of the samples indicates the completing of phase inversion. The solvent of the dissolved sample F was coated onto a silicon slice and then observed by SEM.

2.3.4. Rubber network and rheological behavior measurements. The rubber network of the BIIR/PA 12 samples were measured by a rubber process analyzer (RPA 2000, Alpha Technologies, USA).²¹ A strain sweep from 1% to 1255% was carried out at 180 °C and a frequency of 0.2 Hz and the samples were pre-heated up to 180 °C and kept for 5 min.

The rheological property of samples was also investigated by the RPA and a frequency sweep from 0.01 to 33.3 rad s⁻¹ was performed at 180 °C and a strain of 0.98%. The samples were pre-heated up to 180 °C and kept for 5 min.

2.3.5. Tensile and tensile recovery tests. The mechanical property of the samples was studied by tensile tests on a tensile tester (SANS CMT4204, China) at a crosshead speed of 500 mm min⁻¹ and the elasticity of the samples was studied by tensile recovery tests at 200 mm min⁻¹. Prior to the tests, samples were compression molded into 1 mm films and then punched to 25 × 6 × 1 mm³ dumbbell-shaped tensile bars. Samples were first deformed up to a total strain of 50% and then the tensile force was relaxed to zero during tensile recovery tests, as both the rubber and the matrix of the TPV deform elastically at low strains (≤50%) and the corresponding deformation was almost recovered completely.³⁸ The residual strain is defined as the permanent set. The hysteresis loss at 50% deformation was calculated by subtracting the area under the force–retraction curve from the area under the force–deformation curve.³⁹ At least three tests were carried out for each sample and the median value was used. The measurements were performed at room temperature.

2.3.6. Gas permeability tests. Gas permeability tests were carried out on a gas permeability instrument (SP-2100A, China) at 40 °C with N₂ atmosphere (0.57 MPa). The 13 × 11 × 1 mm³ films were first dried for 24 h. The gas permeability of pure PA 12 and statically vulcanized BIIR were also measured, and the dosage of crosslinking agents in BIIR vulcanizate is the same as that in the BIIR phase in BIIR/PA 12 TPV. The gas permeability of samples with different recycle times was also measured. At least three tests were carried out for each sample and the median value was used.

2.3.7. Recyclability. The changes in mechanical property and gas permeability of sample F after different recycle times are used to measure the recyclability of the BIIR/PA 12 TPV. In order to simulate the recycle condition, the sample F after tensile test was collected and stored at room temperature with a constant humidity of 50% for 16 h, and then dried in a vacuum oven at 60 °C for 8 h. The dried sample F was melt-blended in the Haake Rheomix at 180 °C for 2 min and compression molded into 1 mm film again. The mechanical property and gas permeability of samples with different recycle times were measured. All the tests were performed at room temperature under dry conditions. At least three tests were carried out for each sample and the median value was used.

3. Results and discussion

3.1. Variation of crosslinking degree of rubber phase during DV

The crosslinking degree of the rubber phase has a significant effect on the phase morphology of TPVs. To study the variation of crosslinking degree during DV, six samples were selected according to the torque–time curve and the temperature–time curve, representing the crosslinking of the rubber phase and the morphological evolution of TPV. At the initial stage, the blend melted quickly and correspondingly the torque decreases dramatically to A and further decreases to a minimum at B, where sample A represents the initial state of the BIIR/PA 12 blend and sample B represents the completion of the melting. With the rapid vulcanization of the BIIR phase, the torque rises largely until a peak at C and then declines, ascribed to the phase inversion.⁴ With the further increase in DV time, the torque declines to D and then levels off. Sample E was selected at 15 min (mixing time) and sample F was selected at 20 min.

The torque–time curve of pure BIIR mixed with crosslinking agents at 180 °C during static vulcanization was shown in Fig. 1, indicating the crosslinking progresses of the BIIR with time at 180 °C. The amount of the crosslinking agents in the BIIR/crosslinking agents mixture is the same with that in the BIIR phase in the BIIR/PA 12 TPV. It can be seen that the torque of the BIIR almost keep constant before 2.5 min during static vulcanization, indicating that the crosslinking degrees of the BIIR phase in sample A and B at the dynamic vulcanization time of 0.75 min and 2.5 min (with temperatures lower than 180 °C) are low. The torque increases rapidly with the time increases from 2.5 min to 12 min during static vulcanization in Fig. 1, indicating that the crosslinking degree largely increase in this stage. The torque reaches a plateau with the time further increases, indicating that the crosslinking degree levels off. The variation of crosslinking degree of the BIIR phase in BIIR/PA 12 TPV during DV was characterized by the reciprocal volume swell ratios 1/Q³⁶ of samples, and the results are shown in Fig. 2. A high 1/Q value represents a high crosslinking degree. It can been seen from Fig. 2 that the crosslinking degree increases rapidly at the early stage of DV (A to C) and increases slightly from C to D, and then reaches a plateau at the late stage of DV (D to F), indicating that the crosslinking of rubber phase mainly occurs at the early stage of the DV process, consistent well with that reported in previous studies.^20,21

Fig. 1 Torque–time curve of pure BIIR mixed with crosslinking agents at 180 °C during static vulcanization.

Fig. 2 Variations of torque, reciprocal swell ratio 1/Q and temperature with mixing time during DV for the BIIR/PA 12 blends.

3.2. Phase morphology and morphological evolution of blend during DV

To study the morphological evolution of the BIIR/PA 12 blend during DV, the phase morphology of samples A to F was investigated by using PF-AFM and the results are shown in Fig. 3. In the premix of BIIR/PA 12 and the blend at the initial state of DV, the PA 12 phase is dispersed in the continuous BIIR phase because of the low content of the PA 12 and the low crosslinking degree of the BIIR. Thus, the darker regions represent the PA 12 phase and the lighter regions represent the BIIR phase according to Fig. 3(a). The breakup of the BIIR phase at the early stage of DV is similar to that of the unvulcanized BIIR in the BIIR/PA 12 blend because of the low crosslink degree of the BIIR phase at the early stage of DV. In the unvulcanized BIIR/PA 12 immiscible blend, the breakup and coalescence of the BIIR phase occurred simultaneously under shear.²⁰ As the DV proceeds, the crosslinking degree of the BIIR phase increases rapidly (see Fig. 2), leading to the large increase in the viscosity of the BIIR phase.^16,17 Thus, some crosslinked BIIR phase could not coalesce, resulting in a co-continuous morphology as is shown in Fig. 3(b). As the further increase in the crosslinking degree, all the BIIR phase could not coalesce and is transformed into dispersed phase in the continuous PA 12 matrix in sample C, indicating the completion of the phase inversion, as shown in Fig. 3(c). Thus, the BIIR/PA 12 blend has been transformed in to a TPV which is of easy processability and recyclability because of the continuous PA 12 matrix. With the further increase in DV time, the size of the dispersed BIIR phase decreases from 4 μm in sample C (Fig. 3(c)) to 2 μm in sample F (Fig. 3(f)) and becomes more uniform, ascribed to the breakup of the BIIR phase under shear. In addition, we can see that most of the BIIR particles are of irregular shape, indicating that the BIIR particles are formed by the breakup of larger BIIR particles under shear during DV.⁴⁰ Interestingly, we can see that some PA 12 phase embedded in the dispersed crosslinked BIIR phase in samples C to F ascribed to the graft reaction of the PA 12 and the BIIR, as previously reported.⁴¹

Fig. 3 AFM micrographs of BIIR/PA 12 samples (the darker regions represent the PA 12 phase and the lighter regions represent the BIIR phase): (a) sample A; (b) sample B; (c) sample C; (d) sample D; (e) sample E; (f) sample F.

The size of the dispersed BIIR phase and the thickness of the PA 12 ligaments have significant effect on the rheological property, elasticity, mechanical property and gas barrier property of BIIR/PA 12 TPV. Thus, the number-averaged diameter (d_n), volume-averaged diameter (d_v) and polydispersity index (PDI) of the dispersed BIIR phase in sample C to F and the interparticle distance (ID_poly) were summarized, and the results are shown in Table 2. The ID_poly represents the thickness of the PA 12 ligaments between the dispersed BIIR particles. It can be seen that d_n, d_v, PDI and ID_poly all decrease with increasing DV time from C to F, suggesting that the density of the dispersed BIIR particles increases. The decrease of PDI implies that the BIIR particles become more uniform and the decrease of ID_poly implies the decrease of the thickness of the PA 12 ligaments.

Table 2 Size and size distribution of rubber phase in samples C to F

Samples	C	D	E	F
dn (μm)	4.1	2.9	2.2	2.0
dv (μm)	8.2	6.7	4.2	3.5
PDI	2.0	2.3	1.9	1.8
IDpoly (nm)	960	640	440	390

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To further confirm the phase morphology of the samples and the phase inversion during DV, disintegrating tests were carried out and the photographs of samples A to F immersed in hexafluoroisopropanol (HFIP) at room temperature for 24 h or less are shown in Fig. 4. It should be noted here that PA 12 can be dissolved in HFIP at room temperature within 1 h while BIIR does not dissolve in HFIP. Fig. 4(a) shows that sample A does not disintegrate even after 24 h, indicating that the PA 12 phase is the dispersed phase. Partial disintegration at the surface of sample B after immersion in HFIP for 24 hours was shown in Fig. 4(b), indicating that partial phase inversion has occurred in sample B. Samples C to F are completely disintegrated after immersion in HFIP for 5 h (sample C) to 6 h (sample F), respectively, indicating that the BIIR phase is dispersed in the continuous PA 12 phase and the complete phase inversion of the blend has occurred in sample C. The SEM micrograph of BIIR particles in samples F is shown in Fig. 4(g). We can see that these BIIR particles are all single microparticles with a diameter of about 2 μm, obviously different with the dispersed EPDM phase agglomerated by nanoparticles in EPDM/PP TPVs in our previous studies.^20,21 In addition, these BIIR microparticles are of irregular shapes, demonstrating that the BIIR particles are formed by the breakup of larger BIIR particles under shear during DV.⁴⁰ The disintegrating results are consistent well with the AFM results in Fig. 3. Besides, the increasing disintegration time of samples from C to F suggests that the rubber network of the samples is strengthened as the DV proceeds from C to F, consistent well with the AFM results.

Fig. 4 Final states of samples immersed in hexafluoroisopropanol (HFIP) at room temperature for 24 h or less: (a) sample A, not disintegrated; (b) sample B, partially disintegrated; (c to f) sample C to F, totally disintegrated; (g) SEM micrograph of BIIR particles in sample F.

In addition, a rubber process analyzer (RPA) was used to further demonstrate the change in the rubber network in the BIIR/PA 12 blend during DV. As previously reported, the BIIR particles with high crosslinking degree and elastic modulus in samples C to F can be regarded as fillers dispersed in PA 12 matrix,²⁰ and the phase inversion indicates that samples C to F are TPVs. Thus, the rubber network of samples C to F and neat PA 12 was measured by using RPA, and the results are shown in Fig. 5. The change between the maximum and the minimum of the storage modulus G′ (ΔG′) represents the rubber network and a higher ΔG′ implies a stronger rubber network.^42,43 It can be seen from Fig. 5 that ΔG′ increases as the DV proceeds from C to F, indicating the strengthening of the rubber network. The strengthening of the rubber network consists well with the AFM results and the disintegrating results, ascribed to the increase in the density of the dispersed BIIR phase and the decrease in the thickness of the PA 12 ligaments. Besides, the rubber network has significant effect on the rheological property, elasticity, and mechanical property of BIIR/PA 12 TPV as discussed later.

Fig. 5 Storage modulus (G′) versus strain with a frequency of 0.2 Hz for samples C to F and the neat PA 12.

3.3. Mechanism for the formation of phase morphology

The schematic illustration for the morphological evolution of the BIIR/PA 12 TPV during DV is shown in Fig. 6, where the yellow regions represent BIIR phase and the blue regions represent the PA 12 phase. At the initial state, the PA 12 phase is dispersed in the continuous BIIR phase because of the low content of the PA 12, as is shown in Fig. 6(i). The breakup and coalescence of the BIIR phase occur simultaneously under shear when the crosslinking degree of the BIIR phase is low. As the DV proceeds, the crosslinking degree of the BIIR phase increases rapidly, leading to the large increase in the viscosity of the BIIR phase. Because of the inhomogeneity of the crosslinking degree of the rubber phase, some rubber droplets with low crosslinking degree can still coalesce, whereas some rubber particles with high crosslinking degree cannot coalesce after broken up, resulting in a co-continuous morphology (see Fig. 6(ii)). With the further increase in the crosslinking degree, all the BIIR particles cannot coalesce, and thus the BIIR phase is transformed into the dispersed phase in the continuous PA 12 matrix, as shown in Fig. 6(iii). Therefore, the phase inversion in the BIIR/PA 12 TPV depends on the crosslinking and the breakup/coalescence of the BIIR phase, similar with that reported in EPDM/PP TPV.^20,21 During the subsequent DV process, these BIIR particles break up into smaller and irregular particles under shear. As a result, most of these BIIR particles have the diameter of 2 μm at the end of DV. The difference for the formation mechanism of the dispersed rubber phase in BIIR/PA 12 TPV from that in EPDM/PP TPV is that the dispersed EPDM microparticles in EPDM/PP TPV²⁰ are actually the agglomeration of EPDM nanoparticles formed by in situ crosslinking, whereas the dispersed BIIR microparticles in the BIIR/PA 12 TPV are formed by the breakup of the bigger BIIR microparticles under shear during DV. The minimum size of BIIR particles in BIIR/PA 12 TPV is of about 2 μm, whereas the minimum size of EPDM particles in EPDM/PP TPV is of 40–60 nm.

Fig. 6 Schematic illustration of morphological evolution of BIIR/PA 12 TPV during DV (the yellow regions represent the BIIR phase and the blue regions represent the PA 12 phase).

The difference in the formation mechanism of the dispersed rubber phase is ascribed to the different compatibility between rubber and plastic of the two kinds of TPVs, as demonstrated by the micrographs of the unvulcanized BIIR/PA 12 blend and the unvulcanized EPDM/PP blend with the same rubber/plastic ratio of 30/70, as shown in Fig. 7. Both the EPDM phase and the BIIR phase in the two unvulcanized blends are the dispersed phase because of the low content of the rubber phases. The smaller size of the dispersed rubber phase represents better compatibility between the rubber phase and the plastic phase. Although there is chemical reaction between the BIIR phase and the PA 12 phase,⁴¹ the size of the dispersed BIIR phase (1.8 μm) in Fig. 7(a) is much bigger than the dispersed EPDM phase (0.4 μm) in Fig. 7(b), demonstrating that the compatibility between the BIIR phase and the PA 12 phase is worse than that between the EPDM phase and the PP phase. Thus, the size of the dispersed BIIR phase in the BIIR/PA 12 TPV is not agglomerates of rubber nanoparticles while the dispersed EPDM phase is in the EPDM/PP TPV.²⁰

Fig. 7 (a) SEM micrograph of unvulcanized BIIR/PA 12 blend and (b) TEM micrograph of unvulcanized EPDM/PP blend (the holes and the darker regions represent the rubber phase and the lighter regions represent the plastic phase).

3.4. Properties of BIIR/PA 12 TPV and the microstructure–property relationship

The properties of the BIIR/PA 12 TPV including the rheological property, the elasticity, the mechanical property, and the gas barrier properties depend on the microstructure of the BIIR/PA 12 TPV. Specifically, the size of rubber phase,¹⁰ the rubber network structure,¹¹ and the thickness of the plastic ligaments¹² dominate the properties of the BIIR/PA 12 TPV.

3.4.1. Rheological property. The phase morphology and morphological evolution, especially the size of the dispersed rubber phase, have significant effects on the properties of TPVs.¹² Rheological property, dominated by the continuous plastic matrix, plays an important role in the melt processability, the recyclability and the production efficiency of TPVs. The rheological property during DV was studied by using RPA and the results of samples C to F are shown in Fig. 8. Low storage modulus (G′) and complex viscosity (η*) represents easy processability, easy recyclability, and high production efficiency of TPVs. It can be seen from Fig. 8 that both the complex viscosity (η*) and the storage modulus (G′) increase at a given frequency, and the increase of η* is slight, indicating that the rheological property of the BIIR/PA 12 TPV deteriorates slightly as DV proceeds from C to F. The slight deterioration of rheological property is ascribed to the decreasing in the thickness of the PA 12 ligaments and the strengthening in the rubber network caused by the decrease in d_n. Importantly, all these TPV samples C to F show easy processability, facilitating the industrial production of the BIIR/PA 12 TPV. In addition, compared with traditional thermoset BIIR used in tire inner liners, our BIIR/PA 12 TPV can save raw materials by recycling the leftover materials during production because of the good melt processability of TPVs.

Fig. 8 Storage modulus (G′) and complex viscosity (η*) versus angular frequency (ω) at 180 °C of samples C to F.

3.4.2. Elasticity. Elasticity caused by the high content and high crosslinking degree of the BIIR phase in TPV, has significant effects on the hardness and the dynamic fatigue-resistance property of TPV products. The tensile recovery tests of samples C to F were carried out to study the elasticity of the samples. The permanent set and the hysteresis loss summarized according to the tensile recovery curves are two important parameters to represent the elasticity of TPVs. Low permanent set and hysteresis loss represents high elasticity.^39,44 It can be seen from Fig. 9 that both the permanent set and the hysteresis loss decrease as the DV proceeds from C to F, indicating the increase in the elasticity. The decrease in the thickness of the PA 12 ligaments and the strengthening of the rubber network lead to the decrease in the stress required for bending or buckling,^44–47 and thus improving the elasticity of the BIIR/PA 12 TPV. In addition, according to ASTM D1566-07a, our BIIR/PA 12 TPV is elastomer with high elasticity because the permanent set of the BIIR/PA 12 TPV samples range from 15.0% to 17.8%. The good elasticity of TPVs can result in the low hardness and high dynamic fatigue-resistance of the TPV products. In addition, the elasticity of the BIIR/PA 12 TPV can be further improved by controlling the formula during production.

Fig. 9 (a) Tensile recovery curves and (b) variations of permanent set and hysteresis loss of samples C to F.

3.4.3. Mechanical property. Fig. 10 shows the stress–strain curves of samples C to F and the variations of the tensile stress and the elongation at break as the DV proceeds from C to F. It can be seen that the tensile stress increases from 14.9 MPa to 16.5 MPa and the elongation at break increases from 324% to 348% as the DV proceeds from C to F, indicating the improvement of mechanical property. The increase in the tensile stress and the elongation at break is ascribed to the decrease in d_n (the increase in the interfacial surface area of rubber phase) and the strengthening in the rubber network.¹² Compared with traditional thermoset BIIR products as previously reported,³¹ our BIIR/PA 12 TPV shows higher tensile stress due to the continuous PA 12 phase and lighter weight due to the reduction of reinforcing fillers. Thus, our lighter BIIR/PA 12 TPV used as tire inner liners can reduce the weight of the rubber, and subsequently reduce the fuel consumption and the carbon emission.

Fig. 10 (a) Stress–strain curves and (b) variations of tensile stress and elongation at break of samples C to F.

3.4.4. Gas barrier property. Gas barrier property of the tire inner liners has significant effect on the fuel consumption and the carbon emission of a car. Low gas permeability represents high gas barrier property and low pressure loss. And low pressure loss of tire inner liners represents low fuel consumption and carbon emission of automobiles. Fig. 11 shows the gas permeability of neat BIIR, neat PA 12 and TPV samples C to F. It can be seen that the gas permeability of samples decrease as the DV proceeds from C to F because of the decrease in d_n, indicating the improvement of the gas barrier property. As the d_n decreases, the density of the BIIR particles increases and the thickness of the PA 12 ligaments decreases correspondingly, leading to the increase in the area of the PA 12 ligaments. The increase in the area of the PA 12 ligaments facilitates the improvement of the gas barrier property.⁴⁸ The gas permeability of our BIIR/PA 12 TPV is lower than that of the neat BIIR, indicating that our BIIR/PA 12 TPV shows better gas barrier property than traditional thermoset BIIR. Therefore, our BIIR/PA 12 TPV facilitates the reduction of the pressure loss of tire inner liners and consequently the fuel consumption and carbon emission of automobiles.

Fig. 11 Gas permeability of neat BIIR, neat PA 12 and samples C to F.

3.4.5. Recyclability. Recyclability of the TPVs used in tire makes contributions to reduce the black pollution formed by the waste tires and thus can protect the environment because the tires made by traditional thermoset rubber materials are hard to be recycled. Meanwhile, recyclability of the TPVs used in tire can save resources. Thus, the recyclability of the BIIR/PA TPVs was studied. As an example, the recyclability of sample F after recycling for 4 times was investigated and the variations of the tensile stress, the elongation at break and the gas permeability are shown in Fig. 12. It can be seen that the tensile stress, the elongation at break and the gas permeability almost keep constant, suggesting that the properties of the BIIR/PA 12 TPV after recycling for 4 times are still good. Thus, our BIIR/PA 12 TPV shows good recyclability, facilitating the reduction of the black pollution formed by the waste tires.

Fig. 12 Variations of tensile stress, elongation at break and gas permeability of sample F after recycling for 4 times.

3.4.6. Discussion on the microstructure–property relationship. With the increase in DV time from C to F, the size of the dispersed BIIR particles in BIIR/PA 12 TPV decreases and the thickness of the PA 12 ligaments decreases (see Table 2). The decrease in d_n of the dispersed BIIR particles results in the increase in the density of the BIIR particles and the strengthening in the rubber network, facilitating the absorption of more energy of BIIR particles during the stretching of the TPV samples.¹² Thus, the decrease in d_n of the BIIR particles facilitates the increase in the tensile stress and the elongation at break. In addition, the strengthening of the rubber network and the decrease in the thickness of the PA 12 ligaments lead to the decrease in the stress required for bending or buckling of the PA 12 matrix, and thus increasing the elasticity of the BIIR/PA 12 TPV. Meanwhile, the decreases in d_n, the increase in the density of the BIIR particles and the thickness of the PA 12 ligaments leads to the increase in the area of the PA 12 ligaments, facilitating the increase in the gas barrier property of TPVs. On the other hand, the decrease in the thickness of the PA 12 ligaments and the strengthening in the rubber network results in the slight deterioration of rheological property of BIIR/PA 12 TPV.

4. Conclusions

We successfully prepared BIIR/PA 12 TPV by DV and studied the morphological evolution during DV and the properties of the BIIR/PA 12 TPV. The as-prepared BIIR/PA 12 TPV exhibits good mechanical property, good elasticity, easy processability and good gas barrier property. More importantly, our BIIR/PA 12 TPV still exhibits good properties after recycling for several times. The mechanism for the formation of the phase morphology and morphological evolution during dynamic vulcanization, and the microstructure–property relationship of BIIR/PA 12 TPV were deeply studied. The results indicate that the formation of the dispersed BIIR microparticles in the BIIR/PA 12 TPV depends on the breakup of the bigger BIIR microparticles under shear during DV. As the DV proceeds, the size of the dispersed BIIR particles and the thickness of the PA 12 ligaments in BIIR/PA 12 TPV decrease, leading to the increase in the density of the BIIR particles and the strengthening in the rubber network. As a result, the rheological property was slightly deteriorated, and the elasticity, the mechanical property and the gas barrier property of TPVs were obviously improved as the DV proceeded.

5. Acknowledgements

We gratefully acknowledge the National Science Fund for Distinguished Young Scholars of China (Grant No. 51525301), the National Basic Research Program of China (Grant No. 2011CB606003), the National Natural Science Foundation of China (Grant No. 51221002) and the Doctoral Science Research Foundation of the Education Ministry of China (Grant No. 20130010110005) for financial supports.

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