Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
pubs.acs.org/journal/ascecg
Mechanically Robust and Recyclable EPDM Rubber Composites by a Green Cross-Linking Strategy Ganggang Zhang,† Xinxin Zhou,*,† Kuan Liang,† Baochun Guo,*,‡ Xiaolin Li,† Zhao Wang,† and Liqun Zhang*,† †
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Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *
ABSTRACT: Covalently cross-linked rubbers are indispensable in many important fields due to their unique entropic elasticity. For rubbers cross-linking, the addition of toxic curing agents, release of toxic volatile organic compounds (VOCs), and recycling of waste rubber are three important issues. A combination of green curing chemistry and efficient recycling into commercial polyolefin rubbers is of great importance. Herein, we demonstrate a facile and promising way to incorporate dynamic covalent bonds into ethylenepropylene-diene monomer (EPDM)/carbon black (CB) composites. The epoxy-functionalized EPDM (e-EPDM) was prepared using an in situ epoxidation reaction and then cured with biobased decanedioicacid (DA) through the reactions between the epoxy groups in e-EPDM and the carboxylic groups in DA. Because of the existence of exchangeable β-hydroxyl ester, the covalently crosslinked networks in e-EPDM/CB composites were able to rearrange their topological structure at high temperature, endowing the composites with recycled and reshaped abilities. More importantly, the recycled e-EPDM/CB composites still exhibit outstanding mechanical properties which can meet the needs of practical applications. This strategy may provide an efficient, green, and sustainable way to address the problems brought from rubbers cross-linking. KEYWORDS: EPDM rubber, Epoxy functionalization, Green cross-linking, Vitrimer, Recycling
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INTRODUCTION Almost all rubber products, such as pneumatic tires, are manufactured through a curing process because this process can make linear rubber chains be cross-linked via covalent bonds to form a three-dimensional network for achieving high elasticity.1 Actually, the cross-linked rubbers are a class of thermosetting materials which are insoluble and infusible. Therefore, the covalently cross-linked rubbers are inherently difficult to reshape and recycle due to the irreversibility of the cross-linked networks, resulting in serious resource waste and environmental problems at their end of life.2,3 Today, there are some methods, such as desulfurization and pyrolysis, to make the cross-linked rubbers able to be recycled, and these recycling methods cause a deterioration of mechanical properties and are unfriendly to the environment.4 In addition to the above-mentioned problems, the sulfur curing system and peroxide curing system, which are the two most widely used curing techniques, have other environmental problems in the rubber industry. In sulfur curing, many of the curing additives are toxic or release some toxic substances.5 Since April 2004, the European Commission has classified ZnO as toxic to aquatic organisms and legislated that its application in rubber © 2019 American Chemical Society
technology must be controlled, according to the dangerous substances directive (2004/73/EC).6,7 Moreover, the accelerant diphenyl guanidine (DPG) is toxic, and the accelerant thiurams would generate nitrosamine which is a class of carcinogens.8 Furthermore, curing additives under heating tend to release toxic volatile organic compounds (VOCs) during the curing process. The VOCs give off unpleasant smells because of the existence of amines and sulfur-containing organic compounds (with very low odor thresholds).5 Similarly, the peroxide curing system such as commonly used dicumyl peroxide (DCP) generally produces unpleasant volatile decomposition products.9 Not only for the environmental problems but also for the workers health, the mentioned issues urgently need to be solved. To tackle the above serious issues, exploration of green and effective curing strategies for rubbers is of great importance. We have proposed that a new and green route was put forward for the cross-linking of polyolefin rubbers.10 First, the rubber Received: April 3, 2019 Revised: May 29, 2019 Published: June 6, 2019 11712
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ACS Sustainable Chemistry & Engineering
bonds, so they are also known to undergo oxidative degradation or cross-linking at elevated temperatures, leading to degraded performance or irreversible permanent cross-links. The irreversible permanent cross-links is a disadvantage to the recyclable nature of the cross-linked rubbers. However, the ethylene-propylene-diene monomer (EPDM) could be highly resistant to oxygen and heat because of its almost saturated backbone. EPDM could be the best choice to be used as the matrix material for constructing a vitrimer-like structure. Incorporating active fillers such as carbon black into rubbers is of significant commercial importance since the active fillers not only enhance the mechanical properties of the rubber products but also decrease the cost of the rubber products. Therefore, we attempt to find an easily processable and economical way to incorporate the active fillers into the vitrimeric elastomer to prepare vitrimeric composites. More significantly, we introduced dynamic covalent bonds into the vitrimeric composites by macromolecular design and chemical modification, aiming to prepare recyclable rubber materials with green cross-linking agents.40 Herein, we report a simple and green curing strategy to prepare elastomeric vitrimer composites from a commercial EPDM. First, the epoxy groups were introduced into EPDM chains using in situ epoxidation and serve as cross-linking sites. Then, the epoxy-functionalized EPDM (e-EPDM) was crosslinked by dicarboxylic acids, producing exchangeable βhydroxyl ester bonds. To realize the high mechanical performance, e-EPDM was reinforced by carbon black in an easily processable and economical way with filler contents up to 80 phr (parts per hundreds of rubber). In the presence of a catalyst (such as zinc acetate), the resulting covalent crosslinking networks can undergo network rearrangements via transesterification reactions at high temperatures and acquire reshaped and recycled abilities.
macromolecules with new functional groups rather than double bonds as cross-linking sites need to be synthesized or chemically modified. Then, the green organic molecules with di- or multifunctionality need to be developed as cross-linking agents instead of the sulfur system. Recently, epoxidized natural rubber (ENR) was cross-linked by dicarboxylic acids reacting with oxirane groups, producing β-hydroxy esters along with chains.11,12 In the presence of a catalyst (zinc acetate, Zn(Ac)2), the dicarboxylic acid-cross-linked ENR exhibited vitrimer-like properties; that is to say, the resulting cross-linked networks could relax stress at high temperatures, endowing them with reshaped and recycled abilities.13,14 Inspired by these findings, we set out to incorporate dynamic covalent bonds into the new cross-linking systems, aiming to achieve recyclable rubber materials with green cross-linking agents. Vitrimers,15 first reported by Leibler and his co-workers in 2011, are a kind of chemically cross-linked structure which contain dynamic covalent bonds and can rearrange their topological structures by exchange reactions. Because of the associative nature of the exchange mechanism, these vitrimers behave like thermosets, and their cross-link density stays constant. In addition, the viscosity of vitrimers, which follows the Arrhenius law, decreases gradually along with thermally triggered exchange reactions.16 In other words, the exchange reactions are fast enough to alter the topology of the network at high temperature. The plasticity is introduced to the covalently cross-linked thermosets. The first vitrimer was prepared by adding a suitable transesterification catalyst into an epoxy−acid system.17 Until now, many different types of dynamic covalent chemistries including Diels−Alder chemistry,18 imine bond,19−21 disulfide bond,22,23 siloxane equilibration,24,25 transamination,26−28 transcarbamoylation,29,30 transalkylation,31,32 olefin metathesis,33 and boronic ester34,35 are utilized in the preparation of cross-linked polymers. Recently, by introducing exchangeable β-hydroxyl ester linkages into cross-linked elastomers, Guo et al.14 extended the realm of vitrimers to the most widely used diene rubbers. When epoxyfunctionalized rubber such as epoxidized natural rubber (ENR) was covalently cross-linked with a carboxyl-functionalized carbon nanodot (CD), transesterification reactions in the ENR-CD interphase endowed the cross-linked rubbers with recycled, reshaped, and welded abilities at elevated temperatures. In the traditional rubber industry, nanoparticles are incorporated into rubbers to prepare composites with improved mechanical properties and hardness. Generally, the tensile strength of the composites is about 10 times that of the neat rubber system. However, extending the reinforcing concept to vitrimeric composites poses a challenge because of restriction of polymer chains at the surface of the fillers, which would hinder topology rearrangements of the polymer chains and further affect the recycling efficiency of the vitrimeric composites. Legrand et al.36 found that constructing exchangeable interfacial linkages between the vitrimer matrix and the filler interface contributes to speeding up the relaxation process. Guo and his co-workers37−39 incorporated the surfacefunctionalized fillers into rubbers to prepare the vitrimeric rubber nanocomposites with high mechanical performance and good recyclable nature. However, the surface functionalization of fillers will complicate the preparation of rubber nanocomposites and make costs increase, which is not suitable for large-scale industrial production of rubber products. In addition, diene rubber chains contain plenty of double
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EXPERIMENTAL SECTION
Materials. EPDM (Kelton 6950C, Mooney viscosity ML 1+4 at 125 °C, 65, ethylene content 44%, 5-ethylidene-2-norbornene (ENB) content 9%) was obtained from LANXESS Chemical Co., Ltd. (China). Formic acid (98%) was purchased from Aladdin Industrial Corporation (Shanghai, China). Hydrogen peroxide (30%) and toluene (analytical reagent, AR) were purchased from Beijing Chemical Reagents Co., Ltd. (China). Decanedioicacid (DA, 99%) and Polysorbate 80 (Tween 80) were purchased from Alfa Aesar Chemicals (Shanghai, China), and 1,2-dimethylimidazole (DMI, 99%) and zinc acetate (Zn(Ac)2,99.99%) were purchased from Sigma-Aldrich Corporation. Carbon black (CB, N330, primary particle size ∼35 nm) was purchased from Cabot Corporation. All of the chemicals were commercial chemicals and used as received without further purification. Synthesis of Epoxy-Functionalized EPDM (e-EPDM). Epoxyfunctionalized EPDM was synthesized by in situ epoxidation reaction. First, EPDM was dissolved in toluene at 50 °C in a three-necked glass flask equipped with a mechanical stirrer and a thermometer. Second, formic acid (100 mol % relative to the diene group content) and Tween 80 were added into the EPDM solution, and the mixed solution was stirred for 10 min. Then, hydrogen peroxide (300 mol % relative to the diene group content) was dropwise added into the mixed solution within 30 min to start the epoxidation reaction. After 8 h, the reaction mixture was coagulated in ethanol and thoroughly washed with and soaked in distilled water for 24 h. Finally, the wet product was dried to a constant weight under vacuum at 50 °C. Preparation of e-EPDM/CB Composites. e-EPDM desired amounts of CB, Zn(Ac)2, and DA were successively mixed with a 6-in. two-roll mill for about 10 min until well dispersed. DA loadings were 7 phr, corresponding to closely 50 mol % relative to the epoxy groups. 11713
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ACS Sustainable Chemistry & Engineering The quantity of Zn(Ac)2 was 20 mol % relative to the carboxyl group. In this study, the sample code of CB-x refers to the e-EPDM/CB composite with x being phr CB. The preparation of the control sample was the same as that of DA-cured e-EPDM/CB composites. The basic recipe for the sulfur-cured sample is as follows: CB 60 phr, zinc oxide 3 phr, stearic acid 1 phr, N-cyclohexyl-2-benzothiazole sulphonamide (CZ) 1.0 phr, and sulfur 1.2 phr. The basic recipe for the DCP-cured sample is as follows: CB 60 phr and DCP 3 phr. After mixing, the compounds were subjected to compression under a pressure of 15 MPa at 180 °C for the optimum curing time as determined on a rotorless rheometer. Similarly, according to the optimum curing time, the sulfur-cured sample and DCP-cured sample were subjected to compression molding at 160 and 170 °C, respectively. In order to optimize the recycling process and increase the fluidity of the recycling samples, the recycled samples were prepared by a combination of uncured samples and cured samples. First, uncured samples and cured samples are successively compounded on a tworoll mill by a certain ratio, and then, the mixtures were kneaded for 5 min. Finally, the uniform recycled samples were remolded at 180 °C for 60 min. The e-EPDM/CB composite would be recycled three times, and each recycling process is the same as the method mentioned above. Characterizations. Fourier transform infrared (FTIR) spectra were collected using a Bruker Tensor 27 spectrometer equipped with a heating cell. Spectra were recorded with wavenumbers ranging from 4000 to 400 cm−1 for 32 scans at a resolution of 2 cm−1. Samples of eEPDM/DA were prepared by directly dropping the solution (eEPDM/DA in toluene) onto KBr films. 1 H nuclear magnetic resonance (1H NMR) measurements were recorded on a Bruker AV400 spectrometer, and CDCl3 was the solvent. Cross-linking density was measured by the equilibrium swelling experiment.41 The vulcanizates were immersed in toluene at room temperature for 72 h. Then, the samples are removed from the solvent and immediately weighed and then dried in a vacuum oven at 60 °C until a constant weight. The sol fraction is calculated according to (m0 − m2)/m0, and the swelling ratio is defined as (m1 − m2)/m2, where m0 is the weight of the sample before swelling, and m1 and m2 are the sample masses before and after drying, respectively. The cross-linking density was determined by the classical Flory−Rehner equation.42 The tensile mechanical properties of the e-EPDM/CB composites were investigated on the dumbbell specimens (ca. 60 mm × 6 mm × 1 mm and a gauge length of 25 mm) with a SANS CMT 4104 electrical tensile instrument at room temperature and a tensile rate of 500 mm/min according to ASTM D412. Five specimens were measured for each sample, and the median value and standard deviation were calculated. The stress relaxation experiments were performed on a rectangular specimen (20 mm × 6 mm × 1 mm) by using a TA DMA Q800. The sample was initially preloaded by 1 × 10−3 N force to maintain straightness. After reaching the testing temperature, the samples were equilibrated for 5 min. As known, the strain experienced in the recycled sample is very large in the remolding process upon hotpressing. Therefore, a large strain amplitude should be chosen in the stress relaxation experiments to simulate the conditions of the recycling process. Herein, the sample was stretched to a constant strain of 10%, and then, the stress decay was recorded over time.
bonds of EPDM reacting with the performic acid, which is in situ generated for safety reasons using hydrogen peroxide and formic acid. Herein, we synthesized epoxy-functionalized EPDM by an in situ epoxidation reaction according to Scheme 1(a). In the FTIR spectra of EPDM and e-EPDM (Figure Scheme 1. (a) Preparation of Epoxy Group-Functionalized EPDM by in Situ Epoxidation Reaction. (b) Proposed Cross-Linked Structure of e-EPDM Cured by Epoxy-Acid Reactions
1(a)), the absorption peaks at 808 cm−1 are due to the bending vibrations of the double bond in the diene component (ENB). Compared with pristine EPDM, e-EPDM exhibits a new characteristic absorption peak at 871 cm−1, which originates from the asymmetric epoxide ring stretching deformation vibration of the specific epoxide band. In order to understand the epoxidation process of EPDM, the course of the epoxidation reaction was monitored by FTIR spectra. The different reaction times of the absorption intensity of epoxy groups at 871 cm−1 in e-EPDM are shown in Figure S1. During the first 3 h, the double bonds of ENB can be rapidly transformed into epoxy groups, and then, the conversion rate of the reaction gradually increases over 3 h. After 6 h, the relative content of epoxy groups is almost unchanged, and the absorption peaks of the double bonds nearly disappear. Typical 1 H NMR spectra of EPDM and e-EPDM are shown in Figure 1(b). The peaks at 5.01 and 5.24 ppm in the pristine EPDM are derived from olefinic resonances of substituted methine protons of ENB in EPDM. For the spectrum of e-EPDM, new proton signals at 3.02 and 3.08 ppm are observed, which are ascribed to the protons attached to epoxy groups. Similarly, the course of the epoxidation reaction was recorded by 1H NMR spectra (Figure S2), whose results are in agreement with those of the FTIR spectra. According to the peak integrals, the maximum conversion ratio of double bonds is about 90%. Collectively, the results of 1H NMR spectra confirm that epoxy groups are successfully introduced to EPDM chains via an in situ epoxidation reaction.
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RESULTS AND DISCUSSION Covalent Cross-Linking of e-EPDM with Dicarboxylic Acid. Among all the well-known chemical modifications of polymers, epoxidation is a simple yet efficient method for introducing a new functional group into the polymer backbone. Furthermore, epoxidation is one of the most promising and advantageous modification methods due to its relatively mild reaction conditions.43 The epoxidation of EPDM can be economically accomplished by the double 11714
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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Figure 1. (a) FTIR spectra and (b) 1H NMR spectra of EPDM and e-EPDM.
Figure 2. Evolutions of (a) carbonyl (1850−1550 cm−1) and (b) epoxy (885−860 cm−1) regions of FTIR spectra during curing at 180 °C.
Figure 3. Typical stress−strain curves of e-EPDM/CB composites (a) and temperature dependence of the storage moduli (b) and tan δ (c) of eEPDM/CB composites.
chemical reactions between the epoxy groups and the biobased DA. Through monitoring the evolutions of the absorption peaks of ester and epoxy groups involved in the reaction at 180
Biobased DA was used as a green and nontoxic cross-linking agent to cross-link e-EPDM. The e-EPDM cross-linked networks containing a β-hydroxyl ester are formed through 11715
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ACS Sustainable Chemistry & Engineering
Figure 4. (a) Normalized stress relaxation for CB-50 samples at various temperatures ranging from 180 to 200 °C with a constant strain of 10%. (b) Fitted Arrhenius equation (red line) according to the experimental data (black dots). (c) Normalized stress relaxation for CB samples at 190 °C with a constant strain of 10%.
°C by FTIR spectra (Figure 2), the absorptions related to epoxy groups gradually disappear. Simultaneously, the absorptions related to ester bonds at 1710 cm−1 appear, and the absorption intensity increases as the reaction proceeds. These results reveal the addition of carboxyl groups on epoxy rings of e-EPDM, as illustrated in Scheme 1(b). In addition, the chemical reactions are completed within 80 min. The curing behavior of the e-EPDM/DA compounds was also confirmed by monitoring the torque at 180 °C in a rheometer (Figure S3). The curing profiles of DA-curing e-EPDM exhibit a classic curing behavior, indicating the occurrence of crosslinking reactions. Also, the curing profile can reach equilibrium within 90 min, indicating that the cross-linking reactions are completed. In addition to being efficiently cross-linked, the DA curing system is environmentally friendly. Generally speaking, the toxic volatile organic compounds (VOCs) released by the conventional curing systems (curing system and peroxide curing system) originated from the curing additives (such as accelerants and sulfur). In addition, the odor of the rubber products is characteristic of amines and sulfur-containing organic compounds (with very low odor thresholds). 5 Compared with the conventional curing systems, the DA curing system does not generate hazardous substances and gives off an unpleasant smell. Furthermore, some curing additives have poor solubility in rubbers and tend to migrate to the surface of rubber products over time, resulting in blooming phenomenon and unsatisfactory cross-linking performance. However, DSC measurements were performed on the uncured e-EPDM/DA blends to understand the compatibility of eEPDM and DA (Figure S4). The crystal melting peak of DA is not visible, indicating that most of the DA was dissolved in the e-EPDM matrix. In other words, DA is compatible with e-
EPDM, which could avoid the migrating and blooming phenomena. Mechanical Properties of e-EPDM/CB Composites. The influence of CB loadings on the tensile properties of eEPDM/CB composites was investigated. Typical stress−strain curves of e-EPDM/CB composites are shown in Figure 3(a), and their mechanical properties are summarized in Table S1. It can be directly observed that an increase in CB loadings leads to significant improvements on the modulus of e-EPDM/CB composites, which should be attributed to the hydrodynamic effect of CB and increased constrain on chain mobility due to the formation of interphase between CB and EPDM. However, the high CB loading greatly restricts the slippage of the polymer chains when an external force is applied; hence, the elongation at break and the tensile strength decreases. For example, sample CB-60 has a tensile strength of 21.9 MPa, elongation at break of 337%, and residual strain of lower than 10%. The temperature dependence of the storage moduli (E′) and the loss factor (tan δ) of the e-EPDM/CB composite contents is displayed in Figure 3. As the CB loading increases, the storage modulus (E′) continuously increases, while the peak value of tan δ gradually decreases, which can be attributed to the decreased gum concentration and restricted chain mobility. With the same filler loading and cross-linked density, the tensile strength of DA-cured e-EPDM/CB composites is similar to those of the sulfur-cured e-EPDM composites (Table S2). However, the tensile strength of e-EPDM/CB composites based on the DA curing system is better than that of the peroxide curing system (Figure S5). The reason why is that the DA curing system generates more homogeneous networks in comparison with the DCP-based system.44 11716
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ACS Sustainable Chemistry & Engineering Network Topological Rearrangement and Shape Memory Ability in e-EPDM/CB Composites. Because of the formation of the exchangeable β-hydroxyl ester and the transesterification reactions in the cross-linked networks, the DA-cured e-EPDM rubbers are expected to relax stress to adapt applied force at high temperature. In order to study the network topological rearrangement and flow behavior of the eEPDM/CB composites, stress relaxation experiments were carried out. A comparison of normalized stress relaxation curves for sample CB-50 at different temperatures (namely, 180, 185, 190, 195, and 200 °C) is shown in Figure 4. The characteristic relaxation time τ* is defined as the time required to relax to 37% (1/e) of the initial stress, which follows the Arrhenius law as a function of temperature. Obviously, the obtained τ* decreases with a temperature increase. In other words, the relaxation rate is accelerated with a temperature increase. The activation energy is calculated (Ea ∼ 74.16 kJ/ mol) according to relaxation time at different temperatures. Figure 4(c) shows that the relaxation rate of the e-EPDM/CB composites decreases with an increase in CB loading. The mobility of the rubber chain may be restricted because of the adsorption of the rubber chains on the CB surface, resulting in slowing of the transesterification rate. In other words, the relaxation time τ* dramatically increases compared with those of unfilled CB samples (Table 1, Figure S6).
Figure 5. Shape recovery process and reconfiguration by bending or twisting flat e-EPDM/CB samples at 180 °C.
fine powder and then remolding at high temperature in a heating press.14,22,37,45 However, those methods could not only make the recycling process complicated but also increase the formation of permanent cross-linked bonds under the action of oxygen at high temperature. To solve this problem, we set out to improve the recycling technology. The uncured sample and cured sample were successively kneaded on a two-roll mill and then remolded by the hot-pressing process to prepare the recycled samples. The introduction of the uncured sample could increase the fluidity of the recycling samples and avoid side reactions generating permanent bonds. Furthermore, this recycling method is similar to the traditional processing technology of rubber and avoids grinding the recycling sample into a fine powder. To achieve optimum recovery efficiency, the remolded time and the ratio of the uncured sample to cured sample were investigated (Figure S7). Finally, the ratio of the uncured sample to cured sample was determined to 1:4. After the recycling samples were remolded for 60 min at 180 °C, most of the initial mechanical properties can be recovered (Figure 6 and Figure S8). For example, recycled CB-70 exhibits a recovery ratio of 77.7%, 105%, and 85.1% for the ultimate strength, modulus at 200%, and elongation at break, respectively. More strikingly, the samples can be recycled many times. After multiple cycles, the typical tensile curves of the samples are almost overlapped. In particular, the recovery efficiency for the modulus at 200% is close to 100%, which could be attributed to the almost saturated backbone of EPDM and the improved recycling process. Table S3 compares the mechanical properties of the recycled sample produced in this work with those of previously reported vitrimer elastomers. It can be seen that the mechanical properties of the recycled sample are much higher than those reported in the literature. More importantly, the tensile strength and the elongation at break are greater than 15 MPa and 200%, respectively, which can meet the needs of practical applications. The FT-IR and the cross-linking densities before and after recycling were also performed to monitor the structural changes of the recycled samples. As shown in the FT-IR spectra of the original and recycled samples (Figure S10(a)), the absorption related to ester bonds at 1710 cm−1 did not change after three times of recycling, indicating that the number of ester bonds did not change through the transesterification reactions. In addition, the cross-linking density of the recycled samples is the same as that of the original samples (Figure S10(b)). In a word, the DA-cured e-EPDM/CB composites exhibited considerable stability of the networks. As illustrated in Figure 7, two main types of reactions would occur: cross-linking reactions and transesterification reactions
Table 1. Swelling Ratio, Cross-Linking Density, and Relaxation Time of e-EPDM/CB Samples Samples
Sol fraction (%)
Swelling ratio (%)
Cross-linking density (10−5 mol/cm3)
Relaxation time (min)
CB-50 CB-60 CB-70 CB-80
4.99 4.18 4.17 3.77
501.66 432.75 407.46 333.89
7.89 7.31 7.20 6.83
43.8 59.6 74.1 110.4
As is well known, the covalently cross-linked polymers inherently cannot be recycled and reshaped because of the viscosity, with an abrupt drop for the thermoplastics and dissociative covalent adaptable networks (CANs) in a narrow temperature range. As revealed above, e-EPDM/CB composites contain β-hydroxyl esters, which are assigned to associative dynamic covalent bonds. Therefore, the materials exhibit gradual Arrhenius-like viscosity variations with respect to temperature; the individual properties dramatically induce deformation in a wide temperature range. As proof of concepts, the complex artifacts were shaped by simply bending or twisting in a wide temperature range. The newly permanent shapes (such as fusilli-shaped) are obtained from planar ribbon e-EPDM/CB samples under the deformation and recovery temperature of 180 °C. After this, the original shape can be recovered and successively reconfigured into another permanent shape (Figure 5). It should be emphasized that the crosslinked elastomers containing associative CANs can be easily endowed with an individual character, which could improve the potential applications in smart polymer fields. Recycling of e-EPDM/CB Composites. Vitrimer can alter network topology and relax stress at high temperature; therefore, this feature could enable the recycling of chemically cross-linked rubbers containing dynamic covalent bonds. As a proof of concept, the recyclable rubber is demonstrated by cutting the samples into small pieces or even grinding into a 11717
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ACS Sustainable Chemistry & Engineering
Figure 6. (a) Recovery ratio of the modulus at 200% of the e-EPDM/CB samples after multiple recycling cycles. (b) Typical stress−strain curves of CB-70 after multiple reprocessing cycles.
Figure 7. Schematic illustrating cross-linking reaction and topological rearrangements via exchange reactions.
of the β-hydroxy ester. The cross-linkers in recycled samples were able to react with the epoxy groups in e-EPDM chains to form a cross-linked structure. Meanwhile, the β-hydroxy ester moieties of recycled samples enable the topological rearrangement via catalyzed transesterification reaction, which endows the cross-linked rubber with malleability. Therefore, the eEPDM/CB recycled samples exhibited the ability to be recycled and an efficient mechanical recovery. To further verify transesterification reactions, we prepared a recycled sample with the DCP curing system. As shown in Figure S9, the recovery ratio of the recycled sample with the DCP curing system was much worse than that of the recycled sample with the DA curing system. In addition, the SEM micrographs of the tensile failure section of the original samples and different recycled samples are shown in Figure S11. The tensile failure section of the recycled samples with the DA curing system was similar to that of the original sample. It indicates that the recycled samples have good fluidity because of the existence of dynamic covalent bonds and the introduction of the uncured sample. However, the tensile failure section of the recycled sample with the DCP curing system was rough and contained lots of holes, which can be attributed to poor chemical adhesion between particles. However, the recovery efficiency of tensile strength and elongation at break are relatively lower. The reasons are summarized as follows: (1) The steric hindrance effect of β-hydroxyl esters retard transesterification reactions. (2) The adsorption of rubber chains on CB is inclined to affect the chain mobility, which contributes to the impaired relaxation rate in the present system. (3) The EPDM rubber has poor self-adhesion and low cohesion, which makes
it difficult to obtain a coherent sample from small rubber pieces.
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CONCLUSIONS
In summary, a new and green curing strategy is put forward to prepare e-EPDM/CB composites with high mechanical performance, efficient recyclability, and a shape memory effect by engineering β-hydroxyl ester bonds into cross-linked networks. In addition, this strategy could avoid the use of toxic curing agents and the release of toxic VOCs. Specifically, epoxy groups are successfully introduced into the EPDM chains via an in situ epoxidation reaction, and the modified EPDM rubbers can be effectively cross-linked by biobased DA. Furthermore, the resulting network topology can undergo dynamic shuffling via transesterification at elevated temperatures, which confers the composites recyclable nature. Meanwhile, the introduction of the uncured composites could significantly improve the recovery efficiency. Also, the tensile strength and the elongation at break of the recycled eEPDM/CB composites are greater than 15 MPa and 200%, respectively. The mechanical properties of the recycled sample are much higher than those reported in the literature and exhibit potential application in the rubber industry. We envision that this work provides a successful paradigm that combines green curing chemistry, recyclability, and reshaped ability into commercially available polyolefin rubber networks. 11718
DOI: 10.1021/acssuschemeng.9b01875 ACS Sustainable Chem. Eng. 2019, 7, 11712−11720
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01875. FTIR spectra and 1H NMR spectra of e-EPDM at different reaction times, curing profiles under 180 °C, mechanical properties of e-EPDM/CB composites, DSC curves of e-EPDM/DA before curing, cross-linking density of e-EPDM/CB samples with different curing systems, typical stress−strain curves of e-EPDM/CB composites based on different curing systems, stress relaxation for cross-linked e-EPDM at various temperatures and fitting of characteristic relaxation time to the Arrhenius equation, typical stress−strain curves at different remolding times, typical stress−strain curves after multiple cycles of reprocessing, comparison of the mechanical performance between other vitrimer elastomers and our vitrimer composites, typical stress−strain curves of different curing systems after reprocessing, FTIR spectra and cross-linking density of original and recycled samples, and SEM results of the microstructure of the tensile failure section of original and recycled samples. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X. Zhou). *E-mail:
[email protected] (B. Guo). *E-mail:
[email protected] (L. Zhang). ORCID
Baochun Guo: 0000-0002-4734-1895 Liqun Zhang: 0000-0002-8106-4721 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51790501 and 51825303), the National Key Research and Development Program of China (2017YFB0306900, 2017YFB0306904), and the National Basic Research of China (2015CB654700, 2015CB654705).
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