The Viable Fabrication of Gas Separation Membrane Used by Reclaimed Rubber from Waste Tires

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Article The Viable Fabrication of Gas Separation Membrane Used by Reclaimed Rubber from Waste Tires

Yu-Ting Lin 1, Guo-Liang Zhuang 1, Ming-Yen Wey 1,* and Hui-Hsin Tseng 2,3,*

1 Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan; [email protected] (Y.-T.L.); [email protected] (G.-L.Z.) 2 Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan 3 Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan * Correspondence: [email protected] (M.-Y.W.); [email protected] (H.-H.T.); Tel.: +886-4-22840441 (ext. 533) (M.-Y.W.); +886-4-24730022 (ext. 12115) (H.-H.T.) Received: 14 September 2020; Accepted: 28 October 2020; Published: 30 October 2020

Abstract: Improper disposal and storage of waste tires poses a serious threat to the environment and human health. In light of the drawbacks of the current disposal methods for waste tires, the transformation of waste material into valuable membranes has received significant attention from industries and the academic field. This study proposes an efficient and sustainable method to utilize reclaimed rubber from waste tires after devulcanization, as a precursor for thermally rearranged (TR) membranes. The reclaimed rubber collected from local markets was characterized by thermogravimetric analyzer (TGA) and Fourier transfer infrared spectroscopy (FT-IR) analysis. The results revealed that the useable rubber in the as-received sample amounted to 57% and was classified as styrene–butadiene rubber, a type of synthetic rubber. Moreover, the gas separation measurements showed that the C7-P2.8-T250 membrane with the highest H2/CO2 selectivity of 4.0 and sufficient hydrogen permeance of 1124.61 GPU exhibited the Knudsen diffusion mechanism and crossed the Robeson trade-off limit. These findings demonstrate that reclaimed rubber is an appealing, cost effective, and sustainable alternative, as a precursor for TR membranes, for application in gas separation. The present approach is useful in the selection of a suitable reclaimed rubber precursor and related membrane preparation parameters, leading to the advancement in the recycling value of waste tires.

Keywords: gas separation; thermally rearranged membrane; waste material; sustainable development; circular economy

1. Introduction Rapid population growth and industrialization are responsible for the significant increase in production and accumulation of solid waste. Advances in the automobile industry and increased use of vehicular modes of transport have led to an increase in the number of waste tires [1]. Currently, the global annual consumption of tires is up to 1 billion units [2,3]. Furthermore, the consumption rate of tires is expected to grow exponentially with their demand growing in developing countries. In Taiwan, the annual recovery market for waste tires is approximately 100 thousand tonnes, reaching approximately 140 thousand tonnes in 2017 according to the statistics of Environmental Protection Administration Executive Yuan, Republic of China (R.O.C.) (Taiwan) [4]. The most prevalent disposal methods for waste tires include landfilling and using them as auxiliary fuel for energy recovery [5,6]. However, landfilling is not the preferred option due to the large volumes of tires. Furthermore, the doughnut shape of tires retains stagnant water, which becomes a breeding habitat for mosquitoes, thereby resulting in the spread of vector-borne diseases [7], especially in humid environments found in

Polymers 2020, 12, 2540; doi:10.3390/polym12112540 www.mdpi.com/journal/polymers Polymers 2020, 12, 2540 2 of 15 countries like Taiwan. The incineration of waste tires emits pollutants including carbon monoxide (CO), carbon dioxide (CO2), sulfur oxides (SOx), toxic dioxins, polycyclic aromatic hydrocarbons, and particulate matter [8]. Therefore, the improper disposal and storage of waste tires can have adverse effects on the environment and human health. This necessitates the development of sustainable and efficient recovery and recycling methods for waste tire management. One of the best alternative solutions for dealing with waste tires is to transform them from waste material into valuable substances such as membranes. Considering the number of tires being discarded globally, rubber consumption is facilitating the exhaustion of limited petrochemical resources. Hence, in view of the circular economy and the drawbacks of current disposal methods for discarded tires, investigating technologies that can utilize waste materials are an inevitable tendency worldwide. For countries whose energy is imported, such as Taiwan, the reutilization of waste materials could boost their economic development because of the reduction in trade deficit. For example, the Taiwanese government is pressing ahead with the policy for a circular economy to provide financial support for companies and academic scientists to encourage and promote reutilization technology. Recently, thermally rearranged (TR) membranes have been widely applied in the field of gas separation, such as hydrogen recovery, carbon dioxide capture, and natural gas purification, due to their unique microstructure and superior chemical/thermal stability in harsh environments [9,10]. TR membranes can be fabricated through a simple thermal rearrangement process after the heat treatment of a polymeric membrane, which facilitates the transformation of interconnected microcavities with a narrow cavity [11]. The thermal rearrangement process begins when the polymer aromatic chains undergoes a heating process by reaction of ortho-substituted atoms, creating a new ring. The buckling of the ring creates free volume elements in the membrane and enhances gas diffusion. Consequently, the microporous structure of TR membranes is obtained, which exhibits high gas permeance and great separation property. The frequently used polymer precursors for TR membranes mainly consist of polyimide, polybenzoxazoles, and polypyrrolone [10–12]. All the above-mentioned precursors are thermosetting polymers that are priced exorbitantly. The main constituent (approximately 47%) of the tires is rubber [13,14], usually synthetic, such as styrene–butadiene rubber and butadiene rubber [15]. All rubber materials are thermosetting polymers, except for thermoplastic elastomers. Thus, the rubber material in waste tires is viewed as a valuable resource, given its potential as a membrane precursor [16]. To date, no research has investigated tire-derived TR membranes for application in gas separation, which is likely restricted by their three-dimensional cross-linked structure, thereby resulting in inferior processability. Recently, methods for reclaiming rubber by physical, chemical, biotechnological, and de-link processes have been developed. The reclaimed rubber, with enhanced process flexibility, is converted from a three-dimensional cross-linked structure into a one-dimensional thermosetting polymer by destroying the rubber/sulfur bond [17]. This study was conducted using reclaimed rubber as the precursor for TR membranes. The effects of precursor concentration, incorporation of additives, and thermal treatment temperature on as-prepared TR membranes derived from reclaimed rubber were investigated.

Research Signiﬁcance A huge amount of waste tires are produced worldwide due to the expansion of the automobile industry, and their disposal is an issue faced by all countries. The most common method to handle waste tires is landﬁlling or using them as auxiliary fuel for energy recovery. However, the published studies showed that scrapped rubber tires could not be decomposed under environmental conditions and pose a serious threat to the environment and human health. Moreover, the incineration of waste tires is accompanied with emissions of dioxins, volatile organic compounds and particulate matter. Consequently, recycling waste tire rubber by transforming it into a valuable material (e.g., membranes) has become the preferred solution to dispose of waste tires. From the circular economy perspective, reclaimed rubber-derived membrane is a reutilization alternative that contributed to waste reduction. Therefore, the goals of the present study were to provide a viable and green precursor (reclaimed Polymers 2020, 12, 2540 3 of 15

Polymersrubber) 20 with20, 12 which, x FOR toPEER fabricate REVIEW a TR membrane applied in the field of gas separation, thus boosting3 of the 16 circular economy via prolonging the life cycle of rubber tires. In order to further extend understanding were conducted to understand its characteristics and performance in related membranes. The of the reused rubber from waste tire, a series of characterizations were conducted to understand membrane fabrication parameters, including precursor concentration, heating temperature, and the its characteristics and performance in related membranes. The membrane fabrication parameters, incorporation of a polyphenylene oxide (PPO) modifier, as well as the corresponding gas transport including precursor concentration, heating temperature, and the incorporation of a polyphenylene measurements, were also studied to produce an experimental evaluation of this new precursor for a oxide (PPO) modifier, as well as the corresponding gas transport measurements, were also studied to gas separation membrane. To the best of our knowledge, there are no other existing studies on rubber- produce an experimental evaluation of this new precursor for a gas separation membrane. To the best derived TR membranes applied in gas separation. of our knowledge, there are no other existing studies on rubber-derived TR membranes applied in gas separation. 2. Experiment 2. Experiment 2.1. Materials 2.1. MaterialsThe crumb rubber sample and the reclaimed rubber sample selected as polymer precursors in this workThe crumb wererubber supplied sample by andInternational the reclaimed Quality rubber Materials sample selectedNano Tech. as polymer Co., Ltd. precursors (Taipei inCity, this workTaiwan), were as suppliedshown in byFigure International 1. Toluene Quality (extra pure Materials grade Nano—99% Tech. purity, Co., Union Ltd. Chemical (Taipei City, Works Taiwan), Ltd., Hsinchuas shown City, in Figure Taiwan)1. Toluene was (extrasourced pure as grade—99% a solvent for purity, the Unionpreparation Chemical of the Works casting Ltd., solution. Hsinchu City,The Taiwan)polyphenylene was sourced oxide as(PPO, a solvent Cas. forNo. the 25134 preparation-01-4) purchased of the casting from solution. Sigma-Aldrich The polyphenylene (Saint Louis,oxide MO, (PPO,USA) was Cas. used No. as 25134-01-4) an additive purchased to modify fromthe microstructure Sigma-Aldrich of (Saint the TR Louis, membrane. MO, USA) Commercial was used ceramic as an embryos,additive to with modify an average the microstructure pore size of of 74 the TR, thickness membrane. of 1.4 Commercial mm, and ceramicdiameter embryos, of 23 mm with were an obtainedaverage porefrom size Ganya of 74 Fine Å, thicknessCeramics ofCo. 1.4 (New mm, Taipei and diameter City, Taiwan). of 23 mm They were were obtained calcined from at 1400 Ganya °C Å withFine Ceramicsa heating Co.rate (New of 2 ° TaipeiC/min City, for 2 Taiwan). h holding They time were in our calcined laboratory at 1400 to◦ Cenhance with a heatingtheir physical rate of strength.2 ◦C/min forThey 2 hwere holding used time as ain bare our substrate. laboratory The to enhancetested gases their (99.999% physical pure) strength.—H They2, CO2 were, O2, N used2, and as CHa bare4—were substrate. obtained The from tested Toyo gases Gas (99.999% Co. (Taichung pure)—H City,2, CO Taiwan).2,O2,N 2All, and chemical CH4—were reagents obtained were fromused Toyodirectly Gas without Co. (Taichung purification. City, Taiwan). All chemical reagents were used directly without purification.

Figure 1. OpticalOptical image image of of as as-received-received rubber sample provided from local market. 2.2. Synthesis of Rubber-Derived TR Membrane 2.2. Synthesis of Rubber-Derived TR Membrane Initially, the diﬀerent concentrations at 4, 5, 6, and 7 wt % of casting solutions were obtained by Initially, the different concentrations at 4, 5, 6, and 7 wt % of casting solutions were obtained by dissolving the desired amounts of collected reclaimed rubber in toluene solvents at a ﬁxed temperature dissolving the desired amounts of collected reclaimed rubber in toluene solvents at a fixed of 160 C for 24 h. Similarly, casting solutions with various additive contents within the range of 0.7–2.8% temperature◦ of 160 °C for 24 h. Similarly, casting solutions with various additive contents within the were prepared. Afterwards, the as-prepared casting solution was applied onto an alumina substrate range of 0.7–2.8% were prepared. Afterwards, the as-prepared casting solution was applied onto an via the spin coating technique at a speed of 2000 rpm for 16 s to obtain the uniform rubber polymeric alumina substrate via the spin coating technique at a speed of 2000 rpm for 16 s to obtain the uniform membrane. The fabricated rubber membranes were left overnight in an open environment at room rubber polymeric membrane. The fabricated rubber membranes were left overnight in an open temperature to allow the rest of the solvent in the membrane to evaporate. Subsequently, the two-stage environment at room temperature to allow the rest of the solvent in the membrane to evaporate. thermal rearrangement procedure was performed in a tubular furnace (Chang-Hua Electric Heating Subsequently, the two-stage thermal rearrangement procedure was performed in a tubular furnace Co., Ltd., Changhua city, Taiwan) in terms of a dried polymeric membrane. The polymeric membrane (Chang-Hua Electric Heating Co., Ltd., Changhua city, Taiwan) in terms of a dried polymeric membrane. The polymeric membrane was placed in the central zone of a quartz tube equipped with a tubular furnace, and the heating environment was under a vacuum at less than 10−6 atm. The curing step was conducted at 200 °C with a ramping rate of 5 °C/min and then maintained for 6 h to stabilize

Polymers 2020, 12, 2540 4 of 15 was placed in the central zone of a quartz tube equipped with a tubular furnace, and the heating 6 environment was under a vacuum at less than 10− atm. The curing step was conducted at 200 ◦C with a ramping rate of 5 ◦C/min and then maintained for 6 h to stabilize the polymer chains. Next, the target temperatures (250 and 350 ◦C) were reached at the same heating rate (5 ◦C/min) and were maintained for 2 h. Finally, the TR membrane was naturally cooled to room temperature. To prevent aging, the pure gas permeation value of the resultant TR membrane was measured within 24 h after the conduction of the thermal rearrangement procedure. The TR membranes fabricated by diﬀerent parameters were denoted as Cx-Py-Tz, where x is the precursor concentration, y is the content of PPO additive, and z is the heating temperature. For example, a C7-P0.7-T250 TR membrane means the polymeric membrane, with 7 wt % rubber precursor and 0.7 wt % PPO, was heated at 250 ◦C. Moreover, a C4-P0-T350 TR membrane refers to a polymeric membrane prepared with a 4 wt % rubber precursor, without the incorporation of PPO ﬁller and heated at 350 ◦C.

2.3. Characterization of Collected Rubber Samples and Fabricated TR Membranes The thermal behavior of the collected rubber samples from the recycling company was studied using a thermogravimetric analyzer (TGA, Perkin Elmer, simultaneous thermal analyzer 6000, Waltham, MA, USA). Prior to the analysis, the as-received rubber sample was placed in the oven at 60 ◦C for 24 h to remove any trapped moisture. The TGA testing of the rubber sample was conducted following the guidelines by Perkin Elmer [18]. To evaluate the polymer content in the sample, 6–8 mg of the dried sample was first heated in a nitrogen atmosphere, with temperatures increasing from 50–700 ◦C at the rate of 20 ◦C/min. Subsequently, the sample underwent second heating at 700–900 ◦C, at a fixed heating rate of 20 ◦C/min, in the air atmosphere to measure the carbon black content. The residue that remains undecomposed after heating until 900 ◦C was considered the other filler. Fourier transfer infrared spectroscopy (FT-IR, FT/IR-4100 spectrophotometer, Jasco Corporation, 1 Tokyo city, Japan) in the wavenumber range of 4000 to 400 cm− was performed to identify the functional group and to confirm the composition of the collected rubber sample. 3 The cross-linking density (Ve, mol/cm ) of the rubber sample, presented in Equation (1), was obtained by combining the American Society for Testing and Materials (ASTM) D6814 standard and the Flory–Rehner equation [19,20].

[ln(1 V ) + V + X V2 = r r 1 r ve − − 1 (1) 3 V V Vr /2 1 r − where Vr refers to the volume fraction of the swollen sample in the equilibrium with the pure solvent (toluene), X1 refers to the polymer–solvent interaction parameter (0.446) [20], and V1 refers to the molar volume of the toluene (106.35 cm3/mol). The degree of devulcanization (%) of the as-received reclaimed rubber sample was evaluated using Equation (2): Vr,reclaimed rubber Devulcanization(%) = 100% (2) Vr,crumb rubber × where Vr,crumb rubber and Vr,reclaimed rubber refer to the cross-linking densities of the rubber sample before and after the devulcanization process, respectively. Scanning electron microscopy (SEM) micrographs of the rubber-derived TR membrane were obtained by a JEOL JSM-6700F scanning electron microscope (JEOL Ltd., Tokyo city, Japan) at an accelerating voltage of 3 kV to observe the membrane morphology. Prior to morphological analysis, the outer surface of each sample was sputtered with gold to improve its conductivity. Atomic force microscopy (AFM, BRUKER Dimension Icon, Billerica, MA, USA) was conducted to evaluate the topography and surface roughness of the as-obtained TR membrane. Roughness measurements in this work were performed by surface analysis on a ﬁxed area (5 5 µm2) taken from × each sample. Polymers 2020, 12, 2540 5 of 15

2.4.Polymers Permeation 2020, 12,Test x FOR PEER REVIEW 5 of 16

2.4.The Permeation pure gas Test permeation value of each as-fabricated TR membrane was measured by using lab-scale permeation measurement equipment, with an effective membrane area of 0.785 cm2 and an The pure gas permeation value of each as-fabricated TR membrane was measured by using lab- operation temperature of 28 2 C, by the constant volume/variable pressure approach (Figure2). scale permeation measurement± ◦equipment, with an effective membrane area of 0.785 cm2 and an Before the permeation test, a vacuum pump was used to maintain the system in a vacuum state to avoid operation temperature of 28 ± 2 °C, by the constant volume/variable pressure approach (Figure 2). atmospheric gas interference. Afterwards, the membrane was mounted on a stainless-steel housing Before the permeation test, a vacuum pump was used to maintain the system in a vacuum state to (Millipore Corp., Cat. No. XX4404700, Burlington, MA, USA) with a silicone O-ring. Two pressure avoid atmospheric gas interference. Afterwards, the membrane was mounted on a stainless-steel transducers with a range of 1 to 9 bar (JPT-131S, Jetec Electronics Co., Ltd., Taichung City, Taiwan) housing (Millipore Corp., Cat.− No. XX4404700, Burlington, MA, USA) with a silicone O-ring. Two werepressure used transduce to measurers thewith feed a range and of permeate −1 to 9 bar pressures. (JPT-131S, A gasJetec with Electronics a pressure Co., of Ltd. about, Taichung 1.96 bar City, was fedTaiwan into the) were upstream used to ofmeasure the membrane the feed and and permeate the downstream pressures. pressure, A gas with which a pressure increased of about with time1.96 (dpbar/dt), was was fed recordedinto the upstream with a computer. of the membrane Then, the and gas the separation downstream performance pressure, ofwhich the membraneincreased with was obtained.time (dp/dt), To minimize was recorded the investigational with a computer. error, Then, each the membrane gas separation sample performance was measured of the threemembrane times andwas the obtained. average To value minimize was reported.the investigational The detailed error, permeation each membrane process sample has been was describedmeasured inthree our previoustimes and work the [average21,22]. Thevalue tested was reported. gases in theThe present detailed study permeation contained process hydrogen has been (2.89 described Å), carbon in dioxideour previous (3.24 Å), work oxygen [21,22]. (3.46 The Å), tested nitrogen gases (3.64 in the Å), present and methane study contained (3.80 Å)—the hydrogen values (2.89 in parentheses ), carbon corresponddioxide (3.24 to the), oxygen kinetic (3.46 diameter. ), nitrogen The study(3.64 inferred), and methane the gas (3.80 separation )—the mechanism values in parentheses of the TR Å membranecorrespond based to the on kinetic the measured diameter. permeation The study resultsinferred and the related gas separation characterizations. mechanism Additionally, of the TR Å Å Å Å puremembrane gas permeance based on was the defined measured and permeation calculated byresults Equation and related (3) and characterizations. expressed according Additionally, to the gas 3 pure gas permeance was defined6 cm and(STP calculated) by Equation (3) and expressed according to the gas permeation unit (GPU = 1 10− cm2 s cmHg ). × ( ) permeation unit (GPU = 1 × 10 · · ). 3 " # −6 𝑐𝑐𝑐𝑐 𝑆𝑆𝑆𝑆𝑆𝑆 dp V T0 2 P = · (3) 𝑐𝑐𝑐𝑐 ∙𝑠𝑠∙𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 dt A ∆p P P = [ ] · · 0 (3) where P represents the gas permeation value𝑑𝑑𝑑𝑑 (GPU),𝑉𝑉 ∙ 𝑇𝑇0dp/dt represents the increased rate of the downstreamwhere P represents pressure the at a gas steady permeation state (cm value Hg/s),𝑑𝑑𝑑𝑑 (GPU),V𝐴𝐴refers∙ Δ𝑝𝑝 ∙dp𝑃𝑃 to0/dt the represents downstream the volumeincreased (cm rate3), Tofis the the 3 permeationdownstream temperature pressure at (K), a steadyA is the state eff ective(cm Hg/s), area ofV therefers membrane to the downstream (cm2), ∆p represents volume (cm the), pressureT is the permeation temperature (K), A is the effective area of the membrane (cm2), represents the difference between the feed side and the permeation side of the membrane (cm Hg), P0 is 76 cm Hg, pressure difference between the feed side and the permeation side of the membrane (cm Hg), is and T0 is 273 K. Δ𝑝𝑝 76 cm Hg, and is 273 K. The equation for selectivity calculation is as follows: 𝑃𝑃0 The equation0 for selectivity calculation is as follows: 𝑇𝑇 Pi αi/j = (4) / =Pj (4) 𝑖𝑖 𝑖𝑖 𝑗𝑗 𝑃𝑃 where αi/j represents the ideal selectivity of pure𝛼𝛼 gas i over𝑗𝑗 j. Pi and Pj are the permeance value of gas where / represents the ideal selectivity of pure gas𝑃𝑃 i over j. and are the permeance value of i and gas j, individually. gas i and gas j, individually. 𝛼𝛼𝑖𝑖 𝑗𝑗 𝑃𝑃𝑖𝑖 𝑃𝑃𝑗𝑗

FigureFigure 2. 2.Schematic Schematic ofof gasgas permeationpermeation apparatus.

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3. Results and Discussion

3.1. Analysis of As-Received Rubber Sample The TGA analysis of commercial styrene–butadiene rubber (SBR) and reclaimed rubber were investigated by the TGA and their proximate compositions are shown in Figure3. As expected, prior to heating to a temperature of 700 ◦C, commercial SBR suffered the greatest weight loss and finally decomposed completely at approximately 500 ◦C, which suggests that its whole composition consisted of polymer, that is, rubber. As presented in Figure3a, for reclaimed rubber, the initial decomposition temperature (Td) was 343 ◦C and the weight-loss curve displayed a two-stage degradation process. The primary weight-loss was observed at approximately 350 ◦C, while the secondary weight loss was at approximately 750 ◦C, which implied at least two different compositions existing in the reclaimed rubber sample (i.e., polymer and carbon black). When the temperature reached 900 ◦C, approximately 11% of the sample weight remained, which was indicative of the presence of another filler. In addition, the initial decomposition temperature for reclaimed rubber is lower than that of commercial SBR, which suggests the presence of an impurity such as carbon black or another filler. As a result, based on the weight loss percentages corresponding to the degradation temperature, the approximate composition of reclaimed rubber was determined (Figure3b). As anticipated, polymer accounted for 57% of the reclaimed rubber sample. Thus, using reclaimed rubber as a membrane precursor was feasible because of the abundant rubber polymer in the overall sample. Carbon black, which accounted for 32% of the reclaimed rubber, was the minor ingredient. A tire generally consists of 40–48% rubber polymer [13,14], and our findings show that the rubber polymer content is increased in reclaimed rubber. Additionally, a change in the functional group of the as-received rubber samples before and after devulcanization was detected using the FTIR technique to gain insight into the rubber sample properties (Figure4). For the infrared spectroscopy of crumb rubber, the characteristic peaks at 1 3100–2800, 1540, 1500–1380, 990–910, 780–660, and 700–590 cm− were associated with the stretching vibration of C–H bonds, the stretching vibration of C=C bonds, C–H vibration, =C–H bands, aromatic C–H bonds, and C–S bonds, respectively [23]. The spectra for reclaimed rubber revealed that the characteristic peaks of C–H bonds and C=C bonds were increasingly apparent, and the intensity of C–S bonds was negligible, which mirrored the validity of the devulcanization procedure and validated the increase in the rubber polymer content of reclaimed rubber. In addition, the rubber type of the collected specimen from the local market was shown to be styrene–butadiene rubber because the detected functional groups of the rubber sample were similar to those of styrene–butadiene rubber from previous studies [24,25]. Due to the existence of carbon black and other fillers in the adopted precursor (reclaimed rubber), the light scattering degree would be affected by the black substance. Therefore, the typical peak of sulfur bridges is not easy to detect by the FTIR technique. As a result, the effect of devulcanization on reclaimed rubber was evaluated by the change in cross-linking density between rubber and reclaimed rubber. The cross-linking densities of both rubber samples, as reported in Table1, were tested by the combination of the swelling method in the ASTM D6814 standard and the Flory–Rehner equation [19,20]. The devulcanization degree for reclaimed rubber, calculated by Equation (2), was 45.5%. PolymersPolymers2020 2020, ,12 12,, 2540 x FOR PEER REVIEW 77 of 1516 Polymers 2020, 12, x FOR PEER REVIEW 7 of 16 120 (a) styrene-butadiene rubber (b) 120 reclaimed rubber (a2) 100 (a) styrene-butadiene rubber (b) reclaimed rubber (a2) 100 80 Td:343.68 °C

80 Td:343.68 °C 60 carbon black 60 40 carbon black Residual weight (%)

40 Residual weight (%) 20 polymer 20 0 polymer 0 100 200 300 400 500 600 700 800 900 0 Temperature (°C) 0 100 200 300 400 500 600 700 800 900

Figure 3. (a) TGA curveTemperature of commercial (°C) styrene–butadiene rubber and reclaimed rubber and (b) FigureapproximateFigure 3. 3.(a )( weighta TGA) TGA curve composition curve of of commercial commercial of reclaimed styrene–butadiene styrene rubber–butadiene sample. rubber rubber and and reclaimed reclaimed rubber rubber and and (b )(b) approximateapproximate weight weight composition composition of reclaimedof reclaimed rubber rubber sample. sample.

FigureFigure 4.4. FTIRFTIR spectraspectra ofof rubberrubber samplessamples beforebefore andand afterafter devulcanization.devulcanization. TableFigure 1. 4.Property FTIR spectra of rubber of rubber samples samples before before and after and devulcanization.after devulcanization. Table 1. Property of rubber samples before and after devulcanization. 7 3 Sample Cross-Linking Density (10− mol/cm ) Devulcanization (%) Sample Table 1. PropertyCross-L ofinking rubber D samplesensity (10before−7 mol/cm and after3) devulcanization.Devulcanization (%) Crumb Rubber 2.233 - Crumb Rubber 2.233 −7 3 - ReclaimedSample RubberCross-Linking 1.217 Density (10 mol/cm ) 45.50Devulcanization (%) ReclaimedCrumb RRubberubber 1.2172.233 45.50- 3.2. EﬀRecteclaimed of Reclaimed Rubber Rubber Precursor Concentrations1.217 on Gas Separation Performance 45.50 3.2. Effect of Reclaimed Rubber Precursor Concentrations on Gas Separation Performance As shown in Figure5, the gas permeance of all TR membranes with di ﬀerent precursor 3.2.As Effect shown of R eclaimedin Figure Rubber 5, the Precursor gas permeance Concentrations of all on TRGas membranesSeparation P erformancewith different precursor concentrations decreased in the following order: H2 (2.89 Å) > CH4 (3.80 Å) > N2 (3.64 Å) > concentrationsAs shown decreased in Figure in the5, thefollowing gas permeance order: H2 (2.89of all )TR > CHmembranes4 (3.80 ) >with N2 (3.64different ) > Oprecursor2 (3.46 O2 (3.46 Å) > CO2 (3.2 Å), where the values in parentheses represent the kinetic diameter. Evidently, )concentrations > CO2 (3.2 ), decreased where the in values the following in parentheses order: H represent2 (2.89 ) the> CH kinetic4 (3.80 diameter.) > N2 (3.64 Evidently, ) > O 2 the(3.46 the change in gas permeance for all rubber-derived TR membranesÅ was uncorrelatedÅ withÅ the kinetic change) > COin gas2 (3.2 permeance ), where for the all values rubber in-derived parentheses TR membranes represent thewas kineticuncorrelated diameter. with Evidently, the kinetic the diameterÅ of testedÅ gas. Moreover, all TR membranes exhibitedÅ higher selectivityÅ overÅ H2/CO2, diameterchange ofin testedgas permeance gas. Moreov forer, all all rubber TR membranes-derived TR exhibited membranes higher was selectivity uncorrelated over H with2/CO the2, H kinetic2/N2, H2Å/N2, and H2/CHÅ4 separations than CO2/N2 and CO2/CH4 separation. The dominant gas separation anddiameter H2/CH of4 separationstested gas. Moreov than COer, 2all/N TR2 and membranes CO2/CH 4exhibited separation. higher The selectivity dominant over gas H 2separation/CO2, H2/N 2, mechanism for the membrane depends on the magnitude of the membrane pore. The Knudsen mechanismand H2/CH for4 separationsthe membrane than depends CO2/N 2on and the CO magnitude2/CH4 separation. of the membrane The dominant pore. Thegas Knudsenseparation diffusionmechanism mod el,for as the a kind membrane of gas transport depends mechanism,on the magnitude governs of the the gas membrane separation pore.process The when Knudsen the diffusion model, as a kind of gas transport mechanism, governs the gas separation process when the

PolymersPolymers 20202020, ,1122, ,x 2540 FOR PEER REVIEW 8 8of of 16 15 Polymers 2020, 12, x FOR PEER REVIEW 8 of 16 effective pore size of the membrane is smaller than the mean free path of penetrates, thus resulting ineffectivedi theffusion linear pore model, relationship size as of a the kind betweenmembrane of gas transportgas is permeance smaller mechanism, than and the the governsmean square free theroot path gas of theof separation penetrates, reciprocal process gasthus molecular whenresulting the weightineff theective linear [26,27 pore relationship]. size To of further the between membrane investigate gas is permeance smaller the gas than and separation the the mean square freemechanism root path of ofthe penetrates, reciprocalof rubber gas thus-derived molecular resulting TR membranes,weightin the linear [26,27 we relationship]. comparedTo further between the investigate gas gas permeance permeance the gasto and theseparation thesquare square rootmechanism root of ofthe the reciprocal reciprocalof rubber gas gas-derived molecular molecular TR weightmembranes,weight (Figure [26,27 we]. 6). To compared As further expected, investigate the correlation gas permeance the gass were separation toobserved the square mechanism between root oftheof rubber-derived gasthe permeancereciprocal TR gasvalue membranes, molecular and the squareweightwe compared root(Figure of the the6). gasAsreciprocal ex permeancepected, gas correlation tomolecular the squares were weights root observed of for the all reciprocal between TR membranes the gas gas molecular permeance fabricated weight value at (Figuredifferent and the6). precursorsquareAs expected, root concentrations of correlations the reciprocal (4 were–7 wtgas observed %). molecular Their between coefficients weights the gasoffor determination, permeanceall TR membranes value labeled and fabricated theR2, were square inat rootthe different range of the ofprecursorreciprocal 0.98–0.99. concentrations gas As molecular a result, weights (4 the–7 wtpredominant for%). allTheir TR membranescoefficients gas separation of fabricated determination, mechanism at different labeled for precursorall Rrubber2, were concentrations- derivedin the range TR 2 membranesof(4–7 0.98 wt– %).0.99. was Their As confirmed a coe result,fficients theas ofKnudsen predominant determination, diffusion. gas labeled separationThe permeation R , were mechanism in of the hydrogen range for ofall of 0.98–0.99. rubberall rubber-derived As-derived a result, TR membranesmembranesthe predominant waswas confirmed gashigher separation than as Knudsenthat mechanism of carbondiffusion. for dioxide, all The rubber-derived permeation indicating of TRthat hydrogen membranes the rubber of all was rubberpolymer confirmed-derived was as transformedmembranesKnudsen diff intousion.was thehigher The unique permeation than microporous that ofof hydrogen carbon structure ofdioxide, all of rubber-derived TR indicatingmembrane membranesthatafter thethermal rubber was treatment higher polymer than due was that to thetransformedof carbonpresence dioxide, ofinto aromatic the indicating unique chains microporous that in as the-received rubber structure polymerreclaimed of wasTR rubber membrane transformed (SBR confirmed after into thermal the in unique previous treatment microporous section), due to ratherthestructure presence than of dense TR of membranearomatic rubber chains polymeric after thermalin as -mreceivedembrane. treatment reclaimed With due to regard therubber presence to (SBR the oflowerconfirmed aromatic experimental in chains previous in as-receivedselectivity section), thanratherreclaimed theoretical than rubber dense values, (SBR rubber confirmed this polymeric may inbe previous mascribedembrane. section), to Withthe presence rather regard than toof the densesurface lower rubber diffusion experimental polymeric because membrane.selectivity of the permeationthanWith theoretical regard temperature to the values, lower this(28 experimental ±may 2 ° C).be ascribedThe selectivity presence to thanthe of presence theoreticala surface of diffusionvalues, surface this effectdiffusion may weakens be because ascribed the ofother to the the presence of surface diffusion because of the permeation temperature (28 2 C). The presence of a mechanismpermeation (Knudsentemperature diffusion). (28 ± 2 °C). The presence of a surface diffusion± effect◦ weakens the other mechanismsurface diffusion (Knudsen effect diffusion). weakens the other mechanism (Knudsen diffusion). 5000 5.0 (b) (a) C4-P0-T350 C4-P0-T350 5000 4.55.0 C5-P0-T350 (b) C5-P0-T350 (a) C4-P0-T350 C4-P0-T350 4000 C6-P0-T350 4.04.5 C6-P0-T350 C5-P0-T350 C5-P0-T350 C7-P0-T350 C6-P0-T350 C7-P0-T350 4000 ) 3.54.0 C6-P0-T350 C7-P0-T350 a C7-P0-T350

) 3.5

3000 a 3.0

3000 2.53.0

2000 Selectivity ( 2.02.5 Selectivity (

Gas Permeance (GPU) 2000 1.52.0

Gas Permeance (GPU) 1000 1.01.5

1000 0.51.0

0 0.00.5 H 2 CO2 O2 N2 CH4 H2/CO2 H2/N2 H2/CH4 O2/N2 CO2/N2 CO2/CH4 0 0.0 H 2 CO2 O2 N2 CH4 H2/CO2 H2/N2 H2/CH4 O2/N2 CO2/N2 CO2/CH4 Figure 5. Gas separation performances obtained from resultant thermally rearranged (TR) Figure 5. Gas separation performances obtained from resultant thermally rearranged (TR) membranesFigure 5. Gas with separation different performances precursor concentration obtained froms resultant. (a: single thermally gas permeance; rearranged b (TR): selectivity membranes of membranes with different precursor concentrations. ( : single gas permeance; : selectivity of differentwith diﬀ erentgas pairs) precursor. concentrations. (a: single gas permeance;a b: selectivity of diﬀberent gas pairs). different gas pairs). CO O N CH H 5000 2 2 2 4 2 CO O N CH H 5000 C4-P0-T3502 2 2 R²4 = 0.9883 2 C4-P0-T350 R² = 0.9883 4000 C5-P0-T350 R² = 0.9987 C6-P0-T350C5-P0-T350 R²R² = = 0.99 0.9987

(GPU) 4000 C6-P0-T350 R² = 0.98920.99

(GPU) C7-P0-T350 3000 C7-P0-T350 R² = 0.9892 3000 2000 2000

Gas permeance permeance Gas 1000

Gas permeance permeance Gas 1000 0 00.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 Square root of the inverse of gas molecular weight, Square root of the inverse of gas molecular weight, FigureFigure 6. Single gasgas permeance permeance of of di ﬀdifferenterent gases gases obtained obtained from from resultant resultant TR membranes TR membranes with di ﬀwitherent differentFigureprecursor 6. precursor concentrationsSingle gas concentration permeance as a functions ofas adifferent function of square gasesof root square ofobtained inverse root of from of inverse gas resultant molecular of gas molecularTR weights. membranes weights. with different precursor concentrations as a function of square root of inverse of gas molecular weights.

PolymersPolymers 20202020,, 1122,, x 2540 FOR PEER REVIEW 9 9of of 16 15

3.3. Effect of Heating Temperature on Gas Separation Performance 3.3. Effect of Heating Temperature on Gas Separation Performance The results of pure gas permeation obtained at different heating temperatures showed no apparentThe resultsdifference of pure when gas permeationcompared to obtained that obtained at different from heating a different temperatures precursor showed concentration, no apparent becausedifference the when gas comparedpermeation to order that obtained was the from same a di(Figurefferent 7). precursor Consequently, concentration, the gas because transport the mechanismgas permeation of TR order membranes was the was same still (Figure Knudsen7). diffusion Consequently, when the heating gas transport temperature mechanism shifted of from TR 350membranes to 250 °C. was We still also Knudsen observed diff usionthat the when gas the permeance heating temperature of resultant shiftedmembranes from 350at a to heating 250 ◦C. temperatureWe also observed of 350 that °C, regardless the gas permeance of precursor of resultant concentration, membranes were higher at a heating compared temperature with membranes of 350 ◦C, atregardless a heating of temperature precursor concentration, of 250 °C. In were the higherheating compared process at with 350 membranes °C, the bulking at a heating rings create temperature more spaceof 250 to◦C. provide In the gases heating diffusion process because at 350 ◦ ofC, the the higher bulking thermal ringscreate conversion; more spacetherefore, to provide the effect gases of heatingdiffusion temperature because of theon higherthe as-prepared thermal conversion; membrane therefore, is a crucial the factor. effect ofIn heatingaddition, temperature the selectivity on theof theas-prepared H2/CO2 gas membrane pair obtained is a crucial for all factor. TR membranes In addition, theas a selectivity function of theprecursor H2/CO 2concentrationgas pair obtained and heatingfor all TR temperature membranes is as plotted a function in Figure of precursor 8. As shown concentration in Figure and 8, the heating selectivity temperature of all membranes, is plotted in regardlessFigure8. As of shown heating in temperature, Figure8, the selectivitywas in the ofrange all membranes, of 2.4–3.5, which regardless indicates of heating that the temperature, influence of alteringwas in the the range precursor of 2.4–3.5, concentration which indicates or heating that the temperature influence of alteringon membrane the precursor permselectivity concentration is insignificant.or heating temperature This inferson that membrane the pore permselectivitysize distinguishing is insignificant. between bigger This gases infers (CO that2) theand pore smaller size gasesdistinguishing (H2) would between not be bigger substant gasesially (CO shrunk2) and or smaller enlarged gases with (H2 )the would changes not be in substantiallyboth parameters. shrunk The or lowerenlarged selectivity with the of changes the TR inmembrane both parameters. was observed The lower when selectivity the precursor of the TRconcentration membrane was observedtoo high (6when and the7 wt precursor %); this concentrationcould be correlated was too with high the (6 derived and 7 wt membrane %); this could thickness. be correlated Based withon the the wor derivedk by D.membrane Grosso [28], thickness. the degree Based of on heat the for work the by membrane D. Grosso surface [28], the and degree the ofmembrane heat for the interior membrane was uneven surface inand the the carbonization membrane interior process was and uneven non-selectivity in the carbonization pores were process produced and as non-selectivity a result. Consequently, pores were whileproduced the membrane as a result. was Consequently, fabricated whilewith higher the membrane concentratio wasn fabricated and higher with heating higher temperature, concentration a suppressionand higher heating in H2/CO temperature,2 selectivity awas suppression observed. in H2/CO2 selectivity was observed.

FigureFigure 7.7. SingleSingle gasgas permeance permeance of differentof different gases gases obtained obtained from resultantfrom resultant TR membranes TR membranes with different with differentheating temperatures. heating temperature (a: 4 wt %s. precursor;(a: 4 wt % bprecursor;: 5 wt % precursor; b: 5 wt %c :precursor; 6 wt % precursor; c: 6 wt d%: precursor; 7 wt % precursor) d: 7 wt. % precursor).

Polymers 2020, 12, 2540 10 of 15 Polymers 2020, 12, x FOR PEER REVIEW 10 of 16

4 T=250 T=350

) 3 α

2 selectivity ( selectivity 2 /CO 2 H 1

0 3 4 5 6 7 8 Polymer conc. (wt. %)

Figure 8. SSelectivityelectivity over H 2/CO/CO22 gasgas pair pair of of resultant resultant TR TR membranes membranes fabricated fabricated with with different different heating temperatures.temperatures. (The icon in black represents the temperature at 250 ◦°C;C; the icon in red represents the temperature atat 350350 ◦°C)C).. 3.4. Effect of PPO Additive Content on Gas Separation Performance 3.4. Effect of PPO Additive Content on Gas Separation Performance Figure9 shows the influence of incorporating PPO additive into the rubber precursor on the Figure 9 shows the influence of incorporating PPO additive into the rubber precursor on the morphological structure of the derived TR membrane. No observable cracks were observed on the morphological structure of the derived TR membrane. No observable cracks were observed on the membrane surface of C7-P2.8-T250 compared with other TR membranes. Furthermore, the reduction membrane surface of C7-P2.8-T250 compared with other TR membranes. Furthermore, the reduction of membrane surface roughness from 147 nm to 49.8 nm indicated a significant densification of the of membrane surface roughness from 147 nm to 49.8 nm indicated a significant densification of the membrane surface after incorporating the PPO additive at 2.8 wt %. As expected, by adding the membrane surface after incorporating the PPO additive at 2.8 wt %. As expected, by adding the PPO PPO modifier into membrane matrix, the membrane surface became smoother. The AFM results modifier into membrane matrix, the membrane surface became smoother. The AFM results corroborated this finding, indicating that the membrane was successfully modified by the presence corroborated this finding, indicating that the membrane was successfully modified by the presence of PPO. Polymersof PPO. 20 20, 12, x FOR PEER REVIEW 11 of 16 As shown in Figure 10, the C7-P2.8-T250 TR membrane exhibited the highest selectivity for all gas pairs. Specifically, H2/CO2 selectivity differed nearly by a factor of two and was observed to be 4 and 2.68 for C7-P2.8-T250 and C7-P0-T250, respectively. These findings indicate that the incorporation of PPO additive enhanced the permselectivity of the derived TR membrane for H2/CO2. These results can be attributed to the dense membrane structure obtained by blending a thermally stable polymer, in this case PPO, and are in accordance with previous studies [29,30]. According to the work of Ozaki [31], the pore structure of a carbon membrane is strongly influenced by polymeric precursors, and their thermal properties are vital in determining the derived carbon membrane performance. During thermal treatment, a thermally liable polymer precursor tends to decompose into volatile gas. This provides a diffusion path for gas, leading to an increase in permeance and a decrease in selectivity. The opposite trend in terms of gas permeance and selectivity was observed for the thermally stable polymer in this study. Hence, the addition of a thermally stable polymer (PPO, in this study) improved the resultant TR membrane’s selectivity because of the thermal stable property of the PPO during the heating procedure.

FigureFigure 9. 9. SEMSEM images images ( (11)) of of rubber rubber-derived-derived TR TR membranes membranes as as a a function function of of polyphenylene polyphenylene oxide oxide ((PPO)PPO) additive additive content content and and corresponding corresponding atomic atomic force force microscopy microscopy ( (AFM)AFM) images images ( (22).). ( (aa: :C7 C7-P0-T250;-P0-T250; bb:: C7 C7-P0.7-T250;-P0.7-T250; cc:: C7 C7-P2.8-T250).-P2.8-T250).

Figure 10. Gas separation performances obtained from rubber-derived TR membranes as a function of PPO additive content. (a: single gas permeance; b: selectivity of different gas pairs).

3.5. Comparison of Rubber-Derived TR Membrane with Other Published Literature Table 2 compares the gas separation performance of the TR membrane derived from reclaimed rubber to that of TR membranes derived from expensive precursors [9,10,32–35]. The rubber-derived membrane at a lower thermal rearrangement temperature (250 °C) was comparable to other TR membranes fabricated with expensive polymer precursors at higher heating temperatures because of its acceptable selectivity, remarkable hydrogen permeability, and competitive and feasible

Polymers 2020, 12, x FOR PEER REVIEW 11 of 16

Polymers 2020, 12, 2540 11 of 15

As shown in Figure 10, the C7-P2.8-T250 TR membrane exhibited the highest selectivity for all gas pairs. Specifically, H2/CO2 selectivity differed nearly by a factor of two and was observed to be 4 and 2.68 for C7-P2.8-T250 and C7-P0-T250, respectively. These findings indicate that the incorporation of PPO additive enhanced the permselectivity of the derived TR membrane for H2/CO2. These results can be attributed to the dense membrane structure obtained by blending a thermally stable polymer, in this case PPO, and are in accordance with previous studies [29,30]. According to the work of Ozaki [31], the pore structure of a carbon membrane is strongly influenced by polymeric precursors, and their thermal properties are vital in determining the derived carbon membrane performance. During thermal treatment, a thermally liable polymer precursor tends to decompose into volatile gas. This provides a diffusion path for gas, leading to an increase in permeance and a decrease in selectivity.

The opposite trend in terms of gas permeance and selectivity was observed for the thermally stable polymerFigure in this9. SEM study. images Hence, (1) of the rubber addition-derived of a thermallyTR membranes stable as polymer a function (PPO, of polyphenylene in this study) oxide improved the resultant(PPO) additive TR membrane’s content and selectivitycorresponding because atomic of force the microscopy thermal stable (AFM property) images of(2). the (a: C7 PPO-P0 during-T250; the heatingb: C7 procedure.-P0.7-T250; c: C7-P2.8-T250).

Figure 10. Gas separation performances obtained from rubber-derived TR membranes as a function of FigurePPO additive 10. Gas content. separation (a: singleperformances gas permeance; obtainedb :from selectivity rubber of-derived diﬀerent TR gas membranes pairs). as a function of PPO additive content. (a: single gas permeance; b: selectivity of different gas pairs). 3.5. Comparison of Rubber-Derived TR Membrane with Other Published Literature 3.5. Comparison of Rubber-Derived TR Membrane with Other Published Literature Table2 compares the gas separation performance of the TR membrane derived from reclaimed rubberTable to that 2 compares of TR membranes the gas separation derived from performance expensive of precursors the TR membrane [9,10,32– derived35]. The from rubber-derived reclaimed rubbermembrane to that at of a lowerTR membranes thermal rearrangementderived from expensive temperature precursors (250 ◦ C)[9,10,32 was– comparable35]. The rubber to other-derived TR membranemembranes at fabricated a lower withthermal expensive rearrangement polymer precursorstemperature at higher(250 °C) heating was comparable temperatures tobecause other TR of membranesits acceptable fabricated selectivity, with remarkable expensive hydrogen polymer permeability, precursors at and higher competitive heating temperatures and feasible commercial because of itsproperties. acceptable As theselectivity, hydrogen remarkable capacity of thehydrogen demonstration permeability, plant in Japanand competitive is 50 Nm3/h [and36], thefeasible plant needs a membrane with high ﬂux but a reasonable separation factor for hydrogen enrichment. The C7-P2.8-T250 membrane synthesized during this work, with a hydrogen permeability of 1333 Barrer, exhibits a similar hydrogen permeance to membranes applied in a biogas plant [37]. Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

commercial properties. As the hydrogen capacity of the demonstration plant in Japan is 50 Nm3/h [36], the plant needs a membrane with high flux but a reasonable separation factor for hydrogen enrichment. The C7-P2.8-T250 membrane synthesized during this work, with a hydrogen permeability of 1333 Barrer, exhibits a similar hydrogen permeance to membranes applied in a biogas plant [37]. Hence, the as-prepared membrane with high flux has potential for application in the commercial H2/CO2 gas-separation field. The binary gas mixture test will be performed in our future work to better understand the separation capability of rubber-derived membranes. In addition, for membrane scientists, the Robeson upper bound set the appealing target to aim for and is a commonly used standard with which to evaluate the gas separation performance in the field of polymeric membranes. To compare our work with other studies in the field of TR membranes, the optimum gas separation result in this work (C7-P2.8-T250) and other published studies were illustrated in Robeson upper bound plots for H2/CO2 separation. The comparison clearly indicates that the gas transport capability of the rubber-derived membrane (C7-P2.8-T250) lies in the upper- right quadrant of Figure 11 and crosses the upper limit for H2/CO2 applications, showing that it can be recognized as an attractive candidate for H2/CO2 gas separation. The practical feasibility for the membrane’s commercial application was preliminarily evaluated from the perspective of price per unit membrane area (USD/m2). The price per unit membrane area Polymers 2020, 12, 2540 12 of 15 (USD/m2) was determined by the cost of the raw material, including the polymer precursor, and the used solvent and additive, etc. Table 3 summarizes the data and assumptions employed in this study. Hence,The cost the of as-preparedrequired solvent membrane per ton withrubber high equals flux USD has potential50,309.4/ton, for applicationwhich was calculated in the commercial from the Hsolvent2/CO2 pricegas-separation multiplied field. by the The rubber binary solubility gas mixture in solvent. test will The be cost performed of raw materials in our future ranged work from to betterUSD 51,864.40 understand to USD the separation 52,975.40/ton, capability which ofwas rubber-derived obtained by summing membranes. the price of reclaimed rubber [38] Inand addition, the cost forof membranerequired solvent scientists, and thesubtracting Robeson the upper value bound of national set the appealingaid. The price target per to aimunit formembrane and is a area commonly (USD/m used2) in standardthis work with was whichcalculated to evaluate to be 2.1 the–3.2 gas USD separation/m2, which performance was acquired in the by fieldmultiplying of polymeric the cost membranes. of raw materials To compare and ourthe workrequired with polymer other studies per unit in the membrane. field of TR According membranes, to theRichard optimum W. Baker gas separation[39], if the resultadopted in thisprecursor work (C7-P2.8-T250)is commercial polymer, and other these published membrane studies module weres 2 illustratedvary in price in Robesonfrom 10 to upper 100 ( boundUSD/m plots), which for His2 /farCO more2 separation. expensive The than comparison our figures. clearly It is clear indicates that thatthe membr the gasane transport prepared capability with reclaimed of the rubber-derived rubber is competitive membrane in (C7-P2.8-T250) the capital market. lies in We the upper-rightbelieve this quadranttechnology of is Figure worthy 11 of and development crosses the in upper the future limit and for H could2/CO contribute2 applications, to a significant showing that reduction it can bein recognizedtire waste. as an attractive candidate for H2/CO2 gas separation. 1000

Robeson's 2008 upper bound 100

C7-P2.8-T250 10 permselectivity 2 PANI/PBI [35] PBI [34] /CO 2 1 H HAB-6FDA [33]

6FDA-HAB5DAM5 [32] 0.1 0.01 0.1 1 10 100 1000 10000 100000 H2 permeability (Barrer)

2 2 FigureFigure 11. Robeson 11. Robeson plot for plot H /CO for H separation2/CO2 separation compar comparinging the rubber the-derived rubber-derived TR membrane TR membrane with other with TR membraneother TRprepared membrane by different prepared precursors. by diﬀerent precursors.

The practical feasibility for the membrane’s commercial application was preliminarily evaluated from the perspective of price per unit membrane area (USD/m2). The price per unit membrane area (USD/m2) was determined by the cost of the raw material, including the polymer precursor, and the used solvent and additive, etc. Table3 summarizes the data and assumptions employed in this study. The cost of required solvent per ton rubber equals USD 50,309.4/ton, which was calculated from the solvent price multiplied by the rubber solubility in solvent. The cost of raw materials ranged from USD 51,864.40 to USD 52,975.40/ton, which was obtained by summing the price of reclaimed rubber [38] and the cost of required solvent and subtracting the value of national aid. The price per unit membrane area (USD/m2) in this work was calculated to be 2.1–3.2 USD/m2, which was acquired by multiplying the cost of raw materials and the required polymer per unit membrane. According to Richard W. Baker [39], if the adopted precursor is commercial polymer, these membrane modules vary in price from 10 to 100 (USD/m2), which is far more expensive than our ﬁgures. It is clear that the membrane prepared with reclaimed rubber is competitive in the capital market. We believe this technology is worthy of development in the future and could contribute to a signiﬁcant reduction in tire waste. Polymers 2020, 12, 2540 13 of 15

Table 2. Comparison of rubber-derived TR membrane with other reports with respect to hydrogen

permeability and H2/CO2 selectivity.

Permeability (Barrer) * Thermal Rearrangement H2/CO2 Polymer Reference Protocols H2 CO2 Selectivity

6FDA-HAB5DAM5 450 ◦C for 1 h 1318 1607 0.82 [32] HAB-6FDA 450 ◦C for 1 h 530 410 1.29 [33] Aromatic Polyimides 450 ◦C for 1 h 738 295 2.50 [9] Poly(benzoxazole-co-imide) 400 ◦C for 2 h 222.1 172.8 1.29 [10] Polybenzimidazole (PBI) 450 ◦C for 1 h 1779 1624 1.1 [34] PANI/PBI 300 ◦C for 3 h 3.69 0.242 15.2 [35] C7-P2.8-T250 250 ◦C for 2 h 1333 333.25 4.0 This work *: Barrer = 1 10 10 cm3(STP)cm cm 2 s 1 cmHg; the C7-P2.8-T250 membrane thickness is 1.116 µm. × − · − − Table 3. Summary of material price data and assumptions in this work.

Item The price of reclaimed rubber [38] USD 1555–2666/ton The required polymer per unit membrane 4–6 10 5 ton/m2 × − The price of solvent (toluene) USD 0.52/100 mL The rubber solubility in solvent 1.04 10 5 ton reclaimed/100 mL solvent × − National aid USD 100/ton reclaimed rubber

4. Conclusions Our main conclusions are as follows:

(a) The as-received reclaimed rubber was intensively investigated by TGA and FTIR analysis. The results revealed that the rubber type of reclaimed rubber is styrene–butadiene rubber, which accounted for more than half of the total components (57%). (b) The presence of carbon black and other filler was also found, and their presence decreased the thermal stability of rubber. (c) We measured the degree of devulcanization and the results showed that reclaimed rubber exhibited 45.5% devulcanization. (d) According to the gas permeation tests, all rubber-derived TR membranes exhibited the Knudsen diffusion mechanism. In particular, the C7-P2.8-T250 membrane, with an ideal H2/CO2 selectivity of four and hydrogen permeability of 1333 Barrer, demonstrated a superior gas transport capability, mainly due to the conversion of the reclaimed rubber layer into a microporous membrane structure and the incorporation of a thermal stable filler.

In conclusion, these conditions showed that reclaimed rubber has significant potential as a precursor for fabricating TR membranes for the hydrogen enrichment industry.We have provided a viable approach to adopt reclaimed rubber as a precursor for preparing TR membranes with high performance for H2/CO2 separation.

Author Contributions: Conceptualization, M.-Y.W. and H.-H.T.; validation, G.-L.Z.; investigation, G.-L.Z.; data curation, Y.-T.L. and G.-L.Z.; writing—original draft preparation, Y.-T.L.; writing—review and editing, M.-Y.W. and H.-H.T.; visualization, Y.-T.L.; supervision, M.-Y.W. and H.-H.T.; project administration, M.-Y.W. and H.-H.T.; funding acquisition, M.-Y.W. and H.-H.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Environmental Protection Administration Taiwan (EPAT), Project No. EPA-104-X05. Acknowledgments: The authors would like to thank the Environmental Protection Administration Taiwan (EPAT) (No. EPA-104-X05) for the encouragement and financial support. Conflicts of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Polymers 2020, 12, 2540 14 of 15

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