Carbon Dioxide-Based Polyols as Sustainable ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Carbon Dioxide-Based Polyols as Sustainable Feedstock of Thermoplastic Polyurethane for Corrosion-Resistant Metal Coating Prakash Alagi,† Ravindra Ghorpade,† Ye Jin Choi,† Umakant Patil,‡ Il Kim,*,§ Joon Hyun Baik,*,∥ and Sung Chul Hong*,† †

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 143-747, Republic of Korea ‡ School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea § Department of Polymer Science and Engineering, Pusan National University, 2 Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Republic of Korea ∥ Climate and Energy Research Group, Research Institute of Industrial Science & Technology, 67 Cheongam-ro, Nam-gu, Pohang 37673, Republic of Korea S Supporting Information *

ABSTRACT: Abundant, inexpensive, renewable, and nontoxic carbon dioxide (CO2) has become an attractive feedstock for chemical and polymer syntheses. The use of CO2 as a sustainable precursor for polyurethane has become prominent in polymer industry. In this study, polyols produced from CO2 were successfully incorporated into thermoplastic polyurethanes (TPUs). The thermal, mechanical, shape memory, and anticorrosion properties of the TPUs were investigated. TPUs with CO2-based polyols appeared as hard plastics with relatively high Tg and tension set values. The rigid carbonate units of the CO2based polyols reduced the softness of the polyol chains. The CO2-based polyols also afforded TPU with excellent shape memory characteristics, exhibiting shape fixity and shape recovery values of almost 100%. Interestingly, the incorporation of CO2-based polyols into TPUs improved the anticorrosion characteristics, regardless of the corrosive media. The improved anticorrosion characteristics stemmed from the robust, hydrophobic, and blocking properties of the carbonate units. This allows the TPU to be used in hard coatings for high-performance applications. CO2-based polyols are promising alternatives to conventional petroleum-based polyols and can be used for the fixation of waste CO2 and decreasing the carbon footprint of chemical processes. KEYWORDS: Carbon dioxide, Polyol, Thermoplastic polyurethane, Shape memory, Corrosion resistance

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INTRODUCTION Chemical fixation and transformation of abundant, inexpensive, and nontoxic carbon dioxide (CO2) to value-added resources has attracted considerable attention, because of increasing concerns over environmental and sustainable development.1−3 In this context, CO2 has become a crucial feedstock for chemical and polymer syntheses,4,5 where the fixation of waste CO2 as a recyclable resource is particularly desired (see Figure S1 in the Supporting Information).6−8 Polyols are essential raw materials for producing polymers. Polyols are categorized into three compositions, i.e., ester-, ether-, and carbonate-based polyols.9 The conventional polyols such as polyether-based polyols (e.g., polyethylene glycol, polypropylene glycol, polytetrahydrofuran), polyester-based polyols (e.g., polycaprolactone diol, polybutylene adipate glycol), and polycarbonate-based polyols (e.g., polyhexamethylene carbonate diol) are mainly obtained from petroleum resources.10 Interest in the development of an eco-friendly process to prepare polyols from renewable resources has © 2017 American Chemical Society

recently increased. The chemical transformation of vegetable oil to polyols is a representative example of such an endeavor.11−16 However, the use of edible resources for industrial products is controversial. CO2 is an attractive alternative resource for preparing eco-friendly polyols. The production of polyols with CO2 is essentially a copolymerization between CO2 and epoxides.17,18 In CO2based polyols, CO2 is directly incorporated into the carbonate units of polyols during copolymerization without requiring energy-intensive CO bonds cleavage.6,10,19 These polyols comprise more than 40 wt % CO2 and can be produced at 20− 30% lower production costs than conventional polyether-based polyols.20,21 The CO2-based polyols are highly sustainable in terms of the greenhouse gas emission and the depletion of fossil resources. The life cycle assessment studies indicate that the Received: December 14, 2016 Revised: March 30, 2017 Published: April 12, 2017 3871

DOI: 10.1021/acssuschemeng.6b03046 ACS Sustainable Chem. Eng. 2017, 5, 3871−3881

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ACS Sustainable Chemistry & Engineering

Preparation of TPUs from CO2-Based Polyols and PTMEG. TPUs with CO2-based polyols were prepared through a two-step reaction in a three-necked flask equipped with a mechanical stirrer, a condenser, a nitrogen inlet, and a thermometer. In the first step, isocyanate-terminated PU prepolymers were prepared by reacting the polyol and a predetermined excess amount of MHI in DMF at 80 °C for 2 h. BD and DBTDL (0.01 wt %) were then added to the reaction mixture as a chain extender and a catalyst, respectively. The reaction was continued at 80 °C for an additional 8−12 h. The detailed recipe for the preparation of TPU with a CO2-based polyol is provided in Table S1 in the Supporting Information). After a sufficient increase in viscosity, methanol was added to quench the final reaction mixture. The TPU was recovered by precipitating the reaction mixture in a 10fold excess of methanol, followed by drying under vacuum at 40 °C for 24 h. TPU with PTMEG and mixed polyols (i.e., a mixture of the CO2-based polyol and PTMEG diol) were also prepared, following the same procedure. Characterization. Proton nuclear magnetic resonance (1H NMR) was recorded on a Bruker 500 MHz Avance system to elucidate the structural characteristics of the CO 2-based polyols and the corresponding TPUs. The 1H NMR spectra were obtained at room temperature using deuterated chloroform as a solvent. Size exclusion chromatography (SEC) was performed on a Shimadzu Model LC-20A that was equipped with PSS columns (Styragel HR 2, 4, and 5) and a Shimadzu Model RID-10A refractive index detector. THF was used as an eluent at a flow rate of 1.0 mL/min. A calibration curve to determine the molecular weight was obtained using a polystyrene standard. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) spectra were obtained on a Voyager mass spectrometer (Applied Biosystems). The instrument was equipped with a pulsed nitrogen laser (337 nm) and a timedelayed extracted ion source. The samples were prepared in THF, using dithranol as a matrix. Spectra were recorded in positive ion mode using reflection and an accelerating voltage of 20 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Thermo Nicolet 380 FTIR spectrometer at a resolution of 4 cm−1 over a wavenumber range of 4000−500 cm−1. The mechanical properties of the samples were investigated by subjecting dumbbell-shaped specimens to tensile tests. The TPU samples were compressed and molded between Teflon sheets using a hot press (Model QM900 M, Qmesys) at 100−110 °C under a pressure of 15 MPa for 15 min to yield sample sheets (150 mm × 150 mm × 1 mm). Dumbbell-shaped specimens were then obtained from the film sheet following ASTM Standard D412. The tensile tests were performed using a universal testing machine (Model AGS-5kNX, Shimadzu) at room temperature, following ASTM Standard D412. The results from at least five samples were averaged to determine the mechanical property values. Tension set values were also determined, following ASTM Standard D412. The dumbbellshaped specimens were elongated to 100% for 10 min. Permanent changes in the tensile dimensions were measured 10 min after releasing the loads. The tension set values were determined using the following equation:

global warming impact of CO2-based polyols is 11%−19% lower than that of conventional polyether-based polyol.20 In addition, the use of fossil resources can be reduced by 13%− 16% through the utilization of CO2.20 The production of polyols from CO2 has been commercialized by Novomer in the United States. Other companies, such as Covestro, Sumitomo, and China Blue Chemical, are also in the process of commercialization.22 Polyurethanes (PUs) are one of the most important classes of polymers prepared from polyols and have a wide range of characteristics, varying from thermoplastic to thermoset.23−26 Recently, CO2-based polyols have been employed for the preparation of PUs.6,22 The production of PU foams with CO2based polyols has been demonstrated, because of their favorable glass-transition temperature and low viscosity.19 Wang et al. reported on the properties of waterborne PUs with CO2-based polyols, which exhibited superior hydrolysis and oxidation resistances, compared with PUs with polyether- and polyesterbased polyols.27 Orgilés-Calpena et al. prepared a PU adhesive from CO2-based polyols that satisfied footware quality requirements.10 However, the utilization of CO2-based polyols for the preparation of PUs remains limited. The characteristics of PUs with CO2-based polyols must be widely investigated to determine their potential applications for market. Thermoplastic polyurethanes (TPUs) are one of the most interesting types of PUs, because they are recyclable and thermally processable via extrusion, injection molding, and thermoforming.28 Over 400 000 metric tons of TPUs were produced in 2010.29 TPUs are widely used in various applications, such as automobile parts, sporting goods, and electronic/medical devices, because of their superior flexibility, elasticity, strength, transparency, and abrasion resistance.30 TPUs produced from conventional petroleum-based polyester polyols generally possess good mechanical properties. However, their usage is often limited due to poor hydrolysis resistance.31 TPUs produced from petroleum-based polyether polyols typically show good hydrolysis resistance, whereas their oxidation resistance and mechanical properties are generally poor.32 The incorporation of CO2-based polycarbonate− polyether polyols into TPUs is expected to address these issues by enhancing the thermal and mechanical performances and oxidative degradation resistance.20,27,33 In this study, the use of CO2-based polyols for preparing TPUs was explored. The mechanical and thermal properties of the TPUs were investigated and compared with those of TPUs produced from conventional petroleum-based polyether polyols. The electrochemical resistances of the TPUs were also investigated under various corrosive media to evaluate their degradation resistance.

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E (%) = 100 ×

L − L(0) L(0)

(1)

where E is the tension set value (%), L(0) the original length of the specimen, and L the permanent length of the specimen after elongation and load release. Average tension set values were determined for at least five tests for each sample. The shape memory properties of TPUs with CO2-based polyols and PTMEG were tested according to reported procedures.34−36 The samples were first placed in a chamber at a specified temperature (Tg + 10 °C) for 10 min (where Tg is the glass-transition temperature), followed by clamping of the samples. The distance between the clamps was marked with a pencil and recorded as L0. The samples were stretched to the length required (L1) for specific stretching (100%) and were subsequently placed in a low-temperature chamber (Tg − 20 °C) for 10 min. The clamps were released and the samples were kept in the chamber for 10 min to reach a fixed length (L2). The samples were moved to another

EXPERIMENTAL SECTION

Materials. CO2-based polyol [Converge Polyol 212−20, Novomer, USA, number-average molecular weight (Mn) of ∼2000 g/mol], poly(tetrahydrofuran) diol (PTMEG, Aldrich, St. Louis, MO, USA, Mn ∼2000 g/mol), 4,4′-methylenebis(cyclohexyl isocyanate) (MHI, 90%, Aldrich, St. Louis, MO, USA), dibutyltin dilaurate (DBTDL, 95%, Aldrich, St. Louis, MO, USA), and 1,4-butanediol (BD, 99%, Aldrich, St. Louis, MO, USA) were used as received. N,N′-Dimethylformamide (DMF, 99%, Samchun Chemicals, Seoul, Korea), tetrahydrofuran (THF, >99%, Daejung Chemicals, Seoul, Korea), and methanol (99.5%, Samchun Chemicals, Seoul, Korea) were also used without further purification. 3872


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Figure 1. (a) 1H NMR spectrum, (b) representative MALDI-TOF MS spectrum, and (c) SEC trace of CO2-based polyol. chamber with a higher temperature (Tg + 20 °C) for 10 min. The length between the marked lines was finally recorded as L3. The shape fixity (Rf) and shape recovery (Rr) ratios were calculated using the following equations:

⎛ L − L0 ⎞ R f (%) = ⎜ 2 ⎟ × 100 ⎝ L1 − L0 ⎠

(2)

⎛ L − L3 ⎞ R r (%) = ⎜ 2 ⎟ × 100 ⎝ L 2 − L0 ⎠

(3)

specimens were used as the reference, counter, and working electrodes, respectively. A 4.0 cm2 area of the working electrode SS plate was exposed to the corrosive solution. Before the electrochemical measurements, the working electrode was stabilized, and its open circuit potential (OCP) was recorded as a function of time for 600 s. After the OCP was stabilized, impedance measurements were performed at respective corrosion potentials (Ecorr) over a frequency range of 100 kHz to 0.1 Hz, with a signal amplitude perturbation of 10 mV. The PDP tests were carried out in the potential range of ±250 mV (with respect to OCP) at a 10 mV/s scan rate. The anticorrosion behavior of the TPU coatings was estimated based on Tafel parameters and Nyquist plots recorded when the coated and bare SS plates were immersed in 3.5 wt % HCl, NaOH, and NaCl solutions at room temperature. Electrochemical corrosion rates were determined by a Tafel extrapolation method, in which high cathodic and anodic polarizations provided cathodic and anodic polarization curves for the respective corrosion processes. The Ecorr and corrosion current density (Icorr) values were evaluated by extrapolating these curves in their linear regions to the point of intersection.37 The polarization resistance (Rp) values were determined from Tafel plots by using the Stearn− Geary equation. Detailed experimental setup and procedures are presented in Figure S2 in the Supporting Information.

The thermal behavior of the TPUs was observed using differential scanning calorimetry (DSC) (Model DSC4000, PerkinElmer) in a nitrogen atmosphere. Briefly, 7−10 mg samples were placed in aluminum pans and heated to 180 °C to eliminate any thermal history. Tg values were determined by heating the samples from −90 °C to 200 °C at a heating rate of 10 °C/min. The Tg values were determined using the midpoint temperature in the heat capacity change of the DSC scans. Thermogravimetric analyses (TGA) were performed on a Model TGA4000 system (PerkinElmer). Briefly, 7−10 mg samples were heated from room temperature to 800 °C under a 20 mL/min nitrogen flow at a heating rate of 10 °C/min. Electrochemical corrosion resistance were investigated through potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). PDP and EIS were used to evaluate the barrier properties of the TPU coating against basic, acidic, and salt solutions. Stainless steel (SS) plates were polished with fine abrasive papers, followed by consecutive washing and drying with deionized (DI) water and acetone. The SS plates were then coated with different TPU solutions using a brush. The thicknesses of the coatings were controlled to be ∼20 μm, as determined by a digital outside micrometer (MDM-25N). Six different TPU coatings were prepared for each sample and three coatings with similar thickness (∼20 μm) were selected among the coatings for anticorrosion testing (see Table S2 in the Supporting Information). PDP and EIS tests of the coated and bare SS plates were performed by using a three-electrode glass cell (ZIVE SP2 LAB analytical equipment, South Korea) in 3.5 wt % HCl, NaOH, and NaCl solutions at room temperature. Ag/AgCl, platinum, and the test

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RESULTS AND DISCUSSION The chemical structure of a typical CO2-based polyol was investigated using 1H NMR and matrix-assisted laser desorption ionization−time-of-flight mass spectroscopy (MALDI-TOF MS), as shown in Figures 1a and 1b. The signals at 4.8−5.0 ppm, 3.9−4.3 ppm, and 1.3−1.5 ppm in Figure 1a represented the protons of CH, CH2, and CH3 groups of the carbonate units, respectively. The signal at 1.2−1.3 ppm was assigned to the protons of the terminal CH3 group. The composition of the carbonate unit in the CO2-based polyol was calculated to be 93.4 wt %, whereas that of the terminal propylene oxide unit was determined to be 6.6 wt %. The molecular weight of the CO2-based polyol was determined to be 1890 g/mol, using 3873


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ACS Sustainable Chemistry & Engineering Table 1. Preparation and Characteristics of TPUs with CO2-Based Polyol and PTMEG (a) Characteristics

a b

designation

[CO2-based polyol]

TPU-N-1 TPU-NP-2 TPU-NP-3 TPU-NP-4 TPU-P-5

1.0 1.0 1.0 1.0

[PTMEG]

[MHI]

1.0 2.0 3.0 1.0

3.0 6.0 9.0 12.0 3.0

designation

glass-transition temperature,c Tg (°C)

TPU-N-1 TPU-NP-2 TPU-NP-3 TPU-NP-4 TPU-P-5

37 34 33 29 11

[BD]

[NCO]/[OH]

hard segmenta (wt %)

number-average molecular weight,b Mn (g/mol)

2.0 1:1 32.6 4.0 1:1 32.6 6.0 1:1 32.6 8.0 1:1 32.6 2.0 1:1 32.6 (b) Thermal and Mechanical Properties Young’s modulus (MPa) 290 346 95 65 7.0

± ± ± ± ±

15.0 16.0 4.0 8.0 2.0

22 35 27 28 26

tensile strength (MPa) 8.1 12.5 7.7 6.3 8.1

± ± ± ± ±

1.0 2.0 0.2 0.3 1.0

600 300 600 000 500

elongation at break (%) 118 349 358 337 850

± ± ± ± ±

20 24 14 31 25

tension set (%) 28.5 19.3 13.8 13.1 6.3

± ± ± ± ±

2.0 1.2 1.1 1.1 0.7

Content of hard segment = [(mass of MHI + mass of BD)/(mass of MHI + mass of BD + mass of PTMEG + mass of CO2-based polyol)] × 100. Determined by SEC analyses. cTg determined by DSC analyses.

−CO at 1670−1700 cm−1, −NH−CO at 1500−1530 cm−1, C−O at 1220−1230 cm−1, and −O−CO at 1040−1115 cm−1),42 indicating successful preparations of the TPUs. The presence of the carbonate unit in TPUs was also confirmed by the presence of additional carbonyl peaks at 1745 cm−1 in TPU-N-1 and TPU-NP-2 (Figure 2). The characteristic 1H NMR peaks of the TPUs also supported the successful incorporation of CO2-based polyols and PTMEG into the TPUs (peaks a, c, d, e, and f in Figure 3). The peaks at 1.3 and 2.0 ppm corresponded to the protons of −CH2 at MHI and the protons of −CH2 adjacent to the urethane linkage, respectively (see Figure 3). The stress−strain curves of the TPUs are presented in Figure 4a, and the corresponding mechanical property parameters are listed in Table 1. TPUs with the CO2-based polyol appeared as hard plastics with high Young’s modulus values (290 ± 15 MPa) and low elongation at break values (118% ± 20%). The specific carbonate structure of the CO2-based polyol was responsible for the rigidity of the materials. The carbonate unit of the CO2-based polyol directly reduced the flexibility of the polyol chain, resulting in TPUs with more plastic character.19,27,33,43 The CO2-based polyol exhibited relatively high Tg value (6 °C), because of the presence of rigid carbonate groups (see Figure S4 and Table S3 in the Supporting Information). The TPU with the CO2-based polyol (TPU-N-1) also exhibited a relatively higher Tg value (37 °C) than that of the TPU with PTMEG (TPU-P-5, 11 °C), again indicating the high rigidity and polarity of the carbonate units (see Figure S4, as well as Table 1).19,27,33,43 Decreased carbonate unit contents in the mixed polyols resulted in TPUs with reduced Tg values. When the CO2-based polyol content decreased in the mixed polyols, the Tg values of the TPUs (TPU-NP-2−TPU-NP-4) decreased from 37 °C to 29 °C, because of the incorporation of soft PTMEG (see Figure S4 and Table 1). The Young’s modulus and elongation at break values of TPU-NP-2, in which equal proportions of soft PTMEG and CO2-based polyol were used, increased to 346 ± 16.0 MPa and 349 ± 24%, respectively, indicating a hard plastic with elastomeric properties. Additional increases in the PTMEG content in the TPUs (TPU-NP-3 and TPU-NP-4) resulted in greater elastomeric properties (i.e., softer), because of the increased amount of flexible PTMEG, a lower Young’s modulus

MALDI-TOF MS (Figure 1b). The Mn value of CO2-based polyol, determined by SEC analysis, was 2600 g/mol, which was a slightly higher value than that determined by MALDITOF analyses (1890 g/mol), probably due to different hydrodynamic volumes of the PS standards and CO2-based polyol in SEC analyses (see Figure 1c, as well as Table S3 in the Supporting Information). The molecular weight difference between the peaks in Figure 1b was 102 m/z. This clearly corresponded to the carbonate repeating unit of the CO2-based polyol. A minor amount of cyclic carbonate was noticed in the polyol (Figure 1a), which probably originated from a backbiting side reaction of epoxide and CO2 (see Figure S3 in the Supporting Information).38,39 However, the extent of the side reaction and other degradation reactions was not significant, as evidenced by the low signal intensity of the cyclic carbonate species in 1H NMR analysis (Figure 1a) and narrow PDI value of the polyol (1.04 in Figure 1c).40,41 To elucidate the effects of different compositions of polyols on the characteristics of TPUs, TPUs employing MHI and different types of polyols were prepared. The TPUs were prepared through two-step polyaddition reactions of MHI and the polyols at 80 °C in the presence of DBTDL. A TPU with a CO2-based polyol was designated as TPU-N-1 (see Table 1). Mixtures of polyols composed of CO2-based polyol and conventional petroleum-based PTMEG ([CO2-based polyol]/ [PTMEG] = 1/1−1/3) were also used for the preparation of TPUs (TPU-NP-2−TPU-NP-4 in Table 1). A TPU with PTMEG was also prepared as a reference (TPU-P-5 in Table 1). The overall reaction schemes and recipes for the preparation of the TPUs are presented in Scheme 1 and Table 1, respectively. Aliphatic diisocyanate (MHI) was employed to minimize potential yellowing and carcinogenic degradation issues of the product.9 For all TPUs, the solid contents were adjusted to 15%−20% to avoid gel formation. The contents of hard segments in the TPUs were controlled to the same value (i.e., 32.6 wt %). The Mn values of TPUs were carefully adjusted to be similar to each other (28 000 ± 4600 g/ mol; see Table 1) through a frequent monitoring over the molecular weight values of the TPUs by using SEC during the polymerization. FT-IR spectroscopy demonstrated the disappearance of the isocyanate band (2262 cm−1; see Figure 2) and the development of urethane linkages in the TPUs (−NH at 3330 cm−1, 3874


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Scheme 1. Schematic Representation of the Preparation of (a) TPUs with CO2-Based Polyol and (b) the Mixture of CO2-Based Polyol and PTMEG

(95 ± 4.0 MPa for TPU-NP-3 and 65 ± 8.0 MPa for TPU-NP4), and higher elongation at break values (358% ± 14% for TPU-NP-3 and 337% ± 31% for TPU-NP-4). TPU-P-5 with conventional PTMEG appeared as a soft elastomer with a low Young’s modulus value of 7 ± 2.0 MPa and a high elongation at break value of 850% ± 25%. The elasticities of the TPUs were also confirmed by determining the tension set values. The tension set value of TPU-N-1 was 28.5% ± 2.0%, which was obviously higher than those of the TPUs with PTMEG. The tension set values of TPU-NP-2−TPU-NP-4 were 13.1%− 19.3%, indicating good elastomeric properties (Figure 4b and Table 1).

The TGA curves (Figure 5a) and derivative curves (Figure 5b) of the TPU with the CO2-based polyol (TPU-N-1) indicated a two-step degradation process. The first degradation step at 210−250 °C represented the decomposition of carbonate units. The weight loss in the second degradation step at 270−340 °C corresponded to the decomposition of the urethane linkage of the TPUs. TPUs with PTMEG also exhibited a two-step degradation process, which originated from the degradations of urethane linkage (270−340 °C) and PTMEG (400−450 °C) of the TPUs, respectively.13,14 TPUs with the mixture of CO2-based polyol and PTMEG (TPU-NP2−TPU-NP-4) exhibited three-step degradation processes, 3875


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for shape memory polymers.50 The presence of a short and rigid carbonate unit in the TPUs with the CO2-based polyol likely reduced the softness of the polymer chain, resulting in a hard network structure and improved shape recovery of the TPUs (Rr = 100%). The TPUs with mixed polyols (TPU-NP-2, TPU-NP-3, and TPU-NP-4) still exhibited good shape recovery values (Rr ≈ 100%; see Figure S5 and Table S4 in the Supporting Information) and decreased shape fixity values with increasing amounts of PTMEG in the mixed polyols. The Rf values of the TPUs with mixed polyols (TPU-NP-2−TPU-NP4) decreased from 95% to 36% with increasing amounts of PTMEG in the mixed polyols. This was likely due to the flexible PTMEG requiring lower temperatures to attain segmental vitrification. One of the advantages of incorporating carbonate units into a TPU is improved oxidative degradation resistance, which may afford TPU coatings with enhanced anticorrosion protection. Electrochemical corrosion rates were investigated to evaluate the anticorrosion characteristics of the TPUs. A high Icorr value and a low Rp value typically indicate high corrosion rates, whereas a low Icorr value and a high Rp value indicate morefavorable anticorrosion properties.51 Figure 7 and Table 2 show the PDP measurements for the TPU-coated SS specimens in different corrosive media (3.5 wt % HCl, 3.5 wt % NaOH, and 3.5 wt % NaCl). Overall, the TPU-based coatings showed higher corrosion rates in the aqueous solutions of HCl and NaCl than in the aqueous NaOH solution, likely due to the presence of Cl− ions, which enhances the corrosion of metal.52 Decreasing Icorr values were observed in the order of bare SS and SS coated with TPU-P-5, TPU-NP-2, and TPU-N-1 (see Figure 7 and Table 2), regardless of the corrosive media. This indicated that the anticorrosion characteristics improved with increasing amounts of carbonate units in the coatings. These results were confirmed by the lower corrosion rate values of the coatings containing carbonate units, as shown in Table 2. These results reflect the high ability of TPU-N-1 to reduce SS corrosion in various media, because of the robust, hydrophobic, and blocking nature of the carbonate coatings.53 Effective corrosion prevention was achieved by shielding the metal surface from water and oxygen contact. The increased hydrophobic nature of TPU-N-1 in the presence of carbonate units prevented the penetration of aqueous corrosive medium through the coating. These results were supported by the finding that TPU-N-1 had a higher water contact angle value

Figure 2. FT-IR spectra of TPU-N-1 (TPU with CO2-based polyol), TPU-NP-2 (TPU with the mixture of CO2-based polyol and PTMEG), and TPU-P-5 (TPU with PTMEG).

originating from the degradations of the CO2-based polyol, urethane linkage, and PTMEG. Interestingly, TPU-N-1 degraded gently under nitrogen with lower amounts of ash residue (0.34%) than those for TPU-P-5 (1.45%). The CO2 and epoxide copolymers showed characteristic burning properties without producing toxic materials or ash residues.44 The chain rigidity and Tg values of the TPUs affected their mechanical properties, such as the shape memory properties. Polymers that show shape memory behavior have a sensitive response to external stimuli, such as temperature, pH, humidity, light, and electricity. Among shape memory polymers, thermoresponsive polymers have gained attention, because of their good processability and high recovery rates at relatively low temperatures.45−47 For TPUs, the soft segments are responsible for temporary shape fixing, whereas the hard segments are responsible for permanent shape fixing.48,49 To assess the thermally activated shape memory properties of the TPUs, thermomechanical cyclic tensile tests were performed. Figure 6 and Table S4 depict the shape memory cycles and characteristics of TPUs under stress-controlled conditions. The TPU with the CO2-based polyol (TPU-N-1) exhibited Rf and Rr values of 95% and 100%, respectively, which are excellent values and superior to those of the TPU with PTMEG (for TPU-P-5, Rf = 75% and Rr = 97%). A high recovery ratio and a high fixity are the most desirable properties

Figure 3. 1H NMR spectra of TPU with the mixture of CO2-based polyol and PTMEG (TPU-NP-2 in Table 1). 3876


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Figure 4. (a) Tensile stress−strain curves and (b) tension set data of TPUs with CO2-based polyol and PTMEG.

Figure 5. (a) TGA curves and (b) weight loss derivative curves of TPUs with CO2-based polyol and PTMEG.

Figure 6. Strain-time−temperature diagram (third cycle) of TPU-N-1 and TPU-P-5 with a thermally induced shape memory effect under stresscontrolled conditions.

(71°) than TPU-NP-2 (66°) and TPU-P-5 (59°). The enhanced anticorrosion performance of TPU-N-1 was also attributed to the rigid and compact nature of the coating, as evidenced from the high Tg value, which prevented the corrosive media from contacting the SS.54 TPU-based coatings on SS were further evaluated to investigate the anticorrosion behavior using EIS.55 The EIS results in different corrosive media are presented in Table 2 and

Figure 8. The shapes of the Nyquist plots revealed the formation of a typical single semicircle. The equivalent circuit consisted of a coating capacitance (Cc), a pore resistance (Rpore), and a solution resistance (Rs).53 The Cc and Rpore values represent the formation of ion-conducting paths in the polymer coatings. Figure 8 shows that the Nyquist plots have only one capacitive arc. This indicated that the coatings act as an intact 3877


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Figure 7. PDP curves of bare and TPU-coated SS in (a) 3.5 wt % NaOH, (b) 3.5 wt % NaCl, and (c) 3.5 wt % HCl media.

Table 2. Electrochemical Parameters Obtained from PDP and EIS Studies for Uncoated and TPU-Coated SS under Different Corrosive Conditions at Room Temperaturea sample SS TPU-N-1 TPU-NP-2 TPU-P-5 SS TPU-N-1 TPU-NP-2 TPU-P-5 SS TPU-N-1 TPU-NP-2 TPU-P-5

mediumb NaOH NaOH NaOH NaOH NaCl NaCl NaCl NaCl HCl HCl HCl HCl

Icorr (A cm−2) 2.019 1.852 4.098 5.826 4.737 1.131 9.265 1.231 8.830 1.412 1.470 3.105

× × × × × × × × × × × ×

−6

10 10−8 10−8 10−8 10−7 10−9 10−9 10−8 10−4 10−6 10−6 10−6

Rp (Ω) 1.623 4.297 3.407 2.020 5.656 2.638 1.095 4.050 3.628 3.382 3.050 1.061

× × × × × × × × × × × ×

corrosion rate (mm/yr) 4

10 105 105 105 104 106 106 106 101 103 103 103

2.345 2.151 4.759 6.766 5.502 1.310 1.075 1.428 1.025 1.639 1.708 3.606

× × × × × × × × × × × ×

−2

10 10−4 10−4 10−4 10−3 10−5 10−4 10−4 101 10−2 10−2 10−2

Rpore (Ω) 1.246 1.885 1.782 1.251 1.390 2.100 1.753 1.306 4.560 2.294 1.755 4.234

× × × × × × × × × × × ×

Cc (Farad) 4

10 105 105 105 105 1010 108 107 101 103 103 102

2.063 1.004 1.714 2.008 8.153 6.069 5.930 1.207 6.547 2.034 2.210 8.079

× × × × × × × × × × × ×

10−4 10−5 10−5 10−5 10−5 10−7 10−6 10−6 10−4 10−5 10−5 10−5

a Icorr = corrosion current density, Rp = polarization resistance, Rpore = pore resistance, and Cc = coating capacitance. bThe concentration of each medium is 3.5 wt %.

carbonate units of the CO2-based polyol reduced the softness of the polyol chain. The chain rigidity of TPU-N-1 resulted in excellent shape memory characteristics, exhibiting Rf and Rr values of ∼95% and 100%, respectively. The incorporation of more PTMEG into the mixed polyols (TPU-NP-2−TPU-NP4) produced TPUs with softer elastomeric properties, exhibiting relatively low Young’s modulus, low Tg, low tension set, and high elongation at break values. In the presence of the CO2-based polyols, the TPUs exhibited improved anticorrosion characteristics, regardless of the corrosive media, as evidenced by low Icorr, corrosion rate, and Cc values and high Rpore values. The improved anticorrosion characteristics likely stemmed from the robust hydrophobic blocking nature of the carbonate coatings. These properties further extend the use of TPU as hard coatings for high-performance applications. CO2-based polyols offer a range of chemical and mechanical attributes to

capacitor preventing the permeation of corrosive entities, such as H2O, O2, and other corrosive ions, toward the surface of the metal substrate.56 Compared with bare SS, the TPU-coated SS showed increased Rpore values and decreased Cc values in the order of TPU-P-5, TPU-NP-2, and TPU-N-1. These behaviors were attributed to the formation of a rigid and hydrophobic protective film of TPU on the SS/medium interface, suggesting improved anticorrosion resistance in the presence of carbonate units.

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CONCLUSION TPUs with CO2-based polyols were successfully prepared through two-step polyaddition reactions. The thermal, mechanical, shape memory, and anticorrosion characteristics of the TPUs were investigated. TPU-N-1 was a hard plastic with a relatively high Tg and a high tension set value. The rigid 3878


Research Article


Figure 8. EIS curves of bare and TPU-coated SS in (a) 3.5 wt % NaOH, (b) 3.5 wt % NaCl, and (c) 3.5 wt % HCl media.

PUs, which can be utilized for the fixation of waste CO2 and reduce the carbon footprint of chemical production.

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(NRF) funded by the Ministry of Education (No. 2015R1D1A1A09061172).

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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.6b03046. Recipe for the preparation of TPU, thickness of TPU coatings, characteristics of the CO2-based polyol, shape memory characteristics of TPUs, schematic representation of the utilization of CO2 as a feedstock, the experimental setup of electrochemical resistance test, plausible mechanism for the formation of cyclic carbonate, DSC traces of CO2-based polyol/TPUs, and strain-time−temperature diagram of the TPUs; detailed description on PDP/EIS parameters (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-2-3408-3750. Fax: +82-2-3408-4342. E-mail: [email protected]. *Tel.: +82-51-510-2466. Fax: +82-51-513-7720. E-mail: ilkim@ pusan.ac.kr. *Tel.: +82-54-279-6781. Fax: +82-54-279-6239. E-mail: [email protected]. ORCID

Sung Chul Hong: 0000-0003-0961-7245 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea 3879


Research Article

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