Preparation and Characterization of Water-borne Polyurethane Based on Benzotriazole as Pendant Group with Different N-Alkylated Chain Extenders and Its Application in Anticorrosion

A series of novel anti-corrosive coatings were synthesized successfully. Water-borne polyurethane (WPU) was synthesized using polyethylene glycol and modified by grafting benzotriazole (BTA) as a pendant group (WPU-g-BTA) and N-alkylated amines (ethylene diamine (A), diethylene triamine (B), triethylene tetramine (C)) as side-chain extenders. Fourier-transform infrared spectroscopy, thermogravimetry, and dynamic mechanical analyses were used to characterize the structural and thermomechanical properties of the samples. A gas permeability analyzer (GPA) was used to evaluate molecular barrier properties. The corrosion inhibition performance of WPU-g-BTA-A, WPU-g-BTA-B, and WPU-g-BTA-C coatings in 3.5 wt% NaCl solution was determined by electrochemical measurements. WPU-g-BTA-C coating synthesized with a high cross-linking density showed superior anticorrosive performance. The as-prepared coatings exhibited a very low icorr value of 0.02 µA.cm−2, a high Ecorr value of −0.02 V, as well as excellent inhibition efficiency (99.972%) and impedance (6.33 Ω) after 30 min of exposure.


Introduction
Corrosion is responsible for economic losses, and time consumption is one of the major concerns in architectural engineering. Polymer paints and coatings, such as epoxy, polyurethane, polyacrylate, alkyd, polyester, and phenolic resins are the simplest and cheapest compounds to protect against corrosion [1,2]. Polyurethanes (PUs) are one of the popular materials used in different applications, such as coatings, adhesives, sealants, elastomers, foams, etc. [3]. Recently, water-borne polyurethanes (WPU) have gained research attention due to their unique properties of environment friendliness and easy processing [4]. WPUs are used in strain sensors [5], implantable biomaterials [6], cancer therapy [7], anti-bacterial agents [8], and coatings, adhesives, paints, leather, and composites.

Characterization
The as-synthesized compound was characterized through FTIR analysis. The representative IR spectra of EV 80, HDI trimer, and WPU-g-BTA are shown in Figure 1. The characteristic band for the NCO-group of HDI trimer at 2260 cm −1 disappeared in the corresponding polymer. The absorption band at 3338 cm −1 indicated the hydrogen-bonded N-H group of the polymer. The absorption band for the C-H group was detected at 2870 cm −1 . The strong band at 1638 and 1690 cm −1 were assigned to the C=O groups.

Gel Contents
The as-synthesized anti-corrosive coatings had different gel contents (%). Gel content is correlated with crosslinking density [51] (Figure 2).

Gel Contents
The as-synthesized anti-corrosive coatings had different gel contents (%). Gel content is correlated with crosslinking density [51] (Figure 2). For example, WPU-g-BTA-A had 13.57% gel, which was increased to 18.79% with 'B' as the chain extender. Meanwhile, WPU-g-BTA-C showed the highest gel content (21.68%) with the longest chain length (Table 1). Table 1. Gel content, O2 permeability, and thermal stability measured from TGA and DMA results of WPU-g-BTA coatings. For example, WPU-g-BTA-A had 13.57% gel, which was increased to 18.79% with 'B' as the chain extender. Meanwhile, WPU-g-BTA-C showed the highest gel content (21.68%) with the longest chain length (Table 1). PDP analysis was used for quantitative measurements of the inhibition efficiency (IE) of WPU-g-BTA-A, WPU-g-BTA-B, and WPU-g-BTA-C coatings under 3.5 wt% NaCl environment for 30 min and three days respectively.

Short Term Corrosion Study (30 Min)
The thickness of the coating was set at 060 ± 2 µm. The Tafel plots for the uncoated and coated CRS are presented in Figure 3 and the kinetic corrosion parameters including corrosion potential (Ecorr), corrosion current density (icorr), and IE (%) are summarized in Table 2. corrosion potential (Ecorr), corrosion current density (icorr), and IE (%) are summarized in Table 2. The inhibition efficiency (IE) of the as-prepared coatings was calculated using Equation (1) [15]: where icorr(0) and icorr(i) denote the corrosive current densities for the uncoated and coated CRS electrodes, respectively. Ecorr and icorr are substantial parameters that measure the tendency for corrosion and corrosion inhibition as the corrosion rate is directly proportional to Ecorr and inversely proportional to icorr. Thus, the coating with the lowest icorr and the highest Ecorr exhibited the best anticorrosive performance. The coated CRS electrode demonstrated better corrosion protection than the uncoated sample ( Figure 3).   The inhibition efficiency (IE) of the as-prepared coatings was calculated using Equation (1) [15]: where i corr (0) and icorr (i) denote the corrosive current densities for the uncoated and coated CRS electrodes, respectively. Ecorr and icorr are substantial parameters that measure the tendency for corrosion and corrosion inhibition as the corrosion rate is directly proportional to Ecorr and inversely proportional to icorr. Thus, the coating with the lowest icorr and the highest Ecorr exhibited the best anticorrosive performance. The coated CRS electrode demonstrated better corrosion protection than the uncoated sample ( Figure 3).
The Ecorr and icorr values of the uncoated CRS (black curve) were −0.811 V and 71.77 µA.cm −2 , respectively. The icorr value decreased (0.079 µA.cm −2 ) in WPU-g-BTA-A (red curve), which exhibited excellent barrier property. With respect to CRS, a drastic shift in Ecorr to more positive potential (−0.3311 V) was observed. The Ecorr value for WPU-g-BTA-B was further moved to −0.252 V with a smaller icorr value of 0.068 µA.cm −2 (green curve). Eventually, CRS coated with WPU-g-BTA-C (blue curve) exhibited a very low icorr value of 0.020 µA.cm −2 , which is about 4 and 3.4 orders of magnitude lower than those of WPU-g-BTA-A and WPU-g-BTA-B, respectively. However, the Ecorr for the coating was −0.1983 V, which is more positive than that of WPU-g-BTA-A and WPU-g-BTA-B. Hence, CRS coated with WPU-g-BTA blocked the diffusion of electrolytes. The results indicated that the corrosion inhibition performance of WPU-g-BTA coatings increased with increasing cross-linking density from sample A to C. Overall, CRS-coated samples showed significantly higher corrosion inhibition performance than bare CRS. The IE % values are listed in Table 2. The corrosion inhibition efficiency of the coated CRS was higher and increased as the length of the chain extender increased (A to C). The CRS coated with WPU-g-BTA-C showed extremely higher corrosion inhibition efficiency (99.972%). In the present study, the order of inhibition efficiency was found to be WPU-g-BTA-A < WPU-g-BTA-B < WPU-g-BTA-C Long Term Anti-Corrosion Study (3 Days) In Figure 4 and Table 3, it can be seen that, compared with the bare CRS, self-corrosion potentials of the samples, which were covered with organic coating, shifted more significantly to the right and their corrosion currents also had a downward trend.
WPU-g-BTA-A < WPU-g-BTA-B < WPU-g-BTA-C Long Term Anti-Corrosion Study (3 days) In Figure 4 and Table 3, it can be seen that, compared with the bare CRS, self-corrosion potentials of the samples, which were covered with organic coating, shifted more significantly to the right and their corrosion currents also had a downward trend.   Table 3. Long term anti-corrosion study for raw CRS and CRS electrode coated with WPU-g-BTA-A, WPU-g-BTA-B, and WPU-g-BTA-C in 3.5 wt% NaCl (aq) at 298 K. The corrosion potentials of the samples shifted from −0.634 V for the bare steel to −0.633 V for WPU−g−BTA−A, −0.576 V for WPU−g−BTA−B, and −0.550 V for WPU−g−BTA−C. The corrosion current (icorr) decreased from 4.98 µA.cm −2 for CRS to 4.17 µA.cm −2 for WPU−g−BTA−A, 4.08 µA.cm −2 for WPU−g−BTA−B, and 3.41 µA.cm −2 for WPU−g−BTA−C, where WPU−g−BTA−C had the smallest corrosion current. Further, it is also noted that the CRS exhibited Rp value of 2.587 kΩ whereas the Rp value for WPU−g−BTA−C coated CRS is 3.127 kΩ. This increase in Rp value clearly indicates that the coating acts as a physical barrier and appreciably resists the corrosion of the CRS from corrosion medium. Moreover, the highest corrosion protection efficiency (P EF ) % achieved for WPU−g−BTA−C coating was 31.56% after 3 days' immersion in 3.5 wt% NaCl (aq) .

Morphological Studies
The cross-sectional images of before (top) and after (bottom) the corrosion test in the 3.5 wt% NaCl solution for three days are shown in Figure 5. The cross-sectional image of WPU−g−BTA−A in shows that the corroded layer was uniform and without pits. The image of WPU−g−BTA−B in also shows a rough corroded layer with a few pits, while the WPU−g−BTA−C coating shows a relatively less rough surface. The specimens with higher cross-linking density showed complex corrosion morphologies.

Morphological Studies
The cross-sectional images of before (top) and after (bottom) the corrosion test in the 3.5 wt% NaCl solution for three days are shown in Figure 5. The cross-sectional image of WPU−g−BTA−A in shows that the corroded layer was uniform and without pits. The image of WPU−g−BTA−B in also shows a rough corroded layer with a few pits, while the WPU−g−BTA−C coating shows a relatively less rough surface. The specimens with higher cross-linking density showed complex corrosion morphologies.  SEM images of freshly polished surface (top) as well as specimens retrieved from 3.5 wt% NaCl solution (bottom) after 3 days of immersion are shown in Figure 6. The freshly polished WPU−g−BTA coatings showed a relatively smooth surface as shown in Figure 6a,b, while the longest chain extender C due to high crosslinking density strengthened the coating in WPU−g−BTA−C film showed a coarse surface, as shown in Figure 6c. The WPU−g−BTA specimens retrieved from corrosive media showed a rough surface with corrosion products. The WPU−g−BTA−C coating with high crosslinking density seemed compact, tight, and wrapped around the corrosion products, which is beneficial to shield them from corrosive medium.

Electrochemical Impedance Spectroscopy (EIS)
EIS is an effective technique used to evaluate the anticorrosion performance of asprepared coatings. The impedance spectrum was recorded on uncoated CRS and CRS coated with WPU−g−BTA−C, WPU−g−BTA−B, and WPU−g−BTA−C after 30 min of immersion in 3.5 wt% NaCl solution ( Figure 4 and Table 2) at 298 K.
It can be found from Figure 7 that for CRS, the Nyquist plot consisted of a flattened capacitive loop. The Nyquist plot consisted of a single semicircle over the immersion period, suggesting a complete capacitive and good barrier property. The radius of the semicircle increased with increasing chain length from WPU−g−BTA−A to WPU−g−BTA−C. The radius of the capacitive arc indicated the corrosion rate. The capacitive arc with larger radius had a slower corrosion rate, indicating better corrosive inhibition. The diameter of the Nyquist curve for WPU-g-BTA coatings increased with various chain extenders from A to C. The WPU−g−BTA−C coating had the largest radius, indicating excellent corrosion resistance.
polished WPU−g−BTA coatings showed a relatively smooth surface as shown in Figure  6a,b, while the longest chain extender C due to high crosslinking density strengthened the coating in WPU−g−BTA−C film showed a coarse surface, as shown in Figure 6c. The WPU−g−BTA specimens retrieved from corrosive media showed a rough surface with corrosion products. The WPU−g−BTA−C coating with high crosslinking density seemed compact, tight, and wrapped around the corrosion products, which is beneficial to shield them from corrosive medium.

Electrochemical Impedance Spectroscopy (EIS)
EIS is an effective technique used to evaluate the anticorrosion performance of as-prepared coatings. The impedance spectrum was recorded on uncoated CRS and CRS coated with WPU−g−BTA−C, WPU−g−BTA−B, and WPU−g−BTA−C after 30 min of immersion in 3.5 wt% NaCl solution ( Figure 4 and Table 2) at 298 K.
It can be found from Figure 7 that for CRS, the Nyquist plot consisted of a flattened capacitive loop. The Nyquist plot consisted of a single semicircle over the immersion period, suggesting a complete capacitive and good barrier property. The radius of the semicircle increased with increasing chain length from WPU−g−BTA−A to WPU−g−BTA−C. The radius of the capacitive arc indicated the corrosion rate. The capacitive arc with larger radius had a slower corrosion rate, indicating better corrosive inhibition. The diameter of the Nyquist curve for WPU-g-BTA coatings increased with various chain extenders from A to C. The WPU−g−BTA−C coating had the largest radius, indicating excellent corrosion resistance.  BODE plots are one of the most commonly used tools in corrosion science for monitoring local corrosion damages. Figure 8 is the BODE diagram of coated and un-coated CRS dip in 3.5 wt% NaCl for 30 min at 298 K. A small-amplitude signal was applied over a range of frequencies for EIS measurement. The impedance modulus (log Z) in the BODE plot is usually related to the coatings barrier performance; that is, a large impedance value indicates better corrosion protection. The /Z/ ( Table 2)  BODE plots are one of the most commonly used tools in corrosion science for monitoring local corrosion damages. Figure 8 is the BODE diagram of coated and un-coated CRS dip in 3.5 wt% NaCl for 30 min at 298 K. A small-amplitude signal was applied over a range of frequencies for EIS measurement. The impedance modulus (log Z) in the BODE plot is usually related to the coatings barrier performance; that is, a large impedance value indicates better corrosion protection. The /Z/ (Table 2) of uncoated CRS was 2.22 Ω, which was increased to 5.99 Ω and 6.21 Ω for WPU−g−BTA−A and WPU−g−BTA−B, respectively. WPU−g−BTA−C, which had the largest impedance modulus, exhibited the best corrosion protection. The trend in the BODE plot was similar to those observed in Tafel and Nyquist plots.

O2 Permeability
WPU-g-BTA coatings with film thicknesses of 60 ± 2 µm were used to evaluate molecular barrier properties. The O2 gas permeability of the as-prepared coatings with different cross-linking densities at 298 K is presented in Figure 9. The O2 permeation across different coatings decreased with changes in the length of the chain extender. Compared with WPU-g-BTA-A, WPU-g-BTA-B film had 60% reduction in gas permeability ( Table  1). The best barrier performance was recorded for the WPU-g-BTA-C coating with longer chain length. The incorporation of 'C' into the WPU-g-BTA coating with higher cross-linking density can create a tortuous path for O2 gas molecules, thereby increasing the total path of the gas. The effect of amine-based chain extenders (A, B, C) with various lengths may result in greater interaction, leading to higher efficiency in restricting the movement of polymer chains [52,53].

O 2 Permeability
WPU-g-BTA coatings with film thicknesses of 60 ± 2 µm were used to evaluate molecular barrier properties. The O 2 gas permeability of the as-prepared coatings with different cross-linking densities at 298 K is presented in Figure 9. The O 2 permeation across different coatings decreased with changes in the length of the chain extender. Compared with WPU-g-BTA-A, WPU-g-BTA-B film had 60% reduction in gas permeability ( Table 1). The best barrier performance was recorded for the WPU-g-BTA-C coating with longer chain length. The incorporation of 'C' into the WPU-g-BTA coating with higher cross-linking density can create a tortuous path for O 2 gas molecules, thereby increasing the total path of the gas. The effect of amine-based chain extenders (A, B, C) with various lengths may result in greater interaction, leading to higher efficiency in restricting the movement of polymer chains [52,53].

Thermomechanical Properties
The thermal degradation behavior is shown in Figure 10. The TGA curves showed the weight loss as a function of temperature under N 2 atmosphere. All the samples had the same structural units and almost the same degradation pattern. The degradation temperature was slightly higher for WPU−g−BTA−C. The thermograms showed two-step degradation; first, there was a decomposition stage (around 230-270 • C) due to the urethane bond degradation of hard segment; second, there was a degradation of the soft segment polyol. For WPU−g−BTA−C, 5 wt% loss occurred in the range of 222-261.19 • C followed by 215-258 • C for WPU−g−BTA−A and WPU−g−BTA−B (Table 1).

Thermomechanical Properties
The thermal degradation behavior is shown in Figure 10. The TGA curves showed the weight loss as a function of temperature under N2 atmosphere. All the samples had the same structural units and almost the same degradation pattern. The degradation temperature was slightly higher for WPU−g−BTA−C. The thermograms showed two-step degradation; first, there was a decomposition stage (around 230-270 °C) due to the urethane bond degradation of hard segment; second, there was a degradation of the soft segment polyol. For WPU−g−BTA−C, 5 wt% loss occurred in the range of 222-261.19 °C followed by 215-258 °C for WPU−g−BTA−A and WPU−g−BTA−B (Table 1).

Thermomechanical Properties
The thermal degradation behavior is shown in Figure 10. The TGA curves showed the weight loss as a function of temperature under N2 atmosphere. All the samples had the same structural units and almost the same degradation pattern. The degradation temperature was slightly higher for WPU−g−BTA−C. The thermograms showed two-step degradation; first, there was a decomposition stage (around 230-270 °C) due to the urethane bond degradation of hard segment; second, there was a degradation of the soft segment polyol. For WPU−g−BTA−C, 5 wt% loss occurred in the range of 222-261.19 °C followed by 215-258 °C for WPU−g−BTA−A and WPU−g−BTA−B (Table 1). The mechanical properties of the WPU−g−BTA coatings with different cross-linking densities were determined (Table 1). DMA demonstrated the effect of temperature on various anticorrosive coatings. The storage modulus of all the samples is shown in Figure 11.
The modulus of WPU-g-BTA-C was 171.4 MPa, which is higher than those of WPUg-BTA-A and WPU-g-BTA-B at room temperature (20 • C). The hardening mechanism may be attributed to the various cross-linking density in the given samples. The asprepared coatings showed a sharply decreased modulus. However, the WPU-g-BTA-C coating had a slightly reduced decreasing tendency of storage modulus, providing a good thermomechanical stability up to 50 • C. As shown in Table 1, the 100% modulus, stress at break, and elongation at break of the WPU-g-BTA films increased with increasing chain length of the extenders. The tensile-strain behavior showed a stress of 51.2-47.56 kgf/cm 2 , elongation at break of 178.85%-283.41%, and tensile modulus of 42.2-46.5 kgf/cm 2 . This increase in properties may be associated with the increased cross-linking density due to the WPU-g-BTA chain end reaction with A, B, and C. The mechanical properties of the WPU−g−BTA coatings with different cross-linking densities were determined (Table 1). DMA demonstrated the effect of temperature on various anticorrosive coatings. The storage modulus of all the samples is shown in Figure 11. The modulus of WPU-g-BTA-C was 171.4 MPa, which is higher than those of WPU-g-BTA-A and WPU-g-BTA-B at room temperature (20 °C). The hardening mechanism may be attributed to the various cross-linking density in the given samples. The as-prepared coatings showed a sharply decreased modulus. However, the WPU-g-BTA-C coating had a slightly reduced decreasing tendency of storage modulus, providing a good thermomechanical stability up to 50 °C. As shown in Table 1, the 100% modulus, stress at break, and elongation at break of the WPU-g-BTA films increased with increasing chain length of the extenders. The tensile-strain behavior showed a stress of 51.2-47.56 kgf/cm 2 , elongation at break of 178.85%-283.41%, and tensile modulus of 42.2-46.5 kgf/cm 2 . This increase in properties may be associated with the increased cross-linking density due to the WPU-g-BTA chain end reaction with A, B, and C.

Gel Content
The amount of crosslinking can be measured by a standard tetrahydrofuran insoluble test. The insoluble tetrahydrofuran (THF) of the polyurethane dispersion was measured by mixing 1 g of the polyurethane dispersion with 30 g of THF in a pre-weighed centrifuge tube. After the solution was centrifuged for 2 h at 17,000× g rpm, the top liquid layer was poured out and the non-dissolved gel in the bottom was left. The centrifuge tube with the non-dissolved gel was re-weighed after drying in an oven at 110 • C for 2 h.

%THF insoluble of WPU =
Weight of tube and non − dissolved gel − wight of the tube sample weight * WPU solid%

Synthesis of WPU-g-BTA
A 500 mL round-bottom flask with a magnetic stirrer, condenser, and nitrogen inlet was used as reactor. EV 80 (0.1 mole) and HDI trimer (0.3 mole) were first charged and reacted at about 75 • C for 3 h to obtain NCO-terminated prepolymer-1. PEG-300 (0.07 mole) and DMBA (0.03 mole) were placed in the flask and heated for 3 h (prepolymer-2) at 70 • C. Acetone was then added at 40 • C to reduce the viscosity of the prepolymer-2. Then the prepolymers were cooled to 60 • C and neutralized with DMIPA for 30 min. An aqueous dispersion was obtained by adding water (35 • C) to the mixture. Then, the modified HEC was added and stirred for about one hour to homogenize the mixture. Subsequently, amine chain extenders (A, B, C) were added and stirred for the next 1.5 h. Then the mixture was cast onto a Teflon plate and partially dried for two days at 35 • C. Finally, the film was dried for two days at 70 • C. The reaction procedure is shown in Figure 12 [51].  Figure 12. Synthetic route of WPU-g-BTA with side chain extenders.

Conclusions
WPU was prepared by covalent grafting of BTA on the polymer backbone as a pendant group (WPU-g-BTA). Using this WPU-g-BTA, we developed a series of WPU-g-BTAs containing N-alkylated amine cross-links (WPU-g-BTA-A, WPU-g-BTA-B,

Conclusions
WPU was prepared by covalent grafting of BTA on the polymer backbone as a pendant group (WPU-g-BTA). Using this WPU-g-BTA, we developed a series of WPU-g-BTAs containing N-alkylated amine cross-links (WPU-g-BTA-A, WPU-g-BTA-B, and WPU-g-BTA-C). The successful synthesis of WPU-g-BTAs was confirmed by FTIR. The following conclusions were drawn: 1.
The gel content of WPU-g-BTAs increased from 13.57% to 21.68% with increasing chain length from WPU-g-BTA-A to WPU-g-BTA-C. Further, increasing the chain length of WPU-g-BTAs significantly increased the crosslink density, thereby improving the thermal and mechanical properties. 2.
WPU-g-BTAs improved the corrosion resistance performance of the as synthesized material. The superior inhibition efficiency of 99.972% and 31.16% was achieved by WPUg-BTA-C after 30 min and 3 days of immersion in 3.5 wt% NaCl (aq) respectively. Hence, WPU-g-BTA coatings are considered promising materials for anti-corrosion applications.