Studies on Biobased Non-Isocyanate Polyurethane Coatings with Potential Corrosion Resistance

: This work deals with the development of non-isocyanate polyurethane (NIPU) composites with an aniline oligomer, viz., tetraniline (TANI) for corrosion-resistant coatings. Firstly, epoxidized-soyabean oil was converted to carbonated oil by inserting CO 2 under high temperature and pressure into the epoxy ring. Then, varying weights of oligoaniline—0.5, 1, 2 and 4 wt % (based on the weight of CSBO)—were added to CSBO and cured with tetraethylenepentamine (TEPA, 25 wt % based on CSBO) at 80 ◦ C for 12–15 h. The effects of oligomer loading on the thermal, mechanical and surface wetting properties of the free standing ﬁlms were studied. The ATR-FTIR spectra of the ﬁlms exhibited peaks corresponding to the bis-carbamate linkages, conﬁrming the formation of urethane linkages. TGA analysis showed that the addition of oligoaniline did not alter the initial degradation temperature much; however, the residue increased with increasing loading of tetraniline. Both tensile strength and elongation (at break) increased with increasing oligoaniline content. While the glass transition temperature of the ﬁlms was observed at approximately room temperature, i.e., 20–30 ◦ C, for all the compositions, the contact angles of the composites were found to be less than that of the bare NIPU ﬁlms. However, all the compositions were hydrophobic, exhibiting contact angles in the 98–110 ◦ range. SEM analysis of the cross-sections of the ﬁlms conﬁrmed the uniform distribution of tetraniline particles and surface morphology showed that the roughness increased with the loading of tetraniline from 0.5 to 4%. MS panels coated with NIPU exhibited good barrier properties and as loading of TANI increased, the rate of corrosion decreased. Salt spray tests indicated that oligoaniline improved the adhesion of the coating to MS and corrosion resistance compared to the neat NIPU coating.


Introduction
Biobased coatings are gaining importance due to strict environmental legislations such as restrictions on VOC emission and the need for biodegradability. In addition, depleting fossil fuel resources have now driven both academic and industrial research towards utilization of natural resources for developing novel materials which can substitute petroleum-based materials. Among the various renewable sources, vegetable oil is advantageous in terms of its availability, cost, functionality and performance. Soyabean oil has been widely used for several industrial applications in its epoxidized form as raw materials for coatings, co-plasticizer for PVC processing [1], pharmaceutical plastic packaging material [2], precursors for synthesis of acrylates [3], polyols [4] and more recently non-isocyanate PUs. ESBO is also used in coatings and adhesives [5] because of its low cost, strong photosensitivity, excellent coated film performance and low viscosity. Epoxidizedsoyabean oil is commercially available and it is less expensive, highly functional (with %oxirane oxygen content of approximately 6.5-7), colorless, odorless, and less viscous. It 4,4 -diamino-diphenylamine sulfate (7 g, 0.023 mol) and N-phenyl-p-phenylenediamine (4.33 g, 0.023 mol) were dissolved in aq. HCl (1.0 molar, 300 mL) containing 23 g NaCl. Ammonium persulfate solution (5.306 g, 0.023 mol) in aqueous HCl (1.0 mol/L, 100 mL) was added dropwise to the above solution(5 mL/min) maintained at −5 • C. The reaction mixture was stirred at −5 • C for 2 h and the resulting precipitate was collected by filtration, then washed with aq. HCl (1.0 molar, 300 mL) followed by distilled water. The precipitate was stirred for 1 h by adding 200 mL of 5 M NH 4 OH solution, filtered and dried overnight in a vacuum oven at 50 • C. The resulting compound was characterized for its structure using 1 H NMR, FT-IR and mass spectroscopy (yield-80%, Scheme 1) [34].

Synthesis of Carbonated Soyabean Oil
Carbonization of soybean oil was carried out in a high-pressure reactor (Amar Equipments Pvt. Ltd., Mumbai, India, 1 L Volume, −1 to 100 Bar Vessel Design Pressure and −10 to 250 °C Vessel Design Temperature) as shown in Scheme 2. To the reaction vessel, approximately 250 g of ESBO and a catalytic amount of tetrabutyl ammonium bromide (TBAB, 8.8 weight %) were added with respect to the weight of ESBO. Then, the reaction mixture was heated to 120 °C with continuous stirring. CO2 was passed into the reactor, pressurized to 10 bar and the pressure was maintained during the entire course of reaction, i.e., for 24 h. After the completion of the reaction, clear and brownish viscous oil, CSBO, was collected and the sample (Figure 1) was subjected to epoxy value Scheme 1. Synthesis of TANI.

Synthesis of Carbonated Soyabean Oil
Carbonization of soybean oil was carried out in a high-pressure reactor (Amar Equipments Pvt. Ltd., Mumbai, India, 1 L Volume, −1 to 100 Bar Vessel Design Pressure and −10 to 250 • C Vessel Design Temperature) as shown in Scheme 2. To the reaction vessel, approximately 250 g of ESBO and a catalytic amount of tetrabutyl ammonium bromide (TBAB, 8.8 weight %) were added with respect to the weight of ESBO. Then, the reaction mixture was heated to 120 • C with continuous stirring. CO 2 was passed into the reactor, pressurized to 10 bar and the pressure was maintained during the entire course of reaction, i.e., for 24 h. After the completion of the reaction, clear and brownish viscous oil, CSBO, was collected and the sample (Figure 1) was subjected to epoxy value determination to confirm the conversion of epoxy to 5-membered cyclic carbonate ring. CO 2 insertion was also confirmed by IR, 1 H NMR and 13 C NMR.

Synthesis of NIPU and Its Composite
TEPA as a curing agent (25 weight % based on CSBO) and stannous octoate as a catalyst (2 weight % based on CSBO) were added to a given weight of CSBO. Varying amounts (0, 0.5, 1, 2, and 4%) of TANI were used in the formulation to enhance the corrosion resistance of the NIPU films. TANI was sonicated in DMF for ½ h, prior to mixing with CSBO and PETA. The blend was made by first mixing CSBO and TEPA for 5-6 min followed by the addition of sonicated TANI dispersion. The composites were casted in silicon mold and cured at 80 °C for 12-15 h.

Scheme 2. Synthesis of carbonated SBO and NIPU.
Sustain. Chem. 2023, 4, FOR PEER REVIEW 4 determination to confirm the conversion of epoxy to 5-membered cyclic carbonate ring. CO2 insertion was also confirmed by IR, 1 H NMR and 13 C NMR.

Synthesis of NIPU and Its Composite
TEPA as a curing agent (25 weight % based on CSBO) and stannous octoate as a catalyst (2 weight % based on CSBO) were added to a given weight of CSBO. Varying amounts (0, 0.5, 1, 2, and 4%) of TANI were used in the formulation to enhance the corrosion resistance of the NIPU films. TANI was sonicated in DMF for 1 /2 h, prior to mixing with CSBO and PETA. The blend was made by first mixing CSBO and TEPA for 5-6 min followed by the addition of sonicated TANI dispersion. The composites were casted in silicon mold and cured at 80 • C for 12-15 h.

Characterization of ESBO, CSBO and Cured Film
Epoxy value, iodine value and acid value were determined as per standard procedures (refer supplementary information).

Viscosity
The viscosity of ESBO and CSBO was determined using a strain-controlled rheometer (102 series modular compact rheometer, Anton Paar, Graz, Austria) using cone/plate geometry with an adjusted gap of 1 mm.

Fourier Transform Infrared Spectroscopy (FTIR)
Functional groups present in ESBO and CSBO were identified using a PerkinElmer (Spectrum 1000, Waltham, MA, USA) Fourier transform-infrared (FT-IR) spectrometer, where the material was smeared on the KBr pallets and IR recorded. Attenuated Total Internal Reflection (ATR) was used for recording the IR of the films(minimum 16 scans) in the 400-4000 cm −1 range.

Nuclear Magnetic Resonance Spectroscopy (NMR)
Nuclear Magnetic Resonance Spectroscopy (NMR) ( 1 H and 13 C) spectra were recorded using a Varian-Inova400 MHz spectrometer in DMSO-d6 (TANI) and a CDCl 3 (CSBO) solution in which tetramethylsilane (TMS) was used as an internal standard.

Electrospray Ionization Mass Spectroscopy (ESI-MS)
Electrospray Ionization Mass (ESI-MS) spectra were used to interpret the molecular weight of TANI, carried out on FINNIGAN LCQ Advance Max in methanol solution.

Water Contact Angle Analysis
The effect of TANI on the wettability of NIPU-coated films was evaluated using G10 Goniometer (KRUSS instrument, Hamburg, Germany) through sessile drop.

Thermogravimetric Analysis (TGA)
TGA was carried out using 5-10 mg of sample on a TA Q500 (TA Instruments, New Castle, DE, USA) thermogravimetric analyzer under N 2 atmosphere at room temperature to 600 • C, with a 10 • C/min heating rate and the sample weight ranged from 5 to 10 mg.

Dynamic Mechanical Thermal Analyzer (DMA)
A DMA (TA Q800) was used to interpret the storage modulus (E ) and glass transition temperature (T g ) of films at 1 Hz frequency in the multi-frequency-strain mode in the temperature range −50 to 80 • C at a heating rate of 10 • C/min.

The Universal Testing Machine (UTM)
The mechanical properties of the coatings were evaluated by a universal testing machine (UTM) DAK-7200, India system, with a speed of 10 mm/min with a load cell and gauge length of 100 kg and 50 mm, respectively, according to ASTM D882. The specimens for UTM (micro tensile specimens) had the following dimensions: thickness 0.7 mm, width and length nearly 10 and 80 mm, respectively.

Scanning Electron Microscope
NIPU films containing oligoaniline were analyzed for their morphology using ascanning electron microscope (SEM), Hitachi-S520 (Oxford link ISISSEM model), Tokyo, Japan. Cross-sections of the films were also scanned.

The Salt Spray Fog Test
The coated MS panels were evaluated for corrosion resistance as per the ASTM B-117 salt spray fog test (Komal Scienitific Pvt. Ltd., Mumbai, India). Cross-cuts were made on the coated surface to perceive the protective action on the coated panel. The coated panels were kept in a salt spray chamber and atomized using a 5% salt solution in water for 144 h.

Characterization of TANI
FTIR spectra ( Figure 2A) exhibited a peak at approximately 3433 cm −1 assigned to the terminal NH 2 groups of TANI. In addition, the bands at 1593 and 1499 cm −1 could be related to quinoid and benzonoidring vibration. In NMR ( Figure 2B), signals at 5.14 ppm are related to the proton of the primary amine. Peaks between 6.4 and 7.5 ppm are mainly related to the quinoid and benzonoidrings present in TANI. Further structure confirmation by HRMS ( Figure 2C) showed a molecular ion peak at 380.18 (M+1) matching with TANI molecular weight. From these spectroscopic data, the structure of TANI was confirmed [34].

Characterization of TANI
FTIR spectra ( Figure 2A)exhibited a peak at approximately 3433 cm −1 assigned to the terminal NH2 groups of TANI. In addition, the bands at 1593 and 1499 cm −1 could be related to quinoid and benzonoidring vibration. In NMR ( Figure 2B),signals at 5.14 ppm are related to the proton of the primary amine. Peaks between 6.4 and 7.5 ppm are mainly related to the quinoid and benzonoidrings present in TANI. Further structure confirmation by HRMS ( Figure 2C) showed a molecular ion peak at 380.18 (M+1) matching with TANI molecular weight. From these spectroscopic data, the structure of TANI was confirmed [34].

Characterization of CSBO
The physicochemical properties of the carbonated soyabean oil is listed in Table 2. Since oxirane oxygen content has dropped from 6.5 to 0.55, the insertion of CO2 into the

Characterization of CSBO
The physicochemical properties of the carbonated soyabean oil is listed in Table 2. Since oxirane oxygen content has dropped from 6.5 to 0.55, the insertion of CO 2 into the epoxy ring has been confirmed. Figure 3 shows the IR spectra of epoxidized and carbonated soyabean oil. The peak at approximately 842 cm −1 in the ESBO spectrum corresponds to the epoxy groups, which have completely disappeared in the CSBO and a new peak corresponding to the C-O and C-H stretching at the alpha-position of carbonate appear at approximately 1806, 1054, and 870 cm −1 , respectively. Further, conversion of epoxy rings into cyclic carbonate was confirmed by 1 H and 13 C NMR spectroscopy. The appearance of the new peaks in CSBO at approximately 4.4-4.9 ppm in Figure 4 is related to protons of the cyclic carbonate rings, while the epoxy ring protons have almost disappeared (2.8-3.2 ppm). In the 13 C NMR ( Figure 5), one can observe the new peak at 154 ppm and 79-82 ppm corresponding to the carbon atom of the cyclic carbonate moiety.

Characterization of NIPU Composites
CSBO was formulated with varying amounts of TANI (0.5, 1, 2 and 4 wt % based on the CSBO) using 25 wt % TEPA (based on the CSBO). TANI was dissolved in the DMF by sonication for 1 /2 h, then mixed with CSBO, TEPA and the catalyst in order to achieve homogeneity of TANI in the resin. This mixture was then poured into a mold and cured in the oven overnight (12-15 h). The solid content in the composite was maintained at approximately 65%, which can be considered as high solid coatings. The cured films with different amounts of TANI are shown in Figure 6. The formation of urethane linkages in cured polyhydroxyurethane films was confirmed by ATR-FTIR.

Characterization of CSBO
The physicochemical properties of the carbonated soyabean oil is listed in Since oxirane oxygen content has dropped from 6.5 to 0.55, the insertion of CO2 epoxy ring has been confirmed. Figure 3 shows the IR spectra of epoxidized a bonated soyabean oil. The peak at approximately 842 cm −1 in the ESBO spectrum sponds to the epoxy groups, which have completely disappeared in the CSBO an peak corresponding to the C-O and C-H stretching at the alpha-position of carbo pear at approximately 1806, 1054, and 870 cm −1 , respectively. Further, conversion o rings into cyclic carbonate was confirmed by 1 H and 13 C NMR spectroscopy. The ance of the new peaks in CSBO at approximately 4.4-4.9 ppm in Figure 4 is re protons of the cyclic carbonate rings, while the epoxy ring protons have almos peared (2.8-3.2 ppm). In the 13 C NMR ( Figure 5), one can observe the new peak at 1 and 79-82 ppm corresponding to the carbon atom of the cyclic carbonate moiety.

Characterization of NIPU Composites
CSBO was formulated with varying amounts of TANI (0.5, 1, 2 and 4 wt % based on the CSBO) using 25 wt % TEPA (based on the CSBO). TANI was dissolved in the DMF by sonication for ½ h, then mixed with CSBO, TEPA and the catalyst in order to achieve homogeneity of TANI in the resin. This mixture was then poured into a mold and cured in the oven overnight (12-15 h). The solid content in the composite was maintained at approximately 65%, which can be considered as high solid coatings. The cured films with different amounts of TANI are shown in Figure 6. The formation of urethane linkages in cured polyhydroxyurethane films was confirmed by ATR-FTIR.

Characterization of NIPU Composites
CSBO was formulated with varying amounts of TANI (0.5, 1, 2 and 4 wt % based on the CSBO) using 25 wt % TEPA (based on the CSBO). TANI was dissolved in the DMF by sonication for ½ h, then mixed with CSBO, TEPA and the catalyst in order to achieve homogeneity of TANI in the resin. This mixture was then poured into a mold and cured in the oven overnight (12-15 h). The solid content in the composite was maintained at approximately 65%, which can be considered as high solid coatings. The cured films with different amounts of TANI are shown in Figure 6. The formation of urethane linkages in cured polyhydroxyurethane films was confirmed by ATR-FTIR. The ATR-FTIR spectra show peaks related to the formation of the urethane linkages ( Figure 7). NH stretching and bending peaks appear at 3326 and 1531 cm −1 ,respectively. The peaks at 1695 and 1705 cm −1 are attributed to the C=O stretching of the urethane bond. Hence, ring opening of cyclic carbonate with amines leading to urethane bond formation is confirmed.

Thermal Properties
The TGA analysis of NIPU composite films was carried to understand its thermal stability and degradation trend with respect to the temperature increase from 30 to 600 • C at a rate 10 • C/min. The initial degradation from 180 to 265 • C may be related to the dissociation of urethane linkages while weight loss from 266 to 440 • C may correspond to decomposition of the long alkyl chains of soyabean oil. Thermal degradation of the polymeric chain due to C-C and C-O bond scission occurs at approximately 440-520 • C. The 5% and 50% degradation temperature (T d ) of the different composite film are shown in Table 3 and it was observed that there is no effect of TANI loading on the thermal stability of NIPU coatings. It was observed that the % residue of the composite film at 550 • C increased with TANI loading.

Fourier Transform Infrared Spectroscopy of NIPU-TANI Composites (FTIR)
The ATR-FTIR spectra show peaks related to the formation of the urethane linkages ( Figure 7). NH stretching and bending peaks appear at 3326 and 1531 cm −1 ,respectively The peaks at 1695 and 1705 cm −1 are attributed to the C=O stretching of the urethane bond Hence, ring opening of cyclic carbonate with amines leading to urethane bond formation is confirmed.

Thermal Properties
The TGA analysis of NIPU composite films was carried to understand its thermal stability and degradation trend with respect to the temperature increase from 30 to 600 °C at a rate 10 °C/min. The initial degradation from 180 to 265 °C may be related to the dissociation of urethane linkages while weight loss from 266 to 440 °C may correspond to decomposition of the long alkyl chains of soyabean oil. Thermal degradation of the polymeric chain due to C-C and C-O bond scission occurs at approximately 440-520 °C. The 5% and 50% degradation temperature (Td) of the different composite film are shown in Table 3 and it was observed that there is no effect of TANI loading on the thermal stability of NIPU coatings. It was observed that the % residue of the composite film at 550 °C increased with TANI loading.

Thermomechanical Properties
Dynamic mechanical thermal analysis (DMA) was used to determine the dynamic viscoelastic properties and glass transition temperatures (T g ) of the films. Figure 8A shows a variation in the storage modulus (E ) with respect to the temperature and the tan δ vs. temperature plot ( Figure 8B) shows the T g values. The thermomechanical parameters of the cured composite films are tabulated in Table 4. The storage modulus vs. temperature profiles of the NIPU and NIPU TANI composites are shown in Figure 8A. It was observed that the composites have a lower storage modulus compared to the bare NIPU film, which may be due to the effect of TANI on the extent of H bonding in the NIPU backbone due to the presence of NH groups in the oligoaniline backbone. The glass transition temperature, as observed from the peak of the tan δ curves of the composites, was slightly higher than the bare NIPU films as a result of the reinforcing effect of TANI. The curves also became much narrower with TANI loading, which shows that the oligoaniline is reducing the extent of physical cross-linking within the NIPU chains. The broadest curve was observed for CT 0.0, where TANI is completely absent, hence H bonding comes into the picture, leading to physical cross-links. Generally, the higher the extent of cross-linking (either chemical or physical), the broader the tan δ peak. In general, the peak area of tan δ curves shows the energy dissipation capability of materials [35].

Static mechanical Properties
The effect of TANI addition on the mechanical properties of the NIPU films was evaluated using the universal testing machine (UTM). The tensile strength and the percentage of elongation at break of the NIPU composite are compiled in Table 4. All the composite films show elastic behavior as observed from the stress vs. stain curve (Figure 9).It was observed that TANI-incorporated films had a higher tensile strength and elongation compared to the bare NIPU films. This may be due to the reinforcing effect of TANI on the PU matrix. However, the optimum loading for maximum strength and elongation was found to be 1%, beyond which both the strength and the elongation (%) dropped.
to the presence of NH groups in the oligoaniline backbone. The glass transition temperature, as observed from the peak of the tan δ curves of the composites, was slightly higher than the bare NIPU films as a result of the reinforcing effect of TANI. The curves also became much narrower with TANI loading, which shows that the oligoaniline is reducing the extent of physical cross-linking within the NIPU chains. The broadest curve was observed for CT 0.0, where TANI is completely absent, hence H bonding comes into the picture, leading to physical cross-links. Generally, the higher the extent of cross-linking (either chemical or physical), the broader the tan δ peak. In general, the peak area of tan δ curves shows the energy dissipation capability of materials [35].

Static mechanical Properties
The effect of TANI addition on the mechanical properties of the NIPU films was evaluated using the universal testing machine (UTM). The tensile strength and the percentage of elongation at break of the NIPU composite are compiled in Table 4. All the composite films show elastic behavior as observed from the stress vs. stain curve (Figure 9).It was observed that TANI-incorporated films had a higher tensile strength and elongation compared to the bare NIPU films. This may be due to the reinforcing effect of TANI on the PU matrix. However, the optimum loading for maximum strength and elongation was found to be 1%, beyond which both the strength and the elongation (%) dropped.   Figure 9. UTM Plot of NIPU Composite Films.

SEM Analysis
Scanning electron microscopy was employed to study the surface morphology and the cross-sectional morphology of the NIPU composite. As shown in Figure 10A, surface

SEM Analysis
Scanning electron microscopy was employed to study the surface morphology and the cross-sectional morphology of the NIPU composite. As shown in Figure 10A, surface roughness increased with the increase in TANI loading and the surface had a microporous flake-like structure. The cross-sectional images ( Figure 10B) confirm that TANI is well dispersed in the NIPU matrix even at higher loading (4%).

SEM Analysis
Scanning electron microscopy was employed to study the surface morphology and the cross-sectional morphology of the NIPU composite. As shown in Figure 10A, surface roughness increased with the increase in TANI loading and the surface had a microporous flake-like structure. The cross-sectional images ( Figure 10B) confirm that TANI is well dispersed in the NIPU matrix even at higher loading (4%).

The Water Contact Angle
The surface wettability characteristics of the composite films (water contact angle) coated on glass surfaces were evaluated by using a KRUSS goniometer ( Figure 11). Basically, a material is said to be hydrophilic if the water contact angle (WCA) is less than 90°, and hydrophobic if it is greater than 90°. It was noticed in the present case that all the films exhibited a hydrophobic nature. However, addition of TANI reduced the water contact angle by approximately 12° to 7°. While NIPU without TANI shows hydrophobic behavior due to the presence of long alkyl groups of the triglyceride units in soybean oil, addition of TANI induced surface roughness (as seen in SEM images) and also hydrophilicity in the PU matrix.

The Water Contact Angle
The surface wettability characteristics of the composite films (water contact angle) coated on glass surfaces were evaluated by using a KRUSS goniometer ( Figure 11). Basically, a material is said to be hydrophilic if the water contact angle (WCA) is less than 90 • , and hydrophobic if it is greater than 90 • . It was noticed in the present case that all the films exhibited a hydrophobic nature. However, addition of TANI reduced the water contact angle by approximately 12 • to 7 • . While NIPU without TANI shows hydrophobic behavior due to the presence of long alkyl groups of the triglyceride units in soybean oil, addition of TANI induced surface roughness (as seen in SEM images) and also hydrophilicity in the PU matrix.

Polarization Studies
Tafel polarization measurement is a technique for analyzing the anti-corrosive properties of polymeric coatings by evaluating the current vs. potential. By this technique, corrosion current (Icorr), corrosion potential (Ecorr) and corrosion rate (CR) on the metal panels were measured [36][37][38]. Icorr and Ecorr will give bisecting extrapolated lines from both the oxidation and reduction parts. The Tafel study ( Figure 12 and Table 5) of NIPU coated on the mild steel panel (3 × 3 cm 2 ) demonstrated that as loading of TANI increased, the rate of corrosion decreased. ECorr values of the coated NIPU panel shifted more towards positive anodic values compared to that of the bare MS panel, which substantiates the superior control of the reaction (anodic and cathodic) because of barrier properties of NIPU coatings. CT 0.0 panel without TANI has showed a corrosion rate of 2.69 × 10 −4 mm/year with a polarization resistance of 5.59 × 10 3 kΩ while NIPU with TANI loaded showed a lower corrosion rate with higher polarization resistance. With 4% loaded TANI, the CR was 1.67 × 10 −5 mm/year with a high polarization resistance of 73.80 × 10 5 kΩ. This trend suggested that TANI reduced the corrosion rate of mild steel with its redox behavior [39].

Polarization Studies
Tafel polarization measurement is a technique for analyzing the anti-corrosive properties of polymeric coatings by evaluating the current vs. potential. By this technique, corrosion current (I corr ), corrosion potential (E corr ) and corrosion rate (C R ) on the metal panels were measured [36][37][38]. I corr and E corr will give bisecting extrapolated lines from both the oxidation and reduction parts. The Tafel study ( Figure 12 and Table 5) of NIPU coated on the mild steel panel (3 × 3 cm 2 ) demonstrated that as loading of TANI increased, the rate of corrosion decreased. E Corr values of the coated NIPU panel shifted more towards positive anodic values compared to that of the bare MS panel, which substantiates the superior control of the reaction (anodic and cathodic) because of barrier properties of NIPU coatings. CT 0.0 panel without TANI has showed a corrosion rate of 2.69 × 10 −4 mm/year with a polarization resistance of 5.59 × 10 3 kΩ while NIPU with TANI loaded showed a lower corrosion rate with higher polarization resistance. With 4% loaded TANI, the C R was 1.67 × 10 −5 mm/year with a high polarization resistance of 73.80 × 10 5 kΩ. This trend suggested that TANI reduced the corrosion rate of mild steel with its redox behavior [39].
that TANI reduced the corrosion rate of mild steel with its redox behavior [39].

The Salt Spray Fog Test
The salt spray fog test helps to investigate the spread of damage by way of the delamination or mechanical failure of the coating. Figure 13 displays the images of the coated  The salt spray fog test helps to investigate the spread of damage by way of the delamination or mechanical failure of the coating. Figure 13 displays the images of the coated panels after 144 h of exposure to salt spray (5% NaCl solution). The salt spray fog test result showed that the coating without TANI started to delaminate early while the TANI composite-coated panel showed good adhesion and corrosion resistance ( Figure S1, SI). With increasing TANI, the adhesion of the coating to the panel increased and CT4.0 had the best adhesion and corrosion resistance. This correlates well with the polarization studies, where CT4.0 exhibited the highest polarization resistance and the lowest corrosion rate, 1.67 × 10 −5 mm/year. Sustain. Chem. 2023, 4, FOR PEER REVIEW 13 panels after 144 h of exposure to salt spray (5% NaCl solution). The salt spray fog test result showed that the coating without TANI started to delaminate early while the TANI composite-coated panel showed good adhesion and corrosion resistance ( Figure S1, SI). With increasing TANI, the adhesion of the coating to the panel increased and CT4.0 had the best adhesion and corrosion resistance. This correlates well with the polarization studies, where CT4.0 exhibited the highest polarization resistance and the lowest corrosion rate, 1.67 × 10 −5 mm/year.

Conclusion
A biobased non-isocyanate free polyurethane was synthesized from carbonated soyabean oil using tetraethylenepentamine (TEPA) as a curing agent. Varying amounts of TANI (0.5, 1.0, 2 and 4%) were loaded into the PU matrix to induce corrosion resistance.

Conclusions
A biobased non-isocyanate free polyurethane was synthesized from carbonated soyabean oil using tetraethylenepentamine (TEPA) as a curing agent. Varying amounts of TANI (0.5, 1.0, 2 and 4%) were loaded into the PU matrix to induce corrosion resistance. It was observed that ATR-FTIR of the films exhibited peaks corresponding to the urethane linkages, thus confirming the formation of the polyhydroxyurethanes (PHU). TGA showed that the loading of oligoaniline did not affect the initial degradation temperature much because the dissociation of the biscarbamate linkages is the rate-determining step during the decomposition. Even though there was not a specific trend, the glass transition temperature of the films (from dynamic mechanical studies) increased with the incorporation of tetraniline, while the tanδ peaks became more intense and narrower, indicating the effect of the oligoaniline on the extent of H bonding between the PU chains. TANI-incorporated films had better tensile properties than the bare NIPU, indicating the reinforcing nature of TANI, and composition with 1% TANI loading had the highest tensile strength and elongation. SEM images showed the uniform distribution of TANI particles accompanied with the increase in surface roughness with increasing TANI content. Further, the incorporation of oligoaniline led to a reduction in the water contact angle of the NIPU-TANI films due to the above reason. The corrosion resistance of the MS panels, as evaluated by polarization studies, improved with incorporation of TANI and CT 4.0 exhibited the best performance. The corrosion data were further substantiated by salt spray analysis, wherein TANI increased both the adhesion properties as well as the corrosion resistance compared to bare NIPU coatings.