Canola Oil based Poly(ester–ether–amide–urethane) Nanocomposite and Its Anti-Corrosive Coatings

The environmental and health hazards associated with petro-based chemicals have motivated the researchers to replace them partially or wholly with renewable resource-based polymers. Vegetable oils serve as an excellent alternative to this end as they are cost effective, eco-friendly, easily available and rich with functional groups amenable to chemical reactions. The aim of the research work is to prepare Canola oil [CANO] derived poly (ester–ether–amide–urethane) (CPEEUA) nanocomposite coating material using N,N-bis (2-hydroxyethyl) fatty amide [CFA] obtained from CANO, Lactic acid [LA], and reinforced with Fumed Silica [FS]. CPEEUA was obtained by esterification, etherification, and urethanation reactions and its structure was confirmed from FTIR and NMR spectral analyses. CPEEUA/FS coatings were found to be scratch resistant, flexible, well-adhered to mild steel panels, and hydrophobic with 2.0–2.5 kg scratch hardness, 150lb/inch impact resistance and >90° contact angle value. They exhibited good corrosion protection in 3.5 wt% NaCl solution as investigated by Potentiodynamic Polarization and Electrochemical Impedance tests. CPEEUA coatings are safe for usage up to 200 °C.


Introduction
Thermosetting resins are an inevitable part of the coatings industry. As these are derived from petrochemicals, their toxicity, nonbiodegradability, and volatility of starting materials are of prime health, environmental, and safety concerns, and require immediate attention. An alternative greener solution has been found in the development of biobased resins. The latter resins are partially or completely derived from biological sources such as starch, cellulose, lignin, furan, rosin, and vegetable oils [VO]. They are eco-friendly, cheaper, and biodegradable and offer a completely viable alternative to meet strict environmental regulations, reduce consumption of finite petrochemicals, and are competitive in price and performance to existing petrobased thermosetting resins [1][2][3].

Synthesis of Canola Diol Fatty Amide (CFA)
CFA was prepared according to our previously published article using vegetable oil and diethanolamine in the presence of sodium methoxide catalyst [44]. The synthesized product was washed using diethyl ether and 15% sodium chloride solution and the structure of CFA was confirmed by FTIR analysis.

Synthesis of Canola Oil based Poly(ester-ether-amide-urethane) (CPEEUA) and CPEEUA/FS Nanocomposite
The synthesis of CPEEUA was carried out by the following steps: Step 1: Synthesis of CANO ester amide [CEA]: CFA (0.05 mol) and LA (0.10 mol) were placed in a four-necked round-bottomed flask fitted with dean stark trap, a nitrogen inlet tube, and thermometer, and heated at 80 • C for 3h, over a magnetic stirrer. The reaction temperature was increased to 120 • C and maintained at this temperature for another 2 h until desired (low) acid value was obtained (Scheme 1). The reaction was monitored by thin layer chromatography (TLC) and FTIR spectrum was recorded.  Step 2: Synthesis of poly(ester-ether) amide (CPEEA). CEA was taken in a reaction flask, and to it was added 25 mL toluene (for azeotropic distillation) and 1 mL H 2 SO 4 (1:1 v/v diluted with water) dropwise. Subsequently, the temperature was increased to 140 • C, and stirring was continued at this temperature, until FTIR supported the formation of ether linkages, i.e., the formation of CPEEA (Scheme 1). The reaction was monitored by TLC and recording FTIR spectra at regular intervals of time.
Step 3: Synthesis of CPEEA/fumed silica nanocomposite (CPEEA/FS): FS (2% w/w on the weight of CFA) was added with gradual mixing to the calculated amount of CPEEA, followed by the addition of TDI (35,40, 45% w/w on the weight of CFA), dropwise under continuous stirring. After the complete addition of TDI, the reaction temperature was increased to 60 • C, and the reaction was continued until the FTIR spectrum indicated the formation of polyurethane (Scheme 2). Different CPEEUA/FS nanocomposites were prepared by adding TDI in different weight percentages (35,40, 45% w/w on the weight of CFA), in similar experimental set-up, to obtain CPEEUA/FS-35, CPEEUA/FS-40 and CPEEUA/FS-45 polyurethane nanocomposites (35,40 and 45 indicate the percent loading of TDI). The reaction was monitored by thin layer chromatography and FTIR.
A similar reaction was also accomplished omitting the addition of FS, producing plain CFA based polyurethane, CPEEUA.

Preparation of CPEEUA/FS Nanocomposite Coatings
Carbon steel [CS] strips in two standard sizes (70 mm × 25 mm × 1 mm and 25 mm × 25 mm × 1 m) and composition (Fe, 99.51%, Mn, 0.34%, C, 0.10% and P, 0.05%) were first polished with silicon carbide paper of different grades, then washed with double distilled water, followed by methanol and acetone for degreasing, and next dried at room temperature. These cleaned CS strips were then coated with 40% (w/v% in toluene) solutions of CPEEUA and CPEEUA/FS by brush and the coated panels were left to dry at room temperature for two weeks for complete curing/drying. The dried coated panels were then subjected to physico-mechanical and corrosion tests by standard methods.

Results and Discussion
CFA was prepared by base catalysed amidation reaction of CANO [37][38][39]. CPEEAU/FS nanocomposite was prepared using CFA, LA, FS, and TDI as raw materials, by esterification, etherification, and urethanation. The aliquots of samples were withdrawn to govern the progress of the reaction by TLC and FTIR (which gave a clue about the proposed structure of the end product, based upon the spots in the TLC plate and appearance or disappearance of corresponding functional groups' absorption bands in the FTIR). After indication from FTIR, the samples were then subjected to NMR spectral analysis for confirmation of respective structures. The synthesized CPEEAU/FS was then used for preparation of coatings.
LA bears both hydroxyl and carboxyl functional groups and thus introduces both ether and ester linkages in CANO polyurethane [20][21][22]. CPEEAU is cured at room temperature due to the presence of urethane linkages and the double bonds of the triglyceride chains that undergo auto-oxidation.

Physico-Mechanical Test of Coatings
The thickness of coatings was found to be 95 micron to 105 micron. Scratch hardness increased from 2.0 kg to 2.2 kg to 2.5 kg, and then decreased to 2.4 kg with increased urethane and aromatic content in CPEEUA-40, CPEEUA/FS-35, CPEEUA/FS-40 and CPEEUA/FS-45, indicating that at 40% w/w loading of TDI in CPEEUA/FS-40, the best scratch hardness resistance could be achieved. Good impact resistance (150lb/inch) and bending ability was also achieved in CPEEUA/FS-40 due to good adhesion of coated films to the substrate and crosslinked polymer chains. Beyond 40% w/w addition of TDI, increased aromatic content and excessive crosslinking conferred by urethanation reaction caused brittleness and the coating properties were deteriorated. Thus, CPEEUA-40 and CPEEUA/FS-40 were selected as the study sample to perform corrosion resistance, surface wettability, and thermal stability tests.

Physico-Mechanical Test of Coatings
The thickness of coatings was found to be 95 micron to 105 micron. Scratch hardness increased from 2.0 kg to 2.2 kg to 2.5 kg, and then decreased to 2.4 kg with increased urethane and aromatic content in CPEEUA-40, CPEEUA/FS-35, CPEEUA/FS-40 and CPEEUA/FS-45, indicating that at 40% loading of TDI in CPEEUA/FS-40, the best scratch hardness resistance could be achieved. Good impact resistance (150lb/inch) and bending ability was also achieved in CPEEUA/FS-40 due to good adhesion of coated films to the substrate and crosslinked polymer chains. Beyond 40 wt% addition of TDI, increased aromatic content and excessive crosslinking conferred by urethanation reaction caused brittleness and the coating properties were deteriorated. Thus, CPEEUA-40 and CPEEUA/FS-40 were selected as the study sample to perform corrosion resistance, surface wettability, and thermal stability tests.

Thermal Stability
DSC thermogram of CPEEUA-40 ( Figure 6) shows that the first endothermic eve starts from 108 °C to 209 °C, centered at 178 °C. While this endothermic event is insi nificant in CPEEUA/FS-40. This first endothermic event may be correlated to the loss moisture. The second endothermic event extends from 226 °C to 321 °C in CPEEUA-4 and CPEEUA/FS-40. The third significant and broad endotherms appear from 322 °C 412 °C, respectively, in CPEEUA-40 and CPEEUA/FS-40, which can be attributed to th configurational changes in polymer backbone due to subsequent thermal degradatio stages as evident in TGA and DTG.     TGA thermograms (Figure 7) show the first 10 wt% weight loss up to 250 °C, which can be attributed to the loss of moisture; both CPEEUA-40 and CPPEUA/FS-40 exhibit the same degradation pattern as evident from TGA and DTG (Figure 7). Beyond this temperature, the degradation can be attributed to the onset of decomposition of the urethane bonds, proceeded by subsequent degradation of other moieties of polymer backbone, under the effect of rising temperature. A significant variation is observed in the degradation pattern of CPEEUA/FS-40 from TGA and DTG thermogram; this showcases its improved thermal stability relative to CPEEUA-40. 20wt%, 30wt%, 40wt%, 50wt%, 60wt%, and 70 wt% losses are observed at 300 °C, 333 °C, 363 °C, 397 °C, 425 °C, 445 °C, and 316 °C, 376 °C, 417 °C, 434 °C, 449 °C, 459 °C, in CPEEUA-40 and CPEEUA/FS-40, respectively. The remarkable variation in thermal degradation temperatures, for each wt% loss, in CPEEA-40 and CPEEUA/FS-40 highlights good thermal stability of the nanocomposite, due to fine and homogenous dispersion of FS as well as good interfacial interactions with the matrix. CPEEUA-40 displays a somewhat single-step degradation pattern contrary to CPEEUA/FS-40 which exhibits a notable two-step degradation pattern, covering 90 wt% loss. TGA indicates that these coatings can be safely used up to 250 °C.

Potentiodynamic Polarization
Tafel polarization curves (Figure 8) for CPEEUA/FS-40 coating were conducted in a 3.5 wt% NaCl solution chosen as a corrosive medium. The data pertaining to corrosion

Potentiodynamic Polarization
Tafel polarization curves (Figure 8) for CPEEUA/FS-40 coating were conducted in a 3.5 wt% NaCl solution chosen as a corrosive medium. The data pertaining to corrosion rate (CR), corrosion potential (Ecorr), corrosion current density (Icorr), and polarization resistance are tabulated (Table 1) by the tafel extrapolation method. CPEEUA/FS-40 showed good corrosion protection for MS. However, it is clear from the table that the corrosion potential of CPEEUA/FS-40 coatings pointedly decreased and corrosion current density increased with increased immersion time in the said corrosive medium. These results indicate that CPEEUA/FS-40 can act as protective layer of MS and improve the corrosion resistance performance up to 7 days, then reduce the coating performance due to the formation of some pores in coatings, resulting in contact of corrosive ions to substrate during the long immersion time (11 days) and corrosion potential of CPEEUA/FS-40 coatings also shift towards the more negative potential during these immersion times [32].
corrosion potential of CPEEUA/FS-40 coatings pointedly decreased and corrosion current density increased with increased immersion time in the said corrosive medium. These results indicate that CPEEUA/FS-40 can act as protective layer of MS and improve the corrosion resistance performance up to 7 days, then reduce the coating performance due to the formation of some pores in coatings, resulting in contact of corrosive ions to substrate during the long immersion time (11 days) and corrosion potential of CPEEUA/FS-40 coatings also shift towards the more negative potential during these immersion times [32].

Electrochemical Impedance Spectroscopy
In Figure 9, the corrosion protection performance of CPEEUA/FS-40 was investigated by EIS during 1, 3, 5, 7, 9, and 11 days of immersion of nanocomposite coated metal strips in a 3.5 wt% NaCl medium, as a function of exposure times. Table 2 distinctly revealed that the Cc increased with the immersion time while Rct decreased. CPEEUA/FS-40 throughout immersion time (5 days) showed only one capacitive loop. As immersion time increased, capacitive loop decreased, as shown in the Warburg diffusion ( Figure 9). The impedance data ( Table 2) was observed at 10 6 Ω, on day 1, which reduced to 10 4 Ω after increasing the immersion time to 3 days. The impedance value seems to be affected by the immersion period; although for first 5 days little change in impedance was observed for CPEEUA/FS-40 coatings. On the eleventh day, comparatively more significant loss in impedance was observed. Thus, the CPEEUA/FS-40 coating obstructs the charge transfer reactions taking place at the metal strip surface and the medium to which it is exposed, up to 7 days. This is expected as with continuous immersion (11 days), CPEEUA/FS-40 coating surface would infuse some corrosive ions through the coating itself.

Electrochemical Impedance Spectroscopy
In Figure 9, the corrosion protection performance of CPEEUA/FS-40 was investigated by EIS during 1, 3, 5, 7, 9, and 11 days of immersion of nanocomposite coated metal strips in a 3.5 wt% NaCl medium, as a function of exposure times. Table 2 distinctly revealed that the Cc increased with the immersion time while Rct decreased. CPEEUA/FS-40 throughout immersion time (5 days) showed only one capacitive loop. As immersion time increased, capacitive loop decreased, as shown in the Warburg diffusion ( Figure 9). The impedance data ( Table 2) was observed at 10 6 Ω, on day 1, which reduced to 10 4 Ω after increasing the immersion time to 3 days. The impedance value seems to be affected by the immersion period; although for first 5 days little change in impedance was observed for CPEEUA/FS-40 coatings. On the eleventh day, comparatively more significant loss in impedance was observed. Thus, the CPEEUA/FS-40 coating obstructs the charge transfer reactions taking place at the metal strip surface and the medium to which it is exposed, up to 7 days. This is expected as with continuous immersion (11 days), CPEEUA/FS-40 coating surface would infuse some corrosive ions through the coating itself.

Conclusions
The manuscript described the preparation of a poly (ester-ether-urethane) amide from Canola oil. CPEEUA was further strengthened by including fumed silica as a nanofiller, yielding nanocomposite, which was further applied as a corrosion protective coating material. The coatings showed good physico-mechanical and corrosion resistance against a 3.5 wt% NaCl solution. The approach paves the path for utilization of vegetable oils by a proposed simple, single-pot derivatization method, to be applied as organic coatings and nanocomposite coatings.

Conclusions
The manuscript described the preparation of a poly (ester-ether-urethane) amide from Canola oil. CPEEUA was further strengthened by including fumed silica as a nanofiller, yielding nanocomposite, which was further applied as a corrosion protective coating material. The coatings showed good physico-mechanical and corrosion resistance against a 3.5 wt% NaCl solution. The approach paved path for utilization of vegetable oils by simple, single-pot derivatization method, to be applied as organic coatings and nanocomposite coatings.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.