Soy polyester urethane/TiO₂ and Ce-TiO₂ nanocomposites: preparation, characterization and evaluation of electrochemical corrosion resistance performance

Abstract

Environment friendly soy polyester urethane nanocomposite coating materials were prepared by dispersing TiO₂ and Ce-TiO₂ nanoparticles in a 3-isocyanatopropyltriethoxysilane (IPTES) modified soy oil (SO) polyester urethane triethoxysilane (PEUTES) matrix via a sonication technique. Fourier Transform Infrared Spectroscopy (FT-IR), ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), UV-visible spectroscopy (UV) were employed to characterize the synthesized coating materials. The anticorrosion ability of nanocomposites and that of PEUTES coatings on carbon steel (CS) in 3.5 wt% NaCl solution was investigated using potentiodynamic polarization (PDP), Electrochemical Impedance Spectroscopy (EIS) techniques and a salt mist test. The effects of dispersion of nano TiO₂ and Ce-TiO₂ fillers in PEUTES matrices on the hydrophobic, physico-mechanical, thermal and anticorrosive properties were also studied. The TiO₂ and Ce-TiO₂ nanofiller dispersed soy polyester urethane nanocomposite coatings have exhibited far superior corrosion resistance properties than those of other such reported systems. These studies revealed the presence of nanofiller in polyester urethane matrix, induces strong barrier and locking effects at the coating–metal interface. The higher impedance value (≈108 Ω) and lower corrosion rate (7.8518 × 10⁻⁶ mpy) confirm the superior protection ability of the nanocomposite.

---

1. Introduction

Metallic corrosion causes catastrophic losses in terms of deterioration of devices, machines, industrial setups, infrastructures and economy.¹ Various protective techniques are being used to save such losses thereby enhancing their service life.² Among these protective techniques, the formulation and application of polymer coatings on the substrate is easy and more convenient.³ The presence of various pendent functional groups in polymer coatings significantly improve the adhesion at coating metal interfaces, which enhances the physico-mechanical and anticorrosive properties of the coatings.⁴ However, the use of organic solvents in polymer coating systems produces volatile organic contents (VOCs), which lead to a number of harmful effects on the environment.³ Moreover, the depletion of the fossil fuel reservoir has forced scientists and technologists to develop sustainable resources based polymers. Among various sustainable precursors, vegetable oils (VOs)⁵ are readily available, which are biodegradable, inexpensive and contain numerous functional groups (ester, double bonds, hydroxyl etc.) available for chemical transformation. Generally, VO polymers have excellent chemical, physical, hydrophobic,⁶ and good fluid properties owing to their low molecular weight.⁵ Nevertheless they exhibit low rigidity, poor mechanical strength, unsatisfactory acid and base resistance properties, which limit their application under stringent environmental conditions.³ In order to enhance the useful properties and to overcome the drawbacks of sustainable polymers, new approaches are being adopted. The dispersion of nanofillers in VO polymers is one of them, which leads to the formation of new generation polymeric materials exhibiting excellent mechanical, thermal and barrier properties.^7,8

Titanium oxide (TiO₂) and Ce doped TiO₂ are considered as the most versatile and technologically important materials, having promising applications in the field of electronics, catalysis and pigments. The presence of higher number of electron–hole pairs in Ce increases the redox potential of Ce doped nano TiO₂ particles which enhances the anticorrosive activity of Ce-TiO₂ fillers.⁹ Electronic, photoelectric, catalytic and anticorrosive properties of nano TiO₂ and Ce doped TiO₂ have previously been reported.⁹ However, their synthesis in presence of castor oil using titanium tetra isopropoxide and cerium ammonium nitrate has not been reported so far.

The present article reports, for the first time, the synthesis of TiO₂ and Ce doped TiO₂ using castor oil (a sustainable resource) has been reported for the first time. The structural, morphological and thermal stability of these coatings were investigated using different analytical techniques. The effect of presence of TiO₂ and Ce-TiO₂ nanoparticles in PEUTES coatings on thermal stability, hydrophobicity, physico-mechanical and anticorrosive properties were also investigated. These studies revealed that the nanocomposite coatings have exhibited far superior properties than those of other such reported systems.

2. Experimental

2.1. Materials

Titanium tetra-isopropoxide (TTIP) 97% was purchased from High Media Laboratories Pvt. Ltd. Mumbai, India. Acetic acid (99.7%), cerium ammonium nitrate (CAN), castor oil, sodium hydroxide, sulphuric acid, ethanol dimethylolpropionic acid (DMPA), 50% hydrogen peroxide, glacial acetic acid (99–100%), were obtained from sd Fine-Chemicals Limited, Mumbai, India. Soy oil (SO) was procured from the Kendriya Bhandar, Jamia Millia Islamia, New Delhi, India, while dibutyl tin dilaurate (DBTDL) and 3-isocyanatopropyltriethoxysilane (IPTES) were supplied by Alfa Aesar, UK. All the chemicals were of AR grades and used as such.

2.2. Preparation of soy polyester and IPTES based polyester urethane (PEUTES) hybrid

The fatty acid polyester (FPE) was synthesised by extracting the fatty acid (FA) from soy oil (ESI†),¹⁰ followed by its epoxidation and hydroxylation, which lead to the formation of soy fatty acid polyol (FPOL).¹¹ The FPOL and DMPA were mixed in 1 : 4 ratio along with 1–2 drops of 1 wt% aqueous solution of p-toluene sulphonic acid (PTSA) were used as acid catalyst under continuous stirring at 120 °C followed by condensation reaction between the hydroxyl groups of FPOL and carboxyl groups of DMPA through the elimination of water resulted in the formation of crosslinked three dimensional fatty acid polyester (FPE) after 2 h as per (Scheme 1a).¹² The progress of the reaction was monitored by acid value and FTIR spectra taken at regular intervals of time. Further, FPE and IPTES were mixed in 4 : 1 ratio by weight, allowed to react then in presence of dibutyltindilaurate (DBTDL) catalyst. The reaction mixture continuously stirred on a magnetic stirrer at 60 °C for 30 min which led to the formation of highly crosslinked hybrid nano silica networked polyester urethane (PEUTES) coating material. The progress of these reactions was periodically monitored by FT-IR spectra. The disappearance of [double bond splayed left]NCO peak at 2276 cm⁻¹ confirmed the formation of PEUTES hybrid material.¹³

Scheme 1 (a) Reaction scheme for the synthesis of FPE. (b) Reaction scheme for the synthesis of PEUTES.

2.3. Synthesis of TiO₂ and Ce-TiO₂ nanoparticles

TTIP (25 mL) was drop wise added to the mixture of acetic acid (48 mL) and castor oil (100 mL), in a 250 mL three necked flat bottom flask fitted with condenser, thermometer and magnetic stirrer, then 50 mL distilled water was slowly added to the reaction mixture under vigorous stirring at room temperature (≈30 °C), the reaction mixture was continuously stirred overnight at room temperature to get a transparent sol, which was further heated at 100 °C for 1 h, followed by its calcination at 600 °C in air for 4 h, which resulted in the formation of TiO₂, the same was doped with the Ce, which was used for the synthesis of Ce-TiO₂-I and Ce-TiO₂-II nanoparticles, respectively. The preparation of Ce-TiO₂ nanoparticles in two different ratios (TTIP : CAN in 3 : 1 and 1 : 1 molar ratio) was carried out using the same procedure and reaction setup that was used for the preparation of TiO₂ nanoparticles. The formation of nano TiO₂ and Ce-TiO₂ was confirmed by XRD and FTIR studies.

2.4. Preparation of PEUTES–TiO₂ and PEUTES–Ce-TiO₂ hybrid nanocomposites

The 1.5 wt% of TiO₂, Ce-TiO₂-I and Ce-TiO₂-II nanoparticles were dispersed in PEUTES ultrasonically using minimum amount of ethanol (solvent) and sonicated for 1 h to obtain the homogeneously dispersed nanoparticle suspended colloidal solutions of PEUTES–TiO₂, PEUTES–Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposites.

2.5. Preparation and testing of coatings

The carbon steel (CS) strips (composition: 2.87 wt% C and 97.13 wt% Fe)³ of two different sizes were polished with different grades of silicon carbide papers (180, 320 and 500) followed by thorough rinsing with water and degreased with alcohol and acetone. PEUTES, PEUTES–TiO₂, PEUTS-Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposites were applied on the surfaces of the CS strips by brush techniques and were kept at room temperature for drying. The coatings of ≈120 μm thickness developed on CS of size 70 mm × 25 mm × 1 mm were used for the physico-mechanical studies and that of 25 mm × 25 mm × 1 mm size were used for electrochemical corrosion and morphological studies.

2.6. Instrumentation

Different analytical techniques were used to characterize the nanoparticles and nanocomposite coating materials. Powder X-ray diffraction (XRD) patterns were obtained by a Rigaku Mini Flex diffractometer (using Cu K λ = 1.5418 Å radiation) at a scanning rate of 0.002° per min in the 20° to 70° range. Fourier transform infrared (FT-IR) spectra were recorded on IR Affinity-1 Shimadzu (in the range of 4000 to 400 cm⁻¹). The ¹H and ¹³C NMR spectra of the PEUTES were obtained on a JEOL GSX 300 MHz FX-100 in CDCl₃ using tetramethylsilane (TMS) as internal standard. UV-visible spectra were recorded in aqueous medium on Perkin Elmer Lambda Spectrophotometer (Model EZ-221). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns of the samples were recorded using a JEOL-2100 TEM instrument, operating at 200 kV. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) examinations were carried out on Zeiss EVO 40 instrument at the Advanced Instrumentation Research Facility (AIRF), Jawaharlal Nehru University, New Delhi.

The dry to touch (DTH) and dry to hard time (DTT) of coatings were recorded, the scratch hardness (SH) [BS 3900], impact resistance (IR) [IS; 101 part 5/sec-3, 1998], bending test (BT) [ASTM-D3281- 84] were performed, while specular gloss (at 45°) and thicknesses of coatings were measured by Gloss meter (Model-RSPT-20) and Elcometer (Model 345; Elcometer Instrument, Manchester, UK), respectively. The cross-hatch test (ASTM D3359) was carried out on coated CS by cross hatch meter.¹⁴ The thermal stabilities of coating materials were studied by thermogravimetric analysis (TGA) using EXSTAR TG/DTA 6000 instrument under nitrogen atmosphere at a heating rate of 10 °C min⁻¹ from 25–800 °C. The glass transition temperatures (T_g) of coating materials were determined by differential scanning calorimetry (DSC) (SII EXSTAR 6000, DSC620, Japan) the heating and cooling of the samples were performed at the rates of 10 °C min⁻¹ under nitrogen atmosphere in the temperature range of 25–200 °C. The contact angle measurements for the hydrophobicity of coatings were evaluated by dropping the deionized water using a Drop Shape Analysis System (model DSA10MK2, Kruss GmbH, Germany) image captured with the help of an attached high speed CCD camera. The contact angles of these coatings were found in the range of 87–95°.¹⁵ Ultrasonic wave sonicator (model no. 1.5L, 50 Hz) was used at 30 °C for the homogeneous dispersion of nanoparticles in the polymeric matrix.

The corrosion resistance performance of PEUTES, PEUTES–TiO₂, PEUTS-Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposite coated and uncoated CS were carried out on Potentiostat/Galvanostat microAutolab type III (μ 3AVT 70762, The Netherland) in 3.5 wt% NaCl, at 30 °C according to ASTM G 59–97 standard using EG & G flat cell of 400 mL capacity consisting of conventional three electrodes, i.e. 1.0 cm² area of coated and uncoated CS as working electrode exposed to the corrosive medium, Pt gauge as auxiliary electrode and Ag/AgCl as reference electrode were used for corrosion studies. The electrochemical setup was supported by software Nova 1.8 (Autolab, UK); enabling the study of electrochemical corrosion behavior of coated and uncoated CS. The impedance and Tafel parameters were obtained by curve fitting procedure available in the software. After stabilizing the equipment for 30 min, the electrochemical corrosion behavior of coated and uncoated CS was taken with reference to their respective open circuit potentials (OCP). The AC (alternating-current) impedance measurements were made at open circuit potentials with 20 mV amplitude of the sinusoidal voltage signal at applied frequencies in the range of 100 kHz to 0.1 Hz using ten points per decade. The polarization curves were recorded by sweeping the potential from −100 mV to +100 mV (with respect to OCP) in the noble direction at a constant scan rate of 0.001 mV s⁻¹. The working electrodes were kept in the test solutions for 30 min to stabilize the electrode and to ensure that no blistering occurs during the incubation period before the impedance run. The impedance spectra obtained for the PEUTES, PEUTES–TiO₂, PEUTS-Ce-TiO₂-I and PEUTES–Ce-TiO₂-II coated CS and uncoated CS can be used to model the electrochemical corrosion behavior of coated substrate by an equivalent circuit and were found to be reproducible upto ±2–3%. The equivalent electrical circuit (EEC) is often used to analyze (ESI Fig. 1, ESI†) the impedance spectra of a metal/coating system and R_s(C_c(R_po(C_dlR_ct))) circuit model to simulate the changes in the properties of the coatings, where, R_s is the solution resistance, R_po is the pore resistance, R_ct is the coating resistance while the C_c and C_dl are the constant phase element and the double layer capacitance of the coating respectively. The EEC consists of two time constants; the first time constant (R_poC_c) in the high frequency region, attributed to the intrinsic properties of the coating and the second time constant (C_dlR_ct) in low frequency region, attributed to the corrosion reaction at coating–metal interface. These coatings were exposed to 3.5 wt% NaCl solution for EIS studies for 144 h.

The polarization resistance (R_p) of test samples was evaluated using the slope of the potential–current plot (Tafel plots), according to the Stern–Geary equation:¹⁶

here I_corr is the corrosion current density, determined by an intersection of the linear portions of the anodic and cathodic curves, while b_a and b_c are anodic and cathodic Tafel slopes (ΔE/Δlog I), respectively.

Salt mist test (ASTM B 117-94) for PEUTES–TiO₂, PEUTS-Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposites coated CS specimen was carried out for a period of 480 h in the salt mist chamber under 5.0 wt% NaCl solution at 90% humidity.

3. Results and discussion

3.1. Synthesis

A sustainable approach was applied to synthesised PEUTES matrix using SO as sustainable precursor, FA was extracted from SO to obviate the effect of steric hindrance,⁶ followed by epoxidation and trans-hydroxylation for the synthesis of FPOL.¹¹ During the formation of FPE number of hydroxyl groups further increases and by the reaction of carboxylic and hydroxyl functionalities of DMPA with FPOL and vice versa (Scheme 1b). The unreacted hydroxyl groups were allowed to react with isocyanate group of IPTES in the presence of DBTDL catalyst to form organic–inorganic hybrid PEUTES matrix (Scheme 1b).

The TiO₂ and Ce-TiO₂ nanoparticles were prepared in castor oil via sol–gel technique using TTIP and CAN. The castor oil acts as a nontoxic and multi-functional reagent, acting as a medium stabilizing and dispersing agents for nanoparticles synthesis.¹⁷ Crystal lattice formed by TiO₂ nanoparticles and Ce atoms are aligned within the crystal lattice at the grain boundaries site of the TiO₂ crystals. On the other hand, due to higher ionic size of Ce ions, Ti ions may enter into the Ce lattice leading to the formation of the mixed phase of Ce_xTiO₂ nanoparticles.¹⁸ Furthermore, TiO₂ and Ce-TiO₂ nanoparticles homogenously dispersed in PEUTES hybrids to form the nanocomposite coating material through sonication.

3.2. X-ray diffraction (XRD) studies

The XRD spectra of TiO₂ and Ce-TiO₂ as shown in Fig. 1a–c. The characteristic diffraction peaks are present at θ = 25.3°, 37.8, 48.1, 62.8 etc. of TiO₂ (Fig. 1a) indexed to the pure anatase phase of TiO₂ (JPCDS no. 89-4921) as reported in the literature.¹⁹ Ce-TiO₂ was transformed to different phase, depending on the oxidation state of cerium. As we know that the ionic radii of Ce³⁺ ion is greater than Ti⁴⁺. SO, the cerium ion could not doped in Ti⁴⁺ lattice, however, cerium ion can fit into the TiO₂ crystal lattice along the grain sides. While due to shorter ionic radii, Ti⁴⁺ will dope inside Ce³⁺ lattice in either case Ce and Ti combine with each other, which leads to the formation of a mixed phase as per Fig. 1.¹⁸ The presence of Ce³⁺ ions in Ce-TiO₂, nanoparticles induces formation of mixed phases such as Ce₂TiO₅, Ce₂Ti₂O₇ and Ce₄Ti₉O₂₄, while CeTiO₄ and CeTi₂O₆, contain Ce⁴⁺ ions.²⁰ XRD analysis of mixed oxides of Ce and Ti indicated that the doping of cerium in TiO₂ (JPCDS no. 49-1606) is an orthorhombic structural system with primitive lattice. According to the diffraction angle of new peak (θ = 28.5°), it can be indexed to CeO₂ phase, suggesting that a significant portion of Ce³⁺ ions were oxidized into Ce⁴⁺ during sintering.¹⁸ The average crystalline size of all these samples were analyzed with the help of lower full-width-at-half-maximum (FWHM) of the anatase (101) diffraction peak and found in accordance with the reported literature using Scherrer's equation:²¹where D is the particle size, λ is the X-ray wavelength (nm), θ is Bragg's angle; β is the excess line broadening (radiant), and the particle size of nanoparticles was found to be 25, 15 and 10 nm for TiO₂, Ce-TiO₂-I and Ce-TiO₂-II, respectively.

Fig. 1 (a) The XRD pattern of (a) TiO₂ nanoparticles JCPDS card no: (b) Ce-TiO₂-I JCPDS card no: 49-1606 nanoparticles (c) Ce-TiO₂-II nanoparticles JCPDS card no: 49-1606.

3.3. UV-visible spectroscopic studies

Fig. 2 shows the UV absorption spectra of TiO₂, Ce-TiO₂-I and Ce-TiO₂-II with the maximum absorption in the UV region. The peak at ≈325 nm increase of Ce-TiO₂ shifted slightly towards the visible region (at ≈340 nm) due to the doping of Ce in TiO₂ matrix.²² Furthermore, the increase in the intensity of peak is also observed with the increased loading of Ce. The synthesized Ce-TiO₂-I and Ce-TiO₂-II were annealed in atmospheric air at 600 °C, resulting in the oxidation of cerium leading to the formation of CeO₂ and Ce₂O₃, respectively.²⁰ The peak broadening was observed in the absorbance spectra of Ce-TiO₂-I and Ce-TiO₂-II at 340 nm which can be assigned to the f-d transition of single electron from 4f to 5d subshells of Ce³⁺ ions, as Ce⁴⁺ ion does not contain any electron in its ground state (4f⁰5d⁰).²³ Hence, the UV spectral analysis revealed that the (+3) valence state is predominantly present in the matrix of the TiO₂ nanoparticles.²⁴

Fig. 2 UV-visible spectra of TiO₂ and Ce-TiO₂ nanoparticles.

3.4. FT-IR spectroscopic studies

The peaks at 2910 and 2854 cm⁻¹ are assigned to the C–H stretching vibration of the aliphatic chain present in the SO (Fig. 3). Besides, the peak at 3009 cm⁻¹ assigned to the –C=C–H stretching vibration present in soy oil FA chain (Fig. 3a), which get disappeared in FPOL and a new peak was observed at 3490 cm⁻¹ in the FT-IR spectrum (Fig. 3b). These results confirmed the conversion of double bonds of FA chain into hydroxyl groups via epoxidation and trans hydroxylation. The appearance of absorption band at 1700 cm⁻¹ in FPE (Fig. 3c) revealed that the hydroxyl groups of FPOL had reacted with carboxyl functionality of DMPA.¹¹

Fig. 3 FT-IR spectra of (a) FA (b) FPOL (C) FPE and (d) PEUTES.

Some new changes were observed in the FT-IR the spectrum of PEUTES as compared to that of FPE, such as the characteristic vibration peak of NCO group at 2276 cm⁻¹ disappeared in the spectrum of PEUTES (Fig. 3d), revealing that the hydroxyl functionality of FPE had reacted with the isocyanate groups of IPTES.²⁵ Apart from this peak, a new peak appeared at 1544 cm⁻¹ during the course of the reaction, which corresponds to the bending vibrations of the amide linkages of urethane.²⁶ The appearance of an additional strong absorption band in the range 1074–1120 cm⁻¹ due to asymmetric vibration modes of the Si–O–Si bonds confirms the in situ formation of silica nanoparticles in the FPE matrix.²⁴

The peak at 471 cm⁻¹ and at 425 cm⁻¹ (Fig. 4) can be attributed to Ti–O–Ti as well as Ti–O–Ce vibrations, respectively. Thus, the shift in peak by 46 cm⁻¹ from 471 cm⁻¹ to 425 cm⁻¹ as observed in Ce-TiO₂-I, while the peak shift from to 410 cm⁻¹ in Ce-TiO₂-II, confirms the formation of Ti–O–Ce bonds.²⁷

Fig. 4 FT-IR spectra of TiO₂, Ce-TiO₂-I and Ce-TiO₂-II samples.

3.5. ¹H-NMR and ¹³C-NMR analysis

In ¹H NMR spectrum of PEUTES (Fig. 5), the peak at δ = 0.94 ppm can be attributed to the terminal methyl groups of the fatty acid chains, and the signal at δ = 1.02–1.20 ppm corresponds to the –CH₂ groups attached to the terminal methyl group and CH₂ groups of the fatty acid chains.²⁸ The peak at δ = 4.17 ppm is assigned to –OH groups, the characteristic peaks for –CH₂–O–CO also appeared in the same region at δ = 3.8–3.9 ppm.²⁹ The chemical shift value at δ = 5.0–5.9 ppm can be assigned to the hydrogen of urethane linkages³⁰ while the peak at δ = 4.72 ppm is due to the methylene protons attached to oxygen linking the urethane functional groups (–CH₂–O–CO–NH–).³¹ The ¹³C NMR spectrum (ESI Fig. 3, ESI†) of PEUTES shows the peak at δ = 174–178 ppm corresponding to the carbonyl carbon of ester linkages. The peak in the region of δ = 30–70 ppm is assigned to the carbon atom directly attached to an oxygen atom while the peak at δ = 34.5 ppm corresponds to the tertiary carbon atoms of DMPA.³² Further, the presence of peak at δ = 66.2 ppm (–CH₂–O–Si–) confirmed the inclusion of IPTES in polymer matrix.³³

Fig. 5 ¹H-NMR spectra of PEUTES.

3.6. Morphological studies

(i) Transmission electron microscopy (TEM) analysis. The TEM images of TiO₂, Ce-TiO₂-I, Ce-TiO₂-II (Fig. 6a–c) revealed the changes in morphologies with the corresponding changes in the average particle size. A cubic morphology was observed in case of the TiO₂ having 10–15 nm particle size whereas the Ce-TiO₂-I exhibits spherical morphology with the particle size of 5–10 nm range, while a mix cubic and spherical morphologies were seen for Ce-TiO₂-II in the 10–15 nm particle size range. Furthermore, the average particle size obtained from the TEM measurements are well in agreement with those obtained from the XRD measurements. The HR-TEMs (Fig. 6d–f) of TiO₂, Ce-TiO₂-I and Ce-TiO₂-II exhibit crystalline lattices for TiO₂, Ce-TiO₂-I, Ce-TiO₂-II respectively, confirming the anatase and rutile (phase) types of the crystal structures given in Section 3.2. TEM micrographs of PEUTES, PEUTES–TiO₂, PEUTES–Ce-TiO₂-I and PEUTES–Ce-TiO₂-II are given in ESI Fig. 4a–c.† In these micrographs (ESI Fig. 4a†), homogeneously dispersed silica nanoparticles formed in situ are clearly visible while the ESI Fig. 4b–d† show the TiO₂, Ce-TiO₂-I and Ce-TiO₂-II dispersion in nanocomposite coatings, which is encapsulated by the polymeric matrix.³⁴

Fig. 6 TEM micrographs of (a) TiO₂ nanoparticles (b) Ce-TiO₂-I nanoparticles (c) Ce-TiO₂-II nanoparticles (d) HR-TEM of TiO₂ nanoparticles (e) HR-TEM of Ce-TiO₂-I nanoparticles (f) HR-TEM of Ce-TiO₂-II nanoparticles.

(ii) Scanning electron microscopy (SEM) studies. The SEM micrographs (Fig. 7a–c) of TiO₂ (a), Ce-TiO₂-I (b) and Ce-TiO₂-II (c) show that the TiO₂, Ce-TiO₂-I and Ce-TiO₂-II consist of well-defined cubical and nanoparticles with an average dimension of less than 15 nm as discussed in previous Section 3.7 (i). The EDX spectra (ESI Fig. 2a–c†) of TiO₂, Ce-TiO₂-I and Ce-TiO₂-II nanoparticles show the composition of their constituents in wt% clearly reveals that the increase in the Ce content in Ce-TiO₂ with the increased loading of CAN in the reaction mixture. The SEM micrographs of PEUTES coated CS (Fig. 7d–e) indicate a cross-linked structure with O–Si–O– acting as a bridging moiety in PEUTES (Fig. 7d–e).

Fig. 7 SEM micrograph of (a) TiO₂ nanoparticles (b) Ce-TiO₂-I nanoparticles (c) Ce-TiO₂-II nanoparticles (d) PEUTES at 1.73 KX (e) PEUTES at 2.55 KX.

3.7. Thermal behavior

Fig. 8 and 9 show thermal behavior of organic–inorganic hybrid and nanocomposite coatings. These coatings show 5.0 wt% loss around 185 °C due to the desorption of physically adsorbed solvent and reacted moieties. The decomposition of labile urethane groups occurred at temperature 285 °C showing 18 wt% loss.³⁵ In the temperature range 350–390 °C, 51 wt% loss was observed due to the degradation of ester moieties present in the coating matrix.³⁶ While the 70 wt% loss at around 465 °C occurred due to the partial decomposition of the silicone entrapped polymeric segments.³⁷ Nanocomposite coatings showed the same degradations pattern, but at higher temperatures i.e., 5 wt% at 220 °C, 20 wt% at 320 °C and 70 wt% loss at 590–660 °C. The improved thermal stability of nanocomposites coatings can be explained by an increase in physical interaction of TiO₂ and Ce-TiO₂ with the polymer matrix.

Fig. 8 TGA thermograms of nanocomposite coatings.

Fig. 9 DSC thermograms of nanocomposite coatings.

The glass transition temperature of PEUTES and nanocomposites were observed in the range of 110–160 °C (Fig. 9). The increased T_g values in PEUTES and nanocomposite coatings of different composition attributed to the formation of highly cross linked structure and the filling of interstitial spaces of the matrix by nanoparticles.³⁸

3.8. Coating properties

(i) Physico-mechanical properties of PEUTES and nanocomposite coatings. The toughness of the synthesized nanocomposite coatings were evaluated by scratch hardness test (SHT). The increase in SHT value from PEUTES to PEUTES–TiO₂, PEUTES–Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposite coatings, can be correlated to the increase in interfacial adhesion between the coating material and the CS substrate. The strong adhesion between coating material and substrate surface, which was further confirmed by cross hatch test. The optical micrographs of the sample before and after cross-cut test are shown in ESI Fig. 5.† The results are specified as a 5B class (ESI Table 1†), representing an excellent adhesion between the nanocomposite coatings and CS substrate as no peeling was observed during the flaking step by tape. This can be attributed to the presence of polar functionalities such as –OH, –COOH; O–Si–O– linkages, TiO₂ and Ce-TiO₂ in hybrid polymer, which enhances the interfacial interactions with the substrate. All the coatings passed the 1/8 inch conical mandrel bend test and 26.8 kg cm⁻¹ impact test summarized in Table S1 (given in ESI†). These results suggest that the presence of nanoparticles and pendant in the matrix are responsible for the flexibility of the coatings, thus coating can bend along with substrate without showing any sign of damage. Further, the presence of O–Si–O, TiO₂ and Ce-TiO₂ in PEUTES has an impact on dissipation of energy during the impact test.

(ii) Potentiodynamic polarization (PDP) studies. Potentiodynamic polarization techniques were used to determine various electrochemical corrosion parameters like corrosion potential (E_corr), corrosion current density (I_corr) and corrosion rate to analyze instantaneous corrosion rate of coated specimens. Better corrosion protection was given by a coating showing lower corrosion rate, which corresponds to a lower I_corr or a higher E_corr values.³⁹ Fig. 10 shows the PDP curves in saline medium, and the parameters are displayed in Table 1. Nanocomposite coatings showed an increase in E_corr values and decrease in I_corr values in comparison to PEUTES (−0.5327 V) coatings and bare CS (−0.66951 V) (Table 1). The superior corrosion resistance performance displayed by PEUTES with respect to bare CS is due to the presence of hydrolytically stable urethane bonds, flexible hydrocarbon chains, formation of in situ siloxane nanoparticles and good adhesion between PEUTES and CS substrate.

Fig. 10 PDP curves of PEUTES, PEUTES–TiO₂ and different composition of PEUTES–Ce-TiO₂.

Table 1 PDP parameters of PEUTES, PEUTES–TiO₂ and different compositions of PEUTES–Ce-TiO₂ coated CS

Resin code	Ecorr (V)	Icorr (A)	Rp (Ω)	Corrosion rate (mpy)
CS^40	−0.66951	42.7	575.44	0.49615
PEUTES	−0.5327	4.970 × 10^−6	13 901	0.057752
PEUTES–TiO2	−0.3631	1.284 × 10^−8	3.4251 × 10^6	0.000149
PEUTES–Ce-TiO2-I	−0.3337	6.757 × 10^−10	3.7608 × 10^7	7.8518 × 10^−6
PEUTES–Ce-TiO2-II	−0.3503	1.035 × 10^−7	254 590	0.001202

---

However, in case of nanocomposite coatings, dispersion of inorganic nano fillers in polymer matrix form strong barrier to the penetration of corrosive ions by providing the torturous path to the corrosive species to enter coating metal interface,⁴⁰ which was further confirmed by the low corrosion rate of nanocomposite coatings in comparison to that of PEUTES coatings (Table 1). The enhanced anticorrosive performance of PEUTES–TiO₂, PEUTES–Ce-TiO₂-I and PEUTES–Ce-TiO₂-II nanocomposite coated CS in comparison to PEUTES coated CS can be correlated to the inclusion of nano-fillers by filling the free volume and interstitial spaces present in the crosslinked network structure and increased adhesion between coating and metal surface.^40,41

Beyond 25% loading of Ce in Ce-TiO₂, causes significant reduction in corrosion inhibition ability which is reflected by a negative shift in corrosion potential and higher corrosion current density as well as higher corrosion rates that may be due to the formation of Ce³⁺/Ce⁴⁺ redox couple,⁹ the PDP study revealed that PEUTES–Ce-TiO₂-I showed best corrosion protection among different compositions (Table 1).

(iii) Electrochemical impedance spectroscopy (EIS) studies. EIS was applied to give better information on electrochemical corrosion resistance performance and corrosion protection mechanism of nanocomposite coatings along with the role of TiO₂ and Ce-TiO₂ loading in PEUTES. The capacitive loop at high frequency is ascribed to the charge transfer resistance (R_ct) and diameter of the capacitive loop related to R_ct in the Nyquist plots representing the impedance of the samples.³⁹ Nanocomposite coatings show higher diameter and hence greater impedance value in Nyquist plots (ESI Fig. 6†) than that of PEUTES coatings. The Nyquist plot of PEUTES (ESI Fig. 6a†) shows phenomenon of diffusion in coatings, acquiring Warburg impedance shape, suggesting that the electrolyte penetrates into the polymeric matrix.⁴² Additionally, at higher frequencies, the surface of PEUTES exhibit charge transfer behavior, a sign of initiation of corrosion process.⁴⁰ Nanocomposite coatings show no charge transfer behavior and diffusion tail even after prolonged exposure to the electrolyte solution for 144 h (ESI Fig. 6b and c†) with the exception to the PEUTES–Ce-TiO₂-II nanocomposite coatings due to the formation of redox couple Ce³⁺/Ce⁴⁺ (ESI Fig. 6d†)as discussed in Section 3.10 (i).⁹

Similar results were also obtained from Bode plots (Fig. 11) where the impedance values for TiO₂ (≈10⁷ Ω), Ce-TiO₂-I (≈10⁸ Ω) and Ce-TiO₂-II (≈10⁶ Ω) dispersed nanocomposite coatings were 2–3 fold higher (Fig. 11a–d) as compared to that of plain PEUTES (10⁵ Ω) coatings. The lowest frequency impedance modulus |Z|_{0.01 Hz}, used to estimate the extent of corrosion activity, was found to be stable in case of nanocomposite coatings, suggesting that the aggressive species could not diffuse and reach the metal surface.⁴²

Fig. 11 Bode plot in 3.5 wt% NaCl solution of PEUTES and nanocomposite coated samples (a) PEUTES (b) PEUTES–Ce-TiO₂ (c) PEUTES–Ce-TiO₂-I and (d) PEUTES–Ce-TiO₂-II.

The enhanced corrosion protective performance can be accredited to the presence of nanoparticles in interstitial spaces, which induced good locking effect as well as good barrier property at coating metal interface. Besides, the enhanced hydrophobic nature of the nanocomposite coatings as evident from contact angle images (Fig. 11) this can be one of the reasons for improved corrosion protective ability of composite coatings in the said medium. The high R_p values of nanocomposite coatings in comparison to PEUTES and bare CS, revealed that the nanocomposite coatings can effectively suppress the diffusion of corrosive electrolyte into the polymer matrix by forming a strong barrier at coating metal interface.

Literature revealed that the VO based polymer coatings do not show satisfactory performance under stringent corrosive environments.^1,43 However it has been observed that the dispersion of metal oxide nano particles in such coating systems induces far superior corrosion protective and physico-mechanical properties,⁴⁴ which is also observed in present case that is the TiO₂, CeTiO₂ dispersed PEUTES systems have exhibited superior corrosion protective performance than those of earlier reported systems (Table 2).^1,9,43,45,46 Further, no such work on TiO₂ and Ce-TiO₂ dispersed soy oil PEUTES coating has been reported till now. Although nano TiO₂ dispersed castor oil based polyurethane-esteramide and commercial polyurethanes coatings have been reported whose performance were found to be inferior to the present nanocomposite coatings. Among different compositions, PEUTES–Ce-TiO₂-I shows best corrosion protection as no sign of degradation was observed at higher impedance value (≈10⁸ Ω) under lower frequency. The EIS results are well in agreement with that of PDP studies.

Table 2 Comparison of corrosion resistance properties of TiO₂ and Ce-TiO₂ polyester urethane hybrid nanocomposites coatings with other earlier reported systems in NaCl medium

S. N	Coatings systems	Icorr (A)	Polarization resistance (Rp, Ω)	Rpore (Ω)	Impedance (Rpo, Ω)	Reference
1	TiO2 and Ce-TiO2 soy oil polyester urethane hybrid nanocomposites coatings	6.76 × 10^−10	3.7 × 10^7	1.0 × 10^7	1.0 × 10^8	Present study
2	Castor oil based poly(urethane-esteramide)/TiO2 nanocomposites coatings	8.27 × 10^−7	—	—	—	41
3	Cerium-doped nano-TiO2 metallic coatings	—	—	4.85 × 10^4	4.58 × 10^7	9
4	TiO2 nanoparticles with 1,3,5-triazine based silane and polyurethane composite coating	2.4 × 10^−10	4.7 × 10^7	—	—	42
5	Electroactive polyimide–TiO2 hybrid nanocomposite coatings	1.64 × 10^−7	9460	0.439 × 10^3	Less than 10^7	39

---

(iv) Salt mist test. Fig. 12 shows the SEM micrograph of coated CS after 480 h of exposure to the salt mist test (in 5.0 wt% NaCl of 90% humidity). Here also nanocomposite coatings (Fig. 12b) show no sign of degradation, only slight deposition of salt was observed on the coating surface, while in case of PEUTES (Fig. 12a) the formation of cracks and pin holes give rise to the corrosion. Hence, the salt mist test further confirms the better corrosion protection efficiency of nanocomposite coatings as compared to those of PEUTES and bare CS. The uniform, homogeneous dispersion of nanoparticles and dense hydrophobic network structure of nanocomposite coatings protect the underlying metal substrate from the corrosive ions attack.

Fig. 12 SEM micrograph after exposure to salt mist chamber of (a) PEUTES (b) PEUTES–Ce-TiO₂-I.

3.9. Corrosion mechanism

The PEUTES nanocomposite coatings provide protection to the CS metal substrate through the (i) electro-static interaction of polar group at coating–metal interface leading to the formation of well adhered and compact coating, (ii) the presence of silica, TiO₂ and Ce-TiO₂ nanoparticles in PEUTES act as strong barrier that inhibit the penetration of corrosive ion at the metal–coating interface, and (iii) the presence of these nanoparticles also induces locking effect at the interstitial spaces and healing the other coating artifacts (micro cracks and voids). Hence, the protection mechanism for PEUTES nanocomposite coatings (Fig. 13) are governed by adhesion and locking effect of nanofillers, that provides barrier to the corrosion, and enhances the corrosion-protection efficiency of PEUTES nanocomposite coatings.

Fig. 13 Mechanism of corrosion protection for PEUTES nanocomposite coatings.

4. Conclusion

The TiO₂ and Ce doped (Ce-TiO₂) nanoparticles were synthesized in the green medium using castor oil via Sol-Gel technique. These nanoparticles were dispersed in soy polyesterurethane to prepare nanocomposite on CS surface. There physico-mechanical tests were performed by standard methods. Their anticorrosive performance of these coatings was tested in 3.5 wt% and 5 wt% NaCl solutions using PDP and EIS techniques and salt spray tests respectively. These studies revealed that the TiO₂ and Ce-TiO₂ dispersed soy polyurethane green nanocomposite coatings have far superior physico-mechanical and corrosion resistance properties in comparison to other such reported systems. These coatings are expected to have a potential scope for their commercialization in the field of paints and coatings.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

One of the authors is thankful to CSIR, India (Obaid ur Rahman) for financial support in the form of CSIR senior research fellowship (vide grant No.: 9/466 (0164) 2K13-EMR-I). The authors acknowledge the ‘Sophisticated Analytical Instrumentation Facility (SAIF)’center, All India Institutes of Medical Sciences (AIIMS), New Delhi for TEM facility.

References

1. Z. Guo, H. Wei, D. Ding and S. Wei, Anticorrosive Conductive Polyurethane Multiwalled Carbon Nanotubes NanoComposites, J. Mater. Chem. A, 2013, 1, 10805–10813.
2. J. C. Williams and E. A. Jr Starke, Progress in structural materials for aerospace systems, Acta Mater., 2003, 51, 5775–5799.
3. S. Pathan and S. Ahmad, Synthesis, characterization and effect of s-triazine ring on physico-mechanical and electrochemical corrosion resistance properties of waterborne castor oil alkyd, J. Mater. Chem. A, 2013, 1, 14227–14238.
4. H. Deveci, G. Ahmetli, M. Ersoz and R. Kurbanli, Modified polystyrenes: Corrosion, physicomechanical and thermal properties evaluation, Prog. Org. Coat., 2012, 73, 1–7.
5. S. S. Narine and X. Kong, Vegetable oils in production of polymers and plastics, Bailey's Industrial Oil and Fat Products, John Wiley & Sons, Inc, 2005.
6. M. Alam, D. Akram, E. Sharmin, F. Zafar and S. Ahmad, Vegetable oil based eco-friendly coating materials: A review article, Arabian J. Chem., 2013, 7, 469–479.
7. A. M. Díez-Pascual and A. L. Diez-Vicente, ZnO-reinforced Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Bionanocomposites with Antimicrobial Function for Food Packaging, ACS Appl. Mater. Interfaces, 2014, 6, 9822–9834.
8. C. Sanchez, B. Julian, P. Belleville and M. Popall, Applications of hybrid organic–inorganic nanocomposites, J. Mater. Chem., 2005, 15, 3559–3592.
9. S. Li, Q. Wang, T. Chen, Z. Zhou, Y. Wang and J. Fu, Study on cerium-doped nano-TiO₂ coatings for corrosion protection of 316 L stainless steel, Nanoscale Res. Lett., 2012, 7, 1–9.
10. B. C. Viana, O. P. Ferreira, A. G. Souza Filho, C. M. Rodrigues, S. G. Moraes, J. Mendes Filho and O. L. Alves, Decorating titanate nanotubes with CeO₂ nanoparticles, J. Phys. Chem. C, 2009, 113, 20234–20239.
11. M. J. Haas, K. M. Scott, W. N. Marmer and T. A. Foglia, In situ alkaline transesterification: an effective method for the production of fatty acid esters from vegetable oils, J. Am. Oil Chem. Soc., 2004, 81, 83–89.
12. E. Sharmin, S. Ashraf and S. Ahmad, Synthesis, characterization, antibacterial and corrosion protective properties of epoxies, epoxy-polyols and epoxy-polyurethane coatings from linseed and Pongamia glabra seed oils, Int. J. Biol. Macromol., 2007, 40, 407–422.
13. E. Malmström, M. Johansson and A. Hult, Hyperbranched aliphatic polyesters, Macromolecules, 1995, 28, 1698–1703.
14. I. E. dell’Erba, C. E. Hoppe and R. J. Williams, Synthesis and Properties of Organic–Inorganic Hybrid Materials Based on Glycerol, Ind. Eng. Chem. Res., 2012, 51, 7793–7799.
15. E. Sharmin, D. Akram, F. Zafar, S. M. Ashraf and S. Ahmad, Plant oil polyol based poly(ester urethane) metallohybrid coatings, Prog. Org. Coat., 2012, 73, 118–122.
16. S. Pathak, A. Khanna and T. Sinha, HMMM cured corrosion resistance waterborne ormosil coating for aluminum alloy, Prog. Org. Coat., 2007, 60, 211–218.
17. T.-C. Yeh, T.-C. Huang, H.-Y. Huang, Y.-P. Huang, Y.-T. Cai, S.-T. Lin, Y. Wei and J.-M. Yeh, Electrochemical investigations on anticorrosive and electrochromic properties of electroactive polyurea, Polym. Chem., 2012, 3, 2209–2216.
18. A. Ghosal, J. Shah, R. K. Kotnala and S. Ahmad, Facile green synthesis of Nickel nanostructures using natural polyol and morphology dependent dye adsorption properties, J. Mater. Chem. A, 2013, 1, 12868–12878.
19. C. Gionco, M. C. Paganini, S. Agnoli, A. E. Reeder and E. Giamello, Structural and spectroscopic characterization of CeO₂–TiO₂ mixed oxides, J. Mater. Chem. A, 2013, 1, 10918–10926.
20. Z. Wang, Y. Liu, D. J. Martin, W. Wang, J. Tang and W. Huang, CuO_x–TiO₂ junction: what is the active component for photocatalytic H₂ production?, Phys. Chem. Chem. Phys., 2013, 15, 14956–14960.
21. J. Xie, D. Jiang, M. Chen, D. Li, J. Zhu, X. Lü and C. Yan, Preparation and characterization of monodisperse Ce-doped TiO₂ microspheres with visible light photocatalytic activity, Colloids Surf., A, 2010, 372, 107–114.
22. K. Nagaveni, M. Hegde and G. Madras, Structure and Photocatalytic Activity of Ti_1−xM_xO₂ ±δ (M = W, V, Ce, Zr, Fe, and Cu) Synthesized by Solution Combustion Method, J. Phys. Chem. B, 2004, 108, 20204–20212.
23. J. Zhang, W. Peng, Z. Chen, H. Chen and L. Han, Effect of Cerium Doping in the TiO₂ Photoanode on the Electron Transport of Dye-Sensitized Solar Cells, J. Phys. Chem. C, 2012, 116, 19182–19190.
24. M. Nasir, J. Zhang, F. Chen, B. Tian and S. Bagwasi, Study of Synergistic Effect of Ce and S Co-Doping on the Enhancement of Visible Light Photocatalytic Activity of TiO₂, J. Phys. Chem. C, 2013, 117, 9520–9528.
25. S. S. Silva, R. A. S. Ferreira, L. Fu, L. D. Carlos, J. F. Mano, R. L. Reis and J. Rocha, Functional Nanostructured Chitosan–Siloxane Hybrids, J. Mater. Chem., 2005, 15, 3952–3961.
26. A. G. Kannan, N. R. Choudhary and N. Dutta, Fluoro-Silsesquioxane-Urethane Hybrid For Thin Film Applications, ACS Appl. Mater. Interfaces, 2009, 1, 336–347.
27. B. Choudhury, B. Borah and A. Choudhury, Extending photocatalytic activity of TiO₂ nanoparticles to visible region of illumination by doping of cerium, J. Photochem. Photobiol., C, 2012, 88, 257–264.
28. C. Toscano-Underwood, J. West, B. Fitt, A. Todd and M. Jedryczka, Development of phoma lesions on oilseed rape leaves inoculated with ascospores of A-group or B-group Leptosphaeria maculans (stem canker) at different temperatures and wetness durations, Plant Pathol., 2001, 50, 28–41.
29. C. Fauhl, F. Reniero and C. Guillou, ¹H NMR as a tool for the analysis of mixtures of virgin olive oil with oils of different botanical origin, Magn. Reson. Chem., 2000, 38, 436–443.
30. H. Bhunia, G. Nando, T. Chaki, A. Basak, S. Lenka and P. Nayak, Synthesis and characterization of polymers from cashewnut shell liquid (CNSL), a renewable resource II. Synthesis of polyurethanes, Eur. Polym. J., 1999, 35, 1381–1391.
31. T. Siyanbola, K. Sasidhar, B. Anjaneyulu, K. Kumar, B. Rao, R. Narayan, O. Olaofe, E. Akintayo and K. Raju, Anti-microbial and anti-corrosive poly(ester amide urethane) siloxane modified ZnO hybrid coatings from Thevetia peruviana seed oil, J. Mater. Sci., 2013, 48, 8215–8227.
32. E.-R. Kenawy, S. Worley and R. Broughton, The chemistry and applications of antimicrobial polymers: a state-of-the-art review, Biomacromolecules, 2007, 8, 1359–1384.
33. M. Trejo-Durán, A. Martínez-Richa, R. Vera-Graziano, E. Alvarado-Méndez and V. Castano, Structural and thermal characterization of hybrid materials based on TEOS and DCN, J. Appl. Polym. Sci., 2009, 111, 794–804.
34. C. Sun, J. S. Lee and M. Zhang, Magnetic nanoparticles in MR imaging and drug delivery, Adv. Drug Delivery Rev., 2008, 60, 1252–1265.
35. D. Chattopadhyay and D. C. Webster, Thermal stability and flame retardancy of polyurethanes, Prog. Polym. Sci., 2009, 34, 1068–1133.
36. L. Erdmann and K. Uhrich, Synthesis and degradation characteristics of salicylic acid-derived poly(anhydride-esters), Biomaterials, 2000, 21, 1941–1946.
37. C. Zhang, Y. Xia, R. Chen, S. Huh, P. A. Johnston and M. R. Kessler, Soy-castor oil based polyols prepared using a solvent-free and catalyst-free method and polyurethanes therefrom, Green Chem., 2013, 15, 1477–1484.
38. X. Pan, P. Sengupta and D. C. Webster, High biobased content epoxy–anhydride thermosets from epoxidized sucrose esters of fatty acids, Biomacromolecules, 2011, 12, 2416–2428.
39. Q. Liu, D. Chen and Z. Kang, One-Step Electrodeposition Process To Fabricate Corrosion-Resistant Superhydrophobic Surface on Magnesium Alloy, ACS Appl. Mater. Interfaces, 2015, 7, 1859–1867.
40. X. Shi, T. A. Nguyen, Z. Suo, Y. Liu and R. Avci, Effect of nanoparticles on the anticorrosion and mechanical properties of epoxy coating, Surf. Coat. Technol., 2009, 204, 237–245.
41. T.-C. Huang, Y.-A. Su, T.-C. Yeh, H.-Y. Huang, C.-P. Wu, K.-Y. Huang, Y.-C. Chou, J.-M. Yeh and Y. Wei, Advanced anticorrosive coatings prepared from electroactive epoxy–SiO₂ hybrid nanocomposite materials, Electrochim. Acta, 2011, 56, 6142–6149.
42. D. Prasai, J. C. Tuberquia, R. R. Harl, G. K. Jennings and K. I. Bolotin, Graphene: corrosion-inhibiting coating, ACS Nano, 2012, 6, 1102–1108.
43. C.-J. Weng, J.-Y. Huang, K.-Y. Huang, Y.-S. Jhuo, M.-H. Tsai and J.-M. Yeh, Advanced anticorrosive coatings prepared from electroactive polyimide–TiO₂ hybrid nanocomposite materials, Electrochim. Acta, 2010, 55, 8430–8438.
44. O. ur Rahman and S. Ahmad, Physico-mechanical and electrochemical behavior of soy alkyd/Fe₃O₄ nanocomposite coatings, RSC Adv., 2014, 4, 14936–14947.
45. M. R. Shaik, M. Alam and N. M. Alandis, Development of Castor Oil Based Poly(urethane-esteramide)/TiO₂ Nanocomposites as Anticorrosive and Antimicrobial Coatings, J. Nanomater., 2015, 2015, 1–10.
46. M. Arun, S. Kantheti, R. G. Ranganathan, R. Narayan and K. V. S. N. Raju, Surface modification of TiO₂ nanoparticles with 1,3,5-triazine based silane coupling agent and its cumulative effect on the properties of polyurethane composite coating, J. Polym. Res., 2014, 21, 600–609.

---

Footnote

† Electronic supplementary information (ESI) available: Fatty acid extraction procedure, equivalent electrical circuit, EDX spectra of nanoparticles, TEM micrographs of PEUTES and nanocomposites, ¹³C-NMR spectra of PEUTES, optical micrographs for cross hatch test and Nyquist plot (ESI Fig. 1–6) and ESI Table 1 physico-mechanical of PEUTES, PEUTES–TiO₂, PEUTES–Ce-TiO₂-I and PEUTES–Ce-TiO₂-II. See DOI: 10.1039/c5ra23928a

---