Hyperbranched Soya Alkyd Nanocomposite: A Sustainable Feedstock

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9725-9734

Hyperbranched Soya Alkyd Nanocomposite: A Sustainable Feedstock-Based Anticorrosive Nanocomposite Coatings Obaid ur Rahman,†,‡,# Shahidul Islam Bhat,† Haibin Yu,‡ and Sharif Ahmad*,† †

Material Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, 110025, India Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Division of surface engineering, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, P. R. China

‡

S Supporting Information *

ABSTRACT: Globular structured oleo alkyds possess low viscosity, good fluidity, and play an important role in the generation of volatile organic compound (VOC) free paints and coatings. Soya oil (SO), an abundant, inexpensive, renewable, and sustainable material is one of the examples of such oleo alkyd precursors that meets the requirement of green chemistry. The present work reports the synthesis of hyperbranched soya alkyd based nanocomposite coatings and their corrosion inhibition efficiency. Hyperbranched alkyd (HBA) was synthesized using SO, pentaerythritol, and phthalic anhydride. The magnetite (Fe3O4) nanoparticles were dispersed via sonication in butylated melamine formaldehyde (BMF) modified HBA (HBA−BMF) to formulate the nanocomposite (HBA−BMF− Fe3O4) anticorrosive coatings. The ASTM methods were used to evaluate structural, morphological, physicomechanical, thermal, electrochemical, and anticorrosive properties of these coatings. The HBA has a globular structure with the good degree of branching (DOB = 0.69). HBA−BMF and HBA−BMF−Fe3O4 nanocomposite coatings showed good flexibility and physicomechanical properties. The inclusion of Fe3O4 nanoparticles enhanced the load bearing capacity of nanocomposite coatings by dissipating the instantaneous energy in scratch and impact tests. Electrochemical corrosion studies revealed that the HBA−BMF−Fe3O4 nanocomposite coatings exhibit superior corrosion resistance performance (impedance = 107 Ω and corrosion rate 1.0 × 10−4 mils per year (mpy) than that of HBA−BMF and other similar reported coating systems. KEYWORDS: Hyperbranched Alkyd, Nanocomposite, Corrosion, EIS

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INTRODUCTION Corrosion is a serious everlasting industrial problem and poses significant economic threats to the society and infrastructure. Owing to the unwanted corrosion of goods, items, and machinery, industries lose billions of dollars every year in the form of deterioration and maintenance.1 In fact, corrosion is a nightmare which creates a huge burden on the economy of a country as a large amount of the gross domestic product (GDP) is being spent by both the developing (3−5%) and under-developed countries (5%) to circumvent this culprit.1,2 This, along with other factors, prompted researchers to study and understand the actual mechanism and develop new methods of corrosion protection to enhance service life of metallic devices, machines, and infrastructures, etc.2 Several methods and techniques were used to combat corrosion such as cleansing of the environment, alloying, compositing 2,3 corrosion inhibitors,4,5 and various types of metallic, inorganic, and polymeric coatings.6−8 Among these, polymeric coatings emerged as a smarter way to protect underlying materials from corrosion by providing strong adhesion and good anticorrosive performances.9 Epoxy © 2017 American Chemical Society

coatings have been commonly used to protect the underlying metallic substrate. Brostow et al. used commercial epoxy on mild steel and studied their curing and cross-linking behavior in view of their corrosion inhibition efficiency.10 Generally, epoxies and other polymeric coating materials are based on nonrenewable precursors from petrochemical sources containing VOC, and thus against the principles and regulations of green chemistry. Green chemistry principals urge extensive efforts to develop materials from biodegradable renewable resources to replace toxic petrobased materials.3,4 Nowadays, different types of biobased materials are being used for this purpose such as chitosan and chitin,5 furans,6 polysaccharides,7 and vegetable oil (VO).8−11 VO-based alkyds are of low cost, abundant, and show an ease of application on various substrates,11,12 with excellent transparency and hard film forming ability.12 However, literature reveals that coatings based on alkyds failed to provide satisfactory performance in Received: May 15, 2017 Revised: September 6, 2017 Published: September 19, 2017 9725

DOI: 10.1021/acssuschemeng.7b01513 ACS Sustainable Chem. Eng. 2017, 5, 9725−9734

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthesis of (a) Monoglyceride, (b) Hyperbranched Alkyd (HBA), (c) HBA−BMF and in Inset 3D View of Hyperbranched HBA

stringent and extreme corrosive environments.12,13 To surpass this obstacle and enhance the anticorrosive properties of alkyd coatings, different types of modifications have been reported. For example, nanocomposite synthesis by dispersing nanofillers such as ZrO2,14 TiO2,15 ZnO,16 and Fe2O3 nanoparticles,17 and conducting polymers, etc.18 It has been observed that the nanocomposites exhibit good mechanical, thermal, ultraviolet (UV) protection and anticorrosive properties and thus offer a promising, smart, and economical way to develop new generation anticorrosive materials.19 Particularly, Fe3O4 nanoparticles have emerged as a potential candidate to be used as anticorrosive nanofillers due to their unique electrical and magnetic properties.20 Electron transfer between ferrous (Fe2+) to ferric (Fe3+)21 on octahedral sites enhances the interaction between nanocomposite coatings and the substrate surface resulting in well-adhered nanocomposite coatings with superior physicomechanical and anticorrosive properties.11,22 During the literature survey, we found that hyperbranched polymers induced more superior properties in the coating

materials.22−26 The hyperbranched alkyds have interesting physicochemical properties and similar molecular weight to the linear counterpart. The hyperbranched polymer with low viscosity and a large number of functional groups enables good solubility, compatibility, and enhances adhesion to the metal substrate which facilitates better anticorrosive performance.22,24 As we know nanoparticles inclusion improves thermal, mechanical, UV, and corrosion protection of alkyds. Hence, in view of this, the present work describes the formulation, characterization, and anticorrosive performance of BMF-cured HBA and Fe3O4 nanoparticles dispersed HBA− BMF−Fe3O4 nanocomposite coatings. The results of physicomechanical, electrochemical corrosion, and salt mist studies depicted that the HBA−BMF−Fe3O4 nanocomposite coatings possess superior properties than other oleo-polymer coating systems. 9726


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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis of HBA−BMF−Fe3O4 Nanocomposite

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infrared (FT-IR) analysis (Scheme 1b).13 The HBA was cross-linked in an acid catalyzed (p-TSA) environment by the butylated melamine formaldehyde (in 1:0.5 weight ratio) at 120 °C for 2 h to formulate the HBA−BMF coating system (Scheme 1c). Preparation of HBA−BMF−Fe3O4 Nanocomposite Coatings. The HBA−BMF−Fe3O4 nanocomposite was prepared by the dispersion of 0.5, 1.5, and 2.5 wt % of the Fe3O4 nanoparticles in 10.0 g of HBA−BMF resin to formulate HBA−BMF−Fe3O4-0.5, HBA−BMF−Fe3O4-1.5, and HBA−BMF−Fe3O4-2.5 nanocomposites, respectively, via sonication for 30 min. (using ultrasonic wave sonicator with the frequency 40 kHz) at 30 °C.11,28 Nanocomposite coating materials were obtained as a homogeneous ferrite suspended colloidal solution as per Scheme 2. HBA−BMF and nanocomposite coating materials were applied to commercially available carbon steel (CS) for physicomechanical as well as corrosion analysis (PDP, EIS, and salt mist test) as reported earlier.11,15

MATERIALS AND METHODS

Materials. Butylated melamine formaldehyde (BMF) and soya oil (SO) [specific gravity, 1.10; mol wt, 2603], were procured from Shankar dyes and chemicals (Delhi, India). Ethanol (C2H5OH) and ferrous sulfate heptahydrate (FeSO4·7H2O) were obtained from Merck (India). Ethylene glycol [HOC2H4OH, mol wt 62.07 g mol−1; density, 1.11gcm−3] and ammonium hydroxide (NH4OH) were obtained from SD Fine Chemicals Pvt. Ltd. (Mumbai, India). Hydrogen peroxide (H2O2 30 wt %) was procured from Fisher Scientific (Mumbai, India). Pentaerythritol [C5H12O4, mol wt 136.15 g mol−1] and phthalic anhydride (C6H4(CO)2O, mol wt, 148.1 g mol−1) were purchased from Sigma-Aldrich, Germany. p-Toluenesulfonic acid (p-TSA) was obtained from Loba Chemie Pvt Ltd., and all the chemicals were used as such without further purification. Preparation of Fe3O4 Nanoparticles, Hyperbranched Alkyd (HBA), and HBA−BMF−Fe3O4 Nanocomposite. Fe3O4 nanoparticles were synthesized via the polyol method using ethylene glycol as solvent as well as a stabilizer as reported earlier.20 Ferrous sulfate heptahydrate (3.00 g, 0.1008 mol) was used as a precursor in 50 mL of ethylene glycol and stirred for 30 min to prepare a homogeneous solution. A 20 mL aliquot of 0.5% aqueous solution of 30% H2O2 was added dropwise, and the pH of the reaction mixture was maintained around 13.0 by mixing 25% aqueous ammonia solution. The reaction proceeded at 50 °C for 4 h in a 250 mL three-necked flat-bottomed flask under nitrogen atmosphere. Synthesis of Hyperbranched Alkyd HBA−BMF Resin. As shown in Scheme 1a, monoglycerides were synthesized by transesterification reaction of soy oil with pentaerythritol in the presence of catalyst sodium hydroxide (in 1:2:0.01 molar ratios, respectively) at 240 °C for 3 h in a three-necked round-bottomed flask under continuous stirring on magnetic stirrer in a silicon oil bath in nitrogen atmosphere. The monoglyceride formation was monitored by the solubility in methanol (by mixing one part of the reaction product in three parts of methanol) and recording the Fourier transfer infrared (FT-IR) spectra at regular intervals of time.12,27 Monoglyceride and phthalic anhydride were reacted (in 1:0.5 wt % ratio) to synthesize the hyperbranched alkyd at 190 °C with continuous stirring for 2 h, and the reaction was stopped on achieving the anticipated acid value (30 mg/KOH). The reaction was further confirmed by Fourier transform

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RESULTS AND DISCUSSION The synthesis of SO-based hyperbranched alkyd was achieved by the A2+B3 popular approach in which PA was used as an A2 type of monomer, while the monoglyceride was used as B3. SO was reacted with the pentaerythritol at 240 °C in the presence of the sodium hydroxide to produce monoglyceride (B3 type monomer) through a transesterification mechanism. PA was reacted at 190 °C with monoglyceride to form HBA (Scheme S1).26,27 Further, BMF was used to cross-link HBA to obtain a highly cross-linked network of HBA−BMF via the condensation reaction,11 which further increases the DOB as shown in Scheme 1c. Later, Fe3O4 was dispersed in the highly crosslinked HBA−BMF matrix where nanoparticles occupied the interstitial sites and voids and thus resulted in the formation of the HBA−BMF−Fe3O4 nanocomposite (Scheme 2).28,29 FT-IR spectroscopy of HBA, HBA−BMF, and HBA−BMF− Fe3O4 was performed to confirm and identify the functional groups present in the final product as shown in Figure 1. Figure 1a shows the FT-IR spectra of HBA and the stretching vibration band of the hydroxyl group (−OH) observed at 9727


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chain next to the methylene group of ester linkages shows a peak at δ = 1.7 ppm. Figure 2b shows the 1H NMR of HBA− BMF which shows the peak at δ = 3.2−3.8 ppm, which corresponds to a proton attached to an ether group (−OCH2),30 and other peaks are similar as in the case of HBA. The presence of the prominent peak of the proton of −OCH2 confirmed the formation of HBA−BMF resin by an etherification reaction of HBA with BMF. In the 13C NMR spectrum of HBA, the carbonyl carbon atom of the ester group present in the polymeric skeleton shows peaks at δ = 170 ppm (Figure S2). Carbon present in the aromatic ring of phthalic anhydride shows a peak at δ = 125−135 ppm.13,32 Moreover, carbon attached to the oxygen atom directly shows a peak in the region of δ = 30−70 ppm. The peaks at δ = 0−35 ppm correspond to methyl and methylene carbon. The unsaturated carbon atom of the fatty acid alkyl chain in the HBA show a peak at δ = 127 ppm. The 13C NMR spectra of HBA−BMF show similar peaks as HBA with two additional peaks around δ = 180 ppm and δ = 90 ppm attributed to the carbon atoms of the symmetrical s-triazine ring and carbon atom sandwiched between the oxygen of the ether group and the nitrogen of the BMF (−OCH2−N), respectively as shown in Figure S3, providing evidence for the reaction of BMF with HBA to form the cross-linked HBA−BMF matrix.11 DOB was also determined with the help of the below given equation:22,26

Figure 1. FT-IR spectra of HBA, HBA−BMF and HBA−BMF−Fe3O4 nanocomposite.

around 3389 cm−1, while ester linkages (−COO) show the prominent peak at 1725 cm−1. Peaks associated with the −C− H asymmetric and symmetric stretching vibrations of the −CH3 and −CH2 group of the fatty acid can be seen at 2915 and 2857 cm−1, respectively. The FT-IR spectrum of the HBA−BMF is shown in Figure 1b; the presence of a band at 1553 cm−1 is due to the plane stretching vibration of the striazine ring which confirmed the introduction of BMF in HBA.30 Further, the reaction between BMF and HBA was also confirmed by the additional peaks present in the range 1125− 1072 cm−1 due to different modes of vibration of the ether linkages (−C−O−C−).13 The peak at 1464 cm−1 can be correlated to the asymmetric −C−H deformations of methyl and methylene groups. The FTIR spectrum of the HBA− BMF−Fe3O4 shown in Figure 1c shows similar peaks with lower intensity as in case of HBA−BMF, an additional prominent peak at 576.30 cm−1 confirms the addition of nano-Fe3O4 in HBA−BMF. Intensity lowering of the functional groups of HBA−BMF in the FT-IR of nanocomposite may be due to the physical interaction of Fe3O4 nanoparticles, which are present within the skeleton of the matrix of the nanocomposite at interstitial and voids position.17 Figure 2 shows the protons nuclear magnetic resonance (1H NMR) of HBA and HBA−BMF. In Figure 2a, the fatty acid terminal methyl group shows the peak around δ = 0.9 ppm.31 The proton of the −CH2− group of the fatty acid backbone shows the peak between δ = 1.20−1.59 ppm.30 The peak at δ = 5.4 ppm is attributed to the protons attached to the olefinic carbon atom (−CHCH−) of the fatty acid;28 this peak provides the evidence of the presence of the fatty acid chain in the HBA resin. The peak in the range δ = 7.3−7.5 ppm is associated with the aromatic protons attached to the benzene ring of the phthalic anhydride of HBA. The peak δ = 4.2 ppm associated with the proton attached to the ester group (−CH2− O−CO) confirmed the formation of the HBA by the reaction between PA and monoglyceride. The unreacted hydroxyl proton of pentaerythritol shows the peak at δ = 4.4 ppm. The sandwiched methylene group in between two olefinic carbon atoms of the fatty acid chain show the peak at δ = 2.8 ppm. The proton of the methylene group next to ester linkages shows a peak at δ = 2.3 ppm and the proton of the fatty acid

DOB =

(no. of dendritic units) + (no. of terminal units) = 0.69 total no. of units (1)

The value of DOB confirms the hyper branching of the alkyd resin with DOB values equal to 0.69 and is considered to be polydisperse in nature,33 which leads to the introduction of branching with in the backbone of the alkyd resin resulting in the formation of HBA resin (Scheme S1).34 Figure 3 shows the morphological study HBA−BMF and HBA−BMF−Fe3O4 nanocomposite. Fe3O4 dispersion and its polydisperse nature were studied by the transmission electron microscopy (TEM), and it was found that Fe3O4 nanoparticles were uniformly dispersed in the HBA−BMF matrix (Figure 3a). The black circle represents uniform dispersion of Fe3O4 nanoparticles (20−50 nm) in HBA−BMF matrix (gray in color),11 and no agglomeration of Fe3O4 nanoparticles was observed.20 Figure 3 panels b and c show the SEM micrograph of HBA−BMF and HBA−BMF- Fe3O4 nanocomposite coatings, respectively. From the scanning electron microscopy (SEM) analysis it was clear that there is no phase separation and both the coatings well-adhered to the metal substrate. Figure 4 shows the thermogravimetric analysis (TGA) thermograms of HBA, HBA−BMF, and HBA−BMF−Fe3O4 nanocomposite coatings, and derivative thermogravimetric (DTG) thermograms are given in the Supporting Information as Figure S6. The trapped solvent molecules were evaporated in the temperature range 180−200 °C, which caused 5 wt % loss. In the first thermal degradation step, 30 wt % weight loss occurs due to de-cross-linking of HBA−BMF around 250−350 °C. HBA−BMF shows a second thermal degradation step around 450 °C with a weight loss of 80 wt %. In this thermal decomposition process, functional groups such as ester, ether, and melamine ring decompose showing a major weight loss.13 After Fe3O4 nanoparticle dispersion, there is an increase in thermal stability and the HBA−BMF−Fe3O4 nanocomposite shows the second weight loss around 490 °C.11 In the case of 9728


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Figure 2. (a) 1H NMR of HBA; (b) 1H NMR of HBA−BMF.

Figure 3. TEM micrograph of (a) HBA−BMF−Fe3O4 nanocomposite, SEM micrograph of (b) HBA−BMF and (c) HBA−BMF−Fe3O4 nanocomposite coatings.

reaches 400 °C HBA−BMF−Fe3O4 shows higher thermal stability than HBA and HBA−BMF. Further, at 750 °C 90% weight loss was observed in the case of the HBA−BMF coatings, while higher residue remained for HBA−BMF− Fe3O4, confirming the incorporation of thermally stable Fe3O4 nanoparticles in the HBA−BMF matrix.36 The improved thermal stability of HBA−BMF−Fe3O4 can also be attributed to the electrostatic interaction between the s-triazine ring of the

HBA−BMF−Fe3O4 coatings, initially there is lowering in thermal stability up to 366 °C due to the to the chain scission of HBA−BMF as reported by Ong et al.35 This behavior may be due to the fact that polymers have a much higher linear coefficient of thermal expansion than the metal oxides, therefore most of the heat absorbed by the HBA−BMF is used in increasing interatomic distance rather than increasing the temperature of the HBA−BMF. After the temperature 9729


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Fe3O4 nanoparticles dispersion being responsible for the enhancement.41 The presence the Fe3O4 nanoparticles improve the impact resistance of the HBA−BMF−Fe3O4 coatings providing a crack healing property by restricting the chain mobility of the nanocomposite coatings.42 Further, the adhesion of these coatings was tested by cross-hatch adhesion test, and it was found that after the test no square peeled out with the adhesion tap revealing good adhesion of coatings with the metal substrate. All the coatings passed the bend test revealing that the coatings were flexible, as no damage or fracture was seen, Figure 8b, due to flexible fatty acid chain acting as an internal plasticizer.43,44 The contact angles (CA) of these coatings were also measured to check the hydrophobic behavior of these coatings. Coatings with CA > 90° are considered to have a hydrophobic surface which repels the corrosive ion to penetrate into the coatings. The CA for HBA− BMF was found to be 95° (Figure S5 e), and it is expected to increase after the Fe3O4 nanoparticles dispersion as reported earlier.11,15 The potentiodynamic polarization (PDP) study for uncoated, HBA−BMF and HBA−BMF−Fe3O4 coated CS in 3.5 wt % NaCl at room temperature was performed as shown in Figure 6 and tabulated in Table 1. Tafel parameters such as corrosion potential (Ecorr), corrosion current density (Icorr) corrosion rate and polarization resistance (Rp) of coated and uncoated specimens were obtained by Tafel extrapolation method using the software Nova 1.8 (Table 1). The promising anticorrosive coatings generally exhibit lower Icorr and higher Ecorr values leading to the fairly low corrosion rates.33 The HBA−BMF−Fe3O4 coatings showed higher Ecorr values and lower Icorr values with respect to HBA−BMF coatings Ecorr (−0.811 V) and Icorr (2.67× 10−7 Acm−2) values, while HBA− BMF has higher values as compared to the uncoated CS (Ecorr = −0.853 V and Icorr = 9.18× 10−5 Acm−2). The decrease in Icorr values of HBA−BMF−Fe3O4 coatings can be correlated to the presence of Fe3O4 nanoparticles which acts as a protective barrier layer which reduces corrosive ion interaction with the metal substrate.34 The corrosion inhibition resistance performance of coated sample has been resulted due to the formation of the well adhered film to the CS substrate. The different polar functional group such as ether and ester linkages present in the HBA−BMF facilitate the good adhesion as discussed above in the SHT and cross-hatch adhesion test.45,46 Well adhered oil based hydrophobic coatings further enhance the anticorrosive behavior by preventing the corrosive ions penetration into the coatings.11 Fe3O4 nanoparticles dispersed nanocomposite coatings form a protective barrier layer which enhances the anticorrosive performance by obviating the penetration of the corrosive ions at the coatings−metal interface. Further, nanocomposite coatings show superior anticorrosive behavior as evident from the Tafel parameters, showing higher Ecorr and Rp values, and lower Icorr and corrosion rate values (Table 1). Superior

Figure 4. TGA thermograms of HBA, HBA−BMF, and HBA−BMF− Fe3O4 nanocomposite.

polymer matrix and Fe3O4 nanoparticles, resulting in well dispersed uniform stable nanocomposite coatings.16,37 The glass transition temperature (Tg) for HBA, HBA−BMF, and HBA−BMF−Fe3O4 were in the range of 90−100 °C, 120− 130 °C, 140−150 °C, respectively (Figure S4). The higher Tg value of HBA−BMF as compared to HBA is due to the crosslinked s-triazine ring which restricted the mobility of the polymer. The HBA−BMF−Fe3O4 shows the highest Tg value because of the uniform dispersion of polar Fe3O4 nanoparticles at the interstitial and void positions of the HBA−BMF crosslinked matrix which restricts its mobility and consequently resulted in high Tg values.11,15,38 Physico-mechanical behavior of HBA−BMF and HBA− BMF−Fe3O4 nanocomposite coatings were studied and tabulated in Table S1. With the loading of Fe3O4 nanoparticles in HBA−BMF, there is a decrease in drying time behavior (Table S1). HBA−BMF show a refractive index (RI) value of 1.48, which is well in accordance to the other reported hyperbranched polymer.39 On inclusion of the Fe3O4 nanoparticles, a slight decrease in RI and gloss values were observed because of the dark color and opaque nature of the Fe3O4 nanoparticles.11 On the other hand, the scratch hardness test (SHT) values increase from HBA−BMF to HBA−BMF−Fe3O4-0.5 and show the highest value for HBA−BMF−Fe3O4-2.5 (Table S1). The Fe3O4 nanoparticles fill the voids and provide electrostatic interaction with the matrix which resulted in high scratch hardness values for the nanocomposite coatings with satisfactory performance.40 From the optical images of the scratched surface of the coatings (Figure 5) it was found that the coatings show strong adhesion and good plastic deformation which bear the weight stress during the SHT, the large number of functional groups in HBA and uniform

Figure 5. Optical micrographs after the scratch test (a) HBA−BMF coatings, (b) HBA−BMF−Fe3O4-0.5, and (c) HBA−BMF−Fe3O4-2.5. 9730


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Figure 6. PDP curves in saline of coated and uncoated CS.

Table 1. Electrochemical Parameters Obtained from PDP Studies for Uncoated, HBA−BMF and HBA−BMF−Fe3O4 Coated CS in 3.5 wt % NaCl at Room Temperature sample

Ecorr (V)

CS HBA−BMF HBA−BMF−Fe3O4-0.5 HBA−BMF−Fe3O4-1.5 HBA−BMF−Fe3O4-2.5

−0.853 −0.811 −0.781 −0.752 −0.724

Icorr (A cm−2) 9.18× 2.67× 5.33× 3.50× 8.88×

10−5 10−7 10−7 10−8 10−8

Rp (Ω)

corrosion (mpy)

472 276230 324340 440080 528350

1.0671 0.0031 0.0027 0.0011 0.0001

found many fold (2−3) higher (Figure 7b−d) than those of virgin HBA−BMF (105 Ω) coatings (Figure 7a). The nanocomposite coatings show stable corrosion protection with the single time constant for a longer time period (7 days) providing evidence that there is no corrosion reaction starting, and the underlying material is well protected. From the EIS result, it was revealed that the HBA−BMF− Fe3O4 nanocomposites coatings provide superior corrosion inhibition as compared to the HBA−BMF coatings, due to the presence of Fe3O4 nanoparticles in the HBA−BMF matrix. Fe3O4 nanoparticles act as a strong barrier and provide a torturous path to the corrosive ions which cause a delay in the corrosion initiation and resulted in the high impedance and Rp values of nanocomposite coatings. The HBA−BMF−Fe3O4-2.5 shows the best anticorrosive performance with no sign of corrosive ion diffusion into the coating material during the exposure of aggressive corrosive environment (3.5 wt % NaCl solution), which is clearly visible by the higher impedance value (≈107 Ω) at lowest frequency, and results were in accordance with the polarization studies.

corrosion protection of HBA−BMF−Fe3O4 coatings are governed by the uniform dispersion of Fe3O4 nanoparticles which provide a torturous path and barrier to the corrosive ions by filling coating artifacts at the coating−metal interface. Electrochemical impedance spectroscopy (EIS) was applied to study protection behavior and the influence of Fe3O4 nanoparticles loading in nanocomposite coatings. The impedance of the coated samples directly represent the corrosion protection behavior, and in the Nyquist plots, impedance is directly proportional to the capacitive loop diameter of charge transfer resistance (Rct) at high frequency. Coatings with the larger diameter will show higher impedance hence better corrosion protection. The HBA−BMF−Fe3O4 nanocomposite coating provides better corrosion protection ability, as is clearly visible from the diameter of the nanocomposite coatings in the Nyquist plot in comparison to HBA−BMF coatings as shown in Figure S5. High impedance values at the lowest frequency (|Z|0.01 Hz) is characteristics of good polymeric coating which protects the metal substrate in the stringent corrosive environment.15 In the case of the nanocomposite coatings, impedance values were 9731


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Figure 7. Bode plot of (a) HBA−BMF, (b) HBA−BMF−Fe3O4-0.5, (c) HBA−BMF−Fe3O4-1.5 and (d) HBA−BMF−Fe3O4-2.5.

that among various oil-based polymer coatings, hyperbranched nanocomposite coatings with 2.5 wt % Fe3O4 nanoparticles loading have superior corrosion protective performance, with high potential in the field of corrosion protection, and have good potential for commercialization. From the physicomechanical, PDP, EIS, and salt mist studies it is clear that the HBA−BMF−Fe3O4 nanocomposite provides good corrosion protection to CS substrate beneath. The corrosion protection to the substrate is governed by a large number of the polar groups present in the hyperbranched polymer, which enhance the adhesion by strong electrostatic interaction to form the uniform anticorrosive coating. The HBA−BMF acts as a hydrophobic surface which inhibits the penetration of corrosive ions into the coating and protects the substrate. Moreover, the Fe3O4 nanoparticles fill the voids and cavities of the HBA−BMF matrix and enhance the corrosion protection by acting as a strong barrier, which provides a torturous path to a corrosive ion and delays the corrosion initiation at the coating−metal interface. Hence, the HBA− BMF−Fe3O4 nanocomposite coatings show good corrosionprotection efficiency and protect the underlying CS substrate for the long periods in, for example, a sewer corrosive environment.

To further confirm the corrosion inhibition performance, HBA−BMF and HBA−BMF−Fe3O4 nanocomposite-coated samples were exposed to the salt mist test in the 5.0 wt % NaCl solution under 90% humid environment for 240 h. The HBA− BMF coated samples show a sign of corrosion (Figure 8a)

Figure 8. SEM micrograph after salt mist exposure: (a) HBA−BMF and (b) HBA−BMF−Fe3O4.

which may be due to penetration of corrosive ions. However, no sign of corrosion initiation was observed on the nanocomposite-coated surface except for a slight deposition of salt ions (Figure 8b). The salt mist test also shows similar corrosion protection as discussed above due to the uniformly dispersed Fe3O4 nanoparticles which protect the underlying metal substrate from the corrosive ions attack. The literature revealed 9732


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CONCLUDING REMARKS The HBA and HBA−BMF matrix was developed via green route using soya oil monoglyceride as the precursor. The hyperbranched structure of HBA was confirmed by 0.69 degrees of branching value, and was cross-linked with BMF. The synthesized polymer matrix was further modified by the dispersion of various percentages of Fe3O4 nanoparticles resulting in the formation of nanocomposites. The nanocomposite coatings were tough, mechanically robust, and yielded outstanding results as corrosion protective coatings. The results of these studies are well in agreement with the values derived from corrosion studies such as PDP, EIS, and salt spray test. The comparison of the present system with earlier reported systems reveals that the present system has potential applications as a low cost, eco-friendly, and VOC-free coatings system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01513. Characterization section, chemical structure of HBA, electrical circuit used in EIS, 13C NMR of HBA and HBA−BMF, Nyquist plot, DSC and DTG thermograms of HBA, HBA−BMF, and HBA−BMF−Fe3O4 nanocomposite coatings and table of physico-mechanical properties of HBA−BMF and HBA−BMF- Fe 3 O 4 coatings (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 91 11 26827508. Fax: +91 11 26840229. ORCID

Obaid ur Rahman: 0000-0001-8085-9410 Sharif Ahmad: 0000-0001-5799-7348 Present Address #

O.U.R.: Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Surface Department, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P. R. China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS O.u.R. thanks the Chinese Academy of Science (CAS) for financial support in the form of CAS-PIFI postdoctoral research fellowship (UCAS: 20161963, dated 4th January 2016) and S.I.B. thanks the University Grants Commission (UGC), New Delhi, India, for financial support.

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REFERENCES

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Hyperbranched Soya Alkyd Nanocomposite: A Sustainable Feedstock

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