Enhancement of Anticorrosive Performance of Renewable Cardanol Based Polyurethane Coatings by Incorporating Magnetic Hydroxyapatite Nanoparticles


 The present investigation demonstrates renewable cardanol based polyol for the formulation of nanocomposite polyurethane (PU) coatings. The functional and structural features of cardanol polyol and nanoparticles were studied by FT-IR and 1H NMR spectroscopic techniques. The magnetic hydroxyapatite nanoparticles (MHAP) were dispersed in PU formulations to develop nanocomposite anticorrosive coatings. The amount of MHAP in PU formulations was varied from 1-5%, increase the percentage of MHAP increases the anticorrosive performance as examined by immersion and electrochemical methods. The nanocomposite PU coatings shows good coating properties viz., gloss, pencil hardness, flexibility, cross-cut adhesion and chemical resistance. Additionally, the coatings also studied for surface morphology, wetting, and thermal properties by scanning electron microscope (SEM), contact angle, and thermogravimetric analysis (TGA), respectively. The hydrophobic nature of PU coatings increased by addition of MHAP and optimum result (1050) was observed in 3% loading. The developed coatings revealed hydrophobic nature with excellent anticorrosive performance.


Introduction
Corrosion is a process in which metal articles get damaged that can be from automobile, construction, aerospace, marines, and industrial sectors. Thus, metal corrosion has appeared as a big problem and results to inured massive losses to economy and human health. [1][2][3] Anticorrosive paints and coatings based on synthetic polymeric resins are used to minimize such types of losses and to improve life span of various metal items. However, maximum polymeric resins used in the present paints and coatings are of petroleum origin. Depletion of petroleum sources and uctuations in their prices made it necessary to nd out some new alternatives such as renewable, readily available, and eco-friendly polymeric materials.
Renewable materials including carbohydrates, lipids, vegetable oils, cardanol, eugenol, etc. have been explored to use in the preparation of monomers and polymers such as alkyd, epoxy, polyurethane, polyesters, polyacrylates, phenol formaldehyde resin, benzoxazines, etc. [4][5][6][7] The resultant renewable source based polymers have found applications in the different elds such as coatings, composites, microencapsulation, reactive diluents, foams, and so on. Among all the renewable materials, cardanol have fascinated the researchers due to its unique chemical structure and possible chemical modi cations through availability of reactive sites such as phenolic hydroxyl group, aromatic ring, and long aliphatic carbon chain with unsaturation. Aromatic ring provides chemical and thermal resistances, while hydroxyl group gives good adhesion. A long aliphatic carbon chain provides good exibility, excellent water resistance, and anticorrosive properties.
Over a past decades, nanocomposite polymer coatings have been used to improve the corrosion, thermal, and mechanical properties of the coating formulations. They are developed from the nano-size llers and polymeric resins. The polymeric resin helps to hold the nano-ller in the matrix that works as a reinforcing material or improves physico-mechanical properties of the composites. 8, 9 Various types of nano-size llers such as SiO 2 , 10 ZnO, 11 TiO 2 , 12 Fe 2 O 3 , 13 ZrO 2 , 14 Al 2 O 3 , 15 V 2 O 5 , 16 graphene, 17 and others. 18 have been used in coating matrix for desired applications like improvement of adhesion, gloss, corrosion, scratch, thermal and antibacterial resistances. The main purpose for nanoparticle incorporation in the coating formulation is to enhance the physico-chemical as well as corrosion resistance properties of metal samples. Additionally, use of corrosion inhibitor is also possible based on precursors used in the formulation of coatings, thickness, and adhesion towards the metal surface. 19 The protective organic/inorganic hybrid composite coatings are prepared by addition of magnetic hydroxyapatite nanoparticle (MHAP) as a reinforcing agent. Presence of MHAP has enhanced the coating properties such as chemical, wetting, and corrosion resistances.Previous reports are available on use of hydroxyapatite (HAP) for bone tissue engineering, controlled drug delivery, and as a ller for composites coatings. Even HAP modi ed with silver was used in the formulation of antibacterial composite coatings. 20 Additionally, magnetically modi ed HAP was utilized in the removal of heavy metals such as uranium (VI), 21 lead ion, 22 cadmium (II), 23 copper, nickel, 24 and other pollutants for the water. 25 As per the literature, till today MHAP has not been used in the designing of anticorrosive coatings for the metal protection.
In the present experimental work, cardanol based Mannich polyol have been synthesized to obtain PU of renewable source based. Structural features of the prepared resin were con rmed by the end group analysis as well as by spectroscopic methods. Simultaneously, the magnetic hydroxyapatite nanoparticles were prepared and modi ed for their magnetic properties in order to improve anticorrosion properties of the PU coatings on mixing with polyol and further treatment with hexamethylene diisocyanate. The prepared PU composite coatings were tested for their physico-chemical and anticorrosive properties.

Materials
Cardanol was provided as a gift sample by Polymer Division of Atul Ltd., India. Hexamethylene diisocyanate (HDI), diethanolamine, and dibutyltin dilaurate (DBTDL) were purchased from Sigma Aldrich, India. Ferrous chloride tetrahydrate, ferric chloride, potassium hydroxide, calcium nitrate tetrahydrate, diammonium hydrogen phosphate, and xylene, were purchased from Loba Chemie, India. All the chemical used as such without any puri cation.

Synthesis of cardanol based Mannich polyol
Synthesis of cardanol Mannich polyol (CMP) was based on our previously reported method. 26 In the rst step, the formation of oxazolidine was done on reacting diethanolamine (0.2 M) with formaldehyde (0.2 M) in three-necked ask equipped with a magnetic stirrer, condenser, and thermometer. The reaction mixture was heated at 65 0 C for 2 h followed by distilling water off reaction to form oxazolidine intermediate. Thereafter, cardanol (0.066 M) was added dropwise in the reaction mixture for 30 min and the reaction was maintained to 95 0 C for 4 h. The progress of the reaction was checked by conducting thin layer chromatography (TLC) in ethyl acetate: hexane (20:80) system. Finally, the deep reddish coloured liquid of cardanol polyol was formed. The reaction for synthesis of CMP is given in the Scheme-1.

Synthesis of magnetic hydroxyapatite nanoparticle
Magnetic hydroxyapatite nanoparticles were synthesised according to the literature procedure with some modi cation. 27,28 In the typical process, ferrous chloride tetrahydrate (1.85 mM) and ferric chloride (3.78 mM) were dissolved in 30 mL DI water taken in 250 mL round bottom ask under stirring at 500 rpm in the presence of nitrogen atmosphere. After 1 h, the complete clear orange colour solution was formed.
Then, 10 mL 30 % ammonia solution was added dropwise into the reaction mixture and kept at 70 0 C temperature for 1 h to form black colour iron oxide nanoparticles. The entire synthesized iron oxide nanoparticles were well dispersed in a solution of calcium nitrate tetrahydrate (33.7mM) and diammonium hydrogen phosphate (20 mM) prepared in 250 mL beaker containing 50 mL DI water.
Afterward, the pH of the solution was adjusted to 11 using ammonia solution. This mixture was stirred for 3 h at 90 0 C temperature under nitrogen atmosphere. Milky white particles formed were centrifuged and washed with water and ethanol to remove impurities. The resulting particles were magnetically separated from the medium using local magnet and dried in an oven at 70 0 C. Then, the particles were grinded in a mortar pestle and ltered using 150 mesh size sieved for their uniform size and utilized for further application.

Formulation of polyurethane nano-composite coatings
Mild steel (MS) panels were used as substrate for application of PU nanocomposites coatings. The MS substrate was pre-treated with sandpaper, degreased with acetone, and dried in an oven for 20 min. The required quantity of MHAP of 0, 1, 2, 3, 4 and 5 wt. % and CMP were dispersed into the xylene with stirring. Then, the calculated amount of hexamethylene diisocyanate (HDI) in the ratio of NCO:OH 1.2:1 and DBTDL as a catalyst were added into the above mixture. After achieving desired viscosity to the resultant formulation, it was applied by brush on pre-treated mild steel (MS) panels of 4 X 6-inch 2 dimension. The prepared coating panels were allowed to cure at room temperature for 48 h. The prepared samples were coded as CMPU, CMPU-1, CMPU-2 CMPU-3, CMPU-4, and CMPU-5 based on the amount of MHAP. The schematic of PUs preparation reaction is represented in the Scheme-2.

End group analysis
Hydroxy functionality is most important parameter in the development of polyurethane. They are deciding the actual quantity of diisocyanates for solid lm development in the nal crosslinked structure of PUs. it was determined by experimentally by following the ASTM D 6342-12 methods.

Structural analysis
Transformation of functional group of cardanol to cardanol mannich polyol and structural con rmation were determined by recording FT-IR spectra on a Perkin Elmer-1750 in the range between 4000-400 cm -1 and 1 H NMR spectroscopic techniques on a Bruker Avance-400 MHz spectrometer in deuterated CDCl 3 as a solvent and TMS as an internal standard.

Gloss Test
The lustrous property of PU coatings was investigated using a digital gloss meter (Model BYK Additives and instruments, Germany) at an angle of 60 0 . For the testing, 10 different positions on the surface of coated MS panels were considered and their average value was recorded as a nal gloss of PU coatings.

Adhesion Test
Adhesion of developed coatings to the mild steel surface was checked using a cross cut adhesion tester (model no. 107, Elcometer U.K.) as per the method ASTM D-3359-02. The tester tool box contained a die of parallel sets of 10 blades, adhesive tape (Scotch brand 810 magic tape), brush and lens. Initially the coated surface was rapidly crashed two times in 90 0 to each other with the help of blade and smoothly cleaned using brush. Then, the adhesive tape was pressed on the crashed surface and pulled within 60 S at 180 0 angle. Finally, the visual con rmation of percentage of squares adhered on the surface of adhesive tape form cross cut MS panel with respect to the total initial number of squares were considered for calculating adhesion of the coatings with metal substrate. All the measurements were taken at room temperature and average of 10 times was considered for reporting.

Corrosion performance
Corrosion performance of developed PU coatings were examined by immersion and electrochemical testings.
Immersion study was used to examine the corrosion performance by deeping the uncoated and PU coated MS samples in 3.5% NaCl aqueous solution for 7 days. After testing the coated samples were compared with control for change in gloss, deterioration, cracking, and partial or complete removal of lm from the surface. The result was in the form of captured images of both before and after testing of all the samples.
The corrosion performance of prepared PU coatings was also checked by electrochemical testing. The analysis was done by an Auto lab PGSTAT30 potentiostat instrument and all analysis was carried at room temperature in an aqueous 3.5 wt.% NaCl solution. The analysis comprises tree electrodes viz. platinum wire as a working, calomel and coated MS strips as counter and working electrodes. The area of the coated panels exposed to the test solution was 1×0.5 cm 2 in all the cases and the range of testing current potential was kept in between −1 to +1 V at the scan rate of 0.01 V/s.

Chemical resistance
The chemical resistance was checked by dipping the coatings into aqueous acid and alkali solutions, Additionally, structural con rmation of cardanol and CMP were carried by 1 H NMR and their spectra shown in the Figure-3. In the spectrum of cardanol, the chemical shift of terminal methylene proton was appeared at 0.9 ppm and all -CH 2 -linkages were observed at 1.25-1.5 ppm. The peaks found between 5.03-5.46 ppm were related to the protons of -C=C-, while the peaks at 2.03 and 2.55 ppm corresponded to the -CH 2 -protons adjacent to the unsaturation and aromatic ring, respectively. The peak at 4.6 and 6.68-7.17 ppm were related to the phenolic protons and aromatic benzene ring. In the spectrum of CMP, new chemical shift was observed at 2.35 and 3.7 ppm, which were corresponded to the hydroxy and methylene protons present in between the nitrogen atom and phenolic aromatic ring, respectively. 29 The appearance of these protons peak in the CMP spectrum evidenced happening of the reaction between oxazolidine with cardanol.

FT-IR of magnetic hydroxyapatite nanoparticles
FT-IR spectrum of MHAP nanoparticles is represented in the Figure-

Magnetic behaviour of MHAP nanoparticles
The magnetic property of the nanoparticles was tested by a simple magnet test. For this purpose, the synthesized nanoparticles were suspended in a water-ethanol solution and sonicated for 5 min to form complete suspension of nanoparticles (Figure-5 a). Afterward, the magnet was connected to bottles as shown in the Figure-5 b. In a few seconds, all the nanoparticles were attracted towards the magnet, which concluded that the synthesized particles were with magnetic behaviour.

Coating properties
Determining the properties of coating is important in order to nd out the suitability the coating formulations. The determined results of coating properties are represented in the  Chemical resistance of the developed cardanol based nanocomposite coatings was studied in 5 % HCl, and 5% NaOH solutions, water and xylene as an organic solvent for 7 days. The obtained results of test are expressed in the Table-2   The results clearly detected that the bared and MHAP nanoparticle added PU coating samples were totally damaged, lm detached from surface, and shown loss in gloss in the water and acid media. On the other hand, PUs added with MHAP shown better resistance against those media with exception of only slight loss in gloss. These results indicated that the presence of MHAP plays role in increasing the adhesion of metal surface. Minor loss in gloss was noted to all the coatings in the alkali medium, while all the prepared coatings shown excellent results against solvent medium, which may be attributed to the good interaction between the MHAP and polyurethane matrix. Thus, all the composite coatings with MHAP shown better chemical resistance as compared to the pristine PU.

Anticorrosive performance by immersion method
Corrosion resistance of the prepared nanocomposite coatings was examined by deeping the coated and uncoated samples in 3.5% NaCl solution. After the test, analysed samples were compared with control samples and captured images are given in the Figure-7. The bared sample fully corroded, as it does not cover PU coating layer and had direct contact with the corrosive medium. From the test results, it was revealed that the MHAP based nanocomposite coatings provide superior corrosion resistance as compared to the bared and without MHAP coatings. the presence of MHAP nanoparticles in the PU matrix provides a strong adhesion over the metal surface and act as a barrier between corrosive media and metal surface, which caused inhibition in the corrosive process.

Anticorrosive study by electrochemical method
Anticorrosive performance was also examined by measuring tafel plots of uncoated, coated, and MHAP added coating samples in 3.5% NaCl solution. The tafel plots of all the PU samples are shown in the Figure-8. The plots were used to estimate corrosion potential (E corr ), corrosion current density (I corr ), polarization resistance (Rp), and corrosion rate (CR). Using Tafel extrapolation method based on the software Nova 1.8, values of these corrosion parameters were calculated and represented in the Table-3  Percent IE was determined from the values of corrosion current density of uncoated and coated samples. In general, higher the Icorr values lower is the inhibition e ciency to the coatings. The graphical presentation of all the coated and uncoated samples is shown in the Figure-9. The corrosion inhibition e ciency of MHAP added coating samples was far better than the blank and CMPU. Higher e ciency was obtained for the 5 % loaded coating due to the well adhesion of PU formulation to the MS substrate. Additionally, the hydrophobic nature of the coatings might have helped to enhance the anticorrosive behaviour by restricting interaction between coatings and corrosive media.
The corrosion rate versus type of coatings is graphically represented in the Figure-10. Corrosion rate of all the PU coated samples was better than the uncoated one. Therefore, it con rmed that the prepared PU formulations acted as an obstacle for said medium to interact substrate and thus decreased the corrosion rate. From all the results, it can be concluded that the developed PU coatings were with good resistance against corrosion, which increased with increase in the percent loading of MHAP nanoparticles in coatings.

Contact Angle
Contact angle of the coated CMPU and all the MHAP embedded coated samples was measured to estimate the surface hydrophobicity and the results are shown in the Figure-11. From the result, it was observed that all the nanocomposite coated MS samples were more hydrophobic than the pristine coated sample. The contact angle of all the nanocomposite coatings was found to be more than 90 0 , which was much higher as compared to MHAP coatings (87 0 ) without nanoparticles. Contact angle of the coating samples increased with increase in the amount of MHAP nanoparticles upto 3%, beyond that declining the values of contact angle was seen. Overall, results of contact angle revealed hydrophobic nature to all the prepared nanocomposite PU coatings.

Thermogravimetric Analysis
Thermograms of all the prepared PU lms are presented in the Figure- well as it was free from phase separation or presence of any voids. Images of MHAP incorporated coatings were also clear and homogeneous with absence of any type of phase separation or cracks over the surface. Therefore, it can be concluded that MHAP was properly dispersed in the PU formulation and interacted with matrix. Furthermore, all the coatings were free from the microcracks, voids, and phase separation.

Conclusion
Cardanol was used as a renewable phenol for the preparation of Mannich type polyol, which was further utilized in the formulation of PU nanocomposite coatings using various percentage of MHAP (1,2,3,4, and 5 %). The synthesised cardanol Mannich polyol was characterised for structural features by FT-IR and 1 H NMR spectroscopic techniques. MHAP was synthesised in the laboratory and characterized by FT-IR and SEM analysis. The developed PU nanocomposite coatings demonstrated good physco-chemical properties. The prepared coatings showed excellent anticorrosion and chemical resistance tests. The hydrophobic character of coatings increased upto 3% loading of MHAP beyond that it decreased as measured by contact angle test. All the coatings showed good thermal stability and smooth surface morphology as studied by TGA and SEM respectively.

Declarations
Con icts of Interest The authors declare that there are no con icts of interest.
Authors' Contributions All authors contributed to data analysis, drafting or revising the article, gave nal approval of the version to be published, and agree to be accountable for all aspects of the work