4.1. Formation of cardanol based polyol
Renewable cardanol based polyol was synthesised by the Mannich condensation of cardanol, diethanolamine, and formaldehyde. Overall process for the synthesis of CMP was considered as eco-friendly due to its solvent-less nature. Initially, diethanolamine and formaldehyde were condensed together to form the oxazolidine intermediate.26 Oxazolidine is present in the equilibrium with cyclic and open chain forms. The open chain form is an iminium cation, which is the well-known intermediate of Mannich reactions.29-31 In the presence of base (i.e. oxazolidine and tertiary amine), cardanol dissociates in the cardonalate anion with the negative charge circulated at equilibrium in resonance hybrids at oxygen and at ortho and para positions of aromatic ring. Finally, it results into a substituted product at ortho and para positions of the phenolic hydroxyl group of cardanol. The mechanism for the synthesis of CMP is presented in the Figure-1.
Structural confirmation of the cardanol based polyol was done by FT-IR and 1H NMR spectroscopies. The FT-IR spectrum of CMP is graphically represented in the Figure-2. The stretching vibration band at 3371 cm-1 was related to the -OH group present in the cardanol and CMP. Absorption bands associated with vibration of symmetric and asymmetric -CH2 group were seen in between 2854-2924 cm-1, while aromatic -C-H stretching vibration was observed at 3009 cm-1. The band at 1651 cm-1 was arise due to the presence of -C=C- bond present in the CMP and cardanol. The new broad peak observed in the FT-IR spectra of CMP at 1041 cm-1 for -C-N- stretching confirmed the reaction of oxazolidine with cardanol formed cardanol Mannich polyol.
Additionally, structural confirmation of cardanol and CMP were carried by 1H 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 -CH2- 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 -CH2- 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.
4.2. FT-IR of magnetic hydroxyapatite nanoparticles
FT-IR spectrum of MHAP nanoparticles is represented in the Figure-4. The broad absorption band at 3570 cm-1 was related to -OH bending vibration. The stretching vibration of carbonyl were observed at 1458, and 871 cm-1 while, phosphate group shown absorption at 1035 and 630 cm-1.21,32,33 The Fe-O bond stretching vibration bands were occurred at 1619 and 530 cm-1. It proved that MHAP nanoparticles were successfully formed.
4.3. 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.
4.4. Coating properties
Determining the properties of coating is important in order to find out the suitability the coating formulations. The determined results of coating properties are represented in the Table-1. The gloss of all the coated samples was observed in the range of 73 to 121. Gloss value of pristine sample was more than all nanocomposite coatings because incorporation of MHAP nanoparticles would have increased opacity as well as roughness of the coatings surface. Thus, as the percentage of MHAP was increased in coating formulations, it decreased the gloss. All the coatings showed 100% adhesion towards metal surface as not a single block was removed from the area of the scratched surface. Hence, it concludes that the formulated coatings are with excellent adhesion to the metal surface. The MHAP nanoparticles incorporated coatings were not scratched to the level of a 5H-grade pencil. The coatings containing 4 and 5 % MHAP nanoparticles presented better pencil hardness (5H) than other coating samples. It may be due to the magnetic nature of MHAP nanoparticles that might have resulted in high interactive forces between metal surfaces and the coating matrix. All the coating samples passed the flexibility test that can offer prevention of crack formation. Addition to that the chemical resistance of all the prepared coatings was checked using methyl ethyl ketone rub test. There was no any defect found such as film removed from the surface, crack formation, and changing colour of coating on the surface up to 200 double rubs. Finally, on the basis of all the coating properties, one can say that the developed coatings are suitable for the coatings on metal surfaces.
Table-1: Coating properties of prepared PUs coatings
PUs code
|
Gloss 60o
|
Cross-cut adhesion (%)
|
Pencil hardness
|
Flexibility
|
MEK double rub test
|
MHPU-0
|
121
|
100
|
3H
|
Pass
|
200
|
MHPU-1
|
96
|
100
|
3H
|
Pass
|
200
|
MHPU-2
|
85
|
100
|
4H
|
Pass
|
200
|
MHPU-3
|
84
|
100
|
4H
|
Pass
|
200
|
MHPU-4
|
74
|
100
|
5H
|
Pass
|
200
|
MHPU-5
|
73
|
100
|
5H
|
Pass
|
200
|
4.5. Chemical resistance study
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 and captured images are given in the Figure-6.
Table-2. Chemical resistance of bared, CMPU, CMPU-1, CMPU-2, CMPU-3, CMPU-4, and CMPU-5
Sample Code
|
Water
|
HCl
|
NaOH
|
Xylene
|
Bared
|
F
|
F
|
F
|
F
|
CMPU
|
B
|
D
|
A
|
A
|
CMPU-1
|
B
|
B
|
A
|
A
|
CMPU-2
|
C
|
C
|
A
|
A
|
CMPU-3
|
B
|
C
|
A
|
A
|
CMPU-4
|
B
|
B
|
A
|
A
|
CMPU-5
|
B
|
B
|
A
|
A
|
(A= Not affected, B= slight loss in gloss, C= Change in colour and loss in gloss, D= film partly removed E=film completely removed F= fully damaged)
The results clearly detected that the bared and MHAP nanoparticle added PU coating samples were totally damaged, film 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.
4.6. 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.
4.7. 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 (Ecorr), corrosion current density (Icorr), 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. Generally anticorrosive coatings are exhibiting higher Ecorr and lower Icorr values corresponding to the lower corrosion rate and vice-versa.34,35 The MHAP nanoparticles added coatings showed lower Icorr values and higher Ecorr values than the blank and pristine (CMPU) coatings. As the percent loading of MHAP in formulations increased, it decreased the Icorr values of the coatings because of increase in adherence of the coatings with the metal surface due to magnetic property of MHAP particles and formation of protective barrier between corrosive ions and metal substrate.
Table-3: Electrochemical corrosion measurement of bare, CMPU, CMPU-1, CMPU-2, CMPU-3, CMPU-4, and CMPU-5 coated samples.
Sample
Code
|
Ecorr (mV)
|
Icorr
(nA)
|
Rp
(kΩ)
|
Corrosion rate (CR) (mm/year)
|
Inhibition efficiency
(% IE)
|
Bare (Uncoated)
|
-656.47
|
968.44
|
4.929
|
0.0442
|
0.00
|
MHPU-0
|
-618.24
|
528.12
|
8.994
|
0.00814
|
45.47
|
MHPU-1
|
-602.31
|
479.09
|
8.954
|
0.00812
|
50.52
|
MHPU-2
|
-544.68
|
465.71
|
10.343
|
0.00709
|
51.91
|
MHPU-3
|
-554.76
|
372.97
|
23.280
|
0.005483
|
61.48
|
MHPU-4
|
-547.84
|
345.10
|
16.059
|
0.00401
|
64.47
|
MHPU-5
|
-535.07
|
181.35
|
116.520
|
0.000582
|
81.28
|
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 efficiency to the coatings. The graphical presentation of all the coated and uncoated samples is shown in the Figure-9. The corrosion inhibition efficiency of MHAP added coating samples was far better than the blank and CMPU. Higher efficiency 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 confirmed 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.
4.8. 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 900, which was much higher as compared to MHAP coatings (870) 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.
4.9. Thermogravimetric Analysis
Thermograms of all the prepared PU films are presented in the Figure-12. The thermal analysis showed three steps of thermal degradation in all the PU films. The first step of degradation was started in the range of 212-2250C and ended at 410-4190C with degradation result of 30-38 % weight losses due to the breakdown of urethane groups in PUs. In the second step, degradation occurred in the range of 416-4250C and ended at 530-5390C with weight losses 35-43% due to the degradation of main backbone chain of PUs. The third step of degradation observed in the range of 534-5360C and completed at 643-6770C with weight losses of 17-19% as a result of residual degradation. Thus, it can be stated that all the prepared composite coatings resulted with excellent thermal stability.
4.10. Surface Morphology of MHAP Composite Coating
The MHAP nanoparticles and their PU coatings were observed under a scanning electron microscope and the selected images are represented in the Figure-13. The morphology of MHAP nanoparticles showed irregular shape agglomers with an average size 38.64 nm. The particles shown smooth surface and separated from each other. The CMPU appeared as a smooth surface as it does not contain MHAP as 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.