Synthesis and characterization of cresol based anticorrosive and theoretical

studies

Abstract

Extensive research has been conducted to address the issue of corrosion in steel
applications for industrial purposes and other fields. This study introduces a polyurethane-
based material utilizing benzoxazine as a monomer, which has undergone thorough
characterization through NMR, UV-visible, and FT-IR spectroscopic methods. The electronic
properties of the synthesized monomer have been elucidated through DFT analysis, including
FMO's analysis and molecular mapping studies. Furthermore, different ratios of polyurethane
precursor have been employed to produce an anti-corrosive co-polymer material, with tests
revealing its hygroscopic nature and gel formation, showcasing promising outcomes. The
Tafel experiment was also conducted to evaluate the activity of the coated polymer,
demonstrating its superior performance compared to bare mild steel.

Keywords: Simple monomer, Co-polymer, Corrosion studies, DFT and MEP.

Introduction

Industrial metals face a great deal of risk from corrosion, which affects both their
overall safety and cost. There are now several strategies for preventing metal corrosion, such
as using organic/inorganic coatings, adding corrosion inhibitors, and adjusting the
composition and smelting processes of metals. [1-8] The application of corrosion inhibitors is
the most practical and economical choice among these. When it comes to the final features of
objects made of metal, stone, wood, and polymers, surface quality is vital. [9] Almost all
commercial items go through finishing processes to get the surface to the correct size, shape,
or texture. One often-used method for this is abrasive machining, where the type of abrasive
tool used has a big impact on the final result. [10] By making sure the tool is constructed and
composed correctly, producers may produce the highest-quality surface layer. [11] Corrosion
of mild steel (MS) is a significant problem across various industries such as petrochemical
engineering, chemical processing, mining, industrial cleaning, and material refining. It can
result in financial losses and pose health and safety risks in corrosive environments. Pipeline
failures that carry natural gas, chemicals, and oil are mainly caused by external corrosion.
Such pipeline damages can cost up to 3.4% of the world's gross domestic product, which
amounts to around US $2.5 trillion (2013) [12]. Corrosive elements in the environment such
as soil acidity, oxygen concentrations in the atmosphere and water, CO 2 concentrations in
brine solutions, ambient temperature, the presence of natural catalysts like copper or lead, and
salt concentrations are responsible for the damage caused to pipelines [13].Organic coatings
are commonly used to prevent corrosion in metals and steel. They act as barriers to the flow
of electric current and prevent harmful substances from causing damage. [14] Resistant
organic coating is a primary method of protecting mild steel from corrosion during industrial
processes. The approach involves creating a barrier to stop corrosive agents from passing
through, which is believed to be an economical and practical solution. [15]It was common
practice to characterize a coating's protective barrier qualities using the rates of moisture and
ion transport through its polymer network [16].

Various polymeric compounds have been utilized in this study to protect mild steel.
One of these compounds is polybenzoxazines, a novel type of phenolic resins derived from
benzoxazine through ring-opening polymerization. [17-18] The advantages of
polybenzoxazines have garnered significant interest due to their ability to address the
limitations of traditional phenolic resins. It is expected that polybenzoxazines will serve
effectively as a thermoset resin in various fields, including electronics. [19-24] The primary
challenges associated with polybenzoxazines are their high curing temperature requirement
(approximately 200°C) and brittleness, which is a common issue with thermoset resins.
Various catalysts have been explored to lower the polymerization temperature.
[25]Polybenzoxazine has a notable drawback, which is its limited degree of polymerization.
In order to enhance the performance of polybenzoxazine, various studies have been
conducted on copolymers [26-29], polymer alloys [30-35], fiber-reinforced composites [36-
37], and clay nanocomposites [38-39]. Additionally, another effective approach for
enhancing performance is through the polymerization of novel monomers that contain
different polymerizable groups, such as ethynyl, nitrile, propargyl, and allyl groups [40,41].

Synthesis of the monomer

O-cresol (1g, 6.572 mmol) and 3-aminoacetophenone were employed as reactants in
the present experimental procedure. These aforementioned substrates were combined with
paraformaldehyde (0.414g) in the presence of 20 mL of CHCl 3 and subjected to reflux for a
duration of one day to synthesize the benzoxazole monomer. The progress of the reaction was
monitored using thin-layer chromatography. Subsequently, the mixture was extracted using
150 mL of chloroform, and any impurities that dissolved in the aqueous phase were
eliminated by washing it with 100 mL of water and 100 mL of brine solution. To remove any
residual water, the organic layer containing the pure monomer was separated and dried using
anhydrous sodium sulfate. The resulting chloroform solution was then evaporated under
reduced pressure to obtain the benzoxazole monomer. This monomer was further utilized to
produce a coating material with polyurethane as its polymeric constituent.

NH2
OH (CH2)nO, CHCl3 O
O
+ N
O 70oC, 24 hr

O-Cresol 3'-Aminoacetophenone

Scheme 1: Chemical synthesis of the monomer

Synthesis of the anti-corrosive agent (polymer synthesis) [42-43]

The MS substrates were prepared for coating by first undergoing mechanical blasting
to Sa 2.1/2 and then being cleaned in acetone using ultrasonic methods. The polymer coating
was applied to the MS plate through dip coating and thermal curing techniques. The
monomer, BTP, was dissolved in a solvent mixture of 1,4-dioxime and toluene in a 7:3 ratio,
with the addition of the isocyanate hardener (BTP-PU 60, BTP-PU 80, or BTP-PU 100) in
similar proportions. The MS plate was then immersed in the BZ solution for one minute and
removed at a constant rate of 100 mm per minute. This process was repeated three times on
the BTP-coated MS samples. After being left at room temperature overnight, the samples
were vacuum-dried for an hour at 100°C to remove any remaining solvent molecules. Finally,
the samples underwent a three-hour heat treatment at 180°C.

Water absorption studies [44]

The water absorption capacity of the samples cured at a specific temperature was
evaluated using the guidelines from the ASTM D570. After being cured and dried at 80
degrees Celsius without any air, the samples were weighed to ensure they were completely
dry. Subsequently, the sample was submerged in water at room temperature for 24 hours.
After that, the samples were dried entirely using tissue paper and weighed once again.We
calculated the percentage of water absorption with confidence using the following equation:
Percentage of water absorption = [(Wa-Wb)/Wb] X 100
Where,
Wa = Weight of the cured sample after removal the exposure to water absorption
Wb = Weight of the cured sample before removal the exposure to water absorption
Gel content studies [45]

Similarly, all cured samples had their gel formation content checked after being
immersed in xylene for a day at room temperature; prior to this, the weight of the bare sample
was recorded. After that, a vacuum oven was used to dry all of the samples, and the following
formulas were used to determine how much gel there was:
Percentage of gel content = (Wa/Wb) X 100
Where Wa and Wb are the weight of cured coating samples after and before the extraction
respectively.
Computational calculations
Frontiers Molecular Orbitals (FMO) study in DFT investigations has shed light on the
synthesized monomer's chemical reactivity. The entire DFT analysis was conducted using the
Gaussian 09W software program. In relation to this investigation, the monomer's 3D structure
was first optimized using 3D chem draw, which involved pasting the structure into Gaussian
software and conducting an analysis utilizing the B3LYP/6.311 G basis set. [23, 46] With the
aid of the Gauss View software, the optimized structure could be seen and utilized for a
variety of analytical tasks, including determining the electron density on the molecule and
determining the molecular electrostatic potential.

Electrochemical studies
Tafel Polarization experiment [47]

By aid of these values, the corrosion efficacy was calculated using following formulae,
Corrosion rate = (Icorr × K × EW)/ρA mmpy
Where, K is corrosion rate constant (K= 3272 mm year -1), EW is equivalent weight of the MS
(27.9 g), ρ is MS density (7.85 g cm-3) and A is an area of the sample 1 cm2.
Corrosion Efficiency (%) = [(Icorr(b)-Icorr(c))/(Icorr(b))] × 100
Where, Icorr(b) is corrosion current values for the bare mild steel, Icorr(c) is
corrosion current values for the material coated mild steel.

Results and discussion

Characterization of the monomer

1
H-NMR analysis
 Proton NMR spectroscopy is one of the crucial instruments that can accurately give
each proton's precise chemical situation is NMR spectroscopy. Henceforth, this analytic
technique was used for the analysis of monomer. Chemically, the synthesized monomer
contains 17 protons, which are present in different chemical environments. The 1H-NMR
spectrum in Figure 1 displays all of these protons. Among them, there are four protons
located in the -CH2- groups attached to the benzoxazole ring. These protons are two separate
singlet peaks at 4.9 and 5.3 ppm. Additionally, another singlet peak at a highly shielded
region, specifically at 2.1 and 2.4 ppm, accounting for three protons each. These protons are
associated with the methyl groups attached to the phenyl ring and ketone group, respectively.
The remaining seven aromatic protons appear in the range of 6.7 to 7.4 ppm with multiple
peaks, which confirms the chemical structure of the monomer. Unfortunately, we were
unable to perform NMR analysis on all of the polymers due to solubility issues.

Figure 1: 1H-NMR spectrum of the monomer in CDCl3 solvent

FT-IR analysis

The chemical structure of the monomer and its co-polymers was determined using
FT-IR spectroscopy. Figure 2 shows the FT-IR spectra of the monomer and its polymeric
materials with varying ratios of polyurethane. The monomer contains ketone, benzoxazole,
aliphatic, and aromatic functional groups, all of which were confirmed by FT-IR
spectroscopy. The FT-IR spectra of the monomer are displayed in Figure 2. The peaks at
2928cm-1 and 2862 cm-1 are attributed to aromatic stretching vibrations. Additionally, the
cyclic amine functional group is associated with a prominent peak at 1143 cm -1 due to C-N
stretching vibration. The peaks at 1235cm -1, 1209cm-1, and 1091 cm-1 indicate the presence of
asymmetric and symmetric stretching vibrations of the C-O-C bond. [48] A peak at 2954 cm -1
indicates the presence of an aromatic-substituted methyl group, and a subsequent peak at
1499 cm-1 firmly establishes the existence of a tri-substituted benzene ring fused with an
oxazine ring. Furthermore, a sharp peak at 1686 cm -1 corresponds to the stretching frequency
of a free ketone carbonyl group. This study shows that the synthesized monomer possesses a
specific functional group, specifically an aromatic group fused to an oxazine ring. The
polymeric composites also exhibit these corresponding peaks, along with additional notable
peaks related to the vibrations of polyurethane. For instance, the very weak broad peaks
observed at 3337 cm-1 and 3070 cm-1 are caused by the -NH- and -CH 2- stretching vibrations,
respectively. [49] The stretching vibrations of the C-N and C-O-C bonds are associated with
a series of high peaks at 1515cm -1 and 1146 cm-1, respectively. Additionally, the presence of
the polymeric amide is observed at 1636 cm-1, corresponding to amide C=O stretching
vibrations. This confirms the presence of the polyurethane unit in the synthetic composites.
Figure 2:Solid state FT-IR spectra of monomer (A), and various ratio of polymeric
composites such as 60 % (B), 80 % (C) and 100 % (D)

UV-visible spectroscopy

UV-visible spectroscopy analysis was performed on the synthesized monomer and its
polymers by recording the spectra at room temperature using water as a solvent. Figure 3
depicts two distinct peaks of the synthesized monomer at 342 nm and 298 nm, representing
the n-π* and π - π * electronic transitions. Moreover, the investigation of polyurethane-
coupled materials revealed an increase in absorbance intensity with a red shift, indicating the
influence of the polyurethane unit on the electronic transitions of the monomer. Specifically,
the n-π* transition band shifted by 20 nm towards higher wavelengths with increasing
polymeric content, leading to a rigid band at 360 nm in the π - π * electronic transition. These
results highlight the significant impact of polyurethane on the characteristics of the monomer,
suggesting that a material containing 100% polyurethane may offer advantages in application
procedures.

Figure 3:Absorption spectra of monomer (pink), and various ratio of polymeric composites
such as 60 % (blue), 80 % (red) and 100 % (black) in solution state at RT.

Tafel experiments

In Figure 4, we can see the Tafel plot used to calculate the corrosion current and
potential of the artificial polymeric material. To predict the corrosion current density, the
corrosion potential was used to overlay a straight line over the linear component of both the
anodic and cathodic curves. These properties greatly affect the corrosion behavior of
synthetic materials. Generally, materials with higher conductivity and more electrons corrode
faster. Conversely, materials with a higher positive corrosion potential corrode at a slower
rate. Therefore, materials require a higher potential to demonstrate their conductivity. [50-51]
The studies on corrosion of co-polymeric materials have been carried out using
electrochemical methods such as the tafel experiment. The outcomes were compared with a
steel plate that was left untreated as a blank, and the estimated parameters are presented in
Table 1. The blank had a higher corrosion current value of 7.28 A, which can be
significantly reduced by increasing the concentration of the polyurethane-containing polymer.
A minimum current value for corrosion was found to be 0.12 A for the highest
 concentration of polyurethane. This indicates that synthesized co-polymers considerably
reduce the formation of crusted iron on the surface of steel plates in comparison to untreated
plates. In this study, it was found that the synthesized co-polymer has a positive potential,
which means that the polyurethane part of the polymer has a lower negative potential. This
indicates that polyurethane can form strong chemical bonds with monomers, which can
prevent iron oxidation and protect against air molecules. The effectiveness of this polymer
against corrosion was measured using Icorr, and it was found that the blank had a corrosion
rate of 84660.14 millimeters per year (mmpy), which was reduced up to 60 times with the
highest content of the polymer. The efficiency of the material in this line is also evaluated,
with the PU component reaching an impressive improvement of 98% when it constitutes 60%
of the material, compared to 100% PU. Additionally, the polarization resistance clearly
demonstrates the positive impact of PU on the polymeric materials, resulting in a resistance
that is 6 times higher. Taking into account all the parameters calculated from the tafel plot, it
is highly recommended to utilize the synthesized materials as an anti-corrosive agent for mild
steel, as it is expected to be more effective.

Table 1: The parameter obtained from the electrochemical experiment.

Corrosion Tafel Constant

Polarization
Sample Icorr Ecorr Corrosion inhibition (mV/dec)
resistance
(PTB:PU) (μA) (mV) Rate (mmpy) Efficiency
(KΩ*cm²) βa βc
(%)
Blank 7.28 -638 84660.14 5.02 0.00 -179 159

100:60 2.00 -584 23258.28 10.1 72.52 -207 171

100:80 0.23 -534 2674.70 20.4 96.84 -15.1 8.7

1395.49
100:100 0.12 -508 30.6 98.35 -23.1 13.2
Figure 4: Tafel experiment of the blank (blue) and various concentrations of poly-urethane
materials (Red – 60 %, green – 80 % and black – 100 %)

Water adsorption and gel content studies

Water diffusion studies in polymeric corrosion materials are vital and may differ
depending on the branching and cross-linkage density of the polymeric materials. The cross
or breach density determines the effectiveness of the binder during coating, leading to greater
hydrogen bonding with the materials. [52] The water adsorption capacity of the monomer and
its polymer was examined, yielding a better result, as shown in Figure 5. In the gel content
analysis, the monomer accounted for just a small portion of the water adsorption. This
demonstrates that the monomer may be highly hydrophobic, and clearly, polyurethane will
improve its co-polymeric nature. The data clearly reveal a reciprocal connection with
polyurethane concentration, with greater concentrations yielding a relatively low percentage
of water adsorption (0.75%). These findings are substantially correlated with gel content,
indicating that it has a promising physical character that could be used for corrosion
investigations. As a result, the gel content of the synthesized monomer and polymeric
material was assessed, and the data are presented in Figure 6. The monomer alone has a
higher gel content of 87%, which is further increased by the production of a co-polymer with
polyurethane. The gel content is continually raised by increasing the amount of polyurethane
substance. Finally, it achieved 94% gel content with an equal quantity of polyurethane and
monomer. These results could be attributed to the presence of a very intricate polymeric
network created by the polyurethane, which could result in a denser bonding structure. Co-
polymerization with polyurethane is an essential aspect of this study; thus, it might be
assumed that increasing concentrations of polyurethane have become the best anti-corrosive
agent.

96
%
g 94
e
l 92

a 90
b
s 88
.
86

84

82
PTB PTB100:PU60 PTB100:PU80 PTB100:PU100

Figure 5: The percentage of gel content analysis of monomer and its co-polymeric material.

2.5
%
w
a 2
t
e
r 1.5
a
b
s 1
.
0.5

0
PTB PTB100:PU60 PTB100:PU80 PTB100:PU100

Figure 6: The percentage of water adsorption analysis of monomer and its co-polymeric
material.

Theoretical analysis

Density Functional Theory (DFT) is a powerful computational technique used in

physics, chemistry, and materials science to study the electronic structure of many-body
systems such as atoms, molecules, and condensed phases. [53] Let's look at its chemical
 applications, which include comprehending chemical processes, surface science and catalysis,
material sciences, homogeneous/heterogeneous reactions, chemical sensors, and
electrochemical reactions. [54-57] Frontier molecular orbitals (FMOs) are formed by
combining higher occupied molecular orbitals (HOMOs) and lower unoccupied molecular
orbitals (LUMO). This FMO could help understand the molecule's electronic transition and
electron distribution on the surface, therefore providing the polarity of the molecule.

The FMO orbital analysis of the synthesized monomer has been thoroughly
investigated to gain insights into the reactivity and electronic distribution of the compound.
The orbitals (HOMO and LUMO) can be observed in Figure 7, clearly illustrating the intra-
molecular charge distribution across the molecule. At the HOMO level, electrons are
dispersed throughout the molecule, particularly in the benzacetamide group. This
transformation could be attributed to the presence of a donor-acceptor system within the
molecule. The amide functional group acts as the acceptor unit, while the toluene group at the
other end serves as the donor, thereby facilitating the ICT of the molecule. Additionally, the
energy levels of these orbitals can be utilized to calculate various parameters such as band
gap, global hardness, global softness, electrophilicity index, and chemical potential. The
values obtained for these parameters are presented in Table 2. A more negative chemical
potential value and a more positive electrophilicity index indicate the monomer's polarity, a
chemical property that could lead to the formation of more cross-linked bonds with
polyurethane. These bonds have the potential to enhance other physical properties, as
previously discussed (gel content and water adsorption), making them crucial for the
development of anti-corrosive materials. The synthesized polymeric material resulting from
this study exhibits improved activity, highlighting the chemical reactivity of the monomer
and its contribution to the physical properties of the co-polymeric material.

Table 2: Data obtained from DFT studies

HOMO Band
Compoun LUMO Chemical Global Global Electrophilicity
Gap
d Name (eV) (eV) potential Hardness Softness Index
(eV)

Monomer
-5.8556 -1.4748 4.3807 -3.6652 2.1903 0.2282 3.0365
Figure 7: FMO analysis of synthesized monomer (a) and its optimized structure (b).

Molecular electrostatic mapping

Molecular Electrostatic Mapping, also known as Electrostatic Potential Maps or ESP

Maps, offers a detailed view of the charge distribution within molecules. These maps help us
understand the different charge regions of a molecule and predict how molecules interact
with each other. [58] They are an excellent tool for viewing the charge distributions of
molecules and gaining insights into charge-related characteristics and molecular behavior.
This study involves overlaying the computed electrostatic potential energy data onto an
electron density model generated from the Schrödinger equation. The resulting data is
visually displayed by a color spectrum, with red denoting the lowest electrostatic potential
energy value and blue signifying the highest. [59] Electrostatic potential maps are essential in
predicting the behavior of complex molecules in organic chemistry, as well as in the study of
molecular polarity, charge distribution, and interactions. To continue DFT research, the
molecular electrostatic mapping is produced from the optimized structure and shown in
Figure 8. The red color electronic may have been produced for the acetamide carbonyl
oxygen, suggesting a higher electron content. On the other hand, the toluene component lacks
electrons and appears green. The center-core component of the molecule, known as the
benzoxazole group, is similarly very pale-yellow, indicating that the oxygen and nitrogen
molecules have a lower electron density. This observation was also obtained via the DFT
calculations, thereby proving that the molecule's polarity.
Figure 8: Molecular electrostatic mapping analysis of synthesized monomer molecule.

Conclusion

Various spectroscopic techniques such as NMR, FT-IR, and absorption spectroscopy

were used to successfully characterize the synthesized monomer. The polymer was made
using high-temperature curing and the characterization results indicated the formation of the
polymer under current reaction conditions. Additionally, the water absorption and gel content
properties of both the monomer and polymeric materials were studied. Results confirmed that
the monomer exhibited moderate properties which were improved through polymerization,
resulting in a highly hydrophobic nature and strong gel content that contributed to a more
cross-linked polymeric structure. These exceptional physical properties of the polymers
demonstrated high resistance to mild steel corrosion, as shown by electrochemical studies.
The electronic and chemical properties of the monomer were also investigated through DFT
studies, which were in line with the corrosion results. These initial studies suggest that both
the monomer and its polymers have the potential to serve as effective anti-corrosive agents
for mild steel.

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