Introduction

Polymerization is a process by which an organic compound reacts with itself to form a high‐molecular‐weight compound composed of repeating units of the original compound ‎[1]. ‎There are two general polymerization reactions: addition polymerization and condensation polymerization. Aldol and crossed-aldol condensations are potent tools for the formation of carbon-carbon bonds in many kinds of carbonyl compounds ‎[2-4]. ‎Numerous reports have been published based on the catalysis of crossed-aldol condensations of ketones and aldehydes with  acid, solid base ‎[5], ‎metal oxide ‎[6], ‎and even nano-catalysts ‎[7, 8].‎

The use of anti-corrosion coatings is mandatory in industries that are in contact with corrosive and chemical items. These coatings prevent the contact of corroded materials by creating a new layer. Therefore, they also increase the structure’s lifespan. In recent years, metal corrosion protection through polymer coatings has received more attention. Polymers have long chains of carbon bonds. Therefore, they can block large areas of metal surfaces after adsorption. The thin film of polymer coatings enable strong adhesion and stability of the metal-polymer interface in the corrosive environment ‎[9].‎

The known allotropes of carbon, such as graphite and carbon active have wide applications in making electrodes, paints, water purification catalysts, fuels, power generation, etc. In recent decades, many more allotropes have been discovered and researched, including buckminsterfullerene, graphene, nanotubes, and nanofibers ‎[10]. ‎Using different forms of carbon, typically fiber or nanotubes ‎[11], ‎to reinforce and significantly modify the properties of polymer materials has been known well ‎[12, 13]. ‎However, the polymerization of carbon compounds including graphite, graphite oxide, and carbon active has not been reported.

This work aims to show a new reaction or deformation of carbon compounds. Carbon compounds such as graphite, carbon active, and cellulose are resistant to chemical change. Exfoliation of graphite layers with concentrated acids increases its reactivity. Weak Vander Waals forces between the graphite layers allow its layers to exfoliate. Exfoliated graphite can be obtained via the conventionally chemical oxidation process using concentrated sulfuric acid as the oxidants and intercalating reagents ‎[14]. ‎Activated carbon is further a carbonaceous material that is commonly used for absorption due to its high porosity, wide surface area, and pore volume ‎[15, 16]. ‎The modification of activated carbon using sulfuric acid to improve the adsorption performance of organic compounds was also known ‎[17, 18]‎. Likewise, the interaction of cellulose with sulfuric acid was studied by Ioelovich in 2012 ‎[19].‎ It showed that in concentrated acid above 65%, cellulose dissolved completely and found an amorphized structure.

In this article, a simple method was applied to polymerize carbon compounds. This reaction is based on acetone aldol condensation in acidic media. Graphite is initially exfoliated, the actived carbon is modified with concentrated sulfuric acid, and cellulose sample is dissolved. The condensation reaction starts with adding the acetone to these compounds. The carbon polymers are soluble in organic solvents. They are black in color, highly viscous, adhesive, and cohesive materials. They can be used as anti-corrosion coatings.

Experimental

Chemicals and materials

Graphite powder, activated carbon, H₂SO₄ (1l =1.84, 98%), potassium permanganate, sodium nitrate, extra pure acetone, and hydrazinium hydroxide were purchased from Merck Co. (Germany).

Instrument

UV–Vis absorption spectra were recorded on Optizen 2120UV- PC spectrophotometer. The XRD pattern was obtained with a diffractometer system, XPERT-PRO Focus (Netherlands), under Cu K-Alfa1 radiation at λ = 1.54060 ˚A with a scanning speed of 4◦/min and a step of 0.04° (2θ) in the range from 5° to 80°. The FT-IR spectra were obtained by Fourier transform infrared spectrometer (FT-IR, Prestige-21, Shimadzu, Japan)) at room temperature. The morphologies of the samples and elemental analysis were investigated by scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDAX) using FESEM Model Phenom Pro X (Netherlands). The thermal behavior was observed by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) on a TA instrument; model SDT Q600 V20.9 Build 20 (USA), at a heating rate of 20 °C/min in an air atmosphere, from 25 to 1000 °C.      

Polymerization of graphite, graphite oxide, and activated carbon

The polymerization of graphite, graphite oxide, activated carbon, and cellulose (cotton) was performed by their introduction in an acid-acetone aldol condensation. In a typical method, 5.0 mL concentrated sulfuric acid was added to 1.0 g dry graphite or carbon compounds powder. Next, the mixture was stirred for 24 hrs and 15.0 mL of acetone was slowly added. After 2 hrs, the color of the solution was turned to dark yellow-red, which is at the result of the self-condensation of acetone. After 24 hrs, water was added to the mixture to dilute the solution, and then it was allowed to the solution to rest for 24 hrs and the formed polymers to precipitate. The black substance with high viscosity was separated from the solution and washed several times with water.

Graphite oxide polymer (GO-polymer) and crystallized graphite oxide (GO-crys) were prepared by the same method, but graphite was initially changed into graphite oxide by modified Hummer’s method ‎[20]. ‎In a typical synthesis, 1 g of graphite, 23 mL of 98% H₂SO₄, 100 mg of NaNO₃ and after 24 hrs slowly 3 g of KMnO₄ in ice bath were added. After being heated to 35-40 °C, the mixtures were stirred for another 30 min. In this stage for GO polymerization, 60 mL, and its crystallization, 20 mL acetone was added to mixtures. After 24 hrs, 200 mL water was added into the above mixtures slowly. After that, mixtures was allowed to rest for 24 hrs. The precipitates were washed with water several times. The excess acid can be neutralized with the Na₂CO₃ addition and washed again. In addition, for comparison, graphene oxide (GO) was synthesized by Hummer's method, and the reduced graphene oxide (rGO) was prepared using hydrazinium hydroxide (HH) as a reducing agent ‎[21].‎

Results and Discussion

Acetone self-aldol condensation

Aldol condensation is a reaction in organic chemistry in which an enol reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone. Self-condensation acid catalyst of acetone is a very complex reaction based on aldol condensation, and numerous products are possible via competitive self-condensation and cross-condensation between the same or different ketones formed in the reaction. The major products of the reaction are diacetone alcohol, mesityl oxide, phorone, mesitylene, isophorone, 3,5-xylenol, and 2,3,5-trimethylphenol ‎[22]. ‎The formation process of the reaction of diacetone alcohol, and mesityl oxide is displayed in Scheme 1.

If graphite, graphite oxide (GO) and CA tested in this study, entered the condensation, these compounds form a sticky or polymeric state similar to bitumen. This reaction was described as the polymerization of carbon compounds. Figure 1a, b displays the graphite polymer that has hardened over time. The samples of GO, CA, and dissolved cellulose polymerized are depicted in Figure 1c, d.

Mechanism of carbon compounds polymerization by aldol condensation

The Proposed mechanism for entering the carbon compounds in this condensation is demonstrated in Figure 2. Graphite was initially exfoliated with sulfuric acid. Exfoliation includes separating the individual layers in a more or less regular manner. Moreover, surfaces of carbon compounds were oxidized in reaction with acids such as Nitric acid, sulfuric acid, and phosphoric acid. Activated carbons with acidic surfaces mainly include oxygen-containing functional groups. Such groups, including carboxylic, chromene, lactone, phenol, quinone, pyrone, carbonyl, and ethers, are generally located on the outer surfaces or edges of the basal plane ‎[23]. ‎CA modified, or graphite exfoliated, and modified appears to act as a substrate to initiate the condensation reaction. Acetone self-condensation reaction is equilibrium. However, the entered carbon compounds into this reaction are not in equilibrium. By diluting with water, graphite, and carbon polymers are separated from the solution.

In the polymerization process, the ratio of acid-acetone was 1:3, and the reaction was done at room temperature and in 24 hrs. If the reaction temperature is raised to 50-60 , the polymerization time is significantly reduced. Furthermore, at the same time, if the ratio of acid-acetone is 1:1, graphite or compounds will be found in a crystallized state. In investigating this process on graphite oxide, it is possible to terminate the polymerization at the first states, and a few of its layers enter into condensation. Finally, it was observed that graphite oxide found in crystallized (GO-crys) state and was not dispersed in water. It made washing and purifying significantly more accessible. To dispersion of GO-crys in water, it needs to be powdered and exposed to ultrasonic waves. This state also was observed for other tested carbon compounds.

Characterization of carbon compounds polymers

UV–vis analysis of GO-crys

The UV–Vis spectrum (Figure 3) of GO and rGO obtained from Hummer's method (H) exhibit maximum peaks about 230 and 274 nm, corresponding to π→π* transitions of aromatic C–C and C=C bonds, respectively ‎[24]. ‎In comparison, GO-crys and reduced GO of it (rGO-Crys) has also red shifts from about 226 to 260 nm. This indicates that the crystallization of graphite oxide does not cause a significant change in its surface properties.

Fourier transform infrared (FT-IR)

The FT-IR spectrum (Figure 4a) of the GO-crys, like GO, illustrates the presence of C-O, C═C, C=O, and C-OH bonds from the peaks at 1048, 1455, 1625, and 3425 cm⁻¹, respectively. However, the comparison between the spectra of GO-crys and GO samples show the intensity of peak at 2359 cm^-1of GO-crys has been increased, which was attributed to the C-H sp² hybridization enol bond.  The FT-IR spectrum of graphite, graphite polymer, CA, CA polymer, and the enone product obtained from acetone condensation are plated in Figure 4b. ‎The spectrum of graphite and CA shows an absorption band at 1600 cm⁻¹, due to the vibrations of carbon atoms (C=C) belonging to the sp² rings, also an inclusive and intense signal observed in the 1300-970 cm⁻¹ region, which is mainly due to the C-C skeleton collective modes [25]‎. At the spectrum of graphite polymer and CA polymer, the bands at 3430 and 1100 cm^-1 were assigned to O-H bonds, and C-OH stretching of phenolic groups. The bands observed in the region between1374 cm^-1 and 1600 cm^-1 were attributed to C=C symmetrical stretching of pyrone groups and C=O of carboxylic groups ‎[26]. ‎In addition, the intensity of weak bound at 2967 cm^-1 significantly increased after the polymerization of graphite and CA. This peak is attributed to C-H bonds with sp³ hybridization, as highlighted in the proposed mechanism (Figure 2).

Scanning electron microscopy (SEM) analysis

The GO crystallization as displayed with micrograph was obtained from scanning electron microscopy (SEM) of GO and GO-crys in Figure 5. GO-crys was seen flat and smooth, and had sharp corners.

The dispersive energy X-ray (EDX) spectrum of graphite polymer and GO polymer are illustrated in Figure 6. The EDX spectrum confirmed the oxygen incorporation on the graphite and GO polymer. The average percentages of the elements from the surface SEM graph is plotted in the background of this figure. GO polymer shows more percentage oxygen than graphite polymer surface.

XRD analysis

The XRD patterns of pure graphite, graphite polymer, GO, GO-crys, GO polymer, rGO, carbon active (CA), CA polymer, and cellulose polymer are exhibited in Figure 7. Graphite shows a characteristic peak at 2θ = 26.5 with a d-spacing of 0.335 nm attributed to the (002) plane ‎[27]. ‎Furthermore, other weaker peaks were observed, such as 42 (001), 54 (004), and 78 (110) in accordance with standard data C-Graphite (JCPDS No. 96-901-2231). In graphite polymer, due to changed orientation and decreased crystallization, the intensity of all graphite peaks decreased. However, a broader beak was observed at 2θ below 20, and its intensity increased. This broader peak also was observed for CA polymer. In graphite oxide, as shown, this broadening increases with the completion of polymerization. GO exhibits a typical sharp diffraction peak centered at 2θ = 9.8°, corresponding to the interlayer distance of~0.89 nm and the typical diffraction peak of GO nanosheets. In GO-crys and GO polymer, this peak shifts to 12.1° and 12.6° with d-spacing of 0.73 and 0.70 nm, respectively. This could be due to GO polymerization, which increases the attraction interactions between the layers, and thus reduces the distance between layers. Moreover, the change and increase of the full width at half maximum (FWHM) according to Debby Scherrer's Equation refer to a decrease in the crystallites size ‎[28]‎. Likewise, two peaks around 2θ = 24° (002) and 42 (001) were observed again for GO-crys and GO polymer.

Based on the XRD graph pattern for CA, small peaks appeared with a maximum at 2θ = 27.43°, with d =0.32 nm corresponding to the diffraction of (002), which can be related to the impurity of graphite ‎[23, 28]. ‎CA polymer and cellulose polymer exhibit very broad diffraction peaks under 2θ = 20°. The absence of a sharp peak reveals a predominantly amorphous structure  such as bitumen ‎[29]. ‎

Thermal analysis

The polymerizations effect on storage and thermal stability of the graphite, CA and GO were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC). The TGA is the easiest method of thermal analysis, which is based on the measurement of the sample weight in the heating. This method provides valuable information when the materials decompose in the heating or react with the gaseous surroundings. In the DSC method, the unknown and control samples are kept at the same temperature. The difference in the energy required for being at the same temperature is plotted based on the temperature change. In other words, the unknown and control samples use some energy to stay under the same thermal conditions ‎[30]. ‎Figure 8 shows the combination of TGA and DSC curves for each sample. A sample weighing between 3-5 mg was sealed in an aluminum pan and heated from current temperature to 1000 °C at a rate of 20 °C/min, in air, and 50 minutes.

According to the results of the TGA test, the graphite loses about 94.02% of weight in the temperature range from 700 to 900 °C. The change in weight of CA, such as graphite, occurs in a step at a temperature range of 400 to 600 °C. CA loses 75.61% weight. Similarly, the GO has been found to degrade in three stages with 97.16% weightless. It seems that water is removed at temperatures below 100 °C. The weight loss that occurs between 100 and 300 °C is related to the removal of oxygen functional groups, and the weight loss at 360 to1000 °C can be linked to the oxidative pyrolysis of carbon framework ‎[31, 32].‎

The remarkable thing about polymers is their gradual weight loss. The gradual weight loss of 86.98% of the graphite polymer is in the temperature range from 200 to 800 °C. CA polymer shows a gradual weight loss of 97.5% in two stages, from start to 400 °C and from 400 to 600 °C and 95% weight loss for the GO polymer further occurred in two stages. The resistance of polymers to heat is less. Under thermal effect, the end of the polymer chain departs. According to the chain reaction mechanism, the polymer loses the monomer one by one. In comparison, the weight loss of polymer graphite occurred in a longer time than that of carbon polymer and GO polymer. In other words, polymer graphite has more excellent thermal stability.

Based on Figure 8, most DSC results show the endothermic transition during the heating of samples at a high temperature near 600 °C. The endothermic transition occurred due to the heat absorption for dehydration of water and the decomposition of samples. In DSC graphite, this endothermic transition occurred around 800 °C. It was observed for graphite polymer at 528 °C with the reaction enthalpy of 2149 J/g. The DSC curve of CA and CA polymer shows an endothermic reaction at a peak temperature of 566 °C (2679 J/g) and 547 °C (2068 J/g), respectively. In GO, there are two endothermic transitions at temperatures around 185 °C (1261 J/g) and 600 °C (4729 J/g), respectively. Besides, GO polymer shows a maximum peak at 608 °C with reaction enthalpy of 2065 J/g. It can be mentioned that heat absorption is less for the decomposition of polymers.

Application of polymers as an anti-corrosion coating

The corrosion behavior of the polymer coatings was evaluated by Tafel plots with a potentiostat/galvanostat (model IVIUM made in Netherland) ‎[33].‎ For corrosion measurements, the coated sample was immersed in 3.5% NaCl, and a three-electrode system was applied by coated specimen as working electrode; a platinum wire and saturated calomel electrode were used as counter and reference electrodes. The 1 cm² Cu electrodes were soldered to copper wire and insulated with epoxy resin. After polishing the electrode surface, two drops of polymer solution diluted with acetone were placed on the electrode surface. The electrodes were then polarized cathodic to the anodic direction from the OCP (open circuit potential) at a scan rate of 1 mV/s. Figure 9 depicts the Tafel curves of the bare and graphite polymer coating. The potential and corrosion current densities were determined by extrapolating the linear portion of the anodic and cathodic curves. The potential and current corrosion for bare electrodes were -0.07 V and 3.55 μA, respectively. However, the coated sample does not show potential and corrosion current and acts like a non-conductive coating.

In addition, a comparison was made between the corrosion resistance of bare specimen and polymer-covered carbon steel (1×1.5 cm²) by weight loss method. The entire surface of the carbon steel sheets was covered with a thin layer of polymers diluted with acetone. The covered sheets were immersed in a 10% sulfuric acid solution. In equal time, the specimens were taken out, cleaned, and weighted. The polymers were dissolved and washed in acetone. The results obtained from weightless are presented in Table 1. The bare sample indicated about 68% weightless, while the covered specimens showed almost no weight loss. The inhibition efficiency (IE) was computed using the following equation:

Where, w₀ is the weight loss bare specimen and w is the weightless covered specimen.

Conclusion

In this study, a simple method was applied to polymerize of carbon compounds based on aldol condensation in acid media. Concentrated sulfuric acid acts as an intercalating and oxidizing agent of graphite surfaces and carbon active. Photograph images confirm the formation of the bitumen-like polymers. The FT-IR spectrum was showed an increase in the intensity of CH₂ bound with sp³ hybridization by polymerization, XRD showed an increase in peak width in proportion to a decrease in crystallinity and increase in particle size due to polymerization, and the result of the thermal analysis was a reduction in thermal stability and stepwise decomposition of polymers. Finally, corrosion experiments have shown the successful use of polymers as non-conductive and anti-corrosion coatings.

Acknowledgments

This research was supported by the Nano-Structured Coatings Institute of the Payam-e-Noor University of Yazd.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

How to cite this manuscript: Fatemeh Banifatemeh, Polymerization of graphite and carbon compounds by aldol ‎condensation as anti-corrosion coating. Asian Journal of Green Chemistry, 7(1) 2023, 25-38. DOI: 10.22034/ajgc.2023.1.4