Leucaena leucocephala oil-based poly malate-amide nanocomposite coating material for anticorrosive applications

: This article describes the synthesis of polyester-amide (PEA) resin from Leucaena leucocephala oil (LLO) obtained from seeds of L. leucocephala tree, locally grown in King Saud University Campus. LLO was transformed into amide diol by based catalyzed amidation reaction, followed by esteri ﬁ cation reaction with malic acid (MA), that resulted in LLO-based PEA (LPEA). The synthesis was performed without using any solvent or catalyst. Fourier-transformation infrared spectroscopy and nuclear magnetic resonance con ﬁ rmed the formation of LPEA by the introduction of amide and ester moieties. LPEA was further reinforced with nano graphene oxide (GO) and fabricated into nanocomposite corrosion protective coatings (LPEA/GO). LPEA/GO coatings obtained were tough, ﬂ exibility retentive and showed good corrosion resistance performance toward 3.5 w/w% NaCl medium. Thermogravimetric analysis con-ﬁ rmed good thermal stability of coatings with safe usage up to 200°C.


Introduction
Plant crops and products are utilized as food crops, feed crops, ornamental crops, industrial crops, and others. The hazards and expenses associated with petro-based chemicals have motivated the researchers to substitute sustainable resource-based raw materials for the synthesis of monomers and polymers. In this context, the industrial crops are distinguished substitutes for petro-based chemicals. The industrial seed oils are rich in functional groups that can be transformed into monomers and polymers with applications as biodiesel, lubricants, inks, coatings, and paints. Leucaena leucocephala is an agro-industrial crop, belonging to the family Fabaceae (sub-family Mimossoideae). It has found bioenergetic applications in biodiesel, biogas, ethanol, char, activated carbon, and others, utilizing seeds, leaves, bark, wood, and legumes of the tree (1). L. leucocephala seed oil (LLO) is rich in linoleic, oleic, palmitic, and stearic acids, with the highest composition of linoleic acid (2,3).
Polyesteramide (PEA) resins contain both ester and amide functional groups in their backbone. They are transformed into corrosion-resistant, high-performance coatings. However, mostly they are synthesized at high temperatures, in the presence of solvents, in several steps, from synthetic diols and dicarboxylic acids (4). An alternate feasible method is to synthesize PEA from sustainable resource-based raw materials, i.e., a vegetable oil (VO)-derived diol and a naturally available dicarboxylic acid with the inclusion of a nanofiller for reinforcement, at lower temperatures without the use of any solvent (3,5).
Graphene oxide (GO) has been used as modifier component for coatings, toward corrosion protection of substrates such as mild steel, carbon steel, copper, aluminum, and others (10). GO, used as nanofiller (up to 0.5-1 wt%) in alkyd-based coatings, has dramatically improved the anticorrosion properties of nanocomposite coatings (11)(12)(13). GO-dispersed waterborne soy alkyd nanocomposite, synthesized through solventless approach, has shown superior corrosion protection performance compared to the plain soy alkyd coatings (11). Sunflower alkyd/GO coatings have shown high level of durability, superior physico-mechanical performance, and corrosion protection ability (12). GO-dispersed Garcinia gummigutta VO nanocomposite coating on mild steel has shown efficient anticorrosion performance in 3.5 wt% NaCl medium, by the dispersion of 0.3 wt% GO (14).
This article describes the synthesis of PEA resin (LLObased PEA; LPEA) from Leucaena oil amide diol (LOAD) and MA. The synthesized LPEA was reinforced with GO and developed into anticorrosive coatings for mild steel. The structure of LPEA was established by Fourier-transformation infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) techniques and the interaction of GO with LPEA matrix was investigated by FTIR. The morphology of the synthesized resin was studied by scanning electron microscopy (SEM) and the thermal stability was assessed by thermogravimetric analysis (TGA).
The research work is focused on value-addition to a locally grown plant crop, by an environmentally safe and benign method, selecting sustainable resource-based monomers and opting for solventless synthesis strategy.
Seeds of L. leucocephala tree were collected from King Saud University campus in the month of March. L. leucocephala seed oil was extracted using Soxhlet apparatus as reported in our previously published article (3).

LPEA
LOAD (0.08 mol) was placed in a four-necked flat-bottomed conical flask; the temperature was raised to 50°C with continuous agitation. MA (0.08 mol) was added in pinches slowly, under continuous stirring, to the flask containing LOAD, and after this addition was completed, the temperature was raised to 90°C. The contents were stirred, and this temperature was maintained until a clear and transparent resin was obtained, after which the heating was cut-off, the contents were allowed to cool to room temperature. The reaction was monitored by FTIR, by carefully observing the change in the absorption band for the hydroxyl group (3,370 cm −1 ). The reaction was carried out without any solvent, and this required vigilant monitoring of viscosity changes during the course of reaction. The other pre-requisites of solvent-less synthesis were the optimization of reaction temperature as well as the amount of MA.

LPEA/GO nanocomposite
To the pre-determined amount of LPEA, GO was added in different weight percentages (0.25, 0.50, and 0.75%, w/w, on the weight of LPEA) and was dispersed in LPEA via a mechanical mixer (Dispers Master, Sheen Instruments Ltd, UK) at 30°C, mixed at 3,000 rpm for 2 h with to form LPEA/0.25GO, LPEA/0.50GO, and LPEA/0.75GO. The nanocomposites were placed undisturbed for 14 days to assure that no phase separation, agglomeration, or abnormal viscosity changes occurred (11).

Preparation of LPEA and LPEA/GO nanocomposite coatings
Before the application of coating material on the panels, surface preparation of the panels (composition: Fe, 99.51%; Mn, 0.34%; C, 0.10%; and P, 0.05%) of standard sizes was carried out for deburring and refinishing the panels with silicon carbide paper and degreasing with methanol and acetone. LPEA and LPEA/GO were diluted with 40% (w/w) toluene and were applied by brush on panels of standard sizes (70 mm × 25 mm × 1 mm) for the evaluation of their physico-mechanical performance, gloss measurements, and corrosion tests in 3.5% (w/w) NaCl medium. For SEM analysis, another set of circular panels (diameter 1 cm, thickness 150 μm) was prepared. To optimize the baking temperature and time, the coated panels were placed in hot air oven for different temperatures and time periods. The most adequate curing temperature and time were found as 150°C for 40 min (LPEA) and 140°C for 40 min (LPEA/GO).

Characterization
The structural elucidation of LPEA was carried out by FTIR (FTIR spectrophotometer; Spectrum 100, Perkin Elmer Cetus Instrument, Norwalk, CT, USA) and nuclear magnetic resonance (NMR) ( 1 H NMR and 13 C NMR; JEOL DPX400MHz, Japan) using deuterated chloroform and dimethyl sulfoxide as solvents and tetramethylsilane as internal standard. For corrosion resistance test, an exposed surface area of 1.0 cm 2 was fixed by PortHoles electrochemical sample mask, with Pt electrode as counter electrode, and 3 M KCl filled silver electrode as reference electrode (Auto lab Potentiostat/galvanostat, PGSTAT204-FRA32, with NOVA 2.1.6 software; Metrohom Autolab B.V. Kanaalweg 29-G, 3526 KM, Utrecht, Switzerland), while the specimens were attended as working electrode.

Results and discussion
The synthesis of LPEA was carried out without any solvent and at reduced temperature compared to other VO-based PEA resins (4,16). In solvent-less synthesis, the reaction temperatures are reduced due to most favorable kinetics that allows for complete conversions, often without catalysis (17). The hydroxyl functional group of LOAD reacted with the carboxylic functional group of MA, by esterification reaction producing LPEA (Scheme 1). LPEA on dispersion of GO produced LPEA/GO nanocomposite. Both LPEA and LPEA/GO rendered mechanically strong and chemically resistant coatings.

Morphology
SEM micrograph (Figure 4a) of GO showed wrinkled sheetlike structure due to exfoliation and restacking, as reported previously (11,18,19). The nanocomposite morphology has been given in Figure 4b. SEM micrograph showed the presence of GO sheets in the matrix. LPEA/0.05GO nanocomposite showed   non-uniform, crumpled surface as a result of dispersion of GO. However, no pin-holes or cracks were evident. EDX (Figure 4c) peaks due to C (58.31%), N (8.39%), and O (33.24%) were evident and confirmed the presence of GO in LPEA nanocomposite. TEM image of GO ( Figure 5) showed crumpled edges and, in some regions, sharp edges were evident. The dark regions were also visible for the piles of GO sheets. LPEA/0.5GO showed dark contrast, due to the LPEA layer covering the GO sheets.

Coating properties 4.3.1 Physico-mechanical properties
The coatings were prepared at temperature 140-150°C for 40 min; the nanocomposite coatings showed lower drying temperature compared to the plain LPEA coatings. The coatings obtained were tough and glossy with uniform   (Table 1). However, at higher loading of GO, these characteristics displayed a deteriorating trend. At higher amount of GO, agglomerates were formed, and viscosity increased abnormally and deteriorated the overall performance of coatings. The contact angle measurements ( Figure 6) showed that the contact angle increased from 71°in LPEA to 78°i n LPEA/GO. This indicated a slight improvement in hydrophobicity after the inclusion of GO, which also supported better corrosion protection performance of the nanocomposite coating.

Corrosion resistance performanceelectrochemical studies (EIS)
For LPEA/0.5GO, Nyquist plots were obtained for various immersion times (1,3,6,9,12,15, and 18 days) in 3.5 wt% NaCl solution. These plots displayed Rs, electrolyte resistance; Cc, coating capacitance; and Rc, coating resistance. As evident (Table 2), with an increase in the number of days of immersion of coated panels in the saline medium, Rs is decreased (645-554 Ω·cm 2 ) while Rc is increased (1.22-2.62 MΩ·cm 2 ), as a consequence of coating surface impairment. As the coated panels spend more time in a corrosive saline medium, their surface becomes weaker with ease of diffusion by corrosive ions. In the case of coatings that are not corrosion resistant, a sharp decrease in Rs might be witnessed with the passage of time; however, in the present case, a sluggish decrease is observed. Rs reached its least value after 18 days of exposure. GO provided barrier protection against the diffusion of corrosive ions and prevented the corrosive ions to reach the metal surface and deteriorate it. Coating performance also depended upon the distribution of GO sheets in matrix and their method of dispersion (21). Figure 7 shows that the impedance value decreased as the days of immersion in the medium increased. The phase angle value for 1-day immersion was 95°, decreased to 90°for 6-day immersion,  and remained unaffected at 90°even up to 18-day immersion. Thus, the pronounced deterioration occurred up to 6 days of exposure to the corrosive media. The coated panel of LPEA/0.5GO was immersed in 3.5 wt% NaCl and was subjected to SEM analysis (Figure 4d). The panel was found to be unaffected after immersion in the medium for 18 days. The salt deposition was evident on the panel surface while there were no cracks and no pore formation visible under SEM.

TGA
DSC thermogram (Figure 8) of LPEA and LPEA/0.50GO showed an endotherm from 365 to 475°C, respectively, followed by an exothermic event. TGA thermogram (Figure 9a) of LPEA showed two steps and LPEA/0.50GO depicted somewhat a single-step degradation pattern, which is also evident in the DTG thermogram ( Figure 9b). 5 wt% loss had occurred until 258°C while the rest 75 wt% decomposition had occurred until 474°C (LPEA) and 490°C (LPEA/0.50GO). Among the nanocomposites, LPEA/0.50GO showed higher thermal stability, compared to LPEA, while at 0.75 wt% inclusion of GO, it had deteriorated. This conformed with the fact that higher inclusion of GO leads to impaired physico-mechanical properties, reduced gloss, and lowered thermal stability. Thus, improved thermal stability could be achieved until the dispersion of 0.50 wt% GO in LPEA/0.50GO, due to fine dispersion of GO and good interaction with the matrix (11). Beyond >0.50 wt%, the excess GO caused agglomeration and the thermal stability deteriorated (20).

Conclusion
This article described the synthesis of L. leucocephala oilbased poly malate-amide/GO nanocomposite coating material. The synthesis was carried out without any organic solvent, which resulted in reduced reaction temperature without catalysis. The coatings obtained were flexible, scratch resistant, impact resistant, and thermally stable, with the dispersion of GO up to 0.5 wt% loading of GO. Above 0.75 wt% inclusion of GO, the coatings lost their integrity and homogeneity, and overall performance was impaired. The coatings showed good corrosion protection performance in the saline medium. The approach is simple and environmentally safe and provides a value-addition pathway for a locally available plant crop.