Mechanically Strong, Hydrophobic, Antimicrobial, and Corrosion Protective Polyesteramide Nanocomposite Coatings from Leucaena leucocephala Oil: A Sustainable Resource.

The aim of this research work is to develop polyesteramide [LMPEA] nanocomposite coating material [LMPEA/Ag] using N,N-bis(2-hydroxyethyl) fatty amide obtained from non-edible Leucaena leucocephala [LL] seed oil [LLO], and maleic anhydride, reinforced with silver nanoparticles [SNPs], biosynthesized in Leucaena leucocephala leaf extract. UV, XRD, TEM, and particle size analyses confirmed the biosynthesis of NP (37.55 nm). FTIR and NMR established the structure of LMPEA formed by esterification reaction, without any solvent/diluent. Coatings were mechanically strong, well adherent to substrate, flexibility retentive, hydrophobic, and antimicrobial as evident from good scratch hardness (2-3 kg), impact resistance (150 lb per inch), bend test (1/8 inch), high water contact angle measurement value (109°) relative to pristine LMPEA coating (89°), and broad-spectrum antimicrobial behavior against MRSA, P. aeruginosa, E. coli, A. baumannii, and C. albicans. LMPEA and LMPEA/Ag exhibited high corrosion protection efficiencies, 99.81% and 99.94%, respectively, in (3.5% w/v) NaCl solution for 20 days and safe usage up to 200 °C. The synthesized nanocomposite coatings provide an alternate pathway for utilization of non-edible Leucaena leucocephala seed oil through a safer chemical synthesis route, without the use/generation of any harmful solvent/toxic products, adopting "Green Chemistry" principles.

Introduction

Paints and coatings are applied on surfaces such as metals, walls, equipments, and others to enhance durability, improve performance such as adhesion to substrate, scratch resistance, corrosion resistance, and hydrophobicity, and impart decorative finish. They are composed of organic and inorganic resins, solvents, diluents, and other components that are often petro-based, costly, toxic, and hazardous to the environment. The environmental challenges, stringent regulations, dwindling fossil fuels, associated health concerns, and escalating prices of petro-based chemicals are some of the challenges faced by paints and coatings industries today. Thus, it becomes imperative to substitute renewable and cheaper, environmentally friendly, and chemically rich alternatives such as vegetable oils (VOs) in this area through a benign route as a solution to this gigantic problem, also keeping in mind the performance and durability of the end products, i.e., paints/coatings, for targeted applications. There are several oils that are non-edible, non-medicinal, and chemically rich in terms of their functional attributes, amenable to chemical transformations, however being underutilized to date. Keeping this in mind, the main target of this research work is to prepare polyesteramide [PEA] nanocomposite corrosion protective coatings from a non-edible oil, that is, Leucaena leucocephala seed oil, through a simple, eco-friendly route.[1−3]Leucaena leucocephala belongs to family Fabaceae and sub-family Mimossoideae. Native to Central America and Southern Mexico, it is found in most of the tropical and sub-tropical areas of the world. In India, it is a well-known fodder plant in villages of Tamil Nadu. The tree is draught-resistant and well adapted to a wide range of soils and has found several applications as firewood, animal forage, construction poles, shades in plantations, erosion control, and several other uses. The parts of tree are used as food or salad in Indonesia, Thailand, and India. Different parts of plant are also used as folk medicine.[4]

The seed oil of Leucaena leucocephala [LLO] is rich in linoleic, oleic, palmitic, and stearic acid, which comprise more than 92% of total fatty acids of LLO, and also has a higher content of tocopherols.[5] LLO has also shown antibacterial and antifungal properties.[6] The leaves of Leucaena leucocephala plant [LL] are rich in phytochemicals such as alkaloids, flavonoids, tannins, and cardic glycosides and are useful as antioxidant, antimicrobial, anticancer, and antiparasitic, as pesticide and nematicide, and in other applications.[7,8] Leaves’ extract of Leucaena leucocephala [LLE] has been utilized in the biosynthesis of nanoparticles by phytoreduction due to its phytochemical constituents.[9,10]

PEAs are amide-modified alkyds, with both ester and amide moieties in their backbone. They have been prepared from linseed, karanj, castor, mahua, and several other VOs with applications as protective coatings. The starting materials used in the synthesis of a PEA resin are as follows: an acid/anhydride and VO amide diol. VO amide diols are obtained by base-catalyzed amidation reaction of VO, triglycerides of fatty acids, with diethanolamine in the presence of sodium methoxide. A VO amide diol bears an amide functional group flanked by two −CH2–CH2–OH groups on each side, and attached with carbonyl is one hydrocarbon chain of VO, saturated or unsaturated, depending on the fatty acid composition of the starting VO. It reacts with an anhydride/acid via esterification reaction producing a PEA resin.[11]

VO-based PEAs are generally synthesized at higher temperatures (above 150 °C) in the presence of solvents. Such coatings release solvents on drying and thus pose a threat to the environment. Often, PEAs lack the desirable mechanical strength and chemical resistance required for their application as protective coatings.[11,12] Therefore, they are modified as nanocomposites by reinforcement with nanoparticles [NP] such as nanoclay, silver, zinc oxide nanoparticles, and others[1,11] (Table ). The introduction of NP improves adhesion and mechanical strength of coatings through matrix–NP interactions. It also enhances corrosion resistance performance of coatings in different corrosive media by providing a tortuous path to corrodents as well as enhancing hydrophobicity of coatings. While hydrophilic surfaces attract water molecules and provide easy passage of corrosive media, hydrophobic surfaces act as a barrier, not allowing access to corrosive ions.[13,14] Thus, to overcome these drawbacks, in this manuscript, we report the synthesis of PEA from LLO [LMPEA], without using any solvent, and PEA coatings obtained therefrom have been reinforced with silver nanoparticles [SNPs], biosynthesized using LLE. LLO-based PEA was obtained by esterification reaction between LLO-derived amide diol, prepared by sodium methoxide-catalyzed amidation reaction of LLO, and maleic anhydride. The approach follows “Green Chemistry” principles,[24] which are as follows:

Table 1

Comparison Studies of Polyesteramide/Nanocomposite Coatings

VO	materials	nanofiller	physicomechanical properties	ref.
Mesua ferrea L. seed oil, polyester	phthalic anhydride, nanoclay, poly(amido amine)	clay	gloss (117), scratch hardness (11 kg), curing time, 120° (1.75)	(15)
linseed oil, alkyd	adipic acid, manganese octoate, cobalt octoate, lead octoate	Cu2O	T bending <5, impact >18 J, cross- hatch (5B)	(16)
linseed oil, polyesteramide	phthalic anhydride, bake coating	OMMT clay	gloss (100), scratch hardness, (4 kg), bending (1/8 inch) pass	(17)
palm oil, polyesteramide	oxalic acid, epoxy resin	ZnO,Al2O3, MWCNT	gloss (94), scratch hardness, (3.1 kg), pencil hardness (4H), bending (3 mm) pass	(18)
castor oil, polyesteramide urethane	terephthalic acid, toluene-2,4-diisocyanate	TiO2	gloss (91), scratch hardness, (1.7 kg), pencil hardness (4H), bending(1/8 inch) pass, corrosion test in HCl (IE-99), tap water (IE-98), NaCl (IE-99), NaOH (IE-94%)	(19)
tung oil, polyesteramide	pyromellitic dianhydride, toluene-2,4-diisocyanate	Ce	vertical burning test	(20)
sunflower oil, alkyd	sebacic acid, drying agents (octoates of cobalt, calcium and zirconium	graphene oxide	impact resistance <18, T bending <5, adhesion (cross-hatch) 5B	(21)
soya oil, polyesteramide	phthalic anhydride, poly(melamine-co-formaldehyde)isobutylated solution	reduced graphene oxide	scratch hardness(11.5 kg) and impact resistance 150 lb per inch) and adhesion (cross-hatch 0% peeling), electrochemical anticorrosive properties (impedance modulus |Z| B 107 ohm cm2 and phase angles B 85.61	(13)
sunflower oil, alkyd	sebacic acid, manganese octoate	ZnO	Impact resistance>18, T bending <5, adhesion(cross hatch) pass	(22)
Jatropha curcas oil, alkyd	phthalic and maleic anhydride, Co octoate	graphite	tensile strength (MPa) 43, elongation (%), 20 when filler loading (5 wt %)	(23)
Leucaena leucocephala, seed oil, polyesteramide	maleic anhydride	silver (biosynthesized in Leucaena leucocephala leaf extract)	described in the manuscript	present work

Prevention: no side/waste product is formed, thus eliminating further treatment or cleaning-up step

Less Hazardous Chemical Syntheses: using/producing materials that are not toxic to the health and environment

Safer Solvents and Auxiliaries: by solvent-less synthesis

Use of Renewable Feedstocks: using non-edible LLO

Reduce Derivatives: devoid of derivatization step

Inherently Safer Chemistry for Accident Prevention:[25,26] safe approach.

The properties of VOs and polymers derived therefrom are governed by the fatty acid composition of parent oil. VOs differ in their fatty acid composition depending on the climate and soil conditions.[27,28] Thus, direct comparison with reference from the literature is not possible as these oils do not have an exact fatty acid composition. Also, modifications have been done with different curing agents (acids, anhydrides, and isocyanates) and modifiers/nanofillers (silver , clay, zinc oxide, and others) as can be seen in the table, which summarizes comparison studies of alkyd and PEA (amide-modified alkyd) coatings. However, it can be inferred from the table that the said LMPEA/Ag nanocomposite coatings compete well with their other counterparts in terms of their coating properties.

Our findings reveal that the approach provides an alternate route toward utilization of LLO as antimicrobial corrosion protective coatings. It also unfurls opportunities for other contemporary non-edible, non-medicinal oils that are unexplored, under/unutilized, and go as national waste in spite of their rich functional attributes. Such oils can be chemically transformed into corrosion protective coatings, thus adding value to a waste, but “green” material.

Methods and Materials

Maleic anhydride [MA] (Riedel-de Haen, Germany), silver nitrate, sodium metal, methanol, toluene (BDH Chemicals Ltd. Poole, England), and diethanolamine (WinLab, UK) were used as received. Ripe legumes and fresh leaves from Leucaena leucocephala plant (University Campus, King Saud University) were collected in the month of March 2019. Seeds were separated and powdered, and oil was extracted from powdered seeds through a Soxhlet apparatus using petroleum ether as solvent. The fatty acid composition (linoleic acid 52%, oleic acid 21%, palmitic acid 14%, and stearic acid 6%) of LLO was determined using methyl ester on gas chromatography with an FID detector.[5] Deionized water [DW] was used throughout the experimental steps.

Preparation of LLE and Biosynthesis of SNPs Using LLE

Leaves were plucked, cleaned with DW, and were dried to remove water that adhered to leaves’ surface. They were then cut into small pieces, and a weighed amount of cut leaves was boiled in measured quantity of DW (10% w/v) for 5 min. The solution was cooled to room temperature and was then filtered through Whatman filter paper. Leucaena leucocephala leaves’ extract [LLE] thus obtained was refrigerated for use later. A 0.01 M silver nitrate solution was prepared in DW.

LLE (1 mL) was dropwise added to a 0.01 M silver nitrate solution (9 mL) in a flask, and the contents were left to react under ambient conditions. The solution was visually observed for any color changes after the addition of LLE to silver nitrate solution. The color of the solution changed from colorless to dark brown, as observed by the naked eye, and this indicated the biosynthesis of SNPs by phytoreduction, periodically monitored by UV–Vis technique. The synthesis of SNPs was accomplished in 125 min as confirmed by UV spectroscopy.[9,10,29]

Synthesis of N,N-Bis(2-hydroxyethyl) Leucaena Oil Fatty Amide (HELuA)

HELuA was prepared according to a previously reported method.[30]

Synthesis of LMPEA and LMPEA/Ag Nanocomposite

HELuA (0.02 mol) was placed in a four-neck conical flask equipped with a nitrogen inlet tube, thermometer, and mechanical stirrer, at a temperature of 50 °C. MA (0.18 mol) was added in small portions, and the contents were mixed thoroughly. The reaction temperature was then increased to 120 °C. The reaction was monitored by acid value determination and by recording FTIR spectra, at regular intervals, until the formation of the final product. LMPEA was cooled and then subjected to structural determination by spectral analysis. To the flask containing LMPEA was added SNP solution. The contents were thoroughly mixed over a magnetic stirrer for 60 min to obtain LMPEA/Ag nanocomposite.

Preparation of LMPEA/Ag Nanocomposite Coatings

LMPA and LMPEA/Ag were diluted with xylene (80% w/v) and were applied by brush on mild steel panels of required size and shape (70 mm × 25 mm × 1 mm rectangular panels for physicomechanical and gloss measurements of coatings; 25 mm × 25 mm × 1 mm square shaped panels for corrosion tests; and circular panels, diameter 1 cm, thickness 150 μm for SEM analysis). Before coating application, the panels were thoroughly washed with double distilled water followed by methanol and acetone for degreasing and then dried properly. LMPEA and LMPEA/Ag nanocomposites were applied on these degreased and dried panels, and the coated panels were placed in a hot air oven for drying at different temperatures for different time periods. The ideal curing/drying temperature and time period was found as 200 °C, 15 min for LMPEA and 180 °C, 15 min for LMPEA/Ag nanocomposite.

Characterization

FTIR spectra were recorded on an FTIR spectrophotometer (Spectrum 100, Perkin Elmer Cetus Instrument, Norwalk, CT, USA). A UV–vis spectrophotometer (Lambda 35) Perkin Elmer lambda, Waltham, MA, USA) was used for the optical measurements. X-ray analysis was performed on an X-ray powder diffractometer ULTIMA IV, RIGAKU Inc., Japan. The particle size distribution and surface charge of SNPs were determined using dynamic light scattering (DLS) analyzer (Zetasizer Nano, ZSP, Malvern Instruments Ltd., U.K.). A FE-SEM (JSM 7600F JEOL, Japan) and EDX (Oxford) attached to the SEM were employed to study the composition and morphology of the film. The sample was sputter coated with Pt prior to observation under the microscope, and the microscope was operated at 15 kV. TEM was observed by a transmission electron microscope (JEM-2100F) JEOL, Japan. 1H NMR and 13C NMR spectra (Jeol DPX400MHz, Japan) were recorded using deuterated dimethyl sulfoxide (DMSO) as a solvent and tetramethylsilane (TMS) as an internal standard. Thermal analysis of the samples was carried out by TGA (Mettler Toledo AG, Analytical CH-8603, Schwerzenbach, Switzerland) under nitrogen atmosphere and at heating rate of 10 °C/min. Acid (ASTM D555-61) and hydroxyl values (ASTM D1957-86) of HELuA, LMPEA, and LMPEA/Ag nanocomposite were determined by standard methods. Coating properties were assessed by the following test methods: refractive index (ASTM D1218), scratch hardness test (BS 3900), cross-hatch adhesion test (ASTM D3359-02), pencil hardness test (ASTM D3363-05), impact test (IS 101 part 5 s–1,1988), flexibility/bending test (ASTM D3281-84), gloss (by a gloss meter, model: KSJ MG6-F1, KSJ Photoelectrical Instruments Co., Ltd. Quanzhou, China), thickness (ASTM D 1186-B), and hydrophobicity (from contact angle measurements by a CAM 200 Attention goniometer). The coated mild steel specimens attended as a working electrode; an exposed surface area of 1.0 cm2 was fixed by PortHoles electrochemical sample mask, platinum electrode as a counter electrode, and KCl filled silver electrode as a reference electrode with Autolab potentiostat/galvanostat, PGSTAT204-FRA32, with NOVA 2.0 software (Metrohom Autolab B.V. Kanaalweg 29-G, 3526 KM, Utrecht, The Netherlands). Each corrosion test was performed in triplicate for reproducibility purposes.

Bacterial Strains and Growth Conditions

All tests were assessed for their antimicrobial potential against both Gram-positive and Gram-negative bacteria as well as fungus Candida albicans. Methicillin-resistant Staphylococcus aureus (MRSA) (ATCC 43300), Pseudomonas aeruginosa (ATCC 27853), Escherichia coli (ATCC 35218), Acinetobacter baumannii (ATCC BAA747), and C. albicans (ATCC 12013) were used in the present investigation. All bacteria were grown in nutrient broth, while the Candida strain was cultured in potato dextrose broth. Susceptibility of the test bacteria to compounds was investigated using the agar-well diffusion method. Overnight grown cultures (100 μL) were spread onto Mueller-Hinton agar plates. Wells of 8 mm were punched into the plates, and 100 μL of the test compounds was loaded and incubated at 37 °C. Halo zones around the wells were measured to determine the antimicrobial activity.

MIC of test compounds was determined against each of the bacteria and fungus using the micro-broth dilution method with concentration tested ranging from 256 to 0.0625 μg/mL. MBC against all test pathogens was assessed using a macro-broth dilution method. Twofold dilutions of varying concentrations (256–0.0625 μg/mL) were added, and the tubes were incubated. Overnight grown cultures of bacteria and fungi from each treated concentration were streaked on nutrient agar plates to determine MBC.[31]

Results and Discussion

Biosynthesis of SNPs in LLE and Synthesis of LMPEA and LMPEA/Ag

As LLE was added to the silver nitrate solution, color change became evident, which was indicative of the biosynthesis of SNPs through phytoreduction by LLE constituents as also confirmed by UV, XRD, TEM, and particle size analyses, respectively. Biosynthesis of SNPs was studied by UV spectra recorded at regular intervals of time (Figure a), which showed an absorbance peak at 428 nm due to surface plasmon resonance of nanosilver after 125 min of phytoreduction, providing evidence for the formation of SNPs. XRD patterns (Figure b) showed peaks at 2 theta values 38.15, 44.50, 64.66, and 77.70 corresponding to (111), (200), (220), and (311) planes, respectively, according to JCPDS file no. 04-0783. The size of silver nanoparticles calculated using the Debye–Scherrer equation[32] peaks (2θ) at 38.15°, 44.50°, 64.66°, and 77.70° are 35.15, 28.47, 27.29, and 59.23 nm and an average size of 37.55 nm. The size and shape of SNPs were also studied by TEM (Figure c) that revealed the presence of spherical silver nanoparticles of size 10–79 nm. All the particles showed distinct boundaries, while no agglomeration was visible. The size distribution and zeta potential of SNPs observed by DLS analysis (Figure a,b) revealed that SNPs have an average diameter of 70 nm and the corresponding average zeta potential of −24.20 mV. The negative potential value suggested good colloidal nature, high dispersity, and long-term stability of SNPs.[32] No solvent, surfactant, stabilizer, capping agent was used in SNP formation. LLE itself served as a reducing, stabilization/capping agent due to its phytochemical constituents.[32]

Figure 1

(a) UV–vis absorption spectra recorded at regular intervals of time evidence for the formation of SNPs, (b) XRD pattern, (c) TEM image, and (d) HR-TEM images and a single Ag nanocrystal showing the separation between individual lattice planes of as-synthesized SNPs. The corresponding SAED patterns are shown in (e).

Figure 2

(a) Zeta size and (b) zeta potential of biosynthesized SNPs in deionized water at room temperature.

HELuA was prepared by base-catalyzed amidation reaction of LLO with diethanolamine (see this scheme in the Supporting Information). It was then reacted with MA by esterification reaction between hydroxyl groups of HELuA and anhydride groups of MA to form LMPEA (Scheme ). The synthesis of LMPEA was carried out without using any organic solvent and at temperature and time period much lower than that typical for VO-based PEA synthesis.[11,12] Advantage was taken of fluidity characteristic of HELuA, which served as a diluent, obviating the use of any solvent, during the synthesis.[12,33,34] LMPEA was loaded with biosynthesized SNPs producing an LMPEA/Ag coating material.

Scheme 1

Synthesis of LMPEA and LMPEA/Ag Nanocomposite

Spectral Analysis

FTIR, 1H NMR, and13C NMR spectra of HELuA and LMPEA confirmed the formation of PEA resin. The important absorption bands and peak values have been mentioned below.

FTIR (υ, cm–1)

In HELuA (Figure ), absorption bands at 3348 (−OH), 3008 (C=C), 2853, 2924 (−CH3, −CH2 stretching), 1619 (>C=O amide), and 1463–1364 (−CH3, −CH2 bending) are observed. FTIR spectrum of LMPEA (Figure ) shows absorption bands at 3376 (−OH), 3008 (C=C), 2853, 2924 (−CH3, −CH2 stretching), 1732 (>C=O ester), 1619 (>C=O amide), 1462–1360 (−CH3, −CH2 bending), and 1259–1065 (−(C=O)–O–C), −C–O– as typical for the functional groups present in VO-based PEA. The absorption band of OH present at 3348 cm–1 in HELuA appears suppressed and at a lower absorption value compared to LMPEA where the −OH band is observed at 3376 cm–1 and an additional (ester) band appears at 1732 cm–1. The suppression of the −OH absorption band and the appearance of the ester absorption band confirm (i) the consumption of −OH and, at the same time, (ii) the introduction of the ester band by esterification reaction between −OH of HELuA and −COOH of MA, thus confirming the formation of LMPEA. FTIR spectrum of LMPEA/Ag (Figure ) shows that the absorption bands at 3392 (OH), 1735 (>C=O ester), 1624 (>C=O amide), 1465, 1377 (−CH3, −CH2 bending), and 1295 cm–1 typical for (−(C=O)–O–C), −C–O– have increased in intensity and also appear at higher absorption band frequency values than their counterpart functional groups in LMPEA. This is because LMPEA/Ag also reveals, on the surface of silver nanoparticles, the presence of functional groups from biomolecules of LLE, responsible for bioreduction of silver nitrate, Ag+, and stabilization of silver nanoparticles.[32] Thus, by inclusion of SNPs in the PEA matrix, these biomolecules are also introduced within the PEA matrix and act as capping/stabilizing agents for SNPs.

Figure 3

FTIR spectra of HELuA, LMPEA, and LMPEA/Ag nanocomposite.

1H NMR (DMSO, δ, ppm)

In HELuA (Figure S1), peaks appear at 0.839 (−CH3); 1.088–1.294 (−CH2); 1.448 (>N–CO–CH2–CH2); 1.90–2.016 (−CH2–CH2—CH=); 2.264–2.553 (>N–CO–CH2−); 2.564–2.727 (=CH—CH2—CH=); 3.261–3.494 (>CH2–CH2–OH); 5.254–5.348 (−CH2–OH, —CH=CH—). Additional peaks in LMPEA (Figure S1) are recorded at 4.058–4.198 (>N–CH2–CH2–O–CO−); 4.209–4.225 (>N–CH2–CH2–O–CO−); 5.989 (—CH=CH—COOH); 6.488 (—CH=CH—COOH), confirming the introduction of MA moiety in PEA by esterification reaction with MA.

13C NMR (DMSO, δ, ppm)

In HELuA (Figure S2), peaks appear at 13.928–15.177 (−CH3); 22.035–22.159 (−CH2–CH3); 24.945–28.912 (−CH2−); 30.963–31.373 (−CH2–CH2–CH3) 32.279 (=CH—CH2—CH=); 48.371–51.680 (>N–CH2–CH2–OH); 58.939–60.427 (−CH2–OH); 127.758–129.742 (—CH=CH—); and 172 (>C=O, amide). Additional peaks in LMPEA (Figure S2) at 45.662 (>CH2–CH2–OCOO−); 58.233–59.206 (−CH2–OCOO−); 136.123 (—CH=CH—COO); 167.304 (CO, ester), confirm the introduction of MA moiety in LMPEA.

The nanocomposite was soluble in xylene, toluene, benzene, butanol, n-hexane, tetrahydrofuran, acetonitrile, ethanol, methanol, chloroform, carbon tetrachloride, dimethyl sulfoxide, and dimethyl formamide, sparingly soluble in ethyl methyl ketone, 1,4-dioxane, ethyl acetate, formamide, and insoluble in water.

Coating Properties

Physicomechanical Characterization

Coatings of LMPEA (thickness 200 μm) were found to cure/dry at a lower temperature and time duration (180 °C, 15 min) compared to plain PEA coatings due to the introduction of MA, which is known to facilitate the drying characteristic of PEA coatings,[33,34] while the LMPEA/Ag nanocomposite cured at a still lower temperature compared to LMPEA. The scratch hardness of LMPEA was found to be 2 kg, while LMPEA/Ag nanocomposite showed scratch hardness value of 3 kg. Both LMPEA and LMPEA/Ag nanocomposite coatings passed bend/flexibility test and impact resistance tests, exhibiting good flexibility and adhesion of coatings to the substrate. Pencil hardness test values were found as 4H and 5H, while cross-hatch adhesion test values obtained were 98% and 100%, respectively, for LMPEA and LMPEA/Ag nanocomposite. LMPEA backbone consists of −OH (both alcoholic and carboxylic), amide −N, and >C=O, double bonds (saturation or unsaturation level depending on the fatty acid composition of parent VO), and long hydrocarbon chains. The polar groups of LMPEA and good matrix–nanofiller interactions in LMPEA/Ag improve adhesion of coatings to the metal substrate and strengthen impact resistance of coatings, while cross-linking through polymerization at hydrocarbon chains improves scratch hardness. The dangling hydrocarbon chains, C-18 long, also augment the flexibility of LMPEA coatings.[11,35] The contact angle value of LMPEA/Ag nanocomposite coating (Figure ) was found to be much higher (109.89 and 107.25) compared to that of LMPEA (89.41 and 89.40) because of the introduction of a nanofiller in nanocomposite coating, which provides nano/micro-level roughness, due to entrapped air bubbles, which help to maintain the round shape of the water droplet by minimizing liquid–solid contact area.[21,22,35−37]

Figure 4

Contact angle of LMPEA and LMPEA/Ag nanocomposite.

Figure a–c shows the micrograph for LMPEA/Ag nanocomposite film and bare mild steel (Figure S3). The absence of pinholes and cracks was evident from FE-SEM pictures, validating the highly uniform and homogeneous nature of the film (Figure a). Figure b shows uniform distribution of SNPs in LMPEA, and nanosized compact structure grains are seen throughout the film. A high-resolution image (Figure c) shows small globule-like structures with homogeneous distribution of SNPs embedded in PEA matrix.

Figure 5

FE-SEM image of (a) LMPEA/Ag nanocomposite coated MS (b), (c) high-resolution image of LMPEA/Ag nanocomposite, and (d) EDX profile of LMPEA/Ag nanocomposite; inset table indicates the elemental composition.

Measured EDX profile and elemental mapping for LMPEA/Ag nanocomposite film are exhibited in Figures d and 6, respectively. In Figure d, the energy peaks concerning Ag, C, N, and O are clearly visible, which confirm the Ag element present in LMPEA/Ag nanocomposite thin film, and their content was found to be ∼1.93, 49.49, 21.22, and 48.36 wt %, respectively. The presence of Ag (1.93%) in LMPEA confirmed the inclusion of SNPs in LMPEA/Ag nanocomposite. Additionally, the color mapping for each element was represented as Ag (yellow), C (red), N (pink), and O (green) in LMPEA/Ag nanocomposite thin film, displayed in Figure . From these elemental mapping images corresponding to Ag, C, N, and O, it can be seen that spreading of all elements throughout the film is immense. The absence of any impurities was evident from the mapping profile pictures, validating the highly uniform and homogeneous nature of the film. Furthermore, a line scan profile of the film was performed from the captured electron image with different colors corresponding to each elements’ presence in the film, which is displayed in Figure .

Figure 6

Elemental mapping images Ag (1.93%), N (21.22%), O (48.36%), and C (28.49%) and line scan profile of the LMPEA/Ag nanocomposite film.

Corrosion Study

Corrosion resistance performance of LMPEA, LMPEA/Ag nanocomposite, and bare mild steel (MS) panels was scrutinized electrochemically using potentiodynamic polarization (PDP) for a period of 20 days in 3.5% w/v NaCl solutions at room temperature (Figure a,b). Corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), linear polarization resistance (LPR), and corrosion rate (CR) were derived from Tafel plots as furnished in Table . Corrosion protection efficiency (PE%) of LMPEA and LMPEA/Ag coated MS panels was also evaluated, and the values are summarized in Table .[38] The measured value of corrosion current (Icorr) for the bare MS surface is 3.509 × 10–05 A, whereas Icorr values of LMPEA and LMPEA/Ag nanocomposite after 20 days immersion is 6.025 × 10–09 and 2.288 × 10–08 A, respectively. CR increases and LPR of the coated panel decreases during immersion periods, but as compared to bare MS, corrosion rate decreases and LPR increases. On comparing the value of Icorr of bare MS with LMPEA and LMPEA/Ag nanocomposite coated panels, it is clearly observed that the corrosion current of LMPEA and LMPEA/Ag nanocomposite coated specimens are almost three to four orders of magnitude less than the bare MS (Table ). The occurrence of a significantly low Icorr for LMPEA and LMPEA/Ag nanocomposite coated specimens indicates the presence of an effective barrier film that inhibits the ingress of electrolyte to the coating and MS interface. Accordingly, the corrosion rate values are also decreased for LMPEA and LMPEA/Ag nanocomposite coated specimens. The bare MS panel is very prone toward corrosion, and the rate of corrosion increases in the presence of chloride ions. The application of LMPEA and LMPEA/Ag nanocomposite coatings offers effective corrosion protection to the MS substrate by forming a good protective layer that separates metal surface from corrosive environments and also due to a tortuous pathway provided to water molecules by the presence of a nanofiller.[13] A higher water contact angle value for nanocomposite coating suggests that the coating has good hydrophobicity rendered by the inclusion of nanofiller, which renders efficient barrier property to the coating surface to hinder the diffusion of corrosive electrolytes, thus conferring superior corrosion resistance.[39] The barrier property of coatings in our study remained unaffected for 20 days period of exposure. Tafel polarization curves, drawn for the coated/MS panel, after 20 days of immersion in 3.5% NaCl solution, clearly show that LMPEA/Ag coating protects the underlying MS surface during aforementioned immersion periods, relatively better than LMPEA.

Figure 7

Tafel plot of LMPEA (a) and LMPEA/Ag nanocomposite (b) in 3.5 wt % NaCl solution at room temperature.

Table 2

Corrosion Parameters for Bare MS, LMPEA, and LMPEA/Ag Nanocomposite Coated MS after Immersion in 3.5% NaCl Solution

immersion time	materials	Ba (V/dec)	Bc (V/dec)	Ecorr (V)	Icorr (A)	CR (mm/y)	LPR (Ω·cm2)	PE (%)
1 h	Bare MS	0.109	0.661	–0.591	3.509 × 10–05	0.408	1196.5
4 days	LMPEA	0.350	0.289	–0.191	3.921 × 10–09	4.556 × 10–05	1.814 × 1007	99.98
8 days	LMPEA	0.307	0.369	–0.212	4.452 × 10–09	5.173 × 10–05	1.619 × 1007	99.98
12 days	LMPEA	0.345	0.338	–0.512	5.054 × 10–09	5.874 × 10–05	1.467 × 1007	99.98
16 days	LMPEA	0.330	0.332	–0.527	5.470 × 10–09	5.708 × 10–05	1.962 × 1007	99.98
20 days	LMPEA	0.324	0.330	–0.561	6.025 × 10–09	6.353 × 10–05	1.407 × 1007	99.81
4 days	LMPEA/Ag	0.361	0.301	0.009	8.946 × 10–09	4.039 × 10–03	7.952 × 1006	99.97
8 days	LMPEA/Ag	0.284	0.342	–0.029	7.146 × 10–09	5.304 × 10–05	8.689 × 1006	99.97
12 days	LMPEA/Ag	0.337	0.350	–0.165	6.288 × 10–09	6.496 × 10–05	5.791 × 1006	99.98
16 days	LMPEA/Ag	0.294	0.278	–0.329	4.288 × 10–08	6.781 × 10–05	5.249 × 1006	99.87
20 days	LMPEA/Ag	0.330	0.324	–0.650	2.288 × 10–08	6.898 × 10–05	3.379 × 1006	99.94

The hydrophobic structure of the coating surface acts as an efficient barrier layer to impede the diffusion of electrolytes, inducing superior corrosion resistance properties. However, the barrier property is usually compromised with the lapse of time, with formation of defects and pores, that act as channels for the percolation of the electrolyte. Under these conditions, a galvanic cell is created underneath the coating and charge transfer reaction takes place at coating/metal interface.[40−42] The situation leads to the initiation of corrosion, followed by propagation of corrosion under the coating.

Thermogravimetric Study

DSC thermogram of LMPEA (Figure S4) reveals that the first endotherm starts from 104 to 295 °C and is centered at 171 °C, while the second endotherm starts from 295 to 348 °C and is centered at 318 °C. In LMPEA/Ag nanocomposite, the first endotherm starts from 150 to 221 °C and is centered at 180 °C, and the second endotherm starts from 223 to 347 °C and is centered at 291 °C.

TGA thermogram (Figure S5) shows that 5 wt % loss in SNPs, LMPEA and LMPEA/Ag occurs at 279 °C, 275 °C, and 215 °C, respectively. 50 wt % loss is observed at 400 °C in LMPEA and 407 °C in LMPEA/Ag, while 25 wt % loss occurs between 300 and 375 °C in LMPEA and LMPEA/Ag nanocomposite, respectively, and at 800 °C in Ag nanoparticles. Thus, the degradation ramp differs in temperature range up to approximately 400 °C, while at a higher degradation temperature, beyond 400 °C, both LMPEA and LMPEA/Ag show a similar degradation pattern.

DTG thermograms (Figure S6) of LMPEA and LMPEA/Ag nanocomposite clearly shows decomposition. In LMPEA/Ag nanocomposite, one endotherm starts from 143 to 241 °C and is centered at 188 °C, corresponding to moisture loss and onset of degradation. Another second endotherm runs from 268 to 515 °C and is centered at 407 °C, corresponding to 50 wt % loss, while in LMPEA, only one endotherm starts from 281 to 515 °C and is centered at 400 °C, corresponding to 50 wt % loss in TGA thermogram due to ester, amide, and hydrocarbon chains, collectively. Good thermal stability can be attributed to homogeneous dispersion of SNPs in LMPEA matrix.

Antimicrobial Activity

Antimicrobial potential of test materials was explored against MRSA, P. aeruginosa, Escherichia coli, A. baumannii, and C. albicans. The zone of inhibition of SNPs, LMPEA, and LMPEA/Ag against each microbial strain is presented in Table . Amongst bacteria, MRSA was found to be the most sensitive with an inhibition zone of 21 mm, while the least was recorded for A. baumannii (15 mm) against LMPEA/Ag. C. albicans showed the highest zone of growth inhibition (24 mm) among all tested strains (Figure . It was interesting to note that the zone of inhibition of LMPEA/Ag was greater against all test strains as compared to SNPs and LMPEA alone. Observed MIC values of LMPEA/Ag against each of the microbial strains are presented in Table . The MIC value against MRSA was found to be 16 μg/mL. E. coli was found to be the most sensitive, displaying an MIC of 8 μg/mL, while A. baumannii was the most resistant among test pathogens with an MIC of 64 μg/mL. The MIC value of the LMPEA/Ag against C. albicans was 16 μg/mL. MBC is the lowest concentration of any antimicrobial agent that kills 100% of the microbial population, and no growth is shown upon streaking on agar plates. Recorded MBC values were 16, 64, 16, 64, and 32 μg/mL against MRSA, P. aeruginosa, Escherichia coli, A. baumannii, and C. albicans, respectively. Increased antimicrobial activity the LMPEA/Ag could be credited to their large surface area, which facilitates enhanced surface contact with microbes. Moreover, the synergistic action of LMPEA and SNPs could also be responsible for the elevated antibacterial and antifungal activity. Microbial action of SNPs is either by the release of Ag+ ions in the microbial cells and/or by generation of intracellular ROS that causes oxidative stress and eventually cell death.[43] Our results on the antimicrobial action of LMPEA/Ag nanocomposite find support from the observations made with various nanocomposites of silver.[44]

Table 3

Antimicrobial Activity Screening Data for Ag NPs, LMPEA, and LMPEA/Ag Nanocomposite

	microorganisms (zone of inhibition in mm)
sample code (concentration)	MRSA	P. aeruginosa	E. coli	A. baumannii	C. albicans
Ag (100 μg/mL)	15	15	16	ND	13
LMPEA (100 μg/mL)	16	17	16	11	15
LMPEA/Ag (100 μg/mL)	21	19	20	15	24
ampicillin (positive control)	26	22	25	21	30 (nystatin)
LMPEA/Aga	16/16a	32/64a	8/16a	64/64a	16/32a

(MIC/MBC values in μg/mL).

Figure 8

Antimicrobial activity of LMPEA/Ag against test pathogens: (a) MRSA, (b) P. aeruginosa, (c) E. coli, (d) A. baumannii, and (e) C. albicans.

Conclusion

The manuscript described the biosynthesis of SNPs using LLE and inclusion of these biosynthesized SNPs in LLO-based polyesteramide to prepare nanocomposite coatings. Good matrix–nanofiller interactions are supported by improved scratch hardness values, hydrophobicity, corrosion protection efficiency, and antibacterial performance of the nanocomposite over pristine LMPEA. LMPEA/Ag nanocomposite coatings showed good corrosion resistance in 3.5% NaCl solution up to 20 days immersion, good thermal stability, and the highest zone of growth inhibition (24 mm) against C. albicans, among all tested strains. Thus, these coatings may be employed as scratch-resistant and impact-resistant, thermally stable, hydrophobic antimicrobial coatings in future.