Bio-Based Alkyd–Polyesteramide–Polyurethane Coatings from Castor, Neem, and Karanja Oils with Inherent Antimicrobial Properties for Enhanced Hygiene

Abstract

Background: One of the foremost causes of microbial infections and propagation is improper sanitation and hygiene maintained in public places. Accumulation of stains and microbes results in the spread of infections. Also, due to the extensive use of non-renewable materials like petrochemicals, etc., there is an increasing demand for sustainable growth in the coating industries. Currently, there is no such technology that tackles this problem. Methods: Our present work aims to find a prolonged solution for these problems for the first time by synthesizing and formulating bio-based coatings with inherent antimicrobial properties and durable surface properties with a fast air-curing system. A formulation of alkyd and polyesteramide resins from castor, neem, and karanja oils was crosslinked with isocyanates to form the surface coatings. An esterification reaction of castor oil monoglyceride and phthalic anhydride synthesized the castor oil alkyd resin. The corresponding neem and karanja oil polyesteramides were synthesized by amidation with diethanolamine, followed by an esterification reaction. Results: The coatings exhibit an antimicrobial efficacy of 74%–84% against both Gram-positive and Gram-negative bacteria and contain 76.5% bio-based content. Factors such as thermal stability, physicochemical properties, and chemical and solvent stability were studied. After 24 h of inoculation with 40% polyesteramide resin (AMRESN-4), E. coli and S. aureus CFU values decreased from 6 × 10⁵ to 0.28 × 10⁵ CFU/g and from 5.7 × 10⁵ to 0.26 × 10⁵ CFU/g, respectively. These bio-based coatings are particularly suited for environments requiring high durability and antimicrobial protection, such as food-processing facilities, healthcare settings, and public restrooms.

1. Introduction

The transmission of infectious agents through contaminated surfaces is a critical issue in healthcare environments and public spaces, where frequent contact with shared objects significantly increases the risk of spreading harmful pathogens. In healthcare settings, surfaces contaminated by microbes play a significant role in the indirect transmission of infections. High-touch surfaces near patients are particularly vulnerable, enabling pathogens to spread from people to others through contaminated objects. Bacteria, bacterial spores, viruses, and yeasts in healthcare settings are primarily transmitted from infected and/or colonized patients, but they can also spread through staff and, in some cases, visitors. These pathogens frequently contaminate the inanimate hospital environment, particularly areas adjacent to patients and surfaces frequently touched by hands, referred to as “high-touch surfaces” [1]. Similarly, public transportation systems, including buses and trains, experience high passenger turnover, leading to frequent contact with shared surfaces such as handrails, seats, and buttons. Due to continuous operation and challenges in maintaining adequate cleaning protocols, these surfaces accumulate microbial contamination. The crowded nature of public transit further amplifies the risk of pathogen spread, making it a public health concern [2,3]. To mitigate microbial transmission, antimicrobial coatings have been developed using petroleum-based synthetic polymers, which provide prolonged activity by disrupting bacterial membranes. Nanoparticles [4], such as titanium dioxide (TiO₂), have been incorporated into these coatings to reduce bacterial viability. Among synthetic polymers, polyurethane (PU) is widely used due to its flexibility, durability, and chemical resistance [5,6]. The polyurethane (PU) segments are frequently used for these applications because of their remarkable qualities, which include flexibility, durability, and resistance to abrasion and chemicals. They are derived from petroleum-based chemicals like polyols and isocyanates. The widespread use of non-biodegradable, petroleum-based coatings has raised serious environmental concerns. However, the reliance on petroleum-based materials raises environmental concerns as their production depletes natural resources and contributes to pollution through carbon emissions, black carbon release, and oil spills [7,8,9]. To address these sustainability challenges, researchers are exploring bio-based alternatives [10,11] for polymer synthesis. Renewable feedstocks such as vegetable oils (e.g., castor oil, neem oil, and karanja oil), proteins, starches, carbohydrates, and polysaccharides offer a promising solution. Vegetable oils, in particular, are attractive due to their renewability, low cost, low toxicity, and biodegradability. Advances in green chemistry have enabled the development of bio-based monomers and polymers with properties comparable to petroleum-derived counterparts, for instance, alkyd resins [12,13,14]. Early synthetic polymers used in surface coatings have successfully incorporated fatty acids from vegetable oils into polyester structures, enhancing mechanical strength, drying speed, and durability [15,16,17]. Polyurethane coating systems are widely used in paint and coating industries due to their exceptional resistance against chemical and corrosive damage, mechanical strength, and flexibility [18]. Given the high demand for polyurethane-based coatings, transitioning from petrochemical sources to renewable raw materials is imperative. Vegetable-oil-based polyurethane systems, particularly those derived from castor oil, have demonstrated excellent hydroxyl content for effective crosslinking, ensuring strong mechanical and chemical performance. Polyesteramide-based PU systems have also emerged as viable alternatives due to their ease of synthesis and superior durability. This shift toward sustainable materials aligns with global efforts to reduce environmental impact while maintaining the high-performance characteristics required in antimicrobial coatings [19,20].

Castor oil consists of up to 90% ricinoleic, 4% linoleic, 3% oleic, 1% stearic, and less than 1% linolenic fatty acids [21]. Neem seed oil is a major source of fatty acids and is mainly composed of oleic acid (50%–60%), palmitic acid (13%–15%), stearic acid (14%–19%), linoleic acid (8%–16%), and arachidic acid (1%–3%). Karanja contains 11.6% palmitic acid, 51.5% oleic acid, and 16% linoleic acid [22]. The hydroxyl functionality of ricinoleic acid (RA) makes castor oil a natural polyol, providing oxidative stability and a longer shelf life than other oils by preventing peroxide formation. The hydroxyl group in ricinoleic acid and its derivatives serves as a functional site for various chemical reactions, such as halogenation, dehydration, alkoxylation, esterification, and sulfation. This unique property allows castor oil to be widely used in industrial applications, including paints, coatings, inks, and lubricants [23].

Neem and karanja oils are well known for their inherent antimicrobial properties [24,25,26,27,28]. The incorporation of these oils into the resin system will provide excellent resistance against microbial activity [29,30,31,32]. Neem oil, derived from the seeds of Azadirachta indica, exhibits a broad spectrum of medicinal and biological properties, including antibacterial, antifungal, antifertility, immunostimulant, antipyretic, and acaricidal effects. These properties are attributed to the active phytochemicals present in neem, such as azadirachtin, gedunin, isomargolonone, margolone, margolonone, nimbidin, nimbin, nimbolide, and salannin [33]. Karanja seed oil, sourced from the seeds of the Pongamia tree, displays a wide range of medicinal and biological properties, particularly its antibacterial and antifungal capabilities. These effects stem from various active phytochemicals in P. pinnata, such as karanjin, pongamol, beta-sitosterol, and linoleic acid. These compounds are effective against various antibiotic-resistant pathogens, highlighting their potential use in human healthcare. Neem oil and karanja oils, which have inherent antimicrobial properties, can be potential sources for antimicrobial surface applications. To obtain the needed set qualities, it is worthwhile to investigate the production of polyesteramide from neem and karanja oil (

Table 1. Oils used for the coatings.

Oil	Fatty Acid Profile	Key Components	Role Current Research
Castor Oil	90% Ricinoleic, 4% Linoleic, 3% Oleic, 1% Stearic, <1% Linolenic	Hydroxyl functionality of ricinoleic acid, oxidative stability, prevents peroxide formation	To provide high crosslinking density with hardners. The long aliphatic chains provide good hydrophobic characterisics
Neem Oil	50%–60% Oleic, 13%–15% Palmitic, 14%–19% Stearic, 8%–16% Linoleic, 1%–3% Arachidic	Azadirachtin, gedunin, isomargolonone, margolone, margolonone, nimbidin, nimbin, nimbolide, salannin	Antibacterial, antifungal, antifertility, immunostimulant, antipyretic, acaricidal, antimicrobial coatings
Karanja Oil	11.6% Palmitic, 51.5% Oleic, 16% Linoleic	Karanjin, pongamol, beta-sitosterol, linoleic acid	Antibacterial, antifungal, antimicrobial surface coatings, human healthcare applications

). Due to their suitable biodegradability, biological compatibility, and advantageous physical properties, polyesteramides with a combination of aliphatic and aromatic moieties have recently attracted substantial interest in surface coating and material applications [34,35]. The combined effect of the qualities of polyesters and polyamides is responsible for the desired structural properties. It is caused by the regular arrangement of chains of esters and amide links in the polymer network. The rigidity from the amide groups due to the double-bond nature and substantial hydrogen bonding effects influence the ordering of polyesteramide, which, in turn, enhances the surface’s physiological and thermal durability [36]. This enables the coating to endure the demanding conditions of “high-touch surfaces”, where surfaces are regularly exposed to abrasion, moisture, and various cleaning agents. Additionally, the improved thermal stability ensures that the coatings retain their integrity under varying temperature conditions, while their antibacterial properties contribute to reducing the transmission of infections.

In this study, we have attempted, for the first time, to replace conventional petrochemical-based coatings with antimicrobial bio-based coatings using an alkyd–polyesteramide–polyurethane system derived from castor, neem, and karanja oils. Castor-oil-based alkyd resin was synthesized due to its high hydroxyl content, ensuring effective crosslinking density. Neem and karanja-oil-based polyesteramides were synthesized to impart inherent antimicrobial properties to the coatings, while the amide linkage contributed to their durability. The synthesized oil-based alkyd-polyesteramide polyols were then crosslinked with a combination of hexamethylene diisocyanate (HDI) and methylene diphenyl diisocyanate (MDI) to produce air-cured polyurethane coatings suitable for application in hospitals and sanitation facilities. Regarding application, the nature of the alkyd-polyurethane-polyesteramide system resin is highly versatile. The formulation of the resin system can be modified as per the requirements. Depending on the nature of the substrate on which the coat is to be applied, such as ceramic, metal, wood, etc., a wide variety of adhesion promoters can be incorporated into the resin system. Long-chain fatty acids or short-chain fatty acids can be added to the system to increase flexibility or tensile strength, respectively. The long chains with high crosslinking density to PU provide the coat with omniphobic, anti-stain characteristics. The amount and nature of solvents can be adjusted according to the desired curing time of the coating system.

The unique aspect of this antimicrobial coating lies in its multifunctional applications and bio-based composition. Unlike standard coatings that often rely on synthetic chemicals, this coating is created from natural, renewable resources, which makes it environmentally benign and sustainable. Furthermore, because of its antimicrobial properties, which effectively prevent the growth of dangerous germs and organisms, this coating is a potent way to improve hygiene in various settings, including public sanitation and healthcare.

This combination of bio-based innovation and antimicrobial effectiveness addresses the growing need for safer and more sustainable materials in modern manufacturing. The coating offers an environmentally friendly substitute without sacrificing quality because it stops microbial growth without affecting surface performance or durability.

2. Materials and Methods

Castor seed oil (CSO) 99% extra pure was purchased from LOBA Chemie Pvt. Ltd., Mumbai, India. Karanja oil (98% pure) and neem oil (98% pure) were obtained from Mangalam Agro, Maharashtra, India. Xylene (99.5% AR), methyl ethyl ketone (99.5%), and acetone (99%) were purchased from LOBA Chemie Pvt. Ltd. Methanol (99%), and phthalic anhydride (98%), diethanolamine (99% AR), and glycerol (98% extra pure) were procured from Cisco Research Laboratories Pvt. Ltd., Mumbai, India. Methyl diphenyl diisocyanate (MDI, 98% pure) was sourced from Sigma-Aldrich, Bangalore, India, while hexamethylene diisocyanate (HDI, 98% pure) was obtained from TCI Chemicals India Pvt. Ltd., Tamil Nadu, India. Toluene diisocyanate (TDI) was purchased from SD Fine Chemicals, dibutyltin dilaurate (DBDTL) was acquired from Otto Chemie India Pvt. Ltd., Maharashtra, India, and nutrient broth and agar-agar were purchased from HiMedia Laboratories, Maharashtra, India.

3. Experimental Section

3.1. Alkyd Resin Preparation

Oil-based alkyd resin was prepared by reacting castor oil, glycerol, phthalic anhydride, and xylene acting as an azeotropic solvent using a dean stark setup and continuous stirring through an overhead stirrer. This was carried out through a monoglyceride process. The first step was the synthesis of oil-based monoglyceride, and the second step was the synthesis of alkyd resin by polycondensation reaction of monoglyceride and polybasic acid.

3.2. Synthesis of Castor-Oil-Based Monoglyceride (COMG)

The synthesis of monoglyceride was conducted in a four-necked round-bottom flask with a 1:2 molar ratio (oil–glycerol). The oil (100 g) was charged with lead monoxide (1 g) at 1% weight of the oil until the temperature reached 140 °C. Glycerol (20 g) was then added dropwise until the temperature rose to 180 °C. The reaction was conducted at 180 °C with constant reflux maintained throughout the reaction. A methanol solubility test was conducted to monitor the progress of the reaction. A sample was checked by pipetting out in a 1:2 (volume/volume) ratio of monoglyceride to methanol, and a clear, uniform solution indicates the completion of the reaction.

3.3. Preparation of Castor-Oil-Based Alkyd Resin (COAR)

The prepared monoglyceride was heated to 210 °C, and phthalic anhydride (13 g) was added in a ratio of 1:0.8 by moles of oil. The reaction was performed in a four-neck round-bottom flask (RBF) using a Dean–Stark setup, with continuous nitrogen sparging through a nitrogen purging tool in an inert environment with constant stirring. Xylene (5 mL) was added as an azeotropic solvent to remove water from the reaction. This mixture was collected in the Dean–Stark setup. The volume of water collected indicated the progress of the reaction. Continuous nitrogen sparging and reflux condensing were maintained. The reaction was conducted for 4 h and monitored using the acid value test. Once the acid value was less than 5 mg of KOH g⁻¹ of the sample, the conversion reached 99%.

3.4. Polyesteramide Resin

The synthesis of neem prepared the oil-based polyesteramide resin and karanja-oil-based fatty diethanolamide, followed by its reaction with phthalic anhydride to obtain the desired resin. Xylene was added as an azeotropic solvent to eliminate water.

3.5. Synthesis of Neem Oil Fatty Diethanolamide (NOFA)

The reaction was performed in a four-neck round bottom flask in an inert atmosphere with constant stirring through an overhead motor and a Dean–Stark setup. Oil (100 g) and diethanolamine (43.5 g), in a mole ratio of 1:3, respectively, were charged into the reaction mixture along with 0.5% wt of sodium methoxide (0.5 g) (based on the oil) at 85 °C for 30 min. Neem oil was slowly added to the mixture using an addition funnel. During this addition, the temperature was gradually increased to 120 °C, ensuring homogeneous mixing by the slow addition of the oil. After the complete addition, the reaction temperature was increased to 125 °C, and the reaction was continued for another 3 h. The amine value was determined every 30 min to monitor the reaction. The by-product glycerol was removed by washing with petroleum ether and brine solution. The karanja oil fatty amide (KOFA) preparation followed the same SOPs as above (Figure 1, Figure 2, Figure 3 and Figure 4).

3.6. Preparation of Neem Oil Polyesteramide Resin (NOPA)

In the reaction, the fatty diethanolamide synthesized was reacted with phthalic anhydride (11 g) in a molar ratio of 1:0.5. Xylene was used as an azeotropic solvent to facilitate the reaction. Constant stirring was provided in an inert environment, and sparging with nitrogen gas was carried out along with a reflux condenser on a Dean–Stark apparatus. The reaction was conducted at 180 °C for 3 h. The reaction progress was monitored using the acid value test. An acid value of less than 5 mg KOH/g of oil confirmed the 99% conversion of the resin. The karanja oil polyesteramide resin (KOPA) preparation followed the same SOPs as above (Figure 1, Figure 2, Figure 3 and Figure 4).

3.7. Development of PU Coatings

The coating formulations were developed through a comprehensive study of independent, dependent, and control variables, with a detailed investigation of their influence on coating behavior. Each formulation was systematically evaluated based on Mechanical Properties, Thermal Properties, Chemical Resistance, Surface Properties, Barrier Properties, Curing and Crosslinking Efficiency, and Antimicrobial Efficacy (

Table 2. Experimental design.

Category	Variable	Details
Independent Variables	Concentration of COAR	Castor-oil-based alkyd resin
	Concentration of NOPA	Neem-oil-based polyesteramide resin
	Concentration of KOPA	Karanja-oil-based polyesteramide resin
	Concentration of HDI Isocyanate	Hardener
	Concentration of MDI Isocyanate	Hardener
Dependent Variables	Mechanical Properties
	Adhesion Strength	Cross-cut adhesion test, pull-off test
	Scratch Resistance	Pencil hardness test
	Flexibility	Conical Mandrel test
	Impact Hardness	Impact hardness test
	Thermal Properties
	Glass Transition Temperature	Differential scanning calorimetry (DSC).
	Thermal Stability	Thermogravimetric analysis (TGA)
	Chemical Resistance
	Solvent Resistance	Immersion tests in different chemicals
	Acid/Alkali Resistance	Resistance against acidic and alkaline environments
	Surface Properties
	Contact Angle	Hydrophobicity/hydrophilicity
	Drying Time	Time taken for complete drying
	Surface Roughness	SEM analysis
	Gloss Test	Reflectivity measurements
	Barrier Properties
	Water Absorption	ASTM D570
	Curing and Crosslinking Efficiency
	Gel Content	Percentage of insoluble fraction in solvent extraction test, indicating degree of crosslinking
	Crosslink Density	Measurement of crosslinking efficiency
	Antimicrobial Efficacy
	Antimicrobial Activity	JIS Z 2801 assay
Control Variables	Coating Technique	Brush coating
	Number of Coats	1
	Temperature	25 °C
	Mixing Time	2 min
	Solvent Volume	Xylene, MEK, and acetone in a 50:25:25 w/v ratio with respect to total resin weight

). The castor, neem, and karanja-oil-based polyurethane coatings (AMRESN) were prepared using three resins, namely, COAR, NOPA, and KOPA. The resins were cured with HDI and MDI isocyanates, using xylene, MEK, and acetone as solvents in a 50:25:25 w/v (total weight of resins/volume of solvent) ratio with respect to resin (

Table 3. Formulation of antimicrobial PU coatings from COAR, NOPA, and KOPA.

Resin Formulation				Hardner Isocyanates (Weight% of Resin)
PU COATING	COAR (wt%)	NOPA (wt%)	KOPA (wt%)	HDI (wt%)	MDI (wt%)
AMRESN-1	100	0	0	46.6	-
AMRESN-2	90	5	5	36	2
AMRESN-3	80	10	10	26	6
AMRESN-4	60	20	20	21	9.6

). The choice of solvents as xylene (true solvent), MEK (auxiliary solvent), acetone (diluent/fast-evaporating solvent) was based on the affinity of the solvent to resin and the volatility of the solvent. Using both HDI and MDI results in an optimum balance with respect to the flexibility and hardness/rigidity of the cured coating film. HDI having a linear aliphatic chain provides more flexibility and greater impact resistance, whereas, on the other hand, the usage of MDI introduces aromaticity into the molecular structure, which results in greater hardness and brittleness. The curing rate of individual isocyanates is also a reason for using a combination. Contrary to the general rule, in our experiments, HDI reacted, rapidly resulting in a foam, whereas MDI had a comparatively slower reaction rate. This could be explained by the additional steric hinderance offered by MDI as opposed to HDI. Hence, using a combination of both isocyanates provided an optimum curing time [37]. The dibutyltin dilaurate performs the role of a catalyst in the curing reaction. Initially, it forms a N-coordination complex with the isocyanate resulting in the activation of the isocyanate carbon atom shifting it from a linear sp-hybridized state to a tetrahedral sp²-hybridized state, further leading it to react with the hydroxyl group of polyol to form urethane linkage [38].

The equivalent ratio of resin to isocyanate (OH:NCO) was 1:0.9 of MDI and 1:1.2 of HDI with DBTDL (0.05 wt% of polyol) as a catalyst. AMRESN-1 served as the control, consisting of 100% COAR and 46.6% HDI. The modified formulations included AMRESN-2 (90% COAR, 5% NOPA, 5% KOPA, 36% HDI, and 2% MDI), AMRESN-3 (80% COAR, 10% NOPA, 10% KOPA, 26% HDI, and 6% MDI), and AMRESN-4 (60% COAR, 20% NOPA, 20% KOPA, 21% HDI, and 9.6% MDI). The ideal resin-to-solvent ratio was found using the Daniel flow point technique. The resin and diisocyanate needed to be combined quickly for the reaction between the diisocyanate and polyols to be fully completed, and the endpoint was confirmed by the continuous Daniel flow. The Daniel flow point method helps achieve optimal pigment dispersion in polyurethane coatings by determining the right balance of resin, solvent, and pigment. It identifies the point where pigment particles are effectively coated by the polyurethane resin, minimizing inter-particle attraction and improving flow properties. This process relies on urethane bonding, hydrogen bonding, and Van der Waals forces. Proper dispersion prevents flocculation and settling, ensuring a uniform coating with strong adhesion, durability, and protective performance [39]. Initially, the steel panels were cleaned with sandpaper and then removed with acetone before paint application. Alkali and phosphating mixture were applied to the panels to achieve waterline break-free application. The curing process proceeded at room temperature (~28 °C to 30 °C) and at atmospheric pressure.

Castor-oil-based alkyd resin (COAR), rich in ricinoleic acid, enhances crosslinking density and durability. Neem (NOPA-1) and karanja (KOPA-1) polyesteramides improve antimicrobial efficacy and coating strength. Polyurethane hardeners enable fast-drying and efficient application. This synergistic alkyd–polyesteramide–polyurethane formulation enhances durability, flexibility, thermal resistance, and antimicrobial properties, making it a high-performance, sustainable alternative to conventional petrochemical-based coatings for industrial and commercial applications.

4. Method of Characterization

4.1. Instrumentation Techniques

Using a transmittance technique on a Fourier-transform infrared spectroscopy (FTIR) 8400 (Shimadzu, Kyoto, Japan) with a resolution of 4 cm⁻¹ and a range of 4000–400 cm⁻¹, the functional identification of synthesized COAR, NOPA, and KOPA was examined. On an Agilent Technologies 500 MHz instrument with solvent (CDCl3) medium, proton nuclear magnetic resonance (¹H NMR) spectra of COAR, NOPA, and KOPA were obtained using Tetramethyl silane (TMS) as the internal standard. Chemical shift analysis was reported in ppm. The surface characteristics and morphology were studied using a field emission scanning electron microscope (FE-SEM) (Model No.: Nova Nano SEM 450; Make: FEI, Lausanne, Switzerland). The cured PU films were subjected to thermogravimetric analysis (TGA) with TA-Discovery 55, USA, under a nitrogen environment. The desired 20–600 °C was attained by applying a heating rate of 10 °C min⁻¹. The DSC 60 model in Shimadzu, Japan, was used to carry out differential scanning calorimetry (DSC) analysis of cured films in Shimadzu Japan at a scanning rate of 10 °C min⁻¹. On the Kyowa Interface Science Co., Ltd. (Saitama, Japan), the contact angle (CA) was measured using the sessile drop technique. For greater accuracy, the average of five CA measurements was provided.

4.2. End Group Analysis

4.2.1. Acid Value

The acid value indicates the free fatty acid content present in a sample. The acid value is obtained by titrating the sample against a basic solution of known normality till neutralization. The empirical formula is as follows:

$$AV={\displaystyle \frac{56.1\times N\times V}{W}}$$

(1)

where N = Normality of KOH solution used for titration.

V = Volume of KOH solution required to neutralize the sample.

W = Weight of sample taken [40].

Note: 56.1 represents the molecular weight of potassium hydroxide (KOH).

Percentage conversion of esterification reaction (consumption of phthalic anhydride):

$$\%Conversion={\displaystyle \frac{\left(\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{c}\mathrm{i}\mathrm{d} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}-\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{c}\mathrm{i}\mathrm{d} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}\right)\times 100}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{c}\mathrm{i}\mathrm{d} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}}}$$

(2)

4.2.2. Amine Value

The amine value of an oil sample is the measure of the amine functional group present in it. The amine value is obtained by various methods, with the basic principle being titration to achieve neutralization and obtain a quantitively accurate value. The empirical formula used for calculation is given below.

$$Amine Value={\displaystyle \frac{56.1\times V\times N}{W}}$$

(3)

where N = Normality of HCl solution used for titration.

V = Volume of HCl solution required to neutralize the sample.

W = Weight of the sample taken.

Percentage conversion of amidation reaction:

$$\%Conversion={\displaystyle \frac{\left(\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}-\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}\right)\times 100}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e} \mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{M}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}}}$$

(4)

4.2.3. Hydroxyl Value

The concentration of the hydroxyl groups on the polyol is indicated by the hydroxyl number (OH).

$$HV={\displaystyle \frac{56.1\times \left(A-B\right)\times V}{W}}$$

(5)

where

A = Amount (mL) of KOH required for blank titration.

B = Amount of KOH required for sample titration.

N = Normality of KOH.

W = Weight (g) of sample.

4.3. Cured Film Characterization

The cured coatings of the alkyd–polyesteramide–polyurethane resin system were analyzed by various physiological and surface tests. A dry film thickness gauge was used to ascertain the film thickness of the coat. The film gloss at an angle of 60° was determined with the help of a digitally calibrated gloss meter. The impact resistance of the film was tested as per ASTM D-2794 by dropping a load of 2 kg. The mechanical performance of the cured coat system was evaluated with the help of a Universal Testing Machine with the operating parameters. A conical mandrel tester was used in the range of 45–180° to test the flexibility of the cured film as per the ASTM D-522. The coating’s pencil hardness was assessed as described in ASTM D-3363 by using a pencil hardness tester. The coating’s scratch hardness (IS-104) was assessed by applying a needle with increasing pressure over multiple runs until the film experienced mechanical failure. The coating’s adhesiveness was also tested by performing a cross-cut test (ASTM D-3359). A 1 cm² lattice was marked over which an adhesive tape was applied. The tape was removed, and the adhesion was checked according to scientific standards. Chemical resistance tests against acidic and basic solutions were conducted as per ASTM D-870 and ASTM D-1308, respectively. The panels were submerged in the 5% w/w sodium hydroxide and aqueous hydrochloric acid solutions for 24 h. The solutions were prepared in ambient conditions. The panels were continuously checked for any indication of blister formation, film weakening, film detachment from the panel, and gloss loss. The solvent resistance against the xylene and methyl ethyl ketone of the coated panels was also analyzed as described in ASTM D-4752. The panels were scrubbed using a saturated cotton swab for up to 200 cycles. ASTM D 2765-01 was referred to assess the cured films’ crosslink density by using the Soxhlet apparatus. The 1 g film samples were dried at 60 °C for two days before testing. P1 was the weight of the pouch containing the dried film for extraction in the Soxhlet apparatus, and F was the weight of the filter paper bag. Following the staples, the pouch was weighed once again (P2). Then, continuous reflux was performed using this sealed pouch in the Soxhlet apparatus for 24 h using xylene as the solvent. The bag was dried at 150 °C in the vacuum oven before being weighed (P3). Three trials were conducted, and the results were calculated as the average of three trials. The gel content (Gc) was calculated by the equation below.

$$GC\left(\%\right)=100-\left({\displaystyle \frac{P2-P3}{P1-F}}\times 100\right)$$

(6)

The water absorption capacity of the cured coatings was assessed by referring to ASTM D-570. Before being immersed in water for 24 h at room temperature, the panel was dried in the oven until it attained a constant weight (W1). After 24 h, the panels were weighed (W2) and cleaned with dry tissue paper. The below equation was used to determine the ability of coatings to absorb water.

$$Water Absorbed\left(\%\right)={\displaystyle \frac{W2-W1}{W1}}\times 100$$

(7)

4.4. Antimicrobial Testing

The coating formulations’ antimicrobial efficacy was tested per the JIS Z 2801 assay. The coatings were tested against both Gram-negative Escherichia Coli (E. coli) and Gram-positive Staphylococcus Aureus (S. aureus) test inoculums [41]. The coated test samples were cut into 5 × 5 cm sections and placed on sterile Petri dishes. Pieces coated with 0% polyesteramide content (AMRESN-1) were used for control samples. Two test pieces from each formulation were then inoculated with the test inoculum. The test inoculum from one set of test pieces was immediately pipetted out and added to sterile Petri dishes, followed by the addition of nutrient agar. This set of agar-filled Petri dishes was then incubated for 48 h at 37 °C. The second set of inoculated test pieces was initially incubated for 24 h, and after the initial incubation period, the same procedure as for the first set was followed, and they were incubated for 48 h at 37 °C [42]. After the incubation period, the bacterial colonies on the plate were evaluated by counting the colony-forming units on the Petri plates. The following formula determined the antimicrobial efficacy.

R = [log(B/A) − log(C/A)].

R = Value of antimicrobial activity.

A = Average of the number of viable cells of bacteria immediately after inoculation on the untreated test piece.

B = Average of the number of viable cells of bacteria on the untreated test piece after 24 h.

C = Average of the number of viable cells of bacteria on the antimicrobial test piece after 24 h.

4.5. Statistical Analysis

A one-way classification of ANOVA and student’s t-test (two-tailed) was performed where differences between data were regarded as statistically significant with p-values < 0.05.

5. Results

5.1. Physicochemical Characteristics

The physicochemical characteristics of monoglycerides, fatty amides, polyesteramides, and alkyd resins derived from castor oil, neem oil, and karanja oil were analyzed to evaluate their end-group properties (

Table 4. Monoglyceride, fatty amide, polyesteramide polyol and alkyd resin physicochemical characteristics.

End Group	Acid Value (mg of KOH g−1 of Sample)	Hydroxyl Value (mg of KOH g−1 of Sample)	Conversion (%)
COMG monoglyceride	5	320	99
NOFA fatty amide	15	270	98
KOFA fatty amide	16	260	97

,

Table 5. Physicochemical properties of alkyd resin polyesteramides polyols.

End Group	Acid Value (mg of KOH g−1 of Sample)	Hydroxyl Value (mg of KOH g−1 of Sample)	Equivalent Weight (gm)
COAR alkyd resin	2.5	260	215.76
NOPA polyesteramide polyol	4.5	184.3	304.39
KOPA polyesteramide polyols	6.2	182.2	307.90

and

Table 6. Curing performance of PU coatings.

Sample	Surface Drying (min)	Tack Free (min)	Hard Drying (h)
AMRESN-1	15	45	6
AMRESN-2	6	60	7
AMRESN-3	18	75	7.1
AMRESN-4	19	80	7.5

). The study focused on determining the acid value, hydroxyl value, and conversion efficiency of these compounds, which are critical indicators of their reactivity and suitability for further applications. In the synthesis of castor oil alkyd resins, the first step involves reacting castor oil, which has a hydroxyl value of 170 mg KOH/g, with glycerol in a process known as the monoglyceride reaction. This reaction produces a monoglyceride and increases the hydroxyl value to 320 mg KOH/g. This step is crucial because the increased hydroxyl functionality enables further crosslinking with phthalic anhydride during alkyd resin formation. Similarly, in the synthesis of polyesteramide resins from neem or karanja oil, the first step involves reacting the oil with diethanolamine. This reaction introduces hydroxyl groups into the oils, raising the hydroxyl value from 19 mg KOH/g to 270 mg KOH/g for neem oil and from 17 mg KOH/g to 260 mg KOH/g for karanja oil. This modification prepares the oils for subsequent polymerization, facilitating the formation of the desired polyesteramide resin. In both cases, the increase in hydroxyl value enhances the reactivity of the oils, thereby improving the efficiency of the polymerization process.

Table 4 shows the end-group analysis of monoglycerides and fatty amides derived from castor oil, neem oil, and karanja oil. The acid content in phthalic anhydride contributed to the initial acid value of 86 in the reaction mixture, as shown in Figure 5. The acid value significantly decreases while completing the reaction, which implies the consumption of phthalic anhydride to produce alkyd resin in the required reaction. Table 5 shows the end-group analysis of polyesteramides and alkyd resins derived from neem, karanja, and castor seed oil, respectively. The amino groups in diethanolamine contributed to the initial amine value of 175 in the reaction mixture, as shown in Figure 6. The amine value significantly decreases while completing the reaction, which implies the consumption of diethanolamine to produce fatty amides in the required reaction. Figure 7 shows the rate at water evolved in the polycondensation reaction.

5.2. FTIR Analysis of Castor Oil Alkyd, Neem, and Karanja Oil Polyesteramide Polyols

The functional molecules and spectra of the castor-oil-modified alkyd resin are displayed in Figure 8. In castor-oil-modified alkyd resin, the broad bands of about 3600 cm⁻¹ were attributed to the hydroxyl (-OH) group of unsaturated fatty acid. The peak value represents the cyclic ester of saturated fatty acid at 1740 cm⁻¹. A broad absorption peak for -OH stretching was visible around 3600 cm⁻¹ for the NOPA FTIR spectra. The NOPAs cause distinctive peaks in the 2917 cm⁻¹ and 2921 cm⁻¹ range -CH₂ symmetric and asymmetric stretching. Ester carbonyl (COOR), amide linkages (C-N), and amide carbonyl (-CON) stretching frequencies were determined by looking for distinctive peaks at 1740 cm⁻¹, 1462 cm⁻¹, and 1638 cm⁻¹ in that order. Moreover, the stretching vibration of -CO was matched by peaks at 1163 cm⁻¹ in the NOPA spectra. In contrast, the effective ester linkage formation is shown by the absence of this absorption peak in the NOFA FTIR spectra. A broad absorption peak for -OH stretching was visible in the 3600 cm⁻¹ area of the KOPA FTIR spectra. The distinctive peaks in the 2917 cm⁻¹ and 2921 cm⁻¹ range are caused by the NOPAs -CH₂ symmetric and asymmetric stretching. Ester carbonyl (COOR), amide linkages (C-N), and amide carbonyl (-CON) stretching frequencies were identified by looking for distinctive peaks at 1740 cm⁻¹, 1462 cm⁻¹, and 1638 cm⁻¹, respectively.

5.3. ¹H NMR Analysis of Alkyd Resin, Fatty Amides, and Polyesteramide Polyols

Figure 9 shows the ¹H NMR spectra of the alkyd resins made from castor oil. The duplet peak at 7.5–7.73 ppm in the COAR alkyd resin spectrum (Figure 9) represents the aromatic protons from phthalic anhydride. The highest points at 5.39–6.24 ppm are linked to the unsaturations of fatty acid chains. The peaks at 3.5–4.5 (i, j) ppm are the protons of -CH₂ bound to the monoglyceride’s -OH groups. The peaks at 2.71 and 2.87 ppm are thought to be caused by a bis-allyl proton between two bonds inside the acid 9,12-octadecadienoic. The protons of each internal CH₂ group in the fatty acid chains are represented by peaks at 1.32–2.33 ppm (k, l). The peaks observed at 0.87–0.92 ppm correspond to the protons of the terminal methyl group in the fatty acid chain. However, the peaks (5.4–5.83 ppm) connected to the protons of the unsaturation in the fatty acid chain were found to be less intense. Additionally, the peak corresponding to the protons of the carbon attached to the OH group in the fatty acid chains of -CO was observed at 3.6 ppm.

Figure 10 displays the fatty amide ¹H NMR spectra. The signal at 2.04 ppm is associated with a double bond, and the proton of the -CH₂ group is attached to the terminal -CH₃ group, while the signal at 0.82 ppm is attributed to the terminal -CH₃ group. The representation of the groups in the fatty acid chain is carried out by the peak found at 1.61 ppm and the interior (-CH₂-CH₂) groups, which are represented by 1.3 ppm (h) and are connected to the carbonyl amides adjacent to -CH₂. Protons from the -CH₂ group associated with carbonyl functionality were assigned to 2.29 ppm, while protons from the -CH₂ group adjacent to double bonds were assigned to 2.79 ppm. At 3.8 ppm (i), protons of the -CH₂ group associated with terminal hydroxyl groups were found, and at 3.58 ppm (k), protons of the -CH₂ group associated with amide functionality. The unsaturation (olefinic) in the fatty acid chain caused the peak responsible for the proton of -HC=CH to be observed at 5.39 ppm. Thus, the structural analysis of the diethanolamide derived from neem and karanja seed oil was carried out using the ¹H NMR spectra.

The ¹H NMR spectra of oil-based polyesteramide polyol NOPA-I and KOPA are displayed in Figure 11. Protons of the -CH₂ group are displayed between the terminal -CH₃ group and the double bond at 2.01 ppm, whereas protons of the terminal -CH₃- group are represented at 0.83 ppm. The hydrocarbon chain’s interior -CH₂ groups are indicated by a 1.37 ppm (e). In the carbonyl amide group, the peak seemed to be at 1.63 ppm (n), which is associated with the -CH₂ group. The proton of the -CH₂ group tied to two adjacent double bonds displays a peak at 2.81 ppm; however, the peak at 2.3 ppm (f) is associated with the -CH₂ group connected to carbonyl functionality. Protons of the -CH₂ group associated with amide nitrogen were assigned at 3.54 ppm (h). The peak appearance at 3.81 ppm is caused by protons of the internal -C=O group and the -CH₂ group coupled to the double bond. The peaks at 4.23 ppm (k) and 4.32 ppm (j) indicate that the hydroxyl group of -OH is responsible for the proton of -OH, and the oxygen linked with the carbonyl group is responsible for the protons of -CH₂ group. In the fatty acid chain, unsaturated hydrocarbons are responsible for the protons at 5.38 ppm.

5.4. Water Absorption Properties of Coatings

Determining the water absorption of PU coatings was essential to assess their durability in high-moisture environments. The water absorption of coatings was determined by weighing coated plates before and after being submerged in water for 24 h. Compared to the water absorption of petrochemical-based coatings, the current polyol-polyesteramide-based PU coatings showed contrasting results. The results of the test are shown in Table 4. AMRESN-1 displayed the most significant gain in weight; here, a relatively high hydroxyl value can be attributed to this compared to the other formulations. The presence of free hydroxyl groups results in hydrogen bonding with water molecules and causes water absorption [43]. The extent of water absorption showed a decreasing trend with an increase in the polyesteramide content of coatings due to a high degree of crosslinking and relatively lower hydroxyl values.

5.5. Mechanical Properties of Coatings

Crosslinking density, polymeric network structure, the degree of unsaturation, the chain length of fatty acids, and the presence of aromatic rings are the factors responsible for determining the mechanical properties of oil-based PU coatings. Tensile strength and elongation were used to examine the mechanical properties of the coatings [44,45,46]. The coating formulations displayed increasing tensile strength as the percentage of polyesteramide content in the formulation increased. This reason for the observed trend is due to the composite resonance structure of amide functionality, which inhibits flexibility and a higher degree of crosslinking. The presence of aromatic phthalic anhydride is also responsible for the toughness exhibited by the coating systems.

In contrast with the results of tensile strength, the elongation property of coating formulations decreased with increased polyesteramide content (

Table 7. Mechanical properties of the coatings.

Property	AMRESN-1	AMRESN-2	AMRESN-3	AMRESN-4
Gloss [°]	133	128	122	119
Thickness [μm] [%]	51.11	52.21	47	47
Flexibility test (conical mandrel)	Pass	Pass	Pass	Pass
Scratch hardness [kg]	4.3	4.4	4.8	5.2
Impact resistance [kg/cm]	90	98	110	115
Pencil hardness	3H	3H	4H	4H
Contact angle [°]	102.7	103.1	104.2	104.9
Gel content [%]	98.15	98.3	98.34	98.4
Water absorption [%]	0.25	0.24	0.22	0.21

). The AMRESN-1 formulation exhibited the highest elongation at break. The relatively lower rigidity of polyesters when compared with polyesteramides contributes to this trend. The adequate flexibility of coating formulations despite aromaticity, ester, and amide functionalities is due to the aliphatic long chains present, which impart plasticity due to the rotation of C-C sigma bonds. The results from the above tests confirm the excellent mechanical performance of coating systems due to aliphatic long-chain fatty acids and aromatic, ester, and amide functionalities. The balance between flexibility and durability depends on the application surface. For metal surfaces, such as hand railings, a high degree of flexibility along with moderate durability is preferred. Since metals expand and contract with temperature changes, a flexible coating film can accommodate these volume fluctuations without cracking. As the polyesteramide content in the formulation increases, the hydrophobicity of the cured film also increases due to the reduced availability of free hydroxyl groups. Since hydroxyl groups promote hydrophilicity by forming hydrogen bonds with water molecules, their decreased presence leads to lower water adhesion and a higher contact angle. Simultaneously, the increase in polyesteramide content introduces heterogeneity during the curing process, resulting in uneven curing of the coating film. This uneven curing increases surface roughness, causing greater light scattering and a reduction in gloss. The lower hydroxyl group content in polyesteramide resins, compared to castor oil alkyd resin, contributes to this effect. Castor oil alkyd resin, rich in ricinoleic acid, has a higher degree of hydroxyl functionality, which promotes uniform curing and better gloss retention. The gel content (GC%) values indicated the extent of crosslinking in the coatings, which directly influences their performance characteristics. Typically, a higher GC% corresponds to improved chemical resistance, solvent resistance, and mechanical durability due to the formation of a denser crosslinked network. AMRESN-4 exhibited the highest gel content value as it contained a higher amount of polyesteramide, which led to increased crosslinking and enhanced coating durability. The surface morphology of the cured coatings was analyzed using scanning electron microscopy (SEM). The SEM images of AMRESN-1, AMRESN-2, AMRESN-3, and AMRESN-4 are presented in Figure 12. The SEM image of AMRESN-1 exhibited a smooth and uniform surface, whereas a progressive decrease in surface smoothness was observed from AMRESN-2 to AMRESN-4, with AMRESN-4 displaying the highest surface roughness. This trend can be correlated with the increasing incorporation of polyesteramide linkages into the alkyd resin matrix. The presence of amide (-CONH-) groups enhances hydrogen bonding, which alters the structural arrangement of the cured polymer. This modification promotes phase separation, leading to non-uniform surface formation. Additionally, a higher crosslinking density further disrupts polymer chain packing, resulting in an increase in surface irregularities. Among the samples, AMRESN-4, with the highest polyesteramide content, exhibited the most noticeable roughness due to the combined effects of hydrogen bonding, phase separation, and increased crosslinking. These results indicate that adding polyesteramide linkages influence the surface texture of the cured coatings.

5.6. DSC Thermograms of PU Coatings

The glass transition temperature (Tg) reflects the strength of the polymer’s chain segments and its thermal characteristics. The crosslinking density influences Tg, which also depends on the structure and performance of the curing agent and resin. In polyurethanes, intermolecular hydrogen bonding induces crosslinking, significantly raising Tg while reducing chain mobility [47]. The presence of the aromatic ring in the polyesteramide structure may also influence the increased Tg and crosslinking density. Tg values were obtained from the second heating DSC curves of the cured polyurethane coatings based on the endothermic peaks.

The Tg values of AMRESN-1, AMRESN-3, and AMRESN-4 coatings are displayed in Figure 13 as 54.69, 61.07, and 67.08 °C, correspondingly. Simultaneously, AMRESN-1-based polyurethane coating had lower Tg values due to its low amount of polyesteramide content; greater Tg values of AMRESN-4-based polyurethane coating were influenced by increased crosslinking density, which may be attributed to its rigid structure and higher reactivity.

5.7. Thermo-Gravimetric Analysis of PU Coatings

Thermogravimetric analysis (TGA) is a widely used technique for determining the content of low-boiling compounds in a material by monitoring the change in sample mass as it is subjected to a controlled temperature increase. It is believed that thermal degradation of polyurethane (PU) is a complex phenomenon because PU breaks down thermally and produces a range of compounds during the process [48]. The isocyanate and polyol structures had the greatest impact on the thermal stability of PU coatings. Figure 14 shows a TGA assessment of the cured polyurethane coating. For each PU film sample, the TGA curve revealed three deterioration stages. The disintegration of urethane linkages was the primary cause of the first step of deterioration, which exhibited a 5–10% degradation rate at 221–263 °C [49].

The second stage of degradation, occurring at 315–400 °C, was attributed to the hard segments in the cured PU structure, which resulted in a weight loss of 31%–36%. All cured PU film samples experienced a 50% weight loss, initiating the final stage of degradation at temperatures ranging from 345 to 455 °C. It was associated with the hydrocarbon chain decomposition of polyesteramide polyol. The char residue for the AMRESN-1, AMRESN-3, and AMRESN-4 coatings at 600 °C is 0.07%, 2.11%, and 11.02%, respectively.

Due to the increased reactivity of the aromatic ring and the higher polyesteramide content, AMRESN-4 exhibits greater crosslinking density. PU coatings based on AMRESN-4 demonstrated higher thermal stability than those based on AMRESN-3 and AMRESN-1. In contrast, AMRESN-3 and AMRESN-1-based PU coatings have lower thermal stability because of their lower polyesteramide crosslinking content and increased aliphatic structure. They also experience weight loss before 200–210 °C due to trapped air and residual volatile materials. This work confirmed through thermal analysis that the coatings, which have three stages of degradation, are more stable than petro-based coatings, which only have two stages of degradation [50]. As a result, NSO-based coatings can be used to develop high-performance coatings.

5.8. Chemical Resistance Properties of PU Coatings

The chemical resistance of the cured coated panels was tested using acid (5% HCl) and alkali (5% NaOH) solutions for 48 h. Any changes in gloss or adhesion properties were noted. A loss in gloss was observed in AMRESN-1, while the coatings from AMRESN-2 to AMRESN-4 showed no effects from the acid and alkali solutions. This lack of change is attributed to the higher polyurethane content in the AMRESN-2, AMRESN-3, and AMRESN-4 coatings, which provides enhanced resistance to acid and alkali due to the structure of the crosslinked polyurethane. Furthermore, polyesteramide polyol’s long-chain fatty acids might have improved the chemical stability by offering resistance to chemicals [50].

5.9. Solvent Resistance Properties of PU Coatings

PU coatings were subjected to solvent resistance testing (200 cycles) using various solvents, including MEK and xylene. Differences in the coatings’ appearance were observed. In AMRESN-1 and AMRESN-2, the films were slightly removed, whereas the coatings in AMRESN-3 and AMRESN-4 were unaffected. This is attributed to the higher polyurethane content in these coatings and aromatic rings from phthalic anhydride. Additionally, the higher polyesteramide content in AMRESN-3 and AMRESN-4 contributes to improved solvent resistance due to the strong hydrogen bonding and enhanced cohesive strength within the polymer matrix (

Table 8. Chemical and solvent resistance of PU coatings. X—unaffected, Y—loss in gloss, Z—film slightly removed.

Sample	Solvent Rub Test		Acid-Alkali Resistance (5% HCl, 5% NaOH)
	Methyl Ethyl Ketone	Xylene
AMRESN-1	Z	Y	Y
AMRESN-2	Y	X	X
AMRESN-3	X	X	X
AMRESN-4	X	X	X

).

5.10. Antimicrobial Testing

Control samples consisted of AMRESN-1, which contained 0% polyesteramide content and were used for comparison against the test samples. The values of colony-forming units (cfu) for the control plates of E. coli immediately after inoculation and 24 h after inoculation were 6 × 10⁵ cfu/g and 1.9 × 10⁵ cfu/g, respectively, whereas for S. aureus, the values were 5.7 × 10⁵ cfu/g and 1.9 × 10⁵ cfu/g, respectively. At polyesteramide resin content of 20% (AMRESN-3) and 24 h after inoculation, the cfu values for E. coli and S. aureus were 0.34 × 10⁵ cfu/g and 0.30 × 10⁵ cfu/g, respectively. At polyesteramide resin content of 40% (AMRESN-4) and 24 h after inoculation, the cfu values for E. coli and S. aureus were 0.28 × 10⁵ cfu/g and 0.26 × 10⁵ cfu/g, respectively. The range of antimicrobial efficacy against E. coli strain was from 75% to 84%, whereas the range against S. aureus strain was from 77% to 84% as can be seen in Figure 15, Figure 16, Figure 17 and Figure 18. The results indicated that higher concentrations of neem and karanja oil polyesteramide resin increased efficacy. The increased concentrations of neem and karanja oil contribute to the increased antimicrobial activity of the coatings. These data support the potential of these coatings as a viable alternative to current petrochemical-based antimicrobial coatings to provide antimicrobial resistance against pathogens on high-touch surfaces. Unlike existing nanoparticle-encapsulated coatings that can penetrate the skin [51], these coatings are bio-based and derive their antimicrobial properties from vegetable seed oils, making them safe for human applications (

Table 9. The comparison of antimicrobial coatings.

Existing Antimicrobial Coatings	Bio-Based Antimicrobial Coatings (AMRESN)
Derived from non-biodegradable petrochemical sources.	Derived from natural sources that are biodegradable.
Antimicrobial properties can only be achieved by using additives.	Inherent antimicrobial properties.
Coatings encapsulated with nanoparticles can penetrate the human skin and have toxic effects [51].	The coating has bio-based substrates, namely, neem and karanja oil, that provide inherent antimicrobial properties that are safe for human use.

). The innovation of this coating lies in its capacity to deliver long-lasting antimicrobial protection, providing an efficient alternative to the labor-intensive and repetitive cleaning of surfaces with disinfectants. By inhibiting the growth of microorganisms, the coating ensures continuous protection over time. Additionally, it improves surface durability, extending the service life of materials by resisting wear and tear. This coating is safe for human use and is an ideal solution for high-touch environments where maintaining hygiene is a priority, such as public transportation, healthcare facilities, and commercial spaces.

5.11. Bio-Based Content

One important factor in assessing the compound’s number of green components is its bio-based content. The weight percentage of total organic carbon in the material has been estimated using the bio-based content provided by the US Department of Agriculture. According to the mentioned criteria, HDI and MDI are 0% bio-based, while castor seed oil, neem seed oil, karanja seed oil, glycerol, and phthalic anhydride are 100% bio-based. HDI-B and MDI contribute 57.14% and 76.5% carbon content, and from the equivalent ratio of HDI (1:1.2 of OH:NCO), MDI (1:0.9 of OH:NCO), 2.5 g of COAR, 0.5 g of NOPA, 0.5 g of KOPA (equivalent weight, COAR; 215.76, NOPA; 304.39, KOPA; 307.90), 1.77 g of HDI-B, and 0.15 g of MDI (equivalent weight; 221.35 and 18.87, respectively), the bio-based content of green PU is 76.5% ($={\displaystyle \frac{2.65\times 100\%+0.5\times 100\%+0.5\times 100\%}{2.65\times 100\%+0.5\times 100\%+0.5\times 100\%+1.77\times 57.14+0.15\times 76.5}}$). Comparing PU, which is derived from petroleum, on the other hand, has no bio-based material. The bio-based composition of the product now establishes its eco-friendliness and greener nature, leading to the evaluation and comparison process. This demonstrates that green PU is an environmentally preferable and more sustainable alternative to PU derived from petroleum.

6. Conclusions

This study developed bio-based antimicrobial polyurethane (PU) coatings using castor oil, neem oil, and karanja oil. The coatings were evaluated for their chemical, mechanical, thermal, and antimicrobial properties, with the long-chain fatty acids contributing to flexibility, hydrophobicity, and durability. Among the formulations, AMRESN-4 demonstrated the best performance, achieving 84% antimicrobial efficacy (JIS Z 2801 standard), strong mechanical properties, and high thermal stability (Tg = 67.08 °C). The increased polyesteramide content enhanced both durability and antimicrobial effectiveness, making it ideal for high-touch surfaces in healthcare, food processing, and public spaces.

With 76.5% bio-based content, this coating system minimizes the use of hazardous chemicals while providing a sustainable and cost-effective alternative to petroleum-based coatings. Its hydrophobic properties further enhance moisture resistance, making it suitable for industrial and outdoor applications.

Applications for this coating are mostly found in places where high durability and antimicrobial protection are needed, such as food-processing facilities, healthcare facilities, and public restrooms. It is also used on high-touch surfaces like handrails, door handles, and bathroom fixtures. Its hydrophobic qualities, which help it repel moisture, can also be used in industrial settings where improved chemical resistance and long-lasting performance are required. As a result, it is appropriate for outdoor applications and protective coatings on various surfaces exposed to challenging environmental conditions. Hence, the surface coating system described is a potential economical and sustainable solution for the persisting problem of the transmission of infections through high-touch surfaces in public spaces. Future research should aim to assess the long-term performance of these coatings in real-world applications, examining their durability, resistance to microbial contamination, and effectiveness under different environmental conditions. Further studies can also explore the potential of alternative bio-based raw materials, such as lignin-derived polyols or microbial oils, to improve both the sustainability and functional properties of the coatings. Additionally, research into scalable production techniques and cost efficiency will be essential to facilitate commercial adoption and widespread use.