Development of Biochar-Based Sustainable Corrosion-Resistant Coating ^†

Abstract

Conventional protective coatings based on petroleum raw materials have certain limitations in terms of their availability, environmental pollution, and sustainability. Therefore, this research successfully investigates the potential of sheep wool-derived biochar to develop a sustainable, high-performance protective coating. Two variants of biochar, namely SW800 and SW1000, were developed by pyrolyzing sheep wool at 800 °C and at 1000 °C for 1 h, respectively. The prepared samples were characterized using FTIR, FESEM-EDX, and XRD analyses to confirm the structural and elemental differences between both biochar samples. Furthermore, biochar-based epoxy coatings were developed by varying the concentration of prepared biochar from 1% to 5%. The coating performance was evaluated for its aesthetic, mechanical, chemical resistance, and hydrophobicity. Crucially, this study demonstrated that biochar inclusion did not compromise critical mechanical and chemical properties like adhesion (5B), flexibility (7 mm), scratch hardness (3500 gms), pencil hardness (3H), acid-alkali resistance, and solvent rub test (rating 5). However, a key finding of this research is that the incorporation of biochar into an epoxy coating resulted in a significant improvement in hydrophobicity, which is measured using water contact angle. The incorporation of SW800 and SW1000 into coating formulations at varying concentrations resulted in an increase in water angle of approximately 18% and 20%, respectively. The outcomes of this project establish biochar-based coatings as a promising solution for eco-friendly and high-performance protective applications.

1. Introduction

Coatings are applied to surfaces mainly for protection, aesthetics, and functional enhancements of materials across various industries [1]. Conventional coatings are prepared using petrochemical-derived binders, toxic solvents, and other non-renewable raw materials like pigments and additives. All solvent-based coatings emit ‘volatile organic compounds’ (VOCs) into the environment, causing severe health challenges [2]. Special-purpose raw materials of coatings, like biocides or heavy metals, can affect the ecosystems and human health. Therefore, these challenges encourage the development of sustainable coatings based on renewable raw materials. Sustainable coatings are prepared using bio-based or recycled materials. These coatings reduce the dependency on conventional petrochemical raw materials and reduce the emission of greenhouse gases. Several research works are being carried out to replace the toxic solvents from coating formulation with water as a universal solvent. Use of water in coating synthesis is one of the milestone achievements by coating formulators [3]. Now is the time to replace other paint raw materials with sustainable alternatives. Therefore, this research is carried out to study the potential of biochar as a promising and sustainable raw material for coating formulations.

Biochar is a rich carbon material obtained from the pyrolysis/carbonization of biomass (like wood, crop residue, farm waste, etc.) in an inert atmosphere [4]. Carbonization of organic precursors is carried out at higher temperatures ranging from 400 °C to 1000 °C. The thermal decomposition of biomass produces both high and low-weight molecular vapors called bio-syngases while leaving solid residue called biochar [5]. The properties of biochar are greatly affected by the type of precursor, temperature, heating rate, and duration of pyrolysis [6,7,8]. Biochar produced under identical conditions but with different biomass shows different physical and chemical properties [9]. Physicochemical properties of biochar, such as specific area, pore structure, elemental composition, functional groups, and surface morphology, need to be studied before selecting the biochar for specific applications [10]. In recent years, biochar has gained tremendous importance in the field of materials science; however, its applications in the coating field are still emerging [11].

The use of biochar in the coating field helps to achieve the objectives of green chemistry and sustainable development, thereby reducing dependence on non-renewable raw materials, lowering the carbon footprint of coatings, and also improving their performance. Incorporating biochar into coating enhances desirable properties such as improved thermal resistance, UV resistance, and reduced permeability to gases and moisture. Moreover, the use of biochar also supports waste valorization, transforming otherwise discarded biomass into high-value industrial inputs. Recently, coating formulators have been exploring the use of biochar in various coating systems such as epoxy, polyurethane, and acrylic resins. The performance results of these coatings are encouraging for further study in this field. However, challenges remain in achieving uniform dispersion of biochar particles within the resin, ensuring compatibility with other coating components, and optimizing concentration levels to achieve maximum performance without compromising coating integrity. Surface modification of biochar also needs to be explored to improve the performance of biochar-based coatings.

A super-hydrophobic coating can be prepared using rice straw biochar modified with cobalt sulphate and nickel sulphate [12]. Modification of biochar with cobalt and nickel significantly improved the hydrophobicity of the coating, which is measured using water contact angle and water sliding angle. The contact angle of these coatings was improved to 161° and 165°, respectively. The electrochemical impedance spectroscopy results reveal that metal panels coated with nickel-modified biochar show excellent protective properties in a 0.5 M NaCl solution. The UV stability of these coatings was also measured to study the effect of this modification, and it was observed that steel coated with nickel-modified cobalt biochar (Ni@Co-BC@SA) remains stable for up to 95 h, whereas steel coated with nickel-modified biochar (Ni@BC@SA) remains stable for up to 65 h. Super-hydrophobic coating can be prepared using corn straw-based biochar. The hydrophobicity of the coating can be improved by modifying this biochar with KOH and Fe₃O₄ [13], silane-based additives. The modification of biochar with KOH and Fe₃O₄ significantly improved the hydrophobicity of the coating, where water contact angles were increased up to 154.49°. The modification of biochar with silane-based additives is also a technique, in which the author has experience, to improve the hydrophobicity of the coating [14]. Modifying the biochar with silane-based additives increases the water contact angle up to 143.99° [14]. Biochar modified with carboxyl-methyl chitosan and 8-hydroxyquinoline can be used as a corrosion inhibitor [15]. Rice straw biochar can also be employed for the synthesis of an anti-icing coating for power generation machinery. Barrier properties and corrosion resistance of the coating can be improved using eco-friendly lamellar biochar [16,17]. Biochar nano-particles can also be synthesized using spruce wood and wheat straw [18]. The addition of biochar promotes electrochemical reactions and the barrier effect of zinc-rich epoxy coating. The super-hydrophobicity of the coating also enhances the self-cleaning properties of the coating [19].

The present research focuses on the development of epoxy-based coatings incorporated with biochar at varying concentrations. The biochar used in this research is prepared using pyrolysis at a temperature of 800 °C and 1000 °C. This research also attempts to study the effect of temperature on biochar properties and subsequently its effect on coating properties. The effect of biochar loading is systematically evaluated using contact angle measurement. This study provides a comprehensive understanding of the potential of biochar as a sustainable alternative for coating raw materials.

2. Materials and Methods

2.1. Materials

In this study, sheep wool was used as a precursor for biochar synthesis. Sheep wool was bought from the local market of Pune, Maharashtra, India. Epoxy resin and polyamide hardener were purchased from M/s Ajay Scientific Chemical Company, Pune. The epoxy resin used for coating synthesis had an epoxy equivalent weight of 185 g/epoxy equivalent. The polyamide hardener used as a curing agent for epoxy resin had an amine value of 140 mg KOH/gm. All other reagents used were of analytical grade and used as received.

2.2. Synthesis of Biochar

Raw sheep wool was cleaned with distilled water to remove any traces of impurities and dried in open air for 24 h. Then, sheep wool was filled into the pyrolyzer tube. The pyrolyzer tube was placed in the tube furnace and fitted with a nitrogen gas supply, equipped with a pressure regulator. Two variants of biochar were prepared by pyrolyzing sheep wool at 800 °C and 1000 °C for 1 h. The obtained biochar material was ground using a mortar and pestle, then sieved using a sieve of mesh size no. 500. The final superfine products were named SW800 and SW1000. The yield of biochar was calculated using the following formula:

$$\mathrm{\%}Yeild={\displaystyle \frac{WeightofBiochar}{Weightofsheepwoolbeforeheating}}\times 100$$

(1)

2.3. Synthesis of Coating Material

Synthesis of a coating based on biochar derived from sheep wool is the first attempt of this kind. So, establishing the baseline for coating formulation, epoxy-based coatings were synthesized using SW800 and SW1000 biochar by varying concentrations ranging from 1% to 5%. A total of eleven coating formulations were prepared as per

Table 1. Biochar–epoxy coating formulations.

	Epoxy Resin % by Weight	SW800 % by Weight	SW1000 % by Weight	Solvent % by Weight	Total
Batch-1 (Biochar-free coating)	90	-	-	10	100
Batch-2	90	0.9	-	9.1	100
Batch-3	90	1.8	-	8.2	100
Batch-4	90	2.7	-	7.3	100
Batch-5	90	3.6	-	6.4	100
Batch-6	90	4.5	-	5.5	100
Batch-7	90	-	0.9	9.1	100
Batch-8	90	-	1.8	8.2	100
Batch-9	90	-	2.7	7.3	100
Batch-10	90	-	3.6	6.4	100
Batch-11	90	-	4.5	5.5	100

. These formulations are named Batch 1 to Batch 11. Batch 1 formulation represents the biochar-free coating system. Batch 1 was used as a benchmark formulation to study the effect of biochar on coating properties.

Once the coatings are formulated, the pre-mixture of raw materials is added into a sand mill containing zirconium sand of 1 mm diameter. Grinding was carried out for 1 h to get a degree of dispersion up to 7+ on the Hegman gauge scale.

2.4. Preparation of Coating Panels

Mild steel panels (Grade ASTM A36) of dimensions 150 mm × 75 mm × 0.8 mm were properly degreased and de-rusted with degreasing and de-rusting chemicals, respectively, as per ASTM D 609. Prepared coating samples of batches were mixed with hardener in a 4:1 ratio. The surface prepared mild steel panels were coated uniformly using a conventional spray gun to achieve a dry film thickness of 60–70 microns. The applied panels were kept for curing in a dust-free environment for 168 h to ensure complete curing of the epoxy resin with polyamide hardener.

3. Result and Discussion

3.1. Yield of Biochar

The yield of SW800 was found to be 25.77%, while that of SW1000 was 23.65%. These results confirm that the yield of biochar decreases with an increase in temperature. Temperature and duration of pyrolysis are two main factors influencing the yield of biochar. The association of the yield of biochar with temperature is inversely proportional. This occurs due to higher thermal conditions, which accelerate processes such as aromatization, dehydrogenation, decarboxylation, deoxygenation, and dehydration, which collectively promote more extensive carbonization and the release of volatile compounds [20]. Temperature not only decreases the yield of biochar but also increases the ash content of biochar. This is attributed to increasing the pyrolysis temperature, causing biochar to accumulate more cations and carbonates, resulting in an insignificant increase in the ash of biochar [21,22]. For example, a study on the effect of temperature on ash content during the preparation of sugarcane bagasse biochar found that ash content at 800 °C is two to three times higher than that of biochar obtained at lower temperatures [23].

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Prepared biochar samples were analyzed using FTIR for their structural analysis and characterization of their functional groups.

For SW800, the peak obtained at 3000–3500 cm⁻¹ corresponds to the presence of O-H stretching vibrations (Figure 1). This might be due to adsorbed water (or moisture) in the sample; however, in the case of SW1000, the region from 3000 to 3500 cm⁻¹ is relatively flat and does not show a distinct O-H absorption. This indicates less adsorbed water or fewer hydroxyl groups. For SW800, the peak observed at around 1592.93 cm⁻¹ represents likely C=C stretching (aromatic/alkene), C=O stretch, or N-H bending, whereas for SW1000, this stretching is absent. This reveals that an increase in temperature damages the C=C stretch or N-H bending of the compound. Peaks at around 1032.53 cm⁻¹ and 1006.70 cm⁻¹ of SW800 represent C-O or Si-O-Si stretching. This stretching of C-O or Si-O-Si also corresponds to SW1000 at peaks around 1005.99 cm⁻¹. SW1000 shows a peak at 1377.67 cm⁻¹, which is often associated with C-H bending vibrations (e.g., symmetric bending of CH₃ groups). FTIR analysis of both the biochar exhibits strong and increasing absorption below 700–800 cm⁻¹, indicating similar inorganic or complex skeletal structures/materials. Both samples of biochar show similar core structural elements. Whereas SW800 shows a higher % transmission (%T) compared to SW1000, revealing SW1000 is cleaner, more concentrated, and thicker material compared to SW800.

3.3. Field Emission Scanning Electron Microscopy with EDX (FESEM-EDX)

FESEM-EDX is a tool used for understanding both the detailed surface structure (morphology) and the elemental composition of materials. FESEM-EDX visualizes surface morphology at very high resolution and simultaneously determines elemental composition. The prepared biochar samples were analyzed using FESEM-EDX to study the effect of temperature on their morphology and chemical composition, and it was revealed that the surface of both biochar samples appeared dense with a smooth texture, which positively contributes to the mechanical and adhesive properties of coatings. For elemental analysis, both spectra show a very large peak for carbon (Figure 2). This indicates carbon is a fundamental element of both materials. Similarly, in both biochar samples, oxygen is the second most prominent component of the material; however, in the case of SW1000, the concentration of oxygen increased to 15.9%. This indicates the formation of ash content at higher temperatures.

3.4. X-Ray Diffraction (XRD)

XRD is a fundamental, non-destructive technique widely used in materials science for crystal structure analysis. Both prepared biochars, SW800 and SW1000, were analyzed using XRD to study the effect of temperature on their crystalline properties. XRD patterns of both biochars exhibit broad peak characteristics of amorphous or semi-crystalline materials. This indicates that both samples likely possess an amorphous nature or are composed of very small crystallinity. Both graphs show similar overall intensity ranges, with the maximum intensity reaching around 400–450 counts for SW800 and slightly higher (around 500 counts) for SW1000 (Figure 3). Both biochars exhibit a prominent broad peak centered roughly between 20 and 35 degrees 2θ. This indicates a slightly amorphous nature of the material. The main peak of SW800 appears more prominently at around 27–28 degrees 2θ, leading to a somewhat bimodal appearance within this broad region. SW1000 also exhibits a similar broad peak in this region, but it appears more defined and shifted to a slightly higher 2-theta (around 28–29 degrees). The secondary features of both biochars are present in almost a similar fashion, showing subtle variation in their intensity and definition. This indicates minor differences in the atomic arrangements or the presence of short-range ordered structure between the biochars.

3.5. Study on the Effect of Biochar on Coating Performance

3.5.1. Gloss of the Coating

Different coating formulations were made as per Table 1 to study the effect of biochar on coating performance. Batch 1 formulation represented the coating without biochar. The subsequent batches, i.e., batch 2 to 11, are prepared by varying the concentration of SW800 and SW1000. The gloss results of the prepared coatings are mentioned in

Table 2. Gloss performance with biochar.

Biochar Loading (%)	Batch SW800 (Gloss in GU)	SW1000 (Gloss in GU)
1%	79.2	81.4
2%	75.6	77.7
3%	74.0	75.4
4%	73.8	74.6
5%	72.5	73.8

. Batch 1, which is free from biochar, gave the highest light reflection and highest gloss (85.5 GU). The particle size, shape, and dispersion of biochar within the resin matrix create surface roughness, which reduces the specular gloss of the coating. It was observed that the gloss of the coating is reduced as the concentration of both biochars increased in their respective batches. This is a typical trend for coatings, as the concentration of filler increases, surface irregularities increase, and the resin film gets interrupted by biochar particles. However, coatings with SW1000 biochar show higher gloss than those with SW800 biochar at the same concentration. This may be explained as SW1000 has a significantly larger crystallite size (113.7 Å) than SW800 (12.69 Å). Larger crystallite size generally implies a more condensed and organized carbon structure. Even though both are somewhat disordered, the larger domains in SW1000 suggest a potentially smoother or less porous particle surface, which could lead to better dispersion in the resin and less disruption of the coating film. At the same time, smaller particles have a higher surface-to-volume ratio. Better dispersion and uniform coating film over the surface resulted in improved gloss of the coating.

3.5.2. Mechanical Properties of Coating

Mechanical properties of the coatings were measured, including adhesion, pencil hardness (ASTM D 3363), scratch hardness (ASTM D 2197), and flexibility (ASTM D 522). The adhesion was rated as 5B for all the batches, which was attributed to strong bonding of the coating with the metal substrates, with no detachment or flaking off during the testing. These results indicate the coatings maintained their integrity to the metal substrate, even after being subjected to mechanical stress (Figure 4). Adhesion was not affected by the type of biochar or its concentration. This reveals that biochar is well compatible with epoxy resin matrices, and it does not affect curing and maintains bonding with the resin, hardener, and substrate. This occurs because biochar affects surface aesthetics due to roughness or scattering of light, but it does not interfere with mechanical interlocking or chemical bonding; therefore, adhesion remains unaffected. The addition of SW800 and SW1000 resulted in a slight reduction in pencil hardness from 4H to 3H, while the remaining coatings maintained a pencil hardness of 3H, reflecting good surface durability. All coatings, including biochar-free or biochar-loaded, exhibited excellent scratch hardness of up to 3500 g. This signifies high resistance to scratch, abrasion, and mechanical damage. Similarly, all coatings exhibited identical flexibility at 7 mm bending when tested using a conical mandrel. This is a positive outcome of the study, which indicates that the incorporation of biochar does not make the epoxy resin coating more brittle or less flexible. This is important and crucial for applications where the substrate might undergo some deformation during its operational life.

3.5.3. Chemical Properties of Coating

Determining the chemical resistance of coatings is extremely critical when coatings are applied for metal protection. Prepared biochar-based coatings were evaluated for their stability in 5% HCl and 5% NaOH. The fully cured and aged coated panels were dipped into a 5% solution of HCl and NaOH for 24 h, as per ASTM D 1308. The coated panels were evaluated visually for their performance, and it was noticed that the addition of biochar to the coating did not affect the chemical resistance of the coating. This reveals that the chemical structure of the coating matrix remained intact and stable even under corrosive conditions. Furthermore, the coated panels were also tested for the MEK solvent rub test to evaluate the degree of cure and the chemical resistance of the coating. The rating of “5” representing the coating is passed to 50 double rubs as per ASTM D 4752 and ASTM D 5402. All coatings, irrespective of the type and concentration of biochar in coating formulation, exhibited outstanding solvent rub resistance. This attributes the effective curing of epoxy with polyamide hardener, ensuring chemical stability and resistance to degradation from solvent rub.

3.5.4. Effect of Biochar on Water Contact Angle of Coating

The water contact angle (WCA) of coatings is a crucial parameter for estimating the surface hydrophobicity of developed coatings. A higher water contact angle indicates that the coated surface repels water. This property is important for protective coatings as it improves water resistance, corrosion resistance, and self-cleaning behavior of the coating. For both SW800 and SW1000 biochars, the water contact angle consistently increases as the concentration of biochar increases (Figure 5). It was also noticed that coatings containing SW1000 biochar consistently exhibit higher contact angles compared to coatings with SW800 at equivalent concentrations. This suggests that a higher pyrolysis temperature contributes more significantly to the hydrophobicity of the coating than biochar synthesized at lower temperatures.

The contact angle of coatings also reveals that SW1000 has a significantly larger crystallite size compared to SW800. Larger and more ordered, condensed, and organized carbon structures lead to greater inherent hydrophobicity. When these particles are incorporated into the coating, it improves the surface hydrophobicity of coatings. The higher pyrolysis temperature leads to a greater degree of graphitization. At higher temperatures, volatile compounds and oxygen-containing functional groups (like hydroxyl, carboxyl, and carbonyl) are removed from the surface. Loss of functional groups is evident from FTIR and EDX data as discussed earlier. These polar functional groups are responsible for the formation of hydrogen bonds with surface moisture or water molecules present on the surface, making the surface hydrophilic in nature.

4. Conclusions

This study comprehensively investigated the feasibility of developing sustainable, corrosion-resistant epoxy coatings by incorporating sheep wool-derived biochar. Two variants, SW800 and SW1000, were synthesized and characterized, with SW1000 exhibiting a lower yield (23.65% vs. 25.77% for SW800). Structural analysis via XRD revealed that SW1000 possessed a significantly larger crystallite size as compared to SW800, indicating a more condensed and organized carbon structure at higher pyrolysis temperatures. FTIR confirmed structural differences, and EDX analysis further highlighted elemental variations, including a higher oxygen content in SW1000.

The evaluation of coating performance with biochar loadings from 1% to 5% demonstrated significant functional benefits. This study affirmed that biochar inclusion did not compromise critical mechanical properties. All biochar-loaded coatings, irrespective of type or concentration, achieved an excellent 5B adhesion rating, indicating strong bonding to the metal substrate. Moreover, all coatings exhibited identical mechanical and chemical properties, such as scratch, pencil, flexibility, acid, alkali, and solvent rub test. A major advantage of the incorporation of biochar is that it consistently enhances the hydrophobicity of the coatings. The water contact angle of SW800 and SW1000-based coatings progressively form 68.2 to 81.1 and 85.2 degrees, respectively, with increasing biochar concentration. Coatings containing SW1000 biochar consistently exhibited higher contact angles compared to SW800 at equivalent concentrations.

In conclusion, this research successfully establishes sheep wool-derived biochar as a promising and sustainable alternative raw material for high-performance epoxy coatings. These findings support the development of sustainable protective coatings that contribute to waste valorization and reduce reliance on petrochemical-derived materials.

For future scope, various aspects of biochar need to be explored to improve the performance of the coating. This includes surface functionalization of biochar with silane, amine, or acid groups, etc. Integration of biochar with nanoparticles will also be an interesting field for coating formulators. Though the coating based on biochar materials has opportunities to become a sustainable solution, it is associated with certain challenges that limit its application in broader coating sectors. These challenges include variation in input biomass quality, coloration of coating as biochar mainly available on dark colors, compatibilities with resin matrix, etc. Addressing these challenges is crucial to unlock the potential of biochar in the paints and coatings field.