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Article

Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings

by
Pieter Samyn
*,
Chris Vanheusden
and
Patrick Cosemans
Department of Innovations in Circular Economy and Renewable Materials, SIRRIS, 3001 Leuven, Belgium
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(5), 511; https://doi.org/10.3390/coatings15050511
Submission received: 31 March 2025 / Revised: 18 April 2025 / Accepted: 23 April 2025 / Published: 24 April 2025
(This article belongs to the Section Functional Polymer Coatings and Films)
Browse Figures
Versions Notes

Abstract

:
In view of sustainable-by-design issues, there is an urgent need for replacing harmful coating ingredients with more ecological, non-toxic alternatives from bio-based sources. In particular, fluorine derivatives such as polytetrafluoroethylene (PTFE) powders are frequently applied as coating additives because of their versatile role in rendering hydrophobicity and lubrication. In this research, a screening study is presented regarding the performance of alternative micronized biowax powders, produced from various natural origins, when used as functional additives in protective epoxy coatings for wood. The micronized wax powders from bio-based sources (carnauba wax, rice bran wax, amide biowax) and reference fossil sources (PE wax/PTFE, PE wax, PTFE), of large (8 to 11 µm) and small sizes (4 to 6 µm), were added into fully bio-based epoxy clear coat formulations based on epoxidized flaxseed oil and proprietary acid hardener. Within concentration ranges of 0.5 to 10 wt.-%, it was observed that rice bran micropowders present higher hardness, scratch resistance, abrasion resistance, and hydrophobicity when compared to the results for PTFE. Moreover, the proprietary mixtures of biowax combined with PTFE micropowders provide synergistic effects, with PTFE mostly dominating in regards to the mechanical and physical properties. However, the granulometry of the micronized wax powders is a crucial parameter, as the smallest biowax particle sizes are the most effective. Based on further analysis of the sliding interface, a more ductile surface film forms for the coatings with rice bran and carnauba wax micropowders, while the amide wax is more brittle in parallel with the synthetic waxes and PTFE. Infrared spectroscopy confirms a favorable distribution of biowax micropowders at the coating surface in parallel with the formation of a protective surface film and protection of the epoxy matrix after abrasive wear. This study confirms that alternatives to PTFE for the mechanical protection, gloss, and hydrophobicity of wood coatings should be critically selected among the available grades of micronized waxes, depending on the targeted properties.

1. Introduction

In view of sustainable development and reducing the carbon footprint in the coatings industry, the performance and sustainability of protective coatings should be enhanced by the incorporation of bio-based additives. Environmentally harmful products such as PTFE are frequently used because of their versatility and combination of unique properties, providing hydrophobicity, chemical resistance, thermal stability, non-stick properties, scratch resistance, dry lubrication, and abrasion protection [1]. The epoxy/PTFE-wax composite coatings demonstrated better wear resistance and self-healing properties owing to the low melting point of the PTFE-wax, undergoing phase changes and migration [2]. Depending on the degree of crystalline and amorphous phases of the fluoro-wax compounds, the hydrophobicity of the coatings was controlled through the mobility of the wax molecules [3]. Due to its natural persistence and regulatory concerns regarding the potential hazards of by-products generated during production and/or the particles generated during usage and wear (i.e., PFAS components like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS)), their use is restricted, and substitutes are explored.
Alternatively, the polymer waxes are non-toxic, non-corrosive, white or slightly yellowish solids with a relative molecular mass of 1800 to 8000 g/mol [4]. The waxes of synthetic or bio-based origin can offer a valid replacement when mixed as a dry powder in coating formulations: they exhibit high chemical stability, temperature resistance, chemical resistance, and electrical properties at room temperature. The waxes offer diverse applications as a raw material for mold lubrication, a modifier for plastics, a coating agent for textiles, and an additive for regulating oil viscosity. In many coating formulations, waxes are added under the form of an aqueous emulsion to improve smoothness and scratch resistance [5], while they may control hydrophobicity and introduce matting effects in non-glossy paints [6]. The wax-based additives in wood coatings are also effective for preservation and contain antimicrobial properties [7]. However, the effectiveness of the wax properties highly depends on origin and particle size. The developments in wax technology have allowed for the production of wax micropowders with controllable dimensions in the 1 to 50 µm size ranges by micronization using supercritical fluids [8,9], in combination with extrusion [10], spraying [11], air-jet and ball milling [12], or melt dispersion [13]. Today, many technologies have been industrialized and patented, principally based on processing by spraying a hot wax melt [14] or high-shear mechanical grinding [15]. The effects of micronized wax based on PTFE-modified PE for improving scratch resistance properties in solvent-borne marine coating systems have been demonstrated [16]. The micronized PE waxes could be dispersed at high solid content into water-based wood furniture top coatings with protective properties, offering slip, scratch, and abrasion resistance, as well as a uniform matte finish, clarity, and a smooth feel [17]. In a first step towards more sustainable additives, the dispersion of micronized bio-PE waxes in a polymer matrix via mixing is the most critical process for optimizing coating performance [18].
The selection of appropriate bio-based wax additives is critical, as they enhance the wood protective coatings to different extents, depending on their intrinsic properties [19]. The wax-based coatings for wood protection are mostly available through waterborne emulsions including, e.g., carnauba wax [20], beeswax, and paraffin waxes [21]. Carnauba wax is historically frequently used, complemented by ethylene-bis-stearamide and hydrogenated castor oil [22]. Carnauba wax is one of the hardest natural waxes, producing durable hydrophobic coatings [23], but is more difficult to process and is slightly colored [24]. Stearamide wax typically contains approximately 90% stearic acid, which can be derived from both animal and plant sources [25], and has the highest melting point, providing good lubrication properties [26]. Among others, natural beeswax, candelilla wax, shellac wax, wood wax, and tung wax are currently of interest for wood protection [27].
In this study, the role of micronized biowaxes of different origins and sizes is evaluated when added in 0.5 to 10 wt.-% concentration ranges as micropowder additives in an epoxy coating. This innovation in the systematic screening of biobased waxes may offer a decision-making tool for sustainable PTFE alternatives, depending on the user case, in addition to the availability of specific case studies offered by single industrial suppliers. Indeed, the broad application range of PTFE with benefits for hydrophobicity, abrasion resistance and/or gloss can often not be simulated by a single substituent. With potential applications as wood protective coatings, the mechanical and physical properties are evaluated and benchmarked against those of micronized synthetic wax powders (e.g., PTFE and PE wax/PTFE mixtures), For better understanding on the protective roles of biowaxes, additional analysis of the surface properties by infrared spectroscopy is presented for native and worn coatings. The presented data may contribute to a more dedicate selection of biowaxes serving as potential alternatives for PTFE in coatings with equal performance.

2. Materials and Methods

2.1. Materials

The substrates include 10 × 10 cm2 samples of hardwood beech (Fagus sylvatica), with a thickness of 5 mm and a planed-top surface, as obtained from a local shop (Martens Hout, Leuven, Belgium). The wood samples were dried overnight in a hot circulating air oven at 60 °C to control moisture content before the coatings were applied.
The epoxy resin coating consists of an epoxidized flaxseed oil (Component A), and a proprietary hardener mix comprising bio-based aliphatic organic acids and esters of citric acid, tartaric acid, and ethyl lactate (Component B), as obtained from Orineo BV (Kortenberg, Belgium).
The micronized wax powders, including biowaxes (carnauba, rice bran, stearamide), mixed fossil-based waxes with a proprietary ratio of synthetic PE wax versus PTFE, and the reference wax powders of synthetic PE wax and PTFE, were obtained from Shamrock Technologies (Tongeren, Belgium). The different types of micronized waxes, with selected particle size distributions (d10, d50, d90 percentile values) and melting temperature ranges, are detailed in Table 1. The differences in particle size and size distribution might have an impact on the packing density, surface area available for interaction, and ultimately, the performance of the additives. Among the bio-based waxes, carnauba wax is known to have the highest melting point, being harder and more insoluble in water and ethanol as compared to rice bran wax. A morphology image, obtained by confocal laser scanning microscopy (VK-X3000, Keyence, Mechelen, Belgium, see below) or scanning electron microscopy (Tabletop TM3000, Hitachi, Krefeld, Germany), of the micronized wax grades is shown in Figure 1, depicting variations in morphologies and shapes. All bio-based waxes have regular spherical shapes, while PTFE particles are more plate-like, and PE-wax particles are more cubic. The different particle shapes might particularly influence the distribution, packing, and stress concentrations in the coating. However, targeting the practical performance of the additives in the coatings in this study, the latter microscale effects were not further quantified.

2.2. Coating Formulation and Deposition

The epoxy coatings were made by mixing Component A and Component B above, in a given 2:1 (wt.—wt.) ratio under manual stirring of small batch sizes of 15 g. Given the relatively high viscosity at room temperature of both components (i.e., 1100 mPa·s for Component A; 10,000 mPa·s for Component B), they were preheated to approximately 35 °C (Component B) and 30 °C (Component A) before mixing. The viscosity of the mixed liquid sample was around 5000 mPa·s (room temperature) and dropped towards 1800 mPa·s after mild heating (35 °C), at which time the respective micronized wax powders were added in concentrations of 0.5, 1, 2, 5, 7.5, and 10 wt.-%. A broad range of concentrations was logically selected, while the concentrations below 0.5 wt.-% did not result in significant effects, and the higher concentrations caused agglomeration. The final coating formulations were manually mixed for about 5 min under mild conditions, while minimizing air inclusions. The open time of the liquid coating was about 45 min at room temperature, which is sufficient for fluent processing. A reference coating (R) with the pure epoxy matrix and no wax fillers was simultaneously prepared; other coatings are referred to with the acronyms in agreement with the wax types, as in Table 1.
The coating resin was spread by doctor blading over the substrates into a wet layer thickness of 500 µm, corresponding to a dry coating thickness of 485 to 490 µm. The coatings were cured for 1 h at 60 °C to flow the liquid sample into a homogeneous film, followed by 3 h at 90 °C for full crosslinking. The crosslinking of epoxidized vegetable oils in the presence of appropriate acids via the opening reaction of the oxirane ring in a liquid state was generally described previously [28,29], under established thermal curing conditions [30]. The final coated samples were stored for one month under a controlled atmosphere (25 °C, 60% RH) before further testing.

2.3. Characterization Methods

The mechanical properties of the coatings were tested according to standardized testing methods of the wood coating industry. The abrasion wear resistance was determined on a circular Taber tester with a dual rotary platform (Model 5135, Taber Industries, New York, NY, USA), following ASTM D4060-10(2015), applying a rotational speed of 72 r/m and CS-10 Calibrase abrasive wheels (Taber Industries, New York, NY, USA) under a load of 250 or 500 g. The abrasive wheels were reconditioned against an S-11 resurfacing disc after each testing sample running over 1000 cycles. The abrasive wear was determined as the weight loss after 1000 cycles, using an analytical balance with an accuracy of 0.1 mg (Sartorius, Göttingen, Germany). Microhardness measurements were performed with a handheld Shore D durometer (micro hardness tester) according to ASTM D2240(2021), using a hardened steel tip with a 30 ± 0.5° conical point and 0.100 ± 0.012 mm tip radius. The micro-scratching was performed using a sclerometer type 3092 (Elcometer, Aalen, Germany) with a 0.75 mm radius tungsten carbide tip and loaded under 10 or 20 N by exchange of an appropriate spring constant, following ISO 1518-1(2023).
The determination of physical properties included specular gloss measurements with a micro-TRI-gloss meter (BYK-Gardner Instruments, Geretsried, Germany) under 60° following ISO 2813(2014). The static contact angles with deionized water were measured on an OCA 50 goniometer (Dataphysics Instruments GmbH, Filderstadt, Germany), using 3 µL droplets that were fitted by the Laplace–Young procedure following ISO 19403-2(2024). The optical microscopy observations were made on a stereomicroscope MZ12 (Leica, Wetzlar, Germany) at magnifications of 20× or 50×. The topographical images were obtained on a VK-X3000 laser interferometer (Keyence, Mechelen, Belgium) at magnifications of 50× or 150×.
The Fourier transform infrared (FTIR) spectra were recorded in the attenuated total reflection mode using a diamond crystal with HeNe laser on a Nicolet iS10 spectrometer (Thermo Fischer, Breda, The Netherlands). The spectra were collected in the wavenumber range of 4000 to 5000 cm−1 with a resolution of 4 cm−1 and averaged over 32 scans.

3. Results

3.1. Mechanical Coating Performance

The effect of different types and concentrations of wax micropowders on the mechanical and protective properties of the epoxy coatings on wood was evaluated by abrasive wear testing, hardness measurements, and scratch-resistance testing.
The results of abrasive wear loss, as shown in Figure 2, are expressed as a weight loss of the coatings after comparative testing over 1000 cycles under low loads (250 g) and high loads (500 g). As compared to the native epoxy coatings, the wax-filled coatings show better abrasive wear resistance under both low and high loads; however, there are significant differences between the wax grades, depending on their origin, size, and concentration. For all waxes, there is a trend showing that the smaller-sized micronized powders (4 to 6 µm) provide better wear resistance than the larger powder sizes (8 to 11 µm). Alternatively, the high wax concentrations and high loads may lead to mechanical overload, as also observed in other composites [31], while mechanical overload does not occur for coatings with rice bran wax and PTFE powders. It is known that the improvement in wear resistance is not proportional to the filler concentration, and only small amounts of wax might be needed to reduce the wear loss [32]. For the bio-based wax types, rice bran wax provides lower wear rates than carnauba wax and stabilizes wear loss under both low and high loads at high wax concentrations. The other studies indeed confirmed the good lubricating properties of rice bran wax in comparison with other wax lubricants during the compression of the tablets [33], whereas the present data complement the reported findings in determining the effects of rice bran concentrations and particle sizes. The amide biowax provides low wear rates under very specific conditions of intermediate concentrations and low loads, resulting in a smaller operational window regarding appropriate coating formulations as compared to that for the rice bran and carnauba wax. The amide waxes are typically hard and more brittle, rendering local cracking and fracture under load. The present data are in line with previous evaluations of the tribological properties of amide wax relatively to carnauba wax, where the long carbon chains and high hardness of amide wax result in low friction and less wear under low loads, as well as higher wear under high loads [34], while the differences in wear rates did not correspond to any variations in crystalline properties. Under the highest concentrations of rice bran wax, the abrasive wear remains low and comparable to that of the fossil PTFE powders. For fossil-based wax types, the abrasive wear of synthetic PE wax is higher as compared to that of rice bran wax, while the coatings with PE wax/PTFE provide lower wear rates as compared to those of the pure PE wax. In the latter case, however, the wear profile of the wax mixtures at higher concentrations is dominated by the wear profile of synthetic PE wax, particularly in regards to overload situation under high loads. Indeed the PE wax is often used as an admixture in oil lubricants; however, it cannot be applied under high loads due to softening [35]. For industrial applications, however, the formulations with a low wax content are the most preferred, where PTFE provides unique properties and can partially be replaced by a synthetic wax in PE wax/PTFE mixtures. It is known that the abrasion resistance for wax-filled coatings may be generally enhanced through the reduced friction and surface slip, where the spacer effect of the hard and smooth wax particles at the surface may also reduce the stresses on the surface [36]. In conclusion, the proper selection of coating formulations with the given biowax types and concentrations provides similar abrasive wear resistance as compared to that of the PTFE powders in the epoxy coatings.
The results for mechanical hardness are shown in Figure 3, where intermediate concentrations of wax may increase the mechanical hardness as compared to that for the unfilled epoxy coating. Other studies have also reported a very slight mechanical reinforcement of wax powders in acrylic paint, with the maximum results obtained at concentrations of around 1 wt.-%, with a higher hardness of carnauba-wax filled paints as compared to those using PE wax [32]. Indeed, the lowest wax concentrations up to 0.5 wt.-% in the present study do not influence the intrinsic mechanical properties of the coatings, while the higher concentrations, above 2 wt.-% of wax, are intrinsically softer as compared to the pure epoxy coating. Although it is known that the waxes and PTFE powders are relatively soft materials, they may apparently provide synergistic effects when mixed into an epoxy coating, with improved mechanical properties owing to the effects of the steric hinderance of the particles. For all waxes, there is a trend that the smaller powder sizes (4 to 6 µm) introduce a higher hardness as compared to the results for the larger powder sizes (8 to 11 µm), in parallel with a better abrasive wear resistance for the smaller powders. For bio-based wax types, the hardness increase for coatings containing rice bran is the highest in parallel with the observed best improvement in abrasive wear resistance. For fossil-based wax types, PE wax provides softer coatings at concentrations above 1 wt.-% in parallel with a steady increase in abrasive wear at higher concentrations, while the coatings with mixed PE wax/PTFE proved intermediate hardness between that of both constituents. Although there is a trend that higher hardness provides better abrasive resistance, there is no obvious one-to-one correlation between coating hardness and abrasive wear resistance for all wax types, as the lubricating properties of waxes exposed in the interface may dominantly contribute to reducing the abrasive wear. The formation of a lubricating transfer film dominates the abrasion response more than does the bulk hardness, especially for bio-waxes like rice bran, as shown below.
The scratches incurred on the coatings under a low load (10 N) and a high load (20 N) are illustrated by optical microscopy in Figure 4. According to the literature, the different scratch deformation modes for polymers include ductile ploughing, brittle ploughing, rubber-like or elastomeric deformation, ironing, and elastic responses of the polymeric surfaces [37]. The differences in ductile and brittle behavior are observed for the epoxy coatings with bio-based versus fossil-based waxes, depending on the normal load and wax concentration. Under low loads, all coatings exhibit a ductile behavior, with high recovery of the viscoelastic deformation, except for the epoxy coatings with the highest concentrations of amide wax, which show severe cracks attributed to brittle fracture. Under high loads, coatings with rice bran wax remain ductile up to the highest wax concentrations, while coatings with carnauba wax start to present a more brittle behavior at high wax concentrations. The high hardness of the epoxy coatings with rice bran wax corresponds to a high scratch resistance, without visible deformation, up to 2 wt.-% concentrations, while the lower hardness for coatings with carnauba wax corresponds to ductile properties, except for the case of the highest wax concentration. It is known from previous literature that the addition of soft fillers improves scratch resistance and reduces visibility of the track due to the viscoelastic recovery of the coating [38]. The same trend is presently observed for rice bran wax that has a lower Tg as compared to that of carnauba wax. The coatings with amide wax initially reflect more ductile properties at concentrations up to 2 wt.-%, with strong plastic deformation at the edges of the scratching track, while the higher concentrations of amide wax introduce ductile ploughing and crack formation. The crack formation and tearing effects may be introduced by internal stresses near the scratching tip at higher wax concentrations. The wax types with a higher Tg, such as amide wax, are harder and may change the local scratch deformation mode from ductile ploughing to a brittle failure, with crack formation. The scratched coatings with synthetic PE wax or PE wax/PTFE more frequently show brittle failure with chip formation. The reduced ductility for fossil-based compared to bio-based coatings has reported previously [39]. However, the brittleness of coatings is not uniquely determined by the hardness, but is also influenced by other microstructural factors such as compatibility between the additives and the matrix, introducing eventual stress concentrations. Regarding the effect of wax on scratch resistance, the lubricated friction dominates an improved scratch resistance in a single track, while the reduced hardness counteracts scratch resistance [40]. In summary, the better scratch resistance for bio-based waxes with high hardness for rice bran wax prevails over that for fossil-based wax additives.

3.2. Physical Coating Performance

The water repellence of epoxy coating formulations with different types of wax micropowders is characterized by static water contact angles measured before and after abrasive wear testing, as presented in Figure 5. As the hydrophobicity of wood coatings is a direct consequence of the exposure of wax at the surface, in combination with eventual texturing effects of the wax at the surface [41], an increase in water contact angles for the wax-filled coatings is observed relative to the epoxy coating. However, the reference epoxy already exhibits a relatively high water contact angle (over 95°) as compared to that of the fossil-based DGEBA epoxy coatings (traditionally 70 to 75°) [39] owing to the presence of epoxidized oil with long aliphatic chains. Overall, the coatings with micronized biowax present a higher hydrophobicity than the coatings with reference PE wax or PTFE and introduce the highest water contact angles, mainly for the smallest powder sizes.
After exposure of the wax in combination with the changes in surface roughness within the abrasive wear track, the water contact angles on the worn coatings significantly increase, and the hydrophobic protection remains, particularly in the presence of the biowax micropowders. Owing to the nature of bio-based waxes, with triglycerides containing long aliphatic chains, higher hydrophobicity than that of synthetic PE waxes and PTFE can be expected [42]. In general, the hydrophobicity is a combination of intrinsic hydrophobic properties of the wax micropowders in combination with the migration effects of the wax towards the surface (see the FTIR analysis of worn coatings shown below). In this context, the improved hydrophobicity of the coatings with smaller micronized powders can be better understood.
The variations in surface gloss for the epoxy coatings with different micronized waxes are shown in Figure 6, indicating a reduction in gloss values for the filled coatings as compared to those for the reference epoxy coating. The role of waxes as matting agents is known for different coatings [43], but the particle sizes and wax types exert specific influences on gloss. The gloss reduction is obviously higher for the large wax particles as compared to the small wax particles, as likely explained by the distribution of the wax particles at the coating surface causing interplay between surface roughness and light scattering effects. Within the present range of coating thicknesses, however, the thickness of the coating does not affect the gloss of the coating [44]. For industrial use, the variations over more than 10 gloss units are visually noticeable. Depending on the type of natural waxes, it is known that they are primarily used as matting agents to reduce the surface gloss [45], while they may also provide additional mechanical properties, as previously quantified [45]. The efficiency for creating matting effects with different PE waxes in powder coatings was reported previously, where the larger particle sizes result in a lower specular gloss [46]. However, some waxes are not efficient matting mechanisms when compared to traditional matting agents [47] due to their inherent particle morphology and size. The variations in gloss may be related to changes on the surface, resulting in increased light scattering on the coating [48]. It is particularly known that amide waxes only slightly reduce gloss, while imparting a specific textured pattern on the coatings [49].

3.3. Microscopic Evaluation

The abrasive wear tracks of epoxy coatings with different wax micropowders are evaluated by confocal laser scanning microscopy, as presented in Figure 7.
The protecting properties of the wax additives under abrasion rely on the formation of a film in the sliding interface, with smooth morphology and intrinsic lubricating properties. Depending on the morphologies of the worn surfaces, it can be observed that the smooth surface films mainly formed on epoxy coatings with the micronized biowaxes (rice bran wax, carnauba wax). For rice bran wax, a continuous surface film, with superficial surface scratches, develops at low wax concentrations, and a thicker film, with some pits, develops at higher wax concentrations. For carnauba wax, a thinner film mainly develops at higher wax concentrations. For amide wax, the irregular surface pattern depicts a lack of coherence and local fracture of the surface film, in parallel with the previously reported brittleness. The formation of a smooth sliding film for the bio-based waxes (rice bran, carnauba wax) may be attributed to the intermediate range of Tg of the pure biowaxes (see Table 1), offering a favorable combination of ductility and mechanical cohesion when biowaxes are exposed at the surface [50]. The typical abrasive wear mechanisms for carnauba wax were indeed previously associated with the spreading of the wax through plastic deformation [32]. The alternative wear mechanisms for the harder waxes wax may include material removal by local scratching [34]. For fossil-based waxes, the PTFE powder is known to provide smooth sliding surfaces due to its lubricating properties provided by the mechanical shear of its layered structures [51], which, in the present study, is shown to mainly occur at high PTFE concentrations. In contrast, the coatings with bio-based waxes already developed smooth sliding surfaces at low concentrations. For the coatings with PE wax, no coherent surface film was formed in parallel with the high abrasive wear loss, as determined previously. The coatings with PE wax/PTFE mixtures only form a coherent film at low concentrations, while the film fractures at higher concentrations are in parallel with the increase in abrasive wear loss. The limited capacity for bearing mechanical loads in the surface films with high amount of soft fillers may be a reason for failure due to mechanical overload and fracture of fossil-based waxes, in contrast with the higher ductility and smoothening of the surface films for bio-based waxes.
The observations for formation of protective surface films, as discussed above, are also evidenced in the topographical surface scans, as presented in Figure 8. The smooth surface films are preferentially formed on the coatings with biowax micropowders.

3.4. Spectroscopic Evaluation

The FTIR spectra for epoxy coatings with micronized waxes of various types and concentrations were captured at the top surface before wear (Figure 9), indicating the presence of characteristic peaks for the epoxy and wax components. The epoxy resin based on epoxidized flaxseed oil and an acid crosslinker dsiplays spectral bands at 2950 (CH3 stretching), 2850 cm−2 (C-H stretching), 1744 cm−1 (C=O stretching), 1460 cm−1 (CH2 bending), 1368 cm−1 (CH3 bending), 1162 cm−1, 1100 cm−1 (C-O-C stretching), and 720 cm−1 (CH2 deformation) [52]. The oxirane group at around 850 cm−1 has disappeared after the successful crosslinking of the samples through the ring-opening reaction of the epoxy group with the acid [53], while for the residual vinyl groups (C=C) at around 3020, 1655 cm−1 have almost completely been removed after epoxidation of the flaxseed oil. Based on the epoxy-related bands and constant intensity of the hydroxyl bands (3400 cm−1) and the methoxy groups (1744 cm−1) for the pure and filled epoxy coatings, no interactions between the waxes and the epoxy are detected, as the waxes are chemically inert and do not interfere with the crosslinking mechanism, as also noted in the previous studies with encapsulated wax additives [54].
In plant-based waxes, the presence of carbonyl stretching vibrations within the triglycerides is a unique feature, together with presence of methylene stretching vibrations within the aliphatic chains. Both the carnauba wax and the rice bran wax display very similar spectra representing their composition. Carnauba wax mostly consists of fatty acid esters and fatty alcohols derived from the C26–C30 range [55], including diesters of 4-hydroxycinnamic acid, ω-hydroxycarboxylic acids. Rice bran wax contains aliphatic acids (palmitic acid (C16), behenic acid (C22), lignoceric acid (C24)), alcohol esters (ceryl alcohol (C26), melissyl alcohol (C30)), free fatty acids, squalene, and phospholipids [56]. The typical spectral bands are at 2925 cm−1 and 2848 cm−1 (asymmetric and symmetric stretching of CH2), 1744 cm−1 (C=O stretching), 1456 cm−1 (CH2 bending, with a red shift of wax compared to that of epoxy, resulting in a double peak), and 717 cm−1 (CH2 deformation of the repeating methylene groups (-CH2)n, with a red shift of wax compared to that of epoxy, resulting in double peak) [57]. The carnauba wax also exhibits fewer detectable bands at 1634, 1607, 1515, and 834 cm−1 (skeletal ring breathing modes of aromatic groups [58]), or at 1170 cm−1 (C-O-C stretching of the wax esters [59]).
The amide wax displays additional spectral bands related to the amide groups CONH2 at 3390 and 3180 cm−1, the carbonyl C=O group stretching at 1607 cm−1 (amide I), the N-H bending of the secondary amide group at 1515 cm−1 (amide II), and the CH2 rocking and wagging band of CH2 at 717 cm−1 [60].
The PE wax is characterized by the spectral bands dominated by the presence of strong stretching, in-plane bending, and out-of-plane bending vibrations of the aliphatic CH2 and CH3 groups, without any additional functional groups, appearing at 2956 cm−1 (asymmetric axial deformation of CH3), 2925 cm−1, and 2848 cm−1 (symmetric axial deformations of CH2), 1471 cm−1 and 1463 cm−1 (symmetric angular deformations of CH2), 1377 cm−1 (symmetric angular deformation of CH3), and 717 cm−1 (doublet, asymmetric in-plane CH2 deformations) [61].
The PTFE powders show spectral bands at around 1210 and 1150 cm−1, which overlap between the C–C band at about 1240 cm−1, and strong bands of the epoxy matrix. The bands at 637 cm−1, 552 cm−1, and 503 cm−1 represent CF2 wagging, bending, and rocking vibrations, intensely changing with the crystallinity degree of PTFE [62].
Based on the intensity ratio of the selected spectral bands, information on the relative distribution of wax additives within the epoxy coating could be distinguished, as illustrated in Figure 10.
By using the ATR-FTIR technique, information on the composition within the top 2 µm depth of the coating layer is obtained. For bio-based waxes (Figure 10a), the intensity for rice bran waxes at the surface is higher as compared to that for carnauba waxes, indicating that the rice bran additives are more exposed at the surface. For both the rice-bran and carnauba waxes, the small powders (RBW-5, CBW-5) are more exposed at the surface as compared to the results for the larger powders (RBW-9, CBW-9). There is only a slight increase in the surface concentration of the waxes as a function of the overall wax concentration in the coating. The distribution of the amide wax (ABW-5) at the surface is significantly lower with a slight decrease in concentration as a function of the overall wax concentration in the coating. The high concentration of rice bran wax at the surface is in agreement with the low abrasive wear and hydrophobicity relatively to carnauba wax, but the lower concentrations of amide wax at the surface present a higher hydrophobicity. For fossil-based waxes (Figure 10b), the pure PE wax is poorly presented at the surface, and the PTFE is highly present at the surface, based on the respective band intensities. The trends for PTFE are completely different as compared to those for the PE/wax, with the higher surface concentrations as a function of the overall PTFE concentration in the coating. The latter corresponds to the low abrasive wear, while slightly lowering the microhardness. For the mixed PE wax/PTFE, there is a higher surface presentation of the PE wax relative to the PTFE. In contrast to the bio-based wax powders, however, the intensity of the large PE wax/PTFE powders (PW-9) is higher at the surface as compared to the results for the small PE wax/PTFE powders (PW-5), while the PTFE portion is the most exposed at the surface of coatings with small PE wax/PTFE particles (PW-5). In particular, the ratio of wax to PTFE at the surface of the epoxy coatings with mixed PE wax/PTFE decreases with higher additive concentrations (Figure 10c), while wax exposure remains the highest for the large PE wax/PTFE particles. The latter relationship with particle size does not necessarily introduce a higher hydrophobicity for the largest PE wax/PTFE particles, while it strongly reduces gloss and displays no unique relationship to abrasive wear and surface film formation. Overall, the performance of the coatings with PE wax/PTFE powders remains mainly controlled by the presence of PTFE at the surface. Therefore, the main synergies of PE in the wax mixtures can be mainly attributed to a diluent effect. By comparing the abrasive wear results and water contact angles, it can be observed that the presence of PTFE dominates the wear properties of coatings with mixed waxes. In conclusion, there is a complex distribution of the waxes in the coatings, resulting in different surface exposures. Mainly, the smallest rice bran wax micropowders are highly exposed at the surface, in parallel with previous coating properties.
In order to evaluate the role of waxes and coating degradation after abrasive wear, the chemical composition of the wear tracks was evaluated by comparing the FTIR spectra within the wear track to that of the original coating (Figure 11, note: the spectra of the wear track are manually shifted over 20 cm−1 for better visualization of the changes in peak intensities, while peak positions occurred at the same wavenumber position). The behavior of waxes in the coating under sliding may be complex, as either (i) the heating created by friction and pressure may plastify and liquefy the wax, inducing the its migration (except for PTFE, which fibrillates under mechanical shear), or (ii) the protrusions of single wax particles above the surface bear the loading and shear within localized points [63].
The degradation of the pure epoxy coating after abrasive wear can be noticed in a reduction of the C=O carbonyl group (1744 cm−1), and an increase in the CH2 groups (2950, 2850 cm−1) (Figure 11a), confirming that the ester groups are attacked after wear, likely by mechanical chain scission. The latter can also be confirmed by a higher intensity of the CH2 deformation band (720 cm−1) and a higher intensity of CH3 bending (1368 cm−1) in the molecular backbone of the epoxidized flaxseed oil, attributed to the formation of short end-chain groups and higher freedom of the molecular chains. The previous thermal degradation studies on epoxidized oils crosslinked with anhydride indicated a similar reduction in C=O after aging and degradation [64]. For the epoxy coatings with bio-wax additives CBW-5 (Figure 11b), RBW-5 (Figure 11c), and ABW-5 (Figure 11d), a reverse trend was noted, with growth in C=O and stabilization or reduction in bands associated with the CH2 and CH3 groups (2950, 2850, 720, 1368 cm−1), indicating the protective action of the biowax micropowders exposed in the wear track, while preventing chemical degradation of the epoxy matrix. The spectral analysis indeed confirms that a stable wax-rich surface film could form on the surface, possibly masking the underlying (potentially degraded) epoxy-related FTIR signal. Besides oxidation, the radical reaction can alternatively lead to polymerization reactions and the formation of C-C, C-O-C, C-O-O-C bonds between the epoxidized fatty acid chains through mechanical action and mild local heating effects [65]; these effects were not observed in the case of biowax lubrication. The adverse trends, with degradation near the C=O and C-O-C groups in the epoxy matrix, are noticed in the presence of PE wax (Figure 11e), while the protective properties of PTFE, preventing degradation of the epoxy coating, are also confirmed (Figure 11f). After abrasive wear, the wax-related spectral bands at 717 cm−1 remain present for carnauba wax and rice bran wax in similar intensities, reducing in intensity for amide wax and PE wax, indicating that bio-waxes remain more stable.
Although the mechanisms of wax behavior in a coating are complex, the present data may be associated with the theoretical theories that bio-based waxes are homogeneously distributed through the coating (ball-bearing theory) with constant intensity, while they may migrate to the surface (cold flow theory) with decreasing intensity after wear [66]. In conclusion, the micronized bio-wax powders are chemically present in the wear track and provide comparable protection against chemical degradation of the epoxy matrix when compared to that of the PTFE additives, as opposed to the results for the synthetic PE wax additives.

4. Conclusions

The performance of micronized wax powders in a bio-epoxy coating, including different types and concentrations of bio-based waxes (carnauba, rice bran, amide biowax), or fossil-based waxes (PE wax, PE wax/PTFE) and PTFE additives, was evaluated.
The proper selection of coating formulations with given biowax types and concentrations provides similar abrasive wear resistance as compared to that of PTFE powders in the epoxy coatings. For bio-based waxes, the rice bran micropowders provide lower wear results than carnauba micropowders and stabilize wear loss under both low and high loads at higher wax concentrations, while the small particles are more effective than the large particle sizes. There is no overall trend illustrating that the coatings with higher hardness correspond to a lower abrasive wear, but the coatings with smallest rice bran wax powder sizes provide the highest hardness and the best scratch resistance. Based on mechanical coating properties, there is good potential for replacing PTFE additives with rice bran wax, forming coatings with comparable abrasive wear resistance, high hardness, scratch resistance, and hydrophobicity.
The protective properties of bio-wax versus fossil-based wax micropowders in coatings were further confirmed by the formation of a smooth sliding film. While carnauba wax and rice bran wax additives illustrate ductile properties, amide wax additives show a different behavior, with more brittle properties, and fossil-based waxes show no coherent surface film. Based on spectroscopic data, it is indeed confirmed that smaller biowax micropowders are more exposed to the surface, while the formation of a smooth surface film prevents degradation of the epoxy matrix after abrasive wear in contrast to the results for PE waxes, PTFE, and their mixtures.
This study indeed provides good evidence and guidance for selecting appropriate micronized wax powders for use in protective epoxy coatings.

Author Contributions

Conceptualization, P.S.; methodology, P.S.; validation, P.S.; formal analysis, P.S.; investigation, P.S.; resources, P.C.; data curation, P.S.; writing—original draft preparation, P.S.; writing—review and editing, C.V. and P.C.; visualization, P.S.; supervision, P.C.; project administration, P.C.; funding acquisition, P.C. and P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Flanders Innovation & Entrepreneurship (VLAIO), grant number HBC.2019.2493 (BioCoat) and HBC.2023.0479 (AddBio).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within the article.

Acknowledgments

We thank Guido Heunen for providing technical assistance with the FTIR measurements and Olivier Malek and Felipe Baroni for their support with the microscopic analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Microscopic images of different micronized wax powders, using confocal laser scanning microscopy of (a) CBW-5, (b) CBW-9, (c) RBW-5, (d) RBW-9, (e) ABW-5, and scanning electron microscopy of (f) PTFE and (g) PE.
Figure 1. Microscopic images of different micronized wax powders, using confocal laser scanning microscopy of (a) CBW-5, (b) CBW-9, (c) RBW-5, (d) RBW-9, (e) ABW-5, and scanning electron microscopy of (f) PTFE and (g) PE.
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Figure 2. Abrasive wear loss of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%), under low loads (250 g, green bars) and high loads (500 g, blue bars); error bars determined from n = 3 experiments.
Figure 2. Abrasive wear loss of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%), under low loads (250 g, green bars) and high loads (500 g, blue bars); error bars determined from n = 3 experiments.
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Figure 3. Mechanical hardness (shore D) of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%); error bars determined from n = 10 measurements.
Figure 3. Mechanical hardness (shore D) of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%); error bars determined from n = 10 measurements.
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Figure 4. Mechanical scratching (sclerometer, 10 N and 20 N normal loads) of epoxy coatings with different micronized wax types and concentrations (1, 2, 5, 7, 10 wt.-%) evaluated by optical microscopy under different magnification, with a left-to-right scratching direction.
Figure 4. Mechanical scratching (sclerometer, 10 N and 20 N normal loads) of epoxy coatings with different micronized wax types and concentrations (1, 2, 5, 7, 10 wt.-%) evaluated by optical microscopy under different magnification, with a left-to-right scratching direction.
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Figure 5. Water contact angles of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%), measured on the coatings before wear (blue bars) and after wear under the highest load (orange bars); error bars determined from n = 10 measurements.
Figure 5. Water contact angles of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%), measured on the coatings before wear (blue bars) and after wear under the highest load (orange bars); error bars determined from n = 10 measurements.
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Figure 6. Gloss values under a 60° angle measured on surfaces of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%); error bars determined from n = 10 measurements.
Figure 6. Gloss values under a 60° angle measured on surfaces of epoxy coatings with different micronized wax types and concentrations (0.5, 1, 2, 5, 7, 10 wt.-%); error bars determined from n = 10 measurements.
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Figure 7. Laser interference microscopy of abrasive wear tracks on epoxy coatings with different micronized wax additives at 5, 7, 10 wt.-% after wear under highest load (500 g); (a) magnification 20×; (b) magnification 50×.
Figure 7. Laser interference microscopy of abrasive wear tracks on epoxy coatings with different micronized wax additives at 5, 7, 10 wt.-% after wear under highest load (500 g); (a) magnification 20×; (b) magnification 50×.
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Figure 8. Detailed topographical images (magnification 50×) by confocal laser scanning microscopy of abrasive wear tracks on epoxy coatings with different micronized wax additives at 5, 7, 10 wt.-% after wear under highest load (500 g).
Figure 8. Detailed topographical images (magnification 50×) by confocal laser scanning microscopy of abrasive wear tracks on epoxy coatings with different micronized wax additives at 5, 7, 10 wt.-% after wear under highest load (500 g).
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Figure 9. Fourier-transform infrared (FTIR) spectra of epoxy coatings with micronized wax additives in different concentrations of 0 wt.-% (red), 2 wt.-% (blue), 5 wt.-% (grey), 7 wt.-% (purple), and 10 wt.-% (green), including selected wax types of (a) carnauba wax, (b) rice bran wax, (c) amide wax, (d) PE/wax mixture, (e) PTFE, and (f) PE wax.
Figure 9. Fourier-transform infrared (FTIR) spectra of epoxy coatings with micronized wax additives in different concentrations of 0 wt.-% (red), 2 wt.-% (blue), 5 wt.-% (grey), 7 wt.-% (purple), and 10 wt.-% (green), including selected wax types of (a) carnauba wax, (b) rice bran wax, (c) amide wax, (d) PE/wax mixture, (e) PTFE, and (f) PE wax.
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Figure 10. Distribution of wax and reference additives at the surface of epoxy coatings, based on intensity ratios of characteristic absorption bands in the FTIR spectra: (a) ratio of wax (718 cm−1) to epoxy (720 cm−1) for bio-based waxes, i.e., CBW-5 (Δ), CBW-9 (▲), RBW-5 (o), RBW-9 (●), ABW-5 (■); (b) ratio of wax (718 cm−1) to epoxy (o, ●) and PTFE (637 cm−1) to epoxy (Δ,▲) for fossil-based additives, i.e., PW-5 (o, Δ), PW-9 (●, ▲), PE wax (+), PTFE (x); (c) ratio of wax to PTFE for PW-5 (☐), PW-9 (■).
Figure 10. Distribution of wax and reference additives at the surface of epoxy coatings, based on intensity ratios of characteristic absorption bands in the FTIR spectra: (a) ratio of wax (718 cm−1) to epoxy (720 cm−1) for bio-based waxes, i.e., CBW-5 (Δ), CBW-9 (▲), RBW-5 (o), RBW-9 (●), ABW-5 (■); (b) ratio of wax (718 cm−1) to epoxy (o, ●) and PTFE (637 cm−1) to epoxy (Δ,▲) for fossil-based additives, i.e., PW-5 (o, Δ), PW-9 (●, ▲), PE wax (+), PTFE (x); (c) ratio of wax to PTFE for PW-5 (☐), PW-9 (■).
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Figure 11. FTIR spectra captured on the coatings, before wear (red) and after wear (blue, with manual shift over 20 cm−1 for better visibility), for pure coatings including (a) unfilled epoxy and coatings with wax additives including (b) 7 wt.-% CBW-5, (c) 7 wt.-% RBW-5, (d) 7 wt.-% ABW-5, (e) 7 wt.-% PE wax, and (f) 7 wt.-% PTFE.
Figure 11. FTIR spectra captured on the coatings, before wear (red) and after wear (blue, with manual shift over 20 cm−1 for better visibility), for pure coatings including (a) unfilled epoxy and coatings with wax additives including (b) 7 wt.-% CBW-5, (c) 7 wt.-% RBW-5, (d) 7 wt.-% ABW-5, (e) 7 wt.-% PE wax, and (f) 7 wt.-% PTFE.
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Table 1. Micronized wax powders used as additives in epoxy coating formulations.
Table 1. Micronized wax powders used as additives in epoxy coating formulations.
Micropowder AcronymWax TypePowder Size Range
(µm)		Melting Temperature Range
(°C)
d10d50d90
CBW-5Carnauba biowax4.05.36.082–86
CBW-9Carnauba biowax8.09.511.082–86
RBW-5Rice bran biowax4.85.46.078–82
RBW-9Rice bran biowax8.09.511.078–82
ABW-5Stearamide biowax3.54.85.5142
PW-5PE wax/PTFE2.35.36.458–60
PW-9PE wax/PTFE6.59.212.058–60
PTFEPTFE5.07.09.0(>300)
PEPE wax5.47.09.8110
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MDPI and ACS Style

Samyn, P.; Vanheusden, C.; Cosemans, P. Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings. Coatings 2025, 15, 511. https://doi.org/10.3390/coatings15050511

AMA Style

Samyn P, Vanheusden C, Cosemans P. Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings. Coatings. 2025; 15(5):511. https://doi.org/10.3390/coatings15050511

Chicago/Turabian Style

Samyn, Pieter, Chris Vanheusden, and Patrick Cosemans. 2025. "Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings" Coatings 15, no. 5: 511. https://doi.org/10.3390/coatings15050511

APA Style

Samyn, P., Vanheusden, C., & Cosemans, P. (2025). Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings. Coatings, 15(5), 511. https://doi.org/10.3390/coatings15050511

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