# Feather keratin hydrolysates as sustainable binders for high-density fiberboards

Nidal del Valle Raydan, Anita Wronka, Grzegorz Kowaluk, Eduardo Robles

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Nidal del Valle Raydan, Anita Wronka, Grzegorz Kowaluk, Eduardo Robles. Feather keratin hydrolysates as sustainable binders for high-density fiberboards. Composites Part C: Open Access, 2025, 18, pp.100668. 10.1016/j.jcomc.2025.100668. hal-05367750

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https://univ-pau.hal.science/hal-05367750v1

Submitted on 16 Nov 2025

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# Feather keratin hydrolysates as sustainable binders for high-density fiberboards

Nidal Del Valle Raydan<sup>a</sup>, Anita Wronka<sup>b</sup>, Grzegorz Kowaluk<sup>b</sup>, Eduardo Robles<sup>a,*</sup> <sup>1,2</sup>

## ARTICLE INFO

Keywords:

Fiberboard

Feather

Keratin

Soybean

Urea formaldehyde

Binder

Physical properties

Mechanical properties

## ABSTRACT

This study evaluates the potential of feather keratin hydrolysates, obtained through ultrasound-assisted alkaline hydrolysis at low and high temperatures, as sustainable binders for high-density fiberboards (HDFs). The performance of keratin-based adhesives was compared with that of current binders, namely urea-formaldehyde (UF) and soy protein isolate (SPI). Adhesives were applied at a content of 12%. Additionally, a second formulation using 15% keratin hydrolysate extracted at high temperature (KHT 15) was tested to assess the influence of binder loading. The mechanical and physical properties of the HDF panels—including modulus of rupture, modulus of elasticity, internal bond strength, screw withdrawal resistance, thickness swelling, water absorption, and surface wettability—were systematically evaluated. Both keratin formulations outperformed SPI and matched or surpassed UF in key performance indicators. Among them, keratin extracted at high temperature exhibited superior mechanical strength and moisture resistance, which may be related to the formation of stronger disulfide bonds. In particular, KHT 15 increased internal bond strength by 74% compared to UF and 96% compared to SPI, and reduced 24 h water absorption by 36% and 58% respectively. Keratin-based panels also retained higher water contact angles after 60 s, demonstrating improved surface hydrophobicity. Although keratin extracted at high temperature at 12% fulfilled the EN 622-5 standard for dry-use panels, increasing the content to 15% enabled compliance with the requirements for humid conditions, with TS below 30%. These results position keratin hydrolysates, particularly at high temperature, as viable, bio-based alternatives to synthetic and food-derived adhesives in engineered wood composites aligned with circular bioeconomy principles.

## 1. Introduction

The transition toward safer and more sustainable wood-based composites has gained momentum in recent years, driven by both societal demand for eco-friendly products and increasingly restrictive legislation on formaldehyde emissions [1–4]. In the European Union, new limits set under REACH Annex XVII (entry 77) will take effect on August 6, 2026, establishing a maximum of 0.062 mg m⁻³ for furniture and wood articles and 0.080 mg m⁻³ for other products. Materials exceeding these values will no longer be permitted on the market [5]. Fiberboards are wood-based panels produced by breaking down softwood and hardwood material into fibers and mixing them with wax and an adhesive resin. High-density fiberboards (HDF, density > 800 kg m⁻³) [6] is a high-quality that is a versatile and widely used wood-based composite material known for its higher density, exceeding that of both regular particleboard and even ordinary plywood; this characteristic makes HDF

the ideal choice for various applications, such as premium furniture, laminate flooring, decorative wall panels, shelving, door skins, and specialized packaging [7]. HDF panels offer several advantages that contribute to their widespread adoption. Their combination of smooth surfaces, ease of machining, high strength, and excellent screw-holding capacity underpins their widespread industrial adoption, particularly in sectors where precision and durability are essential [8].

In the wood panel industry, synthetic resins such as UF, phenol-formaldehyde (PF), and melamine-formaldehyde (MF) remain the predominant binders, valued for their high bonding strength, water resistance, rapid curing under heat, and cost-effectiveness [2,9,10]. Despite these advantages, the petrochemical origin of these products and the release of volatile organic compounds—most notably free formaldehyde and phenolic derivatives—during manufacturing, transportation, and service raise significant environmental and health concerns [11]. Additionally, they are not biodegradable. Formaldehyde, identified as a

<footnote><sup>a</sup> University of Pau and the Adour Region, E2S UPPA, CNRS, IPREM, Mont de Marsan, France</footnote>

<footnote><sup>b</sup> Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences – SGGW, Nowoursynowska St. 159, 02-776 Warsaw, Poland</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: eduardo.robles@univ-pau.fr (E. Robles).</footnote>
human carcinogen by the International Agency for Research on Cancer in 2004, has been the subject of progressively tighter regulatory limits [12]. This regulatory pressure has driven two complementary strategies: the incorporation of formaldehyde scavengers into existing resin systems and the development of alternative binders derived from renewable, biodegradable resources [4,9,10,13]. Consequently, there is now a clear trend in research and industrial practice toward replacing conventional formaldehyde-based adhesives with bio-based systems capable of delivering comparable performance while meeting strict emission standards.

A wide range of renewable feedstocks—among them proteins, starch, tannins, and lignin—have been investigated as sustainable precursors for bio-based adhesives in engineered wood products [2,14–22]. For instance, tannin-based adhesives [23,24], lignosulfonate-modified UF resins [25], starch-derived binders [26], and urea glyoxal systems modified with ionic liquids [27] have shown promise as partial or full alternatives to conventional synthetic adhesives. In recent years, proteins have attracted particular attention because they are abundant, inexpensive, and sourced from renewable biomass. Nevertheless, their use in wood bonding still faces well-known limitations, including high initial viscosity, insufficient water resistance, and comparatively low bonding strength, which restrict their direct application without further modification [9,28–30]. In the last decade, soy-based adhesives have garnered attention for their effectiveness, but are limited by regional soy scarcity and poor water resistance, necessitating the combined use of crosslinkers, including phenolics and epoxies [31–34]. However, specific formulations may still pose toxicity risks akin to conventional adhesives [35]. Another approach is to search for different protein sources, such as peas, lentils, and chickpeas, which share a similar amino acid profile with soybeans [15,36,37]. Despite these challenges, the growing market for plant-based meat replacements, driven by health and sustainability concerns, is driving food science advancements, with predictions suggesting extensive growth by 2030 [38]. Consequently, the search for non-food protein sources, such as keratin waste, is becoming progressively promising. However, to truly unlock this potential, it is necessary to develop affordable extraction methods for keratin that enhance wood bonding without costly solvents or processes.

The current study builds upon previous research. An earlier investigation into feather hydrolysis via ultrasound-assisted alkaline hydrolysis demonstrated a quick, cost-effective, and self-exothermic method of extracting keratin, which yielded about 70 % keratin recovery compared to conventional thermal alkaline hydrolysis [39]. Further analysis of temperature-controlled keratin ultrasound-assisted alkaline hydrolysis (25-85°C) revealed significant chemical and thermal variations that affected adhesive mechanical properties (Raydan et al., 2025, under revision). Specifically, keratin processed at lower temperatures retained its original structure and disulfide bridges, similar to those found in feathers. In contrast, high-temperature processing reconstructed these bridges, enhancing wood bonding through increased exposure of functional groups. This bonding resulted in keratin with greater thermal stability, enhanced mechanical strength under stress, and a finer morphology, which improves interlocking (Raydan et al., 2025, under revision). The most effective keratin adhesive, with superior properties, was derived from the ultrasound process without temperature control and tested as a cold-set adhesive, utilizing NaOH rehydrolyzation, without any crosslinkers or additives.

This work advances the state of the art by being the first to apply ultrasound-extracted keratin binders—fully derived from poultry feather waste—to the production of high-density fiberboards (HDF) under hot-press conditions (180 °C). Previous work with keratin adhesives has been limited to cold-set or low-temperature bonding tests without full panel manufacturing or only partial substitution of synthetic resins. Likewise, recent bio-based adhesive developments for HDF have focused mainly on thermoplastics (PLA, PCL, starch) [16] or hybrid systems combining urea-formaldehyde with bio-additives such as lignosulfonates [40], but no study has yet demonstrated a fully bio-based

protein adhesive matching or surpassing UF in performance for humid conditions. The panels were fabricated under laboratory conditions using a dry-process approach. The keratin binder was recovered from raw Mullard duck feathers via ultrasound-assisted alkaline hydrolysis. The main aim was to compare the mechanical and physical performance of HDFs bonded with keratin to those made with a plant-derived protein (soy) and with conventional formaldehyde-based resins.

## 2. Materials

Cleaned duck feathers were supplied by Plum'Export (Saint-Sever, France). The lignocellulosic fraction consisted of industrially harvested pine fibers (Pinus sylvestris L.) provided by IKEA Industry Poland sp. z o. o. (Orla, Poland), with a moisture content close to 4 %. Reagents included sodium hydroxide (NaOH, CAS 1310-73-2) and citric acid (HOC(CO₂H)(CH₂CO₂H)₂, CAS 77-92-9), both obtained from Sigma-Aldrich. For the reference formulations, soybean protein isolate (SPI) was purchased from Bulk TM, while the urea-formaldehyde (UF) resin (65 % solids) was sourced from Silekol Sp-z o.o. (Kędzierzyn-Koźle, Poland). The UF adhesive was combined with a 10 % aqueous solution of ammonium sulfate ((NH₄)₂SO₄) as a hardener, following a weight ratio of 50:5:1.5 (resin:water:hardener). Under these conditions, the curing time at 100°C was approximately 82 s and served for preparing the reference panels. For keratin- or soy-based boards, the protein flour was mixed with wood fibers and sprayed with a 5 % NaOH solution, applying a weight ratio of 50:5:100 (protein:NaOH:water). No hydrophobic additives were included in any of the board formulations to valorize keratin's intrinsic hydrophobicity and enable fair comparison with UF and SPI under equal conditions.

## 3. Methods

### 3.1. Ultrasound-assisted alkaline hydrolysis of raw duck feather

Keratin was extracted from raw duck feathers by ultrasound-assisted alkaline hydrolysis, following a modified version of previously reported protocols [39]. Briefly, the feathers were milled to approximately 5 mm and hydrolyzed in a 3 wt% NaOH solution (solid-to-liquid ratio 10:100 w/v) using a 19.7 kHz ultrasonic homogenizer (UP200St, Hielscher, Germany) at 80% amplitude in a 500 mL jacketed vessel. Two processing temperatures were used: 25°C (temperature-controlled) and ~75°C, the latter reached by the self-heating effect of ultrasound. After filtration to remove insoluble residues, keratin was precipitated at pH 4.5 with citric acid, dialyzed for three days (6–8 kDa MWCO tubing, Spectra/Por 1, Spectrum Labs, USA), and lyophilized (Alpha 1-4, Martin Christ GmbH, Germany). Samples obtained at 25°C and 75°C are hereafter referred to as KLT and KHT, respectively. The keratin powder obtained in this way was stored in sealed containers under dry conditions until further use.

### 3.2. Preparation of the panels

High-density fiberboards measuring 320 × 320 mm with a nominal thickness of 3 mm and a target density of 900 kg·m⁻³ were produced. Adhesive content was set at 12% and 15% for KHT (denoted KHT 12 and KHT 15, respectively) and 12% for KLT, SPI, and UF controls. The adhesive content selection followed standard practice and recommendations for HDF manufacturing, as detailed by Irle and Barbu [41]. For UF-based panels, wood fibers were blended in a drum mixer, and the resin (see Section 2) was added directly into the mixer to the fibers using a 50:5:1.5 resin:water:hardener weight ratio. For protein-based panels (SPI, KLT, KHT), wood fibers were blended in a drum mixer, and a 5% NaOH solution was sprayed using an air gun to slightly moisten the fibers before gradually adding the protein flour to promote adhesion. The remainder of the NaOH solution was added after full incorporation of the protein. This alkaline treatment served two purposes: it partially
activated the fiber surfaces by increasing their reactivity, and it improved the dispersion and anchoring of the protein flour. Alkaline conditions promote protein unfolding, exposing functional groups (e.g., carboxylates, amines, thiols) capable of interacting with hydroxyl groups on cellulose and lignin. Upon heating, these interactions can lead to hydrogen bonding, ionic interactions, and potentially covalent bond formation (e.g., disulfide-related crosslinking), depending on the protein source. Prior to hot-pressing, mat surfaces were lightly sprayed with water to improve heat transfer. Panels were pressed at 180°C under a maximum pressure of 2.5 MPa for 18 s·mm⁻¹, based on nominal thickness, as recommended for HDF panels [41]. Finished boards were conditioned at 20°C and 65% relative humidity until constant mass before testing.

### 3.3. Characterization of the panels

#### 3.3.1. Mechanical properties

The mechanical performance of the boards was evaluated according to the relevant European standards: bending strength and stiffness (EN 310), tensile strength perpendicular to the panel surface (EN 319), and resistance to screw withdrawal (EN 320). Tests were carried out on an Instron 3369 universal testing machine (Instron, Norwood, MA, USA). For each board type, at least ten specimens were examined for every property. Whenever applicable, the obtained values were compared with the reference requirements established for dry-formed fiberboards [42].

#### 3.3.2. Physical properties

The physical characterization of the panels included color, density profile, wettability, surface morphology, and water resistance. Panel color was determined with a Ci62S spectrocolorimeter (X-Rite Inc., MI, USA), pre-calibrated against white and black references, and expressed in the CIE Lab* color space ($L^* = brightness, a^* = red–green, b^* = yellow–blue). Density distribution across the panel thickness was obtained by X-ray scanning densitometry (DA-X, GreCon, Alfeld, Germany) at 0.05 mm s⁻¹ with a resolution of 0.02 mm; three replicates were collected for each binder type, and the representative profile reported. Wettability was evaluated via sessile-drop water contact angle using a PHOENIX 300 analyzer (SEO, Suwon, Korea). The surface microstructure of the boards was examined with scanning electron microscopy (Quanta 200, FEI, Hillsboro, OR, USA). Water absorption (WA) and thickness swelling (TS) were determined following EN 317 after 2 h and 24 h of immersion. For these tests, at least eight specimens per formulation were assessed.

#### 3.3.3. Statistical analysis

Statistical evaluation of the data was carried out by analysis of variance (ANOVA) and pairwise t-tests, considering $p < 0.05$ as the threshold for significance. Calculations were performed with OriginPro 2024 (OriginLab Corp., Northampton, MA, USA). Graphical results are presented as mean values with their corresponding standard deviations.

## 4. Results and discussion

### 4.1. Keratin precipitation and recovery

Keratin recovery, calculated using Eq. 1 by comparing the recovered keratin amount to the initial content in raw feathers (82.97 ± 0.98

<caption>**Table 1**</caption>

Summary of ultrasound-assisted alkaline hydrolysis conditions and absolute keratin yield.

<table><thead><tr><td>Hydrolysis Temperature (°C)</td><td>Hydrolysis time (min)</td><td>Absolute yield (%)</td></tr></thead><tbody><tr><td>25</td><td>180</td><td>85.68</td></tr><tr><td>75</td><td>18</td><td>65.71</td></tr></tbody></table>

[43]), is detailed in Table 1, including the hydrolysis time. A standard deviation of approximately ±5 was observed in a subset of tested conditions.

### 4.2. Mechanical properties

Fig. 1 presents the mechanical properties of the different HDF samples as box plots with whiskers representing the standard deviation. Fig. 1a illustrates the variation in modulus of rupture (MOR) values among different samples. The keratin hydrolyzed at high temperature with a 12% adhesive content (KHT 12) shows the highest average MOR at 54.16 N·mm⁻². In contrast, SPI-bonded boards register the lowest average MOR at 34.75 N·mm⁻². Notably, all samples meet the MOR requirements for general-purpose boards used in dry conditions, as specified in EN 622-5 (type MDF) at 23 N·mm⁻², and the MOR requirements for general-purpose boards used in humid conditions, as specified in EN 622-5 (type MDF.H) at 27 N·mm⁻². The modulus of elasticity (MOE) values in Fig. 1b show that KHT 12 exhibits the highest average MOE at 4292.71 N·mm⁻². The UF adhesive sample presents the lowest average MOE at 3095.96 N·mm⁻². All the samples meet and exceed the EN 622-5 (type MDF.H) requirement for MOE of 2700 N·mm⁻².

In the internal bonding (IB) strength tests shown in Fig. 1c, KHT 15 demonstrated the highest average IB value at 3.11 N·mm⁻², while SPI registered the lowest at 1.59 N·mm⁻². The differences among the samples were not statistically significant, and all exceeded the EN 622-5 standards for both type MDF (minimum of 0.65 N·mm⁻²) and type MDF.H (minimum of 0.7 N·mm⁻²). Fig. 1d presents the screw withdrawal resistance (SWR), where KHT 12 recorded the highest average SWR at 193.71 N·mm⁻¹, and KLT the lowest at 159.09 N·mm⁻¹. The SWR values across all samples showed no significant variation.

The results from Fig. 1 indicate that KHT 12 generally presented the highest mechanical properties among the tested samples. KHT 15 followed closely and even surpassed KHT 12 in terms of internal bonding strength. The superior mechanical performance of KHT compared to KLT can be linked to the stronger reconstituted disulfide bridges in KHT, which result from higher hydrolysis temperatures. This process leads to the cleavage and subsequent reformation of disulfide bridges, as supported by previous research (Raydan et al., 2025, under revision). The reformed disulfide bridges are more capable of handling stress, and the higher hydrolysis temperature also leads to greater exposure of functional groups that can bond with wood, contributing to improved mechanical properties. KHT is characterized by a more amorphous alpha-helix structure and a finer microstructure than KLT, which further enhances its performance as it can fill in better between fibers.

There are statistically significant differences among the panels bonded with UF, SPI, KLT, and KHT; however, the difference between KHT 12 and KHT 15 is not significant. The decreased MOR and MOE values for KLT, in contrast to KHT, can be explained by examining the HDF's appearance in Fig. 2. HDF panels bonded with KLT exhibited remnants of white keratin on their surface, indicating lower solubility and improved compatibility with wood fibers. This low compatibility aligns with findings that KLT has more complex structures with fewer available functional groups, resulting in reduced dispersion in NaOH solutions and fewer interactions with wood. For the SPI-bonded panels, the absence of disulfide bridges, which are prevalent in keratin, leads to a less stiff adhesive matrix. SPI also tends to exhibit higher viscosities than keratin adhesives with similar formulations, which could negatively affect the bonding process and mechanical properties of the panels.

Examining the HDF panels after being tested for internal bonding, as shown in Fig. 2, it can be observed that breakage predominantly occurs within the core for all samples following IB test, suggesting that the fibers are adequately saturated with resin, including KLT. This uniform resin distribution might be attributed to improved heat transfer during hot-pressing, potentially enhanced by spraying the mat surfaces with water before pressing. However, it is worth considering that in the case
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Mechanical performance of HDF panels bonded with different adhesives: (a) MOR, (b) MOE, (c) IB, and (d) SWR. Values are expressed as boxplots with mean and median. Different letters above the boxes indicate statistically significant differences between groups (p < 0.05). Dashed lines represent the minimum requirements according to EN 622-5 for MDF and MDF.H panels.</figcaption></figure>

of KLT, no white keratin remnants were observed in the core, which could be due to moisture evaporating faster on the surface, potentially hindering the solubilization and reaction of the more complex KLT as intended.

### 4.3. Physical properties

The visual and colorimetric data are presented in Fig. 3 and Table 2. The HDF panels bonded with protein-based adhesives—SPI, KLT, and KHT—are noticeably darker than those with UF adhesive. The color parameters show that protein-based panels have lower L\* values (indicating darkness) and higher a\* values (indicating redness), with KHT panels registering the darkest colors and most intense redness. This color composition suggests a greater degree of disulfide bridge reforming and rearranging during high-temperature ultrasound-assisted alkaline hydrolysis in KHT, leading to more exposed chromophores and hydrophilic groups that reexpose and dissolve in mild NaOH conditions [44]. In contrast, KLT panels, retaining more of the feather’s original disulfide bridges, exhibit a less hydrophilic nature and fewer exposed functional groups [45].

The color difference, denoted by ΔE\*, quantifies the extent of the reaction. The most significant color change is observed between UF and

SPI, which are fundamentally different. A minor difference (ΔE\* of 3.17) between SPI and KLT indicates a lack of significant disulfide bridge reformation in KLT. A larger difference between KLT and KHT panels signifies a substantial change, whereas the minimal difference (ΔE\* of 0.76) between KHT 12 and KHT 15 is barely distinguishable to the human eye. It is generally acknowledged that ΔE\* values above 1.0 are noticeable, and those exceeding 6.0 are considered significant. The darker color of KHT panels implies enhanced exposure of functional groups, which likely contributes to the stronger mechanical properties observed, demonstrating a link between the adhesive’s molecular structure and its performance in HDF applications.

Keyence VHX-7000/VHX-970 microscopy imaging was utilized further to analyze the morphology and structure of KHT 15 panels, as shown in Fig. 4. At lower magnifications (20 × to 400 ×), the panel surface appears uniform, compact, and well-coated, highlighting effective binder distribution and adhesion to the wood fibers. This homogeneous structure correlates with the improved hydrophobicity and mechanical properties of KHT 15. The 3-D view provides additional insights into the interaction between the keratin binder and wood fibers. The images reveal keratin penetration up to 70 µm into the wood matrix, forming bridges and filling voids, which contribute to a cohesive and interlocking fiber network. This structural arrangement enhances the
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Representative failure patterns observed after IB testing of HDF panels prepared with different binders: UF, SPI, KLT, KHT 12, and KHT 15.</figcaption></figure>

panel's overall strength, stability, and water resistance, consistent with the superior performance of KHT 15 observed in other tests.

**Fig. 5** summarizes the density profiles of the HDF panels, with mean values converging around 900 kg·m⁻³. While UF, SPI, and keratin panel profiles differ, the keratin variants have no significant differences (KLT, KHT 12, and KHT 15). The wood composite panel profiles typically exhibit denser surface layers than the core, attributed to the pressing process that evenly compresses the fibers across the panel's cross-section [46]. This effect can be more or less pronounced in thinner boards where the press's heat quickly reaches the core, facilitating binder hardening or liquefaction [47]. In the UF sample, the average surface layer density starts at 910 kg·m⁻³ and increases by about 12% to 1017 kg·m⁻³ at the core. The observed inversion in density between the face and the core in the UF-bonded panels, where density increases towards the core, is unusual for HDF. It may result from the mobility of the UF adhesive during the pressing process at 180°C, combined with the heat flow from the press shelves inward toward the panel's core. This mobility, along with the rapid curing and solidification typical of thermosetting UF

resins, is facilitated by the thinness of the panel, which enhances heat transfer efficiency.

In contrast, protein-based adhesives—such as keratin and soybean—do not exhibit thermosetting behavior. Instead, they remain fluid during pressing, facilitating better fiber coating and penetration. Their bonding mechanism relies on forming hydrogen bonds with the cellulose in the wood rather than heat-induced curing. As these protein adhesives cool and dehydrate, they solidify to form an integrated protein–wood matrix. Notably, panels bonded with SPI display a surface-to-core density reduction of approximately 17%, which is likely attributable to SPI's higher viscosity impairing its uniform distribution and penetration within the fiber mat. This phenomenon aligns with observations that excessive viscosity in adhesives can hinder even application and compromise bonding uniformity [48].

Keratin samples have the most consistent density profiles, with about 12% variation from the surface to the core. The standard deviation among KLT, KHT 12, and KHT 15 is approximately 6 kg·m⁻³ between their respective surface and core densities, suggesting that the hydrolysis temperature or resin content has a minimal impact on their density uniformity. This similarity can be attributed to the behavior of the three keratin adhesives, which all remain fluid for longer during pressing, allowing for a more even distribution before setting as the board cools. This phenomenon also means that the protein adhesive's “curing” is less about a chemical reaction triggered by heat and more about the evaporation of water and the subsequent cooling process, which allows the proteins to form a rigid structure as they dry out.

**Fig. 6** presents the results of the water contact angle (WCA) measurements of the different HDF panels, indicating differences in hydrophobicity over 60 s. After 1 s, KHT 12 panels exhibit the highest WCA at 96.5°, followed by KHT 15 at 94.5°, and UF at 91.7°, indicating high initial hydrophobicity. Throughout the duration, the keratin-adhesive panels (KHT 12, KHT 15, and KLT) maintain relatively higher WCAs after 60 s, with KHT 12 showing a WCA at 52.2°, KHT 15 at 37.7°, and KLT AT 18.9°. These values demonstrate a higher resistance to water penetration in keratin-containing adhesives compared to UF and SPI. For UF and SPI panels, the contact angles drop to the point of total water absorption within approximately 45 seconds, indicating a transition to hydrophilic behavior, as the water droplets are no longer visible on the panel surfaces.

The sustained resistance to wettability observed for the keratin-treated KHT panels can be associated with the keratin's molecular structure, which is rich in nonpolar amino acids that repel water. Following the reformation of disulfide bridges in high-temperature hydrolysis, these nonpolar groups are likely to reorient towards the interior of the protein matrix. In contrast, the polar amino acids and functional groups that remain exposed form bonds with the wood, creating a water-resistant barrier due to the dense disulfide crosslinking. Interestingly, a higher resination percentage in KHT 15 does not correspond to increased surface hydrophobicity, which may be due to the hygroscopic nature of the excess keratin resin when mixed with the NaOH solution used. Additionally, the keratin adhesive's liquefaction during pressing may allow for deeper penetration into the core, as reflected in the improved internal bonding, but not in the MOR or MOE.

On the other hand, SPI and KLT panels exhibit initial WCAs below 90°, with a more pronounced decline over time, suggesting a more hydrophilic surface. This decline can be attributed to the chemical structure of SPI, which is enriched in hydrophilic amino acid residues and lacks the extensive disulfide crosslinking network typical of keratin. Disulfide bridges make proteins more rigid and less accessible to water molecules. Without these bridges, soy protein has a more open structure, allowing more water to penetrate and interact with the protein molecules. Conversely, KLT panels exhibit lower WCAs due to their weaker binding affinity with wood, which leaves voids that facilitate water absorption, as indicated by the visible unbound keratin on the HDF surface in **Fig. 3**. However, KLT panels present relatively higher WCAs after 45 seconds compared to UF and SPI, implying some degree of water
<figure><img src="image_4.png" /><figcaption>Fig. 3. Visible differences in the surface texture of HDF panels as a function of the binder formulation (UF, SPI, KLT, KHT 12, and KHT 15).</figcaption></figure>

<table><caption>**Table 2**
Colorimetric Analysis of HDF Panels Bonded with Various Adhesives.</caption><thead><tr><th>Sample</th><th>L*</th><th>a*</th><th>b*</th><th>ΔE*</th><th>Color Reference</th></tr></thead><tbody><tr><td>UF</td><td>66.47 ± 0.38</td><td>6.81 ± 0.19</td><td>26.84 ± 0.37</td><td>-</td><td></td></tr><tr><td>SPI</td><td>57.25 ± 0.56</td><td>8.39 ± 0.28</td><td>27.43 ± 0.26</td><td>9.38</td><td></td></tr><tr><td>KLT</td><td>60.29 ± 0.78</td><td>7.53 ± 0.18</td><td>28.03 ± 0.14</td><td>3.17</td><td></td></tr><tr><td>KHT 12</td><td>53.59 ± 0.31</td><td>9.87 ± 0.08</td><td>28.18 ± 0.46</td><td>7.04</td><td></td></tr><tr><td>KHT 15</td><td>54.09 ± 0.63</td><td>9.90 ± 0.21</td><td>28.75 ± 0.18</td><td>0.76</td><td></td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Microscopic analysis of KHT 15 HDF panel: (a) Uniform surface morphology at 20 × (blue bar is 2 mm), indicating effective binder distribution; (b) Detailed adhesion of keratin to wood fibers at 400 × (blue bar is 100 µm); (c) 3-D view showing keratin penetration and interlocking network for enhanced cohesion at 400 ×.</figcaption></figure>

This eventual hydrophobicity can be attributed to KLT's retention of its original, more hydrophobic feather-like structure with intact disulfide bridges. Unlike KHT, where high-temperature hydrolysis promotes the reformation of disulfide bridges and the resultant exposure of hydrophilic groups on the surface, KLT's low-temperature hydrolysis preserves the native hydrophobicity of the keratin structure.

Fig. 7 presents the thickness swelling (TS) and water absorption (WA) of the HDF panels after 2 and 24 h of immersion, with error bars representing standard deviation. For TS in Fig. 6a, KHT 15 displays the slightest swelling at only 15.7% after 2 hours, followed by UF and KHT 12 with TS values of around 28.9% and 29.3%, respectively. KLT shows a higher swelling at 41.8%, whereas SPI experiences the most swelling after 2 hours at 54.8%. In commercial MDF production, about 1% paraffin emulsion is typically added to improve water resistance; however, no hydrophobic agents were used in this study [16]. Keratin panels achieved lower TS after 24h than UF, aligning with results from WCA

measurements. Increasing resination enhances water resistance, likely because the excess keratin resin liquefies under pressure and diffuses into the core, as previously mentioned. After 24 hours of immersion, the UF and KHT 15 panels show TS values of 32% and 27.5%, respectively, meeting the requirements for dry condition use (type MDF). The KHT 15 panel also meets the requirements for humid conditions (type MDF.H), with a maximum TS of 30% after 24 hours. KLT outperforms SPI in TS, recording 51.8% compared to SPI's 70.3% after 24 hours, reflecting KLT's higher inherent hydrophobicity and corroborating the WCA test results despite its weaker wood affinity.

In the WA analysis (Fig. 6b), the graph indicates that KHT 15 has the lowest WA at 29.7% after 2 hours, which is half the WA of KHT 12 at 61.8%, despite only a 3% difference in resination. UF has WA at 76.3%, and KLT with WA at 85.2%, indicating that KHT offers better water absorption resistance than UF. SPI panels show the highest hydrophilicity with 116.3% WA, almost twice the absorption of KHT 12 and four
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Density profiles of HDF panels bonded with UF, SPI, KLT, KHT 12, and KHT 15. All panels show typical high-density surface layers and lower-density core regions, with SPI exhibiting the most pronounced surface-core density gradient.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Evolution of water contact angle over 60 s for HDF panels prepared with different binders (UF, SPI, KLT, KHT 12, KHT 15). Error bars indicate standard deviation.</figcaption></figure>

times that of KHT 15. After 24 h, KHT 15 remains the least absorptive at 52%, while KHT 12 and UF show similar WA levels at 77.9% and 80.8%, respectively. KLT absorption increases to 94%, and SPI further absorbs water up to 123.7%. These results reinforce the order of hydrophilicity observed in earlier tests, with KHT panels being the most hydrophobic, followed by KLT, then UF, and finally SPI, which exhibits the highest hydrophilicity.

The SEM images provide a detailed view of the microstructure of keratin hydrolysate and HDF panels bonded with KHT 15. Red arrows in the images highlight areas of interest in Fig. 8. The analysis shows the distribution of keratin hydrolysate at magnifications ranging from 300 × to 10,000 × in the first row, illustrating the distinct morphology of

compact clusters. The higher magnification images, particularly those at 10,000 ×, show keratin particles with a spherical structure indicated by a red arrow, characteristic of keratin obtained from ultrasound-assisted alkaline hydrolysis at high temperatures.

Face analysis of HDF bonded with KHT 15 at magnifications of 50 × to 10,100 × reveals the surface morphology and the distribution of keratin on the panel surface—the red arrow in 200 × magnification points to regions where wood fibers appear encapsulated by the keratin adhesive. At increasing magnifications, keratin can be seen adhering to the wood fibers, indicating successful bonding (red arrow in 800 × magnification). In addition, distinct spherical imprints are observable, consistent with the typical appearance of keratin particles, suggesting
<figure><img src="image_8.png" /><figcaption>Fig. 7. Physical properties of the HDF panels: (a) TS and (b) WA after 2 h and 24 h of immersion in water. Error bars indicate standard deviation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Scanning electron microscope images of Keratin hydrolysate (KHT) and HDF with KHT 15 (Red arrows in the images highlight areas of interest).</figcaption></figure>
that they originate from the keratin hydrolysate (KHT) binder (red arrow in 500 × magnification).

Cross-sectional analysis at magnifications ranging from 300 × to 10,000 × reveals the internal structure of the HDF panels bonded with KHT 15. The red arrow in the 6,000 × magnification image indicates spherical imprints within the core wood matrix, representing a characteristic physical form of the keratin binder. The 10,000 × magnification reveals wood fibers closely bonded, highlighting the adequate adhesion provided by the keratin binder within the HDF core. The interlocked arrangement of fibers and the effective coating by the KHT binder likely contribute to the mechanical and physical strength of the HDF panels.

## 5. Conclusions

This study has comprehensively demonstrated the potential of using keratin hydrolysates (KHT and KLT) as effective binders in the production of high-density fiberboards, presenting a promising alternative to conventional urea-formaldehyde resins and food protein-based adhesives. The examination of mechanical properties, water contact angles, thickness swelling, and water absorption, complemented by detailed SEM analysis, has provided significant insights into the successful performance of HDF panels bonded with keratin. KHT, derived from high-temperature hydrolysis obtained by the ultrasound process without temperature control, showed improvements in mechanical strength, hydrophobicity, and water resistance compared to its lower-temperature counterparts, KLT, UF, and soybean-based adhesives. This enhancement is attributed to the formation of stronger disulfide bridges during high-temperature hydrolysis in KHT, which results in stronger wood-adhesive bonds through the exposure of hydrophilic functional groups, while also creating a water-resistant barrier due to the dense disulfide crosslinking. The SEM images further confirmed the solid fiber binder interaction, with uniform keratin covering and interlocking with wood fibers, indicating robust adhesion.

Furthermore, KHT panels exhibited lower thickness swelling and water absorption, nearing the benchmarks set for usage in humid conditions as per EN 622-5 standards. These results suggest that keratin extracted from waste feathers presents an environmentally friendly adhesive option, potentially outperforming traditional synthetic adhesives in terms of water resistance. In light of these findings, keratin-based adhesives, especially KHT, emerge as a sustainable and high-performance alternative in HDF manufacturing. They not only leverage feather wastes but also offer enhanced properties that could extend the application range of HDF. Future research directions could include refining keratin adhesive formulations, particularly by optimizing the resin content, and exploring its efficacy across various wood composite materials, such as MDF and particleboards, thereby broadening the scope of sustainable wood composite technologies.

## CRediT authorship contribution statement

**Nidal Del Valle Raydan:** Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. **Anita Wronka:** Writing – original draft, Investigation, Formal analysis, Data curation. **Grzegorz Kowaluk:** Writing – review & editing, Validation, Supervision, Conceptualization. **Eduardo Robles:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors acknowledge the tenure-track position “Bois: Biobased Materials” as part of E2S UPPA, supported by the Investissements d'Avenir French program, managed by ANR (ANR-16-IDEX-0002). N.R. acknowledges financial support from the New Aquitaine Region (AAPR2020-8631210). Part of this research was conducted with the support of the Student Furniture Scientific Group (Kolo Naukowe Meblarstwa), Faculty of Wood Technology, Warsaw University of Life Sciences – SGGW. The authors also thank Aleksandra Jeżo for her assistance with panel preparation and selected mechanical tests.

## Data availability

Data will be made available on request.

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