Biopolymer-Based Solutions for Sustainable Wood Modification: A Review of Current Advancements

Abstract

Wood modification using biopolymers has emerged as a sustainable alternative to conventional chemical treatments, enhancing wood’s durability, moisture resistance, and mechanical properties while reducing environmental impact. This review provides a comprehensive overview of the latest advancements in biopolymer-based wood modification, focusing on commonly used biopolymers such as furfuryl alcohol, polylactic acid, caprolactone, polybutylene adipate terephthalate, polybutylene succinate, zein, lignin, tannin, chitosan, alginate, gums, fatty acids, rosin, and sorbitol + citric acid. Future perspectives highlight the need for interdisciplinary collaboration between academia, research institutions, and industry to accelerate innovation and commercialization. This review aims to provide valuable insights for researchers and industry professionals working toward the development of high-performance, eco-friendly modified wood products.

1. Introduction

In recent years, the research and application of biopolymers have gained significant attention due to the increasing global concerns over environmental sustainability and plastic pollution. Conventional petroleum-based plastics, while versatile and cost-effective, pose severe ecological challenges, including long degradation periods, microplastic accumulation, and adverse effects on marine and terrestrial ecosystems. As a result, there has been a growing emphasis on developing biodegradable and renewable alternatives that can reduce the environmental footprint of industrial and consumer products [1,2,3].

Biopolymers offer a promising solution to these challenges. They are compounds produced by living organisms. They can be divided into groups based on their monomer unit, such as polysaccharides (cellulose, hemicellulose, chitin, chitosan, glucan, and starch), proteins (gelatin, zein, fibroin, and casein), derived polypeptides (gelatin and collagen peptides), polyphenols (tannin and lignin), and so on. Vegetal or animal-originated waxes and resins with molecular masses close to those of oligomers (carnauba, beeswax, rosin, dammar, copal, and shellac), vegetable oils with at least one double bond in the alkyl chain (tall oil, tung oil, and linseed oil), copper soaps with carboxylic acid groups of unsaturated fatty acids (of maize oil, sunflower oil, and rosin), and macromolecular synthetic compounds obtained by polymerization or polycondensation of monomers from bioresources (furfurylation) can all also be fitted into the category of biopolymers [4].

Unlike traditional plastics, biopolymers can decompose naturally, minimizing waste accumulation and reducing dependence on fossil fuels. In bioactive environments, microorganisms like bacteria, fungi, and microflora degrade biopolymers during their enzymatic processing and chemical hydrolysis [5].

The potential applications of biopolymers span across various industries, including packaging, agriculture, medicine, and construction. However, despite their environmental benefits, biopolymers often exhibit mechanical limitations, such as lower strength and durability compared to conventional plastics [6]. To overcome these drawbacks, researchers have explored composite materials that enhance the performance of biopolymers while maintaining their sustainability [7]. This limitation has driven growing interest in natural, biodegradable reinforcements such as wood fibers, lignin, and other bio-based fillers, which can improve biopolymer performance without compromising environmental sustainability. Other conventional reinforcing materials like glass or carbon are not biodegradable nor incinerable [4].

One particularly promising approach is the combination of biopolymers with wood-based materials, which has been studied since the middle of the 20th century [8,9]. Wood fibers and particles are abundant, biodegradable, and possess excellent mechanical properties, making them ideal reinforcements for biopolymer matrices.

As industries seek viable alternatives to traditional plastics, the study of biopolymer–wood composites presents an exciting avenue for innovation. In this article, the currently most used biopolymer-based wood modification techniques are reviewed, explaining their process, mechanism, and changes in material properties based on the corresponding literature. They are organized according to chemical nature and function: furfurylation as a benchmark treatment, biodegradable synthetic polyesters (PLA, PCL, PBAT, and PBS), natural macromolecules (zein, lignin, tannin, chitosan, alginate, and gums), and small bio-based additives (fatty acids, rosin, and sorbitol + citric acid).

The objective of this article is to provide a comprehensive overview of the latest advancements in biopolymer-based wood modification, aiming to offer valuable insights for researchers and industry professionals by highlighting sustainable alternatives that enhance wood properties and reduce environmental impact.

2. Wood Modification Techniques with Biopolymers

2.1. Furfurylation

2.1.1. Process Description

Furfurylation involves impregnating wood with furfuryl alcohol (C₅H₆O₂), its derivative, or its prepolymer. Furfuryl alcohol (FA) is a bio-based polymer derived from agricultural waste such as sugar cane and corn cobs. It is non-toxic and non-flammable [10,11].

The process begins with partially drying the wood, which is then impregnated in an autoclave under vacuum and pressure with a mixture containing furfuryl alcohol, catalysts (primarily tartaric acid), buffering agents, surfactants, and water. After the resin penetrates the wood and causes it to swell, the wood is transferred to a curing chamber. Here, steam is injected to control the formation of volatiles, and in situ polymerization of the chemicals takes place. The ventilation gas is then cooled, and, finally, the wood is treated in a kiln dryer to achieve the desired moisture content and minimize emissions [11]. The reaction scheme is presented in Figure 1.

The furfurylation of wood has been studied since 1959 [8,9]. Early developments were made using cyclic carboxylic anhydrides [13].

Research and development began in 1997, leading to the commercial availability of furfurylated Scots pine and Radiata pine in 2009 under the brand name Kebony™ AS, based in Skien, Norway. In 2016, it was expanded with an additional production facility in Flanders, Belgium [14].

Furfurylated radiata pine is produced by Foreco Dalfsen as well, under the brand name Nobelwood™ (Dalfsen, Netherland), using prepolymerized furfuryl alcohol resin [15].

Other wood species have been furfurylated, although only at the laboratory scale: pine species [11,16,17,18,19,20,21], beech [10,11,22], birch [10], ash [11], poplar [23,24,25], teakwood [26], maple [20], and eucalyptus [23]. The process is generally more effective for wood species with open pits and loosely ordered cell structures [23].

2.1.2. Changes in Material Properties

Leaching tests show that furfuryl alcohol can be effectively fixed within the wood matrix [26]. Chemical analyses reveal that the cellulose content decreases, while hemicellulose and lignin contents increase [26]. This is due to crosslinking reactions between furfuryl alcohol (and its derivates) and lignin, as confirmed by FTIR analysis [26].

The wood density increases after furfurylation because the cell walls become filled with polymerized furfuryl alcohol [27]. For example, Radiata pine has an average density of 480 kg/m³, while furfurylated Radiata pine (Kebony™ Clear) reaches 670 kg/m³. The density of Scots pine increases from 490 kg/m³ to 570 kg/m³ after furfurylation (Kebony™ Character). This represents a 16%–40% increase in density, which can positively influence other properties like strength, wear resistance, hardness, and durability [14,17].

The resin uptake enhances both the hydrophobicity and dimensional stability [17,18,21,23,25,28]. Furfurylated wood can achieve an anti-swelling efficiency of up to 60% [26,29].

Furfurylated wood demonstrates enhanced mechanical properties compared to untreated wood [11,17,18,25,26,28,30], although this does not necessarily include increases in the modulus of rupture (MOR), modulus of elasticity (MOE), and impact resistance.

Furfurylated wood can achieve Durability Class 1 (according to EN 350) [31] against fungal decay and insect attack [17,18,26,28,30,32,33,34,35,36]. It also shows promising resistance against marine borers [19,37,38], although long-term resistance is only reliable at high WPG levels exceeding 50% [39]. The resistance is attributed to low moisture content, which inhibits fungal growth, and physical barriers formed by the penetration and crosslinking of the resin within the wood cell walls [19,40,41].

After treatment, the material becomes darker and takes on a brownish hue [21,23,26]. This is due to the conjugated structure of the poly-furfuryl alcohol, which masks the natural wood color [42]. In weathering tests, furfurylated wood exhibits reduced distortion, fewer surface and edge cracks, no signs of decay or presence of molds, and color change. However, color uniformity can be an issue, with gray or black spots and stains potentially appearing on the surface due to residual treatment chemicals and/or high catalyst concentrations [20].

No ecotoxicity was reported regarding furfurylated wood and its leachates [11]. Burning furfurylated wood released no harmful volatile organic compounds or polyaromatic hydrocarbons beyond the levels produced by regular wood combustion. It also has higher thermal stability compared to untreated wood, owing to the increased thermal stability of remaining carbonized lignin and the furfuryl alcohol polymer [25,26,28,43].

2.2. Polylactic Acid (PLA)

2.2.1. Process Description

PLA is a widely used biodegradable and renewable thermoplastic biopolymer. It is derived from renewable resources such as corn starch, wheat, or rice [44]. It requires 25%–55% less energy to be produced compared to petroleum-based polymers [45]. It is produced through the polycondensation reaction between hydroxyl groups and carboxylic acid groups of lactic acid monomers [46].

However, PLA is very brittle and has low toughness [47]. Its tensile strength and elastic modulus are similar to those of PET [48]. It has a higher affinity toward natural cellulosic fibers compared to conventional hydrophobic polymers [49]. Although biodegradable, it degrades slowly, which must be considered in its applications [50]. Due to its semi-crystalline structure and hydrophobic nature, PLA exhibits good oxygen and moisture barrier properties, making it suitable for use in wood biocomposites [51]. It also has good grease and oil resistance [52].

PLA was first discovered in 1932 by Carothers (DuPont – Wilmington, Delaware, US), who produced a low-molecular-weight product by heating lactic acid under vacuum [53].

It is widely used in textiles, packaging, pharmaceuticals, drug delivery, sutures, orthopedics, tissue engineering, and composite industry [46]. For the production of environmentally friendly composites, it has been combined with natural fibers such as jute, flax, hemp, and wood fibers [53,54].

The first report on wood impregnation with PLA appeared in 2009 [55]. In this study, lactic acid oligomers were synthesized by dehydrating aqueous solutions and polycondensing lactic acid without a catalyst under vacuum using a rotary evaporator with an oil bath. The polymerization was conducted at 120 °C for 1 h after boiling, and the molten prepolymers were cooled at ambient temperature. Dried wood samples were then immersed in these oligomers, with or without a catalyst, subjected to alternating vacuum and standard pressure cycles, and finally heated at 120 °C for varying durations to ensure proper impregnation. The reaction scheme of PLA and wood is presented in Figure 2.

At present, there are no commercial products available from PLA-impregnated solid wood. However, the following wood species have been studied in the laboratory: Scots pine [55,57], beech [55,57,58], oak [58], cedar, Douglas fir, maple, birch, aspen, spruce, and balsam wood [52].

2.2.2. Changes in Material Properties

Scots pine sapwood and beech heartwood were impregnated with lactic acid oligomers to create composites for flooring and indoor applications in the work of [55]. The oligomers polymerized within the wood cell walls but were not chemically grafted onto the wood’s hydroxyl groups. The best results were obtained without using a catalyst, as catalysts caused intermediate softening of both species. Without a catalyst, lactic acid oligomer impregnation improved anti-swelling efficiency, bending strength, compression strength, and hardness, while shear strength remained unchanged. However, the treated material became more brittle than the untreated wood. Only the untreated samples were attacked by fungi; there was no visible decay on the treated samples due to their acidity and low moisture content [57].

A similar study by [58] on oak and beech included longitudinal densification before PLA impregnation in half of the samples. Oakwood was poorly impregnated due to the large PLA oligomer size and the presence of tyloses. On the other hand, beechwood was well impregnated in the whole cross-section, showing increased mass (64%–73%) and density (36%–43%) and lower water uptake. It also showed a darker, more saturated brown color.

Chemical modification was combined with PLA and densification on various wood species such as cedar, Douglas fir, maple, birch, aspen, spruce, and balsam wood in the study of [52]. This approach resulted in an improved modulus of rupture, water absorbance, and hardness, emphasizing the synergistic benefits of combining densification and polymerization.

Although PLA is not inherently biocidal, antifungal or antimicrobial agents can be added to extend its service life and enhance durability [2].

2.3. Polycaprolactone (PCL)

2.3.1. Process Description

PCL is one of the most systematically studied semi-crystalline, hydrophobic, and biodegradable synthetic polyesters. It is used to impregnate wood through in situ polymerization of ε-caprolactone [59].

During the modification process, the added alcohol initiates the polymerization of caprolactone monomers, leading to the formation of covalent bonds with the hydroxyl groups in the wood cell wall [60,61]. The polyester chains attach to the cell wall polymers and fill the cell wall cavities [62]. The reaction scheme is presented in Figure 3.

The base material, ε-caprolactone, is low-cost compared to other biodegradable alternatives, and the process itself is waste-free [62]. It can be enzymatically degraded [63]. While solvent curing is typically used, oven-curing has shown promising results, allowing multiple uses of the monomer solution and generating less waste [64].

PCL impregnation of wood was first reported in 2014 [62], although its application in enhancing the water resistance of laminated wood papers dates back to 1994 [65]. No commercial products are currently available. So far, PCL modification has been performed on Paulownia, poplar, eucalyptus [66,67], Scots pine [64,68], and spruce wood [62,64,69,70].

2.3.2. Changes in Material Properties

Paulownia, poplar, and eucalyptus wood were modified by grafting PCL in the work of [66]. They observed an increase in ash content, hot water solubility, and holocellulose content after modification. On the other hand, lignin and alpha-cellulose content decreased. The chemical modification of the wood constituents was associated with an increase in carbonyl group absorption in the FTIR analysis.

Ermeydan and colleagues carried out various studies on PCL-modified Scots pine wood. They reported that at 15% WPG, water absorption decreased by 70% and ASE was 40% [62,71]. These improvements were more prominent with higher catalyst concentrations [64]. Poplar with 15% WPG had an ASE of 42%, while Paulownia with 2.4% WPG and eucalyptus with 6.2% WPG were less effectively modified [67]. According to SEM analysis, there were no noticeable cell wall deformations after modification [68]. The compression strength parallel to the grain did not decrease significantly [64]. In a 672 h artificial weathering test, PCL-modified wood showed a lower tendency for color change (∆E* color difference decreased from 8 to 4). It also had fewer macro- and microcracks. FTIR analysis confirmed the presence of PCL on weathered wood surfaces, having high absorption at 1724 cm⁻¹ wavenumber, corresponding to carbonyl stretching of polycaprolactone. The contact angle of water decreased after weathering from 65° to 55° on reference samples and from 105° to 65° on PCL-treated wood [68]. Reusing recovered ε-caprolactone monomer can improve the weathering performance of wood, but only in the initial periods, not in prolonged exposure [69]. In malt agar tests, Scots pine with 15.5% WPG resisted fungal attacks by C. puteana and T. versicolor [68], as did spruce [70]. However, Siberian pine wood with 9.3% WPG did not resist fungal decay [71]. In soil contact decay tests, a 70% ε-caprolactone concentration was more effective against brown rot fungi, whereas lower concentrations worked better against white rot fungi [70].

2.4. Polybutylene Adipate Terephthalate (PBAT)

2.4.1. Process Description

PBAT is a synthetic, biodegradable polymer, an aliphatic-co-aromatic random copolyester, produced via chemical synthesis from fossil fuels. It is widely used in the agricultural, packaging, and medical industries [72]. In terms of plastic production, its major disadvantages are its low strength, low heat resistance, and high production cost. However, it offers several key advantages: high flexibility, processability, tensile strength, elongation at break, ductility, and 100% degradability [73,74,75].

By increasing its terephthalate content and optimizing process parameters, its mechanical properties can be increased, such as Young’s modulus and elongation at break [5,76].

PBAT is produced via polycondensation of 1,4-butanediol with adipic acid and terephthalic acids (or butylene adipate). Catalysts can be added such as rare earth compounds or zinc acetate. PBAT production requires long reaction times, high vacuum, and temperatures above 190 °C to drive condensation and remove by-products like water [77,78,79]. The chemical structure of PBAT is presented in Figure 4.

The PBAT market is expected to reach 112 million tons by 2025 [81], with an estimated market value of USD 2.07 billion by 2031 [73].

PBAT-based composites can be produced via in situ polymerization, melt mixing, or solvent casting [82]. However, most fillers have low compatibility with PBAT; their surface needs to be modified for improved wetting and interfacial compatibility [83]. PBAT–wood composites have been in development since the 2010s [84], generally using non-specific wood flour (WF).

PBAT is sold under the trademarks Ecoflex^® (Ludwigshafen, Germany) (the first certified biodegradable plastic derived from petrochemicals) [85] and Ecovio^® (Ludwigshafen, Germany) (a blend of Ecoflex^® and bio-derived polylactic acid) [86], both produced by BASF. It has been commercially available since 1998 [87]. PBAT can replace polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and low-density polyethylene (LDPE) plastics due to its outstanding thermal properties [7].

2.4.2. Changes in Material Properties

PBAT is widely used in composite production. Cellulose fibers have incompatible surface chemistry with PBAT. In order to improve the adhesion between the fiber and PBAT matrix, mercerization (alkali treatment) [88,89], acetylation [89,90], mercerization followed by acetylation [84], silylation [90,91], and coupling agent treatment [89] can be carried out.

Melt-mixed with 20 wt% lignocellulosic fibers (e.g., peach palm), it improves mechanical and thermomechanical performance [91]. Similar reinforcement levels in PBAT–PLA–wood fiber blends enhance dimensional stability and creep resistance but reduce stiffness [92]. This can be addressed with biopolymer fillers.

Chemically modified fibers, like acetylated curauá, greatly boost tensile strength, elongation, fiber adhesion [90], flexural properties, impact bending strength, and thermal stability [93].

PBAT strengthens PLA–wood foam by 797% and improves PLA melt strength and crystallization [94]. PBAT can be used in WF–PBAT–PLA composites for cost-effective 3D printing material [95].

Biodegradable composite films were produced by esterification of chemically modified WF blended in a PBAT matrix, showing competitive mechanical properties to commercially available films [96].

PBAT and thus PBAT–wood composites are environmentally safe and non-toxic. There is no environmental risk of introducing them into composting systems [81,97]. However, PBAT does not degrade in marine or freshwater environments [98].

2.5. Poly(Butylene Succinate) (PBS)

2.5.1. Process Description

PBS or PBSu or poly(tetramethylene succinate) and its copolymers are biodegradable, thermoplastic aliphatic polyesters made via polycondensation of glycols (e.g., ethylene glycol or 1,4-butanediol) with aliphatic dicarboxylic acids (e.g., succinic or adipic acid) [99].

It has a white crystalline structure. Its glass transition temperature and mechanical properties are comparable to polyethylene and polypropylene. It offers good processability [100] and thermal stability [101] and the highest tensile strength compared to its copolymers [102]. As it has higher crystallinity than its copolymers and it degrades more slowly [102]. The chemical structure of PBS is presented in Figure 5.

The first report of a polyester derived from ethylene glycol and succinic acid dates back to 1863 [103]. Later, PBS was developed in 1990 by Takiyama and colleagues, and commercial production began under the trade name Bionolle^® by Showa High Polymer (Tokyo, Japan) in 1991 [99]. Since 2017, it has been sold under the brand BIOPBS by PTT MCC Biochem Company Limited. Around 2020, its global production volume was estimated to be approximately 100,000 tons. At that time, fossil-based PBS was priced USD 4.0–4.5 per kg, while PBS from renewable resources cost between USD 5.5 and 6.0 per kg [104].

PBS can be used in medical applications, although its biocompatibility needs to be improved by plasma treatment, like H₂O or NH₃ plasma immersion ion implantation in the work of [105]. It is also widely used in other fields where biodegradable plastics are needed, such as disposable consumer products and sanitary products [106].

Currently, PBS–wood composites and PBS-impregnated wood are not commercially available.

PBS is valued for its well-balanced combination of tensile strength, flexibility, durability, impact performance, and notable thermal stability. However, it has certain drawbacks, including a relatively low Young’s modulus and susceptibility to thermal degradation during melt processing—especially at elevated temperatures. These issues have led to efforts to improve PBS properties by blending it with other polymers, reinforcements, or additives to enhance processability, stiffness, and mechanical strength, such as in PBS–PLA blends [104].

PBS can be tailored by incorporating different co-monomers via copolymerization, enabling fine-tuning of its physical, thermal, and degradability characteristics. A notable example is poly(butylene succinate-co-butylene adipate) (PBSA), which offers enhanced elasticity and a more rapid biodegradation rate than pure PBS, making it a promising candidate for compostable packaging applications [107].

PBS oligomers can be made by melt-polymerizing dimethyl succinate with excess 1,4-butanediol (25% more) using titanium (IV) butoxide as a catalyst. The reaction is performed under nitrogen, heated to 180 °C over 130 min at reduced pressure, and then cooled to form solid white blocks [108].

The testing and production of PBS–WF composites and PBS-impregnated solid wood have been topics of research since 2003 [109].

2.5.2. Changes in Material Properties

WF-PBS composites (50/50, w/w) showed an increase in tensile strength and Young’s modulus from 17 and 28 MPa to 814 to 1007 MPa, respectively, in the work of [109]. They also found that the elongation at break increased from 3 to 6%.

WF is highly hydrophilic, while most thermoplastics like PBS are nonpolar and hydrophobic, leading to poor compatibility and weak interfacial adhesion in their composites [110]. This can be improved by adding a suitable compatibilizer [111], like cellulose fatty acid ester [112], maleated PE [113], and silane coupling agents, such as 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, and N1-3-trimethoxysilylpropyldiethylene triamine [114]. On the other hand, the environmental pollution caused by using organic solvents such as dimethyl sulfoxide, tetrahydrofuran, and toluene is to be questioned. Adding a diol-based polyurethane prepolymer compatibilizer to a PBS–WF composite can improve its tensile strength by 52.1%, elongation at break by 125%, and thermal performance and reduce its EMC to 3.57% [115].

Too high WF content can decrease its melt flowability and the quality of the wood–plastic composite [116]. Interfacial adhesion and tensile strength can be increased by adding polymeric diphenylmethane diisocyante (pMDI) [117,118]. The melt flowability can be increased by adding kraft lignin (KL) [119]. Here, WF and KL negatively impacted properties when used alone, but pMDI mitigated these effects by enhancing compatibility and reinforcing composite structure. In the study of [120] about PBS–DLA (dilinoleic acid)–WF composites, those that contained WF up to 30% exhibited enhanced surface degradation, increased roughness, microcracks, porosity, chemical changes, and a noticeable decline in mechanical properties over time.

With chemical modification, [108] impregnated wood with PBS oligomers (OBS). The highest dimensional stability (ASE 60+%) was achieved by using an impregnation temperature of 90 °C, then processing in the wet state (water soaking or wet heat) at 100 °C, and then dry heating at a higher temperature of 120 °C for 4 days. Treating the wood with PBS oligomers (at 160 °C for 2 days and leached) increased its durability according to their 1-year-long weathering test. Here, no cracks, no mold, and less discoloration were observed. In a fungi resistance test against Coriolus versicolor, PBS oligomer impregnation reduced its weight loss from 17% to 3.7%.

PBS and OBS oligomers, used as lumen-filling treatments, were polymerized inside beech and radiata pine veneer samples in the work of [121]. They caused slight changes in wood stiffness (from 23 GPa to a maximum of 25 GPa), with softening depending on the polymer melt temperature. However, they did not significantly reinforce the wood, though they did lower the glass transition temperature of wood components. In [122], thermogravimetric analysis showed that PBS polymerized effectively inside beech wood and improved the wood’s thermal stability. In a similar study of [123], it was found that around 55% of the oligomer could be extracted, indicating partial fixation. While these treatments do not initially improve dimensional stability, extended water leaching appears to shift hydrolyzed oligomers into the cell wall, increasing ASE to about 40%. Despite minimal polymerization, it maintains stable mechanical properties after leaching, making it promising for durable wood modification. Higher treatment temperatures are expected to further enhance polymer fixation and performance. It was also confirmed in [124] that under dry conditions, OBS does not enter wood cell walls, so no improvement in ASE is observed. However, adding humidity helps PBS penetrate the cell walls, likely due to water-induced swelling, achieving ASE values of 60%–70%. Without hydrothermal treatment, OBS remains mostly in the lumens with little interaction with wood polymers. Dynamic Mechanical Thermal Analysis confirmed limited polymer–cell wall interaction in dry-treated samples, while hydrothermal conditions enhanced diffusion and property changes.

PBS is biodegradable and compostable [125]. In the research of [126], it was highlighted that esterases and lipases are crucial enzymes in the degradation of PBS under mesophilic conditions, where they promote the hydrolytic cleavage of ester bonds and thereby enhance the breakdown of the polymer. Its degradation rate can be accelerated by adding WF or rice husk flour [120,127,128].

2.6. Zein

2.6.1. Process Description

Zein is a hydrophobic protein derived from corn or maize, specifically a prolamin fraction [129]. It is non-toxic, combustible, and appears as a white to light yellow powder, without any odor or taste [4]. Zein is produced as a by-product of corn processing [130,131]. It makes up approximately 44%–79% of total corn protein content [132]. It is soluble in diluted alcohol and alkylimidazolium ionic liquids [133] and is known for its excellent film-forming properties [129]. The chemical structure of the zein polymer is presented in Figure 6.

Zein has been utilized for various purposes, including antimicrobial films for food packaging [135,136], cell culture substrates in medical applications [137], and as a hydrophobic, stain-resistant coating of cotton textiles [138]. Although it is less suitable for wood adhesives compared to gluten-based adhesives, it can still be used in applications where water contact is not expected [139,140].

In wood treatments, zein is typically used as a dispersion in ionic liquids by immersing it at around 40 °C. After that, the impregnating agent (1-ethyl-3-methylimidazolium chloride ionic liquid) is washed out with water (immersion), and the treated wood material is dried at 105 °C and conditioned. Zein-based wood treatments have been studied since 2007 [141] but are not commercially available and are rarely the topic of research.

2.6.2. Changes in Material Properties

Spruce wood treated with combinations like zein and Acetem (acetylated mono- and diglyceride), Acetem with Natamax (natamicin antimicrobial), or glycerol-plasticized zein exhibited antifungal properties by forming a barrier against moisture and oxygen, both essential for mold development [141]. The acidic nature of Acetem further enhanced its antifungal activity.

A mild impregnation method was developed by [142] using a 5% zein solution in alkylimidazolium-based ionic liquids at 40 °C. These ionic liquids dissolve more zein than traditional solvents and produce a low-viscosity solution that quickly penetrates the wood (uptake ~520 kg/m³). When water is added, the ionic liquid is removed, causing zein to precipitate inside wood tracheids and on surfaces, forming a physical barrier against oxygen, moisture, and microbes. This treatment also increased wood hardness by about 10%, enhancing wear resistance.

In another study [133], this process increased the Brinell hardness from 41.60 to 54.60 MPa at 16.22% WPG. Its water contact angle increased from 36.85 to 50.72° and the total surface energy decreased from 43.66 to 41.11 mN/m (with the dispersive component decreasing from 30.22 to 10.89 mN/m and the polar component increasing from 6.02 to 10.89 mN/m). It has a dominating acidic character, probably due to the residual isoleucine from zein [143,144]. FTIR studies showed stronger absorption bands assigned to amide groups (3200, 1650, and 1512 cm⁻¹), and a band at 1410 cm⁻¹ assigned to gelatin. Due to zein’s low molecular mass and high diffusivity, the relative coating thickness was low. However, it absorbed ~80% less water than untreated wood and showed ~80% better dimensional stability.

2.7. Lignin

2.7.1. Process Description

Lignin is one of the three main components of wood, along with cellulose and hemicellulose, and serves as the natural “glue” that holds wood fibers together. This complex polymer gives wood its structural integrity and contributes to its natural durability and strength. Traditionally, lignin has been viewed as a waste product in the paper and pulp industries, where it is removed during processing and often burned for energy. A comprehensive review was conducted on lignin and lignin-derived compounds in [145].

However, researchers have recognized lignin’s potential as a valuable resource for wood modification due to its natural compatibility with wood structures and its ability to enhance various wood properties. Lignin molecules (Figure 7) are naturally too large to permeate the wood cell walls in their original state. Therefore, for applications in chemical wood modification, lignin must be cleaved or depolymerized via hydrolysis, pyrolysis, and liquefaction. The main products resulting from most lignin depolymerization methods are phenolic compounds such as phenol, catechol, guaiacol, ortho-cresol, and para-cresol. Studies have shown that some of these cleaved products, like guaiacol, o-, m-, p-cresol, and 4-ethylphenol, are suitable for replacing phenol in phenol-formaldehyde (PF) resins. However, other compounds like catechol, 4-methylguaiacol, and syringol can have negative impacts on resin performance [145].

So there are several challenges associated with applying lignin in wood modification: the size of lignin molecules, not all lignin cleavage products are suitable, lignin is generally less reactive, and wood properties can have significantly higher variability. Although lignin and lignin-derived compounds are widely investigated for use in wood adhesives (e.g., as phenol substitutes in PF resins), the sources indicate that lignin-based wood modification products are currently not commercially available [145].

2.7.2. Changes in Material Properties

Wood modification using lignin impregnation employs several methods.

Vacuum impregnation was used with aqueous lignin solutions in [147]. In this paper, Scots pine was modified by vacuum impregnation with low-concentration (0.5%–1.0%) aqueous solutions of various lignins to improve its durability: alkali lignin, kraft lignin (Curran), hydrolysis lignin (HL), and industrial lignosulfonate (LS-DP), as well as certified lignosulfonates (LS-8 and LS-52). Alkali, kraft, hydrolysis lignins, and industrial lignosulfonate showed the highest efficacy, leading to negligible mass losses for brown rot fungi (%). The durability was proposed to increase because the phenols leached from lignin and they were adsorbed by wood and acted as a biocide. Additionally, the penetration of lignin nanoparticles into microvoids formed “plugs” that hindered the diffusion of fungal enzymes to the cell wall surface. Hydrophilic properties did not increase significantly or did not change at all. In the study of [148], black pine (Pinus nigra) and beech (Fagus sylvatica L.) were impregnated using a vacuum technique at room conditions with three forms of lignin, kraft lignin (L), acetylated kraft lignin (AL), and lignin nanoparticles (LNPs), and post-treatment with atmospheric plasma. All treatments significantly improved the hydrophobicity of wood surfaces, with higher water contact angles (WCAs), particularly in pine wood, which achieved hydrophobic values (WCA > 90°) initially. Kraft lignin showed the highest initial WCA for pine (111.1°) and beech (92.3°). Plasma post-treatment further amplified hydrophobicity, with pine achieving WCAs exceeding 110° after 60 s and maintaining these characteristics over time. Treated samples exhibited better UV stability and lower color change after 300 h of exposure. For pine, pine–LNPs exhibited the best UV stability with the lowest color change of 12.2, significantly outperforming the pine reference at 23.0. For beech, beech–L showed the most significant improvement in color stability, with a color change of 5.4, compared to the beech reference at 9.9. During thermal decomposition, in an oxidative atmosphere (O₂), treated samples showed lower degradation peaks and significantly higher residual mass at 800 °C, pine–L had 21.1% (vs. 14.3% for reference) and beech–L had 22.7% (vs. 17.3% for reference), suggesting enhanced oxidative stability and reduced flammability. Pine–AL achieved the highest retention after leaching at 77.99%. Pine–L maintained hydrophobic behavior (WCA of 90.80° after 60 s) even after the leaching test, indicating long-lasting water resistance. In terms of hygroscopic behavior, pine–AL exhibited stable performance across the entire moisture range, while pine–LNPs showed stability from 75% relative humidity (RH) onwards. For beech wood, beech–AL treatment displayed minor differences in EMC within the 35%–65% relative humidity (RH) range, and beech–LNPs showed similar EMC behavior from 65% RH onwards. In contrast, beech–L treatment did not lead to significant improvements in dimensional stability, exhibiting changes similar to untreated samples.

Wood was impregnated with enzymatic hydrolysis lignin (EHL) in [149]. Here, EHL was dissolved in various solvents: ethanol, dimethyl formamide (DMF), tetrahydrofuran (THF), and dioxane. This was incorporated into Douglas fir wood cells via impregnation and drying. DMF pre-swelling was particularly effective in facilitating penetration into cell walls. EHL successfully colonized in both cell lumens and cell walls. The anti-swelling efficiency reached up to 99.4%, and moisture absorption decreased to 0.55%. Mass loss after brown rot decay decreased significantly to 7.22%. Cell wall elasticity increased by 8.7%, and hardness increased by 10.3%. Nanoindentation data indicated a slight mechanical enhancement of the cell walls, and the wood’s mechanical property was not significantly impaired.

Rubberwood was subjected to a two-step process in [150]: impregnation with maleated lignin (ML), followed by densification. The modification resulted in a more uniform structure and increased density. Scanning electron microscopy (SEM) confirmed fully compressed cell walls and cell lumina filled with ML. Water immersion and moisture absorption experiments showed a significant reduction in thickness swelling. It was only 27.7% and 20.9%, representing a nearly 76% and 55% reduction, respectively, compared to densified wood treated only with water. The MOR improved by nearly 54% and the MOE improved by nearly 200% compared to untreated wood. Lignin-based treatments could provide sustainable solutions for wood protection based on these findings.

2.8. Tannin

2.8.1. Process Description

Tannins are plant-derived polyphenolic compounds, found in various plant parts such as bark, wood, leaves, galls, fruits, and seeds, where they serve as a natural defense mechanism against pathogenic bacteria, fungi, and insects. Historically, the most prominent industrial application of tannins has been leather tanning. Beyond that, tannins are now employed in a variety of fields: the food and beverage industry, pharmaceutical and medical applications, and also as corrosion inhibitors for metal surfaces, forming protective complexes, and as chemically self-expanding fireproof and insulating foams [151,152,153].

For wood protection, tannins naturally contribute to a tree’s resistance to decay. However, their inherent high water solubility previously limited their long-term efficacy as wood preservatives. To overcome this challenge, modern wood modification strategies involve combining tannins with crosslinkers such as hexamine (Figure 8), boric acid, or furfuryl alcohol. These combinations, often applied through in situ polymerization, significantly improve the fixation of tannins within the wood structure [151,154,155,156,157,158].

Research into tannin-based wood modification for enhancing durability has been ongoing for several decades. In the 1970s, it was discovered that condensed tannins could undergo polymerization reactions similar to phenolics, especially when combined with hexamine to create stable, water-resistant networks for bio-based adhesives. In the 1980s, tannin-based wood preservatives began to be prepared, though their high water solubility was a recognized drawback. Since, 1993 the reaction between tannins and furfuryl alcohol for wood modification has been studied [154,155,157]. Tannin-modified solid wood is not yet widely on the market [154].

2.8.2. Changes in Material Properties

The durability of Pinus elliottii wood was investigated after being impregnated with “quebracho colorado” bio-protectives (containing tannin, named Colatan) and a CCA salt against white rot and brown rot fungi in [160]. Wood samples were impregnated using a vacuum–pressure method in a lab autoclave. Working conditions included an initial vacuum of 600 mm Hg for 30 min, a 5.5 kg cm⁻² pressure cycle for 30 min, and a final vacuum of 580 mm Hg for 20 min. Compared to the 38.80% weight loss of untreated samples, treatment with Colatan showed 0 to 29.26% weight loss against brown rot fungi, depending on different retentions. Compared to the 25.84% weight loss of untreated samples, treatment with Colatan showed 0 to 7.94% weight loss against white rot fungi, depending on different retentions.

Tannin from Acacia mearnsii was tested as a fire retardant for Simarouba amara Aubl. wood in [152]. The tannin was prepared at a concentration of 50 g L-1 and tannin was applied via immersion (IT) for 48 h, vacuum impregnation (VT) for 2 h at −40 kPa, brushing (BT) in three coats, and incorporation into water-based wood varnish (BTV). Tannin treatments exhibited the highest color changes, significantly darkening the wood (e.g., VT L* 54.98, varnish L* 78.26, and control L* 81.37). At 500 °C, tannin applied by immersion (IT) and vacuum (VT) retained residual masses of 34% and 24%, respectively, exceeding brushing (6%) and control (1.50%). Immersion (IT) retained 57.25%, vacuum (VT) 45.87%, brushing (BT) 41.58%, and tannin varnish (BTV) 52.52% of wood mass. The application of tannins facilitated char formation, forming a protective layer. While less efficient than commercial flame retardants, tannin demonstrated efficacy in reducing fire spread.

Mimosa tannin extracts were used for impregnation of Scots pine and beech wood with 10% and 20% w/w tannin, with pH adjusted to 9.0 using NaOH and containing 6.0% hexamine, in the work of [157,161]. In leaching tests, 17%–35% of the original tannin was lost depending on test severity. Additionally, 20% tannin-treated samples showed almost complete resistance, with a 3.3% survival rate against Hylotrupes bajulus. However, this increased to 10% after leaching. Leaching also compromised the photostability of treated samples in artificial and natural weathering tests, and all samples turned gray by the end of the test. Compression strength increased by 35% for Scots pine treated with 10% tannin and by 15% for beech wood treated with 20% tannin. MOE of Scots pine increased from 99.2 ± 4.3 MPa to 111.8 ± 12.5 MPa (10% tannin) and 120.9 ± 10.8 MPa (20% tannin). Tannin resin made the wood surface smoother, reducing roughness and decreasing the penetration of water-based PVAc adhesives, with Scots pine showing no change and beech showing lower shearing resistance (from 12.64 to 7.99 N/mm²). Additionally, 20% tannin increased ignition time from 12 s to above 75 s and reduced flame time from 140 s to 15–20 s. Adding 5% boric acid or 5% phosphoric acid to 20% tannin formulations significantly decreased ember time. In a long-time exposure fire test, tannin impregnation allowed samples to resist burning for at least twice the time needed for complete burning of untreated wood.

Spruce, pine, poplar, and beech wood were impregnated with quebracho tannin and hexamine concentration to improve their mechanical and leaching resistance in [154]. They were subjected to vacuum-dipping impregnation in a desiccator, applying a vacuum of 100 mBar for 30 min, followed by 24 h of dipping at atmospheric pressure. The tannin–hexamine formulation was then in situ-polymerized by oven-drying at 102 ± 2 °C for 24 h. Tannin concentrations of 5%, 10%, 15%, and 20% were used, with hexamine added at 2.5%, 5.0%, 7.5%, and 10% by weight of tannin. The findings suggest that 10% quebracho tannin formulations containing 2.5%–5% of hexamine are sufficient for improving mechanical properties and leaching resistance (increased by 20%–30%). Tannin concentrations did not significantly affect compression strength, MOE, MOR, and Brinell hardness. The tannin solution completely penetrated beech specimens, whereas in spruce, only partial penetration from the grains was visible. In a similar work of [157], penetration occurred longitudinally through tracheids with open bordered pits and radially through parenchyma rays in Scots pine. On the other hand, penetration occurred almost exclusively in the longitudinal direction through large and easily accessible vessels in beech wood.

Beech wood was chemically modified using a copolymer of hydrolysable chestnut tannins and furfuryl alcohol (FA), with tartaric acid (TA) as a catalyst. The samples were impregnated via a vacuum/atmospheric pressure cycle (20 mbar vacuum for 30 min and 1 h at atmospheric pressure) and then cured at 120 °C for 24 h. Solutions contained 50% or 25% organic matter, with 5% tartaric acid as a catalyst and various ratios of hydrolysable tannin (TH) to furfuryl alcohol (FA). Tannin-based treatments showed that WPG values reached a maximum of 55%. Higher tannin concentrations generally led to higher WPGs. Treatment (25)TH (tannin only, 25% organic concentration) showed the highest mass loss, with over 50% of the mass gained. All treatments improved the dimensional stability of the wood. Treatments (50)TH(2)FA(1) and (25)TH (mostly tannin) showed the lowest ASE. All treated samples showed improved thermal stability with lower weight loss up to 600 °C compared to untreated wood. The (50)TH(1)FA(1) treatment had the highest percentage of remaining mass, 30.5% of the original mass. Treatment with tannin alone was not sufficient to increase hydrophobicity. All treatments were more effective against brown rot fungi than white rot fungi. Beech samples modified with tannin and FA at 50% organic concentration showed degradation of less than 5%. Results indicated that tannins could replace two-thirds of FA for superior protection compared to conventional furfurylation. The weight loss of the tannin-only treatment (25)TH increased from 10.6% to 34.8% after leaching, which highlights tannin’s antifungal potential but also its problem with leachability.

In a similar research, beech wood was impregnated with furanic or tannin–furanic solutions under vacuum (100 kPa, 1 h) and pressure (1200 kPa, 30 min), followed by curing at 103 °C for 48 h in the work of [155]. Catalysts included adipic acid (AA), succinic acid (SA), tartaric acid (TA), and glyoxal. Treatment H (glyoxal-based solution with tannin) presented the highest average WPG values at 50.8%. The addition of tannins decreased water uptake, lowering swelling and increasing anti-swelling efficiency (ASE) values. Additionally, the Brinell hardness increased. The wood modified with the tannin–furanic polymer presented higher durability (weight losses 0.42%–0.77%) against decay than wood modified with only the furanic polymer (weight losses 15.65%–20.28%). The modified samples maintained their FTIR characteristics after vigorous leaching, indicating good fixation of the polymer. SEM analysis revealed that wood modified with the tannin–furanic-based polymer showed higher deposits not only in the wood cell walls (bulking effect) but also in the wood lumen cells.

Southern yellow pine (SYP) and yellow-poplar (YP) samples were impregnated with a mixture of condensed quebracho tannins and disodium octaborate tetrahydrate (DOT) via a vacuum–pressure cycle in [158]. The specimens were then heat-treated under a nitrogen atmosphere for four hours at 190 °C. The tannin solution was 80 g in 800 mL deionized water, heated at 60 °C, with 74%–77% purity. Here, tannins were effective in preventing boron leaching (reduced by 34.5% for YP and 46.5% for SYP). Tannins significantly increased resistance against white rot, brown rot, and termites.

Tannin impregnation followed by heat treatment showed varied effects on physical and mechanical properties, depending on the extract type (valonia, galex, and pine bark extract) in the study of [162].

2.9. Chitosan

2.9.1. Process Description

Chitosan is a natural, renewable, biodegradable, and non-toxic biopolymer [151,163,164]. Chemically, chitosan is a polysaccharide primarily composed of repeating units of beta (1–4) 2-amino-2-deoxy-D-glucose (or D-glucosamine) [165]. It is structurally comparable to cellulose [151,164]. Unlike chitin, which is completely insoluble in water, chitosan is generally water-soluble under acidic conditions [164,166]. A key advantage of chitosan is the presence of modifiable amino and hydroxyl groups in its chemical structure (Figure 9) [164,167].

It is derived from chitin, one of the most abundant polysaccharides in nature, through a process called N-deacetylation [151,163,164,165,166,167,169]. Chitin itself is a 1–4 linked polymer of 2-acetamido-2-deoxy-β-D-glucose, found extensively in the exoskeletons of marine crustaceans like shrimp and crabs, as well as in insect exoskeletons and fungal cell walls [151,164,165,166].

Chitosan is widely utilized across various industries, including wood and wood-based products (for improving stiffness, dimensional stability, water, decay, mold, and fire resistance, as well as for consolidation), medical/pharmaceutical/health (for drug carriers, artificial skin and bones, wound dressings, contact lenses, and antimicrobial applications), waste management (as a chelating agent for wastewater purification), agriculture (for plant protection and extending fruit life), paper (as a sizing agent and to enhance mechanical properties and water resistance), textiles (for improving water resistance), cosmetics (as an additive), food (as an additive and preservative), battery manufacturing (for solid-state batteries), and tobacco (for improving bonding properties) [151,164,165,166,167,169,170,171,172,173,174].

2.9.2. Changes in Material Properties

In the beginning of 2000s, [175] screened chitosan as a potential environmentally benign antimicrobial component for wood protection against wood-deteriorating fungi. Studies on chitosan-amended nutrient agar media showed total inhibition of Poria placenta, Coriolus versicolor, and Aspergillus niger using a 1% (w/v) concentration. In decay studies using small wood blocks, higher-molecular-weight chitosan proved more efficient against decay fungi. For unleached Scots pine wood samples treated with 5% and 2.5% chitosan solutions, the decay caused by P. placenta and Coniophora puteana was below 5% mass loss, with some formulations achieving below 3% mass loss. However, leaching decreased the efficacy of chitosan. In a similar study [166], chitosans with different molecular weights were compared. Scots pine mini-block samples were impregnated with 5%, 2.5%, and 1% (w/v) aqueous solutions of commercial chitosan products with varying molecular weights. Antifungal activity was tested against brown rot fungi, Poria placenta, and Coniophora puteana for 8 weeks. For unleached chitosan-treated samples, concentrations of 5% and 2.5% resulted in fungal decay below 5% mass loss. Chitosan with high molecular weight demonstrated superior fixation in wood and better resistance to leaching. The relative recovery of chitosan after leaching was approximately 60% for high concentrations (5%) and approximately 90% for low concentrations (1%). Torr and colleagues [169] investigated the chemical modification of Radiata pine with chitin- and chitosan-hexamethyl methylol melamine (HMMM) copolymers. Low-molecular-weight oligosaccharides of chitin and chitosan were reacted with HMMM to form aqueous “prepolymers” for treating Pinus radiata veneers. Treatment with the chitosan oligomer HMMM copolymer resulted in an average veneer stiffness enhancement of 20%. In contrast, chitin oligomer HMMM treatment showed no improvement. Microscopy revealed that the chitosan oligomer HMMM copolymer was concentrated in the S2 and S3 layers of the wood cell wall, suggesting that increased hydrogen bond density in these layers contributed to enhanced composite stiffness. Scots pine and beech wood were treated with 5% (w/v) solutions of modified chitosan of different molecular weights in [171]. The treated wood showed enhanced hydrophobation and some antifungal and fire-retardant properties. The modulus of elasticity of heat-modified, chitosan-treated wood increased by 27% compared to untreated wood. For Scots pine, the average mass loss after exposure to C. puteana and P. placenta was 4.9% and 1.6%, respectively, compared to 37.7% and 42.7% for untreated samples. For beech wood exposed to T. versicolor, the mass loss was 2.8% compared to 30.2% for untreated wood after eight weeks.

In the 2010s, the antifungal activities and fixation in wood of various forms of chitosan oligomers (average degree of polymerization = 4) were investigated by [176]. Wood was treated with a mixture of chitosan oligomers and boric acid. Wood decay results confirmed their antifungal activity against basidiomycetes but also highlighted their susceptibility to leaching upon water exposure. In a different study [163], the antifungal activities of silver nanoparticles, chitosan oligomers, and propolis ethanolic extract were evaluated. Here, Populus × euramericana I-214 clone wood blocks were impregnated with chitosan oligomers (60–130 kDa, 90% deacetylation) at concentrations of 10, 20, 40, and 80 mg/mL via a vacuum–pressure method following. The study found that while chitosan oligomers initially provided a protective effect against the white rot fungus Trametes versicolor, their efficacy gradually decreased over the 16-week exposure period. Untreated control samples experienced a 24.79% weight loss.

In the 2020s, chitosan oligomers and related nanoparticles (nano-chitosan–TPP particles) were evaluated as environmentally friendly wood preservatives in the work of [164], focusing on their bulking effects and leachability. Low-molecular-weight chitosan was depolymerized into oligomers (degree of polymerization = 4), which were then quaternized or left non-quaternized and mixed with tripolyphosphate (TPP) to form nanoparticles. Southern pine wood samples were treated via vacuum impregnation. Quaternized nano-chitosan–TPP-treated samples exhibited higher mass gain (up to 30.54%) and volume gain (up to 19.55%) compared to non-quaternized and control samples. However, these quaternized particles also showed increased mass loss after leaching (up to 22.81%), indicating poor fixation to the cell walls. Consequently, these nanoparticles were deemed suitable primarily for non-leaching interior applications. In the follow-up article [170], the fungal decay and fire resistance properties were evaluated. Quaternization of the nano-chitosan reduced mass loss in pine when exposed to Trametes versicolor under leached conditions, while non-quaternized nano-chitosan reduced mass loss when exposed to Gloeophyllum trabeum under unleached conditions. The nano-chitosan–TPP particles also significantly improved the fire retardant activity of the treated wood.

The paper by Papadopoulos et al., 2020 [167] investigated the sorption behavior of water vapor in pine wood treated with a high-molecular-weight chitosan polymer (3.55 × 10⁵ g/mol, 81% deacetylation). Pine sapwood specimens were immersed in a 5% (v/v) aqueous acetic acid solution of chitosan for 60 s. The Hailwood–Horrobin model was used to analyze the sorption isotherms. The treatment successfully reduced the hygroscopicity of the pine wood, leading to a reduction in equilibrium moisture content at all relative humidities. Specifically, the application of chitosan reduced total sorption by 23.8%, polymolecular sorption by 20.6%, and monomolecular sorption by 35.3% at saturation. This reduction was attributed to the formation of a chitin film encapsulating the wood, acting as a water barrier, and blocking hydrophilic hydroxyl groups. Manii wood (Maesopsis eminii Engl.) samples underwent double impregnation with boric acid and either chitosan or glycerol, followed by heating at 70 °C or 140 °C for 4 h in the work of [177]. The best treatment was found to be double impregnation with boric acid and chitosan, followed by heating at 140 °C, which significantly improved the wood’s resistance to white rot fungi, dry wood termites, and subterranean termites. Treated manii wood showed lower weight loss compared to untreated control wood, which had a 3.96% weight loss, with the optimal treatment achieving a 0.48% weight loss. Scots pine sapwood samples were impregnated with 1% caffeine solution, 1% solutions of medium- (90–310 kDa) or high-molecular-weight (310–375 kDa) chitosan, and combinations thereof in the study of [172]. The aims of this study were limiting caffeine leaching and resisting decay of brown rot fungus Coniophora puteana. Wood treated with caffeine alone or in combination with chitosan showed high resistance, with mass loss around 0.5%. Chitosan alone provided no resistance, resulting in mass loss between 18.75% and 28.10%. While caffeine-treated wood lost significant antifungal activity after leaching (mass loss around 21%), chitosan–caffeine treatments showed higher resistance, with mass loss approximately 10% for medium-MW chitosan and 8% for high-MW chitosan after leaching. Chitosan, especially with a high molecular weight, effectively limited caffeine leaching.

A cinnamaldehyde chitosan emulsion was developed by [174] to reduce the volatility of cinnamaldehyde and resist mold (Aspergillus niger). Poplar wood samples were impregnated with emulsions containing varying molar ratios of cinnamaldehyde aldehyde groups (CA-aldehyde) to chitosan amino groups (CH-amino). The emulsion significantly reduced cinnamaldehyde losses. The mold control effectiveness on wood treated with the emulsion at a 3.0:1.0 molar ratio was 95.8%. While cinnamaldehyde alone completely inhibited A. niger growth on PDA, 1 wt% chitosan showed no antifungal activity at low concentrations.

2.10. Alginate

2.10.1. Process Description

Alginate is a natural anionic polysaccharide that functions as a hydrocolloid. It is abundant in nature and can be found as a component in brown seaweeds (Phaeophyceae) [178,179]. It has a linear (unbranched) structure composed of two monomeric units, β-D-mannuronic acid and α-L-guluronic acid, linked by (1,4) glycosidic bonds (Figure 10) [178,179]. The specific arrangement of these monomers, along with their molecular weight, significantly affects the physical and chemical properties of the polysaccharide [178]. Alginates contain numerous free hydroxyl (-OH) and carboxyl (-COOH) groups, enabling the formation of intramolecular hydrogen bonds. The presence of carboxyl groups contributes to its sensitivity to ambient pH [178,179]. It is known for its low toxicity, biocompatibility, and biodegradability [178,179,180].

Sodium alginate (SA) is a prominent natural anionic polysaccharide and a type of hydrocolloid, widely recognized as a member of the hydrogel group. Hydrogel is a water-swollen and crosslinked polymeric network formed by the reaction of monomers. The alginate extracted from seaweed is typically sodium alginate. It is also found as a capsular polysaccharide in some soil bacteria. It was first identified in Kelp in 1883 [178].

Alginate is utilized in diverse industries such as food (as thickeners, stabilizers, and encapsulants), medicine and pharmaceuticals (for drug delivery, tissue engineering, and wound dressings), wastewater treatment (as an adsorbent), building materials (for insulation and flame retardancy), textiles and paper (as thickeners and consolidants), and cultural heritage conservation (for stabilizing archeological wood) [173,178,179,180,182,183,184,185,186].

The applications of alginate (usually SA) with solid wood and wood-based composites are currently primarily in the research and development phase.

2.10.2. Changes in Material Properties

Sodium alginate was used as an adhesive binder in biocomposites made from wood fibers and textile waste fibers for building insulation applications in the work of [184]. Various wood/textile waste ratios (e.g., 100/0, 50/50, 60/40, 70/30, and 0/100 by weight) were compared. During the process, aldehyde-based crosslinking agents such as glyoxal and glutaraldehyde were also used. The new biocomposites exhibited low thermal conductivity and high thermal capacity. An average thermal conductivity of 0.078–0.089 W/mK was reported. The semi-rigid biocomposites, particularly those with a 60/40 wood/textile waste ratio and glutaraldehyde crosslinking agent, had a mechanical strength of 0.84 MPa under bending and 0.44 MPa under compression.

Sodium alginate natural fiber biological composites were investigated by [183], aiming to use them as sustainable building energy-saving wall insulation materials. Here, different ratios of wood (0%–50%–60%–70%) and rice straw (0%–50%–60%–70%) fibers were mixed. A sodium alginate solution, containing glycerin and a glyoxal crosslinking agent, was prepared. The process was fiber immersion in the solution, stirring, hot-pressing (5 MPa), heat treatment (2 h, 70 °C), conditioning (10 days), and final drying. In their results, the flexural strength, elastic modulus, and compression strength increased with increasing wood fiber content. At 100% wood fiber content, the flexural strength reached 0.573 MPa, the elastic modulus reached 17.580 MPa, and the compressive strength reached 1.410 MPa. Adding the glyoxal crosslinking agent increased the elastic modulus by 21% and compressive strength by 16% in certain samples. SEM analysis showed excellent adhesion and wettability between wood fiber and the sodium alginate binder, particularly in samples with 100% wood fiber, with no microcracks observed. The lowest thermal conductivity of 0.078 W/mK) was observed at 60% wood fiber content. Diffusivity decreased from 310 × 10⁻⁷ m²/s (without wood fiber) to 174 × 10⁻⁷ m²/s (with 100% wood fiber). This material exhibited strong thermal insulation capabilities, making it suitable as an insulating material for energy-efficient building walls. Findings indicate that the composite’s characteristics are enhanced by increasing the wood fiber content and incorporating an appropriate amount of glyoxal as a crosslinking agent.

A bamboo–alginate composite was developed by [185], using delignified bamboo. The process was impregnation with a sodium alginate solution (reduced pressure), ionic and chemical crosslinking, adding CaCl₂ and glutaraldehyde, and then hot-pressing (unfilled voids were removed). The hot-pressed delignified bamboo (HP-DCB) composites achieved a tensile strength of 1.1 GPa and a flexural strength of 679 MPa.

In the study of [186], they focused on developing an eco-friendly delignified wood–calcium alginate (CaA) aerogel with enhanced mechanical properties, efficient thermal insulation, and improved flame retardancy. For this, Chinese fir wood was delignified and then crosslinked with calcium alginate. Its density decreased to 89 kg/m³ (compared to control wood’s 120 kg/m³). The tensile strength, elongation at break, bending strength, and compressive strength were significantly enhanced by 128.4%, 109.1%, 31.8%, and 241.7%, respectively. The thermal conductivity of the aerogel was lower than that of concrete and control wood. The Limiting Oxygen Index (LOI) reached 59.2%, classifying it as self-extinguishing.

Sodium alginate is also a promising consolidant for archeological wood [173,179], having potential for dimensional stabilization, good penetration, compatibility with wood structure, and ability to improve the thermal stability of lignin, especially when forming beneficial network structures through methods like slow drying or freeze-drying

2.11. Natural Gums

2.11.1. Process Description

Natural gums are naturally occurring substances primarily derived from plants. They can be exudate gum from tree trunks or natural resins obtained by distilling resins from pines [187,188].

In the first case, the tree trunk is injured to stimulate the release of the gum. For analysis or specific applications, the essential oil from such gums can be extracted through hydrodistillation, where the gum is heated with distilled water [188].

Natural resins can be obtained by distilling resins from pine trees, like gum rosin (colophony) [187].

They are used in the pharmaceutical and food industries as medicine, in pest control as essential oils, and in adhesives, varnishes, and sealing wax production [189].

2.11.2. Changes in Material Properties

Poplar wood sapwood specimens were modified with rapeseed oil heat treatment at different temperatures (180, 200, and 220 °C) and durations (2 and 4 h) in the work of [188]. Following thermal modification, the wood specimens were cooled directly in rapeseed oil containing Pistacia atlantica gum at concentrations of 0%, 5%, and 10% (w/w) at 25 °C for 30 min. Oil heat treatment resulted in a WPG range of 60.7% to 77.6%. The WPG increased proportionally with the concentration of P. atlantica gum. Oil heat treatment improved resistance to Trametes versicolor (16 weeks). The use of P. atlantica gum provided a slight improvement in decay resistance only for specimens modified at 180 °C. Oil heat treatment improved mold resistance (4 weeks, visual rating of mold coverage), with the effect being more pronounced at higher temperatures. The addition of P. atlantica gum significantly reduced mold coverage, which was 76%–100% for untreated and 51%–75% for OHT samples, decreasing to 26%–50% after the addition of P. atlantica gum. These findings were attributed to its monoterpene content and improved moisture exclusion. Increased heat treatment temperature proved more beneficial than extended treatment time for fungal resistance.

Low-quality young (15-year-old) teak wood was modified using gum rosin (from Pinus merkusii Jungh. et de Vries) to enhance its dimensional stability and strength and to reduce its hygroscopicity in the study of [187]. Here, three nonpolar solvents were employed: turpentine oil, petroleum oil, and n-hexane. Gum rosin solutions were prepared at 7.5% and 15% concentrations in each of them. The process was conditioning (to 12%–15% MC), pre-vacuum (15 min, 1 atm), impregnation (1 h, 10 atm), wiping off excess solution, and conditioning (2 weeks). It was found that the gum rosin solution (at 15% concentration) with turpentine (WPG 3.2%) and petroleum oil (WPG 3.5%) as solvents may have found it more difficult to enter the wood cells than with n-hexane (WPG 4.2%). Gum rosin impregnation was largely ineffective in reducing water uptake, showing no significant difference in EMC compared to untreated wood. The treatment did not significantly reduce the total tangential or radial shrinkage of teak wood. The lowest tangential (4.7%) and radial shrinkages (2.9%) were achieved with a 15% gum rosin solution in petroleum oil. Generally, the treatment did not significantly impact bending MOE and MOR, except for the 7.5% gum rosin solution with petroleum oil as the solvent, showing an MOE of 10 GPa and an MOR of 78 MPa. The 15% gum solution with n-hexane also exhibited a high MOR of 9.7 GPa. The best dry bonding strength (6 MPa) was observed with 15% gum rosin in turpentine oil.

2.12. Fatty Acids

2.12.1. Process Description

Fatty acids, derived from natural oils like linseed, sunflower, maize, soybean, olive, canola, cottonseed, etc., can be used to treat wood to increase hydrophobicity and protect against moisture-related degradation.

Fatty acids chemically bond with the cellulose fibers in wood, forming a hydrophobic layer that repels water and reduces the wood’s tendency to absorb moisture. They are promising replacements for petroleum oil [190]. There are more than 200 different fatty acids found in plants, which are renewable lipids, triacylglycerols.

The proportion of fatty acids can be from 1 to 60% of the total dry weight of the caster seed. Its composition depends on the plant species and the extraction process conditions. Their function is to be an energy source and protective agent of the plant against chemical and biological degradation. The highest resistance are in the lipids of the bark [191].

In wood modification research, fatty acids from the following seeds or beans are typically used: rapeseed, soyabean, linseed, castor bean, sunflower seed, tung oil, and safflower or thistle seed. Unlike oils, fatty acids can polymerize in the wood and chemically modify it, improving its durability, dimensional stability, and hydrophobicity.

Fatty acid derivatives are called waxes, and the most commonly used are carnauba wax and beeswax, but these are non-drying substances, which do not polymerize in the wood and are rather used for a moisture barrier and esthetic enhancement indoors.

Rapeseed oil consists mainly of oleic acid (64.1%), linoleic acid (18.7%), and linolenic acid (7.8%). The rest are palmitic, palmitoleic, stearic, arachidic, gadoleic, behenic, and lignoceric acids (<4%) [192]. It has favorable properties like low viscosity and good compatibility with common timber species. However, high retentions (>150 kg/m³) are experienced because of overfilling lumens. It lacks intrinsic fungicidal or termiticidal activity and relies solely on moisture exclusion; without added biocides or chemical modification, it offers limited protection in continuously wet or biologically aggressive environments. Combustibility also rises with residual oil, raising fire safety considerations [193].

Soybean oil consists mainly of linoleic acid (53.2%), oleic acid (23.4%), palmitic acid (11%), and linolenic acid (7.8%). The rest are myristic, palmitoleic, stearic, arachidic, and behenic acids (<4%) [192]. A patent was issued in 2003 about the use of soybean oil in wood impregnation processes [194]. Here, the process begins by partially oxidizing soybean oil through heating (preferably >150 °C) and airflow to initiate polymerization while keeping the oil fluid for penetration. The wood is placed in a pressure chamber, vacuum-treated (81.3 kPa for 15 min) to open pores, and then impregnated with heated soybean oil (74 °C) under pressure (345–550 kPa) for about 30 min. In situ polymerization is promoted by continued heating (74 °C) inside the chamber to fix the oil within the wood and reduce leakage. Finally, a more oxidized oil layer is applied to the surface and cured with UV light, heat, and air to form a hard sealing coat with self-healing ability if surface breaches occur. During oxidation and heating, the double bonds (like carbon–carbon) in the fatty acids break and reform, allowing the fatty acids to crosslink with each other. This results in a polymerized oil network that solidifies inside the wood. Polymerization can occur between fatty acids on the same or different triglyceride molecules or between free fatty acids, enhancing the fixation of the oil within the wood structure. Soybean oil makes the wood material suitable for structural and outdoor applications such as utility poles, railroad ties, decking, and marine structures, where long-term durability and reduced environmental impact are important. Biocides can be added for enhanced protection outdoors [194].

Linseed oil consists mainly of linolenic acid (47.4%), linoleic acid (24.1%), and oleic acid (19.0%). The rest are palmitic, palmitoleic, stearic, and arachidic acid (<6%) [192]. Linseed oil penetrates deeply into cell walls and lumens, enabling improved dimensional stability and mechanical properties. However, raw linseed oil cures slowly and tends to yellow or darken over time. It offers no intrinsic fungicidal or termiticidal properties, so high retentions or co-treatments with biocides (e.g., boron compounds) are required for long-term durability. Additionally, oil-soaked rags pose a recognized spontaneous combustion hazard during drying. Oil heat treatments (OHTs) combine thermal modification with linseed oil impregnation to further enhance wood properties. Vacuum–pressure impregnation of linseed–oil emulsions has also been used to fix inorganic preservatives like boric acid, bolstering decay resistance against brown rot fungi and termites even after leaching tests [195].

Castor oil is made from the seeds of the castor plant (Ricinus communis), containing about 50% castor oil. There is about 89.5% ricinoleic acid in it, and the rest is palmitic, stearic, oleic, and linoleic in small amounts. It has high kinematic viscosity (15.25 mm²s⁻¹ at 40 °C), high solubility, and a low melting point. There were 400 k tons produced in 2003, which increased to 700 k tons in ten years, and this has kept growing. Castor oil-based bio-binders or biopolymers are usually in the form of polyurethanes, polyamides, polyethers, and polyesters. Their main properties (density, tensile strength, tensile modulus, and elongation) define their field of use and application [196].

Sunflower oil consists mainly of linoleic acid (68.2%), oleic acid (18.6%), and palmitic acid (6.8%). The rest are lauric, myristic, palmitoleic, stearic, linolenic, and arachidic acid (<5%). Its unsaturated triglycerides fill cell lumens and walls during treatment and a natural film is formed by air-curing triglycerides. However, it has a slow curing time, susceptibility to decay, insects, and weather, and increased flammability due to remaining oil residue. Sunflower seed oil-treated timber finds use in exterior claddings, decking, outdoor furniture, interior joinery, etc. Increased durability can be reached with co-impregnation of biocides or mineral salts [192].

Tung oil is a non-edible plant-based oil derived from the seeds of the tung tree (Vernicia fordii), and it is predominantly produced in China, accounting for over 100,000 tons annually—more than 80% of global output. It consists of alpha-eleostearic acid (82.0%), linoleic acid (8.5%), palmitic acid (5.5%), and oleic acid (4.0%). Its renewable nature and affordability have made it a widely adopted raw material in the polymer manufacturing sector. The oil’s rich conjugated unsaturated bonds enable swift polymerization, hydrophobicity, and deliver superior drying properties, making it highly suitable for numerous industrial uses, such as wood varnishes, biodiesel, toughening agents, and pesticides [197].

Oil impregnation of wood has been in use since the 20th century, but it has been a highlight in research since the 21st century, especially in the topics of oil heat treatment and supplementing oil with other biocides [198,199]. A wide variety of wood species have been tested; the most common are pine, spruce, poplar, and beech wood. These are widely available and of low density and low durability, making them suitable candidates for enhancement with oil impregnation.

2.12.2. Changes in Material Properties

Rapeseed oil was used for oil heat treatment (OHT) of Scots pine, Norway spruce, and aspen at 180 °C, 210 °C, and 240 °C. Aspen and pine sapwood showed higher oil uptake than spruce, especially when cooled in oil after heating. At 180 °C, wood mass increased due to oil absorption, while at 240 °C, thermal degradation caused mass loss. Though OHT improved fungal resistance, rapeseed oil showed poor long-term stability, as it leached out under heat and moisture, reducing its effectiveness over time [200]. Poplar wood heat-treated in rapeseed oil showed improved water resistance compared to untreated wood. After 24 h, water absorption dropped to 33.07% and volumetric swelling to 7.31% (vs. 110.14% and 17.13% in control). After 96 h, values increased to 69.04% and 13.85%, respectively. The water contact angle after 300 s was 44.5° (unleached) and 30° (leached), indicating moderate hydrophobicity [193]. In the study of [201], Pannonia poplar wood treated with rapeseed oil at 200 °C showed the best color stability during 12 months of outdoor weathering. Total color change (ΔE*) remained below 12 for up to 10 months, outperforming treatments at 160 °C with rapeseed oil and treatments with other oils. Combining rapeseed oil impregnation and densification at 200 °C increased the density (+13%–17%) and hardness on the transversal section (+25%) and tangential section (+200%–350%) of Scots pine and silver birch in [202].

Soybean oil was used to improve fast-growing poplar wood for construction. Here, samples were impregnated with raw and epoxidized soybean oil and then densified and heat-treated. Epoxidized oil reduced set recovery more effectively than raw oil, and heat treatment further improved dimensional stability (set recovery down to 46% and 63%). Oil impregnation lowered water absorption in non-densified samples, especially with epoxidized oil. Although densified wood absorbed more water, heat treatment reduced this effect, and SEM/FTIR analyses supported the observed improvements in stability [203]. In the study of [204], soybean oil was in situ-epoxidized using peracetic acid to obtain various epoxidation levels. However, none of them showed any reactivity with vinyl acetate (VAc) under the tested conditions, and so no wood treatment was carried out. Treatment of fir wood with a mixture of soybean oil and maleic anhydride (OHT-MA) [205] significantly improved physical properties (increased density and reduced water absorption and swelling) and enhanced mechanical properties (bending strength, modulus of elasticity, and compression strength), with the best results observed with OHT at 160 °C for 60 min. Compared to soybean oil alone, OHT-MA was more effective and allowed for lower treatment temperatures and shorter times, though impact load resistance slightly decreased.

Linseed oil impregnation enhances the wood material’s durability and hydrophobicity. It is a sustainable alternative to synthetic agents. Uptake and distribution are critical to treatment success and have been studied macroscopically, microscopically, and via near-infrared spectroscopy [206,207]. Sapwood retains more linseed oil than heartwood [206,208,209] and earlywood retains more than latewood [208,209]. Higher moisture content (above 30%) promotes linseed oil uptake, likely due to oil-in-water emulsion formation at 60–140 °C [208]. SEM studies showed lumen filling, pit occlusion, and microcracks at high retention, impairing mechanical properties [210,211]. Linseed oil reduces water uptake, but only high retention and WPG (68%–88%) yield lasting hydrophobicity [212,213,214,215,216,217]. Some tests show no dimensional stability improvement or even increased swelling [218,219]. Efficacy against decay fungi is mixed, but linseed oil with nano-CuO, ZnO, or boron improves performance [195,220,221,222]. Linseed oil also decreases discoloration from UV exposure [223], improves corrosion resistance [224], and enhances performance when combined with thermal modification [225] or densification [226,227,228]. Linseed oil shows moderate biodegradability with minimal environmental risk [229]. In the study of [201], heat treatment in linseed oil produced the darkest initial wood color compared to rapeseed and sunflower oil, especially at 200 °C. Although the total color change appeared lower during weathering, this was mainly due to the already darkened starting color. Japanese cedar and beech were treated with a boric acid aqueous solution (1.0% w/w) and then heat-treated in two steps with linseed oil, which showed full protection against Coptotermes formasanus termites and brown rot fungus Pilatoporus palustris and white rot fungus Trametes versicolor [195]. An extensive review was conducted on linseed oil research of the 21st century by [230].

Fatty acid impregnation was carried out in the work of [231] using Japanese cedar, Japanese larch, soft maple, and Mongolian oak with the following fatty acids: castor oil (CAO), hydrogenated castor oil (HCO), soybean oil (SBO), and waste cooking oil (WCO). Methods included spreading, immersion, vacuum, and vacuum–pressure treatments. Japanese cedar, having the lowest density (430 kg/m³), showed the most optimal fatty acid uptake (97.74%), leachability (0.12%), and total volumetric swelling (−0.07%) and the lowest moisture uptake after 2 weeks of weather exposure (1%). This was achieved by using hydrogenated castor oil with vacuum–pressure treatment. This type of oil had the highest thermal stability and lowest moisture absorption among the tested fatty acids. Similar results were found in [232], where Western hemlock showed the highest castor oil uptake, and vacuum–pressure treatment yielded the best dimensional stability and durability. In [233], yellow-poplar, Japanese cedar, and Douglas fir were pressure-treated with castor oil. Yellow-poplar had the lowest leaching ability, and the impregnation with castor oil significantly increased their dimensional stability without a reduction in mechanical properties. In termite resistance tests, Acacia nilotica sapwood and heartwood treated with castor oil under vacuum–pressure showed enhanced durability, especially when pre-seasoned (sun drying for 1 to 60 days, or oven drying for 60 to 100 °C for 15 days, or submerging in water for 20 to 60 days), which improved oil penetration [234].

Sunflower oil is available commercially. It is used in oil heat treatment to enhance wood durability and stability by heating it at 180–260 °C in an oxygen-free closed vessel for 2–4 h [235]. In the study of [201], heat treatment in sunflower oil helped reduce the color change of untreated wood, but not as much as linseed oil or rapeseed oil. In the study of [236], it was reported that hot sunflower oil treatment of beech wood at 190–220 °C significantly improved dimensional stability and reduced moisture-related properties more than hot air treatment due to better heat transfer and oxygen exclusion. Additionally, even room temperature oil treatment lowered equilibrium and overall moisture content.

Tung oil treatment was reported to improve the energy absorption, elastic modulus, and frictional characteristics of lignum vitae (Mesua ferrea L.), while reducing both the friction coefficient and impact damage, thereby boosting its resistance to wear and impact [237]. More recently, [238] reported that the introduction of flexible tung oil-based polymers into the wood matrix led to a notable increase in impact bending strength—by up to 23% in poplar (Populus tomentosa Carr.) and 21% in spruce (Picea asperata Mast.). Promising results were acquired by oil heat treatment in tung oil: elmwood exhibited a great reduction in hygroscopicity and Young’s modulus after treatment at 180 °C [239,240]. Impregnation and heat treatment with tung oil increases the thermal degradation temperature and expands the wood cell walls [240,241]. Heat treatment in tung oil further increased wood properties compared to traditional heat treatment due to the formation of a polymer network between the wood components and tung oil [239].

2.13. Rosin

2.13.1. Process Description

Rosin is a resin used as a natural adhesive and preservative. It is extracted from pine resin or tall oil and contains sesquiterpenoids and oligomer esters of abietic, primaric, levopimaric-derived acids, among others [242,243]. It is renewable, abundant, low-cost and highly hydrophobic. It is yellow and becomes translucent when molten at 60–120 °C [243]. Traditionally used in the paper industry as a sizing agent [244], it has also been studied for its interactions with copper and wood components [245,246]. The chemical structure of rosin is presented in Figure 11.

Rosin bonds well with wood. Studies show that copper–rosin soaps dissolved in ethanol and impregnated into wood enhance resistance to fungi and termites. Additionally, non-solvent rosin–copper treatments have demonstrated good leaching resistance, though they required double impregnation. Rosin-based sizing agents have been shown to reduce the moisture absorption of wood and improve its decay resistance [248]. Wood can be impregnated with rosin by dissolving it in ethanol [249].

Currently, rosin-treated wood is commercially not available. It was first reported for wood impregnation in 1972 [250], used as an alternative to polyethylene glycol (PEG) to preserve waterlogged oak, especially when the wood is very soft or heavily saturated. The process involved dehydration with acetone followed by rosin impregnation, which helped stabilize and preserve the degraded wood structure. Similar success was reported with waterlogged archeological pine, beech, and elm wood materials [251].

Rosin treatments have been conducted on poplar species [6,247,249,252,253], pine species [243,245,246,254], and beech [255,256].

2.13.2. Changes in Material Properties

According to [2], rosin improves moisture resistance and the mechanical strength of the dry-welded joints. This applies to both linearly welded and dowel-welded joints. As a result, these joints become particularly strong and competitive in terms of performance. Ref. [242] reported that linear vibration welding of Scots pine (which naturally contains rosin) resulted in joints with improved water resistance due to molten rosin in the wood, protecting the weld area by repelling water. These joints, although of only moderate strength, had very high wood failure rates, suggesting that the rosin helped strengthen the weld line more than the surrounding wood itself. In a study [255], where diluted rosin solution was applied to beech surfaces before welding, strong outdoor durability and strength was found.

However, rosin’s hydrophobic film can degrade above its melting point. Also, rosin does not react chemically with the wood constituents; it only fills the lumens, cell corners, and middle lamella, while penetrating only partially the wood cell walls. As a result, its long-term effects on hydrophobicity and mechanical performance are limited [249].

By impregnating wood with a rosin–ethanol solution, significant improvements can be achieved. A 3% rosin solution (tall oil rosin) enhanced the water repellency but only slightly improved dimensional stability in pine decking [254]. A 20% rosin solution increased the density from 340 to 440 kg/m³, increased the ASE to 36%, decreased the EMC by 42.7%, and increased the MOR, MOE, and compression strength by 12.8%, 18.9%, and 31.6%, respectively [249].

To reduce the leaching of rosin from wood and improve permanence, chemical bonding is needed between the rosin and the wood components, or in situ polymerization. Impregnating poplar sapwood with rosin-based derivatives improved dimensional stability, hydrophobicity, surface hardness, and leaching resistance [247].

Rosin acids were found to be effective and durable wood preservatives for ground contact applications [245,246]. Long-term field tests (25 years) confirmed resistance to termites and fungi via self-polymerization, co-reaction with lignin’s C=C double bonds, and co-reaction with lignin’s aromatic rings, further enhancing fixation and durability.

Rosin sizing agents have been studied for wood impregnation and to stabilize copper to protect against decay. Poplar wood treated with a 3% CuSO₄ solution and 1%–4% rosin sizing agent decreased the weight loss from fungal degradation from 60% (untreated) to <4%. The treatment halved copper ion leaching, likely due to complex formation with abietic acid [252]. The water contact angle increased significantly [257]. The treatment reduced moisture absorption, water uptake, and swelling while improving water repellency and anti-swelling efficiency by ~40% after 30 days of water immersion. Although compression strength parallel to the grain and Brinell hardness increased, the MOR and MOE values were lower than those of the untreated samples [6].

Incorporating a rosin sizing agent and copper sulfate in micronized copper preservatives also reduced fungal decay and copper loss significantly in [258].

A combination of rosin, aluminum sulfate, and boron compounds improved the leachability, color stability, and decay resistance of poplar wood. It reduced boron leaching by ~30%, with boron retained in cell lumens even after leaching and decay, indicating effective fixation [259]. Similar success was observed in Styrax wood [260].

Rosin–copper treatment of Styrax wood showed enhanced weather resistance, photostability, and compression strength, although MOR and MOE were lower [261].

A superhydrophobic surface (157°) was achieved by impregnating poplar wood with maleic rosin and then coating with TiO₂ particles in two steps. After water immersion, irradiation, and boiling, the superhydrophobicity was preserved (>150°) [253].

Rosin can be a promising additive for enhancing the fire resistance of wood. In a study [256], beechwood was treated with rosin combined with boron compounds such as borax and boric acid. Rosin improved leaching resistance and extended the combustion period while lowering flame ignition temperatures and weight loss in most cases. However, in some cases, rosin slightly reduced boron retention and increased weight loss.

2.14. Sorbitol and Citric Acid

2.14.1. Process Description

Sorbitol is described as a sugar alcohol [262]. It is also identified as D-Sorbitol [263] and is available as a laboratory-grade crystal with approximately 99% purity [264]. Sorbitol has six alcoholic hydroxyl groups, meaning it has six potential reaction sites on one molecule [263]. Sorbitol is industrially manufactured from starch through the enzymatic hydrolysis of starch to dextrose, followed by the catalytic hydrogenation of dextrose [263]. It can also be produced from cellulose found in biomass, and its commercial production often uses biotechnological approaches based on feedstocks like corn starch [263] or beet and sugarcane molasses [262,265]. It is widely used in the food and beverage industry [262].

Citric acid (CA) is primarily described as a bio-based chemical, a natural preservative and flavor enhancer in food and beverages, and a common chemical in the food and beverage industries [262,265]. It is available as powdered citric acid or technical-grade citric acid crystals with approximately 97% purity [262,263,264,265]. Citric acid has three acidic groups and one alcoholic hydroxyl group. It is largely produced through microbial fermentation using Aspergillus niger [263]. It is used as an adhesive for the production of particleboards and WF-based moldings [263].

Polyesters created from sorbitol and citric acid (SCA polyesters) have been investigated for a variety of applications, including controlled-release hydrogels, biomedical tissue scaffolds, antiscalants in detergents, and surface coatings [265].

The use of sorbitol and citric acid (SCA) together for solid wood modification has been gaining research attention over the past decade [263]. Their advantages include their low cost, bio-based nature, non-toxic material and process, ready availability, and easy solubility [262,263,265]. The chemicals are easily soluble in water, allowing for aqueous-based modification processes. No catalyst is required for the reaction, and the only by-product is water. The impregnating solution is also stable and can be reused [262,263,264,265]. The reaction scheme is presented in Figure 12.

In the modification process, citric acid and sorbitol are usually combined in a 3:1 molar ratio. These are often powdered, food-grade or technical-grade chemicals dissolved in deionized water to create an aqueous solution. The stock solution can have a concentration as high as 56% w/w and typically exhibits a low pH (e.g., pH 2 for a 56% w/w solution). The wood samples are oven-dried and then impregnated with the SCA solution using a vacuum–pressure process. A typical sequence includes a pre-vacuum phase followed by a pressure phase. After impregnation, excess solution is removed, and samples are typically pre-dried at room temperature for a period to minimize drying defects. Then, during heat curing, the samples are subjected to elevated temperatures (140–160°) for a specific duration. During this polyesterification process, water is released as a by-product and evaporates [262,263,264,265].

The modification involves the polyesterification of citric acid and sorbitol, forming a crosslinked network of SCA polyesters within the wood cell wall matrix. The presence of these polyesters bulks the cell wall, restricting its ability to shrink and swell [262,263,264,265].

2.14.2. Changes in Material Properties

Scots pine sapwood was modified with powdered citric acid and D-sorbitol in a 3:1 molar ratio, dissolved in deionized water (8 g water to 10 g solids), in the study of [263]. Their process was pre-vacuum (30 min, 40 mbar), pressure phase (1 h, 8 bars), and heat curing (18 h, 130–140 °C). Samples cured at 140 °C showed very low leaching of citric acid and sorbitol; they also showed a permanent increase in dimension and no significant change in weight or dimensions after soaking. Samples cured at 103 °C showed large changes and significant mass loss after leaching, indicating poor chemical fixation. Both 103 °C- and 140 °C-cured samples showed significant resistance to white rot (Trametes versicolor) and brown rot (Postia placenta) fungi after leaching, with almost no decay evident in treated samples. Both unleached and leached samples cured at 103 °C and 140 °C had significantly less surface growth of Aureobasidium pullulans, Sydowia polyspora, and a fungal mix compared to untreated samples. The low pH (pH 2) of the impregnation solution was suggested as a contributing factor to limited fungal growth.

Similarly, Scots pine sapwood was modified with powdered citric acid and D-sorbitol in a 3:1 molar ratio, stock solution of 56% w/w concentration, with various concentrations (14%–56%) in the study of [265]. Their process was pre-vacuum (30 min, 40 mbar), pressure phase (2 h, 8 bars), and heat curing (18 h, 140 °C). ASE ranged from 23% to 43% and decreased at higher weight percentage gains (WPGs) exceeding 50%. SCA-modified samples resisted brown rot (Rhodonia placenta) and white rot (Trametes versicolor) decay. A potential decay protection threshold of 50% WPG was identified for brown rot. White rot fungi appeared more aggressive, causing some mass loss even at the highest WPGs.

Beech sapwood was modified with technical-grade citric acid crystals (approx. 97% purity) and laboratory-grade sorbitol crystals (approx. 99% purity) in a 3:1 molar ratio, with various concentrations (10%–20%–30%–55%), in the study of [264]. Their process was drying (24 h, 40–60–80–103 °C), pre-vacuum (1 h, 5–10 kPa), pressure phase (1 h, 1200 kPa), and heat curing (24 h, 140–160 °C). Swelling at saturation state was the lowest (1%–2%) for 30% and 55% SorCA solutions. WPG increased linearly until 30% SorCA (e.g., 22% for 30% solution) and then drastically increased to 73% at 55% SorCA, potentially due to chemical deposition in the cell lumina. ASE values increased with SorCA concentration and curing temperature. They increased linearly up to 30% SorCA (e.g., 53% for 30% at 140 °C and 58% for 30% at 160 °C) and then increased slightly or not at all at 55% concentration (e.g., 58% for 55% at 160 °C). SorCA 30% was identified as the optimum concentration due to saturation of the wood cell wall. MOE values increased slightly (approx. 9%) at optimum conditions (SorCA 30%) compared to untreated wood. For 30% concentration, MOE was 15,200 N/mm² (140 °C) and 15,206 N/mm² (160 °C), compared to 13,988 N/mm² for untreated wood. MOR values decreased considerably. For 30% concentration, MOR was 106 N/mm² (140 °C) and 102 N/mm² (160 °C), compared to 124 N/mm² for untreated wood. Work to maximum load in bending (WMLB) decreased considerably (up to 80% at the highest SorCA concentration), indicating increased brittleness. For 30% concentration, WMLB was 0.73 KN mm² (140 °C) and 0.63 KN mm² (160 °C), compared to 3.15 KN mm² for untreated wood. During thermogravimetric analysis, SorCA 30%-modified wood exhibited lower mass losses (53%–55%) compared to the wood control (72%) or untreated beech (68%), indicating increased char formation. SorCA 30% w/w solution showed good stability for 4 months (no precipitation or fungal growth) and was classified as “very durable” against white rot (Trametes versicolor, 4.40% weight loss), brown rot (Coniophora puteana, 1.38%), and soft-rotting microfungi (0.46% weight loss) at 160 °C.

In another similar study [266], Scots pine sapwood and Norway spruce were modified with powdered citric acid and D-sorbitol in a 3:1 molar ratio, stock solution of 56% w/w concentration, with various WPGs (25%–100%). In a resistance test against subterranean termites (Reticulitermes grassei), SCA WPG 100% showed nearly no mass loss and no attack. No termite survived the test within the first week at every WPG level. A concentration threshold for effective termite treatment was assumed to be between 25% and 70% WPG. In the resistance test against marine wood borers, all WPG levels showed no marine borer attack after four months of exposure in the marine environment.

Kurkowiak and colleagues carried out several projects in this field. In one of their work [267], Scots pine sapwood was modified with an aqueous SorCA solution at different treatment levels. A permanent increase in oven-dry dimensions was observed compared to untreated specimens. The modified wood showed a reduced moisture content in comparison to untreated reference samples. In a different study [268], SorCA was combined with a commercial phosphorous-based fire retardant (FR). This resulted in significantly improved fire-retardancy, a lower thermal decomposition temperature, a reduced heat release rate, and delayed ignition. In another study [269], Scots pine sapwood was modified with citric acid and sorbitol in a 3:1 molar ratio as an aqueous solution with a solid content of 30%. After the vacuum–pressure process, the samples were dried (20 °C, 168 h) and then cured (24 h, 120–140 °C). A higher curing temperature (140 °C) resulted in higher fixation of impregnation chemicals compared to 120 °C. The peak at 89 ppm (C-4 in crystalline cellulose) decreased, and a higher amount of levoglucosan (a pyrolysis product of glucose) was observed in modified wood, suggesting that cellulose becomes more amorphous and thus more prone to degradation due to the treatment. A different study [262] aimed to develop a quality control (QC) method using electromagnetic radiation-based techniques (FTIR, NIR, and X-ray densitometry) to determine the degree and homogeneous distribution of fixated chemicals in SorCA-treated wood. Scots pine sapwood was modified with food-grade CA monohydrate (approx. 97% purity) and technical-grade sorbitol (approx. 98% purity) in a 3:1 molar ratio, with various concentrations (2.5%–50%). Their process was pre-vacuum (1 h, 0.98 bar), pressure phase (3 h, 1200 kPa), drying (168 h, stepwise from 20 to 103 °C), and heat curing (24 h, 140 °C). WPG varied from 1.65 to 66.67%. WPG and cell wall bulking showed a positive correlation with the solid content of the impregnation solution. X-ray density profiling revealed a density gradient within the samples, with a higher concentration of impregnation chemicals closer to the surfaces than in the middle. Solution uptake appeared higher for earlywood than for latewood. A virtual WPG (vWPG) method was proposed to project the density profile of untreated samples onto treated ones, showing potential for QC. A recent study [270] aimed to determine the chemical loading required for effective protection against wood-destroying basidiomycetes and to enhance understanding of the wood’s moisture behavior. Scots pine sapwood was modified with technical-grade CA monohydrate (approx. 97% purity) and technical-grade sorbitol (approx. 98% purity) in a 3:1 molar ratio, with various concentrations (10%–50%). Their process was pre-vacuum (1 h, 5 kPa), pressure phase (2 h, 1200 kPa), drying (168 h, stepwise from 20 to 103 °C), and heat curing (24 h, 140 °C). WPG ranged from 9.9 to 57.0%. The highest WPG of 57% was required to achieve the highest durability class (DC 1) against all three tested fungi: Rhodonia placenta, Coniophora puteana, and Trametes versicolor. Untreated Scots pine sapwood showed 27% ML for R. placenta, 43% ML for C. puteana, and 17% ML for T. versicolor. For samples with 57% WPG, ML was reduced to merely 5% for R. placenta, 2% for C. puteana, and 3% for T. versicolor. Lower treatment levels (<57% WPG) were classified as DC 4 due to high ML. Liquid water uptake and water vapor uptake decreased for all treated specimens, which increased after leaching but remained lower than untreated controls. The MC of treated wood was lower compared to untreated wood: it was 10.63 (<59.29%) after 144 h of floating tests, it was 13.74% (<69.75%) after 144 h of the submersion test, and it was 20% (<27%) after three weeks above water. A WPG of 57% corresponded to a CWB of approximately 11%.

3. Comparative Analysis

In the table below, the most important properties of the discussed biopolymers are compared to natural wood (

Table 1. Comparison of different biopolymer-impregnated or modified wood properties. Outstanding advantages are marked with bold.

Name of Biopolymer	Physical and Mechanical Properties	Wood-Water Relations and Hydrophobic Properties	Durability and Color Properties	Other Remarks
FA	Density increases by 16%–40%, improving strength, hardness. Does not consistently increase MOR, MOE, impact resistance.	ASE up to 60%.	DC1 against fungi and insects. Resistance to marine borers above 50% WPG. Darkens wood to brownish.	Non-toxic, non-flammable. No ecotoxicity reported for leachates. Higher thermal stability. Commercial (Kebony™, Nobelwood™).
PLA	Brittle, low toughness. Improved bending, compression strength, and hardness without catalyst. With densification, more improved MOR, hardness.	Improved ASE. Lower water uptake.	No visible fungal decay due to acidity and low moisture content. Not inherently biocidal; agents can be added. Darker, more saturated brown color.	Biodegradable, renewable (corn starch). Requires 25%–55% less energy to produce. No commercial solid wood products.
PCL	Compression strength parallel to grain did not significantly decrease.	Water absorption decreased by 70%, ASE of 40%. Water contact angle decreased from 105° to 65° after weathering.	Resisted fungal attacks at 15.5% WPG. Color changes less (ΔE* decreased from 8 to 4).	Low-cost base material, waste-free process. No commercial products. No noticeable cell wall deformations.
PBAT	Low strength, low heat resistance. High flexibility, processability, tensile strength, elongation at break, ductility. Strengthens PLA–wood foam by 797%.	Enhances dimensional stability in PBAT–PLA–wood fiber blends.	Does not degrade in marine or freshwater.	Environmentally safe and non-toxic. Synthetic, but biodegradable. Increasing market.
PBS	Tensile strength increased to 28 MPa, Young’s modulus to 1007 MPa in composites.	EMC reduced to 3.57% with compatibilizer. ASE 60%–70%	No cracks, no mold during one-year weathering. Reduced weight loss against Coriolus versicolor (from 17% to 3.7%).	Biodegradable and compostable. Good processability, thermal stability. Not commercially available for solid wood.
Zein	Brinell hardness increased by 10% at 16.22% WPG.	Absorbed ~80% less water than untreated wood. Showed ~80% better dimensional stability. Water contact angle increased from 36.85° to 50.72°.	Exhibited antifungal properties by forming a barrier against moisture and oxygen. Increased wear resistance.	Hydrophobic protein from corn. Non-toxic, combustible. Not commercially available, rarely researched.
Lignin	Cell wall elasticity increased by 8.7%, hardness by 10.3%. MOR improved by nearly 54%, MOE by nearly 200%.	Contact angle up to 111.1°. ASE 99.4%. Moisture absorption decreased to 0.55%.	Highest efficacy against brown rot fungi, negligible mass losses. Better UV stability and lower color change.	Not commercially available for modification. Challenges with molecule size, reactivity, and variability. Reduced flammability.
Tannin	Compression strength increased by 35% (10% tannin). MOE increased from 99.2 to 120.9 MPa (20% tannin). Brinell hardness increased.	Reduced roughness. Decreased water uptake, increased ASE. High water solubility, historically limited efficacy.	Reduced weight loss against brown rot (0%–29.26%) and white rot (0%–7.94%).	Plant-derived polyphenolic compounds. Natural defense against pathogens. Not widely on market. Leaching loss of 17%–35%. Increased ignition time from 12 s to >75 s, flame time from 140 s to 15–20 s.
Chitosan	Average veneer stiffness enhancement of 20%. MOE of heat-modified wood increased by 27%. Mass gain up to 30.54%, volume gain up to 19.55%.	Total sorption reduced by 23.8%, monomolecular sorption by 35.3%. Reduced hygroscopicity, lower EMC. Enhanced hydrophobation.	Total inhibition of white rot, brown rot fungi, and mold (1% conc.). Resistance to termites (0.48% vs. 3.96% WL). 95.8%	Natural, renewable, biodegradable, non-toxic. Structurally comparable to cellulose. Leaching decreased efficacy. Improved fire-retardant activity.
Alginate	Tensile strength 1.1 GPa, flexural strength 679 MPa. Tensile, elongation, bending, compressive strength (aerogel) enhanced by 128.4%, 109.1%, 31.8%, 241.7%, respectively.	Diffusivity decreased from 310 to 174 × 10−7 m2/s (100% wood fiber).	Biodegradable.	Natural anionic polysaccharide from brown seaweeds. Low toxicity, biocompatibility. Low thermal conductivity (0.078–0.089 W/mK). Limiting Oxygen Index reached 59.2% (self-extinguishing). Improves thermal stability of lignin.
Natural Gums	Gum rosin with n-hexane exhibited high MOR of 9.7 GPa. Bending MOE 10 GPa, MOR 78 MPa with 7.5% gum rosin in petroleum oil. Dry bonding strength 6 MPa.	P. atlantica gum improved moisture exclusion. Gum rosin largely ineffective in reducing water uptake/shrinkage.	P. atlantica gum slightly improved decay resistance. Significantly reduced mold coverage (from 76%–100% to 26%–50%).	From tree trunks or pine resins.
Fatty Acids	Hardness on tangential section increased by 200%–350% (rapeseed oil + densification). Tung oil increased impact bending strength by up to 21%–23%.	Rapeseed oil reduced water absorption to 33.07%, volumetric swelling to 7.31%. H-castor oil had lowest moisture uptake (1%) after 2 weeks. Increased dimensional stability.	Treatment with rapeseed oil at 200°C showed best color stability (ΔE* < 12 for 10 months). Full protection against termites/fungi (boric acid + linseed oil). Tung oil boosts wear/impact resistance.	Derived from natural oils, renewable lipids. Chemically bond with cellulose. Oils can polymerize and modify wood.
Rosin	Improves mechanical strength of dry-welded joints. 20% rosin increased density to 440 kg/m3, MOR by 12.8%, MOE by 18.9%, compression strength by 31.6%.	Increased ASE to 36%, decreased EMC by 42.7% (20% rosin). Enhanced water repellency. Superhydrophobic surface (157°).	Long-term (25 years) resistance to termites and fungi. Improved leaching resistance and extended combustion period.	Natural adhesive and preservative. Renewable, abundant, low-cost, highly hydrophobic. Not commercially available.
SCA	MOE increased slightly (approx. 9%) at optimum conditions. MOR decreased considerably. Increased brittleness (work to max load decreased up to 80%).	Swelling at saturation state lowest (1%–2%). ASE ranged from 23% to 58%. Liquid/vapor water uptake decreased.	Very low leaching at 140 °C curing. Low weight loss when tested against white rot (4.40%), brown rot (1.38%), soft rot (0.46%). No mass loss/attack by subterranean termites (WPG100%). No marine borer attack.	Low-cost, bio-based, non-toxic. No catalyst required, water as by-product. Solution stable, reusable. Forms crosslinked network. Improved fire-retardancy.

).

While biopolymers offer a sustainable alternative for wood modification, they have some limitations, which hinder their widespread utilization. PLA suffers from brittleness, slow degradation, and no biocidal effect, while PBAT shows low strength, poor heat resistance, high cost, and no aquatic degradability. PBS is prone to thermal and has low stiffness and limited composite compatibility. Furfurylation gives inconsistent strength gains and requires very high WPG for marine resistance. Lignin is too large to penetrate cell walls without depolymerization, showing low reactivity and variable results. Tannins leach easily, reducing stability, while chitosan and natural gums show poor fixation and little effect after leaching. Zein has limited application in adhesives due to low coating thickness. Fatty acids, oils, and rosin mostly act as moisture barriers without strong chemical bonding, leading to poor long-term performance, while sorbitol–citric acid systems cure poorly at low temperatures, causing mass loss and brittleness. Future research should address these issues, and a primary focus should be placed on bridging the gap between laboratory research and commercial application, as many promising biopolymer treatments (e.g., PLA, PCL, PBAT, PBS, zein, and rosin) are currently not commercially available for solid wood impregnation. Furthermore, comprehensive life cycle assessments would check the validity of the fact that biopolymer-modified wood truly offers a reduced environmental footprint across its entire life cycle.

4. Conclusions

Wood modification using biopolymers offers a sustainable and eco-friendly alternative to conventional chemical treatments, aiming to enhance wood’s durability, moisture resistance, and mechanical properties while reducing environmental impact. Various biopolymers, such as furfuryl alcohol, polylactic acid (PLA), polycaprolactone (PCL), zein, rosin, lignin, tannin, chitosan, alginate, natural gums, fatty acids, and sorbitol with citric acid, are being explored.

Research demonstrates promising enhancements across these methods, including improved resistance to fungal decay and insect attack, better dimensional stability, increased density and hardness, and, in some cases, enhanced mechanical strength.

However, challenges persist, such as biopolymers sometimes exhibiting mechanical limitations compared to conventional plastics, variable efficacy or leachability after treatment, and limited commercial availability for many biopolymer-modified solid wood products. Compatibility issues arise from specific processing needs, like high-temperature curing or densification.