Polyurethane adhesives have gained significant attention in various industries due to their excellent mechanical properties, versatility, and potential for sustainability. These adhesives are synthesized through the reaction of polyols with isocyanates, resulting in a polymer exhibiting strong adhesion, flexibility, and durability across various applications, including wood bonding, textiles, and automotive components [1–3]. The unique properties of polyurethane adhesives can be tailored by modifying their chemical composition, such as the type of polyol used, the ratio of isocyanate to hydroxyl groups, and the incorporation of additives like fillers or chain extenders [4,5].

Recent advancements in the formulation of polyurethane adhesives have focused on enhancing their environmental sustainability. For instance, bio-based polyols derived from renewable resources, such as vegetable oils and lignin, have been explored to reduce reliance on petroleum-based materials [6,7]. These bio-based formulations contribute to lower environmental impact and maintain or improve adhesive performance. Studies have shown that polyurethane adhesives made from modified polysaccharides or lignin exhibit superior bonding strength and flexibility compared to traditional formulations [8–10]. Furthermore, developing waterborne polyurethane adhesives has emerged as a promising alternative to solvent-based systems, offering lower volatile organic compound emissions and reduced toxicity [11].

The transition from isocyanate-based polyurethanes (PUs) to non-isocyanate polyurethanes (NIPUs) is driven by the need for safer, more environmentally friendly alternatives. Isocyanates, while effective in PU synthesis, pose significant health risks due to their high reactivity and toxicity, leading to severe respiratory and ocular damage upon exposure [12,13]. Furthermore, the production of isocyanates often involves the use of phosgene, a highly toxic compound, which raises additional safety concerns [14,15]. In contrast, NIPUs utilize alternative raw materials and synthesis methods that mitigate these hazards, making them a compelling option for industrial and academic research [16,17].

Beyond safety and environmental benefits, NIPUs—particularly those formulated with bio-based components such as tannins also offer enhanced functional properties. Tannin-based polyurethane adhesives exhibit remarkable fire-resistance. Research has shown that tannin foams have higher thermal and fire resistance and can self-extinguish the flame, making them a potential alternative to polyurethane foams for fire protection in building engineering and automotive industries [18]. Incorporating tannins into polyurethane formulations enhances their thermal stability and reduces flammability, making them suitable for applications where fire resistance is critical [19]. For instance, a study highlighted the dual functionality of tannin-based coatings that exhibit intumescent and fire-retardant properties, depending on tannin concentration [19]. This versatility allows for tailored applications in various industries, including construction and automotive sectors, where fire safety is paramount.

In addition to their fire-resistant properties, tannin-based polyurethanes can be engineered to achieve super-hydrophobic characteristics. The super-hydrophobicity arises from the unique micro/nano-structural features of the coatings, which can be enhanced by the incorporation of nanoparticles or by manipulating the surface roughness [20,21]. NIPU has also gained much attention because of its improved porosity, water absorption, and thermal and chemical resistance compared to conventional polyurethanes [22]. For example, the surface coating of a medium-density fiberboard (MDF) panel was implemented at 170°C under mild pressure to maintain the minimum filming temp so it could produce good hydrophobicity on the surface [23]. The super-hydrophobic nature of these materials is attributed to the hydrophobic groups in tannins and the structural modifications that create a rough surface texture, which collectively reduce the adhesion of water droplets [24–26].

To better understand the direction and scope of research in this area, a bibliometric analysis was conducted based on 20 publications indexed by Scopus. Fig. 1 presents the visualization of the relationship network between keywords, revealing 32 interconnected terms grouped into several colored clusters. Research on tannin-based materials in the red, blue, and green clusters illustrates an integrated approach to developing environmentally friendly NIPUs with specific applications. The red cluster focuses on applying tannin NIPUs as wood adhesives, utilizing the natural adhesive properties of tannins that can reduce the emission of volatile organic compounds (VOCs). In addition, this cluster highlights the use of analytical techniques, such as Fourier Transform Infrared Spectroscopy (FTIR) and MALDI-TOF, to study the chemical structure and curing mechanism of the adhesive to improve performance.

On the other hand, the blue cluster emphasizes the development of tannin-based polyurethane materials that have superior thermal, mechanical, and fire-resistance properties. The natural content of polyphenols in tannins provides fire-resistance properties, making them attractive for construction and thermal insulation applications. The green cluster complements this with a focus on the use of environmentally friendly raw materials such as dimethyl carbonate (DMC) and hexamethylenediamine (HMDA) for NIPU synthesis, replacing hazardous isocyanates. This combined focus creates a versatile, sustainable, and high-performance tannin-based material solution for modern industrial applications.

Tannins are plant-derived polyphenols known for their amphiphilic nature, enabling interactions with hydrophilic and hydrophobic molecules. In tannin-based NIPU resins, hydrophobicity is a key functional attribute that contributes to water resistance and durability. This property emerges from several structural and chemical factors inherent to the polymer network. Hydrophobicity in tannin-based NIPU resins arises from multiple synergistic mechanisms. First, the cross-linked polymer network provides mechanical strength and stability, ensuring long-term water resistance [55,56]. Second, hydrophobic functional groups such as long alkyl chains or fluoroalkyl segments lower surface energy and minimize moisture absorption [57,58]. DMC and Hexamethylene Tetraamine (HMTA) serve as non-isocyanate substitutes, while tannin-glyoxal replaces polyols, maintaining performance with reduced environmental impact.

The use of DMC and HMTA as isocyanate-free curing agents, along with tannin-glyoxal as a polyol substitute, not only enhances sustainability but also introduces aliphatic chains and stable methoxy (-OCH₂) linkages that improve polymer cohesion and hydrophobic performance [59,60]. Urethane groups (-NHCOO-) further reduce water interactions, reinforcing the hydrophobic effect [61]. Additionally, hierarchical micro- or nanoscale surface roughness increases the contact angle, mimicking natural super-hydrophobic surfaces like lotus leaves, enabling self-cleaning properties [62].

In theory, introducing fluoroalkyl and long alkyl chains into NIPU polymers could significantly enhance hydrophobicity by lowering surface energy and creating a physical barrier to moisture penetration. Fluoroalkyl groups, characterized by strong carbon–fluorine bonds, reduce the attraction of water and air molecules to the polymer surface, contributing to both chemical stability and water repellency [36,63]. Meanwhile, long alkyl chains provide a nonpolar layer that prevents interaction with polar substances such as water. However, such modifications have not yet been widely explored or reported in tannin-based NIPU systems. Most existing studies on fluorinated NIPU resins refer to petroleum-based or synthetic polyol systems, and their direct applicability to tannin-based formulations remains unverified. Therefore, while fluorinated NIPUs exist in synthetic systems, their compatibility and direct transfer to tannin-based formulations remain unverified and represent an open area for future research.

Fig. 5 shows the structure of a polymer modified with fluoroalkyl groups (marked with purple fluorine atoms) and long alkyl chains [36,64]. However, while these modifications are effective, they are not without drawbacks. Fluoroalkyl compounds are costly and environmentally persistent, raising concerns about sustainability and long-term ecological impact. Moreover, achieving uniform distribution of these groups within the polymer matrix can be technically challenging, potentially affecting surface consistency. Despite the enhancements these functional groups provide, it is important to recognize that tannin contributes to the overall hydrophobic behavior. Its aromatic and polyphenolic structure offers reactive sites for modification and can inherently reduce polarity through esterification or cross-linking. Therefore, the synergy between tannin’s native structure and the added hydrophobic components is crucial in achieving durable and water-resistant NIPU materials.

Lastly, the structural integrity of the polymer network sustains hydrophobic performance over time. Rough surface textures amplify water resistance by increasing water contact angles, preventing absorption, and enabling self-cleaning effects [65,66]. This interplay between chemical modification, surface engineering, and polymer cross-linking ensures durable hydrophobicity in tannin-based NIPU adhesives [67].

Tannin-based NIPU resins have emerged as a promising alternative to conventional polyurethanes, primarily due to their environmentally friendly nature and the ability to be derived from renewable resources like plant tannins. The fire-resistance properties of these resins are of significant interest, particularly in applications where flame retardancy is critical, such as in coatings, adhesives, and construction materials. Tannin-based NIPU foams exhibit a limiting oxygen index (LOI) value of 24.45%, which is significantly higher than the LOI values of traditional PU foams (17%–19%) [68]. This indicates better fire resistance. The cross-linked structure of NIPU resins can result in a more thermally stable material that is less prone to rapid degradation under high temperatures [69]. This makes it more resistant to ignition and heat propagation than traditional polyurethanes.

Tannin-based NIPU resins exhibit enhanced fire-resistance through multiple mechanisms. Tannins, as polyphenolic compounds, promote the formation of a protective char layer upon exposure to heat, acting as a thermal barrier that slows down heat transfer and material decomposition [70]. Additionally, the cross-linked polymer network formed between tannin and monomers such as cyclic carbonates and amines improves thermal stability by reducing polymer chain mobility and limiting flame propagation [28]. However, the effectiveness of char formation can vary depending on formulation parameters such as the tannin content, curing temperature, or the type of cross-linking agent used. Under suboptimal conditions, incomplete char formation may reduce fire performance. Compared to conventional flame retardants such as halogenated compounds or phosphorus-based additives, tannin-based systems offer a more sustainable and non-toxic alternative. Nevertheless, their performance may still require optimization to match the efficiency of industrial-grade fire retardants, particularly in high-risk applications. Incorporating a systematic formulation approach could help maximize their fire-retardant potential while maintaining their environmental advantages.

Tannin-based NIPU resins also degrade at higher temperatures than conventional polyurethanes, releasing non-flammable gases such as carbon dioxide during thermal decomposition, which dilutes flammable gases in the environment and reduces fire spread [71]. To further enhance fire resistance, flame retardants such as phosphorus-based or halogen-free compounds can be incorporated, promoting char formation and improving flame retardancy. Furthermore, adding nanomaterials or inorganic fillers can create synergistic effects, strengthening the char layer, improving heat barrier properties, and aiding in smoke suppression.

Figs. 6 and 7 collectively illustrate how the chemical structure of tannin-based NIPU resins translates into enhanced thermal and fire-resistant properties. Fig. 6 outlines the synthesis of NIPUs from tannin-based cyclic carbonates and diamines. When a polyphenolic compound combines tannin with cyclic carbonates and diamines, it forms a cross-linked polymer structure. The cyclic carbonate groups react with the amine functional groups of diamines, creating stable, cross-linked networks [72,73]. This cross-linking process enhances the material’s structural integrity, making it more resistant to thermal degradation and the spread of fire [28]. The resulting network acts as a barrier to heat and flame, improving the material’s fire-resistance. Furthermore, the presence of tannin, which can produce a protective char layer during combustion, contributes to the overall fire-retardant behavior of the resin [74]. The high degree of cross-linking, facilitated by these reactions, stabilizes the polymer, preventing it from breaking down easily under heat and thus offering superior fire-resistance compared to traditional polyurethanes. This structure is key in developing bio-based materials with enhanced thermal stability and fire-resistance, particularly in applications requiring sustainable and safe alternatives to conventional polymers.

Fig. 7 illustrates the Thermomechanical Analysis (TMA) traces for resin A (tannin–humin NIPU) and resin B (humin NIPU) [75]. Resin A (green line) shows a higher modulus of elasticity (MOE) at high temperatures than resin B (red line), especially at the second peak around 220°C. This indicates that resin A has a more stable structure against increasing temperatures. In contrast, resin B has a smaller increase in MOE and tends to decrease after 150°C, indicating that its polymer network is more susceptible to deformation and degradation. The higher thermal stability of resin A can be associated with better fire-resistance, since materials that can maintain their stiffness at high temperatures are generally less likely to ignite or experience rapid thermal degradation [76].

Resin A has more cross-linking than resin B due to its highly reactive tannin content. Tannin is essential in increasing thermal stability due to its structure rich in aromatic rings, which tend to form a carbon layer (char) when burning [77]. This layer is a barrier to heat and oxygen, slowing fire spread. In contrast, resin B, which is based solely on humin, has fewer reactive sites; therefore, its polymer network is weaker and more easily degraded at high temperatures. Therefore, resin A, with a tannin-humin composition, has the potential to have better fire resistance than resin B based on pure humin. The graph shows that chemical hardening (cross-linking) occurs at 150°C–160°C, indicating that the adhesive must be heated to a reasonably high temperature to achieve complete stability. After the temperature reaches 200°C, wood degradation occurs, which can be a significant factor in the decrease in MOE after the second peak. Adhesives with a more stable structure at high temperatures can help slow down the wood degradation process, thereby contributing to increased fire-resistance by reducing the burning rate and maintaining material integrity for longer.

TGA and DTG analyses provide insights into the thermal degradation behavior of the tannin-based NIPU resins. The higher thermal stability and greater char residue observed in the TGA curves indicate that the resin possesses inherent fire-retardant characteristics. The formation of a stable char layer during decomposition acts as a thermal barrier, reducing heat release and slowing down the combustion process. Although TGA does not directly assess flammability, the data strongly correlate with improved fire-resistance observed in many char-forming polymer systems. The relatively high char yield observed at 790°C, as reported in Fig. 8, suggests that the T/G-F foams form stable carbonaceous residues upon thermal decomposition. This char acts as a physical barrier that insulates the underlying material from heat and oxygen, thereby slowing down further degradation and combustion. Such behavior is characteristic of materials with intrinsic fire-retardant properties. Although TGA is not a direct flammability test, the multi-step degradation profile and significant char formation indicate that tannin-glucose-based NIPU foams may offer improved flame resistance through enhanced thermal stability and char formation mechanisms.

Castor oil-based polyurethane (COPU) has been investigated as an alternative adhesive for manufacturing oriented strand boards (Fig. 9). Data shows pure COPU has very low water absorption and minimal swelling, making it a moisture-resistant material. In addition, when used in a composite with luffa mats, COPU provides excellent resistance to water absorption because its matrix can coat and protect the luffa fibers effectively. In contrast, commercial OSB exhibits higher water absorption and swelling levels because the adhesive matrix does not completely encase the wood fragments. This suggests that COPU can be a superior adhesive alternative in improving the water resistance of OSB.

The reaction between castor oil and isocyanate produces a polyurethane structure with a high degree of cross-linking, forming a dense polymer network that is difficult for water molecules to penetrate [78]. This structure contributes to increased resistance to water absorption, making it a more stable material in humid conditions. Conventional polyurethanes often contain hydrophilic groups, such as unreacted hydroxyl groups (-OH), which can attract and retain water in their polymer structure [79]. However, in COPU, most hydroxyl groups react with isocyanate during the polymerization process, resulting in a more hydrophobic polyurethane chain that can interact less with water. This makes COPU an alternative adhesive that is more moisture-resistant than conventional polyurethane.

Sari et al. have studied the use of PVOH-Tannin-Hexamine (PTH) based adhesives for plywood applications [80]. The results showed that the F2 adhesive formulation, which has higher tannin and hexamine content, can produce plywood with strength that meets Japanese standards. As shown in Fig. 10, increasing tannin and hexamine content tends to increase plywood’s moisture content (MC), while increasing pressing time decreases the MC value. The control plywood has a lower moisture content than plywood using PTH-based adhesives, which is 5.3%. Based on the JAS No. 233:2003 standard, the maximum moisture content allowed for all MC levels is 14.0%, so the plywood produced is still within the specified standard limits [81].

The moisture content of plywood using polyurethane adhesive is lower due to the hydrophobic nature of the chemical structure of PU itself. PU is formed from the reaction between polyol and isocyanate, producing a highly cross-linked polymer network that is less able to absorb water than tannin or PVOH-based adhesives that still contain hydrophilic groups. In PU adhesives, most hydroxyl groups (-OH) have reacted to form urethane bonds (-NHCOO-), reducing interactions with water molecules. In addition, polyurethane forms a denser and more uniform adhesive layer between wood fibers, reducing porosity and inhibiting water penetration into the plywood structure. Better dimensional stability is also an advantage of polyurethane adhesives, because low water absorption can prevent swelling and shrinkage due to changes in environmental humidity.

The results of particleboard testing with experimental NIPU adhesives in Table 2 show that all samples meet the European norm requirements for internal bond strength (IB), with values above the minimum limit of 0.35 MPa [75]. Resin A has the highest IB value of 0.70 MPa, likely due to the simultaneous carbonation of tannins and humins, which results in a more extensive urethane bonding in the resin structure. Resin D shows a pretty good IB performance (0.74 MPa), which can be attributed to a more even distribution of urethane bonds between tannins and humins due to the mixing procedure of humin and diamine premix before reaction with tannins. Meanwhile, resin C has a lower IB value (0.65 MPa), which is still above the standard. This can be explained by its preparation procedure, in which tannins are carbonated first before reacting with humins and diamines, causing most humins not to react optimally to form urethane bonds [44]. Resin B had the lowest IB value (0.44 MPa).

In addition to IB strength, elastic modulus, and bending strength values also showed variations between resins. Resins A and D had higher bending strength values (14.1 and 14.4 MPa), indicating that the simultaneous carbonation method and mixing of humin-diamine premix contributed to the increase in the mechanical strength of the particleboard. In contrast, resin B had the lowest bending strength and elastic modulus values, indicating that humin alone could not provide a strong adhesion structure. Overall, these results confirm that the combination of tannin and humin in the NIPU formulation plays a vital role in improving adhesive performance, with the simultaneous carbonation approach and mixing of premix providing more optimal results than other methods.

PU adhesives are known to have good water resistance and high adhesion performance, but their use still relies on non-renewable petroleum-based compounds [82]. Therefore, more environmentally friendly alternatives are needed, such as biomass-based non-isocyanate polyurethanes (NIPU) [22,83,84]. The use of natural polyols, such as tannins, castor oil, lignin, or humin, is a potential solution to replace fossil-based polyols [78,80,84]. However, one of the main challenges in developing biomass-based NIPUs is to improve the reactivity of the raw materials to form a strong and stable polymer network.

To overcome this challenge, modification of tannins through carbonation with dimethyl carbonate (DMC) has been proposed as a strategy that can increase the reactivity of hydroxyl groups (-OH) in polyols, allowing for more efficient urethane bond formation [59]. In addition, selecting polyols with sufficient hydroxyl groups is essential to ensure optimal reaction with the hardener. With this approach, biomass-based NIPU formulations can be further developed to produce adhesives that have equivalent performance to conventional PU and are more sustainable and environmentally friendly.

While laboratory-scale studies have demonstrated the promising performance of tannin-based NIPU resins, scaling up these materials for industrial applications remains a critical challenge. One of the main obstacles is the variability and limited availability of tannin sources, which may affect the consistency and reactivity of the adhesive formulations. In addition, the synthesis of NIPUs often involves reagents such as dimethyl carbonate (DMC) and multifunctional amines, which, although safer than isocyanates, can increase production costs when used in large quantities. Process parameters such as reaction time, curing temperature, and mixing efficiency also need to be optimized for high-throughput manufacturing without compromising performance. Furthermore, the slower curing rate of some NIPU systems compared to conventional polyurethane adhesives may hinder their direct integration into fast-paced industrial lines. Despite these challenges, the potential for reduced environmental impact, lower VOC emissions, and improved safety makes tannin-based NIPU resins an attractive option for industries aiming to adopt greener materials. Collaborative efforts between academic research and industry, including pilot-scale production trials and techno-economic assessments, are essential to bridge the gap between laboratory innovation and commercial implementation.

Regarding scalability, the biggest challenge is ensuring the NIPU synthesis process can be well integrated into existing adhesive production lines. Current production technology is still mostly tailored for isocyanate-based polyurethanes, so the transition to biomass-based systems requires investment in equipment and modification of manufacturing processes. Therefore, further research is needed to develop more efficient synthesis methods, utilize more economical raw materials, and improve the reactivity of reagents so that NIPU production can be carried out at a more competitive cost and suitable for industrial scale.

The synthesis of tannin-based NIPUs, employing various methods like polyaddition, polycondensation, ring-opening polymerization, and rearrangement, allows fine-tuning of mechanical and thermal properties to meet the specific demands of different industrial applications. Recent studies demonstrate that these bio-based adhesives surpass international internal bonding strength, flexibility, and dimensional stability standards, particularly in oriented strand boards and other wood composite products. Despite these promising results, several challenges remain—most notably, the scalability of production, the cost of raw materials, and the adaptation of existing industrial manufacturing systems.

Tannin-based NIPU resins represent a significant advancement in developing sustainable, eco-friendly adhesives for wood-based composites. These materials offer a safer alternative to conventional polyurethanes, eliminating toxic isocyanates while maintaining comparable or superior mechanical strength, thermal stability, and water resistance. The natural polyphenolic structure of tannins contributes to the resins’ inherent fire resistance, forming a protective char layer during combustion that slows heat transfer and reduces flammability. Additionally, their ability to achieve super-hydrophobic properties through chemical modifications, such as incorporating fluoroalkyl groups or enhancing surface roughness, improves moisture resistance and durability.