Greening the Bond: A Narrative and Systematic Literature Review on Advancing Sustainable and Non-Toxic Adhesives for the Fiberboard Industry

Abstract

The fiberboard industry remains heavily reliant on synthetic, formaldehyde-based adhesives, which, despite their cost-effectiveness and strong bonding performance, present significant environmental and human health concerns due to volatile organic compound (VOC) emissions. In response to growing sustainability imperatives and regulatory pressures, the development of non-toxic, renewable, and high-performance bio-based adhesives has emerged as a critical research frontier. This review, conducted through both narrative and systematic approaches, synthesizes current advances in green adhesive technologies with emphasis on lignin, tannin, starch, protein, and hybrid formulations, alongside innovative synthetic alternatives designed to eliminate formaldehyde. The Evidence for Policy and Practice Information and Coordinating Centre (EPPI) framework was applied to ensure a rigorous, transparent, and reproducible methodology, encompassing the identification of research questions, systematic searching, keywording, mapping, data extraction, and in-depth analysis. Results reveal that while bio-based adhesives are increasingly capable of approaching or matching the mechanical strength and durability of urea–formaldehyde adhesives, challenges persist in terms of water resistance, scalability, cost, and process compatibility. Hybrid systems and novel crosslinking strategies demonstrate particular promise in overcoming these limitations, paving the way toward industrial viability. The review also identifies critical research gaps, including the need for standardized testing protocols, techno-economic analysis, and life cycle assessment to ensure the sustainable implementation of these solutions. By integrating environmental, economic, and technological perspectives, this work highlights the transformative potential of green adhesives in transitioning the fiberboard sector toward a low-toxicity, carbon-conscious future. It provides a roadmap for research, policy, and industrial innovation.

1. Introduction

The global fiberboard industry is undergoing a major transformation driven by growing awareness of environmental sustainability, occupational health, and consumer safety. Urea–formaldehyde (UF) adhesives have long dominated fiberboard production due to their low cost, strong bonding, and ease of application. However, extensive research has revealed serious drawbacks, particularly formaldehyde emissions linked to health risks and environmental burdens [1,2,3]. Formaldehyde, classified as a human carcinogen by the International Agency for Research on Cancer [4], is associated with respiratory irritation, asthma, and nasopharyngeal cancer. Studies show that fiberboard products containing UF adhesives release volatile organic compounds (VOCs) during use, degrading indoor air quality [5,6]. Given the industry’s key role in construction and furniture manufacturing, the continued reliance on formaldehyde-based adhesives remains a significant environmental and public health concern.

Formaldehyde, a colorless and pungent-smelling gas, is classified as a human carcinogen. Prolonged exposure to formaldehyde can result in cancer, including nasal and sinus cancer and leukemia. These cancers take several years or decades to develop [7,8]. Working eight hours daily for 40 years at the level of the government standard would give you a risk of about 2 in a thousand of getting cancer [4]. Repeated exposure to formaldehyde may cause bronchitis, skin, and asthma-like allergies. Some people are susceptible to formaldehyde, whereas others do not react to the same level of exposure. Recent studies illustrate the scale of the problem. For example, Cheung et al. [9], and H’ng et al. [10], reported that particleboards bonded with UF resins released between 0.12–0.50 mg/m³ of formaldehyde, often exceeding the World Health Organization’s safe indoor limit of 0.10 mg/m³. Similarly, Frihart et al. [11], and Du et al. [12], observed that formaldehyde emissions from UF-bonded panels remained detectable more than five years after production, confirming their long-term risks to human health.

From an environmental perspective, adhesives contribute significantly to the overall ecological footprint of wood-based panels. The synthesis of UF adhesive relies heavily on non-renewable petrochemical feedstocks, and their life cycle is marked by high carbon emissions, limited biodegradability, and toxic by-products [13]. End-of-life disposal poses particular challenges. Incineration of UF-bonded panels releases nitrogen oxides, carbon monoxide, and free formaldehyde vapors [14]. Landfilled residues pose a risk of leaching harmful compounds into soil and groundwater, creating long-term ecological hazards. These environmental impacts are equally concerning. UF adhesives contribute to volatile organic compound (VOC) emissions, which lead to indoor air pollution and contribute to environmental degradation. The persistence of these emissions poses long-term ecological risks, necessitating a shift towards more sustainable practices in the industry.

In response, green adhesives have emerged as viable and sustainable alternatives. Derived from renewable sources such as lignin, tannins, soy protein, starch, and cashew nut shell liquid, these adhesives offer several significant advantages, including reduced toxicity, lower VOC emissions, renewable sourcing, biodegradability, and compatibility with circular economy principles. Several studies indicate that soy-based adhesives achieved formaldehyde emissions near zero, corresponding only to the volatile organic compounds from the wood itself and met the formaldehyde emission regulation, effectively eliminating the hazard while maintaining mechanical strength comparable to UF panels [15,16]. Dunky [17], and Mantanis et al., [18], reported that tannin-based adhesives achieved bonding strengths of 0.8–1.0 MPa, which falls within industrial standards for medium-density fiberboard (MDF).

Global regulatory frameworks are accelerating this transition. The European Union enforces strict formaldehyde emission limits (E1: ≤0.124 mg/m³; E0: ≤0.05 mg/m³), while the California Air Resources Board (CARB Phase II) sets a threshold of ≤0.05 ppm for MDF, pushing manufacturers toward safer adhesives. Consumer demand also drives change, over 70% of buyers in Europe and North America now prefer eco-labeled furniture [19]. The industry is responding, with pilot trials of soy- and lignin-based adhesives in Europe and Asia showing good performance, though cost and moisture resistance remain concerns [20]. Recent advances in nanocellulose reinforcement and enzyme-assisted curing further enhance bio-based adhesives, improving strength, sustainability, and curing precision [20,21,22].

Against this backdrop, the concept of “Greening the Bond”, advancing sustainable and non-toxic adhesives for the fiberboard industry, has gained both scholarly and industrial significance. Transitioning from UF-based adhesives to green adhesives addresses urgent health and environmental concerns, aligning with global sustainability agendas such as the United Nations Sustainable Development Goals (SDGs 3, 9, 12, and 13) [23]. This study, therefore, explores empirical evidence, technological advancements, and industrial opportunities surrounding the adoption of green adhesives, providing both academic insights and practical guidance for stakeholders in the fiberboard sector.

2. Materials and Methods

2.1. Narrative Literature Review

A narrative literature review was conducted to identify the use of various green adhesives in fiberboard production that meet or exceed international industrial standards. A narrative review, as noted by several studies, synthesizes and summarizes existing research across diverse methodologies and theoretical perspectives [24]. Literature was sourced primarily from academic databases such as Google Scholar and ResearchGate, complemented by other relevant publications within the thematic scope of the review. These platforms were selected for their comprehensive access to peer-reviewed articles, books, conference papers, preprints, and gray literature, which support in-depth analysis and identification of emerging research gaps. Google Scholar also facilitates efficient literature mapping through citation tracking and related works, while ResearchGate and Academia enhance access to current debates and scholarly collaboration.

Hence, search terms were developed based on relevant keywords and Boolean operators, using combinations such as: “bio-based adhesives” AND “fiberboard” AND “sustainability”, “non-toxic” AND “wood-based panels” AND “green adhesive technologies”, “formaldehyde-free resins” AND “industrial application” AND “environmental performance”. The search and selection process followed the four EPPI stages: identification, screening, eligibility, and inclusion, ensuring a transparent and replicable pathway from literature retrieval to synthesis.

According to Baumeister [25], narrative reviews are particularly suitable for integrating findings from studies employing varied methods or conceptual frameworks. Similarly, Siddaway et al. [24] and Campbell et al. [26] emphasized that narrative synthesis allows for the integration of quantitative results without focusing on statistical significance, providing a means to connect, reinterpret, and extend theoretical insights [27].

2.2. Systematic Literature Review

This method was adopted to collect literature on green adhesives and harmful adhesives from published studies and literature that utilizes these two groups of adhesives in the production of fiberboards. Siddaway et al. [24] advocated that systematic reviews of scientific research aim to answer specific review questions from published research reports by identifying relevant studies, characterizing such studies to form a systematic map of research in the area, extracting relevant data to establish the value of the findings, and synthesizing and reporting the outcomes.

The Systematic literature review in this study adopted the Evidence for Policy and Practice Initiative (EPPI) [28] method, which the Organization for Economic Co-operation and Development (OECD) 2002 report emphasized that it builds up the methodologies for scientific reviews and exploits the results for future research, which are the most critical efforts currently needed for accumulating knowledge on educational research. Bennett et al. [29] reiterated that the [28] review method tends to contain studies with a wider variety of research designs and draws extensively on those systematic reviews undertaken in other areas. The main phase of the [28] method used is outlined in

Table 1. Phases of the EPPI systematic literature review method.

N°	Review Phases	Critical Activities Performed
1	Identification of the review research question	Consultation with Review Group members to develop and refine the review research question
2	Developing inclusion/exclusion criteria	Developing inclusion and exclusion criteria to enable decisions to be made about which studies are to be included in the review
3	Producing the protocol for the review	Producing an overall plan for the review, describing what will happen in each of the phases
4	Searching	Search of literature for potentially relevant reports of research studies, to include electronic searching, hand searching, and personal contacts
5	Screening	Applying inclusion and exclusion criteria to potentially relevant studies
6	Keywording	Applying adhesives in fiberboard production core keywords, and review-specific keywords to include studies to characterize their main contents
7	Producing the systematic map	Using keywords to generate a systematic map of the area that summarizes the work that has been undertaken
8	Identifying the in-depth review question	Consultation with Review Group members to identify area(s) of the map to explore in detail, and develop the in-depth research review question
9	Data extraction	Extracting the key data from studies included in the in-depth review, including reaching judgements about quality
10	Producing the report	Writing up the research review in a specified format
11	Dissemination	Publicizing the findings of the review, including the production of summaries by users

Source: Bennett et al. [29], pages 391–392.

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The [28] method is essential because it provides a systematic, transparent, and structured approach to reviewing literature. It ensures clarity of focus through well-defined research questions, applies rigorous inclusion and exclusion criteria to minimize bias, and uses a clear review protocol to enhance reproducibility. Comprehensive searching, keywording, and mapping help organize and identify knowledge gaps, while in-depth review and quality assessment strengthen reliability. Ultimately, structured reporting and dissemination enhance the credibility, accessibility, and utility of the findings for both researchers and policymakers, thereby supporting evidence-based decision-making.

3. Results and Discussion

The results in Figure 1 presents the results of a keyword search conducted across 1107 published articles, including 54 from MDPI Sustainability (2023–2025), 23 from Academia, 66 from Elsevier, 37 from ResearchGate, and 128 from MDPI Polymers, as well as contributions from international scientific conference proceedings, policy documents, theses, handbooks, and public health publications spanning 1992 to 2025.

As illustrated in Figure 2, the largest share of publications appeared in MDPI Polymers (19.34%). A significant proportion of globally influential papers emphasized concerns regarding formaldehyde emissions and their link to cancer risk, as well as the development of eco-friendly fiberboards with reduced formaldehyde content, sustainable bio-based adhesives for wood composites, and formaldehyde-free bio-adhesives for plywood, particleboards, and the entire fiberboard industry. Notably, 98% of the reviewed literature was published between 2008 and 2025, compared to only 2% between 1992 and 2007, indicating a sharp increase in scholarly attention over the past two decades.

The urgent need for research and policy engagement in this area stems from several interrelated drivers. Formaldehyde-based adhesives, long used in composite wood production, are now recognized as major indoor air pollutants and human carcinogens, prompting global health and regulatory concern. Simultaneously, the shift toward sustainable materials and green chemistry has accelerated the search for renewable, bio-based, and non-toxic adhesive alternatives consistent with circular economy goals. Rising demand for engineered wood products further underscores the need for scalable, eco-friendly adhesive systems with minimal environmental impact. Policymakers, industry, and researchers increasingly view the transition to non-toxic adhesives as both a health necessity and an opportunity to enhance competitiveness, satisfy consumer preferences, and meet stringent emission standards (e.g., E0, Super E0). Thus, the discourse has evolved beyond laboratory innovation to encompass socio-economic, environmental, and regulatory dimensions.

3.1. Narrative Literature Review

3.1.1. The Paradigm Shift in the Fiberboard Adhesive Industry

Replacing petrochemical, formaldehyde-based adhesives such as urea–formaldehyde (UF), phenol–formaldehyde (PF), and melamine–formaldehyde (MF) remains a central focus in the fiberboard industry due to indoor air-quality concerns, tightening emission regulations, and global sustainability objectives. Recent studies highlight a broad range of biomass-derived and residue-based materials explored as primary binders, co-binders, or performance enhancers for engineered wood composites [30,31]. These bio-adhesive systems are generally categorized according to their biochemical class and raw material origin, with performance evaluations emphasizing key technological properties such as dimensional stability, static bending, compression, hardness, internal bond (IB), and tensile strength [32,33].

This focused narrative review synthesizes findings from 2008–2025, drawing from peer-reviewed studies and reports indexed in major scientific databases. Search combinations included terms such as soy protein adhesive particleboard, lignin adhesive MDF, tannin-citric acid adhesive, starch adhesive particleboard, chitosan wood adhesive, cellulose nanofibril adhesive, and bio-polyurethane particleboard. Emphasis was placed on high-quality empirical studies that assessed both laboratory-scale and pilot-scale performance indicators relevant to fiberboard production.

Among the bio-based adhesive candidates, starches derived from cassava, corn, and potato are widely studied owing to their low cost, renewability, and availability. Native starch exhibits limited bonding strength and poor water resistance; however, chemical and physical modifications, such as oxidation, esterification, grafting, and blending with polyvinyl alcohol (PVA), have been shown to markedly improve adhesive performance [34,35]. Modified starch-based adhesives commonly achieve moderate internal bond strengths (0.5–0.7 MPa) suitable for non-structural applications, along with acceptable bending and density values [36]. Despite these advances, hydrophilicity and biodegradation under humid conditions remain the main limitations, often necessitating chemical crosslinking or hydrophobization treatments to ensure durability and dimensional stability in fiberboards [37]. Whereas starch-based adhesives demonstrate promising bonding performance and renewability, their moisture sensitivity and limited structural strength have encouraged continued exploration of alternative bio-polymers with higher functional reactivity. Among these, plant-derived proteins such as soy, cottonseed, and wheat gluten have attracted considerable attention due to their abundant reactive amino and carboxyl groups, which enable stronger crosslinking and improved water resistance in fiberboard applications.

3.1.2. Plant-Derived Protein Sources

Soy Protein (Soymeal, Soy Protein Isolates)

Soy protein is the most extensively studied bio-protein adhesive for agroforestry panel boards (e.g., particleboard, fiberboard) [38]. When denatured and chemically crosslinked (e.g., via tannins, polycarboxylic acids, epoxy agents), soy adhesives can exhibit internal bond (IB) strengths in the 0.6–0.9 MPa range, approaching that of commercial urea–formaldehyde (UF) resins for non-structural panels, while achieving near-zero formaldehyde emissions [39]. However, unmodified soy adhesives are inherently moisture-sensitive; to meet industrial benchmarks, they require additives (e.g., crosslinkers, hydrophobic modifiers) and optimized process conditions (e.g., elevated hot-press temperatures/pressures) [40].

Bacigalupe and Escobar [38], emphasized that soymeal, a by-product of oil extraction, is abundant and low-cost in major soy-producing regions, making it an attractive feedstock for bio-adhesives. Its valorization helps convert a residue stream into higher-value adhesives. Nonetheless, trade-offs arise when agricultural demands or food-industry uses compete with adhesive applications in certain regions.

Beyond soy protein, polyphenolic compounds such as tannins have been explored as formaldehyde-free adhesive alternatives. Tannin-citric acid adhesives exhibit strong bonding and improved water resistance: citric acid promotes esterification with tannins, yielding covalent networks that enhance mechanical performance and dimensional stability. In many reports, IB and stability metrics are comparable to or exceed those of phenol–formaldehyde adhesives [41]. Some formulations have progressed into pilot-scale trials, signaling promise toward industrial readiness [42].

However, industrial deployment of tannin adhesives is hindered by supply constraints and chemical variability. Dunky [43] emphasized that the composition and concentration of tannins vary by species, geographic origin, and extraction method, complicating consistent adhesive performance. The author reiterated that low solid content in tannin adhesives often leads to elevated moisture in produced composites, which can degrade performance unless careful formulation measures are taken [43].

Lignin, an abundant aromatic biopolymer from pulping or biorefinery operations, can partially replace phenol in phenol–formaldehyde (PF) resins or be transformed into reactive adhesives (via phenolation, methylolation, depolymerization). Partial substitution (commonly 30–50% lignin in PF formulations) often maintains acceptable adhesive performance while reducing fossil phenol use and lowering formaldehyde emissions [44]. Chemical modification increases reactive sites, improving strength and stability in lignin-based polymers [43]. However, Dunky [43] observed that lignin’s structural heterogeneity, variation in molecular weight, functional groups, and chemical origin (e.g., Kraft lignin vs. lignosulfonates), creates formulation challenges and batch-to-batch inconsistency. Recent work has explored crosslinking lignin with citric acid; for example, a hardwood lignin- crosslinked adhesive achieved shear strengths up to ~1.07 MPa, meeting certain national standards in Korea [45]. Such strategies offer a pathway toward fully bio-based adhesives but require further optimization of supply chains, consistency, and performance.

Plant-derived protein adhesives, especially soy-based systems, remain the benchmark for bio-adhesives in panel production once suitably modified and crosslinked. Tannins and lignin offer complementary or hybrid routes toward formaldehyde-free adhesives, but their industrial viability depends on overcoming feedstock variability, supply constraints, and formulation stability.

Marine and Animal By-Products—Chitosan (From Chitin)

Chitosan (deacetylated chitin) from crustacean shells has adhesive and film-forming properties. Recent studies have shown that chitosan-based adhesives crosslinked with bio-aldehydes, such as vanillin, or combined with epoxies, can produce MDF and particleboard with competitive IB and improved mechanical and water resistance. Formulations with chitosan, where epoxy ratios are adjusted for viscosity and cure time, have been tested with positive results [46]. Additional benefits include inherent antimicrobial properties and flame-retardant hybrid formulations (with ammonium polyphosphate) in some studies [47]. Mohan et al. [48] confirmed that the cost and supply of chitosan (from seasonal shellfish waste) and the need for crosslinkers and curing agents can raise costs and complicate the ‘fully green’ adhesive trajectory.

Gelatin and Animal Proteins

Animal gelatin and collagen have adhesive properties (historically used as glues). Gelatin-based adhesives exhibit good initial bonding but have poor water resistance unless chemically modified; hence, their use is typically limited or combined with crosslinkers [49].

3.1.3. Microbial and Biotech-Derived Materials Bacterial Cellulose

Bacterial cellulose (from Gluconacetobacter and Komagataeibacter spp.) forms strong nanofibrous networks with high tensile strength. Studies have explored bacterial cellulose as a reinforcement or as an adhesive component (after chemical modification) to enhance mechanical properties and improve interface adhesion [50]. Bacterial cellulose can be functionalized to enhance adhesion to lignocellulosic fibers. While bacterial cellulose is promising, current production costs and scale limitations restrict its immediate industrial uptake [51].

3.1.4. Microbial Polyesters and Exopolysaccharides

Compounds such as polyhydroxyalkanoates (PHAs) and certain microbial exopolysaccharides have been evaluated for use as composite binders; however, most of the existing work remains exploratory/pioneering [52,53]. Research works focus on improving their physical and mechanical properties, addressing high production costs, and developing new applications, particularly for biomedical and sustainable materials, which is highly recommended [54,55].

3.1.5. Plant Oils and Bio-Polyols (Bio-Polyurethanes)

Vegetable oils (epoxidized soybean oil, castor oil derivatives) have been used as polyols in bio-polyurethane adhesives. Scientific trials have shown that vegetable polyurethane adhesives can effectively bond particleboards, sometimes as complete replacements for urea formaldehyde in non-structural boards or as partial replacements [56]. These systems can offer good water resistance and rapid curing when combined with suitable isocyanates or cross-linkers. However, many such adhesives still rely partly on petrochemical isocyanates (unless fully bio-isocyanates are available), creating hybrid sustainability profiles [57].

3.1.6. Nano-And Micro-Reinforcements/Performance Enhancers—Cellulose Nanofibrils (CNF) and Cellulose Nanocrystals (CNC)

Cellulose nanofibrils (CNF) and nanocrystals (CNC) used at low loadings (1–5% w/w) can dramatically improve adhesive cohesion and interface strength, enhance mechanical properties of boards, and reduce thickness swelling by creating dense interphases [58,59]. Numerous experimental studies show that adding CNF to tannin or starch matrices enhances IB and stiffness; CNF can also act as a rheology modifier, improving application and penetration into wood particles [60].

Nanoclays, silica from rice husk, and other fillers—Inorganic fillers (nano-silica, modified clays) used sparingly improve dimensional stability and sometimes fire performance. Rice husk silica has been studied as a low-cost additive that can increase hardness and water resistance [61].

3.1.7. Crosslinking Strategies and Benign Catalysts

A recurring theme in several studies is the use of benign crosslinkers (citric acid, vanillin, glyoxal substitutes, enzymatic crosslinking) and reactive blends (protein + tannin, lignin + polycarboxylic acids) to improve water resistance and thermal stability without reintroducing toxic formaldehyde-releasing agents [62]. Enzymatic treatments (laccase, peroxidase) have been explored to catalyze oxidative coupling of phenolics such as those found in lignin or tannin systems, offering low-temperature curing routes. A study shows such strategies can markedly improve performance while maintaining low toxicity [41].

3.1.8. Life-Cycle and Environmental Evidence

Several life-cycle studies and review work on LCAs (reviewed across bio-adhesive literature) show substantial reductions in cradle-to-gate greenhouse gas emissions for adhesives derived from residues (soy meal, pulping lignin, tannin from bark) compared to petrochemical urea formaldehyde and phenol formaldehyde adhesives, commonly reported reductions range widely but can be in the order of 30–60% depending on system boundaries and feedstock sourcing [63,64]. However, LCAs also highlight trade-offs: energy-intensive chemical modifications, use of non-renewable crosslinkers, and land-use (if feedstocks are grown specifically) can reduce or eliminate the advantage unless residues are prioritized. Empirical LCA work, therefore, emphasizes the use of industrial residues and minimal additional processing [61].

3.1.9. Evidence of Industrial Readiness, Economics, and Scalability

Bamidele et al. [65] and Jayalath et al. [66], confidently confirmed that soy protein and hybrid Emulsion Polymer Isocyanate (EPI) and Polyurethane (PU) systems are the closest to industrial adoption, with pilot and some commercial implementations. Soy systems often need formulation tailoring and blending to meet pressing cycle requirements. Tannin-citric acid adhesives have advanced to pilot trials and show promise as near-market solutions where tannin supply is available [63,67]. Mateo et al. [68] observed that lignin has the highest feedstock availability; however, the cost of consistent modification and variable chemistry requires further research and development, as well as supply chain standardization. Hence, Correa-Guillen et al. [69] and Islam et al. [70] emphasized that despite its availability and potential as a sustainable alternative to fossil fuels, the heterogeneity of lignin necessitates more work to create consistent, value-added products and fully unlock its potential in the bioeconomy.

High-value bio-materials (chitosan, bacterial cellulose, cellulose nanofibrils (CNF), and nanocrystals (CNC)) offer excellent performance improvements but are constrained by cost; they are typically viable as additives or for specialty panels rather than stand-alone adhesives at current prices [71]. While they offer significant performance benefits, including improved mechanical strength and sustainability, Islam et al. [70] and Oliveira et al. [71], opined that economic viability is often restricted to niche applications or blending with other materials to reduce overall cost. The authors further emphasized that future development would focus on cost reduction and improving properties such as durability and mechanical strength to enable broader applications, including the replacement of synthetic adhesives.

3.1.10. Fiberboard-Specific Performance and Industrial Implications of Green Adhesives

The transition from petrochemical binders to sustainable, non-toxic adhesives in fiberboard production is driven by environmental regulations, occupational health concerns, and the global pursuit of circular bioeconomy materials. Current research demonstrates that several bio-based adhesive systems, including starch-, protein-, lignin-, chitosan-, and tannin-derived formulations, can deliver mechanical and physical properties that approach or meet industrial interior-grade standards while significantly reducing emissions and toxicity, as exhibited in

Table 2. Performance benchmarks and industrial relevance of green adhesives system in fiberboard manufacturing.

Adhesive Type	Performance	Dimensional Stability	Industrial Application	Practical Relevance	Sources
Cassava starch-based	IB: 0.45–0.60 MPa; MoR: 11–16 MPa; MoE: 1800–2500 MPa (modified starch)	Moderate (improved with citric acid, PVA, or tannin modification)	Interior-grade particleboard and furniture panels	Low-cost, renewable, compatible with UF production lines; needs moisture resistance enhancement	[72,73,74,75,76,77,78,79]
Protein-based (soy, other plant)	IB: 0.50–0.65 MPa; MoR: 14–18 MPa; MoE: 2000–3000 MPa	Moderate to good (crosslinked forms improved	Interior-grade MDF and particleboard	High bonding strength and zero formaldehyde; slower curing and higher viscosity limit high-speed production	[80,81]
Lignin-derived	IB: 0.45–0.60 MPa; MoR: 12–17 MPa; MoE: 1900–2800 MPa (hybrid formulations)	Moderate to good (improved with phenolation/glyoxalation)	Partial PF replacement in structural boards	Low-cost valorization of industrial lignin waste; needs standardization and faster curing catalysts	[82,83,84,85]
Chitosan and polysaccharide-based	IB: 0.40–0.55 MPa; MoR: 12–15 MPa; MoE: 1700–2300 MPa	Moderate (enhanced with citric acid or epoxy crosslinkers)	Interior MDF, decorative and specialty panels	Biodegradable, antimicrobial, non-toxic; cost and viscosity constrain scale-up	[86,87,88]
Tannin-based	IB: 0.55–0.75 MPa; MoR: 15–20 MPa; MoE: 2200–3000 MPa	Good (semi-exterior grade possible with glyoxal/furfuryl crosslinkers)	Interior and semi-exterior structural boards	Technically mature; easily integrated in PF lines; curing speed optimization needed	[21,89]

Source: Designed by the authors with data from the review. NOTE: All data presented in Table 2 are drawn from peer-reviewed publications and international standards.

and

Table 3. International minimum standard for some properties of manufactured fiberboard.

	Properties
International Standard minimum	Internal Bond (MPa)	Modulus of Elasticity (MPa)	Modulus of Rupture (MPa)	Hardness (MPa)
ANSI A208.1 M1	0.40	1550	10.0	2.80
EN 312 Type 2	0.40	1800	11.50	3.00
JIS Type 8	0.15	2000	8.0	2.00

EN 312 Type 2 = Fiberboards for interior fitments (including furniture) intended for use in dry environments. JIS Type 8 = Fiberboards not classified for moisture-resistant or structural load-bearing exterior applications. Sources: ANSI (1999) Mat-Formed Wood Particleboard. ANSI A 208.1.1993. National Particleboards Association, Gaithersburg [90]. European Committee for Standardization (CEN). EN 312:2010, Particleboards—Specifications. Brussels, Belgium: CEN; 2010 [91]. Japan Standards Association (JSA). JIS A 5905:2003, Fiberboards. Japanese Industrial Standard. Tokyo, Japan: Japan Standards Association; 2003. (Superseded by later editions) [92].

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3.1.11. Cassava-Starch Adhesives

Cassava (Manihot esculenta) starch has gained prominence as a renewable, formaldehyde-free adhesive. Chemical modifications such as oxidation, carboxymethylation, and dialdehyde conversion, or hybridization with polyvinyl alcohol (PVA), tannin, or citric acid, markedly improve bond strength and moisture resistance compared to native starch. Modified systems achieve internal bond (IB), modulus of rupture (MoR), and modulus of elasticity (MoE) values consistent with interior-grade particleboards [72,73,74]. However, unmodified starch remains highly hydrophilic, resulting in elevated thickness swelling (TS) and water absorption (WA). Crosslinkers and hybrid additives improve dimensional stability but wet durability remains the major technical constraint [75,76]. Industrial relevance lies in cassava’s wide availability in tropical regions, low feedstock cost, and compatibility with existing UF production lines with minimal adjustments. Its zero-formaldehyde emissions make it attractive for interior furniture and low-emission boards, though economic viability depends on low-cost modification routes and standardization of starch quality [77,78].

3.1.12. Protein-Based Adhesives

Plant-derived protein adhesives, especially soy, are the most extensively studied bio-binders. Chemical denaturation, enzymatic treatment, or crosslinking with glyoxal, tannin, or polycarboxylic acids significantly improve adhesion and water resistance, yielding interior-grade mechanical performance (IB, MoR, MoE) comparable to UF panels [79,80]. These adhesives exhibit near-zero formaldehyde emissions and good thermal stability, making them ideal for indoor applications. Nevertheless, high viscosity and slower curing rates limit throughput on industrial lines. Adjusting pressing temperature and dwell time mitigates these issues, while ongoing research targets enhanced wet strength and shelf-life stability [81,82].

3.1.13. Lignin-Derived Adhesives

Lignin, abundant as a pulping by-product, offers an aromatic phenolic structure enabling partial or full substitution of phenol in phenol–formaldehyde systems. Chemically activated lignins (phenolated, methylolated, glyoxalated) or lignin-synthetic hybrids exhibit excellent dry-state mechanical performance and, when crosslinked, improved moisture resistance [83,84,85]. Lignin adhesives are cost-effective and environmentally benign, valorizing industrial waste streams. However, feedstock heterogeneity and slower curing kinetics necessitate formulation control and catalyst optimization. Partial lignin substitution currently represents the most practical industrial pathway, balancing performance, cost, and process compatibility [84].

3.1.14. Chitosan and Other Polysaccharide Adhesives

Chitosan-based adhesives, derived from crustacean or fungal sources, provide high dry bonding strength and intrinsic antimicrobial activity. Crosslinking with citric acid, epoxy, or polycarboxylic acids significantly enhances wet strength and dimensional stability, meeting interior-grade specifications [86,87]. These adhesives are biodegradable, non-toxic, and compatible with conventional pressing systems after minor formulation tuning. Industrial scalability is currently constrained by high raw-material costs and solution viscosity, yet chitosan-epoxy hybrids present promising potential for low-emission MDF and specialty interior panels [86].

3.1.15. Tannin-Based Adhesives

Condensed tannins (from mimosa, quebracho, or pine bark) are among the most mature bio-adhesives, exhibiting strong phenolic reactivity. When crosslinked with furfuryl alcohol, glyoxal, or hexamine alternatives, they achieve excellent IB, MoR, and acceptable water resistance for interior and semi-exterior boards [21,89]. Tannin adhesives integrate smoothly into existing PF resin lines with minimal modification, offering reduced VOCs and renewable sourcing advantages. Their main industrial challenges include variability in tannin purity and the need for faster curing systems compatible with continuous hot-press operations [89].

These adhesive systems collectively demonstrate that sustainable, non-toxic formulations can deliver mechanical and physical performance aligned with industrial interior standards, substantially reducing environmental and health impacts. Cassava- and protein-based systems offer immediate applicability for interior furniture panels; lignin and tannin systems provide stronger structural potential with minor process adaptation; and chitosan and hybrid polysaccharide adhesives show promise for specialty low-emission markets. Future industrial success depends on achieving consistent raw-material quality, low-cost modification, enhanced moisture resistance, and scalable production compatible with existing high-speed lines.

3.1.16. The Urgent Need for a Paradigm Shift in Adhesive Utilization in the Fiberboard Industry

Intensified regulatory scrutiny and growing concern over indoor air quality are accelerating the shift toward sustainable, low-emission adhesives in the wood-based panel industry (fiberboard, MDF, particleboard, OSB). Conventional urea–, phenol–, and melamine–formaldehyde (UF, PF, MF) resins, though cost-effective and high-performing, emit formaldehyde, a Group 1 carcinogen linked to respiratory and mucosal irritation [42,93]. Stricter emission standards (e.g., CARB, TSCA Title VI) and market demand for low-VOC materials now compel industries to adopt safer adhesive systems [94].

Bio-based adhesives from soy protein, tannin–citric acid, lignin, starch, chitosan, and hybrid systems (EPI, PU) show strong potential as eco-friendly substitutes. Studies consistently report near-zero formaldehyde emissions and significant reductions in worker exposure [42,95]. Replacing petrochemical resins also reduces VOC emissions and occupational hazards, aligning with global green manufacturing goals [96].

Life-cycle and techno-economic assessments highlight notable environmental and economic advantages, particularly when using agricultural or industrial residues such as soymeal, pulping lignin, and tannin by-products. Reported reductions in GHG emissions and fossil energy use often exceed 30%, while the “residue advantage” converts waste into value and lessens dependence on virgin petrochemicals [97,98]. However, excessive chemical modification may offset these benefits, reinforcing the need for low-energy, residue-based processing [99,100].

At scale, bio-adhesive costs can approach those of specialty petrochemical adhesives when optimized for feedstock integration and process efficiency [101,102]. Additional benefits include reduced compliance costs, improved workplace safety, and access to green-labeled markets (LEED, EPD, low-VOC certifications), where eco-panels attract premium value [103]. Although UF resins remain economically dominant, internalizing health and environmental externalities strengthens the long-term viability of bio-based alternatives [72,104].

Denatured and crosslinked soy protein, tannin–citric acid, and lignin-derived adhesives have achieved internal bond (IB) strengths comparable to industrial standards, though improvements in water resistance and curing kinetics are still required [105,106]. Additives such as nanocellulose, nano-silica, and benign crosslinkers (citric acid, vanillin, enzymatic coupling) enhance mechanical strength and moisture tolerance, narrowing the gap with petrochemical counterparts [102,107].

Bio-based adhesives deliver major health (formaldehyde-free), environmental (low GHG), and economic (residue valorization, compliance savings) benefits [93,108]. Key challenges remain in feedstock standardization, water resistance, and scaling production. Continued research, pilot-scale validation, and policy support that internalize environmental costs are essential to accelerate adoption [109].

The fiberboard industry stands at a critical transition point. While UF resins persist for their low cost and curing efficiency, they impose high environmental and occupational risks [67,110]. PF resins offer lower emissions but remain fossil-dependent. Emerging systems based on soy protein, tannin–citric acid, and lignin provide viable near-term alternatives, with hybrid formulations serving as transitional solutions. Collaboration among researchers, industry, and policymakers will be decisive in advancing large-scale commercialization of sustainable, non-toxic adhesive technologies [111].

3.2. Systematic Literature Review

The fiberboard industry continues to rely predominantly on urea–formaldehyde (UF) adhesives despite their toxicity, formaldehyde emissions, and reliance on non-renewable feedstocks. In response to increasing environmental, health, and regulatory pressures, research on bio-based and non-toxic adhesive alternatives has intensified. This systematic review synthesizes scientific studies from 1992 to 2025 on green adhesives for fiberboard, with emphasis on performance metrics, environmental impact, and industrial readiness. Using the PRISMA methodology to ensure a transparent and reproducible process for identifying, screening, and selecting the relevant literature. Findings indicate that protein-based (soy), tannin-citric acid, and chemically modified lignin adhesives show the most promise for scale-up. However, they often lag UF in terms of water resistance and curing speed. Hybrid bio-synthetic adhesives bridge this gap in performance but reduce biodegradability. Life-cycle assessments consistently show lower greenhouse gas emissions and reduced toxicity for bio-based systems. The review identifies persistent barriers to adoption, cost, curing kinetics, and feedstock variability, and highlights pathways for industrial integration, regulatory alignment, and future research (Figure 3).

Wood-based panels (WBPs), including particleboard, medium-density fiberboard (MDF), and oriented strandboard, are among the most widely produced engineered wood products globally (Figure 4). Adhesives are essential to their manufacture, with urea–formaldehyde (UF) adhesive dominating due to low cost, fast curing, and satisfactory mechanical performance. However, UF adhesives are major sources of formaldehyde emissions, a Group 1 human carcinogen linked to respiratory irritation, asthma, and cancer risks. Phenol–formaldehyde (PF) and melamine–formaldehyde (MF) resins offer improved durability and lower emissions but remain petrochemical-based, energy-intensive, and partially toxic. Consequently, research and industry are increasingly focused on bio-based, low-toxicity adhesives derived from proteins, lignins, tannins, and polysaccharides for their renewability, low emissions, and biodegradability potential. Driven by population growth, economic expansion, and consumer acceptance, global WBP consumption is rapidly increasing, with particleboard demand projected to double or triple between 2020 and 2025 [112].

Global production of wood-based panels reached approximately 381 million m³ in 2023, showing a ~1% increase over the previous year and ~4% growth over the observed period [114]. The Asia-Pacific region accounted for about 60% of this output (≈230 million m³), with Europe contributing ~23% (≈86 million m³), North America ~11% (≈43 million m³), Latin America & the Caribbean ~5% (≈20 million m³), and Africa ~1% (≈4 million m³) [114].

The fastest-growing segment has been particleboard, driven by robust growth in the Asia—Pacific region [113]. Mirindi [116] emphasized that market demand is supported by factors such as affordability, ease of installation, high density and dimensional uniformity of the panels.

Lee [117] observed that manufacturing of these panels is heavily reliant on petrochemical-based thermosetting adhesives: urea–formaldehyde (UF), melamine–formaldehyde (MF), phenol–formaldehyde (PF), melamine–urea–phenol–formaldehyde (MUPF), methylene diphenyl diisocyanate (MDI), polyurethanes (PU), and resorcinol-based systems. The author opined that UF and related resins account for the majority of adhesive usage in wood-based panel production [117].

Although these adhesives provide strong performance and low cost, their use raises significant health and environmental concerns: formaldehyde emissions from UF, MF and PF resins are key contributors to indoor air pollution, occupational exposure and possible carcinogenic risk; isocyanate-based systems (e.g., MDI, PU) carry risks of occupational asthma; and the reliance on petroleum feedstocks and non-biodegradable adhesives undermines sustainability goals [99].

As a result, there is growing industrial and academic emphasis on bio-based, low-toxicity adhesives, derived from sources such as proteins, lignins, tannins, saccharides and other biomass by-products, which offer potential for reduced emissions, renewable sourcing and improved biodegradability [93]. Given the rising demand for particleboard, MDF and OSB (with annual growth rates estimated in the 5–7% range) combined with abundant biomass raw materials, the expansion of panel-manufacturing industries using sustainable adhesives is both urgent and strategically relevant.

The results presented in

Table 4. Characterization of adhesive types, resources, performance, environmental impact, and industrial readiness.

Adhesive Type	Source/Composition	Bond Strength (Internal Bond, MPa)	Water Moisture Resistance	Formaldehyde Emission	Industrial Readiness/Application	Key References
Urea Formaldehyde (UF)	Synthetic adhesive	0.75–1.00	Moderate	High	Widely used, standard in fiberboard	[118,119]
Starch-based	Corn, potato, cassava, wheat, oil palm	0.65–0.90	Moderate	Low	Pilot and lab-scale, some commercial MDF applications	[58,119]
Lignin-based	Wood or industrial byproducts	0.70–0.95	Good	Very low	Pilot and niche commercial applications	[59,118]
Tannin-based	Quebracho mimosa, Cashew residue extracts.	0.68–0.92	Excellent	Near zero	Small-scale commercial particleboards and MDF	[120]
Soya/Protein-based	Soy, casein	0.60–0.85	Moderate	Near zero	Limited commercial adoption, ongoing research	[121]
Hybrid bio-based adhesive	Starch lignin, tannin, furfural blends	0.70–0.95	Good-Excellent	Near zero	Pilot industrial trial; scalable potential	[72,122]
Synthetic formaldehyde-free adhesive	Bio-derived monomers	0.75–1.00	Good	Near zero	Ready for industrial adoption; emerging markets	[67]

Source: Designed by the authors with data from the review.

indicate that several green adhesives, including lignin- and tannin-based formulations, as well as hybrid blends, achieve bond strength and durability levels comparable to those of conventional urea–formaldehyde (UF) adhesives. Notably, green adhesives consistently exhibit near-zero formaldehyde emissions, thereby significantly reducing associated health risks. Although UF adhesives currently dominate the market, hybrid and synthetic formaldehyde-free alternatives are emerging as commercially viable options, as illustrated in Figure 5. Nonetheless, further optimization of bio-based adhesives is required, particularly in terms of moisture resistance, curing time, and cost-effectiveness, to facilitate their broader adoption in industry.

The comparative visualization of adhesive types in terms of environmental impact and industrial readiness is presented in Figure 5. Urea–formaldehyde adhesives demonstrate high industrial readiness but are associated with significant environmental and health hazards. Bio-based adhesives, though classified as eco-friendly and renewable, currently exhibit moderate performance in both environmental impact reduction and industrial scalability. Hybrid and advanced synthetic alternatives achieve a balance between sustainability and large-scale applicability, thereby representing the most promising direction for future adhesive development. Given the rising market demand, tightening environmental regulations, and consumer preference for eco-friendly products, the transition to green adhesive technology is not only a sustainability imperative but also a strategic industrial opportunity. The wood composites industry must accelerate this paradigm shift to ensure long-term competitiveness, health, safety, and ecological balance.

In contrast, green adhesives, developed from renewable resources such as lignin, tannins, soy protein, and starch, have demonstrated a reduction of up to 80–90% in VOC emissions compared to UF-based resins. Life-cycle assessments (LCA) show that bio-based adhesives can reduce carbon footprints by 30–60%, depending on the raw material and processing technology. Furthermore, their biodegradability and non-toxic nature make them safer for end-users and compliant with stricter emission standards such as E0 and super E0 classifications in Japan, Korea, and certain parts of Europe.

Table 5. Adhesive Categories and Characterization.

Adhesive Categories	Characterization	Key References
Protein-Based Adhesives	Soy protein is the most extensively studied bio-adhesive. Denaturation and crosslinking enhance internal bonding (IB) strength (0.6–0.9 MPa), but water resistance remains lower than that of UF. Commercial trials (e.g., Columbia Forest Products) demonstrate industrial viability in non-structural panels. Other proteins (blood meal, casein, egg albumin) show promising adhesion but lack scalability.	[22,40,121]
Tannin-Based Adhesives	Tannin-citric acid (TCA) adhesives achieve IB values >0.8 MPa and reduced WA/TS compared to starch-based adhesives. Pilot studies demonstrate durability comparable to phenol formaldehyde (PF) adhesives, without the use of toxic reagents. Extracted mainly from mimosa and quebracho bark; scalability linked to forestry residues.	[44,121,123]
Lignin-Based Adhesives	Lignin substitution for phenol in PF resins has reached up to 50% replacement without significant loss of performance. Modified lignins (phenolated, methylolated) show enhanced reactivity. Challenges: heterogeneity of industrial lignin and higher curing temperatures	[71,121]
Polysaccharide-Based Adhesives	Starch-based adhesives remain hydrophilic; however, oxidation or esterification can improve performance. IB ~0.5–0.7 MPa reported, still below UF benchmarks. Chitosan adhesives offer antimicrobial benefits, but are restricted by high costs	[34,82,124]
Hybrid and Low-Emission Synthetic Systems	Emulsion polymer isocyanate (EPI) and bio-polyurethane systems combine bio-based polyols with petrochemicals, achieving high IB values (>1 MPa) and excellent water resistance. However, partial reliance on fossil inputs reduces sustainability.	[125,126]

Source: Designed by the authors with data from the review.

categorizes the adhesive types and characterizes them. Bio-based adhesives, such as those derived from soy, tannin, lignin, and polysaccharides, offer sustainable alternatives to formaldehyde resins; however, most face challenges related to water resistance, cost, or scalability. Soy and tannin systems show the most significant promise, while lignin works well as a phenol substitute when modified, and starch/chitosan remain limited. Hybrid systems, such as emulsion polymer isocyanate (EPI) and bio-polyurethanes, achieve strong, durable bonds but still rely partly on fossil inputs.

The environmental implications of UF adhesives are concerning, as Elcosh [127] emphasized that UF adhesives release formaldehyde gas into the atmosphere, contributing to indoor air pollution and posing health risks to occupants. Whereas Pérez-de-Mora [128] observed that the persistence of UF resins in the environment leads to long-term contamination of soil and water sources, as Dorieh et al. [129] and Thetkathuek et al. [130] informed that recycling UF-bonded fiberboard is complicated due to the chemical stability of the resin, hindering the recovery and reuse of materials. Several studies, including that of the American Cancer Society, have confirmed that formaldehyde, a key component of UF adhesives, is classified as a human carcinogen by the International Agency for Research on Cancer (IARC). The authors emphasized that prolonged exposure has been linked to cancers such as nasopharyngeal cancer and leukemia [129,130]. US DL [131] and Thetkathuek et al. [130] informed that occupational exposure to formaldehyde vapors can lead to respiratory symptoms, including nasal inflammation, asthma, and bronchitis. Benítez-Andrades et al. [132], and Goossens & Aerts [133] confirmed that direct contact with formaldehyde can cause dermatitis, eye irritation, and allergic reactions. Long-term and chronic exposure to formaldehyde has been associated with impaired lung function and other long-term health issues [1].

3.2.1. The Influence of Temperature Fluctuations on Emission Dynamics and Adhesive Performance

Temperature plays a critical role in determining the rate and magnitude of toxic emissions from adhesives. References [134,135] observed that at lower external temperatures, the volatilization and diffusion of toxic compounds, particularly volatile organic compounds (VOCs) such as formaldehyde and phenolic derivatives, tend to decrease due to reduced molecular mobility and slower evaporation kinetics. Conversely, higher temperatures accelerate emission rates, as increased thermal energy enhances diffusion and chemical degradation of adhesive components. These temperature-dependent dynamics have been reported in several recent studies, which demonstrate that adhesives exhibit significantly lower VOC release under cooler ambient conditions compared to warm or curing environments [136,137]. Accordingly, [136] and [138] emphasized that environmental temperature is a key factor influencing the toxic release profile and environmental safety performance of adhesive systems during manufacturing and in-service applications.

3.2.2. Temperature Resistance Properties of Different Adhesive Types

(

Table 6. Comparative Thermal performance indicators of major adhesive types.

Adhesive Type	Primary Resin Base	Thermal Degradation Onset (°C)	Maximum Stability Range (°C)	Key Thermal Resistance Features	Sources
Urea–Formaldehyde (UF)	Amino resin (formaldehyde-based)	120–150	150–180	Moderate stability; degrades rapidly above 150 °C; sensitive to humidity.	[142,143,144]
Phenol–Formaldehyde (PF)	Phenolic polymer network	220–250	250–300	Excellent crosslinking; high char yield and heat resistance.	[145,146,147]
Melamine–Urea–Formaldehyde (MUF)	Co-condensed amino–formaldehyde resin	160–200	200–230	Improved stability over UF; suitable for interior-semi-exterior use.	[148,149]
Epoxy Resin Adhesive	Epoxide crosslinked polymer	180–220	220–250	High bonding strength; stable under moderate heat and moisture exposure.	[150,151,152]
Bio-Based Adhesive (Lignin, Soy, or Tannin-based)	Natural polyphenol/polyamide blends	150–190	190–220	Enhanced by bio-fillers; moderate thermal tolerance; lower toxicity.	[153,154,155,156]
Polyurethane (PU)	Polyether or polyester-based urethane	170–200	200–230	Flexible with good heat resistance; may release VOCs at higher temps.	[157,158]

) illustrates the comparative thermal behavior of different adhesive systems. The data indicate that urea–formaldehyde (UF) adhesives typically exhibit thermal degradation onset around 120–150 °C, while phenol–formaldehyde (PF) adhesives show higher stability up to 250 °C, owing to their stronger crosslinked phenolic structure. Bio-based and epoxy-modified adhesives display intermediate resistance, maintaining structural integrity up to 180–220 °C, depending on filler and resin formulation. These distinctions reflect the influence of polymer backbone chemistry and curing mechanisms on adhesive thermal tolerance [42,139,140,141]. The summarized data thus provide a clear comparative view of temperature-dependent performance and application suitability of various adhesive systems.

3.2.3. Technological Advancements in Green Adhesives for the Fiberboard Industry

With innovative formulations, Kumar et al. [44] and Gonçalves et al. [99], have confirmed that recent research has focused on hybrid adhesives, which combine bio-based materials (starch, lignin, and tannin) with small percentages of synthetic resins to optimize bond strength and moisture resistance. Whereas Wang et al. [51] and Iswanto et al. [159] emphasized that nanomaterials, such as cellulose nanocrystals and nanoclays, have been incorporated to enhance adhesive performance, thereby improving internal bond strength and dimensional stability. Akhil et al. [160] and Faheem & Khan [161] observed that advanced curing methods, such as microwave-assisted or hot-press curing, accelerate polymerization and reduce formaldehyde emission during panel production. These techniques also enable energy-efficient manufacturing, aligning with sustainability goals. Automation and controlled adhesive application systems have been observed to minimize overuse and waste, enhancing panel uniformity and lowering production costs, and continuous monitoring of moisture content and temperature during pressing ensures optimal adhesive curing, further improving panel quality [57].

3.2.4. Green Adhesives, the Sustainable Alternatives

Several studies emphasized that materials such as lignin, tannin, starch, and proteins derived from renewable sources have been explored as alternatives to UF adhesives. These bio-based adhesives are biodegradable and exhibit lower toxicity [21,30,42]. Furthermore, advancements in resin technology have led to the development of adhesives that do not emit formaldehyde, reducing health and environmental risks [162,163]. However, while green adhesives may offer comparable performance in terms of bonding strength and durability, challenges remain regarding their cost-effectiveness and scalability for industrial applications [13,88].

3.2.5. Challenges in Adopting Green Adhesives

The review outlined some technical challenges that could impede the adoption of green adhesive in the fiberboard industry. Ashori and Kuzmin [88], observed that some bio-based adhesives exhibit lower or inconsistent bonding strength under high humidity conditions. Gonçalves et al. [7] and Dunky [17], emphasized that many green adhesives require longer curing times compared to UF resins, which affects production efficiency. It has also been observed that existing industrial machinery may require modification to handle bio-based adhesives effectively [164]. Other challenges may include economic considerations, as confirmed by Arias et al. [32] and Aristri et al. [165] who noted that certain bio-based feedstocks, such as tannin or lignin extracts, are more expensive than conventional chemicals. At the same time, Rasche [166] opined that small-scale laboratory successes often face hurdles in industrial-scale production due to supply chain constraints. Gonçalves et al. [7] again emphasized that regulatory and market barriers, as well as a lack of unified international standards for green adhesives, can delay market acceptance. The authors further acknowledged that manufacturers accustomed to conventional resins may be hesitant to adopt new technologies without clear economic incentives.

3.2.6. Future Directions and Research Opportunities

Studies indicate that one of the areas of interest in research opportunities is material innovation exploration of novel renewable polymers such as hemicellulose derivatives or algae-based adhesives [167]. Hence, emphasis has been laid on the development of hybrid adhesives that combine bio-based and synthetic components to optimize performance while minimizing toxicity [168]. Collaboration among materials scientists, chemists, engineers, and environmental experts to enhance adhesive performance and sustainability is essential. Sala et al. [168] revealed that the urgent need for life cycle assessments (LCA) to quantify environmental benefits and guide policy decisions need not be overemphasized. Hence, Shan and Ji [169] and Jensen [170] hinted that encouraging government subsidies or tax incentives for manufacturers adopting green adhesives could be a great impetus. Brenton et al. [171] and Clapp et al. [172] emphasized that developing international standards and certifications for low-emission panels to facilitate market acceptance is of high importance.

To disseminate this knowledge, Campbell et al. [26] observed that workshops and training programs are required to educate stakeholders on safe, sustainable, and cost-effective alternatives. Whereas academic-industry collaborations to translate laboratory research into commercial applications are imminent [173]. Hence, extensive evidence indicates that conventional UF adhesives present serious health and environmental concerns, whereas bio-based and formaldehyde-free alternatives provide sustainable and often comparable performance; however, addressing the technical, economic, and regulatory challenges will require continued research, innovation, and strong collaboration among academia, industry, and policymakers to enable a complete transition to safer and eco-friendly fiberboard production.

Sandberg [174] noted that the global wood industry is the largest user of adhesives, with approximately 80% of all wood and wood-based products involving some form of bonding. Moreover, 70% of the total volume of adhesives produced is consumed by the woodworking industry. Several studies indicate that other structural adhesives for composites include epoxies, polyurethanes, methacrylates (MMA), and MS polymers. The specific adhesive choice depends on factors like the desired bond strength, environmental exposure, curing time, and whether the composite is for interior or exterior applications [175].

Table 7. Adhesive characteristics and utilization.

Adhesives	Characteristics	Utilization	Sources
Oil palm starch	highest internal-bonding strength	Bond rubberwood particleboard	[176]
Wheat starch	Good internal bonding strength, but requires additive enhancement.	Bond rubberwood particleboard, rice husks	[176]
Soybean protein	Bonding strengths have exceeded commercial UF adhesives	Production of plywood, blockboard, and engineering flooring substrates	[39,40]
Acrylated epoxidized soybean oil (AESO)	Superior mechanical properties, water resistance, and high-temperature resistance	Bamboo particleboards	[22]
Palm-oil-based dimethacrylate	Superior mechanical properties, water resistance, and high-temperature resistance	Bamboo particleboards	[177]
Gum Arabic	Particleboard is recommended to be used for construction to eliminate the health hazards resulting from high formaldehyde emissions from urea formaldehyde resin-based particleboards	Macadamia nutshells, rice husk, sawdust.	[178]
melamine-, phenol-, Urea- formaldehyde	Acceptable mechanical and physical properties performance, strong bonding performance	Strawboards and non-wood-based particleboard	[179]
Epoxy	Heat-curable single composite. Provide high-strength bonds to many composite materials	Fiber composite industry	[180]
Structural acrylic	Form very high-strength bonds to a composite that has high peel strength, providing gap-filling properties	Ideal for bonding of rough surfaces. High fiber-content composite	[51,180]
Cyanoacrylate/instant adhesive	Create strong bonds very quickly in applications that do not require high impact or peel resistance	Can be used in place of clamps or jigs to hold the assembly in place while a longer curing two-component adhesive bonds	[180]
UV curable	Inkjet coating on the substrate surface to bond the composite to clear glass or plastic	They also coat composites, wood-based substrates, and MDF	[22]
MS polymer	Reduce water absorption (WA) and thickness swelling in fiberboards	Wood fibers, Agro-Forest residues, Kenaf fiber	[181]
Methyl methacrylate	high strength and water resistance.	Rice straw and natural wood particles, oil palm trunk bagasse	[158,182]
Polyurethane	Bond fiber well in exhibiting high-performance properties performance	Wood and other non-wood fibers	[82,104,166]
Urethane	Excellent impact resistance and good adhesion to most plastics	Bonds well to woods, concrete, and rubber with reduced resistance to solvents and high temperatures	[183]
Cassava starch	Excellent static bending strength, hardness, and internal bond	Bonds banana fiberboard, Ceiba pentandra, Cocoa stem, Elephant grass particleboards	[73,74,75]

Source: Designed by the authors with data from the review.

exhibits some of the prominent adhesives, their characteristics, and utilization.

Several studies indicate that in the fiberboard industry, static bending characteristics, internal bond strength, thickness swelling, water absorption, hardness, compression, screw withdrawal resistance, and resistance to fungi deterioration (durability) are the most prominent panel properties evaluated for industrial utilization [73,152]. Hence, if a particular adhesive bond meets these requirements according to international standards, it must be adopted for industrial use. However, if otherwise, then additives could be employed to enhance the performance of the adhesive. SGE [184], Srivaro & Jantawee [185] and Suarez [186] noted that these adhesives are designed to meet the requirements of different properties and achieve the targets for products’ shear, peel, and fatigue resistance within the same products. The author further noted that the rapid development of hybrid technologies is one of the prevailing trends in the construction chemicals sector today [186]. According to reports published by FEICA [187], hybrid products are among the fastest-growing product categories. It is therefore recommended that a hybrid adhesive be the preferred choice for the fiberboard industry; hence, all research, as well as graduate and postgraduate projects, should focus on the development of a hybrid adhesive for the fiberboard industry.