Use of Bio-Based Adhesives in Wood-Based Panel Production

Bio-based Sustainable Adhesives and Their Use in Wood-based Panel Production Bio-based materials are gaining increasing importance due to their improved ecological footprint and independence from petroleum. Natural origin products offer multifaceted applications such as the development of biopolymer-based adhesives, adhesive synthesis from renewable monomers, and the use of renewable materials as additives in adhesive formulations. These possibilities have attracted the attention of both academia and various industrial sectors employing bio-based adhesives. Moreover, changing environmental regulations, voluntary standards, and rising public awareness encourage the use of sustainable products across all areas of daily life. In particular, emission limits imposed on formaldehyde-based aminoplast and phenoplast adhesives are affecting sectoral dynamics in the wood-based panel industry. Increasing environmental awareness among the public and the need to reduce dependence on petroleum sources have accelerated research into the use of polymeric products derived from bio-based, renewable sources across various sectors [1, 2]. This situation has revived interest in traditional starch and renewable rubber-based binders previously used extensively in adhesive technology and has created an opportunity for new technologies such as the use of modified vegetable oils and lignin and its derivatives—a byproduct of the paper industry—to come into focus in binder synthesis research and development [3, 4]. Currently, two factors maintain the trend of moving away from formaldehyde-based synthetic resins in the wood-based panel sector. The first is formaldehyde emission regulations; the second is the objective of achieving sustainable products both in raw materials and finished goods. In 2004, when the International Agency for Research on Cancer (IARC) reclassified formaldehyde from Group 2A (possibly carcinogenic to humans) to Group 1 (carcinogenic to humans), it was reported that formaldehyde emissions from wood panels, particularly in indoor applications, posed a threat to human health [5]. With the IARC formaldehyde report, formaldehyde emissions have been subject to strict limitations worldwide, and composite wood panel manufacturers have been affected. Although the future of formaldehyde emission restrictions globally remains uncertain, panel manufacturers have begun using low-emission formaldehyde resins, formaldehyde scavengers, and formaldehyde-free synthetic or bio-based adhesives. [caption id="attachment_130828" align="aligncenter"] Table 1: Key factors in bio-based product development studies [6][/caption] The emission of formaldehyde, a volatile organic compound (VOC), occurs largely during the production of wood-based panels, with the majority stemming from adhesives used in manufacturing. Formaldehyde emission can be controlled through the use of formaldehyde scavenging materials or through product formulations with lower free formaldehyde content [7]. While urea is the most commonly preferred raw material for formaldehyde scavenging in industry, ammonia and other ammonium salts are also utilized [8].

Wood-based Panels and Adhesives

Wood-based panels are composite materials produced by combining wood particles, fibers, wood dust, and veneers with various adhesives. According to their applications, they are classified into two groups: structural and non-structural panels. The types of wood and adhesive from which they are derived directly affect their use in outdoor and indoor applications due to their moisture sensitivity. Urea formaldehyde (UF) resins are preferred in the production of medium density fiberboard (MDF), particleboard, and plywood. Worldwide, 68% of UF resins produced are used in MDF and particleboard manufacturing, while 23% is used in plywood production. Additionally, phenol formaldehyde (PF), melamine urea formaldehyde (MUF), and polymeric 4,4-diphenylmethane diisocyanate (pMDI) are other synthetic adhesive derivatives used in industry. [caption id="attachment_130829" align="aligncenter"] Table 2: Advantages and disadvantages of adhesives used in wood-based panel production [9][/caption]

Use of Sustainable Adhesives

The increasing environmental awareness of end consumers, regulations introduced as a result, and fluctuations in oil prices have made sustainable products and production processes the primary focus of the chemical industry in recent years. The transition to product formulations with higher active substance content and the reduction of solvent use are core focuses of the adhesive industry, yet interest in traditional natural adhesive materials based on polysaccharides and proteins has also increased [10]. Additionally, the shift in perception of businesses and institutions worldwide toward a circular economy centered on low-carbon footprint, bio-renewable products encourages the development of environmentally friendly adhesives. However, since the majority of bio-based products have not been fully commercialized and their technology readiness levels are insufficient, most adhesives produced globally remain petroleum-based. The wood-based panel industry primarily uses heat-curing adhesives that are nearly entirely synthetic, produced through the condensation reaction of formaldehyde with petroleum-derived raw materials such as urea, melamine, and phenol. Due to their low cost and ease of use, they remain considerably practical compared to bio-based alternatives such as lignin, tannin, starch, and protein. The bio-based adhesive to be used in the sector is expected to be economical and have a wide distribution network, like its petroleum-derived competitors, and the resulting panel should have acceptable levels of mechanical strength and moisture resistance. Sustainable Adhesives for Wood-based Panel Production Several strategies are currently used in the development of adhesives from renewable materials. The first is the use of biopolymers such as protein, which already possess adhesive properties. The synthesis of bio-based polymers from monomers derived from renewable sources presents another alternative. Although this technique initially requires more effort than others during the synthesis stage, application compatibility can be achieved with similar equipment. Another approach to using bio-based materials is their direct use as additives in synthetic adhesive formulations.

1. Lignin

Lignin, a waste product of the paper industry, was previously used as fuel within the same sector as a macromolecule. Since lignin does not possess a chemically homogeneous structure, advanced modification and purification processes have been developed to enable its use in the adhesive sector. Commercially available lignin derivatives today are examined in two categories based on the process type from which they are derived. Sulfur-containing lignin (kraft lignin obtained from black waste generated in kraft paper production and lignosulfonate obtained from sulfite waste) and kraft-free biorefinery lignin (soda, organosolv, steam production, hydrolyzed, and acidified lignin derivatives) constitute these categories. Lignin molecules composed of cross-linked phenolic groups contain hydroxyl, methoxyl, carboxyl, and carbonyl groups, showing promise through their susceptibility to advanced chemical modifications. Kraft lignin is insoluble in water and dissolves only in various solvents in an alkaline environment. Soda lignin and organosolv lignin derivatives are generally insoluble in water, while lignosulfonate derivatives with appropriate counter ions exist in water-soluble form [11, 12]. Biorefinery lignin is anticipated to have industrial-scale future applications in biomass-biofuel and biomass-sugar cycles. Partial or complete substitution of lignin for phenol formaldehyde resins used in plywood production constitutes the primary application area in industry. However, due to its chemical structure, lignin has lower reactivity compared to phenol formaldehyde resin, so resins with high lignin substitution continue to show disadvantages, particularly regarding bonding quality, in sectors where rapid curing is demanded. Intensively studied modification types of lignin are methylolation, phenolation, and methylation stages. The use of lignin derivatives subjected to pre-methylolation modification in the presence of PF and pMDI is considered an important step in the wood-based panel sector. According to this technique, hydroxymethyl groups are incorporated into the aromatic lignin structure and increase the reactivity of the aromatic ring.

2. Tannin

Tannin, a natural material found in plant bark, wood, and leaves, is currently widely used in the production of inks and textile dyes and in the preparation of corrosion-preventing chemicals. Although they are found in the structure of many plant species, plant-derived species with high concentrations worth extracting are found in South Africa and South America. This situation results in limited access to raw materials and places them at a disadvantage in competition with synthetic, petroleum-based adhesives. Due to their high water solubility, most commercial applications include highly reactive condensed tannin derivatives, but the use of hydrolyzed tannin derivatives in wood-based panel production is today a new focus area. Hydrolyzed tannins have potential for use as a replacement for phenol in PF resins [13]. Tannin-derived binders, which have higher reactivity toward formaldehyde compared to phenol, present themselves as an obstacle due to their significantly high viscosity and limited shelf life, making them a primary target for technology application studies [14]. Additionally, tannin-based adhesives used in particleboard production are known to provide higher hydrolytic stability compared to UF resins. Commonly used with lignin, tannin can undergo a condensation reaction with formaldehyde or glyoxal and can be used with hexamine as a hardener. Tannins containing phenolic functional groups are used as formaldehyde scavengers in the wood-based panel sector due to their high reactivity [15].

3. Soy Protein

Proteins are generally extracted from soy, palm, canola, cottonseed, and sunflower seeds through mechanical or solvent removal of oils, leaving behind a residue that is widely used in various industrial sectors. Soy oil, the most extensive member of this family, has the largest volume among edible oils and accounts for 52% of global oil production, centered in the United States and Brazil. After oil is removed from soy seeds, the remaining meal contains approximately 45-50% soy protein, and this protein is widely used not only in the food sector but also in paints, paper, and wood sectors due to its binding properties. Its use as an adhesive dates back to antiquity, and the commercial use of soy protein in plywood production began in the 1930s [16]. After denaturation with sodium hydroxide, soy protein-based adhesives used in plywood production have limited application only in interior settings due to their short shelf life, low biological stability, low solids content, slow press times, and poor hydrolytic stability [17]. Although developing technology has improved their moisture resistance and binding capability, the current limitations of soy-based adhesives continue to restrict their application areas. The complex structural and chemical nature of soy protein directly affects the solubility, dispersion capability, viscosity, moisture resistance, and thermal behavior of the raw material. To this end, most research and development efforts have focused on modifications aimed at improving these properties. Studies on substituting soy protein for UF and MUF resins in wood-based panel production found that resin reactivity declined and viscosity increased. In studies conducted as a substitute for PF resins, it was determined that soy protein did not achieve the desired reactivity at the basic pH of resol-type PF resins, preventing the expected advantages from being realized [18]. Particleboard samples obtained with a 40% soy protein-substituted PF formulation conducted by Wescott and colleagues showed high moisture resistance, with the resulting panels succeeding in a 2-hour boiling test [17]. Additionally, soy proteins primarily undergo cross-linking reactions in the presence of polyamides. Isocyanate, amine, and amide derivative chemicals are commercially sold as cross-linking systems for industrial soy protein applications. In honey moon systems for finger-joint production, a type of engineered wood panel, soy protein-based adhesives are used as a two-component system together with phenol resorcinol formaldehyde (PRF) resin [19].

4. Starch

Starch is an industrially valuable biopolysaccharide found in plant leaves, roots, and seeds. As the plant's glucose—and therefore energy—depot, starch is found in high quantities, generally in corn, wheat, potato, tapioca, and rice. Chemically, it is composed of amylopectin and amylose structures. Amylose is formed by linear bonding of glucose monomers through 1-4 glycosidic bonds, while amylopectin structure consists of 1-6 bonds containing a high degree of branching. Starch-based adhesives used in the wood sector are obtained through etherification, oxidation, and esterification reactions of starch [20]. The use of starch in the wood sector has focused on starch-polyvinyl alcohol, starch-isocyanates, and starch-tannin-based adhesives [21-23]. Due to high intramolecular and intermolecular hydrogen bonding, they are insoluble in cold water. During hot pressing in wood-based panel manufacturing, water evaporation causes crystallization of starch molecules, leading to deterioration in bonding performance, necessitating advanced optimization efforts in technology for performance to reach tolerable levels. One of the strategies studied for the use of starch in the wood sector is the use of cross-linkers. Isocyanate derivatives, capable of reacting with hydroxyl, amino, and carboxyl groups, have shown successful results in plywood production when used with starch in both wet and dry environments [24]. Isocyanate derivatives offer promise due to their high bonding capability, moisture resistance in the finished product, zero formaldehyde and VOC emissions, and ease of application; however, the high cost of the raw material and its poor water resistance present disadvantages that hinder technology development. Other chemicals usable as cross-linkers—sodium borate, epichlorohydrin, hexamethoxymethylmelamine, and formaldehyde—can produce water-resistant panels with increased bonding strength [21]. Polyvinyl acetate (PVAc) grafted starch derivatives can be used with epoxy resins, starch derivatives modified with vinyl-functional silane agents, and polyvinyl alcohol (PVOH)-starch systems have been documented in literature [25-27]. [caption id="attachment_130830" align="aligncenter"] Table 3: Comparison of bio-based adhesives used in wood-based panel production [28][/caption]

Future of Technology and Expectations

Bio-based adhesives that can be used in wood-based panels discussed in this work will help reduce the industry's fossil fuel consumption and create a sustainable alternative to increase indoor air quality. Several issues prevent the use of bio-based adhesives in the sector. Challenges include limited availability and accessibility of tannins, poor adhesion performance for starch, weak moisture resistance of natural materials containing hydroxyl groups, and high viscosity of long-chain molecules. Increasing the use of these adhesives in the wood-based panel sector will be possible through collaborative work of different disciplines to address these obstacles. Unlike other bio-based adhesives, tannins can provide high moisture tolerance and adhesion properties without requiring any reinforcement from synthetic petroleum-based adhesives. Moreover, unlike other bio-based alternatives, tannins can be used with new types of alternative cross-linkers to formaldehyde due to their high reactivity. Conversely, tannins are obtained through extraction methods from tree species with limited global availability, have high viscosity, and have limited applications due to their dark color. To increase tannin use in the sector, modifications have focused on reducing viscosity for easier use, extending application time, and increasing cross-linking [14]. Lignin-derived adhesives, the bio-material most researched for wood sector adhesive applications, find limited industrial use due to lignin's low reactivity. Industrial lignin has achieved commercialization in phenol formaldehyde resins through partial substitution of up to 30% for phenol, establishing itself in the sector. Examples of tannin or lignin combinations replacing phenol and different cross-linkers replacing formaldehyde appear in literature, but industrial applicability of these examples has not yet been established [29]. Current lignin technology proves inadequate—both in terms of cost and performance—when its use in sectors expecting rapid curing is considered. Soy protein-based adhesives show promise through the development of new cross-linkers and hardeners. Although until now they have only been used in expensive, premium "green" panels, soy protein's relatively low price and easy availability as a byproduct offer significant potential for future use. The sector benefits from their ease of use, environmental friendliness, and lower production costs due to low-temperature curing, while low biological stability, short shelf life, and insufficient hydrolytic stability present major disadvantages. Starch-based adhesives offer many advantages for solid wood and plywood industries due to their easy processing, low cost, and low formaldehyde emissions. However, the low reactivity, poor adhesion strength, poor storage stability, and weak water resistance of starch-based adhesives complicate their use in industrial panel applications. This requires appropriate modifications together with cross-linking to achieve the desired bonding strength. To date, economically suitable bio-based cross-linkers are not available on the market, and starch-based adhesive use requires synthetic cross-linkers such as isocyanates and epoxies. Bio-based adhesives typically require the cross-linker itself to be bio-based; however, current technology cannot provide this economically. For other bio-based adhesives, common problems identified based on all research conducted to date are long curing times due to low reactivity, weak mechanical properties due to poor adhesive performance, and the lack of economically viable cross-linkers to improve panel hydrolytic stability. Among potential synthetic alternative cross-linkers to formaldehyde, isocyanates emerge as the most popular alternative for bio-based applications approaching commercialization. However, their high cost and the need for production modifications for safety reasons in Europe represent the greatest obstacles to this technology. When addressing the current limitations of the lignin, tannin, protein, and starch-based adhesives examined throughout this work, they do not compete at the level of synthetic adhesives. To find industrially applicable bio-based solutions, research must focus on developing new reactive cross-linkers that can be used with these products. When sustainable design strategies are considered, formaldehyde as a reactive cross-linker maintains its current advantages. Through its low cost, high performance, enabling reuse of recycled wood, and producing long-lasting and durable panels, formaldehyde contributes to a circular economy. Its continued use in industry should persist without compromising indoor air quality and emission regulations. With advancing technology, ongoing research on formaldehyde production from biological sources and atmospheric carbon dioxide shows promise, and the concept of bio-based formaldehyde is expected to find a place in our daily lives in the future [30].

References

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Berkay Demiralp Research and Development Specialist, Polisan Kimya Research and Development Center

Tolga Kaptı Research and Development Manager, Polisan Kimya Research and Development Center