Wood adhesives play a crucial role in the engineered wood industry, and their development and production standards are essential to ensuring the quality of engineered wood products. Currently, the wood-based panel industry primarily relies on “three aldehyde” adhesives—urea formaldehyde resin, phenol-formaldehyde resin, and melamine-formaldehyde resin—among which urea-formaldehyde resin and its modified products account for more than 70% of total adhesive consumption in the wood industry.
Urea formaldehyde resin offers many advantages, such as low cost, rapid curing, high panel bonding strength, a simple production process, abundant and readily available raw materials, and ease of use. However, it cannot be overlooked that the biggest concern with urea-formaldehyde resin at present is its high formaldehyde emission.
Formaldehyde is a colorless, highly volatile gas with a pungent odor. It is a highly toxic gas with carcinogenic and teratogenic properties. Engineered wood panels are primarily used in furniture and interior decoration materials; during their use, they continuously release free formaldehyde, leading to indoor formaldehyde levels exceeding safety standards. Therefore, it is imperative to modify urea-formaldehyde resins to produce adhesives with low or non-toxicity, high bonding strength, and good water resistance.
Adding additives to adhesives is a simple and effective modification method that can increase the resin’s initial tack, improve panel pre-pressability, prevent glue bleed-through, reduce resin shrinkage, and lower free formaldehyde content.
In industrial production, when using urea-formaldehyde resins to manufacture plywood, wheat flour is typically added as a filler in the adhesive, generally at a rate of 20% to 30%. According to incomplete statistics, over 2 million tons of wheat flour are used annually in plywood production, resulting in a significant waste of China’s edible resources.
Consequently, researchers have been actively seeking low-cost alternatives to wheat flour in the wood adhesive industry, including tea waste, bark powder, corn cobs, wood fiber, bamboo powder, and inorganic additives. The emergence of these new eco-friendly additives will help promote the green development of the engineered wood and wood products industry.
Montmorillonite is a naturally occurring, hydrated silicoaluminate nanoclay mineral with a unique layered structure. It is currently the most abundant, inexpensive, and readily available source of nano-additives.
Modification of urea formaldehyde resin adhesives with montmorillonite can be categorized into two methods based on the order and manner of addition: during urea-formaldehyde resin synthesis and during adhesive formulation.
When montmorillonite is added during the first stage of resin synthesis (the addition stage or hydroxymethylation stage), the reaction system is at a lower reaction state, allowing the montmorillonite to achieve a high degree of dispersion. Good dispersion enables full utilization of montmorillonite’s nanoscale layered structure and high specific surface area, thereby ensuring excellent physical adsorption properties. Additionally, the tetrahedral and octahedral crystal forms of montmorillonite exhibit isomorphous substitution (i.e., they readily undergo ion-exchange reactions), giving it ion-adsorption characteristics. Therefore, the addition of montmorillonite can effectively reduce the free formaldehyde content in urea-formaldehyde resins.
Research has found that introducing organic montmorillonite during the first stage of urea-formaldehyde resin synthesis (the addition reaction stage) yields relatively better formaldehyde-reducing effects for the modified urea-formaldehyde resin adhesive; introducing it during the second stage (the condensation stage) yields relatively better reinforcing effects for the modified urea-formaldehyde resin adhesive.
When montmorillonite is added during the formulation of urea-formaldehyde resin adhesives, raw montmorillonite does not significantly adsorb free formaldehyde in the resin. Organically modified montmorillonite, however, increases the interlayer spacing, leading to exfoliation. This results in uniform nanoscale dispersion of montmorillonite within the resin. Due to its large surface area, it exhibits superior adsorption capacity, thereby reducing the free formaldehyde content in the urea-formaldehyde resin. Furthermore, organically modified montmorillonite interacts with the active groups in the urea-formaldehyde resin, resulting in a more robust molecular structure in the cured adhesive and reducing the release of formaldehyde during resin decomposition.
Bentonite is an aluminosilicate mineral composed primarily of montmorillonite. It possesses strong hygroscopicity, swelling properties, and ion exchange capacity, and exhibits adsorption capabilities for gases and liquids.
Calcium-based and sodium-based bentonites are more alkaline than organic bentonites, leading to prolonged curing times for adhesives; water absorbed by bentonite exists in the form of crystalline water, and simultaneously, its siloxy and magnesium-oxygen bonds can form associative interactions with residual hydroxymethyl groups in the resin, thus the addition of bentonite improves the water resistance of bonded products.
Although bentonite shares some properties with flour, such as water absorption, swelling, thickening, and binding, adding bentonite as a filler directly during adhesive formulation does not guarantee good bonding performance. The main reason is that calcium-based bentonite has poor water absorption and a thickening effect inferior to that of flour; using it as a substitute for flour often requires double the amount, and excessive addition of inorganic fillers can cause sedimentation and stratification of the adhesive solution when left to stand.
Although sodium-based bentonite has strong water absorption and a thickening effect twice that of flour, the adhesive layer loses water rapidly after application, leading to an excessively dry film. This results in poor pre-pressing performance during plywood production and failure to bond during hot pressing. To assess the thickening and water-retention capabilities of bentonite, the experiment used a 1:1 mass ratio mixture of calcium- and sodium-based bentonite to modify a urea-formaldehyde resin. This effectively controls the rate of water loss from the adhesive film after application and, while maintaining bonding strength, can replace at least 50% of the wheat flour.
Attapulgite is a crystalline hydrated magnesium-aluminum silicate clay mineral with a chain-layered structure. Its rod-shaped crystals constitute a natural one-dimensional nanomaterial, featuring a large number of honeycomb-like micro- and mesoporous structures internally. The unique zeolite channel structure and large specific surface area of attapulgite confer excellent adsorption properties, while its special crystal structure provides good reinforcing properties.
When organically modified attapulgite is added during the condensation stage of urea-formaldehyde resin synthesis, the wet shear strength of plywood doubles at an addition level of 7.5% (the ratio of attapulgite to the total mass of pure formaldehyde and urea).
The purity and particle size of attapulgite significantly affect the formaldehyde emission from urea-formaldehyde resin adhesives. Unpurified raw attapulgite, due to its high impurity content, lacks excellent adsorption properties and is less effective at reducing formaldehyde than flour; regardless of whether the attapulgite is purified, the smaller the particle size, the larger the specific surface area, the stronger the adsorption capacity, and the more pronounced the formaldehyde-reducing effect.
Sepiolite is a fibrous, hydrated magnesium silicate clay mineral with zeolite-like channels running through its entire layered structure. With a specific surface area of up to 900 m²/g, it is the clay mineral with the largest specific surface area and the strongest adsorption capacity among non-metallic minerals. When sepiolite was used to partially replace wheat flour as a filler to modify urea-formaldehyde resin, the results showed that sepiolite’s unique pore structure provided a pathway for formaldehyde release, resulting in higher formaldehyde release levels measured after 24 hours. However, the formaldehyde release levels measured after 30 days of storage were significantly lower than those observed with wheat flour as the filler.
The addition of sepiolite provides a pathway for formaldehyde release from the urea-formaldehyde resin, thereby shortening the formaldehyde release cycle in the bonded products and reducing formaldehyde emissions during their subsequent use. However, when sepiolite completely replaces wheat flour, the urea-formaldehyde adhesive exhibits poor pre-compression properties and fails to meet requirements.
Research has found that sepiolite can replace up to 80% of the wheat flour, and at this substitution level, the strength of the resulting plywood increases by 39% compared to that of plywood using only wheat flour as a filler.
The addition of sepiolite not only conserves edible resources but also reduces the manufacturing costs of engineered wood products.
Zeolite is a hydrated aluminosilicate mineral with a framework structure formed by silicon-oxygen and aluminum-oxygen tetrahedra. This framework structure creates numerous cavities and channels within the molecular structure, offering advantages such as strong adsorption capacity, regenerability, low cost, and good high-temperature stability.
Research indicates that zeolite enhances fiber interlacing in fiberboard. The addition of zeolite increases the internal bond strength of medium-density fiberboard (MDF) produced with urea-formaldehyde resin by 163%. Furthermore, the microporous structure of zeolite fills the voids in the veneer, making it difficult for water molecules to adsorb onto the wood fibers, thereby reducing the water-induced thickness swelling rate and water absorption rate of the fiberboard.
Diatomaceous earth consists primarily of silicon dioxide and possesses a unique natural porous structure and a large specific surface area, giving it strong adsorption capacity. Research has found that when urea-formaldehyde resin is used as an adhesive in particleboard production, adding diatomaceous earth directly does not yield a significant formaldehyde-reducing effect.
This is because the main component of diatomaceous earth is inert quartz, which cannot undergo effective chemical reactions with formaldehyde; it can only physically adsorb formaldehyde, and its adsorption capacity is limited. However, by utilizing the unique nanomesostructured diatomaceous earth and loading it with 3% urea, the synergistic effect of the two in reducing formaldehyde is significant, resulting in a 45% reduction in formaldehyde emissions from the produced particleboard.
Nano-titanium dioxide possesses photocatalytic properties; when exposed to ultraviolet or visible light, it can catalyze the degradation of toxic gases in the air. It can oxidize and decompose organic macromolecules into smaller molecules such as carbon dioxide and water, and it can effectively sterilize by decomposing bacterial toxins. Therefore, applying nano-titanium dioxide to catalyze the degradation of formaldehyde is a very promising approach.
Under light exposure, electrons in the valence band of nanoscale titanium dioxide are excited and transition to the conduction band. This creates a shortage of electrons in the valence band, generating highly oxidative photogenerated holes. These holes can decompose the reducing free formaldehyde in the resin into carbon dioxide and water, and the efficiency of this decomposition is higher under ultraviolet light; However, the electron-hole pairs generated by nano-titanium dioxide react with formaldehyde to produce hydrophilic hydroxyl radicals, which adversely affect the water resistance of the plywood.
Nano-silica exhibits a flocculent and reticulated microstructure, featuring high surface activity and a high specific surface area, thereby possessing a strong ability to adsorb free formaldehyde; the surface of nano-silica contains a large number of highly reactive unsaturated bonds that can react with active groups in urea-formaldehyde resin, thereby enhancing the bonding strength of urea-formaldehyde resin adhesives.
When nano-silica is added during the first stage of resin synthesis (the addition stage), it participates in the resin’s condensation reaction. This results in insufficient cross-linking of the resin, leading to increased free formaldehyde and a simultaneous decrease in bonding strength. When nano-silica is added at a later stage of urea-formaldehyde resin synthesis (after the endpoint), it reacts with free formaldehyde and some active groups in the resin, thereby enhancing the cohesion of the cross-linked system.
This ensures increased plywood bonding strength while simultaneously reducing formaldehyde emissions. However, adding nanoparticles during the high-temperature stage of the reaction increases the likelihood of agglomeration, preventing the full utilization of silica’s nanoscale properties and resulting in poor resin modification.
During the resin adhesive formulation stage, the addition of nano-silica via intermittent ultrasonic agitation resulted in the lowest free formaldehyde content and the highest bonding strength in the modified adhesive. XPS analysis further confirmed the presence of chemical bonds between nanosilica and the urea-formaldehyde resin. Consequently, at a resin molar ratio of 1.05, the addition of 1.5% (by mass) of nano-silica doubled the bonding strength and effectively reduced the content of free formaldehyde.
Carbonates: Inorganic carbonate additives primarily consist of nanoscale calcium carbonate and modified calcium carbonate. The nanoscale nature of these materials confers strong adsorption capacity on nanoscale calcium carbonate. Studies indicate that the free formaldehyde content in urea-formaldehyde resins decreases as the amount of nanoscale calcium carbonate added increases; the higher the molar ratio, the faster the decrease.
The addition of nano-calcium carbonate prolongs curing times for urea-formaldehyde resins; the lower the molar ratio, the longer the curing time, and, in some cases, the resin may fail to cure at all. When the nano-calcium carbonate content reaches 10%, the adhesive viscosity becomes very high, making application difficult. For resins with low molar ratios, the nano-calcium carbonate content must be controlled below 5% by mass.
Rare Earth Elements: Rare earth elements exhibit strong chemical reactivity, numerous energy-level transitions, and excellent interfacial properties. They can enhance the heat resistance and mechanical properties of composite materials and improve interfacial performance, making them widely used in steel, alloys, and composite materials.
Research has shown that when lanthanum oxide is added during the addition and condensation stages, it primarily acts as a catalyst, promoting more complete resin synthesis reactions and thereby increasing the resin solids content. However, when added during the late condensation stage—where reaction temperatures are relatively low—the rare earth primarily functions to capture free formaldehyde, yielding ideal modification results in reducing the resin’s free formaldehyde content.
1) Modifying urea formaldehyde resins with inorganic additives is a relatively simple and effective method. It not only ensures the adhesive’s bonding performance but also reduces formaldehyde emissions. At the same time, it enhances the product’s value by imparting new functionalities to bonded products, such as flame-retardant and wear-resistant properties. Therefore, inorganic additives hold significant application potential in the wood adhesive industry.
2) Due to their unique nano-effects and large specific surface area, nanoscale inorganic additives can enhance the hardness and wear resistance of composite materials while simultaneously providing reinforcement and toughening effects. However, the smaller the additive particles, the more difficult they are to disperse; therefore, further exploration and improvement of dispersion processes for inorganic additives are of great significance for their application in wood adhesives.
3) To ensure good interfacial bonding between inorganic phase additives and the resin matrix, the resin liquid must fully wet the surface of the inorganic additives. Therefore, researching surface modification techniques for inorganic additives to alter their surface tension and enhance interfacial bonding strength between the inorganic and organic phases is of great significance, aiming to obtain composite materials with superior overall performance.
4) Although there are many types of inorganic additives for urea-formaldehyde resins, they generally have a negative impact on the pre-compression properties of the resins. The addition of nanomaterials often increases the cost of the adhesives; therefore, there remains a certain gap between research findings and practical industrial applications.
5) As essential materials for furniture, flooring, and interior decoration, engineered wood panels are indispensable in national production and daily life. With improvements in living standards and growing environmental awareness, the green development and transformation of the engineered wood panel and products industry have become inevitable. Consequently, the modification of traditional urea-formaldehyde resins to reduce toxicity, as well as the research, development, and widespread application of formaldehyde-free adhesives, will become the primary development directions for adhesives in the engineered wood panel industry.
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