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TURHAN ET AL .
• Lower curing temperature offers energy savings in production.
KEYWORDS adhesive, carboxylation, lignocellulose, wood composites, wood modification
1 | INTRODUCTION
operations, complicating production, and increasing costs. 11 Additionally, pMDI has limited effectiveness in filling irregular spaces, which restricts its performance in applications requiring a continuous and uniform bond- line. 12,13 Furthermore, pMDI's low molecular weight con- tributes to excessive penetration into the porous structure of wood substrates. While some degree of penetration is necessary to achieve mechanical interlocking, over- penetration leaves insufficient adhesive at the bondline, resulting in a weak and discontinuous interface com- pared with UF-resin systems. This reduces the strength and durability of plywood and laminated veneer lumber, particularly in applications that demand uniform adhe- sive coverage. 13 Addressing these issues often requires complex modifications, such as using organosolvent, 14 fillers or extenders 12,15 to the adhesive or processing methods, which can increase both costs and operational complexity. 6,12,16 – 21 Thus, UF resins, despite formalde- hyde emissions and the presence of less reactive groups, remain a cost-effective and efficient option for achieving consistent fiber bonding in wood composites. To address the weak bonding between urea-formal- dehyde and wood fibers, wood modification techniques have emerged as a promising approach to improve the bonding of lignocellulosic fibers with adhesive, thus sig- nificantly reducing the use of formaldehyde. Various chemical modification techniques, including etherifica- tion, oxidation, esterification, and carboxymethylation, have been demonstrated to oxidize hydroxyl groups from cellulose derived from diverse biomass sources. 22,23 Among other functional groups, carboxylic acid groups enhance self-binding and fiber interactions, mak- ing composites more resilient. The hydrophilic surface of modified wood fibers improves the curing process with urea-formaldehyde adhesives, enhancing wetting and promoting uniform adhesive distribution. This leads to stronger intermolecular forces between fibers and adhe- sives, improving chemical compatibility. 24 Importantly, the modified hydrophilic surface provides greater stabil- ity without the need for extra crosslinking agents com- pared with the hydrophobic surfaces due to superior mechanical interlocks, molecular-level interactions, and secondary force interactions. 25 However, typical pretreat- ment methods to create the carboxylic acid groups involve solvent treatment, alkali pretreatment, and bleaching of the wood chips or fibers. The tetramethyl
Wood panels have gained widespread recognition for their versatility, affordability, and eco-friendly nature. Over the years, significant efforts have been made to enhance their strength and resilience to harsh environ- mental conditions by improving both the mechanical and chemical aspects of their performance by using protective treatments, including biocides, coatings, or water repel- lents. While these treatments improve the lifespan of wood products, they face some environmental challenges because of toxic chemcials. 1 Thus, there have been regu- lations against biocidal systems to remove and ban them from the European Market based on the Biocidal Product Regulation. 2 Moreover, the development of wood prod- ucts, including particleboard, medium-density fiberboard (MDF), and plywood, depends highly on the use of formaldehyde-based adhesives due to their high reactiv- ity, chemical adaptability, and cost-effectiveness. 3,4 Form- aldehyde is a volatile organic compound (VOC) and can be slowly released into the ambient from adhesives, rais- ing concerns due to its potential adverse effects on indoor air quality and human health. Strict regulations, like European E emission classes, US CARB standards, and Japanese F*** and F**** classes, 5,6 have been enforced to limit formaldehyde emissions; therefore, developing new formulations with low-emission or completely formalde- hyde-free and environmentally friendly solutions to pro- tect wood without sacrificing the mechanical properties and dimensional stability of the wood products is urgently needed. 5,7 Meanwhile, polymer adhesives like diphenylmethane diisocyanate (pMDI), a formaldehyde- free adhesive, have gained increasing attention due to their exceptional reactivity, curing characteristics, water resistance, and bonding properties. However, challenges with pMDI are not related to its performance but rather to its practical application and cost. This cost disparity 8 coupled with its potential health risks to humans makes pMDI less attractive to industries prioritizing cost- efficiency, despite its superior properties and environ- mental benefits. 9,10 The primary factor contributing to these challenges is the high reactivity of the isocyanate groups in pMDI compared with the UF resins. While this reactivity is beneficial for rapid curing, it creates signifi- cant issues during the manufacturing process, such as a tendency to stick to press surfaces during hot-pressing
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