![]() ![]() 16 In the phenolation process, the number of phenolic groups is increased using phenol in acidic media. 13- 29 Demethylation is used to modify the chemical structure of lignin by removing one or two methoxyl groups from ortho positions in lignin structure, thus, increasing the number of reactive sites in lignin for reaction with formaldehyde. Many researchers have used different modification techniques to improve the low reactivity of lignin toward formaldehyde such as: demethylation, phenolation, and methylolation. 7 In addition to the cost benefit of replacing phenol with lignin, they reported that application of lignin reduced dust in manufacturing facilities and improved environmental conditions on the site. 4- 12 At the commercial scale, in 1990 an OSB producer in North America was the first that blended 5% to 25% organosolv lignin with phenolic resin (powder) and reported no negative impact on properties of the produced OSB panels. 4, 5 However, up to now, to the best of our knowledge, phenol has been only replaced partially up to 50% when unmodified lignin was used, and addition of more than 50% lignin has been reported to have negative impact on the performance of prepared wood composites. Lignin as the most abundant natural aromatic polymer in the world, and with the back bone of phenyl propane units has exceptional capability to replace petroleum-based phenol in PF resin. 4 Fluctuations in price of phenol as a result of changes in oil price and concerns about chronic exposure of workers to phenol during the manufacturing process are the main reasons why over the last three decades many researchers have studied replacing phenol with biobased materials. Formaldehyde is made of methanol and phenol from benzene using the cumene process. 3, 4 PF resin is currently produced through condensation polymerization of petroleum derived compounds. 2 Accounting for more than 54% of worldwide consumption, North America is the largest market for PF resin in wood adhesive applications. In contrast, if the molar ratio of F/ P is less than 1 and the reaction between phenol and formaldehyde occurs under acidic condition, novolacs will be formed. If formaldehyde reacts with phenol under basic condition and the molar ratio of F/ P is more than 1, the production will be resols. 1 Depending on the molar ratio of formaldehyde to phenol ( F/ P) and the type of catalyst used in the reaction (acid or base), the final resin is either resols or novolacs. PF adhesives are known to have exceptional performance such as: excellent moisture resistance, thermal resistance, chemical durability, and bonding strength. Phenol formaldehyde (PF) is the most commonly used adhesive in the production of exterior grade wood products like plywood, oriented strand board (OSB), laminated veneer lumber (LVL), and other engineered wood products. According to two-way ANOVA results, statistically there was no significant difference between the shear strengths of plywood samples made with 100% lignin-based adhesive and those made with the commercial PRF resin. The plywood samples were pressed under exactly the same conditions (time, temperature, and pressure) as what recommended for the commercial PRF formulation. The developed 100% lignin-based adhesive and a commercially formulated phenol resorcinol formaldehyde (PRF, as reference) were used to prepare single-lap-joint samples for mechanical testing. Chemical, physical, and thermal properties of the isolated lignin, PF resin and adhesive were measured using advanced analytical techniques such as Fourier transformed infrared spectroscopy (FTIR), size exclusion chromatography (SEC), phosphorous nuclear magnetic resonance spectroscopy ( 31P NMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The PF resol resin was formulated using isolated lignin under alkaline condition. In this study, the phenol portion of phenol formaldehyde (PF) resin has been replaced by an agricultural-based lignin, which was produced as a byproduct of a cellulosic bioethanol process through dilute-acid pretreatment and enzymatic hydrolysis from corn stover. Lignin, produced as a byproduct of pulp and paper and bioethanol industries, is a polyphenolic compound that has excellent potential to be used as phenol replacement in phenolic adhesive formulation. ![]()
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