The Potential of Visible Spectroscopy as a Tool for the In-Line Monitoring of Lignin Methylolation

Out of the 50 to 70 million tons of lignin that are produced annually, only 1 to 2% are used for value-added products. Currently, 90% of the total market of this compound corresponds to lignosulphonates (LS). The most successful industrial attempts to use lignin for wood adhesives rely on using it as a partial substitute in phenol-formaldehyde or urea-formaldehyde resins. However, lignin's aromatic ring presents a low number of reactive sites. Several methods have been proposed to improve its reactivity, such as prior methylolation with formaldehyde. Off-line methods are commonly applied to monitor this reaction's progress, but this introduces a significant delay in the analysis. This study proposes a new method for in-line monitoring of the methylolation reaction using visible spectroscopy. In order to monitor the reaction progress, principal component analysis was applied to the spectra, and the obtained scores were analyzed. When these results were plotted against those obtained by the off-line methods, a satisfactory regression was obtained at 50 degrees C (R-2 = 0.97) and 60 degrees C (R-2 = 0.98) for two different LS samples. Therefore, it was concluded that visible spectroscopy is a promising technique for studying lignin methylolation.

Automated summary

Out of the total annual production of 50 to 70 million tons of lignin, only 1 to 2% are utilized for value-added products. The majority of the market, 90%, is occupied by lignosulphonates (LS). Lignin is commonly used as a partial substitute in phenol-formaldehyde or urea-formaldehyde resins for wood adhesives, but its low reactivity due to a low number of reactive sites in the aromatic ring limits its applications. This study proposes the use of visible spectroscopy as a promising technique for monitoring the methylolation reaction of lignin in-line, providing a new method to study lignin methylolation.