Evaluating efficacy of different UV-stabilizers/absorbers in reducing UV-degradation of lignin

Saeid Nikafshar,Mojgan Nejad

doi:10.1515/hf-2021-0147

Abstract

Abstract Susceptibility of wood to UV degradation decreases the service life of wood products outdoors. Organic UV absorbers (UVAs) and hindered amine light stabilizers (HALSs), as well as inorganic UVAs, are added to coatings to improve the UV stability of coated-wood products. Although about 85% of UV radiation is absorbed by lignin in the wood, it is unclear which UV stabilizers can minimize lignin degradation. In this study, the photodegradation of softwood organosolv lignin was monitored over 35 days of UV exposure. Changes in lignin properties were assessed using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR). It was found that the aromatic rings of lignin underwent significant degradation, resulting in increased glass transition temperature and molecular weight of lignin. Subsequently, 18 different additives were mixed with lignin and exposed to UV irradiation. The analysis of samples before and after UV exposure with FTIR revealed that inorganic UVAs (cerium oxide and zinc oxide) and a mixture of organic UVAs and HALSs (T-479/T-292, T-5248, and T-5333) were the most effective additives in reducing lignin degradation. This study can help coating scientists to formulate more durable transparent exterior wood coatings.

Highlights

Wood products used outdoors are susceptible to photodegradation (Evans 2012), which usually causes yellowing, discoloration, loss of gloss, increased roughness, and diminished mechanical and physical properties (Hon 1984; Nejad and Cooper 2011; Turkulin and Sell 1997)
The analysis of samples before and after UV exposure with Fourier transform infrared spectroscopy (FTIR) revealed that inorganic UV absorbers (UVAs) and a mixture of organic UVAs and hindered amine light stabilizers (HALSs) (T-479/T-292, T-5248, and T-5333) were the most effective additives in reducing lignin degradation
To confirm that the lignin structure was not changed after dissolving it in THF and solvent was entirely removed, the sample was analyzed with 31 nuclear magnetic resonance spectroscopy (31P NMR)

Summary

Introduction

Wood products used outdoors are susceptible to photodegradation (Evans 2012), which usually causes yellowing, discoloration, loss of gloss, increased roughness, and diminished mechanical and physical properties (Hon 1984; Nejad and Cooper 2011; Turkulin and Sell 1997). Wood is mainly composed of three compounds: cellulose, hemicellulose, and lignin; these compounds have different sensitivities to UV light during photodegradation. Cellulose and hemicellulose only absorb 5–20% of UV light, while lignin absorbs about 80–95% of the UV light due to the presence of chromophores and aromatic rings, making it more prone to decomposition by photooxidation reactions (Davidson 1996; Hayoz et al 2003). Free phenolic radicals are generated immediately under UV irradiation. This way, the radicals delocalization favors the formation of o- and p-quinonoid structures after demethylation and cleavage of the side chain (Hon and Shiraishi 2000). As shown in Scheme 1, the newly formed carbonyl groups in o- and p-quinonoid are considered chromophoric groups that cause significant color changes on the wood surface (Hon and Shiraishi 2000)

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