Discoloration of α-Carbonyl-Free Lignin Model Compounds Under UV Light Exposure

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Abstract
A series of α-carbonyl-free lignin model compounds was prepared and adsorbed onto bleached chemical pulp. The rates of discoloration of these materials, under simulated solar light exposure, were established and compared to that of peroxide bleached methylated (alkali and dimethyl sulfate) and nonmethylated chemithermomechanical pulps. Phenolic phenylcoumarone and phenolic stilbenes were found to be the most sensitive chromophores under UV irradiation, both inducing a strong yellowing. Also, monophenolic biphenyl and biphenylmethane entities were shown to be prone to discolouration in contrast to the biphenolic and dimethylated ones. Moreover, the catechol structures which are known to be easily oxidizable into ortho quinones, display significant yellowing only when their absorption spectra are shifted above 300 nm where the light source emits. The behaviour of the phenolic stilbenes and the biphenyl catechol is reminiscent of the behaviour of CTMP. This indicates the possible involvement of such structures as being mainly responsible for the discoloration of lignocellulosic materials under UV irradiation. A mechanism is proposed to account for the photoyellowing of the models involving a cyclohexadienone hydroperoxide as intermediate, which is formed by UV irradiation from a complex between the phenol and ground state oxygen.