Thermomechanical Pulp and Light -Photoactivity of α-Carbonyl Group in Solid Lignin

Abstract

Interactionof sunlight and UV light with thermomechanical (TMP) pulps, milled wood lignin (MWL) and various lignin model compounds in solid state was studied. It was found that TMP fibers were sensitive to these lights. During exposure, TMP lost its brightness as a function of irradiation time and increased its yellowness. The a-carbonyl group functioned as a light absorption center to promote photocleavage of C a -C fi and 6-0-4 linkages. Electron spin resonance (ESR) studies demonstrated that free radicals were generated in the interim.Phenoxy radicals appeared to be important radical intermediates that ultimately transformed into oquinonoid structures. INTRODUCTIONRapid development and utilization of thermomechanical (TMP) and chemithermomechanical (CTMP) pulps have attracted considerable attention in the pulp and paper industry in the last twenty years. Much research has been conducted to improve the quality, such as strength and color, of these products. Nonetheless, overcoming discoloration and brightness reversion of TMP and CTMP appear to be a perplexing task. The causes of

Review: Light-Induced Yellowing of Lignocellulosic Pulps – Mechanisms and Preventive Methods

“…Kringstad and Lin 1970;Brunow and Eriksson 1971;Brunow and Sivonen 1975;Francis et al 1991 A special type of carbonyl group in lignin is the group in phenacyl aryl ethers (2-aryloxy-1-arylpropanones), made of lignin units with side-chains carrying both an -carbonyl group and a -O-4 linkage. Phenacyl aryl ethers have been widely studied as lignin model compounds irradiated both in solution and in solid state (Gierer and Lin 1972;Vanucci et al 1988;Castellan et al 1989;Netto-Ferreira et al 1990;Fukagawa and Ishizu 1991;Hon 1992;Argyropoulos and Sun 1996;Parkås et al 2004a;Lanzalunga and Bietti 2000). This type of structure can, when excited, undergo homolytic -ether bond cleavage forming a phenacyl-phenoxyl radical pair (Fig.…”

“…These types of rearrangement reactions have been shown to occur in lignin model compounds irradiated both in solution and in solid state (Fukagawa and Ishizu 1991;Argyropoulos and Sun 1996;Parkås et al 2004a). Gierer and Lin 1972;Vanucci et al 1988;Castellan et al 1989;NettoFerreira et al 1990;Fukagawa and Ishizu 1991;Hon 1992;Argyropoulos and Sun 1996;Parkås et al 2004a Phenacyl aryl ether cleavage can potentially result in a depolymerization of the lignin and the formation of new phenolic groups. Depolymerization (decrease in average molecular weight) of milled wood lignin isolated from softwood mechanical pulp upon irradiation in the presence of oxygen has been reported (Koch et al 1994;Destiné et al 1996;Wang et al 1998).…”

Photoyellowing of chemically modified chemithermomechanical pulps (CTMP) from Eucalyptus globulus under various atmospheres

“…-The phenoxyl pathway is promoted by direct excitation or free radical scavenging by phenolic groups (Castellan et al 1990;Jaeger et al 1993;Li and Ragauskas 1999). -The phenacyl pathway is initiated by direct excitation of carbonyl groups (Gratzl 1985;Vanucci et al 1988;Castellan et al 1989;Hon 1992). -The phenoxyl quinone redox cycle is also possible (Castellan et al 1993;Ragauskas 1993;Lennholm et al 1994).…”