Degradation of wood constituents during the self-agglomeration of an oak sawdust. II. Infrared study of two lignins

Abstract

Structural changes in lignins isolated from particle-boards are studied by infrared spectrophotometry, in relation to an endothermic trend which characterizes the self-agglomeration process.Changes in intensity of various infrared bands are reported and are attributed to the following: (a) variation in the nature and distribution of substituents of the aromatic ring and conjugation of the aromatic ring with carbonyl C=O or C=C, which indicate an increasing state of condensation of the lignin, (b) simultaneous division of the macromolecule of lignin, partially caused by breakage of ether linkages between its lateral chains, (c) cleavage of bonds between lignin and carbohydrate material, and (d) insolubilization, with the acid lignin, of an increasing amount of carbohydrate material which is also increasingly condensed.The origin of the endothermic trend cannot be explained in terms of these reactions of lignin.Canadian Journal of Chemistry, 46, 1893 (1968) Introduction The self-agglomeration of woody material to form particle-board can be realized under certain conditions of pressure, temperature, and moisture content (1). A strong endothermic trend accompanies this process and its appearance and development characterize the properties of the resultant product.In an earlier paper (2), the chemical alterations of wood constituents of oak sawdust (Quercus pedunculata Ehrh.) during such self-agglomeration were reported. This paper is a continuation of that work and is concerned with the study by infrared (i.r.) spectrophotometry of structural changes in lignin during this process.

Characterization of aspen exploded wood lignin

“…In one case (spectrum A) two bands at 1710 cm-I and 1745 cm-' were observed while the latter band was very weak or not observed in the other (B). These bands are assigned to unconjugated carbonyl groups (11,12). Saponification of EXWL with 0.1 N NaOH for 24 hat room temperature decreased the intensities of these bands but significant absorptions remained at these frequencies (C).…”

“…This indicates the presence of unconjugated ketone and/or aldehyde groups as well as carboxyl ester groups in aspen EXWL. In the spectra of lignin preparations including MWL and solvolysis lignins, the bands observed at 1710-1750 cm-' are attributed to unconjugated carboxyl ester groups (6,13) or unconjugated aliphaticor P-ketone groups (12,(14)(15)(16)(17)(18). It should be noted that in the reported lignin spectra the bands due to conjugated carbonyl groups are observed at 1660-1670cm-' whereas only background absorption was detected in this region of the spectrum of EXWL.…”