Wood lignin absorptivity

The native Bjorkman milled wood lignin has a very slight yellow color and a low absorption in the visible part of the spectrum (Figure 2). If quinonemethides are present in the native lignin, as suggested by Harkin (20), the amount of such structures must be exceedingly small. 

Infrared spectra of isolated lignins exhibit an absorption band in the region 1700-1720 cm.-1 While doubt still exists as to precisely which groups in dioxane or milled wood lignins are responsible for this absorption, clearly 16) there is at least some contribution from nonconjugated keto groups in the 0-position of the phenylpropane side chain. 

Recently, 2-hydroxypropyl cellulose (HPC) was demonstrated to be a very suitable matrix for UV/Vis absorption study of photoyellowing of milled-wood lignin in the solid state [10] where clean kinetics of discolouration were established after different chemical treatments of the lignin in solution. This indicates the importance in the discolouration process of phenolic structures without carbonyl groups and conjugated double bonds. The present communication describes a similar photochemical study on quinones and hydroquinones incorporated in HPC films under comparative concentrations ( 6.5xl0"5 mol/g HPC). For comparison, we also use UV/Vis reflectance spectroscopy to examine the behaviour of these compounds adsorbed on filter paper after light-exposure. 

Castellan, A., Nourmamode, A., Fomier de Violet, P., Colombo, N. and Jeager, C., "Photoyellowing of Milled Wood Lignin and Peroxide-Bleached Milled Wood Lignin in Solid 2-hydroxypropyl cellulose after Borohydride Reduction Catalytic Hydrogenation in Solution An UV-Visible Absorption Spectroscopic Study", J. Wood Chem. Technol., 1992,12(1) 1. 

When isolated lignins, such as milled wood lignin and hydrochloric acid lignin, are subjected to the Maule reaction, the colored solutions may be collected by filtration immediately after the reaction and their absorption spectra measured. Typical visible spectra for milled wood lignin often after undergoing the Maule reaction are shown in Fig. 2.1.1. 

Fig. 2.1.1. Absorption spectra of beech milled wood lignin after Maule reaction (1% KMn04 followed by 3% HC1 for 1 min (---), 5 min (-----) and 20min (...)...

Although, the ultraviolet spectroscopy of softwood lignin and guaiacyl model compounds has been thoroughly studied, the same cannot be said about hardwood lignins. It has been shown that the absorptivity of hardwood milled wood lignins at 280 nm decreases linearly with the methoxyl/carbon ratio (Sarkanen et al. 1967), but the effects of side chain structures on the ultraviolet absorption properties of hardwood lignins have yet to be determined. 

Note 5. Insolubles are present when Klason lignin or wood meal is used. After a 40-min ozonation of wood meal, examination of the insoluble residue (in KBr) by FTIR showed the absence of characteristic lignin absorption bands. 

A second derivative of the absorbance improves the resolution and discrimination of individual peaks in a spectrum consisting of several overlapping absorption bands. Lin [27] resolved the spectra of milled-wood lignin and lignosulfonates into... 

Imsgard et al. [62] noted that spruce milled-wood lignin contained, in addition to 0.7 quinone groups, 1% catechol groups. Lignin-like catechols form complexes with ferric ions that have a broad absorption from 470-700 nm with around 560-800 nm and s in the range of 1-2 x 10 L mol cm. ... 

Lundquist et al. [162,165,166] reported that the peak shapes and positions in emission spectra of spruce milled-wood lignin in dioxane/water were independent of excitation wavelength. The emission maximnm occurred at 358 nm. The excitation spec-trnm was consistent with the absorption spectrum of milled-wood lignin, indicating... 

H Norrstrom and A Teder. Absorption Bands in Electronic Spectra of Lignins. Part V. Milled-Wood Lignins. Svensk Papperstidn. 76 458 62, 1973. 

In solution, lignin is most conveniently analyzed qualitatively and quantitatively by uv spectroscopy. Typical absorptivity values D at 280 nm for milled wood lignins MWL(s) and other types of lignins are listed in Table 4. These values are used for quantitative determination of the lignins in suitable solvents. 

Particle size distribution analysis was performed using a laser diffractometer (Mastersizer 2000, Malvern Instmments). The film samples (com starch, fir tree needles, and beech wood lignin, respectively) were diluted in water (concentration of 0.05 %) at 2000 rpm until an obscuration rate of 12.06 % was obtained. Three samples were measured in quintuplicate. The Mie theory was applied by considering the following optical properties R1 for starch of 1.527, and RI for lignocel-lulose of 1.36, respectively, and absorption of 0.001. 

---