Soluble Lignins

The absorbance at either 205 or 280 nm is the basis of several techniques for the quantitative determination of lignin. The extinction coefficient depends on species and solvent, and varies from 10 to about 26 L g cm. Summaries of extinction coefficients are available in reviews by Dence [18], Lin [2], and Fengel and Wegener [19]. Lin [2] has provided guidelines for measurement of lignin absorption spectra, while Dence [18] provides instructions for determination of acid-soluble lignin and lignin solubilized by the acetyl bromide method. 

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 

7-13 second derivative peaks between 200 and 300 nm. The technique was unable to resolve any maxima in the tailing absorption beyond 300 nm. The position and intensity of the second derivative peaks differ significantly from the peaks reported by the modeling procedures. 

Difference spectroscopy has been useful in interpreting the UV-visible spectra of lignins. In this technique, the absorption spectrum measured before a given chemical treatment is subtracted from that recorded after the treatment (or vice versa). In the simplest case, a compound, C, is transformed to a product P. At any time during the reaction, these quantities are related by mass balance. Equation 3.4, where Cq is the initial concentration of C. The difference in 

Absorption difference spectra have been used to estimate the content of phenolic groups, carbonyl groups, ethylenic double bonds, noncondensed phenolic groups and phenylcoumarans in soluble lignins, and to study the photochemistry of lignin solutions. These experiments are summarized briefly in the following paragraphs. 

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Attempts to isolate lignin by degrading wood with brown rot fungi afforded only degraded lignins of low molecular weight (<1000) [cf. 33, 94, 132)] similar to Brauns soluble lignin. This is probably due... 

Where do soluble lignins fit with respect to conformation They seem to be rather compact molecules in solution—the opposite of the highly expanded cellulose molecule. They are not as compact as a simple solid sphere. Yet, the chains of the lignin macromolecules in solution are more densely packed than those of a linear flexible polymer such as polystyrene... 

Before leaving the topic of conformation, it should be noted that there do not appear to be much interest in this property today. In earlier times, several schools included conformational studies in their characterization of soluble lignins (19). Recently, only Pla and his group (20) seem to be taking this property seriously. Yet it is important, both to the understanding of delignification and to the use of lignins as colloids and dispersants. 

The water-insoluble lignin amounted to about one-third, and the hexane-dioxane soluble lignins (which were mostly monomers) to about 20%, of the lignins dissolved. 

MEISTER et AL. Water-Soluble Lignin Graft Copolymers... 

In the sulphite pulping process wood cut into pieces is subjected to kiering in a calcium bisulphite solution under a pressure of 3-8 kg/cm2, at 120-145°C for 7-30 hr. The higher the temperature, the shorter the time of boiling. Substances accompanying cellulose dissolve in the liquor. Lignin is converted into a soluble lignin sulphonic acid. Cellulose is isolated from the solution, known as sulphite cellulose lye, and washed with water. 

Somewhat variable results were obtained with Norway spruce since optimum conditions were not used on all runs. However, the fractions of soluble lignin were roughly as above, although the yields of total soluble lignin were lower. 

Soluble and insoluble lignin were determined with the NREL standard biomass analytical methods for the analysis of acid-insoluble and acid-soluble lignin inbiomass, LAP-003 and LAP-004, respectively (13-14). NREL s laboratory analytical procedure for the determination of carbohydrates in biomass, LAP-002, without correction for hydrolysis losses was performed to determine the sugar content (15). 

Index Entries Reaction kinetics glucose decomposition dilute acid hydrolysis kinetic modeling acid-soluble lignin acid-base catalysis rules. 

Organic Extenders. Organic extenders are primarily of two types (1) fillers derived from organic materials and (2) low-cost, naturally occurring or synthetic resins. Of the first type, wood flour, shell flour, and other cellulosic fillers are the most common. They also provide a margin of mechanical property reinforcement because of their relatively high aspect ratio. Of the resinous types these are petroleum-based derivatives as well as soluble lignin and scrap synthetic resins. 

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