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Handbook of Size Exclusion Chromatography and Related ...

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13<br />

Molar Mass <strong>and</strong> <strong>Size</strong><br />

Distribution <strong>of</strong> Lignins<br />

Bo Hortling, Eila Turunen, <strong>and</strong> Päivi Kokkonen<br />

KCL<br />

Espoo, Finl<strong>and</strong><br />

1 INTRODUCTION<br />

Lignin is a heterogenous material with respect both to chemical structure <strong>and</strong><br />

molecular size (1–3). The structure <strong>of</strong> lignin varies with its origin, both according<br />

to species, site in the tree, <strong>and</strong> site in the cell wall. Also, the way <strong>of</strong> isolating lignin<br />

from the raw material affects its molar mass distribution (MMD).<br />

S<strong>of</strong>t wood lignins are mostly built <strong>of</strong> guaiacyl units together with small<br />

amounts <strong>of</strong> p-hydroxyphenyl units, which are enriched in compression wood. The<br />

lignins from hardwoods contain about equal amounts <strong>of</strong> guaiacyl <strong>and</strong> syringyl<br />

units <strong>and</strong> a lower content <strong>of</strong> p-hydroxyphenyl units. Lignins from grasses contain<br />

guaiacyl, syringyl <strong>and</strong> p-hydroxyphenyl units. The main linkage between the<br />

different units in native lignins is the b O 4 linkage.<br />

The native lignins are mainly isolated by milling dry wood (grass) powder in<br />

a vibrational ball mill after which the isolation is performed by extraction using<br />

dioxane–water <strong>and</strong> purification. By performing enzymatic hydrolysis <strong>of</strong> cellulose<br />

<strong>and</strong> hemicelluloses in ball-milled wood (grass) powder enzymatic lignin (EHL) is<br />

obtained. Technical lignins are isolated from pulps or spent liquors obtained<br />

during cooking <strong>and</strong> bleaching processes <strong>and</strong> during other technical treatments <strong>of</strong><br />

© 2004 by Marcel Dekker, Inc.

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