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7.1.
LIGNIN AND
LIGNOSULPHONATES
Lignins are essential components of plant cell walls. In pine and spruce
(softwood) 26-32% of the wood is lignin. In hardwood the lignin content is
about 20-25%, but can exceed 30% in some tropical hardwoods.
Lignin is found as an amorphous, insoluble substance, primarily located in the
middle lamella between the cells, and with a low amount in the secondary cell
wall.
Lignin is a family of polymers based on 3 monomers, all based on phenyl
propane (right). These are:
Important characteristics include phenolic hydroxyl in
para
-position, C=C
double bond between the
α
and
β
carbons, hydroxyl on the
γ
carbon, and
methoxyl groups on carbon 3 and 5.
Coniferyl alcohol (sometimes called guaiacyl type) is the dominant lignin
monomer in softwoods.
p-coumaryl alcohol
HC
CH
CH
2
OH
OH
OCH
3
Coniferyl alcohol
HC
CH
CH
2
OH
OH
OCH
3
Sinapyl alcohol
H
3
CO
HC
CH
CH
2
OH
OH
α
β
γ
1
2
3
4
5
6