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268
Linkage type
Softwood
Hardwood
β
-O-4
50%
60%
α
-O-4
2-8%
7%
β
-5
9-12%
6%
5-5
10-11%
5%
Little is known about the exact sequence of lignin monomers or the location of
different linkages within the lignin molecule. It is believed that lignin has both
branches and cross-links to form a giant molecule. Covalent linkages with
hemicelluloses found in association with lignin possibly exist.
Lignosulphonates
Lignosulphonates are formed in the sulphite process during production of
cellulose, and lignosufonate is thus a byproduct. The process involves
reaction of wood or pulp in the presence of sulphite (HSO
3
-
) at low pH. The
following reactions occur:
1)
Linkages between lignin monomers are partially hydrolysed (-C-O-
ether linkages only, -C-C- linkages are stable), resulting in smaller
lignin fragments with molecular weights in the range 10.000 – 100.000
Da.
2)
The monomers become sulphonated (average 0.25 sulphonate groups
per monomer). The figure shows sulphonation at the
α
-carbon.
Sulphonic acid (not the same as sulphuric acid) is a
very strong acid (R-SO
3
H
↔
R-SO
3
-
+ H
+
).
Lignosulphonates are therefore polyelectrolytes over a
wide pH-range. The industry produces
lignosulphonates as Na
+
or Ca
++
salts.
Low molecular weight and introduction of charges is the
basis for the good solubility of lignosulphonates in
water. It can therefore easily be removed from cellulose
during its production.
The lignosulphonates are very compact molecules,
similar to globular proteins. This is reflected in very low intrinsic viscosities (3-
10 ml/g) for the M
w
range 10.000 – 400.000 Da.
The Kraft process (‘sulphate process’ or ‘soda process’) is an alternative
method for pulping involving high pH and no or little sulphonation. Kraft lignins
C
C
CH
2
OH
OH
OCH
3
O
SO
3