Lig-L2nd

[mmol g–1]

Aliphatic OH 1.56 3.52 4.14 3.87 1.87 2.66

Condensed phenolic

OH

1.02 0.77 0.52 0.55 0.91 1.10

Guaiacyl phenolic

OH

0.92 0.55 0.32 0.30 0.57 0.60

p -hydroxyphenolic

OH

– 0.09 0.06 0.07 0.13 0.22

Carboxyl OH 0.43 0.68 0.30 0.32 0.69 1.27

Carbohydrate (%) 0.9 10.4 9.8 9.5 4.8 4.2

RL: residual lignin; 1st: first stage; 2nd: second stage; 3rd: third

stage of oxygen delignification, Lig-L1st: lignin from liquor after

first stage of oxygen delignification.

636 7Pulp Bleaching

The number of aliphatic saturated methylene and methyl groups increases significantly

in residual and dissolved lignin [90], followed by a further increase during

oxygen bleaching [105].

Investigations into the effect of side-chain constituents on the reactivity of

model compounds to oxygen [90,100,102] revealed that structures containing

methylene or methyl groups are quite reactive, followed by alcohols, carboxylic

acids, aldehydes and ketones, whereas the latter three groups of compounds are

only slightly reactive under oxygen bleaching conditions [90].

The frequency of linkages in kraft residual lignin (see Tab. 7.7) and the reactivity

of model compounds containing these linkages under oxygen bleaching conditions

(see Tab. 7.9) are presented. Though the exact nature of each linkage in kraft

residual lignin has yet to be fully determined [90], and the importance of each

linkage to the susceptibility of lignin to degradation by oxygen is not clear [90].

The different numbers of linkages found by researchers are mainly due to the

different procedures used.

The degradation acids found in kraft pulp lignins before and after oxygen

delignification presented in Tab. 7.6 show slightly higher amounts of condensed

lignin structures of the 5,5′ type. A similar tendency can be seen when comparing

the lignin dissolving late in the kraft cook [61]. This occurrence was also reported

by Fu and Lucia [80], who concluded that p -hydroxyphenyl and 5,5′ biphenolic

units are quite stable and tend to accumulate during oxygen delignification. Additionally,

these authors identified oxalic acid and succinic acid [80].

Tab. 7.6 Relative frequencies of degradation acids obtained from

oxidative degradation with permanganate of various pulps and

lignins (number per 100 aromatic units) (from Ref. [61]).

OH

OCH3

COOH

OH

COOH

OH

OCH3

COOH

H3CO

OH

OCH3

COOH

HOOC

OH

OCH3

COOH

OH

H3CO

COOH

OH

OCH3

COOH

O

H3CO

COOH

Kraft pulp

Residual lignin

2.6

1.4

42.3

40.4

16.1

16.7

6.0

6.5

20.0

22.2

12.1

12.0

O-delig. Kraft pulp

Residual lignin

1.2

1.2

38.4

33.5

18.8

19.1

6.0

5.9

24.3

26.5

11.4

13.3

Diss. lignin, 85–93%

Diss. lignin, 93–95%

0.7

0.6

44.8

40.0

19.5

20.5

4.2

4.5

19.9

22.0

10.4

11.9

7.3 Oxygen Delignification 637

The b-O-4 linkage, in being the most abundant in native lignin, is significantly

cleaved during kraft pulping [59] and contributes to a better bleachability [29]

(Tab. 7.7).

The determined numbers of diphenylmethane (DPM) -type structures formed

during alkaline cooking varied over a wide range, and the validity of the method

used in determining the high values has been questioned [90,106].

Tab. 7.7 Frequency of linkages in kraft residual lignin (from Ref. [90]).

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