Conv. Kraft EMCC Kraft

Kappa number 26.8 26.0

Isolation yielda) % 14.2 49.5 46.7

Elemental composition

C % 61.0 65.2 62.8

H % 5.7 5.7 5.7

O % 32.9 27.9 29.8

S % 1.2 1.7

OCH3 % 15.3 12.1 10.5

Carboxylic groups mmol g–1 0.15 0.32 0.54

Hydroxyl Units

Aliphatic hydroxyls mmol g–1 4.27 2.14 2.15

Phenolic hydroxyls mmol g–1 1.15 2.71 2.50

Type A mmol g–1 0.02 0.02 0.01

Catechol (type B) mmol g–1 0.05 0.21 0.17

Guaiacol (type C) mmol g–1 0.62 0.23 0.56

Type D mmol g–1 0.06 0.38 0.27

Type E mmol g–1 0.35 1.64 1.32

a) Continuous dioxane acidolysis (dioxane-water = 85:15, 0.1 mol/l HCl).

b) Milled wood lignin from spruce.

R

OH

R

OH

OH

R

OH

OCH3

R

OH

OCH3

R

R

OH

R OCH3

(A) (B) (C) (D) (E)

262 4 Chemical Pulping Processes

The content of b-O-4-structures in residual lignin decreases with the extent of

delignification. The residual lignin in the EMCC pulp with kappa number 17.9

contained less b-O-4 structures and a higher content of C5 condensed structures

as compared to the residual lignin of conventional kraft pulp with kappa number

27.4 [85]. This is in accordance with the results obtained from the characterization

of residual lignins isolated from MCC and Super-Batch pulping technologies [86].

George et al., however, made different observations comparing residual lignins

isolated from spruce kraft pulps using also a dioxane acidolysis procedure [84].

The number of alkyl-O-aryl linkages, determined by 13C-NMR, was higher in the

modified residual lignin than in the conventional residual lignins. This observation

is in accordance with the higher amount of free phenolic groups present in

the conventional lignin. In a recent comparative study of conventional and laboratory-

simulated EMCC kraft pulps produced from Pinus elliottii, the residual lignin

of the latter had a higher content of b-O-4-structures and carboxylic groups. At

comparable kappa number, the amount of condensed structures was, however,

similar for both residual lignins [87].

The total phenolic hydroxyl content in the residual lignin continuously

increases during kraft pulping due to progressive cleavage of the b-O-4 bonds.

The guaicol-type of phenolic unit (type C) gradually decreases in parallel with the

progress in delignification. Conditions favoring the formation of unreactive carbon–

carbon bonds prevail, especially during conventional kraft cooking [88]. As

shown in Tab. 4.32, the amount of phenolic units substituted at the C5 position

(type E) continuously rise in both the dissolved and residual lignins. The Ca-C5

and the diphenylmethane units are described as the predominant C5 condensed

structures [89]. The formation of the diphenylmethane moieties has been

described as a considerably more facile reaction under soda pulping conditions as

compared to kraft pulping conditions. This may be one of the reasons why the

bleaching of soda pulps is more difficult compared to a kraft pulp at a given kappa

number [90]. Recently, the accumulation of completely unreactive 5–5′-biphenolic

hydroxyl groups was detected using quantitative 31P-NMR [91]. The final concentration

of the 5–5′ structures after softwood kraft pulping was approximately

0.6 mmol g–1, and thus more than three-fold higher than the corresponding value

of 0.2 mmol g–1 detected for the milled wood lignin.

The molecular weight of the residual lignin increases slightly towards the end

of the cook, which may be an indication of progressive condensation reactions

[83]. Lignin from pulps and corresponding spent liquors during kraft pulping of

Pinus sylvestris covering the kappa number range between 116 and 17 were isolated

by acidic dioxane extraction and characterized by GPC, UV and IR spectroscopy

and oxidative degradation methods [72]. The average molar mass of both lignin

precipitated from the spent liquor and lignins isolated from pulps increases

with the progress in cooking. The lignins extracted from pulps showed a higher

molar mass as compared to the spent-liquor lignins.

In accordance with the higher content of phenolic hydroxyl groups, the conventional

kraft residual lignin exhibits a lower molecular mass than the modified residual

lignin at a given kappa number [84]. In extending the cook from kappa

4.2 Kraft Pulping Processes 263

number 30 to kappa number 15, the molecular weight of the modified residual

lignin continues to decrease, whereas that of the conventional residual lignin is

not influenced [84]. Since the number of phenolic hydroxyl groups in the case of

the residual lignins of both pulps remained constant, it may be assumed that rupture

of the ether bonds immediately leads to lignin dissolution. Extending the

conventional cook results in a significant decrease in the number of methoxyl

groups. This trend is less pronounced with modified cooks. The loss in methoxyl

groups may also be accounted for by a slight enrichment in p -hydroxyphenyl units

toward the end of the cook. It is known that the cleavage of alkyl-aryl ether linkages

is favored by the presence of methoxyl groups. Consequently, guaiacyl units

can be assumed to be removed prior to p -hydroxyphenyl units. In contrast to the

modified residual lignin, the content of quaternary carbons is significantly

reduced in case of the conventional residual lignin, which may be attributed to

the enrichment in p -hydroxyphenyl units.

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