Introduction

Environmental restrictions for bleach plant effluents and the necessity to reduce

the amount of organochlorine compounds (OX) in the pulp have driven the pulp

628 7Pulp Bleaching

industry to develop new environmentally benign delignification and bleaching

technologies. In this context, oxygen delignification has emerged as an important

delignification technology. The benefits of introducing an oxygen delignification

stage are manifold, and include a lower demand for bleaching chemicals in the

subsequent stages, a higher yield as compared to the final part of the cooking

stage, and the possibility of recycling the liquid effluents from an oxygen delignification

stage to the chemical recovery system to reduce the environmental impact

with respect to color, COD, BOD and toxic compounds (e.g., organochlorine) of

the bleach plant effluents. However, one of the major drawbacks of oxygen

delignification is its lack of selectivity for delignification in particular beyond 50%,

as this results in excessive cellulose damage which appears as a decrease in viscosity

and a loss of pulp strength. The significantly lower selectivity of oxygen-alkali

bleaching compared to conventional chlorine-based prebleaching sequences was

one of the reasons why the introduction of oxygen delignification to industrial

bleaching technology was not widely accepted by the industry. Figure 7.22 illustrates

the superior selectivity performance in terms of a viscosity–kappa number

relationship of a treatment with molecular chlorine followed by alkaline extraction

(CE) of a softwood kraft pulp as compared to oxygen delignification with and without

the addition of magnesium carbonate [1].

The potential of lignin degradation into water-soluble fragments by treatment

with oxygen in alkaline solution first became apparent during the 1950s [2–6].

Oxygen delignification was successfully applied to delignify and bleach birch and

spruce sulfite dissolving pulp. Several processes were patented during this early

0 10 20 30 40

0

800

1000

1200

unbleached pine kraft pulp

C-E pre-bleaching

oxygen bleaching at 100 ºC

Intrinsic viscosity [ml/g]

Kappa number

Fig. 7.22 Selectivity of oxygen delignification at 100 °C in

comparison to chlorination followed by alkaline extraction

(according to Hartler et al. [1]). The broken line denotes oxygen

delignification with no MgCO3 added.

7.3 Oxygen Delignification 629

phase of research, but were not commercialized because of the observed extensive

depolymerization of carbohydrates [7–9]. The poor selectivity has been explained

by the formation of reactive oxygen-based radicals (e.g., hydroxy radicals) generated

by oxygen attack on lignin structures [10]. A major breakthrough in oxygen

delignification occurred during the 1960s, when Robert and colleagues discovered

that the addition of small amounts of magnesium carbonate resulted in preservation

of the strength properties of paper-grade kraft pulp [11–13]. This opened the

door to the commercial development of oxygen as a delignifying agent. The first

installation was a high consistency oxygen delignification plant built in South

Africa at SAPPI’s Enstra mill in 1970. This investment was based on a successful

pilot plant operation in 1968 in Sweden, and was constructed as a cooperative

effort among SAPPI, Kamyr Inc., and Air Liquide. Reported high investment

costs and safety problems with the handling of combustible gases certainly

retarded the acceptance and implementation of this new technology.

The development of medium consistency, high-shear mixers during the early

1980s led to a rapid increase in the installation of oxygen delignification plants

due to its beneficial effects on the environment, process economy and energy savings

[14,15]. This process is also more amenable to retrofit in existing mills than

high consistency processes, and can be easily incorporated as intermediary stage

in the sequence, for example as combined with an E-stage [16]. In 1996, there

were more than 185 oxygen-delignification installations throughout the world,

with a combined daily production of about 160 000 t oxygen-delignified kraft pulp

[17,18]. The data in Fig. 7.23 show that 80% of these installations have come onstream

during the past 10 years, mainly driven by the stricter emission limits prescribed

by regulatory authorities.

As mentioned previously, oxygen delignification also provides an economic

attractive alternative to chlorine-based bleaching stages. It is reported that roughly

5 kg of oxygen can replace about 3 kg of chlorine dioxide. At a price differential of

0.61SEK kg–1 versus 8.6 SEK kg–1, respectively, the cost of using oxygen is about

one-eighth that of using chlorine dioxide [19]. Moreover, the energy requirement

to separate oxygen from the atmosphere is significantly less as compared to the

generation of chlorine dioxide, and this is a favorable prospect for the future [20].

Presently, oxygen delignification has become a well-established technology.

Because of selectivity advantages and lower investment costs, the medium consistency

technology (MC, 10–18%) has dominated mill installations for the past 10

years, although high consistency installations (HC, 25–40%) are also in use.

Recently, the industry has adopted the installation of two-stage oxygen delignification

systems to increase both the selectivity and efficiency of the treatment. A typical

50% delignification level has thereby been increased to about 65% for a softwood

kraft pulp with an unbleached kappa number between 25 and 30.

A detailed study of representative oxygen delignification installations worldwide

clearly indicates the advantage of a two-stage oxygen delignification system over a

one-stage system. The good performance of the high-consistency systems mainly

results from better washing before the oxygen stage.

630 7Pulp Bleaching

1970 1975 1980 1985 1990 1995

0

20

40

60

80

100

120

140

160

180

Capacity*103 [adt/d]

Year

one-stage two-stage

Fig. 7.23 Daily production capacity of oxygen-delignified pulp

on a worldwide basis [18].

For softwoods, evaluation of the database provides an average of 47.5% delignification,

ranging from 28% to 67%. The incoming kappa number to the oxygen

stage ranges from 32 to 22, and the outgoing kappa number from 22 to 8.5. For

hardwoods, the performance of oxygen delignification varied from 19% to 55%,

with an average of 40%. Kappa number variation is significantly reduced across

the oxygen stage, from a range of 12–22 at the inlet to 7.5–13.5 at the discharge.

The reason for these differences in unbleached kappa numbers of hardwood kraft

pulps depends on the greater variability of hardwoods with respect to optimum

yield and final pulp properties. For example, birch is often cooked to 18–20 kappa

number, while many eucalypt species are only cooked to 12–14 kappa number

[18]. In accordance with recent developments and the results from detailed investigations,

there appears to be a lower limit of kappa number in the bleach plant

for softwood kraft pulps of 8–10 and for hardwoods of 6–8 [19]. With continuing

progress in oxygen delignification technology, it is expected that in future the

cooking kappa will be raised to levels higher than 30, again because considerable

wood yield can be preserved [21]. The yield loss during the residual cooking phase

is significantly higher than during oxygen delignification. With the new highly

efficient multi-stage medium consistency technology available, the overall yield

can be increased by about one percentage point by increasing the cooking kappa,

for example from 20 to 25.

Delignification in the oxygen stage means a smaller decrease in yield than

delignification in cooking, as long as the degree of delignification in the oxygen

stage remains moderate. As a rule of thumb, the yield decrease in the oxygen

stage equals 0.1–0.2% on wood per 2 units of kappa number decrease, while in

7.3 Oxygen Delignification 631

cooking the yield decrease corresponds to 0.3% on wood for the same kappa number

reduction [22]. Thus, oxygen delignification is more selective in terms of yield

preservation than kraft cooking at kappa numbers corresponding to the final

phase of a low kappa kraft cook [23].

It is generally acknowledged that an O- or OO-stage can remove 35–50% of the

residual lignin in hardwood kraft pulp and 40–65% in softwood kraft pulp, without

significantly impairing the selectivity of delignification and the physical pulp

properties. The results of extended oxygen delignification studies indicate that distinct

yield benefits can be accomplished by interrupting the cook at a high kappa

number (e.g., 40–50) in the case of softwood kraft pulps include reference. The

subsequent oxygen delignification of the high-kappa number pulps has been

shown to provide 3–4% yield benefits over conventional cooking and bleaching

technologies. These observed yield benefits are then further amplified by reducing

the organic load on the recovery furnace

7.3.2

Chemistry of Oxygen Delignification

Manfred Schwanninger

Among different pulping techniques, kraft pulping is the most important process,

consisting of wood treatment with a solution of sodium hydroxide and sodium

sulfide at high temperature. This results in wood delignification through the degradation

of lignin (and also carbohydrates) and its dissolution in pulping liquor.

Although a major fraction of wood lignin (~97%) can be removed in kraft pulping,

the remainder of the lignin (residual lignin) is rather resistant under the pulping

conditions. In order to remove the residual lignin from pulp, oxidative lignin degradation

with bleaching reagents such as dioxygen, hydrogen peroxide, ozone,

and chlorine dioxide is required.

According to the general concept of the chemistry of delignification [1,2], the reactions

of lignin during pulping and bleaching can be divided into two categories:

_ Nucleophilic additions and displacements, which are involved in

pulping processes, in later phases of lignin-degrading bleaching,

and in lignin-retaining bleaching.

_ Electrophilic additions and displacements, initiating the lignindegrading

bleaching processes.

Depending on the nature of the reagent(s), the reactions can be further divided

into categories of nucleophilic and electrophilic which frequently, but not always,

conform to a reduction-oxidation classification.

Carbonyl carbons or the vinylogous carbon atoms in intermediates of the enone

type (quinone methide intermediate; see Section 4.2.4, Chemistry of kraft pulping,

Scheme 3) are the sites where the nucleophiles, which are present in pulping

liquors, begin the attack [1,2]. Additionally, nucleophilic groups in the a– (or c-)

position of the side chain attack the b-carbon atom in a neighboring group participation-

type of reaction which, in b-aryl ether structures, leads to fragmentation

632 7Pulp Bleaching

[1,2]. The initial attack by electrophiles, which are present in bleaching liquors,

takes place on the aromatic rings and side chains, which are activated by free or

etherified phenolic hydroxyl groups [1–4].

In order to emphasize the principal difference in delignification during pulping

and bleaching, it should be stressed that delignification during pulping occurs

exclusively due to nucleophilic reactions [1,2,5], whereas delignification during

bleaching is primarily initiated by electrophilic reactions, which may be followed

by nucleophilic processes [6–9].

This initial step of oxygen-alkali bleaching will be briefly described here. In alkaline

media, the phenolic hydroxyl group (1) (Scheme 7.1) is deprotonated to produce the

phenolate anion (2) that furnishes the high electron density needed to initiate a oneelectron

transfer. The reactive electrophilic (d-) sites marked in Scheme 7.1( 2) are

situated at alternating carbons. Scheme 7.2 (left) depicts theHOMOof the phenolate

ion of coniferyl alcohol. The size of the orbitals’ nodes correspond to centers of high

electron density, and hence to sites of preferred attack of electrophiles; thus, they

determine the pathway of the subsequent reaction. The resultant electron density distribution

is shown in Scheme 7.2 (right), where red zones denote centers of high electron

density. Oxygen attacks at an electrophilic (d-) site and abstracts an electron,

leaving a phenoxyl radical (3) and/or a mesomeric cyclohexadienonyl radical,

while oxygen itself is reduced to the superoxide anion radical.

OH

OCH3

CH

CH

CH2OH

- -

-

-

O-

OCH3

CH

CH

CH2OH

+ OH - , - H2O

O

OCH3

CH

CH

CH2OH

O2 O2

-

1 2 3

Scheme 7.1 The initial step of oxygen-alkali bleaching at

electrophilic (d-) sites.

Scheme 7.2 HOMO-distribution (left) and electron density

distribution (right) of the phenylpropene unit 2 shown in

Scheme 7.1 (PM3calculation with Spartan 4.0).

7.3 Oxygen Delignification 633

7.3.2.1 Bleachability

Beside the differences between hardwood and softwood, it is well known that process

parameters such as temperature [10–23], alkali charge and pH [10,13–

15,17,19,20,22,24–34], kappa number [35], transition metal ions [34,36], surfactants

[37], age of the trees [38,39], wood storage [18,40], pretreatment with chemicals or

enzymes [12,19,28,31,41–45], and the formation of hexenuronic acid [10,15,46] have

an impact on the bleachability of the pulp due to structural changes in lignin [47–61].

Moreover, the efficiency of delignification depends on structural features such as free

phenolic hydroxyl groups, methoxyl groups, carboxyl groups, and linkages between

the phenylpropane units (e.g. b-aryl ether linkages) in lignin contribute to a

better bleachability of the pulp [29], and on the composition of the residual lignin–

carbohydrate complex (RLCC) [62–67]. Notably, a new method to determine

kappa number and the bleachability was recently published [68,69].

7.3.2.2 Lignin Structures and their Reactivity

7.3.2.2.1 Composition of Lignin, Residual Lignin after Cooking and after Bleaching

The limitation of the extent to which dioxygen delignification can be used is well

known in practice and research, and many investigations have focused on an elucidation

of responsible lignin structures, namely those that are stable or only react

slowly under dioxygen bleaching conditions. Therefore, information on the structures

of the residual and dissolved lignins, isolated from pulp and the pulping solution

[70,71], is of primary importance for a better understanding of the underlying

mechanisms of, for example, kraft delignification and the reactivity of residual

lignins in bleaching. Although extensive investigations into the isolation [72–75]

and characterization of residual [76,77] and dissolved lignins using different analytical

techniques have provided valuable information, their structures are not yet

well established. An excellent review of the procedures used for residual lignin has

recently been published [78]. Further progress in the characterization of lignins and

lignin–carbohydrate complexes requires the application of advanced techniques such

as nuclear magnetic resonance (NMR) [12,35,79–89] and others [77].

Based on the results of different procedures [78] applied, theoretical models for

residual lignin structures have been developed, and upon these presumed structures

functional groups and linkages have been selected for the study of model

compounds. Although the rate of degradation in actual lignin systems is much

slower and the extent of degradation is much lower [90], this might be due to the

different matrices and accessibility [26,31,63,66,80,91–93]. Moreover, despite the

selectivity of the chemical agents used [94], the results obtained with model compounds

are valid when describing lignin degradation, as comparisons with lignin

studies have revealed [90].

An excellent review of lignin model compound reactions under oxygen bleaching

conditions has recently been published [90]. This presents a compilation of

published data relating to functional group contents in residual kraft lignins (Tab.

7.3) and the relative reactivity of functional groups of lignin model compounds

634 7Pulp Bleaching

with oxygen (Tab. 7.4). The content of free phenolic hydroxyl groups, which is

important for lignin solubility and reactivity, was increased to about 25/100 C9 in

residual lignin and to about 65/100 C9 in dissolved kraft lignins [90].

Tab. 7.3 Functional group content in residual lignin (from Ref. [90]).

Functional group Amount relative to native lignina Amount Reference

Free phenolic hydroxyl ~20% higher 25–35/100 C9 50, 95

Methoxyl group ~20% lower Variable 60

Catechol Formed in pulping 3/100 C9 96

Aliphatic hydroxyls ~60% lower 40/100 C9b 81, 83

Aliphatic carbonyls Destroyed in pulping Negligible 82, 97

Aliphatic carboxyls Formed in pulping 5/100 C9b 81, 98

Aliphatic reduced units Higher Variable 97

a. Approximate differences for a residual kraft lignin from a 30 kappa pulp.

b. Converted literature values to groups/100 C9 using 185 g mol–1 as C9 unit.

Tab. 7.4 Relative reactivity of lignin model compounds with

oxygen (from Ref. [90]).

Functional group Relative reactivity to oxygen Reference

Phenolic hydroxyl

Methoxyl

Side chain

OH

R3 R2

R1

OCH3

R3 R2

R1

OH

OH

R

OCH3

H3CO OCH3

R

OCH3

OCH3

R

OH

R

OH

OCH3

CH2

OH

OCH3

HC

OH

OCH3

C

O

OH

C

OCH3

OH OH O

100

100–102

100–102

7.3 Oxygen Delignification 635

Due to lack of reactivity of nonphenolic compounds, studies on ring cleavage by

oxygen have focused on compounds with free phenolic hydroxyl groups. The former

can be degraded in the presence of a compound containing a free phenolic

hydroxyl group that produces oxygen radical species in the reaction with oxygen

[99]. Depending on the raw material (hardwood or softwood), the number of

methoxyl groups varies and is also affected by alkaline demethylation reactions

that form catechol groups during pulping. Although the catechol-containing compounds

are by far the most reactive, their number present in residual lignin following

oxygen bleaching is not significantly changed, indicating that these groups

are also formed during oxidation [90].

Carboxyl groups, not present in native lignin, are formed during kraft pulping

(Tab. 7.3) and oxygen bleaching through side chain and ring cleavage reactions. In

contrast, muconic acid structures are the primary ring cleavage products that

should be present in only minor quantities after oxygen bleaching, due to their

high reactivity [90,103].

The quantity of aliphatic hydroxyl groups in softwood kraft pulp (Tab. 7.5) [80]

increased after oxygen alkali treatment, while the carboxyl group content

decreased after an initial increase in the residual lignin, and the quantity of all

hydroxyl groups increased in the effluent lignin. Both were accompanied by a

drastic increase in carbohydrates. Others have also found comparable changes in

the hydroxyl group content [104,105].

Tab. 7.5 Quantities of reactive groups (e.g., aliphatic hydroxyls,

phenolic hydroxyls, carboxylic acid groups) and carbohydrate

content in lignin samples (from [80]).

Group RL

[mmol g–1]

Lig-1st

[mmol g–1]

Lig-2nd

[mmol g–1]

Lig-3rd

[mmol g–1]

Lig-L1st

[mmol g–1]

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]).

Linkage MWL

[% of linkages]

Reference Residual lignina

[relative to MWL]

Reference

b-O-4 48 107 85% lower 52

b–5 7–8 108, 109 Slightly greater 49, 96

9–12 110

b–1<2 107

3.5 111

5–5 10–11 110 49, 96

(16)b 109 Slightly greater

(24–26)b 112

4-O-5 4–5 109, 113 Slightly greater 49 ,96

b-b

Stilbenes Negligible ~3% of linkages 114

Vinyl ethers Negligible 0.5–1% of linkages 52

DPMc Negligible Debatable amountsd 106, 113, 115

a. Approximate differences for a residual kraft lignin from a 30 kappa pulp.

b. Values represent % of phenyl propane units containing 5,5′linkages.

c. Diphenylmethane structures of various linkages.

d. Amounts ranging from ~5% to >60% have been reported.

Gellerstedt and Zhang [61] summarized some of the residual kraft lignin features,

as follows:

_ A low remaining amount of b-O-4 structures [52].

_ Linkages between lignin and polysaccharides.

_ The presence of reduced structures such as methylene and methyl

groups [48].

_ A high degree in discoloration [116].

638 7Pulp Bleaching

_ A successive increase of “condensed” structures with high degree

of delignification [49].

_ An uneven distribution of lignin across the fiber wall.

As noted, a successive cleavage of b-O-4 structure takes place in the kraft cook

(Tab. 7.8) ([61]). Moreover, the data in Tab. 7.8 highlight changes in the number of

substructures during pulping and bleaching. An increasing number of b-O-4 linkages

during oxygen delignification (Tab. 7.8) was also observed by Balakshin et al.

[79].

Tab. 7.8 Number of substructures per 100 C9 in some isolated

lignin samples (adapted from Ref. [61]) prepared by acid

hydrolysis [117].

Lignin sample b-O-4 b–5 b-b

MWL 39a 11 2

Residual lignin, kappa = 30 9–10 5 2

Residual lignin, kappa = 18 5–7 3 1

Dissolved kraft lignin 5 2 2

Residual lignin, from a commercial pulp, kappa = 26 11 6 2

Residual lignin, after an oxygen stage, kappa = 9.3 18 8 2

a. Includes a-hydroxy-b-O-4 and dibenzodioxocin structures.

Comparison of the relative stability and susceptibility of different structures of

dimeric model compounds to degradation by oxygen is difficult due to the different

conditions used, because the oxidation rate in these reactions is highly

affected by system parameters such as pH, temperature, oxygen charge, and reactant

charge [90,118]. Stilbene structures (Tab. 7.9) are rapidly degraded under oxygen-

alkali conditions [118]. Phenolic stilbenes and vinyl ethers degrade across the

double bond, whereas the stilbenes oxidize over one hundred times faster [90].

Under oxygen-alkali conditions, the model compounds in the second row of Tab.

7.9 react an order of magnitude slower than the vinyl ethers, to form phenolic

aldehydes, alcohols, ketones, and carboxylic acids along with aliphatic acids as the

main degradation products [90].

7.3 Oxygen Delignification 639

Tab. 7.9 Relative susceptibility of model compounds to oxygen (from Ref. [90]).

Model compounds tested (approximate order of reactivity) Reference

Very reactive

OH

OCH3

CH

CH OH

OCH3

(1) Stilbene

OH

OCH3

CH

CH OH

OCH3

(2) Vinyl ether

O

118

Somewhat reactive

OH

OCH3

HC

CH OH

OCH3

(3) -1

OH

CH2OH

OR

OCH3

C

HC OH

OCH3

(4) -O-4, -carbonyl

O

CH2OH

R = H, CH3

OH

H3CO CH2

CH2

CH2

(5) DPM

CH3

OH

OCH3

CH2

CH2

CH3

OH

OCH3

HC

HC OH

OCH3

(6) -O-4, -hydroxyl

OH

CH2OH

O

OH

H3CO

CH2

CH2

(7) 5-5

CH3

OH

OCH3

CH2

CH2

CH3

OH

H3CO

C

C

O

OCH3

(8) -5

CH3

H

H

(CH2)2CH3

O

118–124

Non-reactive

OCH3

OCH3

HC

HC OH

OCH3

(9) -O-4, -hydroxyl

OH

CH2OH

O

OCH3

H3CO

C

C

O

OCH3

(10) -5

CH3

H

H

(CH2)2CH3 122, 124

7.3.2.2.2 Composition of RLCC Before and After Bleaching

The composition of the RLCC from two pulps, namely a conventional kraft pulp

(CK) and a polysulfide/anthraquinone pulp (PSAQ), isolated with enzymatic

hydrolysis and further purified is shown in Tab. 7.10 before and after oxygenalkali

bleaching [62]. The number of methoxyl groups decreased due to demethylation,

and the number of phenolic hydroxyl groups also decreased (see Tabs. 7.3

and 7.5). Moreover, an increase in the proportion of a-conjugated phenolic

640 7Pulp Bleaching

Tab. 7.10 Composition of the purified RLCC of two pulps prior

and after oxygen alkali treatment (adapted from Ref. [62]).

Kappa number

OCH3 (% of C9)

PhOH(% of C9)

Proportion of

a-conjugated PhOH(%)

Proportion of weakly

acidic PhOH(%)

Molecular weight

[Da]

Arabinosec

Galactosec

Glucosec