basics of kraft pulping

Lignin
Wood is composed of many chemical components, primarily extractives, carbohydrates,
and lignin, which are distributed nonuniformly as the result of anatomical structure.
Lignin is derived from the Latin term lignum, which means wood.1
Anselme Payen
(1838) was the first to recognize the composite nature of wood and referred to a carbon-
rich substance as the “encrusting material” which embedded cellulose in the wood.
Schulze (1865) later defined this encrusting material as lignin. Lignin has been described
as a random, three-dimensional network polymer comprised of variously linked
phenylpropane units.2
Lignin is the second most abundant biological material on the
planet, exceeded only by cellulose and hemicellulose, and comprises 15-25% of the dry
weight of woody plants. This macromolecule plays a vital role in providing mechanical
support to bind plant fibers together. Lignin also decreases the permeation of water
through the cell walls of the xylem, thereby playing an intricate role in the transport of
water and nutrients. Finally, lignin plays an important function in a plant’s natural
defense against degradation by impeding penetration of destructive enzymes through the
cell wall. Although lignin is necessary to trees, it is undesirable in most chemical
papermaking fibers and is removed by pulping and bleaching processes.
1.1.1 Biosynthesis
Plant lignins can be broadly divided into three classes: softwood (gymnosperm),
hardwood (angiosperm) and grass or annual plant (graminaceous) lignin.3
Three different
phenylpropane units, or monolignols, are responsible for lignin biosynthesis.4
Guaiacyl
lignin is composed principally of coniferyl alcohol units, while guaiacyl-syringyl lignin
contains monomeric units from coniferyl and sinapyl alcohol. In general, guaiacyl lignin
is found in softwoods while guaiacyl-syringyl lignin is present in hardwoods.
Graminaceous lignin is composed mainly of p-coumaryl alcohol units. These three lignin
precursors are shown in Figure 1. The principles employed in Figure 1 to denote carbon
atoms in the side chain will hold throughout this paper.

2
R1=OMe, R2=H: Coniferyl alcohol/guaiacyl
R1=R2=OMe: Sinapyl alcohol/syringyl
R1=R2=H: p-Coumaryl alcohol
R1
OH
R2
CH
CH
CH2OH
1
2
3
4
5
6
α
β
γ
Figure 1. The three building blocks of lignin.
Lignin polymerization is initiated by oxidation of the phenylpropane phenolic hydroxyl
groups. Freudenberg has shown that lignin precursors undergo dimerization through
enzymatic dehydrogenation, which is initiated by an electron transfer and yields
resonance-stabilized phenoxy radicals. Figure 2 shows an example of a phenoxy radical
formed from coniferyl alcohol and its resonance forms. Stabilization of the radical
occurs by coupling to another radical in any of the positions of the unpaired electron.
CH
CH
CH2OH
OCH3
OH
CH
CH
CH2OH
OCH3
O
CH
CH
CH2OH
OCH3
O
CH
CH
CH2OH
OCH3
O
CH
CH
CH2OH
OCH3
O
e- + H+
Coniferyl alcohol
Figure 2. Formation of resonance-stabilized phenoxyl radicals by enzymatic
dehydrogenation of coniferyl alcohol.
A monolignol free radical can then undergo radical coupling reactions, producing a
variety of dimers, termed dilignols. The combination of monomeric radicals through
only β-O-4 and β-5 coupling would lead to a linear polymer. However, branching of the

3
polymer may take place through subsequent nucleophilic attack by water, alcohols or
phenolic hydroxyl groups on the benzyl carbon of the quinone methide intermediate. The
dilignols then undergo further endwise polymerization, instead of combining with one
another.. Figure 3 shows an example of endwise polymerization during lignin
biosynthesis.

4
CH
CH
CH2OH
OCH3
O
OCH3
O
+
OCH3
O
HCOR
HC
HOH2C
OCH3
OH
1
2
4
OCH3
O
HCOR
HC
HOH2C
OCH3
O
OCH3
O
HCOR
HC
HOH2C
OCH3
O
H
CH
CH
CH2OH
OCH3
O
+
5 6 1
3
ROH
OCH3
O
CH
HC
HOH2C
OCH3
O
HC
HC
HOH2C
OCH3
O
OCH3
O
HCOR
HC
HOH2C
OCH3
O
H
HC
HC
HOH2C
OCH3
OH
OCH3
O
HCOR
HC
HOH2C
OCH3
O
7 8
Figure 3. Endwise polymerization during lignin biosynthesis.

5
1.1.2 Structure
After many years of study, the structure of native lignin still remains unclear. However,
the dominant structures in lignin have been elucidated as the methods for identification of
the degradation products and for the synthesis of model compounds have improved. The
results from these numerous studies have yielded what is believed to be an accurate
representation of the structure of lignin. Examples of the elucidated structural features of
lignin include the dominant linkages between the phenylpropane units and their
abundance, as well as the abundance and frequency of some functional groups. Linkages
between the phenylpropane units and the various functional groups on these units give
lignin a unique and very complex structure.
Figure 4 shows some of the common linkages found in softwood lignin. The dominant
linkage is the β-O-4 linkage. In 1995, Karhunen et al. discovered a new 8-membered
ring linkage in softwood lignin called dibenzodioxocin.5 This linkage was found through
advanced 2-D NMR techniques and is now proposed to be the main branching point in
softwood ligniN. The percent abundance of this and other linkages found in softwood
lignin has been determined and is shown in Table 1.
The lignin macromolecule also contains a variety of functional groups that have an
impact on its reactivity. Lignin mostly contains methoxyl groups, phenolic hydroxyl
groups, and few terminal aldehyde groups. Only a small proportion of the phenolic
hydroxyl groups are free since most are occupied in linkages to neighboring
phenylpropane linkages. Carbonyl and alcoholic hydroxyl groups are incorporated into
the lignin structure during enzymatic dehydrogenation. Table 2 illustrates the frequency
of some common functional groups found in lignin.

6
C
C
C
O
C
O
C
C
C
O
C
C
C
O
C
C
O
O
C
C
C
C
O
C
C
C
O
C
C
C
O
O
C
C
C
O
C
C
O
O
C
C
C
O
C
C
C
O
C
O
C
C
C
O
β-O-4 α-O-4 Dibenzodioxocin
β-5
β−β
5-5
4-O-5 β-1
Figure 4. Common phenylpropane linkages in lignin.

7
Table 1. Linkages found in softwood lignin.
Linkage Type Dimer Structure Percent of
Total Linkages (%)
β-O-4 Phenylpropane β−aryl ether 45-50
5-5 Biphenyl and Dibenzodioxocin 18-25
β -5 Phenylcoumaran 9-12
β-1 1,2-Diaryl propane 7-10
α-O-4 Phenylpropane α−aryl ether 6-8
4-O-5 Diaryl ether 4-8
β - β β-β-linked structures 3
Table 2. Functional groups in softwood lignin per 100 phenyl propane units.
Functional Group Abundance per 100 C9 units
Carbonyl 10-15
Benzyl alcohol 15-20
Phenolic hydroxyl (free) 15-30
Methoxyl 92-96
The nature of the lignin polymerization reactions results in the formation of a three-
dimensional, highly-branched, interlocking network of essentially infinite molecular
weight. Figure 5 is a schematic representation of a softwood lignin proposed by Adler6
and later modified by Karhunen et al. This model was constructed based on the analysis
of various linkages and functional groups. The phenylpropane (C9 or C6C3) units in lignin
are connected by C-C and ether (C-O-C) linkages. The frequency of such linkages is
believed to have significant consequences on the lignin’s overall reactivity toward the
delignification process. It is important to note that the model proposed by Adler and
Karhunen et al. does not depict the actual structure of lignin. Instead, it serves as a tool
to visualize the linkages and functional groups believed to occur in lignin.

8
Figure 5. Structural model of softwood lignin.

9
Studies have also suggested that covalent linkages between lignin and hemicelluloses
exist in native wood.7
These structures are typically referred to as lignin-carbohydrate
complexes or simply LCCs. The lignin is covalently bound to the hemicellulose which,
in turn, is bound to cellulose through extensive hydrogen bonding. Lignin-carbohydrate
complexes could be very important when considering pulping of wood since lignin is
chemically bound to the cellulose.
Kraft Pulping
Brief Overview
Chemical pulping is a means of removing lignin from wood (delignification) by
chemically altering it to produce soluble fragments of the polymer. The removal of
lignin allows individual fibers to be freed from the wood matrix with mild mechanical
treatment. Pulping must be able to remove lignin from fibers through chemical
degradation, while minimizing damage to the cellulosic portion of the fibers to maintain
strength. In 2001, the world pulp production was over 180 million metric tons. The
United States produced 53 million metric tons; 85 percent of this was from chemical
pulping. The kraft pulping process was the most commonly used chemical pulping
process, accounting for 98 percent of chemical pulp production in the United States and
92 percent of chemical pulp production in the world.8
In an effort to help industrialize the modern papermaking process, Hugh and Burgess
developed and patented an alkaline pulping process in 1854 that utilized wood as the raw
material, since wood was rather inexpensive and was in ample supply. This alkaline
pulping process involved treating wood shavings with sodium hydroxide under elevated
temperature conditions. Since the source of sodium hydroxide was from caustic soda,
this alkaline pulping process was called the soda process. C.F. Dahl, a German chemist,
later developed the sulfate or kraft process in 1879 when he replaced the sodium
carbonate or soda ash in the recovery cycle with sodium sulfate. The sodium sulfate was
reduced by the burning of organic matter to sodium sulfide and was then used in this

10
form to prepare the pulping liquor. The pulp produced from this alkaline pulping
process, although dark in color, made a paper that was far stronger than any paper up to
this time. The paper, and the process for making it, was hence given the name “kraft”
which means strength or power in Swedish and German.
Although in use since 1879, the kraft pulping process remains the dominant commercial
pulping process today. One major reason for its predominance is that the kraft process
produces stronger pulps than any other pulping process. In addition, it is insensitive to
the presence of bark and high amounts of extractives and has been found to be adaptable
to both hardwood and softwood tree species. The kraft process requires shorter cooking
times than other chemical pulping processes. Finally, efficient recovery of pulping
chemicals and the production of heat and valuable materials from by-products such as tall
oil and turpentine from pine species has been a strong asset to the kraft pulping process.
However, there are several disadvantages to kraft pulping which include high capital
investments, high reaction temperatures, low pulp yields, the foul odor generated by
degradation products, and the dark color of the resulting pulps.
During the kraft pulping process, wood chips are treated with a mixture of sodium sulfide
(Na2S) and sodium hydroxide (NaOH), also known as white liquor, at elevated
temperatures to produce a pulp suitable for the manufacture of paper and related
products. The white liquor and the wood chips are charged to a large pressure vessel
called a digester. The temperature in the digester is increased to approximately 170°C for
roughly two hours, depending on the degree of delignification desired. The hydroxide
and hydrosulfide anions react with lignin during this treatment, causing he polymer to
fragment into smaller alkali/water soluble fragments. The sulfide accelerates the pulping
process; therefore, the wood chips are exposed to high temperatures and alkaline
conditions for a shorter period of time than in other alkaline pulping processes like the
soda process, thereby producing a pulp that is much stronger than soda pulp.9

11
Kraft delignification occurs by modifying the structure of lignin in two different ways to
enhance its dissolution. The first is to degrade the lignin into smaller units by cleaving
interunit linkages. The second is to introduce hydrophilic groups into the polymer and
cleaved fragments, making the lignin more soluble in the cooking liquor. These
phenomena occur when linkages holding the phenylpropane units together are cleaved,
thereby generating free phenolic hydroxyl groups. The presence of these hydroxyl
groups increases the hydrophilicity of the lignin and lignin fragments. Thus, the
solubility of the lignin in the cooking liquor is increased. Meanwhile, the carbon-carbon
linkages, being more stable, tend to survive the kraft pulping process.10
There are three fairly distinct phases of delignification during kraft pulping which include
the initial phase, bulk phase, and residual or final phase. The three phases of
delignification are shown in Figure 6.11
The initial phase of delignification, taking place
at temperatures below 150°C, has been characterized as a rather unspecific alkaline
extraction leading to the dissolution of approximately 15-20% lignin and 20-25% of the
carbohydrates present in the wood. The delignification rate during this phase is first
order with respect to lignin concentration and independent of the hydroxide and
hydrosulfide ion concentrations, provided minimum threshold amounts of these ions are
used. The activation energy of the initial delignification (61 kJ/mol) indicates that the
rate of the process is diffusion- rather than chemically controlled. As the cook proceeds
above 150°C, the bulk phase of delignification begins. The bulk phase includes the
heating period from 150°C to 170°C and the cooking treatment at 170°C, and results in
the dissolution of the main portion (about 60%) of the lignin present in wood. Bulk
delignification is first order with respect to lignin concentration, almost linearly
dependent on the hydroxide, but only slightly dependent on the hydrosulfide ion
concentration. The activation energy of this phase of lignin degradation was found to be
150 kJ/mol, which is of the magnitude typically encountered in chemical reactions. The
residual phase of delignification includes the final treatment at 170°C and leads to
dissolution of roughly 10-15% of the lignin originally present in wood. This process is

12
very slow, the rate being dependent on the temperature and hydroxide ion concentration,
but almost independent of the hydrosulfide ion concentration. The activation energy was
found to be about 120 kJ/mol. The selectivity for lignin in this phase is poor, as
evidenced from an increased degradation of carbohydrates relative to the rate of
delignification. The cook should be terminated at this point to prevent significant loss of
pulp strength, and the remaining residual lignin should be removed using alternative
delignification techniques, such as bleaching.
Figure 6. Lignin content (% on wood) versus reaction time for conventional
softwood kraft pulping [45].
The residual or remaining lignin content of a typical conventional softwood kraft cook is
in the range of 4-5% or 27-33 kappa number. Pulping to lower lignin content under these
conditions causes severe degradation of the carbohydrate fraction, resulting in a pulp with
poor papermaking qualities. The low rate of lignin dissolution could possibly be ascribed
to the fact that while reactions in the residual phase require high alkalinity, the
concentration of alkali at this stage of pulping is considerably lower than during the
preceding phases due to neutralization reactions with various degradation products,
particularly those arising from carbohydrates. Acids formed during the degradation of
carbohydrate degradation consume 60-70% of the charged alkali while 10% of the added

13
alkali is used to neutralize uronic and acetic acids and 20-30% is consumed in
solubilizing the degradation products of lignin. It is also possible that the degree of
swelling decreases as a result of alkali shortage. A fiber wall in an unswelled state may
resist diffusion. It has also been shown that dissolved lignin is precipitated inside the
fibers when the alkalinity is lowered. It is therefore possible that a partially reversible
precipitation of degraded but not dissolved lignins occurs when the alkalinity is lowered.
Another reason for the poor selectivity of the residual phase of delignification is that the
structure of the residual lignin that remains in the fiber at the end of the cook has a low
reactivity toward the pulping chemicals, thereby making fragmentation and dissolution
difficult [49, 50]. Alternatively, several researchers have proposed that lignin dissolution
is hindered by the attachment of the residual lignin to carbohydrates.12
1.1.3 Delignification Chemistry
The primary reactions of lignins during kraft pulping can be divided into two classes,
degradation and condensation reactions. Degradation reactions lead to the liberation of
lignin fragments and ultimately to their dissolution. These degradation reactions are
therefore desirable to kraft pulping. Condensation reactions lead to the formation of
alkali-stable linkages, thereby increasing the molecular size of lignin fragments and may
result in their precipitation. Although having an opposite effect, these two types of
reactions are intimately connected with each other by proceeding via common
intermediates. There are other types of reactions occurring during kraft pulping. These
reactions do not involve any net fragmentation of the lignin macromolecule, yet yield
alkali-stable structures that make additional fragmentation difficult. Carbohydrates
undergo similar reactions to lignin during the kraft pulping process. The reactions
undergone by lignin and carbohydrates during the kraft pulping process allow both
polymers to become partially hydrophilic, thereby enabling solubilization of polymer
fragments into the alkaline liquor during kraft pulping. The following is a general
overview of the degradation, condensation, and other types of reactions that occur during
kraft pulping.

14
1.1.3.1 Degradation reactions
Degradation or fragmentation reactions are the most important reactions occurring during
kraft pulping because these reactions assist in the fragmentation and dissolution of the
lignin polymer. The most prevalent degradation reactions occurring during kraft pulping
include the cleavage of α-aryl ether and β-aryl ether bonds. However, the reactivity of
such linkages is sensitive to the type of moiety (i.e., free or etherified phenolic group)
present at the para position relative to the propane side chain. The α-aryl ether linkage is
readily cleaved, the reaction involving an alkali-assisted rearrangement of the phenolate
structure to the corresponding quinone methide structure with elimination of the α-aroxy
substituent. Figure 7 illustrates this type of cleavage. Since no hydrosulfide ions are
involved in the cleavage of α-aryl ether bonds, the rate of reaction is also independent of
the concentration of these ions.
Figure 7. Alkaline cleavage of α-aryl ether bonds in phenolic phenylpropane
units.
As is true for the cleavage of α-aryl ether bonds, the cleavage of β-aryl ether bonds in
phenolic units proceeds rapidly, with the initial conversion into the quinone methide
constituting the rate determining step in the overall reaction. Phenolic β-aryl ether
structures can also undergo an alkali-assisted transformation into a quinone methide,
provided a suitable leaving group exists at the α-position of the propane side chain. The
HC
HC
CH2
HO
H3CO
H3CO
O
O
HC
HC
CH2
HO
O
H3CO
OR
H3CO
O
-
OR

15
quinone methide intermediate can then react in different ways to restore the aromaticity
of the aromatic ring. The quinone methide could be converted into an alkali-stable enol
ether structure through the elimination of the terminal hydroxymethyl group as
formaldehyde. This reaction is more prominent in the absence of hydrosulfide ions, as is
the case during soda pulping. However, enol ether structures have been detected in kraft
residual lignin and this reaction pathway is therefore possible.
Under kraft pulping conditions, the hydrosulfide ion, which is a stronger nucleophile than
hydroxide ion, readily reacts with the quinone methide intermediate to restore the
aromaticity of the aromatic ring. The reversible addition of hydrosulfide ions results in
the formation of a benzyl mercaptide structure. This mercaptide anion can then attack the
β-carbon, through a neighboring group participation reaction, thereby forming a thiirane
intermediate and eliminating the β-aroxy substituent. The intermediary thiirane structure
can then lose elemental sulfur, generating a coniferyl alcohol-type structure. Although
the presence of hydrosulfide ions is believed to influence the reaction pathway resulting
in the fragmentation of the aryl ether linkage, enol structures have been detected in kraft
residual lignin, suggesting that both reaction pathways in Figure 8 can occur during kraft
pulping. The initial phase of delignification is characterized by diffusion-control and low
activation energy (61 kJ/mol). Therefore, any fragmentation reactions that occur during
this phase must occur at the same or higher rate than diffusion. This requirement is met
by the cleavage of α- and β-aryl ether linkages in phenolic units. Thus, the degradation
of lignin during the initial phase, involving only phenolic units, may continue until it
reaches units which are not of the α- or β−aryl ether type (peeling of lignin).

16
Figure 8. Sulfidolytic cleavage of β-aryl ether bonds in phenolic phenylpropane
units and conversion into enol-ether units.
When all phenolic units of the α- and β−aryl ether types, both those originally present
and those liberated during the initial phase, have reacted, further lignin degradation
requires more drastic conditions such as those prevailing during the bulk phase of
H3CO
O
HC
O
H3CO
HC
H3CO
O
-CH2O
-H
+
+HS
-
- H
+
-
Enol Ether
HC
HC
CH2
HO
O
H3CO
H3CO
O
HC
HC
CH2
HO
O
H3CO
OR
H3CO
O
- OR
CH
C
CH2
HO
H3CO
H3CO
O
S
H
O
CH
CH
CH2
HO
H3CO
O
S
- S0
CH
CH
CH2
HO
H3CO
O

17
delignification. Cleavage of α− and β-aryl ether linkages in phenolic units, proceeding
much faster, cannot be rate-determining during this phase. Instead, on account of
similarities between the kinetic data, the cleavage of β-aryl ether bonds in non-phenolic
units may be considered as the rate determining reaction in the overall bulk
delignification. The cleavage of β-aryl ether linkages in non-phenolic structures is a
relatively slow reaction that is dependent on the phenylpropane units having a hydroxyl
group on the α− or γ-carbon. This group can dissociate under extreme alkaline
conditions, forming an alkoxide anion. A nucleophilic attack on the β-carbon, through a
neighboring group participation reaction, results in an oxirane intermediate with
concomitant elimination of the β−aroxy substituent. Subsequent attack by the
nucleophiles found in the cooking liquor (hydroxide and hydrosulfide ions) breaks the
epoxide ring, producing diols or thioglycol-type structures as shown in Figure 9. Since
hydrosulfide ions do not participate to an appreciable extent, the rate of cleavage of non-
phenolic β-aryl ether linkages is independent of their concentration.
Figure 9. Alkaline cleavage of β-aryl ether bonds in non-phenolic phenylpropane
units.
Cleavage of β-aryl ether linkages in non-phenolic units also liberates new phenolic
structures that may constitute the starting point for the two types of cleavage reactions
operating during the initial phase, i.e., cleavage of α- and β-aryl ether bonds in phenolic
O
H3CO
CH
C
CH2
HO
H3CO
O
H
O
O
H3CO
CH
CH
CH2
HO
O
O
H3CO
CH
HCOH
CH2
HO
O
H3CO
O
OH
-

18
units. As a result of these reactions, lignin degradation during the bulk phase, initiated by
the rate determining cleavage of β-aryl ether bonds in non-phenolic units, is extended to
the point where dissolution takes place. Thus the subsequent fast reactions have no
influence on the rate, but a great influence on the extent of lignin degradation. In this
way, they contribute extensively to the rate of lignin dissolution during the bulk phase.
Unlike the fragmentation reactions of ether linkages, the cleavage of carbon-carbon
bonds during kraft pulping accounts for only a small amount of the total lignin
fragmentation. This cleavage occurs in side chains and between side chains and aromatic
nuclei, often resulting in the formation of new carbonyl linkages. An example of the
cleavage of carbon-carbon linkages is illustrated in Figure 10 with the retrograde aldol
reactions.
Figure 10. Alkaline cleavage of carbon-carbon linkages by retrograde aldol
reactions.
C
O
H3CO
O
H
C
O
H3CO
O
H3CO
C O
+H2O
- H
+
+H2O
- H
+
C
C
C O
HC
HC
C O

19
1.1.3.2 Condensation reactions
The degradation and fragmentation reactions are believed to be counter-balanced by
condensation reactions during kraft pulping. Primary condensation reactions occur when
quinone methide intermediates are formed by the elimination of an α-substituent while
secondary condensation reactions occur with quinone methide structures formed after an
initial ether cleavage. Condensation reactions may be viewed as conjugate additions
(Michael reactions) in which quinone methides, extended quinone methides or side chain
enone structures function as acceptors, and carbanions from phenolic or enolic units serve
as adding nucleophiles.
Figure 11 illustrates the alkali-promoted condensation of phenolic units. In these
reactions, formaldehyde, liberated from quinone methide intermediates, constitutes
another important acceptor for carbanions from phenolic structures. A hydroxybenzyl
alcohol is generated from the addition of a carbanion to a quinone methide intermediate.
This primary addition product is converted to the corresponding o-quinone methide.
Another carbanion reacts with the o-quinone methide, affording the corresponding
diarylmethane structure. The role of formaldehyde in cross-linking lignin units has been
recently question in several publications.
The external nucleophiles (hydroxide and hydrosulfide ions) present in the cooking liquor
have to compete with internal nucleophiles, in particular carbanions from phenolic and
enolic structures, for quinone methide structures. The addition step of the nucleophile to
the quinone methide is reversible and is therefore dependent on not only the
nucleophilicity of the species, but also the ability of the addition product to undergo a fast
irreversible reaction. Thus, in structures containing a good leaving group (e.g. the aroxy
substituent or R=OAr) at the β−carbon, fragmentation by neighboring group participation
reactions will outweigh condensation reactions by virtue of the greater nucleophilicity of
the external nucleophile (hydroxide and hydrosulfide ions) compared to the internal
competitors or carbanions. However, if the quinone methide structure has a β-substituent

20
that cannot be readily eliminated (e.g. alkyl or aryl groups), the addition of an internal
nucleophile, like cyclohexadienone, is followed by a fast, irreversible proton abstraction
and subsequent rearomatization reaction step. This competitive addition between
external and internal nucleophiles is outlined in Figure 12. Thus, the external
nucleophiles, although stronger in some cases than their internal counterparts, do not
prevent, but possibly retard condensation reactions.
H3CO
HC
CH2OH
O
H3CO
HC
O
H3CO
HC
O
H +CH2O
H3CO
HC
O
CH2
H3CO
HC
O
H
HC
OCH3
O
CH2
H3CO
HC
O
-H
+
-OH
H3CO
HC
CH2O
O
H
HC
OCH3
O
CH2
H3CO
HC
O
H
Figure 11. Alkali-promoted condensation reaction of phenolic units.

21
Figure 12. Competitive addition of external and internal nucleophiles to quinone
methide intermediates. OAr=aroxy group; Ar=aryl group.
In general, condensation reactions counteract lignin degradation and fragmentation
reactions. However, in certain instances, the initial condensation may be followed by a
fragmentation step. Figure 13 depicts possible fragmentation reactions that occur after
condensation reactions. Gierer et al. found that primary condensation products may
subsequently undergo neighboring group participation reactions, resulting in cleavage of
the β-aryl ether linkage.13
There are several types of fragmentation reactions that have
been observed with condensation products from β-aryl ether structures. In these
products, the original condensed phenolic structure, in its ionized form, attacks the β-
carbon atom and eliminates the β-aroxy substituent. As a result these aryl (upper reaction
in Figure 13) and phenolate ion (lower reaction in Figure 13) participation reactions give
rise to stilbene and phenylcoumaran, respectively.
O
OCH3
HC
HC R
O
OCH3
HC
HC
S
O
OCH3
HC
HC R
S
O
OCH3
HC
HCR
O
OCH3
H
O
OCH3
H
OCH3
HC
HCR
O
OCH3
O
-R
-
R=aroxyl group
+HS
-
, -H
+
-H
+
R=aryl group

22
Figure 13. β-Aryl cleavage with participation of condensed phenolic structures.
Gierer et al. have shown that fragmentation reactions may follow the initial condensation
reactions through model compound studies and by cooking extracted spruce wood meal
in the presence of xylenol under conditions similar to a kraft cook (white liquor, heated to
170°C). They found that there are several requirements to be met if the neighboring
group participation reactions after condensation are to occur. First, the structure must
contain a free phenolic hydroxyl group and an α-substituent that can be eliminated to
give a quinone methide intermediate. Second, the added phenol has to compete
successfully with external nucleophiles present in the cooking liquor for the quinone
methide intermediate. Finally, the phenolic β-aryl ether structure must not be condensed
to an adjacent phenyl propane unit at the C5 position.
H3CO
O
-
O
H3CO
HC O
OCH3
HC O
H3CO
CH2
HO
O
H3CO
HC
HC
CH2
O
O
OCH3
H
O
H3CO
HC
HC O
OCH3
O
H3CO
HC
OCH3
O
HC
CH2
HO
O
H3CO
HC
HC
OCH3
O
O
H3CO
CH2
HO
- H+, - CH2O
H3CO
O
-

23
Condensation reactions in kraft pulping have been the subject of much debate. Axegard
and Wiken knew of the possibility that formaldehyde, which is formed during a kraft
cook, might cause lignin condensation, as depicted in Figure 11.14
These researchers
tested this hypothesis by soaking wood chips in formaldehyde. Their results indicated
that residual lignin is not caused by formaldehyde condensation. Meanwhile, Robert et
al. [58] investigated structural changes in dissolved kraft lignins to further elucidate the
chemistry of the kraft cooking process. In this research, it was not possible to find any
clear indication of condensation reactions assumed to contribute to the slow rate of
delignification in the residual phase of the kraft cook. Studies by Gellerstedt and
Lindfors on the structural changes of lignin during kraft pulping showed that no
comprehensive condensation involving the aromatic rings of the residual lignin had taken
place during the cook.15
The same conclusion seemed also valid for a soda pulp that gave
analytical data very similar to those for the kraft pulps. Gellerstedt suggests that electron
transfer reactions are taking place between the residual fiber lignin and dissolved phenols
present in the pulping liquor during the residual phase of the kraft cook. These reactions
could increase cause an increase in molecular weight of the residual lignin, as observed
during the residual phase of kraft pulping.
1.1.3.3 Other lignin reactions
Besides degradation and condensation reactions, there are a variety of other reactions that
occur in kraft pulping that are believed to contribute to the low reactivity of residual
lignin. These reactions do not assist in the fragmentation and dissolution of lignin. Enol
ethers and stilbenes can also be formed during pulping, as shown in Figure 14. These
structures are, to a large extent, alkali-stable and are not desirable in kraft pulping. The
formation of these structures involves the elimination of the terminal hydroxymethyl
group as formaldehyde.16
Stilbenes may be formed from phenolic phenylcoumaran
structures by alkali-promoted opening of the five-membered coumaran ring, followed by
formaldehyde elimination from the resulting quinone methide as shown in Figure 15.
Stilbene structures should constitute important structural elements in dissolved as well as

24
residual lignins. These ring-conjugated structures are important to consider when
studying the structure of residual and dissolved lignins because their presence, although
in small quantities, may contribute to the dark color of kraft pulp. A variety of reduced
structures have been suggested to be present in the residual and dissolved lignins, as
evidenced by the increase in the number of methylene and methine groups in the residual
and kraft lignins. However, the formation of these structures is not well understood.
HC
HC
O
H3CO
R
-CH2O
Stilbene when R=Ar
Enol ether when R=OAr
HC
HC
O
H3CO
CH2 O-
R
Figure 14. Formation of enol ethers/stilbene by the elimination of formaldehyde
from a quinone methide intermediate structure.
Another reaction that occurs in kraft pulping involves the cleavage of methyl aryl ether
bonds. Hydrosulfide ions, possessing a strong nucleophilic character, bring about this
type of cleavage. The products of this reaction, generated after demethylation, are methyl
mercaptan and a catechol. The methyl mercaptide ion may cleave another methyl aryl
ether bond (methoxy group) to generate dimethyl sulfide, or it may get oxidized to yield
dimethyl disulfide. An example of these demethylation reactions by hydrosulfide and
methyl mercaptide ions is illustrated in Figure 16.

25
Figure 15. Formation of stilbenes from phenolic phenylcoumaran moieties.
+ HS
-
+ HO
-
O CH3
O [O
-
]
O
O [O
-
]
+ CH3S
-
+ H2O
+ CH3S
-
O CH3
O [O
-
]
O
O [O
-
]
+ CH3SCH3
2 CH3S
-
+ 1/2 O2 + H2O CH3S-SCH3 + 2 HO
-
Figure 16. Demethylation by hydrosulfide and methyl mercaptide ions.
HC
HC
HOH2C
OCH3
O
OCH3
O
HCOR
HC
HOH2C
OCH3
O HC
HC
OCH3
OH
OCH3
O
HCOR
HC
HOH2C
OCH3
OH
HC
HC
H2C
OCH3
O
OCH3
O
HCOR
HC
HOH2C
OCH3
O
O
H
HO-
[HS-]
+ CH2O

26
1.1.3.4 Carbohydrate reactions
The process of delignification during kraft pulping is accompanied to varying extents by
degradation of the carbohydrate constituents, i.e., cellulose and hemicelluloses. One type
of reaction that is responsible for this degradation, namely the peeling reaction, starts at
temperatures of about 100°C and may lead to considerable losses in pulp yield. The
peeling reaction, like the cleavage of α-aryl ether linkages in phenolic units, commences
under the relatively mild conditions of the initial phase. In the peeling reaction, a
stepwise depolymerization of the carbohydrate occurs at the reducing end sites of the
polymer chain, as shown in Figure 17. The reaction generates a monosaccharide that
undergoes a benzilic acid rearrangement (BAR) to form an isosaccharinic acid. The
reaction also forms a new reducing end on the remainder of the polymer, which can
undergo further peeling reactions. The carbohydrate material lost in the peeling reaction
is converted to various hydroxy acids that consume alkali and reduce the effective
concentration of the pulping liquor.
The peeling reaction continues in carbohydrates until the introduction of a carboxyl group
at the reducing end. This stopping reaction stabilizes the carbohydrate against further
peeling. During the stopping reaction, the diketone intermediate rearranges via a BAR
scheme to create a carboxylic acid group on the terminal end of the polymer chain. The
carbohydrate chain no longer has a terminal carbonyl group, and the peeling reaction
cannot proceed, as outlined in Figure 18.

27
Figure 17. Peeling of 1,4-linked polysaccharides such as cellulose, xylans, or
glucomannan.
Considering all these analogies between the alkaline degradation reactions of both lignin
and carbohydrate polymers, the selectivity of alkaline delignification could be interpreted
in terms of differences in the state of order or crystallinity of these substrates. However,
there are purely chemical reasons for this selectivity. Namely, aroxy anions, due to their
polarizability and weaker basicity, constitute more efficient leaving groups in the
cleavage via oxiranes than do alkoxide anions. In addition, lignin degradation during
both the initial and bulk phase is partly due to the sulfidolytic cleavage of β-aryl ether
linkages, a reaction that has no parallel in carbohydrate degradation reactions. Finally,
the alkaline cleavage of a-aryl ether bonds does not have to compete with a stabilizing
reaction, as the peeling reaction competes with the stopping reaction for carbohydrates.17
OH
OH
O
R O
H
R'
OH
R = Polysaccharide chain
O
OH
O
O
R
H
R'
OH
R' = CH2OH (Cellulose or Glucomannan)
or = H (Xylan)
OH
OH
O
R OH
H
R'
OH
OH
O
O
R OH
H
R'
OH
O
OH
H
O
OH
R'
O
OH
H
OH
R'
OH
O
R
O
OH
H
HO
OH
R'
CH2OH
+ H2O
- H
+
Benzylic Acid
Rearrangement
Peeling
OH
O
O
R OH
H
R'
OH

28
Figure 18. The formation of metasaccharinic acid during the stopping reaction
involving carbohydrates.
Another important reaction involving carbohydrates is the formation of hexenuronic acids
during kraft pulping. Clayton initially postulated the formation of hexenuronic acids
during kraft pulping.18
He proposed that the removal of the 4-O-methyl-α-D-glucuronic
acid groups could be initiated by β-elimination of methanol during the alkaline pulping of
wood. Later, Johansson and Samuelson employed 2-O-(4-O-methyl-α-D-gluco-
pyranosyluronic acid)-D-xylitol as a model compound to provide evidence for the
formation of hexenuronic acids upon treatment with alkali at elevated temperatures, as
well as its slow degradation with time.19
Although it has been clear from the literature that 4-O-methyl-D-glucuronic acid must
undergo β-elimination during kraft pulping, the occurrence of hexenuronic acid in kraft
pulps or in the dissolved xylans was not verified until 1995.20
After these investigations,
it became readily apparent that hexenuronic acids contributed to pulp bleachability,
influenced the retention of non-process elements in kraft pulps, and impacted pulp
brightness values.21 In addition, the amount of hexenuronic acids in kraft pulp is
strongly affected by the pulping conditions and attention has therefore been given to
OH
OH
O
R O
H
R'
HO
R = Polysaccharide chain
O
OH
O
O
R
H
R'
HO
R' = CH2OH (Cellulose or Glucomannan)
or = H (Xylan)
OH
O
R O
H
R'
OH
- H
+
+ H2O
Benzylic Acid
Rearrangement
OH
O
R O
H
R'
O
OH
O
R O
OH
R'
OH
- HO
-
OH
OH
O
R O
H
R'
HO

29
monitoring the existence and changes of hexenuronic acid during kraft pulping.22
The
formation of hexenuronic acids during kraft pulping is illustrated in Figure 19. During
this reaction, β-elimination of methanol results in the formation of hexenuronic acid.
Figure 19. Formation of hexenuronic acids during kraft pulping.
1.1.3.5 Lignin-Carbohydrate Complexes
Lignin-carbohydrate complexes (LCCs) are believed to be present in the native wood
lignin. There are possibly several different linkage types between lignin and
carbohydrates. These linkage types can be generalized as either alkali-stable or alkali-
sensitive. The alkali-sensitive linkages are readily cleaved under the harsh alkaline
conditions of the kraft cook while the alkali-stable linkages survive the kraft cook and
have been suggested to be present in kraft pulps.23
By surviving the kraft cook, these
alkali-stable linkages may be enriched and contribute to the difficulty in removing lignin
at the end of the cook. In addition to the native alkali-stable LCCs surviving the cook,
alkali-stable LCCs may be formed during the cook.24
This has been demonstrated in
model compound studies of both phenolic and non-phenolic model compounds and by
analysis of the lignin-carbohydrate bonds present in dissolved kraft lignins. Iversen et al.
performed a methylation analysis of a material enriched in residual lignin and isolated by
enzyme-catalyzed hydrolysis of the polysaccharides.25
The results of these studies
indicated the presence of alkali-stable lignin bonds, mainly to cellulose, but also to other
polysaccharides.
Hexenuronic acid
O
CO2H
O
OH
OH
O
H3CO
O
OH
CO2H
OH
-
OH CH3OH
+

30
Alkaline cleavage of β-aryl ether bonds in non-phenolic units, the dominant reaction
during kraft pulping, may also result in the formation of covalent ether bonds between
lignin and carbohydrate constituents of wood. Such bonds may prevent lignin fractions
from being dissolved in the final or residual phase of the kraft cook. The formation of an
LCC with a non-phenolic β-aryl ether-type structure is believed to occur by a
carbohydrate hydroxyl group, acting as a nucleophile, reacting with an epoxide that is
formed during the elimination of the β-aroxy substituent, as shown in. Thus, the reaction
has to compete with the alkaline-mediated opening of oxirane intermediates in lignin.
The lignin fragment of the native LCCs is believed to be linked exclusively with
hemicelluloses, while the lignin fragment of the LCCs that are formed during pulping
may be more frequently linked to cellulose.
The formation of lignin-carbohydrate complexes requires predissociation of alcoholic
hydroxyl groups followed by nucleophilic attack to form the oxirane intermediate. The
presence of a nucleophilic species stronger than alkoxide ions, such as the hydrosulfide
ion, will inhibit or reduce the formation of LCC linkages. Thus, the hydrosulfide ions not
only facilitate lignin fragmentation via sulfidolytic cleavage of the β-aryl ether units, but
also scavenge reactive intermediates of the oxirane type and prevent these from
undergoing coupling reactions with carbohydrates. Thus, a high sulfidity during the
transition stage from the initial to the bulk phase is desired. The reason for this is
because at this stage of the kraft cook, extensive lignin fragmentation takes place through
cleavage of β-aryl ether linkages in non-phenolic units. Thus, the conditions for the
formation of lignin-carbohydrate complexes involving the reaction between oxirane
intermediates and ionized hydroxyl groups in carbohydrates are favorable at this stage of
the kraft cook.

31
O
H3CO
CH
CH
CH2
HO
O
O
H3CO
HC
HC
CH2
HO
H3CO
O
OH
O
H3CO
+
O
CH2
OH
O
O
O
O
H3CO
HC
HC
CH2
HO
OH
OH
+ Carbohydrate
-OH
Figure 20. Formation of an LCC under kraft pulping conditions by the reaction
between a carbohydrate and a non-phenolic β-aryl ether lignin model.

32
References
1 Sarkanen, K.V. and Ludwig, C.H. Lignin: Occurrence, Formation, Structure and
Reactions. ed. Sarkanen, K.V. and Ludwig, C.H. Wiley-Interscience: New York.
916 pp. (1971).
2 Sjöström, E. Wood Chemistry: Fundamentals and Application. Academic Press:
Orlando. 293 pp. (1993).
3 Pearl, I.W. The Chemistry of Lignin. Marcel Dekker, Inc.: New York. 339 pp. (1967).
4 Freudenberg, K. and Neish, A.C. Constitution and Biosynthesis of Lignin. ed. Springer,
G.F. and Kleinzeller, A. Springer-Verlag: New York. 129 pp. (1968).
5 Karhunen, P.; Rummakko, P.; Sipilä, J.; Brunow, G. and Kilpeläinen,
I.Dibenzodioxocins; A Novel Type of Linkage in Softwood Lignins. Tetrahedron
Letters 36 (1). p. 167-170. (1995).; Karhunen, P.; Rummakko, P.; Sipilä, J.;
Brunow, G. and Kilpeläinen, I.; The Formation of Dibenzodioxocin Structures by
Oxidative Coupling. A Model for Lignin Biosynthesis. Tetrahedron Letters 36
(25). p. 4501-4504. (1995); Karhunen, P.; Mikkola, J.; Pajunen, A. and Brunow,
G.; The behavior of dibenzodioxocin structures during alkaline pulping processes.
Nordic Pulp and Paper Research Journal 14 (2). p. 123-128. (1999); Kukkola,
E.M.; Koutaniemi, S.; Gustafsson, M.; Karhunen, P.; Ruel, K.; Lundell, T.K.;
Saranpaeae, P.; Brunow, G.; Teeri, T.H. and Fagerstedt, K.V.; Localization of
dibenzodioxocin substructures in lignifying Norway spruce xylem by
transmission electron microscopy-immunogold labeling. Planta 217 (2). p. 229-
237. (2003); Kukkola, E.M.; Koutaniemi, S.; Poellaenen, E.; Gustafsson, M.;
Karhunen, P.; Lundell, T.K.; Saranpaeae, P.; Kilpelaeinen, I.; Teeri, T.H. and
Fagerstedt, K.V.; The dibenzodioxocin lignin substructure is abundant in the inner
part of the secondary wall in Norway spruce and silver birch xylem. Planta 218
(3). p. 497-500. (2004).
6 Adler, E.; Lignin - Past, Present and Future. Wood Science and Technology 11 (3). p.
169-218. (1977).
7 Björkman, A.; Studies on Finley Divided Wood Part 3. Extraction of Lignin-
Carbohydrate Complexes with Neutral Solvents. Svensk Papperstidning 60 (7). p.
243-251. (1957); Erikkson, O. and Lindgren, B.O.; About the Linkage Between
Lignin and Hemicelluloses in Wood. Svensk Papperstidning 80 (2). p. 59-63.
(1977); Minor, J.L.; Chemical Linkage of Pine Polysaccharides to Lignin. Journal
of Wood Chemistry and Technology 2 (1). p. 1-16. (1982); Obst, J.R.; Frequency
and Alkali Resistance of Lignin-Carbohydrate Bonds in Wood. Tappi Journal 65
(4). p. 109-112. (1982); Iversen, T.; Lignin-Carbohydrate Bonds in a Lignin-

33
Carbohydrate Complex from Spruce. Wood Science and Technology 19 (3). p.
243-251. (1985).
8 PPI's International Fact and Price Book. Paperloop. (2003).
9 Smook, G.A.; 2nd ed. Handbook for Pulp and Paper Technologists. Angus Wilde
Publications: Vancouver, Canada, pp. (1992).
10 Sjöström, E. Wood Chemistry: Fundamentals and Application. Academic Press:
Orlando. 293 pp. (1993).
11 Smook, G.A.; 2nd ed. Handbook for Pulp and Paper Technologists. Angus Wilde
Publications: Vancouver, Canada, pp. (1992); Axegard, P. and Wiken, J.;
Delignification Studies - Factors Affecting the Amount of Residual Lignin.
Svensk Papperstidning 86 (15). p. R178-R184. (1983).
12 Iversen, T. and Wannstrom, S.; Lignin-Carbohydrate Bonds in a Residual Lignin
Isolated from Kraft Pulp. Holzforschung 40 (1). p. 19-22. (1986); Gierer, J. and
Wannstrom, S.; Formation of Ether Bonds between Lignins and Carbohydrates
during Alkaline Pulping Processes. Holzforschung 40 (6). p. 347-352. (1986).
13 Gierer, J. and Wannstrom, S.; Formation of Ether Bonds between Lignins and
Carbohydrates during Alkaline Pulping Processes. Holzforschung 40 (6). p. 347-
352. (1986); Gierer, J.; Reactions of lignin during pulping. Description and
comparison of conventional pulping processes. Svensk Papperstidning 73 (18). p.
571-596. (1970); Gierer, J. and Pettersson, I.; Studies on the Condensation of
Lignins in Alkaline Media (2). Formation of Stilbene and Arylcoumaran
Structures Through Neighboring Group Participation Reactions. Canadian
Journal of Chemistry 55 (4). p. 593-599. (1977); Gierer, J. and Lindeberg, O.;
Studies on the Condensation of Lignins in Alkaline Media. (3). Formation of
Stilbenes, Arylcoumarans and Diarylmethanes on Treatment of Spruce Wood
Meal with Alkali and White Liquor in the Presence of Xylenols. Acta Chemica
Scandinavica 33B (8). p. 580-582. (1979); Gierer, J.; Noren, I. and Wannstrom,
S.; Formation of Condensation Products on Treatment of Nonphenolic Lignin
Units of the Beta-Aryl Ether Type with Alkali. Model Studies on a Novel Mode
of Alkaline Lignin Condensation. Holzforschung 41 (2). p. 79-82. (1987).
14 Axegard, P. and Wiken, J.; Delignification Studies - Factors Affecting the Amount of
Residual Lignin. Svensk Papperstidning 86 (15). p. R178-R184. (1983).
15 Gellerstedt, G. and Lindfors, E.; Structural Changes in Lignin During Kraft Cooking
(6). On the Formation of Enol Ether Structures in Lignin During Kraft Pulping.

34
Nordic Pulp and Paper Research Journal 2 (2). p. 71-75. (1987); Gellerstedt, G.
and Lindfors, E.; Structural Changes in Lignin during Kraft Pulping (2).
Characterization by Acidolysis. Svensk Papperstidning 9 p. R61-R67. (1984);
Treatment of Spruce Wood Meal with Alkali and White Liquor in the Presence of
Xylenols. Acta Chemica Scandinavica 33B (8). p. 580-582. (1979); Gellerstedt,
G. and Lindfors, E.-L.; Structural Changes in Lignin during Kraft Pulping.
Holzforschung 38 (3). p. 151-158. (1984); Gellerstedt, G. and Lindfors, E.;
Structural Changes in Kraft Pulping (4). Phenolic Hydroxyl Groups in Wood and
Kraft Pulps. Svensk Papperstidning 87 (15). p. R115-R118. (1984); Gellerstedt,
G. and Gustafsson, K.; Structural Changes in Lignin During Kraft Cooking Part 5.
Analysis of Dissolved Lignin by Oxidative Degradation. Journal of Wood
Chemistry and Technology 7 (1). p. 65-80. (1987); Gellerstedt, G. and Lindfors,
E.; On the Structure and Reactivity of Residual Lignin in Kraft Pulp Fibers. In
Proceedings of the International Pulp Bleaching Conference. Stockholm,
Sweden. p. 73-88. (1991).
16 Gierer, J.; Chemistry of Delignification. (1). General Concept and Reactions during
Pulping. Wood Science and Technology 19 (4). p. 289-312. (1985).
17 Johansson, M.H. and Samuelson, O.; The Formation of End Groups in Cellulose
during Alkaline Cooking. Carbohydrate Research 34 p. 33-43. (1974); Gentile,
V.M.; Schroeder, L.R. and Atalla, R.H.; Physical Structure and Alkaline
Degradation of Hydrocellulose. In The Structure of Cellulose: Characteristics of
the Solid State, Atalla, R.H., Editor. Washington, D.C.: American Chemical
Society. ACS Symposium Series: p. 272-291 (1987); Gratzl, J.; Reactions of
Polysaccharides and Lignins in Bleaching with Oxygen and Related Species. In
Proceedings of the TAPPI Oxygen Delignification Symposium. Atlanta, Georgia.
p. 1-21. (1990).
18 Clayton, D.W.; The Alkaline Degradation of Some Hardwood 4-O-methyl-D-
Glucuronoxylans. Svensk Papperstidning 66 p. 115-124. (1963).
19 Johansson, M.H. and Samuelson, O.; Epimerization and Degradation of 2-O-(4-O-
methyl- -D-glucopyranosyluronic acid)-D-xylitol in Alkaline Medium.
Carbohydrate Research 54 (2). p. 295-299. (1977).
20 Teleman, A.; Harjunpaa, V.; Tenkanen, M.; Buchert, J.; Hausolo, T.; Drakenberg, T.
and Vuorinen, T.; Characterization of 4-deoxy- -L-threo-hex-4-
enopyranosyluronic Acid Attached to Xylan in Pine Kraft Pulp and Pulping
Liquor by 1
H and 13
C NMR Spectroscopy. Carbohydrate Research 272 (1). p. 55-
71. (1995).

35
21 Vuorinen, T.; Buchert, J.; Teleman, A.; Tenkanen, M. and Fagerstrom, P.; Selective
Hydrolysis of Hexenuronic Acid Groups and it Application in ECF and TCF
Bleaching of Kraft Pulps. In Proceedings of the International Pulp Bleaching
Conference. Washington, D.C. p. 39-42. (1996); Vuorinen, T.; Fagerstrom, P.;
Rasanen, E.; Vikkula, A.; Henricson, K. and Teleman, A.; Selective hydrolysis of
hexenuronic acid groups opens new possibilities for development of bleaching
processes. In Proceedings of the 9th International Symposium on Wood and
Pulping Chemistry. Montreal, Quebec. p. M4-1 thru M4-4. (1997); Presley, J.;
Hill, R.; Chauveheid, E. and Deveyns, J.; New metals-control technique improves
bleaching performance. Journal of Pulp and Paper Science 71 (10). p. 129-131.
(1997); Deveyns, J. and Chauveheid, E.; Uronic acids and metals control. In
Proceedings of the 9th International Symposium on Wood and Pulping Chemistry.
Montreal, Quebec. p. M5-1 thru M5-4. (1997); Vuorinen, T.; Fagerstrom, P.;
Buchert, J.; Tenkanen, M. and Teleman, A.; Selective hydrolysis of hexenuronic
acid groups and its application in ECF and TCF bleaching of kraft pulps. Journal
of Pulp and Paper Science 25 (9). p. 306-311. (1999).
22 Buchert, J.; Tenkanen, M.; Ek, M.; Teleman, A.; Viikari, L. and Vuorinen, T.; Effects
of Pulping and Bleaching on Pulp Carbohydrates and Technical Properties. In
Proceedings of the International Pulp Bleaching Conference. Washington, D.C.
p. 39-42. (1996); Buchert, J.; Laine, J.; Tenkanen, M.; Vuorinen, T. and Viikari,
L.; Characterization of Uronic Acids During Kraft and Super Batch Pulping. In
Proceedings of the 9th International Symposium on Wood and Pulping Chemistry.
Montreal, Quebec. p. M3-1 - M3-3. (1997); Gustavsson, C.A.-S.; Sjostrom, K.
and Al-Dajani, W.W.; The influence of cooking conditions on the bleachability
and chemical structure of kraft pulps. Nordic Pulp and Paper Research Journal
14 (1). p. 71-81. (1999); Gustavsson, C.A.S. and Al-Dajani, W.W.; The influence
of cooking conditions on the degradation of hexenuronic acid, xylan,
glucomannan, and cellulose during kraft pulping of softwood. Nordic Pulp &
Paper Research Journal 15 (2). p. 160-167. (2000).
23 Yamasaki, T.; Hosoya, S.; Chen, C.-L.; Gratzl, J.S. and Chang, H.-M.;
Characterization of Residual Lignin in Pulp. In Proceedings of the International
Symposium on Wood and Pulping Chemistry. Stockholm, Sweden. p. 34-42.
(1981).
24 Gierer, J. and Wannstrom, S.; Formation of Alkali-Stable C-C Bonds between Lignin
adn Carbohydrate Fragments During Kraft Pulping. Holzforschung 38 (4). p. 181-
184. (1984).

36
25 Iversen, T. and Wannstrom, S.; Lignin-Carbohydrate Bonds in a Residual Lignin
Isolated from Kraft Pulp. Holzforschung 40 (1). p. 19-22. (1986).

basics of kraft pulping

More Related Content

What's hot (20)

Similar to basics of kraft pulping (20)

More from Audrey Zahra (16)

Recently uploaded (20)

basics of kraft pulping