Heat- and Light-Induced Brightness
Reversion of Bleached Chemical Pulps
C. CHIRAT and V. DE LA CHAPELLE
Elemental chlorine free (ECF) and totally chlorine free (TCP) bleached pulps of different types (kraft, bisulphite, sofnvood and hardwood I
were analyzed and submitted to heat and light aging. TCF pulps had residual kappa numbers between 2 and 4, and higher carboxyl content
than the corresponding ECF pulps. TCFcould be observed among pulps. Some oxidized groups were created selectively on cellulose and
their effect on heat and light exposure was studied. Ketone groups impaired brightness reversion upon heat significantly, whereas aldehyde
groups at C2-C3 had only a minor effect. Carboxyl groups at C2-C3 were shown to affect brightness stability to heat. Brightness stability
upon light exposure, however, was affected neither by ketones and aldehydes, nor by carboxyl groups. Finally it was shown than for a
given brightness loss, cellulose was less degraded when exposed to light than to heat, suggesting different mechanisms.
Des pates de divers types, blanchies sans chlore elementaire et sans aucun chlore (kraft, bisulfite, resineux, feuillus), out ete analysees et
soumises au vieillissement par la lumiere et par la chaleur. L'indice Kappa residuel des pates sans aucun chlore se situait entre 2 a 4. et
leur teneur en carboxyles etait plus elevee que celle des pates sans chlore elementaire correspondantes. Les pates sans aucun chlore etaient
en general moins stables a la chaleur que celles sans chlore elementaire. L'effet de I'exposition a la lumiere etait moins evident, bien qu 'i!
y avait des differences importantes entre les p^ates. Certains groupes oxydes ont ete crees defacon selective avec de la cellulose, et leur effet
sur l'exposition a la lumiere et a la chaleur a ete etudie. Les groupes de cetones ont nui de facon importante a la perte de blancheur en
raison de la chaleur, tandis que les groupes aldehydes a C2-C3 n'ont eu qu'un effet mineur. Les groupes carboxyles a C2-C3 ont nui a la
stabilite de la blancheur en raison de la chaleur. Les cetones, les aldehydes et les groupes carboxyles n 'ont cependant eu aucun effet sinla stabilite de la blancheur lors de I'exposition a la lumiere. Enfin, il a ete demontre que, pour une perte de blancheur donnee, la cellulose
se degradait moins a la lumiere qu 'a la chaleur, ce qui suggere des mecanismes differents.
INTRODUCTION
Brightness stability of bleached
chemical pulps is an important characteristic
for many paper grades. Most of the earlier
investigations, made in the 60-70s, were
devoted to brightness reversion upon heat
exposure. Many parameters were reported
to potentially affect brightness stability on
heat exposure: pulp type, wood species,
presence of lignin, hemicellulose, oxidized
groups in cellulose or metal ions [ 1 -11 ]. The
Jp
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conclusions obtained were sometimes contradictory. One explanation could be that the
methods employed to make the brightness
reversion test are numerous, the parameters
of which (temperature, time, presence of
oxygen) have a significant effect on the
result [1,2]. Therefore it is almost impossible to link the conclusions to each other and
no general mechanism has been proposed so
far.
The changes which have occurred
these last years in the bleaching of chemical
pulps have renewed the interest in studying
the yellowing of pulp. The use of oxygen
based reagents, like O2, H2O2 or O3, in elemental chlorine free (ECF) or totally chlorine free (TCF) sequences can lead to the
formation of additional oxidized groups in
cellulose (carbonyl and carboxyl groups).
Furthermore, the use of hydrogen peroxide,
which is an excellent whitening agent, may
produce pulps of high brightness (above
88% ISO) but still containing some residual
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 25 NO. 6 JUNE 1999
lignin.
The objectives of this work were to
study the effect of selected parameters on
brightness reversion upon heat and light
exposure, to understand better the mechanisms of pulp yellowing, and finally to propose ways to minimize it.
The work presented here, which constitutes the first part of the study, deals with
the characterization of several commercial
pulps, including their brightness stability,
and with a more fundamental approach in
which new functional oxidized groups are
intentionally created on carbohydrates and
their effect on brightness stability is studied.
MATERIAL AND METHODS
Material
Industrial pulps were received in
dried form. Mill A pulp was a spruce softwood kraft pulp bleached with either an
ODED sequence or with an OZEZP sequence. The same types of sequences were
201
TABLE CHARACTERIZATION AND BRIGHTNESS
REVERSION OF INDUSTRIAL KRAFT PULPS
Mill
Pulp
Bleaching Stages
involved
Brightness, %
Kappa number
Carbonyl, meq/100 g
Carboxyl, meq/100g
Brightness reversion
upon heat exposure*
Brightness reversion
upon light exposure **
* Heat exposure: 48 h;
** Light exposure: 1 h
A
Softwood kraft
ECF
TCF
O,D
O,Z,P
88.2
87.0
B
Birch kraft
ECF
TCF
0,D
0,Z,P
88.2
88.0
7.2
5.4
2.1
6.3
7.4
<1
6.1
6.8
3.6
6.8
14
5.3
8.6
4.4
8.9
3.4
5.0
5.4
5.8
<1
used for mill B pulp (birch kraft pulp). Mill
C pulps were spruce and mixed hardwood
bisulphite pulps. They were both bleached
with an ECF sequence (ODED type) and
with a TCP sequence (OA(EOP)).
The pulp used for the second part is a
softwood kraft pulp, bleached with a DEDED
sequence (v = 16.4 mPa-s, brightness =
89.6% ISO).
Methods
Handsheets for brightness measurement and brightness reversion tests were
prepared from pulp suspensions in deionized water at a pH around 6. The basis
weight of the handsheets was 100 g/m2.
Viscosity was measured according to
TAPP1 Standard T 230 om 89. Viscosity was
always measured after reduction of the pulp
with sodium borohydride because of the alkali sensitivity of the carbonyl-containing
pulp. Brightness was an ISO brightness
measured according to ISO Standard 2470.
Carbonyl groups were measured according to the sodium cyanide method
described by Lewin and Epstein [12]. Carboxyl groups were determined by the
methylene blue absorption method as described by Davidson [13]. Aldehyde groups
were determined by oxidizing the samples
with sodium chlorite and acetic acid, and
then redetermining the carboxyl content of
the oxidized samples. The difference between this value and the original value gives
the aldehyde content of the sample.
Brightness reversion upon heat exposure was carried out in an oven at 105°C,
under dry conditions, for 24 to 72 h. Brightness reversion upon light exposure test was
performed in a Suntest apparatus equipped
with an UV filter, at 113 W/h/m2, for 30 min
TABLE II CHARACTERIZATION
AND BRIGHTNESS REVERSION OF INDUSTRIAL
BISULPHITE PULPS
Mill
Pulp
Bleaching
C
Softwood
ECF
TCF
(0,D)
(O.P)
91.3
86.4
<1
2.2
Brightness, %
Kappa number
Carbonyl, meq/100g
Carboxyl, meq/100g
Brightness reversion
upon heat exposure*
Brightness reversion
upon light exposure**
* Heat exposure: 48 h; **
Light exposure: 1 h
to l h.
Brightness reversion is calculated as
shown in Eq. (1).
RESULTS
Brightness Reversion of
Industrial Pulps
Tables I and II give the characteristics
of mill pulps, bleached with ECF and TCF
sequences.
The TCP bleached pulps have still
relatively high final kappa numbers compared to the ECF pulps, despite their high
final brightness. This can be explained by
the fact that hydrogen peroxide is used in
almost all TCP sequences (peroxide-based
sequences of the OQPQ(PO) type, or ozonebased sequences of the OZEZQP type).
Another difference between the ECF
and TCP pulps is that TCF pulps have a
higher carboxyl content, more particularly
in the case of the birch kraft pulps. One
explanation could be that carboxyl groups
are created during bleaching, which is not
surprising, since oxygen, ozone and hydrogen peroxide are capable of creating such
groups. Another hypothesis is that the birch
kraft pulp already has a high carboxyl content after cooking, the fate of which depends
on the bleaching agents used [14]. As far as
carbonyl content is concerned, the differences observed among the pulps are small.
In the case of the kraft pulps, TCF
bleaching led to less stable pulps on heat
exposure than ECF. Brightness stability to
light exposure was either better in the case
of ECF (mill A) or not influenced by the
bleaching type (mill B).
The softwood bisulphite pulp from
mill C showed high but comparable brightness reversion upon heat exposure for both
EQUATION (1)
brightness before exposure - brightness after exposure
Hardwood
ECF TCF
(0,D) (0,P)
92.5 90.5
<1
1.8
6.7
4.8
6.0
6.2
6.7
4.5
6.5
6.0
7.8
7.7
5.5
8.5
2.0
4.3
2.0
3.3
ECF and TCF sequences, contrary to what
was observed for the kraft pulps. On the
other hand, the bisulphite hardwood pulp
behaved similarly to the kraft pulps. This
result could be explained by the fact that
softwood bisulphite pulps have a high extractives content which strongly influences
the brightness stability to heat. Indeed, the
removal of the extractives with dichloromethane (DCM) improved the brightness
stability to heat exposure significantly (5%
brightness drop after DCM treatment compared to 8% for the control). Brightness
stability to light was much better for all the
bisulphite pulps than for the kraft pulps.
These results show that the type of
bleaching sequence has an effect on the
brightness stability to heat or light exposure
to a certain extent only. TCF pulps from one
mill might be more stable than ECF pulps
from another mill. The contrary is also true
in other cases. No general statement can be
made.
Effect of Oxidized Groups in
Cellulose
Oxidized groups in cellulose and
more particularly carbonyl groups have
been claimed to be responsible for brightness reversion upon heat exposure [1,4-7].
Carbonyl groups are also assumed to favour
light yellowing in various substrates [3]. The
results presented in the previous part
showed that industrial pulps actually contain
different quantities of carbonyl and carboxyl
groups. The effect of specific functional
groups (aldehyde, ketone, carboxyl, located
on different carbon atoms in carbohydrates)
on brightness stability to both heat and light
exposure is, however, not well known.
The study presented here deals with
the effect of carbonyl groups created by specific oxidation reactions of carbohydrates,
on brightness reversion upon heat and light
exposure. A softwood kraft pulp bleached
with a conventional DEDED sequence was
chosen as a model for the carbohydrates in
pulps.
brightness before exposure
202
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 25 NO. 6 JUNE 1999
Fig. 1. Formation of functional groups In cellulose during perlodate oxidation (perlodate oxidation was carried out at room
temperature, 2% pulp consistency, with 0.05 mol/L sodium periodate, for 15 min to 2 h).
Fig. 2. Formation of functional groups in cellulose during ozonation (ozonation was carried out at low pulp consistency (3.5%)
and room temperature).
Fig. 3. Effect of the amount and nature of carbonyl groups on
brightness reversion upon heat exposure (24 h).
Fig. 4. Effect of the amount and nature of carbonyl groups on
brightness reversion upon light exposure (30 min).
Effect of Carbonyl Groups in
Cellulose
Three ways were chosen to create
carbonyl groups on cellulose or related
materials:
— Periodate oxidation, leading to the for
mation of aldehyde groups at carbons C2
and C3 (Fig. 1);
— Ozonation, leading to the formation of
ketone groups at carbons C2 and/or C3[7]
(Fig. 2);
— Hypochlorite oxidation in neutral or
acidic conditions leading to the forma
tion of both ketone and aldehyde groups
(mainly at the C1and C6 position) [12]
(Table III).
Figure 3 presents the effect of the different types of carbonyl groups on brightness reversion upon heat exposure. It can be
concluded that:
— Dialdehyde groups at C2-C3 affect only
slightly brightness reversion upon heat
exposure;
TABLE III EFFECT OF HYPOCHLORITE TREATMENT AT
DIFFERENT pH ON CELLULOSE CHARACTERISTICS
Reference
COOH,meq/100g
Carbonyl, meq/100 g:
Ketone groups
Aldehyde groups
None
4.9
2.5
2.3
0.2
H1
4.4
16.2
7.7
8.5
Hypochlorite treatment at 5% consistency, 20°C, during
NaCIO: 0.1 mol/L. H1 : initial pH 4 ; H2: initial pH 7; H3:
buffer pH 7.
— Ketone groups at C2 or C3 are
detrimental
to brightness stability upon heat expo
sure, the reversion increasing with the
amount of ketone groups;
— Presence of ketone at C2 or C3 and at the
same time of aldehyde groups (at C1 and
or C6) is more detrimental to brightness
reversion upon heat exposure than ketone
groups alone, for a given amount of carbonyl groups in cellulose.
H2
6.4
25 14
11
H3
5.4
8.3
4.7
3.6
120 min; initial concentration of
Contrary to the results of brightness
reversion on heat exposure, it was found that
the amount and the nature of carbonyl
groups in cellulose do not significantly impair brightness stability upon light exposure
(Fig. 4). For highly oxidized cellulose.
brightness stability upon light exposure is
even improved, which could be explained by
the removal of residual chromophores in the
pulp. These results are rather surprising.
203
since it is generally thought that brightness
reversion upon light exposure is initiated by
UV absorption by unsaturated compounds
such as carbonyl groups [3].
Effect of Carboxyl Groups in
Cellulose
Carboxyl groups were created on cellulose by oxidation with periodate to create
dialdehyde groups, and then by further oxidation with chlorite. A strong reduction with
sodium borohydride was then performed to
make sure that no carbonyl groups were left.
Washing was performed after each of these
steps.
The results presented in Table IV
show that carboxyl groups at C2-C3 can
significantly impair brightness stability to
heat exposure, whereas no effect is detected
on brightness stability to light.
Effect of Heat and Light
Exposure on Cellulose
Characteristics
The control pulp and one oxidized
pulp (periodate-treated pulp) were exposed
to heat (for 24 and 48 h) and light (for 30 and
60 min). The relative brightness drop was
plotted against pulp viscosity after aging
(Fig. 5). These results show that pulp viscosity decreases when the pulp is exposed to
either heat or light. For a given brightness
reversion value, however, it can be noticed
that heat exposure leads to a higher viscosity
loss than light exposure. These results suggest differences in the mechanisms of yellowing upon heat and light exposure.
Carbonyl and carboxyl groups were
measured after aging of the bleached softwood kraft pulp (Table V). The functional
group determination was performed directly
after aging, with no washing of the pulp, to
avoid the dissolution and elimination of low
molecular chains which might result from
cellulose degradation during aging.
Table V results show that exposure of
pulps to heat leads to an increase in carbonyl
groups, mainly of the ketone type. By performing a strong reductive stage with
sodium borohydride, only part of the brightness loss could be recovered, which implies
that the carbonyl groups formed during heat
exposure contribute only partly to the yellow colour, other structures being involved
Fig. 5. Effect of heat and light exposure on viscosity of the control and periodateoxidized pulps.
in the development of colour, or that some
carbonyl-containing groups, in very low
quantities, cannot be reduced under the conditions used here. Further investigations are
needed to answer this question.
As far as the effect of light exposure
is concerned, only a minor increase in carbonyl groups could be detected, and no
change in carboxyl groups was observed.
CONCLUSIONS
The following conclusions can be
drawn from this study:
— TCF bleaching yields pulps with higher
kappa and higher carboxyl groups, which
in some cases lead to higher brightness
reversion upon heat or light exposure
compared to the corresponding ECF
bleached pulps. However, a TCF pulp
from one mill can be more stable than the
ECF pulp from another mill.
— Pulps bleached with sequences giving a
ketone content higher than 10 meq/100 g
are likely to have poor brightness stabil
ity upon heat exposure.
— The simultaneous presence of ketone and
aldehyde groups at C1 or C6 may be even
more detrimental than ketone groups
alone.
— Carbonyl groups of the dialdehyde type
at C2-C3 have no significant effect on
brightness reversion upon heat exposure.
Carboxyl groups at C2-C3 , on the other
hand, seem to have a negative effect.
— Neither carbonyl nor carboxyl groups
were found detrimental to brightness
reversion upon light exposure.
— For a given brightness reversion, light
exposure leads to lower pulp degradation
than heat exposure, suggesting different
mechanisms.
Further investigation is needed to
complete the study on the effect of oxidized
groups on brightness reversion.
ACKNOWLEDGEMENTS
The authors wish to thank ADEME
French Agency and the DGXII of the European Commission.
REFERENCES
1. RAPSON, W.H. and SPINNER, I.H.,
"Brightness Reversion in Bleached Pulps" in
The Bleaching of Pulp, by R.P.Singh, Ed.,
TAPPI PRESS, 357-391 (1962).
2. SHAFIZADEH, F., "Thermal Degradation of
Cellulose" in Cellulose Chemistry and its Applications, T.P. Nevell and S.H. Zeronian,
Eds., Ellis Horwood Ltd, 266-289 (1985).
3. PHILLIPS, G.O., "Photochemistry and Ra
diation Chemistry of Cellulose" in Cellulose
Chemistry and its Applications, T.P. Nevell
and S.H. Zeronian, Eds., Ellis Horwood Ltd,
290-311(1985).
4. GURNAGUL, N., HOWARD, R.C., ZOU,
204
JOURNAL OF PULP AND PAPER SCIENCE: VOL. 25 NO. 6 JUNE 1999
X., UESAKA, T. and PAGE, D.H., "The Mechanical Permanence of Paper: A Literature
Review",/ Pulp Paper Sci. 19(4):J160-J166
(1993).
5. SJOSTROM, E. and ERIKSSON, E., "The
Influence of Carboxyl and Carbonyl Groups
on the Brightness Stability of Bleached
Pulps", TappiJ. 51(1):16-19 (1968).
6. RAPSON, W.H. and HAKIM, K.A., "Carbonyl Groups in Cellulose and Color Rever
sion", Pulp Paper Mag. Can. 58(8):151-157
(1957).
7. CHIRAT, C. and LACHENAL, D., "Effect of
Ozone on Pulp Components. Application to
Bleaching of Kraft Pulps", Holzforschung
48:T133-139(1994).
8. LORAS, V., "Factors Influencing Loss in
Brightness During Heating of Chemical
Pulps", Norsk Skog. 21(10):368-375 (1967).
9. GULLICHSEN, J. and SODERHJELM, L.,
"On the Color Reversion of Bleached Kraft
Pulp", Paperija Puu 1:34-37 (1984).
10. GERMGARD, U. and KARLSSON, R.M.,
"Bleaching of Birch Kraft Pulp with Different
Fractions of Chlorine Dioxide in the Pre-
REFERENCE: CHIRAT, C. and DE LA CHAPELLE, V., Heat- and Light-Induced Brightness Reversion of Bleached Chemical Pulps. Journal of Pulp and Paper Science,
25(6):201-205 June 1999. Paper presented at the 9th Intl. Symposium on Wood and
Pulping Chemistry of the Technical Section, Canadian Pulp and Paper Association, cosponsored by PAPTAC, Paprican, TAPPI, Japan TAPPI, EUCEPA, Appita, CTAPI and the
Canadian Society for Chemistry in Montreal, QC, on June 9-12, 1997. Not to be
reproduced without permission from the Pulp and Paper Technical Association of Canada.
Manuscript received May 1, 1997; revised manuscript approved for publication by the
Review Panel February 9,1999.
KEYWORDS: BLEACHED PULPS, CHLORINE FREE BLEACHING, KRAFT PULPS,
BISULFITE PULPS, HARDWOODS, SOFTWOODS, COLOR REVERSION, BRIGHTNESS, AGING, HEAT, LIGHT, KETONES, ALDEHYDES, CARBOXYL GROUPS.
bleaching", Nordic Pulp Paper Res. J. 4:166-171
(1988).
11. SMIT, D., "Brightness Reversion", Paper
Southern Africa 34-39 (Aug. 1993).
12. LEWIN, M. and EPSTEIN, J.A., "Functional
Groups and Degradation of Cotton Cellulose
Oxidized by Hypochlorite", J. Polymer Sci.
58:1023-1037(1962).
13. DAVIDSON, G., "The Acidic Properties of
Cotton Cellulose and Derived Oxycelluloses. Part
II. The Absorption of Methylene Blue". J.
Textile
Inst.
39:T65-85
(1948).
14.
VUORINEN, T., TELEMAN. A.. FAGERSTROM, P., BUCHERT. J. and TEUKANEN. M.,
"Selective Hydrolysis of Hexenuronic Acid
Groups and Its Application in ECF and TCP
Bleaching of Kraft Pulps". Proc. Intl. Pulp
Bleaching Conf.. WA. 43-51 (1996).