PA P E R C O L O U R
During storage and shipping, nitrogen oxides
can cause rapid yellowing and degradation
of pulp and paper products
By X. Zou
Abstract: Rapid yellowing and degradation of pulp and paper products have usually been associated with the effect of light or extreme heat. In this study, we found that exposure to NOx, even
at very low concentrations can cause rapid yellowing and degradation of pulp or paper. This result
explains the rapid yellowing and degradation often observed during overseas shipping or shortterm storage. To prevent this, it is recommended that NOx concentration should be monitored and
controlled, and/or paper should be properly wrapped during shipping and storage.
and degradation of
pulp and paper products are usually
due to the effect of light or extreme
heat [1,2]. However, rapid yellowing
of mechanical pulps and newsprint,
and significant degradation of market pulps have
often been observed during short-term storage in
warehouses or during overseas shipping. These
phenomena cannot be explained by the effect of
light or heat because it happens in the dark, and
in some cases, even in a temperature and humidity controlled atmosphere.
Air pollutants (e.g., SO2 and NOx) have always
been a concern to long-term degradation of
materials such as paper, film, plastics and metal
[3]. Zinn et al. found that the air pollutants in
storage atmospheres cause degradation of colour
photographs [4]. Wan and Depew observed that
lignin-containing paper suffers a very rapid
yellowing and degradation when exposed to extremely high concentrations of NOx [5]. A more
recent study on the effect of air pollutants showed
that at a low concentration of 10-20 ppm, air pollutants (mainly NOx) can cause significant degradation and reduction in brightness in lignincontaining paper [6-8]. These results may explain
the yellowing and degradation of paper products
without light and heat. The objective of this
paper is to show that NOx is indeed the culprit for
rapid yellowing and degradation, even at low NOx
concentrations occurring in practical situations.
R
APID YELLOWING
oxides such as N2O3 and N2O4 are formed at
room temperatures [9]. Since NO is always in
equilibrium with NO2, the total concentration of
nitrogen oxides is the sum of the concentrations
of NO2 and NO ([NOx] = [NO2] + [NO]). The
absorption of NO2 results in significant acidification of paper which, in turn, leads to acidcatalyzed hydrolysis of cellulose.
Oxidation: Both cellulose and lignin can be
attacked directly by strong oxidants such as NO
and NO2. The attack on cellulose leads to depolymerization of cellulose and loss of mechanical
strength while the attack on lignin can lead
to yellowing.
EXPERIMENTAL
NO2 is the most stable nitrogen oxide in the exhaust from fuel burning at very high temperatures
(>1,100°C). It is a strong oxidant and an acidic
gas. NO2 can interact with paper in two ways:
Acidification and acid-catalyzed hydrolysis: Since
paper usually contains a certain amount of moisture (5-10% of total paper weight), NO2 can readily react with the water to form acids according to
the following overall reaction [4]:
Apparatus — environmental chamber
An automated environmental chamber was used
to generate desired concentrations of NOx, as
previously described in details [7,8]. The temperature, relative humidity and concentration of
NOx in the chamber can be controlled.
Samples
Standard handsheets were made from lignincontaining pulps (TMP and BCTMP) and ligninfree pulp (cotton fibres). Handsheets of BCTMP
and cotton were also made with the addition of
2% calcium carbonate. The initial properties
of the above handsheets such as pH, degree of
polymerization (DP), brightness and zero-span
tensile strength, are summarized in Table I. More
detailed information can be found in the literature [8].
Exposure and analysis
Paper samples were suspended in the environmental chamber and exposed to various concentrations of NOx for different time periods at 23°C
and 50% RH. After each exposure experiment,
brightness, strength, and chemical properties of
the exposed papers were measured. Details of the
testing procedures can be found in previous publications [6-8].
3NO2(g) + H2O(l) ⇔ 2HNO3(l) + NO(g)
RESULTS
CHEMISTRY ASPECT
(1)
For every 3 moles of NO2 that absorb and
react, 1 mole of NO will be produced and desorbed, while 2 moles of HNO3 will accumulate in
paper. No significant amounts of other nitrogen
Pulp & Paper Canada T 75
I. Effect of NOx exposure on yellowing
The effect of NOx concentration on yellowing is
shown in Fig. 1 for spruce BCTMP and in Fig. 2
for spruce TMP. As can be seen, increasing NOx
concentration significantly increases the reduc-
X. ZOU
Pulp and Paper Research
Institute of Canada
Pointe Claire, QC
e-mail: [email protected]
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PA P E R C O L O U R
TABLE I.Initial properties of handsheets.
Sample
pH
DP
Brightness
(%)
Zero-span
tensile
strength(km)
BCTMP (spruce)
BCTMP (aspen)
TMP (spruce)
Cotton fibre
BCTMP (aspen)
with 2% CaCO3
Cotton fibre
with 2% CaCO3
6.9
6.7
6.2
6.6
1247
1985
1240
1932
71.9
81.0
55.6
85.3
12.4
11.9
11.7
15.3
9.1
—
78.4
12.7
9.6
—
86.1
15.2
TABLE II. Properties of handsheets after exposure to 20
ppm of NOx for 3 days.
Sample
pH
Brightness
(%)
Increase in nitrate
content (mg/g)
BCTMP (spruce)
BCTMP (aspen)
TMP (spruce)
Cotton fibre
BCTMP (aspen)
with 2% CaCO3
Cotton fibre
with 2% CaCO3
3.5
3.5
3.0
3.6
55.8
65.2
36.4
82.6
4.38
4.63
4.02
1.87
8.5
60.4
4.11
9.0
85.0
3.22
tion in brightness (yellowing). Even at a concentration as low as
2 ppm, exposure to NOx can cause a brightness drop of about
2-4 points in 24 hours. This indicates that exposure to NOx, even
at a very low concentration, causes significant yellowing of
mechanical pulps. The yellowing is accompanied by a significant
increase in acidity and nitrate content in the pulps, Table II.
Plotted in Fig. 3 are the results on the yellowing of handsheets from different pulps (cotton, aspen BCTMP, spruce
BCTMP and spruce TMP) after exposure to 20 ppm NOx. As can
be seen there is a similar, but significant, brightness drop for all
three mechanical pulps. However, the brightness of the sheets
from cotton fibre remains constant after exposure to NOx. This
indicates that the presence of lignin is responsible for the yellowing by NOx. NOx is known to be a very strong oxidant and its
reaction with lignin causes yellowing [5]. This may explain the
rapid yellowing of mechanical pulps and newsprint during shortterm storage in warehouses or shipping. In such situations, the
air pollutants come from gas and oil burning engines (e.g., forklifts in warehouses, gas or oil-burning heating systems, or
engines in the ship). This was confirmed from two case studies
discussed later.
Although literature results showed that an alkaline reserve
such as CaCO3 in paper could protect the paper from acidcatalyzed degradation [2], the addition of 2% CaCO3 has no
effect on the yellowing caused by NOx, as shown in Fig. 4. This
is probably because CaCO3 does not stop the direct attack by
NOx on lignin.
II. Effect of NOx exposure on cellulose degradation and
strength loss
Exposure to NOx causes not only yellowing, but also cellulose
degradation and strength loss. As shown in Figs. 5 and 6, zerospan tensile strength is reduced with exposure time for both
BCTMP and cotton sheets. It is interesting to note that the initial reduction of zero-span tensile strength (an indication of
fibre strength) is more rapid for cotton sheets. This suggests that
the lignin in BCTMP seems to protect cellulose from degradation, by preferentially reacting with NOx. However, after 20 days
of exposure, both BCTMP and cotton sheets retain only about
50% of the original strength.
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FIG. 1. Brightness as a function of exposure time at different NOx concentrations (spruce BCTMP).
FIG. 2. Brightness as a function of exposure time at different NOx concentrations (spruce TMP).
FIG. 3. Comparison of different pulps in yellowing caused
by NOx.
FIG. 4. Effect of 2% CaCO3 addition on yellowing (aspen
BCTMP).
T 76 Pulp & Paper Canada
PA P E R C O L O U R
TABLE III. Results from a case study of BCTMP yellowing.
Sample
Brightness
(%)
Nitrate
content (mg/g)
85
1.36
77
1.92
60
4.40
BCTMP
(no yellowing)
BCTMP
(yellowed during storage)
BCTMP
(after artificial exposure)
The loss of fibre strength is caused by the degradation of cellulose, as demonstrated in Figs. 7 and 8. The results are plotted
as (1/DPt – 1/DP0) vs. t, based on a previous kinetic model for
paper degradation [10]:
1 – 1
=k.t
DP0
DPt
(2)
where DPt and DP0 are the degree of polymerization at time
t and 0, and k is the rate constant. Results, in Figs. 7 and 8, show
that exposure to NOx causes rapid degradation of cellulose. In
the case of cotton sheets, the cellulose is quickly degraded to the
point of levelling-off DP (in the first few hours, as shown in
Fig. 8). The cellulose degradation is more rapid for cotton
sheets than BCTMP sheets. This indicates again that the reaction
of lignin with NOx (yellowing) causes yellowing, but helps protect cellulose from degradation.
The addition of 2% CaCO3 helps reduce the degradation of
cellulose, as also shown in Figs. 7 and 8. This confirms previous
findings that an alkaline reserve such as CaCO3 can protect
cellulose from acid-catalyzed hydrolysis [2,6]. The implication is
that sheets buffered with an alkaline reserve will suffer less
degradation in an atmosphere containing NOx.
III. Case Studies
Two case studies were carried out to link the yellowing and
degradation of pulp and paper products observed in storage and
shipping to the presence of NOx.
Case 1: Rapid yellowing of BCTMP during warehouse storage
Company A observed significant yellowing of aspen BCTMP
pulp stored in a warehouse during winter months. This pulp was
packed in paper boxes, but part of it (outer layer) yellowed substantially after a four-month period of storage (a brightness drop
from 85 to 77 in the outer layer). Analysis of the yellowed samples showed an increase in nitrate content, Table III, indicating
the absorption of NOx by BCTMP. A subsequent artificial exposure of this BCTMP sample to 20 ppm NOx for 3 days confirmed
that this BCTMP indeed showed a significant reduction in
brightness, and an increase in the nitrate content in the pulp,
also in Table III.
Forklifts used in the warehouse are possible sources of NOx
in this case, because typical NOx concentrations from engines
burning fuels are 25-150 ppm for gas, and 60-1,000 ppm for oil.
Without good air circulation (particularly in winter), NOx can
accumulate to some extent. This was confirmed by a field study
showing the presence of NOx inside the warehouse during the
winter. Vapour wrapping was recommended and adopted by this
company. The yellowing problem then disappeared.
Case 2: Yellowing and degradation of market pulps
Company B reported that a portion of their market kraft pulp
was seriously degraded after a 45-day overseas trip. Pulp viscosity
(an indication of cellulose DP) dropped from 20 to 4 cp, and
brightness also decreased. The yellowing and degradation
occurred to samples that were close to the engine room and that
had their wrapping broken.
An analysis of the degraded pulp showed a significant increase in nitrate content from 3.55 to 6.60 mg/g, indicating that
NOx is present and absorbed by the pulp. The NOx concentra-
Pulp & Paper Canada T 77
FIG. 5. Effect of 2% CaCO3 addition on zero-span tensile
strength (aspen BCTMP).
FIG. 6. Effect of 2% CaCO3 addition on zero-span tensile
strength (cotton fibre).
FIG. 7. Effect of 2% CaCO3 addition on cellulose degradation (aspen BCTMP).
FIG. 8. Effect of 2% CaCO3 addition on cellulose degradation (cotton fibre).
105:3 (2004) ❘❘❘
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PA P E R C O L O U R
tion is usually higher near the engine
room. When the vapour wrapping is broken, NOx can easily reach the pulp. This
indicates again that NOx is the culprit.
CONCLUSIONS
Exposure of pulp and paper to NOx, even
at very low concentrations, can cause
rapid yellowing and degradation. Lignincontaining sheets suffer rapid yellowing,
while lignin-free sheets suffer significant
strength degradation. This explains the
yellowing of mechanical pulps and newsprint, as well as the degradation of market
kraft pulps often observed during overseas shipping or short-term storage. The
addition of 2% CaCO3 protects paper
from rapid degradation caused by NOx.
To prevent yellowing, the following measures are recommended:
• Monitor and control the level of NOx in
warehouses (e.g., good air circulation).
• Use vapour wrapping to stop the NOx
penetration into the pulp products or
paper roll.
LITERATURE
1. LEARY, G.J., Recent Progress in Understanding and
Inhibiting the Light-induced Yellowing of Mechanical
Pulps, J. Pulp Paper Sci. 20(6): J154-J160 (1994).
2. GURNAGUL, N., HOWARD, R., ZOU, X., UESAKA, T., PAGE, D.H. The Mechanical Permanence of
54
❘❘❘ 105:3 (2004)
Paper: A Literature Review. J. Pulp Paper Sci. 19(4):
J160-166 (1993).
3. GURNAGUL, N., ZOU, X. The Effect of Air Pollutants on the Mechanical Permanence of Paper: A Literature Review. Tappi J. 77(7): 199-204 (1994).
4. ZINN, E., REILLY, J.M., ADELSTEIN, P.Z.,
NISHIMURA, D.W. Preservation of Colour Photographs: the Danger of Atmospheric Oxidants in the
Storage Environment, Proceedings of the 2nd ASARG
Conference on Environment and Conservation, Paris,
France, pp. 325-330 (1994).
5. WAN, J.K.S., DEPEW, M.C. Effects of NOx Exposure
on Paper: the Role of Free Radicals. J. Pulp Paper Sci.
22(5): J174-177 (1996).
6. BEGIN, P., DESCHATELETS, S., GRATTON, D.,
GURNAGUL, N, IRACI, J., KAMINSKA, E., WOODS,
D., ZOU, X. The Effect of Air Pollutants on Paper Stability. Restaurator 20: 1-21 (1999).
7. ZOU, X., DESCHATELETS, S., GURNAGUL,
N., BEGIN, P., IRACI, J., GRATTON, D., KAMINSKA,
E., WOODS, D. Canadian Co-operative Permanent
Paper Research Project: The Impact of Lignin on
Paper Permanence, Final Report (January 1998).
8. ZOU. X., DESCHATELETS, S., STONE, R., MURRAY, T. The Impact of Lignin on Paper Permanence.
Part II Apparatus and Protocol for Studying the
Effects of Air Pollutants, Paprican PPR 1315 (November 1997).
9. NELLI, C.H., ROCHELLE, G.T. Nitrogen Dioxide
Reaction with Alkaline Solids. Ind. Eng. Chem. Res.
35(4): 999-1005 (1996).
10. ZOU, X., UESAKA, T., GURNAGUL, N. Prediction
of Paper Permanence by Accelerated Ageing. Part I.
Kinetic Analysis of the Ageing Process. Cellulose J. (3):
243-267 (1996).
Résumé:
Le jaunissement rapide et la dégradation des produits de pâte et de papier ont
habituellement été associés à l’effet de la lumière ou de la chaleur extrême. La présente étude
nous a permis de déterminer que l’exposition aux NOx, même à de très faibles concentrations,
peut entraîner le jaunissement rapide et la dégradation de la pâte ou du papier. Ce résultat
explique le jaunissement rapide et la dégradation souvent observés lors des livraisons outre-mer
ou du stockage de courte durée. Pour éviter cette situation, il est recommandé de surveiller ou de
contrôler la teneur en NOx ou d’emballer le papier de manière appropriée durant le transport et
le stockage.
Reference: ZOU, X. During storage and shipping, Nitrogen oxides can cause rapid yellowing
and degradation of pulp and paper products. Pulp & Paper Canada 105(3): T75-78 (March, 2004).
Paper presented at the 88th Annual Meeting in Montreal, QC, Canada, January 28-31, 2002. Not
to be reproduced without permission of PAPTAC. Manuscript received October 17, 2001. Revised
manuscript approved for publication by the Review Panel on October 3, 2003.
Keywords: YELLOWING, PULPS, PAPER, NITROGEN OXIDES, DEGRADATION, CONCENTRATION.
T 78 Pulp & Paper Canada