|Título/s:||Optimizing alkaline sizing in sugar cane bagasse paper recycling|
|Autor/es:||Molina Tirado, Liliana Beatriz; Area, María Cristina; Vélez, Hugo Enrique|
|Institución:||PROCYP - UNAM. Posadas, AR |
INTI-Celulosa y Papel. Buenos Aires, AR
|Editor:||Cellulose Chemistry and Technology|
|Palabras clave:||Reciclaje; Bagazo; Caña de azúcar; Papel; Aditivos; Dosaje; Alcalinidad; Propiedades físicas; Ensayos; Humedad; Temperatura; Absorción de agua|
| Ver+/- |
CELLULOSE CHEMISTRY AND TECHNOLOGY
Cellulose Chem. Technol., 43 (4-6), 179-187 (2009)
OPTIMIZING ALKALINE SIZING
IN SUGAR CANE BAGASSE PAPER RECYCLING
LILIANA BEATRIZ MOLINA TIRADO, MARIA CRISTINA AREA* and
HUGO ENRIQUE VÉLEZ
INTI – Celulosa y Papel, Av. General Paz 5445, B1650KNA San Martín,
Buenos Aires, Argentina
*PROCYP – UNAM, Posadas, Misiones, Argentina
Received March 11, 2009
The objective of this work was the variation of additives dosage in the sizing treatment of sugar cane bagasse paper
for liners and flutings, on taking into account the cycles of use, for reaching the same sizing degree in each cycle. The
degradation of the physical properties under different conditions of relative humidity and temperature for each stage
of the papermaking cycle was also studied.
The sizing agent utilized was the alkyl ketene dimer (AKD). Cationic starch and a retention agent (a modified high
molecular weight polyethylenimine) were also added. The experimental design applied was a Central Composite
Design (CCD). Once the optimum dosage of additives found, the sheets were submitted to different conditions of
humidity and temperature, and their physical properties were tested. Three papermaking cycles were carried out: a
papermaking cycle, followed by two recycles.
It was observed that a lower amount of AKD was required to reach the water absorption objective (measured by the
Cobb120 test), as it progresses during the papermaking cycles, and also that good properties in liner and fluting paper
could be attained in spite of the recycles, although, at 75% relative humidity (RH), the liner and fluting properties
Keywords: recycling, alkaline sizing, AKD, bagasse
The use of recycled paper as a raw material is
nowadays a fundamental economic factor,
although it is generally accepted that the quality
of secondary fibers is lower than that of virgin
pulp.1-5 Mechanical and chemical treatments
contribute to the reduction of this difference, yet
without wholly recovering the total quality of
Whatever the origin of the raw material, a
sizing treatment has to be carried out in a stock
preparation stage for liner and fluting
papermaking, to limit the penetration rate of
aqueous liquids into the paper sheet (in this case,
especially water absorption). This is a
fundamental condition for corrugated cardbox,
since it should maintain its resistance with respect
to variation in humidity. To this end, a sizing
agent is added, besides other additives, such as
cationic starch and a retention agent, to improve
its performance. The sizing mechanism of AKD
has been thoroughly studied.9-21 The AKD forms
covalent ester bonds with the cellulose hydroxyl
groups, although it does not cover the complete
fiber surface and does not interfere with interfiber
bonding.22,25 Nevertheless, some investigators
question the existence of covalent unions between
AKD and fibers.26 It is also known that the sizing
effect is lost when sized fibers are re-slushed. The
influence of the previous treatment on the new
sizing has been studied by Sjöström and Ödberg,27
in a bleached softwood kraft pulp. These authors
found out a residual effect of the previous sizing
Liliana Beatriz Molina Tirado et al.
treatment, as the fibers were hydrophobized to
Sugar cane bagasse fibers are more and more
frequently found in recycled pulp used for liner
and fluting paper, in national productions. This
work is focused on the behavior of bagasse pulp
subjected to sizing treatments during recycling.
No minerals were added to each papermaking
cycle, for minimizing their interference in the
sizing treatment, although an industrially recycled
pulp could be obtained.
Even when some studies suggest that the use
of large quantities of secondary fibers requires
higher amounts of additives, some mills that use
great amounts of recycled fibers have noticed no
changes in the consumption of sizing and
The influence of relative humidity on the
mechanical strength of sized paper was described
by different authors, using other kinds of fibrous
A sample of sugar cane bagasse semichemical pulp
was used as raw material. This pulp was screened on a
Weverk screen equipment (0.15 mm slots). In the first
stage, the screened pulp was characterized as to fiber
length, refining degree and percentage of fines passing
through a 200-mesh, after which the pulp was
mechanically treated in a PFI refiner, recording the
energy consumption, in accordance with the refining
degree industrially applied for this raw material (35
For the sizing treatment, an aqueous dispersion of a
fatty alkyl ketene dimer with a medium cationic charge
(Basoplast 4118 MC) was used. To achieve a better
performance, the sizing agent was accompanied by
cationic starch and a retention agent (high molecular
weight polyethylenimine, Polymin SK).
A solution of 1% cationic starch was prepared after
cooking it at 90-95 ºC for 15 min. According to the
manufacturer’s recommendations, the aqueous
solutions of the sizing and retention agent were
formulated during the addition to the furnish.
The additives were added to the pulp at 1%
consistency, under stirring for 30 s at high speed
(TAPPI disintegrator), to ensure a correct mixing. The
addition was carried out observing the following order:
1) cationic starch, 2) AKD, and 3) retention agent.
The pH of the medium for the chemical treatment
was fixed at 7.5 with H2SO4 1N, according to
manufacturer’s specifications, in relation to the
optimum pH range of the sizing treatment, and also to
the performance of the retention agent.
The optimization of the chemical treatment was
carried out with a three-factor experimental design
called Central Composite Design (CCD) involving: (I)
a two-level full factorial or fractional factorial design;
(II) a star design in which the experimental points
occur at an α distance from its center; and (III) an
experimental point in the center.33 The center runs
provide information on the existence of a curvature in
the system. By these means, the design displays
properties, such as rotability or orthogonality, for
fitting the quadratic polynomials. CCD allows the
estimation of the main effects, as well as of the
interactions and quadratic effects. Usually, CCD
consists of a 2k factorial runs with 2k axial runs and C0
center point runs.
The concentration range studied for each additive
was taken from the literature in the case of cationic
starch: 0.25 to 0.50% on oven dry pulp base (odp), and
from the specifications sheet in the case of the sizing
agent (1 to 2% odp) and of the retention agent (0.1 to
0.25% odp), the last two being expressed as
percentages of commercial products. The levels
studied are shown in Table 1.
Particle charge was determined through the Total
Cationic Demand (TCD), for checking a -0.5 to -0.05
meq/L medium charge in each point of the experiment.
This range was recommended by the sizing and
retention agent supplier, to ensure the good
performance of the additives.
Testing sheets of 120 g/m2 were formed in a Rapid
Köethen from the furnish, as indicated by each point of
the experimental design. The sheet-former equipment
has a drying system operating under vacuum, at a
temperature of 120 ºC, for 5 min, for AKD curing.
Water absorption was measured on the formed
sheets by the Cobb120 method. The optimum condition
was defined as a Cobb120 = 35 ± 1 g H2O/m2.
Once the optimal dosage to achieve the water
absorption objective was established, sufficient sheets
were formed to obtain the raw material for the
following papermaking cycles. The use of the material
was simulated by subjecting the sheets to different
conditions of relative room humidity and temperature,
for 24 h at each environmental stage (Table 2).
Ring Crush Test, Concora Medium Test, Tensile
Index and Burst Index tests were carried out on air-
conditioned paper in sequences of 50, 75 and 90%
relative humidity (RH), in the water adsorption stage,
and at 75 and 50% RH in the desorption stage. These
values were selected as the most representative ones
for the environmental conditions to which a corrugated
cardboard package can be subjected.
Following the humidity adsorption and desorption
stages, the sheets were disintegrated in water, at room
temperature, in a 15 L pulper. The characterization of
the recycled pulp involved the measuring of fiber
length, percentage of fines passing 200 mesh, pulp
refining degree (ºSR) and water absorption (Cobb120
method), on 120 g/m2 sheets.
Further on, the pulp was refined in a PFI mill, to
the initial refining degree (35 ºSR), and the energy
necessary for achieving it was measured. The
optimization study of the additive dosages (Table 1)
was repeated on the refined pulp, to obtain the same
quality of paper (Cobb120 = 35 ± 1 g H2O/m2), and the
sheets were formed again under the same conditions.
Two recycling stages were simulated, called from now
on second (first recycling) and third papermaking cycle
Treatment combinations (tc) in the CCD experimental design (as real variables)
Nº A B C
1 -1 -1 -1 1.0 0.10 0.25
2 1 -1 -1 2.0 0.10 0.25
3 -1 1 -1 1.0 0.25 0.25
4 -1 +1 -1 2.0 0.25 0.25
5 -1 -1 +1 1.0 0.10 0.50
6 +1 -1 +1 2.0 0.10 0.50
7 -1 +1 +1 1.0 0.25 0.50
8 +1 +1 +1 2.0 0.25 0.50
9 No additives No additives No additives 0 0 0
10 - α 0 0 0.66 0.175 0.375
11 + α 0 0 2.3 0.175 0.375
12 0 - α 0 1.5 0.05 0.375
13 0 + α 0 1.5 0.3 0.375
14 0 0 - α 1.5 0.175 0.165
15 0 + α 1.5 0.175 0.585
16 0 0 0 1.5 0.175 0.375
Environment parameters in acclimatization stages
Stage RH (%) T (ºC)
1 50 23
2 75 27
3 90 30
4 75 27
5 90 30
RESULTS AND DISCUSSION
Pulp characterization and preparation
Pulp characterization prior to mechanical and
sizing treatments and the refining energy for each
papermaking cycle are shown in Table 3. The
pulp used in the first papermaking cycle shows
the quality of the virgin pulp. The pulp was
refined in each papermaking cycle for opening the
fibrous structure to generate new binding points,
by subjecting the fiber to the same treatment that
it would receive in a mill.
The energy supplied per unit of refining
degree gained in the recycled pulp was lower than
that supplied for the virgin pulp. In the first
papermaking cycle, fiber fibrillation is the major
cause of the increase in the refining degree while,
Liliana Beatriz Molina Tirado et al.
in the subsequent uses, cutting is believed to be
the most important effect of refining, caused by
In the second papermaking cycle, the initial
degree of refining was lower than the objective,
and fibers were refined to 35 ºSR while, in the
third papermaking cycle, the initial degree of
refining was higher than the objective. As
expected, the additions in previous cycles had
altered the Schöpper value, so that the recycled
pulp was refined by increasing the Schöpper value
to 5 points.
The first part of this work demonstrates that,
during recycling, fiber length presents a mean
decrease of 30% in the second papermaking cycle
versus the first one, as due to fiber cutting. The
third papermaking cycle presented no significant
differences in relation to the second one. Fines in
the bagasse pulp, in the first and second
papermaking cycles, occur in the range of 15%,
and for the third one – in the range of 20% (Table
Optimization of sizing treatment (application
of CCD design)
The statistical treatment of the CCD design is
based on the Analysis of Variances (ANOVA).
The experimental results were fitted to a second-
order polynomial model including only the
significant variables at a 95% level of significance
(P < 0.05). Thus, the best equations obtained for
water absorption in different cycles were as
1) For the first papermaking cycle:
Water absorption (Cobb120) = 40.8 – 5.8 × Sizing agent – 41.1 × Starch + 3.3 × Sizing agent2 – 35.6 × Sizing
agent × Starch
2) For the second papermaking cycle:
Water absorption (Cobb120) = 42.7 – 11.3 × Sizing agent – 133.5 × Retention agent + 2.6 × Sizing agent 2 +
263.5 × Retention agent2 + 64.7 × Starch2 + 144.8 × Retention agent × Starch
R2 = 64.5%
3) For the third papermaking cycle:
Water absorption (Cobb120) = 36.0 – 2.0 × Sizing agent + 4.9 × Retention agent + 4.8 × Starch
R2 = 60.5%
Pulp characterization before and after mechanical treatment
1st cycle 1360 ± 458 18 35 17.3 2742
2nd cycle 980 ± 489 26 35 16.2 807
3rd cycle 893 ± 361 39 44 20.2 1841
Note: Fiber length expressed as average ± standard deviation
The optimum (minimum) dosage of additives
obtained by the equations above and the cost of
each treatment (combination of additives) are
presented in Table 4. The theoretical and
experimental results of the optimum additive
dosage, and the effects of the additives for
different uses are shown in Table 4 and Figures 1
to 5, respectively. At the beginning of recycling,
the effect of the sizing treatment was lost. It is
believed that this effect appears during the tap
water reslushing stage, when the chemicals had
been partially desorbed from the fiber surface.27
The analysis of the overall results, within the
same additives dosage limits, for the three
papermaking cycles shows that water absorption
(Cobb’s method) decreases with recycling, from
46-42 in the first to 36-35 in the third
papermaking cycle (Figs. 1 to 5). The bagasse
recycled pulp did not seem to require so many
additives as the virgin pulp to achieve the same
sized quality. Otherwise said, each new recycling
will require fewer additives for the sizing
treatment, which might be due to fiber
hornification (and its consequent hydrophobicity),
in accordance with Sjöstrom et al.,27 rather than to
a sizing residual effect, since the secondary fiber
sheets formed without sizing treatment had no
capacity of water absorption. Sjöstrom et al.
suggested that the recycled fibers obtained from
sized sheets are hydrophobized to some extent for
subsequent uses and that, even if major desorption
occurs at high ionic strength of the solution, the
sizing treatment in a recycled pulp evidences no
differences between an alkaline and an aqueous
In the first papermaking cycle, the sizing
agent, the starch and their interaction greatly
influence the response (Figs. 1 and 2). The
retention agent (high molecular weight
polyethylenimine) has no significant influence
over the working range proposed by the
manufacturer, its activity being primarily based
on its ionic strength. It is also assumed that the
ionic nature of the fibrous mass is sufficiently
anionic to promote the reaction between fiber,
sizing agent and cationic starch. In this case, the
mean value of the cationic demand for each
combination of the experimental design was -0.11
meq/L for virgin pulp and -0.05 meq/L for
recycled pulp, respectively.
The effects of the variables and the
interactions between them upon the second-cycle
pulp are illustrated in Figures 3 and 4, while the
effects of the variables upon the third-cycle pulp
are presented in Figure 5. The sizing and retention
agents are the most influential factors in the
response to the second papermaking cycle. The
linear effect of starch is not significant, although
it presents a second-grade polynomial relationship
in the response. The sizing agent and, secondarily,
the starch, are the factors that influenced the most
the response to the third papermaking cycle (Fig.
The effect of the sizing agent (AKD) on water
absorption presented a similar behavior in all
three papermaking cycles (Figs. 1, 3 and 5),
which agrees with the result of the Cobb120 test
carried out at the beginning of each cycle, as the
paper lost all the sizing in the repulping process.
It seems that the ageing of the handsheets under
high humidity conditions caused the hydrolysis of
the unreacted AKD, which contributes to sizing.34
In the first two papermaking cycles, the effect of
AKD is represented by a parabola, which
confirms the optimal range of work recommended
by the suppliers. It is not possible to assure the
same response in the third papermaking cycle, not
even beyond the studied range. The effect of both
starch and retention agent on water absorption has
moved from being directly proportional (first
cycle) to indirectly proportional (third cycle),
presenting a minimum in the second cycle. This
demonstrates the gradual saturation of the system
in relation to these additives. Sjöstrom and
Ödberg27 demonstrated that the adsorption
capacity of the cationic polymers diminishes with
an increasing number of papermaking cycles.
However, it cannot be neglected that the presence
of retention agents and cationic starch from
previous uses contributes to the formation of
microflocs which, in turn, diminish the exposed
Optimum dosage of additives and sizing treatment cost for a stock close to neutral charge
Pulp Sizing agent
(US $/ton pulp)
1st cycle 2.45 0.10 0.27 -0.17 6.4
2nd cycle 2.15 0.42 0.175 -0.06 23.9
3rd cycle 1.60 0.17 0.35 -0.15 21.1
Note: DCT (Total Cationic Demand)
Liliana Beatriz Molina Tirado et al.
Figure 1: Effects of sizing agent, cationic starch and
retention agent on water absorption in the first
Figure 2: Interaction between cationic starch and
sizing agent in the first papermaking cycle
Figure 3: Effects of sizing agent, cationic starch and
retention agent on water absorption in the second
papermaking cycle or first recycling
Figure 4: Interaction between cationic starch and
retention agent in the second papermaking cycle or
Figure 5: Effects of sizing agent, starch and retention agent on water absorption
in the third papermaking cycle or second recycling
It should also be taken into account that, due to
the methodology used, our system is free of the
fines and anionic trash occurring in industrially
recycled pulps. The large specific surface of these
pulps would be the main responsible for the
higher consumption of additives in the mills. In
future studies, the addition of additives will be
avoided, to observe whether the development of
sizing is possible using only the residual and
auxiliary agents from previous papermaking
The behavior of cationic starch with respect to
water absorption varies with the papermaking
cycles. However, it always influences each cycle,
in an either linear or quadratic form. It also
interacted with the other additives, presenting a
synergetic effect with the sizing agent from the
virgin pulp. It did not interact with the retention
agent in the second papermaking cycle, because
of the interference produced by the retention
agent, which has cationic characteristics.1
In a neutral medium, the ionic nature of the
retention agent plays an important role.35 Unlike
other retention agents, polyethylenimine presents
a short molecular chain, its action being based on
the formation of microflocs. On the other hand, it
favors the decrease of water viscosity, which
improves drainage in the formation stage.
The physical properties of sized papers in the
adsorption–desorption cycles, as a function of
relative humidity (RH), are presented in Figures 6
to 9. The properties of the sheets obtained from
bagasse pulp recommend them for their use as
liner and fluting material, in comparison with
those obtained from industrial paper, which can
be partly due to the use of a PFI refiner, known as
producing fewer fiber cuts and higher internal and
external fibrillation than industrial refiners.
The tensile and burst indices are significantly
better in virgin pulp than after the subsequent
papermaking cycles (Figs. 8 and 9). According to
the variance analysis, after the adsorption and
desorption of humidity, the paper properties from
virgin pulp (first papermaking cycle) generally
returned to the values of the standard test
conditions (50% RH).
The burst index classifies liners into three
different qualities: Kraftliner, Testliner and
Biclass.36 According to this classification, the
virgin bagasse pulp sheets (first papermaking
cycles) presented the quality of a Kraftliner, while
the secondary fiber paper (second and third
papermaking cycles) evidenced the quality of a
Generally, pulps behave within the same
range of test conditions (50% RH), even after
having undergone the adsorption–desorption
cycle. Among the properties assayed for
cardboards, paper mechanical compression in the
first papermaking cycle was highly diminished,
between 75-80% RH and, in spite of some
recovery, cardboards did not return to the same
quality. The three pulps present the same behavior
as class-B fluting.36 In all cases, the quality of the
paper formed from bagasse pulps recommends it
for the manufacture of liner and fluting.
Figure 6: CMT (Tappi 809 om – 99) variation with RH
for each papermaking cycle
Figure 7: RCT (Tappi 822 om – 93) variation with
RH for each papermaking cycle
Liliana Beatriz Molina Tirado et al.
Figure 8: Tensile index (Tappi 220 sp – 96) variation
with RH for each papermaking cycle
Figure 9: Burst index (TAPPI 220 sp – 96) variation
with RH for each papermaking cycle
The effect of the sizing treatment is lost during
repulping with tap water. However, it is noticed
that the sizing effect in papermaking cycles is
more easily attained when the sludge has no
mineral charges and the level of anionic trash is
Both cationic starch and the retention agent
(high molecular weight polyethylenimine)
continue to be used, in spite of all mechanical
effects suffered by the fiber during pulp
For a given sizing, the paper produced from
sugar cane bagasse semichemical pulp, destined
for liner or fluting, did not modify its behavior
with respect to the papermaking cycles, under
standard test conditions. On the contrary, during
the first papermaking cycle, its properties had
drastically diminished at a relative humidity of
around 75%. In relation to the other papermaking
cycles, the deterioration is uniform with respect to
the increase in humidity.
ACKNOWLEDGEMENTS: The authors thank
the INTI-Celulosa y Papel staff and engineers
Daniel Grecco (Basf Argentina), Ricardo Jereb
and Felipe Mussi (Papelera Tucumán) for their
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