Biomineral Eggs

Biomineral Eggs
JONNA KUUSISTO* AND THAD MALONEY
Aalto University, School of Chemical Technology,
Department of Forest Products Technology
PO Box 16300, FI-00076 AALTO, FINLAND
*[email protected]
1
INTRODUCTION
Many types of paper and board products are
composed of a mixture of mineral pigment and
cellulosic fibers. In these compositions, the
cellulosic fibers bond to one another via hydrogen
bonding and provide the strength of the material.
Pigments give other important properties such as
optics, surface properties and improved cost
structure, but do not bond to each other or to the
fibers. Thus, mineral pigments have a strong
negative effect on paper strength. Although paper
manufacturers would often desire to increase
pigment content, the loss of strength limits the
maximum amount of pigment in paper. [1]
If a significant hydrogen bonding capability could
be incorporated into a mineral pigment, this would
be an extremely useful technology. The pigment
would be expected to contribute to the overall
strength of the paper, thus allowing for increased use
of precipitated calcium carbonate (PCC) and
corresponding improvement in the cost structure.
Furthermore, the pigment would have the extended
range of applications in paper, board and other
products.
Starch is a common material to enhance paper
strength and provide additional hydrogen bonding,
which originates from the large number of hydroxyl
groups it has in its structure. Starch has a granular
structure consisting of glucose units, which are
linked together either linearly (amylose) or by
forming a branched molecule (amylopectin). [2]
To exploit its bonding potential, starch needs to be
dissolved i.e. gelatinized [3]. When starch is heated
in excess water, intermolecular hydrogen bonds
within the granule are disrupted, water penetrates
into the granule and granule swells. When the
temperature is further increased, starch gelatinizes.
Starch can also be dissolved in alkaline solutions. It
is known [4], that alkali breaks the intermolecular
hydrogen bonds of starch and thus enhances its water
solubility. On the other hand, alkali also accelerates
the molecular degradation of starch. Thus the
temperature, nature and concentration of the alkali
needs to be controlled to find the optimum
conditions for complete solubilization, but with
minimum degradation.
If starch and PCC can be combined in a suitable
way, the resulting pigment could have a bonding
capability. However, if starch is adsorbed onto the
surface of PCC, the amount of starch that can be
added is limited by the monolayer adsorption to
about 2-5% of PCC. Furthermore, the adsorbed
starch can easily desorb in the paper machine wet
end and loose its effect. In practice, the amount of
starch that can be added in the wet end on a typical
paper machine is about 1-2% [5] of the mass of
produced paper before the system overloads with
starch and dewatering and runnability problems are
encountered.
Target of the study was to combine PCC with
native corn starch in a true composite structure,
which would retain a significant hydrogen bonding
capability. The composite pigments were used in
paper in the fashion as traditional PCC and paper
properties were examined. The amount of starch in
paper was greatly increased without encountering
wet end dewatering properties. Additionally, paper
sheets with excellent strength properties and
improved cost structure were formed.
2
2.1
EXPERIMENTAL
Materials
La Mède lime i.e. calcium oxide was provided by
Lhoist Ltd and native corn starch by Roquette Ltd.
Bleached pine and birch kraft pulps were obtained
from Stora Enso, Varkaus mill (Finland). Cationic
polyacrylamide (C-PAM), provided by Kemira Ltd.,
was used as a retention aid in handsheet preparation.
As a reference, scalenohedral PCC with the average
particle size of 4.0 µm was obtained from Omya AG.
Sodium hydroxide (Titripur®) was from Merck
KGaA. Deionized water was used in all experiments.
2.2
Methods
The procedure of composite preparation is
presented in Figure 1. Granular starch was
suspended in water and treated either thermally or
chemically with NaOH to partially swell the
granules and make their surface more accessible for
calcium ions.
Lime and water were mixed with a high-shear
mixer to form calcium hydroxide (Ca(OH)2), i.e.
slaked lime. After slaking, the Ca(OH)2 slurry was
filtered through 100 mesh screen to remove
impurities.
Precipitation of calcium carbonate (CaCO3) was
performed in the presence of swollen starch by
feeding CO2 gas through Ca(OH)2 slurry at the pH of
11.5-12.0 until the pH dropped to 8. A quantity of
Ca(OH)2 was added to the slurry sufficient to form
PCC equal to the amount of starch. The composite
was cooked after precipitation to dissolve the starch
and exploit its bonding capability. After cooking the
composite was mixed with a high-shear mixer and
screened.
remained bonded in the composite structure. The
average particle size of the final composite was more
than double the average size of the untreated
granules. However, the size distribution remained
narrow indicating both the adsorption of PCC on
starch, because no residual PCC with small particle
size was detected, and the absence of large
aggregates.
Microscopy images revealed that the alkaline
treatment of starch resulted in more severe
morphological changes or even in the dissolution of
granules. Unfortunately, the alkaline treatment was
non-uniform leaving part of the granules intact. After
precipitation, only a portion of the granules were
found to be covered with PCC. The particle size
distribution of the composites was wide and a large
fraction of smaller particles (<10 µm) was also
present.
Figure 1. The
preparation.
procedure
of
composite
Bleached softwood and hardwood pulps were
refined in a Valley beater to Schopper Riegler
numbers (°SR) 27 and 21, respectively. A mixture
(70/30) of hardwood and softwood pulp and 200 g/t
of C-PAM was used to prepare hand sheets (80 g/m2)
with a Moving Belt Former (MBF). Target PCC
contents of the hand sheets were 5, 10 and 15%.
Hand sheets were wet-pressed, dried between the
drying-plates at ambient temperature and
conditioned before measuring paper properties.
Morphology of the freeze-dried and Au-coated
samples was examined with the scanning electron
microscope (SEM, Zeiss Sigma VP). Particle size
was measured with Malvern Mastersizer 2000. The
adsorption of starch was evaluated by vigorously
washing the composite three times with deionized
water and measuring the amount of starch left in the
composite. Hand sheet properties were analyzed
according to the following standards: T569 pm-00
(internal bond strength), ISO 1924-2 (tensile
strength) and ISO 5636-3 (air permeability).
3
3.1
RESULTS AND DISCUSSION
Composite pigments
The average size of untreated starch granules was
14 µm (Figure 2) and their shape was angular (Table
1). After thermal treatment, the size of the granules
increased and their structure changed to ring-shaped
indicating that gelatinization started at the hilum of
the granule. These observations are in agreement
with the earlier findings [6,7]. After precipitation,
the surfaces of thermally swollen granules were
covered with CaCO3. Even after cooking, starch
Figure 2. Particle size distribution of starch and
starch-PCC composites during the preparation
procedure. In a) starch was swollen thermally
and in b) chemically with NaOH.
Traditionally, when adsorbing dissolved starch
onto the pigment surface, the adsorption is limited to
roughly a monolayer capacity and already at low
addition levels, a significant amount of free starch is
in solution leading to dewatering and runnability
issues. After washing three times with water, the
composite prepared from thermally swollen starch
showed almost 100 % adsorption of starch (Table 2).
With alkali treated starch, approximately half of the
starch remained in the composite.
Table 1. Morphology of samples during the composite preparation procedure. Scale bars in images
correspond to 5 µm.
Sample
Starch treatment
Thermal
Alkaline
Dry starch granules
Swollen starch
Precipitated composite
Cooked composite
Table 2. The amount of starch left in the
composite after washing with deionized water.
Starch treatment
Sample
Thermal
Alkaline
Added starch
50
50
(% of composite)
Starch in
composite after
49
23
washing
(% of composite)
methods. At the same time, dewatering was
improved (high solids content after sheet couching),
which might be attributed to the large particle size of
composite pigments. In addition, paper had
extremely low air permeability, which indicates
excellent barrier properties. When thermally swollen
starch was used in composite preparation instead of
alkali swollen starch, higher bonding strength and
lower air permeability could be achieved.
4
3.2
Paper properties
When applied in paper, the composite pigment
increased the strength properties (Figure 3). A
significant rise was seen in the bonding strength of
paper and increasing the amount of composite
pigment further enhanced the bonding strength.
More than 10% of starch could be introduced into the
paper, which is much more than with traditional
CONCLUSIONS
The composite gave excellent strength and bonding
properties to paper and has a potential production
cost far below that of kraft pulp. This would allow it
to partly replace fiber in various paper grades
improving thus the cost structure of paper in a
substantial manner. In addition, unique paper
property combinations, such as low air permeability
together with good dewatering, could be achieved
when adding composite to paper. With
Figure 3. a) Solids content after couching, b) tensile strength, c) air permeability and d) internal bond
strength of hand sheets without PCC (X), with reference PCC (), with thermally treated starch-PCC
composites (∆) and with chemically treated starch-PCC composites (O). The sample containing thermally
treated starch-PCC composite pigment at the highest PCC content in d) could not be measured, because it
was outside the measurement range of device.
the composite, the amount of starch in paper could
be significantly increased and cationic wet-end
starch replaced with the low-cost native starch. Since
the composite gave high surface strength to paper
and the surface sizing of paper with starch is done
with the expensive and slow size press, the
composite could even be utilized as a technology to
eliminate size press at the paper machine.
ACKNOWLEDGEMENTS
Tuyen Nguyen is acknowledged for carrying out
the experimental work. Microscopy work made use
of the Aalto University Nanomicroscopy Center
(Aalto-NMC) premises.
REFERENCES
[1] J. Husband et al. Pigments, Papermaking science
and technology vol 11, Pigment coating and surface
sizing of paper. Ed. J. Paltakari, Paper Engineers'
Association/Paperi ja Puu Oy, edition 2, 76-190
(2009).
[2] H. Zobel, Molecules to granules: A
comprehensive starch review, Starch‐Stärke
40(2):44-50 (1988).
[3] D. Lund and K. J. Lorenz, Influence of time,
temperature, moisture, ingredients, and processing
conditions on starch gelatinization, Crit Rev Food
Sci. 20(4):249-273 (1984).
[4] J. Han and S. Lim, Structural changes in corn
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[5] B. Krogerus, Papermaking additives,
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[6] L, Jing‐ming, Z. Sen‐lin, Scanning electron
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(1990).
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B. Singh Gill, Morphological, thermal and
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(2003).

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