Proceedings of the 5th Asian Textile Conference, 1999-10-2, Kyoto, Japan Vol.1 464-467(1999)

APPLICATION OF HOLLOW CALCIUM CARBONATE PARTICLESTO PAPERMAKING
Toshiharu Enomae
Assistant Professor
e-mail enomae@pulpgxa.fp.a.u-tokyo.ac.jp

Graduate school of Agricultural and Life Sciences
The University of Tokyo
Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, JAPANABSTRACT
Spherical particles made of calcium carbonate were reported to be available by the interfacial
reaction method. When a potassium carbonate solution / benzene (W/O) emulsion is poured in acalcium chloride solution, spherical and sometimes hollow particles of calcium carbonate form,
maintaining the shape of emulsified droplets. The surfactant concentration and emulsifying
properties resulting from dispersion and homogenization determined particle shape, size anddistribution. Prepared spherical particles were tried to be applied to paper making as a filler. They
increased brightness of handsheets more than commercial coagulated particles. However, aggregatesof calcium carbonate present in the paper hindered improvement of opacity.

INTRODUCTION
One of the disadvantages of recycled paper when compared to paper from virgin pulp is difficulty togain high brightness. To improve brightness, papermakers make efforts to develop recycling
processes such as introducing a strong surfactant in floatation and intensive bleaching. But, theseprocesses demand more water and energy, causing adversity to environment. Another possible idea
proposed here is to hide gray appearance with an efficient coating or internal loading for highopacity. Hollow spheres made of calcium carbonate could be suitable for this purpose because
hollow structure scatters more light resulting in higher brightness and opacity.
This work was aimed at approaching this goal, but at the present time spherical (not always hollow)
particles of calcium carbonate was tried to be prepared and was compared to another type of calcium
carbonate in fundamental characterization and in application to loading to paper.
Nakahara1) reported that spherical and hollow particles of calcium carbonate were available by theinterfacial reaction method. Figure 1 represents the mechanism predicted by the author. At thesurface of emulsified droplets, potassium carbonate and calcium chloride react to form a spheresometimes with a hollow structure. Hollow structure can also be beneficial to light weight.

EXPERIMENTAL
Preparation of calcium K2CO3 aqueous
carbonatesolution

W/O emulsion

BenzeneremovalBenzene

BenzeneCalcium carbonate wasprepared by theinterfacial reaction
CaCl2 aqueous
method. First, asolution
Hollow calcium
surfactant
carbonateparticlepolyoxyethylene sorbitanmonoolate (Tween-80)
Fig.1 Predicted mechanism to produce hollow calcium carbonate
was dissolved in benzene
particles by the interfacial reaction method.
at 0.5 wt. %. Second, apotassium carbonate solution of 1 mol/L was added to the benzene solution in a W/O volume ratio of1

Proceedings of the 5th Asian Textile Conference, 1999-10-2, Kyoto, Japan Vol.1 464-467(1999)

3:7. Then, this mixture was emulsified by ultrasonic irradiation (Ultrasonic generator U0300FB, 26
kHz / 300 W, Kokusai Electric, Japan) for 10 min and then by homogenization with a homogenizer(Physcotron NS-20, Nihon Seimitsu, Japan) for 10 min at 15,000 min-1. Last, this emulsion waspoured into a stirred calcium chloride solution of 0.2 mol/L which contained twice as many calcium
ions as carbonate ions. The amount of prepared calcium carbonate in one batch was 0.62 g on aregular basis, but for loading to paper it was 3.74 g. The mean yield was ca 80 %.

Paper making
Handsheets of paper were prepared according to ISO 5269-1. The prepared spherical particles ofcalcium carbonate was added to pulp stock of bleached kraft hardwood at 30 wt. % on dry pulp.
Commercially available filler of calcium carbonate, PCX-850, Shiraishi Kogyo co. ltd., was also
added to another stock for comparison. As well, unloaded handsheets were prepared. In advance,cationic polymer, polyamideamine-epichlorohydrin resin (WS-570, Japan PMC Co., Japan) wasadded at 0.1 wt. % on dry pulp to increase the retention of calcium carbonate.

Sheet properties
Apparent sheet density was calculated from basis weight and thickness for 5 handsheets each.
Optical properties, namely, brightness and specific light scattering coefficient were determined fromtwo reflectance values; one for single sheet with a backing of black cavity and the other for the sameplace of the same sheet with a backing of a white standard (the brightness is 93.0). Equations (1)and (2) transformed from the Kubelka-Munk equation by Hamada2) was used.
SW =( RRg R∞ − RR∞ − Rg + R∞ )1ln
1 R∞ − R∞ ( RRg R∞ − R − Rg R∞ 2 + R∞ )R∞ =( Rg 1 + R2 )( R1 Rg 2 − 1) − ( R1 + Rg 2 )( R2 Rg1 − 1)c − c2 − 4, where c =2R1 Rg 2 − R2 Rg12

Eq.(1)

andR1 < R2Eq.(2)In the equations, the symbols used represent those in below:
R∞ : brightness of a handsheet = reflectance of the paper so thick that further increase in thickness
does not change the reflectance = reflectivity
S: specific light scattering coefficientRi : Reflectance of the handsheet which has behind it a surface (background) with a reflectance ofRgi, where black cavity for i = 1 and the white standard for i = 2 were used.
Rgi : Reflectance of the surface of the i-th backgroundW : basis weight

RESULTS AND DISCUSSION
Characterization of spherical particles of calcium carbonate
Figure 2-a is a scanning electron micrograph of the spherical particles of calcium carbonate. The
mean particle diameter was a little less than 2 µm in appearance including many small particles
around 0.5 µm. Figure 2-b shows a close-up of a particle surface. The surface appears to be closely
packed with primary scalenohedral (spin-shaped) and rhombohedral (cubic) particles both 50 to 100
nm in diameter as well as bent-string-shaped particles ca 50 nm in diameter and ca 300 nm long (notin this picture).
To obtain spherical particles at high yields, the concentration of surfactant (Tween-80) seemed to be
one of the most important factors because it is related to the stability of a potassium carbonate /benzene emulsion. At 1.5 wt. %, an excess of the surfactant seemed to lose the stability of the2Proceedings of the 5th Asian Textile Conference, 1999-10-2, Kyoto, Japan Vol.1 464-467(1999)

emulsion as there were large drops of separate phases of benzene and the potassium carbonate
solution. In this condition, fine primary particles were not observed. Instead, much larger squaredplatelets as primary particles comprised sphere-like aggregates 3 to 5 µm in diameter. The surfactant
added at 1.0 % and less than 0.8 % gave ca 30 % and ca 95 % of spherical particles, respectively.

1 µm

100 nm

Fig.2 Scanning electron micrographs of spherical particles of calcium carbonate prepared by theinterfacial reaction method (a) and a close-up of a particle surface (b).

Agitating intensity in emulsifying had important effects on the particle size distribution of sphericalparticles. Homogenization at 15,000 min-1 increased the ratio of the spherical particles less than 1µm in diameter up to ca 20 %. Ultrasonic irradiation cannot render the particle diameter any more,
but relatively large particles to 3 µm or less.

X-ray counts

There are three crystal forms for calcium carbonate; calcite, aragonite and vaterite. Vaterite is not
usually synthesized in reactions of aqueous
Calcite 100 %solutions at room temperatures and atatmospheric pressures. The interfacial
reaction method, however, provided
appreciably high percentage of vaterite, therest of which was calcite as shown in Figure
Spherical calcium
3. In this X-ray diffraction pattern, somecarbonatepeaks are in accordance with those of purecalcite, but the others are all attributed tothose of vaterite. According to Nakahara3),
20 22 24 26 28 30 32 34 36 38 40
the vaterite ratio can amount to ca 50 %, but
2 θ , degree
it depends on the concentrations of calcium
Fig. 3 X-ray diffraction patterns of sphericalchloride and potassium carbonate. Also
particles of calcium carbonate and pure
those concentrations were reported to affectthe synthesis ratio of spherical particles.

Properties of handsheets loaded with spherical calcium carbonate particles
Table 1 summarizes the physical and optical properties of the three kinds of handsheets loaded withcalcium carbonate; one loaded with spherical particles; another loaded with coagulated particles; andthe other unloaded. The density of the paper loaded with the spherical particles was lower than thatof the unloaded one, as is often the case with loading because of acting as an obstruction to interfiber
bondings, but was higher than that for coagulated particles. Figure 4, a scanning electron
micrograph of a surface of a sheet loaded with the spherical particles, explains that there were morelarge aggregates for this type. The agglomerates seems to have become larger around the cubicparticles as a core to ca 30 µm in diameter. Such a large aggregate was not observed for coagulated3Proceedings of the 5th Asian Textile Conference, 1999-10-2, Kyoto, Japan Vol.1 464-467(1999)

ones.

Table 1

Physical and optical properties of the handsheets

Brightness for theBasis weight, Density, Brightness, Specific scattering
Filler type232
spherical calciumkg/m%
coefficient, m /kgg/m
carbonate was theSpherical particles
65.9
0.675
87.0
35.1
highest of the three
Coagulated particles
65.6
0.659
82.5
43.9
types of handsheets,
Unloaded
62.0
0.684
79.0
32.6
but specific lightscattering coefficient for it was lower than that for the coagulated one and was a little higher thanthat for the unloaded. The brightness could be extraordinarily high maybe because the residual
fluorescence due to the surfactant per se or an impurity in it. The structure of the coagulated particles
is also expected to act as an excellent lightscatterer. Conventional rhombohedral particles
would show a lower value of specific lightscattering coefficient.
For further researches, to hollow out sphericalparticles of calcium carbonate is necessary for
additional functions. If attained, this type of
calcium carbonate should be potential tofunction in many other ways in paper making
industry. Possible value-added uses are, for
10 µm
example, microcapusules of odor extinguisher orcolorant, and adsorbent.
Fig.4 Spherical particles and aggregates ofrhombohedral particles on a paper surface.

CONCLUSIONSpherical particles of calcium carbonate was successfully obtained by the interfacial reaction method.
As a filler for internal loading to paper, they increased brightness of paper more than commercial
coagulated particles. However, aggregates hindered improvement of opacity.

ACKNOWLEGEMENT
This work was financially supported by Kurita Water and Environment Foundation. The author
wishes to thank Ms. Hitomi Hamada sincerely for great contribution to this work and usefulsuggestions, and also thank Mr. Koji Tsujino for assistance with the experiments.

REFERENCES
1) Nakahara, Y., Tazawa, T. and Miyata, K., “Properties of calcium carbonate prepared by interfacial
reaction method”(in Japanese), Nippon Kagaku kaishi, Chemical Society of Japan 1976(5),732-736
2) Hamada, H., Enomae, T., Onabe, F. and Saito, Y., “Colloidal properties of hollow latices and theirroles in controlling colorimetric parameters of coated paper surface”, Proceedings ofPre-symposium of the 10th ISWPC, Korea, 309-314(1999)
3) Nakahara, Y., “Preparation of functional inorganic fine powders by reaction on W/O interface”(inJapanese), Funtai Kogaku kai-hi (Journal of the Society of Powder Technology, Japan), 32(2),97-104(1995)

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