中空球状有機顔料の特性 （第１報）−塗工紙の光学特性と印刷物の色彩科学的因子の制御−
東京大学大学院 農学生命科学研究科 濱田仁美、江前敏晴、尾鍋史彦
日本ゼオン株式会社 斉藤陽子

Characteristics of Hollow Sphere Plastic Pigment (PartⅠ)- Optical Properties of Coated Paper and Colorimetric Control in Printing -Hitomi Hamada, Toshiharu Enomae and Fumihiko Onabe,
Graduate School of Agricultural and Life Sciences, The University of Tokyo
Yoko Saito,
Nippon Zeon Co., Ltd.

ABSTRACTWith a view to investigate the influence of hollow sphere plastic pigments upon the printed color on coated paper surface,
the hollow sphere pigments were compared with filled ones in a variety of experimental approaches.
Coated papers containing the hollow sphere pigment in their coatings improved optical properties like gloss andbrightness. Optical parameters solely of the coating accounted well for this finding. An equation derived from the
Kubelka -Munk equation provides optical parameters from two measurements of the reflectance of a coated paper over two
substrates of different reflectances. This method permits to predict the brightness of coated paper having the same kind of
coating with the known one at a different coat weight.
The colorimetric parameters of solid-printed surfaces of the coated papers are closely related to the optical and structuralproperties of coated papers. The color of printed surfaces was dominated by the smoothness and brightness of coated papers.
The hollow sphere pigments proved to improve optical properties of coated paper and to control minutely colorimetricparameters of printed surfaces.

KEYWORDS
Brightness, Coated papers, Coatings, Colorimetric parameters, Hollow sphere pigments, Kubelka -Munk equation, Latex,
Optical properties, Printed surfaces, Smoothness

INTRODUCTION
Latex, namely, an aqueous dispersion of polymers such as synthetic rubbers, is generally produced by emulsion
polymerization. Hollow sphere pigments of latex developed recently 1-3) began to be applied as a plastic pigment to paper
coating to take advantages of light weight, compliance with calendering4) and good optical properties 5,6). For these reasons, theauthor expects that its unique optical characteristics will improve color reproduction in printing. Those advantages stem mainly
from the hollow structure. Hollow sphere pigment has a structure with a core of water surrounded by a shell of hard polymer.
On drying, the inside water evaporates, leaving an air-filled core that further scatters light at the air-polymer interface. In thecase of hollow sphere pigments, both particle size and void volume have positive effects on coating performance. Brown 7)
reported that the direction of the particle size effect on sheet gloss after calendering is in the opposite direction to the filled
sphere pigment. As the particle size of the filled sphere pigments increased, the gloss decreased significantly. However, in thecase of hollow sphere pigments, the gloss increased with increasing particle size. Brightness and opacity tended to exhibit apeak at around 500nm. In our experiments, latex samples with the optimum particle size for optical performance were selected.
In this study, with a view to elucidate mechanism of occurrence of the advantages of hollow sphere pigments over thefilled ones, colloidal and structural properties of the latices, optical properties of the latices, the coatings and the coated papers
containing the latices, and colorimetric parameters of solid-printed coated paper were examined.

EXPERIMENTAL

Latex samples
Two kinds of pigment latices, i.e. hollow and filled, were used in this work as shown in Table 1.

Colloidal and structural properties of latices
Zeta potential and particle size distribution
Zeta potential and particle size distribution were measured as a function of pH, using ZETA SIZER 3000 (Malvern
Instruments, UK). pH was adjusted by adding a solution of either HCl or NaOH.

Microscopic observation
For the purpose of observing the shape of latex particles, scanning electron microscope (SEM) was applied. A
0.01% -solids emulsion of these latices was dropped on a grid precovered with a collodion film. The grid had been etched with
an ion coater in advance for providing hydrophilicity to get the emulsion spread over spontaneously. Then, the emulsion on thegrid was freeze-dried overnight. After the grid was osmium-coated for providing electric conductivity, it was observed by SEM(S-4000, Hitachi, Japan) at an accelerating voltage of 10 kV.
Figures 1 and 2 are SEM photographs of particles of anionic hollow and anionic filled sphere pigments of latex,
respectively. The particle shape of either of the latices was a sphere. The particle diameter of the latices was estimated to be1000 nm and 350 nm, respectively.

Optical properties of coated papers
Coating
The coating color was preliminaly formu lated with 70 pph of kaolin clay (UW-90, Engelhard, USA), 30 pph of calcium
carbonate (Brilliant-15, Shiraishi kogyo, Japan), 0.2 pph of a dispersing agent (Aron T-40, Toagosei, Japan), 0.25 pph of
NaOH and 10 pph of binder latex (LX407G, Nippon Zeon, Japan). Two types of pigment latex, anionic hollow sphere pigment
and anionic filled one, were added separately to this color. The amount of the pigment latex was 5, 10 and 20 pph. The
coatings were drawn down on a base sheet (Woodfree paper, 65 g/m2 , Nippon Paper Industries, Japan) with a wire bar using asheet-fed coater equipped with a synchro-starting dryer (PM9040MC, SMT, Japan). The coat weight was targeted at 9 g/m2 .
The coated papers were dried at 120℃ for 10 sec, and supercalendered at 50℃ under a linear pressure of 49.1 kN/m.
Oken smoothness（Modified Bekk method with a water column, Asahi Seiko, Japan）, sheet gloss (Spectrophotometer
PF10, Murakami Color Research Laboratory, Japan) and reflectance (Spectrophotometer TC-1800, Tokyo Denshoku, Japan)of the coated papers were measured. The measurement conditions of reflectance were D65 diffuse illuminant / 2°normal
observer. The surfaces of these coated papers were observed using SEM. The cross-section of the coated paper was preparedwith a focused ion beam and observed by Scanning Ion Microscope (SIM, SMI-9800, SII, Japan). The SIM observation
followed this procedure: chemical vapor deposition of tungsten for surface protection, coarse cross-sectioning with gallium
ions, fine cross-sectioning with gallium ions to 30 µm below the surface and the SIM observation at a tilt of 60 degrees.
Prediction of brightness of coatings 8-12)
An equation was derived to predict brightness solely of a coating. Equation (1) is the basic Kubelka -Munk equation
relating R (reflectance) to R∞ (brightness) and SW (scattering coefficient multiplied by coat weight). Transforming equation(1) in terms of SW gives equation (2).

R=

( R g − R∞ ) R ∞ − R∞ (R g − 1 R∞ ) e SW (1 R∞ − R∞ )( R g − R ∞ ) − (R g − 1 R ∞ ) e SW (1 R∞ − R∞ )SW =( RR g R∞ − RR ∞ 2 − R g + R ∞ )1ln
1 R∞ − R ∞( RR g R∞ − R − R g R ∞ 2 + R ∞ )Eq.(1)

Eq.( 2)

Two kinds of reflectance of coated paper over two substrates with different reflectances (Rg1 and Rg2 ) were measured andsubstituted into equation (1). Transforming the two resultant equations in terms of R∞ gives equation (3).
c − c2 − 4Eq.( 3)2( R g1 + R2 )( R1 R g2 − 1) − ( R1 + Rg 2 )( R2 Rg1 − 1)
where c =R1 Rg 2 − R2 R g1

R∞ =and R1 < R2

Using this equation, we can predict the brightness solely of coating.

Definition of symbols
Ri : the reflectance of a layer (coating in this work) which has behind it a surface (background) with a reflectance of Rgi,
where black cavity for i = 1 and sufficient number of base paper sheets for i = 2 were used in this work.
Rgi : the reflectance of surface of the i-th backgroundR∞ : the brightness of coating = the reflectance of ideal coating ,whose the layer is thick enough that further increase inthickness does not change the reflectance = reflectivity
S : specific scattering coefficient
W : coat weight

Colorimetric parameters of printed papers
Printing on coated papers were conducted on a RI printing tester (Ishikawajima Sangyo Kikai, Japan) with a conventional
process cyan ink for off-set. RI printing tester permits every sample within the same run to be compared under the same
condition of nip pressure, speed and ink volume on a plate. The colorimetric parameters such as lightness (L* ), a * , b * , chroma(C* ab ), hue (H* ab ) and the differences in each parameter between printed and non-printed surfaces were measured using thespectrophotometer.
Quantification method of ink amount on paper13)
The content of copper included in a cyan ink printed on paper was measured as an intensity of the peak in an X-ray
fluorescence spectrum measured by an X-ray fluorescence analyzer (MESA-500, HORIBA, Japan). The proportional
relationship between the copper content and the transferred ink amount measured gravimetrically was confirmed. Applying thisrelationship, the amounts of the cyan ink printed on the prepared coated papers were determined.
RESULTS AND DISCUSSION
Zeta potential and particle size distribution
Figure 3 shows the effect of pH on the value of zeta potential and the particle diameter. For both of the anionic latices,
regardless of either filled or hollow, the zeta potential showed a negative value in the entire pH range. The negative zeta
potential is probably due to adsorption of hydroxyl ions and ionization of carboxyl groups on the latex polymer. The particle
size distribution for both of the anionic latices was very stable with elapse of time. The measured particle diameters wereconsistent with the results from the SEM photographs.

Microscopic observation of coated papers
Figures 4 and 5 are SEM photographs showing a particle shape in coated paper surfaces. For the filled sphere pigment,
many concave particles were observed. The particles of the hollow sphere pigment appear to be circular and black. To appear
black assumes that surfaces of hollow sphere pigment particles tend to be flattened during calendering, as customarily seen on
coated paper surfaces.
Figure 6 is a SIM photograph of a cross-section of coated paper containing the hollow sphere pigment. The particles of
the hollow sphere pigment in the coating were perfectly spherical. This suggests that the hollow structure of the hollow spherepigment in coating was maintained even after calendaring. However, the particles of the hollow sphere pigment near coatedpaper surface were a little collapsed and the surface was flattened. This result was consistent with the SEM photographs ofcoated paper surfaces.

Optical properties of coated papers
Figure 7 shows that Oken smoothness increased with increasing amount formulated in the coating for the hollow spherepigment, but decreased for the filled sphere pigment. It seems that the surface of the coated paper with the hollow spherepigment became smoother during calendering because hollow particles deform easily to gain flat surfaces. Figures 8 and 9
show that the sheet gloss and the reflectance of the coated paper increased with increasing the formulated amount of pigment.
The hollow sphere pigment showed higher values than those of the filled one.
Figure 10 shows the relation between the brightness (R∞ ) and the specific scattering coefficient (S) of the coatings. Bothof the parameters increased with increasing formulated amount of pigment latices. The hollow sphere pigment showed higher
values than the filled one in both of the parameters. The total number of hollow sphere pigment in the coating wastheoretically about one sixteenth of the number of the filled one at the same formulated amount. However, because the hollow
sphere pigment had an air-filled core surrounded by a shell of hard polymer even in the dry state, the total surface area of thehollow sphere pigment in the coating was about 14% larger than that of the filled one. Considering that a larger interface area
provides more light scattering, a coating with the hollow sphere pigment showed higher light scattering efficiency resulting ina higher reflectance of the coated paper. This means the brightness of the coated paper would be higher as well. Figure 11
shows the relation between the coat weight and the reflectance of the coated papers that would be attained if they were
formulated at 20 pph. It can be predicted that the hollow sphere pigment will show a higher reflectance than the filled one atany coat weight.

Colorimetric parameters of printed papers
Figure 12 shows that lightness (L* ) increased slightly with increasing amo unt for the hollow sphere pigment formulated
in the coating, but decreased slightly for the filled sphere pigment. The chroma increased with increasing formulated amountfor both of the latices. In comparison between the latices, these parameters were always higher for the hollow sphere pigment
than for the filled one. Figure 13 shows changes in color difference (ΔE* ab ) and hue difference (ΔH* ab ). The color difference
increased slightly with increasing formulated amount, and the hue difference decreased both for the two latices.
Figures 14 and 15 show the tendency of changes in the parameters (L* , C* ab , ΔE* ab and ΔH* ab ) of several strips ofcommercial coated paper with a basis weight of 104.7 g/m2 and a coat weight of 10 g/m2 printed all in the same manner but
ink amount. The changes in every colorimetric parameter with increasing ink amount were roughly the same as the changes
with increasing formulated amount of filled sphere pigment. This finding suggests that the volume of transferred ink increased
and the lightness decreased as a result with increasing formulated amount of filled sphere pigment.
Figure 16 shows that ink deposit on the coated paper decreased with increasing formulated amount of the hollow spherepigment in the coating, but increased for the filled sphere pigment. The ink deposit was determined by the X-ray fluorescencemethod.
From these results, the changes in lightness of solid-printed surfaces could be explained in terms of the ink

transfer-smoothness relation for both hollow and filled sphere pigments. For the coated papers containing filled spherepigments, the smoothness decreased with increasing formulated amount. Therefore, the transferred ink volume increased andthe lightness decreased as a result. For the coated papers containing hollow sphere pigments, the smoothness increased withincreasing formulated amount, the transferred ink volume decreased and the lightness increased. Furthermore, specificscattering coefficient increased with increasing the formulated amount, thus inducing high lightness. So, this increase inlightness of the coated paper is considered to help increase lightness of a printed surface, maintaining a contrast to a
non-printed surface. From these results, it is concluded that color of a printed surface is dominated by smoothness andbrightness of the underlying coated paper.

CONCLUSIONSThe effects of hollow sphere pigments of latex on the properties of the coated papers and their prints were examined andcompared to the filled type.
Addition of the hollow sphere pigment increased smoothness, gloss and light reflectance of the coated paper. This effectwas more striking than that of the filled one. This is because the surface of coated paper containing the hollow sphere pigment
is flattened during calendering, as shown by the SEM and SIM photographs. However, the cross-section of coated paper in the
SIM photograph showed that the particles of the hollow sphere pigment in coating were close to perfectly spherical. It wassuggested that the hollow structure of the hollow sphere pigment was maintained even after calendering. Therefore, light isscattered at the air-polymer interface, the hollow sphere pigment show a higher value than the filled one in both brightness andspecific scattering coefficient solely of the coatings, which leads to higher brightness of the coated paper.
As for colorimetric parameters of printed surfaces of coated papers, lightness increased with increasing formulated
amount for the hollow sphere pigment, but decreased for the filled one. The changes in lightness could be explained in termsof the ink transfer-smoothness relation for both the hollow and the filled sphere pigments. It was concluded that color of a
printed surface, in terms of lightness, was dominated by smoothness and brightness of the underlying coated paper.

ACKNOWLEGEMENTS
The author wishes to thank Mr. Kenzo Miyamoto, Nippon Zeon Co., Ltd., for useful suggestions. We are also grateful to
Mr. Kazutaka Takahashi, Seiko I Techno Research Co., Ltd., for his help in the SIM observation of the cross-section of thecoated paper.

REFERENCES
1. Okubo M. and Mori H.: Colloid Polym. Sci. 275 (7) 634 (1997)
2. Okubo M. and Minami H.: Colloid Polym. Sci. 275 (10) 992 (1997)
3. Okubo M., Konishi Y. and Minami H.: Colloid Polym. Sci. 276 (7) 638 (1998)
4. Izawa H., Saito Y. and Kasai J.: Japan Tappi J. 53 (4) 458 (1999)
5. Kaji T.: Japan Tappi J. 46 (2) 271 (1992)
6. Shimokawa Y., Ishizu O. and Shimazoe T.: Japan Tappi J. 52 (2) 171 (1998)
7. Brown J. T.: Tappi Coating Conf. Proc., 1991, p113-122
8. Hamada H., Enomae T., Onabe F. and Saito Y.: Sen-i Gakkai Ann. Meeting Preprints, Chiba, 1999, pG-2
9. Hamada H., Enomae T., Onabe F. and Saito Y.: Pre-symp. 10th ISWPC Proc., Seoul, Korea, 1999, p309-314
10. Kubelka P.: J. Opt. Soc. Am. 38 (5) 448 (1948)
11. Robinson J. V.: Tappi 58 (10) 152 (1975)
12. Lepoutre P., Potier Y. and Alince B.: Pulp Paper Mag. Can. (3) 9 (1982)
13. Hamada H., Enomae T. and Onabe F.: 67th Pulp Paper Res. Conf. Proc., Japan Tappi, Tokyo, 2000, p164-169

Table 1

Properties of latices
Particle diameterpH(nm)

Monomercomponents
Anionic hollow
Styrene and1000
9(HP-1055*)
Acrylate
Anionic filled
Styrene and320
8.5(V1004**)
Butadiene
Manufacturers: *Rohm and Haas Japan and ** Nippon Zeon
Latex

1µ m
Fig. 1 Scanning electron micrograph of anionic hollow
sphere pigment

1µ m
Fig. 2 Scanning electron micrograph of anionic filled
sphere pigment

1200

Zeta Potential (mV)

-10

1000

Hollow

-20

800

-30
600
-40
Filled400-50Filled-60

200

Hollow

-70

0
2

4

6

8

10

12

pH
Fig. 3

Effect of pH on zeta potential
and particle diameter

Particle Diameter (nm)

0

1µ m
Fig. 4 Surface of coated paper containing 20 pph ofanionic hollow sphere pigment

1µ m
Fig. 5 Surface of coated paper containing 20 pph ofanionic filled sphere pigment

1µ m

Fig. 6 Cross section of coated paper containing 20
pph of hollow sphere pigment observed in Scanningion microscope after cross-sectioning by a focusedion beam

1400

Oken smoothness (s)1300

Hollow

1200
1100
1000
900
800

Filled

700
600
0

10

20

30

Formulated amount of latex (pph)Fig. 7 Oken smoothness vs formulated amount of latex

80

Sheet gloss (%)

75
Hollow

70

65

Filled

60
0

5

10

20

Formulated amount of plastic pigment (pph)Fig. 8

Sheet gloss vs formulated amount of plastic pigment

85.5

Reflectance : R (%)

85
Hollow

84.5

2

84
83.5
83

Filled

82.5
0

5

10

20

Formulated amount of plastic pigment (pph)Fig. 9 Reflectance of coated paper backed by sufficient number ofsheets of bas paper (R2) vs formulated amount of plastic pigment

Number represents formulated amount200
20

Hollow(m /kg)175
2

Specific scattering coefficient :S

225

150

10

5
0

125

10

Filled
20

5

100
89

90

91

92

93

Brightness of coating : R ∞ (%)

Fig. 10 Specific scattering coefficient (S ) vsbrightness (R ∞ ) both of the coatings

Reflectance of coated paper (%)

90
88
Hollow
Filled86
84

No Plastic Pigment

82
80
0

5

10

15

20

Coat weight (g/m2)

Fig.11 Predicted reflectance of coated
paper
52

55.5
Hollow

50

54.5
53.5

FilledHollow

49

52.5

48

51.5
Filled

47

50.5
0

5

10

20

Formulated amount of plastic pigment (pph)Fig. 12Lightness (L* ) and Chroma (C* ) ofprinted surface of coated paper

Chroma : C* (%)

Lightness : L* (%)

51

26.5

Hollow

72.5

25.5

Color difference :ΔE

FilledFilled

71.5
70.5

24.5
23.5

Hollow

69.5

22.5

68.5

21.5

0
5
10
20
Formulated amount of plastic pigment (pph)Fig. 13 Color difference (Δ E ) and Hue
difference (Δ H ) (difference between
printed paper and non-printed) of printed
surface of coated paper

Lightness and Chroma (%)

60
Chroma

55
50
45
40
35

Lightness

30
0

1

2

3

4

Amount of ink on paper (g/m2)
Fig. 14
Changes in lightness and chromawith ink amount on the commercial coatedpaper

Hue difference :ΔH

73.5

16

75

15

65

14

55

13

45

12

35

11

25

10
0

1

2

3

Hue difference : ΔH

Color difference : ΔE

85

4Amount of ink on paper (g/m2)
Fig. 15 Changes in color and hue
difference with ink amount on thecommercial coated paper2

Ink amount on paper (g/m )

1.85

1.8

FilledHollow

1.75

1.7

1.65

1.6
0

5

10

15

20

25

Formulated amount of latex (pph)Fig. 16 Ink amount on paper vs formulated
amount of latex