Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

COLLOIDAL PROPERTIES OF HOLLOWLATICES AND THEIR ROLES IN
CONTROLLING COLORIMETRICPARAMETERS OF COATED PAPER SURFACE

synthetic rubbers, is generally produced by emulsion
polymerization. Hollow sphere pigments of latex developed
recently1-3) began to be applied as a plastic pigment to paper
coating to take advantages of light weight, compliance withcalendering4) and good optical properties5-7), the last of which
are reported to improve color reproduction in printing. Thoseadvantages stem all from the hollow structure. Hollow spherepigments have a structure with a core of water surrounded by ashell of hard polymer. On drying, the inside water evaporates,
leaving an air-filled core that further scatters light at the
air-polymer interface.
In this study, with a view to elucidate the advantages of
the hollow sphere pigments over the filled ones, colloidal
properties of the latices, optical properties of their coatings andthe coated papers, and colorimetric parameters of thesolid-printed surfaces were examined.

Hitomi HAMADA,
Toshiharu ENOMAE and Fumihiko ONABE
Paper Science Laboratory, Department of
Bio-material Science, The University of Tokyo,1-1-1
Yayoi, Bunkyo-ku, Tokyo, 113-8657 JAPANYoko SAITO
Nippon Zeon Co., Ltd., 1-2-1 Yako, Kawasaki-ku,
Kawasaki-shi, Kanagawa, 210-8507 JAPANABSTRACTWith a view to seek the influence of hollow spherepigments of latex upon the printed color on coated paper
surface, the hollow sphere pigments were compared with filled
ones in a variety of experimental approaches.
Colloidal properties of latices were determined bymeasuring zeta potential and particle size distribution. For theamphoteric filled sphere pigment of latex, the polarity was
reversed from the negative side to the positive side withdecreasing pH. An extraordinarily high peak in the particle size
distribution of the amphoteric filled evidenced aggregation
between latex particles near the isoelectric point, depending on
the electrolyte concentration and pH of the suspending
medium.
Coated papers containing the hollow sphere pigment intheir coating improved optical properties like gloss andbrightness. Optical parameters solely of the coating could
account for this finding. An equation derived from the
Kubelka-Munk equation calculated them from twice
measurements of reflectance of a coated paper over two
substrates of different reflectances. This method permitted to
predict brightness of coated paper of which coat weight would
be different from the actual one.
The colorimetric parameters of solid-printed surfaces of
the coated papers closely related to optical and structuralproperties of the coated papers. The color of the printed
surfaces was dominated by the brightness and the smoothnessof the coated papers.
The hollow sphere pigments were proved to improve
optical properties of coated paper and to control minutely
colorimetric parameters of printed surfaces.

EXPERIMENTAL
Latices
Table 1 shows the properties of the latices used in theexperiments.
Table 1

Properties of latices

Monomercomponents
Styrene andAnionic hollow
1000
9
Acrylic acid(HP-1055*)
Styrene andAnionic hollow
500
8.5
Acrylic acid
(MH5055**)
Anionic filled
Styrene and320
8.5(V1004**)
Butadiene
Styrene andAmphoteric filled
135
9
Butadiene(LX407K**)
Manufacturers: *Rohm and Haas Japan and ** Nippon Zeon
Latex

Particlediameter (nm)pH

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 and ionic strength of thesuspending medium, using ZETA SIZER 3000 (Malvern
Instruments, UK). pH was adjusted by adding a solution ofeither HCl or NaOH. For controlling the ionic strength, NaCl
was used. Table 1 lists all the latices used.
SEM observation
For the purpose of visualizing the shape of the latex
particles, the latices were observed using scanning electron
microscopy (SEM). The latices used were the anionic filled andthe anionic hollow (MH). A 0.01%-solids emulsion each ofthese latices was dropped on a grid precovered with a collodion

INTRODUCTION
Latex, namely, an aqueous dispersion of polymers such as1Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

1µ
µm

1µ
µm

Fig. 1 Secondary electron micrograph of anionic hollow
sphere pigment (MH) fixed with osmium tetroxide.

Fig. 2 Secondary electron micrograph of anionic filled
sphere pigment fixed with osmium tetroxide.

film. The grid had been etched by an ion coater in advance forproviding hydrophilicity to get the emulsion spread overspontaneously. Then, the emulsion on the grid was exposed to
osmium tetroxide vapor in a dessicator for 2 hours and dried
with silica gel in a dessicator over night. Before the SEM
observation, the grid was platinum-coated for providing
electric conductivity.
Figures 1 and 2 are SEM photographs of particles of theanionic hollow and the anionic filled sphere pigments of latex,
respectively. The particle diameter of the latices was apparently
500 nm and 350 nm, respectively. For the anionic hollow, 20 to
30 % of the particles were concave. The shape of the anionic
filled was not a perfect sphere, but a sphere with wrinkled
surfaces. Both of the apparently deformed shapes are perhaps
due to shrinkage during drying.

measurement conditions of reflectance were D65 diffuse
illuminant / 2°normal observer. Those coated papers wereobserved using SEM.
Prediction of brightness of coatings 8-11)
An equation was derived to predict brightness only of acoating. Equation (1) is the basic Kubelka-Munk equation
relating R (reflectance) to R∞ (brightness) and SW (scattering
coefficient multiplied by coat weight). Solving equation (1) interms 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∞ )( RR g R∞ − RR ∞ − R g + R∞ )1ln
2
1 R∞ − R∞( RR g R∞ − R − R g R∞ + R∞ )Eq.(1)

2SW =Eq.( 2)

Two kinds of reflectance of coated paper over two substrates
of different reflectances (Rg1 and Rg2) were measured andOptical properties of coated papers
Coating
The coating color was preliminaly formulated with 70
pph of kaolin clay (UW-90, Engelhard), 30 pph of calcium
carbonate (Brilliant-15, Shiraishi kogyo), 0.2 pph of adispersing agent (Aron T-40, Toagosei), 0.25 pph of NaOH
and 10 pph of binder latex (LX407G, Nippon Zeon). Twotypes of pigment latex, anionic hollow sphere pigment (HP)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 (NPi, Nippon Paper
Industry) with a sheet-fed coater equipped with ansynchro-starting dryer (PM9040MC, SMT) using a wire bar.
The coat weight was targeted at 9 g/m2. The coated paper weredried at 120℃ for 10 sec, and supercalendered at 50℃ and49.1 kN/m.
The sheet gloss (Spectrophotometer PF10, Murakami
Color Laboratory), dynamic roughness （ Micro-topo-graph,
Toyo Seiki ） and reflectance (Spectrophotometer TC-1800,
Tokyo Denshoku) of the coated papers were measured. Thec − c2 − 4
Eq.(3)
2( R g1 + R2 )( R1 R g 2 − 1) − ( R1 + R g 2 )( R2 R g1 − 1)
where c =R1 R g 2 − R2 R g1

R∞ =

and

R1 < R2

substituted into equation (1). Solving the two resultant
equations in terms of R∞ gives equation (3).
Using this equation, we can predict the brightness of coating.
Definition of symbols
Ri : 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 basepaper sheets for i = 2 were used in this work.
Rgi : reflectance of the surface of the i -th backgroundR∞ : brightness of coating = reflectance of coating so thickthat further increase in thickness does not change thereflectance = reflectivity
S : specific scattering coefficient2Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

side to the positive side at ca. pH 5. At the isoelectric point of
this pH, the ionization of both the negatively charged groups(carboxyl groups) and the positively charged groups wasbalanced on the surface. The negative zeta potential in thebasic range of pH is undoubtedly due to the dominant
ionization of the carboxyl groups on the latex polymer. The
isoelectric point of a cationic latex generally appears at pH
two to three higher than that of an amphoteric latex. 12)
The particle size distribution for all of the anionic latices
was very stable with time. However, for the amphoteric filled,
the distribution curve had a peak at ca. pH 5 that corresponds
to the isoelectric point. The particle size at this peak increased
with time after pH adjustment. It seems that the latex
aggregation proceeded at this pH. The particle diameters
normally measured agreed with the results from the SEM
photographs.

W : coat weight
Colorimetric parameters of printed papers
The coated papers were printed on a RI printing testerwith cyan ink. The colorimetric parameters such as lightness(L), a, b, chroma (C), hue (H) and the differences in eachparameter between printed and non-printed surfaces were
measured using the spectrophotometer.
RESULTS AND DISCUSSION
Colloidal properties of latices
Effect of pH on zeta potential and particle size
Figures 3 and 4 show the effect of pH on the value of zeta
potential and the particle diameter. For all of the anioniclatices, regardless of either filled or hollow, the zeta potential
showed the negative value in the entire pH range. However,for the amphoteric filled, the zeta potential increased withdecreasing pH and the polarity was reversed from the negative

Effect of ionic strength on zeta potential and particle size
Figure 5 shows the effect of ionic strength on zeta
0

30

-10

20

-20

Zeta Potential (mV)

Zeta Potential (mV)

100Amphotericfilled

-10
-20
-30
-40

Anionic
hollow(MH)

-50
-60
-70

Anionic filled

4

-30
-40

-60

-80
6

Anionic filled-50

Anionic
hollow(MH)-70Anionic
hollow(HP)2Amphoteric filled

8

10

10-5

12

10-4

10-3

10-2

10-1

NaCl conc. (mol / L)pH

Fig. 5 Effect of ionic strength on the valueof zeta potential.

Fig. 3 Effect of pH on the value of zeta potential.
1200

800

Anionic hollow(HP)Particle diameter (nm)Particle diameter (nm)

1000
800
600

Anionic hollow(MH)400Anionic filled

200
0

600

Anionic hollow(MH)400Anionic filled

200

Amphoteric filled

Amphoteric filled

2

4

6

8

10

0

12

10-4

10-3

10-2

10-1

NaCl conc. (mol / L)pH

Fig. 4 Effect of pH on the average value of particle
diameter.

10-5

Fig 6.

3

Effect of ionic strength on the averagevalue of particle diameter.

Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

1µ
µm

1µ
µm

Fig. 7 Coated paper surface containing 20 pph ofanionic hollow sphere pigment (HP).

Fig. 8 Coated paper surface containing 20 pph ofanionic filled sphere pigment.

potential. The negative values of zeta potential of both of theanionic latices and the amphoteric latex decrease withincreasing ionic strength at concentrations of NaCl higher than
10-3 mol/L. However, reversal of the polarity of the zeta
potential was not observed even at the highest concentrationsof NaCl (e.g., 10-1mol/L). Effects of the increasing ionicstrength on the zeta potential of the latices may shield thecharges of the ionic groups on the surface, changes in
distribution of the counterions and the coions in the electric
double layer, and a decrease in thickness of the electric double
layer adjacent to the surface. It is normally accepted incolloidal systems that the charge reversal phenomena occur at
higher concentrations of inorganic electrolyte when thepolarization energy of counterions for the interaction with thefixed charged species is sufficiently high.13) In view of this, thepolarization energy of the counterions from NaCl (i.e., Na+ orCl-) was not high enough to reverse the polarity of the chargeof the latices even at the highest concentration of NaCl.
Figure 6 shows the particle size distribution for both theanionic latices was very stable to the increased ionic strength.

However, the amphoteric latex, the particle size distribution
showed an extraordinarily high value at a concentration of
10-1mol/L, where the diameter increased over time. pH of thelatex emulsion with NaCl solution at this concentration is ca.5,
which is equivalent to that at the isoelectric point. So, it seems
that the latex proceeded to be aggregated at this point.
SEM observation of coated papers
Figures 7 and 8 are SEM photographs showing theparticle shape in coated papers. The particle diameter of thehollow sphere pigment of latex (HP) and the filled one wasapparently about 1 µm and 350 nm, respectively. These values
agreed with the results measured by ZETA SIZER. For thefilled sphere pigment, many concave particles were observed.
The particles of the hollow sphere pigment appeares to be
circular and black. This finding assumes that hollow spherepigment particles tend to become collapsed and flat during
calendering, as generally seen on coated paper surfaces.
Optical properties of coated papers
80

Filled1

75
Sheet gloss (%)

Dynamic roughness : Rp (μm)

1.25

0.75
Hollow

0.5

0.25

Hollow

70

65

Filled

60
0
5
10
20
Formulated amount of plastic pigment
(pph)

0

5

10

20

Formulated amount of plastic pigment(pph)

Fig. 9 Dynamic roughness (Rp) vsformulated amount of plastic pigment

Fig. 10 Sheet gloss vs formulated
amount of plastic pigment

4Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

225
Specific scattering coefficient : S
2(m /kg)85.5

Hollow

84.5
84
83.5

Filled

83
82.5

150
125

Hollow

49

89

48

5

90

91

92

93

Colorimetric parameters of printed papers
Figure 13 shows that the lightness (L) increased slightly
with increasing amount of both the sphere pigment latices
formulated in the coating, but decreased slightly for the filled
sphere pigment of latex. The chroma increased with increasing
formulated amount for both of the latices. In comparison
between the latices, these parameters were higher for thehollow sphere pigment than for the filled one. For the coatedpapers containing the hollow sphere pigment, the specificscattering coefficient increased with increasing formulated
amount, thus inducing high lightness. So, this increase inlightness of the coated paper is considerd to help increase thelightness of the printed surface. In the case of the coatedpapers containing the filled sphere pigment, the smoothness
73.5

54.5

72.5

52.5

10

100

55.5

53.5

Filled0efficiency resulting in a higher reflectance of the coated paper.
This means the brightness of the coated paper would be higher
as well.

26.5

Hollow

51.5

70.5

25.5

FilledFilled71.5
ΔELightness : L (%)

50

Filled20

Fig. 12 Specific scattering coefficient (S)vs brightness of coating (R ∞)Chroma : C (%)

Hollow

105Brightness of coating : R ∞ (%)

Figures 9, 10 and 11 show that the sheet gloss and thereflectance of the coated paper increased, and the dynamic
roughness decreased with increasing the amount of pigmentlatex formulated. The hollow sphere pigment showed higher
values for sheet gloss and reflectance, and lower values indynamic roughness compared with the filled one. It seems thatthe surface of the coated paper with the hollow sphere pigment
became smoother during calendering because hollow particles
deform easily to gain flat surfaces.
Figure 12 shows the relation between the brightness (R∞)
and specific scattering coefficient (S) of the coating. Both ofthe parameters increased with increasing formulated amount of
the pigment latices. The hollow sphere pigment showed higher
values than the filled one in both of the parameters. Since thehollow sphere pigment has an air-filled core, even in the dry
state, surrounded by a shell of hard polymer, light is scatteredat the air-polymer interface. Therefore, the coating with thehollow sphere pigment showed higher light scattering

51

20

Hollow

175

0
5
10
20
Formulated amount of plastic pigment(pph)
Fig. 11 Reflectance of coated paper
backed by sufficient number of sheets ofbase paper (R 2) vs formulated amount ofplastic pigment

52

Number represents
formulated amount.

200

Hollow

24.5
23.5

69.5

22.5

68.5

21.5

Filled

47

50.5
0

5

10

20

Formulated amount of plastic pigment (pph)Fig. 13 Lightness (L) and Chroma (C) of
printed surface of the coated paper

5

0
5
10
20
Formulated amount of plastic pigment(pph)
Fig. 14 Color difference (ΔE) and Hue
difference (ΔH) (difference between
printed paper and non-printed) of printed
surface of the coated paper

ΔH

Reflectance : R 2 (%)

85

Pre-symposium of the 10th ISWPC -Recent Advances in Paper Science and Technology-, Korea Tappi, Seoul, Korea, 309-314(June 2-3, 1999)

increased with increasing formulated amount. Therefore, it
seems that the transferred ink volume increased and thelightness decreased as a result. The rule of changes in theparameters (L, a, b, C, ΔE and ΔH) with decreasing the ink
amount found with the same coated paper holds with the case,although the exact volumes of the transferred ink are unknownwith the samples in this work.
Those results conclude that the color of a printed surface
is dominated by the brightness and smoothness of the coatedpaper as a printing medium and that the major factor out of thetwo properties, brightness and smoothness, is dependent oneach level.

REFERENCE
1. M. Okubo and H. Mori, Colloid and Polymer Science,
275(7): 634(1997)
2. M. Okubo and H. Minami, Colloid and Polymer Science,
275(10): 992(1997)
3. M. Okubo, Y. Konishi and H. Minami, Colloid and Polymer
Science, 276(7): 638(1998)
4. H. Izawa, Y. Saito and J. Kasai, Japan Tappi Journal 53(4):458(1999)
5. J. T. Brown, Tappi Coating Conference Proceedings,
113(1991)
6. T. Kaji, Japan Tappi Journal 46(2): 271(1992)
7. Y. Shimokawa, O. Ishizu and T. Shimazoe, Japan Tappi
Journal 52(2): 171(1998)
8. H. Hamada, T. Enomae, F. Onabe and Y. Saito, Sen’I
Gakkai Annual Meeting Preprints(1999)
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38(5): 448(1948)
10. J. V. Robinson, Tappi 58(10): 152(1975)
11. P. Lepoutre, Y. Potier and B. Alince, Pulp and Paper
Magazine of Canada, (3): 9(1982)
12. T. Sugimura, Y. Kataoka, S. Suzuki and K. Kasahara,
Application of Synthetic Latices (in Japanese; Gosei latex
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13. F. Onabe, Journal of Applied Polymer Science, 23: 2909
(1979)

CONCLUSION
The effects of the hollow sphere pigment of latex on
properties of the coated papers and its prints were estimatedand compared to the filled type.
Firstly, colloidal properties were measured. For theamphoteric filled sphere pigment of latex, the polarity was
reversed at pH where the ionization degree of both thenegatively and positively charged groups are balanced on thesurfaces. For the effect of ionic strength, the reversal of thepolarity of the zeta potential was not observed even at aconcentration of NaCl solution as high as 10-1mol/L. The
extraordinarily high particle diameter observed at theisoelectric point for the amphoteric filled suggests that theparticle aggregation proceeded. The zeta potential and particle
size distribution of both the anionic hollow and filled ones
were very stable against changes in pH and ionic strength.
Addition of the hollow sphere pigment decreasedroughness and increased gloss and light reflectance of thecoated paper. This effect was more striking than that of thefilled one. This is because the particles of the hollow spherepigment in the coated paper surface are collapsed during
calendering, as shown by the SEM photographs. The brightness
solely of the coatings was calculated using the Kubelka-Munktheory. The hollow sphere pigment showed a higher value than
the filled one in both brightness and specific scattering
coefficient, which leads to higher brightness of the coatedpaper.
As for colorimetric parameters of the printed surfaces of
the coated papers, lightness increased with increasing
formulated amount for the hollow sphere pigment, but
decreased for the filled one. The former behavior is explainedby higher brightness of the coated paper; the latter byincreased ink volume due to higher smoothness. The major
factor out of the two properties, brightness and smoothness, isdependent on each level.

6