Ink dot formation in coating layer of ink-jet
paper with modified calcium carbonate
Dmitry Ivutin∗, Toshiharu Enomae and Akira Isogai
Graduate School of Agricultural and Life Sciences, the University of Tokyo
Bunkyo-ku, Tokyo 113-8657, JAPAN
Abstract
Commercial ink-jet papers are made from silica oralumina as a coating pigment, both of which give highimage quality, but are expensive. The present work aims at
efficient use of calcium carbonate as an alternative of silica
for ink receptive coating layers. Commercial calcium
carbonate was ground mechanically or dissolved partiallywith hydrochloric acid to reduce and equalize theaggregates size. Trial ink-jet papers were prepared andsome of them were plasma-etched to give a hydrophilic
property. The trial ink-jet papers were evaluated in terms ofdynamic behavior of ink droplets ejected from a test ink-jet
head visualized by a microscopic high-speed video camera
system and of ink penetration depth determined by confocal
laser scanning microscope (CLSM). Particle size reductionof calcium carbonate pigment by mechanical grinding andhydrochloric acid dissolution decreased ink dot area and dot
roughness on the papers. This result suggests that particle
size reduction improves image quality. Based on thecriterion standard that the smaller the dot area, the better theimage quality, the efficient processes were mechanicalgrinding, hydrochloric acid dissolution and plasma-etchingin the decreasing order. It is concluded that particle size
reduction decreases pore size of the coating structures andconsequently ink droplets tend to penetrate deep in coatings
rather than spread laterally along surfaces.

Keywords
Coating layer, confocal laser scanning microscope, inkjet printing, grinding, plasma-etching

Introduction
Nowadays, ink jet printing systems are widely used not
only for personal printing, but for advanced commercial
printing capable of outputting variable information
continuously, prepress of offset printing and non-contact
printing on curved surfaces of packages. Moreover, they are
spreading to various applications in other areas likemicrocontact printing to form a self-assembled monolayer
and micro- to nano-patterning for semiconductors.
Ink-jet paper, as a representative of ink-jet media, israpidly developing and now capable of reproducing photo∗

Affiliated currently with Fuji Xerox Co. Ltd.

like images. But, ink-jet paper does not seem to keep pacewith improvements in printing speed and size reduction of
ink droplets developed by novel ink-jet head technology.
On the other hand, a part of market is demanding
inexpensive versatile ink-jet paper compatible with mostprinters even at current print quality grades.
Most of commercial ink-jet papers are manufactured
from silica or alumina as a coating pigment because of
resultant high water absorbency and high optical density of
ink dots. However, silica and alumina are expensive. In
place of these expensive pigments, calcium carbonate may
be an alternative, but general types of calcium carbonate
cannot give a high print quality. Adequacy as an alternative
pigment is probably related to hydrophilicity, particle sizeand dispersibility. To evaluate calcium carbonate from these
aspects, commercial calcium carbonate was modified andthe relationship between characteristics of the particles andgeometry of ink dots formed on ink-jet papers was
examined. Parameters of pigment required for high image
quality were discussed in terms of dynamic penetration
behavior of ink droplets obtained using a microscopic highspeed video camera system1 and the three-dimensional
shape of ink dots in paper obtained using a CLSM2-4.

Experimental
Samples
Processing of calcium carbonate
To provide commercial precipitated calcium carbonate(PCC), PZ, Shiraishi Kogyo Kaisha. Ltd., with ink-jet
printability, it was modified in terms of wettability,
reduction and equalization of particle size, and porosity ofpigment coatings. PCC tends to form heterogeneous
aggregates in size. First, we tried to break down aggregates
into smaller ones by mechanical grinding. Calcium
carbonate PZ was dispersed into water to 40 % consistency.
This pigment slurry was poured into a cylindrical container
with six balls of about 10 mm diameter all made of agate.
The container was set to the ball mill, Planetary Ball Mill P7, Fritsch-Japan and the pigment was ground. Table 1
shows three grinding conditions.
Another size reduction of calcium carbonate wasconducted by dissolution with hydrochloric acid. The
concentration of the hydrochloric acid was altered with aconstant ratio of the dilute hydrochloric acid volume tocalcium carbonate mass. The concentration used was 0.1,
0.2 and 0.4 M to dissolve about 5, 10 and 20 % of calcium
carbonate.
Table 1 Grinding conditions of calcium carbonate and their surface

Grinding condition
Specific surface area, m2/g
(Nitrogen adsorption
Revolution, Time,
Repetition method)
min
rpm
850
15
3
19.8 (Intensive Grinding)550
103
17.6 (Medium Grinding)250
53
15.4 (Light Grinding)00013.5 (Untreated)
Preparation of trial ink-jet paper and plasma etching
Trial ink-jet paper was prepared from untreated, groundpartially-dissolved calcium carbonates. Calcium carbonateof 100 pph (parts per hundred of pigment) with sodium
polyacrylate of 0.8 pph as a dispersant were dispersed for 5
minutes using a disperser, Hybrid Mixer HM-80, Keyence
Corp. and defoamed for 10 seconds. Polyvinyl alcohol of 10
pph as a binder, poly-DADMAC of 20 pph as an ink
fixative were added and mixed to a coating color at 40 %solids. This color was coated on a base (copy paper) with amotor-driven wire bar coater followed by hot air drying for
60 s.
Some of the trial ink-jet papers prepared from untreatedcalcium carbonate were plasma-etched to givehydrophilicity. Plasma ions emitted by alternating current
corona discharges strike a surface of the trial ink-jet papers
and generate positive ions. The positive ions and hydroxylions of water bind together and the surface becomeshydrophilic. Practically, polyvinyl alcohol formulated as abinder is considered to become hydrophilic rather thancalcium carbonate. Plasma etching was conducted in theatmosphere of 20 Pa by applying 400 V alternating current.
Three levels of etching strength used were, in the increasingorder, [1] 6 to 8 mA for 30 s, [2] 6 to 8 mA for 60 s and [3]18 mA for 120 s.
Properties of processed pigments and trail ink-jet
papersProcessed pigments were subjected to analyses such as
scanning electron microscopic (SEM) observation,
determination of BET specific surface area by the nitrogen
adsorption method, Nova 4000, Quantachrome Instruments,and measurement of particle size distribution by the laser
dynamic light scattering method, Zetasizer 3000, Malvern
Instruments. For trial ink-jet papers, contact angle between
a paper surface and water was measured as well as the ink
penetration analysis described in the following section.
Microscopic high-speed video recording and dynamic
analysis of ink penetration
In ink-jet printing, penetration of ink-jet inks withrelatively low viscosities is a very rapid phenomenon. So,efforts to capture ink-jet ink penetration using a high-speed
camera have been made by several researchers5, 6. Photo 1 isan overview of the video recording device developed in thiswork. A test ink-jet
head, HEK-1, Konica
Minolta, was used forprinting. Voltage data
for ejecting ink droplets
were adjusted in orderto minimize satellite
occurrence. The volumeof one ink droplet is
about
30
pL,
corresponding to about
40 µm in diameter.
Photo 1 Microscopic highAqueous black inks for
speed video camera system for
ink-jet,BCI-6BK,
dynamic ink penetration
Canon, were used.
A high-speed video camera, VFC-1000 black-andwhite model, For-A, was fitted into a common optical
microscope with a tenfold objective lens, Olympus via
tenfold attachment lens MA44S, Elmo. Ink behavior wasrecorded at a speed of 500 frames/s and a shutter speed ofless than 1/1000 s. The ink-jet head was fixed at an about 45
degree angle from the paper sheet plane. Cool lightillumination was introduced at an about 20 degree angle.
The distance between the objective lens and a paper surface
must be about 5 mm for optimum focus, but it is too shortfor the head to be inserted in-between. The head was set
about 20 mm distant from the landing location, thus
reducing the collision speed of collision considerably incontrast to about 1 mm for practical printers. The video
images were decomposed to individual images, each ofwhich was subjected to the following image analysis.
General print quality-related parameters calculated byimage analysis applied to printed characters are area,
perimeter and intensity, namely, optical density7. Area,
perimeter and optical density reflect print sharpness, degreeof wicking and the uniformity of the text, and ease of visual
perception, respectively. In this work, the dot images werefirst binarized based on the dynamic threshold level definedby the moving average method. Ink dot area was filled andclosed when there were holes made apparently by lightreflection, secondly. Pattern region (ink dot region in thiscase) analysis was made to calculate geometrical parameterslike ink dot area and these parameters were saved in the
CSV format for statistical evaluation using Excel, Microsoft.
Parameters calculated for each individual ink dot were ink
dot area and ink dot roughness. The former was calculated
in square micrometers by converting from the pixel scale.
The latter is defined as the ratio of true perimeter to envelop
perimeter indicating the degree of perimeter roughness ofthe pattern region, where the envelop perimeter means thecurve connecting peripheral pixels externally without any
dent. Dot roundness, a ratio of area to perimeter squared, isalso often used to indicate print quality8, but for non-coatedpaper with fiber orientation, elliptic dots do not always
mean poor print quality. Therefore, dot roughness was
adopted here. These two parameters were averaged from 5
ink dots per paper sample. The series of image analysis wasdone using software, PopImaging Ver.3.40, Digital Being
Kids.
Measurement of ink penetration depthDistance of penetration of ink droplets ejected from thetest ink-jet head into paper was measured with the CLSM.
Fluorescence at around 667 nm wavelengths emitted fromthe black ink by irradiation of laser at the 633 nm
wavelength permits to slice ink dots in paper optically by
the confocal system and the three-dimensional shapes were
measured. For each slice image, the ink component was
judged to be present when the fluorescence intensity (graylevel of the dot) was over a certain value that was common
to every slice image. This threshold value was determinedas the most suitable threshold gray level to binarize thevertically central slice image. The penetration depth was
calculated from the slice thickness of 1 µm. This CLSM
method is one of the most useful techniques for measuring
ink penetration depth although there are some othermethods like simulation of ink penetration effects onspectralreflectanceandchromatic
appearance9,
experimental evaluation of chroma and hue of printed
colors10, time-of-flight secondary ion mass spectroscopy(TOF-SIMS) to evaluate effects of polymers on binding
ink-jet dyes11 and Raman spectroscopy without fluorescenceinterference12.

Results and discussion

(b)

(a)

10 µm

2 ms after the landing. Mean values of several ink dots areplotted although the ink droplets taken into the meancalculation were not always ejected from the identicalnozzle. The leveled-off area was the largest for theuntreated carbonate pigment. Even light grinding was foundto have an effect on dot area reduction. Comparison
between commercial ink-jet papers coated with silica
pigments, in our previous work1, smaller dot area resulted inbetter image quality. Therefore, the finding that dot area
decreased as grinding intensity increases implies thatparticle size reduction leads to a good image quality. The
dot area for commercial photo-quality papers ranges
approximately between 1100 and 1300 µm2 and that for thecoated paper prepared from the carbonate ground the mostintensively also falls into this range. But, the time until dot
area has leveled off was 8 to 20 ms for the commercial inkjet papers while it was as long as 40 to 60 ms for the groundcarbonate pigments, as is meaning slow ink absorption. Ink
absorption slowness would cause ink mixture resulting inpoor image quality in high-speed printing. Ink absorption
rate is considered to be related to porosity and binder
distribution of coatings as well as hydrophilicity of pigment
surfaces.
Figure 3 shows changes in ink dot roughness that
represents the degrees of ink bleeding and heterogeneous
ink penetration due to heterogeneous microstructure ofcoatings deriving, for example, from a pigment aggregate
shape. The dot roughness values for the ground carbonate
pigments were lower than those for untreated carbonate
pigment, but all the values for the carbonate pigments weremuch higher than for commercial photo-quality ink-jet
papers. It took about 60 ms for the dot roughness values tolevel off for the carbonate pigments. This fact suggests that

Properties of ground calcium carbonate
Photo 2 is scanning electron micrographs of calcium
carbonate pigment before and after grinding. Because water
was evaporated in the atmospheric conditions to prepare theSEM samples, it is noted that the carbonate tended tocoagulate. Considering this drying method, the untreatedcarbonate is found to have formed heterogeneous
aggregates of up to about 10 µm in diameter while theintensively-ground carbonate shows reduction andequalization of particle size in spite of large aggregatespresent partly. Figure 1 shows normalized particle size
distributions for the carbonate pigments before and after
intensive grinding. The grinding process decreased larger
particles and increased smaller particles both relatively.
Table 1 includes BET specific surface areas measured by
the nitrogen adsorption method. Itshows that the specific surface area
14
increased as the grinding intensity
Before grinding
12
increased.
After intensive grinding

2000

Dot area,µm2
Dot Area, µm

1800

2

10

Intensivity
Intensity

Dynamic
analysisofinkpenetration into trial ink-jet paper
Figure 2 shows changes of ink
dot area on the trial ink-jet papers
prepared from the ground carbonate
pigments. Time scale for thehorizontal axis was calculated fromthe number of image frames. The
initial frame including the ink
droplet on paper was assumed to be

8

10 µm

Photo 2 Scanning electron micrographs of calcium
carbonate before (a) and after (b) grinding

Untreated PCC
Intensive Grinding

6
4
2

1600
1400
1200

Untreatedcarbonate
Intensive Grinding
PCC
LightUntereatedgrinding
Medium Grinding
Mediumgrinding
Low Grinding
Intensive grinding

1000
800
600
400

0
10

100

1000

10000

0

20

40

60

80

Time,msTime,
msFig. 1 Particle size distribution of calcium Fig. 2 Dot area changes with time forcarbonate before and after grinding
trial ink-jet papers coated withcalcium carbonate ground at three

Particle size, nm
Particle Size,nm

100

2000

1.20

Dot Area, µm 2

1.16

1500

1.14
Intensive Grinding
Light grinding
Medium Grinding
Grinding
MediumLow
grinding
Photo Quality IJ Paper
Intensive grinding
Photo-quality IJ paper

1.12
1.10
1.08
1.06

Ink dot depth, µm

DotRoughness
roughness
Index

1.18

1000

5% Dissolv ed
10% Dissolv ed
20% Dissolv ed

500

1.04

0

20

40

60

80

Time,msTime,
msFig. 3 Ink dot roughness changes withtime for trial ink-jet papers
coated with calcium carbonate
ground at three levels

100

5

10
15

0

1.02

0

per
per
g
J pa
IJ pa
ding
g
de I
indin
alit y ium gra
e gr ium grin t grindin
u
iv
q
s
t
n
Pho
M ed
I nte
M ed
Ligh

0

10

20

Time, ms

30

40

Fig. 4 Dot area changes with time for Fig. 5 Ink penetration depth for trial ink-jet
trial ink-jet papers coated withpapers coated with ground calcium
calcium carbonate dissolved
carbonate pigments compared topartially with hydrochloric acidthose for commercial ink-jet papers

ink spread very slowly on the dot periphery although dot
area had hardly increased any more well before this time.
Figure 4 shows changes of ink dot area on the trial inkjet papers prepared from the partially-dissolved carbonate
pigments. Dot area tended to decrease as the amount ofdissolved carbonate increased to be small particles. It ishighly likely that coating structures prepared from pigmentswith smaller particle sizes include pores with shorter radiiand consequently lower the penetration speed with lateral
spread suppressed, regardless of whether the particle size is
reduced by grinding or dissolution.
Contact angle between water and plasma-etchedsurfaces of the trail ink-jet papers prepared from theuntreated carbonate pigment decreased as the etchingstrength was raised. The etching process hardly changes theporous structure, so surface chemical modification of coatedpaper is expected to improve image quality.
Measurement of ink penetration depthDistance of ink penetration into paper was measured
with the CLSM technique. Figure 6 shows the comparisonin the ink penetration depth of trial ink-jet papers from theground carbonate pigments with photo-quality and medium
grades of commercial ink-jet papers. For commercial ink-jet
papers using silica pigment, ink penetrated to 15 µm in
depth. In contrast, it was 8 to 11 µm for the trial ink-jet
papers using the ground carbonate pigments. Between thetrial ink-jet papers, the ink penetration depth tended to begreater for smaller particles processed by more intensive
grinding.

Conclusion
Particle size reduction of calcium carbonate pigment bymechanical grinding and hydrochloric acid dissolution
decreased ink dot area and ink dot roughness in ink-jet
printing. This finding implies that these processes improveimage quality of ink-jet. Assuming that smaller ink dot area
provides higher image quality, mechanical grinding had thegreatest effect, hydrochloric acid dissolution the second andplasma-etching the last. As for the coating pore structure, it
was experimentally elucidated that smaller pores due toparticle size reduction tends to make an ink penetrate deep

into a coating structure rather than spread laterally along thesurface.

Acknowledgement
Authors wish to thank Mr. Fumihiro Fujimaki,
Nakagawa Mfg. Co. Ltd. for providing commercial ink-jet
papers and Mr. Kazumasa Matsumoto and Mr. Kenzo
Nakanishi, Konica Minolta Technology Center, Inc. forpermitting to use the test ink-jet head, HEK-1.

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Biography
Toshiharu Enomae graduated from The University of Tokyo in 1984, andreceived M.Sc. in 1986 and Ph.D. in 1993 regarding "Evaluation of coatedpaper structure". For 1993-1995, post-doctorate fellow for Dr. Pierre
LePoutre at Univ. of Maine, USA. Backgrounds are paper coating, paper
physics etc. Current interests are "Application of new minerals developedby hybridization with inorganic compounds to paper coating" etc.