Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

In-Plane Distribution of Paper Absorbency
Measured by Liquid Absorption Profilometer
Toshiharu Enomae*1, Hiroshi Kataoka*2 and Fumihiko Onabe*1*1

The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-8657*2

ISI-Dentsu of America, Inc., 1114 Avenue of Americas, New York, NY10036, USA

Abstract: A novel apparatus for quantitative evaluation of a liquid, particularly, water
absorption profile of paper was developed. The underlying principle for this method issimilar to that of Bristow's method. This system is characterized by an applicator headwith a small opening scanning a paper surface and by the detection system using an
optical sensor pursuing the meniscus of water filled in a glass capillary connected withthe applicator head. The level of sheet formation evidently affected the variation ofwater absorption rate profile, but the effect was observed only when the contact time
was sufficiently long. The coefficient of variation for the worse formation was 36 %higher than that for the better formation at a contact time of 3 s, while it was almost thesame at a contact time of 0.2 s. However, no distinct correlation such as accordance ofpeaks was found between the transmitted light profile and the absorption rate profile.
The density variation inducing pore size distribution near the surface layer may affectthe absorption rate variation. Additionally, the influence of sizing degree was discussed.
This novel system might be applied to clarify effects of water absorbency of basepaperon coating and adequate ink absorption in inkjet printing.

吸液変動測定装置による紙の面内吸液不均一性測定

江前敏晴*1, 片岡裕史*2 , 尾鍋史彦*1

*1

東京大学

*2

ISI-Dentsu of America, Inc., 1114 Avenue of Americas, New York, NY10036, USA〒113-8657 東京都文京区弥生 1-1-1

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

1. Introduction
Recently mottling has been one of the significant problems associated with print quality of coatedpapers. The term "mottling" is generally defined as in-plane visual unevenness of ink density. Thisphenomenon occurs not only to coated grades, but also to uncoated ones. Uncoated paper with roughsurfaces results in uneven distribution of ink density in an excessive calendering operation that forms
dense and bulky regions. Regarding coated paper, the microscopic structure of coatings is supposed
to govern unevenness of ink absorption.
It has been pointed out that binder migration is one of the mechanism associated with occurrence ofmottling. Binder migration is the phenomenon where binders, such as starch and latex, migrate
towards the surface of coating layer during drying. However, Arai[1] reported that coated paper withhigh latex concentration at the surface does not always cause mottling. This explains that bindermigration does not directly cause it, but that unevenness of surface latex concentration tends to cause
mottling as shown by Eklund[2], Norrdahl[3] and Whalen-Shaw[4].
In-plane unevenness of coat weight has also been pointed out as one of the causes for mottling.
Engström[5] reported that the binder distribution correlated with coat weight distribution, and thiscorrelation depended on the basepaper and on the coating color. Saito et al.[6] showed that the Croda
ink density correlated much better with coat weight distribution than with the surface latex
concentration. Kohno[7] found that the basis weight (not coat weight) profile of coated paper,measured with β-ray, correlated well with the ink density profile and suggested that the printingpressure variation was its cause. So, this should be another factor as well as coat weight and surface
latex concentration.
Coat thickness distribution has been also pointed out as a cause for mottling. Kent[8] examined thecoat thickness variations using a mathematical analysis based on Walsh functions. He concluded thathigh coat thickness areas were mottled areas with less ink absorption because, he speculated, (a) it
took longer time to dry in those areas, (b) migrated latex filmed and sealed pores of the surface, and(c) this prevented rapid ink absorption.
All the morphological parameters of coated paper described above, such as distribution of latex
surface concentration, coat weight and coat thickness, are supposed to originate from unevenness ofthe basepaper in various ways. Concerning latex surface concentration, if basepaper has regions ofdifferent absorbency from place to place, wet coating consolidates quickly and reaches theimmobilization point early in absorbent regions. Therefore, those regions have a lower latex
concentration near the surface than less absorbent regions. It follows that occurrences of ink mottlingindirectly stems from the non-uniform absorbency distribution in the basepaper. More roughening
resulting from fiber swelling in absorbent regions of the basepaper is considered to tend to givehigher levels of coat weight and thickness. Finally, those morphological non-uniformity should leadto non-uniform ink density.
Internal loading and pre-coating with latex or starch have been performed as a means to make theabsorbency of basepaper uniform[10, 11]. However, the evaluation of these effects was restricted tosubjective ratings over trials of coating and printing practice. Although these may be suitable for2

Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

product management or quality control, the basic mechanisms need to be systematically investigated.
Howarth and Schindler[12] reported on the uniformity of the wetting time over the surface of a paper
sheet by rising a stained water surface and making it contact with the sheet horizontally held. They
measured the area where water oozed over the opposite side of the paper. But this method does notexactly simulate coating processes because transudation of water occurs near the surface layer in
actual coating processes. Furthermore, it should be difficult to wet a paper sheet uniformly over thewhole surface because water rising through the cylinder, when overflows, is not flat at the surface.
From the standpoint of the development of apparatus, spiral scan type Bristow tester named DSA(Dynamic Scanning Absorptometer) was developed by Kuga et al[13]. In this apparatus, a Bristow
wheel lies horizontally and a circular side of the wheel is used as a sample stage instead of thecircumference. Just one spiral scan is long enough to obtain data to draw a whole Bristow curve.
In Pulp and Paper Research Institute of Canada, a new apparatus called water absorption tester isunder development to investigate inkjet print quality in terms of feathering and bleed through[14,15]. The tester has a similar applicator head and a scan mechanism as the absorption profilometer inthis study. But, their tester was designed to estimate lateral penetration of printed inkjet dots.
In the present study, we aimed to evaluate the in-plane unevenness of absorption rate of basepaper.
The quantitative evaluation achieved by our new technique allowed to compare it between different
papers easily and gave a clue to the correlation of ink mottling with basis weight and density
distribution of basepaper, which are fundamental and likely to affect the coating distribution[12].

2. Experimental
2.1 Development of Absorption Profilometer
An original apparatus for absorption profilometry, that is, for measurement of in-plane
non-uniformity of liquid transfer rate of paper was developed. The liquid used in this work waswater. The principle of measurement is based on that of the Bristow method[16]. Figure 1 shows theschematic diagram of this apparatus. It consists of the sample stage, applicator head, glass capillary(Iwaki Glass Inc. ,Japan) and the meniscus sensor controlled by a personal computer (PC-9801XL,
NEC Inc., Japan). A paper sample mounted on the stage moves horizontally at a precisely constantand variable speed.
Figure 2 illustrates an enlargement of the applicator head. It is made of a stainless tube for liquid
chromatography having an inside diameter(ID) of 1.0 mm. A tube of 0.5 mm ID was also tried for ahigh resolution. A flexible Teflon tube connects the applicator tube with the glass capillary having
an ID of 0.5 mm. Water flows continuously from the meniscus present in the glass capillary to thetip of the applicator head. Movement of the meniscus in accordance with water transfer to paper ispursued by the optical sensor to detect the meniscus location. The volume of transferred water iscalculated from the pursuit distance of the sensor. The sensor is driven by a step motor with a unit
step of 0.2 mm, that is, about 0.04 µL in water volume. The sensor consists of a light-emitting diode3

Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)(LED) and a phototransistor (TLN101A and TPS601A, respectively, Toshiba Inc., Japan), both ofwhich works in the infrared range of light, thus are suitable for detecting a water meniscus with highdirectivity.
The unique and excellent features of this apparatus are as follows: The situation of the contact of theapplicator head with a paper surface, hence the way of water transfer is constant along a scan on thesurface. Such advantage could not be achieved by the method of wetting the whole surface area at
one time. This characteristic, to be exact, cannot be available with the Bristow applicator head,
where residual liquid in it decreases with transfer and the transferred volume was slight at the end of
the trace. In addition, this method facilitates comparison between different samples because it
evaluates absorption unevenness quantitatively.
The tip of the applicator head was polished carefully not to scratch the paper surface. The external
force applied to the applicator head to press the paper surface was 0.12 N and then the pressure wasabout 0.15 MPa, which is close to the pressure prescribed in Bristow's method, i.e. 0.1 MPa.
To adjust the pressure of water at a wetting point, the effect of interfacial tension at the meniscus in
the glass capillary is not negligible because of its very narrow passage. Therefore, the glass capillary
was held about 50 mm above the applicator head so that no external pressure would be developed towater at a wetting point.

2.2 Formation Measurement
If a local basis weight distribution is assumed to be sheet formation, the formation is conventionallyevaluated using several kinds of radiography like β-ray and soft X-ray. For safety and simplicity,
visible light was used in this study to determine the formation level although it has someinaccuracies[17-19] to identify the transmitted light distribution with the basis weight distribution.
However, for uncalendered handsheets without filler, light transmittance is approximately a functionof basis weight. Enomae and Kuga[20] found that the pressure in wet-pressing hardly affected thelight transmittance. This implies that, even if there happens non-uniform wet-pressing over ahandsheet, the non-uniform density distribution caused by it hardly affects the non-uniformity of thetransmitted light image. So, "formation profile" referred-to here means the profile of transmitted
light intensity, that is, gray level in its images.
A paper sample was placed on the frosted glass through which a fluorescent lamp illuminated thesample. The frosted glass served to scatter light and homogenize the illumination. An anti-reflective
transparent glass was placed on the sample. Transmitted light images were taken with a CCD video
camera (TI-22A, NEC Inc., Japan) and the dynamic range of input signal was adjusted in an image
analyzer (LA-500, PIAS corp., Japan) so that the difference between a maximum and a minimum of
the transmitted light intensity (gray level) would be expanded and optimized. The actual size of 1
mm × 1 mm on the paper sample is equivalent to 6 pixels × 6 pixels in the image. The transmitted
light images of the sample papers were saved ahead of the subsequent water absorption test. Afterthe water absorption test, the scanned location was sought from the saved image by visualcomparison. Then, the corresponding line profile of gray level was extracted from the whole image

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

in such a way that the mean gray was calculated sequentially along the scan line from adjacent 5
pixels in the cross-scan direction, equivalent to 0.8 mm on the actual paper sample.

2.3 Materials and Sample Preparation
Commercially available woodfree paper, a copy paper, was mainly used. The basis weight and the
Stökigt sizing degree were 64 g/m2 and 34 s, respectively. Machine-made woodfree papers withdifferent sizing degrees were also used to examine the effect of sizing degree on the absorption curve.
The sizing degree was varied by adding the different amount of alkyl ketene dimer (AKD) to thefurnish. The Stökigt sizing degree was about 0, <1, and 48.6 s for weakly-, intermediately-, andhard-sized paper, respectively. The basis weight of the three samples was about 75 g/m2. Scans weremade by the absorption profilometer along the machine direction to reduce the risk of elongation
during a scan.
Handsheets were prepared to examine the effect of sheet formation. The furnish consisted of
hardwood bleached kraft pulp beaten for 6000 revolutions in PFI mill. A 1.3 wt-% rosin size to thatof pulp and aluminum sulfate to adjust pH to 4.5 were added. Sheets of two different levels offormation were prepared according to JIS in most procedures. Sheets of the worse formation wereprepared in the following modified manner. The pulp slurry was allowed to stand for 5 minutes withthe slurry surface level at 100 mm (normally 350 mm) above the wire, then drained. Every sheet wasdried according to the ring-restraint drying method. The Stökigt sizing degree was about 25 s.

3. Results and Discussion
3.1 Effect of Scanning Speed on Water Transfer
Figure 3 shows water transfer behavior basically obtained using the absorption profilometer at
different scanning speed of the applicator head. The horizontal axis is the scan distance of theapplicator head on the paper surface. The vertical axis is the transferred water volume accumulatedwith the scan. Every relationship was, in average, linear regardless of the scanning speed. At higher
scanning speeds, the slope of the curves came close while a small difference in the scanning speed
made a lot of differences in the slope at lower scanning speeds.
The final goal of this work is to determine the degree of variation in absorption rate from point topoint. The level of linearity of the relationship found in Fig. 3 represents the degree of
non-uniformity of paper absorbency. In other words, the higher the level of linearity, the more
uniform the in-plane paper absorbency. More discussion regarding unevenness of paper will begiven later. First of all, the data from this new technique were compared with that of Bristow's
method to confirm that this profilometer provides reliable results.
3.2 Comparison with Bristow's Method
When only the average slope of the curves shown in Fig. 3 is taken into account, all the slopes

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

together at each contact time provides a Bristow's curve. The contact time was calculated by
dividing the inside diameter, that is, 1 mm by a scanning speed. However, the contact time, strictly
speaking, is varied along the 1 mm diameter perpendicular to the scan direction because the opening
is circular. In this sense, the precise contact time is difficult to calculate. However, this deviation was
actually ignored because the longest contact time located in the center of the opening was assumed
to govern the practical contact time. This assumption was found to be allowed as mentioned below.
Figure 4 shows a water transfer curve determined by the conventional Bristow's method and by theabsorption profilometry. The fact that the data points by the two methods are well plotted on theidentical curve suggest that the contact time could be calculated with no problem by assuming thatthe longest contact time governed the mean contact time. Another problem inherent to the absorption
profilometry is that the narrow passage of glass capillary might cause viscous drag in water flow.
However, the comparison of the present method with the conventional Bristow's method shows thatthe values measured by this method was sufficiently adequate even at the highest scan speed.
Consequently, the reliability of the absorption profilometry was empirically guaranteed.

3.3 Effect of Sizing Degree on Water Transfer Curve
The relationship between the transferred water volume and the scan distance shown in Fig. 3 isaffected by sheet properties such as sizing degree, surface roughness, basis weight, and density. The
factors of the profilometer such as scanning speed and inside diameter of the applicator head are also
supposed to affect this relationship.
Figure 5 shows the changes in accumulated transferred water volume with scan distance at aconstant scanning speed (200 mm/min) for commercial woodfree papers of three different sizing
degrees. It was observed that the mean transfer rate, that is, the slope of the curves decreased withthe sizing degree.

3.4 Definition of Absorption Profile
Curves of accumulated transferred water volume versus scan distance as plotted in Figs 3 and 5 canbe converted into an absorption profile. Figure 6 illustrates schematically an example of theabsorption profile obtained by differentiating the transferred water volume with regard to the scandistance. The rate of water absorption at position A corresponds to that (Va) of the inset Bristow’s
curve representing the absorbency around position A. In the inset Bristow’s curve, contact time T isdetermined by the scanning speed and the inside diameter of the applicator. The absorbency aroundposition B is assumed like the other inset Bristow’s curve and Vb is plotted in the absorption profile.
The shapes of Bristow’s curve in the insets are practically unknown except values Va and Vb.
3.5 Effect of Sheet Formation on Absorption Profile
Figure 7 shows an absorption profile (solid line) of a handsheet of the better formation. The
formation profile (the profile of transmitted light intensity) just on the same location was also shown(dotted line). Figure 8 shows the results of the same experiment for a handsheet of the worse

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

formation. A comparison between Figs. 7 and 8 exhibited that the worse formation leads to a larger
variation as found in the absorption profile. However, the distinct correlation such as accordance ofpeaks between them was not found even with the sample of the worse formation. Although theformation profile was supposed to influence the absorption profile considerably, this result did notgive its enough evidence. Otherwise, the density distribution especially close to the surface may
have possibly affected the absorption profile considering that it governs the pore size distribution
closely related to the absorption rate. Besides, distributions of fines and size are generally potential
to affect it, but those effects are less likely involved in the data for the handsheets. Anyway, theabsorption profilometer clearly showed an influence of basis weight variation on the absorption rate.
Figure 9 shows absorption profiles at different scanning speeds for the two levels of formation.
Obviously, at least the extent of variation of the absorption profiles was affected by the formation.
Therefore, it follows that the basis weight had some influence on the absorption rate profiles. Table
1 shows the statistical values of the water absorption rate at different scanning speeds for thehandsheets of the different formation. There was little difference among the coefficient of variation
between the two levels of formation at a scanning speed of 300 mm/min. On the contrary, significant
differences were observed at 20 mm/min. The contact time was 0.2 and 3 s at 300 and 20 mm/min,
respectively. From Bristow's theory or in view of wetting delay, it was considered that penetration ofwater had not yet occurred for the initial period of 0.2 s. Therefore, the variation of the profiles at
300 mm/min depended, probably, only on the surface roughness variation. The difference in thisvariation between the two levels of formation was presumably too small to be detected. On the other
hand, water had already started to penetrate before 3 s passed. In summary, the variation ofpenetration rate was considered to be influenced by the variation of basis weight as mentioned above,
but details of the mechanism should be investigated in further researches.

4. Conclusions
A novel technique was developed for measuring an in-plane profile of liquid transfer volume byassembling a prototype applicator head with a small opening and an optical sensor pursuing themeniscus. As a result of the data obtained using this water absorption profilometer, the in-planevariation of an absorption profile of a handsheet was influenced by the basis weight variation at a
sufficiently longer contact time. The coefficient of variation for the worse formation was 36 %higher than that for the better formation at a contact time of 3 s, while it was almost the same at acontact time of 0.2 s. However, no distinct correlation was found between the absorption profile andthe profile of transmitted light intensity that represents approximately the basis weight variation.
This suggests that not only the basis weight variation but also the density variation of paper affected
the absorption profile. The novel technique developed in this study could give a clue to solve ink
mottling particularly if the lateral scanning is combined to expand the scan range to three
dimensional mapping. Additionally, it is expandable to ink absorption behavior in inkjet printing.

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This work was partly presented in 1993 TAPPI Advanced Coating Fundamentals Symposium,
Minneapolis, MN, USA

REFERENCES
1.

Arai, T., Yamasaki, T., Suzuki, K., Ogura, T., and Sakai, Y., 1988 TAPPI Coating Conference

Proceedings: 187-192(1988)
2.

Eklund, D. E., and Palsanen, J. A., Tappi 53(10): 1925-1930(1970)

3.

Norrdahl, P. C., 1991 TAPPI Coating Conference Proceedings: 417-435(1991)

4.

Whalen-Shaw, M., and Eby, T., Tappi 74(12): 188-194(1991)

5.

Engström, G., and Rigdahl, M., Kline, J., and Anlroos, J., 1991 TAPPI Coating Conference

Proceedings: 169-177(1991)
6.

Saito, Y., Matsubayashi, H., Takagishi, Y., Miyamoto, K., and Kataoka, Y., Pan-Pacific Pulp &Paper Technology Conference Proceedings: 45- 52(1992)
7.

Kohno, M., and Hamada, T, Kamipa Gikyoshi (Japan Tappi J.), 40(3): 295-303(1986)

8.

Kent, H. J., Climpson, N. A., Coggon, L., Hopper, J. J., and Gane, P. A. C., Tappi 69(5):78-83(1986)
9.

Howarth, P., and Schindler, M. K., "The aerial distribution of liquid penetration of paper", F. R.

C. at Oxford, England: 483-496(1985)
10. Lepoutre, P., Bichard, W., and Skowronski, J., Tappi J. 69(12): 66-70(1986)
11. Dahlquist, R. W., Welch, L. J., and Hardy, R. E., Tappi J., 73(12): 151-157(1990)
12. Kataoka, H., Enomae, T., Onabe, F., and Usuda, M., Sen'i Gakkai Preprints 1992(S), Tokyo: S228(1992)
13. Kuga, S., Kataoka, H., Enomae, T., Onabe, F., Yagi, N. and Shinkai, O., Kamipa Gikyoshi(Japan Tappi J.) 48(5), 730-734 (1994).
14. Inoue, M., Private communication (1998)
15. Tsai, Y.-G. and Sakamoto, M., 1997 Japan Tappi Annual Meeting Proceedings, 261-269(1997)
16. Bristow, J. A., Svensk Papperstid., 70(19): 623-629(1967)
17. Tomimasu, H., Suzuki, K., Ogura, T., and Luner, P., Tappi J. 73(5): 179-187(1990)
18. MacGregor, M. A., and Johansson, P-A, Tappi 73(12): 161-168(1990)
19. MacGregor, M. A., and Johansson, P-A, Tappi 74(1): 187-194(1991)
20. Enomae, T. and Kuga, S., J. Wood Science, submitted for publication (1999)

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Table 1 Statistical values of water transferred volume for papers of the better andworse formations.
Scanning speed, mm/min

300

Formation

Water transferred
volume, mL/m2

20

BetterWorse

Better

Worse

Mean

5.03

5.69

19.68

25.65

Standard deviation

2.49

2.75

8.28

14.64

Coefficient of variation

0.49

0.48

0.42

0.57

Optical Sensor
Meniscus

Glass capillary tube φ0.5mm

Step
Applicator head φ1.0mm

Motor

Paper sample
Transient
Sample Stage

MemoryPersonal
ComputerFig. 1

Schematic diagram of water absorption profilometer.

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

Stainlesssteel tube

Movablevertically

Glass capillary

Teflontube

Fixed

Accumulated transferredwater volume, mL

Fig. 2 Schematic diagram of liquid applicator head fixed so that it moves
only vertically in scanning.

1.2
8

1.0
16

0.8

160
80
200
400
800

40

0.6
0.4
0.2

Scanning speed in mm/min

0.0
0

20

40
60
80
Scan distance, mm

100

120

Fig. 3 Transferred water volume accumulated with scandistance at various constant scanning speeds.
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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

Transferred water
volume, mL/m2

20
15
10
5

Absorption profilometry
Bristow's method

0
0

20
40
60
Square root of contact time, ms1/2

80

Fig. 4 Water transfer cuve measured by Bristow's method and by absorption
profilometry. Inside diameter of applicator was 1.0 mm.

Accumulated transferredwater volume, µ L

3.0
Scanning speed: 200 mm/min
Inside diameter: 1.0 mm

2.5
2.0

Weakly sized

1.5
Intermediately sized

1.0
0.5

Hard sized

0.0
0

50

100

150

Scan length, mm
Fig. 5 Water transferred volume accumulated with scan lemgth at aconstant scanning speed for variously sized papers. Deviation fromlineality indicates in-plane unevenness of paper absorbency.
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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

Transferred water volume

Bristow's curve at A

Bristow's curve at B
Vb

Va
T

T

A
B
Relative position, mm

Transferred water volume, mL/mTransmitted light intensity

2

Fig. 6 Schematic absorption profile at contact time T. Va and Vb are transferred water volumes
at position A and B, respectively. Inset Bristow's curves represent absorption properties at
specified positions.

140
120
100
80
60
40
20
0

Basis weight profile

Absorption profile

0

20

40

60

Relative position, mm
Fig. 7 Comparison between water absorption profile and formation profile (gray level oftransmitted light) for a handsheet of the better formation. Scanning speed was 40 mm/min.
Inside diameter was 1.0 mm.

12Transferred water volume, mL/mTransmitted light intensity

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Sen'i Gakkaishi（繊維学会誌）55(2), 301-308(1999)

160
140
120
100
80
60
40
20
0

Basis weight profile

Absorption profile

0

20

40

60

Relative position, mm
Fig. 8 Comparison between water absorption profile and formation profile (gray level oftransmitted light) for a handsheet of the worse formation. Scanning speed was 40 mm/min.
Inside diameter was 1.0 mm.

80
Formation : better

Transferred water volume, mL/m2

60

300 mm/min

20 mm/min

40
20
0
80

0

20

40 300 mm/min
60

80

100
Formation120: worse 140
20 mm/min

60
40
20
0
0

20

40

60

80

100

120

140

Fig. 9 Comparison of absorption profile variation between papers of better andworse formations at a scanning speed of 20 or 300 mm/min.

13