3May 6, Friday
13:45-17:00

T. Enomae

Evaluation of water absorption rate of paperby an automatic scanning absorptometer

Instructor: Toshiharu Enomae (Laboratory of Paper Device and Ecofriendly Material Sciences,Biomaterial Engineering)
Gathering place: 2D110 in Bldg.2D
Experiment place: SogoA618
1. General
Paper composed only of pulp superbly absorbs water because cellulose, which constitutes paper,is hydrophilic. However, sizing agents are always added to paper used for printing and writing
because this paper moderately absorbs water. In offset printing, for example, water designated as
dampening solution is simultaneously transferred to paper with ink. If the dampening solution is notabsorbed by the paper at the proper rate, the succeeding ink cannot be applied to the paper. In thecase of paper for inkjet printing, because the transverse spreading of ink results in a blur, a fastabsorption rate of ink is required to ensure satisfactory printing quality without blur. Accordingly,
water absorbency is an important factor affecting printability; for quality control of paper, it isnecessary to accurately evaluate and control the sizing degree (water repellency).
Simple methods for testing the sizing degree include the Stöckigt method and the Cobb test,
which test the water absorbency of paper that has contacted water for many hours. The Bristow
method is used to quickly measure the water absorbency of the surface of paper immediately after it
contacts water under a condition similar to printing. In this experiment, initial water absorption rate
will be measured with an automatic scanning absorptometer, which is based on an improved Bristow
method.
1-1 Bristow method
Bristow testerThe Bristow method was Liquid supply head, filled with a certain
devised to rapidly measure water amount of liquid, scans paper at various
Liquid absorption rate can beabsorbency immediately after speeds.
measured rapidly.
contact with water. Figure 1
shows a liquid supply head and aspecimen mounted on the wheelof a Bristow tester. In this▪Contact time
method, a certain amount (V) of
Scan speed
liquid is poured into a head box
Slit widthwith a narrow slit, this slit is▪Transferredbrought into contact with paper
volumeLiquid volumemounted on a wheel rotating at aSlit length
certain speed (v), and the area
Trace length(wL) of transferred liquid is
Figure 1 Bristow tester for measuring liquid absorption ratemeasured. Because contact time(t) is given by dividing slit width(b) (length parallel to the scanning direction) by scanning speed (v), behavior can be rapidly
observed (on the order of milliseconds).
The Bristow method is a unique, powerful analysis method, but has the following disadvantages:i)
It requires extensive time and effort because it is necessary to prepare multiple strips ofpaper as specimens for every measurement point, perform scanning by applying liquid on
the specimen, and repeatedly measure the lengths of the strips until one absorbency curve
is obtained (Bristow’s plot).
ii)
It is difficult to measure volatile liquids and viscous liquids by using this method.
iii)
Errors can be produced due to uneven liquid application.

1-2 Automatic scanning absorptometerTo overcome these disadvantages, an automatic scanning absorptometer (ASA) was developed by
drastically improving the measuring system, although it is based on the same principle. Figure 2
shows the ASA and sensor that measure water absorbency. In this experiment, the water absorption
behavior of several types of paper will be investigated by using the ASA.

2. Experiment
Specimens: Four types ofpaper(copypaper,
high-quality paper for
offset printing, high-grade
coated paper for inkjet
printing,
andsuper-high-grade
coatedpaper for inkjet printing).

To meniscus sensor

Automatic scanning absorptometer (spiral-scanBristow tester)
Liquid supply head, filled with a certain amount ofliquid, scans paper at various speeds. Liquid
absorption rate can be measured rapidly.

Liquid supply head

Optical sensor
Glass capillary

Meniscus

Apparatus:
Automaticscanningabsorptometer (Kumagai Riki
Kogyo).

Amount of water
absorption (g/m2)

Stepper motor
Automatic scanning absorptometer
Test liquid: 0.1% toluidine
blue solution (this solution
Figure 2 Automatic scanning absorptometer (improved Bristow tester)is sufficiently dilute that its
absorption behavior of this solution
mimics that of water; it is coloredto track observations).

Ka2-1 Water absorbency measurement test
Ir(1) Place an A4-sized specimen on
the stage of the tester and restrain
the specimen with magnets.
tW(2) Define a file name and scanningspeed on a program. Generally,
Square root of contact time (ms1/2)
use the “hard-sized paper” (paperwith high water repellency) mode.
Figure 3 Absorbency curve obtained using Bristow tester(3) After verifying that the glass tube
is completely filled with thesolution, begin scanning.
(4) Create a Bristow’s plot on the basis of the generated data file.
(5) The data file includes approximately 20 sets of data on contact time (ms) and amount ofwater absorption (g/m2) below several lines of experimental conditions (this may be ignored).
The data file will be uploaded to the “Laboratory” menu on the left side bar of
http://www.enomae.com/; download this file.
(6) Create separate graphs for the front and back sides of the specimens. Curves of the dataobtained from the same side of the same paper will be drawn in one graph.
2-2 Analysis
It is known that the amount of liquid permeating into a porous body is approximated to first orderand is proportional to the square root of contact time; the following Lucas-Washburn equation is-2-

applicable. If the data are plotted with the square root of contact time (calculate the square root fromthe downloaded data by entering an appropriate function in the cells) on the abscissa and the amountof liquid absorbed per unit area on the ordinate, a linear line will be obtained.

h=rtcos 2

: Surface tension, r: Radius of capillary, t: Contact time,
: Contact angle, : Viscosity of liquid.

However, when water is absorbed in the paper, sized paper usually changes because fiber swells
or contact angle decreases due to the size effect.
As shown in Figure 3, the slope of an upward straight line in the Bristow’s plot is designated theabsorption coefficient (Ka). This upward straight line corresponds to the actual permeation process.
When water is absorbed in hard-sized paper, the appearance of such behavior is delayed for acertain period. This period is designated the wetting time (or wetting delay) (tw). Actually, because asection corresponding to wetting time is not always a horizontal line, the section that is visuallyalmost horizontal can be defined as the wetting time. If the surface is uneven, the amount of liquid
required to compensate for this unevenness is always reflected in the transferred amount, eventhough the head contacts rapidly. Therefore, the graph shifts upward. The value of this shift isdesignated the roughness index (Ir). Because swelling specimens and paper for inkjet printing usedin this experiment absorb liquid extremely quickly at the initial stage, they often display no wettingtime.
3. Discussion
The three previously described values of each paper are obtained and their significance isdiscussed. No wetting time will be accepted if wetting time is not defined.

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