INFLUENCE OF COATING PROPERTIESON PAPER-TO-PAPER FRICTION OF COATED PAPER
Toshiharu Enomae* , Naoya Yamaguchi and Fumihiko Onabe
Paper Science Laboratory, Department of Biomaterial Sciences, The University of Tokyo
Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, JAPAN*e-mail enomae@psl.fp.a.u-tokyo.ac.jp

ABSTRACT
Coated paper-to-coated paper friction properties were examined in relation to printing runnability
troubles like erroneous double feeding of paper sheets. Higher calcium carbonate (PCC) ratio to clay
in mixed pigment coatings resulted in higher static and kinetic COFs. Micro-roughness in the order ofpigment particle size is considered to relate to it, as cube-shaped particles of PCC resist sliding.
Calendering decreased COF at larger amounts of PCC, but did not change at all COF of the sole clay
formulation. Addition of GCC decreased COF. The rate of decrease in kinetic COF with increasing thenumber of sliding for the GCC-rich formulation was higher than that for the PCC-rich formulation,
presumably because protruding parts, characteristic of the GCC-rich formulation, in the surface wasselectively flattened. Addition of SB-latex up to 14 pph decreased COF, but static COF had the highest
value at 18 pph. An anti-slip property as a rubber of SB-latex developed only in the static mode.
Among lubricants used, the wax type decreased COF the most remarkably with more effects on kinetic
COF than on static COF.

INTRODUCTION

coefficient of friction between rubber roll andpaper and between paper sheets, attraction

Paper-to-paper and paper-to-other material
frictions are related to many sorts of issues
regarding printing runnability, print quality andprinting press design. High friction is desired inhigh-speed printing to maintain good register.
Erroneous double or multiple feeding of sheetsof paper is a serious trouble involved in copiersand personal printers as well as in commercial
printing presses and industrial OCR. In every
case, anyway, it is very important to keep paper

occurring between paper sheets due to static
electricity and so on. On the other hand,demands for coated paper in market are growing
with the aim of high print quality. As quantity of
consumed coated paper increases, it has more
occasions to be processed through a variety ofsheet feeders. This study focuses on coefficientof friction (COF) between coated papers, so
coated paper properties that are likely to
influence the COF were chosen and examined.

friction properties constant and stable because
modification of hardware require much more
efforts to adjust the printing system.

As for uncoated paper, several properties
influencing COF were reported. For pulping andbleaching, Kappa number did not affect friction

As far as erroneous double feeding is concerned,
there are many possible factors involved;

properties despite differences in surface energy1).
For papermaking and paper properties, beatingdoes not significantly affect friction, but theJapan) is a calcium salt of higher fatty acid.

author demonstrated that surface rigidity

Stearate B (SN-Cote 231) is also a calcium salt
2)enhanced by beating increased COF slightly .

of fatty acid with a shorter hydrocarbon chain.

Rough/rough surfaces showed a lower static

The wax type (SN-Cote 950) is an polyolefin.

COF than smooth/smooth surfaces for

They are not originally developed to reduce COF,

paperboard handsheets, as indicates that thebut to improve releasing from rolls, wet ink

interlocking of surface asperities was not a

acceptance, antidusting at supercalender rolls.

factor 3). For fillers, common fillers such as

Coating colors and coated paperhydrous kaolin and talc decreased kinetic COF;
calcined kaolin and, in particular, synthetic
precipitated silicas and silicates increased it.
Filler size and shape were considered to be
dominant factors4). For surface chemistry, wood
extractives like long-chain saturated fatty acids,
fatty alcohols and tristearin reduced COF mostefficiently 5).

Four series of coated paper was prepared. Table
1 shows the formulation of three series ofpigment mixture of the coating color. The firstseries (#1-#5) contains different ratios of clay
and PCC. Coat weight ranged from 10 to 13
g/m2 . The second series (#6-#9) contains
different ratios of PCC and GCC at a constantlevel of clay. Coat weight ranged from 18 to 24

EXPERIMENTAL
Materials
Coating colors were prepared from following
materials. Clay (UltraWhite-90, Engelhard,USA) has a mean diameter of ca. 1.5 µm.
Precipitated calcium carbonate (PCC,
Brilliant-15, Shiraishi Kogyo, Japan) has a mean
diameter of ca.0.4 µm. Ground calcium
carbonate (Softon-1500, Shiraishi Kogyo) has amean diameter of ca. 1.5 µm with a broad
distribution. Styrene-Butadiene (SB)-latex

g/m2 . The third series (#10-#13) contains
different contents of SB-latex. Coat weight
ranged from 10 to 13 g/m2 . Table 2 shows thatfor different types of lubricants all at 2 pph.
Every material but SB-latex was dispersed witha centrifugal disperser (two combined rotations
on the container’s and the disperser’s axes,
hybrid mixer HM-500, Keyence Corp., Japan)
for 20 to 40 min in total at adequate intervals toprevent temperature rise. SB-latex, which tends
to be cohered easily by hard dispersion, wasadded last and dispersed for 10 s.

(LX407G, Nippon Zeon, Japan) has a mean
diameter of 100 nm and a glass transition
temperature of 15 °C. Starch (phosphoric ester
type, P-140, Oji corn starch, Japan) as a
co-binder, dispersant (polyacrylate T-40
Toagosei, Japan) and sodium hydroxide (special
grade, Wako Pure Chemical Industries, Japan)
were also used.

All of the coating colors were applied towoodfree base paper of 63.7 g/m2 basis weightwith a wire bar #7 using a motor-driven
draw-down coater (YOA-B, Yoshimitsu Seiki,
Japan). Then, they were hot air-dried for 1 min.
All of the coated paper, unless mentioned, werecalendered with a laboratory calender (No.2232,
KRK, Japan) equipped with a pair of a steel andLubricants used are three types of dispersions.
Stearate A (Nopcote C-104-HS, San Nopco,

a neoprene rubber rolls at a linear pressure of
14.7 kN/m and a speed of 2.5 m/min. Surface

temperature of the steel roll was ca. 60 °C.
Measurement
Surface roughness was determined as acenterline average roughness calculated frompiece. Every friction test was performed under
the standard condition, 23 °C and 50 % r.h.
RESULTS AND DISCUSSION
Ratio of Clay, PCC and GCC

surface profiles measured by a confocal laser
scanning microscope (CLSM) (VF-7500,
Keyence Corp., Japan) or by a stylus
profilometer (SE-3, Kosaka laboratory, Japan).
Also, Bekk smoothness was measured using
Oken type tester (modified Bekk characterized
by hydraulic pressure difference practicallymeasured in place of time). Gloss was measured
by a portable-type glossmeter (Multigloss-268,
Minolta, Japan). Micrographs were taken with ascanning electron microscope (S-4000, Hitachi,
Japan) and with the CLSM.

Figure 2 shows that static and kinetic COFs
increased with increasing the amount of PCC,
for both the calendered and the uncalenderedsamples. Figure 3 shows roughness measured
by the CLSM and 85 º ISO gloss. Only from astandpoint of roughness, there seems to be atrend that rougher surfaces provided lower COF
for the calendered samples. But, calendering did
not only smoothened the surface, but made thesurface structure closely packed. Note that highdensity and rigidity resulting from the packing
might be a more dominant factor. Another

Friction tests were made with two types ofapparatus. One was a friction sensitivity tester
originally designed for fabric texture (KES-SE,
KatoTech, Japan). The dimension of a test pieceon the sled was 70 mm in length and 35 mm inwidth. That on the table was 40 mm in width.
The sliding speed, the vertical pressure and thesliding distance were 10 mm/s, 1.98 ｋPa and 35
mm. The other apparatus was an automatic
friction coefficient testing machine (Sagawa
Manufacturing Inc., Japan). This machine isdesigned in conformity with ISO 15359. All of
the testing conditions in measurements followed
also the ISO standard. In results, the first and thethird static COF and the third kinetic COF will
be mainly shown. The sliding speed was 0.2
mm/s for the static COFs and 20 mm/s for the
kinetic COF. Figure 1 represents the sled andthe table of this machine. A step motor drives thetable accurately and precisely. Setting of a test
piece onto the sled is automated and can be
performed easily without any touch of the test

possible factor is pigment particle shape.
Cube-shaped particles of the rhombohedral PCC
might have resisted sliding. Practically, larger
oscillations, namely, more intensive stick-slick
behavior was observed in the friction change
during sliding, characteristically of PCC-richsamples. To paraphrase it, roughness in theparticle size range what is calledmicro-roughness is closely related to COF. With
regard to calendering effect, calendering
decreased COF more remarkably at larger
amounts of PCC, but no effect was observedwith the sole clay formulation. Importance of
microstructure was stated in the reference6). In
the reference, the use of clay particles withdifferent average diameters had no marked effecton the friction properties; a great difference washowever noted between papers coated withkaolin and with CaCO3 ; and significant
differences were also detected when using
calcium carbonate particles of different shapes.

It concluded that it was the particle shape and its

seems to be very important as micrographs ofinfluence on the surface microstructure that was

the surfaces imply.

of importance.

Figure 6 is a scanning electron micrograph

Figure 4 shows a GCC effect on COF. The

showing a surface of the coated paper (20 pph ofautomatic friction coefficient testing machine

GCC) after friction measurement. The test piece

was used to measure COF for only this series of

was set on the table and subjected to three times

coated paper. So, static COF was much lowerof sliding, one measurement sequence. Some

than those shown for the other series of coatedscratches due to sliding are observed to run inpaper, because the sliding speed was as low as

the horizontal direction, namely, the sliding

0.2 mm/s for this series in spite of 10 mm/s for

direction. The center of the surface indicated by

the rest. Addition of GCC decreased the third

an arrow seems to have been scarified probably

static COFs and the kinetic COFs, though it didby large particles on the counter test piece set on

not affect the first static COF significantly. The

the sled. Scarification is likely to occur more

first to the third kinetic COFs indicate that

often with coated surfaces with more GCC

kinetic COF decreased with increasing theadded. But, more commonly, scratched andnumber of sliding for the GCC-rich formulation,

deformed parts had flatter surfaces like an area

but, in contrast, it was unchanged or rather

in the lower right-hand corner, suggesting that it

surprisingly increased for the PCC-richwould permit smooth sliding afterwards.

formulation. The GCC contains large particles

Practically, Bekk smoothness increased afterand one might predict these large particles

friction measurement for all these coated paper

would obstruct smooth sliding. Figure 5, in

samples.

practice, shows that centerline averageroughness increased and gloss decreased bothwith increasing the amount of GCC, but
unexpectedly, GCC addition facilitated sliding.
About roughness, there are some articles thatstated it decreased COF3); and there are others
that stated it increased COF. With uncoatedsandpaper-molded handsheets prepared so all of
the properties other than roughness would beequal, roughness increased COF with the
exception of the very rough sheets2). Morediscussion must be made after serious
consideration on contact area between two
facing sheets of paper tested, plastic and elastic
deformability of surfaces and adsorption ofwater molecules on surfaces and so on. For
coated paper, plastic deformability of surfaces

Figure 7 shows surfaces of the coated paper (10pph of GCC) after friction measurement. The
test piece had been set on the sled. Image (A)was acquired with a conventional CCD camera.
Considerable number of scratches due toabrasive sliding can be observed at the upper left
near the center of the image and some abrasion
also can be seen at the upper right. Image (B)captured by the CLSM shows a contour map of
the identical location of the sample. In thisimage, whiter areas represent higher elevations.
The two images confirmed that the location ofscratches agreed well with that of zones at highelevations. Actual contact generating friction insliding occurs with higher or protruding parts ofeach test piece. The rest of it that occupies mostof the surface is not contacted or looselycontacted.
Content of SB-latex

the PCC-rich formulation, presumably because
protruding parts; characteristic of the GCC-richformulation, in the surface was selectively

SB-latex is a kind of rubber, so addition ofSB-latex was likely to increase COF. Figure 8,
however, shows that addition of SB-latex up to
14 pph decreased COF. The static COFs had thehighest COF at 18 pph. The property as a rubber
appeared only in the static mode. Regarding
roughness as shown as Bekk smoothness, theflattened. Addition of SB-latex up to 14 pph
decreased COF, but static COF had the highest
value at 18 pph. An anti-slip property as a rubberof SB-latex developed only in the static mode.
Among lubricants used, the wax type decreased
COF the most remarkably with more effects on
kinetic COF than on static COF.

third kinetic COF, in particular, correlated wellwith smoothness. This result reversed therelationship with the previous series of the GCCcontaining coated paper. To discuss roughness
effects on COF further will require more dataand consideration.
Lubricant effects
Figure 9 shows lubricant effects on COF. The
wax type reduced COF most of the three. When
compared between static and kinetic COFs, thelubricants reduced kinetic COF more effectively.
In particular, the wax type reduced it by 44 %.
The effects of lubricants were well explainedand discussed in the past research5).

ACKNOWLEDGEMENT
The authors are grateful to Mr. Hirofumi Sagawa
and Mr. Yuta Kobayashi at Sagawa
Manufacturing, Inc. for revising the automatic
friction coefficient testing machine to our
purpose.
REFERENCES
1) Bäckström M., Fellers. C., and Htun, M.,“The influence of kappa number and surface
energy on paper-to-paper friction”, Nordic
Pulp Paper Res. J. 14(3): 204-208(1999)
2) Enomae, T., “Overview on tribology of
paper” (in Japanese), J. Japanese Soc

CONCLUSIONS
Coated paper-to-coated paper friction properties

Tribologists 46(10): 741-746(2001)
3) Inoue, M., Gurnagul, N. and Aroca, P. ,were examined. Higher PCC ratio to clay in“Static friction properties of linerboard”,

mixed pigment coatings resulted in higher static

Tappi J. 73(12): 81-85(1990)

and kinetic COFs. Micro-roughness in the order

4) Withiam, M. C., “The effect on fillers onof pigment particle size is considered to relate topaper friction properties”, Tappi J. 74(4):it, as cube-shaped particles of PCC resist sliding.

249-256(1991)

Calendering decreased COF at larger amounts of5) Fellers. C., Bäckström M., and Htun, M.,

PCC, but did not change at all COF of the sole

“Paper-to-paper friction – paper structure

clay formulation. Addition of GCC decreased

and moisture”, Nordic Pulp Paper Res. J.

COF. The rate of decrease in kinetic COF with

13(3): 225-232(1998)

increasing the number of sliding for the
GCC-rich formulation was higher than that for6) Rättö, P., Barbier, C. and Rigdahl, M., “Aninvestigation of the friction properties ofcoated paper”, Nordic Pulp Paper Res. J.

15(5): 351-356(2000)

Table 1 Coating color formulation of the series of pigment mixture
Ingredient<Color #>
Clay
PCC
GCC
SB-latex
StarchDispersant
Sodium hydroxide
Solids, %

#1
#2
#3
#4
0 30
50
70
100 70
50
30
0
0
0
0
10 10
10
10
2
2
2
2
0.1 0.1 0.1 0.1
0.01 0.01 0.01 0.01
45 47.5
50 52.5

Added amount, pph
#5
#6
#7
#8
#9 #10 #11 #12 #13
100
70
70
70
70
70
70
70
70
0
30
20
10
0
30
30
30
30
0
0
10
20
30
0
0
0
0
10
10
10
10
10
6
10
14
18
2
0
0
0
0
2
2
2
2
0.1 0.1 0.1 0.1 0.1
11
1
1
0.01 0.1 0.1 0.1 0.1 0.01 0.01 0.01 0.01
55
45 47.5
50 52.5 54.7
55
55 54.2

Table 2 Coating color formulation of the series of lubricantsIngredient
Clay
PCC
SB-latex
StarchLubricant- Wax type
- Calcium stearate A- Calcium stearate B
Dispersant
Sodium hydroxide
Solids, %Added amount, pph
70
70
70
30
30
30
10
10
10
2
2
2
2
0
0
0.1
0.01
55

0
2
0
0.1
0.01
55

0
0
2
0.1
0.01
55

Fig. 1 Sled and table of the automatic
friction coefficient testing machine0.70
Uncalendered
0.65

Calendered

Coefficient of friction

0.60
Static, 1st

0.55
0.50

Static, 3rd

0.45
0.40
0.35
0.30

Kinetic, 3rd

0.25
0

20

40

60

80

100

Added amount of PCC, pph

Fig. 2 Influence of PCC / Clay ratio on COF(Bars indicate 95 % confidence level in every graph.)
1.5

80
Calendered

60

1.0

85 °ISO gloss

Centerline average roughness, µm

Uncalendered

40

0.5
0

20

40

60

80

20
100

Added amount of PCC, pph

Fig. 3 Roughness and gloss of PCC/Clay series of coated paper

0.45
PCC = 30 pph
0.44
Static, 1st

Coefficient of friction

0.43
0.42
0.41
0.40
0.39

Kinetic, 1st
2nd

Static, 3rd

0.38

3rd

0.37
0.36
0

10

20

30

Added amount of GCC, pph

1.5

15

1.0

10

0.5

5
0

10

20

30

Added amount of GCC, pph

Fig. 5 Roughness and gloss of GCC containing coated paper

60° ISO gloss

Centerline average roughness, µm

Fig. 4 Influence of GCC content on COF

10 µmFig. 6 Surface of on-table test piece of coated paper (20pph of GCC) subjected to three times of sliding
obtained by scanning electron microscopy.

10 µm(A) CCD-camera image

10 µm(B) Profile image

Fig. 7 Surface of on-sled test piece of coated paper (10 pph of GCC)
subjected to three times of sliding. Images (A) and (B) are of theidentical location.

0.65

450
Calendered

0.60

0.55

300

Static, 1st

250
0.50
200
Static, 3rd

0.45

150
100

0.40

Kinetic, 3rd

50

0.35

0
6

10

14

18

Added amount of SB-latex, pph

Fig. 8 Influence of SB-latex content on COF

0.6
0.5
0.4
0.3
0.2
0.1

1st
Static,0.0

3rd
Static,
3rd
Kinetic

ax
W

eB
rat
a
e
St

A
te
a
No
r
a
e
St
Kind of lubricant

nt
ca
i
r
lub

Fig. 9 Influence of lubricants on COF

Coefficient of friction

Coefficient of friction

350

Bekk (Oken) smoothness, s

400

Smoothnes