J Wood Sci (2006)
DOI 10.1007/s10086-005-0792-x© The Japan Wood Research Society 2006

ORIGINAL ARTICLE
Toshiharu Enomae · Naoya Yamaguchi
Fumihiko Onabe

Influence of coating properties on paper-to-paper friction of coated paper

Received: September 14, 2005 / Accepted: December 14, 2005

Abstract Coated paper-to-coated paper friction properties
were examined in relation to printing runnability difficulties
like erroneous double feeding of paper sheets. Higher ratiosof precipitated calcium carbonate (PCC) to clay in mixed
pigment coatings resulted in higher static and kinetic
coefficients of friction (COFs). Microroughness in the orderof pigment particle size is considered to relate to COF,
because cube-shaped particles of PCC resist sliding. Calendering decreased COF at larger amounts of PCC, but did
not change COF of the sole clay formulation at all. Additionof ground calcium carbonate (GCC) decreased COF. The
rate of decrease in kinetic COF with increasing number ofsliding for the GCC-rich formulation was higher than thatfor the PCC-rich formulation, presumably because protruding parts, characteristic of the GCC-rich formulation, on thesurface were selectively flattened. Addition of styrene–butadiene (SB)-latex up to 14 pph decreased COF, but
static COF had the highest value at 18 pph. The antislip
property (as a rubber) of SB-latex developed only in thestatic mode. Among lubricants formulated, the wax typedecreased COF the most remarkably with more effect on
kinetic COF than on static COF.
Key words Calcium carbonate · Coated paper · Friction ·Surface roughness · Lubricant

T. Enomae (*) · N. Yamaguchi · F. Onabe
Paper Science Laboratory, Department of Biomaterial Sciences,
The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657,
Japan
Tel. +81-35-841-8199; Fax +81-35-841-5271
e-mail: enomae@psl.fp.a.u-tokyo.ac.jp
Part of this report was presented at the 11th International Printing andGraphic Arts Conference, Bordeaux, France, October 2002

Introduction
Paper-to-paper and paper-to-other material frictions are
related to many sorts of issues regarding printing
runnability, print quality, and printing press design. Highfriction is desired in high-speed printing to maintain goodregister. Erroneous double or multiple feeding of sheets ofpaper is a serious difficulty that occurs in copiers and personal printers as well as in commercial printing presses andindustrial optical character recognition (OCR). In every
case, it is very important to keep paper friction properties
constant and stable because modification of hardware requires much effort in adjusting printing systems. As far as
erroneous double feeding is concerned, there are many possible factors involved: coefficient of friction between rubberroll and paper and between paper sheets, attraction occurring between paper sheets due to static electricity, and soon. On the other hand, demands for coated paper in themarket are growing with the aim of high print quality. As
the consumption of coated paper increases, it has more
occasion to be processed through a variety of sheet feeders.
This study focuses on the coefficient of friction (COF) between coated papers, so coated paper properties that arelikely to influence the COF were chosen and examined.
As for uncoated paper, several properties influencing
COF have been reported. For pulping and bleaching, kappa
number did not affect friction properties despite differencesin surface energy.1 For papermaking and paper properties,
beating does not significantly affect friction, but the author
demonstrated that surface rigidity that was enhanced bybeating increased COF slightly.2 Rough/rough surfaces
showed a lower static COF than smooth/smooth surfaces forpaperboard handsheets, which indicates that the interlocking of surface asperities was not a factor.3 For fillers, common
fillers such as hydrous kaolin and talc decreased kinetic COF;
calcined kaolin and, in particular, synthetic precipitated silicas and silicates increased it. Filler size and shape wereconsidered to be dominant factors.4 For surface chemistry,wood extractives like long-chain saturated fatty acids, fatty
alcohols, and tristearin reduced COF most efficiently.5

2
Table 1. Coating color formulation of the series of pigment mixtures
Ingredient

Added amount (pph)

Color
Clay
PCC
GCC
SB-latex
StarchDispersant
Sodium hydroxide

1
0
100
0
10
2
0.1
0.01

2
30
70
0
10
2
0.1
0.01

3
50
50
0
10
2
0.1
0.01

4
70
30
0
10
2
0.1
0.01

5
100
0
0
10
2
0.1
0.01

6
70
30
0
10
0
0.1
0.1

7
70
20
10
10
0
0.1
0.1

8
70
10
20
10
0
0.1
0.1

9
70
0
30
10
0
0.1
0.1

10
70
30
0
6
2
1
0.01

11
70
30
0
10
2
1
0.01

12
70
30
0
14
2
1
0.01

13
70
30
0
18
2
1
0.01

Solids (%)

45

47.5

50

52.5

55

45

47.5

50

52.5

54.7

55

55

54.2

pph, parts per hundred; PCC, precipitated calcium carbonate; GCC, ground calcium carbonate; SB, styrene–butadiene

Experimental
Materials
Coating colors were prepared from following materials: clay
(UltraWhite-90, Engelhard, USA), mean diameter of ca.
1.5 mm; precipitated calcium carbonate (PCC, Brilliant-15,
Shiraishi Kogyo, Japan), mean diameter of ca. 0.4 mm; andground calcium carbonate (GCC, Softon-1500, Shiraishi
Kogyo), mean diameter of ca. 1.5 mm with a broad distribution. Figure 6 shows each type of carbonate particle present in the coating layer. Styrene–butadiene (SB)-latex(LX407G, Nippon Zeon, Japan) has a mean particle diameter of 100 nm and a glass transition temperature of 15°C.
Starch (phosphoric ester type, P-140, Oji Corn Starch,
Japan) as a cobinder, dispersant (polyacrylate T-40
Toagosei, Japan), and sodium hydroxide (special grade,
Wako, Japan) were also used.
The lubricants used were three types of dispersions.
Stearate A (Nopcote C-104-HS, San Nopco, Japan) is a
calcium salt of higher fatty acids. Stearate B (SN-Cote 231)
is also a calcium salt of fatty acid, but with a shorter hydrocarbon chain. The wax type (SN-Cote 950) is a polyolefin.
They were not originally developed to reduce COF, but
to improve release from rolls, wet ink acceptance, andantidusting at supercalender rolls.

Table 2. Coating color formulation of the series of lubricantsIngredient

Added amount (pph)

Clay
PCC
SB-latex
StarchLubricantWax type
Calcium stearate A
Calcium stearate B
Dispersant
Sodium hydroxide

70
30
10
2

70
30
10
2

70
30
10
2

2
0
0
0.1
0.01

0
2
0
0.1
0.01

0
0
2
0.1
0.01

Solids (%)

55

55

55

latex, which tends to coher easily by hard dispersion, wasadded last and dispersed for 10 s.
All of the coating colors were applied to wood-free basepaper of 63.7 g/m2 basis weight with a wire bar no. 7 using
a motor-driven draw-down coater (YOA-B, Yoshimitsu
Seiki, Japan). The papers were then hot air-dried for
1 min. All of the coated paper, unless otherwise mentioned,
were calendered with a laboratory calender (No. 2232,
KRK, Japan) equipped with a pair of steel and neoprenerubber rolls at a linear pressure of 14.7 kN/m and a speed of
2.5 m/min. The surface temperature of the steel roll wasca. 60°C

Coating colors and coated paper
Measurement
Four series of coated paper were prepared. Table 1 shows
the formulation of three series of pigment mixtures of thecoating color. The first series (1–5) contained different
ratios of clay and PCC. Coat weight ranged from 10 to
13 g/m2. The second series (6–9) contained different ratiosof PCC and GCC at a constant level of clay. Coat weight
ranged from 18 to 24 g/m2. The third series (10–13) contained different contents of SB-latex. Coat weight ranged
from 10 to 13 g/m2. Table 2 shows the series for different
types of lubricants, all at 2 pph. All materials except SBlatex were dispersed with a centrifugal disperser (two combined rotations on the container and the disperser axes,
hybrid mixer HM-500, Keyence, Japan) for 20–40 min in
total at adequate intervals to prevent temperature rise. SB-

Surface roughness was determined as a centerline averageroughness calculated from surface profiles measured by aconfocal laser scanning microscope (CLSM) (VF-7500,
Keyence) or by a stylus profilometer (SE-3, Kosaka Laboratory, Japan). Also, Bekk smoothness was measured using
an Oken type tester (modified Bekk characterized byhydraulic pressure difference practically measured in placeof time). Gloss was measured by a portable glossmeter(Multigloss-268, Minolta, Japan). Micrographs were takenwith a scanning electron microscope (S-4000, Hitachi,
Japan) and with the CLSM.
Friction tests were made using two types of apparatus.
One was a friction sensitivity tester originally designed for3
0.70
Uncalendered
0.65

(a)

Calendered

(b)

(d)

(c)

Fig. 1. Sled (a), table (b), elevator (c), and load detection arm (d) of
the automatic friction coefficient testing machine

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 coefficient of friction. PCC,
precipitated calcium carbonate. Bars indicate 95% confidence interval

80

1.5Uncalendered
Calendered

60
1.0

85 °ISO gloss

Centerline average roughness, mm

fabric texture (KES-SE, KatoTech, Japan). The dimensionsof a test piece on the sled were 70 mm in length and
35 mm in width. The table was 40 mm in width. The sliding
speed, the vertical pressure, and the sliding distancewere 10 mm/s, 1.98 kPa, and 35 mm, respectively. The otherapparatus was an automatic friction coefficient testing machine (Sagawa Manufacturing, Japan). This machine is designed in conformity with ISO 15359. All of the testing
conditions for the measurements also followed the ISO
standard. As results, the first and third static COF andthe third kinetic COF, which are specified in the standard,
are mainly shown. The sliding speed was 0.2 mm/s for thestatic COFs and 20 mm/s for the kinetic COF. Figure 1
shows the sled and the table of this machine. A step motor
drives the table accurately and precisely. Setting of a test
piece onto the sled was automated on the special clamping
device (not shown) and could be performed easily without
touching the test piece. Every friction test was performed
under the standard conditions, of 23°C and 50% relative
humidity.

40

0.5
0

20

40

60

80

20
100

Added amount of PCC, pph

Results and discussion

Fig. 3. Roughness and gloss of coated papers with different PCC/clay
ratios. Arrows show relevant axes

Ratio of clay, PCC, and GCC
Figure 2 shows that the static and kinetic COFs increased
with increasing amount of PCC, for both the calendered
and the uncalendered samples; 1–5. Figure 3 shows roughness measured by the CLSM and 85° ISO gloss. From thestandpoint of roughness alone, there seems to be a trendthat rougher surfaces provided lower COF for the calendered samples. However, calendering not only smoothened
the surface, but also partially compressed the surface structure. Note that high density and rigidity resulting from thecompression might be a more dominant factor. Anotherpossible factor is pigment particle shape. Cube-shaped particles of the rhombohedral PCC might have resisted sliding.
Practically, larger oscillations, namely, more remarkable
stick–slick behavior was observed in the friction change
during sliding, characteristic of PCC-rich samples. To
paraphrase it, roughness in the particle size range calledmicroroughness is closely related to COF. With regard tocalendering effect, calendering decreased COF more re-markably at larger amounts of PCC, but no effect was observed with the sole clay formulation. Importance of microstructure has been stated previously.6 Rättö et al.6 foundthat the use of clay particles with different average diameters had no marked effect on the friction properties; however, a great difference was noted between papers coatedwith kaolin and those coated with CaCO3. Significant differences were also detected when using calcium carbonate
particles of different shapes. It concluded that it was theparticle shape and its influence on the surface microstructure that was of importance.
Figure 4 shows the effect of GCC on COF. The automatic friction coefficient testing machine was used tomeasure COF only for this series of coated paper; 6–9.
Measured COF values are fairly comparable between thismachine and the friction sensitivity tester under identicalmeasurement conditions. However, the static COF wasmuch lower than those shown for the other series of coated

4
0.45

GCC PCC

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

60° ISO gloss

Centerline average roughness, mm

Fig. 4. Influence of GCC/PCC ratio on coefficient of friction. Total
carbonate amount is 30 pph in addition to 70 pph clay. GCC, groundcalcium carbonate

5
0

10

20

30

Added amount of GCC, pph

Fig. 5. Roughness and gloss of GCC-containing coated papers. Arrows
show relevant axes

paper, because the sliding speed was as low as 0.2 mm/s forthis series compared with 10 mm/s for the rest. Addition of
GCC decreased the third static COF and the kinetic COFs,
although it did not affect the first static COF significantly.
The first to the third kinetic COFs indicate that kinetic
COF decreased with increasing number of slides for the
GCC-rich formulation, but, in contrast, it was unchanged orrather surprisingly increased for the PCC-rich formulation.
GCC contains large particles and one might predict thatthese large particles would obstruct smooth sliding. Figure
5, in practice, shows that the centerline average roughness
increased and gloss decreased with increasing amount of
GCC, but unexpectedly, GCC addition facilitated sliding.
Concerning roughness, there are some articles that statethat it decreases COF;3 and there are others that state
it increases COF. With uncoated sandpaper-molded
handsheets prepared so that all of the properties other thanroughness are equal, roughness increased COF with the
exception of the very rough sheets.2 More discussion must
be made after serious consideration of contact area between
the two facing sheets of paper tested, plastic and elastic

10 mm
Fig. 6. Scanning electron micrograph of surface of on-table test pieceof coated paper (GCC/PCC/Clay = 20/10/70 pph) subjected to threetimes of sliding. Large arrow, scarification probably caused by large
particles on counter test piece

deformability of surfaces, adsorption of water molecules on
surfaces, and so on. For coated paper, plastic deformabilityof surfaces seems to be very important as the micrographsof the surfaces imply.
Figure 6 is a scanning electron micrograph showing asurface of the coated paper (GCC/PCC/Clay = 20/10/
70 pph) after friction measurement. The test piece was set
on the table and subjected to three slides, as one measurement of sequence. Some scratches due to sliding are observed to run in the horizontal direction, namely, the sliding
direction. The center of the surface indicated by an arrowseems to have been scarified probably by large particles on
the counter test piece set on the sled. Scarification is likely
to occur more often with coated surfaces when more GCC
is added. However, more commonly, scratched and deformed parts had flatter surfaces like in the dark area in thelower right corner of Fig. 6, suggesting that it would permit
smooth sliding afterward. Practically, Bekk smoothness
increased after friction measurement for all these coatedpaper samples.
Figure 7 shows surfaces of the coated paper (10 pph of
GCC) after friction measurement. The test piece had
been set on the sled. Image A was acquired with a conventional charge coupled device (CCD) camera. A considerable number of scratches due to abrasive sliding can beobserved at the upper left near the center of the image
and some abrasion can also be seen at the upper right.
Image B captured by the CLSM shows a contour map of thesame location of the sample. In this image, whiter areas
represent higher elevations. The two images confirmedthat the location of scratches agreed well with that of zones
at high elevations. Actual contact generating friction insliding occurs with higher or protruding parts of each test
piece, while the remainder is not contacted or is looselycontacted.

Fig. 9. Influence of lubricantson coefficient of friction

1st
Static,
3rd
Static ,3rdte
Ki n i c

0.6
0.5
0.4
0.3
0.2
0.1
0.0

Coeffic ient offric tion5t
B ica nA tera x at e ara l ub
r
W a
e No
e
St
St

Kind of lubricant

10 mm

10 mm

(A) CCD-camera image(B) Profile image

Fig. 7A,B. Surface of on-sled test piece of coated paper (GCC/PCC/
Clay = 10/20/70 pph) subjected to three times of sliding. A CCD camera
image, B profile image. Images (A) and (B) are of the same location

tween static and kinetic COFs, the lubricants reduced
kinetic COF more effectively. In particular, the wax typereduced it by 44%. The effects of lubricants are well explained and discussed in past research.5

Conclusions
0.65

450
Calendered

0.60

Coefficient of friction

350
300

0.55
Static, 1st

250
0.50
200
0.45

150

Static, 3rd

100
0.40

Kinetic, 3rd

Bekk (Oken) smoothness, s

400

Smoothnes

50

0.35

0
6

10

14

18

Added amount of SB-latex, pph

Fig. 8. Influence of styrene–butadiene (SB)-latex content on coefficient of friction. Arrows show relevant axes

Content of SB-latex
SB-latex is a kind of rubber, so addition of SB-latex was
likely to increase COF. Figure 8, however, shows that addition of SB-latex up to 14 pph decreased COF for the series
10–13. The static COFs had the highest COF at 18 pph.
The property as a rubber appeared only in the staticmode. Regarding roughness shown as Bekk smoothness,
the third kinetic COF, in particular, correlated well withsmoothness. This result reversed the relationship of theprevious series of GCC-containing coated paper. To discussroughness effects on COF further will require more dataand consideration.
Lubricant effects
Figure 9 shows lubricant effects on COF. The wax typereduced COF the most of the three. When compared be-

Coated paper-to-coated paper friction properties were examined. High PCC to clay ratio in mixed pigment coatings
resulted in high static and kinetic COFs. Microroughness inthe order of pigment particle size is considered to relate toit, because cube-shaped particles of PCC resist sliding. Calendering decreased COF at large amounts of PCC, but did
not change COF of the sole clay formulation at all. Additionof GCC decreased COF. The rate of decrease in kinetic
COF with increasing number of slidings for the GCC-richformulation was higher than that for the PCC-rich formulation, presumably because protruding parts, characteristic of
the GCC-rich formulation, in the surface were selectively
flattened. Addition of SB-latex up to 14 pph decreased
COF, but static COF had the highest value at 18 pph. The
antislip property (as a rubber) of SB-latex developed onlyin the static mode. Among lubricants formulated, the wax
type decreased COF the most remarkably with more effecton kinetic COF than on static COF.
Acknowledgments The authors are grateful to Mr. Hirofumi Sagawa
and Mr. Yuta Kobayashi at Sagawa Manufacturing, Inc. for revising
the automatic friction coefficient testing machine for our study.

References
1. Bäckström M, Fellers C, Htun M (1999) The influence of kappa
number and surface energy on paper-to-paper friction. Nord Pulp
Pap Res J 14:204–208
2. Enomae T (2001) Overview on tribology of paper (in Japanese).
J Jpn Soc Tribol 46:741–746
3. Inoue M, Gurnagul N, Aroca P (1990) Static friction properties oflinerboard. TAPPI J 73:81–85
4. Withiam MC (1991) The effect on fillers on paper friction properties. TAPPI J 74:249–256
5. Fellers C, Bäckström M, Htun M (1998) Paper-to-paper friction –paper structure and moisture. Nord Pulp Pap Res J 13:225–232
6. Rättö P, Barbier C, Rigdahl M (2000) An investigation of thefriction properties of coated paper. Nord Pulp Pap Res J 15:351–
356