JPPS 23(7):J1-J7(1997)

SURFACE ROUGHENING BY WATER:GLOSS RELAXATIONPROCESSESTOSHIHARU ENOMAE* AND PIERRE LEPOUTRE
PAPER SURFACE SCIENCE PROGRAM
DEPARTMENT OF CHEMICAL ENGINEERING
UNIVERSITY OF MAINE
ORONO, ME 04469
USA

ABSTRACTA new instrument is used to measure simultaneously the gloss and the moisture content of a sample ofpaper as it is subjected to changes in relative humidity. Measurements made with five different papers
confirm the overall inverse relationship between gloss and moisture content reported in the literature,
although there are reversals (gloss increasing with moisture content) which are unexplained. A
significant result of this work is that gloss lags behind moisture content after a step change in RH,
apparently reflecting the continuing structural rearrangement of the fibers in the bulk of the sheet as
moisture diffuses into fiber walls. This explains why, in order to stabilize sheet gloss to moistureeffects, it is not enough to stabilize the surface only, as is done in surface treatment processes
involving a thermal gradient.

INTRODUCTION
Surface roughening is a prevailing issue for coaters and printers. It inevitably occurs when paper iscontacted with water in processes such as coating and offset printing or simply when it is exposed tohigh relative humidity. Surface roughening can be detrimental to print quality. Aspler’s review[1]introduces publications about sheet roughening in heatset offset printing of higher quality mechanicalpaper grades, water-based gravure and water-based flexography.
Gloss is one of the most sensitive properties indicative of surface roughening. Air-leak methods arerather less sensitive because, during the test, the surface is subjected to a load and is compressed anddeformed. This compression and deformation increase with the moisture content and roughness
measurements may become misleading. A contactless gloss measurement on the other hand, is quite
suitable and is frequently used to characterize surface structural changes.
This paper describes a special glossmeter, referred to as "GlossMachine", which allows the concurrent
measurement of gloss and moisture content of a paper sample subjected to varying relative humidity.
Since moisture sorption-desorption processes are the cause of the changes in surface structure, theconcurrent measurement of these two parameters provides useful information on the roughening
processes in addition to that on the roughened surface.
* Now with: Paper Sci. Lab., Biomaterial Sci. Dept., Grad. School of Agricul. and Life Sci., The
University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan‑ 1 ‑JPPS 23(7):J1-J7(1997)

EXPERIMENTAL
Design of the GlossMachine
Figure 1 shows a sketch of the sample chamber of the GlossMachine. A paper sample is attached to ametal stage with two-sided adhesive tape. The whole sample stage is directly supported on a toploading balance so that sample weight can be converted into moisture content. The chamber ceiling isequipped with a light source (LED of visible red light) and detector for gloss measurement set at
equidistant angles (75 °). 75 ° was selected in accordance with TAPPI Test Method T480. The
illuminated area on the sample used for gloss reading is about 10 mm diameter. Humid air is suppliedby small openings made on both the sides of the chamber. This air inlet is connected to an air
generator providing air at a controlled relative humidity (RH). Figure 2 shows a sketch of the air
generator. Bubbles coming from the sparger are saturated. The air-flow rate is 1.0 L/min. At this rate it
takes only three seconds to supply a volume of air equivalent to the sample chamber capacity. Afterthe sparger cell, the saturated air goes through a reheater cell to raise its temperature to the sample
chamber temperature. All air ducts are well insulated to avoid condensation. But, for more exact RH,
the temperature of the sparger cell is adjusted to that in the sample chamber.
The ratio of water vapor pressure at temperature T1 in the sample chamber to that at T0 in the sparger
cell determines the RH. The RH was calculated based on the vapor pressure table of the atmosphericpressure. Practically, a target RH is attained by lowering only T0. T1 was basically room temperature,
within 2.5 °C. For 0% RH, air was run through a column of Silica gel.
Data acquisition of the GlossMachine gloss (GM-gloss), weight and RH are all made methodically bya single software. Resistance temperature detectors and Peltier junction modules are used to trace thetemperature changes quickly and to raise or lower the temperatures immediately through feedback
loops.
A metal plate is used as a reference to compensate for possible electrical drift in the voltage (GMgloss) output. This reference system also permits to eliminate the change in the detector output
resulting from the absorption of light in the near infrared band (The LED emits near infrared light inpart.) by moisture. The reference plate is moved back and forth across the light path by a step motor.
The ratio of paper gloss to that of the reference plate provides a relative gloss value that was stable
over a long period of time.
Samples
To check the reproducibility of the GlossMachine, a sample of supercalendered, uncoated and filled
wood-containing paper (A) was used. Additionally, softcalendered wood-containing base paper for
LWC (B), softcalendered wood-free base(C), softcalendered wood-containing coated paper (D) andsupercalendered wood-containing coated paper (E) were used. Table Ι lists TAPPI gloss, Parker PrintSurf roughness and basis weight of the papers.
The measurements were made on the felt side, unless mentioned. The crossmachine direction wasaligned parallel to the plane of incidence and reflection .

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Measurement Procedure
At first, the sample stage is tared with an adhesive tape, without paper. Then, a paper sample isattached, folded around the edge of the sample stage to cover most of the backside. This is to provide
as large a sample surface as possible to gain sensitivity in weighing. The sample chamber and thebalance are enclosed in a plastic bag until the end of the measurement to prevent cooling drafts. For
about 1 min, at the beginning of the run, the sample was weighed with no air supplied. Then, air set atthe desired RH is sent at a rate of 1.0 L/min. This causes a deflection of 0.04 g due to air flow pressure
for which a correction was made to the weight measurements. Gloss and weight were measured
continually every 6 s for the first 10 min after every RH switch, and then every minute. The RH
schedule most often used was 4 h at 0 % RH, 3 h at 90 %, and 3 h at 0 % sequentially. To look at RH
cycling effects, the last two steps were repeated. Moisture content of paper was calculated on the drybasis as follows:

MC (%) =where,

W − W0 − Wa
× 100
W0

MC: moisture content (dry basis)
W: sample weight under measurement
W0: sample weight after running dry air (through a silica gel column) for 4 h
Wa: increase in weight due to pressure from air flow

TAPPI gloss (75 °) was also measured. Measurements were made while the sample was still held on
the stage as it was in the GlossMachine.

RESULTS AND DISCUSSION
Reproducibility of GlossMachine
Figure 3 shows two typical successive runs of sample A, where gloss and moisture content aremonitored versus time. Gloss is expressed as a relative value (see experimental), so the values are notcomparable to TAPPI gloss. The RH history was: 0 % for initial 4 h, subsequently, 90 % for 3 h andthen 0 % for 3 h. The results of the two runs agree fairly well in terms of both gloss and moisture
content and the small difference in the gloss curve is attributed to a small difference in the initial gloss
values (36.9 versus 38.0). Also, the relative vertical positions of the sample surface, the reference
surface, the light source and the detector may not have been always exactly the same. This small
variation from measurement to measurement may cause additional error of up to about 4 %. Therefore,
it would not be accurate to compare details of the gloss change between samples unless the gloss
could be connected to the absolute gloss as TAPPI gloss. However, the gloss change with time of asample, once mounted, is considered to be well scaled because the relative locations of thecomponents remain constant during the measurement.
Correspondence between GlossMachine gloss and TAPPI Gloss
Figure 4 shows the relationship between gloss as measured by GM- and TAPPI gloss. TAPPI gloss
was measured on every sample used both before and after GlossMachine runs. The two values werevery consistent, even if there were marginal deviations because of varying relative positions of the‑ 3 ‑JPPS 23(7):J1-J7(1997)

GlossMachine components are assumed to occur at random. Namely, for example, the position of thelight source and detector varies marginally relative to that of the sample every time the sample
chamber lid is closed. The relationship suggests that approximate conversion to TAPPI gloss would be
easy.

Gloss: surface or bulk property
Figure 5 shows the gloss relaxation and the moisture content change as a function of time for thesupercalendered, uncoated and filled wood-containing paper (A). The sample was dried at 0% RH for
initial 4 h then air at 90% RH was sent for 12 h. After switching to 90% RH, the moisture content
quickly increased and continued to increase even after 12 h. Gloss decreased also but its initial rate ofdecrease was slower than the rate of moisture pickup, as Figure 6 shows. Gloss is predominantly
determined by the refractive index and the surface shape[2]. Considering this, one might expect thatchanges in the surface structure of this uncoated paper would take place immediately as watermolecules condense on the surface fibers. In other words, one might have expected surface changes totake place ahead of the bulk moisture pickup. The fact that gloss lags behind moisture pickup and is
still decreasing well after the paper surface can be considered to be at equilibrium with the new RH ( afew seconds considering that humidity measured by a sensor using a thin cellulose acetate film
stabilizes within 10 seconds) suggests that the surface continues to reflect the reconfiguration of fibers
across the bulk even after a long time. This roughening has been ascribed to fiber swelling, stressrelaxation and shape recovery of the fibers in the bulk as water diffuses into fiber walls[3]. In thissense, gloss relaxation can be considered, paradoxically, as a "bulk" process. This finding is novel andhelps explain the rather poor efficiency of calendering processes using a thermal gradient(TG), such as
TG calendering or softcalendering, in stabilizing the gloss to moisture effects[4].
Effect of RH cycling
Figure 7 shows the gloss relaxation and moisture content change as RH was varied cyclically (thicklines). The sample used for these data was also sample A. In the first drying step, the gloss decreased
as the moisture content decreased. (Indeed, the initial drop in gloss, during the very first drying step,
may seem abnormal since gloss normally decreases with increasing humidity. It should be noted,
however, that with the other 4 papers tested, gloss always increased with drying and decreased withmoistening.) In the subsequent exposure to 90 % RH, the gloss dropped substantially. In the drying
step from 90 % to 0 % RH, it increased and leveled off quickly, but to a value (86.9) that was well
below the initial gloss (111.9). After the second cycle, the gloss level was even lower (80.4) than after
the first one, though the loss in gloss was less than in the first cycle. Such irreversibility is often
observed in cellulosic-water phenomena even though it is still incompletely understood[5]. Thus, gloss
reduction was accompanied by irreversible changes as if mechanical relaxation of internal stresses had
occurred.
The thin lines in Fig. 7 show the results of an experiment where the exposure to 90 % RH was of thesame total length (6 h), but in only one cycle. The drying periods were the same, 4 h for the initial
drying, 3 h for the redrying. In this case, the gloss recovered to 81.3, a value lower than that after the
first cycle but higher than after the second cycle of the other run, even though the initial gloss was
lower (109.1). This indicates that 2 cycles of shrinkage and expansion of the fibers caused greater
irreversible roughening (28 % gloss reduction), than one cycle (25 % gloss reduction), for the same
total exposure time.

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Gloss versus moisture content
Figure 8 shows the relationship between gloss and moisture content using the same experimental data
as in Fig. 7. It is not a simple relationship of gloss decreasing when moisture content increases. As
pointed out earlier, in the first drying step, the gloss unexpectedly decreases first as moisture content
decreases. It then increases slightly as moisture content increases up to about 5% (the first 4 or 5 min).
Then it decreases linearly with increasing moisture content. The same phenomenon is observed in the
succeeding two cycles, but the slope is lower in each successive cycle. Note that during the drying
from 90% RH to 0% RH, the gloss first decreases with decreasing RH (instead of increasing) and after
the moisture content decreased to about 8%, it increases continuously until the sample is dry, in asigma-shaped curve. We do not understand these apparent reversals of the expected gloss-moisture
content relationship but suspect that they depend on the conditioning history. Since gloss accompanies
dimensional changes, it is interesting to compare our results with those of Uesaka [6] who observed asimilar hysteresis in hygroexpansion strain with moisture content.

Other types of paper.
Figures 9 and 10 show results obtained with the other four papers (samples B to E) examined. The
same behavior is observed - the gloss decreasing with increasing moisture, with the gloss lagging
behind moisture pickup - except for the distinct anomalous reversals. It was difficult to discuss theinfluence of the coating and fiber sources.

CONCLUSION
A new instrument is used to measure simultaneously the gloss and the moisture content of a sample ofpaper as it is subjected to changes in relative humidity. Measurements made with five different papers
confirm the overall inverse relationship between gloss and moisture content reported in the literature,
although there are reversals (gloss increasing with moisture content) which are unexplained.
A significant result of this work is that gloss lags behind moisture content after a step change in RH,
apparently reflecting the continuing structural rearrangement of the fibers in the bulk of the sheet as
moisture diffuses into fiber walls. This explains why, in order to stabilize sheet gloss to moistureeffects, it is not enough to stabilize the surface only, as is done in surface treatment processes
involving a thermal gradient.

REFERENCES
1. ASPLER, J. S. and BÉLAND, M.-C., “A Review of Fibre Rising and Surface Roughening Effects
in Paper”, J. Pulp Paper Sci. 20(1): J27-32(1994)”.
2. LEE, D. I., “A Fundamental Study on Coating Gloss”, Proc. 1986 TAPPI Coating Conf., Atlanta,
31-34
3. SKOWRONSKI, J., “Surface roughening of pre-calendered basesheets during coating”, J. Pulp
Paper Sci. 16(3): J102-110 (1992)
4. ENOMAE, T., HUANG, T. and LEPOUTRE, P., “Softcalendering: Effect of Temperature,‑ 5 ‑JPPS 23(7):J1-J7(1997)

Pressure and Speed on Sheet Properties” , Proc. 1995 TAPPI Finishing and Converting Conf.,
Atlanta, 169-174
5. SALMÉN, L., “Responses of Paper Properties to Changes in Moisture Content and Temperature”,
Products of Papermaking, Trans. 10th Fundamental Res. Symp. Vol 1, Pira International, Surrey,
UK 369-430(1993)
6. UESAKA, T., MOSS, C. and NANRI, Y., “The Characterization of Hygroexpansivity of Paper”,
J. Pulp Paper Sci. 18(1): J11-16(1992)

ACKNOWLEDGEMENT
The authors would like to thank Bob Bessette, Amos Cline and Professor John C. Hassler, Chemical
Engineering, University of Maine, for their contribution to the development of the GlossMachine.
Randy D. Charette, Madison Paper Industries is thanked for supplying the paper sample. The financial
support of the industrial sponsors of the University of Maine Paper Surface Science Program is
gratefully acknowledged.

Table I properties of samples used
Kind of paper

TAPPI Glossa) Roughnessb) Basis weight(g/m2)(%)(µm)

Supercalendered, uncoated andfilled wood-containing paper (A)35.3 (CD)38.9 (MD)

1.6

51.1

Softcalendered wood-free basepaper (B)

28.9

2.8

77.9

Softcalendered wood-containingbase paper (C)

17.1

2.9

34.7

Supercalendered woodcontaining coated paper (D)

37.9

1.6

44.3

Softcalendered wood-containing
coated paper (E)

45.7

1.4

46.7

a)

75 ° gloss on the feltside in the crossmachine direction.

b)

By Parker Print-Surf at a load of 0.1 MPa with soft backing.

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Air out

ReferenceLight
Air in atcertain RH

Detector
Air in atcertain RH

Paper
Sample stage

Shaft connectedwith balance
Figure 1 Sample chamber of Glossmachine.

Sparger cell

Reheater cell

Air inSample chamber
Air out at
controlled RH

Water
Bubbles
Sparger

T1
Insulated

T1

T0

Figure 2 Air generator to provide controlled relative humidity.

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120

60

100

50

80

40

60

30

40

20

20

10

0

0
0

1

2

3

4

5

6

7

8

Moisturecontent, %GM-Gloss, %JPPS 23(7):J1-J7(1997)

9 10

Time, hour

GM-Gloss, %Fig. 3 Two Glossmachine runs exhibiting suffcient
reproducibility. Same line pair was obtained at one run. RH
was 0 % for the initial 4 hours; subsequently 90% 3 hours; 0 %3 hours.

140
120
100
80
60
40
20
0

GGM= 2.54 GTAPPI + 9.65
R2 = 0.983

0

10

20

30

40

TAPPI Gloss, %Fig.4 Relationship between Glossmachine gloss and Tappi
gloss both at 75 ｰ.

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50

120

60

100

50

80

40

60

30

40

20

20

10

0

0

Moisture content, %GM-Gloss, %JPPS 23(7):J1-J7(1997)

0

2

4

6

8

10 12 14 16 18

Time, hour

110

10

105

7.5

100

5

95

2.5

90

0
4.1

4.2

4.3

4.4

4.5

Time, hourFig. 6 Gloss relaxation and moisture content change on
exposure to high humidity (blow-up of the previous figurearound the humidity change).

‑ 9 ‑Moisturecontent, %GM-Gloss, %Fig. 5 Gloss relaxation and moisture content change with tme.
RH was switched from 0 to 90 %.

120

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50

80

40

60

30

40

20

20

10

0

0
0

2

4

6

8

10

12

14

Moisture content,%

GM-Gloss, %JPPS 23(7):J1-J7(1997)

16

Time, hourFig. 7 Influence of humidity cycling on gloss relaxation vs.
continuous humidifying.

GM-Gloss, %

110
100
90
80
70
60
0

2

4

6

8

10

12

Moisture content, %Fig. 8 Gloss change with moisture content forsupercalendered, uncoated and filled wood-containing paper(A).

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14

120

60

100

50

80

40

WF

60

30
WC

40

20

WC

20

10

WF

0

Moisture content,%

GM-Gloss, %JPPS 23(7):J1-J7(1997)

0
0

1

2

3

4

5

6

7

8

9 10

Time, hour

120

60

100

50
SOFT

80

40

60

SUP

30

40

SUP

20

20

Moisturecontent, %GM-Gloss, %Fig. 9 Gloss relaxation and moisture content change forwood-free(WF) and wood-containing(WC) basepapers.

10

SOFT

0

0
0

1

2

3

4

5

6

7

8

9 10

Time, hourFig. 10 Gloss relaxation and moisture content change forsoft(SOFT)- and super(SUP)calendered coated woodcontaining papers.

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