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Influence of fibre - Fibre joint properties on the dimensional stability of paper

Article  in  Cellulose · August 2008

DOI: 10.1007/s10570-008-9203-y

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1

Influence of Fibre-Fibre Joint Properties on the Dimensional

Stability of Paper

Per A Larsson*,**, Lars Wågberg*

* Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden.

** BIM Kemi Sweden AB, Box 3102, SE-443 03 Stenkullen, Sweden.

Author for correspondence: Per A Larsson, telephone: +46 8 790 81 02, fax: +46 8 790 81 01,

e-mail: per.larsson@polymer.kth.se

Keywords

Dimensional stability, drying, hornification, hygroexpansion, multilayer, polyelectrolytes, shrinkage

Abstract

Measurements have been performed to clarify the connection between fibre-fibre joint properties

and dimensional stability using laboratory sheets prepared from never-dried fibres, from heavily

hornified fibres presenting a low molecular contact area between the fibres, and from both hornified

and never-dried fibres treated with a polyelectrolyte multilayer (PEM) technique to increase the

molecular contact area in the fibre-fibre joint. The influence of the drying mode, i.e. whether the

sheets are dried freely or under restraint, was also evaluated. The results showed that neither paper

strength nor fibre-fibre joint contact area had any significant influence on the dimensional stability

of sheets dried under restraint. On the other hand, when the sheets were dried freely, the PEM-

treated sheets expanded to the same extent as, or to an even greater extent than the non-PEM-treated

sheets, even though they adsorbed less water for a given change in relative humidity. There was

also a correlation between drying shrinkage and dimensional stability, where greater shrinkage was

associated with a greater hygroexpansion in the freely dried sheets.

perl5Text BoxNB. This is the final manuscript before publication. The final published version is found in Cellulose (2008) 15:515–525, which is available at Springer via http://dx.doi.org/10.1007/s10570-008-9203-y

http://dx.doi.org/10.1007/s10570-008-9203-y

2

Introduction

In printing, copying and converting operations, dimensional stability is often a critical parameter.

Significant dimensional changes may cause misregister during multicolour printing, generate curl,

cockle and wavy edges during printing or copying, and also lead to substantial problems in

packaging machines and during storage.

Although the effects of fibre swelling on fibre dimensions during papermaking have been

thoroughly studied (cf. Stone and Scallan 1967, Lindström and Carlsson 1978, Grignon and Scallan

1980, Laine et al. 2003), the fundamental mechanism(s) controlling the dimensional changes of

both individual fibres and whole fibre networks after paper formation are not yet fully understood.

As the fibres absorb liquid water, hydrogen bonds within and between the fibres are broken and the

fibrils are separated from each other in the fibre wall, resulting in a swelling of the whole fibre, and

this dimensional change is transferred throughout the whole fibre network. For a free single fibre

the transverse shrinkage, and the subsequent re-swelling, from the wet to the dry state is of the order

of 10 to 30 per cent, and the longitudinal dimensional change is a few per cent (Tydeman et al.

1966). Inter-fibre interactions can however induce greater shrinkage in the fibres. Page and

Tydeman (1962) showed that this forced longitudinal shrinkage of a single fibre, usually between a

few per cent and up to about 12 per cent, is equal to the mean shrinkage of the entire paper.

Depending on the final paper application, the time scale on which the dimensional changes occur

may be more or less important. In multi-colour offset printing, rather large amounts of water are

applied during a very short time. Hence, the rate of expansion is important and it would be possible

to minimise register errors if the rate of expansion could be reduced. On the other hand, if paper is

to be used as e.g. a packaging material in boxes the equilibrium dimensional change over a long

3

period of time may be important. It is therefore obvious that different strategies are needed to

improve the dimensional stability quality of printing papers and that of e.g. packaging paper.

A common way to study the network dimensional change is to study the equilibrium

hygroexpansion, i.e. how a sheet in equilibrium at a given atmospheric relative humidity changes

shape when subjected to moist air until it reaches a new equilibrium at the new humidity level. The

main advantage of this approach, compared to the study of hydroexpansion, where liquid water is

used, is that measurement is easier and it is, to some extent, easier to control the moisture uptake by

controlling the relative humidity. The water uptake is much slower when a paper is subjected to

moist air than to liquid water, and this makes it easier to register the ongoing dimensional change. It

can take several hours for paper to reach an equilibrium moisture content when subjected to a large

humidity change (Jarrell 1927, Leisen et al. 2002). Leisen et al. (2002) concluded that moisture

adsorption is the rate-determining factor of the moisture uptake since the rate of inter-fibre diffusion

was much greater than the rate of the sorption process. The coupling, i.e. the exchange of heat and

moisture, between the paper and the surrounding air, also has a significant influence on the rate of

sorption, and thus on the rate of dimensional change (Brecht and Hildebrand 1960). The

hygroexpansion of fibres and paper has been studied by several authors (e.g. Niskanen et al.,

Salmén et al. and Uesaka et al.), and is of interest not only for the paper industry but also when

fibres are to be used as reinforcement in composites (cf. Neagu et al. 2005).

Nordman (1958) found that there is a linear relation between the hygroexpansion when the ambient

atmosphere is changed from 65 to 50 per cent relative humidity (% RH) and sheet shrinkage during

drying. Salmén et al. (1987) later published data indicating that this is true only when the sheets are

wet-pressed to the same density, and that the density and fines content have a significant influence

on the dimensional stability of freely dried sheets.

4

Another factor which increases the dimensional instability of paper is the built-in stress developed

when sheets are dried under restraint and the fibres are physically prevented from moving during

the drying process. This results in an irreversible shrinkage as soon as the fibre network is again

exposed to moisture, and the magnitude of the permanent shrinkage is greater the higher the relative

humidity to which the paper is exposed (Uesaka et al. 1992). This phenomenon is not as

pronounced in freely dried sheets, but the magnitude of the dimensional change for a given

humidity change is much higher for freely dried, or cross-machine direction, sheets (cf. Uesaka et

al. 1992, Nanri and Uesaka 1993, Uesaka and Qi 1994, Niskanen et al. 1997).

Several thorough reviews have been written on the subject of dimensional stability (e.g. Gallay

1973; Uesaka 1991, Kajanto and Niskanen 1998, Uesaka 2002) and a complete literature review

will not therefore be given in the present work.

The object of this study was to investigate whether there is a relation between fibre-fibre joint

properties, such as fibre-to-fibre contact area and adhesion between the surfaces, and dimensional

stability. As Uesaka and Qi (1994) mention, it is generally believed that the lower the degree of

fibre-to-fibre contact, the greater is the dimensional stability. Through finite element analysis,

Uesaka and Qi (1994) propose that this is true for freely dried sheets, but that for sheets dried under

restraint the transfer of the thickness expansion to the in-plane expansion is more or less negligible.

According to Eklund (1969), the hygroexpansivity is reduced after a couple of drying cycles or re-

use of the pulp, i.e. as the fibre ages and becomes hornified. Torgnysdotter and Wågberg (2006)

showed that drying, i.e. hornification, reduces the fibre-fibre joint contact area, but that the contact

area can be increased by adsorbing polyelectrolyte multilayers (PEMs) onto the fibre surface. This

treatment also enhances the mechanical properties, i.e. increases the tensile strength and the strain at

5

break, of paper, not only because of the increase in contact area but also because of a greater

adhesion between the fibres (Wågberg et al. 2002, Eriksson et al. 2005, Torgnysdotter and

Wågberg 2006). By comparing the hygroexpansional behaviour of paper made from both hornified

and PEM-treated fibres, it should thus be possible to assess the influence of the fibre-fibre joint

properties on the magnitude of the dimensional change, and thus increase the understanding of the

mechanism(s) of dimensional stability.

Materials and Methods

Fibres

Unbeaten virgin softwood kraft pulp (SCA, Östrand Mill, Sweden) bleached according to a

(OO)Q(OP)(ZQ)(PO) sequence, was used throughout this work. Fines were removed by spray

screening through a wire with a mesh size of 75 µm in an equipment developed at STFI-Packforsk.

The pulp was washed and the fibres’ carbonyl groups were converted to their sodium form

according to an earlier described procedure (Wågberg and Hägglund 2001).

Chemicals

Polyallylamine hydrochloride (PAH, Aldrich, prod. No 28,321-5) with a molecular weight of

15,000 Da and polyacrylic acid (PAA, Aldrich, prod. No 19,203-1) with a molecular weight of

7,000 Da were used to build multilayers onto the fibres. The PAH was received as powder and the

PAA was delivered as a 50 % aqueous solution. Both were dissolved in/diluted with deionised

water prior to use. The information regarding the molecular masses of the polymers was provided

by the supplier, and the polymers were used as received without further purification. The

hydrochloric acid, sodium hydroxide, sodium chloride and sodium bicarbonate were of analytical

grade.

6

Hornification

One part of the pulp was hornified according to the following procedure. An aqueous suspension of

fibres was dewatered leaving a small amount of water at the surface of the filter cake to minimise

the effect of the capillary forces during pulp dewatering. Thereafter, the filter cake was left to dry at

30C in a fan dryer in order both to minimise cleavage of the cellulose chains (Kato and Cameron

1999) during the subsequent temperature treatment and to minimise fibre-fibre joint formation

during drying. After being dried the fibres were cured for 24 hours at 105C. This procedure

permits a simple, standardised reslushing of the fibres in water without the extra mechanical

treatment that would otherwise make it difficult to compare dried and never-dried fibres.

Polyelectrolyte multilayers

The fibres were treated consecutively with PAH and PAA according to an earlier described

procedure (Wågberg et al. 2002). The adsorptions were performed in a 0.4 per cent fibre suspension

with a background electrolyte concentration of 0.01 M sodium chloride. Amounts of 30 mg/g fibre

of the two polyelectrolytes were added separately and were allowed to adsorb for 20 minutes. After

each polyelectrolyte adsorption step, the material was thoroughly rinsed with deionised water. PAH

and PAA were adsorbed at pH values of 7.5 and 3.5, respectively as recommended by Eriksson et

al. (2005) to optimise the adsorption. Five layers of polyelectrolytes, i.e. three layers of PAH and

two layers of PAA, were adsorbed onto the fibres prior to sheet forming.

Preparation of sheets dried under restraint

Sheets with an average grammage of 110 g/m2 were prepared using tap water in a “Rapid Köthen”

equipment (Paper Testing Instruments, Pettenbach, Austria). The sheets were dried at 93C and a

negative pressure of 95 kPa for 10 minutes. The PAH/PAA-treated sheets were further heat-treated

for 30 minutes at 160C to induce cross-links in the multilayer structure, and thus increase the

7

tensile strength of the sheets (Eriksson et al. 2006). Since hygroexpansion is highly history-

dependent (Uesaka et al. 1989), the sheets were stored at 23C and 50 % RH until testing.

Preparation of freely dried sheets

Sheets with an average grammage (after shrinkage) of about 110 g/m2 were formed in the “Rapid

Köthen” sheet former using tap water, but the sheets were dried for only 1 minute at 93C and a

negative pressure of 95 kPa. Instead of conventional blotting papers, two 105 µm mesh PTFE wires

were used to ease the transfer to the specially designed drying-frame shown in Figure 1. After one

minute of pressurised drying, the sheet was transferred to the drying-frame and placed between the

two stretched PTFE wires separated by four 0.92 mm thick spacers to prevent the sheet from curling

and buckling. The sheet was dried to equilibrium dryness at 23C and 50 % RH and kept there

during storage. The PAH/PAA-treated sheets were further heat-treated for 30 minutes at 160C to

induce cross-links in the multilayer structure and possibly between the PAH and the fibre (Eriksson

et al. 2006) before storage.

Figure 1: Drying-frame equipped with two stretched PTFE wires to give as little restraint as possible during

drying. The sheet between the wires has a diameter of 20 cm.

8

Shrinkage measurements

Before transferring the sheet from the sheet former to the drying-frame two pairs of holes were

made perpendicular to each other (140 mm apart within each pair) with a marking tool. After

equilibration at 23C and 50 % RH, the sheet was flattened by a PMMA-disc. The disc was

equipped with marks similar to those made by the marking tool used for calibration. A digital

camera was used in combination with image recognition in Matlab to measure the shrinkage of the

sheet during drying. The main advantage of this approach was that it gave an unbiased and

systematic measurement of the sheet shrinkage, but it also made it possible to better study the

shrinkage as a function of time and moisture content. The error of a single measurement was

estimated (taking four times two pictures of un-shrinked paper) to be about 0.10 per cent over a

width of about 140 mm, i.e. a value less than the natural scatter in shrinkage among freely died

sheets.

Mechanical testing

After storage at 23C and 50 % RH, sheet grammage was measured and tensile testing was

performed in accordance with the SCAN-P 67:93 standard. At least 12 strips were used per data

point. Sheet thickness and density were evaluated by measuring the thickness according to a method

developed at STFI-Packforsk (Schultz-Eklund et al. 1992).

Dimensional stability measurements

To study the dimensional movement and dimensional stability of the sheets, a dimensional-stability-

meter developed at STFI-Packforsk was used. 15 mm wide paper strips were mounted horizontally

between two clamps, 1201 mm apart, where one of the clamps was connected to a LVDT-sensor

capable of measuring movement with an accuracy of one micrometre. Six strips representing each

fibre treatment and each drying mode were measured, i.e. total of 48 strips (unfortunately, one or

9

two results had to be discarded due to technical problems). The apparatus is described in more

detail in Lyne et al. (1994).

To control the relative humidity, a moisture generator mixing saturated and dry air streams, was

connected to the dimensional-stability-meter-box. Due to the rather large volume of the measuring

compartment, a change from 20 to 85 % RH, and vice versa, took between 15 and 30 minutes when

loaded with samples. The moisture content of the strips inside the humidity-controlled box was

gravimetrically estimated by also placing test pieces of the papers to be tested within the chamber.

Since the dimensional-stability-meter-box does not have room for a balance, the weighings were

made outside the box.

To ensure that equilibrium had been reached, the strips were equilibrated for at least 7 hours.

The hygroexpansion coefficient is here defined as the relative change in length divided by the

change in equilibrium moisture content (l/(lMC)) after all built-in stresses have been released.

Sorption isotherms

A DVS, dynamic vapour sorption, equipment from Surface Measurement Systems Ltd. was used to

obtain near equilibrium sorption isotherms at a temperature of 331 C. To achieve the desired

relative humidity, dry and saturated air currents were mixed in the appropriate proportions. A

microbalance continuously measured the weight of the sample and the weight at 0 % RH was used

to calculate the moisture content of the sample. The equipment was set to equilibrate until the slope

was below 0.3 g/min for 10 minutes or for a minimum of 10 minutes and a maximum of

120 minutes at each humidity step.

10

Results

Both the hornification process and the PEM technique had, as shown in Figure 2, a significant

influence on the mechanical properties of the sheets. This is in agreement with previous results that

have reported a significant increase in the strength properties of the paper (Wågberg et al. 2002;

Eriksson et al. 2005) and hence in the fibre-fibre joint contact area (Torgnysdotter and Wågberg

2006) when PEM is applied onto wood fibres.

0

50

100

150

200

250

300

Tensile strengthindex (kNm/kg)

Strain at break (%)

Tensile stiffnessindex (MNm/kg)

Rel

ativ

e di

ffere

nce

(%)

VirginHornifiedVirgin + PEMHornified + PEM

27.0 ± 0.5

8

2.35 ± 0.16

4.6

1 ± 0.08

11.7 ± 0.23

59.5

± 1.8

8

30.9 ± 0.62 0.95

± 0.0

3

4.65 ± 0

.22

3.38 ± 0

.16

2.37 ± 0

.05

5.4

7 ± 0.11

3.73 ± 0

.05

(a)

0

50

100

150

200

250

300

Tensile strengthindex (kNm/kg)

Strain at break (%)

Tensile stiffnessindex (MNm/kg)

Rel

ativ

e di

ffere

nce

(%)

VirginHornifiedVirgin + PEMHornified + PEM

8.78

± 0.29

5.42 ± 0

.55

1.02 ± 0

.04

3.70 ± 0.2

9

2.05 ± 0.1

9

0.53 ± 0.06

21.91 ±

1.27

8.64 ± 0.43

1.51

± 0.10

8.96 ± 0.79

3.98 ± 0.57

0.95 ± 0

.08

(b)

Figure 2: Mechanical properties of (a) restraint-dried and (b) freely dried sheets. All differences are

normalised with respect to the virgin pulp and the bars indicate 95 % confidence limits. The absolute values

for the different sheet properties are given above the respective bars.

The shrinkage of freely dried sheets during drying was a fast process as soon as the moisture

content reached about 50 per cent (Figure 3a) and the never-dried pulps shrank to a greater extent

when dried freely and became more consolidated, i.e. they displayed a higher density, when dried

under pressurised and restrained conditions (Figure 3b). The improvement in mechanical properties

of the PEM-treated and never-dried sheets was thus partly a consequence of an increase in density.

11

93

94

95

96

97

98

99

100

0 40 80 120 160 200 240 280 320 360 400 440 480Time (min)

She

et s

ize

(%)

0

10

20

30

40

50

60

70

80

90

Moi

stur

e co

nten

t (%

)

Sheet sizeMoisture content

(a)

-1

0

1

2

3

4

5

6

200 250 300 350 400 450 500 550 600 650 700Density (kg/m3)

Shr

inka

ge (

%)

VirginHornifiedVirgin +PEMHornified + PEM

Freely dried

Dried under restraint

(b)

Figure 3: (a) Sheet size and moisture content as a function of time for a freely dried sheet made from virgin

pulp when dried at 50 % RH and (b) the shrinkage from sheet formation until equilibrium dryness at 50 % RH

and density of freely dried and restraint-dried sheets given with 95 % confidence limits.

When the sheets were exposed to humidity cycles between 20 and 85 % RH, no major difference in

amplitude of either dimensional change or moisture adsorbance was seen for the sheets dried under

restraint. However, the PEM-treated fibres dried under restraint seemed, in absolute numbers, to

adsorb somewhat more moisture in this humidity interval (Figure 4a). The amplitude of the

dimensional change was, as shown in Table 1 where the data from the last desorption in Figure 4 is

presented, somewhat lower for the hornified pulps than for either the virgin or the multilayer-treated

pulps when dried freely. These results agree with the lower shrinkage of the hornified pulps.

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.450 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 50

Relative humidity (%)

Dim

ensi

onal

cha

nge

(%)

0

4

8

12

16

20

24

28

Moi

stur

e co

nten

t (%

)

Virgin Hornified Virgin + PEM Hornified + PEM

(a)

-1.2

-0.9

-0.6

-0.3

0

0.3

0.6

0.9

50 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 65 85 65 40 20 40 50

Relative humidity (%)

Dim

ensi

onal

cha

nge

(%)

0

4

8

12

16

20

24

28M

oist

ure

cont

ent (

%)

Virgin Hornified Virgin + PEM Hornified + PEM(b)

Figure 4: Equilibrium hygroexpansional strain and equilibrium moisture content as a function of relative

humidity for (a) restraint-dried and (b) freely dried sheets. The bars indicate 95 % confidence limits. Filled

symbols indicate dimensional change and open symbols indicate moisture content.

12

The restraint-dried sheets presented a significant permanent shrinkage after a few relative humidity

cycles. This shrinkage was not as pronounced for the freely dried sheets, i.e. when the built-in stress

was smaller. Due to the higher moisture content after cycling the multilayer-treated sheets presented

a somewhat smaller permanent shrinkage (Figure 4).

Table 1: Amplitude of dimensional change, change in moisture content (MCat 85 % RH - MCat 20 % RH) and

hygroexpansion coefficient after all built-in stresses were released. The values are taken from the last

desorption from 85 to 20 % RH in Figure 4, but calculated as an expansion. The expansion values are given

with 95 % confidence limits.

Expansion (%) Change in moisture content (%)

Hygroexpansion coefficient (%/%)

Restraint-dried Freely dried Restraint-dried Freely dried Restraint-dried Freely dried

Virgin 0.469 0.027 0.890 0.062 7.76 8.57 0.060 0.104

Hornified 0.503 0.017 0.797 0.020 7.83 8.33 0.064 0.096

Virgin + PEM 0.528 0.043 0.936 0.047 8.13 7.41 0.065 0.126

Hornified + PEM 0.466 0.016 0.829 0.027 7.61 7.45 0.061 0.111

Figure 5, where the values in Figure 4 have been replotted as dimensional change vs. moisture

content, clearly shows that, for restraint-dried sheets, the hygroexpansional strain followed a non-

linear relationship during the first moisture cycle but that, as the built-in stresses are released, the

behaviour becomes linear. Freely dried sheets on the other hand presented a linear relationship for

all cycles. This is in agreement with the earlier findings (cf. Uesaka et al. 1992).

13

-0.6

-0.4

-0.2

0

0.2

0.4

0 2 4 6 8 10 12 14 16Moisture content (%)

Dim

ensi

onal

cha

nge

(%)

VirginHornifiedVirgin + PEMHornified + PEM

(a)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12 14 16Moisture content (%)

Dim

ensi

onal

cha

nge

(%)

VirginHornifiedVirgin + PEMHornified + PEM

(b)

Figure 5: Equilibrium hygroexpansional strain as a function of equilibrium moisture content for (a) restraint-

dried and (b) freely dried sheets. The figure shows the first three cycles of Figure 4. The bars indicate 95 %

confidence limits.

Kinetics measurements of the hygroexpansional strain during adsorption/desorption (Figure 6 and

Figure 7) showed no significant difference in expansion rate between the different fibre treatments.

The time to reach equilibrium dimensional change was about the same for the two drying strategies,

i.e. the freely dried sheets expanded at a faster absolute rate than those dried under restraint. The

same behaviour could be seen during both adsorption (Figure 6) and desorption (Figure 7). On the

other hand, there was no clear difference in the relative rate of dimensional change between

restraint-dried and freely dried sheets, as is shown in Figure 8, but the papers made from hornified

fibres and dried under restraint showed the slowest relative change when subjected to the change in

relative humidity.

14

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100 120 140 160 180 200Time (min)

Dim

ensi

onal

cha

nge

(%)

2

4

6

8

10

12

14

16

18

20

22

24

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(a)

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140 160 180 200Time (min)

Dim

ensi

onal

cha

nge

(%)

2

4

6

8

10

12

14

16

18

20

22

24

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(b)

Figure 6: Kinetics of hygroexpansional strain and moisture content changes in (a) restraint-dried and (b)

freely dried sheets subjected to a humidity change from 20 to 85 % RH after all built-in stresses were

released. Every fourth value is given with 95 % confidence limits. Filled symbols indicate dimensional

change and open symbols indicate moisture content.

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 20 40 60 80 100 120 140 160 180 200Time (min)

Dim

ensi

onal

cha

nge

(%)

0

2

4

6

8

10

12

14

16

18

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(a)

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 20 40 60 80 100 120 140 160 180 200Time (min)

Dim

ensi

onal

cha

nge

(%)

0

2

4

6

8

10

12

14

16

18

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(b)

Figure 7: Kinetics of hygroexpansional strain and moisture content changes in (a) restraint-dried and (b)

freely dried sheets subjected to a humidity change from 85 to 20 % RH after all built-in stresses were

released. Every fourth value is given with 95 % confidence limits. Filled symbols indicate dimensional

change and open symbols indicate moisture content.

15

-40

-20

0

20

40

60

80

100

0 10 20 30 40 50 60Time (min)

0

20

40

60

80

100

120

140

160

180

200

220

VirginHornifiedVirgin + PEMHornified + PEMRelative humidity

Fra

ctio

n of

the

equl

ibriu

m s

trai

n (%

)

Rel

ativ

e h

um

idity

(%

)

Figure 8: Relative hygroexpansional strain as a function of time for sheets subjected to a humidity change

from 20 to 85 % RH after all built-in stresses were released. Filled symbols indicate restraint-dried sheets and

open symbols indicate freely dried sheets. Every fourth value is given with bars indicating 95 % confidence

limits.

Sorption isotherms, Figure 9, revealed no clear differences between the different fibre treatments.

The virgin pulp treated with PAH/PAA tended to be desorb water somewhat less readily. After

moisture cycling, the hysteresis area between the sorption curves decreased and the moisture

content at higher humidity became lower, even for the already hornified fibres.

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90 100Relative humidity (%)

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(a)

Adsorption

Desorption

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90 100Relative humidity (%)

Moi

stur

e co

nten

t (%

)

VirginHornifiedVirgin + PEMHornified + PEM

(b)

Desorption

Adsorption

Figure 9: Sorption isotherms for restraint-dried sheets, (a) before and (b) after all in-built stresses were

released.

16

Discussion

The decrease in moisture content upon cycling (Figure 9a vs. 9b) is a result of a recrystallisation at

higher relative humidities of the cellulose crystallites, which have been partly decrystallised during

the pulping process (Caulfield and Steffes 1969). As a consequence, this will also result in a change

in the cellulose free volume (Pekarovicova et al. 1997). Interestingly, the already hornified fibres

showed both the same sorptivity and the same decrease in equilibrium moisture content as the

virgin fibres after being subjected to several moisture cycles (Figure 9); an observation in

accordance with Lundberg and de Ruvo (1978), who argued that hornification does not affect the

equilibrium moisture content under moist conditions. This recrystallisation also indicates that the

permanent shrinkage shown Figure 4a is at least partly due to a relaxation of the fibre.

Figure 10: Degree of contact defined as the non-accessible area (for rosanilin) in the fibre-fibre joint as a

function of the amount of adsorbed PAH in PEMs and polyelectrolyte complexes (PECs). Figure reprinted by

permission from Torgnysdotter and Wågberg (2006).

When PEMs were applied, the mechanical properties of paper were improved. According to

Torgnysdotter and Wågberg (2006), see Figure 10, this is due both to an increased contact area in

the fibre-fibre joint and to an increase in the molecular adhesion in the contact zone. The use of

hornified fibres dramatically decreased both the fibre-fibre joint contact area and the tensile strength

of the sheets. Nevertheless, there was no major difference in the in-plane dimensional movement

17

between sheets prepared with the differently treated fibres (Figure 4). This implies, in line with

Uesaka and Qi (1994), that neither the fibre to fibre contact area, nor the adhesion properties, i.e.

both the existence and absence of covalent linkages, has any significant influence on the

dimensional stability of paper as long as the sheet has been dried under restraint. However, for

freely dried sheets it is shown in Table 1 that the PEM-containing sheets show a greater expansion

when exposed to a given change in relative humidity, despite the fact that the change in moisture

content for the given change in relative humidity was lower for the PEM-treated sheets when dried

freely, i.e. freely dried sheets show a greater hygroexpansion coefficient. There may naturally be

several explanations of these phenomena, but the difference can probably be traced back to how the

joints are formed in the freely dried PEM-treated sheets. Since the density is significantly lower in

the freely dried sheets, the fibre-fibre joints will have a greater extension in the z-direction,

schematically shown in Figure 11. The PEM-treatment will also create a larger contact zone, not

just a larger degree of contact, between the fibres when PEMs are applied on dried fibres (personal

communication with Torgnysdotter and Wågberg 2006), as is also indicated in Figure 11. Once the

fibres expand due to moisture adsorption, the joints with a greater extension in the z-direction will

transfer a larger fraction of the expansion in the in-plane direction.

Figure 11: Schematic representation of the fibre-fibre joint configuration depending on drying strategy and

whether or not PEMs are adsorbed onto the fibre surface.

18

The restraint-dried and freely dried sheets show major difference in dynamic hygroexpansion when

studied as function of sheet moisture content. In the first part of the adsorption process both freely

dried and restraint-dried sheets expand with a similar slope (Figure 12). At a certain point, the

restraint-dried sheets cease to expand while, at more or less the same moisture content around 11

per cent, the freely dried ones start to expand with a higher slope. This behaviour was not seen by

Niskanen et al. (1997) when using their vertical experimental setup for hygroexpansion

measurements. However, with a horizontal set-up, and assuming the same moisture sorption rate

(moisture content was not monitored), their machine-direction samples ceased to expand much

earlier than the cross-machine samples, i.e. a behaviour similar to the present results. The

explanation may be that, like the restraint-dried sheets, the freely dried sheets showed a linear in-

plane expansion in the fibre’s longitudinal direction and later, at higher moisture contents, a

straightening of microcompressions and kinks in the free sections between the fibre-fibre joints.

These compressions and kinks are formed only in the machine direction of the sheet and not in the

machine-direction, i.e. when the sheets are dried under restraint (Page and Tydeman 1962). Since

the microcompressions are of the order of micrometres, it would be possible to test this hypothesis

using a high-resolution optical microscope in a climate chamber. However, this is not within the

scope of this study.

0

0.1

0.2

0.3

0.4

0.5

0.6

2 4 6 8 10 12 14Moisture content (%)

Dim

ensi

onal

cha

nge

(%)

VirginHornifiedVirgin + PEMHornified + PEM

Time

(a)

0

0.2

0.4

0.6

0.8

1

2 4 6 8 10 12 14Moisture content (%)

Dim

ensi

onal

cha

nge

(%)

VirginHornifiedVirgin + PEMHornified + PEM

Time

(b)

Figure 12: Hygroexpansion as a function of moisture content in (a) restraint-dried sheets and (b) freely dried

sheets when subjected to a one-step humidity change from 20 to 85 % RH after all built-in stresses were

released. The bars indicate 95 % confidence limits. The broken line serves as a guide to the eye.

19

The trend seen by Niskanen et al. (1997) and Lavrykov et al. (2004) for machine direction (MD)

expansion to take place much faster, in relative terms, than cross direction (CD) expansion when

subjected to moisture sorption, cannot be seen when restraint-dried and freely dried isotropic

laboratory sheets are compared (Figure 8). The fact that sheets change dimensions much faster in

MD thus seems to be due to anisotropy, i.e. that the MD-expansion of the sheet evolves faster since

more fibres are oriented in the MD of the paper, and the net expansion force is much stronger in this

direction.

Is should be stressed that these results are valid for unbeaten fibres. The effect of beating should be

investigated from a dimensional stability point of view, to clarify the potential of using e.g. never-

dried unbeaten PEM treated fibres in combination with beaten fibres at a certain strength level.

Conclusions

By comparing sheets from virgin fibres and hornified fibres and also sheets from untreated and

PEM-treated fibres, the present work has shown no correlation between the tensile properties of

paper and dimensional stability in the case of unbeaten fibres. The degree of contact was both

increased with PEM and decreased by hornification and contrary to general opinion; there was no

correlation between the dimensional stability of the sheets and the fibre-fibre contact area for

laboratory sheets dried under restraint. However, in freely dried sheets an increased contact area led

to a decrease in the dimensional stability by means of a higher hygroexpansion coefficient, thus

making the sheets more sensitive to absolute changes in moisture content. It is suggested that this is

due to an increased extension in the z-direction of the joint.

The different fibre treatments did not affect either the kinetics of the sheet moisture sorption or the

rate of sheet expansion for the two drying modes. Nor was there any significant difference in

20

relative expansion rate as earlier observed between MD and CD sheets. However, the freely dried

sheets showed a non-linear expansion with respect to moisture content and continued to expand

long after the moisture content had reached its equilibrium level.

Acknowledgements

BIM Kemi Sweden AB and the Knowledge Foundation through its graduate school YPK are

acknowledged for financial support.

STFI-Packforsk is acknowledged for granting access to their facilities and Sune Karlsson, STFI-

Packforsk, and Jarmo Tulonen, TJT-Teknik AB, are thanked for helping with the dimensional

stability equipment. Jarmo Tulonen is also acknowledged for substantially contributing to the

design of the drying-frame.

Mats Rundlöf, AB Capisco, is acknowledged for the aid with the fibre-fibre joint illustration.

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