PA P E R P R O P E R T I E S
Systematic changes
in paper properties
caused by fines
By J. Sirviö and I. Nurminen
Abstract: Paper properties can be tailored within some ranges by modifying the properties of
fibres, but the influence of fines quality on structure, strength and optical properties of paper can
be even greater. Fibrillar particles increase bonding between fibres, and thus also tensile index of
paper, while flake-like particles promote a light-scattering coefficient of paper. The results can be
used in planning new fibre processing strategies for tailoring paper properties.
ithin the structure of paper the fines
particles may act as small fibres, fill
voids between fibres, block fibre-fibre
bonding, assist in forming fibre-fibre
bonds, or just lie on free fibre surfaces [1, 2, 3]. Fibrillar mechanical pulp fines are
shown to improve strength properties, while
flake-like fines are favourable for the light-scattering coefficient of paper, when mixed into a
fines-free mechanical pulp [4, 5]. Our objective
was to clarify the contribution of fines quality on
sheet structure, strength and optical properties,
with different mechanical pulp long fibres.
Under constant preparation conditions, pulp
fibres having specified dimensions and conformability result in a specific network porosity and density. The thickness of a fibre network is a linear
function of network grammage, when the density
is constant. If pulp fines were capable in shrinking
the fibre network structure, adding a small
amount of fines into it should be seen as decreased thickness of fibre network. Correspondingly, if we prepared sheets with constant grammage and varying fines content, this should be
seen as lower sheet thickness at the sheet grammage equivalent to the amount of pure fibres.
W
MATERIAL & METHODS
2
We prepared 65-g/m handsheets using the long
fibre fractions of some distinct thermomechanical pulps. The +14/+28 fractions were separated
using a BauerMcNett fractionator. Fractions differed from each other by fibre dimensions and
flexibilities, thus resulting in handsheets of varying porosities and densities, Table I. The lengthweighted mean fibre length and fibre coarseness
were determined using a Kajaani FS200, and fibre
width and thickness by applying CLSM and image
analysis to dry fibres. Fibre stiffness was determined using the method developed by Tam Doo
and Kerekes [6].
In addition, we made mixture sheets containing the long fibre fractions and different types of
pulp fines at varying mass proportions, as well as
pure fines sheets. The fines were separated from
the original pulps using a DDJ (dynamic drainage
jar) with wire hole diameter corresponding to a
200-mesh wire. The exact amount of fines in each
mixture sheet was determined by separating them
Pulp & Paper Canada T 193
using a DDJ from some undried laboratory sheets
prepared between the actual test sheets. Basic
data of pure fines sheets is shown in Table I.
The pulp fines differed from each other in
their fibrillar content (measured using fines analyzer, see [4]), i.e., the mass proportion of fibrillike and flake-like particles varied between the
fines samples. The fibrillar contents were 42%,
56% and 61% for fines of TMP-A, TMP-C and
kraft pulps, respectively. Naturally, fines differed
from each other also in case of other properties.
Sheet properties were determined using standard ISO-methods (ie., grammage ISO 536:1995,
bulk and thickness ISO 534:1998, tensile index
ISO 1924-2:1994, light-scattering coefficient ISO
9416:1998, and Bendtsen air permeance ISO
5636-3: 1992). Sheet porosity was determined by
using cross-sectional cuts from sheets, taking
SEM-produced images of them and then image
analysis to measure the images. The resolution in
images taken from relatively thick long fibre
sheets was 1.2 µm, so pores smaller than that were
not detected. In other images the resolution was
at least 0.6 µm.
RESULTS
Effect of Fines on Sheet Structure: Sheet thickness is heavily influenced by the sheet grammage.
Although the target was to prepare 65-g/m2
sheets, this appeared to be rather difficult due to
low retention of fines on pure long fibre fractions. Thus, all measured sheet thickness values
were scaled to the target grammage before analysis. The exact amount of retained fines was determined using a DDJ.
Figure 1 shows that even a small addition of
fines into a mechanical pulp long fibre fraction
results in a sheet thickness below the expected
one (indicated by the solid line). The expected
sheet thickness was calculated based on the mass
proportion of fibres within the sheet and the
sheet density of pure long fibre fraction. DDJmeasured fines contents of mixtures are shown in
Table II. The fines type having the highest fibrillar content (kraft) had the greatest compacting
effect. Respectively, the lowest fibrillar content
having fines (TMP-A) yielded in smallest, but still
clear, change. Similar behaviour was observed for
other long fibre fractions.
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39
PA P E R P R O P E R T I E S
FIG. 1. Apparent sheet thickness as a function of fibre
grammage within mixture sheets containing mechanical
pulp long fibres and varying amounts of distinct fines.
FIG. 2. Apparent sheet thickness as a function of fibre
grammage within mixture sheets containing distinct
mechanical pulp long fibres and varying amounts of kraft
fines.
TABLE I. Fibre and sheet properties of long fibre (L) and fines (F) fractions examined.
Sample
Fibre
length
[mm]
Fibre
coarseness
[mg/m]
Fibre
width
[µm]
Fibre
thickness
[µm]
TMP-A L
TMP-B L
TMP-C L
TMP-A F
TMP-C F
Kraft F
2.3
2.3
2.5
n.a.
n.a.
n.a.
0.27
0.26
0.22
n.a.
n.a.
n.a.
24.0
23.8
21.5
n.a.
n.a.
n.a.
11.8
10.2
10.2
n.a.
n.a.
n.a.
All the sheet thickness curves approach in a similar manner
the thickness of corresponding pure fines sheet, which seems to
be a good indicator for the compacting effect of fines. At high
fines contents the sheet thickness is higher than expected on the
basis of fibres, because fines totally cover the fibrous backbone.
If we then compare the influence of fibre type on this compacting tendency of fines, we see the lower the density of pure
fibre network is, the higher is the “shrinking” effect of certain
kind of fines, Fig. 2. At kraft fines content 40%, all the very different fibrous backbones show similar thickness.
For example, air permeance of a paper depends heavily on its
pore structure. Figure 3 shows that even a small amount of fines
can have a remarkable influence on pore height distribution
within bulk of paper. Air permeance of a pure long fibre fraction, TMP-B L, even with the lowest kraft fines addition, was out
of measurable range, i.e., it was above 5,000 mL/min. With 15%
kraft fines addition air permeance dropped to 529 mL/min.
With 40% kraft fines the sheet had a porosity of only 17.6%, and
zero air permeance was observed.
Effect of fines on Tensile Strength and Light Scattering: High
fibrillar content fines have been reported to influence positively sheet tensile index and negatively on the sheet light-scattering coefficient [4, 5]. The effect of flake-like fines is opposite. These were verified within our study, Fig. 4. Unlike in
studies mentioned above, we explored the functioning of distinct fines also with several different kinds of mechanical pulp
long fibres. The influence of fines quality seems to be essentially the same irrespective of the fibrous backbone, resulting
in a wing-like spread of tensile index-light-scattering coefficient
combinations (Fig. 4). The fines contents of the mixtures are
shown in Table II.
With increasing content of kraft fines tensile index increases,
but the light-scattering coefficient-remain combinations quite
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❘❘❘ 105:8 (2004)
Fibre
Sheet
stiffness
grammage
[10-12 Nm2]
[g/m2]
46.0
37.3
19.8
n.a.
n.a.
n.a.
65.1
67.7
68.2
63.8
66.6
81.6
Sheet
density
[kg/m3]
Sheet
thickness
[µm]
Sheet
porosity
[%]
203
275
380
498
658
875
321
246
180
128
101
93
72.3
68.0
60.6
36.1
26.9
5.5
TABLE II. Mass proportion of fines within mixture sheets.
Fibres
Mixture
TMP-A
TMP-A
TMP-A
TMP-B
TMP-B
TMP-B
TMP-C
TMP-C
TMP-C
TMP-C
* Targeted dosage;
TMP-A
1
10.0*
2
—
3
80.0*
1
6.0
2
13.1
3
45.8
1
6.9
2
11.0
3
20.9
4
36.4
not measured.
Fines [%]
TMP-C
Kraft
10.0*
20.0*
50.0*
6.8
13.7
43.9
7.1
14.9
29.4
—
6.3
14.3
42.3
4.2
15.0
39.9
6.2
15.1
67.4
—
unchanged, until a certain addition limit. This limit appears to
be around 15% of fines (Fig. 4 and Table II).
In the case of mechanical pulp fines, the light-scattering coefficient seems to be highly associated with the amount of fines, and
not this quality. This can be seen at fines contents below 15%: the
same amount of fines results in the same light-scattering coefficient
with constant fibre quality. At higher fines contents the quality of
fines may become more important (cf. TMP-C with 20-30% fines
of varying quality; Fig. 4 and Table II). The tensile index, however,
shows clear dependence on fines quality, too. The fines fraction
having higher fibrillar content yields in higher tensile index.
Fibrillar fines may improve bonding in three ways:
1. Specific bond strength (like glue);
2. Bonded area within fibre crossing (accumulation on bond
borders); and
T 194 Pulp & Paper Canada
PA P E R P R O P E R T I E S
FIG. 3. Pore height distribution within mixture sheets containing mechanical pulp long fibres and varying amounts
of kraft fines.
TABLE III. Surface area of fibre particles. Length (L) and
perimeter (P), and density (D of cylindrical particles taken
as given dimensions.
Particle
L
[mm]
P
[µm]
Long fibre
Short fibre
Fibril
Flake
2.5
0.8
0.05
0.03
80
60
4
50
D
#/g
[kg/m3] [106/g]
1,300
1,300
1,500
1,500
0.6
3.4
10427
112
Surface
Area
[m2/kg]
121
163
2,121
212
3. Number of bonds between fibres (pulling fibres into more
close contact).
Here we did not try to test the two first options, but there are
studies that indicate such phenomena occur, see [3]. In addition
to these, Fig. 1 suggests that the third option realizes, too,
because fines pull fibres into closer contact with each other.
Thus, it is not surprising the addition of fines (containing also
fibrillar fines particles) into fibres increases the tensile index.
The mechanical pulp fines did not increase tensile index as
much as kraft fines, probably due to their lower fibrillar content.
Specific surface area of fines is much greater than that of
fibres. The light-scattering power of fines is so much higher than
that of fibres, Table III, that it dominates the change in light
scattering of the mixture sheet. On the other hand, based on
total surface area of fines particles per gram, the fibrils should
promote a much greater light-scattering effect than the flakes.
However, this was not the case in our experiment (Fig. 4).
Due to their extremely high conformability, the fibrils probably settle down so closely to fibre surfaces that they in fact do
not contribute to the light-scattering coefficient. The light-scattering coefficient of a sheet consisting of long fibres of mechanical pulp cannot be predicted using purely the surface area of
fibres (cf. Table III and values in Fig. 4 for pure fibres). Fibre
surfaces are so rough that ideal light scattering is not possible.
DISCUSSION & CONCLUSIONS
It seems obvious that fines fractions of both mechanical and
chemical pulps are capable in pulling fibres closer to each other
within a fibre network, thus decreasing thickness and porosity
and increasing the density of the fibrous backbone of a paper
sheet. In addition they naturally decrease sheet porosity and
increase sheet density by filling the voids between the fibres. This
is not necessarily only due to high shrinking of (fibrillar) fines
during drying. During forming the water flowing through the
Pulp & Paper Canada T 195
FIG. 4. The effect of amount and quality of fines on sheet
tensile index and light scattering; fibrous backbone consisting of distinct mechanical pulp long fibres. The mass
proportions of fines within mixtures are shown in Table II.
sheet experiences higher flow resistance due to high specific surface area of fines, possibly compacting the fibrous backbone. The
reason might also be related to restrained recovery of thickness
after pressing due to the presence of (fibrillar) fines. Whatever
the basic mechanisms, the fact is that at least the fibrillar fines are
detrimental to the bulk of paper, thus emphasizing the importance of monitoring the fines quality.
Increased proportion of fibril-like particles within the fines
fraction causes a greater sheet condensing effect. Thus, it is
believed that the fibrils are the acting particles in this respect.
The flake-like fines particles, namely, broken pieces of fibres and
fibre walls, may also contribute here, but their influence is probably minor compared to fibril-like particles. Low sheet densities
of pure mechanical pulp fines indicate that these can improve
105:8 (2004) ❘❘❘
41
PA P E R P R O P E R T I E S
the bulk of paper when very conformable
long fibres form the fibrous backbone.
The sheet density of pure long fibre fraction of a refined softwood kraft pulp can
be well above 600 kg/m3.
With an increasing content of kraft
fines, the tensile index of paper increases,
but the light-scattering coefficient remains
unchanged, until a certain addition limit.
This limit appears to be around 15% of
fines. In typical chemical pulps the fines
content is below that. Thus, in practice,
kraft fines do not have an influence on light
scattering, neither positively nor negatively.
In case of mechanical pulp fines the
light-scattering coefficient seems to be
highly associated with the amount of fines,
not their quality, at least with fines contents
below 15%. In typical mechanical pulps the
fines content may be even 40%, and in
these kinds of pulps the quality of fines
may also be important for the light-scattering coefficient. Tensile index however,
shows clear dependence on fines quality,
too. The fines fraction having higher fibrillar content yields in higher tensile index.
The fibril-like particles within the fines
fraction are not necessarily the only ones
capable of pulling fibres into closer contact with each other. In addition, the fibril-like particles existing in middle fractions and long fibre fractions, might have
similar effects.
Porosity and density are the most relevant structural parameters that influence
the mechanical properties of paper.
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❘❘❘ 105:8 (2004)
Thus, it is important to recognize the possible pulp and fibre properties influencing these. The particle size distribution
and conformability of fibres have been
thought to be the main contributors.
Based on the results presented here, it
seems also that parameters quantifying
the quality of fines particles are needed
for reliable estimation of resulting sheet
porosity and density under constant sheet
preparation conditions.
The quality of fines and fibres within a
pulp can be tailored by the wood raw
material and processing conditions. Exact
knowledge on these and their relations to
paper quality provide new insights into
process and product development.
LITERATURE
1. De SILVEIRA, G., ZHANG, X., BERRY, R., WOOD,
J.R. Location of Fines in Mechanical Pulp Handsheets
Using Scanning Electron Microscopy. JPPS 22(9):J315J320 (1996).
2. GÖRRES, J., AMIRI, R., WOOD, J.R., KARNIS, A.
Mechanical Pulp Fines and Sheet Structure. JPPS
22(12):J491-J496 (1996).
3. RETULAINEN, E. The Role of Fibre Bonding in
Paper Properties. Dissertation, Helsinki University of
Technology, Laboratory of Paper Technology, Espoo,
Finland, Reports Series A7, 63 pp (1997).
4. LUUKKO, K. Characterization and Properties of
Mechanical Pulp Fines. Dissertation, Helsinki University of Technology, Laboratory of Paper Technology,
Espoo, Finland, Acta Polytechnica Scandinavica,
Chemical Technology Series No 267, 66 pp + seven
original articles (1999).
5. ALINCE, B., PORUBSKÁ, J., Van de VEN, T.G.M.
Effect of Model and Fractionated TMP Fines on
Sheet Properties. Proc., 12th Fundamental
Research Symposium, Oxford, 1343-1355 (September 2001).
6. TAM DOO, P.A., KEREKES, R.J. A Method to
Measure Wet Fiber Flexibility. Tappi J. 64(3):113-116
(1981).
Résumé: On peut adapter les propriétés du papier à un certain degré en modifiant les propriétés des fibres, mais l’effet de la qualité des fines sur la structure, la résistance et les propriétés
optiques du papier peut être encore plus important. Les particules fibrillaires accroissent la cohésion des fibres et, ainsi, le degré de traction, tandis que les particules floconneuses ont une incidence sur le coefficient de diffusion de la lumière. On peut utiliser les résultats pour planifier les
nouvelles stratégies de traitement de la fibre afin d’adapter le papier au produit final.
Reference: SIRVIÖ, J., NURMINEN, I. Systematic changes in paper properties caused by fines.
Pulp & Paper Canada 105(8): T193-196 (August, 2004). Paper presented at the 2003 Intl. Mechanical Pulping Conference in Quebec City, Quebec, Canada, on June 2 to 4, 2003. Not to be reproduced without permission of PAPTAC. Manuscript received on October 22, 2003. Revised
manuscript approved for publication by the Review Panel on February 3, 2004.
Keywords: PAPER PROPERTIES, FINES, MECHANICAL PROPERTIES, OPTICAL PROPERTIES.
T 196 Pulp & Paper Canada