Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH-STRETCH SACKPAPER
TECHNICAL FIELD
The present disclosure relates to the field of highly stretchable sack paper.
BACKGROUND
During filling and handling of powdery material, such as cement, paper sacks
are required to meet high standards.
Firstly, the paper sacks need to hold a considerable material weight, i.e.
have
high strength. For this purpose, Kraft paper is a suitable sack wall material.
The sacks typically have two or more walls, i.e. layers of paper material, to
further strengthen the sack construction. A wall layer of a sack is often
referred to as a ply. Production of ply material (i.e. sack paper) is for
example
disclosed in WO 99/02772.
Secondly, the paper sack should vent air during filling. In detail, the air
that
accompanies the powdered material shall efficiently vent from the sack as the
filling machines that delivers the material run at high throughput rates.
Often, the venting capability of the sack is the actual limiting factor for
the
filling rate. Efficient venting also prevents air from being trapped in the
sack.
Such trapped air may otherwise cause under-weight packs, sack rupture and
problems when sacks are stacked for transportation. The "venting" is also
referred to as "deaeration".
During the filling process, the only way for air to escape from the interior
of
the sack has, in many sack constructions, been through the walls of the sack.
Kraft paper of high porosity is often used in the walls to achieve air
permeability. However, an increased porosity of the paper normally results in
a decrease in the overall strength. In particular, the strength may be
significantly reduced if the paper material is mechanically perforated to
achieve sufficient air permeability.
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SUMMARY
To prevent sack rupture, e.g. when the sack is dropped, high tensile strength
is not the only desired property for a sack paper. High stretchability has
been
shown to be equally important for preventing sack rupture. By carefully
adapting a creping/compacting process using a Clupak device, the present
inventors have managed to produce a sack paper that exhibits exceptional
stretchability and maintains other properties, such as tensile strength and
porosity (as measured by the Gurley test), at acceptable levels.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a simplified schematic illustration of a pressing section and a
drying
section part comprising a Clupak unit.
Fig 2 is a more detailed schematic illustration of a Clupak unit.
The table of Fig 3 belong to the Examples section below and presents the
speed of the paper web at various positions on the paper machine from the
pressing section to the pope reel winding the paper into a paper roll.
DETAILED DESCRIPTION
As a first aspect of the present disclosure, there is provided a sack paper. A
sack paper normally has a grammage (according to ISO 536) of 50-140 g/m2.
As discussed below, the stretchability of the sack paper of the first aspect
is
exceptionally high. The present inventors have found that generally, higher
stretchabilities can be reached for sack papers of higher grammages.
Accordingly, the grammage of the sack paper of the first aspect is preferably
70-140 g/m2, such as 80-140 g/m2, such as 90-140 g/m2. Grammages above
130 g/m2 are rather unusual. Accordingly, the grammage of the sack paper of
the first aspect may be 70-130 g/m2, such as 80-130 g/m2, such as 90-130
g/m2.
The air resistance according to Gurley (ISO 5636-5) is a measurement of the
time (s) taken for 100 ml of air to pass through a specified area of a paper
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sheet. Short time means highly porous paper. As discussed above, highly
porous paper enables high filling rates.
As the paper is compacted in a Clupak unit, it could have been expected that
the increase in stretchability of the present disclosure would come at the
expense of an unacceptable reduction of the porosity. However, the present
inventors have shown that such a drastic decrease of porosity is not obtained.
Accordingly, the sack paper of the present disclosure has a Gurley value (ISO
5636-5) of 15 s or lower. Preferably, it is 13 s or lower, such as 12 s or
lower. A
sack paper having a Gurley value below 3 s often has an insufficient strength.
Typical Gurley value ranges for the present disclosure are thus 3-15 s,
preferably 3-13 s, such as 3-12 s.
The present inventors have realized that there is a need for improved
stretchability in the machine direction. The inventors' efforts have resulted
in
the sack paper of the first aspect, which is characterized by a stretchability
(according to ISO 1924-3) in the MD of above 10 %, preferably above ii %,
more preferably above 12 %. How to obtain such a stretchable paper is
described in connection with the second aspect and in EXAMPLES section
below.
The stretchability (according to ISO 1924-3) in the CD of the sack paper of
the first aspect is typically above 6 %, preferably above 7 %, more preferably
above 8 % or 9 %. A typical (practical) upper limit for the stretchability in
the
CD is 11 %. As shown in the EXAMPLES section below, the treatment
substantially increasing the MD stretchability also results in a slight
increase
in CD stretchability.
Tensile energy absorption (TEA) is often considered to be the paper property
that best represents the relevant strength of the paper sack wall. This is
supported by the correlation between TEA and drop tests. When a sack is
dropped, the filling goods move inside the sack when it hits the floor. This
movement strains the sack wall. To withstand the strain, the TEA should be
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high; a combination of high tensile strength and good stretch in the paper
absorbs the energy.
The tensile strength is the maximum force that a paper will withstand before
breaking. In the standard test ISO 1924-3, a stripe having a width of 15 mm
and a length of loo mm is used with a constant rate of elongation.
As stated above, the tensile strength is one parameter in the measurement of
the TEA and the other parameter is stretchability. The tensile strength, the
stretchability and the TEA value are obtained in the same test. The TEA index
is the TEA value divided by the grammage. In the same manner, the tensile
index is obtained by dividing the tensile strength by the grammage.
To provide high tensile strength, the sack paper of the first aspect is
preferably a Kraft paper, which means that is formed from a pulp prepared
according to the Kraft process. For the same reason, the starting material
used for preparing the pulp that is used for forming the sack paper preferably
comprises softwood (which has long fibers and forms a strong paper).
Accordingly, the sack paper is preferably formed from a paper pulp
comprising at least 50 % softwood pulp, preferably at least 75 % softwood
pulp and more preferably at least 90 % softwood pulp. The percentages are
based of the dry weight of the pulp.
Further, the sack paper may comprise at least one dry strength agent to
improve the tensile strength. The at least one dry strength agent preferably
comprises starch, preferably cationic starch. In addition to cationic starch,
the sack paper may comprise anionic and/or amphoteric starch. For example,
the added amount of starch may be 2-15 kg/ton paper, such as 3-14 kg/ton
paper.
The tensile index of the sack paper of the first aspect is preferably at least
50
kNm/kg, such as at least 55 kNm/kg, in the MD and at least 40 kNm/kg, such
as at least 45 kNm/kg, in the MD.
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The exceptional stretchability of the sack paper of the first aspect results
in
high TEA values. The TEA according to ISO 1924-3 in the MD of the sack
paper of the first aspect may for example be at least 300 J/m2, such as at
least
330 J/m2, such as at least 350 J/m2. Further, the TEA index according to ISO
5 1924-3 in the MD of the sack paper of the first aspect may for example be
at
least 3.4 J/g, such as at least 3.5 J/g.
The TEA index according to ISO 1924-3 in the CD of the sack paper of the
first aspect may for example be at least 2.4 J/g, such as at least 2.6 J/g.
The sack paper of the first aspect may be bleached, which means that its
brightness may be least 78 % or at least 80 % according to ISO 2470-1.
Preferably, the brightness of a bleached sack paper according o the first
aspect is at least 83 %.
Sack paper is normally printed. Accordingly, the sack paper of the first
aspect
preferably provides a satisfactory printing surface. Satisfactory printing
properties are for example reflected by a relatively low surface roughness. It
might have been expected that the heavy creping/compacting in the Clupak
unit described below would drastically increase the surface roughness, but
the present inventors have demonstrated that this is not the case. In fact,
the
surface roughness of the wire side (particularly suitable for printing) was
increased by only 7 % (from 895 ml/min to 957 ml/min) when the
stretchability in the MD was more than doubled (see table 1 of the
EXAMPLES section).
The Bendtsen roughness according to ISO 8791-2 of at least one side of the
sack paper of the first aspect may thus be below 1200 ml/min, preferably
below 1100 ml/min, such as below 1000 ml/min.
In one embodiment, the sack paper of the first aspect may be treated to form
a barrier, for example on a surface of the paper. The barrier is preferably a
moisture barrier and/or a water barrier. The barrier may also be a grease
barrier. In such a treatment, a barrier chemical or barrier composition is
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applied, e.g. by blade coating, curtain coating or spraying. It is also
possible
to add a barrier-forming agent to the pulp.
As a second aspect of the present disclosure, there is provided a method of
producing a sack paper according to the first aspect. The method comprises
drying a paper web in a drying section comprising a Clupak unit and a dryer
group arranged downstream the Clupak unit.
A dryer group refers to a drying arrangement comprising at least one dryer
screen and at least one drying cylinder against which the dryer screen(s)
holds the paper web passing through the drying group. The components of a
drying group are coupled or such that the paper web moves with substantially
constant speed through the dryer group.
Typically, a plurality of dryer groups are arranged in series upstream the
Clupak unit of the second aspect. The second aspect is not limited to any
particular design of such dryer groups as long as the moisture content of the
paper web entering the Clupak unit is 30-40 %.
In one embodiment of the second aspect, a plurality of dryer groups are
arranged in series downstream the Clupak unit.
The drying section is typically arranged downstream a pressing section.
Figure 1 illustrates a pressing section loo comprising two press nips 101 and
102. Further, figure 1 illustrates a drying section part 103 comprising a
first
dryer group 104 arranged directly upstream a Clupak unit 105 and a second
dryer group 106 arranged directly downstream the Clupak unit 105.
As mentioned above, the moisture content of the paper web entering the
Clupak unit is 30-40 % according to the method of the second aspect. The
inventors have found that such relatively high moisture levels facilitate the
increase in stretchability. Preferably, the moisture content of the paper web
entering the Clupak unit is 32-40 %. Further, the inventors have found that
the increase in stretchability is facilitated by a relatively high nip bar
line
load, i.e. at least 20 kN/m, in the Clupak unit. Preferably, the nip bar line
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load is at least 21 kN/m or at least 22 kN/m. If the line load is too high,
the
Gurley value is increased too much (i.e. the porosity is reduced too much). A
typical upper limit may be 30 kN/m or 28 kN/m. In the Clupak unit, the nip
bar line load is controlled by the adjustable hydraulic cylinder pressure
.. exerted on the nip bar. The nip bar is sometimes referred to as the "nip
roll".
The present inventors have shown that the stretchability in the machine
direction to a large extent depends on the relative speed over the Clupak
unit.
In detail, the inventors have found that the speed of the paper web in the
dryer group arranged downstream the Clupak unit should be 8-14 % lower
than the speed of the paper web entering the Clupak unit. Preferably, the
speed of the paper web in the dryer group arranged downstream the Clupak
is 9-14 % lower, such as 9-13 % lower, than the speed of the paper web
entering the Clupak unit.
In one embodiment, the rubber belt tension in the Clupak unit is at least 5
kN/m (such as 5-9 kN/m), preferably at least 6 kN/m (such as 6-9 kN/m),
such as about 7 kN/m. In the Clupak unit, the rubber belt tension is
controlled by the adjustable hydraulic cylinder pressure exerted on the
tension roll stretching the rubber belt.
The Clupak unit typically comprises a steel cylinder. When the paper web is
compacted by the contraction/recoil of the rubber belt in the Clupak unit, it
moves relative the steel cylinder. To reduce the friction between the paper
web and the steel cylinder, it is preferred to add a release liquid. The
release
liquid may be water or water-based. The water-based release liquid may
comprise a friction-reducing agent, such as polyethylene glycol or a silicone-
based agent. In one embodiment, the release liquid is water comprising at
least 0.5 %, preferably at least 1 %, such as 1-4 %, polyethylene glycol.
Fig 2 illustrates a Clupak unit 205, comprising an endless rubber belt 207
(sometimes referred to as a "rubber blanket") contacted by two blanket rolls
208, 209, a guide roll 210, a tension roll 211 and a nip bar 212. A first
.. hydraulic arrangement 213 exerts pressure on the tension roll 211 to
stretch
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the rubber belt 207. A second hydraulic arrangement 214 exerts pressure on
the nip bar 212 to press the rubber belt 207, which in turns presses the paper
web 217 against a steel cylinder 215. A release liquid spray nozzle 216 is
arranged to apply a release liquid to the steel cylinder 215.
.. As a third aspect of the present disclosure, there is provided a sack
comprising a ply composed of the sack paper of the first aspect.
The sack of the third aspect may comprise a hydraulic binder, such as a
hydraulic binder for the production of a cement slurry, a mortar, a concrete,
a
plaster paste or a slurry of hydraulic lime.
Further, the sack of the third aspect may comprise a chemical product, a
mineral or mineral mixture, a garden fertilizer, a foodstuff, animal feed or
pet
food.
The sack of the third aspect may for example be a multiple-ply sack.
Preferably, at least two, such as all, plies of such a multiple ply sack may
be
composed of a sack paper of the first aspect.
In one embodiment of the third aspect, the sack is a one-ply sack, wherein the
only ply is composed of a sack paper of the first aspect having a grammage of
90-140 g/m2, such as 95-130 g/m2. Such a one-ply sack could replace prior
art sacks having two plies of 70-80 g/m2 sack paper and thus lower costs.
In another embodiment of the third aspect, the sack is a two-ply sack,
wherein the plies are composed of a sack paper according to any one of the
preceding claims having a grammage of 70-130 g/m2, such as 70-110 g/m2,
such as 80-110 g/m2, such as 80-100 g/m2. Such a two-ply sack would be
stronger (e.g. have a higher TEA value) than corresponding prior art sacks.
The dimensions of the sack of the third aspect may for example be such that it
has a volume of 8-45 liters, preferably 12-45 liters in a filled
configuration.
When the sack of the present disclosure contains a hydraulic binder, such as
cement, the amount of the hydraulic binder may for example be 17-60 kg,
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such as 40-60 kg. 25 kg sacks, 35 kg sacks and 50 kg sacks are demanded on
the market and may thus be prepared according to the present disclosure.
The dimensions of a filled 25 kg sack may for example be 400x450x110 mm.
A "25 kg sack" typically can be filled with about 17.4 liters of material,
while a
"50 kg sack" is typically can be filled with about 35 liters of material.
EXAMPLES
Five full-scale trials were carried out to produce sack paper of different
stretchability in the machine direction.
In all five trials, the sack paper was produced as described below.
.. A bleached softwood Kraft pulp was provided. The pulp was subjected to high
consistency (HC) refining (180 kWh per ton paper) at a consistency of about
35 % and low consistency (LC) refining (20 kWh per ton paper) at a
consistency of about 4 %. Cationic starch (7 kg per ton paper), anionic starch
(3 kg per ton paper), rosin size (2.2 kg per ton paper) and alum (3.5 kg per
ton paper) were added to the pulp. In the headbox, the pH of the
pulp/furnish was about 6.0 and the consistency of the pulp/furnish was
about 0.2 %. A paper web was formed on a wire section. The dry content of
the paper web leaving the wire section was about 20 %. The paper web was
dewatered in a press section having two nips to obtain a dry content of about
40 %. The dewatered paper web was then dried in a subsequent drying
section having 9 dryer groups and one Clupak unit arranged in series. The
Clupak unit was arranged between dryer group six and dryer group seven.
When entering the Clupak unit, the moisture content of the paper web was 33
%. The hydraulic cylinder pressure exerted on the nip bar was set to 20 bar,
resulting in a line load of 22 kN/m. The hydraulic cylinder pressure
stretching the rubber belt was set to 31 bar, resulting in a belt tension of 7
kN/m. To reduce the friction between the paper web and the steel cylinder in
the Clupak unit, a release liquid (1% polyetylene glycol) was added in an
amount 250 litre/hour.
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Five trials were carried out to obtain sack papers of different stretchability
in
the machine direction (MD stretch). In the first, second, third, fourth and
fifth trial, the target MD stretch was 6 %, 8 %, 10 %, 12 % and 14 %,
respectively. To reach the respective MD stretch value, the speed of the paper
5 web in the press section and the drying section was adapted (see figure
3). In
particular, the speed of the paper web in the dryer group directly downstream
the Clupak unit relative the speed of the paper web entering the Clupak unit
was changed between the trials. In the first trial (aiming for 6 % MD
stretch),
the relative speed was -4.4 %, while in the fifth trial (aiming for 14 % MD
10 stretch), the relative speed was -11.0 %.
The properties of the sack papers obtained in trials 1-5 are presented in
table
1 below.
Table 1.
Trial 1 2 3 4 5
Target stretchability, MD (%) 6 8 10 12 14
Relative speed* (%) -4.4 -6.3 -7.9 -9.9 -11.0
Grammage (g/m2) 100.3 101.7 100.1 101.6 102.6
Thickness ( m) 149 154 152 156 156
Density (kg/m3) 675 660 659 652 660
Tensile strength, MD (kN/m) 6.46 6.56 5.86 5.71 5.63
Tensile strength, CD (kN/m) 4.91 4.64 4.85 4.84 4.62
Tensile index, MD (kNm/kg) 64.5 64.5 58.6 56.2 54.9
Tensile index, CD (kNm/kg) 49.0 45.6 48.4 47.6 45.0
Stretchability, MD (%) 6.o 8.2 9.6 11.6 13.3
Stretchability, CD (%) 8.5 8.4 8.5 8.7 9.2
TEA, MD (J/m2) 240 310 318 355 386
TEA, CD T (J/m2) 283 268 279 285 285
TEA index, MD (J/g) 2.4 3.0 3.2 3.5 3.8
TEA index, CD (J/g) 2.8 2.6 2.8 2.8 2.8
Tear strength, MD (mN) 1 871 1 752 1 830 1 936 1 814
Tear strength, CD (mN) 2 024 1 962 2 015 2 019 1 911
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Tear index, MD (Nm2/kg) 18.7 17.2 18.3 19.1 17.7
Tear index, CD (Nm2/kg) 20.2 19.3 20.1 19.9 18.6
Burst strength (kPa) 508 542 517 488 467
Burst index (mN/kg) 5.1 5.3 5.2 4.8 4.6
Gurley value (s) 9.6 9.8 10.0 10.8 12.0
Bendtsen roughness, TS** 1 569 1 873 2223 2 526 2 915
(ml/min)
Bendtsen roughness, WS 895 864 863 901 957
(ml/min)
* The speed of the paper web in the dryer group arranged directly
downstream the Clupak unit relative the speed of the paper web entering the
Clupak unit.
' Top side
*** Wire side
Table 1 shows a significant increase in MD stretch by lowering the relative
speed. The Gurley values of Table 1 show that the compacting resulting from
the lower relative speeds did not close the pores of the sack paper (only a
moderate increase in Gurley values was observed). Further, the compacting
only slightly increased the surface roughness of the wire side of the sack
paper (that is intended for printing).
