Note: Descriptions are shown in the official language in which they were submitted.
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1
KRAFT PAPER AND METHOD FOR MAKING THE SAME
Field of the Invention
The present invention relates to the field of kraft
paper and more specifically to a new method for making
such paper, which method has been found to provide kraft
paper with a unique combination of physical properties.
Accordingly, the invention also relates to this new paper
as well as to so-called valve sacks made of such a kraft
paper.
Background of the Invention
Kraft paper is a general term for paper with above
all good strength properties, for which an important
application area is the manufacture of sacks. A represen-
tative example of such sacks are so-called valve sacks,
e.g. for cement, which must meet high standards primarily
with respect to tensile strength (an important part of
tensile energy absorption) and porosity (high air perme-
ability). High porosity is required in paper intended for
such sacks to enable the sack to let out the air which
accompanies the filling material when the sack is being
filled. In other words, the sack should retain, and be
strong enough to hold, the filling material and at the
same time let out said air. In the case of valve sacks,
in this context, the air can only escape through the
paper, which to this end is provided with perforations.
However, this high porosity is not required in open
sacks, such as garbage bags and the like, since air can
escape through the opening at the top while the sack is
being filled.
Obviously, in connection with the manufacture of
kraft paper, one has been aware of the desirability of
a good combination of high strength and high porosity,
but in general, in this respect, these two properties
have been considered to be mutually opposite, so that
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measures taken to increase the strength have led to a
corresponding reduction in porosity and vice versa.
The measures taken according to the prior art for
the purpose of achieving an optimisation of the above
mentioned primary properties of kraft paper can, in
short, be described as follows. The pulp intended for
kraft paper has been subjected to a first fibre process-
ing with energy being supplied to the fibres when the
fibre suspension has a high consistency, generally over
15 % by weight, (HC refining) followed by a second beat-
ing with energy being supplied at a low fibre consis-
tency, typically about 4o by weight (LC beating), the
energy supplied in connection with said LC beating being
at least 100 kWh/tonne of paper. HC refining is an opera-
tion which provides kneading of fibre against fibre and
micro compressions in the fibre and consequently imparts
good stretching properties to the fibre, while LC beat-
ing provides shredding of the fibre wall and consequently
improves its tensile strength.
Moreover, in order further to improve its tensile
strength, starch, for example, has been added as a
strengthening agent to the stock obtained from the beat-
ing at one or more points in the process, before the
stock is fed onto the wire of the paper machine. The
total amount of strengthening agent added has been up to
a maximum of approximately 6 kg/tonne of finished paper,
in order to maintain satisfactory porosity in the finish-
ed paper.
According to this prior art, the normal combination
of tensile strength and porosity has generally been a
tensile energy absorption index of 2.9 J/g both in the
machine direction (longitudinally) and in the cross
direction (transversely), distributed over a tensile
index of 75 Nm/g longitudinally and 60 Nm/g transverse-
ly with a stretching of up to 8.0%. It has been possible
to achieve this at a Gurley value of 18 seconds. Higher
tensile strength or higher porosity has been achievable
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only at the cost of an unacceptable value for the latter
of these two properties.
According to the present invention, it has been
found possible to achieve a unique combination of
important properties in one and the same paper. In other
words, the paper provided according to the invention
exhibits both high porosity and a high tensile energy
absorption index, which properties can, in addition, be
combined with e.g. good runnability in connection with
the production of sacks. A more detailed definition of
these properties is given below.
A first object of the invention is thus to provide a
method for making kraft paper with an exceptional
combination of tensile energy absorption.index and
porosity.
Another object of the invention is to provide a
method which produces kraft paper which, in addition,
exhibits good runnability in connection with sack pro-
duction.
A further object of the invention is to reduce, with
the aid of the above-mentioned beneficial properties, the
grammage of the kraft paper made by means of the method
according to the invention, which in turn can lower costs
to the manufacturer and the end consumer and which, more-
over, reduces environmental impact with better utilisa-
tion of raw materials and fewer shipments.
In addition to providing a method of the type men-
tioned above, the invention also has for its object to
provide finished kraft paper exhibiting in itself the
above-mentioned beneficial properties.
A further object of the invention is to provide
valve sacks made or formed of the above-mentioned kraft
paper, a major advantage of the good porosity or air
permeability being that the sacks do not need be provid-
ed with perforations. This means, first of all, one less
operation in the manufacturing of the sack itself and,
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secondly, a better work environment for the customer when
filling and handling the sack in question:
Another of object of the invention is to provide
valve sacks of the above-mentioned type, wherein the
improved tensile strength can be utilised for making the
sacks smaller and/or for manufacturing sacks formed of
fewer layers than was previously the case.
Detailed Description of the Invention
More specifically, the method according to the pre-
sent invention is a method for making kraft paper com-
prising the steps of subjecting sulphate or kraft pulp
to high consistency refining (HC refining) only or HC
refining in combination with low consistency beating (LC
beating), wherein, if LC beating is utilised, the energy
supply in connection with LC beating is maintained at a
value below 80 kWh per tonne of finished paper, calculat-
ed as 100% dry paper, and adding a strengthening agent
to the stock obtained in this connection at one or more
separate points in the process, before the stock is fed
onto the wire of the paper machine, so that the total
amount of strengthening agent added will correspond to at
least 8 kg of strengthening agent (calculated as starch
with DS 0.035) per tonne of paper (calculated as 100% dry
paper) .
What is fundamental in connection with the method
according to the invention is thus that the LC beating
is carried out at a considerably lower energy supply than
was previously the case, or that this LC beating is even
omitted, and that the strengthening agent is added in a
dose amount which is considerably greater than according
to the prior art.
The method as it will be described herein is a
method for making kraft paper with a grammage which
is generally within the 50 - 140 g/m2 range. Where one
chooses to be within this range in connection with kraft
paper production depends primarily on the application
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area intended for the paper. A major advantage of the
method according to the invention is, of course, that it
is possible to make kraft paper exhibiting good tensile
strength and good porosity despite a very low grammage,
5 e.g. in the order of 50 - 70 g/m2, while in other cases
it may, for example, be of great value to have paper
with a relatively high grammage, e.g. in the order of
120 - 140 g/mZ, and to utilise it as a single layer in
a bag rather than having to make a similar sack with a
plurality of layers of a different grammage with equally
good physical properties.
A preferred application area for the method accord-
ing to the invention is for making sack kraft paper,
since this represents a very large market and since the
paper obtained according to the invention exhibits a
unique combination of properties which are particularly
advantageous in connection with sacks. Primarily, this
applies to valve sacks, in which good air permeability
is required, as described above, and where the method
according to the invention has even made it possible to
omit the perforations which previously had to be used in
sacks of this type.
With respect to the term kraft paper, in connection
with the invention this refers to bleched or unbleched
paper made primarily of sulphate pulp (kraft pulp). For
example, in this context, it may refer to unbleached
sulphate softwood pulp.
Naturally, with respect to the method according to
the invention, it is understood that further steps are
included in the production of the paper in question if
one looks at the whole process from the sulphate pulp,
which is the starting point, up to the finished paper.
One such step is for instance the use of a micro-creping
equipment in order to enhance the stretching of the
paper. To achieve a stretching of more than 4% such an
equipment is generally used. However, these steps are
conventional and, consequently, it should not be
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necessary to provide a more detailed description of these
in this specification.
With respect to the steps essential to the method
according to the invention, the following applies.
According to a first alternative, the sulphate pulp,
which forms the starting point, is subjected to HC refin-
ing only, which is generally carried out at an energy
supply in the range of 150 - 400 kWh per tonne of finish-
ed kraft paper, which in this case as in every other case
in connection with the invention is calculated as 100%
dry paper, unless otherwise indicated. A particularly
preferred range for HC refining is 200 - 300 kWh per
tonne of paper.
HC refining is generally carried out at a fibre sus-
pension consistency exceeding 15% by weight and usually
at an upper limit of 40% by weight, i.e. suitably in the
range of 15 - 40% by weight. A preferred consistency of
the fibre suspension in connection with HC refining is
28 - 40% by weight, most preferably 30 - 34% by weight.
According to the second alternative of the method
according to the invention, HC refining can be combined
with low consistency beating (LC beating), provided that
the energy supply in connection with the LC beating is
maintained at a value below 80 kWh per tonne of paper.
With respect to HC refining, the above-mentioned general
and preferred values concerning energy supply and fibre
suspension consistency apply to this alternative as well.
With respect to LC beating, it should preferably be car-
ried out at a maximum energy supply of 50, more prefer-
ably a maximum of 30, most preferably a maximum of 20,
kWh per tonne of finished paper. In the present case, LC
beating refers to a fibre suspension consistency in the
range of 2 - 10% by weight, preferably 3 - 6% by weight,
a specially preferred value being about 4% by weight.
Moreover, with respect to both the HC refining and
the LC beating, these operations can be carried out inde-
pendently of one another with the aid of conventional
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refining and beating apparatus, such as refiners. Never-
theless, a preferred embodiment of the invention involves
carrying out at least one, or both, of said operations in
a refiner. Furthermore, these two operations must not
necessarily be carried out in the form of a single HC
refining or a single LC beating. Thus, the terms HC
refining and LC beating in connection with the method
according to the invention also comprise the cases where
either or both of these operations are carried out in
several sequentially arranged refining or beating
devices. Finally, in connection with the HC and LC ope-
rations, it should be added that the energy supply values
stated for these operations refer to net values with the
idling effect for the respective device having been sub-
tracted.
It can be seen from the above that another impor-
tant feature of the method according to the invention
is the adding of a strengthening agent in a certain mini-
mum amount before the stock is fed onto the wire of the
paper machine. In this connection, starch is a suitable
strengthening agent, the term starch being interpreted
broadly as comprising all conceivable types or fractions
of starch which provide the desired effect. However, the
choice of strengthening agent as such is not of primary
importance in connection with the invention, which means
that the choice per se can be made on the basis of prior
art in the field of strengthening agents. Specifically,
this agent is chosen among charged polymers in accordance
with common practice.
In principle, the adding of a strengthening agent to
the stock can be carried out at any point in the process
from the time the latter leaves the refining or the beat-
ing until it is fed onto the wire of the paper machine.
In addition, it can be carried out at one or more sepa-
rate points, the use of two points having been found par-
ticularly advantageous. In this context, for a more or
less optimal effect, it has been found particularly
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advantageous to carry out a first addition of a strength-
ening agent in connection with the machine chest of the
paper machine. An advantageous point for the second addi-
tion of a strengthening agent has been found to be at or
in connection with the mixing pump utilised for the paper
machine, preferably on its suction side. In this context,
the function of the strengthening agent at the latter
point is not only strength enhancing but the agent in
question is also, as is known per se in the field, active
in retaining and distributing the fine material in the
sheet.
According to the above, the total minimum amount of
the strengthening agent in question is 8 kg per tonne of
paper and the maximum amount is usually a total of at
most 20 kg per tonne of paper, the amount being calculat-
ed on the basis of starch with DS 0.035, as can be seen
above.
According to the preferred embodiment where the add-
ing is carried out at two or more separate points, 4-10
kg of strengthening agent per tonne of paper are prefer-
ably added in a first addition and an amount of the agent
in question in the same range, i.e. 4-10 kg per tonne of
paper, is added in a second addition. Particularly pre-
ferred ranges in both of these cases are 5-8 kg per tonne
of paper.
A variant of the method according to the invention
comprises the adding of essentially an equal amount by
weight of the strengthening agent, within the range stat-
ed above, in connection with two additions of said agent.
Another embodiment of the method comprises adding a
maximum of 6 kg per tonne of paper in a first addition
and more than 6 kg per tonne of paper in a second addi-
tion, i.e. a greater amount of strengthening agent in the
second addition than in the first. In this connection, a
particularly preferred range for the first addition is
5-6 kg per tonne of paper, while a particularly preferred
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range for the second addition is more than 6 kg and up to
8 kg per tonne of paper.
As mentioned above, the kraft paper obtained in con-
nection with the above method exhibits a unique combina-
tion of above all tensile strength and porosity or more
specifically tensile energy absorption index and poro-
sity. In this field, the tensile energy absorption index
is defined as energy absorbed per weight of paper during
stretching to initial rupture in connection with tensile
testing, (at standard testing conditions). In this
context, the porosity is measured in the Gurley unit,
which is defined as the time required for 100 ml of air
to pass a circle of the sample with a diameter of 28.7
mm.
More specifically, it has been found possible by
means of the method according to the invention to achieve
the following combination of values for tensile energy
absorption index and porosity: tensile energy absorption
index 2.5-3.5 J/g, especially 2.7-3.5 J/g, at a Gurley
porosity of less than 10 seconds, preferably less than 7
seconds. More specifically, at a Gurley value of 5
seconds, a paper having a tensile energy absorption index
of 3.1 longitudinally and 3.0 transversely has been
achieved. However, if not specifically stated, the
reference to a single tensile energy absorption index
value means the average of the longitudinal and
transverse values, i.e. MD(machine direction)-value +
CD(cross direction)-value divided by 2.
In addition, a kraft paper is provided with the aid
of the method according to the invention, which in itself
exhibits good runnability in connection with sack produc-
tion. In this context, "runnability" refers to the fact
that the sack manufacturer's tube and bottom machines can
be run at a high speed and produce high quality sacks. If
the paper is flimsy, pulls to one side, etc., one talks
of poor runnability. This advantageous characteristic is
in itself a result of good tensile strength at a suitable
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grammage, but according to a preferred embodiment of the
method according to the invention, it has also been found
that this runnability can be improved considerably if, in
connection with the drying of the paper web leaving the
5 wire of the paper machine, the paper web is subjected to
embossing with the aid of embossing equipment comprising
e.g. an embossing wire.
According to a preferred variant of this embossing,
the embossing wire utilised is driven separately in rela-
10 tion to the paper web, the embossing wire preferably
being driven slower or faster than the paper web.
In this way, according to the invention, it has
proved possible to achieve equally good runnability of,
i.e. equally high speed (sacks/min) when making sacks
from embossed 60 g/m2 paper as with unembossed "ordinary"
(=previously known) 80 g/m2 paper.
The embossing is carried out by the wire being
pressed against the paper and resulting in a pattern. In
this connection, the paper is lying against a soft under-
lay, which means that the pattern being pressed into the
paper increases the thickness of the same. The pattern
redistributes tensions in the paper and makes it more
"tensionless". Because of the high strength and the high
porosity of the paper according to the invention, the
embossing can be utilised to a maximum extent on lower
grammages; i.e. to lower the grammage of the paper uti-
lised by the customer.
According to a second aspect, since the invention
has made it possible to provide kraft paper with a new,
unique combination of properties, the invention also
relates to this new paper per se.
This new paper is characterised by the following
combination of physical data:
tensile energy absorption index 2.5-3.5 J/g at a Gurley
porosity of less than 10 seconds, preferably less than
7 seconds. More preferably, especially for unbleched
paper, the tensile energy absorption index is 2.7-3.5 J/g
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at said Gurley porosity of less than 10 seconds,
preferably less than 7 seconds. Most preferably, said
tensile energy absorption index is about 3 J/g at a
Gurley value of about 5 seconds.
Generally, it can also be added that the tensile
energy absorption index value is somewhat lower for a
bleched than for an unbleched kraft paper, such as
approximately l0a lower.
In addition, this kraft paper preferably exhibits
equally good runnability as previously known 70 g/mz
paper, most preferably as 80 g/m2 paper.
In other words, the above-mentioned kraft paper
according to the invention is producible with the aid
of the method described above. However, the invention
obviously relates to this new paper per se, irrespective
of how it is produced.
A third aspect of the invention relates to a valve
sack made of one or more layers of the kraft paper defin-
ed above, since valve sacks in particular have proven an
exceptionally advantageous application of the new kraft
paper because of its combination of properties.
As mentioned above, a preferred embodiment of such a
valve sack, for e.g. 50 kg cement, means that it can be
made entirely without perforations.
Another preferred variant of a valve sack, specifi-
cally for a content of 50 kg, is that it can be made of
only two layers of kraft paper with a low grammage, spe-
cifically a maximum grammage not exceeding 70 g/m2.
Another interesting variant of the valve sack is the
kind formed of a single layer of the kraft paper in ques-
tion with a relatively low grammage value for such a
single layer construction, specifically not exceeding
120 g/m2.
Drawings
The invention will be described in more detail below
in the form of concrete embodiments of the method, which
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also comprise comparative examples, with reference to the
accompanying drawings, to which the following applies.
Fig. 1 shows a flow chart relating to the part of
the method comprising the essential features of
refining/beating and addition of a strengthening agent,
more specifically, starting with a sulphate pulp and up
to the operation where the stock is to be fed to a paper
machine (not shown); and
Fig. 2 shows an outline diagram of an embodiment of
an embossing device in connection with a drying cylinder
included in the drying section of a paper machine.
More specifically, Fig. 1 shows an arrow 1 indi-
cating the feeding of sulphate pulp into a so-called HC
(high consistency) tower 2, in which the pulp is stored
before being fed into an HC press 3 for adjusting (in-
creasing) its consistency to the desired value. From this
press 3, the pulp moves on to an HC refiner 4 for high
consistency refining. The pulp suspension is then fed by
the intermediary of a buffer vat 5 to a first set of LC
refiners 6 in which it is subjected to a first LC beat-
ing. By the intermediary of a second buffer vat 7, a
new LC beating follows in a second set of LC refiners 8
(machine beaters). The purpose of the storing in the buf-
fer vats 5 and 7 is to enable equalisation of any flow
fluctuations from prior steps in the process.
The beaten pulp leaving the second set of LC beaters
8 then goes to a machine chest 9, in which the pulp is
mixed with starch added at the arrow 10. In the machine
chest, sulphuric acid is also added for pH regulation
according to prior art. By the intermediary of a pump 11,
a grammage box 12, and a centricleaner 13, the pulp is
then conducted to a mixing pump 14, on whose suction side
further starch is added (indicated by the arrow 15). Sub-
sequent to this mixing, the stock is fed onwards by the
intermediary of a pressurised screen 16 to a paper
machine (indicated by the arrow 17). This paper machine
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is of the conventional type for making kraft paper for
e.g. sacks and, consequently, need not be described here.
Moreover, the Figure shows the addition of, in this
context, the conventional additives alum (auxiliary
chemical) at 18, rosin size (hydrophobising) at 19 and WS
agent (wet strength) at 20.
The embossing device shown in Fig. 2 comprises an
embossing roller 21 with an associated embossing wire 22.
The figure also shows a number of drying cylinders 23
included in the drying section of the paper machine. A
drying wire 24 and a paper web 25 resting thereupon run
above these drying cylinders.
In the embodiment shown, the embossing wire 22 is
driven separately in relation to the paper web 25 and
preferably with a speed which deviates from the speed
of the paper web. By means of this embossing device, the
paper is given the desired embossing pattern, which in
connection with the invention has been found to provide
exceptionally good runnability when producing sacks from
the kraft paper according to the invention.
EXAMPLES
A number of test were carried out with the aid of
the device shown in Fig. 1, viz. three tests with respect
to the method according to the invention (= test Nos. 4,
5, and 6) and three comparative tests (test Nos. 1, 2 and
3) carried out in connection with LC beating other than
according to the invention and with a different addition
of strengthening agent than according to the invention.
The test conditions were as follows (with the refe-
rence numerals for the respective devices indicated in
brackets).
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Table 1
Test No.
HC-ref. 1 2 3 4 5 6
(4)
energy ~ by
kWh/tonneweight220 30 230 30 23030 240 30 220 31 21030
LC beating
(6)
kWh/tonne& by
weight60 9 50 3.9 55 4 0 4 0 4 0 4
LC beating
(8)
kWh/tonne~ by
weight35 4 40 3.9 40 4 35 4 20 4 0 3.9
Starch
(9) 2.0 2.5 0 7.0 5 5
kg/tonne
Starch
(15) 4.0 4.5 4.5 5.0 7.0 6.5
kg/tonne
The results obtained in these tests are compiled
in Table 2 below. The properties indicated have been
measured in accordance with the methods stated in the
right-hand column of the Table.
CA 02295075 1999-12-23
WO 99/02772 PCT/SE98/01340
N N N ~ H
\ \ \ \ OD
Q Wit' ~T Wit' ~?' l4
lO N N N l~ tnt-~M [-i
M 01 01 a1 01 M 00 lD
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16
The table shows that the paper made according to
the invention exhibits a unique combination of tensile
energy absorption index and Gurley value in relation to
the comparative examples at otherwise comparable proper-
ty values. A lowering of the Gurley value from about 18 s
to about 5 s at comparable, or even somewhat enhanced,
values for the tensile energy absorption index thus
represents an exceptionally valuable development in this
field.
