CA2295075A1

CA2295075A1 - Kraft paper and method for making the same

Info

Publication number: CA2295075A1
Application number: CA002295075A
Authority: CA
Inventors: Bjorn Nilsson; Ing-Marie Ryden; Stefan Hakansson
Original assignee: Individual
Current assignee: BILLERUD SKARBLACKA AB
Priority date: 1997-07-09
Filing date: 1998-07-07
Publication date: 1999-01-21
Also published as: EE04531B1; SE9702656D0; ATE219536T1; SE9702656L; CZ297129B6; DE69806168D1; RU2208079C2; CZ476399A3; HUP0003123A3; HUP0003123A2; PL187952B1; EE200000009A; PL337972A1; SE510506C2; ZA986082B; WO1999002772A1; AU8365898A; ES2147169T1; DE1012377T1; PT1012377E

Abstract

A method for making kraft paper, wherein a sulphate pulp is subjected to HC
refining only or to HC refining in combination with LC beating, the LC
beating, if applicable, being carried out at a relatively low energy supply, and wherein a strengthening agent, especially a charged polymer, is subsequently added to the stock, preferably at two separate points in the process, in a total amount of at least 8 kg per tonne of paper. Kraft paper with a tensile energy absorption index of 2.5-3.5 J/g at a porosity (Gurley) of less than 10 seconds. A valve sack made of one or more layers of said kraft paper.

Description

2 PCT/SE9$/01340 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 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 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, 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 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 WO 99/02772 PCT/SE98l01340 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 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 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 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 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 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 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 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).

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.

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

In H e-I r-I rl u~N ~.1~r1 O O O O O O O O

C/~U7 C!~ CO U~ UaCO UJ( H H H H H H H H H

M

O d1O ~ ~' O ~ crM 01O f'~l0 f~ l~~ '"~rl N .

l~00 M N ~ ~,O --I

I I

u7 61V' ~'~ H ~ M M O O N f~

l0 !'~lD '-i~-tM

~ f"-M N ~ ~'O

N

M

O a1~ ~ O M H (,-)W' d1O h lU

l0 I'~~ ''~c-iN

~ 00 M M ~-.-Il~O

O II U
_ z o Cl) M

O Wit'61 OD O t'~ H N

fn ~,,~O N O ~.,~ct'01 Cp N N CO I"~l0 ~ t'-N N rlrl N r, ,-I0 ~

H x 0 o c~

_ U O

H O O O O 00 01 00 ~,-~M 01O In~ S-I

N I~ l~~ I~.t~ N C rir-IN ~ '-i~ -rl N

N

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O O N O O H N O O O ~p ~ O

~ t~a~ M M ~ ~ M r; N o U

II

a H a H f-l H a H ~

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~ N

t~ N b O~O \ ~ ~ o~oV7 x c~,z ~ b, ~ ro ~

a~

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+~ I ~ o x ~ -~ o -~ ' ~ ~o b x 't3 N O ~ O II
H wl w ~ ~ ~

,~ ~n ~ ~ a v >, a~ .~ o -o a~ ~ ,~
+~

+~ ~ a~ .~ a~ s~ o ~ o .~ cn ~-I c~ r-I U .-1 H to~ '-,-rl ~1.

~ 0 Cl. ~ N cn s-I .LZ~ r U
O -I!~

O rd t~ ~-I ~ rtf .L1-~-IS-a-.-I
~ N

~ N .t-.~ O N O O ~ S-i .C7 -rl w C~ E-~ v3 En H U E C7C~
r.~ U

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.

Claims

1. A method for making kraft paper, especially with a grammage of about 50-140 g/m2, preferably sack paper, such as for valve sacks for e.g. cement, characterised by comprising the steps of subjecting a sulphate pulp to high consistency refining (HC refining) only or to HC refining in combination with low consistency beating (LC beating), if LC beating is utilised, the energy supply in connection with LC beating being maintained at a value below 80 kWh per tonne of paper, calculated as 100% dry paper, and adding a strengthening agent, especially a charged polymer, e.g. starch, 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 a paper machine, so that the total amount of strengthening agent will be least 8 kg (calculated for starch) per tonne of paper (calculated as 100% dry paper).

2. A method according to claim 1, characterised by adding said strengthening agent at two or more, preferably two, separate points.

3. A method according to claim 1 or 2, characterised by carrying out a first addition of said strengthening agent in connection with the machine chest of the paper machine.

4. A method according to claim 1, 2 or 3, characterised by carrying out a second addition of said dry strengthening agent at or in connection with the mixing pump of the paper machine, preferably on its suction side.

5. A method according to any one of the preceding claims, characterised by carrying out the HC
refining at a fibre suspension consistency exceeding 15%
by weight, preferably 15-40% by weight.

6. A method according to claim 5, characterised by maintaining the consistency in connection with said HC refining at 28-40% by weight, preferably 30-34% by weight.

7. A method according to any one of the preceding claims, characterised by carrying out the LC
beating at a fibre suspension consistency in the range of 2-10% by weight, preferably 3-6% by weight, especially about 4% by weight.

8. A method according to any one of the preceding claims, characterised by carrying out the HC
refining and/or LC beating in a refiner.

9. A method according to any one of the preceding claims, characterised by maintaining the energy supply in connection with said LC beating at a value not exceeding 50, preferably not exceeding 30, especially not exceeding 20, kWh per tonne of paper.

10. A method according to any one of the preceding claims, characterised by maintaining the energy supply in connection with said HC refining at a value in the range of 150-400, preferably 200-300, kWh per tonne of paper, calculated as 100% dry paper.

11. A method according to any one of the preceding claims, characterised by adding a maximum of 20 kg in total of said strengthening agent per tonne of paper.

12. A method according to any one of the preceding claims, characterised by adding 4-10, preferably 5-8, kg of said strengthening agent per tonne of paper in a first addition.

13. A method according to any one of the preceding claims, characterised by adding 4-10, preferably 5-8, kg of said strengthening agent per tonne of paper in a second addition.

14. A method according to any one of the preceding claims, characterised by adding an essentially equal amount by weight of said strengthening agent per tonne of paper in two additions of said strengthening agent.

15. A method according to any one of claims 1 - 13, characterised by adding a maximum of 6 kg in a first addition of said strengthening agent and adding more than 6 kg per tonne of paper in a second addition of said strengthening agent, preferably 5-6 kg and >6 and up to 8 kg per tonne of paper, respectively.

16. A method according to any one of the preceding claims, characterised by subjecting, in connection with drying of the paper web leaving the wire of the paper machine, the paper web to embossing by means of embossing equipment comprising an embossing wire.

17. A method according to claim 16, characterised by driving the embossing wire separately in relation to the paper web, the embossing wire preferably being driven slower or faster than the paper web.

18. Kraft paper, especially with a grammage of 50-140 g/m2, preferably sack paper, such as for valve sacks for e.g. cement, 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.

19. Kraft paper according to claim 18, characterised by a tensile energy absorption index of 2.7-3.5 J/g at a Gurley porosity of less than 10 seconds, preferably less than 7 seconds, especially for an unbleched paper.

20. Kraft paper according to claim 19, characterised by exhibiting a tensile energy absorption index of about 3 J/g at a Gurley porosity of about 5 seconds.

21. Kraft paper according to claim 19 or 20, producible by means of the method according to any one of claims 1-17.

22. A valve sack made of one or more layers of kraft paper according to any one of claims 18-21.

23. A valve sack according to claim 22, characterised in that it lacks perforations.

24. A valve sack according to claim 22 or 23, characterised in that it is formed of two layers of said kraft paper, the layers each having a maximum grammage of 70 g/m2.

25. A valve sack according to claim 22, 23 or 24, characterised in that it is formed of one layer only of said kraft paper, the layer having a maximum grammage of 120 g/m2.