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. 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 l 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/m ² . 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/m ² , such as 80-140 g/m ² , such as 90-140 g/m ² . Grammages above 130 g/m ² are rather unusual. Accordingly, the grammage of the sack paper of the first aspect may be 70-130 g/m ² , such as 80-130 g/m ² , such as 90-130 g/m ² .

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 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 11 %, 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 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 100 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. 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/m ² , such as at least 330 J/m ² , such as at least 350 J/m ² . Further, the TEA index according to ISO 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 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 100 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 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 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/m ² , such as 95-130 g/m ² . Such a one-ply sack could replace prior art sacks having two plies of 70-80 g/m ² 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/m ² , such as 70-110 g/m ² , such as 80-110 g/m ² , such as 80-100 g/m ² . 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, 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. 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 %, io %, 12 % and 14 %, respectively. To reach the respective MD stretch value, the speed of the paper 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 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/m ² ) 100.3 101.7 100.1 101.6 102.6

Thickness (μιτι) 149 154 152 156 156

Density (kg/m ³ ) 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 (%) 6.0 8.2 9.6 11.6 13-3

Stretchability, CD (%) 8-5 8.4 8-5 8-7 9.2

TEA, MD (J/m ² ) 240 310 318 355 386

TEA, CD T (J/m ² ) 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 Tear index, MD (Nm ² /kg) 18.7 17.2 18.3 19.1 17.7

Tear index, CD (Nm ² /kg) 20.2 19-3 20.1 19.9 18.6

Burst strength (kPa) 508 542 517 488 467

Burst index (n N/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).