ES2932151T3 - Production method of a high-stretch paper sack - Google Patents

Abstract

A sack paper is provided, in which: the grammage according to ISO 536 is 50-140 g/m2, such as 70-130 g/m2; the Gurley value according to ISO 5636-5 is 15 s or less, such as 13 s or less; and the stretchability according to ISO 1924-3 in the machine direction is greater than 10%, as greater than 11%, as greater than 12%. A method for producing the sack paper is also provided. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Production method of a high-stretch paper sack

technical field

The present description relates to the field of highly stretchable paper sacks.

Background

During the filling and handling of powdery material, such as cement, paper bags are required to meet high standards.

First of all, paper bags must support a considerable material weight, that is, have a high resistance. For this purpose, Kraft paper is a suitable bag wall material. The bags usually have two or more walls, ie, layers of paper material, to further strengthen the bag construction. A wall layer of a bag is often referred to as a strand. The production of strand material (ie paper sack) is described, for example, in WO 99/02772.

Second, the paper bag must ventilate air during filling. In detail, the air accompanying the powdered material will be efficiently vented from the bag as the filling machines that deliver the material operate at high throughput rates. Often the ventilation capacity of the bag is the real limiting factor for the fill rate. Efficient ventilation also prevents air from getting trapped in the bag. Trapped air can cause underweight packages, bag rupture, and problems when bags are stacked for shipping. "Venting" is also called "deaeration".

During the filling process, the only way for air to escape from the interior of the bag has, in many bag constructions, been through the bag walls. High porosity Kraft paper is often used on walls to achieve air permeability. However, higher paper porosity usually results in a decrease in overall strength. In particular, the resistance can be significantly reduced if the paper material is mechanically perforated to achieve sufficient air permeability.

EP0124496 describes a process for the manufacture of Kraft paper, especially Kraft paper sacks, on a multi-wire machine in which the reel is dried by means of combined roll drying and free drying and is optionally crimped or micro-crimped and optionally also it is enameled By forming the coil into two or more layers that are formed together in the machine wire portion and shrinking to obtain a stretch at break of at least 2.5% in the machine direction and of at least 5 % in the transverse direction, improved strength properties are provided.

Short description

To avoid rupture of the bag, for example, when the bag is dropped, high tensile strength is not the only property desired for a paper bag. High stretchability has been shown to be equally important in preventing bag rupture. By carefully adapting a creping/compaction process using a Clupak device, the present inventors have succeeded in producing a paper sack 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 figures

Figure 1 is a simplified schematic illustration of a pressing section and a drying section part comprising a Clupak unit.

Figure 2 is a more detailed schematic illustration of a Clupak unit.

The table in Figure 3 is from the Examples section below and presents the speed of the paper reel at various positions on the paper machine from the press section to the Pope winder winding the paper into a paper roll.

Detailed description

As a first unclaimed aspect of the present disclosure, a paper sack is provided. A paper sack typically has a grammage (according to ISO 536) of 50-140 g/m2 As discussed below, the stretchability of the first aspect paper sack is exceptionally high. The present inventors have found that, in general, higher stretch capacities can be achieved for paper sacks of higher grammages. Accordingly, the grammage of the paper bag of the first aspect is preferably 70-140 g/m2, such as 80-140 g/m2, such as 90-140 g/m2.

Grammages greater than 130 g/m2 are quite unusual. Accordingly, the grammage of the paper bag of the first aspect may be 70-130 g/m2, such as 80-130 g/m2, such as 90-130 g/m2.

Air resistance according to Gurley (ISO 5636-5) is a measure of the time(s) required for 100 ml of air to pass through a specified area of a sheet of paper. Short time means highly porous paper. As discussed above, highly porous paper allows for high fill rates.

Since the paper is compacted in a Clupak unit, it might have been expected that the increase in stretchability of the present disclosure would come at the expense of an unacceptable reduction in porosity. However, the present inventors have shown that such a drastic decrease in porosity is not obtained. Therefore, the paper sack of the present description has a Gurley value (ISO 5636-5) of 15 s or less. Preferably, it is 13 s or less, such as 12 s or less. A paper sack that has a Gurley value of less than 3 s often has insufficient strength. Typical Gurley value ranges for the present description are therefore 3-15 s, preferably 3-13 s, such as 3-12 s.

The present inventors have realized that a need exists for improved stretchability in the machine direction. The inventors' efforts have resulted in the paper sack of the first aspect, which is characterized by a stretchability (according to ISO 1924-3) in the MD of more than 10%, preferably more than 11%, plus preferably more than 12%. The way of obtaining the stretchable paper is described in connection with the second aspect and in the EXAMPLES section below.

The stretchability (according to ISO 1924-3) in the CD of the paper sack of the first aspect is usually greater than 6%, preferably greater than 7%, more preferably greater than 8% or 9%. A typical (practical) upper limit for CD stretchability is 11%. As shown in the EXAMPLES section below, treatment that substantially increases MD stretchability also results in a slight increase in CD stretchability.

Tensile Energy Absorption (TEA) is often considered to be the property of paper that best represents the relevant strength of the paper sack wall. This is supported by the correlation between TEA and drop tests. When a bag is dropped, the filler products move within the bag as it hits the ground. This movement tightens the wall of the sac. To withstand the stress, the TEA must be high; a combination of high tensile strength and good stretch in the paper absorbs energy.

Tensile strength is the maximum force a piece of paper will withstand before breaking. In the ISO 1924-3 standard test, a belt having a width of 15 mm and a length of 100 mm with a constant rate of elongation is used.

As stated above, tensile strength is one parameter in the TEA measurement and the other parameter is stretchability. Tensile strength, stretchability and TEA value are obtained in the same test. The TEA index is the TEA value divided by the grammage. In the same way, the tensile index is obtained by dividing the tensile strength by the grammage.

In order to provide high tensile strength, the paper sack of the first aspect is preferably a Kraft paper, which means that it is formed from a pulp prepared according to the Kraft process. For the same reason, the starting material used to prepare the pulp that is used to form the paper sack preferably comprises softwood (which has long fibers and forms a strong paper). Accordingly, the paper sack 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% wood pulp. soft. The percentages are based on the dry weight of the pulp.

Furthermore, the paper sack may comprise at least one dry strength agent to improve tensile strength. The at least one dry strength agent preferably comprises starch, preferably cationic starch. In addition to cationic starch, the paper sack may comprise anionic and/or amphoteric starch. For example, the amount of starch added may be 2-15 kg/ton of paper, such as 3-14 kg/ton of paper.

The tensile index of the paper bag 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 first aspect paper sack results in high TEA values. The TEA according to ISO 1924-3 in the MD of the paper bag of the first aspect can be, for example, at least 300 J/m2, such as at least 330 J/m2, such as at least 350 J/m2 Furthermore, the TEA index according to ISO 1924 3 in the MD of the paper bag of the first aspect may be, for example, at least 3.4 J/g, such as at least 3.5 J/g.

The TEA index according to ISO 1924-3 on the paper sack CD of the first aspect can be, for example, from at least 2.4 J/g, such as at least 2.6 J/g.

The paper bag of the first aspect can be bleached, which means that its brightness can be at least 78% or at least 80% according to ISO 2470-1. Preferably, the brightness of a paper sack bleached according to the first aspect is at least 83%.

Normally the paper sack is printed. Therefore, the paper sack of the first aspect preferably provides a satisfactory printing surface. The satisfactory printing properties are reflected, for example, in a relatively low surface roughness. It might have been expected that heavy crimping/compaction in the Clupak unit described below would dramatically increase surface roughness, but the present inventors have shown that this is not the case. In fact, the surface roughness of the wire side (particularly suitable for printing) increased by only 7% (from 895 ml/min to 957 ml/min) when the drawability on 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 paper sack of the first aspect may therefore be below 1200 ml/min, preferably below 1100 ml/min, such as by below 1000ml/min.

In one embodiment, the paper sack of the first aspect can 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 can also be a fat barrier. In the treatment, a barrier composition or chemical barrier composition is applied, for example, by sheet 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, a method for producing a paper sack in accordance with the first aspect is provided. The method comprises drying a reel of paper in a drying section comprising a Clupak unit and a drying unit arranged downstream of the Clupak unit.

A drier group refers to a drier arrangement comprising at least one drier filter and at least one drier cylinder against which the drier filters support the paper reel passing through the drier group. The components of a dryer group are coupled or so that the paper reel moves at a substantially constant speed through the dryer group.

Typically, a plurality of dryer groups are placed in series upstream of the Clupak unit of the second aspect. The second aspect is not limited to any particular design of dryer groups, as long as the moisture content of the paper reel entering the Clupak unit is 30-40%.

In an embodiment of the second aspect, a plurality of dryer groups are arranged in series downstream of the Clupak unit.

The drying section is usually arranged downstream of a pressing section. Figure 1 illustrates a pressure section 100 comprising two pressure rollers 101 and 102. Furthermore, Figure 1 illustrates a drying section part 103 comprising a first drying group 104 arranged directly upstream of a Clupak unit 105 and a second drying group 106 arranged directly downstream of the Clupak unit 105.

As mentioned above, the moisture content of the paper reel entering the Clupak unit is 30-40% according to the second aspect method. The inventors have found that relatively high moisture levels facilitate increases in stretchability. Preferably, the moisture content of the paper reel entering the Clupak unit is 32-40%. In addition, the inventors have found that the increase in drawability is facilitated by a relatively high pressure bar line load, ie, at least 20 kN/m, in the Clupak unit. Preferably the pressure bar line load is at least 21 kN/m or at least 22 kN/m. If the line charge is too high, the Gurley value increases too much (ie, 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 adjustable hydraulic cylinder pressure exerted on the nip bar. The pinch bar is sometimes referred to as the "pinch roller."

The present inventors have shown that stretchability in the machine direction is highly dependent on relative velocity over the Clupak unit. In detail, the inventors have found that the speed of the paper reel in the dryer group arranged downstream of the Clupak unit should be 8-14% less than the speed of the paper reel entering the Clupak unit. Preferably, the speed of the paper reel in the dryer group arranged downstream of the Clupak is 9-14% less, such as 9-13% less, than the speed of the paper reel entering the Clupak unit.

In one embodiment, the tension of the rubber band 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 tension of the rubber band I is controlled by the adjustable pressure of the hydraulic cylinder exerted on the tension roller that stretch the rubber band.

The Clupak unit usually comprises a steel cylinder. When the paper web is compacted by the shrinkage/recoil of the rubber band in the Clupak unit, it moves relative to the steel cylinder. In order to reduce the friction between the paper reel and the steel cylinder, it is preferred to add a release liquid. The release liquid can be water or water based. The water-based release fluid 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.

Figure 2 illustrates a Clupak unit 205, comprising an endless rubber band 207 (sometimes called a "rubber blanket") in contact with two blanket rollers 208, 209, a guide roller 210, a tension roller 211 and a pressure bar 212. A first hydraulic arrangement 213 exerts pressure on the tension roller 211 to stretch the rubber band 207. A second hydraulic arrangement 214 exerts pressure on the pressure bar 212 to press the rubber band 207, which then in turn presses the paper reel 217 against a steel cylinder 215. A release liquid spray nozzle 216 is provided to apply a release liquid to the steel cylinder 215.

As a third unclaimed aspect of the present disclosure, there is provided a sack comprising a yarn made up of the paper sack of the first aspect.

The bag 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.

Furthermore, the sack of the third aspect may comprise a chemical, mineral or mineral mixture, garden fertilizer, food product, animal feed or pet food.

The sack of the third aspect can be, for example, a multi-ply sack. Preferably, at least two, such as all, the threads of the multi-thread sack may be composed of a paper sack of the first aspect.

In an embodiment of the third aspect, the sack is a single ply sack, wherein the single ply is made up of a first aspect paper sack having a grammage of 90-140 g/m2, such as 95-130 g /m2 The single ply sack could replace prior art sacks which have two plies of 70-80 g/m2 paper sack and therefore lower costs.

In another embodiment of the third aspect, the sack is a two-ply sack, wherein the plies are made up of a paper sack according to any 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. The two-ply sack would be stronger (eg, have a higher TEA value) than the corresponding prior art sacks.

The dimensions of the third aspect bag may, for example, indicate that it has a volume of 8-451 liters, preferably 12-45 liters in a filled configuration.

When the bag of the present description 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 required on the market and therefore can be prepared according to the present description. The dimensions of a full 25 kg bag can be, for example, 400X450x110 mm. A "50 pound bag" can typically be filled with approximately 5 gallons of material, while a "100 pound bag" can typically be filled with approximately 10 gallons of material.

examples

Five full-scale trials were carried out to produce paper sacks of different stretchability in the machine direction.

In all five trials, the paper sack 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 of paper) to a consistency of approximately 35% and low consistency (LC) refining (20 kWh per ton of paper) to a consistency of approximately 4%. . Cationic starch (7 kg per ton of paper), anionic starch (3 kg per ton of paper), rosin size (2.2 kg per ton of paper) and alum (3.5 kg per ton of paper) were added to pulp. In the headbox, the pulp/paste pH was approximately 6.0 and the pulp/paste consistency was approximately 0.2%. A coil of paper was formed on a section of wire. The dry content of the paper reel exiting the wire section was approximately 20%. The paper reel was dewatered in a press section having two rolls to obtain a dry content of approximately 40%. The dehydrated paper reel was then dried in a post-drying section having 9 dryer groups and a Clupak unit arranged in series. The Clupak unit was arranged between drying group six and drying group seven. Upon entering the Clupak unit, the moisture content of the paper reel was 33%. The Hydraulic cylinder pressure exerted on the pressure rod was set to 20 bar, resulting in a line load of 22 kN/m. The pressure of the hydraulic cylinder stretching the rubber band was set to 31 bar, resulting in a band tension of 7 kN/m. To reduce the friction between the paper reel and the steel cylinder in the Clupak unit, a release liquid (1% polyethylene glycol) was added in an amount of 250 litres/hour.

Five tests were carried out to obtain paper bags of different stretchability in the machine direction (MD stretch). In the first, second, third, fourth, and fifth trials, the target MD stretch was 6%, 8%, 10%, 12%, and 14%, respectively. In order to achieve the respective MD stretch value, the speed of the paper reel in the press section and the dryer section was adapted (see figure 3). In particular, the speed of the paper reel in the dryer group directly downstream of the Clupak unit with respect to the speed of the paper reel entering the Clupak unit changed between trials. In the first trial (targeting 6% MD stretch), the relative velocity was -4.4%, whereas in the fifth trial (targeting 14% MD stretch), the relative velocity was -11.0%.

The properties of the paper bags obtained in tests 1-5 are presented in Table 1 below.

* The speed of the paper reel in the dryer group located directly downstream of the Clupak unit with respect to the speed of the paper reel entering the Clupak unit.

**Top side

*** Wire side

Table 1 shows a significant increase in MD stretch by reducing relative velocity. The Gurley values in Table 1 show that the compaction resulting from the lower relative velocities did not close the pores of the paper bag (only a moderate increase in Gurley values was observed). Furthermore, compaction only slightly increased the surface roughness of the wire side of the paper sack (which is intended for printing).

Claims (11)

1. A method of producing a paper sack, comprising drying a paper reel in a drying section comprising a Clupak unit and a drying unit arranged downstream of the Clupak unit, wherein: - the moisture content of the paper reel entering the Clupak unit is 30-40%; - the line load of the pressure bar in the Clupak unit is at least 20 kN/m; Y - the speed of the paper reel in the dryer group arranged downstream of the Clupak unit is 8-14% lower than the speed of the paper reel entering the Clupak unit; Y wherein the grammage according to ISO 536 of the paper bag is 50-140 g/m2, such as 70-130 g/m2; the Gurley value according to ISO 5636-5 of the paper bag is 15 s or less, such as 13 s or less; and the stretchability according to ISO 1924-3 in the machine direction of the paper bag is greater than 10%, such as greater than 11%, such as greater than 12%. The method according to claim 1, wherein the tension of the rubber band in the Clupak unit is 5-9 kN/m, such as 6-9 kN/m, such as about 7 kN/m. The method according to claim 1 or 2, wherein the pressure bar line load in the Clupak unit is 20-30 kN/m, such as 21-28 kN/m, such as about 22 kN/m. The method according to any of claims 1-3, wherein the water comprising at least 0.5% polyethylene glycol is added as a release liquid in the Clupak unit. The method of any of the preceding claims, wherein the paper sack is Kraft paper. The method of any of the preceding claims, wherein the paper sack is bleached. The method of any of the preceding claims, wherein the stretchability according to ISO 1924-3 in the transverse direction of the paper bag is greater than 6%. The method of any of the preceding claims, wherein the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction of the paper bag is at least 3.4 J/g , such as at least 3.5 J/g. The method of any of the preceding claims, wherein the tensile energy absorption (TEA) index according to ISO 1924-3 in the transverse direction of the paper bag is at least 2.4 J/g such as at least 2.6 J/g The method of any of the preceding claims, wherein the stretchability according to ISO 1924-3 in the transverse direction of the paper sack is greater than 7%, such as greater than 8%. The method of any of the preceding claims, wherein the Bendtsen roughness according to ISO 8791-2 of at least one side of the paper sack is below 1200 ml/min, such as below 1100 ml/min. min, such as below 1000 ml/min.