DE69806168T2 - FUEL PAPER AND METHOD FOR THE PRODUCTION THEREOF - Google Patents

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

Field of the invention

The present invention relates to the field of kraft paper and in particular to a new process for producing kraft paper, which process has been found to create 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 bags made from such kraft paper.

Background of the invention

Kraft paper is a general term for paper that has good strength properties and for which the manufacture of bags is an important application. A representative example of such bags are so-called valve bags, for example for cement, which have to meet high requirements, especially in terms of tensile strength (an important part of tensile energy absorption) and porosity (high air permeability). High porosity is required in paper intended for such bags so that the bag can release the air that is carried along with the filling material when the bag is filled. This means that the bag should hold the filling material and be strong enough to absorb it, while at the same time releasing the air. In valve bags, the air can only escape through the paper, which is perforated for this purpose. However, this pronounced porosity is not necessary for open bags, such as garbage bags and the like, since air can escape through the opening at the top when the bag is filled.

It is obvious that in the manufacture of kraft paper, it is recognized that a good combination of high strength and high porosity is desirable, but these two properties have generally been considered to be contradictory, so that measures to improve the strength led to a corresponding reduction in porosity and vice versa.

The measures taken according to the state of the art to optimise the above-mentioned main properties of kraft paper can be briefly described as follows. The pulp intended for kraft paper has been subjected to a first fibre processing, in which energy is added to the fibres when the fibre suspension has a high consistency, generally more than 15% by weight (HC refining), followed by a second refining in which energy is added when the fibre consistency is low, normally around 4% by weight (LC beating), and the energy added during this thin beating is at least 100 kWh per tonne of paper. Thick pulp grinding is a process in which fiber is kneaded against fiber, resulting in micro-compactions in the fiber, giving the fiber good stretch properties, while thin pulp grinding involves breaking up the fiber wall, improving its tensile strength.

In addition, in order to further improve its tensile strength, starch, for example, has been added as a reinforcing agent to the raw material resulting from refining at one or more points in the process before the raw material is discharged onto the wire of the paper machine. The total amount of reinforcing agent added was up to a maximum of approximately 6 kg per tonne of finished paper in order to achieve a satisfactory porosity of the finished paper.

This state of the art is exemplified in Pulp and Paper Canada, 98: 5 (1997) p. 4548.

According to this state of the art, the normal combination of tensile strength and porosity has generally been a tensile energy absorption index of 2.9 J/g in both the machine direction (longitudinal) and the transverse direction (transverse), distributed over a tensile index of 75 Nm/g in the longitudinal direction and 60 Nm/g in the transverse direction with an elongation 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 could only be achieved at the cost of an unacceptable value for the latter of the two properties. Therefore, for example, FI 81643 discloses a tensile strength index of maximum 2.22 kJ/kg at a Gurley value of 4 seconds.

According to the present invention, it has been found possible to achieve a unique combination of important properties in one and the same paper. That is, the paper created according to the invention has both high porosity and a high tensile energy absorption index, which properties are further combined with, for example, good processability in the manufacture of sacks. A detailed definition of these properties is given below.

A first object of the invention is therefore to provide a method for producing kraft paper with an exceptional combination of tensile energy absorption index and porosity.

A further object of the invention is to provide a method for producing kraft paper which also has good processability in sack production.

A further object of the invention is to use the above-mentioned advantageous properties to reduce the weight per square metre of the kraft paper produced by the process according to the invention, which in turn reduces the costs for the manufacturer and the end user and reduces the environmental impact with better use of raw materials and with less transport effort.

In addition to creating a process of the type listed above, the aim of the invention is further to create finished kraft paper which itself has the advantageous properties listed above.

A further object of the invention is to provide valve bags made of the above-mentioned kraft paper, wherein a main advantage of the porosity or air permeability is that the bags do not have to be provided with perforations. This has the main consequence that a process in the production of the bag itself and, secondly, better working conditions for the customer when filling and transporting the bag in question.

Another object of the invention is to provide valve bags of the type described above, whereby the improved tensile strength can be used to reduce the size of the bags and/or to produce bags consisting of fewer layers than has previously been the case.

Detailed description of the invention

The process according to the present invention is in particular a process for producing kraft paper with a square meter weight of 50-140 g/m² and a tensile energy absorption index of 2.5-3.5 J/g at a Gurley porosity of less than 7 seconds, in particular sack paper, for example for valve bags such as for cement, comprising the steps of subjecting a sulphate pulp to HC refining only or LC beating, and a reinforcing agent, in particular a charged polymer, e.g. B. Starch is added to the raw material thus produced before the raw material is fed onto the wire of a paper machine, characterized in that the energy input in connection with the high-density refining is kept at a value of at least 150 kWh per ton of paper, the energy input in connection with the low-density refining, if used, is kept at a value not exceeding 50 kWh per ton of paper, in both cases calculated on 100% dry paper, and the reinforcing agent is added to the raw material at two or more separate points, namely at least 5 kg of the same at a first point and at least 5 kg of the same at a second point (calculated for starch) per ton of paper (calculated as 100% dry paper), so that the total amount of reinforcing agent is at least 10 kg (calculated for starch) per ton of paper (calculated as 100% dry paper).

The decisive factor in the method according to the invention is therefore that the thin material grinding is carried out at a considerably lower energy input than was previously the case, or that this thin material grinding is even omitted, and that the enhancing agent is added in a dosage amount which is considerably higher than in the prior art.

The method as described here is a method for producing kraft paper with a weight per square metre in the range 50-140 g/m². Which value from this range is chosen in connection with the production of kraft paper depends mainly on the field of application of the paper. An important advantage of the method according to the invention is of course that it is possible to produce kraft paper which has good tensile strength and good porosity despite a low weight per square metre, for example in the order of 50-70 g/m², while in other cases it may be of considerable advantage, for example, to have paper with a relatively high weight per square metre, for example in the order of 120-140 g/m², and to use it as a single layer for a bag, instead of having to produce a similar bag with a plurality of layers with a different weight per square metre and equally good physical properties.

A preferred field of application for the method according to the invention is the production of kraft paper for bags, since this is a large market and since the paper produced according to the invention has a unique combination of properties that are particularly advantageous in bags. This applies in particular to valve bags, where good air permeability is required, as described above, and where the method according to the present invention even makes it possible to omit the perforations that had to be used in bags of this type until now.

As regards the term "kraft paper", in the context of the invention it refers to bleached or unbleached paper consisting mainly of sulphate pulp (kraft pulp). In this context it can refer, for example, to unbleached softwood pulp.

It is of course understood that, as far as the method according to the invention is concerned, further steps are involved in the production of the paper in question, if one considers the entire process from the sulphate pulp, which is the starting point, to the finished paper. One such step is, for example, the use of Microcreping devices to improve the elongation of the paper. To achieve an elongation of more than 4%, such devices are generally used. However, these are conventional steps and therefore it should not be necessary to explain them in more detail in the present patent specification.

As regards the decisive features of the process according to the invention, the following applies: According to a first alternative, the sulphate pulp which forms the starting point is only subjected to slurry refining, which is generally carried out at an energy input in the range of 1150 - 400 kWh per ton of finished kraft paper, which, unless otherwise stated, is calculated as 100% dry paper, as in all other cases in connection with the invention. A particularly preferred range for slurry refining is 200-300 kWh per ton of paper.

Thickness grinding is generally carried out at a consistency of the fibre suspension of over 15 wt.% and generally at an upper limit of 40 wt.%, i.e. suitably in the range 15-40 wt.%. A preferred consistency of the fibre suspension in connection with thickness grinding is 28-40 wt.%, most preferably 30-34 wt.%.

According to the second alternative of the method according to the invention, high-consistency beating can be combined with low-consistency beating (LC beating) if the energy input in connection with low-consistency beating is kept at a value not exceeding 50 kWh per ton of paper. As regards high-consistency beating, the general and preferred values listed above regarding the energy input and the consistency of the fiber suspension also apply to this alternative. As regards low-consistency beating, it should preferably be carried out at a maximum energy input of 30, even better a maximum of 20 kWh per ton of finished paper. In the present case, low-consistency beating refers to a consistency of the fiber suspension in the range of 2-10 wt.%, preferably 3-6 wt.%, with a particularly preferred value being approximately 4 wt.%.

Furthermore, as far as both thick and thin grinding are concerned, these processes can be carried out independently of each other using conventional grinding equipment, such as conical mills. However, a preferred embodiment of the invention involves carrying out at least one or both of the operations in a conical mill. Furthermore, these two operations do not necessarily have to be carried out in the form of a single high-viscosity grinding or a single low-viscosity grinding. Therefore, the terms high-viscosity grinding and low-viscosity grinding in connection with the method according to the invention also include the cases in which one or both of these operations are carried out in several grinding devices arranged one after the other. Finally, in connection with the high-viscosity grinding and low-viscosity grinding operations, it should be added that the energy input values given for these operations are net values from which the idle time of the corresponding device has been deducted.

From the above it can be seen that another important feature of the process according to the invention is that a reinforcing agent is added in a certain minimum amount before the raw material is fed onto the wire of the paper machine. In this context, starch is a suitable reinforcing agent, the term "starch" being interpreted broadly and encompassing all conceivable types or classes of starch that achieve the desired effect. However, the selection of a reinforcing agent per se is not of decisive importance in the context of the invention, i.e. the selection per se can be made on the basis of the state of the art in the field of reinforcing agents. That is, this agent is selected from the charged polymers according to common practice.

In principle, a reinforcing agent can be added to the raw material at any point in the process, from the time it leaves the refining until the time it is fed onto the wire of the paper machine. Furthermore, this can be carried out at two or more separate points, the use of two points having proved particularly advantageous. In this context, in order to achieve a more or less optimal effect, it has proved particularly advantageous to make a first addition of a reinforcing agent at the machine chest of the paper machine. An advantageous point for the second addition of a reinforcing agent has proved to be the mixing pump used for the paper machine, preferably its suction side. The function of the reinforcing agent at the latter point is not only to increase the strength, but the agent in question, as is known in the art, helps to hold and distribute the fine material in the sheet.

According to the above, the minimum total amount of the reinforcing agent concerned is 10 kg per tonne of paper and the maximum amount is normally not more than 20 kg in total per tonne of paper, the amount being calculated on the basis of starch with DS 0.035 as seen above.

According to the preferred embodiment, where the addition is carried out at two or more separate points, 5-10 kg of reinforcing agent per ton of paper are preferably added at the first addition, and an amount of the agent in question in the same range, i.e. 5-10 kg per ton of paper, is added at a second addition. Particularly preferred ranges in both of these cases are 5-8 kg per ton of paper.

A variant of the method according to the invention comprises adding substantially an equal amount by weight of the enhancing agent in the range listed above when adding the agent twice.

A further embodiment of the method comprises adding a maximum of 6 kg per ton of paper in a first addition and more than 6 kg per ton of paper in a second addition, i.e. a larger amount of the reinforcing agent is added in the second addition than in the first. A particularly preferred range for the first addition is between 5 and 6 kg per ton of paper, while a particularly preferred range for the second addition is more than 6 kg and up to 8 kg per ton of paper.

The kraft paper produced by the process described above has, as mentioned above, a unique combination of tensile strength and porosity, or more precisely, tensile energy absorption index and porosity. In this context, the tensile energy absorption index is defined as the energy absorbed during the tensile test (under standard test conditions) per weight of paper at Stretching until it breaks is absorbed. The porosity is measured in Gurley units, which is defined as the time required for 100 ml of air to pass through a circle of the sample with a diameter of 28.7 mm.

That is, it has been found possible to achieve the following combination of tensile energy absorption index and porosity values with the method according to the present invention: tensile energy absorption index 2.5-3.5 J/g, in particular 2.7-3.5 J/g, at a Gurley porosity of less than 7 seconds. That is, at a Gurley value of 5 seconds, a paper was produced with a tensile energy absorption index of 3.1 in the longitudinal direction and 3.0 in the cross direction. However, unless specifically stated, reference to a single tensile energy absorption index value means the average of the longitudinal and cross values, i.e. MD(machine direction) value + CD(cross direction) value divided by 2.

Furthermore, by means of the method according to the invention, a kraft paper is created which in itself has good processability in sack production. In this context, "processability" refers to the fact that the bag manufacturer's tube and bottom machines run at high speed and can produce high-quality sacks. If the paper is thin, pulls to one side, etc., this is said to be poor processability. This advantageous property is in itself due to the good tensile strength at a suitable square meter weight, but according to a preferred embodiment of the method of the invention, it has been found that this processability can be significantly improved if, when drying the paper web leaving the wire of the paper machine, the paper web is embossed by means of an embossing device comprising, for example, an embossing wire.

According to a preferred variant of this embossing process, the embossing screen used is driven separately from the paper web, wherein the embossing screen is preferably driven slower or faster than the paper web.

In this way, according to the invention, it has been found possible to obtain equally good results in the production of embossed paper bags with a weight of 60 g/m². Processability, i.e. to achieve the same high speed (bags/minute) as with non-embossed "normal" (= previously known) paper with a weight of 80 g/m².

Embossing is carried out by pressing the screen onto the paper so that a structure is created. The paper is resting on a soft base, i.e. the structure pressed into the paper leads to an increase in the thickness of the same. The structure leads to a redistribution of stresses in the paper and makes it "stress-free". Due to the high strength and the high porosity of the paper according to the invention, embossing can be used to its maximum at lower grammages, i.e. to reduce the grammage of the paper used by the customer.

In general, it should also be added that the value of the tensile energy absorption index for bleached paper is slightly lower than for unbleached paper, i.e. approximately 10% lower.

In addition, this kraft paper preferably has the same good processability as previously known paper with a square meter weight of 70 g/m², and best of all as paper with a square meter weight of 80 g/m².

A second aspect of the invention relates to a valve bag consisting of one or more layers of the kraft paper produced as defined above, since valve bags in particular have proven to be an extremely advantageous field of application for the new kraft paper due to its combination of properties.

This means, as mentioned above, that in a preferred design of such a valve bag, for example for 50 kg of cement, it can be manufactured completely without perforations.

A further preferred variant of a valve bag, in particular for a content of 50 kg, is that it can be made from only two layers of kraft paper with a low weight per square meter, in particular a maximum weight per square meter of not more than 70 g/m².

Another interesting variant of the valve bag is the type made from a single layer of the kraft paper in question with a relatively low weight per square metre for such a single-layer construction, i.e. not more than 120 g/m².

drawings

The invention is described in more detail below in the form of concrete embodiments of the method, which also include 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 process which includes the basic features of refining and adding a reinforcing agent, i.e. starting from a sulphate pulp up to the operation where the raw material is fed to a paper machine (not shown); and

Fig. 2 shows a schematic representation of an embodiment of an embossing device on a drying cylinder that is integrated into the drying section of a paper machine.

In detail, Fig. 1 shows a pipe 1 indicating the feeding of sulphate pulp into a so-called HC (high consistency) tower 2, where the pulp is stored before being fed to a high consistency press 3 to regulate (increase) its consistency to the desired value. From this press 3, the pulp moves on to a high consistency mill 4 for high consistency grinding. The pulp suspension is then fed via a buffer tank 5 as an intermediate station to a first set of low consistency mills 6, in which it undergoes a first low consistency grinding, which is then followed by a new low consistency grinding in a second set of low consistency mills 8 (machine mills). The storage in the buffer tanks 5 and 7 serves to compensate for any flow fluctuations resulting from previous steps in the process.

The ground pulp emerging from the second set of thin stock mills 8 then passes to a machine chest 9 where the pulp is mixed with starch which is added at arrow 10. Sulphuric acid is also added in the machine chest in accordance with the state of the art to regulate the pH. Via a pump 11, a grammage box 12 and a Centricleaner 13 as an intermediate station, the pulp is then passed to a mixing pump 14 at the suction side of which further starch is added (indicated by arrow 15). After this mixing, the raw material is passed on via a pressure screen 16 to a paper machine (indicated by arrow 17). This paper machine is of the conventional type for the production of kraft paper, e.g. for sacks, and therefore need not be described here.

Furthermore, the figure shows in this context the addition of the conventional additives alum (auxiliary chemical) in 18, resin glue (hydrophobization) in 19 and wet strength agent in 20.

The embossing device shown in Fig. 2 comprises an embossing roller 21 with an associated embossing wire 22. The figure also shows a series of drying cylinders 23 which are included in the drying section of the paper machine. A drying wire 24 and a paper web 25 lying thereon run over these drying cylinders.

In the embodiment shown, the embossing screen 22 is driven separately from the paper web and preferably at a speed that differs from the speed of the paper web. This embossing device gives the paper the desired embossed structure, which, as has been found in connection with the invention, ensures extremely good processability when sacks are made from the kraft paper according to the invention.

EXAMPLES

A series of tests were carried out using the device shown in Fig. 1, i.e. three tests relating to the process according to the invention (tests No. 4, 5 and 6) and three comparative tests (tests No. 1, 2 and 3) which were carried out in connection with thin-ground grinding other than according to the invention and with a different addition of reinforcing agent than in the invention.

The experimental conditions were as follows (the reference symbols for the corresponding devices are given in parentheses). Table 1

The results obtained in these tests are summarized in the following Table 2. The properties indicated were measured according to the procedures in the right column of the table. Table 2

L = machine direction; T = cross direction; TS = top side; WS = wire side Conditioning and test atmosphere, 50% relative humidity The table shows that the paper produced according to the invention has a unique combination of tensile energy absorption index and Gurley value compared to the comparative examples with otherwise comparable property values. A reduction in the Gurley value from about 18 s to about 5 s with comparable or even slightly better values for the tensile energy absorption index thus represents an extremely valuable development in this field.

Claims (21)

1. Process for producing kraft paper with a weight of 50-140 g/m² per square meter and a tensile energy absorption index of 2.5-3.5 J/g at a Gurley porosity of less than 7 seconds, in particular sack paper, for example for valve bags, e.g. for cement, which comprises the steps in which a sulphate pulp is subjected to high-volume grinding only or high-volume grinding in combination with low-volume grinding and a reinforcing agent, in particular a charged polymer, e.g. starch, is added to the raw material thus produced before the raw material is fed onto the screen of a paper machine, characterized in that the energy input during the high-density refining is maintained at a value of at least 150 kWh per ton of paper, the energy input during the low-density refining, if used, is maintained at a value not exceeding 50 kWh per ton of paper, which in both cases is calculated as 100% dry paper, and the reinforcing agent is added to the raw material at two or more separate points, namely at least 5 kg of the same at a first point and at least 5 kg of the same at a second point, calculated for starch, per ton of paper, calculated as 100% dry paper, so that the total amount of the reinforcing agent is at least 10 kg, calculated for starch, per ton of paper, calculated as 100% dry paper. 2. Method according to claim 1, characterized in that a first addition of the reinforcing agent is carried out in connection with the machine chest of the paper machine. 3. A method according to claim 1 or 2, characterized in that a second addition of the dry reinforcing agent is made on or in connection with the Mixing pump of the paper machine, preferably on its suction side. 4. Method according to one of the preceding claims, characterized in that the slurry grinding is carried out at a consistency of the fiber suspension that is above 15% by weight, preferably between 15 and 40% by weight. 5. Process according to claim 4, characterized in that the consistency during the slurry grinding is kept at 28-40% by weight, preferably 30-34% by weight. 6. Method according to one of the preceding claims, characterized in that the thin-material grinding is carried out at a consistency of the fiber suspension in the range of 2-10 wt.%, preferably 3-6 wt.% and in particular approximately 4 wt.%. 7. Method according to one of the preceding claims, characterized in that the thick material grinding and/or the thin material grinding is carried out in a conical mill. 8. Method according to one of the preceding claims, characterized in that the energy supply in connection with the thin stock grinding is kept at a value not exceeding 30, preferably not exceeding 20 kWh per ton of paper. 9. Method according to one of the preceding claims, characterized in that the energy supply during nitrogen grinding is kept at a value in the range of 150-400, preferably 200-300 kWh per ton of paper, calculated as 100% dry paper. 10. Process according to one of the preceding claims, characterized in that a maximum of 20 kg of the reinforcing agent per ton of paper is added. 11. Method according to one of the preceding claims, characterized in that 5-10, preferably 5-8 kg of the reinforcing agent are added per ton of paper in a first addition. 12. Method according to one of the preceding claims, characterized in that in a second addition 5-10, preferably 5-8 kg of the reinforcing agent are added per ton of paper. 13. Method according to one of the preceding claims, characterized in that when two additions of the reinforcing agent are made, a substantially equal proportion by weight of the reinforcing agent is added per ton of paper. 14. Method according to one of the preceding claims, characterized in that a maximum of 6 kg of the reinforcing agent is added during a first addition and more than 6 kg, preferably up to 8 kg per ton of paper, are added during a second addition of the reinforcing agent. 15. Method according to one of the preceding claims, characterized in that during drying of the paper web leaving the screen of the paper machine, the paper web is embossed with an embossing device comprising an embossing screen. 16. Method according to claim 15, characterized in that the embossing screen is driven separately in relation to the paper web, wherein the embossing screen is preferably driven more slowly or more quickly than the paper web. 17. Method according to one of the preceding claims, characterized by producing a kraft paper with a tensile energy absorption index of 2.7-3.5 J/g at a Gurley porosity of less than 7 seconds, preferably at least 3 J/g at a Gurley porosity of at most 5 seconds, in particular for an unbleached paper. 18. A valve bag consisting of one or more layers of kraft paper manufactured according to any one of claims 1-17. 19. Valve bag according to claim 18, characterized in that it has no perforations. 20. Valve bag according to claim 18 or 19, characterized in that it consists of two layers of kraft paper, the layers each having a maximum square meter weight of 70 g/m². 21. Valve bag according to claim 19 or 20, characterized in that it consists of only one layer of kraft paper, the layer having a maximum square meter weight of 120 g/m².