Method for producing multi-ply paperboard

Abstract

A method of forming a multi-ply paperboard in a full-size paperboard machine, the multi-ply paperboard comprising at least a first ply and a second ply, is provided, wherein the method comprises the steps of: - forming a first ply web and a second ply web in a wire section of the paperboard machine; - spraying water, which is essentially free of bonding chemicals, onto a surface of the first ply web or the second ply web to obtain a wetted surface in the wire section; and - merging the wetted surface with a surface of the other web by couch roll pressure bonding.

Description

Technical Field

[0001] This disclosure relates to the field of production of multi-layer paperboard. Background Technology

[0002] Paperboard is typically made from one layer or two or more layers, the latter being referred to as multilayer paperboard. Multilayer paperboard is used, for example, in packaging due to its mechanical properties.

[0003] The mechanical properties between the layers, including the bonding strength, are important characteristics of multilayer plates.

[0004] In addition to mechanical properties, optimizing the production process of multi-layer paperboard is also important. Summary of the Invention

[0005] One objective is to provide a method for producing high-strength multilayer cardboard in a full-size cardboard machine at a reduced cost.

[0006] Therefore, this disclosure provides a method for forming a multi-layered paperboard in a full-size paperboard machine, the multi-layered paperboard comprising at least a first layer and a second layer, wherein the method includes the following steps: - A first sheet web and a second sheet web are formed in the wire section of the paperboard machine; - Spraying water, which is substantially free of adhesive chemicals, onto the surface of the first or second layer of web to obtain a wetted surface in the web section; and - The wetted surface is combined with the surface of another sheet by rolling. Attached Figure Description

[0007] Figure 1 A box plot is shown showing the amount of water sprayed on the back side of the top sheet before pressing with the intermediate sheet roll for a final basis weight of about 175 g / m², as referenced in the embodiment of the present invention (IE).

[0008] Figure 2 A box plot is shown showing the amount of water sprayed on the back side of the top sheet before pressing with the intermediate sheet rolls for a final basis weight of about 248 g / m², as referenced in Embodiment (IE) of the present invention.

[0009] Figure 3 A box plot is shown showing the amount of water sprayed on the top surface of the bottom layer, as referenced in the embodiment (IE) of the present invention, before being pressed with the intermediate sheet roll for a final basis weight of about 175 g / m².

[0010] Figure 4A box plot is shown showing the amount of water sprayed on the top surface of the bottom layer, as referenced in the embodiment (IE) of the present invention, before being pressed with the intermediate sheet roll for a final basis weight of about 248 g / m².

[0011] Figure 5 Box plots are shown for the internal bond strength (Scott bond) of paperboard with a final basis weight of about 175 g / m², as well as for paperboard produced according to embodiments (IE) of the present invention.

[0012] Figure 6 Box plots are shown for the internal bond strength (Scott bond) of paperboard produced with reference to and according to embodiments (IE) of the present invention for a final basis weight of approximately 248 g / m².

[0013] Figure 7 Box plots are shown of the z-direction tensile strength of paperboard produced with reference to and according to embodiments (IE) of the present invention for a final basis weight of approximately 175 g / m².

[0014] Figure 8 Box plots are shown of the tensile strength in the z-direction for paperboard with a final basis weight of about 248 g / m², as well as for paperboard produced according to embodiments (IE) of the present invention. Detailed Implementation

[0015] This disclosure provides a method for forming a multi-layered paperboard in a full-size paperboard machine, the multi-layered paperboard comprising at least a first layer and a second layer, wherein the method includes the following steps: - A first sheet web and a second sheet web are formed in the wire section of the paperboard machine; - Spraying water, which is substantially free of adhesive chemicals, onto the surface of the first or second layer of web to obtain a wetted surface in the web section; and - The wetted surface is combined with the surface of another sheet by rolling.

[0016] As understood by the technicians, in a multi-layered paperboard, the first layer web forms the first layer, and the second layer web forms the second layer.

[0017] The first and second web layers are formed in the wire section of the paperboard machine. The webs are then supplied by the headboxes to the forming wires.

[0018] A full-size paperboard machine includes a machine chest, an approach flow system, at least one headbox, a wire section, a press section, a drying section, a calender, and a pope reeler. The paperboard machine used in this disclosure includes at least two headboxes. The wire section may contain one or more forming wires. In this disclosure, where there is more than one headbox, there are several forming wires; each forming wire has its own headbox. In the forming section, the forming wire is an auxiliary filtration means during dewatering of the stock, resulting in a wet web that clearly shows the surface characteristics of the wire. In the press section, this formed web is mechanically dewatered under pressure. The full-size paperboard machine operates at a forming wire speed of several hundred meters per minute (e.g., at least 400 m / min).

[0019] The water is substantially free of binding chemicals. Specifically, the water preferably does not contain at least the binding chemicals starch, microfibrillated cellulose (MFC), and carboxymethyl cellulose (CMC). Binding chemicals refer to substances that improve the lamination bond strength in multi-layer paperboard. For the avoidance of doubt, this does not include starch added to any stock upstream of the headbox, i.e., not starch added to the stock. Lamination strength can be further improved by adding starch to the stock. Starch is preferably added in an amount of 1.5–7.0 kg / ton based on the dry weight of the stock, such as 2.0–6.0 kg / ton based on the dry weight of the stock.

[0020] Water is typically sprayed onto the surface of the first or second web at a rate of at least 35 g / m² to achieve a wetted surface in the web section. The amount of water sprayed can be even higher, such as at least 40 g / m², or at least 45 g / m². As understood by those skilled in the art, the water is sprayed evenly onto the surface of the first or second web. One method of evenly spraying water is to use a nozzle of model HU-9515 from Spraying Systems AB.

[0021] Typically, the first and second layer webs are dehydrated to a dry content of less than 30% before being sprayed with water. This is, for example, by partially dehydrating the webs by passing them through absorbent boxes. The inventors have recognized that even if the layers have been partially dehydrated, applying a certain amount of water to the first or second layer web according to this disclosure is sufficient to provide satisfactory mechanical properties in the z-direction. Therefore, a convenient process is provided because absorbent boxes are typically located downstream of the headbox in full-size paperboard machines, and these absorbent boxes must be disconnected or closed to provide a uniform web moisture content, which in turn can cause problems such as downstream dehydration during production.

[0022] The wetted surface is bonded to the surface of another web by roll bonding. Roll bonding is a process well-known to those skilled in the art, in which webs are pressed together to adhere to each other before final drying. Roll bonding is typically carried out at temperatures below 50°C, such as below 40°C. Therefore, roll bonding differs from hot pressing, and such a process is advantageous in terms of energy efficiency. The inventors have recognized that by spraying water, no external heat is required to bond the wetted surface to the other web.

[0023] Importantly, different lamellar adhesions are necessary to ensure satisfactory mechanical properties, particularly in the z-direction. Furthermore, unsatisfactory adhesion between lamellars (i.e., lamellar bond strength) can lead to at least partial lifting of the outer lamellar from one or more intermediate lamellars, causing surface defects in addition to other drawbacks.

[0024] Typically, when water is applied to the surface of the first or second layer of web to obtain a wetted surface in the web section, the moisture content of the web with the wetted surface increases by 0.5-2.0%. The moisture content of the web can be measured online, for example, by a near-infrared (NIR) sensor. The increase in moisture is measured by measuring the moisture content before and after water spraying.

[0025] The first layer of web may include chemical pulp. A preferred type of chemical pulp is kraft pulp, such as cork kraft pulp. Typically, the proportion of chemical pulp in the first layer of web is 15-80% by dry weight, such as 15-50% by dry weight. Including kraft pulp in the first layer of web is advantageous in terms of strength properties.

[0026] The first layer web may include chemithermomechanical pulp (CTMP). CTMP is a bulky mechanical pulp. CTMP can be bleached or unbleached. Even though CTMP is a relatively weak type of pulp, including chemithermomechanical pulp balances the strength because chemithermomechanical pulp is a relatively strong type of pulp. Therefore, it is preferable that if the first layer web includes CTMP, then the first layer web also includes chemithermomechanical pulp. Including CTMP in the first layer web is advantageous because it reduces the density of the paperboard, i.e., increases the bulk of the paperboard. High bulk is preferred because less fiber is used per gram of paperboard. In this disclosure, CTMP also includes high-temperature chemithermomechanical pulp (HT-CTMP). When producing HT-CTMP, a higher preheating temperature, typically above 150°C, is used to produce the pulp. In embodiments where the first layer web includes chemithermomechanical pulp (CTMP), CTMP is provided in proportions of 0-45% by dry weight, such as 0-40% by dry weight. When the first layer web includes CTMP, the first layer web typically forms the middle layer in a multilayer paperboard. In such cases, the second layer web typically forms the top layer in a multilayer paperboard.

[0027] Surprisingly, the inventors realized that if water, which is substantially free of binding chemicals, is sprayed according to this disclosure, mechanical properties in the z-direction, such as the ZD tensile strength and Scott binding, are provided at least to a level similar to that when a conventional amount of sheet-bound starch is applied between the sheets, which is typically higher, for example, about 1 g / m² or more.

[0028] Furthermore, the inventors have recognized that when measuring the Scott bond value of the z-direction properties of multilayer paperboard having at least three layers, failure typically occurs at or near the interface between the top and middle layers, or at or near the interface between the bottom and middle layers. Given that failure typically occurs at or near the interface between the top and middle layers, it is concluded that providing a strong interface between the top and middle layers to provide multilayer paperboard with good z-direction (ZD) tensile strength and Scott bond value is of paramount importance. This applies to multilayer paperboard produced on the paperboard machine used in the experimental section of this disclosure.

[0029] The internal bond strength (Scott bond) of multi-layer paperboard according to Tappi T 569 is typically at least 175 J / m², such as at least 190 J / m². The z-direction (ZD) tensile strength of multi-layer paperboard according to Tappi T 541 is typically at least 230 kPa, such as at least 300 kPa, such as at least 330 kPa.

[0030] A third layer web can be further formed in the wire section of the paperboard machine, and said third layer web forms the bottom layer in a multi-layer paperboard. That is, in such a case, the first layer forms the middle layer and the second layer forms the top layer. Water, free of adhesive chemicals, is typically sprayed onto the non-wet surface of the second or third layer web to obtain a second wet surface in the wire section, which is then combined with the non-wet surface of the second or third layer web by pressing with a crease roll. The amount sprayed is typically at least 35 g / m². The amount of water can be even higher, such as at least 40 g / m², or at least 45 g / m². Typically, in such a case, the amount of water sprayed onto the non-wet surface of the second or third layer web is the same as the amount of water sprayed onto the surface of the first or second layer web.

[0031] Waste pulp is formed from scraps and / or produced but damaged (or otherwise wasted) boards. The first-layer web may include waste pulp. In such cases, the proportion of waste pulp in the first-layer stock is typically 20-60% by dry weight, such as 25-50% by dry weight. A suitable Schopper-Riegler (SR) number for the waste pulp is 20-40, such as 22-38. Such an SR number can be obtained by adjusting the beating degree of the waste pulp. The SR number is measured according to ISO 5267-1:1999. Typically, the waste pulp in the first-layer stock is beated at 15-50 kWh / t to provide a specific SR number. The waste pulp is typically obtained from the methods of this disclosure, meaning it has the same fiber composition as multilayer paperboard.

[0032] The density of multi-layered paperboard according to ISO 534:2011 is typically 680-820 kg / m³, such as 690-810 kg / m³, such as 690-780 kg / m³.

[0033] The basis weight of multi-layered paperboard according to ISO 536:2020 is typically 130-500 g / m², such as 150-480 g / m².

[0034] The thickness of multi-layered cardboard, according to ISO 534:2011, is typically 220-800 µm, such as 250-750 µm.

[0035] This method may include the step of coating a multilayer paperboard with at least one coating layer. The at least one coating layer is typically applied to the top layer of the multilayer paperboard forming the printed side of the paperboard. The at least one coating layer is typically pigment-based. The total coating weight (dry weight) can be 12-30 g / m². 2 The top sheet can be coated with more than one layer, such as a two-layer or three-layer coating. Suitable pigments are clay and / or calcium carbonate and / or talc. In the case of a two-layer coating, drying can occur between the application of the first coating layer and the application of the second coating layer. For a single-layer coating, drying occurs after coating. Drying is typically performed using non-contact drying such as IR and / or hot air, or contact drying such as using (one or more) drying cylinders, or a combination of non-contact and contact drying. This disclosure is not limited to any particular coating technique, and therefore several different types of techniques can be used. Techniques include: blade coating, bar coating, air knife coating, rotary gravure printing coating, and / or curtain coating. All coatings can be applied in-line (also known as on-line). In such cases, productivity is increased by eliminating processing operations associated with offline processing and by eliminating or at least reducing the amount of waste. Alternatively, coating can also be applied offline.

[0036] Multi-layer paperboard can be used as containerboard, such as linerboard, or cartonboard, such as liquid packaging paperboard (LPB).

[0037] Example Three-layer cardboard is manufactured using varying amounts of sprayed water. In a full-size board machine, 3-layer board is produced from top layer stock, middle layer stock, and bottom layer stock. This composition of the board is referred to as "board 1". Board 1 is produced with two different substrate basis weights: 175 g / m² ("board 1a") and 248 g / m² ("board 1b"). The board is then coated with a coating weight of 19 g / m².

[0038] The pulp comprises softwood kraft pulp (SW kraft), hardwood kraft pulp (HW kraft), chemithermomechanical pulp (CTMP), and waste pulp, as presented in Table 1 below.

[0039] Table 1. Composition of each layer in cardboard 1.

[0040]

[0041] The top sheet stock, intermediate sheet stock, and bottom sheet stock are each supplied from separate headboxes onto separate forming wires to provide the top sheet web, intermediate sheet web, and bottom sheet web, respectively. Water is sprayed onto the back side of the top sheet web that will come into contact with the intermediate sheet web. Subsequently, the top sheet web and intermediate sheet web are bonded together by pressing with a crease roll.

[0042] For paperboard 1a with a basis weight of 175 g / m², the amount of water sprayed varies from about 25 g / m² (reference) to about 45 g / m² (Embodiment (IE) of the present invention), and for paperboard 1b with a basis weight of 248 g / m², the amount of water sprayed varies from about 25 g / m² (reference) to about 65 g / m² (Embodiment (IE) of the present invention). For each basis weight, the composition of the paperboard layers remains unchanged between the reference and the embodiment of the present invention.

[0043] The water sprayed on the back of the top sheet web appears as... Figure 1 and Figure 2 In. Figure 1 The image shows water sprayed on the back of the top layer of cardboard 1a, and... Figure 2 The image shows water sprayed on the back of the top layer of cardboard 1b.

[0044] When the amount of water sprayed is 25 g / m², the amount of starch sprayed is approximately 1 g / m² (reference), and as the amount of water sprayed increases, the amount of starch decreases to 0 g / m² (Example (IE) of the present invention). Since no starch is added to the aqueous dispersion, the amount of water increases. This is because the total flow rate remains the same, but the starch no longer constitutes a portion of the flow rate.

[0045] In addition, water is sprayed on the top surface of the bottom sheet web that will come into contact with the intermediate sheet web. Subsequently, the bottom sheet web and the intermediate sheet web that has been previously pressed with the top sheet web by the bottom roll are joined together by the bottom roll.

[0046] For paperboard 1a, the amount of water sprayed remains constant at approximately 25 g / m² (reference; IE), and for paperboard 1b, the amount of water sprayed varies from approximately 25 g / m² (reference) to approximately 40 g / m² (Example (IE) of the present invention). The sprayed water on the top surface of the film web is presented as... Figure 3 and Figure 4 In. Figure 3 The image shows water sprayed onto the top surface of the film layer in cardboard 1a, and... Figure 4 The image shows water sprayed onto the top surface of the film layer in cardboard 1b.

[0047] exist Figure 5 In the example, for cardboard 1a, the Scott bonding value measured according to TAPPI T 569 is presented. As mentioned above, in the reference, approximately 25 g / m² of water and approximately 1 g / m² of starch are sprayed onto the back side of the top sheet layer. In the embodiment of the present invention (IE), approximately 45 g / m² of water and 0 g / m² of starch are sprayed onto the back side of the top sheet layer. Figure 6 In the example, for cardboard 1b, the Scott bonding value measured according to TAPPI T 569 is presented. As mentioned above, in the reference, approximately 25 g / m² of water and approximately 1 g / m² of starch are sprayed onto the back side of the top sheet layer. In the embodiment of the present invention (IE), approximately 65 g / m² of water and 0 g / m² of starch are sprayed onto the back side of the top sheet layer.

[0048] Surprisingly, the Scott bonding value even increased when the amount of sprayed starch was reduced to 0 g / m² (i.e., in IE) and the amount of sprayed water was increased. It is known that sprayed starch is used for lamellar bond strength. The inventors therefore realized that if the amount of sprayed water is increased, the sprayed starch can be omitted. The moisture content of the top lamellar web was measured online before and after spraying with starch-free water, and the moisture content increased by approximately 1% by spraying with starch-free water.

[0049] To avoid any doubt, no other binding chemicals were added to the water to replace the starch; therefore, when water containing 0 g / m² starch was sprayed, only water was sprayed.

[0050] In addition, Figure 7 The paper presents the z-direction (ZD) tensile strength of paperboard 1a as measured according to TAPPI T 541, and the tensile strength of paperboard 1b in the z-direction (ZD) is shown. Figure 8 As shown in the figure, ZD tensile strength did not decrease due to reduced starch spraying, just as it did when starch was sprayed. On the contrary, it remained at roughly the same level as when starch was sprayed.

[0051] Manufacturing 3-layer paperboard with varying amounts of CTMP and starch in the intermediate layers. In a full-size board machine, 3-layer board is produced from top layer stock, middle layer stock, and bottom layer stock. The composition of the board is referred to as "board 2-4". The grammage range of the produced board is shown in Table 2 below.

[0052] The pulp comprises softwood kraft pulp (SW kraft), hardwood kraft pulp (HW kraft), chemithermomechanical pulp (CTMP), and waste pulp, as presented in Table 2 below.

[0053] Table 2. Composition of each layer in cardboard 2-4.

[0054]

[0055] All cardboard is produced with varying amounts of sprayed starch or no starch at all, as summarized in Table 3 below.

[0056] The amount of water sprayed is the same as that of paperboard 1, i.e., about 25 g / m² for reference, and about 45 g / m² or 65 g / m² for embodiments with reduced or no starch, depending on the paperboard basis weight. Paperboard with a lower basis weight (about 175 g / m²) is sprayed with 45 g / m² of water, while paperboard with a higher basis weight (about 248 g / m²) is sprayed with 65 g / m² of water.

[0057] Table 3 below summarizes the amount of starch sprayed on the back of the top sheet web and the top surface of the bottom sheet web.

[0058] For these paperboards, the mechanical properties were evaluated in terms of the z-direction properties, measured by the Scott bond according to TAPPI T 569 and the z-direction (ZD) tensile strength according to TAPPI T 541. Subsequently, for each type of paperboard, comparisons were made with their respective individual references (in which 25 g / m² water and 1 g / m² starch were sprayed onto the back side of the top sheet web).

[0059] Table 3. Starch addition and comparison of mechanical properties in the z-direction.

[0060]

[0061] *Based on the following estimates: 55% CTMP (paperboard 4) provided unsatisfactory z-direction mechanical properties without starch, while 40% CTMP (paperboard 2) provided similar z-direction mechanical properties with and without starch spraying.

[0062] These tests concluded that when the sheets contain CTMP and the CTMP content is higher than 45%, at least some amount of starch is required to provide satisfactory z-axis mechanical properties.

Claims

1. A method for forming a multi-layered paperboard in a full-size paperboard machine, the multi-layered paperboard comprising at least a first layer and a second layer, wherein the method comprises the following steps: - A first sheet web and a second sheet web are formed in the wire section of the paperboard machine; - Spray water, which is substantially free of adhesive chemicals, onto the surface of the first or second layer web to obtain a wetted surface in the web section; as well as - The wetted surface is combined with the surface of another sheet by rolling.

2. The method according to claim 1, wherein, The water is sprayed at a rate of at least 35 g / m², such as at least 40 g / m², to obtain the wetted surface in the mesh section.

3. The method according to claim 1 or 2, wherein, The water is substantially free of at least the binding chemicals starch, microfibrillated cellulose (MFC), and carboxymethyl cellulose (CMC).

4. The method according to any one of the preceding claims, wherein, The roll pressing is performed at a temperature below 50°C, such as below 40°C.

5. The method according to any one of the preceding claims, wherein, The moisture content of the web with the wetted surface increases by 0.5-2.0% by spraying water.

6. The method according to any one of the preceding claims, wherein, The first and second sheet webs are dehydrated to a dry content of less than 30% before being sprayed with water.

7. The method according to any one of the preceding claims, wherein, The first sheet web comprises chemical pulp, such as kraft pulp, such as cork kraft pulp.

8. The method according to claim 7, wherein, The proportion of chemical pulp in the first sheet web is 15-80% by dry weight, such as 15-50% by dry weight.

9. The method according to any one of the preceding claims, wherein, The first sheet web comprises 0-45% by dry weight, such as 0-40% by dry weight, of chemothermal mechanical pulp (CTMP).

10. The method according to any one of claims 7-9, wherein, The first sheet web forms the middle sheet in the multi-layered paperboard.

11. The method according to claim 10, wherein, The second sheet web forms the top sheet layer in the multi-layered paperboard.

12. The method according to claim 10 or 11, wherein, A third layer web is further formed in the wire section of the paperboard machine, and wherein the third layer web forms the bottom layer in the multi-layer paperboard.

13. The method according to claim 12, wherein, Water, which is essentially free of adhesive chemicals, is also sprayed onto the non-wet surface of the second or third layer web to obtain a second wet surface in the web section, and the second wet surface is then combined with the non-wet surface of the second or third layer web by roll pressing.

14. The method according to any one of the preceding claims, wherein, The Scott bond strength of the multi-layered paperboard according to Tappi T569 is at least 175 J / m², such as at least 190 J / m².

15. The method according to any one of the preceding claims, wherein, The multi-layered paperboard has a tensile strength in the z-direction (ZD) according to Tappi T541 of at least 300 kPa, such as at least 330 kPa.

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