JP2006265817A - Method for making low density multi-ply paperboard with high internal bond strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply. <P>SOLUTION: In the method, additives (a cationic fixative, an anionic starch, a cationic starch or the like) are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry containing cellulose fibers at less than zero. The paperboard having high ZDT, Scott Bond and Taber Stiffness but having a low density is obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

This application claims priority to US Application No. 60 / 664,440, filed March 22, 2005.
Field This application relates to increasing the bond strength of multilayer paperboard having a number of crosslinked cellulose fibers in at least one of the plurality of layers.
SUMMARY This application relates to improving the internal bond strength of paperboard having more than 25% crosslinked fibers in at least one layer. In this method, additives are added to the slurry in various combinations and sequences while maintaining the ion demand of the slurry below zero. A paperboard with high ZDT, Scott bond and Taber stiffness is obtained.
DETAILED DESCRIPTION Single layer or multilayer paperboard in which the inner layer contains more than approximately 25% crosslinked cellulosic fibers, the layer density will drop below 0.4 g / cc. As a result, its internal bond strength is not only well below the level required to process paperboard into a packaged product, but also requires processing for processing by conventional methods that increase internal strength. It can drop below a level that cannot provide a sufficient increase to meet the minimum level. The present application provides a method for increasing the internal bonding of low density paperboard to the extent that it can be used for processing.

  This application has demonstrated the use of high concentrations of wet end additives during the manufacture of low density paperboard.

  A distinctive feature of this application is that at least one layer of paperboard, whether single layered or multilayered, contains cross-linked cellulose fibers and strength enhancing additives such as anionic and cationic starches. Thus, the strength of the paperboard lost by adding the cross-linked cellulose fibers is compensated. Cross-linked cellulose fibers increase the bulk density, which is an insulating paperboard property of paperboard. Such paperboard also contains chemical pulp fibers. As defined herein, chemical pulp fibers that can be used in this application are derived primarily from wood pulp. Wood pulp suitable for use in the present application can be obtained from well-known chemical methods such as kraft and sulfite methods, with or without subsequent bleaching. Both coniferous and hardwood materials can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. For example, suitable cellulosic fibers made from Southern Pine that can be used in this application are available from a number of vendors including Wehrhauser, Inc. under the names C-Pine, Chinook, CF416, FR416 and NB416. . Bleached kraft Douglas fir pulp, KKT, Prince Albert coniferous wood and ground prairie coniferous wood (all from Werehauser) are examples of northern policy leafwood that can be used. When used with cross-linked cellulose fibers, mercerized fibers such as HPZ, mercerized flash-dried fibers such as HPZ III (both from Buckey Technologies, Inc., Memphis, Tennessee), and Leoneer Performance Fiber, Jessup, Georgia • Porosinier-J-HP available from the division is also suitable for use in this application. Other non-crosslinked cellulose fibers include Chemi-thermomechanical pulp fiber (CTMP), Bleached chemi-thermomechanical pulp fiber (BCTMP), Thermomechanical pulp fiber (TMP), Refiner groundwood pulp fiber, groundwood pulp fiber, Federal Way, Washington Wemphauser TMP (thermomechanical pulp) and CTMP (chemithermomechanical pulp fiber) sold as CTMP NORPAC Newsprint Grade obtained from Norpac, Longview, Washington, filed on August 20, 2004 Jet-dried cellulose fibers and treated jet-dried cellulose fibers manufactured by Wehrhauser, Inc. by the method described in US patent application Ser. No. 10 / 923,447. These fibers are twisted, twisted and curled. Additional fibers include flash dried fibers and treated flash dried fibers as described in US Pat. No. 6,837,970.

Suitable crosslinking agents for producing crosslinked fibers include carboxylic acid crosslinking agents such as polycarboxylic acids. US Pat. Nos. 3,526,048; 4,820,307; 4,936,865 for polycarboxylic acid crosslinkers (eg, citric acid, propanetricarboxylic acid, and butanetetracarboxylic acid) and catalysts. No .; and 5,221,285. The use of C ₂ -C ₉ polycarboxylic acids containing at least three carboxyl groups (eg, citric acid and oxydisuccinic acid) as crosslinking agents is described in US Pat. Nos. 5,137,537; 5,183, 707; 5,190,563; 5,562,740; and 5,873,979.

  High molecular weight polycarboxylic acids are also suitable crosslinking agents for producing crosslinked fibers. These include U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; No. 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981, And high molecular weight polycarboxylic acid crosslinking agents described in No. 739. Polyacrylic acid and related copolymers as crosslinking agents are described in US Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in US Pat. No. 5,998,511 and US application Ser. No. 09 / 886,821. In this study, CHB405, which is a citrate-crosslinked cellulose fiber, and CHB505, which is a polyacrylic acid-crosslinked cellulose (both are commercially available from Warehauser, Federal Way, WA) were used.

  Single or multi-layer paperboard structures use a mixture of wood pulp fibers and crosslinked cellulose. In one embodiment, the crosslinked cellulose fibers are present in at least one layer at a level of 25-80% based on the total fiber weight of that layer. In another aspect, the cross-linked cellulose fibers are present at a level of 40-75% based on the total fiber weight of the layer, and in yet another aspect, at a level of 50-70% based on the total fiber weight of the layer. To do.

  Single layer handsheets designed to mimic the middle layer of low density multilayer paperboard were produced. In these studies, a slurry having a consistency of 0.015% to 0.035% was used. The handsheet making machine was a standard 8 "x 8" (20.3 cm x 20.3 cm) sheet type modified with an enlarged headbox, so it used twice the normal stock volume did. This change is necessary to improve handsheet formation when using materials designed to provide high bulk, such as crosslinked cellulose fibers. Fiber weight is expressed as weight percent based on the dry weight of all fibers. The additive is based on the weight of the dry fiber.

To demonstrate the level of internal bond strength provided by the starch-added web, a series of handsheets were made using different levels of wet end additives and different order of addition. Additives were added to the slurry in order across each sample row, and the slurry was stirred for each addition. Avebe® AP25, an anionic starch, is obtained from Carolina Starches, Inc., North Charleston, South Carolina, both of which are cationic starches, Stay-Lok® 300 and Stay-Lok®. 330, a subsidiary of Tate and Lyle, Inc. of Decatur, Illinois. E. Obtained from Stayley, Kymene (R) was obtained from Hercules, Wilmington, Delaware, and RediBOND (R) 3050 was obtained from National Starch, Indianapolis, Indiana. Each handsheet was then coated with Celvol V24203 supplied by Celanese, Houston, Texas, which is a polyvinyl alcohol (PVA) coating. The total coating weight was about 50 g / m ² and was divided equally on each side of the sheet. Since the low density structure without coating tends to separate with tape rather than inside the sheet, a coating was added to the surface to facilitate testing of Z-direction pull (ZDT) and internal Scott bonds. Each sheet was evaluated for several physical properties including basis weight, ZDT, internal Scott bond and Taber stiffness (15 °). Table I below shows the results for these important features.

  ZDT (tensile in the Z-direction) strength was measured by TAPPI method T541 om-05, Scott bond strength was measured by TAPPI 569 om-00, and Taber stiffness was measured by TAPPI T489 om-04.

  Sample 1 is a fiber formulation designed to give low density paper and uses conventional wet chemistry. The result is a very low ZDT and Scott bond. Sample 2 shows that adding 4% total starch to the composition essentially doubles ZDT and Scott Bond. Sample 3 doubles the amount of anionic starch, maintains the cationic starch constant, and has a higher ionic demand than cationic starch (+0.3 meq / g, Table III) (+2.2 meq). / G) when Kymeme® 557H is used to balance the charge of the additional anion, the internal bond is further increased, the ZDT is increased by 500% over the control, and the Scott bond is approximately 2 It shows that it doubles.

  The data show that anionic starch combined with a cationic fixative allows effective retention of starch in the wet end. As defined herein, a fixative is a charged polymer that binds by ionic bonds to molecules of opposite charge. Examples of fixatives include, but are not limited to, cationic starch, polyalanine, polyaluminum chloride® (PAC), polyDADMAC (polydiallyldimethylammonium chloride), EPEDMA, Kymene®, PAAE. (Polyamidoamine-epichlorohydrin), PASS (polyaluminum silica slate), PEI (polyethyleneimine), and cationic polyacrylamide.

  In one embodiment, a cationic fixative is added to the slurry, followed by the addition of anionic starch and then the cationic starch. In each case, after adding the additive, the ion demand of the slurry is less than zero. In another embodiment, the cationic starch is added to the slurry before the anionic starch and the cationic fixative is added after the anionic starch. In yet another embodiment, a cationic fixative is added to the slurry and anionic starch is added to the slurry after the cationic fixative is added. In each case, after adding the additive, the ion demand of the slurry is less than zero. In yet another embodiment, the anionic starch is added to the slurry prior to the addition of the cationic fixative. In each case, after the addition of starch or sticking agent, the ionic demand of the slurry is less than zero. A combination of anionic and cationic starches can also be used. In one embodiment, cationic starch is added to the slurry, followed by the anionic starch. In each case, after adding starch, the ionic demand of the slurry is less than zero. In yet another embodiment, anionic starch is added to the slurry, followed by cationic starch. After adding starch, the ionic demand of the slurry is less than zero.

  In one embodiment, Kymene® is added at a level of 2.5 lb / t to 20 lb / t (1.1 kg / t to 9.1 kg / t). In another aspect, 5 lb / t to 15 lb / t (2.3 kg / t to 6.8 kg / t), and in yet another aspect, 8 lb / t to 10 lb / t (3.6 kg / t to 4.5 kg / t). ) Add at the level.

In general, the effect of starch addition on Taber stiffness at 15 ° bend is small. For single-layer handsheets, this is reasonable because thickness is the dominant variable that provides bending stiffness. The effect of starch addition on the density is small enough that the increase in sheet modulus with starch addition compensates for the small change in thickness. In a multi-layer web, the same response is expected.
Comparison of different anionic and cationic starches Table II shows the basic formulation and conditions used in making handsheets in this study.

  In these studies, the addition levels of both anionic and cationic starches were varied as well as the ionic demand of starch. By ion demand is meant the number of charges (both positive or negative) required to zero the net charge for a given volume of composition. By cation demand is meant the amount of cation charge required to bring the sample to a net zero charge. Samples with a cationic demand are net anionic. By anion demand is meant the amount of anionic charge required to bring the sample to a net zero charge. Samples with anionic demand are net cationic.

Aniofax® AP25 (−0.25 meq / g ion demand) was replaced with Hercules RediBOND® 3050 with a low ion demand of ˜−0.19 meq / g. In addition, Sta-Loc® 300 with an ion demand of +0.30 meq / g was replaced with Sta-Loc® 330 with an ion demand of about +0.41 meq / g. The results shown in Table II indicate that the ZDT of RediBOND® 3050 with the same mass as Aniofax® AP25 is about 10% lower. This reduction is believed to be due to the lower retention of starch addition due to the low charge contribution of RediBOND® 3050. However, the Scottbond and Taber stiffness results are approximately equal (sample numbers 5 to 6, 7 to 8 and 9 to 10).
Ion Demand Balance This technology depends on the ability to balance the ion demand at the wet end of the paper machine: 1) Anionic high molecular weight material is retained on the fibers and fine particles without remaining excessively in the water system 2) The fibers and system do not pass through a zero charge point that destabilizes retention and drainage, and 3) Since the pulp fibers are anionic, cationic materials can be added, Adding too much cationic material without balancing the excess anion demand will agglomerate the fibers to reduce formation and / or reduce drainage and affect production capacity.

Each component used in the paperboard containing cross-linked fibers in the present disclosure typically has a specific charge density measured by ionic demand titration. A Mutek PCD-titrator was used in conjunction with a PCD 02 particle charge detector to measure the ion demand of the component or fiber structure. The method is described in A. A., a subsidiary of Tate and Lyle, Inc. of Decatur, Illinois. E. The procedure was performed according to the staying manufacturing company. The method is as follows.
1. Activate the mutex using the power switch on the back of the device.
2. Place 10 mL wells with mixed sample in sample container. Insert the plunger and washer into the container. The sample consistency is 0.83 or less. If the stock sample is dark, dilute it.
3. When the device is activated, the plunger moves up and down and the mV potential is displayed. The potential symbol (+ or-) indicates whether the sample is cationic (+) or anionic (-).
4). The sample is titrated with a suitable titrant until the mV potential is 0 mV (polyDADMAC is a cationic polymer and is used for titration of anionic samples; PVSK or PESNa is an anion used for titration of cationic samples Polymer). A burette or syringe can be used to inject the titrant into the sample. Titration should not be done with titrants above 4 mL. This is because the measurement becomes inaccurate with a larger volume. If the sample requires more than 4 mL of titrant, dilute the sample or use a higher concentration titrant.
5. Record the amount of titrant used to titrate the sample. To calculate system demand, use the following equation:
“Ion demand” (ueq / L) = (mL titrant) × (% titrant dilution) × (sample dilution)
Specific components obtained by measuring the ion demand by the mutex method are shown in Table III. Other component values are from suppliers.

  Referring to the table, the difference between the “total” and “available” ion demand represents the amount of charge that is inaccessible to a polymer that is inside the fiber and has a molecular weight above 300,000 g / mol. In the case of papermaking, the available ion demand is more representative than the total ion demand among the results actually obtained.

  The situation is even more complex at the wet end of the paper machine, where dilution water from external sources and / or wash water from the pulp mill bleaching stage carries ionic substances (both dissolved and dispersed). Contains and is used to control the consistency of the pulp slurry. In an integrated plant where there is an excess of ionic material, the material added to the pulp slurry to increase the internal bond strength will be consumed by the excess ionic material. Also, the available ionic sites on the pulp will depend on how much refining has been done and the basic fiber morphology, i.e. the smaller the fiber or partial fiber, the more available The area required increases, so the available ion demand is high.

In general, it can be said that the fiber slurry starts with anionic but should remain anionic throughout the papermaking process, ie the ionic demand of the slurry should be less than zero.
Balance Calculation For the purposes of this study, the ion demand for the formulations shown in Tables I and II were calculated using the ion demand values in Table III. Ionic materials in the slurry other than those shown were ignored.

  The order and amount of addition of ionic additives should determine the ionic demand of the stock composition at any point during the forming process. For these experiments, the estimated ion demand is shown in Tables IV and V.

  Table VI shows ion demand measurements for similar combinations of additives at 0.5% slurry consistency using mill white water for dilution.

  When all of the cationic material was added before the additional anionic starch, the ionic demand of the system crossed the zero charge point (Table VI 12 and 14 and FIG. 1). This is known to destabilize particulate retention and affect agglomeration and drainage at the wet end of the paper machine, resulting in a decrease in ZDT. In FIG. 1, the addition points are as follows: A is the pulp stock structure, B is the addition of Kymene®, and C is Hercules RediBOND (Registered for 13, 15 and 16). (Trademark) 3050 is added, 12 and 14 are Sta-Loc (R) 300, and D is Hercules RediBOND (R). Samples 11-16 were manufactured with CHB405, and Samples 17-19 were manufactured with CHB505. Samples 16 and 19 each contained 0.5 lb / t (0.2 kg / t) of PSM Particol® BX added after the addition of HRB® 3050. Comparing a sample made with CHB405 (citric acid cross-linked fiber) and a sample made with CHB505 (polyacrylic acid cross-linked fiber) (Table VI 15 and 16 vs. 18 and 19, FIGS. 2 and 3) It can be seen that there is a slight difference in ion demand between the crosslinked cellulose fiber grades. The net result has no specific effect on the number of sites available to retain binding material. This is supported by the fact that the addition of the same amount of Kymene® results in the same ion demand.

Single or multi-layer paperboards are made using cross-linked fibers such as polyacrylic acid cross-linked fibers (CHB505) with a slurry consistency of 3 to 3.2% and a dry weight ratio of 50/50 and Douglas fir fibers. Can do. A slurry consistency of 0.05% to 4% can also be used for the production of paperboard. The term “consistency” means the percent solids of the solid and liquid mixture, for example, 2% consistency means that there are 100 g of fiber and 2 g of cellulose fiber in the liquid. The dry weight as defined herein means that a representative fiber is dried at 105 ° C. ± 2 ° C., and the weight is measured every hour until the weight becomes constant. In practice, the water content of commercial grade fibers is approximately 9%. Douglas fir can be refined 500 CSF or lower before adding to the paper machine chest. A standard 4 mm barrier screen can be used in front of the headbox. The addition points for various additives such as Kymene (R), anionic starch and cationic starch are shown in FIG. Paperboard to be formed is able to have a basis weight of 200-500 g / ^{m 2,} 15-25% by weight based on the dry weight of each of the surface layers the total fiber and the center layer is the total fiber dry weight of 50% to 70% It is. In one embodiment, a single layer paperboard is made. In another aspect, a paperboard having at least two layers is made. Cross-linked cellulose fibers can be in both layers or in only one layer. In yet another aspect, a paperboard having three layers can be made. These layers may all be the same, all different, or a combination of various layers.

  Starch and other additives can be added at the locations shown in FIG.

  As shown in FIG. 5, the surface layer can be formed separately from the intermediate layer with a stock consistency of ~ 0.6% using conventional long web technology.

  The intermediate layer can be formed directly on the bottom layer on the long web table using a beloit top former. Dehydration can be performed by a top wire drainage device.

  The top layer can be combined with the bottom layer and the intermediate layer via a pick-up roll. The combined web is then subjected to a typical paper machine unit operation, wet pressing in a conventional felted wet press, steam can drying, up to 2 to 2.5% starch based on fiber dry weight. It is processed by a size press that can be applied to the web, followed by steam can drying to 6.5% moisture, wet rolling, dry rolling and winding.

FIG. 1 shows the effect of various chemical additions on the ion demand of 3-3.2% slurry. FIG. 2 shows the effect of various chemical additions on the ion demand of the intermediate layer simulation using polyacrylic acid crosslinked cellulose fibers. FIG. 3 shows the effect of various chemical additions on the ion demand of the intermediate layer simulation using citrate-crosslinked cellulose fibers. FIG. 4 shows a schematic diagram of a paper machine chest and additive entry point suitable for carrying out the method of the present application. FIG. 5 shows a multilayer board making machine suitable for multilayer products.

Explanation of symbols

1 Middle Layer Paper Machine Chest 2 Middle Layer Paper Line-Refined 3 Chemical Additive Inlet 4 Middle Layer Paper Machine Chest Recirculation Pump 5 Stock Line-Cross-linked Fiber 6 Stock Line-Combined Intermediate Layer Paper Material 7 Main barrier screen for intermediate layer 8 Fan pump for intermediate layer 9 Head box for bottom layer 10 Drain device for bottom layer 11 Head box for intermediate layer 12 Drain device for intermediate layer 13 Head box for top layer 14 Drain device for top layer 15 Combined web continues to wet press section

Claims (21)

Next steps:
Forming a slurry of cellulose fibers including crosslinked fibers;
Adding a cationic fixative and mixing with the slurry;
Adding anionic starch after adding said cationic fixative;
Adding cationic starch after adding said anionic starch;
Here, after each addition step, the ion demand of the slurry is less than zero;
Forming a fibrous web layer by removing liquid from the slurry;
A method for forming paperboard, comprising drying the web to form paperboard.   The method of claim 1, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 25-80% of the total fiber weight.   The method of claim 2, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 40-75% of the total fiber dry weight.   The method of claim 1, wherein the total starch level is 50-120 lb / t (22.7-54.4 kg / t).   The method of claim 1, wherein the total starch level is 60-100 lb / t (27.2-45.4 kg / t).   The method of claim 1, wherein the total starch level is 80-90 lb / t (36.3-40.8 kg / t).   The method of claim 1, wherein the cationic starch is added to the composition before the anionic starch and the cationic fixative is added after the anionic starch. Next steps:
Forming a slurry of cellulose fibers including crosslinked fibers;
Adding a cationic fixative and mixing with the slurry;
Adding anionic starch after adding said cationic fixative;
Here, after each addition step, the ion demand of the slurry is less than zero;
Forming a fibrous web layer by removing liquid from the slurry;
A method for forming paperboard, comprising drying the web to form paperboard.   9. A process according to claim 8, wherein anionic starch is added to the slurry before the cationic fixative.   9. The method of claim 8, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 25-80% of the total fiber weight.   9. A method according to claim 8, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 40-60% of the total dry fiber weight.   9. The method of claim 8, wherein the total starch level is 50-120 lb / t (22.7-54.4 kg / t).   9. The method of claim 8, wherein the total starch level is 60-100 lb / t (27.2-45.4 kg / t).   9. The method of claim 8, wherein the total starch level is 80-90 lb / t (36.3-40.8 kg / t). Next steps:
Forming a slurry of cellulose fibers including crosslinked fibers;
Adding cationic starch and mixing with the slurry;
Adding anionic starch after adding said cationic fixative;
Here, after each addition step, the ion demand of the slurry is less than zero;
Forming a fibrous web layer by removing liquid from the slurry;
A method for forming paperboard, comprising drying the web to form paperboard.   The method of claim 15, wherein anionic starch is added to the slurry prior to the cationic fixative.   16. The method of claim 15, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 25-80% of the total fiber weight.   16. The method of claim 15, wherein the crosslinked fibers are present in at least one layer of paperboard at a level of 40-60% of the total dry fiber weight.   16. The method of claim 15, wherein the total starch level is 50-120 lb / t (22.7-54.4 kg / t).   16. The method of claim 15, wherein the total starch level is 60-100 lb / t (27.2-45.4 kg / t).   16. The method of claim 15, wherein the total starch level is 80-90 lb / t (36.3-40.8 kg / t).

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