Sizing method for board

Abstract

A method for managing the water sorption of a board by layered sizing is provided, such that the sizing of a central layer is effected with a hydrophobic and fast-curing adhesive, and the surface sizing in preparation for calendering is effected with an adhesive which is hydrophilic during its curing period, such that, during a surface wetting process, water sorption occurs substantially only into a topliner.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase application of PCT/FI02/01021; filed 13 Dec. 2002, and claims priority to Finnish Application No. 20012511; filed 19 Dec. 2001, which are incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the management of a surface moisture level required for example in a finishing process for liquid packaging board by using two or more different adhesives for the treatment of various layers.

2. Description of Related Art

Surface Sizing in General

Surface sizing is usually effected in a paper machine in a film size press, which is positioned at such a distance on a dryer section that the paper will have a sufficient dryness to withstand the sizing process. A size press can be used for applying a starch solution, CMC, an adhesive or other chemicals to the surface of paper. Even fillers can be admixed in the solution. Coating with these substances, as opposed to admixing within the pulp, results in a total retention and a dense material layer for web surfaces. Thus, surface strength is improved and dusting is reduced. Paper or board will have more strength and smoothness, and the subsequent coating will be more successful. Surface sizing is used for fine papers, coating raw stock, and board.

Surface Sizing in Principle

In surface sizing, a web surface is supplied with a mixed adhesive (e.g. starch or AKD) by using a film size press. A function of the adhesive is to bond raw materials contained in the web with itself. For example, the adhesive qualities of starch are based on its basic structure, on numerous OH groups contained in glucose units and capable of forming hydrogen bonds. In solution, the hydrogen bonds form between water molecules and starch. Thus, the starch functions as an adhesive. The sizing of liquid packaging boards is naturally effected with insoluble adhesives, for example resin adhesives, ASA (alkylsuccinic anhydride), or AKD adhesives.

Surface Sizing Equipment

In practice, the moisture of paper downstream of a size press is 40-50%, its strength being seriously impaired and its susceptibility to breaking being increased. The prior used size presses are currently replaced with sizing equipment making use of a film transfer method. The film size press is used to apply no more than the exactly desired amount of adhesive with applicator devices for a thin film on roll surfaces, and the rolls are used to transfer the adhesive to paper. Thus, there will be no puddle, the amount of adhesive can be reduced, and consequently the need for second drying is diminished. Hence, in the inventive solution, the sizing is performed two or even more times. After the sizing of inner layers, the board no longer becomes soaked through, which is why it has a high wet strength in the calendering process.

Objective Of Surface Sizing

Primarily, an improvement regarding the surface strength of paper, and properties of paper consistent therewith (e.g. non-dusting). In addition, changing also other properties of paper in a desired direction: Improving co-planar strengths (with the exclusion of tearing strength), reduction of porosity,

• • modification of absorption properties
  • improvement of dimensional stability
  • improvement of smoothness
  • improvement of glazing properties
    Surface Sizing Technique (On-Machine) Can also be Used for:
  • pigmenting of paper
  • coating of paper (precoating)
  • manufacture of special grades.

As far as liquid packaging boards are concerned, important aspects are of course liquid tightness, reduction of absorption, printability of surface.

Coating

Coating is used for improving the printing qualities of paper and board—primarily smoothness, gloss and brightness. However, the properties of base paper are also crucial for those of coated papers and boards.

Coating Process

Finishing methods for paper and board can be divided according to the amount of a material transferring to the surface and the composition of a finishing or glazing agent in three different methods: surface sizing, pigmenting, and coating. In a coating process, to the surface of paper or board, either on one or both sides, is applied a coating paste, which contains pigment (e.g. kaolin, talc, carbonate) and a binder (e.g. starch, latexes). Paste is spread, i.e. applied, to the surface of paper or board in excess. The coating amount is adjusted as desired by removing excess paste. After adjusting the amount of coating, the coating is still dried. Printing quality can be improved by coating paper or board several times. High-grade printing papers can be coated up to three times. Surface adhesive does not contain any pigment at all, and only a very small amount (1.0-2.0 g/m²) of coating transfers to the surface of paper or board in the pigmenting process.

Coating Methods

The most common coating method is blade coating in its various forms. In addition to blade coaters, the coating can also be effected e.g. with a rod coater, a film transfer coater, and an air brush coater.

On-Machine And Off-Machine Coating

Pigment coating can be performed at the mill either with a paper or board machine (on-machine coating) or with a separate coating machine (off-machine coating).

Stuff sizing is effected by admixing adhesive in wet-end pulp. Thus, the web will be sized throughout its thickness. In the inventive method, the sizing of a topliner and the sizing of a central layer are thus performed by joining for example three layers together, the central position being occupied by a fast drying layer which is hydrophobic after drying. The surface layers are sized with a hydrophilic adhesive, which is uncured during wetting and calendering. The inventive method can naturally be also used for making just a one-sided surface layer. In this case, the reverse side is hydrophobic during wetting. The inventive method enables an improvement of calendering and coating properties and an improvement of runnability by virtue of reduced susceptibility to breaking. The surface is quickly absorbed with water, and the water-soluble coating layers have a high adhesion. At the same time, however, wetting is almost nonexistent with regard to the already cured base and/or back element sized with a quick-setting adhesive and, thus, it retains a high strength despite wetting.

Calendering

A web emerging from the dryer section of a board machine is not applicable as such to its ultimate application, but it requires further treatment. The most common further treatment is calendering. Calendering is performed by passing the web through a nip system established by two or more rolls. Board can be calendered:

• • in board machine, with on-machine calender
  • in finishing, with off-machine calender
    Purpose of Calendering

Calendering could be said to have three principal functions:

• • improvement of board surface properties (smoothness, gloss) for improving its printing or other processing qualities
  • adjustment of board thickness for producing board of a desired density
  • equalization of board thickness profile for a slitter to produce uniform rolls.
    Calender Types
    Machine Calendering

The type of calendering effected on a board machine immediately after drying is referred to as machine calendering. In machine calendering, the web is passed through one or more nips established by hard-surfaced cast iron or steel rolls.

Intermediate Calendering

In addition to a machine calender, the board machine can be provided with an intermediate calender. It is located on the dryer section, generally upstream of the final dryer group. The intermediate calender includes a single hard nip.

Soft Calendering

Soft calender refers to a mechanism, wherein at least one of the rolls establishing a nip is an elastic, polymer-coated roll, and each nip is separately loaded. Indeed, soft calenders are mostly on-line machines, i.e. integrally built in a paper, board or coating machine. The number of nips in soft calenders ranges from one to four.

Long Nip Calendering

Long nip calendering is based on the utilization of very soft coating belts and shoe press technology. Long nip calendering is particularly applicable to board calendering, wherein saving of bulk is especially important. In a long nip calender, one of the rolls is a so-called shoe roll or belt solution and the other is a thermal roll. In long nip calendering, the width of a nip does not depend on loading or roll diameters, but it is constant. This enables the optimization of a nip time and a nip pressure regardless of each other.

General Information Regarding Manufacture of Liquid Packaging Boards

Liquid packaging boards are sized with wet strength adhesives for limiting the liquid absorption thereof. Typically, the sizing is effected with ASA, AK13, and resin adhesives, as well as combinations thereof. ASA and resin sizing develop to an almost full strength as early as on the dryer section of a paper machine. Thus, in practice, paper reaches a full sizing degree at the end of a dryer section. As a result of this the product's water sorption potential falls substantially.

AKD sizing does not develop fully until a few days after the manufacture of paper. Downstream of the dryer section, paper is practically unsized, and its water sorption can be at the level of 200 Cobb.

If it is desirable to enhance pre-calendering of board, this is feasible by raising the moisture of paper. Surface wetting of board has been found a particularly effective way of doing this. Surface wetting is preferably performed with a spray device or e.g. with a surface-sizing apparatus.

In terms of surface wetting it is naturally important that water be retained in the surface layer or topliner of board. As the grammage of board and the proportion of mechanical pulp contained therein increase, the amount of required wetting water will also increase. Consequently, the containment of water in a topliner (30-60 g/m²) is not self-evident.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for managing the surface wetting of liquidproof board in a controlled fashion in anticipation for pre-calendering. One problem is that, in preparation for calendering, the paper surface should be enabled to absorb water for more effective calendering, and a sizing method generally applied in the process of making liquid packaging board practically blocks the absorption of water in paper surface. Therefore, it is difficult to achieve a good calendering result. It is characteristic of the inventive solution that the sizing of a central layer and/or a back layer is performed with a hydrophobic and fast-setting adhesive and the sizing of a topliner with an adhesive which is hydrophilic during its setting period, such that, during the process of surface wetting, the water sorption essentially occurs into the desired topliner only. Thus, the topliner is enabled to absorb water, and the hydrophobic sizing of the inner ply blocks the absorption of water deeper than the board surface. At the same time, the strength is improved during a calendering process and the susceptibility to breaking is reduced, since only the surface is moist.

The inventive method is used for example for making three-layer board, such that each layer emerges from its own wet end. The central layer pulp is admixed for example with a fast-setting resin adhesive and the topliners with a slow-curing AKD adhesive (alkylketyldimer). The layers are pressed together and, at that point, the resin adhesive of the central layer has had time to set prior to wetting and has developed into a highly hydrophobic central layer in board. The AKD adhesive of the topliner is still undeveloped and hence hydrophilic. In this case, the adhesive for all layers is thus admixed within the pulp. Wetting can be preferably effected for example as a spray wetting. Thus, during the wetting process, water is well absorbed only in the topliner, and the subsequent calendering, by virtue of a high moisture content, provides a high-quality smoothness and calendering result by using a temperature and nip pressure lower than those applied in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

The inventive procedure is used to block the penetration of water deeper than the topliner of liquid packaging board.

The central and back layers of board are sized with ASA or ASA-resin sizing. The adhesive can be admixed in pulps at the initial stage of manufacture. Thus, the central and back layers are totally hydrophobic during a surface wetting process. The board surface is sized with AKD (or some other corresponding stuff sizing agent of the type AK13 and/or a derivative thereof). Hence, the topliner will be hydrophilic during a surface wetting process. Thus, the topliner absorbs water very quickly. As the amount of water is increased, the topliner has its saturation degree rising, yet water is not able to penetrate deeper than the above-mentioned top layer. Since the central layer remains dry, the susceptibility to breaking and process control are good. Since the surface is saturated with water, a good calendering result is obtained with a lower nip pressure and temperature. This procedure improves essentially the surface properties and bulk of board at a constant level of PPS (indicator of surface smoothness). This is particularly important with regard to liquid packaging boards as it provides a high adhesion for a very thin coating foil. For example, the adhesion of a foil, having a thickness expressed in microns, to a bottom layer necessitates a smooth surface layer.

Since the inventive method enables the use of lower pressures, it is respectively possible, by virtue of smaller masses in the machine and improved management thereof, to increase the machine speed, while improving the quality of resulting board.

Claims

1. A method for managing water sorption of board by layered sizing, wherein sizing of at least one of a central layer and a back layer is effected with a hydrophobic and fast-curing adhesive, and sizing of a topliner is effected with an adhesive, the adhesive being hydrophilic during a curing period thereof, such that, during a surface wetting process, water sorption occurs substantially only into the topliner.

2. A method as set forth in claim 1, wherein calendering is effected after the wetting process.

3. A method as set forth in claim 1, wherein the method is used for manufacture of liquid packaging board.

4. A method as set forth in claim 1, wherein the adhesive used as a topliner adhesive comprises a slow stuff sizing adhesive.

5. A system for the manufacture of board, wherein the system sizes at least one of a central layer and a back layer with a hydrophobic, fast-curing stuff sizing adhesive, and sizes a topliner with a stuff sizing adhesive, the stuff sizing adhesive being hydrophilic during a curing period thereof, and, during a surface wetting process, water sorption occurs substantially only into the topliner.

6. A system as set forth in claim 5, wherein the wetting mechanism comprises a spray device.

7. Board manufactured by layered sizing, wherein at least one of a central layer and a back layer is sized with a hydrophobic and fast-curing adhesive, and surface sizing of the board is effected with an adhesive, the adhesive being hydrophilic during a curing period thereof.