WO2008092997A2

WO2008092997A2 - Method of producing paper

Info

Publication number: WO2008092997A2
Application number: PCT/FI2008/050033
Authority: WO
Inventors: Petri Silenius
Original assignee: M-Real Oyj
Priority date: 2007-01-31
Filing date: 2008-01-31
Publication date: 2008-08-07
Also published as: FI20075061A; FI119443B; WO2008092997A3; FI20075061A0

Abstract

Method of producing at least two different paper webs on one paper machine. According to the method during a first period of time, a first paper grade is produced and during a second period of time, a second paper grade is produced, said paper grades differing from each other with respect to their distribution of filler in direction Z. According to the invention, a filler-containing pigment slurry is applied on a wet paper web before drying and the feeding point of the pigment slurry is selected based on the desired distribution of the filler. By means of the invention, a change of quality can be achieved by changing the application point of the pigment particle suspension during the run, which provides for fast change of quality and small loss of material.

Description

Method of producing paper

The present invention concerns a method according to the preamble of claim 1 for producing at least two different paper webs on one paper machine.

According to a method of such kind, a first paper grade is produced during a first period of time and a second paper grade during a second period of time, said paper grades differing from each other with respect to the distribution of filler in direction z.

Paper has been produced, filled, coated and sized as well as calendered in may different ways. By filling a paper, the aim has generally been to increase its opacity, i.e. non- transparency, and by coating the aim has been mainly to improve printing properties of the surface.

With one and the same paper machine several paper grades can be produced by varying the fiber composition, the degree of fiber refining, the colouring agents, the filler, i.e. the filling pigment, the surface sizing, the surface coating and the degree of calendaring. A change of materials will generally consume considerable lengths of machine time and give rise to considerable amounts of reject.

When paper is coated it has often at least once already been calendered, i.e. smoothened between rolls, at some stage before coating. There are problems related to this. The coating colour is, viz., typically a dispersion or slurry of pigment, binder and water. Water applied on the paper web along with the coating colour thereby causes a considerable degree, up to 45 to 70 %, of swelling of the portions that have already once been smoothened by the calendar so that they will resume their original form. Further, the water which is brought together with the coating slurry has to be evaporated off a paper which has already once been dried.

There are also solutions known from before for adding filler or coating to paper on the wire portion or after it, but before drying by the application of heat, at a stage when the paper is still moist or wet. Solutions of this kind, where new material is brought onto the wet end of the paper machine have been presented e.g. in the article Alince B & al, Journal of Pulp and Paper Science, vol 28 (2002), no. 9 pp.315 - 319 and in the patent literature: US Patent Specification No. 2,112,540 discloses how modifying agents, for improving grease repellancy and strength are added to grease proof paper or similar, highly refined paper pulp on the wire of a paper machine.

US Patent Specification No. 3,560,334 teaches how paper strength can be improved by adding from a second headbox onto the surface of a web, which still is moist, dispersions of additives for improving strength.

In the solution according to FI Patent No. 49439, filler pigments having a size of 0.01 - 0.6 μm are sprayed by using a pressure of 1.2 to 2.4 kp/cm² onto the surface of a web at a position between the headbox of the paper machine and the first suction unit, whereby a uniform distribution of the pigment in direction z is attained.

In the "Hydra-Sizer" method of the company Black Clawson-Kennedy, additives are applied on the surface of the forming paper web in the wire section of a paper machine. As examples of the additives, starch can be mentioned. The Published International Patent Application No. WO 96/00816 concerns a corresponding method in which the coating colour which is being sprayed upon a forming paper web is sprayed using a pressure which is specifically limited to the range of 0.1 -10 psig.

US Patent No. 4,008,121 discloses an interesting arrangement wherein, using a curtain coater, kaolin is added as a pigment and starch as a binder between the layers of a multilayered paper or cardboard.

International Published Patent Application WO 99/32721 of Jefferson Smurfit Co discloses how uncooked starch mixed with water is fed to the wire area in such a way that its dosage on the edge parts is greater than at the middle. In this way the strength profiles of the web can be leveled out so that the edge parts are no longer weaker the middle part of the paper web. hi EP Patent No. 0 963 478 Bl by the same company it has been disclosed how starch is added to the wet end uncooked because then it can be fed in large amounts compared to starch dissolved by cooking which greatly increases the viscosity. Starch is fed in the form of an aerated suspension with a dry matter content of 2 - 10 % to a web held at a temperature of about 60 to 70 °C so that the structure of the web is not disturbed. The application takes place using a nozzle having the width of the web and which is positioned very close to the web.

No suggestion that the afore-mentioned methods could be applied to a change of paper grade can be found in the publications.

It is an aim of the present invention to solve at least a part of the problems relating to a change of paper grade. It is also an aim of the invention to provide a novel method for producing a paper which contains filler and which optionally is coated, wherein an economically inexpensive pigment is used both as a filler and in coating.

The invention is based on the concept of applying a pigment slurry which contains a filler on a wet paper web before drying and the feeding point of the pigment slurry is selected based on the desired distribution of the filler. The grade of the filler-containing paper can thus rapidly be changed by changing the feeding point of the additive or by changing the composition of the pigment slurry, whereby penetration in z-direction of the additive can be regulated by adjusting the selected point of application and/or the composition of the slurry. According to the invention, preferably a low-consistency slurry is used in order to achieve segregation of the pigment, whereby a good printing substrate is provided on the surface of the paper.

More specifically, the method according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by means of the invention. Thus, according to the invention, a change of grade can be performed by changing the application point of the pigment particle suspension merely by turning some valves, as a result of which the change of grade does not take any time or no material is lost. The pigments which are applied on the wet paper web can have a relatively large particle size distribution because by means of the invention it is possible to utilize the classification of the pigment which takes place on the surface of the paper and inside it, when the pigment is applied at a suitable position to the web on the wire. In this case, the smallest particles are retained on the surface which is desirable as far as printing is concerned if no separate coating is performed. A change of grade can be carried out for example when a change of roll is performed at the end of the paper machine.

Normally the paper is machine calendered before coating and calendered even after coating. This machine calandering reduces paper strength - as all calandering does - and a great part of the increased smoothness obtained therewith is lost in the following coating steps where the surface is swollen by the influence of water. This problem can, however, be avoided with the invention.

Normally, the aim is to carry out coating at as high a dry matter content as possible, usually > 60 %, the aim being even a dry matter content of up to 70 %.

So high dry matter content requires different, expensive dispersing agents which are applied in amounts of 1 % of the pigment. When coating is carried out by means of the method according to the present invention, the dry matter content of the pigment mixture can be much lower (at maximum about 50 weight-%, preferably at maximum 30 weight-%, in particular at maximum 20 weight-%, for example 0.5 to 15 weight-%) and the low dry matter content is used for achieving segregation in the pigment slurry.

The method gives, in particular, a special advantage when changing on the fly paper grade with respect to filler and coating. The method can be utilized in particular in cases where the pigment is supplied to the paper mill in dry form or when they are produced at the mill or in the direct vicinity thereto, such as at a PCC satellite plant, in which case it is easy to control the composition and the consistency of the pigment slurry at the paper mill.

In the following, the invention will be examined more closely with the aid of a detailed description.

hi the connection with the present invention "paper" stands generally for a planar "fibrous web" which is formed from a fibrous slurry, and for the corresponding sheets produced from such webs (i.e. "fibrous sheets"). The concept therefore also includes products which in other connections are called "cardboard" or "cardboard web" or "cardboard sheet". Generally the grammage of the paper can therefore vary within the range of 30 - 500 g/m². In the present invention, the concept "paper machine" stands for an apparatus which is suitable for producing the afore-described products and the concepts therefore includes both paper machines and cardboard machines.

As already stated, the invention provides a method of producing at least two different paper webs on the same paper machine. During a first period of time, a first paper grade is produced in the method, and during a second period of time, a second paper grade, the paper grades differing from each other with respect of the filler distribution in direction z. The filler is fed into the paper on the paper machine in such a way that the feed takes place to a moist paper web before drying. According to the invention a change of quality from the first paper grade to the second and vice versa can be performed by changing the feeding point of the pigment slurry or by modifying the composition of the filler slurry, or by a combination thereof, in order to regulate the penetration of the filler slurry into the paper.

According to a first embodiment, the paper grade is changed by adjusting the feeding point of the pigment slurry. In this case, it is particularly preferred to proceed by using, as far as pigment is concerned, the same kind of pigment slurry for producing different kinds of paper grades. The feeding point is transferred in order to obtain the desired material distribution in direction Z and depending on the drainability of the fibrous material.

In a second embodiment, the paper grades are produced from the same paper web and the same pigment slurry, the additives of the pigment slurry being adjusted depending on the desired distribution of the filler. In particular, the consistency or viscosity of the slurry is thereat being adjusted.

According to the invention up to 100 weight- % of the amount of filler or coating pigment is applied onto a moist paper web. The pigment slurry is then applied in toto on the surface of the paper web, whereby the composition of the slurry is made to correspond to , preselected surface properties by adjusting the species of the pigment or particle size distribution of it or by varying the addition of additives to the slurry.

It is also possible to feed a part of the filler in a conventional fashion, viz. by mixing it into the headbox of the paper machine. Similarly, a part of the pigment coating can be done after drvin_r. Even in these cases tvoicallv about 20 to 60 weisht-% of the filler is brouεht onto a moist paper web according to the invention. This amount allows for efficient adjustment of the composition of the paper and provides the change of grade aimed at by the invention.

There can be one or several application stations of the kind which will be described in more detail below, hi practice, on the same paper machine there are at least two fixed or two mobile or one fixed and one mobil application station for the pigment, binder or surface sizing agent on the same paper machine, but there can be several fixed and mobile stations. The position of them can be varied using interlinked piping.

According to a preferred embodiment of the invention at least one - preferably all application stations are mobile within specific limits in the longitudinal direction of the web (i.e. the direction of the movement of the web), whereby for different base paper webs fine tuning of the application points can be carried out. A greater degree of refining or a larger initial concentration in the fibre requires application earlier than a material which is less refined and which is fed from the headbox at a lower concentration to the wire.

hi order for the filler, coating, surface sizing or corresponding substance to be preferably changed on the fly, it is preferred according to the invention to have two pigment mixing and applications units per application unit, in particular if it is desired to change pigment grade. The units may comprise different mixing containers. When several application units are used simultaneously at the wet end, different pigments can be used as fillers for the pulp and different pigments in the coating.

At the application stations, the pigment can be applied as a filler in particular when the fibres have already started to "bond" together so that their physical position is not changed when the hydrogen bonds finally attach the fibres to each other.

According to a preferred embodiment, the filler used comprises particles which are roughly spherical, or even ball-formed. With these controlled penetration into the paper is achieved. The roughly spherical or ball-formed pigments used are for example ground calcium carbonate or precipitated calcium carbonate. According to the invention, the filler slurry is fed onto the moist paper web before the drying section.

The position of the application device or applicator is defined by the properties of the basic pulp. The paper web is preferably dried before the first feeding point to such a dry matter concentration that bonding of the fibres has taken place. This means that the fibres are already so well organized that the addition of the pigment does not essentially weaken the strength of the dried paper. Typically the paper web is dried to a dry matter concentration of at least 5 weight-%, in particular at least 15 weight-% or at least 20 weight-% before addition of the pigment slurry.

The location of the application point depends on, for example, the size of the pigment applied, the particle size distribution, the particle shape and density. Further, it is also influenced by the viscosity of the pigment slurry which depends on temperature and any binder polymers in the aqueous phase of the pigment slurry. For example, when cooked starch is used in the pigment slurry, the pigments can be kept very close to the surface and hardly any segregation, i.e. classification in direction z based on particle size, takes place.

In the wet end of the paper machine there is a so-called waterline, which is a point where the surface gloss of the water film ends and there is no free water film any more on the surface of the pulp. Application of a pigment layer intended only for coating or surface sizing is preferably performed at the latest close to this waterline or immediately before it. The position of the waterline depends on the degree of refining and the fibre composition of the paper. Thus, the physical position of the application station can be and advantageously also is modified when the basic grade of the paper is changed. A particularly preferred position is, depending on the velocity of the web, about 0.1 to 1.0 m, in particular about 0.3 to 0.7 m, e.g. about 0.5 m from the waterline towards the headbox.

The coating can be carried out so early that the pigment behaves as a filler. The coating can also be performed close to the water line, whereby the pigment preferably along with the binder acts as a coating. For minimizing any disturbance of the slurry on the formation of the web it is thereby advantageous to apply the slurry without contact onto the paper web, i.e. to apply by a nonimpact method. Examples of non-impact methods include spraying and curtain coating. Coating using a falling curtain is particularly preferred because then the whole amount of the coating / filler is evenly and with an efficiency of 100 % applied on the surface or inside the fibrous pulp, without splashes and any need for cleaning of the ambient. By curtain coating the dry matter of the coating slurry can be better adjusted than in spraying and then the ambient does not become dirty nor is there any need for cleaning.

The invention allows for free adjustment of the properties of the web. In a first application position, it is possible to apply pigment onto the web which at least partially penetrates into the fibrous network, whereby pigment and pigment binder which form a coating for the fibrous product are applied at a second application position. Preferably the fineness of the pigments used for the first and the second applications of the pigment products varies or the pigments are of different kinds with respect both to fineness and quality.

Tradition chemicals can be employed as additives for the filler/pigment slurry, whereby ^■ typically the slurry is a mixture of mineral pigment and polymeric organic material. In a preferred embodiment, at the feeding point located in the drier end of the wire section a material is applied onto the web which contains at least one component formed by an organic high molecular weight material which binds pigments or other solid substances dustlessly to the paper web. Examples of such materials include different starches and cellulose derivatives.

It is preferred to use said binder polymers, either soluble or dispersions or mixtures thereof, when the pigments are applied close to the end of the wire section because the pigments are retained at the surface. Surface sizing of the paper can be and is preferably performed with the same apparatus also at the wet end of the paper machine, because then it is also possible to do away with the need of more than one drying.

The paper web can be treated according to the invention either on one side or - preferably - on both sides. Thus, according to a preferred embodiment of the invention, the pigment slurry is applied on both sides of the web in order to achieve a symmetric filler profile in z- direction. In all of the afore-described embodiments, penetration of the pigment slurry inside the web can be enhanced by reduced pressure or centrifugal forces.

The invention can be used for producing an uncoated base web containing filler. By means of the method according to the invention it is also possible suitably to form a precoating on the paper web which is dried at the drying section of the paper machine.

Examples of paper grades that can be thus produced include uncoated and coated paper grades containing mechanical or chemical pulp. Specific examples are SC and LWC grades, folding boxboard and liner and other cardboard products.

According to a preferred embodiment of the invention, e.g. graphic base papers are produced wherein both filling of the paper and coating thereof can be carried out at the same time to obtain

- advantageous drying economy;

- the benefit of eliminating machine calendering,

- advantages with respect to the composition of the coating colour and

- the advantage of increased strength, all at the same time.

According to another preferred embodiment, cardboard having a grammage of about 120 - 500 g/m is produced.

It should finally be pointed out that by means of the method according to the invention it is possible from one unit to apply on the surface of the paper or to incorporate into the web any substance or chemical, polymer, adhesive or additive, which gives rise to special effects, which is water-borne or which can be formed into an aqueous emulsion. As examples of these, substances for pigment colouring or substances for emulsion dying or marking fibres can be mentioned. With the aid of the method it is therefore possible to produce bank note or other value paper materials. With the coating stations according to the invention, more fibres and pigments can naturally be applied on the base web, either together or separately, and the fibres can be coloured or uncoloured. Example

When pigment was applied from nozzles at low pressure in a consistency of 0.3 % to the point of a web where the consistency of the slush was also 0.3 % (initial consistency 0.2 %) a 30 % improvement of tensile index was attained compared to conventional filler application at the same value of refractive index. At the tests, the web speed of the machine was 900 rn/min, the pulp consisting of 50 % birch kraft pulp beaten to a degree of SR 24.2 and 50 % softwood pulp beaten to a degree of SR 24.2.

Claims

Claims:

1. A method of producing at least two different paper webs on one paper machine, according to which method - during a first period of time a first paper grade is produced and during a second period of time a second paper grade, said paper grades differing from each other with respect to their distribution of filler in direction z, characterized in that

- a pigment slurry, which contains a filler is applied on the wet paper web before drying and

- the feeding point of the pigment slurry is selected based on the desired distribution of the filler.

2. The method according to claim l,characterizedin that a change between different paper grades is attained by changing the feeding point of the pigment slurry.

3. The method according to claim 2, characterized in that for producing different paper grades a pigment slurry is being used which, with respect to the pigment, is similar.

4. The method according to any of the preceding claims, characterized in that the paper grades are produced from the same paper web and from the same pigment slurry, whereby the additives of the pigment slurry are selected based on the desired distribution of the filler.

5. The method according to any of the preceding claims, characterized in that at least 20 - 60 % of the amount of filler and coating pigment of the paper web are applied on a wet paper web.

6. The method according to any of the preceding claims, characterized by using a pigment slurry which contains at least essentially spherical pigments.

7. The method according to claim 6, characterized in that the spherical pigments used are ground calcium carbonate or precipitated calcium carbonate.

8. The method according to any of the preceding claims, characterizedin that the paper web is dried to a dry matter content of at least 5 weight-%, preferably at least 15 weight- %, before adding the pigment slurry.

9. The method according to any of the preceding claims, characterized in that the pigment slurry is applied to the wet paper web using an non-impact-method, e.g. by spraying or by curtain coating.

10. The method according to any of the preceding claims, characterized in that the pigment slurry is applied to both sides of the web in order to provide a symmetric distribution of the filler in direction z.

11. The method according to claim 9 or 10, characterized in that migration of the pigment slurry into the web is promoted by reduced pressure or centrifugal forces.

12. The method according to any of the preceding claims, characterized in that a pre-coating is formed on the paper web, which is dried in the drying section of the paper machine.

13. The method according to any of the preceding claims, characterizedin that uncoated or coated paper grades are produced, containing mechanical pulp or chemical pulp.

14. The method according to claim 13, characterized in that SC- or LWC-qualities, folding boxboard or liner or other cardboard qualities are produced.

15. The method according to claim 1, characterized in that when the entire pigment slurry is applied to the surface of the paper web, the composition of the slurry is regulated such that it corresponds to predetermined surface properties by adjusting the species of the pigment or the particle size distribution or by regulating the composition of the additives of the slurry.

16. The method according to any of the preceding claims, characterizedin that marking agents are added to the pigment slurry.

17. The method according to any of the preceding claims, characterized in that the dry matter concentration of the pigment slurry is at maximum about 50 weight-%, preferably at maximum 30 weight-%, in particular, at maximum 20 weight-%, e.g.0.5 - 15 weight-%.

18. The method according to any of the preceding claims, characterized in that a paper having a grammage of 30 - 500 g/m is produced.