WO1999061701A1

WO1999061701A1 - Method for forming of a multi-ply paper or board product

Info

Publication number: WO1999061701A1
Application number: PCT/SE1999/000897
Authority: WO
Inventors: Bengt NORDSTRÖM
Original assignee: SCA Research AB
Current assignee: SCA Research AB
Priority date: 1998-05-26
Filing date: 1999-05-26
Publication date: 1999-12-02
Anticipated expiration: 2000-11-26
Also published as: SE9801846D0; SE9801846L; SE512414C2; AU4541399A

Abstract

A method of forming a multi-ply paper or board product having a favourable geometric mean value of mechanical properties such as tensile strength, tensile stiffness and compression strength, comprising forming at least one ply with a relatively low fibre orientation anisotropy and forming at least one other ply with a relatively higher fibre orientation anisotropy, said plies being combined into a multi-ply web in which the difference in fibre orientation anisotropy defined as the ratio of major to minor axes b/a of the distribution pattern profile between the individual plies is at least 0.3, and the ply having a relatively low fibre orientation anisotropy is formed from a furnish having a relatively lower shrinkage tendency as compared to the furnish used for forming said ply having a relatively higher fibre orientation anisotropy.

Description

METHOD FOR FORMING OF A MULTI-PLY PAPER OR BOARD PRODUCT

Technical field

The present invention refers to a method of forming a multi-ply paper or board product having a favourable geometric mean value of mechanical properties such as tensile strength, tensile stiffness and compression strength.

Background of the invention

The contraction of a web in the cross direction (CD) during the conventional cylinder drying operation in the paper manufacturing process is a well-known problem. The web contracts both as a result of the Poisson effect related to the web tension in the machine direction (MD), and due to shrinkage of the network as the web dries. The shrinkage is most pronounced at the edges where a minimum of forces counteract the web contraction and the conditions resembles free drying. The conditions in the centre of the web on the other hand resembles restrained drying in CD.

The uneven shrinkage across the width of the web causes an uneven grammage profile if no corrections are made. Usually however the feed of fibre material from the headbox is regulated in the cross-machine direction, i e less fibre material is fed at the edges than in the centre of the web. Besides the problems related to the control of grammage variations, the CD-shrinkage causes a drop at the edges in the CD-components of mechanical properties such as tensile strength, tensile stiffness and compression strength.

One way of minimizing shrinkage and thus equalizing the profile of the mechanical properties is to have a low fibre orientation anisotropy. This is a common measure when the mechanical properties in CD are of a main interest. If however the MD- properties are of principal interest the effect of shrinkage is of less importance and the strategy will be to have a high fibre orientation anisotropy. The present invention is of special interest in those cases where both MD- and CD- properties are of importance. This is e.g. the case for linerboards for use in boxes having substantially square sections. Linerboard is a heavy weight paper that is used for the manufacture of corrugated cartons and the like. Medium or fluting is the configured material that is placed between the walls of the linerboard to make the corrugated structure. The box compression strength is one of the most important properties for corrugated boxes. This is the ability of a box to resist forces applied in a compressive manner, such as when the boxes are stacked in warehouses and during shipment. The box compression strength for a box with essentially square sections is then related to the geometric mean of MD- and CD-values, MD CD of tensile strength, tensile stiffness and compression strength.

Object and most important features of the invention

An object of the present invention is to provide a method for producing a multi-ply paper or board product having a favourable geometric mean value of tensile strength and possibly other mechanical properties such as tensile stiffness, bending stiffness and compression strength. This is important for linerboard and other grades. This object has according to the invention been achieved by forming at least one ply with a relatively low fibre orientation anisotropy and forming at least one other ply with a relatively higher fibre orientation anisotropy, said plies being combined into a multi-ply paper or board product in which the difference in fibre orientation anisotropy defined as the ratio of major to minor axes b/a of the distribution pattern profile between the individual plies is at least 0.3, and the ply having a relatively low fibre orientation anisotropy is formed from a furnish having a relatively lower shrinkage tendency as compared to the furnish used for forming said ply having a relatively higher fibre orientation anisotropy.

The invention further refers to a multi-ply paper or board product produced with the above method.

Further important features of the invention are found in the subclaims and in the following description. Description of the invention

A ply-differentiated fibre orientation anisotropy is according to the invention suggested in order to achieve an improved geometric mean value of tensile strength, especially at the edges with essentially free drying in CD. This is based on the following findings:

1. The effect of wet straining, i e the stretching of the wet paper web in the paper machine, increases with an increase in the fibre orientation anisotropy in the straining direction.

2. The shrinking stress, i e the stress required to avoid shrinkage, decreases with an increased fibre orientation anisotropy (Htun and de Ruvo, 1977, Relation between drying stresses and internal stresses and the mechanical properties of paper, Fibre- water interactions in paper-making. J.D.Peel (ed.) London; Tech.Div.BPBIF) even if the shrinkage for free drying increases with increasing fibre orientation anisotropy (Byrd, 1981, Fibre orientation and drying restraint, Svensk Papperstidning

(15):R105).

3. Chemical pulps give rise to higher shrinking stresses (Htun and de Ruvo, 1977) and increased shrinkage than mechanical pulps (Htun et al, 1987, Torkningens inverkan pa papperets mekaniska egenskaper, STFI-report D281).

In a sheet structure comprising two or more plies, it is proposed to form at least one ply, ply A, with a low fibre orientation anisotropy, giving a ply with a low shrinkage in CD. Ply A is preferably formed from a of a furnish having a relatively low shrinkage at free drying, such as mechanical pulp furnish, thermomechanical pulp furnish, chemithermo- mechanical pulp furnish (CTMP), furnish from chemically stiffened cellulosic fibres and/or furnish from recycled fibres. Ply A should have a fibre orientation anisotropy in the form of the b/a ratio (defined below) of 2 or less, preferably 1.7 or less and most preferably 1.5 or less. At least one other ply, ply B, is formed with a relatively high fibre orientation anisotropy giving low shrinking forces which are counteracted by ply A. Ply B is preferably formed of a furnish having a high tensile strength potential, such as chemical pulp furnish (kraft pulp). Such pulp usually has a relatively higher shrinkage at free drying than the above mentioned furnish used for ply A.

By this the utilization of the strength potential of ply B can be improved especially at the edges as compared to the conventional case where one has a similar fibre orientation degree in all plies. Firstly, the effect of the MD-strain is maximized with a high degree of fibre orientation, which simultaneously minimizes the shrinking stresses at the edges. Secondly, the support ply (plies) enhances the CD-properties at the edges by counteracting the shrinkage of the chemical pulp ply.

The difference in the a/b ratio between plies A and B should be at least 0.3, preferably at least 0.5 and most preferably at least 0.8.

The test results for a two-ply paper comprising a kraft ply and a recycled ply and which are presented below, suggest that the geometric mean increases over the whole CD- width with increasing fibre orientation anisotropy applying a ply-differentiated fibre orientation strategy.

The multi-ply web according to the present invention may be produced according to various known multi-ply forming technologies, such as forming each ply in a separate forming unit before couching the plies together or forming the web plies on top of each other in a sequential mode, i e forming the second ply on top of the first ply and the third ply on top of the second ply etc. Preferably the last mentioned type of multi-ply forming technology is used, especially the one disclosed in SE 9702978-9.

Fibre orientation anisotropy (b/a ratio) The fibre orientation anisotropy is defined as the ^atio of major (b) to minor axes (a) of the distribution pattern ellipse. This distribution pattern ellipse is experimentally determined using image analysis as described by Erkkila, A.L., Pakarinen, P., Odell, M. in "Sheet forming studies using layered orientation analysis". Pulp & Paper Canada 99:1, T39 (1998). Each sheet ply or sheet layer is split in several sublayers in this test. The fibre orientation anisotropy for the respective layer is then calculated as a weighted average of the fibre orientation anisotropy of the sublayers in said layer.

Example

The combined effect of the differentiated fibre orientation and drying conditions was studied for a two-ply paper with kraft pulp in one ply and recycled fibres in the other ply. Each ply represented one half of the grammage. Forming and pressing of the paper was performed on a pilot machine and drying carried out off-line. Separate forming of the plies was applied using roll forming for the kraft ply and fourdrinier forming for the ply containing recycled fibres.

As a reference a low fibre orientation anisotropy was applied in both plies as a starting point at a machine speed of 500 m/min and a total grammage of about 107 g/m². Fibre orientation was then selectively increased in the kraft ply by means of an increase in the jet-to-wire speed difference through a reduced jet speed at a constant wire speed. For comparison, the fibre orientation anisotropy was increased simultaneously in both plies, again starting from the point with a low fibre orientation anisotropy in both plies. The wire speed for both plies was then gradually increased to 550 m/min at a constant jet speed, corresponding to a reduction of grammage to 94 g/m².

Furnish Bleached softwood kraft pulp was used for one of the plies in the trial, whereas rolls of corrugated medium made from recycled fibres were used for the second ply. The kraft ply was slushed and refining was carried out with a specific energy input of 182 kWh/ton using a specific edge load of 1.5 Ws/m. The corrugated medium rolls were slushed and no refining was applied. Results from a furnish evaluation are given in the table below. Table 1. Furnish evaluation.

Pulp MSR Fines¹' Ash²) WRV Density Tensile type (%) (%) (%) (kg/m³) strength (kNm/kg)

Kraft 72-74 11-13 <0.5 159-162 793-832 59-67

Recycled 71-74 30-31 12-13 1 19-124 632-670 32-36

1. <200 mesh from Bauer-McNett fractionation

2. Burning temperature ₅7₅ °C

The shrinking tendency of the two furnishes was evaluated on handsheets dried between blotters, accomplishing essentially free during. The kraft furnish showed a shrinkage of 4.4% whereas the furnish of recycled fibres showed a 2.5% shrinkage.

Roll forming unit The roll former unit comprised a hydraulic headbox, and a headbox consistency of

0.5% was employed. The jet speed was set for a minimum fibre orientation anisotropy at a wire speed of 500 m/min. producing a paper comprising only the kraft ply. The in- plane anisotropy, reflecting the fibre orientation anisotropy, was checked with an offline ultrasonic tensile stiffness tester. During the trial, a selective increase of the fibre orientation anisotropy in the kraft ply was accomplished by a successive reduction in the jet speed.

Fourdrinier unit

Also the fourdrinier unit comprised a hydraulic headbox and the consistency was 0.8- 0.9%. Similarly to the roll former unit, the jet speed was set for a minimum fibre orientation anisotropy at a wire speed of 500 m/min. The in-plane anisotropy was then checked with an off-line ultrasonic tensile stiffness tester as only the fourdrinier ply was run through the machine. The jet speed was kept constant in the fourdrinier unit through the whole experiment. Press section

The press section comprised three nips. The first nip was a conventional double felted roll nip and the second and third nips were both single-felted shoe press nips. The paper web was winded up after the shoe press section at a dryness of 43-44%.

Drying

The paper was dried off-line according to three different methods:

A: 2% initial wet strain in MD and restrain in CD, simulating a mid position on a commercial paper machine,

B: 2% initial wet strain in MD and free drying in CD, simulating an edge position on a commercial paper machine,

C: Restrained drying in both MD and CD as a reference condition.

Analysis

Tensile testing was performed with a strip length of 50 mm between the clamps and with a straining rate of 99 m/min. Sheet density was determined according to the STFI- method (Tellers, C, H. Anderson, et al. (1986). The definition and measurement of thickness and density. Paper structure and properties. J. A. Bristow and P. Kolseth (eds.). New York, Marcel Dekker, p.151). Otherwise SCAN standards were followed.

Results

Results for an individual increase of the fibre orientation anisotropy in the kraft ply (1- ply adjustment) are shown against a case of a simultaneous increase of the fibre orientation anisotropy in both plies (2-ply adjustment). In the example below the tensile ratio for restrained drying in MD and CD has been used for characterization of the fibre orientation anisotropy, assuming that the MD/CD ratio substantially corresponds to the b/a ratio as defined above. Table 2 below shows the anisotropy ranges for the individual plies, evaluated with only one of the plies run through the machine. Table 2. Fibre orientation anisotropy in the different plies

Fig. 1 shows the shrinkage in CD as a function of the fibre orientation amsotropy for the case with 2% MD-strain and free drying in CD.

Fig. 2 shows the geometric mean of the tensile strength in MD and CD vs. fibre orientation anisotropy for an adjustment of the fibre orientation anisotropy in one ply and two plies during different drying conditions A and B. Tensile strength is presented as the machine-to-handsheet ratio. This means the ratio between the tensile index for the pilot machine sheet and the average tensile index of the handsheets prepared of kraft pulp and recycled fibres.

Fig. 3 shows the geometric mean of the tensile strength in MD and CD vs. tensile strength in MD for an adjustment of the fibre orientation anisotropy in one ply and two plies during drying conditions A and B as above. Tensile strength is presented as the machine-to-handsheet ratio .

For drying condition A, the geometric mean of tensile strength increased with increasing fibre orientation anisotropy (Fig. 2) or MD strength (Fig. 3) at a slightly higher rate for one-ply adjustment than for two-ply adjustment of the fibre orientation anisotropy.

For one-ply adjustment of the fibre orientation anisotropy employing drying condition B, the geometric mean increased with increasing fibre orientation anisotropy (Fig. 2) or MD-strength (Fig. 3) at a rate similar to the case of drying condition A. On the other hand, as the fibre orientation anisotropy was changed in both plies for drying condition

B, the geometric mean showed essentially no effect of an increasing fibre orientation anisotropy or MD-strength. The results suggest that the geometric mean of tensile strength may be improved at the mid position as well as at the edges by an individual increase of the fibre orientation anisotropy in the kraft ply keeping the fibre orientation anisotropy in the support ply at a minimum. The result thus indicate that the difference in the geometric mean between mid-position and edges increases with a simultaneous increase of fibre orientation anisotropy in both plies, whereas this is avoided by means of a one-ply adjustment. For a ply-differentiated fibre orientation, the rate of increase in the geometric mean with increasing anisotropy was similar for the edge condition and the centre position. A ply- differentiated fibre orienta-tion thus enables an increase in the geometric mean without causing a more severe CD-variation in the geometric mean.

The shrinkage of the support ply sets the under level of shrinkage for the two-ply structure. It is determined by the fibre material as well as by the fibre orientation anisotropy. In order to minimize the shrinkage in CD one strives for a random fibre orientation in the support ply.

Using a kraft furnish for the top ply or a furnish of other type with higher shrinkage than the furnish for the support ply, the shrinkage of the two-ply paper becomes higher than the shrinkage of the support ply. The shrinking stresses exerted by the kraft ply onto the support ply are of course related to the ply grammage and they decrease with increasing fibre orientation anisotropy. The stiffness in CD of the support ply, i.e. the ability of the support ply to resist shrinking stresses imposed by the kraft ply, should increase with a more random fibre orientation and increasing ply grammage.

Besides the possibility to an improved and more even geometric mean over the CD- profile with a ply-differentiated fibre orientation, there are other beneficial effects that may be mentioned. A reduced shrinkage at a given fibre orientation anisotropy or MD- strength of course facilitates profile control of other sheets properties such as e g grammage. Moreover, at least up to grammages of about 150 g/m², web runnability is considered important. It is well known that runnability related properties such as wet strength, fracture toughness or tensile strength in MD improves with increasing fibre orientation anisotropy. Since it is possible to increase the geometric mean for mid position as well as for edge conditions by means of ply-differentiated fibre orientation anisotropy, it is then possible to improve the whole CD-profile simultaneously with an improved web runnability.

Other mechanical properties such as tensile stiffness and compression strength may also be improved by the present invention.

In order to reinforce the ability of the support layer to prevent CD-shrinkage of the top layer the drying of the support layer can be accelerated. This can be made in conventional cylinder drying in so called single-tier in which the support layer is in contact with the heated cylinders. Further possibilities are drying by means of electromagnetic radiation such as IR radiation.

Claims

1. A method for forming of a multi-ply paper or board product, characterized in forming at least one ply with a relatively low fibre orientation anisotropy and forming at least one other ply with a relatively higher fibre orientation anisotropy, said plies being combined into a multi-ply paper or board product in which the difference in fibre orientation anisotropy defined as the ratio of major to minor axes b/a of the distribution pattern profile between the individual plies is at least 0.3, and the ply having a relatively low fibre orientation anisotropy is formed from a furnish having a relatively lower shrinkage tendency as compared to the furnish used for forming said ply having a relatively higher fibre orientation anisotropy.

2. A method as claimed in claim 1, characterized in that the difference in the b/a ratio between the individual plies is at least 0.5 and preferably at least 0.8.

3. A method as claimed in claim 1 or 2, characterized in that the ply having a relatively low fibre orientation anisotropy has a b/a ratio of 2 or less, preferably 1.7 or less and most preferably 1.5 or less.

4. A method as claimed in any of the preceding claims, characterized in that the ply having a relatively higher fibre orientation anisotropy is formed from a furnish having a relatively high tensile strength potential as compared to the furnish in the ply having a relatively low fibre orientation anisotropy.

5. A method as claimed in any of the preceding claims, characterized in that the ply having the relatively lower shrinkage tendency is mainly formed from mechanical pulp furnish, thermomechanical pulp furnish, chemithermomechanical pulp furnish (CTMP), furnish from chemically stiffened cellulosic fibers and/or furnish from recycled fibers.

6. A method as claimed in any of the preceding claims, characterized in that the ply having a relatively higher fibre orientation anisotropy is mainly formed from chemical pulp furnish.

7. A multi-ply paper or board product, characterized in that it comprises at least one ply having a relatively low fibre orientation anisotropy and that it further comprises at least one ply with a relatively higher fibre orientation anisotropy, said plies being combined into said multi-ply paper web in which the difference in the fibre orientation anisotropy defined as the ratio of major to minor axes b/a of the distribution pattern profile between the individual plies is at least 0.3, and the ply having a relatively low fibre orientation anisotropy is formed from a furnish having a relatively lower shrinkage tendency as compared lo the furnish used for forming said ply having a relatively higher fibre orientation anisotropy.

8. A multi-ply paper or board product as claimed in claim 7, characterized in that the difference in the b/a ratio between the individual plies is at least 0.5 and most preferably at least 0.8.

9. A multi-ply paper or board product as claimed in claim 7 or 8, characterized in that the ply having a relatively low fibre orientation anisotropy has a b/a ratio of 2 or less, preferably 1.7 or less and most preferably 1.5 or less.

10. A multi-ply paper or board product as claimed in any of claims 7-9, characterized in that the ply having a relatively higher fibre orientation anisotropy is formed from a furnish having a relatively high tensile strength potential as compared to the furnish in the ply having a relatively low fibre orientation anisotropy.

11. A multi-ply paper or board product as claimed in any of claims 7-10, characterized in that the ply having a relatively low shrinkage tendency is mainly formed from mechanical pulp furnish, thermomechanical pulp furnish, chemithermomechanical pulp furnish (CTMP), furnish from chemically stiffened cellulosic fibers and/or furnish from recycled fibers.

12. A multi-ply paper or board product according to claim 10, characterized in that the ply having a relatively high tensile strength potential is mainly formed from chemical pulp furnish.

13. A multi-ply paper or board product according to any of claims 7-12, characterized in that the multi-ply product is linerboard.