Patent Description:
A machine for producing multi-layered paper- or linerboard has different sections. In the wet end of the paper machine, furnishes are prepared from pulps and additives (added to improve properties such as retention, dewatering and strength). In the forming/wire section, a multi-layered web is then formed from the furnishes. The multi-layered web is dewatered in the press section and dried in the drying section. Downstream the drying section, the multilayered web may be coated and/or calendered. When the multilayered web is both coated and calendered, calendering can be carried upstream or downstream the coating step(s) or even between coating steps.

The pulps that can be used in the paperboard machine include virgin pulps, recycled pulps (i.e. pulps prepared from recycled paper and/or paperboard) and broke pulps (pulps of fibres recycled from downstream portions of the same process). Further, virgin pulps are typically divided into chemical pulps and mechanical pulps. Sulfate (kraft) pulp and sulfite pulp are examples of chemical pulp. Groundwood pulp, thermomechanical pulp (TMP) and chemi-thermomechanical pulp (CTMP) are examples of mechanical pulp. All these pulps can be made from hardwood, softwood or mixtures thereof. Further, the pulps can be bleached or unbleached.

A furnish for a particular layer typically comprise a mixture of different pulps. The properties of the final board product are not only a result of the pulp mixture of each furnish; they also depend on the degree of refining of the pulps, the types and amounts of additives and other operating conditions. However, the process for making paperboard cannot be designed with only the final properties in mind; runnability in the paper machine and energy consumption must also be taken into consideration.

In <CIT> a light weight linerboard with a low CTMP content for corrugated board was produced, wherein the linerboard is having a high bending resistance and high compression strength.

In <CIT> a machine for producing a fibrous web comprising at least one, preferably two, double-felted shoe presses arranged one behind the other in the web running direction and at least one single-felted shoe press arranged behind the two double-felted shoe presses in the web running direction is disclosed.

In <CIT> a ply of a linerboard for corrugated board, which ply exhibits high strength properties and which enables the production of lightweight linerboard was produced with addition microfibrillated cellulose (MFC).

In CN <NUM><NUM><NUM> A a dark grain anticounterfeit cardboard produced from chemical pulp and hot-grinding pulp is disclosed.

In <CIT> a method for the production of a paper, board, tissue or another fiber web having a mass per unit area of less than <NUM>/m2 is disclosed. By using an individual circulation of water and starch, the starch can be transferred to a web by felts and thereby enrichment of starch in the whitewater is avoided.

The present invention aims to provide an efficient method of producing a paper- or linerboard of satisfactory properties.

Accordingly, the present invention provides a method of producing a paper- or linerboard having a grammage measured according to ISO <NUM>:<NUM> of <NUM>-<NUM>/m<NUM> and comprising a print layer, a middle layer and a back layer, wherein the middle layer is formed from a middle layer furnish comprising at least <NUM>% by dry weight CTMP and at least <NUM>% by dry weight broke pulp and the back layer is formed from a back layer furnish comprising an unbleached kraft pulp, said method comprising the step of subjecting a multi-layered web at a speed of at least <NUM>/min to pressing in a press section comprising a first and a second double-felted shoe press nip arranged in series, wherein the total press impulse of the double-felted shoe press nips is at least <NUM> kPa*s, wherein CTMP and broke pulp together constitute at least <NUM>% by dry weight of the middle layer furnish.

Compared to the prior art, this press impulse is high, which increases the press dryness (i.e. the dryness of the multilayered web exiting the press section). As a result, less steam is consumed in the drying section. The skilled person making paperboard is often reluctant to use high press impulses as they can densify the web to such an extent that inferior bending stiffness is obtained. However, the selection of an unbleached pulp for the back layer furnish and the inclusion of CTMP in the middle layer furnish make the web of the method of the present disclosure relatively insensitive to high press impulses. The press dryness is further increased by both shoe press nips being double-felted.

The present disclosure provides a method of producing a paper- or linerboard having a grammage measured according to ISO <NUM>:<NUM> of <NUM>-<NUM>/m<NUM>.

The paper- or linerboard comprises a print layer, a middle layer and a back layer. In one embodiment, the paper- or linerboard comprises more than one middle layer.

The middle layer is formed from a middle layer furnish comprising at least <NUM>% by dry weight CTMP. In case of more than one middle layer, each middle layer furnish preferably comprise at least <NUM>% by dry weight CTMP. The CTMP of the middle layer(s) preferably has a Canadian Standard Freeness (CSF) measured according to ISO <NUM>-<NUM>:<NUM> of <NUM>-<NUM>, such as <NUM>-<NUM>. To reach such a CSF, the CTMP may be subjected to refining. If the CSF is too low, dewatering is impaired and density increases. If the CSF is too high, strength properties may be insufficient.

According to the invention, the middle layer furnish comprises at least <NUM>% by dry weight CTMP.

In addition to CTMP, the middle layer comprises at least <NUM>% by dry weight broke pulp, such as at least <NUM>% by dry weight broke pulp. The broke pulp of the middle layer furnish preferably has a Schopper-Riegler number (°SR) of less than <NUM> (such as <NUM>-<NUM>), more preferably less than <NUM> (such as <NUM>-<NUM>). To reach such a °SR, the broke pulp may be subjected to refining. In the present disclosure, °SR is measured according to ISO <NUM>-<NUM>:<NUM>.

According to the invention, the CTMP and the broke pulp together constitute at least <NUM>% by dry weight of the middle furnish.

The middle layer furnish may further comprises kraft pulp, such as softwood kraft pulp. This kraft pulp may have a relatively low °SR, such as <NUM>-<NUM>, preferably <NUM>-<NUM>.

The head box consistency of the middle layer furnish may be <NUM>% - <NUM>%, such as <NUM>% - <NUM>%.

The back layer is formed from a back layer furnish comprising an unbleached kraft pulp. The unbleached pulp of the back layer preferably has a °SR of <NUM>-<NUM>, such as <NUM>-<NUM>. To reach such a °SR, the unbleached pulp may be subjected to refining. If the °SR is too high, dewatering is impaired. If the °SR is too low, strength properties may be insufficient.

The unbleached pulp of the back layer is typically a softwood pulp.

In one embodiment, the back layer furnish comprises at least <NUM>% by dry weight of the unbleached pulp, such as at least <NUM>% by dry weight of the unbleached pulp.

In addition to the unbleached pulp, the back layer furnish may comprise broke pulp. The broke pulp of the back layer furnish preferably has a °SR of less than <NUM> (such as <NUM>-<NUM>), more preferably less than <NUM> (such as <NUM>-<NUM>). To reach such a °SR, the broke pulp may be subjected to refining.

In one embodiment, the unbleached pulp and broke pulp together constitute at least <NUM> % by dry weight of the back layer furnish, such as at least <NUM> % by dry weight of the back layer furnish.

The head box consistency of the back layer furnish may be <NUM>% - <NUM>%.

The print layer furnish preferably comprises a mixture of hardwood kraft pulp and softwood kraft pulp, such as a mixture of bleached hardwood kraft pulp and bleached softwood kraft pulp. However, the print layer may also be unbleached.

Each of the kraft pulps of print layer furnish preferably has a °SR of <NUM>-<NUM>. To reach such a °SR, the kraft pulps are typically subjected to refining.

In one embodiment, hardwood kraft pulp and softwood kraft pulp together constitute at least <NUM>% by dry weight of the print layer furnish, such as at least <NUM>% by dry weight of the print layer furnish.

The head box consistency of the print layer furnish may be <NUM>% - <NUM>%.

As understood by the skilled person, the above-mentioned furnishes form a multi-layered web in a forming section. A top former may be arranged to aid the dewatering of the middle layer in the forming section. Further, the formation of the middle layer preferably comprises the use of a breast roll shaker.

The method of the present disclosure comprises the step of subjecting the multi-layered web to pressing in a press section at a speed of at least <NUM>/min, such as at least <NUM>/min, such as at least <NUM>/min, such as at least <NUM>/min.

The press section comprises a first and a second double-felted shoe press nip arranged in series. As understood by the skilled person, the second double-felted shoe press nip is arranged downstream the first double-felted shoe press nip. Preferably the line load of the second double-felted shoe press nip is higher than the line load of the first double-felted shoe press nip.

The total press impulse of the double-felted shoe press nips is at <NUM> kPa*s. An upper limit may be <NUM> kPa*s or <NUM> kPa*s.

Since the grammage is relatively high, i.e. <NUM>-<NUM>/m<NUM>, the speed of the multi-layered web is typically lower, which means that the press impulse is at least <NUM> kPa*s.

In a preferred embodiment, the press section comprises a further nip arranged downstream the double-felted shoe press nips. The further nip is preferably a non-felted nip or a single-felted nip, such as a non-felted hard nip or a single-felted shoe press nip. When the further nip is a non-felted hard nip, its line load may be in the range of <NUM>-<NUM> kN/m, such as <NUM>-<NUM> kN/m, such as <NUM>-<NUM> kN/m. The main purpose of such a hard nip is to smoothen the print side, which was in contact with a felt in the first and the second double-felted shoe press nip. When the further nip is a single-felted shoe press nip, the felt is preferably contacting the back side. Thereby, the single-felted shoe press nip not only further dewaters the web, it also smoothens the print side, which - as mentioned above - was in contact with a felt in the first and the second double-felted shoe press nip.

The print layer of the paper- or linerboard may be coated with a composition comprising at least one pigment and at least one binder. Accordingly, the method of the present disclosure may further comprise a coating step. The coating step may comprise a plurality of substeps, each applying a sublayer.

When pigment-coated, the print side of the paper- or linerboard of the present disclosure may have a PPS <NUM> roughness of below <NUM>, such as below <NUM>. A lower limit for this PPS <NUM> roughness may be <NUM>. In the present disclosure, PPS <NUM> roughness is measured according to ISO <NUM>-<NUM>:<NUM> (soft backing & <NUM> kPa clamping pressure).

In an embodiment, the method of the present disclosure comprises no calendering step, which typically saves bulk.

The density of the paper- or linerboard of the present disclosure may be below <NUM>/m<NUM>. If not coated with a pigment coating, the density may be below <NUM>/m3. In the present disclosure, density is measured according to ISO <NUM>:<NUM>.

A three-layer white-top linerboard (WTL) was produced in a full-scale paperboard machine. The layer design of the WTL was as follows: a print layer (<NUM>/m<NUM>), a back layer (<NUM>/m<NUM>), a middle layer (<NUM>/m<NUM>) and a pigment coating (<NUM>/m<NUM>, applied onto the print layer).

To prepare a print layer furnish, bleached hardwood kraft pulp (NBHK) and bleached softwood kraft pulp (NBSK) were mixed in a <NUM>:<NUM> dry weight ratio. Before the mixing, the NBHK and the NBSK were subjected to LC refining (<NUM> and <NUM> kWh/tonne, respectively) such that both pulps obtained a °SR value of <NUM>. Before the print layer headbox, rosin size (<NUM>/tonne), alum (<NUM>/tonne), strength agent (cationic starch, <NUM>/tonne), retention starch (<NUM>/tonne), retention polymer (<NUM>/tonne), silica (<NUM>/tonne) and clay (<NUM>/tonne) were added. In the print layer headbox, the consistency was <NUM>% and the pH was <NUM>.

To prepare a back layer furnish, unbleached softwood kraft pulp (UBK) and broke pulp were mixed in a <NUM>:<NUM> dry weight ratio. Before the mixing, the UBK and the broke pulp were subjected to LC refining to obtain °SR values of <NUM> and <NUM>, respectively. Before the back layer headbox, AKD (<NUM>/tonne), alum (<NUM>/tonne), strength agent (cationic starch, <NUM>/tonne), retention starch (<NUM>/tonne), retention polymer (<NUM>/tonne) and silica (<NUM>/tonne) were added. In the bottom layer headbox, the consistency was <NUM>% and the pH was <NUM>.

To prepare a middle layer furnish, broke pulp, NBSK and CTMP were mixed in a <NUM>:<NUM>:<NUM> dry weight ratio. Before the mixing, the pulps were subjected to LC refining to obtain the following values: °SR <NUM> for the broke pulp; °SR <NUM> for the NBSK and CSF <NUM> for the CTMP. Before the middle layer headbox, AKD (<NUM>/tonne), alum (<NUM>/tonne), strength agent (cationic starch, <NUM>/tonne), retention starch (<NUM>/tonne), retention polymer (<NUM>/tonne) and silica (<NUM>/tonne) were added. In the middle layer headbox, the consistency was <NUM>% and the pH was <NUM>. The wire used for forming the middle layer included e.g. a top former and a breast roll shaker.

In the wire section, <NUM>/m<NUM> starch was sprayed to each of the print layer web and the back layer web for ply-bond strength. At the end of the wire section, the three individual webs were couched together to form a three-layered web. In the press section arranged downstream the wire section, the three-layered web was pressed in three nips; a first double-felted shoe press nip followed by a second double-felted shoe press nip and a hard nip. The line load of the first double-felted shoe press nip was <NUM> kN/m. The line load of the second double-felted shoe press nip was <NUM> kN/m. The line load of the hard nip was <NUM> kN/m. The web speed in the press section was <NUM>/min, which means that the press impulse of the whole press section was <NUM> kPa*s and that the total press impulse of the double-felted shoe press nips was <NUM> kP*s.

Downstream the press section, the three-layered web was dried in a drying section (the steam consumption in this section was relatively low) and then coated in a coating section according to the following: in a first blade coater, <NUM>/m<NUM> of a pigment coating composition was applied to the surface of the print layer to form a first coating layer; and in a second blade coater, <NUM>/m<NUM> of a pigment coating composition was applied to the first coating layer to form a second coating layer. Hence the total (dry) coat weight on the print layer surface was <NUM>/m<NUM>. Further, a very small amount (~<NUM>/m<NUM>) of starch was applied to the surface of the back layer in the coating section.

The properties of the resulting WTL product are presented in table <NUM> below. The properties are satisfactory.

Claim 1:
A method of producing a paper- or linerboard having a grammage measured according to ISO <NUM>:<NUM> of <NUM>-<NUM>/m<NUM> and comprising a print layer, a middle layer and a back layer, wherein the middle layer is formed from a middle layer furnish comprising at least <NUM>% by dry weight CTMP and at least <NUM>% by dry weight broke pulp and the back layer is formed from a back layer furnish comprising an unbleached kraft pulp, said method comprising the step of subjecting a multi-layered web at a speed of at least <NUM>/min to pressing in a press section comprising a first and a second double-felted shoe press nip arranged in series, wherein the total press impulse of the double-felted shoe press nips is at least <NUM> kPa*s,
wherein CTMP and broke pulp together constitute at least <NUM>% by dry weight of the middle layer furnish.