Patent Application
20250198089
The present disclosure relates to the field of liners for use in corrugated fiberboard.
Kraft pulp is produced according to the kraft process, which is a chemical pulping process well known to the person skilled in the art of pulp and paper. Kraft pulp can be produced from both hardwood and softwood. If the kraft pulp is not bleached, it will produce a brown product. A relatively large proportion of all kraft pulp is however bleached and can hence be used to produce white paper and board. The brightness of the white paper/board depends on the degree of bleaching of the pulp, the type and amount of pigment (and other additives) added to the furnish and the composition of any coating applied.
A kraftliner is formed from predominantly kraft pulp and is typically used in corrugated fiberboard.
A kraftliner can be composed of several layers. In a so called white top kraftliner, the top layer is formed from bleached kraft pulp, whereas the other layer(s) is/are formed from unbleached kraft pulp. There is also white top test liner, in which the back layer is formed from recycled fibers.
The present disclosure provides a single layer white kraftliner having a grammage from 120 to 185 g/m2 when measured according to ISO 536:2019, wherein the kraftliner:
A method of producing a single layer white kraftliner comprising the steps of:
As a first aspect of the present disclosure, there is provided a single layer white kraftliner having a grammage in the range of 120 to 185 g/m2 when measured according to ISO 536:2019. Preferably, the grammage is at least 130 g/m2, such as at least 140 g/m2. In one embodiment the grammage is in the range of 130 to 170 g/m2, such as 140 to 170 g/m2.
The kraftliner comprises bleached hardwood fibers and bleached softwood fibers in a dry weight ratio from 95:5 to 82:18, such as from 94:6 to 88:12, such as from 94:6 to 90:10. Preferably, at least 65% by weight of the hardwood fibers are obtained from maple.
The kraftliner comprises a first surface that is coated with a first coating composition comprising starch and pigment and a second surface that is coated with a second coating composition comprising starch and pigment. The first surface is typically intended to face outwards in a fiberboard.
The dry weight ratio of starch to pigment in the coating compositions may for example be in the range of 30:100 to 50:100.
In one embodiment, the compositions further comprise synthetic binder in such an amount that the dry weight ratio of synthetic binder to pigment is from 5:100 to 15:100. The synthetic binder is preferably a copolymer, such as a styrene-based copolymer, such as a styrene-butadiene copolymer or a styrene-acrylic copolymer.
Said pigment may comprise clay pigment and/or calcium carbonate pigment. When the pigment comprises clay and calcium carbonate, the amount of calcium carbonate may be higher than the amount of clay.
In one embodiment, the first and the second coating composition are the same. In another embodiment, they are different.
The coat weight of each of the first and the second coating composition may for example be from 1 to 5 g/m2, such as from 2 to 5 g/m2. In one embodiment, the coat weight of the first coating composition is higher than the coat weight of the second coating composition.
The Bendtsen roughness of at least one of the surfaces (typically at least the first surface) of the kraftliner is less than 300 ml/min when measured according to ISO 8791-2:2013. In one embodiment, the Bendtsen roughness is less than 250 ml/min, such as less than 215 ml/min. A lower limit may be 140 or 100 ml/min.
The kraftliner has a specific formation of less than 0.7 √g/m when measured according to SCAN-P 92:09. In one embodiment, the specific formation is less than 0.60 √g/m, such as less than 0.52 √g/m. A lower limit may be 0.35 √g/m. The specific formation is preferably measured using equipment from Ambertech.
The kraftliner has a compressive strength index in the cross direction (CD) of at least 21 Nm/g when measured using a short-span compressive tester according to ISO 9895:2008. In one embodiment, the compressive strength index in the CD is at least 23 Nm/g. An upper limit may be 28 Nm/g. The compressive strength index is obtained by dividing the compressive strength (here measured according to ISO 9895:2008) by the grammage (here measured according to ISO 536:2019).
The kraftliner has a geometric tensile stiffness index of at least 5.2 kNm/g when measured according to ISO 1924-3:2005. In one embodiment, the geometric tensile stiffness index is at least 6.0 kNm/g. An upper limit may be 7.2 kNm/g. As known to the person skilled in the art of papermaking, the geometric tensile stiffness is the square root of the product of the tensile stiffness in the machine direction (MD) and the tensile stiffness in the CD. The tensile stiffness index is obtained by dividing the tensile stiffness (here measured according to ISO 1924-3:2005) by the grammage (here measured according to ISO 536:2019).
The kraftliner may have a brightness of at least 80% when measured according to ISO 2470-1:2016. In one embodiment, the brightness is at least 86%, such as at least 89%. An upper limit may be 95%.
The density of the kraftliner may for example be 810-920 kg/m3, such as 850-900 kg/m3, when measured according to ISO 534:2011. Further, the thickness of the kraftliner may be 115-220 μm, such as 155-200 μm, when measured according to the same standard.
In one embodiment, the ash content of the kraftliner is from 3.5% to 8.0% by weight, such as from 4.0% to 7.0% by weight, when measured according to ISO 2144:2015.
As a second aspect of the present disclosure, there is provided a method of producing a single layer white kraftliner, such as a kraftliner according to the first aspect.
The method comprises a step of providing a furnish comprising bleached hardwood fibers and bleached softwood fibers in a dry weight ratio from 95:5 to 82:18.
In one embodiment, the method comprises preparation of the furnish, which preparation comprises:
Here, “freeness” refers to Canadian Standard Freeness measured according to ISO 5267-2:2001.
The preparation may further comprise addition of broke pulp. The amount of broke pulp may be 10% to 20% of the dry weight of the other pulps.
The furnish may further comprise inorganic filler, such as calcium carbonate, in an amount from 30 to 80 kg per tonne of dry fiber, such as from 40 to 70 kg per tonne of dry fiber.
The method further comprises a step of forming a web from the furnish in a wire section comprising a headbox, typically a dilution-controlled headbox. The consistency of the furnish in the headbox may for example be from 0.65% to 0.95%, such as from 0.74% to 0.90%. The wire section preferably comprises a lower wire loop and a top former. Such a combination is sometimes referred to as a hybrid former.
The method further comprises a step of pressing the web from the wire section in a pressing section. In one embodiment, the pressing section comprises a shoe nip and optionally at least one additional nip, such as at least two additional nips. If a shoe nip is used, the line load applied therein may be at least 650 kN/m, such as 700-1150 kN/m.
The method further comprises the step of drying the web from the pressing section in a drying section to obtain a dried web and then optionally the step of calendering the dried web. As an example, a hard nip calender can be used for the calendering step.
The method further comprises applying a first aqueous coating composition comprising starch and pigment to a first surface of the web and a second aqueous coating composition comprising starch and pigment to a second surface of the web.
The applications of the first and the second coating composition may be simultaneous or sequential. In one embodiment, the first coating composition is applied in a first blade coater and the second coating composition is applied in a second blade coater arranged downstream the first blade coater. In such an embodiment, drying is typically carried out between the blade coaters and after the second blade coater.
The applications of the first and the second coating composition are preferably carried out in-line in a coating section belonging to the same machine as the above-mentioned wire section, pressing section and drying section.
In one embodiment, the first and/or the second aqueous coating composition further comprise(s) synthetic latex in such an amount that the dry weight ratio of synthetic latex to pigment is from 5:100 to 15:100, such as from 6:100 to 12:100.
The synthetic latex is preferably styrene-based, such as a styrene-acrylate latex or a styrene-butadiene latex.
Otherwise, the embodiments and examples of the first aspect apply to the second aspect mutatis mutandis.
A single layer white kraftliner was produced in a paper machine normally used for producing graphical papers.
A furnish mixture was prepared by mixing bleached softwood kraft pulp (BSKP), bleached hardwood kraft pulp (BHKP) and broke pulp. Before mixing, the BSKP and the BHKP had been refined to a freeness of 453 ml and 415 ml, respectively. The dry weight ratio of BSKP to BHKP was 7:93. The amount of broke pulp was 15% by dry weight of the other pulps. 12 kg/tonne of PCC and 37 kg/tonne of GCC were added to the furnish.
Further, the following wet end chemistry was added to the furnish:
A web was formed from the furnish in a wire section comprising a Beloit concept IV dilution-controlled headbox (supplied by Valmet) and a hybrid former.
In the headbox, the consistency of the furnish was 0.82%. The jet/wire ratio was 1.0064. The wire speed in the wire section was 732 m/min.
The wire section was equipped with a breast roll shaker that was not operating.
The web formed in the wire section was passed through a pressing section having four nips: 1) a double-felted roll press nip; 2) a single-felted roll press nip; 3) a single-felted shoe press nip having the same central roll as the 2nd nip; and 4) a single-felted stand-alone offset (smoothing) press nip.
The line loads were as follows: 79 kN/m in the 1st; 96 kN/m in the 2nd nip; 744 kN/m in the 3rd nip; and 88 kN/m in the 4th nip.
The web from the press section was dried in a drying section to a moisture content of 7%-8%. The construction of the drying section was such that there were almost no free draws, hence limiting the shrinkage of the web in the cross direction.
The dried web was calendered in a hard nip machine calender at a line load of 22 kN/m.
The calendered web was then coated using blade coaters in a coating section, first on the top side (intended to be printed) and then on the back side. After the first and the second coating step, the coated web was subjected to IR drying and then air drying using air flotation dryers.
The (water-based) coating, which was applied to both sides, had the following composition:
After coating and drying, the coated single layer white kraftliner was reeled up at a reel speed of 749 m/min. The coated product was not calendered.
Relevant properties of the coated single layer white kraftliner and the uncoated product obtained before the coating section were measured. These properties are presented and compared to those of competing liner products in the table shown in
Notably, the coated product produced in the present example has a considerably lower specific formation (0.47 √g/m) than the competing products (≥0.71 √g/m). Further, it has lower roughness (202 ml/min) than the competing products (≥220 ml/min). It also has much higher brightness (92%) than the competing products (≤84.9%).
In conclusion, the coated product produced in the present example has much better surface and visual properties than the competing products. If the comparison is limited to products of similar grammage, the difference is even more striking.
It is expected that the superior surface/visual properties facilitate a superior printing appearance.
Further, it is expected that the product of the present example would have had an even better formation if the breast roll shaker had been used and an even lower roughness if a calendering was performed downstream the coating section.
Turning to the strength properties, it is notable that the coated product produced in the present example has considerably higher compressive strength (SCT) index in the CD (25.3 Nm/g) than the competing products (≤22.5 Nm/g). Further, it has a geometrical tensile stiffness index (6.4 kNm/g) that is comparable to that of the competing products (5.5-8.2 kNm/g). If this comparison is limited to grammages of at least 140 g/m2, all of the competing products have inferior geometrical tensile stiffness index (5.5-6.1 kNm/g).
In
