F IELD

[0001] The present application relates to the field of linerboard, corrugated board comprising linerboard and methods for manufacturing thereof.

B A CK GRO UND

[0002] Corrugated board includes a linerboard and a corrugating medium attached to the linerboard. The corrugated board may include a first linerboard, a second linerboard, and a corrugating medium between the first linerboard and the second linerboard, thus forming a sandwich structure. The corrugated board may include additional linerboard layers and/or additional corrugating mediums. For example, the corrugated board may include a double sandwich structure or triple sandwich structure. Environmental concerns have increased the demand for linerboards with higher strength.

[0003] Calendering has been used for surface property improvements for paperboard. However, calendering has been cited as a negative attribute to developing strength.

[0004] Accordingly, those skilled in the art continue with research and development in the field of linerboard, corrugated board comprising linerboard and methods for manufacturing thereof.

SUMMARY

[0005] In one embodiment, a method for manufacturing a calendered linerboard includes forming a linerboard comprising cellulosic fibers and lignin and calendering the linerboard at a temperature of at least 70 °C. The linerboard includes a lignin content of at least 8%, by weight percent. [0006] In another embodiment, a calendered linerboard includes cellulosic fibers and lignin, wherein the linerboard includes a lignin content of at least 8%, by weight percent, and wherein the linerboard has been calendered at a temperature of at least 70 °C.

[0007] In yet another embodiment, a corrugated board includes a calendered linerboard and a corrugating medium attached to the calendered linerboard. The calendered linerboard includes cellulosic fibers and lignin, wherein the linerboard includes a lignin content of at least 8%, by weight percent, and wherein the linerboard has been calendered at a temperature of at least 70 °C.

[0008] In yet another embodiment, a method for manufacturing a calendered linerboard includes chemically pulping cellulosic material to a kappa number of at least 55 to form cellulosic fibers and lignin, forming a linerboard comprising the cellulosic fibers and the lignin; and calendering the linerboard at a temperature of at least 70 °C.

[0009] Other embodiments of the disclosed linerboard, corrugated board comprising linerboard and methods for manufacturing thereof will become apparent from the following detailed description, the accompanying drawings and the appended claims.

B RIEF DE S C RIP T I ON OF THE DRAW ING S

[0010] Fig. l is a cross sectional view of an exemplary corrugated cardboard according to an embodiment of the present description.

[0011] Fig. 2 is a graph showing a boxplot of STFI (Nm/g) of experimental linerboard manufactured according to the present description.

D E TAILED DE S C RIP T ION

[0012] The present description relates to a calendered linerboard comprising cellulosic fibers and lignin, a corrugated board comprising the calendered linerboard and methods for manufacturing the calendered linerboard.

[0013] The present description relates to providing improved linerboards with high strength. [0014] Calendering has been used for surface property improvements for linerboard. However, calendering has been cited as a negative attribute to developing strength. Although the present description is not limited by theory, it is believed that calendering typically decreases a strength of the linerboard due to damage caused to the cellulosic fibers during a typical calendering process.

[0015] The present description relates to providing improved strength for a linerboard having a high lignin content by calendering the high lignin content linerboard at a high calendering temperature, particularly above a glass transition temperature of the lignin. Although the present description is not limited by theory, it is believed that, calendering above a glass transition temperature of the lignin promotes flow of the lignin and increases a relative bonded area of the cellulose fibers, and that the increased bonding of the cellulose fibers increases a compression and burst strength of the linerboard.

[0016] To produce a linerboard having a high lignin content, a cellulosic material having a high lignin content may be selected. The type of cellulosic material is not limited. In an example, a cellulosic material having high lignin content includes hardwood. In another example, a cellulosic material having a higher lignin content includes softwood. The linerboard of the present description may be produced from virgin cellulosic fibers, recycled cellulosic fibers, or from a combination of virgin cellulosic fibers and recycled cellulosic fibers.

[0017] For the present description, the size of the cellulosic fibers are not limited. Thus, the linerboard may be formed from cellulosic fibers having a high fiber length or may be formed from cellulosic fibers having a low fiber length. In an example, the linerboard of the present description may include a high amount of cellulosic fibers having a high fiber length and a high lignin content. To produce a high amount of cellulosic fibers having a high fiber length and a high lignin content, a cellulosic material having a high lignin content and having cellulosic fibers with a high fiber length may be selected. An exemplary cellulosic material having high lignin content and having high fiber length cellulosic fibers includes softwood.

[0018] The cellulosic fibers of the present description may be produced by chemical pulping, such as a kraft chemical pulping. Two main processes have been used for wood pulping: mechanical pulping and chemical pulping. Pulping is a process that separates fibers from a cellulosic material (e.g., wood chips) to produce pulp. Two main processes have been used for pulping: mechanical pulping and chemical pulping. Mechanical pulping primarily uses mechanical energy to separate pulp fibers from cellulosic material without a substantial removal of lignin. However, mechanical pulps often have poor strength. Other process options have been used in a combination with mechanical energy. Thermomechanical pulping (TMP) grinds pulps under steam at high pressures and temperatures. Semichemical or chemi-thermomechanical pulping (CTMP) uses chemicals to break up pulps prior to a mechanical pulping. Semichemical pulping is used to make corrugating medium. Chemical pulping relies upon chemicals to separate fibers and form pulp. A chemical pulping process results in less fiber damage, and thus greater retention of high fiber length, compared with other pulping processes that rely upon mechanical means. The four common types of chemical pulping processes are kraft pulping, sulfite pulping, neutral sulfite semi-chemical (NSSC) pulping, and soda pulping.

[0019] To achieve a high lignin content, a chemical pulping process, such as kraft chemical pulping, may be performed to a high kappa number. The chemical pulping process reduces a lignin content of the cellulosic material to help with separating fibers. By chemical pulping to a high kappa number, the reduction of the lignin content of the cellulosic material is minimized and a high lignin content is retained. In an aspect, the step of chemically pulping the cellulosic material is performed to a kappa number of at least 55, preferably at least 60, more preferably at least 70, more preferably at least 80, more preferably at least 90, more preferably at least 100, more preferably at least 110, more preferably at least 120, more preferably at least 130, more preferably at least 140, more preferably at least 150. In an aspect, the linerboard may comprise at least 50 wt% kraft fibers, preferably at least 60 wt% kraft fibers, more preferably at least 70 wt% kraft fibers, more preferably at least 80 wt% kraft fibers, and more preferably at least 85 wt% kraft fibers.

[0020] The linerboard preferably includes a high lignin content. In one aspect, the lignin content is preferably at least 8%, by weight percent, more preferably at least 9%, more preferably at least 10%, more preferably at least 11%, more preferably at least 12%, more preferably at least 13%, more preferably at least 14%, more preferably at least 15%, more preferably at least 16%, more preferably at least 17%, more preferably at least 18%, more preferably at least 20%. In another aspect, lignin content is preferably in a range of 8% to 28%, more preferably 9% to 27%, more preferably 10% to 26%, more preferably 11% to 25%. In one expression, the lignin content is preferably in a range of 8% to 20%, more preferably 10% to 18%, more preferably 12% to 16%. In another expression, the lignin content is preferably in a range of 20% to 28%, more preferably 21% to 27%, more preferably 22% to 26%. A desired lignin content may be achieved by, for example, selecting a suitable cellulosic material and a suitable pulping process. A preferred cellulosic material having high lignin content and having high fiber length cellulosic fibers includes softwood. A desired lignin content may also be achieved by blending virgin and recycled cellulosic fibers.

[0021] The high lignin content enables for a high strength of the linerboard after a high temperature calendering step. Referring to Fig. 2, increasing a lignin content decreases strength, but a net improvement in the linerboard strength after the high temperature calendering increases when the lignin content is high.

[0022] In an aspect, the linerboard may include cellulosic fibers having a high fiber length. In one expression, the linerboard may include cellulosic fibers having a fiber length of at least 0.5 mm, preferably at least 1.0 mm, more preferably at least 1.5 mm, more preferably at least 2.0 mm, more preferably at least 2.5 mm, more preferably at least 3.0 mm. In another expression, the linerboard may include cellulosic fibers having a fiber length in a range of 0.5 mm to 10.0 mm, preferably 1 .0 mm to 7.0 mm, more preferably 1.5 to 5.0 mm, more preferably 2.0 to 4.0 mm. A desired fiber length of cellulosic fibers may be achieved by, for example, selecting a cellulosic material having cellulosic fibers with a high fiber length and chemical pulping the cellulosic material to a high kappa number. A preferred cellulosic material having high lignin content and having high fiber length cellulosic fibers includes softwood.

[0023] The linerboard may include a high amount of the high fiber length cellulosic fibers. In one expression, the linerboard may include at least 1%, by volume, of the cellulosic fibers having the high fiber length, preferably at least 5 %, by volume, more preferably at least 10 %, by volume, more preferably at least 20 %, by volume, more preferably at least 30 %, by volume, more preferably at least 40 %, by volume, more preferably at least 50 %, by volume. In another expression, the linerboard may include between 1 % and 99 %, by volume, of the cellulosic fibers having the high fiber length, preferably between 5 % and 95 %, more preferably between 10 % and 90%. A desired amount of the high fiber length cellulosic fibers may be achieved by, for example, selecting a suitable cellulosic material and a suitable pulping process. A preferred cellulosic material having high lignin content and having high fiber length cellulosic fibers includes softwood. A desired amount may also be achieved by blending virgin and recycled cellulosic fibers.

[0024] The thickness and basis weight of the linerboard may depend on requirements for a corrugated board. In an aspect, the linerboard may have a basis weight in a range of between 17 Ibs/MSF (pounds per 1000 ft ² ) and 90 Ibs/MSF.

[0025] In an aspect, the linerboard may comprise at least 50 wt% unbleached fibers, preferably at least 60 wt% unbleached fibers, more preferably at least 70 wt% unbleached fibers, more preferably at least 80 wt% unbleached fibers, and more preferably at least 85 wt% unbleached fibers.

[0026] Conventionally, calendering has been used for surface property improvements for paperboard. In a typical calendering process, paperboard is rewetted and passed through a calendering device having two or more hard rolls. The wet stack calendering process smoothes the paperboard by compressing the fiber network to reduce the pits and crevices. The result is a smooth paperboard with reduced board thickness. However, calendering has been cited as a negative attribute to developing strength. In comparison, linerboard is mostly flexo printed. For the present description, calendering occurs at a high calendering temperature, preferably above a glass transition temperature of the lignin. It is understood that, by calendering above a glass transition temperature of the lignin, lignin flow is promoted, and a relative bonded area of the cellulose fibers is increased to increase a compression and burst strength of the linerboard.

[0027] For the linerboard of the present description, calendering is preferably performed at a temperature of at least 70 °C, more preferably at least 80 °C, more preferably at least 90 °C, more preferably at least 100 °C, more preferably at least 110 °C, more preferably at least 120 °C, more preferably at least 130 °C, more preferably at least 140 °C, more preferably at least 150 °C, more preferably at least 160 °C, more preferably at least 170 °C, more preferably at least 180 °C, more preferably at least 190 °C. In another aspect, calendering is preferably performed in a range of 70 °C to 260 °C, more preferably in a range of 80 °C to 255 °C, more preferably performed in a range of 90 °C to 250 °C, more preferably performed in a range of 100 °C to 240 or

[0028] For the linerboard of the present description, the pressure applied during calendering is preferably at least 50 pli, more preferably at least 100 pli, more preferably at least 150 pli, more preferably at least 200 pli, more preferably at least 250 pli, more preferably at least 300 pli, more preferably at least 350 pli, more preferably at least 400 pli, more preferably at least 450 pli, more preferably at least 500 pli, more preferably at least 550 pli, more preferably at least 600 pli, more preferably at least 650 pli, more preferably at least 700 pli. In another aspect, the pressure applied during calendering is preferably in a range of 50 pli to 3000 pli, more preferably 200 pli to 2500 pli, more preferably 80 pli to 2000 pli, more preferably 110 pli to 1750 pli, more preferably 140 pli to 1500 pli.

[0029] The calendering of the present description may be performed by any method suitable for applying heat and pressure to the linerboard. One method is to pass the linerboard through a nip between one or more heated rolls, and the linerboard is pressed by the heated rolls. The heat from the rolls raises the temperature of the linerboard to the glass transition temperature of the lignin. Alternatively, the linerboard may be pre-heated before being passed between a nip of one or more heated or unheated rolls.

[0030] Moisture (e.g., water or steam) can also be added before the nip to the linerboard. One method of moisture addition is by using waterboxes on at least one calender stack. Another method of moisture addition is by spraying for more precise moisture addition. Other methods of moisture addition are included within the present description. In the case of high levels of moisture addition, drying of the linerboard may be used to obtain a desired moisture content for subsequent processing, e.g., reeling.

[0031] In another embodiment, the present description includes calendered linerboard, which may be manufactured by the method of the present description. The calendered linerboard includes cellulosic fibers and lignin, wherein the linerboard includes a lignin content of at least 8%, by weight percent, and wherein the linerboard has been calendered at a temperature of at least 70 °C. Additional aspects of the calendered linerboard are described above in the description of the method. [0032] Furthermore, calendered linerboard preferably has a high compression strength as measure by STFI compression strength. The resulting STFI compression strength of the calendered linerboard depends on the lignin content of the linerboard.

[0033] In one aspect, the STFI compression strength of the calendered linerboard is preferably at least 1% higher than the STFI compression strength of the uncalendered linerboard, more preferably at least 2% higher, more preferably at least 3% higher, more preferably at least 5% higher, more preferably at least 8% higher, more preferably at least 10% higher, more preferably at least 12% higher, more preferably at least 15% higher, more preferably at least 18% higher, more preferably at least 20% higher. Referring to Fig. 2, in one example, a uncalendered linerboard having a lignin content of 14 percent had STFI compression strength of less than 30.0 Nm/g, and increased up to 35.0 Nm/g after high temperature calendering, evidencing an increased STFI compression strength of 16.6 %. In another example, a uncalendered linerboard having a lignin content of 24 percent had STFI compression strength of less than 17.5 Nm/g, and increased up to 22.5 Nm/g after high temperature calendering, evidencing an increased STFI compression strength of 28.5 %.

[0034] In another aspect, a product of a STFI compression strength of the linerboard times the lignin content of the linerboard is at least 420, more preferably at least 430, more preferably at least 440, more preferably at least 450, more preferably at least 460, more preferably at least 470, more preferably at least 480, more preferably at least 490, more preferably at least 500. In another aspect, a product of a STFI compression strength of the linerboard times the lignin content of the linerboard is in a range of 420 to 540. Referring to Fig. 2, in one example, a linerboard having a lignin content of 14 percent was hot calendered to a STFI compression strength of greater than 30 Nm/g, resulting in a STFI compression strength times the lignin content of at least 420. In another example, a lignin content of 24 percent was hot calendered to a STFI compression strength of greater than 17.5 Nm/g, resulting in a STFI compression strength times the lignin content of at least 420.

[0035] The calendered linerboard may further have a high gloss resulting from the calendering process. Thus, in one aspect, calendered linerboard may have been calendered to a TAPPI T 480 Gloss (75°) of at least 15. In another aspect, calendered linerboard may have been calendered to a TAPPI T 480 Gloss (75°) in a range of 15 to 25. A high gloss is not necessarily a beneficial quality for a linerboard material. However, a high gloss can be an indicator that a linerboard has been subjected to high temperature calendering step.

[0036] In another embodiment, the present description includes a corrugated board comprising a first linerboard according to the present description as previously described, in which a corrugating medium is attached to the first linerboard. The corrugated board may further include a second linerboard, and the corrugating medium being positioned between the first linerboard and second linerboard to form a sandwich structure. The second linerboard may also be formed from a linerboard according to the present description as previously described with respect to the first linerboard, or the second linerboard may be different from the first linerboard. The corrugated board may further include additional linerboard layers and/or corrugating medium layers. For example, the corrugated board may include a double sandwich structure or triple sandwich structure.

[0037] Referring to Fig. 1, an exemplary corrugated board 100 is illustrated. The corrugated board 100 may include a first linerboard 101, a second linerboard 103, and a corrugating medium 105 attached to and disposed between the first linerboard 101 and the second linerboard 103. The first linerboard 101 and second layer 103 may be attached to the corrugating medium 105 at one or more peaks 105a, 105b of the corrugating medium 105 using an adhesive 107.

[0038] Experimental Results

[0039] The experiment was conducted on a laboratory calender stack. Unbleached handsheets were made from two unbleached kraft cooked softwood fibers at lignin contents of 14% and 24%, by weight percent. The sheets were fed through a calender under pressure from 10.9 pli to 75.9 pli at temperatures 167 °F (75 °C) and 392 °F (200 °C) with results shown in FIG. 2. High pressure and high temperature showed the largest change in the short span or measured STFI compression properties. The resulting calendered linerboard had a high compression strength as measure by STFI compression strength. The resulting STFI compression strength of the calendered linerboard depended on the lignin content of the linerboard. Significantly, the experimental data shows strength loss traditionally expected during low temperature calendering, but with high temperature calendering, this strength loss is overcome and even increases above the uncalendered strength.

[0040] Although various embodiments of the disclosed linerboard, corrugated board comprising linerboard and methods for manufacturing thereof have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.