STRETCHABLE PAPER

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of stretchable papers.

BACKGROUND

[0002] Billerud AB (Sweden) has marketed a highly stretchable paper under the name FibreForm^® since 2009. The stretchability of FibreForm^® in both the machine direction (MD) and the cross direction (CD) allows it to replace plastics in many applications.

[0003] In the process of producing FibreForm^®, the paper is microcreped and as a consequence the surface roughness can be unsatisfactory.

SUMMARY

[0004] The present disclosure aims to provide a stretchable paper having a desirable surface roughness.

[0005] Accordingly, the present disclosure provides the following listing of itemized embodiments:

1. Method of producing a paper having a stretchability according to ISO 1924-3:2011 in the machine direction (MD) of at least 8 % and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is 2000 ml/min or lower, wherein the method comprises the following steps:

a) providing a pulp;

b) diluting the pulp into a pulp stock;

c) adding retention aid(s) to the pulp stock, wherein the/each retention aid is added in an amount of 0.03-0.12 kg/tonne based on dry weight of the pulp stock;

d) diluting the retention-aid-containing pulp stock in a headbox with a headbox dilution ratio of 9-13 %, wherein the headbox dilution ratio is the dilution water flow into the headbox divided by the pulp stock flow into the headbox;

e) adding the diluted pulp stock from the headbox to a forming wire to obtain a paper web;

f) pressing the paper web; and

g) drying the pressed paper web into a paper, which drying comprises a step of compacting the paper web in a Clupak unit or an Expanda unit.

2. The method of item 1, wherein the pulp is unbleached pulp.

3. The method of item 1 or 2, wherein the pulp is kraft pulp.

4. The method of any one of the preceding items, wherein the pulp is softwood pulp.

5. The method of any one of the preceding items, wherein the headbox dilution ratio is 9.5-12 %, such as 9.5-11.5 %.

6. The method of any one of the preceding items, wherein the/each retention aid is added an amount of 0.06-0.11 kg/tonne based on dry weight of the pulp stock.

7. The method of any one of the preceding items, wherein one retention aid is anionic polyacrylamide (aPAM).

8. The method of any one of the preceding items, wherein one retention aid is silica microparticles.

9. The method of any one of the preceding items, wherein the headbox has a vertical lip opening of 50-60 mm, such as 52-56 mm.

10. The method of any one of the preceding items, wherein the grammage of the paper according to ISO 536:2020 is 50-200 g/m², such as 75-175 g/m².

11. The method of any one of the preceding items, wherein the diluted pulp stock has a consistency of 0.20-0.30 % based on dry weight when added to the forming wire.

12. The method of any one of the preceding items, wherein the paper has a stretchability according to ISO 1924-3:2011 in the machine direction (MD) of at least 10%, such as at least 11 %, such as at least 12 %.

13. The method of any one of the preceding items, wherein the paper has a stretchability according to ISO 1924-3:2011 in the cross direction (CD) of at least 8%, such as at least 9 %.

14. The method of any one of the preceding items, wherein the Gurley value according to ISO 5636-5:2013 of the paper is at least 15 s, such as at least 20 s.

15. The method of any one of the preceding items, wherein the tensile energy absorption (TEA) index in MD according to ISO 1924-3:2011 is at least 4.0 J/g, such as at least 4.3 J/g.

16. The method of any one of the preceding items, wherein the TEA index in CD according to ISO 1924-3:2011 is at least 3.0 J/g.

17. The method of any one of the preceding items, wherein the grammage of the paper according to ISO 536:2020 is 50-125 g/m² and TEA in MD according to ISO 1924-3:2011 is at least 375 J/m², such as at least 425 J/m².

18. The method of item 17, wherein the Bendtsen roughness of at least one side of the paper is 1800 ml/min or lower according to ISO 8791-2:2013.

19. The method of any one of the items 1-16, wherein the grammage of the paper according to ISO 536:2020 is 126-200 g/m² and TEA in MD according to ISO 1924-3:2011 is at least 600 J/m², such as at least 650 J/m².

20. The method of any one of the preceding items, wherein the density of the paper according to ISO 534:2011 is 650-850 kg/m³.

21. The method of any one of the preceding items, wherein the paper is calendered after being compacted in the Clupak unit or the Expanda unit.

22. An unbleached kraft paper with a grammage according to ISO 536:2020 of 50-200 g/m², wherein the paper is having a stretchability according to ISO 1924-3:2011 in CD of at least 8 % and in MD of at least 11 %, and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper of 2000 ml/min or lower.

DETAILED DESCRIPTION

[0006] As a first aspect of the present disclosure, there is provided a method of producing a paper having a stretchability according to ISO 1924-3:2011 in the machine direction (MD) of at least 8 % and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is 2000 ml/min or lower, wherein the method comprises the following steps:

a) providing a pulp;

b) diluting the pulp into a pulp stock;

c) adding retention aid(s) to the pulp stock, wherein the/each retention aid is added in an amount of 0.03-0.12 kg/tonne based on dry weight of the pulp stock;

d) diluting the retention-aid-containing pulp stock in a headbox with a headbox dilution ratio of 9-13 %, wherein the headbox dilution ratio is the dilution water flow into the headbox divided by the pulp stock flow into the headbox;

e) adding the diluted pulp stock from the headbox to a forming wire to obtain a paper web;

f) pressing the paper web; and

g) drying the pressed paper web into a paper, which drying comprises a step of compacting the paper web in a Clupak unit or an Expanda unit.

[0007] The inventors have realized that surface roughness is improved, i.e. lowered, by using a dilution ratio of 9-13% in the headbox. Preferably, the headbox dilution ratio is 9.5-12 %, such as 9.5-11.5 %. This is an increased dilution ratio compared with what is customary for paper. The dilution of the stock will, thereby, to a greater extent be conducted in the headbox instead of before the headbox. The dilution is typically conducted by using a headbox having actuators as well as several dilution zones in the headbox. Further, the actuator average dilution percentage is typically 56-64 % open, such as 60-62% open. The headbox dilution ratio is the dilution water flow into the headbox divided by the pulp stock flow into the headbox. Typically, the dilution ratio is increased by increasing the dilution water flow without making any significant adjustments of the pulp stock flow. Preferably, the final consistency of the pulp stock in the headbox, that is the pulp stock applied to the forming wire, is 0.20-0.30 % based on dry weight when added to the forming wire. Such consistency is comparable to consistencies regularly used when forming paper. That is, even though the dilution ratio is higher than is customary for paper, the final consistency is not substantially altered.

[0008] The amount of pulp stock that is applied on the wire per second is controlled by a slice opening arrangement of the headbox. The slice opening arrangement consists of two lips that are parallel to each other, a stationary lip and a regulating lip. Depending on the distance between the lips i.e. vertical lip opening, the pulp stock flow from the headbox to the wire can be varied. Typically, the headbox has a vertical lip opening of 50-60 mm, such as 52-56 mm. Combining such vertical lip opening with the headbox dilution ratio and amount of retention aid(s), synergistically further improves roughness.

[0009] Retention aid(s) is/are added to the pulp stock, wherein the/each retention aid is added in an amount of 0.03-0.12 kg/tonne based on dry weight of the pulp stock. Preferably, the/each retention aid is added an amount of 0.06-0.11 kg/tonne based on dry weight of the pulp stock. Typically, one retention aid is anionic polyacrylamide (aPAM). If aPAM is added in such amount, the paper will be less open, which is beneficial for low surface roughness. Typically, one other retention aid is silica microparticles. The inventors have realized that there is a synergistic effect with respect to improved roughness of the addition of retention aid(s) wherein the/each retention aid is added in an amount of 0.03-0.12 kg/tonne, preferably 0.06-0.11 kg/tonne, based on dry weight of the pulp stock and using the specific dilution ratio in the headbox.

[0010] The combination of the method steps of the first aspect is not limited to any particular type of pulp as the improved roughness effect is not limited to a particular pulp. Nevertheless, it is preferred that the pulp is a softwood pulp as the long fibres of softwood provides mechanical strength. Accordingly, the pulp may comprise at least 50 % softwood pulp, preferably at least 75 % softwood pulp and more preferably at least 90 % softwood pulp. The percentages are based of the dry weight of the pulp. Moreover, it is preferred that the pulp is a kraft pulp since such pulp provides mechanical strength. Kraft pulp is also known as sulphate pulp. Typically, the pulp is an unbleached pulp.

[0011] The Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is 2000 ml/min or lower, such as 1800 ml/min or lower. It is desired to be able to produce a stretchable paper with a relatively fine surface.

[0012] The grammage of the paper according to ISO 536:2020 is 50-200 g/m², such as 75-175 g/m². If a stretchable material having a grammage above 200 g/ m² is desired, a laminate can be produced from a plurality of paper layers each having a grammage in the range of 50-200 g/ m². Below 50 g/m² the strength and rigidity are typically insufficient.

[0013] The air resistance according to Gurley, i.e. the Gurley value or the Gurley porosity, is a measurement of the time (s) taken for 100 ml of air to pass through a specified area of a paper sheet. Short time means highly porous paper. The Gurley porosity of the paper of the present disclosure is typically above 15 s, such as above 20 s according to ISO 5636-5:2013.

[0014] The paper obtained by the method has a stretchability of at least 8 % in the machine direction (MD). Compacting in the Clupak unit or the Expanda unit increases the stretchability of the paper, in particular in MD. The Clupak unit typically comprises a steel cylinder or a chromed cylinder and a rubber blanket. When the paper web is compacted by the contraction/recoil of the rubber blanket in the Clupak unit, it moves relative the steel/chromed cylinder and becomes micro-creped, especially in MD. The Expanda unit typically comprises a venturi section formed in a nip between a rubber and a steel roll. On running the rubber roll more slowly than the steel roll, the web will shrink in MD and become micro-creped. Preferably, the stretchability in MD is even higher than 8 %, such as at least 10 %, such as at least 11% or at least 12 %. The stretchability enables formation of three-dimensional (double curvature) shapes in the paper, e.g. by press forming, vacuum forming or deep drawing. The formability of the paper in such processes is further improved if the stretchability is relatively high also in the cross direction (CD). Preferably, the stretchability in CD is at least 8 %, such as at least 9 %. An upper limit for the stretchability in MD may for example be 20 % or 25 %. An upper limit for the stretchability in CD may for example be 15 %. The stretchability (in both MD and CD) is determined according to the standard ISO 1924-3:2011. The paper web is preferably allowed to dry freely after the Clupak unit. During such "free drying", which improves the stretchability, the paper web is not in contact with a dryer screen (often referred to as a dryer fabric). A forced, optionally heated, air flow may be used in the free drying, which means that the free drying may comprise fan drying.

[0015] The density of the paper according to ISO 534:2011 is typically 650-850 kg/m³. Higher density typically means reduced bending stiffness, which is often undesired.

[0016] One or more sizing agents may also be added to the pulp stock. Examples of sizing agents are AKD, cationic starch and rosin size. Rosin size can for example be added in an amount of 0.5-4 kg/tonne, preferably 0.7-2.5 kg/tonne, based on dry weight of the pulp stock. Cationic starch can for example be added in an amount of 4-10 kg/tonne, preferably 5-9 kg/tonne paper, based on dry weight of the pulp stock. AKD can for example be added in an amount of 0.03-0.2 kg/tonne, preferably 0.7-0.15 kg/tonne, based on dry weight of the pulp stock. Typically, if added, rosin size and/or cationic starch and/or AKD, is/are added upstream of the headbox.

[0017] The tensile strength is the maximum force that a paper will withstand before breaking. Tensile energy absorption (TEA), that is the area under the cure tensile strength vs stretch, is a measure of how tough the material is. The TEA index is the TEA value divided by the grammage. The TEA index in MD according to ISO 1924-3:2011 is typically at least 4.0 J/g, such as at least 4.3 J/g. The TEA index in CD according to ISO 1924-3:2011 is typically at least 3.0 J/g. A paper of higher tensile strength and stretch has higher TEA value. Thus, a paper of higher grammage typically has a higher TEA value. For TEA index, on the other hand, this effect is substantially insignificant as a higher TEA value is balanced by a higher grammage and a lower TEA value is compensated for in the division by a lower grammage. Consequently, the TEA index does not vary to any great extent depending on grammage. The TEA value, on the other hand, is typically varied with grammage. Accordingly, the grammage of the paper according to ISO 536:2020 maybe 50-125 g/m² and TEA in MD according to ISO 1924-3:2011 is then least 375 J/m², such as at least 425 J/m². Alternatively, the grammage of the paper according to ISO 536:2020 may be 126-200 g/m² and TEA in MD according to ISO 1924-3:2011 is at least 600 J/m², such as at least 650 J/m².

[0018] The paper may be calendered after being compacted in the Clupak unit or the Expanda unit. In such case, the calender is preferably a soft nip calender. A soft nip calender comprises a soft, resilient, calender roll and a hard backing roll, typically a steel roll. The steel roll may be heated, by e.g. steam or oil. If the paper is calendered, the Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper typically becomes 200-700 ml/min.

[0019] As a second aspect of the present disclosure, there is provided an unbleached kraft paper with a grammage according to ISO 536:2020 of 50-200 g/m², wherein the paper is having a stretchability according to ISO 1924-3:2011 in CD of at least 8 % and in MD of at least 11 %, and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper of 2000 ml/min or lower.

[0020] The examples and embodiments discussed above in connection to the first aspect apply to the second aspect mutatis mutandis.

EXAMPLES

Preparation of paper

[0021] Unbleached softwood kraft pulp was provided. The pulp was diluted into a pulp stock and subjected to high consistency (HC) refining (240-280 kWh per ton pulp) at a consistency of about 33-36 %. To the refined pulp stock, anionic acrylamide (aPAM) was added in a content of 0.15 kg/ton (reference) or 0.08-0.10 kg/ton (inventive example) based on dry weight as well as silica microparticles (SM) in a content of 0.3-0.35 kg/ton (reference) or 0.08-0.10 kg/ton (inventive example) based on dry weight. Cationic starch (7 kg per dry tonne paper), rosin size (0.7 kg per dry tonne paper) and AKD (0.1 kg per dry tonne paper) were also added.

[0022] The retention-aid-containing pulp stock was forwarded into a headbox (Voith Sulzer type ModuleJet SD year 1998), wherein the pulp stock was diluted. The stock flow into the headbox was 2469 l/s, and the dilution water flow was 207 l/s (reference) or 257 l/s (reference/inventive example). 207 l/s provides a dilution ratio of 8.4 % (207/2469^∗100) and 257 l/s provides a dilution ratio of 10.4% (257/2469^∗100). In both cases, the stock was diluted to a final headbox consistency of 0.23-0.25 % based on dry weight.

[0023] The stock was, thereafter, added to a forming wire with a speed of 440 m/min to obtain a paper web. The vertical lip of the headbox was set to 48 mm (reference) or to 52-56 mm (inventive example). The paper web was dewatered in a press section having three nips. The dewatered paper web was then dried in a subsequent drying section including one Clupak unit to obtain a paper. Papers were produced having a grammage of 100 g/m² and 150 g/m².

[0024] The differences in production process between the references and inventive examples (IE) are presented in table 1 below.

Table 1. Production process variables for references (Refs) and inventive examples (IEs).

	Retention aids	Headbox dilution ratio	Headbox lip opening
Ref1	aPAM: 0.15 kg/ton	8.4 %	48 mm
	SM1: 0.3-0.35 kg/ton
Ref2	aPAM: 0.15 kg/ton	10.4 %	48 mm
	SM1: 0.3-0.35 kg/ton
IE1	aPAM: 0.08-0.10 kg/ton	10.4 %	48 mm
	SM1: 0.08-0.10 kg/ton
IE2	aPAM: 0.08-0.10 kg/ton	10.4 %	52-56 mm
	SM1: 0.08-0.10 kg/ton

1 Silica Microparticles

[0025] The properties of the papers produced in the trials are presented in table 2 below.

Table 2. Properties of produced papers.

	100 g/m2			150 g/m2
	Stretch CD (%)	Stretch MD (%)	Bendtsen Roughness print (wire) side (ml/min)	Stretch CD (%)	Stretch MD (%)	Bendtsen Roughness print (wire) side (ml/min)
Ref1	9.7	12.2	2094	9.8	13.4	2597
Ref2	9.3	12.6	2185	9.9	12.9	1934
IE1	9.6	12.8	1407	9.4	13.2	1728
IE2	9.8	12.2	1635	9.7	13.3	1645

[0026] Further mechanical properties of Ref1 were evaluated and presented in table 3 below. Since the stretch properties were in principle unaffected by the process of the inventive examples it can be anticipated that also these mechanical properties will be about the same for the inventive examples.

Table 3. Properties of Ref1.

	100 g/m2	150 g/m2
TEA MD (J/m2)	450	710
TEA CD (J/m2)	280	460
TEA Index MD (J/g)	4.5	4.7
TEA Index CD (J/g)	2.8	3.0

Claims

1. Method of producing a paper having a stretchability according to ISO 1924-3:2011 in the machine direction (MD) of at least 8 % and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is 2000 ml/min or lower, wherein the method comprises the following steps:

a) providing a pulp;

b) diluting the pulp into a pulp stock;

c) adding retention aid(s) to the pulp stock, wherein the/each retention aid is added in an amount of 0.03-0.12 kg/tonne based on dry weight of the pulp stock;

d) diluting the retention-aid-containing pulp stock in a headbox with a headbox dilution ratio of 9-13 %, wherein the headbox dilution ratio is the dilution water flow into the headbox divided by the pulp stock flow into the headbox;

e) adding the diluted pulp stock from the headbox to a forming wire to obtain a paper web;

f) pressing the paper web; and

g) drying the pressed paper web into a paper, which drying comprises a step of compacting the paper web in a Clupak unit or an Expanda unit.

2. The method of claim 1, wherein the pulp is unbleached pulp.

3. The method of claim 1 or 2, wherein the pulp is kraft pulp.

4. The method of any one of the preceding claims, wherein the headbox dilution ratio is 9.5-12 %, such as 9.5-11.5 %.

5. The method of any one of the preceding claims, wherein the/each retention aid is added an amount of 0.06-0.11 kg/tonne based on dry weight of the pulp stock.

6. The method of any one of the preceding claims, wherein one retention aid is anionic polyacrylamide (aPAM).

7. The method of any one of the preceding claims, wherein one retention aid is silica microparticles.

8. The method of any one of the preceding claims, wherein the headbox has a vertical lip opening of 50-60 mm, such as 52-56 mm.

9. The method of any one of the preceding claims, wherein the grammage of the paper according to ISO 536:2020 is 50-200 g/m², such as 75-175 g/m².

10. The method of any one of the preceding claims, wherein the diluted pulp stock has a consistency of 0.20-0.30 % based on dry weight when added to the forming wire.

11. The method of any one of the preceding claims, wherein the paper has a stretchability according to ISO 1924-3:2011 in MD of at least 10%, such as at least 11 %, such as at least 12 %.

12. The method of any one of the preceding claims, wherein the paper has a stretchability according to ISO 1924-3:2011 in the cross direction (CD) of at least 8%, such as at least 9 %.

13. The method of any one of the preceding claims, wherein the tensile energy absorption (TEA) index in MD according to ISO 1924-3:2011 is at least 4.0 J/g, such as at least 4.3 J/g.

14. The method of any one of the preceding claims, wherein the Bendtsen roughness of at least one side of the paper is 1800 ml/min or lower according to ISO 8791-2:2013.

15. An unbleached kraft paper with a grammage according to ISO 536:2020 of 50-200 g/m², wherein the paper is having a stretchability according to ISO 1924-3:2011 in CD of at least 8 % and in MD of at least 11 %, and a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper of 2000 ml/min or lower.