ES2975591T3 - New substrate - Google Patents

Abstract

A high-density paper is provided having a grammage measured according to ISO 536:2012 of 30-75 g/m2 and a density measured according to ISO 534:2011 greater than 1000 kg/m3 and wherein the high-density paper is impregnated on each side with 0.3-4.0 g/m2 , such as 0.5-3.0 g/m2 , of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC), and carboxymethyl starch (CMS). (Automatic translation with Google Translate, no legal value)

Description

DESCRIPTION

New substrate

Technical field

The present disclosure relates to a paper substrate to be used in a multilayer material for packaging oxygen-sensitive products, such as foods.

Background

Cardboard-based packaging laminates for packaging liquid foods typically include an aluminium foil to make the packaging container gas-tight, in particular oxygen-tight.

To facilitate recycling and reduce the carbon footprint, it would be beneficial to find an alternative to aluminium foil. WO 2011/003565 discloses a packaging laminate in which aluminium foil is replaced with a paper or cellulose-based material (a "substrate") that is pre-coated and then covered with a metal vapour deposited layer. The basis weight (grammage) of the substrate is stated to be preferably 20 to 100 g/m2.

A subsequent patent application, WO 2017/089508, discloses additional details about the substrate, namely that it is a compact surface barrier paper having a density of 800 kg/m or more, a surface roughness value of less than 450 ml/min (Bendtsen, ISO 8791-2), a thickness of 60 pm or less, and a basis weight of 60 g/m2 or less. A specific example of the compact surface barrier paper is "Super Perga WS" (Nordic Paper) having a basis weight of 32 g/m2 and a surface roughness value of approximately 200 ml/min.

Brief description

An objective of the present disclosure is to provide a novel cellulose-based substrate that improves the barrier properties of non-aluminum foil packaging. Another objective is to provide a novel cellulose-based substrate that facilitates recycling of a packaging material comprising the substrate. A final and general objective is to reduce the environmental impact of packaging materials.

Accordingly, the following detailed list of embodiments of the present disclosure is provided:

1. A high density paper having a grammage measured in accordance with ISO 536:2012 of 30-75 g/m2 and a density measured in accordance with ISO 534:2011 above 1000 kg/m3 and wherein the high density paper is impregnated on each side with 0.3-4.0 g/m2, such as 0.5-3.0 g/m2, of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS).

2. The high density paper of item 1, which is formed from at least 50% by dry weight of kraft pulp, such as at least 75% by dry weight of kraft pulp, such as at least 85% by dry weight of kraft pulp, such as at least 95% by dry weight of kraft pulp.

3. High-density paper of point 1 or 2, where 20-65% by dry weight, such as 30-60% by dry weight, of the pulp used to form the high-density paper is hardwood pulp.

4. High-density paper of any of the above, wherein 35-80% by dry weight, such as 40-70% by dry weight, of the pulp used to form the high-density paper is softwood pulp, preferably softwood that has undergone high consistency (HC) refining.

5. High-density paper of any of the above points that has a top layer and a bottom layer.

6. The high density paper of item 5, wherein the top layer is formed from at least 50% by dry weight of hardwood pulp, such as at least 65% by dry weight of hardwood pulp, such as at least 75% by dry weight of hardwood pulp.

7. High density paper of point 5 or 6, wherein the bottom layer is formed from at least 50% by dry weight of softwood pulp, such as at least 65% by dry weight of softwood pulp, such as at least 75% by dry weight of softwood pulp.

8. High density paper of any of the above points, where the grammage measured according to ISO 536:2012 is 35-65 g/m2.

9. High density paper of any of the preceding items, wherein at least one side of the high density paper has a Bendtsen roughness measured in accordance with SS-ISO 8791-2:2013 of 7-80 ml/min, such as 8-70 ml/min, such as 10-60 ml/min, such as 30-60 ml/min.

10. High density paper of any of the above having a tear index measured in the machine direction (MD) according to ISO 1924-3:2005 of at least 4.1 mNm2/g, such as at least 4.4 mNm2/g.

11. High density paper of any of the above having a tear index measured in the cross direction (CD) according to ISO 1924-3:2005 of at least 4.9 mNm2/g, such as at least 5.3 mNm2/g.

12. High density paper of any of the above items having an ash content measured in accordance with ISO 2144:2015 of high density paper is less than 5% by dry weight, such as less than 3% by dry weight, such as less than 1% by dry weight.

13. High density paper of any of the above, having a Schopper-Riegler number (°SR) measured in accordance with ISO 5267-1:1999 of 33-50, such as 35-45, after repulping in accordance with ISO 5263-1:2004.

14. High density paper of any of the above, having a Canadian Standard Freeness (CSF) measured in accordance with ISO 5267-2:2001 of at least 200 ml, such as 200-450 ml, such as 200-350 ml, after repulping in accordance with the Valmet repulping method carried out on a Valmet repulper of the HD400 type.

15. High density paper of any of the above, having a fines content measured with an L&W Fibretester (ABB, Lorentzen & Wettre, Sweden) of less than 40%, such as less than 34%, such as less than 32%, after repulping in accordance with ISO 5263-1:2004, where fines are defined as fibrous particles shorter than 0.2 mm.

16. A high density paper having a grammage measured according to ISO 536:2012 of 30-75 g/m2 and a density measured according to ISO 534:2011 above 1000 kg/m3 and wherein the high density paper is impregnated on at least one side with a composition comprising polyvinyl alcohol (PVOH) and glyoxal.

17. The high density paper of item 16, wherein the dry weight ratio of glyoxal to PVOH in the composition is between 3:100 and 12:100, preferably between 4:100 and 9:100, more preferably between 5:100 and 8:100.

18. A method for forming a high density paper having a grammage measured in accordance with ISO 536:2012 of 30-75 g/m2 and a density measured in accordance with ISO 534:2011 of more than 1000 kg/m3, the method comprising the steps of:

- impregnating each side of a paper substrate with 0.3-4.0 g/m2, such as 0.5-3.0 g/m2, of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS) to obtain an impregnated substrate; and

- supercalendering of the impregnated substrate to obtain high-density paper.

19. The method of claim 18, wherein the impregnating step comprises adding an aqueous composition comprising the polymer to each side of the paper substrate.

20. The method of item 19, wherein the viscosity measured at 60 °C of the aqueous composition is 55-90 mPa*s. 21. The method of item 19 or 20, wherein the concentration of the polymer in the aqueous composition is 7.0%-13.0% (w/v), such as 8.0%-12.0% (w/v).

22. The method according to any of items 19-21, wherein the polymer is PVOH and the aqueous composition further comprises a glyoxal crosslinker.

23. The method of any of items 19-22, wherein the dry weight ratio of PVOH to glyoxal crosslinker in the aqueous composition is 100:3 to 100:8, such as 100:4 to 100:7.

24. The method of any of items 18-23, wherein the impregnated substrate entering the supercalendering step has a moisture content of 11.0%-20.0%, preferably 12.0%-19.0%, more preferably 13.5%-18.0%.

25. The method of claim 24, wherein the impregnated substrate is dried after the impregnation step to a moisture content below 11% and then re-wetted before the supercalendering step to the moisture content of 11.0%-20.0%, preferably 12.0%-19.0%, more preferably 13.5%-18.0%.

26. The method of item 25, wherein the impregnated substrate is dried after the impregnation step to a moisture content of less than 10%.

27. The method of item 26, wherein the impregnated substrate is dried after the impregnation step to a moisture content of less than 9%.

28. The method of any of items 18-27, wherein the impregnation step is carried out by means of a size press or a film press.

29. The method of any of 18-28, where the number of contact points of the supercalendering step is 7-19, such as 11-17.

30. The method of any of items 18-29, wherein the surface temperature of the hot rollers of the supercalendering step is 120-160 °C.

31. The method of any of items 18-30, wherein the total nip impulse of the supercalendering step is at least 600 kPa*s.

32. The method of any of items 18-31, wherein the paper substrate is formed from at least 50% by dry weight of kraft pulp, such as at least 75% by dry weight of kraft pulp, such as at least 85% by dry weight of kraft pulp, such as at least 95% by dry weight of kraft pulp.

33. The method of any of items 18-32, wherein the paper substrate is formed from 20-65% dry weight hardwood pulp, such as 30-60% dry weight hardwood pulp.

34. The method of any of items 18-33, wherein the paper substrate is formed from 35-80% dry weight softwood pulp, such as 40-70% dry weight softwood pulp.

35. The method of point 34, where the softwood pulp has been subjected to high consistency (HC) refining.

36. The method of any of items 18-35, wherein the paper substrate has a first layer and a second layer.

37. The method of item 36, wherein a first wire is used to form a first fabric that becomes the top layer and a second wire is used to form a second fabric that becomes the second layer, the first and second bands of which are joined together.

38. The method of claim 37, wherein the first fabric is formed from a first pulp comprising at least 50% by dry weight of hardwood pulp, such as at least 65% by dry weight of hardwood pulp, such as at least 75% by dry weight of hardwood pulp.

39. The method of point 38, where the Schopper-Riegler number (°SR) measured according to ISO 5267-1:1999 of the first pulp is 33-45.

40. The method of any of items 37-39, wherein the second fabric is formed from a second pulp comprising at least 50% by dry weight of softwood pulp, such as at least 65% by dry weight of softwood pulp, such as at least 75% by dry weight of softwood pulp, preferably softwood pulp that has undergone high consistency (HC) refining.

41. The 40-point method, where the Schopper-Riegler number (°SR) measured according to ISO 5267-1:1999 of the second pulp is 25-35.

42. The method according to any of items 36-41, wherein the second layer is impregnated with a greater amount of polymer than the first layer.

43. The method according to any of points 18-42, wherein the density measured in accordance with ISO 534:2011 of the high density paper is greater than 1050 kg/m3.

44. A coated paper comprising a high density paper according to any of items 1-17, wherein a surface of the high density paper is provided with a barrier coating comprising polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), a starch or starch derivative, a nano- or microfibrillar cellulose, polyvinylidene chloride (PVDC) or a polyamide.

45. The coated paper of article 44, wherein the barrier coating comprises PVOH in an amount of 1-3 g/m2.

46. Use of a multilayer material for packaging a food product or other oxygen-sensitive product, wherein one layer of the multilayer material is a high-density paper according to any of items 1-17, provided that the food product is not a liquid, semi-liquid or viscous food product.

47. The use of item 46, wherein the multilayer material comprises a coated paper according to item 44 or 45.

48. High density paper of any of items 1-17, which is not PECVD or PVD coated.

49. The method of any of items 18-43, provided that the method does not involve PECVD or PVD coating of high-density paper.

50. Coated paper of item 44 or 45, which is not PECVD or PVD coated.

51. The use of item 46 or 47, where the multilayer material does not comprise a PECVD or PVD coating. Brief description of the drawings

Figure 1A is a SEM image of a surface portion of the paper substrate produced in Example 1A below.

Figure 1B is a SEM image of a surface portion of the PVOH impregnated paper substrate produced in Example 1B below.

Figure 1C is a SEM image of a surface portion of the high-density paper produced in Example 1C below.

Figure 2 is a SEM image of a cross section of the high-density paper produced in Example 1C below.

Figure 3 is a graph showing the oxygen transmission rate (ml/sample) before and after folding and unfolding of PVOH coated papers described in Example 2 below.

Detailed description

As a first aspect of the present disclosure, there is provided a high density paper having a basis weight measured according to ISO 536:2012 of 30-75 g/m2 and a density measured according to ISO 534:2011 above 1000 kg/m3. This high density is preferably obtained by supercalendering. Accordingly, the high density paper of the present disclosure is preferably supercalendered. In one embodiment, the density is at least 1050 kg/m3, such as at least 1070 kg/m3. A typical upper limit for the density may be 1300 kg/m3. The high density paper is impregnated on each side with 0.3-4.0 g/m2, such as 0.5-4.0 g/m2, such as 0.5-3.0 g/m2 of a polymer. "Impregnated" means that the polymer has to a substantial degree penetrated into the fiber web. However, it does not necessarily mean that the fibre web, through its thickness direction, has been completely saturated with the polymer. Consequently, the high-density paper may comprise unfilled pores, especially in the middle. Reference is also made here to the analysis on Figure 2 below. It can therefore be determined from an SEM image of a cross section of the high-density paper that the polymer has penetrated into the fibre web instead of forming a coating layer on the surface.

The polymer is selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC), and carboxymethyl starch (CMS). PVOH, EVOH, and CMC are preferred examples of the polymer. In one embodiment, the polymer is PVOH or EVOH.

To facilitate application of the polymer, it is preferably water soluble or water dispersible.

The most preferred polymer in the group is PVOH. The degree of hydrolysis of PVOH may be 96%-100%, such as 97%-100%, such as 97%-99%. A PVOH having a high degree of hydrolysis is less sensitive to water and is preferred, both in production and in use. The weight average molecular weight (Mw) of PVOH is preferably less than 100,000 g/mol, such as 10,000-90,000 g/mol, such as 30,000-80,000 g/mol. This relatively low Mw is preferred during impregnation since it has a relatively low viscosity at a relatively high concentration. A low Mw PVOH has a greater tendency to penetrate into the fiber web rather than remain on the paper surface.

The viscosity of PVOH when measured according to DIN 53015 is preferably less than 20 mPa*s, such as 5-16 mPa*s, such as 6-13 mPa*s.

The degree of polymerization (DP) of PVOH is preferably less than 3000, such as 1000-2000. The DP can be determined from the average degree of polymerization viscosity derived from the viscosity in water. In this case, the viscosity is measured in a 4% aqueous solution at 20 °C and determined by a Brookfield synchronous motor rotary type viscometer.

An example of a suitable PVOH is Poval 10/98 from Kuraray, which has a viscosity of 10 mPa*s, a degree of hydrolysis of 98%, a DP of about 1400 and a Mw of about 61,000 g/mol. Another example is Poval 6/98 from Kuraray, which has a viscosity of 6 mPa*s and a degree of hydrolysis of 98%.

The PVOH or EVOH may comprise a crosslinker, such as glyoxal. The dry weight ratio of glyoxal to PVOH may be between 3:100 and 12:100, preferably between 4:100 and 9:100, more preferably between 5:100 and 8:100.

In some applications, it may be beneficial to select EVOH as the polymer. EVOH has high moisture resistance and excellent oxygen barrier properties. An example of an EVOH is Kuraray's Exceval® AQ-4104, which provides low viscosity.

As mentioned above, the polymer can also be NCC, which is a form of nanocellulose, but is not the same as microfibrillar cellulose ("MFC") or nanofibrillar cellulose ("NFC"). While the term "MFC" can be used generally and incorrectly for all types of defibrillated cellulose, there is a more scientific view that "MFC" should denote fibers or fibrils or aggregates of fibrils of nanoscale cellulose particles with at least one dimension of less than 100 nm. Thus, MFC can contain longer particles, so-called "fibrils" that have a width of 10-100 nm, and a length of at least 1 pm, such as up to 10 pm, such as more than 10 pm.

Both MFC and NFC have an aspect ratio of 50 or higher, while NCC can be defined as having an aspect ratio less than 50, for example, according to the TAPPI WI3021 draft standard.

The term "NCC" is used for shorter particles and "rod-like" particles, which have a width of 3-50 nm, and a length of 100 to 1000 nm, such as 100 to 900 nm, such as 100 to 500 nm, such as 100 to 200 nm. The preferred dimensions of NCC for the purpose of impregnating and filling pores in a formed paper, meaning that the majority of NCC particles in the composition should have this dimension, perhaps a length of 100-500 nm, such as 100-200 nm and a width of 3-50 nm.

In one embodiment, the high density paper is formed from at least 50% by dry weight of chemical pulp, such as at least 75% by dry weight of chemical pulp, such as at least 85% by dry weight of chemical pulp, such as at least 95% by dry weight of chemical pulp. The chemical pulp is preferably kraft pulp, but may also be sulfite pulp (i.e., pulp obtained by the sulfite pulp process).

In one embodiment, 20-65% by dry weight, such as 30-60% by dry weight, of the pulp used to form the high density paper is hardwood pulp. In an alternative or complementary embodiment, 35-80% by dry weight, such as 40-70% by dry weight, of the pulp used to form the high density paper is softwood pulp. A benefit of including hardwood pulp is that it collapses relatively easily during refining, while still allowing for efficient dewatering in the wire section of the paper machine. A benefit of including softwood pulp is the improved runnability on the paper machine and beneficial strength/toughness properties in the resulting paper. The latter properties can be improved by subjecting the softwood pulp to high consistency (HC) refining.

In one embodiment, the high-density paper has a top layer and a bottom layer. In this construction, the properties of the top layer can be tailored to receive another barrier layer, while the properties of the bottom layer are tailored for strength/toughness. Alternatively, the top layer can be tailored for printing, while the bottom layer is coated with one or more additional layers.

In one embodiment, the top layer is formed from at least 50% by dry weight of hardwood pulp, such as at least 65% by dry weight of hardwood pulp, such as at least 75% by dry weight of hardwood pulp. The hardwood pulp may provide an improved surface for printing or another barrier layer.

In one embodiment, the bottom layer is formed from at least 50% by dry weight of softwood pulp, such as at least 65% by dry weight of softwood pulp, such as at least 75% by dry weight of softwood pulp. As mentioned above, softwood is associated with improved runnability on the paper machine and provides beneficial strength/toughness properties in the resulting paper product.

In one embodiment, the bottom layer side is impregnated with a higher amount of polymer than the top layer. In one embodiment, the basis weight measured in accordance with ISO 536:2012 is 35-65 g/m2. A relatively low basis weight can be beneficial as it may require less polymer in a packaging application, for example in order to make long lasting, airtight seals when processing the laminated material into filled and sealed cuboid shaped packages. In turn, the use of less polymer facilitates high fibre content recycling streams. There is also evidence that package integrity is improved when the basis weight of the paper substrate is lower. However, the strength and toughness of the paper may be insufficient if the basis weight is too low.

In one embodiment, at least one side of the high density paper has a Bendtsen roughness measured in accordance with SS-ISO 8791-2:2013 of 7-80 ml/min, such as 8-70 ml/min, such as 10-60 ml/min, such as 30-60 ml/min. Preferably, both sides of the high density paper have a Bendtsen roughness measured in accordance with SS-ISO 8791-2:2013 of 7-80 ml/min, such as 8-70 ml/min, such as 10-60 ml/min.

Preferably, the beneficial barrier and recycling properties are obtained without sacrificing strength, such as tensile strength and tear strength.

The tensile index measured in MD according to ISO 1924-3:2005 is preferably greater than 100 Nm/g, such as 110-150 Nm/g. The tensile index measured in CD according to ISO 1924-3:2005 is preferably greater than 50 Nm/g, such as 55-90 Nm/g, such as 65-90 Nm/g.

In one embodiment, the tear rate measured in the machine direction (MD) according to ISO 1924-3:2005 is at least 4.1 mNm2/g, such as at least 4.4 mNm2/g and/or the tear rate measured in the cross direction (CD) according to ISO 1924-3:2005 of at least 4.9 mNm2/g, such as at least 5.3 mNm2/g. Typical upper limits in MD and CD may be 6.5 mNm2/g and 7.5 mNm2/g, respectively.

Preferably, the high density is obtained without any substantial addition of inorganic filler to the pulp. Silica or bentonite used as a retention agent, typically in an amount of less than 1 kg per tonne of dry pulp, is not considered an inorganic filler. Accordingly, the ash content measured in accordance with ISO 2144:2015 of the high density paper is preferably less than 5% by dry weight, such as less than 3% by dry weight, such as less than 1% by dry weight.

The high density paper of the present disclosure can be obtained without extensive low consistency (LC) refining, which improves the speed of the papermaking process (and reduces its energy consumption) and facilitates recycling. This comparatively limited refining can be reflected by the drainability measured after repulping. In one embodiment, the high density paper exhibits a Schopper-Riegler number (°SR) measured according to ISO 5267-1:1999 of 33-50, such as 35-45, after repulping according to ISO 5263-1:2004. Another drainability value is the Canadian Standard Slickness (CSF). In one embodiment, the high density paper exhibits a CSF measured in accordance with ISO 5267-2:2001 of at least 200 ml, such as 200-450 ml, such as 200-350 ml, after repulping according to the Valmet repulping method carried out on a Valmet repulper of the HD400 type. The Valmet pulping method is described below in the examples section. A moderate degree of refining may also be reflected in a relatively low fines content. In one embodiment, the high density paper has a fines content measured with an L&W Fibretester (ABB, Lorentzen & Wettre, Sweden) of less than 40%, such as less than 34%, such as less than 32%, after repulping in accordance with ISO 5263-1:2004. "Fines" are defined as fibrous particles shorter than 0.2 mm. The L&W Fibretester+ operates in accordance with ISO 16065-2:2014. A typical lower limit for fines content is 15% or 20%.

As a configuration of the first aspect, there is provided a high density paper having a grammage measured according to ISO 536:2012 of 30-75 g/m2 and a density measured according to ISO 534:2011 above 1000 kg/m3 and wherein the high density paper is impregnated on at least one side with a composition comprising polyvinyl alcohol (PVOH) and glyoxal.

The embodiments of the first aspect discussed above apply to the configuration mutatis mutandis.

Accordingly, the dry weight ratio of glyoxal to PVOH in the configuration composition may be between 3:100 and 12:100, preferably between 4:100 and 9:100, more preferably between 5:100 and 8:100.

In one embodiment, the high density paper of the first aspect (and its configuration) is not PECVD or PVD coated. However, the high density paper of this embodiment may be intended for PECVD or PVD coating. PECVD stands for Plasma Enhanced Chemical Vapor Deposition. PVD stands for Plasma Vapor Deposition. As a second aspect of the present disclosure, there is provided a method for forming a high density paper having a basis weight measured according to ISO 536:2012 of 30-75 g/m2 and a density measured according to ISO 534:2011 of more than 1000 kg/m3, the method comprising the steps of:

- impregnating each side of a paper substrate with 0.3-4.0 g/m2, such as 0.5-4.0 g/m2, such as 0.5-3.0 g/m2, of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS) to obtain an impregnated substrate; and

- supercalendering of the impregnated substrate to obtain high-density paper.

In one embodiment, the impregnation step comprises adding an aqueous composition comprising the polymer to each side of the paper substrate. The viscosity measured at 60 °C of the aqueous composition may be 55-90 mPa*s. This relatively low viscosity facilitates penetration of the polymer into the fiber web. The concentration of the polymer in the aqueous composition is preferably 7.0%-13.0% (w/v), such as 8.0%-12.0% (w/v).

Viscosity measurement at 60 °C is preferably carried out using a Brookfield rotational viscometer equipped with a No. 3 spindle at 100 rpm.

In one embodiment, the polymer is PVOH or EVOH and the aqueous composition further comprises a crosslinker, such as a glyoxal crosslinker.

dry weight ratio of PVOH or EVOH to glyoxal crosslinker in the aqueous composition may be from 100:3 to 100:8, such as from 100:4 to 100:7.

The aqueous composition may further comprise inorganic particles, preferably in a low amount. To facilitate densification and impregnation, the impregnated substrate entering the supercalendering step preferably has a relatively high moisture content, such as 11.0%-20.0%. More preferably, the moisture content is 12.0%-19.0%, such as 13.5%-18.0%.

In one embodiment, the impregnated substrate is dried after the impregnation step to a moisture content of less than 11%, such as less than 10%, such as less than 9%. It is then re-wetted prior to the supercalendering step, for example, to a moisture content in the range of 11.0%-20.0%, 12.0%-19.5%, or 13.5%-18.0%.

The impregnation step is preferably carried out by means of a size press or a film press. The most preferred equipment is a film press. The film press can be an OptiSizer film (Valmet) or a SpeedSizer (Voith).

The number of contact points of the supercalendering step can be 7-19, preferably 11-17. The surface temperature of the heated rollers of the supercalendering step can be 120-160 °C.

The total nip impulse of the supercalendering step can be at least 600 kPa*s.

In one embodiment of the second aspect, the headbox consistency or the headbox consistencies in case of a multiple paper is/are in the range of 0.06%-0.60%, such as 0.10%-0.40%, such as 0.10%-0.30%. These relatively low consistencies facilitate the production of a low porosity paper, meaning a high density.

In one embodiment, the paper substrate of the second aspect is formed from at least 50% by dry weight of chemical pulp, such as at least 75% by dry weight of chemical pulp, such as at least 85% by dry weight of chemical pulp, such as at least 95% by dry weight of chemical pulp. The chemical pulp is preferably kraft pulp, but may also be sulfite pulp.

In one embodiment, the paper substrate of the second aspect is formed from 20-65% dry weight hardwood pulp, such as 30-60% dry weight hardwood pulp and/or 35-80% dry weight softwood pulp, such as 40-70% dry weight softwood pulp.

When softwood pulp is used, it may have been subjected to high consistency (HC) refining, i.e. refined to a consistency of 20%-40%, such as 25%-38%. The specific energy of the HC refining step may be at least 100 kWh/tonne, such as at least 150 kWh/tonne, such as 150-300 kWh/tonne. The "tonne" in the unit means tonne of dry fibre.

The effects of selection pulps are described above in relation to the first aspect.

In one embodiment, the paper substrate of the second aspect has a first layer and a second layer. A first wire may be used to form a first web that becomes the top layer and a second wire may be used to form a second web that becomes the second layer, which first and second webs are bonded together.

The first fabric may be formed from a first pulp comprising at least 50% by dry weight of hardwood pulp, such as at least 65% by dry weight of hardwood pulp, such as at least 75% by dry weight of hardwood pulp. The headbox consistency of the first pulp may be 0.12%-0.60%, such as 0.18%-0.35%.

The Schopper-Riegler number (°SR) measured according to ISO 5267-1:1999 of the first pulp in the headbox can be 33-45. This SR number can facilitate a sufficiently high density without causing dewatering and/or recycling problems and can be obtained by adjusting the low consistency (LC) raffinate degree.

The second fabric may be formed from a second pulp comprising at least 50% by dry weight of softwood pulp, such as at least 65% by dry weight of softwood pulp, such as at least 75% by dry weight of softwood pulp. This softwood pulp has preferably been subjected to high consistency (HC) refining (suitable specific energies are discussed above). The headbox consistency of the second pulp may be 0.06%-0.40%, such as 0.10%-0.25%.

In one embodiment, the headbox consistency of the second paste is lower than the headbox consistency of the first paste.

The Schopper-Riegler number (°SR) measured according to ISO 5267-1:1999 of the second pulp in the headbox can be 25-35. This SR number can facilitate a sufficiently high density without causing dewatering and/or recycling problems and can be obtained by adjusting the low consistency (LC) raffinate degree.

Preferably, the raw materials comprise less than 2% by weight of inorganic filler, such as less than 1% by dry weight of inorganic filler, such as substantially no inorganic filler.

In one embodiment, the second layer side is impregnated with a greater amount of polymer than the first layer. Otherwise, the embodiments of the first aspect discussed above apply to the second aspect mutatis mutandis.

As demonstrated in the examples section below, the high density paper of the first aspect is an excellent substrate for a coating, in particular an oxygen barrier coating. As a third aspect of the present disclosure, there is therefore provided a coated paper comprising a high density paper according to the first aspect, wherein a surface of the high density paper is provided with a barrier coating, for example comprising polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), a starch or starch derivative, a nano- or microfibrillar cellulose, polyvinylidene chloride (PVDC) or a polyamide. A preferred barrier coating comprises PVOH and/or EVOH, for example in an amount of 1-3 g/m2.

In one embodiment, the coated paper of the third aspect is not PECVD or PVD coated.

In one embodiment, the coated paper of the third aspect comprises several barrier layers, for example, including a water vapor barrier layer.

The coated paper of the third aspect may be used to package oxygen-sensitive products, such as dry and/or fatty foods. Examples of fatty foods are cheese, butter and spreads. This packaging may be continuous wrap packaging or form-fill-seal (FFS) packaging, for example in pouches. It may also be packaging in a jar, a tray, a spreadable container with a lid, a collapsible tube, a clamshell package, a sleeve, an envelope or a wrapper. Another application is use as a packaging window. In these applications, the packaging material is typically subjected to folding or a similar type of stress (e.g. folding, stretching), which makes the high-density paper-based packaging material of the present disclosure particularly suitable.

As a fourth aspect of the present disclosure, there is provided a use of a multilayer material for packaging a food product or other oxygen-sensitive product, wherein one layer of the multilayer material is a high-density paper according to the first aspect, provided that the food product is not a liquid, semi-liquid or viscous food product. In the context of the present disclosure, "liquid food" includes water.

Applications of the fourth aspect are discussed above in relation to the third aspect.

The multilayer material of the fourth aspect may comprise a coated paper according to the third aspect.

In one embodiment, the multilayer material of the fourth aspect does not comprise a PECVD or PVD coating.

Examples

Example 1

1A: Paper substrate production

Two pulps were provided: i) a softwood ECF-bleached kraft pulp (i.e. a blend of pine and spruce); and ii) a hardwood ECF-bleached kraft pulp (i.e. birch).

The softwood pulp was refined using high consistency (HC) refiners at a specific energy of 225 kWh/tonne. The HC-refined pulp was then mixed in a mixing box with a broken pulp comprising a mixture of bleached softwood and hardwood pulps (most of the broken was obtained from the same paper production). The proportion of broken in this softwood-based mixture was 30%. The softwood-based mixture was then refined by low consistency LC refining at a specific energy of 75 kWh/tonne. This LC refining resulted in a Schopper-Riegler (°SR) according to ISO 5267-1:1999 of ~30 °SR.

The hardwood pulp was mixed separately with the same type of breakage and then refined by low-consistency refining using a specific energy of 85 kWh/ton. The proportion of breakage in the hardwood-based mixture was 20%. The hardwood-based mixture subjected to LC obtained a Schopper-Riegler (°SR) value of ~38 °SR.

For each of the two fibre streams, papermaking chemicals (4 kg/tonne cationic starch, 0.2 kg/tonne silica and 0.4 kg/tonne AKD) were added. The softwood-based mixture was pumped into a bottom-layer headbox of a two-layer Fourdrinier machine, whereas the hardwood-based mixture was pumped into the top-layer headbox of the same Fourdrinier machine. The dry mass flow through each of the headboxes was the same and was adjusted to achieve a total basis weight of 60 g/m2 before coating (i.e. 30 g/m2 per layer). The vertical cut-off shoulder was 34 mm for the bottom layer headbox and 16 mm for the top layer headbox, reflecting relatively low headbox consistencies (approximately 0.12% for the bottom layer and approximately 0.25% for the top layer). The wire speed was 600 m/min. On a paper machine specifically adapted for this product, the wire speed can be considerably higher.

The two layers formed in the Fourdrinier machine were compressed together to ~10% dryness and further dehydrated using vacuum foil boxes to ~20% dryness before being subjected to wet pressing in a press section having two unique felt press nips, where the first press had the felt on the top side and the second press had the felt on the bottom side.

After wet pressing, the fabric was dried in a conventional multi-cylinder dryer to form a paper substrate having a moisture content of ~5%. Prior to winding, the paper substrate was soft nip calendered at a linear load of 20 kN/m. The properties of the paper substrate are presented in Table 1 below.

Figure 1A shows a SEM image of a surface portion of the paper substrate.

1B: Impregnation

The 1A paper substrate was impregnated off-line with an aqueous polyvinyl alcohol (PVOH) composition from both sides on a conventional film press. The PVOH type was Poval 10/98 (Kuraray) and its concentration in the composition was 10% (in another test the concentration was 8%, which also worked). The composition further comprised glyoxal (Cartabond TSI) in an amount of 6% by weight compared to the amount of PVOH. Glyoxal acted as a crosslinker. The viscosity of the composition was 74 mPa*s (measured at 60 °C). The applied amount of PVOH was 1 g/m2 on the top side and 2 g/m2 on the reverse/bottom side. The reason for using a higher amount of PVOH for the reverse/bottom side was that the pulp used to form the bottom layer had a lower SR number (and therefore the reverse/bottom side had a less dense surface compared to the top side). The PVOH-impregnated paper substrate was dried using hot air to a moisture content of approximately 8%. The properties of the dried PVOH-impregnated paper substrate are presented in Table 1 below.

Figure 1B is a SEM image of a surface portion of the PVOH-impregnated paper substrate. As shown in Figure 1B, the PVOH has not formed a film on the surface portion. Instead, it has penetrated into the fiber web.

In another trial, the applied amount of PVOH was 1.5 g/m2 on each side instead of 1 g/m2 on the top side and 2 g/m2 on the reverse/bottom side.

1C: Supercalendering

The 1B impregnated paper substrate was rewetted to 15%. The rehydrated paper was fed to an off-line multi-nip calender also called a supercalender (the number of nips was 12). Supercalendering was carried out using a surface temperature of 140 °C on the heated rolls, which could be obtained by means of external induction heaters, to obtain a high-density paper. The line load at each nip was 450 kN/m. The total impulse of the supercalendering nip was ~800 kPa-s [# of nips x line load/fabric speed]. Heating of the heated rolls dried the high-density paper. The moisture content at the time of winding was 8%. The properties of the high-density paper are presented in Table 1 below.

An SEM image of a surface portion of the high-density paper is shown in Figure 1C. In addition, Figure 2 is an SEM image of a cross section 20 of the high-density paper. The dark gray areas 21 are PVOH and the light gray areas 22 are fibers. There are also unfilled pores 23. Consequently, the high-density paper is not saturated with PVOH. However, Figure 2 shows that most of the PVOH is within the fiber web. Only a minor portion of the PVOH is found on the surface.

1D: 1st reference supercalendering

For reference, a paper substrate produced according to 1 above, but with softwood kraft pulp and cracked as the only pulps in both layers, was supercalendered as in 1C above (but not impregnated). The resulting properties are presented in Table 1 below.

1E: 2nd reference supercalendering

For reference, a machine glazed (MG) paper formed from a blend of hardwood pulp and softwood pulp (dry weight ratio 40:60) was supercalendered as in 1C above, but the total nip impulse was about 10% lower. The resulting properties are presented in Table 1 below. Resulting Properties

For Table 1, the following applies:

The grammage was measured according to ISO 536:2012 and has the unit g/m2. The density was measured according to ISO 534:2011 and has the unit kg/m3. Roughness means Bendtsen roughness, was measured according to ISO 8791-2:2013 and has the unit ml/min. The tensile index was measured in the MD and CD according to ISO 1924-3:2005 and has the unit Nm/g. The tear index was measured in the MD and CD according to ISO 1974:2012 and has the unit mNm2/g. °SR was measured according to ISO 5267-1:1999 after repulping according to ISO 5263-1:2004. Canadian Standard Freeness ("CSF") has the unit ml and was measured in accordance with ISO 5267-2:2001 after repulping according to a Valmet repulping method using a Valmet repulper type HD400. The Valmet repulping method is described in further detail below. The fines content was measured with an L&WFibretester (ABB, Lorentzen & Wettre, Sweden) after repulping in accordance with ISO 5263-1:2004. Fines are defined as fibrous particles shorter than 0.2 mm. "Somerville residue" means residues retained on a Somerville shiver and flake content analyser having a slot plate width of 0.15 mm. Residue content was calculated as % dry weight of the dry material originally fed (into the repulper). Dry has the meaning of having 0% moisture content in the tested material, which is consequently oven dried before weighing. The Somerville method is described in further detail below. The oxygen transmission rate ("OTR") has the unit cm3/m2/24h, 20.26 kPa (0.2 atm) (21%) oxygen. It was measured according to ASTM F1927-14 after lamination with 20 g/m2 LDPE on the top side of the paper. Super Perga 1 and 2 are commercial greaseproof papers from Nordic Paper. Super Perga 1 was used as a paper substrate in WO 2017/089508.

Table 1.

§ According to the supplier's data sheet

x Tested on high density paper that had been impregnated with 1.5 g/m2 PVOH on each side

* The weight was 38 g/m2 instead of 32 g/m2

** After supercalendering according to example 1C (without PVOH impregnation)

As shown in Table 1 above, neither supercalendering (high density) nor PVOH impregnation alone result in really low OTR values. As an example, supercalendering had very little effect on the OTR value of the unimpregnated Super Perga 2 paper. In contrast, supercalendering of the PVOH impregnated paper substrate from Example 1B reduced the OTR value (50% RH) by ~70% to well below 10 cm3/m2/24h. Valmet repulping method: Repulping was carried out on a Valmet repulper which is designed for material preparation, i.e. fibre disintegration, of the type HD400. Agitation was performed with an impeller with three radial and toothed blades with dimensions of 30 by 40 mm rotating at a speed of 3000 rpm. The material to be repulped was cut into 90 by 90 mm pieces. 0.5 kg of air-dried pieces was mixed with 10 litres of water, i.e. to a consistency of 5%, and repulped after 2.5 minutes at a temperature of 57 °C. Then 5 litres of water were added, giving a consistency of 3.3%, and further repulping was carried out for a further 17.5 minutes at a temperature of 57 °C. The total repulping time was therefore 20 minutes.

Somerville method: The Valmet repulping method described above was first carried out to obtain a pulp. Quantification of the proportion of Somerville residues, retained on the Somerville shives and scales content analyser with a slot plate width of 0.15 mm, was performed by diluting the pulp to less than 1% consistency. The diluted pulp was then analysed on the Somerville analyser to obtain the proportion of scale residues as % by weight calculated on the oven-dried material (i.e. 0% moisture content), initially introduced into the repulping operation.

Example 2

A single-ply paper made for a different purpose than Example 1, but by a process similar to that of Example 1, was provided. The single-ply paper was prepared from a pulp blend comprising Kraft softwood pulp, Kraft hardwood pulp, and a minor amount of CTMP pulp, in an approximate dry weight ratio of 45:45:10.

During production, the single-ply paper was impregnated with PVOH from the top and subsequently supercalendered to a density of approximately 1100 kg/m3. The grammage of the single-ply paper was 57 g/m2. The surface of the top side had a smoothness of 15 ml/min Bendtsen.

In addition, an unimpregnated reference paper was provided. It had a top side surface smoothness of approximately 20 ml/min Bendtsen, a grammage of 39 g/m2 and a density of 978 kg/m3.

The single-ply paper and the unimpregnated reference paper were coated twice with 1 g/m2 PVOH on the top side of the paper substrate and dried after each coating operation. They were then laminated to packaging materials having the following layer structure:

/LDPE 12 g/m2/cardboard (geometric bending strength 80 mN)/LDPE 20 g/m2/paper substrate with 2x PVOH at 1 g/m2/EAA adhesive copolymer 6 g/m2/LDPE+m-LLDPE blend 19 g/m2/

The oxygen transmission of laminated materials was measured by a fluorescent method using a PSt9 oxygen probe from PreSens GmbH, Germany. According to this method, a flat sample to be tested is placed in a cell, which is flushed with dry nitrogen, in which the probe is also located. The area of the circular cell section is 68 cm2 (0.0068 m2). The surface of the sample not facing the cell is oriented to ambient air, i.e. 21% oxygen, at 23 °C and 50% RH. Using the oxygen concentration reading from the probe, an oxygen transmission rate is calculated according to ASTM F3136-15. The unit is given as ml/sample.

Using this alternative method of measuring oxygen transmission, the oxygen barrier properties of a flat material can be investigated before and after it has been folded and then unfolded. The folding angle was 165 degrees and the barrier layer was directed to be on the outside of the fold. The measured values are the average of five measured samples.

The results of these measurements are presented in Figure 3, which shows that the unfolded laminated packaging material comprising the PVOH-coated (non-impregnated) reference paper substrate provides an oxygen barrier. However, this oxygen barrier is lost after one round of folding and unfolding. In contrast, the oxygen barrier provided by the laminated packaging material comprising the impregnated paper is much less affected by the folding-unfolding action. This “fold-resistant” oxygen barrier is an important property since the laminate will be folded in the packaging process.

Example 3

The high-density paper obtained in Example 1C above was coated twice with intermediate and subsequent drying operations to provide a PVOH coating weight of 3 g/m2 and then metallized to an optical density of approximately 2. A laminated packaging material

was produced according to the following layer structure:

/LDPE 12 g/m2/cardboard (geometric bending strength 80 mN)/LDPE 20 g/m2/high density paper PVOH+met./EAA copolymer adhesive 6 g/m2 29 g/m2 LDPE mLLDPE blend /

The packs were produced on a Tetra Pak® E3/CompactFlex filling machine. This type of filling machine has the capacity to fill portion packs at a rate of 9000 packs/hour and a flexibility that allows for quick changeover between different pack formats. The packs were in the Tetra Brik® format with a volume of 200 ml.

No major issues were identified with respect to packaging integrity (i.e. tightness of the package versus the surrounding environment) and sealing performance during the trials, which were therefore considered successful.

Laminates comprising a PVOH-coated greaseproof paper from Nordic Paper, identified as "Super Perga® WS Parchment" and having a grammage of 32 g/m2, were used as comparative examples:

/LDPE 12 g/m2/ cardboard (260 mN geometric bending strength)/LDPE 20 g/m2/ paper substrate (32 g/m2)+PV0H+met./LDPE 20 g/m2/LDPE+mLLDPE 20 g/m2/; and

/LDPE 12 g/m2/cardboard (geometric bending strength 80 mN)/LDPE 20 g/m2/paper substrate (32 g/m2)+PV0H+met./LDPE 40 g/m2/

These reference laminates are further described in Table 2 below.

The oxygen transmission rate (OTR) of the flat packaging material was measured using a coulometric detector according to ASTM F1927-14. Relative humidity (RH) was either 50% or 80%. The unit was cm3/m2/24h, with the option of using 20.265 kPa (0.2 atm) or 101.325 kPa (1 atm) oxygen pressure. In order to compare OTR values measured at 101.325 kPa (1 atm) with OTR values measured at 20.265 kPa (0.2 atm), the former values can be multiplied by 0.2.

The oxygen transmission rate (OTR) of the packages (filled, empty and dry) was measured according to ASTM F1307-14, at 0.2 atm (surrounding air containing 21% oxygen). The unit is cm3/package/24h. The OTR test was carried out 2-3 weeks after the production of the filled and sealed packages.

The package was mounted on a special holder; nitrogen was purged inside the package; the outside of the package was exposed to the environment surrounding the instrument. When oxygen permeated through the package into the nitrogen carrier gas, it was transported to the coulometric sensor. The sensor read the amount of oxygen that leaked into the nitrogen gas inside the package.

Table 2.

Although there is a difference between the comparative examples and the HD paper-based laminate of Example 1C in the amount of polyethylene in the layers facing the inside of the package, i.e. 40 g/m2 and 35 g/m2, respectively, this has no practical influence for the comparison of the oxygen transmission rate since polyethylene is a poor oxygen barrier relative to the HD paper and the applied coating. The typical oxygen transmission rate for 40 pm thick LDPE is 600-900 cm3/m2/24h/20,265 kPa (0.2 atm) at 23 °C.

As shown in Table 2, the 200 mL filled package made of the laminate comprising the impregnated high-density paper according to the present disclosure has a very low level of oxygen transmission (0.03 cm3/m2/day/20,265 kPa (0.2 atm) at 23 °C and 50% RH). Furthermore, the oxygen barrier remained just as effective in a more humid environment (23 °C and 80% RH). In the reference laminate comprising the Super Perga paper substrate, the OTR value was 0.075 at 23 °C and 50% RH, meaning a 2 to 3 times shorter shelf life for an oxygen-sensitive product in a 200 mL package. Therefore, it is shown that the impregnated high-density paper according to the present disclosure allows for a higher fiber-based content in a packaging material for packaging oxygen-sensitive products.

The OTR of a laminate based on the high-density paper impregnated in accordance with the present disclosure is at least as good as similar paper-based barrier laminates of the prior art. More importantly, it does not exhibit the same level of loss of oxygen barrier properties when converted into a filled, formed and heat-sealed packaging container. Thus, it is shown above that the high-density paper impregnated in accordance with the present disclosure provides robustness against the stresses that a packaging material typically experiences during conversion and use.

Example 4

The proportion of coarse reject, i.e. the non-fibrous recyclable part of the laminated materials (polymers, aluminium foil and some non-releasable fibres), was determined after repulping with a Valmet pulper. Repulping was carried out in the same way as described above in connection with the CSF and Somerville measurements, except that the laminated packaging material to be repulped and analysed was first cut into 30 by 90 mm pieces. The coarse reject was sieved (separated) using a plate with holes (diameter: 10 mm) and then dried to a moisture content of 0%. The proportion of coarse reject was then calculated as a percentage by weight of dry material (0% moisture) fed into the repulper.

Coarse reject as determined by a contracted global industrial supplier of fiber processing and recycling equipment was performed in a similar manner. In this alternative method, however, 20 g of laminated material was mixed in 21 g of water and disintegration was carried out to a consistency of approximately 1% over 18-minute time cycles. The water temperature was maintained at 57 °C also in this test.

The results are presented in Table 3 below.

Table 3.

*/LDPE 12 g/m2/cardboard (geometric bending strength 80 mN)/LDPE 20 g/m2/aluminium foil 6.3 pm/EAA adhesive copolymer 6 g/m2 19 g/m2 LDPE mLLDPE blend/

**LDPE 12 g/m2/cardboard (geometric bending strength 80 mN)/LDPE 20 g/m2/paper substrate PVOH+met/EAA copolymer adhesive 6 g/m2 19 g/m2 LDPE mLLDPE blend/

The results in Table 3 indicate that impregnation of the HD paper of the present disclosure does not result in worse repulpability/recyclability than the previously tested unimpregnated Super Perga paper.

Claims (15)

1. A high density paper having a basis weight measured in accordance with ISO 536:2012 of 30-75 g/m2 and a density measured in accordance with ISO 534:2011 above 1000 kg/m3 and wherein the high density paper is impregnated on each side with 0.3-4.0 g/m2, such as 0.5-3.0 g/m2, of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS). 2. The high density paper of claim 1, which is formed from at least 50% by dry weight of kraft pulp, such as at least 75% by dry weight of kraft pulp, such as at least 85% by dry weight of kraft pulp, such as at least 95% by dry weight of kraft pulp. 3. The high density paper of claim 1 or 2, wherein 20-65% by dry weight, such as 30-60% by dry weight, of the pulp used to form the high density paper is hardwood pulp. 4. The high density paper of any of the preceding claims, wherein 35-80% by dry weight, such as 40-70% by dry weight, of the pulp used to form the high density paper is softwood pulp, preferably softwood that has undergone high consistency (HC) refining. 5. The high density paper of any of the preceding claims having a top layer and a bottom layer. 6. The high density paper of any of the preceding claims, wherein the grammage measured according to ISO 536:2012 is 35-65 g/m2. 7. The high density paper of any of the preceding claims, having a Schopper-Riegler number (°<s>R) measured according to ISO 5267-1:1999 of 33-50, such as 35-45, after repulping according to ISO 5263-1:2004. 8. The high density paper of any of the preceding claims, having a fines content measured with a L&W Fibretester (ABB, Lorentzen & Wettre, Sweden) of less than 40%, such as less than 34%, such as less than 32%, after repulping according to ISO 5263-1:2004, where fines are defined as fibrous particles shorter than 0.2 mm. 9. A method for forming a high density paper having a grammage measured in accordance with ISO 536:2012 of 30-75 g/m2 and a density measured in accordance with ISO 534:2011 of more than 1000 kg/m 3, the method comprising the steps of: - impregnating each side of a paper substrate with 0.3-4.0 g/m2, such as 0.5-3.0 g/m2, of a polymer selected from the group consisting of polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), carboxymethyl cellulose (CMC), nanocrystalline cellulose (NCC) and carboxymethyl starch (CMS) to obtain an impregnated substrate; and - supercalendering of the impregnated substrate to obtain high-density paper. 10. The method of claim 9, wherein the polymer is PVOH having a weight average molecular weight (Mw) less than 100,000 g/mol. 11. The method of claim 9 or 10, wherein the impregnating step comprises adding an aqueous composition comprising the polymer to each side of the paper substrate and the viscosity measured at 60 °C of the aqueous composition is 55-90 mPa*s. 12. The method of any of claims 9-11, wherein the impregnated substrate entering the supercalendering step has a moisture content of 12.0%-19.0%, preferably 13.5%-18.0%. 13. The method of any of claims 9-12, wherein the impregnation step is carried out by means of a film press. 14. A coated paper comprising a high density paper of any one of claims 1-8, wherein a surface of the high density paper is provided with a barrier coating comprising polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), a starch or starch derivative, a nano or microfibrillar cellulose, polyvinylidene chloride (PVDC) or a polyamide. 15. Use of a multilayer material for packaging a food product or other oxygen-sensitive product, wherein one layer of the multilayer material is a high-density paper according to any of claims 1-8, provided that the food product is not a liquid, semi-liquid or viscous food product.