COATED PAPER SUITABLE FOR USE IN A SACK

Abstract

 There is provided a coated paper comprising a kraft paper substrate, a precoating layer provided on the kraft paper substrate and a water vapour barrier coating layer provided on the pre-coating layer. The grammage according to ISO 536:2019 of the coated paper is 65-155 g/m², the pre-coating layer comprises inorganic filler and binder in a dry weight ratio of between 100:25 and 100:5 and the water vapour barrier coating layer comprises clay pigment and styrene-butadiene co-polymer (SBR) binder in a dry weight ratio between 100:30 and 100:80. The particle size distribution (% < 2 µm) of said clay pigment is above 90 and the shape factor of said clay pigment is below 20, such as below 10.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of paper for use in a sack, such as a cement sack.

BACKGROUND

[0002] During filling and storage of powdery material, such as cement, paper sacks are required to meet high standards.

[0003] Firstly, the paper sacks need to hold a considerable material weight, i.e. have high tensile strength. For this purpose, kraft paper is a suitable sack wall material. The sacks typically have two or more walls, i.e. layers of paper material, to further strengthen the sack construction. A wall layer of a sack is often referred to as a ply. Production of ply material (i.e. sack paper) is for example disclosed in WO 99/02772.

[0004] Secondly, a material such as cement is sensitive to moisture contamination during storage. Hence, the contents of sacks often require protection against atmospheric water vapor that may penetrate the sack plies. Such protection is typically achieved by a moisture barrier incorporated as an intermediate layer in the sack, i.e. between two plies. The moisture barrier is typically a plastic film ("free film"), e.g. of polyethylene (PE), that is impermeable to water vapour. The free film may also improve resistance to grease and prevent contamination by microorganisms.

[0005] Thirdly, the paper sack should vent air during filling. In detail, the air that accompanies the powdered material shall efficiently vent from the sack since the filling machines that delivers the material run at high throughput rates. The venting capacity of the sack is often the factor limiting for the filling rate. Efficient venting also prevents that air is trapped in the sack and causes under-weight packs, sack rupture and problems when sacks are stacked for transportation.

[0006] During the filling process, the only way for air to escape from the interior of the sack is in many sack constructions through the walls of the sack. Kraft paper of high porosity is often used in the walls to facilitate air permeability. However, an increased porosity of the paper normally results in a decrease in the overall strength. In particular, the strength may be significantly reduced if holes must be made in the paper material to achieve sufficient air permeability. Furthermore, the use of a free film may reduce deaeration during filling, since most such films are impermeable to air. Therefore, the free film layers have been provided with slits or openings to facilitate deaeration.

[0007] In WO 2016/001029, the free film is replaced by two coating layers provided on the outer paper ply: first a pre-coating layer and then a moisture barrier coating layer. In the examples disclosed in this document, the moisture barrier layer is formed from latex and hyper-platy clay. The two-layer coating structure of WO 2016/001029 not only provides a barrier against moisture/water vapour, but it also facilitates disintegration of the sack in a cement mixer.

SUMMARY

[0008] The hyper-platy clay used in WO 2016/001029 is expensive and may be complicated to handle in the paper mill.

[0009] Accordingly, an objective of the present disclosure is to provide a barrier concept of reduced cost that still runs well in a coating station and provides sufficient protection against water vapour. While it is not necessary that this coating concept facilitates disintegration in a cement mixer, it should allow for recycling of the coated paper according to industry standards.

[0010] There is thus provided a coated paper comprising a kraft paper substrate, a precoating layer provided on the kraft paper substrate and a water vapour barrier coating layer provided on the pre-coating layer, wherein the grammage according to ISO 536:2019 of the coated paper is 65-155 g/m², the pre-coating layer comprises inorganic filler and binder in a dry weight ratio of between 100:25 and 100:5 and the water vapour barrier coating layer comprises clay pigment and styrene-butadiene co-polymer (SBR) binder in a dry weight ratio between 100:30 and 100:80, wherein the particle size distribution (% < 2 µm) of said clay pigment is above 90 and the shape factor of said clay pigment is below 20, such as below 10.

BRIEF DESCRIPTION OF THE FIGURES

[0011] 

Fig 1 shows a sack according to the present disclosure configured to allow "top deaeration".

Fig 2 shows the sack of Fig 1 provided with a top patch for reinforcement.

DETAILED DESCRIPTION

[0012] As a first aspect of the present disclosure, there is provided a coated paper comprising a kraft paper substrate, a precoating layer provided on the kraft paper substrate and a water vapour barrier coating layer provided on the pre-coating layer.

[0013] The kraft paper substrate (also referred to as base paper) is preferably formed from a pulp comprising at least 75% by dry weight of virgin fibres. These virgin fibres are typically softwood fibres.

[0014] The kraft paper substrate may be bleached or unbleached.

[0015] For sufficient strength when used in a sack, the geometric tensile energy absorption (TEA) index of the coated paper is preferably at least 2.0 J/g. In the present disclosure, TEA is measured according to ISO 1924-3:2005. The TEA index is obtained by dividing the TEA value by the grammage. The geometric TEA index is the geometric mean of the TEA index in the machine direction and the TEA index in the cross direction.

[0016] The grammage of the coated paper is 65-155 g/m². Preferably it is 65-135 g/m², such as 75-115 g/m². In the present disclosure, grammage is measured according to ISO 536:2019.

[0017] The coat weight of the pre-coating layer is preferably 4-20 g/m², such as 4-12 g/m², such as 5-10 g/m² and the coat weight of the water vapour barrier coating layer is preferably 4-20 g/m², such as 4-12 g/m², such as 5-10 g/m².

[0018] Accordingly, the grammage of the kraft paper substrate is typically 50-140 g/m² and preferably 50-120 g/m², such as 60-100 g/m².

[0019] The Cobb 60s value of both sides of the kraft paper substrate may be below 40 g/m², such as below 35 g/m². Accordingly, the Cobb 60s value of the side the coated paper that is not provided with the pre-coating and the barrier coating is preferably below 40 g/m², such as below 35 g/m². The Cobb 60s values of the present disclosure are measured according to ISO 535:2014. To obtain lower Cobb 60s values, hydrophobic size, such as AKD, ASA and/or rosin size may be added in the wet end during production of the kraft paper substrate.

[0020] In one embodiment, the kraft paper substrate comprises a wet strength agent. However, the amount of the wet strength agent should be controlled such that the coated paper is still recyclable.

[0021] The pre-coating layer comprises inorganic filler and binder in a dry weight ratio of between 100:25 and 100:5, preferably between 100:20 and 100:8, and more preferably between 100:16 and 100:8. In one embodiment, the dry weight ratio is between 100:16 and 100:10. The binder of the pre-coating layer may for example be a styrene-butadiene co-polymer (SBR). The SBR is typically provided in the form of a latex when the coating composition is prepared and coated onto the kraft paper substrate. The inorganic filler of the pre-coating layer is preferably a relatively coarse pigment, such as a pigment having a particle size distribution (% < 2 µm) below 70. In a particularly preferred embodiment, the inorganic filler is a calcium carbonate pigment having a particle size distribution (% < 2 µm) below 70. The person of skill in the art of pigments for paper coating layers is familiar with particle size distribution values expressed as the percentage of particles having a size below 2 µm. As an example, this type of values is frequently found on data sheets for pigment products.

[0022] The water vapour barrier coating layer comprises clay pigment and styrene-butadiene co-polymer (SBR) binder in a dry weight ratio between 100:30 and 100:80, wherein the particle size distribution (% < 2 µm) of said clay pigment is above 90, preferably above 95. The SBR is typically provided in the form of a latex when the water vapour barrier coating composition is prepared and coated onto the pre-coating layer.

[0023] Further, the shape factor of the clay pigment of the water vapour barrier coating layer is preferably below 20, more preferably below 10.

[0024] The dry weight ratio of clay pigment to SBR binder in the water vapour barrier layer is preferably between 100:40 and 100:70, more preferably between 100:40 and 100:60.

[0025] As a second aspect of the present disclosure, there is provided a sack comprising a ply formed from the coated paper of the first aspect. The sack is preferably a sack for a hydraulic binder, such as cement. The contents of the sack is typically used in mortar or tile fix.

[0026] In one embodiment, the sack comprises at least two plies and the ply formed from the coated paper of the first aspect is an outer ply. Such a sack may comprise an inner ply formed from a kraft paper having a Gurley permeance (also referred to as Gurley value or air permeance) of 2-10 s, such as 4-8 s, such as 4-7 s, such as 5-6 s. In the present disclosure, Gurley permeance is measured according to ISO 5636-5:2013.

[0027] The kraft paper of the inner ply typically has a grammage of 60-90 g/m², preferably 60-85 g/m², such as 60-80 g/m².

[0028] The coated paper of the sack typically has a grammage of 75-105 g/m², such as 75-95 g/m². Accordingly, the paper substrate of the coated paper of the sack typically has a grammage of 60-90 g/m², such as 60-80 g/m².

[0029] As understood by the skilled person, there typically is no free film in the sack.

[0030] The sack is typically configured to allow air to escape from an interspace between the inner paper ply and the outer paper ply during filling of the sack.

[0031] This is facilitated by the permeability of the inner paper ply of the sack, which allows air to pass from the inside of the sack to the interspace between the inner paper ply and the outer paper ply. Such a sack is also configured to allow air to escape from the interspace between the inner paper ply and the outer paper ply (to the ambient air) during filling of the sack.

[0032] Preferably, the sack design is such that air can escape from the interspace between the inner paper ply and the outer paper ply through a top end of the sack during filling of the sack.

[0033] For example, a top end of the sack may be formed by folding and gluing the plies such that a portion of the top end is not sealed and air can escape from the interspace through the non-sealed portion during filling of the sack. Such an embodiment is further discussed below with reference to figures 1 and 2.

[0034] The sack of the present disclosure is preferably a valve sack. Valve sacks are well known to the skilled person. A valve sack is provided with a valve through which it may be filled. Such a valve is normally provided at a folded top end of the sack. A valve is further discussed below with reference to figures 1 and 2.

[0035] Figure 1 illustrates an embodiment of a multi-ply sack 100 having a top end 111 and a bottom end 112. The sack comprises an inner paper ply 101 and an outer paper ply 102. To create a water vapour barrier, the paper of the outer ply 102 is coated as described above. To facilitate deaeration, the Gurley permeance of the inner paper ply 101 is 10 s or less.

[0036] The sack 100 is configured to allow air to escape (the air escape is illustrated by the arrow 103) from an interspace between the inner paper ply 101 and the outer paper ply 102 through the top end 111 of the sack 100 during filling of the sack 100. Such a deaeration is achieved by a non-sealed portion 104 forming an opening between the inner paper ply 101 and the outer paper ply 102 at the top end 111. The non-sealed portion 104 may be flanked by sealed portions 105, i.e. portions in which the outer paper ply 102 is sealed (preferably glued) to the inner paper ply 101. For a 25 kg sack 100 having a width of 400-420 mm, the width of the non-sealed portion may for example be 150-160 mm and for a 35 kg having a width of 440-460 mm, the width of the non-sealed portion may be 190-200 mm.

[0037] The top end 111 of the sack 100 of figure 1 further has a filling valve 106 into which a filling spout may be inserted. The arrow 107 illustrates how the filling spout is inserted into the valve 106. A ceiling of the valve 106 is reinforced by a valve reinforcement 108, which preferably is composed of paper. Because of the reinforcement provided by the valve reinforcement 108, the sack 100 can hang on the filling spout during filling without breaking. When fully opened, the opening of the valve 106 may be approximatively diamond-shaped.

[0038] Figure 2 illustrates the sack 100 of figure 1 onto which a top patch 201 has been applied. The top patch reinforces top end 111 of the sack 100. A slit 202 in the top patch 201 ensures that the air still can escape through the non-sealed portion 104 during filling. The slit 202 is thus substantially aligned with the non-sealed portion 104. Further, the width of the slit 202 is preferably approximately the same as the width of the non-sealed portion 104. Again, the escape of air from the interspace between the inner paper ply 101 and the outer paper ply 102 through the top end 111 of the sack 100 is illustrated by the arrow 103.

EXAMPLES

[0039] In this examples section, "parts" means parts by dry weight. All base papers used in the examples section are formed from pulps in which all cellulose fibres are virgin softwood fibres.

Reference trial 1

[0040] A bleached sack kraft paper having a grammage of 70 g/m², a Cobb 60s value of 30 g/m², a Gurley value of 16 s and an MD stretchability of about 2.3 % was used as the base paper.

Coating the top side (but not the wire side)

[0041] The top side of the base paper was blade coated with a pre-coating composition comprising 100 parts of Hydrocarb 60 (a relatively coarse calcium carbonate pigment commercialized by OMYA), 13.5 parts of Ligos P 1217 (an SBR binder commercialized by TRINSEO), 0.62 parts of Finnfix 10 (a CMC rheology modifier commercialized by Nouryon), 0.01 parts of an anti-foaming agent, 0.001 parts of colour and 0.07 parts of NaOH. The (dry) coat weight of the pre-coating was 8 g/m².

[0042] After drying, the precoated side was blade coated with a coating composition comprising 100 parts of a 50/50 mixture of Hydragloss 90 (an ultra-fine (% < 2 µm = 96-100) clay pigment commercialized by KaMin) and Hydrocarb 95 (an ultra-fine calcium carbonate pigment commercialized by OMYA), 13 parts of Ligos P 1217, 0.65 parts of Finnfix 10, 0.01 parts of an anti-foaming agent and 0.07 parts of NaOH. The (dry) coat weight of this coating was 7 g/m².

[0043] After drying, the double-coated base paper was calendered in a soft nip calender at a temperature of 140°C using a line load of 160 kN/m.

Coating the wire side (but not the top side)

[0044] The wire side of the base paper was coated and calendered in the same way as the top side.

Results

[0045] The WVTR of the coated papers was measured at 23°C/50% RH and at 38°C/90% RH. The results are presented in table 1 below.

Table 1. WVTR values (g/m²day) obtained in reference trial 1.

Sample	WVTR (23°C/50% RH)	WVTR (38°C/90% RH)
Double-coated top side	507	3760
Double-coated wire side	541	4081

Reference trial 2

[0046] A coated paper was produced on several occasions over a production period of about one year. In this production, a bleached sack kraft paper having a grammage of 80 g/m², a Cobb 60s value of about 80 g/m² and an MD stretchability of 6% was used as the base paper.

Coating the wire side (but not the top side)

[0047] The wire side of the base paper was blade coated with a pre-coating composition comprising 100 parts of Hydrocarb 60 (a relatively coarse calcium carbonate pigment commercialized by OMYA), 13.5 parts of Litex P6115 (an SBR binder commercialized by Eka Synthomer), 0.62 parts of Finnfix 10 (a CMC rheology modifier commercialized by Nouryon), 0.024 parts of an anti-foaming agent and 0.097 parts of NaOH. The (dry) coat weight of the pre-coating was 8 g/m².

[0048] After drying, the precoated side was blade coated with a barrier coating composition comprising 100 parts of Barrisurf LX (a high aspect ratio (shape factor = 60) clay pigment commercialized by IMERYS), 50 parts of Litex P6115, 0.6 parts of Rheocoat 27 (a rheology modifier commercialized by ARKEMA) and 0.13 parts of NaOH. The (dry) coat weight of the barrier coating was 7 g/m².

[0049] After drying, the double-coated base paper was calendered in a soft nip calender at a temperature of 80°C using a line load of 70 kN/m.

Results

[0050] The WVTR of the coated papers was measured at 38°C/90% RH. The average result from the whole production period is presented in table 2 below. Before calculating the average result, outliers (suspiciously high values) were deleted.

Inventive trial 1

[0051] The same base paper as in reference trial 2 was used.

Coating the wire side (but not the top side)

[0052] The wire side of the base paper was blade coated with a pre-coating in the same way as in reference trial 2.

[0053] After drying, the precoated side was blade coated with a barrier coating composition comprising 100 parts of Hydragloss 90, 50 parts of Litex P6115, 0.16 parts of Rheocoat 27 and 0.13 parts of NaOH. As in reference trial 2, the (dry) coat weight of the barrier coating was 7 g/m².

[0054] After drying, the double-coated base paper was calendered in a soft nip calender at a temperature of 80°C using a line load of 70 kN/m.

Results

[0055] The WVTR of the coated paper was measured at 38°C/90% RH. The result (an average value) is presented in table 2 below.

Inventive trial 2

[0056] A coated paper was produced on several occasions over a period of about nine months. The same base paper as in reference trial 2 and was used.

Coating the wire side (but not the top side)

[0057] The wire side of the base paper was blade coated with a pre-coating composition comprising 100 parts of Hydrocarb 60 (a relatively coarse calcium carbonate pigment commercialized by OMYA), 9 parts of Ligos P1217, 0.16 parts of Rheocoat 27 and 0.13 parts of NaOH. The (dry) coat weight of the pre-coating was 8 g/m².

[0058] After drying, the precoated side was blade coated with a barrier coating composition comprising 100 parts of Hydragloss 90, 50 parts of Ligos P 1217, 0.16 parts of Rheocoat 27 and 0.13 parts of NaOH. As in reference trial 2, the (dry) coat weight of the barrier coating was 7 g/m².

[0059] After drying, the double-coated base paper was calendered in a soft nip calender at a temperature of 80°C using a line load of 70 kN/m.

Results

[0060] The WVTR of the coated paper was measured at 38°C/90% RH. The average result from the whole production period is presented in table 2 below.

Table 2. WVTR values (g/m²day) obtained in reference trial 2, inventive trial 1 and inventive trial 2.

Sample	WVTR (38°C/90% RH)
Reference trial 2	1116
Inventive trial 1	1124
Inventive trial 2	1174

[0061] Barrisurf LX (used in reference trial 2) is an expensive hyper-platy clay pigment specifically designed to improve the water vapour barrier properties of coatings of fibre-based substrates. Surprisingly, table 2 shows that the relatively inexpensive clay pigment Hydragloss 90 that was used in inventive trials 1 and 2 provided WVTR values on par with that for Barrisurf when used together with a SBR binder on top of a pre-coating. A comparison to the data in table 1 shows that the WVTR values at 38°C/90% RH for inventive trials 1 and 2 are much lower than that for reference trial 1.

Inventive trial 3

[0062] In this trial, four different base papers were used:

(i) a bleached sack kraft paper produced in BillerudKorsnäs' Karlsborg mill and having a grammage of 70 g/m², a Cobb 60s value of 30 g/m², a Gurley value of 15 s and an MD stretchability of 6%;

(ii) an unbleached sack kraft paper produced in BillerudKorsnäs' Skärblacka mill and having a grammage of 70 g/m², a Cobb 60s value of 30 g/m², a Gurley value of 10 s and an MD stretchability of 6%;

(iii) a bleached sack kraft paper produced in BillerudKorsnäs' Pietarsaari mill and having a grammage of 75 g/m², a Cobb 60s value of 21 g/m², a Gurley value of 18 s and an MD stretchability of 6%; and

(iv) an unbleached sack kraft paper produced in BillerudKorsnäs' Pietarsaari mill and having a grammage of about 70 g/m², a Cobb 60s value of about 23 g/m², a Gurley value of about 19 s and an MD stretchability of 6.7%.

[0063] Base papers (iii) and (iv) comprised wet strength agent. Further, these base papers comprised more AKD than (i) and (ii), which is reflected by lower Cobb values.

[0064] Inventive trial 3 was carried out in three rounds.

Coating the smoothest side

[0065] The smoothest side of each of the base papers (i)-(iv) was blade coated with a pre-coating composition comprising 100 parts of Hydrocarb 60, 13.5 parts of Ligos P1217, 0.62 parts of Finnfix 10 and 0.07 parts of NaOH. The (dry) coat weight of the pre-coating was 8 g/m².

[0066] After drying, the precoated side was blade coated with a barrier coating composition comprising 100 parts of HG90, 50 parts of Ligos P1217, 0.16 parts of Rheocoat 27 and 0.13 parts of NaOH. The (dry) coat weight of the barrier coating was 7 g/m².

[0067] After drying, the double-coated base paper was calendered in a soft nip calender at a temperature of 80°C using a line load of 70 kN/m.

Results

[0068] The WVTR of the coated papers was measured at 23°C/50% RH and at 38°C/90% RH. The results are presented in table 3 below.

Table 3. Average WVTR values (g/m²day) obtained in inventive trial 3.

Round	Base paper	WVTR (23°C/50% RH)	WVTR (38°C/90% RH)
1	(i)	36	789
1	(iv)	N/A	662
1	(iii)	34	699
2	(i)	45	862
3	(i)	62	940
3	(ii)	59	866

[0069] Notably, the coated papers of inventive trial 3 exhibit much better water vapour barrier properties than those of the coated paper of reference trial 1. Further, the WVTR values at 38°C/90% RH are lower in inventive trial 3 than in reference trial 2.

[0070] The inventors further note that HG90 (i.e. a clay pigment having particle size distribution (% < 2 µm) of 96-100 and a shape factor below 20) was uncomplicated to handle in full-scale operation and did not cause any particular runnability problems.

[0071] It is also notable that the friction properties are satisfactory. Friction is needed in most processes of conversion of the paper to a sack.

[0072] Further, the recyclability of coated papers produced according to inventive trial 3 was tested in a BillerudKorsnäs laboratory according to the standard method PTS-RH:021/97 Category II. The results are shown in table 4 below.

Table 4. Recyclability of the double-coated base papers of inventive trial 3.

Base paper		(i)	(ii)	(iii)	(iv)
Disintegrat-ability	Total reject	0.08%	0.64%	0.38%	0.98%
	Recyclability percentage	99.92%	99.36%	99.62%	99.02%
Sheet formation	Adhesive impurities	None	None	None	None
(from accept)	Optical inhomogeneities	None	None	None	None
Overall recyclability rating		Recyclable	Recyclable	Recyclable	Recyclable

[0073] Notably, also the base papers of lower cobb values (i.e. (iii) and (iv)) were recyclable

[0074] Finally, the recyclability of coated base papers (i) and (ii) produced according to inventive trial 3 was tested by an external laboratory (PROPAKMA, Germany) according to the standard method PTS-RH 021:2012. Both coated papers were found to be recyclable according to the standard. Details of the test results are shown in table 5 below.

Table 5. Recyclability of the double-coated base papers (i) and (ii) of inventive trial 3 according to PTS-RH 021:2012.

Base paper	(i)		(ii)
Parameter	Result	Assessment (per parameter)	Result	Assessment (per parameter)
Loose or easy to remove material	None	Recyclable	None	Recyclable
Sheet adhesion test (from total stock)	Negative		Negative
Residue of defibration test	0%	Recyclable	0%	Recyclable
Sheet adhesion test (from screened stock)	Negative	Recyclable	Negative	Recyclable
Visual assessment of screened stock	Dirt specks	Recyclable	Dirt specks	Recyclable

Claims

1. A coated paper comprising a kraft paper substrate, a precoating layer provided on the kraft paper substrate and a water vapour barrier coating layer provided on the pre-coating layer, wherein the grammage according to ISO 536:2019 of the coated paper is 65-155 g/m², the pre-coating layer comprises inorganic filler and binder in a dry weight ratio of between 100:25 and 100:5 and the water vapour barrier coating layer comprises clay pigment and styrene-butadiene co-polymer (SBR) binder in a dry weight ratio between 100:30 and 100:80, wherein the particle size distribution (% < 2 µm) of said clay pigment is above 90 and the shape factor of said clay pigment is below 20, such as below 10.

2. The coated paper of claim 1, wherein the grammage according to ISO 536:2019 of the kraft paper substrate is 50-140 g/m², such as 50-120 g/m², such as 60-100 g/m².

3. The coated paper of claim 1 or 2, wherein the grammage according to ISO 536:2019 of the coated paper is 65-135 g/m², such as 75-115 g/m².

4. The coated paper of any one of the preceding claims, wherein the coat weight of the pre-coating layer is 4-20 g/m², preferably 4-12 g/m², such as 5-10 g/m².

5. The coated paper of any one of the preceding claims, wherein the coat weight of the water vapour barrier coating layer is 4-20 g/m², preferably 4-12 g/m², such as 5-10 g/m².

6. The coated paper of any one of the preceding claims, wherein the dry weight ratio of inorganic filler to binder in the pre-coating layer is between 100:20 and 100:8, such as between 100:16 and 100:8.

7. The coated paper of any one of the preceding claims, wherein the dry weight ratio of clay pigment to SBR binder in the water vapour barrier layer is between 100:40 and 100:70, such as between 100:40 and 100:60.

8. The coated paper of any one of the preceding claims, wherein the particle size distribution (% < 2 µm) of said clay pigment is above 95.

9. A sack comprising a ply formed from the coated paper of any one of the preceding claims.

10. The sack of claim 9, wherein the sack comprises at least two plies and the ply formed from the coated paper is an outer ply.

11. The sack of claim 10, wherein the sack comprises an inner ply formed from a kraft paper having a Gurley permeance measured according to ISO 5636-5:2013 of 2-10 s, such as, such as 4-8 s, such as 4-7 s, such as 5-6 s.

12. The sack of claim 11, wherein the kraft paper of the inner ply has a grammage according to ISO 536:2019 of 60-90 g/m², such as 60-80 g/m².

13. The sack of any one of claims 10-12, wherein the coated paper has a grammage according to ISO 536:2019 of 75-105 g/m², such as 75-95 g/m².

14. The sack of any one of claims 10-13, wherein the paper substrate of the coated paper has a grammage according to ISO 536:2019 of 60-90 g/m², such as 60-80 g/m².

Drawing