EP4382666A1

EP4382666A1 - Heat sealable coated paper product

Info

Publication number: EP4382666A1
Application number: EP22211942.2A
Authority: EP
Inventors: Johan Larsson; Wouter PROSPER; Thomas GILLGREN
Original assignee: Billerud AB
Current assignee: Billerud AB
Priority date: 2022-12-07
Filing date: 2022-12-07
Publication date: 2024-06-12
Also published as: WO2024121097A1

Abstract

The present disclosure relates to heat sealable coated paper product comprising a cellulosic fibrous substrate comprising a first and a second side, the first side comprising a single coating layer being applied in direct contact with the fibrous substrate, the coating layer comprises ethylene (meth)acrylic acid copolymer (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, in an amount of at least 51 % by weight based on the dry weight of the coating layer and a pigment being talc and/or calcium carbonate (CaCO<sub>3</sub>). The present disclosure furthermore relates to a method of producing the heat sealable coated paper product.

Description

TECHNICAL FIELD

The present disclosure relates to the field of heat sealable coated paper products.

BACKGROUND

Traditionally food or food service packages use plastic wrapping material for packaging material which needs to be heat sealed and provide some barrier properties. More recently, paper materials with multiple coating layers of plastic materials and metal foil layers are used as heat sealable barrier material, replacing the previously used plastic materials.
For environmental reasons there is an increasing requirement to minimize the use of plastic materials and to minimize the amount of plastic material used for coating fibrous substrates as such materials provides higher carbon footprint and are more cumbersome to recycle. However, reducing the amount of plastic material may negatively impact the heat sealability and render the process of applying the coating more difficult, resulting in a higher number of defective products and a higher amount of waste in production and higher costs.
In view of these requirements there is a need to develop heat sealable paper materials which reduces the carbon footprint, are easier to recycle while not compromising the heat sealability and processability of the paper product.

SUMMARY

It is an object of the present disclosure to provide a paper that at least partially overcome the above-described deficiencies. This is achieved by a heat sealable coated paper product according to claim 1 and by a method of producing a heat sealable paper product according to claim 17.
A heat sealable coated paper as disclosed herein comprises a fibrous substrate comprising a first and a second side, the first side comprising a single coating layer being applied in direct contact with the fibrous substrate, the coating layer comprises ethylene (meth)acrylic acid copolymer (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, in an amount of 51 % or more by weight based on the dry weight of the coating layer and a pigment, the pigment being talc and/or calcium carbonate (CaCO₃).
The heat sealable paper product according to the present invention, comprising only one single coating layer, is an environmentally friendly and easily recyclable paper product which still provides satisfying heat sealability. The heat sealable paper product is furthermore easy to produce and keeps the number of defective products at a low level, which is both an environmental and cost advantage.
The single coating layer according to the present invention, comprising a combination of at least 51 % by weight of EAA/EMAA,EAAMAA preferably within the range of from 51% by weight to 85 % by weight of EAA/EMAA/EAAMAA, and a pigment being talc and/or CaCO₃ has beneficial density properties. The beneficial density properties have been found to support uniform coating of relatively thin layers onto fibrous substrates, without pre-coating layers, with conventional coating techniques such as blade coating.
The EAA/EMAA/EAAMAA may thus be present in an amount within the range of from 51 to 85 % by weight based on the dry weight of the coating layer, preferably within the range of from 53 to 85 % by weight based on the dry weight of the coating layer, more preferably within the range of from 60 to 80 % by weight based on the dry weight of the coating layer.
The pigment may be present in an amount within the range from 15% to 45 % by weight, based on the dry weight of the coating layer, preferably within the range of from 20% to 40 % by weight, based on the dry weight of the coating layer, most preferably within the range of from 25% to 35 % by weight, based on the dry weight of the coating layer. This has been found to induce advantageous density properties of the coating mixture and thus an improved processability of the single coating layer during coating of the fibrous substrate.
The pigment may have a shape factor of 40 or greater, preferably 50 or more preferably 75 or greater. The fact that the pigment has a relatively high aspect ratio has been found to improve the coating properties of the coating layer.
The pigment may be talc. Talc has, in the context of the present disclosure, due to its platy nature been found to provide beneficial coating properties to the EAA/EMAA/EAAMAA and pigment mixture when used in a single coating layer. The pigment may preferably be talc having a shape factor of 40 or greater, preferably 50 or more preferably 75 or greater.
The fibrous substrate is a cellulosic fibrous substrate.
The fibrous substrate according to the present disclosure only comprises one coating layer on a first side thereof and thus no pre-coating layer or protective coating layers. The fact that the talc has a platy shape with a high aspect ratio may assist in covering the pores of the fibrous substrate and balancing the amount of the coating mixture on the surface and in the pores, thereby facilitating a uniform covering of the fibrous substrate.
At least 90% by weight of the dry weight of the coating layer may consist of EAA/EMAA/EAAMAA and the pigment, preferably at least 95% by weight of the dry weight of the coating consists of EAA/EMAA/EAAMAA and the pigment. A coating mixture wherein at least 90% by weight of the dry weight of the coating layer may consist of EAA/EMAA/EAAMAA and the pigment has, in the context of the present invention, been found to provide a superior heat sealability, a uniform covering of the single coating layer onto the fibrous substrate and facilitated processability.
The dry coating weight of the coating layer may be within the range of from 4 g/m² to 9 g/m². Preferably, the dry coating weight of the coating layer may be within the range of from 5 g/m² to 8 g/m². It has been found by the present inventors that such amounts provide for an improved coverage of the covering layer on the paper substrate, which is of particular importance for a single coated paper product.
The single coating layer may comprise a surfactant, such as ionic or non-ionic surfactants. Preferred surfactants are non-ionic and anionic surfactants. The single coating layer may also contain defoamer, blocking agents and rheology modifiers.
The single coating layer may comprise the additives in an amount of 0,01% to 10%. For example, including a surfactant into the single coating layer may improve runability of the coating machines and improve the interaction with the underlying fibrous substrate. The fact that the interaction with the underlying fibrous substrate is improved enhances the film formation of the coating mixture on the fibrous substrate, thereby inducing a more homogenous coating layer even at lower coating amounts.
The coated layer may preferably have a heat seal strength at 120° of 2.5 N/15 mm or more, the maximum heat seal strength being measured according to ASTM F88 technique A with unsupported 90 degree angle at 200mm/min, as measured on a 15 mm test strip sealed for 0.5 s at 120 °C and 3 bar. This means that 2.5 N is required to separate the sealed paper strip.
The second side of the fibrous substrate may have been subjected to a treatment of water to reduce curl of the finished product.
The heat sealable coated paper may have a recyclability score within the range of from 80 to 100%, as measured according to the Recyclability Testing Method PTS-RH 021:2012 - Cat II.
The heat sealable coated paper may have Friction Kinetic within the range of from 0.15 and 0.25, as measured in the coated side and according to the standard method for Determination of the kinetic coefficients of friction ISO 15359:2011, which is advantageous when forming the coated paper into a packaging suitable for the present disclosure. A higher Friction Kinetic may risk abrading the coating when being formed into packages, which is particularly unwanted for a single coated paper. A lower Friction Kinetic may indicate that the coated paper is too slippery for the type of packages that the heat sealable coated paper product is intended for.
The second side may be free from heat sealable coating layers.
The grammage of the fibrous substrate may be within the range of from 30 to 150 g/m², as measured according to the standard method ISO 536:2020.
The fibrous substrate may be a machine-glazed (MG) paper substrate, or a machine finished (MF) paper substrate. Typical applications for these papers are tissue wraps, dry food packaging, confectionary and baked goods, cosmetics and toiletries, toys packaging and home textiles or the like, which packages need to be heat sealed while not necessary needing as high barrier properties as for example vacuum packed food.
The fibrous substrate may for example be an MG fibrous substrate and have a grammage within the range of from 30 to 120 g/m².
The fibrous substrate may be a Machine glazed (MG) paper and have a tensile energy absorption index (TEA index) within the range of from 1 J/g to 6 J/g, preferably 1.2 J/g to 3.0 J/g, as measured according to the standard method ISO 1924-3.
The fibrous substrate may be an MG paper substrate and have a Bendtsen roughness value of the first side of the fibrous substrate is 1400 ml/min or less, preferably 1300 ml/min or less, such as within the range of from 170 ml/min to 1200 ml/min, as measured according to the standard method ISO 8791-2:2013.
It has been found that an MG paper substrate with a Bendtsen roughness value, as measured on the first side of the fibrous substrate, within the range of 170 ml/min to 1400 ml/min, in particular within the range of from 190 ml/min to 1350 ml/min, such as within the range of from 200 ml/min to 1200 ml/min, provides a sufficiently smooth surface to prevent the coating mixture to penetrate down into the paper substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties.
The Gurley porosity of an MG paper substrate according to the present disclosure may be 25 s or more, preferably 30 or more, such as within the range of from 25 and 2000 s.
The Gurley porosity is determined according to ISO 5636-5. The air resistance according to Gurley, i.e. 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 and a high Gurley porosity value indicates that the paper substrate has a low porosity. In the context of the present disclosure, a relatively low porosity may be preferred since this implies that the coating has a lower tendency to bleed through a paper of low porosity. An environmentally friendly coated paper product with a good heat sealability is thus obtained by reducing the Gurly porosity of the paper substrate in combination with the high amount of EAA/EMAA/EAAMAA as disclosed herein.
The fibrous substrate may alternatively be a MF paper substrate and have a grammage within the range of from 50 g/m² to 150 g/m², optionally within the range of from 55 g/m² to 130 g/m².
The fibrous substrate may be a MF paper substrate and have a Bendtsen roughness value on the first side of the fibrous substrate within the range of from 200 ml/min to 550 ml/min, preferably within the range of from 220 ml/min to 500 ml/min, such as within the range of from 200 ml/min to 460 ml/min, as measured according to the standard method ISO 8791-2:2013.
It has been seen that a MF paper substrate with a Bendtsen roughness value of the first side of the fibrous substrate within the range of from 200 ml/min to 550 ml/min, preferably within the range of from 220 ml/min to 500 ml/min, such as within the range of from 200 ml/min to 460 ml/min, provides a sufficiently smooth surface to prevent the coating mixture to penetrate down into the paper substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties.
The Gurley porosity of an MF paper substrate according to the present disclosure may be 30 s or more, such as 32 s or more, such as within the range of from 30 and 2000 s. The Gurley porosity is determined according to ISO 5636-5.
The fibrous substrate may be a MF paper substrate and have a tensile energy absorption index (TEA index) within the range of from 1 J/g to 6 J/g, as measured according to the standard method ISO 1924-3.
An MF paper substrate for use in packages with a higher fill-weights may suitably have a TEA index within the range of from 2 to 3 J/g, while an MF paper for use in high-end packages for lower fill-weight may suitably have a TEA index within the range of from 1 to 2 J/g.
In one embodiment, the fibrous substrate is a stretchable paper substrate; i.e. a paper substrate having a stretchability according to ISO 1924-3:2005 of at least 7% in the machine direction (MD) and at least 7% in the cross direction (CD). Preferably, the stretchability according to ISO 1924-3:2005 is at least 8% in the machine direction (MD) and at least 8% in the cross direction (CD).
In embodiments, the stretchable paper substrate may have a stretchability according to ISO 1924-3:2005 of at least 9% in at least one of the machine direction (MD) and the cross direction (CD). The stretchable paper may have a stretchability in MD or CD of at least 10%.
The stretchable paper substrate may have a stretch in the CD direction within the range of from 7 to 15%, optionally within the range of from 8% to 15%, as measured according to the standard method ISO 1924-3, optionally the fibrous substrate is an MF paper substrate. The fibrous substrate may furthermore have a stretch in the MD direction within the range of from 7 to 25%, optionally within the range of from 8% to 20%, as measured according to the standard method ISO 1924-3.
Such stretchable paper substrate is particularly suitable for preparing molded items with the paper substrate, such as for replacing plastic item and to reducing the coating layer amount, while still providing heat sealeability properties is highly relevant.
Such stretchable paper substrate may comprise at least 50 % softwood pulp, preferably at least 75 % softwood pulp and more preferably at least 90 % softwood pulp and wherein the fibrous substrate has a stretchability of at least 7% in the machine direction (MD) and at least 7% in the cross direction (CD), optionally, the fibrous substrate has a stretchability of at least 9% in the machine direction (MD) and at least 9% in the cross direction (CD), as measured according to the standard method ISO 1924-3:2005.
A stretchable paper substrate according to the present disclosure may have a grammage within the range of from 80 to 300 g/m².
A stretchable paper substrate according to the present disclosure may have a Bendtsen roughness value of the first side of the fibrous substrate within the range of from 1200 to 2000 ml/min.
The tensile energy absorption (TEA) index of the stretchable paper substrate may be at least 3.5 J/g in the machine direction (MD) and/or at least 2.9 J/g in the cross direction (CD), as measured according to the standard method ISO 1924-3:2005.
The Gurley porosity of the stretchable paper substrate according to the present disclosure may be above 15 s, preferably above 20 s.
According to a second aspect, the present disclosure relates to a method of producing a heat sealable coated paper product comprising the steps of:

providing a fibrous substrate comprising a first and second side; and
coating the first side of the fibrous substrate with a coating layer being applied in direct contact with the fibrous substrate, the coating layer comprises ethylene (meth)acrylic acid (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, in an amount of 51 % or more by weight based on the dry weight of the coating layer and a pigment, the pigment being talc and/or calcium carbonate (CaCO₃), the coating layer being the single coating layer on the first side of the fibrous substrate.

The method optionally comprises a step of treating the second side with water or dilute starch solution approximately 1-3% to for curl control. The treatment may be carried out by steam, spray or coating.

DETAILED DESCRIPTION

The present disclosure thus relates to a single coated heat sealable fibrous substrate wherein the single coating layer comprises ethylene (meth)acrylic acid copolymer (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, and a pigment being talc and/or calcium carbonate (CaCO₃), the single coating layer comprising the EAA/EMAA/EAAMAA in an amount of at least 51 % by weight based on the dry weight of the coating layer.
The coated paper product may be heat sealable as the coating layer comprises EAA/EMAA/EAAMAA in an amount of 51 % by weight or more based on the total dry coating weight. Consequently, the amount of pigment is at most 49 % by weight. A higher talc content has been found to impair the heat sealability. Preferably, the maximum heat seal strength measured according to ASTM F88 technique A with unsupported 90 degree angle at 200mm/min of the coated paper product is 2.5 N or greater, as measured on a 15 mm test strip sealed for 0.5 s at 120 °C and 3 bar. This means that 2.5 N is required to separate the sealed paper strip.
The coating layer may thus comprise ethylene acrylic acid copolymer (EAA) and/or ethylene methacrylic acid copolymer (EMAA) and/or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA). The coating layer may comprise only one of these polymers or a mixture of two of the polymers or a mixture of all three of these polymers. The EAA/EMAA/EAAMAA may be biobased and/or recycled.
The EAA/EMAA/EAAMAA may be present in the coating layer in an amount within the range of from 51 to 90 % by weight based on the dry weight of the coating layer, preferably within the range of from 53 to 85 % by weight based on the dry weight of the coating layer, more preferably within the range of from 60 to 70 % by weight based on the dry weight of the coating layer.
The pigment may be present in an amount within the range from 15% to 45 % by weight based on the dry weight of the coating layer, preferably within the range of from 20% to 40 % by weight based on the dry weight of the coating layer, most preferably within the range of from 25% to 35 % by weight based on the dry weight of the coating layer.
The pigment may have a shape factor of 40 or greater, preferably 50 or more preferably 75 or greater. Optionally, the pigment has a shape factor within the range of from 75 to 100. The fact that the pigment has a relatively high aspect ratio has been found to improve the coating properties of the coating layer.
The pigment may be talc and/or CaCO₃. However, talc has, in the context of the present disclosure, and due to its platy nature been found to provide beneficial coating properties to the EAA/EMAA/EAAMAA and pigment mixture when used in a single coating layer. The pigment may preferably be talc having a shape factor of 40 or greater, preferably 50 or more preferably 75 or greater.
The dry coating weight of the coating layer may be within the range of from 4 to 9 g/m². The ratio of the dry weight of the fibrous substrate and the dry weight of the coating layer may preferably be within the range of from 5 to 8 g/m².
The single coating layer may comprise the additives in an amount of 0,01% to 10%. For example, including a surfactant into the single coating layer may improve runability of the coating machines and improve the interaction with the underlying fibrous substrate. The fact that the interaction with the underlying fibrous substrate is improved enhances the film formation of the coating mixture on the fibrous substrate, thereby inducing a more homogenous coating layer even at lower coating amounts. The surfactant may be a nonionic surfactant or an ionic surfactant, preferably a nonionic or anionic surfactant The heat sealable paper product is a paper-based product that may, for example, be used for replacing plastic wrapping material for packaging material which needs to be heat sealed. The heat sealable paper product may be suitable for preparing packaging containers for powders and pellets, wherein the packaging containers are prepared in a vertical form fill and seal process. The packaging material may furthermore be suitable for preparing packages for articles, tablets and bars which are prepared in a horizontal form fill and seal process.
The second side of the fibrous substrate have been subjected to a treatment of water to reduce curl of the finished product.
The cellulosic fibrous substrate may be a machine-glazed (MG) paper or a machine finished (MF) paper.
The grammage measured according to ISO 536:2020 of the fibrous substrate is typically 35 to 150 g/m², such as 40-120 g/m², such as 42-100 g/m². A suitable thickness (measured according to ISO 534:2011) of the fibrous substrate is 40-200 µm, such as 48-115 µm.
In one embodiment the fibrous substrate is an MG paper substrate. It is preferably a kraft paper and typically at least 80%, preferably at least 90%, by dry weight of the fibres used to produce the MG paper substrate are never-dried fibres (i.e. virgin fibres). An MG paper has glazed side and a non-glazed side. The glazed side is the side that faced the Yankee cylinder (a polished metal cylinder sometimes referred to as a MG cylinder) used for drying the paper web in the MG papermaking machine. The contact with the polished metal surface during drying makes the glazed side smoother than the non-glazed side.
Typically, the coating layer is applied to the less smooth side, non-glazed side of the fibrous substrate. The opposite side, i.e. glazed side, in such case is typically printed. It is beneficial to apply the coating on the non-glazed side to provide the glazed side for printing. The glazed side may be coated with a thin layer of water or starch solution (≤ 1 g/m²) for curl control. A lacquer may be provided on the optional print, e.g. to modify gloss, friction and/or release properties.
The fibrous substrate may be a Machine glazed (MG) paper substrate and have a tensile energy absorption index (TEA index) within the range of from 1 J/g to 6 J/g, preferably 1.2 J/g to 3.0 J/g, as measured according to the standard method ISO 1924-3.
The tensile strength is the maximum force that a paper will withstand before breaking. In the standard test ISO 1924-3, a stripe having a width of 15 mm and a length of 100 mm is used with a constant rate of elongation. Tensile energy absorption (TEA) is sometimes considered to be the paper property that best represents the relevant strength of a paper. The tensile strength is one parameter in the measurement of the TEA and another parameter is stretchability. The tensile strength, the stretchability and the TEA value are obtained in the same test. The TEA index is the TEA value divided by the grammage. In the same manner, the tensile index is obtained by dividing the tensile strength by the grammage.
The fibrous substrate may be an MG paper substrate and have a Bendtsen roughness value of the first side of the fibrous substrate is 1400 ml/min or less, preferably 1300 ml/min or less, such as within the range of from 170 ml/min to 1200 ml/min, as measured according to the standard method ISO 8791-2:2013.
It has been found that an MG paper substrate with a Bendtsen roughness value of the first side of the fibrous substrate within the range of 170 ml/min to 1400 ml/min, in particular within the range of from 190 ml/min to 1350 ml/min, such as within the range of from 200 ml/min to 1200 ml/min, provides a sufficiently smooth surface to prevent the coating mixture to penetrate down into the paper substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties.
The Gurley porosity of an MG paper substrate according to the present disclosure may be 25 s or more, preferably 30 or more, such as within the range of from 25 and 2000 s. The Gurley porosity is determined according to ISO 5636-5. In the context of the present disclosure, a relatively low porosity may be preferred since this implies that the coating has a lower tendency to bleed through a paper of low porosity. An environmentally friendly coated paper product with a good heat sealability is thus obtained by reducing the Gurly porosity of the paper substrate in combination with the high amount of EAA/EMAA/EAAMAA as disclosed herein.
The air resistance according to Gurley, i.e. 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 and a high Gurley porosity value indicates that the paper substrate has a low porosity.
In an alternative embodiment, the fibrous substrate is an MF paper substrate. For an MF paper substrate, it is preferably a kraft paper and typically at least 80%, preferably at least 90%, by dry weight of the fibres used to produce the MF paper are never-dried fibres (i.e. virgin fibres). An MF paper substrate has an optionally calendered side or is uncalendared on both sides. The drying is done via a multiple of drying cylinders. Optionally the paper can be compacted by using a Clupac or an expanda unit to add stretchability properties in the machine direction.
Typically, the coating layer is applied to the less smooth, non calendered, side of the fibrous substrate. If the opposite side is calendered, i.e., a smooth side, this side is typically printed. The smooth side may be coated with a thin layer of water or starch solution (≤ 1 g/m²) for curl control. A lacquer may be provided on the optional print, e.g. to modify gloss, friction and/or release properties.
The MF paper substrate according to the present disclosure may have a grammage within the range of from 50 g/m² to 150 g/m², optionally within the range of from 55 g/m² to 130 g/m².
The Gurley porosity of an MF paper substrate according to the present disclosure may be 30 s or more, such as 32 s or more, such as within the range of from 30 and 2000 s. The Gurley porosity is determined according to ISO 5636-5.
The MF paper substrate may have a Bendtsen roughness value of the first side of the fibrous substrate is within the range of from 200 ml/min to 550 ml/min, preferably within the range of from 220 ml/min to 500 ml/min, such as within the range of from 200 ml/min to 460 ml/min, as measured according to the standard method ISO 8791-2:2013. It has been seen that a MF paper substrate with a Bendtsen roughness value of the first side of the fibrous substrate within the range of from 200 ml/min to 550 ml/min, preferably within the range of from 220 ml/min to 500 ml/min, such as within the range of from 200 ml/min to 460 ml/min, provides a sufficiently smooth surface to prevent the coating mixture to penetrate down into the paper substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties.
The MF paper substrate may have a tensile energy absorption index (TEA index) within the range of from 1 J/g to 6 J/g, as measured according to the standard method ISO 1924-3.
An MF paper substrate for use in packages with a higher fill-weights may suitably have a TEA index within the range of from 2 to 3 J/g, while an MF paper for use in high-end packages for lower fill-weight may suitably have a TEA index within the range of from 1 to 2 J/g.
In a still further embodiment, the fibrous substrate is a stretchable paper; i.e. a paper having a stretchability according to ISO 1924-3:2005 of at least 7% in the machine direction (MD) and at least 7% in the cross direction (CD). Preferably, the stretchability according to ISO 1924-3:2005 is at least 8% in the machine direction (MD) and at least 8% in the cross direction (CD).
In embodiments, the stretchable paper may have a stretchability according to ISO 1924-3:2005 of at least 9% in at least one of the machine direction (MD) and the cross direction (CD). The stretchable paper may have a stretchability in MD or CD of at least 10%.
Preferably the paper is stretchable in both MD and CD. An upper limit for the stretchability in MD may for example be 20 percent or 25 percent. An upper limit for the stretchability in CD may for example be 15 percent.
The stretchability (in both MD and CD) is determined according to the standard ISO 1924-3:2005.
Such a stretchable fibrous substrate is particularly suitable for molding to prepare paper items which may replace plastic items. For such paper items, replacing plastic items, there may be a particular interest to reduce the amount of coating to minimum while still providing heat-sealable properties to the coated paper product used to mold the paper items.
The tensile strength and the tensile energy absorption (TEA) index of the paper are preferably high in the stretchable paper. In embodiments, the tensile energy absorption (TEA) index of the stretchable paper according to ISO 1924-3:2005 is at least 3.5 J/g in the machine direction (MD) and/or at least 2.9 J/g in the cross direction (CD). For example, the TEA index may be from 3.5 to 7.5 J/g in the machine direction (MD) and from 2.9 to 3.9 in the cross direction (CD).
The stretchable paper substrate may have a Gurley porosity of above 15 s, preferably above 20 s, as measured according to the standard method ISO 5636-5. In contrast to many sack papers, which may be highly stretchable, the stretchable paper substrate as disclosed herein is not particularly porous. Instead, relatively low porosity may be preferred. In the context of the present disclosure, where the fibrous substrate comprises a single layer coating, the coating has a lower tendency to bleed through a paper of low porosity.
In embodiments, the Gurley porosity of the stretchable paper substrate is at least 25 s, such as at least 35 s. An upper limit may for example be 300 s or 250 s. The Gurley porosity is determined according to ISO 5636-5.
The pulp used to form the stretchable paper substrate of the present disclosure may be a virgin pulp, such as a sulphate pulp (sometimes referred to as a "kraft pulp"), which provides high tensile strength. Accordingly, the paper of the present disclosure is preferably a kraft paper.
For the same reason, the starting material used for preparing the pulp preferably comprises softwood (which has long fibers and forms a strong paper). Accordingly, the pulp used to form the paper of the present disclosure 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. Preferably, the paper of the present disclosure is a softwood kraft paper formed from 100% virgin fibers.
The stretchable paper substrate suitable for use in the present disclosure may be produced according to a process disclosed in WO2018/185213 , WO2018/185215 or WO2018/185216 , the entire contents of which are incorporated herein by reference.
The stretchable paper substrate may be a paper commercialized by Billerud under the trademark FibreForm^®.
The second side of the fibrous substrate may be a non-coated side or may be coated only with a thin layer of starch (≤ 1 g/m²) for curl control. If the second side is intended to be provided with a print, a lacquer may be provided on such optional print, e.g. to modify gloss, friction and/or release properties, i.e. the second side may be a non-coated side, or at least a non-heat sealable side.

EXAMPLES

Samples were prepared by coating two different kinds of fibrous substrates. Fibrous substrate 1 (FS1) is a calendered fiber substrate produced from pure bleached pulp with a grammage of 80 gsm and Fibrous substrate 2 (FS2) is a calendered unbleached kraft paper 80gsm.
Each of the samples were coated with a coating mixture comprising ~70 wt.% of EEA and ~30 wt % of talc. Additionally, small amounts of thickening agents, defoaming agents and surfactants were added. Table 1

Sample Pulp Grammage [gsm] Coat weight [gsm] EAA [wt.%] Talc [wt.%]

1 FS1 80 5,4 70 30

2 FS2 80 6,0 70 30

3 FS2 80 6,6 70 30

4 FS2 80 6,2 70 30

The results are illustrated below in table 2.

Table 2

Sample	Seal (120°C) [N/15mm]	Friction Kinetic	WVTR (23/50) [g/m2day	Bendtsen [ml/min] First side	Bendtsen [ml/min] Second side	Gurley porosity [s]
1	3,2	0,16	49,6	424	267	39
2	4,5	0,18	30,1	255	194	27
3	3,5	0,19		231	161	32
4	3,2	0,19		275	171	34

The heat seal strength, Friction Kinetics, Bendtsen rougness and Gurley porosity were measured for the four product samples. The WVTR were measured for samples 1 and 2. The Bendtsen roughness was measured for the first and the second side of the paper substrate according to the standard method ISO 8791-4 and the Gurley porosity was measured for the first side according to the standard method ISO 5636-5. As, may be seen in Table 2 a combinatory effect of a relatively high Gurley porosity with a coat weight of least 5.4 g/m² provided an advantageous heat seal of above 2.5 N/15mm. The relatively low Bendtsen roughness on the first side was also found to improve the heat seal strength of the coated paper products according to the present disclosure. The Friction Kinetic was measured on the coated side of the paper and were within the range of from 0,16 and 0.19 for each of the samples, which is advantageous when forming packages of the heat sealable coated paper product.

Claims

A heat sealable coated paper product comprising a cellulosic fibrous substrate comprising a first and a second side, the first side comprising a single coating layer being applied in direct contact with the fibrous substrate, the coating layer comprising ethylene (meth)acrylic acid copolymer (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, in an amount of at least 51 % by weight, based on the dry weight of the coating layer, and a pigment being talc and/or calcium carbonate (CaCO₃).
The heat sealable coated paper product according to claim 1, wherein the pigment is present in an amount within the range from 15% to 45 % by weight based on the dry weight of the coating layer, preferably within the range of from 20% to 40 % by weight based on the dry weight of the coating layer, most preferably within the range of from 25% to 35 % by weight based on the dry weight of the coating layer.
The heat sealable coated paper product according to claim 1 or 2, wherein at least 90% by weight of the dry weight of the coating layer consists of EAA/EMAA/EAAMAA and the pigment, preferably at least 95% by weight of the dry weight of the coating consists of EAA/EMAA/EAAMAA and the pigment.
The heat sealable coated paper product according any one of the preceding claims, wherein the dry coating weight of the coating layer is within the range of from 4 g/m² to 9 g/m², preferably within the range of from 5 g/m² to 9 g/m².
The heat sealable coated paper product according to any one of the preceding claims, wherein the grammage of the fibrous substrate is within the range of from 30 to 150 g/m², as measured according to ISO 536:2020.
The heat sealable coated paper product according to claim 5, wherein the fibrous substrate is a machine-glazed (MG) paper, or a machine finished (MF) paper.
The heat sealable coated paper product according to any one of claims 1 to 5, wherein the fibrous substrate is an MG paper and has a Bendtsen roughness value of 1400 ml/min or less, preferably 1300 ml/min or less, such as within the range of from 170 ml/min to 1200 ml/min, as measured on the first side of the fibrous substrate and according to the standard method ISO 8791-2:2013.
The heat sealable coated paper product according to any one of the claims 1 to 5 or claim 7, wherein the fibrous substrate is an MG fibrous substrate and has a Gurley porosity of 25 s or more, preferably 30 s or more, as measured according to the standard method ISO 5636-5.
The heat sealable coated paper product according to any one of claims 1 to 5, wherein the fibrous substrate is an MF paper substrate and has a Bendtsen roughness value within the range of from 200 ml/min to 550 ml/min, preferably within the range of from 220 ml/min to 500 ml/min, such as within the range of from 200 ml/min to 460 ml/min, as measured on the first side of the fibrous substrate and according to the standard method ISO 8791-2:2013.
The heat sealable coated paper product according to any one of claims 1 to 5 or claim 9, wherein the fibrous substrate is an MF paper substrate and has a Gurley porosity of 30 s or more, preferably 32 s or more, as measured according to the standard method ISO 5636-5.
The heat sealable paper according to any one of claims 1 to 5, wherein the fibrous substrate has a stretch in the CD direction of at least 7%, preferably at least 9%, as measured according to the standard method ISO 1924-3:2005.
The heat sealable paper according to any one of claims 1 to 5 or claim 11, wherein the fibrous substrate has a stretch in the MD direction of at least 7%, preferably at least 9%, as measured according to the standard method ISO 1924-3:2005.
The heat sealable paper according to any one of claims 11 or 12, wherein the Gurley porosity of the fibrous substrate is above 15 s, preferably above 20 s.
The heat sealable paper according to any one of claims 11 to 13, wherein the grammage of the fibrous substrate is within the range of from 80 to 300 g/m², preferably within the range of from 90 to 290 g/m², as measured according to ISO 536:2020.
The heat sealable coated paper product according to any one of the preceding claims, wherein the second side of the fibrous substrate is free from coating or alternatively is coated only with a single layer comprising or consisting of starch.
The heat sealable coated paper product according to any one of the preceding claims for use as a packaging material.
A method of producing a heat sealable coated paper product comprising the steps of:
- providing a fibrous substrate comprising a first and second side; and

- coating the first side of the fibrous substrate with a coating layer being applied in direct contact with the fibrous substrate, the coating layer comprises ethylene (meth)acrylic acid (EAA/EMAA) or ethylene-acrylic acid-methacrylic acid terpolymer (EAAMAA) or mixtures thereof, in an amount of at least 51 % by dry weight of the coating layer and a pigment being talc and/or Calcium Carbonate (CaCO₃), the coating layer being the single coating layer on the first side of the fibrous substrate; and

- optionally treating the second side of the fibrous substrate with water or starch.