Patent Application
20250215647
The present disclosure relates to the field of heat sealable coated paper products and in particular to coated paper for use in packaging of food products.
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.
The present disclosure aims to provide a heat sealable paper product for
use in packaging, where heat-sealability as well as water-vapour barrier and water-resistance properties are important.
Moreover, a reduction of the amount of plastics regularly used in packaging is desirable.
According to a first aspect, a coated paper product is provided, the coated paper product comprising
Optionally, the coat weight of the barrier coating is at least 4 g/m2, such as 4-15 g/m2, such as 5-10 g/m2, or such as 5-9 g/m2.
Optionally, the polyolefin-based binder is a polyethylene (PE)-based binder.
Optionally, the PE-based binder comprises ethylene-co-alkene copolymers.
Optionally, the PE-based binder comprises a copolymer of ethylene and a carboxylic acid monomer.
The paper sheet substrate is a single ply paper product, in contrast to paperboard which are generally multiply paperboard products.
Optionally, the paper sheet substrate has a density within the range of from 700 to 1400 kg/m3 as measured according to ISO 534: 2011. Examples of paper sheets having densities in the higher end of the range are glassine paper as well as high-density paper (HD-paper). The latter for example disclosed in WO2021/069696 A1.
Optionally, the second side of the paper sheet substrate may have been subjected to a treatment of water or starch to reduce curl of the finished product.
Optionally, the second side of the paper sheet substrate is free from coating or alternatively is coated only with a single layer print layer comprising pigments or a single layer comprising or consisting of starch. The second side may thus be a non-heat sealable side.
A print layer may comprise a binder and a pigment, the amount of pigment in the print layer may be within the range of from 70 and 95 wt-%, such within the range of from 80 to 90 wt-%, based on the dry weight of the print layer. The binder may for example be a styrene-acrylate copolymer (SA) and/or styrene-butadiene copolymer (SB).
Optionally, the coated paper product comprises a pre-coating arranged between the paper sheet substrate and the barrier coating.
Optionally, the pre-coating comprises binder and pigment.
Optionally, the binder of the pre-coating is selected from the group consisting of ethylene-acrylic acid copolymer (EAA), ethylene-methacrylic-acid-copolymer (EMAA), maleic acid ethylene copolymer (MAE), vinyl acetate acrylate copolymer (VAcA), styrene-acrylate copolymer (SA) and styrene-butadiene copolymer (SB).
Optionally, the amount of pigment in the pre-coating is 10-50 wt-%, such as 15-40 wt-%, based on the dry weight of the pre-coating.
Optionally, the amount of pigment in the pre-coating is 70-95 wt-%, such as 80-90 wt-%, based on the dry weight of the pre-coating.
Optionally, the amount of talc in the barrier coating is 20-40 wt-%, such as 20-35 wt-% based on the dry weight of the barrier coating.
Optionally, the amount of polyolefin-based binder in the barrier coating is at least 55 wt-%, such as 55-85 wt-%, such as 60-80 wt-%, such as 65-80 wt-%, based on the dry weight of the barrier coating.
Optionally, the recyclability measured according to PTS Method PTS-RH 021/97 of the coated paper product is at least 80%.
The grammage of the paper sheet substrate may be within the range of from 10 to 150 g/m2, such as within the range of from 30 to 120 g/m2, such as from 30 to 110 m2, as measured according to the standard method ISO 536: 2020.
The paper sheet substrate may be a machine-glazed (MG) paper sheet substrate, or a machine finished (MF) paper sheet 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.
The paper sheet substrate may for example be an MG paper sheet substrate and have a grammage within the range of from 30 to 120 g/m2.
The paper sheet 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 paper sheet substrate may be an MG paper sheet substrate and have a Bendtsen roughness value of first side of the paper sheet 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 sheet substrate with a Bendtsen roughness value, as measured on the first side of the paper sheet substrate, here the non-glazed side, 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 sheet substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties.
Alternatively, the paper sheet substrate is an MG paper sheet and has a Bendtsen roughness value on 200 ml/min or less on the first side, i.e, the paper sheet substrate is coated on the glazed side, as measured according to the standard method ISO 8791-2: 2013.
The Gurley porosity of an MG paper sheet 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: 2013. 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, i.e. long time, indicates that the paper sheet 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 increasing the Gurley porosity of the paper sheet substrate in combination with the high amount of EAA/EMAA/EAAMAA as disclosed herein.
The paper sheet substrate may alternatively be a MF paper sheet substrate and have a grammage within the range of from 40 g/m2 to 150 g/m2, optionally within the range of from 55 g/m2 to 130 g/m2.
The paper sheet substrate may be a MF paper sheet substrate and have a Bendtsen roughness value on the first side of the paper sheet substrate within the range of from 200 ml/min to 1200 ml/min, preferably within the range of from 220 ml/min to 550 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 sheet substrate with a Bendtsen roughness value of the first side of the paper sheet substrate within the range of from 200 ml/min to 1200 ml/min, preferably within the range of from 220 ml/min to 550 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 sheet 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 sheet 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: 2013.
The paper sheet substrate may be a MF paper sheet 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: 2005.
An MF paper sheet 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 paper sheet substrate is a stretchable paper sheet substrate; i.e. a paper sheet 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 sheet 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 and/or CD of at least 10%.
The stretchable paper sheet 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 paper sheet substrate is an MF paper sheet substrate. The paper sheet 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 sheet substrate is particularly suitable for preparing molded items with the paper sheet substrate, such as for replacing plastic item and to reducing the coating layer amount, while still providing heat sealability properties is highly relevant.
Such stretchable paper sheet 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 paper sheet substrate has a stretchability of at least 7% in the machine direction (MD) and at least 7% in the cross direction (CD), optionally, the paper sheet 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 sheet substrate according to the present disclosure may have a grammage within the range of from 80 to 300 g/m2.
The paper sheet substrate may thus either have a grammage of within the range of from 10 to 150 g/m2, such as within the range of from 30 to 120 g/m2 or be a stretchable paper sheet substrate according to the present disclosure and have a grammage within the range of from 80 to 300 g/m2.
A stretchable paper sheet substrate according to the present disclosure may have a Bendtsen roughness value of the first side of the paper sheet substrate within the range of from 1200 to 2000 ml/min.
The tensile energy absorption (TEA) index of the stretchable paper sheet 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 sheet substrate according to the present disclosure may be above 15 s, preferably above 20 s.
The paper can also be a Glassine paper or a HD-paper. In such case, the paper typically has one or more of the following properties:
Grammage is measured according to ISO 536: 2012 and density and thickness are measured according to ISO 534: 2011. Gurley value is determined according to ISO 5636-5: 2013. Bendtsen roughness is measured according to SS-ISO 8791-2: 2013.
The glassine paper and the HD paper may be supercalendered papers. Such papers are thin with a smooth surface and have advantageous barrier properties.
The barrier coated side may have a heat sealing strength of at least 2.5 N/15 mm, as measured at 160° C./0.5 s/3 bar, according to the standard method ASTM F88, EN 868-5* and on a 15 mm test strip. Preferably at least 3 N/15 mm.
According to a second aspect, a method of producing a coated paper product is provided, the method comprising the steps of:
Optionally, the barrier coating is applied in-line.
Optionally, the method further comprises the step of:
Optionally, the method comprises drying between the pre-coating step and the barrier-coating step.
Optionally, the pre-coating is applied in-line.
Optionally, the paper sheet substrate in the method is any one of the paper sheet substrates according to the first aspect of the present disclosure.
Optionally, the paper sheet substrate is calendered or supercalendered.
Optionally, the barrier coating and/or the pre-coating is according to the disclosure of the first aspect.
According to a third aspect, the present disclosure relates to use of a coating comprising talc and a polyolefin-based binder as a water-resistant barrier on a paper sheet substrate, wherein the amount of talc in the barrier coating is 15-40 wt-% based on the dry weight of the barrier coating.
The inventors have realized that coating a paper sheet substrate with a barrier coating comprising 15-40 wt-% talc and a polyolefin-based binder a coated paper is obtained that has barrier properties combined with heat-sealability.
Moreover, the need for an extrusion-coated PE-layer at least on the outside of a package produced from the coated paper is omitted.
When the paper is used for packaging, the packaging is typically produced via form fill sealing (FFS) of the paper, such as vertical form fill sealing (VFFS) or horizontal form fill sealing (HFFS), or via sealing of a self-opening satchel (SOS) bag of the paper. Examples of packaging formed via form fill sealing (FFS) are a gusseted packaging or a pillow packaging, having a longitudinal seal and each end portion sealed by a fin seal. FFS is either produced as vertical form fill sealing (VFFS) or horizontal form fill sealing (HFFS).
A filled gusseted paper packaging has a longitudinal seal adhering two overlapping ends of the paper material to each other to form a lap seal. Further, the packaging has a top end sealed by a fin seal and a bottom end sealed by a fin seal.
A self-opening satchel (SOS) bag is typically delivered in a folded form a bag machine. To fill it, it is opened fully and, lastly filled and sealed. An SOS bag, when fully opened, is free-standing. Products packed in these bags comprise basic foodstuffs, for example flour, sugar, cereals, tea, coffee, biscuits and confectionery, but also food mixes for both retail and catering requirements. Numerous other industries are also served, including cat litter, small pet-food packs, agrochemicals and horticulture.
According to a first aspect of the present disclosure there is provided a coated paper product comprising:
The polyolefin-based binder typically comprises a base polymer, a dispersant and a neutralizing agent. A binder is a polymer dispersed in an aqueous or solvent-based medium, as opposed to a polymer applied by melt extrusion or melt lamination. The base polymer is typically a copolymer of an olefin together with a suitable comonomer that allows the olefin to be dispersed in water. Examples of such a comonomers are alkene and/or carboxyl acid. Accordingly, the base polymer is typically a copolymer of an olefin and an alkene or a copolymer of an olefin and a carboxylic acid or mixtures thereof.
An example of a suitable polyolefin-based binder is a PE-based binder, such as the PE-based binder described in WO2021/225764 A1. In such case, the base polymer is typically an ethylene-co-alkene copolymer comprising non-functionalized ethylene-co-alkene copolymers wherein the weight-to-weight ratio of the structural units of ethylene to alkene is typically in the range of from 99.8:0.2 to 50:50. The dispersant copolymer typically has an acid value of 130 or less and comprises structural units of ethylene and a carboxylic acid monomer. The dispersant copolymer typically has a melt flow index in the range of from 50 to 2000 g/10 min at 190° C./2.16 kg, and a weight-to-weight ratio of structural units of ethylene to carboxylic acid monomer in the range of from 95:5 to 70:30. The neutralizing agent is typically a hard base and the concentration of the neutralizing agent is sufficient to neutralize at least half of the carboxylic acid groups present in the PE-based binder.
It has surprisingly been found by the inventors that talc is a particularly suitable pigment due to its synergistic functions with the polyolefin-based binder in dry as well as humid and wet environments. The talc and polyolefin-based binder in combination displays beneficial properties with respect to water-vapour resistance, water-resistance and heat-sealability. It has furthermore been found that the provision of talc in the herein specified amounts prevent, or at least reduces, the formation of ruptures in the coating layer upon folding of the paper. The amount of talc in the barrier coating may be 20-40 wt-%, such as 20-35 wt-% based on the dry weight of the barrier coating. The amount of polyolefin-based binder in the barrier coating is typically at least 55 wt-%, such as 55-85 wt-%, such as 60-80 wt-%, such as 65-80 wt-% based on the dry weight of the barrier coating. Typically, additives, such as rheology modifiers, may also be added to the dispersion.
The water resistance according to the Cobb 1800 s test (measured according to ISO 535) is typically below 2.5 g/m2 on the coated paper product. Such Cobb 1800 value is obtained by measuring on a barrier coated paper without any PE-layer. Thus, by the application of the barrier coating, at least an outer PE-layer can be omitted compared with conventional papers for applications requiring higher barrier properties, and thereby the PE ratio as determined by the ratio of PE of the whole coated paper is advantageously decreased.
The weight of the barrier coating is typically at least 4 g/m2, such as 4-15 g/m2, such as 5-10 g/m2 or such as 5-9 g/m2. From an economical and environmental perspective, it is advantageous to keep the coat weight as low as possible and in a full-scale coating machine, the coat weight can be kept lower than in a laboratory equipment.
The coated paper product may comprise a pre-coating arranged between the paper sheet substrate and the barrier coating. If the surface of the paper sheet substrate is rough, it is beneficial to apply a pre-coating to level out the surface. The pre-coating typically comprises binder and pigment. The amount of pigment in the pre-coating is typically 10-50 wt-%, such as 15-40 wt-%, based on the dry weight of the pre-coating. Such an amount of pigment is beneficial for water vapour barrier properties. Alternatively, the amount of pigment in the pre-coating is 70-95 wt-%, such as 80-90 wt-%, based on the dry weight of the pre-coating. The fact that the amount of pigment in the pre-coating is 70-95 wt-%, such as 80-90 wt-%, based on the dry weight of the pre-coating has been found to improve adhesion between the pre-coating layer and the barrier coating layer. The barrier coating is typically almost optically transparent and a higher amount of pigment in the pre-coating is beneficial when a higher brightness of the coated paper product is desired.
The binder in the pre-coating is typically selected from the group consisting of ethylene-acrylic acid copolymer (EAA), ethylene-methacrylic-acid-copolymer (EMAA), maleic acid ethylene copolymer (MAE), vinyl acetate acrylate copolymer (VAcA), styrene-acrylate copolymer (SA) and styrene-butadiene copolymer (SB). The invention is not limited to a specific type of binder in the pre-coating and other binders of similar properties are also suitable.
The paper sheet substrate may be surface sized, typically with a starch-based surface sizing to prevent, or at least decrease, the risk of curl and dust.
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.
Preferably, the paper is a non-paperboard product.
Optionally, the paper sheet substrate has a density within the range of from 700 to 1400 kg/m3 as measured according to ISO 534: 2011. Examples of paper sheets having densities in the higher end of the range are glassine paper as well as high-density paper (HD-paper). The latter for example disclosed in WO2021/069696 A1. The cellulosic paper sheet substrate may be a machine-glazed (MG) paper, a machine finished (MF) paper or a glassine paper sheet.
The grammage measured according to ISO 536: 2020 of the paper sheet 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 paper sheet substrate is 40-200 μm, such as 48-115 μm.
In one embodiment the paper sheet substrate is an MG paper sheet 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 sheet 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 barrier coating layer is applied to the less smooth side, non-glazed side of the paper sheet 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. However, the barrier coating layer may alternatively be applied to the smooth side.
The paper sheet substrate may be a Machine glazed (MG) paper sheet 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 paper sheet substrate may be an MG paper sheet substrate and have a Bendtsen roughness value of the first side of the paper sheet 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 sheet substrate with a Bendtsen roughness value of the first side of the paper sheet 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 sheet 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 sheet 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 porosity of the paper sheet substrate in combination with the PE-based binder 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 sheet substrate has a low porosity.
In an alternative embodiment, the paper sheet substrate is an MF paper sheet substrate. For an MF paper sheet 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 sheet 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 paper sheet 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 sheet substrate according to the present disclosure may have a grammage within the range of from 40 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 sheet 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 sheet substrate may have a Bendtsen roughness value of the first side of the paper sheet substrate is within the range of from 200 ml/min to 1200 ml/min, preferably within the range of from 220 ml/min to 550 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 sheet substrate with a Bendtsen roughness value of the first side of the paper sheet substrate within the range of from 200 ml/min to 1200 ml/min, preferably within the range of from 220 ml/min to 550 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 sheet substrate while still allowing a sufficiently high coating as disclosed herein to provide the paper product with advantageous heat sealing properties, in particular when providing a pre-coating layer.
The MF paper sheet 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 sheet 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 paper sheet 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 paper sheet 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 sheet 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 sheet substrate as disclosed herein is not particularly porous. Instead, relatively low porosity may be preferred. In the context of the present disclosure, where the paper sheet 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 sheet 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 sheet 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 sheet 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 sheet substrate may be a paper commercialized by Billerud under the trademark FibreForm®.
The paper sheet may have a basis weight of less than 120 g/m2, optionally less than 110 g/m2 and be a non-paperboard paper product.
The paper may be a Glassine paper or a HD-paper. In such case, the paper typically has one or more of the following properties:
A second side of the paper sheet 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. By non-heat-sealable herein may mean a sealing strength of less than 0.8 N/15 mm or less than 0.5 N/15 mm, such as less than 0.2 N/15 mm, as measured at 160° C./0.5 s/3 bar, according to the standard method ASTM F88, EN 868-5* and on a 15 mm test strip.
According to a second aspect of the present disclosure there is provided a method of producing a coated paper product comprising the steps of:
The method may further comprise the step of:
The barrier coating and pre-coating may be applied in-line (also referred to as on-line). In such case, the productivity is increased by eliminating the handling operations linked to off-line treatment and by eliminating, or at least reducing, the amount of waste. In one embodiment, the method comprises drying between the application of the pre-coating layer and the application of the barrier coating layer. Drying is typically performed with non-contact drying, such as IR and/or hot air, or contact drying, such as a drying cylinder, or a combination of non-contact and contact drying.
The examples and embodiments discussed above in connection to the first aspect apply to the second aspect mutatis mutandis.
The paper sheet substrate in the method may be any one of the paper sheet substrates described for the first aspect of the present disclosure.
According to a third aspect of the present disclosure there is provided use of a coating comprising talc and a polyolefin-based binder as a water-resistant barrier on a paper sheet substrate, wherein the amount of talc in the barrier coating is 15-40 wt-% based on the dry weight of the barrier coating.
The examples and embodiments discussed above in connection to the first and second aspect apply to the third aspect mutatis mutandis.
The aspects of the present disclosure will now be described with reference to the accompanying drawings, in which certain embodiments of the invention are shown. Like numbers refer to like elements throughout the description.
Various amounts of pigment (talc (Finntalc C15B2), kaolin clay (Barrisurf LX) or CaCO3 (Setacarb HG-ME 75%)) were dispersed in a polyolefin-based binder (the binder is further described in WO2021/225764 A1 by Dow Global Technologies) having a solids content of about 50%.
For evaluation of heat-sealing, the different barrier coatings were coated with a pilot-scale blade coater at a coat weight of each barrier coating of about 8 g/m2 on MF-paper sheets (about 80 g/m2).
For evaluation of Cobb, water vapour transmission rate, a baseboard (268 g/m2) having 3 plies: a top-ply of bleached kraft pulp, a centre ply with of a mix of CTMP, kraft fibres and broke pulp, and a bottom inner ply of unbleached kraft pulp was used as paper sheet substrate. The baseboard was coated with a pre-coating containing 95 parts CaCO3 (HydroCarb 75), 5 parts kaolin clay (Kaovit), 16 parts styrene-acrylate (SA) latex (SA 95085.05), and 0.23 parts rheology modifier (CartaCoat RM15) using a pilot-scale blade coater at a coat weight of each pre-coating of about 13 g/m2. Even though the tests are performed for paperboard substrates the results are relevant for evaluating these properties also for a non-paperboard barrier paper.
On top of the pre-coated baseboards, the different barrier coatings were coated with a pilot-scale blade coater at a coat weight of each barrier coating of about 8 g/m2.
The properties of the barrier coated baseboard is shown in Table 1 below.
The following parameters were evaluated, and the standard used for each evaluation is listed below:
As indicated in Table 1, no or only 10% pigment is insufficient for runnability.
25% and 30% talc did not fail any of the criteria in Table 1. A further increase of the proportion of talc to 50%, however, affected both Cobb1800 and heat sealing negatively.
Regarding clay as a pigment, the addition of more than 10% negatively affected the water resistance (Cobb1800).
