WO2021111357A1

WO2021111357A1 - Water-resistant cellulose-based substrate with improved repulpability

Info

Publication number: WO2021111357A1
Application number: PCT/IB2020/061439
Authority: WO
Inventors: Susanne HANSSON; Raija BÅDENLID
Original assignee: Stora Enso Oyj
Priority date: 2019-12-04
Filing date: 2020-12-03
Publication date: 2021-06-10
Also published as: SE1951393A1

Abstract

The present invention relates to a water-resistant cellulose-based substrate comprised of one or more plies, wherein said substrate has a basis weight in the range of 120-500 g/m², and a density below 1000 kg/m³, wherein said substrate has been subjected to grafting with a fatty acid halide through the entire thickness of said substrate such that the substrate has a Cobbeo value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m² and an edge wick index (Lactic acid 1% solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h. The present invention further relates to a method for manufacturing the water-resistant cellulose-based substrate.

Description

WATER-RESISTANT CELLULOSE-BASED SUBSTRATE WITH IMPROVED

REPULPABILITY

Technical field The present disclosure relates to water-resistant cellulose-based substrates, such as paper, paperboard or containerboard, for use in wet or damp environments.

Background

Water resistance is an important property in many paper, paperboard or containerboard applications. Some examples include packaging, such as boxes, corrugated board and other containers; fresh and aseptic liquid packaging; boxes, corrugated board, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction material.

Paper, paperboard or containerboard for use in wet or damp environments are usually treated with sizing agents to enhance certain qualities; and above all, to increase the water-resistance (i.e. resistance to penetration of water and other liquids into the cellulose-based substrate). There are two main types of sizing: internal sizing and surface sizing. For internal sizing, chemicals are added to the pulp at the wet end, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) or rosin sizing agent. In surface sizing, chemicals for providing water-resistance are applied onto the surface of the paperboard/containerboard. Common surface-sizing agents include, e.g., starch or acrylic co-polymers.

Coating of paper, paperboard or containerboard with plastics is often employed to combine the mechanical properties of the paperboard with the barrier and sealing properties of a plastic film. Also in plastic coated paperboard, the board is often treated with a hydrophobic sizing agent to prevent so-called edge wick, i.e. absorption of liquid at the cut edges (or so-called raw edges) of the paperboard. Edge-wick resistance is an important parameter in many packaging applications. A problem with internal sizing agents, such as AKD, is that they interfere with the hydrogen bonding between the cellulose fibers, giving a debonding effect and hence a weaker material. To compensate for the weaker material, the grammage of paper and board is increased leading to higher carbon footprint due to overuse of wood fibers and higher transport weight at all stages downstream the production.

To improve the wet strength of the material, the internal sizing agent can be combined with a wet strength agent. A wet-strength agent improves the tensile properties of the paper or paperboard in the wet state by for example covalently binding to the cellulose fibers and also form a crosslinking network between the fibers that do not break upon wetting. Common wet strength agents include urea- formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE). Other wet strength agents can give wet-strength by other mechanisms, and some of these wet strength agents can also have a temporary wet-strength function. A problem with the addition of wet strength agents is that the repulpability of the paperboard is severely reduced.

Thus, there remains a need for improved solutions to render cellulose-based substrates such as paper, paperboard or containerboard water resistant, without weakening the material and without reducing the repulpability of the material.

Description of the invention

It is an object of the present disclosure to provide a water-resistant cellulose- based substrate with high repulpability.

It is an object of the present disclosure to provide a method for rendering a cellulose-based substrate water-resistant and edge-wick resistant, without weakening the material and without reducing the repulpability of the material.

It is a further object of the present disclosure to provide a water-resistant cellulose- based substrate with improved wet strength and similar repulpability as compared to a corresponding non water-resistant cellulose-based substrate. It is a further object of the present disclosure to provide a water-resistant cellulose- based substrate which can reduce the grammage of the cellulose-based substrate and still meet strength requirements. It is a further object of the present disclosure to provide a water-resistant cellulose- based substrate which is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin-sizing agent. It is a further object of the present disclosure to provide a water-resistant cellulose- based substrate which is free from added wet strength agents, particularly crosslink-forming wet strength agents, for example urea-formaldehyde (UF), melamine-formaldehyde (MF) and/or polyamide-epichlorohydrin (PAE). The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a water-resistant cellulose-based substrate comprised of one or more plies, wherein said substrate has a basis weight in the range of 120-500 g/m², and a density below 1000 kg/m³, wherein said substrate has been subjected to grafting with a fatty acid halide through the entire thickness of said substrate and that the substrate has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m², and an edge wick index (Lactic acid 1 % solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.5 kg/m²h.

The cellulose-based substrate is preferably a sheet or web of material formed from a pulp of wood or other fibrous substances comprising cellulose fibers. The cellulose-based substrate is preferably paperboard, containerboard or high- grammage paper having a basis weight in the range of 150-500 g/m², and a density below 1000 kg/m³. Preferably, the cellulose-based substrate is a paperboard, containerboard or high-grammage paper having a basis weight in the range of 150-400 g/m².

Paper generally refers to a material manufactured in sheets from the pulp of wood or other fibrous substances comprising cellulose fibers, used for e.g. writing, drawing, or printing on, or as packaging material. Paper used in the present disclosure is paper having a basis weight in the range of 150-500 g/m², preferably 150-400 g/m². Examples of containerboard or high-grammage paper types for use in the present disclosure include, but are not limited to, paper qualities used in the manufacture of corrugated board, such as fluting, liner and testliner. Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example in boxes and other types of packaging. Paperboard can be comprised of one or more plies. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements.

The cellulose-based substrate is suitable for providing water-resistance and edge- wick resistance to a product made by the material, suitable for use in wet or damp environments, e.g. protecting against rain, water and condensation, and providing splash-proof package. In some embodiments, the cellulose-based substrate is for use in packaging, such as boxes, corrugated board and other containers; fresh and aseptic liquid packaging; boxes, corrugated board, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction material. In some embodiments, the cellulose-based substrate is for use in the manufacture of corrugated board.

The present disclosure is based on the inventive realization that a cellulose-based substrate of certain basis weight and density can be conveniently subjected to grafting with a fatty acid halide such that grafting of fatty acids to the substrate material is achieved through the entire thickness of the substrate. Hence, according to the inventive method there is achieved both water-resistance and protection against water edge penetration (i.e. edge-wick resistance). These properties (water-resistance and edge-wick) are achieved even for substrates having an air permeance below 500mI/m in, which is surprising.

Besides improved hydrophobicity (e.g. water contact angle, Cobb₆₀ value, and wick index), the fatty acid halide grafting through the entire thickness of the substrate has been found to also provide a range of other properties especially useful in thicker paperboard, such as paperboard with thickness >150μm, preferably >200μm and more preferably >250μm. Improved properties include significantly improved wet strength compared to an untreated cellulose-based substrate, and at the same time all of: tensile strength; bending resistance; and Z-strength prove to be similar to an untreated cellulose-based substrate. Also, a significantly improved wet strength has been observed for the grafted material according to the invention compared to the same cellulose-based substrate with added internal sizing agent (AKD). In fact, the wet tearing strength of the grafted cellulose-based substrate has been found to be considerably higher than the wet tearing strength of the same cellulose-based substrate with added internal sizing agent (AKD) only, as well as compared to the same cellulose-based paper with added internal sizing agent (AKD) and wet strength agent (PAE).

Furthermore, the repulpability of the grafted cellulose-based substrate was found to be on the same level as the untreated cellulose-based substrate and significantly improved compared to the same cellulose-based substrate with added wet strength agent (PAE). The combination of improved wet strength and high repulpability is surprising and of course very useful in paper, paperboard and containerboard products. The fatty acid halide grafting according to the invention results in a cellulose-based substrate having a Cobb₆₀ value below 30 g/m². In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a

Cobb₆₀ value below 20 g/m², preferably below 10 g/m². The cellulose-based substrate useful in the present disclosure should have a basis weight in the range of 150-500 g/m², preferably 150 - 400 g/m², and a density below 1000 kg/m³. Higher density and basis weight prevent grafting through the entire thickness of the substrate. Cellulose-based substrates of lower basis weight is typically of limited use in the intended applications.

In preferred embodiments, the basis weight of the cellulose-based substrate is in the range of 150-400 g/m². In some embodiments, the density of the cellulose-based substrate is below 800 kg/m³ or below 400 kg/m³.

In some embodiments, the thickness of the cellulose-based substrate is above 150 μm, preferably above 200 μm or 250 μm.

In some embodiments, the cellulose-based has an air permeance below 1000 ml/min, preferably below 700ml/min or below 500ml/min.

In some embodiments, the fatty acid halide grafted on the cellulose-based substrate has an aliphatic chain length of 8-22 carbon atoms. Examples of fatty acid halides include octanoyl chloride (C8), lauroyl chloride (C12), myristoyl chloride (C14), palmitoyl chloride (C16), and stearoyl chloride (C18), and/or a mixture thereof. Grafting of the fatty acid halide to the cellulose-based substrate having available hydroxyl group can be achieved by applying a fatty acid halide in liquid state to the surface of the substrate, followed by penetration of the reagent upon heating, which also promotes the formation of covalent bonds between the fatty acid halide and the hydroxyl groups of the substrate. The reaction between the fatty acid halide, e.g. fatty acid chloride, and the hydroxyl groups of the substrate results in ester bonds between the reagent and the substrate. Ungrafted and thereby unbound fatty acids may also be present to a certain extent. Upon the reaction with the hydroxyl groups in the substrate, and/or with water in the substrate and/or in the air, hydrohalic acid, e.g. hydrochloric acid, is formed as a reaction byproduct. The grafting may preferably be followed by removal of the formed hydrohalic acid, and optionally by removal of the ungrafted residues. One example of a chromatogenic grafting process which could be used in production of the water-resistant cellulose-based substrate of the present disclosure is described in detail in the international patent application WO2012066015A1. In accordance herewith, a suitable equipment for producing a material according to the present invention include an application device for applying the reagent (the fatty acid halide) in liquid state onto a surface of said cellulose-based substrate, and a heating arrangement (e.g. a heating roll) for the development of the grafting reaction on said surface. Said application device could be e.g. an anilox roller, allowing for control of the density and quantity of the applied grafting reagent. Other application devices for applying the reagent in liquid state onto the substrate are conceivable. It is also to be understood that, according to the present invention, the reagent can be applied as a coating by means of a coating roller, or as liquid spray droplets dispersed from e.g. a nozzle directed towards the substrate surface.

Another example of a grafting process, which could be used in production of the water-resistant cellulose-based substrate in the present disclosure, is described in detail in the international patent application WO2017002005A1.

In order to achieve fatty acid halide grafting through the entire thickness of the cellulose-based substrate, it has been found that the cellulose-based substrate should preferably, but not always, be subjected to grafting with a fatty acid halide at least twice. In some embodiments, the cellulose-based substrate has a front surface and a back surface and said substrate has been subjected to grafting with a fatty acid halide in consecutive steps on both the front surface and the back surface, or subjected to grafting with a fatty acid halide at least twice on the same surface. It has been shown that double grafting (i.e. grafting in consecutive steps) by applying liquid reagent onto a cellulose-based substrate followed by heating so that the reagent transfers into gas phase, leads to efficient hydrophobic properties of the substrate as well as edge-wick protection. It is to be understood that “liquid state” include that the reagent, when being applied onto the substrate, can be in the form of a coating or as spray droplets. The at least two consecutive grafting steps may be performed by means of double coating, double spraying or as coating and spraying combined.

The fatty acid halide grafting preferably results in a total amount of grafted and free fatty acids in the cellulose-based substrate in the range of 0.5-10 kg/ton of the total dry weight of the substrate.

A surface of the cellulose-based substrate subjected to grafting with a fatty acid halide preferably has a water contact angle above 90°, preferably above 100°.

The fatty acid halide grafting through the entire thickness of the cellulose-based substrate removes the need for a hydrophobic sizing agent. Thus, in preferred embodiments the substrate is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin- sizing agent.

The fatty acid halide grafting through the entire thickness of the cellulose-based substrate also removes the need for an added wet strength agent to improve the wet strength of the substrate. Thus, in some embodiments the cellulose-based substrate is free from added wet strength agents. In some embodiments the cellulose-based substrate is free from added crosslink-forming wet strength agents, for example polyamide-epichlorohydrin (PAE).

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has an edge wick index (Lactic acid 1 % solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.5 kg/m²h.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has an edge wick index (hydrogen peroxide 35 % solution, 10 min at 70 °C) below 5 kg/m²h, preferably below 1.5 kg/m²h.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has an edge wick index (warm water, 90 min at 55 °C) below 3 kg/m²h, preferably below 1 kg/m²h. In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a relative wet strength (in machine direction and cross direction, determined according to standard ISO 3781 :2011 ) of at least 5%, preferably at least 10%.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a relative wet strength (in machine direction and cross direction, determined according to standard ISO 3781 :2011 ) of at least two times higher, preferably at least four times higher, as compared to the same cellulose- based substrate not subjected to said grafting.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a relative wet tearing strength (in machine direction and cross direction, determined according to TAPPI T 496 sp-13 (T 496 cm-85) and ISO 1974:2012) of at least 30%, preferably at least 50%.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a relative wet tearing strength (in machine direction and cross direction, determined according to standard ISO 3781 :2011 ) of at least two times higher, preferably at least four times higher, as compared to the same cellulose- based substrate not subjected to said grafting.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a tensile strength (in machine direction and cross direction, determined according to standard ISO 1924-3:2005) of at least 90%, preferably at least 95%, of the same cellulose-based substrate not subjected to said grafting.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a L&W bending resistance (in machine direction and cross direction, determined according to standard ISO 2493-1 :2010, bending length 50 mm, bending angle 15°) of at least 90%, preferably at least 100%, of the same cellulose-based substrate not subjected to said grafting. In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a Z-strength (determined according to standard ISO 15754:2009) of at least 90%, preferably at least 100%, of the same cellulose- based substrate not subjected to said grafting.

In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a repulpability characterized by a reject (as determined according to the PTS RH 021/97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1%. This is a reject rate which is comparable to a substrate without any added wet strength agent.

According to a second aspect illustrated herein, there is provided a method for manufacturing a water-resistant cellulose-based substrate, said method comprising: a) providing a cellulose-based substrate comprised of one or more plies, wherein said substrate has a basis weight in the range of 150-500 g/m² and a density below 1000 kg/m³, and b) subjecting said substrate to grafting with a fatty acid halide in liquid state on both the front surface and the back surface thereof, or at least twice on the same surface, such that said substrate is subjected to grafting with the fatty acid halide through the entire thickness of said substrate and the substrate has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m² and an edge wick index (Lactic acid 1% solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.5 kg/m²h.

The fatty acid halide grafting in step b) results in a cellulose-based substrate having a Cobb₆₀ value below 30 g/m². In some embodiments, the cellulose-based substrate subjected to grafting with a fatty acid halide has a Cobb₆₀ value below 20 g/m², preferably below 15 g/m².

The cellulose-based substrate in step a) should have a basis weight in the range of 120-500 g/m², and a density below 1000 kg/m³. Higher density or basis weight prevents grafting through the entire thickness of the substrate. Cellulose-based substrates of lower basis weight is typically of limited use in the intended applications. In some embodiments, the basis weight of the cellulose-based substrate is in the range of 150-400 g/m².

In some embodiments, the density of the cellulose-based substrate is below 800 kg/m³ or below 400 kg/m³.

In some embodiments, the grafting in step b) involves contacting the cellulose- based substrate with fatty acid halide as a liquid state (e.g. coating or spray). In one embodiment the fatty acid halide is applied as a coating onto the substrate by means of an application roll contacting the substrate. Said application roll may be an anilox roll arranged to transfer reagent onto the substrate upon contact. In another embodiment the fatty acid halide is applied as liquid droplets (spray) applied by means of an applicator, e.g. a spray nozzle.

In order to achieve fatty acid halide grafting through the entire thickness of the cellulose-based substrate having a basis weight in the range of 120-500 g/m², and a density below 1000 kg/m³, it has been found that the substrate should preferably be subjected to grafting with a fatty acid halide at least twice. In some embodiments, the cellulose-based substrate has a front surface and a back surface and said substrate has been subjected to grafting with a fatty acid halide on both the front surface and the back surface, or subjected to grafting with a fatty acid halide at least twice on the same surface.

In one embodiment, the cellulose-based substrate has been subjected to grafting with a fatty acid halide in liquid state on the front surface as well as the back surface, and the fatty acid halide reagent is applied onto the respective surfaces by means of reagent-transferring rolls contacting the respective substrate surfaces. The fatty acid halide grafting preferably results in a total amount of grafted and free fatty acids in the cellulose-based substrate in the range of 0.5-10 kg/ton of the total dry weight of the cellulose-based substrate. A surface of the cellulose-based substrate subjected to grafting with a fatty acid halide preferably has a water contact angle above 90°, preferably above 100°.

The fatty acid halide grafting through the entire thickness of the cellulose-based substrate removes the need for a hydrophobic sizing agent. Thus, in preferred embodiments the cellulose-based substrate is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent.

The fatty acid halide grafting through the entire thickness of the cellulose-based substrate also removes the need for an added crosslink-forming wet strength agent to improve the wet strength of the cellulose-based substrate or to compensate for a reduced tensile strength caused by the presence of a hydrophobic sizing agent. Thus, in some embodiments the cellulose-based substrate is free from added wet strength agents. In some embodiments the cellulose-based substrate is free from added crosslink-forming wet strength agents, for example polyamide-epichlorohydrin (PAE).

The water-resistant cellulose-based substrate may further comprise at least one protective polymer layer disposed on a surface thereof. The protective polymer layer preferably comprises a thermoplastic polymer. The polymer layer may for example comprise any of the polymers commonly used in paper-based or paperboard-based packaging materials in general. Examples include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polylactic acid (PLA) and polyvinyl alcohol (PVOH). Polyethylenes, especially low-density polyethylene (LDPE) and high-density polyethylene (HOPE), are the most common and versatile polymers used.

Thermoplastic polymers, and particularly polyolefins are useful since they can be conveniently processed by extrusion coating techniques to form very thin and homogenous films with good barrier properties. In preferred embodiments, the polymer layer comprises a polyethylene, more preferably LORE or HOPE.

The basis weight (corresponding to the thickness) of the protective polymer layer is preferably less than 50 g/m². In order to achieve a continuous and substantially defect free film, a basis weight of the polymer layer of at least 8 g/m², preferably at least 12 g/m², is typically required. In some embodiments, the basis weight of the polymer layer is in the range of 8-50 g/m², preferably in the range of 12-50 g/m². According to a third aspect illustrated herein, there is provided a carton blank comprising a water-resistant cellulose-based substrate according to the first aspect.

According to a fourth aspect illustrated herein, there is provided a container, comprising a water-resistant cellulose-based substrate according to the first aspect.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Examples

Three-plies bleached boards at 240 g/m² were produced containing: no sizing (sample BO), 4 kg/ton AKD sizing (sample Ref-BA) and both 2.1 kg/ton AKD sizing and 1.5 kg/ton wet-strength agent PAE (Ref-BAW), respectively. The thickness of the board was around 370 μm and the density was around 650 kg/m³.

Sample B0 was grafted by subjecting both sides of the board (i.e. both front and back surfaces) to palmitoyl chloride as a liquid and thereafter it was heat treated at 190 °C. The total amount of fatty acids in the board (sample BOG) after the reaction was 2.4 kg/ton.

The method for measurement of contact angle (CA) is based on the standard ISO TC 6/SC 2/WG 41 : Paper and board - Measurement of water contact angle by optical methods. Contact angle was measured for 10 seconds and values for each second and at 0.1 s were noted. The values are an average from 5 drops. The liquid used was Milli-Q water, drop size was 4 pi and drops were evaluated by the software calculation program Circle.

The contact angle values for all boards except B0 were above 120° with no bigger differences between the boards. B0 absorbed the droplet.

Cobb₆₀ analyses were performed by ISO 535:2014 for 60 seconds.

Edge-wick penetration testing was performed with a lactic acid (LA) solution (1 %) for 1 h at conditioned climate of 23 °C and 50% RH. The thickness of the sheets was determined, and the surfaces were masked with a plastic film on both sides, prior to cutting them into five pieces to reveal raw edges. These were thereafter immersed into the LA bath for 1 h, and the amount of absorbed liquid was subsequently weighed. Thereafter the edge penetration wick index can be calculated in kg/m²h.

Edge-wick penetration testing was also performed with hydrogen peroxide and with warm water, respectively. For the hydrogen peroxide (HP) edge wick, a 35% solution was utilized at 70 °C for 10 min. For the warm water edge wick, water at 55 °C was utilized for 90 min.

The Cobb₆₀ and edge-wick values of the four samples are presented in Table 1.

Table 1. Cobb₆₀ and edge wick values.

The unsized sample B0 gave very poor results with either high or unmeasurable values. Compared to the AKD and AKD/WS-containing samples, the Cobb₆₀ value became slightly better for the grafted sample (BOG), and its LA wick value was at a good level at 0.45 kg/m²h. As a comparison, when sample B0 was grafted only on one side of the board with palmitoyl chloride the edge penetration protection was not sufficient showing an LA Wick value over 5 kg/m²h. A surprising difference could also be seen for the HP and warm water wick, where the uptake of the solution was surprisingly low for the grafted board; despite the fact that the amount of fatty acids were lower than the amounts of AKD and/or WS.

The tensile strength and tensile stiffness were measured according to standard IS0 1924-3:2005.

The L&W bending resistance 50 mm 15° MD was measured according to standard 180 2493-1 :2010. The Z-strength was measured according to standard ISO 15754:2009. It can clearly be seen in Table 2 that the addition of the AKD has a negative impact on the strength and stiffness compared to the unsized board BO. The same damaging effect cannot be seen for the grafted board, and here the stiffness was also slightly improved. This trend also held for the bending resistance and the Z- strength.

Table 2. Tensile properties.

The wet strength was measured according to standard ISO 3781 :2011 for the boards without sizing and with fatty acid grafting, AKD sizing and/or WS agent, respectively. The relative wet strength is the difference between the tensile strength and the wet strength in percentage (in accordance with the same standard).

The wet tearing strength was measured based on TAPPI T 496 sp-13 (T 496 cm- 85). Test species was cut out and subsequently soaked in distilled water for 60 min. Excess water was removed by couching. A small cut was made prior to subjecting the samples for tearing according to ISO 1974:2012. The relative wet tearing strength is the difference between the tearing strength and the wet tearing strength in percentage (in accordance with the same standard).

Table 3 summarizes the wet strength and wet tearing strength in MD as well as the relative wet strength and the relative wet tearing strength in MD of the different boards. The grafting of fatty acids to the unsized board had a positive impact on the wet strength compared to when only AKD was utilized, and it was almost on the same level as when wet-strength agent also was added. Surprisingly, the wet tearing strength was much higher for the grafted sample compared to the others, with a relative wet tearing strength of 90 %.

Table 3.

Claims

1. Water-resistant cellulose-based substrate comprised of one or more plies, wherein said substrate has a basis weight in the range of 120-500 g/m², and a density below 1000 kg/m³, wherein said substrate has been subjected to grafting with a fatty acid halide through the entire thickness of said substrate such that the substrate has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m², and an edge wick index (Lactic acid 1 % solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h.

2. Cellulose-based substrate according to any one of the preceding claims, wherein the basis weight of said substrate is in the range of 150-400 g/m².

3. Cellulose-based substrate according to any one of the preceding claims, wherein the density of said substrate is below 800 kg/m³ or below 400 kg/m³.

4. Cellulose-based substrate according to any one of the preceding claims, wherein the thickness of said substrate is above 150 μm, preferably above 250 μm.

5. Cellulose-based substrate according to any one of the preceding claims having an air permeance below 1000 ml/min, preferably below 700ml/min or below 500ml/min.

6. Cellulose-based substrate according to any one of the preceding claims, wherein the total amount of grafted and free fatty acids in the substrate is in the range of 0.5-10 kg/ton of the total dry weight of the substrate.

7. Cellulose-based substrate according to any one of the preceding claims, wherein a surface of said substrate subjected to grafting with a fatty acid halide has a water contact angle above 90°, preferably above 100°.

8. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent.

9. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate is free from added wet-strength agents.

10. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 20 g/m², preferably below 10 g/m².

11. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has an edge wick index (Lactic acid 1% solution, 1 h at 23 °C and 50 % relative humidity) below 0.5 kg/m²h.

12. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a relative wet strength (in machine direction and cross direction, determined according to standard ISO 3781 :2011 ) of at least 5%, preferably at least 10%.

13. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a relative wet tearing strength (in machine direction and cross direction, determined according to TAPP I T 496 sp-13 (T 496 cm-85) and ISO 1974:2012) of at least 30%, preferably at least 50%.

14. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a tensile strength (in machine direction and cross direction, determined according to standard ISO 1924-3:2005) of at least 90%, preferably at least 95%, of the same cellulose-based substrate not subjected to said grafting.

15. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a L&W bending resistance (in machine direction and cross direction, determined according to standard ISO 2493-1 :2010, bending length 50 mm, bending angle 15°) of at least 90%, preferably at least 100%, of the same substrate not subjected to said grafting.

16. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a Z- strength (determined according to standard ISO 15754:2009) of at least 90%, preferably at least 100%, of the same substrate not subjected to said grafting.

17. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate subjected to grafting with a fatty acid halide has a repulpability characterized by a reject rate (as determined according to the PTS RH 021/97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1%.

18. Cellulose-based substrate according to any one of the preceding claims, wherein said substrate is for use in wet or damp environments.

19. A method for manufacturing a water-resistant cellulose-based substrate, said method comprising: a) providing a cellulose-based substrate comprised of one or more plies, wherein said substrate has a basis weight in the range of 120-500 g/m²and a density below 1000 kg/m³, and b) subjecting said substrate to grafting with a fatty acid halide on both the front surface and the back surface thereof, or at least twice on the same surface, such that said substrate is subjected to grafting with the fatty acid halide through the entire thickness of said substrate and the substrate has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m² and an edge wick index (Lactic acid 1 % solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.5 kg/m²h.

20. The method according to claim 19, wherein the basis weight of said cellulose-based substrate is in the range of 150-400 g/m².

21. The method according to any one of claims 19-20, wherein the density of said cellulose-based substrate is below 800 kg/m³, preferably below 400 kg/m³.

22. The method according to any one of claims 19-21 , wherein the grafting involves contacting the cellulose-based substrate with fatty acid halide in a liquid state, such as in the form of a coating or spray droplets.

23. The method according to any one of claims 19-22, wherein the total amount of grafted and free fatty acids in the cellulose-based substrate is in the range of 0.5-10 kg/ton of the total dry weight of the substrate.

24. The method according to any one of claims 19-23, wherein a surface of said cellulose-based substrate subjected to grafting with a fatty acid halide has a water contact angle above 90°, preferably above 100°.

25. The method according to any one of claims 19-24, wherein said cellulose- based substrate is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent.

26. The method according to any one of claims 19-25, wherein said cellulose- based substrate is free from added wet strength agents.