A method for producing a barrier substrate, and a barrier substrate

Abstract

The present invention relates to a method for producing a barrier substrate, comprising the steps of: providing a fibrous web comprising a first layer, which comprises at least 50 weight-% highly refined cellulose having a Schopper Riegler value of 70-98; applying a first composition comprising a first polymer dispersed or dissolved in an aqueous medium on the fibrous web, the first polymer being selected from the group of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof, to provide a first PVOH coating layer; and drying the first PVOH coating layer by means of a single type of drying treatment, which consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web. The invention relates also to a barrier substrate, a barrier substrate laminate with a polymer layer and a packaging material comprising the barrier substrate.

Description

[0001] A METHOD FOR PRODUCING A BARRIER SUBSTRATE, AND A BARRIER SUBSTRATE

[0002] Technical field

[0003] The present disclosure relates to a method for producing a barrier substrate, e.g., for a paper or paperboard based packaging material, which barrier substrate comprises a layer comprising a high amount of highly refined cellulose, such as microfibrillated cellulose (MFC), and a PVOH coating layer. In addition, the present disclosure relates to a barrier substrate, a barrier substrate laminate, and a paper or paperboard-based packaging material laminate comprising the barrier substrate or the barrier substrate laminate.

[0004] Barrier substrates comprising cellulose fibers, including films or papers comprising high amounts of highly refined cellulose, such as microfibrillated cellulose (MFC), are known in the art. Depending on how they are produced the cellulose-based barrier substrates may have particularly advantageous strength and / or barrier properties, whilst being biodegradable and recyclable or repulpable. Such barrier substrates may be used in, for example, the manufacture of packaging materials and may be laminated or otherwise provided on the surface of paper or paperboard based packaging materials. Use of cellulose-based barrier substrates in packaging materials facilitate repulping and recycling of the used packaging materials.

[0005] However, the gas and oil / grease barrier properties of cellulose-based barrier substrates may be excellent, but sensitive to moisture or (higher) relative humidity. In particular, the gas barrier properties of such barrier substrates tend to deteriorate at high temperatures and high humidity, such as when exposed to tropical conditions or conditions allowing condensation.

[0006] Many approaches for improving the barrier properties towards oxygen, air, water vapour and aromas at high relative humidity have been investigated and described. For example, various chemical solutions, such as coatings, lamination and surfacetreatments, have been tested for improving the gas barrier properties of cellulose- based barrier substrates at high relative humidity or to provide the barrier substrates with improved barrier against water vapor. In order to retain the repulpability of the barrier substrate, aqueous-based barrier coatings may be preferred. Such aqueousbased barrier coatings may be formed by means of a liquid film coating process, whereby an aqueous solution or dispersion comprising a barrier chemical is spread out on the substrate and thereafter dried.

[0007] One solution is to utilize a coating of polyvinyl alcohol (PVOH) as a barrier coating of cellulose-based barrier substrates, wherein the PVOH coating is applied as an aqueous-based barrier coating. It is known that PVOH coatings provide improved barrier properties, in particular oxygen, water vapour and aroma barrier properties, to cellulose-based barrier substrates. Also, it is known that PVOH coatings provide improved mechanical properties, such as improved tensile strength and reduced brittleness, to cellulose-based barrier substrates. Furthermore, it is known that PVOH coatings provide improved surface characteristics, such as improved printability, reduced surface roughness and improved adhesion, to cellulose-based barrier substrates. In addition, PVOH coatings may improve the flexibility of cellulose-based barrier substrates and make them more pliable and easier to process.

[0008] However, when the PVOH coating is applied as an aqueous-based PVOH coating and then dried to form a dried PVOH coating by common drying methods, defects in the dried PVOH coating may appear. For example, the drying may be associated with formation of bubbles, which then may burst and provide defects in the dried PVOH coating. Also, skin-forming may appear. Common drying methods include drying of the PVOH coating by means of, for example, infrared drying equipment, electric dryers and drying cylinders.

[0009] Thus, there is still room for improvements of methods for producing a cellulose- based barrier substrate, such as a film or paper, which method comprises application of an aqueous-based PVOH coating to a fibrous web comprising a high amount of highly refined cellulose, such as MFC.

[0010] Description of the invention

[0011] It is an object of the present disclosure to provide an improved method for producing a barrier substrate, e.g. for a paper or paperboard based packaging material, which method comprises application of an aqueous-based PVOH coating to a fibrous web comprising a high amount of highly refined cellulose, such as MFC, and which method eliminates or alleviates at least some of the disadvantages of the prior art methods.

[0012] The above-mentioned object, 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.

[0013] According to a first aspect illustrated herein, there is provided a method for producing a barrier substrate, wherein the method comprises the steps of: providing a fibrous web comprising a first layer, wherein the first layer comprises at least 50 weight-% highly refined cellulose having a Schopper Riegler (SR) value in the range of 70-98 according to standard ISO 5267-1 :1999, based on the total dry weight of the first layer, wherein the fibrous web has a first side and a second side, wherein the fibrous web has a moisture content of 2-12 weight-%; applying, in a first coating step, a first composition comprising a first polymer dispersed or dissolved in an aqueous medium on the first side of the fibrous web, wherein the first polymer is selected from the group consisting of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof, to provide the first side of the fibrous web with a first PVOH coating layer having a first outer side and a second inner side being in contact with the fibrous web, thereby forming a coated fibrous web having a coated side and a non-coated side, and

[0014] - drying, in a first drying step, the first PVOH coating layer of the coated fibrous web to form a dried first PVOH coating layer, wherein the drying of the first PVOH coating layer of the coated fibrous web to form the dried first PVOH coating layer is performed by means of a single type of drying treatment, wherein the single type of drying treatment consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web.

[0015] Thus, the method of the first aspect provides a barrier substrate comprising a fibrous web comprising a first layer, wherein the first layer comprises at least 50 weight-% highly refined cellulose having an SR value in the range of 70-98, based on the total dry weight of the first layer, wherein the fibrous web is coated with a first PVOH coating layer on one side. It was surprisingly found that by only actively drying the aquoeus-based first PVOH coating layer from the non-coated side of the fibrous web and only by means of blowing of hot air and / or steam, i.e., by only drying the aquoeus-based first PVOH coating layer by means of a single type of drying treatment consisting of blowing of hot air and / or steam against the non-coated side of the coated fibrous web, a first PVOH coating layer comprising less defects, due to e.g., less bursting of bubbles during drying and less skin-formation, is possible to obtain.

[0016] The drying of the aquoeus-based first PVOH coating layer by means of a single type of drying treatment consisting of blowing of hot air and / or steam against the noncoated side of the coated fibrous web implies that the temperature, and thus solids content, are increased from the inside to the outside (i.e., from the second side of the first PVOH coating layer being in contact with the fibrous web to the first side of the first PVOH coating layer being in contact with the surroundings) by thermal convection. The hot air and / or steam blown against the non-coated side of the coated fibrous web will provide heat which is transported through the fibrous web into the wet first PVOH coating layer. The wet first PVOH coating layer will then start drying from the second side (i.e., inside) of the first PVOH coating layer towards the first side (i.e., outside). Thus, the first side of the first PVOH coating layer will remain in liquid form until heat transported through the first PVOH coating layer reaches the first side (which is in contact with the surroundings). This will allow water, water vapour and air in the wet first PVOH coating layer to easier evaporate and penetrate through the first side of the first PVOH coating layer. Since the first side of the first PVOH coating layer is in contact with the surroundings, the evaporation is not hindered whereby bursting of bubbles and skin-formation are prevented or reduced. Also, the utilization of hot air and / or steam implies an efficient heat transfer and a possibility to control the drying of the PVOH coating efficiently.

[0017] The term “barrier substrate” as used herein generally refers to a thin continuous sheet formed material with low permeability for gases and / or liquids. Depending on the composition of the pulp suspension, the barrier substrate can be defined as a barrier film or as a thin barrier paper. Preferably, the barrier substrate provides a barrier against at least one of gas, such as oxygen, oil, grease, liquid and water vapor. As mentioned above, the method of the first aspect of the present disclosure comprises a step of providing a fibrous web comprising a first layer, wherein the first layer comprises at least 50 weight-% highly refined cellulose based on the total dry weight of the first layer. In some embodiments, the first layer of the fibrous web comprises at least 70 weight-%, preferably at least 80 weight-%, more preferably at least 90 weight-%, highly refined cellulose based on the total dry weight of the first layer. In some embodiments, the first layer of the fibrous web comprises 50-99 weight-%, such as 70-99 weight-%, preferably 80-99 weight-%, more preferably 90- 99 weight-%, highly refined cellulose based on the total dry weight of the first layer.

[0018] The highly refined cellulose of the first layer of the fibrous web used in the method of the first aspect has a Schopper Riegler value (SR) of 70-98 SR, preferably in the range of 80-98 SR, more preferably in the range of 85-95 SR, as determined by standard ISO 5267-1 :1999. The SR value is determined for a pulp of the fibers without additional chemicals, thus the fibers have not consolidated into a substrate or started e.g., hornification.

[0019] In some embodiments, the highly refined cellulose of the first layer of the fibrous web used in the method of the first aspect has a water retention (WRV) value of >200%, more preferably >300%. In addition, the WRV value is preferably <400%, more preferably <380% or <370% or <350%. In some embodiments, the highly refined cellulose of the first layer of the fibrous web used in the method of the first aspect has a WRV value of 250-400%, or 250-380%, or 250-350%, or 300-350%. The WRV value may be determined by standard ISO 23714 with the use of a 200 mesh wire.

[0020] The highly refined cellulose of the first layer of the fibrous web used in the method of the first aspect can be produced in many different ways using methods known in the art to achieve the desired Schopper Riegler value and the desired WRV value.

[0021] The highly refined cellulose of the first layer of the fibrous web used in the method of the first aspect may for example be produced from softwood or hardwood or a mix thereof, such as 5-95, 10-90, 15-95, 20-80 or 25-75 (weight-% softwood - weight-% hardwood). It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It can also be made from broke or recycled paper. For example, the highly refined cellulose may be produced from mechanical pulp, thermochemical pulp, chemical pulp such as sulphate (Kraft) or sulphite pulp, dissolving pulp, organosolv pulp or chemi- thermomechanical pulp (CTMP), or combinations thereof. Preferably, the cellulose fiber material is chemical pulp, such as Kraft pulp. The pulp is preferably delignified and processed according to known methods in the art. One preferred source of fiber is an EOF or TCF bleached Kraft pulp.

[0022] In some embodiments, the highly refined cellulose is microfibrillated cellulose (MFC). Thus, in some embodiments, the first layer of the fibrous web is a microfibrillated cellulose (MFC) barrier paper or film comprising at least 50 weight-% of MFC based on the total dry weight of the MFC barrier paper or film. In some embodiments, the MFC barrier paper or film comprises at least 70 weight-%, preferably at least 75 weight-%, and more preferably at least 80 weight-%, of MFC based on the total dry weight of the MFC barrier paper or film.

[0023] Microfibrillated cellulose (MFC) shall in the context of the patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm. Various methods exist to make MFC, such as single or multiple pass refining, prehydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

[0024] MFC is produced from wood cellulose fibers, both from hardwood and softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and / or thermomechanical pulps. It can also be made from broke or recycled paper.

[0025] In some embodiments, the MFC has a Schopper Riegler (SR) value in the range of 80-98, preferably in the range of 85-98, more preferably in the range of 90-98, according to standard ISO 5267-1 :1999.

[0026] In some embodiments, the MFC has a water retention value (WRV) value of at least 230%, preferably at least 280%, according to standard ISO 23714:2014.

[0027] In some embodiments, the first layer of the fibrous web comprises one or more further cellulose pulp fractions in addition to the highly refined cellulose, which one or more further cellulose pulp fractions have been refined to different refining degrees than the highly refined cellulose. In some embodiments, the first layer of the fibrous web comprises a fraction of normal fibers. The first layer of the fibrous web may comprise, for example, 1-30 weight-%, more preferably 2-30 weight-%, most preferably 5-30 weight-%, of normal fibers.

[0028] By normal fibers is meant normal pulp fibers of a conventional length and fibrillation for papermaking. Normal fibers may include mechanical pulp, thermochemical pulp, chemical pulp such as sulphate (Kraft) or sulphite pulp, dissolving pulp, recycled fiber, organosolv pulp or chemi-thermomechanical pulp (CTMP), or combinations thereof. The pulp may be bleached or unbleached. The normal fibers can be vegetable fibers, such as wood derived (e.g. hardwood or softwood) or agricultural sources including straw, bamboo, etc. The normal fibers may have a beating degree, i.e. Schopper Riegler value, in the range of 15 to 50 SR or more preferably in the range of 18 to 40 SR. The normal fibers may preferably be chemical pulp, such as Kraft pulp. The normal fibers may have an average length in the suspension of 1 mm to 5 mm, more preferably in the range of 2 to 4 mm.

[0029] The first layer of the fibrous web may, in addition to highly refined cellulose and optional further cellulose pulp fraction(s), comprise any conventional paper making additives or chemicals such as fillers, pigments, wet strength chemicals, retention chemicals, cross-linkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, colorants, fluorescent whitening agents, de-foaming chemicals, hydrophobizing chemicals such as AKD, ASA, waxes, resins, bentonite, stearate, wet end starch, silica, precipitated calcium carbonate, cationic polysaccharide, etc. In some embodiments, the first layer of the fibrous web comprises less than 10 weight-%, preferably less than 8 weight-% or less than 5 weight-% of fillers, based on the total dry weight of the first layer of the fibrous web.

[0030] In some embodiments, the fibrous web consists of the first layer.

[0031] In other embodiments, the fibrous web comprises the first layer and a second layer. In such embodiments, the second layer may, similar to the first layer, comprise at least 50 weight-% highly refined cellulose based on the total dry weight of the second layer. In some embodiments, the second layer of the fibrous web comprises at least 70 weight-%, preferably at least 80 weight-%, more preferably at least 90 weight-%, highly refined cellulose based on the total dry weight of the second layer. In some embodiments, the second layer of the fibrous web comprises 50-99 weight- %, such as 70-99 weight-%, preferably 80-99 weight-%, more preferably 90-99 weight-%, highly refined cellulose based on the total dry weight of the second layer. The highly refined cellulose of the second layer may be further defined as set out above with reference to the highly refined cellulose of the first layer. In embodiments, the second layer may have the same composition as the first layer. In alternative embodiments, the second layer may comprise cellulose fibers, such as normal fibers as defined above, in an amount of between 50 - 99 weight-%, or 60 - 99 weight-% or 70 - 99 weight-%, but less than 30 weight-% or less than 20 weight-%, or less than 10 weight-%, or less than 5 weight-% of highly refined cellulose, based on the total dry weight of the second layer.

[0032] The first layer of the fibrous web used in the method of the first aspect may be formed by wet laid techniques, such as similar to a papermaking process, or by casting techniques. In wet laid techniques, the first layer of the fibrous web may be formed by the application of an aqueous suspension comprising the highly refined cellulose on a porous wire whereafter it is dewatered and / or dried in a drying section to form the first layer of the fibrous web. In casting technologies, the first layer of the fibrous web may be formed by application of an aqueous suspension comprising the highly refined cellulose onto a non-porous casting substrate, such as a polymeric or metal substrate. The thereby formed layer may thereafter be dewatered, e.g. by pressing, and / or dried by evaporation to form the first layer of the fibrous web.

[0033] The fibrous web used in the method of the first aspect has a first side and an opposite second side. The moisture content of the fibrous web when used in the method of the first aspect (i.e., the provided fibrous web before being coated with the first PVOH coating layer) is between 2-12 weight-%, preferably 3-8 weight-%, most preferably 4-7 weight-%. Unless specifically stated otherwise, moisture content in the present disclosure is determined according to the standard ISO 287:2017.

[0034] In some embodiments, the fibrous web used in the method of the first aspect (i.e., the provided fibrous web before being coated with the first PVOH coating layer) has a grammage of less than 90 g / m2, preferably less than 80 g / m2, more preferably less than 75 g / m2, such as a grammage between 25-90 g / m2, or 25-80 g / m2, or 30-75 g / m2. Unless specifically stated otherwise, grammage in the present disclosure is determined according to the standard ISO 536:2019.

[0035] In some embodiments, the fibrous web used in the method of the first aspect (i.e., the provided fibrous web before being coated with the first PVOH coating layer) has a density of above 600 kg / m3, preferably above 650 kg / m3, or above 750 kg / m3, or above 850 kg / m3, such as 600-1200 kg / m3, preferably 650-1200 kg / m3, most preferably 750-1200 kg / m3. The higher densities can be achieved by e.g., calendaring of the fibrous web or by using fillers. Unless specifically stated otherwise, density in the present disclosure is determined according to the standard ISO 534:2011.

[0036] In some embodiments, the fibrous web used in the method of the first aspect (i.e., the provided fibrous web before being coated with the first PVOH coating layer), has a thickness of less than 100 pm, preferably less than 80 pm, most preferably less than 70 pm or less than 60 pm, such as a thickness of between 25 - 100 pm, or 25 - 80 pm or 25 - 70 pm. Unless specifically stated otherwise, thickness in the present disclosure is determined according to the standard ISO 534:2011 .

[0037] As mentioned above, the method of the first aspect comprises a step of applying, in a first coating step, a first composition comprising a first polymer dispersed or dissolved in an aqueous medium on the first side of the fibrous web to provide the first side of the fibrous web with a first PVOH coating layer, which has a first outer side and a second inner side being in contact with the fibrous web. Thereby a coated fibrous web having a coated side (i.e., coated with the first PVOH coating layer) and an, opposite, non-coated side is formed.

[0038] The first polymer is selected from the group consisting of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof.

[0039] The polyvinyl alcohol may be a single type of PVOH, or it can comprise a mixture of two or more types of PVOH, differing e.g. in degree of hydrolysis or viscosity. The PVOH may for example have a degree of hydrolysis in the range of 80-99.9 mol%, preferably in the range of 88-99.9 mol%. The PVOH can also be modified with alkyl substituents such as ethylene groups, or anionic groups such as carboxylic acid groups, or other functional groups such as cationic or silanol groups. In some embodiments, the PVOH has an average molecular weight Mw in the range of 15 000-150 OOO g / mol.

[0040] In some embodiments, the first composition applied on the first side of the fibrous web in the first coating step comprises the first polymer dispersed or dissolved in an aqueous medium in an amount of at least 40 wt%, or at least 60 wt%, such as between 50 - 100 wt% or 50 - 99 wt% or 60 - 100 wt% as calculated on the total solids content of the first composition.

[0041] In some embodiments, the first composition has a solids content of between 1-50 weight-%, preferably between 1-20 weight-%, more preferably between 2-15 weight- %.

[0042] In some embodiments, the first PVOH coating layer has a coat weight of 0.1-12 g / m2, preferably 0.5-8 g / m2, calculated as dry weight.

[0043] As mentioned above, the method of the first aspect comprises also drying, in a first drying step, the first PVOH coating layer of the coated fibrous web to form a dried first PVOH coating layer. The drying of the first PVOH coating layer of the coated fibrous web to form the dried first PVOH coating layer is performed by means of a single type of drying treatment. The single type of drying treatment consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web. By the phrase “single type of drying treatment” it is herein referred to that no further active drying treatment of the first PVOH coating layer is performed than the blowing of hot air and / or steam against the non-coated side of the coated fibrous web. The phrase “no further active drying treatment” is herein intended to refer to the fact that no further drying treatment / process / equipment generating heat or energy applied to the first PVOH coating layer is utilized in the first drying step.

[0044] Thus, the aquoeus-based first PVOH coating layer is actively dried only from the non-coated side of the fibrous web and only by means of hot air and / or steam, i.e., only by means of a single type of drying treatment consisting of blowing of hot air and / or steam against the non-coated side of the coated fibrous web. The second side of the first PVOH coating layer is in contact with the surroundings (i.e., not covered by any layer, coating or other means) during the first drying step.

[0045] Preferably, the first drying step is performed to provide a moisture content of 3-6 weight-% of the coated fibrous web.

[0046] In some embodiments, hot air is utilized in the first drying step. In embodiments in which hot air is utilized in the first drying step, the hot air may have a temperature of 80-300 °C. In some embodiments, steam is utilized in the first drying step. In embodiments in which steam is utilized in the first drying step, the steam may have a temperature of 80-200 °C. In some embodiments, hot air and steam are utilized in the first drying step.

[0047] In some embodiments, hot air is utilized in the first drying step, wherein the hot air is blown against the non-coated side of the coated fibrous web by means of an air dryer, such as a through air dryer (TAD).

[0048] In some embodiments in which hot air is utilized in the first drying step, the drying of the first PVOH coating layer of the coated fibrous web in the first drying step is performed in a first drying section of a production machine and is optimized by controlling the drying in different zones in the machine direction and cross direction of the first drying section. For example, the drying in edge zones and / or middle zones of the first PVOH coating layer may be controlled. By using a through air dryer for blowing hot air against the non-coated side of the coated fibrous web, efficient control of the drying may be obtained since fast temperature changes of the hot air may be obtained by the through air dryer.

[0049] The method of the first aspect may further comprise a step of applying, in at least one second coating step, a second composition comprising a second polymer dispersed or dissolved in an aqueous medium on the dried first PVOH coating layer. The second polymer is selected from the group consisting of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof. The first outer surface of the dried first PVOH coating layer is thereby provided with at least one second PVOH coating layer. Each second PVOH coating layer has a first outer side and a second inner side. The at least one second PVOH coating layer of the coated fibrous web is dried, in at least one second drying step, to form at least one dried second PVOH coating layer. The drying of each second PVOH coating layer of the coated fibrous web is performed in one second drying step of the at least one second drying step by means of a single type of drying treatment, wherein the single drying treatment consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web.

[0050] In embodiments in which the method comprises more than one second coating step, thereby providing the coated fibrous web with more than one second PVOH coating layer of the at least one second PVOH coating layer, each provided second PVOH coating layer is dried in one second drying step of the at least one second drying step before application and drying of any next second PVOH coating layer. Thus, if more than one second PVOH coating layer is applied to the coated fibrous web, one second PVOH coating layer is applied and dried by means of the single type of drying treatment before a next second PVOH coating layer is applied and dried by means of the single type of drying treatment. The drying of each second PVOH coating layer may be performed in a respective separate second drying section.

[0051] Thus, the second polymer of the second composition may be selected from the same group of polymers as the first polymer of the first composition. The second polymer and the second composition may be further defined as set out above with reference to the first polymer and the first composition, respectively. Preferably, the first composition and the second composition (s) are the same.

[0052] However, the first composition and the second composition(s) may be different.

[0053] The first composition and the second composition(s) may be applied by any suitable coating method, such as contact or non-contact coating methods. Examples of useful coating methods include, but are not limited to, rod coating, curtain coating, film press coating, cast coating, transfer coating, size press coating, flexographic coating, gate roll coating, twin roll HSM coating, blade coating, such as short dwell time blade coating, jet applicator coating, spray coating, gravure coating or reverse gravure coating.

[0054] In some embodiments, each second composition comprises the second polymer dispersed or dissolved in an aqueous medium in an amount of at least 40 wt%, or at least 60 wt%, such as between 50 - 100 wt% or 50 - 99 wt% or 60 - 100 wt% as calculated on the total solids content of the second composition.

[0055] In some embodiments, each second composition has a solids content of between 1- 50 weight-%, preferably between 1-20 weight-%, more preferably between 2-15 weight-%.

[0056] In some embodiments, each second PVOH coating layer has a coat weight of 0.1-12 g / m2, preferably 0.5-8 g / m2, calculated as dry weight.

[0057] Preferably, each second drying step is performed to provide a moisture content of 3- 6 weight-% of the coated fibrous web.

[0058] In some embodiments, hot air is utilized in the second drying step. In embodiments in which hot air is utilized in the second drying step, the hot air may have a temperature of 80-300 °C. In some embodiments, steam is utilized in the second drying step. In embodiments in which steam is utilized in the second drying step, the steam may have a temperature of 80-200 °C. In some embodiments, hot air and steam are utilized in the second drying step. In some embodiments, hot air is utilized in the second drying step, wherein the hot air is blown against the non-coated side of the coated fibrous web by means of an air dryer, such as a through air dryer (TAD).

[0059] In some embodiments in which hot air is utilized in the second drying step, the drying of the second PVOH coating layer of the coated fibrous web in the second drying step is performed in a second drying section of a production machine and is optimized by controlling the drying in different zones in the machine direction and cross direction of the second drying section. For example, the drying in edge zones and / or middle zones of the second PVOH coating layer may be controlled. By using a through air dryer for blowing hot air against the non-coated side of the coated fibrous web, efficient control of the drying may be obtained since fast temperature changes of the hot air may be obtained by the through air dryer.

[0060] The barrier substrate can be used as such, or it can be combined with one or more other layers. The barrier substrate is for example useful as a barrier layer in a paper or paperboard based packaging material. The barrier substrate may also be or constitute a barrier layer in a multiply product comprising a base such as glassine, greaseproof paper, barrier paper or bioplastic films. Alternatively, the barrier substrate can be comprised in at least one layer in a multiply sheet such as a liquid packaging board.

[0061] Paper generally refers to a material manufactured in thin sheets from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as packaging material. Paper can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end use requirements. Paper may be a single ply material, or a multiply material comprised of two or more plies.

[0062] Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for boxes and other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end use requirements. Paperboard may be a single ply material, or a multiply material comprised of two or more plies. According to a second aspect illustrated herein, there is provided a barrier substrate obtainable by the method according to the first aspect.

[0063] In some embodiments, the barrier substrate has an oxygen transfer rate (OTR), measured according to the standard ASTM F1927 - 20 at 50% relative humidity and 23 °C, of less than 100 cc / m2 / 24h, preferably less than 50 cc / m2 / 24h, more preferably less than 25 cc / m2 / 24h, most preferably less than 10 cc / m2 / 24h, even more preferably less than 5 cc / m2 / 24h.

[0064] In some embodiments, the barrier substrate has a water vapor transmission rate (WVTR) value vapor transfer rate (WVTR), measured according to the standard ASTM F1249 - 20 at 50% relative humidity and 23 °C, of less than 50 g / m2 / 24h, preferably less than 35 g / m2 / 24h, more preferably less than 20 g / m2 / 24h, most preferably less than 15 g / m2 / 24h, even more preferably less than 10 g / m2 / 24h.

[0065] According to a third aspect illustrated herein, there is provided a method of producing a barrier substrate laminate, e.g. for a paper or paperboard based packaging material, comprising the barrier substrate obtainable by the method according to the first aspect, wherein the method comprises the steps of. performing the method according to the first aspect so as to produce the barrier substrate, and laminating the barrier substrate with at least one additional polymer layer or extruding at least one additional polymer layer to the barrier substrate so as to form the barrier substrate laminate.

[0066] For example, the one or more additional polymer layers may be constituted by any suitable polyolefin, such as polyethylene, high-density polyethylene (HD-PE), low- density polyethylene (LD-PE), polypropylene, low-density polypropylene (LD-PP), biaxially-oriented polypropylene (BO-PP), polyethylene terephthalate (PET), etc. or mixtures or modifications thereof that could readily be selected by a skilled person. The one or more additional polymer layer(s) may also be constituted by bio-derived or recyclable and / or compostable versions, such as polylactic acid (PLA), polyglycolic acid (PGA), polyhydroxyalkanoates (PHA), etc. The additional polymer layer(s) may comprise any of the thermoplastic polymers commonly used in paper or paperboard based packaging materials in general or polymers used in liquid packaging board in particular. Polyethylenes, especially low-density polyethylene (LDPE) and high- density polyethylene (HDPE), are the most common and versatile polymers used in liquid packaging board.

[0067] The additional polymer layer(s) can be provided e.g. by extrusion coating, film coating or dispersion coating. The coating can be formed by the extruded plastic itself, or the molten plastic can be used as an adhesive to laminate a solid plastic film onto the substrate.

[0068] This laminate structure may provide for even more superior barrier properties and may be biodegradable and / or compostable and / or repulpable. In one embodiment, the barrier substrate according to the present disclosure can be provided between two coating layers, such as between two layers of polyethylene, with or without a tie layer.

[0069] The basis weight of each additional polymer layer is preferably 6-40 g / m2, more preferably 8-30 g / m2, most preferably 10-25 g / m2.

[0070] According to a fourth aspect of the present disclosure there is provided a barrier substrate laminate obtainable by the method of the third aspect.

[0071] According to a fifth aspect of the present disclosure there is provided a method of producing a paper or paperboard based packaging material laminate, comprising the steps of:

[0072] - performing the method according to the first aspect so as to form the barrier substrate or the method according to the third aspect so as to form the barrier substrate laminate, and

[0073] - laminating the barrier substrate or the barrier substrate laminate with a paper or paperboard based base material so as to form the paper or paperboard based packaging material laminate.

[0074] According to a sixth aspect of the present disclosure there is provided a paper or paperboard based packaging material laminate obtainable by the method according to the fifth aspect. Generally, while the products, polymers, materials, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, layers and processes can also “consist essentially of’ or “consist of’ the various components and steps.

[0075] 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.

Claims

CLAIMS1 . A method for producing a barrier substrate, wherein the method comprises the steps of: providing a fibrous web comprising a first layer, wherein the first layer comprises at least 50 weight-% highly refined cellulose having aSchopper Riegler (SR) value in the range of 70-98 according to standard ISO 5267-1 :1999, based on the total dry weight of the first layer, wherein the fibrous web has a first side and a second side, wherein the fibrous web has a moisture content of 2-12 weight-%; applying, in a first coating step, a first composition comprising a first polymer dispersed or dissolved in an aqueous medium on the first side of the fibrous web, wherein the first polymer is selected from the group consisting of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof, to provide the first side of the fibrous web with a first PVOH coating layer having a first outer side and a second inner side being in contact with the fibrous web, thereby forming a coated fibrous web having a coated side and a non-coated side, and- drying, in a first drying step, the first PVOH coating layer of the coated fibrous web to form a dried first PVOH coating layer, wherein the drying of the first PVOH coating layer of the coated fibrous web to form the dried first PVOH coating layer is performed by means of a single type of drying treatment, wherein the single type of drying treatment consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web.

2. The method according to claim 1 , wherein said first drying step is performed to provide a moisture content of 3-6 weight-% of the coated fibrous web.

3. The method according to claim 1 or 2, wherein the fibrous web has a grammage of less than 90 g / m2, preferably less than 80 g / m2.

4. The method according to any one of the preceding claims, wherein the fibrous web has a density of above 600 kg / m3, preferably above 650 kg / m3.

5. The method according to any one of the preceding claims, wherein the first layer of the fibrous web comprises at least 70 weight-% highly refined cellulose having a Schopper Riegler (SR) value in the range of 70-98 according to standard ISO 5267-1 : 1999, based on the total dry weight of the first layer.

6. The method according to any one of the preceding claims, wherein the highly refined cellulose is microfibrillated cellulose (MFC).

7. The method according to any one of the preceding claims, wherein the first composition has a solids content of between 1-50 weight-%, preferably between 1-20 weight-%.

8. The method according to any one of the preceding claims, wherein the first PVOH coating layer has a coat weight of 0.1-12 g / m2, preferably 0.5-8 g / m2, calculated as dry weight.

9. The method according to any one of the preceding claims, wherein the hot air of the first drying step has a temperature of 80-300 °C.

10. The method according to any one of the preceding claims, wherein the hot air of the first drying step is blown against the non-coated side of the coated fibrous web by means of an air dryer, such as a through air dryer (TAD).11 . The method according to claim 9 or 10, wherein the first drying step is performed in a first drying section and is optimized by controlling the drying in different zones in the machine direction and cross direction of the first drying section.

12. The method according to any one of the preceding claims, wherein the steam of the first drying step has a temperature of 80-200 °C.

13. The method according to any one of the preceding claims, wherein the method further comprises:applying, in at least one second coating step, a second composition comprising a second polymer dispersed or dissolved in an aqueous medium on the dried first PVOH coating layer, wherein the second polymer is selected from the group consisting of: a polyvinyl alcohol (PVOH), a modified polyvinyl alcohol and a combination thereof, to provide the first outer side of the dried first PVOH coating layer with at least one second PVOH coating layer, each second PVOH coating layer has a first outer side and a second inner side, and- drying, in at least one second drying step, the at least one second PVOH coating layer of the coated fibrous web to form at least one dried second PVOH coating layer, wherein the drying of each second PVOH coating layer of the coated fibrous web is performed in one second drying step of said at least one second drying step by means of a single type of drying treatment, wherein the single type of drying treatment consists of blowing hot air and / or steam against the non-coated side of the coated fibrous web.

14. The method according to claim 13, wherein the method comprises more than one second coating step, thereby providing the coated fibrous web with more than one second PVOH coating layer of said at least one second PVOH coating layer, wherein each provided second PVOH coating layer is dried in one second drying step of said at least one second drying step before application and drying of any next second PVOH coating layer.

15. The method according to claim 13 or 14, wherein said second drying step is performed to provide a moisture content of 3-6 weight-% of the coated fibrous web.

16. The method according to any one of claims 13-15, wherein the second composition has a solid content of between 1-50 weight-%, preferably between 1-20 weight-%.

17. The method according to any one of claims 13-16, wherein the second PVOH coating layer has a coat weight of 0.1-12 g / m2, preferably 0.5-8 g / m2, calculated as dry weight.

18. The method according to any one of claims 13-17, wherein the hot air of the second drying step has a temperature of 80-300 °C.

19. The method according to any one of claims 13-18, wherein the hot air of the second drying step is blown against the non-coated side of the coated fibrous web by means of an air dryer, such as a through air dryer (TAD).

20. The method according to any one of claims 13-19, wherein the drying of each second PVOH coating layer of the coated fibrous web is performed in a second drying section and is optimized by controlling the drying in different zones in the machine direction and cross direction of the second drying section.

21. The method according to any one of claims 13-20, wherein the steam of the second drying step has a temperature of 80-200 °C.

22. A barrier substrate obtainable by the method according to any one of claims 1-21.

23. A method of producing a barrier substrate laminate comprising the steps of: performing the method according to any one of claims 1-21 so as to produce said barrier substrate, and laminating said barrier substrate with at least one additional polymer layer or extruding at least one additional polymer layer to said barrier substrate so as to form said barrier substrate laminate.

24. A barrier substrate laminate obtainable by the method according to claim 23.

25. A method of producing a paper or paperboard based packaging material laminate, comprising the steps of: performing the method according to any one of claims 1-21 or 23 so as to form said barrier substrate or said barrier substrate laminate, andlaminating said barrier substrate or said barrier substrate laminate with a paper or paperboard base material so as to form said paper or paperboard based packaging material laminate.

26. A paper or paperboard based packaging material laminate obtainable by the method according to claim 25.