EP4638868A1

EP4638868A1 - A method for manufacturing a pha coated fiber-based substrate with a first and a second pha aqueous coating compositions and a fiber-based substrate coated with the method

Info

Publication number: EP4638868A1
Application number: EP23906213.6A
Authority: EP
Inventors: Isto Heiskanen; Kaj Backfolk; Katja LYYTIKÄINEN; Åsa NYFLÖTT
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2022-12-22
Filing date: 2023-12-18
Publication date: 2025-10-29
Also published as: SE546269C2; CN120344739A; SE2230441A1; AU2023409780A1; WO2024134456A1; JP2025541007A

Abstract

The present invention relates to a method for manufacturing a PHA (polyhydroxyalkanoate) coated fiber-based substrate, said method comprising the steps of: a) providing a fiber-based substrate having a first main surface and a second main surface; b) forming a first PHA layer by applying a foamed first aqueous coating composition comprising a first PHA and a foaming agent on the first main surface and drying the foamed first aqueous coating composition, wherein the foamed first aqueous coating composition is a foam having a foam density of 0.75 g/cm3 or less; c) forming a second PHA layer by applying a second aqueous coating composition comprising a second PHA on the first PHA layer and drying the second aqueous coating composition. The present invention further relates to a PHA coated fiber-based substrate and a packaging container comprising a PHA coated fiber-based substrate.

Description

A METHOD FOR MANUFACTURING A PHA COATED FIBER-BASED SUBSTRATE WITH A FIRST AND A SECOND PHA AQUEOUS COATING COMPOSITIONS AND A FIBER-BASED SUBSTRATE COATED WITH THE METHOD

Technical field

The present disclosure relates to methods for preparing coated fiber-based substrates, specifically polyhydroxyalkanoate (PHA) coated paper or paperboard, for use as packaging materials.

Background

Coating of paper and paperboard with plastics is often used to combine the mechanical properties of the paper or paperboard with the barrier and sealing properties of a plastic film. Paper or paperboard provided with even a relatively small amount of a suitable plastic material can provide the properties needed to make the paper or paperboard suitable for many demanding applications, for example as liquid or food packaging board. In liquid or food packaging board, extrusion coated polyolefin coatings are frequently used as liquid barrier layers, heat sealing layers and adhesives. However, the recycling of such polymer coated board is difficult since it is difficult to separate the polymers from the fibers.

In the prior art, attempts have been made to replace extrusion coated polyolefin coatings with more environmentally friendly and/or easier to recycle solutions, but so far with no real success. In many cases some, but not all, of the properties of the extrusion coated polyolefin coatings are achieved by the alternative solutions.

Dispersion barrier coatings for paper and paperboard is an interesting alternative to extrusion coating for improving repulpability and recyclability of barrier coated fiber-based substrates. Dispersion coating is especially useful as it can be implemented on-line in paper or paperboard machines. Many dispersions or emulsions such as styrene/acrylate or styrene/butadiene emulsions are, however, not biodegradable or compostable. Dispersion coating barriers based on polyhydroxyalkanoates (PHA), on the other hand, are both compostable and biodegradable. The challenge with PHA dispersions is to find a suitable particle size distribution and compositions, which enables good coater runnability, good coating coverage (hold-out) especially at low coat weights, and good barrier performance. Another challenge is to efficiently cure or film-form the wet coating, particularly avoiding the potential negative effects of co-additives, for example surfactants, on barrier performance. Coating coverage, and subsequent barrier performance, is very dependent on substrate roughness and coat weight, whereas higher coat weights have a negative impact on recyclability, drying efficiency, and cost.

Many of the aforementioned problems can be avoided by applying the PHA using melt extrusion coating. However, extrusion coating of PHA is unfortunately not implementable on-line on full scale paper or paperboard machines. Also, extrusion coating usually requires relatively high coat weights, and extrusion coated grades are more difficult to re-pulp than dispersion coated grades.

Thus, there remains a need for improved solutions to replace conventional plastic coatings, especially polyolefin coatings, in paper and paperboard based packaging materials, while maintaining acceptable liquid, water vapor, and oxygen barrier properties. At the same time, there is a need for liquid barrier layers for paper or paperboard based packaging materials that facilitate repulping and recycling of the used packaging materials as compared to packaging laminates using conventional plastic films.

Description of the invention

It is an object of the present disclosure to provide an alternative to the plastic films commonly used as barrier layers for providing liquid barrier properties in paper or paperboard based packaging materials, such as liquid or food packaging board.

It is a further object of the present disclosure to provide a liquid barrier layer for a paper or paperboard based packaging material, such as a liquid or food packaging board, which is based on renewable raw materials. It is a further object of the present disclosure to provide a liquid barrier layer for a paper or paperboard based packaging material, such as a liquid or food packaging board, which facilitates re-pulping of the packaging material as compared to packaging laminates using conventional plastic films.

It is a further object of the present disclosure to provide an improved method for coating a fiber-based substrate with a PHA coating, particularly a method which can be implemented on-line in full scale paper or paperboard machines.

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.

The present invention is based on the understanding that the problems associated with dispersion coating of PHA dispersions on fiber-based substrates, and particularly on fiber-based substrates having a relatively high surface roughness, can be overcome by applying PHA in a two-step coating process, wherein the first step comprises applying a foamed aqueous coating composition comprising PHA to the substrate.

According to a first aspect illustrated herein, there is provided a method for manufacturing a PHA (polyhydroxyalkanoate) coated fiber-based substrate, said method comprising the steps of: a) providing a fiber-based substrate having a first main surface and a second main surface; b) forming a first PHA layer by applying a foamed first aqueous coating composition comprising a first PHA and a foaming agent on the first main surface and drying the foamed first aqueous coating composition, wherein the foamed first aqueous coating composition is a foam having a foam density of 0.75 g/cm³ or less; c) forming a second PHA layer by applying a second aqueous coating composition comprising a second PHA on the first PHA layer and drying the second aqueous coating composition. The inventive method may be implemented on-line in full scale paper or paperboard machines.

The 2-layer PHA coating structure allows for more efficient barrier and reduced risks of pinholes.

The composition of each layer can be tailored to achieve desired properties. For example, the first aqueous coating composition can be formulated using a PHA which allows for optimizing foam forming, adhesion, and/or repulpability. The second aqueous coating composition can instead be formulated with another PHA, preferably a hydrophobic PHA, which allows for to optimizing liquid barrier surface properties, such as hydrophobicity. Foaming of the second dispersion comprising a hydrophobic PHA using a surfactant is not desired, as it could cause the resulting dispersion coated surface to be less water repellent.

The dispersion coated substrate is preferably repulpable and reusable according to the PTS standard. The 2-layer PHA coating structure comprising a foaming agent, in the first PHA layer allows for easier release of the coating layer since wetting of the layer, and also wetting of the fiber-based substrate, is enhanced by the foaming agent, typically in the form of a surfactant.

The fiber-based substrate (also referred to herein as “the substrate”) is preferably a sheet or web of material mainly formed from pulp of wood or other fibrous substances. The fiber-based substrate is preferably paper or paperboard.

Paper generally refers to a material manufactured in sheets or rolls 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 can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements.

Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and/or other types of packaging. Paperboard can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements.

In some embodiments, the fiber-based substrate is comprised of two or more cellulose-based plies. Each of the cellulose-based plies can have a certain composition of pulp fibers, such as bleached and/or unbleached Kraft pulp, sulfite pulp, dissolving pulp, thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), high-temperature CTMP (HT-CTMP), broke, and/or mixtures thereof. The different plies can have different grammages and/or thicknesses and may contain different amounts of additives, such as internal sizing agents.

As an example, the fiber-based substrate can be built up of one top-ply consisting of bleached or unbleached Kraft pulp, a mid-ply consisting of a mixture of bleached or unbleached Kraft pulp and CTMP, and a bottom-ply consisting of bleached or unbleached Kraft pulp, wherein the mid-ply has a higher thickness and/or lower density than both the top and bottom plies, respectively.

A preferred fiber-based substrate is a paper or paperboard having high content of unbleached fibers, such as at least 50 wt% in at least one of the plies, thus forming rough surface but with good mechanical performance and a natural look.

In some embodiments, the basis weight of the fiber-based substrate is in the range of 20-800 g/m² In some embodiments, the fiber-based substrate has a grammage of at least 100 g/m². In some embodiments, the fiber-based substrate has a grammage of at least 150 g/m², 200 g/m², 250 g/m², 300 g/m², 350 g/m², or 400 g/m². The grammage of the fiber-based substrate is preferably below 1000 g/m², 800 g/m², or 600 g/m². Unless otherwise stated, the grammage is determined according to the standard ISO 536.

The fiber-based substrate may also be surface sized or impregnated. In some embodiments, the fiber-based substrate is surface sized or impregnated on one or both sides with a surface sizing composition, preferably comprising a starch derivative, a cellulose derivative, or polyvinyl alcohol (PVOH) or a combination of thereof. The starch derivative may for example be a slightly modified, such as oxidised or cationised, starch. The cellulose derivative may for example be a sodium carboxymethyl cellulose with a degree of substitution higher than 0.4 such as in the range of 0.5-1 .5. The PVOH may be fully or partly hydrolysed. The surface sizing or impregnation may facilitate the release of the PHA coating structure from the fiber-based substrate during repulping.

In some embodiments, the grammage of the surface sizing composition is 0.2-10 g/m², preferably 0.4-8 g/m², and more preferably 0.8-5 g/m² per side, based on dry weight.

The fiber-based substrate itself, before PHA coating, may have a relatively high permeability for liquids, such as water, oil and grease, water vapor and gases, such as oxygen, air and carbon dioxide. In some embodiments In some embodiments, the fiber-based substrate has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 - 20 at 50% relative humidity and 23 °C, of at least 200 g/m²/24h. In some embodiments, the fiberbased substrate has a Cobb60 value as measured according to the standard ISO 535 of less than 100 g/m², preferably less than 80 g/m², less than 60 g/m², or less than 40 g/m².

The inventive method is particularly useful for coating of fiber-based substrate having a relatively high surface roughness. In some embodiments, first main surface of the fiber-based substrate has a PPS surface smoothness at 1 .0 MPa in the range of 1 -20 pm, preferably in the range of 1 .5-10 pm, and more preferably in the range of 2-8 pm, as determined according to ISO 8791 -4:2007.

The first PHA layer is formed by applying a foamed first aqueous coating composition comprising a first PHA and a foaming agent on the first main surface and drying the foamed first aqueous coating composition, wherein the foamed first aqueous coating composition is a foam having a foam density of 0.75 g/cm³ or less, preferably 0.6 g/cm³ or less, or 0.5 g/cm³ or less.

The term foam, as used herein, refers to a liquid comprising air or gas bubbles dispersed therein. Typically, the volume of gas is much larger than that of the liquid, with thin films separating gas pockets. Three requirements must be met in order for foam to form. Mechanical work is needed to increase the contact area between the gas and the liquid. This can occur by agitation, dispersing a large volume of gas into a liquid, or injecting a gas into a liquid. The second requirement is that a foam forming agent, typically an amphiphilic substance, a surfactant or surface active component, must be present to decrease surface tension. Finally, the foam must form more quickly than it breaks down.

Foams may be open-cell or closed-cell in nature. Pores connect the gas regions in open-cell foams, while closed-cell foams have enclosed cells. The cells are usually disordered in their arrangement, with varying bubble sizes. The cells present minimal surface area, forming honeycomb shapes or tessellations.

Foam coating is advantageous as it allows for coating at higher solids content compared to a non-foamed coating. The lower water content of a foam coating also reduces the problems with rewetting of the fiber-based substrate.

The foamed first aqueous coating composition may be prepared by mixing the foaming agent with a PHA dispersion, and then shearing or mixing the mixture into a foam using for example a high shear mixer or a foam generator.

The present inventors have identified that certain hydrophobic PHAs, which form hydrophobic coatings, are difficult to form into a stable foam. Thus, the foamed first aqueous coating composition comprises a first PHA, which allows for the preparation of a stable foam. The inventors have found that the first PHA should preferably be a PHA co-polymer, i.e. a PHA wherein two or more different types of monomers are linked in the same PHA polymer chain. Such PHA co-polymers are well known to the skilled person.

The inventors have further identified that PHAs suitable for the foamed first aqueous coating composition are characterized by forming films having a relatively low water contact angle and a relatively high melting point, whereas PHAs suitable for the second aqueous coating composition are characterized by forming films having a relatively high water contact angle and a relatively low melting point. The second PHA thus has distinctly different features as compared to the first PHA.

In some embodiments, the first PHA is a PHA co-polymer.

The type of PHA co-polymer suitable for use in the first aqueous coating composition may be characterized by its melting point. In preferred embodiments, the first PHA has a melting point in the range of 100-170 °C preferably in the range of 120-160 °C.

In some embodiments, the first PHA is selected from the group consisting of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-co- 3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHOHH), poly(3- hydroxyoctanoate-co-3-hydroxydecanoate) (PHOHD), and poly(3- hydroxyoctanoate-co-3-hydroxydodecanoate) (PHDHDD), or a mixture thereof. The skilled person understands that the PHA co-polymers suitable for use in the first aqueous coating composition are not limited to those listed here.

The first PHA is preferably the main ingredient of the foamed first aqueous coating composition based on dry weight of the composition. In some embodiments, the foamed first aqueous coating composition comprises the first PHA in an amount of 70-99.9 wt%, preferably in an amount of 90-99.9 wt%, based on the total dry weight of the foamed first aqueous coating composition.

The foaming agent is a compound capable of forming and/or stabilizing a foam in an aqueous composition. The foaming agent is typically an amphiphilic substance, i.e. a chemical compound possessing both hydrophilic and hydrophobic (lipophilic) properties. A foaming agent reduces the work needed to create the foam by reducing the surface tension of the liquid and increases the colloidal stability of the foam by inhibiting coalescence of bubbles.

The foaming agent of the solid composite may be any foaming agent suitable for facilitating the formation of a foam in an aqueous PHA dispersion and for stabilizing the formed foam. In other words, the foaming agent should be capable of forming a stable foam in an aqueous PHA dispersion. By “stable foam” as used herein is meant that the foam of the foamed first aqueous coating composition is sufficiently stable to remain in foam form at least until the foamed first aqueous coating composition has been applied on the first main surface.

The foaming agent is typically a surfactant, such as SDS, or a surface active polymer, or a combination thereof. In some embodiments, the foaming agent is a non-polymeric or polymeric surfactant, or a combination thereof. The surfactant may be anionic, non-ionic, or zwitter-ionic.

In some embodiments, the foaming agent is a non-polymeric surfactant, preferably sodium dodecyl sulfate (SDS).

The polymeric foaming agent is preferably an amphiphilic polymer, i.e. a polymer possessing both hydrophilic and hydrophobic (lipophilic) properties. In some embodiments, the foaming agent is water-soluble. The polymeric foaming agent may for example be a water-soluble polymer with hydrophobic moieties, such as a hydrophilic polymeric backbone provided with hydrophobic sidechains, or a block copolymer comprised of hydrophilic and hydrophobic sections.

In some embodiments, the foaming agent is selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate and mixtures thereof. The optional hydrophobic modification typically comprises one or more hydrophobic groups, e.g. alkyl groups, covalently attached to the foaming agent.

In some embodiments, the foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose, starch, hemicellulose and mixtures thereof.

In some embodiments, the polymeric foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose acetate (CA), ethyl(hydroxyethyl)cellulose (EHEC), methylcellulose (MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), sodium carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), sulfoethylcellulose, starch, and mixtures thereof.

In some embodiments, the polymeric foaming agent is selected from the group consisting of ethyl(hydroxyethyl)cellulose, hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), hydroxyethylcellulose, hydrophobically modified hydroxyethyl cellulose (HM-HEC), methylcellulose (MC), hydrophobically modified methylcellulose (HM-MC), hydrophobically modified carboxymethylcellulose (HM-CMC), and hydrophobically modified starch (HM- starch). Examples of useful hydrophobically modified starch derivatives include, but are not limited to dialdehyde starch, hydroxypropylated starch, octenyl succinic anhydride (OSA) starch, and dodecyl succinic anhydride (DDSA) starch.

In some embodiments, the polymeric foaming agent is an optionally hydrophobically modified methyl cellulose.

In some embodiments, the polymeric foaming agent is a hydrophobically modified polyvinyl alcohol (PVOH), such as ethylene modified PVOH. In some embodiments, the polymeric foaming agent is a polyvinyl alcohol containing at least 2% acetate groups, more preferably at least 10% acetate groups, and even more preferably at least 15% acetate groups.

In some embodiments, the foamed first aqueous coating composition comprises the foaming agent in an amount of 0.1-10 wt% based on the total dry weight of the foamed first aqueous coating composition.

In some embodiments, the foamed first aqueous coating composition comprises SDS in an amount of 0.1-2 wt% based on the total dry weight of the foamed first aqueous coating composition. In some embodiments, the foamed first aqueous coating composition further comprises a polymeric co-additive, preferably selected from the group consisting of carboxymethyl cellulose (CMC), m icrofibrillated cellulose (MFC), and starch, in an amount of 0.1-20 wt% based on the total dry weight of the foamed first aqueous coating composition.

The foamed first aqueous coating composition is a foam having a foam density of 0.75 g/cm³ or less. In some embodiments, the foamed first aqueous coating composition has a foam density of 0.6 g/cm³ or less, preferably 0.5 g/cm³ or less.

In some embodiments, the foamed first aqueous coating composition has a viscosity in the range of 50-3500 mPas determined using a Brookfield viscosimeter at rotational speed of 100 rpm.

The first PHA layer is preferably formed by means of a liquid film coating process, i.e. in the form of a foamed aqueous dispersion of the first PHA which, on application, is spread out to a thin, uniform layer on the substrate and thereafter dried.

In some embodiments, the foamed first aqueous coating composition is applied by a non-contact application method. In some embodiments, the foamed first aqueous coating composition is applied by an application method selected from the group consisting of roller coating, spray coating, curtain, blade coating, slot coating, immersion coating, gravure roll coating, reverse direct gravure coating, rod coating, soft-tip blade coating, short dwell, and soft-tip rod coating, and combinations thereof. The foamed first aqueous coating composition may be applied directly onto the fiber-based substrate or indirectly, for example via a transfer roll or belt.

To minimize the risk of pinholes in the first PHA layer, the foamed first aqueous coating composition may be applied in at least two different coating steps with drying of the coated film between the steps. In some embodiments, the drying comprises subjecting the foamed first aqueous coating composition to heating. In some embodiments, the drying comprises subjecting the foamed first aqueous coating composition to at least one noncontact drying step, such as infrared radiation, electron beam radiation, ultraviolet radiation, microwave radiation, hot air or a combination thereof. Optionally, the foamed first aqueous coating composition is then subjected to at least one additional drying step, which can be a hot air drying step or a contact drying step, e.g. using a heated belt or heated cylinders.

In some embodiments, the drying of the foamed first aqueous coating composition causes the foam to collapse. This means that the resulting first PHA layer preferably has a structure which is free from, or substantially free from bubbles of air and other gases.

In some embodiments, the first PHA layer has a grammage in the range of 0.3-8 g/m², preferably in the range of 0.5-5 g/m²

The formed first PHA layer is preferably characterized by a relatively low water contact angle and a relatively high melting point.

In some embodiments, the first PHA layer has a water contact angle below 80 degrees, preferably below 70 degrees, as determined after 3 seconds according to ASTM D7490-13. In some embodiments, the first PHA layer has a water contact angle in the range of 30-80 degrees, preferably in the range of 30-70 degrees, as determined after 3 seconds according to ASTM D7490-13.

In some embodiments, the first PHA layer has a melting point above 100 °C, preferably above 110 °C, as determined according to ASTM E794-06(2018). In some embodiments, the second PHA layer has a melting point in the range of 100- 170 °C, preferably in the range of 110-170 °C, as determined according to ASTM E794-06(2018).

The second PHA layer is formed by applying a second aqueous coating composition comprising a second PHA on the first PHA layer and drying the second aqueous coating composition. Forming the second PHA layer on top of the dried first PHA layer prevents absorption of the second aqueous coating composition into the fiber-based substrate and serves to cover pinholes possibly present in the first PHA layer.

In some embodiments, the second PHA is a PHA homopolymer, i.e. a PHA wherein the polymer chain ids formed from a single type of monomer. Preferably the second PHA is a PHA homopolymer having a side chain length in the range of 2-14 carbon atoms, and more preferably a PHA homopolymer having a side chain length in the range of 5-14 carbon atoms.

In some embodiments, the second PHA has a melting point in the range of 50-100 °C, preferably in the range of 70-100 °C.

In some embodiments, the second PHA is selected from the group consisting of poly(3-hydroxyoctanoate) (PHO), poly(3-hydroxydecanoate) (PHD), poly(3- hydroxyhexanoate) (PHH), and poly(3-hydroxyvalerate) (PHV), or a mixture thereof, preferably selected from the group consisting of PHO, PHD, and PHH, or a mixture thereof.

The second PHA is preferably the main ingredient of the second aqueous coating composition based on dry weight of the composition. In some embodiments, the second aqueous coating composition comprises the second PHA in an amount of 70-99.9 wt%, preferably in an amount of 90-99.9 wt%, based on the total dry weight of the second aqueous coating composition.

In some embodiments, the second aqueous coating composition is free from added foaming agents, such as non-polymeric or polymeric surfactants.

In some embodiments, the second aqueous coating composition is de-aerated.

Thus, in some embodiments, the second aqueous coating composition is preferably free from, or substantially free from bubbles of air and other gases. In some embodiments, the second aqueous coating composition has a density of 0.9 g/cm³ or higher, preferably 0.95 g/cm³ or higher, and more preferably 0.98 g/cm³ or higher.

The second PHA layer is preferably formed by means of a liquid film coating process, i.e. in the form of an aqueous dispersion of the second PHA which, on application, is spread out to a thin, uniform layer on the substrate and thereafter dried.

In some embodiments, the second aqueous coating composition is applied by a non-contact application method. In some embodiments, the second aqueous coating composition is applied by an application method selected from the group consisting of roller coating, spray coating, curtain, blade coating, slot coating, immersion coating, gravure roll coating, reverse direct gravure coating, rod coating, soft-tip blade coating, short dwell, and soft-tip rod coating, and combinations thereof.

To minimize the risk of pinholes in the second PHA layer, the second aqueous coating composition may be applied in at least two different coating steps with drying of the coated film between the steps.

In some embodiments, the drying comprises subjecting the second aqueous coating composition to heating. In some embodiments, the drying comprises subjecting the second aqueous coating composition to at least one non-contact drying step, such as infrared radiation, electron beam radiation, ultraviolet radiation, microwave radiation, hot air or a combination thereof. Optionally, the second aqueous coating composition is the subjected to at least one additional drying step, which can be a hot air drying step or a contact drying step, e.g. using a heated belt or heated cylinders.

In some embodiments, the second PHA layer has a grammage in the range of 2- 30 g/m², preferably in the range of 5-20 g/m². The formed second PHA layer is preferably characterized by a relatively high water contact angle and a relatively low melting point.

In some embodiments, the second PHA layer has a water contact angle above 90 degrees, preferably above 100 degrees, as determined after 3 seconds according to ASTM D7490-13. In some embodiments, the second PHA layer has a water contact angle in the range of 100-140 degrees, preferably in the range of 105-125 degrees, as determined after 3 seconds according to ASTM D7490-13.

In some embodiments, the second PHA layer has a melting point below 100 °C, preferably above 90 °C, as determined according to ASTM E794-06(2018). In some embodiments, the second PHA layer has a melting point in the range of 40- 100 °C, preferably in the range of 50-90 °C, as determined according to ASTM E794-06(2018).

In some embodiments, the first or second aqueous coating composition further comprises a pigment in an amount of 0.1 -20 wt% based on the total dry weight of the aqueous coating composition. The pigment may for example comprise inorganic particles of talcum, silicates or phyllosilicates, carbonates, alkaline earth metal carbonates and ammonium carbonate, or oxides, such as transition metal oxides and other metal oxides. The pigment may also comprise nano-size pigments such as nanoclays and nanoparticles of layered mineral silicates, for instance selected from the group comprising montmorillonite, bentonite, kaolinite, hectorite and hallyosite.

In some embodiments, the second main surface is uncoated.

In some embodiments the second main surface is coated with a 2-layer PHA coating structure as described herein with reference to the first main surface. The 2-layer PHA coating structures on the first and second main surfaces may have identical or different composition. In other words, the coating composition for each layer may be selected independently. In some embodiments, the second main surface is only coated with the foamed aqueous coating composition. In some embodiments, the method further comprises: d) forming a third PHA layer by applying a foamed third aqueous coating composition comprising a third PHA and a foaming agent on the second main surface and drying the foamed third aqueous coating composition, wherein the foamed third aqueous coating composition is a foam having a foam density of 0.75 g/cm³ or less.

The step of forming the third PHA layer, the third aqueous coating composition, and the components thereof, and the properties of the formed third layer, may be further defined as described herein with reference to the step of forming the first PHA layer.

In some embodiments, the method further comprises: e) forming a fourth PHA layer by applying a fourth aqueous coating composition comprising a fourth PHA on the third PHA layer and drying the fourth aqueous coating composition.

The step of forming the fourth PHA layer, the fourth aqueous coating composition, and the components thereof, and the properties of the formed fourth layer, may be further defined as described herein with reference to the step of forming the second PHA layer.

The method according to the first aspect described herein allows for the preparation of improved PHA coated fiber-based substrate. According to a second aspect illustrated herein, there is provided a PHA (polyhydroxyalkanoate) coated fiber-based substrate comprising: a fiber-based substrate having a first main surface and a second main surface, a first PHA layer comprising a first PHA and a foaming agent on the first main surface, a second PHA layer comprising a second PHA on the first PHA layer. The PHA coated fiber-based substrate according to the second aspect described herein, and the components thereof, including the first PHA layer, the first PHA and the foaming agent, may be further defined as described with reference to the first aspect.

In some embodiments, the PHA coated fiber-based substrate has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 - 20 at 50% relative humidity and 23 °C, of less than 10 g/m²/24h, preferably less than 5 g/m²/24h.

In some embodiments, the PHA coated fiber-based substrate has a Cobb600 value as measured according to the standard ISO 535 of less than 15 g/m², preferably less than 10 g/m².

In some embodiments, the PHA coated fiber-based substrate has a grease resistance as measured according to the standard ASTM F119-82 of at least 0.5 h, preferably at least 1 h.

The coated fiber-based substrate provides an alternative to conventional packaging materials using polyolefin layers, which can more readily be repulped and recycled. In some embodiments, the coated fiber-based substrate has a reject rate according to PTS RH 021/97 of less than 30 %, preferably less than 20 %, more preferably less than 10%.

According to a third aspect illustrated herein, there is provided a packaging container comprising a PHA coated fiber-based substrate according to the second aspect. The PHA coated fiber-based substrate is well suited for trays, plates, bowls, cups, and lids, but also for other liquid or food packaging materials. The PHA coated fiber-based substrate is especially well suited for trays, plates and bowls formed by thermoforming, deep drawing or press-forming.

The 2-layer PHA coating structure of the inventive PHA coated fiber-based substrate has been found useful as a liquid barrier for use on the inside surface of food and liquid packaging containers. Thus, in some embodiments, the PHA coated first main surface forms an inside surface of the container.

The 2-layer PHA coating structure of the inventive PHA coated fiber-based substrate has also been found useful as a moisture/liquid barrier for use on the outside surface of food and liquid packaging containers. Thus, in some embodiments, the PHA coated first main surface forms an outside surface of the container.

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.

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 can 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

1 . A method for manufacturing a PHA (polyhydroxyalkanoate) coated fiberbased substrate, said method comprising the steps of: a) providing a fiber-based substrate having a first main surface and a second main surface; b) forming a first PHA layer by applying a foamed first aqueous coating composition comprising a first PHA and a foaming agent on the first main surface and drying the foamed first aqueous coating composition, wherein the foamed first aqueous coating composition is a foam having a foam density of 0.75 g/cm³ or less; c) forming a second PHA layer by applying a second aqueous coating composition comprising a second PHA on the first PHA layer and drying the second aqueous coating composition.

2. The method according to claim 1 , wherein the first PHA is a PHA co-polymer.

3. The method according to any one of the preceding claims, wherein the first PHA has a melting point in the range of 100-170 °C preferably in the range of 120- 160 °C.

4. The method according to any one of the preceding claims, wherein the first PHA is selected from the group consisting of poly(3-hydroxybutyrate-co-3- hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3- hydroxyoctanoate-co-3-hydroxyhexanoate) ( P H O H H ) , poly(3-hydroxyoctanoate- co-3-hydroxydecanoate) (PHOHD), and poly(3-hydroxyoctanoate-co-3- hydroxydodecanoate) (PHDHDD), or a mixture thereof.

5. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition comprises the first PHA in an amount of 70-99.9 wt%, preferably in an amount of 90-99.9 wt%, based on the total dry weight of the foamed first aqueous coating composition.

6. The method according to any one of the preceding claims, wherein the foaming agent is a non-polymeric or polymeric surfactant.

7. The method according to any one of the preceding claims, wherein the foaming agent is a non-polymeric surfactant, preferably sodium dodecyl sulfate (SDS).

8. The method according to any one of claims 1 -6, wherein the foaming agent is an amphiphilic polymer, preferably an amphiphilic polymer selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol (PVOH), and partially hydrolyzed polyvinyl acetate (PVOH/Ac), and mixtures thereof.

9. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition comprises the foaming agent in an amount of 0.1 -10 wt% based on the total dry weight of the foamed first aqueous coating composition.

10. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition further comprises a polymeric coadditive, preferably selected from the group consisting of carboxymethyl cellulose (CMC), m icrofibrillated cellulose (MFC), and starch, in an amount of 0.1 -10 wt% based on the total dry weight of the foamed first aqueous coating composition.

11 . The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition has a foam density of 0.6 g/cm³ or less, preferably 0.5 g/cm³ or less.

12. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition has a viscosity in the range of 50-3500 mPas determined using a Brookfield viscosimeter at rotational speed of 100 rpm.

13. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition is applied by a non-contact application method.

14. The method according to any one of the preceding claims, wherein the drying comprises subjecting the foamed first aqueous coating composition to heating.

15. The method according to any one of the preceding claims, wherein the drying of the foamed first aqueous coating composition causes the foam to collapse.

16. The method according to any one of the preceding claims, wherein the first PHA layer has a grammage in the range of 0.3-8 g/m², preferably in the range of 0.5-5 g/m².

17. The method according to any one of the preceding claims, wherein the first PHA layer has a water contact angle below 80 degrees, preferably below 70 degrees, as determined after 3 seconds according to ASTM D7490-13.

18. The method according to any one of the preceding claims, wherein the first PHA layer has a melting point above 100 °C, preferably above 110 °C, as determined according to ASTM E794-06(2018).

19. The method according to any one of the preceding claims, wherein the second PHA is a PHA homopolymer, preferably a PHA homopolymer having a side chain length in the range of 2-14 carbon atoms, and more preferably a PHA homopolymer having a side chain length in the range of 5-14 carbon atoms.

20. A PHA (polyhydroxyalkanoate) coated fiber-based substrate comprising: a fiber-based substrate having a first main surface and a second main surface, a first PHA layer comprising a first PHA and a foaming agent on the first main surface, a second PHA layer comprising a second PHA on the first PHA layer.