Method of making PHA coated fiber-based substrates with first and second PHA aqueous coating compositions and fiber-based substrates coated with method

Abstract

The present invention relates to a method of making a PHA (polyhydroxyalkanoate) coated fiber-based substrate, the method comprising the steps of: a) providing a fiber-based substrate having a first major surface and a second major surface; b) forming a first PHA layer by applying a foamed first aqueous coating composition comprising a first PHA and a blowing agent on the first major 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 invention also relates to a PHA coated fiber-based substrate and a packaging container comprising a PHA coated fiber-based substrate.

Description

Technical Field

[0001] The present disclosure relates to a method for preparing a coated fiber-based substrate for use as a packaging material, in particular a polyhydroxyalkanoate (PHA)-coated paper or cardboard. Background Art

[0002] Coating paper and cardboard with plastics is commonly used to combine the mechanical properties of paper or cardboard with the barrier and sealing properties of plastic films. Paper or cardboard provided with even a relatively small amount of a suitable plastic material can provide the properties required to make the paper or cardboard suitable for many demanding applications, such as liquid or food packaging cardboard. In liquid or food packaging boards, an extruded polyolefin coating is often used as a liquid barrier layer, a heat-sealing layer, and an adhesive. However, recycling of such polymer-coated boards is difficult because it is difficult to separate the polymer from the fibers.

[0003] In the prior art, attempts have been made to replace the extruded polyolefin coating with more environmentally friendly and / or more easily recyclable solutions, but so far without real success. In many cases, some but not all of the properties of the extruded polyolefin coating are achieved by alternative solutions.

[0004] Dispersion barrier coating for paper and cardboard is an interesting alternative to extrusion coating for improving the repulpability and recyclability of fiber-based substrates coated with a barrier. Dispersion coating is particularly useful because it can be carried out online in a paper or cardboard machine. However, many dispersions or emulsions, such as styrene / acrylic or styrene / butadiene emulsions, are not biodegradable or compostable.

[0005] On the other hand, polyhydroxyalkanoate (PHA)-based dispersion coating barriers are both compostable and biodegradable. The challenge with PHA dispersions is to find a suitable particle size distribution and composition that enables good coater runnability, good coating coverage (retention), especially at low coating weights, and good barrier properties. Another challenge is to effectively cure or film the wet coating, especially to avoid potential negative effects of co-additives, such as surfactants, on the barrier properties. Coating coverage and subsequent barrier properties are highly dependent on substrate roughness and coating weight, while higher coating weights have a negative impact on recyclability, drying efficiency, and cost.

[0006] By applying PHA using melt extrusion coating, many of the above problems can be avoided. However, unfortunately, extrusion coating of PHA cannot be carried out online on a full-scale paper or cardboard machine. In addition, extrusion coating generally requires a relatively high coating weight, and extrusion-coated grades are more difficult to repulp than dispersion-coated grades.

[0007] Accordingly, there is still 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 a liquid barrier layer for paper or paperboard-based packaging materials that facilitates the repulping and recycling of used packaging materials compared to packaging laminates using conventional plastic films. Detailed Description

[0008] An object of the present disclosure is to provide an alternative to plastic films that are commonly used as barrier layers for providing liquid barrier properties in paper or paperboard-based packaging materials, such as liquid or food packaging paperboard.

[0009] Another object of the present disclosure is to provide a liquid barrier layer for paper or paperboard-based packaging materials (such as liquid or food packaging boards) that is based on renewable raw materials.

[0010] Another object of the present disclosure is to provide a liquid barrier layer for paper or paperboard-based packaging materials (such as liquid or food packaging paperboard) that facilitates the repulping of the packaging material compared to packaging laminates using conventional plastic films.

[0011] Another object of the present disclosure is to provide an improved method for coating a fiber-based substrate with a PHA coating, particularly a method that can be implemented online in a full-scale paper or paperboard machine.

[0012] The above objects and other objects that will be appreciated by those skilled in the art from the present disclosure are achieved through various aspects of the present disclosure.

[0013] The present invention is based on the understanding that problems associated with the dispersion coating of a PHA dispersion on a fiber-based substrate, and particularly on a fiber-based substrate having a relatively high surface roughness, can be overcome by applying the PHA in a two-step coating process, where the first step includes applying a foamed aqueous coating composition containing the PHA to the substrate.

[0014] According to a first aspect shown herein, there is provided a method for manufacturing a PHA (polyhydroxyalkanoate)-coated fiber-based substrate, the method comprising the following steps:

[0015] a) providing a fiber-based substrate having a first major surface and a second major surface;

[0016] b) forming a first PHA layer by applying a foamed first aqueous coating composition containing a first PHA and a foaming agent to the first major 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 / cm 3 or less;

[0017] c) A second PHA layer is formed by applying a second aqueous coating composition comprising a second PHA onto the first PHA layer and drying the second aqueous coating composition.

[0018] The method of the present invention can be implemented online in a full-scale paper or board machine.

[0019] The 2-layer PHA coating structure allows for more effective barrier and a reduced risk of pinholes.

[0020] The composition of each layer can be customized to achieve the desired properties. For example, the first aqueous coating composition can be formulated with a PHA that allows for optimization of foam formation, adhesion, and / or repulpability. The second aqueous coating composition can alternatively be formulated with an additional PHA, preferably a hydrophobic PHA, which allows for optimization of liquid barrier surface properties such as hydrophobicity. It is not desirable to foam the second dispersion containing the hydrophobic PHA using a surfactant as it can result in a lower water repellency of the surface coated with the resulting dispersion.

[0021] According to the PTS standard, the substrate coated with the dispersion is preferably repulpable and reusable. The 2-layer PHA coating structure containing a foaming agent in the first PHA layer allows for easier release of the coating as the wetting of this layer as well as the fiber-based substrate is enhanced by the foaming agent (usually in the form of a surfactant).

[0022] The fiber-based substrate (also referred to herein as "substrate") is preferably a sheet or web of a material mainly formed from the pulp of wood or other fibrous substances. The fiber-based substrate is preferably paper or board.

[0023] Paper generally refers to a material made from the pulp of wood or other fibrous substances containing cellulose fibers in sheets or rolls, used for, for example, writing, painting, or printing, or as a packaging material. Paper can be bleached or unbleached and is produced in various thicknesses depending on the requirements of the end use.

[0024] Board generally refers to a strong, thick paper or cardboard containing cellulose fibers, which is used, for example, as a flat substrate, tray, box, and / or other types of packaging. Board can be bleached or unbleached and is produced in various thicknesses depending on the requirements of the end use.

[0025] In some embodiments, the fiber-based substrate consists of two or more cellulose plies. Each cellulose ply may 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. Different plies may have different grammages and / or thicknesses and may contain different amounts of additives, such as internal sizing agents.

[0026] For example, the fiber-based substrate can be constructed from a top ply consisting of bleached or unbleached kraft pulp, an intermediate ply consisting of a mixture of bleached or unbleached kraft pulp and CTMP, and a bottom ply consisting of bleached or unbleached kraft pulp, where the intermediate ply has a higher thickness and / or lower density than the top and bottom plies, respectively.

[0027] Preferred fiber-based substrates are paper or board that have a high content of unbleached fibers, such as at least 50 wt% unbleached fibers in at least one ply, thus forming a rough surface but having good mechanical properties and a natural appearance.

[0028] In some embodiments, the basis weight of the fiber-based substrate is in the range of 20 - 800 g / m 2 . In some embodiments, the grammage of the fiber-based substrate is at least 100 g / m 2 . In some embodiments, the grammage of the fiber-based substrate is at least 150 g / m 2 , 200 g / m 2 , 250 g / m 2 , 300 g / m 2 , 350 g / m 2 or 400 g / m 2 . The grammage of the fiber-based substrate is preferably 1000 g / m 2 , 800 g / m 2 or 600 g / m 2 or less. Unless otherwise stated, the grammage is determined according to standard ISO 536.

[0029] The fiber-based substrate can 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, which preferably comprises a starch derivative, a cellulose derivative, or polyvinyl alcohol (PVOH), or a combination thereof. The starch derivative can be, for example, a slightly modified starch, such as oxidized or cationized starch. The cellulose derivative can be, for example, sodium carboxymethyl cellulose, having a degree of substitution higher than 0.4, for example, in the range of 0.5 - 1.5. The PVOH can be fully or partially hydrolyzed. Surface-sizing or impregnation can facilitate the release of the PHA coating structure from the fiber-based substrate during repulping.

[0030] In some embodiments, based on dry weight, the grammage of the surface-sizing composition is 0.2 - 10 g / m per side 2 , preferably 0.4 - 8 g / m 2 , and more preferably 0.8 - 5 g / m 2 .

[0031] Before PHA coating, the fiber-based substrate itself can have a relatively high permeability to liquids (such as water, oil, and grease), water vapor, and gases (such as oxygen, air, and carbon dioxide). In some embodiments, measured according to standard ASTM F1249 - 20 at 50% relative humidity and 23 °C, the fiber-based substrate has a water vapor transmission rate (WVTR) of at least 200 g / m 2 / 24h. In some embodiments, the fiber-based substrate has a Cobb60 value of less than 100 g / m 2 , preferably less than 80 g / m 2 , less than 60 g / m 2 or less than 40 g / m 2 , as measured according to standard ISO 535.

[0032] The method of the present invention is particularly suitable for coating fiber-based substrates having a relatively high surface roughness. In some embodiments, the PPS surface smoothness of the first major surface of the fiber-based substrate at 1.0 MPa is in the range of 1 - 20 μm, preferably in the range of 1.5 - 10 μm, and more preferably in the range of 2 - 8 μm, as determined according to ISO 8791 - 4:2007.

[0033] 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 major surface and drying the foamed first aqueous coating composition, wherein the foamed first aqueous coating composition has a foam density of 0.75 g / cm 3 or less, preferably 0.6 g / cm 3 or less, or 0.5 g / cm 3Or smaller foams.

[0034] As used herein, the term foam refers to a liquid containing air or gas bubbles dispersed therein. Generally, the volume of the gas is much larger than the volume of the liquid, with thin films separating the gas pockets. To form a foam, three requirements must be met. Mechanical work is needed to increase the contact area between the gas and the liquid. This can occur by agitation, dispersing a large amount of gas into the liquid, or injecting gas into the liquid. The second requirement is that a foam former must be present, typically an amphiphilic substance, surfactant, or surface-active component, to reduce the surface tension. Finally, the foam must form faster than it decomposes.

[0035] Foams can be open-cell or closed-cell in nature. The pores connect the gas regions in open-cell foams, while closed-cell foams have enclosed pores. The arrangement of the pores is usually disordered, with different bubble sizes. The pores present a minimum surface area, forming a honeycomb shape or tessellations.

[0036] Foam coating is advantageous because it allows coating at a higher solids content compared to non-foamed coatings. The lower water content of the foam coating also reduces the rewetting problem of the fiber-based substrate.

[0037] The first aqueous coating composition for foaming can be prepared by mixing a 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.

[0038] The inventors have determined that certain hydrophobic PHAs that form hydrophobic coatings are difficult to form stable foams. Thus, the first aqueous coating composition for foaming contains a first PHA that allows the preparation of stable foams. The inventors have found that the first PHA should preferably be a PHA copolymer, i.e., a PHA in which two or more different types of monomers are linked in the same PHA polymer chain. Such PHA copolymers are well known to those skilled in the art.

[0039] The inventors have further determined that PHAs suitable for the first aqueous coating composition for foaming are characterized by forming a film with a relatively low water contact angle and a relatively high melting point, while PHAs suitable for the second aqueous coating composition are characterized by forming a film with a relatively high water contact angle and a relatively low melting point. Thus, the second PHA has significantly different characteristics compared to the first PHA.

[0040] In some embodiments, the first PHA is a PHA copolymer.

[0041] The type of PHA copolymer suitable for the first aqueous coating composition can be characterized by its melting point. In a preferred embodiment, the melting point of the first PHA is in the range of 100 - 170 °C, preferably in the range of 120 - 160 °C.

[0042] In some embodiments, the first PHA is selected from 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 mixtures thereof. Those skilled in the art will understand that the PHA copolymers suitable for the first aqueous coating composition are not limited to those listed herein.

[0043] Based on the dry weight of the composition, the first PHA is preferably the main component of the foamed first aqueous coating composition. In some embodiments, based on the total dry weight of the foamed first aqueous coating composition, the foamed first aqueous coating composition comprises the first PHA in an amount of 70 - 99.9 wt%, preferably 90 - 99.9 wt%.

[0044] A foaming agent is a compound capable of forming and / or stabilizing foam in an aqueous composition. Foaming agents are generally amphiphilic substances, i.e., chemical compounds having hydrophilic and hydrophobic (lipophilic) properties. Foaming agents reduce the work required to generate foam by lowering the surface tension of the liquid and increase the colloidal stability of the foam by inhibiting the coalescence of gas bubbles.

[0045] The foaming agent for the solid composite material can be any foaming agent suitable for promoting the formation of foam in the aqueous PHA dispersion and stabilizing the formed foam. In other words, the foaming agent should be able to form a stable foam in the aqueous PHA dispersion. As used herein, "stable foam" means that the foam of the foamed first aqueous coating composition is stable enough to remain in foam form, at least until the foamed first aqueous coating composition has been applied to the first major surface.

[0046] Foaming agents are generally surfactants, such as SDS, or surface-active polymers, or combinations thereof. In some embodiments, the foaming agent is a non-polymeric or polymeric surfactant, or a combination thereof. The surfactant can be anionic, non-ionic, or zwitterionic.

[0047] In some embodiments, the foaming agent is a non-polymeric surfactant, preferably sodium dodecyl sulfate (SDS).

[0048] The polymer foaming agent is preferably an amphiphilic polymer, i.e., a polymer having hydrophilic and hydrophobic (lipophilic) properties. In some embodiments, the foaming agent is water-soluble. The polymer foaming agent can be, for example, a water-soluble polymer having a hydrophobic moiety, such as a hydrophilic polymer backbone provided with hydrophobic side chains, or a block copolymer composed of hydrophilic and hydrophobic moieties.

[0049] In some embodiments, the foaming agent is selected from optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate, and mixtures thereof. Optional hydrophobic modification typically involves covalently attaching one or more hydrophobic groups, such as alkyl groups, to the foaming agent.

[0050] In some embodiments, the foaming agent is an optionally hydrophobically modified polysaccharide selected from optionally hydrophobically modified cellulose, starch, hemicellulose, and mixtures thereof.

[0051] In some embodiments, the polymer foaming agent is an optionally hydrophobically modified polysaccharide selected from 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.

[0052] In some embodiments, the polymer foaming agent is selected from ethyl(hydroxyethyl)cellulose, hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), hydroxyethylcellulose, hydrophobically modified hydroxyethylcellulose (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.

[0053] In some embodiments, the polymer foaming agent is an optionally hydrophobically modified methylcellulose.

[0054] In some embodiments, the polymer foaming agent is a hydrophobically modified polyvinyl alcohol (PVOH), such as ethylene-modified PVOH. In some embodiments, the polymer foaming agent is a polyvinyl alcohol containing at least 2% acetate (acetate) groups, more preferably at least 10% acetate groups, and even more preferably at least 15% acetate groups.

[0055] In some embodiments, based on the total dry weight of the first aqueous coating composition to be foamed, the first aqueous coating composition to be foamed contains a foaming agent in an amount of 0.1% to 10% by weight.

[0056] In some embodiments, based on the total dry weight of the first aqueous coating composition that is foamed, the first aqueous coating composition that is foamed comprises SDS in an amount of 0.1 wt% to 2 wt%.

[0057] In some embodiments, the first aqueous coating composition that is foamed further comprises a polymeric co-additive, which is preferably selected from carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC), and starch, in an amount of 0.1 - 20 wt% based on the total dry weight of the first aqueous coating composition that is foamed.

[0058] The first aqueous coating composition that is foamed is a foam having a foam density of 0.75 g / cm 3 or less. In some embodiments, the foam density of the first aqueous coating composition that is foamed is 0.6 g / cm 3 or less, preferably 0.5 g / cm 3 or less.

[0059] In some embodiments, the viscosity of the first aqueous coating composition that is foamed is in the range of 50 mPas to 3500 mPas, measured using a Brookfield viscometer at a rotational speed of 100 rpm.

[0060] The first PHA layer is preferably formed by a liquid film coating method, i.e., in the form of an aqueous dispersion of the first PHA that is foamed, which spreads into a thin, uniform layer on a substrate upon application and is then dried.

[0061] In some embodiments, the first aqueous coating composition that is foamed is applied by a non-contact application method. In some embodiments, the first aqueous coating composition that is foamed is applied by an application method selected from: roll coating, spraying, curtain coating, knife coating, slot coating, dip coating, gravure roll coating, reverse direct gravure coating, bar coating, soft tip knife coating, short dwell, and soft tip bar coating, and combinations thereof. The first aqueous coating composition that is foamed can be applied directly to a fiber-based substrate or indirectly, e.g., via a transfer roll or belt.

[0062] To minimize the risk of pinholes in the first PHA layer, the first aqueous coating composition that is foamed can be applied in at least two different coating steps, with the coated film being dried between the steps.

[0063] In some embodiments, drying comprises subjecting the foamed first aqueous coating composition to heating. In some embodiments, drying comprises subjecting the foamed first 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 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, such as using a heated belt or a heated cylinder.

[0064] In some embodiments, drying of the foamed first aqueous coating composition results in foam collapse. This means that the resulting first PHA layer preferably has a structure that is free or substantially free of air and other gas bubbles.

[0065] In some embodiments, the grammage of the first PHA layer is 0.3 - 8 g / m 2 and preferably 0.5 - 5 g / m 2 .

[0066] The formed first PHA layer is preferably characterized by a relatively low water contact angle and a relatively high melting point.

[0067] In some embodiments, the first PHA layer has a water contact angle of less than 80 degrees, preferably less than 70 degrees, as measured according to ASTM D7490 - 13 after 3 seconds. 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 measured according to ASTM D7490 - 13 after 3 seconds.

[0068] In some embodiments, as measured according to ASTM E794 - 06 (2018), the melting point of the first PHA layer is above 100 °C, preferably above 110 °C. 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 measured according to ASTM E794 - 06 (2018).

[0069] The second PHA layer is formed by applying a second aqueous coating composition comprising the 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 the second aqueous coating composition from being absorbed into the fiber-based substrate and serves to cover pinholes that may be present in the first PHA layer.

[0070] In some embodiments, the second PHA is a PHA homopolymer, i.e., a PHA in which the polymer chains are formed from a single type of monomer. Preferably, the second PHA is a PHA homopolymer with a side chain length in the range of 2 - 14 carbon atoms, and more preferably a PHA homopolymer with a side chain length in the range of 5 - 14 carbon atoms.

[0071] In some embodiments, the melting point of the second PHA is in the range of 50 - 100 °C, preferably in the range of 70 - 100 °C.

[0072] In some embodiments, the second PHA is selected from poly(3-hydroxyoctanoate) (PHO), poly(3-hydroxydecanoate) (PHD), poly(3-hydroxyhexanoate) (PHH), and poly(3-hydroxypentanoate) (PHV) or mixtures thereof, preferably selected from PHO, PHD, and PHH or mixtures thereof.

[0073] Based on the dry weight of the composition, the second PHA is preferably the main component of the second aqueous coating composition. In some embodiments, based on the total dry weight of the second aqueous coating composition, the second aqueous coating composition comprises an amount of 70 - 99.9 wt%, preferably an amount of 90 - 99.9 wt%, of the second PHA.

[0074] In some embodiments, the second aqueous coating composition does not contain added blowing agents, such as non-polymeric or polymeric surfactants.

[0075] In some embodiments, the second aqueous coating composition is degassed. Thus, in some embodiments, the second aqueous coating composition preferably does not contain or is substantially free of air and other gas bubbles.

[0076] In some embodiments, the density of the second aqueous coating composition is 0.9 g / cm 3 or higher, preferably 0.95 g / cm 3 or higher, and more preferably 0.98 g / cm 3 or higher.

[0077] The second PHA layer is preferably formed by a liquid film coating method, i.e., in the form of an aqueous dispersion of the second PHA, which spreads into a thin and uniform layer on the substrate upon application and is then dried.

[0078] 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: roll coating, spraying, curtain coating, knife coating, slot coating, dip coating, gravure roll coating, reverse direct gravure coating, bar coating, soft tip knife coating, short dwell and soft tip bar coating, and combinations thereof.

[0079] To minimize the risk of pinholes in the second PHA layer, the second aqueous coating composition can be applied in at least two different coating steps, with the coated film dried between these steps.

[0080] In some embodiments, drying comprises subjecting the second aqueous coating composition to heating. In some embodiments, 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 subjected to at least one additional drying step, which may be a hot air drying step or a contact drying step, such as using a heated belt or a heated cylinder.

[0081] In some embodiments, the basis weight of the second PHA layer is 2-30 g / m 2 , preferably 5-20 g / m 2 .

[0082] The formed second PHA layer is preferably characterized by a relatively high water contact angle and a relatively low melting point.

[0083] In some embodiments, the second PHA layer has a water contact angle of more than 90 degrees, preferably more than 100 degrees, as measured 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 measured after 3 seconds according to ASTM D7490-13.

[0084] In some embodiments, the melting point of the second PHA layer is below 100 °C, preferably above 90 °C, as measured 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 measured according to ASTM E794-06 (2018).

[0085] In some embodiments, based on the total dry weight of the aqueous coating composition, the first or second aqueous coating composition further comprises a pigment in an amount of 0.1-20 wt%. The pigment may comprise, for example, inorganic particles of talc, silicate or phyllosilicate, carbonate, alkaline earth metal carbonate and ammonium carbonate or oxides (such as transition metal oxides and other metal oxides). The pigment may also comprise nano-sized pigments, such as nanoparticles of nano-clay and layered mineral silicates, such as selected from montmorillonite, bentonite, kaolinite, lithium montmorillonite and halloysite.

[0086] In some embodiments, the second major surface is uncoated.

[0087] In some embodiments, the second major surface is coated with a 2-layer PHA coating structure as described herein with reference to the first major surface. The 2-layer PHA coating structures on the first and second major surfaces may have the same or different compositions. In other words, the coating compositions for each layer can be selected independently. In some embodiments, the second major surface is coated only with an expanded aqueous coating composition.

[0088] In some embodiments, the method further comprises:

[0089] d) forming a third PHA layer by applying an expanded third aqueous coating composition comprising a third PHA and a foaming agent on the second major surface and drying the expanded third aqueous coating composition, wherein the expanded third aqueous coating composition is a foam having a foam density of 0.75 g / cm 3 or less.

[0090] The step of forming the third PHA layer, the third aqueous coating composition and its components, and the properties of the formed third layer can be further defined as described herein with reference to the step of forming the first PHA layer.

[0091] In some embodiments, the method further comprises:

[0092] 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.

[0093] The step of forming the fourth PHA layer, the fourth aqueous coating composition and its components, and the properties of the formed fourth layer can be further defined as described herein with reference to the step of forming the second PHA layer.

[0094] The method according to the first aspect described herein allows for the preparation of an improved PHA-coated fibrous substrate. According to the second aspect shown herein, there is provided a PHA (polyhydroxyalkanoate)-coated fibrous substrate comprising:

[0095] a fibrous substrate having a first major surface and a second major surface,

[0096] a first PHA layer on the first major surface comprising a first PHA and a foaming agent,

[0097] a second PHA layer on the first PHA layer comprising a second PHA.

[0098] The PHA-coated fibrous substrate according to the second aspect described herein and its components, including the first PHA layer, the first PHA and the foaming agent, can be further defined as described with reference to the first aspect.

[0099] In some embodiments, measured at 50% relative humidity and 23 °C according to ASTM F 1249-20 standard, the water vapor transmission rate (WVTR) of the PHA-coated fibrous substrate is less than 10 g / m 2 / 24 h, preferably less than 5 g / m 2 / 24 h.

[0100] In some embodiments, measured according to ISO 535 standard, the Cobb600 value of the PHA-coated fibrous substrate is less than 15 g / m 2 , preferably less than 10 g / m 2 .

[0101] In some embodiments, as measured according to standard ASTM F119-82, the PHA-coated fibrous substrate has a grease resistance of at least 0.5 h, preferably at least 1 h.

[0102] The coated fibrous substrate provides an alternative to conventional packaging materials using polyolefin layers, which can be more easily repulped and recycled. In some embodiments, the coated fibrous substrate has a reject rate of less than 30%, preferably less than 20%, more preferably less than 10% according to PTS RH 021 / 97.

[0103] According to the third aspect described herein, there is provided a packaging container comprising a PHA-coated fibrous substrate according to the second aspect. The PHA-coated fibrous substrate is very suitable for trays, plates, bowls, cups and lids, but is also suitable for other liquid or food packaging materials. The PHA-coated fibrous substrate is particularly suitable for trays, plates and bowls formed by thermoforming, deep drawing or press forming.

[0104] It has been found that the 2-layer PHA coating structure of the PHA-coated fibrous substrate of the present invention can be used as a liquid barrier on the inner surface of food and liquid packaging containers. Therefore, in some embodiments, the first major surface coated with PHA forms the inner surface of the container.

[0105] It has also been found that the 2-layer PHA coating structure of the PHA-coated fibrous substrate of the present invention can be used as a moisture / liquid barrier on the outer surface of food and liquid packaging containers. Therefore, in some embodiments, the first major surface coated with PHA forms the outer surface of the container.

[0106] Generally, although products, polymers, materials, layers and methods are described in a manner of "comprising" various components (assemblies) or steps, products, polymers, materials, layers and methods may also "consist essentially of various components (assemblies) and steps" or "consist of various components (assemblies) and steps".

[0107] Although the present invention has been described with reference to various exemplary embodiments, those skilled in the art will understand that various changes can be made and elements thereof can be replaced with equivalents without departing from the scope of the present invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present invention without departing from the basic scope thereof. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A method for manufacturing a PHA (polyhydroxyalkanoate)-coated fibrous substrate, the method comprising the following steps: a) providing a fibrous substrate having a first major surface and a second major surface; b) forming a first PHA layer by applying a foamed first aqueous coating composition comprising a first PHA and a blowing agent on the first major 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 / cm 3 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 copolymer.

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

4. The method according to any one of the preceding claims, wherein the first PHA is selected from 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.

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

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 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, Based on the total dry weight of the foamed first aqueous coating composition, the foamed first aqueous coating composition comprises the foaming agent in an amount of 0.1-10% by weight.

10. The method according to any one of the preceding claims, wherein the foamed first aqueous coating composition further comprises a polymeric co-additive, the polymeric co-additive being preferably selected from carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC), and starch, and the amount of the polymeric co-additive is 0.1-10% by weight 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 / cm 3 or less, preferably 0.5 g / cm 3 or less.

12. The method according to any one of the preceding claims, wherein the viscosity of the foamed first aqueous coating composition is in the range of 50 mPas to 3500 mPas, measured using a Brookfield viscometer at a 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 drying of the foamed first aqueous coating composition causes collapse of the foam.

16. The method according to any one of the preceding claims, wherein the gram weight of the first PHA layer is in the range of 0.3 - 8 g / m 2 preferably in the range of 0.5 - 5 g / m 2 .

17. The method according to any one of the preceding claims, wherein the first PHA layer has a water contact angle of less than 80 degrees, preferably less than 70 degrees, measured 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 of greater than 100 °C, preferably greater than 110 °C, measured 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 to 14 carbon atoms, and more preferably a PHA homopolymer having a side chain length in the range of 5 to 14 carbon atoms.

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