A heat-sealable flexible packaging material, a method of making such and a use thereof

Abstract

A heat-sealable flexible packaging material and a method of making such, the method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, by extruding the plasticised lignin or from an aqueous dispersion of the plasticised lignin. Also disclosed is a method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, by a hot melt method; and a heat-sealable flexible packaging material produced by the same.

Description

[0001] A HEAT-SEALABLE FLEXIBLE PACKAGING MATERIAL, A METHOD OF MAKING SUCH AND A USE THEREOF

[0002] Field

[0003] The present invention relates to a heat-sealable flexible sheet material, to a method of making such, and to a use of such. The heat-sealable flexible sheet material comprises a flexible sheet material and a heat seal layer on a surface thereof, the heat seal layer comprises a lignin and a plasticiser; wherein the heat seal layer has been deposited on the flexible sheet material from an aqueous dispersion or an extrusion.

[0004] Background

[0005] Traditional plastics, derived from fossil fuels, have given rise to environmental concerns, including pollution, resource depletion, and climate change. The longevity of plastic waste in landfills and ecosystems poses threats to biodiversity and human health, necessitating a shift toward more sustainable packaging materials. Consumer demand for eco-friendly products, coupled with evolving regulations aimed at curbing plastic pollution, is causing industries to adopt more sustainable packaging materials.

[0006] Flexible packaging refers to a type of packaging that is made from materials that can easily conform to the shape of the product being packaged. Flexible packaging can be easily bent, folded, or shaped, providing versatility in design and functionality. Flexible packaging is widely used for products across different industries due to its numerous advantages. It is typically a thin sheet or film material able to be bent or folded into the desired form.

[0007] Many flexible packaging materials comprise a heat seal layer, also known as a heat-sealable layer, which is a component in packaging materials to bond or seal when subjected to heat. This layer creates secure closures for various types of packaging such as pouches, bags, and sachets. The heat seal layer is typically positioned between layers of packaging material, and when heated, it adheres to itself to another heat-sealable surface, forming a tight seal.

[0008] The choice of materials for the heat seal layer depends on the specific requirements of the packaging application, including the type of product being packaged and the desired level of seal strength. Conventionally, non-biodegradable, fossil fuel-derived polymers are used as heat seal layers, including Polyethylene (PE), Polypropylene (PP) and Polyester (PET). Consequently, conventional heat seal layers may reduce a packaging material’s biodegradability and increase the carbon dioxide emissions associated with its use. The utilization of biodegradable natural polymers (polymers that are the result of a polymerisation process that has taken place in nature) for material applications has been extensively investigated and has gained momentum due to efforts to increase sustainability. However, natural polymers are generally not suitable for use in heat seal layers as they do not have suitable thermoplastic properties. For example, many natural polymers are moisture sensitive, have low thermal stability, and are brittle, and also face barriers relating to cost, availability and processing challenges.

[0009] The present invention aims to provide improvements generally over the prior art methods and apparatus. In addition, the present invention may optionally seek to provide one or more of the following advantages: i) to provide a biodegradable and / or natural heat seal; ii) to provide a low-cost heat seal; and / or iii) to provide a heat seal having high heat seal strength.

[0010] Summary of the Invention

[0011] In the first aspect of the present disclosure, there is a method of making a heat sealable flexible packaging material, the method comprising: compounding lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, by extruding the plasticised lignin or from an aqueous dispersion of the plasticised lignin. There is also a method of making a heat-sealable flexible packaging material, the method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, wherein the plasticised lignin is applied using a hot melt method.

[0012] In a second aspect of the present disclosure, there is a heat sealable flexible packaging material, comprising a flexible substrate and a heat seal layer on a surface thereof, the heat seal layer comprising: lignin and a plasticiser; wherein the heat seal layer has been deposited on a surface of the flexible substrate from an aqueous dispersion or extrusion. There is also a heat sealable flexible packaging material, comprising a flexible substrate and a heat seal layer on a surface thereof, the heat seal layer comprising: lignin and a plasticiser; wherein the heat seal layer has been deposited on a surface of the flexible substrate by a hot melt method.

[0013] In a third aspect of the present disclosure, there is provided the use of a heat sealable flexible packaging material according to the second aspect. The use may be for sealing a portion of the heat sealable flexible packaging material to a portion of further packaging material with the application of heat and pressure.

[0014] Lignin is a naturally occurring biopolymer which forms the support structure in plants. It can be readily broken down by microorganisms meaning it does not contribute to plastic pollution. Lignin is also a waste material from the paper industry, where it is often burned as a heat source. This means the heat seal of the present invention is made from low-cost components.

[0015] The present disclosure does not modify the lignin or react it with agents to form an adhesive, instead, the heat seal layer of the present invention is a lignin-based layer with thermoplastic characteristics. By not modifying the lignin or reacting it with agents, the biodegradability is improved. Moreover, by not modifying the lignin or reacting it with agents, the product can be reasonably priced to be competitive in the high-volume market of heat seal layers. Further, by maintaining lignin’s natural molecular structure, its feasibility for natural bioconversion can be assured (even when considering present and potential future legislations / certifications).

[0016] Brief Description of the Figures

[0017] Figure 1 shows sealing measurements according to ASTM F88 for plasticised lignin 1 (TL26) and plasticised lignin 2 (TL30), before (dry powder) and after dispersion on standard paper.

[0018] Figure 2A shows sealing force for several plasticised lignin formulations using a range of different plasticisers. All samples were compounded on a heat plate, except for those marked F (= filament extruder). A heat seal above 1 N / 15mm is considered a suitable heat seal strength. Further details of the formulations used in Figure 2A are provided in Figure 2B.

[0019] Figure 3 is a 3D plot showing the range of sealing parameters (sealing temperature (°C), dwell time (s) and sealing pressure (kPa)) for plasticised lignin 1 (TL26) and plasticised lignin 2 (TL30).

[0020] Figure 4 provides the sealing force generated under varying seal parameters for lignin formulation TL26 as compared to benchmark PE, on a 50 gsm paper (PAP030).

[0021] Figure 5 shows a differential scanning calorimetry (DSC) graph for two formulations, plasticised lignin 1 (TL26) and plasticised lignin 2 (TL30), compared to pure lignin (TO). For both formulations, sample mass has been indicated, and glass transition temperatures were calculated, as shown in the Figure. Figure 6 shows a melt flow test measurement (MVR) for plasticised lignin 1 (TL26) across several replicates.

[0022] Figure 7 shows a particle size distribution analysis, demonstrating particle size before (A) and after (B) optimization.

[0023] Figure 8 shows a viscosity analysis of the dispersion before (A) and after (B) the addition of an antifoaming agent.

[0024] Figure 9 shows the chemical structure of common lignin monomers.

[0025] Detailed Description of the Invention

[0026] An object of the invention is to identify improved naturally occurring biopolymers for use in a heat seal layer, e.g. for use in a heat sealable flexible packaging material, that provides a sustainable product which overcomes existing barriers relating to the use of natural polymers in material applications and which also represents a drop-in solution to existing industry processes. The inventors have identified a novel biopolymer compound with thermoplastic properties capable of forming a heat seal layer, which provides a holistically sustainable, economically and functionally competitive drop-in solution for existing industrial infrastructure.

[0027] Flexible packaging

[0028] Flexible packaging may refer to packaging made of non-rigid materials. Flexible packaging may be considered any packaging material or part of a package whose shape can readily be changed during conversion into a packaging product when filled or during use. Flexible packaging is different to rigid packaging which is a preshaped product that has sufficient rigidity to hold a three-dimensional shape.

[0029] Flexible packaging may typically have a bending resistance of from 1 to 95 mNm (CD), and / or 3 to 130 mNm (MD), or from 3 to 50 (CD), and / or from 7.5 to 75 (MD), or any range made from any of these endpoints. Bending resistance may be determined by ISO 2493.

[0030] Flexible packaging may predominantly be thin and planar in form i.e. before conversion to a three-dimensional packaging product. Flexible packaging may have a thickness from 0.01 mm to 0.5 mm, or from 0.05 mm to 0.25 mm, or from 0.07 to 0.015 mm or any range made from any of these endpoints.

[0031] The flexible packaging may be a multilayered material. The flexible packaging may comprise a flexible substrate, and any number of barrier layers, metallised layers, tie layers, primer layers, print layers, heat seal layers, and / or over-print varnish layers, amongst others.

[0032] The heat-sealable flexible packaging material is a flexible packaging material comprising a heat-seal layer. The heat seal layer is typically a layer that exhibits thermoplastic properties which enables the flexible packaging material to be bonded to itself or further packaging material with the application of heat and pressure.

[0033] The heat-sealable flexible packaging material of the present disclosure may meet one or more of the following compostability standards: EN 13432, ASTM D6400. The heat-sealable flexible packaging material of the present disclosure may meet one or more of the following Biodegradability standards: ASTM D6691-09 (marine), ASTM D5988-12 (soil). The heat- sealable flexible packaging material of the present disclosure may meet one or more of the following Recycling standards CEPI (Confederation of European Paper Industries) Recyclability Test Method Version 2.

[0034] Biopolymer

[0035] Thermoplasticity is an important feature for polymers used in packaging applications to ensure processability and effectiveness, particularly for use as heat-seal adhesives.

[0036] As noted above, natural polymer-based technologies are generally not suited as thermoplastic materials. Firstly, the hydrophilic nature of many natural polymers makes them sensitive to moisture, which can cause swelling and loss of mechanical stability, limiting their real life applications in environments of varying humidity (e.g. in the packaging industry). Moreover, their hydrophilic nature does not allow the use of natural polymers for applications in dispersion systems, which are required in some industrial coating processes to achieve low coating thicknesses. Further, many natural polymers exhibit low thermal stability, which restricts their processing temperatures and limits their use in high-temperature applications, such as in a heat seal layer. Additionally, the production of natural polymers and their processing usually is more expensive compared to conventional petroleum-based thermoplastics, affecting their market competitiveness, which is particularly important for high volume applications in the packaging sector. Moreover, natural polymers are often inherently brittle, which compromises the mechanical performance of thermoplastic materials and makes these less suitable as an adhesive. Finally, the processing of natural polymers into thermoplastics usually requires specific additives (such as plasticisers) to achieve desired properties.

[0037] Lignin is a naturally occurring biopolymer (also referred to herein as a natural polymer) which forms the support structure in many plants. It can be readily broken down by microorganisms meaning it does not contribute to plastic pollution. Lignin is also a waste material from the paper industry, where it is often burned as a heat source and is therefore readily available from low-cost components.

[0038] The inventors have discovered that lignin can address several limitations of using natural polymers for processing into thermoplastic materials as it is water insoluble, thermally stable available at low cost and is readily available. Thus, in some embodiments, the lignin is water insoluble.

[0039] Lignin, a heterogeneous material with up to three phenyl propane monomers, varies based on the source. Softwood lignins are mainly coniferyl alcohol-based, hardwood lignins have a higher syringyl alcohol content, while grass lignins have a higher paracoumaryl alcohol content.

[0040] The lignin may derive from softwoods, hardwoods, and perennial plants (including grasses amongst others). In some preferred embodiments, the lignin may comprise lignin derived from perennial plants or annual plants, including but not limited to grasses, wheat straw and bagasse. Optionally this may be lignin derived from grasses, optionally miscanthus grasses, optionally from Miscanthus giganteus. In some embodiments, the lignin may have a paracoumaryl alcohol content of greaterthan 5 %, or 10 %, or 15 %, or 20 %, or 25%, or 30%, or 35%. In some embodiments, the lignin may have a paracoumaryl alcohol content not exceeding 50%, or 45%, or 40%, or 35%, or 30% or 25% or 20%. The inventor of the present disclosure has found that lignin derived from grasses, and / or lignin with a high paracoumaryl alcohol content may result in further improved heat seal properties. The lignin may comprise a blend of lignin from different sources. Preferably a blend of lignin will have an overall paracoumaryl alcohol content as stated above or comprise predominantly lignin from grasses.

[0041] Market-available technical lignins fall into four main groups, lignosulphate lignin, kraft lignin, organosolv lignin and soda lignin. A fifth group, Biorefinery lignin, differs in origin, linked to biorefining processes. In some embodiments, the lignin may comprise any of: biorefinery lignin, lignosulphate lignin, kraft lignin, organosolv lignin or soda lignin.

[0042] Lignosulphonates account for a large share of the total market of commercial lignin. Lignosulphonates are water-soluble and therefore may be unsuitable for aqueous dispersion of packaging materials. Thus, in some embodiments, the lignin is not derived from lignosulphonate when the plasticised lignin is applied by aqueous dispersion.

[0043] Without being bound by theory, a polymer material suitable for applying as a heat-seal layer has a certain rigidity and inflexibility to hold / withstand a certain load, but should not be brittle. Rigidity is also advantageous for the dispersion process, so that the lignin can be milled into particles small enough to be able to be dispersed. This target mechanical behaviour was tested by the inventors by observation of samples and mechanical stress testing by hand. The skilled person will also be familiar with the use of elastic modulus following injection moulding or heat press to assess these properties. By further testing lignins on heat-sealing standard paper and conducting seal strength measurements (according to ASTM F88), the inventors were able to identify highly suitable lignins for use in the context of the invention. In particular, lignin produced from the soda pulping process provided considerable tensile strength values above 1 N / 15mm, and was identified as being suitable for dispersion.

[0044] Thus, in some preferred embodiments, the lignin comprises lignin from the soda pulping process, i.e. soda lignin. The lignin may comprise at least 20%, at least 40%, at least 80%, at least 90%, at least 95%, at least 99%, or comprise entirely of soda lignin. The inventor of the present disclosure has found that soda lignin may result in further improved heat seal properties.

[0045] It is known in the context of polymer chemistry that the linearity and degree of crosslinking significantly influence the thermoplasticity of polymers. Higher crosslinking density typically results in thermosetting instead of thermoplastic behaviour. Conversely, lower crosslinking allows for more flexibility and thermoplastic characteristics. The arrangement of polymer chains (linear vs. branched) affects the degree of entanglement and mobility. Linear polymers tend to exhibit better thermoplastic properties due to their ability to flow upon heating. The highly cross-linked structure of commonly used lignins makes it a very brittle material, even if it is plasticised to become thermoplastic, which limits its utilization as stand-alone material.

[0046] Thus, without being bound by theory, the advantages of soda lignin may be associated with its linear structure. Thus, in some embodiments, the lignin has a linear molecular structure. In some embodiments, the lignin does not have a cross-linked molecular structure. The skilled person will be familiar with methods for assessing the degree of cross-linking in a molecular structure, for example, by use of C13 NMR analysis.

[0047] The inventors have discovered that the thermoplastic properties of lignin may be influenced by the feedstock and extraction method used for producing the lignin. Without being bound by theory, temperature and monomer identity may influence lignin thermoplasticity / crosslinking properties. For example, lignin derived from grasses show the highest content of the monomer p-hydroxyphenol, which can only create linear polymers (see Figure 9). In contrast, hardwood contains a high proportion of Sinapyl monomers, which comprises four functional groups and allows for two potential cross-linking interactions per monomer. Thus, in some embodiments, the lignin (e.g. soda lignin) derived is from grasses, optionally miscanthus grasses or wheat straw. In other embodiments, the lignin (e.g. soda lignin) is derived from mixed biomass (e.g. a mixture of wood waste and wheat straw). Advantageously, lignin produced from grasses is highly sustainable as they remove large amounts of carbon dioxide from the environment on an annual basis.

[0048] The inventors have discovered additional factors which may influence the suitability of a lignin for use in the invention. In particular, a higher molecular weight of the lignin (e.g. soda lignin) is associated with increased functionality. Without being bound by theory, higher molecular weight lignin is associated with lower melt flow and associated with higher mechanical stability, which is required for higher seal strength. In contrast, a lower molecular weight prevents polymer chain entanglement, causing lower mechanical stability and increased brittleness, yielding materials with low strength. Thus, in some embodiments, the lignin (e.g. soda lignin) may have weight average molecular weight of from 2000 to 12000 Daltons, or from 4000 to 10000 Daltons, or from 5000 to 8000 Daltons, or from 6000 to 7000 Daltons. In a preferred embodiment, the lignin (e.g. soda lignin) may have a weight average molecular weight of greater than about 5000 Daltons. In some embodiments, isolation of lignin with suitable molecular weight may be achieved by fractionation of the lignin sample by molecular weight (for example, by ultrafiltration). In some embodiments, at least 50% (e.g. at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, 99.5%, 99.9% or 100%) of the lignin (e.g. soda lignin) has a molecular weight of greater than about 5000 Daltons. In some embodiments, at least 70% (e.g. at least 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, 99.5%, 99.9% or 100%) of the lignin (e.g. soda lignin) has a molecular weight of greater than about 5000 Daltons. In some embodiments, at least 90% (e.g. at least 90%, 95%, 98%, 99%, 99.5%, 99.9% or 100%) of the lignin (e.g. soda lignin) has a molecular weight of greater than about 5000 Daltons. Without being bound by theory, certain pulping processes tend to produce low molecular weight lignin. For example, Kraft and Organosolve pulping processes tend to result in lignin with a molecular weight below 5000 Da. Thus, in some embodiments, the lignin is not derived from Kraft or Organosolve pulping processes.

[0049] Further, the inventors have discovered that the extraction temperature during the production of lignin (e.g. soda lignin) may influence the functionality of the lignin for use in the invention. In particular, decreasing the extraction temperature to 120°C or below is associated with decreased functionality. Without being bound by theory, it is understood that the polymers extracted at low temperatures do not show thermoplastic behaviour. Thus, in some embodiments, the extraction temperature for the production of the lignin (e.g. soda lignin) is greater than 120°C. In some embodiments, the extraction temperature is greater than 125°C, greater than 130°C, greater than 140°C, greater than 150°C, greater than 160°C, greater than 170°C, greater than 180°C, or greater than 190°C. In some embodiments, the extraction temperature is between 120°C and 200°C, or more preferably, between 120°C and 180°C. In a preferred embodiment, the extraction temperature is about 160°C.

[0050] The lignin may have a particle size of 40 / zm or less, or 30 / zm or less, or 20 / zm or less, or 10 / zm or less. Optionally the lignin may comprise a particle size greater than 1 / zm, or 5 / zm, or 10 / zm. In some embodiments, the lignin may have a particle size of about 7 / zm. This may be measured by passing the particles through sieves of known hole sizes. Smaller particle sizes may be measured using laser diffraction or dynamic light scattering.

[0051] Plasticiser

[0052] Known plasticisers include polyols, such as glycerol (and deep eutectic solvents derived thereof) or sorbitol and other sugars, as well as water; natural oils, such as olive oil and soybean oil, their fatty acids and derivatives thereof; and ionic liquids, such as 1 -Butyl-3- methylimidazolium acetate.

[0053] The plasticiser of the invention may comprise one or more selected from the group of polyols (including glycerol, glycerol-based deep eutectic solvent, sorbitol, propanediol, polypropylene glycol 4000 and polyethylene glycols, including PEG 400 or PEG 4000), polyethylene glycol ethers, polyethers, glycol esters (including diethyleneglycol dibenzoate and di(propylene glycol) dibenzoate), oil derivatives (including epoxidized soybean oil), fatty acids and / or derivatives (including undecanoic acid, lauric acid, oleic acid, ricinoleic acid and 1 ,2- hydroxystearic acid), hydrogenated sugars, phthalates and / or acids (including carboxylic acids such as adipic acid, vanillic acid and lactic acid; and dicarboxylic acids such as pimelic acid, sebacic acid and azelaic acid), keto acids and / or derivatives (including lactic acid, levulinic acid, and methyl levulinic acid), parabens (including Ethyl Paraben, Butyl paraben, and Propyl paraben), aromatic acids (including 4-phenyl butyric acid, mandelic acid and phenylpropionic acid), phenolic aldehydes (including 3-ethoxy-4-hydroxy-benzaldehyde and Vanillin), aromatic ketones (including 4'-hydroxy-3'-methoxyacetophenone), acrylic polymers, polyvinyl alcohol, polyurethane dispersions, ethylene carbonate, propylene carbonate, lactones (including gamma-Valerolactone), lactams, lactides, acrylic-based polymers, carbonates, including cyclic carbonate esters (such as ethylene carbonate and propylene carbonate), carbonic acid esters (including dimethyl adipate and dimetyl succinate), benzoate esters (including methyl benzoate), lactones, lactams, lactides, compounds with a structure similarto lignin like vanilin or vanilic acid, gum resin, acetosyringone and / or solvents used as coalescing agents like alcohol ethers or polyvinyl alcohol. In some embodiments, the plasticiser of the invention comprises more than one plasticiser.

[0054] Without being bound by theory, certain plasticisers are hygroscopic in nature ( / .e., moisture sensitive), which consequently affects their performance in humid conditions by ‘attracting’ water that can act as an additional plasticizer, thus decreasing the mechanical stability of the material. Some plasticisers (e.g. polyols)usually suffer from plasticiser migration, which is also preferably avoided. Further, certain plasticisers suffer from limited performance, toxicity, and high costs. One approach to address these limitations is internal plasticisation by chemical modification of the polymer, usually by the covalent binding of the plasticiser to the polymer (for example by esterification reaction). However, such an approach induces extra costs to the process and the final material is no longer considered natural, which affects biodegradation and green certification.

[0055] The inventors have discovered that the use of plasticisers which are water soluble and / or have high polarity, when combined with polymers which are applied to a flexible substrate by dispersion, tends to exacerbate plasticiser migration and results in reduced functionality in the final product. The inventor of the present disclosure has found that carboxylic acid plasticisers, in particular, dicarboxylic acid plasticisers, result in better heat seal properties when used in an aqueous dispersion.

[0056] Thus, in some embodiments, the plasticiser is not water soluble. In some embodiments, the plasticiser is not hygroscopic. In some embodiments, the plasticiser does not suffer from migration. In some embodiments, the plasticiser is suitable for aqueous dispersion (e.g. not water soluble). The skilled person will be familiar with plasticisers which are not water soluble, are hygroscopic, suffer from migration, and / or are suitable for aqueous dispersion (for example, plasticisers which are not water soluble), and suitable methods for detecting same. Examples of such plasticisers are set out in Table 1 .

[0057] Without being bound by theory, plasticiser migration may be associated with weak hydrogen bonding. Thus, particularly suitable plasticisers for use in the invention may be capable of forming strong hydrogen bonds (e.g., hydrogen bond formation between a lignin hydroxyl group and a plasticiser carboxyl group); and / or formation of multiple hydrogen bonds (e.g., presence of hydroxy, carboxyl or carbonyl groups) to create a network of plasticizer molecules that facilitates more effective plasticisation. Thus, in some embodiments, the plasticiser comprises a hydroxyl group and a carboxyl or carbonyl group. In some embodiments, the plasticiser comprises a hydroxyl group. In some embodiments, the plasticiser comprises a carboxyl group. In some embodiments, the plasticiser comprises a carbonyl group. In some embodiments, the plasticiser comprises a hydroxyl group and a carboxyl group. In some embodiments, the plasticiser comprises a carboxyl group and a carbonyl group. In some embodiments, the plasticiser comprises a hydroxyl group and a carbonyl group. Fourier transform infrared (FTIR) spectroscopy is one suitable method for identifying hydrogen bond formation between plasticiser molecules and / or between a plasticiser and a lignin.

[0058] Migration may also be dictated by the state of aggregation (melting point) and / or evaporation pressure. For example, without being bound by theory, liquid or evaporating plasticisers tend to migrate more than solids do. Thus, in some embodiments, the plasticiser is a solid plasticiser (e.g. solid at normal operating temperatures, e.g. up to 60°C). The inventors have also identified that soda lignin denatures (e.g. formation of cross-linking induced by condensation reactions) if exposed to temperatures greater than approximately 120°C during compounding. The melting point of the plasticiser may therefore be paired accordingly. Thus, in some embodiments, the melting point of the plasticiser does not exceed 190°C, or 170°C, or 150°C, or 130°C, or 120°C, or 100°C. In some preferred embodiments, the melting point of the plasticiser does not exceed 120°C.

[0059] In some embodiments, the plasticiser has a similar solubility to the polymer ( / .e. lignin). Advantageously, this may result in a strong plasticiser (low Tg value). In some embodiments, the solubility is measured by d value (cm2 / s). The skilled person is capable of identifying solubility parameters of a given plasticiser and polymer. The inventors screened for plasticisers which are suitable for use in the invention (Table 1). In some embodiments, the plasticiser may be selected from one or more of a polyol, dicarboxylic acid, a keto acid or derivative thereof, a paraben, an aromatic acid, a phenolic aldehyde, an aromatic ketone, a lactone, a cyclic carbonate ester, a carbonic acid ester, and / or a gum resin. In some preferred embodiments, the plasticiser may be selected from one or more of a dicarboxylic acid, a paraben, an aromatic acid, a phenolic aldehyde, a carbonic acid ester and / or a gum resin. The inventors have discovered that these plasticiser classes are particularly advantageous as they are capable of plasticising lignin, do not suffer from migration, are not hygroscopic, and are suitable for aqueous dispersion. However, the skilled person will be aware that other plasticisers with similar properties may also be suitable for use in the context of the invention. Similarly, the skilled person would understand that plasticisers which plasticise lignin but which are not suitable for dispersion, are hygroscopic, and / or suffer from migration, may in some embodiments still be useful in the context of the invention.

[0060] In some embodiments, the plasticiser may comprise an acid. The plasticiser may optionally comprise a dicarboxylic acid. The plasticiser may comprise a saturated dicarboxylic acid. The plasticiser may comprise an acid with the structure HOOC(CH2)xCOOH where X is 1-50, or 3-30, or 4-15, or 5-10, or 6-8, or 7. In some embodiments, the dicarboxylic acid comprises pimelic acid, sebacic acid, and / or azelaic acid. Typically, the plasticiser comprises azelaic acid. Thus, in some embodiments, the plasticised lignin comprises lignin (e.g. soda lignin) in combination with a dicarboxylic acid (e.g. azelaic acid). In some embodiments, the plasticised lignin comprises lignin (e.g. soda lignin) with a molecular weight of at least 5000 Da in combination with a dicarboxylic acid (e.g. azelaic acid).

[0061] In some embodiments, the plasticiser may comprise an aldehyde.

[0062] In some embodiments, the polyol comprises one or more of glycerol, glycerol-based deep eutectic solvent, sorbitol, propanediol, PEG 400 and / or PEG 4000. In some embodiments, the keto acids or keto acid derivative comprises one or more of lactic acid, levulinic acid and / or methyl levulinic acid. In some embodiments, the paraben comprises one or more of ethyl paraben, butyl paraben and / or propyl paraben. In some embodiments, the aromatic acid comprises one or more of 4-phenyl butyric acid, mandelic acid, and / or phenylpropionic acid. In some embodiments, the phenolic aldehyde comprises one or more of 3-ethoxy-4-hydroxy- benzaldehyde and / or vanillin. In some embodiments, the aromatic ketone comprises 4'- hydroxy-3'-methoxyacetophenone. In some embodiments, the lactone comprises gamma- Valerolactone. In some embodiments, the cyclic carbonate ester comprises one or more of propylene carbonate and / or ethylene carbonate. In some embodiments, the carbonic acid ester comprises dimethyl adipate.

[0063] In some embodiments, the plasticiser does not comprise one or more of a glycol ester (such as diethyleneglycol dibenzoate and / or di(propylene glycol) dibenzoate), an oil derivative (such as epoxidized soybean oil), a fatty acid or fatty acid derivative (such as undecanoic acid, lauric acid, oleic acid, ricinoleic acid, and / or 1 ,2-hydroxystearic acid), and / or a benzoate ester (such as methyl benzoate).

[0064] The plasticiser may be non-reactive and may not form covalent bonds with the lignin. The plasticiser may be hydrophilic and / or may be polar. The plasticiser may be miscible with water and optionally soluble in water.

[0065] Flexible substrate

[0066] The flexible substrate may be any flexible substrate commonly used in flexible packaging. This may include polymeric films or cellulosic substrates. Preferably the flexible substrate is a cellulosic substrate. A cellulosic substrate may be any substrate that predominantly comprises cellulose. The cellulosic substrate may comprise paper and / or paper board. The basis weight of the cellulosic substrate may range from 20 g / m2to 1000 g / m2. The higher end of this range may be deemed paper board for the purposes of this present disclosure, i.e. a cellulosic substrate in the thickness range of 200 g / m2to 1000 g / m2. A basis weight from 20 g / m2to 200 g / m2may be deemed paper.

[0067] In some embodiments, the basis weight of the flexible substrate may be from about 20g / m2to 200 g / m2, or from about 40 g / m2to about 120 g / m2, or from about 50 g / m2to about 100 g / m2, or from about 60 g / m2to 85 g / m2. Or a range formed from any combination of these endpoints. The basis weight may also be referred to as areal weight or grammage and is a measure of mass per unit area. Basis weight may be determined according to ISO 536. The thickness of a substrate may be determined using ISO 534. Unless specified the basis weight will be determined in ambient conditions, after drying if necessary.

[0068] A paper substrate may comprise kraft paper, uncoated paper, blotting paper, bond paper, gloss paper, copy paper, matte paper, silk paper or book paper. A paper substrate may comprise virgin and recycled fibre, grass paper, or cardboard. A paper board substrate may comprise a coated unbleached kraft board, solid bleached sulphate board, clay-coated news backboard or folding box board.

[0069] Pre-coating of the flexible substrate is a common step in material applications. The inventors have surprisingly demonstrated that the use of lignin (e.g. plasticised lignin) as a heat seal layer does not require pre-coating of the paper substrate to yield high seal strength. Without being bound by theory, it is considered that pre-coating is not necessary due to the natural compatibility between the cellulose in the paper and the lignin. Thus, in some embodiments, the flexible substrate (e.g. paper substrate) is not pre-coated prior to application of the plasticised lignin.

[0070] Plasticised lignin

[0071] The lignin (e.g. soda lignin) and plasticiser may be compounded to form the plasticised lignin. This may occur prior to dispersing into an aqueous dispersion or extrusion. The inventor has found compounding may improve the interaction between the plasticiser and lignin prior to dispersion, resulting in an improved heat seal layer after drying. Compounding may comprise the mixing of the plasticiser and lignin under heat and / or pressure and shear forces. Compounding may convert the lignin and plasticiser to a single phase. Compounding may occur in a heater chamber and the mixing may comprise, amongst others, mixers, rollers, kneaders or screws. Optionally compounding may be performed by twin-kneading apparatus or twin-screw apparatus. In some embodiments, compounding may be performed on a heat plate. In some embodiments, compounding may be performed by filament extruder. The kneaders or screws may be co- or counter-rotating. Compounding may occur at a temperature of at least 120°C, or at least 110°C, or at least 100°C, or at least 90°C, or at least 70°C or at least 50°C. Compounding may occur at a temperature of not exceeding 190°C, or 170°C, or 150°C or 130°C or 120°C or 100°C. The inventors have identified that soda lignin denatures (e.g. formation of cross-linking induced by condensation reactions) if exposed to temperatures greater than approximately 120°C during compounding. Thus, in some preferred embodiments, compounding may occur at a temperature not exceeding 120°C. The compounded lignin and plasticiser may be extruded following compounding or as part of the compounding process. Extrusion may be defined as pushing the plasticised lignin through a die. The plasticised lignin may be formed into pellets or elongate strings.

[0072] During compounding, water may be present in the lignin or added to decrease the viscosity of the lignin. Typically 0-15%, or 1-12%, or 5-10%, or 0-5% of water to lignin by weight may be present before compounding, or any range formed from any of these endpoints. Optionally the lignin may be dried to reduce water content prior to compounding.

[0073] The compounded lignin and plasticiser form a plasticised lignin. The plasticised lignin may be a lignin that exhibits thermoplastic properties. For example, it may be a rigid and hard material below a Tg (glass transition temperature) and above that temperature it may be rubbery, viscous and / or flowable. The transition between these two states may be flowable. Thus a plasticised lignin may also be capable of being extruded.

[0074] A plasticised lignin may have a Tg of at least 120 °C, or at least 110 °C, or at least 100 °C, or at least 90 °C or at least 80 °C, or at least 60 °C. The plasticised lignin may have a Tg not exceeding 120 °C, or 130 °C, or 140 °C, or 160 °C or 180 °C.

[0075] The inventors have identified that high lignin content may contribute to the functionality of the heat seal layer. Thus, the percentage by mass (wt. %) of the lignin in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be from 60 wt. % to 95 wt. % or from 70 wt. % to 90 % or from 75 wt. % to 85 wt. %. In some embodiments, the lignin comprises from about 70% to about 99% concentration by weight of the compound. In some embodiments, the lignin comprises greater than about 70%, greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, or greater than about 95%. The plasticised lignin may comprise a ratio of lignin to plasticiser from 3:5 to 19:20, or from 7:10 to 9:10, or from 15:20 to 17:20. The plasticised lignin may comprise only lignin and a plasticiser.

[0076] Thus, the percentage by mass (wt. %) of the plasticiser in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be from about 1 wt. % to about 30 wt. % or from 5 wt. % to about 30 wt. % or from about 10 wt. % to about 30% or from about 15 wt. % to about 25 wt. %. In some embodiments, the plasticiser in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be about 20 wt %. In some embodiments, the plasticiser in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be about 10 wt. %. In some embodiments, the plasticiser in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be about 15 wt. %. In some embodiments, the plasticiser in the compounded lignin and plasticiser ( / .e. the plasticised lignin) may be about 25 wt. %.

[0077] The plasticised lignin may be applied to the surface of the flexible substrate. The surface of the flexible substrate may refer to a planar surface on a sheet of flexible substrate. The plasticised lignin may be applied to one or both surfaces of the flexible substrate. The plasticised lignin may be applied directly to the flexible substrate or may be applied onto a layer previously deposited on the flexible substrate.

[0078] In some preferred embodiments, the plasticised lignin may be applied to the surface of the flexible substrate by an aqueous dispersion of the plasticised lignin. Alternatively, the plasticised lignin may be applied directly, or by extruding the plasticised lignin onto the surface of the flexible substrate.

[0079] The method may comprise heating and extruding the plasticised lignin to apply a layer of the plasticised lignin to the surface of the flexible substrate to form a heat seal layer.

[0080] The heat seal layer when the plasticised lignin has been extruded onto the surface of the flexible packaging may have a thickness of from 5 to 50 / / m, or from 7 to 40 / zm or from 10 to 30 / / m, or 12 to 20 / / m, or any range formed from any of these endpoints. The thickness of the layer may be measured after cooling to 23°C (ambient temperature). Measurement may be done by deducting the known thickness of the substrate and any preceding layers. Or it may be measured from a section of the packaging material with microscopy.

[0081] The plasticised lignin may be extruded by any known appropriate extrusion method, in particular one that produces sheets appropriate for direct application to packaging materials. Extrusion may comprise guiding thermoplastic material under heat and / or pressure through a die with a hole. In particular, the plasticised lignin may be extruded via slot die extrusion. Alternatively, the plasticised lignin may be subject to blown film extrusion, calendering, injection moulding, compression moulding or solvent casting, amongst other known hot melt or thermoforming methods e.g. a hot melt roller coater.

[0082] The application of the plasticised lignin to a surface of the flexible substrate by an aqueous dispersion of the plasticised lignin may comprise dispersing the plasticised lignin in an aqueous liquid to form the aqueous dispersion. In some embodiments, the plasticised lignin may be dispersed in water and no other solvent to form the aqueous dispersion. No other solvent may refer to an absence in the water of organic solvents, alcohols, ketones ammonia, polar aprotic solvents, including but not limited to dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP), Tetrahydrofuran (THF) and other commonly used solvents. Thus, the aqueous dispersion may be free from any non-aqueous solvents. Specifically, the dispersion may be free of ammonia, ammonia hydroxide and formaldehydes. The lignin may be insoluble in the aqueous dispersion. The aqueous liquid into which the plasticised lignin is dispersed may be weakly acidic, weakly alkaline or neutral before the addition of the lignin. Preferably the aqueous liquid has a pH from 5 to 9, or 6 to 8, or 6.5 to 7.5, or 6.75 to 7.25, or 7.1 to 7.25, or 6.9 to 6.75, or is 7, or any range formed from any combination of any of these endpoints. The plasticised lignin may be added to an aqueous liquid at ambient temperature (optionally 23°C). The aqueous dispersion may not undergo any heating.

[0083] Without being bound by theory, the application of plasticised lignin by dispersion can increase the seal performance due to optimal wetting of the fibres. In some cases, this may be preferable to the uniform film which is produced by extrusion coating. The application of plasticised lignin by dispersion may also advantageously improve heat seal properties because the active surface area of the heat seal layer is increased, when compared to a film produced from application by extrusion coating. The increase in surface area may result in further improved heat seal performance due to faster thermal softening and increased heat seal strength.

[0084] The aqueous dispersion may be applied to the flexible substrate by any of the following, nonlimiting exemplary methods: gravure printing, flexographic printing, offset printing, inkjet printing, Mayer bar coating, air knife coating, reverse roll coating, size press, curtain coating or dip coating.

[0085] The presence of a layer of plasticised lignin that has been deposited via a dispersion to a flexible substrate can be determined with a microscopy examination. Particles of the milled plasticised lignin can be seen under a microscope after evaporation of the aqueous liquid. The size of the particles visible under microscopy examination accords with the milled size of the plasticised lignin. Because dispersions of the present invention retain the lignin as a dispersion and do not dissolve the lignin into a solution, particles are retained in the heat seal layer, rather than presenting as a uniform film as would occur from a solution. This may advantageously improve heat seal properties because the particles may increase the surface area of the heat seal layer compared to a film deposited from a solution. The increase in surface area may result in further improved heat seal performance from faster thermal softening and increased heat seal strength. Thus, the plasticised layer may not be formed as a uniform film after drying of the dispersion but as a layer of particles.

[0086] The method may comprise the step of drying the aqueous dispersion. Drying may take place once the aqueous dispersion has been applied to the surface of the flexible substrate. Drying may be performed using air drying, infrared drying, hot air drying, (microwave drying, rIR radiation, a heated air source, and microwave radiation, amongst others. Drying may be performed by directing the heat source at the surface of the flexible substrate with the aqueous dispersion on, or on the reverse side of the flexible substrate, or both.

[0087] After a first layer of plasticised lignin has been applied to the flexible substrate and dried, subsequent layers may be applied using the same process. Thus, a method of making a heat- sealable flexible packaging material may comprise applying and drying two, three, or more layers of an aqueous dispersion to form the heat-seal layer. Consequently, the heat-sealable flexible packaging material may comprise a bulk layer of plasticised lignin that has been deposited from multiple individual layers of an aqueous dispersion. Individual layers may be visible in the bulk layer by microscopy inspection of a section through the packaging material.

[0088] The aqueous dispersion may comprise at least 10 wt.%, or at least 20 wt.%, or at least 30 wt.%, or at least 40 wt.% or at least 50 wt.%, or at least 60 wt.%, or at least 70 wt.%, or at least 80 wt.% of lignin and plasticiser ( / .e. the plasticised lignin).

[0089] The aqueous dispersion may have a pH of from 4 to 10, or from 5-9, or from 6-8 or any range formed from any combination of any of these endpoints. The aqueous dispersion may have a pH of from 5 to 10, or from 6-9, or from 7-8 or any range formed from any combination of any of these endpoints. A basic substance may be added to the aqueous liquid to neutralise it. Basic substances may include, amongst others oxides, for example, metal hydroxides such as sodium hydroxide, potassium hydroxide, aluminium hydroxide and magnesium oxide. Strongly acidic or alkaline aqueous dispersions may increase wear on the apparatus for handling the dispersion. The dispersion may comprise a basic substance at from 0.01 to 10 wt.%, or from 0.05 to 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0090] The plasticised lignin may be converted into micron-scale particles for dispersing in an aqueous liquid. The compounded lignin and plasticiser, i.e. the plasticised lignin, may be milled, to produce small particles. This may produce a particulate from the compounded plasticised lignin suitable for dispersion. Milling may take place after compounding / extruding. Milling may comprise any mechanical method where solid polymer material is subjected to grinding or shearing forces to break it into micron-sized particles. Milling may produce micronsized particles of the plasticised lignin of not exceeding 100 / / m, or 80 / / m, or 60 / / m, or 40 / / m, or 30 / / m, or 20 / / m, or 10 / / m. Milling may produce micron-sized particles of the plasticised lignin greater than 0.1 / / m, or 0.5 / / m, or 1 / / m, or 2 / / m, or 5 / / m, or 10 / / m, or 15 / / m, or 20 / / m. Preferably milling of the plasticised lignin produces particles greater than 2 / zm and not exceeding 40 / / m. In some particularly preferred embodiments, the plasticised lignin produces particles greater than 2 / zm and not exceeding 15 / zm (e.g., not exceeding 10 / / m, or even more preferably, not exceeding 7 / zm). The size of a milled particle may be determined by passing the particles through sieves of known spacing sizes. Smaller particle sizes may be determined by laser diffraction or dynamic light scattering.

[0091] The inventors further optimized the milling process to reduce the average particle size to provide a more stable dispersion. This particle size enables the use of usual printing technologies to be applied for dispersion coating on paper substrates. In particular, the inventors surprisingly found that the optimisation of the milling process may reduce the average particle size to a desired level.

[0092] Milling may be performed by any appropriate milling apparatus capable of achieving the desired particle size. Milling may be performed by, amongst others, ball milling, basket milling, hammer milling, jet milling / fluid energy milling, attrition milling, roller milling, colloid milling, cone milling, vibratory milling, knife milling, pin milling, cryogenic milling or rotary cutter milling. Some of the methods above may be performed either wet ( / .e. in the presence of a liquid) or dry ( / .e. in the absence of a liquid). Wet milling in an aqueous liquid may have the advantage the milled plasticised lignin may be dispersed into the aqueous liquid during the milling process.

[0093] Alternatively, the compounded plasticised lignin may be converted to micron-sized particles by using spinning (including electrospinning) to create micron-sized fibers that can later be processed into particles; or by laser ablation, hydrodynamic cavitation, high-pressure homogenization or ultrasonication.

[0094] After milling, the milled particles may be dispersed in an aqueous liquid. Alternatively, the milled particles may be applied directly to the flexible substrate. This may be directly by spreading, or the particles may be combined with a polymeric binder (e.g. PVA) and then applied, or may be deposited by any known particle deposition method (for example spraying, fluidised bed deposition, or electrostatic powder coating). The milled particles may alternatively be melted and applied to the substrate using known hot melt or thermoforming techniques e.g. a hot melt roller coater.

[0095] The method of making a heat-sealable flexible packaging material may comprise dispersing the compounded and milled lignin and plasticiser ( / .e. the milled, plasticised lignin) in an aqueous liquid. For example, the milled plasticised lignin is dispersed, or scattered, in waterbased liquid. This is different to a solution where the particles are dissolved in a liquid solvent. Mechanical agitation may be employed to disperse the particles in the aqueous liquid. Any appropriate stirrer, mixer or homogeniser known in the art may be used. Examples include but are not limited to ultrasonic homogenizers, rotor-stator mixers and magnetic stirrers. Alternatively, mechanical agitation may be done as part of wet milling when used, as explained above.

[0096] Stabilizing agents may be added to the dispersion. These are often used to prevent the agglomeration or coalescence of particles and to maintain the stability of the dispersion. Stabilizing agents may include the following non-limiting examples: surfactants: such as nonionic surfactants (e.g. Triton X-100, Tween 80), or anionic surfactants (e.g. sodium dodecyl sulfate (SDS), sodium lauryl sulfate), cationic surfactants (e.g. cetyltrimethylammonium bromide (CTAB), benzalkonium chloride); polymeric stabilizers: such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG)); electrostatic atabilizers (including polyelectrolytes such as polyacrylic acid (PAA) or polydiallyldimethylammonium chloride (PDADMAC)); steric stabilizers (including hydrophilic polymers (e.g. hydroxyethyl cellulose (HEC); hydrophobic polymers (poly(methyl methacrylate) (PMMA), polyethylene oxide (PEG); particulate stabilizers such as silica nanoparticles, and clay nanoparticles (e.g. bentonite, montmorillonite); biopolymers, such as starch and gum arabic; alcohols: ethanol, isopropanol; polymeric surfactants, such as pluronic series, polymer-coated nanoparticles; such a polymer-coated metal or oxide nanoparticles or any combination of the above. The dispersion may comprise a stabilizing agent at from 0.1 to 20 wt.%, or from 0.5 to 15 wt.%, or from 1 to 10 wt.%, or from 2 to 5 wt.% or any range formed from any combination of these endpoints.

[0097] Dispersants may be added to the aqueous dispersion to facilitate the uniform distribution of particles or components throughout the dispersion. Suitable dispersants may include but are not limited to surfactants, polymeric dispersants, phosphates, amino polymers, cellulose derivatives, acrylic polymers, silica dispersants, sulfonates and carboxylates. Specific examples may include but are not limited to sodium dodecyl sulfate (SDS), polyacrylic acid, sodium tripolyphosphate, polyethyleneimine, hydroxyethyl cellulose, tetramethylammonium hydroxide, dodecylbenzenesulfonic acid and disodium EDTA. The inventors surprisingly found that certain dispersing agents, which are not traditionally used for polymer dispersions were specifically compatible with lignin dispersion due to their use in water waste treatment, in particular Dow Acumer 9210 (sodium salt of an acrylic homopolymer), and Fennodispo A41 (Kemira). Thus, in some embodiments, the dispersing agent is selected from Acumer 9210 and Fennodispo A41. Moreover, the inventors surprisingly found that such dispersing agents are functional at very low concentration (<0.5 wt%). Thus, the dispersion may comprise a dispersant from 0.01 to 10 wt.%, or from 0.05 1 o 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0098] Antifoaming agents may be added to the dispersion to reduce the viscosity of formulation. The viscosity of the dispersion is important in any kind of processing / application method. In particular, viscosity of the formulation in some cases may be too high based on microfoam present in the dispersion (caused by the wet milling process), which can be addressed by the addition of a specific antifoaming agent at low concentration.

[0099] Buffers may be added to the dispersion to resist changes in pH. Suitable buffers may include phosphate buffers, acetate buffers, citrate buffers, tris buffers, Good's buffers, and phthalate buffers amongst others. The dispersion may comprise a buffer from 0.01 to 10 wt.%, or from 0.05 to 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0100] Antioxidants may be added to the dispersion to delay the oxidation of the dispersion by reacting with free radicals. Suitable antioxidants may include but are not limited to, ascorbic acid (vitamin C), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tocopherols (vitamin E), quercetin, propyl gallate, trolox, gallic acid and catechins. The dispersion may comprise an antioxidant from 0.01 to 10 wt.%, or from 0.05 to 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0101] Anti-bacterial agents may be added to the dispersion to inhibit the growth or kill bacteria. Suitable anti-bacterial agents may include but are not limited to: Silver Nanoparticles, Triclosan, Quaternary Ammonium Compounds, Chlorhexidine, Iodine Compounds, Zinc Pyrithione, Trisodium Nitrilotriacetate, Isothiazolinones, Benzoyl Peroxide and Polyhexamethylene Biguanide. The dispersion may comprise an antibacterial agent from 0.01 to 10 wt.%, or from 0.05 to 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0102] Rheology modifiers may be added to the dispersion to achieve specific viscosity, stability, and texture properties. Rheology modifiers may include polysaccharides (e.g. Xanthan Gum, Guar gum, alginate, carrageenan), polymers (e.g. Carbomers, acrylics, PVA), cellulose derivatives (e.g. Methylcellulose, Hydroxypropyl Methylcellulose, Hydroxyethyl Cellulose) and Polyethers (e.g. Polyethylene Glycol, polypropylene glycol), amongst others. The dispersion may comprise a rheology modifier from 0.01 to 10 wt.%, or from 0.05 1o 5 wt.%, or from 0.1 to 2.5 wt.%, or from 0.5 to 1 wt.% or any range formed from any combination of these endpoints.

[0103] In some embodiments, the lignin has not been reacted with an oxidising agent prior to dispersing in an aqueous liquid to form an aqueous dispersion. Lignin is acidic and can result in an acidic dispersion. The aqueous liquid may be neutralised by the addition of a basic substance as explained above. The basic substance may cause some incidental oxidation of the lignin but oxidation of the lignin may have minimal effect on the functioning of the present disclosure.

[0104] The method may comprise applying a layer of the plasticised lignin to the surface of the flexible substrate by a hot melt method. Hot melt methods comprise applying melted plasticised lignin to the flexible substrate and / or melting plasticised lignin on the flexible substrate. The skilled person can readily identify suitable means for melting the plasticised lignin (e.g. using an electric heater). Likewise, the skilled person can readily identify suitable means for applying melted plasticised lignin to the flexible substrate (e.g. by pouring, spreading or extruding).

[0105] The lignin may not have undergone a chemical modification. Chemical modification may comprise oxidisation by an oxidization agent, grafting of functional groups or any other reaction between the lignin and a reactive agent intended to substantially change the properties of lignin such as LA. solubility. Preferably, the lignin does not undergo any covalent reaction to alter its molecular structure. Preferably, the lignin may not undergo any covalent reaction to alter its molecular structure prior to addition into the aqueous dispersion.

[0106] In embodiments where a basic substance is added to the aqueous dispersion to alter the pH, may not be considered a covalent reaction to alter the lignin molecular structure as any reaction with a basic substance of this nature may result in an incidental change to the lignin molecular structure.

[0107] The heat seal layer may have a thickness after drying of from 2 to 30 / / m, or from 3 to 25 / / m, or from 4 to 20 / / m, or from 5 / zm to 15 / / m, or any range formed from any of these endpoints. As explained above the heat seal may be deposited on the flexible substrate via extrusion or from an aqueous dispersion

[0108] The heat seal layer may have a basis weight of 2 to 35 g / m2, or from 3 to 30 g / m2or from 4 to 25 g / m2or from 5 g / m2to 20 g / m2after drying, or any range formed from any of these endpoints. The heat seal layer may be deposited on the flexible substrate in one, two, three, or more layers. Each layer may be dried before a subsequent layer is applied on top. The thickness of a heat seal layer when deposited in multiple layers may be calculated after the final layer has dried.

[0109] The heat sealable flexible packaging material may be sealed via the heat seal layer to a further packaging material with a heat seal strength greater than 2.5 N / 15mm, or 2.9 N / 15mm, or 3.4 N / 15mm, or 3.9 N / 15mm. Heat seal strength may be measured using ASTM F88. In a preferred embodiment, the heat sealable flexible packaging material may be sealed via the heat seal layer to a further packaging material with a heat seal strength greater than 1 N / 15mm.

[0110] A heat seal may provide a seal from the melting and solidifying of the heat seal layer. This is distinct from curing where covalent bonds form over time, or a pressure-sensitive adhesive where bonds form from contact with a tacky surface.

[0111] A heat sealable flexible packaging material may be capable of being sealed with high throughput. Therefore, the heat seal may be sealable with heat and pressure applied for less than 10 seconds, less than 8 seconds, less than 6 seconds, less than 5 seconds, less than 4 seconds, less than 3 seconds, less than 2 seconds, or less than 1 second. In some embodiments, the heat seal may be sealable with heat and pressure applied for less than 3 seconds. In some embodiments, the heat and pressure may be applied for greater than 0.25 seconds, greater than 0.5 seconds, greater than 0.75 seconds or greater than 1 second. This may be necessary to allow for a large number of packaging items to be sealed on a production line. In preferred embodiments, the heat sealable flexible packaging material may be sealed for between 0.5 and 3.5 seconds (s).

[0112] A further packaging material may comprise a separate packaging material or a further portion of the heat-sealable flexible packaging material. The separate packaging material may include paper, paper board, polymeric sheets or films, polymeric, wood, fibre or pulp rigid packaging products. The separate packaging material may include additional functional layers thereon, which may include primers, barrier layers and / or a heat seal layer thereon. Where the heat sealable flexible packaging material may be sealed to a further portion of the heat-sealable flexible packaging material, this may be to either a surface with the heat seal layer or a surface free from the heat seal layer. The heat sealable flexible packaging material may be sealed with the application of heat of at least 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C. The heat sealable flexible packaging material may be sealed with the application of heat of no greater than 200°C, 170°C, 140°C, or 110°C. In preferred embodiments, the heat sealable flexible packaging material may be sealed with the application of heat of between 110°C and 200°C.

[0113] The heat sealable flexible packaging material can be sealed with the application of pressure of 50 kPA, or 150 kPA, or 200 kPa, or 250 kPA, or 350 kPA, or 400 kPA, or 450 kPa or greater. The application of pressure may not exceed 1500 kPa, or 1000 kPA, or 800 kPA, or 600 kPA, or 500 kPA, or 400 kPA, or 300 kPA. In preferred embodiments, heat sealable flexible packaging material can be sealed with the application of pressure of between 200 and 500 kPa, more preferably between 200 and 450 kPa.

[0114] The heat seal layer may be free from cross-linking agents. As noted above, without being bound by theory, lower levels of cross-linking in a lignin molecular structure may advantageously improve thermoplastic properties. Moreover, the absence of cross-linking agents increases the overall sustainability of the product and reduces the cost of production. Cross-linking agents may be defined as any agent that is capable of forming a plurality of covalent bonds with the lignin. Cross-linking agents may include p-hydroxyalkylamide-cross- linkers, oxazoline-cross-linkers, aldehydes, isocyanates, phenolic hydroxyls, epoxy-based cross linkers and / or novolacs.

[0115] The method may comprise bringing a portion of a heat-sealable flexible packaging material in contact with a further portion of a flexible packaging material and applying heat and pressure, to seal the further flexible packaging material to the heat-sealable flexible packaging material via the heat seal layer.

[0116] Heat seal apparatus may comprise, amongst others, an impulse heat sealer, constant heat sealers, hot bar sealers, jaw sealers, vacuum sealers, ultrasonic sealers and induction sealers where the heat sealable packaging material comprises a metallic or otherwise inducible component.

[0117] The use may comprise cutting a series of blank shapes from a roll of the heat-sealable flexible packaging material. The reverse side of the heat sealable flexible packaging material may be printed on and / or varnish may be applied. The heat sealable flexible packaging material may be used to form an overwrapper, a pouch, a sachet, a doy pack, a pillow bag, a gusseted bag, quad seal bag, a shaped pouch, a zipper bag or any other packaging form requiring heat sealing in the manufacture thereof. Assembling any of these packaging forms may require arranging one or more blanks of the heat sealable flexible packaging material, and / or folding one or more blanks of the heat sealable flexible packaging material and heat sealing to retain the desired shape. The packaging forms may be assembled around a product, or an intermediate form may be created (e.g. a tube), filled with a product, and then sealed.

[0118] The preceding description is provided for the purposes of summarizing some embodiments to provide a basic understanding of aspects of the subject matter described herein. Accordingly, the above-described features are merely examples and should not be construed to narrow the scope or spirit of the subject matter described herein in any way. Moreover, the above and / or proceeding embodiments may be combined in any suitable combination to provide further embodiments. Features described in one aspect of the disclosure are to be construed as applicable to other aspects of the disclosure.

[0119] Additional Embodiments

[0120] Embodiment 1. A method of making a heat-sealable flexible packaging material, the method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, by extruding the plasticised lignin or from an aqueous dispersion of the plasticised lignin.

[0121] Embodiment 2. A method of making a heat-sealable flexible packaging material according to Embodiment 1 , wherein the plasticised lignin is dispersed in an aqueous liquid to form the aqueous dispersion.

[0122] Embodiment 3. A method of making a heat-sealable flexible packaging material according to Embodiment 1 or Embodiment 2, wherein the aqueous dispersion comprises at least 10 wt.%, or at least 20 wt.%, or at least 30 wt.%, or at least 40 wt.% or at least 50 wt.%, or at least 60 wt.%, or at least 70 wt.%, or at least 80 wt.% of lignin and plasticiser.

[0123] Embodiment 4. A method of making a heat-sealable flexible packaging material according to any preceding Embodiment, comprising the step of drying the aqueous dispersion. Embodiment 5. A method of making a heat-sealable flexible packaging material according to any preceding Embodiment, comprising applying and drying two, three or more layers of an aqueous dispersion to form the heat seal layer.

[0124] Embodiment 6. A method of making a heat-sealable flexible packaging material according to any preceding Embodiment, wherein the heat seal layer has a thickness of from 2 to 30 / / m, or from 3 to 25 / zm or from 4 to 20 / zm or from 5 / zm to 15 / zm after drying.

[0125] Embodiment 7. A method of making a heat sealable flexible packaging material according to any preceding Embodiment, wherein the heat seal layer has a basis weight of 2 to 35 g / m2, or from 3 to 30 g / m2or from 4 to 25 g / m2or from 5 g / m2to 20 g / m2after drying.

[0126] Embodiment 8. A method of making a heat-sealable flexible packaging material according to Embodiment 2 or any Embodiment dependent thereon, wherein the lignin and a plasticiser are dispersed in water and with no additional solvents to form the aqueous dispersion.

[0127] Embodiment 9. A method of making a heat-sealable flexible packaging material according to Embodiment 2 or any Embodiment dependent thereon, wherein the aqueous dispersion is free from any non-aqueous solvents.

[0128] Embodiment 10. A method of making a heat-sealable flexible packaging material according to Embodiment 2 or any Embodiment dependent thereon, wherein the aqueous dispersion has a pH of from 5 to 10, or from 6-9, or from 7-8.

[0129] Embodiment 11 . A method of making a heat-sealable flexible packaging material according to any preceding Embodiment, comprising milling the compounded lignin and plasticiser.

[0130] Embodiment 12. A method of making a heat-sealable flexible packaging material according to Embodiment 11 , comprising dispersing the compounded and milled lignin and plasticiser in an aqueous liquid.

[0131] Embodiment 13. A method of making a heat-sealable flexible packaging material according to Embodiment 2 or any Embodiment dependent thereon, wherein the lignin is not reacted with an oxidising agent prior to dispersing in an aqueous liquid to form an aqueous dispersion. Embodiment 14. A method of making a heat-sealable flexible packaging material according to Embodiment 1 , comprising extruding the plasticised lignin to apply a layer of the plasticised lignin to a surface of the flexible substrate to form a heat seal layer.

[0132] Embodiment 15. A method of making a heat-sealable flexible packaging material according to Embodiment 14, wherein the heat seal layer has a thickness of from 5 to 50 / / m, or from 7 to 40 / zm or from 10 to 30 / zm.

[0133] Embodiment 16. A method of making a heat-sealable flexible packaging material according to any of Embodiment 14 or Embodiment 15, wherein the plasticised lignin is extruded via slot die extrusion.

[0134] Embodiment 17. A method of making a heat-sealable flexible packaging material according to any preceding Embodiment, comprising bringing a portion of a heat-sealable flexible packaging material in contact with a further portion of a flexible packaging material, and applying heat and pressure, to seal the further flexible packaging material to the heat- sealable flexible packaging material via the heat seal layer.

[0135] Embodiment 18. A heat-sealable flexible packaging material, comprising a flexible substrate and a heat seal layer on a surface thereof, the heat seal layer comprising: lignin and a plasticiser; wherein the heat seal layer has been deposited on a surface of the flexible substrate from an aqueous dispersion or an extrusion.

[0136] Embodiment 19. A heat sealable flexible packaging material according to Embodiment 18, wherein the heat seal layer has been deposited on the flexible substrate from an aqueous dispersion and has a thickness of from 2 to 30 / / m, or from 3 to 25 / zm or from 4 to 20 / zm or from 5 / / m to 15 / / m after drying.

[0137] Embodiment 20. A heat sealable flexible packaging material according to Embodiment 18 or Embodiment 19, wherein the heat seal layer has been deposited on the flexible substrate in one, two, three, or more layers.

[0138] Embodiment 21 . A heat sealable flexible packaging material according to Embodiment 20, wherein the heat seal layer has been deposited on the flexible substrate as an extrusion, and has a thickness of from 5 to 50 / / m, or from 7 to 40 / zm or from 10 to 30 / zm after cooling. Embodiment 22. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the heat sealable flexible packaging material can be sealed via the heat seal layer to a further packaging material with a heat seal strength greater than 2.5 N / 15mm, or 2.9 N / 15mm, or 3.4 N / 15mm, or 3.9 N / 15mm, measured with ASTM F88.

[0139] Embodiment 23. A method or a heat sealable flexible packaging material according to Embodiment 22, wherein the heat sealable flexible packaging material can be sealed with the application of heat of 130°C for 4 seconds.

[0140] Embodiment 24. A method or a heat sealable flexible packaging material according to Embodiment 23, wherein the heat sealable flexible packaging material can be sealed with the application of pressure of 450 kPa for 4 seconds.

[0141] Embodiment 25. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the flexible substrate is a cellulosic substrate, optionally wherein the flexible substrate is a paper or paper board.

[0142] Embodiment 26. A method or a heat sealable flexible packaging material according to any preceding Embodiment, wherein the flexible substrate has a basis weight of 20g / m2to 200 g / m2, or from 40 g / m2to 120 g / m2, or from 50 g / m2to 100 g / m2, or from 60 g / m2to 85 g / m2.

[0143] Embodiment 27. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the lignin comprises lignin from the soda pulping process.

[0144] Embodiment 28. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the lignin comprises lignin derived from grasses, optionally wherein the lignin is derived from miscanthus grasses.

[0145] Embodiment 29. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the lignin has a paracoumaryl alcohol content of 10% or more.

[0146] Embodiment 30. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the plasticiser comprises an acid.

[0147] Embodiment 31 . A method or a heat sealable flexible packaging material according to Embodiment 30, wherein the plasticiser comprises a dicarboxylic acid. Embodiment 32. A method or a heat sealable flexible packaging material according to Embodiment 31 , wherein the plasticiser comprises a saturated dicarboxylic acid.

[0148] Embodiment 33. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the plasticiser comprises an acid with the structure HOOC(CH2)XCOOH where X is 1-50 or 3-30 or 4-15 or 5-10 or 6-8 or 7.

[0149] Embodiment 34. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the % of lignin in the compounded lignin and plasticiser is from 60% to 95% or from 70 % to 90 % or from 75% to 85%.

[0150] Embodiment 35. A method or a heat sealable flexible packaging material according to any previous Embodiment, wherein the lignin comprises a particle size of 40 / zm or less, or 30 / zm or less, or 20 / zm or less, or 10 / m or less.

[0151] Embodiment 36. A method or a heat sealable flexible packaging material according to any preceding Embodiment wherein the heat seal layer is free from cross-linking agents.

[0152] Embodiment 37. A method or a heat sealable flexible packaging material according to any preceding Embodiment wherein the lignin has not undergone a chemical modification, optionally wherein the chemical modification includes oxidisation by an oxidization agent or grafting of functional groups.

[0153] Embodiment 38. A method of making a heat-sealable flexible packaging material according to any of Embodiments 1 to 17 or any Embodiment dependent thereon, wherein the plasticised lignin is not dissolved or dispersed with a non-aqueous solvent.

[0154] Embodiment 39. A method of making a heat-sealable flexible packaging material according to any of Embodiments 1 to 17 or any Embodiment dependent thereon, wherein the lignin is not reacted with an oxidising agent.

[0155] Embodiment 40. Use of a heat sealable flexible packaging material according to any of Embodiments 18 to 37 or any Embodiment dependent thereon, for sealing a portion thereof to a portion of further packaging material with the application of heat and pressure. Examples

[0156] Before describing examples of the present disclosure, it is to be understood that the present disclosure is not limited to the details of construction or process steps set forth in the Examples below. It will be apparent to those skilled in the art having the benefit of the present disclosure that the embodied examples are capable of being practised or being carried out in various ways.

[0157] Example 1 : Determining the suitability of lignin for functionalization towards a heat seal

[0158] In this example, the inventors used lignin from grasses (chosen preferentially due to the high content of the monomer p-hydroxyphenol, which creates linear polymers), which was extracted using the soda pulping process (providing higher average MW).

[0159] In the present example, the inventors specifically sought to produce a material suitable for dispersion. Soda lignin is not water soluble, and therefore was deemed to be suitable for dispersion. Furthermore, the inventors sought to ensure that the material has suitable rigidity to be applied as an adhesive. This target mechanical behaviour was tested by observation of samples and mechanical stress testing by hand. Further, those candidates that passed the manual examination were subsequently tested by means of heat-sealing it with standard paper and conducting seal strength measurements according to ASTM F88. Soda lignin obtained from grasses was deemed by the inventors to be functionalizable, producing a heat seal with strength of 3.34 N / 15mm after plasticisation (SD = 0.25). Soda lignin obtained from mixed biomass (a combination of wood waste and wheat straw) was also deemed by the inventors to be functionalizable, producing a heat seal with strength of 1.74 N / 15mm after plasticisation (SD = 0.24).

[0160] Example 2: Assessing plasticisers for suitability for compounding with lignin

[0161] The inventors tested the suitability of multiple known plasticisers, including polyols (and deep eutectic solvents thereof), glycol esters, oils, their fatty acids and derivates thereof), as well as other candidates not previously used as plasticisers. Table 1 provides a list of these plasticiser candidates, as well as indicating their suitability for dispersion and functionality.

[0162] Table 1 : List of molecules examined for lignin plasticisation suitability

[0163] The inventors have further tested the influence of using different plasticisers on the plasticised lignin polymer seal strength. As seen in Figure 1 , a formulation including a mixture of azelaic acid and PEG 400 with lignin (TL30) was dispersed and coated on paper, which resulted in a decrease in seal strength as compared to dry powder form; in comparison, a formulation which contains only azelaic acid and lignin (TL26) increased seal strength following dispersion as compared to dry powder form. The decrease in seal strength of TL30 was almost proportional to the reduction in amount of azelaic acid that was included in comparison to TL26 sample. This data therefore indicates that formulations which contain only azelaic acid and lignin (TL26) are considered preferred for dispersion application techniques. Formulations including a mixture of azelaic acid and PEG 400 with lignin (TL30) may be better suited for alternative thermoplastic application techniques, e.g., hot melt application.

[0164] Example 3: Plasticised lignin as a heat seal adhesive

[0165] Materials:

[0166] Lignin 1 : Soda lignin, from WEPA, Germany, a lignin derived from miscanthus grass. Plasticiser 1 : Technical grade Azelaic acid containing 80% Azelaic acid (Azelainsaure tech. 80%, technische Qualitat VWR, Germany).

[0167] Plasticiser 2: Polyethylenglycol 400 (GPR RECTAPUR® | VWR, Germany).

[0168] Paper 1 is an 80 gsm bleached, coated paper from Brigl & Bergmeister, Germany.

[0169] Paper 2 is a 50 gsm bleached and uncoated paper from Griinewaldt, Germany.

[0170] Paper 3 is a 150 gsm paper made from agricultural waste from PaperWise, Netherlands.

[0171] Method:

[0172] Plasticised lignin was produced by compounding the lignin with the plasticiser. Compounding was performed using a Noztek filament single-screw extruder (Noztek Touch, Noztech Extrusion Systems, UK). The plasticisers and the lignin were added to the single-screw extruder. Extrusion took place at 115°C. Two plasticised lignins were produced:

[0173] Plasticised lignin 1 : comprising 80 wt.% lignin 1 and 20 wt.% plasticiser 1.

[0174] Plasticised lignin 2: comprising 75 wt.% lignin 1 , 15 wt.% plasticiser 2 and 10 wt.% plasticiser 1.

[0175] Plasticised lignin was milled using a blade mill (Multidrive, IKA, UK). This produced a typical particle size of < 45 / zm (mesh size of the utilised sieve).

[0176] To make the dispersion, the milled lignin particles were added to water at 50 wt%. A dispersant (DOW Acumer 9210) was added to the water at 0.1 wt%. The lignin particles were dispersed in the aqueous using a disperser (T-18 Ultra-Turrax, IKA, UK).

[0177] The plasticised lignin was applied to papers 1 to 3. The plasticised lignin was applied from the dispersion described above at 9-16.5 gsm or was laminated at 40 gsm. Dispersion was performed using a SUMET roll-to-roll coater in filmpress or rod mode at ICP in Ljubljana, Slovenia. Lamination was performed by applying the milled lignin directly to paper by weighting an amount calculated for a defined surface to reach a certain grammage and spreading the milled lignin particles onto the defined surface by hand.

[0178] The coated paper was sealed to itself using a Labthink heat sealer (Labthink GmbH, Germany) at 130°C for 3s at 450kPa. The resulting heat seal strength was analysed using a Labthink tensile tester (Labthink Gmbh, Germany) and ASTM F88. The results of the heat seal testing are shown in Table 2 below.

[0179] The results of Table 2 show that the plasticised and milled lignin provides an effective heat seal to paper.

[0180] Example 4: Successful plasticisation using plasticisers including dicarboxylic acids

[0181] Samples have further been tested for heat-sealing paper (in dry state, without dispersing, by applying powder manually). Sealing force results are provided in Figure 2. The results indicate the best performance for dicarboxylic acids, in particular, azelaic acid (TL26; plasticised lignin 1) and azelaic acid combined with PEG400 (TL30; plasticised lignin 2).

[0182] As azelaic acid was determined to be suitable for dispersion and economically feasible, this compound was selected for larger-scale trials, including industrial compounding, dispersing and coating.

[0183] The heat seal strength for lignin plasticised with azelaic acid was then tested across a variety of paper types with differing weights. The results are shown in Table 3. The results indicate that strong seal strength can be achieved across a range of paper types and weights and is not limited by the cohesive force between the sealer and the paper.

[0184] Table 3: Testing plasticised lignin as heat seal on different papers at optimised sealing conditions Further details of the paper types tested in Table 3 are provided in Table 4, below:

[0185] Table 4: Details of paper types tested

[0186] Example 5: Assessing seal force

[0187] As described above, the inventors have found that plasticised lignin demonstrates a very good seal force for low coating grammages, when compared to benchmarks in the same market segment (single use plastics; natural heat seals). A seal force of up to 4 N / 15mm for gsm values below 10 (AA seal) was generated, mostly limited by fibre tear of the paper.

[0188] Sealing conditions for applying the heat seal to the paper substrate were derived from industrial standards. When reduced to a minimum of 0.5 sec and 200 kPa, a seal force of up to 3 N / 15mm was successfully maintained. The inventors also found that low temperature heat seal starting from 110°C was also possible. Figure 3 demonstrates the sealing parameters tested.

[0189] Figure 4 provides the sealing force generated under varying seal parameters for lignin formulation TL26 as compared to benchmark PE, on a paper type with 50gsm (paper030). The results indicate that TL26 is capable of generating comparable sealing force to the control under a range of different sealing conditions.

[0190] Example 6: Working examples of plasticised lignin

[0191] As described above, the inventors produced two samples of plasticised lignin using an industrial compounder (plasticised lignin 1 and 2). These represent samples for use with either dispersion or hotmelt applications. These samples were chosen among the candidates based on their good seal strength, potential biological origin and low cost. In order to validate the functionalization of nonthermoplastic lignin into a thermoplastic material, these samples were tested by differential scanning calorimetry (DSC). Figure 5 demonstrates that no phase separation between polymer and plasticiser was observed, as only one glass transition temperature (Tg) per sample was recorded, whereas for the pure lignin (TO) no glass transition temperature was detected. This confirms that while lignin alone is thermoplastic, both plasticised lignins are thermoplastic.

[0192] The materials were further tested for their thermoplasticity, using the melt flow index (MFI) which indicates melt flow viscosity. 37 replicates were tested at 120°C with 5kg test weight, and the results presented in Table 5 and Figure 6.

[0193] Table 5: MVR (Melt Volume-Flow rate (MVR; cm3 / 10min) for plasticised lignin 1 (TL26); n= 37.

[0194] The average MVR was 30.43 cm3 / 10min. The average MFR was 40.17 g / 10min. average density was 1 .320 g / cm3. Thus, Table 5 and Figure 6 demonstrates that the plasticised lignin is suitably thermoplastic and defines the processing window of compounding and demonstrates the applicability of the plasticised lignin for hot melt applications.

[0195] Example 7: Optimizing dispersion

[0196] The inventors also tested numerous dispersing agents. The inventors surprisingly found that the dispersing agents shown in T able 6, which are not traditionally used in polymer dispersions but are rather used in waste-water treatment, were specifically compatible with lignin formulations. Advantageously, these are also available at low cost and are functional at very low concentrations:

[0197] Table 6: Identified dispersants to optimise dispersion

[0198] The inventors further optimized the milling process to reduce the average particle size (<7 pm) to provide a more stable dispersion. This particle size enables the use of usual printing technologies to be applied for dispersion coating on paper substrates. In particular, the inventors optimized the process of wet milling in a bead / pearl mill (by adapting e.g., bead size, milling time, initial particle size (by pre-grinding) and dispersing agent), surprisingly reducing the average particle size to 6.99 pm. This is shown in Figure 7, in which the narrow peak without a shoulder to the higher particle size direction indicates the stability of the dispersion.

[0199] The inventors further optimised dispersion by the addition of a specific antifoaming agent at low concentration, which reduced the viscosity of formulation. The viscosity of the dispersion is important in any kind of processing / application method of the dispersion to coat it on paper. Viscosity of the formulation in some cases may be too high based on microfoam present in the dispersion (caused by the wet milling process), which can be decreased by the addition of a specific antifoaming agent at low concentration, as demonstrated in Figure 8.

[0200] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps than those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0201] Numeric ranges are inclusive of the numbers defining the range. Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within this disclosure. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within this disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in this disclosure.

[0202] “About” may generally mean an acceptable degree of error for the quantity measured given the nature or precision of the measurements. Exemplary degrees of error are within 20 percent (%), typically, within 10%, and more typically, within 5% of a given value or range of values. Preferably, the term “about” shall be understood herein as plus or minus (±) 5%, preferably ± 4%, ± 3%, ± 2%, ± 1%, ± 0.5%, ± 0.1%, of the numerical value of the number with which it is being used.

[0203] The term "consisting of' refers to compositions, methods, and respective components thereof as described herein, which are exclusive of any element not recited in that description of the invention. Embodiments described herein as “comprising” one or more features may also be considered as disclosure of the corresponding embodiments “consisting of’ such features of the corresponding embodiments "consisting of' such features.

[0204] Unless otherwise explicitly stated as incompatible, or the physics or otherwise of the embodiments, example or claims prevent such a combination, the features of the foregoing embodiments, examples, and of the following claims may be integrated together in any suitable arrangement, especially ones where there is a beneficial effect in doing so. This is not limited to only any specified benefit and instead may arise from an “ex post facto” benefit. This is to say that the combination of features is not limited by the described forms, particularly the form (e.g. numbering) of the example(s), embodiment(s), or dependency of the claim(s). Moreover, this also applies to the phrases “in one embodiment”, “according to an embodiment” and the like, which are merely a stylistic form of wording and are not to be construed as limiting the following features to a separate embodiment to all other instances of the same or similar wording. This is to say, a reference to ‘an’, ‘one’ or ‘some’ embodiment(s) may be a reference to any one or more, and / or all embodiments, or combination(s) thereof, disclosed. Similarly, the reference to “the” embodiment may not be limited to the immediately preceding embodiment.

[0205] The preceding description of one or more implementations provides illustration and description but is not intended to be exhaustive or to limit the scope of the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from the practice of various implementations of the present disclosure.

Claims

Claims1. A method of making a heat-sealable flexible packaging material, the method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, by extruding the plasticised lignin or from an aqueous dispersion of the plasticised lignin.

2. A method of making a heat-sealable flexible packaging material according to claim 1 , wherein the plasticised lignin is dispersed in an aqueous liquid to form the aqueous dispersion.

3. A method of making a heat-sealable flexible packaging material according to claim 1 or claim 2, wherein the aqueous dispersion comprises at least 10 wt.%, or at least 20 wt.%, or at least 30 wt.%, or at least 40 wt.% or at least 50 wt.%, or at least 60 wt.%, or at least 70 wt.%, or at least 80 wt.% of lignin and plasticiser.

4. A method of making a heat-sealable flexible packaging material according to any preceding claim, comprising the step of drying the aqueous dispersion.

5. A method of making a heat-sealable flexible packaging material according to any preceding claim, comprising applying and drying two, three or more layers of an aqueous dispersion to form the heat seal layer.

6. A method of making a heat-sealable flexible packaging material, the method comprising: compounding a lignin and a plasticiser to form a plasticised lignin, and applying a layer of the plasticised lignin to a surface of a flexible substrate to form a heat seal layer thereon, wherein the plasticised lignin is applied using a hot melt method.

7. A method of making a heat-sealable flexible packaging material according to any preceding claim, wherein the heat seal layer has a thickness of from 2 to 30 / / m, or from 3 to 25 / zm or from 4 to 20 / zm or from 5 / zm to 15 / zm after drying.

8. A method of making a heat sealable flexible packaging material according to any preceding claim, wherein the heat seal layer has a basis weight of 2 to 35 g / m2, or from 3 to 30 g / m2or from 4 to 25 g / m2or from 5 g / m2to 20 g / m2after drying.

9. A method of making a heat-sealable flexible packaging material according to claim 2 or any claim dependent thereon, wherein the lignin and a plasticiser are dispersed in water and with no additional solvents to form the aqueous dispersion.

10. A method of making a heat-sealable flexible packaging material according to claim 2 or any claim dependent thereon, wherein the aqueous dispersion is free from any non-aqueous solvents.11 . A method of making a heat-sealable flexible packaging material according to claim 2 or any claim dependent thereon, wherein the aqueous dispersion has a pH of from 5 to 10, or from 6-9, or from 7-8.

12. A method of making a heat-sealable flexible packaging material according to any preceding claim, comprising milling the compounded lignin and plasticiser.

13. A method of making a heat-sealable flexible packaging material according to claim 12, comprising dispersing the compounded and milled lignin and plasticiser in an aqueous liquid.

14. A method of making a heat-sealable flexible packaging material according to claim 2 or any claim dependent thereon, wherein the lignin is not reacted with an oxidising agent prior to dispersing in an aqueous liquid to form an aqueous dispersion.

15. A method of making a heat-sealable flexible packaging material according to claim 1 , comprising extruding the plasticised lignin to apply a layer of the plasticised lignin to a surface of the flexible substrate to form a heat seal layer.

16. A method of making a heat-sealable flexible packaging material according to claim 15, wherein the heat seal layer has a thickness of from 5 to 50 / / m, or from 7 to 40 / zm or from 10 to 30 / zm.

17. A method of making a heat-sealable flexible packaging material according to any of claim 15 or claim 16, wherein the plasticised lignin is extruded via slot die extrusion.

18. A method of making a heat-sealable flexible packaging material according to any preceding claim, comprising bringing a portion of a heat-sealable flexible packaging material in contact with a further portion of a flexible packaging material, and applying heat and pressure, to seal the further flexible packaging material to the heat-sealable flexible packaging material via the heat seal layer.

19. A heat-sealable flexible packaging material, comprising a flexible substrate and a heat seal layer on a surface thereof, the heat seal layer comprising: lignin and a plasticiser; wherein the heat seal layer has been deposited on a surface of the flexible substrate from an aqueous dispersion or an extrusion.

20. A heat-sealable flexible packaging material, comprising a flexible substrate and a heat seal layer on a surface thereof, the heat seal layer comprising: lignin and a plasticiser; wherein the heat seal layer has been deposited on a surface of the flexible substrate by a hot melt method.

21. A heat sealable flexible packaging material according to claim 19, wherein the heat seal layer has been deposited on the flexible substrate from an aqueous dispersion and has a thickness of from 2 to 30 / / m, or from 3 to 25 / zm or from 4 to 20 / zm or from 5 / / m to 15 / / m after drying.

22. A heat sealable flexible packaging material according to any one of claims 19 or 20, wherein the heat seal layer has been deposited on the flexible substrate in one, two, three, or more layers.

23. A heat sealable flexible packaging material according to claim 22, wherein the heat seal layer has been deposited on the flexible substrate as an extrusion, and has a thickness of from 5 to 50 / / m, or from 7 to 40 / zm or from 10 to 30 / zm after cooling.

24. A method or a heat sealable flexible packaging material according to any previous claim, wherein the heat sealable flexible packaging material can be sealed via the heat seal layer to a further packaging material with a heat seal strength greater than 2.5 N / 15mm, or 2.9 N / 15mm, or 3.4 N / 15mm, or 3.9 N / 15mm, measured with ASTM F88.

25. A method or a heat sealable flexible packaging material according to claim 24, wherein the heat sealable flexible packaging material can be sealed with the application of heat of 130°C for 4 seconds.

26. A method or a heat sealable flexible packaging material according to claim 25, wherein the heat sealable flexible packaging material can be sealed with the application of pressure of 450 kPa for 4 seconds.

27. A method or a heat sealable flexible packaging material according to any previous claim, wherein the flexible substrate is a cellulosic substrate, optionally wherein the flexible substrate is a paper or paper board.

28. A method or a heat sealable flexible packaging material according to any preceding claim, wherein the flexible substrate has a basis weight of 20g / m2to 200 g / m2, or from 40 g / m2to 120 g / m2, or from 50 g / m2to 100 g / m2, or from 60 g / m2to 85 g / m2.

29. A method or a heat sealable flexible packaging material according to any previous claim, wherein the lignin comprises lignin from the soda pulping process.

30. A method or a heat sealable flexible packaging material according to any previous claim, wherein the lignin comprises lignin derived from grasses, optionally wherein the lignin is derived from miscanthus grasses.31 . A method or a heat sealable flexible packaging material according to any previous claim, wherein the lignin has a paracoumaryl alcohol content of 10% or more.

32. A method or a heat sealable flexible packaging material according to any previous claim, wherein the plasticiser comprises an acid.

33. A method or a heat sealable flexible packaging material according to claim 32, wherein the plasticiser comprises a dicarboxylic acid.

34. A method or a heat sealable flexible packaging material according to claim 33, wherein the plasticiser comprises a saturated dicarboxylic acid.

35. A method or a heat sealable flexible packaging material according to any previous claim, wherein the plasticiser comprises an acid with the structure HOOC(CH2)XCOOH where X is 1-50 or 3-30 or 4-15 or 5-10 or 6-8 or 7.

36. A method or a heat sealable flexible packaging material according to any previous claim, wherein the % of lignin in the compounded lignin and plasticiser is from 60% to 95% or from 70 % to 90 % or from 75% to 85%.

37. A method or a heat sealable flexible packaging material according to any previous claim, wherein the lignin comprises a particle size of 40 / zm or less, or 30 / zm or less, or 20 / zm or less, or 10 / zm or less.

38. A method or a heat sealable flexible packaging material according to any preceding claim wherein the heat seal layer is free from cross-linking agents.

39. A method or a heat sealable flexible packaging material according to any preceding claim wherein the lignin has not undergone a chemical modification, optionally wherein the chemical modification includes oxidisation by an oxidization agent or grafting of functional groups.

40. A method of making a heat-sealable flexible packaging material according to any of claims 1 to 18 or any claim dependent thereon, wherein the plasticised lignin is not dissolved or dispersed with a non-aqueous solvent.41 . A method of making a heat-sealable flexible packaging material according to any of claims 1 to 18 or any claim dependent thereon, wherein the lignin is not reacted with an oxidising agent.

42. Use of a heat sealable flexible packaging material according to any of claims 19 to 39 or any claim dependent thereon, for sealing a portion thereof to a portion of further packaging material with the application of heat and pressure.

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