Background

Plastic packaging materials are commonly used to package foods and beverages. Plastics are inexpensive to manufacture and to transport, and are effective barriers against moisture and oil and grease. However, plastics have a significant environmental impact. For example, most plastics are produced from non-renewable resources and are not biodegradable. There is an increasing desire to reduce the amount of plastic waste.

Glass and metal packaging may be used as alternatives to plastics. These materials have good barrier properties, and can readily be recycled. However, glass and metal suffer from the drawbacks that they are expensive to manufacture and to transport.

Cellulosic materials, such as paperboard, are attractive materials from an environmental standpoint since they are manufactured from renewable materials and are biodegradable. However, cellulosic materials are porous and absorbent, and do not have adequate moisture and oil and grease barrier properties for use as e.g. food packaging. Therefore, cellulosic materials used for these purposes are coated, most commonly with fossil-based and non- biodegradable polymer films.

There remains a need in the art for cost-effective packaging materials with less environmental impact.

Summary

In one aspect, there is provided a material comprising a cellulosic substrate and a barrier layer arranged on a surface of the substrate. The barrier layer is a continuous film comprising a film-forming polymer and a lignosulfonate. It has been found that the inclusion of a lignosulfonate in a barrier layer may reduce the permeability of the barrier layer to water vapour, and/or may increase the resistance of the barrier layer to oil and grease. The film-forming polymer may comprise a carbohydrate or a carbohydrate derivative. The carbohydrate or carbohydrate derivative may be selected from glucans; hemicelluloses; and pectins.

The film-forming polymer may comprise a glucan.

The glucan may comprise a p-glucan. Examples of p-glucans include cellulose; cellulose derivatives; and laminarin.

The cellulose derivative may be a cellulose ether. The cellulose ether may be a hydroxyalkyl cellulose bearing hydroxyalkyl groups selected from hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, and combinations thereof. The cellulose ether may be hydroxyethyl cellulose.

The glucan may comprise an a-glucan. The a-glucan may be selected from dextran, glycogen, pullulan, amylose, amylopectin, starch or a modified starch. In particular, the a-glucan may be starch or a modified starch. The modified starch may be dextrin.

The film-forming polymer may comprise a hemicellulose. The hemicellulose may be xylan, glucuronoxylan, arabinoxylan, glucomannan or xyloglucan.

The film forming polymer may comprise a pectin. The pectin may be galacturonan or rhamnogalacturonan.

The film-forming polymer may be present in the barrier layer in an amount of 25 to 40 %, optionally 30 to 35 %, or about 33 % by weight based on the weight of the barrier layer, on a dry matter basis.

The lignosulfonate may be obtainable by sulfite pulping of wood, e.g. Norway spruce. In this process, wood chips are treated with sulfite and/or bisulfite salts, under acidic, neutral or alkaline conditions at elevated temperatures. Lignin in the wood is depolymerised and sulfonated, and forms water-soluble lignosulfonate polymers which are dissolved in the spent liquor. Examples of sulfite and bisulfite salts useful in this process include sodium, calcium, potassium, magnesium and ammonium salts.

The lignosulfonate may be present in the barrier layer in an amount of 30 to 50 % by weight based on the weight of the barrier layer, on a dry matter basis.

The barrier layer may further comprise a plasticiser. The plasticiser may be glycerol, for example. The plasticiser may be present in the barrier layer in an amount in the range 5 to 30%, optionally lOto 25 %, by weight based on the weight of the barrier layer, on a dry matter basis.

The barrier layer may comprise carbon from a renewable source in an amount of at least 50 % of the total amount of carbon present in the barrier layer. The substrate may comprise carbon from renewable sources in an amount of at least 50 % of the total amount of carbon present in the substrate. Both the barrier layer and the substrate may comprise carbon from renewable sources.

The barrier layer may have a thickness of less than or equal to 50 pm, optionally less than or equal to 40 pm, further optionally less than or equal to 30 pm. For example, the barrier layer may have a thickness in the range 20 to 50 pm.

The cellulosic substrate may comprise paperboard. By way of illustration, the paperboard may have a grammage (i.e., mass per unit area) in the range 200 to 300 g rrr ² , optionally 240 to 260 g nr ² , and a thickness in the range 450 to 550 pm, optionally 480 to 500 pm.

The material may have at least one crease, fold, or seam. For example, the material may be folded to form a packaging container, such as a box.

In another aspect, there is provided a method of manufacturing the material. The method mixing the film-forming polymer, the lignosulfonate, and a solvent to form a liquid composition; applying the liquid composition to the surface of the cellulosic substrate; and drying the liquid composition to form the barrier layer. A still further aspect provides the use of a lignosulfonate to increase the resistance of a polymer film to oil and/or grease; and/or to decrease the permeability of a polymer film to water vapour; wherein the polymer film is in the form of a barrier layer on a surface of a cellulosic substrate.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Nor is the claimed subject matter limited to implementations that solve any or all of the disadvantages noted herein.

Brief Description of the Drawings

To assist understanding of embodiments of the present disclosure and to show how such embodiments may be put into effect, reference is made, by way of example only, to the accompanying drawings in which:

Fig. 1 is a schematic cross-section of an example material; and

Fig. 2 is a flow diagram outlining a method of manufacturing a material of the type illustrated in Fig. 1.

Detailed Description

The verb 'to comprise' is used herein as shorthand for 'to include or to consist of'. In other words, although the verb 'to comprise' is intended to be an open term, the replacement of this term with the closed term 'to consist of' is explicitly contemplated, particularly where used in connection with chemical compositions.

Where used in connection with a numeric value, the term "about" contemplates variations of ± 5 % of the stated value. It has surprisingly been found that applying a coating comprising a film-forming polymer and a lignosulfonate to a cellulosic substrate may increase resistance to oil and grease, and may reduce permeability to water vapour.

An example material 100 will now be described with reference to Fig. 1. Fig. 1 is a schematic cross-section of the material 100.

Material 100 comprises a cellulosic substrate 102 and a barrier layer 104 arranged on a surface of the cellulosic substrate 102. The barrier layer 104 increases the resistance of the material 100 to oil and grease and reduces the permeability of the material 100 to water vapour, compared to the cellulosic substrate alone. Cellulosic substrate 102 supports the barrier layer 104 and provides the material with structural strength.

The nature of the cellulosic substrate 102 is not particularly limited. The cellulosic substrate 102 may, for example, comprise paper, paperboard, fibreboard, or a textile such as cotton fabric. In implementations where the cellulosic substrate comprises paper or paperboard, the material may be used as packaging for food or beverages.

Cellulosic substrates are generally biodegradable. The cellulosic substrate may be biobased. By biobased is meant that at least 50 %, and preferably all of the carbon in the cellulosic substrate is derived from a renewable source. Carbon from renewable sources may be distinguished from carbon from fossil fuel sources by isotope analysis. Fossil fuel sources will be substantially free of ¹⁴ C. Renewable sources will include ¹⁴ C in a proportion approximately equal to the proportion present in the atmosphere, i.e. 1 to 2 ¹⁴ C atoms per 10 ¹² atoms of total carbon. Thus, a biobased material comprises at least 0.5 ¹⁴ C atoms per 10 ¹² atoms of total carbon, and preferably 1 to 2 ¹⁴ C atoms per 10 ¹² atoms of total carbon.

The cellulosic substrate may be in the form of a sheet.

A barrier layer 104 is arranged on a surface of the cellulosic substrate 102. The barrier layer 104 may also be referred to as a coating. The barrier layer 104 is a continuous film, covering the surface of the substrate 102. The barrier layer 104 is obtainable by the use of a coating technique such as rod coating, spraying, or curtain coating. A barrier layer or coating is not a sizing. Barrier layer 104 is not a fibre treatment and is not obtained by the use of a sizing press.

The barrier 104 has a thickness t. The thickness of the barrier layer 104 is not particularly limited and may be selected as appropriate. Typically, the thickness of the barrier layer is less than or equal to 50 pm. Since the barrier layer 104 is formed in situ on the cellulosic substrate and is supported by the cellulosic substrate, in contrast to a free-standing film there is no particular lower limit on the thickness of the barrier layer provided that a continuous layer is obtained. For example, the thickness t of barrier layer 104 may be in the range 20 to 50 pm.

The barrier layer 104 comprises a film forming polymer, and a lignosulfonate. Typically, the barrier layer 104 further comprises a plasticiser.

The film-forming polymer may be selected as appropriate. The film-forming polymer is preferably biobased. Blends of two or more film-forming polymers may be used.

The film-forming polymer may, in particular, be selected from carbohydrates such as glucans, and derivatives thereof. A "derivative" is a polymer which has been chemically modified to include additional substituents.

The film-forming polymer may comprise a p-glucan. Examples of p-glucans include cellulose and cellulose derivatives. Examples of cellulose derivatives include cellulose esters and cellulose ethers.

The film-forming polymer may comprise a cellulose ether made up of units of general formula: where each R group is independently selected from H; Cl to C4 alkyl; Cl to C4 hydroxyalkyl; and Cl to C4 carboxyalkyl groups. The film-forming polymer may comprise a mixture of different such units. Where carboxyalkyl groups are present, the cellulose ether may be provided in the form a salt with an appropriate counterion, e.g. sodium.

The film-forming polymer may be a hydroxyalkyl cellulose or a carboxyalkyl cellulose.

The film-forming polymer may comprise a hydroxyalkyl cellulose ether bearing hydroxyalkyl groups selected from hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, and combinations thereof. Particularly preferably, the film-forming polymer may comprise hydroxyethyl cellulose.

The film-forming polymer may comprise a carboxyalkyl cellulose ether bearing alkyl substituents selected from Cl to C4 carboxyalkyl groups and combinations thereof. One example of a useful carboxyalkyl cellulose ether is carboxymethyl cellulose.

The viscosity average molecular weight of the cellulose or cellulose derivative may be in the range 50,000 to 400,000 Da, optionally 70,000 to 120,000 Da, further optionally 80,000 to 100,000 Da, still further optionally 85,000 to 95,000 Da.

Other useful glucans include a-glucans such as dextran, glycogen, pullulan, amylose, amylopectin, starch and starch derivatives. Starch derivatives may also be referred to as modified starches. The term "starch derivatives" may refer in particular to an a-glucan composition selected from: dextrin, alkaline-modified starch, bleached starch, oxidized starch, enzyme-treated starch, maltodextrin, cyclodextrin, monostarch phosphate, distarch phosphate, acetylated starch, hydroxypropylated starch, hydroxyethyl starch, starch sodium octenyl succinate, starch aluminium octenyl succinate, cationic starch, and carboxymethylated starch.

The film-forming polymer may, for example, be a dextrin, optionally a corn dextrin. Other carbohydrates may be used as film-forming polymers. For example, the film-forming polymer may comprise a hemicellulose and/or a pectin. Examples of hemicelluloses include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan. Examples of pectins include galacturonan and rhamnogalacturonan.

Further examples of film-forming polymers include acrylics; polyphenols; polyureas; polyisocyanates; polyolefins, such as polyethylene or polypropylene; polyesters, such as poly(lactic acid); polyamides; epoxy polymers; and polyvinyl acetate.

The film-forming polymer may be present in the barrier layer in an amount in the range 25 to 40 %, optionally 30 to 35 %, or about 33 % based on the total dry weight of the barrier layer.

In addition to the film-forming polymer, the barrier layer 104 includes a lignosulfonate. Lignosulfonates are polymers obtainable by sulfonating lignin.

For example, lignosulfonates are often produced as a side-product of sulfite pulping. Lignosulfonates may be isolated by ultrafiltration of spent sulfite pulping liquor.

An alternative route to lignosulfonates is to obtain lignin from a lignocellulosic raw material, and then sulfonate the lignin. Lignin may be obtained from, for example, kraft pulping, organosolv pulping, hydrolysis, or other fractionation processes.

Lignosulfonates are negatively-charged polymers. The negative charge originates from sulfonic acid and carboxylic acid groups. Lignosulfonates may be water-soluble.

Lignosulfonates may be polydisperse, i.e., they may have a wide molar mass distribution. Lignosulfonates typically have molar masses in the range 1,000 to 400,000 g/mol.

Suitable lignosulfonates are commercially available, and include those obtainable as a sideproduct of sulfite pulping of Norway spruce. The lignosulfonate may be present in the barrier layer in an amount in the range 30 to 70 %, optionally 40 to 60 %, further optionally 45 to 55 %, or about 50 % by weight based on the total dry weight of the barrier layer.

The ratio of amounts of the film-forming polymer and the lignosulfonate may be selected as appropriate. For example, the amount of lignosulfonate by weight based on the combined dry weight of the lignosulfonate and film-forming polymer may be in the range 30 % to 80 %, optionally 40 % to 70 %, further optionally 50 % to 70 %, further optionally 55 to 65 %, or about 60 %.

The barrier layer may further comprise a plasticiser. The plasticiser may increase the flexibility of the barrier layer. The plasticiser may reduce or avoid cracking of the barrier layer.

The plasticiser may be selected based on the nature of the film-forming polymer. The plasticiser may be a polyol, a monosaccharide, a disaccharide, or an oligosaccharide. Examples of plasticisers include glycerol, triethyl citrate, tributyl sebacate, polyethylene glycol, sorbitol, and xylitol. In particular, the plasticiser may be glycerol.

The plasticiser may include carbon from a renewable source. For example, at least 50 % of carbon present in the plasticiser may be from a renewable source.

The amount of plasticiser may be selected as appropriate. For example, the plasticiser may be present in the film in an amount in the range 10 to 25 % by weight, based on the weight of the barrier layer on a dry matter basis.

Various modifications may be made to the described material.

In the illustrated example, the barrier layer is present on one face of a sheet of cellulosic material. As will be appreciated, more than one face of the substrate may be coated if desired. In implementations where the substrate is in the form of a sheet, one or both sides of the sheet may be coated. In Fig. 1, the material depicted as a planar sheet. The material may be used for packaging, in particular for packaging for foods and beverages. In such use cases, the material is often folded, e.g. to assemble a box, tray, or other form of container. It has been found that the materials retain their favourable resistance to oil and grease even after folding or creasing.

An example method of manufacturing the material will now be described with reference to Fig. 2. Fig. 2 is a flow diagram outlining the method.

At block 201, a film forming polymer, a lignosulfonate, a solvent and optionally a plasticiser are mixed to form a liquid composition. The liquid composition may be a solution, dispersion, emulsion, or suspension.

As will be appreciated, the film forming polymer, lignosulfonate, and plasticiser may be as described above with reference to Fig. 1. The solvent is most preferably water for environmental reasons, though other solvents may be used.

Forming the liquid composition may comprise forming a solution of the film forming polymer, forming a solution of a lignosulfonate, and mixing the solutions together. The film forming polymer and lignosulfonate may be dissolved, dispersed or suspended using any appropriate procedure. For example, the film forming polymer or lignosulfonate may be stirred and optionally heated with the solvent.

Subsequently, at block 202, the liquid composition is applied to the surface of a cellulosic substrate. Any appropriate coating technique may be used. Examples include rod coating, spraying, casting, and curtain coating.

Subsequently, at block 203, the liquid composition is dried to form the barrier layer.

Various modifications may be made to the method of Fig. 2. It is contemplated that the method may be implemented as either a batch process or a continuous flow process. As will be appreciated, in a continuous flow process, the operations of blocks 201, 202, and 203 may overlap in time.

If desired, the operations of blocks 202 and 203 may be iteratively repeated to increase the effective thickness of the barrier layer. In other words, two or more barrier layers may be applied to the substrate.

It may be desirable for the liquid composition to have a solids content which is as high as possible while also having a viscosity which is compatible with the chosen coating technique. This may allow increase the thickness of the barrier material deposited per iteration of blocks 202 and 203.

After forming the material, the material may be used in the manufacture of, for example, packaging. This may comprise printing onto the material; cutting the material; creasing or folding the material, etc.

The concepts described herein will now be explained in more detail with reference to illustrative worked examples.

Examples

Example 1. Preparation of Coating

Two 20 wt% solutions of two different commercially-available lignosulfonates (LSI and LS2) were prepared as follows. Deionized water was measured into a beaker and placed on a magnetic stirrer. The lignosulfonate was then added gradually while stirring. The resulting mixture was then stirred for 30 minutes.

A 10 wt% solution of a hydroxyethyl cellulose (HEC) having a viscosity average molar mass of 90,000 g mol ¹ was prepared by measuring cold deionized water into a beaker and then gradually adding the HEC while stirring with a magnetic stirrer. The resulting mixture was then stirred for 1.5 hours.

A 14 wt% dextrin solution was prepared by adding a commercially-available corn dextrin to room temperature deionized water and heating the resulting solution to 94-98 °C for 30 minutes. The solution was then allowed to cool down to room temperature under stirring.

The coating compositions set out in Table 1, below, were then prepared using the solutions. To prepare a coating formula, the lignosulfonate solution was first weighed into a beaker, followed by the film-forming polymer (i.e., HEC or dextrin) solution. Finally, glycerol was added as a plasticiser. After weighing out the desired amounts of each component, the formula was stirred for 30 minutes using a magnetic stirrer.

Table 1. Coating compositions

*Samples 1.3 and 2.3 are comparative examples.

Example 2. Viscosity measurements

The viscosity of the coating compositions obtained in Example 1 was measured at room temperature, i.e. 22 ± 1 °C, using a Brookfield LV DV-II+ Pro EZ viscometer operating at a spindle speed of 100 revolutions per minute. A suitable spindle was selected based on the viscosity of the measured solution. The results are presented in Table 2. Table 2. Viscosities of the coating compositions

Example 3. Formation of barrier films

A film applicator (TQC Baker film applicator 80 mm) was used to apply the coating formulations obtained in Example 1 onto a paperboard substrate. The paperboard substrate had a grammage of 248 g rrr ² and a thickness of 491 pm. The use of a film applicator allows a film to be applied to the substrate quickly and without final adjustments to the rheology of the coating formula.

Approximately 10 grams of coating solution was applied in front of the film applicator. The film applicator was then drawn across the paperboard, with a 120 pm gap between the film applicator and the surface of the paperboard. The coated papers were dried at room temperature. Before carrying out any measurements, the studied samples were kept in a climate-controlled room (23±1°C and 50±2 % relative humidity) for minimum of 4 hours, following the ISO 187:1990 standard, published December 1990.

Example 4. Analysis of coating properties

Various properties of the coatings obtained in Example 3 were measured. 4.1 Thickness

The thicknesses of the coatings obtained in Example 3 were determined with an L&W Micrometer using 100 kPa pressure, 2 cm ² measuring area and 1.0 mm/s speed. An uncoated board was used as a calibration standard.

4.2 Grease barrier properties: KIT test

The grease barrier properties of the coated samples were studied using the KIT-method as set out in TAPPI Standard T 559 cm-12 (published 2012). In this method, a sample is tested with a series of numbered oil compositions of varying aggressiveness, i.e. surface tension and viscosity.

In total, 12 test solutions containing different amounts of castor oil, toluene and n-heptane were used. In each test, a drop of test solution was applied to the coated side of the sample, and wiped off after 15 seconds. All of the test solutions tested through from smallest to highest number, i.e. least aggressive to most aggressive. If the drop penetrated to the sample, a return to previous solution was made.

The KIT number was determined to correspond to the solution with highest number that left the surface undamaged. The test was repeated five times for each sample, applying the drops to different parts of the sample. The highest possible KIT rating is 12.

4.3 Water vapor transmission rate, WVTR

The water vapour transmission rate (WVTR) was determined with the Systech Model M 7002 Water Vapour Permeation Analyzer. One measurement was done for each sample. The results are presented as g/m ² /day. 4.4 Results

The results of the analyses described above are summarized in Table 3, below.

Table 3. Properties of the coated substrates

As may be seen from the table above, the inclusion of a lignosulfonate in the barrier composition decreased the permeability of the barrier layer to water vapour.

In compositions comprising dextrin as the film-forming polymer, the inclusion of a lignosulfonate dramatically increased the resistance of the film to grease.

Example 5. Barrier properties after folding or creasing

A 10 wt% aqueous solution of hydroxyethyl cellulose and a 20 wt% solution of a commercially- available lignosulfonate were prepared as described in Example 1. A barrier layer formulation as identified in Table 4 was then prepared by mixing the solutions and adding glycerol.

Table 4. Example barrier layer formulation. A comparative formulation, comprising hydroxyethyl cellulose and glycerol but lacking the lignosulfonate, was also prepared.

The example formulation had a dry content of 17.8 wt% and viscosity 346 mPas at a spindle speed of 100 RPM. The comparative formulation had a dry content of 12.4 wt% and a viscosity of 1770 mPas. Viscosity was measured as described in Example 2.

Sheets coated with each of the formulations were prepared as follows. Two layers of either the example formulation or the comparative formulation were applied to the back side of respective paperboard sheets by rod coating. The rod coating was performed using a RK K Control Coater with an infrared dryer. The drying time for the first layer was 60 seconds. The drying time for the second layer was increased to 90 seconds to minimize any unwanted interactions between the coating layers.

After rod coating, the paperboard sheets were die-cut to remove uncoated parts of the paper. The sheets, along with an uncoated control sheet, were conditioned by storage in a climate- controlled room (23±1°C and 50±2 %RH) for minimum of 4 hours, following the ISO 187:1990 standard.

After conditioning, the weight and dimensions of the die-cut sheets were measured. Coat weight was calculated from conditioned samples as the difference in grammage, measured in g nr ² ("gsm") of the coated sheet and the uncoated control.

Samples of the coated sheets were creased using a Cyklos GPM-315 creasing and perforation unit, operating in a machine direction and a cross-machine direction. Kit testing was performed using TAPPI Standard T 559 cm-12 (published 2012) as described in Example 4.2, above. The results are shown in Table 5. Table 5. Coat weights and kit values before and after creasing.

As may be seen, the barrier layer including lignosulfonate maintained its resistance to oil and grease after creasing. In contrast, a decrease in oil and grease resistance after creasing was observed for the comparative sample. Without wishing to be bound by theory, it is believed that barrier layers including lignosulfonate may have a reduced likelihood of cracking along crease lines.

It will be appreciated that the above embodiments have been described by way of example only.

Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The scope of the disclosure is not limited by the described embodiments but only by the accompanying claims.