OF MANUFACTURING A BARRIER FILM

TECHNICAL FIELD

The present disclosure relates in general to the field of barrier film for a cellulose-based material, said barrier film comprising at least one layer comprising microfibrillated cellulose (MFC).

BACKGROUND

An effective gas and/or aroma barrier and particularly oxygen barrier is required in packaging industry for shielding products that are oxygensensitive, thereby extending their shelf-life. These include many food products, in particular, but also pharmaceutical products and electronic industry products. Known packaging materials with oxygen barrier properties may be comprised of one or several polymer films or of a fibrous paper or board coated with one or several layers of an oxygen barrier polymer, usually as part of a multilayer coating structure.

More recently, microfibrillated cellulose (MFC) films, in which fibrillated cellulosic fibrils have been suspended e.g. in water, re-organized and rebonded together forming a film that is predominantly continuous good gas barrier properties have been developed.

Such films may be made by applying an MFC suspension on a porous substrate forming a wet web or film followed by dewatering of the web by draining water through the substrate for forming the film. This can be accomplished e.g. by use of a paper- or paperboard machine type of process. US201229839A teaches a method of manufacturing of an MFC film by applying a furnish comprising MFC directly on porous substrate thus allowing the MFC to be dewatered and filtered. Alternatively, the film can be made by use of casting technologies, including applying an MFC dispersion onto a non-porous cast substrate, such as a polymeric or metal substrate, and drying said film by evaporation. The publication EP 2771390 A4 describes preparation of MFC films, in which an aqueous cellulose nanofiber dispersion is coated on a paper or polymeric substrate, dried and finally peeled off as a nanofiber film sheet.

Fibrillating of cellulose fibers into MFC however results in that certain properties are lost. Examples include reduced tear strength and poor elasticity. This causes problems upon lamination and/or converting operations since MFC films tend to break and tear which destroys the barrier function.

Films made from MFC thus often comprise quite a large amount of plasticizers in order to exhibit the required stretchability. There is however a need to limit the amount of plasticizers, especially in films to be used in connection with food packages in order to comply with stipulated laws and regulations.

Moreover, a high amount of plasticizers may deteriorate the barrier properties of the MFC film.

SUMMARY

It is an object of the present disclosure to provide a barrier film with good stretchability and barrier function for use e.g. in paper and board laminate materials for packaging purposes, while avoiding the aforementioned problems connected to the converting of the film and the use of a large amount of plasticizers.

According to the invention, there is provided a barrier film for a cellulose- based material, wherein said barrier film comprises at least one microfibrillated cellulose layer (MFC layer), which MFC layer in its turn comprises a three-dimensional (3D) pattern, and wherein said MFC layer further comprises:

- an elongation at break of at least 6 %, preferably at least 8%, and more preferred above 10% measured according to standard ISO 1924-3:2005; and

- an oxygen transfer rate (OTR.) below 500 cc/m2/24h/atm, measured according to the standard ASTM F1927-20 at 50 percent relative humidity and 23°C.

According to the invention, the 3D pattern is arranged to provide an improved elongation at break (strain at break) compared to a corresponding MFC layer void of such a 3D pattern. Preferably, said 3D pattern is a micro-scale 3D pattern. Examples of 3D-patterns include discrete patterns, continuous patterns, undulations including spots, corrugated pattern, ridges, furrows, crests, peaks and valleys, grids, recesses, depressions, indentations, cavities, sags, knobs and imprints.

A suitable 3D pattern which provides increased elasticity to the material is creped pattern, or microcreped pattern. Creping involves a step of compacting the MFC layer resulting in that densely distributed small wrinkles/undulations/compactions in microscale are formed across the material forming a crinkly texture. Creped material comprises a number of irregularities per distance in machine direction (MD). In one embodiment, the crepe ratio of the creped barrier film is 5-70%, or 7- 35% or 7-30%, wherein "crepe ratio" is a measure of how much the film has shortened during creping treatment.

It is to be understood that the "cellulose-based substrate" herein refers to e.g. a paper or paperboard material or any other cellulose-based sheet, web and/or fiber article including e.g. 3D-shaped trays or bowls. It is further understood that "micro-scale" refers to dimensions ranging from 0.1-1500pm.

Thanks to the invention, there is provided a way of providing an MFCbased film that provides gas-, oil-, grease- and aroma barrier function which thanks to its improved stretchability may be subjected to lamination and converting operations with low risk of tearing and/or breaking, and thus retaining a good barrier function even after such lamination/converting.

According to one aspect of the invention, said MFC layer provided with a three-dimensional (3D) pattern has a thickness between 10-1000 pm, such as between 10-250 pm. Said thickness may be measured with standard method or with microscope such as cross section imaging.

According to another aspect of the invention, both the upper and lower (i.e. top- and back-) sides of the MFC layer undergo transformation in 3D when formed into said pattern or when subjected to mechanical deformation. An example is said process of creping whereby the layer to be 3D formed is buckled and bent, or compressed under heat and friction into an undulating and/or creased pattern where such pattern extends along a cross direction (CD).

According to yet another aspect of the invention, said 3D pattern has a neighboring peak-to-peak distance in a longitudinal and/or transverse direction of the film in the interval of 0.1-1500pm.

According to yet another aspect of the invention, said 3D pattern has a neighboring peak-to-valley distance in a machine direction of the film in the interval of 0.1-1500pm. According to yet another aspect of the invention, the MFC layer comprises at least 50 wt%, preferably at least 70 wt%, more preferably at least 80 wt% MFC, based on the total dry weight of the MFC layer.

According to yet another aspect of the invention, said MFC is coarse fibril, derived from fibrillated chemical pulp, said fibril coarse having a Schopper Riegler (SR) value between 50-95°, more preferably between 60-93°, ISO 5267-1.

According to yet another aspect of the invention, the MFC layer comprises hemicellulose at a content between 5-25wt% based on the total dry weight of the MFC layer. Hemicellulose may be provided as an additive and/or be part of the MFC used for forming the MFC layer.

According to yet another aspect of the invention, the MFC layer comprises lignin at a content between 5-30wt%, preferably between 10-30wt% based on the total dry weight of the MFC layer. Lignin may be provided as an additive and/or be part of the MFC used for forming the MFC layer.

According to yet another aspect of the invention, the water retention value of the MFC in the MFC layer (WRV) is >180%, such as 200-350%, ISO 23714.

In its simplest form, the barrier film according to the invention comprises only said MFC layer. However, the barrier film may also comprise further additional layers. For example, the barrier film may comprise at least a second MFC layer. Such a barrier film (i.e. comprising a double MFC film layer) can be advantageous when post-converting a laminate comprising said barrier. According to one example, such a barrier film comprises a first MFC layer with a basis weight between 10-40 gsm, and a second MFC layer comprising a basis weight between 5-30 gsm. According to another example, in addition to said MFC layer one layer in said barrier film may be based on refined pulp with an SR. value between 20-40°.

According to yet another aspect of the invention, the barrier film further comprises at least one of: a coating layer, a pre-coating layer, a varnish, a protective coating, a laminated cellulose-based material, a polymer layer, CMC layer, starch layer, PVOH layer, a humectant or lubricant /stearate, wax, and a metal layer or mixtures thereof.

According to yet another aspect of the invention, the MFC layer comprises PVOH in an amount between 2-50wt%, preferably 4-40wt% or 4-30wt% of the total weight of the MFC film. Preferably, PVOH has a degree of hydrolysis between 85-100 such as 88-99 mol%. The PVOH may also be modified with ethylene-, silanol-, or carboxyl groups.

According to yet another aspect of the invention, the MFC layer has an oxygen transmission rate (OTR) <100 cc/m2/24h/atm and most preferably <50 cc/m2/24h/atm measured according to the standard ASTM F1927-20 at 50% relative humidity and 23°C.

According to yet another aspect of the invention, said MFC layer has a KIT value of the barrier film of at least 10, preferably 12, as measured according to standard ISO 16532-2.

According to yet another aspect of the invention, said MFC layer has a specific surface weight between 20-100 gsm, such as 25 - 80 gsm, and a density between 100-700 kg/m3, preferably between 200-700 kg/m3.

According to yet another aspect of the invention, said MFC layer comprises less than 10 pinholes/m2, preferably less than 8 pinholes/m2, more preferably less than 2 pinholes/m2, as measured according to standard EN13676:2001. According to yet another aspect of the invention, said MFC layer has a water vapor transmission rate (WVTR) >50, preferably >100 at 23°C and 50% relative humidity according to ASTM F-1249.

The present invention also relates to a method for manufacturing a barrier film comprising at least one layer of microfibrillated cellulose (MFC). Said method comprises at least the steps of:

- providing an aqueous composition comprising MFC;

- forming a wet layer of said MFC composition,

- dewatering said wet MFC layer; and

- subjecting the layer comprising MFC to a patterning treatment resulting in that the layer acquires a three-dimensional (3D) pattern leading to that the stretchability of the resulting patterned layer is increased.

According to one aspect of the invention, said MFC layer comprises a solid content of at least 50wt%, preferably 55-98wt%, more preferably between 65-95wt% based on the total weight when subjecting it to said patterning treatment.

According to one aspect of the invention, the wet layer is a wet web layer.

The invention also relates to a cellulose-based material comprising a cellulose-based layer and a barrier film according to the invention.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 shows a schematic principle of operation of Clupac compaction method. DETAILED DESCRIPTION

In order to better understand the present invention, further aspects of the product and method are now to be described. It is to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

Microfibrillated cellulose (MFC) shall in the context of the patent application be understood to mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 1000 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils typically have a diameter less than 1000 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as its large surface area or its ability to form a gel-like material at low solids (for example, 1- 5 wt%) when dispersed in water.

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps are usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp to be utilized may thus be pre-treated, for example enzymatically or chemically, to hydrolyse or swell the fibers or to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, such that the cellulose molecules contain other (or more) functional groups than found in the native cellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), quaternary ammonium (cationic cellulose) or phosphoryl groups. After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or nanofibrils.

The nanofibrillar cellulose may contain some hemicelluloses, the amount of which is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose, or other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

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

The oxygen transmission rate (OTR.) as used in the patent claims and in the description is measured in accordance with (ASTM F1927-20, in 24 hours at 23°C, 50% R.H. Tensile tests of the MFC films for measuring strain at break (elongation at break) were measured according to standard ISO 1924-3:2005.

The term "plasticizers" as used herein is meant additives that increase the plasticity of the film. Plasticizers used in the process of the invention can e.g. be chosen from the group of sugar alcohols such as sorbitol, xylitol polyols, glycerol, or polyethers such as polyethylene glycol (PEG), cellulose derivatives such as carboxy methyl cellulose (CMC), low molecular weight cellulose derivatives, or polysaccharides such as dextrin, or a combination of any of these.

The term "film" as used herein refers generally to a thin continuous sheet formed material. Depending on the composition of the pulp suspension, the film can also be considered as a thin paper or even as a membrane.

The MFC layer (sometimes herein also referred to as MFC film) according to the invention can be manufactured by applying suspension comprising microfibrillated cellulose as the main component onto a non-porous substrate. The suspension comprises at least 50 wt% MFC calculated on the total solids content of the suspensions, the remainder being conventional additives such as e.g. longer fibers, fillers (such as clay), binders, such as PVOH or PVOH/Ac, dispersing agents or softeners etc. The suspension building up the MFC layer preferably is applied at a consistency between 0.05-10 wt%.

After the application of the wet layer, the layer is dried to a final moisture content preferably of between 0.1 - 20wt% to form said barrier film. The dewatering/drying of the barrier film, both in-between the application of subsequent webs and the final drying, may be accomplished by noncontact drying using e.g. hot air, steam, impingement drying, IR. or microwaves or by contact drying.

According to the invention, said MFC layer is arranged in a three- dimensional (3D) pattern such as in a microscale 3D pattern, preferably microcreped pattern. Thanks hereto, an improved stretchability may be obtained with only a limited amount (such as less than 10wt% based on total solid amount), or no addition of plastic additives or plasticizers. The method of the invention also enables the use of different kind of fibers in different additional layers (in addition to the MFC layer), which opens up the possibility to build up an optimized barrier structure.

According to the invention, the MFC layer comprises a 3D pattern which makes the layer more flexible and/or stretchable. One particularly suitable alternative is creping and/or microcreping. Any previously known creping process may be used without departing from the scope of the present disclosure. Creping is a process wherein a layer or film is provided with densely distributed small wrinkles/undulations/compactions resulting in some degree of shortening the film during the process. It can be made by doctoring (using a creping blade) a moist fiber- or MFC-containing web from a supporting cylinder. A suitable alternative is dry creping wherein the web is substantially dry, for example having a solid content between 90-95wt%. According to the present invention, a suitable solid content of the film is at least 50wt%, such as between 65-95wt%. Creping increases the elongation or stretch compared to a corresponding non-creped film or layer. Preferably, the creping may suitably be made to provide wrinkles or undulations of a size in the range of microns, so called microcreping. It is also plausible to perform creping such as to provide wrinkles or undulations of a size in the range of millimetres. Creping of the film is made before the film is used in a laminated structure. In one embodiment, the crepe ratio of the creped barrier film is 5-70%, or 7- 35% or 7-30%, wherein "crepe ratio" is a measure of how much the film has shortened during creping treatment.

As stated above, compacting or creping procedure or other patterning procedures may be performed in various ways known per se. Hereafter follows some non-limiting examples.

-Clupak® method is an in-plane compacting treatment of moist fiber layer resulting in improved extensibility of the material. A schematic principle of operation of Clupak compaction unit is seen in Fig. 1. Herein an MFC layer 1 is fed into the junction between a nip roll 3 and a heated steel drying cylinder 4. A rubber blanket 2 is moving along with the MFC layer and is compressed between the nip roll 3 and the drying cylinder 4 with a certain pressure P. Said rubber blanket 2 is stretched before entering the junction S, and before contacting the MFC layer 1. Once the rubber blanket 2 and the MFC layer 1 passes the nip 3, the rubber blanket 2 recoils since the straining force is released. Due to frictional forces, the MFC-layer 1 adheres to the rubber blanket 2 and therefore shrinks and gets compacted after passing the junction S resulting in that it acquires a 3D pattern such as a creped pattern. The MFC layer having passed through the Clupak equipment and thus acquired a compacted 3D pattern is referred to as 1' in Fig. 1. Instead of a rubber blanket 2, one or several rubber rolls may be used. The drying cylinder 4 is preferably heated to a temperature between 50-250°C. Clupak can be off- or online and is preferably performed on moist MFC films/layers such as with a solid content between 55-85wt%. Re-moisturing can be done in the Clupak machine before compacting the material.

-Creping the film/layer from a cylinder which is integrated in a paper machine by using a doctor blade. In the methods using doctor blades, the adhesion to the metal surface (or coated metal surface) can be adjusted by using adhesive and release agents.

-Creping the film/layer by using a doctor blade when releasing said film/layer from a metal belt in cast forming process. Possibly, the metal belt comprises a temperature between 120-250°C to warm up the substrate.

According to a preferred aspect of the invention, the waves obtained by means of creping the material is in microscale, preferably with a wavelength below 1500pm, or below 1200pm. According to a preferred aspect of the invention, the MFC layer has a grammage between 25-50 gsm before subjected to patterning treatment.

The barrier film according to the invention may be used in a multilayer laminate. Thus, the film may be applied onto a fibrous base, such as a paper, paperboard or cardboard made of semichemical, chemical- termomechanical, chemical or (chemi-) mechanical pulp. When applying the creped MFC film to the paperboard, a water based adhesive or extruded tie layer can be used to provide adhesion between the MFC layer and the substrate. The paper or paperboard can further be surface sized, or a dispersion barrier may be coated on at least one side. In one embodiment, the paperboard is coated on the print side with a mineral coating and then on inside with at least one dispersion barrier coating layer such as starch, styrene/acrylate latex, CMC or PVOH based coating.

Preferably the fibrous base is paperboard of a weight of 130 to 450 g/m2, preferably of 170 to 250 g/m2, or paper of a weight of 40 to 130 g/m2. The laminate may further comprise polymer layers, e.g. of polyethylene, or further barrier layers. Such laminates are useful e.g. for heat-sealable packages of food or liquids. In that respect, the barrier film according to the invention may be laminated with paper or paperboard using e.g. water based adhesive or extruded tie layer. Examples of such structures include:

-board/water-based adhesive/microcreped sheet;

-board/tie PE layer/microcreped sheet;

-board/water-based adhesive/microcreped sheet/dispersion barrier; and

-board/tie PE layer/microcreped sheet/PE.

The above-mentioned thermoplastic layers is preferably added on the print side of a material, opposite the barrier film side. The thermoplastic layers may be extruded of dispersion coated. The barrier film according to the invention, i.e. including an MFC layer arranged in a 3D pattern where such 3D pattern improves elasticity of the layer, is as such useful for materials intended for packaging foods or liquids, with or without coating and/or as part of a laminate. For example, the barrier film is suitable as a direct food contact wrapping material or laminate. Such wrapping materials for food contact have good grease resistance and the 3D pattern will also provide the desired flexibility that is required for wrapping purposes. The barrier film according to the invention may also be used for PET food packaging, dry snacks and/or cereals.

The method according to the invention includes at least the following steps:

-providing an aqueous composition comprising MFC; -forming a wet layer of said MFC composition;

-dewatering said wet MFC layer; and -subjecting the layer comprising MFC to a patterning treatment resulting in that the layer acquires a three-dimensional (3D) pattern leading to that the stretchability of said barrier film is increased.

The MFC layer may be part of a multilayer barrier film which in addition to said MFC layer comprises further MFC layers, films or coatings such as CMC, starch or PVOH coatings, or humectants, lubricants, stearate or wax. As an example, each such additional coating can comprise a grammage between 0.1 - 10 gsm in one or several layers on one or both sides of the MFC layer. Furthermore, such additional layers and/or coatings may be applied onto the MFC layer before arranging it into said three-dimensional pattern.

The MFC layer comprises a solid content of at least 50wt%, preferably 55- 98wt%, more preferably between 65-95wt% based on the total weight when subjecting it to said patterning treatment. This means the MFC layer comprises a certain moisture content. It is possible to perform a wetting of the MFC layer before e.g. creping operation using e.g. steam or moisturing unit, so that it acquires the suitable moisture content. Furthermore, the microcreping may be performed at high temperatures such as >120°C, or between 140-240°C which is caused by heat treatment or/and friction or friction heating.

After arranging the MFC layer into a 3D pattern, it may be used as a laminate layer in a packaging material such as a paperboard. Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and/or other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements.

As understood by those skilled in the present field of art, numerous changes and modifications may be made to the above described and other embodiments and aspects of the present invention, without departing from the scope of the present invention as defined in the appending claims.

It should be noted that the above described aspects may be the subject for its own protection, as such in a separate divisional application. Hence, it is foreseen that some aspects of the invention may require a protection by its own, e.g. since it may be applicable per se also in other concepts than that defined by the independent claims in this application.