SE2330085A1

SE2330085A1 - Solid cellulose foam comprising discrete units of cellulose foam embedded in a cellulose foam matrix

Info

Publication number: SE2330085A1
Application number: SE2330085A
Authority: SE
Inventors: Torron Daniel Hult; Martin Sterner; Maryam Ghanadpour; Oruç Köklükaya
Original assignee: Stora Enso Oyj
Priority date: 2023-02-16
Filing date: 2023-02-16
Publication date: 2024-08-17
Also published as: WO2024171124A1; SE546347C2; MX2025008286A; CN120615107A

Abstract

Abstract The present invention relates to a method for producing a solid cellulose foam, where the method comprises: providing discrete units of a first cellulose foam on a surface to obtain a first foam deposition, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm; depositing a second wet cellulose foam between the discrete units to obtain a subsequent foam deposition; and drying the second wet cellulose foam in the subsequent foam deposition to obtain a solid cellulose foam wherein discrete units of cellulose foam are embedded in a cellulose foam matrix. The present invention also relates to a solid cellulose foam comprising discrete units of a cellulose foam embedded in a cellulose foam matrix, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm.

Description

SOLID CELLULOSE FOAM COIVIPRISING DISCRETE UNITS OF CELLULOSE FOAM EMBEDDED IN A CELLULOSE FOAM IVIATRIX Field of the invention The present invention relates to a method for producing a solid cellulose foam, and to a solid cellulose foam comprising discrete units of a first foam deposition, embedded in a cellulose foam matrix of a subsequent foam deposition. The solid cellulose foam of the present invention may for example be used as a cushioning material in packaging applications.

Background Different porous materials, such as foams, are commonly used in applications such as insulation in buildings and vehicles and as packaging materials that are used to protect various goods during storage and transportation.

Depending on the item to be protected, different types of protective packaging materials can be used. For many items, a low-weight cushioning material that reduces impact shock and vibrations is used. Common examples of such cushioning materials are petroleum-based polymer foams such as polyurethane, polyethylene and expanded polystyrene. The foams used should be low-weight, stable and easy to manufacture.

Today, there is an increasing interest in replacing petroleum-based polymers with polymers from renewable resources, i.e. biobased polymers. Cellulose is the most abundant renewable natural polymer on earth and is therefore of special interest. For a cellulose foam, recycling of the material in regular recycling streams may be possible, depending on the composition of the foam.

There are several examples of cellulose foams, prepared using different methods. Drying the wet foam composition is often a critical step. Since the stability of the wet foam is typically low, moulds are commonly used to prevent the foam from collapsing during drying. WO20200011587 A1 describes a porous material that is prepared by aerating a paste comprising cellulose fibres and gluten and depositing the aerated paste in a mould where it is dried. The dried porous material has the shape of the mould. WO2015036659 A1 describes a moulded fibrous product prepared by foaming an aqueous suspension of natural fibres in combination with synthetic fibres and surfactant, feeding the fibrous foam to a mould and drying the foam by first mechanically withdrawing a part of the water followed by evaporating water to produce a dry fibrous product.

When drying a cellulose foam without restrictions, the cellulose foam will shrink in all directions, due to collapse of the foam, as tension forces pull the cellulose fibres together. Drying shrinkage is an inherent property of cellulose, as swollen cellulose fibres will collapse onto each other when water is removed from the system. Even in more complicated drying systems such as combined air impingement and lR-dryers, a shrinkage in the thickness direction of above 10% is expected. This is because of the capi||ary pressure build-up inside the material as the water level recedes during drying, leading to menisci forming between particles and as a result attractive inter particle forces. increasing the dry component of foam having less affinity to water and/or hydrophobic character reduces the attractive forces occurring due to capi||ary pressure between particles. However, the network strength forming during drying is reduced due to reduction in the fiber-fiber joint strength as well as the number average of fiber-fiber joint.

To avoid or reduce shrinkage during drying, the cellulose foam needs to dry under tension, such as in a frame or mould. The use of such restriction means will to a great extent prevent shrinkage of the foam in the width and length directions. When drying an object with a large surface area the tension obtained by the frame or mould is however limited to the regions closest to the mould. Typically, the thickness of the foam will be reduced due to shrinkage in the middle of the object, with a gradual increase in thickness towards the edges, as illustrated for a prior art foam in fig. 1. Thus, shrinkage during drying is a problem, especially when drying foam objects with large surface areas since the shrinkage is non-uniform and the thickness of the dried foam may vary along the width and length of the dry foam.

The drying time of the foam is typically long since both the wet foam and dry foam are heat insulating. A short drying time is desired to enable a cost-efficient process.

Thus, there is still a need for alternative methods for the preparation of a cellulose foam. ln addition, the prepared foam should have a high impact resistance when used as a packaging material to enable protection also of heavier objects.

Summary of the invention lt is an object of the present invention to provide solid cellulose foam, which is recyclable and made from renewable sources, and which eliminates or alleviates at least some of the disadvantages of the prior art materials. lt is a further object of the present invention to provide an improved method of obtaining a solid cellulose foam with uniform thickness and minimized non-uniform shrinkage during drying, also when producing large foam articles. lt is a further object of the present invention to provide a cost-efficient method of obtaining a solid cellulose foam where the drying time of the foam is reduced.

The above-mentioned objects, as well as other objects as will be realized by the skilled person in light of the present disclosure, are achieved by the various aspects of the present disclosure.

The present invention is defined in the independent claims. Further embodiments of the invention are defined in the dependent claims.

According to a first aspect, the present invention relates to a method for producing a solid cellulose foam, wherein the method comprises: - providing discrete units of a first cellulose foam on a surface to obtain a first cellulose foam deposition, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm; - depositing a second wet cellulose foam between the discrete units to obtain a subsequent foam deposition; - drying the second wet cellulose foam in the subsequent foam deposition to obtain a solid cellulose foam wherein discrete units of cellulose foam are embedded in a cellulose foam matrix. lt has surprisingly been found that non-uniform shrinkage of a wet cellulose foam object during drying can be significantly reduced by the method according to the first aspect, involving two foam depositions instead of only one. By providing the first wet cellulose foam deposition in the form of discrete units, the surface area of each discrete unit is sufficiently small so as to prevent non-uniform shrinkage in the height direction during drying. The dried discrete units of the first cellulose foam deposition provide support for the second wet cellulose foam during drying of the subsequent foam deposition, so that no shrinkage will occur. Thus, the obtained solid cellulose foam has a uniform height. By placing the discrete units relatively close to each other, so that the distance between adjacent discrete units is in the range of from 2 to 20 mm, the top surface of the obtained solid cellulose foam is smooth and uniform. lt has also surprisingly been found that the drying time of the wet cellulose foam is decreased by the method according to the first aspect. When drying a large object of wet cellulose foam, drying is slow due to the foam being thermally insulative. When drying the discrete units of foam, the surface area in contact with air is increased and drying is faster. Since the discrete units are placed close to each other, the total width of the subsequent wet cellulose foam deposition is small. This enables a shorter total drying time for the cellulose foam according to the present two-step deposition method, as compared to a cellulose foam having the same properties but deposited using one deposition step. A short drying time is beneficial from a cost perspective.

According to a second aspect, the present invention relates to a solid cellulose foam comprising discrete units of a cellulose foam embedded in a cellulose foam matrix, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm.

The solid cellulose foam according to the second aspect has a high impact resistance and excellent cushioning properties, and can be used in as a packaging material in various protective packaging applications. lt can also be used as a building material, or as a thermal or acoustic insulation material. The solid cellulose foam may also be used as a hydroponic plant growth media.

According to a third aspect, the present invention relates to a use of the solid cellulose foam according to the second aspect as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media.

Brief description of the drawings Figure 1 schematically illustrates a prior art method where a wet cellulose foam sheet is deposited and dried, with non-uniform shrinkage in the height direction.

Figure 2 schematically illustrates an embodiment of the two-step deposition method of the present invention, comprising: i) a first deposition of cellulose foam as discrete units, placed a distance d from each other, ii) an intermittent drying step drying the discrete units to create iii) self-standing discrete units of cellulose foam, iv) a subsequent deposition of cellulose foam between the discrete units of cellulose foam, and v) a second drying step to obtain vi) a cellulose foam material comprising discrete units of cellulose foam.

Figure 3 shows a schematic representation of a solid cellulose foam sheet produced according to the method of the present invention, where A) illustrates the solid cellulose foam with a densified layer at the top and at the bottom in black colour, and looking externally similar to foam sheets produced with other techniques, B) illustrates the bulk of the material beneath the densified top and bottom, the bulk containing discrete units of cellulose foam (black cuboids), where each discrete unit is distinguished from the surrounding foam matrix by a densified layer of cellulose, and C) illustrates one discrete unit surrounded by a densified layer of cellulose (left), illustrated in black, and the homogenous cellulose foam inside the densified layers (right).

Figure 4 shows the drying time for the subsequent foam deposition depending on the distance between discrete units in the first foam deposition. A wet cellulose foam was deposited between dry discrete units placed at distances of 5 mm (o), 8 mm (n), 10 mm (A), and 20 mm (o) from each other. The samples were dried, and the weight was measured at several time points during drying.

Figure 5 shows drying curves obtained during drying of a wet cellulose foam deposited in one single step (o), or in two steps with a first foam deposition (n) being air cut into discrete units prior to drying, and a second deposition (A) filling the gaps between discrete units formed during air cutting.

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

The term "cellulose foam", as used herein, refers to a foam comprising cellulose, and other components such as thickeners, surfactants and additives. The main component of the cellulose foam is cellulose, such that cellulose constitutes at least 70 wt% of the dry content of the cellulose foam. Cellulose is in the form of fibres, and the foam can thus also be defined to be a fibrous foam or a cellulose fibre foam. The cellulose foam may be wet or solid.

The term "wet foam", or "wet cellulose foam", as used herein, refers to a wet foam comprising cellulose, and other components such as thickeners, surfactants and additives. Gas bubbles are present within the wet foam. The wet foam is freestanding and behaves as a viscoelastic solid. This means that the wet foam has both viscous and elastic properties. The wet foam will behave as a solid, and thus be freestanding, unless a large enough force is applied so that it starts to flow and instead behave as a viscous material. Depending on the magnitude and timescale of any applied shear stress, the wet foam can show a predominantly viscous or elastic behaviour.

The term "solid cellulose foam", or "dry cellulose foam", as used herein, refers to a dry porous cellulose material that has been formed from a wet cellulose foam, i.e. a foam formed material. During the drying process, a closed wet cellulose foam is transformed into an open solid cellulose foam. The network of cellulose fibres is prevented from collapsing during drying. The solid cellulose foam will as a result have a shape that to a large extent corresponds to that of the wet cellulose foam. The dry content of the solid cellulose foam is at least 95 wt% as calculated based on the total weight of the solid cellulose foam. The shape and density of the solid cellulose foam is retained also in a non-confined state. The solid cellulose foam has an open cell structure, allowing air to occupy the pores within the foam. The solid cellulose foam can also be described as a porous material or a low-density material.

The method according to the first aspect of the present invention involves a first wet cellulose foam used in the discrete units of the first foam deposition, and a second wet cellulose foam used in the second foam deposition. ln some embodiments, the first and second wet cellulose foam are identical. ln some embodiments, the first and second wet cellulose foams are different, such as in terms of having a different composition or a different density.

The cellulose foams used in the present invention may comprise cellulose fibres in a range of from 71 to 95 wt%, such as from 75 to 95 wt%, based on the total dry weight of the cellulose foam.

Cellulose fibres suitable for use in the present invention can originate from wood, such as softwood or hardwood, from leaves or from fibre crops (including cotton, flax and hemp). The cellulose fibres suitable for use in the present invention can also originate from regenerated cellulose such as rayon and Lyocell. The cellulose fibres suitable for use in the present invention may include lignin or hemicellulose or both, or the cellulose fibres may be free from lignin and hemicellulose. Preferably, the cellulose fibres originate from wood, more preferably the cellulose fibres are pulp fibres obtained by pulping processes which liberates the fibres from the wood matrix. Pulp fibres can be liberated by mechanical pulping, obtaining mechanical pulp such as thermomechanical pulp (Tl\/IP) or chemical thermomechanical pulp (CTMP), or by chemical pulping such as Kraft pulp or pulps obtained by the sulphite process, soda process or organosolv pulping process. l\/lore preferably, the cellulose fibres are pulp fibres liberated by chemical pulping processes. The different characteristic of each cellulose fibre will affect the properties of the final cellulose foam. A cellulose fibre is significantly longer than it is wide. Cellulose fibres can have a mean width of 0.01 to 0.05 mm. The fibre length of softwood can be from 2.5 to 4.5 mm, while hardwood can have a fibre length from 0.7 to 1.6 mm, and Eucalyptus from 0.7 to 1.5 mm. However, the fibre length can vary considerably with different growing place etc. The cellulose fibres in the cellulose foam disclosed herein can have a length from 0.1 mm to 65 mm, or from 0.1 mm to 10 mm, or from 0.5 mm to 65 mm, or from 0.5 mm to 10 mm, or from 0.5 mm to 7mm. The fibre lengths may provide different mechanical Characteristics to the foam. Due to the length of fibres, they can entangle with each other and impart fibre to fibre interbonds that bring strength to the foam. The aspect ratio, i.e. the ratio of the fibre length to the fibre width, of the cellulose fibres in the cellulose foam according to the present invention can be at least 10, at least 25, at least 50, at least 75, or at least 100, which provides for preservation and stabilization of the foam structure during the drying procedure, making it possible to dry the wet cellulose foam with retained shape. The aspect ratio can be up to 6500, or preferably up to 2000.

The cellulose fibres may be modified to provide different properties to the final cellulose foam. For example, phosphorylated fibres or periodate oxidized fibres could also be used when producing a cellulose foam according to the present invenﬁon.

Preferably, the cellulose fibres are selected from wood pulp, such as softwood Kraft bleached pulp, hardwood pulp, chemical-thermomechanical pulp, and from dissolving pulp, or a combination of one or more of these. l\/lore preferably the cellulose pulp fibres are from softwood pulp, chemical-thermomechanical pulp, or dissolving pulp. Most preferably the cellulose pulp fibres are from softwood pulp, such as softwood Kraft bleached pulp.

The cellulose foams used in the present invention preferably comprises cellulose fibres in a range of from 71 to 95 wt%, such as from 75 to 95 wt%, based on the total dry weight of the cellulose foam, a water-soluble thickener in a range of from 4 to 24 wt%, such as from 5 to 20 wt%, based on the total dry weight of the cellulose foam, and at least two surfactants.

The water-soluble thickener may have a molecular weight of from 80 000-250 000 g/mol, or from 83 000-197 000 g/mol. Exemplary water-soluble thickeners are selected from carboxy methyl cellulose (GMC), methyl cellulose (MC), hydroxyethyl cellulose (HEC), ethyl hydroxyethyl cellulose (EHEC), methyl hydroxypropyl cellulose (MHPC), starch, xanthan, guar gum, and xyloglucan, or mixtures thereof.

The fact that the thickener is water-soluble facilitates recycling of the cellulose foam.

The water-soluble thickener may improve the fibre-fibre bonding strength, primarily through hydrogen bonding, in the cellulose foam. Therefore, the amount of water- soluble thickener will influence the mechanical performance of the cellulose foam, and especially the bulk of the material. A higher content of water-soluble thickener provides for a stiffer material. Thus, the water-soluble thickener enables tailoring of the mechanical properties.

The cellulose foam may also comprise a mixture of at least two surfactants. One of the at least two surfactants is preferably a fast-acting surfactant, a suitable surfactant for this purpose is an anionic surfactant, preferably a low-molecular weight anionic surfactant. The anionic surfactant may have an apparent pKa of from 3.2 to 3.8, preferably from 3.4 to 3.6, or an apparent pKa of 3.5 in a solution having a pH of from 7 to 9, preferably a pH of 8. The low-molecular weight anionic surfactant may be selected from sodium dodecyl sulphate (SDS); potassium dodecyl sulphate, sodium laureth sulphate (SLES); sodium dodecylbenzenesulphonate; sodium cocoyl sarcosinate; sodium lauroyl sarcosinate. The low-molecular weight anionic surfactant is preferably selected from sodium dodecyl sulphate (SDS); sodium p-n-dodecylbenzenesulphonate; sodium cocoyl sarcosinate; and sodium lauroyl sarcosinate. l\/lore preferably the low-molecular weight anionic surfactant is sodium cocoyl sarcosinate. The anionic surfactant may be biodegradable.

The other one of the at least two surfactants is preferably a co-surfactant. The co- surfactant may be selected from the group comprising surfactants having an apparent pKa of at least 8, or at least 9, in a surfactant solution having pH of from 7 to 9, preferably having a pH of 8; and amphoteric betaines. The co-surfactant may have maximum apparent pKa of 10. The co-surfactant preferably has a long carbon chain, more preferably a carbon chain with 14 carbon atoms (C14). The co- surfactant may be selected from high pKa fatty acids, such as from plant derived feedstock, e.g. tetradecanoic acid (myristic acid), sodium oleate, lauric acid, palmitic acid, and stearic acid; glucose based co-surfactants with an aliphatic carbon tail, such as alkyl glycosides, alkylpolyglucosides, alkyl thio-glycosides, and alkyl maltosides; amphoteric betaines, such as cocamidopropyl betaine (CAPB), and sodium cocoiminodipropionate (CADP); polyethylene glycol sorbitan monolaurate, i.e. tween® (e.g. tween® 20, tween® 80 and tween® 85); and polyoxyethylene lauryl ethers, such as polyethylene glycol dodecyl ether, pentaethylene glycol monododecyl ether and octaethylene glycol monododecyl ether.

Thus, the at least two surfactants used in the cellulose foam preferably comprise a mixture of an anionic surfactant and a co-surfactant. The molar ratio between anionic surfactant to co-surfactant may be from 0.2:1 to 3:1, preferably from 0.5:1 to 2:1. The total amount of the at least two surfactants together in the cellulose foam may be 0.6-5 wt%, or 0.8-2.0 wt%, as calculated on the total weight of the cellulose foam.

The solid cellulose foam can be re-dispersed in water and as a result be recyclable in regular paper recycling streams.

The wet cellulose foams used in the present invention may be prepared using a method comprising the following steps: - disintegrating cellulose fibres in water to obtain a slurry of cellulose fibres; - adding a water-soluble thickener to the slurry to obtain a mixture of thickener and cellulose fibres in water; - adding at least two surfactants to the mixture to obtain a fibre suspension; and - aerating the fibre suspension to obtain a wet foam, wherein the wet cellulose foam comprises 10-38 wt% cellulose fibres, 0.5-10 wt% of the water-soluble thickener, and 0.1-2 wt% surfactants, as calculated on the total weight of the wet foam, and wherein the wet cellulose foam has a density of from 70-600 kg/ms, and a yield stress of at least 80 Pa.

Addition of a water-soluble thickener increases the viscosity of the slurry and enables incorporation of enough air to generate a densely packed foam during aeration. Since the cellulose fibres are mixed in high concentrations a drainage step is not needed, which enables the use of a water-soluble bio-based thickener in high concentrations.

Addition of a fast-acting surfactant will contribute to the formation of a cellulose foam with a high density and a high viscosity as it will quickly settle at the air-water interphase during aeration. This enables a free-standing wet cellulose foam. Addition of a co-surfactant along with the fast-acting surfactant will further improve the properties of the cellulose foam since it will facilitate the action of the fast-acting surfactant. A co-surfactant having a suitable pKa and a long carbon chain further contributes to a stable fibre suspension and a stable wet cellulose foam. 11 Upon aeration the composition comprising cellulose fibres, thickener and at least tvvo surfactants will form a highly stable wet fibre foam. The aeration may be performed by mechanical agitation, and a substantial amount of air is incorporated into the material. The formation of a foam will be promoted by the surfactants. By adjusting the stability of the wet foam with the use of thickeners and surfactant combinations, a free-standing cellulose foam can be made without the use of a cross-linker or fibrillated cellulose. A good stability of the foam prevents ripening, i.e. change in bubble size, and drainage. The obtained wet foam is free-standing and does not require a mould or a forming fabric to retain its shape upon drying. The wet foam can thus be formed into a free-standing foam that is stable enough to be dried in the absence of a supporting mould without collapsing. ln some embodiments, the yield stress of the wet cellulose foams used in the present invention may be at least 80 Pa, or at least 100 Pa, or at least 150 Pa, or from 80 to 500 Pa, or from 100 to 500 Pa, or from 150 to 500 Pa. ln some embodiments, the density of the wet cellulose foams used in the present invention may be from 70 - 600 kg/ms, or from 100 - 500 kg/ms, or from 100 - 400 kg/ms, or from 125 - 375 kg/ms, or from 140 - 375 kg/ms. ln some embodiments, the wet cellulose foam used in the present invention comprises at least 10 wt% cellulose, as calculated on the total weight of the wet cellulose foam. ln some embodiments, the wet cellulose foam may comprise 10 - 40 wt%, 11 - 40 wt%, 10 - 30 wt%, 11 - 30 wt%, 10 - 20 wt%, or 11 - 20 wt%, cellulose fibres, as calculated on the total weight of the wet cellulose foam.

Because of the high solid content, the wet foam does not need to be dewatered before it is dried. The foam may be dried by evaporation at room temperature or at an elevated temperature, such as a temperature of from 40°C to140°C. After drying, the solid cellulose foam may have a density of from 10 to 80 kg/ms, or from 10 to 60 kg/ms or from 20 to 50 kg/ms. The solid cellulose foam may after drying have a solid content in the range of from 95 to 100 wt%, preferably from 98 to 100 wt%, as calculated on the total weight of the solid cellulose foam. 12 During drying a densified layer is formed on the outer surface of the wet cellulose foam and remain on the outer surface of the dried cellulose foam. The densified layer comprises cellulose fibres that are packed more tightly and partly oriented differently compared to the bulk. The densified layers have improved mechanical stability and strength as compared to the core of the cellulose foam. The core of the cellulose foam comprises a homogenous open-cell fibre network. The core is highly porous, and even though the densified layer has a denser structure than the core, it is still porous. The densified layer thus provides the cellulose foam with increased stability and mechanical strength.

The cellulose foam described above is the preferred foam to use in the cellulose foam depositions of the present invention. Alternatively, other cellulose foams, such as those disclosed in WO2016068771 A1, WO2016068787 A1, and WO2020011587 A1 may be used.

The method according to the first aspect involves providing discrete units of a first cellulose foam on a surface to obtain a first cellulose foam deposition. The distance between adjacent discrete units is in the range of from 2 to 20 mm, or from 5 to 20 mm, or from 2 to 15 mm, or from 2 to 10 mm, or from 2 to 8 mm. The discrete units may be provided on any suitable surface. For example, the surface may be a perforated metal tray or a conveyor belt. The tray may be fitted with an outer frame to provide additional support for the deposited wet foams.

The term "discrete unit" as used herein refers to an individual three-dimensional foam unit within a solid cellulose foam. ln the solid cellulose foam, the discrete unit is separated from other adjacent discrete units by a certain distance. The space between the discrete units in the solid cellulose foam is filled with a subsequent foam deposition, such that the discrete units are embedded in a foam matrix. The discrete units are distinguishable from each other and from the foam matrix.

The term "adjacent" as used herein in expression such as "adjacent discrete units", refers to discrete units that are next to each other. Adjacent discrete units may be aligned, or may be offset from each other. The distance between adjacent discrete units is measured from one discrete unit to the discrete unit that is its closest neighbour in the first foam deposition. 13 ln some embodiments, the discrete units are obtained by providing discrete units of a first wet cellulose foam on a surface; and drying the first wet cellulose foam in the discrete units. Thus, in one embodiment, the method according to the first aspect involves providing discrete units of a first wet cellulose foam on a surface to obtain a first cellulose foam deposition, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm; and drying the first wet cellulose foam in the discrete units.

The discrete units may be obtained by dispensing a first wet cellulose in discrete units on to a surface, followed by drying of the first wet cellulose foam. Dispensing may be made using any suitable means, such as by extrusion through a nozzle, or by passing the wet cellulose foam through a stencil surface.

The discrete units may be obtained by depositing a first wet cellulose foam on a surface, providing cutting lines extending through the height of the first wet cellulose foam so as to obtain discrete units, and drying the first wet cellulose foam. The first wet cellulose foam may be deposited in the shape of a plank. Cutting may be performed using a blade or knife, by using a mould, or by air cutting. During drying of the first wet cellulose foam, the foam will contract to some extent and gaps will form along the cutting lines. After drying, the discrete units of the first cellulose foam deposition will be separated by the gaps having a distance in the range of from 2 to mm.

Preferably, cutting lines are formed by air cutting. Air cutting utilizes cutting with an air jet of compressed air to provide cuts in a material. Compressed air is provided through a nozzle, the movements of which may be controlled by a computer. This enables high precision of the method. Several nozzles may be provided to increase the speed of the method by enabling simultaneous cutting. The pressure used may be in the range of from 0.1 to 5 bar, or from 0.25 to 4 bar. To cut discrete units in a wet cellulose foam, the air jet must have a sufficient speed so that the air penetrates through the height of the wet cellulose foam. The speed of the air in the jet depends on parameters such as air pressure, size of the nozzle and the density and thickness of the wet cellulose foam. As realized by a person skilled in the art, a sufficient air speed can be obtained by selection of a suitable air pressure for the wet cellulose foam to be cut. The provided cutting lines are typically thin, the size will depend on the size of the nozzle and the air pressure. The width of a cutting line 14 may typically be in the range of from 0.5 mm to 2.0 mm. Since the wet cellulose foam of the present invention is a soft material it can be cut with high precision using only a jet of compressed air. No particies, such as abrasive particies, need to be included in the airjet.

A large number of small bubbles provides stability to the wet cellulose foam and provides for a low density. The wet cellulose foam used in the present method has a sufficiently high viscosity and low density to enable the formation of discrete units that do not collapse before they are dried. Each discrete unit may thus stand by itself without collapsing before being dried.

The first wet cellulose foam in the discrete units is dried prior to depositing the second wet cellulose foam in the subsequent foam deposition. Drying of the wet discrete units may be carried out by evaporation at room temperature or at an elevated temperature, such as a temperature of from 40°C to140°C. Any suitable equipment may be used. ln some embodiments, the discrete units of wet cellulose foam are at least partially dried when the second wet cellulose foam of the subsequent foam deposition is deposited. ln some embodiments, the discrete units of wet cellulose foam are completely dried when the second wet cellulose foam of the subsequent foam deposition is deposited.

Each discrete unit may be self-standing during drying without collapsing. Drying of the discrete units of wet cellulose foam is at least made until a crust, i.e. a thin densified layer of cellulose fibres, is formed on an outer surface of the discrete unit, such as on each of the faces of the discrete unit. The densified layer is a very thin layer that is formed on the very outer surface of the cellulose foam during drying. The densified layer is made up of cellulose fibres that are mainly oriented in a two- dimensional plane (x-y-plane), while the fibres in the bulk of the cellulose foam comprises clusters of fibres oriented in a three-dimensional space with more empty space in between clusters. The two-dimensional structure of cellulose fibres in the densified layer transitions rapidly, but gradually, to the three-dimensional structure found in the bulk of the cellulose foam. The thin thickness of the densified layer implies that it practically does not affect the overall density of the cellulose foam, while it still contributes to the good mechanical properties of the discrete units.

When the discrete units of cellulose foam fibres have been dried, their core consists of a homogeneous fibre network having a density, and their outer surfaces, such as their bottom, top and side faces, consists of a more densely packed fibre network, i.e. the densified layer. The formation of a densified layer on the faces of the discrete units in the first cellulose foam deposition makes the discrete units stronger and prevents them from being demolished during the subsequent foam deposition, when a wet cellulose foam is deposited between the discrete units. ln an alternative embodiment, the discrete units may be obtained by depositing a first wet cellulose foam, drying the first wet cellulose foam, and cutting the dried first cellulose foam into discrete units. The wet cellulose foam may be deposited by extrusion or casting. The dried discrete units are deposited on a surface. Preferably the wet foam is extruded into a board, plank, bar or rod. The board, plank, bar or rod can be cut into discrete units, which may be deposited on to a surface. ln this embodiment, the discrete units will not have a densified layer on the faces of the discrete unit, since the discrete units are cut from a dry foam and not dried individually. lt may in some applications be advantageous to provide discrete units without a densified layer, since the fibre-fibre joint strength between cellulose fibres in the discrete units and in the subsequent foam deposition will be improved.

The solid cellulose foam in the discrete units of the present invention may have a density of from 10 to 80 kg/ms, or from 10 to 60 kg/ms or from 20 to 50 kg/ms. The dry content of the solid cellulose foam in the discrete units may be at least 95 wt%, as calculated based on the total weight of the solid cellulose foam.

Each discrete unit may have a three-dimensional shape. Preferably, all the discrete units in the first deposition have the same shape. The discrete units may have any suitable three-dimensional shape, such as a cylinder or a polyhedron. Each discrete unit has a height, a width, a length, a top surface, and a bottom surface. The height of the discrete unit is measured perpendicular to the surface on which the unit has been deposited. The width and length are measured at the top part of the discrete unit. For cylindrical discrete units, the width corresponds to the diameter. ln some embodiments, each discrete unit has the shape of a polyhedron, such as a cuboid, cube, or prism, such as a hexagonal prism. Small variations in the symmetry of the discrete units may exist without changing their main purpose to impart stability 16 to the foam. For example, the cube, cuboid, or prism may be slightly distorted so that their opposite bases are not always exactly parallel and over each other. The top surface of each discrete unit may for example be in the form of a rectangle, square, rhombus, octagon or hexagon.

Depending on the shape of the discrete unit, the width of the discrete unit may be the same along the entire height of the discrete unit, or it may be different. ln some embodiments, the width is smaller at the top part of the discrete unit than at the bottom part of the discrete unit, due to contraction of the wet cellulose foam during drying. This means that the side walls of the discrete unit may be slanting.

The distance between adjacent discrete units is in the range of from 2 to 20 mm, or from 5 to 20 mm, or from 2 to 15 mm, or from 2 to 10 mm, or from 2 to 8 mm. The distance is measured perpendicularly from an edge of the top surface of one discrete unit to an edge of the top surface of an adjacent discrete unit, as illustrated in figure 2. Preferably, the distance between adjacent discrete units is the same in all parts of the solid cellulose foam. The discrete units may be provided in any suitable pattern. Adjacent discrete units may be placed parallel to each other or offset from each other. For example, the discrete units may be placed in a pattern selected from a chevron pattern, a honeycomb pattern, a diamond pattern, a ribbed pattern, or a square pattern. ln embodiments where the width of the discrete unit is smaller at the top part of the discrete unit than at the bottom part, the distance between adjacent discrete units will be larger at the top part than at the bottom part. ln one embodiment, the width of each discrete unit is in the range of from 0.9 to 1.3, or from 0.9 to 1.2, or from 0.9 to 1.1 times its height. When the width of each discrete unit is similar to its height, non-uniform shrinkage during drying of the discrete units in the first foam deposition is minimized.

The length of each discrete unit may be similar to the width, or it may be significantly longer. ln one embodiment, each discrete unit extends along the entire length or width of the solid cellulose foam, creating a ribbed structure. ln other embodiments, the length of each discrete unit is similar to its width, and a plurality of discrete units are placed both in the direction of the width and length of the solid cellulose foam. 17 A second wet cellulose foam is deposited between the discrete units of the first foam deposition to obtain a subsequent foam deposition. The second wet cellulose foam may in some embodiments be identical to the first wet cellulose foam used in the discrete units. This facilitates processing and ensures that the obtained solid cellulose foam has similar properties in the entire foam. ln other embodiments, the second wet cellulose foam may be different from the first wet cellulose foam, for example in terms of composition or density. For example, the density of the first wet cellulose foam may be higher than that of the second wet cellulose foam. The final properties of the solid cellulose foam can be tailormade by providing wet cellulose foams with different properties.

To ensure that no voids are present within the solid cellulose foam, it is important to ensure that the second wet cellulose foam completely fills the gaps separating the discrete units of the first cellulose foam deposition. The second wet cellulose foam may be deposited using any suitable means, such as by extrusion. ln one embodiment, vacuum is applied during deposition of the second wet cellulose foam, to ensure that the gaps between adjacent discrete units are entirely filled with the second wet cellulose foam. ln such embodiments, the air pressure on the bottom side of the first foam deposition is decreased when the second wet cellulose foam is deposited. This means that the wet cellulose foam will be forced into the gaps by the difference in air pressure. The difference in air pressure must be large enough to overcome the yield stress of the wet cellulose foam and initiate flow, and also large enough to force the wet foam into the narrow gaps. To ensure that the vacuum does not cause expansion and subsequent collapse of bubbles in the wet cellulose foam, the vacuum must be weak. lf the applied vacuum is too high, the gas in each bubble in the foam would expand with great force in order to occupy the volume that the gas would have in equilibrium at such low pressure. The foam would break due to the strain of such expansion. Vacuum-assisted deposition is facilitated by the wet cellulose foam being impermeable to air, while the solid cellulose foam is air- permeable.

As an alternative to applying vacuum during deposition of the second wet cellulose foam, the wet foam may be pushed down in the gaps with increased air pressure, with mechanical means or by an increase in pressure within the wet foam itself. 18 ln some embodiments, an increased air pressure is applied on top of the second wet cellulose foam to force the foam down into the gaps. ln some embodiments, mechanical means, such as a scraper or roller, is used to mechanically force the second wet cellulose foam down into the gaps. ln some embodiments the second wet cellulose foam has an increased pressure. When the second wet foam exits the depositor, it is pressurized which will facilitate filling of the gaps between the discrete units of the first deposition. Filling the gaps is further facilitated if other paths than going down into the gaps are at least partially restricted. One way to partially restrict other paths is to put a pipe collar on an exit pipe from the depositor and position the exit pipe close to the gaps to be filled.

The height of each discrete unit in the first foam deposition may be from 0.7 to 1.0, or from 0.7 to 0.95 or from 0.75 to 0.9 times the height of the second wet cellulose foam in the subsequent foam deposition. ln embodiments where the height of the discrete units is lower than that of the surrounding foam matrix of the subsequent foam deposition, a smooth top surface of the solid cellulose foam is obtained. ln such embodiments, the discrete units are not visible from the top surface of the solid cellulose foam since they are covered by the foam matrix. The surface of the subsequent deposition of wet foam may be scraped before drying to provide an even surface.

The second wet cellulose foam in the subsequent foam deposition is dried in order to obtain a solid cellulose foam wherein discrete units of cellulose foam are embedded in a cellulose foam matrix. The discrete units are formed in the first deposition step, and the cellulose foam matrix is formed in the subsequent deposition step.

Drying of the wet cellulose foam in the subsequent deposition may be carried out by evaporation at room temperature or at an elevated temperature, such as a temperature of from 40°C to140°C. Any suitable equoment may be used. After drying, the solid cellulose foam in the cellulose foam matrix may have a density of from 10 to 80 kg/ms, or from 10 to 60 kg/ms or from 20 to 50 kg/ms. The dry content 19 of the solid cellulose foam in the cellulose foam matrix may be at least 95 wt%, as calculated based on the total weight of the solid cellulose foam. ln embodiments where the wet cellulose foam in the discrete units was only partially dried prior to deposition of the second wet cellulose foam in the subsequent deposition, the discrete units will be dried along with the second wet cellulose foam. lt is however preferred to completely dry the discrete units prior to the subsequent deposition. ln a wet cellulose foam, resistance forces keep the cellulose fibres in place. During drying, the water level between the fibres recedes causing capillary forces to build up inside the cellulose foam material, and when the capillary forces exceed the resistance forces, the fibres slip. As water evaporates, the resistance forces increase and causes the fibres to get stuck in a position closer to each other than before drying, which causes the material to shrink. On a macroscopic level, the geometry of the wet cellulose foam influences the direction and magnitude of tension vectors that build up in the material during drying. Points of contact, such as a frame or a perforated mould, causes tension in the opposite direction and will impact the net tension forces. Deformation, such as shrinkage, will occur when the net tension forces, i.e. the tension vector, dominate in any particular direction. Thus, the ratio of the width to the height of the wet cellulose foam deposition affects the distribution of the net tension forces in the foam upon drying, and the greater the ratio, the greater the net tension forces that arise.

The method according to the first aspect of the present invention reduces the net tension forces arising in the wet cellulose foam upon drying since the discrete units each have a low width to height ratio. Non-uniform shrinkage of the wet cellulose foam in the discrete units in the height direction may thus be minimized or avoided. ln comparison to drying of a large wet cellulose foam deposition with a large surface area, the surface area of the wet cellulose foam deposition in the present invention is divided into a multitude of discrete units, each having a relatively small surface area, with a favourable width to height ratio. Figure 2 illustrates one embodiment of the method of the present invention, wherein a wet cellulose foam is deposited on a surface as discrete units to obtain a first foam deposition (i). The discrete units are placed a distance d from each other. All discrete units have the same size and shape. Each discrete unit has a low aspect ratio of height to width, which eliminates or significantly reduces non-uniform shrinkage of each discrete unit when the foam is dried (ii). When the discrete units are dried, they will consist of a core comprising a homogeneous fibre netvvork, and densified outer faces (i.e. the top, bottom and sides) (iii). A subsequent deposition of a second wet cellulose foam is then made on the surface between the already dried discrete units (iv). When the wet foam of the subsequent deposition is being dried (v), the discrete units of the first deposition that are already distributed on the surface provide for a low width to height ratio of the wet cellulose foam in the subsequent deposition. The tension forces of each discrete unit will act against each other thus reducing the net tension forces in the wet foam of the subsequent deposition, which restrains build-up of the tension during drying and as a result mitigates the effect of shrinkage on the outer dimensions of the obtained solid cellulose foam (vi). The method of the present invention thus provides for formation of a solid cellulose foam object with a uniform height, since non- uniform shrinkage in the thickness (i.e. height) direction is minimized. Further, the present method allows for the formation of a foamed object without having to use a mould with walls, which implies that very large objects can be produced with this method, such as boards or planks for use in large constructions, such as buildings, and other large structures.

A solid cellulose foam obtained by a method according to one embodiment of the present invention is illustrated in figure 3. Discrete units of cuboid shape and surrounded by a densified layer (black) are present in the bulk of a solid cellulose foam, and surrounded by a foam matrix (B). The top and bottom layers of the solid cellulose foam also comprise densified layers (A). Apart from the densified layers, the foam in the discrete units and in the foam matrix is composed of a homogenous foam.

When drying a large object of wet cellulose foam deposited as a single foam deposition, drying is slow due to the foam being thermally insulative. ln the method according to the present invention, instead of depositing one single foam deposition, two foam depositions are provided. When drying the discrete units of the first foam deposition, the surface area in contact with air is increased and drying is relatively fast. Thus, a shorter total drying time for a cellulose foam is enabled by the present two-step deposition method, as compared to a cellulose foam of the same size obtained by a traditional one-step deposition method. A short drying time is beneficial from a cost perspective. The term "total drying time" as used herein, refers 21 to the sum of the drying time for the first foam deposition and the drying time for the subsequent foam deposition. For a one-step foam deposition, the total drying time is the time required to dry the entire wet foam. lt has been found that the drying time of the first foam deposition is generally faster than the drying time of the subsequent foam deposition. By placing the discrete units of the first foam deposition close to each other, as in the present invention, the drying time of the subsequent deposition, and thus the total drying time of the foam, is reduced since the width of the subsequent foam deposition become thinner. A wet foam deposition of thin width dries faster than a wet foam deposition of thick width. By selecting a narrow distance between discrete units, the width of the subsequent foam deposition become even thinner, and the total drying time is further reduced.

The height of the solid cellulose foam may be in the range of from 1 to 20 cm, or from 1 to 10 cm, or from 1 to 6 cm, or from 4 to 6 cm. The width and length of the solid cellulose foam are not particularly limited, but may both be in the range of from 60 cm to 400 cm, such as from 100 cm to 300 cm, depending on the equipment used for production and the desired dimensions. The height of the solid cellulose foam corresponds to the height of the cellulose foam of the subsequent deposition. ln one example, the solid cellulose foam is in the shape of a plank having a thickness of 5 cm.

The solid cellulose foam may have a density of from 10 to 80 kg/ms, or from 10 to 60 kg/ms or from 20 to 50 kg/ms. The dry content of the solid cellulose foam may be at least 95 wt%, as calculated based on the total weight of the solid cellulose foam.

The number of deposited discrete units within the solid cellulose foam depends on the size of the solid cellulose foam to be produced, the size of the discrete units and the distance between adjacent discrete units, as well as on the deposition method. With the two-step deposition method according to the present invention foamed objects can be produced with reduced non-uniform shrinkage compared to similar foamed objects obtained with a single-step deposition. The present method also enables a reduced drying time as compared to a foam object of similar size and obtained with a one-step deposition method. The present method also allows for a process with continuous formation of a foamed object, such as a continuously foamed web. 22 The present invention provides for a low-density cellulose foam comprising discrete units of cellulose foam, with stiffer densified cellulose fibre walls, embedded in a cellulose foam matrix. The incorporation of the discrete units as structural elements in solid foams, enables the formation of stiffer foams while maintaining the same low density. The discrete units constitute from 30 to 90%, or from 40 to 90%, or from 50 to 90%, or from 60 to 90% of the total volume of the foamed material comprising the discrete units and the surrounding foam matrix.

By the distance between adjacent discrete units in the solid cellulose foam being in the range of from 2 to 20 mm, it is ensured that an even surface is obtained, with no depressions in the interfaces between the discrete units and the foam matrix in the top or bottom surface of the solid cellulose foam. The cross section of the solid cellulose foam is also more homogenous. At larger distances between adjacent discrete units, depressions in the top and bottom surface of the solid cellulose foam may be formed in the interface between the discrete units and the foam matrix. ln some embodiments, the density of the first foam deposition is higher than the density of the subsequent foam deposition. For example, the density of the first foam deposition may be at least 110 %, such as 130%, or 150% or 200%, higher than the density of subsequent foam deposition. ln one embodiment, the density of the first foam deposition may be in the range of from 105 to 500%, or 110 to 330%, or 110 to 250%, or 110 to 200%, or 150 to 330% higher than the density of the subsequent foam deposition.

When the density of the first foam deposition is higher than the density of the subsequent foam deposition, it has been found that the total drying time of the foam decreases, thus enabling a more efficient process. ln addition, the higher density regions of the foam (i.e. the discrete units) will have different properties, for example stiffness, compared to the lower density regions (i.e. the foam matrix). ln some embodiments, the density of the discrete units varies in different parts of the first foam deposition. The density and properties of the final solid cellulose foam can be adjusted by using discrete units with different densities. For example, a solid cellulose foam with regions of different stiffnesses can be obtained. 23 The density of the wet cellulose foam depends on how much air that is included during foaming. lf a low density is desired, relatively more air should be included. lf a high density is desired, relatively less air should be included. The density of the wet cellulose foam will have a direct influence on the density of the solid cellulose foam after drying. Thus, density differences between the wet foams of the first deposition and the subsequent deposition will remain also in the solid cellulose foam. ln some embodiments, a coating may be applied to any surface of the solid cellulose foam, and/or to the first or subsequent foam deposition(s). The coating is preferably applied in the form of a liquid coating composition, and one or several coating layers can be applied. The composition of the coating layers may be the same or different. The coating may comprise at least one particulate material and at least one film- forming material. The particulate material may be selected from at least one of microfibrillated cellulose (MFC), cellulose fibres or mineral particles such as clay or calcium carbonate. l\/IFC shall in the context of the present application mean a cellulose particle, fibre or fibril having a width or diameter of from 20 nm to 1000 nm. The film-forming material may be selected from at least one of carboxymethyl cellulose (GMC), cellulose ethers, starch, polyvinyl alcohol or synthetic latexes, such as acrylic or styrene-butadiene latexes. The coating may comprise at least one hydrophobic agent, for example selected from at least one of a wax, such as bee's wax or carnauba, alkyl ketene dimer (AKD) or alkyl succinic anhydride (ASA).

By application of a coating, the air permeability of the solid cellulose foam decreases since pores on the surface of the foam are closed by the coating. This facilitates various processing and converting operations involving vacuum. ln addition, and depending on the type of coating, properties such as strength and hydrophobicity of the solid cellulose foam may be altered by application of a coating. The coating is preferably applied to a surface of the foam comprising a densified layer.

According to a second aspect, the present invention relates to a solid cellulose foam comprising discrete units of a cellulose foam embedded in a cellulose foam matrix, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm. The solid cellulose foam according to the second aspect may be produced by the method according to the first aspect. The solid cellulose foam according to the second aspect may be further defined as set out above with reference to the first aspect 24 According to a third aspect, the present invention relates to a use of the solid cellulose foam according to the second aspect as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media. The solid cellulose foam of the present invention has excellent cushioning properties which are desirable in the field of protective packaging. lt also has thermal and acoustic insulation properties.

Examples EXAMPLE 1 Drvind time, subseduent foam deposition A wet cellulose foam having a dry content of 15.8 wt% was prepared. The cellulose foam comprised 10 wt% GMC, 1 wt% surfactants (mixture of myristic acid and sodium cocoyl sarcosinate) and 89 wt% cellulose fibres (softwood bleached Kraft pulp), all weights based on the total weight of the solid content of the foam. Cellulose fibres (125 g, softwood bleached Kraft pulp fibres) were disintegrated in 700 mL water using a Kenwood Chef XL Titanium mixer equipped with the K beater. Cl\/IC (13.5 g) was added as a dry powder after achieving adequate pulping of the cellulose fibre suspension and mixed with the K beater until reaching a homogenous mixture. Thereafter, a surfactant solution (20 wt%) containing 1:1 molar ratio of sodium cocoyl sarcosinate:myristic acid was added (8 ml) to the cellulose fibre/Cl\/IC solution mixture. The mixture was then aerated using the balloon whipper of Kenwood mixer until the desired amount of air was mechanically introduced to the mixture. After mixing, foam was collected in a 250 ml plastic cup and density was measured (target density 188 kg/ms).

Discrete units of the wet cellulose foam were deposited on a perforated tray and dried in a convection oven at 120°C. The discrete u1its had a height of 5 cm, a width of 5 cm and a length of 14 cm. The discrete units were placed so that the distance between adjacent units were either 5 mm, 8 mm, 10 mm, or 20 mm.

Next, a second deposition of a wet cellulose foam was made. The wet cellulose foam of the second deposition was the same foam as used for the discrete units. Wet foam was deposited between the discrete units so as to obtain a foam matrix surrounding the discrete units.

After deposition of the wet cellulose foam, the samples were dried in an oven at 120°C. The samples were weighed at several time pohts (see figure 4). lt is evident that the drying time decreases when the distance between adjacent discrete units decreases.

EXAMPLE 2 Drvino time of foam manufactured with two-step deposition A wet cellulose foam (comprising (based on dry weight) 88 wt% softwood bleached kraft pulp, 10 wt% Cl\/IC and a surfactant mixture of myristic acid and cocoyl sarcosinate) having a dry content of 15 wt% and a density of 154 kg/ms was deposited in a wooden frame (2.1x24.5x43.5 cm) on a perforated tray. The surface was made even by scraping the top with a scraper resting on the frame. Afterwards the foam was cut with the help of 16 air nozzles facing downwards perpendicular to the foam surface. The nozzles where fixated in a row with 20 mm in between each nozzle exit. The exit pipe of each nozzle had a diameter of 0.75 mm and was sufficiently long to achieve a directed air beam. The nozzles were connected to pressurized air with the pressure of between 1 and 1.5 bar, and placed at a height 3 mm above the foam. The foam was moved in the horizontal plane along its width direction, with a speed of 0.2 m/s, passing under the row of nozzles, forming parallel cuts 20 mm apart in the wet foam. The cuts were perpendicular to the side of the foam. The foam was then turned 90° and cut again wth the same row of nozzles. For the second cutting the foam was thus moved along its length direction, with the same procedure as for the first cutting, forming parallel cuts that were crossing the first cuts. The cut-out pattern was a square grid pattern, and the cuts penetrated the foam completely. Both the first set of cuts and the second set of cuts were continuous, extending from one side of the wet foam to the opposite side. The cut out discrete units were of cuboid shape and completely separated from each other. The distance between adjacent discrete units, as measured from the edge of the top of one discrete unit to the edge of the top of an adjacent discrete unit, was 4 mm.

The obtained first foam deposition was dried in an oven at 120°C. During drying, the foam was weighted at several points in time. The obtained drying curve is shown in figure 5 (air cut first step 2x2 cm). The drying time of the first foam deposition was 50 minutes. 26 After drying of the first foam deposition, a second wet cellulose foam with the same composition and properties as the foam used for the first foam deposition was applied on top of the discrete units of the dry first foam deposition. Vacuum was applied from below to ensure that the wet foam filled the gaps between adjacent discrete units of the first foam deposition. The wet foam, still confined within the same wooden frame, was scraped to make the surface even. The resulting foam plank was dried using the same conditions as for the first foam deposition. Again, the foam was weight at several points in time. The obtained drying curve is shown in figure 5 (subsequent filling after air cutting). The drying time of the subsequent foam deposition was 49 minutes.

The total amount of water that evaporated in the two consecutive drying steps was 390 g. The total drying time of the foam was 99 minutes. After both drying steps the density of the foam was 38 kg/ms.

EXAl\/IPLE 3 (comparative example) Drvinq time of foam manufactured with single-step deposition A reference foam with higher foam density was used to make a reference foam plank in the same frame as described in example 2. The composition and dry content of the reference foam was the same as described for the foam in example 2.

The wet cellulose foam used in example 3 contained a similar amount of water as the amount of water evaporated during drying of the foam in example 2. The wet cellulose foam had a density of 206 kg/ms. The reference plank was dried as described in example 2, the foam plank was weighted at several point in time. The obtained drying curve is shown in figure 5 (single step deposited plank). The density of the dry foam was 39 kg/m3. The drying time was 124 minutes.

Thus, the total time required for removing the same amount of water is shorter when using a two-step deposition method than a single-step deposition method. ln view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims A method for producing a solid cellulose foam, wherein the method comprises: - providing discrete units of a first cellulose foam on a surface to obtain a first foam deposition, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm; - depositing a second wet cellulose foam between the discrete units to obtain a subsequent foam deposition; - drying the second wet cellulose foam in the subsequent foam deposition to obtain a solid cellulose foam wherein discrete units of cellulose foam are embedded in a cellulose foam matrix. The method according to c|aim 1, wherein the discrete units are obtained by providing discrete units of a first wet cellulose foam on a surface; and drying the first wet cellulose foam in the discrete units. The method according to c|aim 1, wherein the discrete units are obtained by extruding a first wet cellulose foam, drying the extruded first wet cellulose foam, and cutting the dried first cellulose foam into discrete units. The method according to any one of the preceding claims, wherein the wet cellulose foam used in the first and subsequent foam depositions comprise at least 10 wt% cellulose, as calculated based on the total weight of the wet cellulose foam. The method according to any one of the preceding claims, wherein the wet cellulose foam used in the first and subsequent foam depositions has a density in the range of from 70 to 600 kg/ms. The method according to any one of the preceding claims, wherein vacuum is applied during deposition of the second wet cellulose foam between the discrete units of the first foam deposition.The method according to any one of the preceding claims, wherein the width of each discrete unit is in the range of from 0.9 to 1.3 times its height. The method according to any one of the preceding claims, wherein the height of each discrete unit is from 0.7 to 1.0 times the height of the wet cellulose foam in the subsequent foam deposition. A solid cellulose foam comprising discrete units of a cellulose foam embedded in a cellulose foam matrix, wherein the distance between adjacent discrete units is in the range of from 2 to 20 mm. The solid cellulose foam according to claim 9, wherein the solid cellulose foam comprises at least 75 wt% cellulose fibres as calculated on the total weight of the foam. The solid cellulose foam according to any one of claims 9 or 10, wherein the solid cellulose foam has a density in the range of from 10 to 80 kg/ms. The solid cellulose foam according to any one of claims 9-11, wherein the width of each discrete unit is in the range of from 0.9 to 1.3 times its height. The solid cellulose foam according to any one of claims 9-12, wherein the height of each discrete unit is from 0.7 to

1. 0 times the height of the wet cellulose foam composition in the subsequent foam deposition. The solid cellulose foam according to any one of claims 9-13, wherein each discrete unit is surrounded by a densified layer of foam. Use of the solid cellulose foam according to any one of claims 9-14 as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media.