Method for producing a solid cellulose foam

Abstract

The present invention relates to a method for producing a solid cellulose foam the method comprises the steps of: providing a wet cellulose foam; depositing the wet cellulose foam on a supporting surface to obtain a wet cellulose foam deposition; subjecting the wet cellulose foam deposition to an initial drying step so as to form partially dried cellulose foam sections, wherein the initial drying is carried out by means of heat transferring spikes extending at least partially through the wet cellulose foam from a first surface towards a second surface, and wherein the partially dried cellulose foam sections are formed adjacent to the heat transferring spikes; subjecting the wet cellulose foam deposition to a final drying step so as to obtain a solid cellulose foam, wherein the partially dried cellulose foam sections provide support for the wet cellulose foam during the final drying step; removing the heat transferring spikes so as to obtain a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam, wherein the cavities are formed by the heat transferring spikes.

Description

[0001] METHOD FOR PRODUCING A SOLID CELLULOSE FOAM

[0002] Field of the invention

[0003] The present invention relates to a method for producing a solid cellulose foam, and a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam. The solid cellulose foam of the present invention may for example be used as a cushioning material in packaging applications.

[0004] Background

[0005] 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.

[0006] 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, dimensionally stable and easy to manufacture.

[0007] 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.

[0008] There are several examples of cellulose foams, prepared using different methods. Drying the wet foam 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. W020200011587 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.

[0009] 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 IR-dryers, a shrinkage in the thickness direction of above 10% is expected. This is because of the capillary 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 capillary 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.

[0010] 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 a foam 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 foam object, with a gradual increase in thickness towards the edges. 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.

[0011] The drying time of a cellulose foam is typically long since both wet cellulose foams and dry cellulose foams are heat insulating. During drying, removal of water from the core of the foam is slowed down by formation of a heat insulating layer on the surface of the foam. The heat insulating layer grows gradually thicker during drying, reducing mass transfer of water further. Cellulose foams typically comprise thickeners to provide stability to the foam, and during drying the thickeners may form a film on the surface of the foam, further impairing mass transfer of water and slowing down drying. A short drying time is desired to enable a cost-efficient process, since drying of the foam is typically the most energy-consuming, and thus costly, step of the production process.

[0012] Thus, there is still a need for alternative methods for the preparation of a cellulose foam. In addition, the obtained foam should have a sufficiently high impact resistance for use as a cushioning material in packaging applications.

[0013] Summary of the invention

[0014] It is an object of the present invention to provide a 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.

[0015] It is a further object of the present invention to provide an improved method of obtaining a solid cellulose foam with a substantially uniform thickness and minimized non-uniform shrinkage during drying, also when producing large foam articles.

[0016] It 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.

[0017] 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.

[0018] The present invention is defined in the independent claims. Further embodiments of the invention are defined in the dependent claims.

[0019] According to a first aspect, the present invention relates to a method for producing a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam from a first surface of the solid cellulose foam towards a second surface of the solid cellulose foam, wherein the method comprises the following steps: providing a wet cellulose foam; - depositing the wet cellulose foam on a supporting surface so as to obtain a wet cellulose foam deposition comprising a first surface and a second surface;

[0020] - subjecting the wet cellulose foam deposition to an initial drying step so as to form partially dried cellulose foam sections, wherein the initial drying is carried out by means of heat transferring spikes extending at least partially through the wet cellulose foam deposition from the first surface towards the second surface, and wherein the partially dried cellulose foam sections are formed adjacent to the heat transferring spikes;

[0021] - subjecting the wet cellulose foam deposition to a final drying step so as to obtain a solid cellulose foam, wherein the partially dried cellulose foam sections provide support for the wet cellulose foam during the final drying step;

[0022] - removing the heat transferring spikes so as to obtain a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam, wherein the cavities are formed by the heat transferring spikes.

[0023] It has surprisingly been found that non-uniform shrinkage in the height direction of a wet cellulose foam deposition during drying can be significantly reduced by the method according to the first aspect, where heat-transferring spikes are used during drying of the wet cellulose foam. Typically, a wet cellulose foam deposition would experience non-uniform shrinkage during drying, with the resulting dry foam having a lower thickness in the middle than around the edges of the foam. The heattransferring spikes are distributed along the wet cellulose foam deposition and during drying the wet cellulose foam adjacent to the heat-transferring spikes will become partially dry during an initial drying step. Thus, partially dried foam sections are formed. These partially dried foam sections will provide support for the remaining wet cellulose foam during the final drying step, so that shrinkage in the height direction is minimized.

[0024] It has also been found that the drying time of the wet cellulose foam deposition decreases by the method according to the first aspect. Typically, drying of a wet cellulose foam deposition is slowed down by the formation of a thermally insulating layer on the surfaces of the cellulose foam. By use of heat-transferring spikes during drying, heat is provided not only from the outer surfaces of the foam, but also at several locations within the foam where the heat-transferring spikes are present. A short drying time is beneficial from a cost perspective.

[0025] According to a second aspect, the present invention relates to a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam from a first surface of the solid cellulose foam towards a second surface of the solid cellulose foam, and wherein the solid cellulose foam is formed from a single deposition of wet cellulose foam. The solid cellulose foam according to the second aspect has a substantially more uniform height in all parts of the solid foam except for where the cavities are situated, as compared to a solid cellulose foam formed from a single deposition of wet cellulose foam where no heat transferring spikes are used during drying.

[0026] The solid cellulose foam according to the second aspect provides impact resistance and cushioning properties, and is therefore a good choice as packaging material in various protective packaging applications. It 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. The solid cellulose foam is made from renewable resources and can be re-dispersed in water and as a result be recyclable in regular paper recycling streams.

[0027] 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.

[0028] The cellulose foam of the present invention preferably comprises in the range of from 71 to 95 wt% cellulose fibres, as calculated on the total weight of solid content in the foam, in the range of from 4 to 24 wt% of a water-soluble thickener, as calculated on the total weight of solid content in the foam, and at least two surfactants.

[0029] Brief description of the drawings

[0030] Figure 1 shows the drying times of a wet cellulose foam (20x20x5 cm) with no heat transferring spikes used during drying (■), or with 66 (•), 90 (A) or 126 (♦) heat transferring spikes used respectively. The water content of the drying cellulose foam was measured at several points in time during drying.

[0031] Detailed description

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The term “solid cellulose foam”, or “solid 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.

[0036] The cellulose foam preferably used in the method according to the present invention will now be described in detail.

[0037] The cellulose foam 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.

[0038] 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 (TMP) 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. More 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 to 65 mm, or from 0.1 to 10 mm, or from 0.5 to 65 mm, or from 0.5 to 10 mm, or from 0.5 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.

[0039] 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 invention.

[0040] 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. More 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.

[0041] The cellulose foam 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.

[0042] 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 (CMC), 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.

[0043] 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.

[0044] 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. More preferably the low-molecular weight anionic surfactant is sodium cocoyl sarcosinate. The anionic surfactant may be biodegradable.

[0045] The other one of the at least two surfactants is preferably a co-surfactant. The cosurfactant 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 cosurfactant 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.

[0046] 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 dry weight of the cellulose foam.

[0047] The solid cellulose foam can be re-dispersed in water and as a result be recyclable in regular paper recycling streams.

[0048] The wet cellulose foam may be prepared using a method comprising the following steps:

[0049] - disintegrating cellulose fibres in water to obtain a slurry of cellulose fibres;

[0050] - adding a water-soluble thickener to the slurry to obtain a mixture of thickener and cellulose fibres in water;

[0051] - adding at least two surfactants to the mixture to obtain a fibre suspension; and

[0052] - 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 120-500 kg / m3, or from 120-400 kg / m3, and a yield stress of at least 80 Pa.

[0053] 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. The mixture of cellulose fibres and thickener preferably has a Schopper-Riegler (SR) value of less than 40, or less than 30. The SR value is measured prior to addition of surfactant and aeration of the fibre suspension.

[0054] 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.

[0055] 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. Upon aeration the composition comprising cellulose fibres, thickener and at least two 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.

[0056] In 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.

[0057] In some embodiments, the density of the wet cellulose foams used in the present invention may be from 70 - 600 kg / m3, or from 100 - 500 kg / m3, or from 100 - 400 kg / m3, or from 125 - 375 kg / m3, or from 140 - 375 kg / m3.

[0058] In 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. In 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. The wet cellulose foam may have a total solid content ranging from 12-40 wt%, or from 12-25 wt%, or from 13-20 wt%, as calculated on the total weight of the wet cellulose foam.

[0059] 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 to 140°C. After drying, the solid cellulose foam may have a density of from 10 to 80 kg / m3, or from 10 to 60 kg / m3or from 20 to 50 kg / m3. 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. 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 provides the cellulose foam with increased stability and mechanical strength.

[0060] The cellulose foam described above is the preferred foam to use in the present invention. Alternatively, other cellulose foams, such as those disclosed in WO2016068771 A1 , WO2016068787 A1 , and W02020011587 A1 may be used.

[0061] In the method according to the first aspect of the present invention, a wet cellulose foam is provided. The details of the wet cellulose foam are further described above.

[0062] The method according to the first aspect further involves a step of depositing the wet cellulose foam on a supporting surface so as to obtain a wet cellulose foam deposition comprising a first surface and a second surface. The term “surface” as used herein in expressions such as “first surface”, and “second surface”, refers to outermost surfaces of the wet cellulose foam deposition and the solid cellulose foam. The first and second surfaces of the wet cellulose foam deposition corresponds to the first and second surfaces of the solid cellulose foam after drying of the wet cellulose foam deposition.

[0063] The wet cellulose foam deposition further has a height (corresponding to the thickness of the foam), a length and a width.

[0064] The supporting surface may be any suitable surface, for example a metal tray or a conveyor belt. Preferably, the supporting surface is perforated to facilitate removal of water from the wet foam during drying. The supporting surface may be fitted with an outer frame to provide additional support for the wet cellulose foam during deposition and drying. The wet cellulose foam may be deposited using any suitable means, such as by extrusion. The height of the wet cellulose foam deposition 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 dimensions of width and length of the wet cellulose foam deposition 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 dimension of the solid cellulose foam to be produced. The wet cellulose foam is preferably deposited such that the wet cellulose foam deposition has the shape of a sheet or plank. The term “plank” as used herein, refers to an object where the dimensions of length and width are significantly larger than the height, and where the length may be significantly larger than the width. The first surface may be any surface of the wet cellulose foam deposition, such as a top surface, a bottom surface or a side surface.

[0065] In a preferred embodiment, the second surface is opposite the first surface. For example, if the first surface is the bottom surface of the wet cellulose foam deposition, the second surface will be the top surface of the wet cellulose foam deposition.

[0066] In an alternative embodiment, the second surface is adjacent to the first surface. For example, the first surface may be the bottom surface of the wet cellulose foam deposition, and the second surface may be a side surface of the wet cellulose foam deposition.

[0067] The top, bottom and side surfaces of the wet cellulose foam deposition corresponds to the top, bottom and side surfaces of the solid cellulose foam obtained by drying of the wet cellulose foam deposition.

[0068] The method according to the first aspect further involves a step of subjecting the wet cellulose foam deposition to an initial drying step so as to form partially dried cellulose foam sections, wherein the initial drying is carried out by means of heat transferring spikes extending at least partially through the wet cellulose foam deposition from the first surface towards the second surface, and wherein the partially dried cellulose foam sections are formed adjacent to the heat transferring spikes.

[0069] Using heat transferring spikes during drying will shorten the drying time of the wet cellulose foam deposition, and also minimize non-uniform shrinkage in the height direction of the foam during drying. After drying, the obtained solid cellulose foam will comprise cavities corresponding in size to the heat transferring spikes. The obtained solid cellulose foam comprising cavities still has sufficient cushioning properties so that it can be used in packaging applications. The cavities induced by the heat transferring spikes therefore do not impact the foam properties negatively, whereas the use of heat transferring spikes significantly shortens the drying time of the foam, thus enabling a more cost-efficient process.

[0070] The term “heat transferring spikes” as used herein refers to a long narrow object made of a material with good heat transferring properties, such as a metal. The heat transferring spikes may for example be in the shape of a needle. Preferably, all the heat transferring spikes used in the present invention have the same properties, such as length, width, shape and appearance. It is however also within the scope of the present invention to use heat transferring spikes with different properties in different parts of the foam.

[0071] The heat transferring spikes preferably have a circular cross-section, since such spikes have been found to be easy to remove from the dried foam. However, the heat transferring spikes may also have any suitable cross-section, such as a triangular, oval or rectangular cross-section. The diameter, or width, of the heat transferring spikes is preferably in the range of from 0.1 to 20 mm, such as from 0.1 to 6 mm, or from 0.5 to 5 mm, or from 0.5 to 3 mm. When using spikes with a small diameter, the formed cavities will be more narrow and the impact on the properties of the solid foam are reduced. However, the reduction in drying time may be more pronounced using spikes with a larger diameter since the heat transfer to adjacent sections of wet foam is more efficient. The partially dried foam section adjacent to each heat transferring spike will thus be larger and provide more support during the final drying step, ensuring that non-uniform shrinkage is minimized. The heat transferring spikes may have the same diameter along the entire length of the spike. Alternatively, the spikes may be tapered, such that the diameter is smaller at one end than at the other. A tapered configuration may facilitate insertion of the heat transferring spikes into the wet cellulose foam. When using a tapered configuration, the end of the spike with the smallest diameter is extending into the foam such that the end with the largest diameter is adjacent to the first surface.

[0072] The heat transferring spikes may comprise an end section with a larger diameter compared to the rest of the spike. The length of the end section is typically short compared to length of the rest of the spike. The end section is preferably not inserted into the wet cellulose foam but facilitates handling of the heat transferring spikes. For example, the heat transferring spikes may be attached to a supporting substrate by means of the end sections.

[0073] The length of the heat transferring spikes depends on the dimensions of the wet cellulose foam deposition, and how far into the wet cellulose foam the heat transferring spikes are intended to extend.

[0074] The heat transferring spikes may comprise any suitable material with good heat transfer properties. Preferably, the heat transferring spikes comprise metal, such as aluminum, copper, brass, or stainless steel. The heat transferring spikes may be made entirely of metal, or may comprise a metal coating. It is preferable that the outer surface of the heat transferring spikes is smooth, since that facilitates removal of the spikes from the dried foam.

[0075] The number of heat transferring spikes used during drying depend on, among other things, the dimensions of the wet cellulose foam deposition, the diameter of the spikes, how far into the wet foam the spikes extend, the distance between the heat transferring spikes, and on foam properties such as density. The distance between adjacent heat transferring spikes may for example be in the range of from 1 to 20 cm, or from 1 to 15 cm, or from 1 to 10 cm. If the distance between adjacent spikes is too large, such as larger than 20 cm, or larger than 15 cm, the distance between partially dried foam sections will be too large to be able to support the wet foam during drying and as a result shrinkage in the height direction may occur between the partially dried foam sections.

[0076] A large number of heat transferring spikes will shorten the drying time since the foam is heated at several places simultaneously. There will also be a short distance between adjacent partially dried foam sections, which will reduce the risk for non- uniform shrinkage in the height direction during drying further. On the other hand, using a large number of heat transferring spikes will create more cavities in the dried foam which may have an impact on the cushioning properties of the foam. Therefore, as realized by a person skilled in the art, the number of heat transferring spikes should be selected such that a short drying time and reduced non-uniform shrinkage in the height direction is obtained while not impairing the cushioning properties of the obtained dried foam. Preferably, the heat transferring spikes are extending through the wet cellulose foam deposition in a direction perpendicular to the first and second surfaces of the wet cellulose foam deposition. When the heat transferring spikes are extending in a direction perpendicular to the first and second surfaces, the partially dried foam sections will also extend in a direction perpendicular to the first and second surfaces, which has been found to provide excellent support for the wet foam during drying.

[0077] In an alternative embodiment, the heat transferring spikes are extending through the wet cellulose foam deposition at an angle to the first surface, such as at an angle between 5 and 75°, as measured from the first surface to the heat transferring spike inside the foam.

[0078] The heat transferring spikes may extend through from 50 to 100%, such as from 60 to 100%, or from 70 to 100% or from 80 to 100% of the distance between the first surface and the second surface of the wet cellulose foam deposition. The extension of the heat transferring spikes into the wet foam is measured from the first surface towards the second surface. The extension of the heat transferring spikes may equal the length of the heat transferring spikes. Alternatively, the spikes may be longer than the extension into the wet foam, such that some parts of the spikes are not extending into the foam. In embodiments where the heat transferring spikes extend through the entire distance between the first surface and the second surface, the partially dried foam sections will also extend through the entire distance. In such embodiments, excellent support is provided during the final drying of the wet foam and non-uniform shrinkage is minimized.

[0079] In some embodiments, the heat transferring spikes extend through from 50 to 90%, such as from 60 to 90%, or from 70 to 90%, or from 80 to 90% of the distance between the first surface and the second surface of the wet cellulose foam deposition. In such embodiments, the second surface will not comprise cavities and thus have a flat and smooth appearance which is desired in many applications. The cushioning properties of the foam may also be improved by the cavities not extending all the way through the foam.

[0080] If the heat transferring spikes does not extend a sufficient distance, for example if they extend through less than 50%, or less than 60%, or less than 70%, of the distance between the first surface and the second surface, the partially dried cellulose foam sections will not have a height sufficient to support the wet foam during drying. The amount of wet foam on top of the partially dried foam sections may be too large for the partially dried foam sections to support. This may lead to non-uniform shrinkage of the foam in the height direction during drying.

[0081] The extension of the heat transferring spikes into the wet cellulose foam deposition may be controlled for example by the height of the wet cellulose foam deposition, by the length of the heat transferring spikes used, and / or by how much of the length of the heat transferring spike that is inserted into the wet cellulose foam deposition.

[0082] To facilitate handling of the heat transferring spikes, all spikes may be mounted on a carrier substrate. The carrier substrate is preferably flat and made of metal, or provided with a metal coating. During drying, the carrier substrate may also provide heat to the surface of the wet cellulose foam that is adjacent to the carrier plate. This further reduces the drying time of the wet cellulose foam.

[0083] In some embodiments, a perforated plate is placed adjacent to the first surface of the wet cellulose foam deposition, and the heat transferring spikes are extending through the perforations of the perforated plate into the wet cellulose foam deposition. The perforated plate facilitates removal of the heat transferring spikes from the solid cellulose foam after the final drying step, since it prevents the foam from sticking to the spikes. In some embodiments, the perforated plate may also be the supporting surface on which the wet cellulose foam is deposited. In other embodiments, the perforated plate is not the same as the supporting surface. In some embodiments, the heat transferring spikes are attached to the perforated plate, so that the perforated plate also acts as the carrier substrate. In other embodiments, the heat transferring spikes are inserted through the perforations of the perforated plate while still attached to the carrier substrate.

[0084] The heat transferring spikes may be provided before deposition of the wet cellulose foam, simultaneously with deposition of the wet cellulose foam or after deposition of the wet cellulose foam.

[0085] In some embodiments, the heat transferring spikes are provided before deposition of the wet cellulose foam. Preferably, the heat transferring spikes are provided on the supporting surface on which the wet cellulose foam is deposited, and the first surface is thus a bottom surface of the wet cellulose foam deposition. The heat transferring spikes may in such embodiments extend from the bottom surface of the wet cellulose foam deposition towards a top surface of the wet cellulose foam deposition. Since the spikes are already present when the wet foam is deposited, the number of process steps are decreased which is beneficial in terms of efficiency. However, care must be taken during deposition so that no voids are formed in the wet cellulose foam deposition.

[0086] In other embodiments, the heat transferring spikes are provided after deposition of the wet cellulose foam on the supporting surface. The heat transferring spikes may be inserted into the wet cellulose foam deposition, for example through a perforated plate, from a first surface of the wet cellulose foam and extending towards a second surface. For example, the first surface may be a top surface of the wet cellulose foam deposition and the heat transferring spikes may extend from the top surface towards a bottom surface; or the first surface may be a side surface of the wet cellulose foam deposition and the heat transferring spikes may extend towards an opposite side surface; or the first surface may be a bottom surface of the wet cellulose foam deposition and the heat transferring spikes may extend towards a top surface. By providing the heat transferring spikes after deposition of the wet cellulose foam, depositing the wet foam may be facilitated. The presence of the heat transferring spikes may limit for example the deposition methods that can be used. It may also be difficult to ensure that the wet foam completely fills the space between the spikes, with no formation of voids.

[0087] It is also within the scope of the present invention to provide heat transferring spikes extending from more than one surface of the wet cellulose foam deposition. For example, heat transferring spikes may extend both from the bottom surface and from the top surface.

[0088] The initial drying step and the final drying step may be discrete steps or may be carried out as one step in direct sequence. The temperatures during the initial drying step and the final drying step may be the same or may be different. Drying may be carried out at one or more temperatures in the range of from 40 to160°C. Any suitable equipment may be used. During the drying steps, heat is provided to the heat transferring spikes which in turn will transfer heat to the wet cellulose foam so that the wet foam will begin to dry. Heat may be provided to the heat transferring spikes using any suitable means. In one embodiment, the initial drying step is carried out in an oven, such as a convection oven. The wet cellulose foam deposition is placed in an oven together with the heat transferring spikes. In the oven, the heat transferring spikes along with other metal objects such as the supporting surface, if made of metal, and if present, the perforated plate and the carrier substrate, will be heated much faster than the foam due to their heat transfer properties. The heat transferring spikes will provide heat inside the wet foam, such that the wet foam is not only dried by heated air in contact with its outer surfaces, and will thus shorten the drying time of the wet cellulose foam deposition. If no heat transferring spikes were present, the wet foam would be dried only by convection from heated air within the oven, and the drying would progress from the other surfaces towards the middle of the foam.

[0089] In one embodiment, the initial drying step is carried out by providing heat directly to the heat transferring spikes, or to the carrier plate in embodiments where the heat transferring spikes are attached to a carrier plate. For example, a heated plate may be put in contact with the heat transferring spikes or the carrier plate. Heat will be transferred from the heated plate to the heat transferring spikes, in some embodiments by means of the carrier plate, and the heat transferring spikes will in turn transfer heat to the foam. In another example, the heat transferring spikes are provided with electric heating means such that they are heated by means of electricity. For example, wires may be provided within each heat transferring spike. In embodiments where heat is provided directly to the heat transferring spikes, the drying steps may be carried out in room temperature.

[0090] During the initial drying step, partially dried foam sections are formed adjacent to the heat transferring spikes. The term “partially dried foam section” as used herein refers to a section of the cellulose foam that has been partially dried, i.e. that has a lower water content than the rest of the foam. The partially dried foam sections may also comprise sections of the foam that are completely dried, e.g. the sections that are directly adjacent to the heat transferring spikes, since those sections are the first to become completely dried. The partially dried foam sections will provide support for the wet foam during the final drying step.

[0091] Drying of the wet cellulose foam deposition will proceed from the sections of the foam closest to the heat transferring spikes, and thus those sections will be dried first. There will be a smooth transition within the foam from sections that are partially dried to those that are not dried at all. Drying will proceed also from the outer surfaces from the foam, but that drying process is much slower than the drying proceeding from the heat transferring spikes.

[0092] The method according to the present invention further involves a step of subjecting the wet cellulose foam deposition to a final drying step so as to obtain a solid cellulose foam, wherein the partially dried cellulose foam sections provide support for the wet cellulose foam during the final drying step.

[0093] The final drying step may preferably be carried out in direct sequence with the initial drying step, using the same drying means and settings as described above for the initial drying step. In such embodiments, there is no sharp transition between the initial drying step and the final drying step. Also, the heat transferring spikes are used during the final drying step in such embodiments.

[0094] Alternatively, the final drying step may be a discrete step using other drying means and settings than what is used for the initial drying step. For example, the heat transferring spikes may be removed after the initial drying step, such that the final drying is carried out by means other than the heat transferring spikes.

[0095] The time required for drying the wet cellulose foam to a solid cellulose foam depends on for example the temperature(s) used during the drying steps, the thickness of the wet cellulose foam deposition, the number of heat transferring spikes and how far into the wet foam the heat-transferring spikes extend.

[0096] During the drying steps, water is evaporated from the wet cellulose foam, such that after the final drying step, a solid cellulose foam is obtained. After drying, the solid cellulose foam may have a density of from 10 to 80 kg / m3, or from 10 to 60 kg / m3or from 20 to 50 kg / m3. 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.

[0097] In 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 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 width to 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.

[0098] The method according to the first aspect of the present invention reduces the net tension forces arising in the first wet cellulose foam upon drying by providing a number of additional contact points in the form of heat transferring spikes. During the initial heating step, the wet cellulose foam directly adjacent to the heat transferring spikes will begin to dry, and partially dried foam sections will be formed. The width to height ratio of the partially dried foam sections will be favourable in terms of preventing shrinkage. During the final drying step, the wet cellulose foam between the partially dried foam sections will be dried along with the partially dried foam sections until the entire cellulose foam is completely dried. Once completely dried, it is not possible to distinguish the former partially dried foam sections from the rest of the foam.

[0099] When the wet cellulose foam between the partially dried foam sections is being dried, the already formed partially dried foam sections provide for a low width to height ratio of the wet cellulose foam. The tension forces of each partially dried foam section will act against each other thus reducing the net tension forces in the wet cellulose foam between the partially dried foam sections, 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. The method of the present invention thus provides for formation of a solid cellulose foam with substantially uniform height since non-uniform shrinkage in the 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. Thus, the present invention provides for a method where a cellulose foam having a uniform thickness can be obtained from a single deposition of wet foam. Only making one foam deposition will facilitate processing since the number of process steps is reduced. The obtained foam product will also be more uniform and coherent in appearance. The use of heat transferring spikes in the method according to the present invention will both decrease the drying time of the wet cellulose foam and minimize non-uniform shrinkage of the foam in the thickness direction.

[0100] During drying of the wet cellulose foam, a densified layer will form on the outer surface of the foam, as well as on the surfaces of the foam adjacent to the heat transferring spikes, i.e. on the surfaces of the partially dried foam sections. The outer surfaces of the wet cellulose foam deposition are dried by air, either at room temperature or at an elevated temperature. The densified layer is a very thin layer that is formed on the very outer surfaces 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. The formation of a densified layer on the outer surfaces of the partially dried foam sections will increase the stability of the foam sections and improve their ability to provide support during the final drying step.

[0101] The method according to the present invention further involves a step of removing the heat transferring spikes so as to obtain a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam, wherein the cavities are formed by the heat transferring spikes.

[0102] The heat transferring spikes are preferably removed from the cellulose foam once the cellulose foam is dried. Alternatively, the heat transferring spikes may be removed after the initial heating step.

[0103] The cavities in the solid cellulose foam will have a shape corresponding to that of the heat transferring spikes. The cavities preferably have a circular cross-section. However, the cavities may also have any suitable cross-section, such as a triangular, oval or rectangular cross-section. The diameter, or width, of the cavities is preferably in the range of from 0.1 to 20 mm, such as from 0.2 to 6 mm, or from 0.5 to 5 mm, or from 0.5 to 3 mm. To maintain sufficient cushioning properties of the solid cellulose foam, it is preferable that the diameter of the cavities is small. The cavities may have the same diameter along their entire depth. Alternatively, the cavities may be tapered, such that the diameter is smaller at the bottom of the cavity than at the top of the cavity. The top part is the part located in the first surface, and the bottom part is the part located either at the second surface or inside the foam.

[0104] The number of cavities depend on the number of heat transferring spikes used during drying of the wet cellulose foam. The distance between adjacent cavities may for example be from 1 to 20 cm. A large number of cavities may have a negative impact on the cushioning properties of the solid cellulose foam.

[0105] Preferably, the cavities are extending through the solid cellulose foam in a direction perpendicular to the first and second surfaces of the solid cellulose foam. In an alternative embodiment, the cavities are extending through the solid cellulose foam at an angle to the first surface, such as at an angle between 5 and 75°, as measured from the first surface to the hole.

[0106] The cavities may extend through from 50 to 100%, such as from 60 to 100%, or from 70 to 100% or from 80 to 100% of the distance between the first surface and the second surface of the solid cellulose foam. The extension of the cavities is measured form the first surface towards the second surface.

[0107] In some embodiments, the cavities extend through from 50 to 90%, such as from 60 to 90%, or from 70 to 90%, or from 80 to 90% of the distance between the first surface and the second surface of the solid cellulose foam. In such embodiments, the second surface will not comprise cavities and thus have a flat and smooth appearance which is desired in many applications. The cushioning properties of the foam may also be improved by the cavities not extending all the way through the foam.

[0108] The walls (i.e. the outer surface of the solid cellulose foam inside the cavities) of the cavities will comprise a densified layer. The densified layer is formed during drying of the wet cellulose foam, as further outlined above. The densified layers may improve the mechanical properties of the solid cellulose foam since they are stiffer than the core of the foam.

[0109] According to a second aspect, the present invention relates to solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam from a first surface of the solid cellulose foam towards a second surface of the solid cellulose foam, and wherein the solid cellulose foam is formed from a single deposition of wet cellulose foam. The term “single deposition” as used herein refers a process where all the wet foam needed to create a solid foam product has been deposited at the same time, i.e. that no section of the foam has dried before other sections are deposited. For example, the solid foam may have been formed from one coherent wet foam deposition, or from several wet foam depositions that have been merged into one deposition prior to drying.

[0110] By the solid cellulose foam being formed from a single deposition of wet cellulose foam, a coherent and uniform solid foam is obtained. The strength, such as the tensile strength and the tear strength, of the solid foam is improved, since there are no interfaces between different foam depositions. The height of the solid cellulose foam is uniform, or substantially uniform, along the length and width of the foam. The height is taken to be the distance between the bottom surface and the top surface of the solid cellulose foam, and is measured at places of the foam where there are no cavities. The non-uniform shrinkage during drying of the wet cellulose foam deposition is minimized.

[0111] The solid cellulose foam according to the second aspect may be obtained by the method according to the first aspect, and may be further defined as set out above with reference to the first aspect.

[0112] According to a third aspect, the present invention relates to a use of the solid cellulose foam according to the second aspect, or the solid cellulose foam obtainable with the method according to the first aspect, as a packaging material or a building material. The solid cellulose foam of the present invention has excellent cushioning properties which are desirable in the field of protective packaging. In addition, it may be used as a building material, for example to provide thermal insulation or acoustic insulation. Examples

[0113] Example 1 - preparation of wet cellulose foam

[0114] Wet cellulose foams having a dry content of 15.8% were prepared. The wet foam compositions all comprised 10 wt% CMC, 1 wt% surfactants and 89 wt% cellulose fibres, based on the total weight of the solid content of the wet foam composition. 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. CMC (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 / CMC solution mixture. The mixture was then aerated using the balloon whipper of the 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 was 188 kg / m3).

[0115] The wet cellulose foam of example 1 was used to evaluate the drying time, with and without heat transferring spikes. A wooden frame was used with a height of 5 cm and length and width of 20 cm. In experiments where heat transferring spikes were used, those were placed in the bottom of the frame prior to deposition of the wet foam. The heat transferring spikes were made of metal and had a circular cross section with a diameter of 2.35 mm, and a length of 44 mm. Either 66, 90 or 162 spikes were used. The spikes were placed with a regular spacing across the bottom of the frame. The wet foam was deposited inside the frame using a spatula, the height of the wet foam deposition was 5 cm. The top of the wet foam was scraped so that a flat surface was obtained. The frame with the wet foam and the heat transferring spikes was placed in on oven and dried at 120°C. The amount of water in the foam was measured at several points in time during drying.

[0116] In a comparative experiment, a foam was prepared and dried as described above, except that no heat transferring spikes were used during drying. The amount of water in the foam was measured at several points in time during drying. The results for drying with 66 (•), 90 (A) or 126 (♦) heat transferring spikes, or without heat transferring spikes (■) are shown in figure 1 . It was found that the drying time decreases with an increased number of heat transferring spikes, and that drying is faster when heat transferring spikes are used.

[0117] Example 3 - shrinkage

[0118] The wet cellulose foam of example 1 was used to fill a frame of dimensions 42 by 62 cm and a height of 4 cm. The frame was placed on a perforated plate. Heat transferring spikes (same dimension as described in example 2) were placed on the bottom of the frame prior to deposition of the wet foam inside the frame. The heat transferring spikes extended through the entire thickness of the wet foam. After deposition, the wet foam was scraped to obtain an even top surface. The wet foam was dried in the convection oven at 120°C. The heat transferring spikes were removed after drying. After drying, the height of the obtained solid foam was measured at regular intervals. The height of the foam was at all places equal to or higher than the height of the frame.

[0119] In a comparative example, a foam was prepared and dried as described above except that no heat transferring spikes were used. The height of the obtained solid foam was lower than the height of the frame in the middle of the foam. Thus, shrinkage had occurred during drying.

[0120] In 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

Claims1 . A method for producing a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam from a first surface of the solid cellulose foam towards a second surface of the solid cellulose foam, wherein the method comprises the following steps:- providing a wet cellulose foam;- depositing the wet cellulose foam on a supporting surface so as to obtain a wet cellulose foam deposition comprising a first surface and a second surface;- subjecting the wet cellulose foam deposition to an initial drying step so as to form partially dried cellulose foam sections, wherein the initial drying is carried out by means of heat transferring spikes extending at least partially through the wet cellulose foam deposition from the first surface towards the second surface, and wherein the partially dried cellulose foam sections are formed adjacent to the heat transferring spikes;- subjecting the wet cellulose foam deposition to a final drying step so as to obtain a solid cellulose foam, wherein the partially dried cellulose foam sections provide support for the wet cellulose foam during the final drying step;- removing the heat transferring spikes so as to obtain a solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam, wherein the cavities are formed by the heat transferring spikes.

2. The method according to claim 1 , wherein the wet cellulose foam comprises cellulose fibres in a range of from 71 to 95 wt% based on the total dry weight of the wet cellulose foam, a water-soluble thickener in a range of from 4 to 24 wt% based on the total dry weight of the wet cellulose foam, and at least two surfactants.

3. The method according to any one of claims 1 or 2, wherein the heat transferring spikes extend through from 50 to 100% of the distance between the first surface and the second surface of the wet cellulose foam deposition.

4. The method according to any one of the preceding claims, wherein the heat transferring spikes are provided on the supporting surface before the wet cellulose foam is deposited.

5. The method according to any one of claims 1-3, wherein the heat transferring spikes are provided after the wet cellulose foam has been deposited on the supporting surface.

6. The method according to any one of the preceding claims wherein the heat transferring spikes have a circular cross-section.

7. The method according to any one of the preceding claims wherein the second surface is opposite the first surface.

8. The method according to any one of the preceding claims wherein the heat transferring spikes are extending through the wet cellulose foam deposition in a direction perpendicular to the first and second surfaces of the wet cellulose foam deposition.

9. A solid cellulose foam comprising cavities extending at least partially through the solid cellulose foam from a first surface of the solid cellulose foam towards a second surface of the solid cellulose foam, and wherein the solid cellulose foam is formed from a single deposition of wet cellulose foam.

10. The solid cellulose foam according to claim 9, wherein the solid cellulose foam comprises cellulose fibres in a range of from 71 to 95 wt% based on the total dry weight of the solid cellulose foam, a water-soluble thickener in a range of from 4 to 24 wt% based on the total dry weight of the solid cellulose foam, and at least two surfactants.11 . The solid cellulose foam according to any one of claims 9 or 10, wherein the cavities extend through from 50 to 100% of the distance between the first surface and the second surface.

12. The solid cellulose foam according to any one of claims 9-11 , wherein the cavities have a circular cross-section.

13. The solid cellulose foam according to any one of claims 9-12, wherein the second surface is opposite the first surface.

14. The solid cellulose foam according to any one of claims 9-13, wherein the cavities extend in a direction perpendicular to the first and second surfaces of the solid cellulose foam.

15. 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.

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