A method for forming an imprinted region in a solid cellulose foam

Abstract

The present invention relates to a method for forming at least one imprinted region in a solid cellulose foam, the method comprising the steps of: a) providing a solid cellulose foam; b) applying moisture at least to a region of the solid cellulose foam where the at least one imprinted region is to be formed, so as to provide the solid cellulose foam with at least one moistened region; c) pressing at least one forming tool into the at least one moistened region on the solid cellulose foam such that at least one imprinted region is formed in the solid cellulose foam; and d) drying the solid cellulose foam.

Description

[0001] A METHOD FOR FORMING AN IMPRINTED REGION IN A SOLID CELLULOSE FOAM

[0002] Field of the invention

[0003] The present invention relates to a method for forming at least one imprinted region in a solid cellulose foam. The invention further relates to a solid cellulose foam comprising at least one imprinted region, and a use of said solid cellulose foam.

[0004] Background

[0005] Practically all consumer goods need protective packaging to cushion the goods during storage and transportation. It is recognized that there are numerous solutions for packaging and cushioning various goods depending on the physical properties of the goods to be protected and the degree of protection required relating to its application. Examples of these protective packaging materials are polymeric foam materials for packaging, such as polyurethane foam (PU), polyethylene foams (PE), expanded polystyrene (EPS) or expanded polypropylene (EPP). Porous materials for this type of use must be stable, low-weight and easy to manufacture. Due to the increased awareness of the need to use renewable materials, it is highly motivated to replace petroleum-based polymers with polymers from renewable resources.

[0006] There are many challenges with finding foam materials from renewable sources. Many bio-based foams have higher cost of production and lower mechanical performance compared to established foams from petroleum-based sources. Biobased protective materials need to have the same characteristics and properties as petroleum-based protective materials for being the first choice over petroleum-based protective materials.

[0007] Low weight and good shock absorption of the bio-based foams are examples of crucial characteristics. Also, the ability to customize the shape and the form of the bio-based foams to the shape and form of the goods to be protected is of uttermost importance. Cellulose has been shown to have a special potential, as the most abundant renewable natural polymer on earth, with its crystalline structure, and the availability of methods for preparing large volumes on an industrial scale. Die-cutting is commonly used to convert conventional polymeric foams to create shapes and patterns in the foam. It has however been proven difficult to convert a rigid cellulose foam with die-cutting, since the process typically results in an irreversible height reduction also of the foam surrounding the area subjected to diecutting, which is typically not desired due to loss in cushioning ability. As an alternative to die-cutting, foam layers can be individually shaped by cutting and subsequently assembled, e.g. by gluing, to the final product. That however require several process steps, and also involves material waste since foam is cut away and discarded.

[0008] Thus, there still exists a need for protective and cushioning materials that are natural, bio-based and recyclable and that are easy to convert for use as packaging materials. It must be possible to easily adapt the packaging materials for use as packages for products of various sizes and shapes while maintaining the protective and cushioning properties.

[0009] Summary of the invention

[0010] 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.

[0011] It is a further object of the present invention to provide a method for shaping a solid cellulose foam such that it can accommodate various products while maintaining its protective and cushioning properties.

[0012] 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.

[0013] According to a first aspect, the present invention relates to a method for forming at least one imprinted region in a solid cellulose foam, the method comprising the steps of: a) providing a solid cellulose foam; b) applying moisture at least to a region of the solid cellulose foam where the at least one imprinted region is to be formed, so as to provide the solid cellulose foam with at least one moistened region; c) pressing at least one forming tool into the at least one moistened region on the solid cellulose foam such that at least one imprinted region is formed in the solid cellulose foam; and d) drying the solid cellulose foam.

[0014] It has surprisingly been found that by moistening the solid cellulose foam prior to pressing at least one forming tool into the foam, a shape can be imprinted into the foam. The solid cellulose foam is partly disintegrated / dissolved and softened by the applied moisture and will easily be deformed by the forming tool that is pressed into it. The shape of the imprinted region will correspond to the shape of the forming tool. By selection of the shape of the forming tool, the shape of the imprinted region can be controlled, thus enabling a versatile forming method. The regions of the solid cellulose foam surrounding the imprinted region will provide the foam with cushioning so that the protective properties are maintained.

[0015] If a forming tool is instead pressed into a solid cellulose foam with no prior application of moisture, the foam will be deformed but will to some extent resume its original shape once the forming tool has been removed. The imprinted region formed by the method according to the first aspect will instead retain its shape also when the forming tool is removed due to permanent deformation of the foam structure in the moistened foam.

[0016] An additional advantage with the method according to the first aspect is that the imprinted region after drying of the applied moisture will have a smooth outer surface. This provides the foam with a nice uniform appearance. During drying, a densified layer will be formed on the moistened regions of the outer surface, such that the outer surface of the imprinted region in the moistened region will comprise a densified layer which improves the protective and mechanical properties of the imprinted region of the solid cellulose foam so that it will be more resistant to damages. This is of particular importance for example when objects to be protected have sharp edges that may otherwise penetrate through the open cell structure of the core of the solid cellulose foam.

[0017] According to a second aspect, the present invention relates to a solid cellulose foam comprising at least one imprinted region, wherein an outer surface of the solid cellulose foam in the imprinted region comprises a densified layer. The solid cellulose foam of the second aspect may be obtained with the method according to the first aspect. By forming an imprinted region by the method according to the first aspect, the foam in the imprinted region is permanently deformed into the shape of the forming tool. The foam in the imprinted region is further partly dissolved during the method, which changes its structure as compared to the solid cellulose foam as provided. The surface of the foam in the imprinted region is smooth and homogenous, and is due to the moistening followed by drying provided with a densified outer layer which improves its mechanical properties.

[0018] The solid cellulose foam is made from renewable resources and can be redispersed in water and as a result be recyclable in regular paper recycling streams.

[0019] The solid cellulose foam according to the second aspect is particularly suitable to use as a packaging material in various protective packaging applications, with a formed imprinted region corresponding to a product to be protected. The solid cellulose foam may 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.

[0020] According to a third aspect, the present invention relates to a use of the solid cellulose foam according to the second aspect or obtainable by the method according to the first aspect, as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media.

[0021] 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.

[0022] Detailed description

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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. After drying, 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 cellulose foam preferably used in the method according to the present invention will now be described in detail.

[0027] 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.

[0028] 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 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 invention.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] The solid cellulose foam can be re-dispersed in water and as a result be recyclable in regular paper recycling streams. The wet cellulose foam 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 120-500 kg / m3, or from 120-400 kg / m3, and a yield stress of at least 80 Pa.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] The bubble size in the wet foam is typically below 100 pm. This provides for a homogenous wet foam with good stability that does not flocculate during processing. During processing, and also during the subsequent drying step, the average bubble size is maintained to a large extent and the cellulose fibres remain well dispersed. The resulting solid cellulose foam obtained by drying the wet foam will be homogenous in structure, strong, have good mechanical properties, a smooth surface and no defects.

[0042] In comparison, a wet cellulose foam with low stability has a larger average bubble size (i.e. typically above 100 pm) and the bubbles will coalesce faster during processing and drying such that larger bubbles are formed. In addition, the cellulose fibres will form clusters during processing and drying. This results in the wet foam collapsing during drying. The resulting solid cellulose foam will not have a homogenous structure and will also contain defects in the form of cavities resulting from the coalesced bubbles in the wet foam. Such a solid cellulose foam is, due to the defects, weak and has a rough surface.

[0043] 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.

[0044] 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.

[0045] 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. 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.

[0046] 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. In preferred embodiments the cellulose foam 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. A wet cellulose foam having such a composition is homogenous in structure and has a good stability as discussed above. Such a wet cellulose foam can also be dried without prior dewatering.

[0047] 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.

[0048] 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.

[0049] The cellulose foam may be prepared by a two-step deposition such as described in WO2023119215 A1 . In a first deposition, a wet foam is deposited as discrete units on a surface and at least partially dried. During drying, a densified layer is formed on the outer surface of the discrete units. In a second deposition a wet foam, preferably having the same composition as the wet foam in the first deposition, is deposited so that it fills the spaces surrounding the discrete units of the first deposition. After drying, a solid cellulose foam comprising discrete units of foam embedded in a foam matrix is obtained. The cellulose foam may also be prepared by assembly of individually formed complimentary dry cellulose foam pieces. For example, dry foam pieces of complimentary shape, each formed from a wet foam, may be assembled to form a foam substrate of any desired shape, such as a plank.

[0050] The cellulose foam described above is the preferred solid cellulose foam to use in the method of the present invention and is further described in WO2023119213 A1. Alternatively, other cellulose foams, such as those disclosed in WO2016068771 A1 , WO2016068787 A1 , and W02020011587 A1 may be used.

[0051] Step a) of the method according to the first aspect of the present invention involves providing a solid cellulose foam that is to be imprinted. The solid cellulose foam is preferably the foam as described above. The solid cellulose foam is rigid. The term “rigid” as used herein, refers to a solid cellulose foam that is substantially stiff and non-flexible and not possible to bend to a large extent without tearing and / or creasing the foam. When a rigid cellulose foam is compressed, it does not resume its original shape, or only resumes it to a small extent.

[0052] The solid cellulose foam provided in step a) may have any shape, such as a block, a cube, a cylinder or any irregular shape. Preferably, the solid cellulose foam has at least one flat surface. The term “flat” as used herein refers to a level surface with no indentations or protruding portions. The imprinted region is preferably formed in a flat surface of the solid cellulose foam.

[0053] In some embodiments, the cellulose foam may have at least one non-flat surface, such as a surface having protruding portions.

[0054] In one embodiment, the solid cellulose foam is provided in the form of a plank having a thickness in the range of from 1 to 20 cm, preferably from 1 to 10 cm, more preferably from 2 to 6 cm. The length and width dimensions of the plank are typically in the range of from 100 to 300 cm. The planks may be cut into smaller pieces but preferably having the same thickness also after cutting. In embodiments where the foam is provided in the form of a plank, the foam will have a top surface, a bottom surface and side surfaces.

[0055] In some embodiments, the solid cellulose foam provided in step a) comprises a densified layer on at least one outer surface. In embodiments where the cellulose foam is provided in the form of a plank, the solid cellulose foam may comprise densified layers on at least the top surface and bottom surface, and optionally on the side surfaces.

[0056] In some embodiments, the solid cellulose foam provided in step a) does not comprise a densified layer on any outer surface. Due to the stiffness of the densified layer, it may be easier to form an imprinted region on a surface that does not comprise a densified layer, in particular if imprinted regions, such as a repeated pattern, is to be formed on a large portion of the solid cellulose foam. In such an example, moisture may be applied along the entire surface and during drying a densified layer will form along said surface. Also for that reason, in such embodiments a densified layer is not needed on the solid cellulose foam provided in step a).

[0057] In embodiments where the solid cellulose foam comprises a densified layer on at least one outer surface, the imprinted region may be formed on an outer surface comprising a densified layer. This is advantageous in embodiments where moisture is not applied along the entire surface of the solid cellulose foam, such that the regions surrounding the imprinted region will also comprise a densified layer.

[0058] The densified layer comprises cellulose fibres that are packed more tightly and partly oriented differently compared to the bulk. The densified layer is formed on the outer surface of a wet foam during drying of the wet foam and remain on the outer surface of the cellulose foam once dried. 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. Since the densified layers are very thin compared to the rest of the solid cellulose foam, they have a negligent influence on the overall density of the foam.

[0059] A solid cellulose foam with outer surfaces comprising densified layers is easier to handle since it has improved mechanical properties due to the densified layer acting as a protective layer.

[0060] A densified layer will be formed in the imprinted region, as further discussed below, by the method according to the present invention. When the surface of the solid cellulose foam into which the imprinting is performed comprises a densified layer, the solid cellulose foam after applying moisture, imprinting and drying will have a uniform surface comprising a densified layer both in imprinted regions and in other regions. Alternatively, if moisture is applied on the entire surface of the solid cellulose foam, a densified layer will be formed across the entire surface during the drying step.

[0061] Step b) of the method according to the first aspect involves applying moisture at least to regions of the solid cellulose foam where the at least one imprinted region is to be formed, so as to provide the solid cellulose foam with at least one moistened region. By providing moisture to the solid cellulose foam, the foam is partly disintegrated and softened which facilitates imprinting. Moisture can be applied to the entire solid cellulose foam, or only to regions of the foam that are to be imprinted. Typically, moisture is applied to a top surface of the solid cellulose foam. Preferably, the moisture is applied to a surface comprising a densified layer.

[0062] Applying moisture may involve applying water or an aqueous solution, aqueous dispersion or aqueous suspension to the solid cellulose foam. If an aqueous solution, dispersion or suspension is used, it can comprise any suitable additive so that it in addition to moisture provides the foam with any desired properties. The additive may for example be selected from hydrophobation agents or sizing agents, wet-strength agents, dry-strength agents, barrier forming agents, grease-proofing agents, pigments, dyes and / or surface structure agents.

[0063] Moisture can be applied by any suitable means, such as spraying, injecting, coating and passing the foam through a humid atmosphere. Coating may involve for example roll coating, blade coating or curtain coating.

[0064] Moisture may be applied prior to pressing the foam to form the at least one imprinted region, and / or simultaneously as pressing the foam to form the at least one imprinted region.

[0065] In some embodiments, moisture is provided by the forming tool. The forming tool may for example comprise at least one nozzle through which moisture is applied to the solid cellulose foam. In such embodiments, moisture is typically only provided to regions of the foam that are to be imprinted.

[0066] When applying moisture to the solid cellulose foam, moistened regions are formed.

[0067] In such moistened regions, the foam is partly disintegrated. This means that the foam becomes softer and is much easier to shape. It is believed by the present inventors that the applied moisture will interfere with the fiber-fiber hydrogen bonds in the cellulose foam such that the bonds are at least partly weakened and / or broken. Since the bonds are loosened, fibres may disentangle more easily, reducing the stiffness of the foam and rendering the foam more malleable and mouldable.

[0068] The amount of moisture applied depends on the shape of the imprinted region to be formed. As realized by a person skilled in the art, more moisture is needed if a deep imprint is formed and less if a shallow imprint is to be formed. Moisture may be applied to a top surface of the solid cellulose foam and will penetrate into the foam such that the moistened regions extend into the foam from the top surface in a direction downwards into the core of the foam. Alternatively, moisture may be injected, e.g. with a needle, into the core of the foam. In such embodiments, injections can be made where needed to form a moistened region of suitable shape.

[0069] In embodiments where a shallow imprint is formed, for example a surface pattern, the moistened region does not have to extend deep into the foam. For example, the depth of the moistened region as measured from the top surface of the foam and extending towards the bottom surface of the foam may be in the range of from 5 to 10 mm.

[0070] In embodiments where a deep imprint is formed, for example a cavity for holding a product to be protected, the moistened region preferably extends at least to a depth corresponding to the depth of the cavity. By ensuring that the moistened region extends sufficiently deep, imprinting is facilitated.

[0071] In some embodiments, the moistened region extends throughout the thickness of the foam. If moisture is also applied all over the surface of the foam from which the imprinted regions is to be formed, the entire solid cellulose foam will be moistened. Thus, in some embodiments the moistened region will extend throughout the solid cellulose foam.

[0072] In some embodiments, at least one moistened region is formed in the solid cellulose foam, such that each moistened region is surrounded by parts of the foam to which no moisture has been applied. Due to diffusion of moisture from the moistened region to the surrounding regions, there will not be a sharp line between a moistened region and its surrounding regions, but instead a gradual decrease in moisture content from the moistened region towards the surrounding regions.

[0073] The moisture content of the foam in the at least one moistened region may be in the range of from 20 to 85 wt%, or from 40 to 85 wt%, or from 60 to 85 wt%, or from 40 to 60 wt%, based on the total weight of the solid cellulose foam in the moistened region. The moisture content refers to the amount of moisture in the foam in the moistened region at ambient conditions, i.e. a relative humidity of from 30-50% and a temperature of from 20-25°C. The moisture content of the solid cellulose foam prior to applying moisture is typically less than 10 wt%, depending on the conditions the foam is exposed to. If the moisture content of the moistened region is too small, it will not be possible to form an imprinted region. If the moisture content of the moistened region is too high, the foam will be disintegrated to a large extent and will not be stable enough for the pressing step. The moisture content of the solid cellulose foam to which no moisture has been applied remains the same. In regions surrounding the moistened region the moisture content of the cellulose foam may increase slightly due to diffusion of moisture from the moistened regions.

[0074] If no moisture is applied to the solid cellulose foam, it is not possible to form a shape by pressing if not simultaneously cutting the foam and / or applying heat. To permanently deform the solid cellulose foam, moisture and / or heat is required. By simply cutting and pressing, the deformation is not permanent, and the foam will at least to some extent resume its original shape.

[0075] Step c) of the method according to the first aspect involves pressing at least one forming tool into the at least one moistened region on the solid cellulose foam such that at least one imprinted region is formed in the solid cellulose foam. The forming tool is pressed into a surface of the solid cellulose foam comprising the moistened region, and is then pressed further into the moistened region extending though the thickness of the foam. The area of the moistened region on the surface of the solid cellulose foam may correspond to the surface of the forming tool pressed into the moistened region, or the area of the moistened region may be larger than the surface of the forming tool pressed into the moistened region.

[0076] One or several imprinted regions may be formed in the same solid cellulose foam substrate. The at least one imprinted region is surrounded by regions of the solid cellulose foam that have not been imprinted and thus retain their original properties. Thus, after step c) of the method according to the first aspect, the solid cellulose foam comprises at least one imprinted region and at least one region which has not been imprinted.

[0077] The forming tool may have a shape corresponding to the shape of the imprinted region. One or several forming tools may be used. In embodiments where more than one forming tool is used, the forming tools may have the same shape or different shapes, so as to form imprinted regions of the same shape or different shapes at different locations of the solid cellulose foam.

[0078] In embodiments where more than one forming tool is used, each forming tool may be pressed into one moistened region separated from other moistened regions by regions of the foam where no moisture has been applied. In some embodiments, at least two forming tools are pressed into the same moistened region. For example, moisture may have been applied all over the top surface of the solid cellulose foam. If more than one forming tool is used, these may be pressed into the foam simultaneously or in sequence. More than one forming tool may be used for forming one imprinted region, such that the imprinted region is formed by first one forming tool and then at least one additional forming tool.

[0079] In embodiments where one forming tool is used, it may be used for providing several imprinted regions in sequence at different locations of the solid cellulose foam substrate. The different locations may be located within the same moistened region, or in different moistened regions.

[0080] Preferably, the forming tool is pressed perpendicularly into the surface of the solid cellulose foam. Alternatively, the forming tool may be pressed at an angle to the surface of the solid cellulose foam.

[0081] The pressing depth of the forming tool depends on the shape of the imprinted region. The pressing depth preferably does not extend beyond the depth of the moistened region.

[0082] Once the imprinted region has been formed in the solid cellulose foam, the forming tool is removed. In some embodiments, the solid cellulose foam is heated when the pressing step is carried out. The solid cellulose foam may be heated before the steps of applying moisture and pressing, for example by placing it on a heated substrate, or in an oven. The solid cellulose foam may also be heated by applying heated moisture, such as steam. The solid cellulose foam may also be heated by using a heated forming tool. When heat is used, forming of an imprinted region is facilitated, and the drying time of the moistened foam after pressing may also be reduced since moisture is removed to some extent already in the pressing step.

[0083] The forming tool may be made of any suitable material, such as a metal or metal alloy, or a plastic material. It may be perforated such that moisture can be drained from the moistened region of the foam through the forming tool.

[0084] In one embodiment, moisture is provided by the forming tool. For example, the forming tool may comprise nozzles through which moisture can be applied to the solid cellulose foam so as to form the at least one moistened region. The moisture may be applied through the nozzles either prior to pressing, or while the forming tool is pressed into the foam such that applying moisture and pressing is carried out simultaneously. In embodiments where heat is applied during pressing, steam may be provided through the nozzles of the forming tool. In such embodiments, moisture may be applied to the solid cellulose foam both prior to pressing by means different than from the forming tool, and by the forming tool.

[0085] During pressing of the moistened region of the solid cellulose foam, the foam is to some extent compressed by the pressure exerted by the forming tool such that the foam after pressing assumes a more compacted structure. The density of the solid cellulose foam is locally increased. The foam located below the imprinted region is compressed and has a higher density compared to surrounding regions to which no pressure has been applied. The compression leads to some loss in cushioning properties of the compressed foam. No foam material is removed during the pressing step. The regions of the foam surrounding the imprinted region are not compressed and thus retain its original structure and thus also its cushioning properties. Due to the present method where moisture is applied, it is possible to very specifically apply pressure and form imprinted regions at a certain location of the foam without also affecting the structure, and thus the cushioning properties, of the surrounding regions of the foam. Therefore, the overall cushioning properties of the foam is not impaired rendering the solid cellulose foam with the at least one imprinted region suitable for use in protective packaging applications. If not applying moisture, it is not possible to form imprinted regions without also compressing the surrounding regions and reducing the overall cushioning properties of the foam.

[0086] In a preferred embodiment of the present invention, the imprinted region in the solid cellulose foam has the shape of a cavity. Such a cavity extends from a first surface, preferably the top surface, of the solid cellulose foam such towards a second surface, preferably the bottom surface, of the solid cellulose foam. The cavity typically does not extend completely through the foam, such that the second surface of the foam remains intact. The shape of the cavity is selected depending on the product intended to be placed in the cavity. Different parts of the cavity may extend to different extents through the thickness of the foam, such that the depth of the cavity varies. In some embodiments, the shape of the forming tool corresponds to the shape of the cavity to be formed. In other embodiments, more than one forming tool is used such that parts of the cavity is formed by different forming tools.

[0087] In an alternative embodiment of the present invention, the imprinted region is in the form of a pattern imprinted in the surface of the solid cellulose foam. Such an imprinted region typically does not extend very deep into the solid cellulose foam. For example, it may have a maximum depth corresponding to 10%, or 20% of the thickness of the foam. The pattern may be a repeating pattern extending along the complete surface of the solid cellulose foam. For example, the repeating pattern may be in the form of text or a logo, or in a pattern intended to provide friction or any other suitable pattern. It may be advantageous to provide solid cellulose foam intended for use in thermal or acoustic insulation applications with a surface pattern to increase friction to facilitate adherence to other materials, and / or to improve the acoustic properties.

[0088] In embodiments where more than one imprinted region is formed in the solid cellulose foam, all imprinted regions may be formed by pressing into the same surface of the foam. Alternatively, imprinted regions may be formed in more than one surface of the foam. In such embodiments, the corresponding moistened regions also extend from more than one surface of the foam, or the entire foam may be moistened. In one embodiment of the present invention a thin layer of a fibrous substrate is applied to the surface of the foam in the at least one moistened region of the foam. Such a fibrous substrate may for example be a tissue. The fibrous substrate becomes attached to the moistened region of the foam during drying. The fibrous substrate may be attached prior to pressing or after pressing. It will be attached to the surface of the foam in the moistened region, i.e. also in the imprinted region. By selecting a fibrous substrate with desired properties, such as a particular surface structure, colour or pattern, such properties are conveyed to the solid cellulose foam.

[0089] Step d) of the method according to the first aspect involves drying the solid cellulose foam. In step d) the moistened regions in the solid cellulose foam are dried, together with drying of any other regions of the foam into which moisture has diffused. Step d) is preferably carried out after the forming tool has been removed from the foam. If the forming tool remains during drying, there is a risk that the foam is attached to the surface of the forming tool and that the imprinted region is damaged if subsequently removing the tool. Drying 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. After drying, the moisture content is uniform throughout the solid cellulose foam.

[0090] During drying, a densified layer is formed in the outer surfaces of the moistened regions as described above with reference to the wet cellulose foam being dried to a solid cellulose foam. Thus, the surface of the at least one imprinted region of the solid cellulose foam will comprise a densified layer, which provides improved mechanical properties to the imprinted region. A surface comprising a densified layer will be more resistance to impact from e.g. sharp objects and will also provide overall structural integrity to the foam.

[0091] In an embodiment of the present invention, the solid cellulose foam provided in step a) comprises a densified layer on at least one surface. If the imprinted region is formed as extending from such a surface, said surface will after applying moisture, pressing to form the imprinted region and drying will comprise a densified layer on the entire surface, also in the imprinted region. If moisture is applied along an entire surface of the solid cellulose foam, a densified layer will be formed all over said surface regardless if the solid cellulose foam as provided comprised a densified layer or not.

[0092] When moisture is applied to a surface of the solid cellulose foam comprising a densified layer, the densified layer is to some extent disintegrated along with the rest of the foam. A densified layer is however formed again during the drying step.

[0093] After drying, the solid cellulose comprising at least one imprinted region will have a moisture content of less than 5 wt%.

[0094] According to a second aspect, the present invention relates to a solid cellulose foam comprising at least one imprinted region, wherein the outer surface of the imprinted region comprises a densified layer. The solid cellulose foam of the second aspect is further characterized as set out above with regards to the first aspect. The outer surface of the imprinted region is the surface area of the cellulose foam in the imprinted region.

[0095] In some embodiments, the at least one imprinted region has the shape of a cavity. Such a solid cellulose foam is particularly suitable for use in protective packaging applications. For example, the solid cellulose foam may be used as a protective packaging insert, where an object to be protected is placed in the imprinted region in the shape of a cavity.

[0096] In some embodiments, the at least imprinted region has the shape of a regular surface pattern. Depending on the pattern, friction properties or acoustic properties may be provided to the foam. Such solid cellulose foam is particularly suitable for use as insulation material or building materials.

[0097] According to a third aspect, the present invention relates to use of the foam obtained by the method according to the first aspect as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media.

[0098] Examples A solid cellulose foam substrate in the form of a plank having a thickness of 45 mm was provided. The foam was dry, had a density in the range of from 32 to 35 kg / m3 and comprised from 83 to 88 wt% cellulose fibres (softwood bleached Kraft pulp), from 10 to 15 wt% thickener (CMC) and about 2 wt% surfactant (mixture of myristic acid and sodium cocoyl sarcosinate).

[0099] Moisture was applied to the foam substrate by spraying water on the top surface. The top surface comprises a densified layer. A metal mould was pressed into the foam while continuously applying more moisture. The mould was removed and the imprinted region had obtained the same shape as the mould. The foam was then dried and the imprinted region remained, retaining the same shape as when wet. The outer surface of the imprinted region was covered with a densified layer. 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 forming at least one imprinted region in a solid cellulose foam, the method comprising the steps of: a) providing a solid cellulose foam; b) applying moisture at least to a region of the solid cellulose foam where the at least one imprinted region is to be formed, so as to provide the solid cellulose foam with at least one moistened region; c) pressing at least one forming tool into the at least one moistened region on the solid cellulose foam such that at least one imprinted region is formed in the solid cellulose foam; and d) drying the solid cellulose foam.

2. The method according to claim 1 , wherein the solid cellulose foam provided in step a) comprises cellulose fibres in a range of from 71 to 95 wt% based on the total dry weight of the solid cellulose foam.

3. The method according to any one of claims 1 or 2, wherein the solid cellulose foam provided in step a) 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.

4. The method according to any one of the preceding claims, wherein the solid cellulose foam provided in step a) has a density in the range of from 10 to 80 kg / m3.

5. The method according to any one of the preceding claims, wherein the solid cellulose foam provided in step a) comprises a densified layer on at least one outer surface.

6. The method according to claim 5, wherein the at least one imprinted region is formed on an outer surface of the solid cellulose foam comprising a densified layer.

7. The method according to any one of the preceding claims, wherein step b) involves applying water or an aqueous solution to the solid cellulose foam.

8. The method according to any one of the preceding claims, wherein the moisture content of the foam in the at least one moistened region is in the range of from 20 to 85 wt%, based on the total weight of the foam in the at least one moistened region.

9. The method according to any one of the preceding claims, wherein moisture is applied prior to pressing and / or simultaneously with pressing.

10. The method according to any one of the preceding claims, wherein moisture is provided by the forming tool.11 . The method according to any one of the preceding claims, wherein the solid cellulose foam is heated during the pressing step.

12. The method according to claim 11 , wherein heating is provided by using a heated forming tool.

13. The method according to any one of claims 11 or 12, wherein heating is provided by using heated moisture.

14. A solid cellulose foam comprising at least one imprinted region, wherein an outer surface of the solid cellulose foam in the imprinted region comprises a densified layer.

15. The solid cellulose foam according to claim 14, 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.

16. The solid cellulose foam according to any one of claims 14 or 15, 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.

17. The solid cellulose foam according to any one of claims 14-16, wherein the solid cellulose foam has a density in the range of from 10 to 80 kg / m3.

18. Use of the solid cellulose foam according to claims 14-17, or the solid cellulose foam obtainable by the method according to any one of claims 1-13 as a packaging material, a building material, a thermal insulation material, an acoustic insulation material or as a hydroponic plant growth media.