SE2230300A1

SE2230300A1 - Preparation of a cellulose foam comprising discrete units of foam embedded in a foam matrix

Info

Publication number: SE2230300A1
Application number: SE2230300A
Authority: SE
Inventors: Torron Daniel Hult; Martin Sterner; Oruç Köklükaya
Original assignee: Stora Enso Oyj
Priority date: 2022-09-23
Filing date: 2022-09-23
Publication date: 2024-03-24
Also published as: SE546065C2

Abstract

The present invention relates to a method for the preparation of a solid foam, wherein the method comprises depositing discrete units of a foam on a surface to obtain a first foam deposition, followed by depositing a wet foam between the discrete units to obtain a subsequent foam deposition, and drying the wet foam, and wherein the density of the foam in the discrete units is higher than the density of the foam matrix The invention further relates to a solid foam comprising discrete units of foam embedded in a foam matrix.

Description

FIELD OF THE INVENTION The present invention relates to a method for the preparation of a solid foam, wherein the method comprises depositing discrete units of a foam on a surface to obtain a first deposition, followed by depositing a wet foam between the discrete units to obtain a subsequent deposition, and drying the wet foam to obtain a solid foam wherein discrete units of a foam are embedded in a foam matrix. The invention further relates to the solid foam comprising discrete units of foam embedded in a foam matrix. The density of the discrete units of foam is higher than the density of the foam matrix.

TECHNICAL BACKGROUND Today different techniques are used to deposit wet foam material and produce low- density thick foam materials. ln WO2020011587 A1 a porous material of cellulose fibres and gluten is prepared by depositing at once an aerated wet foam of cellulose fibres and gluten in a mould, followed by drying, to obtain a dried porous material with the shape of the mould and a homogeneous fibre network through the whole bulk. ln WO2015036659 A1 a wet fibrous foam is fed into a mould, where part of the water contained in the foam is mechanically withdrawn to produce a solidified, moist fibrous composition, and evaporating water to produce a dry fibrous product. These techniques are similar in that the final foam sheet might present a densified layer on the faces of the sheet, while the core of the foam sheet is a homogeneous fibre network of lower density.

Collapse of a particulate or fibrous foam causes the thickness of a foam sheet to shrink, as tension forces pull the particles, or fibres, together. Drying shrinkage is an inherent property of cellulose, as 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, shrinkage above 10% is expected.

Similar to conventional papermaking techniques, foam-forming needs to be drained onto a screen. ln this case, there are limits to the shape and size ofthe material as it will level during draining. The density and thickness ofthe sample is determined by the wet fibrous foam concentration, which is usually from 1-4 wt%, and the amount of draining before drying. Generally dry content is about 1% to 8% after draining. For efficient draining to occur, the viscosity of the suspension needs to be kept rather low for efficient extraction of water. Any soluble binder would be limited to low concentrations and being able to retain on the fibre to avoid excessive losses. Therefore, draining limits the number of additives that could be used in this type of process. The strength ofthick low-density foam-formed paper is primarily controlled by the bulk of the material. The primary means of improving strength here is to increase density, with some limited control on fibre orientation, based on draining characteristics.

Higher dry-content techniques such as foams made from large quantities of protein- based foaming-agents, such as in WO2020011587 A1, hinders recyclability due to large fractions of the material not being water soluble or easily washed out from the product. The method also suffers from poor wet-foam stability, as protein particles start to agglomerate and bubbles coalesce, leading to gradual collapse of foam in the wet-state. This fact makes it a non-suitable candidate for free-standing wet foam deposition.

To avoid shrinkage, the foam needs to dry under tension, but when drying a very large surface area the tension obtained by the frame or mould is limited to the regions closest to the mould. Thus, shrinkage is a problem, especially when drying large surface areas. The drying time ofthe foam is typically long since both the wet foam and dry porous material are heat insulating. A short drying time is desired to enable a cost-efficient process. ln addition, the prepared foam should have a high impact resistance when used as a packaging material to enable protection also of heavier objects. Thus, there is still need for alternative methods for foam preparation.

SUMMARY OF THE INVENTION lt is an object of the present invention to provide a method suitable for the preparation of a foam where the shrinkage of the foam is reduced, and where the method enables a shorter drying time ofthe foam. The method enables a structural control of thick low-density foam materials, through a controlled wet-deposition technique and is particularly suitable for the preparation of cellulose foam sheets.

A further object is to provide a lightweight solid foam with good dimensional stability.

To be more specific, this invention relates to cellulose foam materials comprising discrete units of a cellulose foam embedded in a cellulose foam matrix, wherein the cellulose foam material has a low density. The cellulose foam matrix surrounding the discrete units may be composed of the same foam com position as the discrete units. The density ofthe discrete units of foam is higher than the density ofthe foam matrix. The discrete units may be distinguished from the matrix by a densified layer. Furthermore, this foam material can be made by two or more individual deposition- steps with a drying step after each deposition. The method allows for the creation and control of densified cellulose fibre walls. When dry, the fibres can be re-dispersed in water and as a result the cellulose foam can be recyclable in regular paper recycling streams. When the wet foam density of foam used for the discrete units is higher than the wet foam density of foam used for the surrounding cellulose foam matrix, it has been found that the total drying time of the cellulose foam decreases, thus enabling a more efficient process. ln addition, the higher density regions ofthe cellulose foam (i.e. the discrete units) will have different properties, for example stiffness, compared to the lower density regions (i.e. the foam matrix). This can be used to provide a cellulose foam with different properties in different regions.

Thus, this invention pertains both to a cellulose foam having a characteristic macrostructure with discrete units of cellulose foam embedded in a cellulose foam matrix, where the density ofthe foam in the discrete units is higher than the density of the foam in the matrix surrounding the discrete units, and the method to obtain the foam, which comprises a multistep deposition and drying method.

The cellulose foams are expected to contribute to technologies in protective packaging as a cushioning material, thermal insulation for cold chain logistics, and as a construction material, acoustic insulation panels, as a hydroponic plant growing media, and other applications that need lightweight, high-performance bio-based materials. Due to the excellent impact resistance, the cellulose foams ofthe present invention are particularly suitable for protection of heavy objects.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a two-step deposition with i) a first deposition of cellulose foam as discrete units, ii) an intermittent drying step drying the discrete units to create iii) self- standing discrete units of cellulose foam, iv) a second deposition of cellulose foam between the discrete units of cellulose foam, and v) a second drying step to obtain vi) a cellulose foam material comprising discrete units of cellulose foam.

Figure 2 shows a schematic representation of a solid foam sheet produced according to the method of the present invention, where A) illustrates the solid foam with a densified layer at the top and at the bottom in black colour, and looking externally similar to foam sheets produced with other techniques, B) illustrates the bulk ofthe material beneath the densified top and bottom, the bulk containing discrete units of cellulose foam (black cuboids), where each discrete unit is distinguished from the surrounding foam matrix by a densified layer of cellulose, and C) illustrates one discrete unit surrounded by a densified layer of cellulose (left), illustrated in black, and the homogenous cellulose foam inside the densified layers (right).

Figures 3-5 shows the cushioning performance of a solid foam according to the present invention during impact, measured by drop testing the foam with a load containing an accelerometer on top ofthe foam. Results of peak acceleration for drop 1 (figure 4) and for the average of drops 2-5 (figure 5) and degradation, measured as the relative compression after 5 drops (figure 3) is shown. Cellulose foam samples composed of discrete units of the same or different density as compared to the ce||u|ose foam matrix are tested; and compared to single step deposited foam samples.

DETAILED DESCRIPTION OF THE INVENTION ln a first aspect the present invention provides for a method for the preparation of a solid foam, wherein the method comprises depositing discrete units of a foam on a surface to obtain a first foam deposition, depositing a wet foam between the discrete units to obtain a subsequent foam deposition, and drying the wet foam in the subsequent foam deposition to obtain a solid foam wherein discrete units of a foam are embedded in a foam matrix, and wherein the density of the foam in the discrete units is higher than the density ofthe foam matrix.

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. I\/|echanical work is needed to increase the surface area. This can occur by agitation, dispersing a large volume of gas into a liquid, or injecting a gas into a liquid. The second requirement is that a foam forming agent, typically an amphiphilic substance, a surfactant or surface active component, must be present to decrease surface tension. Finally, the foam must form more quickly than it breaks down. The foam may be liquid or solid.

The term "ce||u|ose foam", as used herein, refers to a foam comprising ce||u|ose, and other components such as thickeners, surfactants and additives. The main component of the ce||u|ose foam is ce||u|ose, such that ce||u|ose constitutes at least 70 wt% of the dry content of the ce||u|ose 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 dry.

The term "solid cellulose foam", or "dry cellulose foam", as used herein, refers to a dry porous cellulose material that has been formed from a wet cellulose foam, i.e. a foam formed material. During the drying process, a closed wet cellulose foam is transformed into an open solid cellulose foam. The network of cellulose fibres is prevented from collapsing during drying. The solid cellulose foam will as a result have a shape that to a large extent corresponds to that of the wet cellulose foam. The dry content of the solid cellulose foam is at least 95 wt% as calculated based on the total weight of the solid cellulose foam. The shape and density ofthe 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 wet foam used in the discrete units may be a fibrous foam. The foam may comprise at least 10 wt% cellulose fibres, or at least 11 wt% cellulose fibres. The wet 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 foam. The cellulose fibres may be selected from wood pulp; regenerated cellulose fibres; or plant fibres, such as fibres from bamboo, cotton, hemp, flax, and jute. Preferably, the cellulose fibres are selected from wood pulp, such as softwood kraft bleached pulp, chemicaI-thermomechanical pulp, dissolving pulp (such as bleached wood pulp or cotton linters), and hardwood pulp; more preferably from softwood kraft bleached pulp and chemicaI-thermomechanical pulp; and most preferably softwood kraft bleached pulp.

The discrete units may be made by dispensing a wet foam in discrete units on to a surface followed by drying of the discrete units. The wet foam forming the discrete units may have a density from 150 - 600 kg/m3, or from 150 - 500 kg/m3, or from 200 - 500 kg/m3, or from 200 - 400 kg/m3. The density of the wet foam forming the discrete units is selected such that it is higher than the density of the wet foam forming the surrounding foam matrix. A large number of small bubbles provides stability to the foam and provides for a low density. The wet foam used in the present method has a sufficiently high viscosity and low density to enable the formation of discrete units that do not collapse before they are dried. Each discrete unit may thus stand by itself without collapsing before being dried. Further, each discrete unit may be self-standing during drying without collapsing. Drying of the discrete units of the wet foam is at least made until a crust, i.e. a thin densified layer of cellulose fibres, is formed on an outer surface ofthe discrete unit, such as on each ofthe faces of the discrete unit. The densified layer is a very thin layer that is formed on the very outer surface ofthe 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 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 ofthe foam. The thin thickness of the densified layer implies that it practically does not affect the overall density of the foam, while it still contributes to the good mechanical properties of the discrete units.

Optionally, further foam depositions of discrete units may be made on the surface, preferably between the discrete units of the first deposition. The wet foam forming the discrete units in the further deposition may have a density of from 150 - 600 kg/m3, or from 150 - 500 kg/m3, or from 200 - 500 kg/m3, or from 200 - 400 kg/m3. The density ofthe wet foam used in the further deposition may be the same for the wet foam in the first deposition, or it may be different. The density of the wet foam used in the further deposition is higher than the density ofthe wet foam in the foam matrix.

When discrete units of a foam comprising cellulose fibres have been dried, their core consists of a homogeneous fibre network having a density, and their outer surfaces, such as their bottom, top and side faces, consists of a more densely packed fibre network. The formation of a densified layer on the faces of the deposited discrete units in the first foam deposition makes the discrete units stronger and prevents them from being demolished during subsequent foam depositions, when wet foam is deposited between the discrete units. The dried discrete units may have an overall density of from 20 - 80 kg/m3, or from 20 - 60 kg/m3, or from 30 - 50 kg/m3. ln an alternative embodiment, the discrete units may be made by extruding or casting a wet foam, drying the wet foam to obtain a dry foam, cutting said dry foam into discrete units, and depositing said discrete units on to a surface. Preferably the wet foam is extruded into a board, plank, bar or rod. The board, plank, bar or rod can be cut into discrete units, which may be deposited on to a surface.

Each discrete unit may have a three-dimensional shape, such as a cylinder, or a polyhedron. Examples of symmetric polyhedrons are cubes, cuboids, and hexagonal prisms. Small variations in the symmetry ofthe discrete units may exist without changing their main purpose to impart stability to the foam. For example, the cylinder, cube, cuboid, and hexagonal prism may be slightly distorted so that their opposite bases are not always exactly parallel and over each other. For example, the discrete unit may have the shape of a truncated cone. Preferably, each discrete unit has the shape of a cylinder. The term "cylinder" is used herein for the geometrical figure that is commonly defined as a closed solid with two principally parallel, congruent, and circular or oval, bases that are connected by a curved surface. Discrete units obtained in further foam depositions may have the same or a different three-dimensional shape as the discrete units obtained in the first deposition.

As used herein, the height ofthe discrete unit is measured perpendicular to the surface on which the unit has been deposited. As used herein, the width, or average of the length and width, is measured at the bottom part of the discrete unit. For discrete units having a circular shape, the width corresponds to the diameter. The width of a discrete unit may be from 0.5 to 3 times its height, or from 0.5 to 2, or from 0.5 to 1.5, or from 0.8 to 3, or from 0.8 to 2, or from 0.8 to 1.5, or from 1 to 3, or from 1 to 2, or from 1 to 1.5 times its height. ln a preferred embodiment, the width of a discrete unit may be less than 1.3 times its height, such as less than 1.2, or from 0.5 to 1.3, or from 0.5 to 1.2, or from 0.7 to 1.3, orfrom 0.7 to 1.2, or from 0.9 to 1.3, orfrom 0.9 to 1.2 times its height. The total drying time ofthe foam is shortened when the width ofthe discrete units is less than 1.3 times its height.

The term "total drying time" as used herein, refers to the time it takes for the foam to dry, i.e. the sum of the drying time for the discrete units and the drying time for the foam matrix.

A subsequent deposition of wet foam is made between the already dried discrete units. The height of the wet foam in this subsequent deposition may be slightly higher or equal to the height of the discrete units. After said deposition of foam between the discrete units, the height of the discrete units may be from 90 to 100 %, or from 95 to 100 %, or from 98 to 100 %, of the height ofthe foam matrix surrounding said discrete units. ln some embodiments a deposition of wet foam may also be made on top ofthe discrete units, either simultaneously with the previously mentioned deposition ofthe wet foam between the discrete units or in a successive deposition. The surface ofthe subsequent deposition of wet foam may be scraped before drying to provide an even surface.

The wet foam in the subsequent deposition may be a fibrous foam. The wet foam may comprise at least 10 wt% cellulose fibres, or at least 11 wt% cellulose fibres. The wet foam may comprise 10 - 40 wt%, 11 - 40 wt%, 10 - 30 wt%, 11 - 30 wt%, 12 - 30 wt%, 10 - 20 wt%, or 11 - 20 wt%, or 12-20 wt% cellulose fibres, as calculated on the total weight ofthe wet foam. The cellulose fibres used in the subsequent deposition are suitably selected from different kinds of wood pulp; regenerated cellulose fibres; or plant fibres, such as fibres from bamboo, cotton, hemp, flax, and jute. Preferably, the cellulose fibres are selected from wood pulp, such as from softwood kraft bleached pulp, chemicaI-thermomechanical pulp (CTMP), dissolving pulp (such as bleached wood pulp or cotton linters), and hardwood pulp; more preferably from softwood kraft bleached pulp and chemical-thermomechanicaI pulp; and most preferably softwood kraft bleached fibres. The cellulose fibres used in the subsequent deposition may be of the same sort as the cellulose fibres used in the discrete units of earlier depositions, i.e. the first and optionally further depositions.

The wet foam used in the subsequent deposition may have a density from 50 - 400 kg/m3, or from 50 - 300 kg/m3, or from 50 - 200 kg/m3, or from 60 - 150 kg/m3. The density of the wet foam used in the subsequent deposition is selected so that it is lower than the wet density of the foam used in the first deposition.

The solid foam prepared with the method according to the present invention may have a density of from 10 - 80 kg/m3, or 10 - 60 kg/m3, or from 20 - 50 kg/m3. ln a wet 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 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 surface, causes tension in the opposite direction and will impact the net tension forces. Deformation, such as shrinkage, will occur when the net tension forces, i.e. the tension vector, dominate in any particular direction. Thus, the ratio of the width, or surface area, to the height of cellulose foam planks affects the distribution of the net tension forces in a foam material upon drying, and the greater the ratio, the greater the net tension forces that arise.

Preparation of a foamed material according to the method of the present invention reduces the net tension forces arising in the material upon drying and the shrinkage of the material may thus be mitigated. Figure 1 illustrates one embodiment of the method ofthe present invention, wherein a wet fibrous foam is deposited on a surface as small discrete units to obtain a first deposition (i). Each discrete unit has a low aspect ratio of width to height, which eliminates or significantly reduces shrinkage of each discrete unit when the units are dried (ii). When the discrete units are dried, they will consist of a core comprising a homogeneous fibre network, and densified outer faces (i.e. the top, bottom and sides) (iii). A subsequent deposition of a wet foam is then made on the surface between the already dried discrete units (iv). The density of the wet foam in the subsequent deposition is lower than the density of the foam in the first deposition. When the wet foam of the subsequent deposition is being dried (v) the discrete units of the first deposition that are already distributed on the surface provides for a low width to height ratio of the wet foam in the subsequent deposition. The tension forces of each discrete unit will act against each other thus reducing the net tension forces in the foam of the subsequent deposition, which restrains build-up ofthe tension during drying and as a result mitigates the effect of shrinkage on the outer dimensions of the obtained solid foam (vi). The method of the present invention thus provides for formation of a solid foam object with a reduced shrinkage, especially in the z-direction. 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. The size of the solid foam object to be produced may depend on the number of individual discrete units that can be placed in the first and optionally further depositions and the distance between them. The present method allows for the manufacturing of foam formed planks that are at least 60 * 60 cm, or at least 100 * 100 cm, or at least 200 * 200 cm, or at least 11 300 * 300 cm. With the multi-step deposition method according to the present invention foam formed objects can be produced with reduced shrinkage compared to similar foam formed objects obtained with the single-step deposition. The present method also allows for a process with continuous formation of a foam formed object, such as foam forming on a continuous belt. The present method also allows for a shorter total drying time, as compared to a foam with the same total density but made from depositions of wet foam having the same density.

The present invention provides for a low-density cellulose foam comprising discrete units of cellulose foam, with stiffer densified cellulose fibre walls, embedded in a cellulose foam matrix. The incorporation of the discrete units as structural elements in solid foams, enables the formation of stiffer foams while maintaining the same low density. The discrete units constitute from 30 to 90 % of the total volume, or from 30 to 80%, or from 40 to 80 %, or from 50 to 80 % ofthe total volume or from 60 to 80% ofthe total volume, or from 40 to 78 % of the total volume, or from 50 to 78 % ofthe total volume, or from 40 to 75 % ofthe total volume, or from 50 to 75% ofthe total volume ofthe foam formed material comprising the discrete units and the surrounding foam matrix.

The density ofthe discrete units of foam is higher than the density ofthe foam matrix. ln one embodiment, the density of the discrete units in the solid cellulose foam may be at least 110 %, such as 130%, or 150% or 200%, higher than the density ofthe cellulose foam matrix. ln one embodiment, the density of the discrete units in the solid cellulose foam may be in the range of from 105 to 500%, or 110 to 330%, or 110 to 250%, or 110 to 200%, or 150 to 330% higher than the density of the cellulose foam matrix. ln the solid cellulose foam, the density will thus vary within the solid cellulose foam such as that it alternates between a higher and lower value. By providing a solid cellulose foam comprising discrete units of cellulose foam having a higher density than the surrounding cellulose foam matrix, properties, such as stiffness and air permeability, 12 that depends on the density can be made to vary between different locations in the solid cellulose foam.

The wet foams used for the depositions in the method according to the present invention may be prepared by mixing cellulose fibres and one or more thickeners in water to obtain a flowable fibre mixture, adding a surfactant mixture and agitating to obtain a wet foam. The mixture of cellulose fibres and one or more thickeners in water may form an adequately flowable non-flocculated fibre paste. The mixture can be aerated by the addition of a surfactant mixture and agitation, such as by mechanical agitation. The aeration may form fine micron-sized air bubbles separating the fibres. The micron-sized air bubbles will stabilize the foam, which contributes to the preservation of the shape during drying of the discrete units.

Since cellulose fibres are mixed in high concentrations a drainage step is not needed. This may reduce the overall time and costs for the method and prevent leakage of water-soluble substances added during manufacturing, thereby allowing higher concentrations of water-soluble additives in in the final cellulose foam.

The performance ofthe dry cellulose foam can be tailored by the amount ofthickener used and hence the dry material fibre-fibre bonding strength. The amount of thickener may be from 4 to 24 % or from 5 to 20 %, as calculated per weight of solid content in the foam. The method according to the present invention allows adjustment ofthe stability ofthe wet foam with the use of thickeners and more stable surfactant combinations, enabling the provision of a free-standing cellulose foam. Further, the method allows for simple inclusion of different types of additives.

The density ofthe wet foam depends on how much air that is included during foaming. lf a low density is desired, relatively more air should be included. lf a high density is desired, relatively less air should be included. The density of the wet foam will have a direct influence on the density ofthe solid foam after drying. Thus, density differences 13 between the wet foams of the first deposition and the subsequent deposition will remain also in the solid foam.

During drying ofthe discrete free-standing foam units, a densified layer is formed on the face ofthe foam of each discrete unit, which helps to preserve the shape of the discrete unit. After the free-standing discrete units of foam has been dried, new wet foam can be added to fill in the gaps between the dried discrete units (as illustrated by (iv) in Figure 1). This allows for the creation of low-density cellulose foam divided into discrete units having stiffer densified cellulose fibre walls. By the incorporation of the densified thin layers the final foams can be made more rigid while maintaining the same light weight. Furthermore, this process provides for many units of a wet foam having a lower aspect ratio of width to height, minimizing the effect of tension buildup during drying and as a result mitigating the effect of shrinkage on the outer dimensions ofthe plank.

A foam composition suitable for the preparation of a solid foam with the method according to the present invention is a foam that is self-standing and do not collapse. Other foams might be used for extrusion and cutting into discrete units, or for deposition between the discrete units, such as foams and cellular solid materials described in WO2016068771 A1, WO2016068787 A1, and WO2020011587 A1. ln another aspect, the present invention relates to a solid foam comprising discrete units of a foam, and a foam matrix surrounding said discrete units. The density of the foam in the discrete units is higher than the density ofthe foam matrix surrounding said discrete units. Each discrete unit may be surrounded by a densified layer ofthe foam. The total solid foam, as well as the discrete units and the foam matrix may comprise from 75 to 95 wt%, or from 80 to 95 wt%, or from 85 to 92 wt% or from or from 85 to 90 wt% cellulose fibres as calculated on the total weight of the dry foam.

The height of the discrete units may be from 90 to 100 %, or from 95 to 100 %, or from 14 98 to 100 %, or equal to the height ofthe total solid foam prepared. The solid foam may have a density of 10 - 60 kg/m3, or from 20 - 50 kg/m3.

The density and properties of the final solid foam can be adjusted by using discrete units with higher densities than the matrix. The discrete units may also have densities that may differ from each other to provide the solid foam with different densities at different locations. The density ofthe discrete units in the solid foam may be from 110 %, such as 130%, or 150%, or 200%, higher than the density of the cellulose foam matrix. ln one embodiment, the density of the discrete units in the solid cellulose foam may be in the range of from 105 to 500%, or 110 to 330%, or 110 to 250%, or 110 to 200%, or 150 to 330% higher than the density ofthe cellulose foam matrix. Each discrete unit may have a three-dimensional shape, such as a cylinder or a polyhedron. Examples of symmetric polyhedrons are cubes, cuboids (such as rectangular cuboids), and hexagonal prisms. Small variations in symmetry of the discrete units may exist without changing their main purpose to impart stability to the foam. Preferably, each discrete unit has the form of a cylinder. The diameter, or width, of the discrete unit may be from 0.5 to 3 times its height, or from 0.5 to 2, or from 0.5 to 1.5, or from 0.8 to 3, or from 0.8 to 2, or from 0.8 to 1.5, or from 1 to 3, or from 1 to 2, or from 1 to 1.5 times its height. ln a preferred embodiment, the width of a discrete unit may be less than 1.3 times its height, such as less than 1.2, orfrom 0.5 to 1.3, or from 0.5 to 1.2, or from 0.7 to 1.3, or from 0.7 to 1.2, or from 0.9 to 1.3, orfrom 0.9 to 1.2 times its height. The total drying time of the foam is shortened when the width of the discrete units is less than 1.3 times its height. The width ofthe discrete units may also differ from each other in order to provide a solid foam with different properties at different locations.

The total drying time of the foam is also influenced by the density of the wet foam in the first and subsequent depositions. ln the inventive method, the density of the wet foam in the first deposition is higher than the density of the wet foam in the subsequent deposition. lf a cellulose foam with density D is desired, this can be provided by letting the foam in the first deposition as well as in the subsequent deposition have the same density that after drying achieves a solid cellulose foam with density D. Alternatively, as in the inventive method, the density D can be obtained by using a wet cellulose foam having a higher density for the first deposition and a wet foam having a lower density for the subsequent deposition, selected so that the average density ofthe solid cellulose foam will be equal to D after drying. The total drying time of the cellulose foam is shorter when the wet foam in the first deposition has a higher density than the wet foam in the subsequent deposition, as compared to a foam prepared using a wet foam with the same density for both the first deposition and the subsequent deposition. The drying time of the first deposition is always significantly faster than the drying time ofthe second deposition due to a large portion ofthe surface ofthe discrete units of the first deposition being exposed to air. The drying is faster for a wet foam with a low density so by using a wet foam with a relatively lower density for the second deposition, the drying time of the second deposition, and thus also the total drying time of the cellulose foam can be reduced. Therefore, it is advantageous to use a wet foam with a high density for the first deposition, with a relatively short drying time, and a wet foam with a low density for the second deposition. ln a further aspect, the present invention relates to a solid foam prepared by the method according to the present invention. A foam according to the present invention may be used in large sheets.

The invention will now be described by the following examples which do not limit the invention in any respect. All cited documents and references mentioned herein are incorporated by reference in their entireties. 16 EXAMPLES EXAMPLE 1 A uniform wet paste comprising 12 wt% cellulose pulp in water and a thickener was prepared. The paste was aerated with a surfactant mixture until a desired wet foam density was obtained. Wet foams of different densities (see table 1) were prepared. Cylindric moulds with the dimensions of 5 cm in height and 6.6 cm or 9.2 cm in diameter were used to deposit four discrete units of the foam on a flat surface (oven tray with a frame with dimensions of 20*20*5 cm). Four depositions with 6.6 cm diameter moulds covers approximately 1/3 of the frame volume, while 9.2 cm diameter moulds cover 2/3 ofthe frame volume. The moulds were placed in a symmetric fashion with equal distance between each mould and to the frame wall. The moulds were used only for depositing the foam in the desired shape and dimension and were removed before drying. The discrete units were dried in an ordinary convection oven at 120 °C until completely dry. When the first deposited discrete units were dry and had cooled down to room temperature for an extended time, a second deposition was applied filling up the void in between the discrete units. The frame was completely filled, scraped and dried in oven at 120°C until completely dry.

The foam in the second deposition typically had a different density than the foam in the first deposition. The aim was to reach approximately similar end density ofthe dry material and therefore wet foam densities were used to always put approximately the same total amount of material in the frame. The wet foam densities that were used are described in table 1. 17 Table 1 Share of total frame volume Density in Density in deposited in first step first deposition second deposition (6.6 or 9.2 cm diameter (kg/m3) (kg/m3) mould) 1/3 84 268 1/3 116 248 1/3 208 208 1/3 312 152 1/3 384 116 2/3 268 84 2/3 248 116 2/3 208 208 2/3 152 312 2/3 116 384 lt was found that the total drying time of the foam within the frame was shorter when the foam in the first deposition had a higher density than the foam in the second deposition.

Sample pieces were also made with the same frames 20x20x5 cm with single step deposited foam. These pieces were used as reference material to compare the cushioning properties of two step deposited material with single step deposited material. These test pieces were fabricated in a range of densities.

The sample pieces were conditioned in 23°C and 50% RH (relative humidity) for 3 days and used as sample pieces for drop testing.

The results ofthe drop testing, degradation and peak acceleration as function of sample density, is found in Figure 3 (degradation), figure 4 (peak acceleration for drop 1) and figure 5 (peak acceleration for drop 2-5). The different foams used in the tests are further explained in table 1. To illustrate the symbols used, (n) in the ﬁgure 18 represents a cellulose foam formed from a first deposition of a foam having a density of 84 kg/m3 deposited in discrete units with a diameter of 6.6 cm, and a second deposition of a foam having a density of 268 kg/m3. Dotted symbols are the reverse, for the same example the dotted white square represents a cellulose foam formed from a first deposition of a foam having a density of 268 kg/m3deposited in discrete units with a diameter of 9.2 cm, and a second deposition of a foam having a density of 84 kg/m3.

EXPERIMENTAL METHODS Characterization The cellulose foam produced according to Example 1 presents a densification on the top, bottom and side faces of the sheet (as i||ustrated in Figure 2A) while the bulk of the material contains discrete units of cellulose foam that are distinguished from the cellulose foam matrix by border walls of densified cellulose fibre material (Figure 2B). Depending on the densities ofthe foam in the depositions (see table 1), the densities ofthe discrete units and the cellulose foam matrix may be the same or different.

Drop test data was generated using a "TrueDrop-160" free fall drop tester in which a 20x20x5 cm piece ofthe sample material is tested. The material is placed in the bottom of a corrugated box with supporting PE-foam on the sides to hold it in place in horizontal direction. On top ofthe sample material a metal weight of 4.8 kg is placed. The metal weight has a 20x20 cm bottom side, and thus distribute a load of 12 g/cm2 on the sample. The metal weight is also held in place in the horizontal direction by the supporting PE foam. On top ofthe metal weight an accelerometer that records the acceleration while performing drop tests is placed. When a drop test is performed, the box containing the sample and the metal weight is dropped in straight vertical direction and allowed a free fall of 76 cm before hitting a metal floor. This is performed five times per sample and two results are registered: 1: Peak acceleration: Which is the maximum acceleration that the accelerometer placed on the metal weight is registering. The high acceleration occurs after the impact 19 when the sample material is using its cushioning ability to break the fall ofthe metal weight. A low peak acceleration is considered a positive result for a cushioning material such as the cellulose foam according to the present invention. 2: Degradation: Which is the difference between starting height ofthe sample pieces before dropping and its height after 5 consecutive drops. The height was measured with a calliper measuring in five positions, one on each side and one in the centre to calculate the average height.

Degradation = 1 - (height after 5 drops / Starting height).

For a piece that is first 5 cm heigh and after five consecutive drops 4 cm heigh the degradation is then: 1 - (4/5) = 20% The sample pieces were conditioned in 23°C and 50% RH (relative humidity) for 3 days before any measurements were performed on them.

The results (figures 3-5) show that the cellulose foam produced using a two-step deposition method has a better cushioning performance than a cellulose foam produced from a single-step deposition. The foams produced using a two-step deposition method according to the present invention, behaves similarly to cellulose foams produced with a lower density in the first deposition compared to the second deposition, or foams produced with similar density in both depositions. This proves that the decrease in drying time does not have an impact on the cushioning performance of the cellulose foam.

All combinations of different densities for first and second deposition performed better than single step deposited cellulose foam when it came to reducing degradation (relative compression after 5 drops). All combinations of different densities for first and second deposition also performed similar to single step deposited planks for peak acceleration of first drop and reduced the peak acceleration for the average of drop 2- compared single step deposited material for drop 2-5. These comparative statements refer to comparing materials with similar density.

Claims

1. A method for the preparation of a solid foam, wherein the method comprises depositing discrete units of a foam on a surface to obtain a first foam deposition, depositing a wet foam between the discrete units to obtain a subsequent foam deposition, and drying the wet foam to obtain a solid foam wherein discrete units of a foam are embedded in a foam matrix, and wherein the density of the foam in the discrete units is higher than the density ofthe foam matrix. The method according to c|aim 1, wherein the height of the discrete units is from 90 to 100 % of the height of the wet foam in the subsequent foam deposition. The method according to c|aim 1 or 2, wherein the wet foam in the subsequent foam deposition comprises at least 10 wt% ce||u|ose, as calculated on the total weight of the wet foam. The method according to any one of claims 1-3, wherein the discrete units are obtained by dispensing a wet foam as discrete units on to a surface, preferably in the form of cylinders, followed by drying of the wet foam. The method according to any one of claims 1-3, wherein the discrete units are obtained by extruding a wet foam, drying the foam, cutting the dried foam into discrete units, and depositing said discrete units on to a surface. The method according to any one of claims 4-5, wherein the wet foam used in the discrete units comprises at least 10 wt% ce||u|ose, as calculated on the total weight of the wet foam. The method according to any one of claims 1-6, wherein the wet foam has a density from 70 - 600 kg/m The method according to any one of claims 1-7, wherein the solid foam comprises 75-95 wt% cellulose fibres, as calculated on the total weight ofthe foam, and has a density of 10 - 80 kg/m The method according to any one of claims 1-8, wherein the density ofthe foam in the discrete units is at least 110 % ofthe density ofthe foam matrix. The method according to any one of claims 1-9, wherein the width of each discrete unit is less than 1.3 times its height. A solid foam characterized in that it comprises discrete units of a foam embedded in a foam matrix, wherein the density ofthe foam in the discrete units is higher than the density ofthe foam matrix, and wherein the solid foam comprises at least 75-95 wt% cellulose fibres as calculated on the total weight of the foam. A solid foam according to claim 11, wherein the height of the discrete units is from 90 to 100 % of the height of the foam matrix. A solid foam according to claim 11 or 12, wherein the solid foam has a density of 10 - 80 kg/m A solid foam according to any one of claims 11-13, wherein each discrete unit is surrounded by a densified layer of the foam. A solid foam according to any one of claims 11-14, wherein each discrete unit is in the form of a cylinder. A solid foam according to any one of claims 11-15, wherein the density ofthe foam in the discrete units is at least 110 % of the density of the foam matrix. A solid foam according to any one of claims 11-16, wherein the width of the discrete units is less than 1.3 times its height. A solid foam prepared by the method according to any one of claims 1- Use of a solid foam according to any one of claims 9-17 in packaging or large constructions, or as a hydroponic plant growing media.