SE2151607A1 - Preparation of a foam comprising discrete units of foam embedded in a foam matrix - Google Patents

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. The invention further relates to a solid foam comprising discrete units of foam embedded in a foam matrix.

Description

PREPARATION OF A FOAM COMPRISING DISCRETE UNITS OF FOAM EMBEDDED IN A FOAM MATRIX 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.

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 ofthe 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WO2015036659 A1 a wet fibrous foam is fed into a mould, where part of the water contained in the foam is mechanically withdrawn to produce a solidified, moist fibrous composition, and evaporating water to produce a dry fibrous product.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, and there is still need for alternative methods for foam preparation.

SUMMARY OF THE INVENTION lt is an object ofthe present invention to provide a method suitable for the preparation of a foam where the shrinkage ofthe foam is reduced. The method enables a structural control ofthick 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 ofthe same foam composition as the discrete units. 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 foam can be recyclable in regular paper recycling streams.

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, 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.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 A) shows a schematic representation of a prior art foam plank with densification on top and bottom and B) shows a schematic representation of the inner bulk in such foam plank, which comprises a homogeneous fibre network with a lower density through the whole bulk.

Figure 2 is a schematic representation of cross-section of a comparative foam after drying showing influence of aspect ratio of width to height on shrinkage.

Figure 3 a) shows a graph obtained from drying of a single large block of a comparative cellulose foam, displaying the dry thickness of the densified layer, i.e. crust, (-) on the left Y-axis, and the shrinkage profile (O) on the right y-axis, as a function of % conversion of liquid water to steam during drying, and b) illustrates the cross section of two fibres (white circles) in a wet environment (grey area) and during drying. Steps A), B) and C) in Figure 3b) corresponds to the situation in zone A), B) and C) in figure 3a).

Figure 4 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 5 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 of the material beneath the densified top and bottom, the bulk containing discrete units of cellulose foam (black cuboids), where each discrete unit is distinguished from the surrounding foam matrix by a densified layer of cellulose, and C) illustrates one discrete unit surrounded by a densified layer of cellulose (left), illustrated in black, and the homogenous cellulose foam inside the densified layers (right).

Figure 6 illustrates differences in tension development (higher tension in darker area) during drying of (a) single block and (b) multistep deposition.

Figure 7 shows an example of compression curves showing single-step deposition (lower dashed curve), as prepared according to Example 2, and multistep deposition (upper solid curve), as prepared according to Example 1, for the same density foam (30 kg/m3).

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.

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 ofthe 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 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. 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 ofthe discrete units ofthe wet foam is at least made until a crust, i.e. a thin densified layer of cellulose fibres, is formed on an outer surface of the discrete unit, such as on each of the faces of the discrete unit. The densified layer is a very thin layer that is formed on the very outer surface 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 of the 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 ofthe 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 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. The density of the wet foam used in the further deposition may be the same for the wet foam in the first deposition, or it may be different.

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 10 - 60 kg/m3, or from 20 - 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 of the 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. 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. The diameter, or the average ofthe length and 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, orfrom 0.8 to 2, or from 0.8 to 1.5, orfrom 1 to 3, or from 1 to 2, or from 1 to 1.5 times its height.

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 ofthe 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 of the 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, chemical-thermomechanicaI 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 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. The solid foam prepared with the method according to the present invention may have a density of 10 - 60 kg/m3, or from 20 - 50 kg/m3. ln a wet foam, resistance forces keep the cellulose fibres in place (illustrated in Figure 3b, A). 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 (Figure 3b, B). 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 (Figure 3b, C). On a macroscopic level, the geometry ofthe 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 which is illustrated for a comparative foam without discrete units in Figure 2. Thus, the ratio of the width, or surface area, to the height of cellulose foam planks affects the distribution ofthe 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 4 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). When the wet foam of the subsequent deposition is being dried (v) the discrete units ofthe 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 of the tension during drying and as a result mitigates the effect of shrinkage on the outer dimensions ofthe 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 ofthe 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 foamed planks that are at least 60 * 60 cm, or at least 100 * 100 cm, or at least 200 * 200 cm, or at least 300 * 300 cm. With the multi-step deposition method according to the present invention foamed objects can be produced with reduced shrinkage compared to similar foamed objects obtained with the single-step deposition. The present method also allows for a process with continuous formation of a foamed object, such as continuously foamed web.

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 80 % of the total volume, 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 % ofthe total volume, or from 50 to 78 % ofthe total volume, or from 40 to 75 % of the total volume, or from 50 to 75% ofthe total volume ofthe foamed material comprising the discrete units and the surrounding foam matrix.

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 ofthe 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 foam.

The performance of the dry foam can be tailored by the amount ofthickener used and hence the dry material fibre-fibre bonding strength. The amount ofthickener 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 of the 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.

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 ofthe 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 4). 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 of the plank (Figure 6).

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 foa ms 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. 11 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. 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 98 to 100 %, or equal to the height of the 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 ofthe final solid foam can be adjusted by using discrete units with the same or different densities as the foam 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 of the discrete units in the solid foam may be from 83 % to 500 %, or from 60 % to 150 %, or from 90 % to 110 % ofthe density of the foam matrix surrounding said discrete units. Preferably, the discrete units have a density that is from 90 % to 110 % of the density ofthe foam matrix surrounding said discrete units. 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, ofthe 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 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. 12 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.

EXAMPLES EXAMPLE 1 IVIuIti-step deposition For a first deposition of wet foam in discrete units, a uniform wet paste was prepared comprising 12 wt% cellulose pulp in water and a thickener. The paste was aerated with a surfactant mixture until a wet foam density of 222 kg/m3 was obtained.

Cylindric moulds with the dimensions of 5 cm in height and 6.6 cm in diameter were used to deposit 12 discrete units ofthe foam on a flat surface (oven tray with a frame with dimensions of 27*37*5 cm). The moulds were used only for depositing the foam in the desired shape and dimension and are removed before drying). The discrete units were dried in an ordinary convection oven at 120 °C for 1-2 hours.

For the second deposition, a new batch of wet foam was prepared following the above description for the wet foam used in the first deposition. The wet foam was filled in the space between the discrete units, leaving out no voids in the oven frame. The surface was scraped to remove any possible extra foam and levelling out the surface to the height ofthe frame. Finally, the foam was dried in the oven at 120 °C for 8 hours. The density of the dried discrete units as well as the density of the final foam object was "30-31 kg/m3. The final thickness of the foam object was "5 cm and no shrinkage was observed.

EXAM PLE 2 (Comparative example) Single-step deposition 13 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 wet foam density of 222 kg/m3 was obtained.

A mould of 27*37*5 cm was filled with the aerated wet foam and the surface was scraped to remove any possible extra foam and levelling out the surface to the height of the frame. Finally, the foam is dried in an ordinary convection oven at 120 °C for 8 hours.

The density ofthe dried foam object was "30 kg/m3 and a shrinkage of 20% was observed in the centre of the foam object. The shrinkage is calculated based on the difference ofthe thickness in the centre ofthe foam and near the edges of the foam.

EXAMPLE 3 Preparation of a foam obiect A uniform wet paste comprising 12 wt% cellulose pulp in water and a thickener was prepared. A surfactant mixture was added to the paste and the obtained mixture was extruded and simultaneously aerated to obtain an extruded wet foam with a density of 222 kg/m3. The extruded foam was dried to obtain a foam object. The foam object was cut into square pieces of 4*4 cm and a height of "5 cm. The square pieces were distributed throughout a 27*37*5 mould. The mould was filled with wet foam prepared according to the description in example 1. The surface was scraped afterwards to remove any possible extra foam and levelling out to the height of the frame. The foam object was then dried in an ordinary convection oven at 120 °C for 8 hours. The density of the dried cut discrete units as well as the density of the final foam was "30-31 kg/m3. The final thickness ofthe foam object was "5 cm and no shrinkage was observed. 14 EXPERIMENTAL METHODS Characterization The comparative foam prepared according to Example 2 presents a densification on the top, bottom and side faces ofthe sheet (as illustrated in Figure 1A) , while the core of the foam sheet is a homogeneous fibre network of lower density (Figure 1B).

The foam produced according to Example 1 also presents a densification on the top, bottom and side faces of the sheet (as illustrated in Figure 5A) while the bulk ofthe material contains discrete units of foam that are distinguished from the foam matrix by border walls of densified cellulose fibre material (Figure 5B).

Compression curves were obtained for the multistep deposition foam according to Example 1, and the single-step deposition foam prepared according to Example 2 (Figure 7). Dried solid foams were cut into 10 cm square test pieces with heights of 5 cm. Compression tests were performed with an lnstron 5969 universal testing machine in a conditioned room at 23 °C and 50 % relative humidity. The samples were conditioned at 23 °C and 50 % relative humidity for 48 hours prior being tested. A 500 N load cell, 15 cm in diameter, was used with a compression rate of 100 % of the original sample thickness per min. The final strain was chosen to 70 % ofthe original specimen height. The multistep deposition foam provides for an improved energy absorption as is demonstrated by the larger area under the solid curve compared with the area under the dashed curve obtained for the single-step deposition foam.

Claims (1)

1.Claims 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. The method according to c|aim 1, wherein the height of the discrete units is from 90 to 100 % of the height ofthe 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 of the foam, and has a density of 10 - 60 kg/m3. A solid foam characterized in that it comprises discrete units of a foam embedded in a 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 9, 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 9 or 10, wherein the solid foam has a density of- 60 kg/m A solid foam according to any one of claims 9-11, wherein each discrete unit is surrounded by a densified layer of the foam. A solid foam according to any one of claims 9-12, wherein each discrete unit is in the form of a cylinder. 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 9-14 in packaging or large constructions, or as a hydroponic plant growing media.