WO2025158038A1 - Wet-stable foam - Google Patents

Abstract

This disclosure provides a foam comprising at least one structuring component selected from lignin, cellulose fibres (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), polyelectrolyte, multivalent ions, and closed pores comprising gas, wherein at least some of the polyelectrolytes are present in non-covalent complexes with the multivalent ions, providing a network structure in the foam. There is also provided several manufacturing methods for manufacturing such foams.

Description

WET-STABLE FOAM

TECHNICAL FIELD

The present invention relates to a wet-stable foam, a method of manufacturing a wet-stable foam, and products and uses related to such foam.

BACKGROUND

Synthetic polymer foams are presently used for multiple purposes. Foamed materials are suitable for use when a structure is to be both relatively rigid and lightweight. They may also provide certain characteristics while using less material, by weight, compared to alternative materials. The porous structure of a foam may also provide good thermal and physically protective properties.

A foam comprising synthetic and/or fossil-based components may however pose problems in areas such as environmental impact, human health, and recycling. Efforts are being made to address these issues by exploring alternative sustainable materials and environmentally friendly manufacturing processes.

Bio-based materials, i.e., materials at least partly derived from biomass, such as plants, trees, or animals, may be suitable sources to use as the production and the use of such materials generally is more environmentally friendly. In this respect, cellulose and lignin have great potential as they are the first and the second most abundant renewable natural polymers on earth and possess suitable structural characteristics for use in bio-based materials.

One objective of the present invention is to obviate at least some of the problems in the prior art and provide a foam with improved characteristics as well as up-scalable processes for manufacturing such foams.

SUMMARY

It is an objective of the present invention to obviate at least some of the disadvantages in the prior art by providing a wet-stable bio-based foam and a method of manufacturing such a foam that is convenient, sustainable and environmentally friendly. According to a first aspect, the present disclosure provides a foam comprising i) at least one structuring component selected from the group consisting of lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), ii) at least one polyelectrolyte, and iii) multivalent ions with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte, wherein the multivalent ions are present in non-covalent complexes with the at least one polyelectrolyte, and wherein the at least one polyelectrolyte is present in a polymeric gel structure, wherein the concentration of the polyelectrolyte in the foam is at least 1 wt%, based on dry weight of the polyelectrolyte and the dry weight of the foam, wherein the poly electrolyte has a charge density of at least 1 charge per 3 Bjerrum -lengths, and wherein the polyelectrolyte has a molecular weight (Mv) of 500.000 to 15.000 000 g/mol.

The material is renewable and bio-degradable and thereby solves several of the problems of the prior art. Wet-stability, shape-recovery and buoyancy of the product provides a broad variety of application areas.

The polyelectrolyte may be alginate. The multivalent ions may be multivalent metal ions such as Ca²⁺.

According to a second aspect, there is provided a method of manufacturing a foam, comprising the steps of i) mixing water, at least one polyelectrolyte (403), at least one foaming agent, and at least one structuring component (401) selected from lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), thereby obtaining a pre-foam mixture, wherein the concentration of the polyelectrolyte is at least 1 wt% of the dry weight of the prefoam mixture, the poly electrolyte has a charge of at least 1 charge per 3 Bjerrum -lengths, the poly electrolyte has a molecular weight (Mv) of 500 000 to 15 000 000 g/mol, ii) introducing gas into the pre-foam mixture (200), thereby obtaining a wet foam (201), and iii) subjecting the at least one poly electrolyte (403) to multivalent ions (404) with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte (403) so that a non-covalent complex form which stabilize the structure of the foam (100).

Different ways of providing the multivalent ions are disclosed as embodiments one to three below.

The method is simple to apply and does not involve unnecessary heat treatments, chemical modifications or organic solvents.

According to a third aspect, there is provided a product comprising the foam, wherein the product is selected from the group consisting of a smolt guide, a fish guide, a pool noodle, a wet-resilient packaging material, an insulation material, an isolation material, a floating island for vegetation, a filler, a packaging material, shock absorber, a medical device or orthopaedic product, a building material, sports equipment, and gardening product.

It is an advantage of the invention that wet stable foams can be obtained with environmentally friendly ingredients.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of which the invention is capable will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which:

Fig. 1 illustrates the schematic dimensional relationship between cellulose fibres (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF),

Fig. 2 illustrates a step of introducing air into a pre-foam mixture by stirring, and a resulting volume increase of an obtained wet foam,

Fig. 3 illustrates a step of subjecting a wet foam to a solution comprising multivalent ions and subsequent drying the resulting crosslinked foam to obtain a dry wet-stable foam,

Fig. 4 shows a cross-section of a foam, where three closed pores are seen, surrounded by thin walls where structural components are intertwined with polyelectrolytes, and said polyelectrolytes are linked together by non-covalent crosslinks comprising multivalent ions. DETAILED DESCRIPTION

Before the invention is disclosed and described in detail, it is to be understood that this invention is not limited to particular compositions, process steps and materials disclosed herein as such compositions, process steps and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof. A person skilled in the art would recognize numerous variations to the described embodiments that would still fall within the scope of the appended claims.

If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those being skilled in the art to which this invention pertains.

It must be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

As used herein, the terms “comprise/comprises” do not exclude the presence of other elements or steps. Furthermore, although individual features may be included in different claims (or embodiments), these may possibly advantageously be combined, if not incompatible.

A foaming agent is a component such as a surfactant or a blowing agent that facilitates the formation of foam. Surfactants, short for surface-active agents, are compounds that lower the surface tension between two substances, typically a liquid and a solid or another liquid. Surfactants may contribute to creating bubbles in liquids by reducing the resistance to deformation of the liquid surface, allowing the formation and stabilization of bubbles. These versatile compounds are widely used in various applications, including in detergents, emulsifiers, and foaming agents.

Cellulose fibres (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF) are fibrous raw materials derived from cellulose, a complex polysaccharide naturally found e.g. in plants. Their dimensional relationship is illustrated by Fig. 1. The definitions for these different forms vary in the literature. For the purpose of the invention, the following definitions are made. Cellulose-rich fibres, herein referred to as cellulose fibres, are long and thread-like structures composed of structurally organized cellulose macromolecules, which are the primary structural components of the cell walls in plants. These cellulose-rich fibres provide strength and rigidity to plant cells and are commonly used in the production of paper, textiles, and various other materials. Generally, cellulose fibres in plants range from less than a millimeter to several centimeters in length, while not being limited to this range. Cellulose fibers may be 0.1-10 mm long, such as 0.3-10 mm, and 2.5-500 pm in diameter, such as 10-80 pm.

Cellulose microfibrils (also referred to as microfibrillated cellulose, MFC) are smaller, fine- scale structures that make up cellulose fibres. They are typically in the sub -micrometer range in terms of width and ranges from nanometers to millimetres in length. Cellulose microfibrils are produced through breaking down cellulose fibres through mechanical or chemical disintegration or a combination of both, involving separating the cellulose into micro-sized fibrils. Cellulose microfibrils may be 1-500 pm long, such as 10-50 pm, and 1-100 nm in diameter, such as 20-50 nm.

Cellulose nanofibrils (CNFs) (also referred to as nanofibrillar cellulose, cellulose nanofibre, NFC) are even smaller structures derived from cellulose microfibrils through breaking cellulose fibres into nanoscale dimensions, typically to within the range of from less than one nanometer to hundreds of nanometers in width, and a length that may typically be around hundred nanometers to several micrometers. Due to their nanoscale dimensions and the organisation of the cellulose molecules in the fibrils, cellulose nanofibrils exhibit unique properties such as high surface area, high aspect ratio, and exceptional mechanical strength. Cellulose nanofibrils may be 100-2000 nm long, such as 400-1500 nm, and 0.3-6 nm in diameter, such as 0.4-2 nm.

Dry weight is the weight of the components for instance in the foam, calculated without water. If another solvent than water is used, then it dry weight refers to the weight without solvent.

Lignin is a complex amorphous, non-water soluble, organic polymer found in the cell walls of plants, providing structural support and rigidity to wood tissue. It is a matrix polymer in the wood holding cellulose and hemicellulose together and can be said to be responsible for the woody and fibrous characteristics of plant materials, such as wood, and is the second most abundant component in common plant cell walls after cellulose. Since native lignin is not water soluble at pHs below approximately 11, lignin will precipitate into small nanodimensional entities that will further associate into larger colloids of larger size as the pH is decreased below 10. Such lignin self-association can be controlled to create nanoparticles in a size-range from around 10 nm to several hundred nms. Both the precipitated lignin or different types of lignin nanoparticles can be used to create stable foams with the methodologies described in the present invention.

Carbohydrates make up a broad category of organic compounds encompassing a variety of molecules containing carbon, hydrogen, and oxygen, often but not always following the general formula C_m(H2O)_n. Carbohydrates include simple sugars (monosaccharides) as well as more complex structures such as disaccharides, oligosaccharides, and polysaccharides. Polysaccharides are large carbohydrate molecules created from multiple monosaccharide units joined by glycosidic bonds. Examples include starch, cellulose, and glycogen. Not all carbohydrates are water-soluble; for instance, cellulose is insoluble in water due to its unique suprastructural arrangement.

Polyelectrolytes are macromolecules consisting of repeating units that have ionic groups (charged groups), either permanently charged (strong polyelectrolytes) or with a charge that is pH dependent (weak polyelecytrolytes).

Proteins are complex macromolecules consisting of amino acid chains. They are ampholytic in nature, meaning that they can carry both a net cationic or a net anionic charge in aqueous media depending on the pH.

Certain carbohydrates and proteins, when they possess ionizable groups along their chains, behave as polyelectrolytes in solution due to their charged nature. Whether a protein behaves as a polyelectrolyte depends on its amino acid composition and the pH of the solution.

Closed pores are enclosed voids that generally contain an entrapped fluid, usually a gas, which was present during the forming of the pores. Closed pores in a foam result in a floating ability of the foam in a liquid, such as water, since the total density of the foam is lower than that of water due to the entrapped gas. Closed pores may be denoted as closed cell structures, closed voids, or bubbles. Air comprises a mixture of gases, and is in this context considered a gas.

As used herein, a foam is a material comprising closed pores. A foam may be in liquid (wet) or solid (dry) form. When a foam is dried, its liquid content decreases and it becomes more solid. The size and quantity of pores may differ, affecting the total density of the foam. The total volume of gas may be greater than that of the material of the foam. The foam may comprise thin walls surrounding pores. In other cases, there are smaller and/or fewer pores, surrounded by thicker areas of material.

Wet resilience, wet integrity, and wet stability refers to a material withstanding disintegration in aqueous media. A method for measuring wet stability is disclosed herein.

Good shape-recovery of a material means that the material maintains its overall structure well upon mechanical stress and that the material is able to recover parts of its original shape once the mechanical stress is removed.

Non-covalent interactions are a broad category of interactions that hold molecules together based on, for example, electrostatically driven interactions without the formation of covalent bonds. Examples include ionic interactions, and non-ionic interactions such as hydrogen bonds, van der Waals interactions and dipole-dipole interactions. These interactions are generally weaker than covalent bonds but play a crucial role in determining the structure, stability, and properties of molecules and molecular assemblies and self-assembly of colloidal particles. Non-covalent crosslinks refer to the linking or bridging of different molecules or between different parts of a molecule based on non-covalent interactions without the involvement of covalent bonds. These interactions can be very strong given that the molecular structure of interacting molecules show excellent matching on a molecular scale.

Polymer gelation (gel transition) is the formation of a gel from a system with polymers in a dissolved or well-dispersed state. Polymeric gel structures are created through the introduction of cross-links between polymer chains, which lead to progressively larger molecular assemblies. As the interaction continues, larger branched structures are obtained and at a certain extent of the reaction, the links between the polymer result in the formation of a macroscopic continues phase. At that point in the association, which is defined as gel point, the system loses fluidity and there is a rapid and sometimes dramatic increase in the viscosity of the system. Crosslinks are connections between different or similar polymer chains that provide mechanical strength and stability to the gel structure. There are in general two main types of crosslinks that may be found in polymer gels: physical and covalent. Physical crosslinks are reversible interactions between polymer chains that are non-covalent in nature. These interactions allow the polymer chains to form a network, yet to maintain some degree of flexibility. Covalent bonds between polymer chains are typically stronger and more permanent than physical crosslinks.

Gelation may be promoted by so-called gelling agents. Common methods for introducing chemical crosslinks include the use of covalent crosslinking agents, which can react with functional groups on the polymer chains to form covalent bonds. Another option is to use a gelling agent capable of creating non-covalent interactions with a polymer, forming a non- covalent complex. The formation of these complexes affects the polymer in a manner such that further non-covalent crosslinks are formed within the same polymer and between different polymers. The utilization of physical crosslinks is attractive since there is a rich toolbox for the formation of these interactions and these types of interactions generally do not need harsh reaction conditions.

Multivalent ions function well as gelling agents for polymer gelation of polyelectrolytes, when the ions have an opposite sign of charge compared to the net charge of the polyelectrolyte.

According to a first aspect, there is provided a foam 100 comprising i) at least one structuring component (401) selected from the group consisting of lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), ii) at least one polyelectrolyte (403), and iii) multivalent ions (404) with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte, wherein the multivalent ions (404) are present in non-covalent complexes with the at least one polyelectrolyte (403), and wherein the at least one polyelectrolyte (403) is present in a polymeric gel structure, wherein the concentration of the polyelectrolyte (403) in the foam (100) is at least 1 wt%, based on dry weight of the polyelectrolyte and dry weight of the foam, wherein the poly electrolyte (403) has a charge density of at least 1 charge per 3 Bjerrum- lengths, and wherein the poly electrolyte (403) has a molecular weight (Mv) of 500 000 to 15 000 000 g/mol. The foam 100 can be provided through a simple, inexpensive and efficient procedure where lignin particles, CF, CMF and/or CNF is mixed with polyelectrolytes 403 into a liquid prefoam mixture, pores 402 are formed in the mixture in a foaming process, and polymers 403 are non-covalently complexed with multivalent ions 404, leading to polymer gel formation.

Other possible processes for manufacturing of wet-stable bio-based foams tend to be tedious, complicated, and expensive. They may, for example, involve chemically modified nanocellulose, a raw material that is complicated to scale up and commercialize due to the use of organic solvents and expensive chemicals needed for modification, as well as time- and energy-consuming manufacturing procedures.

The multivalent ions 404 forms complexes with polymers based on non-covalent interactions. When these interactions are formed during manufacturing, polymer gelation is induced, wherein the polyelectrolytes 403 create a gel structure based on non-covalent crosslinks. Non-covalent crosslinks may be formed between polymer chains within the same polymer and between different polymers.

In general, single covalent bonds are more stable than single non-covalent bonds. It has, however, surprisingly been found that a foam 100 according to the first aspect shows desirable characteristics such as maintained wet-stability, low density, and high buoyancy in water or similar liquids. The non-covalent complexes create a wet-resilient gel network in the foam 100 that maintains the structures of the pores 402, at least to a large extent, even when the foam 100 is subjected to prolonged liquid exposure or to physical stress. The material demonstrates a good shape-recovery in that it is not easily deformable and if deformed it mainly regains its original structure when stress is released. No covalent crosslinking agent needs to be present in the manufacturing of the foam 100. Covalent crosslinking generally requires more costly and complex procedures involving chemical modification, heat treatment and non-aqueous solvents, which are not needed in the manufacturing of the inventive foam 100.

The foam 100 is highly porous. The foam comprises closed pores that contribute to the floatability of the foam, as discussed above. Aside from the closed pores 402, there may also be some open pores present in the foam. A suitable way to create the closed pores is by introducing gas into a pre-foam mixture comprising structuring component and poly electrolytes as well as a foaming agent, such a surfactant, which facilitates the formation of bubbles (closed pores). The final foam may therefor comprise surfactant. The foam 100 may for example comprise 0.2-10 wt%, such as 0.3-5 wt% of surfactant, which is a foaming agent. The foaming process and additional steps of a manufacturing method for producing the foam is further described below.

The foam 100 maintains its floating ability even after 24 h in water under mechanical stress, e.g., shaking or stirring. Excellent wet stability of the material and the fact that the foam maintains its buoyancy over extended time indicates the existence of closed voids that enclose gas over extended time. The density of water is 1000 kg/m³. Since the density of cellulose and lignin is higher, the material would sink over time without the presence of closed voids with entrapped gas.

It has also been shown that the non-covalently crosslinked foam 100 has such high initial wet stability that it can withstand oven drying at temperatures around 60 °C, and also much higher, such as 100 °C, without collapsing or being subjected to Ostwald ripening. This is important for the up-scalability of the manufacturing procedure.

The foam 100 also has excellent potential for upscaling due to the potential simple manufacturing procedure and the use of commercially available bio-derived raw materials. The raw materials of the foam 100 are much more sustainable and environmentally friendly than their fossil-based counterparts. The polyelectrolytes may be bio-based polymers (polyelectrolytes), which are polymers derived from renewable biological resources, such as plants, crops, or microorganisms. Examples are carbohydrates and proteins. One example of a suitable carbohydrate is alginate. Bio-based polymers offer an environmentally friendly alternative to traditional petroleum-based polymers, significantly contributing to improved sustainability of the materials by reducing dependence on fossil based raw materials in the production of different plastic and polymeric materials.

Polyelectrolytes 403 with a high charge density possess a strong interaction potential with certain multivalent ions. The polyelectrolytes may contain ionizable groups, such as carboxylic acids, sulfonic acids, amino groups, or ammonium groups, along their molecular chains. The presence of these ionizable groups imparts a significant number of charged sites along the polymer structure. The polyelectrolytes may be cationic or anionic and the multivalent ions acting as gelling agents may be counterions with an opposite charge to these charges on the polyelectrolytes. The polyelectrolyte has a molecular weight Mv of 500 000 to 15 000 000 g/mol. Molecular weight as used herein refers to Viscosity Average Molecular Weight (Mv). For the polyelectrolyte this is measured by measuring the viscosity. The viscosity is related to the molecular weight and the molecular weight is calculated from the viscosity.

For the invention is has been unexpectedly found that when using a fairly high molecular weight, such as above Mv 500 000 g/mol, a good wet stability can be obtained even for moderate additions of a polyelectrolyte such as alginate. A high molecular weight above 500 000 g/mol gives a good structural integrity since the high molecular weight polyelectrolyte contributes to keeping the structure together. The use of a high molecular weight polyelectrolyte also has the following effects: a) It induces a high viscosity in the liquid film formed between the bubbles in the foam to prevent a too rapid liquid drainage when the foam is dewatered, b) Due to overlap between the polyelectrolyte chains, the use of the polyelectrolyte induces a physical crosslinking, i.e. gelling, between the polyelectrolyte chains and multivalent counterions to the charges which is a prerequisite for the wet stability of the dried foams. Thus the molecular weight Mv should be higher than 500 000 g/mol.

At the same time a very high molecular weight Mv above 15 000 000 g/mol may still work, but it gives practical difficulties since such high molecular weight polyelectrolytes will have very high viscosity making it more difficult to handle. Further, such extremely high molecular weight polyelectrolytes may be difficult and/or time consuming to dissolve. Thus a molecular weight Mv above 15 000 000 g/mol is generally not practically suitable.

In view of this the lower limit for the viscosity average molecular weight (Mv) can be 500 000, 600 000, 700 000, 800 000, 900 000, 1 000 000, 1 100 000, 1 200 000, 1 300 000, 1 400 000, 1 500 000, 1 600 000, 1 700 000, 1 800 000, 1 900 000, 2 000 000 g/mol. The upper limit can be 8 000 000, 8.500 000, 9 000 000, 9.500 000, 10 000 000, 11 000 000, 12 000 000, 13 000 000, 14 000 000 and 15 000 000 g/mol. All lower limits are freely combinable with all upper limits to arrive at all possible intervals including but not limited to 500 000 - 15 000 000, 600 000 - 10 000 000, 700 000 - 10 000 000, 800 000 - 9 000 000, 700 000 - 8 000 000, 900 000 - 8 000 000, 2 000 000 - 10 000 000, 1 000 000 - 8 000 000, 1.500 000 - 8 000 000 g/mol. Molecular weight and viscosity are related since a larger molecular weight corresponds to a larger excluded volume of the solvent which in turn creates a larger resistance to flow. In liquids, a higher molecular weight leads to higher viscosity at a certain concentration by weight and for polyelectrolytes, which have an extended conformation, due to the charges within the polymer, this effect is even larger.

The viscosity is measured at a solution of 1 wt%, at a temperature of 21 °C using a rotational type viscometer at 30 rpm. For measurements of the viscosity of alginate, the solution is prepared by dissolving the alginate in water for 30 minutes and then allowing the alginate solution to equilibrate for at least 30 minutes. For high molecular weight poly electrolytes, the viscosity is highly dependent on how long the solution has been equilibrated. It is thus important to adhere to this method. This method for the viscositiy measurement is used throughout the examples and the description.

In one embodiment, the foam looses maximum 50 wt% of its dry weight during immersion in deionized water for 6 days on a shaking table set to 115 rpm, and wherein a cylinder with 5 cm diameter of the foam is able to withstand a load of more than 0.5 kg without cracking.

The poly electrolyte has a charge density of at least 1 charge per 3 Bjerrum-lengths. The polyelectrolyte may have a higher charge density, such as for instance at least 1 charge per 2 Bjerrum-lengths, at least 1 charge per 1 Bjerrum length, at least 1 charge per 0.8 Bjerrum lengths and at least 1 charge per 0.5 Bjerrum-lengths. In one embodiment the polyelectrolyte has at least 1 charge per 1 Bjerrum length. A high charge density gives a good strength and wet stability of the foam.

Charged polyelectrolytes have a very extended conformation in aqueous media which means that they occupy a large volume even at very low concentrations provided the charges are dissociated and that the salt concentration is not too high. This property of the polyelectrolytes is usually defined as an overlap concentration which is determined by the number of monomers in the chain, the charge of the monomers and the ionic strength of the medium. The absolute values of the overlap concentration are complex to calculate where the chemical properties of the polyelectrolyte are taken into consideration but the overlap concentration is very low for polyelectrolytes at low to medium ionic strengths and suitable ranges may be between 1 g/1 and 40 g/1 (considered a very high value). This means that highly charged polyelectrolytes with somewhat lower molecular weight still have a high viscosity which is helpful for the first purpose mentioned above.

For the polyelectrolytes to form strong complexes with each other they must have a significant overlap, which is suitably combined with a lower level of concentration. This in order to fulfil purpose b) above. To create a strong complex, the charge density of the poly electrolyte should be close to the Bjerrum length or a few Bjerrum lengths as defined from equation 1. wherein e = electronic charge so = Permittivity of vacuum

Sr = Relative permittivity of the solvent used k = Boltzmann’s constant

T = Absolute temperature

In water at 20 °C this length is 7 A. For polyelectrolytes this distance can be calculated from the chemical structure of the polyelectrolytes. Experimentally the charge can be safely determined from either potentiometric titration, conductometric titration or polyelectrolyte titration.

The lowest level of the viscosity of the polyelectrolyte to be used is naturally linked to the charge and molecular weight of the polyelectrolyte. When determining the molecular weight of the polyelectrolyte from viscosity measurements the influence of the charges should be minimized. This is done by performing the measurements at high ionic strengths and relating the intrinsic viscosity to the molecular weight by using the well-known Mark-Houwink equation:

[r|] = K Mv^a wherein [r|] = Intrinsic viscosity of the polyelectrolyte solution

Mv= The average molecular weight (viscosity-average M_v)

K and a = experimentally determined constants.

The number average molecular mass Mn of a polymer can also be determined by viscometry via the Mark-Houwink equation.

For alginates it has been found that this relationship can be used with the constants K=2- 10'⁵ and a=l using 0.1 M NaCl as a solvent at 20 °C and where [q] is expressed in a 100 ml/g scale. This means that a determination of the intrinsic viscosity can be used to evaluate the molecular weight of a polyelectrolyte.

When using alginate, the detailed composition of the alginate with different combinations of mannuronic (M) acid residues and guluronic (G) acid residues is important for the binding and complexing of multivalent ions. The fraction of G-residues may suitably be at least 0.2, such as at least 0.3, at least 0.4 or at least 0.5. Examples of M:G ratio are 57:43 and 45:55. In one embodiment, the M:G ratio is in the interval 80:20 - 20:80. In another embodiment, the M:G ratio is in the interval 70:30 - 30:70. The M:G ratio is believed to contribute to the wet stability. Furthermore, there may be a limiting DP (Degree of polymerization) of the alginate for achieving a good complexing with the multivalent ions.

The polyelectrolytes 403 may be of one type or of more than one type of polyelectrolyte. The herein mentioned examples of polyelectrolytes may be present in the foam alone or in combination. Other polyelectrolytes with suitable properties in light of this disclosure may also be considered.

Non-covalent crosslinks can for example be formed between carboxylic acid groups present in carbohydrates. Some water-soluble carbohydrates show a strong interaction with and a macroscopic complexing ability with multivalent ions, resulting in the creation of waterinsoluble complexes. Water-solubility of the polyelectrolytes facilitates the manufacturing of the foam 100.

The polyelectrolyte may be polysaccharides. Examples of suitable carbohydrates are alginate, chitosan, carrageenan, pectin, gellan gum, xanthan gum, glucan, agarose and agaropectin. It may also be any combination of these. A preferred example is alginate. Alginate forms strong water-insoluble complexes with multivalent ions such as multivalent metal ions and specifically calcium ions (Ca²⁺). The alginate may for example be sodium alginate. Proteins can be added to the mixture in form of gelatine, a material that consists mainly of proteins.

Carbohydrates as well as proteins are in general abundant, renewable and biodegradable elements, making them a sustainable source for and component in the material, compared to synthetic materials that could be used for similar applications.

The multivalent ions 404 form strong non-covalent complexes with polymers such as carbohydrates or proteins and especially polyelectrolytes. The multivalent ions may be multivalent metal ions. The multivalent ions may be divalent ions such as Ca²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Sr²⁺, CO²⁺, Zn²⁺, or trivalent ions, such as Al³⁺, Fe³⁺, Nd³⁺, or multinuclear species of these latter ions. The ions may preferably be Ca²⁺ or Fe³⁺. A high valency provides the opportunity of directly forming non-covalent crosslinks between parts within a polymer or between different polymers, strengthening the network structure. For cationic polyelectrolytes typical multivalent ions could be sulphate²' or phosphate³' ions, such as SO4² and PO4³ '.

The foam 100 may for example comprise non-covalent calcium alginate complexes, created by the polyelectrolyte sodium alginate and the multivalent metal ion Ca²⁺. Such complexes provide a strong gel network.

The foam 100 may further comprise solid calcium carbonate (CaCCh) that might contribute to the stability of the foam. If the crosslinking step of the manufacturing is not performed directly after the mixing and foaming of the foam, phase separation may occur and pores coalesce. CaCCh may be added as it prevents (delays) phase separation and stabilizes the pores, until the structure is fixed in the gelation step. It may also contribute somewhat to complexing due to Ca²⁺ ions present on the surface of the solid material. The dry foam may for example comprise 0.01 - 25 wt%, 0.1 - 50 wt%, 1-5 wt% or around 0.5 wt% of CaCCh. The concentration may depend on the manufacturing method performed. If an acidic treatment has been performed, the level of remaining CaCCh may be lower than if a ion solution or ester hydrolysis has been used.

The foam 100 may additionally or alternatively comprise a plasticizer such as glycerol, xylitol, sorbitol, erythritol, mannitol, maltitol, triacetin, or triethyl citrate. A plastiziser may also be added during manufacturing to prevent phase separation, and further acts as a plasticizer, providing a softer final product. The dry foam may comprise 0.01 - 5, such as 0.1 - 2, such as around 1 wt% of plasticizer. It is also possible that the dry foam comprises a higher concentration of plasticizer, such as up to 50. 70 or 80 wt% of the weight of the dry foam. A high concentration of plasticizer results in a foam with characteristics that may be desirable such as softness.

The structuring component may be any combination of the types mentioned in step i). It may be only lignin particles, CF, CMF or CNF, or any combination thereof. Lignin and cellulose fibres (from which CMF and CNF may be derived) are renewable and biodegradable. They are non-expensive and abundant organic substances, making them excellent raw material sources as a main component. The structuring component provides structure to the foam, a structure that is maintained by aid of the gel complexes. The complexes and the structuring components in the foam are at least partly overlapping or intertwined (as illustrated by Figure 4).

Micro- and nanofibrils have higher aspect ratios than cellulosic fibres, which leads to increased volumetric overlapping of the micro or nanofibrils at a certain weight concentration. For this reason, the total concentration of cellulosic material in the foam may be lower when it comprises a higher content of CMF and/or CNF compared to CF.

The following exemplary concentrations are based on the dry weight of the foam. The dry weight excludes water that may still be present in this so-called dry foam. It may also be possible to use concentrations outside of these ranges. It is to be understood that the ranges are to be combined such that the total concentration of these components amount to 100 wt%.

The dry foam 100 may comprise 1-99.5 wt%, such as 10-99.5 wt%, such as 50-99.5 wt%, such as 50-99 wt%, such as 40-97.5 wt%, such as around 30, 40, 50, 60, 70, 75, 80, 85 or 90 wt%, of structuring component, as calculated with respect to the dry weight of the foam. By around is meant ±10% so that around 50 wt% is 45-55 wt%. In cases where the glycerol content is high, the concentration of structuring component may be lower, such as below 50 wt%, below 40 wt% or below 30 wt%, such as 20 wt% or 10 wt%.

The structuring component 401 may comprise lignin, such as precipitated lignin or lignin nanoparticles, cellulose fibres (CF), cellulose microfibrils (CMF), or cellulose nanofibrils (CNF), or any combination thereof. The structuring component may comprise 0-100 wt% of lignin, such as 10-90, 20-80, 30-70, 40-60 or around 50 wt%, as calculated with respect to the dry weight of the foam. In one example, lignin constitutes 70-100 wt% of the structuring component. The structuring component may comprise 0-100 wt% of CF, such as 10-90, 20- 80, 30-70, 40-60 or around 50 wt%, as calculated with respect to the dry weight of the foam. In one example, CF constitutes 70-100 wt% of the structuring component. The structuring component may comprise 0-100 wt% of CMF, such as 10-90, 20-80, 30-70, 40-60 or around 50 wt%, as calculated with respect to the dry weight of the foam. In one example, CMF constitutes 70-100 wt% of the structuring component. The structuring component may comprise 0-100 wt% of CNF, such as 10-90, 20-80, 30-70, 40-60 or around 50 wt%, as calculated with respect to the dry weight of the foam. In one example, CNF constitutes 70- 100 wt% of the structuring component. In one example, 1-10 wt% of the structuring component is CNF and the remaining is CF. In another example, 1-50 wt% of the structuring component is CMF, and the remaining is CF.

The dry foam may comprise 1-50 wt% of poly electrolyte, as calculated with respect to the dry weight of the foam. It may for example comprise 2.5-40, such as 5-30 wt%, 5-25 wt%, or 5-15 % of polyelectrolyte. The concentration of the polyelectrolyte in the foam is at least 1 wt%, based on dry weight of the polyelectrolyte and dry weight of the foam. In one embodiment, the concentration of the polyelectrolyte in the foam is 1 wt% or more, based on dry weight of the polyelectrolyte and dry weight of the foam. In one embodiment, the concentration of the poly electrolyte in the foam is 2.5 wt% or more, based on dry weight of the polyelectrolyte and dry weight of the foam. In one embodiment, the concentration of the polyelectrolyte in the foam is 5 wt% or more, based on dry weight of the polyelectrolyte and dry weight of the foam. In one embodiment, the concentration of the polyelectrolyte in the foam is 10 wt% or more, based on dry weight of the poly electrolyte and dry weight of the foam. A higher percentage of polyelectrolyte, such as alginate, gives a better wet stability as evidenced by the examples. When measuring the amount of polyelectrolyte in practice it is important to consider that polyelectrolytes such as alginate often comprises about 14-15 wt% water although it looks dry. This amount of water should be considered when calculating the dry weight of polyelectrolyte.

The weight ratio of structuring component:polyelectrolyte in the foam may be from 1:1 - 40:1, for example 4: 1 or 9:1, with respect to the dry material of those components. For example, there may be a ratio of CMF: sodium alginate in the foam from 8:2 to 9: 1 or a ratio of CF : CMF : sodium alginate in the foam of 8 : 1 : 1.

The dry foam 100 may comprise from 0 0005 wt% to 40 wt% of multivalent ions, or from 0.5 wt% to 10 wt%, or from 1 wt% to 5 wt%, or from 1.5 wt% to 2.5 wt%, as calculated with respect to the dry weight of the foam. In one example, the dry foam 100 comprises 89 wt% of cellulosic material, 10 wt% of alginate, and 1 wt% of foaming agent. In another example, the dry foam 100 comprises 88 wt% of cellulosic material, 9 wt% of alginate, 1 wt% of foaming agent and 2 wt% of CaCCh. In another example, the dry foam 100 comprises 27 wt% of cellulosic material, 3 wt% of alginate, 0.5 wt% of foaming agent and 69.5 wt% of glycerol. In another example, the dry foam 100 comprises 91 wt% of lignin, 8 wt% of alginate, and 1 wt% of foaming agent. In all these examples, the foaming agent may be a surfactant. In all these examples, alginate can be exchanged for another suitable polyelectrolyte.

The foam may comprise esters, as these may be used in the manufacturing of the foam. The amount of esters in the dry foam will depend on the hydrolysis degree during the heating and drying step in the production of the dry foam. A suitable concentration of ester may be 0.1- 50% of the dry material of the foam. During the drying step, the esters will hydrolyse to their corresponding carboxylic acids and alcohols, at least partially dissolving the gelling compound and releasing multivalent ions causing the poly electrolyte to gel. The final amount of ester and their corresponding carboxylic acids and alcohols in the dry foam may be from 0.1% to 50%. The foam may further comprise enzymes such as lipases and esterases, as these may be used in the manufacturing of the foam. Examples and concentrations of esters and enzymes correspond to those disclosed in relation to of the method described below. For example, there may be a concentration of 0.5 mg active lipase/1 kg cellulose fiber to 10 g active lipase/1 kg cellulose fiber. This corresponds to a lipase concentration of 0 00005 wt%- 1 wt%. The enzymes may be both active and inactive in the dry foam due to enzyme deactivation during a drying step.

In one embodiment the foam (100) comprises 10-99 wt% of structuring component, 1-50 wt% of poly electrolyte, 0.2-10 wt% of foaming agent, 0-50 wt% of CaCCh, and 0-80 wt% of glycerol.

It should be noted that the sum of all components of the foam 100 add upp to 100 wt%. Thus it is clear that the entire intervals of all component cannot be combined, instead the amount of all components have to be adjusted with regard to the total content of all ingredients.

The dry foam 100 may have a density of less than 1000, 750, 500, 400, 300, 200 or 100 kg/m³. It may for example be around 10-100 kg/m³. The density includes the solid (and possible liquid) material of the foam as well as the closed pores. Introducing more air during manufacturing lowers density. The foam may additionally contain non-wood micro or nanoparticles. The number of closed pores can be increased by incorporating certain micro- and/or nanoparticles in the foam mixture during manufacturing with the ability to adsorb surfactants from the mixture of structuring component, polyelectrolytes and surfactants. These accumulate at the air/water interface together with the structuring component and will increase the number of closed pores by filling gaps between the structuring components in the foam lamellae. The added amount of these particles must naturally be tuned to the type of structuring components used and the floating power needed for the foams. The particles should have a width equal to or smaller than of the structuring components of the foam and have a high anisotropy, i.e. thickness to width, since the will give a more efficient filling of the gaps as they will orient along the lamellae. Potential particles are e.g. starch granules from different raw materials, clay particles and preferably montmorillonite clays with a known high anisotropy, and bentonite. The starch granules may be collected from different plants. They may be spherical in nature and have typical dimensions between 2 and 35 pm, depending on the starch origin. The montmorillonite clays are composed of anisotropic flakes with thin crossections from 1.5 nm thickness and upwards and an in-plane dimension around 1 pm and upwards.

The properties of the foam 100 mean that it can be used in many different areas where fossilbased polymer foams are used today. The final product may for example be a smolt (fish) guide in hydroelectric power plants. The product may be a floating product used for swim training of beginners, such as a so-called pool noodle, today often comprising fossil-based foams. The product may be a wet-resilient packaging material, insulation material, filler, floating island of vegetation that can host plants, insects, birds and fish at hydroelectric reservoirs, or packaging material such as shock absorbing material in different types of packaging.

There is thereby also provided a use of a product comprising the foam according to the disclosure in such circumstances, such as use as a guide for fish, pool noodle, wet-resilient packaging material, insulation material, isolation material, floating islands, packaging material, as shock absorber, in medical devices or orthopaedic products, in building material, sports equipment, in gardening.

According to a second aspect, there is provided a method of manufacturing a foam (100), comprising the steps of i) mixing water, at least one polyelectrolyte (403), at least one foaming agent, and at least one structuring component (401) selected from lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), thereby obtaining a pre-foam mixture (200), wherein the concentration of the polyelectrolyte (403) is at least 1 wt% of the dry weight of the pre-foam mixture (200), the poly electrolyte (403) has a charge of at least 1 charge per 3 Bjerrum-lengths, the poly electrolyte (403) has a molecular weight (Mv) of 500 000 to 15 000 000 g/mol, ii) introducing gas into the pre-foam mixture (200), thereby obtaining a wet foam (201), and iii) subjecting the at least one poly electrolyte (403) to multivalent ions (404) with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte (403) so that a non-covalent complex form which stabilize the structure of the foam (100).

Through this method, a foam such as the foam 100 described herein may be provided. It is a simple, inexpensive and efficient procedure to prepare low density, resilient, wet stable foams with a high buoyancy over a prolonged time in water. No covalent crosslinking agents need to be added in the manufacturing of the foam 100. No other chemical modification is needed to maintain the foam structure.

Characteristics of and examples of the components used in the method are described in relation to the foam 100 of the first aspect above. Some characteristics or examples may only be mentioned with regards to either the method or the foam, but it is understood that those apply for both, if not stated otherwise or clearly incompatible. For example, at least some of the polyelectrolytes of the method may be carbohydrates and/or proteins. The structuring component may comprise lignin, CF, CMF or CNF or any combination thereof. The lignin may be precipitated lignin or lignin nanoparticles. Possible combinations and concentrations of these correspond to the concentrations in the final foam 100 disclosed above.

The foaming agent may be a surfactant, which may be cationic, anionic, non-ionic, or any combination thereof. It may for example comprise sodium dodecyl sulfate (SDS), sodium lauryl ether sulfate (SLES), sodium oleate, and potassium oleate, polysorbate (Tween 20), polysorbate (Tween 80), triton X-100, sorbitan monolaurate and/or polyvinylalcohol (PVA), alkyl glucosides, decyl glucoside, dodecyl glucoside, sodium coco-sulfate, and/or sodium stearate. The surfactant may be provided in a commercially available mixture. It is preferably chosen to be environmentally friendly, bio-based, renewable and biodegradable. As the surfactant is usually a minor component it may also be chosen not to have all these properties. It may nevertheless be selected to be a completely non-toxic and bio-safe material. Foam boosters such as cocamidopropyl betaine (CAPB), lauryl ethanolamide, lauryldimethylamine oxide and/or lauryl isopropanolamide may additionally be added in step i).

The step ii) of introducing gas to the mixture 200 (the foaming step) may involve agitating the mixture, by for example stirring, as illustrated in Figure 2, whipping or beating the mixture in the presence of gas, such as the surrounding air. The gas may additionally/alternatively be introduced into the material by being blown/sprayed into the mixture. Such introduction, in the presence of a foaming agent, creates bubbles (closed pores) in the material and thereby foam, and may be referred to as a foaming procedure. The introduction may be performed mechanically. It may be performed for at least 5 seconds, such as for at least 30 seconds, such as around 1 min or 5 minutes or more. It may be performed until the volume of the components increases by at least 25%, such as at least 50% or 100% or such as from 100% to 1000% or more. The time needed for volume increase is amongst other factors dependent on the amount of foaming agent present. A high-volume increase may increase the risk of collapsing of the material at a drying stage. A volume increase of up to 200 % may be suitable in order for a large part of the closed pores to be maintained through the process. The extensive volume increase of the mixture of 200 occurs due to the entrapment of air (or another gas present in the surrounding environment) and is directly correlated to the foaming agents, their adsorption at the air/water interface, the amount of gas introduced and the association of the polyelectrolytes and structuring components at the air water interface. The foams obtained need to be dried to remove the water from the system, without collapsing the network. Hence, a very stable porous network that can withstand the capillary pressure-driven collapse is needed, which is provided by the non-covalent complexes.

The density and volume increase of the foam 201 compared to the pre-foam mixture 200 may for example be determined by determining the amount of water (with known density) used to fill a vessel (thereby determining the volume of the vessel), followed by determining the weight of pre-foam mixture 200 needed to fill the vessel (thereby determining its density) and then the weight of wet foam 201 needed to fill the vessel (thereby determining its density and the volume increase). If the amount of wet foam 201 needed to fill the vessel is half of the amount of pre-foam mixture 200 needed to fill the same vessel, the volume increase is 100%. The wet foam 201 may have a density of less than 600, 500, 400, 300 or 200 kg/m³. The density includes the solid (and wet) material and the content of the pores.

The polyelectrolytes provide for an excellent dispersion of the gelling compounds (for example CaCCh), lignin and/or cellulose components, both in the aqueous dispersion and also in the foam. When the multivalent ions are introduced to the material in step iii), the polyelectrolytes form complexes with multivalent ions wherever they are present, inducing crosslinking and gel formation of the polyelectrolytes. The result is a gelated network of polyelectrolytes that are at least to some extent intertwined with the structuring components. The polyelectrolytes and structuring components are thereby firmly associated in the final foam 100. As the complexes are water resistant, the structure is maintained in liquid.

The multivalent ions may be any of those disclosed in relation to the foam 100 disclosed herein, such as multivalent metal ions. For example, if the multivalent metal ion is Ca²⁺, it may be provided in a solution 302 comprising a calcium salt, such as CaCh, or calcium acetate.

In a first embodiment of the method, at least one polyelectrolyte (403) is contacted with multivalent ions (404) in step iii) by subjecting the pre-foam mixture (200) or the wet foam (201) to a solution comprising the multivalent ions (404).

In a second embodiment of the method, a gelling compound with the ability to react with an acid and thereby release multivalent ions (404) with opposite sign of charge compared to the net charge of the at least one polyelectrolyte (403) is added in step i), and the at least one poly electrolyte (403) is contacted with the multivalent ions (404) in step iii) by subjecting the pre-foam mixture (200) or the wet foam (201) to acidic conditions, and thereby release the multivalent ions (404).

In a third embodiment of the method, a gelling compound with the ability to react with an acid and thereby release multivalent ions (404) with opposite sign of charge compared to the net charge of the at least one polyelectrolyte (403) is added in step i), and the at least one polyelectrolyte (403) is contacted with the multivalent ions (404) in step iii) by the ester undergoing hydrolysis, such that an acid is formed, which acid reacts with the gelling compound which thereby releases the multivalent ions (404). While step ii) is always performed after step i), step iii) may be performed at any time of the process, although it is often suitable to perform it after step ii).

When step iii) is performed after step ii) of foaming, the emerged porous foam structure is locked in place during gelation and the pores are kept intact. No premature gelation of polyelectrolytes is present to hinder foaming. The complexing reaction, in general, causes the material to solidify in a way that makes foaming difficult after the addition of multivalent ions and that is more suitable after the pore structure is in place. Alternatively, or additionally, step iii) may be performed at least partly simultaneously as step ii).

In the first embodiment of the method, the step iii) may be performed by immersing the wet foam 201 in a solution 302 comprising multivalent ions. It is additionally or alternatively possible that a solution comprising multivalent ions is added to the mixture during foaming, wherein complex formation is initiated already at this step. It may for example be successively added, e.g. by dripping it into the mixture. Alternatively, or additionally, step iii) may be performed before step ii), by adding the multivalent ions to the pre-foam mixture 200 in step i). Using Ca²⁺ as an example of multivalent ion, step iii) may involve transferring a wet foam 201 to a bath containing a CaCh solution.

While there is no specific minimum or maximum time period, immersion of wet foam in a solution 302 comprising multivalent ions may be performed for at least five minutes. The immersion may for example be performed overnight or over 2 nights. It may be performed for 8-48, such as 12-26 hours, or for 1, 2, 4, 6, 8, 10, 14, or 16 hours or more.

The wet foam 201 may be immersed in a solution 302 with a concentration of a compound comprising the multivalent ion (such as CaCh) of 0.05 - 5 M, such as 0.1 - 5 M, 0.1 - 0.3 M or 0.2-0.7 M.

In the second embodiment of the method, the multivalent ions released are ions capable of forming non-covalent complexes with polyelectrolytes. When the multivalent ions are released in step ii), the polyelectrolytes form complexes with the ions wherever they are present, inducing gel formation (gelation) of the polyelectrolytes-based formation of on non- covalent interactions within and between poly electrolytes. The result is a gelated network of polyelectrolytes that are at least to some extent intertwined with the structuring components. The polyelectrolytes and structuring components are thereby firmly associated in the final foam 100. As the complexes are wet stable, the structure is maintained in liquid. Release of multivalent ions 404 from the gelling compound where they are present requires some stimulation, i.e. certain environmental conditions that differ from the storing conditions of the gelling compound. This may involve a change in pH (such as lowering the pH). It could also or instead involve temperature change or radiation (such as UV-radiation). The gelling compound may be a salt or may be a solid particle coated with multivalent ions. It may be a metal carbonate capable of releasing multivalent ions in acidic conditions, such as CaCCh, MgCCh, FeCCh, CuCCh, ZnCCh, or MnCCh. It may be a zeolite coated with a salt of multivalent ions. A zeolitic imidazolate framework, such as ZIF-8, combined with multivalent ions may be used. The gelling compound may be organic or inorganic.

Subjecting the gelling compound to a stimulus may involve subjecting the gelling compound to acidic conditions, such as contacting it with an acidic liquid or acid gas.

In the second embodiment of the method, the wet foam 201 obtained in step ii) may in step iii) be immersed in an acidic fluid such as an acidic liquid. It is beneficial if the acidic liquid it is immersed in leads to the dissolution of CaCCh particles generating calcium salts that are water soluble. The acidic liquid may for example comprise acetic acid, or hydrochloric acid allowing for a release of Ca²⁺. The acidic liquid may have a pH of lower than 7, such as lower than 6, or lower than 5, such as around 2 or 3. Such a step may also lead to production of gas in the wet foam 201, such as carbon dioxide gas, further contributing to the porous structure of the foam 201, 100. The wet foam 201 may alternatively or additionally be subjected to a gas. It may be a gas that reacts with water in wet foam 201 in a way that lowers the pH of wet foam. One example is carbon dioxide (CO2). When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3), which can further dissociate into bicarbonate (HCO3 ) and hydrogen ions (H⁺). The presence of hydrogen ions in the solution lowers the pH, making it more acidic. Other gases, such as sulfur dioxide (SO2) and nitrogen oxides (NOx), can also contribute to the acidity by forming sulfuric acid (H2SO4) and nitric acid (HNO3) when reacting with water. Other examples are sulfur hexafluoride (SFe) and chlorine (Ch). It may also be a gas that is an acid gas. Acid gas is a type of natural gas or any other gas mixture that contains significant quantities of hydrogen sulfide, carbon dioxide, sulfur oxides, nitrogen oxides, hydrogen halides, or similar acidic gases. Acid gases form acidic solutions when dissolved in water.

It is, alternatively or additionally, possible to provide acidic conditions in step ii) by introducing the acidic conditions at least in part simultaneously as the foaming, by addition of acidic liquid or acidic gas. It is for example possible to facilitate release of multivalent ions 404 from the gelling compound by using a foaming gas that when introduced into the prefoam mixture 200 lowers the pH of the pre-foam mixture 200. Examples of such gases are named above. In this case, the step iii) of subjection to acidic conditions coincides with step ii). As an alternative to acidic conditions, change of temperature, or radiation such as UV- radiation may be used as stimuli for release of ions.

The multivalent ions 404 are capable of forming non-covalent crosslinks with polyelectrolytes and, thereby, form non-covalent complexes. Examples of multivalent ions are provided in connection with the foam 100 of the first aspect. The non-covalent complexes provide stability to the structure of the foam, such that the closed pores created when introducing gas to the mixture 200 are maintained when the foam is subjected to water or physical stress.

Salts of multivalent ions may require acidic conditions for release of multivalent ions. For example, CaCC does not provide sufficient Ca²⁺ ions by dissolution in water and does not readily undergo complex formation with organic molecules such as polyelectrolytes. The solubility of CaCC in water is quite limited. The dissociation of CaCCE in alkaline water is minimal, and the species in alkaline solution are: very few carbonate ions (CCE² ), and calcium ions Ca²⁺, along with the major part as undissociated CaCCE. However, under acidic conditions, the release of Ca²⁺ from CaCCE is increased. Lowering the pH, to for example under 7, gives an increased fraction of dissociated CaCCE and hence the release of Ca²⁺. Calcium ions for complex formation are thereby provided to the foam 100 subjecting the wet foam, and thereby the CaCCE, to an acidic liquid. This causes release of Ca²⁺ from CaCCE within the material, which then act as gelling agents forming complexes with poly electrolytes.

In the third embodiment of the method, an ion solution bath or acid bath for introduction of or release of ions may be avoided by the addition of esters to the mixture. The esters hydrolyse in the presence of water to form an acid, especially during hot conditions. Step iii) may thus comprise a step of subjecting the wet foam to a higher temperature. The drying step described herein that may be performed to dry the foam may serve to act as this step of increased temperature. The formed acid will react with gelling compound in the mixture and release ions that will solidify the wet foam structure. The dry foam structures will have similar wet stable properties as if step i) or ii) is performed. Step iii) may be considered as a version of step ii), wherein the subjection to acidic conditions is performed by the initial addition of esters, which hydrolyse at least during step iii). When performing step iii), hydrolysis of the ester may be actively induced by heat treatment. The ester hydrolysis can be further activated by lipase enzymes catalysing the ester bond cleavage. The heat treatment may correspond to the drying step that may be performed in combination with any version of the method, described above.

Many esters can be used in this formulation together with polyelectrolytes and gelling compounds such as described above. The corresponding organic acid calcium salt should preferably be water soluble or at least partly water soluble. For example, oxalic acid-based esters may not be suitable as calcium oxalate is non-water soluble. Non-water-soluble calcium salts will not as optimally solidify the polyelectrolyte which is present in the foam formulation.

A positive feature of the ester hydrolysis is the slow reaction that will slowly solidify the wet foam mixture. Many lactones and anhydrides will hydrolyse too fast and cause uneven solidification of the wet foam mixtures. The solidification should not start before or during the foaming procedure as this will prevent optimal pore structure formation and lead to aggregation of the components. The slow hydrolysis rate of esters is thus suitable for this method.

The hydrolysis rate of the ester can be further adjusted by choosing more reactive esters that hydrolyse more quickly. The hydrolysis rate can also be increased by certain enzymes, such as lipases and esterases.

The chosen esters may be at least partly soluble in water. The boiling point of the ester are preferably above 100 °C to avoid evaporation of the esters during the drying stage.

The esters may be carboxylic esters. Examples of suitable esters where the corresponding calcium carboxylate salt is soluble or partly water soluble are: Ethyl acetate, triacetin, triethyl citrate (TEC), tributyl citrate, ethyl lactate, methyl salicylate, ethyl salicylate, pentyl acetate, butyl acetate, vinyl acetate, benzyl acetate, propyl acetate, Iso-propyl acetate, 1,4 butanediol diacetate, ethyl pyruvate.

Ethyl lactate forms lactic acid when hydrolyzed, which is a much stronger acid than, for example, acetic acid which is formed when triacetin is hydrolyzed, which probably gives more dissolution of CaCCh particles. Thus, the use of ethyl lactate can be advantageous. The esters can be both monoesters, diesters, triesters or higher esters. Example of monoesters are ethyl lactate, ethyl acetate, benzyl acetate, ethyl salicylate and propyl acetate or any ester made from an alcohol and a carboxylic acid.

Examples of diesters are 1,4 butanediol diacetate or any other ester made from a diol or dicarboxylic acid.

Suitable triesters are triacetin and triethyl citrate or any ester made from a triol or tri carboxylic acid.

Tetraesters such as peracetylated erytritol or peracetylated monosaccharides and sugar alcohols and higher esters are also possible to use in this invention.

Esters where the corresponding carboxylic acid is more acidic can also be used for example lactic acid-based esters instead of acetic acid-based esters. More acidic carboxylic acids and their corresponding esters can further increase the neutralization of calcium carbonate and increase the alginate gelling. The esters can also have different electron withdrawing groups at the alpha position and thereby can the hydrolysis rate be further increased. The electron withdrawing groups can be ketones, halogens, nitro groups or cyano groups.

Short chain triglycerides, medium chain triglycerides and long chain triglycerides are also included. These esters are not water soluble, but may be used with water oil emulsions. Hydrolysis of triglycerides can be facilitated by enzymes such as lipases.

Low boiling point esters such as ethyl acetate can also be used especially in combination with lipases or esterases as catalysts. Reactions can be run at lower temperatures and evaporation of the ester can be reduced during the reactions.

The esters contribute to wet stability and homogeneity of the foam.

The concentration of ester used may be the amount required to dissolve all the calcium carbonate in the formulations if complete hydrolysis occurs. A higher amount may also be used. The concentration of ester may be 0.1-50%, such as 0.5-40% or 0.5-10%, based on the total dry weight of polyelectrolyte and structuring component. Using an enzyme such as lipase or esterase makes the hydrolysis of the ester more efficient and lowers the amount of ester required, to for example half of the concentration required without enzymes.

The activity of the enzyme should be such that is may hydrolyze all ester in a few hours. The concentration needed may be less than 1 wt% of the dry weight of the foam or less than 0.1 wt% or 0.01 wt% or less than 0 0001 wt%, such as 0 00005 wt% - 0.7 wt% or 0 00005 wt% - 1 wt%.

The three embodiments of the method may be combined.

The method may comprise a step of adding plastiziser to the foam, suitably to the pre-foam mixture 200. The amount in the final product, i.e. the dry foam 100, may be 0.01 - 5, such as 0.1 - 2, such as around 1 wt%, with regards to the dry weight of the foam. As discussed above the amount may also be higher, such as up to around 70 or 80 wt%. Adding an amount of plastiziser during the manufacturing such that the pre-foam mixture 200 comprises around 10 wt% of plastiziser (including the mass of water) may result in such high amount in the dry foam 100 after evaporation of water from the material. Plastizisers may prevent phase separation of water and solids and stabilizes the bubbles of the wet foam 201 during the manufacturing process. The addition results in a softer, flexible final product. Plasticizers that may be added to the mixture are for example glycerol, xylitol, sorbitol, erytritol, mannitol, maltitol, triacetin, or triethyl citrate. For all ingredients except the plastiziser the contents are based on the dry weight in relation to the dry weight of the foam, i.e. without any water. For the plastiziser all percentages are instead based on all components of the foam including water.

Using a relatively high concentration of polyelectrolyte (such as alginate) or a high-viscosity poly electrolyte may also stabilize the mixture 201 and prevent phase separation. By increasing the amount or viscosity of e.g. alginate added, the amount of CaCCh or glycerol needed to prevent phase separation is decreased. At a certain viscosity/concentration, usage of CaCCh or glycerol may not be necessary. A viscosity of above 700 mPas of a 1 wt% alginate solution may for example be considered a high viscosity alginate in this context, such as 900-1500 mPas.

To facilitate the step of gelation, the wet foam 201 obtained in step ii) may be transferred to a container 301 that allows for the passage of ions and/or molecules, such as a perforated mold. The container 301 may then be sealed such that a desired shape of the foam is formed based on the shape of the container 301, and optionally such that no large openings are present from where foam may exit. The container 301 is then submerged into a solution 302 comprising multivalent ions, or such a solution is flushed through the container. Such a performance is illustrated in Figure 3. The solution may for example comprise a salt of a metal ion such as Ca²⁺ or Fe³⁺. It may for example be a CaCh solution. This allows for an ion-exchange in the encapsulated foam 201 and a non-covalent crosslinking of the poly electrolytes by the multivalent ions.

Using water-soluble polyelectrolytes facilitate the formation of a homogenous dispersion of poly electrolytes in the pre-foam mixture 200 and thereby the foam 100. It may be suitable to dissolve the polyelectrolytes in water before mixing with the structuring component and foaming agent. They may for example be dissolved in water such that they constitute 0.5-5 wt% of that solution, before mixing with structuring component and foaming agent. The water to be added in step i) may be added in this fashion.

In the pre-foam mixture 200, the content of water may be 50-99.9 wt%, such as 70-99.8, 90- 99.7 or 95-99.6 wt%, 95-97.5 wt%. It is also possible to replace an amount of the water with glycerol. If a large amount of glycerol is used in the manufacturing, the amount of water may be decreased. For example, the pre-foam mixture 200 may comprise 10 wt% of glycerol and 85 wt% of water.

After step iii), a step of drying the wet, crosslinked foam 100 may be performed. It may be performed at a temperature of 20-90 °C, such as at room temperature, at 40-70 °C, 50-80 °C, 60-70 °C, or at a higher temperature such as 100 °C. This may be performed for 20 minutes to 100 hours, such as for 12 or 48 hours. Gentle heating methods may be used to avoid degradation or undesirable side reactions, typically using temperatures under 100 °C. The temperature does not have to be consistent over time, but may, for example, be increased after a period of time. When drying, water evaporates from the foam, leaving a dry foam with a greater solid content. The foam 100 may be dried until it reaches a dry weight of 85, 90, 95, or 99 wt%. The non-covalent complexes of the foam generally withstand such a drying step and the structure comprising closed pores is largely maintained. Although a portion of the closed pores may collapse, a sufficient number of closed pores remain.

Wet stability can be increased by utilizing irreversible association of cellulosic material using heat treatment, also known as hornification of cellulose-rich materials. Such treatment reduces reswelling of the material. The treatment may be performed at for example 100 to 140 °C for Ih to 24h. In one example, it is performed at 120 °C for 5-20 h; in another example, it is performed at 130 °C for 1-5 h. Higher temperatures require shorter treatment time to achieve a similar result. This step is suitably performed after the drying step. A hornification step may be particularly appropriate when the complexes of the polymers are weak. The concentrations used in the method, of e.g. structuring component, polyelectrolyte, multivalent ions, foaming agent and CaCCh, correspond to those disclosed with regards to the foam 100. The relative dry weights are similar when added in step i) as in the final product.

The amount of foaming agent added in step i) may be such that it constitutes 0.2 - 10 wt% , such as 0.2-5 %, such as 0.3-5wt% of the pre-foam mixture 200. The foaming agent may be added as a component in an aqueous solution comprising pure foaming agent, such as a commercial surfactant mixture. Residuals of the foaming agent (likely a surfactant) may be maintained in the foam.

As discussed above in relation to the foam, non wood nanoparticles or microparticles with the ability to adsorb surfactants may be added to the foam, suitably to the pre-foam mixture in step i). Such particles may be starch granules, clay particles or bentonite.

EXAMPLES

Example 1 - Viscosity measurements

Different alginate solutions were used throughout the experiments to develop different foams. 11.5 g of sodium alginate (containing 15% of moisture) was dissolved in 988.5g of deionized water under stirring, resulting in a final concentration of 1 wt%. The alginates had 1.3-1.7 charges per Bjerrum length. The solutions were allowed to rest for an extended period of time (1-5 weeks) prior to measurement. The viscosity of the solution (T = 21.5°C ± 0.9 ) was determined using a rotational viscometer (Anton Paar, ViscoQC) with a suitable free spindle at a speed of 30 rpm. The viscosity was determined after 60 seconds once the reading was stabilized. The values are averaged over several replicates and rounded to the nearest hundredth and are reported in Table 1.

Table 1. The viscosity of the different alginates used.

*It is important to note that this value for AO only covers the alginate solution that has been equilibriated for at least 1 week. For solutions that has been equilibrated for only 30 minutes after the solution was prepared, the viscosity was measuresd to around 2000-3000 mPas. Hence, the viscosity is highly dependent on the equilibrium time for high molecular weight/viscosity polyelectrolytes, such as alginate.

Example 2 - Effect of concentration of gelling compound CaCCh

Dry chemo-thermomechanical pulp (CTMP) fibres (50 g) was soaked in water (2-24 h) before disintegration using a Kenwood mixer. Then, a sodium alginate solution (A0, 1 wt%, 555 g solution) was added to the fibers and further mixedThe alginate had 1.3-1.7 charges per Bjerrum length. Once a homogenous mixture was obtained, different amounts of CaCCh particles was added (Table 2) to the mixture and further homogenized. A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. The volume was increased 2 times (2x) in the wet foam. The mixture was then transferred into dialysis tubing or perforated molds and soaked in an acetic acid bath (20 Vol%) to allow for the dissolution of the CaCCh particles and crosslinking. The gels were then rinsed with water to remove excess acid and dried in an oven overnight (80 °C). The amounts of the components added are listed in Table 2. All components, except the surfactant, are listed as pure components. It is assumed that the dry weight composition of the ingredients also reflects the composition of the finished foam.

Table 2, Final amounts added of the components in the wet foam.

* The dry weight of the commercial mixture was maximum 20 wt% before dilution. Example 3 - Effect of volume increase in the wet foam

Dry CTMP fibres (100 g) were soaked in water (2-24 h) before disintegration using a Kenwood mixer. Then, a sodium alginate solution (AO, 1 wt%, 1110 g solution) was added to the fibers and further mixed. The alginate had 1.3-1.7 charges per Bjerrum length. Once a homogenous mixture was obtained, CaCOs particles (8.1g) were added to the mixture and further homogenized. A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. The volume was increased by 2x, 3x and 5x times in the wet foam. The mixture was then transferred to dialysis tubing or perforated molds and soaked in an acetic acid bath (20 Vol%) to allow for the dissolution of the CaCCh particles and crosslinking. The gels were then rinsed with water to remove excess acid and dried in an oven overnight (80 °C). The amounts of the components added are listed in Table 3. All components, except the surfactant, are listed as pure components. It is assumed that the dry weight composition of the ingredients also reflects the composition of the finished foam.

Table 3, Final concentration of the components in the wet foam used for different volume increase.

* The dry weight of the commercial mixture was maximum 20 wt% before dilution.

Example 4 - Effect of alginate concentration

Dry CTMP fibres (50 g) were soaked in water (2-24 h) before disintegration using a Kenwood mixer. Then, a sodium alginate solution (A0, 1 wt% or 5 wt% solution) was added to the fibers and further mixed (Table 4). The alginate had 1.3-1.7 charges per Bjerrum length. Once a homogenous mixture was obtained, different amounts of CaCCh particles were added to the mixture in order to keep the ratio between CaCOs and alginate fixed (Table 4). A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. The volume was increased 2x in the wet foam. The mixture was then transferred to dialysis tubing or perforated molds and soaked in an acetic acid bath (20 Vol%) to allow for the dissolution of the CaCCh particles and crosslinking. The gels were then rinsed with water to remove excess acid and dried in an oven overnight (80 °C). The final concentration of alginate in the dry foams correspond to 0wt% (0_Alginate), ~1 wt% (l_Alginate), ~2 wt% (2_Alginate), ~2.5 wt% (2.5_Alginate), ~5 wt% (5_Alginate), ~9 wt% (9_Alginate), ~17wt % (17_Alginate), and ~30 wt% (30_Alginate). The amounts of the components added are listed in Table 4. All components, except the surfactant, are listed as pure components. It is assumed that the dry weight composition of the ingredients also reflects the composition of the finished foam.

Table 4, The mass of different components added.

* The dry weight of the commercial mixture was maximum 20 wt% before dilution.

**Due to the high viscosity of the mixture as a result of the high amounts of the alginate solution added, more water and surfactant was required to be added to reach a 2 times volume increase.

Example 5 - Effect of different alginates

Dry CTMP fibres (50 g) were soaked in water (2-24 h) before disintegration using a Kenwood mixer. Then, different sodium alginate solutions (1 wt%) with different viscosities were added (Table 1 and 5). The alginate had 1.3-1.7 charges per Bjerrum length. Once a homogenous mixture was obtained, different amounts of CaCOs particles were added to the mixture (Table 5). A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. The volume was increased 2x in the wet foam. The mixture was then transferred to dialysis tubing or perforated molds and soaked in an acetic acid bath (20 Vol%) to allow for the dissolution of the CaCCh particles and crosslinking. The gels were then rinsed with water to remove excess acid and dried in an oven overnight (80 °C). The amounts of the components added are listed in Table 5. All components, except the surfactant, are listed as pure components. The final concentration of alginate in the dry foams correspond to ~2.5 wt% (2.5_A_X), ~5 wt% (5_A_X), and ~9 wt% (9_A_X), where x is 1, 2, 3, or 4. Table 5, The mass of different components added.

* The dry weight of the commercial mixture was maximum 20 wt% before dilution.

Results from experiments 2, 3, 4, and 5

The foams based on different concentrations of CaCCh particles (Table 2), different volume increases in the wet foam (Table 3), different concentrations of alginate (Table 4), and different alginate viscosities (Table 5) were evaluated regarding density (Table 6), and wet stability (Table 7). The wet stability of the dry foams was evaluated by mechanical agitation in water for 6 days at 115 rpm in separate beakers, followed by mechanical loading.

Table 6, The densities, water absorption, and the percentage of open and closed pores in the foams.

Table 7. Summary of wet stability tests for the foams. The weigths before and after water imersion are rounded to the first decimal.

* A weight tolerance of <x means that the sample did not pass the test for x kg, but passed the test for a weight lighter than x kg. For x = 0.5 kg, it means that it did not pass the test for any weight. A weight tolerance of >x kg means that it passed the test for this weight. For instance >10 means that the sample passed the test for a 10 kg weight, and no heavier weight was tested, and the foam thus tolerates at least 10 kg. This nomenclature applies to all examples. The samples with 0 wt% alginate and 0 wt% CaCO₃ did not pass the wet stability test.

Conclusion from examples

The results show that higher viscosity/Mv alginate are needed in order to obtain excellent wet stability at lower concentrations of alginate. This means that excellent wet stability is obtained at different lower amounts for different viscosity/Mv alginate solutions. For the lowest viscosity alginate foams (A4), i.e. 200 mPas for a 1 wt% solution in both water and 0.1 M NaCl, the foam samples consisting of -2.5 wt% alginate, lost 48.8% in weight after the shaking board test, and the remainder of the foam body could withstand only less than 1 kg in load. However, for the highest viscosity alginate samples (AO), i.e. -5000-6000 mPas for a 1 wt% solution in water and 3500 mPas for a 1 wt% solution in 0.1 M NaCl, a foam sample consisting of -2.5 wt% of alginate lost only 10.3% of its mass after the shaking board, while withstanding up to 4 kg in load. These results indicate that the foam samples with higher viscosity alginate become more wet stable at the same concentrations as foam samples with lower viscosity alginate at said concentration regime.

Example 6 - Foams with esters

Foams with esters

50 g of CTMP fibers (dry weight) was mixed with water to reach a total amount of 282 g water. The mixture was disintegrated using a Kenwood kitchen mixer. A sodium alginate solution (A0, 1 wt%, 555 g solution) was added to the fibres and mixed. Then, CaCCh (4.2 g) was added and the mixing was continued until a homogenous mixture was obtained before triacetin was added under continuous mixing (see amounts in Table 8). A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam followed by complete drying overnight at 80 °C. The foams were not treated with any ion solution, or with acidic conditions to cause release of ions from CaCCh. The amounts of the components added are listed in Table 8. All components, except the surfactant, are listed as pure components. It is assumed that the dry weight composition of the ingredients also reflects the composition of the finished foam.

Foams with esters and lipases

In this experiment, two different enzymes, i.e. lipases (sample names: E or IE), was added to the mixtures to catalyze the hydrolysis of the ester.

50 g of CTMP fibers (dry weight) was mixed with water to reach a total amount of 282 g water and the mixture was disintegrated using a Kenwood kitchen mixer. A sodium alginate solution (A0, 1 wt%, 555 g solution) and CaCCh (4.2 g) was added and the mixing was continued until a homogenous mixture was obtained before triacetin was added under continuous mixing (see amounts in Table 8). Then, the mixture was cooled to about 10 °C on ice before different lipases were added. A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. All wet foams were incubated for either 4 hours at 40 °C or 6 hours at 70 °C, which was the temperature optimimum for the different enzymes, followed by complete drying overnight at 80 °C. The foams were not treated with any ion solution, or with acidic conditions to cause release of ions from CaCCh. The amounts of the components added are listed in Table 8. All components, except the surfactant, are listed as pure components. It is assumed that the dry weight composition of the ingredients also reflects the composition of the finished foam.

Table 8. Foams with different amounts of triacetin and lipase.

* The dry weight of the commercial mixture was maximum 20 wt% before dilution.

Results of Example 6

The results of the wet stability tests with triacetin and different lipases from Table 8 can be seen below in Table 9. The wet stability of the dry foams was evaluated by mechanical agitation in water for 6 days at 115 rpm in separate beakers, followed by mechanical loading.

Table 9. Summary of wet stability tests for foams containing triacetin with or without lipase.

* A weight tolerance of <x means that the sample did not pass the test for x kg, but passed the test for a weight lighter than x kg. For x = 0.5 kg, it means that it did not pass the test for any weight. A weight tolerance of >x kg means that it passed the test for this weight. For instance >10 means that the sample passed the test for a 10 kg weight, and no heavier weight was tested, and the foam thus tolerates at least 10 kg. This nomenclature applies to all examples.

Increasing triacetin amount produced more wet stable foams. High amount of triacetin was necessary to produce wet foams (H-tri-2x) that could withstand weights of 10 kg. By introducing lipases, only low amount of triacetin was required to produce wet foams that could withstand 10 kg (L-tri-IE-2x).

Example 7 - Foams with higher concentrations and different esters

Foams with higher concentrations and triacetin or ethyl lactate as esters

The objective was to produce wet foams with higher dry content to reduce the drying time and cost. 100 g CTMP was wetted with 564 g water overnight. Then, a sodium alginate solution (A0, 2 wt%, 555 g solution) was added. The mixture was mixed using a Kenwood mixer before the addition of 8.2 g of CaCCh particles. Then, triacetin (12.2 g) or ethyl lactate (19.84 g) was introduced to the mixture. A commercially available surfactant mixture was added to the mixture and air was introduced using the whipping device fitted to the Kenwood mixer to obtain a wet foam. The volume was increased by 2.5 (2x) and 5 (5x) times in the wet foam. The foams were dried overnight at 80 °C.

Results of Example 7

The results of the wet stability tests with triacetin and ethyl lactate can be seen below in Table 10. The wet stability of the dry foams was evaluated by mechanical agitation in water for 6 days at 115 rpm in separate beakers, followed by mechanical loading.

Table 10. Wet stability results for the high dry content triacetin and ethyl lactate foams.

The foams became wet stable with ethyl lactate and triacetin at higher concentrations.

Methods for measurements

Density Measurements

The density of the cylindrical foams was then calculated according to Eq. 1 :

P = (Eq. 1)

Where p is the density (g/cm³), m (g) is the mass of the foam and V (cm³) is the volume of the foam sample. The volume was measured using a caliper, based on a average diameter and average height of the sample.

Wet Stability

The wet stability test can be divided in two steps, where most competing materials, including cardboard, break in step 1. For wet stability a sample must pass both tests.

1. The wet stability of a material is defined by said material’s ability to remain intact under mechanical agitation in water for more than 6 days. Sample foams were immersed in containers containing deionized water. The volume of water was several times the volume of the samples. The water was pure deionized water. The containers were put on a shaking table that was set to 115 RPM, which ensured vigorous mechanical agitation of the samples in the containers. The time which different foams disintegrated in the water was noted to record the degree of wet stability. The test lasts for 6 days, i.e. 144 hours.

2. For samples for step 2, the size was cylindrical with an approximate diameter ot 5 cm and an approximate height of 5 cm. The top and the base of the samples were flat. Samples can be cut into the correct shape. The samples were first subjected to step 1 and then lifted out of the beakers after 6 days, and those that broke during this procedure were considered not intact and discarded as non-wet stable. The wet samples directly taken from step 1 were then subjected to different weights (0.5, 1, 1.5, 2.5, 4, 5, 7.5 and 10 kg). The weight was very carefully placed on top of the sample. The size of the weight always exceeded the size of the sample so that the entire top of the sample was covered by the weight, i.e. so that the pressure from the weight was evenly distributed over the top of the sample. During load the sample is compressed by the weight. After letting the sample be compressed for at least 10 seconds, the weight was removed and the sample was evaluated. The samples were evaluated visually for cracks. If the sample has cracked, i.e. if there are any visible cracks in the sample, then the sample is not considered to be wet-stable. The weight that the samples could resist without breaking were noted. To claim excellent wet stability, the sample had to resist more than 0.5 kg. The pieces were then dried and the dry mass was recorded. Thereby the weight loss during the immersion in water during 6 days could be measured. The weight loss and the tolerance for pressure can be measured separately on separate samples if desired.

The weight loss measurements are in summary performed according to the following method. The weight loss in x% as measured by immersing the sample in deionized water for 6 days at 20 °C in a container subjected to shaking at 115 RPM, wherein the dry weight of the dry sample before and after the immersion is measured and the difference is calculated and divided by the dry mass after immersion (Eq.2).

(Eq. 2)

Good wet stability is defined as a weight loss not exceeding 50 wt% according to the above measurement method. Good wet stability further requires that the sample can resist a load of more than 0.5 kg as measured according to the above method.

Claims

1) A foam (100) comprising i) at least one structuring component (401) selected from the group consisting of lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), ii) at least one polyelectrolyte (403), and iii) multivalent ions (404) with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte, wherein the multivalent ions (404) are present in non-covalent complexes with the at least one polyelectrolyte (403), and wherein the at least one polyelectrolyte (403) is present in a polymeric gel structure, wherein the concentration of the polyelectrolyte (403) in the foam (100) is at least 1 wt%, based on dry weight of the polyelectrolyte and dry weight of the foam, wherein the poly electrolyte (403) has a charge density of at least 1 charge per 3 Bjerrum- lengths, wherein the poly electrolyte (403) has a molecular weight (Mv) of 500 000 to 15 000 000 g/mol.

2) The foam (100) according to claim 1, wherein the foam looses maximum 50 wt% of its dry weight during immersion in deionized water for 6 days on a shaking table set to 115 rpm, and wherein a cylinder with 5 cm diameter of the foam is able to withstand a load of more than 0.5 kg without cracking.

3) The foam (100) according to any of the preceding claims, wherein the foam is wet stable, in that it remains substantially intact after mechanical agitation in water for at least 6 days.

4) The foam (100) according to claim 1, wherein the at least one polyelectrolyte (403) is at least one selected from carbohydrates and proteins. 5) The foam (100) according to any of the preceding claims, wherein the at least one polyelectrolyte (403) comprises at least one selected from chitosan, carrageenan, pectin, gellan gum, xanthan gum, glucan, agarose and agaropectin alginate and polyvinylamine.

6) The foam (100) according to any of the preceding claims, wherein at least some of the multivalent ions are multivalent metal ions.

7) The foam (100) according to any of the preceding claims, wherein the multivalent ions are selected from the group consisting of Ca²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Sr²⁺, CO²⁺, Zn²⁺, Fe²⁺, Ai³+, F_e ³+, Nd³ , SO₄ ²’ and PCh³' or any combination thereof.

8) The foam (100) according to any of the preceding claims, further comprising an ester.

9) The foam (100) according to any of the preceding claims, further comprising CaCCh.

10) The foam (100) according to any of the preceding claims, further comprising a plasticizer.

11) The foam (100) according to any of the preceding claims, wherein the foam (100) comprises 10-99 wt% of structuring component, 1-50 wt% of polyelectrolyte, 0.2-10 wt% of foaming agent, 0-50 wt% of CaCCh, and 0-80 wt% of glycerol.

12) The foam (100) according to any of the preceding claims, wherein the foam (100) comprises non wood nanoparticles or microparticles with the ability to adsorb surfactants, such as starch granules, clay particles or bentonite.

13) A method of manufacturing a foam (100), comprising the steps of i) mixing water, at least one polyelectrolyte (403), at least one foaming agent, and at least one structuring component (401) selected from lignin, cellulose fibers (CF), cellulose microfibrils (CMF), and cellulose nanofibrils (CNF), thereby obtaining a pre-foam mixture (200), wherein the concentration of the polyelectrolyte (403) in the foam (100) is at least 1 wt%, based on dry weight of the polyelectrolyte and dry weight of the foam, wherein the polyelectrolyte (403) has a charge density of at least 1 charge per 3 Bjerrum-lengths, and wherein the poly electrolyte (403) has a molecular weight Mv of 500 000 to 15 000 000 g/mol, ii) introducing gas into the pre-foam mixture (200), thereby obtaining a wet foam (201), and iii) subjecting the at least one poly electrolyte (403) to multivalent ions (404) with opposite sign of the charge compared to the net charge of the at least one polyelectrolyte (403) so that a non-covalent complex form which stabilize the structure of the foam (100).

14) The method according to claim 13, wherein the finished foam looses maximum 50 wt% of its dry weight during immersion in deionized water for 6 days on a shaking table set to 115 rpm, and wherein a cylinder with 5 cm diameter of the finished foam is able to withstand a load of more than 0.5 kg without cracking.

15) The method according to any one of claims 13-14, wherein the at least one poly electrolyte (403) is contacted with the multivalent ions (404) in step iii) by subjecting the pre-foam mixture (200) or the wet foam (201) to a solution comprising multivalent ions (404).

16) The method according to any one of claims 13-14, wherein a gelling compound is added in step i), which gelling compound has the ability to release multivalent ions (404) with opposite sign of charge compared to the net charge of the at least one polyelectrolyte (403) upon reaction with an acid, and wherein the at least one polyelectrolyte (403) is contacted with the multivalent ions (404) in step iii) by subjecting the pre-foam mixture (200) or the wet foam (201) to acidic conditions, and thereby releasing the multivalent ions (404).

17) The method according to any one of claims 13-14, wherein a gelling compound is added in step i), wherein an ester is added in step i), and wherein step iii) comprises subjecting the wet foam (201) to an increased temperature, thus inducing hydrolysis of the ester, such that an acid is formed, which acid reacts with the gelling compound, leading to release of multivalent ions (404).

18) The method according to claim 17, wherein step iii) comprises subjecting the wet foam (201) to a temperature of 30-100 °C for 20 min to 100 h.

19) The method according to claim 15, wherein the step i) involves immersing the wet foam 201 obtained in step ii) in a solution comprising multivalent ions (404) for a time period of at least 5 min.

20) The method according to any of claims 16-17, wherein the gelling compound is a metal carbonate capable of releasing multivalent ions in acidic conditions, such as CaCCh, MgCCh, FeCCh, CuCO₃, ZnCCh, or MnCCh.

21) The method according to claim 16, wherein step iii) involves immersing the wet foam (201) in an acidic fluid with a pH of 5 or lower.

22) The method according to claim 16, wherein step iii) involves subjecting the gelling compound to at least one selected from the group consisting of: acetic acid, hydrochloric acid, carbon dioxide, sulfur dioxide, nitrogen oxide, sulfur hexafluoride, and chlorine.

23) The method according to any of claims 16-17, wherein the gas used in step ii) is a gas that when introduced into the pre-foam mixture (200) lowers the pH of the pre-foam mixture (200), causing release of multivalent ions (404) from the gelling compound if present.

24) The method according to any one of claims 17-18, wherein the ester is at least one ester selected from the group consisting of ethyl acetate, tributyl citrate, ethyl lactate, methyl salicylate, ethyl acetate, butyl acetate, vinyl acetate, benzyl acetate, iso-propyl acetate, 1,4 butanediol diacetate, ethyl pyruvate, ethyl salicylate propyl acetate, triacetin, and triethyl citrate (TEC).

25) The method according to any one of claims 17-18, wherein the ester is selected from the group consisting of a) an ester made from an alcohol and a carboxylic acid, b) an ester made from a diol or dicarboxylic acid, c) an ester made from i) a triol, ii) a tri carboxylic acid, iii) a peracetylated erytritol iv) a peracetylated monosaccharide, v) a sugar alcohol, or vi) a triglyceride.

26) The method according to any of claims 17, 18, 24 or 25, wherein an enzyme being a lipas or esteras is added in step i) at a concentration of 0 00005 - 1 wt%.

27) The method according to any of claims 13-26, wherein CaCCh is added in step i).

28) The method according to any of claims 13-27, wherein a plastiziser is added in step i).

29) The method according to any of claims 13-27, wherein step ii) involves aerating, stirring, whipping, or beating the mixture in the presence of a gas until at least one of the following conditions is satisfied: a) the introduction has been performed for at least 15 seconds, and b) the volume of the mixture is increased by 25 -1000%.

30) The method according to any of claims 13-29, comprising a step of drying the foam (100) at a temperature of 20-100 °C for 20 min to 48 h and optionally a step of heating the foam (100) at a temperature of 100-140 °C for 30 min to 24 h.

31) The method according to any of claims 13-30, wherein the wet foam (201) obtained in step ii) is transferred to a container that allows for the passage of ions and/or molecules, optionally followed by sealing the container, before step iii).

32) A product comprising the foam (100) according to any of claims 1-13, wherein the product is selected from the group consisting of a fish guide, a pool noodle, a wet- resilient packaging material, an insulation material, an isolation material, a floating island for vegetation, a filler, a packaging material, shock absorber, a medical device or orthopaedic product, a building material, sports equipment, gardening product.