Preparation of foams containing individual units of foam embedded in a foam matrix

Abstract

The present invention relates to a method for the preparation of a solid foam comprising depositing individual units of foam on a surface to obtain a first foam deposit, followed by depositing wet foam between the individual units to obtain a subsequent foam deposit, and drying the wet foam.The present invention further relates to a solid foam comprising individual units of foam embedded in a foam matrix.

Description

[Technical field]

[0001] The present invention relates to a method for the preparation of a solid foam comprising depositing individual units of foam on a surface to obtain a first deposit, followed by depositing wet foam between the individual units to obtain a subsequent deposit, and drying the wet foam to obtain a solid foam, wherein the individual units of foam are embedded in a foam matrix.The present invention further relates to a solid foam comprising individual units of foam embedded in a foam matrix. [Background technology]

[0002] Today, various techniques are used to deposit wet foam materials and produce low density thick foam materials. In WO2020011587, a porous material of cellulose fibers and gluten is prepared by depositing an aerated wet foam of cellulose fibers and gluten in a mold at once, followed by drying to obtain a dry porous material with the shape of the mold and a uniform fiber network throughout the bulk. In WO2015036659, the wet fiber foam is fed into a mold where a portion of the water contained in the foam is mechanically extracted to produce a solidified wet fiber composition, and the water is evaporated to produce a dry fiber product. These techniques are similar in that the final foam sheet may present densified layers on the faces of the sheet, but the core of the foam sheet is a uniform fiber network with a lower density.

[0003] When particle or fiber foam collapses, the thickness of the foam sheet shrinks as tension forces pull the particles or fibers together. Drying shrinkage is an inherent property of cellulose because the fibers collapse onto each other when water is removed from the system. Shrinkage of over 10% can also be expected with more complex drying systems such as a combination of air impingement and IR dryers.

[0004] As with traditional papermaking techniques, the foam formation needs to be drained onto a screen. In this case, the material is leveled during drainage, limiting the shape and size of the material. The density and thickness of the sample are determined by the wet fiber foam concentration, which is usually 1-4% by weight, and the amount of drainage before drying. Generally, the dry content is about 1%-8% after drainage. For efficient drainage, the viscosity of the suspension needs to be kept fairly low to extract the water efficiently. Any soluble binders can be limited to low concentrations and retained on the fibers to avoid excessive losses. Thus, drainage limits the number of additives that can be used in this type of process. The strength of paper formed with thick low density foams is mainly controlled by the bulk of the material. Here the main means of improving strength is to increase density by restricting fiber orientation to some extent, based on drainage characteristics.

[0005] Higher dry content techniques, such as foams made from large amounts of protein-based foaming agents, such as WO2020011587, impede recyclability because most of the material is not water soluble or easily washed out of the product. The method also suffers from poor wet foam stability, as the foam gradually collapses in wet conditions as protein particles begin to aggregate and gas bubbles coalesce. This fact makes the method a poor candidate for freestanding wet foam deposition.

[0006] To avoid shrinkage, the foam needs to be dried under tension, but when drying very large surface areas, the tension provided by the frame or mold is limited to the area closest to the mold. Thus, shrinkage is a problem, especially when drying large surface areas. Since both wet and dry foams are insulating, foam drying times are typically long. To allow for a cost-efficient process, short drying times are desirable. Thus, there remains a need for alternative methods for preparing foams. Furthermore, the prepared foams should have high impact resistance when used as packaging materials that also allow for the protection of heavier objects. Summary of the Invention

[0007] The object of the present invention is to provide a method suitable for the preparation of foams with reduced foam shrinkage. The method allows for structure control of thick, low density foam materials by controlled wet laying techniques and is particularly suitable for the preparation of cellulose foam sheets. Preferably, the method also allows for shorter drying times of the foam.

[0008] A further object is to provide a lightweight solid foam having good dimensional stability.

[0009] More specifically, the present invention relates to a cellulose foam material comprising individual units of cellulose foam embedded in a cellulose foam matrix, the cellulose foam material having a low density. The cellulose foam matrix surrounding the individual units may be composed of the same foam composition as the individual units. The individual units may be differentiated from the matrix by a densified layer. Furthermore, the foam material may be made by two or more individual deposition steps with a drying step after each deposition. The method allows for the creation and control of densified cellulose fiber walls. Upon drying, the fibers can be redispersed in water, resulting in the foam being recyclable in normal paper recycling streams.

[0010] The present invention thus relates to both a cellulose foam having a distinct macrostructure comprising individual units of cellulose foam embedded in a cellulose foam matrix, and to a process for obtaining the foam comprising a multi-step deposition and drying process.

[0011] Cellulose foam is expected to contribute to technologies such as protective packaging as cushioning, insulation for cold chain logistics, and in building materials, acoustic panels, growing media for hydroponic plants, and other applications requiring lightweight, high-performance bio-based materials. [Brief description of the drawings]

[0012] [Figure 1] A) shows a schematic of a prior art foam board with top and bottom densification, and B) shows a schematic of the inner bulk of such a foam board which contains a less dense uniform fiber network throughout the bulk. [Diagram 2] FIG. 1 is a schematic diagram of a cross section of a comparative foam after drying, illustrating the effect of width-to-height aspect ratio on shrinkage. [Diagram 3] a) shows a graph obtained from the drying of a single large block of comparative cellulose foam, showing the dry thickness of the densified layer, i.e. the skin (-) on the left y-axis and the shrinkage profile (○) on the right y-axis as a function of the % conversion of liquid water to vapor during drying, while b) illustrates the cross section of two fibers in a humid environment (grey area) and during drying (open circles). Steps A), B) and C) in Fig. 3b) correspond to the situation in zones A), B) and C) in Fig. 3a). [Figure 4] Illustrated is a two-step deposition having i) a first deposition of cellulose foam as individual units, ii) an intermittent drying step to dry the individual units to create iii) free-standing individual units of cellulose foam, iv) a second deposition of cellulose foam between the individual units of cellulose foam, and v) vi) a second drying step to obtain a cellulose foam material comprising individual units of cellulose foam. [Diagram 5] Schematic diagrams of solid foam sheets produced according to the methods of the present invention are shown: A) illustrates a solid foam in black with densified layers on top and bottom, similar in appearance to foam sheets produced by other techniques; B) illustrates the bulk of material below the densified top and bottom, which contains individual units of cellulose foam (black cuboids) with each individual unit differentiated from the surrounding foam matrix by a densified layer of cellulose; and C) shows one individual unit surrounded by a densified layer of cellulose illustrated in black (left) and homogenous cellulose foam inside the densified layer (right). [Figure 6]Illustrates the difference in tension development (higher tension in darker areas) during drying of (a) a single block and (b) a multi-step stack. [Figure 7] FIG. 1 shows an example of compression curves showing a single-step stack prepared according to Example 2 (lower dashed curve) and a multi-step stack prepared according to Example 1 (upper solid curve) for the same density foam (30 kg / m3). [Figure 8] Figures 8a-b show the drying time of a cellulose foam board with individual units of different sizes and compare it to the drying time of a single step foam stack. 8a shows the drying time of individual units of varying widths and 2.1 cm in height. 8b illustrates the drying time of a subsequent wet foam stack surrounding the individual unit of 8a, where the subsequent wet foam stack is 2.1 cm in height. [Figure 9] The cushioning performance during impact of a solid foam according to the present invention is shown, which is measured by drop testing the foam with a load containing an accelerometer on top of the foam. [Figure 10] The cushioning performance during impact of a solid foam according to the present invention is shown, which is measured by drop testing the foam with a load containing an accelerometer on top of the foam. [Figure 11] Figure 1 shows the cushioning performance during impact of a solid foam according to the invention, measured by drop testing the foam with a load containing an accelerometer on top of the foam. Results are shown for peak acceleration for drop 1 (Figure 10) and the average of drops 2-5 (Figure 11) and degradation, measured as relative compression after 5 drops (Figure 9). Cellulose foam samples composed of individual units of foam having the same or different density compared to the cellulose foam matrix were tested and compared to foam samples deposited in a single step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In a first aspect, the present invention provides a method for the preparation of a solid foam comprising depositing individual units of foam on a surface to obtain a first foam deposit, depositing wet foam between the individual units to obtain a subsequent foam deposit, and drying the wet foam in the subsequent foam deposit to obtain a solid foam, wherein the individual units of foam are embedded in a foam matrix.

[0014] The term "foam", as used herein, refers to a material created by entrapment of air or gas bubbles inside a solid or liquid. Typically, the volume of gas is much greater than the volume of the liquid or solid, with a thin film separating the gas pockets. Three requirements must be met to form a foam. Mechanical work is required to increase the surface area. This can occur by agitation, dispersion of a large amount of gas into the liquid, or by injecting gas into the liquid. The second requirement is that a blowing agent, typically an amphiphile, surfactant or surface-active component, must be present to reduce the surface tension. Finally, the foam must be formed more quickly than it can be destroyed. Foams can be wet or dry, i.e., solid.

[0015] The term "discrete units" as used herein refers to individual foam units within a solid foam. The discrete units are deposited as separate foam units. The space between the discrete units in the solid foam is filled with foam such that the discrete units are embedded in the foam matrix. The discrete units are distinguishable from each other and from the foam matrix.

[0016] In one embodiment, the solid foam is a cellulose foam, and both the foam in the individual units and the foam matrix are cellulose foams. The term "cellulose foam" refers to a foam that includes cellulose and other components such as thickeners, surfactants, and additives. The main component of cellulose foam is cellulose, which constitutes at least 70% by weight, or 75-95% by weight, of the solid content of the cellulose foam. The cellulose is in the form of fibers, and thus the foam can also be defined as a fiber foam or a cellulose fiber foam. The foam can be wet or dry.

[0017] The wet foam used in the individual units may be a fiber foam. The wet foam used in the individual units may contain at least 10% by weight of cellulose fiber, or at least 11% by weight of cellulose fiber, calculated on the total weight of the wet foam. The wet foam may contain 10-40%, 11-40%, 10-30%, 11-30%, 10-20%, or 11-20% by weight of cellulose fiber, calculated on the total weight of the wet foam. The cellulose fiber may be selected from wood pulp, regenerated cellulose fiber, or vegetable fiber, such as fiber from bamboo, cotton, hemp, flax, and jute. Preferably, the cellulose fiber is selected from wood pulp, such as softwood kraft bleached pulp, chemical thermomechanical pulp, dissolving pulp (e.g. bleached wood pulp or cotton linters), and hardwood pulp, more preferably softwood kraft bleached pulp and chemical thermomechanical pulp, and most preferably softwood kraft bleached pulp.

[0018] In one embodiment, the individual units can be made by distributing the wet foam in the individual units on a surface, followed by drying the individual units. The wet foam forming the individual units has a density of 70 to 600 kg / m 3 , or 100-500kg / m 3, or 100-400kg / m 3 , or 125-375 kg / m 3 , or 140-375 kg / m 3 The foam may have a density of 0.1 to 1.0 mm. The large number of small air bubbles provides stability and low density to the foam. The wet foam used in the present method has a high enough viscosity and low density to allow the formation of individual units that do not collapse before being dried. Thus, each individual unit can stand on its own without collapsing before being dried. Furthermore, each individual unit can stand on its own without collapsing during drying. Drying of the individual units of the wet foam is carried out at least until a skin, i.e., a thin densified layer of cellulose fibers, is formed on the exterior surfaces of the individual units, e.g., on each face of the individual units. The densified layer is a very thin layer that forms on the very outer surface of the foam during drying. The densified layer is composed of cellulose fibers that are primarily oriented in a two-dimensional plane (xy plane), while the fibers in the bulk of the foam include clusters of fibers oriented in a three-dimensional space with more free space between the clusters. The two-dimensional structure of the cellulose fibers in the densified layer rapidly but gradually transitions to the three-dimensional structure found in the bulk of the foam. The low thickness of the densification layer means that it does not substantially affect the overall density of the foam, but still contributes to good mechanical properties of the individual units.

[0019] Optionally, further foam stacks of individual units may be created on the surface, preferably between the individual units of the first stack. The wet foam forming the individual units in the further stacks has a density of 70 to 600 kg / m 3 , or 100-500kg / m 3 , or 100-400kg / m 3 , or 125-375 kg / m 3 , or 140-375 kg / m 3 The density of the wet foam used in the further deposition may be the same as or different from the wet foam in the first deposition.

[0020] When the individual units of foam containing cellulose fibres are dried, their core consists of a uniform fibre network with density, and their outer surfaces, such as their bottom, top and sides, consist of a more densely packed fibre network. The formation of a densified layer on the faces of the individual units deposited in the first foam deposit makes the individual units stronger and prevents them from collapsing during subsequent foam deposits when wet foam is deposited between the individual units. The dried individual units have a densification strength of 10-60 kg / m 3 Or 20~50kg / m 3 In one embodiment, the dry individual units may have an overall density of 10 to 80 kg / m 3 The total density may be 0.01 to 0.01.

[0021] In an alternative embodiment, the individual units may be made by extruding or casting a wet foam, drying the wet foam to obtain a dry foam, cutting the dry foam into individual units, and depositing the individual units onto a surface. Preferably, the wet foam is extruded into a board, plate, bar or rod. The board, plate, bar or rod may be cut into individual units that may be deposited onto a surface.

[0022] In some embodiments, the individual units of foam are at least partially dried when the wet foam of the subsequent foam stack is deposited, hi some embodiments, the individual units of foam are completely dried when the wet foam of the subsequent foam stack is deposited.

[0023] Each individual unit may have a three-dimensional shape, such as a cylinder or a polyhedron. Examples of symmetric polyhedrons are cubes, rectangular prisms, and hexagonal prisms. Small variations in the symmetry of the individual units may exist without changing their primary purpose of providing stability to the form. For example, cylinders, cubes, rectangular prisms, and hexagonal prisms may be slightly distorted such that their opposing bases are not always exactly parallel and overlap each other. In some embodiments, each individual unit has the shape of a cylinder. The term "cylinder" is used herein for a geometric figure generally defined as a closed solid having two primarily parallel congruent circular or elliptical bases connected by a curved surface. In some embodiments, each individual unit has the shape of a rectangular prism.

[0024] The individual units obtained in the further foam deposition may have the same or different three-dimensional shape as the individual units obtained in the first deposition.

[0025] As used herein, the height of an individual unit is measured perpendicular to the surface on which the unit is deposited. As used herein, the width, or the average of the length and width, is measured at the bottom of the individual unit. The bottom of the individual refers to the part of the individual unit closest to the surface on which the individual unit is deposited. For circular individual units, the width corresponds to the diameter of the individual unit. Depending on the shape of the individual unit, the width of the individual unit may be the same or different along the entire height of the individual unit.

[0026] The width of an individual unit may be 0.5 to 3 times its height, or 0.5 to 2 times, or 0.5 to 1.5 times, or 0.8 to 3 times, or 0.8 to 2 times, or 0.8 to 1.5 times, or 1 to 3 times, or 1 to 2 times, or 1 to 1.5 times its height.

[0027] In some embodiments, the width of an individual unit is less than 1.3 times its height, such as 1.2 times its height, or 0.5-1.3 times, or 0.5-1.2 times, or 0.7-1.3 times, or 0.7-1.2 times, or 0.9-1.3 times, or 0.9-1.2 times. When the width of an individual unit is less than 1.3 times its height, the total drying time of the wet foam relative to the solid foam is reduced.

[0028] The term "total drying time" as used herein refers to the total time it takes for the foam to dry, i.e., the sum of the drying time of the individual units and the drying time of the foam matrix.

[0029] In some embodiments, the width of the individual units is greater at the bottom of the individual units than at the top. For example, the individual units may have the shape of a cone, a truncated cone, a pyramid, or a truncated pyramid. In some embodiments, each individual unit has the shape of a pyramid.

[0030] In some embodiments where the width of the individual units is greater at the bottom of the individual units than at the top of the individual units, the bottom portions of adjacent individual units may be partially in contact with each other. The contact may be along the entire perimeter of the bottom portions of the individual units, or along a portion of the perimeter. In the remaining height, i.e., the portion of the height of the individual units located above the bottom portions, the individual units are separated from each other, i.e., not in contact. For example, an individual unit may be partially in contact with one or several adjacent individual units along less than 30%, or less than 20%, or less than 10%, or less than 5% of the height of the individual units, as measured perpendicularly from the surface on which the individual units are deposited. The size and shape of the individual units depend on the deposition technique, although the individual units are deposited as separate units. The bottom portions of the deposited individual units may be made to flow outward when a shear stress large enough to overcome the yield stress of the wet foam in the individual units is applied. Thus, the bottom portions of adjacent individual units may be in contact with each other. Also, in embodiments where the individual units are partially in contact, the individual units are distinguishable from each other both before and after drying. No mixing occurs between the wet foam from the individual units at different contact points.

[0031] A subsequent deposition of wet foam is performed between the already dried individual units. The height of the wet foam in this subsequent deposition may be slightly higher than or equal to the height of the individual units. In some embodiments, after said deposition of foam between the individual units, the height of the individual units may be 90-100%, or 95-100%, or 98-100% of the height of the foam matrix surrounding said individual units.

[0032] In some embodiments, the height of the individual units may be 70-100%, or 70-95%, or 75-90% of the height of the foam matrix surrounding said individual units. In embodiments where the height of the individual units is less than the height of the surrounding foam matrix, a smooth upper surface of the solid foam is obtained. In such embodiments, the individual units are not visible from the upper surface of the solid foam because they are covered by the foam matrix.

[0033] In some embodiments, a deposition of wet foam may also be performed on top of the individual units, either simultaneously with the preceding deposition of wet foam of the individual units, or in successive depositions. The surface of the subsequent deposition of wet foam may be scraped to provide a flat surface prior to drying.

[0034] The wet foam in the subsequent deposition may be a fiber foam. The wet foam may comprise at least 10% by weight of cellulose fibers, or at least 11% by weight of cellulose fibers, calculated on the total weight of the wet foam. The wet foam may comprise 10-40%, 11-40%, 10-30%, 11-30%, 12-30%, 10-20%, or 11-20%, or 12-20% by weight of cellulose fibers, calculated on the total weight of the wet foam. The cellulose fibers used in the subsequent deposition are suitably selected from different types of wood pulp, regenerated cellulose fibers, or plant fibers such as fibers derived from bamboo, cotton, hemp, flax, and jute. Preferably, the cellulose fibers are selected from wood pulps, such as softwood kraft bleached pulps, chemical thermomechanical pulps (CTMPs), dissolving pulps (e.g. bleached wood pulps or cotton linters), and hardwood pulps, more preferably softwood kraft bleached pulps and chemical thermomechanical pulps, and most preferably softwood kraft bleached fibers. The cellulose fibers used in the subsequent stacks may be of the same type as the cellulose fibers used in the previous stacks, i.e. the individual units of the first and optionally further stacks.

[0035] The wet foam used in the subsequent deposition is 70-600 kg / m 3 , or 100-500kg / m 3 , or 100-400kg / m 3 , or 125-375 kg / m 3 , or 140-375 kg / m 3 The solid foams prepared by the method according to the invention may have a density of 10 to 60 kg / m 3 , or 20-50kg / m 3 In one embodiment, the solid foam prepared by the process according to the invention may have a density of from 10 to 80 kg / m 3 The density of the sintered body may be 0.01 to 0.01.

[0036] In wet foams, resisting forces hold the cellulose fibers in place (illustrated in FIG. 3b, A). During drying, the water level between the fibers drops and capillary forces build up inside the foam material, causing the fibers to slip when the capillary forces exceed the resisting forces (FIG. 3b, B). As the water evaporates, the resisting forces increase, packing the fibers closer together than before drying and causing the material to shrink (FIG. 3b, C). At a macroscopic level, the foam geometry influences the direction and magnitude of the tension vectors that build up in the material during drying. Contact points such as frames or perforated surfaces introduce tensions in opposing directions, affecting the net tension. Deformations such as shrinkage occur when the net tension, i.e., tension vector, prevails in any particular direction as illustrated for the comparative foams without discrete units in FIG. 2 and FIG. 6a. Thus, the ratio of width or surface area to height of the cellulose foam board influences the distribution of net tension in the foam material as it dries, the larger the ratio, the greater the net tension that is produced.

[0037] When the foam material is prepared by the method of the present invention, the net tension that occurs in the material during drying is reduced, and therefore the shrinkage of the material can be mitigated. Figure 4 illustrates one embodiment of the method of the present invention, in which wet fiber foam is deposited on a surface as small individual units to obtain a first deposition (i). Each individual unit has a low aspect ratio of width to height, which eliminates or greatly reduces the shrinkage of each individual unit when the individual units are dried (ii). When the individual units are dried, they consist of a core containing a uniform fiber network and a densified outer surface (i.e., top, bottom, and side portions) (iii). Thus, a subsequent deposition of wet foam is made on the surface between the already dried individual units (iv). When the wet foam of the subsequent deposition is dried (v), the individual units of the first deposition already distributed on the surface reduce the width to height ratio of the wet foam in the subsequent deposition. The tensions of each individual unit act against each other, thus reducing the net tension in the foam of the subsequent stack, reducing tension build-up during drying and thus mitigating the effect of shrinkage on the overall dimensions of the resulting solid foam (vi). Thus, the method of the present invention results in the formation of solid foam bodies with reduced shrinkage, particularly in the z-direction. Furthermore, the method allows for the formation of foams without the need to use molds with walls, meaning that very large objects such as boards or planks for use in large buildings and other large structures can be produced by this method.

[0038] The size of the solid foam body produced may depend on the number of individual separate units and the distance between them that can be arranged in the first and, optionally, further stacks. The method allows the production of foam boards of at least 60*60 cm, or at least 100*100 cm, or at least 200*200 cm, or at least 300*300 cm, or at least 400*400 cm. The thickness of the foam board may range from 1 to 20 cm, or 1 to 10 cm, or 1 to 5 cm. In one example, the foam board has a thickness of 5 cm. The thickness of the foam board corresponds to the height of the deposited foam. The number of deposited individual units depends on the size of the foam board produced, the size and spacing of the individual units, and the deposition method. For example, a 100*100 cm foam board may contain at least 100 individual separate units. The multi-step deposition method according to the invention allows the production of foams with reduced shrinkage compared to similar foams obtained in a single-step deposition. The method also allows processes involving continuous formation of foams, such as continuous foam webs.

[0039] The present invention provides a low density cellulose foam comprising individual units of cellulose foam having stiffer densified cellulose fiber walls embedded in a cellulose foam matrix. Incorporation of the individual units as structural elements in a solid foam allows for the formation of a stiffer foam while maintaining the same low density. The individual units comprise 30-80% of the total volume of the foamed material including the individual units and the surrounding foam matrix, or 40-80% or 50-80% of the total volume, or 60-80% of the total volume, or 40-78% of the total volume, or 50-78% of the total volume, or 40-75% of the total volume, or 50-75% of the total volume. In one embodiment, the individual units comprise 10-90% of the total volume of the foamed material including the individual units and the surrounding foam matrix.

[0040] In one embodiment, the density of the individual units of the foam is greater than the density of the foam matrix. For example, the density of the individual units in the solid foam may be at least 110%, such as 130%, or 150% or 200% greater than the density of the foam matrix. In one embodiment, the density of the individual units in the solid foam may be in the range of 105-500%, or 110-330%, or 110-250%, or 110-200%, or 150-330% greater than the density of the foam matrix.

[0041] It has been found that if the wet foam density of the foam used for the individual units is higher than the wet foam density of the foam used for the surrounding foam matrix, the total drying time of the foam is reduced, thus allowing for a more efficient process. Furthermore, the high density regions of the foam (i.e., the individual units) have different properties, such as stiffness, compared to the low density regions (i.e., the foam matrix). This can be used to provide foams with different properties in different regions.

[0042] In embodiments of the present invention where the density of the wet foam in the first deposition is higher than that of the wet foam in the subsequent depositions, it has been found that the total drying time of the foam is influenced by the density of the wet foam in the first and subsequent depositions. If a cellulose foam with density D is desired, this can be provided by making the foam in the first deposition and the subsequent depositions the same density to achieve a solid cellulose foam with density D after drying. Alternatively, density D can be obtained by using a wet cellulose foam with a higher density for the first deposition and a wet cellulose foam with a lower density for the subsequent depositions, selected such that the average density of the solid cellulose foam is equal to D after drying. The total drying time of the cellulose foam is shorter when the wet foam in the first deposition has a higher density than the wet foam in the subsequent depositions, compared to a foam prepared using a wet foam with the same density for both the first and subsequent depositions. The drying time of the first deposition is always significantly faster than the drying time of the subsequent depositions, since most of the surfaces of the individual units of the first deposition are exposed to air. Because lower density wet foams dry faster, using a lower density wet foam for subsequent deposits can reduce the drying time of the subsequent deposits, and therefore the total drying time of the solid cellulose foam. Thus, in some cases, it is advantageous to use a higher density wet foam for the first deposit and a lower density wet foam for subsequent deposits, with a relatively short drying time.

[0043] The wet foam used for deposition in the method according to the invention can be prepared by mixing cellulose fibers and one or more thickening agents in water to obtain a flowable fiber mixture, adding a surfactant mixture, and stirring to obtain a wet foam. The mixture of cellulose fibers and one or more thickening agents in water can form a fully flowable, non-agglomerated fiber paste. The mixture can be aerated by adding a surfactant mixture and agitating, such as by mechanical stirring. The aeration can form fine, micron-sized bubbles that separate the fibers. The micron-sized bubbles stabilize the foam and contribute to maintaining the shape during drying of the individual units.

[0044] Because the cellulose fibers are mixed at a high concentration, no drainage step is necessary, which reduces the overall time and cost of the process and prevents leakage of water-soluble materials added during production, thereby allowing a higher concentration of water-soluble additives in the final foam.

[0045] The performance of the dry foam can be adjusted by the amount of thickener used and thus the fiber-fiber bond strength of the dry material. The amount of thickener can be 4-24% or 5-20%, calculated per weight of solids in the foam. The method according to the invention allows the adjustment of the stability of the wet foam using a combination of thickeners and more stable surfactants, making it possible to provide a self-supporting cellulose foam. Furthermore, the method allows the simple inclusion of different types of additives.

[0046] The density of the wet foam depends on the amount of air included during foaming. If a low density is desired, relatively more air should be included. If a high density is desired, relatively less air should be included. The density of the wet foam directly affects the density of the solid foam after drying. Thus, the density difference between the wet foam of the first and subsequent deposits remains in the solid foam.

[0047] During drying of the individual free-standing foam units, a densified layer forms on the surface of the foam of each individual unit, helping to maintain the shape of the individual units. After the free-standing individual units of foam are dried, new wet foam can be added to fill the gaps between the dried individual units (exemplified by (iv) in Figure 4). This allows for the creation of low density cellulose foam divided into individual units with stiffer densified cellulose fiber walls. By incorporating densified thin layers, the final foam can be made stiffer while maintaining the same light weight. Additionally, this process results in many units of wet foam with a lower aspect ratio of width to height, minimizing the effects of tension build-up during drying and, consequently, mitigating the effects of shrinkage on the board's overall dimensions (Figure 6).

[0048] The foam composition suitable for preparing the solid foam by the method according to the present invention is a self-supporting, non-disintegrating foam. Other foams, such as those described in WO2016068771, WO2016068787, and WO2020011587, and the porous solid materials can be used for extrusion and cutting into individual units, or for stacking between individual units.

[0049] In one embodiment, a coating is applied to the exterior surface of the individual units prior to depositing the subsequent foam stack. The coating is applied to the exterior surface of the individual units when the individual units are at least partially dried, such that when the coating is applied, the exterior surface of the individual units includes a densified layer. Alternatively, the coating may be applied to the exterior surface of an individual unit that is completely dried. The coating may be applied to at least one exterior surface of a solid foam. Thus, in one embodiment, the coating is applied to at least one exterior surface of the individual units and / or the solid foam.

[0050] The coating is preferably applied in the form of a liquid coating composition and one or several coating layers can be applied. The composition of the coating layers can be the same or different. If applied to individual units, the coating is preferably dried before depositing the subsequent foam deposit. The coating may comprise at least one granular material and at least one film-forming material. The granular material can be selected from at least one of microfibrillated cellulose (MFC), cellulose fibers, or mineral particles such as clay or calcium carbonate. MFC means, in the context of the present application, cellulose particles, fibers or fibrils having a width or diameter between 20 nm and 1000 nm. The film-forming material can be selected from at least one of carboxymethyl cellulose (CMC), cellulose ethers, starch, polyvinyl alcohol, or synthetic latexes such as acrylic or styrene-butadiene latexes. The coating may include at least one hydrophobic agent selected from at least one of a wax, such as, for example, beeswax or carnauba, an alkyl ketene dimer (AKD) or an alkyl succinic anhydride (ASA).

[0051] Application of the coating reduces the breathability of the foam as the pores on the surface of the foam are blocked by the coating. This facilitates various processing and conversion operations, including vacuum. Additionally, depending on the type of coating, application of the coating may alter properties such as strength and hydrophobicity of the foam. The coating is preferably applied to the surface that includes the densified layer.

[0052] In one embodiment, a coating composition comprising the MFC is applied to the exterior surface of the at least partially dried individual units and a coating composition comprising the hydrophobic agent is applied to the exterior surface of a solid foam comprising the individual units embedded in a foam matrix, i.e., after subsequent deposition and drying steps.

[0053] The coating composition can be applied using any suitable method used for coating, such as roller coating, blade / knife coating, brushing, flexographic roller, and spray coating.

[0054] In another aspect, the invention relates to a solid foam comprising individual units of foam and a foam matrix surrounding said individual units. Each individual unit may be surrounded by a densified layer of foam. The entire solid foam, as well as the individual units and the foam matrix, may comprise 75-95% by weight, or 80-95% by weight, or 85-92% by weight, or 85-90% by weight of cellulose fibres, calculated on the total weight of the dry foam. The height of the individual units may be 90-100%, or 95-100%, or 98-100% or equal to the height of the entire solid foam prepared. In one embodiment, the height of the individual units may be 70-100%, or 70-95%, or 75-90% of the height of the entire solid foam prepared. The solid foam may have a mass of 10-60 kg / m 3 , or 20-50kg / m 3 In one embodiment, the solid foam may have a density of 10 to 80 kg / m 3 The density of the sintered body may be 0.01 to 0.01.

[0055] The density and properties of the final solid foam can be adjusted by using individual units having the same or different density as the foam matrix. The individual units may also have densities that may differ from each other to provide a solid foam with different densities at different locations. The density of the individual units in the solid foam may be 83% to 500%, or 60% to 150%, or 90% to 110% of the density of the foam matrix surrounding said individual units. Preferably, the individual units have a density of 90% to 110% of the density of the foam matrix surrounding said individual units.

[0056] In one embodiment, the density of the individual units of the foam is higher than the density of the foam matrix. For example, the density of the individual units in the solid cellulose foam may be at least 110%, such as 130%, or 150% or 200% higher than the density of the cellulose foam matrix. In one embodiment, the density of the individual units in the solid cellulose foam may be in the range of 105-500%, or 110-330%, or 110-250%, or 110-200%, or 150-330% higher than the density of the cellulose foam matrix.

[0057] Each individual unit may have a three-dimensional shape, such as a cylinder or polyhedron. Examples of symmetric polyhedrons are cubes, cuboids (such as rectangular cuboids), and hexagonal prisms. Small variations in the symmetry of the individual units may exist without changing their primary purpose of imparting stability to the form. In some embodiments, each individual unit has the form of a cylinder. The diameter, or width, of an individual unit may be 0.5 to 3 times its height, or 0.5 to 2 times its height, or 0.5 to 1.5 times its height, or 0.8 to 3 times its height, or 0.8 to 2 times its height, or 0.8 to 1.5 times its height, or 1 to 3 times its height, or 1 to 2 times its height, or 1 to 1.5 times its height.

[0058] In one embodiment, the width of each individual unit is less than 1.3 times its height, such as less than 1.2 times its height, or 0.5-1.3 times, or 0.5-1.2 times, or 0.7-1.3 times, or 0.7-1.2 times, or 0.9-1.3 times, or 0.9-1.2 times. If the width of each individual unit is less than 1.3 times its height, the drying time of the foam in the individual units is reduced, and thus the total drying time of the foam is also reduced. The widths of the individual units in the foam may also differ from each other to provide a solid foam with different properties at different positions. However, it is still preferred that the width of each individual unit in the foam is less than 1.3 times its width.

[0059] In some embodiments, the width of the individual units is greater at the bottom of the individual units than at the top. For example, the individual units may have the shape of a cone, a truncated cone, a pyramid, or a truncated pyramid. In such embodiments, the bottom portions of adjacent individual units may be partially in contact. The contact may be along the entire perimeter of the bottom portion of the individual units, or along a portion of the perimeter. In the remaining height, i.e., the portion of the height of the individual units located above the bottom portion, the individual units are separated from each other, i.e., not in contact. For example, an individual unit may be partially in contact with one or several adjacent individual units along less than 30%, or less than 20%, or less than 10%, or less than 5% of the height of the individual units, as measured perpendicularly from the surface on which the individual units are deposited. The individual units are visually distinguishable from each other in the solid form.

[0060] In one embodiment, at least one exterior surface of the individual units and / or solid foam is provided with a coating, the presence of which can affect properties such as strength, hydrophobicity, breathability and surface gloss of the solid foam.

[0061] In a further aspect, the present invention relates to a solid foam prepared by the process according to the invention.The foam according to the invention can be used in large sheets.

[0062] The present invention will now be illustrated by the following examples, which are not intended to be limiting in any way. All citations and references mentioned herein are incorporated by reference in their entirety. EXAMPLES

[0063] Example 1 Multi-step deposition For the first deposition of wet foam in the individual units, a homogeneous wet paste was prepared containing 12% by weight of cellulose pulp and thickener in water. 3 The paste was aerated with the surfactant mixture until a wet foam density of 0.01 g was obtained.

[0064] Twelve individual units of foam were deposited on a flat surface (oven tray with a frame of dimensions 27*37*5 cm) using a cylindrical mold with dimensions 5 cm height and 6.6 cm diameter. The mold is used only to deposit the foam in the desired shape and dimensions and is removed before drying. The individual units were dried in a conventional convection oven at 120°C for 1-2 hours.

[0065] For the second stack, a new batch of wet foam was prepared as described above for the wet foam used in the first stack. The wet foam was filled into the spaces between the individual units, not eliminating any voids in the oven frame. The surface was scraped to remove excess foam and to level the surface to the height of the frame. Finally, the foam was dried in an oven at 120°C for 8 hours.

[0066] The density of the dry individual units and the final foam mass is approximately 30-31 kg / m 3 The final thickness of the foam body was about 5 cm and no shrinkage was observed.

[0067] Example 2 (Comparative Example) Single-step deposition A homogeneous wet paste was prepared containing 12% by weight of cellulose pulp in water and thickener. 3 The paste was aerated with the surfactant mixture until a wet foam density of 0.01 g was obtained.

[0068] A 27*37*5cm mould is filled with aerated wet foam, the surface is scraped to remove excess foam and the surface is levelled to the height of the frame. Finally, the foam is dried in a conventional convection oven at 120°C for 8 hours.

[0069] The density of the dry foam is approximately 30 kg / m 3and 20% shrinkage was observed in the center of the foam body. Shrinkage is calculated based on the difference in thickness between the center of the foam and near the edge of the foam.

[0070] Example 3 Preparation of foam bodies A homogeneous wet paste was prepared containing 12% by weight of cellulose pulp in water and a thickener. A surfactant mixture was added to the paste and the resulting mixture was extruded and simultaneously aerated to produce a cellulose pulp having a density of 222 kg / m 3 The extruded wet foam was obtained. The extruded foam was dried to obtain a foam body. The foam body was cut into 4*4cm square pieces and about 5cm high. The square pieces were distributed over a 27*37*5 mold. The mold was filled with the wet foam prepared as described in Example 1. Then, the surface was scraped to remove excess foam and flattened to the height of the frame. The foam body was then dried at 120°C for 8 hours in a conventional convection oven.

[0071] The density of the dry cut individual units and the density of the final foam is approximately 30-31 kg / m 3 The final thickness of the foam body was about 5 cm and no shrinkage was observed.

[0072] Experimental Method, Examples 1-3 Characterization The comparative foam prepared according to Example 2 exhibits densification on the top, bottom and sides of the sheet (as illustrated in FIG. 1A), while the core of the foam sheet is a uniform fiber network of lower density (FIG. 1B).

[0073] The foam produced according to Example 1 also exhibits densification on the top, bottom and sides of the sheet (as illustrated in FIG. 5A), but the bulk of the material contains individual units of foam that are distinguished from the foam matrix by boundary walls of densified cellulose fiber material (FIG. 5B).

[0074] Compression curves were obtained for the multi-step deposited foam according to Example 1 and the single-step deposited foam prepared according to Example 2 (Figure 7). The dried solid foam was cut into 10 cm square specimens with a height of 5 cm. Compression tests were performed using an Instron 5969 universal testing machine in a conditioned room at 23°C and 50% relative humidity. The specimens were conditioned for 48 hours at 23°C and 50% relative humidity before testing. A 500 N load cell with a diameter of 15 cm was used with a compression rate of 100% / min of the original specimen thickness. The final strain was selected to be 70% of the original specimen height. The multi-step deposited foam provides improved energy absorption as demonstrated by the larger area under the solid curve compared to the area under the dashed curve obtained for the single-step deposited foam.

[0075] Example 4 Individual unit width-to-height ratio A homogeneous wet paste was prepared containing 12% by weight of cellulose pulp and thickener in water. 3 The paste was aerated with the surfactant mixture until a wet foam density of 43*24.5cm was obtained. The foam was filled into a frame on a perforated tray and the surface scraped to remove excess foam and level the surface to the height of the frame. A height setting frame was used to prepare foams 2.1cm high. Squares of wet foam were cut and removed from the foam, leaving a pattern of deposited material in a precise square pattern with every second square unfilled, filling only the white squares of the chessboard. Square widths used were 2.5cm, 5cm and 10cm. The frame was large enough to allow several repetitions of the pattern 43*24.5cm.

[0076] The wet foam of Example 4 was used in a drying study in which three scenarios were tested. (1) The material is divided into uniform square pieces and piled up and dried. (2) Fill each of the gaps between the squares with new foam, scrape the surface again, and allow the second stack to dry. (3) The entire frame is filled with one single layer, scraped, and allowed to dry (Comparative Example).

[0077] The foam was weighed as it dried at regular intervals and plotted as drying curves. These drying curves can be seen in Figures 8a-b. From Figure 8a it is clear that the drying time decreases as the width of the individual units decreases. This is due to the smaller volume of the individual units. It is also clear that the drying time of the single step stack is significantly longer. From Figure 8b it is clear that the drying time of the subsequent foam stack surrounding the individual unit is also faster than the drying time of the single step stack. Thus, the total drying time, i.e. the combined drying time of the first and subsequent foam stacks, is shorter when individual units having a width less than 1.3 times their height are used compared to the single step stack.

[0078] Example 5 Foam density and foam matrix of individual units A homogeneous wet paste was prepared containing 12% by weight cellulose pulp and thickener in water. The paste was aerated with surfactant mixture until the desired wet foam density was obtained. Different densities of wet foam (see Table 1) were prepared. Four individual units of foam were deposited on a flat surface (oven tray with frame of dimensions 20*20*5 cm) using cylindrical molds with dimensions 5 cm height and 6.6 cm or 9.2 cm diameter. Four deposits with 6.6 cm diameter molds cover approximately 1 / 3 of the frame volume, while the 9.2 cm diameter mold covers 2 / 3 of the frame volume. The molds were symmetrically positioned with equal distances between each mold and the frame wall. The molds were used only to deposit the foam in the desired shape and dimensions and were removed before drying. The individual units were dried in a conventional convection oven at 120° C. until completely dry. Once the first layer of individual units had dried and were allowed to cool to room temperature for an extended period, a second layer was applied to fill the gaps between the individual units. The frame was then completely filled, scraped, and dried in a 120°C oven until completely dry.

[0079] The foam in the second pile typically had a different density than the foam in the first pile. The aim was to reach a similar end density of the dry material, so the wet foam density was used to always put approximately the same total amount of material in the frame. The wet foam densities used are listed in Table 1. TIFF2024545323000002.tif80170

[0080] It has been found that the total drying time of the foam in the frame is shorter when the foam in the first pile has a higher density than the foam in the second pile.

[0081] Specimens were also made in the same frame 20x20x5cm with foam deposited in a single step. These specimens were used as reference materials to compare the cushioning properties of the two-step deposited material with the single-step deposited material. These specimens were made in a range of densities.

[0082] The specimens were conditioned at 23° C. and 50% RH (relative humidity) for 3 days and used as specimens for the drop test.

[0083] The results of the drop tests, degradation and peak acceleration as a function of specimen density can be seen in Figure 9 (degradation), Figure 10 (peak acceleration for drop 1) and Figure 11 (peak acceleration for drops 2-5). The different foams used in the tests are further explained in Table 1. To explain the symbols used, the (□) in the figures represents a 84 kg / m2 load on a discrete unit of 6.6 cm diameter. 3 and a first layer of foam having a density of 268 kg / m 3 The dotted symbols are the reverse, and in the same example, the dotted white squares represent cellulose foam formed from a first deposit of 268 kg / m2 foam with a density of 1.0 kg / m3 deposited in a separate unit with a diameter of 9.2 cm. 3 and a first layer of foam having a density of 84 kg / m 3 and a second deposit of foam having a density of 100.degree.

[0084] Experimental Method, Example 5 Drop test data was generated using a "TrueDrop-160" free-fall tester testing a 20x20x5cm piece of material. The material is placed in the bottom of a cardboard box with supporting PE foam on the sides to hold it in place horizontally. A 4.8kg metal weight is placed on top of the material. The metal weight has a base of 20x20cm, thus distributing a load of 12g / cm2 to the specimen. The metal weight is also held in place horizontally by the supporting PE foam. An accelerometer is placed on top of the metal weight to record the acceleration while the drop test is performed. When performing the drop test, the box containing the specimen and metal weight is dropped in a straight vertical direction, allowing a free fall of 76cm before impacting the metal floor. This is performed five times per specimen, with two results recorded. 1: Peak Acceleration: The maximum acceleration recorded by the accelerometer placed on the metal weight. High acceleration occurs when the test material uses its cushioning capacity to break the fall of the metal weight after impact. Low peak acceleration is considered a positive result for a cushioning material such as cellulose foam according to the present invention. 2: Degradation: The difference between the starting height of the specimen before the drop and the height of the specimen after five consecutive drops. The height was measured with a caliper measuring in five positions, one on each side and one in the center, to calculate the average height. Degradation = 1-(height after 5 drops / starting height).

[0085] For a piece with an initial height of 5cm and a height of 4cm after 5 consecutive drops, the degradation will be: 1-(4 / 5)=20%

[0086] Measurements were performed on the specimens after conditioning them at 23° C. and 50% RH (relative humidity) for 3 days.

[0087] The results (FIGS. 9-11) show that the cellulose foam produced using the two-step deposition method has better cushioning performance than the cellulose foam produced from one-step deposition. The foam produced using the two-step deposition method according to the invention behaves similarly to the cellulose foam produced using a first deposition with a lower density compared to the second deposition, or to foams produced with similar densities in both depositions. This demonstrates that the reduction in drying time does not affect the cushioning performance of the cellulose foam.

[0088] All combinations of different densities for the first and second deposits performed better than single step deposited cellulose foam when it came to reducing degradation (relative compaction after 5 drops). All combinations of different densities for the first and second deposits also performed similarly to the single step deposited plates for peak acceleration of the first drop, and reduced the average peak acceleration for drops 2-5 compared to the single step deposited material for drops 2-5. These comparative statements refer to comparing materials having similar densities.

[0089] Example 6 Preparation of foam board A composition containing 14% pulp in water, 1.6% CMC and 0.08% surfactant (all amounts based on the total weight of the composition) was foamed to a wet foam using mechanical agitation. The wet foam had a foaming capacity of 182 kg / m 3and a final dry content of 15.7%. The first deposition of foam was deposited using a depositor capable of distributing the foam as individual units onto a flat metal tray. The individual units of foam were deposited in a pyramidal shape. The individual units were deposited in rows along the tray, spaced 75 mm apart widthwise and 37.5 mm apart lengthwise (45° offset between rows). The flow profile of the foam within the individual units was adjusted to flow outwards or build height on the tray by adjusting the depositor output pressure and the distance to the tray. High pressure was applied during deposition to cause the foam at the bottom portion of each individual unit to flow and begin to spread and contact adjacent individual units. After the first deposition, the wet foam was dried using a convection dryer at 80° C. until the foam was completely dry. The individual units were distinguishable from each other after drying, even at the contact points between adjacent individual units. No intermixing was observed at the contact points. After the first drying step, the voids between the dried individual units were filled with wet foam having the same composition as used for the individual units. The same stacker was used to form a wet foam board, adjusting the pressure so that all gaps between the individual units were filled. The stacked wet foam boards were then scraped to give the boards a flat surface. The size of the foam board was 80*120cm. The thickness of the foam board was 5cm, measured from the tray. The foam board was then dried at 80°C using a convection dryer until it was completely dry.

Claims

1. 1. A method for preparing a solid foam, comprising depositing individual units of foam on a surface to obtain a first foam deposit, depositing wet foam between the individual units to obtain a subsequent foam deposit, and drying the wet foam to obtain a solid foam, wherein the individual units of foam are embedded in a foam matrix.

2. 10. The method of claim 1, wherein the height of the individual units is 70 to 100% of the height of the wet foam in the subsequent foam stack.

3. 10. The method of claim 1, wherein the wet foam in the subsequent foam deposition comprises at least 10% by weight of cellulose, calculated relative to the total weight of the wet foam.

4. 10. The method of claim 1, wherein the discrete units are obtained by dispensing the wet foam as discrete units onto a surface and subsequently drying the wet foam.

5. 10. The method of claim 1, wherein the individual units are obtained by extruding a wet foam, drying the foam, cutting the dried foam into individual units, and depositing the individual units onto a surface.

6. 5. The method of claim 4, wherein the wet foam used in the individual units comprises at least 10% by weight of cellulose, calculated relative to the total weight of the wet foam.

7. Wet foam: 70 to 600 kg / m ３ The method of claim 1 , wherein the granular material has a density of

8. 10. The method of claim 1, wherein the solid foam comprises 75 to 95% by weight of cellulose fibers, calculated relative to the total weight of the foam.

9. Solid foam: 10 to 60 kg / m ３ The method of claim 1 , wherein the granular material has a density of

10. 10. The method of claim 1, wherein the density of the foam in the individual units is greater than the density of the foam matrix.

11. 10. The method of claim 1, wherein the width of each individual unit is less than 1.3 times its height.

12. The method of claim 1 , wherein the coating is applied to at least one exterior surface of the individual units and / or solid forms.

13. 1. A solid foam comprising individual units of foam embedded in a foam matrix, characterized in that the solid foam comprises at least 75-95% by weight of cellulose fiber, calculated on the total weight of the foam.

14. 14. A solid foam according to claim 13, wherein the height of the individual units is 70 to 100% of the height of the foam matrix.

15. Solid foam: 10 to 60 kg / m ３ 14. The solid foam of claim 13 having a density of

16. 14. The solid foam of claim 13, wherein each individual unit is surrounded by a densified layer of foam.

17. 14. The solid foam of claim 13, wherein the density of the foam in the individual units is greater than the density of the foam matrix.

18. 14. The solid foam of claim 13, wherein the width of each individual unit is less than 1.3 times its height.

19. 14. A solid foam according to claim 13, wherein the individual units and / or the outer surface of the solid foam are provided with a coating.

20. A solid foam prepared by the method of claim 1.

21. 14. Use of the solid foam of claim 13 in packaging or large structures or as a growing medium for hydroponic plants.

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