Improved composites for making coffee capsules

Abstract

The invention relates to a method for producing a fibre-based composite material, to the fibre-based composite material thus obtained, and to the use of the fibre-based composite material for producing a capsule for use in a capsule coffee machine.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a fiber-based composite material, the fiber-based composite material obtained therefrom, and the use of the fiber-based composite material in the preparation of capsules for use in capsule coffee machines. Background Technology

[0002] The sol-gel method has been extensively studied in the packaging field because it offers precise control over film thickness, composition, and functional properties, making it attractive for packaging applications. It enables the fabrication of films and coatings with enhanced barrier properties, optical transparency, and additional functionalities such as antimicrobial activity and self-cleaning capabilities (Applied Surface Science, Vol. 351, October 1, 2015, pp. 897-903).

[0003] Sol-gel coatings have the potential to contribute to sustainable packaging solutions. Compared to traditional coating methods, sol-gel methods are generally more environmentally friendly because they typically operate at lower temperatures and avoid the use of harmful organic solvents or harsh processing conditions (Encyclopaedia of Physical Science and Technology (3rd Edition), 2003, pp. 257-276). Furthermore, sol-gel coatings are compatible with bio-based (Progress in Organic Coatings, Vol. 77, No. 6, June 2014, pp. 1111-1116) or recyclable packaging materials, supporting the development of environmentally friendly packaging systems.

[0004] This method has been extensively studied for developing barrier coatings for packaging materials. By depositing thin sol-gel films on substrates, their barrier properties against oils, moisture, oxygen (Polymer Vol. 195, May 8, 2020, 122437) and other gases can be enhanced. These coatings help extend the shelf life of packaged products by reducing permeation and maintaining product quality.

[0005] Sol-gel coatings can also be customized to provide additional functionality in packaging applications. For example, antimicrobial properties can be incorporated by adding specific nanoparticles or organic compounds to the sol-gel formulation. This helps inhibit the growth of bacteria or fungi on the packaging surface, aiding in food preservation and reducing the need for chemical preservatives (FoodChem. Dec 15, 2014; 165:342-7).

[0006] The sol-gel process also offers the advantage of producing transparent coatings and films. This property is particularly useful in packaging applications where visibility or light transmission is critical, such as for display packaging (transparent packaging) or UV-blocking films for photosensitive products.

[0007] The adhesion between sol-gel coatings and substrates is crucial for ensuring the durability and performance of packaging materials and the safety of packaged foods. Researchers have explored various strategies to improve the adhesion of sol-gel coatings, including surface pretreatment, modification of sol-gel formulations, and the use of adhesion promoters. Other applications involve adding varnishes to protect the coating. Such varnishes can be based on polystyrene acrylate materials. However, when the inventors subjected packaging materials made using this method to overnight hexane-acetone extraction, they observed that such methods could lead to material leaching. Residues were found, indicating poor grafting of cellulose.

[0008] Capsule coffee machines, such as Nespresso ® The Original and Verturo series extraction systems are essentially composed of four main components: an aluminum capsule and its aluminum membrane, an extraction cage, puncture needles, and a pyramidal plate. When the capsule is inserted into the extraction chamber, the lever action of the cap forces the capsule into the chamber, and the capsule is held in place between two flanges to prevent it from loosening. Simultaneously, multiple needles puncture the top of the capsule, compressing the bottom aluminum membrane against the pyramidal plate. High-pressure hot water is then pumped into the chamber, submerging the capsule and entering through the holes created by the needles. Water flushes from the outside in to balance the hydrostatic pressure in the container and prevent bursting. It extracts the coffee grounds bed, and when it reaches the bottom membrane, it causes the aluminum membrane to rupture on the pyramidal plate, thus expelling the coffee from the system into the cup, along with its distinctive crema. Therefore, pressure only helps to break the membrane within the pyramidal plate, but there is no overall Δp form outside / inside the capsule. This means the entire capsule is exposed to water at a 90-degree angle.

[0009] When using pure paper capsules instead of aluminum capsules, different things happen. Due to the flexibility of the paper fibers, needle penetration is hindered, and when hot water is injected into the system, the paper capsule swells as the fibers absorb a large amount of water. When water enters through an improperly pierced hole, it begins to extract the coffee, and when it reaches the bottom membrane, the membrane cannot be properly pierced. The expansion of the capsule, combined with the unpierced membrane, forces the membrane to delaminate, resulting in a beverage dispensed from the machine containing entrained coffee particles and less appealing crema. Furthermore, the swollen capsule remains trapped in the extraction chamber and cannot be ejected. Therefore, using compostable paper capsules no longer maintains the overall consumer experience expected with aluminum capsules; the cup is not clean, the crema is inconsistent, and the capsule requires a finger to eject.

[0010] Therefore, the object of the present invention is to provide a fiber-based composite material that resists the expansion of the material due to hot water, thereby allowing capsules made of said material to pop out of the capsule coffee machine without assistance and without being weakened during extraction, thus providing a clean coffee cup.

[0011] Because the inner side of the can is laminated with a polymer, the outer part should also be protected from water to avoid the aforementioned problems. For this purpose, an external inert protection should be applied. In this case, this is achieved by applying a very stable grafted polycrystalline silicon oxide portion to the fiber-based material, which remains firmly attached to the fibers even when exposed to highly corrosive food liquid simulants. This versatile and durable material can be used in a variety of applications, and the present invention covers methods for manufacturing this material and its use in the preparation of capsules for use in capsule coffee machines. Furthermore, due to the very low dosage, properties such as paper recyclability or biodegradability are unaffected. Summary of the Invention

[0012] Therefore, in a first aspect, the present invention provides a method for manufacturing fiber-based composite materials, the method comprising the following steps:

[0013] a) Provide a substrate containing cellulose fibers;

[0014] b) Prepare a mixture comprising one or more alkoxysilane monomers and an acid catalyst in a solvent;

[0015] c) Apply the mixture to the substrate;

[0016] d) React the mixture with the cellulose fibers of the substrate;

[0017] e) subject the mixture to a sol-gel process;

[0018] f) Dry the treated substrate to obtain the composite material.

[0019] In another aspect, the present invention relates to fiber-based composite materials obtained by the method of the present invention described herein.

[0020] In another aspect, the present invention relates to a fiber-based composite material comprising a fiber-based substrate and a silane coating, wherein the coating imparts improved hot water resistance and strength to the fiber-based substrate.

[0021] In another aspect, the present invention relates to the use of the fiber-based composite material described herein for the preparation of capsules used in capsule coffee machines. Detailed Implementation

[0022] This invention provides a fiber-based composite material, which is prepared by chemically bonding a substrate containing cellulose fibers and one or more alkoxysilane monomers using a sol-gel method according to the following steps to add a highly stable polycrystalline silicon oxide layer:

[0023] a) Provide a substrate containing cellulose fibers;

[0024] b) Prepare a mixture containing one or more alkoxysilane monomers and an acid catalyst in water;

[0025] c) Apply the mixture to the substrate;

[0026] d) React the mixture with the cellulose fibers of the substrate;

[0027] e) subject the mixture to a sol-gel process;

[0028] f) Dry the treated substrate to obtain the composite material.

[0029] According to a preferred aspect of the invention, the cellulose fibers are selected from: soft / hardwood cellulose fibers, jute, bagasse, hemp, straw, bamboo, and combinations thereof.

[0030] The bonding occurs by grafting alkoxysilane monomers onto the hydroxyl groups at C2, C3, and C6 of the cellulose monomer, glucose.

[0031] According to a preferred aspect of the invention, the molecular ratio of alkoxysilane to cellulose is in the range of 2:1 to 0.5:1. In an even more preferred aspect of the invention, the molecular ratio of alkoxysilane to cellulose is 3:1.

[0032] The sol-gel method enables the synthesis of solid materials from small molecules. This involves the formation of silanols from alkoxysilanes, and then the formation of polysiloxanes. Initially, a sol is formed, which is a colloidal solution of alkoxysilane particles. These particles begin to aggregate, forming a macroscopic structure. This process leads to gel formation, which is a two-phase system containing both a liquid phase and a solid phase. It involves several steps to transform the sol into a gel and ultimately into a solid material.

[0033] Sol formation :

[0034] A precursor solution, known as a sol, is prepared by dissolving an alkoxysilane in a solvent. This precursor solution may also contain organic compounds or polymers.

[0035] According to a preferred aspect of the invention, the precursor solution comprises an alkoxysilane monomer and an aqueous acid catalyst selected from hydrochloric acid, sulfuric acid, nitric acid, acetic acid, and formic acid.

[0036] gelation :

[0037] Sols undergo gelation to form a three-dimensional network of interconnected particles or polymers. This is achieved through a number of chemical steps, such as hydrolysis (Journal of Applied Polymer Science 108, 1958-1968) and condensation.

[0038] According to the invention, this is achieved by hydrolyzing the alkoxy functional group (R1-O as described below) in the alkoxysilane monomer with an acid catalyst to produce a silanol, and then polycondensing the silanol with itself and with a cellulose-based substrate added as wet pulp or as flat paperboard.

[0039] aging :

[0040] The gel is aged or subjected to further chemical reactions, which leads to the strengthening and stabilization of the gel structure. This step can be carried out at ambient temperature or elevated temperature, depending on the desired properties of the final material.

[0041] According to the invention, the aging of the gel structure is preferably carried out at room temperature or in a cold environment (i.e., provided by an ice bath). In each case, the gel formation rate is affected by temperature.

[0042] dry :

[0043] The wet gel is dried to remove the solvent from its structure. This can be accomplished through evaporation, supercritical drying, or other drying techniques. The drying process is crucial for transforming the gel into a solid material while preserving its structure.

[0044] According to a preferred aspect of the invention, the drying of the sol-gel, releasing ethanol or methanol (1-10 and 11-20 respectively, depending on the type of alkoxysilane monomer used), is preferably carried out in an oven heated to 100°C, more preferably 110°C, and most preferably 120°C. Temperatures above 150°C should be avoided, as they may cause degradation of the cellulose-based substrate. In the case of a continuous coating process, drying is performed via an infrared lamp.

[0045] According to another preferred aspect of the invention, drying the treated substrate to obtain the composite material is carried out over a period of 10 seconds to 5 minutes, more preferably over a period of 2 minutes in a static environment (oven) or over a period of 30 seconds in a dynamic environment (continuous coating), and most preferably over a period of 3 seconds to 5 seconds.

[0046] The alkoxysilane monomers that can be used to produce the fiber-based composites according to the invention are preferably selected from:

[0047] 1. Tetraethyl orthosilicate, 2. Triethoxy(methyl)silane, 3. Triethoxy(ethyl)silane, 4. Triethoxy(propyl)silane, 5. Butyltriethoxysilane, 6. Triethoxy(isopentyl)silane, 7. (3,3-dimethylbutyl)triethoxysilane, 8. Triethoxy(vinyl)silane, 9. 3-(triethoxysilyl)prop-1-amine, 10. 3-(triethoxysilyl)propyl methacrylate, 11. Tetraethyl orthosilicate, 12. Trimethoxy(methyl)silane, 13. Trimethoxy(ethyl)silane, 14. Trimethoxy(propyl)silane, 15. Butyltrimethoxysilane, 16. Trimethoxy(isopentyl)silane, 17. (3,3-dimethylbutyl)trimethoxysilane, 18. Trimethoxy(vinyl)silane, 19. 3-(trimethoxysilyl)prop-1-amine and 20-(trimethoxysilyl)propyl methacrylate.

[0048]

[0049] Applying an acid catalyst to the alkoxysilane monomer prior to reaction with cellulose enhances grafting with cellulose, as demonstrated by reduced leaching. The preferred time for applying the acid catalyst is several minutes, but no more than two hours, before reaction with the substrate containing cellulose-based fibers. Beyond this timeframe, inappropriate grafting occurs due to the significant advancements in sol-gel methods, primarily due to steric hindrance hindering proper grafting of the hydroxyl groups to cellulose. Additionally, cooling the alkoxysilane monomer in solution with acid prolongs the application time, providing a longer window before inappropriate grafting occurs.

[0050] The level of silicon-cellulose grafting is crucial for achieving the desired modification of the physicochemical properties of substrates containing cellulose fibers. The purpose of the treatment in this invention is to enhance the hydrophobicity of substrates containing cellulose fibers via a sol-gel method. This modification enables their application in moisture-proof packaging materials.

[0051] For example, composite materials made of cellulose with enhanced hydrophobicity can be used in water-retaining packaging, providing improved moisture protection. This type of packaging material is suitable for a variety of applications, such as paper cups, paper trays, paper forks, paper spoons, paper lids, and paper products.

[0052] Another aspect of the present invention relates to the use of fiber-based composite materials obtained by the methods covered by the present invention for the preparation of paper capsules used in capsule coffee machines.

[0053] When considering the use of such composite materials in direct contact with food, it is essential to ensure that the silicon is properly grafted onto the cellulose to prevent any potential risks of material leaching into the food. This ensures the safety and compliance of the packaging materials used in food applications.

[0054] To determine the safety of such packaging materials, we referred to Annex 1 of Regulation (EU) 10 / 2011 applicable to plastic materials and articles. This regulation addresses situations where there is no specific migration limit (SML), which is the maximum amount of a substance permitted to migrate from the packaging material into food. In the case of paper-based packaging, there is no specific migration limit, and the default migration limit for a substance is set at 60 mg / kg of food. In the following example, instead of using a specific food simulant, a mixture of hexane and acetone was used, which is considered an extreme condition. If the migration of a substance from the packaging into this mixture is less than 60 mg / kg of food, the packaging is considered safe for food contact. As for food simulants, six types are typically used: water, 3% acetic acid, 10% ethanol, olive oil, 50% ethanol, and hexane. These simulants are selected based on their ability to represent different food types and conditions of use.

[0055] When silane monomers are mixed with an acid catalyst and applied to cellulose after one day or one week, the resulting composite material exhibits limited grafting. This is demonstrated by the residue obtained after overnight extraction with a common food simulant, such as hexane / acetone.

[0056] Conversely, when alkoxysilane monomers were contacted with an acid catalyst at room temperature for a few minutes to a maximum of 2 hours, the resulting composites exhibited a higher degree of grafting. This was evidenced by the smaller amount of residue recovered after overnight extraction with hexane / acetone.

[0057] Therefore, according to a preferred aspect of the invention, the reaction of a mixture comprising one or more alkoxysilane monomers and an acid catalyst in water is carried out at a temperature of 21°C or lower for a period of 2 minutes to 2 hours.

[0058] Furthermore, by maintaining a mixture containing one or more alkoxysilane monomers and an acid catalyst in water at a temperature below room temperature, the solution can be applied to a substrate containing cellulose fibers even after 2 hours without causing additional leaching of the material.

[0059] Less material leaching was observed when higher temperatures were applied during the curing or drying of the composite material. Furthermore, less material was extracted when the final composite material was left to stand for up to a week after the drying process.

[0060] Different leaching behaviors were observed by applying different amounts of catalyst and water ratios relative to the alkoxysilane monomers. Minimal leaching was achieved when using a small amount of catalyst and a high curing temperature. This can be explained by a sol-gel formation mechanism, where the rates of hydrolysis and polycondensation are strongly influenced by many factors, such as water content, catalyst type, and pH.

[0061] Water plays a crucial role in the sol-gel process because it initiates hydrolysis and triggers polycondensation. The stoichiometry of water required for complete hydrolysis depends on the structure of the precursor (the number of alkoxy groups) and, when provided, ensures complete hydrolysis of the reagent throughout the reaction. When the water ratio is below stoichiometry, the reaction may be hindered, allowing only a portion of the total amount available for polymerization. This type of reaction-limited cluster aggregation is described, for example, by Roucher, A., Biofunctional materials made by integrative chemistry, Elaboration de matériaux biofonctionnels par chimie intégrative, Université de Bordeaux, (2018).

[0062] On the other hand, using a water ratio greater than the stoichiometric ratio appears to accelerate the hydrolysis and condensation reactions. However, it should be emphasized that the condensation process is diffusion-limited, and a decrease in reactant concentration slows down the process (diffusion-limited clustering).

[0063] Therefore, according to a preferred aspect of the invention, the molecular ratio of alkoxysilane to water is 1:1.

[0064] Catalysis can increase the rates of hydrolysis and polycondensation, leading to a faster reaction. It is important to note that catalysts can alter not only the rate but also the final structure of the solid. There are two options: adding an acid, such as HCl, acetic acid, or citric acid, or a base, such as different hydroxides (e.g., NaOH, KOH). Acidic catalysis accelerates hydrolysis more than basic catalysis, and polycondensation occurs after this initial process. The main difference between the two is the reaction mechanism; under acidic conditions, the driving force is the well-developed leaving group generated during the protonation of hydroxyl groups, while under basic conditions, the key factor is the nucleophilicity of the deprotonated hydroxyl groups in silica.

[0065] The isoelectric point of silica, measured at approximately 2.1 (Chemical Reviews 65, 177-198), is crucial for understanding the total charge that forms the structure. Values ​​below or above this point result in particles that are either positively or negatively charged and will repel each other, thus inhibiting cluster aggregation. Maintaining the pH between 7 and 10 prevents gel formation and promotes particle growth until flocculation occurs.

[0066] Therefore, according to a preferred aspect of the invention, a mixture comprising one or more alkoxysilane monomers and an acid catalyst is reacted with the cellulose fibers of the substrate in water at a pH of 7 to 10.

[0067] Another key consideration is the reaction portion during polycondensation under different pH conditions. The pKa of silanols varies with particle size, ranging from approximately 9.5 to 10.7 for small oligomers and approximately 6.8 for particles as small as 1 nm (The FEBS journal 279, 1710-1720, 2012). Reactions occurring below the isoelectric point mean that there is more positive charge relative to the negative charge (protonated silanols). The most fundamental hydroxyl groups, present in small oligomers or monomers, will be the most reactive units. This also applies above the isoelectric point, where the most reactive portion (deprotonated hydroxyl groups) will be on larger particles.

[0068] All of the aforementioned factors will alter the progress of the reaction, subsequent interactions with cellulose after coating, and the final form of the solid.

[0069] In another aspect, the present invention relates to a fiber-based composite material comprising a fiber-based substrate and a silane coating, wherein the coating imparts improved hot water resistance and strength to the fiber-based substrate.

[0070] In a preferred aspect of the invention, the coating penetrates up to 70% of the total thickness of the fiber-based substrate.

[0071] When paper packets are produced from the fiber-based composite material of this invention, the resulting composite packets exhibit increased hydrophobicity and increased strength. This results in the packets not expanding as much during hot water extraction and retaining their structural integrity after extraction, thus allowing them to be easily removed from the machine and providing a clean coffee cup.

[0072] Example

[0073] Material :

[0074] Trimethoxymethylsilane (MTMS) and 1M hydrochloric acid were purchased from Sigma Aldrich. The water used was softened. The 3D paper capsules used for coating were compostable Nespresso. ® A sac, which may be filled with coffee or empty.

[0075] Formulation preparation and coating on capsules :

[0076] The coating formulation was prepared by combining all components in a 125 mL glass beaker containing 30 mL of TEOS. First, a silane precursor was added, followed by 8 mL of water and 50 μL of HCl. Initially, phase separation was observed, which disappeared after 10 minutes. Once the solution became homogeneous, it was coated onto paper bags by dip coating or spray coating. The resulting material was dried at 150 °C for 10 minutes. After this drying step, the coating could be reapplied to the coated bags using the same method.

[0077] Applying the coating after dip coating

[0078] Table 1 summarizes the coating amount applied after one and two coats. Using a dip coating technique, each coating step results in a coating of approximately 10% of the fiber weight.

[0079]

[0080] Water absorption at room temperature :

[0081] Add a pre-weighed reference capsule (untreated) and two pre-weighed treated capsules to the top of a Schott bottle filled with 20 mL of room temperature water. Seal the bottle with the stopper and invert each bottle so that the capsule is completely submerged in water. After 15 minutes, return the bottle to its original position and remove the capsule. Remove excess water by shaking and weigh the capsule again to determine the amount of water adsorbed.

[0082] Table 2 shows the amount of water adsorbed by each capsule. It can be seen that the untreated capsules absorbed over 104% of their weight, while the capsules coated once and coated twice absorbed only 14% and 9% of their respective weights. This indicates that the application of the silane coating increases the hydrophobicity of the capsules because they absorb less water.

[0083]

[0084] Moisture content after extraction :

[0085] Using Nespresso ® The extraction system extracts reference capsules (untreated) and two treated capsules (single-coated and double-coated).

[0086] Figure 1 Shown in Nespresso ® The moisture content of the capsules after hot water extraction in the system. The decrease in moisture content of the coated capsules clearly indicates that the coating imparts hydrophobic properties to the capsules. The difference between single and double coatings is small, indicating that a single coating is sufficient to ensure improved hydrophobicity.

[0087] The improved hydrophobic properties of the coated capsules can also be determined by measuring the contact angle of a water droplet placed on the coated capsule over time. In short, a 2 μL droplet is placed on top of the capsule, and the contact angle is measured every 2 seconds for 80 seconds using a Kruss instrument. Figure 2 The evolution of the droplet contact angle over time is shown. In the case of the uncoated capsule, the contact angle decreases sharply, indicating that the water droplet is absorbed by the capsule, while the contact angles of the two coated capsules remain unchanged, indicating that the coating imparts increased hydrophobic properties.

[0088] Mechanical resistance

[0089] Compression tests were performed between two metal plates on a Zwick instrument to evaluate the effect of the coating on the mechanical strength of the coated capsule. Figure 3 The average compressive forces of dry capsules (A) and wet capsules (B) are shown. For both dry and wet capsules, the coating introduces increased rigidity due to the high compressive forces in each case. The increased rigidity of the dry-coated capsule facilitates puncture during insertion, while the increased rigidity of the wet capsule facilitates smooth ejection.

[0090] Comparison of biopolymer coatings and silane coatings

[0091] Another way to demonstrate the additional benefits of silane coatings to capsule integrity is to compare them with biopolymer coatings. A PHA layer can be added to the polymer by dissolving the biopolymer, such as polyhydroxyalkanoate (PHA), in an organic solvent, such as chloroform, and then immersing the capsules in the solvent as explained above.

[0092] like Figure 4 As can be seen in Nespresso ® The compressibility of the PHA capsules after extraction in the system was almost twice that of the reference capsules, but half that of the silane-coated capsules after extraction, clearly demonstrating the benefits imparted by the silane coating. This is explained by the fact that the silanes not only create a thin, invisible outer layer, but they also penetrate the fibers and cross-link them, thereby increasing fiber strength.

[0093] Residue after hexane-acetone extraction

[0094] When considering the use of such composite materials in direct contact with food, it is essential to ensure that the silicon is properly grafted onto the cellulose to prevent any potential risks of material leaching into the food. This ensures the safety and compliance of the packaging materials used in food applications.

[0095] To determine the safety of such packaging materials, we referred to Annex 1 of Regulation (EU) 10 / 2011 applicable to plastic materials and articles. This regulation addresses situations where there is no specific migration limit (SML), which is the maximum amount of a substance permitted to migrate from the packaging material into food. In the case of paper-based packaging, there is no specific migration limit, and the default migration limit for the substance is set at 60 mg / kg of food. In the following example, instead of using a specific food simulant, a mixture of hexane and acetone was used, which is considered an extreme condition. If the migration of a substance from the packaging into this mixture is less than 60 mg / kg of food, the packaging is considered safe for food contact.

[0096] Figure 5 The levels of residue recovered from the two coated capsules are shown, expressed in mg / kg food. Both showed levels below 60 mg / kg food, and therefore can be considered safe.

[0097] at last, Figure 6 The cross-section and penetration extent of the silane coating are shown. It can be seen that the coating penetrates up to 330 micrometers within a 500-micrometer capsule thickness, representing up to 66% of the total capsule thickness.

[0098] This invention aims to prevent the negative effects of capsule swelling during hot water extraction, which leads to weakened capsule strength and reduced coffee cup cleanliness. The inventors have developed a method for obtaining a silane coating using a sol-gel process. When this coating is applied to cellulose in the presence of water and an acid catalyst, it produces a composite material with increased hydrophobicity and strength. This invention also relates to the resulting composite fiber-based packaging material with enhanced water resistance and strength, and its safety for use in food contact applications.

Claims

1. A method for manufacturing a fiber-based composite material, the method comprising the following steps: a) Provide a substrate containing cellulose fibers; b) Prepare a mixture containing one or more alkoxysilane monomers and an acid catalyst in water; c) Apply the mixture to the substrate; d) React the mixture with the cellulose fibers of the substrate; e) subject the mixture to a sol-gel process; f) Dry the treated substrate to obtain the composite material.

2. The method according to claim 1, wherein the cellulose fiber is selected from: cellulose fiber, jute, bagasse, hemp, and combinations thereof.

3. The method according to claim 1 or claim 2, wherein the one or more alkoxysilane monomers are selected from the group consisting of: tetraethyl orthosilicate, triethoxy(methyl)alkoxysilane, triethoxy(ethyl)alkoxysilane, triethoxy(propyl)alkoxysilane, butyltriethoxyalkoxysilane, triethoxy(isopentyl)alkoxysilane, (3,3-dimethylbutyl)triethoxyalkoxysilane, triethoxy(vinyl)alkoxysilane, 3-(triethoxysilyl)prop-1-amine 3-(triethoxysilyl)propyl methacrylate, tetraethyl orthosilicate, trimethoxy(methyl)alkoxysilane, trimethoxy(ethyl)alkoxysilane, trimethoxy(propyl)alkoxysilane, butyltrimethoxyalkoxysilane, trimethoxy(isopentyl)alkoxysilane, (3,3-dimethylbutyl)trimethoxyalkoxysilane, trimethoxy(vinyl)alkoxysilane, 3-(trimethoxysilyl)propyl-1-amine and 3-(trimethoxysilyl)propyl methacrylate.

4. The method according to any one of claims 1 to 3, wherein the molecular ratio of alkoxysilane to cellulose is in the range of 2:1 to 0.5:

1.

5. The method according to any one of claims 1 to 4, wherein the molecular ratio of alkoxysilane to cellulose is 3:

1.

6. The method according to any one of claims 1 to 5, wherein the acid catalyst is selected from hydrochloric acid, sulfuric acid, nitric acid, acetic acid and formic acid.

7. The method according to any one of claims 1 to 6, wherein the molecular ratio of the alkoxysilane to water is 1:

1.

8. The method according to any one of claims 1 to 7, wherein the reaction of the mixture with the cellulose fibers of the substrate is carried out at a temperature of 21°C or lower for a period of 2 minutes to 2 hours.

9. The method according to any one of claims 1 to 8, wherein the reaction of the mixture with the cellulose fibers of the substrate is carried out at a pH of 7 to 10.

10. The method according to any one of claims 1 to 9, wherein drying the treated substrate to obtain the composite material is carried out at a temperature of 100°C to 150°C, preferably at a temperature of 120°C.

11. The method according to any one of claims 1 to 10, wherein drying the treated substrate to obtain the composite material is carried out over a time period of 1 minute to 10 minutes, preferably over a time period of 2 minutes.

12. A fiber-based composite material obtained by the method according to any one of claims 1 to 11.

13. A fiber-based composite material comprising a fiber-based substrate and a silane coating, wherein the coating imparts improved hot water resistance and strength to the fiber-based substrate.

14. The fiber-based composite material of claim 13, wherein the coating penetrates at most 70% of the total thickness of the fiber-based substrate.

15. Use of the fiber-based composite material according to any one of claims 12 to 14 for the preparation of capsules used in capsule coffee machines.

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