Improved composite materials for the preparation of coffee capsules

Abstract

The present invention relates to a method of manufacturing a fibre-based composite material, to the fibre-based composite material thus obtained, and to the use of the fibre-based composite material for the preparation of capsules for use in capsule coffee machines.

Description

[0001] IMPROVED COMPOSITE MATERIALS FOR THE PREPARATION OF COFFEE CAPSULES

[0002] Field of the Invention

[0003] The present invention relates to a method of manufacturing a fibre-based composite material, to the fibre-based composite material thus obtained, and to the use of the fibrebased composite material for the preparation of capsules for use in capsule coffee machines.

[0004] Background of the invention

[0005] The sol-gel process has been extensively studied in the field of packaging as it offers precise control over film thickness, composition, and functional properties, making it attractive for packaging applications. It enables the fabrication of thin films and coatings with enhanced barrier properties, optical transparency, and additional functionalities like antimicrobial activity and self-cleaning capabilities (Applied Surface Science Volume 351 , 1 October 2015, Pages 897-903).

[0006] Sol-gel coatings have the potential to contribute to sustainable packaging solutions. Compared to traditional coating methods, the sol-gel process is often more environmentally friendly, as it typically operates at lower temperatures and avoids the use of harmful organic solvents or harsh processing conditions (Encyclopaedia of Physical Science and Technology (Third Edition), 2003, Pages 257-276). Furthermore, sol-gel coatings can be compatible with bio-based (Progress in Organic Coatings Volume 77, Issue 6, June 2014, Pages 1111-1116) or recyclable packaging materials, supporting the development of eco-friendly packaging systems.

[0007] The process has been extensively studied for the development of barrier coatings for packaging materials. By depositing thin sol-gel films on substrates, it is possible to enhance their barrier properties against grease, moisture, oxygen (Polymer Volume 195, 8 May 2020, 122437), and other gases. These coatings help extend the shelf life of packaged products by reducing permeation and maintaining product quality. Sol-gel coatings can also be tailored to provide additional functionalities in packaging applications. For instance, antimicrobial properties can be incorporated by adding specific nanoparticles or organic compounds to the sol-gel formulation. This can help inhibit the growth of bacteria or fungi on the packaging surface, contributing to the preservation of food and reducing the need for chemical preservatives (Food Chem. 2014 Dec 15; 165:342-7).

[0008] 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 essential, such as for display packaging (transparent packaging) or UV- blocking films for light-sensitive products.

[0009] Adhesion between the sol-gel coating and the substrate is crucial for ensuring the durability and performance of packaging materials and to ensure the food safety of the packaging. Researchers have explored various strategies to improve the adhesion of sol-gel coatings, including surface pre-treatments, modification of the sol-gel formulation, and the use of adhesion-promoting agents. Other applications have involved the addition of varnish to protect the coating. Such varnish can be a polystyrene acrylate-based material. However, such approach can lead to leaching of material as observed by the inventors when they subjected packaging materials made from this approach to a hexane acetone extraction overnight. Residues were found indicating poor grafting to the cellulose.

[0010] Capsule coffee machines, such as the Nespresso® Original line and Verturo line extraction systems, are basically composed of four main elements. The aluminium pod with its aluminium membrane, an extraction cage, piercing needles and the pyramid plate. When a pod is inserted into the extraction cavity, the lever movement of the lid forces the pod into the chamber, and the pod is blocked between two flanges to keep it tight. At the same moment a plurality of needles pierce the top of the pod while compressing the pod’s bottom aluminium membrane onto the pyramid plate. High pressure hot water is then pumped into the chamber, submerges the pod and enters through the holes produced by the needles. The water is flushed from outside to inside in order to equilibrate the hydrostatic pressure in the pot to avoid bursting. It extracts the coffee bed and when it reaches the bottom membrane it causes this aluminium membrane to crack on the pyramid plate leading to the discharging the coffee out of the system into the cup with its distinctive crema. So the pressure only helps to break the membrane in the pyramid plate, but overall there is no A p form outside / inside the pod. This means that the full capsule is exposed to water at 90 degrees.

[0011] When a pure paper pod is used instead of an aluminium pod, a different phenomenon occurs. The piercing of the needles is impaired due to the flexibility of the paper fibres and when hot water is injected into the system, the paper pod swells as the fibre absorbs a substantial amount of water. When the water enters through the improperly pierced wholes, it starts to extract the coffee and when it reaches the bottom paper membrane the latter does not properly pierce. The swelling of the pod coupled to the unpierced membrane forces the delamination of the latter resulting in beverage that is dispensed out of the machine with entrained coffee particles and a less appealing crema. Moreover, the swelled pod remains in the extraction chamber and cannot be ejected. As a result, the entire consumer experience that is expected with an aluminium pod is no longer maintained with a compostable paper pod; the cup is not clean, the crema not consistent, and the pod needs a push of a finger to be ejected.

[0012] It is therefore an object of the present invention to provide a fibre-based composite material resistant to swelling of the material with hot water, thereby allowing a pod made from said material to be ejected from a capsule coffee machine without help and not being weakened during the extraction, thus delivering a clean coffee cup.

[0013] As the inner side of the pot is laminated with a polymer, Also the external part should be protected from water to avoid the aforementioned issue. For this an external inert protection should be applied. In this case, this object is achieved by applying a very stable grafted poly-silicon oxide moiety onto a fibre-based material, which remains strongly attached to the fibres even when exposed to highly aggressive food liquid simulants. This versatile and durable material can be used for various applications, and the invention encompasses the method of manufacturing this material as well as its use for the preparation of capsules for use in capsule coffee machines. In addition to that, due to the very low amount, characteristics such as paper recyclability or biodegradability are not influenced.

[0014] Summary of the Invention

[0015] Accordingly, the present invention provides in a first aspect a method for manufacturing a fibre-based composite material, comprising the steps of: a) providing substrate comprising cellulosic fibres; b) preparing a mixture comprising one or more alkoxy silane monomers and an acid catalyst in a solvent; c) applying said mixture to said substrate; d) reacting said mixture with the cellulosic fibres of said substrate; e) subjecting said mixture to the sol-gel process; f) drying the treated substrate to obtain the composite material.

[0016] In another aspect, the present invention relates to the fibre-based composite material obtained by the inventive method described herein.

[0017] In yet another aspect, the present invention relates to a fibre-based composite material comprising a fibre-based substrate and a silane-based coating, wherein said coating confers an improved hot water resistance and strength to the fibre-based substrate.

[0018] In a further aspect, the present invention relates to the use of the inventive fibre-based composite material described herein for the preparation of capsules for use in capsule coffee machines.

[0019] Detailed Description of the Invention

[0020] This invention provides a fibre-based composite material made by chemically bonding a substrate comprising cellulosic fibres and one or more alkoxy silane monomers to add a highly stable poly silicon oxide layer using the sol-gel process in accordance with the following process steps: a) providing substrate comprising cellulosic fibres; b) preparing a mixture comprising one or more alkoxy silane monomers and an acid catalyst in water; c) applying said mixture to said substrate; d) reacting said mixture with the cellulosic fibres of said substrate; e) subjecting said mixture to the sol-gel process; f) drying the treated substrate to obtain the composite material. In accordance with a preferred aspect of the present invention, the cellulosic fibres are selected from the group consisting of soft / hard wood cellulose fibres, jut, beggase, hemp, straw, bamboo and combinations thereof.

[0021] The bonding occurs by grafting alkoxy silane monomers to the hydroxyl groups of the cellulose monomers, glucose on C2, C3, and C6.

[0022] In accordance with a preferred aspect of the present invention, the alkoxy silane-to- cellulose molecular ratio is in the range of 2:1 - 0.5:1. In an even more preferred aspect of the present invention, the alkoxy silane-to-cellulose molecular ratio is 3:1.

[0023] The sol-gel process enables the synthesis of solid materials from small molecules, which involves the formation of silanols and then polysiloxanes from an alkoxy silane. Initially, a sol is formed, which is a colloidal solution of alkoxy silane particles. These particles begin to aggregate, forming a macrostructure. This process leads to gel formation, which is a biphasic system comprising both a liquid and a solid phase. It involves several steps to transform the sol into a gel and eventually into a solid material.

[0024] Sol Formation:

[0025] A precursor solution, called a sol, is prepared by dissolving alkoxy silanes in a solvent. The precursor solution may also contain organic compounds or polymers.

[0026] In accordance with a preferred aspect of the present invention, the precursor solution comprises the alkoxy silane monomers and an aqueous acid catalyst selected from the group consisting of hydrochloric acid, sulphuric acid, nitric acid, acetic acid, and formic acid.

[0027] Gelation:

[0028] The sol undergoes gelation, forming 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.

[0029] In accordance with the present invention, this is achieved with the hydrolysis of the alkoxy function in the alkoxy silane monomer (Ri-0 as depicted below) by the acid catalyst, leading to a silanol, followed by the polycondensation of said silanol with itself and with the cellulosic substrate that is added as wet pulp or as a flat paper board.

[0030] Aging:

[0031] The gel is allowed to age or undergo further chemical reactions, which leads to the strengthening and stabilization of the gel structure. This step can take place at ambient or elevated temperatures, depending on the desired properties of the final material.

[0032] In accordance with the present invention, the aging of the gel structure is preferably done at room temperature or in a cold environment, i. e. provided by an ice bath. In each case, the rate of formation of the gel is affected by the temperature.

[0033] Drying:

[0034] The wet gel is subjected to drying to remove the solvent from the gel structure. This can be done by evaporation, supercritical drying, or other drying technigues. The drying process is essential for converting the gel into a solid material while preserving its structure.

[0035] In accordance with a preferred aspect of the present invention, the drying of the sol-gel, releasing ethanol or methanol depending on the type of alkoxy silane monomer used (1- 10 and 11-20, respectively) is preferably performed in an oven, heated at 100°C, more preferably 110°C, most preferably 120°C. Temperatures higher than 150°C should be avoided as they can lead to the degradation of the cellulosic substrate. In case of a continuous coating process, the drying is done via infrared lamps.

[0036] In accordance with another preferred aspect of the present invention, drying the treated substrate to obtain the composite material is done over a period of time from 10 seconds to 5 minutes, more preferably over a period of time of 2 minutes in a static environment (oven) or 30 seconds in a dynamic environment (continuous coating), most preferably 3 to 5 seconds.

[0037] The alkoxy silane monomers that can be used for producing the fibre-based composite material in accordance with the present invention are preferably selected from the group consisting of: 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, 93-(triethoxysilyl)propan-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)propan- 1 -amine , and 20 3-(trimethoxysilyl)propyl methacrylate.

[0038] Applying the acid catalyst to the alkoxy silane monomers just before reacting with cellulose enhances the grafting to cellulose, as evidenced by reduced leaching. The preferred timing for applying the acid catalyst is a few minutes before reacting with the substrate comprising cellulosic fibres, but no more than two hours before. Beyond that timeframe, improper grafting occurs due to significant advancement of the sol-gel process, which hinders proper grafting to the hydroxyl groups of cellulose, mainly due to steric hindrance. Additionally, cooling the alkoxy silane monomers in solution with the acid extends the application time, providing a longer window before improper grafting takes place.

[0039] The level of silicon-to-cellulose grafting is crucial for achieving the desired modifications in the physicochemical properties of the substrate comprising cellulosic fibres. The purpose of the treatment of the present invention is to enhance the hydrophobicity of the substrate comprising cellulosic fibres through the sol-gel process. This modification enables its application in moisture-protective packaging materials.

[0040] For instance, a composite material composed of cellulose with enhanced hydrophobicity can be utilized in water retention packaging, offering improved moisture protection. This type of packaging material is suitable for various applications, such as paper cups, paper trays, paper forks, paper spoons, paper lids, paper sheets.

[0041] Another aspect of the present invention relates to the use of the fibre-based composite material, obtained through the method encompassed by the invention, for the preparation of paper capsules for use in capsule coffee machines.

[0042] When considering the use of such composite materials in direct contact with food, it is essential to ensure proper grafting of silicon to cellulose to prevent any potential risks of material leaching into the food. This ensures the safety and compliance of the packaging material for food applications.

[0043] To determine the safety of such packaging material, we refer to Annex 1 of Regulation (Ell) No 10 / 2011 , which applies to plastic materials and articles. This regulation states that in the absence of a specific migration limit (SML, which refers to the maximum amount of a substance that is allowed to migrate from packaging materials into food. In the case of paper-based packaging, there is a lack of specific migration limits.), the default migration limit for substances is set at 60 mg / kg of food. In the following examples, instead of using a specific food simulant, a hexane acetone mixture was utilized, which is considered an extreme condition. If the migration of substances from the packaging into this mixture is lower than 60 mg / kg of food, the packaging is considered to be safe for food contact. As for food simulants, there are typically six types 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.

[0044] When the silane monomer was mixed with the acid catalyst and applied to cellulose after a day or a week, the resulting composite material exhibited limited grafting. This was evidenced by the residues obtained after an overnight extraction with common food simulant such as hexane / acetone.

[0045] Conversely, the resulting composite material demonstrated a higher degree of grafting when the alkoxy silane monomer was in contact with the acid catalyst for a few minutes to a maximum of 2 hours at room temperature. This was evidenced by the smaller amount of residues recovered after an overnight extraction with hexane / acetone.

[0046] Therefore, in accordance with a preferred aspect of the present invention, reacting the mixture comprising one or more alkoxy silane monomers and an acid catalyst in water is performed at a time of from 2 minutes to 2 hours at a temperature of 21 °C or below.

[0047] Furthermore, by keeping the mixture comprising one or more alkoxy silane monomers and an acid catalyst in water at temperatures below room temperature, it is possible to apply the solution to the substrate comprising cellulosic fibres even after 2 hours without causing additional leaching of materials.

[0048] By applying higher temperatures during the curing or drying of the composite material less leaching of material is observed. Moreover, when the final composite material is allowed to rest after the drying process for up to a week, less material is extracted.

[0049] By applying different amounts of catalyst and water ratios with respect to the alkoxy silane monomer, different leaching behaviours were observed. The least amount of leaching was achieved when low amounts of catalyst were used and high curing temperatures. This can be explained by the sol-gel formation mechanism where the rate of hydrolysis and polycondensation are strongly influenced by many factors such as the water content, the type of catalyst and the pH. Water plays a crucial role in the sol-gel process as it initiates hydrolysis and triggers polycondensation. The stoichiometric quantity of water required for full hydrolysis depends on the precursor's structure (the number of alkoxy groups), and when provided, it ensures complete hydrolysis of the reagent throughout the reaction. With a water ratio lower than stoichiometric, the reaction may be hindered, allowing only a portion of the overall moieties to be available for polymerization. A reaction limited cluster aggregation of this kind is described e.g. by Roucher, A., Biofunctional materials made by integrative chemistry, Elaboration de materiaux biofonctionnels par chimie integrative, Universite de Bordeaux, (2018).

[0050] On the other hand, employing a water ratio greater than stoichiometric may appear to speed up both hydrolysis and polycondensation reactions. However, it should be emphasized that the process of polycondensation is constrained by the diffusion, and a reduced concentration of reactants decelerates this process (diffusion limited cluster aggregation).

[0051] Therefore, in accordance with a preferred aspect of the present invention, the alkoxy silane-to-water molecular ratio is 1 :1.

[0052] The catalytic effect can enhance the hydrolysis and polycondensation rates, leading to a faster reaction. It is important to note that a catalyst can not only alter the rate but also the final structure of the solid. There are two options: adding an acid, such as HCI, acetic acid, or citric acid, or a base, such as different hydroxides (e.g., NaOH, KOH). Acidic catalysis accelerates hydrolysis more than the basic one, and polycondensation occurs after this initial process. The primary difference between the two cases is the reaction mechanism; under acidic conditions, the driving force is the good leaving group generated upon protonation of the hydroxyl, while the key factor in basic conditions is the nucleophilicity of the deprotonated hydroxyl group in silica.

[0053] The isoelectric point of silica, measured at approximately 2.1 (Chemical Reviews 65, 177-198), is of utmost importance in understanding the overall charge of the forming structure. At values below or above this point, the particles will be positively or negatively charged and will repel each other, inhibiting cluster aggregation. By maintaining a pH between 7 and 10, it's possible to impede gel formation and promote particle growth until flocculation occurs. Therefore, in accordance with a preferred aspect of the present invention, reacting the mixture comprising one or more alkoxy silane monomers and an acid catalyst in water with the cellulosic fibres of the substrate is performed at a pH between 7 and 10.

[0054] Another crucial consideration is the reacting moieties during polycondensation under different pH conditions. The pKa of silanols varies depending on the particle size, around 9.5-10.7 for small oligomers, and approximately 6.8 for small particles of 1nm (The FEBS journal 279, 1710-1720, 2012). A reaction carried out at a pH lower than the isoelectric point implies the presence of more positive charges (protonated silanols) with respect to negative charges. The most basic hydroxyls are those present in small oligomers or monomers which will be the most reacting units. The same applies above the isoelectric point, where the most reactive moieties (deprotonated hydroxyls) will be on big particles.

[0055] All factors described before will modify the evolution of the reaction, the successive interaction with the cellulose after the coating, and the final morphology of the solid.

[0056] In another aspect, the present invention relates to a fibre-based composite material comprising a fibre-based substrate and a silane-based coating, wherein said coating confers an improved hot water resistance and strength to the fibre-based substrate.

[0057] In a preferred aspect of the present invention, the coating penetrates up to 70 % of the total thickness of the fibre-based substrate.

[0058] When producing paper pods from the inventive fibre-based composite material, the resulting composite pods show an increased hydrophobicity as well as an increased strength. This results in pods that do not swell as much during the hot water extraction and that retain their structural integrity after extraction, thereby allowing them to be easily expelled from the machine and delivering a clean coffee cup.

[0059] Examples

[0060] Materials:

[0061] Trimethoxymethylsilane (MTMS) and hydrochloric acid 1 M were purchased from Sigma Aldrich. The water used was demineralized. The 3D paper pods used for the coating were the compostable Nespresso® pods, either filled with coffee or empty. Preparation of Formulations and Coatings on the Pods:

[0062] The formulations for the coating were prepared by combining all ingredients in a 125 mL glass beaker containing 30 mL of TEOS. First, the silane precursor is added, followed by the addition of 8 ml of water and 50 pL of HCI. Initially, a phase separation is observed that disappears after 10 minutes. Once the solution gets homogenous the latter is coated on the paper pod either by dip coating or spray coating. The resulting material is dried for 10 minutes at 150°C. After this drying step, the coating can be reapplied to the coated pod using the same approach.

[0063] Coating application after dip coating

[0064] Table 1 summarizes the amount of coating that is applied after one time and after two times. With the dip coating technigue, each coating step leads to a coating of approximately 10% of the weight of the fiber.

[0065] Table 1

[0066] Water uptake at room temperature:

[0067] A pre-weighed reference pod (untreated) and two pre-weighed treated pods were added on top of a Schott bottle that was filled with 20ml of water at room temperature. The bottle was closed with the stopper and each bottle was put upside down to have the pod fully emerged in water. After 15 minutes, the bottles are put back in their original position and the pods are removed. The excess water is removed by a shake and the pod is weighed once more to determine the amount of water that was adsorbed.

[0068] Table 2 shows the amount of water adsorbed per pod. As can be seen, the untreated pod absorbs over 104% of its weight while the pod coated once and the pod coated twice only absorbed 14% and 9% of their respective weights. This suggests that the application of the silane-based coating increases the hydrophobicity of the pod as less water is absorbed by the pod.

[0069] Table 2

[0070] Moisture content after extraction:

[0071] A reference pod (untreated) and two treated pods, one coating and two coatings, were extracted using a Nespresso® extraction.

[0072] Figure 1 shows the moisture content of the pods after hot water extraction in a Nespresso® system. The reduced moisture content of the coated pods clearly shows that the coating is bringing a hydrophobic character to the pod. There is a little difference between one coating and two coatings, suggesting that one coating is already enough to ensure an improved hydrophobicity.

[0073] The improved hydrophobic character of the coated pods can also be determined by measuring the contact angle over time of a water droplet placed on the coated pods. In brief, a 2pl drop is placed on the top of a pod and the contact angle is measured every 2 seconds for 80 seconds with the help of a Kruss machine. Figure 2 shows the evolution over time of the contact angle of the droplet. The contact angle in the case of the uncoated pod drops drastically indicating that the water droplet is being absorbed by the pod, while the contact angle of both coated pods remains unchanged suggesting an increased hydrophobic character that is conferred by the coating Mechanical Resistance

[0074] The impact on the coating on the mechanical strength of the coated pods was assessed with compression test performed between two metal plate on a Zwick instrument. Figure 3 shows the average compression force of dry pods (A) and wet pods (B). For both the dry pods and the wet pods, the coating brings an increased rigidity as the compression forces were great for each case. The increased rigidity of the dry coated pods helps the piercing during pod insertion and the increased rigidity of the wet pods helps the proper ejection of the pods.

[0075] Comparison with a Bio Polymer Coating vs Silane Coating

[0076] Another means to show the added benefit of the silane coating to the pod’s integrity is to compare it with a bio-polymer coating. By dissolving a bio-polymer, such as a polyhydroxy alkanoate (PHA), in an organic solvent, such as chloroform, and dip coating the pod in the solvent as explained before, a PHA layer is added in the polymer.

[0077] As can be seen in Figure 4, the compression force of the PHA pod after extraction in the Nespresso® system is almost twice higher than the reference pod but twice less than the compression force of the silane coated pod after extraction, clearly showing the benefit conferred by the silane coating. The explanation is that not only silane creates a thin external invisible layer, but they penetrate the fibre and crosslink it with an increase in fibre strength.

[0078] Residues after Hexane Acetone Extraction

[0079] When considering the use of such composite materials in direct contact with food, it is essential to ensure proper grafting of silicon to cellulose to prevent any potential risks of material leaching into the food. This ensures the safety and compliance of the packaging material for food applications.

[0080] To determine the safety of such packaging material, we refer to Annex 1 of Regulation (Ell) No 10 / 2011 , which applies to plastic materials and articles. This regulation states that in the absence of a specific migration limit (SML, which refers to the maximum amount of a substance that is allowed to migrate from packaging materials into food. In the case of paper-based packaging, there is a lack of specific migration limits.), the default migration limit for substances is set at 60 mg / kg of food. In the following examples, instead of using a specific food simulant, a hexane acetone mixture was utilized, which is considered an extreme condition. If the migration of substances from the packaging into this mixture is lower than 60 mg / kg of food, the packaging is considered safe for food contact.

[0081] Figure 5 shows the residue levels recovered and expressed in mg / kg food for the two coated pods. Both show levels below the 60 mg / kg food so they can be considered as safe.

[0082] Finally, Figure 6 shows the cross-section and the extent of the penetration of the silane- based coating. As can be seen, the coating penetrates up to 330 microns within the 500 microns pod thickness representing up to 66% of the total thickness of the pod.

[0083] The present invention aims to prevent the negative effects of pod swelling during hot water extraction, resulting in a weakened pod and a less clean coffee cup. The inventors have developed method using a sol-gel process to obtain silane-based coatings that, when applied to cellulose in the presence of water and an acid catalyst, create a composite material with increased hydrophobicity and strength. The invention also relates to the obtained composite fibre-based packaging material with enhanced water resistance and strength, and to safe for food contact applications using the same.

[0084] RECTIFIED SHEET (RULE 91 ) ISA / EP

Claims

CLAIMS:

1. A method for manufacturing a fibre-based composite material, comprising the steps of: a) providing a substrate comprising cellulosic fibres; b) preparing a mixture comprising one or more alkoxy silane monomers and an acid catalyst in water; c) applying said mixture to said substrate; d) reacting said mixture with the cellulosic fibres of said substrate; e) subjecting said mixture to the sol-gel process; f) drying the treated substrate to obtain the composite material.

2. The method of claim 1 , wherein said cellulosic fibres are selected from the group consisting of cellulose fibres, jut, beggase, hemp, and combinations thereof.

3. The method of claim 1 or claim 2, wherein the one or more alkoxy silane monomers are selected from the group tetraethyl orthosilicate, triethoxy(methyl)alkoxy silane, triethoxy(ethyl)alkoxy silane, riethoxy(propyl)alkoxy silane, butyltriethoxyalkoxy silane, triethoxy(isopentyl)alkoxy silane, (3,3-dimethylbutyl)triethoxyalkoxy silane, triethoxy(vinyl)alkoxy silane, 3-(triethoxysilyl)propan-1 -amine, 3- (triethoxysilyl)propyl methacrylate, tetraethyl orthosilicate, trimethoxy(methyl)alkoxy silane, trimethoxy(ethyl)alkoxy silane, trimethoxy(propyl)alkoxy silane, butyltrimethoxyalkoxy silane, trimethoxy(isopentyl)alkoxy silane, (3,3-dimethylbutyl)trimethoxyalkoxy silane, trimethoxy(vinyl)alkoxy silane, 3-(trimethoxysilyl)propan-1 -amine, and 3- (trimethoxysilyl)propyl methacrylate.

4. The method of any of claims 1 to 3, wherein alkoxy silane-to-cellulose molecular ratio is in the range of 2:1 - 0.5:

15. The method of any of claims 1 to 4, wherein alkoxy silane-to-cellulose molecular ratio is 3:1.

6. The method of any of claims 1 to 5, wherein the acid catalyst is selected from the group consisting of hydrochloric acid, sulphuric acid, nitric acid, acetic acid, and formic acid.

7. The method of any of claims 1 to 6, wherein the alkoxy silane-to-water molecular ratio is 1 :

18. The method of any of claims 1 to 7, wherein reacting said mixture with the cellulosic fibres of said substrate is performed at a time of from 2 minutes to 2 hours at a temperature of 21 °C or below.

9. The method of any of claims 1 to 8, wherein reacting said mixture with the cellulosic fibres of said substrate is performed at a pH between 7 and 10.

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

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

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

13. A fibre-based composite material comprising a fibre-based substrate and a silane-based coating, wherein said coating confers an improved hot water resistance and strength to the fibre-based substrate.

14. The fibre-based composite material of claim 13, wherein the coating penetrates up to 70 % of the total thickness of the fibre-based substrate.

15. Use of the fibre-based composite material of any of claims 12 to 14 for the preparation of capsules for use in capsule coffee machines.

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