Improved composite materials for food application

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 preparing an item.

Description

[0001] IMPROVED COMPOSITE MATERIALS FOR FOOD APPLICATION

[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 preparing an item.

[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 ((REF)). 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 in 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.

[0008] 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.

[0009] 2014 Dec 15; 165:342-7).

[0010] The sol-gel also process 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 or UV-blocking films for lightsensitive products.

[0011] 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.

[0012] It is therefore an object of the present invention to provide a fibre-based composite material having improved adhesion between a cellulosic fibrous-based substrate and a coating applied thereon by using the sol-gel process.

[0013] Summary of the Invention

[0014] To achieve the above-defined object, 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. In another aspect, the present invention relates to the fibre-based composite material obtained by the inventive method described herein.

[0015] 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.

[0016] In a further aspect, the present invention relates to the use of the inventive fibre-based composite material described herein for the preparation of an item selected from the group consisting of paper cups, paper trays, paper forks, paper spoons, paper lids, and paper sheets.

[0017] Detailed Description of the Invention

[0018] This invention provides a fibre-based composite material made by chemically bonding a substrate comprising cellulosic fibres and one or more alkoxy silane monomers 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.

[0019] In accordance with a preferred aspect of the present invention, the cellulosic fibres are selected from the group consisting of cellulose fibres, jut, beggase, hemp, and combinations thereof.

[0020] The bonding occurs by grafting alkoxy silane monomers to the hydroxyl groups of the cellulose monomer, glucose on C2, C3, and C6.

[0021] 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. The sol-gel process enables the synthesis of solid materials from small molecules, which involves the formation of silicon oxide from a 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.

[0022] Sol Formation:

[0023] 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.

[0024] 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, sulfuric acid, nitric acid, acetic acid, and formic acid.

[0025] Gelation:

[0026] The sol undergoes gelation, forming a three-dimensional network of interconnected particles or polymers. This can be achieved through various methods, such as chemical reactions, hydrolysis (Journal of Applied Polymer Science 108, 1958-1968), or condensation.

[0027] 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 in Error! Reference source not found.) 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 pulp or as a paper board.

[0028] Aging:

[0029] 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. 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.

[0030] 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 techniques. The drying process is essential for converting the gel into a solid material while preserving its structure.

[0031] 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.

[0032] 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 1 minute to 10 minutes, more preferably over a period of time of 2 minutes.

[0033] 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:

[0034] 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)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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] Therefore, in another aspect the present invention relates to the use of the fibre-based composite material obtained by the method of the present invention for the preparation of an item selected from the group consisting of paper cups, paper trays, paper forks, paper spoons, paper lids, and paper sheets.

[0039] 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.

[0040] 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 hexane / acetone.

[0041] 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 X hours at room temperature. This was evidenced by the smaller amount of residues recovered after an overnight extraction with hexane / acetone.

[0042] 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.

[0043] 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. 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.

[0044] By applying different amounts of catalyst and water ratios with respect to the alkoxy silane monomer, different leaching behaviors 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.

[0045] 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 silanes), 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).

[0046] 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).

[0047] Therefore, in accordance with a preferred aspect of the present invention, the alkoxy silane-to-water molecular ratio is 1 :1.

[0048] 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, LiOH). 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. Regardless of the fundamental factor, both reactions involve an SN2 mechanism with inversion of configuration.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] In a preferred aspect of the present invention, the coating penetrates up to 70 % of the total thickness of the fibre-based substrate.

[0054] 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. Examples

[0055] Materials:

[0056] Trimethoxymethylsilane (MTMS) and hydrochloric acid 1 M were purchased from Sigma Aldrich. The water used was demineralized. The paper sheets used for the coating were Natural Board 260 gsm from Metsa.

[0057] Preparation of Formulations and Coating:

[0058] The formulations for the coating were prepared by combining all ingredients in a 125 mL glass beaker containing 65 mL of MTMS. First, the alkoxy silane precursor was added, followed by a gradual addition of the catalyst and water using a funnel, while stirring.

[0059] At different reaction time a small amount of the formulation, approximately 1.6 g, was taken and coated onto a paper sheet using a close wound bar with a wire diameter of 0.30 mm. The coated paper was then immediately placed in an oven set to varying temperatures and heated for 2 minutes.

[0060] For evaluating the stability of the coating, an extraction was carried out using a 1 :1 acetone: hexane solution, either the day after or a week after (samples kept at room temperature) the coating process. For each coated sheet, three samples measuring 34mm in diameter were taken and soaked in pre-weighed vials containing the solvent. The extraction was left for approximately 10 hours after which the samples were removed, and the vials were dried. The amount of residue was estimated by calculating the difference between the final and initial weight.

[0061] All test conditions are reported in the table below: Different formulations were tried and coated on paper to see the effect of the parameters on the residues. The data reported are obtained through the extraction reported in the experimental part.

[0062] Example 1 (Effect of water)

[0063] Two different formulations with the same amount of catalyst and different amount of water were deposited on the paper which is then cured at 100° for 2 minutes.

[0064] The results are depicted in Figure 1 .

[0065] The amount of residues of the stoichiometric ratio is lower with respect to the 1 :1 ratio probably because of the complete hydrolysis. The gel time is reduced to 2 hours for the formulation with higher water.

[0066] Example 2 (effect of the catalyst)

[0067] Two different formulations with the same amount of water and different amount of catalyst (see experimental part) were deposited on the paper which is then cured at 100° for 2 minutes.

[0068] The results are depicted in Figure 2.

[0069] The formulation with low amount of catalyst presents a reduced amount of residues with an increasing trend increasing the time of reaction. This probably is explained with the slow reaction of the monomers / dimers whose kinetics is slow down by the pH conditions.

[0070] Example 3 - (effect of the temperature)

[0071] Two different formulations with the same amount of catalyst and water were deposited on the paper which is then cured at 100° or 150° for 2 minutes.

[0072] The results are depicted in Figure 3.

[0073] The higher temperature of curing creates a stronger interaction between the cellulose and the alkoxy silane probably inducing a higher percentage of covalent bonding. Example 4 (effect of ageing)

[0074] Two different formulations with the same amount of catalyst and water were deposited on the paper which is then cured at 150° for 2 minutes, the extraction is carried the day after the coating or one week later.

[0075] The results are depicted in Figure 4. Considering all the different factors previously seen, the results are summarised in Figure 5.

[0076] In summary, the experiments revealed that timely application of the alkoxy silane monomer and acid catalyst to cellulose is beneficial for improving effective grafting, with shorter contact times and temperatures below room temperature yielding better results.

Claims

CLAIMS:

1. 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 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, sulfuric 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 an item selected from the group consisting of paper cups, paper trays, paper forks, paper spoons, paper lids, and paper sheets.

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