Technical field

The invention relates to a method of preparing a continuous layer consisting of porous carbon fibres which is capable of being sterilized by Joule heat. The invention further relates to a continuous layer consisting of porous carbon fibres functionalized with copper particles, which is prepared by this method.

In addition, the invention relates to an air filter and personal protective equipment for filtering inhaled and/or exhaled air comprising at least one continuous layer consisting of porous carbon fibres and functionalized with copper particles prepared by this method.

Background art

Currently, numerous functional materials based on (active/activated) carbon are known, which can capture and eliminate various biological contaminants, especially bacteria and fungi, and due to sorption properties and specific surface area, also a number of chemical contaminants in gaseous and liquid state, including odours of different origin. The disadvantage of these materials is the fact that they contain carbon in the form of a disjointed and discontinuous layer, for example, in the form of powder or granules, whereby such a layer only has a limited use and does not allow to utilize the full potential of this material.

The object of the invention is to propose a method of preparing a continuous layer consisting of porous carbon fibres which would be in terms of practical use more suitable than a discontinuous layer. In addition, it is an object of the invention to provide this layer and an air filter, as well as personal protective equipment comprising at least one such layer. Principle of the invention

The method of preparing a continuous layer consisting of porous carbon fibres according to the invention is based on carbonization of a continuous fibrous structure consisting entirely or at least of 80 % by weight of acrylic fibres under controlled heating conditions (carbonization temperature, heating rate and holding time). The initial structure used is preferably a fibrous structure, which is otherwise unusable waste from various textile technologies. Suitable initial fibrous structures include especially different types of non-woven structures, such as non-woven fabrics and webs - reinforced, bonded (e.g., needle-punched), cross-laid, vertically laid and/or laid in folds (e.g., by the method according to CZ 306111), having a total surface weight of 1200 to 36000 g/m ² and a thickness of preferably 1 to 50 mm or more. Carbonization of such a structure takes place under the layer of charcoal or in an atmosphere of an oxidation-inhibiting gas, such as CO2, nitrogen, etc., at a temperature between 1200 and 1500 °C. The initial fibrous structure is gradually heated to this temperature at a rate of 300 to 400 °C/min and is kept at it for 3 to 10 minutes. This process results in carbonization during which hydrogen and part of the nitrogen and oxygen are removed from the structure of acrylic fibres and a structure consisting of 80 to 95 % by weight of carbon is produced; in some parts of the structure, under these conditions, graphitization occurs, during which the last nitrogen atoms are removed from the acrylic fibrous structure, resulting in the formation of more perfect carbon microcrystals and the formation of a more organized carbon structure - the so-called turbostratic structure, with controlled electrical conductivity, which consists of at least 99 % by weight of carbon. The resulting continuous layer thus consists of microporous carbon fibres and retains the fibrous character and morphology of the initial fibrous structure; at the same time, it keeps approximately 45 to 60 % of its original weight and thickness and is thermally and electrically conductive, which allows, for example, its heating by Joule heat, e.g., for the purpose of sterilization (see below). Moreover, the resulting continuous layer suitably combines rugged interfibrous spaces with a diameter of approximately 50 nm to 3 pm, depending on the initial structure, with porous fibres with pores having a diameter of 2 to 50 nm formed by the escaping of gaseous products of carbonization from the fibrous material, which makes it possible for the continuous layer to reliably capture not only mechanical impurities, but also microorganisms, especially bacteria and some viruses from 50 nm in size, chemical contaminants in gaseous and liquid form, as well as odours of biological origin or generated by some chemicals, such as ammonia, formaldehyde, volatile organic compounds, etc., while having its original or even higher breathability.

In order to prevent the internal structure of the initial continuous layer from collapsing during carbonization, it is advantageous to stabilize this layer before carbonization. A suitable method of stabilization is to expose the layer to a temperature of 250 to 320 °C in a pre-stressed state (under tensile stress) for 10 to 30 minutes in an oxidizing medium (e.g. in presence of air). During stabilization, in the acrylic fibrous structure, the bonds in the macromolecule chain cyclize and crosslink the macromolecules by oxygen bridges, which is accompanied by a change in colour, a reduction in weight, and gas escape, whereby individual fibres become non-meltable. In a preferred stabilization process, the initial layer is heated to stabilization temperature at a rate of 35 °C per minute.

To improve the antimicrobial effect and electrical conductivity of the resulting carbon structure, at least one copper precursor with a positive redox potential, such as copper hydroxide (Cu(OH) ₂ ), which is easily reduced to metal particles due to its positive redox potential, is incorporated into the initial structure. The metal particles then bind to the fibres of the carbon structure being formed by means of van der Waals forces and complex bonds with nitrile groups (-CºN) and carboxyl groups (-COOH). Preferably, this precursor is introduced into the structure of the initial layer in the form of an aqueous reaction mixture using one of the known methods - for example, by immersion in a bath, by spraying, padding, etc. In a preferred variant of embodiment, the reaction mixture contains, per 1 litre, 3 to 10 g of at least one divalent copper salt, such as copper sulphate pentahydrate (CuS0 ₄ .5H ₂ 0), which subsequently forms copper hydroxide Cu(OH) ₂ in the alkaline aqueous environment of the reaction mixture, 0.5 to 1 g of a reducing agent from the group consisting of sodium borohydride (NaBH ₄ ), glucose, furfural and formaldehyde, 10 to 18 g of triethanolamine (TEA) acting as a complexing agent preventing copper separation from the reaction mixture, 5 to 10 g of salt containing sulphure and sodium from the group consisting of sodium thiosulphate (Na ₂ S ₂ 0 ₃ ) and sodium sulphite (Na ₂ S0 ₃ ), which stabilizes the reaction mixture and ensures the deposition of copper only on the surface of the fibres, and an addition of a base which adjusts the pH of this mixture to pH « 9.5. A preferred base is, for example, sodium hydroxide (NaOH), which at the same time also loosens the compact surface of acrylic fibres, helps to open pores and partially hydrolyzes nitrile groups (-CºN) in the structure of acrylic fibres and converts them to amide (-NH ₂ ) and carboxyl groups (-COOH), resulting subsequently in increased copper deposition.

This structure with the reaction mixture applied is then heated to a temperature of 80 °C to 95 °C for 10 to 30 minutes, reducing the copper precursor and attaching the formed copper particles to the fibrous surface of the initial layer. During the reduction of the copper precursor, copper nanoparticles are formed, aggregating into larger units, and the agglomerates thus formed are deposited mainly on the surface of individual fibres and possibly also in their pores (see Fig. 3).

If necessary, this process can be performed simultaneously or successively with two or more precursors of the same or different metal/metals. A suitable combination is, for example, a combination of a copper precursor with a nickel precursor, e.g. nickel hydroxide (Ni(OH) ₂ ), which in the alkaline environment of the reaction mixture is formed from a divalent nickel salt, such as nickel sulphate hexahydrate (NiS0 ₄ .6H ₂ 0), wherein during the reduction of these precursors, the nickel particles bind by means of van der Waals forces and complex bonds via nitrile groups (-CºN) and carboxyl groups (-COOH) by physical forces to the fibres of the fibrous structure, part of the copper particles subsequently binding to the nickel particles. Nickel also acts as a fibre surface activator and copper reduction catalyst. The amount of the copper precursor applied corresponds to 2 to 15 mg of elemental metal per 1 g of the continuous layer consisting of porous carbon fibres. With respect to the fact that nickel has a lower electrical conductivity than copper, the amount of the nickel precursor in the reaction mixture is preferably equal to or less than the amount of the copper precursor in the reaction mixture, i.e., 1 to 15 mg (with a content of 1 to 10 g of divalent nickel salt in the reaction mixture).

The continuous layer consisting of porous carbon fibres according to the invention has a basis weight of 540 to 21600 g/m ² and a thickness of 0.45 to 50 mm, wherein the copper particles are attached to the carbon fibres of the continuous layer in an amount of 2 to 15 mg per 1 g of the continuous layer.

In another variant, the copper particles are attached to the carbon fibres of this continuous layer in an amount of 2 to 15 mg per 1 g of the continuous layer and nickel particles in an amount of 1 to 15 mg, wherein at least some of the copper particles are bound to the fibres of the layer through the nickel particles.

The thus prepared continuous layer consisting of porous carbon fibres has a wide range of uses. Due to its properties and high breathability, it can be used mainly as an active layer (i.e., layer with antimicrobial properties) of various types of gas filters, especially air filters, from air conditioning filters and similar systems of buildings or vehicles to filters of personal protective equipment for filtration of inhaled and/or exhaled air, including masks, half masks, surgical masks, face masks, etc. Thanks to a suitable combination of fibres with pores having a diameter of 2 to 50 nm and interfibrous spaces having a diameter of approximately 50 nm to 3 pm with high fragmentation

(curvature), the continuous layer can reliably capture and eliminate microorganisms, especially bacteria, fungi and viruses in sizes from 50 nm, due to the action of copper with antimicrobial effect, its breathability being sufficient for comfortable breathing of the user, even at thicknesses above 30 mm. What is more, however, in addition to capturing microorganisms, due to its porous structure, this layer can adsorb some gaseous and liquid chemicals and remove odours of different origins, such as those generated by some chemicals, e.g., by ammonia, formaldehyde, volatile organic compounds, etc. Its electrical conductivity enables its sterilization (regeneration) by so-called dry heat - Joule heating, in which electrical voltage from any source of electrical energy (power bank, electrical network, batteries, mobile phone, etc.) is applied to the continuous layer of porous carbon fibres. Sterilization is carried out efficiently even with a power input of only a few units of W for a period of only a few units of minutes, and the continuous layer consisting of porous carbon fibres is heated to a temperature above 100 °C, preferably around 140 °C, at which the adjacent layers of the filter, or protective equipment, are also effectively sterilized. For this purpose, this layer is preferably provided with two electrical contacts on opposite sides, such as metal strips, flat or longitudinal textiles made of electrically conductive fibres (silver, carbon, copper, stainless steel of Inox type, etc.). The form of attachment can typically be by conductive adhesives or encapsulation in a package with conductive contacts.

The air filter based on the use of the continuous layer of porous carbon fibres according to the invention comprises at least one such layer with a thickness of 1 to 50 mm, or even more, in combination with at least one layer of any filter material (preferably a HEPA type material or a filter material based on polymer fibres and/or nanofibres) with a melting point preferably above 170 ° C. The layer of filter material serves to trap the mechanical impurities contained in the filtered air and at the same time provides mechanical support and protection to the continuous layer consisting of porous carbon fibres. In a preferred variant of embodiment, the continuous layer consisting of porous carbon fibres arranged between two layers of filter material (the same or different), wherein it is mechanically attached to at least one of them, preferably by means of dots of a suitable binder - either circumferentially or over the entire surface. The binder used and the material of the other layers of the air filter must then have a softening point higher than the temperature to which the continuous layer consisting of porous carbon fibres is heated during sterilization.

A face mask for filtration of inhaled and/or exhaled air based on the use of the continuous layer consisting of porous carbon fibres according to the invention comprises at least one such layer with a thickness of 1 to 50 mm, optionally even more, in combination with at least one layer of any filter material (preferably a HEPA type material). The layer of filter material serves to capture mechanical impurities contained in the inhaled and/or exhaled air and at the same time provides mechanical support and protection to the continuous layer consisting of porous carbon fibres. In a preferred variant, the continuous layer consisting of porous carbon fibres is arranged between two layers of filter material (identical or different layers), wherein it is mechanically attached to at least one of them, preferably by means of dots of a suitable binder - either circumferentially or over the entire surface. The binder used and the material of the other layers of the air filter must then have a softening point higher than the temperature to which the continuous layer consisting of porous carbon fibres is heated during sterilization.

Description of the drawings

In the accompanying drawings, Fig. 1 is a photograph of a needle punched non-woven fabric made from acrylic fibres, Fig. 2 is a photograph of a continuous layer consisting of porous carbon fibres prepared by carbonizing the non-woven fabric of Fig. 1 , and Fig. 3 is an SEM image of this continuous layer with indication of resolution.

Examples of embodiment For illustrative purposes, 6 specific examples of preparing the continuous layer consisting of porous carbon fibres by the method according to the invention are shown below.

Example 1 A needle-punched non-woven fabric made from acrylic fibres having a basis weight of 1300 g/m ² (Fig. 1) was immersed for 1 minute in a reaction mixture which contained, per 1 litre, 14 g of triethanolamine, 0.5 g of sodium borohydride (NaBFU), 5 g of sodium sulphite (Na ₂ S0 ₃ ), 1 g of sodium hydroxide (NaOFI), copper hydroxide (Cu(OFI)2) formed from 5 g of copper sulphate pentahydrate (CUSO4.5FI2O) and nickel hydroxide (Ni(OFI)2) formed from 1 g of nickel sulphate hexahydrate (NiSC .OFhO). After removal, the non-woven fabric was heated to a temperature of 85 °C, at which temperature it was kept for 20 minutes, whereby the copper and nickel precursors were reduced and the formed nickel and copper particles attached to the surface of acrylic fibres, wherein some of the copper particles attached to the nickel particles on the surface of the acrylic fibres.

This was followed by stabilization, during which the thus treated non- woven fabric, in a pre-stressed state, at a rate of 35 °C/min was heated to a temperature of 280 °C, at which temperature it was kept for 20 minutes. It was then heated at a rate of 300 °C/min under the layer of charcoal to a temperature of 1250 °C, at which temperature it was kept for a period of 5 minutes, during which it was carbonized and partially graphitized (Fig. 2 and Fig. 3).

The surface area of the thus prepared layer consisting of porous carbon fibres was 210 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power source, it was heated to a temperature of 140 °C in 3 minutes.

Example 2

The same non-woven fabric made from acrylic fibres as in Example 1 , after immersion in the same bath for 1 minute in a pre-stressed state, was stabilized at a temperature of 320 °C, at which temperature it was kept for 10 minutes. This layer was then heated at a rate of 300 ° C/min under the layer of charcoal to a temperature of 1300 °C, at which temperature it was kept for 5 minutes, during which it was carbonized and partially graphitized.

The surface area of the thus prepared layer consisting of porous carbon fibrous structures was 278 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power source, it was heated to a temperature of 170 °C in 3 minutes.

Example 3 The same non-woven fabric made from acrylic fibres as in Example 1 , after immersion in the same bath for 1 minute in a pre-stressed state, was stabilized at a temperature of 320 °C, at which temperature it was kept for 30 minutes. The layer was then heated at a rate of 310 °C/min under the layer of charcoal to a temperature of 1200 °C, at which temperature it was kept for a period of 7 minutes, during which it was carbonized and partially graphitized. The surface area of the thus prepared layer consisting of porous carbon fibrous structures was 190 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power supply, it was heated to a temperature of 110 °C in 3 minutes. Example 4

An acrylic fibre web with a basis weight of 2700 g/m ² was immersed for 1 minute in a reaction mixture which contained, per 1 litre, 16 g of triethanolamine, 1 g of NaBH ₄ , 10 g of Na ₂ SC> ₃ , 1 g of NaOH and Cu(OH) ₂ formed from 3 g of CuS0 ₄ .5H ₂ 0. After removal, the web was heated to a temperature of 80 °C, at which temperature it was kept for 10 minutes, during which time the copper precursor was reduced and the formed copper particles attached to the surface of the acrylic fibres.

This was followed by stabilization, during which the thus treated web, in a pre-stressed state, was heated at a rate of 35 °C/min to a temperature of 300 °C, at which temperature it was kept for 15 minutes. It was then heated at a rate of 350 °C/min under the layer of charcoal to a temperature of 1450 °C, at which temperature it was kept for a period of 4 minutes, during which it was carbonized and partially graphitized.

The surface area of the thus prepared layer consisting of porous carbon fibres was 257 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power supply, it was heated within 3 minutes to a temperature of 125 °C.

Example 5

A needle-punched non-woven fabric made from acrylic fibres with a basis weight of 2300 g/m ² was immersed for 1 minute in a reaction mixture which contained, per 1 litre, 10 g of triethanolamine, 1 g of glucose, 7 g of Na ₂ SC> ₃ , 1 g of NaOH, CU(OH) ₂ formed from 8 g of CuS0 ₄ .5H ₂ 0 a Ni(OH) ₂ formed from 3 g of NiS0 ₄ .6H ₂ 0. After removal, the non-woven fabric was heated to a temperature of 90 °C, at which temperature it was kept for 30 minutes, whereby the copper and nickel precursors were reduced and the formed nickel particles attached to the surface of acrylic fibres and the copper particles attached to the nickel particles.

This was followed by stabilization, in which the non-woven fabric thus treated and in a pre-stressed state was heated at a rate of 35 ° C/min to a temperature of 250 °C, at which temperature it was kept for 30 minutes. It was then heated at a rate of 320 °C/min under the layer of charcoal to a temperature of 1250 °C, at which temperature it was kept for a period of 8 minutes, while it was carbonized and partially graphitized.

The surface area of the thus prepared layer consisting of porous carbon fibres was 272 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power supply, it was heated to 120 °C in 3 minutes.

Example 6

A non-woven fabric made from acrylic fibres folded by the method according to CZ 306111 with a basis weight of 36000 g/m ² was immersed for 5 minutes in a reaction mixture which contained, per 1 litre, 18 g of triethanolamine, 0.7 g of NaBH ₄ , 10 g of Na ₂ S ₂ C> ₃ , 1 g of NaOH, Cu(OH) ₂ formed from 10 g of CuS0 ₄ .5H ₂ 0 and Ni(OH) ₂ formed from 4 g of NiS0 ₄ .6H ₂ 0. After removal, the fabric was heated to a temperature of 95 °C, at which temperature it was kept for 30 minutes, wherein the copper and nickel precursors were reduced and some of the nickel and copper particles formed attached to the surface of acrylic fibres, whereby some of the copper particles attached to the nickel particles on the surface of the acrylic fibres.

This was followed by stabilization, in which the non-woven fabric thus treated, in a pre-stressed state, was heated at a rate of 35 ° C/min to a temperature of 320 ° C, at which temperature it was kept for 30 minutes. It was then heated at a rate of 400 °C/min in a CO2 atmosphere to 1500 °C, at which temperature it was kept for 10 minutes, during which it was carbonized.

The surface area of the thus prepared layer consisting of porous carbon fibres was 326 m ² /g. The layer was electrically conductive, wherein, when connected to a 4 W power supply, it was heated to 140 °C in 3 minutes.