FR3114514A1 - Use of airgel based on metallic nanowires as filter material for air treatment, Associated electrode cartridge, Associated air treatment system. - Google Patents

Abstract

Use of airgel based on metallic nanowires as filter material for air treatment, Associated electrode cartridge, Associated air treatment system. The invention essentially consists in proposing an airgel based on metallic nanowires whose intrinsic properties of biological air filtration (effective porous structure, biocidal material) have been demonstrated by the inventors, this airgel also being able to be thermally activated by Joule effect when a controlled electrical voltage is applied, in order to regenerate it. Figure for the abstract: Fig.2

Description

Use of airgel based on metallic nanowires as filter material for air treatment, Associated electrode cartridge, Associated air treatment system.

The present invention relates to the use of an airgel based on metallic nanowires as a filtering material for treating, and in particular decontaminating air, in particular air through personal protective equipment such as a mask , or habitable enclosed spaces (homes, offices, vehicle interiors, etc.).

The present invention aims to improve currently existing air treatment equipment.

Air quality is an important subject in the field of public health.

In particular, the air can contain human pathogenic microbes transmitted by the respiratory tract, or “pathogens”. These pathogenic agents can cause a pathology in a human being, in particular in the respiratory tract.

In particular, viruses, bacteria and fungi which can be transmitted by air are known. We can mention the Influenza viruses and the coronaviruses, in particular SARS, SARS-CoV-2. These pathogens can be attached to airborne particles which, depending on their size, can enter the gas exchange regions of the lungs, or even pass through the lungs to affect other organs.

Air filtration systems have been the subject of numerous developments based, for example, on low porosity materials, the oxidation of volatile organic compounds or the absorption/adsorption on active supports (in particular carbon-based) .

In particular, filtration with a fiber filter is widely used to retain particles in suspension because such a filter generally presents a good compromise between filtration efficiency and energy consumption. Collective or individual protective equipment or industrial dust collectors are composed of non-woven fibrous media, i.e. a veil or sheet of fibers oriented directly or randomly and linked by friction, cohesion or adhesion. . These systems must be regularly renewed.

In addition, personal protective equipment conventionally uses paper or fabric filters. However, these filters are for single use only. They therefore generate waste in large quantities. They also require an uninterrupted manufacturing and distribution process to avoid shortage.

The air can also contain pollutants.

In particular, human activities, such as the burning of fossil fuels in vehicles and various industrial processes, generate significant amounts of harmful particles. Typical air pollutants in homes and workplaces can include, for example, particulate matter, nitrogen oxides (NOx) or sulfur oxides (SOx), organic compounds including formaldehyde and volatile organic compounds similar (VOC).

Activated carbon is widely used for the treatment of molecules by adsorption. Its mechanical resistance is limited. In addition, it needs to be replaced regularly.

Fiber filter filtration is also widely used to separate airborne pollutant particles.

During the recent SARS-CoV-2 coronavirus epidemic, it became apparent that the issue of personal protective equipment with filtering capacity was of prime importance to prevent the spread of the virus. Viruses are between a few tens and a few hundred nanometers in size. In the case of SARS-CoV-2, a diameter of approximately 60-140 nm has been reported: https://www.ncbi.nlm.nih.gov/books/NBK554776//.

Like other coronaviruses, it is sensitive to ultraviolet rays and heat.

It is known that microbes and in particular viruses are destroyed by the action of heat. For example, the SARS coronavirus in 2003 was strongly destroyed at 56°C (https://www.who.int/csr/sars/survival_2003_05_04/en/), and that of SARS-CoV-2 is very quickly inactivated at 70 °C: https://www.thelancet.com/action/showPdf?pii=S2666-5247%2820%2930003-3.

There is a need to improve the systems for decontamination and/or depollution of the air, and in particular of the air intended to be inhaled.

There is a particular need to provide personal protective equipment with filtration material(s) that can be heated, in order to ensure the destruction of any virus or microbe that would be trapped in the said material(s). ).

An object of the invention is to meet, at least partially, this (these) need(s).

To do this, the invention firstly relates to the use of electrically and thermally conductive metallic airgel based on metallic nanowires, as a filter material for the treatment of air, the airgel having an electrical conductivity greater than or equal to 50 S/m and a density less than or equal to 300 mg/cm ³ .

Preferably, the metallic airgel has one or more of the following characteristics:

• the nanowires are made of silver and/or copper or a metallic mixture comprising at least 50% by mass of at least one of these two metals;
• the airgel has an electrical conductivity between 50 and 5000 S/m, preferably between 100 and 2000 S/m;
• the density of the metallic airgel is less than or equal to 100 mg/cm³, preferably less than 50 mg/cm³ ;
• the porous structure of the metallic airgel can be modified as desired by modulating the dimensions of the objects (mainly length) and their concentration at the time of manufacture of the airgel. Process conditions such as freezing processes, e.g. freezing rate, especially radial vs. longitudinal freezing axis or drying processes (freeze-drying or supercritical drying) also have an influence on the porous structure. All these parameters will be chosen by those skilled in the art to best optimize the porous structure of the metallic airgel with regard to the target filtration system in which it is intended to be implanted in order to obtain the desired preferential filtration;
• the pressure drop induced by the metallic airgel is less than 150 Pa measured at a speed of 5.33 cm/s, preferably less than 80 Pa;
• the nanowires have an average diameter of less than 500 nm, preferably less than 200 nm, and an average length between 1 and 500 μm, preferably between 5 and 50 μm.

Advantageously, the airgel can comprise one or more polymers to help with the structuring. Thus, according to an advantageous embodiment variant, the metallic airgel comprises one or more polymers, preferably based on EDOT (3,4-ethylenedioxythiophene) or cellulose derivatives such as hydroxyethylcellulose, carboxymethylcellulose or hydroxypropylmethylcellulose .

Preferably, the mass proportion of metallic nanowires in the polymer(s) being between 1 and 1000%, preferably between 30 and 300%, preferably between 60 and 140%.

In order to give the desired shape to the metallic airgel, so that it corresponds exactly to the location provided in a filtration system, a mold of specific size, for example in Teflon or in metal (stainless steel Inox 316 coated or not PTFE (Teflon®) film, can be used.

The invention also relates to an air treatment system, comprising:

- an electrical voltage generator;

- at least one heating element comprising:

• an electrically and thermally conductive metallic airgel based on metallic nanowires, the airgel having an electrical conductivity greater than or equal to 50 S/m and a density less than or equal to 300 mg/cm ³ ,

• two contact recovery electrodes, each arranged in contact with the metallic airgel allowing a flow of air to be treated to pass, and connected to the voltage generator, so that they can apply an electrical voltage in the airgel.

Advantageously, the two electrodes are arranged on two opposite faces of the metallic airgel. Making electrical contact on the metallic airgel falls within the competence of a person skilled in the art. A point contact, typically less than 1 mm² is possible but, preferably, the resumption of contact is established on a larger surface, at least 10 mm² to ensure an optimal passage of the current in the airgel. The structure of the electrodes can have any shape such as a grid of the surface, a square, a zig-zag shape or a circular shape.

A device for applying the electrodes against the airgel based on a compression spring can also be considered to limit the pressure on the airgel.

Preferably, the electrodes can be deposited in contact with two opposite faces of a conductive airgel material of cylindrical shape, for example, the heating element then being functional by applying a voltage between the two electrodes. They can be made, for example, from a metal deposit. They can, for example, be produced from a conductive varnish, ink or lacquer (preferably based on silver or copper) and/or metal wires/films (copper, silver, indium, tin), carbonaceous materials (carbon nanotubes or graphene for example), and/or conductive polymers and/or a metal eutectic (for example gallium-indium).

The contact resumptions can also be carried out according to the usual physical techniques, for example by chemical vapor deposition (CVD for “Chemical Vapor Deposition”) or by physical vapor deposition (PVD for “Physical Vapor Deposition”).

The contact recovery electrodes are connected to an external voltage generator. The power supply can be fixed or nomadic, for example it can be a battery, or a cell, supplied continuously or discontinuously, thus making the filtration system autonomous.

Advantageously again, the electrical resistance of the heating element is between 1 and 500 Ω and preferably between 5 and 50 Ω.

According to an advantageous embodiment, the heating element is in the form of a cartridge comprising a rigid body of electrically insulating material housing the metallic airgel and supporting the two electrodes.

Since the airgel is relatively mechanically fragile, wrapping it in a body that can withstand strong mechanical stresses ensures its physical integrity. By "rigid body" is meant here and in the context of the invention, an envelope made of a material that is rigid (resistance to deformation), hard (resistance to deformation under a high compressive load) and solid (ability to absorb energy during impact).

The material of the cartridge body is electrically insulating in order to avoid any short circuit, the two contact points being present on the outside of the cartridge.

Also, the body should be light as much as possible and be airtight and waterproof.

With a view to producing regenerable cartridges, the body of the cartridge is adapted to withstand temperatures above 220°, preferably above 250° C. and this without undergoing any notable alteration, because the regeneration cycles of the metallic airgel are preferably made above 200°C.

Considering the requirements, the wall of the cartridge body has a thickness of at least 2 mm.

Preferably, the body of the cartridge is made of thermoplastic material, more preferably chosen from polyimides, polyamide-imides, polybenzimidazoles, polyetheretherketones, polyphenylene sulphides, polyethers, perfluoroalkoxy or any mixture thereof. The polymer must not be electrically conductive, it preferably has a surface resistance greater than 10 ⁷ ohm/square.

According to an advantageous characteristic, the voltage generator is suitable for generating a supply voltage up to 48V, preferably between 0 and 20V, and more preferably between 0 and 12V.

According to an advantageous mode, the filtration system comprises a device for measuring the electrical resistance of the heating element.

The resistance measurement is easily carried out periodically.

Preferably, the resistance after regeneration is identical to that measured before use of the filter cartridge. A significant increase in this resistance indicates that the active airgel has deteriorated and that the cartridge must be changed.

Thus, a resistance measurement is advantageously carried out before the cartridge is used. The measured value called R0 is characteristic of the initial state of the active airgel. After regeneration and/or at regular intervals, typically from 1 min to 100 h, preferably from 10 min to 1 h, this resistance value Ri is measured and compared with R0 in order to determine the viability of the cartridge.

For example, if Ri < 1.2 R0, then the system is considered still effective, and if Ri > 1.2 R0 then the system is no longer considered effective and must be changed. The measurement can be performed manually with an ohmmeter.

Preferably, the resistance measuring device is autonomous and comprises for this purpose a power source which is integrated either in the cartridge or preferably in the filtration system. The resistance detection and/or quantification apparatus of the invention has a simple structure which enables it to be produced at low cost and on a large scale. This device can be very small in size, requiring little power to operate, thus promoting its portability. If the authorized resistance threshold is exceeded, an alert will be issued, for example by means of a light signal, via a red LED for example, a vibration, an audible signal, an electromagnetic signal or any combination of these signals.

The invention also relates to an air treatment method comprising the following steps:

a) filtration of a quantity of air by means of an electrically and thermally conductive airgel based on metallic nanowires, as filter material for the treatment of air, the airgel having an electrical conductivity greater than or equal to 50 S/m and a density less than or equal to 300 mg/cm3;

b) heating the airgel for a predetermined time.

Steps a) and b) can be carried out simultaneously or step b) can be carried out once step a) has been completed.

The heating according to step b) can be carried out according to different protocols, including these preferentially:

b1) the heating is maintained before, until a certain temperature stabilizes, for example at 60°C and for at least the time of use of the filtration system.

b2) heating is carried out at the end of use to clean the filter (destruction of any living species), for example up to a temperature of 100°C. In the case where the material comprises only nanowires and no associated polymer, it can also be envisaged to heat to a higher temperature, for example 250°C, in order to definitively eliminate any trace of organic residue with a view to a complete regeneration of the airgel filter material.

b3) the heating is activated intermittently by controlled temperature rises and falls, with the possibility of plateaus at all temperatures greater than or equal to the ambient temperature.

The method is advantageously implemented by a filtration system as described above, step b) being carried out by applying a voltage to the contact electrodes in order to achieve heating by Joule effect.

The air treatment system according to the invention can be a device that actively filters the air, for example an air conditioner or a heater blowing air, in particular an electrical device. It then comprises an air circulator configured to generate a flow of air to be treated through the metallic airgel.

The treatment system can be used in particular for filtering the air in habitable enclosed spaces such as homes, offices or vehicle interiors.

The treatment system can also be personal protective equipment such as a mask.

Thus, the invention essentially consists in proposing an airgel based on metallic nanowires whose intrinsic properties of biological air filtration (effective porous structure, biocidal material) have been demonstrated by the inventors, this airgel also being able to be thermally activated by Joule effect when a controlled electrical voltage is applied, in order to regenerate it.

Surprisingly, no biological air filtration system has implemented a metallic airgel, combining the following numerous properties of interest:

- an intrinsically antimicrobial, biocidal material thanks to the carefully selected metals of the nanowires;

-a submicron porosity preventing the diffusion of microdroplets potentially carrying viruses or other microbes within the material;

- a low pressure drop allowing the flow of air to be treated to pass easily;

- a low weight of active material, which limits the overall mass of a filtration system, which is particularly advantageous, for example for personal protective masks;

- an electrical conductivity making it possible to be able to heat the airgel by applying a preferably low voltage (<48V, preferably <20V) to the terminals thereof, continuously or discontinuously depending on the intended use, in particular to regenerate it at high temperature. For the application of a voltage on the conductive airgel, in order to heat the element in a controlled manner, reference may be made to the patent application FR3070973.

An air treatment system according to the invention can use several heating elements, preferably in the form of cartridges, for example in series or in fluidic parallel.

It is quite possible to combine an air treatment system according to the invention with another filtering system, with an oxidation system, by plasma for example, with an ionizer, or with a UV treatment system or any another system to purify the air.

A cartridge according to the invention can in particular be placed in fluidic series, preferably after a first filtering system. For example, a paper filter impregnated to improve resistance to humidity, arranged upstream will increase the performance of the treatment system according to the invention and extend the life of the metal airgel cartridge. A filter upstream of the metal airgel cartridge can also contain materials known to those skilled in the art such as, and without limitation, activated carbon, plastic fibers or foams, cotton, glass fibers , silica gels or aerogels.

Other advantages and characteristics will emerge better on reading the detailed description, given by way of illustration and not limitation, with reference to the following figures.

there is a perspective view of a metal airgel according to the invention, in the form of a cylinder, used to carry out the treatment of an air flow.

there is a perspective view of a metal airgel according to , further comprising two contact electrodes in order to heat the airgel by the Joule effect during the air treatment.

there is a perspective view of a metal airgel according to integrated in the body of a cartridge integrating the two contact recovery electrodes.

detailed description

There shows a metallic airgel cylinder 1 based on silver metallic aerogels which is used to carry out the air treatment.

To carry out the manufacture of this airgel with the expected dimensions, the following procedure was carried out.

Silver nanowires are prepared according to the process described in patent application WO2015/177717 and dispersed in water at a concentration of 40 g/L.

CMC (carboxymethylcellulose) 250K is dissolved in water at a concentration of 1.5% by weight.

The nanowires are dispersed in the CMC solution to obtain the total mass concentrations of 3 wt.% (Ag:CMC ratio = 1:1).

The mixture is stirred for 30 min then left like this for 5 hours before being introduced into a cylindrical mold made of PTFE (Teflon®) with a diameter of 3 cm and a height of 3 cm.

The whole is cooled by contact on cold thermoregulated trays at -20°C, then the solid obtained is freeze-dried.

An airgel in the form of the cylinder 1 is thus obtained with the expected dimensions.

Cylinder 1 can be used as is as an air filtration device.

The cylinder 1 described can also be part of a filtration device implementing Joule effect heating.

An example of an electrical conduction filtration device will now be described.

On the two opposite faces of the cylinder, the airgel 1, as shown in , contact electrodes 10, 11 are made by depositing flat copper wires in the shape of a hollow disc which are glued with a conductive glue on each side, as shown in .

By way of example, the ring of an electrode 10, 11 may have a circumference width of approximately 2 mm.

The whole of the airgel 1 with its electrodes 10, 11 is placed in a receptacle 2 made of PEEK (polyetheretherketone) which is closed, leaving only the two contact wires to come out for connection to a low voltage generator. An air filtration cartridge 3 is thus formed, as shown in .

The resistance R0 across the terminals of cartridge 3 is measured equal to 10 ohm.

As an example of the use of the filtration cartridge 3, a flow of air is circulated at the speed of 50 L.min ^-1 for 120 min through the cartridge 3.

At the end of this filtration step, the resistance R0 across the terminals of the cartridge 3 is measured equal to 11 ohm.

A voltage of 20V is applied to the electrodes 10, 11 and therefore in the airgel 1. The application of this voltage allows a rise in temperature to 200° C., which is a surface temperature, this temperature then being stabilized for 10 min .

The resistance R0 across the terminals of cartridge 3 is measured equal to 10 ohm.

The cartridge 3 is again reused with an air flow at a speed of 50 L.min ⁻¹ for 120 min without significant modification of the pressure drop measurement, since it is less than 3%.

Other advantages and improvements may be made without departing from the scope of the invention.

The synthesis of metallic nanowires, for example silver or copper, which constitute a metallic airgel according to the invention can also be carried out as described respectively in the articles Nanotechnology 24 (2013) 215501 and Nano Research 2014, 7(3): 315 –324.

The invention is not limited to the examples which have just been described; it is in particular possible to combine together characteristics of the examples illustrated within variants not illustrated.

Claims (19)

Use of electrically and thermally conductive metallic airgel based on metallic nanowires, as a filtering material for the treatment of air, the airgel having an electrical conductivity greater than or equal to 50 S/m and a density less than or equal to 300 mg/cm ³ . Use according to claim 1, the nanowires being made of silver and/or copper or of a metallic mixture comprising at least 50% by weight of at least one of these two metals. Use according to claim 1 or 2, the airgel having an electrical conductivity between 50 and 5000 S/m, preferably between 100 and 2000 S/m. Use according to one of the preceding claims, the density of the metallic airgel being less than or equal to 100 mg/cm ³ , preferably less than 50 mg/cm ³ . Use according to one of the preceding claims, the pressure drop induced by the metallic airgel being less than 150 Pa measured at a speed of 5.33 cm/s, preferably less than 80 Pa. Use according to one of the preceding claims, the nanowires having an average diameter of less than 500 nm, preferably less than 200 nm, and an average length between 1 and 500 μm, preferably between 5 and 50 μm. Use according to one of the preceding claims, the metallic airgel comprising one or more polymers, preferably based on EDOT (3,4-ethylenedioxythiophene) or cellulose derivatives such as hydroxyethylcellulose, carboxymethylcellulose or hydroxypropylmethylcellulose . Use according to claim 7, the mass proportion of the metal nanowires in the polymer(s) being between 1 and 1000%, preferably between 30 and 300%, preferably between 60 and 140%. Air treatment system, including:

• an electric voltage generator;
• at least one heating element (3) comprising:

an electrically and thermally conductive metallic airgel (1) based on metallic nanowires, the airgel having an electrical conductivity greater than or equal to 50 S/m and a density less than or equal to 300 mg/cm³,
two contact electrodes (10, 11), each arranged in contact with the metallic airgel by allowing a flow of air to be treated to pass, and connected to the voltage generator, so that they can apply an electric voltage in the airgel. Treatment system according to claim 9, the two electrodes being arranged on two opposite faces of the metallic airgel. Treatment system according to claim 9 or 10, the electrical resistance of the heating element being between 1 and 500 Ω and preferably between 5 and 50 Ω. Treatment system according to one of Claims 9 to 11, the heating element being in the form of a cartridge comprising a rigid body (2) of electrically insulating material housing the metallic airgel and supporting the two electrodes Treatment system according to claim 12, the body of the cartridge being adapted to withstand temperatures above 220°, preferably above 250°C. Treatment system according to claim 12 or 13, the body of the cartridge being made of thermoplastic material, preferably chosen from polyimides, polyamide-imides, polybenzimidazoles, polyetheretherketones, polyphenylene sulphides, polyethers, perfluoroalkoxy or any mixture of these. Processing system according to one of Claims 9 to 14, the voltage generator being adapted to generate a supply voltage of up to 48V, preferably between 0 and 20V, and more preferably between 0 and 12V. Treatment system according to one of Claims 9 to 15, comprising a device for measuring the electrical resistance of the heating element. Air treatment process comprising the following steps:

1. filtration of a quantity of air by means of an electrically and thermally conductive airgel based on metallic nanowires, as a filtering material for the treatment of air, the airgel having an electrical conductivity greater than or equal to 50 S/ m and a density less than or equal to 300 mg/cm ³ ,

b) heating the airgel for a predetermined time. Treatment process according to claim 17, according to which steps a) and b) are carried out simultaneously or step b) is carried out once step a) has been carried out. Treatment method according to one of Claims 17 or 18, implemented by a system according to one of Claims 9 to 16, step b) being carried out by applying a voltage to the contact electrodes .