GB2601381A

GB2601381A - Air filtration apparatus

Info

Publication number: GB2601381A
Application number: GB2018833.0A
Authority: GB
Inventors: Kant Anil
Original assignee: Hardshell Uk Ltd
Current assignee: Hardshell Uk Ltd
Priority date: 2020-11-30
Filing date: 2020-11-30
Publication date: 2022-06-01
Also published as: GB202018833D0

Abstract

A filtration layer suitable for use in an air filtration apparatus is provided by a layer of nonwoven nanofibers. The filtration layer may be produced by providing a polymer-containing liquid and forming the nanofibers from the liquid through a spinning process. The filtration layer may include an antiviral or antibacterial agent selected from nano zinc oxide, nanosilver and chitosan and mixtures thereof. The filtration layer may form part of an air filtering apparatus suitable for use as a protective face covering. The face covering may take the form of a face mask comprising at least three superimposed layers, and the filtration layer (layer 1) is situated between cover layers (layer 2, layer 3). The nanofibers of the filtration layer may have a smooth surface, to minimise light reflection, and therefore increase transparency. This ensures the suitability for use in an air filtration apparatus, such as a face mask, when it is desirable to see a wearer’s face.

Description

AIR FILTRATION APPARATUS

Field of the Invention

The present invention relates to a filtration layer, and methods of production thereof More particularly, the present invention relates to an air filtration apparatus comprising the filtration layer, and methods of production thereof.

Background

Personal protective equipment, such as face masks, are commonly worn by medical personnel. This can be stressful for patients, particularly children, as they cannot see their doctor or nurse's facial expression. The majority of our daily interactions involve visual communication. The inability to see a person's face due to a face mask can result in miscommunication, and can make conversation difficult. This may be particularly relevant in a medical setting, when medical personnel are required to have important conversations with patients regarding their treatment or diagnosis.

As a result of the global pandemic, face masks have become compulsory in many public places. However, this can make identification of individuals difficult, and hence pose a security risk There exists a need for face masks, or other suitable air filtering apparatus, that protect the wearer, whilst also allowing their face to be seen.

Summary of the Invention

According to a first aspect of the present invention, there is provided a filtration layer suitable for use in an air filtration apparatus, wherein the filtration layer is a layer of non-woven nanofibers.

The filtration layer may act as an air filtering layer, which is capable of allowing air to pass through, whilst filtering out particles, viruses and/or bacteria. The filtration layer may be suitable for use in an air filtration apparatus, for example a face mask. The filtration layer may allow the wearer of the mask to breathe unhindered.

Optionally, the nanofibers are between 10-500 nm in diameter.

Optionally, the nanofibers comprise at least one polymer having a molecular weight in the range of 10,000 to 300,000 g/mol and polydispersity index lower than 1.5.

Optionally, the nanofibers comprise at least one polymer having a monomer molecular weight in the range of 50 to 350 g/mol.

Optionally, the nanofibers comprise at least one polymer which is semi crystalline or amorphous, characterised in that said polymer has a degree of crystallinity equal to or less than 55 prior to processing.

The polymers selected for use may have a low molecular weight and low crystallinity, which means they can be hydrophobic in nature and may be easily dissolved.

Optionally, the nanofibers comprise one or more polymers selected from the group comprising: polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), poly(styrene-butadiene-styrene), polylactic acid (PLA), poly(styrene-isoprene-styrene) (SIS), poly(styeneethylene/butylene-styrene) (SEBS) or any mixture thereof.

Optionally, the filtration layer has a pore volume between 70-85%.

Optionally, the filtration layer has a transparency of at least 50%, as defined by ASTM D1746-15.

Optionally, the filtration layer further comprises at least one antiviral or antibacterial agent.

Optionally, the antiviral or antibacterial agent is selected from the group comprising: nano zinc oxide, nanosilver and chitosan and mixtures thereof.

According to a second aspect of the present invention, there is provided a method of producing a filtration layer according to the first aspect of the present invention, comprising at least the steps of providing a polymer-containing liquid and forming the nanofibers from the liquid through a spinning process.

Optionally, the liquid is provided by dissolving a polymer in a solvent, or melting a polymer, or using a resin polymer.

Optionally, the method comprises the step of forming the nanofibers through electrospinning.

Optionally, the method comprises the step of forming the nanofibers through electroblowing.

Optionally, during the electrospinning step the distance from needle to collector lies in the range 10 -30 cm.

Optionally, the electrospinning step is carried out at a flow rate in the range of 0.1 to 5 ml/h per needle.

Optionally, the electrospinning step is carried out at an applied voltage in the range 20 to 35 kV.

Preferably, the electrospinning step is carried out at an applied voltage in the range 30 to 35 kV.

Optionally, the method comprises the step of depositing the nanofibers on a collector surface, wherein a metal plate is provided on or behind the collector surface.

Conductive metal plates may be used to align the nanofibers. Aligning the nanofibers allows for the provision of a porous fibrous structure that can also be transparent. This may be achieved by aligning the nanofibers such that light reflection is reduced.

Optionally, the metal plate comprises copper.

Optionally, the method comprises the step of depositing the nanofibers on a collector roll, wherein the speed of rotation of the collector roll is in the range 1000-2000 rpm.

This speed range facilitates alignment and stretching of the nanofibers, in order to achieve transparency of the nanofiber layer.

Optionally, the liquid is provided by dissolving a polymer in a mixture of a first solvent and a second solvent, the first solvent having a boiling point of less than 100°C, and the second solvent having a boiling point greater than 100°C and at least one of said first and second solvents having electrical conductivity equal to or greater than 10' S/m.

Using more than one solvent can help to break down polymer crystallites and agglomerates. This may reduce light reflection and increase transparency of the nanofiber layer.

Optionally, the method further comprises the step of annealing the nanofibers at 10°C -30°C below their melting point.

The annealing step may smooth the surface of the nanofibers and thereby increase transparency of the nanofiber layer.

Optionally, the method further comprises the step of cooling the nanofibers at a temperature range of -5°C -0°C.

According to a third aspect of the present invention, there is provided an air filtering apparatus suitable for use as a protective face covering comprising at least two superimposed layers, the two layers being provided by a filtration layer according to the first aspect of the present invention, and a cover layer.

Optionally, the air filtering apparatus comprises at least three superimposed layers, wherein the filtration layer is situated between the cover layer and a further cover layer.

Optionally, the cover layer is a fibrous layer, wherein the fibres of the fibrous layer are selected from the group comprising: spun-bonded polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), polylactic acid (PLA), poly(styrene-butadiene-styrene), poly(styrene-isoprene-styrene) (SIS), poly(styrene-ethylene/butylene-styrene) (SEBS), or any mixture thereof.

Optionally, the cover layer comprises non-woven microfibers.

Optionally, the cover layer is formed by melt blowing the microfibers.

Optionally, the cover layer comprises mesh.

Optionally, the mesh cover layer is formed by 3D printing.

Optionally, the mesh cover layer is formed by a sintering method.

According to a fourth aspect of the present invention, there is provided a method of producing an air filtering apparatus according to a third aspect of the present invention, wherein the filtration layer is formed through electrospin-coating of nanofibers onto the cover layer.

Optionally, the filtration layer is formed onto the cover layer by placing the cover layer on the collector and electrospin-coating the filtration layer onto it.

This may provide a filtration layer which is inseparable from the cover layer. This may improve breathability of the air filtering apparatus.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a representative view of an air filtration apparatus comprising 3 layers, according to an embodiment of the present invention.

Figs. 2A-2C provide representative views of a single nanofiber of an air filtration layer, according to various embodiments of the present invention.

Fig. 20 is an SEM image of nanofibers with nanopores.

Fig. 3A and 3B are SEM images of layer 1 of a filtration apparatus according to

Example 1.

Figs. 4A and 4B are SEM images of layer 1 of a filtration apparatus according to Example 2.

Figs. 5A, 5B, 5C and 5D are Computer Aided Design (CAD) images of the air filtration layer.

Fig. 6 is an SEM image of a nylon nanofiber layer (layer 1) on top of a PP microfiber layer (layer 2).

Detailed Description

Fig. 1 provides an air filtration apparatus having three layers -layer 1, layer 2 and layer 3. Layer 1 is present between layers 2 and 3. Layer 1 is a layer of nanofibers. Layer 1 is an air filtration layer.

In accordance with the Examples of the present invention, layer 1 can comprise the following polymers: polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), poly(styrenebutadiene-styrene), polylactic acid (PLA), poly(styrene-isoprene-styrene) (SIS), poly(styrene-ethylene/butylene-styrene) (SEBS), or any mixture thereof.

The polymers may have molecular weight in the range of 10,000 to 300,000 g/mol and polydispersity index lower than 1.5.

The polydispersity index (PI) is a measure of the heterogeneity of a sample based on size, and is calculated by dividing the weight average molecular weight by the number average molecular weight.

The polymers may be semi crystalline or amorphous. The polymers may have a degree of crystallinity which is 55 or less in raw form, prior to processing.

The degree of crystallinity refers to the degree of structural order of the polymer.

Layer 1 can be formed by providing a liquid containing at least one of the polymers listed, and forming nanofibers from the liquid using a spinning process.

The liquid can be provided by dissolving a polymer in a solvent. Alternatively, the liquid can be provided by melting a polymer. Alternatively, a resin polymer may be used.

The liquid can be provided by dissolving a polymer in a mixture of two distinct solvents. One solvent may have a lower boiling point than the second solvent. At least one of the solvents is electrically conductive. A higher evaporation rate can therefore be achieved, as well as increased stretching of the fibers. This reduces fiber size and allows them to align in a manner that reduces light reflection.

Various spinning methods may be used to form the nanofibers, including electrospinning and electroblowing.

Electrospinning is a fibre production method which uses electric force to draw charged threads of polymer solutions or polymer melts up to fibre diameters in the order of some hundred nanometres. Electrospinning is a method that would be commonly understood by those of skill in the art.

Nanofibers are formed on a collector during electrospinning. It is widely accepted that the collector can have a significant effect on the productivity and arrangement of the nanofibers being formed, and their final structure. The distance from the needle to the collector can lie between 10 and 30 cm.

Electrospinning can be carried out at flow rate in the range of 0.1 to 5 ml/h per needle. Fibres of different diameters can be obtained using different flow rates. A lower flow rate is preferred as when a lower flow rate is used, less solution exits the needle, which results in the formation of thinner nanofibers and reduced combination of fibres. Singular fibres are therefore achieved at each stretching.

The number of needles present will depend on the scale of production, for example, hundreds of needles may be used in a manufacturing plant, whereas tens of needles may be used in a laboratory apparatus.

Electrospinning can be carried out an applied voltage in the range 20 to 35 kV, more preferably 30 to 35 kV. The applied voltage also influences the diameter of the nanofibers. In this range, thinner nanofibers formed, with increased transparency.

The nanofibers formed during electrospinning are deposited on a collector surface.

The collector surface may comprise a metal plate, such as a copper plate, which can be present on or behind the collector surface. A metal plate is used due to its conductivity, to facilitate alignment of the nanofibers.

The nanofibers may be deposited on a collector roll. The collector roll can rotate, and the speed of rotation can be in the range 1000-2000 rpm.

Alternatively, the nanofibers can be formed by electroblowing. Electroblowing is a gas-assisted electrospinning method that would be commonly known to those of skill in the art.

After the spinning step has taken place, the nanofibers may be annealed at 10°C -30°C below their melting point. The nanofibers may then be cooled to within a temperature range of -5°C -0°C.

The methods of production described herein can be used to produce layer 1, which is a nanofibrous and nanoporous air filtration layer, suitable for use in an air filtration apparatus, such as that depicted in Fig. 1.

The air filtration apparatus may be suitable for use as a protective face covering. The apparatus may comprise at least two superimposed layers. At least one layer is a filtration layer (layer 1). At least one layer is a cover layer.

The apparatus may comprises three superimposed layers, wherein the filtration layer (layer 1) is situated between the cover layer and a further cover layer (layers 2 and 3) This specific embodiment is depicted in Fig. 1.

The cover layer may be a fibrous layer. The fibres may be selected from the group comprising: spun-bonded polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), polylactic acid (PLA), poly(styrene-butadiene-styrene), poly(styrene-isoprene-styrene) (SIS), poly(styrene-ethylene/butylene-styrene) (SEBS), or any mixture thereof.

The cover layer may be a hydrophobic or superhydrophobic layer.

Alternatively, the cover layer may comprise non-woven microfibers. The cover layer may be formed by melt blowing the microfibers.

Melt blowing is a conventional fabrication method of micro-and nanofibers where a polymer melt is extruded through small nozzles surrounded by high speed blowing gas. A person skilled in the art would be familiar with this method.

Alternatively, the cover layer may comprise mesh. The mesh may be formed by 3D printing method or by a sintering method. The mesh may be porous and fibrous, or porous and non-fibrous.

The air filtration apparatus may be formed by electrospin-coating nanofibers onto the cover layer. The filtration layer may be formed onto the cover layer by placing the cover layer on the collector and electrospin-coating the filtration layer onto it.

Figs. 2A-2C provide representative views of a single nanofiber of layer 1.

Layer 1 may be a nanofibrous and nanoporous layer. The nanofibers may have a diameter between 10-500 nm.

Layer 1 may comprise nanopores having a pore volume between 70-85%.

Layer 1 may have a transparency of at least 50%, as defined by ASTM D1746-15.

Layer 1 may further comprise at least one antiviral or antibacterial agent, which can be selected from the group comprising: nano zinc oxide, nanosilver and chitosan and mixtures thereof.

The antiviral or antibacterial agent(s) may be present within the nanopores of the nanofibers of layer 1.

As depicted in Figs. 2A-C, each single nanofiber may have nanopores at its surface.

Antiviral nanostructures such as chitosan, titanium dioxide and/or zinc oxide can be present, as shown in Fig. 2A. These structures are present to kill viruses trapped within the nanopores.

Submicron or nanoparticles of silver may be mechanically trapped in or at the surface of or between the nanofibers. Viruses can spread via droplets. The nanofibers may prevent the absorption of such droplets, thereby preventing viruses from travelling through the filtration layer.

The nanofibers may be hollow, as shown in Figs. 2A and 2B, in order to improve breathability of the air filtration layer, and filtration efficiency. This makes the layer suitable for use in an air filtration apparatus, such as a face mask.

The presence of pores can improve filtration, as well as antiviral and antibacterial efficiency.

Fig. 20 is an SEM image of nanofibers with nanopores.

Fig. 3A and 3B are a scanning electron microscopy images of layer 1 (air filtration layer) according to Example 1. The layer in Fig. 3A may comprise TPU, PLA or PCL nanofibers, or any combination thereof. The layer in Fig. 39 may comprise nylon (polyamide).

As demonstrated by Figs. 3A and 3B, the nanofibers of layer 1 have a smooth morphology.

Rough nanofiber surfaces reflect light from the nano and micro crazes and lines from their surface. The nanofibers of the present invention may have a smooth surface, in order to minimise light reflection, and therefore increase transparency. This ensures their suitability for use in an air filtration apparatus, such as a face mask, when it is desirable to see the wearer's face.

Fig.s 4A and 4B are SEM images of layer 1 (air filtration layer) of a filtration apparatus according to Example 2.

As outlined by the figure, this layer has a porous and fibrous structure. This contributes to the filtration efficiency of the apparatus according to Example 2.

The smoothness of the fibers contributes to the transparency of layer 1.

Figs. 5A and 5B provide an air filtration layer with hexagonal cuts in its centre. These hexagonal cuts improve breathability.

Fig. 5C provides an air filtration layer with computer-generated triangle-shaped cuts. Fig. 5D provides an air filtration layer with computer-generated cross-shaped cuts. The cut shape of 5C and 5D may further increase pore size.

The air filtration layers outlined by these CAD images may be produced by a 3D printing method.

The nanofiber layer is the air filtration layer, and the microfiber layer is a cover layer.

EXAMPLES

Example 1 Example 2 Example 3 Example 4 Example 5 Layer 1 Electrospun Nanofibrous Transparent filter ENTF@ Antibacterial Antibacterial Antibacterial Electrospun Nanofibrous Transparent filter ENTF@ and antiviral and antiviral and antiviral Electrospun Nanofibrous transparent filter Electrospun Nanofibrous transparent filter Electrospun Nanofibrous transparent filter AAENTF@ AAENTF@ AAENTF@ Layer 2 Hydrophobic Semi- Hydrophobic Electrospun Fibrous Transparent Filter AHEMNTF@ Hydrophobic Semi- Hydrophobic Electrospun Fibrous Transparent Filter AHEMNTF@ 3D printed transparent transparent porous and Fibrous Spun Fibrous Spun transparent filter cover 3DPTFC@ bonded PP bonded PP (lower GSM) (lower GSM) Layer 3 Hydrophobic Semi- Hydrophobic Electrospun Fibrous Transparent Filter AHEMNTF@ No layer No layer Electrospun Nanofibrous Transparent filter ENTF@ transparent Fibrous Spun bonded PP (lower GSM) Efficiency Type II Type II Type I Type I Type II (Antiviral) (Antiviral) (Antiviral) Transparency >50% >60% >70% >75% >70% Methods: According to Example 1 of the present invention, layer 1 is made of nanofibrous and nanoporous structure of polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), and polyvinyl alcohol (PVA) or a mixture thereof. According to a specific embodiment, layer 1 is made out of 5-10 wt% TPU dissolved in Dimethylformamide (DMF) and mixed for 2 hours at 80 °C. The solution is injected into the blowing or spinning machine where the nanofibers are deposited on a rotating or moving collector. In order to reach to the maximum transparency (>50%) and a filtration efficiency equal to Type II face masks (according to BS EN 14683:2019) the nanofibers are designed to have an optimised combination of characteristics. These characteristics include: fibre size, which should be below 400 nm in order to minimise reflection; pore volume, should be more than 70% and less than 85%; pore size, which should be as small as possible (<100 nm preferably); and the surface of the nanofiber should have a smooth morphology. In this embodiment, TPU is dissolved in DMF at an optimum concentration, and electrospinning is carried out at optimised parameters (including distance from needle to collector, flow rate, and applied voltage). The layer produced contributes to more than 50% of the filtration efficiency.

According to Example 2 of the present invention, layer 1 possesses additional antiviral and antibacterial agents. These agents are added to layer 1 by being added to the base polymeric solution In a specific embodiment, nanosilver (2-3 wt%) is added to the TPU composition.

According to Example 2, layer 2 is a hydrophobic layer. The polymer solution was prepared by dissolving PLA in a combination of solvents such as chloroform (CF) and Dimethylformamide (DMF). In a specific embodiment, the polymer PLA (10-16 wt %) is dissolved in the mixture of CF and DMF (ratio of CF to DMF= 3:1). The membranes are dipped and submerged in a polystyrene (PS) bath at 45°C in order to ensure that layer 2 is superhydrophobic. This hydrophobicity contributes to water splash resistance of the air filtration apparatus/face mask.

Claims

CLAIMS1. A filtration layer suitable for use in an air filtration apparatus, wherein the filtration layer is a layer of non-woven nanofibers.
2. A filtration layer according to claim 1, wherein the nanofibers are between 10- 500 nm in diameter.
3. A filtration layer according to claim 1 or claim 2, wherein the nanofibers comprise at least one polymer having a molecular weight in the range of 10,000 to 300,000g/mol and polydispersity index lower than 1.5.
4. A filtration layer according to any one of the preceding claims, wherein the nanofibers comprise at least one polymer which is semi crystalline or amorphous, characterised in that said polymer has a degree of crystallinity equal to or less than prior to processing.
5. A filtration layer according to any one of the preceding claims, wherein the nanofibers comprise one or more polymers selected from the group comprising: polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), poly(styrene-butadiene-styrene), polylactic acid (PLA), poly(styrene-isoprene-styrene) (SIS), poly(styene-ethylene/butylenestyrene) (SEBS) or any mixture thereof.
6. A filtration layer according to any one of the preceding claims, wherein the filtration layer has a pore volume between 70-85%.
7. A filtration layer according to any one of the preceding claims, wherein said filtration layer has at transparency of at least 50%, as defined by ASTM D1746-15.
8. A filtration layer according to any one of the preceding claims, further comprising at least one antiviral or antibacterial agent.
9. A filtration layer according to claim 8, wherein the antiviral or antibacterial agent is selected from the group comprising: nano zinc oxide, nanosilver and chitosan and mixtures thereof.
10. A method of producing a filtration layer according to any one of the preceding claims, comprising at least the steps of providing a polymer-containing liquid and forming the nanofibers from the liquid through a spinning process.
11. A method of producing a filtration layer according to claim 10, wherein the liquid is provided by dissolving a polymer in a solvent, or melting a polymer, or using a resin polymer.
12. A method according to claim 11, comprising the step of forming the nanofibers through electrospinning.
13. A method according to claim 11, comprising the step of forming the nanofibers through electroblowing.
14. A method according to claim 12 or claim 13, wherein during the electrospinning step the distance from needle to collector lies in the range 10 -30 cm.
15. A method according to any one of claims 12-14, wherein the electrospinning step is carried out at a flow rate in the range of 0.1 to 5 ml/h per needle.
16. A method according to any one of claims 12-15, wherein the electrospinning step is carried out at an applied voltage in the range 20 to 35 kV, more preferably in the range 30 to 35 kV.
17. A method according to any one of claims 12-16, comprising the step of depositing the nanofibers on a collector surface, wherein a metal plate is provided on or behind the collector surface.
18. A method according to any one of claims 12-17, comprising the step of depositing the nanofibers on a collector roll, wherein the speed of rotation of the collector roll is in the range 1000-2000 rpm.
19. A method according to claim 11, wherein the liquid is provided by dissolving a polymer in a mixture of a first solvent and a second solvent, the first solvent having a boiling point of less than 100°C, and the second solvent having a boiling point greater than 100°C and at least one of said first and second solvents having electrical conductivity equal to or greater than 10-5 S/m.
20. A method according to any one of claims 11-19, further comprising the step of annealing the nanofibers at 10°C -30°C below their melting point.
21. A method according to claim 20, further comprising the step of cooling the nanofibers at a temperature range of -5°C -0°C.
22. An air filtering apparatus suitable for use as a protective face covering comprising at least two superimposed layers, the two layers being provided by a filtration layer according to any one of claims 1-10 and a cover layer.
23. An air filtering apparatus according to claim 22, comprising at least three superimposed layers, wherein the filtration layer is situated between the cover layer and a further cover layer.
24. An air filtering apparatus according to claim 22 or claim 23, wherein the cover layer is a fibrous layer, wherein the fibres of the fibrous layer are selected from the group comprising: spun-bonded polypropylene (PP), polyvinylidene fluoride, polyvinylidene difluoride (PVDF), polyethylene (PET), polyester (PS), polyurethane (TPU), polycaprolactone (PCL), polyamide (PA), polyvinyl alcohol (PVA), polylactic acid (PLA), poly(styrene-butadiene-styrene), poly(styrene-isoprene-styrene) (SIS), poly(styrene-ethylene/butylene-styrene) (SEBS), or any mixture thereof.
25. An air filtering apparatus according to any of claims 22 to 24, wherein the cover layer comprises non-woven microfibers.
26. An air filtering apparatus according to claim 25, wherein the cover layer is formed by melt blowing the microfibers.
27. A method of producing an air filtering apparatus according to any one of claims 22-26, wherein the filtration layer is formed through electrospin-coating of nanofibers onto the cover layer.
28. A method according to claim 27, wherein the filtration layer is formed onto the cover layer by placing the cover layer on the collector and electrospin-coating the filtration layer onto it.20 25 30