WO2019058292A1

WO2019058292A1 - Nano-fiber based filter media and methods of preparation thereof

Info

Publication number: WO2019058292A1
Application number: PCT/IB2018/057240
Authority: WO
Inventors: Ashwini Kumar AGRAWAL; Manjeet Jassal; Deepika Gupta; Tushar Vyas; Prateek Sharma; Jatin KEWLANI
Original assignee: Indian Institute Of Technology Delhi; Nanoclean Global Pvt Ltd
Priority date: 2017-09-20
Filing date: 2018-09-20
Publication date: 2019-03-28

Abstract

This invention relates to nanofiber-based filter media and method of preparation thereof. The nanofiber-based filter media of the present invention has better adhesion properties and provide improved filtration efficiency of particulate matter (PM) of size between 1 and 2.5 micron in the range of 50 to 99.999% at the level of pleural pressure drop of. 0.05 - 1 mbar.

Description

"NANO FIBER BASED FILTER MEDIA AND METHODS OF

PREPARATION THEREOF"

FIELD OF INVENTION

The present invention encompasses the area of nanofiber technology. More specifically, the present invention relates to the development of nanofibers-based filter media with enhanced filtration efficiency and reduced pressure drop.

BACKGROUND PRIOR ART

In the prevalent adverse environmental conditions, there exists a need of filtration devices capable of removing pollutants, particulate matter and other environmental elements. With the advent of nanofiber technology, use of nanofibers as filter media has greatly enhanced filtration efficiency and lowered the pressure drop. Several air/ fluid filtration technologies are available commercially employing use of nanofibers. Some standard configuration of filter media composed of nanofibers is as follows:

6073/DELNP/2009 discloses nanofiber-based filter configured for insertion as a filter structure, panel or cylindrical cartridge into a filtration unit. The filter comprises a high efficiency substrate layer or combination of layers with at least one layer of nanofiber or fine fiber formed on the substrate layer or combination of layers. The nanofiber layer and the high efficiency substrate are selected to obtain a balanced set of properties that permits the user to remove submicron particles efficiently at a relatively low-pressure drop. A high efficiency substrate (either a single layer or a layered substrate structure) has a particulate efficiency exceeding 80%.

WO 2010127634 Al describes various configurations of making multilayered filter assembly with nanofibers. The method includes coating a layer of nanofibers on single or both sides of substrate medium to obtain a composite filter medium and folding the composite filter medium in a serpentine arrangement to form the multilayer filter. Alternatively, the method includes coating a layer of nanofibers on both sides of a substrate medium to obtain a composite filter medium and stacking up a plurality of sheets of the composite filter medium to form a multilayer filter. The multilayer filter comprises a top layer, a bottom layer and at least one unit between the top layer and the bottom layer. Each unit comprises two layers of nanofibers sandwiched between two layers of the substrate medium. The nonwoven material constitutes the substrate.

US 9421707 B2 describes method of manufacturing filter media for personal protection and face mask incorporating nanofibers made from electrospinning process, deposited on convex moulds and inner or outer shell of protective mask. Nanofibers were also functionalized for capturing gases. Structurally, the filter media comprise of an upper cover made of spunbond materials and nanofibers are deposited on the inner side of this covering layer. Similarly, a lower covering material is deposited with nanofiber and is placed on the upper cover such that nanofiber layer is sandwiched between the two spunbond layers.

2552/DEL/2013 relates to a multi-layered web-shaped filter material for filter elements for the filtration of gases and/or liquids, with a fleece layer, with a nanofiber layer and with a cellulose layer. A reduced flow resistance is obtained when the nanofiber layer is formed with nanofibers through a coating of die fleece layer and when die cellulose layer is glued onto the nanofiber layer by means of an adhesive.

WO 2014143039 Al discloses a face mask comprising of a nanofiber layer having a configuration where in an inner layer comprising nonwoven fiber material configured to contact a wearer's nose and mouth, a middle layer disposed on the inner layer and comprising a nanofiber material, an outer layer disposed on the middle layer and comprising nonwoven fiber material.

However, aforementioned multilayered configurations lead to high pressure drop. Moreover, substrates on which the nanofibers are deposited are non-woven synthetic materials. Air leakage from the edges of the face/ nasal masks is another major problem and to overcome the same, these have been further modified to cover only the lower portion of the nose or to fit tightly inside the nostrils. US5740798A describes a nasal band filter to be worn on user's nose and surrounding user's nostrils, comprising a filter element made up of a swatch of thermal undergarment material having an upper, a lower, a right and a left outer edge for surrounding user's nostrils, two sheer material pieces having an adhesive coating on one side wherein each of said sheer material pieces is attached to an opposite end of said filter element and at least one backing strip covering said sheer material pieces to protect the adhesive coating prior to use of nasal band filter.

US20040194784A1 describes non-insertable type respiratory particulate filter that is composed of synthetic meshed filter region which can be adhered to the nose with the use of adhesives at the edges of the filter media. The filter layer is composed of triangular synthetic melt blown woven nylon fabric material with mesh size between 20-25μ.

However, over the past decade increasing developments have been made in the area of nanofiber filters. US20080110342A1 describes particle filter system incorporating nano fibers of diameters less than 1μ formed into a fiber mat by electrospinning technique deposited on a support mesh thereby making a layered filter media. Adhesion of the nanofibers to the support was improved via the application of an adhesive to the mesh directly prior to electrospinning. The adhesive typically is a slow drying adhesive permitting enhanced adherence of the electro spun fibers. Alternately, in another embodiment, the fibers of the mesh can be coated with a very thin layer of polymer that has surface groups which interact with the polymer fibers being deposited on the mesh, via non-covalent interactions such as van der Waals, hydrogen-bond, dipole, electrostatic attraction, etc.

US6872311 B2 describes formation of nanofiber filter media by depositing a nanofiber layer on to High Efficiency Particulate Air (HEP A) filter blended with lyocell and/or fiberglass. Filter media had thickness of 0.25 mm, FoM= 0.075, efficiency= 99.9% for PM -0.18 μ, pressure drop -40 mm of water column at flow rate of 32 1/min. The performance was increased by increasing the FoM of the filter media. KR20110131665, CN 201420422404, US20170216634 Al and KR20170000747U discuss various polymer-based nanofiber filter media/ devices. All of these nasal filters describe insertable devices that fit properly inside the nasal wall. However, insertable nasal filters may prove irritable and uncomfortable to the user. Further, the filter media is also non-biodegradable non-woven material.

2832/DEL/2014 relates to composite nanofiber technology-based hybrid water filter element. More specifically, it discloses a water filtration element and assembly, wherein a water permeable barrier comprises a composite nanofiber-based assembly, sequentially packed in the form of a bed.

Another major issue involved in the development of nanofiber-based filters is to optimize the filtration efficiency and pressure drop without compromising on the adherence of the nanofibers on the support material. US20080110342A1 has demonstrated adhesion of the nanofibers to the support mesh via the application of an adhesive to the mesh directly prior to electrospinning.

WO2016171331 Al claims mask pack with improved attachment of nanofibers layer to the substrate by electrospinning of low melting point polymer prior to spinning of polymer nanofiber followed by heat treatment.

JP2015040366 A describes adhesion of nanofiber layer with substrate by spraying of adhesive before spinning of nanofibers.

Afore mentioned adhesion techniques do not improve the attachment of nanofibers with each other and the nanofibers can easily separate from each other due to abrasion/ shear forces on handling. Therefore, these techniques do not suffice for the nanofibrous layer to endure mechanical handling of the nanofibrous layer either by users during the intended use or while undergoing various processes during manufacturing. Accordingly, the present invention provides nanofiber-based filter media having better adhesion properties among nanofibers within nanofiber web and of the nano web with the substrate. The filter media of the present invention provides high filtration efficiency in the range of 50-99.999% of particles of average size between 1 to 2.5micron PM while ensuring reduced pressure drop.

OBJECTIVES OF THE INVENTION

The objective of the present invention is to provide a novel nanofiber-based filter media.

Another objective of the present invention is to provide abrasion resistant layer of nanofiber-based filter media having high filtration efficiency at low pressure drop.

Another objective of the present invention is also to devise a method for the preparation of the above filter medium where in electrospun polymeric nanofiber layer is deposited/coated over a microfiber substrate layer in the form of woven, non- woven or knitted material of areal density (i.e. grams per square meter) of 10 to 150 followed by drying and curing to generate covalent bonds among the fibers at crossover points to obtain a mechanically stable and abrasion resistant structure.

Yet another objective of the present invention is to provide an efficient method for improving adhesion of nanofibers within the nanofiber layer and of nanofiber layer with the microfiber substrate layer in one step by infiltration of a reactive fluid on to the microfiber substrate layer coated with nanofiber layer.

SUMMARY OF INVENTION

The present invention discloses nanofiber-based filter media. The filter media of the present invention incorporates composite nanofibers by electrospinning blends of polymer, adhered to a microfiber substrate layer via a reactive fluid followed by curing to reinforce adhesiveness and methods of preparation thereof.

The nanofiber-based filter media of the present invention provides efficient filtration with reduced pressure drop.

In an embodiment, the polymers used for the purpose of the invention are as it is or combination of synthetic and/or natural polymers that may also be biodegradable and biocompatible. Examples of the polymers include but are not limited to polyamides, polyesters, poly styrene, cellulose or its derivatives, gelatin, polyure thanes, poly-ε- caprolactone, poly (vinyl acetate), vinyl-based polymers, ether-based polymers, silk derived polymers, soy protein, zein protein, polylactides, polyglycolides, polygalacturonic acid, alginates etc.

In an embodiment, the nanofiber-based filter media of the present invention provides efficient filtration of particulate matter 1 to 2.5 micron at low pressure drop at the level of pleural pressure drop, i.e. 0.05 - 1 mbar.

The invention further provides a method for the preparation of filter media comprising the use of a substrate which is made up of microfibers of natural, regenerated or synthetic polymeric fibers having diameters in the range of 1 micron to 100 microns and may be in the form of woven, non-woven or knitted material of areal density (i.e. grams per square meter) of 10 to 150. The said microfiber substrate layer is then coated with electrospun nanofibers of diameter in the range of 50 to 400 nm, which is made up of synthetic and/or natural polymers that may be also biodegradable and biocompatible. The said polymers can be used as it is in various concentrations or as a blend of two or more polymers in various compositions and concentrations to achieve nanofibers of uniform diameter. The as-formed structure comprising of nanofiber layer supported on the microfiber substrate layer is treated with a reactive fluid by infiltration method to create a coating/layer of reactive material around the nano and the microfibers. The treated structure is then subjected to curing by known methods such as heat treatment and UV irradiation to form covalent bonds at the cross-over points among all the fibers.

In an embodiment, the polymers used in the formation of the microfiber substrate layer are made up of either synthetic, natural or derivatives of natural polymers including cellulosics, cellulose acetates, polyesters, nylons, polypropylene, wool, silk, etc. or their blends. The said microfiber layer used for the preparation of the filter media is made up of fibers having diameters in the range of 1 micron to 100 microns and may be in the form of woven, non-woven or knitted material of areal density (i.e. grams per square meter) of 10 to 150.

In an embodiment, the polymers for the purpose of preparing nanofibers are used as it is or in combination of two or more synthetic or natural polymers that may also be biodegradable and biocompatible. Blending of two or more polymers alters the rheology of a polymeric solution and is thus used to vary morphology and diameter. The composition and concentration of polymers or its blends are selected to achieve good spinnability, with uniform fiber diameter in the range of 50-400 nm. The rheology of the polymer solution can also be altered by using more than one solvents. Adhesion among the nanofibers, within the nanofiber layer and of the nanofiber layer with the microfiber substrate layer is achieved by creating a chemically reactive layer on the nanofibers of the nanofiber layer and on microfibers of the substrate layer followed by curing by known methods such as heat treatment, UV radiation, or plasma treatment etc., to form covalent bonds at the cross-over points between the fibers. The creation of reactive layer on the fibers is achieved using methods, such as by infiltration of solution of a reactive material (termed as reactive fluid), blending the reactive material with the electrospinning solution, coating a reactive material using sheath-core spinning system during formation of fibers. The reactive coatings on nanofibers and microfibers of the substrate may be formed in one step or separately in two steps. The reactive fluid may consist of solutions of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds having multiple reactive functional groups optionally blended with initiators and catalysts as known in the art. Colour may be optionally added to the nano and microfiber layer for improving the aesthetic appearance.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates nanofibers morphology before and after blending polymer with rheology modifier.

Figure 2 illustrates different morphology and diameter of nanofibers on varying polymer composition. Figure 3 provides different extent of depositions of nanofibers onto substrate.

Figure 4 provides schematic representation of reactive liquid infiltration methodology to create a thin reactive layer on fibers.

Figure 5 illustrates delamination of nanofiber layer from the substrate without creating a reactive layer on fibers (both nano and micro).

Figure 6 provides a schematic of the nanofiber-filter media joined together by creating a reactive layer around the fibers (a) in side view and (b) top view.

Figure 7 provides scanning electron micrographs showing (a) joined nanofibers of the nanofiber layer (b) joined nanofibers with microfibers of substrate using reacted coating at cross-over points.

Figure 8 provides scanning electron micrograph of a filter media obtained after deposition and adhesion of nanofibers on a fibrous substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses nanofiber-based filter media comprising composite nanofibers of natural or synthetic polymer adhered to a microfiber substrate layer via a thin coating of reactive material. The nanofiber-based filter media of the present invention provides efficient filtration. In an embodiment, the nanofiber-based filter media of the present invention provides efficient filtration of PM 1 to 2.5 micron at the level of pleural pressure drop, i.e. 0.05 - 1 mbar. The process of preparing the filter media of the present invention comprises use of methods known in the art including but not limited to continuous electro spinning a polymer melt or solution comprising blend of polymers to form nanofibers of various diameter ranging from 50-400 nm over a woven, knitted or non-woven substrate made up of natural, synthetic polymer or their blends. The nanofibers of the nanofiber layer are adhered among themselves and to the micro fibers of substrate via a coated layer of a reactive material followed by curing to induce adhesion. The treatment with the reactive fluid to form a layer around the fibers followed by curing results in joining of the nano fibers with each other at the cross over points or at the junction points within the microfiber substrate layer and nanofiber layer.

In an embodiment, the nanofiber-based filter media comprises at least one nanofiber layer and at least one microfiber substrate layer, wherein 50% or more of the fibre surface in all of the said layers is coated with a reactive material and said fibers are covalently bonded with each other at the cross-over points to form a mechanically stable and abrasion resistant structure.

In another embodiment, the microfiber substrate layer used is in the form of woven, knitted or nonwoven.

In an embodiment, the polymers for the purpose of preparing nanofibers are used as it is or in combination of two or more synthetic or natural polymers that may also be biodegradable and biocompatible. Blending of two or more polymers alters the rheology of a polymeric solution and is thus used to vary morphology and diameter.

The concentration of a polymer and composition and concentrations of more than one polymer as a blend is optimized and selected to achieve good spinnability, with uniform fibers of diameter in the range of 50 - 400 nm. The rheology of the polymer solution is modified by blending specific amounts of other polymers and/or more than one type of solvent. The total polymer concentration is used in the range of 3- 25 wt%. In case of polymer blends, one of the polymer components is used in lower concentration, which is in the range of 0.005 to 10 wt%. The solvent system can be composed of either a single solvent or blend of more than one solvent. Certain exemplary solvent/ solvent mixtures used for the purpose of the present invention include but are not limited to a mixture of acetone, dimethylsulfoxide and dimethylacetamide in the ratio 2: 1 :1 or a mixture of acetone and dimethylsulfoxide in the ratio 2:1 or a mixture of water and dimethylsufoxide in the ratio 2:1 ratio or a mixture of organic acids such as formic and acetic acids in the ratio 1 :2. In another embodiment, the microfiber substrate layer is made up of natural, regenerated or synthetic polymeric fibers or their blends and are in the form of woven, nonwoven or knitted structure.

In yet another embodiment, the microfiber substrate layer has an areal density of 10 to 150 grams per square meter (gsm).

In an embodiment, the diameter of nanofibers in the nanofiber layer is in the range of 50-400 nm and has an areal density in the range of 0.01 to 5 gram per square meter (gsm).

In another embodiment, the reactive fluid consists of solutions of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds optionally blended with initiators and catalysts.

In an embodiment, the filter media has filtration efficiency of particulate matter (PM) of size between 1 and 2.5 micron between 50 to 99.999% at the level of pleural pressure drop of 0.05 - 1 mbar at a flow rate of 32 L/min through a filter surface area of 100 cm².

Adhesion of nanofibers in the nanofiber layer as well as the nanofibers onto the microfiber layer is achieved by infiltration of a reactive fluid to create a reactive material coating/layer around the nano and micro fibers. The reactive material layer is created using a reactive fluid, which may consist of solutions of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds having multiple reactive functional groups optionally blended with initiators and catalysts as known in the art. Colour may be optionally added to the nano and microfiber layer for improving the aesthetic appearance in case of face/ nasal masks.

Nanofibers are prepared using polymer solutions of varying concentrations using methods known in the art including but not limited to electrospinning. Nanofibers of different morphologies are obtained by varying the concentration and composition of the polymers and solvents. In an embodiment, colour is added to the nanofiber layer to improve the aesthetic properties of the nanofibers.

The electrospun nanofibers are deposited over a woven, knitted or non-woven microfiber substrate layer in varying amounts with areal density ranging from 0.01 to 5 gsm. Preferably, the microfiber substrate layer is a woven or nonwoven material made-up of a natural polymer.

The microfiber substrate layer thus coated with the nanofiber layer is treated with reactive fluid of concentration varying from 0.1-15 wt% by applying the reactive fluid in the range from 5% on the weight of the microfiber substrate layer to 300% on the weight of the microfiber substrate layer, for achieving adhesion of the nanofibers with the microfibers. In an embodiment, the reactive fluid used consists of a solution of curable acrylic based binder. A reactive coating/layer is created around the nano and microfibers, followed by curing by known methods such as heat treatment and UV irradiation to form covalent bonds at the cross-over points between fibers. The reactive layer around the fiber surfaces is achieved using methods known in the art, such as by infiltration of a reactive fluid, blending the reactive fluid with the electrospinning solution, or using sheath-core spinning geometry while forming the nanofibers.

In an embodiment, the method of preparing a nanofiber-based filter medium comprises: a) Depositing nanofibers of diameter between 50 nm to 400 nm by continuous electrospinning from a melt or solution of a polymer or a blend of polymers at an areal density of 0.01 to 5 gsm over a microfiber layer of areal density of 10 to 150 gsm; b) Creating a coating of a reactive fluid around the nano and micro fibers present in the structure obtained from step (a) by infiltration of a reactive fluid; and c) Drying and curing the product obtained from step (b) to generate covalent bonds among the fibers at cross-over points to obtain a mechanically stable and abrasion resistant structure.

In another embodiment, the polymer is non-water soluble and biodegradable.

In an embodiment, the reactive fluid consists of solutions of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds optionally blended with initiators and catalysts.

In another embodiment, colour is added/imparted to nano and micro fiber layers optionally for improving the aesthetic appearance.

In another embodiment, the microfiber substrate layer coated with the nanofiber layer is dipped in the solution of the reactive fluid followed by drying and curing at 100°- 150°C.

In another embodiment, the microfiber substrate layer is first padded with the reactive fluid, dried and coated with nanofiber layer followed by curing.

In yet another embodiment, the reactive fluid was sprayed from the top onto the microfiber substrate layer with nanofiber layer facing upwards.

The filtration efficiency and pressure drop of the nanofiber filter media thus obtained are then determined.

The methods of preparation and examples provided herein are for the purpose of illustration of the invention and are not intended in any way to limit the scope of the invention.

Example 1

Preparation of the filter media

A biodegradable, dyed, woven microfiber substrate layer of GSM 70 was chosen for deposition of the nanofiber layer. Nanofibers composed of a polymer which is a cellulose derivative of 10 wt% blended with another polymer which is a biodegradable vinyl-based polymer of 0.2 wt% were prepared in solvent mixture of acetone, dime thylsu If oxide and dimethylacetamide in the ratio 2:1 :1 and electrospun on the said microfiber substrate layer. Blending of another polymer and more than one solvent (act as a rheology modifier) alters the rheology of the said solutions and helps in obtaining uniform spinning and fiber morphology (fig 1 ).

The said microfiber substrate layer was coated with nanofibers using electrospinning at a flow rate of 2 ml/h, spinneret to collector distance of 18 cm, spinning voltage of 12 kV and -10 kV and winding speed of 6 m/min to obtain a deposition with areal density of 0.1 gsm. The fiber diameter obtained was ~ 340 ± 21 as observed under scanning electron microscope (SEM).

Example 2

In another embodiment, a biodegradable, woven microfiber substrate layer of GSM 40 was taken. Nanofibers composed of a polymer which is a cellulose derivative of 6 wt% blended with another polymer which is a biodegradable vinyl based polymer of 1.0 wt% were prepared in solvent mixture of acetone and dimethylsulfoxide in the ratio 2:1 and electrospun at a flow rate of 3 ml/h, spinneret to collector distance of 18 cm, spinning voltage of 14 kV and -9 kV and winding speed of 4 m/min over the said microfiber substrate layer to achieve an areal density of 0.2 gsm. The fiber diameter obtained was ~ 290 ± 15 as observed under SEM.

Example 3

In yet another embodiment, a cellulose based non-woven substrate of GSM 50 was selected. Nanofibers composed of polyamide polymer of 9 wt% blended with another polymer, which is a biodegradable ether-based polymer of 0.5 wt% were electrospun at a flow rate of 1 ml/h, spinneret to collector distance of 18 cm, spinning voltage of 10 kV and -8 kV and winding speed of 8 m/min and deposited over the said microfiber substrate layer to achieve an areal density of 0.07 gsm. The fiber diameter obtained was ~ 400 ± 17 as observed under SEM.

Example 4

In yet another embodiment, a cellulose based non-woven substrate of GSM 50 was selected. Nanofibers composed of vinyl-based polymer of 11 wt% in a mixture of water and dimethylsufoxide (2:1 ratio) were electrospun at a flow rate of 5 ml/h, spinneret to collector distance of 20 cm, spinning voltage of 15 kV and -8 kV and winding speed of 5 m/min and deposited over the said microfiber substrate layer to achieve an areal density of 0.25 gsm. The fiber diameter obtained was ~ 400 ± 17 as observed under SEM.

Example 5

Adhesion of the nanofibers with substrate

The structure comprising of nanofiber layer and microfiber substrate layer was treated with reactive liquid to achieve adhesion of all the layers of nanofibers in the nanofiber layer and of the nanofibers with the microfibers of the substrate layer without blocking pores by the method known in the art of liquid infiltration (fig 3).

In an embodiment, said structure comprising of nanofiber layers and microfiber substrate layer was infiltered with 5 wt% reactive liquid composed of acrylic based polymer by dip-padding and cured at 150 °C for 3 min.

Example 6

The structure comprising of nanofiber layers and microfiber substrate layer was infiltered with 3 wt% reactive fluid composed of multifunctional polymer by spraying thereby creating a thin reactive coating/layer around the nano and microfibers. After curing at 130 °C for 5 min, the thin reactive layer forms crosslinks at the crossover points thereby adhering all the nanofibers within the nanofiber layer and nanofiber layer with the microfiber of the substrate layer (Figure 7).

Example 7

In yet another embodiment, the structure comprising of nanofiber layers and the microfiber substrate layer was infiltered with liquid having no reactive fluid resulting in no adhesion of nanofibers with microfibers (Figure 5).

Example 8

In another embodiment, the microfiber substrate layer was treated with 10 wt% reactive fluid, dried and then was coated with electrospun nanofibers followed by wetting and curing at 150 °C for 3 min.

Example 9

In yet another embodiment, 10 wt% of the reactive fluid was mixed in the polymer solution for electrospinning and nanofibers were electrospun on the microfiber substrate layer followed by wetting and curing at 130 °C for 5 min.

Characterization of nanofibers and the filter media

Characterization of some of the samples created using the methodology mentioned in this document are given below:

(a) Morphology

Table 1 shows polymer-modifier concentrations used for some of the sample and their respective fiber diameters obtained. Table 1: Polymer- modifier concentrations and fiber diameters

Some examples of SEM analysis of the prepared nanofibers with different diameters are shown in Figure 2 and that of a typical filter media is shown in Figure 8.

(b) Analysing the filtration efficiency

The filter media created using the methodology mentioned herein were tested for their efficiency for filtration of various particle sizes. The filtration efficiency and pressure drop was measured as per the standard EN799-initial efficiency using instrument Topas GmbH (model LAP 340) at a flow rate of 32 L/min and net effective filtering area of 100 cm² at temperature and humidity conditions of 19 ^°C and 36% respectively. Test for filtration efficiency and pressure drop of known filter media (consisting of more than 2 layers) were conducted as shown in Table 2 and compared with the nanofiber filter media as formed herein. Results of some of the filter media sample with varying areal density of nanofibers (gsm) in the nanofiber layer is shown in Table 3. Table 2: Filtration efficiency and pressure drop of known filter media Known filter Pressure Efficiency (%) Quality media Drop factor samples

mbar* PM PM PM 1 PM 5 For PM

0.3 0.5 1.0 Romson filter 2.26 98.26 99.99 99.99 99.99 2.03 (6 layered) 3M-N95 filter 1.85 96.65 99.99 99.99 99.99 2.48 (5 layered) Surgical mask 0.80 0.15 9.87 29.25 48.76 4.8

Table 3: Filtration efficiency and pressure drop of filter media Sample Pressure Efficiency (%) Quality

Drop (Δρ) factor mbar PM PM PM 1 PM 5 for PM

0.3 0.5 1.0

Base woven- 0.20 0.04 10.33 28.96 48.71 16.65 without

nanofiber Woven- 0.5 3.078 65.20 88.15 98.17 8.46 nanofiber -0.5

gsm Woven- 0.40 26.77 50.93 62.62 80.28 10.4 nanofiber- 0.3

gsm

Base 0.12 0.95 23.28 33.56 50.44 nonwoven- without

nanofiber Non-woven- 0.16 3.77 48.20 66.277 86.70 26.12 nanofiber 0.09

gsm Non- woven- 0.17 5.01 49.05 66.69 87.055 24.7 nanofiber 0.1

gsm The nanofiber-based filter media shows good overall performance expressed in terms of the filtration efficiency and quality factor which is given by 1η(1/Ρ)/Δρ, which is fractional penetration (i.e. 1 -fractional filtration efficiency) and Δρ is the pressure drop when compared with the known filter media.

Claims

The Claims:

1. A nanofiber-based filter media comprising at least one nanofiber layer and at least one microfiber substrate layer, wherein 50% or more of the fibre surface in all of the said layers is coated with a reactive material and said fibers are covalently bonded with each other at the cross-over points to form a mechanically stable and abrasion resistant structure.

2. The nanofiber-based filter media as claimed in claim 1 , wherein the nanofiber layer is made up of natural polymers or their derivatives, synthetic polymers or combination thereof.

3. The nanofiber-based filter media as claimed in claim 1, wherein the microfiber substrate layer is made up of natural, regenerated or synthetic polymeric fibers or their blends and are in the form of woven, nonwoven or knitted structure.

4. The nanofiber-based filter media as claimed in claim 1 , wherein the microfiber substrate layer has an areal density of 10 to 150 grams per square meter (gsm).

5. The nanofiber-based filter media as claimed in claim 1, wherein the diameter of nanofibers in the nanofiber layer is in the range of 50-400 nm and has an areal density in the range of 0.01 to 5 gram per square meter (gsm).

6. The nanofiber-based filter media as claimed in claim 1, wherein the reactive material consists of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds optionally blended with initiators and catalysts.

7. The nanofiber-based filter media as claimed in claim 1 , wherein the filter media has filtration efficiency of particulate matter (PM) of size between 1 and 2.5 micron between 50 to 99.999% at the level of pleural pressure drop of 0.05 - 1 mbar at a flow rate of 32 L/min through a filter surface area of 100 cm².

8. A method of preparing a nanofiber-based filter medium comprising:

a) Depositing nanofibers of diameter between 50 nm to 400 nm by continuous electro spinning from a melt or solution of a polymer or a blend of polymers at an areal density of 0.01 to 5 gsm over a microfiber layer of areal density of 10 to 150 gsm;

b) Creating a coating of a reactive material around the nano and micro fibers present in the structure obtained from step (a) by infiltration of a reactive fluid; and

c) Drying and curing the product obtained from step (b) to generate covalent bonds among the fibers at cross-over points to obtain a mechanically stable and abrasion resistant structure.

9. The method as claimed in claim 8, wherein the polymer is non-water soluble and biodegradable.

10. The method as claimed in claim 8, wherein the microfiber substrate layer is made up of a natural, regenerated or synthetic polymer.

11. The method as claimed in claim 8, wherein the microfiber substrate layer may be in the form of woven or non- woven, knitted structure.

12. The method as claimed in claim 8, wherein curing involves formation of covalent bonds at the cross-over points between the fibers.

13. The method as claimed in claim 8, wherein the reactive fluid consists of solutions of curable acrylic, polyurethane and other polymers having multiple reactive functional groups or compounds optionally blended with initiators and catalysts.

14. The method as claimed in claim 8, wherein a colour is added/imparted to nano and micro fiber layers optionally for improving the aesthetic appearance.