FIELD OF THE INVENTION
The invention relates to electrospun polymeric nanofiber and a method of preparation thereof by an electrospinning process which utilizes electrical force to produce said nanofibers from polymer solutions or melts.
More particularly, the invention relates to Polyacrylonitrile (PAN) based nanofibers deposited on filtration media in automotive filters. The present invention also provides an application of the electrospun polymeric nanofiber obtained from PAN solution in air filter, fuel filter and oil filter.
BACKGROUND AND PRIOR ART OF THE INVENTION
Nanofibres combined with other non-woven products are used in a wide range of applications such as textile, sensors, drug delivery system, air filtration, protective clothing, in preparation of nanotubes etc. The use of nanofibres have increased because of the properties of the nanofibres that have ultra fine fibres with a high surface area per unit mass that increases the efficiency of the applications. Nanofibres have significant use in the area of filtration since the surface area is substantially greater and comprises of very small micropores. Highly porous structure makes it suitable for the use in variety of filtration techniques. (Doshi, J., and Reneker, D.H., "Electrospinning process and applications of Electrospun fibers", Journal of Electrostatics, Vol. 35, 1995, pp. 151- 160.)
Electrospinning is a process involving polymer science, applied physics, fluid mechanics, electrical, mechanical, chemical, material engineering and rheology. It is defined as the process by which a charged liquid polymer solution is introduced into an electric field. In this electrostatic technique, a high electric field is generated between a

polymer liquid contained in a spinning dope reservoir with a capillary tip or spinneret and a metallic fibre collection ground surface. When the voltage reaches a critical value, the charge overcomes the surface tension of the deformed drop of the suspended polymer solution formed on the tip of the spinneret and a jet is produced. This stretching process is accompanied by the rapid evaporation of the solvent molecules that reduces the diameter of the jet, in a cone shaped volume called the "envelope cone" typically referred to as Taylor cone After the jet Hows away from the droplet in a nearly straight line, it bends into a complex path and other changes in shape occur, during which electrical forces stretch and thin it by very large ratios (Reneker, D. H., and Chun, I., "Nanometre Diameter Fibres o[ Polymer, Produced by Electrospinning", Nanotechnology, Volume 7, 1996, pages 216-233; Yarin, A. L., and D.H. Reneker, Taylor cone and jetting from liquid droplets in electrospinning of nanofibers' - Journal of Applied Physics. 90 (2001) 4836-4846; Kowalewski, T. A, A.L. Yarin, and S. BJonski. 'Electrospinning of Polymer Nanofibers' Paper presented at The 5th Euromech Fluid Mechanics Conference, Toulouse, France. August 24-28, 2003).
In electrospinning, the fibre morphology is controlled by the experimental parameters and is dependent upon solution conductivity, concentration, viscosity, polymer molecular weight, applied voltage etc. The process can be adjusted to control the fibre diameter by varying the electric field strength and polymer solution concentration (Gu, S.Y., J. Ren and G. J. Vancso, "Process optimization for electrospun polyacrylonitrile (PAN) nanofibers precursor of carbon nanofibers", European Polymer Journal, Vol. 41, 2005, pp. 2559-2568).
The structure, morphology and geometry of nanofibre assemblies and the porosity and tensile properties of nanofibre materials can be investigated through
conventional techniques and equipments after modification. The surface characterization involves standard surface techniques such as x-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), scanning tunneling microscope (STM), scanning electron microscope (SEM), High resolution transmission electron microscope (HRTEM) and low energy electron diffraction (LEED). The morphology of the nanofibres produced can be determined by using techniques such as SEM, HRTEM and Atomic Force Microscope (AFM) (Theron S.A., E. Zussman and A.L. Yarin, "Experimental Investigation of the governing parameters in the electrospinning of polymer solutions", Polymer 45, 2004, pp. 2017-2030; M. Ziabari, V. Mottaghitalab, Scott T. McGovern, A. K. Haghi, "A New Image Analysis Based Method for Measuring Electrospun Nanofiber Diameter" Nanoscale Res Lett (2007) 2:597-600).
Polyacrylonitrile (PAN) is the polymer used mostly for the development of nanofibres in electrospinning. Apart from Polyacrylonitrile (PAN), Polycaprolactone (PCL) is the polymer widely used in the electrospinning process for the enhancement of nanofibres. PCL provides this with its non toxic and biodegradable nature. In addition, PCL is a good electrical conductor that is essential for electrospinning (Gu, S.Y., J. Ren and G. J. Vancso, "Process optimization for electrospun polyacrylonitrile (PAN) nanofibers precursor of carbon nanofibers", European Polymer Journal, Vol. 41, 2005, pp. 2559-2568).
In industrial factory, working office and hygienic surgical operation room, air purification is essential requirement to protect people and precision equipment. Filter media is utilized to purify air which contains solid particles (virus, mine dust and anther dust etc.) and liquid particles (smog, evaporated water and chemical solvents etc.). However, mesh pore size must be small or thick mesh is required to remove ultra
fine particles, which means that a filtration fun needs to blow the air with high pressure. In contrast, air blowing with lower pressure leads to poor ventilation through a filter media. This kind of property is called "pressure drop" and lower pressure drop is required to an excellent filter media. On such background, electrospun nanofibre membranes have gained the large potential and it was estimated that future filtration market (X. Hong, S. Wang, "Filtration Properties of Electrospinning Nanofibers" Published online in Wiley Inter Science; Timothy Grafe and Kristine Graham, "Polymeric Nanofibers and Nanofiber webs: A new class of nonwovens". International Nonwoven Technical conference (Joint INDA- TAPPI Gonference), Atlanta, Georgia, September 24-26, 2002, pp. 1-15; Timothy Grafe, Mark Gogins, Marty Barris, James Schaefer and Ric Canepa "Nanofibers in filtration applications in transportation", Filtration 2001).
Nanofibers combined with other non-woven products are used in a wide range of applications such as textile, sensors, drug delivery system, air filtration, protective clothing, in preparation of nanotubes etc. The use of nanofibers have increased because of the properties of the nanofibers that have ultra fine fibers with a high surface area per unit mass that increases the efficiency of the applications. Nanofibers have significant use in the area of filtration since the surface area is substantially greater and comprises of very small micropores. Highly porous structure makes it suitable for the use in variety of filtration techniques.
US Patent No. 7,134,857 teaches an apparatus for producing libers, comprising: a spray head having a longitudinal axis and including, at least one electrospinning element having a tip, said tip extending in a radial direction from a peripheral wall of the spray head, said peripheral wall encircling the longitudinal axis, said
electrospinning element having a passage by which a substance from which the fibers are to be composed is provided to an interior of the tip of the electrospinning element, and said electrospinning element configured to electrospin said fibers by electric field extraction of the substance from the tip of the electrospinning element.
U.S. Patent No. 705,691, describes a simple spray head as described above. U.S. Patent Application Publication No. U.S. 2003/0106294 teaches an apparatus for electrospinning fibers that utilize a disc like spray head having multiple orifices being rotated about its center in which fibers are emitted from a face surface of the disk-like spray head. In this technique the emission of fibers from a face surface of a rotating spray head results in twisting and contorting of the extruded fibers due to the centripetal forces existing between the free end of the fiber and the end still attached to the extruding medium.
U.S. 2002/0122840A1 discloses an apparatus for electrospinning in which two conductor boards 26 and 30 make electrical contact to each needle 32. A high voltage is applied to each needle 32 through the conductor boards 26 and 30 that are in direct contact with the needles. U.S. Pat. Publication Appl. No. U.S. 2002/0175449A1, teaches electrospinning of polymer solutions through one or more charged conducting nozzles arranged on at least one conducting plate. The method or the techniques employed in the prior art produces nanofiber but are suitable only for small scale production. There are also limitations in the processing condition and such methods are not advantageous to produce nanofibers on large scale.
U S Patent No. 5114631, US Patent no. 6114017 and US Patent No. 5260003 utilize relative high pressures, temperature and velocities to achieve the small fiber diameter. The above method have not been used to make webs with coated nanofibers.
US patent No. 7291300 discloses a method of forming nonwoven web comprising coated fibers. The method of forming the nonwoven web comprises forming fibers from a melt fibrillation process forming atleast one fluid stream containing a coating substance, applying the coating substance onto the surface of the fiber and depositing the coated fibers on a surface to form web. This invention uses melt fibrillation process utilizing a single phase polymer melt. Single phase include a dispersion but do not include solvent based melt as in electro spinning. Typical single melt fibrillation processes include melt blowing, melt film fibrillation, spun bonding, melt spinning and combination thereof. The process may include a two step process where the fiber is first made and then split or a part is dissolved to make smaller diameter fibers after the fiber has solidified.
The prior art discloses several methods to make nanofiber nonwoven webs, still there are drawbacks to each of the methods and there is a requirement to produce cost effective nanofibers using the most effective and direct method. The use of nanofibers have increased not only in biological/chemical protective clothings, biomedical use and energy storage etc but also in filtration techniques. Therefore, the techniques for speedy and large production of nanofiber with improved properties for filtering particulate materials and fine particulate materials in microns which are emitted from industrial plume, vehicles etc. with high temperature, high humidity and high flow rates is in demand.
In the present manufacturing scenario, the development of new filters in the automotive industry has been growing in view of technological challenges. The performance of filter in terms of efficiency and life plays an important role in determining the optimum wear protection of the injection systems. The demand for new development of engine management systems has increased and to prevent the injection systems from untimely failure. Filters with increased filter efficiency and life are needed. In automotive sector, the emission norms are more stringent and due to that the working tolerance between the moving parts in the engine are reduced. This needs very high efficiency of filtration of finer particles in automotive filtration, especially fuel filtration to reduce wear of the engine and also it requires longer change over of filters. The development of filtration media with gradient density layers is very important. There is a constant need for development of finer and ultra fine fibers such as meltblown fibers and nanofibers.
The generation of such nanofibers is also being achieved by the application of spinnerets. The process of developing the single and multi hole spinneret for generating the nanofibers is being carried out using different polymer solution. Preliminary experiments were carried out by varying the process parameters such as bore diameter, flow rate, electrode spacing distance and concentration for optimizing the nanofiber diameter on filter media in terms of air, oil and fuel for improving the performance. The generation of nanofibers using single and multi hole spinneret with different bore diameter in the range of 0.5 mm to 2.5 mm were varied during the parametric study. Based on these observations, it was found that single hole spinneret in the range of 0.8 mm to 1.5 mm bore diameter deposits uniformly over the surface which enhance the performance in terms of efficiency and life.
During the initial stage, conventional approach is to enhance the efficiency of the particular filtration media through reducing the mean pore by increasing the paper thickness, which has an adverse effect on dust holding capacity. The generation of nanofibers using Polyacrylonitrile (PAN) polymer solution is one of the most significant characteristics in automotive filters which show the performance in terms of pressure drop, efficiency and contaminant holding capacity.
In the present invention, multilayer filtration concept has been adopted by using nanofibers generated by Polyacrylonitrile (PAN) polymer solution with high particle retention efficiency. The nanofibers generated are sandwiched between a pre-filtering meltblown media with high dust-holding capacity and a fine supporting cellulose filter media This approach has significantly improved particle retention efficiency and water separation efficiency in comparison to standard filter media with enhanced dust holding capacity in liquid applications.
The process for manufacturing the multilayer composite media is developed with the unique features. The process combines both PUROLATOR INDIA LTD developed state of the art electrospinning process and the process of joining the electrospun nanofibers generated by using PAN polymer solution with the other layers of the composite media in terms of meltblown and cellulose media.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a process for producing nanofibers from PAN solution for use in automotive filters for filtering air, oil and fuel.
It is another object of the present invention to provide a method to check the compatibility of PAN polymeric nanofibers in composite air, oil and fuel filter media to improve the performance in terms of efficiency of filter by minimizing the pressure drop.
It is yet another object of the present invention to provide a method for production of electrospun polymeric nanofibers for mass production.
It is still another object of the present invention to provide electrospun polymeric nanofibers obtained from PAN polymer solution using electrospining technique, wherein said fibers are spun under high voltage electrical field.
It is yet another object of the present invention to join the web of PAN nanofiber with the meltblown and the Phenol formaldehyde (PF) impregnated cellulose media to form multi layer composite media.
SUMMARY OF THE INVENTION
According to one embodiment the present invention provides a method for producing polyacronylonitrile nanofiber for multilayer filter medium comprising the steps of:
(i) preparing a solution of polyacronylonitrile;
(ii) storing said solution of step (i) in a syringe wherein said syringe is
provided with needle for delivering said solution; (iii) applying an electric field to the Up of the needle so that the charge
overcomes the surface tension of the deformed drop of polymer solution
can be discharged into nanaofibers; (iv) collecting said charged jet stream of step (iii) on a substrate to form
nanofibers;
In another embodiment the polyacronylonitrile solution comprises polyacronylonitrile and solvent to dissolve the polyacronylonitrile. The solvent is N-N-di-methyl formamide.
In yet another embodiment of the invention the elcctrospinning process utilizes an electric field of strength 20 kV to lOOkV which cause the polymer solution charged and converts into nanofibers which are drawn towards a substrate/collector plate. The charged polymer solution generates in the form of nanofibers. These nanofibers are collected on a substrate or a grounded collector plate. The substrate may have a web of conventional filter media on whih nanofibers are collected.
In yet another embodiment of the invention nanofibers are in the diameter range of 50nm to 800 nm.
In still another embodiment of the invention the nanofibers are used for the manufacture of filter medium comprising:
first layer of Phenol formaldehyde (PF) resin impregnated cellulose media; second layer of polyacronylonitrile nanofiber;
The second layer comprises polyacronylonitrile nanofibers coated on cellulose media in the range of 0.01 GSM to 2 GSM. The nanofiber filter medium is air filter medium. The efficiency of air filter medium is in the range of 98.5% to 99.98% on 0.1 micron particle size.
In another embodiment of the present invention the filter medium comprising: second layer of polyacronylonitrile nanofibers; third layer of meltblown media;
The second layer comprises PAN nanofibers coated cellulose media in the range of 0.01 GSM to 2 GSM.
In yet another embodiment the layers are affixed by adhesive and laminated by passing through rolls and winders. The filter media acts as multi layer media.
In another embodiment the nanofiber filter media is oil filtration media. The efficiency of oil filter media is in the range of 94 % to 98% on 1 5 micron particle size.
In yet another embodiment the nanofiber filter media is fuel filtration media. The efficiency of fuel filter media is in the range of 97 % to 99.5 % on 3 micron particle size.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows the schematic representation of electrospinning process. The description of the reference numerals is as follows:
(1) Gear
(2) Needle
(3) Collector plate
(4) Supply voltage
(5) High voltage generator
(6) Charged jet generating nanofiber
With reference to Figure 1, an electrospinning process makes use of electrical force to produce nanofibers. A high voltage (4) source is applied to the solution contained in a syringe with a needle (2). The solution gets charged and ejects a charged jet stream of nanofibers (6) which are collected over a substrate/collector plate (3). The dispensing rate through a needle is controlled by lead screw driven by a high voltage generator (5) with a gear (1). The process is described below in detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The development of new engine for the next generation of automotive vehicles based on emission norms has been growing in view of technological challenges. Engines are subjected to contaminant from two different sources such as contaminants produced from combustion and outside environment. Contaminants are produced when the engine is in operating condition. This needs very high efficiency of filtration of finer particles in automotive filtration to reduce wear of the engine and also it requires longer
change over of filters. Hence, the development of filtration media with gradient density
layers became very important and need for development of finer and ultra fine fibers such as meltblown fibers and nanofibers are essential.
Multi layer media approach is used to make the media with gradient density pores. The multi layer media contains three layers joined together in which the primary layer consists of more porous melt blown that helps in increasing the dust holding capacity and the secondary layer of nanofibers coated on cellulose media that helps in attaining the efficiency targets. The two medias were joined with special adhesives, which did not alter the filtration properties and also had the desired liquid resistant properties.
The Multi layered composite filtration concept is adapted using a pre-filtering meltblown media with high dust-holding capacity followed by a very fine nanofiber media with high particle retention efficiency and then a fine supporting Phenol formaldehyde (PF) resin impregnated cellulose filter media, i.e. nanofiber web using PAN polymer is sandwiched between the meltblown and the cellulose media. This approach has significantly improved the particle retention efficiency and water separation efficiency in comparison to standard filter media. The use of electrospun nanofiber in air, oil and fuel filtration has given cutting edge performance in efficiency targets, which is in the range of 98.5% to 99.98% of 0.1 micron particle size in terms of air, 97 % to 99.5% of 3 micron particle size in terms of fuel and 94% to 98% of 15 micron particle size in terms of oil filtration media.
The process for manufacturing the multilayer composite media has been developed with the unique features. This process also incorporates electrospinning process for generation of polymeric nanofiber in the diameter range of 100 nm to 800 nm
that provide dramatic increases in filtration efficiency. Nanofiber deposited filter media has also given demonstrated improvement in filter life and more contaminant holding capacity.
The process of producing polymeric nanofibers will now be described in detail and with reference to the accompanying illustrative figure 1 .
The electrospining process comprises: motor (5) with a gear (1) to drive the lead screw and variable frequency drive which is connected for controlling the motor speed. The purpose of the lead screw is to control the dispensing rate through the needle. This arrangement is based on mechanical displacement for fixing a set of needles (2) between the plates connected to the lead screw and a fixed plate which moves towards syringes. The collector plate (3) which is a fixed plate is grounded and kept normal to the syringe needles. High voltage panel is provided besides the syringe to supply voltage (4) to the solution through the needle. When the high voltage from the high voltage generator (5) is applied to the syringe needle having solution, the solution gets charged and ejects a charged jet which generates nanofiber (6) and collects on the substrate over electrically grounded metal sheet.
The electrospinning process for generating the nanofibers is developed in the laboratory. Electrospinning is a process by which a charged liquid polymer solution is introduced into an electric field. In this electrostatic technique, a high electric field is generated between a polymer liquid contained in a spinning dope reservoir with a capillary tip or spinneret and a metallic fiber collection ground surface. When the voltage reaches a critical value, the charge overcomes the surface tension of the
deformed drop of the suspended polymer solution formed on the tip of the spinneret and a jet is produced. This stretching process is accompanied by the rapid evaporation of the solvent molecules that reduces the diameter of the jet, in a cone shaped volume called the "taylor cone".
The electrospinning process uses an electric field to draw a polymer solution from the tip of a capillary to a collector plate. A high voltage in the range of 20 kV to 100 kV is applied to the polymer, which causes a jet of the solution to be drawn toward a grounded collector. The fine jets converts into several fibres of fine diameter in nanometer range, which are collected on conventional filter media or meltblown media passing over the grounded collector.
This process is incorporated in to the process of joining the composite layers, where the adhesive is applied to the one side of the moving web of cellulose/meltblown through roll coating and the nanoweb containing meltblown/cellulose web comes in contact with the adhesive coated web. The composite web is then passed though the pressing rolls and to the winders. The other embodiments of the invention are further illustrated by the following non limiting examples.
EXAMPLES
Example 1
Preparation of PAN solution
PAN solution is prepared by two methods stated below:
Method 1
N, N-di-methyl formamide (DMF) of 250 to 300 gms and 25 to 50 gms of washed PAN fiber is taken in a beaker with a magnet. The beaker is then covered with para film and placed on a magnetic stirrer plate for 3 to 8 hours. A brown pale coloured liquid is formed when all the fibers get dissolved in DMF.
Method 2
The required amount of N, N-dimethyl formamide is taken in a dry reactor fitted with a condenser and a stirrer. Complete sealing is done with the vaccum grease. The reactor is then heated slowly with stirring. The washed PAN fiber is added slowly from one neck of the reactor with continuous stirring. The temperature is slowly increased to 60°C and maintained in the range of 40°C to 80°C. When all the PAN fiber get dissolved a brown pale coloured liquid is formed. The obtained solution is allowed to cool for one to three hours.
Example 2
Washing the PAN fiber
The required amount of PAN fiber is taken in Soxhlet extractor attached to a condenser. The extractor is then fitted to a round bottom flask containing acetone. The temperature is maintained at the range of 40° C to 80° C. The flow rate of water is adjusted such that condensation of the solvent takes place in the condenser. The fiber is washed for 1 to 3 hours. Heating is stopped and PAN fibers are removed form the Soxhlet Extractor and dried in oven at a temperature in the range of 60°C to 80°C for 3 hours.
Example 3
Formation of multilayer composite media for use in Air Filter
In air filter applications, the efficiency of 0.1 micron particle size is increased to close level 99.98 % from 99.32% and 0.3 micron particle size is increased to 100% as per ISO 5011 after depositing the nanofibers generated using PAN polymer over the air filtration media. This corresponds to higher improvement in efficiency in depositing the nanofibers.
Example 4
Formation of multilayer composite media for use in Fuel Filter
In fuel filter applications, the efficiency of 3 micron particle size is increased to close level of 99% from 87% and 5 micron particle size is increased to 100% as per ISO TR 13353 after depositing the nanofibers generated using PAN polymer over the fuel filtration media. This corresponds substantial improvement in efficiency in depositing the nanofibers. It is due to the fact that the media forms a higher surface area and hence larger void volume is generated and forms uniform pore size distribution by use of nanofibers.
Example 5
Formation of multilayer composite media for use in Oil filter
In oil filter applications, the efficiency of 15 micron particle size is increased to close level in the range of 94% to 98% from 85% and 5 micron particle size is increased to 100% as per ISO 4548 after depositing the nanofibers generated using PAN polymer over the fuel filtration media. This corresponds to substantial improvement in efficiency in depositing the nanofibers.
ADVANTAGES OF THE INVENTION
1. The preparation of PAN polymer nanofibcr using a simple technique which is cost effective.
2. Multilayer composite filter of the present invention increases high capacity for dust holding and is highly efficient for filtration of finer particles.
3. Filtration media has its application in oil/fuel filtration.
4. Media forms a higher surface area with uniform pore size distribution which increases the efficiency of the filter.
5. Composite filter media of the present invention is more stable than the conventional filter.
REFERENCES
1. Doshi, J., and Reneker, D.H., "Electrospinning process and applications of Electrospun fibers", Journal of Electrostatics, Vol. 35, 1995, pp. 151- 160.
2. Reneker, D. H., and Chun, I., "Nanometre Diameter Fibres of Polymer, Produced by Electrospinning", Nanotechnology, Volume 7, 1996, pages 216-233.
3. Yarin, A. L., and D.H. Reneker, Taylor cone and jetting from liquid droplets in electrospinning of nanofibers' - Journal of Applied Physics. 90 (2001) 4836-4846.
4. Kowalewski, T. A, A.L. Yarin, and S. Blohski. 'Electrospinning of Polymer Nanofibers' Paper presented at The 5th Euromech Fluid Mechanics Conference, Toulouse, France, August 24-28, 2003.
5. Gu, S.Y., J. Ren and G. J. Vancso, "Process optimization for electrospun polyacrylonitrile (PAN) nanofibers precursor of carbon nanofibers", European Polymer Journal, Vol. 41, 2005, pp. 2559-2568.
6. Theron S.A., E. Zussman and A.L. Yarin, "Experimental Investigation of the governing parameters in the electrospinning of polymer solutions", Polymer 45, 2004, pp. 2017-2030.
7. M. Ziabari, V. Mottaghitalab, Scott T. McGovern, A. K. Haghi, "A New Image Analysis Based Method for Measuring Electrospun Nanofiber Diameter" Nanoscale Res Lett (2007) 2:597-600.
8. X. Hong, S. Wang, "Filtration Properties of Electrospinning Nanofibers" Published online in Wiley Inter Science.
9. Timothy Grafe and Kristine Graham, "Polymeric Nanofibers and Nanofiber webs: A new class of nonwovcns". International Nonwoven Technical conference (Joint INDA- TAPPI Gonfercncc), Atlanta, Georgia, September 24-26, 2002, pp. 1-15.
10.Timothy Grafe, Mark Gogins, Marty Barris, James Schaefer and Ric Canepa "Nanofibers in filtration applications in transportation", Filtration 2001.

We. claim
1. A method for producing polyacronylonitrile nanofibers for multilayer filter medium
comprising the steps of :
(i) Preparing a solution of polyacronylonitrile;
(ii) storing said solution of step (i) in a syringe wherein said syringe is
provided with needle for delivering said solution; (iii) applying an electric field to the tip of the needle to form a plurality of
charged jet stream; (iv) collecting charged jet stream of step (iv) on a substrate to form
nanofibers;
2. The method as claimed in claim 1 wherein said polyacronylonitrile solution is prepared by dissolving said polyacronylonitrile in a solvent.
3. The method as claimed in claim 2 wherein said solvent is N-N-di-methyl form amide.
4. The method as claimed in any preceding claim wherein said polyacronylonitrite solution is exposed to electric field of strength 20 kV to l00 kV.
5. The method as claimed in claim 1 or claim 4 wherein electric field produce charged jet stream of polyacronylonitrile solution wherein said stream is drawn toward a substrate/collector plate.
6. The method as claimed in claim 5 wherein said substrate/collector comprises conductive material at 0V.
7. The method as claimed in claim 5 wherein charged polymer solution is generated in the form nanofibers.
8. The method as claimed in any preceding claim wherein said nanofibers are collected on a web of conventional filter media over the said substrate/collector plate.
9. A polyacronylonitrile (PAN) nanofiber prepared by a method as claimed in any preceding claim.
10. A Polyacronylonitrile nanofiber as claimed in claim 9 wherein said nanofiber is in
the diameter range of 50 nm to 800 nm.
11. A filter medium comprises two layers wherein
first layer comprising phenol formaldehyde resin impregnated cellulose media; second layer comprising polyacronylonitrile nanofiber fibers as claimed in claim
i;
12. A filter medium as claimed in claim 11 wherein said second layer comprises
polyacronylonitrile nanofibers coated on cellulose media in the range of 0.01 GSM to 2 GSM.
13.A filter medium as claimed in claim 11 and 12 wherein said medium is air filtration medium.
14.A filter medium as claimed in claim 13 wherein efficiency of said air filter media is in the range of 98.5% to 99.98% on 0.1 micron particle size.
15. A filter medium as claimed in claim 1 1 comprising:
second layer of polyacronylonitrile nanofibers as claimed in claim 1; third layer of meltblown media;
16.A filter medium as claimed in claim 15 wherein said third layer comprises polyacronylonitrile nanofibers coated cellulose media in the range of 0.01 GSM to 2 GSM.
17.A filter medium as claimed in claims 11 and 15 wherein said layers are affixed by adhesive.
18. A filter medium as claimed in claims 11 and 15 wherein said layers are laminated by passing through rolls and winders.
19.A filter medium as claimed in claims 15 and 16 wherein said medium is oil filtration medium.
20. A filter medium as claimed in claim 19 wherein efficiency of said oil filter media is in the range of 94 % to 98% on 15 micron particle size.
21. A filter medium as claimed in claim 15 and 16 wherein said medium is fuel filtration medium.
22. A filter medium as claimed in claim 21 wherein efficiency of said fuel filter media is in the range of 97 % to 99.5 % on 3 micron particle size.
23.A filter medium wherein said medium is prepared by a method as claimed in any preceding claim.
24.An electrospinning apparatus for producing polyacronylonitrile nanofibers as claimed in claim 1 comprises:
a reservoir for storing polyacronylonitrile solution;
a single horizontal setup which is connected to said reservoir for delivering said solution;
a power source generating high voltage for transmitting said voltage to said syringe setup; a collector/substrate for collecting said polyacronylonitrile nanofibers.
25.A nanofiber substantially as described herein before and with reference to the drawing.
26.A nanofiber filter media substantially as described herein before and with reference to the foregoing examples.
27.An electrospinning apparatus substantially as described herein before with reference to the drawings.