Method for filtering particles from an airflow

Abstract

A method for filtering particles from an airflow is provided comprising the steps of i) providing a filter medium comprising a nonwoven fabric comprising fibers comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, ii) electrostatically charging the nonwoven fabric comprising the charging additive, iii) electrostatically charging the particles in the airflow, and iv) passing the airflow comprising the electrostatically charged particles through the filter medium comprising the electrostatically charged nonwoven fabric comprising the charging additive enables to obtain a high filtration efficiency, in particular at a low pressure drop.

Description

[0001] Method for Filtering Particles from an Airflow

[0002] Description:

[0003] The application pertains to a method for filtering particles from an airflow and to filter media for filtering particles from an airflow.

[0004] There is a high demand for filter media for filtering particles from an airflow, in particular for filter media having a high filtration efficiency in combination with a low pressure drop.

[0005] Meltblown nonwoven fabrics are for example used as a filter medium to obtain a high filtration efficiency. However, meltblown nonwoven fabrics have a relatively high pressure drop over the filter medium.

[0006] To achieve an even higher filtration efficiency nanofiber nonwovens are utilized. However, nanofiber nonwovens generally have an even higher pressure drop over the filter medium than meltblown nonwovens.

[0007] To reduce the pressure drop over a filter medium, a filter medium composed of relatively coarse fibers having a linear density of 5 dtex or higher could be used. However, a high filtration efficiency can generally not be achieved by such a filter medium comprising relatively coarse fibers, not even when the filter medium is electrostatically charged.

[0008] A higher pressure drop over a filter medium results in a higher energy consumption required to pass an airflow through the filter medium, which is disadvantageous from an ecological point of view. A higher energy consumption to pass an airflow through a filter medium is disadvantageous in many applications, including for example in heating, ventilation and air conditioning (HVAC) applications, and air filtration for vehicles (cabin air filters), in particular for electrical vehicles. As an example, the ventilation system of a car may provide up to 300,000 liters of air from the environment into the interior of the car during a drive of several hours, wherein the air is likely to contain particles, for example pollutants and allergens, such as e.g. pollen, which have to be filtered from the airflow. A high energy consumption to pass an airflow through a filter medium may significantly reduce the driving range of electrical vehicles. The total airflow to be filtered in the ventilation system of a car may reach up to 600,000 to m3 / year or even more.

[0009] There thus remains a need for improved methods for filtering particles from an airflow, in particular providing a high filtration efficiency, and in particular in combination with a low pressure drop over the filter medium.

[0010] The object of the invention is thus to provide a method which mitigates the disadvantages of the prior art.

[0011] The object is solved by the method for filtering particles from an airflow according to claim 1 and by the use of the nonwoven according to claim 15.

[0012] Advantageous further improvements of the method for filtering particles from an airflow and the use of nonwovens are provided by the dependent claims.

[0013] A method for filtering particles from an airflow comprising the steps of i) providing a filter medium comprising a nonwoven fabric comprising fibers comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, ii) electrostatically charging the nonwoven fabric comprising the charging additive, iii) electrostatically charging the particles in the airflow, and iv) passing the airflow comprising the electrostatically charged particles through the filter medium comprising the electrostatically charged nonwoven fabric comprising the charging additive enables to obtain a high filtration efficiency. In certain embodiments, a high filtration efficiency is obtained in combination with a relatively low pressure drop over the filter medium.

[0014] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, wherein the nonwoven fabric is composed for at least 50 wt.% of thermoplastic polymer based on the total weight of the nonwoven fabric, preferably for at least 75 wt.%, or preferably for at least 85 wt.%, or for at least 50 wt.% for at least 95 wt.% of thermoplastic polymer based on the total weight of the nonwoven fabric.

[0015] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer having a melting temperature of at most 300°C, preferably of at most 290°C, or preferably of at most 270°C, or preferably of at most 260°C, enabling to process the thermoplastic polymer without requiring unconventional equipment.

[0016] The filter medium preferably can be operated at temperatures up to 100°C, enabling filtration of air flows in hot environments, such as in a desert or in industrial installations.

[0017] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, wherein the nonwoven fabric excludes metal fibers and / or ceramic fibers to enable recycling of the filter medium.

[0018] The charging agent comprised in the nonwoven fabric may be any chemical compound suitable enabling electrostatic charging of the nonwoven fabric. The charging agent may be an organic-based additive. The charging agent may be an organic (hindered) amine, such as for example bis(1 , 2,2,6, 6-pentamethyl-4- piperidyl) sebacate, a metal titanate salt, such as for example barium titanate, a silicate salt, such as for example tourmaline, a hindered amine light stabilizer, a fatty acid amid, such as for example ethylene bis stearamide (EBS), a metal fatty acid salt, such as for example calcium stearate, copper stearate or magnesium stearate, a N-substituted amino aromatic compound, particularly tri-amino substituted compounds, such as for example 2,4,6-trianilino-p-(carbo-2’- ethylhexyl-1’-oxy)-1 ,3,5-triazine (llvinul T-150), or 2,4,6-tris-(octadecylamino)- triazine, or poly[[6-(1 , 1 ,3, 3, -tetramethylbutyl) amino]-s-triazine-2,4-diyl][[(2, 2,6,6, - tetramethyl-4-piperidyl)imino] hexamethylene [(2,2,6,6-tetramethyl-4- piperidyl)imino]]) (Chimassorb 944).

[0019] The method for filtering particles from an airflow may comprise the step of electrostatically charging the nonwoven fabric comprising fibers comprising a thermoplastic polymer, comprising the charging additive, wherein the step of electrostatically charging the nonwoven fabric is performed separately before providing the nonwoven fabric in a filter medium in the method for filtering particles from an airflow. Within the scope of the invention of electrostatically charging the nonwoven fabric before providing the nonwoven fabric in a filter medium in the method for filtering particles from an airflow is also referred to as pre-charging of the nonwoven fabric. Pre-charging of the nonwoven fabric may for example be performed by corona charging of the nonwoven fabric, preferably at 1 to 100 kV / cm, by hydrocharging of the nonwoven fabric or by triboelectric charging of the nonwoven fabric, such as by mechanical needling or by friction rollers.

[0020] The method for filtering particles from an airflow may comprise the step of electrostatically charging the nonwoven fabric comprising the charging additive, wherein the step of electrostatically charging the nonwoven fabric is performed after providing the nonwoven fabric in a filter medium in the method for filtering particles from an airflow. Within the scope of the invention of electrostatically charging the nonwoven fabric after providing the nonwoven fabric in a filter medium in the method for filtering particles from an airflow is also referred to as in- situ charging of the nonwoven fabric. In-situ charging of the nonwoven fabric may for example be performed by providing an electrostatic field over the filter medium comprised in a filter element. The method for filtering particles from an airflow may comprise the step of electrostatically charging the particles in the airflow with the same polarity as the step of electrostatically charging the nonwoven fabric comprising the charging additive.

[0021] The method for filtering particles from an airflow may comprise the step of electrostatically charging the particles in the airflow with the opposite polarity as the step of electrostatically charging the nonwoven fabric comprising the charging additive.

[0022] The method for filtering particles from an airflow may comprise the steps of providing a filter medium comprising a nonwoven fabric, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of mono-component fibers, and wherein the charging additive is comprised in the mono-component fibers.

[0023] In an embodiment, the method for filtering particles from an airflow comprises the step of providing a filter medium comprising a nonwoven fabric, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric consists of a nonwoven layer of mono-component fibers comprising a thermoplastic polymer, and wherein the charging additive is comprised in the mono-component fibers.

[0024] The mono-component fibers comprising a thermoplastic polymer have an average linear density which may be varied to obtain a high filtration efficiency or to obtain a combination of a high filtration efficiency and a low pressure drop over the filter medium. The mono-component fibers comprising a thermoplastic polymer may have an average linear density of 20 dtex or less, preferably 15 dtex or less, or preferably 10 dtex or less, or preferably 5.0 dtex or less, or preferably 4.0 dtex or less, or preferably 3.0 dtex or less, or preferably 2.5 dtex or less.

[0025] The mono-component fibers may be staple fibers or filaments. Staple fibers are fibers which have a specified, relatively short length, generally in the range of 2 to 200 mm. Filaments are fibers having a length of more than 200 mm, preferably more than 500 mm, more preferably more than 1000 mm. Filaments may preferably be virtually endless, for example when formed by continuous extrusion and drawing of a filament through a spinning hole in a spinneret. In an embodiment, the mono-component fibers comprising a thermoplastic polymer are filaments.

[0026] The mono-component fibers may have any cross sectional shape, including round or non-round, in particular oval, trilobal, multi-lobal or rectangular. In an embodiment, the mono-component fibers have a non-round cross sectional shape to further improve the filtration efficiency, which is believed, without being bound to theory, to be the result of an increased surface area of the fibers.

[0027] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of mono-component or multicomponent fibers, wherein the charging additive is comprised in the monocomponent or in the multi-component fibers, wherein the mono-component or the multi-component fibers comprise a component comprising a thermoplastic polymer which can be electrostatically charged, such as for example a polyolefin, such as a polyethylene or a polypropylene, a polyester, such as a polybutylene terephthalate or a polyethylene terephthalate, a copolymer of a polyolefin, such as a copolypropylene comprising monomers of ethylene, butylene, pentylene, hexene, 4- methyl-1 -pentene and / or (iso-)octene, a copolymer of a polyester, such as copolyesters polymerized from ethylene glycol or diethylene glycol with terephthalic acid and a third monomer, wherein the third monomer may be isophthalic acid, adipic acid or a butanediol monomer, or any mixture thereof.

[0028] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of multi-component fibers, preferably bicomponent fibers, wherein the charging additive is comprised in the multi-component fibers, preferably in the bicomponent fibers.

[0029] The multi-component fibers comprising a thermoplastic polymer, preferably the bicomponent fibers, may be selected from the side-by-side fibers, core / sheath fibers, segmented-pie fibers, or islands-in-the-sea fibers.

[0030] The charging additive may be comprised in one or more of the components of the side-by-side fibers. In an embodiment, the charging additive is comprised only in the one component of the side-by-side fibers.

[0031] The charging additive may be comprised in the core component of the core / sheath fibers and / or in the sheath component of the core / sheath fibers. In an embodiment, the charging additive is comprised only in the sheath component of the core / sheath fibers for further improving the filtration efficiency.

[0032] The charging additive may be comprised in one or more of the components of the segmented-pie fibers. In an embodiment, the charging additive is comprised only in one component of the segmented-pie fibers.

[0033] The charging additive may be comprised in the island components of the islands- in-the-sea fibers and / or in the sea component of the islands-in-the-sea fibers. In an embodiment, the charging additive is comprised only in the sea component of the islands-in-the-sea fibers for further improving the filtration efficiency.

[0034] The multi-component fibers, preferably the bicomponent fibers, comprising a thermoplastic polymer have an average linear density which may be varied to obtain a high filtration efficiency or to obtain a combination of a high filtration efficiency and a low pressure drop over the filter medium. The multi-component fibers, preferably the bicomponent fibers, may have an average linear density of 25 dtex or less, preferably 20 dtex or less, or preferably 15 dtex or less, or preferably 10 dtex or less, or preferably 7 dtex or less. The multi-component fibers, preferably the bicomponent fibers, may be staple fibers or filaments. In an embodiment, the multi-component fibers, preferably the bicomponent fibers, are filaments.

[0035] The multi-component fibers, preferably the bicomponent fibers, comprising a thermoplastic polymer may have any cross sectional shape, including round or non-round, in particular oval, trilobal, multi-lobal or rectangular. In an embodiment, the multi-component fibers, preferably the bicomponent fibers, have a non-round cross sectional shape to further improve the filtration efficiency, which is believed, without being bound to theory, to be the result of an increased surface area of the fibers.

[0036] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of mono-component or multi-component fibers comprising a thermoplastic polymer, wherein the charging additive is comprised in the monocomponent or in the multi-component fibers, wherein the mono-component or the multi-component fibers comprise a component comprising a thermoplastic polymer which can be electrostatically charged, such as for example a polyolefin, such as a polyethylene or a polypropylene, a polyester, such as a polybutylene terephthalate or a polyethylene terephthalate, a copolymer of a polyolefin, such as a copolypropylene comprising monomers of ethylene, butylene, pentylene, hexene, 4- methyl-1 -pentene and / or (iso-)octene, a copolymer of a polyester, such as copolyesters polymerized from ethylene glycol or diethylene glycol with terephthalic acid and a third monomer, wherein the third monomer may be isophthalic acid, adipic acid or a butanediol monomer, or any mixture thereof.

[0037] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of multi-component fibers, preferably of bicomponent fibers, comprising a component comprising a polyester and a component comprising a copolyester. In an embodiment, the multicomponent fibers are core / sheath bicomponent fibers comprising a polyester, preferably a polyethylene terephthalate, in the core component of the core / sheath bicomponent fibers and a copolyester in the sheath component of the core / sheath bicomponent fibers, or a lower melting polyester in the sheath component of the core / sheath bicomponent fibers having a melting temperature which is lower than the melting temperature of the polyester comprised in the core component of the core / sheath bicomponent fibers.

[0038] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of multi-component fibers, preferably of bicomponent fibers, comprising a component comprising a polyester and a component comprising a polyolefin. In an embodiment, the multi-component fibers are core / sheath bicomponent fibers comprising a polyester, preferably a polyethylene terephthalate, in the core component of the core / sheath bicomponent fibers and a polyolefin, preferably a polypropylene, a polyethylene or a copolypropylene, in the sheath component of the core / sheath bicomponent fibers.

[0039] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of multi-component fibers, preferably of bicomponent fibers, comprising a component comprising a polyolefin and a component comprising a polyolefin or a copolymer of a polyolefin. In an embodiment, the multi-component fibers are core / sheath bicomponent fibers comprising a polyolefin, preferably a polypropylene, in the core component of the core / sheath bicomponent fibers and a polyolefin, preferably a polyethylene, or a copolymer of a polyolefin, preferably a co-polypropylene, in the sheath component of the core / sheath bicomponent fibers. The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric comprises a nonwoven layer of meltblown fibers, and wherein the charging additive is comprised in the meltblown fibers.

[0040] In an embodiment, the method for filtering particles from an airflow comprises the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, wherein the nonwoven fabric consists of a nonwoven layer of meltblown, and wherein the charging additive is comprised in the meltblown fibers.

[0041] The meltblown fibers may have an average diameter of at least 0.1 and at most 10 pm, preferably at most 7 pm, more preferably at most 5 pm.

[0042] The meltblown fibers may comprise a thermoplastic polymer which can be electrostatically charged, such as for example a polyolefin, such as a polyethylene or a polypropylene, a polyester, such as a polybutylene terephthalate or a polyethylene terephthalate, a copolymer of a polyolefin, such as a copolypropylene comprising monomers of ethylene, butylene, pentylene, hexene, 4- methyl-1 -pentene and / or (iso-)octene, a copolymer of a polyester, such as copolyesters polymerized from ethylene glycol or diethylene glycol with terephthalic acid and a third monomer, wherein the third monomer may be isophthalic acid, adipic acid or a butanediol monomer, or any mixture thereof. In an embodiment, the meltblown fibers comprise, more preferably consist of, a polyolefin, such as a polyethylene or a polypropylene, preferably a polypropylene, a polyester, such as a polybutylene terephthalate, a polycarbonate or a polystyrene.

[0043] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a nonwoven layer of bicomponent fibers, preferably filaments, and a nonwoven layer of mono-component fibers, preferably filaments comprising the charging additive in the mono-component fibers, preferably filaments. The nonwoven layer of mono-component fibers may be a nonwoven layer of mono-component fibers as described herein above. The nonwoven layer of bicomponent fibers, preferably filaments, enables to provide improved pleatability to the filter medium. The bicomponent fibers, preferably filaments, may have an average linear density of at least 5 dtex, preferably of at least 7 dtex, or preferably of at least 10 dtex enabling to prevent a too high pressure drop over the filter medium. The bicomponent fibers, preferably filaments, may have an average linear density of 25 dtex or less, preferably 20 dtex or less, or preferably 15 dtex or less to provide good pleatability to the filter medium.

[0044] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, the nonwoven fabric comprising a nonwoven layer of bicomponent fibers, preferably filaments, and a nonwoven layer of meltblown fibers, preferably filaments comprising the charging additive in the mono-component fibers, preferably filaments. The nonwoven layer of meltblown fibers may be a nonwoven layer of meltblown fibers as described herein above. The nonwoven layer of bicomponent fibers, preferably filaments, enables to provide improved pleatability to the filter medium. The bicomponent fibers, preferably filaments, may have an average linear density of at least 5 dtex, preferably of at least 7 dtex, or preferably of at least 10 dtex enabling to prevent a too high pressure drop over the filter medium. The bicomponent fibers, preferably filaments, may have an average linear density of 25 dtex or less, preferably 20 dtex or less, or preferably 15 dtex or less to provide good pleatability to the filter medium.

[0045] The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, wherein the nonwoven fabric does not comprise, i.e. excludes, a nonwoven layer of meltblown fibers, enabling to obtain a combination of a high filtration efficiency and a low pressure drop over the filter medium. The method for filtering particles from an airflow may comprise the step of providing a filter medium comprising a nonwoven fabric comprising a thermoplastic polymer, wherein the nonwoven fabric does not comprise, i.e. excludes, a nonwoven layer of nanofibers, enabling to obtain a combination of a high filtration efficiency and a low pressure drop over the filter medium.

[0046] The method for filtering particles from an air flow may comprise the step of filtering particles from an air flow at a filter efficiency of at least 60%, preferably at least 70%, more preferably at least 80%, most preferably at least 90%.

[0047] The method for filtering particles from an air flow may comprise the step of filtering particles from an air flow at a filter efficiency of at least 60%, preferably at least 70%, more preferably at least 80%, most preferably at least 90%, and a pressure drop, in particular an initial pressure drop, of at most 20 Pa, preferably at most 15 Pa, more preferably at most 10 Pa, most preferably at most 5 Pa.

[0048] A flat sheet of filter medium providing a high filter efficiency, preferably with a low (initial) pressure drop, is particularly suitable for use in a filter element, wherein the filter medium may be pleated.

[0049] The filter efficiency of filtering sodium chloride particles having a diameter of 0.3 pm at an air flow speed of 20 cm / s, and the associated pressure drop over the filter medium, are determined on a flat sheet of filter medium according to the test procedure of EN 779:2012, but using a vertical test bench according to ISO 11155 with an effective sample size of 500 mm by 500 mm for the flat sheet of filter medium. By determining the filter efficiency of a flat sheet, any disturbing effects, which may for example caused by pleating the filter medium, may be avoided.

[0050] The method for filtering particles from an air flow may comprise the step of filtering particles from an air flow at a quality factor, defined as the filtration efficiency divided by the (initial) pressure drop, of at least 2 Pa’1, preferably at least 5 Pa’1, more preferably at least 10 Pa’1, most preferably at least 15 Pa’1. Example 1

[0051] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a copolyester (coPET) in a weight ratio of 75 / 25. The PET / coPET core / sheath bicomponent filaments had a linear density of 10 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 100 g / m2The PET / coPET core / sheath bicomponent filaments fibers comprised 3% by weight of a charging additive in the sheath. The charging additive was Arguflame 686, which was provided in a polypropylene masterbatch, available from Argus Additive Plastics GmbH. The filter medium was electrostatically charged by corona charging from a distance of 5 cm at 29 kV, 0.8 mA and at a speed of 5 m / min.

[0052] Comparative Example 1a

[0053] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a copolyester (coPET) in a weight ratio of 75 / 25. The PET / coPET core / sheath bicomponent filaments had a linear density of 10 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 75 g / m2. The filter medium did not comprise a charging additive.

[0054] Comparative Example 1 b

[0055] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a copolyester (coPET) in a weight ratio of 75 / 25. The PET / coPET core / sheath bicomponent filaments had a linear density of 10 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 100 g / m2The PET / coPET core / sheath bicomponent filaments fibers comprised 3% by weight of a charging additive in the sheath. The charging additive was Arguflame 686, which was provided in a polypropylene masterbatch. The filter medium was not electrostatically charged. The filtration efficiency for filtering out NaCI particles having a diameter of 0.3 pm had been determined, both for charged NaCI particles and for non-charged NaCI particles, as summarized in Table 1.

[0056] Table 1 Table 1 shows that a filter medium of Comparative Example 1a, which is composed of PET / coPET core / sheath bicomponent filaments having a relatively high linear density of 10 dtex and which does not comprise an additive has a very low filtration efficiency below 5%, both for charged NaCI particles and for noncharged NaCI particles. Table 1 shows that a filter medium of Comparative Example 1 b, which is composed of PET / coPET core / sheath bicomponent filaments having a relatively high linear density of 10 dtex, which comprises an additive, but which is not electrostatically charged, has a very low filtration efficiency less than 1 % for noncharged NaCI particles, and still a very low filtration efficiency of 6.0 % for charged NaCI particles.

[0057] Table 1 shows that a filter medium of Example 1 in accordance with the invention, which is composed of PET / coPET core / sheath bicomponent filaments having a relatively high linear density of 10 dtex, which comprises an additive, and which has been electrostatically charged by corona charging, already has a filtration efficiency of 14% for non-charged NaCI particles, and a high filtration efficiency of 78%, which is surprisingly high in view of the relatively high linear density of the core / sheath bicomponent filaments of 10 dtex. As the filter medium of Example 1 has a low pressure drop, the filter medium of Example 1 also has a high quality factor of 13 Pa-1for filtering out charged particles.

[0058] Example 2

[0059] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a polypropylene (PP) in a weight ratio of 60 / 40. The PET / PP core / sheath bicomponent filaments had a linear density of 10 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 100 g / m2. The PET / PP core / sheath bicomponent filaments fibers comprised 3% by weight of a charging additive in the sheath. The charging additive was Arguflame 686, which was provided in a polypropylene masterbatch. The filter medium was electrostatically charged by corona charging from a distance of 5 cm at 29 kV, 0.8 mA and at a speed of 5 m / min.

[0060] Comparative Example 2a

[0061] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a polypropylene (PP) in a weight ratio of 60 / 40. The PET / PP core / sheath bicomponent filaments had a linear density of 15 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 75 g / m2. The filter medium did not comprise a charging additive.

[0062] Comparative Example 2b

[0063] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a polypropylene (PP) in a weight ratio of 60 / 40. The PET / PP core / sheath bicomponent filaments had a linear density of 10 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 100 g / m2. The PET / PP core / sheath bicomponent filaments fibers comprised 3% by weight of a charging additive in the sheath. The charging additive was Arguflame 686, which was provided in a polypropylene masterbatch. The filter medium was not electrostatically charged.

[0064] The filtration efficiency for filtering out NaCI particles having a diameter of 0.3 pm had been determined, both for charged NaCI particles and for non-charged NaCI particles, as summarized in Table 2.

[0065] Table 2 Table 2 shows that a filter medium of Comparative Example 2a, which is composed of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex and which does not comprise an additive is unable to filter out non-charged NaCI particles, and has a very low filtration efficiency of 1.9% for charged NaCI particles.

[0066] Table 2 shows that a filter medium of Comparative Example 2b, which is composed of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex, which comprises an additive, but which is not electrostatically charged, has a very low filtration efficiency less than 1 % for non-charged NaCI particles, and still a low filtration efficiency of 20% for charged NaCI particles.

[0067] Table 2 shows that a filter medium of Example 2 in accordance with the invention, which is composed of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex, which comprises an additive, and which has been electrostatically charged by corona charging, already has a filtration efficiency of 15% for non-charged NaCI particles, and a high filtration efficiency of 77% for charged NaCI particles, which is surprisingly high in view of the high linear density of the core / sheath bicomponent filaments of 15 dtex. As the filter medium of Example 2 has an even lower pressure drop than the filter medium of Example 1 at comparable filtration efficiency, the filter medium of Example 2 has an even higher quality factor of 16 Pa-1for filtering out charged particles.

[0068] Example 3

[0069] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a polypropylene (PP) in a weight ratio of 60 / 40. The PET / PP core / sheath bicomponent filaments had a linear density of 15 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 75 g / m2. The nonwoven fabric comprised in the filter medium further comprised a nonwoven layer of meltblown fibers composed of a polypropylene (PP), which was bonded to the nonwoven layer of core / sheath bicomponent filaments by print calendaring. The nonwoven layer of meltblown PP fibers had a weight of 15 g / m2. The meltblown PP fibers comprised 3% by weight of a charging additive. The charging additive was Arguflame 686, which was provided in a polypropylene masterbatch. The filter medium was electrostatically charged by hydrocharging.

[0070] Comparative Example 3

[0071] The filter medium of Example 3 had been de-charged by treatment in an iso-propyl alcohol (IPA) vapor for a period of 24 hours at 20°C.

[0072] The filtration efficiency for filtering out NaCI particles having a diameter of 0.3 pm had been determined, both for charged NaCI particles and for non-charged NaCI particles, as summarized in Table 3.

[0073] Table 3

[0074] Table 3 shows that a filter medium of Comparative Example 2a, which is composed of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex and which does not comprise an additive is unable to filter out non-charged NaCI particles, and has a very low filtration efficiency of 1.9% for charged NaCI particles.

[0075] Table 3 shows that a filter medium of Comparative Example 3, which is composed of a nonwoven layer of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex and a nonwoven layer of meltblown fibers composed of a polypropylene which was bonded to the nonwoven layer of core / sheath bicomponent filaments by print calendaring, which comprises an additive, but which has been de-charged, has a very low filtration efficiency of 6.6% for noncharged NaCI particles, and still a low filtration efficiency of 29% for charged NaCI particles.

[0076] Table 3 shows that a filter medium of Example 2 in accordance with the invention, which is composed of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex, which comprises an additive, and which has been electrostatically charged by corona charging, already has a filtration efficiency of 15% for non-charged NaCI particles, and a high filtration efficiency of 77% for charged NaCI particles, which is surprisingly high in view of the high linear density of the core / sheath bicomponent filaments of 15 dtex. As the filter medium of Example 2 has an even lower pressure drop than the filter medium of Example 1 at comparable filtration efficiency, the filter medium of Example 2 has an even higher quality factor of 16 Pa-1for filtering out charged particles.

[0077] Table 3 shows that a filter medium of Example 3 in accordance with the invention, which is composed of a nonwoven layer of PET / PP core / sheath bicomponent filaments having a high linear density of 15 dtex and a nonwoven layer of meltblown fibers composed of a polypropylene which was bonded to the nonwoven layer of core / sheath bicomponent filaments by print calendaring, which comprises an additive, and which has been electrostatically charged by hydrocharging, already has a filtration efficiency of 46% for non-charged NaCI particles, and a very high filtration efficiency of 92% for charged NaCI particles. Comparative Example 4

[0078] A filter medium was provided comprising a nonwoven fabric comprising a nonwoven layer of core / sheath bicomponent filaments having a core consisting of a polyethylene terephthalate (PET) and a sheath of a polyam ide-6 (PA6) in a weight ratio of 75 / 25. The PET / PA6 core / sheath bicomponent filaments had a linear density of 15 dtex. The nonwoven layer of core / sheath bicomponent filaments had a weight of 75 g / m2. The filter medium did not comprise a charging additive.

[0079] The filtration efficiency for filtering out NaCI particles having a diameter of 0.3 pm had been determined, both for charged NaCI particles and for non-charged NaCI particles, as summarized in Table 4.

[0080] Table 4

[0081] Table 4 shows that a filter medium of Comparative Example 3, which is composed of PET / PA6 core / sheath bicomponent filaments having a high linear density of 15 dtex and which does not comprise an additive is unable to filter out non-charged NaCI particles, and has a very low filtration efficiency of 0.9% for charged NaCI particles.

Claims

Method for Filtering Particles from an AirflowClaims:

1. A method for filtering particles from an airflow, the method comprising the steps of i. providing a filter medium comprising a nonwoven fabric comprising fibers comprising a thermoplastic polymer, the nonwoven fabric comprising a charging additive, ii. electrostatically charging the nonwoven fabric comprising the charging additive, iii. electrostatically charging the particles in the airflow, and iv. passing the airflow comprising the electrostatically charged particles through the filter medium comprising the electrostatically charged nonwoven fabric comprising the charging additive.

2. The method for filtering particles from an air flow according to claim 1 , wherein the nonwoven fabric comprises a nonwoven layer of mono-component fibers, preferably filaments, comprising a thermoplastic polymer and wherein the charging additive is comprised in the mono-component fibers.

3. The method for filtering particles from an air flow according to claim 2, wherein the mono-component fibers have an average linear density of 20 dtex or less, preferably 15 dtex or less, or preferably 10 dtex or less, or preferably 5.0 dtex or less, or preferably 4.0 dtex or less, or preferably 3.0 dtex or less, or preferably 2.5 dtex or less.

4. The method for filtering particles from an air flow according to claim 1 , wherein the nonwoven fabric comprises a nonwoven layer of bicomponent fibers, preferably bicomponent filaments, more preferably core / sheath bicomponent fibers / filaments, comprising a thermoplastic polymer and wherein the charging additive is comprised in the bicomponent fibers, preferably in the sheath of the bicomponent fibers / filaments.

5. The method for filtering particles from an air flow according to claim 4, wherein the bicomponent fibers have an average linear density of 25 dtex or less, preferably 20 dtex or less, or preferably 15 dtex or less, or preferably 10 dtex or less, or preferably 7 dtex or less.

6. The method for filtering particles from an air flow according to any one or more of claims 4 to 5, wherein the bicomponent fibers are core / sheath fibers comprising a core of a polyethylene terephthalate and a sheath of a copolyester.

7. The method for filtering particles from an air flow according to any one or more of claims 4 to 5, wherein the bicomponent fibers are core / sheath fibers comprising a core of a polyethylene terephthalate and a sheath of a polyolefin, preferably a polypropylene, a copolymer of a polypropylene or a polyethylene.

8. The method for filtering particles from an air flow according to any one or more of claims 4 to 5, wherein the bicomponent fibers are core / sheath fibers comprising a core of a polypropylene and a sheath of a polyolefin, preferably a copolymer of a polypropylene or a polyethylene.

9. The method for filtering particles from an air flow according to claim 1 , wherein the nonwoven fabric comprises a nonwoven layer of meltblown fiberscomprising a thermoplastic polymer comprising the charging additive in the meltblown fibers.

10. The method for filtering particles from an air flow according to any one or more of claims 1 to 9, wherein the nonwoven fabric comprises a nonwoven layer of bicomponent filaments and a nonwoven layer of mono-component filaments comprising the charging additive in the mono-component filaments and / or in the bicomponent filaments.11 . The method for filtering particles from an air flow according to any one or more of claims 1 to 9, wherein the nonwoven fabric comprises a nonwoven layer of bicomponent filaments comprising a thermoplastic polymer and a nonwoven layer of meltblown fibers comprising a thermoplastic polymer comprising the charging additive in the meltblown fibers.

12. The method for filtering particles from an air flow according to any one or more of claim 1 to 11 , wherein the step of electrostatically charging the particles in the airflow is performed with the same polarity as the step of electrostatically charging the nonwoven fabric comprising the charging additive.

13. The method for filtering particles from an air flow according to any one or more of claim 1 to 11 , wherein the step of electrostatically charging the particles in the airflow is performed with the opposite polarity as the step of electrostatically charging the nonwoven fabric comprising the charging additive.

14. The method for filtering particles from an air flow according to any one or more of claim 1 to 8, 10 or 12 to 13, wherein the nonwoven fabric excludes a nonwoven layer of meltblown fibers.

15. Use of an electrostatically charged nonwoven fabric comprising fibers comprising a thermoplastic polymer, the nonwoven comprising a chargingadditive, in a filter medium for filtering electrostatically charged particles from an airflow.

Images