WO2010109273A1

WO2010109273A1 - Process for producing a nonwoven air filter medium

Info

Publication number: WO2010109273A1
Application number: PCT/IB2009/051286
Authority: WO
Inventors: Asis Patanaik; Catherine Tsehloane; Haydon Whitebooi; Lydia Boguslavsky; Rajesh D Anandjiwala
Original assignee: Csir
Priority date: 2009-03-27
Filing date: 2009-03-27
Publication date: 2010-09-30

Abstract

One aspect of the invention concerns a continuous process for producing a nonwoven air filter medium. The process includes fibre bonding by hydroentanglement. According to the invention, a web (10) of fibres is fed continuously from a delivery station (11 ) to a first bonding station (12) at which the web is needle-punched, and from the first bonding station to a second bonding station (15) at which the web is subjected to hydroentanglement to produce a porous fabric. Further according to the invention, the fabric is fed continuously from the second bonding station to a coating station (27) at which the fabric is chemically coated to produce an air filter medium having a desired pore size. Another aspect of the invention concerns the apparatus used in the process.

Description

"PROCESS FOR PRODUCING A NONWOVEN AIR FILTER MEDIUM"

FIELD OF THE INVENTION

THIS invention relates to a continuous process for producing a nonwoven air filter medium. The invention relates in particular to a process for producing a nonwoven air filter medium using hydroentanglement as a bonding technique in the process. The invention also relates to apparatus used in the process.

BACKGROUND TO THE INVENTION

Nonwoven filters are widely used for both dry and wet filtration applications. The distinctive porous structure of nonwoven fibrous media makes them a preferred medium for filtration applications when compared to woven media.

A high efficiency air filter should be capable of preventing small particles from entering a controlled environment or preventing escape of such particles from an environment. Furthermore such filters should be capable of removing a high percentage of particulate matter from the air with a relatively low pressure drop and energy consumption.

It is known to use hydroentanglement to produce filter media with a controlled porous structure. Hydroentanglement is a mechanical technique in which a nonwoven fibrous web is bonded using high pressure water jets. Compared to conventional processes in which intensive needle punching is used to achieve fibre bonding in non-woven media, an advantage of hydroentanglement is that it causes little or no fibre shearing or other fibre damage. Also, relatively short fibres can be bonded by the hydroentanglement technique whereas this is generally not possible with intensive needle-punching. Various prior art documents disclose techniques for manufacturing nonwoven media using hydroentanglement. One example is US 3,485,706 which discloses the application of high pressure water jets to fibrous webs such that the fibres are rearranged and entangled by the influence of the water impingement, thus creating a fabric with coherence and integrity. The web is positioned over a perforated or forming surface as it is subjected to the impingement of the water jets. This forming surface drains excess water from the web and may also have specific features which are used to create a desired pattern in the fabric.

Another example is US 7,015,158 which discloses the use of hydroentanglement to produce filter media comprising predominantly polyester fibers. It is claimed that the above media can be used as a direct replacement for needle punched felts in applications such as in air and liquid filtration and other areas.

US 4,612,237 also applies a hydroentanglement technique to produce filter media. This document discloses a hydroentanglement felt comprising a uniformly dispersed sheet-like structure containing PTFE (polytetrafluroethylene) fibres and glass fibres hydraulically entangled onto a supporting scrim.

US 5,028,465 discloses a filter element which comprises a wound sheet of apertured material formed by hydroentangling webs of disparate glass or polyester fibres. It is claimed that the filter element can serve as a principal component of a filter for automotive lubricant.

US 4,556,601 discloses hydroentangled nonwoven fabrics which can be used as heavy duty gas or air filter media.

US 2,862,251 also discloses the formation of a hydroentangled or apertured gauze-like web which may be useful as a filter medium. In all the previously described hydroentanglement techniques very high pressure water jets are utilized to entangle the web of fibres. The use of very high pressure water jets involves undesirably high energy consumption and accordingly high cost.

SUMMARY OF THE INVENTION

According to this invention, there is provided a process for producing a nonwoven air filter medium, the process including hydroentanglement to bond fibres, characterised in that a web of fibres is fed continuously from a delivery station to a first bonding station at which the web is lightly needle- punched and from the first bonding station to a second bonding station at which the web is subjected to hydroentanglement to produce a porous fabric.

The light needle-punching of the web prior to hydroentanglement is advantageous in that it achieves a level of loose fibre entanglement which in turn reduces the water jet pressure required to achieve final entanglement of the fibres at the second bonding station. The reduction in required water pressure correlates directly with a reduction in the overall cost of production of the air filter medium.

In the preferred process the fabric produced by hydroentanglement is fed continuously from the second bonding station to a coating station at which the fabric is chemically coated to produce an air filter medium having a desired pore size, most preferably a pore size in the range 37.6μm to 60.16μm for efficient air filtration duties.

The preferred coating is an acrylic ester-acrylonitrile copolymer, most preferably Acronal® 32D available from BASF. Other suitable coating materials include Acrodur® DS3558, Acrodus® 950L, Emuldur®, Styrofan® and Lufofan®. The invention envisages multi-stage hydroentanglement with incrementally increasing water jet pressure at sequential stages. For example the web may be subjected to hydroentanglement at a first pressure by water jets of a first manifold and thereafter to hydroentanglement at a second pressure by water jets of a second manifold. The web may then be subjected to water jets of a third manifold, downstream of the second manifold. In one arrangement, the water jets of the second manifold act on one side of the web while the water jets of the third manifold, which may be at the same pressure as the water jets of the second manifold, act on an opposite side, i.e. the back or reverse side, of the web. The first pressure is selected to ensure effective de-aeration of the web and is typically in the range 2 bar to 10 bar, preferably 10 bar. The second pressure is selected to ensure adequate compaction of the web with an appropriate pore size and is typically in the range 60 bar to 200 bar, preferably 60 bar or 120 bar.

Although specific mention is made of a three stage hydrotentanglement procedure with three separate manifolds, one of which acts on the back or reverse side of the web, it will be understood that hydroentanglement could be carried out in any desired number of stages, depending on the properties that the filter fabric is required to have. In each case the relatively low pressures used in the various stages can reduce the overall consumption of power, and hence the cost of production of the air filter medium.

Preferably the web is lightly needle-punched at the first bonding station by barbed needles which penetrate the web, typically having a thickness in the range 10mm to 100mm, to a depth in the range 3mm to 6mm. According to another preferred parameter, the web is needle-punched by needles which penetrate the web at a stroke frequency in the range 236 strokes/min to 256 strokes/min, The web may be fed continuously at a speed in the range 1.2m/min to 3.2m/min, preferably about 2.7m/min, corresponding to a high rate of production of the air filter medium In a case where the fibres of the initial fibrous web are of polypropylene the produced fabric typically has a thickness in the range 1.31 mm to 1.51 mm.

Another aspect of the invention provides an apparatus for producing a non- woven air filter medium continuously from a fibrous web, the apparatus comprising hydroentanglement means for hydroentangling a fibrous web to form a porous fabric, characterised by needle-punching means for needle- punching the web and web supply means for continuously feeding the web through the needle-punching means to the hydroentanglement means.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing which diagrammatically illustrates the process of the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The drawing illustrates an apparatus for practising the process of the present invention for producing a nonwoven air filter fabric or medium. Although specific reference is made to the production of a filter medium for air, it will be appreciated that the medium produced by the process described below could equally well be used for filtering gases other than air. It is accordingly to be understood that the term "air" is used for convenience only in this specification and that this term should be interpreted to extend to other gases as well.

The filter medium is produced from polypropylene fibres, in this example having a fineness of 2.2 dtex and a length of about 40 mm. In other, non- limiting examples the fibres could be polypropylene fibres have a fineness of in the range 2 dtex to 6.6 dtex and a length in the range 40mm to 60mm. As a further, non-limiting example, polyester fibres having a fineness in the range 1.5 dtex to 3.6 dtex and a length in the range 38mm to 60mm could also be used.

Referring to the drawing, the polypropylene fibers are initially subjected to carding whereafter the carded web P is oriented in a cross machine direction using a cross lapper. These steps are carried out at a carding and cross-lapping station 11 from which the web is -delivered continuously and on-line through other stages, as described below. The carded and cross- lapped web 10 is fed from the carding and cross-lapping delivery station to a needle punching machine 12 situated at a first bonding station indicated generally by the numeral 13. The needle punching machine 12 uses barbed needles to pre-bond the fibres of the web. This achieves light consolidation of the fiber web as well as a level of loose fiber entanglement.

In this example the web has a thickness in the range 10mm to 100mm and the depth of needle penetration is selected to be 4mm and the needle penetration frequency is selected to be 242 strokes/min with the speed of the web being about 2.7 m/min. In other examples, operating at the same web speed and also with a web having a thickness in the range 10mm to 100mm, the needle penetration depth could for instance be 3, 5 or 6mm with corresponding penetration frequencies of 236, 248 or 254 strokes/min. These parameters are selected to ensure that an adequate level of pre- bonding is achieved without undue breakage or shearing of the fibres of the web. In this regard it is to be noted that a very high degree of pre-bonding may result in undesirably tight consolidation of the fibres of the web, with higher fibre entanglement increasing the chances of fibre shearing or other damage.

Even with a web thickness of 10mm and a needle penetration depth of 6mm it will be understood that the needle penetration depth is substantially less than the web thickness, in this case 60% of the web thickness, resulting in a light pre-needling or needle punching of the web. In other cases with smaller needle penetration depths and/or greater web thicknesses, the ratio needle penetration:web thickness will be even less than 60% so the needle punching operation in such cases will be even lighter. Other parameters, such as the web speed, typically varying in the range 1.2m/min to 3.2m/min, and needle stroke frequency, typically varying in the range 236 strokes/min to 256 strokes/min, can also be varied to control the intensity of the needle punching operation and hence the properties of the web which is subsequently hydroentangled.

The light pre-bonding which is achieved by the needle punching operation also results in desirable changes to the fibre orientation resulting from the initial carding and cross-lapping steps.

The initial bonding or entanglement of the fibres by light needle punching in the machine 12 subsequently plays a major role in energy saving and in the creation of a required nonwoven filter medium with a distinctive pore configuration. As described below the web is subjected after light needle punching by the machine 12 to hydroentanglement. In the hydroentanglement step water jets are caused to impinge on the continuously moving fibrous web. The pre-bonding or initial entanglement of the fibres by needle punching reduces the required water jet pressure and accordingly reduces the overall consumption of energy in the hydroentanglement step.

After passage through the first bonding station 13, the web 10 passes over a water-permeable backing member 20 which may in the form of a moving conveyor. The web passes initially beneath a web compression roller 14 which applies light compression to the web. Thereafter hydroentaglement is carried out in multiple stages at a second bonding station indicated generally by the numeral 15.

In the hydroentanglement station 15, high pressure, columnar water jets are produced by pumping water through an array of nozzles placed in a jet strip and clamped into different manifolds or pressure heads. In each manifold there may be one or more rows of several nozzles in the jet strip. Generally multiple manifolds with different water jet pressure are used to produce the desired nonwoven fabric. In the first stage of hydroentanglement, the web 10 passes over the member 20 beneath a first manifold 16. This manifold operates at a relatively low pressure of 10 bar, with the water jets serving inter alia to pre-wet the web and achieve initial hydroentanglement of the fibres. In the next stage the web moves over the member 20 beneath a second manifold 18 operating at a higher pressure of 60 bar. Here the water jets achieve further fibre entanglement. De- energised water which passes through the web and the member 20 is collected by a suction unit 22 for recycling and reuse.

Final entanglement of the fibers is achieved in a third stage in which the back or reverse side of the web is subject to water jet impingement by a manifold 24 while passing around a roller 26 having a perforated, water permeable surface. The water pressure at the manifold 24 may be the same as that at the manifold 18, i.e. 60 bar. In other, non-limiting examples, the water jet pressure at either or both of the manifolds 18 and 24 could be 120 bar.

Final hydroentanglement by the water jets emanating from the manifold 24 produces a fabric which is passed around a carrier roller 28 where the fabric is coated in-line, at a coating station indicated generally by the numeral 29, with an acrylate chemical binder. This is preferably Acronal® 32D, available from BASF, at a 25% concentration and is achieved by a foam bonding process. The coating which is applied configures the fabric pores to a size, typically in the range 37.6μm to 60.16μm, suitable for the duty which the eventual air filter medium will be required to perform, i.e. to configure the pores such that the medium is able to capture particulate matter of a required, small size.

Thereafter the coated fabric is passed through a drying and winding station indicated generally by the numeral 29. The fabric passes initially through a nip between opposed squeezing rollers 30 which squeeze out some of the water content. The fabric is then dried and cured in an oven 32 maintained at an elevated temperature, in this case 150°C. Finally the dried fabric is wound onto a roller at a winding station 34.

Test Results

In comparative tests two different sets of hydroentangled filter media or fabrics are produced. One set of filter media are produced without needle punching while another set of media are produced by the process described above, i.e. with in-line pre-bonding before hydroentanglement.

In both cases different water jet pressures and different manifold process parameters were used. The selected water jet pressures in the first, second and third manifolds were 10, 60 and 60 bar, and 10, 120 and 120 bar respectively, i.e. filter media were produced at 60 and 120 bars for each set.

In all cases the fabrics produced by hydroentanglement were coated with the aforementioned chemical binder and were dried and cured. The average weight per unit area of the resultant media in all cases was 130 g/m².

The prepared filter media were subjected to performance evaluations in terms of difference in pore size, filtration efficiency, pressure drop and tensile breaking strength. Various types of through pores were present in the nonwoven fabrics. The sizes of these pores were classified in terms of smallest pore diameter, largest or maximum pore diameter, and mean flow pore diameter. The smallest and maximum pore diameters were the diameters of the smallest and largest pores, respectively. The mean flow pore diameter is the diameter of the majority of the pores. Different pore sizes were measured by a capillary flow porometer. Filtration parameters were measured by ASHRAE standard 52.2. A dust filtration device was used to evaluate filtration performance. Dust particles having a size in the range 0.6μm to 180μm were fed at a constant rate to the filtration device and deposited on samples of the filter media having an area of 0.0095m².

The tensile strengths of the filter media were measured according to the ASTM D5034 standard by using a lnstron tensile testing instrument operating on the principle of constant rate of extension

Measured comparative data are represented in Tables 1 and 2 below.

TABLE 1

Pre- bonding needle punching process parameters

4 242 2 7

5 248 2.7

6 254 2 7

FABLE 2

Structural/Performance Properties

Breaking

2 10, 120, 120 17 62 52 28 115 43 78 23 19 5 101 20 185 4"

With pre- bonding by needle punching

3 10, 60, 60 10 63 60 16 1 12 23 87 1 19 5 93 51 231 T

4 10, 60, 60 7 48 56 92 95 18 90 9 19 5 95 94 232 2

5 10, 60, 60 8 19 52 93 91 27 91 3 19 5 97 22 230 0'

6 10, 60, 60 7 69 50 11 97 45 91 0 19 5 96 22 233 9^'

7 10, 120, 120 7 10 37 60 82 13 91 8 12 0 108 36 252 7'

8 10, 120, 120 6 95 38 54 78 16 93 7 12 0 110 12 245 U 10, 120, 120 7.42 39.23 75.23 92.3 12.0 109.27 250.09 10, 120, 120 6.88 37.73 76.81 92/7 12.0 108.29 251.34

Thickness of example numbers 1 and 2 (mm): 1.62, 1.43. Thickness of example numbers 3-

6 (mm): 1.51, 1.48, 1.46, 1.46. Thickness of example numbers 7-10 (mm): 1.32, 1.36, 1.31, 1.34.

At the foot of Table 2, the term "thickness" refers to the thickness of the final nonwoven filter fabric or medium and the term "filtration efficiency" refers to the percentage by mass of particles captured by the filter medium.

The data shown in the accompanying Table 2 compares the performance of the filter media. Notably, the use of in-line needle punching prior to hydroentanglement (examples 3 to 10), was shown to produce filter media with higher filtration efficiency, lower pressure drop, and higher tensile strength in comparison to media produced without prior needle punching (examples 1 and 2). It can be seen that the performance of the filter media produced with prior needle punching and hydroentanglement at a water jet pressure of 60 bar is superior to that of filter media produced by hydroentanglement at a water jet pressure of 120 bar without prior needle punching. This indicates that at a low water jet pressure better performance can be achieved if prior needle punching, in accordance with the invention, is carried out.

It can be expected that the need for reduced water pressure will in turn contribute to a reduction in the energy requirement of the production process and hence to an overall reduction in the cost of filter media production.

Table 2 also shows that when prior needle punching is carried out in accordance with the invention, an increase in water jet pressure to 120 bar provides only a marginal increase in the performance characteristics of the filter media.

It is envisaged that filter media produced in accordance with the inventioin will find application in environments where a high level of air purity is required in the air, for example in hospitals. Referring again to Table 2 it can be seen that with an increase in water jet pressure there is a decrease in pore size, an increase in filtration efficiency and a decrease in pressure drop across the filter medium in use. There is also an increase in strength in the machine direction (MD) and cross direction (CD). Skilled persons will recognize that the CD strength is generally higher than the MD strength because of the fibre alignment in that direction which contributes to the strength of the medium.

Claims

1.

A continuous process for producing a nonwoven air filter medium, the process including hydroentanglement to bond fibres, characterised in that a web of fibres is fed continuously from a delivery station to a first bonding station at which the web is needle-punched and from the first bonding station to a second bonding station at which the web is subjected to hydroentanglement to produce a porous fabric.

2.

A process according to claim 1 characterised in that the fabric is fed continuously from the second bonding station to a coating station at which the fabric is chemically coated to produce an air filter medium having a desired pore size.

3.

A process according to claim 2 wherein the fabric is chemically coated to produce an air filter medium having a pore size in the range 37.6μm to

60.16μm.

4.

A process according to claim 2 or claim 3 characterised in that the fabric is coated, at the coating station, with an acrylic ester-acrylonitrile copolymer.

5.

A process according to any one of the preceding claims characterised in that, at the second bonding station, the web is subjected to hydroentanglement by water jets at a first, lower pressure and thereafter to hydroentanglement by water jets at a second, higher pressure.

6.

A process according to claim 5 characterised in that the web is subjected to hydroentanglement at the first pressure by water jets of a first manifold and thereafter to hydroentanglement at the second pressure by water jets of a second manifold.

7.

A process according to claim 6 characterised in that after hydroentanglement at the second pressure by water jets of the second manifold, the web is subjected to hydroentanglement by water jets of a third manifold downstream of the second manifold.

8.

A process according to claim 7 characterised in that the pressure of the water jets of the third manifold is the same as that of the water jets of the second manifold.

9.

A process according to claim 7 or claim 8 characterised in that the water jets of the second manifold are applied to one side of the web and the water jets of the third manifold are applied to an opposite side of the web.

10.

A process according to any one of claims 5 to 9 characterised in that the first pressure is in the range 2 bar to 10 bar.

11.

A process according to claim 10 characterised in that the first pressure is

10 bar.

12.

A process according to any one of claims 5 to 11 characterised in that the second pressure is in the range 60 bar to 200 bar.

13.

A process according to claim 12 characterised in that the second pressure is 60 bar.

14.

A process according to claim 12 characterised in that the second pressure is 120 bar.

15.

A process according to any one of the preceding claims characterised in that the web has a thickness in the range 10mm to 100mm and is needle- punched at the first bonding station by needles which penetrate the web to a depth in the range 3mm to 6mm.

16.

A process according to claim 15 characterised in that, at the first bonding station, the web is needle-punched by needles which penetrate the web at a stroke frequency in the range 236 strokes/min to 254 strokes/min.

17.

A process according to any one of the preceding claims characterised in that the web is fed continuously at a speed in the range 1.2m/min to

3.2m/min.

18.

A process according to claim 17 characterised in that the web is fed continuously at a speed of 2.7m/min.

19.

A process according to any one of the preceding claims characterised in that the fibres are polypropylene fibres.

20.

A process according to claim 19 characterised in that the fabric has a thickness in the range 1.31 mm to 1.51mm.

21.

An apparatus for producing a non-woven air filter medium continuously from a fibrous web, the apparatus comprising hydroentanglement means for hydroentangling a fibrous web to form a porous fabric, characterised by needle-punching means for needle-punching the web and web supply means for continuously feeding the web through the needle-punching means to the hydroentanglement means.

22.

An apparatus according to claim 18 characterised by coating means for chemically coating the fabric formed by the hydroentanglement means to produce an air filter medium having a desired pore size.