24
160 © Woodhead Publishing Limited, 2010 9 The use of nonwovens as filtration materials S. Z O B E L and T. G R I E S, RWTH Aachen University, Germany Abstract: Textile fabrics are used as filter media. Depending upon the filtration application, different requirements have to be fulfilled. A number of standards exist for the development of filters and filter media for different applications. Sometimes it is necessary to combine different filtration media to best fit the application’s requirements (e.g. textile filter and membrane). As well as describing the standards, the structural design of the filters and their manufacturing technologies are discussed. Some technological priorities that have arisen due to the introduction of stringent environmental regulations are discussed and future trends are documented. Key words: filter, filtration, depth filtration, surface filtration, nonwoven filter, filter cell, cartridge filter, bag filter, filter application. 9.1 Introduction This chapter deals with filtration and specifically the use of nonwovens in filtration. The term ‘filtration’ has been defined as: The separation of particles from a fluid–solid suspension of which they are a part by passage of most of the fluid through a septum or membrane that retains most of the solids on or within itself. The septum is called a filter medium, and the equipment assembly that holds the medium and provides space for the accumulated solids is called a filter. The fluid may be a gas or a liquid (Anon., 2007). The particles may be solid, liquid or gaseous substances. There is a huge variety of filter media available. Textile fabrics, porous foams, films and sands can be used as filter media. Depending upon the filtration application different requirements have to be fulfilled. Sometimes it is necessary to combine different filtration media to fit best the application’s requirements (e.g. textile filter and film). The choice of the filter medium depends on the properties of the particles that need to be separated (e.g. particle size, potential for agglomera- tion, particle concentration) and the surrounding medium (e.g. temperature, flow velocity, etc.). When nonwovens are used as filters, they offer a range of advantages above

Applications of Nonwovens in Technical Textiles || The use of nonwovens as filtration materials

  • Upload
    s

  • View
    229

  • Download
    12

Embed Size (px)

Citation preview

160

© Woodhead Publishing Limited, 2010

9The use of nonwovens as filtration materials

S. Z O B E L and T. G R I E S, RWTH Aachen University, Germany

Abstract: Textile fabrics are used as filter media. Depending upon thefiltration application, different requirements have to be fulfilled. A number ofstandards exist for the development of filters and filter media for differentapplications. Sometimes it is necessary to combine different filtration mediato best fit the application’s requirements (e.g. textile filter and membrane). Aswell as describing the standards, the structural design of the filters and theirmanufacturing technologies are discussed. Some technological priorities thathave arisen due to the introduction of stringent environmental regulations arediscussed and future trends are documented.

Key words: filter, filtration, depth filtration, surface filtration, nonwovenfilter, filter cell, cartridge filter, bag filter, filter application.

9.1 Introduction

This chapter deals with filtration and specifically the use of nonwovens infiltration.

The term ‘filtration’ has been defined as:

The separation of particles from a fluid–solid suspension of which theyare a part by passage of most of the fluid through a septum or membranethat retains most of the solids on or within itself. The septum is called afilter medium, and the equipment assembly that holds the medium andprovides space for the accumulated solids is called a filter. The fluid maybe a gas or a liquid (Anon., 2007).

The particles may be solid, liquid or gaseous substances.There is a huge variety of filter media available. Textile fabrics, porous foams,

films and sands can be used as filter media. Depending upon the filtrationapplication different requirements have to be fulfilled. Sometimes it is necessaryto combine different filtration media to fit best the application’s requirements (e.g.textile filter and film). The choice of the filter medium depends on the propertiesof the particles that need to be separated (e.g. particle size, potential for agglomera-tion, particle concentration) and the surrounding medium (e.g. temperature, flowvelocity, etc.).

When nonwovens are used as filters, they offer a range of advantages above

The use of nonwovens as filtration materials 161

© Woodhead Publishing Limited, 2010

9.1 Nonwoven applications (EDANA; Anon., 2006b).

other filter media. For example, nonwovens offer large and adjustable surfaceproperties and can be adapted to different filtration requirements. Depending uponthe filter requirements, different textile or plastic grid structures can be combinedtogether to form a sandwich structure (e.g. backing fabric and nonwoven).Compared to other filtration media like membranes, wire cloth and monofilamentfabrics, nonwovens offer a thicker cross-section and bulk (Gregor, 2004). Thisprovides the opportunity to use nonwovens as a structure that can fulfil therequirements and boundary conditions of all types of applications. To influence thestructure of the nonwoven media, different manufacturing methods are used tomanufacture filters for diverse applications. Nonwovens offer high permeabilityand surface area, which are further enhanced by pleating of the material. Also thewide range of fibre materials available offers good mechanical, chemical andphysical thermal properties. Thus, the production of nonwovens for filtrationapplications is very efficient and can be very economic depending on the fibrematerial and process steps used. According to Gregor (2004), nonwoven filters arethe material of choice when large quantities of particulate loading, long life orwhere general clarification of a liquid or gas stream is required.

In 2005, 4000 t of nonwovens were produced worldwide (Anon., 2006a). In

Unidentified 0.50%

Others 4.70%

Hygiene 33%

Medical/surgical3.10%

Wipes for personal care7.70%

Wipes – others 7.20%

Garments 1%

Interlinings 1.90%

Coating substrates1.80%

Upholstery/table linen/household 5.90%

Floor coverings 2.10%

Liquid filtration 3.90%

Air and gas filtration2.40%

Building/roofing12.70%

Civil engineering/underground 4.50%

Automotive 3.50%

Agriculture 1.40%

Shoe/leather goods 1.80%

162 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

9.2 Trend of world filter demand (Anon., 2008).

Europe a total of 1399 t of nonwovens was produced for various applications(Schumann and Erth, 2006) including 3.9% for liquid filtration and 2.4% for gasand air filtration (see Fig. 9.1). Increasing demand and ongoing development ofnew applications continues to fuel an increase in growth of the use of nonwovensin filtration. As can be seen from Fig. 9.2, growth is expected not only in Americaand Europe but also in Asia and other regions (Anon., 2008).

Rigby (2003) showed that the annual growth rate of nonwovens in filterapplications in 2005 was expected to be 8% and to reach a growth of 8.6% by 2010.For comparison the growth rate of general nonwovens production, independentfrom application was expected to increase by about 4.7% by 2005 and about 5% in2010 (David Rigby Associates, 2002). Thus, the use of nonwovens for filterapplications is one of the fastest growing sectors in the nonwoven market.

9.2 Classification of filters

When choosing the appropriate filter for an application the properties of the fluidsurrounding the filter have to be considered. The following features of thesurrounding fluid are important:

• temperature• humidity• flow condition• mass flow• chemical composition.

These qualities affect the filter’s performance. In addition the fibre material used,the assembly and the forces and stress exercised on the filter during operation needto be considered. Finally, also the particle properties, for example particle size,particle size distribution and particle material, have to be considered. Taking intoaccount these manifold types of requirements and boundary conditions the avail-able filter media can be classified.

2001 2006 2011

16000

12000

8000

4000

0

(Mill

ion U

S$)

North America West Europe Other regionsAsia-Pacific region

The use of nonwovens as filtration materials 163

© Woodhead Publishing Limited, 2010

For the classification of filter media different methodologies are used. Accord-ing to the Filters and Filtration Handbook (Dickenson, 1997), filters can beclassified in four main categories. These categories are solid–gas separation,solid–fluid separation, liquid–liquid separation and solid–solid separation. Albrechtet al. (2000) add one more separation type, the gas–gas separation, so that filtermedia are categorised into five different types.

The most common methodology for the classification of filter media, which isdescribed in this chapter, is as follows. The filter media are classified dependingon:

• the nature of the surrounding medium (dry and wet filtration)• surface filters or depth filters• particle size to be filtered (e.g. micro-, ultra-filtration).

9.2.1 Dry and wet filtration

Dry filtration deals with the separation of solid, liquid or gaseous substances froma solid or gaseous medium. These substances are dispersed in the solid or gaseousmedium. For example, for solid–solid separation the finer particles are separatedfrom the larger particles by means of a multiple stage sieving process. Thisprocedure determines the grain size distribution of different soils used. For theseparation of solids or liquids from a gaseous medium, filter fabrics (e.g. nonwovens,wovens) are used. The fluid with the substances to be filtered out is passed throughthe filter. Depending on the structure of the filter the particles can be deposited onthe surface (surface filtration) or inside (depth filtration) the filter medium instal-lations. Dry filters are usually voluminous structures.

The air and gas filtration market includes domestic filters, industrial filters andautomotive filters. Domestic filters are used in heating, ventilation and air condi-tioning (HVAC), cooking, vacuum cleaners and various portable filters in themarket. Industrial filters are in general used in (HVAC), high efficiency particulateairfilter (HEPA) and ultra low penetration air (ULPA) filters, dust removal forpower stations, incinerators, paint spray house and many industrial processes,where air is contaminated and needs to be cleaned, or a very clean environment isrequired for the production of, for example, electronic components. Automotivefilters include engine air filters (intake and exhaust) and cabin air filters.

Wet filtration deals with the separation of solid, liquid or gaseous substancesfrom a liquid medium. The materials to be filtered are usually suspended in themedium.

In the case of the separation of a liquid–liquid mixture the boiling point of thedifferent liquids is taken into consideration. By evaporating one of the componentsof the liquid mixture, the separation can take place.

Solid–liquid separation by deposition occurs due to the deposition of the solidparticles at the bottom of the container (e.g. sewage treatment). In addition to

164 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

separation by deposition, filter fabrics (e.g. nonwovens, wovens) are used for wetfiltration. Wet filters offer the possibility for fluid permeability and at the sametime provide the impermeability for particles that need to be filtered. Nonwovenfilter media offer the possibility of collecting the particles on the filter surface(surface filtration) and in the filter medium installation (depth filtration). Wetfiltration media are usually very thin and compacted media.

Liquid filtration is a fast growing market for nonwovens. It includes waterfiltration (tap and waste water), food and beverage filtration, pharmaceutical andelectric processes, blood filtration, tea bags and coffee and juice filters, cooking oilfilters and oil/fuel filters for automotives. Nonwovens have been successfully usedin the industry as membrane support for micro-filtration, ultra-filtration andreverse osmosis filtration.

9.2.2 Surface filters and depth filters

Filter media can be classified into surface filter and depth filter media. It has to bementioned that mostly, both surface and depth filtration occur in the filter medium.The classification of whether the filter medium is a surface or depth filtrationmedium depends on the preferred deposition area of the particles.

Surface filtration is characterised by a deposition of particles or aerosols, whosediameter is greater than the pore size of the filter, on the filter surface (see Fig. 9.3).On one hand the particles can clog and block the filters, which would result in highfluid resistance across the filter, at which point the filter would need to be eithercleaned or changed. On the other hand the deposited particles on the surface of thefilter can result in the formation of a layer of substrate that has lower pore size thanthe filter itself, thus facilitating filtering. This layer of substrate is commonlyknown as a filter cake. Many surface filters function most effectively when thefilter cake is developed on the filter. A filter cake is compressible and its filtrationefficiency decreases with increasing pressure and reduction of the pore volume.This effect increases the separation until the filter cake is completely blocked(Hoeflinger and Pongratz, 2000).

The filtration through a filter cake functions as a depth filter, where the filteredparticles are mechanically held or adsorbed into the the cake. For surface filtermedia only a few particles penetrate into the interior of the filter and remain there.Hence surface filters can be cleaned and reused multiple times. Surface filtersusually have a smooth, paper-like surface and are very thin. Filters are generallycompared based on their filtration area and the degree of separation possible.

According to the Filters and Filtration Handbook (Dickenson, 1997) surfacefilters have the following properties:

• low pressure loss compared to depth filters• high filtration reproducibility with a narrow pore size distribution.

In the case of depth filters, the filtered particles of different dimensions settle and

The use of nonwovens as filtration materials 165

© Woodhead Publishing Limited, 2010

9.3 Surface filtration.

9.4 Depth filtration.

deposit themselves within the pores of the filters (see Fig. 9.4). Depth filters arenormally used for applications where there is a large difference in particle size.They are usually very thick and can have a progressive density of pore structure.This leads to the interception of very coarse particles in the upper layers of thefilter. In subsequent layers, finer particles can be intercepted. Thus coarserparticles are separated mechanically and finer particles are filtered due to theiradsorption. The pressure difference and the fluid flow rate remain almost con-stant. Depth filters are difficult to clean and can be reused only under specificcircumstances. Criteria for comparison are pore volume, the filter thickness anddegree of separation. Depth filters are characterised using the following proper-ties:

• suitability for the filtration of difficult filterable solids (e.g. particles of differentdimensions);

• high filtration efficiency over a wide range of particle sizes.

Through the combination of surface and depth filters, it is possible to separatecoarser particles by surface filters and the finer particles through depth filters fromthe fluid stream. This results in high endurance and maintains the throughputperformance of the filter media.

Cakeformation

Filter

Blockedfilter

Suspension

Filtrate

Filter cake

Filter

Particleabsorption

Filter

Mechanicalfiltration

Suspension

Filtrate

Filter

166 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

9.5 Filter media categorised by particle size.

9.2.3 Particle size

When selecting filter media the particle size, particle size distribution, particle typeand particle concentration must be taken into account. Large particles are normallyintercepted on the surface and finer particles within the filter medium or a filtercake. Based on the particle size, one can categorise currently available filter mediauseful for different types of filtration processes (see Fig. 9.5). These include forexample filter media for reverse osmosis (particle size <0.007 microns),nanofiltration (particle size <0.01 microns), ultra-filtration (particle size <0.1microns), micro-filtration (particle size <10 microns) and conventional filtration(particle size <900 microns) (Gasper, 2000). In particular, for the filtration ofparticles in air or exhaust gases, special guidelines and standards help to classifythe filter medium. Therefore, the filtration efficiency of the filter medium ismeasured and classified. In Europe 17 filter classes exist. The higher the class, thehigher is the degree of separation of particles with size greater than 0.1–0.3 µm.Thus, the filters are classified in classes G1–G4 (large particle filter) and in classesF5–F9 (fine particle filter) according to DIN EN 779. The remaining filters areclassified using EN 1822-1 in H10–H14 (HEPA) and U15–U17 (ULPA) filters. InNorth America, the filter efficiency is determined using ASHRAE’s MinimumEfficiency Reporting Value (MERV). The MERV value lies between 1 and 20.

9.3 Filtering mechanisms, technical requirements

and standards for nonwoven filtration

Depending upon different filtering mechanisms, particles with different sizes andweight can be separated. The parameters such as the size, size distribution andshape of the particles and the pore diameter and pore size distribution of the filter

The use of nonwovens as filtration materials 167

© Woodhead Publishing Limited, 2010

medium play a major role. Using numerous empirical and theoretical equations,the relationship between the porosity and the pressure drop of a filter medium canbe predicted. The basic equation of filtration is the Darcy equation. For theselection of filter media different standards for filter media and filter tests haveemerged. This section deals briefly with the filter mechanisms, the criteria that areused for the selection of the filter medium, and their testing.

9.3.1 Filtration mechanisms

There are three basic filtration mechanisms (see Fig. 9.6):

• direct interception• inertial impact• diffusion

Direct interception describes the attachment or interception of a particle on thefibre surface. If the particle follows a fluid flow direction at a distance that is equalto or smaller than the particle diameter, it is attached to the fibre surface. If particlesflow at a distance greater than the particle diameter, they are no longer attracted bythe fibre and the interception mechanism does not function.

If the particle, due to its size and weight, does not follow the change in the fluidflow pattern, inertial impact occurs. This phenomenon occurs for fluids that arefiltered at high speeds and also with filter media that are very dense. If the numberof particles and the particle size increase, the probability of collision and attach-ment to the fibre surface in the filter increases. In doing so, the particles themselves

9.6 Basic filtration mechanisms.

Particles

Inertial impact

Diffusion

Direct interception

Fibre

Streamlines

168 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

form part of the filter and thereby increase the filter efficiency. The filter efficiencyalso increases due to the thickness (filter depth or particle travel distance) of thefilter medium used. Thus the efficiency of filtration by impact depends on theparticle size, particle concentration, the fluid flow conditions and filter thickness/density.

In the case of lower fluid flow velocities and smaller particles (diameter<0.1µm), a different mechanism applies. The small particles in this case follow arandom zigzag motion pattern because they collide with the molecules of the fluidand interact with them. This motion of the particles can be attributed to theBrownian motion of particles: the smaller the particle, the slower is the flowvelocity of the fluid. The particles have more time to carry out the zigzag motionand require longer to actually collide with the fibres. The more the particles attachto the fibres, the higher is the probability that particle deposition will occur(increasing the filter efficiency). This results in a higher pressure drop across thefilter.

There are other filter mechanisms that are related to the mechanisms describedabove, for example electrostatic filtration and cake filtration. For charged parti-cles, an electrostatic charged filter can help to give good filter performance. In thiscase the particles are attracted to filter media that have an opposite charge to theone on the particles. For example, dust particles are always negatively charged,therefore if the filter medium carries a positive charge, the dust particles would bedeposited on the surface or within the filter.

In case of the cake filtration mechanism the filtered particles are deposited oneither the surface of the filter or within the filter. These particles form a thin layerof their own. This layer possesses finer pore sizes compared to the filter mediumitself. This thus facilitates the filtration of finer particles.

The filtration mechanisms can be understood by the basic equation presented byDarcy:

dV A • ∆p • k—– = ———–– [9.1]dt η • h

where dV/dt is the volume flow, A the filter area, ∆p the pressure loss, η the fluidviscosity, k the filter permeability and h the filter thickness. This equation gives therelationship between the pressure loss in fluid flow across a filter medium and thefilter thickness. k is the permeability and must be determined experimentally. k canalso be determined from the deposits (from the filter cake). The relationshipbetween the permeability, porosity and specific surface of the filter is given by theKozeny equation:

ε 3

k = ————— [9.2]K(1 – ε)2 S2

V

where ε is the filter porosity, SV the specific filter surface and K the Kozenyconstant. The specific filter surface is defined as the surface that is formed by the

The use of nonwovens as filtration materials 169

© Woodhead Publishing Limited, 2010

fibres within the filter medium. Thus if specific filter surface increases (e.g.because of the use of finer fibres), the permeability of the filter would reduce. TheKozeny constant describes the ability of a fluid to move through the porousstructure. Its value depends on the porosity of the filter medium. For porositiesbetween 0.3 and 0.65 the Kozeny constant is constant and has a value of approxi-mately 4. For varying porosity values the Kozeny constant has to be validated byexperiment.

9.3.2 Requirements for nonwoven filtration

Filters are used for many applications in different areas. In order to achieve the fullpotential of nonwoven filtration, the correct filter needs to be chosen and placedcorrectly depending upon the process conditions (e.g. pressure on the filter surface,cleaning interval, cleaning method). It is necessary to accurately adjust the filter tothe requirements. For this purpose, the material used for filter media and also thetype of filter needs to be taken into account. For the selection of fibre material tobe used in nonwoven filters the following parameters need to be considered:

• Basis weight: the fibre content (by weight) per square area of nonwoven.Depending on the thickness of the textile structure and the fibre fineness thepore size and pore size distribution vary.

• Pore size/distribution: the pore size and its shape depend on the fibre finenessand the compaction of the nonwoven medium. The size and the distribution alsodepend on the manufacturing process and the consolidation and finishingprocess of the nonwoven.

• Thickness: depends on the basis weight and the consolidation process. Thethickness also plays a role in filtration.

• Solid volume fraction (SVF): the amount of fibres in a volume element inpercentage.

• Porosity (P): related to the SVF (P = 1 – SVF).• Density: determined as the weight per cubic metre. Relevant to the processing

behaviour of different fibre materials, e.g. in a nonwoven that comprises a blendof fibres.

• Permeability: describes the fluid’s ability to pass through the filtration me-dium. Highly permeable filter media decrease the pressure drop across thefiltration medium. Coarse filter media give an increase in permability becauseof the larger pores.

• Surface texture: influences the particles’ ability to adhere to the surface of thefiltration medium. Influenced by the fibre shape and the manufacturing process,e.g. the formation, consolidation and finishing of the nonwoven material.

• Moisture absorption capacity: depending on the moisture absorption of amaterial and the filter structure, the filtration efficiency may be negativelyinfluenced.

170 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

• Flammability behaviour: relevant when used in hot applications. The filtermedium should remain rigid enough to be able to perform the filtration. Usageof filtration media above glass transition temperature should be avoided.

• Strength and drape: the strength of a medium defines, for example, whether thefilter is appropriate for high pressure filtration applications. The drapeabilityenables the fitting of the filter to a special design.

• Electrostatic behaviour: relevant for the application of air and gas filtration.Some particles are charged with an opposite charge so that the filter efficiencyincreases.

• Chemical, thermal and biological stability of the materials: the materials usedshould be able to perform their filtration function permanently, e.g. if thematerial dissolves in an acid environment it should not be used.

For each of the properties mentioned, the different characteristics provide differ-ences in filter performance.

9.3.3 Technical standards for nonwovens

There are different standards and testing norms, for use of filters in differentapplications. In order to test filter media, prescribed test aerosols need to be usedaccording to the norms. This is an important requirement for the determination ofthe fractionating potential of the filters. Often the total fractionating potential orseparation potential, which depends on the particle size distribution in the gas andthe particle concentration, is used to characterise the properties of the filter used.

To determine particle size and particle size distribution, particle size measuringdevices according to the VDI 3489 can be chosen. These measuring devices useaerosols for the determination of filter efficiency. In VDI guideline 3491 concepts,definitions and the production of test aerosols are explained. VDI guideline 2066directive describes the arrangement of sampling techniques for the measurementof dust in flowing gases and the classification of filter media according to theirintended use. Thus, for example, air filters are classified according to DIN EN24185, colloidal solution filters according to DIN EN 1822, waste treatment filtersaccording to VDI 3926, air filters for internal combustion engines and compressorsin accordance with ISO 5011, filters for the automotive interior according to DIN71460 and indoor particulate filter according to DIN EN 779.

There are also standards to characterise the properties of the nonwovensthemselves. For example, basis weight (ISO 9073-1), bursting strength (DIN53861-3), breaking extension (ISO 9073-3/-18), abrasion resistance (DIN 53863-1/-2), electrostatic behaviour (DIN 54345-4) and air permeability (ISO 9073-15).Air permeability is particularly important because it can define the pressure drop,ease of cleaning and separation potential. The tensile strength of the filter mediadepends on the density, the consolidation and the fibre length and material.

According to the aforementioned standards and testing norms commercially

The use of nonwovens as filtration materials 171

© Woodhead Publishing Limited, 2010

available test machines exist to determine the air permeability, the filter efficiency,the cleaning and the reuse of the filter medium using different types of particles. Alot of companies have developed their own test equipment and test machines.Thus, the comparison between different filter media data is not always possible orappropriate.

All the laboratory tests of filtration media data have to be scaled up to theperformance of the filter during the process. The air permeability and the amountof particles in the cleaned fluid during the real usage of the filter have to bemeasured and controlled. This is important for new filtration media.

9.4 Design of nonwoven filters

The type of filter medium and the filter-medium structure are influenced by themethod of manufacture used. The method of manufacture can affect the filtermedia properties such as pore size, thickness, basis weight, air permeability, etc.Depending on the method of manufacture the particles can penetrate into theinterior of the filter medium (depth filter), thus the filter must be completelydisposed of, or the particles can be distributed only superficially on the filtermedium (surface filter) and can be cleaned and reused where possible. Theproperties also depend on the design of the filter itself. The filtration efficiency andthe pressure drop differ if the structure of the filter is, for example, a bag orcartridge filter.

9.4.1 Material variables

Depending on the application a suitable filter has to be chosen. The appropriatefilter materials are selected depending on the type of particles, particle size, particlesize distribution and surrounding fluid. In particular, the choice of fibre propertiesand the design of the filter medium influence the performance of a filter, and thecosts of a fibre material and the filter medium also influence the design of the filtermedium and the filter. The following parameters have a great influence on the filtermedia properties:

• fibre materials (e.g. density, costs)• fibre fineness (e.g. pore size)• fibre cross-sectional shape (e.g. pore size, surface texture)• chemical properties (e.g. acid resistance)• physical properties (e.g. abrasion resistance)• thermal properties (e.g. operating temperature)• biological properties (e.g. biocompatibility).

Nearly all fibres, natural, inorganic, metallic and synthetic, have been used infiltration. These include cotton, wool, flax, asbestos, glass, ceramics, carbon, steel,polypropylene, polyethylene, polyester, aramids and many more. Every year the

172 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

journal Chemical Fibres International publishes the Man-Made Fibre Year Book,where a list of fibre materials and their end-use is given (Anon., 2008).

For the filtration of micro and fine particles, filter media should provide a largesurface area with many fine pores in order to reduce the particle throughput. Largesurface areas can be achieved using micro- and nanofibres. These fibres are usuallypositioned on the input/feed side of the filter. The use of special fibre cross-sections provides a further opportunity to increase fibre-specific area or filtermedia surface area. There are a large number of different cross-section shapes (e.g.trilobal, multilobal, snow flake). There are also fibres that are either hydrophilic orhygroscopic and fibres to which the particles adhere better or worse.

Among physical properties, electrical conductivity in particular plays a majorrole. Under extreme conditions the electrostatic charge of the filter can lead to filterfires. In such cases, conductive fibres (carbon, metal fibres) are used for dispersingthe charge developed.

Thermal properties must be taken into account in hot gas filtration and otherprocesses involving high temperatures. The temperature should be below the glasstransition temperatures of the filter medium. If the temperature exceeds the glasstransition temperature, the dimensional stability of the filter and therefore itsproperties could change to an unacceptable extent. The processing temperatureshould also be maintained below the ignition temperature to avoid the possibilityof a fire. For high temperature applications, mineral, ceramic or metal fibres areused as filter materials.

9.4.2 Production of nonwoven filters

Several methods can be used to produce nonwovens for filtration. Differentproduction methods provide filter nonwovens with a range of properties suitablefor particular applications. Composite filter media are made by combining mat-erials from two or more manufacturing processes. The range of nonwovenmanufacturing routes includes drylay (airlay, carding), wet (wetlay) and extrusionprocesses (meltblown, spunlaid, flashspun). Table 9.1 summarises the variousprocedures for nonwovens production to be considered. Basically, the manufac-ture of filter media can be split in four major process steps:

• raw material preparation• nonwoven formation• nonwoven consolidation• nonwoven finishing.

Raw material preparation

For both dry and wet processed nonwovens, bales of natural or synthetic staplefibres are opened on an opening line. The opening line normally includes baleopener, fibre flock cleaners, mixers and fine openers. Depending on the fibre

The use of nonwovens as filtration materials 173

© Woodhead Publishing Limited, 2010

Table 9.1 Overview of nonwoven production

Dry processed Wet processed Extrusion processnonwovens nonwovens

Raw material Staple fibre Staple fibre Endless filament,staple fibre

Raw material Bale opening, fibre Fine opening, form- Polymer drying,preparation flock cleaning, fine ing of fibre, water preparation for

opening, mixing slurry extrusionNonwovenformation

Method Carding/air-laid Wetlaid Flashspun, electro-spun, spunlaid,meltblown

Mechanism Mechanical web Suspension of fibres Polymer extrusionformation/aero-dynamic webformation

Fibre Parallel fibres Randomly placed Randomly placedorientation Randomly placed fibres fibres

fibres Bi-directional placed Bi-directional placedfibres fibres

Nonwoven Mechanical, Mechanical, Mechanical,consolidation thermal, chemical thermal, chemical thermal, chemical

consolidation consolidation consolidation

Nonwoven Dyeing, drenching, Dyeing, drenching, Dyeing, drenching,finishing printing, impregnat- printing, impregnat- printing, impregnat-

ing, finishing ing, finishing ing, finishing

material type and quantity of production, machines can be added or removed fromthe processing sequence for the staple fibre nonwoven opening line. The aim ofthis is to provide an appropriate fibre opening line for the opening of fibres that canthen be further used for the manufacture of mechanical and aerodynamic formednonwovens.

In the case of the wetlay process, the opened fibres need to be further processedto form fibre water slurry by using additives. For the extrusion processes, thepolymers need to be dried and prepared for the extrusion to fibres.

Nonwoven formation

For the production of filters using the drylay process, the nonwoven process iscarding (mechanical nonwoven formation) or airlaying (aerodynamic nonwovenformation). For both processes staple fibres are processed. Both natural as well assynthetic fibre materials can be used. The carding process helps produce parallelwebs from fibres 10–200 mm long having anisotropic tensile properties. Inairlaying, fibres are deposited randomly on a surface to form a much more three-dimensional random web. Such webs have isotropic properties. Mechanically

174 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

produced nonwovens are popular for filter applications because of their durabilityand also their tensile properties. Airlaid nonwovens are more voluminous struc-tures and have the potential to absorb higher proportions of dust particles. They arenormally used as prefilter media in order to separate out very large dust particlesin applications for household air filtration applications (Gregor, 2004).

The wetlay process comprises the suspension of fibres in water with chemicalsadded to facilitate the process, e.g. wetting agents. The fibres are then transportedin this water solution to a conveyor belt that also functions as a sieve. This sievehelps in the separation of the water and additive solute. The resultant fibres laid onthe sieve form a three-dimensional nonwoven having a certain proportion of fibresaligned in the direction of sieve conveyor motion. The fibres used for this processcan be either natural or synthetic fibres with a length between 0.3 and 15 mm.Filters manufactured using this technology have a poor potential to absorb dustparticles but are comparatively strong (Gregor, 2004).

The manufacture of nonwovens using extrusion techniques can be furthersubdivided into different technologies including meltblown, spunlaid, flashspunand electrospun technologies. All these processes are based on the principle ofconversion of polymers into fibres from a polymer melt or a polymer solution. Theresulting fibres are then laid onto a perforated belt (continuous or discontinuous)or on a perforated suction cylinder. The fibres thereby form the nonwovenstructure. Thus using these technologies, nonwovens can be made using endlessfilament, as in the case of spunlaid, or made using shorter filaments, as in the caseof meltblown, electrospun and flashspun. Depending on the process, the nonwovensproduced can have a wide range of tensile properties.

In the spunlaid process, the filaments are drafted during the process sequenceand are deposited on a suction bed (cylinder or belt). The delivery speeds of thesuction bed influences the filament orientation and also the thickness of thenonwoven.

The meltblown process produces nonwovens having very fine fibres, i.e. 1–10 µm in diameter. The fibres are generally extruded, drafted and formed intononwovens simultaneously. The filaments form an interlaced web because of theway they are placed on the belt and the fact that they are deposited while stillsufficiently hot to fuse at their contact points. These nonwovens have excellentdust removal properties in addition to good tensile properties and a uniform poredistribution (Gregor, 2004).

The electrospinning process provides another possibility for the manufacture ofnonwovens. These nonwovens comprise very small fibres in the nanometer range(fibre diameter < 500 nm). They are produced within an electric field from a fibre-forming substance. The fibres are collected on a collector plate and have a broadrange of diameters. For the production of electrospun fibres and nonwovens, thefibre-forming liquid is normally a solution of polymer using a suitable solvent.

The flashspun process also involves the use of spinning solutions. Flashspinning is the most complex and difficult method of manufacturing nonwoven

The use of nonwovens as filtration materials 175

© Woodhead Publishing Limited, 2010

fabrics because of the need to spin a heated and pressurised solution under preciseconditions. In flash spinning a polymer, typically polyethylene, is blended with asolvent (typically methylene chloride) under high temperature (about 25 °C ormore above the boiling point of the solvent) and under high pressure. The blendedsolution is then released under controlled conditions. The solvent flashes off toproduce a thee-dimensional network of thin, continuous interconnected ribbons,many of which are less than 4 µm thick (Bhat and Malkan, 2007).

The technologies described above are often combined to produce a composite(e.g. a sandwich) nonwoven structure that can help form an effective filter. Norm-ally, electrospun nonwovens are combined with spunlaid and meltblown structures.This combination provides a way to combine the properties of different nonwovenstogether to form a composite structure with enhanced properties. The durability ofspunlaid and meltblown structures can be used to reinforce electrospun structures,which themselves have low tensile strength but have excellent filtration character-istics.

In the combination of meltblown (M) and spunlaid (S) nonwovens (SMS,SMMS or SSMMS), the spunlaid component provides the required strength andabrasion properties and the meltblown component act as a barrier for liquids orparticles. Another method to create a composite or sandwich structure is tocombine woven fabrics with nonwoven fabrics (so called felts). The woven fabricis placed directly after the web formation process of the nonwoven fabrics,between two nonwoven layers. Woven fabrics with different properties andadvantages can be used. The woven fabrics take the forces during the filtration.

Nonwoven consolidation

Dry and wetlaid webs need to be interlaced further during manufacture in order toproduce a durable and strong nonwoven. Consolidation and interlacing is achievedusing various technologies including:

• mechanical consolidation• thermal consolidation• chemical consolidation

The most conventional method of consolidating a web is mechanical consolida-tion. Barbed needles or water jets are used to entangle the fibres and reorient them.These processes, known as needle punching and hydroentanglement respectively,work by increasing the friction between individual fibres (increasing the strengthof the felt). The mechanical bonding processes can also be used for furthermodification of the web, with respect to grip or softness. In order to form micro-fibres, bi-component fibres can be split using these methods.

The other form of web consolidation used for nonwovens involves the use ofheated calender rollers. This form of consolidation can only be used for thermo-plastic fibres. The heated roller melts the fibres and the fibre junctions are weldedtogether.

176 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

The third method of consolidation requires the addition of additives such aspowder binders to the web. These binders can be activated to help the fibres adhereto one another thus providing strength to the nonwoven.

The particular method of consolidation affects the web properties, for example,handle, in different ways.

Nonwoven finishing

In the case of filters, not only the finishing of the nonwovens but also the tailoringof the nonwoven for desired applications plays a very big role. The cutting,pleating and tailoring of the filtration media is mostly an automated process. It isvery important that the filter medium fits perfectly into the shape of the filterhousing or frame (e.g. length, width, stitching amount, welding temperature). Ifthere are any small holes, which enable the air stream to bypass the filter medium,the filtration efficiency will decrease. If the filter media are too big or there isinsufficient tension for the filter medium in the filter, the filter medium will bedamaged because of friction or faults. Depending on the application, filters areeither pleated, draped or/and placed in a frame or are supported by a fabric or acage. Typical forms used for surface filters are tube filters, compact filters andcartridge/series filters. For depth filters, filter nonwovens such as filter mats, filtercells, bag filters and compact filter elements are produced (see Figs 9.7, 9.8 and9.9).

Filter nonwovens are processed chemically, mechanically, thermally and bio-logically. By such processes the fibres are thermobonded, welded, filtersimpregnated, singed or even calendered or coated (Dietrich, 2004). By singeingthe surface of the nonwoven filter medium, the mechanical properties (e.g.

9.7 Viledon filter cell (Freudenberg, 2009).

The use of nonwovens as filtration materials 177

© Woodhead Publishing Limited, 2010

9.8 Viledon WinAir pocket filter (Freudenberg, 2009).

9.9 Viledon MaxiPleat cassette filter (Freudenberg, 2009).

abrasion resistance) are enhanced, although cleaning off the filter cake of thefilters surface is easier. By using the calendering process, not only the thicknessand the air permeability of the filter medium are influenced but also the surfacetexture. Using nano particles for dyeing or coating the filter media, excellentwater and oil repellent properties can be achieved. By impregnating or coating

178 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

the nonwoven filter media, the surface properties of the nonwovens can besignificantly enhanced. This broadens the spectrum of application of these fil-ters (Schmalz and Jolly, 2008). Coating can be carried out using membranes.The membranes are coated onto the nonwoven layer to avoid the penetration ofvery fine particles into the filter medium. It also supports the build up of a filtercake. Another new technique is the Sol-Gel process. Sol-Gel processing allowsthe application of a coating by spraying or impregnating the nonwoven in afoulard. When the filtration material is impregnated the solvent evaporates andthe sol particles start to aggregate and form a strong gel. Because of the smallsize of the sol particles produced, nano particles can be produced. The filtersmade using this technology possess a smaller pressure drop, and are moreenergy efficient and durable.

9.5 Common filter designs and applications

Filters are being used in a number of applications. Some of the typical designs forfilter applications, in air filtration as well in liquid filtration, are tube filters,compact filters, cartridge/series filters and filter candles. In this section, some filterdesigns will be briefly discussed.

9.5.1 Tube filters

Tube filters are normally produced using needle punched nonwovens becausethey can easily be combined with other textile structures. In Europe and theUSA a significant quantity of needle punched nonwovens are used as filterswith supportive fabrics. The supporting textile fabric offers the filter gooddurability and also provides a small amount of elongation, which is an advan-tage for the cleaning of the filters. Using different production methods, tubefilters with a range of properties can be made, e.g. filters with different weightper unit area (50–2500 g/m2), varying thickness, porosity, etc. The fibres usedfor these filters need to possess a wide range of properties, for example, resist-ance to chemical, mechanical and biological effects, which might arise fromeither the particles or the surrounding fluid. If rigid media are needed, thisproperty can be achieved by chemical or thermal bonding. During processing,the surface of the filters can be finished using chemical treatments to increasedurability, filter cake formation and also the ease of cleaning of the filter.Depending upon the usage environment, a filter surface can either be madesmooth or rough. The efficiency of these filtration media increases with increas-ing build up of a filter cake. There is a need for the filter to be cleaned by asimple process, for example, pressure pulses or vibration, even when the filteredmaterial adheres strongly to the filter.

The use of nonwovens as filtration materials 179

© Woodhead Publishing Limited, 2010

9.5.2 Cartridge filters

For applications in cartridge filters, extremely thin and stiff nonwovens are used.The thickness of these filter media varies between 0.1 and 5 mm and their basisweight is lower than for tube filters. They also have a paper-like appearancebecause they are mostly wetlaid and spunlaced. These filters are pleated and foldedin a star form which helps to provide higher filter stiffness and higher filter surfacearea. This in turn works positively towards the pressure drop (lower pressuredrop).

9.5.3 Bag filters

Bag filters are used for the clarification/filtration of fluids that have a relativelysmall loading of particles to be removed. The particle suspension passes throughand the particles settle in the bag. Filtration occurs from inside to outside with adelivery speed of about 100 m³/m² h. Therefore bag filters are normally used witha supporting vessel. In situations where no support is used, the pressure drop acrossthe filter needs to be lower. In situations where bag filters are used, industrystandards normally define the size of the bag house. The supporting vesselprovides mechanical support for filters with high throughput rates and prevents thefilter material from elongating. Bag filters are normally made from needle punchednonwovens and act as depth filters. The pore volume varies between 70–90%. Thefilter bags are normally stitched at the edges. Bypass filtration can be avoided bywelding the seams, however, this is only possible if the material used is thermo-plastic e.g. polypropylene or polyester.

9.5.4 Filter candles

Filter candles can be used for multiple applications. These filters are normallyunidirectional and are placed in a filter support vessel. Depending upon theapplication the filters can be either surface filters or depth filters. These filtersmainly comprise spunbonded nonwovens or meltblown nonwovens. They aregenerally used for the filtration of suspensions that have only a small amount ofsolid particles. The flow of suspension is from the outer surface to the innersurface. Systems exist where very long candles are placed horizontally or in theform of a cage. The main advantage of having candle filters in the form of a cageis that they can be replaced quickly. Depending upon the filter material and filtertreatments, the filter can be used under different processing conditions, e.g.processing temperatures, particle size and pressure. The filters are changed whena particular pressure drop value is reached. Candle filters are mostly pleated, i.e.they are folded to increase the functional surface area. With a higher surface areait is possible to have higher throughputs with lower start pressure losses. Incomparison to bag filters, candle filters offer a much lower throughput, however,they are used in the same application areas.

180 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

9.5.5 Common filter applications

The following sections describe applications for both air and gas filtration, andliquid filtration and include specialist application areas, e.g. filter media in cars,electronics manufacture and hot melt filtration.

Filters in automobiles

In automobiles, there are 8 to 15 different filter elements. A wide range ofspecifications are needed depending on exactly where in the automobile or enginethe filters are used, and what directives are in place in the country of use. It isimportant to note that some of the filter media are not changed throughout thewhole life of the car. There are also a large number of filter elements, which requireroutine maintenance. Studies in 2006 assumed that an annual replacement ofapproximately 1.93 billion filter elements could be expected. The filter elementsare used for filtration of, for example, oils, fuels and air. Their working tempera-tures vary between 40 °C and 1000 °C. The filter media themselves are made ofimpregnated papers on a cellulose base and nonwovens made from syntheticfibres, ceramics, metal, fabrics and porous sintered metals. In addition to filtration,some filter elements in automobiles provide acoustic dampening and energyabsorption in a crash. Filters are also used for the filtration of impurities such aspollen grains, dust particles, etc. from the air that is supplied to the passenger cabinof an automobile. The filters used in these sectors need to possess the property ofhigh particle separation efficiency using less pressure drop along with goodmechanical and thermal properties. Filters in the automotive sector are a combina-tion of active carbon and conventional filter media. These hybrid filters compriseup to 500 g active carbon per m2 (Sievert, 2004).

Filters for the manufacture of electronic components

For the production of electronic components, for example, semiconductor chips,strict particle contamination regulations need to be followed. The particles thatneed to be filtered have a diameter between 0.3 and 0.4 µm and must be separatedto a degree of 99.9%. Commercially available filters are made of(polytetrafluoroethylene (PTFE) fibres or glass fibre paper. The degree of separa-tion can be increased by using active carbon filters or nanofibre filter media withfibre diameters lesser than 1 µm or by the use of electrostatic filters. However,nanofibre filters cannot be commercially and economically produced yet. Electro-static filters possess a charge on their surface. This is achieved using coronacharging or by the use of electrostatic fibres (Sievert, 2004).

Hot melt filtration

Fibres made from high nickel–chromium steels are shaped as a random fibre

The use of nonwovens as filtration materials 181

© Woodhead Publishing Limited, 2010

nonwoven and sintered to form a nonwoven. The fibres typically have a diameterbetween 2–40 microns. In comparison to sinter powder metal filters they providebasic benefits such as a high degree of flexibility, a high proportion of pores (up to80%) and a high throughput filtrate. These metal filters are used in the fields oflubricants and detergents, the food and beverage industry, oils and fuels.

9.6 Future trends

A growing consciousness among people towards the use of filters in variousapplications has raised the functionality of filters benchmark. Filters these daysneed to be reusable, and durable as well as being biodegradable or recyclable andshould be able to filter chemical vapours.

The demand of filter media will rise in the coming years. According to thepredictions of the Association of the Nonwoven Fabrics Industry (INDA), therewill be a significant rise in the consumption of air filters in the fields of industrialdust filtration (bag house filter and cartridge filters), consumer/residential HVACfilters, HEPA/ULPA, disposable face masks and in-cabin automotive air filters.Including cellulosic media, the air filtration industry consumed 108,735 tons offilter media in 2007 with a value of US$643 million. The transportation and(HVAC) segments were the largest air filtration markets, accounting for almost80% of the total air filtration volume. The demand for air filtration media willincrease almost 14% over the five year period through 2012, to 120,314 tons(109,147 tonnes), equivalent to US$754 million. One of the fastest growingsignificant markets is the consumer/residential HVAC market. The higher effi-ciency and higher profit margin filters are forecast to increase almost 8% per yearthrough to 2012 (INDA, 2009).

In the sectors of wet filtration a growth is expected in the fields of processfiltration, water filtration and applications in automotive and life sciences with thehighest in water filtration (waste and desalination) and life sciences (laboratory,diagnostics, medical). In the fields of process filtration the sectors of chemistry andpharmacy are of special importance (Barrillon, 2008).

Due to the increased awareness of global environmental conditions, the marketdemand for filter applications will increase. In Europe the growth in nonwovens ispartly driven by European Union (EU) regulations. For example, in 1999 EUregulation 99/30/CE was passed for protecting the civil population against expo-sure to fine particulates in the atmosphere. The regulation placed a limitation on theamount of PM 10 concentration (particles < 10 µm) released in the environment.The PM 10 concentration was limited to 40 µg/m³ in 2005 and this limit will bemade more stringent and set at 20 µg/m³ released per day from 2010 onwards(Europäische Union, 2008). This is expected to present a considerable challenge tomany industrial regions.

In order to meet new requirements and new application areas, finer fibres will beproduced and/or processes for the production of filter media will be combined.

182 Applications of nonwovens in technical textiles

© Woodhead Publishing Limited, 2010

There is a trend towards the application of finer fibres for filter applications, whichwill help in the filtration of finer particles. Depending upon the different processcombinations, fine particles with a size less than one µm can be filtered out. Inorder to achieve this, filters are formed from very fine fibres. These fine fibres canbe produced using different processes. Fibres such as bi-component fibres, whichhave a higher number of island fibres, can be formed. With this island-in-the-seaformation, 1200 fine island fibres can be produced. Apart from the island-in-the-sea technology, the segmented pie technology also helps in producing fine fibresand has become more popular in recent years.

In the field of electrospinning the challenges to be met are the manufacture ofmass production machines for the production of fibres that would facilitate anincreased filter-specific surface area. Another challenge is the spinning of fibreswithout solutions, and environment friendly production of filter media. Thus, itcan be said that nonwovens and nonwoven structures will play a very significantrole in the filtration sector due to their properties.

9.7 References

Albrecht W, Fuchs H and Kittelmann W (2000), Vliesstoffe, Weinheim (Germany), Wiley-VCH Verlag GmbH.

Anon. (2006a), ‘ITMA 2003 … 2007’, Allgemeiner Vliesstoffreport, 4, 23–24.Anon. (2006b), EDANA (www.edana.org), ‘Discover Nonwovens; Facts amd Figures,

Markets’, http://www.edana.org/objects/4/images/Graph B.gif.Anon. (2007), The Columbia Electronic Encyclopedia®, Copyright © 2007, Columbia

University Press, Licensed from Columbia University Press.Anon. (2008), Man-Made Fiber Year Book 2008, Frankfurt am Main.Barrillon J (2008), ‘Trends in Liquid Filtration Media’, Nonwovens Industry Webinar, 2

December 2008.Bhat G S and Malkan S R (2007), ‘Polymer-laid Web Formation’, in Russell S J, Handbook

of Nonwovens, Cambridge, Woodhead, 193.David Rigby Associates (2002), Nonwoven End-use Products: World Market Forecasts to

2010, David Rigby Associates.Dickenson T C (1997), Filters and Filtration Handbook, Oxford (United Kingdom), Elsevier

Advanced Technology.Dietrich H (2004), ‘Der Entstauber-Markt’, Allgemeiner Vliesstoffreport, 2, 32–34.Europäische Union (2008), ‘Richtlinie 2008/50/EG des Europäischen Parlaments und des

Rates vom 21. Mai 2008 über Luftqualität und saubere Luft für Europa’, Amtsblatt derEuropäischen Union, 11.06.2008, L152, 5.

Freudenberg (2009), Freudenberg Filtration Technologies, http://www.freudenberg-filter.com/en/products/industrial-air-filtration/commercial-and-industrial-hvac.

Gasper H (2000), ‘Analyse des Filtrationsproblems’ in Gasper H, Dietmar Oechsle andPongratz E, Handbuch der industriellen Fest/Fluessig-Filtration, Weinheim (Germany),WILEY-VCH Verlag GmbH,11.

Gregor E C (2004), ‘Versatile nonwoven filtration media’, Allgemeiner Vliesstoffreport, 2,26–27.

Hoeflinger W and Pongratz E (2000), ‘Theoretische Grundlagen der Fest/Fluessig-Filtration’

The use of nonwovens as filtration materials 183

© Woodhead Publishing Limited, 2010

in Gasper H, Dietmar Oechsle and Pongratz E, Handbuch der industriellen Fest/ Fluessig-Filtration, Weinheim (Germany), WILEY-VCH Verlag GmbH, 24.

INDA (2009), ‘Association of the Nonwovens Fabrics Industry’, INDA Press Releases,http://www.inda.org/press/2008/AirFilterReport.html.

Schmalz E and Jolly M (2008), ‘Teil 2: Zukunft Spunlace … oder auf der Suche nachinnovativen Materialien’, Allgemeiner Vliesstoffreport, 1, 45.

Schumann A and Erth H (2006), ‘Neue Anwendungsbereiche und Entwicklungen auf demGebiet der Vliesstoffbeschichtung’, Allgemeiner Vliesstoffreport, 5, 22.

Sievert J (2004), ‘Der Einsatz von vollsynthetischen Vliesstoffen in der Luftfiltration-Entwicklungen und Trends’, Allgemeiner Vliesstoffreport, 2, 28–30.