The use of nonwovens as filtration materials

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 agglomeration, 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 160 © Woodhead Publishing Limited, 2010

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Others 4.70% Agriculture 1.40%

Unidentified 0.50%

Automotive 3.50% Civil engineering/ underground 4.50%

Hygiene 33%

Building/roofing 12.70%

Air and gas filtration 2.40% Liquid filtration 3.90%

Medical/surgical 3.10%

Floor coverings 2.10%

Wipes for personal care 7.70%

Upholstery/table linen/ household 5.90% Coating substrates 1.80%

Wipes – others 7.20%

Garments 1% Shoe/leather goods 1.80% Interlinings 1.90%

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

other filter media. For example, nonwovens offer large and adjustable surface properties and can be adapted to different filtration requirements. Depending upon the filter requirements, different textile or plastic grid structures can be combined together to form a sandwich structure (e.g. backing fabric and nonwoven). Compared to other filtration media like membranes, wire cloth and monofilament fabrics, nonwovens offer a thicker cross-section and bulk (Gregor, 2004). This provides the opportunity to use nonwovens as a structure that can fulfil the requirements and boundary conditions of all types of applications. To influence the structure of the nonwoven media, different manufacturing methods are used to manufacture filters for diverse applications. Nonwovens offer high permeability and surface area, which are further enhanced by pleating of the material. Also the wide range of fibre materials available offers good mechanical, chemical and physical thermal properties. Thus, the production of nonwovens for filtration applications is very efficient and can be very economic depending on the fibre material and process steps used. According to Gregor (2004), nonwoven filters are the material of choice when large quantities of particulate loading, long life or where general clarification of a liquid or gas stream is required. In 2005, 4000 t of nonwovens were produced worldwide (Anon., 2006a). In

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(Million US$)

16000 12000 8000 4000 0 North America

West Europe 2001

Asia-Pacific region 2006

Other regions

2011

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 gas and air filtration (see Fig. 9.1). Increasing demand and ongoing development of new applications continues to fuel an increase in growth of the use of nonwovens in filtration. As can be seen from Fig. 9.2, growth is expected not only in America and Europe but also in Asia and other regions (Anon., 2008). Rigby (2003) showed that the annual growth rate of nonwovens in filter applications 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, independent from application was expected to increase by about 4.7% by 2005 and about 5% in 2010 (David Rigby Associates, 2002). Thus, the use of nonwovens for filter applications 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 fluid surrounding the filter have to be considered. The following features of the surrounding 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 need to be considered. Finally, also the particle properties, for example particle size, particle size distribution and particle material, have to be considered. Taking into account these manifold types of requirements and boundary conditions the available filter media can be classified.

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For the classification of filter media different methodologies are used. According to the Filters and Filtration Handbook (Dickenson, 1997), filters can be classified in four main categories. These categories are solid–gas separation, solid–fluid separation, liquid–liquid separation and solid–solid separation. Albrecht et al. (2000) add one more separation type, the gas–gas separation, so that filter media are categorised into five different types. The most common methodology for the classification of filter media, which is described in this chapter, is as follows. The filter media are classified depending on: • 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 from a solid or gaseous medium. These substances are dispersed in the solid or gaseous medium. For example, for solid–solid separation the finer particles are separated from the larger particles by means of a multiple stage sieving process. This procedure determines the grain size distribution of different soils used. For the separation 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 through the filter. Depending on the structure of the filter the particles can be deposited on the surface (surface filtration) or inside (depth filtration) the filter medium installations. Dry filters are usually voluminous structures. The air and gas filtration market includes domestic filters, industrial filters and automotive filters. Domestic filters are used in heating, ventilation and air conditioning (HVAC), cooking, vacuum cleaners and various portable filters in the market. Industrial filters are in general used in (HVAC), high efficiency particulate airfilter (HEPA) and ultra low penetration air (ULPA) filters, dust removal for power stations, incinerators, paint spray house and many industrial processes, where air is contaminated and needs to be cleaned, or a very clean environment is required for the production of, for example, electronic components. Automotive filters include engine air filters (intake and exhaust) and cabin air filters. Wet filtration deals with the separation of solid, liquid or gaseous substances from a liquid medium. The materials to be filtered are usually suspended in the medium. In the case of the separation of a liquid–liquid mixture the boiling point of the different liquids is taken into consideration. By evaporating one of the components of the liquid mixture, the separation can take place. Solid–liquid separation by deposition occurs due to the deposition of the solid particles at the bottom of the container (e.g. sewage treatment). In addition to

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separation by deposition, filter fabrics (e.g. nonwovens, wovens) are used for wet filtration. Wet filters offer the possibility for fluid permeability and at the same time provide the impermeability for particles that need to be filtered. Nonwoven filter media offer the possibility of collecting the particles on the filter surface (surface filtration) and in the filter medium installation (depth filtration). Wet filtration media are usually very thin and compacted media. Liquid filtration is a fast growing market for nonwovens. It includes water filtration (tap and waste water), food and beverage filtration, pharmaceutical and electric processes, blood filtration, tea bags and coffee and juice filters, cooking oil filters and oil/fuel filters for automotives. Nonwovens have been successfully used in the industry as membrane support for micro-filtration, ultra-filtration and reverse 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 be mentioned that mostly, both surface and depth filtration occur in the filter medium. The classification of whether the filter medium is a surface or depth filtration medium depends on the preferred deposition area of the particles. Surface filtration is characterised by a deposition of particles or aerosols, whose diameter 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 high fluid resistance across the filter, at which point the filter would need to be either cleaned or changed. On the other hand the deposited particles on the surface of the filter can result in the formation of a layer of substrate that has lower pore size than the filter itself, thus facilitating filtering. This layer of substrate is commonly known as a filter cake. Many surface filters function most effectively when the filter cake is developed on the filter. A filter cake is compressible and its filtration efficiency 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 filtered particles are mechanically held or adsorbed into the the cake. For surface filter media 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 filters usually have a smooth, paper-like surface and are very thin. Filters are generally compared based on their filtration area and the degree of separation possible. According to the Filters and Filtration Handbook (Dickenson, 1997) surface filters 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

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Suspension

Filter Filter cake Filter

Blocked filter Filtrate

9.3 Surface filtration. Suspension Mechanical filtration

Filter

Filter

Particle absorption Filtrate

9.4 Depth filtration.

deposit themselves within the pores of the filters (see Fig. 9.4). Depth filters are normally 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 the filter. In subsequent layers, finer particles can be intercepted. Thus coarser particles are separated mechanically and finer particles are filtered due to their adsorption. The pressure difference and the fluid flow rate remain almost constant. Depth filters are difficult to clean and can be reused only under specific circumstances. Criteria for comparison are pore volume, the filter thickness and degree of separation. Depth filters are characterised using the following properties: • suitability for the filtration of difficult filterable solids (e.g. particles of different dimensions); • high filtration efficiency over a wide range of particle sizes. Through the combination of surface and depth filters, it is possible to separate coarser particles by surface filters and the finer particles through depth filters from the fluid stream. This results in high endurance and maintains the throughput performance of the filter media.

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9.5 Filter media categorised by particle size.

9.2.3

Particle size

When selecting filter media the particle size, particle size distribution, particle type and particle concentration must be taken into account. Large particles are normally intercepted on the surface and finer particles within the filter medium or a filter cake. Based on the particle size, one can categorise currently available filter media useful for different types of filtration processes (see Fig. 9.5). These include for example filter media for reverse osmosis (particle size <0.007 microns), nanofiltration (particle size <0.01 microns), ultra-filtration (particle size <0.1 microns), micro-filtration (particle size <10 microns) and conventional filtration (particle size <900 microns) (Gasper, 2000). In particular, for the filtration of particles in air or exhaust gases, special guidelines and standards help to classify the filter medium. Therefore, the filtration efficiency of the filter medium is measured and classified. In Europe 17 filter classes exist. The higher the class, the higher 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 classes F5–F9 (fine particle filter) according to DIN EN 779. The remaining filters are classified using EN 1822-1 in H10–H14 (HEPA) and U15–U17 (ULPA) filters. In North America, the filter efficiency is determined using ASHRAE’s Minimum Efficiency 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 and weight can be separated. The parameters such as the size, size distribution and shape of the particles and the pore diameter and pore size distribution of the filter

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medium play a major role. Using numerous empirical and theoretical equations, the relationship between the porosity and the pressure drop of a filter medium can be predicted. The basic equation of filtration is the Darcy equation. For the selection of filter media different standards for filter media and filter tests have emerged. This section deals briefly with the filter mechanisms, the criteria that are used 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 the fibre surface. If the particle follows a fluid flow direction at a distance that is equal to or smaller than the particle diameter, it is attached to the fibre surface. If particles flow at a distance greater than the particle diameter, they are no longer attracted by the fibre and the interception mechanism does not function. If the particle, due to its size and weight, does not follow the change in the fluid flow pattern, inertial impact occurs. This phenomenon occurs for fluids that are filtered at high speeds and also with filter media that are very dense. If the number of particles and the particle size increase, the probability of collision and attachment to the fibre surface in the filter increases. In doing so, the particles themselves Streamlines

Inertial impact

Particles

Fibre

Diffusion Direct interception

9.6 Basic filtration mechanisms.

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form part of the filter and thereby increase the filter efficiency. The filter efficiency also increases due to the thickness (filter depth or particle travel distance) of the filter medium used. Thus the efficiency of filtration by impact depends on the particle 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 a random zigzag motion pattern because they collide with the molecules of the fluid and interact with them. This motion of the particles can be attributed to the Brownian motion of particles: the smaller the particle, the slower is the flow velocity of the fluid. The particles have more time to carry out the zigzag motion and require longer to actually collide with the fibres. The more the particles attach to the fibres, the higher is the probability that particle deposition will occur (increasing the filter efficiency). This results in a higher pressure drop across the filter. There are other filter mechanisms that are related to the mechanisms described above, for example electrostatic filtration and cake filtration. For charged particles, an electrostatic charged filter can help to give good filter performance. In this case the particles are attracted to filter media that have an opposite charge to the one on the particles. For example, dust particles are always negatively charged, therefore if the filter medium carries a positive charge, the dust particles would be deposited on the surface or within the filter. In case of the cake filtration mechanism the filtered particles are deposited on either the surface of the filter or within the filter. These particles form a thin layer of their own. This layer possesses finer pore sizes compared to the filter medium itself. This thus facilitates the filtration of finer particles. The filtration mechanisms can be understood by the basic equation presented by Darcy: dV A • ∆p • k —– = ———–– dt η•h

[9.1]

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

ε3 k = ————— K(1 – ε)2 S2V

[9.2]

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

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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. The Kozeny constant describes the ability of a fluid to move through the porous structure. Its value depends on the porosity of the filter medium. For porosities between 0.3 and 0.65 the Kozeny constant is constant and has a value of approximately 4. For varying porosity values the Kozeny constant has to be validated by experiment.

9.3.2

Requirements for nonwoven filtration

Filters are used for many applications in different areas. In order to achieve the full potential of nonwoven filtration, the correct filter needs to be chosen and placed correctly depending upon the process conditions (e.g. pressure on the filter surface, cleaning interval, cleaning method). It is necessary to accurately adjust the filter to the requirements. For this purpose, the material used for filter media and also the type of filter needs to be taken into account. For the selection of fibre material to be 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 the pore size and pore size distribution vary. • Pore size/distribution: the pore size and its shape depend on the fibre fineness and the compaction of the nonwoven medium. The size and the distribution also depend on the manufacturing process and the consolidation and finishing process of the nonwoven. • Thickness: depends on the basis weight and the consolidation process. The thickness also plays a role in filtration. • Solid volume fraction (SVF): the amount of fibres in a volume element in percentage. • 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 blend of fibres. • Permeability: describes the fluid’s ability to pass through the filtration medium. Highly permeable filter media decrease the pressure drop across the filtration medium. Coarse filter media give an increase in permability because of the larger pores. • Surface texture: influences the particles’ ability to adhere to the surface of the filtration 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 a material and the filter structure, the filtration efficiency may be negatively influenced.

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• Flammability behaviour: relevant when used in hot applications. The filter medium should remain rigid enough to be able to perform the filtration. Usage of filtration media above glass transition temperature should be avoided. • Strength and drape: the strength of a medium defines, for example, whether the filter is appropriate for high pressure filtration applications. The drapeability enables 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 efficiency increases. • Chemical, thermal and biological stability of the materials: the materials used should be able to perform their filtration function permanently, e.g. if the material dissolves in an acid environment it should not be used. For each of the properties mentioned, the different characteristics provide differences in filter performance.

9.3.3

Technical standards for nonwovens

There are different standards and testing norms, for use of filters in different applications. In order to test filter media, prescribed test aerosols need to be used according to the norms. This is an important requirement for the determination of the fractionating potential of the filters. Often the total fractionating potential or separation potential, which depends on the particle size distribution in the gas and the particle concentration, is used to characterise the properties of the filter used. To determine particle size and particle size distribution, particle size measuring devices according to the VDI 3489 can be chosen. These measuring devices use aerosols for the determination of filter efficiency. In VDI guideline 3491 concepts, definitions and the production of test aerosols are explained. VDI guideline 2066 directive describes the arrangement of sampling techniques for the measurement of dust in flowing gases and the classification of filter media according to their intended use. Thus, for example, air filters are classified according to DIN EN 24185, colloidal solution filters according to DIN EN 1822, waste treatment filters according to VDI 3926, air filters for internal combustion engines and compressors in accordance with ISO 5011, filters for the automotive interior according to DIN 71460 and indoor particulate filter according to DIN EN 779. There are also standards to characterise the properties of the nonwovens themselves. For example, basis weight (ISO 9073-1), bursting strength (DIN 53861-3), breaking extension (ISO 9073-3/-18), abrasion resistance (DIN 538631/-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 media depends on the density, the consolidation and the fibre length and material. According to the aforementioned standards and testing norms commercially

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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. A lot of companies have developed their own test equipment and test machines. Thus, the comparison between different filter media data is not always possible or appropriate. All the laboratory tests of filtration media data have to be scaled up to the performance of the filter during the process. The air permeability and the amount of particles in the cleaned fluid during the real usage of the filter have to be measured 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 the method of manufacture used. The method of manufacture can affect the filter media properties such as pore size, thickness, basis weight, air permeability, etc. Depending on the method of manufacture the particles can penetrate into the interior of the filter medium (depth filter), thus the filter must be completely disposed of, or the particles can be distributed only superficially on the filter medium (surface filter) and can be cleaned and reused where possible. The properties also depend on the design of the filter itself. The filtration efficiency and the pressure drop differ if the structure of the filter is, for example, a bag or cartridge filter.

9.4.1

Material variables

Depending on the application a suitable filter has to be chosen. The appropriate filter materials are selected depending on the type of particles, particle size, particle size distribution and surrounding fluid. In particular, the choice of fibre properties and the design of the filter medium influence the performance of a filter, and the costs of a fibre material and the filter medium also influence the design of the filter medium and the filter. The following parameters have a great influence on the filter media 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 in filtration. These include cotton, wool, flax, asbestos, glass, ceramics, carbon, steel, polypropylene, polyethylene, polyester, aramids and many more. Every year the

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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 large surface area with many fine pores in order to reduce the particle throughput. Large surface areas can be achieved using micro- and nanofibres. These fibres are usually positioned on the input/feed side of the filter. The use of special fibre crosssections provides a further opportunity to increase fibre-specific area or filter media 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 or hygroscopic and fibres to which the particles adhere better or worse. Among physical properties, electrical conductivity in particular plays a major role. Under extreme conditions the electrostatic charge of the filter can lead to filter fires. In such cases, conductive fibres (carbon, metal fibres) are used for dispersing the charge developed. Thermal properties must be taken into account in hot gas filtration and other processes involving high temperatures. The temperature should be below the glass transition temperatures of the filter medium. If the temperature exceeds the glass transition temperature, the dimensional stability of the filter and therefore its properties could change to an unacceptable extent. The processing temperature should also be maintained below the ignition temperature to avoid the possibility of a fire. For high temperature applications, mineral, ceramic or metal fibres are used as filter materials.

9.4.2

Production of nonwoven filters

Several methods can be used to produce nonwovens for filtration. Different production methods provide filter nonwovens with a range of properties suitable for particular applications. Composite filter media are made by combining materials from two or more manufacturing processes. The range of nonwoven manufacturing routes includes drylay (airlay, carding), wet (wetlay) and extrusion processes (meltblown, spunlaid, flashspun). Table 9.1 summarises the various procedures for nonwovens production to be considered. Basically, the manufacture 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 staple fibres are opened on an opening line. The opening line normally includes bale opener, fibre flock cleaners, mixers and fine openers. Depending on the fibre

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Table 9.1 Overview of nonwoven production Dry processed nonwovens

Wet processed nonwovens

Extrusion process

Raw material Staple fibre

Staple fibre

Endless filament, staple fibre

Raw material Bale opening, fibre preparation flock cleaning, fine opening, mixing Nonwoven formation Method Carding/air-laid

Fine opening, forming of fibre, water slurry

Polymer drying, preparation for extrusion

Wetlaid

Flashspun, electrospun, spunlaid, meltblown

Mechanism Mechanical web formation/aerodynamic web formation

Suspension of fibres Polymer extrusion

Fibre Parallel fibres orientation Randomly placed fibres

Randomly placed fibres Bi-directional placed fibres

Randomly placed fibres Bi-directional placed fibres

Nonwoven Mechanical, consolidation thermal, chemical consolidation

Mechanical, thermal, chemical consolidation

Mechanical, thermal, chemical consolidation

Nonwoven finishing

Dyeing, drenching, printing, impregnating, finishing

Dyeing, drenching, printing, impregnating, finishing

Dyeing, drenching, printing, impregnating, finishing

material type and quantity of production, machines can be added or removed from the processing sequence for the staple fibre nonwoven opening line. The aim of this is to provide an appropriate fibre opening line for the opening of fibres that can then be further used for the manufacture of mechanical and aerodynamic formed nonwovens. In the case of the wetlay process, the opened fibres need to be further processed to form fibre water slurry by using additives. For the extrusion processes, the polymers 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 is carding (mechanical nonwoven formation) or airlaying (aerodynamic nonwoven formation). For both processes staple fibres are processed. Both natural as well as synthetic fibre materials can be used. The carding process helps produce parallel webs from fibres 10–200 mm long having anisotropic tensile properties. In airlaying, fibres are deposited randomly on a surface to form a much more threedimensional random web. Such webs have isotropic properties. Mechanically

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produced nonwovens are popular for filter applications because of their durability and also their tensile properties. Airlaid nonwovens are more voluminous structures and have the potential to absorb higher proportions of dust particles. They are normally used as prefilter media in order to separate out very large dust particles in applications for household air filtration applications (Gregor, 2004). The wetlay process comprises the suspension of fibres in water with chemicals added to facilitate the process, e.g. wetting agents. The fibres are then transported in this water solution to a conveyor belt that also functions as a sieve. This sieve helps in the separation of the water and additive solute. The resultant fibres laid on the sieve form a three-dimensional nonwoven having a certain proportion of fibres aligned in the direction of sieve conveyor motion. The fibres used for this process can 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 dust particles but are comparatively strong (Gregor, 2004). The manufacture of nonwovens using extrusion techniques can be further subdivided into different technologies including meltblown, spunlaid, flashspun and electrospun technologies. All these processes are based on the principle of conversion of polymers into fibres from a polymer melt or a polymer solution. The resulting fibres are then laid onto a perforated belt (continuous or discontinuous) or on a perforated suction cylinder. The fibres thereby form the nonwoven structure. Thus using these technologies, nonwovens can be made using endless filament, as in the case of spunlaid, or made using shorter filaments, as in the case of meltblown, electrospun and flashspun. Depending on the process, the nonwovens produced can have a wide range of tensile properties. In the spunlaid process, the filaments are drafted during the process sequence and are deposited on a suction bed (cylinder or belt). The delivery speeds of the suction bed influences the filament orientation and also the thickness of the nonwoven. The meltblown process produces nonwovens having very fine fibres, i.e. 1– 10 µm in diameter. The fibres are generally extruded, drafted and formed into nonwovens simultaneously. The filaments form an interlaced web because of the way they are placed on the belt and the fact that they are deposited while still sufficiently hot to fuse at their contact points. These nonwovens have excellent dust removal properties in addition to good tensile properties and a uniform pore distribution (Gregor, 2004). The electrospinning process provides another possibility for the manufacture of nonwovens. These nonwovens comprise very small fibres in the nanometer range (fibre diameter < 500 nm). They are produced within an electric field from a fibreforming substance. The fibres are collected on a collector plate and have a broad range of diameters. For the production of electrospun fibres and nonwovens, the fibre-forming liquid is normally a solution of polymer using a suitable solvent. The flashspun process also involves the use of spinning solutions. Flash spinning is the most complex and difficult method of manufacturing nonwoven

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fabrics because of the need to spin a heated and pressurised solution under precise conditions. In flash spinning a polymer, typically polyethylene, is blended with a solvent (typically methylene chloride) under high temperature (about 25 °C or more above the boiling point of the solvent) and under high pressure. The blended solution is then released under controlled conditions. The solvent flashes off to produce 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. Normally, electrospun nonwovens are combined with spunlaid and meltblown structures. This combination provides a way to combine the properties of different nonwovens together to form a composite structure with enhanced properties. The durability of spunlaid and meltblown structures can be used to reinforce electrospun structures, which themselves have low tensile strength but have excellent filtration characteristics. In the combination of meltblown (M) and spunlaid (S) nonwovens (SMS, SMMS or SSMMS), the spunlaid component provides the required strength and abrasion properties and the meltblown component act as a barrier for liquids or particles. Another method to create a composite or sandwich structure is to combine woven fabrics with nonwoven fabrics (so called felts). The woven fabric is placed directly after the web formation process of the nonwoven fabrics, between two nonwoven layers. Woven fabrics with different properties and advantages 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 to produce a durable and strong nonwoven. Consolidation and interlacing is achieved using various technologies including: • mechanical consolidation • thermal consolidation • chemical consolidation The most conventional method of consolidating a web is mechanical consolidation. 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 strength of the felt). The mechanical bonding processes can also be used for further modification of the web, with respect to grip or softness. In order to form microfibres, bi-component fibres can be split using these methods. The other form of web consolidation used for nonwovens involves the use of heated calender rollers. This form of consolidation can only be used for thermoplastic fibres. The heated roller melts the fibres and the fibre junctions are welded together.

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The third method of consolidation requires the addition of additives such as powder binders to the web. These binders can be activated to help the fibres adhere to 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 tailoring of 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 is very important that the filter medium fits perfectly into the shape of the filter housing or frame (e.g. length, width, stitching amount, welding temperature). If there 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 is insufficient tension for the filter medium in the filter, the filter medium will be damaged because of friction or faults. Depending on the application, filters are either pleated, draped or/and placed in a frame or are supported by a fabric or a cage. Typical forms used for surface filters are tube filters, compact filters and cartridge/series filters. For depth filters, filter nonwovens such as filter mats, filter cells, bag filters and compact filter elements are produced (see Figs 9.7, 9.8 and 9.9). Filter nonwovens are processed chemically, mechanically, thermally and biologically. By such processes the fibres are thermobonded, welded, filters impregnated, singed or even calendered or coated (Dietrich, 2004). By singeing the surface of the nonwoven filter medium, the mechanical properties (e.g.

9.7 Viledon filter cell (Freudenberg, 2009).

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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 the filters surface is easier. By using the calendering process, not only the thickness and the air permeability of the filter medium are influenced but also the surface texture. Using nano particles for dyeing or coating the filter media, excellent water and oil repellent properties can be achieved. By impregnating or coating

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the nonwoven filter media, the surface properties of the nonwovens can be significantly enhanced. This broadens the spectrum of application of these filters (Schmalz and Jolly, 2008). Coating can be carried out using membranes. The membranes are coated onto the nonwoven layer to avoid the penetration of very fine particles into the filter medium. It also supports the build up of a filter cake. Another new technique is the Sol-Gel process. Sol-Gel processing allows the application of a coating by spraying or impregnating the nonwoven in a foulard. When the filtration material is impregnated the solvent evaporates and the sol particles start to aggregate and form a strong gel. Because of the small size of the sol particles produced, nano particles can be produced. The filters made using this technology possess a smaller pressure drop, and are more energy efficient and durable.

9.5

Common filter designs and applications

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

9.5.1

Tube filters

Tube filters are normally produced using needle punched nonwovens because they can easily be combined with other textile structures. In Europe and the USA a significant quantity of needle punched nonwovens are used as filters with supportive fabrics. The supporting textile fabric offers the filter good durability and also provides a small amount of elongation, which is an advantage for the cleaning of the filters. Using different production methods, tube filters with a range of properties can be made, e.g. filters with different weight per unit area (50–2500 g/m2), varying thickness, porosity, etc. The fibres used for these filters need to possess a wide range of properties, for example, resistance to chemical, mechanical and biological effects, which might arise from either the particles or the surrounding fluid. If rigid media are needed, this property can be achieved by chemical or thermal bonding. During processing, the surface of the filters can be finished using chemical treatments to increase durability, filter cake formation and also the ease of cleaning of the filter. Depending upon the usage environment, a filter surface can either be made smooth or rough. The efficiency of these filtration media increases with increasing build up of a filter cake. There is a need for the filter to be cleaned by a simple process, for example, pressure pulses or vibration, even when the filtered material adheres strongly to the filter.

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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 basis weight is lower than for tube filters. They also have a paper-like appearance because they are mostly wetlaid and spunlaced. These filters are pleated and folded in a star form which helps to provide higher filter stiffness and higher filter surface area. This in turn works positively towards the pressure drop (lower pressure drop).

9.5.3

Bag filters

Bag filters are used for the clarification/filtration of fluids that have a relatively small loading of particles to be removed. The particle suspension passes through and the particles settle in the bag. Filtration occurs from inside to outside with a delivery speed of about 100 m³/m² h. Therefore bag filters are normally used with a supporting vessel. In situations where no support is used, the pressure drop across the filter needs to be lower. In situations where bag filters are used, industry standards normally define the size of the bag house. The supporting vessel provides mechanical support for filters with high throughput rates and prevents the filter material from elongating. Bag filters are normally made from needle punched nonwovens and act as depth filters. The pore volume varies between 70–90%. The filter bags are normally stitched at the edges. Bypass filtration can be avoided by welding the seams, however, this is only possible if the material used is thermoplastic e.g. polypropylene or polyester.

9.5.4

Filter candles

Filter candles can be used for multiple applications. These filters are normally unidirectional and are placed in a filter support vessel. Depending upon the application the filters can be either surface filters or depth filters. These filters mainly comprise spunbonded nonwovens or meltblown nonwovens. They are generally used for the filtration of suspensions that have only a small amount of solid particles. The flow of suspension is from the outer surface to the inner surface. Systems exist where very long candles are placed horizontally or in the form of a cage. The main advantage of having candle filters in the form of a cage is that they can be replaced quickly. Depending upon the filter material and filter treatments, the filter can be used under different processing conditions, e.g. processing temperatures, particle size and pressure. The filters are changed when a 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 area it is possible to have higher throughputs with lower start pressure losses. In comparison to bag filters, candle filters offer a much lower throughput, however, they are used in the same application areas.

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9.5.5

Applications of nonwovens in technical textiles

Common filter applications

The following sections describe applications for both air and gas filtration, and liquid 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 of specifications are needed depending on exactly where in the automobile or engine the filters are used, and what directives are in place in the country of use. It is important to note that some of the filter media are not changed throughout the whole life of the car. There are also a large number of filter elements, which require routine maintenance. Studies in 2006 assumed that an annual replacement of approximately 1.93 billion filter elements could be expected. The filter elements are used for filtration of, for example, oils, fuels and air. Their working temperatures vary between 40 °C and 1000 °C. The filter media themselves are made of impregnated papers on a cellulose base and nonwovens made from synthetic fibres, ceramics, metal, fabrics and porous sintered metals. In addition to filtration, some filter elements in automobiles provide acoustic dampening and energy absorption in a crash. Filters are also used for the filtration of impurities such as pollen grains, dust particles, etc. from the air that is supplied to the passenger cabin of an automobile. The filters used in these sectors need to possess the property of high particle separation efficiency using less pressure drop along with good mechanical and thermal properties. Filters in the automotive sector are a combination of active carbon and conventional filter media. These hybrid filters comprise up 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 that need to be filtered have a diameter between 0.3 and 0.4 µm and must be separated to a degree of 99.9%. Commercially available filters are made of (polytetrafluoroethylene (PTFE) fibres or glass fibre paper. The degree of separation can be increased by using active carbon filters or nanofibre filter media with fibre diameters lesser than 1 µm or by the use of electrostatic filters. However, nanofibre filters cannot be commercially and economically produced yet. Electrostatic filters possess a charge on their surface. This is achieved using corona charging 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

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nonwoven and sintered to form a nonwoven. The fibres typically have a diameter between 2–40 microns. In comparison to sinter powder metal filters they provide basic benefits such as a high degree of flexibility, a high proportion of pores (up to 80%) and a high throughput filtrate. These metal filters are used in the fields of lubricants 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 various applications has raised the functionality of filters benchmark. Filters these days need to be reusable, and durable as well as being biodegradable or recyclable and should be able to filter chemical vapours. The demand of filter media will rise in the coming years. According to the predictions of the Association of the Nonwoven Fabrics Industry (INDA), there will be a significant rise in the consumption of air filters in the fields of industrial dust filtration (bag house filter and cartridge filters), consumer/residential HVAC filters, HEPA/ULPA, disposable face masks and in-cabin automotive air filters. Including cellulosic media, the air filtration industry consumed 108,735 tons of filter media in 2007 with a value of US$643 million. The transportation and (HVAC) segments were the largest air filtration markets, accounting for almost 80% of the total air filtration volume. The demand for air filtration media will increase 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 growing significant markets is the consumer/residential HVAC market. The higher efficiency and higher profit margin filters are forecast to increase almost 8% per year through to 2012 (INDA, 2009). In the sectors of wet filtration a growth is expected in the fields of process filtration, water filtration and applications in automotive and life sciences with the highest in water filtration (waste and desalination) and life sciences (laboratory, diagnostics, medical). In the fields of process filtration the sectors of chemistry and pharmacy are of special importance (Barrillon, 2008). Due to the increased awareness of global environmental conditions, the market demand for filter applications will increase. In Europe the growth in nonwovens is partly driven by European Union (EU) regulations. For example, in 1999 EU regulation 99/30/CE was passed for protecting the civil population against exposure to fine particulates in the atmosphere. The regulation placed a limitation on the amount 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 be made 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 to many industrial regions. In order to meet new requirements and new application areas, finer fibres will be produced and/or processes for the production of filter media will be combined.

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There is a trend towards the application of finer fibres for filter applications, which will help in the filtration of finer particles. Depending upon the different process combinations, fine particles with a size less than one µm can be filtered out. In order to achieve this, filters are formed from very fine fibres. These fine fibres can be produced using different processes. Fibres such as bi-component fibres, which have a higher number of island fibres, can be formed. With this island-in-the-sea formation, 1200 fine island fibres can be produced. Apart from the island-in-thesea technology, the segmented pie technology also helps in producing fine fibres and has become more popular in recent years. In the field of electrospinning the challenges to be met are the manufacture of mass production machines for the production of fibres that would facilitate an increased filter-specific surface area. Another challenge is the spinning of fibres without solutions, and environment friendly production of filter media. Thus, it can be said that nonwovens and nonwoven structures will play a very significant role in the filtration sector due to their properties.

9.7

References

Albrecht W, Fuchs H and Kittelmann W (2000), Vliesstoffe, Weinheim (Germany), WileyVCH 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 der Europäischen Union, 11.06.2008, L152, 5. Freudenberg (2009), Freudenberg Filtration Technologies, http://www.freudenbergfilter.com/en/products/industrial-air-filtration/commercial-and-industrial-hvac. Gasper H (2000), ‘Analyse des Filtrationsproblems’ in Gasper H, Dietmar Oechsle and Pongratz 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’

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in Gasper H, Dietmar Oechsle and Pongratz E, Handbuch der industriellen Fest/ FluessigFiltration, 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 nach innovativen Materialien’, Allgemeiner Vliesstoffreport, 1, 45. Schumann A and Erth H (2006), ‘Neue Anwendungsbereiche und Entwicklungen auf dem Gebiet der Vliesstoffbeschichtung’, Allgemeiner Vliesstoffreport, 5, 22. Sievert J (2004), ‘Der Einsatz von vollsynthetischen Vliesstoffen in der LuftfiltrationEntwicklungen und Trends’, Allgemeiner Vliesstoffreport, 2, 28–30.

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