Non Woven Filters

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Abstract:

The advancements in science and technology have always leads to the new developments and inventions of the products. There is continuously evolution of technology in textile from weaving, knitting, braiding, non woven, etc. This article highlights the significant growth of non woven technology and use of the same in various filters.

Introduction:

The term nonwoven is an interesting word. To most, the word nonwoven means “not a woven” or “not a knit”, but nonwoven fabrics are much more. Nonwoven fabrics are broadly defined as sheet or web structures bonded together by entangling fiber or filaments (and by perforating films) mechanically, thermally or chemically. They are flat or tufted porous sheets that are made directly from separate fibres, molten plastic or plastic film. They are not made by weaving or knitting and do not require converting the fibres to yarn. Typically, a certain percentage of recycled fabrics and oil-based materials are used in nonwoven fabrics. The percentage of recycled fabrics varies based upon the strength of material needed for the specific use. In addition, some nonwoven fabrics can be recycled after use, given the proper treatment and facilities. For this reason, some consider non-woven a more ecological fabric for certain applications, especially in fields and industries where disposable or single use products are important, such as hospitals, schools, nursing homes and luxury accommodations. Nonwoven fabrics provide specific functions such as absorbency, liquid repellence, resilience, stretch, softness, strength, flame retardancy, washability, cushioning, thermal insulation, acoustic insulation, filtration, use as a bacterial barrier and sterility. Filtration is an important process in separating the unwanted dust particles during the process.  A filter medium is defined as a permeable material used for a filter that separates the particles moving from it. Textile filter fabrics are essential parts of countless industrial processes, contributing to the product purity, saving in the production cost and cleaner environment. Till the beginning of 1950, textiles used in filtration were based on woven fabrics of cotton, wool, and glass fibers. The development of synthetic and non woven fabric technology substantially modified the use of textiles in filtration. Textile materials play a vital role in the industrial filters. Filtration process consists of two main processes namely filtration and cleaning of filters. Woven, knitted and nonwoven textiles materials are generally used as filter fabrics. Nonwoven material yields better filtration efficiency compared to other materials.

  For any fabric or material used as a filter their primary functions are:

  • To retain and stabilize the particle size structure.
  • To allow free through flow of liquid / air / gas in the long term.

Brief History of Non Woven filters

The Prologue of Introduction to the nonwoven technology provides a historical listing of possible beginnings of nonwovens. These hypothetical beginnings range from historical legends to early technology developments. The actual roots of nonwovens may not be clear but in 1942 the term “nonwoven fabrics” was coined and were produced in the United States. These early “nonwoven fabrics” were created by adhesively bonding fiber webs. The first written definition of nonwoven fabrics came from the American Society for Testing and materials in 1962 which defined them as “textile fabrics made of carded web or fiber web held together by adhesives”. Currently, INDA defines a nonwoven as “sheet or web structures bonded together by entangling fiber or filaments (and by perforating films) mechanically, thermally or chemically. They are not made by weaving or knitting and do not require converting the fibers to yarn” (INDA). Technical definitions express the fundamental basis for the nonwoven processes, but due to the wide variety of production techniques, a general description of nonwoven fabrics is not enough. As with woven or knitted fabrics, each process possesses unique characteristics. The resulting fabrics do not have much in common aside from being categorized as nonwoven. Nonwoven components such as fiber selection, web formation, bonding, and finishing techniques can be altered to manipulate fabric properties or reverse engineer fabrics based on functional requirements. Due to its assortment of achievable characteristics nonwoven fabrics penetrate a wide range of markets including medical, apparel, automotive, filtration, construction, geotextiles, and protective. While manufacturing nonwovens, some conventional textile operations such as carding, drawing, roving, spinning, weaving or knitting, are partially or completely eliminated. For this reason the choice of fiber is very important for nonwoven manufacturers. The commonly used fibers include natural fibers (cotton, jute, flax, wool), synthetic fibers (polyester (PES), polypropylene (PP), polyamide, rayon), special fibers (glass, carbon, nanofiber, bi-component, superabsorbent fibers). Two or more types of fibers are typically utilized. The fibers are usually blended or mixed in order to improve performance properties of nonwovens, such as strength and other properties. The fiber blend or mix can be natural/natural, synthetic/synthetic, or natural/synthetic. 

NON WOVEN FILTERS

Nonwovens offer many advantages in all types of filtration. Non Woven Filter Fabric is thin layers of material made up by compressing fibers and filaments together. They are porous and air and water can pass through them. They filter out micro and nano solid particles from crossing them. Nonwoven filters are designed to remove ultrafine dust, aerosols, and viable organisms to meet highest requirements to the cleanliness of air in a wide range of applications such as clean rooms, hospital operating theatres, microelectronics, optical and precision industries, and the pharmaceutical and food industries.

Natural randomness of textile structure increases as probability of the particle being caught by fibers. Therefore, nonwoven fabric, in general provide higher filtration efficiency than woven or knit fabric. Woven and knit fabric is considered to have a two- dimensional structure while nonwoven fabrics may give a three-dimensional structure with a larger thickness which increases the distance for the particle to travelAlso nonwoven can be constructed in layers. A coarse open fabric removes larger particles. As the fluid progresses through the filter, the fibers and pores become finer to trap finer particles. Although, filtration plays a critical role in our day-to-day life, there is not a single type of fabric used in all the applications. The usage of the filter fabrics varies according to their end-use. This depends on the properties the filters have which ultimately depends on the characteristics of the raw material used for the manufacturing of the filter fabric. A filter fabric intended to use for heavy chemical filtration may or may not be used at high temperatures. Similarly, a filter fabric intended to use at the high temperatures may not be a good chemical resistant. Hence, it is totally depending on the type of filter fabric intended to use and the specific end use. Filter material are generally used in solid-gas separation and solid –liquid separation. Needle felt nonwoven are mostly used as filtration fabrics.

Advantages of nonwoven filter over the woven filters are

  • High filtration efficiency.
  • High permeability.
  • Less blinding tendency.
  • No yarn slippage as in woven media.
  • Good cake discharge.
  • There is no limitation for thickness.
  • High production rate.
  • Continuous process line.

As fiber diameter increases, filtration efficiency decreases. Also, void volume, which is required to reduce the pressure drop is more in case of nonwoven (98%) than that of woven or knitted (70%), thus nonwoven are advantageous to use. 

Nonwovens technologies used in filtration

Applications :

Air filtration

  • HVAC – industrial heating, ventilation and air conditioning
  • Industrial
  • Consumer Products (vacuum cleaners, cooker hoods, PCs, etc.)
  • Clean Rooms

Liquid filtration

  • Food & Beverage (milk, wine, tea etc.)
  • Pharmaceutical/Medical
  • Water
  • Blood
  • Hydraulic

Automotive filtration

  • Engine air
  • Oil
  • Fuel
  • Cabin air

Speciality filtration

  • Antimicrobial
  • Biopharmaceutical
  • Dust
  • Odour

Air Filtration

In case of air filtration, the filter medium is the material upon which or within which, the solid suspensions are deposited while allowing the air to pass through. The separation of airborne particulate from an air stream is called air filtration and is accomplished through mechanical, aerodynamic, and electrostatic phenomenon. The filter media which are mainly used for air filtration purpose can be of following types: metal cloths, ceramic porous plates, oil bath, venturi scrubber, water spray, cyclone and fibrous porous media.

Polypropylene (PP), polyethylene (PE), and polyester (PET) fibres are the three major synthetic fibres for making filter media of large quantities. Both PP and PE fibres, which have high electrical resistance and do not absorb moisture from environment to retain the stability of E-charges, are usually employed to make electret filters to achieve lower pressure drops. Other polymers used to make electret fibres include polybutylene terephthalate, polytetrafluoroethene (PTFE), and PC. The electret filters are widely used in HVAC filters, respiratory masks, cabin air filters, vacuum filters, high-efficiency particulate air (HEPA)/Ultra low particulate air (ULPA) filters, dust removals, and engine intake air filters, etc. However, such electrostatic filters are greatly influenced by air humidity, which causes charge dissipation; therefore, such electro statically -treated filters usually perform well initially, but their performance drops with the gradual accumulation of particles captured and the decay of the electrostatic charge.

Polyester wet-laid fabrics are used as a support layer for membrane filters, and needle-punched nonwoven fabrics made from polyester staple fibres are widely used as baghouse filters and dust collection cartridges in the production processes of cement, kaolin, and abrasive particles. 4DG fibres and P84 fibres are typical examples of fibres having irregular cross sections for air filtrations. 4DG fibres have deep grooves on the surface of the polyester fibres, and P84 are high-performance polyimide fibres having trilobal/star cross sections for high-temperature filtrations.

Single layer needle punched nonwoven materials in air filtration application include non uniformity of web and distribution size of pores which result in not all the dust particles being intercepted. A multi layer homogeneous nonwoven fabric can be produced cost effectively with improved filtration performance, cotton and other natural fibres are suitable fibre for making multi layer nonwoven fabric filter. The structural anisotropy of different fibres leads to improve various physical and filtration property of the multilayer nonwoven filter fabrics. Multi layer nonwoven fabrics were used for gas turbine intake filter and it results better performance than the single layer filters.

Liquid Filtration

In liquid filtration, composite structures are often used to provide gradient density filtration. They are also for prefilters to membrane filtration and reverse osmosis. Needle felts for liquid filter applications are usually scrim reinforced to provide adequate strength.

Nano filtration is a membrane filtration based method that uses nanometer-sized cylindrical through-pores that pass through the membrane at a 90°. Nanofiltration membranes have pore sizes from 1-10 Angstrom, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis. Membranes used are predominantly created from polymer thin films. Materials that are commonly used include polyethylene terephthalate. Pore dimensions are controlled by pH, temperature and time during development with pore densities ranging from 1 to 106 pores per cm2. Membranes made from polyethylene teriephthalate and other similar materials are referred to as “track-etch” membranes, named after the way the pores on the membranes are made.

Application FieldUses
Fine chemistry and PharmaceuticalsNon-thermal solvent recovery and management room temperature solvent exchange
Oil and Petroleum chemistryRemoval of tar components in feed

Purification of gas condensates

Natural Essential Oils and similar productsFractionation of crude extracts, Enrichment of natural compounds Gentle Separations
MedicineAble to extract amino acids and lipids from blood and other cell culture.

The typical particle size used for microfiltration ranges from about 0.1 to 10 µm. In terms of approximate molecular weight, these membranes can separate macromolecules generally less than 100,000 g/mol. The filters used in the microfiltration process are specially designed to prevent particles such as sediment, algae, protozoa or large bacteria from passing through a specially designed filter. 

Micro-filtration applications

A) Water Treatment

The most important use of microfiltration membranes is for the treatment of water supplies. The membranes are a key step in the primary disinfection of the uptake water stream. Such a stream might contain pathogens such as the protozoa Cryptosporidium and Giardia lamblia which are responsible for numerous disease outbreaks. Both species show a gradual resistance to traditional disinfectants i.e. chlorine. The use of MF membranes presents a physical means of separation (a barrier) as opposed to a chemical alternative. In this sense, both filtration and disinfection take place in a single step, negating the extra cost of chemical dosage and the corresponding equipment. Similarly, the MF membranes are used in secondary wastewater effluents to remove turbidity but also to provide treatment for disinfection.

RO (Reverse Osmosis) is another technology used for water treatment. RO is a water purification technology that uses a semipermeable membrane. This membrane technology is not properly a filtration method. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential, a thermodynamic parameter.

B) Sterilization

Another crucial application of microfiltration membranes lies in the cold sterilization of beverages and pharmaceuticals. Historically, the heat was used to sterilize refreshments such as juice, wine, and beer in particular, however, a palatable loss in flavor was clearly evident upon heating. Similarly, pharmaceuticals have been shown to lose their effectiveness upon heat addition. Micro filtration membranes are employed in these industries as a method to remove bacteria and other undesired suspensions from liquids, a procedure termed as ‘cold sterilization’, which negate the use of heat.

C) Petroleum Refining

Furthermore, microfiltration membranes are finding increasing use in areas such as petroleum refining, in which the removal of particulates from flue gasses is of particular concern. The key challenges/requirements for this technology are the ability of the membrane modules to withstand high temperatures i.e. maintain stability, but also the design must be such to provide a very thin sheeting to facilitate an increase in flux. In addition, the modules must have a low fouling profile and most importantly, be available at a low-cost for the system to be financially viable.

D) Dairy Processing

Aside from the above applications, microfiltration membranes have found dynamic use in major areas within the dairy industry, particularly for milk and whey processing. The MF membranes aid in the removal of bacteria and the associated spores from milk, by rejecting the harmful species from passing through. This is also a precursor for pasteurization, allowing for an extended shelf-life of the product. However, the most promising technique for MF membranes in this field pertains to the separation of casein from whey proteins i.e. serum milk proteins.

Medical and Healthcare

Nonwovens offer many advantages in the medical/healthcare market.

Here are a couple of examples,

Operating room personnel wear single-use nonwoven surgical gowns when they are in the operating room. Nonwovens are also used to make surgical drapes. The advantage for using nonwovens is protection from the patient’s blood, bodily fluids, and they are sterile.

Millions of people all over the world acquire HAI (Hospital Acquired Infection) resulting in tens of thousands of fatalities every year. There are many causes for acquiring infections in the hospitals, one of them being during surgery (commonly referred to as SSI – Surgical Site Infection). Nonwoven medical drapes and gowns can help prevent the spread of bacteria and microbes during surgery, thereby reducing the risk of SSIs/HAIs.

The fibers used in medical nonwovens can be classified in natural and synthetic categories. The natural fibers used are wood-pulp, cotton and rayon. Wood pulp is used for its obvious absorbency, bulk and low cost. Cotton and rayon are good to be used directly on wounds. They have good absorbency and make excellent nonwovens. The synthetic fibers mostly used in this application are: polypropylene for its excellent rheological characteristics, hydrophobicity which is desired in some systems where barrier properties are required, low cost, bicomponent fibers which are widely used in thermal bonding and added functionality and polyester when strength, mechanical properties and ease of sterilization are of prime importance. Synthetic fibers also account for the products strength, solvent resistance, static dissipation and many other desirable properties.

Examples of Nonwovens in the Medical and Healthcare Industries

  • Bandages
  • Cast paddings & covers
  • Surgical drapes
  • Dressings
  • Surgical gowns
  • Packs
  • Sterile packaging
  • Sterile overwraps
  • Surgical masks
  • Swabs
  • Underpads

Hydraulic Filters

Hydraulic filters protect your hydraulic system components from damage due to contamination of oils or other hydraulic fluid in use caused by particles. Every minute, approximately one million particles larger than 1 micron (0.001 mm or 1 μm) enter a hydraulic system. These particles can cause damage to hydraulic system components because hydraulic oil is easily contaminated. Thus maintaining a good hydraulic filtration system will increase hydraulic component lifetime.

Nonwoven filtration media are widely used in fluid power applications. Many different fluids are used under a wide range of operating conditions. Nonwoven filtration media are thermally bonded or resin bonded with polymers that are susceptible to thermal and chemical attack.
The polymers can swell, fracture and/or soften which may change the filtration performance. The change in filtration performance can be gradual or sudden based on the concentration, exposure time, temperature and additives. Various different nonwovens such as Wetlaid nonwovens in which short-cut man-made, fiberglass and/or cellulosic fibers are used in manufacturing hydraulic filters

Automotive Filtration

Whether a filter is designed to keep the air in your home clean, an operating room sterile, or to remove dirt and grit from the oil in your car before it reaches the engine, nonwoven fabrics get the job done. Nonwoven filter media also play an important role in today’s vehicles. In total, there are more than 50 filters in an average passenger car, contributing to functions ranging from the performance of the engine and oil and fuel consumption through to the quality of the air in the cabin.

The application of nonwoven filter media in the fuel and oil filter segment of the transportation industry is affected significantly by governmental regulations related to the environment and the desire for longer intervals between engine filter replacements.

Nonwoven filter media employed for filtering fuel and oil must demonstrate good chemical and temperature resistance. The filtration system is also constantly subjected to a great deal of vibration and shock, so the media must withstand extreme operating conditions. And since the filter media represent the last line of defense for engines against contaminant-related wear and tear, the performance of the nonwoven must, in addition, be consistent and predictable during the service cycle.

Oil filters are an important component in your car. The nonwoven fabrics in the filters help remove contaminants from the engine oil, hydraulic oil, transmission oil and lubricating oil.

Most of the filter media used in fuel and oil filtration are manufactured by wet-laid processes using cellulosic fibres. Recent technological advances in extrusion-based nonwovens manufacturing have led to increased demand for spunbond or meltblown media in high-end fuel and oil applications. Filter media manufacturers have developed unique fibre and processing combinations to improve automotive oil and fuel filtration efficiency.

One of the drawbacks of wet-laid-based nonwoven filter media is shedding. Small fibre particles can become dislodged from the filter and travel downstream with the filtrate. Since tolerances on contaminant levels in the filtrate are tightening, the segment is shifting towards use of extrusion-based nonwoven filter media.

The ability of extruded spunbond and meltblown nonwoven media to easily conform to a desired shape and size is resulting in their acceptance in the oil and fuel filtration segments.

Specialty Filtration

An antimicrobial nonwoven material composed of two or more distinct fibers that are entangled to form a three-dimensional nonwoven web. The individual fiber components of the web are chosen to have properties that impart both excellent filtering capacity and antimicrobial activity to the web. At least one fiber of the composite web is selected to have excellent filtering characteristics. Moreover, at least one fiber of the composite web is selected to be able to incorporate an active agent, such as an iodinated resin, with a sufficient loading capacity and diffusivity such that the active agent can migrate to the surface of the fiber and exert its antimicrobial effect. Mostly these filters are used as air filters in Heating Ventilating and Air Conditioning (HVAC) systems.  HVAC system components usually operate in a warm, dark and humid environment, which makes it an ideal breeding ground for microbes such as bacteria and/or fungi. The microbiological contamination cause odour in their mildest form, but generally may cause much graver issues. Besides HVAC, hospitals and generally facilities sensitive to microbiological pathogens have been increasingly facing issues with microbiological contamination of the air. An antimicrobial filter plays an important role in such cases by cleaning the air and removing all the impurities.

CONCLUSION

As people are looking continuously for cheap and reliable alternatives of the existing products, the use of non woven filters will definitely increase in the upcoming years. Usage of woven and knitted filters will be minimized as much more modifications can be made easily in the properties of non woven filters than that of the woven and knitted. Some nonwoven fabrics can be recycled after use, given the proper treatment and facilities. For this reason, some consider non-woven a more ecological fabric for certain applications, especially in fields and industries where disposable or single use products are important, such as hospitals, schools, nursing homes and luxury accommodations. Besides, the technology of some new cars, sensors can monitor parameters such as operating hours, engine speed, oil temperature range, and number of cold starts, and use this data to develop service life software models. Ultimately, the user receives a service indication for the oil or filters change only if it is actually required, instead of changing the filters at regular intervals regardless of the operating cycle.

This technology will be adapted to future filtration systems not only in automotive industries but in Industry 4.0 where technologies such as Internet of things (IOT), cloud computing, etc will be implemented artificial intelligence plays vital role, thus required that filters should meet the necessary standards. In such cases nonwoven filters will be an ideal choice. The trend to improve machine reliability will continue and will be supported by the need for information and sophisticated monitoring and control algorithms. Even machines at remote locations are warned of imminent machine damage to prevent downtime and reduce operating costs.

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By- Varqha Bagwan (B.Text) Textile Plant Engineering,

Prof S.G.Kulkarni, M.Tech (IIT,Delhi)

D.K.T.E‘ Textile & Engineering Institute, Ichalkaranji.