Nonwoven filter fabrics

A.L. Rollin, R. Denis, L. Estaque, J. Masounave, Hydraulic behaviour of synthetic nonwoven filter fabrics. Can J Chem Eng (1982) 226—234. 

Where clean hquids are constantly required and abraded particles are produced, varying in size, shape and material, an automatic band filter with a nonwoven filter fabric will as a rule provide a cost-effective solution. 

American Nonwovens Amoco Fabric Filters Adas Corp. 

Filter fabrics, especially nonwoven fabrics, which are produced from short staple fibres, invariably possess surfaces with protruding fibre ends. Since such protrusions may inhibit the cake release by clinging to the dust, it is a common practice to remove them. This is achieved by singeing, a process in which the fabric is passed, at relatively high speed, over a naked gas flame, or, in another technique, over a heated copper plate. The heat of the flame causes the fibres to contract to the surface of the fabric where, in the case of thermoplastic fibres, they form small hard polymer beads (Fig. 3.8). Singeing conditions (i.e. speed and gas pressure) will normally be adjusted according to polymer type and the intensity required by either the end-use application or the individual manufacturer s preference. 

Coating is a basic and important form of finishing for nonwoven bonded fabrics. The way in which the coating is carried out depends on the substrate, the machinery available, the substance that is to be applied, and also the effect desired. Crushed foam coatings can modify the porosity of filter fabrics from zero upwards, depending on the add-on of resin and the severity of cmshing. Foam collapsed into the fabric will present the least resistance and is more suitable for high airflow requirements. 

Although widely used in dust collection, nonwoven fabrics have found only limited use in liquid filtration because their thickness and density render them prone to blinding in many applications. One area where they have found some success, however, has been in the filtration of metal ore concentrates such as copper on horizontal vacuum belt filters. These applications tend to be very aggressive on the filter fabric, and hence a suitably designed and finished nonwoven fabric is often more cost effective than a considerably more expensive woven fabric, especially if required in lengths of around 80 m and widths up to 6 m. The solids which are captured in such applications quickly form a cake on the surface and, should some penetration occur, as with woollen spun yams, the bulky nature of the material provides scope for the particles to escape. For such arduous applications, nonwoven fabrics are generally in the area density range of 800-1000 gm- ... 

Whilst this amount covers all filtration media, the textile fabrics used for the filter media have worldwide sales of more than US 3 billion. Woven filter fabrics made from both multifilaments and monofilaments have dominated the filtration market for both wet and dry applications. However, the recent developments in nonwoven technology have allowed the process to gain market share due to cost, disposal, and production advantages. ... 

Filtration properties are effected by the nonwoven fabric production properties. For the filtration purposes depending on the size of the particulate to be filtered, fabric thickness plays quite important role moret titan other properties. 

Wang, X.Y., Gong, R.H., 2006b. Thermally bonded nonwoven filters prepared using bicomponent PP/PET fibre II. Relationships between fabric area density, air permeability and pore size distribution. J. Appl. Polym. Sci. 102 (3), 2264-2274. 

Hutten IM. The handbook of nonwoven filter media. Oxford, UK Elsevier Ltd 2007. Horrocks AR, Anand SC. Handbook of technical textiles. Woodhead Publishing Ltd 2000. The Journal of Engineered Fibers and Fabrics (JEFF) (http //www.JeffJoumal.org). International Nonwovens Journal (http //www.inda.org/INJ/index.html). 

Wood pulp fibres are frequently less than 1 pm in fibre size and dominate wet-laid nonwoven filter media they are able to provide higher filtrafion efficiency and are environmentally fiiendly. Synthetic fibres (eg, polyolefin, polyester, and polycarbonate (PC) fibres) could work as electret fibres to provide additional electrostatic filtration mechanism, and also could be made binder-fitee, thermobond nonwoven fabrics if they are made Ifom thermoplastic polymers. 

Filtration of particles through nonwoven filters is depth filtration rather than surface filtration. The particle capture capacities of a nonwoven filter are based on the interactions between targeted particles, individual fibres of the filter, and the fluid molecules, and the dust particles captured by nonwoven filters are described by using single fibre particle collection theory. The filter efficiency of nonwoven filter, which is composed of many individual fibres, depends on the particle collection efficiency of a single fibre. The equation defining overall filter efficiency of a nonwoven fabric in dry air filtrations, Y(d(), for any particle size, (L, and the set of conditions is as follows ... 

The pitfall of the filter efficiency of a nonwoven fabric for particles of the diameter between dpi and dp2 is unacceptable but inevitable. To design a nonwoven filter with high filter efficiency, nonwoven fabrics composed of fibres of more than one diameter... 

Air filters have many different configurations in end use markets for both general dust filtration and high-efficiency filtration. The principal air filtration media for HVAC include melt blown and spunbond nonwoven fabric filters (around 825— 850 million m per year in North America) HEPA wet-laid fibreglass nonwoven filters account for another additional 90—100 million m. ... 

Examples of such nonwoven filters comprising one or more layers of microfibres and nanofibres for microfiltration of specific biological contaminants were reported in US patents 2004/0038014, 2007/0075015, and 2007/0018361. Prefilters are commonly pleated or wound filter fabrics. Prefilters have a large band of retention ratings. The most common retention rating of these filters is 20 or 50 nm, and it can be engineered to all necessary apphcations. 

DE4114952 Nonwoven material for use as filter fabric, etc.-has two layers consisting of spun-bounded or nonwoven material on which carded staple mat is placed. 1992. 

In the past 10 years, electrospun nanofibrous membranes have been spotlighted as an effective filter media to capture fine particles. In addition to the basic studies of electrospinning process to better understand the membrane construction process, researchers from all over the world focus on the study of the relationships between the structure characteristics of electrospun nanofibrous membranes (fiber diameter, pore size, porosity, surface area, etc.) and filtration performances (filtration efficiency, pressure, air permeability, etc.). In this chapter, recent advances in fabricating nanofibrous filter media via electrospinning process have been reviewed. In particular, filtration performances and relevant mechanical properties are discussed in detail. It is interesting that the electrospun nanofibrous membranes have been able to outperform conventional nonwoven membranes fabricated essentially by using the meltblown or spunbonded process. 

The use of polypropylene in textiles is steadily becoming more prevalent due to its lower cost basis. Polypropylene is commonly used in the fabrication of textile (nonwoven) filters for medical applications. In its native state, polypropylene nonwoven filtration media can only wet-out with liquids which possess surface tensions less than 32 mN/m. As such, water filtration requires high pressure to pass through the membrane. The use of an oxidative atmospheric pressure plasma treatment significantly increases fiber surface hydrophilicity and therefore filtra-... 

In the case of fabric (woven or nonwoven) filters for air intake duties, detailed design of the filter element may differ appreciably from the standard pleated form. An example is shown in Figure 6.34, where the fabric is mounted on wire gauze and folded into a star-shaped formation. This results in a high surface area, without pinching at the base of the folds, and consequently low flow velocities over the surface of the febric. A fabric element may be preferred to a paper element for heavy duty industrial filters. 

As binders for fiherfill and nonwovens, the emulsions are applied to a loose web or mat, then heated to form a film that sticks the loose fibers together. Polyester (188—191), glass (192), and rayon (193) mats are bonded in this manner for a variety of end uses including quilting, clothing, disposable diapers and towels, filters, and roofing (see Nonwoven fabrics). 

In 1992, as much as two-thkds of all spunlace fabrics were used in medical products other appHcations included wipes, industrial apparel, intedinings, absorbent components, filtration, and coating. Medical product appHcations also accounted for about one-thkd of all wet-laid nonwovens other appHcations included tea bags, meat casings, filter media, battery separators, and wipes. Most bonded-pulp fabrics were used as wipes or absorbent components. 

Nonwoven wipe categories include products for babies and adults, the food service and electronics industries, medical and clean room appHcations, industrial cleaning, computer diskettes, and household products such as dusters, tea towels, shoe cleaning cloths, towelettes, and hand towels. Nonwoven fabrics are used to filter air, water, petroleum (qv), food, and beverages. Nonwovens loaded with abrasives, cleansers, or finishes can be found in a variety of products used by many industries and in many homes to scour or poHsh. Also, a majority of garments designed to protect industrial workers and consumers from hazardous environments are made from nonwoven fabrics. 

Polyolefin—ceUulose composites also are used in nonasbestos flooring felts, waUpapers, filter media, labels, embossable papers, and other nonwoven fabrics that are made on paper machines. Use of synthetic fibers in paper has been reviewed (103,104). 

Air Permeability. Air permeabiUty is an important parameter for certain fabric end uses, eg, parachute fabrics, boat sails, warm clothing, rainwear, and industrial air filters. Air permeabiUty of a fabric is related to its cover, or opacity. Both of these properties are related to the amount of space between yams (or fibers in the case of nonwovens). The most common method for specifying air permeabiUty of a fabric involves measuring the air flow per unit area at a constant pressure differential between the two surfaces of the fabric. This method, suitable for measuring permeabiUty of woven, knitted, and nonwoven fabrics, is described in ASTM D737. Units for air permeabiUty measured by this method are generally abbreviated as CFM, or cubic feet per square foot per minute. 

Filter-medium selection embraces many types of construction fabrics of woven fibers, felts, and nonwoven fibers, porous or sintered solids, polymer membranes, or particulate solids in the form of a permeable bed. Media of all types are available in a wide choice of materials. 

Media made from woven or nonwoven fabrics coated with a polymeric film, such as Primapor, made by SCAPA Filtration, and Gore-Tex, made by W. L. Gore and Associates, combine the high retentivity charac teristics of a membrane with the strength and durability of a thick filter cloth. These media are used on both continuous and batch filters where excellent filtrate clarity is required. 

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