Acrylic Fibers
Acrylic fibers are man made fibers, in which the fiber forming substance is any long chain polymer composed of at least 85% acrylonitrile units, and the remainder a copolymer. Acrylic is non-thermoplastic. Orlon® (Dupont), Acilian® (Monsanto), Creslan® (American Cyanimid), Crylor® (Crylor SA), Zefran® (BASF), and Draylon-T® (Bayer) are trade names used by various producers of acrylic fibers.
Draylon-T® (or equivalents), a homopolymer (100% acrylonitrile), is a widely used acrylic, for high-pressure cleaned needled felts. Fiber cross sections are dumbbell-shaped, and surfaces are striated.
Diameters are typically 15 to 35 microns. Where polyesters are not suitable, because of potential hydrolysis, acrylics offer a combination of abrasion resistance and resistance to wet heat degradation, particularly under acid conditions. Homopolymer felt is a candidate for hot gas applications of less than 284 degrees F. Temperature resistance of copolymers is less, 250°F degrees.
Acrylic felts are used in drying raw flour, coal, gold and copper ores, galvanizing, and low temperature flue gas applications. Polyester is superior for most dry heat applications.
Aromatic polymide (Nomex®)
Nomex® nylon was a proprietary (patent has expired) aromatic-polymide (aramide) linked structure, developed by E.I. duPont de Nemours, for applications requiring dimensional stability and high heat resistance. Nomex® is a non-thermoplastic so it does not melt, but at temperatures above 700 degrees F, degradation sets in rapidly. In dry heat up to and including 375 degrees F, this fabric may be used satisfactorily as long as there is no acid dew point problem.
Nomex® is unaffected by small amounts of water vapor and high temperatures. When exposed to saturated steam at high temperatures, Nomex® will progressively lose strength. However, it withstands these conditions better than many other fibers.
Nomex® withstands the attack by mild minerals and inorganic acids, mild alkali, and most hydrocarbons. It is unaffected by florines and gases from metallurgical and rock processing operations. However, high active oxidizing agents, such as sulphur oxides, will rapidly degrade Nomex®.
The single biggest use for Nomex®, needled felt, is asphalt batch plant dryers and drum-mix plants. Other uses include raw and finish mill grinding on cement plants, carbon bake, clinker coolers, and many other hot gas processes.
Polyester
A manufactured fiber in which the fiber forming substance is any long chain synthetic polymer composed of at least 85% by weight of an ester of a dihydric alcohol and terephtalic acid. This material is a thermoplastic.
Fiber is available under various trade names: Dacron® (duPont), Enka Polyester® (American Enka), Fortrel® (Fiber Industries/Celanese), and Kodel® (Eastman Chemical). Polyester is the most widely used needled felt for pulse jet applications.
Polyester (PE) is superior to most synthetics in dry heat installations but it is not comparable to Teflon® nor Nomex®.Recommended operating temperature is 275 degrees F maximum.
Under moist, near-saturation, heat conditions, polyester is inferior to other synthetics. In the presence of a saturated vapor, the fibers will hydrolyze, weakening the fabric.
PE provides good resistance to most oxidizing agents, mineral acids, and most organic solvents except high concentrations of sulfuric, carbolic and nitric acids. It resists weak alkalis. However, strong alkalis, at high temperatures, will dissolve the fabric.
Polyester fiber makes an excellent filter fabric, with efficient filtration and good energy absorption characteristics.
The fabric is used in agricultural, woodworking, chemical and other applications where chemical and mechanical characteristics are compatible. As needled felt, it is available in 12, 16, and 18 oz./yd² versions. The use of 12 oz./yd² should be reserved for non-critical applications, handling large particulate. Spunbonded POLIPLEETTM media is available in plain, water/oil resistant (TR), metalized-antistatic (ME), and with a laminated PTFE membrane.
Polypropylene – (polyolefin)
It is a manufactured fiber in which the fiber forming substance is any long chain synthetic polymer, composed of at least 85% by weight of olefin units.
Herculon® (Hercules) and Reevon® (Phillips) are trade names used by various producers of polypropylene (PP) fibers.
The fibers combine excellent resistance to most acids and alkalis, plus high strength. It has one of the lowest specific gravities of any synthetic fiber, and is one of the most economical synthetics, (about the same price as polyester). PP is a good choice for replacement of cotton in low temperature applications.
PP absorbs no moisture and provides excellent cake discharge and resistance to blinding. Filtration efficiency is not quite as good as polyester.
Polypropylene has very low heat resistance, even less than cotton, and should not be exposed to prolonged temperatures of over 165 degrees F. Since it does not absorb moisture, its degradation characteristics in dry heat and moist heat are virtually the same.
Within its limited temperature range, polypropylene provides good resistance to mineral and organic acids. It resists alkalis, reducing agents and organic solvents. It is, however, soluble at 160 degrees F in chlorinated hydrocarbons.
P84 (Polyimide)
P84 is a proprietary fiber produced by Lenzing in Austria. P84 is a non-thermoplastic, and in 100% form may be used for temperatures of 500 degrees F. Its fiber is highly convoluted, having a high surface area-to diameter ratio.
Extensive testing has shown that P84 fabric approaches Gortex® in efficiency, but has all of the advantages of a needled felt. It may be layered on top of lower cost fibers, to provide the benefits of the base fiber, but at lower costs than a 100% version of the P84 felt. Composites generally have a 4 oz.yd² P84 fiber layer, needled to a 12 oz.yd² carrier base. 100% P84 is readily available in 14 oz. weight, but other weights are available.
P84 needled felt, and composites, are used in many industrial applications where high efficiency is required. Examples are: gold and copper ore processing, incinerators, boilers, and various chemical processes.
Teflon® –
It is a proprietary fluorocarbon fiber, manufactured by E.I. duPont. It is composed of long chain carbon molecules in which all of the available bonds are completely saturated with fluorine. These strong carbon-to-fluorine bonds create fibers that are exceptionally stable to both heat and chemicals. Teflon® is the most chemically resistant fiber used in conventional dust filtration.
Teflon® is not affected by any known solvents except some prefluorinated organic liquids at temperatures above 570 degrees F. Exposure to temperatures above 550 degrees F. will cause some decomposition, although it is slow to develop. Teflon® bags shrink when exposed to high temperatures, especially in length.
The low friction properties of Teflon® fibers provide excellent cake discharge. In addition, Teflon® fibers’ chemical inertness and resistance to dry and moist heat degradation makes it ideal for use under severe conditions.
Teflon® needled felt is extremely expensive. Recently a lower cost version, Tefaire®, has been introduced. This felt is a blend of 85% Teflon® and 15% fiberglass fibers. Commercial uses are limited to extreme chemical environments where the advantages of Teflon fibers’ great chemical resistance outweighs cost disadvantages.
Some boilers, carbon black plants, soil remediation systems, and incinerators have been equipped with Teflon® products.
Glass
Glass fiber is a product of fusion, a non-crystalline silicate analogous to other fiber polymeric materials. Selected silica sands, limestone, soda ash, and borax or other ingredients are melted at about 2500 degrees F. and the mixture is extruded through spinnarets. The resulting filaments may be drawn while still molten and later twisted and plied into filament yarn.
Or, as the extruded glass may be drawn and broken by jets of compressed air into staple of lengths 8 to 15 inches. The fibers are then treated with a lubricant which is of great importance in the durability of the eventual fabric. Following drying, the fibers are process much like the more conventional fibers.
Woven fiberglass and felted fiberglass medias are available for high energy cleaned fabric filters. These are specialty products and are used for very specific applications; involving high (up to 500 degrees F.) temperatures, usually in the presence of oxidizing agents.
Some common trade names for woven fiberglass are GL65 Tri-LoftTM (BHA), and FL57 Hi-LoftTM (BHA); other companies have similar products. Available weights are 16 to 22 oz./yard². Huyglas® (Air Purator Corporation) felted fiberglass medias are available in a variety of weights, form 14 to 27 oz./yd². Bag/cage fit & support is very critical – check with the vendors involved for specific recommendations.
Woven fiberglass, in particular, is very easy to damage, and is not as efficient as other medias. Felted fiberglass tends to be heavy and difficult to handle. Suitable applications for this material are limited.
HEPA
High Efficiency Particulate Air filter is the maximum efficiency available in particulate filtration. Rated for temperatures up to 275°F. Usually offered as a static after filter following a dust collector; however, a pulse cleaned version is available.
Ryton® (Polyphenylene Sulfide)
Ryton® (Phillips) is a long chain synthetic polysulfide, with at least 85% of the sulfide linkages attached directly to two aromatic rings. The resin was developed by Phillips Petroleum, in 1973.
Ryton® is resistant to sulphur oxides, and is used for high temperature gas streams, up to 360 degrees F. Gas stream oxygen content should not exceed 15%. Ryton® does not hydrolyze and has flame retarding characteristics.
Typical applications are industrial/municipal solid waste incinerators and coal-fired boilers.
Other fibers and medias are available for very special temperature, chemical, pressure drop, or efficiency requirements.
