yarns have relatively regular and uniform pore sizes. "Some engineers believe 

 that because of their simple pore structure, the monofilament fabrics are more 

 reliable filter materials and use them in critical installation, where their 

 higher cost can be justified" (Bell and Hicks, 1980). The fabric is thin. 



2_ Multifilament Cloths . Multifilament cloths are woven 

 of yarns containing many fine filaments, except fibri Hated yarns which are 

 produced from synthetic sheets. Fibrillated yarns are formed of fibers 

 from sheet plastic film. All multifilament fabric can be produced with 

 higher tensile strengths than monofilaments. With the exception of fib- 

 rillated fabrics, multifilament cloths also have a simple, relatively 

 regular and uniform pore size, and generally are thin. Fibrillated fabrics 

 have a slightly more irregular pore system and generally are thicker. 



3 Mono-Multifilament Combination Fabrics . Mono-multi- 

 filament combination fabrics contain monofilament yarn in one direction and 

 multifilament in the other. The pore sizes are consistent and controlled 

 by the weaving process. The openings are oblique to the plane of the 

 fabric. The cloth is slightly thicker than monofilaments. 



4 Slit-Film . The term as used herein refers to a fiber 

 which has a width many times its thickness. Such fabrics are also called 

 ribbon, split-film, slit-tape, and split-tape. Because of the poor dis- 

 tribution and uneven sizes of the pores, there is a great variation in 

 their retention and filtration capabilities. The fabrics are thin. Slit- 

 film fabrics are not recommended for use as filters. 



(b) Nonwoven Fabrics . Nonwoven fabrics include all materials 

 which are not woven or knitted. They consist of discrete fibers, which may 

 have a preferred orientation or may be placed in a random manner and do not 

 form a regular or simple pattern as do wovens. Nonwoven fabrics are 

 composed of either continuous filament or staple filament fibers. Con- 

 tinuous filaments are extruded, drawn and laid in the fabric as one con- 

 tinuous fiber. Staple filaments are cut to length before being laid in the 

 fabric. The engineering properties of nonwoven fabrics are controlled by 

 the fiber type, the geometric relationships of the fibers, and the methods 

 of bonding. Four methods of bonding are described below. 



1 Needle Punched . Barbed needles are punched through 

 the fabric web, perpendicular to the plane of the fabric and withdrawn, 

 drawing filaments with them. This causes the fabric to become mechanically 

 entangled. These fabrics have a very complex pore structure and the fabrics 

 are compressible, so the nature of the pore structure changes. This results 

 in a different in-situ filtration performance than might be indicated by 

 isolated permeability and particle retention tests. The fabric is relatively 

 thick and has the appearance of felt. 



2_ Heat Bonded . The fabric is subjected to a high 

 temperature, which results in the filaments welding themselves together at 

 the contact points. These fabrics have a relatively discrete and simple, 

 though irregular, pore structure and are thin. 



3^ Resin Bonded . The fabric web is impregnated with a 

 resin which coats and cements the fibers together. Pore structure and 



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