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Sec. 1.2 Overview of Geotextiles <br />type of manufacture at lower fabric weights (2 to 12 ounces per square yard) than for <br />needle-punched fabrics, owing to the fiber bonding utilized in the process. The bonding <br />operations, in fact, differ between the commercially available fabrics, depending on the <br />basic fiber characteristics. The bonding methods include (1) homofil bonding, in which <br />all filaments are composed oi a single polymer type, but some of the filaments have dif­ <br />ferent melting characteristics, bonding being achieved by a high-temperature calendering <br />operation, accomplished by passing fabric between two counter-rotating hot rollers; and <br />(2) heterofil bonding, in which some of the filaments comprise two types of polymers <br />having different melting points (heterofilaments i, whereas others have only one polyrre" <br />(homofilaments). The heterofilaments have different softening characteristics from the <br />homofilaments, and strong flexible bonds can therefore be formed at the heterofilament <br />crossover points by controlling the application of heat and pressure to fuse only the <br />lower-melting-point polymer forming the sheath, while leaving the core and homofila­ <br />ments unaffected. <br />1.2.2.3 Resin-Bonding Process <br />Usually, acr>'lic resin is sprayed onto, or impregnated into, a fibrous web. .After <br />curing and/or calendering, strong bonds are formed between filaments. Often a forced- <br />air drying operation is used to reestablish the fabrics* open-pore structure. <br />1.2.2.4 Needle-Punched Process <br />A needle-punched fabric is produced by introducing a fibrous web — already <br />formed by cards, gametts, or air laying — into a machine equipped with groups of spe ­ <br />cially designed needles. While the web is trapped between a bed plate and a stripper <br />plate the needles punch through it and reorient the fibers so that mechanical bonding is <br />achieved among the individual fibers. Often, the batt of fibers is carried into the needle- <br />punching section of the machine on a lightweight support material or substrate. This is <br />done to improve finished fabric strength and integrity (see Figure 1.5). <br />The needle-punching process is generally used to produce fabrics that have high <br />density yet retain considerable bulk. Fabric weights usually range from 1.7 to 20 ounces <br />Needlepunched Process <br />Needle board <br />Formed web iiiiiiiniiiiiiiiliRi <br />windup <br />/ \ <br />Needlepunching <br />Figure 1.5 Schematic diagram of needle-punched manufactunnc process. lAfier IS'DA <br />113}.> <br />UAiBE