Filaments spun by melting resin chips are spread on a net directly from a spinning nozzle and randomly deposited to form a web and then bonded in the form of a sheet through thermal bonding or by other means.
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3. BOND AREA AND SIZE EFFECTS:
The bonding temperature increased the strength and elongation values increased, till the optimum, with webs having higher bond sizes showing higher strength values compared to the webs of smaller bond size. As one can expect, the higher bond area fabrics were Stiffer and had lower breaking elongation, indicating that very high bond area may not be suitable for producing webs suitable for certain applications.
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4. COMPOSITE STRUCTURES:
Whereas spun bond fabrics are strong, melt blown fabrics are weak, but have very good filtration characteristics. Many of the applications need the balance of these properties. As a result, one of the growing trends is to make composite structures from spun bond and melt blown. These are popularly making composite structures from spun bond and melt blown. These are popularly known as SMS structures, in which thin melt blown layer is sandwiched between two spun bond layers, producing strong fabrics with good barrier properties.
Electro spinning has been gaining a lot of attention these days because of extremely high surface area of the nanofibres. However, the nanofibre webs are very difficult to handle. One of the approaches of taking advantage of barrier properties of nanofibres is to incorporate them with spun bond or melt blown nonwoven webs12. In nanofibres are shown on melt blown web towards left and on spun bond web on the right side. In both the cases, because of the small size, the nano web looks like a film. It is clear that by adding less than 10 percent nanofibres, a large improvement in filtration efficiency of the webs can be accomplished.
5. SPUN - BONDED AND MELT-BLOWN:
The spun-bonding and melt-blowing processes of manufacturing nonwoven have been the fastest growing systems for the past fifteen years, each showing a growth rate of 10-15 % per year. Ever since the commercial production of polyester (Reemay&#;), polypropylene (Typer) and polyethylene (Tyvek) spun-bonds by DuPont, in the early sixties, numerous companies have entered the production of spun-bonded fabrics. The spun-bonding process avoids the process of first converting melt-spun filaments into staple and then carding and bonding them to form fabrics.
The process is shown, when the polymer is extruded through a spinneret and sometimes is drawn by rollers, crimped, and then passed through an air gun (aspirator jet) before being spread on a conveyor belt. To keep the filaments on the conveyor belt a vacuum is pulled through the porous belt. The web thus formed can then be bonded either thermally, be needle punching, by application of latex, or by any other desired system. Sometimes two streams of extruded filaments, one that has a lower melting point than the other, are mixed to achieve thermal bonding. The most commonly used polymers are the thermoplastic type such a polyesters, polypropylene and polyamides.
Tyvek, which is a trademark of DuPont, is produced by exploding the polymer (polyethylene) that is dissolved in a solvent ad flash freezing the fibrillated polymer on a colleting screen. The process produces a fabric with very fine fibres and the fabrics are bonded by heat. The fabric thus produced is primarily used in protective clothing, packaging, labelling and filtration.
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