Drylaid, wetlaid and polymer-laid web formation systems have their roots in textile, paper making and polymer extrusion processes.

Needlepunching Technology

Needlepunching is the most common web bonding method. It is the method of consolidation of fibrous webs by the repeated insertion of barbed needles into the web as shown in Figure 1.21. The needling can be done either from one side or from both (top and bottom) sides of the web. This process consolidates the structure of fibrous web without any binder by interlocking of fibres in the third or ‘Z’ dimension. Continuous filaments or short staple fibres are initially arranged in the form of a fibrous web in various orientations (random, cross, parallel, or composite). This forms a three-dimensional intermingled structure which fulfils the necessary requirements of geotextiles.

Needlepunched nonwoven geotextiles are extensively used in civil engineering applications including road and railway construction, landfills, land reclamation and slope stabilization. Such applications require geotextiles to perform more than one function including filtration, drainage, and separation. The properties of needle-punched nonwoven depend on parameters like fibre type, web aerial density, needle penetration depth, punch density (number of punches/cm²) and number of needling passages.

Hydroentanglement Technology

Spunbond and meltblown fabrics belong to the class of polymer-laid nonwovens. In spunbonding process, fluid polymer is converted into finished fabric by a series of continuous operations (Figure 1.23). Polymer is first extruded into filaments and then the filaments are attenuated. While the filaments are being attenuated, they remain under tension. After attenuation, the tension is released and the filaments are forwarded to a surface where the web is formed. The web is then subjected to the bonding process which can be done by chemical and/or thermal process. A binder may be incorporated in the spinning process or applied subsequently (e.g., a latex). Polypropylene and polyester are commonly used for spun-bonding process. Spun-bended nonwovens have high strength but lower flexibility.

Melt blown technology is unique in the sense that the web is very fine along with very fine pore sizes which no other nonwoven technology can match. The polymer is fed into the die tip and the resulting fibre is attenuated by hot air, which is blown near the die tip as shown in Figure 1.24. Air and fibre are expanded into the free air. Due to the mixture of high speed air and fibre with ambient air, the fibre bundle starts its movement forward and backward creating ‘form drag’. This form drag appears with every change of fibre direction. Therefore the meltblown fibres usually do not have a constant diameter. The fibres in the melt blown web are laid together by a combination of entanglement and cohesive sticking. The ability to form a web directly from a molten polymer without controlled stretching gives melt blown technology a distinct cost advantage over other systems.

Melt blown webs offer a wide range of product characteristics such as random Fibre orientation, very fine fibre and low to moderate web strength. The largest end-uses for monolithic melt blown materials are filtration media (surgical mask filters, liquid and gaseous filtration), surgical disposable gown, sterilization wrap, disposable absorbent products, oilsorbents. About 40% of meltblown material is used in the uncombined (monolithic) state. The laminated SMS structures (Spunbond-meltblown-spunbond) are ideal for gradient filtration as the material shows excellent barrier properties combined with mechanical strength. The filtration efficiency often is increased by applying electrostatic loads to the filament to improve filtration efficiency. With this standard meltblown technology fibres with diameter of 1-3 µm can be produced. Several filter applications using meltblown fibres are already in the market today.