Geotextiles thermal bonding

Olefin fiber is an important material for nonwovens (77). The geotextile market is stiU small, despite expectations that polypropylene is to be the principal fiber in such appHcations. Disposable nonwoven appHcations include hygienic coverstock, sanitary wipes, and medical roU goods. The two competing processes for the coverstock market are thermal-bonded carded staple and spunbond, both of which have displaced latex-bonded polyester because of improved strength, softness, and inertness. 

Nonwoven geotextiles. Nonwoven fabrics are defined as a sheet, web, or batt of directionally or randomly oriented fibres/filaments, bonded either by friction, and/or cohesion, and/or adhesion. In general, nonwoven fabric formation can be considered as a two-step process web formation (aligning the fibres with certain orientation characteristics) and bonding these fibres by mechanical, thermal, or chemical means. This two-step process has formed the classification of nonwoven structures, i.e. carded, airlaid, spunlaid, meltblown, needlepunched, hydroentangled, adhesive bonded, thermal bonded, stitch bonded, etc. Some of the important processes that are used for the manufacture of nonwoven geotextiles are discussed below. 

Table 8.2 Comparison of needle-punched and thermally bonded non-woven geotextile properties...

Although the above illustration is of needle-punched non-wovens, a similar trend with confining pressures can be reasoned for thermally bonded geotextiles. 

Nonwoven geotextiles are composed of directionally or randomly oriented fibers which are mechanically (needle punching), chemically, or thermally bonded into a loose web. 

The degree of fusion, line speed, and the engraved pattern on the roll affect the finished geotextile pore size distribution. The pore size distribution will also depend on the fibre type and the density of fibres in the geotextUes. Thermal bonded nonwovens are thinner and have greater stiffness. 

Both woven and nonwoven geotextiles are made from four polymeric materials (polyamide, polyester, polyethylene, and polypropylene) and natural materials. Woven geotextiles may be fabricated with monofilament yams, muldfilament yams, or a combination of each. Nonwoven geotextiles are bonded by one of the several methods thermal, mechanical, or chemical. With the many combinations of materials and processes, it is not difficult to see that the physical properties of geotextiles will vary greatiy among products (Yeo, 2008). 

Nonwoven geotextiles first form a loose web or layers of fibers or filaments, arranged in either an oriented or random pattern, prior to fixing them by bonding (by an adhesive or thermal fixation) and in this way a cohesive planar stmcture is developed. 

The third stage in this process consists of bonding the web into a geotextile possessing the physical and mechanical characteristics that approach those of the finished product. The bonding process can be done using mechanical, thermal or chemical means. These are the principle operations of this stage of the process ... 

After bonding of the geotextile using mechanical means, some thermal or chemical processes, as shown in Section 3.2.1, can be applied to give the product its final physical, mechanical and hydraulic properties. Once the product has attained its final characteristics, it will be inspected, cut and packaged. These procedures can be more or less automated, as was explained in the previous section. 

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