Webs, other than spunlaid, have little strength in their unbonded form. The web must therefore be consolidated in some way. This is effected by bonding, a vital step in the production of nonwovens. The choice of method is at least as important to ultimate functional properties as the type of fibre in the web.
There are three basic types of bonding:
Chemical bonding mainly refers to the application of a liquid based bonding agent to the web. Three groups of materials are commonly used as binders-acrylate polymers and copolymers, styrene-butadiene copolymers and vinyl acetate ethylene copolymers. Water based binder systems are the most widely used but powdered adhesives, foam and in some cases organic solvent solutions are also found.
There are many ways of applying the binder. It can be applied uniformly by impregnating, coating or spraying or intermittently, as in print bonding. Print bonding is used when specific patterns are required and where it is necessary to have the majority of fibres free of binder for functional reasons.
This method uses the thermoplastic properties of certain synthetic fibres to form bonds under controlled heating. In some cases the web fibre itself can be used, but more often a low melt fibre or bicomponent fibre is introduced at the web formation stage to perform the binding function later in the process.
There are several thermal bonding systems in use:
Calendering uses heat and high pressure applied through rollers to weld the fibre webs together at speed.
Through-air thermal bonding makes bulkier products by the overall bonding of a web containing low melting fibres. This takes place in a carefully controlled hot air stream.
Drum and blanket systems apply pressure and heat to make products of average bulk.
Sonic bonding takes place when the molecules of the fibres held under a patterned roller are excited by high frequency energy which produces internal heating and softening of the fibres.
In mechanical bonding the strengthening of the web is achieved by inter-fibre friction as a result of the physical entanglement of the fibres.
There are two major types of mechanical bonding:
can be used on most fibre types. Specially designed needles are pushed and pulled through the web to entangle the fibres. Webs of different characteristics can be needled together to produce a gradation of properties difficult to achieve by other means.
is mainly applied to carded or wetlaid webs and uses fine, high pressure jets of water to cause the fibres to interlace. Hydroentanglement is sometimes known as spunlacing, as the arrangement of jets can give a wide variety of aesthetically pleasing effects. The water jet pressure used has a direct bearing on the strength of the web, but system design also plays a part.
Stichbonding is a third type of mechanical bonding. It can be done with or without the addition of a thread. When no thread is added, the process is often referred to a loop formation.