How are SMS nonwovens made?
Nonwovens manufacturing starts by the arrangement of fibres in a sheet or web. The fibres can be staple fibres or filaments extruded from molten polymer granules.
Illustrations of some of the methods used to form a web:
Spunmelt is a generic term describing the manufacturing of nonwoven webs directly from thermoplastic polymers. It encompasses 2 processes, spunlaid and meltblown, often run in combination.
Polymer granules are extruded into filaments through so called spinnerets. The continuous filaments are stretched and quenched before being deposited on conveyor belt to form a uniform web. The spunlaid process results into nonwovens with an increased strength compared to carding, due to the attenuation of the filaments. The downside is that the choice of raw materials is more restricted. Co-extrusion of two components leads to bico fibres, either adding more properties to the web or allowing air-through bonding. Please note that the word spunbonded is reserved for thermo bonded spunlaid.
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 high 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.
Ultrasonic bonding is a technology in which molecules of the fibres are being ‘excited’ under a patterned roller by high frequency movement of a ‘sonotrode’ which produces internal heating and softening of the fibres.
The opportunity to combine different raw materials and different technologies accounts for the diversity of the industry and its products.
This diversity is further enhanced by a range of finishing treatments. By finishing the nonwoven can be tailored or functionalized to meet specific properties. Finishing treatments can be either mechanical (stretching, perforating, crimping etc) or chemical. With the latter one can modify the surface of the fibres and the nonwoven to change the haptics or the repellency of the nonwoven.
Nonwovens can be made conductive, flame retardant, water repellent, porous, antistatic, breathable, absorbent and much more. They can also be coated, printed, flocked, dyed or laminated to other materials.
Nonwoven manufacturing ends usually with large rolls of product. Converters convert -as the word says- this roll good into a consumer product.
Sometimes converting is done in 2 steps. Before manufacturing the finished product one might want to bring the rolled good one step closer to the final product by slitting, cutting, folding, sewing or heat sealing.