Nonwovens
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nonwovens".

Felt: a non-woven cloth

Nonwovens is the term used in the textile manufacturing industry to denote fabrics, such as felt, which are neither woven nor knitted. Non-woven materials typically lack strength unless densified or reinforced by a backing. In recent years, nonwovens have become an alternative to polyurethane foam.

Nonwovens are typically manufactured by putting small fibers together in the form of a sheet or web, and then binding them either mechanically (as in the case of felt, by interlocking them with serrated needles such that the inter-fiber friction results in a stronger fabric), with an adhesive, or thermally (by applying binder (in the form of powder, paste, or polymer melt) and melting the binder onto the web by increasing temperature).

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Applications

Non-woven materials are nowadays mainly produced from man-made fibers. Two synthetic polymers dominate the market: polypropylene (PP) and polyesters (mainly PET). Nonwovens are often application-designated as either durable or disposable. For example, nonwovens used as housewraps to prevent water infiltration are durable nonwovens. Nonwovens used as facings on baby diapers are disposable or single-use nonwovens. Horticultural applications include both frost and insect protection.

Non-woven materials are used in numerous applications, including:

Hygiene

Medical

  • isolation gowns
  • surgical gowns
  • surgical drapes and covers
  • surgical scrub suits
  • caps

Filters

  • gasoline, oil and air - including HEPA filtration
  • water, coffee, tea bags

Geotextiles

  • soil stabilizers and roadway underlayment
  • frost protection
  • agriculture mulch
  • pond and canal water barriers
  • sand infiltration barrier for drainage tile

Other

Manufacturing processes

Staple nonwovens

Staple nonwovens are made in 2 steps. Fibers are first spun, cut to a few centimeters length, and put into bales. These bales are then dispersed on a conveyor belt, and the fibers are spread in a uniform web by a wetlaid process or by carding. Wetlaid operations typically use 1/4" to 3/4" long fibers, but sometimes longer if the fiber is stiff or thick. Carding operations typically use ~1.5" long fibers. Rayon used to be a common fiber in nonwovens, now greatly replaced by PET and PP. Fiberglass is wetlaid into mats for use in roofing and shingles. Synthetic fiber blends are wetlaid along with cellulose for single-use fabrics. Staple nonwovens are bonded by using either resin or thermally. Bonding can be throughout the web by resin saturation or overall thermal bonding or in a distinct pattern via resin printing or thermal spot bonding. Coforming with staple fibers usually refers to a combination with meltblown, often used in high-end textile insulations. Melt Blown non wovens are produced by extruding melted polymer fibers through a spin net or die consisting of up to 40 holes per inch to form long thin fibers which are stretched and cooled by passing hot air over the fibers as they fall from the die.The resultant web is collected into rolls and subsequently converted to finished products.The extremely fine fibers typically polypropylene differ from other extrusions paticularly spun bond in that they have low intrinsic strength but much smaller size offering key properties.Often melt blown is added to spun bond to form SM or SMS webs, which are strong and offer the intrinsic benefits of fine fibers such as fine filtration, low pressure drop as used in face masks or filters and physical benefits such as acoustic insulation as used in dishwashers.

Spunlaid nonwovens

Spunlaid nonwovens are made in one continuous process. Fibers are spun and then directly dispersed into a web by deflectors or can be directed with air streams. This technique leads to faster belt speeds, and cheaper costs. Several variants of this concept are available, but the leading technology is the REICOFIL machinery1. PP spunbonds run faster and at lower temperatures than PET spunbonds, mostly due to the difference in melting points. Spunbond has been combined with meltblown nonwovens, coforming them into a layered product called SMS (spun-melt-spun). Meltblown nonwovens have extremely fine fiber diameters but are not strong fabrics. SMS fabrics, made completely from PP are water-repellent and fine enough to serve as disposable fabrics. Meltblown is often used as filter media, being able to capture very fine particles. Spunlaid is bonded by either resin or thermally.

Other

Nonwovens can also start with films and fibrillate, serrate or vacuum-form them with patterned holes. Fiberglass nonwovens are of two basic types. Wet laid mat or "glass tissue" use wet-chopped, heavy denier fibers in the 6 to 20 micrometre diameter range. Flame attenuated mats or "batts" use discontinuous fine denier fibers in the 0.1 to 6 range. The latter is similar, though run at much higher temperatures, to meltblown thermoplastic nonwovens. Wet laid mat is almost always wet resin bonded with a curtain coater, while batts are usually spray bonded with wet or dry resin. An unusual process produces polyethylene fibrils in a Freon-like fluid, forming them into a paper-like product and then calendering them to create Tyvek.

Bonding

Both staple and spunlaid nonwovens would have no mechanical resistance, per se, without the bonding step. Several methods can be used:

  • thermal bonding
    • using a large oven for curing
    • calendering through heated rollers (called spunbond when combined with spunlaid), calenders can be smooth faced for an overall bond or patterned for a softer, more tear resistant bond
  • hydro-entanglement: mechanical intertwining of fibers by water jets (called spunlace)
  • ultrasonic pattern bonding, often used in high-loft or fabric insulation/quilts/bedding
  • needlefelt: mechanical intertwining of fibers by needles
  • chemical bonding (wetlaid process): use of binders (such as latex emulsion or solution polymers) to chemically join the fibers. A more expensive route uses binder fibers or powders that soften and melt to hold other non-melting fibers together
  • one type of cotton staple nonwoven is treated with sodium hydroxide to shrink bond the mat, the caustic causes the cellulose-based fibers to curl and shrink around one another as the bonding technique
  • meltblown is very weakly bonded from the air attenuated fibers intertangling with themselves during web formation as well as the temporary tackiness when they are forming
  • one unusual polyamide spunbond (Cerex) is self-bonded with gas-phase acid

References

  1. ^ manufactured by Reifenhäuser REICOFIL GmbH & Co. KG (Germany)

External links
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