Nonwovens

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Felt
Felt

Nonwovens are textiles which are neither woven nor knit, such as felt. General use hyphenates the word, but industrial use spells it as one word. Non-wovens are typically not strong (unless reinforced by a backing or densified). In recent years, non-woven material has become an alternative to polyurethane foam.

Non-woven fabric is 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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[edit] 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:

[edit] Hygiene

[edit] Medical

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

[edit] Filters

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

[edit] Geotextiles

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

[edit] Other

[edit] Manufacturing processes

[edit] Staple non-wovens

Staple non-wovens are made in two 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.

[edit] Spunlaid non-wovens

Spunlaid non-wovens 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 machinery[1]. 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.

[edit] 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.

[edit] Bonding

Both staple and spunlaid non-wovens 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

[edit] References

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

[edit] External links