5. Fibres

1
5. Fibres

• The fibre industry can be divided
• into natural and synthetic sources,
• the latter divided into cellulosics
• and non-cellulosics.
• Natural includes proteins that
• constitute silk and wool, and
• vegetable products such as
• cotton.
• Cellulosics cellulose-derived
• materials produced through the
• treatment of wood pulp.
• Non-cellulosics synthetic, fibre-forming
  polymers.

1993 US Production Of Synthetic Fibres Billions
of Pounds Cellulosics Rayon
0.28 Acetate
0.23 Noncellulosics Polyester
3.56 Nylon 2.66 Olefin
2.14 Acrylic 0.43
TOTAL 9.30 Chemical and
Engineering News, April 11, 1994.
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Natural Fibrous Materials

• The aesthetic properties afforded by natural
  products (drape, hand, moisture regain) are
  difficult to duplicate due to the complexity of
  fibre structure.
• Silk A protein derived of glycine and alanine
  amino acids
• produced by silkworms as a solidified viscous
  liquid.
• Wool Complex fibrous structure whose polymeric
  components are
• proteins, but are made up of corticle and
  cuticle cells
• separated by a membrane.
• CottonWhile essentially 95 cellulose (same as
  rayon), cotton is
• a microfibrous material with pores, channels
  and twists that
• result in a unique feel/hand.

3
Fibre Properties General

• The most general requirement of a fibre is a
  length-to-diameter ratio of at least 1001.
• Staple generally 2-6 cm in length, used yarn
  manufacture through spinning.
• Continuous essentially unending fibres formed by
  
• commercial production of synthetic
  materials
• Mechanical Properties
• High tensile strength, pliability and abrasion
  resistance
• 200C apparel and melt processing
• Chemical Properties
• Acceptance of Dyes/pigments
• Appropriate moisture retention to the application

4
Textile Fibre Properties
5
Textile Fibre Properties Moisture Regain

• Response of a material to water determines in
  large part the comfort of a garment. Below is
  the weight percent of moisture gained by a
  previously dried sample when exposed to different
  relative humidity at 20C.

6
Fibre Synthesis Spinning

• Spinning is the process through which
• bulk polymers are processed into
• a thread and/or yarn. Three options
• are used, depending on material
• properties.
• Melt Spinning Thermally stable materials are
  extruded at high temperature through a spinneret,
  passing through a cooling air stream.
• Wet Spinning Polymer solution is extruded into a
• bath of non-solvent, causing precipitation of the
  polymer into a fibre.
• Dry Spinning Solution is extruded into an air
  bath, wherein solvent is flashed to generate a
  fibre.

7
Fibre Synthesis Cold Drawing

• Successive stages in the drawing of a polymer,
  showing the necking down and subsequent neck
  growth resulting in increased chain alignment.

A C D
8
Fibre Synthesis Cold Drawing and Crystallinity

• Fibre drawing results in extensive changes to the
  organization of polymer chains.
• Sperulites are broken apart, and the number of
  chain folds decreases
• crystallization is enhanced, and the material
  becomes anisotropic
• Questions remain
• concerning whether
• crystals melt and reform
• or if crystals themselves
• rotate.

9
Fibre Synthesis Draw Ratio and Chain Orientation

• Drawing induces the orientation of crystallites
  within semi-crystalline materials as well as the
  chain segments located in the amorphous phase.
• The amorphous phase of a fibre will elongate
  under an applied load, making the orientation of
  this component critical to efforts to improve
  modulus.
• Poly(acrylonitrile) is spun to create acrylic
  fibres whose strength is derived from strong
  dipole association encouraged by drawing, not
  crystallinity.
• Shown here is the transformation of high density
  polyethylene during solid- state extrusion. Note
  the change in crystallite morphology as the draw
  ratio (DR) is increased.

10
Fibre Synthesis Draw Ratio and Mechanical
Properties

• Effect of draw temperature on the
• maximum draw ratio attainable
• (solid line) for high molecular
• weight (Mw8 x I05) linear
• poly(ethylene).
• Also shown (broken line) the
• room temperature modulus of
• samples drawn to the maximum
• draw ratio at each temperature.
• Note that higher draw
• temperatures increase the
• attainable draw ratio, but reduce
• the extent of chain alignment
• as the melting point of the resin
• is approached.

11
Fibre Synthesis Ultra-high Modulus Fibres

• Polymers crystallized from the melt have, in
  general, a random orientation of chains at the
  macroscopic level.
• While chain segments within crystals are
  oriented, the multiple crystallites in the
  material are not.
• Polymers with rigid, rod-like
• backbones show a pronounced
• tendency towards ordering
• in solution (lyotropic) or in
• a melt state (thermotropic).
• The nematic liquid-crystalline
• state is most common.
• Examples include
• Vectra A (thermotropic) Kevlar (lyotropic)

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Fibre-Forming Polymers

• Fibre-forming polymers must support strong
  interchain association and close packing when
  drawn into an oriented condition.
• Usually linear, symmetric materials of
  10,000-15000 g/mole
• Physical properties are optimized from
  measurements of tenacity, toughness, initial
  modulus and permanent set.

13
Fibre-Forming Polymers Cellulosics

• Wood pulp and cotton remnants are processed to
  yield cellulose-based fibres, rayon and cellulose
  acetate.
• Cellulose is strongly H-bonded and highly
  crystalline.
• Insoluble and infusible
• Rayon is solubilized cellulose that
• is wet spun to generate a clean fibre.
• Valued for high tenacity, it has largely
• been replaced by synthetic fibres.
• Cellulose acetate is a acetylated derivative of
  rayon, valued for its whiteness before dyeing,
  attractive appearance.
• Apparel use limited due to poor
• abrasion resistance
• Used widely in cigarette filters

14
Fibre-Forming Polymers Polyesters

• Polyester fibres are melt-spun, and drawn at
  elevated temperatures due to their relatively
  high Tg
• drawing develops orientation of the amorphous
  phase, and induces crystallinity in the rather
  slow-crystallizing material.
• Highest tonnage polyester is
• poly(ethylene terephthalate),
• with Tg69C, Tm265C and
• a molecular weight in the
• 10,000 g/mole range.
• Polyesters are widely used in apparel, tire cord,
  home furnishings.
• Solvent, abrasion and oxidative resistance is
  high.
• Tend to be difficult to dye, requiring
  copolymerization to promote effective
  colouration.

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Fibre-Forming PolymersPolyamides and Aramids

• Aliphatic polyamides are nylons, while polyamides
  with high aromatic content are commonly referred
  to as aramids.
• Valued for abrasion resistance, tenacity
• Used widely in carpets and apparel
• Aramids form ultra-high modulus fibres used in
  specialty applications including
• advanced composites,
• tire cords.
• Kevlar, shown below, will
• not melt but only
• decompose at 400C.
• Its modulus is greater
• than that of steel,
• yet it is 6 times lighter.

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Fibre-Forming Polymers Acrylics and Polyolefins

• Fibres based on poly(acrylonitrile) are produced
  by wet spinning due to thermal instability.
• Polarity of the nitrile (R-C?N) group provides
  strong intermolecular bonding, especially when
  drawn
• Products are valued for their wool-like
  characteristics and UV stability, while abrasion
  and tenacity are inferior to nylon.
• Copolymerization improves dye characteristics
• isotactic-Poly(propylene) can be drawn to
  generate a cheap fibre of good tenacity, and
  weathering characteristics when stabilized.
• Used in rope, fishnets (low density), as well as
  indoor-outdoor carpeting.
• Negligible water uptake and low Tm limit use in
  apparel.