S.M. Desai Fineotex Chemical Limited 42, 43 Manorama Chambers; S. V. Road; Bandra (W) Mumbai 400 050. INDIA
FABRIC construction, consisting micro filament synthetic yarns are commonly referred to as MICROFIBRES in Europe & USA and SHIN GOSEN in Japan (meaning new generation synthetics).
Fabrics from these sources use a combination of: Conventional, fine and / or superfine filaments, filaments of widely varying shrinkage characteristics, very high and low twist yarns, variations in cross sectional shape to produce a very wide range of high density materials which have novel characteristics and properties which demand very specialized processing.
Effect fabrics available till date include:
- Imitation silks
- Peach skins
- Suedes (light to heavy weight)
- Air and moisture permeable water proof fabrics.
- Staple blends and yarn mixtures with viscose, cotton and
Popular outlets for those rapidly growing effect fabrics are:
- Apparel by virtue of very soft, silk-like handle
- Sportswear and improved absorbency leisure wear wicking
- and evaporation given by microfibers Furnishing and upholstery fabrics
- Lens wipes.
The fineness of a synthetic filament yarn is usually described in "Decitex" or "Denier". Decitex =The weight in grammes of 10,000 meters of yarn. Denier = The weight in grammes of 9000 meters of yarn. Hence a 167 decitex yarn can also be described as a 150 denier yarn. NOTE: For simplicity, only the decitex nomenclature will be used here. Decitex per filament (dtexpf) is described as the decitex of a yarn divided by the number of filaments within the yarn . 1/167f200 yarn. Thus, this yarn has dtexpf = 167/200= 0.835
Singles yarn decitex of yarn number of filaments within the yarn:
0.5-0.99 dtexpf-> Microfibre
0.5 dtexpf->Super micro
1.2 - 1.6 dtexpf-> Weight reduced polyester silk.
2.0 - 6.0 dtexpf-> Conventional filament.
A" Microfibre" is widely described as a yarn which contains filaments of less than 1 decitex per filament.
Microfibre production methods
1. Conjugate technology
Developed by Kanebo Ltd. Japan, involves spinning of bicomponent filaments comprising of a nylon 6 matrix with polyester fibrilles. Following the spinning and weaving stages, the fabric is subjected to a solvent swelling treatment. The poor cohesion in solvent of the polyester fibrilles and the nylon matrix causes the individual wedge- shaped polyester segments to move outwards. Each polyester segment then acts as an individual super-microfibre filament.
Other Conjugate developments: TORAY - Solvent or alkali swelling KURARAY - Solvent or alkali swelling
TEIJIN - Mechanical Brush / Crush
2. Sea Island Technology
Developed by TORAY IND of Japan, involves the spinning of "islands" of polyester in a "Sea" of polystyrene. Following the spinning and weaving stages, the polystyrene is dissolved out using chlorinated hydrocarbon solvents to leave polyester super-microfibre filaments.
3. Direct melt spinning
Single component filaments are extracted through spinnerettes. Using conventional melt spinning technology, it is possible to produce micro fibres down to 0.4 dtexpf. Development by ASAHI KASEI and UNITIKA of Japan, have enabled spun fibrils of 0.1 dtexpf to be produced.
The greater the surface area of micro fibres, requires a greater addition of lubricants in spinning etc. These lubricants must be removed in pretreatments. The very high surface area associated with closely packed micro fibre filaments means that a greater proportion of incident light is reflected when compared with larger diameter filaments. This results in a visually lighter appearance and necessitates higher percentages of dye needing to be applied to achieve the same relative visual strength as on coarser fabrics than on conventional fibres in the same shade.
The key effects achievable with micro fibres can be categorised as general and special effect.
- Softness from the low profile effect of the fibres
- Absorbency from the capillary action of fine fibres.
- Dimensional stability like normal polyester but unlike cotton.
- As microfibre in polyester/ cotton; compared to regular polyester/cotton - greater durability and softness.
- Use of less cotton for the same absorbency.
- More rapid cooling through moisture transport
- Excellent anti-creasing and stability properties.
- Special surface effects, such as "Peaching" and " Suedeing".
- Special feel like "Micropowder" or "Wet-feel”
- Water barrier effects, while permeable to vapour.
These effects lead to:
- Improved comfort and desirability.
- Texture effects those are durable and dimensionally stable.
Flow chart for microfibre processing:
To establish whether the size used on woven polyester is polyester or polyacrylate based:
Dip sample of gray fabric in solution of 0.5% C. I. Basic red 22 (eg. Astrazone Red F3BL). Wet out thoroughly. Take out fabric and leave for 10 secs. in air. Rinse with cold water. Light to dark red colour indicates pressure of polyacrylate size.
In addition to the removal of waxes and oils present on the fabric, the preparation conditions must be adapted to optimize removal of the particular size (identified as above).
The type of size present determines the pH of the preparation bath.
Polyester size - Usually pH 7-9 Polyacrylate size - pH 10-11
Desize, Scour / Relax:
Woven polyester fabrics will often contain very high twist, high shrinkage yarns incorporated into the fabric. Microfibre based fabrics have a high bulk density and a particularly large surface area.
An efficient preparation stage thus becomes vital to optimize removal of fibre processing aids whiles minimizing creasing and maintaining the desired aesthetics.
Optimising removal of processing aids
In addition to polyacrylate or polyester sizes, large amounts of oils and waxes are frequently applied to aid fibre processing and help protect delicate polyester microfiber fibrilles. Due to the very large surface area of microfiber based fabric, 2 to 5 times more fibre processing aids must be applied compared with conventional polyester based fabrics. To avoid problems with levelling and coverage of the disperse dyes applied in dyeing, the size and fibre processing aids must be efficiently removed.
Minimizing creasing and maintaining desired aesthetics
Relaxation of the fabric must be under minimum tension to allow the controlled shrinkage during scouring. Creasing and "crow-feet mark" will result if sufficient care is not taken at this stage. Shrinkage relaxation of microfibers starts at lower temperatures than conventional polyester filaments.
Problems with oligomer in dyeing polyester yarns and fabrics
Typically polyester fibres contain between 1.5 and 3.5 % by mass of low molecular weight esters, the principal oligomer being tris. (ethylene terephthalate) with smaller quantities of a dimer, pentamer as well as traces of other lower D.P compounds.
Problems caused by oligomer deposits.
- Spinning characteristics impaired.
- Reduced liquor- flow through package of yarn because spindle perforations become blocked and deposits on pump cause improper pump pressure.
- Presence can cause nucleation and growth of dye crystals or agglomeration of dye particles and hence dye spots, unlevelness and poor fastness.
- Deposits on machine guiders at winding or twisting cause high tensions and increased friction on the yarns, leading to poor package build and end-breaks.
- White powdery deposits cause dulling of the yarn/fabric especially in dark shades.
- Variation in rate of temperature rise due to deposits on heat exchanger.
- More often cleaning of dyeing and winding machines needed, hence increased down time and lower efficiency.
Cyclic trimers migrate from PET fibre during dyeing and steam setting and, to a lesser extent during dry setting. The amount of oligomer migrating to the fibre surface increases with increase in temperature and with prolonged dyeing time, therefore the liberation of oligomers during dyeing can be minimized by lowering the dyeing temperature from 130ºC to 120ºC and by using the shortest dyeing time.
Often polyester yarn and fabrics may be dyed with no preparation at all. However, when problems with oligomer deposits are being encountered, we have found that a scouring treatment before dyeing can remove oligomers from the outset. Customers routinely preparing woven polyester fabric on an open width scouring range, mainly to remove size, have also noted a marked reduction in oligomers present after scouring with Finocon 10X and Diquest SNA. (Both from Fineotex Chemical Ltd)
By now we have found that after dyeing, much more oligomer is migrated to the surface.
High temperature dyeing with Finocon DON (Fineotex Chemical Ltd) in the bath to disperse Oligomers:
Cyclic tris (ethylene terephthalate) is insoluble in water, solubility is increased in the presence of carriers, particularly of the methylsalicylate, alkyl phthalate and benzyl benzoate types, and such compounds, present in some levelling agents like Finocon DG New (Fineotex Chemical Ltd.) promote migration of the trimmer to the surface of the fibre, but a dispersing agent should be present to try to prevent crystallization and deposition of the oligomers. The longer the dyeing time, the more oligomers will be released. Dark shades will produce more oligomers than pastel shades because the dyeing time is longer.
Finocon DON is the dispersing agent, stable at high temperature, will do the above mentioned functions for disposing - off oligomers in dyeing. Many of the competitors dispersing agents are "NOT" high temperature stable. Thus when oligomers are released from the polyester fibre, after some time at 130ºC , there will not be any efficient dispersing agent in the bath to prevent crystallization of the oligomers, and prevent small particles of oligomers coming together to form larger agglomerates and depositing on the yarn and the machine.
Finocon DON does not magically remove the oligomers, but it will maintain them in fine dispersion, so that when the bath is drained more oligomers go down the drain. Draining the dye bath at high temperature, if possible, can also reduce oligomer deposition.
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