Manufacturing Process of Microfiber

Methods of Manufacturing Microfibers
Harshani Wijendra
Sri Lanka Institute of Textile & Apparel Technology (SLITA)
Email: harshani_bipasha@yahoo.com




a) Dissolved type
Microfibers of this type are manufactured from bi-component fibres (see below for details) with different types of polymers. Comparatively thick bi-component filaments containing different types of incompatible polymers are spun, and the fabric is made using them. When the fabric is treated chemically with solvent, one component is dissolved and removed, and the other component remains as the microfibre. Polyester and nylon microfibers can be made by this method. Commercial production has been reported to use 20/80% ratios of soluble/insoluble polymers to produce a bi-component filament of up to 2 dtex fineness, and a final dissolved filament with linear density of about 0.50 dtex. The main considerations for selection of suitable polymer component are:
  • High solubility;
  • Stability at extrusion temperature;
  • The polymers’ rheological properties should be compatible at extrusion temperature;
  • It should be recoverable for use, so that the cost of solvent and soluble polymer is affordable;
  • It should be non-toxic, non-corrosive, and non-polluting.
The various combinations of soluble/insoluble polymers reported to form fibres successfully are polystyrene/polyamide and polystyrene/polyester.

b) Split type

The microfibers of this type are obtained by physically or chemically treating the bi-component filaments containing two types of polymers and splitting them into different types of filaments. It is easier to split the Segment in filament from itself than in the fabrics. Suitable polymer combinations for splittable bi-component filament spinning are polyamides/polyester and polyester/polyolefines. The main considerations for selecting the polymer combinations are as follows:
  • The polymers must be incompatible;
  • The polymer should have reasonably similar melt at common extruder temperature;
  • The polymers should have weak adhesivity.
c) Direct spun type
This microfibre is directly manufactured by melt spinning. For this method, highly selected polymerization, polymer, spinning conditions, and drawing conditions are required. Special melt spinning dynamics should be considered for the production of low linear density polyester by the direct extrusion method. When polymers have similar dynamic viscosities at the given temperature, the polymer with the lower dynamic viscosity permits the spinning of finer fibres. This result has been attributed to the lower spin-line tension generated when spinning polymers with lower dynamic viscosity. It is postulated that the important parameter in the production of finer PET fibre is the spin-line tension level, which must be kept low in order to obtain finer fibres. The increase in take-up velocity and the fibre line length between the spinnered and the take-up device increase the spin-line stress level, and therefore the minimum fineness attainable increases. There are the following areas where microfilament spinning requires care:
  • The die-swell formation must be minimized by using the lowest possible melt viscosity at the capillary entrance, and by limitation of the horizontal component of the elastic melt expansion spinnerets’ exit. 
  • In the solidification phase, the quenching air must not stress the filament. Therefore, the hole distance and the arrangement of the spinneret holes must allow the turbulence-free quenching air stream to penetrate the whole filament bundle. 
  • Processing all filaments equally in order to obtain evenness in density. 
  • After solidification, the individual capillaries should be guided side by side in one layer. Due to the large number of fine capillaries which forma microfilament yarn, the stationary guides become problematic, and crossing of capillaries may result in deviations in linear density and drawing failures.
(d) Super-drawing technique
In this technique, no molecular orientation is involved. Staple fibre with linear density less than 0.5 dtex can be produced with high drawing ratios. This technique is based on the principle that yarn can be stretched as much as 10-75 times; much beyond their conventional draw ratios (3-6 times) if the drawing is carried out at a minimum crystallizing temperature and at special selected drawing conditions, including the temperature range and the type of heating the fibre.

(e) Sheath-core spinning method.
In this method, two different polymers are mixed, melted, and mix-annealed under specified conditions. The conjugate fibre comprising of a concentric circular sheath and a core is manufactured, and the sheath portion is removed to form ultra-fine fibres.

(f) Some other methods;
  1. Flash-spinning method
  2. Solution flash-spinning
  3. Emulsion-spinning method
  4. Jet-spinning method 5. Centrifugal-spinning method
  5. Turbulent forming method
  6. Conjugate-spinning method
Fibers for Microfiber:
Bi-component fibers; Are co-extruded with two different polymers in the cross-section. This allows the fibre to use the properties of both materials, and vastly expands the array of possible fibre performance characteristics. Bi-component fibres are available in staple, filament, and microfibre forms. Typical bi-component fibre types are presented in Figure 1.
Bi-component fibres
Fig 1: Bi-component fibers
Specialty Cross-Sections
A modified cross-section can provide added functionality, such as unique luster or moisture transport. These cross-sections are available in staple, filament and microfibre forms, and in most cases, are also available as bi-component fibres. Typical microfibers with special cross-section shape, commonly used, are presented in Figure 2; many applications find especially the hollow fibres.
Fig 2: Specialty Cross-Sections
Melt spinning process diagram
Processing of Microfibers

Carding
It has so far proved impossible to card microfibers at production rates which are comparable with conventional type of fibres, and so the cost per unit weight of production is much greater. In carding it is density of carding wire points.

Winding and warping
All guide surfaces must be very smooth and in the best mechanical condition, as microfilaments are likely to break more easily than regular filament. The frictionless rotating discs type of tension devices are desired to minimize the drug.

Sizing
Warp sizing of microfibres should ideally be done on single-end sizing machines to minimize filament breakage at splitting rods. If single-end sizing is not available, then a pre-dryer is essential. The size pick up on microfibre yarn is higher and it is also more desirable. The size recipe should be decided by trial and error determining.

Weaving
Generally the tensions should be kept as low as possible. Weft yarn for air jet or water jet looms will need some finish to perform at maximum efficiency.

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