In spun yarn applications, microdenier fibres can be used to enhance the quality and process performance of the yarn, and the functional and aesthetic properties of the fabrics made from these yarns, affirm Mohamed F Elkady and Al Said A Elmetwally.

Consumer tastes in the clothing sector have been changed radically over the past two decades. There has been a trend towards finer synthetic filament fibres, and consequently various microfibres have been developed with novel fibre spinning techniques to reduce thickness and alter the cross section shape. Microfibre fabrics have enhanced drapeability, luster, softness, bulkiness, and smoothness, and also high tactile aesthetics and high water absorption and chemical adsorption properties.

Definition of microfibres

In a broad sense, especially in Europe, the term microfibre means fine fibres of less than 1 denier. However, in Japan, where fibre technology is more advanced, fine fibres of 0.04 - 0.4 denier class are generally used in this filament area⁽¹⁾. Most microfibre is made of polyester; however, other polymers are used, including nylon, polyacrylonitrile, polypropylene, cellulose, acetate, and rayon. Mixtures of polymers such as polyester-nylon and polyester-polypropylene are also used ⁽²⁾.

As shown in Figure 1, comparatively, microfibres are ten times finer than silk, up to thirty times finer than cotton, forty times finer than wool, and one hundred times finer than a human hair⁽³⁾. Generally, fibre producer use such term as: >7.0 dtex for coarse fibres; 7.0 to 2.4 dtex for medium fine and normal fibres; 2.4 to 1.0 dtex for fine fibres; 1.0 to 0.3 dtex for microfibre; and < 0.3 dtex for super, ultrafine microfibres. Table 1 compares the fineness of different textile fibres⁽⁴⁾.

Microfibres less than half size of the finest silk are now available commercially and furthermore microfibres as small as 0.001 dpf are produced by Toray of Japan ⁽⁵⁾.

Production methods of microfibre

The production of polyester microfibres started in Japan in the 1970s by size reduction of the fibres in polyester fabrics, using hot alkaline treatment to dissolve their surface polymer. Polyester fabrics were subjected to hot alkaline treatment to remove up to 40 - 50%, 20 - 30% of the fabric weight. The treatment resulted in smaller diameter fibres and a fabric "with an increased liveliness", drape, and covering power, plus a warm, dry, silk-like hand with increased Scroop ^(2,8). In the United States similar treatment of polyester fabric is carried out with lower weight loss typically 8 - 15% ⁽⁶⁾. Rame⁽⁷⁾ has studied the loss in the weight of polyester fibres in alkali and found that it is linear from 15 to 95 Co at 10% concentration of alkali. He found that silk like properties can be imparted to polyester with a weight reduction of 8 - 12%.

Two methods can be employed to produce microfibres: conventional direct spinning and conjugate spinning. With direct spinning, single component filaments are extruded through spinnerets to give highly uniform microfibres. Although all three conventional spinning methods (melt – wet - dry) can be employed and polyester, polyamide, and acrylic microfibres can be obtained, the direct spinning of microfibres is a complex process and involves the modification of spinning methods used to produce conventional denier fibres⁽¹⁰⁾. Generally, there is a limit to the fineness of fibres produced using direct spinning, as exemplified by polyester that cannot be extruded at less than about 0.15 g/min because of breakage of the filaments. Typically, microfibres in the range of 0.2 - 0.9 denier are secured using conventional spinning methods, although filaments of 0.1 denier have been reported⁽⁹⁾.

For conjugate spinning, two methods have been devised, separation and dissolution. The separation technique involves the spinning of a bicomponent filament, typically one comprising nylon 6 and polyester, although polyolefin/polyester and polyolefin/polyamide bicomponent have been developed ⁽⁹⁾. After being woven, the fabric is exposed to solvent/alkali swelling or thermal/mechanical treatment such that the two immiscible components separate, resulting in individual polyamide/polyester microfilaments.

In the dissolution technique, two polymers are extruded through a suitable spinneret to produce a bicomponent filament that comprises either several individuals “islands” of one component embedded in a “sea” of the other component (the sea-island type, Figure 2) or several sectors of one component embedded in a radial type of the other component (the radial type, Figure 3). After weaving or knitting, one of the components is removed by dissolution in a solvent thereby producing microfilaments⁽¹⁾.

With the sea island method, when the sea is dissolved with a certain solvent, the polymer that is the island remains and forms the finest fibres. The number of islands, the ratio of island components to sea components, and the cross-sectional shape of the resulting microfilaments can be varied, as this method is capable of producing super micro polyester filaments of 0.1*10-4 denier as well as hollow microfibres ⁽¹⁾.

Kiang and Cuculo ⁽¹¹⁾ reported melt spinning dynamics for the production of low linear density polyester by the direct extrusion method. When polymers have similar shear viscosities, the polymer with the lower elongational viscosity permits spinning of finer fibres. This result has been attributed to lower spinline tension generated when spinning lower elongational viscosity polymers. It is postulated that the important parameter in the production of finer PET is the spinline tension level, which must be kept low in order to obtain finer fibres. The increase of take up velocity and the thread line length increase the spinline stress level, and therefore the minimum fineness attainable increases.

Figure 4 shows the structure of the spinnerets, which are used to produce microfibre by the island in the sea type. The microfibre fineness can be defined by the following equation ⁽¹²⁾:

d = dL * (R/100) / N
where,
d = microfibre fineness.
dL = extruded fibre fineness.
R = “island” content.
N = number (of “islands” per filament).

The smaller dL and R, and the greater N, the finer the microfibre produced. In practice, fine microfibres can be easily and economically produced by high “island” content. Microfibres in the 0.1 to 0.01 dtex range can be successfully produced on an industrial scale with this system. Toray Industries is the main producer of this fibre type.

Quality of the chips for microfibre

For production of microfibres, demands on polymer quality are very stringent. Polymer quality has to be consistent and it should not contain much of foreign matters and large size particles. The molecular weight distribution should also be narrow and short-term viscosity must be kept as consistent as possible. Polymer chips need to be smooth, tailless and identical in shape/size. As microfibres are high value product, the use of “off quality” chips should be avoided, which otherwise they would cause undue problems in the process⁽¹³⁾. It is advisable to use the best quality chips available for the production of microfibres.

It is also important to maintain the quality of the product in all subsequent steps. The drying process should be as consistent as possible and chips must have even residence time. The moisture regain of the dried chips should be less than 0.005% to minimize the hydrolytic degradation in molten stage. Thus, precise moisture level in the dried chips is very critical for trouble free production of microfibres. For quality chips drying, continuous drying process is better than batch drying⁽¹⁴⁾.

Processing of microfibres

In spun yarn applications, microdenier fibres can be used to enhance the quality and process performance of the yarn, and the functional and aesthetic properties of the fabrics made from these yarns ⁽¹⁵⁾. Because of the delicate nature of the fibres, processing difficulties are likely to occur due to fibre damage and nep formation at carding ⁽¹⁶⁾. Also the increase in number of fibres in the cross-section results in higher frictional forces, which can cause problems during drafting ⁽¹⁵⁾.

Viswanath⁽¹⁷⁾ studied the quality of micropolyester-cotton blended yarns at different processing stages. He stated that it is possible to produce quality-blended yarns using microdenier polyester without any serious processing difficulties. In carding the micropolyester fibre, the interaction of cylinder speed and wire points has a more pronounced effect than their individual contributions.

Texturing of microfibre yarns

The developments in texturing machine for microfibre polyester have been reported in literature. Generally, microfibres are being textured to overcome deficiencies in bulk and handle associated with flat filament fabrics. Texturing microfibre yarns is a challenge because of the many problems faced during the process ⁽¹⁸⁾. Bruske ⁽¹⁹⁾ remarked that the texturing of microfibre requires good uniformity of the textile material; the machine profile must not damage the textile and the surface must present low coefficients of friction. Depeuble ⁽²⁰⁾ suggested the use polyurethane friction discs for microfibre texturing. The high temperature shorter heaters are preferred for microfilament texturing ⁽²¹⁾. Foster et al ⁽²²⁾ have also reported the advantages of using shorter high intensity heater in false-twist texturing.

Phipps ⁽²³⁾ observed that during draw-texturing of microfibre yarn, the yarn tensions before and after the spindle increases with the decrease in linear density per filament. Franz⁽²⁴⁾ reported the false-twist texturing and air-jet texturing of microfibre polyester yarn. Grag⁽²⁵⁾ reported that the crimp contraction is less for microfibre yarn, if the same setting temperatures are used as for normal yarns. It was also suggested that polyurethane friction discs should be preferred.

Dupeuble⁽²⁶⁾ recommended ceramic yarn guides to reduce friction and maintain constant tension in case of microfibre polyester. Anhara et al⁽²⁷⁾ studied the filament migration during false-twist texturing of both POY and fully drawn yarns through measuring yarn tension. Bruske ⁽²⁸⁾ observed that if the texturing machines as well as the process parameters are tailored to special properties, textured microfibre yarns of high quality can be produced. Pal⁽¹⁸⁾ studied the effect of different texturing variables on the properties of textured microfibre polyester yarn. It was found that first heater temperature and draw ratio have a significant impact on tensile, crimp, and dyeing properties of the microfibre yarn.

Dyeing properties

It is well-known that as the diameter of the individual fibrils becomes smaller, or as the denier decreases, the total surface area of the fibre increases exponentially. This results in a paler appearance for microfibres than for higher denier fibres when both fibre types are dye with same amount of dye ⁽²⁹⁾. A greater amount of dye is needed for microfibres to reach the same depth of shade as higher denier fibres ⁽³⁰⁾.

The increased total surface area of the microfibre also affects the fastness performance after heat setting. Unfixed dye on the surface of the fibre at the end of the dyeing process reduces the wet fastness. Even though the unfixed dye can be completely removed from the fibre surface, a through reduction clearing treatment problems may occur with subsequent finishing treatment⁽³¹⁾. The larger amount of dye needed for microfibres results in greater migration to the fibre surface by thermomigration than occurs with conventional polyester, leading to lower wash fastness.

The following precautions should therefore be taken when dyeing microfibres⁽³²⁾:

• Dyeing machines should be set at a higher fabric speed (250 m/min at least).
• Ensure that the fabric rope is plaited down evenly.
• Add rope crease inhibitor to prevent creasing.
• Prevent the fabric from sticking during the dyeing process.
• Reduce the standard temperature (approximately 60 - 70°C).
• Optimum dyestuff selection (identical affinity).

It was reported that the small radius of microfibres causes many problems with the dyeing process and colour fastness of dyed fabrics ⁽³³⁾. The effect of a stagnant solution layer surrounding the fibre on the dyeing rate is greater when dyeing microfibres ^{(37, 38)}. Therefore, an even flow rate of dye liquor through the fibre assemblies is apt to cause uneven dyeing. It was noticed that the visual shade depth of textiles decreases with decreasing fibre radius ⁽³⁵⁾. Studies on the relation between the amount of dye on the fibre and the visual colour depth and the end-use fastness properties of the microfibres have been published^{(34, 36)}. It was shown that microfibres dyed with disperse dye attain sorption equilibrium in a short time, even blow 100°C, which has enabled the researchers to determine the sorption isotherms and the kinetic data of dyeing very accurately⁽³³⁾.

Dye consumption for dyeing fibres of different fineness is shown in Table 2. The percentage dye consumption figures in this table were calculated using the fothergill rule⁽³⁶⁾. A level of 1.0% disperse dye on a 4 dtex per filament polyester fibre was used as a control and the subsequent dye percentages were calculated to give an equal visual depth of shade on the fibre filaments.

Properties of microfibres and microfibre fabrics

Aesthetically, microfibre has a very soft and luxurious hand, with a silken or suede touch, as well as excellent drapeability. From a performance point of view, microfibre is easy to care for, strong, durable, wrinkle resistant, shrink resistant, water repellent, and wind resistant⁽³⁹⁾.

Microfibre fabrics are generally lightweight, resilient or resist wrinkling, have a luxurious drape and body, retain shape, and resist pilling ^{(40, 41)}. Also, they are relatively strong and durable in relation to other fabrics of similar weight⁽⁴²⁾.

Because microfibres are so fine, many fibres can be packed together very tightly. Thus, the fineness of the microfibres produces so dense a fabric structure that the garment are wind and waterproof but also air and water vapour permeable at the same time⁽³²⁾.

Microfibres end-uses

The use of microfibres is now well-established in many apparel markets, as well as in other outlets. microfibres are used either as single-fibre fabrics or in conjunction with coarser synthetic or natural fibres, providing fabrics that have enhanced drapeability, luster, softness, smoothness, and, in many cases, novel tactile and visual aesthetics ^{(43, 44, 45)}.

Microfibres are used in various applications, for example, in high-grade woven and knitted fabrics, such as towels and typewriter ribbons. Wiping clothes, filter clothes, and clean-room garment utilise the large fibre surface^{(46, 47)}. They are also used as moisture-permeable, water-proof, and water-repellent high density woven fabrics. In these end uses, a suitable combination of the fibre assembly structure and the fibre material is important in realising excellent performance⁽⁴⁸⁾.

Trade names of microfibres

Many fibre companies use trade names to identify their microfibre products. A few examples include ^{(48, 49)}:

• Trevira Finess (polyester).
• Fortel Microspun (polyester).
• Shingosen (polyester).
• Tactle Micro (nylon).
• Silky Touch (nylon).
• Microsupreme (acrylic).

Fabric manufacturers also use trade names for microfibre fabrics. They include:

Logantex:

* Charisma-dress weight with suede-like finish.

* Ultima-water repellent finish

Thompson of California:

* Moonstruck-soft finish, silk-like

* Micromist-brushed finish

* Reghal-dry hand

Springs Mills:

* Silkmore-sandwashed silk finish

* Stanza-water repellent microtwill

References

1. Jooneok K: Alkaline Dissolution Montoring of Radial-Type Polyester Microfibre Fabrics by a Cationic Dye-Staining Method; Journal of Applied Polymer Science, Vol 99, 279-285, 2006.

2. Hua Song: Fibre Splitting of Bicomponent melt Blown microfibre Nonwoven by Chemical and Water Treatment; Msc, University of Tennessee, Knoxvlle, August, 2002.

3. http:// www.The-cloth.com/what is.htm

4. Gunter Jerg, and Josef Baumann: Polyester Microfibres: A New Generation of Fabrics; Textile Chemist and Colorist, Vol 22, No 12, 1990.

5. Ohmura K: Microfibres in Nonwoven Production; Nonwoven Industry, pp 58-60, 1992.

6. Goldestine H B: Mechanical and Chemical Finishing of Microfibres; Textile Chemist Colorist, 25 (2), 1993, pp 16-21.

7. Rame S S: Imparting Silk-like Properties to Polyester Fibres; Manmade Textiles India, 35(7) pp 251-253, 1992.

8. Herman B Goldstein: Mechanical and Chemical Finishing of Microfabrics; Textile Chemist and Colorist Vol 25, No 2, February 1993.

9. Burkinshaw S M: Chemical Principles of Synthetic fibre Dyeing; Chapman & Hall: Glasgow, Scotland, 1995.

10. Anonymous, Japan Textile News, 1992,83,81.

11. C T Kiang and J A cuculo: J Appl Polym Sci 1992, 46, 55.

12. Jurg Rupp and Akira Yonenaga: Microfibres the New Man-made Fibre Image, International Textile Bulletin, 4/2000.

13. V Garg: Man-made Textile in Indian; February 1994, 37, 63.

14. Ghandhi R S and Pal S K: Microfibre Polyester; The Indian Textile Journal; February, 1996.

15. Bela Von Falki: Production and Properties of Microfibres and Microfilaments, The Indian Textile Journal, Feb 1991, pp 62-70.

16. P Artzt Ch and Abt H Maidel: Carding Fine Denier Polyester Fibre, Textile Praxis International, Sept 1984, pp 869-876.

17. Viswanath B A and Tasnim S: Micropolyester-cotton Blend, The Indian Textile Journal, March 1998.

18. S K Pal, R S Gandhi and V K Kothari: Draw Texturing of Microfibre Polyester Yarn, Textile Research Journal, 66, (12), 770-776, 1996.

19. J F Bruske: Text. Asia, 1992, 7, No 11, 53.

20. J C Dupeuble, Chemiefaserm/Textilindustrie, 1990, 40/92, No 10, E-108.

21. J C Dupeuble, Textile World, 1994, 144, No 2, 82.

22. P W Foster, S K Mukhopadhyay, and K Greenwood: J Text Inst; 1992, 83, 414.

23. J Phipps: Textile Month, September 1991, 112.

24. G Franz; Chemiefaserm/Textilindustrie, 1992, 42/94, No 10, 802.

25. V Grag, Man Made Textile In Indian 1994, 37, 102.

26. J C Dupeuble: Industrie Text, 1992, 1236, No 10, 45.

27. M Anhara and T Fujita; J Text Masch Soc Japan, 1981, 34, 120.

28. J F Bruske; Chemiefaserm/Textilindustrie, 1992, 42/94, No 10, 797.

29. Kunihiko Imada: Wetfastness of Disperse Dyes on Polyester Microfibre, Textile Chemist and Colorist, Vol 29, No 11, 1997.

30. Wiegner D: Chf (1991), 2, 148.

31. Richter P and B Willi: Melliand Textilberichte, Vol 65, 1984, pp 692.

32. Hilden J: The Effect of Fibre Properties on the Dyeing of Microfibres; ITB, Dyeing/Printing/Finishing 3/91.

33. T Nakamora: Dyeing Properties of Polyester Microfibre; Textile Research Journal, 65(2), 113-118, 1995.

34. Griesser W and Trefenbacher H: PES-Mikrofasern, Textilveredlung 28, 88-96, 1996.

35. Kobsa H, Rubin B and Schulz EM: Using Optical Ray Tracing to Explain the Reduced Dye Yield of Microdenier Yarns, Textile Research Journal, 63, 475-479, 1993.

36. Leadbetter P and Dervan S: The Microfibre Step Change, J Soc Dyers Colour, 108,369-371, September 1992.

37. MCGregor R and Peters R H: The Physics-Chemical Hydrodynamics of Dyeing, J Soc Dyers Colour, 86, 437-445, 1970.

38. Shibosawa T, Endo T and Rys P: Estimation of the Thickness of Diffusional Boundary Layer by Analysing Rate of Dyeing, Seni, Gakkaishi 42, T671-T679, 1986.

39. http:// ianrpubs.unl.edu/textiles/nf47.htm

40. Ricardson M T: Southern Textile News, 20, 1994.

41. Gross D: Knitting Times, 4, 1991, 25.

42. Issacs M: Kalogeridisc. Textile World. 3, 1991, 48.

43. Burkinshaw S M: Chemical Principles of Synthetic Fibre Dyeing; Chapman & Hall: Glasgow, Scotland, 1995.

44. Koh J S and Y G Kim: J P Fibes Polym, 2001, 2, 35.

45. Begmann L: Microfibre and Electrodstatically Charged Nonwovens in Filtration, Nonwoven Industry, Feb 1991.

46. Feroe D M: Microfibres in Nonwoven Production, Nonwoven Industry, pp 58-60, 1992.

47. Wada O: J Text Inst, 1992, 83, 382.

48. Joycee A Smith: Microfibres: Functional Beauty, Ohio State University Extension Fact Sheet, HYG-5546-96.

49. Rosr M T: Micro-Fibres, http://ianrpubs.unl.edu/textiles/nf47.htm

Note: For detailed version of this article please refer the print version of The Indian Textile Journal February 2012 issue.

Mohamed Fathy El Kady,
Textile Engineering Dept
National Research Centre,
Dokki, Cairo, Egypt.
Email: kadym2000@yahoo.com.

Al Said A Elmetwally,
Textile Research Division,
National Research Centre,
Dokki, Cairo, Egypt.