What is Fiber Fineness?
Fiber fineness is one among the three important fiber quality characteristic normally measured in terms of micronaire. In textiles, fineness refers to the thinness of the fiber. Thinner fibers have greater surface-to-weight ratios and are more flexible, giving a softer drape (flexibility) and handle than thicker fibers. Generally speaking, the thinner the fiber, yarn or fabric, the better its quality and the higher its price. Fineness is measured in textiles in several ways, depending, for example, on whether it is in fiber, yarn or fabric form.
Fiber fineness is an important factor in determining the stiffness of a fabric or, alternatively, its softness of handle and its draping quality.
The fineness of fibers also has an important bearing on the properties of yarns and fabrics made from them. There are various ways of representing fineness (the count). It can be expressed as the diameter in microns, where 1 micron, μm = 0.001 mm. Fine fibers have diameters lesser than 18 μm, medium fibers between 18 and 25 μm and coarse fibers greater than 25 μm. In general, the finer the fibers, the better their quality, because they feel softer and are more comfortable next to the skin.
Fiber diameter also has a significant influence on the dyeing properties of the fiber, because the surface area of a given mass of fibers is higher for finer fibers than for coarser ones. The finer fibers are therefore able to take up dye more rapidly. However a disadvantage of finer fibers is that considerably more dye is required to achieve a given depth of shade than is needed for coarser fibers. For example, fibers with a diameter of 25 μm must absorb about twice as much dye per unit mass as fibers of 44 μm diameter to appear the same depth.
Fiber fineness therefore affects the character of the lustre of the fabric. In descriptive terms, fine fibers produce a soft ‘sheen’, whereas coarse fibers give rise to a hard ‘glitter’.
So we can say that Fiber fineness influences primarily the following:
Spinning limit
Yarn strength
Yarn evenness
Yarn fullness
Drape of the fabric
Lustre
Handle
Productivity of the process
Importance of Fiber Fineness: It has been known since long that fiber fineness plays an important rolein determining the quality of resultant yarn and hence that of the resultant fabrics. In general fiber fineness is important due to the following factors:
a) It affects stiffness of the fabric
As the fiber fineness increases, resistance to bending decreases.
It means the fabric made from yarn of finer fiber is less stiff in feel.
It also drapes better.
b) It affects torsional rigidity of the yarn
Torsional rigidity means ability to twist.
As fiber fineness increases, torsional rigidity of the yarn reduces proportionally. Thus, fibers can be twisted easily during spinning operation.
Also there will be less snarling and kink formation in the yarn when fine fibers are used.
c) Reflection of light
Finer fibers also determine the luster of the fabric.
Because there are so many number of fibers per unit area they produce a soft sheen.
This is different from hard glitter produced by coarser fibers.
Also, the apparent depth of the shade will be lighter in case of fabrics made with finer fibers than those made with coarser fibers.
d) Absorption of dyes
The amount of dye absorbed depends on the amount of surface area accessible for dye, out of a given volume of fibers. Thus, a finer fiber leads to quicker exhaustion of dyes than coarser fibers.
e) Ease in spinning process
A finer fiber leads to more fiber cohesion because the number of contact surfaces are more and hence cohesion due to friction is higher.
Also finer fibers lead to less amount of twist because of the same increased force of friction.
This means yarns can be spun finer with the same amount of twist as compared to coarser fibers.
f) Uniformity of yarn and hence uniformity in the fabric
Uniformity of yarn is directly proportional to the number of fibers in the yarn cross-section.
Hence, finer the fiber, more uniform is the yarn. When the yarn is uniform it leads to other desirable properties such as better tensile strength, extensibility and lustre.
It also leads to fewer breakages in spinning and weaving.
Fiber Fineness Measurement Methods: Different principles are used in the measurement of fiber fineness. Popular fiber fineness measurement methods are-
Air flow method
Projection Microscope method
Gravimetric method (Dimensional measure)
Vibrating String Method
Air flow method of fiber fineness measurement: This is an indirect method of measuring fiber fineness which is based on the fact that the airflow at a given pressure difference through a uniformly distributed mass of fibers is determined by the total surface area of the fibers. This test method is based on the measurement of resistance offered to the flow of air through a plug of fibers compressed into a cylindrical container of fixed dimensions. Compressed air is forced through the sample, at a definite pressure for the purpose and the volume (rate of flow) of air is measured by a roto-meter type flow meter. The sample for Micronaire test should be well opened cleaned and thoroughly mixed (by hand fluffing and opening method). The fiber resistance and thereby volume rate of air flow varies in inverse proportion to the specific surface area (s) offered by the fibers plug.
Fig: Air flow method
Where;
Thus finer fibers own higher specific surface area as compared to coarser one. However, the specific surface area which determines the flow of air through a cotton plug is dependent not only upon the linear density of the fibers in the sample but also upon their maturity. Hence the micronaire readings have to be treated with caution particularly when testing samples varying widely in maturity. Air-flow method based instruments, like Sheffield micronaire (old) and now a days HVI are robust in construction, easy to operate and presents little difficulty as regards its maintenance.
Fiberfineness measurement by projection microscope:
The projection microscope is the standard method for measuring wool fibre diameter, and all other methods have to be checked for accuracy against it. The method is also applicable to any other fibers with a circular cross-section. The method involves preparing a microscope slide of short lengths of fiber which is then viewed using a microscope that projects an image of the fibers onto a horizontal screen for ease of measurement. The apparatus is shown diagrammatically in Fig. Techniques are followed that avoid bias and ensure a truly random sample.
Fig: The projection microscope
Method of Test:
A suitable random and representative sample is conditioned for 24 h in a standard testing atmosphere. Using a modified Hardy microtome the fibers are cut to a suitable length (0.4mm for fibers below 27 (im) and a slide is prepared by carefully mixing the fibers into the mountant. The use of short fibers gives a length-biased sample so that proportionally more of the longer fibers will have their diameter measured. The mounting agent should be non-swelling and have a suitable refractive index (for example liquid paraffin). The mixture of fibers and mountant is spread thinly on the slide and covered with a cover glass, carefully avoiding air bubbles and finger prints.
The slide is placed on the stage, coverglass down (microscope inverted) and fibers are selected for measurement in the following way. The slide is traversed in a zigzag fashion, measuring every fiber that complies with the following requirements:
Has more than half its length visible in the 7.5cm circle which is drawn in the centre of the field of view;
Is not in contact with any other fiber at the point of measurement.
The traverse of the slide is continued until the required number of fibers has been measured. The magnification of the microscope is adjusted to be 50Ox so that on the scale used to measure the fibers each millimetre represents 2 um.
For accurate tests three slides should be measured from randomly selected areas of the material and not less than 150 fibers per slide should be measured.
The coefficient of variation of diameter for unblended wool lies between 20% and 28%. From this value the number of tests to give certain confidence limits has been calculated.
References:
Physical Testing of Textiles by B. P. Saville
A Practical Guide to Textile Testing By K. Amutha
Engineering Cotton Yarns with Artificial Neural Networking (ANN) By Tasnim N. Shaikh and Sweety A. Agrawal
Physical Properties of Textile Fibres (Fourth edition) by W. E. Morton and J. W. S. Hearle
An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
Textiles and Fashion: Materials, Design and Technology Edited by Rose Sinclair
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
