The elliptic nature of yarn cross-sections and the interaction between the levels of twists and eccentricity play an important role in characterising the surface properties of the fabrics, aver R N Narkhedkar and Dr C D Kane.
Yarn cross-section is a very important factor to be considered for deciding the physical, mechanical and surface properties of the yarn. The yarn cross section measurement is a very difficult and time consuming task for the textile technologist. There are many parameters of cross section which affect the physical and mechanical properties of the yarns. These parameters are surface area, perimeter, equivalent diameter, large diameter, small diameter, convexity, stiffness, eccentricity, and hydraulic diameter of the yarn. Mainly there are some other factors which decide the yarn cross section which are as mentioned below:
- Yarn ‘Twist Multiplier’.
- Yarn manufacturing method.
- Yarn twisting tension.
- Yarn linear density.
- Type of yarn packing.
Here a very important factor to be considered is that if the yarn diameter at a point on the yarn at different angles is exactly same, then the yarn is having a completely round cross-section shape at that point. So there is a correlation between yarn diameter and the yarn cross-section at that point. In short it can be said that yarn diameter can decide the cross-section shape of the yarn.
Yarn diameter is an important factor for determination of many fabric parameters and properties, eg, cover factor, porosity, thickness, air permeability, etc. Yarn diameter measurement is also a very difficult task for the textile technologist, mainly due to the unevenness of the yarn. As the yarn unevenness increases there is an increase in the difficulties in deciding the yarn diameter. So the methods of measuring the yarn diameter directly are very time consuming as well as tedious.
In order to overcome this problem in regular practice, indirect methods are used to compare the diameter of two different yarns which is called as yarn linear density testing. For testing the linear density of the yarn, weight per unit length of the yarn is taken and from this the diameter of the yarn can be predicted; but in doing so, other different factors are also to be considered, like the type of material, specific density of the material, and the raw material fineness.
In case of the winding machine for removing the yarn faults, principle of yarn diameter measurement is used. But in this case there is online yarn diameter measurement made for checking the yarn faults. In case of winding machine, the following diameter measuring principles are used,
Factors affecting the yarn cross-section shape are:
1. Testing method
The most important factor which is yet not considered during testing the yarn cross-section shape is the testing method. Conventionally, during measuring the yarn cross section there is a need of cutting the yarn which leads to the scattering of the fibres in the yarn which will change the cross section of the yarn.
2. Yarn manufacturing techniques
During ring yarn manufacturing, the material is drafted in the drafting system and then it is converted in the form of yarn. During this process as soon as fibre web comes out of the front roller, the fibre strand is converted from the ribbon form into the round. During twisting, when the fibres from ribbon form are forced to the round form there is a tension variation among the fibres giving dominant migration of the fibres in the ring yarn and the yarn formed is having locked and compact structure.
In case of the rotor spinning, the individualised fibres are collected in triangular rotor groove and then it is converted into round form so there is tension variations among the fibres giving migration as that of the ring yarn at the core, but on the yarn surface there is a folding of the corners of triangle giving very less migration due to lack of fibre tension variations giving bulkier yarns. From the above it is clear that the yarns produced by the different manufacturing techniques may give different types of yarn cross-sections.
3. Yarn twist multiplier
When the yarn ‘Twist Multiplier’ is increasing the transverse forces acting on the yarn from the outer fibre, layers go on increasing and the yarn becomes more circular in cross-section due to the increase in yarn packing density and vice versa when the twist multiplier is reduced.
4. Twisting tension
Twisting tension is a very dominating factor in deciding the fibre migration in the yarn and indirectly affects the yarn cross-section shape. If the twisting tension is increasing then the fibre migration reduces and vise versa.
5. Yarn linear density
When the yarn is finer more will be the twist inserted in it in order to have optimum yarn strength. So when the yarn twist is increases the yarn cross-section becomes more and more circular one due to the increase in transverse forces.
6. Fabric weaving
When the yarn goes into the fabric form then there is a flattening of the yarn at the cross-over points. When this yarn passes through the processing stages in fabric form then also there is a change in the yarn cross-section shape.
Techniques for yarn diameter measurement
1. Uster Evenness Testers
These are widely used in the textile industry for a long time. Uster Tester 4 and 5 are a combination of capacitive type, and optical one. The OM module mounted on model, 4-SX, as described by Tsai et al. (1996), is capable of measuring yarn diameter with dual light beams perpendicular to each other. This design reduces shape error caused by irregular yarn cross-sections. The irregularity of yarn is detected from the variations in electric capacitance generated by the movement of yarn specimen that passes through the gap of a fixed air condenser. On the other hand, using the photoelectric measurement, the irregularity is measured from the fluctuation of the light intensity or shadow on the sensor caused by the beam of light passing across the yarn cross-section.
2. QQM-3 Yarn Quality Analyzer
QQM-3 is a portable device used for evaluation of yarn unevenness characteristics directly on OE & RS machines. It provides measurements, analysis and data source for further investigation. It is a tool for identifying faults on spinning units, provides measurement and analysis of CV% as well as imperfections and Spectrograph. The QQM has 2 optical sensors of 2 mm width, equipped with infra diodes and transistors positioned in the direction of yarn delivery of 10 mm apart, sampling rate is limited to 300 m/min (capability 600 m/min). Sensors are programmed for sampling each 2 mm, data processing, measuring yarn speed. Memory equalizer controls the serial port.
3. Laboratory Measuring Systems
A laboratory method was introduced by Prof B Neckar and Dr D Kremenakova from the Technical University of Liberec, where a near parallel light beam is positioned under a sample of yarn on a microscope equipped by a CCD camera. The image of the yarn is captured, processed and stored in a binary system. Some light beams can pass at distance x without any problems, some others are “hindered” by the fibres. The longest section of black pixels creates the yarn body and is assumed to be the diameter of the yarn. The midpoint represents the yarn axis. The relative frequency of black points at each distance can be found experimentally (usually 800 pictures from different places of a yarn). A double exponential function is fitted to find both the so-called dense and cover diameters. The yarn packing density in the cross section is taken into consideration in this model depending upon twist, fibre orientation and yarn fineness.
Yarn cross section is prepared and a system of annular rings centered on yarn axis (yarn center of gravity) is used. The packing density is then expressed as function of distance from yarn axis. Local packing density is expressed as the ratio of the fibres cross sectional area in annular ring to the total area of annular ring. The diameter of the yarn measured is found at 15% of yarn packing density.
A second method is based on processing of the longitudinal images of the yarn.
Direct yarn cross-section measurement
In case of direct cross section measurement method, the most difficult and essential part is the sample preparation. The literature mentions several ways to preserve the cross section. One of the methods is embedding the fibre into cork (Figure 6). In determining the yarn cross section disadvantage is that the sample has to fit tight into the hole in the cork, and hence the yarn structure is distorted.
Another group of sample preparation methods are the plate methods. In this case the yarn bundle is pulled through a small hole in a metal plate and cut on both ends (Figure 7). A special kind of this method is the Hardy plate (Figure 8). After the sample is prepared in any way mentioned above, it is placed under a microscope and images taken of the cross section of the yarn either by light or electron microscopy and studied by image analysis. The main aim during preparation is to preserve the original state of the yarn against the compression during yarn cutting. That is why another material was found for embedding, the mixture of paraffin and beeswax.
This is much better because if the structure is fixed (eg, glued) beforehand, and the molten mixture is poured on it (then the sample is cooled),the fibres are preserved in their original state. This is the so- called soft method. In case of the hard method the yarn is embedded in a special material called Spurr’s resin, epoxy resin or gelatine, specially designed for cross sectioning. After embedding, thin (typically 15 µm thick) pieces are cut usually with the help of a microtome.
The process of obtaining accurate measurements of yarn cross-sections presents a difficulty because the process itself may be unstable primarily due to the stability of the yarn at a given angle. While it is impractical to cut the yarn at various locations and examine the cross-sections without distorting the yarn, fast and accurate determination of yarn cross-sections can be achieved indirectly by rotating the yarn segment around its center. As in the Figure 10 the projected diameter when light is shed on a strand of yarn with semi-major and semi-minor axes a and b, respectively. The derivation of the diameter d in terms of a, b and a yield the following equation:
Figure 10 shows a typical yarn cross-section that is obtained using a line-scan camera by rotating the yarn along its axis. The dots in Figure 10 mark the measured diameters at various angles. In this case, the yarn’s irregular cross-section is shown to be approximated by a best fitting ellipse with an eccentricity measure of 0.46.
When several yarn cross-sections were examined as described above, the typical yarn was found to have an elliptical profile with varying eccentricity measures along its length while the major axes were rotating periodically with the twists of the yarn. In addition, it was demonstrated that even a regular yarn with elliptic cross sections could have a CV as high as 7.2% due to the rotation of the major axes only. The authors believe that the elliptic nature of yarn cross-sections and the interaction between the levels of twists and eccentricity play an important role in characterising the surface properties of the fabrics.
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Note: For detailed version of this article please refer the print version of The Indian Textile Journal March 2012 issue.
R N Narkhedkar
DKTE’s Textile & Engineering Institute,
M: 098506 23592.
Dr C D Kane
DKTE’s Textile & Engineering Institute,