Yarn Conditioning Process : An Overview
High-speed spinning machines generate more friction thus giving additional heat to the yarn and as a result of such heat transfer the yarn moisture content is vaporized. Rising speeds in spinning result in decreased yarn quality for other processes and it is well known that dry yarns have worse properties. For quality reasons it is absolutely important to have even distribution of this recuperated moisture throughout the entire yarn package. Only the vacuum technology provides the conditions for the required moisture regain. This paper attempts to comprehensively review the yarn conditioning mechanism, process parameters of conditioning and applications of yarn conditioning.
Moisture in atmosphere has a great impact on the physical properties of textile fibres and yarns. A high degree of moisture improves the physical properties of yarn and it helps the yarn to attain the standard moisture regain value of the fibre. Yarns sold with lower moisture content than the standard value will result in monetary loss. Therefore the aim of yarn conditioning is to provide an economical device for supplying the necessary moisture in a short time, in order to achieve a lasting improvement in quality.
In these days there is a dramatic change in the production level of weaving and knitting machines, because of the sophisticated manufacturing techniques. Yarn quality required to run on these machines is extremely high. In order to satisfy these demands without altering the raw material, it is possible to make use of the physical properties inherent in the cotton fibres. Cotton fiber is hygroscopic material and has the ability to absorb water in the form of steam. It is quite evident that the hygroscopic property of cotton fibers depends on the relative humidity. The higher the humidity is, more the moisture absorption. The increase in the relative atmospheric humidity causes a rise in the moisture content of the cotton fiber.
The fibre strength and elasticity increase proportionately with the increase in humidity. If the water content of the cotton fibre is increased, the fibre is able to swell, resulting in increased fibre to fibre friction in the twisted yarn structure. This positive alteration in the properties of the fibre will again have a positive effect on the strength and elasticity of the yarn.
Problems in conventional yarn conditioning method
The standard conventional steaming treatment for yarn is chiefly used for twist setting to avoid snarling in further processing. It does not result in lasting improvement in yarn quality. The steaming process may fail to ensure even distribution of the moisture, especially on cross-wound bobbins (cheeses) with medium to high compactness (Fig. 1).
The absence of vacuum in conventional conditioning chambers prevents Conventional homogeneous penetration. The outer layers of the package are also too moist and the transition from moist to dry yarn gives rise to substantial variations in downstream processing of the package, both with regard to friction data and strength.
Since the moisture is applied superficially in the wet steam zone or by misting with water jets, it has a tendency to become re-adjusted immediately to the ambient humidity level owing to the large surface area. Equipment of this kind also prevents the optimum flow of goods and takes up too much space.
CONTEXXOR conditioning process by Xorella
The thermal conditioning process of the yarn according to the CONTEXXOR process developed by Xorella, is a new type of system for conditioning the yarn package (Fig. 2). Thermal conditioning uses low-temperature saturated steam in vacuum. With the vacuum principle and indirect steam, the yarn is treated very gently in an absolutely saturated steam atmosphere. The vacuum first removes the air pockets from the yarn package to ensure accelerated steam penetration and also removes the atmospheric oxygen in order to prevent oxidation. The conditioning process makes use of the physical properties of saturated steam or wet steam (100% moisture in gas-state). The yarn is uniformly moistened by the gas. The great advantage of this process is that the moisture in the form of gas is very finely distributed throughout the yarn package and does not cling to the yarn in the form of drops. This is achieved in any cross-wound bobbins, whether the yarn packages are packed on open pallets or in cardboard boxes.
Effect of conditioning process parameters on yarn properties
Although yarn conditioning machine can add the moisture of yarn, one must consider different raw material and yarn counts to make different yarn conditioning programs. In spite of higher moisture, if there is high vacuum condition, the low quality of raw cotton, coarse count and higher ratio of short fibre would make yarn over relaxed and thus will have less strength. Thus cotton yarn less than Ne 21s, needs less vacuum, low steam temperature, longer heating up time and constant temperature. For PC or pure polyester fibre as well as combed yarn. It requires high vacuum degree and high steam temperature. The time for heating up and constant temperature is related to the actual twist but maximum should not exceed 70 minutes.
Steam temperature is set according to the types of fibers. If cotton fiber is conditioned under 100 for 20 days, it would have only 92% strength left, but polyester fibre under the same conditions would retain 100% strength. For different fibers, there is a big difference in the flow temperature, fusion, resolution and other index of thermo logy. Maximum temperature for cotton fiber in yarn conditioning machine should not exceed 85 but pure polyester could reach 140. Otherwise it will have negative effects on breakage strength and colors.
P. V. Kadole et al found that, the cycle with first cycle temperature 58C - 05 min. and second cycle temperature 62C - 25 min. (with total time for the cycle 55 min.) gives optimized yarn properties for 20KW (4.26 TM), 20KH (3.78) waxed, 20 CH waxed. (3.6 TM) yarns. They also stated that in case of conditioning waxed and unwaxed yarn simultaneously with same programme, care should be taken while selecting maximum temperature in second cycle. Always it should be less than melting point of the wax. They showed that the combined programme helps in achieving best yarn results at low power cost and higher production rate.
Sibel Sardag et al studied the effects of vacuum steaming process parameters (temperature and duration) on tenacity properties of 1 00 % cotton and 100 % viscose yarns. For this purpose, the yarns with different twist coefficients and numbers were twisted and exposed to vacuum steaming at different temperatures and for durations appropriate to their raw material properties. Tenacity properties of the yarns were measured before and after vacuum steaming. They found that vacuum steaming temperature has significant effect on tenacity properties of 100% cotton and 100 % viscose yarns but vacuum steaming duration has been found to be statistically insignificant on tenacity properties of cotton yarns, and viscose yarns.
In another study, these authors showed that tenacity, elongation at break (in per cent), and work of rupture of 30 tex and 20 tex PES/viscose yarns were to be enhanced due to heat-setting. They also showed that the tenacity and elongation at break values of the yarns decreased after dyeing; however, these values are still high when compared with those of the pre-heat setting. The increase of temperature from 90C to 110C caused a decrease in the strength values of the yarns. For this reason, they considered the heat- setting at 90C to be sufficient to enhance the strength properties of PES/viscose yarns - consisting of 67% PES and 33% viscose.
Yarn conditioning machine does improve the CV of yarn and also it does not make it worse. It is the fact that CV and neps of yarn detected by yarn evenness tester is raised rapidly for yarn, just taken from the yarn conditioning machine. However, leaving it for 24 hours, later the data will remain as before. The reason is after processing, a part of water gets into the inner core of yarn and becomes relatively steady crystal water, and water attached to the surface of yarn becomes unsteady. The difference in each part of the surface water would result in different dielectric coefficient, which makes the yarn evenness tester give inaccurate results.
In case of cotton fiber, absorption of moisture is delayed after processing in the yarn conditioning machine. It is therefore, better to pack or use the yarn after 30 minutes rest in the yarn stock room. This would allow enough time for the surface water to evaporate and also to retain the same moisture level both inside and outside of the yarn. The yarn that has been processed in the yarn conditioning machine should not be mixed with the one which has not been processed; otherwise, there will appear long and narrow shadow on the final product after dyeing, especially for knitted fabric.
Major applications of the yarn conditioning
In spinning process yarn conditioning machine is used to adjust the moisture of yarn and to improve efficiency in the next process. After processing, when the moisture level of yarn would reach about 8.5 %, strength and elongation can be increased greatly, which help the performance at processing line. The dealing of cone yarn would improve the efficiency on winding, doubling and twisting and also reduce the yarn hairiness. It will be also helpful to improve warping efficiency and to reduce the defects from rewinding and warping. Owing to the conformity of moisture of the yarn, the efficiency of sizing and weaving will also improved ensuring the quality of the final fabric.
The yarns after processing by yarn conditioning machine would improve the working process as well as the quality of final fabric. Because of fixed twisting and shaping of the yarn, it will give equal height of rising and looping, reduce the unwinding tension, stable the structure and size of the end fabric and give better appearance. Especially it is the key process for setting the seamless underclothes. The process of conditioning will not only stabilize the twist, but also fully relax the yarn to reduce the shrinkage of final product.
In dyeing industry, processing after yarn conditioning machine can remove the stress on grey fabric, which will be helpful for uniform absorption of dyes to obtain bright color as well as uniform shade of the fabric. In garment industry, yarn conditioning machine can make fabric with stable size, reducing the shrinkage and improving the quality. Processing yarn, especially the chemical fiber, in the conditioning machine can greatly improve the quality and appreciation of product.
Conclusion
Textile market is becoming sensitive buyer's market. Weaver is demanding dimensioned quality with consistency from spinners. Therefore in order to satisfy these demands without altering the raw material; the hygroscopic nature of cotton fibers can be used. Therefore most of the spinning mills are now going for YCP. These modern YCP gives us even penetration of steam into all the layers of yarns on cone & ensures even conditioning effect throughout the package. The yarn conditioning plant supply the yarn with increased strength & elongations, that have reduced snarling of yarn, improved working at post spinning processes like warping, weaving, knitting, etc. Yarn conditioning reduces invisible loss to the spinners. Though its initial cost of investment is high, but its payback period is very less.
This article was originally published in the Textile Review magazine, January, 2013, published by Saket Projects Limited, Ahmedabad.
