Ariful Haque Ashik, Lecturer
Dept. of Textile Machinery Design & Maintenance
Bangladesh University of Textiles (BUTEX)
Denim fabrics are mostly produced using cotton fiber in twill weave with colored warp and white weft yarns that are mainly used for the production of jeans, work clothes and casual wear. Denim garments are subjected to different dry and wet processes to obtain specific appearance and texture. Textile finishing processes include physical or chemical treatments that are mostly used to achieve the desired result of improved appearance and better texture of fabrics, which modernizes clothes to increase their sale in the market. The effects of these processes on the performance and thermal comfort properties of denim fabrics solely depends on the correct selection of both the warp and weft yarn.
Warp yarns plays a significant role on the performance of the denim fabrics. As mostly, the warp yarn is dyed and the weft yarn remains undyed, so the colored textures, different washing procedures, different finishing techniques directly affects the warp yarn. The selection of warp yarns depends on many factors. The two main denim production systems in the world are slasher dyeing-sizing system and rope dyeing systems. Depending on these techniques, the quality of the warp yarns will change though slightly. If the slasher technique is used, then on a single machine the dyeing & sizing will be done consequently on the same go. If the rope dyeing is done, then there will be another stage needed which is long chain beamer to convert from rope form to sheet form, another machine is used solely for sizing. No matter whether slasher or rope dyeing there are some common criteria which are needed to be fulfilled by the warp yarns. These are mentioned below-
Cohesive strength: the warp yarns needed to have a minimum cohesive strength to withhold the tension of the warping, dyeing padding roller tensions, also the tensions of different types of rollers used in sizing machines. Also after sizing its strength increases otherwise it will fail to resist mechanical strains in the weaving process as shedding, picking, beating exerts upon an extra tension on the warp yarns.
Fluff binding: Very important factor to prevent yarns from clinging among yarns. From the practical point of view, it has been seen that ring warp yarn tends to generate more fiber fly than the rotor yarns. This fluff binding property depends on whether the yarn got false twist (Open ended yarn) or real twist (Ring yarn). That means fluff fly depends on the end spinning systems of the warp yarn. Less fluff warps yarns will make shed smoothly in weaving loom.
Dye & Size pick up: Both these plays a key role on selecting the warp yarns. The amount of dye stuffs & sizing materials are calculated on the sample preparation stages for the further bulk production. An ideal dye & Size pick up is pre-determined according to yarn or fabric construction, by which we can formulate a size recipe and set squeeze roll pressure and other mechanical conditions. The dye & size materials should be picked up by yarn sheet or rope evenly.
Smoothness & Softness: Another important factor of warp yarns in terms of production terms, particularly in weaving high-count yarns into high density fabric by a high speed air jet loom. These things are observed during the sample preparations part then for bulk productions these are observed thoroughly.
|Table 1 : A basic chart for ideal Size pick up (SPU) for regular cotton woven cloth|
Size cooking conditions: It is well established that sizing is the king of weaving! So different warp yarns needed different sizing materials cooking conditions and therefore special attentions are needed for determining the right size materials cooking conditions for different warp yarns. The key factors here are the quality of the size solution in terms of viscosity, particle size etc. in other words, a good size solution can be prepared in a good cooking process totally depends on the warp yarns we choose.
Dyeing & Sizing machine conditions: which warp yarns are used, on the basis of that different dyeing rollers pressures, size box temperatures, stretching(draft) in sizing process, drying temperatures (of cylinder dryer) etc. are also determined.
Some practical examples of warp yarn used for denim fabrics are –
Rotor yarns (Open ended, false twisted yarns): 7Ne (Card/Slub) ,8Ne (Card/100 % conta free/100 %OCS/ CMIA) ,9Ne ,10Ne (PC 65%+35%),11Ne ( Organic) , 12 Ne(Conta free with BCI) ,14 Ne ,16 Ne ,20 Ne ( Card / Slub/ OCS)
Ring Yarns (Closed ended, Real twisted yarns) : 7Ne (Card/combed/slub ) , 8Ne ( Slub/ Card S-twist, Card Siro S-Twist,Ring siro) ,9Ne ,10 Ne ,12 Ne (Ring recycled),14 Ne ( Ring CVC), 16 Ne( 50% Cotton+50% Lyocell), 20 Ne ( 50% Viscose+50% cotton) , 20 Ne( 50% Cotton+50% Tencel)
Apart from that some multicount warp yarns are also used such as ,8,10 & 12 Or 7,10&11 etc. Tencel are also used as Warp yarns sometimes as per requirements of the denim fabric.