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1. Introduction
Denim jeans are considered to be one of the most popular apparel items among consumers all over the world. The selection of apparels by the customers gets influenced by the aesthetic appearance, fashion, comfort, cost and durability. Due to the movement of the body of the wearer, the skin stretches. Similarly, the apparel fabric also needs to stretch during body movement to avoid uneasy feeling during the movement. Thus, for imparting comfort related to body movement, the stretching of the fabric is very important. The fabric with higher elastane content shows relatively lower values of stress and a higher level of elastic recovery during extension (Gorjanc and Bukošek, 2008). Therefore, in recent times, the usage of elastane-containing denim fabric has increased (Özdil, 2008).
The extension level in clothing during the body movement generally varies from 20 to 45%. Depending on the stretch levels, there are two categories of stretch-denim fabrics. The fabrics having a stretch level below 30% comes under the category of comfort stretch and above 30% comes under the category of power stretch (Hu and Lu, 2015; AK and Bansal, 2018). For imparting stretch to the denim fabric, various types of elastic yarns are used. These yarns generally have elastane filament in the core and staple fibre as a sheath or covering. Different types of covering fibres are used like cotton, polyester, viscose etc. The spinning technologies used to produce elastic yarns are ring spinning, wrap spinning, intermingling or air covering etc. However, ring-spinning is the traditional system to produce core-spun elastic yarn. Nowadays, other systems like air covering are also being used to produce elastic yarn for denim industries due to its lower cost of production. These elastic yarns are generally used in the weft direction to provide horizontal stretch in stretch-denim garments. It must be added that to provide both horizontal and vertical stretch, bi-stretch-denim is also produced. Such bi-stretch fabric provides high level of skin fit. In such bi-stretch fabrics, elastic yarns are used in both warp and weft direction.
To improve the aesthetic properties of denim garments, denim washing is done. Denim washing is a finishing process that gives a special worn-out appearance to the garment. There are different processes of mechanical and chemical finishing giving the...