Finishing of Denim Fabric
Generally denim fabric is a 100% cotton woven fabric and exceptionally solid and strong. Denim fabric is courser than as usual woven fabric. Normally courser yarn is used for making denim fabric. The look and nature of the denim fabric should be enhance subsequent to coloring. Generally blue denim is twist-confronted cotton fabric with 3 × 1 twill development with the twist coloured in a strong shading and the weft left uncoloured. Denim fabric is widely accepted fabric for its appearance and durability. In this article I will discuss about different finishing process of denim fabric.
A proper finishing process is essential for the performance and the appearance of denim fabric. Denim finishing is critical to profits in that customers who pay the highest prices are very demanding with regard to shrinkage differences between and within fabric rolls. After weaving, the denim fabric should be mechanically and chemically treated to give it a soft and pleasant handle and correct dimensional stability, both in the warp and the weft to prevent shrinkage in subsequent washing. The finishing process can comprise various stages such as brushing, singeing, impregnating, stretching, drying and shrinking. Brushing and singeing eliminate impurities and help to even the surface of denim fabric. Impregnation and stretching regulate the hand and rigidity of the fabric, while compressive shrinking ensures its dimensional stability. The standard width denim fabrics are then sent for making up. In this process, the fabric is cut into the desired width according to the size required.
In addition to these process stages and depending on the required result, further chemical processing can take place such as desizing, overdyeing, coating, retro dyeing, pigment dyeing, reverse dyeing, non uniform dyeing or multicoloured dyeing. The purpose of chemical finishing is to change or improve the aspect and hand feel of the fabric to stiff, soft or drapey, and to gain unique functional properties such as oil or water repellence, wrinkle resistance or flame resistance and also to improve the sewability.
Singeing is a process that uses a gas flame to burn off the fluff or tiny hairs on the surface of denim fabric. This process burns away surface material that makes the fabric look fuzzy. It is carried out to obtain a cleaner and smoother appearance of denim fabrics. This process enhances the colour, and the fabric wettability is also increased.
Progressive shrinkage is a common problem with garments manufactured from denim. In particular, heavy weight twill denim construction is dimensionally unstable after weaving, which requires very high compression at preshrinking or sanforizing. Fabric that does not undergo the sanforizing process and is considered raw is likely to shrink up to 10–15% on the initial wash and continue to shrink up until the third wash, and sanforizing aims to reduce the shrinkage to 1–2%. The cause of progressive shrinkage may be the excessive swelling of the yarn during the washing cycle.
The sanforizing process involves stretching of the fabric before it is washed, and it helps to prevent further shrinkage. Thus sanforizing is a crucial process to set the relevant parameters to adjust the dimensional stability of warp, weft and skew of fabrics, so that the fabric will not shrink during garment production or wear.
Calendering is a processes in which denim fabric is passed between rollers or calenders, usually under controlled heat and pressure, to produce a variety of surface textures or effects in fabric such as compactness, smoothness, glazing, etc. The process involves passing the denim through a calender in which a highly polished, usually heated, steel bowl rotates at a higher speed than the softer bowl against which it works, thus producing a glaze on the face of the fabric that is in contact with the steel bowl.
Mercerisation is an industrial process involving sodium hydroxide for cotton yarns or fabrics to increase the lustre and dyeability. But the mercerisation of denim is usually carried out after the denim is woven, and so it is different from the more common method of mercerising cotton yarn. Mercerisation of denim may be used for achieving ring dyeing, thus keeping the dye on the surface of the yarns or fabrics and to prevent dyes from fully penetrating the fibres. In addition to increasing the fabric lustre, it also improves its strength. As it significantly increases the cost and lead times of denim production, at present, it is a relatively rare process.
Due to the removal of impurities such as wax, paraffin and oil after pretreatment processes, denim fabric loses its natural hand feel and therefore it is necessary to regain its softness. Softeners improve abrasion resistance, increase tearing strength and diminish the risk of stitching thread and needle breakage during garment sewing. A wide range of softeners is used in the aftertreatment of denim fabrics. The major softener types are cationic, anionic, nonionic, silicones and special softeners.
Foam finishing of denim fabric is an environmentally friendly and energy saving finishing and shrinking method for meeting the highest quality demands. In this method, the required finishing agents are added to the foam, where the moisture content is minimum. The foam is usually laid on a rubber conveyor, evenly across the width of the conveyor by an oscillating feed pipe system, and a doctor blade ensures a uniform layer of foam across the width. Foam finishing can result in an 80% savings in water and energy compared with the traditional denim finishing methods.
Traditional indigo dyed denim fabric can be overdyed as part of a finishing process. Overdyeing can take place between desizing and the addition of softeners. The best results, however, are achieved after stone washing and subsequent bleaching. The preparation requirements for overdyeing are the same as for any other dyeing process.
Resin is a chemical solution that fills into the amorphous area of a fibre, penetrates thorougly by drying and curing and polymerises inside the fibres. The resin amount, fixation temperature, pH and process time are the critical parameters of a proper resin application on denim fabric. A post curing can activate the resin after garment production.
Coating is a simple process for covering the surface or back of denim fabric with chemicals or dyestuffs in order to gain or improve various surface properties such as waxy, oily, glossy, paper, leather, silicone, etc. Coating materials can be waxes, rubbers, latex, plastic films, resins, polyurethanes, binders or metal powders. Today several coating layers are applied to denim fabrics to obtain different effects after washing. Knife coating, screen coating and foam coating are the common coating methods.