7.4.4 Finishing in the production process
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Finishing comprises a series of treatments performed on textile materials to improve properties or add new properties. Finishing can take place at 3 points in the process from hemp fibre to textile (cloth).
Overview of finishing processes:
Singeing
Protruding fibres are burned from the fabric. This is mainly applied to fabrics made of cellulose fibres such as cotton, hemp and flax. This makes the fabric smoother and softer and prevents 'pilling' and hairiness during subsequent processes.
Desizing
Removal of sizing agents applied to warp yarns as an aid to weaving (§7.4.1). The sizing agents are removed to facilitate the absorption of dyes and finishing products in subsequent process steps. Enzymes can possibly be used for this purpose.
Twined yarns do not need to be sized and therefore do not need to be desized.
Washing
Washing is performed with detergent (soap) to remove contaminants, spin oils or water-soluble products from the previous processing.
Extraction
Removal of natural substances such as fats, waxes and pectin; by pressure cooking with sodium hydroxide, or by enzymatic hydrolysis.
Bleaching
Removal of natural pigments present in natural fibres. Bleaching is necessary if one want to apply light shade dyes.
Mercerisation
Increasing the lustre or bloom, strength and dyeability of yarns and fabrics made of hemp (and flax and cotton). The fabric is treated under tension with a high concentration of sodium hydroxide.
Dyeing
Dyeing of hemp can be done in the same way as for flax and cotton; one can use the same dye classes (Table below) and dyeing machines.
Several machines can be used for dyeing hemp textiles. The choice of dyeing machine depends on the textile material (yarn, woven fabric, knitted fabric, non-woven), the choice of dye class and the desired end result. Dyeing can be discontinuous, semi-continuous or continuous.
Dyeing is done in several stages:
Preparation of the dye bath
Colouring
Fixing
Washing and rinsing
Dehydrating and drying
Table: Dye classes, dyeing principles and properties of textile dyeing methods.
Dye class | Dyeing principle | Features |
direct | Diffusion of the dyes from an aqueous solution into the fibre. They bind to the fibre by weak secondary bonds. | Poor wet- and sweat resistance. Lightfastness varies depending on colour. Large colour spectrum. |
vat | Insoluble dyes are made temporarily soluble so they can dye the textile material. After penetrating the fibres, the dyes are converted back to the insoluble form. | Good lightfastness. Smaller colour spectrum with dull shades (few red tones). Expensive dyes. |
sulphur | idem as vat | Less good light and chlorine fastness. Narrow colour spectrum with only dull shades but contains a nice black. Cheap dyes. |
reactive | The dyes form a strong chemical bond with the fibre. Also suitable for printing. | Very good washing and rubbing resistance. Moderate light and bleach fastness. Large colour spectrum (slightly less than direct) with brilliant shades. More expensive dyeing process. |
pigments | Pigments are bonded with an agent to the outside of the fibre. They are mainly used in printing. | Good light fastness. Moderate washing and rubbing resistance. |