Dyeing is the fixing of a dye to a substrate, and normally intends to give an even distribution. A dye is a colorant that is dispersed in its application medium before being absorbed, adsorbed, reacted with, or deposited on, a substrate. They are distinct from pigments because they are soluble in water and possess substantivity (attractiveness) for their substrate; pigments lack both of these features.
Terms and Definitions
Finishing is a description of processes applied to a substrate (textiles, in this case) to yield a desired effect. Finishes may be induced physically or chemically. An aesthetic finish is applied to change the appearance of the substrate, whereas a performance finish is applied to alter the functional properties of the substrate.
Finishing processes can be divided into two broad classes: physical and chemical. In most cases finishing comprises 3 stages:
• Washing and Drying
• Pressing and aesthetics
Washing processes are essential to ensure that fabrics are not contaminated and are preparatory for other finishing stages. Processes that are carried out during washing include:
• Bleaching: Whitens the fabric by destroying the colour in the fabric
• Carbonizing: Removing vegetable from wool in an acidic treatment
• Desizing: Removing any sizing in the warp threads
• Scouring: Removing any dirt, wax or grease accumulated in the manufacture process.
• Mercerizing: For cellulosic fabrics, caustic (using a base) treatment to improve strength, lustre and absorbency.
• Milling, Fulling or Walking- agitating the fabric under specific conditions to create friction and tighten up the fabric.
During drying, most performance-enhancing chemical finishes are applied. Chemical finishes can be either subtractive or additive. An additive finish increases the mass of the fabric by absorbing onto the surface or into the fibre. A subtractive finish carefully degrades the fabric surface to reduce its mass. Chemical finishes can be used to modify fabric appearance, alter handle, control fabric dimensions, improve fabric performance, protect the fibre, or impart easy-care properties. Not all finishes last for long; short-lived finishes are known as Transient. Durable finishes last much longer.
It is preferable, both commercially and environmentally, for a chemical finish to be applied in an aqueous, rather than an organic-solvent-based, environment. To make the process as efficient as possible, it is important to use a concentration of finish approximately equal to the critical application value (CAV). Below this value the finish will be uneven, but far above this value the energy costs of removing excess solvent become prohibitive.
Fabric handle is a very important quality in all textiles, and particularly in the Clothing market sector. Fabric handle can be modified by mechanical means (through bending, flexing or abrasion), or by chemical softening agents. Chemical softening agents are applied to almost all fabrics and are there to counteract the inherent harshness of man-made fibres, or the harshness imparted by other finishes. Some fabric softeners, such as those used in industry, are durable, whereas home-application softeners tend to last for one wash before being reapplied.
Antistatic finishes are applied by chemical means. They prevent the build-up of static electricity that can occur due to friction between electrically-insulating fibres. Static electricity can be of annoyance in clothing but is of great concern in the Automotive, Aerospace, PPE? and Geotextiles market sectors, where it can cause sparks, possibly leading to fires or explosions. Antistatic finishes work by increasing the conductivity of fibres, thus preventing the build-up of static charge, and by reducing the friction between fibres.
Water repellent (hydrophobic) and oil repellent (oleophobic) finishes can be applied chemically. Surface Tension? of the fabric is the crucial factor that must be modified for these finishes to work.
Easy-care and durable press finishes are a lucrative market and are largely applied to cotton or cotton/polyester blends. Easy-care implies that the product requires minimal ironing or pressing, while durable press implies that pleats and creases will be resistant to normal use. Easy-care products seek to reduce the shrinkage that cellulosic fibres undergo during washing as this leads to creasing. They work by cross-linking the hydroxyl groups in the cellulosic polymer. Easy-care finishes must be easy to apply, cheap, have minimal environmental impact and cause no change in fabric whiteness. They are applied either when the fabric is wet, moist, or dry; each method produces slightly different results.
Flame-retardant finishes can also be applied by chemical means, as can stain resistant and antimicrobial finishes.
Plasma finishing uses plasma, sometimes called the fourth state of matter, to alter a fabric. Plasma is a gas containing both charged and neutral species and is present in flames, the sun, in lightning and in lasers. It can be used to etch a fibre or fabric surface, to attach new functional groups to a surface, or to change structure. However, plasma finishes are not yet durable as they affect only the surface of the substrate.
Stabilising occurs once the drying and performance finishing has halted. It is important because during wet finishing fabrics are often stretched, so realigning and setting is necessary to limit their unpredictability when working in the finished fabric. Without stabilising, fabrics are vulnerable to shrinking. Stabilising can be carried out in many different ways:
• Calendar compressing: a mechanical process that modifies a fabric surface by passing it through heated rollers. It compacts the spacing in knit fabrics.
• Decatising: For wool fabrics, the fabric is sandwiched between a cotton cloth, the pressure of which will influence the set; this is then placed in an autoclave and heated under pressure.
• Heat setting: Carried out both on natural and synthetic fabrics. It is much-used in the production of carpets because it reduces the tensions in a yarn, which otherwise can detrimentally effect a carpet.
• Sanforising: Fabric is mechanically stabilised with the application of heat and moisture, then ran through a compressive roller.
The conditions experienced during stabilising need to be those which the fabric will not encounter again during its product life.
Physical finishing encompasses many different processes, including brushing, calendering, and heat-setting. Calendering is a mechanical process that modifies a fabric surface by passing it through heated rollers. This can be carried out to increase sheen and lustre, to reduce thickness, or to reduce air permeability. There are many types of calendering, and some can be carried out using embossed rollers to impart patterns on the fabric.
Pressing is the final opportunity to change the finish of a fabric. It is greatly influence by both the fabric fibre content, structure and end requirement. Some fabrics will require a clean, lustrous finish, but that same fabric can be manipulated to look fussy, soft and warm.
• Brushing is used to make a fabric’s handle softer. The fabric is run through a series of wire bristles that lift individual fibres from the fabric, making a soft nap.
• Calendering: the fabric passes through a set of rollers which can add lustre or an embossed effect.
• Singeing: Heat is used to singe away any loose fibres on the fabric surface. It is a dry process, which can be used prior to washing.
• Raising is a physical finish where fibres are lifted to produce a warm-feeling and soft fabric, such as flannelette.
• During emerising, a fabric is passed over a rotating emery-covered roller (or over multiple rollers) to give a suede-like or peach-like finish. It produces a very short pile (protruding fibres) that softens the handle of the fabric.
The HSE has produced a series of documents about the health and safety implications of using chemical finishes and have produced a companion document regarding chemical finishes: http://www.hse.gov.uk/pubns/tis2.htm
DEFRA also produce UK-wide regulations regarding emission controls:(Click here to view)
Marks and Spencer provide a very informative article regarding banned and restricted dyes and finishes: (Click here to view)