Textiles Dyes & Colour Fading
One of the frequently asked questions we receive from customers concerns one of the many aspects of textile colour fading. This is a complex issue. The following sections attempt to describe the types of dyes commonly used in different fibres and some of the ‘fading’ issued associated with each. This is not an exhaustive account only a guide.
Dye Classification: This section include a number of facts taken from various sources regarding dyes commonly used within the textile industry. Dyes may be classified in two ways;
- by virtue of the chemical structure of the dye molecule or
- by virtue of the method of dye application to the textile.
The most popular classification appears to be according to dye application method, so we will use this protocol.
Acid Dyes: So called because they are usually applied to textiles under acidic conditions. Usually applied to fibres from solutions containing sulphuric, formic or acetic acids.
- Mainly used for man-made fibres (acrylic, nylon, spandex, polypropylene & other olefinic fibres and certain modified polyester fibres) and natural protein fibres such as silk, wool & mohair). They are not used for cellulosics.
- Acid dyes fall into three sub-groups;
- High Level acid dyes: Produce bright & even colours but have poor wash fastness but have good light fastness.
- Average Level acid dyes: Difficult to apply evenly. Only fair wash fastness but good light fastness. Mainly used for dyeing woolen fabrics.
- Poor Level acid dyes: Very difficult to apply evenly but have good wash fastness and good light fastness.
- Structurally they are organic acids (contain SO3H group) in the form of a salt. Since they are acids, they easily combine with alkalis (detergents used for washing) and so are readily removed from textiles during washing (High Level Dye most easily Poor Level Dye least easily).
Azoic or Naphthol Dyes: So called because their molecules contain an azo (-N=N-) group
- Used on cellulosics & occasionally on man made fibres
- Colour produced via a reaction within the textile polymer fibre between two components which constitute the azoic dye (namely the Naphthol coupling agent and base diazo component). This makes azoic dyes pretty distinct in that the dye is actually produced within the fibre during the dyeing/printing process.
- Produce brilliant colours (particularly red & orange)
- The azoic dye chromophores are particularly stable and as a consequence these dyes have very good colour fastness to laundering, bleaching, alkalis and light.
- Sometimes referred to as ice colours because they are applied to fabrics in a low temperature (usually room temperature) dye bath (dye baths are usually pretty hot).
Basic Dyes: Also called cationic dyes because in solution the basic dye molecule ionises thereby becoming a cation (positively charged).
- This class of dye is the oldest synthetic dye group. Now only used on acrylics and modacrylic fibres.
- Belongs to the aniline group of dyes.
- Characterised for their brilliant colours on acrylics.
- Very good wash fastness and excellent light fastness
Direct Dyes: Also called substantive colours due to their excellent substantivity for cellulosic fibres. Substantivity means high attraction between the fibre polymer and the dye molecule which makes the fibres easy to dye.
- Largest & most commercially significant group of dyes.
- Water soluble composed of amines & phenols
- Dye application controlled by the addition of salts (usually sodium chloride). These overcome the negative surface charge of the textile fibres enabling the coloured anionic (negatively charged) part of the direct dye to enter into the fibre.
- Mainly applied to cellulosics and occasionally to protein fibres (silk/wool) & polyamides
- Used for bright colours
- Only moderate fastness to sunlight
- Poor colour fastness to washing. These dyes adhere to fabrics by chemical attraction. Whilst they are very easy to apply and have a good attraction to the fibres they are very water soluble (easy in, easy out).
- Many after treatments used to improve wash fastness of cellulosics dyed with direct dyes – diazotisation, copper after-treatment, cationic agents, formaldehyde after-treatment.
Disperse Dyes: These dyes are virtually insoluble in water but can be dispersed (hence the name) using a surface active agent, same sort of thing the backcoating paste uses.
- Used for acetate and polyester fibres. Occasionally used for acrylic & nylon.
- The dye molecules are relatively small. The dyeing process involves forcing (high temperatures & pressures) the dye into the fibre. Once in the fibre they are held by weak forces of attraction to the fibre polymer, but because they are particularly insoluble in water, they show very little tendency to leave the fibres.
- Printing with disperse dyes requires heat setting.
- Have good wash & light fastness.
- Known to discolour when exposed to oxides of nitrogen. Other gas fume sources may also effect this class of dye.
Mordant Dyes: Derived from the Latin mordeo, meaning to bite or take hold of, mordant dyes are attracted to fibres by a mordant which may be organic or inorganic in nature. The most common mordant is chromium and, as a consequence, mordant dyes are often called chrome dyes.
- Most commonly used for natural protein fibres, particularly wool. Sometimes used on modacrylic and nylon.
- The chromium (as a chromate salt) is applied and absorbed by the fibres. When added, the actual dye is attracted to and then attached to the chromium metal. This complex is called a Lake, and this name is occasionally used to describe this type of dye class. Lakes are very insoluble in water – make sense of that if you can!
- Good wash & light fastness (due to large size of lake complex and its insoluble nature)
- Not used that much because they have a limited colour range and tend to produce dull colours. Colour matching is difficult. The long dyeing process causes damage to the fibres.
- Dichromate salt (waste products) are pollutants once released into sewerage.
- These dyes have largely been replaced by Premetallised dyes.
Premetallised Dyes: These are derived from mordant dyes. The metal (usually chromium) is already incorporated into the dye molecule during manufacture. This reduces the dyeing process normally associated with mordant dyes and does less damage to the fibres.
- Wash & light fastness are good.
- Only a limited number of mainly dull colours.
Reactive Dyes: Named because they react with the fibres to form a stable chemical bond between dye molecule and fibre polymer.
- Used mainly on cellulosics, wool, silk and occasionally on acrylic blends.
- Bright colours
- Very good wash and light fastness.
- The formation of the bond between dye and fibre occurs under alkali conditions.
- Requires extensive washing after dyeing to remove loose dye.
This is a class of dye which has been found to be a problem with regard to flame retardancy. Over the years we have encountered isolated problems where fabric has faded following treatment using those chemicals suitable for curtaining (i.e. water soluble inorganic salts applied by padding, more information is available here), we have not experienced fading problems with backcoated fabric. As we know, fabrics can fade simply by exposure to light irrespective of any treatments they may have recieved. However, the fading problems referred to here occurred some months after processing and treated cloth appeared to fade faster than untreated cloth. The following explanation of these observations are speculative but are based on problems known to be associated with this class of dye.
- Reactive dyes are sensitive to acids since this can break the bond between dye and fibre and result in colour loss. Atmospheric pollutants such as oxides of nitrogen or oxides of sulphur produce acids on the surface of fibres after absorption from the atmosphere.
- The flame retardancy process involving the wet padding of water soluble chemicals onto fabrics involves the use of weak acidic solutions which may initiate fading problems when we treat fabrics dyed with these dyes.
- They are also sensitive to chlorine (bleaches/swimming pools/wet towels etc.). Treated items coming in contact with type of agent may well exhibit fading.
- Not all reactive dyes will fade after treatment, but a significant proportion of problems we have encountered have been shown to involve these dyes.
We try to screen fabrics for fading problems prior to treatment. This is only successful with problems which become evident at the instant the flame retardant is applied. Reactive dyes often take many months to develop the fading problem thereby making a viable assessment of potential problems impossible at the time of processing. We therefore recommend avoiding this type of dye since most of the problems we have encountered involving fabric fading has been linked to their use.
Sulphur Dyes: They contain sulphur, hence the name.
- Used on cellulosics.
- Require strong alkali conditions in order to dissolve and therefore are applied under such conditions.
- They are insoluble in water.
- Fair wash and light fastness.
- Produce mainly black, brown blue and olive shades which usually dull.
We have encountered one known fading problem with yarn that had been dyed with is class of dye. The fabric was woven from approximately 5 types of yarn. Only the single yarn dyed with sulphur dye discoloured. The problem became evident approximately 6 months after treatment.
Vat Dyes: Their name comes from the large wooden vessels originally used to apply these dyes.
- Used on all cellulosic fibres.
- Insoluble in water, applied as a dispersion.
- Many steps involved in the dye process.
- VAT dyes are expensive.
- Have a very wide range of colours available
- Have the best colour fastness of all the dyes in common use.
- Excellent light fastness and very good wash fastness.
- Can withstand very hard wear.
Possibly the best dye you could find.
Fluorescent Brighteners: Also called optical brighteners. Fluorescence occurs when irradiated with the Ultra-violet light component of sunlight which reaches the Earths surface.
- They cause textiles to reflect more blue light which we perceive as a brightening of the textile, particularly when used with white fabrics.
- They have fair wash fastness.
- They generally have poor light fastness (with some exceptions such as the types used on polyester and nylon).
- Once broken down by UV-radiation, they stop fluorescing and the textile takes on a yellow colour making it appear as though fading has occurred.