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;

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.

Azoic or Naphthol Dyes: So called because their molecules contain an azo (-N=N-) group

Basic Dyes: Also called cationic dyes because in solution the basic dye molecule ionises thereby becoming a cation (positively charged).

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.

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.

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.

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.

Reactive Dyes: Named because they react with the fibres to form a stable chemical bond between dye molecule and fibre polymer.

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.

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.

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.

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.