Saminathan Ratnapandian

Fastness testing

Coloured textiles are exposed to various influencing factors that may cause decomposition, sublimation, decolourisation or stripping (bleeding) of colour. Singly or in combination, these factors cause fading of the textile. Staining occurs when the colour is transferred to any adjacent textile during washing/rubbing. The most common degrading factors for coloured textiles are exposure to light, rubbing under dry and wet conditions and interaction with water as in perspiration, washing, chlorinated water and sea water during their useful life. Hence, in addition to producing level shades, colouration must also yield materials possessing sufficient resistance or fastness to the above factors during the intended end-use [66].

Test methods and standards have been developed to evaluate coloured textile materials for the above and other possible degrading factors. Each country has its own standards organization; for example, Standards Australia put forth standards equivalent to universal standards developed by organizations such as the International Organization for Standardization (ISO). Countrywise standards take into account local conditions. It should be noted that acceptable performance levels are usually agreed on by the concerned parties (supplier and customer) and do not form part of the test method especially in the case of textiles [19].

3. Present scenario

The textile industry in general and textile colouration in particular are embracing the global ‘go-green’ movement. Sustainability is being promoted in all aspects of colouration [2, 3, 7]. This is reflected as consistent efforts towards obtaining desired results in terms of shade and fastness with a reduction in the quantity of consumables by optimizing the process. Another pathway being explored is reclaiming and reusing consumables from effluent. Substitution with consumables possessing a lower environmental impact is an attractive development. In the above instances, consumables include dyes, chemicals, auxiliaries, water, energy (usually heat) and at times even the textile substrate. These efforts have led to constant reports on zero-
discharge processing [67, 68]. For example, Levi Strauss has gone further by encouraging recycling of denim and promoting efforts to reduce the amount of water required for washing during use [69].

In general, automation is gaining wide acceptance in the dye-house in order to minimise human error, especially in dyeing machine control and dye--dispensing operations. Integration of computer colour matching and recipe formulation has further reduced subjective errors and increased repeatability. Low-liquor and ultra-low-liquor machines are gaining popularity in batch-dyeing. These machines are pressurized to yield consistent results [52, 54]. Semi-continuous dyeing, that is continuous dyeing with intermediate storage, is gaining popularity for dyeing fabric lengths of 1000–5000 metres in view of the 1:1 liquor ratio possible [53].

The worldwide volume and variety of demand for colouring textiles is being met by a large range of synthetic dyes. With an annual growth of 3.9%, the market value for synthetic dyes is estimated to be US$16.2 billion distributed as shown in Figure 1.6 [17]. Synthetic dyes are manufactured using petroleum derivatives, for example benzene, as raw material. The dwindling estimate of long-term availability of petroleum resources, and thereby its derivatives, is encouraging the search for renewable alternatives [70]. With the advent of the environmental age, consumers have become aware of the environmental and health issues associated with some synthetic dyes [71]. Synthetic dyes identified as carcinogens have been banned or replaced. Effluents from dyeing process cause environmental hazards because some dyes and auxiliaries used are non-biodegradable, posing a great threat to marine life [4, 72]; hence they must be treated before discharge.

Figure 1.6 Distribution of global dye demand
Natural dyes are derived from renewable sources and hence it is commonly believed and claimed that they are biodegradable and non-polluting [73]. In some cases they also possess medicinal properties [74]. These features and a growing concern for the environment have created a niche market for the industrial scale use of natural dyes [40, 75].

1.