Several agents can cause significant colour fading in textiles (see Tables 5.2 and 5.4) These include, but are not limited to, light and weathering, washing and laundering, dry cleaning, aqueous agents, atmospheric contaminants, and bleach- ing and heat treatments. Of these, light and washing are the most commonly faced agents, so the colour fastness principles against these elements are explored in the following section.
96 Understanding and improving the durability of textiles
5.5.1 Light and weathering fastness
Light fastness properties are related to the resistance of textile colour to the influence of daylight. Exposure can be achieved using natural light, or by the application of artificial light via a source that simulates natural daylight (for example, a xenon arc lamp simulating D65 light source). When the light agents act in combination with outdoor conditions (e.g. rain) or artificial weathering (e.g.
spraying cycles with water), colour fastness can be more accurately assessed.
Daylight and weathering are agents that can significantly influence the final colour of a textile, so it is necessary to ascertain and measure the resistance or fastness of coloured dyed and/or printed textiles.
Tests to measure the effects of light evolved in parallel with the development of industrial methods for producing artificial daylight sources. As such, in the nineteenth and early twentieth centuries, exposure to natural daylight and weath- ering predominated. While this made it possible to establish reproducible, reliable methods and systems to test colour fastness to daylight (i.e. ISO105-B01 (1994)) and to weathering (i.e. ISO 105-B03 (1994)), the concept of ‘acceleration’ had not yet been incorporated. The tests based on natural sources of light and weather fastness were not practical means of assessment in many cases (for dyers, printers or garment manufacturers, for example) because of the long time needed to ascertain the final quality of a particular textile product or finished garment.
Furthermore, the variability of natural daylight due to seasonal (summer–winter) and weather conditions (sun, clouds or rain), as well as differences from one country to another (hours of sunlight, depending on latitude), meant that exposure time was not a valid reference. Therefore an alternative method to measure the amount of light or energy received by the exposed samples had to be found that would facilitate the reproduction of the result in the same place, as well as making a comparison between different exposure sites possible. To overcome this issue, an alternative scale, the Blue Wool Scale Standard, was established, based on visual scales of coloured wool textile substrates.
Two types of Blue Scale have been proposed: (i) the European ISO Blue Scale developed in Germany by the Germany Colour Fastness Committee (DEK) in 1913; (ii) the USA ISO-L Blue Scale developed about a decade later by the AATCC.
In the European ISO Blue Wool Standards, eight wool fabrics are dyed with selected colorants, each used with increasing levels of light fastness (see Table 5.7). In contrast, for ISO-L Blue Wool Standards just two wool flock samples are dyed, one with an unstable fugitive dye (C.I. Mordant Blue 1) and the other with a maximum fastness dye (C.I. solubilised Vat Blue 8). The dyed flocks are then mixed in eight different proportions and processed to obtain the eight intermediate blue wool standard fabrics (Grades L2 to L9). The design of both Blue Scales (ISO and ISO-L) aim to obtain a scale of eight values in which each standard material takes twice as long to fade as the previous one, i.e. a geometric scale. Whilst this
Colour fastness 97 Table 5.7 Selected dyes used to prepare the ISO Blue Wool Standards scale for testing and assessing light fastness. (Adapted from ISO 105-B02 (1994).) ISO fastness rating Dye in ISO scale Dye in ISO-L scale (ISO-L rating)
8 (L9) C.I. Solubilized Vat Blue 8 C.I. Solubilized Vat Blue 8
7 C.I. Solubilized Vat Blue 5
6 C.I. Acid Blue 23 Intermediate ISO-L Blue
5 C.I. Acid Blue 47 Wool Standards (L3–L8)
4 C.I. Acid Blue 121 obtained mixing different
3 C.I. Acid Blue 83 proportions of the two
2 C.I. Acid Blue 109 wool flocks samples
1 (L2) C.I. Acid Blue 104 C.I. Mordant Blue 1
has more or less been achieved, with both scales corresponding fairly well, they do not completely coincide.
When the ISO/TC38/SC1 scales had to be incorporated into ISO 105, the notations ‘ISO Blue Wool Standards’ for the European scale and ‘ISO-L’ for the USA were established. Both contain eight degrees or levels (1 to 8 for ISO and L2 to L9 for ISO-L), which are not interchangeable. This prevents confusion regard- ing their colour fastness indications.
The ISO 105 Part B (as part of ISO 105-A01 (2010)) includes all test methods in which light is involved as an agent that fades or changes colour: natural or artificial light, acting alone or in combination with rain, water or perspiration, high temperature and temporary actions on textile colour (e.g. photochromism).
There are numerous factors affecting light fastness of coloured textiles (Pugh and Guthrie, 2001). Consequently, in order to assure concordance of accelerated test methods involving the use of artificial light sources (e.g. xenon arc lamp), control of the following variables is crucial and must be take into account: (i) the spectral power distribution of the light to simulate as much as possible the spectral distribution of natural daylight (light source D65), (ii) the temperature of the sample, and (iii) the effective humidity of the air in contact with the exposed samples. Suitable filters are therefore used to adapt spectral light distri- bution, Black Panel Temperature (BPT) and Black Standard Temperature (BST) are introduced to control the temperature of the specimens, and humidity test control fabric (red azoic dyed cotton cloth) is used to control the effective humidity.
Colour fastness to artificial light, for example, can be carried out according to ISO 105-B02 (1994). This describes accelerated test methods for determining resistance of textile colour to light by using an artificial xenon light source to simulate the action of natural daylight in a chamber at a given temperature and humidity. Test specimens are assessed by comparing any change in their colour with that recorded for the Blue Wool reference standards, according to a specific set of exposure conditions. The light fastness rating corresponds to the number of
98 Understanding and improving the durability of textiles
the Blue Wool reference standard (1–8 for ISO or L2–L9 for ISO-L) that shows a similar contrast to the test specimen being assessed.
5.5.2 Washing and laundering fastness
As a key influence on the degradation of colour, it is essential that the resistance of textile colour to modification caused by the effects of domestic and commercial laundering is tested and assessed.
The first wash tests prepared by standard organizations were based on the use of soap as a detergent, and five normalized test conditions were originally proposed.
As technology and textiles have progressed, new test methods have been stabilized and adapted to reflect changes in standard washing conditions (such as lower washing temperatures) and in modern synthetic detergent formulations (non- phosphate detergents and/or incorporation of bleaching activators). Appropriate equipment is used to conduct these tests, consisting of a thermostatically control- led water bath containing a rotatable shaft which supports stainless steel containers.
Non-corrodible steel balls can be used to simulate different levels of abrasive and mechanical action.
Several test conditions can be selected and altered for colour fastness determi- nation, based on the vast range of factors capable of influencing colour fastness to washing. These include:
• Temperature of the washing bath, usually chosen between 40 and 95 ºC.
• The washing liquor to goods ratio.
• The type and composition of the detergent used. The detergent could be a soap or synthetic, and may include phosphate. The synthetic detergent called ECE (European Colourfastness Establishment) has been normalized in Europe, while in the USA, WOB (Without Optical Brighteners) is used.
• The use of additives to the washing bath. Bleaching powers and activators, such as sodium perborate tetrahydrate, tetra-acetylethylenediamine (TAED) or so- dium nonanoyloxybenzene sulfonate (SNOBS) may be used in the washing bath. Furthermore, other oxidizing agents (e.g. sodium hypochlorite) and/or optical brighteners may be included.
• The severity of mechanical action. This can be controlled by changing the duration of the test and the number of steel balls added to the container holding the test specimen.
To include the aforementioned factors in a standardized test, a variety of normal- ized washing methods for testing the colour fastness of domestic laundering have been established. For example, ISO 105-C06 (2010) details 16 washing tests, which include changes to the following factors: temperature, liquor ratio, time, number of steel balls and the addition of bleaching activators (e.g. perborate) and/
or oxidizing agents (e.g. sodium hypochlorite). As an example, the washing test referenced as C2S is carried out for 30 min at 60 ºC, with a liquor ratio of approximately 30:1 (bath volume of 50 mL), including sodium perborate
Colour fastness 99 tetrahydrate and the use of 25 steel balls. Single (S) and multiple (M) type tests can be carried out, according to the selected abrasive action of the test.
A composite specimen composed of the test specimen placed between two single adjacent fabrics, or mounted together with one multifibre adjacent fabric, is placed in the container of a washing machine according to the specified operating conditions (e.g. temperature, time, number of balls, type of detergent and addi- tives). The specimen is treated with a solution of 4 g/L of detergent before being removed from the container, rinsed and dried. The colour change of the sample and the staining on the single or multifibre adjacent fabrics are then evaluated in comparison to the original colour, using the corresponding Grey Scales in each case. For staining assessment, the white adjacent fabric (single or multifibre) must be treated with the same procedure in a separate blank container.
5.5.3 Other colour fastness test methods
Apart from light, weathering and washing fastness tests, a wide variety of different tests have been developed to examine the effects of many additional agents on colour fastness.
Colour fastness to aqueous agent tests (e.g. ISO 105 Part E) examine any change in colour of a test specimen and staining on the adjacent fabric after immersion of the samples in a suitable solution, such as water, sea water or perspiration. The specimens are placed between two glass plates and are kept in an oven under constant pressure and temperature (normally 37 ºC) for some time using an apparatus such as a perspirometer.
The testing of colour fastness to atmospheric contaminants, such as ozone and nitrogen oxides, is important to the production of many textiles. ISO 105 Part G sets out the conditions for these tests, which evaluate the resistance of textile colour to the influence of nitrogen oxides as they occur during incineration processes, and to ozone and nitrogen oxides in the atmosphere. Special gas fading apparatus are used to expose the test samples to the gas until the simultaneously exposed control dyeing specimen has changed to the corresponding standard dyeing shade.
Colour fastness to bleaching agents such as sodium hypochlorite and hydrogen peroxide, has also been standardized (i.e. ISO 105 part N) to assess resistance of colour to these kinds of oxidizing agents. Colour fastness to sodium hypochlorite, for example, is assessed by treating the test specimen with sodium hypochlorite solution (2 g/L available chlorine) for 1h at 20 ºC, and recording resultant colour degradation.
Fastness to rubbing (ISO 105-X12) is commonly tested to assess colour resist- ance. A piece of white cotton rubbing cloth is rubbed against the dyed test specimen, and the extent of the colour transfer is recorded. An apparatus called a Crockmeter (linear or rotator) is used to allow the adjustment of speed and pressure during testing.
Colour fastness to hot pressing (i.e. ISO 105-X11) can be evaluated by treating
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a dyed test specimen with dry heat and assessing both the change in colour and any staining to adjacent cotton fabric. Detailed hot dry treatments for synthetic fibres, and hot wet steam treatments for natural and synthetic fibres have been established in ISO 105-P.
Many others tests have been established to test and assess colour fastness, and further details and information are available in fastness standard methods, includ- ing the ISO 105 and AATCC test method outlines.