Color Fastness Properties | Factors Affecting Color Fastness

What is Color Fastness Properties?
The textile industry consumes thousands of tonnes of colorants in the form of dyes and pigments, which are used to add color to textile goods via dyeing and printing. Dyed materials have various uses. Consequently, they are subjected to treatment with several external agents under diversified conditions. The resistance to such external agents is known as fastness properties. We can also clarify color fastness in the following way; color fastness means the resistance of the color of textiles to the different agents to which these materials may be exposed during manufacture and their subsequent use.

Fastness of dyed textiles is evaluated in regard to natural destructive agents, such as daylight, weather, and atmospheric gases, as well as to various treatments the material is likely to undergo, such as washing, dry cleaning, ironing, steaming, etc.

The fastness or stability of the resultant color is crucial in attaining commercially acceptable colored textiles. Frequently a compromise must be reached between the cost of attaining a particular color of goods and the fastness properties of such goods.

Color fastness standards were fixed by various national standards organizations (Langton, 1984), such as SDC, AATCC, JIS, and, in particular, by ISO, the International Organization for Standardization, which produced the ISO 105 series used worldwide. At present, these ISO color fastness standards are adopted without any technical changes by the European Standard (EN) of the European Committee for Standardization (CEN) and are binding for all the countries members.

The most important fastness properties are:

1. Washing fastness
2. Light fastness
3. Rubbing fastness – wet and dry
4. Fastness to perspiration
5. Fastness to bleaching
6. Fastness to dry heat

1. Washing Fastness:
Unlike other types of fabric testing, such as fammability, there are no mandatory legal requirements for color fastness testing. But color fastness testing is essential to ensure customer satisfaction with fabric products. Color fastness issues will often prompt consumers to reject the product and return it or submit a claim.

Poor washing fastness of dyes may appear in two ways: dyes may come out from the dyed or printed fabric causing lower color strength of the washed sample as com­pared to unwashed sample (termed as “color change”).

The washing fastness ratings are expressed by two numbers: one number represents rating for “color change”, while the other for “staining”. Both assessments are made by measuring color contrast between washed and raw (i.e., unwashed) samples. The contrast is compared visually with the contrast represented by Grey Scale.

2. Light Fastness:
The light fastness of dyed materials is measured by exposing them to daylight under glass or to an artificial illumination in an accelerated fading lamp alongside a set of standard dyed materials.

Currently, the most common standards for light fastness testing are the blue wool standards. At the time of development of the light fastness standards, wool was found to be the substrate that was least effected by humidity. The scale consists of a set of eight woolen fabrics dyed with different dyes numbered 1 to 8 (British system) in order of increasing fastness – 1 having lowest light fastness and 8 hav­ing highest fastness.

3. Rubbing Fastness:
Rubbing or crocking fastness is the transfer of dyed textile material’s color from its surface to an adjacent area by the same fabric or other surface (normally bleached cotton fabric) principally by rubbing. It is tested using a crockmeter. It may be either hand-driven or motorized. This test is designed to determine the degree of color which may be transferred from the surface of a colored fabric to a specific test cloth for rubbing (which could be dry and wet). Fastness to rubbing is a main test which is always required for every colored fabric, be it printed or dyed.

The rubbing is carried out by mov­ing 10 times in 10 seconds with a downward force of 6 N. The color change of the rubbed fabric and the staining of the rubbing white cloth are assessed by the respective Grey Scales.

4. Perspiration Fastness:
Unless dyes are selected carefully, those parts of the cloth or garment which come in contact with the body, where perspiration is heavy, may suffer local discoloration. Two artificial perspiration solutions (BS 1006 E04: 1978) are pre­pared by dissolving in one liter of distilled water 0.5 g l-histidine monohydro­chloride monohydrate (C₆H₉O₂N₃ HCl,H₂O), 5.0 g sodium chloride, 2.5 g (2.2 g for the 2nd solution) disodium orthophosphate (Na₂HPO₄ ,2H₂O); and pH is adjusted to 8 and 5.5 with 0.1 N NaOH and 0.1 N acetic acid for two solutions, respectively.

The material to be tested is placed between two undyed pieces, one of which is of the same fiber and the other of a different fiber as in the case of washing fastness test. The specimens are then placed between glass plates pressed together with a force equivalent to 10 pounds (4.536 kg) and allowed to stand in an oven at 37 ± 2°C for 4 hours. The color change of the dyed fabric and the staining of the white cloth are assessed by the respective Grey Scales.

5. Color Fastness to Peroxide Bleaching:
The fastness to peroxide (BS: 1006 N02: 1978) test is carried out with a specimen sewed with two adjacent fabrics as in case of washing fastness test. The composite specimen is kept inside a test tube by rolling in the direction of the longer side and bleaching is carried out. A refux condenser is fitted with the test tube to reduce evap­oration from the bleaching bath during test. After bleaching, the composite specimen is removed and rinsed for 10 minutes in cold running tap water, squeezed and dried by hanging in air. The change in color of the specimen and the staining of the adja­cent fabrics are assessed with the Grey Scales.

6. Fastness to Dry Heat:
The purpose of dry heat fastness tests is to determine how much the color of dyed or printed textile products resists dry heat (sublimation) effect. This fastness (BS: 1006 P01: 1978) is to be regularly assessed for disperse dyes on polyester, as these dyes are sublimable. For dyed or printed textile specimens sewed with two adjacent fabrics (one of them usually polyester), the time of treatment is 30 seconds and the three temperatures recommended are 150 ± 2°, 180 ± 2° and 210 ± 2°C. The change of color and staining on adjacent fabric are then assessed with the respective Grey Scales.

Factors Affecting Color Fastness:
The color fastness properties of a dyed or printed textile are characteristic of the dye–fiber system. Multiple factors can influence the final fastness properties in a complex manner. However, some factors are known to have a direct influence on color fastness properties:

1. The molecular structure of the dye.
2. The type and structure of the fiber.
3. The dyeing process.
4. The depth of color.

a) The molecular structure of the dye: The molecular structure of a dye is crucial for its fastness properties. The textile industry is familiar with many practical examples of the effect of this factor on fastness.

b) The type and structure of the fiber: The type and structure of the fiber is also key, as in most cases the same dye applied to different types of textile fiber results in different dye fastness properties. Cationic dyes, for example, produce better light fastness on acrylic fibers than on wool, silk or mordanted cellulosic fibers, whilst disperse dyes on polyester exhibit greater resistance to atmospheric contaminants than on polyamide fibers.

c) The dyeing process: The treatments to which the fibers have been subjected can further influence fastness properties. For wool, shrink-resist finish chlorination treatments will affect color fastness, whilst for synthetic fibres, thermosetting treatments can also modify fastness properties. Methods of dyeing and post dyeing treatments can also influence the color fastness of dyed materials. Color fastness may be influenced by both the temperature at which the dyeing is carried out, and the carriers used during the process. In addition, post dyeing treatments can be carried out with the purpose of improving fastness properties.

d) The depth of color: Finally, the fastness of a dye depends on the depth of color. An absolute color fastness value of a dye in relation to a particular agent cannot be established, as such a value is a function of its concentration.

References:

1. Principles of Textile Printing by Asim Kumar Roy Choudhury
2. Understanding and Improving the Durability of Textiles Edited by Patricia A. Annis
3. Textile Dyes By Mansoor Iqbal
4. A Practical Guide to Textile Testing By K. Amutha

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