What is color fastness?

Dyeing fastness refers to the ability of dyes or pigments to maintain their original color state under the influence of various external factors during use or later processing of dyed fabrics.

Dyeing fastness is one of the important quality indicators for measuring dyed finished products. The dyeing fastness that is easy to fade is low, and the dyeing fastness that is not easy to fade is high. The color fastness depends largely on its chemical structure. In addition, the physical state of the dye on the fiber, the degree of dispersion, the combination of the dye and the fiber, the dyeing method and process conditions also have a great impact.

Dyeing fastness is multifaceted. For consumers, the most important ones include: sun exposure, soaping, perspiration, friction, brushing, ironing, smoke and other fastnesses. In addition, textiles have different uses or processing processes, and their fastness requirements are also different. In order to carry out quality inspections on products, textile and commercial departments have formulated a set of test methods and standards for dyeing fastness based on the use of textiles, which are briefly introduced below.

1. Light fastness The sun fading of dyed fabrics is a more complicated process. Under the action of sunlight, the dye absorbs light energy, and the molecules become extremely unstable in an excited state. It is easy to produce certain chemical reactions, which cause the dye to decompose and fade, leading to greater fading of dyed fabrics after exposure to sunlight. The light fastness varies with the dyeing concentration, and the light fastness of the low concentration is worse than the high concentration. The light fastness of the same dye on different fibers is also quite different. For example, the light fastness of indigo on cellulose fiber is only grade 3, but it is grade 7 to 8 on wool. The light fastness is also comparable to that of dyes. Factors such as the aggregation state on the fiber and the dyeing process are related.

The light fastness is divided into 8 grades, grade 1 is the worst and grade 8 is the best.

2. Soaping fastness refers to the degree of fading of dyed fabrics after soaping in soap solution under specified conditions, including the original fading and white cloth staining. The original fading refers to the fading of printed and dyed fabrics before and after soaping. White cloth staining is a situation where the white cloth and the dyed material are stitched together, and after soaping, the white cloth is stained due to the fading of the dyed material.

Soaping fastness is related to the chemical structure of the dye and the binding state of the dye and fiber. In addition, soaping fastness is also related to dye concentration, dyeing process, soaping conditions and so on.

The test conditions of soaping fastness vary with the types of fibers constituting the fabric. Common soaping temperatures can be divided into three types: 40℃, 60℃ and 95℃ (each dyeing factory has its own specific soaping temperature). After the test sample is tested, rinsed, and dried, it will be graded according to the national standard with a "gray faded sample card". The soaping fastness is divided into five grades and nine grades, among which grade one is the worst, grade five is the best, staining is also divided into five grades and nine grades, grade one is the most serious staining, and grade five is non-staining.

3. Rubbing fastness The rubbing of dyed fabrics is divided into dry rubbing and wet rubbing fastness. The former is used for white cloth rubbing fabrics, depending on the staining of the white cloth, the latter rubbing the dyed fabrics with 100% water-containing white cloth to see the staining of the white cloth. Wet friction is caused by external friction and the action of water, and its wet friction fastness is generally lower than that of dry friction.

The rubbing fastness of the fabric mainly depends on the amount of floating color, the combination of dye and fiber and the uniformity of dye penetration. If the dye is covalently bonded to the fiber, its rubbing fastness is higher. The concentration of the dye used in dyeing often affects the rubbing fastness. High dyeing concentration can easily cause floating color, so the rubbing fastness is low. The rubbing fastness is graded by the "stained gray sample card" according to a five-level nine-level comparison system, with the worst grade one and the best grade five.

The fastness to perspiration and the fastness to chlorine bleaching are rated according to five grades and nine grades.

To evaluate the color fastness of dyes, the textiles should be dyed to the specified color concentration for comparison. This is because different dyeing concentrations will make the measured fastness different. The test methods for the color fastness of dyed fabrics should be carried out according to the methods prescribed by the country, and the standards for the fastnesses should also be subject to national standards.