Light fastness refers to the degree to which a colored fabric is discolored by sunlight, also called light fastness.
The test method can be used either by daylight or by sunlight, that is, the sample and a set of blue wool standards (divided into three levels according to the degree of fading under the illumination of light), and placed in the equivalent Under the artificial light source of sunlight, the exposure was carried out according to the specified conditions, and then the discoloration of the sample and the blue wool standard was compared to evaluate the light fastness level of the sample.
The degree of fading of the sample after exposure to sunlight is compared with the standard color sample. The European standard is divided into 8 grades, 8 grades are the best, and grade 1 is the worst. The US standard is divided into 5 levels.
Fabrics with poor light fastness should not be exposed to sunlight for a long time, and should be placed in a ventilated place to dry.
1. Influencing factors of light fastness
It is related to dye structure: generally, anthraquinone dyes, phthalocyanine dyes, sulfurized blue and metal complex dyes have better light fastness; most insoluble azo dyes have higher light fastness; The aniline type azo dye has a low light fastness; the triarylmethane dye is not resistant to the sun.
It is related to the type of fiber: 1 indigo is light fast on wool and has a low light fastness on cellulose; 2 vanamin blue salt B has a higher light fastness on the adhesive than on cotton. .
It is related to the concentration of the dye: the low concentration is generally lower than the high concentration, and this is especially true on the azo dye.
It is related to the influence of external conditions: the level of moisture content of the air and the temperature of the air have a great influence on the light fastness. Generally, when the moisture content is high, the light fastness is low. For example, Van Lamming Blue is easily reddish and faded in the South than in the North.
Related to auxiliaries: There are now many auxiliaries that can effectively improve the light fastness.
2. Measures to improve the light fastness
Dye selection (the most important factor)
1 high light fastness of yellow spectrum: the reactive dyes are pyrazoline and naphthalene trisulfonic acid as the parent structure;
2 high light fastness of blue spectrum: the reactive dyes are made of ruthenium, phthalocyanine and methyl ester;
3 Red spectrum reactive dyes generally have low light fastness, especially light color. Only some azo groups have hydroxyl groups in two adjacent positions, which form a stable chelate ring with copper, which can improve sun exposure. degree;
4 When using dyes for color combination, it is necessary to make the dye fastness level of each component dye is equivalent, as long as any one of the components, especially the least used component, does not reach the light fastness. The requirements of the object will make the final dyeing fastness to the standard.
3. Other measures
1 The effect of dye floating color, after soaping is not thorough, the unfixed dye and hydrolyzed dye remaining on the cloth surface will also affect the light fastness of the dye, their light fastness is significantly lower than the fixed dye. The more fully soaped, the better the light fastness;
2 cationic low molecular or polyamine condensed resin type fixing agent and cationic softening agent applied to fabric finishing, which will significantly reduce the light fastness of the dyed material. Therefore, the use of fixing agents and softeners must pay attention to their effects on the light fastness of dyes;
3 UV absorbers have recently been used in light-colored dyes to improve light fastness, but they must be used in large amounts to have some effect, which not only increases the cost, but also causes yellowing and strong damage to the fabric, so it is not a good measure.
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