The light fastness of textiles has been paid more and more attention at home and abroad. At present, my country's textile industry product standards, especially the new standards promulgated in recent years (except for underwear standards), all take light fastness as one of the assessment standards. In chemical fiber silk fabrics and cotton fabric product standards, color fastness to light is also used as an important assessment index, and some product standards even use color fastness to light as an assessment index.

1 The light fastness

Sunlight has wave-particle duality, and sunlight that transmits energy in the form of photons has a strong impact on the molecular structure of dyes. When the basic structure of the chromophore in the dye structure is destroyed by photons, the color of the light emitted by the dye chromophore will change, usually the color becomes lighter until it is colorless. The more obvious the color change of the dye under sunlight, the worse the light fastness of the dye. In order to improve the light fastness of the dye, dye manufacturers have taken many measures. Increasing the relative molecular weight of the dye, increasing the chance of internal complexation of the dye, increasing the coplanarity of the dye and the length of the conjugated system can relatively improve the light fastness of the dye.

For phthalocyanine dyes that have reached 8 grades of light fastness, adding appropriate metal ions in the dyeing and finishing process to form complexes inside the dye molecules can significantly improve the brightness and light fastness of the dyes. . For textiles, choosing a dye with better light fastness is the key to improving the light fastness grade of the product.

2 Reason analysis

1. Influence of dye/paint structure

The photobleaching mechanism of dyes is very complicated, but the main reason is that the dyes are excited after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The light fastness of textiles mainly depends on the chemical structure of the dye, as well as its aggregation state, bonding state and mixed color combination. Therefore, the rational selection of dyes is very important.

Dye/Paint Solar Fading Mechanism

2. The influence of dyeing depth and floating color

The light fastness of dyed fabrics is related to the depth of dyeing. The greater the dyeing depth, the larger the aggregate particles of the dye on the fabric, the smaller the proportion of the dye per unit weight exposed to the air, and the higher the light fastness. For light-colored fabrics, the proportion of dye aggregates on the fiber is smaller, The light fastness also tends to decrease accordingly. In the factory, the light fastness grades that cannot meet the requirements are generally medium and light colors, and some light brown fabrics. After the light fastness test, the red color light is almost completely lost, and the color change is more obvious. Whether the dyeing process is properly selected, and whether the washing and soaping are thorough after dyeing, will affect the amount of unfixed dyes and hydrolyzed dyes present on the fabric, that is, the amount of floating color. The light fastness of the floating color is obviously lower than that of the fixed reactive dyes. Therefore, improper treatment after dyeing will also affect the light fastness of the fabric.

3. Influence of fixing agent and softener

The use of fixing agent greatly improves the rubbing fastness, washing fastness and staining fastness of reactive dyes. Generally, the washing fastness of fabrics treated with cationic low-molecular or polyamine fixatives is in the 4-5 grade, but the light fastness of fabrics fixed by such fixatives has decreased. Soft finishing of cotton fabrics with cationic softener will reduce the light fastness of reactive dyes, mainly because the softener will turn yellow after being exposed to the sun, so that the color of the fabric will also change.

3 Improvement measures

1. Choice of dye or paint

The photobleaching mechanism of dyes is very complicated, but the main reason is that the dyes are excited after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The light fastness of textiles mainly depends on the chemical structure of the dye, as well as its aggregation state, bonding state and mixed color combination. Therefore, the rational selection of dyes is very important.

First, dyes are selected according to fiber properties and textile applications. For cellulose fiber textiles, dyes with better oxidation resistance should be selected; for protein fibers, dyes with better reduction resistance or containing weak oxidative additives should be selected; for other fibers, dyes should be selected according to the effect on fading. In order to enhance the photo-oxidative stability of the azo group in the dye molecular structure, some strong electron withdrawing groups are usually introduced in the ortho position of the azo group during the dye synthesis process, thereby reducing the electron cloud density of the azo nitrogen atom. In addition, hydroxyl groups can also be introduced into the two vicinal positions of the azo group to use its coordination ability to complex with heavy metals, thereby reducing the electron cloud density of the hydrogen atom of the azo group, and shielding the azo group. light fastness.

Second, the dye should be selected according to the depth of the color. A large number of tests have proved that the light fastness of reactive dyes on cellulose fibers is proportional to the depth of the dyed luster, that is, the darker the color, the better the light fastness. This is because the higher the concentration of the dye on the fiber, the greater the aggregation of the dye molecules, the smaller the surface area of the same amount of dye exposed to air, moisture and light, and the lower the probability of the dye being oxidized by light. On the contrary, the lighter the color, the more highly dispersed the dyes are on the fiber, the higher the probability of being exposed to light, and the color fastness to light will decrease significantly. Therefore, when dyeing light-colored varieties, dyes with higher light fastness should be selected. In addition, many post-finishing agents such as softeners and anti-wrinkle finishing agents are added to the fabric, which will also reduce the light fastness of the product. Therefore, dyes that are not sensitive to these finishing agents should be selected.

Third, dyes with good light resistance stability and compatibility should be selected for color matching. Different dyes have different fading properties and even different photofading mechanisms. Sometimes, the presence of one dye can sensitize the fading of another dye. When color matching, dyes that will not sensitize each other and can even improve light stability should be selected, which is particularly important when dyeing dark varieties such as black. If one of the three primary colors fades too quickly, the dyed fiber or fabric will soon be discolored, and the faded dye residue will also affect the light stability of the other two dyes that are not faded. Reasonable control of the dyeing process, to fully combine the dyes with the fibers, and to avoid the hydrolyzed dyes and unfixed dyes remaining on the fibers is an important way to obtain higher color fastness to light.

2. Improvement of soaping process

In the dyeing process, a reasonable dyeing process should be formulated, fully soaped and washed, and the amount of hydrolyzed dyes and floating color should be minimized to improve the light fastness of the fabric; at the same time, it should also improve the color fastness to washing, water perspiration, friction, etc. .

3. Selection of fixing agent and softener

Most fixatives are quaternary ammonium salts, sulfonium salts or phosphorus salts. Such fixatives and dyes form lakes on the fibers. Although the washing fastness is very good, it often reduces the original light fastness of the dyes. Therefore, when the light fastness requirements are high, try to avoid using this type of fixing agent. Some cationic softeners and amino-modified silicone softeners bring a plump hand to the fabric, but also have disadvantages such as yellowing, discoloration of dyes, and inhibition of fluorescent whitening agents.

4. UV absorbers and light fastness enhancers

If the processing conditions of dyed fabrics have been fixed, such as dyeing and fixing, soft finishing and other steps have been completed, under these conditions to improve the light fastness of dyed fabrics, you can choose a light fastness enhancer or UV absorber. Such additives can directly absorb the ultraviolet rays irradiated on the fabric and prevent the dyes from being damaged by photo-oxidation.

5. Light fastness enhancer

It can significantly improve the UV resistance grade, light fastness to sunlight and perspiration of dyed fabrics such as reactive, direct, disperse, acid, vat dyes, etc. It is suitable for anti-ultraviolet finishing of dyed fabrics such as reactive, direct, disperse, acid and vat dyes.

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