How to improve the fastness of textiles to the sun?

More and more attention has been paid to the color fastness of textiles to the sun. At present, the product standards of China's textile industry, especially the new standards issued in recent years (except for underwear standards), all regard light fastness as one of the assessment standards. The color fastness to light is also taken as an important evaluation index in the standards of chemical fiber and cotton products, and some product standards even take the color fastness to light as an evaluation index.

1. Sunlight fastness

Sunlight has wave-particle duality, and the sunlight which transmits energy in the form of photon has strong impact on the molecular structure of dye. When the basic structure of the chromophobe part of the dye structure is destroyed by photons, the color of the light emitted by the dye chromophobe will change, usually the color becomes lighter, until it is colorless. In the sunshine condition, the more obvious the color change of dye, the worse the fastness to the sun in order to improve the fastness to the sun of dye, dye manufacturers have taken many measures. The sun fastness of dyes can be improved by increasing the relative molecular weight of dyes, increasing the chance of complexation, increasing the coplanar properties of dyes and the length of conjugate system.

For phthalocyanine dyes with grade 8 sun fastness, the brightness and sun fastness of dyes can be obviously improved by adding appropriate metal ions to form complex compounds inside the dye molecules during dyeing and finishing process. For textiles, the selection of dyes with good fastness to the sun is the key to improve the fastness to the sun.

a. Influence of dye/coating structure

The photofading mechanism of dyes is very complex, but the main reason is that dyes are activated after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The color fastness to light of textile mainly depends on the chemical structure of dye, its aggregation state, combination state and mixed color. Therefore, reasonable selection of dyes is very important.

b. Influence of dyeing depth and floating color

The sun fastness of dyed fabrics is related to the dyeing depth. The greater the dyeing depth, the greater the aggregation of dye particles on the fabric, the smaller the proportion of dye per unit weight exposed to the air, the higher the sun fastness for light-colored fabrics, the smaller the proportion of dye aggregates on the fiber, the sun fastness also tends to decrease accordingly. In the factory, the sunfastness grade that cannot meet the requirements is generally medium or light color. Some light brown fabrics, after the sunfastness test, almost completely lose the red light, and the color change is obvious. The amount of unfixed dyes and hydrolyzed dyes on the fabric will be affected by the choice of proper dyeing process and thorough washing and soapping after dyeing. The light fastness of the floating color is obviously lower than that of the fixed reactive dye. Therefore, improper post-dyeing will also affect the light fastness of the fabric.

c. The influence of fixing agent and softener

The use of fixing agent greatly improved the rubbing fastness washing fastness and staining fastness of reactive dyes. The washing fastness of fabrics treated with cationic low molecular or polyamine fixative is at grade 4 ~ 5, but the fastness of fabrics treated with this kind of fixative is decreased. Soft finishing of cotton fabric with cationic softener will decrease the fastness to the sun of reactive dyes, mainly because the softener will yellowing after sun exposure, which will change the color of the fabric.

a. Choice of dye or paint

The photofading mechanism of dyes is very complex, but the main reason is that dyes are activated after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The color fastness to light of textile mainly depends on the chemical structure of dye, its aggregation state, combination state and mixed color. Therefore, reasonable selection of dyes is very important.

First, dyes are selected according to fiber properties and textile uses. For cellulose fiber textiles, dyes with better oxidation resistance should be selected. For protein fiber, should choose good reducibility or containing weak oxidation additives dyes; Other fibers should be selected according to the effect of fading. In order to enhance the photooxidation resistance of azo groups in the molecular structure of dyes, some strong electron-absorbing groups are usually introduced into the adjacent positions of azo groups during the synthesis of dyes, so as to reduce the electron cloud density of azo nitrogen atoms. In addition, hydroxyl groups can also be introduced into the two ortho positions of the azo group, and their coordination ability can be used to complex with heavy metals, thus reducing the electron cloud density of the azo hydrogen atom, and shielding the azo group, and ultimately improving the color fastness to light of the dye.

b. dyes should be selected according to the color depth. A large number of experiments have shown that the color fastness to light of reactive dyes on cellulose fibers is proportional to the depth of color, that is, the darker the color, the better the color fastness to light. This is because the higher the concentration of dye on the fiber, the greater the concentration of dye molecules, the smaller the surface area exposed to air, water and light for the same amount of dye, and the lower the chance that the dye will be photooxidized. On the contrary, the lighter the color, the dye on the fiber is mostly highly dispersed, the probability of exposure to light is higher, and finally the color fastness to light decreased significantly. Therefore, dyed light varieties, should be selected to light fastness higher dye. In addition, the addition of many finishing agents, such as softeners and anti-wrinkle finishing agents, can also reduce the light fastness of the product. Therefore, dyes that are not sensitive to these finishing agents should be selected.

c. should choose light stability, good compatibility of dye color. Different dyes have different fading properties and even different photofading mechanisms. Sometimes the presence of one dye sensitizes the fading of another. Color, should choose mutual sensitization, and can even improve the light stability of the dye, which is particularly important in the dark variety such as black. Fast fading of one dye in the three primary colors will soon lead to discoloration of dyed fibers or fabrics, and the fading residue will affect the photostability of the other two dyes that are not fading. Reasonable control of dyeing process to fully combine dye with fiber and to avoid residue of hydrolyzed dye and unfixed dye on fiber is an important way to obtain high light fastness.

2. Improvement of soap washing process

In dyeing process, reasonable dyeing process should be made, full soaping and washing, and the amount of hydrolyzed dye and floating color should be reduced as much as possible to improve the sun fastness of the fabric. At the same time, it can also improve the color fastness of water washing, water sweat stains and friction.

3. Fixing agent and softener selection

Most fixing agents are quaternary ammonium salts, matte salts or phosphorus salts. This kind of fixing agent and dye generate lake on the fiber, although the washing fastness is very good, but often reduce the dye's original sun fastness. So when the sun fastness requirements are high, try to avoid choosing this kind of fixing agent. Some cationic softeners and amino modified silicone softeners bring plump feel to the fabric, but also have yellowing, dye color change and fluorescent whitening agent.

4. Uv absorbent and sun fastness enhancer

If the treatment conditions of the colored fabric have been fixed, such as dyeing fixing, soft finishing and other steps have been completed, under this condition to improve the sun fastness of the colored fabric, you can choose the sun fastness enhancer or ultraviolet absorbent. These additives can directly absorb ultraviolet light on the fabric and prevent the dye from being destroyed by photooxidation.

5. Sun fastness enhancer

It can significantly improve the uv resistance, light fastness to sun and sweat of reactive, direct, disperse, acid, VAT dyes and other dyed fabrics. It is suitable for anti-uv finishing of fabrics dyed with reactive, direct, disperse, acid and VAT dyes.