The color fastness of textiles to sunlight has attracted 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 underwear standards), all use light and color fastness as one of the assessment standards. In the product standards of chemical fiber fabrics and cotton fabrics, color fastness to light is also used as an important assessment indicator. Some product standards even use color fastness to light as an assessment indicator.
01 Sunlight fastness
Sunlight has wave-particle duality, and sunlight that transfers energy in the form of photons has a strong impact on the molecular structure of dyes. When the basic structure of the color-producing part of the dye structure is destroyed by photons, the color of the light emitted by the dye color body will change, usually becoming lighter in color until it is colorless. The more obvious the color change of the dye is under sunlight conditions, the worse the light fastness of the dye is. In order to improve the light fastness of the dye, dye manufacturers have adopted many methods. Increasing the relative molecular weight of the dye, increasing the opportunity for internal complexation of the dye, and increasing the coplanarity and length of the conjugated system of the dye can relatively improve the light fastness of the dye.
Taking phthalocyanine dyes with level 8 light fastness as an example, adding appropriate metal ions during the dyeing and finishing process to form complexes within the dye molecules can significantly improve the dye’s brightness and light fastness. . For textiles, choosing dyes with better sun fastness is the key to improving the sun fastness grade of the product.
02 Cause Analysis
1. Influence of dye/paint structure
The photofading mechanism of dyes is very complex, but the main reason is that the dye is 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, combination state and mixed color matching. Therefore, the rational selection of dyes is very important.
↑ Mechanism of dye/paint sun fading
2. The influence of dyeing depth and floating color
The light fastness of dyed fabrics is related to the dyeing depth. The greater the dyeing depth, the larger the dye aggregate particles on the fabric, the smaller the proportion of 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. Sunlight fastness also tends to decrease accordingly. In factories, the light fastness grades that cannot meet the requirements are generally medium and light colors. Some light brown fabrics, after the light fastness test, the red color is almost completely lost, and the color change is obvious. Whether the dyeing process is properly selected and whether washing and soaping are thorough after dyeing will affect the amount of unfixed dyes and hydrolyzed dyes, that is, floating colors, present on the fabric. The light fastness of floating colors is significantly lower than that of fixed reactive dyes. Therefore, improper post-dyeing treatment will also affect the light fastness of fabrics.
3. The influence of color fixing agent and softening agent
The use of fixatives greatly improves the rubbing fastness, washing fastness and stain fastness of reactive dyes. Generally, the washing fastness of fabrics treated with cationic low-molecular or polyamine fixing agents is at level 4 to 5, but the light fastness of fabrics fixed by such fixing agents decreases. Using cationic softeners to soften cotton fabrics will reduce the light fastness of reactive dyes, mainly because the softeners will turn yellow after being exposed to the sun, causing the fabric’s color and light to also change.
03 Improvement measures
1. Selection of dye or paint
The photofading mechanism of dyes is very complex, but the main reason is that the dye is 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, combination state and mixed color matching. Therefore, the rational selection of dyes is very important.
First, dyes are selected based on fiber properties and textile use. For cellulose fiber textiles, dyes with better oxidation resistance should be selected; for protein fibers, dyes with better resistance to reduction or containing weak oxidizing additives should be used; for other fibers, dyes should be selected based on their impact on fading. In order to enhance the photooxidation stability of the azo group in the dye molecular structure, during the dye synthesis process, some strong electron-withdrawing groups are usually introduced at the ortho-position of the azo group, thereby reducing the electron cloud density of the nitrogen atom of the azo group. 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, thereby reducing the electron cloud density of the hydrogen atoms of the azo group and shielding the azo group, ultimately improving the dye quality light fastness.
Second, dyes should be chosen based on color depth. A large number of tests proveThe light fastness of reactive dyes on cellulose fibers is proportional to the depth of the dye, 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 chance of the dye being photo-oxidized. On the contrary, the lighter the color, the dye is mostly highly dispersed on the fiber, and the probability of being exposed to light is higher, which ultimately leads to a significant decrease in light fastness. Therefore, when dyeing light-colored varieties, dyes with higher light fastness should be used. In addition, many 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 used.
Third, dyes with good light stability and compatibility should be used 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, you should choose dyes that will not sensitize each other and can even improve light resistance. This is especially important when dyeing dark colors such as black. If one of the three primary colors fades too quickly, it will quickly cause the dyed fiber or fabric to discolor, and the faded dye residue will also affect the light stability of the other two dyes that have not faded. Reasonable control of the dyeing process to fully combine the dye with the fiber and try to avoid hydrolyzed dyes and unfixed dyes remaining on the fiber is an important way to obtain higher light fastness.
2. Improvement of soaping process
During the dyeing process, a reasonable dyeing process should be formulated, soaping and washing should be carried out fully, and the amount of hydrolyzed dyes and floating colors should be reduced as much as possible to improve the sun fastness of the fabric; at the same time, the color fastness to water washing, water perspiration, friction, etc. should be improved. .
3. Selection of color fixing agent and softening agent
Most fixatives are quaternary ammonium salts, sulfonium salts or phosphorus salts. Such fixatives and dyes produce lakes on the fiber. Although the washing fastness is very good, it often reduces the original light fastness of the dye. Therefore, when the requirements for light fastness are high, try to avoid using this type of color fixing agent. While some cationic softeners and amino-modified silicone softeners bring a plump feel to fabrics, they also have drawbacks such as yellowing, discoloration of dyes, and inhibition of fluorescent whitening agents.
4. UV absorbers and sunlight fastness enhancers
If the processing conditions for dyed fabrics have been fixed, such as dyeing, color fixation, softening and finishing and other steps have been completed, to improve the sunlight fastness of dyed fabrics under these conditions, you can choose a sunlight fastness enhancer or ultraviolet absorber. This type of additive can directly absorb ultraviolet rays irradiating the fabric and prevent the dye from being damaged by photooxidation.
5. Sunlight fastness enhancer
It can significantly improve the UV resistance, sun resistance and perspiration light fastness of fabrics dyed with reactive, direct, disperse, acid and vat dyes. It is suitable for improving the UV resistance finishing of fabrics dyed with reactive, direct, disperse, acidic and vat dyes.
04 Postscript
When it comes to improving light fastness, the choice of dye or coating is key. For single colors, try to choose dyes with higher light fastness. For colored fabrics that require color matching, the light fastness levels of each component dye should be equivalent. The poor light fastness of one component will affect the light fastness of the entire mixed color. For dyes with relatively small content in the mixed color components, dyes with high light fastness levels can be selected to ensure the overall color fastness of the fabric to light.
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