Light fastness

Light fastness

Sunlight has wave-particle duality, and the sunlight that transfers energy in the form of photons has a strong impact on the molecular structure of the dye. When the basic structure of the coloring part in the dye structure is destroyed by photons, the color of the light emitted by the dye color body will change, usually the color becomes lighter, until it is colorless. The more obvious the color change of the dye under sunlight conditions, the worse the light fastness of the dye. 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 chance of internal complexation of the dye, increasing the coplanarity of the dye and the length of the conjugated system can relatively improve the fastness of the dye to light.

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

Cause Analysis

1. Dye/paint structure influence

The light-fading 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, leading to 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.

2. The effect 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 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 factories, the light fastness grades that cannot meet the requirements are generally medium and light colors. Some light brown fabrics have almost completely lost the red shade after the light fastness test, and the color change is more obvious. Whether the selection of the dyeing process is appropriate, whether the washing and soaping after dyeing is thorough, will affect the amount of unfixed dyes and hydrolyzed dyes that are floating on the fabric. The light fastness of floating colors is significantly lower than that of the fixed reactive dyes. Therefore, improper handling after dyeing will also affect the light fastness of the fabric.

3. The influence of fixing agent and softener

The use of fixing agent greatly improves the rubbing fastness of reactive dyes, washing fastness and staining fastness. Generally, the washing fastness of fabrics treated with cationic low-molecular-weight or polyamine fixing agents is in the 4 to 5 grades, but the light fastness of fabrics fixed by this kind of fixing agents has decreased. The softening and finishing of cotton fabrics with cationic softeners will reduce the light fastness of reactive dyes, mainly because the softeners undergo yellowing after exposure to the sun, which changes the shade of the fabric.