Factors affecting the light fastness of dyes and improvement methods

Introduction

Light fastness refers to the ability of dyed materials to maintain their original color under sunlight. According to general regulations, the determination of light fastness is based on sunlight. In order to facilitate control in the laboratory, artificial light sources are generally used and corrected when necessary. The most commonly used artificial light sources are xenon lamps, and carbon arc lamps are also used. When dyed materials are exposed to light, the dye absorbs light energy, the energy level increases, the molecules are in an excited state, the color system of the dye molecules changes or is destroyed, resulting in the decomposition of the dye and discoloration or fading.

1. The effect of light on dyes

When a dye molecule absorbs the energy of a photon, it will cause the outer valence electrons of the molecule to transition from the ground state to the excited state.

Depending on the structure, dye molecules can undergo different excitation processes under the action of light waves of different wavelengths, including π →π*, n → π*, CT (charge transfer), S →S (singlet), S → T (triplet), ground state → first excited state and ground state → second excited state. The ground state of the singlet state is written as S0, and the first and second excited singlet states are written as S1 and S2 respectively. The corresponding triplet states are represented by T0, T1, and T2.

During the excitation process, the dye molecules are excited into electronic excited states of various vibrational energy levels, and their vibrational energy levels will rapidly decrease, converting energy into heat and dissipating. This process of lowering energy levels is called vibration passivation. During the vibration passivation process, the S2 excited state with a low vibrational energy level will also be transformed into the S1 excited state with a higher vibrational energy level, and vibration passivation will continue to occur. In this way, the original S2 excited state with a higher energy level is rapidly converted into the S1 excited state with the lowest vibrational energy level. The conversion between S2 and S1 electronic energy states under the condition of equal energy intersection does not include the change of electron spin multiplicity, which is called internal conversion. Conversion also occurs between singlet and triplet states, from S1 to T1 excited state. This kind of electronic energy state conversion accompanied by the change of electron spin multiplicity under the condition of equal energy intersection is called intersystem crossing. Due to the "forbidden" of electron spin selection law, the rate of intersystem crossing is generally relatively low.

The photochemical reaction between the excited dye molecules and other molecules leads to the light fading of the dye and the light brittle loss of the fiber.

2. Factors affecting the light fastness of dyes

1. Light source and wavelength of irradiated light;

2. Environmental factors;

3. Chemical properties and organizational structure of fibers;

4. Bonding strength between dye and fiber;

5. Chemical structure of dye;

6. Dye concentration and aggregation state;

7. The influence of artificial sweat on dye light fading;

8. The influence of auxiliaries.

The influence of dye floating color, incomplete soap washing after dyeing, and the residue of unfixed dyes and hydrolyzed dyes on the fabric will also affect the light fastness of the dyed fabric. Their light fastness is significantly lower than that of fixed reactive dyes. The more thorough the soap washing is, the better the light fastness is. The application of cationic low molecular weight or polyamine condensed resin-type fixing agents and cationic softeners in fabric finishing will significantly reduce the light fastness of the dyed fabric. Therefore, when selecting fixing agents and softeners, attention must be paid to their influence on the light fastness of the dyed fabric.

3. Methods for improving the light fastness of dyes

1. Improve the dye structure so that it can minimize the influence of the dye color development system while consuming light energy, thereby maintaining the original color; that is, the so-called high light fastness dyes. Such dyes are generally more expensive than ordinary dyes. For fabrics with high sun exposure requirements, the first thing to start with is the selection of dyes.

2. If the fabric has been dyed and the light fastness does not meet the requirements, it can also be improved by auxiliaries. Add appropriate auxiliaries during or after dyeing, so that they react before the dye when exposed to light, consuming light energy, thereby protecting the dye molecules. Generally divided into UV absorbers and anti-UV agents, collectively known as sunlight fastness enhancers.

4. Recommended high-sunlight dyes and UV absorbers with better effects:

Offering excellent light fastness, this series meets various fabric requirements. It reduces dyeing costs while maintaining high standards for light exposure. Suitable for fabrics needing durable lightfastness.

Sylic® Anti-UV Finishing Agent

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