What Factors Affect The Lightfastness of Dyes? How Can It Be Improved?

Introduction

Lightfastness refers to the ability of a dyed material to retain its original color under sunlight. Generally, sunlight is used as the standard for determining lightfastness. In the laboratory, artificial light sources are typically used for ease of control, with calibration applied when necessary. The most commonly used artificial light source is a xenon lamp, although carbon arc lamps are also used. When dyed materials are exposed to light, the dye absorbs light energy, increasing its energy level and exciting the molecules. This alters or destroys the color development system of the dye molecules, leading to dye decomposition and discoloration or fading.

1. The Effect of Light on Dyes

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

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

During the excitation process, the dye molecule is excited into electronically excited states with various vibrational energy levels. These vibrational energy levels rapidly decrease, converting energy into heat and dissipating. This process of energy level reduction is called vibrational passivation. During vibrational passivation, the lower vibrational energy level S2 excited state can also transform into the higher vibrational energy level S1 excited state and continue to undergo vibrational passivation. Thus, the previously higher S2 excited state rapidly transforms into the lowest vibrational energy level, the S1 excited state. The transformation between the S2 and S1 electronic states under iso-energy intersection conditions does not involve a change in electronic spin multiplicity and is called an internal transformation. Transformation also occurs between singlet and triplet states, from S1 to T1 excited state. This electronic state transformation accompanied by a change in electronic spin multiplicity under iso-energy intersection conditions is called intersystem crossing. Due to the "forbidden" nature of electron spin selectivity, the rate of intersystem crossing is generally relatively low.

Excited dye molecules undergo photochemical reactions with other molecules, leading to dye fading and fiber photobrittleness.

2. Factors Affecting the Lightfastness of Dyes

(1) Light source and wavelength of illumination;

(2) Environmental factors;

(3) Chemical properties and structure of fibers;

(4) Bond strength between dye and fiber;

(5) Chemical structure of dye;

(6) Dye concentration and aggregation state;

(7) Influence of artificial sweat on dye light fading;

(8) Influence of auxiliaries.

The effect of dye floating, incomplete soaping after dyeing, and the presence of unfixed and hydrolyzed dyes remaining on the fabric surface will also affect the lightfastness of dyed fabrics. Their lightfastness is significantly lower than that of fixed reactive dyes. The more thorough the soaping, the better the lightfastness.

The application of cationic low-molecular-weight or polyamine condensation resin-type fixing agents and cationic softeners in fabric finishing will significantly reduce the lightfastness of dyed fabrics. Therefore, the influence of fixing agents and softeners on the lightfastness of dyed fabrics must be considered when selecting them.

3.Methods to Improve the Light Fastness of Dyes

1. Improve the dye structure to minimize the impact on the dye's color development system while consuming light energy, thus maintaining the original color; this is commonly referred to as high light fastness dyes. These dyes are generally more expensive than ordinary dyes. For fabrics with high light fastness requirements, dye selection should be the first step.

2. If the fabric has already been dyed but the light fastness does not meet the requirements, it can be improved using auxiliaries. Adding suitable auxiliaries during or after dyeing allows them to undergo a photoreaction before the dye upon exposure to light, consuming light energy and thus protecting the dye molecules. These are generally divided into ultraviolet absorbers and ultraviolet stabilizers, collectively known as light fastness improvers.

4. Recommend suitable high-sunlight dyes and UV-resistant finishing agents.

（1）High-Sunlight dyes 

UV Resistant Finishing Agent Sylic FU5681 (CY-659)

• 1. Give polyester and its blended fabric with excellent ultraviolet absorption performance, and have good absorption, conversion, reflection and scattering effects on the ultraviolet in the 280-400nm band.

• 2. It can reduce fading and improve the fastness of some dyes to sunlight.

• 3. Excellent wash resistance, minimal impact on fabric shade and color fastness

  
  

It is suitable for anti-ultraviolet finishing of polyester fabric and improving the fastness to sunlight. Especially textile fabrics exposed to strong light, such as car seat cushions, decorative fabrics, sportswear and swimwear.

Padding process
FU5600 20-40g / L
Liquid preparation → padding (liquid rate is about 70-80%) → baking (160-170 ℃).
The specific process should be adjusted by the user through the sample as appropriate.

If you want to get free samples ,please contact :info@tiankunchemical.com