What is Light Fastness?

Light fastness refers to the ability of dyed textiles to maintain their original color when exposed to sunlight or artificial light sources.

When dyed fabrics are exposed to light, dye molecules absorb light energy and become excited. If the molecular structure of the dye is not sufficiently stable, the absorbed energy may cause changes or degradation of the chromophore system, resulting in fading.

The level of light fastness depends mainly on the chemical structure of the dye, as well as factors such as fiber type, dye concentration, dyeing process, and finishing conditions.

Generally, dyes with stable molecular structures, strong conjugated systems, and good resistance to oxidation show better light fastness performance.

1. Common Standards for Light Fastness Testing

The most commonly used standards for textile light fastness testing include:

• GB/T 8427

• ISO 105-B02

• AATCC 16.3

Among them:

GB/T 8427-2019 "Textiles — Tests for Color Fastness — Color Fastness to Artificial Light: Xenon Arc Fading Lamp Test" officially came into effect on July 1, 2020.

This standard was revised based on GB/T 8427-2008 and evaluates the light fastness of textiles by using a xenon arc lamp to simulate natural daylight conditions.

The main differences between the commonly used standards are shown below:

2. Light Fastness Testing Procedure

2.1 Principle of Exposure Testing

Light fastness testing is usually carried out using a xenon arc weathering tester.

The xenon lamp simulates natural sunlight by providing ultraviolet, visible, and partial infrared radiation. During continuous exposure, the light energy causes aging effects on the dye molecules, allowing the fading resistance of the dyed material to be evaluated.

During the test, the specimen is exposed together with Blue Wool Reference Standards. The color change of the specimen is compared with the blue wool standards to determine the light fastness rating.

2.2 Specimen Preparation

The preparation method depends on the type of textile sample.

Solid Color Fabrics

For solid-colored fabrics, specimens are generally cut along the warp direction:

45 mm × 10 mm

The specimen is then fixed onto an opaque backing card.

The actual size may be adjusted according to the test equipment and specimen holder requirements.

Printed or Multi-color Fabrics

For printed fabrics or textiles containing multiple colors, several specimens should be prepared to ensure that all colors are tested.

Different areas, such as:

• Dark shades

• Light shades

• Different printed patterns

should be evaluated separately because each color may have different light fastness performance.

2.3 Selection of Blue Wool Reference Standards

Light fastness ratings are determined by comparing the tested sample with Blue Wool Standards.

The Blue Wool Scale generally consists of eight grades:

Normally:

• Grades 1–3 are used for products with low light exposure requirements;

• Grade 4 is commonly accepted for general apparel and home textiles;

• Grades 6–8 are required for high-performance applications such as outdoor textiles, automotive interiors, and curtains.

2.4 Exposure End Point Determination

According to GB/T 8427-2019, the exposure process is controlled by monitoring the color change of the Blue Wool Reference Standards.

For example:

First stage:

When the target Blue Wool Standard reaches a color change equivalent to Grade 4 on the Grey Scale, the first exposure stage is completed.

Second stage:

The first exposed area is covered, and the specimen continues to be exposed.

When the target Blue Wool Standard reaches a color change equivalent to Grade 3 on the Grey Scale, the exposure test is stopped.

The final light fastness rating is determined based on the color difference between the exposed specimen and the Blue Wool Standards.

2.5 Light Fastness Rating Method

After exposure, the tested specimen is compared with the Blue Wool Reference Standards.

The corresponding light fastness grade is determined according to the degree of color change.

The results are usually expressed as:

Grade 1, Grade 1-2, Grade 2 … Grade 8

If different exposure stages produce different ratings, the arithmetic average can be taken as the final light fastness rating.

The result should be expressed as the nearest half grade or whole grade.

Note:

Some dyes may show a phenomenon called photochromism.

To avoid incorrect evaluation caused by temporary color changes, the exposed specimen should be kept in darkness at room temperature for approximately 24 hours before final assessment.

3. Main Factors Affecting Light Fastness

3.1 Dye Chemical Structure

The chemical structure of the dye is the most important factor affecting light fastness.

For example:

• Anthraquinone dyes generally show excellent light stability;

• Many high-performance disperse dyes and vat dyes provide outstanding light fastness;

• Some bright shades may have lower light stability due to their molecular structures.

3.2 Fiber Type

Different fibers have different interactions with dyes:

Polyester:

• Mainly dyed with disperse dyes;

• Usually provides good light fastness performance.

Cotton:

• Vat dyes and selected reactive dyes can achieve high light fastness.

Wool:

• High-performance acid dyes are required for demanding applications.

3.3 Dyeing Process

The following factors also influence light fastness:

• Dye concentration

• Dyeing temperature

• Fixation efficiency

• After-treatment process

Optimized dyeing conditions can further improve light fastness performance.

4. High Light Fastness Dye Solutions from Tiankun Chemical

For applications requiring excellent resistance to sunlight, such as outdoor textiles, workwear, automotive interiors, and functional fabrics, Tiankun Chemical provides high-performance dye solutions with outstanding light fastness properties.

4.1 Skycron® High Light Fastness Disperse Dyes

Applications:

• Polyester fabrics

• Polyester/cotton blends

• Automotive textiles

• Outdoor fabrics

Key Features:

✓ Excellent light fastness performance
✓ Good sublimation fastness
✓ Stable batch-to-batch consistency
✓ Suitable for both continuous and exhaust dyeing processes

4.2 High Light Fastness Vat Dyes

Applications:

• Denim fabrics

• Cotton textiles

• Workwear

• Outdoor apparel

Key Features:

✓ Outstanding light fastness
✓ Excellent washing fastness
✓ Superior weather resistance
✓ Ideal for long-term outdoor exposure applications

4.3 High Light Fastness Acid Dyes

Applications:

• Wool

• Nylon

• Cashmere

• High-end textiles

Key Features:

✓ High light fastness grades available
✓ Brilliant and vivid shades
✓ Excellent overall color fastness performance

Conclusion

Light fastness is one of the most important quality indicators for dyed textiles.

With the increasing demand for outdoor textiles, functional apparel, and high-performance fabrics, customers require dyes with improved resistance to sunlight and weathering.

Selecting dyes with stable molecular structures, combined with optimized dyeing processes, is the key to achieving excellent light fastness performance.

Tiankun Chemical continuously provides high-performance disperse dyes, vat dyes, and acid dyes, delivering reliable color solutions to customers worldwide.

Keywords:
Light Fastness | Xenon Arc Test | ISO 105-B02 | GB/T 8427 | Blue Wool Scale | High Light Fastness Dyes | Disperse Dyes | Vat Dyes | Acid Dyes | Textile Dyeing Solutions | Tiankun Chemical