An Overview to Lightfastness Test for Textile

As a textile manufacturer, have you encountered this situation: consumers complain that during use, the product quickly fades in color or turns to darken? Why is this? It might be due to the fabric’s inadequate lightfastness. Hurry and read this article to find a solution.

What is lightfastness?

What does Lightfastness mean? Lightfastness, also known as sunlight fastness, sun exposure fastness, color fastness to light, is important for textiles as they are usually exposed to light. Light can damage dyes, leading to the well-known phenomenon of “color fading,” causing colored textiles to discolor, lighten, or darken. Lightfastness is one aspect that affects the color fastness of textiles.

How to test lightfastness?

To test lightfastness, textiles are exposed to artificial light sources alongside a set of blue wool standards under specified conditions. After exposure, the change in color of the sample is compared with the change in the blue wool standards to assess color fastness.

Most fabrics require several months or even years to fade and darken due to sunlight. As a manufacturer, you obviously cannot wait for mass production and sale of the product and receive consumer complaints before remedying this issue. How can you predict the fastness to light of the fabrics you choose?

Many manufacturers opt for laboratory testing. They simulate natural light to accelerate the testing process, commonly using the xenon arc fading lamp method. This reliable method simulates natural light spectra using various filters to recreate specific optical conditions.

Fastness-to-light testing involves not just simulating sunlight but also adjusting temperature and humidity to create an overall environmental condition. A spray system simulating rain is also part of the environmental simulation.

Standards for Testing Lightfastness

Currently, the commonly used standards for testing lightfastness include the following:

ISO 105—B01: Lightfastness (Sunlight)
This standard specifies a method for determining the lightfastness of various textiles to sunlight. The principle involves exposing the sample with eight blue wool standards under specified conditions without rain, and then comparing the sample with these standards to assess lightfastness.

ISO 105—B02: Lightfastness to Artificial Light (Xenon Arc)
This involves exposure under artificial light that simulates sunlight (D65), followed by comparison with blue wool standards to evaluate color fastness.

ISO 105—B03: Lightfastness to Weathering (Outdoor Exposure)
This test involves outdoor exposure under unshielded conditions, followed by a comparison with blue wool standards to evaluate color fastness.

AATCC TM16: Lightfastness
This is suitable for indoor textiles.

AATCC TM169: Weather Resistance of Fabrics: Xenon Arc
This is applicable to outdoor textiles.

AATCC TM192: Weather Resistance: Carbon Arc Sunlight

Among these, AATCC TM16-3 and ISO 105—B02 are widely used standards.

Introduction to the Test Method

Principle of Lightfastness Test

The test sample is exposed to sunlight along with a blue wool cloth. After the exposure, the color change on the blue wool cloth is used to assess and rate the degree of color change in the test sample due to sunlight exposure.

Tools and Materials for Lightfastness Testing

Blue Wool Cloth: Used as a standard for comparison to measure the color change.
Gray Scale Rating Card: A tool for evaluating the extent of fading or color change.
Carbon Arc Sunlight Tester: An instrument that simulates sunlight exposure using carbon rods.
Opaque Cardboard or Cardboard Covered with Thin Aluminum Foil: Used to partially cover the sample and standards to provide a reference for the original color.
White Cardboard: Used as a background for neutral color comparison.
Gray Cardboard: Also used for background purposes to ensure accurate color assessment.

Lightfastness Testing Process

Explained using the GB/T 8427-2019 Test Method

Sample Preparation
The size of the sample depends on the number of samples and the shape and size of the sample holder of the equipment. For solid color fabrics, a sample is typically cut along the warp direction measuring 45×10mm. For printed or multicolored fabrics, multiple samples may be required. The size of the sample should be the same as the blue wool standard to avoid significant errors in evaluation.
Fabric samples should be closely affixed to a white card; yarn samples should be tightly wound around a white card or arranged in parallel and fixed to the card; loose fiber samples should be organized into a uniform thin layer fixed on the white card. Each exposure and unexposed area should not be less than 10mm×8mm.

Exposure
Determine the required exposure conditions and the specified effective humidity from the table; place the temperature and humidity control standard samples along with the relevant blue wool standards on a white card. The temperature and humidity control standard and the blue wool standards should each be not less than 45mm×10mm.
Cover approximately one-third and two-thirds of the sample and blue wool standard with appropriate covering material. Arrange the samples on the white card as required (the blue wool standards should be limited to the target blue wool standard and one and two levels lower than the target blue wool standard), and cover one-third in the middle of the test card with covering materials ABCD.

Exposure Method Diagram

Lightfastness Exposure Chart

Place the prepared test card in the test chamber, exposing it under the conditions selected from the table, until the color difference between the exposed and unexposed parts of the target blue wool standard reaches grade 4 on the gray scale card (first stage). At this stage, be mindful of potential light-induced color changes. Exposure to white textiles (bleached or with fluorescent whitening) can be stopped at this stage for grading.
Continue exposing the test card in the test chamber until the color difference between the exposed and unexposed parts of the target blue wool standard reaches grade 3 on the gray scale card.

Lightfastness Grading

After exposure, compare the color difference between the sample and the blue wool standard. Grade the sample at each stage accordingly. If different stages yield different evaluations, the arithmetic average of these values is taken as the lightfastness grade of the sample. This grade is expressed in the nearest half or whole grade.

If the arithmetic average of the grades is a quarter or three-quarters, the evaluation should be rounded up to the nearest higher half or whole grade. This approach ensures a more accurate and standardized assessment of the lightfastness of the textile sample.

Reasons for Lightfastness Failure

Dye/Pigment Structure Impact

The photofading mechanism of dyes is complex, involving the dye being excited by absorbing photons, leading to photolytic reactions that damage its structure, causing discoloration and fading.
The lightfastness of textiles primarily depends on the chemical structure of the dye, its state of aggregation, binding state, and how it’s mixed with other colors. Therefore, selecting the right dye is crucial.
Influence of Dyeing Depth and Floating Color

The lightfastness of dyed fabrics relates to the depth of dyeing.

Deeper dyeing means larger dye particles aggregating on the fabric, reducing the proportion of dye exposed to air per unit weight, thus enhancing lightfastness.
For lighter-colored fabrics, the proportion of dye aggregates on fibers is smaller, reducing lightfastness. Middle and light shades often fail to meet lightfastness standards in factories. For example, certain light brown fabrics may lose almost all their red hue after lightfastness testing, showing significant color change.
The choice of dyeing process, thoroughness of post-dyeing washing, and soaping affect the amount of unfixed and hydrolyzed dyes (floating colors). Floating colors have significantly lower lightfastness than fixed reactive dyes. Improper post-dyeing treatment can also affect the fabric’s lightfastness.

Effect of Fixatives and Softeners

The use of fixatives significantly improves the friction, wash, and stain fastness of reactive dyes. Fabrics treated with typical cationic low molecular or polyamine type fixatives generally have a wash fastness of 4 to 5 grades. However, fabrics fixed with such fixatives often experience reduced lightfastness.
Using cationic softeners on cotton fabrics can decrease the lightfastness of reactive dyes. This is mainly because the softeners can yellow upon exposure to sunlight, thus altering the fabric’s hue.

Methods to Improve Lightfastness

  • Select Dyes with High Lightfastness. Opt for dyes with high resistance to sunlight, as they can prevent or reduce the occurrence of photo-oxidation reactions. Generally, dyes with higher overall fastness exhibit better lightfastness.
    Careful Selection in Dye Mixing
  • When mixing dyes, choose components with equivalent lightfastness to ensure consistent color fastness across all sunlight grades.
  • Role of Fixatives. The use of certain fixatives has a close relationship with the lightfastness of dyes. Employing non-surfactant ammonium salt-type fixatives and reactive fixatives can effectively enhance lightfastness. This is particularly relevant for products like nylon, cotton, light colors, and fluorescent colors.
    Post-treatment Agents
  • Post-treatment agents can enhance the lightfastness of dyes. Adding specific agents that improve lightfastness during the post-treatment phase can significantly boost this property.

Conclusion

The lightfastness of textiles is increasingly being emphasized. As a manufacturer, having a Darong Light and Climate Tester in your lab can significantly enhance the efficiency and reduce the cost of testing in your design, production, and quality control stages. This equipment can be a valuable asset in gaining an edge during the initial phases of new product development.

If you’re considering purchasing a Light and Climate Tester for your lab, the Darong Xenon Arc Test Chamber YG(B)611-3C is highly recommended. It’s produced by Darong, a company with 36 years of experience in manufacturing textile testing instruments and a leader in setting industry standards.

FAQ

Q: Brief Summary of Factors Affecting Sunlight Fading
A: The main factors affecting sunlight fading include light, the molecular structure of the dye, the state of dye aggregation, depth of dyeing, dye mixing, additives used in the dyeing process, fiber characteristics, and regional factors.

Q: Why are Blue Wool Standards Made of Wool?
A: Humidity significantly influences lightfastness. Wool is one of the materials least sensitive to humidity changes, making it an ideal choice for creating standard samples for lightfastness testing.

Related Resources

Color Fastness Test

Color Fastness to Rubbing

Color Fastness in Textile Dyeing

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