What if the light fastness failure causes the fabric to fade?

Clothes do fade … this is kind of truth, while, if fading happens too fast, this is not acceptable.

In this article, our textile lab testing engineer Mr. Ho will share his knowledge in predicting the fading performance of fabric to sun light, after washing, from the real-world testing projects.

First, let’s understand “why”.

The reason that causes the fading when exporting to sunlight is due to the bad light fastness testing performance.

What’s light fastness?

Light fastness, also known as light fastness, refers to textiles that are usually exposed to light when in use. Light can destroy dyes and cause the well-known “fading”, which makes colored textiles discolored, generally lighter and darker.

What is the test principle?

The textiles and a set of blue wool standard samples are exposed to the artificial light source under specified conditions, and then the discoloration of the sample is compared with the blue wool standard sample to evaluate the color fastness.

Common testing standards

Currently, there are mainly three commonly used standards for testing light fastness: GB/T 8427, ISO 105-B02, AATCC 16.3. Among them, GB/T 8427 formally implemented the latest method GB/T 8427-2019 “Textile color fastness test color fastness to artificial light: xenon arc” on July 1, 2020. This method is in accordance with GB/T 8427-2008 Revised on the basis. The main technical differences between these methods are shown in the following table:

Lighting condition	GB/T 8427-2019 (New) Cycling A1 General conditions	GB/T 8427-2008General conditions	ISO 105-B02:2014Cycling A1 General conditions	AATCC 16.3-2014Method 3
Black-panel thermometer （BPT）	45 ± 3 0C	5 0C lower than BST	45 ± 3 0C	70 ± 1 0C
Black standard thermometer (BST)	47 ± 3 0C	Max. 50 0C	47 ± 3 0C	–
Testing Chamber Temp.	–	–	–	Light cycle: 43 ± 2 0CDark cycle: 43 ± 2 0C
Relative humidity	40%		40%	Light cycle: 30 ± 5%Dark cycle: 90 ± 5%
Irradiance(at 300 ~ 400 nm)	42 ± 2 W/m2	42 W/m2	42 ± 2 W/m2	48 ± 1 W/m2
Irradiance(at 420 nm)	1.10 ± 0.02 W/m2	1.1 W/m2	1.10 ± 0.02 W/m2	1.10 ± 0.03 W/m2
Reference specimen	Blue wool 1 ~ 8	Blue wool 1 ~ 8 or L2 ~ L9	Blue wool 1 ~ 8	Blue wool L2 ~ L9
Blocking material	Opaque material	Opaque material	Opaque material	Opaque material

Testing procedure （Take GB/T 8427-2019 as example）

• Sample preparation

If the sample is a solid-color fabric, generally cut 45×10mm along the warp direction. You can also fix the sample on an opaque cardboard according to the shape and size of the equipment sample holder; if the sample is a printed or multi-color fabric, Multiple samples need to be cut to ensure that all colors can be tested.

• Sample preparation

Usually the target blue wool sample and the blue wool standard sample that is one and two grades lower than the target blue wool standard sample.

• Instrument: Light Fastness Tester

Put the sample holder with the test specimen into the tester.

• End of exposure

Expose until the first stage on the segmented surface of the target blue wool standard is equal to the discoloration gray card level 4. That is, stop the exposure, then cover the first stage, and continue with the second stage until the target blue wool standard is equal to the discoloration gray Card level 3, stop exposure. As shown below:

• Evaluation

The color difference between the exposed sample and the blue wool standard sample is compared, and each stage corresponds to each stage, and the grade is evaluated.

If different evaluations are obtained on the color difference at different stages, the arithmetic average value can be used as the light fastness of the sample, which is expressed in the nearest half or whole grade. When the arithmetic average of the series is one-quarter or three-quarters, the assessment should be the highest half or first class adjacent to it.

(Note: To avoid misappraisal of light fastness due to photochromism, the sample should be placed in a dark place and kept at room temperature for 24 hours before assessing light fastness.)