Introduction

The phenomenon whereby white or light-colored materials develop a yellowish surface under external conditions such as exposure to light or chemical reagents. We refer to this phenomenon as “Yellowing” 。

When white and dyed textiles yellow, their appearance is compromised and their service life is significantly shortened. Consequently, research into the causes of textile yellowing and strategies for preventing it has long been a prominent topic both domestically and internationally.

I. What are the causes of yellowing in textiles?

1. Light-induced yellowing:

Photoyellowing refers to the yellowing of textile and apparel surfaces caused by molecular oxidative cleavage reactions induced by exposure to sunlight or ultraviolet light. It is most common in light-colored garments, bleached fabrics, and optically brightened textiles. Upon illumination, light energy is transferred to the fabric dyes, leading to the cleavage of dye chromophores and subsequent photodegradation, which results in color fading and a yellowish appearance on the fabric surface. Notably, visible light and ultraviolet light are the primary factors responsible for the fading of azo-dyed and phthalocyanine-dyed fabrics, respectively.

2. Phenol yellowing:

Phenolic yellowing is typically caused by the transfer of NOx and phenolic compounds from packaging materials to fabric surfaces, resulting in yellowing. The primary reactive species are often antioxidants present in the packaging, such as butylated hydroxytoluene (BHT). After garments and footwear leave the factory and undergo prolonged packaging and transportation, BHT in the packaging material can react with atmospheric NOx, leading to yellowing of the textiles.

3. Oxidative yellowing:

Oxidative yellowing refers to the yellowing of fabrics caused by oxidation in the atmosphere or by other oxidizing substances. During dyeing and finishing, textiles and apparel typically use reductive dyes or auxiliaries; when these come into contact with oxidizing gases, redox reactions occur, leading to yellowing.

4. Yellowing of optical brighteners:

Yellowing caused by optical brighteners primarily occurs on light-colored fabrics. Over time, residual brighteners on the fabric surface can migrate due to prolonged storage, leading to localized overconcentration of the brightener and resulting in yellowing.

5. Yellowing of softening agents and other finishing agents:

During the finishing process, cationic softening agents used in apparel can undergo oxidation under conditions such as heat and light exposure, resulting in yellowing of the fabric’s softened areas.

II. What are the testing standards for yellowing of textiles?

1. Test method for yellowing caused by natural light exposure: GB/T 8427—2008 “Textiles—Tests for colour fastness—Colour fastness to artificial light: Xenon arc.” The test principle involves placing the test specimen together with a set of blue wool reference standards in a chamber, exposing them under artificial light according to specified conditions, and then comparing the colour change of the test specimen with that of the blue wool reference standards to determine the colour fastness rating.

2. Test method for yellowing caused by ultraviolet irradiation: GB/T 30669—2014 “Textiles—Tests for colour fastness—Lightfastness to yellowing.” The test principle is as follows: after the test specimen has been exposed to ultraviolet light for a specified period, it is compared with an unexposed reference sample, and the degree of colour change is assessed using a grey scale for colour change.

3. Test method for yellowing caused by phenolic substances: GB/T 29778—2013 “Textiles—Tests for colour fastness—Evaluation of potential phenolic yellowing” and SN/T 2468—2010 “Test method for phenolic yellowing of imported and exported textiles.” The testing principle involves sandwiching each test specimen and a control fabric between test papers containing 2,6-di-tert-butyl-4-nitrophenol (BHT), placing the assembly between glass plates, and then tightly wrapping the entire assembly with a polyethylene film free of BHT to form a test package. The package is then placed in a constant-temperature chamber or oven under specified pressure for a predetermined period. The degree of yellowing of the test specimens is assessed using grey scale cards for staining evaluation, thereby determining the likelihood of phenolic yellowing occurring in the specimens.

4. Test method for yellowing of fabrics caused by chlorine bleaching: FZ/T 01078—2009 “Test Method for Chlorine-Induced Yellowing of Fabrics.” The principle of this test is to subject the test specimen to chlorine bleaching, washing, and drying in a washing machine, then compare it with the original sample to determine the degree of yellowing.

III. How to Prevent Yellowing of Textiles?

1. For varieties that require the use of optical brighteners, select brighteners that exhibit minimal yellowing under various conditions.

2. During fabric finishing and setting, the temperature should not be too high; excessive heat can cause oxidative degradation of dyes or auxiliaries on the fabric surface, leading to yellowing. 3. Before using finishing auxiliaries such as softeners, conduct tests for yellowing resistance and select formulations with low yellowing potential. 4. During packaging, storage, and transportation, use packaging materials with a low BHT content, and maintain ambient temperature and good ventilation in the storage and transport environments to prevent phenolic yellowing. 5. Apply anti-yellowing agents according to the specific cause of yellowing; these are generally categorized as antioxidant yellowing inhibitors and phenolic yellowing inhibitors.

(Source: HQTS-QAI Testing; Supplement: Dyeing and Finishing Encyclopedia)