Yellowing refers to the phenomenon where the surface of white or light-colored materials turns yellow under the influence of external factors such as light and chemicals.When white or dyed textiles yellow, not only is their appearance compromised, but their service life is also significantly reduced. Therefore, research into the causes of textile yellowing and measures to prevent it has long been a major topic of interest both domestically and internationally.
一,What Are the Causes of Yellowing in Textiles?
1. Light-Induced Yellowing:
Light-induced yellowing refers to the yellowing of textile garments caused by molecular oxidation and cleavage reactions triggered by exposure to sunlight or ultraviolet (UV) light. This type of yellowing is most common in light-colored garments, bleached fabrics, and fabrics treated with optical brighteners. When fabric is exposed to light, light energy is transferred to the fabric dyes, causing the conjugated systems of the dyes to break down. This leads to light fading, resulting in a yellowish appearance on the fabric's surface. Specifically, visible light and ultraviolet light are the primary factors causing fading in azo-dyed and phthalocyanine-dyed fabrics, respectively.
2. Phenolic Yellowing:
Phenolic yellowing is generally caused by contact migration between NOx and phenolic compounds, resulting in yellowing on the fabric surface. The primary reactants are typically antioxidants contained in packaging materials, such as butylated hydroxytoluene (BHT). After garments and footwear leave the factory, they undergo prolonged packaging and transportation. During this time, BHT in the packaging materials reacts with NOx in the air, causing the garments to yellow.
3. Oxidative Yellowing:
Oxidative yellowing refers to the yellowing of fabrics caused by oxidation from the atmosphere or other substances. Reducing dyes or auxiliaries are commonly used during the dyeing and finishing of textiles and apparel; upon contact with oxidizing gases, redox reactions occur, resulting in yellowing.
4. Yellowing Caused by Optical Brighteners:
Yellowing caused by optical brighteners primarily occurs on light-colored fabrics. When residual optical brighteners on the garment's surface migrate due to prolonged storage, it leads to localized excess concentrations of these brighteners, resulting in yellowing of the garment.
5. Yellowing Caused by Softeners and Other Finishing Agents:
When softening agents used in the garment finishing process are exposed to heat, light, or other conditions, the cations within them undergo oxidation, causing the areas of the fabric that have undergone softening treatment to turn yellow.
二,What Are the Testing Standards for Textile Yellowing?
1. Test method for yellowing caused by natural light exposure: GB/T 8427-2008 "Textiles-Color Fastness Tests-Resistance to Artificial Light: Xenon Arc." The testing principle involves placing the test specimen alongside a set of blue wool reference standards in a chamber, exposing them to artificial light under specified conditions, and finally grading the discoloration of both the test specimen and the blue wool reference standards to evaluate color fastness.
2. Test method for yellowing caused by ultraviolet (UV) light: GB/T 30669-2014 "Textiles-Color Fastness Tests-Resistance to Yellowing by Light." The test principle involves exposing the test specimen to UV light for a specified period, then comparing it with an unexposed specimen and evaluating the degree of discoloration using a gray scale chart.
3. Test method for yellowing caused by phenolic substances: GB/T 29778-2013 "Textiles-Color Fastness Tests-Evaluation of Potential Phenolic Yellowing" and SN/T 2468-2010 "Test Method for Phenolic Yellowing of Imported and Exported Textiles." The test principle involves sandwiching each test specimen and control fabric between test strips containing 2,6-di-tert-butyl-4 -nitrophenol (BHT), placed between glass plates and stacked on top of one another. They are then tightly wrapped with a polyethylene film free of BHT to form a test package, which is placed in a constant-temperature chamber or oven for a specified duration under prescribed pressure. The degree of yellowing of the samples is assessed using the gray scale card for color transfer evaluation, thereby evaluating the likelihood of phenolic yellowing in the samples.
4. Test method for fabric yellowing caused by chlorine bleaching: FZ/T 01078-2009 "Test Method for Chlorine Absorption and Yellowing of Fabrics." The testing principle involves subjecting the sample to chlorine bleaching, washing, and drying in a washing machine, then comparing it with the original sample to determine the degree of yellowing.
三,How to Prevent Yellowing in Textiles?
1. For products requiring the use of fluorescent whitening agents, select whitening agents that exhibit minimal yellowing under various conditions.
2. During fabric finishing and setting, avoid excessively high temperatures, as high heat can cause the dyes or auxiliaries on the fabric surface to undergo oxidative decomposition, leading to yellowing.
3. Test the yellowing properties of finishing auxiliaries, such as softeners, before use, and select formulations with low yellowing potential.
4. During packaging, storage, and transportation, use packaging materials with low BHT content and maintain a well-ventilated environment at room temperature to prevent phenolic yellowing.
5. Use anti-yellowing agents appropriate for the specific cause of yellowing; these are generally categorized as anti-oxidative yellowing agents and anti-phenolic yellowing agents.
