The biggest headache for fabric merchants! Yellowing, also known as yellowing, refers to the phenomenon of white or light colored materials turning yellow on the surface under environmental conditions such as light and chemicals. Yellowing of textiles during storage, transportation, and wearing can directly lead to economic losses. Therefore, in recent years, fabric manufacturers and buyers have begun to pay increasing attention to this indicator. Let's take a look at the following analysis
Generally speaking, there are two common types of yellowing in textiles: light yellowing and phenol yellowing. The former refers to the yellowing of the surface color of textiles caused by exposure to sunlight or ultraviolet light; The latter refers to the phenomenon of yellowing of textile materials caused by the action of nitrogen oxides or phenolic compounds.
Compared with conventional color fastness testing items such as sweat resistance, water resistance, and friction resistance, the assessment of yellowing started relatively late. Currently, the national standard for assessing light yellowing is GB/T 30669-2014 "Textile Color Fastness Test - Color Fastness to Light Yellowing", and the national standard for assessing phenolic yellowing is GB/T 29778-2013 "Evaluation of Potential Phenolic Yellowing in Textile Color Fastness Test".

White fabrics may sometimes turn yellow during storage, and the yellowed areas will disappear and return to their original white color after acid treatment. If alkaline solution is added again, the yellow color will return. The main reason is the yellowing of phenols.
The main reason for phenolic yellowing is that the antioxidant BHT (2,6-di-tert-butyl-p-cresol) commonly used in packaging materials reacts chemically with nitrogen oxides (NOx) in the air to form DTNP (2,6-di-tert-butyl-p-nitrophenol). DTNP is colorless in an acidic state and easily sublimates and transfers at room temperature. When it encounters alkaline substances, it immediately turns yellow.
1、 Testing method
1.1 GB/T 29778-2013 "Assessment of Potential Phenolic Yellowing in Textile Color Fastness Tests"
Principle: The test sample and a piece of control fabric are wrapped in test paper containing phenol and placed between glass plates to form a composite sample. After applying a certain pressure, they are wrapped tightly with polyethylene film and then placed in a constant temperature box or oven at a specified temperature for a period of time. After cooling, the polyethylene film is removed and the control fabric and sample are taken out. The yellowing level of the sample is immediately evaluated using a gray sample card. If the color of the fabric is controlled to reach the predetermined value, the color level of the sample is the test value, otherwise the test is repeated.

1.2 GB/T 30669-2014 "Test for Color Fastness of Textile Products - Color Fastness to Light Yellowing"
Principle: After being irradiated with ultraviolet light for a certain period of time, the sample is compared with the untreated sample, and the degree of color change is evaluated using a gray sample card to determine the ability of the textile material to resist light yellowing under ultraviolet light irradiation.
2、 Reasons for yellowing
The main reasons for yellowing are as follows:.
2.1 Yellowing caused by raw materials (fabrics) themselves is generally caused by wool, viscose fibers, nylon, spandex, etc. Nylon and spandex are both derived from the same raw material TDI (toluene diisocyanate), and BHT (2,6-di-tert-butyl-p-cresol) is often added to TDI to prevent photo aging during storage and use. The BHT in these raw materials reacts chemically with nitrogen oxides (NOx) in the air, resulting in yellowing, which is called BHT yellowing.
During the production process, yellowing of fabrics can be caused by processing techniques, the use of additives, and the pH value during the processing. During fabric finishing, due to the amphoteric nature of hydrogen peroxide, residual hydrogen peroxide on the fabric can cause yellowing under appropriate reaction conditions. The concentration of fluorescent whitening agents also affects the yellowing of textiles. Usually, in order to achieve satisfactory whiteness in fabrics, fluorescent whitening agent finishing is sometimes required during post-processing. When the concentration of fluorescent whitening agent reaches a certain value, the whiteness will reach its highest value; But when the concentration exceeds this level, the whiteness will decrease with the increase of concentration, leading to yellowing of the fabric. During the processing, uneven steam ironing or shaping drying can also cause the whitening agent to migrate, resulting in excessive local concentration and causing local yellowing. The additives used in the post-treatment process of dyeing may also cause yellowing of the fabric.

If cationic softeners have strong adhesion with fibers, they can achieve excellent softening effects on fabrics. But the disadvantage is that fabrics treated with anionic softeners are more prone to yellowing; When adding antibacterial and deodorizing finishing agents, the fabric may also turn yellow due to the presence of nitrogen atoms in the finishing agent. In addition, many direct dyes can form complexes with heavy metal ions, leading to decreased solubility and changes in dyeing color. Even a small amount of copper or iron ions can suppress fluorescence, resulting in a decrease in the whiteness of fluorescent whitening fiber materials and yellowing.
2.3 Yellowing caused during packaging and storage. The packaging materials for various textiles are polyethylene and other materials. In most cases, these materials use phenolic compounds containing steric hindrance as antioxidants. Among them, 2,6-di-tert-butyl-4-methyl (BHT) is widely used and inexpensive. Due to the low solubility and easy volatilization of BHT in polyethylene (or polypropylene), BHT in polyethylene film will diffuse to the surface of packaging materials. When packaging polyamide fabrics or clothing with polyethylene film containing BHT, BHT can be transferred to the fabric or clothing during storage and undergo complex chemical reactions with nitrogen oxides (mainly NO2) absorbed or stored in the air and packaging materials (such as cardboard), forming a yellow substance.
During the wearing and washing process, yellowing may occur due to exposure to sunlight, changes in temperature, variations in humidity, as well as movement and contact. During the washing process, it is important to pay attention to whether the laundry detergent has been completely removed and whether bleach or pH imbalanced detergents have been improperly used in the cleaning solution. In the presence of nitrogen oxides, any trace amount of phenol based antioxidant can be transferred to the surface of the fabric through volatilization or friction, all of which will eventually cause yellowing.
3、 Judgment of yellowing
There are several methods to determine yellowing:

4、 Anti yellowing measures
4.1 During the wearing process, consumers should pay attention to their own maintenance.
During the production and processing process, strictly control the amount of fluorescent whitening agent to avoid exceeding the yellowing point of the fluorescent whitening agent. The effect of improving textile yellowing and increasing textile whiteness can be achieved by increasing the blue light that can be reflected in textiles. According to chromaticity theory, a certain amount of blue light and yellow light are mixed to form white light, and blue light and yellow light complement each other as complementary colors. Therefore, the reason why textiles appear yellow is due to insufficient blue light in the reflected light. Based on this, textiles with high yellowness have less blue light and lower whiteness in their reflected light; Reflected light with low yellowness has more blue light and high whiteness.
During the post-processing of 4.3, the setting temperature should not be too high. If the setting temperature is too high, it will cause the fabric to turn yellow after setting. The setting temperature of fabrics containing spandex should be below 150 ℃.
During the packaging process, prevent BHT from yellowing. Phenolic and BHT yellowed products are sensitive to pH values. In terms of preventing yellowing, fabrics should be slightly acidic after dyeing and processing. According to testing, when the pH value of the fabric is less than 5, the yellowing time is longer; When the pH value is greater than 6, the time for yellowing to occur will be relatively short. Adding a small amount of non-volatile acid (such as citric acid) in fabric post-processing can greatly reduce the likelihood of yellowing.
4.5 Anti yellowing treatment of packaged products. For packaged products or products stored in the warehouse, if there is yellowing or if the yellowing test is found to be unqualified, and the washing label and hanging tag have been nailed, it is not easy to dispose of them in the water. At this point, the reduction method can be considered for treatment. Place a layer of white paper under the cardboard box, sprinkle a certain amount of reducing powder, and then cover it with a layer of cardboard. Make small holes in the cardboard and stack the products inside the cardboard box. Seal the cardboard box and let it sit for 24-48 hours before spreading it out for more than 6 hours. If there is no odor, repackage. This method does not require water treatment, can maintain the original shape of the product, and does not require removing the washing label or hang tag. However, improper treatment of the reducing powder may cause color changes.
In addition, if there is yellowing during garment testing, it is necessary to avoid using brown cardboard and plastic film packaging fabrics containing phenolic antioxidants. Good ventilation is required during storage.
In short, yellowing is a common problem in textiles, and the reasons for yellowing are diverse. By analyzing the causes of yellowing, making judgments on yellowing, and taking effective measures to prevent yellowing, we can better improve the quality and performance of the product.
Factors causing yellowing of fabrics and preventive measures
The main reasons for yellowing of cotton fabrics are:
[1] Fiber degradation: The degradation of fibers due to the effects of heating, light, other radiation, chemicals, and biodegradation.
[2] Additives: Yellowing and discoloration caused by the addition of softeners, oils, lubricants, resins, fluorescent whitening agents, and metal compounds.
[3] Atmospheric pollutants: yellowing caused by the effects of nitrogen oxides, sulfur dioxide, and ozone.
[4] The transmitted pollutants: The original yellow pigment of the yellowing substance is transmitted from the contact paper, cardboard, plastic film or plastic packaging, and pumice used for stone grinding and washing to the fabric.
[5] Consumer pollutants, such as sweat and residues from household washing and bleaching.
During storage, yellowing of phenols is most common. The original yellow pigment, such as phenol compounds or butylated hydroxytoluene [BHT], transferred from packaging materials such as cardboard, paper, or plastic film to fabrics during storage, and the yellow product formed by nitrophenol compounds or 1,2-stilbene due to nitrogen, oxidation, and chemical reactions, is more favorable in alkaline environments. The yellowing of phenols has a pure bright yellow color, with maximum absorption characteristics at 420-450nm. They will fade when exposed to light or ozone. The yellow product dissolves in hot water and polar solvents such as alcohol, and becomes colorless in acidic media with a pH of 5 or lower.
Measures to prevent yellowing caused by phenols include
[1] Avoid using brown paper, brown cardboard, and plastic film containing phenol antioxidants to package finished products;
[2] Avoid using plastic packaging based on thermal shrinkage;
[3] Provide good ventilation conditions during storage;
[4] Use fewer phenol derivatives as additives;
[5] Avoid organizing finished products that are alkaline.
In addition, after finishing with citric acid as an acid agent, the fabric is also prone to yellowing. The reason behind this is that citric acid is highly prone to dehydration and the formation of anhydrides when heated. This is because citric acid has one more hydroxyl group in its structure than general polyacids and belongs to the hydroxy acid category. When heated and baked, it dehydrates to form anhydrides. At the same time, the hydroxyl groups and adjacent hydrogen atoms in the molecule may be further removed by heat and external environmental influences, generating unsaturated acids (such as acrylic acid), and possibly removing water and CO2 to form itaconic acid. These unsaturated acids can easily cause yellowing of the fabric surface, so it is best not to use citric acid as an acid agent for white fabric.
Analysis of the causes of yellowing of bleached yarn
After bleaching and whitening the yarn for a certain period of time, there will be yellow spots and a decrease in whiteness over time. Analyzing the reasons, the formation of yellow spots is due to poor bleaching water quality and high content of divalent iron ions. Over time, divalent iron ions are oxidized or trivalent iron ions are oxidized, resulting in the formation of yellow spots on the yarn. This type of macula not only affects the appearance quality, but also causes fiber fragility.
The solution to overcome this is to use oxalic acid at 1.5-2.0g/L and acid wash at 80-85 ℃ before bleaching the yarn in case of poor water quality. Because oxalic acid can form complex ions with iron ions and dissolve in water, the iron in the water can be removed, achieving the goal of not yellowing.
The main reason for the decrease in whiteness of bleached and whitened yarns over time is due to insufficient cooking. If the yarn is not thoroughly boiled and impurities in the cotton fiber are not completely removed, immediately bleach and whiten the yarn. Over time, impurities and natural pigments inside the fiber will be exposed, causing a decrease in whiteness. The solution to overcome this is to ensure that the wool effect of bleached yarn reaches 13-15cm/30min, and the wool effect must be uniform and consistent to ensure that bleached and whitened yarn does not turn yellow.
In addition, when bleaching with sodium hypochlorite and dechlorination, excessive use of baking soda or failure to wash thoroughly can also cause the yarn to turn yellow. When drying yarn, if the temperature in the drying room is too high and the drying time is too long, it can also cause the yarn to turn yellow.