Yellowing, also known as “yellowing”, refers to the phenomenon that the surface of white or light-colored materials turns yellow under environmental conditions such as light and chemicals.
Yellowing of textiles during storage, transportation, wearing, etc. will directly lead to economic losses. Therefore, in recent years, fabric manufacturers and buyers have begun to pay more and more attention to this indicator.
Generally speaking, there are two main types of yellowing commonly seen in textiles: photoyellowing and phenolic yellowing. The former refers to the yellowing of the surface of textiles caused by sunlight or ultraviolet light; the latter refers to the 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 later. The current national standard for assessing light yellowing is GB /T 30669-2014 “Color Fastness to Light Yellowing in Textile Color Fastness Test”, and the national standard for assessing phenol yellowing is GB/T 29778-2013 “Evaluation of Potential Phenol Yellowing in Textile Color Fastness Test”.
White fabrics sometimes turn yellow during storage. The yellowed parts will disappear and return to their original white color after acid treatment. If alkali solution is added again, the yellow color will return. The main reason is phenol yellowing.
The main cause of phenol yellow: the antioxidant BHT (2,6-di-tert-butyl-p-cresol) commonly used in packaging materials and nitrogen oxides in the air ( NOx) undergoes a chemical reaction to generate DTNP (2,6-di-tert-butyl-p-nitrophenol). DTNP is colorless in an acidic state and easily sublimates and transfers at room temperature. When encountering alkaline substances, it immediately Yellowing.
1. Test methods
1.1 GB/T 29778-2013 “Textiles Color Fastness Test” Evaluation of potential phenolic yellowing》
Principle: The test sample and a control fabric are wrapped in test paper containing phenol and placed between glass plates and superimposed together. Form a combined sample, apply a certain amount of pressure and wrap it tightly with a polyethylene film, then place it in a constant temperature oven or oven at a specified temperature for a period of time. After cooling, remove the polyethylene film, take out the control fabric and sample, and immediately use a gray sample card. Evaluate the yellowing grade of the sample. If the staining of the control fabric reaches the predetermined value, the staining level of the sample is the test value, otherwise the test is repeated.
1.2 GB/T 30669-2014 “Textile Color Fastness test: Color fastness to light yellowing”
Principle: After the sample is irradiated under ultraviolet light for a certain period of time, compare it with the unirradiated sample, and use the gray sample to evaluate discoloration. The card evaluates the degree of discoloration to determine the ability of textile materials to resist photoyellowing under UV irradiation.
2. Causes of yellowing
The main reasons for yellowing are the following four aspects.
2.1 Yellowing caused by the raw material (fabric) itself
Generally, wool, viscose fiber, nylon, spandex, etc. can easily cause yellowing Change. For example, 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 photoaging 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.
2.2 Yellowing caused during the production process
In the production process, treatment process, use of additives and treatment process The pH value will cause the fabric to turn yellow. For example, during fabric finishing, due to the amphoteric nature of hydrogen peroxide, residual hydrogen peroxide on the fabric will cause the fabric to turn yellow under suitable reaction conditions. The concentration of optical brighteners also affects the yellowing of textiles. Usually, in order to obtain satisfactory whiteness of the fabric, optical brightener finishing is sometimes required in the post-finishing process. When the concentration of fluorescent whitening agent reaches a certain value, the whiteness will reach the highest value; but when it exceeds this concentration, the whiteness will decrease with the increase of concentration, causing the fabric to turn yellow. During the processing, uneven steam ironing or shaping and drying will also cause the whitening agent to migrate, resulting in excessive local concentration and local yellowing. The auxiliaries used in the post-dyeing process may also cause yellowing of the fabric.
If the cationic softener has strong binding force with the fiber, the fabric can achieve excellent softening effect. But the disadvantage is that it is more likely to yellow than fabrics treated with anionic softeners; when adding antibacterial and deodorizing finishing agents, the fabrics will also turn yellow because the finishing agents contain nitrogen atoms. In addition, many direct dyes can form complexes with heavy metal ions, resulting in reduced solubility and changes in dyeing color. Even a small amount of copper or iron ions can inhibit fluorescence, reduce the whiteness of fluorescently whitened fiber materials, and cause yellowing.
2.3 Yellowing caused during packaging and storage
The packaging materials for storing various textiles are polyethylene and other materials. In most cases, these materials use sterically hindered phenolic compounds as antioxidants, whereOriginal yellow pigments such as phenolic compounds or butylated hydroxytoluene [BHT] are transferred to the fabric from packaging materials such as plastic films and nitrophenols are produced by the action of nitrogen, oxidation and The yellow product formed by the compound or 1,2-stilbene is more favorable in an alkaline environment. The yellowing of phenols has a pure bright yellow color, with maximum absorption characteristics at 420~450nm, and 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 phenols from causing yellowing include:
[1] Avoid using brown paper containing phenol antioxidants, Brown cardboard and plastic film packaging finished products;
[2] Avoid using heat shrink-based plastic packaging;
[3] Provide good ventilation conditions during storage;
[4] Use less phenol derivatives as additives;
[5] Prevent the finished product from being alkaline.
In addition, fabrics tend to turn yellow after being treated with citric acid. The reason: citric acid is easily dehydrated to form anhydride when heated. Because citric acid has one more hydroxyl group than ordinary polybasic acids in its structure, it is a hydroxy acid. It dehydrates to form anhydride when heated and baked. At the same time, the hydroxyl group and the hydrogen in the ortho position in the molecule may also be further removed by heat and the influence of the external environment to generate unsaturated acid (acrylonic acid), and may remove water and CO2 to form itaconic acid. These unsaturated acids tend to make the cloth yellow, so it is best not to use citric acid as an acid agent for white cloth.
Analysis of causes of yellowing of bleached yarn
After the bleached and whitened yarn is left for a certain period of time, The first is the formation of yellow spots, and the second is the decrease in whiteness over time. Analysis of the reasons revealed that the yellow spots were caused by poor bleaching water quality and high content of divalent iron ions. The divalent iron ions were oxidized or trivalent iron ions over time, thus forming yellow spots on the yarn. This kind of yellow spots not only affects the appearance quality, but also makes the fibers brittle and damaged. The way to overcome it is: when the water quality is poor, pickle the yarn with 1.5-2.0g/L oxalic acid at 80-85°C before bleaching. Because oxalic acid can form complex ions with iron ions and dissolve in water, the iron in the water can be removed to achieve the purpose of non-yellowing.
The main reason why the whiteness of bleached and whitened yarns decreases over time is due to insufficient scouring. If the yarn is not scoured enough and the impurities in the cotton fiber are not completely removed, the yarn will be bleached and whitened immediately. Over time, the impurities and natural pigments inside the fiber will be exposed, causing the whiteness to decrease. The way to overcome it is to ensure that the wool effect of bleached yarn reaches 13-15cm/30min, and the wool effect must be uniform to ensure that the bleached and whitened yarn does not turn yellow.
In addition, when dechlorinating after bleaching with sodium hypochlorite, too much sodium hypochlorite or insufficient washing can also cause the yarn to turn yellow. When drying yarn, the temperature of the drying room is too high and the drying time is too long, which will also cause the yarn to turn yellow. </p
