As a kind of casual clothing suitable for all ages, denim clothing is deeply loved by people. With the improvement of people’s living standards, consumers are no longer satisfied with the traditional heavy and rough denim clothing, hoping that denim clothing can be lighter, thinner and more comfortable. Demonstrate a unique taste. However, thin denim clothing is prone to product yellowing, which will seriously affect product quality.

Yellowing is a phenomenon in which the surface of a material turns yellow under the single or combined effect of light, heat, oxygen, water, and chemical substances. The yellowing phenomenon is common in the production, storage, transportation and use of light-colored textiles.

◆ Classification of yellowing

① Ozone yellowing

The yellowing of fabrics in the air caused by exhaust gas from automobiles, sulfur dioxide, ozone and other factors.

② Phenol yellowing

The effect of nitrogen oxides or phenolic compounds when fabrics are stored causes the yellowing of textile materials. Such as yellowing and discoloration caused by the addition of softeners, oils, lubricants, resins, optical brighteners and metal compounds.

③ Hot yellowing

Under the influence of light/heat, such as heat setting or lamination, or under sunlight or ultraviolet light, the fabric degrades and causes yellowing.

◆ Common specific causes and solutions for yellowing

① Yellowing caused by the raw material (fabric) itself

1.1 General wool, viscose fiber, nylon, spandex, etc. are easy to cause yellowing. For example, nylon and spandex are 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 light aging during storage and use. The BHT in these raw materials reacts chemically with nitrogen oxides (NOx) in the air, resulting in yellowing.

solution:

The storage place isolates the fabric from contact with the air, or uses some special softeners with ozone function in the finishing process. For example, the anti-yellowing soft oil HS-1510 has very low yellowing, has the effect of anti-oxidation and yellowing, and feels soft and slippery. The choice of softener and the required final result must be carefully considered with the drying and finishing conditions to reduce the probability of yellowing.

1.2 In the spinning and winding process of elastic fibers, a lot of silicone oil and lubricants are used, and many chemicals are added in the weaving or knitting process of elastic blends. Degradation of these chemicals under high temperature, light and heat conditions during long-term storage will cause yellowing, and when it is more serious, the elasticity will decrease.

Solution:

Degrease thoroughly during pretreatment. The anti-dyeing soap oil HS-A207 can not only ensure the high-efficiency desizing effect, but also protect the bombs, and achieve a good anti-dyeing effect.

1.3 In the finishing process, spandex and its blended elastic fiber, soybean fiber and other fabrics that are not resistant to high temperature treatment will turn yellow when the heat treatment temperature exceeds 150 ℃. Steam ironing or uneven drying will also cause the brightener to migrate, resulting in excessive local concentration and local yellowing.

Solution:

In the finishing process, the setting temperature should not be too high. For fabrics containing spandex, the setting temperature should be lower than 150°C.

② Yellowing caused by production process and chemicals

2.1 In the process of finishing and finishing, the cationic softener has a strong binding force with the fiber, which can make the fabric obtain an excellent softening effect. But the disadvantage is that it is more prone to yellowing than fabrics treated with anionic softeners

Solution:

The use of weak cationic softener can reduce the possibility of fabric yellowing. Anti-yellowing soft oil HS-1510 has very low yellowing and has the effect of anti-oxidation yellowing, and the hand feels soft and slippery. The choice of softener and the required final result must be carefully considered with the drying and finishing conditions to reduce the probability of yellowing.

2.2 The anti-dyeing was not done well during desizing, resulting in a large amount of indigo dye remaining on the fabric surface. Because the indigo dye used in the dyeing of denim garments has the characteristic of oxidizing yellowing, the indigo dye on the dyed yarn peeled off during desizing and stained back to the original white. On the yarn, these indigo dyes stained on the fabric are more prone to oxidative yellowing.

Solution:

Use anti-dyeing soap HS-A207 to reduce the contamination of the white yarn during the desizing process. A207 not only guarantees high-efficiency desizing effect, but also protects bullets and achieves a good anti-dyeing effect.

2.3 The concentration of fluorescent whitening agent also has an impact on the yellowing of textiles. Generally, in order to obtain satisfactory whiteness of the fabric, it is sometimes necessary to carry out a fluorescent whitening agent finishing in the finishing process. When the concentration of the fluorescent whitening agent reaches a certain value, the whiteness will reach the highest value; but when the concentration exceeds this concentration, the whiteness will decrease with the increase of the concentration, resulting in yellowing of the fabric.

Solution:

During the processing, strictly control the dosage of the fluorescent whitening agent to avoid exceeding the yellowing point of the fluorescent whitening agent. The effect of improving the yellowing of textiles and increasing the whiteness of textiles can be achieved by increasing the blue light that textiles can reflect. 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 are complementary colors. Therefore, the reason why textiles appear yellow is that there is insufficient blue light in the reflected light. According to this, textiles with high yellowness have less blue light and low whiteness in reflected light; while light yellowness has more blue light and high whiteness in reflected light.

2.4 The N-methylolamide of the non-iron resin finishing agent and some antibacterial and deodorizing finishing agents contain nitrogen atoms. If they encounter the available chlorine or NaOCl in the water during the washing process, the phenomenon of chlorine absorption will occur, and some finishing products will absorb chlorine. Later, it will produce yellowing, which is called chlorine yellowing.

Solution:

Use baking soda or other neutralizers to completely neutralize the chlorine in the water.

2.5 After finishing with citric acid as an acid agent, the fabric is also prone to yellowing. The reason is that citric acid is easily dehydrated to form anhydride when heated. Because citric acid has one more hydroxyl group than general polybasic acids in structure, it is a hydroxy acid. It is dehydrated to form anhydride during heating and baking. At the same time, the hydroxyl and ortho-position hydrogen in the molecule may also be further removed under the influence of heat and external environment to generate unsaturated acid (trimonic acid), and may remove water and CO2 to form itaconic acid. These unsaturated acids tend to make the cloth yellow

Solution:

It is best not to use citric acid as an acid agent for white cloth.

2.6 Denim fabric is not completely neutralized after fierce bleaching or chlorine bleaching, resulting in too much chlorine or fierce residue on the cloth surface, which can easily cause yellowing.

Solution:

Increase the amount of sodium bicarbonate or hydrogen peroxide. If you use potassium permanganate for bleaching, you can use monkey neutralizer instead of pyrosulfonate and oxalic acid to neutralize, debleach more thoroughly and make the fabric clearer.

2.7 The metal content of the water exceeds the standard and the hardness is high. Hard water contains a large amount of calcium, magnesium, and iron ions, and the mineral content varies with the degree of removal in the treatment. Many direct dyes can form complexes with heavy metal ions, resulting in reduced solubility and changes in dyeing shade, even in small amounts. Copper or iron ions can also inhibit fluorescence, reduce the whiteness of the fluorescent whitened fiber material, and cause yellowing of the fabric.

Solution:

In the water washing process, water softeners such as CT powder or chelating dispersant are added to complex the metal ions in the water and soften the water.

③ Yellowing caused during packaging and storage

The packaging materials for storing various textiles are polyethylene and other materials. In most cases, these materials will use sterically hindered phenolic compounds as antioxidants. Among them, 2,6-double is widely used and cheap. Tert-butyl-4-methyl (abbreviated as BHT). Because the solubility of BHT in polyethylene (or polypropylene) is very small and easy to volatilize, the BHT in the polyethylene film will diffuse to the surface of the packaging material. The polyethylene film containing BHT is used to package polyamide fabrics or clothing. During storage, BHT can be transferred to fabrics or clothing, and undergo a complex chemical reaction with nitrogen oxides (mainly NO2) absorbed or stored in the air and packaging materials (such as cardboard, etc.) to form a yellow substance .

Solution:

• Prevent BHT from yellowing during the packaging process. The yellowing of phenols and BHT products is sensitive to pH. In terms of anti-yellowing, the fabric should be slightly acidic after dyeing. According to the test, when the pH value of the fabric is <5, the yellowing time will be longer; when the pH value is >6, the yellowing time will be shorter. Adding a small amount of non-volatile acid (such as citric acid) in the post-processing of the fabric will greatly reduce the probability of yellowing.

• Avoid using packaging materials containing BHT. Second, make the pH of the fabric lower than 6 (fibers can be used to neutralize the acid), which can avoid this problem. Anti-phenol yellowing treatment during the dyeing and finishing process can avoid the problem of yellowing.

④ Yellowing during wearing and washing. Sunlight, temperature changes, dry humidity changes, movement, contact, etc. may cause yellowing. During the washing process, pay attention to whether the washing powder is removed cleanly, and whether bleaching agents and detergents with unbalanced pH value are improperly used in the cleaning agent. In the presence of nitrogen oxides, any trace of phenolic antioxidants will be transferred to the surface of the fabric through volatilization or friction, all of which will cause yellowing over time.