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Solutions & Analysis of the Causes of Yellowing of Nylon Fabric

Solutions & Analysis of the Causes of Yellowing of Nylon Fabric

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Yellowing, refers to the phenomenon that the surface of white or light colored substances turns yellow under the action of external conditions such as light, heat and chemicals. When white textiles and dyed textiles turn yellow, their appearance will be damaged and their service life will be greatly reduced. Therefore, the research on the causes of textile yellowing and measures to prevent yellowing has been one of the hot topics at home and abroad. 

The white or light colored fabrics of nylon, elastic fiber and its blended are particularly to be yellowing, which may occur in the dyeing and finishing process, storage or hanging in the shop window, or even at home. There are many reasons for yellowing. For examples, the fiber itself is easy to be yellowing (material related), or chemicals used on fabrics, such as oil residue and softener (chemical related) cause the yellowing. 

We mainly analyzes the high heat yellowing and storage yellowing of nylon, polyester and elastic fiber blended fabrics such as Lycra, Dorlastan, and Spandex, and summarizes the causes of fabric yellowing as follows: 
(1) Gas Fading: 
NOx flue gas of setting machine
NOx flue gas during storage
Exposure to ozone 

(2) Temperature: 
High heat setting
High temperature pressing
Softening agent and high temperature treatment

(3) Packing&Storage:
Phenol and amidogen related, light related
Color fading of dyes and fluorescein
Degeneration of fiber

(4) Micro-organism: 
Destroyed by bacteria and fungi

(5) Miscellaneous:
Correlation of softening agent and fluorescein

The analysis and countermeasures of problems
1. Setting machine
There are several different types of setting machines used in textile industry, such as directly heated by burning gas and fuel oil or not directly heated by hot oil. The setting machine heated by burning can produce harmful NOx because the heated air is in direct contact with the burning gas and fuel oil, while the setting machine heated by hot oil will not mix the burning gas with the hot air which used to set the fabric. Therefore, the latter type of setting machine can be selected to avoid this kind of situation. 

2.Flue gas and storage
Some fibers and packaging materials, such as plastics, foam and recycled paper, are added with phenolic antioxidants such as BHT (Butylated Hydrogen Toluene) in the process. These antioxidants react with NOx flue gas in stores and warehouses from air pollution (including air pollution caused by traffic, for example). 
Therefore, we can take the following measures: first, avoid the use of packaging materials containing BHT; second, make the pH value of the fabric lower than 6 (fiber can be used to neutralize acid), which can avoid such problems. 

Fading caused by ozone mainly occurs in the garment industry, because some softeners can cause yellowing of fabrics due to ozone. The special anti-ozone softeners can reduce this problem. In particular, cationic amino aliphatic softeners and amine modified silikan softeners (high nitrogen content) are very sensitive to high temperature oxidation, resulting in yellowing. The selection of softeners and the required of final results must be carefully considered with the drying and finishing conditions in order to reduce the occurrence of yellowing. 

4.High temperature
Textile fabrics exposed to high temperature will get yellowing due to oxidation of fibers, fiber and spinning lubricants, and impurities on fibers. Other yellowing problems may occur when pressing synthetic fabrics, especially women's underwear (such as PA/EL bras). It is a great help to the use of our company's high temperature yellowing resistant product 8242D to overcome this kind of problem, as well as can maintain excellent whiteness of white cloth. 

5.Packaging materials
The relationship between the gases containing nitrogen oxide and the yellowing during storage has been confirmed. The traditional method is to adjust the final pH value of the fabric between 5.5 and 6.0, because the storage yellowing only occurs in neutral or alkaline conditions, which can be easily confirmed by acid washing, and the yellowing will disappear under acidic conditions. However, due to the pH value requirements of many nylon fabrics, it is recommended to use anti phenol yellowing agent2191C, a product of our company, which can effectively prevent phenol yellowing in storage. 
There are many reasons for phenolic yellowing, among which the most important reason is the antioxidant used in packaging materials. In most cases, BHT (Butylated Hydroxy Toluene) will be used as the antioxidant of packaging materials. During storage, BHT and nitrogen oxides in the air will form yellow 2,6-di-tert-butyl-1, 4-quinone methide, which is one of the most likely causes of storage yellowing. 
The main reason is that phenolic substances such as (BHT) combine with NOx from air pollution to produce yellowing substances. BHT may exist in plastic bags, recycled paper cartons and medium glue. Plastic bags without BHT may be used to reduce such problems. 

Generally speaking, the fluorescent whitening agents have low light fastness. If the fluorescent whitening fabrics are exposed to the sunlight for too long, they will gradually turn yellow. It is suggested that  fluorescent whitening agents with high sunlight fastness should be used for fabrics with high quality requirements. Sunlight, as an energy source, degrades fibers; glass cannot filter all ultraviolet rays (only light waves below 320nm). Nylon is a kind of fiber which is very easy to yellow, especially semi gloss or dull fiber containing pigment. This kind of light oxidation will cause yellowing and strength loss. If the fiber contains high moisture, the problem will be more serious. 

In addition, fungi and bacteria can also cause yellowing, even change into brown or black pollution. Mold and bacteria need nutrients to grow, such as residual organic chemicals (such as organic acids, levelling agent, interfacial active agents) on fabrics. Moist environment and surrounding temperature will accelerate the growth of microorganisms. 

Cationic softeners will interact with anionic fluorescent whitening agent, and reduce the whiteness value of fabrics. The rate of reduction depends on the type of softeners and the chance of contacting nitrogen atoms. The influence of pH value is also very important, but strong acid conditions should be avoided. If the pH of the fabric is lower than pH 5.0, the color phase of the fluorescent whitening agent will become green. If the fabric must be in acid condition to avoid phenolic yellowing, a suitable fluorescent whitening agent must be selected.
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