Yellowing, also known as “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 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 yellowing of textiles and the measures to prevent yellowing has been one of the hot topics at home and abroad.
White or light colored fabrics of nylon and elastic fiber and their blended fabrics are particularly prone to yellowing. Yellowing may occur in the dyeing and finishing process, may also occur in storage or hanging in the shop window, or even at home. There are many reasons that may cause yellowing. For example, the fiber itself is prone to yellowing (material related), or the chemicals used on the fabric, such as the residue of oil and softening agent (chemical related).
Generally, further analysis is required to know the cause of yellowing, how to set the processing conditions, what chemicals should be used or only what chemicals can be used, and what factors will cause the interaction of yellowing, as well as the packaging and storage of fabrics.
We mainly focus on the high heat yellowing and storage yellowing of nylon, polyester fiber and elastic fiber blended fabrics, such as Lycra, dorlastan, spandex, etc.
Causes of fabric yellowing
Gas fading：
——NOx flue gas of sizing machine
——NOx flue gas during storage
——Ozone exposure
Temperature：
——High heat setting
——High temperature die
——Softener and high temperature treatment
Packaging&Storage：
——Phenol and amine related yellowing sunlight (light):
——Fading of dyes and fluorescein
——Degradation of fibers
Micro-organisms：
——Damaged by bacteria and mold
Miscellaneous：
——Relationship between softener and fluorescein
Source analysis of problems and Countermeasures
Setting machine
There are several different types of setting machines used in the textile industry, including those that are directly heated by burning gas and oil or indirectly heated by hot oil. The shaping opportunity of combustion heating will produce more harmful NOx, because the heated air is in direct contact with the combustion gas and fuel oil; While the setting machine heated with hot oil does not mix the burning gas with the hot air used to set the fabric.
To avoid the excessive NOx produced by the direct heating setting machine during the high-temperature setting process, we can usually use our spanscor to remove it.
Smoke fading and storage
Some fibers and some packaging materials, such as plastic, foam and recycled paper, are added with phenolic antioxidants during the processing of these auxiliary materials, such as BHT (butylated hydrogen toluene). These antioxidants will react with NOx fumes in stores and warehouses, and these NOx fumes come from air pollution (including air pollution caused by traffic, for example).
We can: firstly, avoid the use of packaging materials containing BHT; secondly, make the pH value of the fabric lower than 6 (fiber can be used to neutralize acid), which can avoid this problem. In addition, anti phenol yellowing treatment is carried out in the dyeing and finishing process to avoid the problem of phenol yellowing.
Ozone fading
Ozone fading mainly occurs in the garment industry, because some softeners will cause fabric yellowing due to ozone. Special anti ozone softeners can minimize this problem.
In particular, cationic amino aliphatic softeners and some amine modified silikan softeners (high nitrogen content) are very sensitive to high temperature oxidation, thus causing yellowing. The selection of softeners and the final results required must be carefully considered with the drying and finishing conditions to reduce the occurrence of yellowing.
high temperature
When the textile is exposed to high temperature, it will turn yellow due to the oxidation of the fiber, the fiber and spinning lubricant, and the impure fabric on the fiber. Other yellowing problems may occur when pressing synthetic fiber fabrics, especially women’s intimate underwear (such as PA / El bras). Some anti yellowing products are of great help to overcome such problems.
Packing material
The relationship between the gas containing nitrogen oxide and the yellowing during storage has been proved. The traditional method is to adjust the final pH value of the fabric between 5.5 and 6.0, because the yellowing during storage only occurs under neutral to alkaline conditions. Such yellowing can be confirmed by acid washing simply because the yellowing will disappear under acidic conditions. Anti phenol yellowing of companies like Clariant and Tona can effectively prevent the occurrence of stored phenol yellowing.
This yellowing is mainly caused by the combination of phenol containing substances such as (BHT) and NOx from air pollution to produce yellowing substances. BHT may exist in plastic bags, recycled paper cartons, glue, etc. plastic bags without BHT can be used as far as possible to reduce such problems.
sunlight
Generally speaking, fluorescent whitening agents have low light fastness. If fluorescent whitening fabrics are exposed to sunlight for too long, they will gradually turn yellow. It is recommended to use fluorescent whitening agents with high light fastness for fabrics with high quality requirements. Sunlight, as an energy source, will degrade the fiber; Glass cannot filter all ultraviolet rays (only light waves below 320 nm can be filtered). Nylon is a fiber that is very prone to yellowing, especially the semi gloss or matte fiber containing pigment. This kind of photooxidation will cause yellowing and strength loss. If the fiber has high moisture content, the problem will be more serious.
microorganism
Mold and bacteria can also cause fabric yellowing, even brown or black pollution. Mold and bacteria need nutrients to grow, such as residual organic chemicals (such as organic acids, leveling agents, and surfactants) on the fabric. Humid environment and ambient temperature will accelerate the growth of microorganisms.
Other reasons
Cationic softeners will interact with anionic fluorescent brighteners to reduce the whiteness of fabrics. The rate of reduction is related to the type of softener 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 hue of the fluorescent whitening agent will also become greenish. If the fabric must be under acidic conditions to avoid phenol yellowing, an appropriate fluorescent brightener must be selected.
Phenol yellowing test (aidida method)
There are many reasons for phenol yellowing, among which the most important reason is the antioxidant used in packaging materials. In most cases, hindered phenolic compounds (BHT) are 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 reasons for storage yellowing.
Post time: Aug-31-2022