Discussion on the Causes and Solutions of Nylon Fabric Yellowing

Xanthochroia, also known as "yellowing", refers to the phenomenon that the surface of a white or light-colored material turns yellow under the effect of external conditions such as light, heat, chemicals and so on. The yellowing of white textiles and dyed textiles not only damages their appearance, but also reduces their service life. Therefore, the research on the causes of textile yellowing and anti-yellowing measures has been one of the hot topics at home and abroad.

Nylon and elastane and their blends are particularly prone to yellowing on white or light-coloured textiles. Yellowing can occur during the dyeing and finishing process, in storage or hanging in shop windows, or even at home. Yellowing can be caused by a number of factors, such as the fibers themselves (material-related), or by chemicals used on the fabric, such as residues of oils and softeners (chemical-related).

We focus on blends of nylon and polyester with elastomers such as Lycra, Dorlastan, Spandex. The high heat yellowing and storage yellowing that occurs is analyzed. 

Reasons for the yellowing of the fabric are summarized as follows.

(1) Gas fading:
--NOx fumes from sizing machines
--NOx fumes in storage
--Ozone exposure

(2) Temperature:
--High heat setting
-- High-temperature compression molding
--Softener and high-temperature treatment

(3) Packaging&Storage:

--Phenol and amine-related yellowing of sunlight (Light).
--Fading of dyes and fluorescents
--Degradation of fibres

(4) Micro-organisms:
--Destruction by bacteria and fungus

(5) Other (Miscellaneous):
--Interrelationship between textile softeners and fluorescein

Analysis of the sources of the problem or measures to deal with it

1. Setting machine
There are several different types of styling machines used in the textile industry, either directly heated by combustion of gas and oil or indirectly heated by hot oil. Combustion-heated moulders produce the more hazardous NOx because of the direct contact between the heated air and the combustion gas and fuel oil. Moulding machines heated by hot oil do not mix the burning gas with the hot air used to shape the fabric, so the latter type of machine can be used to avoid such situations.

2. Smoke fading and storage
Certain fibres and some packaging materials, such as plastics, foam and recycled paper, where phenolic antioxidants, such as BHT (Butylated Hydroxy Toluene), are added during the processing of these auxiliary materials. These antioxidants react with NOx fumes in shops and warehouses, which come from air pollution (including from traffic, for example).
We can therefore avoid this problem by either avoiding the use of packaging materials containing BHT or by keeping the pH of the fabric below 6 (by using fibre-neutralising acids).

3. Ozone fading
Ozone fading is a problem mainly in the garment industry. Because some softeners can cause yellowing in fabrics due to ozone, special ozone-resistant softeners can minimize this problem.In particular, cationic amino aliphatic softener and some amine modified silica softener (high nitrogen content), are very sensitive to high temperature oxidation, resulting in yellowing.The choice of softener and the required final result must be carefully considered in conjunction with the drying and finishing conditions in order to reduce the occurrence of yellowing.

4. High temperature
Exposure of the textile to high temperatures can cause yellowing due to oxidation of the fibers, fiber and textile lubricants, and impurities on the fibers. Other yellowing problems can occur during the pressing of synthetic fabrics, especially in women's intimate apparel (e.g. PA/EL bras). The use of our anti-yellowing product 8242D will help to overcome these problems and maintain the excellent whiteness of white fabrics.

5. Packaging material
The relationship between gases containing nitrogen oxides and yellowing in storage has been demonstrated. Traditionally, the final pH of the fabric is adjusted between 5.5 and 6.0, Because storage yellowing occurs only under neutral to alkaline conditions. This type of yellowing can be confirmed simply by pickling, since yellowing disappears when stored in acidic conditions. However, since many nylon fabrics have a pH requirement, it is recommended to use our anti-phenol yellowing agent 2191C, which can effectively prevent the occurrence of stored phenolic yellowing.
There are a number of causes of phenol yellowing, the main one is the antioxidants used in packaging materials. In most cases, a hindering phenolic compound (BHT: Butylated Hydroxy Toluene) is used as an antioxidant in packaging materials. During storage, BHT forms a yellow 2,6-di-di-tert-butyl-1,4-quinone methide with oxides of nitrogen in the air, which is one of the most likely causes of storage yellowing.
It is mainly because substances containing phenol such as (BHT) combine with NOx from air pollution to produce yellowing. BHT may be found in plastic bags, recycled cardboard boxes, glue, etc.. BHT-free plastic bags can be used as much as possible to reduce the occurrence of such problems.

6. Daylight
In general, fluorescent brighteners have a low sunlight fastness. Fluorescent whitening fabrics will gradually turn yellow if exposed to sunlight for too long. Fluorescent brighteners with high sunlight fastness are recommended for fabrics requiring high quality. Sunlight, as an energy source, degrades the fibres; glass cannot filter all ultraviolet light (only light waves below 320 nm). Nylon is a fibre that is very susceptible to yellowing, especially in semi-gloss or matt fibres with pigments. This type of photo-oxidation causes yellowing and loss of strength, a problem that is exacerbated by high moisture content in the fiber.

7. Microorganisms
Molds and bacteria can also cause yellowing and even brown or black contamination of fabrics. Molds and bacteria need nutrients to grow, such as organic chemicals (e.g. organic acids, equalizing agents, surfactants) left on the fabric. A humid environment and ambient temperature can accelerate the growth of microorganisms.

8. Other
Cationic softeners interact with anionic fluorescent whitening agents to reduce the whiteness value of the fabric. The rate of reduction is related to the type of softener and the exposure to nitrogen atoms. The effect of PH is also important, but strong acidic conditions should be avoided. Fabrics with a pH of less than PH 5.0 can also cause the hue of the fluorescent brightener to turn greenish. If the fabric must be subjected to an acidic pH to avoid phenolic yellowing, the appropriate fluorescent whitening agent must be selected.