Nylon and elastane and their blends in white or light-coloured textiles are particularly susceptible to yellowing, which can occur during the dyeing process, in storage or hanging in shop windows, or even at home. There are many possible causes of yellowing, for example the fibres themselves are susceptible to yellowing (material related) or the chemicals used on the fabric, such as residues of oils and softeners (chemical related).
Further analysis is often necessary to understand the causes of yellowing, how to set the processing conditions, which chemicals should be used or only which chemicals should be used, which influences interact with each other to cause yellowing, and how the fabric is packaged and stored.
We focus on high heat and storage yellowing in blends of nylon with polyester and elastane, such as Lycra, Dorlastan, Spandex, etc.
Causes of fabric yellowing.
1.Gas fading: NOx fumes from the sizing machine; NOx fumes from storage; ozone exposure
2.Temperature: High heat setting; high temperature mould pressing; softener and high temperature treatment
3. Packaging&Storage: phenol and amine related yellowing of sunlight; fading of dyes and fluorescent finishes; degradation of fibres
4. Micro-organisms: damage by bacteria and moulds
Analysis of the sources of the problem or countermeasures
1. Shaping machines.
There are several different types of sizing machines used in the textile industry, either directly heated by burning gas and oil or indirectly heated by hot oil. The combustion heated machines produce the more hazardous NOx because the heated air comes into direct contact with the combustion gas and fuel oil, whereas the hot oil heated machines do not mix the combustion gas with the hot air used to set the fabric.
Avoiding excess NOx created by direct heating of the sizer during the high temperature sizing process can often be removed by using our SPANSCOUR.
2. Fume fading and storage
Some fibres and some packaging materials such as plastics, foam, recycled paper, have phenolic antioxidants such as Butylated Hydroxy Toluene (BHT) added during the processing of these auxiliary materials, which react with NOx fumes in shops and warehouses from air pollution (e.g. including traffic). These antioxidants react with NOx fumes in shops and warehouses, which come from air pollution (including traffic, for example).
3. Ozone discolouration
Ozone discolouration is mainly a problem in the garment industry, as some softeners can cause yellowing of fabrics due to ozone, which can be minimised by special ozone-resistant softeners.
In particular, cationic amino-aliphatic softeners and some amine-modified silicone softeners (high nitrogen content) are very sensitive to high temperature oxidation and therefore cause yellowing. The choice of softener and the required end result must be carefully considered with the drying and finishing conditions in order to reduce the chance of yellowing.
We can avoid this problem by, firstly, avoiding the use of packaging materials containing BHT and, secondly, by keeping the pH value of the fabric below 6 (fibre neutralising acids can be used). In addition, anti-phenolic yellowing treatment can be carried out during the dyeing and finishing process to avoid the problem of phenolic yellowing.
4. High temperatures
Exposure of textiles to high temperatures can cause yellowing due to oxidation of the fibres, fibre and spinning lubricants, and impurities on the fibres. Other yellowing problems can occur when synthetic fabrics are moulded, particularly in women's intimate underwear (e.g. PA/EL bras). Some anti-yellowing products can be extremely helpful in overcoming these problems.
5. Packaging materials
The relationship between gases containing nitrogen oxide and yellowing in storage has been well established. Anti-phenolic yellowing from companies such as Clariant and TONA can effectively prevent phenolic yellowing in storage.
BHT can be found in plastic bags, recycled paper cartons, glue etc. Try to use BHT-free plastic bags to reduce this problem.
6. Sunlight
Generally speaking, fluorescent whitening agents have a low fastness to sunlight. Fluorescent whitening fabrics will gradually turn yellow if they are exposed to sunlight for too long, so it is recommended that fabrics with high quality requirements use fluorescent whitening agents with high fastness to sunlight. Sunlight, as an energy source, degrades fibres; glass does not filter all UV rays (only those below 320 nm). Nylon is a very yellowing-prone fibre, especially semi-glossy or non-glossy fibres containing pigments, where photo-oxidation can cause yellowing and loss of strength, exacerbated by the high water content of the fibre.
7. Microorganisms
Mildew and bacteria can also cause yellowing and even brown or black contamination of the fabric. Mildew and bacteria need nutrients to grow, such as organic chemical residues on the fabric (e.g. organic acids, homogenising agents, surfactants). Humid conditions and surrounding temperatures can accelerate the growth of microorganisms.
