Why does cotton fabric always turn yellow?

Analysis of the causes of yellowing of cotton fabrics include:

1. Fiber degradation: Fiber degradation occurs due to the effects of heat, light, other radiation, chemicals and biodegradation.

2. Additives: Yellowing and discoloration caused by adding softeners, oils, lubricants, resins, fluorescent whitening agents and metallic compounds.

3. Atmospheric pollutants: yellowing caused by the effects of nitrogen oxides, sulfur dioxide and ozone.

4. Transmitted pollutants: The original yellow pigment of yellowing matter is transferred to the fabric from the contact paper, cardboard, plastic film or plastic bag and the pumice stone used for stone washing.

5. Consumer contaminants: such as sweat and residues from household washing and bleaching.

During storage, yellowing of phenols is common. Raw yellow pigments such as phenolic compounds or butylated hydroxytoluene [BHT] are transferred to the fabric from packaging materials such as cardboard, paper or plastic film during storage, where they are retained by nitrogen, oxidation and The yellow product formed by nitrophenol compounds or 1,2-stilbene produced by the action of chemicals is more favorable in an alkaline environment. The yellowing of phenols has a pure bright yellow color and has large absorption characteristics at 420~450nm. It will fade when exposed to light or ozone. The yellow product dissolves in hot water and polar solvents such as alcohol, and becomes colorless in acidic media with a pH of 5 or lower.

Measures to prevent phenols from causing yellowing are:

1. Avoid using brown paper and brown cardboard containing phenol antioxidants Paper and plastic film packaging finished products; 2. Avoid using heat shrinkage

3. Provide good ventilation conditions during storage;

4. Use less phenol derivatives as additives;

5. Prevent the finished product from being alkaline.

In addition, after using citric acid as an acid treatment, the fabric will also tend to turn yellow.

The reason: citric acid is easily dehydrated to form anhydride when heated. Because citric acid has one more hydroxyl group than ordinary polyacids in its structure, it is a hydroxy acid. It dehydrates to form anhydride when heated and baked. At the same time, the hydroxyl group and the hydrogen in the ortho position in the molecule may also be further removed by heat and the external environment to generate unsaturated acid (acrylonic acid), and may remove water and CO2 to form itaconic acid. These unsaturated acids can easily cause the cloth to turn yellow, so it is best not to use citric acid as an acid agent for white cloth.