Chemistry Behind Yellowing of Textiles- Causes and Preventive Measures
Intertek India Pvt. Ltd, Bangalore, India
Yellowing is the discoloration of textiles developed during production, processing, usage& storage. It is one of the most common quality problems in natural and synthetic fibers. About 20% of textiles are finished in full white. Yellowing is mostly encountered in white and pastel colors, Even dark shades are also affected and it is not prominent. So it is necessary to know the chemistry behind, various causes and preventive measures of yellowing. In this article the various potential causes of yellowing like fiber ageing, yellowing due to gaseous pollutant in the atmosphere, chemical auxiliary used in processing and finishing, contaminant present in the storage materials & the preventive measures are studied.
Why white garments tend to yellow?
In general a substance appears to white when it reflects red, blue and green light. On usage due to fiber ageing and environmental factors the substance absorbs blue light and it reflects red and green light. By theory of light a substance appears to be white when it reflects red, blue and green light. If it absorbs blue light then it reflects red and green light. The combination of red and green gives yellow light. That’s why the white garments shows Yellowing.
|Yellowing of Textiles|
1. Fiber ageing
Fiber ageing is one of the primary causes of yellowing. The environmental impact on the textile fibers leads to fiber ageing. The continuous exposure of sunlight leads to photo degradation of the textile fibers in particular natural fibers like cotton, viscose, linen etc. In the photo degradation process there is breakage of bond occurs within the polymeric structure of the fibers and leads to strength and color loss etc.. Usually natural fibers like cotton by its origin it is slight yellowish in color. On further photo oxidation it yellowness increase. Manmade fibers like nylon, spandex and polyester are more prone to yellowing.
- We can avoid the fiber ageing by proper storing of fibers.
- Avoid exposure to direct sunlight and high humidity.
Yellowing issues due to the atmospheric pollutants play a vital role. The various environmental pollutants are ozone, NOX, SOX etc..
2.1 Yellowing by ozone
Ozone is a tri-atomic molecule of oxygen. It is a strong oxidizing agent.
2.1.1 Ozone Formation
Ozone is one of the primary pollutants which found naturally in atmosphere. In addition to this, ozone is produced by the combined action of sunlight, oxides of nitrogen and oxygen.
2.1.2 Yellowing of denims
The ozone molecules present in the surface of the earth interact with the denim garments. Ozone oxidizes the Indigo dye into Isatin and Antranalic acid and the complex of the two products. Isatin is yellow in color, so indigo dyed blue garment turns to yellow.
22.214.171.124 Factors affecting the yellowing of denims
Indigo dyeing is ring dyeing technique in which dyes are not allowed to diffuse into yarn structure. On usage the dyes present at the surface of the yarn get abraded and gives faded look. The yellowing of Indigo dyes depends on the unfixed dye particles present on the surface of the yarn. The unfixed dyes are mainly due to back staining occurring during the washing process. Back staining is the re deposition of the loose dyes discharged in to the wash bath.
- Unfixed dyes should be removed at the end of the dyeing process.
- Prevent back staining during de sizing & washing process by using proper anti back staining agents.
Yellowing of denim depend on the humidity of the garment in which it is present. Water film formation occurs at the surface of the fibers and causes swelling. The swelled fibers absorb ozone easily and results in yellowing.
- Avoid storing the garment in humid condition.
- Don’t leave the garment in wet condition for longer period.
For a short term protection we can go by antioxidants like sodium bisulfate and ethylene diamine etc.. These agents undergo decomposition process and protect Indigo dyes from yellowing. For a long term period we can use anti ozone softeners & these are amine based softeners.
How Anti ozone softener works?
The main steps involved are
a. Film formation
The anti ozone softener forms a film over fabric surface thus preventing the Indigo dye from reaction with atmospheric ozone.
b. Self degradation
The anti ozone film will react with atmospheric pollutants and undergoes self degradation process and breakdown in to its colorless compounds. Thus the anti ozone softener undergoes self scarification process in the prevention of indigo dyes from the oxidation.
Oxides of nitrogen are one of the main causes of yellowing. In the combustion process when the temperature of air exceeds 538˚C, nitrogen and oxygen combine to form nitrogen oxide. These nitrogen oxides are released from automobiles, trucks and industrial equipments. These nitrogen oxides react with the residues present on the surface of the fabric. At higher concentration these oxides of nitrogen directly affect the nylon fiber.
- Ware houses and factories should be properly ventilated to avoid the accumulation of oxides of nitrogen.
- Avoid use of diesel trucks instead use electrical trucks in ware houses and storage areas.
One of the main causes of yellowing of white and pastel shades is phenols. BHT (Butyl Hydroxyl Toluene) is an antioxidant used in polythene and polypropylene bags to prevent ageing of olefin. It is used in shoulder pads and interlining materials
3.1 Mechanism of yellowing
BHT undergo oxidation process in the presence of NOX to form quinone and nitrophenol. 2,6 di-tert butyl p-cresol undergoes nitration in the presence of NOX to form 2, 6 di-tert-4-nitrophenol.
In the presence of nitration it forms quinone. These products are yellow in color. Even at the room temperature it can be transferred to the textile materials These kind of yellowing occur in the form of yellow streaks or patches while the garments are stored in poly bags in retail shops. This reaction is base catalyzed and takes place in slightly alkaline medium. These nitro phenols and quinines are colorless in acidic medium and become bright yellow in alkaline medium.
It is found that polyamide fibers shows severe yellowing with BHT. This yellowing is reversible and it can be removed by acid scouring or washing with hot water and methanol.
- It can be prevented by avoiding the usage of BHT poly bags for storage purpose.
- The pH of the final stage of the garment should be slightly acidic to neutral (5.5 to 6.5). Neutralize with non volatile organic acid (Citric and oxalic acid) to achieve core neutralization. Avoid using volatile acid for neutralization process because only ring neutralization can be achieved.
- Avoid the accumulation of exhaust gases to control the reaction between BHT and nitrogen oxides.
- To avoid the reaction between BHT and nitrogen oxides we can use Anti phenolic yellowing agent.
- These agents blocks the reactive sites of BHT ,thus the reaction between the BHT & Atmospheric nitrogen oxides are prevented.
By improper use of chemical additives during processing and finishing like softeners, OBA and salts leads to yellowing.
Softeners are used to impart soft handle to the fabric. Among softeners most commonly used is the cationic and silicone softeners. These are amine based softeners (N-H) and reacts with free chlorine present in the commercial detergents containing chlorine bleaches and also with municipal water containing chlorine as a disinfectant. The chlorine reacts with these amines to form chloramines (yellow in color). So it imparts yellow color to textile garments.
- Use softeners with low amine value.
- Wash with detergents containing non chlorine bleaches and enzymes.
OBA is used to enhance the whiteness of the fabric. The unsaturated structure of OBA absorbs uv light and violet light at 340-370nm and it re-emits the absorbed uv light (short wavelength) in to visible blue light (longer wavelength) at 420-470nm. The emitted blue light changes the hue of the yellowness in the fabric to whiter. These unsaturations are very sensitive to sunlight and loss its ability to work & leads to yellowing
- OBA used for cellulosic fibers not stable at low pH so it leads to yellowing.
- Unfavorable conditions like excessive heat and high humidity leads to oxidation of OBA and it loses its whiteness.
- Over usage of OBA beyond the saturation limits leads to yellowing and it can be controlled by stripping.
- Proper selection of OBA to avoid ionic interactions of anionic brightener with cationic finishes leads to deterioration of brightener.
From the above study it can be concluded that yellowing of textiles is mainly due to fiber ageing, atmospheric pollutants, chemical additives and storage contaminants. The below precautionary measures should be taken at each stage of the garment production until it reaches the final customer.
- pH of the garments should be slightly acidic to neutral.
- Avoid storing in place contain atmospheric pollutants like NOX and SOX.
- Avoid use of poly bags containing BHT.
- Proper selection and usage of OBA.
- “Yellowing of white fabric and garments” by Dr. Naresh M Saraf and Deepak V. Alat, International dyer, page no: 23-25
- “Yellowing of textiles” by Rajesh koul at http://articles.fibre2fashion.com
- “Jack ‘n’ Jill and their Jaded Jeans” by Dr. Naresh M Saraf and Deepak V. Alat, International dyer, page no: 28-30
- “Yellowing of textiles on storage” by David T Parkes at http://articles.fibre2fashion.com
- Denim garment processing by Dr. Ravichandran.L, Fabric Care