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Getting to the Roots of Yellowing

With value-adds soon becoming the market mantra, fabric properties are being enhanced in a number of ways. But, while endowing garments with distinct features certain pertinent problems might arise. Here we find out the hows and whys of such phenomena

1. What is Textile Yellowing and How Does it Happen?

Yellowing of textiles is the yellow discolouration that textiles can develop during processing, use or storage. This is especially apparent on white or pastel shades, as the yellowness, which appears on fabric is of very low intensity and can only be visible on white or pastel backgrounds. Dark shades are also affected, as the colour becomes dull.

Yellowing of fabrics is caused due to various reasons. The most common reasons are listed below:

Use of Softeners: Cationic and silicone softeners are commonly used to impart a good handle to fabrics. Both these softeners generally contain primary, secondary or tertiary amines and sometimes amides also. These groups contain 'H' atom attached to 'N' atom. The former is susceptible to substitution and can be replaced by chlorine and form chloramines. Chloramines being yellow in colour impart yellowness to the fabric. Nowadays the domestic detergents contain chlorine bleaches. So, with each domestic wash the fabric tends to yellow more.

Also, all these nitrogen-based groups (amines or amides) can very well be oxidised to nitrogen oxides if the finished fabric is over heated. Even during normal drying of the fabric/garment these groups can be oxidised and produce different oxides of nitrogen. Nitrogen oxides are brownish in colour and at very low concentrations they seem yellow.

Use of Fluorescent Compounds: Fluorescent whitening agents are used to improve the whiteness of fabrics. These fluorescent whitening agents absorb the UV light and emit it in the visible region. This emitted light is blue-violet in colour and masks the yellow colour of the material (blue is the exact opposite of yellow in the colour co-ordinate).

These fluorescent whitening compounds contain some unsaturation in their structure, which are responsible for absorbing the UV light and then emitting it in the visible regions. But, these unsaturations are very sensitive towards sunlight, oxidation or weathering. Therefore, these compounds are not at all fast and lose their ability to work as fluorescent whitening agents. Hence, the fabric's yellowness becomes more prominent.

Phenolic Yellowing: Phenolic yellowing is perhaps the most common type of textile yellowing and it generally occurs during storage. Phenolic yellowing is characterised by its bright yellow colour and has maximum absorption at 420-450 nm. This yellow colour fades when exposed to ozone. The yellow products are soluble in hot water and polar solvents such as alcohols, and become colourless in an acid medium at pH 5.0 or below.

Plastic wrapping materials, such as poly bags, contain butylated hydroxy toluene (BHT) as an antioxidant to prevent it from ageing. These can be transferred to the textile surface. BHT reacts with nitrogen oxides (NOx) present in the atmosphere and form nitro phenol compounds or stillbene quinone. These are yellow in colour and impart yellowness to the fabric. Alkaline pH is a must for this process to take place. The sources of alkalinity on whites or pale-shaded fabrics are the residual chemicals used in the bleaching or dyeing process, which are not properly neutralised.

2. What do you Mean by Ozone-induced Yellowing?

Ozone-induced yellowing is a major problem in denims. The Ozone layer, found in the upper atmosphere of earth, absorbs the sun's UV radiation. Due to the increased pollution levels from industry as well as vehicles, the concentration of NO x and SO x gases have increased in the atmosphere. These gases can potentially react with Ozone and reduce its concentration tremendously in the Ozone layer. As a result, a portion of the sun's UV radiation penetrates through the atmospheric layer and reaches the earth's surface. This UV light can generate Ozone gas from the atmospheric oxygen.

As a very strong oxidising agent in the atmosphere, Ozone (O3), in the presence of UV light, decomposes indigo dyes in the following manner:

The deteriorating effect O3 has on denim is irreversible. Based on molecular weight calculation, 1 g of ozone could destroy 10.9 g of indigo dye. Since the O3 induced oxidation of indigo produces a compound (isatin), which is yellow in colour, denim garments turn yellow due to Ozone fading.

3. What is the Significance of Moisture Management? Is it a Fibre Property or a Finish Property?

Moisture management can be developed in polyester-cotton-blended fabrics. The only drawback of polyester is its hydrophobicity. To make polyester hydrophilic, it can be treated with some good hydrophilic agent (like Resil HPJC Plus). The phenomenon that polyester has four times less water retention capacity than cotton is exploited in making fabrics, which show exceptional moisture management properties. Once polyester is made hydrophilic, it can absorb water. Now, if we take polyester-cotton-blended fabrics, the cotton part can absorb water from sweat and transport it to its surroundings by wicking. Polyester, now being hydrophilic in nature, can absorb water from cotton and transfer it directly to the atmosphere, much faster as compared to cotton. This will bring comfort to the wearer, as the body remains dry and cool.

In case of 100% cotton fabric, although, sweat absorption will be good, its transfer to the atmosphere will be slow. Hence, the fabric will not dry easily and will be uncomfortable to wear.
Moisture management involves the transfer of moisture from one part of the fabric to another.
This happens predominantly through capillary action. In textile structures, the spaces between fibres effectively form capillaries. The closer fibres are packed together in yarns. It is evident from the basis of capillary action that the smaller the apparent capillary diameter, the more readily wicking can occur. Hence, the basic textile properties like diameter, cross-section, crimp, etc, all play a role in capillary transport and, hence, in moisture management.

4. What do you Mean by a Durable Finish?

Durability refers to the behaviour of finishes against washing or laundering. It also refers to the long-lasting performance of a finish. For eg, when we say a 'durable stain-repellant' finish, the stain repellency effect should be durable up to a certain number of home laundering cycles (typically ten to 20 home launderings).

For any finish to be durable, there should be a chemical bonding between the chemical imparting the finish and the fibres of the fabric. Also, if the chemical, which gives a particular kind of a performance finish to the fabric, remains there as a water-insoluble entity, then too the finish becomes considerably durable to home launderings.

5. Do 'Oil- and Water-repellant' Finish and 'Stain-repellant' Finish Mean the Same?

Yes, 'oil- and water-repellant' finish and 'stain-repellant' finish are the same.

Finishing with fluorochemicals has been, until now, very successful in providing good water- and oil-repellency. The surface tension of fluorochemicals is very less. It is far below the surface tension of water and considerably lower than that of any oil. That is the reason why fluorochemicals are able to repel both water and oil.

Stains can either be water-based or oil-based. So, if a fluorochemical-based finish is able to repel both water and oil, it can very well repel any stain, which is based on either of them.