Who hasn’t carefully stored a pastel-colored seasonal garment only to discover later that it has developed an unsightly yellow stain? Or, gone shopping for a pair of bright white pants and discovered they have a straw-colored stain where they came into contact with the clothes hanger?
In this article we will explore the main factors that contribute to the yellowing of garments, also taking into consideration some special cases, like the yellowing of denim. And we will try to provide some useful advice for prevention.
Why do white garments tend to turn yellow?
To begin, let’s look at the fact that white is a color that contains many different colors. Even sunlight, which we perceive as white, is the sum of many colors, like the colors of the rainbow, apparent in simple experiments with a prism or a drop of water in the atmosphere.
A material is white when all the light that illuminates it is reflected, especially red, blue and green. If, due to some alterations of its properties, the material begins to absorb the blue light, it will reflect only the red and green components. The combination of red and green leads to a yellow light and, on the whole, the material will be perceived as yellowed.
What are the main causes of yellowing?
Various factors can influence the tendency of a material to absorb blue light and thus appear yellow. Following are the three main ones.
- Aging of the fibers
Aging of the fibers is one of the prime causes of yellowing. Prolonged exposure to sunlight or to conditions of excessive heat and humidity can lead to the breakdown of some chemical bonds typical of fibers and consequently to the appearance of the color yellow.
When certain substances are present in the air, even in very small amounts, they can trigger yellowing, sometimes even severe, that shows up most often when garments are put away for storage. The pollutants implicated in yellowing are primarily ozone and nitrogen oxides.
Ozone is responsible for a particular type of yellowing that appears on denim, rather than on white fabrics. The indigo with which the denim is dyed is destroyed by the ozone and transformed into a yellow molecule, easily dissolved during a simple wash cycle. But, after washing, the color of the garment can appear a little or a lot lighter, depending on the amount of indigo destroyed by the ozone.
Nitrogen oxides (designated by the generic NOx symbol) are present in the air and in the workplace, wherever processes of combustion occur (motor vehicles, furnaces/boilers…)
Even in very small amounts, these gases are able to trigger intense reactions of yellowing when some residues remain on the fabric, in particular BHT (butyl-hydroxytoluene), which produces a phenomenon known as phenolic yellowing.
BHT is an antioxidant widely used in materials containing polymers, for example in the plastic bags used to protect garments. It is very volatile, even at low temperatures, and thus migrates easily from the plastic bag to the fabric. If the fabric comes into contact with the polluting NOx gases, the formation of substances with the typical straw-colored yellow are catalyzed. The reaction is further assisted by alkalinity in the fabric, a condition that easily develops in cotton fabrics that are laundered in an alkaline solution.
- Chemical additives
The use of some fabric additives can produce yellowing. For example, some cationic fabric softeners tend to yellow through the action of chlorine contained in laundry detergents or even in the local water supply. Even some optical whiteners, used to create a “super white” effect, can oxidize and then yellow when exposed to sunlight or to unusually hot and humid conditions.
To summarize, yellowing of garments is primarily caused by aging of the fibers due to exposure to light, heat, humidity, the presence of polluting gases, and the improper use of some fabric/laundry additives. The preventative measures to take at each stage of the life of a garment, up to and beyond the end user, are the following:
- ensure fabrics are slightly acid (pH 5.5 – 6.5)
- choose fabric softeners and optical whiteners that are resistant to yellowing
- avoid the accumulation of exhaust gases in the workplace and where garments are stored
- use plastic garment bags that do not contain phenolic antioxidants like BHT
- avoid exposure of white garments to direct sunlight.