Why do dyes fade in the sun

Posted on 11 May by Guy Cookson. I noticed someone posted the question on Reddit, and the answer is pretty fascinating, and way more complex than I realised:. The short answer is that sunlight causes irreversible changes in the compounds and molecules that gave a material its color in the first place, in a process we call photodegradation.

As a result of these changes, the material gradually loses the ability to specifically absorb and reflect different parts of the visible spectrum, creating a faded appearance. To look at things a bit more closely, the color of the dyes used in clothing, colored paper, etc. For example, take a look at the structure of this common dye called Orange II.

When choosing dyes, we need to fit them for their end use. This includes their ability to withstand light if needed. We want happy campers! So, why do dyes fade? To answer this, we need to review the characteristics of dyes that provide color. Simply put, dyes are organic compounds that absorb light in the visible spectrum. These compounds have a color bearing group, called a chromophore, that provides the hue.

They also exhibit resonance of electrons. This point is key to understanding why color fades. It is a well-known fact that sunlight causes colors to fade. It is a less known fact that certain chemicals and pH can have the same effect.

When dyes are exposed to sunlight, the ultraviolet light breaks the azo bond in the dye molecule. This can happen over a short or a long period of time depending on the structure of the dye. It is a natural process that is difficult to avoid. For outdoor signs and such, UV-blocking coatings are often applied to extend the life of the dyes. It's also not necessary for the light to interact directly with the dye molecules to cause bleaching: depending on what other molecules are present with the dye solvents, etc , these other molecules may produce reactive species that then react with the dye.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why does sunlight cause colors to fade? Ask Question. Asked 6 years, 6 months ago. Active 3 years, 11 months ago. The chemical process is known as oxidation. With the help of the sun, even some unwanted stains can be removed from light materials. If the laundry is wet, the bleaching process is started.

In particular, dark laundry should not be hung out in direct sunlight to dry, otherwise, unwanted light stripes can appear along the clothesline. Colors on paintings must also be protected from direct sunlight and the bleaching process that starts with it, otherwise, the appearance and thus the value of the picture could be significantly reduced.

However, not all hair fades equally quickly. There are only two different color pigments in human hair and skin that determine its appearance.

Compared to dark hair, light blonde hair contains significantly fewer pigments.