Why do colors fade in the sun? And not just fabrics but also painted and plastic items?



Why do colors fade in the sun? And not just fabrics but also painted and plastic items?

In: Chemistry

The sun’s energy is radiation. Basically, any objects in the direct path of sunlight is getting blasted with radiation. The radiation breaks down particals and causes deterioration. This is also how people get skin cancer.

Colours in durable products come from specific dye molecules. The shapes of these molecules, the distribution of electric charge within the molecules, and how they interact with other molecules, are all specific properties of the dye molecules, and they are what dictates the colours you see.

Sunlight carries a lot of energy. Every so often, a photon of sunlight will strike one of the dye molecules and cause a change. Maybe it will knock an electron off, or break a bond between two atoms, or change the shape of the molecule or how it interacts with the others. Once this happens, the molecule no longer has the exact properties that give it its colour. If this happens to enough dye molecules, the object will lose its colour overall because there will be fewer molecules left to give it that colour.

Sun is mega hot and super jiggly, the light that comes from the sun is also super hot and super jiggly. It jiggles the colors off of items slowly over time because it’s SOOO jiggly!

I feel like everyone’s giving really good answers to *parts* of your question, but nobody’s really answering your entire question.

**First**, as someone said below, the color of something is determined by the shape of the molecule. The sun’s radiation hits the molecule, and parts of it are trapped while parts of it bounce around and then fly back out. The parts that fly back out are the parts that then hit your eyes, and determine the color you see.

The parts that are trapped can be experienced in other ways. For example, objects that appear black look that way because *none* of the sun’s radiation manages to bounce back out as color that your eyes can see. But where does it go? Well, it all gets trapped, and that’s why black objects get so much hotter in the sun than white ones.

**Second**, as someone else mentioned, not all color molecules are the same. You can make a lot of different molecules that look “red.” The thing is, in order to color a “thing” like a piece of plastic, or fabric, or a car, you need to have your “red” molecule mix well into what you’re making.

If you’re trying to make a red birthday cake, you can pour in red food coloring. But you can’t pour red food coloring into a vat of molten plastic and expect it to work. The plastic will only mix with certain molecules. You need a different “red” to color plastic, versus a red for a cotton t-shirt, versus a red car.

**Third**, because the molecules that make paint red vs. plastic red are different, they have different resistances to solar radiation. As the sun’s radiation is absorbed or bounces around them, some molecules hold up better than others. Occasionally, instead of bouncing right when struck by radiation, the molecule just shatters into pieces that no longer bounce radiation in the right way to make the color you want. That’s fading.

**Fourth**, there’s an expense consideration. Stronger color molecules that don’t shatter are often more expensive to make. They also tend to be less universal — the strongest red for a certain kind of car paint probably can’t be used in, say… plastic. The strongest red for plastic may just be weaker than the strongest red for paint, period. Or there might exist a super-strong red for plastic, but it’s too expensive (or toxic!) for a $20 children’s toy.

Put all of these considerations together in different combinations, and you have the answer to your question why certain colors, in certain materials, fade at different rates.

(Edit: If you’re curious, the science of discovering and creating molecules that reflect different colors, that can be mixed with different materials, and that hold up better or worse to the sun’s radiation, is an *entire industry* of very well-paid chemists and materials scientists. You can make a whole career out of your question.)

“Colors” are due to dyes, molecules that reflect (or re-emit) radiation in the wavelength of the colors you see.

That happens because of the way the electrons in the molecule interact with the light and these light-interacting electrons are usually part of double bonds.

When UV radiation from the sun hits these molecules over time, the double bonds break so the dye molecule doesn’t interact with light anymore (becomes faded).

Some dyes don’t have double bonds (like Van Gogh’s yellow, CdS, which is toxic) and they also fade due to chemical reactions started by the light. However, these are mostly older substances that aren’t really used industrially anymore.

Car paint doesn’t fade as easily because it has fillers, anti-corrosives (molecules that prevent the dyes from fading) and because it demand the use of more durable dyes, especially since car surfaces get hotter (and dyes fade faster if it’s hotter). Car surfaces are also metallic, so they naturally reflect part of light anyway.

Source: I’m a chemical engineer working on light-activated molecules for my dissertation.