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Why does plastic turn yellow?

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📓 The short answer

Plastics typically turn yellow due to photo-oxidation, a process where UV light and oxygen triggers a chain reaction that breaks down the material's chemical bonds. Over time, new molecular arrangements start to absorb more blue and violet light, making the plastic appear yellow.

📚 The long answer

Once upon a time, your retro gaming device, lawn chair, or other favorite older plastic object was brilliantly white. Now? It's yellow and brittle. Let's dig into the chemistry of plastics to explain why some of them turn yellow over time.

Plastic is a polymer, meaning its structure consists of many (poly) repeating smaller molecules (monomers). Think of it like linking paper clips together to form a chain.

For the chain to remain intact, the covalent bonds—where monomers share electrons—must stay strong. If these bonds break, the polymer degrades. This is why plastics can weaken, become brittle, or change color over time.

While there are many causes of polymer degradation, the most common culprit is photo-oxidation (light + oxygen). Specifically, ultraviolet (UV) light is particularly tough on plastic's chemical structure.

Some plastics, like polyethylene (plastic bags) and polypropylene (plastic food containers) break down quickly in sunlight, while others, like polyvinyl chloride (PVC pipes), resist degradation longer—but all break down over time.

Here’s how photo-oxidation drives the process of polymer degradation:

Step 1: Plastic is exposed to light, leading to the formation of free radicals.

Interestingly, plastic itself absorbs little UV light due to its chemical structure. However, the production of plastic often leaves behind impurities, such as hydroperoxide (-OOH) and carbonyl (-C=O), which can absorb more UV energy. As energy is absorbed, it breaks the chemical bonds of plastic, producing unstable particles called free radicals.

Free radicals are highly reactive molecules with an unpaired electron, which makes them seek out nearby molecules to stabilize. In the process, they steal electrons from other molecules, and create new free radicals, causing a domino effect.

Step 2: Free radicals react with oxygen and kick off a chain reaction of degradation.

Once free radicals form, they trigger a chain reaction, creating different types of free radicals along the way. A chain reaction has been set off as these free radicals continue to break more bonds in the plastic and — you guessed it — form even more free radicals.

This is when plastic begins to turn yellow. As the chemical structure of plastic changes, new molecular arrangements called chromophores form. Chromophores are groups of atoms that absorb certain wavelengths of light.

Just as the chromophores found in chlorophyll absorb blue and red light, making plants to appear green, these new chromophores formed in degraded plastic absorb blue and violet light, making the material look yellow or brown over time.

Step 3: Free radicals react with each other and stop the chain reaction.

Eventually, free radicals collide and neutralize each other, forming stable molecules which slows down the chain reaction. By this point, the plastic's chemical structure has already weakened and its color has started to turn yellow.

🧠 Bonus brain points

Why do newer plastics not turn yellow as often?

As more research has been conducted on plastics, chemists have figured out ways to slow photo-oxidation. Plastics now commonly include additives like UV absorbers, which soak up UV radiation before it can break chemical bonds, and antioxidants, which neutralize free radicals.

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

🌐 Wikipedia article of the week

​9,10-Dithioanthracene​

"9,10-Dithioanthracene (DTA) is an organic molecule and a derivative of anthracene with two thiol groups. In 2004, DTA molecules were demonstrated to be able to "walk" in a straight line (reportedly a first) on a metal surface by, in effect, mimicking the bipedal motion of a human being."

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👀 Catch up on other curious questions

• ​Why do we need antioxidants if we need oxygen to survive?​
• ​How are magnets manufactured?​​
• ​Why is there cotton in pill bottles?​​

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