Learn something new every Friday. Join 2,000+ nerds and satisfy your curiosity one question at a time with topics including science, technology, history, and more.
Why does plastic turn yellow?
|
β Why does plastic turn yellow?β
|
For the chain to remain intact, the covalent bondsβwhere monomers share electronsβmust 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.
"Electromagnetic spectrum" by the Center for Disease Control is part of the public domain. |
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 longerβbut all break down over time.
Hereβs 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 β you guessed it β 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.
"Why are plants green" by Nefronus is licensed under CC BY-SA 4.0. |
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.
π§ 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, |
βSources for this week's newsletterβ
|
π Wikipedia article of the week
β9,10-Dithioanthraceneβ
"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."
π Catch up on other curious questions
- βWhy do we need antioxidants if we need oxygen to survive?β
- βHow are magnets manufactured?ββ
- βWhy is there cotton in pill bottles?ββ
P.S. π€ Do you have a question for the newsletter? Click here to submit your question (or reply to this email!)
|
π‘ Today You Should Know
Learn something new every Friday. Join 2,000+ nerds and satisfy your curiosity one question at a time with topics including science, technology, history, and more.
Where does "hunky-dory" come from? This question came from a reader submission (thanks Miguel!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question π The short answer βHunky-doryβ likely stems from Frisian honcke (βhomeβ), later paired with a playful β-doryβ suffix, and it entered everyday speech with the rise of 19th-century blackface minstrel shows. π The long answer If you're not familiar with the Americanism, to say something is...
Why does glass break so easily? This question came from a reader submission (thanks anonymous!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question π The short answer Glass breaks easily because its atoms form a rigid, disordered network rather than an ordered crystal lattice. When under stress, there's no way for planes of atoms to slip past each other, so its molecular bonds break easily and form cracks. π The long answer Glass is...
Do you need to wash dishes with hot water? This question came from a reader submission (thanks anonymous!). Has a curious question stumped you lately? Feel free to submit your own question here: Submit your question π The short answer Cold water with soap and scrubbing removes most grease and germs, but hot water makes cleaning easier by better dissolving oils. You would need to use water hotter than your hands can comfortably tolerate to kill germs, which is one reason why dishwashers are so...