Answering this question is not a minor undertaking. So, I’m going to break this post into several different parts for those that want to dig a little bit deeper than this post. And, I’m going to write the short answer here. Also, I’m sorry/not sorry for this series of posts as they will likely sound more like a technical paper/technical review. I’m pulling from a lot of sources in order to answer this question. I’m also very thankful for my mechanical engineering background from Texas A&M 😊 Go Aggies!
The Short Answer
Plastics yellow because when they are exposed to heat, light, radiation, mechanical action, etc., (and combinations of these things) their chromophores are altered within the plastic. What’s a chromophore? Chromophores are the areas in a molecule that is responsible for giving an object it’s color (yes, I did just use Wikipedia as a reference, but it’s a good straightforward definition). So, to say it again, plastic color ends up changing because as UV light, heat, etc. reacts with the plastic, it changes the chromophore and ends up altering the color of the object.
This is a very “handy-wavy” way of describing the process, which, as an engineer, leaves something to be desired. I’d like to get a little more down in the weeds with this question, so the less “hand-wavy” answer can be found in the following posts. I hope you find them satisfying!
Quick note: I want to give a "hat-tip" to Hey Brit's video post called "The Truth about Retr-brite - Busting Myths with Science". He did an excellent job putting this together and also did a pretty cool experiment. If you get a chance, please watch it. The topics he covers will be similar to what we will discuss in this series of post, though we will discuss things a little bit differently. All that said, you should watch his video!
So, to begin, let's start with Part 2: Chromophores
If you have questions, comment, or feedback on this post, please leave a comment below. We'd love to hear from you!
Part 2: Why Plastics Yellow: Chromophores
Part 3: Why Plastics Yellow: Mechanisms of Degradation
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