Why Do Some Plastics Turn Yellow Over Time?

If you own any plastic item, whether something as simple as a roll of packing tape or a storage box, you may have noticed something strange: these plastics turn yellow over time, leaving a tint that wasn’t previously there. Weird, right?!

So, why does this happen? Why does your once white clock face, storage box, old video game console, or white appliances turn yellow?

Well, it’s because of excess UV light exposure. The more exposed plastic is to UV light, the quicker it changes color. However, the change of color does not happen straight away – it will take a while to change. But the increase in light exposure initiates and accelerates the chemical reaction of the polymer, which will eventually turn the plastic into a yellow color.

But why is this important, and why do we care? Good question. The more we understand about polymers – and their structure – the better plastic products we can create – they’ll last longer, not turn yellow, and cause less environmental damage when ready to recycle. 

Essentially, the more we know, the better products will be made – they’ll be more durable, less likely to change color, and overall, withstand the test of time. 

What happens when polymers are exposed to light?

When polymers are exposed to ultraviolet light, it initiates a reaction that will eventually cause a change of color. This does not happen instantaneously, and the slightest bit of light won’t ruin a product for good. But it is something to be wary about – so be careful where you store your plastic goods.

Despite the color change still being a thing, it’s important to note that we’ve come a long way. Newer polymers typically last longer, are more durable, are better for the environment, and take longer to degrade and turn yellow. 

However, it still happens. So, the better we understand how this reaction takes place, the sooner we can start using new polymers or entirely different materials to create white goods, and other appliances and applications that do not turn yellow at some point after purchasing, albeit, it’s usually years later.

UV light is not the only culprit 

When people think of a yellow plastic – one that was originally white or a similar shade – they blame UV light. Or at least that’s what you’ll now blame. But there are actually a few other culprits you need to keep an eye out for. 

For example, increased exposure to visible light, extreme temperatures and humidity, and even solvents can cause damage, causing the polymer to eventually fail. Once again, this only initiates the reaction – it’s why your games console doesn’t turn yellow the first time it’s in a particularly hot room, is exposed to the sun seeping through your windows, or if smoke happens to fill your home after you leave the oven on for a little too long. 

It’s important to test why the polymer turned yellow. This might not be something you will do yourself, but as a scientist, understanding how polymers react to the environment allows scientists to improve future polymers and search for new materials that do not turn yellow when exposed to different environments.

But with that out of the way, we hate to break it to you, but it probably was the exposure to those UV rays that caused any number of plastic products to turn yellow. And yes, even though the last time you used it, it was white, now collecting dust under the bed or in a cupboard, it very well now could be yellow.

If this isn’t a sign to check on your old belongings, perhaps that old video games console, then now’s your chance. Go and witness some first-hand science – just don’t get too upset about it (it’s just science, after all).

The bottom line 

Over time, plastics may turn yellow. This is usually due to an increased exposure to ultraviolet light, but it can also be due to excess heat, humidity, the cold, and other extreme temperatures and environments.

Usually, though, it’s due to increased light exposure that triggers a reaction in the polymer, which will eventually cause it to turn yellow.

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