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Why Does TPU Filament Turn Yellow Over Time?

TPU filament (Thermoplastic Polyurethane) is a popular 3D printing material. It’s durable, flexible, and shock absorbent, making it great for printing items like phone cases. However, there is a downside: TPU tends to turn yellow with age.

TPU filament yellows over time when exposed to light, heat, friction, and certain chemicals. The material slowly degrades, which is what makes it appear yellow. No matter how much you try to clean the TPU, it won’t become clear again because these chemical reactions are permanent. 

That said, there are some methods that you can use to slow down how long it takes TPU filament to yellow. If you’re planning to use TPU filament to print, you’ll want to know why it turns yellow so that you can prevent it and make sure your prints have a long life.

Why TPU Filament Turns Yellow Over Time

TPU filament becomes yellow when it has a chemical reaction, which can come from a few different causes. Heat, sunlight, and certain chemicals can cause the material to turn yellow slowly. You’ll notice it after a few months when the yellowing increases enough to be noticeable. 

UV light breaks down the chemicals that make up TPU, changing its composition. If you have a clear phone case that’s turning yellow and won’t clean, this is the reason why. It’s not a buildup of dirt or oil on the phone like many people believe—the case itself has degraded.

However, TPU filament stays durable and flexible, even after turning yellow. You don’t really have to replace any prints that turn yellow unless you don’t like how they look.

Overall, heat and sunlight seem to be the most significant factors in how long it takes TPU filament to start to turn yellow. If you handle the print often or keep it in a sunny place, it’s likely that it will start degrading sooner.

Most TPU Isn’t Truly Clear

Most transparent TPU contains very, very small amounts of blue coloring, which makes us perceive the material as clear instead of yellow. As the TPU breaks down, the yellow shows through, and the blue disappears.

TPU is naturally slightly yellow. For this reason, manufacturers need to include blue dye to cancel it out. The blue hides the yellow, but it will still show through as the material starts to age. Overall, there isn’t much you can do to stop yellowing from happening.

Can You Prevent TPU From Yellowing Over Time?

You can’t prevent TPU filament from yellowing over time, but you can make it last longer before it starts to turn yellow. Keeping the print out of the sun helps a lot. You can also keep it in a cool place and handle it less frequently. 

TPU filament experiences a natural chemical degradation, which makes it eventually turn yellow. You can’t reverse the chemical reaction—slowing down the aging process is all you can do for it. 

It’s tough to reduce the yellowing on your TPU. You’ll want to save your slicer files of all your TPU prints in case you need to recreate the object at a later date. Even though yellow TPU is still strong and shock-resistant, many people feel like it’s dirty and don’t want to use it anymore.

In short, you can’t stop TPU from yellowing over time, but you can slow down the process. Keeping the TPU out of the sun will certainly help. Touching it as little as possible will prevent friction, which could cause a chemical reaction with the oils on your hands.

Store TPU Filament the Right Way

Storing TPU filament correctly can prevent yellowing from occurring before you’re done using your print.

Make sure to store the material in an airtight container. Many options for filament storage also come with a desiccant, which keeps it dry. 

If you need an airtight filament container to use at home, I recommend checking out the Sunlu 3D Filament Storage Box from Amazon.com. The box dehumidifies the filament, keeping it safe and durable. It can hold a lot of material and is always completely sealed. The box gives you all the protection your TPU filament needs to not yellow in the short term.

Treating and handling your filament with care is sure to prevent early degradation and yellowing. The best place to start is by changing how you store your materials. Leaving it out is only going to cause damage from moisture, dust, and sunlight.

Make sure that you always store your TPU filament in an airtight case. If you know you won’t need to use it for a very long time, you can seal your TPU in a plastic bag. This will ensure the material doesn’t yellow while in storage, and it’s almost as easy as putting it in a box.

When To Expect Yellowing in TPU

It doesn’t take TPU very long to start to become yellow. With objects that you often handle, such as a TPU phone case, you can expect the transparent material to degrade noticeably within one to three months. 

However, with prints that you don’t often handle and keep out of the sun, you can expect them to last a lot longer before yellowing. If it’s in the best possible conditions, you can get twice as much time out of a print before it starts to yellow.

If you store it in an airtight container and out of the sun, it should never get yellow—even if it stays in storage for a long time.

Final Thoughts

Exposure to heat, sunlight, or chemicals can cause TPU filament to become yellow as time passes. You can’t reverse this effect: if the yellowing is noticeable, it’s because the material has started to degrade.

This is especially noticeable with TPU phone cases. They become yellow after only a few months because the material gets warmer while you’re holding it or when you place it in your pocket. Still, TPU is still an excellent printing material. It’s flexible, durable, and has many different uses—so don’t avoid printing with it!