The life of a smartphone is short, and a mobile phone can only accompany us for a few years. And what's shorter than the phone's flowering period is the transparent case that is put on the outside: it always turns yellow ridiculously fast, even after only a few months.
What is the culprit of yellowing? Is there any salvation for a yellowed phone case? This article will explain it in detail for you.
Yellowed phone case | Photographed by the author
Yellowing: Photoaging of plastics
Many different types of plastics turn yellow over time, a result of the degradation of polymer molecules and a sign of material aging.
The main cause of yellowing is photooxidation. Under the influence of ultraviolet light, polymer molecules react with oxygen in the air, which causes the molecular chains to break and form a new series of molecules. In the products of the oxidation reaction, there are molecules that happen to have chromogenic groups that absorb visible light, and they show color.
In addition to changing the appearance, these oxidation reactions can also affect the mechanical properties of the material, making them increasingly fragile.
Why is the phone case extra yellow?
A lot of soft and transparent phone cases turn yellow very quickly, which has to do with the material they use.
This type of phone case usually uses a material called "TPU", which stands for Thermoplastic polyurethane. TPU can be subdivided into two categories: "aromatic" and "aliphatic", and most of the mobile phone cases are aromatic TPU, which contains a lot of benzene rings in its molecular chain.
Aromatic TPU actually has many advantages, it has high transparency, toughness, anti-wear and oil, and at the same time, it is not expensive, these characteristics are beneficial to make mobile phone cases. However, the material's biggest weakness is its sensitivity to light, and photooxidation always occurs quickly.
The rapid yellowing of aromatic TPU is due to the benzene ring structure in the molecule, which easily absorbs ultraviolet light and easily becomes a chromogenic group in the oxidation reaction. Aliphatic TPU has removed the benzene ring structure and has greatly improved lightfastness – but it is more expensive to produce and is not widely used than aromatic TPUs.
芳香族聚氨酯发生光氧化的反应之一 | Dan Rosu et al.
How do I prevent my phone case from turning yellow?
Light is the main factor affecting the rate of yellowing, and if you can reduce the contact between the phone case and daylight, you can make it yellow a little more slowly. In addition to this, regular cleaning and staying away from heat sources can also help.
However, a more effective way is to improve the case material directly. If you add a light stabilizer to the aromatic TPU, or replace it with a material such as polycarbonate, you can significantly slow down the yellowing rate.
However, replacing materials can also create new problems. For example, polycarbonate is not very resistant to abrasion, and unless it is specially treated, it is easy to get scratches during use.
Replacing TPU with polycarbonate can improve yellowing, but it can also cause scratch troubles Dbrand/The Verge
Is there any salvation for the yellowed shell?
Let's start with the conclusion: this one really doesn't. Polymer aging and yellowing is an irreversible process, and once it happens, there is no good way to restore it to its original state.
There are plenty of tips online that can be found on the internet to fix yellowing phone cases, and most of them are just ordinary cleaning methods, such as scrubbing with detergent, toothpaste, or baking soda. These operations remove dirt from the surface of the case, but they are largely useless for yellowing, which is not caused by dirt in the first place, but by chemical changes in the material itself.
Ordinary cleaning methods have little effect on yellowing of the phone case WikiHow
In addition to this, there are also places where people are advised to bleach yellowed plastic products with hydrogen peroxide and use sunlight or UV lamps to speed up the bleaching process. This operation does make sense in theory: as a strong oxidizing substance, hydrogen peroxide can destroy the structure of many colored substances, causing them to fade their color.
But there are still two obvious problems with bleaching. First of all, it is only effective for a subset of materials. According to netizens, this method is useful for yellowed white ABS plastic, but it has no effect on transparent phone cases. I've also found in my own tests that at least 3% of hydrogen peroxide has little to no bleaching effect on yellowing cases.
After soaking in 3% hydrogen peroxide for a few days, the case doesn't look much the same Photographed by the author
Second, bleach does not repair the oxidative damage of plastics, but can cause more oxidative damage. Even if it does improve the appearance, it will not repair the properties of the aging material and will make it more fragile.
Therefore, it is better to give up the struggle and get a new phone case as soon as possible.
bibliography
[1] https://www.hardwareasylum.com/articles/retro/magi_retrobright
[2] https://www.youtube.com/shorts/HI_GdD8SDcY
[3] https://www.youtube.com/watch?v=gswAw7Z6ess
[4] https://www.sciencedirect.com/science/article/abs/pii/S0141391009000135
[5] https://www.theverge.com/2024/3/27/24113943/dbrand-clear-ghost-case-no-yellow-iphone-galaxy-s-pixel
[6] https://hackaday.com/wp-content/uploads/2021/01/The-Retrobright-Mystery.pdf
[7] https://en.wikipedia.org/wiki/Thermoplastic_polyurethane
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Source | Inner Mongolia Youth Editor| Liu Bo reviewed Ge Shuang