Emir BardakçıThe Xiaomi smartphones, along with Redmi and POCO series phones, come along with a transparent protector case for free. This case is mainly composed of Thermoplastic Polyurethane named TPU. Although it’s great for immediate protection for newly Xiaomi smartphones, most users have stated that it yellows within a few days of usage.
The Chemical Structure of Aromatic TPU
These bundled packs come with a plastic called Aromatic TPU. The reason why this particular form of plastic is chosen for the case relates to its robust nature, flexibility, and very low production costs. The downside, however, comes in the form of a chemical structure referred to as a benzene ring. The benzene rings used in this plastic are very sensitive to the ultraviolet light emitted naturally by the sun. When the case comes into direct exposure to daylight, a process referred to as Photo-Fries rearrangement happens.
This causes the internal structure of the plastic to change. This produces a new molecule that absorbs blue light. As a result, the plastic looks yellow or brown to the human eye. Since the change is in the polymer, the yellow color cannot be washed away by soap and water. It is a permanent degradation of the bond at the molecular level.
How HyperCharge & Heat Accelerate Aging
Xiaomi is a global leader in charging technology with a 67W or 120W HyperCharge capability in some of its models. Even though this is one of the most impressive technologies on a smartphone, it produces a lot of heat during the charging process. Heat is a strong catalyst for oxidation. When your phone is warm from charging it or from excessive gaming, the polymer chains from the TPU material will expand. This makes it easier for oxygen and oils to seep into the material. High temperatures basically “bake” the aromatic compounds, accelerating a process that would take months.
The Effects of Skin Oils and Creams
Light and heat are not the only elements responsible for the quick discoloration of the mixture; other elements also play an important part
- Sebum: The skin produces its own natural oils that can be easily absorbed by the porous TPU.
- Skincare products: Products with Vitamin C or Retinol react very harshly with plastic.
- Oxidation: These materials undergo rapid oxidation when placed between a warmed phone with a Snapdragon 8 Elite processor.
These reactions may account for why a case may appear to stain so easily after only 48 hours of heavy use.
