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Most transparent phone cases are made from a plastic called Thermoplastic Polyurethane (TPU). Manufacturers use TPU because it’s flexible, shock-absorbing, cheap to produce, and good at protecting phones from drops. It’s a very practical material. But TPU has a weakness: it naturally oxidizes over time. When the case is exposed to UV light, heat, oxygen, and oils from your hands, chemical reactions slowly happen in the polymer chains. These reactions change the way the material absorbs and reflects light, which is what causes the yellow or brown tint you eventually see. Even if you kept the case perfectly clean, it would still slowly yellow because the process happens inside the material itself. Manufacturers do try to slow this down. Many cases include UV stabilizers and anti-oxidation additives, but these only delay the reaction; they don’t stop it completely. Another material that stays clear longer is Polycarbonate, which is used in harder transparent cases. Polycarbonate resists yellowing better, but it has its own trade-offs: it’s more rigid, cracks easier under impact, and doesn’t absorb shocks as well as TPU. So companies usually combine materials: a polycarbonate back for clarity and TPU edges for drop protection. Ironically, the part that usually turns yellow first is the TPU bumper around the edges. In other words, it’s not that engineers “can’t” make anti-yellow transparent cases. They can but those materials would either be more expensive, more brittle, or worse at protecting your phone.