Why has my screen protector gone yellow?
Cheap TPU film yellows under UV exposure as the polymer degrades. Tempered glass doesn't. If yours has yellowed, it's film, and the only fix is replacement — ideally with glass.
Yellowing is a property of certain plastics, not of glass, and it isn't a defect. If your protector has gone amber, it's film, and no cleaning will reverse it.
Why it happens
Thermoplastic polyurethane. The material in most film protectors, degrades under ultraviolet light. The polymer chains break down and the breakdown products absorb light at the blue end of the spectrum, which the eye reads as a yellow or amber cast.
Heat accelerates it, which is why phones left on car dashboards or windowsills yellow fastest, and why the effect often appears first around the edges where heat concentrates and where the protector is thinnest.
The change is in the material not on it. Nothing cleans it off, and no amount of scrubbing will do anything except scratch the surface.
Why glass doesn't do this
Tempered glass is inorganic and doesn't photodegrade in any timescale that matters. A glass protector fitted today will be the same colour in five years.
If your protector has yellowed, you can conclude with certainty that it's film, regardless of what the listing called it. Some products described as glass film or hybrid glass are polymer with a thin glass-like coating, and they yellow.
Grades differ substantially
Better hydrogel and TPU use UV-stabilised formulations with additives that absorb ultraviolet before it reaches the polymer backbone. These resist yellowing considerably longer — perhaps two years instead of six months in normal use.
But stabilisers deplete. Longer isn't permanent, and any film should be treated as a consumable with a finite life rather than a fitting you do once.
What to do about it
If your screen is flat, this is a good reason to move to tempered glass. You get better impact protection, easier fitting and no colour change, usually for similar money.
If your screen is curved or folding and film is your only real option, buy a better grade, expect to replace it annually, and keep the phone off dashboards and windowsills. An annual replacement at a few pounds is a reasonable running cost for a phone you can't fit glass to.
The related question
Clear phone cases yellow for exactly the same reason and on a similar timescale. If your case has gone amber alongside the protector, nothing is wrong with either product. Both are TPU reaching the end of its useful life.
Common mistakes
Nothing reaches it, and solvents will damage the surface further. Replacement is the only remedy.
If you need film because the screen is curved, paying more for a UV-stabilised grade genuinely buys months of extra life.
Related questions
Tempered glass or film, which should I buy?
Glass for impact protection and a screen-like feel; film for curved and folding displays where glass can't sit flat. Glass is the right default for the large majority of flat-screen phones and tablets.
What screen protector works on a curved display?
Three options: a case-friendly protector that stops short of the curve, a hydrogel film that wraps it, or a UV liquid-adhesive glass kit that bonds edge to edge. Flat tempered glass doesn't work on a curve.
How do I remove a screen protector safely?
Warm it gently with a hairdryer on low, lift a corner with a plastic card or dental floss, and pull slowly and evenly. Never use a metal blade, and never pull fast on a cracked protector.