In your examples I imagine it is the long term result of UV light rays from the Sun.
gypsum painting pump|used construction machinery
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https://s.yimg.com/hd/answers/v/b430011a3d2449289ea066bf5e8e1023_4.mp4?a=answers&mr=0&x=1409923349&s=298bb047186f5e8fe32d2720b451f4e3
I've got an old wall-telephone that used to be white, but after years in the sun it's all yellowish. Same thing for my old white alarm clock = now it's a beige-yellow instead of white. I've got a white closet light that has a fluorescent tube in it, and while the outside is white, the part on the inside that 'USED' to be white is now all yellow-beige.
Why does white plastics turn yellow in these situations?
