Acetate sheets have to be aged in controlled conditions for a given time (can't remember exactly how long) before working them to make frames. I believe it is at least 12 month.
You can get an idea by reading about storage of actetate films at
https://www.imagepermanenceinstitute.org/webfm_send/299
Read about Vinegar Syndrome.
VINEGAR SYNDROME
I still have about 3500 brand new frames made in France and Germany back in the 1970s and to 1990s which are as flexible and solid as the day they were made. not one has a faded color.
Of course the chinese have a stepped up production as they are in heavy competition. Alibaba says there are 517 of them. They probably use acetate that has not been properly aged.
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