Discussion
Polyester is dyed above one hundred degrees under pressure because
Answer: B
Correct answer: the fibre must be above its glass transition
Polyester is highly crystalline and its chains are essentially immobile below the glass transition, so disperse dye cannot diffuse into it at any useful rate. Above about 100 degrees the amorphous regions become mobile and the dye enters, which is why the process is run at 130 degrees under pressure - or at the boil with a carrier, which lowers the transition chemically instead.
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