PET can be both transparent or opaque depending on its processing and thermal history. If PET is amorphous it will be transparent, if it is semi-crystalline it will appear either white or opaque depending on its crystal structure.
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Polyethylene terephthalate, also called PET, is the name of a type of clear, strong, lightweight and 100% recyclable plastic. Unlike other types of plastic, PET plastic is not single-use -- it is 100% recyclable, versatile, and made to be remade.