PU vs polyurethane foam

PU and polyurethane foam both are nouns.

Word	Noun	Adjective	Verb	Adverb
PU	Yes	No	No	No
polyurethane foam	Yes	No	No	No

As nouns, polyurethane foam is a hyponym of PU; that is, polyurethane foam is a word with a more specific, narrower meaning than PU:

PU: any of various polymers containing the urethane radical; a wide variety of synthetic forms are made and used as adhesives or plastics or paints or rubber
polyurethane foam: a foam made by adding water to polyurethane plastics

Other hyponyms of PU include polyfoam.

PU (noun)	polyurethane foam (noun)
any of various polymers containing the urethane radical; a wide variety of synthetic forms are made and used as adhesives or plastics or paints or rubber	a foam made by adding water to polyurethane plastics