As title, or does it just burn

They're PU, so will kind of melt/blister/blubble and then burn

Any reason your trying to melt them down ?

Trying to make bush-juice ?

PU is just a two pack mix, you vary the % of mixes to give you different shore value this the measure of PU.

PU is thermosetting though^^... it won't melt as such, it will just blister like has already been said