Which pipe is better: Three-layer or Five-layer?
No difference. Just different technologies, no difference in physical and mechanical or operational characteristics. The main thing: if 5 layers are declared, then you should see the middle layer of EVOH - if you do not see it, but they tell you that it is, then you should think about it (and you don't need to smear the end with iodine - you just need to see the layer in the light). In this sense, a three-layer pipe benefits at least in that EVOH can be detected and verified. And if you were invited to the production - do not forget: there should be 5 extruders for a five-layer pipe.