Polypropylene - for small diameters max. 40 mm and not too high flow, approx. 200 m3 / h at 7 bar - the cheapest material, but pay attention to the inner diameter of the pipe (thick walls) - limits the flow of compressed air, requires specialized tools - welding machine - aluminum; definitely yes - certified for compressed air, very quick and easy assembly and disassembly, definitely for large diameters of the area. 40mm and high flows, high material cost for small diameters and small flows, but already comparable for large diameters and large flows, does not require specialized tools. Steel, yes, but only galvanized! Much worse parameters of the flow rate due to the rough internal surface, very time-consuming assembly. Copper - yes, but it is the most expensive material ... it is not dedicated to compressed air, a common mistake is to make installations with too small a diameter