They are some differences as follows.
Different materials: PPR pipes use random copolymers of polypropylene, which have different molecular structures,while HDPE pipe use High-density polyethylene.
Different densities:The density of HDPE pipe is higher than that of PPR pipe.
Different uses：HDPE Pipe is often suitable for municipal engineering and PPR Pipe is commonly used building water supply and domestic water supply
Different installation methods：The welding modes of the two pipes are also different. The connection mode of the HDPE pipe is electric fusion connection, the other is hot fusion butt joint, and the ppr pipe is hot fusion connection.
Different temperature resistance: HDPE pipe is used under normal conditions, the high temperature can reach 70 degrees, while the low temperature is about -30 degrees. While PPR pipe under normal use, high temperature can reach 70 degrees, and its low degree is about -10 degrees. Both have the same high temperature resistance, but the PE tube is better at low temperature.