PEX-A tubing is cross-linked, high-density polyethylene. Apollo EPPW30034 Cross-Linked PEX-A Pipe, 3/4 in, 300 ft L, Opaque Use of Apollo® Expansion PEX pipe in heating systems requires corrosion protection and/or isolation by using a heat exchanger or non-ferrous components throughout the system.. All Apollo® Expansion PEX is opaque in color and ensures easy identification of hot, cold and main water lines through different colored print lines.