Can I separate them with a thin piece of plastic, if required ?
It would be best that they not be touching a slow acting electrolysis can occur and eventually eat away at both the pipe and aluminum, rather than a Piece of thin plastic I would recommend a small piece of PVC pipe. measure the copper Pipe 1 inch, 7/8, 3/4, 5/8, 1/2 what ever it is get a piece of PVC pipe just large enough to fit over it, split the PVC pipe in half and off center so you have a profile of a C the essayist way would be to use a grinder now snap it over the pipe spin it around to insulate it from the aluminum Duct.
Take a piece of black foam pipe insulation and slip around pipe!
