10' PVC Schedule 40 vs ABS sun sail support post. Life expectancy, wind and UV damage
I would never even consider using 1.5" or 2" PVC or ABS piping of any schedule as a structural member; expecially for the application that you describe. These materials can flex and given some aging or overextended stress could fail dramatically. 

Sure you may be able to make 5" or 6" diameter thick walled piping work in this application but the cost of that type of product should easily guide you back to some type of steel tubing whether that be square, round or rectangular cross section.