Silicone and plastic waste pipes

[GMM]

Just got back to this job after a few months of distractions. I tiled the floor in our downstairs bog as part of refitting the whole thing. The old waste pipe for the sink was a copper jobbie, cemented into the floor. I cut the tiles/grouted around a new plastic pipe (with a compression union just above the floor level, to take the new waste) and sealed this around using silicone. Coming back to it after a few months, I expected the silicone to have cured etc but, although it has gone off well and filled the gap on the tile side, it doesn't grip the plastic at all. I rather expected it would grip the thing properly, so it would hold while I tightened the screw thread.

Obviously I now need a Plan B. Is there something I should do to treat the plastic pipe so the silicone adheres to it, or is there another preferred strategy for this situation?

Can't see any potential for an angle grinder here, but you never know.....

[BigWallop]

If you have connected a plastic fitting to the copper then remove it and use the plastic pipe directlly into the copper. Use solvent weld around the plastic, about an inch and half up the pipe, and slowly twist the plastic into the copper. The same depth as you wrapped the solvent.

The water is running from plastic to copper, so the joint is running in the correct direction. The solvent will soften the plastic enough to allow it to shrink into the copper. The sticky goo that comes off the plastic pipe is enough to make a water tight joint between them.

Simples

[gunsmith]

And there's me about to suggest he roughens the surface of the pastic with the angle grinder.