Trex is not made of PVC.
The original Trex was a composite of recycled low density polyethylene and sawdust.
The current line of Trex has the same core but now includes a co-extruded "shell".
Trex does not say what the shell is made of. It might be PVC but the core is still ldpe and sawdust.
"In 2009 Trex introduced a ‘Transcend’ line of decking [17] that is a co-extruded composite decking product that includes a shell around the composite boards on three sides to protect the boards from staining, fading and scratching."
Adhesion test results by West System epoxy;
"Different methods of surface preparation are the variables for this test. With the Trex Decking, any surface preparation (including none at all) results in
substrate failure, an indication the adhesive and adhesion are adequate for the material. Even with no surface preparation, the surface seems to be rough enough to provide a good grip for the epoxy. "
I don't know anything about the new shell but it looks like this stuff needs to be mechanically fastened. The substrate comes apart too easily if a really good adhesive is used. Maybe a flexible adhesive caulk would work.
Here is a high res photo of a cross section of a Trex Transcend plank. It looks like the shell is still made of PE.
If joining two sections of shell then acrylic, urethane, or adhesive caulk should work fairly well.
