Can the amount of heat generated due to welding slide plates affect the composition of the PTFE material?

In general yes. The lip dimension on the slide plate is intended to isolate the PTFE pad from the outer edge of the base plate. This helps to dissipate the heat generated during the welding procedure, however, it is also recommended that a “skip” weld procedure be followed to insure that high temperatures are not seen at the PTFE pad.

Ptfe, 25% Glass Filled Slide Plates Bonded To Stainless Steel Backing Plates
PTFE, 25% Glass Filled Slide Plates Bonded to Stainless Steel Backing Plates