Thank you, unclesyd. Here is what I got from Dupont:
"Recommend you contact the supplier of the expansion joint - they should be experienced in making field splices such as the one they are going to attempt, and should know the best way to make the splice.
The splice CAN be made using uncured FKM compound, sandwiched between two sheets of cured FKM, but the surfaces of the belting to be spliced need to be very clean (wiped clean with a solvent-soaked rag) and the splice needs to be clamped under pressure (ideally, >50psi) during the application of heat to the splice area. Sufficient time must be allowed for the 350F heat to fully penetrate the belting, to the uncured FKM sandwiched between the layers. If the belting is 3/16" thick, and is reinforced with fabric (the Kevlar you mentioned), it might well require at least 4-5 hours, for the temperature to penetrate into the center of the splice and cure the layer of uncured FKM.
Another option is for them to use a FKM adhesive (like Pelmor Laboratories PLV-2000). PLV-2000 is a 2-part adhesive, and will vulcanize at room temperature.
Clean both belt surfaces that are to be spliced together, by wiping surface with a solvent-soaked rag (acetone or toluene)
Apply a thin coat of the mixed, two-part adhesive to both belt surfaces that are to be spliced together and allow to air-dry (they should experiment with the adhesive, to determine how long a time is necessary, to allow the solvent to evaporate, but without allowing the accelerated cement to cure)
Clamp the coated surfaces together and allow the cement to cure (at least 2 hours)."
