I attempted to add new rubber to an already cured urethane rubber mold and the new layer delaminated. What happened and how can I get the rubber to stick?

New liquid urethane rubber does not bond well to fully cured urethane rubber. Some urethane rubbers (such as PMC®-121 30/WET) will not bond to themselves under any circumstances because they exude an oil to aid in the release of plaster and concrete.

Other urethane rubbers (such as our Brush-On® Series, Vytaflex® or Reoflex® DRY) will stick to themselves if conditions are right:

1. Add newly mixed rubber to cured rubber as soon as possible. The longer cured rubber is allowed to cure, chances of a lasting bond diminish. The best chance for a bond between urethane rubber layers is to pour the new liquid urethane over existing urethane before it cures, while it is in its "gel" or "tacky" state.
2. When adding newly mixed rubber to cured rubber, make sure the cured rubber surface is clean. Dust, oils or other contaminants on the surface will prevent rubber from sticking. Wipe the cured surface with acetone and let dry before applying liquid rubber.

Smooth-On offers a flexible adhesive called URE-BOND® II that will repair torn urethane molds. You can also apply URE-BOND® II to cured urethane rubber as a bonding primer before applying new urethane rubber.