PTC rubber is made from polydimethylsiloxane (PDMS) loaded with carbon nanoparticles.
The quantum mechanical tunneling effect inside the material is highly temperature dependent.
At a specific temperature the quantum mechanical tunneling effect current ceases.
The copper is in turn connected to a voltage to provide the electrical field inside the material necessary to trigger the tunneling effect.
These heaters will provide high power when they are cold and rapidly heat up themselves to a constant temperature and remain there virtually unaffected by changes in the ambient conditions.