What chemical that when cured chemically cross-links to already vulcanized rubber
Various elastomers, such as ethylene propylene diene terpolymer (EPDM), can be cured (crosslinked) using ionizing radiation.

The general radiation effects on polymers are 


formation of gas, 
cross-linking of polymer chains, and 
scission of polymer chains.


However, EPDM rubber contains side chains, ring systems and π electrons, which results in a delocalisation of excitation energy and causes a greater stability of the excited states. Therefore, radiation chemical yields for all dissociative radiation chemical reactions on EPDM are decreased.

The mechanism of cross-linking involves the cleavage of a $\ce{C-H}$ bond on one polymer chain to form a hydrogen atom. The free radical released in the initial radiolysis reaction has enough kinetic energy to abstract a second hydrogen atom from a neighbouring chain to produce molecular hydrogen. Different recombination pathways follow the irreversible loss of hydrogen. Double bond formation results from two events occurring on the same chain; the recombination of two separate macroradicals leads to cross-linking. Cross-linking can also result from the reaction of free radicals with double bonds (comparable to free radical addition polymerization). The radiochemical ageing of EPDM shows the formation of unsaturated bonds (trans-vinylene, vinylidene, and vinyl) as well as the consumption of double bonds by radical addition reactions. Since EPDM already contains double bonds which originate in the diene, the last-mentioned reaction predominates.

The overall effect of cross-linking is that the molecular mass of the polymer steadily increases with radiation dose, leading to branched chains until ultimately a three-dimensional polymer network is formed when each polymer chain is linked to another chain. In contrast, scission is the opposite process to cross-linking in which the rupture of $\ce{C-C}$ bonds occurs. Under irradiation in absence of oxygen, EPDM shows some increased cross-linking with a plateau at roughly $10^5\ \mathrm{Gy}$.

However, further exposure to ionizing radiation makes elastomers hard and brittle until disintegration occurs.