C9 Petroleum resins contain many double bonds which are relatively unstable. An effective way to stabilize these resins is to hydrogenate them. C9 Petroleum resins have aromatic ring structures with an overall aromaticity around 40%, measured by proton NMR. Resins are hydrogenated in solution with very specific operating parameters: temperature, pressure, hydrogen concentration, and catalyst level. Changing any one of these operating parameters will bring a change in the degree of hydrogenation of the final resin. During hydrogenation, the aromatic ring structures gradually lose their aromatic nature and become cyclo-aliphatic.

The first hydrogenated C9 petroleum resins were fully saturated producing aliphatic resins with excellent initial color and stability. These resins are ideal for tackifying SIS and SEBS block copolymers. To produce light colored and stable resins with the correct compatibility to tackify SBS block copolymers and to adjust properties in high styrene SIS block copolymers, partially hydrogenated C9 aromatic petroleum resins were introduced. This produces a resin with solubility parameters (cloud points) in the same range of C5/C9 mixed aliphatic/aromatic resins but with significantly improved stability and much lower color.
When the process is allowed to go to completion, the result is a fully hydrogenated hydrocarbon resin with full aliphatic character. The process can also be adjusted so that partially hydrogenated resins are the end result. Partial hydrogenation results in resins for wide use in adhesive formulations and is achieved through sequential, but not total hydrogenation of the rings. This means that partially hydrogenated resins still have some aromatic rings. The ability to be hydrogenated to varying degrees, resulting in various aliphatic/aromatic balances, gives these resins their unique properties. In theory, any degree of hydrogenation can be manufactured. Eastman resins carry a range of hydrogenation levels, varying from 50% to 100%.