As far as the chemical resistance is concerned, data obtained from swelling tests at +30 -C concerning weight and hardness changes are summarized in Table 2. The Sifeli rubber shows least swelling and higher hardness retention after exposure to more polar solvents like M15 or methanol. On the other hand the FEU sample shows the best performance in Fuel C, likely because of the synergistic effect of fluorinated chain and polar urethane linkages, both Bphobic^ with respect to hydrocarbons. It has however a higher sensitivity to methanol again due to the presence of urethane bonds with possible solvent hydrogen bonding. The quite high swelling of FEU in M15 is not easy to be justified according to classical solubility parameters (d) theories, since values calculated for the polymer and the solvent mixture are not very close. Actually, according to tabulated values and rules of admixtures