Intermolecular forces are forces that act between adjacent molecules in a chemical sample. Urethane consists of an ethyl ester on one side and an amine on another side. So the intermolecular force at play here is hydrogen bonding. There are ample opportunities for hydrogen bonding between the ester and the amine. The amino hydrogens are electropositive while the ester carbonyl oxygens are electronegative. So a network of hydrogen bonds can form between these two species. This hydrogen bonding network helps urethane to be a crystalline solid in pure, dried form.