Low viscosity, radiation curable urethane oligomer, preferably a radiation curable polyester urethane oligomer, that can be incorporated into radiation curable ink compositions. Preferred embodiments are suitable for ink jetting applications. The ink jettable embodiments are particularly well-suited for use in piezo ink jet printers. The viscosity characteristics of the compositions are such that conventional solvent is not required in order to satisfy the requisite low ink jet viscosity specifications. The oligomer generally is a reaction product of ingredients comprising an aliphatic polyisocyanate; and a radiation curable, polyester alcohol comprising one or more radiation curable moieties, one or more hydroxyl moieties, and one or more ester moieties.
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