The micro-Vickers hardness measurements showed that the thin surface layer of ABS resin was readily hardened to the extent of the hardness of AS resin by irradiation with ultraviolet light. The Charpy impact strength of ABS resin with such hardened surface layer was remarkably reduced due to the disappearance of plastic deformation. The critical crack length at the ductile-brittle transition of ABS resin was found to be only about 10μm from the observation of the hardened surface layer of the resin. To protect ABS resin from ultraviolet light in outdoor use, a coating on ABS resin is useful, but the ABS resin coated with a conventional thermosetting paint showed a brittle manner in the same way as the ABS resin irradiated with ultraviolet light, due to the early crack initiation in the brittle coat. The ABS resin with a rubber layer as an undercoat and with a thermosetting paint as a topcoat maintained the same ductility as the original ABS resin in a long time outdoor exposure. This behavior of such coated resins can be well explained from the elastic stress distribution analysis by the use of finite element method.
