1.1 This part of ISO 2897 establishes a system of designation for impact-resistant polystyrene (PS-I) thermoplastic materials, which may be used as the basis for specifications. 1.2 The types of impact-resistant polystyrene plastic are differentiated from each other by a classification system based on appropriate levels of the designatory properties a) Vicat softening temperature b) melt flow rate c) Izod impact strength d) flexural modulus and on information about the intended application and/or method of processing, important properties, additives and colourants. 1.3 This part of ISO 2897 is applicable to all impact-resistant polystyrene plastics with a two-phase polymer system comprised of a continuous phase, consisting of polystyrene and/or a copolymer of styrene with an alkyl-substituted styrene, and a dispersed elastomeric phase based on butadiene. It applies to materials ready for normal use, unmodified or modified by colourants, additives, fillers, etc. This part of ISO 2897 does not apply to expandable materials. 1.4 It is not intended to imply that materials having the same designation give necessarily the same performance. This part of ISO 2897 does not provide engineering data, performance data or data on processing conditions which may be required to specify a material for a particular application and/or method of processing. If such additional properties are required, they shall be determined in accordance with the test methods specified in part 2 of this International Standard, if suitable. 1.5 In order to specify a thermoplastic material for a particular application or to ensure reproducible processing, additional requirements may be given in data block 5 (see clause 3, introductory paragraph).