This test involves characterization of the thermoplastic raw material with regard to its thermal properties and is applied accordingly if eventual use of the pipe at elevated temperatures is anticipated.
The VST measured value does not represent a physical quantity, but serves as a reference value for testing and classifying materials under the same test conditions.
The Vicat softening temperature indicates the temperature at which a circular indenter with a cross-section of 1 mm² under a standard load of 10 N or 50 N will penetrate the specimen to a depth of exactly 1 mm. The Vicat softening temperature is defined in ISO 306and ASTM D1525.
Classical VST instruments employ silicone oil as a heat-transfer fluid. This is the most cost-efficient method and can be carried out accurately and precisely. Other methods employing air or other heat-transfer media have also become established, but often tend to produce less accurate values with greater scatters. A reliable method with directcontact heat-transfer from metal surfaces to the specimen is offered by ISO 306. This oil-free method is particularly suitable where the odour generated by heated oil presents a problem and it is necessary to perfom VST tests regularly.