ISO 306 Determination of Vicat Softening Temperature (VST)

Vicat softening temperature of plastics: definition

The Vicat Softening Temperature (VST) test, specified by the ISO 306 standard, measures polymers’ ability to retain their mechanical (in this case surface) properties at high temperatures.

Also known as Vicat Needle test, the VST is becoming increasingly important as plastics continue to replace more traditional materials in all manner of applications.

Vicat Apparatus Test Procedure

The ISO 306 Vicat test is performed by slowly raising the temperature of the specimen while applying a point load. Once the point load has penetrated 1mm into the material, the test is ended and the temperature recorded. The test does not provide results for use in design calculations.

However, it is very useful as a quality control or development tool – the result being a measure of the temperature at which thermoplastics begin to rapidly soften.

One of the useful aspects of this test is that as long as the specimen covers a minimum footprint, it can be of any size or shape. This means that tests run according to this standard can be run using much less material than with other standards.

ASTM D1525 / ISO 306

Similar to ISO 306 are ASTM D1525, and the Heat Deflections Temperature test methods, ISO 75 and ASTM D648. ASTM D1525 is very similar to this standard, but accounts for the initial creep of the sample. Additionally, the tests are operationally similar to the test but using a different head and specimen size.