HDT/Vicat Allround for Convenient Testing
Many plastics applications are placing increasing demands on heat resistance. An important value with regard to the temperature resistance of plastics is the heat deflection temperature. This can be divided into Vicat softening temperature and HDT (heat deflection temperature). In both methods an oil-bath is heated at a defined rate and the temperature is recorded at a deflection or indentation depth specified by the standard.
Vicat softening temperature (VST) provides a value for the temperature at which a thermoplastic material begins to soften rapidly. This temperature is determined using a needle-shaped indenter with a round cross-sectional area of 1 mm2.
The indenter is placed on the plastic specimen and loaded with a defined test weight. The specimen is then heated at a specified rate. The VST is obtained when the needle penetrates to a depth of 1 mm. ISO 306 and ASTM D1525 are identical with regard to the test sequence.
The classical medium used for heat transfer is oil. An alternative is the 'dry method', in which heat transfer takes place via heating blocks. A special version of the Zwick instrument, the Vicat Dry, is available for this method. A detailed description of this method and the instrument used can be found in the 'Vicat Dry' section.
Heat deflection temperature (HDT) indicates the relative behavior of different types of material under load at elevated temperatures. It is determined for materials such as thermoplastics, hard rubber and duroplastic laminates.
A specimen is placed in a 3-point flexure test kit and loaded with the test weight required to achieve the flexural stress specified in the standard. The temperature is then increased at a uniform heating rate of 120K/h. The temperature at which the specimen reaches the deflection specified in the standard is determined.
Deflection is specified by ASTM as an absolute deflection of 0.25 mm; ISO standards define an increase in flexural strain of 0.25%.
The standard also specifies how the specimen is to be positioned on the anvil: flatwise or edgewise.
The standards also specify different flexural stresses:
Accurate measurement of specimens before the test, or the use of precisely identical specimens, is therefore extremely important with this method. Use of the standard HDT weight set is only worthwhile if specimen dimensions are adhered to exactly (±0.05 mm). In all other cases the universal set of weights is used.
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