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 (VST)
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.
Vicat Dry
Once the heating block has reached the starting temperature, the specimens are positioned in the measuring stations, the instrument is closed pneumatically and the test weight specified in the standards is automatically applied.
On completion of preheating under the influence of force, the displacement signals are zeroed (program-controlled) and warming of the heating block at the preset rate is started. As soon as the required penetration is attained at all stations, the heating is switched off and the test is ended.
Re-cooling is started automatically; the specimens can afterwards be removed.
HDT
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
Deflection is specified by ASTM as an absolute deflection of 0.25 mm; ISO standards define an increase in flexural strain of 0.25%.
Specimen positioning
The standard also specifies how the specimen is to be positioned on the anvil: flatwise or edgewise.
Flexural stress
The standards also specify different flexural stresses:
Test weight
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.
