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ISO 306, ASTM D1525, ISO 75, ASTM D648: Heat Deflection Temperature (HDT) & Vicat Softening Temperature (VST)

The ISO 306, ASTM D1525, ISO 75 and ASTM D648 standards describe the determination of heat deflection temperature (HDT) and Vicat softening temperature (VST). The heat deflection temperature and Vicat softening temperature are characteristic values, which under comparable conditions, provide an indication of the maximum service temperature of a plastic.

Thermoplastics, in particular, increasingly lose their stiffness and hardness at higher temperatures. This behavior is measured by a constant load applied to a defined specimen, which with increasing temperature leads to increasing deformation in the form of deflection (HDT) or needle indentation (VST).

Plastics brochure Vicat softening temperature VST Heat deflection temperature HDT Test requirements Heat transfer methods Testing systems

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Vicat softening temperature VST ISO 306 and ASTM D1525

For the Vicat softening temperature (VST), loss of hardness is determined via indentation of a weight-loaded needle at increasing temperature. VST is the temperature at which the needle reaches a specified indentation depth. The method is used in the characterization of molding materials and is part of the single-point data defined in ISO 10350-1. Due to the relatively small size of the specimen, this method is also ideal for measurements on specimens that are removed from components.

Heat Deflection Temperature (HDT) ISO 75 and ASTM D648

For the heat deflection temperature (HDT), the loss in stiffness is measured using a three-point flexure method. This method is normally used in the characterization of molding materials and is part of the single-point data defined in ISO 10350-1, which is usually presented in material cards (see www.campusplastics.com). The test method is described in detail in ISO 75-1, ISO 75-2, ISO 75-3 and in ASTM D648.

Test requirements

Depending on the frequency of the measurements and according to the time available in between, which is used for re-cooling of the heat-transfer fluid, different instrument versions are available. This ranges from relatively simple, manually operated instruments to instruments that support a largely automatic test sequence. Higher quality instruments are equipped with electrically interlocking protective covers and the option of integrated fume extraction, which provide additional convenience and safety for the operator.

The test starts at ambient temperature. ISO 75-1 specifies 27°C as the maximum starting temperature. After the test weight specified in the appropriate standards has been applied, the material begins to undergo time-dependent deformation. This part of the deformation is undesirable and is minimized by zeroing the displacement measurement of the deflection, or needle indentation, after 5 minutes have passed. After that, the test is started, whereby heat transfer fluid is heated at a specified heating rate of 50 K/h or 120 K/h, until the standard-based deflection or indentation depth is reached. The temperature at this point represents the test result.

Heat transfer methods

  • Since heat is transfered at a defined increase in temperature, heat transfer to the specimen is important in this test method.
  • The classical method uses a heat transfer fluid, usually silicone oil. Test results with a high degree of comparison accuracy are achieved through ample contact between the specimen and the transfer medium.
  • Furthermore, various alternative methods for heat transfer have been established in the standards over the years. One of these alternatives has been technically implemented in the Vicat D model. In this method, the specimen is heated by means of contact heat transfer between two heated plates. This method does not require the use of silicone oil.

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