Vicat softening temperature according to ISO 306 and ASTM D1525
The Vicat softening temperature (VST) indicates the temperature at which a flat-ended needle penetrates a specimen to a depth of 1 mm. This is done under a specified load at a uniform rate of temperature rise and usually in oil. The VST corresponds to the temperature value at which thermoplastics begin to soften rapidly.
ISO 306 specifies the general test method for determination of the Vicat softening temperature (VST) of thermoplastic materials. ASTM D1525 specifies the standard test method for the Vicat softening temperature of plastics. The definitions of the method in ISO 306 and ASTM D1525 are almost identical. These standards are therefore considered to be technically equivalent.
An alternative method for determining the thermal load resistance of a plastic is the heat deflection temperature, which is described in more detail in the ISO 75 and ASTM D648 standards.
ISO 2507 specifies the test method and test conditions for determining the Vicat softening temperature of thermoplastic pipes and fittings.
Objective & applications Specimens Test procedure Video Testing instrument Request a consultation Downloads FAQs
Objective and applications for VST testing
The Vicat softening temperature and the heat deflection temperature provide important comparative values for plastics used in high-temperature environments:
- in quality assurance they help in showing differences in the quality of the material during manufacturing, or as a relative comparison value for the evaluation of possible plastics alternatives.
- In the area of product development, they can also serve as a reference point for the operating temperature range or for pre-selection of potentially suitable materials in relation to the temperatures at which they will be used.
Specimens according to ISO 306 and ASTM D1525
In both ISO 306 and ASTM D1525, the preferred dimensions of the specimens are at least 10 x 10 mm or 10 mm in diameter with a thickness between 3.0 mm and 6.5 mm.
It should be taken into account that the results may differ when the dimensions are changed. If the specimen geometry is smaller than required or the Vicat needle is aligned too close to the edge, the material does not provide the needle with enough resistance, allowing it to penetrate the specimen more easily. This can lead to significantly worse results.
Due to the relatively small size of the specimen, this method is also ideal for measurements on specimens that are removed from components.
Test procedure according to ISO 306 and ASTM D1525
Requirements for the test and the test equipment used:
ISO 306 and ASTM D1525 are identical in terms of the test procedure.
ISO 306 defines ≤ 25 °C as the starting temperature, ASTM D1525 specifies 20-23 °C and allows temperatures up to 30 °C.
In the ISO 306 and ASTM D1525 test, a needle-shaped indenter with a round cross-sectional area of 1 mm2 is placed on the plastic specimen and loaded with a defined test weight. After the test weight is applied, the material begins to deform over time. This part of the deformation is undesirable and is minimized by zeroing the displacement measurement of the needle penetration after 5 minutes have passed. The specimen is then heated at a specified heating rate. The Vicat softening temperature (VST) is determined when the needle penetration reaches a depth of 1 mm.
Both ISO and ASTM specify two procedures (force) and two different heating rates, resulting in four different test methods. The force to be applied does not have to be calculated, as is the case with the heat deflection temperature (HDT) according to ISO 75 or ASTM D648. Instead, it is specified for each VST test method:
- Method A50 with a force of 10 N ± 0.2 N and a heating rate of 50 °C/h
- Method B50 with a force of 50 N ± 1 N and a heating rate of 50 °C/h
- Method A120 with a force of 10 N ± 0.2 N and a heating rate of 120 °C/h
- Method B120 with a force of 50 N ± 1 N and a heating rate of 120 °C/h
Video: Vicat Softening Temperature According to ISO 306 and ASTM D1525
Test procedure for determining the Vicat softening temperature according to ISO 306 and ASTM D1525 as well as the heat deflection temperature HDT according to ASTM D648 / ISO 75 with the Amsler Allround heat deflection tester and testXpert testing software.
Vicat Tester
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.
ZwickRoell's testing solution, the Amsler Allround 6-300 heat deflection tester, is a closed instrument for determining Vicat and HDT temperatures up to 300 °C in accordance with all ISO and ASTM standards. Accurate and repeatable results are achieved through the use of advanced displacement measurement and temperature control technology. A user-friendly, safety-oriented and uncompromising design ensures comfort and safety. Either 2, 4 or 6 test stations with automated start of the cooling process, motorized lowering of the specimen and load application can be provided. The heat deflection tester can be operated in standalone mode with touch display or in connection with a PC. With the testXpert testing software, a meaningful analysis of the results can be realized.
More about the heat deflection tester More about the testXpert testing software
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.
Frequently asked questions about Vicat testing / Vicat softening temperature:
Thermoanalytical tests, such as DSC (differential scanning calorimetry) or DMA (dynamic mechanical analysis), for determination of the heat deflection temperature of plastics and their maximum operating temperature are relatively complex. It is simpler and quicker to determine the heat deflection temperature (HDT) and the Vicat softening temperature (VST) instead. Both HDT and VST are used for fast determination of the heat deflection temperature under defined load and at a specific rate of temperature increase. For the Vicat softening temperature (VST), loss of hardness is determined via penetration of a weight-loaded needle at increasing temperature. For the heat deflection temperature (HDT - heat distortion temperature), the loss in stiffness is measured using a three-point flexure method.
- If the differences in results are not due to the specimen geometry or material, a common problem is the correct alignment of the Vicat needle with the specimen. If the needle is aligned too close to the edge of the specimen, the results are lower than they would be with a correctly aligned needle.
- An additional common error is the use of a damaged Vicat needle. While it may not always be visible, it is easily damaged. Even dropping the needle once, for example, can be enough to influence the test results. This instance alone can change the cross-section of the needle.
If the Vicat specimen is separated from the Vicat needle without taking proper care, the needle is easily damaged.
The easiest way to separate the specimen from the needle is to dismantle the needle with the specimen attached and heating them together in an oven or furnace, for example. As soon as the specimen is softened, it can be removed. A temperature of 100 °C is sufficient.