Vicat test

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). In the Vicat test a sample of the plastics material is heated at a specified rate of temperature increase and the temperature is noted at which a needle of specified dimensions indents into the material a specified distance under a specified load. In the most common method (method A) a load of ION is used, the needle indentor has a cross-sectional area of 1 mm, the specified penetration distance is 1 mm and the rate of temperature rise is 50°C per hour. For details see the relevant standards (ISO 306 BS 2782 method 120 ASTM D1525 and DIN 53460). (ISO 306 describes two methods, method A with a load of ION and method B with a load of SON, each with two possible rates of temperature rise, 50°C/h and 120°C/h. This results in ISO values quoted as A50, A120, B50 or B120. Many of the results quoted in this book predate the ISO standard and unless otherwise stated may be assumed to correspond to A50.)... 

Whilst the Vicat test usually gives the higher values the differences are quite modest with many polymers (e.g. those of types A, B and C). For example, in the case of the polycarbonate of bis-phenol A (Chapter 20) the heat distortion temperatures are 135-140°C and 140-146°C for the high and low stress levels respectively and the Vicat softening point is about 165°C. In the case of an acetal homopolymer the temperatures are 100, 170 and 185°C respectively. With nylon 66 the two ASTM heat distortion tests give values as different as 75 and 200°C. A low-density polyethylene may have a Vicat temperature of 90°C but a heat distortion temperature below normal ambient temperatures. 

At the risk of oversimplification it might be said that the Vicat test gives a measure of the temperature at which a material loses its form stability whilst the higher stress level heat distortion temperature (1.82 MPa) test provides a measure of the temperature at which a material loses its load-bearing capacity. The lower stress (0.45 MPa) heat distortion temperature test gives some rather intermediate figures and it is perhaps not surprising that it is today less often quoted than the other two tests. 

In the Vicat test, the polymer sample is subjected to a load of 1 kg on a standard needle. The sample is immersed in a bath and heated, and the softening point is defined as the temperature at which the loaded needle penetrates to a depth of 1.0 mm. 

With the Vicat test a steel needle, at the end flattened to an area of 1 mm2 is pressed into a block of the material with a standard force (1 kgf for Vicat-A, 5 kgf for Vicat-B). The temperature is increased at a standard rate of 50 °C / hour until the needle has penetrated 1 mm into the sample then the Vicat softening point has been reached. As well as we have seen with the Shore hardness ( 7.5.1), this process of penetration is, in fact, also governed by the E-modulus of the material, though in a much more complicated way. Globally, also to the Vicat test a characteristic E-modulus can be ascribed, which is lower than with the ISO bending test, namely about 200 MPa for Vicat-B and 40 MPa for Vicat-A. 

Another standard heat distortion test is the Vicat test where a flat-ended needle (diameter 1 mm) penetrates to the depth of 1 mm under a specific load of 50 N (Vicat A) or 10 N (Vicat B). It will be clear that the penetration depth is again determined by the Young modulus, although in a much more complicated way than in the bending test. A rough estimate yields Vicat softening temperatures where E 1000 MPa (Vicat A) or E 200 MPa (Vicat B). 

Vicat test See test, hardness Vicat test, softening point Vicat. 

Vicat test n. A test for determining softness of a polymer at temperature an indentor under fixed load will penetrate a specified distance into the material. 

In the Vicat test, the samples are placed under a thin cylindrical probe, again at a stress specified in the standards. The apparatus is heated in a similar manner. The temperature at which the probe penetrates the sample is recorded. 

Vicat tests were performed under a weight of 10 N. 

Most fillers are harder than polymers, so filler addition increases the hardness of polymers. The trends are similar to those for modulus and yield strength, namely that more anisotropic fillers are more effective at increasing hardness. As hardness is a surface measurement, similar to a room temperature Vicat test, the filler will only affect the hardness if it is present at or near the surface. If the filler is depleted at the surface, for example due to flow effects during injection molding, then it may have little or no effect on the measured hardness. 

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