Polymers Vicat softening temperature

The Vicat softening temperature is the temperature at which a standard deflection occurs for defined test samples subjected to a given linear temperature increase and a compression loading from a defined indenter of a specified weight. The load used is often ION (Vicat A) or SON (Vicat B) and must be indicated with the results. In either case the polymer cannot be used under this compression load at this temperature. 

All these polymers are very flexible with low melting points, low Vicat softening temperatures, and very low moduli. 

The following polymer characteristics were obtained on the variously irradiated test specimens melt index (ASTM 1238-62T), Vicat softening temperature (modified ASTM 1525-58T) heat distortion (ASTM D 1220-63T) physical strength yield strength ultimate tensile strength percent elongation at break (ASTM D 412-64T) chemical resistance to boiling toluene. 

At elevated temperatures al polymers soften, dependent on their glass-rubber transition points, Tg, and/of their melting points, Tm. These temperatures limit the practical use of plastics. To characterize the softening behaviour, in practice various types of standard tests are being carried out, resulting in values for the softening temperature , defined in different ways. The values mostly used are the ISO Heat Deflection Temperature (HDT) and the Vicat Softening Temperature (VST or... 

Table 2 Vicat Softening Temperatures of Radiation-Sterilizable Medical Grade Molding Polymers...

ASTM D1525 provides a third type of the softening temperature test. A hat-ended needle of 1 mm circular cross-section is pressed into a thick sheet of the polymer with a load of 1.0 kg. The polymer is heated at a rate of either 50 or 120 K/h. The Vicat softening temperature is the temperature at which the needle has penetrated the polymer to a depth of 1 mm. For such a depth of penetration, the material must be very soft, hence the Vicat softening temperature is higher than other HDT data. 

The Vicat softening temperature (VST) is standardized in ISO 306 and ASTM D 1525. It is very useful as a quality control or development tool (ASTM D1525-2009). (This test method is technically equivalent to ISO 306 1987(E)). The result is a measure of the temperature at which thermoplastics begin to rapidly soften. VST describes the temperature at which a circular indenter with a cross section of 1 mm under a standardized loading of 10 N or 50 N penetrates exactly 1 mm into the specimen. VST was introduced to measurement technology as a substitute value for melting point. VST for some common polymers are presented in Table 10.20. 

Table 10.20 Vicat softening temperatures for some common polymers...

The glass transition can be measured by a variety of techniques. The most commonly accepted methods currently are differential scanning calorimetry (dsc) (3,13) and dynamic mechanical analysis (dma) (5,6). Other methods employed in the past have included the approximation methods of Vicat softening temperature and brittle point measurement. More accurate numbers were also determined by methods which rely on the changes manifested in the physical properties of the polymer, such as dilatometry (volume) and refractive index measurements. The Tg values for a large variety of pol3rmers are found in References 6, 7, 14, and 15. 

The dimensional stability of a plastic when exposed to heat is another crucial property. The Vicat method of determining a plastic s softening point and the heat deflection temperature (HDT) method are two ways of rapidly determining which polymers have suitable characteristic values. In both methods the test specimens are heated under defined load with a certain heating rate while deformation is measured. Vicat softening temperature and HDT are each defined as the temperatures at which deformation reaches a given value [8]. 

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. 

Polycarbonates based on tetramethylbisphenol A are thermally stable and have a high Vicat softening point of 196°C. On the other hand they have lower impact and notched impact resistance than the normal polymer. Blends with styrene-based polymers were introduced in 1980, and compared with PC/ABS blends, are claimed to have improved hydrolytic resistance, lower density and higher heat deflection temperatures. Suggested applications are as dishes for microwave ovens and car headlamp reflectors. 

From the first two examples it appears that simple usage of an arbitrary softening temperature, taken from a table, does not provide an answer as to the temperature resistance of a polymer the data for Vicat and ISO-HDT contradict each other ... 

As another example we consider two (amorphous) polymers with strongly different Tg, namely high for A and low for B. If we choose rpu = 0.4, then for each of the three types of dispersion the -modulus can be calculated as a function of temperature from the E(T) curves of the components. The result is shown in Figure 9.13, in which also the values of the ISO and Vicat softening points have been schematically indicated. It again appears that the type of dispersion has a strong effect on the properties (see also Qu. 4.24,9.13,9.15 and 9.23). 

A major disadvantage of PVC is its relatively low service temperature (60-70 °C) and the addition of high levels of some polymers can improve the heat distortion temperature and Vicat softening point. An example of such a polymer is alphamethyl styrene-acrylonitrile (AMSAN) copolymer. AMSAN-ABS polymer can be used as a combined heat and impact modifier. 

Two particular test methods have become very widely used. These are the Vicat softening point test and the test widely known as the heat distortion temperature test (also called the deflection temperature under load test). In the Vicat softening point test a sample of polymer is heated at a specified rate temperature increase and the temperature is noted at which a needle of specified dimensions indents into the polymer a specified distance under a specified load. 

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