Vicat polymers

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

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. 

Over the years many attempts have been made to produce commercial acrylic polymers with a higher softening point than PMMA. The usual approach was to copolymerise MMA with a second monomer such as maleic anhydride or an N-substituted maleimide which gave homopolymers with a higher Tg than PMMA. In this way copolymers with Vicat softening points as high as 135°C could be obtained. 

As might be expected from a consideration of the factors discussed in Section 4.2, the imidisation process will stiffen the polymer chain and hence enhance Tg and thus softening points. Hence Vicat softening points (by Procedure B) may be as high as 175°C. The modulus of elasticity is also about 50% greater than that of PMMa at 4300 MPa, whilst with carbon fibre reinforcement this rises to 25 000 MPa. The polymer is clear (90% transparent) and colourless. 

Table 15.5 Vicat softening of methacrylate polymers from monomers of type CHj = C (CHj) COOR CO...

The nuclear substituted methyl styrenes have been the subject of much study and of these poly(vinyl toluene) (i.e. polymers of m- and /7-methylstyrenes) has found use in surface coatings. The Vicat softening point of some nuclear substituted methyl styrenes in given in Table 16.8. 

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. 

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. 

Polymer Melting point (°C) Glass transition (°C) Vicat A (°C) Young s modulus (GPa) Tensile strength (MPa) Strain (%) Impact strength (Izod) (J/m) Density (g/cm)... 

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. 

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... 

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). 

TABLE 26.2 Softening points in °C of amorphous and semi-crystalline polymers AT = Vicat-B- ISO-A... 

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. 

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. 

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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