Vicat

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. 

ASTM D1525, Vicat S oftening Temperature of Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1991. 

ISO 306, Plastics Thermoplastic Materials-Determination of Vicat Softening Temperature, ISO, Geneva, Swit2eiiand, 1987. 

The alkyl group also produces subde changes in the processing of the PVC, the use level and cost of the stabilizer, and in some cases even the final properties of the article, especially the heat distortion temperature or Vicat softening point. Overall, methyl derivatives are most widely used. Butyls are second and octyls a distant third. 

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. 

Figure II.5. Variation of Vicat softening point (5 kg load) with isotactic index and melt flow index.

Vicat softening point (°C) 80 70 flexible at room temperature... 

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. 

As is the case in the polyolefins, the polymethacrylates with branched side chains have higher softening points and are harder than their unbranched isomers. The effect of branching of Vicat Softening point is shown in Table 75.5.> ... 

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. 

Higher softening point (Vicat temperatures of 110-117°C compared to 89-107°C). 

A glass transition temperature of 215°C, a deflection temperature of 200°C and a Vicat softening point of 219°C. 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

The TMC polycarbonate homopolymer has a glass transition temperature of 238°C, nearly 100°C above that of the bis-phenol A polycarbonate. Therefore, copolymers will have intermediate glass transitions depending on the relative proportions of TMC and bis-phenol A. Commercial grades (marketed by Bayer as Apec HT) have Vicat softening points from 158 to... 

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. 

---