Nylon normal stress difference

Figures 7.12-7.14 show the effect of glass fibers at different loading levels on the normal stress difference behavior of nylon, polycarbonate (PC), and polyst)n-ene (PS). Since the plots have been made as Nj vs. they are independent of temperature. This is specifically clear from Figure 7.12 which shows a uniqueness of data at the three temperatures of 275°C, 285°C and 295°C [95].

Figure 5.16 Coalesced normal stress difference curve for nylon at 2.16-kg test load condition for MFl using data from Refr. 12 and 25. (Reprinted from Ref. 21 with kind permission from Steinkopff Verlag Darmstadt.)...

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. 

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