The above variations correlate well with the chemical reac- tions the binder undergoes during heating. Giese et al.11
mea-
sured the energy released during heating (at a 10C/min (18F/ min) rate) of a 60:40 PUNB sample using differential scanning calorimetry (DSC). The various peaks in the DSC curve were associated with specific changes in the binder composition. Figure 10 shows the findings of Giese et al. superimposed on the fit of the elastic modulus data from Figure 9. It can be seen that the start of solvent vaporization corresponds to the small knee in the elastic modulus curve at about 65C (149F). The end of solvent vaporization coincides with the elastic modulus reaching the 600 MPa value at about 125C (257F). The start of urethane bond breakage at 180C (356F) can be associated with a small kink in the elastic modulus curve. The peak in the elastic modulus around 280C (536F) coincides with the maxi- mum decomposition rate and the end of urethane bond break- age. The linear increase in the elastic modulus above 350C (662F) can be associated with the breakdown of the binder to
polymer aromatics. Effect of Solvent
The results shown in Figure 10 indicate that the solvent in the binder may have an effect on the elastic modulus. There- fore, additional tests were performed where the solvent was removed from the specimens prior to testing by heating the specimens to 140C (284F) and allowing them to cool down to room temperature again. The results of two such tests (at a 8C/min (14.4F/min) heating rate) are shown in Figure 11 and compared to the fit of the elastic modulus data from Figure 11 (without solvent removal). Despite the scatter in the data, it can be seen that the solvent has an effect on the elastic modulus at temperatures below about 150C (302F). While
the room temperature elastic modulus is approximately the same with and without solvent, the elastic modulus between 50C (122F) and 150C (302F) is generally higher with the solvent removed. The differences can be as large as a fac- tor of two. However, at temperatures above 150C (302F), Figure 11 shows that the elastic modulus of the specimens with the solvent removed follows the same variation with temperature as the fit of the elastic modulus data from the six original specimens where the solvent was not removed.
Figure 10. Comparison of the measured elastic modulus variation with the DSC data from Giese et al.11 ratio PUNB sample heated at 10°C/min.
for a 60:40
Figure 9. Measured elastic modulus as a function of temperature for an average heating rate of 8°C/min.
14
Figure 11. Comparison of the elastic modulus variation with temperature for specimens with and without the solvent removed.
International Journal of Metalcasting/Fall 10
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