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gas does not condense when heated to temperatures above about 710C (1310F). However, heating the PUNB bonded sand samples to temperatures not exceeding about 510C (950F) results in partial condensation of the binder gas dur- ing subsequent cooling. This condensation starts to occur at approximately 165C (329F) in all tests.


The present binder gas molecular weight measurements were fit to a set of piecewise polynomials. They were also extrapolated to higher temperatures based on the average molecular weights calculated using the binder gas com- position data from References 13-16, and a final polyno- mial was fit to this extrapolation. The molecular weight polynomial curves and the binder gas molecular weights calculated from the data of References 13-16, 20, and 21 are plotted as a function of temperature in Figure 9. The equations for the binder gas molecular weight polynomials are listed in Table 5. Figure 9 shows that the decreasing be- havior of the present molecular weight measurements be- tween 710C (1310F) and 898C (1648F) is continued by the high temperature molecular weight data calculated from References 13-16. Following the extrapolated curve, it is expected that the binder gas molecular weight continues to decrease with increasing temperature from 898C (1648F) to 1350C (2462F). The molecular weight can be expected to remain approximately constant at 17.4 g/mol for tem- peratures above 1350C (2462F). In addition, the present measurements are consistent with the molecular weight data calculated from References 20 and 21, aside from the high molecular weight of 137 g/mol at 500C (932F) that appears to be an outlier. Any differences with the data in References 20 and 21 could also be due to the fact that those data are for PUCB rather than PUNB.


The measured variation in the binder gas molecular weight with temperature reflects the binder’s thermal degradation mechanisms during heating. Giese et al.34


used differential scanning calo-


rimetry (DSC) to measure the energy released from pure PUNB binder sam- ples (60:40 ratio of Part 1 to Part 2) during heating at a rate of 10°C/min (18°F/min). The various peaks in the DSC curve were associated with spe- cific physical or chemical changes in the binder. Figure 9 shows a portion of the solid binder’s thermal degrada- tion mechanisms superimposed on the fit of the binder gas molecular weight measurements. The high molecular weights measured at temperatures be- low 200C (392F) are likely from va- porized solvents. The breaking of the binder’s urethane bonds corresponds to the plateau in the gas molecular weight between 200C (392F) and 280C (536F). The breakdown of the binder to poly-


36


mer aromatics coincides with the decrease in the binder gas molecular weight between 280C (536F) and 400C (752F). The binder’s thermal degradation mechanisms above 400C (752F) are undetermined.


Figure 9. Piecewise polynomial fit of the binder gas molecular weight measurements as a function of temperature during heating at a rate of 2°C/min. The data fit is extrapolated based on the average molecular weights calculated from the data of Bates et al13-15 Scott et al16


results are also compared with the binder gas molecular weights calculated from the data of McKinley et al20 Lytle.21


60:40 ratio pure PUNB binder sample heated at a rate of 10°C/min (18°F/min) from Giese et al.34 on the data fit.


Table 5. Piecewise Polynomial Fitted to the


Binder Gas Molecular Weight Measurements Obtained During Heating of PUNB Bonded Sand Samples at a Rate of 2°C/min (3.6°F/min).


The partial thermal degradation mechanism for a is superimposed


and


within two minutes after pouring. The fitted and


International Journal of Metalcasting/Spring 2012


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