This page contains a Flash digital edition of a book.
pressures of the argon and binder gases and manipulation of Equation 9 yields the binder gas molecular weight, Mb

, as Eqn. 12

The interpolation of the binder gas mass evolution, volume, and pressure measurements at intervals of 0.1°C (0.18°F) allowed the data sets to be matched together when the ex- periments employed the same heating rate. This allowed Equation 11 to be directly inserted into Equation 12, and the binder gas molecular weight as a function of temperature was straightforwardly calculated.

results and discussion thermogravimetric analysis

Preliminary TGA experiments were performed to compare the decomposition behavior of PUNB bonded sand (55 mg) and pure IC55 silica sand (55 mg) for a heating rate of 100°C/ min (180°F/min) with 25 cm3

/min (1.526 in3 /min) of argon

gas flow. TGA of the pure sand was of interest to determine whether sand decomposition significantly contributes to gas evolution. Figure 2 shows the measured mass percentage re- maining in the bonded sand and pure sand samples as a func- tion of temperature. It can be seen that, as expected, the PUNB bonded sand loses far more mass than the pure sand during heating. The relatively minor mass loss in the pure sand is likely due to trace impurities being volatized. In addition, Fig- ure 2 shows that the bonded sand and pure sand curves show a small mass increase between about 20C (68F) and 115C (239F). This unexpected increase in the sample mass at such low temperatures necessitated additional investigation.

TGA was performed on 61 mg of broken quartz rods for a heating rate of 100°C/min (180°F/min) with 25 cm3 (1.526 in3

/min /min) of argon gas flow. The quartz rods were not

expected to experience any mass change during heating, and the quartz rod sample mass was made to be similar to the mass of the bonded sand and pure sand samples. The measured mass percentage characteristic of the quartz rod samples dur- ing heating is superimposed on the bonded sand and pure sand curves in Figure 2. The increase in the measured quartz rod mass from 20C (68F) to 115C (239F) directly coincides with the increase in the measured bonded sand and pure sand sam- ple masses. The quartz rod mass readings continued to show some variation up to a temperature of 1000C (1832F), and additional quartz rod tests at 100°C/min (180°F/min) showed that the variations were reproducible. Abnormal temperature dependent variations in quartz rod mass readings were also obtained at heating rates of 2°C/min (3.6°F/min) and 10°C/ min (18°F/min). Subsequently, the quartz rod TGA measure- ments were used to account for the temperature dependent bias in the TGA machine’s mass measurements and to correct the TGA mass measurements for PUNB bonded sand. Ad-


ditional information regarding the TGA correction with the quartz rod measurements is presented elsewhere.27

Other preliminary TGA experiments revealed that increas- ing the PUNB bonded sand sample mass from 55 mg to 80 mg or 100 mg does not affect the results. Lowering the bonded sand sample mass below 55 mg was not desirable. This is because samples below 55 mg were more likely to have non-homogeneous distribution of the binder within the sand. Additional results for pure PUNB binder (without sand) decomposition and the effects of purge gas flow rate on the TGA measurements are documented elsewhere.27

The measured fractions of original binder mass remaining in 55 mg PUNB bonded sand samples heated at rates of 2°C/min (3.6°F/min), 10°C/min (18°F/min), and 100°C/min (180°F/ min) with 25 cm3

/min (1.526 in3

/min) of argon gas flow are

plotted as a function of temperature in Figure 3. Because of the TGA mass measurement correction, the fractions re- main very close to unity until binder decomposition begins at approximately 50C (122F). With increasing temperature, the fraction of original binder mass remaining decreases in a complex manner. The curves in Figure 3 are almost horizontal when the decomposition reaction is finished, indicating nearly complete removal of the bias in the TGA machine’s mass measurements. The decomposition ceases at a temperature of about 710C (1310F) for a heating rate of 2°C/min (3.6°F/min) and at somewhat higher temperatures, approaching 800C (1472F), for heating rates of 10°C/min (18°F/min) and 100°C/ min (180°F/min). It should be noted that applying the correc- tion changes the fraction of original binder mass remaining by as much as 10% at the maximum decomposition temperature.

Figure 2. Measured percentage of mass remaining as a function of temperature typical of PUNB bonded sand samples, pure IC55 silica sand samples, and quartz rods heated at a rate of 100°C/min (180°F/min) with a total argon gas flow of 25 cm3

/min. International Journal of Metalcasting/Spring 2012

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91