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freely mix together throughout the casting process, and Ref- erence 20 acknowledges that the evolved binder gas com- ponents will interact with one another and may combine to form new compounds. This means that the gas species sepa- ration and thermal programming required for thorough GC analysis deviates from the conditions experienced during actual casting processes. Furthermore, binder gas condensa- tion during the measurements must be prevented. In addition to these experimental problems, the gas composition analy- sis procedure is quite complex and the associated equipment costs are high. Therefore, determination of the binder gas molecular weight as a function of temperature from addi- tional binder gas composition measurements is impractical.

An alternative method for determining the binder gas mix- ture molecular weight is to use the ideal gas law in conjunc- tion with measurements of the evolved binder gas mass, volume, and pressure as a function of temperature. In this respect, it is important that the bonded sand sample and the gas are at a uniform temperature and that the heating rates are consistent among the various measurements. Thermo- gravimetric analysis (TGA) of bonded sand facilitates deter- mination of the binder gas mass evolution as a function of temperature during controlled heating. Unfortunately, previ- ous experimental techniques15,22-25

for measuring the volume

and pressure of gas evolved from bonded sand are unsuitable for determination of the binder gas molecular weight as a function of temperature. Previous techniques do not ensure a sufficiently uniform temperature, do not fully prevent con- densation of the binder gas, do not allow measurements of the gas volume, pressure and temperature to be performed on the same volume of evolved gas, and provide no means for controlling the heating rates of the bonded sand and binder gas during the experiments. Therefore, it is necessary to develop new tech- niques for measuring the volume, pressure, and temperature of gas evolved from bonded sand in order to determine the binder gas molecular weight as a function of temperature.

In the present study, the mass and molecular weight of the binder gas evolved during de- composition of PUNB bonded sand is mea- sured. TGA is employed to measure the binder gas mass as a function of temperature at vari- ous heating rates. A specially designed quartz manometer, with liquid metal as the working fluid, is used to measure the binder gas volume, pressure, and temperature during heating and cooling. These gas evolution measurements are combined with the ideal gas law to determine the binder gas mixture molecular weight as a function of temperature. The bonded sand is initially immersed in an inert atmosphere in or- der to ensure that the binder is pyrolyzed, rather than combusted. Unlike in previous studies, the evolved binder gas is contained within the gas

International Journal of Metalcasting/Spring 2012

volume measurement apparatus and stays in contact with the bonded sand sample. This is intended to more closely simu- late the conditions inside of a mold during casting. If the evolved gases were continually swept away from the sam- ple, as in GC for example, the binder decomposition behav- ior could be different and the chemical reactions within the evolved gas could not be studied. Furthermore, the present gas evolution device prevents undesired binder gas conden- sation, allows for the measurement of the molecular weight evolution of the evolved gases even after the binder is fully pyrolyzed, and eliminates issues related to non-localized measurement of the gas volume, pressure, and temperature. The present experimental techniques allow for continuous measurement of the binder gas molecular weight variations with temperature.

experimental Methods

The following sub-sections describe the preparation of bonded sand specimens, the TGA experimental procedures, the design of the gas measurement apparatus, and the gas measurement procedures. Additional experimental details may be found elsewhere.27

PunB Bonded sand Preparation

The PUNB bonded sand specimen composition and prepara- tion procedure follow those employed by Thole and Becker- mann.26

Specimens of bonded sand were prepared from IC55

silica lake sand, black iron oxide (BIO), and a PUNB binder system. The values for the binder content (1.25% of total mass), binder ratio (60:40 ratio of Part 1 to Part 2), catalyst

Table 2. Primary Chemical Components Identified by McKinley et al.20

and Lytle21 Flash Pyrolysis of 1.5% PUCB Bonded Sand through GC-MS Analysis during

*Note: Additional Reference to Data in: Starobin, A., Goettsch, D., Walker, M., Burch, D., “Gas Pressure in Aluminum Block Water Jacket Cores, Int. J. Metalcasting, vol. 5, no. 3, pp. 57-64 (Summer 2011)


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