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The results in Table 1 show that the binder gas generated during pouring of the gray iron and steel castings was most- ly comprised of carbon monoxide and carbon dioxide, had varying amounts of nitrogen and hydrocarbons, and low amounts of hydrogen and oxygen. The binder gas from the aluminum castings, poured at significantly lower tempera- tures than the gray iron and steel castings, was comprised almost entirely of oxygen and nitrogen. This indicates that the evolution of binder gas within the first two minutes after pouring the aluminum castings was insufficient to replace the air atmosphere originally in the molds. The hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons evolve primarily at higher temperatures. It is also possible that the binder gas experienced some condensation before it escaped the mold and was collected, making the measured composi- tion not entirely reflective of the actual binder gas composi- tion. In addition, the binder gas samples had the opportunity to cool inside the collection tubes prior to injection of the samples into the gas chromatograph, which can also cause some binder gas to condense before analysis. Regardless of these issues, the molecular weight data calculated from the data (in References 13-16) are useful for comparison against the present molecular weight measurements.


More recently, McKinley et al.20 and analysis available in the work of Lytle21


(with detailed information ) performed


flash pyrolysis of 1.5% PUCB bonded sand and used gas chromatography-mass spectroscopy (GC-MS) to analyze the evolved binder gas. Small samples of PUCB bonded sand were rapidly heated to 500C (932F), 700C (1292F), and 900C (1652F) under a helium atmosphere and held at these temperatures for various periods of time. The pyroly- sis products were swept directly into the gas chromatograph, and gas species ranging from 10 g/mol to 425 g/mol were


detected by the mass spectrometer. The mass fractions of the primary components emitted from the PUCB bonded sand during twenty seconds of pyrolysis at 700C (1292F) and 900C (1652F) and the mixture molecular weights calculated from the composition data are shown in Table 2.


The components in Table 2 comprise more than 99% of the measured gas species at each temperature, with the remain- ing gases being high molecular weight compounds. None of the components listed in Table 2 were detected during pyrolysis at 500C (932F), however the molecular weight of the gas mixture evolved at 500C (932F) was calculated to be 137 g/mol. It can be seen from Table 2 that, besides carbon monoxide, the major species evolved are hydrocarbons. In addition, the binder gas composition experiences significant changes between 700C (1292F) and 900C (1652F). Refer- ence 21 did not detect any oxygen or nitrogen in the binder gas. This further supports the conclusion that the nitrogen and oxygen measured in References 13-16 during all cast- ing experiments were from the residual air in the mold. The significant hydrocarbon content at 700C (1292F) reported by Reference 21 also contradicts the results of References 13-16 for aluminum poured at 750C (1382F). Regardless of such measurement discrepancies, the detailed composi- tion data from References 20 and 21 currently provide the only means to directly obtain reliable binder gas molecular weight data at a few selected temperatures.


Even though coupling GC with any one of the many types of gas detectors provides the means to analytically determine the binder gas composition and molecular weight, this tech- nique is not well suited to generate the magnitude of temper- ature resolved molecular weight data required in binder gas models. The gas components evolved from bonded sand will


Table 1. Time-averaged Mass Fraction and Mixture Molecular Weight of Gas Components Evolved from PUCB and PUNB Bonded Sand within Two Minutes after Pouring during the Experiments of Bates et al.13-15


and Scott et al.16


24


International Journal of Metalcasting/Spring 2012


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