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Determination of novolac resin thermal decomposition products by pyrolysis-gas chromatography-mass spectrometry
Journal of Analytical and Applied Pyrolysis 45 (1998) 121 – 131
Determination of novolac resin thermal decomposition products by pyrolysis-gas chromatography-mass spectrometry C.A. Lytle a, W. Bertsch a,*, M. McKinley b a b
Uni6ersity of Alabama, Department of Chemistry, Box 870336 Tuscaloosa, Alabama 35487, USA Uni6ersity of Alabama, Department of Engineering, Box 870203 Tuscaloosa, Alabama 35487, USA Received 19 December 1997; accepted 20 January 1998
Abstract Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was used to identify the major volatile components produced by pyrolysis of a novolac resin. This resin is frequently used in the metal casting industry as a binder for sand molds. Quantitative analysis data of the pyrolysis products can serve as a model for the foundry industry to predict the amount of volatile organic compounds (VOCs) produced before actually carrying out the casting process. This allows the industry to comply with the Clean Air Act Amendments (CAAA) regarding the emissions of hazardous air pollutants (HAPs). © 1998 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis; Gas chromatography-mass spectometry; Novolac resin; Thermal decomposition; Metal casting
1. Introduction Pyrolysis-gas chromatography (Py-GC) is well established as a method for the analysis of polymers [1]. Pyrolysis has also been used for characterization and for structural analysis of polymers [2 – 4]. More specifically, Py-GC-MS has been used to analyze cured polyfunctional epoxy resins, determine the sequence of phenol units and study mechanistic and kinetic aspects of the thermal degradation of phenol-formaldehyde polycondensates [5–7]. This study focuses on the analysis of a novolac resin binder commonly used in the metal casting industry. * Corresponding author. 0165-2370/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0165-2370(98)00062-X
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Novolacs are thermoplastic, brittle, solid phenolic resins that do not cross-link without the help of a cross-linking agent. The polymers are formed by an acid-catalyzed reaction of excess phenol with formaldehyde. The formaldehyde produces the — CH2OH groups on the phenol rings and these react further with active hydrogen atoms in other phenols under elimination of water and formation of methylene bridges. Fig. 1A shows a general structure of the polymer which has not yet undergone cross polymerization. To connect the phenol units in different novolac chains by new methylene bridges, the resin is heated with additional formaldehyde. This formaldehyde is produced along with ammonia, by decomposition of hexamethylenetetramine (HMTA) which acts as the crosslinking agent [8]. Depending on the temperature at which the crosslinking reaction occurs, the crosslinked polymer can have one of two structures. Fig. 1B shows the general structure of the crosslinked polymer when the polymer is cured at around 130°C. When the cure temperature is raised, the benzylamine group undergoes dehyrogenation, resulting in the structure shown in Fig. 1C [9]. Resin of the latter composition is commonly used by the foundry industry for casting. Casting is a process in which a metal object is cast to the required shape by pouring or injecting liquid metal into a mold. One important characteristic of metal
Fig. 1. General structure of (a) novolac resin. (b) Cross-linked novolac resin cured at 130°C. (c) Cross-linked novolac resin cured at higher temperatures.
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casting is the ability to use a sand core inside the mold to produce hollow parts. The moldmaking and coremaking process involve the coating of sand with a binder, compacting the coated sand into the desired shape and then curing or hardening the compacted mass so that it can be handled [10,11]. The resin binder near the metal interface will decompose during casting due to the large amount of heat transferred from the molten steel. Temperatures are usually in the range of 1400–1600°C, depending on the casting material. The binder decomposes into a variety of chemical compounds, the nature of which depend primarily on the binder composition and the temperature to which the mold is exposed. Some decomposition products condense in the sand mold, but others are emitted into the air during casting, cooling (solidification), and removal of the cast from the sand mold (shakeout). It has been found that small amounts of toxic materials are given off from the resin during resin decomposition [12]. Many of these substances must be eliminated or reduced to a level specified by the Clean Air Act Amendments (CAAA) of 1990. Title III of the CAAA requires more stringent control of the emissions of the 189 HAPs listed in the act [13]. The US Environmental Protection Agency (EPA) schedule calls for iron and steel foundry air emission standards to be in place by the year 2000. At the present time there are no reliable data on the nature and quantity of HAPs generated by specific resins. Therefore, foundries cannot accurately assess what kind and what quantity of HAPs are being emitted, nor can they determine their adherence to the requirements under the CAAA. The objective of this study was to model in the laboratory conditions for the production of volatiles which are typical of metal pouring and casting. Data produced from thermogravimetric analysis (TGA) and Py-GC-MS were compared to assess the volatile organic compounds (VOCs) released during decomposition of the novolac resin. Initial experiments were conducted to determine how each individual component contributed to the total VOCs. Uncoated sand, resin, resin containing 15% HMTA and resin coated sand were all subjected to TGA and PY-GC-MS. The data obtained from TGA produced information on the quantity of volatiles and the temperatures at which these volatiles evolved. These data were used to determine the basic operating parameters for the pyrolysis experiments. Studies were performed to determine what effect, if any, pyrolysis temperature, temperature rise time, and the duration of pyrolysis had on the nature and quantity of VOCs. The identity and quantity of major pyrolysis volatiles are presented.
2. Experimental
2.1. Chemicals The novolac resin (Plasti Flake 1105) was manufactured by Borden, (Westchester, Ill). Coated sand was produced following procedures used by a local foundry (Southern Precision, Birmingham, AL). The resin coated sand was prepared by dissolving 1 gm of HMTA in 7 ml of water. This solution was then mixed
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with the resin and added to a type of sand called Wedron washed sand obtained from a local foundry. The final product contained 3.00% resin, 0.45% HMTA, and 96.55% sand. The mixture was placed in an oven set at 100°C and left until all the water had evaporated. The temperature was then raised to approximately 130°C to cure the coated sand. The curing process was finished when the product turned to a light yellow, corn-flake like color. Although the cured coated sand was hard, it was ground into fine particles using a mortar and pestle. n-Pentadecane, ACS grade (Polyscience, Niles, IL) was diluted with n-hexane, to produce a 1000 ppm stock solution which was further diluted, as needed. A gas standard which contained methane, ethane, ethene, acetylene, carbon monoxide and carbon dioxide, each at 1% in nitrogen was purchased from Supelco (Bellefonte, PA). A 0.1% propane standard was made in house using grade HD-5 propane from a propane torch purchased locally. Cis-2-butene was purchased from Matheson (Atlanta, GA), and a 0.1% standard, in nitrogen was prepared. A retention time standard of HCN was produced in house by dissolving a small amount of NaCN in a 150 ml Erlenmeyer flask and adding dilute HCl. Gaseous HCN was withdrawn from the headspace with a gas tight syringe.
2.2. Thermogra6imetric Analysis (TGA) Weight loss analysis was performed on the uncoated sand, resin containing 15% HMTA and also on the resin coated sand using a TA 2950 thermogravimetric analyzer (TA Instruments, New Castle, DE). The resin coated sand contained approximately 4% resin (wt./wt.). The standard temperature program was from room temperature to 1000°C at a rate of 150°C min − 1. Nitrogen was used as the purge gas at a rate of 40 ml min − 1 to the balance, and 60 ml min − 1 to the furnace, as recommended by the instrument manufacturer.
2.3. Pyrolysis Pyrolysis was performed using a standalone pyrolyzer, CDS Pyroprobe 2000 (Chemical Data Systems, Oxford, PA) equipped with a coil probe. The pyrolyzer used a CDS Pyroprobe 100 interface to the GC/MS. Probe temperatures of 200, 400, 500, 750, and 980oC were employed, each held for 1, 5, 10, and 20 s. The probe was cleaned at 1000°C for 10 s between samples. Blanks were run periodically, to ensure a clean background. Weighed samples were loaded into quartz tubes and packed with a small quantity of silanized glass wool at both ends.
2.4. GC-MS Analysis The separation of the volatiles from the pyrolysate was carried out on a quadrupole type GC/MS instrument (HP GCD, Avondale, PA), using a 30 m×0.25 mm×0.25 mm fused silica capillary column coated with 5% diphenyl/95% dimethylpolysiloxane (HP-5, Hewlett Packard, Avondale, PA). A temperature program of 40 – 280°C at a rate of 10°C min − 1 was found to give the best
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separation within a reasonable amount of time. A 30 m ×0.32 mm fused silica capillary PLOT type column coated with a porous divinylbenzene homopolymer (GS-Q, J&W Scientific, Folsom, CA), and a 30 m× 0.32 mm× 10 mm fused silica capillary molsieve absorbent type column (CP-Molsieve 5A, Chrompack, Raritan, NJ) was used for the separation of low molecular weight components. Standard EI conditions were used.
3. Results and Discussion Initial thermogravimetric analysis was performed on the uncoated sand and the solid resin containing 15% HMTA. The resin coated sand was then investigated to determine how the resin behaved on the sand matrix. Fig. 2 shows the first derivative curve of weight loss with respect to temperature of resin containing HMTA (---) and resin coated sand ( – –). Data produced by repetitive sampling showed a high degree of variation, indicating that the uncoated sand was rather inhomogeneous. The volatiles ranged from 0.3–1.5% (wt./wt.), and were determined to mainly consist of benzene, toluene and cresols. The source of these compounds is suspected to be the washing process the sand endures before being used. The majority of the volatiles from the resin containing 15% HMTA evolved between 150 – 195°C and 450 – 650°C. However, from the thermogram of the resin coated sand, the volatiles seemed to behave differently when novolac resin and coated sand were compared to each other. The thermal profiles of the novolac resin containing HMTA and the novolac resin coated sand behave very similar up to around 700°C. With the novolac resin containing 15% HMTA, nearly 95% of the VOCs have evolved at 700°C, however, only approximately 50% of the VOCs have been emitted with the novolac resin coated sand at the same temperature. The discrepancy can be explained by the high heat capacity of the sand, which is capable of absorbing a large amount of heat before transferring it to the resin. This creates a ‘lag’ in the volatilization of the resin. A comparison of the fraction of the sample volatilized by TGA and pyrolysis was performed to determine the compatibility of the techniques in terms of volatilization. Table 1 presents quantitative data for both TGA and pyrolysis. For these experiments the average RSD was in the range of 5%, indicating that the two techniques were compatible in terms of volatilization. The difference was minimal at temperatures of 750 and 980°C. At 500°C, the thermal energy was not sufficient to volatilize the entire resin. The vapor from the resin condensed on the quartz sample tubes thus generating an artifact. The sample tubes were cut to smaller lengths, but some of the resin sample would still condense on the tube wall and also bubble out onto the probe coils. It was thus not possible to accurately determine the fraction volatilized by pyrolysis at 500°C. The volatiles that were released around 200°C Fig. 3 were low molecular weight gases. They were analyzed on a PLOT column. This column was used to separate ethene, propene, butene, 1,3-butadiene, 2-propenenitrile, 1-pentene, and 1,3-cyclopentadiene. The separated gases
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Fig. 2. First derivative curve of weight loss with respect to temperature of resin containing 15% HMTA (------) and resin coated sand ( – – – – –).
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Fig. 3. Pyrogram at probe temperatures of (top) 500°C, (middle) 750°C, and (bottom) 980°C for 5 s.
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Table 1 Percent of novolac resin volatilized by TGA and Py-GC TGA o
Py-GC
C
%
o
C
%
500 750 980
49 64 68
500 750 980
— 59 71
Average RSD = 5%, N =3.
were identified by mass spectrometry and retention time matching. However, the column was unable to separate the most volatile components, including carbon monoxide from air, an important separation. In this case the molsieve column Table 2 Major volatile components emitted from the novolac resin at 980°Ca Components
Concentration (mg g−1)
Methane Carbon monoxide Ethene Propene HCN 1-butene 1,3-butadiene 2-propenenitrile 1-pentene 1,3-cyclopentadiene 1,3-pentadiene Benzene Toluene Phenol Benzofuran Indene 2-methyl phenol 4-methyl phenol 2,5-dimethyl phenol 2,4-dimethyl phenol Naphthalene 2-methylnaphthalene 1-methylnaphthalene Biphenyl 2-methyl,1-1’-biphenyl Acenaphthylene Dibenzofuran Fluorene Phenanthrene Anthracene
47 000 73 000 10 400 2600 — 500 700 100 100 300 63.8 23.1 6.5 13.8 1.0 1.6 6.0 4.6 0.9 1.8 8.8 1.2 1.2 2.1 3.3 2.1 0.7 0.9 2.9 1.1
a
Only components \0.8% of the total volatiles are listed in this table. Average RSD =15%, N =3.
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Fig. 4. Quantity of selected products as a function of probe temperature.
produced satisfactory results. It was also useful for determining carbon monoxide and methane. The origin of the water in the novolac resin was of interest. Was the water derived from moisture in the atmosphere or produced during pyrolysis? A sample of the coated sand weighing about 10.0 mg was baked in an oven at 100°C for 30 min., then pyrolyzed. The results indicated that the water is a product of pyrolysis and originated from an elimination reaction taking place at high temperature. Due to the strong tailing of the peak, the quantity of water was not determined. Mass spectrometry, standards and retention indices were used to determine the identity and quantity of the VOCs produced by the novolac resin. Table 2 list the components that are greater than 0.8% of the total volatiles emitted at the highest temperature. These conditions most closely resemble the pouring environment. The average RSD for this set of experiments was around 15%. Due to the many variables involved in sample manipulation and pyrolysis, large variations are expected. The amount of VOCs produced was also examined as a function of probe temperature. Fig. 3 shows pyrograms at 500°C (top), 750°C (middle) and 980°C (bottom). The production of phenol peaks at 750°C and then gradually decreases. Emissions of benzene, toluene, and naphthalene are not observed until around 750°C, increasing in amount as the temperature was raised. The total peak areas for
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Table 3 Novolac resin total peak area as a function of probe temperature and pyrolysis time Pyrolysis time (s)
1 5 10 20
Pyrolysis probe temperature 500
750
980
109 261 135 411 146 991
114 313 909 741 1 194 325 1 304 894
274 117 1 510 737 2 056 604 2 424 654
Average RSD =5%, N =3.
these pyrograms are displayed in Fig. 4. The quantity of both carbon monoxide and HCN (not shown) also increase as the probe temperature is increased. The reasons for the relatively high temperature required for the emission for these low molecular weight components are not fully understood. The effect of the length of sample exposure to temperature was also studied. Samples of 10 mg were pyrolyzed at 500, 750 and 980°C for 1, 5, 10 and 20 s. The results of these experiments are summarized in Table 3. As one may expect, the amount of VOCs increases as the probe temperature increases and also as the length of exposure to temperature increases. Experiments were also performed to study the effect of temperature rise time on the distribution of VOCs. A temperature rise time of 300°C ms − 1 was changed to 300°C s − 1. The results indicate that temperature rise time had no significant effect on the nature of VOCs produced under these conditions. One should note that the actual rise in temperature of the sample in the quartz tube is less than that produced by the heating coil. This lag in temperature is expected to be particularly significant as the rate increases.
4. Conclusion Conditions typical of pouring and casting were modeled in the laboratory. Py-GC-MS has shown to be a valuable technique for determining the nature and quality of volatiles produced from thermal decomposition of a commercial resin binder system. The quality of volatiles produced below 500°C is rather insignificant compared to the amounts released at 750 and 980°C. Fixed gases are the major pyrolysis products at all pyrolysis temperatures studied. Except for these fixed gases, phenol is the major pyrolysis product. At 750°C benzene, toluene and naphthalene begin to evolve. At 980°C phenol is no longer the major component. Benzene becomes the major pyrolysis product and the amount of naphthalene increases by a factor of roughly 20. The amount of VOCs determined in the model studies appears to be high. It is important to note that foundry workers are not exposed to these high concentrations, because the molds are continually flushed with fresh air. A small amount of HCN is produced. It was not quantitated due to the difficulties of producing reliable standards.
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References [1] S.A. Liebman, E.J. Levy, Pyrolysis and GC in Polymer Analysis, Marcel Dekker, New York, 1985. [2] L. Prokai, J. Anal. Appl. Pyrolysis 12 (1987) 265 – 273. [3] M.H.P.M. van Lieshout, H.G. Janssen, C.A. Cramers, M.J.J. Hetem, H.J.P.J. Schalk, High Resol. Chromatogr. 19 (1996) 193–199. [4] F.C. Wang, J. Chromatogr. 765 (1997) 279 – 285. [5] P. Branda, J. Zima, J. Anal. Appl. Pyrolysis 21 (1991) 207 – 220. [6] M. Blazso, T. Toth, J. Anal. Appl. Pyrolysis 19 (1991) 251 – 263. [7] F.C. Wang, P.B. Smith, Anal. Chem. 67 (1996) 3033 – 3037. [8] A. Knop, L.A. Pilato, Phenolic Resins, Springer-Verlag, New York, 1985. [9] R.W. Martin, The Chemistry of Phenolic Resins, Wiley, New York, 1956. [10] J.S. Cambell, Casting and Forming Process in Manufacturing, Mc Graw – Hill, New York, 1950. [11] W.C. Stimpson, B.L. Gray, Foundry Work, American Technical Society, Chicago, 1947. [12] M McKinley, A. Jefcoat, W.J. Herz, C. Frederick, AFS Transactions 979 (1993) 93 – 105. [13] J.W. Bradstreet, Hazardous Air Pollutants-Assessment, Liabilities, and Regulatory Compliance, Noyes Publication, New Jersey, 1995.
.
Determination of novolac resin thermal decomposition products by pyrolysis-gas chromatography-mass spectrometry C.A. Lytle a, W. Bertsch a,*, M. McKinley b a b
Uni6ersity of Alabama, Department of Chemistry, Box 870336 Tuscaloosa, Alabama 35487, USA Uni6ersity of Alabama, Department of Engineering, Box 870203 Tuscaloosa, Alabama 35487, USA Received 19 December 1997; accepted 20 January 1998
Abstract Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was used to identify the major volatile components produced by pyrolysis of a novolac resin. This resin is frequently used in the metal casting industry as a binder for sand molds. Quantitative analysis data of the pyrolysis products can serve as a model for the foundry industry to predict the amount of volatile organic compounds (VOCs) produced before actually carrying out the casting process. This allows the industry to comply with the Clean Air Act Amendments (CAAA) regarding the emissions of hazardous air pollutants (HAPs). © 1998 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis; Gas chromatography-mass spectometry; Novolac resin; Thermal decomposition; Metal casting
1. Introduction Pyrolysis-gas chromatography (Py-GC) is well established as a method for the analysis of polymers [1]. Pyrolysis has also been used for characterization and for structural analysis of polymers [2 – 4]. More specifically, Py-GC-MS has been used to analyze cured polyfunctional epoxy resins, determine the sequence of phenol units and study mechanistic and kinetic aspects of the thermal degradation of phenol-formaldehyde polycondensates [5–7]. This study focuses on the analysis of a novolac resin binder commonly used in the metal casting industry. * Corresponding author. 0165-2370/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0165-2370(98)00062-X
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Novolacs are thermoplastic, brittle, solid phenolic resins that do not cross-link without the help of a cross-linking agent. The polymers are formed by an acid-catalyzed reaction of excess phenol with formaldehyde. The formaldehyde produces the — CH2OH groups on the phenol rings and these react further with active hydrogen atoms in other phenols under elimination of water and formation of methylene bridges. Fig. 1A shows a general structure of the polymer which has not yet undergone cross polymerization. To connect the phenol units in different novolac chains by new methylene bridges, the resin is heated with additional formaldehyde. This formaldehyde is produced along with ammonia, by decomposition of hexamethylenetetramine (HMTA) which acts as the crosslinking agent [8]. Depending on the temperature at which the crosslinking reaction occurs, the crosslinked polymer can have one of two structures. Fig. 1B shows the general structure of the crosslinked polymer when the polymer is cured at around 130°C. When the cure temperature is raised, the benzylamine group undergoes dehyrogenation, resulting in the structure shown in Fig. 1C [9]. Resin of the latter composition is commonly used by the foundry industry for casting. Casting is a process in which a metal object is cast to the required shape by pouring or injecting liquid metal into a mold. One important characteristic of metal
Fig. 1. General structure of (a) novolac resin. (b) Cross-linked novolac resin cured at 130°C. (c) Cross-linked novolac resin cured at higher temperatures.
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casting is the ability to use a sand core inside the mold to produce hollow parts. The moldmaking and coremaking process involve the coating of sand with a binder, compacting the coated sand into the desired shape and then curing or hardening the compacted mass so that it can be handled [10,11]. The resin binder near the metal interface will decompose during casting due to the large amount of heat transferred from the molten steel. Temperatures are usually in the range of 1400–1600°C, depending on the casting material. The binder decomposes into a variety of chemical compounds, the nature of which depend primarily on the binder composition and the temperature to which the mold is exposed. Some decomposition products condense in the sand mold, but others are emitted into the air during casting, cooling (solidification), and removal of the cast from the sand mold (shakeout). It has been found that small amounts of toxic materials are given off from the resin during resin decomposition [12]. Many of these substances must be eliminated or reduced to a level specified by the Clean Air Act Amendments (CAAA) of 1990. Title III of the CAAA requires more stringent control of the emissions of the 189 HAPs listed in the act [13]. The US Environmental Protection Agency (EPA) schedule calls for iron and steel foundry air emission standards to be in place by the year 2000. At the present time there are no reliable data on the nature and quantity of HAPs generated by specific resins. Therefore, foundries cannot accurately assess what kind and what quantity of HAPs are being emitted, nor can they determine their adherence to the requirements under the CAAA. The objective of this study was to model in the laboratory conditions for the production of volatiles which are typical of metal pouring and casting. Data produced from thermogravimetric analysis (TGA) and Py-GC-MS were compared to assess the volatile organic compounds (VOCs) released during decomposition of the novolac resin. Initial experiments were conducted to determine how each individual component contributed to the total VOCs. Uncoated sand, resin, resin containing 15% HMTA and resin coated sand were all subjected to TGA and PY-GC-MS. The data obtained from TGA produced information on the quantity of volatiles and the temperatures at which these volatiles evolved. These data were used to determine the basic operating parameters for the pyrolysis experiments. Studies were performed to determine what effect, if any, pyrolysis temperature, temperature rise time, and the duration of pyrolysis had on the nature and quantity of VOCs. The identity and quantity of major pyrolysis volatiles are presented.
2. Experimental
2.1. Chemicals The novolac resin (Plasti Flake 1105) was manufactured by Borden, (Westchester, Ill). Coated sand was produced following procedures used by a local foundry (Southern Precision, Birmingham, AL). The resin coated sand was prepared by dissolving 1 gm of HMTA in 7 ml of water. This solution was then mixed
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with the resin and added to a type of sand called Wedron washed sand obtained from a local foundry. The final product contained 3.00% resin, 0.45% HMTA, and 96.55% sand. The mixture was placed in an oven set at 100°C and left until all the water had evaporated. The temperature was then raised to approximately 130°C to cure the coated sand. The curing process was finished when the product turned to a light yellow, corn-flake like color. Although the cured coated sand was hard, it was ground into fine particles using a mortar and pestle. n-Pentadecane, ACS grade (Polyscience, Niles, IL) was diluted with n-hexane, to produce a 1000 ppm stock solution which was further diluted, as needed. A gas standard which contained methane, ethane, ethene, acetylene, carbon monoxide and carbon dioxide, each at 1% in nitrogen was purchased from Supelco (Bellefonte, PA). A 0.1% propane standard was made in house using grade HD-5 propane from a propane torch purchased locally. Cis-2-butene was purchased from Matheson (Atlanta, GA), and a 0.1% standard, in nitrogen was prepared. A retention time standard of HCN was produced in house by dissolving a small amount of NaCN in a 150 ml Erlenmeyer flask and adding dilute HCl. Gaseous HCN was withdrawn from the headspace with a gas tight syringe.
2.2. Thermogra6imetric Analysis (TGA) Weight loss analysis was performed on the uncoated sand, resin containing 15% HMTA and also on the resin coated sand using a TA 2950 thermogravimetric analyzer (TA Instruments, New Castle, DE). The resin coated sand contained approximately 4% resin (wt./wt.). The standard temperature program was from room temperature to 1000°C at a rate of 150°C min − 1. Nitrogen was used as the purge gas at a rate of 40 ml min − 1 to the balance, and 60 ml min − 1 to the furnace, as recommended by the instrument manufacturer.
2.3. Pyrolysis Pyrolysis was performed using a standalone pyrolyzer, CDS Pyroprobe 2000 (Chemical Data Systems, Oxford, PA) equipped with a coil probe. The pyrolyzer used a CDS Pyroprobe 100 interface to the GC/MS. Probe temperatures of 200, 400, 500, 750, and 980oC were employed, each held for 1, 5, 10, and 20 s. The probe was cleaned at 1000°C for 10 s between samples. Blanks were run periodically, to ensure a clean background. Weighed samples were loaded into quartz tubes and packed with a small quantity of silanized glass wool at both ends.
2.4. GC-MS Analysis The separation of the volatiles from the pyrolysate was carried out on a quadrupole type GC/MS instrument (HP GCD, Avondale, PA), using a 30 m×0.25 mm×0.25 mm fused silica capillary column coated with 5% diphenyl/95% dimethylpolysiloxane (HP-5, Hewlett Packard, Avondale, PA). A temperature program of 40 – 280°C at a rate of 10°C min − 1 was found to give the best
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separation within a reasonable amount of time. A 30 m ×0.32 mm fused silica capillary PLOT type column coated with a porous divinylbenzene homopolymer (GS-Q, J&W Scientific, Folsom, CA), and a 30 m× 0.32 mm× 10 mm fused silica capillary molsieve absorbent type column (CP-Molsieve 5A, Chrompack, Raritan, NJ) was used for the separation of low molecular weight components. Standard EI conditions were used.
3. Results and Discussion Initial thermogravimetric analysis was performed on the uncoated sand and the solid resin containing 15% HMTA. The resin coated sand was then investigated to determine how the resin behaved on the sand matrix. Fig. 2 shows the first derivative curve of weight loss with respect to temperature of resin containing HMTA (---) and resin coated sand ( – –). Data produced by repetitive sampling showed a high degree of variation, indicating that the uncoated sand was rather inhomogeneous. The volatiles ranged from 0.3–1.5% (wt./wt.), and were determined to mainly consist of benzene, toluene and cresols. The source of these compounds is suspected to be the washing process the sand endures before being used. The majority of the volatiles from the resin containing 15% HMTA evolved between 150 – 195°C and 450 – 650°C. However, from the thermogram of the resin coated sand, the volatiles seemed to behave differently when novolac resin and coated sand were compared to each other. The thermal profiles of the novolac resin containing HMTA and the novolac resin coated sand behave very similar up to around 700°C. With the novolac resin containing 15% HMTA, nearly 95% of the VOCs have evolved at 700°C, however, only approximately 50% of the VOCs have been emitted with the novolac resin coated sand at the same temperature. The discrepancy can be explained by the high heat capacity of the sand, which is capable of absorbing a large amount of heat before transferring it to the resin. This creates a ‘lag’ in the volatilization of the resin. A comparison of the fraction of the sample volatilized by TGA and pyrolysis was performed to determine the compatibility of the techniques in terms of volatilization. Table 1 presents quantitative data for both TGA and pyrolysis. For these experiments the average RSD was in the range of 5%, indicating that the two techniques were compatible in terms of volatilization. The difference was minimal at temperatures of 750 and 980°C. At 500°C, the thermal energy was not sufficient to volatilize the entire resin. The vapor from the resin condensed on the quartz sample tubes thus generating an artifact. The sample tubes were cut to smaller lengths, but some of the resin sample would still condense on the tube wall and also bubble out onto the probe coils. It was thus not possible to accurately determine the fraction volatilized by pyrolysis at 500°C. The volatiles that were released around 200°C Fig. 3 were low molecular weight gases. They were analyzed on a PLOT column. This column was used to separate ethene, propene, butene, 1,3-butadiene, 2-propenenitrile, 1-pentene, and 1,3-cyclopentadiene. The separated gases
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Fig. 2. First derivative curve of weight loss with respect to temperature of resin containing 15% HMTA (------) and resin coated sand ( – – – – –).
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Fig. 3. Pyrogram at probe temperatures of (top) 500°C, (middle) 750°C, and (bottom) 980°C for 5 s.
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Table 1 Percent of novolac resin volatilized by TGA and Py-GC TGA o
Py-GC
C
%
o
C
%
500 750 980
49 64 68
500 750 980
— 59 71
Average RSD = 5%, N =3.
were identified by mass spectrometry and retention time matching. However, the column was unable to separate the most volatile components, including carbon monoxide from air, an important separation. In this case the molsieve column Table 2 Major volatile components emitted from the novolac resin at 980°Ca Components
Concentration (mg g−1)
Methane Carbon monoxide Ethene Propene HCN 1-butene 1,3-butadiene 2-propenenitrile 1-pentene 1,3-cyclopentadiene 1,3-pentadiene Benzene Toluene Phenol Benzofuran Indene 2-methyl phenol 4-methyl phenol 2,5-dimethyl phenol 2,4-dimethyl phenol Naphthalene 2-methylnaphthalene 1-methylnaphthalene Biphenyl 2-methyl,1-1’-biphenyl Acenaphthylene Dibenzofuran Fluorene Phenanthrene Anthracene
47 000 73 000 10 400 2600 — 500 700 100 100 300 63.8 23.1 6.5 13.8 1.0 1.6 6.0 4.6 0.9 1.8 8.8 1.2 1.2 2.1 3.3 2.1 0.7 0.9 2.9 1.1
a
Only components \0.8% of the total volatiles are listed in this table. Average RSD =15%, N =3.
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Fig. 4. Quantity of selected products as a function of probe temperature.
produced satisfactory results. It was also useful for determining carbon monoxide and methane. The origin of the water in the novolac resin was of interest. Was the water derived from moisture in the atmosphere or produced during pyrolysis? A sample of the coated sand weighing about 10.0 mg was baked in an oven at 100°C for 30 min., then pyrolyzed. The results indicated that the water is a product of pyrolysis and originated from an elimination reaction taking place at high temperature. Due to the strong tailing of the peak, the quantity of water was not determined. Mass spectrometry, standards and retention indices were used to determine the identity and quantity of the VOCs produced by the novolac resin. Table 2 list the components that are greater than 0.8% of the total volatiles emitted at the highest temperature. These conditions most closely resemble the pouring environment. The average RSD for this set of experiments was around 15%. Due to the many variables involved in sample manipulation and pyrolysis, large variations are expected. The amount of VOCs produced was also examined as a function of probe temperature. Fig. 3 shows pyrograms at 500°C (top), 750°C (middle) and 980°C (bottom). The production of phenol peaks at 750°C and then gradually decreases. Emissions of benzene, toluene, and naphthalene are not observed until around 750°C, increasing in amount as the temperature was raised. The total peak areas for
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Table 3 Novolac resin total peak area as a function of probe temperature and pyrolysis time Pyrolysis time (s)
1 5 10 20
Pyrolysis probe temperature 500
750
980
109 261 135 411 146 991
114 313 909 741 1 194 325 1 304 894
274 117 1 510 737 2 056 604 2 424 654
Average RSD =5%, N =3.
these pyrograms are displayed in Fig. 4. The quantity of both carbon monoxide and HCN (not shown) also increase as the probe temperature is increased. The reasons for the relatively high temperature required for the emission for these low molecular weight components are not fully understood. The effect of the length of sample exposure to temperature was also studied. Samples of 10 mg were pyrolyzed at 500, 750 and 980°C for 1, 5, 10 and 20 s. The results of these experiments are summarized in Table 3. As one may expect, the amount of VOCs increases as the probe temperature increases and also as the length of exposure to temperature increases. Experiments were also performed to study the effect of temperature rise time on the distribution of VOCs. A temperature rise time of 300°C ms − 1 was changed to 300°C s − 1. The results indicate that temperature rise time had no significant effect on the nature of VOCs produced under these conditions. One should note that the actual rise in temperature of the sample in the quartz tube is less than that produced by the heating coil. This lag in temperature is expected to be particularly significant as the rate increases.
4. Conclusion Conditions typical of pouring and casting were modeled in the laboratory. Py-GC-MS has shown to be a valuable technique for determining the nature and quality of volatiles produced from thermal decomposition of a commercial resin binder system. The quality of volatiles produced below 500°C is rather insignificant compared to the amounts released at 750 and 980°C. Fixed gases are the major pyrolysis products at all pyrolysis temperatures studied. Except for these fixed gases, phenol is the major pyrolysis product. At 750°C benzene, toluene and naphthalene begin to evolve. At 980°C phenol is no longer the major component. Benzene becomes the major pyrolysis product and the amount of naphthalene increases by a factor of roughly 20. The amount of VOCs determined in the model studies appears to be high. It is important to note that foundry workers are not exposed to these high concentrations, because the molds are continually flushed with fresh air. A small amount of HCN is produced. It was not quantitated due to the difficulties of producing reliable standards.
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