Analysis of Polymeric Flame Retardants in Engineering

identified by peak pattern recognition through an AED halogen element trace. In this study, a set of thermoplastic resins containing different bromina...
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Anal. Chem. 1999, 71, 2037-2045

Analysis of Polymeric Flame Retardants in Engineering Thermoplastics by Pyrolysis Gas Chromatography with an Atomic Emission Detector Frank Cheng-Yu Wang

Analytical Sciences Laboratory, Michigan Division, The Dow Chemical Company, Midland, Michigan 48667

Halogen-containing polymeric flame retardants in engineering thermoplastics have been analyzed by pyrolysis gas chromatography (Py-GC) with an atomic emission detector (AED) and a mass selective detector (MSD). The atomic emission detector is very effective in monitoring halogen-containing pyrolysates after Py-GC. The type of polymeric flame retardant used in thermoplastics can be identified by peak pattern recognition through an AED halogen element trace. In this study, a set of thermoplastic resins containing different brominated polymeric flame retardants have been studied. After generation of the pyrogram by Py-GC/AED, those pyrolysates produced from polymeric flame retardants were identified by MSD analysis of a complex total ion chromatogram (TIC) generated by the thermoplastic resins. This study demonstrated the effectiveness of the AED in the tasks of specific element monitoring and component pattern recognition. The advantages and disadvantages of using the AED are also discussed.

Flame retardants are common additives in engineering thermoplastics to improve their flame resistance properties. The level of flame retardants present in the polymers is in the range 5-15% depending on the kind of flame retardant used and the type of end application targeted. There are many different families of flame retardants, such as inorganic oxides, organic halogen compounds, and halogen-containing polymers. Depending on the application, one or a combination of multiple types may be used in the thermoplastics. Halogen-containing organic or polymeric flame retardants are widely used in the plastic industry, especially polyamides and polyesters. Generally, these flame retardants are added to or blended into polymers during extrusion or other postpolymerization processes. The main advantage of polymeric flame retardants is that they are less volatile and less migratory, allowing high processor temperatures and application temperatures. Most polymeric flame retardants are repeat monomer units of chlorinated or brominated phenol, bisphenol A, or styrene. These aromatic halogen-substituted monomer units are relatively ther10.1021/ac990100r CCC: $18.00 Published on Web 04/17/1999

© 1999 American Chemical Society

mally stable and ignition resistant compared with aliphatic analogues.1 The separation of flame retardants from their original polymeric matrix is always a challenging task, especially for those engineering thermoplastics targeted for high-temperature applications such as polyesters and polyamides. Most of these polymers hardly dissolve in any solvent at room temperature. Although there may be a solvent that can dissolve these polymers, either the complete dissolution may occur at elevated temperature or the final solution may have relatively high viscosity. This situation makes any further separation (either extraction or precipitation) troublesome. Furthermore, for practical purposes, many engineering thermoplastics are reinforced with fibers to enhance their physical properties. To effectively separate additives from this polymer/fiber matrix is even more difficult. Flame retardants normally have very high boiling points; organic flame retardants may be analyzed by gas chromatography (GC) or liquid chromatography (LC) methods if these compounds can be successfully separated from the original polymer matrix. However, if the flame retardant is in the polymeric form, most GC methods will not be that effective because of high boiling point and high molecular weight. Most LC methods will not be that satisfactory because of high molecular weight. In addition, even if there is an LC method that is capable of separating flame retardants from their original polymeric matrix, detection and identification will still be rigorous tasks. There is no universal inline detection device which can effectively detect and identify these flame retardants. Owing to these separation and detection difficulties, the analytical techniques available to resolve this flame retardant issue are greatly limited. Pyrolysis gas chromatography (Py-GC)2 is one of the techniques which is suitable to attack this problem. Py-GC is a technique that uses thermal energy (pyrolysis) to break down a polymeric chain to monomers, oligomers, and other fragments, followed by the separation of pyrolysates with gas chromatography, and utilizes an appropriate detector for detection. Most of time, the identification relies on mass selective detector (MSD). (1) Troitzsch, H. J. Flame Retardants. In Plastics Additives Handbook; Gachter, R., Muller, H., Eds.; Hanser Publishers: New York, 1993; p 716. (2) Wampler, T. P. Analytical PyrolysissAn Overview. In Analytical Pyrolysis Handbook; Wampler, T. P., Ed.; Marcel Dekker: New York, 1995; pp 1-3.

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One of the advantages of the pyrolysis technique is the simple sample preparation. For a solid sample, the only preparation required is to cut the solid into the appropriate size and weigh the sample to a suitable (approximately 500 µg) amount. The purpose of this preparation is to fit a sample into the pyroprobe while, at the same time, ensuring complete pyrolysis. This technique is extremely useful in the study of a solid sample or additives in the solid where there is hardly any way to separate them. Flame retardants have been studied by pyrolysis for a long time. Most of the studies focused on thermal degradation under certain pyrolysis or fire conditions.3-5 The pyrolysates produced have been evaluated for the safety of humans and the environment around the fire area.6,7 Other studies of flame retardants emphasized the flame resistance capability or the mechanism of flame propagation prevention.8,9 Few reports focused on the analytical identification of the type of flame retardants in the polymers.10,11 In this study, a set of polyesters and polyamides which contain brominated polymeric flame retardants have been investigated by Py-GC with an atomic emission detector (AED) and an MSD. The AED is very effective in monitoring different halogen-containing fragments after Py-GC. The type of polymeric flame retardant used in the thermoplastic resins was identified by peak pattern recognition through an AED halogen element trace. After generation of the pyrogram by Py-GC/AED, those pyrolysates produced from polymeric flame retardants were identified by MSD analysis of a complex total ion chromatogram (TIC) which was mainly generated by the thermoplastic polymer matrix. This study demonstrated the effectiveness of the AED in the tasks of specific element monitoring and component pattern recognition. The advantages and disadvantages of using the AED are also discussed EXPERIMENTAL SECTION A. Samples Sources. Four brominated flame-retardant polymers were used in this study. Poly(dibromostyrene) (PDBS-80) was obtained from Great Lake Chemical Corp. (West Lafayette, IN), brominated polystyrene (Pyrocheck PB68) was obtained from FERRO Corp. (Evansville, IN), pentabromobenzyl polyacrylate (FR-1025P) and brominated epoxy (tetrabromobisphenol A diglycidal ether) (F-3020) were obtained from AmeriBrom, Inc. (New York). A thermoplastic resin, CASTIN-LW9330-FR507, was obtained from DuPont. All other thermoplastic resins, poly(1,4butylene terephthalate) (PBT) (Catalog No. 43,515-5), styreneacrylonitrile copolymer (SAN) (Catalog No. 18,286-9), nylon-4,6 (Catalog No. 44,299-2), nylon-6,6 (Catalog No. 42,917-1), and nylon6,9 (Catalog No. 18,806-9), were purchased from Aldrich Chemical (3) Sato, H.; Kondo, K.; Tsuge, S.; Ohtani, H.; Sato, N. Polym. Degrad. Stab. 1998, 62 (1), 41-48. (4) Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1996, C36 (4), 721-794. (5) Price, D.; Horrocks, A. R.; Akalin, M.; Faroq, A. A. J. Anal. Appl. Pyrolysis 1997, 40-41, 511-524. (6) Weber, L. W. D.; Greim, H. J. Toxicol. Environ. Health 1997, 50 (3), 195215. (7) Neupert, M.; Weis, H.; Thies, J.; Stock, B. Chemosphere 1989, 19 (1-6), 219-224. (8) Danzer, B.; Riess, M.; Thoma, H.; Vierle, O.; van Eldik, R. Organohalogen Compd. 1997, 31, 108-113. (9) Hall, M. E.; Zhang, J.; Horrocks, A. R. Fire Mater. 1994, 18 (4), 231-241. (10) Faroq, A. A.; Price, D.; Milnes, G. J.; Horrocks, A. R. Polym. Degrad. Stab. 1991, 33 (2), 155-170. (11) Dave, V.; Israel, S. C. Polym. Prepr. 1990, 31 (1), 554-555.

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Co. A reference polymer, a 50/50 wt % blend of PBT and SAN, incorporating (approximately 5-10 wt %) different polymeric flame retardant, was prepared to test the detection of polymeric flame retardants in the thermoplastic resin. Three polyamides, nylon4,6, nylon-6,6, and a nylon-6,9 blend with 15% brominated polystyrene, were also prepared to examine the secondary reaction. B. Py-GC/AED and Py-GC/MS Conditions. A sample of the polymer was carefully deposited into a quartz tube. The quartz tube was equilibrated for 5 min in a 300 °C interface connected to the injection port of a Hewlett-Packard (HP) model 5890 gas chromatograph. The sample was pyrolyzed (CDS 2000 Pyroprobe Pt coil) at an actual temperature of 700 °C with probe calibration. The coil was heated to the set temperature at 20 °C/ms and held at the set temperature for a 20-s interval. The pyrolysis products were split in the 300 °C injection port, with 10 psi head pressure, 200:1 split ratio, and separated on a fused-silica capillary column (J & W Scientific DB-5, 30 m × 0.25 mm i.d., 0.25 µm film) using a linear temperature program (40 °C/4 min, 10 °C/min, to 320 °C/18 min). The GC output region of the detector was kept at 300 °C. In Py-GC/AED, the output from the gas chromatograph was connected with an HP 5921A atomic emission detector. The atomic emission lines used for detection of carbon, hydrogen, and bromine were 496, 486, and 478 nm. In Py-GC/MS, the output from the gas chromatograph was connected with an HP 5971 mass selective detector. An electron ionization mass spectrum was obtained every second over the mass range 15-650 Da. C. Safety Considerations. All brominated polymeric flame retardants are considered as a hazardous materials. Any eye and skin contact, inhalation, and ingestion should be avoided. Personal protective devices should be used during the blend extrusion or any mixing processes. The thermoplastic resins are considered to be nonhazardous materials, but eye and skin contact or inhalation of vapor/powder should be avoided. RESULTS AND DISCUSSION Figures 1-4 show the Py-GC/AED results of the reference polymer blend with different brominated polymeric flame retardants as well as the pure brominated polymeric flame retardants. In Figure 1, the flame retardant used is poly(dibromostyrene); the other flame retardants used are brominated polystyrene, pentabromobenzyl polyacrylate, and brominated epoxy as in the order of the figures. The major peaks labeled with numbers have been identified by the MSD. The components are listed in the captions of the figures. On the basis of the AED bromine element trace, the type of flame retardant used can be identified by the peak pattern that matches with the peak pattern of the pure flame retardant. Since the bromine element is almost never present in any other commonly used monomer or additive in thermoplastics, the chance of interference with this peak pattern recognition method is very small. Traditionally, the polymeric flame retardant in a thermoplastic can be directly analyzed by Py-GC/MSD, especially when these flame retardants contain the halogen elements, such as chlorine and/or bromine. Because the chlorine and bromine elements possess characteristic pairs of isotopes with well-known ratios, it is relatively easy to detect chlorine and/or bromine in a component through its mass spectrum. However, when a thermoplastic

Figure 1. Py-GC/AED results (carbon trace and bromine trace) for the reference polymer with poly(dibromostyrene) as the polymeric flame retardant along with those for the pure flame retardant for comparison. On the basis of the peak pattern in the bromine trace, the reference polymer is matched with the pure flame retardant. The identities of the major peaks labeled are (1) bromostyrene, (2) 2,6-dibromostyrene, (3) 2,4-dibromostyrene, and (4) 2,4,6-tribromostyrene. The inset demonstrates the intensity loss of isomers associated with major pyrolysates in the reference polymer (labeled by /).

resin with a polymeric flame retardant is pyrolyzed, there are many other components that may come from thermoplastic resin along with those components from the flame retardant. In addition, there may be superimposed and partially overlapped peaks, such that the mass spectrum of one peak may represent the combination of two or more than two components. Because of the large number of components, interpretation and identification of all components can be rather time consuming. An understanding of which pyrolysates originally came from the polymeric flame retardants may not be straightforward. Definition of the type of polymeric

flame retardant is even more difficult. Sometimes, single-ion monitoring (SIM) may be a way to explore certain fragments in the pyrolysates. However, without knowing the identity of the polymeric flame retardant, there is no simple way to predict which fragment mass/ion to monitor. Even though these flame retardants contain halogen elements, there is no good way to know the identity of the aliphatic or aromatic counterpart to which the halogen element is attached. Most of the commercially available bromine-containing polymeric flame retardants have two major fragment families after Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

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Figure 2. Py-GC/AED results (carbon trace and bromine trace) for the reference polymer with brominated polystyrene as the polymeric flame retardant along with those for the pure flame retardant for comparison. On the basis of the peak pattern in the bromine trace, the reference polymer is matched with the pure flame retardant. The identities of the major peaks labeled are (1) 2,4-dibromostyrene and (2) 2,4,6-tribromostyrene. The inset demonstrates the intensity loss of isomers associated with major pyrolysates in the reference polymer (labeled by /).

pyrolysis: the brominated styrene family and the brominated phenol family. For example, brominated phenol will be produced from the pyrolysis of brominated poly(phenylene oxide), and similar fragments will also result from the pyrolysis of brominated epoxy (bisphenyl A based) and brominated poly(vinylphenol). Additionally, dibromostyrene and tribromostyrene will be produced from the pyrolysis of poly(dibromostyrene) and brominated polystyrene. One or two common brominated fragments (such as with the brominated phenol family or the brominated styrene family) detected may not necessarily identify which brominated 2040 Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

polymeric flame retardant was used in the thermoplastic resin. Instead, a complete or nearly complete identification of all fragments as well as a correct relative peak intensity ratio (pattern) for the flame retardant is necessary to ensure the correct investigation of that brominated polymeric flame retardant. In this situation, the AED may be superior to the MSD for the detection of specific-element-containing fragments and their relative intensity pattern. Depending on the specific atomic emission lines chosen, the AED will detect those components which contain the specific

Figure 3. Py-GC/AED results (carbon trace and bromine trace) for the reference polymer with pentabromobenzyl polyacrylate as the polymeric flame retardant along with those for the pure pentabromobenzyl polyacrylate for comparison. On the basis of the bromine trace, the peak pattern of the test polymer is matched with the peak pattern of the pure flame retardant. The identities of the major peaks labeled are (1) 4-bromo-1-butene, (2) 1,4-dibromobutane, (3) tetrabromobenzene, (4) tetrabromotoluene, (5) pentabromobenzene, and (6) pentabromotoluene.

element of interest (such as bromine). Unlike the MSD, the AED does not have direct identification capability for those components detected. However, the AED can still be used as an identification tool by a peak pattern recognition approach. A unique element trace pyrogram can be used to identify the specific polymeric flame retardant. There is little chance that two different polymeric flame retardants will have the same set of pyrolysates plus similar relative intensity ratios, producing the same GC elution pattern. Even in the case of poly(dibromostyrene) and brominated polystyrene, the number of isomers in dibromostyrene and the

relative peak intensity between dibromostyrene and tribromostyrene can be used to distinguish them. The thermal degradation pathways of polymeric flame retardants may change when they are pyrolyzed in the presence of other polymers. The first evidence of this phenomenon is that the intensities of isomers which are associated with the major pyrolysates from the flame retardant may decrease. Examples are given for the decrease in intensities of dibromostyrene-related isomers in the insert of Figure 1 and for the decrease in isomer intensities of tribromostyrene in the insert of Figure 2. Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

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Figure 4. Py-GC/AED results (carbon trace and bromine trace) for the reference polymer with brominated epoxy as the polymeric flame retardant along with those for the pure brominated epoxy for comparison. On the basis of the bromine trace, the peak pattern of the test polymer is matched with the peak pattern of the pure flame retardant. The identities of the major peaks labeled are (1) 4-bromo-1-butene, (2) 2-bromophenol, (3) 2,6-dibromophenol, (4) 2,4-dibromophenol, (5) 2,4,6-tribromobenzene, and (6) 2,4,6-tribromophenol.

This “intensity decreasing phenomenon” can be attributed to the competition of excited energy release pathways. When the major pyrolysates formed, they were in excited states or carried an extra amount of kinetic/potential energy. This extra energy may initiate self-rearrangement to form isomers. The mechanism can be an internal (rotation and vibration) energy redistribution or energy transfer involving other excited analogous especially when the local concentrations of these pyrolysates are high. However, this self-rearrangement may not take place if their energy can be quickly dissipated to others (different pyrolysates). During the pyrolysis of a flame retardant with a thermoplastic 2042 Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

resin present, the dominant energy release pathway seems to be the one without self-rearrangement. The second evidence of thermal degradation pathway changes is lost or greatly reduced intensities of several specific pyrolysates such as pentabromobenzene in the pentabromobenzyl polyacrylate case, as indicated in peak 5 of Figure 3, and 2,4,6-tribromobenzene in the brominated epoxy case, as displayed in peak 5 of Figure 4. The disappearance of some pyrolysates may be explained by the competition of other degradation pathways or by secondary reactions when pyrolysates from the thermoplastic resin are present.

Figure 5. Py-GC/AED results (carbon trace and bromine trace) for the CASTIN-LW9330-FR507 thermoplastic resin along with the bromine traces for the reference polymer with poly(dibromostyrene) and the reference polymer with brominated polystyrene for comparison. On the basis of the bromine trace, the peak pattern matches with that of brominated polystyrene. This result demonstrates that the brominated polystyrene exists in this resin as the polymeric flame retardant.

For the purpose of identification through peak pattern recognition, what is the most suitable peak pattern that can be used as a standard? As discussed in the pervious paragraphs, when thermoplastic resins are blended with flame retardants, peak intensities may change and peaks may be lost depending on what type of thermoplastics are analyzed. The best standard may not necessarily come from pyrolysis of pure bromine-containing flame retardants. Instead, pyrolysis of polymeric flame retardants in a polymer matrix, which is very close to an unknown polymer with a flame retardant, will produce a better peak pattern for the purpose of identification.

CASTIN-LW9330-FR507 is a commercially available thermoplastic resin which is a fiber-reinforced PBT alloy (a blend of PBT with other polymers). Figure 5 shows carbon and bromine traces from Py-GC/AED of the CASTIN resin along with two bromine traces from the reference polymer containing poly(dibromostyrene) and brominated polystyrene. It is obvious that the brominated polymeric flame retardant present is brominated polystyrene. There are pyrolysates produced from the thermoplastic resin which are reactive, such as the butylene fragment from PBT and aliphatic amines from the aliphatic polyamides. These reactive Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

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Figure 6. Py-GC/AED results (bromine traces) for nylon-4,6, nylon-6,6, and nylon-6,9. All three polyamides contain brominated polystyrene. The extra peak for nylon-4,6 is 4-bromo-1-butene (labeled as peak 1). The extra peak group for nylon-6,6 and -6,9 is 6-bromo-1-hexene (labeled as peak 2) and its isomers.

pyrolysates may react with pyrolysates from brominated polymeric flame retardants to form new components which are not found in of the pyrolysates from either the thermoplastic resin or the brominated polymeric flame retardants. These secondary reaction products may cause some confusion in the peak pattern recognition during the identification of brominated polymeric flame retardants. Figure 6 presents the AED bromine trace pyrograms of different aliphatic polyamides (nylon-4,6, nylon-6,6, and nylon-6,9) with brominated polystyrene as the flame retardant. An extra component (the secondary product) appears at the retention time of 3.88 min in the nylon-4,6 pyrogram and at the retention time of 9.40 min in the nylon-6,6 and -6,9 pyrograms. This component has been identified (by Py-GC/MSD) as 4-bromo-1-butene in the nylon-4,6 case and 6-bromo-1-hexene in the nylon-6,6 and -6,9 cases. On the basis of the repeat unit structure of nylons [nylon4,6 is poly(tetramethyleneadipamide), nylon-6,6 is poly(hexamethyleneadipamide), and nylon-6,9 is poly(tetramethylenenonanediamide)] and the structure of the extra component produced from the secondary reaction, the reactants must come from the aliphatic amine part of the polyamides and hydrogen bromide (HBr) from the brominated polymeric flame retardant. The reaction mechanism can be postulated as the aliphatic amine reacting with HBr 2044 Analytical Chemistry, Vol. 71, No. 10, May 15, 1999

to form the aliphatic bromide where the amine group has been replaced by the bromine during the reaction. However, once the extra product is identified, the reactants and the mechanism of formation can be understood. This reaction product would be that expected upon analyzing for the polymeric flame retardant for that polymer family. However, when one analyzes for the brominated polymeric flame retardant in aliphatic polyamides, extra precautions should be taken. In addition to the conventional pyrolysis peak pattern, extra peaks such as those from the aliphatic bromide should be expected due to the product of the reaction of the aliphatic amine part of the polyamide with HBr. Quantitative analysis of polymeric flame retardants may be achieved by Py-GC/AED if a set of polymeric flame retardant standards of varying concentrations are available or can be prepared in similar thermoplastic resin matrixes. A calibration must be performed with different concentrations of flame retardant standards to ensure the same pyrolysis efficiency as well as the linearity of signal intensity. CONCLUSIONS Py-GC/AED has been successfully applied to the analysis of polymeric flame retardants in engineering thermoplastics. The

most significant advantage of using the Py-GC technique is that there is almost no sample preparation. The advantage of using the AED is the selectivity of detection. The ability to detect specific-element-containing components while at the same time discriminating any other possible complications is the key point of this effective detection. The type of brominated polymeric flame retardant used in the thermoplastics can be identified by peak pattern recognition through an AED halogen element trace. This method can be applied to most polymer systems without any further modification. Sometimes, if there is a secondary reaction occurring after pyrolysis, the peak pattern recognition method may require a little experience to identify any extra peaks that

originate from this secondary reaction. In this study, a set of thermoplastics containing different brominated polymeric flame retardants as well as a commercial thermoplastic resin have been studied. After generation of the chromatogram, the AED and MSD were able to identify those pyrolysates specific to the polymeric flame retardants by analysis of a complex total ion chromatogram. This study demonstrated the effectiveness of the AED in the tasks of specific element monitoring and component pattern recognition. Received for review January 29, 1999. Accepted February 23, 1999. AC990100R

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