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RR-11: the Chemical and Physical Characterization of Recycled Tire Crumb Rubber

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RR-11: the Chemical and Physical Characterization of Recycled Tire Crumb Rubber NTP RESEARCH REPOrt ON THE CHEMICAL AND PHYSICAL CHARACTERIZATION OF RECYCLED TIRE CRUMB RUBBER NTP RR 11 JULY 2019 NTP Research Report on the Chemical and Physical Characterization of Recycled Tire Crumb Rubber Research Report 11 National Toxicology Program July 2019 National Toxicology Program Public Health Service U.S. Department of Health and Human Services ISSN: 2473-4756 Research Triangle Park, North Carolina, USA Chemical and Physical Characterization of Recycled Tire Crumb Rubber Table of Contents About This Report............................................................................................................................v Peer Review .................................................................................................................................. vii Publication Details ....................................................................................................................... viii Abstract .......................................................................................................................................... ix Introduction ......................................................................................................................................1 Materials and Methods .....................................................................................................................3 Materials ......................................................................................................................................3 Optical Microscopy .....................................................................................................................3 Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy............................3 Thermogravimetric Analysis .......................................................................................................3 Inorganic Analysis ......................................................................................................................4 Headspace Gas Chromatography–Mass Spectrometry Analysis for Volatile and Semivolatile Organic Compounds ......................................................................................4 Estimation of Solvent-Extractable Constituents Fraction ...........................................................5 Gas Chromatography–Mass Spectrometry Analysis of Solvent-Extractable Constituents Fraction ..........................................................................................................5 Liquid Chromatography–Mass Spectrometry Analysis of Solvent-Extractable Constituents ........................................................................................................................6 Size Fractionation of Crumb Rubber Lot and Gas Chromatography–Mass Spectrometry Profiling of Fractions ...................................................................................6 Energy Dispersive X-Ray Spectroscopy of Crumb Rubber Fractions ........................................7 Bioaccessibility In Vitro .............................................................................................................7 Results ..............................................................................................................................................9 Optical Microscopy .....................................................................................................................9 Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy............................9 Thermogravimetric Analysis .......................................................................................................9 Inorganics Analysis .....................................................................................................................9 Headspace Gas Chromatography–Mass Spectrometry Analysis for Volatile and Semivolatile Organic Compounds ....................................................................................10 Gas Chromatography–Mass Spectrometry Analysis of Solvent-Extractable Constituents Fraction ........................................................................................................10 Liquid Chromatography–Mass Spectrometry Analysis of Solvent-Extractable Constituents Fraction ........................................................................................................11 Gas Chromatography–Mass Spectrometry Analysis of Crumb Rubber Fractions ...................11 Energy Dispersive X-Ray Spectroscopy of Crumb Rubber Fractions ......................................13 Bioaccessibility In Vitro ...........................................................................................................13 Conclusions ....................................................................................................................................14 References ......................................................................................................................................15 Appendix A. Individual and Mixtures of Standards Procured to Identify Crumb Rubber Constituents ........................................................................................................... A-1 ii Chemical and Physical Characterization of Recycled Tire Crumb Rubber Appendix B. Potential Crumb Rubber Constituents Identified in the Literature and Initial Screening Analyses ................................................................................................B-1 Appendix C. Additional Constituents of Crumb Rubber Identified by Various Techniques ......C-1 Appendix D. Settings Used in XCMS Software ......................................................................... D-1 Appendix E. Bioaccessibility Studies .......................................................................................... E-1 Tables Table 1. Elemental Compositiona (Wt %) of Six Individual Crumb Rubber Grains by Energy Dispersive Spectroscopy .....................................................................................17 Table 2. Estimated Levels of Metals in Crumb Rubber Lota .........................................................17 Table 3. Estimated Levels of Volatile and Semivolatile Organic Compounds in Crumb Rubber Lot by Headspace GC–MSa ................................................................................18 Table 4. Percent of Extractable Crumb Rubber in Various Solventsa ...........................................19 Table 5. Constituents of Crumb Rubber in Methylene Chloride Extract Identified by GC– MS with High Confidencea ..............................................................................................20 Table 6. Constituents of Crumb Rubber in Ethanolic Extract Identified by LC–MS ....................21 Table 7. Average Estimated Levels of Volatile and Semivolatile Organic Compounds in Various Crumb Rubber Fractions by Headspace GC–MS ..............................................22 Table 8. Elemental Composition (Wt %) of Crumb Rubber Fractions by Energy Dispersive Spectroscopya ................................................................................................23 Table 9. Estimated Blank Biofluid Concentration (Equivalent μg in Extract per g of Crumb Rubber Extracted)a...............................................................................................24 Table 10. Estimated Bioaccessible Constituent Concentration (μg in Extract per g of Crumb Rubber Extracted)a ............................................................................................26 Figures Figure 1. Representative Optical Micrographs of Two Crumb Rubber Regions ..........................28 Figure 2. Representative Scanning Electron Micrographs of the Cut Surfaces of Four Dissected Crumb Rubber Grains ....................................................................................29 Figure 3. Representative Thermogram from Thermo Gravimetric Analysis of Crumb Rubber ............................................................................................................................30 Figure 4. Comparison of GC–MS Chromatograms at Varying Incubation Temperatures in Headspace GC–MS Analysis: (A) Full Scale (B) Expanded Scale................................31 Figure 5. GC–MS Spectra for Methyl Isobutyl Ketone: (A) Standard Spectrum, (B) Crumb Rubber Sample Spectrum, and (C) Library Spectrum .................................32 Figure 6. Comparison of GC–MS Chromatograms of Crumb Rubber Following Extraction with Various Solvents: (A) Full- and (B) Expanded-Scale Chromatograms, and (C) Blank Solvents .......................................................................34 Figure 7. GC–MS Chromatogram of Crumb Rubber Following Extraction with Water ..............35 Figure 8. GC–MS Spectra for the n-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD): (A) Standard Spectrum, (B) Crumb Rubber Sample Spectrum, and (C) Library Spectrum .....................................................................................................36 iii Chemical and Physical Characterization of Recycled Tire Crumb Rubber Figure 9. LC–MS Total Ion Chromatograms of Ethanol Extract of Crumb Rubber: Comparison of Blank-Subtracted Extract of Crumb Rubber, (A) Positive Ion Mode and (B) Negative Ion Mode .................................................................................37 Figure 10. Comparison of Headspace GC–MS Chromatograms of Various Crumb Rubber Fractions and Unsieved Bulk Material Following Incubation at 100C:
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