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THERMAL DECOMPOSITION of CYANATE ESTER RESINS September 2001 6 DOT/FAA/AR-01/32 Thermal Decomposition of Office of Aviation Research Cyanate Ester Resins Washington, D.C. 20591 September 2001 Final Report This document is available to the U.S. public through the National Technical Information Service (NTIS), Springfield, Virginia 22161. U.S. Department of Transportation Federal Aviation Administration NOTICE This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The United States Government assumes no liability for the contents or use thereof. The United States Government does not endorse products or manufacturers. Trade or manufacturer's names appear herein solely because they are considered essential to the objective of this report. This document does not constitute FAA certification policy. Consult your local FAA aircraft certification office as to its use. This report is available at the Federal Aviation Administration William J. Hughes Technical Center’s Full-Text Technical Reports page: actlibrary.tc.faa.gov in Adobe Acrobat portable document form (PDF). Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. DOT/FAA/AR-01/32 4. Title and Subtitle 5. Report Date THERMAL DECOMPOSITION OF CYANATE ESTER RESINS September 2001 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. Michael L. Ramirez1, Richard Walters2, Edward P. Savitski3, and Richard E. Lyon4 1Hewlett Packard 3Eastman Chemical Company 10. Work Unit No. (TRAIS) Technical Services Lab P.O. Box 1972 P.O. Box 4048 Kingsport, TN 37662-5150 Rd 110 N Bldg. 3 Aguadilla, PR 00605 4Federal Aviation Administration 11. Contract or Grant No. 2Galaxy Scientific Corporation William J. Hughes Technical Center 2500 English Creek Avenue, Bldg. C Aircraft and Airport Safety Egg Harbor Township, NJ 08234 Research and Development Division Atlantic City International Airport NJ 08405 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered U.S. Department of Transportation Final Report Federal Aviation Administration Office of Aviation Research 14. Sponsoring Agency Code Washington, DC 20591 AIR-100 15. Supplementary Notes 16. Abstract Polycyanurate networks were prepared by thermal polymerization of cyanate ester monomers containing two or more cyanate ester (–O-C≡N) functional groups. The thermal decomposition chemistry of nine different polycyanurates was studied by thermogravimetry and infrared analysis of solid films and analysis of the gases evolved during pyrolysis using infrared spectroscopy and gas chromatography-mass spectrometry. It was found that the thermal stability of the polycyanurates was essentially independent of monomer chemical structure with the major mass loss occurring at about 450°C for all materials. Analysis of the solid-state and gas phase thermal degradation chemistry indicates a thermal decomposition mechanism for polycyanurates which begins with hydrocarbon chain scission and cross-linking at temperatures between 400°-450°C with negligible mass loss, followed by decyclization of the triazine ring at 450°C that liberates volatile cyanate-ester decomposition products. The solid residue after pyrolysis increases with the aromatic content of the polymer and incorporates about two thirds of the nitrogen and oxygen present in the original material. 17. Key Words 18. Distribution Statement Polymer, Polycyanurate, Cyanate ester, Thermoset polymer, This document is available to the public through the National Thermal degradation, Fire, Flammability Technical Information Service (NTIS), Springfield, Virginia 22161. 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price Unclassified Unclassified 20 Form DOT F1700.7 (8-72) Reproduction of completed page authorized ACKNOWLEDGEMENTS The authors are grateful to Dave Shimp for many helpful discussions and to David Esral for determining the char-forming tendency of some of the polycyanurates. Certain commercial equipment, instruments, materials, or companies are identified in this report in order to adequately specify the experimental procedure. This in no way implies endorsement or recommendation by the Federal Aviation Administration. iii/iv TABLE OF CONTENTS Page EXECUTIVE SUMMARYvii 1.INTRODUCTION1 2.EXPERIMENTAL2 2.1Materials2 2.2Thermal Analyses2 2.3Infrared Spectroscopy2 2.3.1 Solid Films 2 2.3.2 Pyrolysis Gases 3 2.4Pyrolysis-Gas Chromatography/Mass Spectrometry4 3.RESULTS AND DISCUSSION4 3.1Polymerization Chemistry4 3.2Solid-State Degradation Chemistry5 3.3Volatile Degradation Products8 4.CONCLUSIONS12 5.REFERENCES12 v LIST OF FIGURES Figure Page 1 Cyanate Ester Polymerization Reaction 1 2 Fourier Transform Infrared Cure Monitoring of B-10 Cyanate Ester Resin 5 3 Thermogravimetic Data for the Nine Polycyanurates 6 4 Mass Loss Rate Versus Temperature for PT-30 6 5 Temperature Resolved FTIR Spectra of B-10 Polycyanurate Film During Heating 8 6 Temperature Resolved Gas Phase FTIR Spectra of B-10 Polycyanurate Volatile Decomposition Products 9 7 Temperature Resolved Gas Phase FTIR Spectra of BPC Polycyanurate Volatile Decomposition Products 9 LIST OF TABLES Table Page 1 Cyanate Ester Monomers 3 2 Thermal Properties of Polycyanurates 5 3 Wavenumbers of the –OCN Group Absorbance Doublet for the Volatile Decomposition Products 10 4 Percentages of Volatile Decomposition Products Identified by Mass Spectroscopy for Six Polycyanurates 11 vi EXECUTIVE SUMMARY Polycyanurate networks were prepared by thermal polymerization of cyanate ester monomers containing two or more cyanate ester (–O-C≡N) functional groups. The thermal decomposition chemistry of nine different polycyanurates was studied by thermogravimetry, infrared analysis of solid films, and analysis of the gases evolved during pyrolysis using infrared spectroscopy and gas chromatography-mass spectrometry. It was found that the thermal stability of the polycyanurates was essentially independent of monomer chemical structure with the major mass loss occurring at about 450°C for all materials. Analysis of the solid-state and gas phase thermal degradation chemistry indicates a thermal decomposition mechanism for polycyanurates which begins with hydrocarbon chain scission and cross-linking at temperatures between 400°-450°C with negligible mass loss, followed by decyclization of the triazine ring at 450°C that liberates volatile cyanate-ester decomposition products. The solid residue after pyrolysis increases with the aromatic content of the polymer and incorporates about two thirds of the nitrogen and oxygen present in the original material. vii/viii 1. INTRODUCTION. Polycyanurates are addition-cured thermoset polymers that exhibit good [1-4] to outstanding [5 and 6] fire resistance. Because the ignition and fire resistance of solid polymers are governed by short term thermal stability and anaerobic degradation chemistry [7-10], the following study was conducted to understand these processes in polycyanurates. Polycyanurates are formed when three cyanate ester monomers containing the –O-C≡N functional group undergo a thermally initiated cyclotrimerization (addition) reaction [11 and 12] to form a six-member oxygen-linked triazine ring (cyanurate) as illustrated in figure 1. R R R R OCN NCO O NO NN OCN cyanurate O R R CYANATE ESTER MONOMERS POLYCYANURATE FIGURE 1. CYANATE ESTER POLYMERIZATION REACTION The cyanurate is a thermally stable cross-link that is responsible for the high mass loss temperature (450°C) of these thermosets. Polycyanurates derived from phenol novolac cyanate esters have a high glass transition temperature Tg > 350°C approaching their thermal decomposition temperature [13 and 14]. In addition to having high thermal stability, polycyanurates form a carbonaceous char during burning that protects the underlying material and further enhances fire resistance [15 and 16]. Because cyanate ester resins polymerize by an addition reaction, no volatiles or by-products are produced during cure which can cause voids and subsequent loss of strength in the final product [14-16]. Thermogravimetric studies of polycyanurates in air have indicated that thermoxidative degradation proceeds via rapid hydrolysis of the ether oxygen bond between the phenyl and triazine rings in the presence of moisture at temperatures of 350°-420°C [17-19]. Purely thermal degradation under anaerobic conditions is claimed at higher temperature (≥ 450°C) via homolytic cleavage of the hydrocarbon backbone over a narrow temperature range (450°-500°C) independent of the chemical structure of the linking groups between the cyanurate rings [18-20]. The present study extends these previous thermogravimetric studies of polycyanurate thermal degradation to include infrared and mass spectroscopic analysis of the solid and gaseous pyrolysis products. 1 2. EXPERIMENTAL. 2.1 MATERIALS. Nine cyanate ester monomers (resins) with different chemical structures were examined in this study. The monomer structures and suppliers are shown in table 1. Five of the resins are difunctional cyanate esters derived from bisphenols, while two of the resins are multifunctional resins derived from phenol novolacs. All of the cyanate ester resins were used as received from the manufacturer without modification, purification, or catalysts. With the exception of the bisphenol C (BPC) cyanate ester, which is a research monomer at present, all of the resins examined in this study are commercial materials. The BPC polycyanurate was included in this study because
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