Composite materials consisting of an organic resin and fiber support are widely used throughout the Department of Defense (DoD) and the adoption of these materials in place of metal and ceramic components is accelerating. Composites provide a number of performance advantages over conventional materials including a significant reduction in weight leading to reduced fuel usage and greater range for tactical vehicles, particularly aircraft. Although increased use of composites provides several environmental advantages, these benefits are offset by the non-sustainable derivation of resins from petroleum products.
The objective of this project was to demonstrate the feasibility of converting diaryl or single-ring polyphenols that are extracted or derived from sustainable and renewable plant sources to cyanate ester resins for use in high-performance polymer composites.
Cyanate esters were prepared from several different renewable sources including mixed extracts from creosote bush leaves and stems, diaryl bis-phenols derived from both the oxidative and reductive coupling of vanillin, resveratrol, and single-ring systems derived from resorcylic acid. These resins were fully characterized by techniques including nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, mass spectrometry, and in some cases, single crystal X-ray diffraction. The resins were further characterized by thermal analysis utilizing techniques including differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and thermomechanical analysis (TMA). The thermal stability and decomposition of resins was monitored with TGA/IR experiments.
Several new extraction procedures were developed to isolate polyphenols from the leaves and stems of the creosote bush. These crude mixtures could be converted to cyanate esters, but upon work-up the mixtures rapidly polymerized, even at room temperature and this system was not studied further. Three synthetic routes were then pursued to evaluate the best route to high performance, renewable cyanate esters. In the first route, vanillin was successfully coupled by both an oxidative and a reductive route to produce two biaryl structures. These bisphenols were then cyanated and fully characterized. The reductively coupled vanillin structure underwent complete cure and the thermomechanical properties of the polymer were assessed. A glass transition temperature (Tg) of 202°C was observed by TMA (tan δ). The thermal stability of the vanillin derived resins was also evaluated with TGA/IR experiments. Both resins showed good thermal stability up to ca. 310°C and then decomposed with evolution of methyl isocyanate. This novel decomposition pathway results from decomposition of the cyanurate ring in conjunction with the ortho-methoxy groups of the aromatic rings, but does not affect the use temperature of these materials as the decomposition proceeds at a temperature far beyond the glass transition temperature. In the second synthetic route, 3,5-dihydroxybenzoic acid was functionalized with different alkyl groups and several new cyanate esters were prepared. The Tg was found to decrease with increasing alkyl chain length. The Tg of the methyl derivative was determined to be 315°C, while for R = ethylhexyl the Tg dropped to 150°C. Water uptake in these resins was inversely proportional to chain length. The propyl derivative was produced on a multigram scale and glass fiber composite flat panels were successfully fabricated. In the final route, resveratrol was hydrogenated and converted to a tricyanate ester. The resin melts at 120°C and on the basis of DSC data is shown to cure completely.
The highly efficient and environmentally favorable synthetic procedures developed through this research will allow for the use of renewable cyanate esters that address the demanding requirements of the warfighter. In addition to reducing net carbon emissions by utilizing renewable carbon as the feedstock for these materials, use of the methods developed through this program have the potential to greatly reduce solvent use and the overall energy footprint of production and fabrication processes.