Synthesis, Characterization, and Cure Chemistry of Renewable Bis(cyanate) Esters Derived from 2‑Methoxy-4-Methylphenol

A series of renewable bis­(cyanate) esters have been prepared
from
bisphenols synthesized by condensation of 2-methoxy-4-methylphenol
(creosol) with formaldehyde, acetaldehyde, and propionaldehyde. The
cyanate esters have been fully characterized by infrared spectroscopy, 1H and 13C NMR spectroscopy, and single crystal
X-ray diffraction. These compounds melt from 88 to 143 °C, while
cured resins have glass transition temperatures from 219 to 248 °C,
water uptake (96 h, 85 °C immersion) in the range of 2.05–3.21%,
and wet glass transition temperatures from 174 to 193 °C. These
properties suggest that creosol-derived cyanate esters may be useful
for a wide variety of military and commercial applications. The cure
chemistry of the cyanate esters has been studied with FTIR spectroscopy
and differential scanning calorimetry. The results show that cyanate
esters with more sterically demanding bridging groups cure more slowly,
but also more completely than those with a bridging methylene group.
In addition to the structural differences, the purity of the cyanate
esters has a significant effect on both the cure chemistry and final Tg of the materials. In some cases, post-cure
of the resins at 350 °C resulted in significant decomposition
and off-gassing, but cure protocols that terminated at 250–300
°C generated void-free resin pucks without degradation. Thermogravimetric
analysis revealed that cured resins were stable up to 400 °C
and then rapidly degraded. TGA/FTIR and mass spectrometry results
showed that the resins decomposed to phenols, isocyanic acid, and
secondary decomposition products, including CO2. Char yields
of cured resins under N2 ranged from 27 to 35%, while char
yields in air ranged from 8 to 11%. These data suggest that resins
of this type may potentially be recycled to parent phenols, creosol,
and other alkylated creosols by pyrolysis in the presence of excess
water vapor. The ability to synthesize these high temperature resins
from a phenol (creosol) that can be derived from lignin, coupled with
the potential to recycle the composites, provides a possible route
to the production of sustainable, high-performance, thermosetting
resins with reduced environmental impact.