Abstract
Cyanate ester resins are essential matrix materials for advanced composites demanding excellent mechanical properties and dimensional stability under varying hygrothermal and radiative environments. However, a critical processing complication arises due to moisture sensitivity during cure which compromizes mechanical and thermal properties. Therefore, a reliable evaluation technique is necessary to ensure the successful use of cyanate ester-based composites. In this report, quality control (QC) signatures of a cyanate ester blend (EX-1515) are evaluated in comparison to those of an unmodified single-component cyanate ester resin (RS3C). Varying degrees of EX-1515 hydrolysis, i.e., carbamate formation, were investigated via FTIR and correlated to decreases in glass transition temperature by DSC and changes in the thermal decomposition profile by TGA. Also flatwise tension (FWT) strengths were observed to decrease with increasing hydrolysis. These results on as-cured EX-1515 were analogous to what was previously reported on RS3C. However, with elevated thermal post-treatments, their thermomechanical properties displayed an opposite trend; While hydrolyzed RS3C exhibited even further progressive reductions in Tg with a post-cure, EX-1515 showed significant increases, likely due to a new polymerization reaction involving carbamates and additional chemical moieties in the blend formulation. Interestingly, the post-cure Tg increase did not translate to any improvement in mechanical properties. On the contrary, the strength showed a dramatic reduction due to thermal degradation of the hydrolysis by-products resulting in increased porosity. The QC signatures of EX-1515 proved to be more complex than RS3C. Therefore, it is necessary to consider multiple QC signatures with carefully chosen thermal treatments rather than relying on a single point Tg measurement without the full thermal history information of the cyanate ester material.
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