Chemistry of Novolac/Furfuryl Alcohol Resins Cured with Hexamethylenetetramine: A Solid-State NMR Study

The chemical processes involved in the reactions between novolac resins, furfuryl alcohol (FA), and hexamethylenetetramine (HMTA) have been studied by solid-state NMR spectroscopy. A highly cross-linked homogeneous network which incorporates both novolac and furan entities is formed. The initial reaction below 90 °C is between the novolac and HMTA and produces mainly benzoxazine and benzylamine intermediates. When the temperature is increased to 90−120 °C, FA reacts with these initially formed intermediates to generate methylene-linked novolac/furan structures as the major products. Minor amounts of nitrogen-containing structures are also formed. In the absence of HMTA, only a minor amount of cross-reactions occur between novolac and FA, and cross-linked structures are not formed. The curing reactivity and chemical structures of the finally cured materials can be controlled through variation of the HMTA ratio, FA content, and curing conditions.