Abstract
As part of our study on the curing reactions between novolac resins and hexamethylenetetramine (HMTA), a model benzoxazine, 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine, was heated under carefully controlled conditions, and the structural changes were studied by 13C and 15N n.m.r. spectroscopy. The benzoxazine structure is relatively stable, and detectable decomposition only occurred about 185°C with the formation of methylene linkages between phenolic rings. Various nitrogen-containing structures, such as amides, amines and imines, together with hydroxybenzyl alcohol, bis(ortho-hydroxybenzyl) ether, hydroxybenzaldehyde and ortho-hydroxybenzoic acid, are also formed as the side-products of the decomposition. The dominant product after heating the sample to 240°C is 2,2′-methylene-4,4′,6,6′-tetramethyldiphenol. These findings when applied to novolac/HMTA systems provide an explanation for reaction mechanisms/reaction pathways from the benzoxazine intermediates to the final cross-linking network. © 1997 Elsevier Science Ltd.
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