THE CHEMISTRY OF NOVOLAC RESINS .5. REACTIONS OF BENZOXAZINE INTERMEDIATES

As part of our study on the curing reactions between novolac resins an d hexamethylenetetramine (HMTA), a model benzoxazine, yl)-6,8-dimethyl -3,4-dihydro-(2H)-1,3-benzoxazine, was heated under carefully controll ed conditions, and the structural changes were studied by C-13 and N-1 5 n.m.r. spectroscopy. The benzoxazine structure is relatively stable, and detectable decomposition only occurred about 185 degrees C with t he formation of methylene linkages between phenolic rings. Various nit rogen-containing structures, such as amides, amines and imines, togeth er with hydroxybenzyl alcohol, bis(ortho-hydroxybenzyl) ether, hydroxy benzaldehyde and ortho-hydroxybenzoic acid, are also formed as the sid e-products of the decomposition. The dominant product after heating th e sample to 240 degrees C is 2,2'-methylene-4,4',6,6'-tetramethyldiphe nol. These findings when applied to novolac/HMTA systems provide an ex planation for reaction mechanisms/reaction pathways from the benzoxazi ne intermediates to the final cross-linking network. (C) 1997 Elsevier Science Ltd.