An approach to develop high-Tg epoxy resins for halogen-free copper clad laminates

A series of advanced epoxy resins (2) were prepared by the nucleophilic addition of cresol novolac epoxy (CNE) with a benzoxazine monomer (1), which is prepared by a one-pot procedure using 2-hydroxybenzaldehyde, 4-aminophenol and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) as starting materials. The nucleophilic addition was monitored by epoxy equivalent weight titration and NMR analysis. Based on this approach, the overall number of functionality of the resulting epoxy resins was retained because a curable benzoxazine linkage was also incorporated after the nucleophilic addition. As a result, high-Tg thermosets qualified for the UL-94 V-0 rating can be achieved after curing. When an UL-94 V-0 rating was achieved, Tg as high as 245 °C (DMA data) was obtained for 4,4′-diaminediphenyl sulfone (DDS)-cured systems. The corresponding phosphorus content for the UL-94 V-0 rating was as low as 0.61 wt%. The flame-retardant nature of oxazine linkage and nitrogen-phosphorus synergistic effect might be responsible for the low phosphorus content required for flame retardancy.