Epoxy resins. II. The preparation, characterization, and curing of epoxy resins and their copolymers

A polymer with high aromatic ring content in the chain backbone usually has high heat and flame resistance. Three diglycidyl ethers of epoxy resins were prepared from bisphenol A (DGEBA), phenolphthalein (DGEPP), and 9,9‐bis(4‐hydroxyphenyl)fluorene (DGEBF) in a study of the relation between the cured polymer structure and properties. The epoxy resin prepared from phenolphthalein was separated by liquid chromatography and three fractions were obtained. The fractions had a basic structure of 3,3‐disubstituted phthalide and differed only in molecular weight. The DGEPP resin changed color from yellow to red after mixing with trimethoxyboroxine (TMB), the curing agent, and to orange after completing the curing cycle. To prepare a highly crosslinked material with good thermal stability, TMB with three active Lewis sites in a molecule was used as the curing agent. The reactivity of the three different resins toward TMB, measured by differential scanning calorimetry (DSC), was DGEBA > DGEBF > DGEPP. For the same curing conditions the order of crosslink density was DGEBA > DGEPP > DGEBF. To modify the flammability of DGEBA, the conventional epoxy resin, it was copolymerized with DGEPP and DGEBF, the higher‐performance epoxy resins. The glass transition temperatures of poly(DGEBA‐co‐DGEPP) and poly(DGEBA‐co‐DGEBF) systems deviated from this relationship. The DGEBF copolymers showed an increased char residue (40 wt % at 700°C) at 20 mole % of DGEBF. This deviation may be due to the lower crosslinking density of this system.