A novel phosphorus/nitrogen-containing polycarboxylic acid endowing epoxy resin with excellent flame retardance and mechanical properties

Abstract

To meet the requirements of application of epoxy resin (EP) in some special fields, the flame retardance must be improved without losing its mechanical properties. To achieve this purpose, a novel phosphorus/nitrogen-containing polycarboxylic acid (TMD) was synthesized successfully via a facile monoesterification and addition reaction between 1,3,5-tris(2-hydroxyethyl)isocyanurate (THEIC), maleic anhydride (MAH) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and used as reactive flame retardant for anhydride-cured epoxy system. The structure of TMD was confirmed by Fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR). Thermogravimetric analysis (TGA) indicated that after the introduction of TMD, cured EP maintained good thermal stability owing to the cross-linking reaction of TMD and EP. Besides, with the addition of 26.0 wt% TMD (2.0 wt% phosphorous loading), the limited oxygen index (LOI) value of cured EP increased from 20.1% of pure EP to 32.8%, and vertical burning (UL-94) V-0 rating was achieved. Compared with pure EP, its peak heat release rate (PHRR) sharply decreased by 59.3% in the cone calorimeter (CC) test. All those results suggested that TMD endowed EP with excellent flame retardance. The investigations on char residue of cured EP and pyrolysis process of TMD further revealed that TMD exerted bi-phase flame-retardant effects. Additionally, because of the formation of heterogeneous network and hydrogen bonds, cured EP possessed good mechanical properties including tensile modulus and strength as well as flexural modulus and strength. Meantime, it presented great transparency due to the excellent compatibility of TMD with EP.